Isolation and Characterization of a Highly Reducing Aqueous Chromium(II) Complex

The highly reducing Cr^II-(1,3-propylenediaminetetraacetate) (CrPDTA) complex (-1.1 V vs SHE) has been isolated from aqueous solution and the solid-state structure is described. The reduced Cr^IIPDTA complex is characterized by single-crystal X-ray diffraction, elemental analysis, infrared spectroscopy, UV-vis spectroscopy, magnetic moment, and density functional theory calculations. The concentration profile, state of charge, and pH of CrPDTA electrolyte were monitored in a flow battery system in situ by absorption spectroscopy and a pH probe. The stability of Cr^IIPDTA in aqueous environments is demonstrated by the ability to isolate CaCrPDTA, despite the common misconception that water spontaneously evolves hydrogen at such potentials. The reduced Cr^IIPDTA prevents water from coordinating to the metal center by maintaining the same coordinatively saturated pseudo-octahedral structure as the oxidized Cr^IIIPDTA, despite experiencing an increased geometric strain from a Jahn-Teller distortion of the high-spin Cr^II ion. The important difference between solvent reactivity and solvent thermodynamic window is examined by comparing the electrochemical behavior of the reduced species of CrPDTA in various organic solvents to its behavior in aqueous solution. When examined in tetrahydrofuran (THF), the reduction potential of CrPDTA is observed to be -1.19 V vs cobaltocene (-2.52 V vs ferrocene). Reduced CrPDTA in aqueous solution is also exposed to atmospheric O₂ without exhibiting any decomposition of the Cr(III) or Cr(II) species. The techniques detailed provide a higher standard method of characterization for flow battery electrolyte species.