Bimetallic Dihydrobis(methimazolyl)borate Coordination: Molecular

Jan 20, 2009 - Ian A. Cade, Anthony F. Hill*, Never Tshabang and Matthew K. Smith. Research School of Chemistry, Institute of Advanced Studies, Austra...
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Organometallics 2009, 28, 1143–1147

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Bimetallic Dihydrobis(methimazolyl)borate Coordination: Molecular Structure [Mo2Au{µ2-H2B(mt)2}(PPh3)(CO)7] (mt ) methimazolyl) Ian A. Cade, Anthony F. Hill,* Never Tshabang, and Matthew K. Smith Research School of Chemistry, Institute of AdVanced Studies, Australian National UniVersity, Canberra, ACT, Australia ReceiVed August 8, 2008

The sequential treatment of Na[H2B(mt)2] (mt ) methimazolyl) with [Mo(pip)2(CO)4] (pip ) piperidine), [Mo(CO)3(L)3] (L ) NCMe, L3 ) η6-C7H8) and [AuCl(PPh3)] provides the diheterotrimetallic complex [Mo2Au{µ-H2B(mt)2}(CO)7(PPh3)] in which each thione group of the H2B(mt)2 ligand bridges two molybdenum centers, one of which is also involved in a 3-center, 2-electron B-H-Mo interaction. This complex is also obtained directly from [Mo(CO)3(η6-C7H8)] and [Au(PPh3){H2B(mt)2}] while the triheterotrimetallic species [MoWAu{µ-H2B(mt)2}(CO)7(PPh3)] results from Na[H2B(mt)2], [W(CO)3(NCMe)3], [Mo(pip)2(CO)4], and [AuCl(PPh3)]. Introduction While sulfur donors are prevalent in the active sites of many metalloenzymes, thiolate ligands in the absence of a prophylactic protein sheath often render small molecule analogues of metalloenzynes prone to oligomerization through thiolato bridges, while thioether coordination is typically labile. It has been suggested1 that the poly(methimazolyl)borate class of anionic ligands HxB(mt)4-x (x ) 1,2; methimazolyl) might have a role to play in bioinorganic chemistry as “tame thiolate” mimics. For most first row biologically significant metals, this is a reasonable working hypothesis. Indeed Vahrenkamp,1 Reglinski,2 Parkin,3 and Rabinovich4 have convincingly demonstrated that the curtailed propensity toward bridging by methimazolyl groups has much to offer biomimetic chemistry. However, with the exception of molybdenum, the transition metals commonly encountered in a bioinorganic context are not especially thiophilic (i.e., “soft”), and to date bridging modes have not generally been a feature of the bioinorganic chemistry of these otherwise intriguing chelates. Rather, e.g., binuclear complexes of the form [M2(µ-X)2{HB(mt)3}2] (X ) halide) with halide rather than sulfur bridges appear the preferred norm. Our own interest in these ligands5 has, in the main, been with respect to organometallic applications, not least because numer* To whom correspondence should be addressed. E-mail: a.hill@ anu.edu.au. (1) (a) Ibrahim, M. M.; Shu, M.; Vahrenkamp, H. Eur. J. Inorg. Chem. 2005, 1388. (b) Seebacher, J.; Shu, M.; Vahrenkamp, H. Chem. Commun. 2003, 2502. (c) Ji, M.; Benkmil, B.; Vahrenkamp, H. Inorg. Chem. 2005, 44, 3518. (d) Benkmil, B.; Ji, M.; Vahrenkamp, H. Inorg. Chem. 2004, 43, 8212. (e) Schneider, A.; Vahrenkamp, H. Z. Anorg. Allg. Chem. 2004, 630, 1059. (f) Seebacher, J.; Vahrenkamp, H. J. Mol. Struct. 2003, 656, 177. (g) Shu, M.; Walz, R.; Wu, B.; Seebacher, J.; Vahrenkamp, H. Eur. J. Inorg. Chem. 2003, 2502. (2) Garner, M.; Reglinski, J.; Cassidy, I.; Spicer, M. D.; Kennedy, A. R. Chem. Commun. 1996, 1975. (b) Reglinski, J.; Garner, M.; Cassidy, I. D.; Slavin, P. A.; Spicer, M. D.; Armstrong, D. R. Dalton Trans. 1999, 2119. (3) (a) Kimblin, C.; Hascall, T.; Parkin, G. Inorg. Chem. 1997, 36, 5680. (b) Kimblin, C.; Bridgewater, B. M.; Hascall, T.; Parkin, G. Dalton Trans. 2000, 891. (c) Melnick, J. G.; Docrat, A.; Parkin, G. Chem. Commun. 2004, 2870. (d) Morlok, M. M.; Docrat, A.; Janak, K. E.; Tanski, J. M.; Parkin, G. Dalton Trans. 2004, 3448. (e) Docrat, A.; Morlok, M. M.; Bridgewater, B. M.; Churchill, D. G.; Parkin, G. Polyhedron 2004, 23, 481. (f) Bridgewater, B. M.; Fillebeen, T.; Friesner, R. A.; Parkin, G. Dalton Trans. 2000, 4494. (g) Kimblin, C.; Bridgewater, B. M.; Churchill, D. G.; Hascall, T.; Parkin, G. Inorg. Chem. 2000, 39, 4240. (h) Parkin, G. Chem. ReV. 2004, 104, 699.

