Reactivity of the Organometallic fac-[(CO)₃Re^I(H₂O)₃]⁺ Aquaion. Kinetic and Thermodynamic Properties of H₂O Substitution

The water exchange process on [(CO)₃Re(H₂O)₃]⁺ (1) was kinetically investigated by ¹⁷O NMR. The acidity dependence of the observed rate constant k_obs was analyzed with a two pathways model in which k_ex (k_ex²⁹⁸ = (6.3 ± 0.1) × 10^-3 s^-1) and k_OH (k_OH²⁹⁸= 27 ± 1 s^-1) denote the water exchange rate constants on 1 and on the monohydroxo species [(CO)₃Re^I(H₂O)₂(OH)], respectively. The kinetic contribution of the basic form was proved to be significant only at [H⁺] < 3 × 10^-3 M. Above this limiting [H⁺] concentration, kinetic investigations can be unambiguously conducted on the triaqua cation (1). The variable temperature study has led to the determination of the activation parameters ΔH^‡_ex = 90 ± 3 kJ mol^-1, ΔS^‡_ex = +14 ± 10 J K^-1 mol^-1, the latter being indicative of a dissociative activation mode for the water exchange process. To support this assumption, water substitution reaction on 1 has been followed by ¹⁷O/¹H/¹³C/¹⁹F NMR with ligands of various nucleophilicities (TFA, Br^-, CH₃CN, Hbipy⁺, Hphen⁺, DMS, TU). With unidentate ligands, except Br^-, the mono-, bi-, and tricomplexes were formed by water substitution. With bidentate ligands, bipy and phen, the chelate complexes [(CO)₃Re(H₂O)(bipy)]CF₃SO₃ (2) and [(CO)₃Re(H₂O)(phen)](NO₃)_0.5(CF₃SO₃)_0.5·H₂O (3) were isolated and X-ray characterized. For each ligand, the calculated interchange rate constants k‘_i (2.9 × 10^-3 (TFA) < k‘_I < 41.5 × 10^-3 (TU) s^-1) were found in the same order as the water exchange rate constant k_ex, the S-donor ligands being slightly more reactive. This result is indicative of I_d mechanism for water exchange and complex formation, since larger variations of k‘_i are expected for an associatively activated mechanism.