Chiral and achiral vanadyl lactates with vibrational circular dichroism: Toward the chiral metal cluster in nitrogenase

A series of chiral and achiral neutral vanadyl complexes with N-heterocycle chelated ligands [V2O2(S-lact)(2)(bpy)(2)] (1), [V2O2(S-lact)(2)(phen)(2)] (2), [V2O2(R-lact)(S-lact)(bpy)(2)] (3), [V2O2(R-lact) (S-lact)(phen)(2)] (4) (H(2)lact = lactic acid, phen = 1,10-phenanthroline, and bpy = 2,2'-bipyridine) have been obtained under hydrothermal condition. The complexes feature bidentate lactate that chelates to vanadium atom through their alpha-alkoxido and carboxylato groups, while the other coordination sites are occupied by N-heterocycle ligand and terminal oxygen without coordinated or crystallized water molecule. Unusual short bond distances are observed for the coordinated lactates with V-(IV)-O alpha-alkoxy and V-(IV)-O-carboxy 1.986(7) angstrom in average, which imply the influence of full deprotonation in the chelated ring compared with those of 2.15 angstrom in iron vanadium cofactor of V-nitrogenase. Enhanced activity and the red-shifts of vibrational circular dichroism (VCD) have been observed for the chiral neutral vanadyl complexes 1 and 2 and supported by B3LYP calculation. The carboxylato vibrations in VCD are observed in 1660, 1649, 1600, 1474, 1443 cm(-1) and 1664, 1653, 1636, 1483, 1431 cm(-1) for 1 and 2 respectively. Their absolute configurations of chiral metal center are assigned as A(s). The EPR spectra of 1 and 3 with bpy ligands appear to be isotropic, while 2 and 4 with phen ligands display textbook rhombic EPR spectra, and the g values obtained are g(xx) 2.4727, g(yy) 1.9975, g(zz) 1.6456 for 2; and g(xx) 2.4585, g(yy) 1.9976, g(zz) 1.6412 for 4 respectively. (C) 2016 Elsevier B.V. All rights reserved.