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

다국어 초록 (Multilingual Abstract)

Synthesis and single‐crystal X‐ray diffraction structures of (3,5‐tBu2‐C6H3)3P, (3,5‐tBu2‐C6H3)3PO·H2O, (3,5‐tBu2‐C6H3)3PS and (3,5‐tBu2‐C6H3)3PSe are reported. The structure of (3,5‐tBu2‐C6H3)3P 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 Ph3P(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 CH3CN/[nBu4N][ClO4] indicated anodic peak potentials of +0.785 V for (3,5‐tBu2‐C6H3)3P, +0.745 for (3,5‐Me2‐C6H3)3P, +0.735 V for (4‐MeO‐3,5‐Me2‐C6H2)3P and +0.535 V for (4‐MeO‐C6H4)3P, all relative to Fc+/0. On this scale, Ph3P 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 1J(P,Se) NMR coupling constants.
MANABEphos, valued as a supporting ligand because of the distal steric bulk from the exo‐oriented tert‐butyl groups, is also rendered more pyramidal than PPh3 through London dispersion forces on and between the endo‐substituents, which results in a raised HOMO energy and reduced basicity.