Multiconfigurational non-adiabatic molecular dynamics towards photochemical-N2-extrusion reactions in borodiazenes

Abstract

We used state-of-the-art quantum-chemical calculations to study the photodenitrogenation of 1-methylborodiazene to borirane. Vertical excitation energy calculations indicate that the S0 → S1 (nN → π*NN) and S0 → S2 (nN → 2pB) transitions are accessible (318 nm and 287 nm, respectively), and the S0 → S1 transition is the bright state. The minimum-energy path indicates near-degeneracy between the S1 and S2 surfaces in the Franck-Condon region, leading to the steepest descent path on the S1 state to be towards fluorescent decay rather than denitrogenation. Our simulations indicate that denitrogenation primarily occurs shortly after an S1/S0 hopping event (79% of trajectories) and proceeds via three distinct pathways. The dominant pathway involves boron pyramidalization and partial πNN isomerization in the S1 state, with N2 elimination shortly after an S1/S0 hop. Minor pathways include either πNN isomerization or boron pyramidalization, with N2 elimination occurring on the S0 and S1 states, respectively. We also observed that some trajectories rearrange to form a diazoborete intermediate. Our results show that the labile N2 group can be used to access base-free boriranes.