Porphyrin complexes containing coordinated BOB ...

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An international journal of inorganic chemistry www.rsc.org/dalton Downloaded by The University of Auckland Library on 25 October 2011 Published on 15 February 2008 on http://pubs.rsc.org | doi:10.1039/B716189A

Number 12 | 28 March 2008 | Pages 1509–1648

ISSN 1477-9226

PAPER Brothers et al. Porphyrin complexes containing coordinated BOB groups: synthesis, chemical reactivity and the structure of [BOB(tpClpp)]2+

FRONTIER Ravoo Nanofabrication with metal containing dendrimers

PAPER

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Porphyrin complexes containing coordinated BOB groups: synthesis, chemical reactivity and the structure of [BOB(tpClpp)]2+ †‡ Warwick J. Belcher,b Michael C. Hodgson,a Kenji Sumida,a Ana Torvisco,c Karin Ruhlandt-Senge,c D. C. Ware,a P. D. W. Boyda and P. J. Brothers*a

Downloaded by The University of Auckland Library on 25 October 2011 Published on 15 February 2008 on http://pubs.rsc.org | doi:10.1039/B716189A

Received 19th October 2007, Accepted 23rd January 2008 First published as an Advance Article on the web 15th February 2008 DOI: 10.1039/b716189a The reactions of boron halides with free base porphyrins under conditions where partial hydrolysis of the boron halides can occur give diboron porphyrin complexes containing BOB moieties in which each boron is bonded to two porphyrin nitrogen atoms. BF3 ·OEt2 with H2 (por) gives B2 OF2 (por) (por = tpp, ttp, tpClpp, oep) which has an asymmetric structure in which one boron lies in the porphyrin plane (Bip ) while the other lies above it (Boop ). BCl3 ·MeCN with H2 (por) gives B2 O2 (BCl3 )2 (por) which contains a four-membered B2 O2 ring and is stable only in the presence of excess BCl3 . BBr3 with Li2 (tpClpp) gives the dicationic complex [B2 O(tpClpp)]2+ as its [BBr4 ]− salt, and is the first example of a boron porphyrin containing three-coordinate boron to be structurally characterised. B2 O2 (BCl3 )2 (por) can be chromatographed on basic alumina to give the hydroxyboron complex B2 O(OH)2 (por), which is deduced from its NMR spectra and DFT calculations to have a structure analogous to B2 OF2 (por). The OH protons are shifted upfield to near d −4 (Boop -OH) and −10 (Bip -OH) by the diamagnetic porphyrin ring current. The reaction of either B2 O2 (BCl3 )2 (por) or B2 O(OH)2 (por) (por = ttp, tpClpp) with alcohols (ROH, R = Et, 4-C6 H4 CH3 ) gives B2 O(OR)2 (por), which can in turn be converted to B2 O(OR)(OH)(por) by repeated chromatography. The reaction of PhBCl2 with H2 (por) (por = ttp, tpClpp) gives B2 O(Ph)(OH)(por) which has been characterised by spectroscopy in concert with DFT calculations. It is a further example of the B2 OF2 (por) structural type, in which the phenyl group is coordinated to the out-of-plane boron and the OH group to the in-plane boron, as are its derivatives B2 O(Ph)(X)(tpClpp) (X = F, OEt). Steric drivers for the facile hydrolysis of haloboron porphyrins relative to their dipyrromethene and expanded porphyrin counterparts are discussed.

Introduction Over the last decade we have demonstrated that the tetrapyrrole porphyrin macrocycle is an effective ligand for the small main group element boron, despite the mismatch in size and preferred coordination geometry.1 Coordination of boron to related ligands containing three (subporphyrins, subpyriporphyrin, subazaporphyrin), four (N-confused porphyrin) or more (expanded porphyrins) moieties has also been established.1c A number of different structural types have been demonstrated for boron porphyrin complexes, with the common features being the coordination of two boron atoms per porphyrin, in which each boron is coordinated to two porphyrin nitrogen atoms, and a marked tetragonal elongation of the porphyrin in the direction parallel a Department of Chemistry, The University of Auckland, Private Bag, 92019, Auckland, New Zealand. E-mail: [email protected]; Fax: +64 9 373 7422; Tel: +64 9 373 7599 b School of Environmental and Life Sciences, The University of Newcastle, University Drive, Callaghan, NSW, 2308, Australia c Department of Chemistry, Syracuse University, 1-014 Center for Science and Technology, Syracuse, New York, 13244-4100, USA † Dedicated to Professor Kenneth Wade on the occasion of his 75th birthday. ‡ CCDC reference number 664478. For crystallographic data in CIF or other electronic format see DOI: 10.1039/b716189a Electronic supplementary information (ESI) available: representative 1 H NMR and UV-visible spectra. See DOI: 10.1039/b716189a

1602 | Dalton Trans., 2008, 1602–1614

to the B · · · B axis. While sharing these common features, a surprising range of different structural types have been elucidated and their molecular structures have been established either by X-ray crystallography or by DFT optimisations, each in concert with spectroscopic studies.2–6 The two boron atoms can be threeor four-coordinate and can lie both in the porphyrin N4 plane, one in-plane and one out-of-plane, both out-of-plane but displaced to opposite faces of the porphyrin, and both out-of-plane on the same face of the porphyrin. Depending on the arrangement of boron atoms, the porphyrin undergoes a complementary out-of-plane distortion.1 These structural types are illustrated in Scheme 1. The first attempts to prepare boron porphyrin complexes were reported in 1965,7 and 19778 and while these indicated boronand oxygen-containing products with a 2 : 1 boron : porphyrin ratio, none were completely characterized. We have subsequently shown that the first-formed products of boron halides (BF3 ·OEt2 or BCl3 ·MeCN) with either H2 (por) or Li2 (por) (por = tpp, ttp, tpClpp, oep)9 are the diboryl complexes (BX2 )2 (por) (X = F, Cl) containing two boryl groups on opposite faces of the porphyrin (Scheme 1, type E).4,6 The benchmarks in boron pyrrole chemistry are the difluoroboron dipyrromethene (dipyrrin) complexes [e.g. BF2 (dpm)] in which the BF2 moiety is complexed to a chelating ligand comprising two pyrrole groups linked through a methine carbon atom.10 Essentially, the diboryl boron porphyrin complexes contain two of these groups constrained in close proximity through This journal is © The Royal Society of Chemistry 2008


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Scheme 1 Structural types observed for boron porphyrin and corrole complexes.

linking of the two dipyrromethene groups to create the porphyrin macrocycle.

This close confinement has some unexpected consequences for both boron and the porphyrin. Unlike the reaction of BF3 ·OEt2 or BCl3 ·MeCN with Li2 (por), described above, the corresponding reaction of the heavier halides BBr3 or BI3 with Li2 (ttp) gives, unexpectedly, the diboranyl porphyrin products of spontaneous reductive coupling, (BX)2 (por) (X = Br, I), which contain a B–B single bond (type F). From DFT computation of thermochemical data and modeling on related boron dipyrromethene complexes we have established that this is an example of sterically induced reduction.6,11 A similar spontaneous reductive coupling reaction is observed when (BCl2 )2 (por) is treated with BuLi, giving the reduced product (BBu)2 (por). The diboranyl complex (BCl)2 (por) can also be prepared directly from H2 (ttp) or Li2 (ttp) with the tetrachlorodiborane B2 Cl4 .4,6 The unexpected consequence for the porphyrin ligand is, firstly, the marked tetragonal in-plane distortion mentioned above. This can be measured by taking the difference between the non-bonded N · · · N distances parallel and perpendicular to the B · · · B axis. ˚ for the boron This value, D(N · · · N), ranges from 0.84 to 1.28 A complexes.1c Porphyrins which are sterically overcrowded on the periphery of the macrocycle have been observed to exhibit this kind of tetragonal, in-plane distortion, but for these the D(N · · · N) ˚ .12 Secondly, when the values are typically of the order of 0.5 A diboranyl complex (BCl)2 (por) is further reduced by the addition of magnesium anthracenide, reduction occurs at the porphyrin ligand rather than the diboron moiety, resulting in isolation of the unusual 20-electron, antiaromatic, isophlorin macrocycle (type C).6 The diboryl and diboranyl complexes are extremely hydolytically sensitive, and must be prepared and handled under rigorThis journal is © The Royal Society of Chemistry 2008

ously anhydrous conditions. Indeed, the first examples of boron porphyrins that we communicated were boron- and oxygencontaining species resulting from partial hydrolysis of the boron halides either during the reaction or workup.2,3,5 These complexes comprised two unusual structural types, B2 OF2 (por) which contains an FBOBF group threaded through the porphyrin cavity (type D), and B2 O2 (BCl3 )2 (por) in which a B2 O2 four-membered ring is inserted into the porphyrin (type A). Our more recent work on the diboryl and diboranyl porphyrins now helps to paint a more complete picture of the interactions of boron with porphyrins, and the links between the boron–halide and boron–oxygen partially hydrolysed porphyrin complexes can be better established. With this broader view, in this article we expand on the chemistry of porphyrin complexes containing coordinated B–O–B groups, with further examples and synthetic details of the first two structural types (types A and D), including the hydroxo-, alkoxo- and aryloxo-boron complexes B2 O(OH)2 (por) and B2 O(OR)2 (por); a new structural type of partial hydrolysis product (type H), [B2 O(por)]2+ , containing three-coordinate trigonal planar boron; and the first instance of the use of a substituted boron halide for insertion of boron into a porphrin, PhBCl2 , which gives the phenyl-substituted complex B2 O(Ph)(OH)(por). The intermediate boron halide porphyrin species are very susceptible to hydrolysis, and water is not added as a reagent to the reactions in which hydrolysis occurs. Either adventitious water in undried solvents, or water on silica and alumina chromatography supports serve as the reagent. We propose a rationale for the facile hydrolysis reactions observed for the boron halide complexes.

Results and discussion B2 OF2 (por) We have previously communicated that the reaction of H2 (por) with excess BF3 ·OEt2 in chlorobenzene at 25 ◦ C (or, for por = oep, in CH2 Cl2 –NEt3 ) followed by chromatography on silica gel, gives B2 OF2 (por) (por = tpp, ttp, tpClpp, oep).2,5 We know from more recent work that these products form from partial hydrolysis during the reaction and workup of the intermediate (BF2 )2 (por) complexes, but one B–F bond is retained at each boron.6 These complexes are readily identified by 1 H NMR in solution because of their C s symmetry which results, for example, in a 2 : 1 : 1 ratio for the resonances of the tolyl methyl protons (por = ttp) or meso protons (por = oep). Two of the complexes, B2 OF2 (tpClpp) and B2 OF2 (oep), were characterised by X-ray crystallography (Fig. 1a).2,5 The molecular structures show that the low symmetry arises because one of the boron atoms is coordinated in the plane of the porphyrin (Bip ) while the other is significantly displaced out

Fig. 1 Molecular structures of (a) B2 OF2 (oep) and (b) B2 O2 (BCl3 )2 (tpClpp). Peripheral ethyl and aryl groups have been removed.

