A boron-transfer mechanism mediating the thermally induced revival of frustrated carbene–borane pairs from their shelf-stable adducts

Chemists have designed strategies that trigger the conformational isomerization of molecules in response to external stimuli, which can be further applied to regulate the complexation between Lewis acids and bases. We have recently developed a system in which frustrated carbene–borane pairs are revived from shelf-stable but external-stimuli-responsive carbene–borane adducts comprised of N-phosphine-oxide-substituted imidazolylidenes (PoxIms) and triarylboranes. Herein, we report the detailed mechanism on this revival process. A thermally induced borane-transfer process from the carbene carbon atom to the N-phosphinoyl oxygen atom initiates the transformation of the carbene–borane adduct. Subsequent conformational isomerization via the rotation of the N-phosphinoyl group in PoxIm moieties eventually leads to the revival of frustrated carbene–borane pairs that can cleave H2. We believe that this work illustrates an essential role of dynamic conformational isomerization in the regulation of the reactivity of external-stimuli-responsive Lewis acid-base adducts that contain multifunctional substituents.

On the association of frustrated Lewis pairs in ionic liquids: a molecular dynamics simulation study

Steric hindered frustrated Lewis pairs (FLPs) have the ability to activate hydrogen molecules, and their reactivity is strongly determined by the geometric parameters of the Lewis acids and bases....

Building new discrete supramolecular assemblies through the interaction of iso-tellurazole N-oxides with Lewis acids and bases

The supramolecular macrocycles spontaneously assembled by iso-tellurazole N-oxides are stable towards Lewis bases as strong as N-heterocyclic carbenes (NHC) but readily react with Lewis acids such as BR3 (R = Ph, F). The electron acceptor ability of the tellurium atom is greatly enhanced in the resulting O-bonded adducts, which consequently enables binding to a variety of Lewis bases that includes acetonitrile, 4-dimethylaminopyridine, 4,4′-bipyridine, triphenyl phosphine, a N-heterocyclic carbene and a second molecule of iso-tellurazole N-oxide.

Conformational studies on heteroleptic trifluoromethyl-substituted phenylboranes

Organoboranes carrying electron-withdrawing substituents are commonly used as Lewis acidic catalysts or cocatalysts in a variety of organic processes. These Lewis acids also became popular through their application in `frustrated Lewis pairs',i.e.combinations of Lewis acids and bases that are unable to fully neutralize each other due to steric or electronic effects. We have determined the crystal and molecular structures of four heteroleptic arylboranes carrying 2-(trifluoromethyl)phenyl, 2,6-bis(trifluoromethyl)phenyl, 3,5-bis(trifluoromethyl)phenyl or mesityl substituents. [3,5-Bis(trifluoromethyl)phenyl]bis[2-(trifluoromethyl)phenyl]borane, C22H11BF12, (I), crystallizes with two molecules in the asymmetric unit which show very similar geometric parameters. In one of the two molecules, both trifluoromethyl groups of the 3,5-bis(trifluoromethyl)phenyl substituent are disordered over two positions. In [3,5-bis(trifluoromethyl)phenyl]bis[2,6-bis(trifluoromethyl)phenyl]borane, C24H9BF18, (II), only one of the twometa-trifluoromethyl groups is disordered. In [2,6-bis(trifluoromethyl)phenyl]bis[3,5-bis(trifluoromethyl)phenyl]borane, C24H9BF18, (III), bothmeta-trifluoromethyl groups of only one 3,5-bis(trifluoromethyl)phenyl ring are disordered. [3,5-Bis(trifluoromethyl)phenyl]dimesitylborane, C26H25BF6, (IV), carries only onemeta-trifluoromethyl-substituted phenyl ring, with one of the two trifluoromethyl groups disordered over two positions. In addition to compounds (I)–(IV), the structure of bis[2,6-bis(trifluoromethyl)phenyl]fluoroborane, C16H6BF13, (V), is presented. None of theortho-trifluoromethyl groups is disordered in any of the five compounds. In all the structures, the boron centre is in a trigonal planar coordination. Nevertheless, the bond angles around this atom vary according to the bulkiness and mutual repulsion of the substituents of the phenyl rings. Also, theortho-trifluoromethyl-substituted phenyl rings usually show longer B—C bonds and tend to be tilted out of the BC3plane by a higher degree than the phenyl rings carryingorthoH atoms. A comparison with related structures corroborates the conclusions regarding the geometric parameters of the boron centre drawn from the five structures in this paper. On the other hand, CF3groups inmetapositions do not seem to have a marked effect on the geometry involving the boron centre. Furthermore, it has been observed for the structures reported here and those reported previously that for CF3groups inorthopositions of the aromatic ring, disorder of the F atoms is less probable than for CF3groups inmetaorparapositions of the ring.

On the behaviour of biradicaloid [P(μ-NTer)]2 towards Lewis acids and bases

The well-known diphosphadiazane-1,3-diyl [P(μ-NTer)]2 (Ter = 2,6-bis(2,4,6-trimethyl-phenyl)-phenyl) was treated with Lewis bases such as N-heterocyclic carbenes and Lewis acids e.g. gold(i) chloride complexes.