SOLVENT EXCHANGE KINETICS AND PROTON RELAXATION IN SOLUTIONS OF COBALT(II) DIOXIMES

The temperature dependence of the solvent proton transverse and longitudinal relaxation rates has been measured in solutions of the low-spin d(7) complexes of bis(dimethylglyoximato)cobalt(II), Co(dmgH(2)), and bis(diphenylglyoximato)cobalt(II), Co(dpgH)(2), in methanol and acetonitrile, and Co(dmgBF(2))(2) in acetone and acetonitrile. There is distinct evidence for solvent exchange controlled transverse relaxation in all systems except Co(dpgH)(2) in acetonitrile. The rate constants are similar in magnitude to those of hexasolvated, high-spin cobalt(II) complexes. However, the cobalt(II) oximes are less favorable for NMR relaxation studies because of their longer electron spin relaxation times, although the chemical shifts are generally larger than those of hexasolvated cobalt(II). The solvent exchange rate constant (25 degrees C), Delta H-double dagger (kcal mol(-1)), Delta S-double dagger (cal mol(-1) K-1) are 2.3 x 10(5), 7.2 +/- 0.2, -9.8 +/- 0.7 for Co(dmgH)(2) in acetonitrile, 2.8 x 10(4), 13.9 +/- 0.2, 8.3 +/- 0.6 for Co(dmgH)(2) in methanol, 1.4 x 10(5), 4.3 +/- 0.3, -20.5 +/- 1 for Co(dmgBF(2))(2) in acetonitrile, 3.5 x 10(5), 3.4 +/- 0.9, -22 +/- 5 for Co(dmgBF(2))(2) in acetone, and 1.2 x 10(6), 3.4 +/- 0.4, -21 +/- 2 for Co(dpgH)(2) in methanol.