1194 J.C.S. Perkin I Colour and Constitution of the Nitro- and Dinitro-p-phenylenediamines and their N-Methyl Derivatives By Kwong-Yung Chu and John Griffiths.' Department of Colour Chemistry, The University, Leeds LS2 9JT The syntheses of new violet to blue-green dinitro-p-phenylenediamines are described. The visible absorption spectra are compared with those of the mononitro-analogues, and the light absorption characteristics have been examined by the PPP-MO procedure. Good agreement between the theoretical predictions and experiment was observed for those derivatives with minimal steric crowding. Bathochromic shifts of up to 100 nm result from the introduction of a second nitro-group into the nitro-p-phenylenediamine chromogen. NITRO-amp;PHENYLENEDIAMINEand various N-alkyl deriv- atives have found extensive use as low molecular-weight dyes for the colouration of hair,l and range from orange to violet in colour.Recently we reported the synthesis of the previously unknown 2,3-and 2,5-dinitro-P-phenylenediamines, and also described a convenient in a sealed tube with methylamine and dimethylamine re~pectively.~ These compounds were prepared more conveniently, and in higher yields, by condensation of the appropriate amines with 4-fluoro-N,N-dimethyl-3-nitroaniline in boiling ethanol. The W-methyl- and N1,N1-dimet hyl-2-nitro-P-phenylenediamines,(1b) and route to the difficultly accessible 2,6-dinitro-i~omer.~ (Id) respectively, were prepared similarly from 4-The large bathochromic shifts resulting from the intro- duction of a second nitro-group into nitro-p-phenyl- enediamine prompted an investigation of the N-alkylated dinitro-compounds, to see how far the colour range of these dyes could be extended, and to examine the light absorption characteristics of these systems.For comparison purposes we have examined the related N-methylated nitro-9-phenylenediamines. Although Corbett has discussed the absorption spectra of these compounds from an empirical ~iewpoint,~ no theoretical treatments have been reported. Thus we have used the PPP-SCF-MO procedure to calculate the visible absorp- tion maxima of the mononitro-compounds, and have extended the calculations to include the dinitro-analo- gues. Substituent, solvent, and steric effects have been interpreted with the aid of these computations.RESULTS AND DISCUSSION N-Methylated Nitro-p-pheny1enediamines.-The eight N-methyl derivatives (1b-i) of nitro-p-phenylenedi-amine (la) were prepared by literature methods, or fluoro-3-nitroaniline and the appropriate amines. NR'R~ R' Ra R3 R4 a b; c; d; e; f; ; H Me H Me H H H H H Me H Me H H H H Me H H H Me H Me Me g; h; i; Me H Me Me Me Me Me Me Me H Me Me N-Methylated Dinitro-p-9henylenediamines.-The syn-theses of 2,3-, 2,5-, and 2,6-dinitro-fi-phenylenediamine (2a), (3a), and (4a), respectively, by nitration and sub- sequent hydrolysis of N1,N4-bis(phenylsulphonyl)-$-phenylenediamine have been reported recently.2 In an attempt to prepare the N1,N4-dimethylated derivatives, nitration of N1,N4-dimethyl-N1,N4-bis(modifications of these.N1,N4,N4-Trimethy1-2-nitro-p-phenylsulphony1)-phenylenediamine (lh) and the N1,N1,N4,N4-tetra-methyl-compound (li) have been prepared by condens- ation of 4-chloro-3-nitro-N,N-dimethylanilineat 180 "C 1 J. F. Corbett, ' The Chemistry of Synthetic Dyes,' Vol. V, ed. K. Venkataraman, Academic Press, New York, 1971, pp.508-510. 2 K. Y. Chu, and J. Griffiths, J.C.S. Pevkin I, 1978, 406. P-phenylenediamine was examined. The latter com-pound proved unreactive, however, and prolonged reaction times at 100 "C afforded only starting materials. The cause of this inertness is probably steric, and similar J. F: Corbett, J. SOC.Dyers and Colourists, 1967, $8, 273; Spectrochzm. Ada, 1967, 23A.2315.J. F. Corbett and A. G. Fooks, J. Chem. SOC.