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 (k298K = 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.

本文报道了四类带有不同羧酸桥联配体（carboxylate-bridging ligands）的新型氧中心三核铑(III)簇（oxo-centered trinuclear rhodium(III) clusters）——即[Rh3(μ3-O)(μ-O2CR)6(OH2)3]+（其中R分别为CH2CH3、CH2CH2Cl、CH2Cl与CHCl2）以及[Rh3(μ3-O)(μ-O2CCH3)6(OH2)3]+——被氘代甲醇（methanol-d4）取代配位水分子的反应速率。
通过改变R基团的烷基链结构，配位水的取代反应速率跨度可达近3个数量级（k298K = 1.2×10^-2 ~ 2.3×10^-5 s^-1），且反应速率遵循如下趋势：R = CH2CH3 > CH3 > CH2CH2Cl > CH2Cl > CHCl2。
取代反应的活化参数指向解离活化途径（活化焓ΔH⧧ = 99~115 kJ·mol^-1；活化熵ΔS⧧ = 48~52 J·mol^-1·K^-1），表明在过渡态络合物形成过程中，与进攻的氘代甲醇分子几乎无缔合作用。
由于全部五个三核簇的取代反应机理均具有显著的解离特性，其取代速率与配位水交换速率大概率极为相近。
上述数据表明，配位水分子的动力学反应活性深受脂肪族取代基的诱导效应影响，但取代反应机理几乎未发生改变。
本文同时报道了四种新型三核铑(III)簇盐的结构数据，以及铑-103核磁共振谱（103Rh NMR）。研究发现，铑-103核磁共振化学位移跨度超过200 ppm，且与配位水取代反应速率呈现相同的反应活性趋势（103Rh δ：9406~9620 ppm；R = CH2CH3 < CH3 < CH2CH2Cl < CH2Cl < CHCl2）。
综合上述结果，本研究提供了一种仅通过化学位移数据即可估算其他氧中心三核铑(III)簇配位水交换速率的方法。