Chemistry of polynuclear metal complexes with bridging carbene or carbyne ligands. Part 87. Docosahedral carbaborane(alkylidyne)tungsten complexes as reagents for the synthesis of compounds with heteronuclear metal–metal bonds: crystal structures of [NEt4][W(CC6H6Me2-2,6)(CO)2(η6-C2B10H10Me2)] and [NEt4][WFe(µ-CC6H3Me2-2,6)(CO)4(η6-C2B10H10Me2)]

Abstract

Addition of thf (tetrahydrofuran) solutions of the salt Na₂[C₂B₁₀H₁₀Me₂] to thf solutions of the compounds [W(CR)Cl(CO)₂L₂](R = C₆H₄Me-4 or C₆H₃Me₂-2,6; L = pyridine or 4-methylpyridine), in the presence of [NEt₄]Cl, affords the complexes [NEt₄][W(CR)(CO)₂(η⁶-C₂B₁₀H₁₀Me₂)] in good yield. An X-ray diffraction study on the product with R = C₆H₃Me₂-2,6 reveals that the tungsten atom is ligated by two CO groups, the alkylidyne fragment [WC 1.84(1)Å], and the carbaborane cage. The open face of the latter is η⁶-co-ordinated to the tungsten but is decidedly non-planar. The requirement of a six-atom [graphic omitted] ring above a pentagonal belt of five boron atoms results in two faces in the cage distorting from a triangular to an essentially square arrangement. Variable-temperature n.m.r. studies showed that in solution the anions undergo a dynamic exchange process. Treatment of the salts [NEt₄][W(CR)(CO)₂(η⁶-C₂B₁₀H₁₀Me₂)] with the reagents [AuCl(PPh₃)] and [Rh(cod)(PPh₃)₂][PF₆](cod = cyclo-octa-1,5-diene) yields the dimetal compounds [WAu(µ-CR)(CO)₂(PPh₃)(η⁶-C₂B₁₀H₁₀Me₂)] and [WRh(µ-CR)(CO)₂(PPh₃)₂(η⁶-C₂B₁₀H₁₀Me₂)], respectively. The tungsten–gold species, and the compound [WRh(µ-CC₆H₄Me-4)(CO)₂(PPh₃)₂(η⁶-C₂B₁₀H₁₀Me₂)], are formed as mixtures of two isomers. The reaction between [NEt₄][W(CC₆H₄Me-4)(CO)₂(η⁶-C₂B₁₀H₁₀Me₂)] and [Fe₂(CO)₉] affords the trimetallic complex [NEt₄][WFe₂(µ₃-CC₆H₄Me-4)(µ-σ:σ′:η⁶-C₂B₁₀H₈Me₂)(CO)₈]. In the latter, the carbaborane fragment has slipped over the WFe₂ triangle so that two boron atoms in the face of the cage form σ bonds with the iron atoms. The p-tolylmethylidyne group caps the other side of the metal triangle. In contrast, the reaction between the compounds [NEt₄][W(CC₆H₃Me₂-2,6)(CO)₂(η⁶-C₂B₁₀H₁₀Me₂)] and [Fe₂(CO)₉] in thf gives the novel bimetallic complex [NEt₄][WFe(µ-CC₆H₃Me₂-2,6)(CO)₄(η⁶-C₂B₁₀H₁₀Me₂)], the structure of which has been established by an X-ray diffraction study. In the anion a very short W–Fe bond [2.512(2)Å] is bridged by the xylylmethylidyne ligand [µ-C–W 2.02(1), µ-C–Fe 1.82(1)Å]. Both metal centres carry two terminally bound CO groups. As expected, the tungsten atom is ligated by the six atoms in the open face of the carbaborane cage, but the latter also bridges the W–Fe linkage via an exopolyhedral B–H⇀Fe three-centre two-electron bond. The n.m.r. data (¹H, ¹³C-{¹H}, ¹¹B-{¹H}, and ³¹P-{¹H}) for the new compounds are reported, and where appropriate are discussed.