1653
NITRO-p-TERPHENYLS
Table 111: A Comparison between the Observed Nitroterphenyl Absorption Band and That Calculated from Charge-Transfer Spectra" Long wavelength max, om-1 X IO-'--Caled Obsd
Compd
3.29 >3.31 3.15 3.13 3.42 2.96 >3.61
' All spectra were taken
-3.13 (sh) -2.99 (sh) 3.03 3.03 ~ 3 . 1 (sh) 3 2.90 2.99
in methylene chloride.
In one case, however, the model seems particularly apt. The spectrum of 4,4"-dinitroterphenyl contains only the band described above. The elementary theory indicates that the second absorption band should be a transition from the highest filled to the second vacant MO. In the point group V h this should be 'A, -+ 'A, which is parity forbidden, accounting nicely for the absence of the band. An interesting possibility for further study in materials with dual charge-transfer properties would be to determine if the nitroterphenyls are capable of simultaneously complexing with both TCNE and DMT. Our attempts to investigate this failed when it was found that the TCNE-DMT complex is not stable.
Nitro-p-terphenyls. 11. The Relation between Charge-Transfer Properties and Polarographic Oxidation and Reduction Potentials
by Richard L. Hansen, P. E. Toren, and R. H. Young Contribution No. $63 from the Central Research Laboratories, Minnesota Mining and Manufacturing Company, St. Paul, Minnesota 66119 (Received Nmember 12, 1966)
Polarographic data have been obtained for seven nitro-p-terphenyls and related compounds. Linear correlations of oxidation and reduction potentials with charge-transfer frequencies and parameters arising from elementary molecular orbital theory have been demonstrated. Steric strain in certain of these molecules can be relieved by preferential twisting of a nitro group out of the aromatic ring plane or by rotation about a carbon-carbon bond.
Introduction The singular ability of several nitro-p-terphenyls to perform both electron donor and acceptor roles in charge-transfer complexes was described in a previous article.' We have measured the polarographic oxidation and reduction potentials of these nitroterphenyls and have obtained similar data for several model compounds in order to relate their charge-transfer and elec-
trochemical properties. We also hoped to determine the sites of steric strain in certain of these molecules and to test the theoretical models which were employed. The energy of a charge-transfer absorption band is given approximately by eq 1, where ID is the ionihvct = (1) Part I: R.
ID
-EA
+C
(1)
L. Haneen, J . Phys. Chem., 70, 1646 (1966). Volume 70, Number 6 May 1966
R. L. HANSEN, P. E. TOREN, AND R. H. YOUNG
1654
zation potential of the donor, EA is the electron affinity of the acceptor, and C should be nearly constant in a series of related complexes with a given donor or acceptor. I11 elementary theory, ionization potentials and electron affinities should also be linearly related to polarographic oxidation and reduction potentials, respectively.2 Thus, one expects that electrochemical and charge-transfer properties will be correlated. Actually, both oxidation potentials3 and reduction potentials* have been correlated with charge-transfer frequencies, but it has not been possible to test both correlations with one series of compounds before. The relations between oxidation and reduction potentials and molecular orbital energy levels are well known in the cast: of aromatic hydrocarbon^,^^^ but few attempts have been made to test these relations for molecules containing heteroatoms. In addition, oxidation potentials of aromatic nitro compounds appear not to have been reported before.
