SOLVENT EXCHANGE AND JAHN-TELLER INVERSION ON CU2+ IN WATER AND N,N'-DIMETHYLFORMAMIDE - A HIGH-PRESSURE O-17 NMR-STUDY

We have measured the longitudinal and transverse O-17 NMR relaxation rates and chemical shifts in solutions of CU2+ in DMF as a function of temperature, pressure, and magnetic field. The temperature and magnetic field dependence of the transverse relaxation rates and chemical shifts yields the following kinetic parameters for DMF exchange on [Cu(DMF)(6)](2+): k(ex)(298) = (9.1 +/- 2.4) x 10(8) s(-1) [lifetime tau(m)(298) = (1.1 +/- 0.4) x 10(-9) s] with activation parameters Delta H-double dagger = 24.3 +/- 1.2 kT mol(-1) and Delta S-double dagger = 8.1 +/- 4.6 J K-1 mol(-1). The pressure dependence of the transverse relaxation rates yielded the activation volume, Delta V-double dagger = +8.4 +/- 0.4 cm(3) mol(-1), which indicates a dissociative activation mode for solvent exchange, The unusually large magnetic field dispersion of the longitudinal relaxation rates at high temperatures is similar to that observed in aqueous Cu2+ solutions and is consistent with a model of relaxation due to a scalar interaction modulated by a dynamic Jahn-Teller distortion. We estimate the characteristic lifetime for inversion of the distortion axis to be tau(i)(298) = (6.0 +/- 2.5) x 10(-12) s, i.e., the process is very much faster than either solvent exchange or rotation of the solvated ion. We have also measured the pressure dependence of the transverse O-17 NMR relaxation rates in aqueous CU2+ solutions. This study complements our earlier variable-temperature and magnetic field study of the same system and yields the activation volume, Delta V-double dagger = +2.0 +/- 1.5 cm(3) mol(-1), consistent with a dissociative activation mode for water exchange. We compare all these results with kinetic data for solvent exchange on other divalent first-row transition metal ions. We conclude that solvent exchange on hexasolvento CU2+ occurs via the dissociatively activated loss of a solvent molecule from an axial position in the tetragonally elongated complex: the high lability of this relatively distant axial site is conferred to all the bound solvent molecules by the rapid inversion of the distortion axis.