Alternating and Direct Current Polarography of Azobenzene in Indifferent Electrolyte in Dimethylformamide Gordon H. Aylward,' John L. Garnett, and John H. Sharp2 School of Chemistry, University of New South Wales, Kensington, Australia
Azobenzene is reduced in two one-electron steps in dimethylformamide with tetraethylammonium perchlorate supporting electrolyte. An electron spin resonance spectroscopic study of the products of coulometric reduction shows that the product of the first electron transfeir is the monoanion of azobenzene and the product of the second electron transfer is diamagnetic. Ultraviolet spectroscopy and the ease of oxidation suggest that the product of the second electron transfer is a stable dianion of azobenzene. Alternating current polarography and faradaic impedance measurement show that the first electron transfer is rapid with an apparent rate constant of 0.5 cm sec-1 and a chargle transfer coefficient of 0.37. The second electron transfer is slow.
ALTHOUGH THE d.c. polarography of a large number of organic compounds in nonaqueous solvents has been reported in the literature, in only a few cases have charge transfer rates been measured. Aten et al. (I) have investigated the a x . polarography of a number of aromatic hydrocarbons in dimethylformamide (DIdF). Phase angles were not measured but were assumed to be 45'. The measurements were supplemented by studies using the a.c. impedance bridge and the resistive and capacitative components of the faradaic impedance were found to be linearly dependent on u - 1 ' 2 . Peover has measured the height and position of the a.c. polarographic waves of quinones (2) and of oxygen (3) in D M F at a single frequency as a test of reversibility of the electrode process. In this paper an investigation of the reduction of azobenzene in D M F is reported. Electron spin resonance (ESR) spectrometry is used to identify the products of controlled potential electrcdysis; and d.c. polarography, a.c. polarography, and faradaic impedance measurements are used to measure rates of the charge transfer processes. The a.c. polarography of azo compounds in aqueousethanol solutions has been investigated (4-6)but the systems are complicated by strong adsorption of both redox forms onto the mercury surface. In D M F the a.c. polarography of
Present address, School of Chemistry, Macquarie University, Eastwood, N.S.W., Australia. Present address, Department of Chemistry, University of Kansas, Lawrence, Kansas.
(1) A. C. Aten, C. Buthker, and G. J. Hoijtink, Trans. Faraday Soc., 55, 324 (1959). (2) M. E. Peover, J. Chem. Soc., 1962, p. 4540. (3) M. E. Peover and B. S . White, Chem. Comrnun., 10, 183 (1965). (4) T. M. Florence and G. H. Aylward, Australian J. Chem., 15, 65, 416 (1962). (5) T. M. Florence and Y. J. Farrer, Ibid., 17,1085 (1964). (6) B. Nygard, Arkiu Kemi, 20,163 (1963).
azobenzene can be investigated without the complication of adsorption of reactants. Also in DMF, electrode processes can be investigated in the absence, or in the presence, of controlled amounts of proton donors. EXPERIMENTAL
Apparatus. The conventional ax.-d.c. polarograph used was described in an earlier communication (7). The alternating voltage was kept at 5 mv rms and phase angles were measured to +2". The impedance bridge was similar in design to that of Tamamushi and Tanaka (8) but incorporated a 74-henry 366-ohm choke to block a.c. from the polarizing circuit. The bridge was balanced as described by these authors. A graphical procedure (9) was used to correct for the contribution of the series resistance and double layer capacity. The series resistance measured at 60,000 c/s was approximately 450 ohms. A water-jacketed all-glass cell fitted with high vacuum taps and ground-glass sockets was used as a combined polarographic/electrolysis cell. A two-way stopcock allows nitrogen to be passed either through or over the solution. The method of generation of radical anions and of transference to the ESR cell or ultraviolet cell has been described previously (IO). ESR spectra were recorded using a Varian V-4502 X-band spectrometer with 100-K cps field modulation and a Varian V-3400 9-inch magnet. Reference Electrode. A silver-silver nitrate electrode was used as a reference electrode in these investigations. The reference electrode consisted of a silver wire spiral (giving an electrode area of 10 cmz) immersed in a solution of silver nitrate (0.1M) and tetraethylammonium perchlorate (TEAP) (O.1M) in DMF. The reference electrode compartment was connected to the polarographic cell by an H-type bridge fitted with sintered glass disks (porosity 3) at each end, and filled with a saturated solution of TEAP in DMF. This electrode, acting as a polarographic anode, was not polarized when currents of less than 20 pa were drawn through it and was stable to + I mv for several hours. The temperature coefficient of the reference electrode was 0.4 mv/OC. Liquid junction potentials in the cell are estimated to be less than 1 mv when the TEAP supporting electrolyte concentration is 0.1M. Maximum current at the end of the drop life was measured, and all reported potentials are corrected for iR drop. The overall resistance of the solution, bridge, and reference electrode at the end of drop life was about 6000 ohms. Reagents. Laboratory reagent grade D M F was purified by the method of Thomas and Rochow (11). After drying
(7) G. H. Aylward and J. W. Hayes, ANAL.CHEM.,37, 195 (1965). (8) R. Tarnamushi and N. Tanaka, Z. Phys. Chem. N.F., 21, 89 (1959). (9) P. Delahay and T. J. Adams, J. Am. Chem. Soc., 74, 5740 ( 1952). (IO) G. H. Aylward, J. L. Garnett, and J. H. Sharp, Chem. Commun. 137 (1966). (11) A. B. Thomas and E. G. Rochow, J. Am. Chem. Soc., 79,1843 (1957). VOL. 39, NO. 4, APRIL 1967
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Figure 1. D.c. and a.c. (60cls, 5 mv rms) polarograms of 0.980 mM azobenzene in 0.1M TEAP
-----a.c.
over molecular sieves for 48 hours, the water content of DMF was shown by Karl Fischer titration to be
