Dimolybdenum complexes containing bridging silyl-substituted acetylenes: crystal and molecular structure of [Mo2(CO)4(µ-Me3SiC2SiMe3)(η-C5H5)2]

Abstract

Reaction of [Mo₂(CO)₄(η-C₅H₅)₂] with the alkyne Me₃SiC₂SiMe₃ in toluene at reflux affords the complex [Mo₂(CO)₄(µ-Me₃SiC₂SiMe₃)(η-C₅H₅)₂]. In hexane, i.r. measurements reveal that the compound exists as an isomeric mixture, the species present having structures with or without semi-bridging CO ligands, in addition to terminally bonded CO groups. The simplicity of the n.m.r. spectra (¹H and ¹³C) shows a rapid interconversion between isomers at room temperature, but low solubility of the complex prevented measurement of limiting spectra. The structure in the solid state was established by an X-ray diffraction study which shows that the molecule has C₂ symmetry in the crystal, and that the carbonyl ligands are all terminal. The alkyne ligand is bonded transversely with respect to the Mo–Mo bond [2.952(1)Å], forming a tetrahedrane type Mo₂C₂ core. The trimethylsilyl ligands bend back from the C–C bond at an angle of 137°. The Mo(CO)₂(η-C₅H₅) moieties are in a skew (trans) relationship to the metal–metal bond; the actual configuration adopted in the crystal arises largely as a result of steric factors. Thus the structure in the solid is different from that established previously for compounds [Mo₂(CO)₄(µ-alkyne)(η-C₅H₅)₂] which have been shown to have semi-bridging CO ligands. Reaction of [Mo₂(CO)₄(µ-HC₂H)(η-C₅H₅)₂] with an excess of the alkynes R₃SiC₂H (SiR₃= SiMePh₂, SiMe₃, or SiEt₃) in octane at reflux affords the complexes [Mo₂(CO)₄(µ-R₃SiC₂H)(η-C₅H₅)₂]. The spectroscopic properties of these compounds are in accord with structures containing a semi-bridging CO ligand. The compound [Mo₂(CO)₄(µ-Me₃SiC₂SiMe₃)(η-C₅H₅)₂] reacts with C₂(CO₂Me)₂ to afford the complexes [Mo₂(CO)₂{C₆(SiMe₃)₂(CO₂Me)₄}(η-C₅H₅)₂] and [Mo₂{C₆(CO₂Me)₂(SiMe₃)₂(CO₂Me)₂}(η-C₅H₅)₂].