Synthesis, X-ray Characterization, and Reactivity of a-Aminoacidato Ethoxide Complexes of Niobium(V) and Tantalum(V)

High yields of the ethoxo-a-aminoacidates of general formula M(OEt)4[C(O)CHRNHR’-O,N] (M = Nb, R = CH2Ph, R’ = H, 2; R = CHMe2, R’ = H, 3; R = CH2CHMe2, R’ = H, 4; R = H, R’ = Me, 5; R = CH2CH2SMe, R’ = H, 6; M = Ta, R = CH2Ph, R’ = H, 7; R = CH2CH2SMe, R’ = H, 8) and M(OEt)4( -O,N) (M = Nb, 9, Ta, 10) have been obtained by the reaction of the L-enantiopure a-amino acid (or sarcosine) with a slight molar excess of M(OEt)5 (M = Nb, 1a; Ta, 1b) in dichloromethane solution. The new complexes 2-10 have been fully characterized by spectroscopic and analytical methods, and by X-ray diffraction in the cases of 2, 7, 8 and 9. Such X-ray structures are the first ones ever reported for niobium and tantalum coordination compounds containing an -aminoacidato ligand. The early-late heterobimetallic derivative Ta(OEt)4[O2CCH(NH2)CH2CH2S(CH3)(AuCl)], 11, has been prepared by the reaction of 8 with the stoichiometric amount of Au(CO)Cl. The bulk polymerization of D,L-lactide initiated by complexes 2 and 7 gave a slightly heterotactic enchainment (Pr = 0.54 and 0.63 respectively) whereas complexes 1a and 1b formed fully atactic PLA with Pr ≈ 0.4.