Dalton Trans., 2008, 1602–1614 | 1603

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of the plane (Boop ). As a result, one of the B–F bonds is directed towards the periphery of the porphyrin, while the other is directed over the centre. This low symmetry is also reflected in the 11 B and 19 F NMR spectra which show two resonances for each of the boron and fluorine atoms in the complexes.4 DFT calculations confirm that this asymmetric transoid geometry is more stable than the alternative, higher symmetry cisoid geometry.2,13 Very recent results have shown that the reaction of BF3 ·OEt2 with the related macrocyle meso-triphenylcorrole, H3 (tpc), gives the equivalent product, [B2 OF2 (tpc)]− but with the cisoid geometry (Scheme 1, type G).14

B2 O2 (BX3 )2 (por) The reaction of BCl3 ·MeCN with H2 (por) (por = tpp, ttp, tpClpp, oep) under similar conditions (although at reflux temperature) gave a product in which all the B–Cl bonds have been hydrolysed.3 This complex, B2 O2 (BCl3 )2 (por), was shown by X-ray crystallography to contain a B2 O2 four-membered ring coordinated in the centre of the macrocycle with both boron atoms lying in the porphyrin plane (Fig. 1b), and as a result exhibits higher symmetry in the 1 H NMR, showing two peaks in a 1 : 1 ratio for the tolyl methyl protons (por = ttp) or meso protons (por = oep). This complex is stable in solution only in the presence of excess BCl3 , and can be crystallised and isolated only under these conditions. The fact that the oxygen atoms of the B2 O2 ring act as donors towards BCl3 acceptors is critical to the stability of the complex. In the absence of excess BCl3 the complex quickly loses boron to form the porphyrin acid salt [H4 (por)]2+ . The corresponding complex containing BBr3 , B2 O2 (BBr3 )2 (tpClpp), can be made either from BBr3 with H2 (tpClpp), or by exchange of the boron halide acceptor, by treating B2 O2 (BCl3 )2 (tpClpp) with BBr3 . Very few structures of complexes containing B2 O2 fourmembered rings have been reported.15,16 [B2 O2 F4 (BF3 )2 ]2− , the most closely related example to the porphyrin complex, contains the same central motif, a B2 O2 ring with BF3 acceptors bonded to the oxygen atoms, but with four F− ligands in place of the porphyrin ligand. A computational study on this molecule indicated that the small size of the B2 O2 ring allows strongly anti-bonding cross-ring O · · · O interactions which destabilise the ring. The BF3 acceptors mediate this interaction and help to stabilise the ring.15 This is in agreement with our own observation that the porphyrin complex is unstable in the absence of excess boron halide.

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We have established that the diboryl complex (BCl2 )2 (ttp) undergoes hydrolysis with two equivalents of water to give B2 O2 (BCl3 )2 (ttp), according to the balanced equation shown in eqn (1),

(1)

suggesting that reaction of the boron halide with the porphyrin may occur prior to the hydrolysis step.6 As mentioned above, B2 O2 (BBr3 )2 (tpClpp) results from the reaction of BBr3 with H2 (tpClpp) under conditions where partial hydrolysis can occur (chlorobenzene with a trace of water). If the reaction of BBr3 with Li2 (ttp) is undertaken in toluene in rigorously dry conditions, then a mixture of the diboryl complex (BBr2 )2 (ttp) and the reduced diboranyl product (BBr)2 (ttp) occurs.6 One attempt to crystallise the products of this reaction resulted, surprisingly, in a novel hydrolysis product illustrating a further kind of porphyrin complex containing a BOB bond. The solvent was removed from the reaction mixture and the product dissolved in toluene-d 8 in order to measure its 1 H NMR spectrum. The sample was then cooled at −20 ◦ C in a plastic-capped NMR tube for several days, and the resulting crystals were determined by X-ray crystallography to contain the cation [B2 O(tpClpp)]2+ as the BBr4 − salt, giving the overall formulation [B2 O(tpClpp)][BBr4 ]2 . A plausible equation is given in eqn (2),

(2)

with the result that the charged complex crystallised from toluene and the neutral free base porphyrin remained in solution. All attempts to reproduce the reaction by addition of just one equivalent (or less) of water to the reaction of BBr3 with the porphyrin were unsuccessful, and resulted in over-hydrolysis. The conditions of slow diffusion of trace amounts of water into the toluene solution must have been optimal for formation of this partially hydrolysed complex. As a result, we were unable to obtain analytical or spectroscopic data for this extremely hydrolytically sensitive complex, and it was characterised by Xray crystallography and DFT calculations. The dication [B2 O(tpClpp)]2+ shows a number of unusual structural features. The other two types of boron porphyrins containing BOB groups have boron atoms either one in-plane, one out-of-plane as in B2 OF2 (por) (Scheme 1, type D), or both boron atoms in-plane as in B2 O2 (BCl3 )2 (por) (type A). The new dication has both boron atoms out-of-plane and displaced to the same face of the porphyrin (type H). As a result the porphyrin is markedly folded to accommodate this arrangement of boron atoms. The [B2 O(tpClpp)]2+ cation is the first structurally characterised example of a boron porphyrin to contain trigonal planar, three-coordinate boron. The reduced species [B2 (ttp)]2+ and B2 (ttp) also contain three-coordinate boron but their structures This journal is © The Royal Society of Chemistry 2008

View Online Table 1 Metrical data observed for the dication [B2 O(tpClpp)]2+ and calculated for the model [B2 O(tpp)]2+ [B2 O(tpClpp)]2+


Molecule Aa (exp) ˚ Bond lengths/A B(1)–O(1) 1.312(14) B(2)–O(1) 1.340(14) ◦ Bond angle/ B(1)–O(1)–B(2) 111.5(9) ˚ Tetragonal elongation/A N · · · N distances 2.394, 3.399 D(N · · · N) 1.005 ˚ Average displacements from mean 24-atom plane/A Boron +0.602 Oxygen +1.337 Nitrogen +0.162 meso-Carbon +0.481, −0.448 a

[B2 O(tpp)]2+ Molecule Ba (exp)

(calculated)b

1.351(13) 1.307(12)

1.355 1.355

112.7(8)

111.9

2.428, 3.420 0.992

2.441, 3.492 1.051

+0.596 +1.341 +0.165 +0.488, −0.428

+0.578 +1.337 +0.154 +0.454, −0.424

Two independent molecules in the unit cell. b B3LYP/6-311G(d,p).

have been determined by DFT optimisations rather than X-ray crystallography.6 The crystallographic asymmetric unit contain two independent [B2 O(tpClpp)]2+ dications, four [BBr3 ]− anions and two molecules of toluene. Very good agreement is obtained between the metrical data measured by X-ray crystallography and calculated by DFT for the model cation [B2 O(tpp)]2+ (Table 1). Fig. 2a shows the molecular structure of the [B2 O(tpClpp)]2+ cation determined by crystallography, while Fig. 2b shows a superposition of the [B2 O(tpClpp)]2+ and calculated [B2 O(tpp)]2+ cations, illustrating the excellent agreement.

A comparison of the molecular structure of [B2 O(tpClpp)]2+ with those of the other two types of complexes containing BOB bonds, B2 OF2 (oep) and B2 O2 (BCl3 )2 (tpClpp) shows that the B–O and B–N bonds in [B2 O(tpClpp)]2+ are shorter than in the other two complexes, consistent with the presence of three-coordinate boron.17 The tetragonal elongation, measured ˚ , less than observed for the by the D(N · · · N) value, is 1.00 A ˚; related complexes (D(N · · · N) values: B2 OF2 (tpClpp) 1.06 A ˚ ; B2 O2 (BCl3 )2 (tpClpp) 1.27 A ˚ ), indicating that B2 OF2 (oep) 1.14 A the porphyrin is less sterically strained when not accommodating one or two borons in the plane. The BOB angle is intermediate (112◦ ), wider than that in the tightly constrained B2 O2 ring in B2 O2 (BCl3 )2 (tpClpp) (88.5◦ ) but narrower than that in the more strained B2 OF2 (oep) complex (116◦ ). The NBN bond angles of [B2 O(tpClpp)]2+ and B2 O2 (BCl3 )2 (tpClpp), are similar, close to 116◦ , whereas B2 OF2 (oep) shows quite different NBN angles (110◦ (av) for the in-plane boron, Bip , 103.5◦ for the out-ofplane boron, Boop ). B2 O2 (BCl3 )2 (tpClpp) has a highly planar porphyrin, in B2 OF2 (oep) the two pyrrole rings coordinated to the in-plane boron are approximately coplanar but the two pyrrole rings bonded to the out-of-plane boron are tilted, while the dication [B2 O(tpClpp)]2+ , with both boron atoms out-of-plane, has the porphyrin folded approximately along the meso–meso axis perpendicular to the B–O–B axis, and can be viewed as two planar dipyrromethene fragments with a 33.4◦ angle between the planes. This folding can also be seen in the displacement of each of the core 24 atoms from the mean 24-atom plane (Fig. 3). B2 O(OH)2 (por)

Fig. 2 (a) ORTEP representation of the dication [B2 O(tpClpp)]2+ , and (b) superposition of the molecular structure of [B2 O(tpClpp)]2+ determined by crystallography and the optimised (B3LYP/6-311G(d,p)) structure of [B2 O(tpp)]2+ (peripheral aryl rings removed for clarity).

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When B2 O2 (BCl3 )2 (por) is dissolved in dichloromethane and chromatographed on basic alumina, the hydroxyboron complex B2 O(OH)2 (por) (por = tpp, ttp, tpClpp, oep) is formed. This complex is an analog of the fluoroboron complex B2 OF2 (por), but with OH groups in place of F, and exhibits the same low symmetry. Chromatography on basic alumina is required because the hydrolysis reaction produces HCl, which leads to demetallation of the porphyrin unless a basic medium is present. Addition of BCl3 to B2 O(OH)2 (por) reforms the precursor. Treatment of B2 O2 (BCl3 )2 (por) with a fluoride source (BF3 ·OEt2 or nBu4 NF) Dalton Trans., 2008, 1602–1614 | 1605


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˚ ) from the mean 24-atom plane measured Fig. 3 Atomic displacements (A for the dication [B2 O(tpClpp)]2+ (for one of the two independent molecules in the unit cell). Filled circles (䊉) lie above the mean plane, open circles (䊊) lie below the plane.

produces B2 OF2 (por). However, the reverse reaction cannot be accomplished, indicating that one B–F bond remains resistant to hydrolysis. The hydroxyboron complex is most conveniently prepared by simply undertaking the reaction of BCl3 ·MeCN with H2 (por) in chlorobenzene, removing the solvent, redissolving in CH2 Cl2 and chromatography on basic alumina, without actually isolating the B2 O2 (BCl3 )2 (por) intermediate. The structures of the model complexes B2 O(OH)2 (tpp) and B2 O(OH)2 (omp)9 were calculated, and two views of B2 O(OH)2 (omp) are given in Fig. 4. The 1 H NMR spectrum of B2 O(OH)2 (por) in very dry CDCl3 allows one or both OH protons to be observed, shifted upfield to d −3.70 and −9.05 (for por = tpp) and d −4.26 and −10.15 (for por = oep) by the porphyrin diamagnetic ring current effect, which causes significant upfield shifts for the resonances of atoms above and below the porphyrin plane. The more upfield resonances at d

Fig. 4 Views of the structure calculated (B3LYP/6-311G(d,p)) for B2 O(OH)2 (omp).