Experimental Section Materials. The preparation and purification of the nitroterphenyls have been described as has the purification of the hydrocarbons.’ The nitrobenzene was distilled while the other nitro compounds were commercial samples used as received. Tetra-n-putylarnmonium perchlorate (TBAP) was prepared from tetrabutylammonium bromide and sodium perchlorate. The electrolytes were dried under vacuum and stored in a drybox. Acetonitrile was purified by method D of Coetzee, et a1.’ Karl Fischer titration indicated that the final product contained less than 0.002% water. The acrylonitrile and methacrylonitrile content combined was less than 5 ppm as determined by vapor phase chromatography. The dimethylformamide was distilled from anhydrous potassium carbonate.* Both solvents were stored in a drybox. Procedure. The polarographic measurements were made on ceoxygenated solutions containing 1-5 mM substrate and 0.1 M supporting electrolyte. The solutions were made up in a drybox. D N F gave somewhat better defined reduction waves than did RleCN, while I\leCS proved to be superior for obtaining the oxidation potentials. Reduction potentials were obtained by conventional three-electrode polarography at a dropping mercury cathode’ The referred to an aqueous saturated calomel electrode. The quantitative reductions were carried out on 10-ml samples using a mercury pool as the cathode. The applied potential corresponded to the plateau of the first polarographic wave and the current was integrated electronically. The Journal
of
Physical Chemistry
c
+ I!O.
I
--voltage
I
c
g 0
.-u 0
t
i Figure 1. The current-voltage curve for 4-nitro-p-terphenyl.
The polarographic oxidations were done at a platinum disk anode having an area of 0.125 cm2. An aqueous sce served as reference in the three-electrode system. A cyclic voltammetric method was used,g and current-voltage curves were obtained at a scan rate of 3 v/min.
Results The half-wave reduction potentials and the oxidation potentials of seven nitroterphenyls displaying dual charge-transfer properties are given in Table I together with supporting data for similar compounds. A typical current-voltage scan used in determining an oxidation potential is shown in Figure 1. It is clear that the oxidation of 4-nitroterphenyl is chemically irreversible since no reaction occurred on the cathodic going scans. This was the case for all of the compounds studied. Electrochemical reversibility was not investigated directly, but if the oxidations are reversible in the electrochemical sense, the E, values will be related to classical half-wave potentials by an additive constant.’O Chemical irreversibility does not necessarily exclude the possibility that the electrode process may be reversible, e.g., the reduction of 2,4,6trinitroterphenyl, vide infra. The diffusion current constants, ID,and Tomes’ criterion are consistent with electrochemically reversible one-electron reductions. Cyclic voltammetry in the case of 2,4,6-trinitroterphenyl, however, showed that (2) A. Streitwieser, Jr., “Molecular Orbital Theory for Organic Chemists,” John Wiley and Sons, Inc., New York, N. Y., 1961, p 173. (3) A. Zweig, W. G. Hodgson, and W. H. Jura, J . Am. Chem. SOC., 86, 4124 (1964).
(4) M. E. Peover, Trans. Faraday SOC.,58, 1656, 2370 (1962). (5) C. J. Hoijtink, Rec. Trav. Chim., 74, 1525 (1955). ibid., 77, 555 (1958), (6) c. J. (7) J. F. Coetsee, G. P. Cunningham, D. K. McGuire, and G . R. Padmanabhan, Anal. Chem., 34, 1139 (1962). ( 8 ) P. H.Rieger, I. Bernal, W. H. Reinmuth, and G . K. Fraenkel, J. Am. Chem. SOC.,85, 683 (1963). (9) Z. Galus, H. Y . Lee, and R. Adams, J. Electroanal. Chem., 5, 17 (1963). (10) H. Matsuda and Y . Ayabe, 2. Elelctrocha., 59, 494 (1955). ~I
NJTRO-p-TERPHENYLS
1655
Table I : Oxidation and Reduction Potentials
Compound
Nitrobenzene (1) m-Dinitrobenzene (2) s-Trinitrobenzene (3) 2-Nitrobiphenyl (4) 4-Nitrobiphenyl ( 5 ) 2-Nitroterphenyl (6) 3-Nitroterphenyl (7) 4-Nitroterphenyl(8) 2,4-Dinitroi erphenyl (9) 2,g-Dinitroi erphenyl ( 10) 4,4”-l)initroterphenyl (11) 2,4,6-Trinitroterphenyl (12) Benzene (13) Biphenyl (14) p-Terphenyl (15)
1.88 1.78 1.76 zk 0.01’ 1.86 1.76 2.06 2.00 2 , 43h 1 .91i 1.78
1.12*0.01 0. 838 0.46 1.16 1.03 1.30 0.98 1.14 0.81 1.04g 1.02 0.67
60 f 4 61 42 52 66 80 80
70 60 72 66 52
3.0 2.6 2.3 2.2 2.4 2.4 2.9 2.2 2.9 2.3 2.4 1.9
a I n MeCN-TBAP. I n DMF-TBAP. This should be 56 mv for a one-electron reversible electrochemical process: J. Tomes, where i d is the diffusion current, C is the substrate conCollection Czech. Chem. Commun., 9, 12, 81, 150 (1937). ID= id/Cm2/at1’e centration, and the quantity m*/3t1/nis characteristic of the dropping mercury electrode. ‘ Second, poorly defined wave a t - 1.27 v. Small polarographic maximum. H. Lund [Acta Chem. Scand., 11, 491, 1323 (1957)l reported a halfSecond wave at 1.95 v. wave potential of :!.OO v ; 0.43 v was added corresponding to the difference in the case of biphenyl. Lundh reported El/, = 1.48 v.
