Di(hydroperoxy)cycloalkane Adducts of Triarylphosphine Oxides: A Comprehensive Study Including Solid-State Structures and Association in Solution.
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Four new di(hydroperoxy)cycloalkane adducts (Ahn adducts) of p-Tol3PO (1) and o-Tol3PO (2), namely, p-Tol3PO(HOO)2C(CH2)5 (3), o-Tol3PO(HOO)2C(CH2)5 (4), p-Tol3PO(HOO)2C(CH2)6 (5), and o-Tol3PO(HOO)2C(CH2)6 (6), have been synthesized and fully characterized. Their single crystal X-ray structures have been determined and analyzed. The 31P NMR data are in accordance with hydrogen bonding of the di(hydroperoxy)alkanes to the PO groups of the phosphine oxides. Due to their high solubility in organic solvents, natural abundance 17O NMR spectra of 1-6 could be recorded, providing the signals for the PO groups and additionally the two different oxygen nuclei in the O-OH groups in the adducts 3-6. The association and mobility of 3-6 were explored by 1H DOSY (diffusion ordered spectroscopy) NMR, which indicated persistent hydrogen bonding of the adducts in solution. Competition experiments with phosphine oxides allowed ranking of the affinities of the di(hydroperoxy)cycloalkanes for the different phosphine oxide carriers. On the basis of variable temperature 31P NMR investigations, the Gibbs energies of activation G for the adduct dissociation processes of 3-6 at different temperatures, as well as the enthalpy H and entropy S of activation, have been determined. IR spectroscopy of 3-6 corroborated the hydrogen bonding, and in the Raman spectra, the (O-O) stretching bands have been identified, confirming the presence of peroxy groups in the solid materials. The high solubilities in selected organic solvents have been quantified.
