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

Dinuclear complexes of formula Nb2Cl9[O2CCH(R)NR′R2-O,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[O2C⌈CH(CH2)3NH⌉], 1f) were prepared by allowing NbCl5 to react in dichloromethane with the appropriate α-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 α-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.

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'R"2][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.