Frustrated Lewis pair briefing

Frustrated Lewis pair briefing
17 July 2008 Updated 11 September

One of these 2008 buzzwords: Frustrated Lewis pair (FLP). This is a Lewis acid - base pair in which steric hindrance precludes regular Lewis acid–base adduct formation - as in for instance ammonia borane - through a coordinate covalent bond. Justly frustrated , these reactive groups turn to other molecules hanging around with unexpected reactivity. The first Lewis pairs to be investigated were based on boron and phosphorus, largely as a result of work by the group of Douglas W. Stephan of the University of Windsor (Latest review Stephan 2008 DOI).

Ever since his pioneering 2006 article on metal-free hydrogenation with a frustrated phosphonium borate (DOI) (48 citations to date) Stephan explained how the catalyst was made in the first place - by thermal rearrangement of phosphine / tris(pentafluorophenyl)boron Adducts (DOI) - and how its Lewis acidity can be tuned (DOI).

That phosphine-borane complexes can expulse hydrogen to form phosphinoborines is known since 1953 (Burg & Wagner DOI). Reversible hydrogen uptake and delivery surely was a surprise.

Stephan then demonstrated the reaction of sterically hindered phosphines and boranes in tricomponent reactions with hydrogen (cleavage!) ( DOI) and with olefines (to zwitterionic phosphonium borates) (DOI) and with nitriles (reduction!) ( DOI)

The tricomponent olefin reaction is unusual because phosphines or boranes are unable to react to the olefin on their own. Therefore a special reaction mechanism must be at work.

In a recent in silico contribution Stirling et al (DOI) shed some light. The reaction is described as truly trimolecular with a single (early) transition state, slightly asynchronous with B-C bond formation preceding P-C bond formation. Further computed characteristics: reaction is strongly exothermic and irreversible and the molecules preorganise themselves for reaction based on a mixture of van der Waals force interactions, weak interactions between hydrogen and fluorine and weak boron - ethylene interactions. A charge transfer complex as postulated by Stephan is not observed.

Another computational study (DOI) also arrives at a concerted reaction mechanism in which ethylene seems to act as a bridge for electron transfer from the Lewis base center P to the Lewis acid center B

In another development, Bertrand et al have demonstrated hydrogen and ammonia cleavage with certain carbenes (DOI).

Hydrogen activation by a singlet carbene is very similar to that of a transition metal as both have a filled orbital and a vacant nonbonding orbital. Unable to interact with each other (or dimerize to an alkene) carbenes too can be considered frustrated Lewis pairs. In this reaction the carbene reacts as a nucleophile, creating a transient hydride which recombines with the positively polarized carbon.

The definition of FLP gets a bit stretched in the Bertrand report (in it FLP does not get a mention but others have made the connection) but a true carbene - borane system was reported in 2008 By Stephan et al. (DOI) and (identical system only 5 days later) by Holschumacher et al. (DOI):

In absence of hydrogen carbene and borane will slowly form an insoluble and unreactive adduct, the reaction of carbene with hydrogen alone was not investigated.

Most recently, the frustrated Lewis pair family was extended to combinations of amines and boranes by Sumerin et al. (DOI). TMP was reacted to B(C6F5)3 in presence of hydrogen gas to form a adduct capable of reducing benzaldehyde:

A B/P system with an ethylene or alkene bridge and capable of simple reductions has been developed by Spies et al (DOI (DOI).