Evidence of boride–borylene ligand-tautomerism leading to a remote C–C-bond and concomitant boryl ligand formation†

The formation of a rhodium pincer-type complex with a boron-based donor ligand and its reactivity are reported. The starting complex contains a formal borylene moiety, stabilised by two pyridine substituents. Quantum chemical investigations indicate the possibility of deprotonation of the central donor group of the type py₂BH in this complex. Efforts to isolate the resulting formal boride species, however, led to a boryl complex with concomitant formation of a new C–C-bond, accompanied by a loss of aromaticity. Mechanistic investigations indicate the presence of tautomerism between two deprotonated species, giving rise to a ligand-stabilised boride and a ligand-stabilised borylene motif.