ous heterogeneous (and indeed biological6) catalysts feature metals in a sulfur-rich environment that we feel might be emulated by these poly(thia)chelates. The supposition that bridging modes might be disfavored is perhaps less apt for ‘soft’ metal centers. This has been illustrated by the unusual bridging mode observed for the complex [Pt2I(Me)5{µ-H2B(mt)2}] (a, Chart 1).5a We would not suggest that the H2B(mt)2 ligand serves such a role in nature, however it is nevertheless of interest that the toplogy is not dissimilar to that found in the organometallic core of [Fe-Fe]-hydrogenases isolated from Clostridium

(4) (a) Mihalcik, D. J.; White, J. L.; Tanski, J. M.; Zakharov, L. N.; Yap, G. P. A.; Incarvito, C. D.; Rheingold, A. L.; Rabinovich, D. Dalton Trans. 2004, 1626. (b) Graham, L. A.; Fout, A. R.; Kuehne, K. R.; White, J. L.; Mookherji, B.; Marks, F. M.; Yap, G. P. A.; Zakharov, L. N.; Rheingold, A. L.; Rabinovich, D. Dalton Trans. 2005, 171. (c) Patel, D. V.; Mihalcik, D. J.; Kreisel, K. A.; Yap, G. P. A.; Zakharov, L. N.; Kassel, W. S.; Rheingold, A. L.; Rabinovich, D. Dalton Trans. 2005, 2410. (d) Graham, L. A.; Fout, A. R.; Kuehne, K. R.; White, J. L.; Mookherji, B.; Marks, F. M.; Yap, G. P. A.; Zakharov, L. N.; Rheingold, A. L.; Rabinovich, D. Dalton Trans. 2005, 171. (e) Alvarez, H. M.; Tanski, J. M.; Rabinovich, D. Polyhedron 2004, 23, 395. (f) Philson, L. A.; Alyounes, D. M.; Zakharov, L. N.; Rheingold, A. L.; Rabinovich, D. Polyhedron 2003, 22, 3461. (g) White, J. L.; Tanski, J. M.; Rabinovich, D. Dalton Trans. 2002, 2987. (h) Bakbak, S.; Incarvito, C. D.; Rheingold, A. L.; Rabinovich, D. Inorg. Chem. 2002, 41, 998. (i) Bakbak, S.; Bhatia, V. K.; Incarvito, C. D.; Rheingold, A. L.; Rabinovich, D. Polyhedron 2001, 20, 3343. (j) Alvarez, H. M.; Krawiec, M.; Donovan-Merkert, B. T.; Fouzi, M.; Rabinovich, D. Inorg. Chem. 2001, 40, 5736. (5) (a) Crossley, I. R.; Hill, A. F.; Willis, A. C. Organometallics 2005, 24, 4889. (b) Foreman, M. R. St.-J.; Hill, A. F.; Tshabang, N.; White, A. J. P.; Williams, D. J. Organometallics 2003, 22, 5593. (c) Foreman, M. R. St.-J.; Hill, A. F.; Smith, M. K.; Tshabang, N. Organometallics 2005, 24, 5224. (d) Abernethy, R. J.; Foreman, M. R.St.-J.; Hill, A. F.; Tshabang, N.; Willis, A. C.; Young, R. D. Organometallics 2008, 27, 4455. (e) Hill, A. F.; Rae, A. D.; Smith, M. K. Inorg. Chem. 2005, 44, 7316. (f) Hill, A. F.; Smith, M. K. Dalton Trans. 2006, 28. (g) Foreman, M. R. St.-J.; Hill, A. F.; White, A. J. P.; Williams, D. J. Organometallics 2003, 22, 3831. (h) Abernethy, R. J.; Hill, A. F.; Neumann, H.; Willis, A. C. Inorg. Chim. Acta 2005, 358, 1605. (i) Hill, A. F.; Smith, M. K. Chem. Commun. 2005, 1920. (j) Foreman, M. R. St.-J.; Hill, A. F.; White, A. J. P.; Williams, D. J. Organometallics 2003, 22, 3831. (k) Hill, A. F.; Owen, G. R.; White, A. J. P.; Williams, D. J. Angew. Chem., Int. Ed. 1999, 38, 2759. (6) (a) Stiefel, E. I. J. Chem. Soc., Dalton Trans. 1997, 3915. (b) Stiefel, E. I.; Murray, H. H. HeaVy Metals in the EnVironment. Sarkar, B., Ed.; Marcel Dekker: New York, 2002; p 503. (c) Maher, M. J.; Santini, J.; Pickering, I. J.; Prince, R. C.; Macy, J. M.; George, G. N. Inorg. Chem. 2004, 43, 402.