’
the reduction was chemically irreversible just as the oxidations were. Sone of the other reductions was investigated in this regard. Quantitative reductions were carried out jn the cases of nitrobenzene, 2,6-dinitro-, 4,4”-dinitro-, and 2,4,6trinitroterphenyl in DMF. The integrated current showed that several electrons per molecule were passed through these solutions with no indication of an approaching end point except in the case of 2,4,6trinitroterphenyl which gave a quantitative oneelectron reduction. The use of sodium perchlorate as the electrolyte in place of the tetrabutylammonium salt gave the sanie results. 2 ,6-D init ro- md 2,4,6-t rinitro terphenyl gave rise to deeply colored solutions upon reduction in DMF, red in the case ,if the trinitro compound and violet in the other. On the basis of other experience, we believed these colors to be due to the free radical anions, the anticipated reduction products.” We were surprised to find that no epr signal could be detected in 0.2 mil4 solutions, whereas 1 mM solutions gave intense epr spectra. Jl’e can only conclude that the DlLIF contained an inipurity in less than 1 mM concentration which reacted with the anion-radical reduction product.
Discussion The relation between the frequencies of the chargetransfer bands in nitroterphenyl-N,N-dimethyl-p-tolui-
dine complexes‘ and the reduction potentials of the nitro compounds was found to be linear with a correlation coefficient, r = 0.90. The least-squares correlation line was vct(crn-l) = (8 f 4) X lo3(-v) (1.7 f 0.2) X lo4 cm-’. The slope is unity when the charge-transfer frequencies are measured in electron volts. A similar correlation (r = -0.93) was found between the reduction potentials of the nitro compounds and the energies of their lowest vacant molecular orbitals calculated by the Huckel method.’ A value for of 7 2 ev was obtained from the slope. Oxidation potentials are expected to be related to the energies of the highest filled molecular orbitals. The oxidation potentials of six compounds listed in Table I, including the three hydrocarbons and the three nitroterphenyls not containing 2-substitution1 were found to be linearly related (T = 0.94) to the energies of their highest filled orbitals. The effective value of JpI in this correlation was 1.8 f 0.5 ev. A Iinear correIation between the oxidation potentiak of the donors and the frequencies of the long wavelength charge-transfer bands in complexes of TCNE with the nitroterphenyls and the three hydrocarbons appears in Figure 2.‘ I n this case r = 0.92 and the least-squares line is E, (v) = (8 f 2) X cm-’
+
*
+
(11) Part 111: R. L. Hansen, R. H. Young, and P. E. Toren, J. Phys. Chem., 70, 1657 (1966).
Volume 70, Number 6 May 1966
R. L. HANSEN,P. E. TOREN, AND R. H. YOUNG
1656
I
2.4
bd
110'
I2
I50
I .6
1.5
T0 6O 9
2.0 2.5 3.0 Charpr -Transfer Maximum X ld'(cI'~i' )
Figure 2. The relation between oxidation potentials and charge-transfer frequencies.
