Dihydrogen Activation by Antiaromatic Pentaarylboroles

Abstract

Facile metal-free splitting of molecular hydrogen (H 2 ) is crucial for the utilization of H 2 without the need for toxic transition-metal-based catalysts. Frustrated Lewis pairs (FLPs) are a new class of hydrogen activators wherein interactions with both a Lewis acid and a Lewis base heterolytically disrupt the hydrogen-hydrogen bond. Here we describe the activation of hydrogen exclusively by a boron-based Lewis acid, perfluoropentaphenylborole. This antiaromatic compound reacts extremely rapidly with H 2 in both solution and the solid state to yield boracyclopent-3-ene products resulting from addition of hydrogen atoms to the carbons R to boron in the starting borole. The disruption of antiaromaticity upon reaction of the borole with H 2 provides a significant thermodynamic driving force for this new metal-free hydrogen-splitting reaction.

Lewis Acid–Base Adducts of 1‐Mesityl‐ and 1‐Chloro‐2,3,4,5‐tetraphenylborole

“…Typical reaction types that cause disruption to the antiaromatic π system include cycloaddition reactions [8,12,21,25] and the activation of small molecules such as H 2 . [22] The essential strategy to isolate a borole molecule is steric and electronic shielding of the backbone in the C 4 B annulus. This has hitherto been achieved by substitution with aryl or thienyl groups.…”

Direct observation of a borane–silane complex involved in frustrated Lewis-pair-mediated hydrosilylations

“…1, inset) is an extremely Lewis acidic compound that binds CO 41 and activates dihydrogen towards H-H bond cleavage. 42,43 In the latter reaction, an adduct of H2 with V is clearly implicated. Borole V also undergoes less well defined reactions with silanes that involve cleavage of the Si-H bond, 44 but due to the compound's low solubility in non-donor solvents, we have been unable to conduct low temperature spectroscopic studies aimed at detecting either an H2 or a silane adduct of V. Accordingly, we recently reported the synthesis of 1,2,3-tris(pentafluorophenyl)-4,5,6,7-tetrafluoro-1-boraindene, 45 1 ( Fig.…”

Reactivity of a Dihydrodiborene with CO: Coordination, Insertion, Cleavage, and Spontaneous Formation of a Cyclic Alkyne

“…[11] This borole was also found to undergo H2 addition to the two endocyclic carbon atoms adjacent to boron. [12] More recently unsymmetrical diboranes have also started to show some promise for TM-free H2 and CO activation. [13] Advances in ligand design have enabled the isolation of numerous electron-rich, low-valent main group species that are capable of activating a wide variety of small molecules.…”