1606 | Dalton Trans., 2008, 1602–1614

−9.05 and −10.15 are assigned to the OH proton attached to the in-plane boron, which lies closer to the centre of the ring. DFT calculations of the chemical shifts for the OH protons confirm this assignment. Chemical shifts calculated for B2 O(OH)2 (tpp) are d −5.09 (Boop OH) and −10.35 (Bip OH) and for B2 O(OH)2 (omp) are d −5.47 (Boop OH) and −11.35 (Bip OH). These are consistent with the observed data in that the more electron rich oep (or omp) complexes show greater upfield shifts than does the tpp complex, and the chemical shift difference (Dd) between the Boop OH and Bip OH resonances compares very closely (for por = tpp, Dd 5.35 (obs.) versus 5.26 (calc.); for por = oep, Dd 5.89 (obs.) versus 5.29 (calc. for por = omp)). The calculated gas phase structures also show a hydrogen bond between Boop OH and the BOB oxygen ˚ for tpp and 2.33 A ˚ for omp), and a (Boop OH · · · O distance 2.35 A further interaction between the Bip OH and the porphyrin nitrogen atoms. B2 O(OR)2 (por) The reaction of B2 O2 (BCl3 )2 (por) with water can be extended to alcohols, and treatment of this complex with sodium ethoxide or sodium p-cresolate produces the corresponding alkoxo and aryloxo complexes B2 O(OEt)2 (por) and B2 O(OC6 H4 CH3 )2 (por) (por = ttp, tpClpp). The 1 H NMR spectra show that these have the same symmetry as B2 O(OH)2 (por) and B2 OF2 (por), and in particular two sets of resonances are seen for the alkoxo (or aryloxo) groups, corresponding to the two chemically different groups attached to the in-plane and out-of-plane boron atoms. The porphyrin diamagnetic ring current shifts the resonances upfield of their normal diamagnetic positions. Like the OH protons in B2 O(OH)2 (por), the more upfield set of resonances is assigned to the OR group bonded to the in-plane boron as it lies more nearly over the porphyrin centroid. For example, in the 1 H NMR spectrum of B2 O(OC6 H4 CH3 )2 (ttp), the resonances of the two p-cresolate groups occur as two doublets for the Hmeta protons (d 5.04, 6.11), two doublets for Hortho (d 0.74, 3.93) and two singlets for the CH3 protons (d 1.27, 1.92) (spectra are given in the ESI‡). The alkoxo or aryloxo group attached to the in-plane boron is more labile than that bonded to the out-of-plane boron, presumably because it occupies a more sterically hindered position, and repeated chromatography of B2 O(OR)2 (por) on basic alumina results in hydrolysis of the in-plane Bip –OR group to give B2 O(OR)(OH)(por). In the 1 H NMR spectrum the more upfield set of resonances for the OR group disappears and is replaced by a resonance for the OH proton near d −9. The 1 H NMR spectrum of B2 O(OC6 H4 CH3 )(OH)(ttp) shows OC6 H4 CH3 peaks at d 6.06 (doublet, Hmeta ), 3.78 (doublet, Hortho ) and 1.88 (singlet, CH3 ) and the OH proton at −8.72. Similar results are observed for the conversion of B2 O(OEt)2 (por) to B2 O(OEt)(OH)(por) (por = ttp, tpClpp). A more direct route to the B2 O(OR)2 (por) complexes involves the reaction of BCl3 ·MeCN with H2 (por) in refluxing chlorobenzene for 6 h, followed by addition of alcohol and stirring for a further 6 h at room temperature, then chromatography on basic alumina. The reaction is quite general for a variety of alkyl and aryl alcohols (methanol, isopropanol, 1,2-ethanediol, phenol, 1,4-tBuC6 H4 OH, 1,2-NH2 C6 H4 OH and 1,2-C6 H4 (OH)2 ) but pure products could only be isolated for ethanol and 1,4CH3 C6 H4 OH. For the reactions with the other alcohols it This journal is © The Royal Society of Chemistry 2008

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proved difficult to cleanly separate mixtures of B2 O(OR)2 (por), B2 O(OR)(OH)(por) and H2 (por), although 1 H NMR evidence could be obtained for the formation of each species. The routes for interconversion of B2 O2 (BCl3 )2 (por) and B2 OX2 (por) (X = F, OH, OR) are summarised in Scheme 2.

Fig. 5 1 H NMR spectrum of B2 O(Ph)(OH)(ttp) (OH proton at d −9.22 not shown).

Scheme 2 Routes for the interconversions of B2 O2 (BCl3 )2 (por) and B2 OX2 (por) (X = F, OH, OR).

B2 O(Ph)(OH)(por) All of the above reactions utilise the boron trihalides for insertion into the porphyrin macrocycle. A variety of other boron reagents were tried, using either Li2 (por) or H2 (por) as the porphyrin precursor. The reaction of PhBCl2 with Li2 (por) under strictly anhydrous conditions did not give a tractable reaction product. This is perhaps understandable, as the expected diboryl product, (BPhCl)2 (por) would be severely sterically crowded. The dichlorodiboryl species (BCl2 )2 (por) is at the limit of what the porphyrin can accommodate (this complex shows the largest D(N · · · N) value of all the boron porphyrins6 ) and replacing one chlorine by a phenyl group appears not to be possible. On the other hand, the reaction of PhBCl2 with H2 (por) under conditions where hydrolysis could occur gave a new product after workup by chromatography on basic alumina, B2 O(Ph)(OH)(por) (por = ttp, tpClpp). The 1 H NMR spectrum of B2 O(Ph)(OH)(ttp) showed the familiar pattern of a 2 : 1 : 1 ratio for the tolyl methyl peaks on the periphery of the porphyin, indicating C s symmetry, and the complex is a further example of the structural type B2 OX2 (por), so far observed for X = F, OH and OR (type D). The phenyl protons appeared at d 6.16 (Hpara ), 6.01 (Hmeta ) and 3.73 (Hortho ) in the 1 H NMR spectrum, well upfield of their usual positions, evidence that the phenyl group is bonded to Boop and is positioned above the plane of the porphyrin. The OH proton resonated at d −9.22 indicating that the OH is attached to the in-plane boron atom Bip (Fig. 5). In related chemistry, PhBCl2 has recently been shown to react with N-confused porphyrin (a porphyrin isomer in which one of the pyrrole rings is inverted) and N-fused porphyrin (a porphyrin contracted through fusing of two adjacent pyrroles across a meso carbon) to give products each containing a single phenylboron This journal is © The Royal Society of Chemistry 2008

group in which the boron coordinates to two or three nitrogens, respectively.18 The 1 H NMR spectra of these two complexes show upfield shifts for the phenyl resonances similar to those observed for B2 O(Ph)(OH)(por). DFT calculations on the model complex B2 O(Ph)(OH)(porphine) indicated that this structure is an energy minimum, ˚ below and Boop and the transoid arrangement, with Bip 0.06 A ˚ above the mean 24-atom plane, was calculated to be 1.21 A 19.4 kJ mol−1 more stable than the cisoid isomer. The optimised structures of both isomers are shown in Fig. 6. Although the 1 H NMR spectrum indicates that the complex has C s symmetry in solution, the calculated transoid structure has lower symmetry due to twisting of the phenyl group. Rapid rotation of the phenyl group on the NMR time scale presumably occurs in solution. The bond lengths and angles show a similar pattern to those observed for ˚ ) significantly B2 OF2 (oep), with the in-plane Bip –O bond (1.421 A ˚ longer than the out-of-plane Boop –O (1.379 A) bond, and the ˚) reverse for the B–N bonds, with the Bip –N(av) bond (1.557 A ˚ ). shorter than the Boop –N(av) bond (1.611 A

Fig. 6 (a) Side and top views of the optimised model of B2 O(Ph)(OH)(porphine), with all hydrogen atoms except the OH proton omitted for clarity; (b) the cis isomer of B2 OPh(OH)(por), with all hydrogen atoms except the OH proton omitted for clarity (B3LYP/6-31G(d)).

Some partial B–N bond delocalisation within the planar Bip NCCCN chelate ring may be the cause of this. All of the Dalton Trans., 2008, 1602–1614 | 1607

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B–N distances calculated for B2 O(Ph)(OH)(porphine) are longer than those observed for B2 OF2 (oep), although the difference is more marked for the Boop –N bonds, which may result from the increased steric crowding caused by the phenyl substituent on this boron atom. Fig. 7 gives the displacements of the core porphyrin atoms from the mean 24-atom plane. It is noticeable that the displacements are much larger for the two pyrrole groups and meso carbon associated with the out-of-plane boron, indicating that this half of the molecule is more distorted. The same effect was noted for B2 OF2 (oep).5

˚ ) from the mean 24-atom plane calculated Fig. 7 Atomic displacements (A for B2 O(Ph)(OH)(porphine). Filled circles (䊉) lie above the mean plane, open circles (䊊) lie below the plane. Non-highlighted atoms are displaced ˚ from the mean plane. less than 0.03 A

The hydroxo group in B2 O(Ph)(OH)(tpClpp) is labile, and further derivatives B2 O(Ph)(F)(tpClpp) and B2 O(Ph)(OEt)(tpClpp) were prepared, the first by reaction of B2 O(Ph)(OH)(tpClpp) with BF3 ·OEt2 in THF at room temperature, and the second by heating B2 O(Ph)(OH)(tpClpp) at reflux temperature in EtOH. The 1 H NMR resonances of the ethyl groups in the three complexes B2 O(OEt)2 (tpClpp), B2 O(OEt)(OH)(tpClpp) and B2 O(Ph)(OEt)(tpClpp) are consistent, with the Bip OEt resonances shifted further upfield than those arising from Boop OEt. B2 O(OEt)2 (tpClpp) shows peaks at d −0.19 (q) and −0.55 (t) for the Boop OEt group, and at −3.33 (m) corresponding to Bip OEt. B2 O(OEt)(OH)(tpClpp) which has only a Boop OEt group has resonances at d −0.31 (q) and −0.64 (t), while the Bip OEt peaks in B2 O(Ph)(OEt)(tpClpp) appear at d −3.10 (q) and −3.23 (t). The same effect can be seen in the 19 F NMR of B2 O(Ph)(F)(tpClpp) in which the Bip –F resonance occurs at d −167.85. This compares with B2 OF2 (tpClpp) for which Bip F and Boop F are observed at d −167.90 and −146.56, respectively.2 Hydrolysis reactions Hydrolysis reactions of boron halide porphyrin intermediates play a large role in the formation of the experimentally observed BOB porphyrin complexes discussed in this article. A number of compounds on the hydrolysis pathway have been identified, [B2 O(por)]2+ , B2 O2 (BX3 )2 (por), B2 OX2 (por) and B2 O(OH)2 (por), but the mechanism and energetics of these reactions are not well understood. For example, in the formation of B2 O(Ph)(OH)(por) from PhBCl2 one B–C(phenyl) bond is hydrolysed but the other remains intact. A possibility for this reaction is that an intermediate [B2 O(Ph)2 (por)] complex forms, but is not observed, which 1608 | Dalton Trans., 2008, 1602–1614

then undergoes hydrolysis. In order to test this idea, the energy of this putative hydrolysis reaction (eqn (3))was probed using DFT

