RESUMO
1,2-Azaborinines are the BN analogues of arynes through exchange of the formal CC triple bond by an isoelectronic BN bond. The BN-arynes are an underexplored class of reactive intermediates. Dibenzo[c,e][1,2]azaborinine (10,9-BN-phenanthryne) 1 was inferred as reactive intermediate by trapping reactions. Here it is shown that 1 can be generated in the gas phase by thermolysis from the pyridine adduct of 9-azido-9-borafluorene by cleavage of the dative bond with pyridine and dinitrogen extrusion. The ionization potential of 1 is 8.2â eV with ionization resulting from the π HOMO. Under cryogenic matrix isolation conditions, 9-azido-9-borafluorene photolysis results in isomerization to the dinitrogen adduct of 1 without involvement of a triplet borylnitrene intermediate. Photochemical nitrogen extrusion from 1 â N2 is not possible and nitrogen fixation is irreversible under cryogenic conditions. In contrast, 2,4,7,9-tetra-tert-butyldibenzo[c,e][1,2]azaborinine can be photogenerated from the corresponding azidoborole precursor under cryogenic matrix isolation conditions, and nitrogen fixation is precluded due to steric hindrance. The BN stretching vibration at about 1750â cm-1 is much weaker than in typical linear diaryl iminoboranes.
RESUMO
The unpaired electron impacts the binding between radicals and ordinary closed-shell molecules in noncovalent complexes. Conversely, the complexation partner can enhance, decrease, or even control the reactivity of the interacting radical. Previously, such radical-molecule (and especially radical-water) complexes were studied by controlled assembly of the interacting partners which mostly leads to formation of the thermodynamically most stable species. Here, we show that UV photolysis of the resonance-stabilized carboxymethyl radical isolated in a cryogenic argon matrix at 4 K leads to the intermediary formation of a metastable, noncovalent complex of the ketenyl radical with a water molecule. In this complex, the ketenyl radical binds water at its terminal carbon atom, although a more stable isomer exists in which water interacts with the C-H bond of the radical. Rigorous W1 theory computations confirm that the ketenyl radical is a stronger donor in C-H···O interactions than ketene itself, while it performs comparably well as an acceptor. We propose that complex formation proceeds via an initial excited-state C-O bond breaking reaction in carboxymethyl under release of an OH radical, which is supported by multireference QD-NEVPT2 computations.
RESUMO
The boryl nitrene CatBN (Cat = catecholato) turns highly reactive toward small inert molecules upon irradiation of its triplet ground state XÌ3A2 with light of wavelength λ > 550 nm. A computational study of a model boryl nitrene using complete active space self-consistent field (CASSCF) theory provides evidence for the population of the highly reactive electronic state aÌ1A1 upon irradiation. Potential energy scans connecting different critical points (minima, minimum energy crossing points, and conical intersections) reveal two possible pathways that could relax photoexcited boryl nitrene from the Franck-Condon region of AÌ3B1 to the aÌ1A1 state minimum. Considering the energy barriers to relaxation from one electronic state to another and the magnitude of spin-orbit couplings, the energetically most favorable pathway involves photoexcitation to AÌ3B2, followed by intersystem crossing to the open-shell singlet state (bÌ1A2) and internal conversion to aÌ1A1. The relevant minimum energy crossing point is about 7-8 kcal mol-1 higher in energy than the Franck-Condon region.
RESUMO
The reaction of a borylnitrene with carbon dioxide is studied under cryogenic matrix isolation conditions. Photogenerated CatBN (Cat=catecholato) reacts with CO2 under formation of the cycloaddition product CatBNCO2 , a 3-oxaziridinone derivative, after photoexcitation (>550â nm). The product shows Fermi resonances between the CO stretching and ring deformation modes that cause unusual 13 C and 18 O isotopic shifts. A computational analysis of the 3-oxaziridinone shows this cyclic carbamate to be less strained than an α-lactone or an α-lactame.
RESUMO
The reaction of dioxygen with nitrenes can have significant energy barriers, although both reactants are triplet diradicals and the formation of nitroso-O-oxides is spin-allowed. By means of matrix-isolation infrared spectroscopy in solid argon, nitrogen, and neon, and through high-level computational quantum chemistry, it is shown herein that a 3-nitreno-1,3,2-benzodioxaborole CatBN (Cat=catecholato) reacts with dioxygen under cryogenic conditions thermally at temperatures as low as 7â K to produce two distinct products, an anti-nitroso-O-oxide and a nitritoborane CatBONO. The computed barriers for the formation of nitroso-O-oxide isomers are very low. Whereas anti-nitroso-O-oxide is kinetically trapped, its bisected isomer has a very low barrier for metathesis, yielding the CatBO+NO radicals in a strongly exothermic reaction; these radicals can combine under matrix-isolation conditions to give nitritoborane CatBONO. The trapped isomer, anti-nitroso-O-oxide, can form the nitritoborane CatBONO only after photoexcitation, possibly involving isomerization to the bisected isomer of anti-nitroso-O-oxide.