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Cite this: Dalton Trans., 2013, 42, 12828 Received 1st May 2013, Accepted 3rd July 2013 DOI: 10.1039/c3dt51138c www.rsc.org/dalton

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A fine tuning of metallaborane to bridged-boryl complex, [(Cp*Ru)2(µ-H)(µ-CO)(µ-Bcat)] (cat = 1,2O2C6H4; Cp* = η5-C5Me5)† R. S. Anju,a Dipak Kumar Roy,a K. Geetharani,a Bijan Mondal,a Babu Vargheseb and Sundargopal Ghosh*a

Room temperature photolysis of [(Cp*RuCO)2BH4(Bcat)], 3, generated from the reaction of arachno-[(Cp*RuCO)2B2H6], 1, with HBcat (cat = 1,2-O2C6H4), yielded a rare homodinuclear bridged-boryl complex, [(Cp*Ru)2(μ-H)(μ-CO)(μ-Bcat)], 4, confirmed by X-ray diffraction.

Boron displays a wide range of bonding types with transition metals; based on this the metal–boron compounds are categorized into different subclasses such as borane, borylene, or boryl.1 Out of these, boryl complexes, representing one of the most broad subclasses, have received significant attention from both structure–bonding and reactivity perspective.2,3 Among the several methods available, oxidative addition of B–H, B–Cl, B–Br and B–B bonds to a low coordinated transition metal centre and salt elimination are the most viable routes for the synthesis of metal boryl complexes.1,3 The first example of a boryl complex was reported by Nöth4 and Schmid5 between 1963 and 1970. Crystallographically characterized examples did not appear until 1990 when the groups of Merola6 and Baker and Marder7 reported a number of complexes that contain a –BR2 moiety, bound to the metal centre. Since there are fewer reports on structurally characterized homo and heterometallic bridged-boryl complexes (Chart 1; I–IV)8–11 than the terminal ones, it is of particular interest to add entries to the metal-bridged-boryl complex library. Recently in our laboratory we have reported a series of lowboron content metallaborane compounds that turned our attention towards synthesizing borylene and boryl complexes of early and late transition metals.12 As a result, we have synthesized numerous triply-bridged borylene complexes, [(Cp*MCO)2(μ3-BH)Hx{M′(CO)4}] (M = Ta and Ru, M′ = Fe and x = 0; M = Ru, M′ = Mn, x = 1).12 Thus, motivated by our earlier

a Department of Chemistry, Indian Institute of Technology Madras, Chennai, 600 036, India. E-mail: [email protected]; Fax: +91 44-22574202; Tel: +91 44-22574230 b Sophisticated Analytical Instruments Facility, Indian Institute of Technology Madras, Chennai 600 036, India † Electronic supplementary information (ESI) available: X-ray analysis of 2 and 4. CCDC 926986 and 926985. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3dt51138c

12828 | Dalton Trans., 2013, 42, 12828–12831

Chart 1

Homo and heterometallic bridged-boryl complexes, I–IV.

Scheme 1

Synthesis of arachno-2, 3 and bridged-boryl 4.

work,12 and the work of Marder,3 Hartwig,2 Braunschweig1 and others2,3 on boryl chemistry, we performed the reaction of 1 with various boranes to generate boryl complexes supported on a metallaborane framework. Reacting with HBpin ( pinacolborane; pin = OCMe2CMe2O) and 9-BBN (9-borabicyclo[3.3.1]nonane) led to decomposition of the starting material. However, monitoring the reaction of HBcat (catecholborane, cat = 1,2-O2C6H4) with 1 via 11B NMR spectroscopy revealed an immediate consumption of the starting material, which led to the formation of new compounds with a concomitant release of BH3 (Scheme 1). The release of BH3 was confirmed by the addition of PPh3 (see ESI†). Following the chromatographic separation using thin layer chromatography (TLC), two new compounds, [(Cp*RuCO)2B2H5(C6H4O2H)], 2 and [(Cp*RuCO)2BH4(B-cat)], 3 have been isolated as air and moisture sensitive solids (see ESI†). Full spectroscopic characterization of this mixture presented us with a structural puzzle. An unambiguous

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Fig. 1 Molecular structure and labeling diagram for 2. Selected bond lengths (Å): Ru1–Ru2 2.926(5), B1–Ru2 2.186(6), B1–Ru1 2.356(6), B1–B2 1.819(12), Ru2–B2 2.225(8).

