Trialkylphosphines Having a Bulky Phosphacyclopentane Backbone: Structural and Redox Properties Depending on the Exocyclic Alkyl Groups and EPR Observation of a Persistent Trialkylphosphine Radical Cation.

Bulky phosphines and their redox properties have received increased attention in the view of useful auxiliary ligands for transition metal catalysts and Lewis-base components of frustrated Lewis pairs for chemical transformations. Herein we report the synthesis, structure, and properties of a series of trialkylphosphines 2R (R = methyl, ethyl, isopropyl, tert-butyl, 1-adamantyl) which possess the bulky 2,2,5,5-tetrakis(trimethylsilyl)-1-phosphacyclopentane as a structural backbone. Among these phosphines, 2Ad that contains an adamantyl moiety has a very large buried volume (%Vbur) for a trialkylphosphine (62.0) and shows a quasi-reversible oxidative wave at a lower oxidation potential (-0.12 V in CH2Cl2, vs. ferrocene/ferrocenium couple) by cyclic voltammetry. The reaction of 2Ad with AgPF6 afforded a cationic silver aquo complex [Ag(2Ad)(H2O)]+[PF6]-, whereas the reaction with NOSbF6 gave a persistent phosphine radical cation [2Ad]•+. Based on the EPR spectra and DFT studies, the spin and positive charge of [2Ad]•+ are localized on the phosphorus atom.