ansa-metallocene derivatives : XXV. Synthesis, crystal structure and reactions of a tetramethylethano-bridged vanadocene dichloride, (CH₃)₄C₂(C₅H₄)₂VCl₂

Abstract

The tetramethylethano-bridged vanadocene complex (CH3)4C2(C5H4)2VCl2 has been made in two ways: (i) from VCl3·3THF by reaction with (CH3)4C2(C5H4MgCl·2THF)2 in THF and subsequent oxidation with PCl3, and (ii) from VCl4·2THF by reaction with (CH3)4C2(C5H4Li)2 or with (CH3)4C2(C5H4MgCl·2THF)2 in a toluene/dimethoxyethane mixture. Its crystal structure is in accordance with expectations. Reduction of (CH3)4C2(C5H4)2VCl2 with tetrakis(dimethylamino)ethene in toluene or with LiAlH4 in THF yields the monochloride (CH3)4C2(C5H4)2VCl; from this VIII compound cationic complexes (CH3)4C2(C5H4)2VL2+ with L = CO or CNtBu are readily obtained by ligand exchange. 1H NMR signals of these diamagnetic complexes have been assigned to H atoms in α and β C5-ring positions by use of selective nuclear Overhauser effects. The ring-bridged VII complex (CH3)4C2(C5H4)2V could not be prepared. Cyclovoltammetry studies indicate that (in contrast to its unbridged counterpart (C5H5)2V) this VII complex decays within seconds after its formation by electrochemical reduction of its chloro derivatives.