Correlation Between NMR Coupling Constants and σ-Donating Properties of N-Heterocyclic Carbenes and Their Derivatives

Is the ¹J_C-H coupling constant for protonated carbene a relevant measure of its σ-donation ability? This paper addresses this question by comparing calculated ¹J_C-H values with various experimental and theoretical approaches across a broad spectrum of carbene compounds. We examined Arduengo-type NHCs based on the 2-imidazolylidene scaffold and many other derivatives with modified frameworks, including carbenes with extended, saturated, or conjugated rings, reduced heteroatom stabilization, alternative heteroatoms, permanently charged carbenes, acyclic carbenes, amidocarbenes, and cyclic amino(alkyl/aryl) carbenes, carbodicarbenes and carbodiphosphoranes. Our findings reveal a nuanced relationship between different parameters associated with σ-interaction, such as ¹J_C-H, Huynh electronic parameter (HEP), σ-donation from ETS-NOCV, and lone pair energy. Notably, the best correlation was observed between ¹J_C-H and the ETS-NOCV method, particularly for mono- and diaminocarbenes, highlighting the utility of ¹J_C-H in comparing σ-donation among structurally similar carbene types. However, the use of ¹J_C-H as a universal measure across all carbene classes appears limited, especially when considering carbenes with significantly different structural frameworks. While HEP is less effective for carbenes with diverse structural backbones, our study suggests that ¹J_C-H has potential across a broader range of systems. Additionally, the analysis demonstrates that lone pair energy reflects basicity rather than σ-donor ability.