Chemistry of polynuclear metal complexes with bridging carbene or carbyne ligands. Part 53. Alkylidyne(carbaborane)dimetal complexes with tungsten–molybdenum and –tungsten bonds; crystal structures of the compounds [MW(µ-CC6H4Me-4)(CO)3(η5-C9H7)(η5-C2B9-H9Me2)][M = Mo (two isomers) or W]

The salts [N(PPh3)2][W(CC6H4Me-4)(CO)2(η5-7,8-C2B9H9R2)](R = Me or H) and [M (CO)2(NCMe)2(η5-C9H7)][BF4](M = Mo or W, C9H7= indenyl) react in CH2Cl2 at ambient temperatures to give the dimetal compounds [MW(µ-CC6H4Me-4)(CO)3(η5-C9H7)(η5-7,8-C2B9H9R2)](2a, M = Mo, R = Me; 2b, M = W, R = Me; 2c, M = Mo, R = H). X-Ray diffraction studies on (2a) and (2b) showed they were isostructural, with the metal–metal bonds bridged by the p-tolylmethylidyne group and by a three-centre two-electron B–H⇀M bond involving the carbaborane group ligating the tungsten atom. The tungsten atom carries two terminally bound CO groups while the other metal centre (2a, Mo; 2b, W) is bonded by one CO ligand and the indenyl group. Structural parameters for (2a) and (2b) are essentially identical; for (2a), Mo–W 2.657(2), µ-C–Mo 1.95(2), µ-C–W 2.03(l), B–µ-H 1.02, and µ-H–Mo 1.719 A. In toluene at 80 °C, compounds (2a) and (2b) isomerise. The isomerisation involves a polytopal rearrangement of the carbaborane ligand, thereby affording the species [MW(µ-CC6H4Me-4)(CO)3(η5-C9H7)(η5-7,9-C2B9H9Me2)](3a, M = Mo; 3b,M = W). Compound (3a) was identified and structurally characterised by an X-ray diffraction study. Apart from migration of the carbon atoms within the carbaborane cage, (3a) retains the structural features of (2a) with the Mo–W bond [2.669(1)A] bridged by CC6H4Me-4 [µ-C–Mo 1.963(4), µ-C–W 2.056(4)A] and B–H⇀Mo [µ-H–Mo 1.93(3)A] groups. Treatment of (2a) with K[BH-(CHMeEt)3] followed by [N(PPh3)2]Cl gives the salt [N(PPh3)2][MoW{µ-CH(C6H4Me-4)}(CO)3(η5-C9H7)(η5-7,8-C2B9H9Me2)](4), with p-tolylmethylidene and B–H⇀Mo groups bridging the Mo–W bond. Reactions of the compounds (2) with PMe3 affords the complexes [MW(µ-CC6H4Me-4)(CO)n(PMe3)(η5-C9H7)(η5-7,8-C2B9H9R2)](5a, M = Mo, R = Me, n= 3; 5b, M = W, R = Me, n= 3; 6, M = Mo, R = H, n= 2). Whereas in (5a) and (5b) the groups M(PMe3)(η5-C9H7) are present and exo-polyhedral B–H⇀M bonds are absent, in contrast, in (6) the PMe3 ligand is attached to the tungsten atom and a three-centre two-electron B–H⇀Mo linkage is present. The latter bridge system is also found in [W2(µ-CC6H4Me-4)(CO)2( PMe3)(η5-C9H7(η5-7,8-C2B9H9Me2)] which contains a W( PMe3)(η5-C9H7) group, and was prepared by displacing the ligand N2C(C6H4Me-4)2 from [W2(µ-CC6H4Me-4)(CO)2{(N2C(C6H4Me-4)2}(η5-C9H7)(η5-7,8-C2B9H9Me2)]. Treatment of (2a) with dmpm (Me2PCH2PMe2) gives the compound [MoW(µ-CC6H4Me-4)(µ-CO)(µ-dmpm)(CO)(η5-C9H7(η5-7,8-C2B9H9Me2)]. The n.m.r. data (1H, 13C-{1H}, 11B-{1H}, and 31P-{1H}) for the new complexes are reported, and discussed in relation to structures proposed or established by X-ray diffraction.