Boron beats nitrogen: strained boron–boron bonds as (molecular) proton sponges

Although boron usually behaves as a Lewis acid, some molecular structures containing B-B bonds can act as electron donors. Inspired in reported crystalline structures, the basicity and the hydrogen bond (HB) acceptor capability of diborane derivatives of dipyrazole 1 have been studied theoretically using M06-2x and CCSD(T) computational methods. The topology of the electron density and molecular electrostatic potential of compound 1 reveal that the richest electron region is located above the B-B bond, making it suitable to be a strong donor and a very effective proton catcher. A key finding is the remarkably high proton affinity of the parent derivative, which exceeds that of very strong nitrogen-containing organic bases such as guanidines. In line with this finding, the hydrogen-bonded complexes exhibit binding energies up to 37 kJ mol^-1, which is a significant interaction considering the electronegativity of boron in comparison with elements typically involved in hydrogen bonds. We have also designed substituted structures where the inductive effect improves the proton affinity and HB acceptor capabilities. The proton affinities of 1b reach 1096 kJ mol^-1, among the highest reported for molecules in the gas-phase.