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 17 O 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 (CF 3 SO 3 - ) 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(H 2 O) 6 ] 3+ and [Ir(H 2 O) 5 (OH)] 2+, respectively. The kinetic parameters obtained are summarized as follows: k 1 298 = (1.1 ± 0.1) × 10 - 10 s - 1, Δ H 1 ⧧ = 130.5 ± 0.6 kJ mol - 1, Δ S 1 ⧧ = +2.1 ± 1.7 J K - 1 mol - 1, and Δ V 1 ⧧ = −5.7 ± 0.5 cm 3 mol - 1; k 2 298 = (1.4 ± 0.6) × 10 - 11 m s - 1, Δ H 2 ⧧ = 138.5 ± 4.5 kJ mol - 1, Δ S 2 ⧧ = +11.5 ± 11.6 J K - 1 mol - 1, and Δ V 2 ⧧ = −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 p K a 298 and the volume change Δ V a 0 associated with the first hydrolysis of [Ir(H 2 O) 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(H 2 O) 5 (OH)] 2+ were estimated to be k OH 298 = 5.6 × 10 - 7 s - 1 and Δ V OH ⧧ = +1.3 cm 3 mol - 1, respectively. These data are supportive of an associative interchange (I a ) mechanism for water exchange on [Ir(H 2 O) 6 ] 3+, but of an interchange (I) mechanism on the deprotonated species [Ir(H 2 O) 5 (OH)] 2+ . These mechanistic results have also been compared to those reported for other trivalent metal ions.