Fig. 2 Molecular structure and labeling diagram for 4. Selected bond lengths (Å): Ru1–Ru2 2.404(3), B1–Ru2 2.230(4), B1–Ru1 2.290(4), B1–O1 1.440(5).

explanation eluded us until an X-ray structure study revealed the geometry of 2 (Fig. 1; ESI†). The core geometry and the basic structural features of compound 2 are very similar to that of 1. The Ru–B bond length of 2.186(6) Å at the hinge position in 2 is significantly shorter than that of 1 (2.25(3) Å). The dihedral angle between the two planes, Ru2–B1–B2 and Ru1–B1–Ru2 of 2 is 117.1°, which is similar to arachno-B4H10 (117.48° by electron diffraction and microwave spectroscopy),13 Ir and Co analogs.14 The FAB mass spectrum of 2 contained the parent ion peak at m/z 664 and the 11B NMR spectrum showed the presence of two boron environments, appearing at δ = 11 and 13 ppm. Whereas, the 11B (δ = 8 and 23 ppm), 1H, and 13C NMR spectra of 3 have the best fit with the proposed structure as shown in Scheme 1. The hydrogen free environment of the boron in 3, appearing at 23.6 ppm in 11B NMR, has been established by 1 H{11B}/11B{1H} HSQC spectroscopy. Further, the computed 11 B and 1H chemical shift values of 2 and 3 found using the gauge-including atomic orbital density functional theory [GIAO-DFT] method at the B3LYP/def2-TZVP level show a satisfactory match with the experimental values (Table S1; ESI†). The pathway for the formation of 2 and 3 is not clear; however, the molecular entity and composition of these compounds show that compound 2 is transformed into 3 with a straightforward release of hydrogen by the reaction of the phenolic hydroxyl group with the B–H bond. This is a process which is related to that involved in the synthesis of catechol borane. Therefore, with this assumption we monitored the 1H and 11B NMR spectra of compound 2 in a sealed NMR tube over regular intervals of time and observed the conversion of compound 2 to 3, via the release of H2 (ESI†).15,16 Interestingly in parallel with this conversion, a very small amount of compound 2 was also converted to [(Cp*RuCO)2(µ-H)2], 517 by the release of boric acid and catechol. The presence of a boryl ligand in 3 inspired us to utilize it as a possible precursor for a bridged-boryl complex under thermal and photolytic conditions. Thermolysis led to the decomposition of the starting material, however, compound 3, under photolytic conditions, yielded compound 4 (ESI†). The single crystal X-ray diffraction study together with spectroscopic studies, established 4 as a homometallic bridged boryl

