Pressure-Induced Valence Tautomerism in Cobalt<i>o</i>-Quinone Complexes: An X-ray Absorption Study of the Low-Spin [Co^III(3,5-DTBSQ)(3,5-DTBCat)(phen)] to High-Spin [Co^II(3,5-DTBSQ)₂(phen)] Interconversion

Abstract

The temperature- and pressure-driven solid state valence tautomeric transformations of [CoII(3,5DTBSQ)2(phen)]·C6H5CH) and nonsolvated [CoII(3,5DTBSQ)2(phen)] have been studied by EXAFS and XANES. When the toluene solvate sample is cooled it converts in the 270−215 K region (Tc = 240 K) from a high-spin CoII complex at higher-temperatues to a low-spin CoIII complex at temperatures below ∼215 K. EXAFS data indicate that the HS-CoII tautomer has bond lengths of Co−O = 2.08 ± 0.02 Å and Co−N = 2.13 ± 0.02 Å, whereas the LS-CoIII tautomeric complex is smaller with Co−O = 1.91 ± 0.02 Å and Co−N = 1.93 ± 0.02 Å. The XANES spectra obtained in the 270−215 K interconversion region were simulated to obtain a plot of the fraction of HS-CoII complexes, n(HS-CoII), vs temperature. This plot of n vs T agrees with that obtained from magnetic susceptibility data for the toluene solvate. In the XANES spectral region transitions characteristic of either a HS-CoII or a LS-CoIII complex could also be used to monitor the valence tautomeric conversion. The nonsolvated phen complex does not convert at low temperatures (2 K) to a LS-CoIII tautomer. However, both complexes could be driven from the larger HS-CoII form to the smaller LS-CoIII form upon application of pressure. EXAFS and XANES data readily detected these changes. For the toluene solvate, this conversion occurred within the 0.075−0.700 GPa range with a midpoint of Pc↑ = 0.370 GPa. The nonsolvated complex required higher pressures of 0.10−2.5 GPa with Pc↑ = 1.1 GPa. These are the first reported examples of valence tautomeric transformations driven by pressure. Both the thermally driven and pressure-driven valence tautomeric transformations are found to be reversible.