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

Glasovac, Zoran; Barić, Danijel; Despotović, Ines; Kovačević, Borislav (2025) Phosphazenyl Phosphine Proton Sponges: Does the Proton-Chelating Effect Enhance Their Basicity?. International Journal of Molecular Sciences, 26 (11). ISSN 1422-0067

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Official URL: http://doi.org/10.3390/ijms26115058

Abstract

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 pKa 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 pKa 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.

Item Type:

Article

Uncontrolled Keywords:

proton sponges; phosphazenyl phosphines; basicity; pKa; quantum chemical calculations

Subjects:

NATURAL SCIENCES

Divisions:

Division of Organic Chemistry and Biochemistry
Division of Physical Chemistry

Projects: