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

Abstract

We here report the rates of water substitution by methanol-d(4) for four new oxo-centered trinuclear rhodium(III) clusters with different carboxylate-bridging ligands, [Rh3(mu3-O)(mu-O2CR)6(OH2)3]+ (R = CH2CH3, CH2CH2Cl, CH2Cl, and CHCl2), and [Rh3(mu3-O)(mu-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 x 10(-2)-2.3 x 10(-5) s(-1)) and reflect the following trend R = CH2CH3 > CH3 > CH2CH2Cl > CH(2)Cl > CHCl2. Activation parameters for substitution point toward a dissociative activation pathway (DeltaH = 99-115 kJ mol(-1); DeltaS = 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 delta (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.

Computational Insight into 103Rh Chemical Shift–Structure Correlations in Rhodium Bis(phosphine) Complexes

“…10 More recently, rates of water substitution processes in rhodium clusters have been shown to correlate with chemical shifts. 11 Similar NMR−reactivity relations have also been observed or predicted in other organometallic systems, e.g., using 59 Co 12 or 51 V NMR. 13 As a whole, transition metal NMR is a useful tool to study structural features as well as reactivities.…”

“…For instance, 103 Rh chemical shifts have been correlated with rate constants of ligand exchange and migration reactions, enantioselectivities, and stability constants . More recently, rates of water substitution processes in rhodium clusters have been shown to correlate with chemical shifts . Similar NMR–reactivity relations have also been observed or predicted in other organometallic systems, e.g., using 59 Co or 51 V NMR .…”

Synthesis and Properties of the Cyano Complex of Oxo-Centered Triruthenium Core [Ru3(μ3-O)(μ-CH3COO)6(pyridine)2(CN)]

“…Oxo−carboxylate-bridged di-and polynuclear metal complexes are known as important structural motifs not only in coordination chemistry but also in biological systems. 1−4 Diand trinuclear units, M 2 (μ-O)(μ-RCOO) n (n = 1, 2) and M 3 (μ 3 -O)(μ-RCOO) 6 , are frequently found in many metal ions. 5−14 Among them, the ruthenium analogues have been the most extensively studied.…”

Cooperation between bound waters and hydroxyls in controlling isotope-exchange rates

“…A dissociative mechanism around Rh(III) is favored because it has a small innercoordination sphere and high charge density. As a result, outer-sphere solvent molecules must overcome a greater electrostatic repulsion when attempting to insert into the inner-coordination sphere and make a seven-ligand transition state, which is unlikely (see (Cusanelli et al, 1996;Drljaca et al, 1996Drljaca et al, , 1998aHouston et al, , 2006). …”