10.1021/om8007647 CCC: $40.75  2009 American Chemical Society Publication on Web 01/20/2009

1144 Organometallics, Vol. 28, No. 4, 2009 Chart 1. Topological Similarities between Binuclear Dihydrobis(methimazolyl)borate Complexes and the Organometallic Core of HoxCO-[Fe-Fe] hydrogenases7

pasteurianum and DesulfoVibrio desulfuricans,7 i.e., two bridging sulfur donors chelated by a third group which is proximal to one metal (b, Chart 1). Given the hydrogenase role of these enzymes, if a similar architecture were to be constructed in which this proximal group was a potential hydride donor, in combination with a ‘protic’ metal hydride, one might envisage interesting reactivity. We have already demonstrated that the mononuclear complex [RuH(CO)(PPh3){κ3-H,S,S′-HB(mt)3}] hydrogenates ethynylbenzene under mild conditions, albeit stoichiometrically,5j to afford styrene and the metallaboratrane [Ru(CO)(PPh3){B(mt)3}].5k Herein, we wish to report that the bridging mode in (1) is not a unique curiosity, but perhaps the fore-runner of a wider class of bi- and polymetallic species that adopt such a bridging mode.

Results and Discussion The complex 1 has the H2B(mt)2 ligand bound to otherwise three- and four coordinate d6-platinum centers.5a We have therefore sought to construct, in a rational manner, a similar architecture based on two d6-molybdenum centers, given that the active sites of molybdoenzymes typically involve two or more sulfur donors. The carbonyl metallates Na[Mo(CO)x{H2B(mt)2}] (x ) 4 Na[2], 3 Na[3])5b are readily obtained under mild conditions via the reaction of Na[H2B(mt)2] with either cis-[Mo(pip)2(CO)4] (x ) 4, pip ) piperidine) or fac-[Mo(CO)3(L)3] (x ) 3, L ) NCMe, L3 ) η6-C6H7). These metal-based nucleophiles provide ready access to thiocarbamoyl,5b stannyl5c and nitrosyl derivatives.5d Their (7) (a) Fontecilla-Camps, J. C.; Volbeda, A.; Cavazza, C.; Nicolet, Y. Chem. ReV. 2007, 107, 4273. (b) Vincent, K. A.; Parkin, A.; Armstrong, F. A. Chem. ReV. 2007, 107, 4366.

