The Synthesis of B₂(SIDip)₂ and its Reactivity Between the Diboracumulenic and Diborynic Extremes

A new compound with the formula L-B2-L wherein the stabilizing ligand (L) is 1,3-bis[diisopropylphenyl]-4,5-dihydroimidazol-2-ylidene (SIDip) has been synthesized, isolated, and characterized. The π-acidity of the SIDip ligand, intermediate between the relatively non-acidic IDip (1,3-bis[diisopropylphenyl]imidazol-2-ylidene) ligand and the much more highly acidic CAAC (1-[2,6-diisopropylphenyl]-3,3,5,5-tetramethylpyrrolidin-2-ylidene) ligand, gives rise to a compound with spectroscopic, electrochemical, and structural properties between those of L-B2-L compounds stabilized by CAAC and IDip. Reactions of all three L-B2-L compounds with CO demonstrate the differences caused by their respective ligands, as the π-acidities of the CAAC and SIDip carbenes enabled the isolation of bis(boraketene) compounds (L(OC)B-B(CO)L), which could not be isolated from reactions with B2(IDip)2. However, only B2(IDip)2 and B2(SIDip)2 could be converted into bicyclic bis(boralactone) compounds.