BISTABILITY IN THE [CO(II)(SEMIQUINONATE)2] TO [CO(III)(CATECHOLATE)(SEMIQUINONATE)] VALENCE-TAUTOMERIC CONVERSION

The preparation and characterization of a series of valence-tautomeric cobalt complexes are presented. The diiminium ligand arc NN in [Co(arc NN)(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(II) 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) angstrom, b = 26.894(10) angstrom, c = 8.472(2) angstrom, and Z = 4. Refinement with 1307 observed [F>3.0sigma(F)] reflections gave R=0.0461 and R(w)=0.0504. The complex [Co(phen)(3,5-DTBSQ)2].C6H5CH3 crystallizes in the spage group P2(1)/c, which at 295 K has a unit cell with a = 10.436(4) angstrom, b = 32.524(14) angstrom, c = 13.563(6)angstrom, beta = 111.44(3)degrees, and Z = 4. Refinement with 2060 observed [F>4.0sigma(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) valence-tautomeric forms of the complex. An electron is transferred from the Co(II) ion to one semiquinonate ligand to give the Co(II) 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].C6H5CH3 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(II)(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.