Cade et al. Scheme 1. Synthesis of Bi- and Trimetallic µ-H2B(mt)2 Complexes

generation requires the careful combination of equivalent amounts of the two reagents. When, however an excess of the molybdenum reagent is employed, a second product Na[4] is obtained and while this may be employed in situ as a sodium salt, metathesis with [PPN]Cl (PPN ) bis(triphenylphosphine)iminium) provides the more tractable salt [PPN][4]. Notably, Na[4] may be prepared either by addition of [Mo(CO)3(L)3] to Na[2] or by addition of [Mo(pip)2(CO)4] to Na[3], i.e., the formation of [4]- is independent of the construction sequence (Scheme 1). In the absence of structural data, the formulation of [4]- rests upon spectroscopic and microanalytical data. While the ESI mass spectrum (-ve ion mode) is devoid of a molecular ion, a major isotope cluster may be identified that corresponds on the basis of isotopic abundances to the fragment [Mo2(CO){H2B(mt)2}]- thereby suggesting a bimetallic complex. Although three carbonyl environments are indicated (13C), the molecule appears to straddle a molecular symmetry plane, given that only one methimazolyl environment is observed. The infrared spectrum provides an indication that one B-H group is involved in 3-center, 2-electron bonding to one molybdenum center (2410 νBH, 2237 νBHMo cm-1). The decrease in the

Bimetallic Dihydrobis(methimazolyl)borate Coordination

Organometallics, Vol. 28, No. 4, 2009 1145 Scheme 2. Synthesis of a µ-H2B(mt)2 Triheterotrimetallic Complex

Figure 1. The molecular structure of 5 in a crystal (50% displacement ellipsoids, phenyl groups simplified, hydrogen atoms omitted). Selected bond lengths (Å) and angles (deg): Au1-P1-2.2869(14), Au1-C6 2.363(6), Au1-C7 2.430(5), Au1-Mo2 2.7537(8), Mo1-S11 2.6262(14), Mo1-S21 2.6267(13), Mo2-S11 2.5401(13) Mo2-S21 2.5593(13), S11-C11 1.730(5), S21-C21 1.733(5), Mo2-B1 2.945(6), P1-Au1-Mo2 166.24(4), S11-Mo1-S21 79.28(4), S11-Mo2-S21 82.17(4), S11-Mo2-Au1 137.84(3), S21-Mo2-Au1 138.28(3), O6-C6-Mo2 167.9(5), O7-C7-Mo2 169.5(5), C11-S11-Mo1 101.71(15), Mo2-S11-Mo1 97.30(4), C21-S21-Mo1 102.40(16), Mo2-S21-Mo1 96.81(4).

frequency of one νBH absorption is typical of such an interaction, e.g., the complex [Mo(η2-SCNMe2)(CO)2{H2B(mt)2}]5b has absorptions at 2407 and 2261 cm-1(Nujol). The presence of a B-H-Mo interaction is further substantiated by the appearance of a broadened resonance to high frequency (δH ) -4.30 ppm) of SiMe4. Accordingly, we assign to [4]-, the structure shown in Scheme 1 (c, Chart 1), akin to that which was crystallographically authenticated for 1.5a Thus the molecule contains two chemically distinct molybdenum centers that do not undergo exchange on the NMR time scale, though rotation of the ‘Mo(CO)3’ group is implicit in the appearance of three carbonyl 13 C resonances. The signal due to three equivalent carbonyl ligands is moved to lower field of the remaining two resonances, suggestive of semi bridging character in the time averaged environment. In the solid state, two of the carbonyl ligands also appear to adopt a semibridging role (νCO ) 1795, 1767 cm-1), though this interpretation is clouded by the high π-basicity of the molybdenum centers which would also lead to a decrease in the value of νCO. Treating a solution of isolated PPN[4] or alternatively Na[4] generated in situ with [AuCl(PPh3)] results in the formation of a neutral complex (5) via salt elimination. A consideration of spectroscopic data for 5 would appear to suggest that only modest structural rearrangement attends the addition of the “AuPPh3” fragment. Thus the pattern of carbonyl infrared absorbances, though moved to higher frequency, is similar to that of PPN[4]. The pattern of 13C resonances is less diagnostic in that due to limited solubility only two carbonyl resonances were reliably observed. However, both 1H and 13C{1H} NMR data indicate that the complex has a molecular plane of symmetry that is once again straddled by the H2B(mt)2 ligand. The B-H-Mo interaction would also appear to be retained in the product (νBHMo(CH2Cl2) ) 2305 cm-1; δBHMo ) -5.0 ppm). The molecular structure of 5 in the solid state was established from a crystallographic study, the results of which are summarized in Figure 1. The molecular geometry is as anticipated from spectroscopic data, confirming the connectivity in addition to the single B-H-Mo interaction. Although the hydrogen position in this B-H-Mo arrangement is characteristically imprecise, being in the vicinity of two heavy atoms, the more accurately