(3)

calculations. Optimised models were computed for each of the species in eqn (3) using porphine as the ligand, and the energy (DE), enthalpy (DH) and free energy (DG) calculated by taking the difference between the products and reactants in eqn (3). The values of DE and DH of the hydrolysis reaction were found to be −177.3 and −172.7 kJ mol−1 , respectively, indicating that the hydrolysis reaction is exothermic in nature, and is energetically favoured. The free energy DG at 298 K was found to be −186.0 kJ mol−1 , showing that the reaction is also spontaneous at room temperature. This result affords a partial explanation as to why the postulated intermediate [B2 OPh2 (por)] is not observed experimentally under conditions in which further hydrolysis can occur. The hydrolysis reaction results in the replacement of a relatively low-energy B–C bond (bond strength 372 kJ mol−1 ) in [B2 OPh2 (por)] with a much stronger B–O bond (bond strength 536 kJ mol−1 ) in B2 OPh(OH)(por), together with the favourable formation of a benzene molecule. However, the phenyl substituent on the out-of-plane boron atom is retained, suggesting that favourable thermodynamics is not the sole driving force of this reaction. Hydrolysis of the in-plane B–C(phenyl) bond may be favoured by the steric crowding in this position in [B2 OPh2 (por)] resulting from the close proximity of the phenyl substituent to the porphyrin ring. The steric effects are less significant for the out-of-plane boron. The hydrolysis of the phenyl group at the inplane boron atom may be due to a combination of the exothermic and spontaneous nature of the B–C bond hydrolysis, and the sterically-compromised environment of the in-plane boron atom in [B2 OPh2 (por)]. The formation of B2 OPh(OH)(por) in which one B–C bond is hydrolysed while the other is retained can be contrasted with the chemistry observed for the boron porphyrins synthesised from BF3 ·OEt2 and BCl3 ·MeCN. The former reagent gives the B2 OF2 (por) compound, in which one B–F bond is retained on each boron atom, while BCl3 ·MeCN gives, after chromatography, B2 O(OH)2 (por) in which all the B–Cl bonds are hydrolysed. DFT calculations were performed on the two reactions shown in eqn (4).The hydrolysis of B2 OF2 (por) to B2 O(OH)2 (por) was

(4)

found to have a computed DE value of +169.9 kJ mol−1 while the corresponding reaction of the [B2 OCl2 (por)] compound (not observed experimentally) to B2 O(OH)2 (por) had a calculated DE of −25.4 kJ mol−1 . This is consistent with the experimental observations that the B–F bonds in B2 OF2 (por) are not hydrolysed, whereas the analogous compound [B2 OCl2 (por)] would not be expected to be isolable if water is present. This difference is most likely driven by the strength of the B–F bonds. This journal is © The Royal Society of Chemistry 2008

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Experimental section


General All solvents were dried and distilled prior to use, except for chlorobenzene which was used as received. Reactions were performed in a nitrogen atmosphere using Schlenk techniques. Subsequent purification and isolation procedures were carried out in air. Where relevant, inert atmosphere manipulations were carried out in a Vacuum Atmospheres dry box under nitrogen. The removal of solvents was carried out at reduced pressure, using a rotary evaporator. Solid samples were dried under vacuum, over P2 O5 , at RT. UV-visible absorption spectra were recorded on a Shimadzu UV-2101PC scanning spectrophotometer with dichloromethane as solvent, except where noted. 1 H, 11 B, 13 C and 19 F NMR spectra were recorded on a Bruker AM 400 spectrometer, operating at 400, 128, 101 and 376 MHz, respectively. Tetramethylsilane (Me4 Si) was used as an internal reference for 1 H and 13 C spectra. BF3 ·OEt2 and CFC13 were used as internal references for the 11 B and 19 F NMR spectra, respectively. Mass spectra were recorded on a VG 70-SE mass spectrometer using the fast atom bombardment (FAB) technique. Microanalyses were carried out at the University of Otago. H2 (oep) and H2 (tpp) were prepared as described in the literature.19,20 H2 (ttp) and H2 (tpClpp) were prepared by a procedure analogous to that used for H2 (tpp). The syntheses of B2 OF2 (ttp), B2 O2 (BCl3 )2 (ttp) and B2 O2 (BCl3 )2 (tpClpp) have been previously communicated.2,3 Syntheses B2 OF2 (ttp). Method (a). BF3 ·OEt2 (4 mL, 32.5 mmol) was added to a solution of H2 (ttp) (2.00 g, 2.99 mmol) in chlorobenzene (25 mL). Instantly, a bright green precipitate, which was identified as [H4 (ttp)]2+ by UV-visible spectroscopy, formed. The reaction mixture was stirred for 12 h under a positive flow of nitrogen. During this period the bright green suspension redissolved to give a blue-green solution. The solvent was removed and the solid was redissolved in CH2 C12 and chromatographed on silica gel. Elution with CH2 C12 removed a pink band containing H2 (ttp). Elution with CH2 C12 –tetrahydrofuran (9 : 1) then removed a green band of the desired product. Repeated chromatography was required to obtain a pure product. The solution was evaporated to dryness and the solid recrystallised from CH2 C12 –hexane to give the product as a purple powder (0.41 g, 18%). Method (b). BF3 ·OEt2 (1 mL, 8.13 mmol) was added to a solution of B2 O(OH)2 (ttp) (0.10 g, 0.14 mmol) in chlorobenzene (15 mL). The dichroic purple-red solution was stirred for 6 h under N2 . The reaction mixture was evaporated to dryness and purified by chromatography as described above (0.063 g, 63%). Anal. found: C, 75.2; H, 5.3; N, 7.3%. C48 H36 N4 B2 OF2 ·H2 O requires: C, 75.6; H, 5.0; N, 7.4; F, 5.0%. kmax (CH2 Cl2 )/nm 427 (log e 5.60), 569 (4.29) and 619 (4.42). d H (400 MHz; CDCl3 , Me4 Si) 9.03 (2H, d, J 4.8, Hb ), 8.99 (2H, d, J 5.0, Hb ), 8.64 (2H, d, J 4.9, Hb ), 8.59 (2H, d, J 5.0, Hb ), 8.30 (2H, d, J 7.8, Ho ), 8.16 (6H, m, Ho ), 7.64 (6H, m, Hm ), 7.55 (2H, d, J 7.7, Hm ), 2.77 (6H, s, CH3 ), 2.70 (3H, s, CH3 ), 2.67 (3H, s, CH3 ). d C (100 MHz; CDCl3 , Me4 Si) 147.80, 147.30, 144.38, 142.02, 141.87, 138.19, 137.81, 137.38, 136.53, 134.82, 134.81, 134.10, 129.04, 126.89, 126.61, 125.80, 123.32, 122.89, 119.25, 21.57, 21.43. d B (101 MHz; CDCl3 , BF3 ·OEt2 ) −15.07, This journal is © The Royal Society of Chemistry 2008

−16.80. d F (376 MHz; CDCl3 , CFC13 ) −146.56, −167.90. m/z (FAB) 744.3043 (M+ , C48 H36 B2 N4 OF2 , −2.3 ppm), 744 (98%, M+ ), 725 (100, M − F), 706 (14, M − 2F), 634 (15, M − C6 H4 CH3 − F). B2 OF2 (tpClpp). Method (a). Prepared as described for B2 OF2 (ttp) (method (a)) using BF3 ·OEt2 (4 mL, 32.5 mmol), H2 (tpClpp) (2.00 g, 2.66 mmol) and chlorobenzene (25 mL). The crude solid was recrystallised from CH2 C12 –hexane to give the product as a purple powder (0.50 g, 23%). Method (b). Prepared as described for B2 OF2 (ttp) (method (b)) using BF3 ·OEt2 (1.00 mL, 8.13 mmol), B2 O(OH)2 (tpClpp) (0.10 g, 0.12 mmol) and chlorobenzene (15 mL) (0.047 g, 47%). Anal. found: C, 62.9; H, 3.2; N, 6.4%. C44 H24 N4 B2 OF2 Cl4 ·H2 O requires: C, 62.7; H, 3.1; N, 6.7%. kmax (CH2 Cl2 )/nm 428 (log e 5.46), 568 (4.28) and 618 (4.28). d H (400 MHz; CDCl3 , Me4 Si) 9.01 (4H, ABq, Hb ), 8.65 (2H, d, J 5.0, Hb ), 8.59 (2H, d, J 5.0, Hb ), 8.34 (4H, m, Ho ), 8.21 (2H, d, J 8.0, Ho ), 8.17 (2H, d, J 8.1, Ho ), 7.86 (2H, d, J 7.8, Hm ), 7.83 (2H, d, J 7.4, Hm ), 7.75 (4H, m, Hm ). d C (100 MHz; CDCl3 , Me4 Si) 147.86, 147.19, 145.14, 141.90, 137.59, 137.52, 128.71, 126.74, 126.40, 126.17, 125.81, 123.62, 121.73, 116.53, 113.97. B2 OF2 (tpp). Prepared as described for B2 OF2 (ttp) (method (a)) using BF3 ·OEt2 (4 mL, 32.52 mmol), H2 (tpp) (2.00 g, 3.26 mmol) and chlorobenzene (25 mL) (0.39 g, 17%). Anal. found: C, 74.7; H, 4.0; N, 8.0%. C44 H28 N4 B2 OF2 .H2 O requires: C, 74.76; H, 4.28; N, 7.93%. kmax (CH2 Cl2 )/nm 425 (log e 5.16), 566 (4.14) and 614 (4.12). d H (400 MHz; CDCl3 , Me4 Si) 8.97 (4H, s, Hb ), 8.68 (4H, ABq, Hb ), 8.53 (2H, d, J 6.4, Ho ), 8.32 (4H, m, Ho ), 8.22 (2H, d, J 7.6, Ho ), 7.85 (12H, m, Hm and Hp ). d C (100 MHz; CDCl3 , Me4 Si) 147.61, 146.94, 142.26, 141.97, 138.02, 137.73, 137.41, 136.52, 135.42, 135.19, 134.72, 134.27, 128.30, 128.17, 126.21, 125.59, 123.77, 122.74, 120.14. B2 OF2 (oep). BF3 ·OEt2 (1.00 mL, 8.13 mmol) was added to a solution of B2 O(OH)2 (oep) (0.05 g, 0.083 mmol) in chlorobenzene (15 mL). The red solution was stirred for 12 h under a positive flow of nitrogen. The reaction mixture was evaporated to dryness and the solid chromatographed on silica gel. Elution with CH2 C12 removed a red band containing H2 (oep). Elution with CH2 C12 – tetrahydrofuran (9 : l) then removed a pink band of the desired product. The solution was evaporated to dryness and the crude solid recrystallised from CH2 Cl2 –hexane to give the product as a red powder (0.042 g, 83%). Anal. found: C, 68.69; H, 7.27; N, 8.47%. C36 H44 N4 B2 OF2 ·H2 O requires: C, 69.0; H, 7.4; N, 8.9%. kmax (CH2 Cl2 )/nm 400 (log e 5.13), 548 (3.92) and 592 (3.64). d H (400 MHz; CDCl3 , Me4 Si) 10.90 (2H, s, Hmeso ), 10.05 (1H, s, Hmeso ), 10.02 (1H, s, Hmeso ), 4.17 (16H, m, CH 2 CH3 ), 1.963 (24H, m, CH2 CH 3 ). d C (100 MHz; CDCl3 , Me4 Si) 147.87, 147.15, 142.03, 139.12, 138.47, 137.52, 102.21, 99.42, 96.31, 20.51, 20.06, 19.17, 18.37, 18.19, 17.96. d B (101 MHz; CDCl3 , BF3 ·OEt2 ) −11.90, −15.70. B2 O2 (BCl3 )2 (ttp). BCl3 ·MeCN (5.00 g, 31.6 mmol) was added to a solution of H2 (ttp) (2.00 g, 2.99 mmol) in chlorobenzene (25 mL). The reaction mixture was refluxed under N2 for 6 h. Over this period the solution changed colour from dark purple to blue-green and a dark solid began to form. The reaction mixture was cooled at 4 ◦ C overnight. The resulting purple crystals were Dalton Trans., 2008, 1602–1614 | 1609

