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

摘要

High yields of the ethoxo-α-aminoacidates of the general formula M(OEt)_4[OC(O)CHRNHR'-κO,κN] (M = Nb, R = CH _2Ph, R' = H, 2; R = CHMe_2, R' = H, 3; R = CH_2CHMe_2, R' = H, 4; R = H, R' = Me, 5; R = CH_2CH_2SMe, R' = H, 6; M = Ta, R = CH_2Ph, R' = H, 7; R = CH_2CH_2SMe, R' = H, 8) and M(OEt)_4(OC(O)CH(CH_2CH_2CH_2)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[O_2CCH(NH_2)CH _2CH_2S(CH_3)(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 (P_r = 0.54 and 0.63, respectively), whereas complexes 1a and 1b formed fully atactic polylactide with P_r ≈ 0.4.