"Hummingbird" Behaviour of N-Heterocyclic Carbenes Stabilises Out-of-Plane Bonding of AuCl and CuCl Units

An N‐heterocyclic carbene substituted by two expanded 9‐ethyl‐9‐fluorenyl groups was shown to bind an AuCl unit in an unusual manner, namely with the AuX rod sitting out of the plane defined by the heterocyclic carbene unit. As shown by X‐ray studies and DFT calculations, the observed large pitch angle (21°) arises from an easy displacement of the gold(I) atom away from the carbene lone‐pair axis, combined with the stabilisation provided by weak CH⋅⋅⋅Au interactions involving aliphatic and aromatic H atoms of the NHC wingtips. Weak, intermolecular Cl⋅⋅⋅H bonds are likely to cooperate with the H⋅⋅⋅Au interactions to stabilise the out‐of‐plane conformation. A general belief until now was that tilt angles in NHC complexes arise mainly from steric effects within the first coordination sphere. Bimodal NHC pincers: Behaving like a flying hummingbird, alkylfluorenyl‐substituted N‐heterocyclic carbenes readily modify their shape in order to optimise bonding to AuCl and CuCl moieties. In the resulting complexes, the metal atom has moved away from the ideal carbenic lone‐pair axis.