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.

Trapping an Elusive Phosphanyl‐Phosphaalumene

“…The Al1−P1 bond length (2.3350(6) Å) is consistent with a typical single bond (∑ r cov (Al−P)=2.37 Å; ∑ r cov (Al=P)=2.15 Å), whilst the P1−P2 bond length (2.0437(5) Å) is in the range expected for a double bond (∑ r cov (P−P)=2.22 Å; ∑ r cov (P=P)=2.04 Å), and consistent with negative hyperconjugation between P1 and P2 [38] . Hering‐Junghans, Braunschweig and co‐workers recently reported the formation of a similar C−H bond activation product on reaction of Al(Nacnac) and Mes*P(PMe 3 ) [39] . The reactivity observed is in accordance with the generation of the desired phosphanyl phosphaalumene, [Al]P[P] , although despite our best efforts, we were unable to isolate this compound.…”

“…[38] Hering-Junghans, Braunschweig and coworkers recently reported the formation of a similar CÀ H bond activation product on reaction of Al(Nacnac) and Mes*P-(PMe 3 ). [39] The reactivity observed is in accordance with the generation of the desired phosphanyl phosphaalumene, [Al]P-[P], although despite our best efforts, we were unable to isolate this compound. It is worth noting that the gallium-containing analogue, A, does not undergo CÀ H bond activation reactions, even upon prolonged heating.…”