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

Abstract A series of chiral and achiral neutral vanadyl complexes with N - heterocycle chelated ligands [V 2 O 2 ( S -lact) 2 (bpy) 2 ] ( 1 ), [V 2 O 2 ( S -lact) 2 (phen) 2 ] ( 2 ), [V 2 O 2 ( R -lact)( S -lact)(bpy) 2 ] ( 3 ), [V 2 O 2 ( 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 α-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 α-alkoxy and V (IV) –O carboxy 1.986(7) A in average, which imply the influence of full deprotonation in the chelated ring compared with those of 2.15 A 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 Λ 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.