(C),1967, 1136. difficulties have been reported for the nitration of N1,N4-diacetyl-N1,N4-dimethyl-p-phenylenediamineIt was.4 found, however, that 2, 6-dinitro-N1 ,N4-bis (phenylsul- phony1)-p-phenylenediaminecould be dimethylated, and removal of the protective phenylsulphonyl groups afforded N1,N4-dime t hyl-2,6-dinitro-P-phenylenedi-amine (4c) (Scheme). Curiously, methylation of the 2,3-, and 2,5-dinitrobis(phenylsulphonyl)-compounds ethanol, and the extinction coefficients for the longest wavelength band were determined in the first solvent. The absorption spectra were calculated by the PPP-SCF-MO method, using a limited configuration inter- action treatment including the first nine singly excited singlet states.The success of this procedure in handling related donor-acceptor substituted benzene chromo-phores indicates its general usefulness and, provided PHO PCHO NHMe NHMe NHMe (2c) (3c) SCHEME gave only monomethylation, and thus the monomethyl- amines (2b) and (3b) were obtained after hydrolysis in concentrated sulphuric acid (see Scheme). More severe methylation conditions, using methyl iodide or dimethyl sulphate and potassium hydroxide in boiling acetone, failed to give any dimethyl derivatives, and resulted only in decomposition of starting material. The required N1,N4-dimethyl derivatives of 2,3- and 2,5-dinitro-$-phenylenediaminewere obtained by direct methylation of the free amines with formaldehyde in sulphuric acid at 50 0C.5 Thus the red (2c) and blue (3c) were obtained in 59 and 89 yields respectively (see Scheme).Interestingly, methylation of the 2,6- dinitro-amine by this procedure afforded only a mono- methyl-derivative, and on steric grounds this was presumed to be the N4-methyl compound (4b). It was found that hydrolysis of the protected bis(pheny1-sulphonyl) derivatives and subsequent methylation with formaldehyde could be achieved in one reaction sequence, thus greatly simplifying the preparation of the methyl- derivatives (2c), (3c), and (4b). Electronic A bsorption Spectra.-The visible absorption spectra of the mono-and di-nitro compounds were measured in cyclohexane, dichloromethane, and absolute A. Halasz, Chem.and Ind., 1969, 1701. * J. Griffiths, M. Lockwood, and B. Roozpeikar, J.C.S. Perkin II. 1977, 1608. strictly planar structures are considered, the results of such calculations can be as useful as the considerably more lengthy all-valence electron procedures. An empirical procedure was adopted for the evaluation of suitable parameters, and it was found that published values7 for the nitro-group did not give satisfactory results for the nitroanilines, which were used as model compounds. Better nitro-parameters are listed in Table 1, and these gave excellent agreement between the predicted and experimental absorption maxima of the nitroanilines. Parameters for the amino-and di-methylamino-group have been described elsewhere,8 and those for the methylamino-group were evaluated empiri- cally (Table 1).For all the compounds studied a planar TABLE1 New parameters used in the PPP calculations Bond type x-Y zya PXYI eV VSIPYI evb YXY It~i Ad C-N 2 -2.75 18.5 9.26 1.38 (methylamino) C-N 2 -2.0 24.8 12.5 1.49 (nitro) N-0 1 -3.05 16.3 1.8 1.21 a Number of x electrons contributed by the atom. Valence state ionisation potential. Electron affinity. Bond length. geometry was assumed, although certain of these are known to deviate appreciably from molecular co-planarity. Hydrogen bonding between mutually ortho amino- and nitro-groups was neglected in these calcul- ations, as this should have only a relatively minor effect on the absorption spectra.H. Labhart and G. WagniGre, Helv. Chim.Acta, 1963, 46, 1314. J. Griffiths and M. Lockwood, J.C.S. Perkin I, 1976, 48. Corbett has examined the electronic absorption spectra of the mon~nitro-$-phenylenediamines,~and by relating the two longest wavelength bands to those of nitrobenzene a set of empirical relationships have been derived for predicting the A,, values of these and other J.C.S. Perkin I evident from a consideration of absorption intensities that mono-methylation of an amino-group causes little steric interaction with an adjacent nitro-group. The hydrogen bond existing between the two groups may serve to preserve molecular coplanarity. TABLE2 Experimental and calculated spectral data for the mononitro-p-phenylenediamines (1) xmax.(exp.)