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filtered off and washed with hexane (2.36 g, 84%). Anal. found: C, 57.; H, 3.84; N, 5.5; C1, 20.1%. C48 H36 N4 B4 O2 C16 ·3H2 O requires: C, 57.1; H, 4.2; N, 5.6; C1, 20.8%. kmax (CH2 Cl2 )/nm 425 (log e 5.58), 559 (4.36) and 610 (4.42). d H (400 MHz; CDCl3 , Me4 Si) 9.12 (8H, ABq, Hb ), 8.31 (4H, d, J 7.7, Ho ), 8.25 (4H, d, J 7.7, Ho ), 7.73 (4H, d, J 7.7, Hm ), 7.59 (4H, d, J 7.7, Hm ), 2.79 (6H, s, CH3 ), 2.72 (6H, s, CH3 ). d B (101 MHz; CDCl3 , BF3 ·OEt2 ) −15.12 (br). B2 O2 (BCl3 )2 (tpClpp). Prepared as described for B2 O2 (BC13 )2 (ttp) using BC13 ·MeCN (5.00 g, 31.6 mmol), H2 (tpClpp) (2.00 g, 2.66 mmol) and chlorobenzene (25 mL) (2.54 g, 92%). kmax (CH2 Cl2 )/nm 422, 556 and 600. d H (400 MHz; CDCl3 , Me4 Si) 9.17 (8H, ABq, Hb ), 8.42 (4H, d, J 7.8, Ho ), 8.30 (4H, d, J 8.2, Ho ), 7.94 (4H, d, J 7.9, Hm ), 7.82 (4H, d, J 7.6, Hm ). B2 O2 (BCl3 )2 (tpp). Prepared as described for B2 O2 (BC13 )2 (ttp) using BC13 ·MeCN (5.00 g, 31.60 mmol), H2 (tpp) (2.00 g, 3.26 mmol) and chlorobenzene (25 mL) (1.67 g, 57%). kmax (CH2 Cl2 )/nm 422, 559 and 608. d H (400 MHz; CDCl3 , Me4 Si) 8.98 (8H, ABq, Hb ), 8.67 (4H, br d, J 7.6, Ho ), 8.31 (4H, br d, J 7.5, Ho ), 7.86 (12H, m, Hm and Hp ). B2 O2 (BCl3 )2 (oep). BC13 ·MeCN (0.50 g, 3.16 mmol) was added to a solution of H2 (oep) (0.10 g, 0.19 mmol) in chlorobenzene (10 mL). The reaction mixture was refluxed under N2 for 6 h. Over this period the solution changed colour from dull red-brown to brilliant red-purple and small red crystals began to form. After cooling at 4 ◦ C overnight, the resulting red crystals were filtered off and washed with small amounts of chlorobenzene (0.086 g, 56%). kmax (CH2 Cl2 )/nm 385, 545 and 587. d H (400 MHz; CDCl3 , Me4 Si) 11.54 (2H, s, Hmeso ), 10.53 (2H, s, Hmeso ), 4.45 (8H, q, J 7.4, CH 2 CH3 ), 4.24 (8H, q, J 7.6, CH 2 CH3 ), 2.06 (12H, t, J 5.2, CH2 CH 3 ), 2.00 (12H, t, J 5.2, CH2 CH 3 ). B2 O2 (BBr3 )2 (tpClpp). Prepared as described for B2 O2 (BC13 )2 (ttp) using BBr3 (1.00 mL, 10.58 mmol), H2 (tpClpp) (2.00 g, 2.66 mmol) and chlorobenzene (25 mL) (1.29 g, 37%). kmax (CH2 Cl2 )/nm 434, 560 and 607. d H (400 MHz; CDCl3 , Me4 Si) 9.37 (8H, ABq, Hb ), 8.40 (4H, d, J 8.3, Ho ), 8.34 (4H, d, J 8.4, Ho ), 7.96 (4H, d, J 8.4, Hm ), 7.85 (4H, d, J 8.4, Hm ). B2 O(OH)2 (ttp). B2 O2 (BC13 )2 (ttp) (2.36 g, 2.47 mmol) was dissolved in the minimum amount of CH2 C12 and chromatographed on basic alumina, resulting in two bands. The first, a minor pink band, was eluted with CH2 C12 and identified as H2 (ttp) by UVvisible spectroscopy. Elution with CH2 C12 –tetrahydrofuran (19 : l) removed a major green band of the desired product. The solvent was removed and the solid recrystallised from CH2 C12 –hexane to give the product as a purple powder (1.23 g, 67%). Anal. found: C, 75.6; H, 5.4; N, 7.1; B, 2.6%. C48 H40 N4 B2 O4 requires: C, 76.0; H, 5.3; N, 7.4; B, 2.9%. kmax (CH2 Cl2 )/nm 430 (log e 5.54), 573 (4.28) and 623 (4.38). d H (400 MHz; CDCl3 , Me4 Si) 8.95 (4H, ABq, Hb ), 8.67 (2H, d, J 4.7, Hb ), 8.63 (2H, d, J 4.8, Hb ), 8.37 (2H, dd, J 1.8 and 7.7, Ho ), 8.20 (2H, d, J 7.8, Ho ), 8.17 (2H, d, J 8.0, Ho ), 8.37 (2H, dd, J 1.7 and 7.6, Ho ), 7.65 (6H, m, Hm ), 7.53 (2H, d, J 7.7, Hm ), 2.77 (6H, s, CH3 ), 2.70 (3H, s, CH3 ), 2.68 (3H, s, CH3 ), −9.10 (1H, br, OH). d C (100 MHz; CDCl3 , Me4 Si) 147.79, 147.14, 144.44, 142.31, 141.94, 138.23, 138.05, 137.76, 137.72, 136.60, 135.42, 135.05, 134.95, 134.78, 134.66, 134.11, 129.35, 129.11, 126.83, 126.62, 125.75, 125.48, 123.62, 122.88, 119.84, 21.56, 21.42. d B (101 MHz; CDCl3 , BF3 ·OEt2 ) −13.84, −16.82. 1610 | Dalton Trans., 2008, 1602–1614

m/z (FAB) 706.3075227 (M − 2OH, C48 H36 B2 N4 O, −6.7 ppm), 706 (100%, M − 2OH), 615 (59, M − C6 H4 CH3 − 2OH). B2 O(OH)2 (tpClpp). Prepared as described for B2 O(OH)2 (ttp) using B2 O2 (BC13 )2 (tpClpp) (2.00 g, 1.93 mmol) (1.32 g, 83%). Anal. found: C, 63.2; H, 3.3; N, 7.3%. C44 H26 N4 B2 O3 Cl4 ·0.5H2 O requires: C, 63.6; H, 3.3; N, 6.8%. kmax (CH2 Cl2 )/nm 432 (log e 5.43), 572 (4.16) and 624 (4.23). d H (400 MHz; CDCl3 , Me4 Si) 8.93 (2H, d, J 5.2, Hb ), 8.88 (2H, d, J 4.6, Hb ), 8.68 (2H, d, J 4.6, Hb ), 8.62 (2H, d, J 4.6, Hb ), 8.40 (2H, d, J 9.0, Ho ), 8.24 (2H, d, J 8.4, Ho ), 8.19 (2H, d, J 8.4, Ho ), 8.11 (2H, d, J 8.0 Ho ), 7.80 (8H, m, Hm ), −9.20 (1H, br, OH). d C (100 MHz; CDCl3 , Me4 Si) 147.84, 147.14, 145.13, 141.84, 137.57, 137.41, 128.71, 126.63, 126.39, 126.08, 125.93, 123.61, 121.74, 116.48, 113.91. B2 O(OH)2 (tpp). Prepared as described for B2 O(OH)2 (ttp) using B2 O2 (BC13 )2 (tpp) (1.50 g, 1.67 mmol) (0.76 g, 67%). Anal. found: C, 77.30; H, 4.86; N, 7.78%. C44 H30 N4 B2 03 requires: C, 77.2; H, 4.4; N, 8.2%. kmax (CH2 Cl2 )/nm 427 (log e 5.13), 570 (4.10) and 619 (4.07). d H (400 MHz; CDCl3 , Me4 Si) 8.94 (4H, s, Hb ), 8.68 (2H, d, J 4.8, Hb ), 8.61 (2H, d, J 4.7, Hb ), 8.49 (2H, br d, J 6.5, Ho ), 8.30 (2H, d, J 6.7, Ho ), 8.28 (2H, d, J 7.9, Ho ), 8.17 (2H, d, J 7.6, Ho ), 7.81 (12H, m, Hm and Hp ), −3.70 (1H, br, OH), −9.05 (1H, br, OH). d C (100 MHz; CDCl3 , Me4 Si) 147.83, 147.04, 142.26, 141.93, 138.17, 137.75, 137.38, 136.51, 135.50, 135.11, 134.84, 134.61, 134.13, 128.28, 128.06, 126.13, 126.04, 125.91, 125.48, 123.76, 122.88, 119.92. B2 O(OH)2 (oep). B2 O2 (BC13 )2 (oep) (0.10 g, 0.12 mmol) was chromatographed on basic alumina, resulting in two bands. The first, a minor red band, was eluted with CH2 C12 and identified as H2 (oep) by UV-visible spectroscopy. Elution with CH2 C12 – tetrahydrofuran (19 : 1) then removed a major purple band. This was dissolved in CH2 C12 and stirred at RT for 7 d to effect complete conversion to the product. The solution was evaporated to dryness and the crude solid recrystallised from CH2 C12 –hexane to the product as a red powder (0.051 g, 69%). Anal. found: C, 69.8; H, 7.4; N, 9.3%. C36 H46 N4 B2 O3 ·H2 O requires: C, 69.4; H, 7.8; N, 9.0%. kmax (CH2 Cl2 )/nm 381 (log e 5.29), 547 (4.22) and 584 (4.30). d H (400 MHz; CDCl3 , Me4 Si) 10.84 (2H, s, Hmeso ), 10.03 (1H, s, Hmeso ), 9.96 (1H, s, Hmeso ), 4.12 (16H, m, CH 2 CH3 ), 1.96 (24H, m, CH2 CH 3 ), −4.26 (1H, br, OH), −10.15 (1H, br, OH). d C (100 MHz; CDCl3 , Me4 Si) 147.31, 145.35, 144.36, 142.66, 140.03, 139.07, 138.49, 137.42, 102.11, 99.21, 96.21, 20.47, 20.04, 19.63, 18.40, 18.16, 17.97. d B (101 MHz; CDCl3 , BF3 ·OEt2 ) −12.46, −15.83. B2 O(OC6 H4 CH3 )2 (ttp). Method (a). NaOC6 H4 CH3 (4.00 g, 30.7 mmol) was added to a solution of B2 O2 (BC13 )2 (ttp) (1.00 g, 1.05 mmol) in tetrahydrofuran (30 mL) and the mixture was stirred under N2 for 6 h. The resulting solution was evaporated to dryness, redissolved in CHC13 and filtered to remove the bulk of the unreacted NaOC6 H4 CH3 and HOC6 H4 CH3 . The filtrate was concentrated and chromatographed on basic alumina. Elution with CHC13 – hexane (9 : l) removed a pink band containing H2 (ttp). Elution with CHC13 removed a green band containing the desired product which was evaporated to dryness and the crude solid recrystallised from CHCl2 –hexane to give a purple powder (0.39 g, 41%). Method (b). BC13 ·MeCN (5.00 g, 31.6 mmol) was added to a solution of H2 (ttp) (2.00 g, 2.99 mmol) in chlorobenzene (25 mL). The reaction mixture was refluxed under a positive flow of nitrogen This journal is © The Royal Society of Chemistry 2008


