Photoreactions of Phenylborylene with Dinitrogen and Carbon Monoxide

Formal removal of two bonding partners from boranes, BR₃, yields borylenes, RB, which have been inferred as reactive intermediates in a number of reactions. Phenylborylene (R = C₆H₅; 1) is accessible from phenyldiazidoborane by photochemical extrusion of dinitrogen under matrix isolation conditions. Concomitantly, the nitrene PhNBN is formed via phenyl rearrangement. Here we used a combination of UV/vis, IR, and ESR spectroscopy under cryogenic matrix isolation conditions to investigate the properties and reactivity of phenylborylene. We detected an absorption band of phenylborylene at 375 nm (S₀ → S₂) and tentatively assigned the S₀ → S₁ transition to a very weak band at 518 nm. We also show for the first time that an electrophilic borylene such as 1 can react with N₂ reversibly and with CO irreversibly under photochemical conditions. The corresponding photoproducts PhBNN and PhBCO have triplet electronic ground states. Their small E values are in agreement with the linear arrangements Ph-B-N-N and Ph-B-C-O obtained by density functional theory computations. The D values decrease in the series PhNBN > PhBNN > PhBCO and approach the value for phenylcarbene (PhCH). Indeed, the boron center in PhBCO is isoelectronic with the carbene center in PhCH. The compounds are the first examples of boron analogues of diazoalkanes (R₂CNN) and ketenes (R₂CCO), and their formation may serve as a demonstration of the high reactivity of phenylborylene.