(0.15 f 0.09) v. With comparable units of energy the slope is 0.6 f 0.2. Since 4,4"-dinitroterphenyl does not display a long wavelength charge-transfer band with TCNE,' the point corresponding to this compound, although shown in Figure 2, was not included in the correlation. Pysh and iang,12 and more recently Neikam and co-workers13 have found linear relationships between half-wave oxidation potentials and photoionization potentials of aromatic hydrocarbons, but the slopes were not unity. It has been pointed out that the slope in these correlations need not be 1 if there are specific interactions a t the electrode-solution interface.13 It has now been reported that aromatic hydrocarbons form charge-transfer complexes with platinum oxide,l 4 which may account for the slopes observed in the relations between oxidation potentials and charge-transfer frequencies or ionization potentials. For aromatic nitro compounds, Geske and his coworkers have shown clearly that steric hindrance which causes decoupling of the nitro group from the aromatic ring shifts the reduction potential to a more negative voltage.16 Aside from the nitro group, the length of the conjugated system has little effect on the reduction potential in the present series of compounds. Compare nitrobenzene, 4-nitrobiphenyl, and 4-nitroterphenyl with reduction potentials of -1.12, -1.03, and - 1.14 v. The Journal of Physical Chemistry
The effects of steric hindrance on oxidation potentials have not been investigated systematically. In the methoxybenzenes, decoupling a methoxy substituent causes a decrease in the oxidation potential.3 In the nitroterphenyls the addition of a nitro group to an otherwise conjugated system seems to have little effect on the oxidation potential. Compare terphenyl, 3-nitroterphenyl, and 4-nitroterphenyl, all having oxidation potentials of 1.77 f 0.01 v. The oxidation potential does reflect the extent of the hydrocarbon portion of the molecule, however, as shown by the oxidation potentials of benzene, biphenyl, and terphenyl. These generalities are consistent with Huckel calculations which indicate that the lowest vacant A 4 0 in these nitro compounds is strongly associated with the nitro groups while the highest filled orbital is essentially a hydrocarbon orbital. The nitroterphenyls which are substituted in the %position are expected to be noncoplanar due to steric hindrance. The strain may be relieved by rotation about the C1-C1t bond, a C-N bond, or both. The first possiblility should be revealed in the oxidation potential and the second in the reduction potential. The nitroterphenyls can be divided into two broad groups on the basis of their oxidation potentials, those similar to biphenyl with oxidation potentials more anodic than about 1.85 v and the remainder similar to terphenyl. 2-Nitro-, 2,4-dinitro-, and 2,4,6-trinitroterphenyl are in the former group, suggesting that they are twisted about the Cl-Clt bond. The effect on reduction potential of rotation about a C-N bond is illustrated by the more cathodic reduction potential of 2-nitroterphenyl as compared to either the 3- or 4-nitro isomers or nitrobenzene, and by the reduction potential of Z16-dinitroterpheny1 compared to rn-dinitrobenzene. The reduction potential of 2,4-dinitroterphenyl is identical with that of m-dinitrobenzene, The Huckel treatment of this molecule indicates that the lowest vacant molecular orbital is a "pure" m-dinitrobenzene level.' The reduction potential substantiates this while suggesting, as does the oxidation potential, that steric strain in this molecule is relieved almost exclusively by rotation about the Cl-Clt bond. In contrast, steric strain in the 2,6-dinitro isomer appears
(12) E. S. Pysh and N. C. Yang, J . Am. Chem. SOC.,85, 2124 (1963). (13) W.C. Neikam, G. R. Dimeler, and M. M. Desmond, J . Etectrochem. SOC.,111, 1190 (1964). (14) I. T. Ernst, J. L. Garnett, and W. A. Sollich-Baumgartner, J . Catalysis, 3 , 568 (1964). (15) D. H.Geske, J. L. Ragle, M. A. Bambenck, and A. L. Balch, J . Am. Chem. Soc., 86,987 (1964).
