Substituent Effects in Five Oxo-Centered Trinuclear Rhodium(III) Clusters

We here report the rates of water substitution by methanol-d₄ for four new oxo-centered trinuclear rhodium(III) clusters with different carboxylate-bridging ligands, [Rh₃(μ₃-O)(μ-O₂CR)₆(OH₂)₃]⁺ (R = CH₂CH₃, CH₂CH₂Cl, CH₂Cl, and CHCl₂), and [Rh₃(μ₃-O)(μ-O₂CCH₃)₆(OH₂)₃]⁺. By varying the R group alkyl chain, water substitution rates were found to span almost 3 orders of magnitude (k^298K = 1.2 × 10^-2−2.3 × 10^-5 s^-1) and reflect the following trend R = CH₂CH₃ > CH₃ > CH₂CH₂Cl > CH₂Cl > CHCl₂. Activation parameters for substitution point toward a dissociative activation pathway (ΔH^⧧ = 99−115 kJ mol^-1; ΔS^⧧ = 48−52 J mol^-1 K^-1), indicating that there is little association with the incoming methanol molecule during the formation of the transition-state complex. Because the mechanism for substitution in all five trimers has a considerable dissociative character, substitution rates are likely very similar to water exchange rates. These data suggest that the kinetic reactivity of the ligated waters is heavily influenced by the inductive ability of the aliphatic substituents, but yet the mechanism of substitution remains virtually unchanged. Structural data are also reported for the four new rhodium(III) trimer salts as well as ¹⁰³Rh NMR spectra. We find that ¹⁰³Rh NMR chemical shifts span more than 200 ppm and mirror the same reactivity trend found for the rates of water substitution (¹⁰³Rh δ (9406−9620 ppm):  R = CH₂CH₃ < CH₃ < CH₂CH₂Cl < CH₂Cl < CHCl₂). Taken together, these data suggest a means for estimating water exchange rates for other oxo-centered rhodium(III) trimers from chemical shift data alone.