/nm Emax 1 mol-1 cm-1 f"I h R' R2 R3 R4 CeH1, EtOH CH2C12 (C6H12) (calc.) H H H H 441 473 465 3 900 461 0.18 Me H H H 472 49 8 500 5 100 473 0.18 H H H Me 459 498 485 3 790 468 0.17 Me Me H H 429 440 461 1 400 485 0.17 H H Me Me 461 484 491 3 300 474 0.16 H Me H Me 489 520 520 4 800 480 0.17 Me Me Me H 435 451 479 1100 49 2 0.16 H Me Me Me 489 505 522 4 500 487 0.16 Me Me Me Me 423 445 468 1200 501 0.16 Oscillator strength.amino-and hydroxy-substituted nitrobenzenes. It is interesting that the bathochromic shift accom- Examination of the PPP calculations for the nitro- and panying methylation, which is due to the increase in dinitro-9-phenylenediamines show, however, that the electron donating strength of the amino-group, depends visible absorption bands are complex transitions, critically on the position of the amino-group involved.involving predominantly electron-density migration Thus methylation of an amino-group ortho to nitro gives TABLE3 Experimental and calculated spectral data for the dinitro-P-phenylenediamines Emax.Amax. (exp.)/nm , 1mol-1 cm-1 * c Compd. R' R2 K3 R4 Camp;12 EtOH CH,C12 (CeH12) ., 2,3-Dinitro H H H H (2a) 472a 491 474 4 200 455 0.31 Me H H H (2b) 499 512 507 6 300 465 0.32 Me H Me H (2c) 530 539 539 6 900 478 0.32 2,5-Dinitro H H H H (3a) 518 560 540 4 600 489 0.29 Me H H H (3b) 558 592 588 4 950 506 0.28 Me H Me H (3c) 596 624 632 5 050 520 0.27 2,6-Dinitro H H H H (4a) 498 516 512 3 280 464 0.33 H H H Me (4b) 519 548 540 6 760 476 0.32 Me H Me H (4c) 504 532 525 5 060 489 0.32 Measured in benzene because of low solubility in cyclohexane.from the amino- to the nitro-group(s). Thus additive wavelength correlations with nitrobenzene probably have little theoretical significance. The experimental and calculated spectral data for the mononitro-9-phenylenediamines (1) are summarised in Table 2. The predicted wavelength values are in good agreement with the experimental values measured in cyclohexane, with the exception of those compounds possessing an N,N-dimethylamino-group ortho to the nitro-group. The latter compounds, i.e. (Id), (lg), and (li), will possess steric crowding, causing the amino- and nitro-groups to rotate out of full conjugation with the benzene ring.Thus hypsochromic shifts occur, and the theoretical A,, values are overestimated. The devi- ations from planarity are clearly indicated by the low extinction coefficients of these compounds. It is also greater shifts than for methylation of a rneta-amino-group. This suggests that the 1-amino-group is the dominant auxochrome, and the theoretical treatment predicts this effect well. The most bathochromic member of the nitro-series is the N1,N4,N4-trimethyl derivative (lh) A,,, (CH,C12) 522 nm which is violet. The red tetramethyl compound (li) absorbs at shorter wavelengths (468 nm) because of steric crowding, and its absorption intensity is correspondingly low.The experimental and calculated spectral data for the dinitro-9-phenylenediamines are given in Table 3. The introduction of a second nitro-group into the mono- nitro compounds (1) produces a general bathochromic shift of the visible band and the magnitude of the shift is dependent on the relative orientation of the two nitro- groups. Thus when the two nitro-groups are mutually 1978 +ara (i.e. 2,5) the shift in cyclohexane is of the order of 77-107 nm, whereas for the meta (2,6) orientation this is 15-57 nm, and for the ortho (2,3) orientation 27-41 nm. Thus for any given series of dinitro-derivatives the A,,, values generally follow the order 2,5 2,6 2,3. For the N1,N4-dimethyl compounds, however, the order is 2,5 2,3 2,6, and this can be attributed to the anomolous 2,6-isomer (4c) which has a bulky methyl- amino-group between two ortho nitro-groups. In this derivative only will loss of planarity occur, thus causing it to absorb at shorter wavelengths.This dependence of wavelength on the relative orientation of the nitro-groups is predicted accurately by the MO calculations (Table 3). Absolute agreement between the theoretical A,,, values and experiment is not as good as in the mononitro- series, however. The most bathochromic member of the dinitro-series, i.e. N1,N4-dimethyl-2,5-dinitro-p-phenylenediamine(3c), is predicted correctly, and this compound is blue-green in colour. The dinitro-compounds in general augment the shades