ELECTRON AFFINITIESOF POLYNUCLEAR ACCEPTORS
2277
Electron Affinities of Polynuclear Acceptors. Dinitro- and Trinitrophenanthrenequinones’
by Tapan K. Mukherjee Energetics Branch, Air Force Cambridge Research Laboratories, Bedford, Massachusetts (Received January 6 , 1967)
Electron-acceptor strengths of 2,4,7-trinitrophenanthrenequinone(I), 3,6-dinitrophenanthrenequinone (II),2,7-dinitrophenanthrenequinone(111),and 2,5-dinitrophenanthrenequinone (IV) have been determined from charge-transfer spectra and from half-wave reduction potentials. The order of electron affinities is found to be 2,4,7- > 3,6- > 2,7- > 2,5-. The energies of the absorption bands of the semiquinone radical ions are linearly related to the electron affinities and half-wave potentials of the quinones. Stable salts of 2,4,7-trinitrosemiquinone (I) radical ions with N-methylquinolinium and lithium cations, respectively, have been isolated. The acceptor property of the 2,5-dinitro isomer is decreased owing to the steric interference of the nitro group a t position 5. From the reported data of the spin densities of phenanthrenequinone radical ion, it is suggested that the 3,6-dinitro isomer will be the strongest acceptor in the dinitro series and l13,6,8-tetranitrophenanthrenequinone will be a very powerful acceptor.
Introduction This study was designed to investigate the complexing properties of organic charge-transfer acceptors of relatively large molecular radii. It is hoped that this work will lead to the discovery of powerful acceptors which will form solid complexes of sandwich configuration and maximum ?r-orbital overlap with polynuclear donor molecules. Such complexes are needed for better understanding of the mechanism of solidstate electrical conduction in donor-acceptor complexes. Most of the known strong acceptors like chloranil, tetracyanoethylene, etc., are relatively small molecules, their acceptor strengths depending on the number and electronegativity of the functional groups which are suitably placed around restricted conjugated systems. Lepley and Thelman2 have set up qualitative criteria of acceptor strengths on the basis of the above-mentioned characteristics. The structural features of electron acceptors have been reviewed by Briegleb3and A n d r e w ~ . ~We have studied the properties of several acceptors derived from nitrofl~orenes.~ Electron-acceptor characteristics of the following nitro derivatives of phenanthrenequinone are reported in this paper.
~~~
~~
(1) A part of this work was presented a t the 4th Carribean Chemical
Symposium held a t Kingston, Jamaica, Jan 3-7, 1967. (2) A. R. Lepley and J. P. Thelman, Tetrahedron, 2 2 , 101 (1966). ( 3 ) G. Briegleb, Angew. Chem., 7 6 , 326 (1964). (4) L. J. Andrews and R. M. Keifer, “Molecular Complexes in Organic Chemistry,” Holden-Day, Inc., San Francisco, Calif., 1964.
Volume 7 1 , Number 7 June 1967
TAPAN K. MUKHERJEE
2278
Experimental Section Synthesis of 2,7-Dinitrophenanthrenequinme and 2,6Dinitrophenanthrenequinone. Nitration of phenanthrenequinone (purified by crystallization and sublimation) with nitric acid (d 1.51) by the method outlined by Schmidt and Karnf6 afforded a mixture of dinitrophenanthrenequinones in 82% yield. Separation of the isomers was effected by fractional crystallization from glacial acetic acid, in which 2,5-dinitro isomer was more soluble. Repeated crystallizations from glacial acetic acid, foIlowed by chromatography over silica gel using benzene as eluent, gave 2,7-dinitrophenanthrenequinone, mp 301-303’, and 2,5-dinitrophenanthrenequinone, mp 230-232’ (lit? 228230’). The purity of the isomers was checked by thin layer chromatography. Since pure materials were needed in this work, no attention was given to the yields of the separated isomers. 