Dalton Transactions

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Non-innocence and mixed valency in triand tetranuclear ruthenium complexes of a heteroquinone bridging ligand† Mohd. Asif Ansari,a Abhishek Mandal,a Katharina Beyer,b Alexa Paretzki,b Brigitte Schwederski,b Wolfgang Kaim *b and Goutam Kumar Lahiri *a The redox-active ligand 5,7,12,14-tetraazapentacene-6,13-quinone = L forms structurally characterised compounds with three (1) or four (2) [Ru(acac)2] complex fragments in which each of the metals is N,O-chelated. The new tris- and tetrakis-bidentate chelate compounds exhibit ruthenium centres bridged at about 4 Å by quinone O atoms which are then situated across the pentacene

Received 19th September 2017, Accepted 27th October 2017 DOI: 10.1039/c7dt03509h rsc.li/dalton

π system at about 6–8 Å distance. Several electron transfer processes were observed by voltammetry (CV, DPV) and the intermediates identified by EPR and UV-Vis-NIR spectroelectrochemistry. TD-DFT calculations were applied to assign the proper oxidation states within the multistep redox systems {(μn-L)[Ru(acac)2]n }k, n = 3 or 4, revealing both metal and ligand based electron transfer.

Introduction The study of mixed valent ruthenium complexes1 has been dominated by dinuclear species, ranging from the CreutzTaube ion ([(NH3)5Ru(μ-pyrazine)Ru(NH3)5]5+)2 via triply halide-bridged systems3 to complexes with bis-bidentate4 and bis-tridentate5 bridging molecules. Tris-bidentate bridges were used to form symmetrical trinuclear compounds6 but there are only few tetraruthenium compounds with noninnocent ligand bridges and the potential for mixed valency. Among these are tetracyano ligand-bridged species {(μ4-TCNX)[Ru(NH3)5]4}k+, k = 6, 7, 8, 10, which include non-chelated pentammineruthenium fragments.7,8 Experimental and theoretical analyses have shown that the systems can be described as pairs of coupled diruthenium entities.7c

A recent study9 employing the heteroquinonoid compound 5,7,12,14-tetraazapentacene-6,13-quinone = L10 has shown that it can coordinate two [Ru(acac)2] groups to form L bridged linkage isomers {(μ-L)[Ru(acac)2]2}k (A and B), k = 2−, −, 0, +, 2+, in which each metal is N,O coordinated to that redox-active ligand.

a

Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India. E-mail: [email protected] b Institut für Anorganische Chemie, Universität Stuttgart, Pfaffenwaldring 55, D-70550 Stuttgart, Germany. E-mail: [email protected] † Electronic supplementary information (ESI) available: X-ray crystallographic files in CIF format for 1 and 2, mass spectra (Fig. S1), DFT optimised structures (Fig. S2), EPR (Fig. S3), exprimental and TD-DFT calculated electronic spectra (Fig. S4, S5), crystal (Table S1), DFT calculated bond lengths and angles (Tables S2–S5), energies and MO compositions (Tables S7–S22), experimental and TD-DFT calculated transitions (Tables S23 and S24). CCDC 1553770 for (1) and 1553120 for (2). For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c7dt03509h

This journal is © The Royal Society of Chemistry 2017

We have now succeeded in adding one or two more [Ru(acac)2] fragments by using the uncoordinated N donor atoms and the then bridging quinone oxygen atoms, leading to the tri- and tetranuclear compounds 1 and 2. Tetrakisbidentate ligands such as L or 3,6-bis(2-pyrimidyl)-1,2,4,5tetrazine = bmtz are rare but interesting bridges which should find applications in structural and magnetic studies.11

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Results and discussion Synthesis, characterisation and structures The earlier reported linkage isomers9 of the dinuclear complexes A and B were prepared via the reaction of a 1 : 2 mixture of 5,7,12,14-tetraazapentacene-6,13-quinone (L) and Ru(acac)2 (CH3CN)2 in refluxing EtOH over a period of 5 h under dinitrogen atmosphere. As a part of the present article, the trinuclear (1) and tetranuclear (2) complexes were synthesized by following a similar procedure as stated above for A and B but by using a higher L : metal ratio (1 : 4) and longer reaction times of about 15 h. The complexes 1 (blue) and 2 (green) were separated on a neutral alumina column (Experimental).

