Redox chemistry of the selenium coronand, 1,5,9,13-tetraselenacyclohexadecane, and a mechanistic study of the electron transfer reaction of its Cu(II) complex

Abstract

The crystal structure of Cu(16Se4)(SO3CF3)2 (1) shows a centrosymmetric complex having tetragonally distorted octahedral coordination about Cu with trans-axial triflate ligands; Cu–O 2.464(5) Å. The stereochemistry of the coronand is c,t,c; Cu–Se 2.4592(9), 2.4553(9) Å. 1: T = 190 K; fw = 845.83; space group P21/n; Z = 2; a = 8.220(2), b = 10.965(4), c = 14.657(5) Å; V = 1273.4 Å3; Rf = 0.037 for 1708 data (Io [Formula: see text] 2.5sigma(Io)) and 152 variables. When recrystallized from MeNO2–Et2O 1 undergoes an electron-transfer reaction to give Cu(I) as well as the intermediate radical cation [16Se4]·+ and the stable dication [16Se4]2+. The crystal structure of a mixed MeCN–CH2Cl2 solvate of [(16Se4)][SO3CF3]2 (2) revealed the [16Se4]2+ cation which displays two transannular Se–Se bonds of 2.5916(15) and 2.6689(15) Å, linking three of the Se atoms in an approximately linear relationship. The central Se atom of this grouping also has a close contact to the fourth Se atom of the molecule of 3.3941(20) Å. 2·solv: T = 195 K; fw = 828; space group P1-; Z = 2; a = 9.015(2), b = 12.850(3), c = 13.835(3) Å; α = 63.98(2), β = 74.71(2), γ = 73.59(2)°; V = 1363.3 Å3; Rf = 0.042 for 2098 data (I0 [Formula: see text] 2.5σ(I0)) and 254 variables. A solid-state 77Se NMR spectrum of 2 shows 4 lines, with isotropic shifts ranging from 173 to 737 ppm. The line widths are all different, and we obtain a tentative assignment by attributing this to differences in dipolar coupling to 19F. Significant differences in chemical shift anisotropy are observed for the various selenium atoms. UV-visible absorption spectroscopy has been used to characterize 1 and its reduction products. 1 absorbs at 560, 464, and 310 nm. Reaction of 1 with the free ligand 16Se4 leads to the disappearance of these peaks, and the growth of a new peak at 320 nm. Oxidation of 16Se4 by NOBF4 produced a transient peak at 320 nm, and subsequently a peak at 256 nm. From the dependence of intensity on 16Se4 concentration, we infer that the former arises from a dimeric species; we assign the lines to the radical cations (16Se4)2+· and 16Se4+·, respectively. Electrochemical studies have been carried out on 1 and on 16Se4. Cyclic voltammetry of 1 shows a two-step reduction to Cu(II)L+· (L = 16Se4) and subsequently to Cu(I)L+. Electrochemical oxidation of 16Se4 leads to 16Se4+· and 16Se42+. Spectroelectrochemical studies showed that oxidation to 16Se4+· gives rise to a band at 256 nm, as seen in chemical oxidation, and at high concentrations a band at 322 nm is also seen, supporting the assignment of this species to the dimeric radical cation. The EPR spectrum of 1 in CH3NO2 solution gave an isotropic g value of 2.053 with hyperfine constants ACuiso = 75 G and ASeiso = 65 G. The low temperature EPR spectrum of 1, measured at -148°C in CH3NO2:toluene (1:1 v/v), gave values of g// = 2.085, ACu// = 160 G; g[Formula: see text] = 2.049, ACu[Formula: see text] = 46 G. An EPR spectrum of 1 in CH3NO2 in the presence of added 16Se4 showed a decrease in intensity of the signals attributable to 1 and the emergence of new signals that are presumed to arise from a species with radical cation character. We have carried out kinetic studies on the reaction between 1 and 16Se4. It is found that the reaction is first order in each of these species, second order overall. The reaction stoichiometry is 2 Cu(16Se4)2+ + 16Se4 –> 2 Cu(16Se4)+ + 16Se42+. These results can be explained by the simple mechanism Cu(II)L2+ + L = L+· + Cu(I)L+, followed by Cu(II)L2+ + L+· –> L2+ + Cu(I)L+. The activation energy is found to be 35 kJ mol-1.Key words: selenium coronands, Cu(II) complex, redox chemistry, mechanism, electron transfer.