Synthesis, Structure, Electrochemistry, and Spectroelectrochemistry of Hypervalent Phosphorus(V) Octaethylporphyrins and Theoretical Analysis of the Nature of the PO Bond in P(OEP)(CH₂CH₃)(O)

A variety of phosphorus(V) octaethylporphyrin derivatives of the type [P(OEP)(X)(Y)]⁺Z^- (OEP:  octaethylporphyrin) (X = CH₃, CH₂CH₃, C₆H₅, F; Y = CH₃, CH₂CH₃, OH, OCH₃, OCH₂CH₃, On-Pr, Oi-Pr, Osec-Bu, NHBu, NEt₂, Cl, F, O^-; Z = ClO₄, PF₆) were prepared. X-ray crystallographic analysis of eleven compounds reveals that the degree of ruffling of the porphyrin core becomes greater and the average P−N bond distance becomes shorter as the axial ligands become more electronegative. Therefore, the electronic effect of the axial substituents plays a major role in determining the degree of ruffling although the steric effect of the substituents plays some role. A comparison of the ¹H NMR chemical shifts for the series of [P(OEP)(CH₂CH₃)(Y)]⁺Z^- complexes with those of the corresponding arsenic porphyrins, which possess a planar core, indicates a much smaller ring current effect of the porphyrin core in the severely ruffled phosphorus porphyrins. The electrochemistry, spectroelectrochemistry and ESR spectroscopy of the singly reduced compounds are also discussed. The OH protons of [P(OEP)(X)(OH)]⁺ are acidic enough to generate P(OEP)(X)(O) by treatment with aq dilute NaOH. X-ray analysis of P(OEP)(CH₂CH₃)(O) reveals that the PO bond length is very short (1.475(7) Å) and is comparable to that in triphenylphosphine oxide (1.483 Å). The features of the quite unique hexacoordinate hypervalent compounds are investigated by density functional calculation of a model (Por)P(CH₂CH₃)(O) and (Por)P(F)(O) (Por:  unsubstituted porphyrin).