determined Mo2-B1 bond length (2.945(6) Å) is comparable to those found in [Mo(η2-SCNMe2)(CO)2{H2B(mt)2}] (2.964 Å)5b and [Mo(NO)(CO)2{H2B(mt)2}] (2.918 Å).5d The Au-1Mo2separationof2.7537(8)Åisunremarkableforgold-molybdenum bonds (e.g., 2.710 Å for [MoAu(CO)3(PPh3)(η-C5H5)]8) other than to note that this bond is supported by two carbonyl ligands that display semibridging character (Au1 · · · C6 2.363(6), Au1 · · · C7 2.430(5) Å). The Mo1-Mo2 separation (3.879 Å) is however beyond what would normally be considered bonding distance. There exist copious structural data for dimolybdenum bis(µ-thiolato) carbonyl complexes of the form (OC)w(L)xMo(µSR)2Mo(CO)yLz for which Mo-Mo bonding may be invoked.9 In such cases the Mo-Mo separations typically span the range 2.9-3.1Å however these generally involve d5-d5 configurations rather than the d6-d6 configuration in 5 (and 1), for which metal-metal bonding is not required. Although spectroscopic details have yet to appear, the crystal structure of the complex [Au(PPh3){H2B(mt)2}] (6) has been previously reported.10 We considered that the AuMo2 core in 5 might be constructed in the reverse order beginning with 6, given that we have previously demonstrated that zerovalent molybdenum reagents can insert into the Sn-S bond of poly(methimazolyl)boratostannanes.5c This is indeed the case, such that the complex 5 results from the direct reaction of [Mo(CO)3(η6-C7H8)] with 6 in remarkably good yield, considering that some carbonyl scrambling is required to arrive at the final stoichiometry. It also proved possible to construct a triheterotrimetallic analogue of 6 via the sequential treatment of Na[H2B(mt)2] with [W(CO)3(NCMe)3] (to provide in situ, the salt Na[W(CO)3{H2B(mt)2}] Na[7]5d), [Mo(pip)2(CO)4] and [AuCl(PPh3)] (Scheme 2). Although the yield obtained was modest and not optimized, the spectroscopic data for [MoWAu{µH2B(mt)2}(CO)7(PPh3)] (6) are directly comparable to those for 5 and are consistent with the retention of the B-H-W interaction (νBHW ) 2250 cm-1; δBHW ) -4.67 ppm), initially present in [7]- (νBHW ) 2252 cm-1). (8) Pethe, J.; Maichle-Mossmer, C.; Strahle, J. Z. Anorg. Allg. Chem. 1997, 623, 1413. (9) Some 750 crystal structure determinations involve compounds with a Mo2(m-SR)2 core: Cambridge Crystallographic Data Centre, Conquest, Version 1.7, November 2007 release. (10) Mohamed, A. A.; Rabinovich, D.; Fackler, J. P. Acta Crystallogr. 2002, E58, m726.

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Cade et al.

