Preparation and characterization of molybdenum and tungsten compounds with diazabutadiene ligands constructed from amino esters and glyoxal: Crystal structures of meso and C2-symmetric isomers of Mo(CO)4(dab-asp(OMe)-OMe)

The title compounds were prepared by heating solutions of ester protected amino acids (H-l-Ala-OEt, H-β-Ala-OEt, H-l-Val-OMe, GABA-OMe, H-l-Asp(OMe)-OMe) and glyoxal in the presence of M(CO)4(pip)2 (M=Mo, W). The resulting novel complexes, M(CO)4(dab-xxx-OR) (dab=diazabutadiene), contain an N,N′-diimine ligand and were characterized by 1H- and 13C-NMR, IR, and UV–vis measurements. The low energy band in the visible portion of the electronic spectrum is assigned to a MLCT transition and exhibits solvatochromism. The valine, alanine and aspartic ester derivatives have C2 symmetry resulting from the C2 symmetry of the ligand. The reaction of the alanine and aspartic amino esters in the presence of NEt3 produces diastereomeric mixtures caused by racemization at the amino acid α-carbon. Racemization is not observed during the formation of the valine derivatives. The crystal structures of (R,S)-Mo(CO)4(dab-asp(OMe)-OMe) (5-RS), and (S,S)-Mo(CO)4(dab-asp(OMe)-OMe) (5-SS), were determined. The structure of 5-RS confirms that racemization at the α-carbon occurred. 5-SS has C2 symmetry.