Dehydrogenation Reactions of Cyclic C₂B₂N₂H₁₂ and C₄BNH₁₂ Isomers

The energetics for different dehydrogenation pathways of C(2)B(2)N(2)H(12) and C(4)BNH(12) cycles were calculated at the B3LYP/DGDZVP2 and G3(MP2) levels with additional calculations at the CCSD(T)/complete basis set level. The heats of formation of the different isomers were calculated from the G3(MP2) relative energies and the heats of formation of the most stable isomers of c-C(2)B(2)N(2)H(6), c-C(2)B(2)N(2)H(12), and c-C(4)BNH(12) at the CCSD(T)/CBS including additional corrections together with the previously reported value for c-C(4)BNH(6). Different isomers were analyzed for c-C(2)B(2)N(2)H(x) and c-C(4)BNH(x) (x = 6 and 12), and the most stable cyclic structures were those with C-C-B-N-B-N and C-C-C-C-B-N sequences, respectively. The energetics for the stepwise loss of three H(2) were predicted, and the most feasible thermodynamic pathways were found. Dehydrogenation of the lowest energy c-C(2)B(2)N(2)H(12) isomer (6-H(12)) is almost thermoneutral with DeltaH(3dehydro) = 3.4 kcal/mol at the CCSD(T)/CBS level and -0.6 kcal/mol at the G3(MP2) level at 298 K. Dehydrogenation of the lowest energy c-C(4)BNH(12) isomer (7-H(12)) is endothermic with DeltaH(3dehydro) = 27.9 kcal/mol at the CCSD(T)/CBS level and 23.5 kcal/mol at the G3(MP2) level at 298 K. Dehydrogenation across the B-N bond is more favorable as opposed to dehydrogenation across the B-C, N-C, and C-C bonds. Resonance stabilization energies in relation to that of benzene are reported as are NICS NMR chemical shifts for correlating with the potential aromatic character of the rings.