Synthesis, Structure, NMR Spectroscopy, and Electrochemistry of the Sterically Congested Triarylarsine Dipp3As: EPR Characterization of its Radical Cation

The synthesis, NMR spectroscopy, single-crystal X-ray structure, and solution electrochemistry of the new compound [2,6-{CH(CH3)}2C6H3]3 As, abbreviated as Dipp3As, is reported. The molecule, prepared by reaction of AsCl3 with a pre-formed aryl copper reagent, Dipp4Cu4, crystallizes in the hexagonal space group R3 as a racemic twin. The sum of angles around As, ∑∠{CAsC}, is 329.13(3)° in the X-ray structure and 329.17° from an R-B3LYP/6-31G(d,p) hybrid density functional theory calculation. The aromatic rings are quite distorted with both the ipso carbon and especially the As atom significantly out of plane by 0.503(3) Å. The ambient temperature NMR spectrum fits for C3 symmetry implying that inversion is slow on the NMR timescale. Cyclic voltammetry on a glassy carbon electrode in CH2Cl2 with 0.4 M [nBu4N][PF6] over scan rates of 0.05–0.8 V s–1 and temperatures of 22 ± 2°C produced one quasi-reversible process with Em1 = +0.43 V at a scan rate of 0.20 V s–1 and a second irreversible process with a peak potential of +1.45 V (v. Fc+/0). The diffusion coefficient has been measured as 3.3 ± 0.1 × 10–6 cm2 s–1 in CH2Cl2 solution containing 0.4 M [nBu4N][PF6]. Chemical oxidation with AgPF6 in CH2Cl2 in degassed solutions in sealed vessels allowed for recording of characteristic EPR spectra; at 293 K, a (75As) = 26.1 mT and g = 2.021. In frozen solution, an almost isotropic spectrum is obtained (g║ = 2.000 mT and g⊥ = 2.003 mT) and the hyperfine splitting constants are a║ = 47.9 and a⊥ = 19.0 mT, leading to an estimate for the structure being slightly pyramidal with ∑∠{CAsC} ≈ 351°.