An N-heterocyclic carbene with a saturated backbone and spatially-defined steric impact

The synthesis and coordination chemistry of a saturated analogue of a 'bulky-yet-flexible' N-heterocyclic carbene (NHC) ligand are described. "SIPaul" is a 4,5-dihydroimidazol-2-ylidene ligand with unsymmetrical aryl N-substituents, and is one of the growing class of 'bulky-yet-flexible' NHCs that are sufficiently bulky to stabilise catalytic intermediates, but sufficiently flexible that they do not inhibit productive chemistry at the metal centre. Here, the synthesis of SIPaul.HCl and its complexes with copper, silver, iridium, palladium, and nickel, and its selenourea, are reported. The steric impact of the ligand is quantified using percent buried volume (%V_bur), while the electronic properties are probed and quantified using the Tolman Electronic Parameter (TEP) and δ_Se of the
corresponding selenourea. This work shows that despite the often very different performance of saturated versus unsaturated carbenes in catalysis, the effect of backbone saturation on measurable properties is very small.