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]

Abstract

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 Å. 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)Å] bridged by CC₆H₄Me-4 [µ-C–Mo 1.963(4), µ-C–W 2.056(4)Å] and B–H⇀Mo [µ-H–Mo 1.93(3)Å] 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.