Stereoelectronic Profiling of Expanded-Ring N‑Heterocyclic Carbenes

Heterobis­(carbene) complexes of palladium­(II) and gold­(I) containing expanded-ring N-heterocyclic carbenes (erNHCs) have been prepared in order to study their electronic properties. erNHCs with mesityl substituents were found to exhibit anisotropic interferences, which hampered ranking of their donicities by ¹³C NMR spectroscopy. The anisotropy effects were found to be stronger in the linear gold complexes, where a smaller coordination number allows the wingtips to spread out more. erNHCs with flexible N-benzyl groups are more suitable, and their donor strengths were found to gradually increase from five- to seven-membered heterocycles. The same trend can also be obtained by comparing the ¹J(C–H) coupling constants of the respective salts, although significant differences between seven- and eight-membered erNHCs could not be detected. The %V_bur values of erNHCs obtained from structures of their palladium and gold complexes revealed that the anisotropic interferences increase with overall steric bulk.