Molybdenum carbonyl complexes with pyridylimino acidato ligands

Abstract

Reactions of [Mo(CO)₄(pip)₂] with pyridine-2-carbaldehyde and the appropriate amino ester or amino acid produce complexes with chelated pyridylimino ligands bearing, respectively, an ester (1) or carboxylate (2a,b, 4, 5a,b) pendant arm, the structures of which have been determined by X-ray crystallography. In the case of α-amino acids, the resulting imino carboxylate complexes are unstable towards decarboxylation, this being complete for (R)-2-phenylglycine. The products of decarboxylation 3a–c were isolated and characterized, including X-ray structure determinations for 3b and 3c. In contrast, the derivatives of β-alanine (4) and 3- and 4-aminobenzoic acids (5a,b) are stable towards decarboxylation. The structure determinations show that the pyridyliminocarboxylate complexes crystallize as salts with piperidinium cations, forming hydrogen-bonded ion-pair dimers featuring twelve- or eight-membered rings. Protonation of carboxylate complexes with 2 M HCl in CH₂Cl₂/water yields the corresponding neutral complexes 6a,b containing a free carboxylic acid functionality.