Chemical and Electrochemical Oxidation of Tris (3,5‐di‐ tert ‐butylphenyl)phosphine – High Z′ Crystal Structures and Conformational Effects Associated with Bulky meta Substituents

Synthesis and single‐crystal X‐ray diffraction structures of (3,5‐ t Bu 2 ‐C 6 H 3 ) 3 P, (3,5‐ t Bu 2 ‐C 6 H 3 ) 3 PO · H 2 O, (3,5‐ t Bu 2 ‐C 6 H 3 ) 3 PS and (3,5‐ t Bu 2 ‐C 6 H 3 ) 3 PSe are reported. The structure of (3,5‐ t Bu 2 ‐C 6 H 3 ) 3 P has Z′ = 4, with both M and P enantiomers of a helical conformation and a conformation with one flat ring in the same lattice. The pyramidality index, Σ{∠CPC}, is smaller for the helical compared to the flat conformers. All four have structures that are more pyramidal than the corresponding Ph 3 P(E) derivatives, which is attributed to stronger intramolecular dispersion forces. The oxide crystallizes with Z′ = 2 as a water‐bridged dimer that is the most separated such dimer amongst 26 known structures, providing evidence for a distal (or perimeter) steric effect. Cyclic voltammetry in CH 3 CN/[ n Bu 4 N][ClO 4 ] indicated anodic peak potentials of +0.785 V for (3,5‐ t Bu 2 ‐C 6 H 3 ) 3 P, +0.745 for (3,5‐Me 2 ‐C 6 H 3 ) 3 P, +0.735 V for (4‐MeO‐3,5‐Me 2 ‐C 6 H 2 ) 3 P and +0.535 V for (4‐MeO‐C 6 H 4 ) 3 P, all relative to Fc +/0 . On this scale, Ph 3 P oxidizes at +1.04 V. The unexpectedly high oxidation potentials for the first three phosphines is attributed to a more pyramidal structure resulting in lowering of the HOMO energy compared to expectations from Hammett constants and 1 J (P,Se) NMR coupling constants.