N-Heterocyclic carbene-carbodiimide (NHC-CDI) adducts are versatile compounds that can be used as ligands and (pre)catalysts, but their systematic structure-property relationships are underexplored. Herein, we investigated how structural electronic variations on both the NHC and CDI affect the inherent kinetic and thermodynamic properties of the adducts. Using in situ carbene trapping and variable-temperature NMR spectroscopy, we measured the rates of dissocia-tion and the equilibrium constants then used Eyring and van ’t Hoff analyses to calculate ΔG‡ and ΔG, respectively. Linear free energy relationships indicate that the CDI substituents have greater influence than variations to the NHC backbone, presumably caused by (de)stabilization of the highest occupied molecular orbital of the adduct. These CDI structural modifications affected the adducts’ thermodynamics (ΔG) more than the kinetics (ΔG‡), and were found to be primarily driven by inductive, rather than resonance, factors. Preliminary results suggest a steric threshold beyond which steric effects dominate electronic effects in governing the strength of the adduct bond. This systematic investigation provides valuable insight into the design of NHC-CDIs for current and future applications.

中文翻译：

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N-杂环卡宾-碳二亚胺 （NHC-CDI） 加合物：电子对动力学和热力学性质影响的系统研究


N-杂环卡宾-碳二亚胺 （NHC-CDI） 加合物是可用作配体和（预）催化剂的多功能化合物，但它们的系统结构-性质关系尚未得到充分探索。在此，我们研究了 NHC 和 CDI 的结构电子变化如何影响加合物的固有动力学和热力学性质。使用原位卡宾捕获和变温 NMR 波谱，我们测量了解离速率和平衡常数，然后使用 Eyring 和 van 't Hoff 分析分别计算 ΔG‡ 和 ΔG。线性自由能关系表明，CDI 取代基比对 NHC 主链的变化具有更大的影响，这可能是由加合物的最高占据分子轨道的（不稳定）引起的。这些 CDI 结构修饰对加合物热力学 （ΔG） 的影响大于动力学 （ΔG‡），并且发现主要由感应因素而不是共振因素驱动。初步结果表明一个空间阈值，超过该阈值，空间效应在控制加合键强度方面主导电子效应。这项系统的研究为当前和未来应用的 NHC-CDI 设计提供了有价值的见解。