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

Abstract

The water exchange process on [(CO)(3)Re(H(2)O)(3)](+) (1) was kinetically investigated by (17)O NMR. The acidity dependence of the observed rate constant k(obs) was analyzed with a two pathways model in which k(ex) (k(ex)(298) = (6.3 +/- 0.1) x 10(-3) s(-1)) and k(OH) (k(OH)(298)= 27 +/- 1 s(-1)) denote the water exchange rate constants on 1 and on the monohydroxo species [(CO)(3)Re(I)(H(2)O)(2)(OH)], respectively. The kinetic contribution of the basic form was proved to be significant only at [H(+)] < 3 x 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 Delta H(++)(ex) = 90 +/- 3 kJ mol(-1), Delta S(++)(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 (17)O/(1)H/(13)C/(19)F NMR with ligands of various nucleophilicities (TFA, Br(-), CH(3)CN, 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)(3)Re(H(2)O)(bipy)]CF(3)SO(3) (2) and [(CO)(3)Re(H(2)O)(phen)](NO(3))(0.5)(CF(3)SO(3))(0.5).H(2)O (3) were isolated and X-ray characterized. For each ligand, the calculated interchange rate constants k'(i) (2.9 x 10(-3) (TFA) < k'(I) < 41.5 x 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.

Ligand exchange and complex formation kinetics studied by NMR exemplified on fac-[(CO)3M(H2O)]+ (M=Mn, Tc, Re)

“…6) [46]. These experiments are often very time-consuming and expensive because considerable amounts of enriched H 2 17 O (10% enrichment) have to be used.…”

“…21 shows that the results are in agreement. The second order rate constant Complex formation reactions on group 7 tricarbonyl triaqua complexes have been studied for DMS and CH 3 CN and thermodynamic and kinetic data are available (Table 4) [43,46]. For all three metals, complexes are less stable with CH 3 CN than with DMS.…”

“…The most rate and equilibrium constants on complex formation on tricabonyl metal triaqua complexes are available on fac-[(CO) 3 Re(H 2 O) 3 ] + [46,62]. A series of N-and S-bonded ligands, neutral and negatively charged ligands have been studied (Table 5).…”

Structure of the Triaquatricarbonylrhenium(I) Cation and Its Conjugate Base

“…One focal point is the search for viable d 6 M(CO) 3 L + (M = 186/188 Re, 99m Tc) compounds for use as therapeutic and imaging agents, respectively, in nuclear medicine. [3,4] Kinetic and thermodynamic details of water exchange and ligand substitution for these aquacarbonyl ions have been reported [5,6] and many syntheses utilizing these important synthons have been published. [7][8][9][10][11] Several crystal structures have been solved that formally result from the deprotonation of 1 + in aqueous solution and subsequent aggregation.…”

A Novel Field of Research: Bioorganometallic Chemistry, Origins, and Founding Principles

“…One shining example of an organometallic pharmaceutical that has been successfully commercialized is technetium-99m sestamibi, a hexakis (alkylisocyanide) complex of Tc-based radiopharmaceuticals [193][194][195][196][197] Tc radiopharmaceuticals. This water-and air-stable aquo-carbonyl complex 30 contains three labile water ligands that can easily be substituted with a wide variety of organic ligands, thus providing a family of carbonyl complexes that can be used in life sciences and nuclear medicine applications [198][199][200][201][202].…”