Phosphazenyl Phosphine Proton Sponges: Does the Proton-Chelating Effect Enhance Their Basicity?

Gas-phase basicity and basicity in acetonitrile solvent were investigated for a series of proton sponges derived from phosphazenyl phosphines. A range of aromatic and aliphatic scaffolds bearing phosphazenyl phosphine substituents were employed to modulate the basicity of these compounds, primarily by varying the distance between the phosphazenyl phosphine units. These proton sponges were shown to be exceptionally strong organic bases, with pK_a values in acetonitrile reaching up to 42.0 units and gas-phase proton affinities (PA) up to 307.0 kcal mol^-1. However, none exhibited higher basicity than the closely related phosphazenylphosphine systems, for which a pK_a value of 43.8 and PA value of 307.5 kcal mol^-1 was previously reportedIt was found that the proton-chelating effect, typically defined as the difference in proton affinity between bis- and mono-substituted systems (ΔPA), moderately influences basicity. However, it was also established that ΔPA should not be attributed exclusively to the elimination of electron-pair repulsion and the formation of hydrogen bond upon protonation, as has been commonly assumed in most previous studies of proton sponges, but must also account for mesomeric and inductive effects, as well as dispersion interactions.