Mechanistic studies of the reductive coupling of isocyanides in seven-co-ordinate molybdenum complexes; X-ray crystal structures of [Mo(CNBut)6] and [Mo{CN(But)(SiMe2But)}(CNBut)5][O3SCF3]

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Abstract
Aspects of the mechanism for reductive coupling of alkyl isocyanide ligands in seven-co-ordinate molybdenum(II) complexes of general formula [Mo(CNR)6X]+(X = halide) have been elucidated. The chemistry is analogous to that previously reported for reductive coupling of adjacent CO ligands in [M(CO)2(dmpe)2Cl] molecules (M = Nb or Ta, dmpe = Me2PCH2CH2PMe2). Reduction of [Mo(CNBut)6l]+ with 1% Na/Hg yields the six-co-ordinate molybdenum(0) species, [Mo(CNBut)6]. An X-ray crystal structure determination of this molecule revealed considerable bending at the isocyanide nitrogen atoms indicative of substantial charge delocalization onto the ligands. Addition of trifluoroacetic acid to [Mo(CNBut)6] results in reductive coupling of two isocyanide ligands to form [Mo(ButHNCCNHBut)(CNBut)4(O2CCF3)]+. Although additional intermediates in this coupling reaction were not isolated, an alkylaminocarbyne is suggested to be another likely species along the mechanistic pathway, by analogy to the CO reductive coupling mechanism. In support of this hypothesis, addition of SiMe2But(O3SCF3) to [Mo(CNBut)6] resulted in the formation of the stable cation [Mo{CN(But)(SiMe2But)}(CNBut)5]+. The structure of this new aminocarbyne complex was elucidated in an X-ray crystal structure determination. When 1 equivalent of trifluoroacetic acid was added to [Mo(CNBut)6] the interesting doubly protonated species [Mo(CHNHBut)(CNBut)5]2+ was obtained, as revealed by solution NMR spectroscopic analyses. The possible mechanistic implications of these results are discussed.

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