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 O-17 NMR spectroscopy. The former compound shows a solvent exchange rate constant k(ex) of (8.6 +/- 0.4) x 10(6) 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 DeltaH#, DeltaS# and DeltaV# for the reaction are 43.0 +/- 1.5 kJ mol(-1), +32 +/- 6 J K-1 mol(-1) and -3.0 +/- 0.1 cm(3) mol(-1), respectively. For [Cu(fz)(2)(H2O)](2-), k(ex) is (3.5 +/- 2.6) x 10(5) s(-1) at 298 K and ambient pressure, which is about an order of magnitude less than for [Cu(tren)(H2O)](2+). The DeltaH#, DeltaS# and DeltaV# 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 cm(3) 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.