Tuning the topologies of Co(II) and Ni(II) complexes with EDTA, 1,3-PDTA and 1,4-BDTA ligands: Synthesis and spectroscopic data of [Mg(H2O)6][Co(1,4-bdta)]·3H2O and [Mg(H2O)6][Ni(1,4-bdta)]·3H2O complexes, and the X-ray structure of their chiral crystals

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.

Poly[[tetra-aquabis-(μ-hydroxy-acetato-κO,O:O,O)-μ(2)-sulfato-κO:O'-dicadmium(II)] monohydrate]

The title compound, {[Cd₂(C₂H₃O₃)₂(SO₄)(H₂O)₄]·H₂O}_n, was obtained unintentionally in a transmetallation reaction. The crystal structure contains a two-dimensional metal–organic framework based on Cd^II–(μ-hydroxy­acetato-κ⁴O¹,O²:O¹,O^1′)–Cd^II zigzag chains joined together by bridging SO₄ anions. The resulting layers are shifted with respect to each other and are stacked along the c axis. Their construction is supported by hydrogen bonds between water molecules and between water molecules and carboxylate or sulfate groups. Neighbouring layers are bridged by hydrogen bonds between the hydroxyl substituent and a sulfate anion. The sulfate anion and solvent water mol­ecule are located on twofold axes. The results demonstrate that care must be taken when preparing ethyl­enediamine­tetra­acetic acid-type complexes by transmetallation, in order to avoid precipitation of metal complexes with the α-hydroxy­acetate ligand.