Of ortho-conjugatively linked reactive intermediates: The cases of ortho-Phenylene-(bis)nitrene, -carbenonitrene, and -(bis)carbene

ortho-Phenylene-bisnitrene (2) and -carbenonitrene (3) were generated in an inert matrix at low temperature, characterized by IR spectroscopy, and ide ntified with the help of DFT calculations. Their thermal and photochemical reactivity was examined in the matrix, and it was found that both 2 and 3 r ing-open to 1,4-substituted butadienes. Calculations (DFT and MCSCF) on the lowest singlet, triplet, and quintet states of 2, 3, and the all-carbon an alogue ortho-phenylene-biscarbene (4) are also reported. All three species are found to have singlet ground states. The S-T splitting is small (2-3 kc al mol(-1)), but the quintet states lie siginificantly higher in energy (20 -25 kcal mol(-1)). The ring-opening reactions of 2-4 (to give 1,4-substitut ed butadienes) as well as the ring-closure reactions (to give benzocyclobut adiene derivatives) were also investigated computationally. The calculated barriers for the ring-opening reactions are in reasonable agreement with th e experimentally obtained activation energies for 2 and 3. Calculations als o suggest that 4, unlike its heteroanalogues 2 and 3, has a stronger prefer ence for the ring-closure reaction (to form benzocyclobutadiene) rather tha n for the ring-opening reaction.