RICHARD L. HANSEN
1646
(NO&] is strongly salted-in by alkali halides. The salt effects in these systems were explained largely in terms of dispersion forces between the complex molecule on the one hand and the ions of the salt on the other. Extending this idea to complex ion electrolytes, one might expect dispersion forces to make a significant contribution to the formation of ion pairs involving a large, highly polarizable cation such as [CO(NH&A]~+. This would explain, among other things, why the extent of association appears to increase in the order C1- < Br- < I- since this is the order of increasing polarizability of the anions. I n this connection, Rosseinsky13has pointed out that, contrary to popular belief, ion-pair formation in salts of
the alkali or alkaline earth metals usually increases with the size of the cation. This lends support to our belief that, while coulombic forces and solvent structure effects are undoubtedly important in ion pairing, dispersion forces may play a much more significant role than has been generally recognized.
Acknowledgment. This work was supported in part by the Research Foundation of the University of Connecticut and by an NSF graduate fellowship awarded to L. H. B. ~
(12) W. L. Masterton and Robert N. Schwartz, J . Chem. Phys., 69, 1546 (1965).
(13) D. R. Rosseinsky, J. Chem. SOC.,785 (1962).
Nitro-p-terphenyls. I. Dual Charge-Transfer Properties and Spectral Correlations
by Richard L. Hansen Contribution No. $60 jrom the Central Research Laboratories, Minnesota Mining and Manujacturing Company, St. Paul, Minnesota 66119 (Receiued November 10, 1966)
Seven nitro-p-terphenyls have been found to form charge-transfer complexes with both electron donors and acceptors. The results of quantitative st,udies of complexes with tetracyanoethylene and with N,N-dimethyl-p-toluidine are reported. A partial interpretation of the absorption spectra of the nitroterphenyls has been made on the basis of their charge-transfer spectra and elementary molecular orbital theory.
Introduction Organic charge-transfer complexes have been extensively investigated for a number of years. Typically, a given organic molecule acts as either an electron donor or an electron acceptor but seldom as both. In addition to intramolecular charge-transfer interactions in certain molecules, a limited number of compounds such as iodine can play a dual role and form self-complexes in which one molecule behaves as an electron donor toward a second molecule as acceptor.’ Examples of materials possessing more general dual The Journal o j Physical Chemistry
charge-transfer properties are quite rare. The “chargetransfer’’ or “a-complex” theories developed by Mulliken,2 by Dewar and L e ~ l e y and , ~ enunciated by Briegleb4 intimate that organic molecules should be capable of functioning as both charge-t,ransfer donors and ac(1) P. A. D.deMaine, J . Chem. Phys., 24, 1091 (1956). (2) R. S. Mulliken, J . Am. Chem. Soc., 72, 600 (1950); 74, 811 (1952); J . Phys. Chem., 56, 801 (1952). (3) M. J. S. Dewar and A. R. Lepley, J. A m . Chem. SOC.,83, 4560 (1961). (4) G. Briegleb, Angew. Chem., 76, 326 (1964).
NITRO-p-TERPHENYLS
ceptors; albeit, one of these roles may be dominant. I n support of these views, Wentworth and Chen have reported that certain polycyclic aromatic hydrocarbons can behave as charge-transfer acceptors. However, the complexes were very weak, the uncertainties were large, and charge-transfer spectra could not be obtained.6 The donor abilities of aromatic hydrocarbons are well known. Proceeding along these lines, we have found that several nitro-p-terphenyls, some of which are new, exhibit well-defined dual charge-transfer properties; e.g., they complex with tetracyanoethylene (TCNE) , a typical acceptor, and with N,N-dimethyl-p toluidine (DMT), a representative electron donor. These complexes are described below. The electronic absorption spectra of the nitroterphenyls are related to their charge-transfer properties. The ability of elementary molecular orbital theory to account for the properties of the nitroterphenyls is tested. Following papers in this series will deal with polarographic studies of the nitroterphenyls and with the epr spectra of the radical anions derived from them.