Dalton Transactions

The electrically neutral and diamagnetic complexes 1 and 2 were characterised by their microanalytical data, molar conductivity and mass spectrometry (Fig. S1† Experimental). Single crystal X-ray structures of 1 and 2 (Fig. 1) established their identities as well as their rac and meso diastereomeric forms, respectively. Selected bond and crystallographic parameters are listed in Table 1 and Tables S1–S6†, respectively. The asymmetric unit of 1 consists of two independent molecules in rac form of the three tris chelated chiral metal units (Molecule A: ΛΛΛ and Molecule B: ΔΔΔ), while the Ru1, Ru2 and Ru3, Ru4 centres in the meso-diastereomeric form of 2 with four tris chelated chiral metal units exist in ΔΔ and ΛΛ forms, respectively. The two molecules in the unit cell of 1 (Molecule A and Molecule B) exhibit some variations in their bond parameters. The nearly planar L is linked to three and four {RuII(acac)2} units in 1 and 2, respectively, through its suitably positioned N,O donors, forming multiple five-membered chelates with bite angles close to 80°. The Ru1 in Molecule A and Ru4 in Molecule B in the crystal of 1 are bonded to one of the oxygen donors (O1 or O15) of the central para-quinone ring of L, while the other oxygen atom of the quinone ring of L, O2 or O16, bridges Ru2, Ru3 (Molecule A) or Ru5, Ru6 (Molecule B), respectively, which leads to an unsymmetric structure with two different C–O and

Fig. 1 Perspective views of (a) asymmetric unit of 1 and (b) Molecule A in the crystal of 1 and (c) 2. Ellipsoids are drawn at 30% probability level. Hydrogen atoms (C–H) and solvent molecules are removed for clarity.

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Dalton Transactions Table 1

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Selected experimental and DFT calculated bond lengths (Å) and intramolecular Ru–Ru-distances for 1 and 2


1 (Molecule A)

1 (Molecule B)

2·2CH3CN

Bond length

X-ray

DFT (S = 0)

Bond length

X-ray

Bond length

X-ray

DFT (S = 0)

Ru1–N1 Ru1–O1 Ru1–O3 Ru1–O4 Ru1–O5 Ru1–O6 Ru2–N2 Ru2–O2 Ru2–O7 Ru2–O8 Ru2–O9 Ru2–O10 Ru3–N3 Ru3–O2 Ru3–O11 Ru3–O12 Ru3–O13 Ru3–O14 Ru1–Ru2 Ru1–Ru3 Ru2–Ru3

1.995(13) 2.002(13) 2.037(13) 2.010(13) 2.034(12) 2.026(13) 2.047(13) 2.036(13) 2.020(14) 1.998(15) 2.019(13) 2.031(15) 2.041(14) 2.044(13) 2.009(13) 2.025(14) 1.992(14) 2.032(14) 6.702 7.756 3.868

2.057 2.054 2.060 2.064 2.069 2.067 2.095 2.113 2.079 2.053 2.055 2.058 2.114 2.095 2.053 2.061 2.078 2.065 6.877 7.977 3.929

Ru4–N5 Ru4–O15 Ru4–O17 Ru4–O18 Ru4–O19 Ru4–O20 Ru5–N6 Ru5–O16 Ru5–O25 Ru5–O26 Ru5–O27 Ru5–O28 Ru6–N7 Ru6–O16 Ru6–O21 Ru6–O22 Ru6–O23 Ru6–O24 Ru4–Ru5 Ru4–Ru6 Ru5–Ru6

2.013(14) 2.000(13) 2.029(13) 2.037(12) 2.008(15) 2.015(13) 2.051(14) 2.040(13) 2.056(12) 2.012(13) 2.010(12) 2.014(13) 2.038(14) 2.040(13) 2.001(14) 2.017(14) 1.998(13) 2.038(12) 6.737 7.832 3.850

Ru1–N1 Ru1–O1 Ru1–O2 Ru1–O3 Ru1–O4 Ru1–O5 Ru2–N2 Ru2–O1′ Ru2–O6 Ru2–O7 Ru2–O8 Ru2–O9 Ru1–Ru2 Ru1–Ru1′ Ru1–Ru2′ — — — — — —

2.044(3) 2.049(2) 2.019(3) 2.001(2) 2.008(3) 2.013(2) 2.033(3) 2.050(2) 2.019(3) 1.998(2) 2.005(3) 2.023(2) 6.770 7.809 3.879 — — — — — —

