B–B vs. B–H Bond Activation in a (μ‐Hydrido)diborane(4) Anion upon Cycloaddition with CO₂, Isocyanates, or Carbodiimides

Abstract

The intriguing (m-hydrido)diboranes(4) with their prominent pristine representative [B 2 H 5 ] À have mainly been studied theoretically.W en ow describe the behavior of the planarizedt etraaryl (m-hydrido)diborane(4) anion [1H] À in cycloaddition reactions with the homologous series of heterocumulenes CO 2 ,i PrNCO,a nd iPrNCNiPr.W es how that a C = Obond of CO 2 selectively activates the B À Bbond of [1H] À , while the m-H ligand is left untouched( [ 2H] À ). The carbodiimide iPrNCNiPr,i nc ontrast, neglects the BÀBb ond and rather adds the B-bonded H À ion to its central Ca tom to generate aformamidinate bridge across the B 2 pair ([3] À ). As ah ybrid, the isocyanate iPrNCO combines the reactivity patterns of both its congeners and gives two products:o ne of them ([4H] À )isrelated to [2H] À ,the other ([5] À )isananalog of [3] À .W ef inally propose am echanistic scenario that rationalizes the individual reaction outcomes and combines them to ac oherent picture of B-B vs.B -H bond activation.