Concluding Remarks

(br, 1 H, BH), 3.63 (s, 6 H, NCH3), 6.83, 6.96 (d × 2, 2 H × 2, NHCdCHN, 3JHH ) 2.1 Hz), 7.48-7.55, 7.65-7.71 (m × 2, 30H, C6H5). δC ) 221.6 (3MoCO), 209.1 (2MoCO), 206.3 (2MoCO), 164.4 (CdS), 134.1 [C4(C6H5)], 132.5 [t, C3,5(C6H5), JPC ) 11.8 Hz], 129.8 [d, C2,6(C6H5), JPC ) 13.1 Hz], 127.4 [d, C1(C6H5), JPC ) 107 Hz], 123.4, 122.0 (NHCdCHN), 35.1 (NCH3), ppm. δP ) 21.7 ppm. δB ) -6.42 ppm. ESI-MS(-ve): m/z ) 462.0(20) [M - 6CO]-. Anal. Found: C, 52.50; H, 3.88; N, 6.14. Calcd for C51H42BMo2N5O7P2S2: C, 52.55; H, 3.63; N, 6.01. Synthesis of [Au{H2B(mt)2}(PPh3)] (6). A mixture of [AuCl(PPh3)]12 (0.50 g, 1.01 mol) and Na[H2B(mt)2]5b (0.26 g, 1.01 mmol) was stirred in tetrahydrofuran (30 mL) at room temperature for 4 h and then freed of volatiles under reduced pressure. The crude residue was extracted with dichloromethane (3 × 10 mL) and the combined extracts filtered through diatomaceous earth to remove the precipitated NaCl. The filtrate was concentrated under reduced pressure and then diluted with diethyl ether (30 mL). The solution was then slowly concentrated under reduced pressure to furnish an off-white precipitate which was isolated by filtration, washed with light petroleum ether and dried in vacuo. Yield ) 0.67 g (96%). IR CH2Cl2: 2383 νBH cm-1. THF: 2388 νBH cm-1. Nujol: 2397 νBH cm-1. 1H NMR (C6D6, 298 K): δ ) 3.46 (s, 6 H, NCH3), 6.69, 6.93 (d × 2, 3JHH ) 2.0 Hz, 2 H, NHCdCHN), 7.38-7.57 (m, 15 H, C6H6) ppm. 31P{1H} NMR (121.4 MHz, CD2Cl2); δP ) 35.5 ppm. Anal. Found: C, 44.74; H, 4.19; N, 7.87. Calcd for C26H27AuBN4PS2: C, 44.71; H, 3.90; N, 8.02%. The crystal structure of this complex has been reported;10 however spectroscopic and microanalytical data were not given. Synthesis of [Mo2Au{H2B(mt)2}(CO)7(PPh3)] (5). Method 1. A mixture of [Mo(CO)3(η6-C7H8)]13 (0.20 g, 0.74 mmol) and [Au{H2B(mt)2}(PPh3)] (6: 0.25 g, 0.37 mmol) in tetrahydrofuran (30 mL) was stirred for 12 h after which time the reaction mixture had turned from deep red to orange. Volatiles were removed under reduced pressure and the residue extracted with dichloromethane (3 × 10 mL). The combined extracts were filtered through diatomaceous earth and concentrated to ca. 10 mL and then diluted with diethyl ether (15 mL). The total volume was further reduced to ca. 10 mL followed by cooling overnight (-18 °C) to furnish an orange powder. Yield ) 0.23 g (74%). Method 2. A solution of Na[4] was prepared as described above: A mixture of [Mo(pip)2(CO)4]14 (0.24 g, 0.64 mmol) and Na[H2B(mt)2]5b (0.17 g, 0.64 mmol) was stirred in THF (30 mL) for 30 min at room temperature to provide a solution of Na[3]. To this was added [Mo(CO)3(η6-C7H8)]13 (0.17 g, 0.64 mmol) and the mixture was left to stir for 1 h after which time it had changed from red to yellow. To this was added [AuCl(PPh3)] (0.32 g, 0.64 mmol) and the mixture left to stir overnight. The solvent was removed and the residue purified as described above to provide an orange powder. Yield ) 0.47 g (68%). IR (Nujol): 2410 w νBH, 2237 w νBHMo, 2022 w, 1930 s, 1911 m, 1895 w, 1853 vs, 1812 vs νCO cm-1. IR (CH2Cl2): 2405 νBH, 2305 νBHMo, 2066 m, 1929 vs, 1871 vs, 1812 vs νCO cm-1. NMR (CD2Cl2, 298 K): δH ) -5.02 (br., 1 H, BHMo), 3.52 (s, 6 H, NCH3), 6.73, 6.75 (d × 2, 2 H × 2, 3JHH ) 1.9 Hz, NHCdCHN), 7.52-7.55 (m, 15 H, C6H5). δC ) 214.9 (µ-CO), 206.6 (MoCO), 150.0 (CdS), 134.5 [d, C3,5(C6H5), JPC ) 14.7 Hz], 132.1 [C4(C6H5)], 129.6 [d, C2,6(C6H5), JPC ) 12.2 Hz], 126.4 [d, C1(C6H5), JPC ) 149 Hz], 120.0, 118.6 (NCHCHN), 35.2 (NCH3) ppm. δP ) 36.6 (s.br.) ppm. Anal. Found: C, 36.18; H, 2.32; N, 5.10%. Calcd for C33H27AuBMo2N4O7PS2: C, 36.49; H, 2.51; N, 5.16%. Crystal data: C33H27AuBMo2N4O7PS4; Mr ) 1086.33; monoclinic; P21/c; a ) 9.330(2); b ) 14.460(3); c ) 28.570(6) Å; β ) 98.69(3)°; V ) 3810.2(13) Å3; Z ) 4; Dc ) 1.894 Mgm-3; µ(Mo KR) ) 4.691 mm-1; T ) 200(2) K, yellow plate, 0.25 × 0.15 × 0.15 mm; 8715 independent measured reflections, F2 refinement, R1 ) 0.0375, wR2 ) 0.0800; 6606 independent observed absorption corrected reflections ([I] > 2σ([I]), 2θ e 55°), 468 parameters, 18 restraints, CCDC 693009.