3,G-Dinitrophenanthrenequinone was synthesized via a multistep reaction sequence described by Schenck and Schmidt-Thombe6,s which involved the preparation of the photoadduct of phenanthrenequinone with sulfur dioxide, its nitration, and the pyrolytic elimination of sulfur dioxide; mp 293-295’ (from AczO). d14,7-Trinitrophenanthrenequinone was synthesized by drastic nitration of 2,7-dinitrophenanthrenequinone.9 The crude product was crystallized three times from benzene when shiny yellow crystals of the benzene complex were obtained, mp 203-205’. Recrystallization from glacial acetic acid afforded pure 2,4,7trinitropherianthrenequinone, mp 212-213’ dec.1° Spectra. The charge-transfer absorption spectra were taken in a Cary Model 14 recording spectrophotometer. The measurements were made at room temperature, using methylene dichloride as a solvent, in stoppered quartz cells of 1-cm path lengths. Solutions of the complexes were prepared by adding a solution of the acceptor to the solution of large excess of the donor. The reference cell contained the solution of the donor. In several cases, mixing of the solution of I with a donor resulted in the separation of solid complexes which were removed by filtration and the filtrate was used for spectral measurement. Analyses of the solids corresponded with 1:l composition for complexes with 1,2-benzanthracene, 3,4-benzopyrene1 and anthracene, respectively. Charge-transfer spectra of a few of the powdered solid complexes in Nujol film were recorded. There was no significant difference between the peaks of the long wavelength absorption maxima of the solids and solutions. The donor hydrocarbons were used in the form obtained from Rutgerswerke-Aktiengesellschaft, Frankfurt-am-Main, West Germany. The Journul of Physicccl Chemistry
Polarography. Both a Sargent Model XV and a Sargent Model XXI polarograph were employed in this research. Spectral grade acetonitrile as solvent and tetra-n-butylammonium perchlorate (TBA) and lithium perchlorate as supporting electrolytes were used. Polarographic data were obtained a t room temperature, using a three-electrode Arthur and Vanderkam” cell. The I R drop in the cell was compensated by a Sargent Model A compensator. Two identical calomel electrodes were used as the electrolysis reference electrode (ERE) and the stable reference electrode (SRE). The electrolyses were carried out under nitrogen atmosphere and the measurements were recorded under conditions of “long imrnersion.”l2 The dropping mercury electrode was used at 71-em pressure and had a drop time of 3.6 sec (open circuit) in acetonitrile; concentration was 1mmole/l.
Results and Discussion Charge-Transfer Spectra. Jlolecules capable of donating an electron react with molecules which can accept the electron and during this process often give rise to new absorption bands at lower frequencies than those of the donors or acceptors. The extent of the reaction ranges from weak orbital interaction to complete transfer of an electron from the highest filled orbital of the donor to the lowest unfilled orbital of the acceptor, and the energy of the resultant chargetransfer band, derived from simple molecular orbital treatment,13is given by the relationship Ex
=
Bj - ( a
+
XiP)
(1)
where B j is the energy of the acceptor orbital involved in the interaction, a is the coulomb integral for carbon, p is the carbon-carbon resonance integral, and xi is the energy coefficient of the highest filled orbital of the (5) (a) T. K. Mukherjee and L. Levasseur, J . Org. Chem., 30, 644 (1965); (b) T . K. Mukherjee, J . Phys. Chem., 70, 12, 3848 (1966); (c) T. K. Mukherjee and A. Golubovic, Abstracts, 149th National Meeting of the American Chemical Society, Detroit, 1965, p 108. (6) J. Schmidt and A. Kamf, Chem. Ber., 35, 3117 (1902). (7) Schmidt and Kamf6 thought that they obtained the 4,bdinitro isomer; however, Christie and Kenner ( J . Chem. Soc., 671 (1926)) later showed that this was, in fact, the 2,bdinitro isomer. (8) G. 0. Schenck and G. A. Schmidt-Thombeb, Ann. Chem., 584, 199 (1953). (9) G. H. Christie and J. Kenner, J . Chem. Soc., 123, 779 (1926). (10) The apparent discrepancy of the melting point with literature prompted us to confirm the structure of the 2,4,7-trinitrophenanthrenequinone by potassium dichromate oxidation leading to the known 4,4’,&trinitrodiphenic acid, mp 290-291. (11) P. Arthur and R. H. Vanderkam, Anal. Chem., 33, 765 (1961). This cell is supplied by E. H. Sargent and Co. (12) J. F. Coetzee and G. R. Padmanabhan, J . Phys. Chem., 66, 1709 (1962). (13) (a) RZ. J. S. Dewar and A. R. Lepley, J . Am. Chem. Soc., 83, 4560 (1961); (b) M. J. S. Dewar and H. Rogers, ibid.,84, 395 (1962).
