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) that possess the bulky 2,2,5,5-tetrakis­(trimethylsilyl)-1-phosphacyclopentane
as a structural backbone. Among these phosphines, 2Ad, which 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.