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for 6 h. Over this period the solution changed colour from dark purple to blue-green and a dark solid began to form. The reflux was stopped and HOC6 H4 CH3 (2.00 g, 18.5 mmol) was added to the reaction mixture. This was stirred at room temperature of 6 h, evaporated to dryness and the crude solid recrystallised from CHCl2 –hexane to give a purple powder (1.75 g, 64%). Anal. found: C, 79.5; H, 5.6; N, 5.9%. C62 H50 N4 B2 O3 ·H2 O requires: C, 79.3; H, 5.6; N, 6.0%. kmax (CH2 Cl2 )/nm 431, 573 and 623. d H (400 MHz; CDCl3 , Me4 Si) 9.01 (4H, m, Hb ), 8.52 (2H, d, J 4.9, Hb ), 8.38 (2H, d, J 4.8, Hb ), 8.21 (4H, d, J 7.7, Ho ), 8.15 (2H, d, J 7.6, Ho ), 8.08 (2H, d, J 7.7, Ho ), 7.71 (2H, d, J 7.9, Hm ), 7.68 (2H, d, J 7.8, Hm ), 7.53 (4H, m, Hm ), 6.18 (2H, d, J 8.3, Hm (OAr)), 5.15 (2H, d, J 8.4, Hm (OAr)), 4.01 (2H, d, J 8.4, Ho (OAr)), 2.79 (6H, s, CH3 ), 2.74 (3H, s, CH3 ), 2.72 (3H, s, CH3 ), 1.97 (3H, s, CH3 (OAr)), 1.33 (3H, s, CH3 (OAr)), 0.82 (2H, d, J 8.5, Ho (OAr)). d C (100 MHz; CDCl3 , Me4 Si) 151.93, 148.17, 147.85, 144.52, 142.44, 141.96, 138.24, 138.07, 137.72, 137.35, 136.77, 135.11, 134.99, 134.85, 134.73, 134.36, 129.14, 129.10, 127.64, 126.80, 126.56, 125.85, 123.20, 122.94, 117.52, 21.56, 21.45, 20.21. B2 O(OC6 H4 CH3 )2 (tpClpp). Method (a). Prepared as described for B2 O(OC6 H4 CH3 )2 (ttp) (method (a)) using NaOC6 H4 CH3 (4.00 g, 30.74 mmol), B2 O2 (BC13 )2 (tpClpp) (1.00 g, 0.96 mmol) and tetrahydrofuran (30 mL). The crude solid was recrystallised from CHCl3 –hexane to give a purple powder (0.26 g, 49%). Method (b). Prepared as described for B2 O(OC6 H4 CH3 )2 (ttp) (method (b)) using BCl3 ·MeCN (5.00 g, 31.60 mmol), H2 (tpClpp) (2.00 g, 2.66 mmol) and chlorobenzene (25 mL) (1.53 g, 57%). Anal. found: C, 67.8; H, 4.3; N, 5.9%. C58 H38 N4 Cl4 B2 03 ·H2 O requires: C, 68.4; H, 4.0; N, 5.5%. kmax (CH2 Cl2 )/nm 425, 557 and 601. d H (400 MHz; CDCl3 , Me4 Si) 8.96 (4H, s, Hb ), 8.45 (4H, ABq, Hb ), 8.26 (2H, d, J 7.8, Ho ), 8.12 (4H, m, Ho ), 7.92 (2H, d, J 7.8, Ho ), 7.67 (8H, m, Hm ), 6.11 (2H, d, J 8.1, Hm (OAr)), 5.04 (2H, d, J 8.4, Hm (OAr)), 3.93 (2H, d, J 8.1, Ho (OAr)), 1.92 (3H, s, CH3 (OAr)), 1.27 (3H, s, CH3 (OAr)), 0.74 (2H, d, J 8.3, Ho (OAr)). B2 O(OC6 H4 CH3 )(OH)(ttp). Method (a). B2 O(OC6 H4 CH3 )2 (ttp) (0.50 g, 1.99 mmol) was repeatedly chromatographed on basic alumina until a pure product was obtained (as evidenced by 1 H NMR). Elution with CHC13 – hexane (9 : l) removed a pink band containing H2 (ttp). Elution with CHC13 removed a green band containing progressively larger amounts of the desired product. Once no B2 O(OC6 H4 CH3 )2 (ttp) remained, as monitored by 1 H NMR, this band was evaporated to dryness and the crude solid recrystallised from CHC13 –hexane to give a purple powder (0.047 g, 11%). Method (b). Repeated recrystallisation of B2 O(OC6 H4 CH3 )2 (ttp) from CHC13 –hexane resulted in a purple powder containing progressively larger amounts of the desired product. Anal. found: C, 79.7; H, 5.3; N, 6.7%. C55 H44 N4 B2 O3 requires: C, 79.5; H, 5.3; N, 6.8%. kmax (CH2 Cl2 )/nm 432, 575 and 623. d H (400 MHz; CDCl3 , Me4 Si) 9.00 (4H, m, Hb ), 8.63 (2H, d, J 4.8, Hb ), 8.52 (2H, d, J 4.7, Hb ), 8.35 (2H, d, J 7.3, Ho ), 8.22 (2H, d, J 7.7, Ho ), 8.20 (2H, d, J 7.3, Ho ), 8.02 (2H, d, J 7.9, Ho ), 7.67 (6H, m, Hm ), 7.53 (2H, d, J 7.6, Hm ), 6.11 (2H, d, J 8.1, Hm (OAr)), 3.88 (2H, d, J 8.1, Ho (OAr)), 2.78 (6H, s, CH3 ), 2.73 (3H, s, CH3 ), 2.71 (3H, s, CH3 ), 1.92 (3H, s, CH3 (OAr)), −9.13 (1H, br, OH). This journal is © The Royal Society of Chemistry 2008

B2 O(OC6 H4 CH3 )(OH)(tpClpp). Method (a). Prepared as described for B2 O(OC6 H4 CH3 )(OH)(ttp) (method (a)) using B2 O(OC6 H4 CH3 )2 (tpClpp) (0.50 g, 0.50 mmol) (0.072 g, 16%). Method (b). Repeated recrystallisation of B2 O(OC6 H4 CH3 )2 (tpClpp) from CHC13 –hexane resulted in a purple powder containing progressively larger amounts of the desired product. Anal. found: C, 65.0; H, 3.3; N, 5.9%. C51 H32 N4 Cl4 B2 O3 ·1.5H2 O requires: C, 65.2; H, 3.8; N, 6.0%. kmax (CH2 Cl2 )/nm 423, 557 and 598. d H (400 MHz; CDCl3 , Me4 Si) 8.96 (4H, s, Hb ), 8.54 (4H, ABq, Hb ), 8.37 (2H, d, J 7.8, Ho ), 8.17 (4H, m, Ho ), 8.00 (2H, d, J 7.8, Ho ), 7.78 (8H, m, Hm ), 6.06 (2H, d, J 8.2, Hm (OAr)), 3.78 (2H, d, J 8.3, Ho (OAr)), 1.88 (3H, s, CH3 (OAr)), −8.72 (1H, br, OH). B2 O(OEt)2 (ttp). Prepared as described for B2 O(OC6 H4 CH3 )2 (ttp) (method (b)) using BC13 ·MeCN (1.00 g, 6.32 mmol), H2 (ttp) (0.30 g, 0.45 mmol), chlorobenzene (10 mL), ethanol (5 mL, 85.2 mmol) with subsequent stirring for 12 h at RT. The reaction mixture was evaporated to dryness and the residue chromatographed on basic alumina to afford three major bands. Elution with CH2 C12 removed a pink band containing H2 (ttp). Elution with CH2 C12 –ethanol (19 : l) then removed two green bands. The second band contained B2 O(OH)2 (ttp), while the first contained the desired product. This band was evaporated to dryness and the residue recrystallised from CH2 Cl2 –hexane– ethano1 to give the product as a purple powder (0.093 g, 23%). kmax (CH2 Cl2 )/nm 434, 576 and 622. d H (400 MHz; CDCl3 , Me4 Si) 8.93 (4H, ABq, Hb ), 8.39 (2H, d, J 4.8, Hb ), 8.36 (2H, d, J 4.8, Hb ), 8.26 (2H, d, J 7.2, Ho ), 8.17 (2H, d, J 7.7, Ho ), 8.11 (2H, d, J 7.6, Ho ), 8.06 (2H, d, J 7.6, Ho ), 7.65 (4H, d, J 8.0, Hm ), 7.59 (4H, d, J 7.6, Hm ), 2.75 (6H, s, CH3 ), 2.69 (3H, s, CH3 ), 2.66 (3H, s, CH3 ), −0.16 (2H, q, J 7.0, CH 2 CH3 ), −0.53 (3H, t, J 6.9, CH2 CH 3 ), −3.21 (2H, q, J 7.0, CH 2 CH3 ), −3.33 (3H, t, J 6.8, CH2 CH 3 ). d C (100 MHz; CDCl3 , Me4 Si) 147.73, 147.06, 144.38, 142.35, 141.88, 137.64, 137.25, 136.52, 135.36, 135.00, 134.68, 134.61, 134.04, 133.87, 129.01, 126.76, 126.56, 125.79, 125.68, 125.43, 123.55, 123.10, 122.80, 122.60, 58.20, 52.76, 21.50, 21.37. m/z (FAB) 813 (31%, M + OH), 798 (100, M+ ), 723 (79, M − 2OCH2 CH3 + OH), 706 (79, M − 2OCH2 CH3 ), 616 (24, M − 2OCH2 CH3 − C6 H5 ). B2 O(OEt)2 (tpClpp). Prepared as described for B2 O(OEt)2 (ttp) using BC13 ·MeCN (1.00 g, 6.32 mmol), H2 (tpClpp) (0.30 g, 0.40 mmol) and chlorobenzene (10 mL) (0.084 g, 24%). Anal. found: C, 63.3; H, 4.1; N, 6.2%. C48 H34 N4 Cl4 B2 O3 ·2H2 O requires : C, 63.2; H, 4.2; N, 6.1%. kmax (CH2 Cl2 )/nm 430, 571 and 623. d H (400 MHz; CDCl3 , Me4 Si) 8.90 (4H, ABq, Hb ), 8.38 (4H, ABq, Hb ), 8.36 (2H, d, J 7.4, Ho ), 8.18 (4H, m, Ho ), 8.02 (2H, d, J 8.2, Ho ), 7.84 (6H, m, Hm ), 7.69 (2H, d, J 8.0, Hm ), −0.19 (2H, q, J 6.9, CH 2 CH3 ), −0.55 (3H, t, J 6.9, CH2 CH 3 ), −3.33 (5H, m, CH 2 CH3 and CH2 CH 3 ). B2 O(OEt)(OH)(ttp). Samples of this complex were obtained by repeated chromatography of B2 O(OEt)2 (ttp) (0.05 g, 0.063 mmol) on basic alumina. Elution with CHC13 –hexane (9 : l) removed a pink band containing H2 (ttp). Elution with CHC13 removed a green band containing progressively larger amounts of the desired product. Once no more B2 O(OEt)2 (ttp) complex remained, as determined by 1 H NMR, this band was evaporated to dryness and the crude solid recrystallised from CHC13 –hexane to give a purple powder (0.021 g, 44%). kmax (CH2 Cl2 )/nm 432, Dalton Trans., 2008, 1602–1614 | 1611

