The stabilization of elusive monomeric species containing multiple boron-chalcogen bonds has motivated the investigation of sophisticated ligand systems in the past few years. Recently, a series of neutral, Lewis acid-free chalcogenboranes were prepared by incorporation of an amido imidazoline-2-imine as the supporting ligand (Frank et al., Angew. Chem. Int. Ed. 2021, 60, 4633), resulting in well-defined molecular entities with pronounced multiple bond character, B=X (X=O, S, Se, Te). In view of the potential use of N-heterocyclic boranes (NHB=X) as ligands in catalysis and fine chemistry, we evaluated in this work the bonding properties of the new B=X compounds based on a π-backdonation model. The electronic properties of systems in question were modulated via systematic modifications of the NHB ring with respect to ligand variation, saturation, size, and heteroatom substitutions. Investigations into the B=X bond length and order, calculated by means of density functional theory methods, reveal enhanced B=X bonding properties for NHB rings with high electron delocalization in the NHB ring and bearing electron-withdrawing substituents; a fact that was also confirmed by computational assessment of electron interactions of the B=X species with a dicarbonyl manganese complex. We expect that the analysis will contribute to the rational optimization of existing ligands as well as the development of new moieties, which would further allow for exploration of new boron chemistry.
Keywords: boranes; chalcogens; density functional calculations; multiple bonds; transition metals.
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