Insertion Routes to Tetrasubstituted Guanidinate Complexes of Ta(V) and Nb(V)

The preparation and characterization of guanidinate-containing complexes of Nb and Ta is described. The direct reactions of M(NMe(2))(5) with either dicyclohexylcarbodiimide (CyN=C=NCy) and diisopropylcarbodiimide ((i)PrN=C=N(i)Pr) proceeded smoothly at room temperature under nitrogen to yield [RNC(NMe(2))NR]M(NMe(2))(4) (M = Ta, Nb; R = Cy, (i)Pr). The spectroscopic characterization of these materials is consistent with a symmetrical chelating bidentate guanidinate anion bonded to a pseudo-octahedral metal center. Confirmation of these details was provided by a single-crystal X-ray diffraction study in the case of [CyNC(NMe(2))NCy]Ta(NMe(2))(4) (1). Delocalization of the lone pair of electrons on the guanidinate NMe(2) group into the ligand N-C-N pi system does not appear to be significant in these species.