2279
ELECTRON AFFINITIES OF POLYNUCLEAR ACCEPTORS
Table I : The ChargeTransfer Absorption Maxima for Hydrocarbon Complexes with the Nitro Derivatives of Phenanthrenequinones and Their Relative Electron Affinities Acceptor (Nitrophenantbrenequinone)
Donor
No.
Xl(p)O
None 1. Anthracene
2. 3. 4. 5. 6. 7. 8. 9. 10. a
(&’*)k
Perylene 3,4Benaopyrene l12-Beneopyrene l,%Benaanthracene 3,CBenaotetraphene Pyrene 1,2,5,6Dibenaanthracene 1,2,3,4Dibensanthracene Decacyclene
0.414 0.347 0.371 0.497 0.452 0.405 0.445 0.473 0.499 0.470
,---2,4,7-Trinitro E,, mp
367,322 6353Z5 7353Z2 680f3 580h5 610i2 650f3 6103~2 600 585 600i2
(I)EAb
1.26 1.26 1.22 1.24 1.26 1.24 1.29 1.24 1.28 1.27
-3,&Dinitro E n , mr
410,317 595i5 705 6453Z3 540f5 572i5 600f5 575f5 555 A 5 535 560f10
(11)EAb
1.15 1.19 1.12 1.08 1.13 1.08 1.17 1.07 1.08 1.11
-2,7-Dinitro E~,mr
385,295 570f. 10 660 615 5 2 3 i 10 550 585 i 2 555i12 537f10 517 560f5
(111)EAb
1.06 1.07 1.03 1.00 1.04 1.03 1.09 1.00 0.99 1.12
--2,5-Dinitro
EA^
375,278 560&8 650It3 605&5 507 ==! 10 537It5 575&5 545zklO 525 505 525It5
1.02 1.04 1.00 0.93 0.99 0.99 1.05 0.94 0.94 0.07
Energy of highest occupied MO; C. A. Coulson and R. Daudel, “Dictionary of Values of Molecular Constants.’’ = EAk - &‘Ai-
2.5
-
*L 0.4
-
0.3-
1.5
2.0
E, ev
2.5
Figure 1. Plots of the transition energies of the chargetransfer bands of polynitrophenanthrenequinonesagainst the energies of the highest occupied molecular orbitals of donor hydrocarbons (Table I): 0, 2,4,7-trinitrophenanthrenequinone (I); X, 3,6dinitrophenanthrenequinone (11); 0,2,7-dinitrophenanthrenequinone, (111); 0, 2,5-dinitrophenanthrenequinone (IV).
donor. This relationship holds reasonably well for large sets of a-ITinteractions between a given acceptor
(IV)--
E,, mp
From (E,)i
-
and a series of axomatic hydrocarbon donors for which xip values are a~ai1able.l~The weakness in the binding of the ?T complexes is shown by the fact that the principle bands of the donors and acceptors are not perturbed. Table I lists the wavelengths (E,) of the charge-transfer absorption bands found for ten donors. The wavelength maxima of the acceptors are recorded in the first row. Since the charge-transfer bands are sufficiently separated from the first singlet transitions of the donors, the latter are not included in the table.’S Plots between E, and xip for the four acceptors are shown in Figure 1 and, as expected from eq 1, close linear relationships are observed. The straight lines are those derived from the method of least squares.16 The slope from eq 1 gives the value of p which has been found by other workers to be constant and is close to -3.01, the p value for benzene. The intercept of the straight line (E, = 0) gives the value of (Bj C Y ) , a measure of acceptor strength. Inspection of Table I reveals that, for a given donor, the absorption maxima for the first C-T transition shifts regularly from lower to higher energy, indicating the following order of the acceptor strengths of the nitro derivatives of phenanthrenequinone; 2,4,7-trinitro (I) > 3,6dinitro (11) > 2,7-dinitro (111) > 2,5-dinitro (IV). However, this order is not retained in the intercepts of the straight lines of I-IV (p = -2.65, -3.02, -3.29, -3.38 and (Bj - CY) = 0.81, 0.87, 0.67, 0.76 for ac(14) C. A. Coulson and R. Daudel, “Dictionary of Values of Molecular Constants,” Mathematical Institute, Oxford, England. A handy compilation is available in ref 13. (16) For convenient tabulation, see A. R. Lepley, J . Am. Chem. Soc., 84, 3577 (1962), Tables I and 11. (16) We wish to thank Miss N. Dimond for her assistance with the computation work carried out at the AFCRL Computation Center.