complex, [(Cp*Ru)2(μ-CO)(μ-H)(μ-Bcat)] (ESI†). The structural characterisation of the bridging coordination of monodentate ligands, such as alkyl and silyl groups, is prevalent along with several bridged borylene complexes.1 However, the accounts on transition metal bridged boryl complexes are very rare,8–11 for example, Norman and co-workers reported the platinum bridged boryl complex, [Pt2(PPh3)(μ-dppm)2(Bcat)(μ-Bcat)]; Marder et al. isolated a mixed Rh(I)/Rh(III) complex containing a semi-bridging Bcat group. Recently Braunschweig and Hartwig reported dinuclear monoboryl complexes [(η5C5Me5)Fe(μCO)2(μBCl2)Pd(PCy3)], and [Cp*2Ru2H3{µ-B(N,N-dimethylphenylenediamine)}] respectively which consist of symmetrically bridging boryl groups (Chart 1). Compound 4 is an additional entry into the library of homometallic bridged-boryl complexes that have been synthesized and structurally characterized. An analytically pure compound of 4 was obtained as brown crystals by slow evaporation from a hexane solution at −10 °C.18 The solid state X-ray structure of 4, shown in Fig. 2, reveals the presence of a bridging Bcat moiety lying on a crystallographic mirror plane and symmetrically bridging the two ruthenium centers. The avg. Ru–B bond length in 4 (2.260 Å) is somewhat shorter than that observed in [Cp*2Ru2H3{μ-B(N,Ndimethylphenylenediamine)}]11 (2.307 Å) and other boryl complexes (Table S3†).3 Although the Ru–Ru bond length of 2.404 (3) Å is significantly shorter when compared to 2 (Ru1–Ru2: 2.926(5) Å), it is similar to that observed in the homodinuclear ruthenium boryl complex, [Cp*2Ru2H3[B(N,N-dimethylphenylenediamine)] (Ru–Ru 2.455(1) Å).11 This may be due to the presence of both bridging CO and boryl ligands, which bring the two metal centers into close proximity. The dihedral angle between the planes of Ru1–B1–Ru2 and O1–B1–O2 is 90.71°, which indicates that the boryl ligand is perfectly orthogonal to the triangular plane made by two ruthenium atoms and one boron atom. Considering 4 as a dinuclear complex, the total electron count (TEC) of 4 is 30. The very short Ru–Ru bond length implies there may be a formal Ru–Ru triple bond which satisfies the eighteen electron count on each metal centre. To understand the electronic structure, DFT studies were carried out on the model complex 4′ (Cp analog of 4, Fig. 2, Fig. S4†).


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Table 1 Ru–Ru bond distance (theoretical and experimental), Wiberg bond indices (WBI) for the complexes 1’–5’


Ru1–Ru2 (Å) Compounds

Theoretical

Experimental

WBI

1′ 2′ 3′ 4′ 4a 5′

2.896 2.897 2.907 2.407 2.434 2.699

2.9258(10) 2.926(5) — 2.404(3) — 2.6920(3)

0.26 0.26 0.25 0.71 0.62 0.40

The structural parameters of 4′ were found to be close to those of the experimental one. To validate the bond multiplicity of 4, the Wiberg bond indices (WBIs) were determined in a family of dinuclear ruthenium compounds 1′, 2′ and 5′ (1′, 2′ and 5′ are the Cp analogs of 1, 2 and 5 respectively) by using natural bond orbital (NBO) analysis. The WBI values for 1′, 2′, 4′ and 5′ are 0.26, 0.26, 0.71 and 0.40 respectively and are listed in Table 1. Based on these comparative WBI values of the dinuclear ruthenium systems and a comparative WBI study by Lupan and King on Re systems,19 the high WBI value of 4′ (0.71) supports the Ru–Ru triple bond. The solution spectroscopic data of 4 are in full agreement with its solid state formulation. The mass spectrum of 4 shows a molecular ion peak at m/z = 619 corroborating the composition of C27H35BO3Ru2. The 11B NMR spectrum shows the presence of one boron resonance which appeared at δ = 53.7 ppm, and is very similar to those known for bridged boryl complexes.1–3 The 1H and 13C NMR spectra support the presence of equivalent Cp* ligands and the aromatic ring. The IR spectrum displayed bands that support the presence of B–O as well as the bridging CO stretching vibrations. Although, the pathway for the formation of 4 has not yet been understood, the formation of 4 from 3 can be proposed by the exclusion of BH3 and one CO ligand. In fact, the release of BH3 was confirmed by trapping it with PPh3. The 11B NMR spectrum of the reaction mixture shows a doublet at δ = −38.5 ppm with a 11B–31P coupling constant value of 56 Hz. In addition, the 31P NMR spectrum shows a peak at δ = 20.5 ppm indicating the formation of a [BH3·PPh3] adduct.20 In order to find if any reversibility exists between 3 and 4, DFT studies have been carried out. The calculations suggest the existence of a low energy barrier (ca. 1.2 kcal mol−1) between 3 and 4, signifying that the conversion of 4 to 3 by adding BH3 is feasible. To support this pathway, we performed the reverse reaction, that is, addition of BH3·THF to 4. Indeed, the room temperature reaction of 4 with BH3·THF instantaneously converts the boryl complex into 3.21 Although the DFT study, performed on 4a (an analog of 4, where CO ligand is replaced by two hydrogens, Fig. S5†) suggested thermodynamic stability (HOMO–LUMO gap = 1.56 and 1.84 eV for 4 and 4a respectively, Table S2†), several efforts to isolate [(Cp*Ru)2(μ-H)3(μ-Bcat)] failed. In summary, we have established that compound 3 serves as the precursor for the generation of the bridged-boryl

