Synthesis of α-Amino Acidato Derivatives of Niobium and Tantalum Pentahalides and Their Conversion into lminium Salts

Dinuclear complexes of formula Nb2Cl9[O2CCH(R)NR'R(2)-kappa O,kappa O] (R = CH2CHMe2, R' = R = H, 1a; R = CH2Ph, R' = R = H, 1b; R = CH2CH2SCH3, R' = R = H, 1c; R = R' = H, R = Me, 1d; R = CH2Ph, R' = R = Me, 1e; Nb2Cl9[O(2)Cinvertered left perpendicularCH(CH2)(3)NHinvertered right perpendicular], 1f) were prepared by allowing NbCl5 to react in dichloromethane with the appropriate a-amino acid in 1:2 amino acid/Nb molar ratio. The 1:1 reactions between MX5 (M = Nb, Ta; X = Cl, Br) and a series of a-amino acids resulted in the formation of the iminium salts [(R)CH-NR'R2][MX6] (R = CH2Ph, R' = R = Me: M = Nb, X = Cl, 2a; M = Nb, X = Br, 2b; M = Ta, X = Cl, 2c; M = Ta, X = Br, 2d; R = CH2Ph, R' = R = H, M = Nb: X = Cl, 3a; X = Br, 3b; R = CH2CHMe2, R' = R = H, M = Nb, X = Cl, 4; R = R' = H, R = Me, M = Nb, X = Cl, 5). The formate/amino acidate derivative NbCl3(O2CH)[O2CCH(CH2Ph)NMe2], 6, was isolated and identified as coproduct of the 1:1 reaction between NbCl5 and N,N-dimethyl-l-phenylalanine, leading to 2a. All of the compounds were characterized by analytical and spectroscopic methods and by X-ray diffractometry in the cases of 2a, 2b, and 2d. Moreover, density functional theory studies were carried out to shed light on mechanistic and structural aspects.