Experimental Section Materials. A mixture of 2-nitro- and 4-nitro-pterphenyl was prepared by the method of France, Heilbron, and Hey.6 Fractional crystallization from either ethanol or benzene followed by chromatography on an alumina column gave 2-nitroterphenyl, mp 129-130" and 4-nitroterphenyl, mp 214-215". The infrared spectra of these compounds displayed typical N-0 stretching bands in KBr. In 4-nitroterphenyl these bands appeared at 1340 and 1517 cm-l but were a t 1349 and 1527 cm-l in the 2-nitro isomer, suggesting that the nitro group is sterically hindered in the latter case.' 4,4"-Dinitro-p-terphenyl, mp 272-273", was obtained by the direct nitration of p-terpnenyl using a mixture of acetic and fuming nitric acids.8 Infrared bands were at 1342 and 1512 em-'. 3-Nitro-pterphenyl was prepared as suggested by Gray and Lewis.* Chromatography on alumina followed by recrystallization from methylene chloride gave nearly colorless plates: mp 176-177" ; infrared bands, 1350 and 1522 cm-'. Nitropterphenyls containing more than one nitro group in a given benzene ring have not been reported previously. The Ullmann biaryl coupling reaction, which in these cases involved heating a melt of 4iodobiphenyl and the appropriate 1-chloronitrobenzene with copper, provided a convenient route to three of these compounds. The general technique has been described adequately.9JO In each case the product
1647
was purified by column chromatography and recrystallization from a methylene chloride-n-hexane mixture. 2,4-Dinitro-p-terphenyl was made at 210". The yield was 6%: yellow needles; mp 150-151"; infrared bands, 1352 and 1530 cm-l. Anal. Calcd for CISH12N204: C, 67.5; H, 3.8; N, 8.8. Found: C, 67.0; H, 4.0; N, 8.6. When 2,4-dinitrobromobenzene was used as the reactant, the yield was increased to 18%. 2,6-Dinitro-p-terphenyl was prepared at 185" in 46% yield. The yellow needles melted at 131-132". Infrared bands were at 1356 and 1529 cm-'. Anal. Found: C, 67.2; H,3.7; N,8.8. The reaction between 4-iodobiphenyl and picryl chloride a t 158" produced 2,4,6-trinito-p-terphenyl in 80% yield. The yellow needles melted at 174180". Infrared bands were at 1352 and 1557 cm-l. Anal. Calcd for CleH1lN3O6: C, 59.2; H, 3.0; N, 11.5. Found: C,59.1; H,3.3; N, 11.3. pTerphenyl and biphenyl were recrystallized and the p-terphenyl was then sublimed. Tetracyanoethylene was crystallized from chlorobenzene and sublimed. N,N-Dimethyl-ptoluidine was distilled under vacuum, saturated with nitrogen, and stored in a refrigerator. Spectro grade methylene chloride was used as received. Equipment and Procedure. Differential absorption spectra were obtained in matched 1- or 5-cm cells on a Beckman hiodel DK-2A spectrophotometer. The cell holder was thermostated to =t0.2", and the cell compartment was flushed with dry nitrogen for work below room temperature. Charge-transfer spectra were measured at 15, 25, and 35". For the series of TCXE complexes, the nitroterphenyl concentration M , and the TCXE covered the range 4-36 X M . I n the case concentration the range 4-26 X of the DMT complexes the nitroterphenyl concentration was held constant at either 1 X or 4 X M . The DAIT concentration was varied in the range to lo-' M . The absorbance was reproducible to at least ~ 7 2 %and in most cases to k l % or better. Spectra of the parent nitroterphenyls were measured M. in the range of about 8-80 X Calculations. I n the case of the TCNE complexes, data taken at ten wavelengths chosen to encompass the charge-transfer band were averaged by the matrix methods of Wallace" and Ainsworth.12 Averaged (5) W. E. Wentworth and E. Chen, J . Phys. Chem., 67,2201 (1963). (6) H.France, I. M. Heilbron, and D. H. Hey. J. Chem. SOC.,1364 (1938). (7) L. H. Bellamy, "The Infrared Spectra of Complex Molecules,'' John Wiley and Sons, Inc., New York, N. Y., 1958,p 300. (8)G. W. Gray and D. Lewis, J . Chem. SOC.,5156 (1961). (9) P. E.Fanta, Chem. Rev., 38, 139 (1946). (10) J. Forrest, J. Chem. SOC.,566 (1960).
