Stereoelectronic Profiling of Expanded-Ring N-Heterocyclic Carbenes

Abstract

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 1 J(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.

Gold(I) Complexes with Eight‐Membered NHC Ligands: Synthesis, Structures and Catalytic Activity

“…During the preparation of this paper, Huynh and co‐workers reported the synthesis of some alkyl‐chain backbone eight‐membered NHC gold(I)‐bromide complexes . Recently, we have developed the first examples of sterically bulky nine‐ and ten‐membered NHC ligands and the synthesis of the corresponding gold complexes.…”

Steric Properties of N‐Heterocyclic Carbenes affect the Performance of Electronic Probes

“…This longer bond results in the NHC having an artificially lower measured σ ‐donation than would be the case in the absence of steric effects, due to physical stretching of the bond [4] . It should be noted that steric effects on HEP have been considered (and explained) before for NHCs with larger rings [22] …”

Sterically Demanding AgI and CuI N‐Heterocyclic Carbene Complexes: Synthesis, Structures, Steric Parameters, and Catalytic Activity

“…Oxazoline II was obtained (93 % yield) as the only product after 8 h, conducting the reaction in chloroform at room temperature. A brief survey on the existing literature reveals only a few examples of gold(I) bromide complexes that spans from six‐ to eight‐membered NHC ligands, including their structural and catalytic features …”