Photophysical, dynamic and redox behavior of tris(2,6-diisopropylphenyl)phosphine

Abstract

The title phosphine, Dipp3P, was synthesized using an aryl copper reagent and the structure determined by X-ray crystallography (R = 2.94%): d(P–C) = 1.852(1) A, ∠C–P–C = 111.88(5)°. In hexane solution, the electronic spectrum displays 3 bands [326 (9.3), 254 (8.7), 205 (11.4) nm (log|e|)] and the fluorescence spectrum has a Stokes shift of 129 kJ mol−1. NMR: (δ) 31P = −49.7 ppm in solution and −49.5 in the solid (CP-MAS). Room temperature 1H and 13C spectra reflect D3 symmetry, changing below −30 °C to C3. A variable temperature NMR study provided an activation enthalpy of 49(±1) kJ mol−1 and entropy of 24–27(±5) J mol−1K−1. An energy surface calculation using HF/3-21G theory discovered a single low-energy path describing pyramidal inversion through a transition state that is close to D3 geometry. The B3LYP/6-31G(d) calculated barrier to planarization is 37.5 kJ mol−1. Voltammetric studies employing cyclic, rotating disk, steady state and Fourier Transform ac methods confirm a fully chemically reversible one-electron oxidation of Dipp3P to Dipp3P+˙ at +0.18 (CH3CN–nBu4NPF6) and +0.09 (CH2Cl2–nBu4NPF6) V vs. Fc+/0 (Fc = ferrocene). The diffusion coefficient for Dipp3P is 1.0–1.2 × 105 cm2 s−1. The electrode process displays quasi-reversible electron transfer kinetics [ks ≈ 0.01 (CH2Cl2) to 0.08 (CH3CN) cm s−1]. Optically transparent thin layer electrolysis reversibly generates Dipp3P+˙ in CH2Cl2–nBu4NPF6 [UV-Vis: 498 (3.31), 456 (3.29), 373 (4.04), 357 (3.84), 341 (3.49), 296 (3.78), 385 (3.91), 251 (3.99) nm (log|e|)]. The EPR spectrum of Dipp3P+˙ in solution is a doublet (a(P) = 23.9 mT, g = 2.008), and in frozen solution is axial (a∥ = 42.6 mT, g∥ = 2.0045; a⊥ = 12.7 mT, g⊥ = 2.0085 mT).