Volume 70,Number 6 Maa, 1966
RICHARD L. HANSEN
16448
absorbances at three widely separated wavelengths were then treated according to the least-squares method of Liptay for the calculation of equilibrium constants and extinction coefficient^.'^ These computations were programmed for an IBM 705 computer. Less elaborate analyses were needed for the series of D l I T complexes. In these cases it was convenient to employ an excess of the amine. Equilibrium constants and extinction coefficients were obtained graphically at several wavelengths using the Scott modification of the Benesi-Hildebrand equation. l4 Huckel molecular orbital calculations were performed for the nitroterphenyls and related molecules on a CDC 1604 computer using standard diagonalization techniques. The heteroatom parameters, kcN = 0.8, kNO = 0.7, h N + = 2.0, and ho = 1.0, suggested by Streitwieser were used where required.15 I n an effort to make the model more realistic, the effects of nonplanarity on energy levels were investigated by varying Hn', the resonance integral connecting the biphenyl and nitrobenzene moieties, between 1.00 and 0.00 in those molecules containing a 2-nitro group.I6 For simplicity, coplanarity was otherwise assumed and no attempt was made to study the effects of simultaneous rotation about a carbon-nitrogen bond, for example.
2.5r2.0
Wavelength (mp)
Figure 2. Charge-transfer spectra of nitroterphenyl-DMT complexes in methylene chloride at 25".
Results The stoichiometry of both series of complexes was * . 7 r
'A'
I
Wavelength ( mp)
Figure 1. Charge-transfer spectra of nitroterphenyl-TCNE complexes in methylene chloride a t 25'.
The Journal of Physical Chemistry
1: 1 with respect to the reactants. I n the case of the TCNE complexes, a matrix ranking method was used which is mathematically accurate but can only be as certain as the spectroscopic I n the case of the DMT complexes, the stoichiometry rests solely on the linear Scott plots obtained. The pitfalls involved in determining the stoichiometry, thermodynamic, and optical properties of complexes are well doc~mented."-~ The charge-transfer spectra of the complexes of six of the nitroterphenyls with TCNE and D M T appear in Figures 1 and 2, respectively. The curves are identified by the compound numbers given in Table I. Parabolic extrapolations were made
(11) R.M. Wallace, J. Phys. Chem., 64, 899 (1960). (12) 9. Ainsworth, ibid., 65, 1968 (1961); 67, 1613 (1963). (13) W. Liptay, 2. Elektrochem., 65, 375 (1961). (14) R. L.Scott, Rec. Trav. Chim., 75, 787 (1956). (15) A. Streitwieser, "Molecular Orbital Theory for Organic Chemists," John Wiley and Sons, Inc., New York, N. Y., 1961,p 117. (16) H. Suzuki, Bull. Chem. SOC.Japan,35,1853 (1962),and previous papers. (17) W. B. Person, J. Am. Chem. SOC.,87, 167 (1965). (18) G.D.Johnson and R. E. Bowen, ibid., 87, 1655 (1965). (19) K.Conrow, G. D. Johnson, and R. E. Bowen, ibid., 86, 1025 (1964). (20) P. R. Hammond, J. Chem. SOC.,479 (1964).
NITRO-p-TERPHENYLS
1649
Table I : The Properties of Complexes with T C N E in Methylene Chloride a t 25”
Compd
2-Nitroterphenyl (1) 3-Nitroterphenyl (2) 4-Nitroterphenyl (3) 2,4-Dinitroterphenyl (4) 2,6-Dinitroterphenyl (5) 2,4,6-Trinitroterphenyl (6) 4,4”-Dinitro terphenyl (7) p-Terphenyl (8) Biphenyl (9) Benzene” (10) a
Ct max, mfi
l./mole om
506 520 -390 516 480
260 f 80 170 f 40 200 f 40 110 f 40 100 f 9
emax, fa
-AH,
- AFx,
A&,
Kx
kcal/mole
kcal/mole
eu
bl/agb X 102
7 3 3
14 f 4 39 f 10
1 . 6 f0 . 2 1 . 7 f0 . 6
1 . 6 f0 . 2 2 . 2 f0 . 2
0f2 2 f 3
2.8 f 0.5 3f1
3 2.2
50 f 18 32 f 3
2 . 6 f0 . 5 1.2f O . 1
2 . 3 f0 . 3 2.1 zto.1
-1 f 3 3 . 1 f0 . 9
4.7 f0.9 4.0 f0.2
x
108
475
60 f 7
1.1
51 f 1’
1.0 f0.2
2 . 3 f0 . 1
4 . 4 f0.5
1 . 6 f0.2
460
90 f 20
1.9
31 f 7
0.4 f0 . 1
2 . 0 f0 . 2
5f1
0.6 f0.2
...
...
...
...
1 . 4 f0.2 1.1f 0 . 2 1 . 7 f0 . 1 1.2fO.l 2.30
1 . 4 f0 . 3
0f2
2.7 f0 . 4
1 . 2 f0 . 2
-2 f 1
2 . 9 i0 . 2
