Water Exchange from the Oxo-Centered Rhodium(III) Trimer [Rh3((mu)3-O)((mu)-O2CCH3)6(OH2)3]+: A High-Pressure 17O NMR Study

Water exchange from the oxo-centered rhodium(III) trimer, [Rh3((mu)3-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 ((plus-minus)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: kex298 = 5 × 10^-3 s^-1, (delta)H++ = 99 ((plus-minus)3) kJ mol6-1, and (delta)S++ = 43 ((plus-minus)10) J K-1mol-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 (delta)V++ = +5.3 ((plus-minus)0.4) cm^3 mol^-1, indicative of an interchange-dissociative (Id) 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).