Chemistry of polynuclear metal complexes with bridging carbene or carbyne ligands. Part 104. Trimetal molybdenum and tungsten complexes containing η5-C2B9H9R′2(R′= H or Me) ligands; crystal structure of [NEt4][Mo2W(µ3-CC6H4Me-4)(µ-CO)(CO)7(PMe3)(η5-C2B9H9Me2)

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Solutions of the salts [NEt4][M(CR)(CO)2(η5-C2B9H9R′2)](M = Mo or W, R = C6H4Me-4 or CCBut, R′= H or Me), in thf (tetrahydrofuran) at room temperature, react with the compound [Mo(CO)3(NCMe)3] to afford the trimetal complexes [NEt4][Mo2M(µ3-CR)(µ-CO)(CO)8(η5-C2B9H9R′2)](M = W, R = C6H4Me-4, R′= H or Me; M = Mo, R = C6H4Me-4, R′= H; M = W, R = C CBut, R′= Me). A single-crystal X-ray diffraction study was carried out on the PMe3 derivative [NEt4][Mo2W(µ3-CC6H4Me-4)(µ-CO)(CO)7(PMe3)(η5-C2B9H9Me2)]. The metal triangle [Mo–Mo 2.968(3), Mo–W 2.881 (3) and 2.867(3)A] in the anion is capped on one side by the p-tolylmethylidyne group [µ3-C–Mo av. 2.26(2), µ3-C–W 2.01 (2)A], and is bridged on the other by the C2B9H9Me2 cage. The latter is η5 co-ordinated to the tungsten atom and linked to the molybdenum atoms via two exopolyhedral B–H⇀Mo bonds. The bond to one molybdenum centre involves a BH fragment α to a CMe group in the CCBBB face of the cage, while the bond to the other molybdenum centre employs a BH fragment β to the CMe groups. Moreover, the molybdenum atom linked to the cage via the β BH group carries the PMe3 ligand [Mo–P 2.489(7)A]. This metal atom is also ligated by two terminally bound CO groups, and by a CO molecule that asymmetrically bridges to the other molybdenum [Mo–C–O 156(2) and 124(2)°]. The latter is co-ordinated by three CO groups, and the tungsten atom by two CO molecules. In the formation of [NEt4][Mo2W(µ3-CC6H4Me-4)(µ-CO)(CO)7(PMe3)(η5-C2B9H9Me2)] there was n.m.r. evidence for the formation of a second isomer (ca. 15%) in which the PMe3 group occupies a different site. Attempts to protonate the salts [NEt4][Mo2M(µ3-CR)(µ-CO)(CO)8(η5-C2B9H9R′2)] in order to obtain stable neutral complexes were unsuccessful, but treatment of [NEt4][Mo2W(µ3-CCCBut)(µ-CO)(CO)8(η5-C2B9H9Me2)] in thf with [AuCl(PPh3)] in the presence of TIPF6 gives [Mo(CO)6] and the dimetal complex [WAu(µ-CCCBut)(CO)2(PPh3)(η5-C2B9H9Me2)]. The latter reacts with [Pt(nb)(PMe2Ph)2](nb = bicyclo[2.2.1]heptene) to afford the trimetal complex [WPtAu(µ3-CCCBut)(CO)2(PPh3)(PMe2Ph)2(η5-C2B9H9Me2)]. The n.m.r. data (1H, 13C-{1H}, 11B-{1H}, and 31P-{1H}) for the new compounds are reported and discussed.