N-Heterocyclic Carbene-Carbodiimide (NHC-CDI) Adducts: A Systematic Investigation of Electronic Effects on Kinetic and Thermodynamic Properties

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.