For soft metal centers, it would appear that coordination of the H2B(mt)2 ligand to one metal does not entirely ‘tame’ the potential nucleophilicity of the thione donors. Thus complexes in which the H2B(mt)2 ligand adopts a κ3-H,S,S′ coordination mode (molybdenum, gold or platinum) are predisposed to act as sulfur chelates to a second metal center. Such complexes might thus be considered metalloligands that serve as building blocks for heterobi- and trimetallic compounds. While the connection to bis(µ-thiolate) bridged [Fe-Fe] and [Fe-Ni] hydrogenase active sites might be tenuous, this perspective does provide a strategy for heterobimetallic assembly in which one metal is ligated by a potentially reactive or hemilabile B-H-M interaction.

Experimental Section General Considerations. All manipulations of air-sensitive compounds were carried out under a dry and oxygen-free nitrogen atmosphere using standard Schlenk and vacuum line techniques, with dry and degassed solvents. NMR spectra were recorded at 25 °C on a Varian Gemini 300BB (1H at 300.75 MHz, 13C at 75.4 MHz, 13P at 121.4 MHz) or a Varian XL-200E (1H at 200 MHz, 13 C at 50.3 MHz, 31P at 81.0 MHz). The chemical shifts (δ) for 1H and 13C spectra are given in ppm relative to residual signals of the solvent and to an external 85% H3PO4 reference for 31P. Mass spectra were obtained on a ZAB-SEQ4F spectrometer by fast-atom bombardment (FAB), APCI or ESI techniques using matrices of either 3-nitrobenzyl alcohol (NBA), 3-nitrophenyl octyl ether (NOPE), or acetonitrile and methanol (ESI) by the mass spectrometry service of the Australian National University. Elemental microanalysis was performed by the microanalytical service of the Australian National University. Data for X-ray crystallography were collected with a Nonius Kappa CCD diffractometer. The compounds Na[H2B(mt)2],5b [Mo(NCMe)3(CO)3],11 [W(CO)3(NCMe)3],11 [AuCl(PPh3)],12 [Mo(CO)3(η6-C7H8)],13 and [Mo(pip)2(CO)4]14 were prepared according to the indicated published procedures. Synthesis of [PPN][Mo2{H2B(mt)2}(CO)7] [PPN] (4). A mixture of [Mo(pip)2(CO)4]14 (0.72 g, 1.91 mmol) and Na[H2B(mt)2]5b (0.50 g, 1.91 mmol) was stirred in THF (30 mL) for 30 min at room temperature to provide a solution of Na[Mo{H2B(mt)2}(CO)4]5b (Na[2] confirmed by infrared spectroscopy (THF: νCO, 1928, 1879, 1860 and 1817 cm-1; νBH, 2336 cm-1). To this was added [Mo(CO)3(η6-C7H8)]13 (0.52 g, 1.91 mmol) and the mixture was left to stir for 1 h during which time it changed from red to yellow. [PPN]Cl (1.10 g, 1.91 mmol) was added and the reaction mixture was left to stir for a further hour to give a bright yellow solution. The reaction mixture was filtered through diatomaceous earth to leave behind a white solid residue (NaCl). The filtrate was reduced to ca. 5 mL and then diluted with diethyl ether to provide a yelloworange oil which was triturated with diethyl ether in an ultrasonic cleaning bath to afford a yellow suspension. The supernatant was decanted off and discarded and the resulting yellow powder washed with diethyl ether and then anaerobically recrystallized twice from a mixture of tetrahydrofuran and diethyl ether. Yield ) 1.39 g (76%). IR Nujol: 2399 w νBH, 2291 νBHMo, 2010 m, 1913 s, 1893 s, 1862 s, 1795 vs br, 1767 cm-1. CH2Cl2: 2409 νBH, 2311 w νBHMo, 2013 m, 1917 vs, 1899 vs, 1865 vs, 1816 vs 1799 vs, 1790 sh cm-1. NMR (CD2Cl2, 298 K) δH ) -4.30 (br, 1H, BHMo), 1.6 (11) Tate, D. P.; Knipple, W. R.; Augl, J. M. Inorg. Chem. 1962, 1, 433. (12) Braunstein, P.; Lehner, H.; Matt, D.; Burgess, K.; Ohlmeyer, M. J. Inorg. Synth. 1990, 27, 218. (13) Abel, E. W.; Bennett, M. A.; Burton, R.; Wilkinson, G. J. Chem. Soc. 1958, 4559. (14) Darensbourg, D. J.; Kump, R. L. Inorg. Chem. 1978, 17, 2680.

