Water exchange from the oxo-centered rhodium(III) trimer [Rh3(mu3-O)(mu-O2CCH3)6(OH2)3]+: a high-pressure 17O NMR study.

Water exchange from the oxo-centered rhodium(III) trimer, [Rh3(mu3-O)(mu-O2CCH3)6(OH2)3]+, was investigated using variable-temperature (272.8-281.6 K) and variable-pressure (0.1-200 MPa) 17O NMR spectroscopy. The exchange reaction was also monitored at three different acidities (pH = 1.8, 2.9, and 5.7) in which the molecule is in the fully protonated form (pKa = 8.3 (+/-0.2), I = 0.1 M, T = 298 K). The temperature dependence of the pseudo-first-order rate coefficient for water exchange yields the following kinetic parameters: k(ex)298 = 5 x 10(-3) s(-1), deltaH(double dagger) = 99 (+/-3) kJ mol(-1), and deltaS(double dagger) = 43 (+/-10) J K(-1) mol(-1). The enhanced reactivity of the terminal waters, some 6 orders of magnitude faster than water exchange from Rh(H2O)6(3+), is likely due to trans-labilization from the central oxide ion. Also, another contributing factor is the low average charge on the metal ions (+0.33/Rh). Variation of reaction rate with pressure results in a deltaV(double dagger) = +5.3 (+/-0.4) cm3 mol(-1), indicative of an interchange-dissociative (I(d)) pathway. These results are consistent with those published by Sasaki et al. who proposed that water substitution from rhodium(III) and ruthenium(III) oxo-centered trimers follows a dissociative mechanism based on highly positive activation parameters (Sasaki, Y.; Nagasawa, A.; Tokiwa-Yamanoto, A.; Ito, T. Inorg. Chim. Acta 1993, 212, 175-182).