Two isomers of a bis(diphenylphosphino)phosphinine, and the synthesis and reactivity of Ru arene/Cp* phosphinophosphinine complexes

Abstract

The reaction of 4,6-di(tert-butyl)-1,3,2-diazaphosphinine (3) with two equivalents of MeCCPPh₂ gave two isomeric products, 2,6-bis(diphenylphosphino)-3,5-dimethylphosphinine (5) and 2,5-bis(diphenylphosphino)-3,6-dimethylphosphinine (6), which were successfully separated and their molecular structures determined by X-ray crystallography. Although the 2,6-bis(iminophosphorano)phosphinine 7 was readily synthesised from 5 using mesityl azide, its coordination to late transition metals was not achieved. The reaction of 2-diphenylphosphino-3-methyl-6-trimethylsilylphosphinine (1) with [{Ru(Cl)(μ-Cl)(p-cymene)}₂] generated two products: cis-[Ru(Cl)₂(1)₂] (2) and the dinuclear species [Ru(μ-Cl)₃(p-cymene)Ru(Cl)(1)] (8), which was characterised by single crystal X-ray diffraction. The reaction of 1 with [{Ru(Cl)(μ-Cl)(C₆Me₆)}₂]/NH₄PF₆ led to cleavage of the SiMe₃ group and addition of H₂O across a PC bond to generate [Ru(C₆Me₆)(1-OH-2-PPh₂-3-MePC₅H₄)][PF₆] (9). The reaction of 1 with [{Ru(Cp*)(μ₃-Cl)}₄] yielded [Ru(Cp*)(Cl)(1)] (10) which readily reacted with H₂O across a PC bond to form [Ru(Cp*)(Cl)(1-OH-2-PPh₂-3-Me-6-SiMe₃PC₅H₃)] (11). Neither 9, 10, 11 or cis-[Ru(Cl)₂(dppm)₂] were effective precatalysts for the transfer hydrogenation (TH) of acetophenone, unlike 2 which in addition was also found to catalyse the TH of benzophenone at 82 °C (0.1 mol% 2 with 0.5 mol% KO^tBu in ⁱPrOH), with much lower activity for 2-fluorobenzaldehyde and 4-methylcyclohexanone. 11 was a competent precatalyst for the hydrogen-borrowing upgrading of EtOH/MeOH to isobutanol, albeit in lower yields compared to 2.