Mechanism of water exchange onfive-coordinate copper(II) complexes

The effects of temperature and pressure on the water exchange reaction of the five-coordinate complexes [Cu(tmpa)(H₂O)]²⁺ and [Cu(fz)₂(H₂O)]^2- (tmpa = tris(2-pyridylmethyl)amine; fz = ferrozine = 5,6-bis(4-sulfonatophenyl)-3-(2-pyridyl)-1,2,4-triazine) were studied by employing ¹⁷O NMR spectroscopy. The former compound shows a solvent exchange rate constant k_ex of (8.6 ± 0.4) × 10⁶ s^-1 at 298 K and ambient pressure, within a factor of three of that for the corresponding process for [Cu(tren)(H₂O)]²⁺ (tren = 2,2′2″-triaminotriethylamine). The activation parameters ΔH^#, ΔS^# and ΔV^# for the reaction are 43.0 ± 1.5 kJ mol^-1, +32 ± 6 J K^-1 mol^-1 and -3.0 ± 0.1 cm³ mol^-1, respectively. For [Cu(fz)₂(H₂O)]^2-, k_ex is (3.5 ± 2.6) × 10⁵ s^-1 at 298 K and ambient presure, which is about an order of magnitude less than for [Cu(tren)(H₂O)]²⁺. The ΔH^#, ΔS^# and ΔV^# values for the water exchange are 25.9 ± 2.2 kJ mol^-1, -52 ± 7 J K^-1 mol^-1 and -4.7 ± 0.2 cm³ mol^-1. The activation volume is modestly negative for both reactions and therefore implies an associative interchange (I_a) mechanism. The results are discussed in reference to data for water exchange reactions of Cu(II) complexes available from the literature.