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

Add time:08/11/2019    Source:sciencedirect.com

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) (H2lact = 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)–Oα-alkoxy and V(IV)–Ocarboxy 1.986(7) Å in average, which imply the influence of full deprotonation in the chelated ring compared with those of 2.15 Å 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 ΛS. 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 gxx 2.4727, gyy 1.9975, gzz 1.6456 for 2; and gxx 2.4585, gyy 1.9976, gzz 1.6412 for 4 respectively.

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