1979 gen atoms of the amide group are involved: one in a Using the at?mic radii listed by Pauling4l (1.40 A for bond to the amide oxygen atom of the anion IFcated at 0 and 1.33 A for K+)o, the van der Waals contact dis- (-x, -y, -2) with an N-0 distance of 2.93 A and the tance would be 2.73 A. The only potassium distance other in a bond to Osof the anion locate; at (- 1 - x, which was found to be shorter than this involves 01,the 1 - y, -2) with an N-0 distance of 2.97 A. amide group oxygen. If this is a charge interaction, (Note: If bond angle C1-C2-C3had a more usual value, it is further evidence of delocalization of the charge on i.e., 122”, while C3, C4, and the nitro groups main- the carbanion from its formal location on C4. In any tained their positions and the bond distances and angles event, the atomic distribution shows that potassium of the amide group remaiped normal, this N-0 separa- 4,4-dinitro-2-butenamide forms an ionic solid. tion would be only 2.81 A. Therefore, intermolecular crowding probably causes angle C1-C2-C3 to assume the Table VI. Shortest Distances to the K+ Ion unusually small value of 118.1 ”.) Anion atom Anion position Distance, A Double hydrogen bonding between amide groups has been found in the following compounds: formamide, l6 (x, Y, 2) 2.633 (-x, 1 - y, 1 - z) 2.726 decanamide, l* tetradecanamide, l9 oxamide, 2o suc- (1 - x, 1 - y, 1 - z) 2.796 cinamide,?l glutaramide, suberamide, 24 benzamide, 25 (x, -1 + Y, z) 2.852 picolinamide,?’ and a-pyrazinamide. 28 Th? N-0 dis- (1 + x, -1 + y, 2) 2.899 tance found for dinitrobutenamide, 2.93 A, is about (1 - x, 1 - y, 1 - 2) 3.024 equal to the aLerage value from these other amide (x, -1 + Y, 2) 3.167 (-x, 1 - y, 1 - z) 3.214 structures, 2.94 A. The nitro group hydrogen bond in potassium dinitrobutenamide has the same N-0 separation as the average value found for the inter- Acknowledgment. The authors are deeply indebted molecular amine to nitro group bonds in the strongly to Dr. Lloyd Kaplan of this laboratory for permission bonded compounds, 1,3-diamino-2,4,6-trinitroben- to use his unpublished results and for his detailed ex- zene3’ and 1,3,5-triamin0-2,4,6-t;initrobenzene.38 planation of their significance. Also the authors wish The potassium cations lie 0.7 A above (+) and below to acknowledge the Foundational Research Fund of the (-) the anion planes (Figure 3). The eight shortest U. S. Naval Ordnance Laboratory, Task FR-44, for distances between anion atoms and the potassium posi- financial support of this work. tion listed in Table IV are listed in Table VI. These (41) L. Pauling, “Nature of the Chemical Bond,” 3rd ed, Cornell atoms do not form an obvious coordination pattern. University Press, Ithaca, N. Y., 1960. The Electrochemical Reduction of Aromatic Azo Compounds Joe L. Sadler and Allen J. Bard’ Contribution from the Department of Chemistry, The University of Texas, Austin, Texas 78712. Received September 11, 1967 Abstract: The electrochemical behavior of a series of aromatic azo compounds in dimethylformamide solutions was studied by polarography, cyclic voltammetry, controlled-potential coulometry, electron spin resonance spec- troscopy, and ultraviolet spectroscopy. The electrochemical reduction of these compounds is generally similar to that of the aromatic hydrocarbons in aprotic media. The reduction occurs in two one-electron steps. The product of the first electron transfer is a stable anion radical. The second electron transfer to the dianion is fol- lowed by a chemical reaction producing a protonated species which is oxidizable to the parent azo compound. The product of the second electron transfer also produces the corresponding arylhydrazine in a following reaction. lthough the electrochemical reduction of azobenzene The aim of this investigation was to study the elec- A and relatedcompounds has been studied extensively, trochemical reduction of a series of aromatic azo com- relatively few studies of the reaction in aprotic media pounds in DMF using a variety of electrochemical have been reported. Aylward, Garnett, and Sharp? re- techniques, so that a general mechanism for the reduc- cently reported the ac and dc polarographic reduction of tion of these compounds could be formulated. A fur- azobenzene in dimethylformamide (DMF) solutions. ther aim is to obtain electrochemical data, e.g., reduc- They proposed that azobenzene is reduced in a rapid tion potentials, which can be correlated with electron one-electron transfer reaction to the anion radical and spin resonance (esr) spectroscopic results and molecular in a second slow electron transfer to produce a stable orbital calculations, as has been accomplished with dianion. aromatic hydrocarbons. (1) To whom correspondence and requests for reprints should be The compounds studied in this investigation are directed. (2) G. G. Aylward, J. L. Garnett, and J. H. Sharp, Anal. Chem., 39, given below. 