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

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[MoΔO2(R-mand)2]·3H2O (1a), (NH4)2[MoΛO2(S-mand)2]·3H2O (1b), (NH4)4[MoΔO2(RS-mand)2][MoΛO2(RS-mand)2]·8H2O (2), (NH4)2[WΔO2(R-mand)2]·2H2O (3a), (NH4)2[WΛO2(S-mand)2]·2H2O (3b) (H2mand=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 Δ or Λ. While the RS racemic reagent yields only mesomeric compound. The ΔR,R-complexes 1a and 3a are enantiomers of ΛS,S-1b 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 α-alkoxyl and α-carboxyl groups, forming a stable five-membered chelated rings. The average Mo(VI)–O bond distances with α-alkoxyl and α-carboxyl are 1.944 and 2.210 Å, respectively. Further comparison indicates that bonds of α-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 α-alkoxyl group in homocitrate ligand.