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complex 4. This work presents an opportunity to improve catalysts with ligands and geometries different from those of the existing catalysts for the borylation of arenes, heteroarenes and alkanes.3 Such studies are in progress in our laboratory. Generous support from the Department of Science and Technology, DST (Project No. SR/S1/IC-13/2011), New Delhi, India, is gratefully acknowledged. We thank Prof. JeanFrancois Halet, CNRS-Universite de Rennes-1 for his help on scientific discussions. R. S. A. is grateful to UGC New Delhi, India, for a Research Fellowship. D. K. R. and K. G. are grateful to CSIR, India, for research fellowships.

Notes and references 1 (a) H. Braunschweig, Angew. Chem., Int. Ed., 1998, 37, 1786; (b) H. Braunschweig and M. Colling, Coord. Chem. Rev., 2001, 223, 1; (c) H. Braunschweig, in Inorganic Chemical Highlights, ed. G. Mayer, D. Naumann and L. Wesemann, Wiley-VCH, Weinheim, 2001, p. 213; (d) S. Aldridge, M. Besora, H. Braunschweig, D. L. Kays, C. Kollann, Z. Lin, A. Lledós and F. Seeler, in Structure and Bonding, ed. T. B. Marder and Z. Lin, Springer-Verlag, Berlin, 2008, ch. 1–3, vol. 130, pp. 1–148; (e) H. Braunschweig, R. D. Dewhurst and A. Schneider, Chem. Rev., 2010, 110, 3924. 2 (a) H. Chen, S. Schlecht, T. C. Semple and J. F. Hartwig, Science, 2000, 287, 1995; (b) J. F. Hartwig, K. S. Cook, M. Hapke, C. D. Incarvito, Y. Fan, C. E. Webster and M. B. Hall, J. Am. Chem. Soc., 2005, 127, 2538; (c) S. Aldridge and D. L. Coombs, Coord. Chem. Rev., 2004, 248, 535; (d) D. Vidovic, G. A. Pierce and S. Aldridge, Chem. Commun., 2009, 1157. 3 (a) G. I. Irvine, M. J. G. Lesley, T. B. Marder, N. C. Norman, C. R. Rice, E. G. Robins, W. R. Roper, G. R. Whittell and L. J. Wright, Chem. Rev., 1998, 98, 2685; (b) I. A. I. Mkhalid, J. H. Barnard, T. B. Marder, J. M. Murphy and J. F. Hartwig, Chem. Rev., 2010, 110, 890; (c) D. N. Coventry, A. S. Batsanov, A. E. Goeta, J. A. K. Howard, T. B. Marder and R. N. Perutz, Chem. Commun., 2005, 2172; (d) I. A. I. Mkhalid, D. N. Coventry, D. Albesa-Jove, A. S. Batsanov, J. A. K. Howard, R. N. Perutz and T. B. Marder, Angew. Chem., Int. Ed., 2006, 45, 489; (e) T. B. Marder and N. C. Norman, Top. Catal., 1999, 5, 63; (f ) T. Ishiyama and N. Miyaura, J. Organomet. Chem., 2003, 680, 3; (g) J.-Y. Cho, M. K. Tse, D. Holmes, R. E. Maleczka Jr. and M. R. Smith III, Science, 2002, 295, 305; (h) G. A. Chotana, M. A. Rak and M. R. Smith III, J. Am. Chem. Soc., 2005, 127, 10539; (i) M. R. Smith III, Prog. Inorg. Chem., 1999, 48, 505. 4 H. Nöth and G. Schmid, Angew. Chem., Int. Ed., 1963, 2, 623. 5 G. Schmid, Angew. Chem., Int. Ed., 1970, 9, 819. 6 J. R. Knorr and J. S. Merola, Organometallics, 1990, 9, 3008.


