PHYSICAL CHEMISTRY

V. A. GLUSHENKOV, V. A. IZMAILSKII, Yu. Sh. MOSHKOVSKII

SPECTRA OF E.D.A. COMPLEXES OF 4-NITRODIPHENYLALKANES CONTAINING A DONOR GROUP IN THE OTHER RING. E.P.R. EFFECT

(Presented by Academician A. N. Terenin, 19 VII 1963)

The coloration and the long-wavelength part of the spectrum of derivatives of p-nitrodiphenylalkanes (I) are a consequence not of endomolecular but of exomolecular interactions, i.e., of the formation of electron-donor–acceptor complexes (e.d.a. complexes \((^{4})\)) through interaction of the electron-donor component \(A^{1}KA^{2}\) with the acceptor \(BKA^{1}\) (I) \((^{1-3})\):

\[ \text{I. }\ \mathrm{O_2N} -\underset{\text{B—K—A}^{1}}{\left\langle\text{phenyl}\right\rangle} -(\mathrm{CH}_2)_n -\underset{\mathrm{A}^{1}\text{—K—A}^{2}}{\left\langle\text{phenyl}\right\rangle} -\mathrm{A}^{2} \qquad n=1,2,4;\ \mathrm{A}^{2}=\mathrm{NH_2},\mathrm{NMe_2},\mathrm{NEt_2}. \]

This supposition was confirmed by the observation that the long-wavelength branch of the spectral curve shifts toward longer wavelengths: 1) upon increasing the concentration and 2) upon adding to a solution of compounds of type (I) an excess of one of the components: \(BKA^{1}\) in the form of p*-nitrotoluene or nitrobenzene, \(A^{1}KA^{2}\) in the form of \(p\)-\(\mathrm{CH_3—C_6H_4—NH_2}\), \(p\)-\(\mathrm{CH_3—C_6H_4—NR_2}\), or \(\mathrm{C_6H_5—NH_2}\), \(\mathrm{C_6H_5—NR_2}\) \((^{2})\). Of particular interest is the use as \(BKA^{1}\) of a simplified model in the form of nitrobenzene (NB), and as \(A^{1}KA^{2}\) of aniline (AN), dimethylaniline (DMA), and diethylaniline (DEA) as solvent, i.e., in the form of a large excess in order to shift the equilibrium toward the e.d.a. complex.

Fig. 1. Absorption spectrum of 4-nitro-4¹-aminodiphenylethane.
\(I\) — calculated sum, \(C=10^{-4}\) mol/l; \(II\) — in n-hexane, \(C=10^{-4}\) mol/l; \(III\) — in alcohol, \(C=10^{-4}\) mol/l; \(3\) — in NB, \(C=10^{-2}\) mol/l; \(3a\) — in NB, \(C=10^{-4}\) mol/l; \(3b\) — in AN, \(C=10^{-2}\) mol/l; \(3v\) — in AN, \(C=10^{-4}\) mol/l.

For derivatives (I), at \(C=10^{-2}\) mol/l in NB solution, as compared with spectra in n-hexane, benzene, and alcohol, there is not only a bathochromic shift of the absorption boundary at \(\lg \varepsilon = 1\), but also a significant increase in \(\varepsilon_{\max}\) of the e.d.a. band (Figs. 1 and 2). At \(C=10^{-4}\) mol/l, besides preservation of the e.d.a. \(\alpha\)-band with a peak at 420 mµ, a clearly pronounced e.d.a. \(\beta\)-band appears with \(\lambda_{\max}\) 440–445 mµ (Table 1, Nos. 6a, 7a, 8a, 9a).

* Method for proving the connection of the spectrum with formation of an e.d.a. complex, proposed by V. A. Izmailskii and V. E. Limanov \((^{5})\).

Table 1*

* The numbers of compounds and solutions in the table correspond to the numbers of the curves in Figs. 1 and 2.
** AN—aniline, NB—nitrobenzene, DMA—dimethylaniline, DEA—diethylaniline.

A similar picture is observed with an excess of \(\mathrm{A^1KA^2}\), i.e., for solutions in aniline (Nos. 2б, 2в, 3б, 3в, 4б) and in dimethylaniline (Nos. 6б, 6в, 7б, 7в, Figs. 1 and 2) or in diethylaniline (8б, 8в, 9б, 9в).

The close position of the observed maxima in the indicated solutions to the maxima of the e.d.a. complexes \([\mathrm{C_6H_5—NMe_2 + C_6H_5—NO_2}]\), \([\mathrm{C_6H_5—NH_2 + C_6H_5—NO_2}]\) according to (5) (Table 1, Nos. 1, 5) confirms the correctness of explaining their origin as the result not of endo-, but of exo-interaction of the systems \(\mathrm{A^1KA^2}\) and \(\mathrm{BKA^1}\) (I).

