On the Reactivity of MesP(PMe3) towards Aluminum(I) Compounds - Evidence for the Intermediate Formation of Phosphaalumenes

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, (Ar)TerP(PMe3) ((Ar)Ter=2,6-Ar-2-C6H3), with (Cp*Al)(4) (Cp*=C-5(CH3)(5)) afforded the phosphaalumenes (Ar)TerPAlCp* as isolable, violet, thermally stable compounds. In here we describe attempts to utilize Mes*P(PMe3) (Mes*=2,4,6-tBu(3)-C6H2) as a phosphinidene source in combination with different Al(I) precursors, namely (Dip)NacnacAl ((Dip)Nacnac=HC[C(Me)NDip](2), Dip=2,6-iPr(2)-C6H3), (Cp*Al)(4) and (CpAl)-Al-3t (Cp-3t=1,2,4-tBu(3)-C5H2). In all cases the formation of phosphaalumenes was not observed, however, their intermediate formation is indicated by formation of the dimer [Cp*Al(mu-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.

Automated summary

We reported the synthesis of isolable and thermally stable phosphaalumenes (Ar)TerPAlCp* by combining a phosphinidene transfer agent (Ar)TerP(PMe3) with (Cp*Al)(4). Attempts to utilize Mes*P(PMe3) as a phosphinidene source with different Al(I) precursors did not yield phosphaalumenes, but the formation of the dimer [Cp*Al(mu-PMes*)](2) and C-H bond activation products indicated their intermediate formation.