Interconversion of oxo and imido ligands at a dimolybdenum centre: molecular and electronic structure of [{Mo(η-C5H4Me)(NPh)(µ-NPh)}2]

Abstract

The tetraimido complexes [{Mo(η-C₅H₄R)(NR′)(µ-NR′)}₂](R = Me, R′= Ph or C₆H₄Me-p; R = Prⁱ, R′= Ph) have been synthesised from the corresponding oxo compounds [{Mo(η-C₅H₄R)O(µ-O)}₂] and the aryl isocyanates R′NCO. The three mixed imido–oxo derivatives [Mo₂(η-C₅H₄Me)₂O_{4 –x}(NPh)_x](x= 1–3), presumed to be intermediates in the synthesis of the tetraimido derivatives, have been synthesised independently. The imido–oxo compounds react with excess of phenyl isocyanate to give [{Mo(η-C₅H₄Me)(NPh)(µ-NPh)}₂], for which a single-crystal X-ray structure analysis reveals a trans disposition of the η-methylcyclopentadienyl ligands and a planar Mo₂(µ-N)₂ core. A molecular-orbital analysis suggests that each NPh ligand acts as a formal three-electron donor. Hydrolysis of [{Mo(η-C₅H₄R)(NPh)(µ-NPh)}₂] proceeds via a stepwise replacement of the two terminal imido groups to give [Mo₂(η-C₅H₄R)₂O_{2 –x}(NPh)_x(µ-NPh)₂](x= 0 or 1). Attempts to replace the bridging imido functionalities of [{Mo(η-C₅H₄Me)O(µ-NPh)}₂] by oxo ligands proved unsuccessful.