Measurement of the electron paramagnetic resonance (e.p.r.) spectra showed that charge transfer can occur only for the substance in the solid crystalline state, whereas ionization in solution does not occur. The e.p.r. effect for the solution [nitrobenzene + dimethylaniline]

is absent at room temperature in the liquid state and upon cooling. No appreciable EPR effect was also found for derivatives of p*-nitrodiphenylmethane in the region corresponding to absorption of a free electron with a \(g\)-factor equal to \(\sim 2.000\). This can be explained as the result

Fig. 2. Absorption spectra of 4-nitro-4′-dimethylaminodiphenylethane.
\(I\)—calculated sum, \(C = 10^{-4}\) mol/l; \(II\)—in n-hexane, \(C = 10^{-4}\) mol/l; \(III\)—in alcohol, \(C = 10^{-4}\) mol/l; \(7\)—in NB, \(C = 10^{-2}\) mol/l; \(7a\)—in NB, \(C = 10^{-4}\) mol/l; \(7b\)—in DMA, \(C = 10^{-2}\) mol/l; \(7в\)—in DMA, \(C = 10^{-4}\) mol/l.

of the inductive interaction of the \(\mathrm{CH_2}\) group-separated electron-acceptor \((\mathrm{BK}^{A^1})\) and electron-donor \((\mathrm{A^1K}^{A^2})\) systems. In a single molecule there is an equalization of the ionization potential of the indicated components, thereby reducing the ability to form EDA complexes \((^{1,2})\) and hindering charge transfer.

With lengthening of the polymethylene chain separating the benzene nuclei, the inductive interactions weaken sharply, and the ionization potential of the electron-donor system should decrease, while the electron-affinity energy of the nitro system should increase; consequently, the appearance of an EPR signal could be expected. In a series of derivatives of p-nitrodiphenylethane \((\mathrm{I}.\ A^2 = \mathrm{NH_2},\ \mathrm{NMe_2}\ \text{and}\ \mathrm{NEt_2})\), only very weak absorption at the noise level was observed. However, for 4-nitro-4′-aminodiphenylbutane with \((\mathrm{CH_2})_4\) in the crystalline state, a distinct absorption line appeared with a width of \(\sim 11\) oersted and a \(g\)-factor coinciding with the \(g\)-factor of the standard (diphenylpicrylhydrazyl) (Fig. 3). Thus, with lengthening of the chain separating the benzene nuclei in derivatives (I), a spin density appears in the crystalline state, which is undoubtedly associated with transfer of an electron from the electron-donor to the electron-acceptor system.

Fig. 3. EPR spectra. \(a\)—diphenylpicrylhydrazyl, \(b\)—4-nitro-4′-aminodiphenylbutane.

* Upon cooling with liquid nitrogen, the light-red liquid turns into colorless crystals, which indicates decomposition of the complex (analogously to \((^{6})\)).

An unexpected increase of $\varepsilon$ in the region of the EDA band is observed when the concentration of solutions in AN, DMA, DEA, and NB is decreased, and at the same time there is a considerable broadening of the band; on the other hand, there is a considerable decrease of $\varepsilon$ of the EDA $\alpha$-band in NB with a hypsochromic shift from 425 to 420 m$\mu$ ($\Delta\lambda = -5$ m$\mu$) and the appearance of a new maximum in the region 440–445 m$\mu$ (Table 1, Figs. 1 and 2, Nos. 2a, 3a, 4a, 6a, 7a, 8a, 9a), the intensity of the peak of the EDA $\alpha$-band varying in the order $\mathrm{NH_2} > \mathrm{NEt_2} > \mathrm{NMe_2}$ (Figs. 1 and 2).

On the basis of the work of Briegleb and Czekalla ($^7$), we put forward the hypothesis that in our case, too, EDA complexes of two types are formed in NB solution: an EDA $\alpha$-complex as a result of layered association of molecules of type (I) without participation of solvent molecules (EDA $\alpha$ band); and an EDA $\beta$-complex as a result of layered association of molecules of type (I) with participation of solvent molecules (EDA $\beta$ band). With decreasing concentration, the EDA $\alpha$ complex decomposes to a considerable extent as a result of a shift of the equilibrium toward nonassociated molecules. As a consequence, the relative fraction of the EDA $\beta$ complex increases because of the very large excess of one of the components (the solvent). In this process the absolute absorption decreases, while the molar absorption increases.

State Scientific Research Institute
of Organic Intermediates and Dyes

Institute of Chemical Physics
Academy of Sciences of the USSR

Moscow State Pedagogical Institute
named after V. I. Lenin

Received
15 VII 1963

CITED LITERATURE

1. V. A. Izmail’skii, V. A. Glushenkov, DAN, 139, No. 2, 373 (1961).
2. V. A. Glushenkov, V. A. Izmail’skii, DAN, 142, No. 4, 820 (1962).
3. V. A. Izmail’skii, V. A. Glushenkov, Journal of the All-Union Chemical Society named after D. I. Mendeleev, 6, No. 1, 111 (1961).
4. G. Briegleb, Elektronen-Donator-Acceptor-Komplexe, Berlin, 1961; V. A. Izmail’skii, V. E. Limanov, in the collection ZhKh, Structure of Matter and Spectroscopy, USSR Academy of Sciences Press, 1960, p. 41.
5. V. E. Limanov, Candidate dissertation, Moscow, 1959.
6. V. A. Izmail’skii, E. A. Smirnov, ZhOKh, 8, 1734 (1938).
7. G. Briegleb, J. Czekalla, Angew. Chem., No. 12, 401 (1960).