Imidazoliumyl-substituted di- and iso-tetraphosphanes and their metal-mediated fragmentation reactions

Abstract

Monocationic imidazoliumyl-substituted phosphanes, L_CP(R)Cl⁺ (L_C = 1,3-diisopropyl-4,5-dimethylimidazol-2-yl), were synthesized as their respective triflate salts 2a–c[OTf] (a: R = Cy; b: R = Ph; c: R = Py). These serve as versatile building blocks for the formation of the corresponding imidazoliumyl-substituted diphosphanes ((L_CPR)₂ 5a–c[OTf]₂) and iso-tetraphosphanes ((L_CPR)₃P[OTf]₃ 6a,b[OTf]₃). The introduction of a cationic (onio) substituent substantially alters the preferred reaction pathways of the P-scaffolds compared to their neutral congeners and electronically predisposes selected P–P bonds toward fragmentation. DFT calculations reveal a strong localization of the LUMO orbitals on the P–P bond, indicating susceptibility to nucleophilic attack. Both 5a,b[OTf]₂ and 6a[OTf]₃ undergo metal-mediated fragmentation reactions. Reactions of 5a,b[OTf]₂ with Pd(PPh₃)₄ afford two distinct dinuclear complexes, while reactions of 6a[OTf]₃ with metal chlorides yield triphosphanide complexes (L_CPCy)₂PM[OTf]₂ 9[OTf]₂: M = Pd(allyl), 10[OTf]₂: M = Au(PPh₃), accompanied by the formation of 2a[OTf]. Treatment of 6a[OTf]₃ with the NHC IPr (1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) provides the free triphosphanide ligand (L_CPCy)₂P[OTf] (11[OTf]).