Enantiomeric and mesomeric mandelate complexes of molybdenum - on their stereospecific formations and absolute configurations

Abstract

The stereospecific formation and absolute configuration of R-homocitrate coordinated FeMo-co in nitrogenase was mimicked through the structural analyses of a collection of enantiomeric and mesomeric mandelato molybdenum complexes, i.e., (NH4)(2)[MODeltaO2(R-mand)(2)] . 3H(2)O (1a), (NH4)(2)[MoLambdaO2(S-mand)(2)] . 3H(2)O (1b), (NH4)(4)[MoDeltaO2(RS-mand)(2)][MoLambdaO2(RS-mand)(2)] . 8H(2)O (2), (NH4)(2)[WDeltaO2(R-mand)(2)] . 2H(2)O (3a), (NH4)(2)[WLambdaO2(S-mand)(2)] . 2H(2)O (3b) (H(2)mand = mandelic acid, C8H8O3), which have been characterized by elemental analyses, optical rotation, circular dichroism, IR, NMR spectroscopes and X-ray single crystal studies. The R and S chiral mandelic acids induce the formations of the enantiomeric pair of chiral complexes, which are supported by the characterizations of optical rotation and circular dichroism. The configuration of the resulted metal center could be assigned as Delta or Lambda. While the RS racemic reagent yields only mesomeric compound. The Delta(R.R)-Complexes 1a and 3a are enantiomers of Lambda(S.S)- and 3b, respectively. Of the five complexes, Mo and W atoms are all hexa-coordinated by two cis-oxo groups and two bidentate mandelate ligands through the deprotonated alpha-alkoxyl and alpha-carboxyl groups, forming a stable five-membered chelated rings. The average Mo(Vl)-O bond distances with alpha-alkoxyl and alpha-carboxyl are 1.944 and 2.210 Angstrom, respectively. Further comparison indicates that bonds of alpha-alkoxyl groups in the hydroxycarboxylato molybdenum complexes are much sensitive to the change in the oxidation state of molybdenum, which support the possible Mo activation model in FeMo-co through the protonation and cleavage of alpha-alkoxyl group in homocitrate ligand. (C) 2004 Elsevier Inc. All rights reserved.