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)A], 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].