Rates of CO exchange in a comparative series of tetrairidium carbonyl clusters Ir₄CO₉L₃ consisting of phosphine ligands of varying steric bulk (diphenylmethylphosphine in 1, triphenylphosphine in 2, and calix[4]arene phosphine in 3) have been investigated in toluene-d₈. The presence of bridging CO ligands and the same phosphine substitution pattern (axial, equatorial, and equatorial) as confirmed by ³¹P NMR spectroscopy enables the rigorous comparison of this series of isoelectronic clusters. Inverse gated decoupling ¹³C qNMR spectroscopy was applied for quantification and assignment of the entire spectrum, the carbonyl region of which was used to characterize CO exchange. A toluene solution of the calixarene-based cluster 3 exhibited no evidence of CO exchange up to 353 K. This included a lack of observed exchange involving apical CO ligands, which underwent scrambling by 323 K for 1 and 2. Activation energies for CO exchange in a toluene solution of 1 were <4.5 kcal/mol based on line-width analysis, whereas they could not be calculated for 2 because resonances were too broad to be analyzed by 353 K. Large differences in phosphine mobility between 1 and 2 relative to 3 were also reflected in the ³¹P NMR spectra, which for the latter remained unchanged up to 353 K, in contrast to significant broadening observed for the former two clusters. The observed trends here reinforce the crucial role of cumulative noncovalent interactions involving sterically bulky calixarene ligands in 3. These interactions are responsible for immobilizing phosphine ligands and encaging CO ligands, in a manner that limits their intramolecular exchange. These observations elucidate a previously observed mechanism of selective molecular recognition involving basal-plane bonding of hydrogen but not hydrocarbon (i.e., catalytic “S” sites) in a silica-supported cluster derived from 3, in particular its electronic rather than steric origin.

中文翻译：

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速率过程的机械控制：三取代四铱簇簇催化剂中配体立体异构体对CO交换的影响

在甲苯中研究了比较系列的四铱羰基簇Ir ₄ CO ₉ L ₃的CO交换速率，该簇由变化的空间体积（二苯甲基膦在1中，三苯膦在2中，杯[4]芳烃膦在3中）组成- d ₈。通过³¹ P NMR光谱证实，桥接的CO配体和相同的膦取代模式（轴向，赤道和赤道）可对这一系列等电子簇进行严格的比较。反向门极去耦¹³C qNMR光谱用于整个光谱的定量和分配，其羰基区域用于表征CO交换。杯芳烃类簇3的甲苯溶液在最高353 K时没有显示出CO交换的证据。这包括缺乏观察到的涉及顶端CO配体的交换，该交换过程受1和2的323 K扰乱。根据线宽分析，在1的甲苯溶液中用于CO交换的活化能<4.5 kcal / mol，而对于2则无法计算，因为共振太宽而无法用353 K进行分析。相对于1和23 P也反映在³¹ P NMR光谱中，后者在353 K之前保持不变，与前两个簇的显着展宽形成鲜明对比。在这里观察到的趋势加强了涉及空间庞大的杯芳烃配体3的累积非共价相互作用的关键作用。这些相互作用以限制它们的分子内交换的方式负责固定膦配体并固定CO配体。这些观察阐明了先前观察到的选择性分子识别的机制，该机制涉及氢的基面键合，而不是衍生自3的二氧化硅支撑的簇中的碳氢化合物（即催化“ S”位），特别是其电子而不是空间起源。