An EXAFS study of Mo(VI) and Mo(V) complexes with proline dithiocarbamate

The reaction between proline and carbon disulfide in a basic medium yields the corresponding dithiocarbamate thereafter dtc pro. Reaction of this comp ound with Na2MoO4. 2H(2)O leads to formation of a coordination compound wit h the formula MoO2(dtc Hpro)(2), while reaction with MoCl5, in an inert atm osphere leads to Mo2O4(dtc Hpro)(4). Mo K XANES data are consistent with th e presence of Mo(VI) in the former compound, while a lower oxidation state of molybdenum in the latter, Mo(V), shifts the absorption edge to lower ene rgies. EXAFS data indicate that, in these complexes, molybdenum ions are in a different coordination environment. In the first it is in a distorted oc tahedral coordination, and oxygen atoms are in cis positions, with an Mo-O distance of 1.68 A and an Mo-S distance of 2.48 Angstrom. However, in the d imer the Mo atoms (Mo-Mo distance 2.55 Angstrom) are bridged by two oxygen atoms (Mo-O distance 1.95 Angstrom), and the Mo-S distance is 2.43 Angstrom . The molybdenum atom in this case is pentacoordinated with a terminal oxyg en atom distance Mo-O of 1.66 Angstrom. The dithiocarbamate unit behaves in all cases as a bidentate ligand, both sulfur atoms being equivalent. Both complexes are diamagnetic, and FT-IR spectroscopy provides evidence that th e carboxylic group is in the acidic form, not being coordinated to the Mo c ations. Only charge transfer bands are detected in the UV-Vis spectra. EPR spectra produced evidence of the presence of a small amount of a paramagnet ic Mo(V) species, probably Mo(dtc Hpro)(3). (C) 1999 Elsevier Science S.A. All rights reserved.