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) Å, ∠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|ε|)] 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 [k_s ≈ 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|ε|)]. 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).