The <i>Slowest</i> Water Exchange at a Homoleptic Mononuclear Metal Center: Variable-Temperature and Variable-Pressure ¹⁷O NMR Study on [Ir(H₂O)₆]³⁺

Abstract

The rate constants and activation parameters for water exchange on hexaaqua and monohydroxy pentaaqua iridium(III) have been determined by 17O 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 = k1 + k2/[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: k1298 = (1.1 ± 0.1) × 10-10 s-1, ΔH1⧧ = 130.5 ± 0.6 kJ mol-1, ΔS1⧧ = +2.1 ± 1.7 J K-1 mol-1, and ΔV1⧧ = −5.7 ± 0.5 cm3 mol-1; k2298 = (1.4 ± 0.6) × 10-11 m s-1, ΔH2⧧ = 138.5 ± 4.5 kJ mol-1, ΔS2⧧ = +11.5 ± 11.6 J K-1 mol-1, and ΔV2⧧ = −0.2 ± 0.8 cm3 mol-1. The value obtained for k1298 corresponds to a residence time of ca. 300 years. The pKa298 and the volume change ΔVa0 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 cm3 mol-1, respectively. Utilizing the relation k2 = kOHKa1, values for the first-order rate constant and the corresponding activation volume for [Ir(H2O)5(OH)]2+ were estimated to be kOH298 = 5.6 × 10-7 s-1 and ΔVOH⧧ = +1.3 cm3 mol-1, respectively. These data are supportive of an associative interchange (Ia) 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.