Torsional Control of Frustrated Lewis Pair Behaviour Involving PNP-type Phosphines

We present convenient syntheses of a range of PNP-type phosphines which feature a pair of ortho-functionalized ArPPh2 moieties linked via a central NMe unit, but which differ in the nature of the additional backbone 'tether' linking the aryl groups. These comprise the acyclic system bis(2-diphenylphosphinophenyl)methylamine, together with related systems centred around 5-, 6- and 7-membered heterocycles. Structurally, contraction of the backbone tether causes the phosphine centres to be projected further apart, and to be more constrained torsionally to the plane containing the NMe unit. In terms of potential frustrated Lewis pair (FLP) reactivity, the conformationally more flexible systems based on an acyclic linker or a cyclic core featuring a 7-membered ring, are capable of achieving P-B bond formation and generating a classical Lewis adduct with B(C6F5)(3). The less flexible carbazole-derived compound (featuring a five membered heterocyclic core) gives rise to an FLP at room temperature, and an equilibrium mixture of frustrated and classical Lewis pairs at lower temperatures. Most interestingly, the PNP system based on a central six-membered acridine ring - which gives rise to the shortest contacts in the solid state between phosphorus and the N-bound methyl group - undergoes C-H activation in the presence of B(C6F5)(3), resulting in the formation of B-H and P-C bonds.