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-containing 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 (α-aminoisobutyric acid, Aib) yielded several Cr(V) EPR signals. For the reaction involving glycine, the
only Cr(V) EPR signals detected were those of the Cr(V)−intermediate methanol complexes, which were also
observed in the absence of amino acids. The reaction involving alanine yielded one Cr(V) signal with a giso value
of 1.9754 (aiso = 4.88 × 10-4 cm-1 and Aiso(53Cr) = 17.89 × 10-4 cm-1). However, a solid product isolated from
the reaction solution was EPR silent and was characterized as a dioxo-bridged dimeric species, [CrV2(μ-O)2(O)2(Ala)2(OCH3)2]2-, by multiple-scattering XAFS analysis and electrospray mass spectrometry. The EPR spectrum
of the reduction reaction of Cr(VI) in the presence of Aib showed several different Cr(V) signals. Those observed
at lower giso values (1.9765, 1.9806) were assigned to Cr(V)−methanol intermediates, while the relatively broad
six-line signal at giso = 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 superhyperfine coupling lines from
six to three when the deuterated ligand was substituted in the reaction. The reduction of Cr(VI) with excess
alanine or Aib ligands resulted in the formation of tris-chelate Cr(III) complexes, which were analytically identical
to complexes formed via Cr(III) synthesis methods. The fac-[Cr(Aib)3] complex was characterized by single-crystal X-ray diffraction.