Synthesis of a pyridinium bis[citrato(2-)]oxochromate(V) complex and its ligand-exchange reactions

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The reaction of citric acid (caH4) with pyridinium dichromate (PDC) in anhydrous acetone yields pyridinium bis[citrato(2-)]oxochromate(V), pyH[CrO(caH2)2], as a mixed salt with the Cr(III) product. The compound persists in the solid state for months, is highly soluble in water (pH 4.0), and gives a sharp electron paramagnetic resonance (EPR) signal in solution (giso = 1.9781, Aiso(Cr) = 17.1 × 10-4 cm-1), which is characteristic of d1 Cr(V). The presence of [CrVO(caH2)2] - in the solid state was confirmed by electrospray mass spectroscopy, X-ray absorption near-edge structure (XANES), and EPR spectroscopy. Solid-state EPR spectroscopy, XANES, and a spectrophotometric assay showed that the solid is a mixture of [CrVO(caH2)2] - and a Cr(III)-citrate complex. The structures of the [Cr VO(caH2)2]- and [Cr III(caH2)2]- components of the mixture were established by multiple-scattering MS analysis of the X-ray absorption fine structure data. The structure of [CrVO(caH 2)2]- is similar to that of other 2-hydroxy acid complexes with Cr=O, Cr-O(alcoholato), and Cr-O(carboxylato) bond lengths of 1.59, 1.81, and 1.90 Å, respectively. The Cr(III) complex has bond lengths typical for ligands with deprotonated carboxylate and protonated alcohol donors with distances of 1.90 and 1.99 Å, respectively, for the Cr-O(carboxylato) and Cr-O(alcohol) bond lengths. In aqueous solution, [CrO(caH2)2]- is short lived, but it is a convenient starting material for ligand-exchange reactions. It has been used to generate short-lived mixed-ligand Cr(V) complexes with citrate and picolinate, iminodiacetate, 2,2′-bipyridine, or 1,10-phenanthroline, which were characterized by EPR spectroscopy. The g values are between 1.971 and 1.974. For the picolinate, 2,2′-bipyridine, and 1,10-phenanthroline mixed-ligand complexes, there is hyperfine coupling (2.2 × 10-4 to 2.4 × 10-4 cm-1) to a single proton of the citrate ligand.

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