NOTES
to be relieved predominantly by rotation of the nitro groups out of the ring plane. Comparison of the reduction potential of 2,4,6trinitroterphenyl with that of s-trinitrobenzene suggests that the 3- and 6-nitro groups in this molecule are also twisted to some extent. Thus, both mechanisms for the relief of steric strain are operative in 2-nitro- and 2,4,6-trinitroterphenyl.
1657
It is not possible from these rather limited data to estimate angular degrees of noncoplanarity, but the intensity of the nitroterphenyl absorption band near 330 mp seems to be decreased more dramatically by nitro group decoupling than by rotation about the C1C1!bond. This band, regarded as an intramolecular charge-transfer band, is much more intense in 2,4dinitroterphenyl than in the 2,bdinitro isomer.
NOTES
Nitro-p-terphenyls. 111. Electron Paramagnetic Resonance Spectra of the Radical Anions by Richard L. Hansen, R. H. Young, and P. E. Toren Contribution No. 343 jrom the Central Research Laboratories, Minnesota Mining and Manufacturing Company, St. Paul, Minnesota 66119 (Received November 12, 1966)
The dual charge-transfer abilities and polarographic properties of several nitro-p-terphenyls can be explained reasonably well in terms of Huckel MO theory.'S2 A consequence of this model is that the lowest vacant MO's of these molecules should be essentially nitrobenzene levels. For example, the LVO of 2,6-dinitroterphenyl should be very similar to the LVO of m-dinitrobenzene with low electron density in the biphenyl portion of the molecule. The lowest vacant molecular orbitals of aromatic nitro compounds have been extensively mapped by epr spectroscopy using the electrochemical anionradical generation technique pioneered by Geske and Makia3 We have used this technique to test the Huckel model. 'We also hoped to learn more about steric strain in these molecules.
Experimental Section The materials which were used have been described.lJ The anion radicals were generated by electrochemical reduction at potentials corresponding to the first polarographic waves of the nitroterphenyls.2 The
solutions in DMF were 1-5 mM in the nitro compound and contained 0.1 M tetrabutylammonium perchlorate. The reductions were conducted in a vessel similar to that described by Rieger and Fraenkel' placed in the dual cavity of a Varian Model 4205 spectrometer. The reference contained peroxylamine disulfonate.
Results and Discussion The epr spectra of seven nitro-p-terphenyls were analyzed in terms of NI4 and proton hyperfine coupling constants. The results are summarized in Table I. The assignments have been made largely on the basis of analogies reported in the literature. The hypefine coupling constants of the mononitro compounds are similar to those reported for nitrobenzene16although the nitrogen coupling constants are smaller. The effect on the N14 coupling constant of rotation about the C-N bond is well documented both experimentally and noncoplanarity causes an increase in a N . The nitrogen coupling constants of the three mononitro terphenyls do not follow the behavior expected on this basis, but vary in a manner (1) Part I: R. L. Hansen, J . Phys. Chem., 70, 1646 (1966). (2) Part 11: R.L. Hansen, P. E. Toren, and R. H. Young, ibid., 70, 1653 (1966). (3) D. H. Geske and A. H. Maki, J. Am. Chem. SOC., 8 2 , 2671 (1960). (4) P. H.Rieger and G . K. Fraenkel, J . Chem.Phys., 39, 609 (1963). (5) P.Ludwig, T.Layloff, and R. H. Adams, J . Am. Chem. SOC.,86, 4568 (1964). ( 6 ) D. H.Geske, J. L. Rrtgle, M. A. Bambenek, and A. L. Balch, ibid., 86, 987 (1964). ( 7 ) D.H.Geske and J. L. Ragle, ibid., 83, 3632 (1961). (8) P. H.Rieger and G . K. Fraenkel, J. Chem. Phys., 39, 609 (1963).
Volume 70, Number 6 May 1968
