Coordination isomerism in dioxophosphorane cyanides

Abstract

The 1,3-phosphaazaallene ^DippTerP = C=NtBu (^DippTer = 2,6-(2,6-iPr₂C₆H₃)₂–C₆H₃) is thermally labile towards iso-butene elimination and formation of the corresponding cyanophosphine ^DippTerP(H)CN (1). In previous work we have shown facile deprotonation of 1 with K[N(SiMe₃)₂ and formation of cyanophosphide [(^DippTerPCN)K]. We now present the alkali metal tethered cyanophosphides [(^DippTerPCN)M(crown)] (M = Na, K; crown = 15-c-5, 18-c-6) and their structural diversity in the solid state depending on the metal (M) and the crown ether. Facile oxidation of [^DippTerPCN][M(crown)] with O₂ yields the formal cyanide adducts of dioxophosphoranes [^DippTerPO₂(CN)]⁻. Surprisingly, [^DippTerPO₂(CN)]⁻ is obtained as a mixture of the cyanide and isocyanide isomers, indicating a coordination isomerism. This phenomenon is corroborated by experimental and theoretical studies revealing the cyanide isomer to be thermodynamically more stable. The oxidation with elemental sulphur gave the corresponding dithiophosphorane cyanide adduct [^DippTerPS₂(CN)]⁻, in which no isomerism was observed. This points to a crucial role of triplet oxygen in the isomerisation process. Monooxidation occurs when [^DippTerPO₂(CN)]⁻ salts were treated with N₂O, giving formal anionic phoshinidene monoxide adducts.