2.110 2.106 2.059 2.054 2.060 2.078 2.110 2.166 2.054 2.059 2.060 2.078 6.874 7.985 3.938 — — — — — —

three different RuII–O(L) distances. Unlike in 1, the bridging mode of both the oxygen atoms of the central para-quinone ring of L in the crystal of 2 resulted in a symmetric chemical arrangement around each of the four ruthenium ions. In the crystal of complex 1, the outer rings of L are slightly twisted by angles of 6.35° and 3.70° along the C3, C8 or C51, C56 bond with respect to the rest of the tetraazapentacene ring system in Molecules A and B, respectively. In the crystal of 2, the outer ring system is twisted by 25.25° along the N1, N2 axis, resulting in a chair-like conformation of the bridge. The O–Ru–N chelate planes yield dihedral angles of 20.15°, 16.53°, 22.21°; 15.65°, 19.53°, 14.31° and 15.13°, 16.28° with the connecting O–C–C–N plane involving the bridge in Molecule A and Molecule B of 1 and in 2, respectively. The C1–O1/C49–O15 (quinone ring of L) and Ru1–O1/Ru4– O15 distances involving non-bridged oxygen of L in 1 of 1.28 Å (Molecule A)/1.27 Å (Molecule B) and 2.002 Å (Molecule A)/ 2.000 Å (Molecule B), respectively, are close to those reported earlier for the dinuclear complex A or B,9 which is, however, appreciably shorter than those involving the bridging oxygen donor(s) of L (C10–O2: 1.33 Å, Ru2–O2: 2.036 Å, Ru3–O2, 2.044 Å (Molecule A)/C58–O16: 1.34 Å, Ru5–O16: 2.040 Å, Ru6– O16: 2.040 Å (Molecule B) in 1 and C1–O1: 1.329 Å, Ru1–O1: 2.049 Å, Ru2–O1′: 2.050 Å in 2). The average Ru–N distances (2.028 Å (Molecule A)/2.034 Å (Molecule B)) and Ru–O–Ru angles (142.90° (Molecule A)/141.33° (Molecule B)) in 1 are similar to those of 2.038 Å and 142.27°, respectively, in 2. The average RuII–O(acac) bond distances of 2.019 Å (Molecule A)/2.0198 Å (Molecule B) in 1 and 2.011 Å in 2 match fairly well with those in the analogous L bridged dinuclear complexes A or B (2.02 Å).9 Both the trinuclear (1) and tetranuclear (2) complexes exhibited two different types of Ru–Ru bond distances. The


Ru–Ru distances across the pentacene bridge of ∼6.5–8 Å are significantly longer as compared to those of the oxygenbridged ruthenium centres of 900 nm) contain more metal–metal charge transfer contributions, reflecting the tendency for mixedvalency in such polynuclear arrangements.1–5


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Dalton Transactions

Fig. 7

UV-vis-NIR spectroelectrochemical response in CH3CN/0.1 M Bu4NPF6.


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Table 5

Dalton Transactions UV-visible-NIR spectroelectrochemical dataa for 1n and 2n

Compound

λmax [nm] (ε/dm3 mol−1 cm−1)

12+ 1+ 1 1− 12− 22+ 2+ 2 2−

654 (21 870), 584 sh, 358 (48 500) 700 sh, 572 (31 250), 329 (62 600) 1023 (7320), 599 (37 800), 325(65 300) 1800 (2500), 691 (29 050), 561 (15 320), 370 sh 976 sh, 811 (20 400), 710 (20 250), 450 sh, 338 (46 400) 1186 (8870), 608 (22 810), 454 (19 340), 350 (69 490) 1700 (br, 3230), 800 sh, 617 (32 610), 331 (59 630) 1290 sh, 673 (39 850), 345 (65 440) 1900 (br, 3960), 940 sh, 743 (46 500), 420 sh, 370 sh, 321 (61 420) 1196 (10 870), 760 (49 580), 435 (36 180), 327 (56 650) 1754 (10 860), 1386 (7240), 998 (13 960), 771 (34 810), 583 sh, 500 sh, 445 (44 220)

22− 23− a

From OTTLE spectroelectrochemistry in CH3CN/0.1 M Bu4NPF6.

