Dinuclear chromium(V) amino acid complexes from the reduction of chromium(VI) in the presence of amino acid ligands: XAFS characterization of a chromium(V) amino acid complex

The first synthesis and characterization of Cr(V) complexes of non-sulfur-c ontaining amino acids are reported. The reduction of Cr(VI) in methanol in the presence of amino acids glycine, alanine, and 2-amino-2-methylpropanoic acid (alpha -aminoisobutyric acid, Aib) yielded several Cr(V) EPR signals. For the reaction involving glycine, the only Cr(V) EPR signals detected we re those of the Cr(V)-intermediate methanol complexes, which were also obse rved in the absence of amino acids. The reaction involving alanine yielded one Cr(V) signal with a gi, value of 1.9754 (a(iso) = 4.88 x 10(-4) cm(-1) and A(iso)(Cr-53) = 17.89 x 10(-4) cm(-1)). However, a solid product isolat ed from the reaction solution was EPR silent and was characterized as a dio xo-bridged dimeric species, [Cr-2(V)(mu -O)(2)(O)(2)(Ala)(2)(OCH3)(2)](2-), by multiple-scattering XAFS analysis and electrospray mass spectrometry. T he EPR spectrum of the reduction reaction of Cr(VI) in the presence of Aib showed several different Cr(V) signals. Those observed at lower g(iso) valu es (1.9765, 1.9806) were assigned to Cr(V)-methanol intermediates, while th e relatively broad six-line signal at g(iso) = 2.0058 was assigned as being due to a Cr(V) complex with coupling to a single deprotonated amine group of the amino acid. This was confirmed by simplification of the superhyperfi ne coupling lines from six to three when the deuterated ligand was substitu ted in the reaction. The reduction of Cr(VI) with excess alanine or Aib lig ands resulted in the formation of tris-chelate Cr(III) complexes, which wer e analytically identical to complexes formed via Cr(III) synthesis methods. The fac-[Cr(Aib)(3)] complex was characterized by single-crystal X-ray dif fraction.