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

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Abstract
The water exchange process on [(CO)3Re(H2O)3]+ (1) was kinetically investigated by 17O NMR. The acidity dependence of the observed rate constant kobs was analyzed with a two pathways model in which kex (kex298 = (6.3 ± 0.1) × 10-3 s-1) and kOH (kOH298= 27 ± 1 s-1) denote the water exchange rate constants on 1 and on the monohydroxo species [(CO)3ReI(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 kex, the S-donor ligands being slightly more reactive. This result is indicative of Id mechanism for water exchange and complex formation, since larger variations of k‘i are expected for an associatively activated mechanism.

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