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Dalton Transactions 7 R. T. Baker, D. W. Ovenall, J. C. Calabrese, S. A. Westcott, N. J. Taylor, I. D. Williams and T. B. Marder, J. Am. Chem. Soc., 1990, 112, 9399. 8 (a) S. A. Westcott, PhD thesis, University of Waterloo, Canada, 1992; (b) S. A. Westcott, T. B. Marder, R. T. Baker, R. L. Harlow, J. C. Calabrese, K. C. Lam and Z. Lin, Polyhedron, 2004, 23, 2665. 9 D. Curtis, M. J. G. Lesley, N. C. Norman, A. G. Orpen and J. Starbuck, J. Chem. Soc., Dalton Trans., 1999, 1687. 10 H. Braunschweig, K. Radacki, D. Rais and G. R. Whittell, Angew. Chem., Int. Ed., 2005, 44, 1192. 11 J. M. Murphy, J. D. Lawrence, K. Kawamura, C. Incarvito and J. F. Hartwig, J. Am. Chem. Soc., 2006, 128, 13684. 12 (a) K. Geetharani, S. K. Bose, S. Sahoo and S. Ghosh, Angew. Chem., Int. Ed., 2011, 50, 3908; (b) K. Geetharani, S. K. Bose, B. Varghese and S. Ghosh, Chem.–Eur. J., 2010, 16, 11357. 13 C. J. Dain, A. J. Downs, G. S. Laurenson and D. W. H. Rankin, J. Chem. Soc., Dalton Trans., 1981, 472. 14 (a) X. Lei, M. Shang and T. P. Fehlner, Chem.–Eur. J., 2000, 6, 2653; (b) J. Feilong, T. P. Fehlner and A. L. Rheingold, J. Organomet. Chem., 1988, 348, C22. 15 The release of hydrogen has been confirmed by 1H NMR spectroscopy (δ = 4.62 ppm in CDCl3).


Communication 16 G. R. Fulmer, A. J. M. Miller, N. H. Sherden, H. E. Gottlieb, A. Nudelman, B. M. Stoltz, J. E. Bercaw and K. I. Goldberg, Organometallics, 2010, 29, 2176. 17 (a) N. J. Forrow and A. R. Knox, J. Chem. Soc., Chem. Commun., 1984, 679; (b) K. Geetharani, S. K. Bose, S. Sahoo, B. Varghese, S. M. Mobin and S. Ghosh, Inorg. Chem., 2011, 50, 5824. 18 Even though the data collection strategy was set for 100% completion, collected data showed incompleteness in pockets of reciprocal space. Thus we have collected the data for 4 many times, but we were unable to get improved data in any of our attempts. 19 A. Lupan and R. B. King, Inorg. Chem., 2012, 51, 7609. 20 S. E. Ashbrook, N. G. Dowell, I. Prokes and S. Wimperis, J. Am. Chem. Soc., 2006, 128, 6782. 21 The formation of 3 from 4 by the addition of BH3·THF was instantaneous and a colour change from brown to yellow was observed. Further the formation of 3 was confirmed by 11 B NMR spectroscopy and thin layer chromatography (TLC). Although we are not able to give the exact stoichiometry of the reaction, it may be possible that from two molecules of 4 one molecule of 3 has been formed, as the reaction leads to formation of 3 along with some decomposition.

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