Cu(II)-α-keto acid complexes as structural models of α-keto acid-dependent enzymes: Syntheses, crystal structure and properties of [Cu(L) (benzoylformate)]X

Three [Cu^II(L) (BF)]⁺ complexes, where L is tris(2-pyridylmethyl)amine (TPA) (1), bis(2-pyridylmethyl)amine (BPA) (2) or bis(2-benzimidazolylmethyl)amine (BBA) (3), and BF is benzoylformate, have been synthesized as structural models for the active site of α-keto acid-dependent enzymes. Single crystals of 1 (P2₁/n, a=11.3332(1), b=9.0159(1), c=40.2883(1) Å, Z=4, V=4114.62(6) ų) were obtained and subjected to X-ray diffraction analysis. Complex 1 has a mononuclear trigonal bipyramidal Cu(II) center with a monodentate BF occupying an axial position. The solid structure of 1 appears to be maintained in solution as probed by IR, Vis, NMR and EPR spectroscopy. Spectroscopic studies of 2 and 3 with tridentate ligands replacing the tetradentate TPA ligand suggest a square pyramidal Cu(II) center in these two complexes, and the BF is proposed to bind in a bidentate manner via its two carboxylate oxygen atoms. The fact that the α-keto carbonyl is not involved in binding to the Cu(II) center suggests that it is not crucial for the formation of Cu(II)-BF complexes. This conclusion supports the working model proposed for the metal site in Cu-TfdA (Cu-substituted 2,4-dichlorophenoxyacetate/α-ketoglutarate dioxygenase) and its interaction with the cofactor α-ketoglutarate.