Reactivity of a phosphaalumene towards heteroallenes

Exposure of ^DipTerPAlCp* (1) to CO₂ at ambient temperature results in the formation of ^DipTerPCO (2) along with the formation of (Cp*AlO) _x . Isolating 2 in crystalline form is challenging, often resulting in mixtures with its dimer (^DipTerPCO)₂ (2dim), which was crystallographically characterized. A [2 + 2] cyclization product of 1 and 2 was crystallographically identified as the cyclic diphosphaurea derivative [(^DipTerP)₂(µ-AlCp*)(µ-CO)] (3), which shows a planar P₂AlC ring. 2 also reacts with an NHC-stabilized boraketene via CO-elimination, giving the oxaborirane 4, featuring a planar BOCP unit with a localized P[double bond, length as m-dash]C double bond. Additionally, the reactivity of 1 towards diisopropylcarbodiimide (DIC) was explored. Treatment of 1 with DIC resulted in the formation of ^DipTerP[C(NiPr)₂]Al[C(NiPr)₂]Cp* (5), featuring a PCNAl four-membered ring with an additional Cp*-substituted amidinate ligand on the Al atom. This amidinate substituent results from insertion of a second DIC molecule into the Al-Cp* bond. Heating a solution of 5 results in a thermodynamically driven rearrangement to aluminium phosphaguanidinate complex 6 with a P[double bond, length as m-dash]C double bond. Compound 6 is best classified as an phosphaguanidinate or as an inversely polarized phosphaalkene, which is supported by NMR data and DFT calculations. Upon hydrolysis 5 and 6 decompose cleanly to give phosphaguanidine ^DipTerP(H)C(NHiPr)NiPr (7) with the concomitant formation of a trimeric aluminoxan. This study demonstrates the potential of phosphaalumene 1 as a synthon for aryl phosphaketenes, which themselves enable the synthesis of novel heterocyclic structures. Moreover, the reactivity towards DIC differs considerably from that of related phosphagallenes, enabled by the lability of the Al-Cp* bond.