Bistability in the [CoII(semiquinonate)2] to [CoIII(catecholate)(semiquinonate)] valence-tautomeric conversion

Article abstract of DOI:10.1021/ja00071a035

The preparation and characterization of a series of valence-tautomeric cobalt complexes are presented. The diiminium ligand Ndown curve signN in [Co(Ndown curve signN)(3,5-DTBSQ)( 3,5-DTBQ )] is 4,4′-diphenyl-2,2′-bipyridine (bpbpy), 4,4′-dimethyl-2,2′-bipyridine (dmbpy), 2,2′-bipyridine (bpy), 1,10-phenanthroline (phen), 2,2′-bipyrimidine (bpym), or 2,2′-bipyrazine (bpyz). It is shown that at room temperature the first three complexes are low-spin Co^III complexes with one semiquinonate(1-) and one catecholate(2-) ligand. On the other hand, the phen, bpym, and bpyz complexes at room temperature are Co^II complexes with two semiquinonate ligands. The complex [Co(bpyz)(3,5-DTBSQ)₂] crystallizes in the space group Ccc2, which at 296 K has a unit cell with a = 16.233(6) ?, b = 26.894(10) A, c = 8.472(2) ?, and Z = 4. Refinement with 1307 observed [F>3.0σ(F)] reflections gave R = 0.0461 and R_w = 0.0504. The complex [Co(phen)(3,5-DTBSQ)₂]·C₆H₅CH ₃ crystallizes in the spage group P2₁/c, which at 295 K has a unit cell with a = 10.436(4) ?, b = 32.524(14) ?, c = 13.563(6) ?, β = 111.44(3)°, and Z = 4. Refinement with 2060 observed [F>4.0σ(F)] reflections gave R = 0.0824 and R_w = 0.0850. At room temperature both of these complexes have Co-O and Co-N bond lengths which indicate high-spin Co^II ions. The bond lengths in the quinone-derived ligands indicate that for both complexes the ligands are semiquinonate ligands. Variable-temperature UV-vis electronic absorption spectra are presented to show that in toluene solution all six of the complexes exhibit an equilibrium involving the Co^II and Co^III valencetautomeric forms of the complex. An electron is transferred from the Co^II ion to one semiquinonate ligand to give the Co^III tautomer. Variable-temperature (2-320 K) magnetic susceptibility data are presented for polycrystalline samples of all of the complexes. The toluene solvate [Co(phen)(3,5-DTBSQ)₂]·C₆H₅CH ₃ is the first complex to undergo the valence-tautomeric transformation in the solid state. Abrubtly in the 220-260 K region this toluene solvate converts between the Co_II and Co^III tautomeric forms in the solid state. X-ray structures at 238 and 173 K are presented to show that, upon cooling, this phen complex does convert from a Co^II(semiquinonate)₂ to a Co^III(semiquinonate)-(catecholate) complex. The chlorobenzene solvate of the phen complex also undergoes the tautomeric transformation, although less abruptly than the toluene solvate. The nonsolvated phen complex remains in the Co^II tautomeric form from 320 down to 2 K. It is shown that the valence-tautomeric transformation is entropy driven.