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

The crystal structure of Cu(16Se4)(SO3CF3)(2) (1) shows a centrosymmetric c omplex having tetragonally distorted octahedral coordination about Cu with trans-axial triflate ligands; Cu-O 2.464(5) A. The stereochemistry of the c oronand is c,t,c; Cu-Se 2.4592(9), 2.4553(9) A. 1: T = 190 K; fw = 845.83; space group P2(1)/n; Z = 2; a = 8.220(2), b = 10.965(4), c = 14.657(5) A; V = 1273.4 A(3); R-f = 0.037 for 1708 data (I => 2.5(I)) and 152 variables. When recrystallized from MeNO2-Et2O 1 undergoes an electron-transfer reacti on 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-CH2C l2 solvate of [(16Se4)][SO3CF3](2) (2) revealed the [16Se4](2+) cation whic h displays two transannular Se-Se bonds of 2.5916(15) and 2.6689(15) A, lin king three of the Se atoms in an approximately linear relationship. The cen tral Se atom of this grouping also has a close contact to the fourth Se ato m of the molecule of 3.3941(20) A. 2,solv: T = 195 K; fw = 828; space group P (1) over bar; Z = 2; a = 9.015(2), b = 12.850(3), c = 13.835(3) A; alpha = 63.98(2), beta = 74.71(2), gamma = 73.59(2)%; V = 1363.3 A(3); R-f = 0.0 42 for 2098 data (I => 2.5 sigma(I)) and 254 variables. A solid-state Se-77 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 assi gnment by attributing this to differences in dipolar coupling to F-19. Sign ificant differences in chemical shift anisotropy are observed for the vario us selenium atoms. UV-visible absorption spectroscopy has been used to char acterize 1 and its reduction products. 1 absorbs at 560, 464, and 310 nm. R eaction 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. Fr om the dependence of intensity on 16Se4 concentration, we infer that the fo rmer arises from a dimeric species; we assign the lines to the radical cati ons (16Se4)(2)(+) and 16Se4(+), respectively. Electrochemical studies have been carried out on 1 and on 16Se4. Cyclic voltammetry of 1 shows a two-ste p reduction to Cu(II)L+ (L = 16Se4) and subsequently to Cu(I)L+. Electroche mical oxidation of 16Se4 leads to 16Se4(+) and 16Se4(2+). Spectroelectroche mical 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 32 2 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 A(iso)(Cu) = 75 G and A(iso)(Se) = 65 G. The low temperature EPR spectrum of 1, measured at -148%C in CH3NO 2:toluene (1:1 v/v), gave values of g = 2.085, A(Cu) = 160 G; g = 2.049, A( Cu) = 46 G. An EPR spectrum of 1 in CH3NO2 in the presence of added 16Se4 s howed a decrease in intensity of the signals attributable to 1 and the emer gence of new signals that are presumed to arise from a species with radical cation character. We have carried out kinetic studies on the reaction betw een 1 and 16Se4. It is found that the reaction is first order in each of th ese species, second order overall. The reaction stoichiometry is 2 Cu(16Se4)(2+) + 16Se4 --> 2 Cu(16Se4)(+) 16Se4(2+). These results can be explained by the simple mechanism Cu(II)L2 + L reversible arrow 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).