Conclusion and outlook Symmetrically tetranuclear complexes with redox-active metals and ligands are very rare. In contrast to tetraruthenium compounds of the redox-active TCNX ligands (TCNE, TCNQ etc.)7 the now presented system 2 (complemented by a trinuclear form 1) contains tetrakis-chelated metals. The heteroquinononoid ligand L has been shown to function as a redox active tetrakis-bidentate chelate ligand for up to four coordinated metal complex fragments. In contrast to the two isomeric dinuclear species A and B, the quinone oxygen centres then serve as atomic bridges (ca. 4 Å) between two metals. The systems appear electronically delocalised with reduced ligand and partially oxidised metals. Thus, pairs of mixed-valent entities [Ru2.5–(C–O−)–Ru2.5] are connected in spin-coupled fashion via the tetraazapentacene π system. Magnetically less interacting centres from the first row of transition metals may provide further interesting results. The multiple electron transfers of the complexes lead to species with DFT-analysed absorption and EPR features. Oneelectron oxidation leads to labile cations with mostly metalbased spin whereas the addition of an electron provides for anions with metal/ligand mixed spin distribution (Schemes 2 and 3). The intense long wavelength absorptions result from various degrees of mixture between π or π* MOs of the condensed heteroaromatic system and the combinations of dπ orbitals at the metals. Optical, magnetic and electron transfer properties of oligonuclear compounds of tetrakis-bidentate L with other metal complex fragments appear to be promising materials for components in information technology and multielectron catalysis.

Experimental section Materials The precursor complex cis-[Ru(acac)2(CH3CN)2],15a and the ligand 5,7,12,14-tetraaza-6,13-pentacenequinone (TAPQ)10 (L)

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were prepared according to the literature procedures. Other chemicals and solvents were of reagent grade and used as received. For spectroscopic and electrochemical studies HPLC grade solvents were used. Physical measurements Electrical conductivity of the complexes in CH3CN was checked using an autoranging conductivity meter (Toshcon Industries, India). The EPR measurements were made in a two electrode capillary tube15b with an X-band (9.5 GHz) Bruker system ESP300 spectrometer. Cyclic voltammetric and differential pulse voltammetric measurements of the complexes were done using a PAR model 273A electrochemistry system. Glassy carbon working, platinum wire auxiliary electrode and saturated calomel reference electrode (SCE) were used in a standard three-electrode configuration with tetraethylammonium perchlorate (TEAP) as the supporting electrolyte (substrate concentration ≈10–3 M; standard scan rate 100 mV s−1). UV-visNIR spectroelectrochemical studies were performed in CH3CN/ 0.1 M Bu4NPF6 at 298 K using an optically transparent thinlayer electrode (OTTLE) cell16 mounted in the sample compartment of a J&M TIDAS spectrophotometer. All spectroelectrochemical experiments were carried out under a dinitrogen atmosphere. 1H NMR spectra were recorded on a Bruker Avance III 400 MHz spectrometer. The elemental analyses were recorded on a PerkinElmer 240C elemental analyser. Electrospray mass spectral measurements were done on Electrospray ionisation (ESI) mass spectrometry (MS) was checked on a Bruker’s Maxis Impact (282001.00081) spectrometer. (Caution! Perchlorate salts are explosive and should be handled with care). Crystallography Single crystals of 1 and 2 were grown by slow evaporation of 1 : 1 : 1 CH2Cl2–CH3OH–n-hexane, and 1 : 1 : 1 : 1 CH2Cl2– CH3CN–CH3OH–n-hexane solutions, respectively. X-ray diffraction data were collected using a Rigaku Saturn-724+ CCD single crystal X-ray diffractometer using Cu and Mo-Kα, radiation respectively. The data collection was evaluated by using the CrystalClear-SM Expert software. The data were collected by the standard ω-scan technique. The structure was solved by direct method using SHELXS-97 and refined by full matrix least-squares with SHELXL-2014, refining on F2.17 All data were corrected for Lorentz and polarisation effects and all nonhydrogen atoms were refined anisotropically. The remaining hydrogen atoms were placed in geometrically constrained positions and refined with isotropic temperature factors, generally 1.2Ueq of their parent atoms. Despite several attempts we failed to generate better quality crystals of 1. Though the crystal was small and twinned (BASF 0.30863), we could solve the structure with reasonable R factor but with highly disordered solvent molecules (MeOH and H2O). These were refined isotropically which in effect reflected in Alert A in the checkcif file along with highly disordered C atom (C94). There were two twin components with the ratio of


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Dalton Transactions

0.69 : 0.31, which were handled at integration. Component 2 rotated by 179.9524° around [0.00 0.00 1.00] (reciprocal) or [0.38 0.33 0.86] (direct) and that .hkl (hklf5 form) file from component 2 was used to solve and refine the structure.

