Photoreactions of Phenylborylene with Dinitrogen and Carbon Monoxide

Abstract

Formal removal of two bonding partners from boranes, BR, yields borylenes, RB, which have been inferred as reactive intermediates in a number of reactions. Phenylborylene (R = CH; 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 (S → S) and tentatively assigned the S → S 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 N 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 (RCNN) and ketenes (RCCO), and their formation may serve as a demonstration of the high reactivity of phenylborylene.

Where silylene–silicon centres matter in the activation of small molecules

“…108 By far, numerous transition metal-N 2 complexes taking advantage of Fe, 109a-h,j,k Mo, 109c-e,g-k W 109d,g,h have been developed for consecutive conversion of N 2 to ammonia, hydrazine and other fundamental nitrogen-containing chemicals. 109 Main-group elements supported fixation and functionalisation of N 2 is scarce, examples like borabenzene intermediate under flashthermolysis condition, 110 as well as phenyl cation 111 and phenylborylene 112 under cryogenic argon matrix condition were observed by IR spectroscopy and isotopic labelling experiments. Very recently, Braunschweig and co-workers have demonstrated that coordination of N 2 to an empty 2p-orbital of a transient borylene complex gives rise to a borylene-N 2 adduct, which either reacts with the second equivalent of borylene to afford the reductive B 2 N 2 product, 113 or is reduced by excess KC 8 to give a nitrene-like radical followed by dimerisation to afford a [N 4 ] 2À coupling product caped with two boron centres.…”

Nitrogen fixation and reduction at boron

“…1A), inspired us to test our borylene fragment as a scaffold to bind N 2 . The suitability of borylenes as candidates for N 2 binding is further underlined by a recent report of N 2 binding by the unstabilized borylene "PhB:" under matrix conditions (18).…”

Boron compounds tackle dinitrogen

“…3,4 This suggested that borylenes might also bind N 2 , albeit using different orbitals than transition metals do (Figure B). Very recent work demonstrated binding of N 2 to an unstabilized borylene under matrix conditions at 10 K. 5 However, even at these extremely low temperatures – which are used to observe the most reactive and unstable molecules – N 2 binding was very weak. In this work, Légaré et al .…”