Synthesis of trifluoroborate functionalised imidazolium salts as precursors to weakly coordinating bidentate NHC ligands

A new class of trifluoroborate functionalised N-heterocyclic carbene precursors have been synthesised, isolated and characterised structurally. The ligands were obtained via a serendipitous one-pot reaction in which deprotection, cyclisation and fluorination of boryl-functionalised diarylethylenediamine derivatives occur concurrently. Deprotonation of the imidazolium salts was found to yield the free carbene, though 18-crown-6 was found necessary to prevent further reactivity of the resulting aryl potassium trifluoroborate salts; in the absence of 18-crown-6, elimination of KF resulted in a cyclic carbene–BF2 arene adduct. Complexation to rhodium was facile, and yielded four-coordinate complexes in which the Rh–BF3 interaction was determined by 19F NMR spectroscopy to be weak.