Computational details Full geometry optimisations were carried out using the density functional theory method at the (U)B3LYP level for 1n (n = 2+, 1+, 1−, 2−), 2n (n = 2+, 1+, 1−, 2−, 3−) and (R)B3LYP for 1n (n = 0), 2n (n = 0).18 All elements except ruthenium were assigned the 6-31G(d) basis set. The LanL2DZ basis set with effective core potential was employed for the ruthenium atom.19 The broken symmetry formalism20 has been applied for 1n (n = 0), 2n (n = 0). All calculations were performed with the Gaussian09 program package.21 Vertical electronic excitations based on (U) B3LYP optimised geometries were computed using the timedependent density functional theory (TD-DFT) formalism22 in acetonitrile using the conductor-like polarisable continuum model (CPCM)23 Chemissian 1.724 was used to calculate the fractional contributions of various groups to each molecular orbital. All calculated structures were visualised with ChemCraft.25 Synthesis of [Ru3(acac)6(L)] 1 and [Ru4(acac)8(L)] 2. TAPQ (L) (28 mg, 0.0897 mmol) and the precursor cis-Ru (acac)2(CH3CN)2 (13.96 mg, 0.3586 mmol) were taken in 30 cm3 ethanol. The reaction mixture was heated at reflux for 15 h under dinitrogen atmosphere. The solvent was removed under reduced pressure and the solid mass thus obtained was purified using a neutral alumina column. Complex 1 (blue) was eluted initially by 15 : 1 CH2Cl2–CH3CN mixture, followed by complex 2 (green) with 10 : 1 CH2Cl2–CH3CN mixture. Evaporation of solvent afforded the pure complexes. 1: Yield: 20 mg, 18.43%. MS (ESI+, CH3CN), m/z calcd for {1 + H}+: 1210.0390; found: 1210.1258. ΛM (Ω−1 cm2 M−1) in acetonitrile at 298 K: 3. 1H NMR (400 MHz, CDCl3, 298 K): δ( ppm, J (Hz)): 8.52 (d, 1H, 6.4), 8.23 (d, 1H, 6.4), 7.59 (m, 2H), 7.44 (m, 2H), 6.65 (d, 1H, 6.8), 6.20 (d, 1H, 6.4), 5.84 (s, 1H), 5.77 (s, 1H), 5.60 (s, 1H), 5.19 (s, 1H), 5.14 (s, 1H), 5.06 (s, 1H), 2.51 (s, 3H), 2.44 (s, 3H), 2.37 (s, 6H), 2.33 (s, 3H), 2.31 (s, 3H), 2.30 (s, 3H), 2.27 (s, 3H), 2.07 (s, 3H), 1.99 (s, 3H), 1.98 (s, 3H), 1.83 (s, 3H). Anal. calcd for C48H50N4O14Ru3: C, 47.64; H, 4.16; N, 4.63; found: C, 47.74; H, 4.10; N 4.45. 2: Yield: 45 mg, 33.33%. MS (ESI+, CH3CN), m/z calcd for + {2} : 1509.0139; found: 1509.4069. ΛM (Ω−1 cm2 M−1) in acetonitrile at 298 K: 10. 1H NMR (400 MHz, CDCl3, 298 K): δ( ppm, J (Hz)): 7.16 (m, 1H), 6.51 (m, 1H), 5.62 (s, 1H), 4.96 (s, 1H), 2.36 (s, 3H), 2.28 (s, 3H), 2.22 (s, 3H), 1.96 (s, 3H); Anal. calcd for C58H64N4O18Ru4: C, 46.15; H, 4.27; N, 3.71; found: C, 46.34; H, 4.16; N 3.83.

Conflicts of interest There are no conflicts to declare.


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Acknowledgements Financial support received from the Department of Science and Technology (DST, SERB), Council of Scientific and Industrial Research (fellowship to A. M.), New Delhi (India), the Land Baden-Württemberg (Germany), is gratefully acknowledged. The help of Dr S. M. Mobin, IIT-Indore for the crystal structure determination of 1 is also gratefully acknowledged.

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20

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