Extending N-heterocyclic carbene ligands into the third dimension: a new type of hybrid phosphazane/NHC system† ‡ †Electronic supplementary information (ESI) available: Methods and additional data. CCDC 1040123 1040125–1040128. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4sc03966a Click here for additional data file. Click here for additional data file. ‡Dedicated to Professor Manfred Scheer on the occasion of his 60th birthday.

N-heterocyclic carbenes have been successfully established as versatile ligands in coordination chemistry, as powerful ancillary ligands in catalysis, as organocatalysts, and for an increasing number of other applications. In view of these applications the ability to vary the steric and electronic character of these ligands systematically, using simple modular synthetic routes, is a central theme in this area. In recent years a range of approaches to modify their s-donor/p-acceptor and coordination properties have been implemented, for instance different ring sizes of the backbone resulting in different NCNangles, variation of the heteroatoms adjacent to carbon, abnormal and remote carbenes, acyclic carbenes, anti-Bredt NHCs, push–pull carbenes, redox-switchable NHCs or even cyclophane-derived carbenes. A less explored approach relies on NHCs comprising “inorganic” or organometallic fragments in their ligand backbones (relevant examples shown in Fig. 1).