Imidazoliumyl-Substituted Di-and Iso-Tetraphosphanes and Their Metal-Mediated Fragmentation Reactions

Abstract

Monocationic imidazoliumyl-substituted phosphanes, LCP(R)Cl+ (LC=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 ((LCPR)2 5a-c[OTf]2) and iso-tetraphosphanes ((LCPR)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 (LCPCy)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 (LCPCy)2P[OTf] (11[OTf]).