Volume 71, Number 7
June 1967
TAPANK. MUKHERJEE
2280
¶
I--
I
1 .a
I
I
I
1.1
1.2
1.3
EN,,
rV
2.4
2.3
2.2
2.1
I
I .o
0.5
I
I
I .I
20
2.0
W k 8'4
Figure 2. Charg+transfer energy as a function of electron afiities, (.EA)t, of acceptors according to eq 2: donor, pyrene; acceptors, (I)2,4,7-trinitrophenanthrenequinone, (11)3,6-dinitrophenanthrenequinone, (111)2,7-dinitrophenanthrenequinone, (IV) 2,5dinitrophenanthrenequinone. (1) Tetracysnoethylene, (2) tetracyanoquinodimethene, (3) chloraail, (4) 2,4,7-trinitrofluorenone, ( 5 ) 1,3,6-trbitrobenaene.
ceptors 1, 11, 111, and IV, respectively). The reason for this discrepancy is mainly owing to the scatter of the poifits and limited number of the complexes studied. A small deviation in the slope induces a large deviation in the intercept. Further, the linear relationship itself is predicted by an oversimplified use of the Huckel theory. l6* l7 Alternatively, the electron affinities of two acceptors can be compared from their charge-transfer transitions with a common donor. Thus, if (E,)i and EAi stand for the chrtrge-transfer energy and electron affinity of the standard acceptor, respectively, then E&, the electron affinity of a second acceptor, is obtained from
(E,)i
- (&)k
=
EAk
- EAi
stricted to the complexes of same bond type and configurati~n.~In the present case, we have used 2,4,7trinitrofluorenone (TNF) as the standard. This preference has no special significance except that the molecular dimension of T N F is comparable with the acceptors used here and it has been used as a standard in the determination of electron affinities of polynitro derivatives of fluorene-AQ"-malononitrile.ls The average electron affinity of 2,4,7-trinitrofluorenone, as determined from charge-transfer spectra, is 1 ev (compared to ~hloranil).~From polarographic half-wave potential the value is 0.94 ev; since the oneelectron reversible reduction potential is more accurate than the C-T bands, EAi = 0.94 ev has been used as standard. The electron affinities of the dinitro- and trinitrophenanthrenequinones as calculated from eq 2 are shown in Table I. An approximately linear relationship exists between the C-T energy and electron affinities of these acceptors with one and the same
(2)
where (&!,)a: is the charge-transfer energy of the complex with the same donor. This relationship is reThe Journal of Pkymical Chemistry
Figure 3. Correlation between the energy of the semiquinone anion radical and (a) half-wave potential, 0, and (b) average electron affinities, 0, of polynitrophenanthrenequinones.
(17) R. 9. Mulliken and W. B. Person, Ann. Rev. Phys. C h m . , 13, 107 (1962). (18) T. K. Mukherjee, submitted.
