Twisting the arm: structural constraints in bicyclic expanded-ring N-heterocyclic carbenes.

A series of diaryl, mono-aryl/alkyl and dialkyl mono- and bicyclic expanded-ring N-heterocyclic carbenes (ER-NHCs) have been prepared and their complexation to Au(i) investigated through the structural analysis of fifteen Au(NHC)X and/or [Au(NHC)]X complexes. The substituted diaryl 7-NHCs are the most sterically encumbered with large buried volume (%V) values of 40-50% with the less flexible six-membered analogues having %V values at least 5% smaller. Although the bicyclic systems containing fused 6- and 7-membered rings (6,7-NHCs) are constrained with relatively acute NCN bond angles, they have the largest %V values of the dialkyl derivatives reported here, a feature related to the fixed conformation of the heterocyclic rings and the compressional effect of a pre-set methyl substituent.