Transition Metal Complexes of Guanidinate Dianions: Reactions between Guanidines and M(NMe₂)₅ (M = Ta, Nb)

The protonation of two metal-amido groups of M(NMe2)5 with trialkylguanidines yielded a series of novel complexes with formulas [RNC(NR)NR]M(NMe2)3 (1−4) (M = Ta, Nb; R = isopropyl, cyclohexyl). These complexes contained dianionic N,N‘,N‘ ‘-trialkylguanidinate ligands which were coordinated in a chelating bidentate mode. A single-crystal X-ray study of [CyNC(NCy)NCy]Ta(NMe2)3 (3) (C25H51N6Ta, triclinic, P1̄, a = 9.4155(2) Å, b = 13.3188(2) Å, c = 13.5215(2) Å, α = 117.075(1)°, β = 101.744(1)°, γ = 98.507(1)°, Z = 2) confirmed the connectivity of these species. These guanidinate ligands exhibited both planarity of the central CN3 group and the correct orientation of the three NR substituents to allow for π conjugation within the ligand core.