ELECTRON AFFINITIESOF POLYNUCLEAR ACCEPTORS
228 1
Table I1 : Polarography of Quinones in Acetonitrile -Supporting B I / ~US. ' sce
Compound
Phenanthrenequinone 2,4,7-Trinitrophenanthrenequinone(I) 3,6Dinitrophenanthrenequinone (11) 2,7-Dinitrophenanthrenequinone( 111) 2,5-Dinitroplienanthenequinone(IV)
Ia
El/4
-0.660
72 75 90 70 110
-0.098
-0.150 -0.195 -0.265
3.68 3.63 4.07 4.01 3.16
donor, which is illustrated in Figure 2 using pyrene as the donor. ,i relative comparison of the acceptor strengths of these quinones with other well-known acceptor molecules can be obtained from the additional data included in Figure 2. Polarography. The one-electron reversible halfwave reduction potential, measured in aprotic solvents, against a calomel electrode is related linearly to the electron affinity of an acceptor according to eq 33p19 =
1.04E11,'"~
+ 1.39
(3)
I n these solvents, quinones are reduced by two oneelectron steps to the semiquinone anions and hydroquine dianions, respectively. In alkali metal perchlorate as supporting electrolyte, the reduction can proceed either by two consecutive one-electron transfers or a one-step two-electron transfer. Lithium cation markedly shifts El/, to more positive potentia1.N The first half-wave potentials for 2,4,7-trinitrophenanthrenequinone and the three dinitro isomers are shown in Table 11. The number of electrons involved in each step are deduced from Tomes' relationship,z1 Ea/,- Ell4= 59 mv for a one-electron reversible electrode process at 25'. Excepting the large deviation of 2,5-dinitro isomer (IV) in TBA, this criterion is approximately fulfilled in both the electrolytes; in the latter, reductions proceed by two-electron single steps. The reversibility of the reduction in TBA is not as good as in lithium perchlorate. However, electron affinities calculated from Ell2 valueszz in TBA fit in the same order as those obtained from charge-transfer spectra. Since aromatic nitro groups are reduced a t more negative potentials, z 3 it is reasonable t o assume that the first electron addition takes place to the lowest unoccupied orbital of the a-diketone function. Neglecting the questionable constancy of the reduction mechanism and of the transfer coefficient, the large negative displacement of the half-wave poten-
-
EA^
EAC
sce
0.69 1.29 1.23 1.19 1.11
0 . 70d
-0.235 0.085 0.062 0.045 0.012
a Limiting current constant I = i ~ / C m Z / 8 t 'where / B , i d is the limiting current in microamperes, liter, m is the mass of mercury flowing in milligrams per second, and t is drop time in seconds. Average values from charge-transfer spectra, Table I. See ref 3, Table VI. taken.
EA
Supporting electrolyte -LiC104E1/g' US. Ea/,
electrolyte, N-t-butyl perchloraEa/, -
1.256 1.118 1.043 0.982
E1/4
30 30 30 30 30
c is concentration in millimoles per From eq 3, positivevaluesof Ell,
tial of the 2,5-dinitro compound (IV) can be attributed to the decoupling of the &nitro group from the ring due to steric hindrance. A space-filling model, together with a simple calculation involving normal bond lengths and angles with allowable distortions, indicates that the nitro group at position 5 is twisted out of the plane of the ring by 75-90'. Geske and coworkersz3 found that in substituted nitrobenzenes, the half-wave potentials shift linearly with the angle of twist of the nitro group. In 2,4,7-trinitro derivative (I) the inductive effect of the third nitro group supersedes the inhibitory influence of the sterically hindered nit0 group at position 4. In the absence of any steric factor, the difference in the resonance energies of I1 and I11 is probably responsible for the differences in electron affinities. Anion Radical. Watson and Matsenz4showed that the half-wave reduction potentials of aromatic hydrocarbons are related to the frequencies of the long wavelength absorption maxima according to
+ constant
v = 2E1/,
(4) Apparently, this relationship has not been extensively examined for T acceptors. In the first place, the task of assignment of energy levels in polysubstituted quinones is difficult owing to the absence of suitable parameters in molecular orbital calculation.26 Berg, (19) R. M. Hedges and F. A. Matsen, J . Chem. Phys., 28, 950 (1958). (20) M. E. Peover and J. D. Davis, J . Electroanal. Chem., 6 , 46 (1963). (21) J. Tombs, Collection Czech. Chem. Commun., 9, 12 (1937). (22) M. E. Peover, Trans. Faraday SOC.,58, 1656 (1962), see footnote, p 1657. (23) D. H. Geske, J. L. Ragle, M. A. Bambenek, and A. L. Blach, J . Am. Chem. SOC.,86, 987 (1964). (24) (a) A. T. Watson and F. A. Matsen, J . Chem. Phys., 18, 1305 (1950); (b) I. Bergman, Trans. Faraday SOC.,50, 829 (1954). (25) M. E. Peover, J . Chem. Soc., 4540 (1962).