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573 and 624. d H (400 MHz; CDCl3 , Me4 Si) 8.93 (4H, ABq, Hb ), 8.64 (2H, d, J 4.6, Hb ), 8.60 (2H, d, J 4.8, Hb ), 8.41 (2H, d, J 7.4, Ho ), 8.18 (4H, m, Ho ), 7.97 (2H, d, J 7.8, Ho ), 7.66 (6H, m, Hm ), 7.53 (2H, d, J 7.5, Hm ), 2.77 (6H, s, CH3 ), 2.70 (3H, s, CH3 ), 2.69 (3H, s, CH3 ), −0.23 (2H, q, J 7.0, CH 2 CH3 ), −0.58 (3H, t, J 6.9, CH2 CH 3 ), −9.13 (1H, br, OH). B2 O(OEt)(OH)(tpClpp). Prepared as described for B2 O(OEt)(OH)(ttp) using B2 O(OEt)2 (tpClpp) (0.05 g, 0.057 mmol) (0.011 g, 23%). Anal. found: C, 62.9; H, 3.1; N, 6.7%. C46 H30 N4 Cl4 B2 O3 ·2H2 O requires: C, 62.7; H, 3.4; N, 6.4%. kmax (CH2 Cl2 )/nm 432, 570 and 623. d H (400 MHz; CDCl3 , Me4 Si) 8.92 (4H, ABq, Hb ), 8.60 (4H, ABq, Hb ), 8.39 (2H, d, J 7.4, Ho ), 8.19 (4H, m, Ho ), 8.03 (2H, d, J 8.2, Ho ), 7.84 (6H, m, Hm ), 7.70 (2H, d, J 8.0, Hm ), −0.31 (2H, q, J 6.8, CH 2 CH3 ), −0.64 (3H, t, J 6.8, CH2 CH 3 ), −9.27 (1H, br, OH). B2 O(Ph)(OH)(ttp). PhBC12 (1.00 g, 6.30 mmol) was added to a solution of H2 (ttp) (0.375 g, 0.56 mmol) in chlorobenzene (50 mL). The reaction mixture was refluxed for 18 h, during which time the dark purple solution changed to a blue-green colour. The reaction mixture was evaporated to dryness and the solid chromatographed on basic alumina. Elution with CH2 C12 removed a pink band containing H2 (ttp). Elution with CH2 C12 – tetrahydrofuran (19 : l) then removed a green band of the desired product. The solvent was removed and the crude solid recrystallised from CH2 C12 –hexane to give the product as a purple powder (0.18 g, 40%). Anal. found: C, 80.0; H, 5.4; N, 6.8%. C55 H44 N4 B2 O2 ·0.5H2 O requires: C, 80.1; H, 5.4; N, 6.9%. kmax (CH2 Cl2 )/nm 438 (log e 5.41), 580 (4.14) and 632 (4.36). d H (400 MHz; CDCl3 , Me4 Si) 8.97 (2H, d, J 4.9, Hb ), 8.88 (2H, d, J 4.9, Hb ), 8.66 (2H, d, J 4.6, Hb ), 8.59 (2H, d, J 4.6, Hb ), 8.51 (2H, d, J 7.5, Ho ), 8.12 (2H, d, J 7.9, Ho ), 8.02 (2H, d, J 7.6, Ho ), 7.92 (2H, d, J 7.9, Ho ), 7.69 (2H, d, J 7.7, Hm ), 7.64 (2H, d, J 7.9, Hm ), 7.53 (4H, m, Hm ), 6.16 (1H, t, J 7.4, Hp (Ph)), 6.01 (2H, t, J 7.5, Hm (Ph)), 3.73 (2H, d, J 7.5, Ho (Ph)), 2.78 (6H, s, CH3 ), 2.68 (3H, s, CH3 ), 2.62 (3H, s, CH3 ), −9.22 (1H, br, OH). d C (100 MHz; CDCl3 , Me4 Si) 148.37, 147.89, 144.55, 142.63, 142.42, 138.20, 137.80, 137.73, 137.43, 136.87, 135.06, 135.01, 134.41, 133.81, 129.04, 128.94, 127.66, 126.93, 126.69, 126.22, 125.90, 125.37, 124.44, 123.03, 122.63, 119.07, 53.38, 29.70, 21.59, 21.39. m/z (FAB) 799 (100%, M − H), 783 (14, M − OH), 723 (21, M − C6 H5 ), 706 (87, M − C6 H5 − OH). as described for B2 O(Ph)(OH)(tpClpp). Prepared B2 OPh(OH)(ttp) using PhBC12 (1.00 g, 6.30 mmol), H2 (tpClpp) (0.50 g, 0.67 mmol) and chlorobenzene (50 mL). The crude solid was recrystallised from CH2 C12 –hexane to give the product as a purple powder (0.37 g, 63%). Anal. found: C, 67.2; H, 3.6; N, 6.8%. C50 H30 N4 B2 O2 C14 ·0.5H2 O requires: C, 67.5; H, 3.5; N, 6.3%. kmax (CH2 Cl2 )/nm 442 (log e 5.33), 450 (sh), 582 (4.07) and 634 (4.29). d H (400 MHz; CDCl3 , Me4 Si) 8.97 (2H, d, J 4.8, Hb ), 8.90 (2H, d, J 5.0, Hb ), 8.69 (2H, d, J 5.0, Hb ), 8.62 (2H, d, J 4.8, Hb ), 8.53 (2H, d, J 8.8, Ho ), 8.15 (2H, d, J 8.4, Ho ), 7.62 (10H, m, Ho and Hm ), 6.20 (1H, t, J 7.4, Hp (Ph)), 6.03 (2H, t, J 7.4, Hm (Ph)), 3.68 (2H, d, J 7.1, Ho (Ph)), −8.88 (1H, br, OH). d C (100 MHz; CDCl3 , Me4 Si) 148.12, 147.98, 145.31, 142.58, 142.31, 137.52, 135.90, 135.35, 135.24, 134.80, 128.58, 126.63, 126.42, 126.14, 125.91, 125.57, 124.82, 123.33, 121.49, 114.30, 107.68, 67.96, 25.60. 1612 | Dalton Trans., 2008, 1602–1614

B2 OPh(F)(tpClpp). BF3 ·OEt2 (2.00 mL, 16.26 mmol) was added to a solution of B2 O(Ph)(OH)(tpClpp) (0.20 g, 0.23 mmol) in tetrahydrofuran (25 mL). The resulting reaction mixture was stirred at RT for 12 h and then evaporated to dryness and the solid chromatographed on silica gel. Elution with CH2 C12 removed two green bands. The second, minor band contained unreacted B2 OPh(OH)(tpClpp). The first band containing the desired product was evaporated to dryness and the crude solid recrystallised from CH2 Cl2 –hexane to give the product as a purple powder (0.16 g, 80%). Anal. found: C, 67.0; H, 3.4; N, 6.3%. C50 H29 N4 B2 OFCl4 ·H2 O requires: C, 66.7; H, 3.5; N, 6.2%. kmax (CH2 Cl2 )/nm 423 (log e 5.37), 557 (4.10) and 598 (3.92). d H (400 MHz; CDCl3 , Me4 Si) 9.04 (2H, d, J 5.1, Hb ), 8.93 (2H, d, J 5.0, Hb ), 8.57 (2H, d, J 5.0, Hb ), 8.51 (2H, br, Ho ), 8.50 (2H, d, J 4.9, Hb ), 8.14 (2H, d, J 8.5, Ho ), 7.82 (10H, m, Ho and Hm ), 6.21 (1H, t, J 7.3, Hp (Ph)), 6.05 (2H, t, J 7.3, Hm (Ph)), 3.78 (2H, d, J 6.8, Ho (Ph)). d C (100 MHz; CDCl3 , Me4 Si) 148.48, 148.32, 145.24, 142.52, 141.63, 137.78, 137.46, 135.93, 135.78, 135.49, 135.31, 135.13, 134.78, 134.63, 134.38, 128.68, 128.43, 126.58, 125.95, 125.58, 124.75, 122.88, 121.42, 119.80, 111.87, 107.62, 67.65, 29.51. d F (376 MHz; CDCl3 , CFC13 ) −167.85. (0.05 g, B2 O(Ph)(OEt)(tpClpp). B2 O(Ph)(OH)(tpClpp) 0.058 mmol) was dissolved in ethanol (20 mL) and the resulting solution refluxed for 8 h. The solution was then evaporated to dryness and the crude solid recrystallised from CH2 C12 –hexane– ethano1 to give a purple powder (0.047 g, 92%). Anal. found: C, 66.7; H, 4.0; N, 6.3%. C52 H34 N4 Cl4 B2 O2 ·H2 O requires: C, 67.4; H, 3.9; N, 6.1%. kmax (CH2 Cl2 )/nm 441, 449 (sh), 582 and 632. d H (400 MHz; CDCl3 , Me4 Si) 8.92 (2H, d, J 5.0, Hb ), 8.82 (2H, d, J 5.0, Hb ), 8.39 (2H, d, J 5.0, Hb ), 8.33 (2H, d, J 5.0, Hb ), 8.14 (2H, d, J 8.4, Ho ), 7.78 (10H, m, Ho and Hm ), 7.52 (2H, d, J 8.0, Hm ), 6.18 (1H, t, J 7.2, Hp (Ph)), 6.03 (2H, t, J 7.3, Hm (Ph)), 3.84 (2H, d, J 6.7, Ho (Ph)), −3.10 (2H, q, J 6.9, CH 2 CH3 ), −3.23 (3H, t, J 6.8, CH2 CH 3 ). Crystal structure determination of [B2 O(tpClpp)][BBr4 ]2 The reaction of BBr3 with Li2 (tpClpp) in toluene was carried out as described previously.6 The solvent was removed and the reaction product dissolved in toluene-d 8 in order to measure its 1 H NMR spectrum. The sample was cooled at −20 ◦ C in a plasticcapped NMR tube for several days, resulting in single crystals of [B2 O(tpClpp)][BBr4 ]2 . Crystal data. C44 H24 B2 Cl4 N4 O·2(BBr4 )·3.5(C7 H8 ), M = ˚, 1765.48, triclinic, a = 15.2434(2), b = 18.2050(3), c = 25.7639(1) A ˚ 3, a = 85.711(1)◦ , b = 82.190(1)◦ , c = 81.760(1)◦ , U = 6999.1(2) A T = 200 K, Dc = 1.550 g cm−3 , lMo = 4.774 mm−1 , space group P1¯

(no. 2), Z = 4, 54354 reflections measured, 21768 unique, (Rint = 0.220) which were used in all calculations. The final R was 0.0943 and wR2 was 0.2826 (all data).

