Magnetic Exchange and Electronic Structure across a Series of NHC-Stabilized Boron Diradicals

The magnetic exchange couplings of N-heterocyclic carbene-stabilized (NHC-stabilized) boron diradicals are very sensitive to the bridging tethers. In this work, we conduct a computational investigation of magnetic exchange for a series of 14 NHC-stabilized dicationic boron diradicals, spanning a coupling range from strongly antiferromagnetic, below -2000 cm-1, to extremely ferromagnetic, approaching +1250 cm-1. These systems differ mostly in the nature of the bridging tether between the boron centers. By employing broken-symmetry DFT, spin-flip time-dependent DFT, complete active space self-consistent field, CASSCF, and N-electron valence state second-order perturbation theory, NEVPT2, we establish the magnetic hierarchy for these systems and show that the bridging topology (electronic structure, sterics and geometry) dictates the magnitude and sign of the exchange coupling across the series. The calculated singlet-triplet gaps were correlated with the spatial overlap integrals, Sαβ, from unrestricted corresponding orbitals, allowing us to evaluate the applicability and limits of the Hay-Thibeault-Hoffmann framework. To rationalize the large ferromagnetic couplings in some systems, we employ the diradical index and bridge partial spin densities to analyze the competition between spatial overlap pathways and spin polarization mechanisms. We describe how structural reorganization and the electronic topology of the bridging tether are correlated with each system's position on the continuum between an open-shell diradical and a quinoidal closed-shell structure. Taken together, these descriptors provide the electronic-structure requirements for designing NHC-stabilized boron-based magnetic materials with controllable magnetic exchange.