Volume 71,Number 7 JUM 1967
TAPAN K. MUKHERJEE
2282
however, found that the first half-wave reduction potentials of quinones (measured in 40% isopropyl alcohol and 60% aqueous buffer a t pH 7) show linear dependencies on the excitation energies of the n-r* transitions.z6 As some uncertainty remains in the identification of r r * and n-a* transitions in the acceptors studied here, the spectra of their anion radicals were correlated with the electron affinities and half-wave potentials. I n order to avoid the formation of diamagnetic ion pairs (commonly encountered when alkali metals are used), the semiquinone radical ions were generated by adding acetonitrile solution of N-methyl-N-triethylammonium iodide to the solutions of the quin o n e ~ . ~ 'Fairly stable semiquinone radical ionsz8* 2D with absorption in 500-600-mp range were obtained. The 2,5-dinitro compound (IV) gave two peaks a t 537 and 508 mp, respectively. Absorption maxima for other radical ions were (I) 572 mp, (11) 555 mp, and (111) 543 mp. In Figure 3, it will be observed that a linear relationship exists between the energy of the semiquinone radical ion and the average electron affinities of the acceptors (from Table I). Similarly, the energy of the radical ion is approximately related to the reduction potential by ( u ) * - = 0.88E,,, constant. Dehl and Fraenke130 found that the odd electron densities in phenanthrenequinone ion radical are larger in positions 1 (0.057) and 3 (0.071)than those at 2 (0.008) and 4 (0.018). Their experimental results
+
The J O U Tof~Physieal Chemistry
were in accord with the Huckel and McLachlan calculations. Highly polar nitro groups at 1,3 and their equivalent 6 and 8 positions will cause enhanced depletion of electrons from the ring; consequently, 1,3dinitrophenanthrenequinone is expected to be the strongest acceptor in the dinitro series. On the same basis, one can predict that 1,3,6,8-tetranitrophenanthrenequinone will be an extremely powerful acceptor.
Acknowledgment. The author is grateful to Dr. R. Payne for helping with the polarographic measurements. Thanks are also due to Dr. Jerry Silverman for helpful discussions. (26) H. Berg and K . Kramarczyk, Ber. Bunsenges. Physik. Chem., 68, 296 (1964). (27) (a) Potassium is known t o produce even a trianion radical from phenanthrenequinone. N. A. Bauld, J . Am. Chem. SOC.,86, 3894 (1964). (b) Iodide ion functions as an electron donor, in turn being oxidized to iodine which is scavenged as Is by excess of iodide present in the solution. Iodine in acetonitrile absorbs at 465 and 380 mp, respectively. For peaks in other solvents, see H. A. Benesi and J. H. Hildebrand, J. Am. Chem. SOC.,71, 2703 (1949). The cation does not absorb in the visible range. (28) Corresponding absorption for mononegative sodium ketyl from unsubstituted phenanthrenequinone absorbs at 475 mp. K. Maruyama, Bull. Chem. SOC.Japan, 37, 553 (1964). (29) Strongly paramagnetic semiquinone radical salts of I have been isolated as black crystals; e.g., (a) N-methylquinolinium salt, mp 237' dec, Anal. Calcd for C~H16N408:C, 59.13; H, 3.10; N , 11.49. Found: C, 59.09; H , 3.43; N , 13.2; (b) lithium salt, mp 308O (explodes), Anal. Calcd for ClaHsLiNsOs: C, 48.02; H, 1.43; N, 12; Li, 1.98. Found: C, 47.84; H, 1.70; N , 12.24; Li, 1.77. Although the free-radical contents of these salts have not been determined, it is almost certain that the lithium salt is contaminated with some diamagnetic dianion. (30) R. Dehl and G. K. Fraenkel, J . Chem. Phys., 39, 1793 (1963).
