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
Authors: Batchelor Raymond J , Einstein Frederick WB , Gay Ian D , Gu Jian-Hua , Pinto B Mario , Zhou Xue-Min 
Published: April 2011

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.