X-Ray crystallographic studies. X-Ray quality crystals were grown as described above. The crystals were removed from the NMR tube under a stream of N2 and immediately covered with a layer of viscous hydrocarbon oil (Paratone N, Exxon). A suitable crystal was selected with the aid of a microscope, attached to a glass fiber, and immediately placed in the low temperature N2 stream of the diffractometer operating at −120 ◦ C. The intensity data set for the compound was collected using a Siemens SMART system, This journal is © The Royal Society of Chemistry 2008


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complete with 3-circle goniometer and CCD detector operating at −54 ◦ C. Graphite monochromated Mo-Ka radiation (k = ˚ ) was employed. The data collection nominally covers 0.71073 A a hemisphere of reciprocal space utilizing a combination of three sets of exposures, each with a different angle, and each exposure covering 0.3◦ in φ. Crystal decay was monitored by repeating the initial frames at the end of the data collection and analyzing the duplicate reflections. In all cases, no decay was observed. An absorption correction was applied for all compounds utilizing the program SADABS.21 The crystal structure was solved by Patterson methods as included in the SHELXTL-Plus program package.22 Missing atoms were located in subsequent difference Fourier maps and included in the refinement. The structure of the compound was refined by full-matrix least-squares refinement on F 2 (SHELX93). Hydrogen atoms were placed geometrically and refined using a riding model, including free rotation about C–C bonds for methyl groups with U(iso) constrained at 1.2 for non-methyl groups, and 1.5 for methyl groups times U(eq) of the carrier C atom. The crystallographic programs used for structure refinement and solution were installed on a Silicon Graphics Indigo2 R10000 Solid Impact or a PC clone. Scattering factors were those provided with the SHELX program system. All non-hydrogen atoms, with the exception of some disordered positions were refined anisotropically. Three toluene molecules of crystallization were severely disordered to an extent that satisfactory modeling was not possible. Consequently, they were removed from the refinement using the Squeeze program, as implemented in PLATON.23 The electron density and hole size agree well with the solvent being toluene. Due to very weak intensity data, the 2h values were limited to 48◦ . Computation Density functional calculations of the molecular structures of the [B2 O(tpp)]2+ cation and model complexes B2 O(OH)2 (tpp) and B2 O(OH)2 (omp)9 were carried out using the Gaussian 03 Program.24 Full geometry optimisations were carried out using the B3LYP density functional with 6-311G(d,p) basis sets for all elements.25 NMR chemical shieldings were calculated using the GIAO method with the B3LYP/6-311+G(2d,p) model. 1 H NMR chemical shifts were then calculated with reference to chemical shieldings, calculated using the same models for tetramethylsilane. The molecular structure of the model complex B2 O(Ph)(OH)(porphine) and the free energy of the boron porphyrin hydrolysis reaction (3) were calculated using the Spartan ’04 program with a B3LYP/6-31G(d) model.26

Conclusions Size does matter. The reaction of BX3 with a porphyrin precursor in anhydrous conditions produces the very strained diboryl complexes (BX2 )2 (por) for X = F and Cl, but BBr3 and BI3 are too bulky and spontaneous, sterically-induced reductive coupling is observed.6 The diboryl complexes have calculated D(N · · · N) ˚ for (BF2 )2 (por) and 1.28 for (BCl2 )2 (por). Hyvalues of 1.15 A drolysis of these complexes leads to B2 OF2 (tpClpp) (D(N · · · N) = ˚ ). The ˚ ) or B2 O2 (BCl3 )2 (tpClpp) (D(N · · · N) = 1.14 A 1.06 A decrease in D(N · · · N) and hence the strain in the porphyrin may provide a partial driving force for the hydrolysis to proceed. This journal is © The Royal Society of Chemistry 2008

The well-known difluoroboron dipyrromethene (dpm) complexes BF2 (dpm) do not undergo hydrolysis of the B–F bonds,10 and neither do the expanded porphyrin complexes (BF2 )2 (amethyrin) and (BF2 )2 (octaphyrin), in which each BF2 group occupies a dipyrromethene-type site within the expanded macrocycle but well-separated from each other.5 Both of these observations support the hypothesis that steric strain within the diboron complexes is a major driver for the chemistry. A number of structural types have now been established for the hydrolysis products, with the three complexes B2 O(OH)2 (por), B2 O2 (BX3 )2 (por) and [B2 O(por)]2+ serving as the examples. This further underscores the conformational flexibility and structural diversity available to the surprising combination of two boron atoms coordinated to the relatively ridgid and constrained porphyrin macrocycle.

Acknowledgements The authors are grateful to the Marsden Fund, administered by the Royal Society of New Zealand, for support of this work. W.J.B. thanks the New Zealand University Grants Committee for the award of postgraduate and William Georgetti scholarships. K.R.-S. thanks the US National Science Foundation (grant CHE0505863) and the Royal Society of New Zealand for an ISAT travel grant.

Notes and references 1 (a) Reviews: P. J. Brothers, Adv. Organomet. Chem., 2001, 48, 289– 342; (b) P. J. Brothers, J. Porphyrins Phthalocyanines, 2002, 6, 259–267; (c) P. J. Brothers, Chem. Commun., 2008, DOI: 10.1039/b714894a, (and references therein). 2 W. J. Belcher, P. D. W. Boyd, P. J. Brothers, M. J. Liddell and C. E. F. Rickard, J. Am. Chem. Soc., 1994, 116, 8416–8417. 3 (a) W. J. Belcher, M. Breede, P. J. Brothers and C. E. F. Rickard, Angew. Chem., Int. Ed., 1998, 37, 1112–1114; (b) M. O. Senge, Angew. Chem., Int. Ed., 1998, 37, 1071–1072. 4 A. Weiss, H. Pritzkow, P. J. Brothers and W. Siebert, Angew. Chem., Int. Ed., 2001, 40, 4182–4184. ¨ 5 T. Kohler, M. C. Hodgson, D. Seidel, J. M. Veauthier, S. Meyer, V. Lynch, P. D. W. Boyd, P. J. Brothers and J. L. Sessler, Chem. Commun., 2004, 1060–1061. 6 A. Weiss, M. C. Hodgson, P. D. W. Boyd, W. Siebert and P. J. Brothers, Chem.–Eur. J., 2007, 13, 5982–5993. 7 R. Thomas, Diplomarbeit, Technische Hochschule Braunschweig, 1965. 8 C. J. Carrano and M. Tsutsui, J. Coord. Chem., 1977, 7, 125–130. 9 Abbreviations: dpm, dipyrromethene anion; por, unspecified porphyrin dianion; oep, dianion of 2,3,7,8,12,13,17,18-octaethylporphyrin; omp, dianion of 2,3,7,8,12,13,17,18-octamethylporphyrin; tpp, dianion of 5,10,15,20-tetraphenylporphyrin; ttp, dianion of 5,10,15,20tetra-p-tolylporphyrin; tpClpp, dianion of 5,10,15,20-tetra-pchlorophenylporphyrin; tpc, trianion of meso-triphenylcorrole. 10 T. E. Wood and A. Thompson, Chem. Rev., 2007, 107, 1831–1861. 11 (a) W. J. Evans, Coord. Chem. Rev., 2000, 206–207, 263–283; (b) W. J. Evans, Chem. Rev., 2002, 102, 2119–2136; (c) W. J. Evans, Inorg. Chem., 2007, 46, 3435–3449. 12 M. O. Senge, C. J. Medforth, T. P. Forsyth, D. A. Lee, M. M. Olmstead, W. Jentzen, R. K. Pandey, J. A. Shelnutt and K. M. Smith, Inorg. Chem., 1997, 36, 1149–1163. ´ F. Espinosa-Leyton and T. L. Sordo, J. Chem. 13 G. I. Cardenás-Jiron, Sci., 2005, 117, 515–524. 14 A. M. Albrett, J. Conradie, P. D. W. Boyd, G. R. Clark, A. Ghosh and P. J. Brothers, J. Am. Chem. Soc., 2008, 130, in press. 15 J. M. Burke, M. A. Fox, A. E. Goeta, A. K. Hughes and T. B. Marder, Chem. Commun., 2000, 2217–2218. 16 (a) H. Borrmann, A. Simon and H. Vahrenkamp, Angew. Chem., Int. Ed. Engl., 1989, 28, 180–181; (b) L. G. Vorontsova, O. S. Chizov, L. S.

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17 18 19 20


21 22 23

Vasil’ev, V. V. Veselovskii and B. M. Mikhailov, Russ. Chem. Bull., 1981, ¨ and U. Wietelman, Chem. Ber., 30, 273–277; (c) E. Hanecker, H. Noth 1986, 119, 1904–1910; (d) M. Wu, T. S.-C. Law, H. H.-Y. Sung, J. Cai and I. D. Williams, Chem. Commun., 2005, 1827–1829. Although B2 OF2 (tpClpp) has been structurally characterised, disorder in the region of the FBOBF group prevents comparisons (see ref. 2). ´ A. Młodzianowska, L. Latos-Gra˙zynski, L. Szterenberg and M. ´ Inorg. Chem., 2007, 46, 6950–6957. Stępien, (a) K. M. Smith, Porphyrins and Metalloporphyrins, Elsevier, Amsterdam, 1964; (b) K. Rousseau and D. Dolphin, Tetrahedron Lett., 1974, 15, 4251–4254. (a) G. Engelsma, A. Yamamoto, E. Markham and M. Calvin, J. Phys. Chem., 1962, 66, 2517–2531; (b) H. W. Whitlock and R. Hanauer, J. Org. Chem., 1968, 33, 2169–2171. G. M. Sheldrick, SADABS, Program for Absorption Correction Using ¨ ¨ Area Detector DataUniversity of Gottingen, Gottingen, Germany, 1996. G. M. Sheldrick, Acta Crystallogr., Sect. A, 2008, 64, 112–122. (a) A. L. Spek, Acta Crystallogr., Sect. A, 1990, 46, C34; (b) A. L. Spek, PLATON, A multipurpose crystallographic tool, Utrecht University, Utrecht, The Netherlands, 1998.

1614 | Dalton Trans., 2008, 1602–1614

24 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, J. A. Montgomery, Jr., T. Vreven, K. N. Kudin, J. C. Burant, J. M. Millam, S. S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P. Hratchian, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. Ochterski, P. Y. Ayala, K. Morokuma, G. A. Voth, P. Salvador, J. J. Dannenberg, V. G. Zakrzewski, S. Dapprich, A. D. Daniels, M. C. Strain, O. Farkas, D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul, S. Clifford, J. Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. L. Martin, D. J. Fox, T. Keith, M. A. AlLaham, C. Y. Peng, A. Nanayakkara, M. Challacombe, P. M. W. Gill, B. G. Johnson, W. Chen, M. W. Wong, C. Gonzalez and J. A. Pople, GAUSSIAN 03 (Revision B.03), Gaussian, Inc., Wallingford, CT, 2004. 25 G. Igel-Mann, H. Stoll and H. Preuss, Mol. Phys., 1988, 65, 1321. 26 SPARTAN, version 4.0., Wavefunction, Inc., Irvine, CA, 1995.

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