On the Reactivity of Mes*P(PMe3) towards Aluminum(I) Compounds – Evidence for the Intermediate Formation of Phosphaalumenes

Abstract

Phosphaalumenes are the heavier isoelectronic analogs of alkynes and have eluded facile synthesis until recently. We have reported that the combination of a phosphinidene transfer agent, ArTerP(PMe3) (ArTer=2,6-Ar2-C6H3), with (Cp*Al)4 (Cp*=C5(CH3)5) afforded the phosphaalumenes ArTerPAlCp* as isolable, violet, thermally stable compounds. In here we describe attempts to utilize Mes*P(PMe3) (Mes*=2,4,6-tBu3-C6H2) as a phosphinidene source in combination with different Al(I) precursors, namely DipNacnacAl (DipNacnac=HC[C(Me)NDip]2, Dip=2,6-iPr2-C6H3), (Cp*Al)4 and Cp3tAl (Cp3t=1,2,4-tBu3-C5H2). In all cases the formation of phosphaalumenes was not observed, however, their intermediate formation is indicated by formation of the dimer [Cp*Al(μ-PMes*)]2 (2) and C−H-bond activation products along the putative P=Al bond, giving unusual 1,2-P,Al-tetrahydronaphtalene derivatives 1 and 4, clearly underlining the role the sterically demanding group on phosphorus plays in these transformations. The reactivity studies are supported by theoretical studies, demonstrating a thermodynamic preference for the C−H activation products. Additionally, we show that there are potential pitfalls in the synthesis of Cp*2AlH, the precursor to make (Cp*Al)4 and give recommendations how to circumvent these.