Synthesis, X-ray characterization, and reactivity of α-aminoacidato ethoxide complexes of niobium(V) and tantalum(V).

High yields of the ethoxo-α-aminoacidates of the general formula M(OEt)4[OC(O)CHRNHR'-κO,κN] (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(OC(O)CH(CH2CH2CH2)NH-κO,κN) (M = Nb, 9; Ta, 10) have been obtained by the reaction of the l-enantiopure α-amino acid (or sarcosine) with a slight molar excess of M(OEt)5 (M = Nb, 1a; Ta, 1b) in a dichloromethane solution. The new complexes 2-10 have been fully characterized by spectroscopic and analytical methods and by X-ray diffraction for 2 and 7-9. Such X-ray structures are the first ones ever reported for niobium and tantalum coordination compounds containing an α-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 a 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 polylactide with P(r) ≈ 0.4.