457 (1967). The voltammetric behavior of these compounds in Sadler, Bard / Electrochemical Reduction of Aromatic Azo Compounds 1980 R-N=N-R a azobenzene R = C6Hs 1,l’-azonaphthalene R = 05 2,2’-azonaphthalene R = 4,4’-azobiphenyl 3,3 ’-azotoluene R= 6- 4,4’-azopyridine R = N3- DMF solutions at mercury and platinum electrodes, in the absence and presence of proton donors, will be de- scribed first. The coulometric reduction of these sub- stances, the analysis of products of the reduction, and the electrochemical oxidation of these products will I then be given, A mechanism consistent with the experi- \I mental results will be proposed. Results L -0 8 -I o -I z -I 4 -I 6 -I e -2 o -2 z Voltammetric Methods. The polarographic reduc- E(V VI SCE) tion of each of the members of this series in a DMF Figure 2. Cyclic voltammograms of azobenzene in DMF con- solution containing 0.10 M tetra-n-butylammonium taining 0.1 M TBAP at (a) hmde and (b) ppde. perchlorate (TBAP) as indifferent electrolyte shows two well-defined waves. The polarogram given in Figure 1, that of 4,4’-azopyridine, is typical of the polarographic lecular size of the compound increases. The Id values for azobenzene are very close to those reported by Ayl- ward, et aln2 The cyclic voltammetric behavior of this series of compounds in DMF containing 0.10 M TBAP at both a hanging mercury drop electrode (hmde) and a planar platinum disk electrode (ppde) is, in general, very similar to that observed for aromatic hydrocarbons. Typical cyclic voltammetric behavior, that of azobenzene at a hmde and ppde, is shown in Figure 2. The first wave exhibits a corresponding anodic peak upon scan reversal about 100 mV past the first cathodic peak, while the second electron-transfer step gives no reversal peak. I 0‘ +. - Typical cyclic voltammetric data at different scan rates, O’Td Y, and concentrations, at both a hmde and ppde, are 02 PO -02 -04 -06 -08 -IO -12 -14 -16 -le -20 -22 -24 given in Table 11. E(Vvs SCEl The cyclic voltammetric behavior of 4,4’-azopyridine Figure 1. Polarogram for the reduction of azopyridine. The solu- in a DMF-TBAP medium at a hmde is distinctly dif- tion was 0.10 M TBAP in DMF containing 0.74 mM azopyridine. ferent from that of other azo compounds examined (Fig- ure 3, curve a). The second reduction wave is clearly behavior exhibited by every member of the series. The followed, upon scan reversal, by an anodic wave corre- half-wave potentials (Ellp)and other polarographic data sponding to oxidation of the reduction product of the are given in Table I. The slope of a plot of E tis. log second step. This anodic wave is absent when a ppde i/(id - i) for the first wave exhibited by these com- is used as the working electrode material (Figure 2, pounds was in every case close to 59 mV, suggesting a curve b). Data which illustrate this contrasting be- reversible, one-electron reaction. A similar plot for havior are given in Table 111. the second reduction wave of each of these compounds The first reduction wave of every compound examined shows slopes ranging from 67 mV for 4,4’-azopyridine exhibits similar cyclic voltammetric behavior. The to 110 mV for 3,3’-azotoluene. The diffusion currents separation of anodic (Ep,) and cathodic (Epc) peak of both waves for every compound varied as the square (3) M. E. Peover in “Electroanalytical Chemistry. A Series of root of the head of the dropping mercury electrode. Monographs on Recent Advances,” Vol. 11, A. J. Bard, Ed., Marcel The diffusion current constants, Id,decrease as the mo- Dekker, Inc., New York, N. Y.,1967, Chapter 1, and references therein Journal of the American Chemical Society / 90:8 / April IO, 1968 1981 Table I. Polarographic Data for the Reduction of Aromatic Azo Compounds” 7 First wave---- Second wave-------- Log plotd Log plotd Compound -El/: Id slope - B/,b Id slope Azobenzene 1.36 2.76 61 2.03 4.78 82 3,3’-Azotoluene 1.38 2.44 63 2.01 4.46 110 1,l ’-Azonaphthalene 1.13 2. IO 58 1.69 3.86 103 2,2’-Azonaphthalene 1.22 2.03 60 1.75 3.47 98 4,4’-Azobiphenyl 1.22 2.06 63 1.77 3.88 109 4,4’-Azopyridine 0.80 2.49 60 1.53 3.95 67 Q The solution was 0.10 M TBAP in DMF.