Bimetallic Dihydrobis(methimazolyl)borate Coordination Synthesis of [MoWAu{H2B(mt)2}(CO)5(µ-CO)2(PPh3)] (8). The reaction of [W(CO)3(NCMe)3]11 (0.25 g, 0.64 mmol) with Na[H2B(mt)2]5a (0.17 g, 0.64 mmol) was carried out in CH2Cl2 (30 mL) for 1 h at room temperature to provide the orange salt Na[W{H2B(mt)2(CO)3] Na[7]. To the reaction mixture was added [Mo(CO)4(pip)2]14 (0.23 g, 0.64 mmol) and reaction mixture was allowed to stir for 12 h. To the resultant mixture was added [AuCl(PPh3)]12 (0.32 g, 0.64 mmol) and the reaction allowed to stir for a further 12 h. The reaction mixture was filtered and ethanol (20 mL) added to the filtrate and then the total solvent volume reduced to ca 15 mL to afford a light yellow precipitate. The precipitate was isolated by filtration, washed with diethyl ether (10 mL) and dried in vacuo and recrystallized as a dichloromethane hemisolvate from a mixture of dichloromethane and light petroleum ether (-18 °C). Yield ) 0.23 g (31%). IR CH2Cl2: 2392 νBH, 2250 νBHW, 2001, 1919, 1906, 1886, 1794, 1750 νCO cm-1. NMR (CD2Cl2, 298 K): δH ) -4.67 (br., 1 H, BHW), 3.49 (s, 6 H, NCH3), 6.68, 6.74 (d × 2, 2 H × 2, 3JHH ) 1.8 Hz, NHCdCHN), 7.41-7.52

Organometallics, Vol. 28, No. 4, 2009 1147 (m, 15 H, C6H5). δC ) 207.8, 197.4, 184.9 (CO), 164.7 (CdS), 127.7-134.3 (C6H5), 124.1, 122.8 (NHCdCHN), 30.0 (NCH3) ppm. δP ) 26.2 ppm. ESI-MS: m/z ) 1147(60) [M - CO]+. Anal. Found: C, 32.88; H, 2.30; N, 4.85. Calcd for C33H27N4O7MoWBPAuS2.(CH2Cl2)0.5: C, 33.07; H, 2.32; N, 4.60. Dichloromethane of solvation confirmed by 1H NMR integration.

Acknowledgment. We thank the Australian Research Council (ARC) for financial support (Grant No. DP0556236) and the University of Botswana fort a studentship (to N.T.). Supporting Information Available: Full details of the crystal structure determinations of 5 in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org. This information is also available from the Cambridge Crystallographic Data Centre (CCDC 693009). OM8007647