Mechanism of water exchange on five-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 kex 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−, kex is (3.5 ± 2.6) × 105 s−1 at 298 K and ambient pressure, 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 (Ia) mechanism. The results are discussed in reference to data for water exchange reactions of Cu(II) complexes available from the literature.