The slowest water exchange at a homoleptic mononuclear metal center: Variable-temperature and variable-pressure O-17 NMR study on [Ir(H2O)(6)](3+)

The rate constants and activation parameters for water exchange on hexaaqua and monohydroxy pentaaqua iridium(III) have been determined by O-17 NMR spectroscopy as a function of temperature (358-406 K) and pressure (0.1-210 MPa) at several acidities (0.5-5.0 m). Noncoordinating trifluoromethanesulfonate (CF3SO3-) was used as the counterion. The observed rate constant was of the form k = k(1) + k(2)/[H+], where the subscripts 1 and 2 refer to the exchange pathways on [Ir(H2O)(6)](3+) and [Ir(H2O)(5)(OH)](2+), respectively. The kinetic parameters obtained are summarized as follows: k(1)(298) = (1.1 +/- 0.1) X 10(-10) s(-1), Delta H-1(double dagger) = 130.5 +/- 0.6 kJ mol(-1), Delta S-1(double dagger) = +2.1 +/- 1.7 J K-1 mol(-1), and Delta V-1(double dagger) = -5.7 +/- 0.5 cm(3) mol(-1); k(2)(298) = (1.4 +/- 0.6) X 10(-11) m s(-1), Delta H-2(double dagger) = 138.5 +/- 4.5 W mol(-1), Delta S-2(double dagger) = +11.5 +/- 11.6 J K-1 mol(-1), and Delta V-2(double dagger) = -0.2 +/- 0.8 cm(3) mol(-1). The value obtained for k(1)(298) corresponds to a residence time of ca. 300 years. The pK(a)(298) and the volume change Delta V-a(0) associated with the first hydrolysis of [Ir(H2O)(6)](3+) were determined by potentiometric and high-pressure spectrophotometric methods to be 4.45 +/- 0.03 and -1.5 +/- 0.3 cm(3) mol(-1), respectively. Utilizing the relation k(2) = k(OH)K(a1), values for the first-order rate constant and the corresponding activation volume for [Ir(H2O)(5)(OH)](2+) were estimated to be k(OH)(298) = 5.6 x 10(-7) s(-1) and Delta V-OH(double dagger) = +1.3 cm(3) mol(-1), respectively. These data are supportive of an associative interchange (I-a) mechanism for water exchange on [Ir(H2O)(6)](3+), but of an interchange (I) mechanism on the deprotonated species [Ir(H2O)(5)(OH)](2+). These mechanistic results have also been compared to those reported for other trivalent metal ions.