provided by the mononitro-compounds, and give violet to blue-green colours on polymeric substrates.All the nitro-dyes show a positive solvatochromism between cyclohexane as solvent and dichloromethane or ethanol, and it is evident that the shifts are dependent on the hydrogen bonding characteristics of the solvent. For example, many of the higher alkylated mononitro- compounds show smaller shifts in ethanol than in dichloromethane. In the dinitro-series the solvato-chromic shifts are more consistent, and are always greater in dichloromethane. The charge-transfer character of the visible transition of the nitro-p-phenyl- enediamines results in an excited state of greater polarity than the ground state, and thus a positive solvato- chromism is to be expected. EXPERIMENTAL N1,N4,N4-Trimethyl-2-nitro-p-phenylenediamine(1 h) .-4-Fluoro-NN-dimethyl-3-nitroaniline ( 1.1 g) and methyl-amine (33 ethanolic solution, 10 ml) in ethanol (30 ml) were heated under reflux for 24 h.Water (15 ml) was added and the ethanol removed under reduced pressure to give black crystals, which were filtered off and dried (0.95 g, 81.5). Recrystallisation from a mixture of benzene-petroleum (b.p. 80-100 "C) gave (lh) as very dark, lustrous crystals, m.p. 97-98 "C (lit.,4 100 "C). N1,N1,N4,N4-Tetramethyl-2-nitro-p-phenylenediamine( li). -4-Fluoro-3-nitro-NN-dimethylaniline (0.8 g) was treated with dimethylamine (33 ethanolic solution, 12 nil) in ethanol (20 ml) as described for (lh).The product (li) (0.76 g, 84) was obtained as a dark red oil, homogeneous by t.1.c. analysis. The physical properties and absorption spectrum in ethanol were identical to those reported by Corbett.4 N4,N4-Dimethyl-2-nitro-p-phenylenediamine( le) .-4-Flu- oro-N,N-dimethyl-3-nitroaniline(0.55 g) and aqueous ammonia (d 0.88; 10 ml) were heated in ethanol (100 ml) at 110-120 "C for 24 h in a steel autoclave. The red solu- tion was evaporated to near dryness and diluted with water (30 ml). The precipitated solid was purified by chromato- graphy (alumina ; benzene-dichloromethane 1 : l), giving 1197 very dark red crystals of (le) (0.31 g, 56), m.p. 108- iiooc(iit.,g 1120~). N1-Methyl-2,5-dinitro-p-fihenylenediamine(3b) .--2,5-Dini- tro-N~,N4-bis(phenylsulphonyl)-~-phenylenediamine(0.2 g) was dissolved in sodium hydroxide solution (1~; 100 ml) and stirred vigorously with an excess of dimethyl sulphate at room temperature.The pH of the mixture was kept above 8-9 by the addition of more sodium hydroxide as required. After 15 h the precipitate was filtered off, washed with water, and dried, giving the lemon yellow monomethylated derivative (0.17 g, say0),m.p. 273-274 "C (decomp.). The methyl derivative was dissolved in concentrated sulphuric acid (25 ml) and kept at 15 "C for 30 h. When the mixture was poured into water a blue precipitate formed, which was filtered off and recrystallised from benzene, giving N1-methyl-2,5-dinitro-p-phenylenedi-amine (3b) as blue crystals (0.053 g, 72), m.p.216 "C (Found: C, 39.6; H, 4.0; N, 26.4. C,H,N,O, requires C, 39.6; H, 3.8; N, 26.4y0), vmx. (KBr) 3 390, 3 425, and 3 500 cm-l. N1-Methyl-2,3-dinitro-p-phenylenediamine (2b).-2,3-Di-nitro-Nl, N4-bis (phenylsulphonyl) -P-phenylenediamine (0.3 g) was methylated as described for the 2,5-dinitro-isomer. Hydrolysis of the monomethylated derivative in concen-trated sulphuric acid gave N1-methyl-2,3-dinitro-p-phenyl-enediamine (2b) as dark red plates (0.051 g, as),m.p. 187 "C (M+212; calc. for C,H,N404, M 212). N1,N4-Dimethyl-2,6-dinitro-p-phenylenediamine(4c) .-2,6- Dinitro-N1,N4-bis (phenylsulphonyl) -p-phenylenediamine was methylated with an excess of dimethyl sulphate as described for the 2,5-dinitro-isomer.The N1,N4-dimethyl derivative (0.13 g, 75) was hydrolysed in concentrated sulphuric acid to give N1,N4-dimethyZ-2, 6-dinitro-p-phenyl- enediamine (4c) as very dark red, lustrous needles (0.056 g, 94y0), m.p. 120 "C (M+226; calc. for C,HloN404, M 226), vrnBx.(KBr) 3 340 and 3 437 cm-l. N1,N4-DimethyZ-2,5-dinitro-p-Phenylenediamine (3c).-(a) 2,5-Dinitro-p-phenylenediamine(0.10 g) was dissolved in concentrated sulphuric acid (10 ml) and treated with methanolic formaldehyde solution (3774, 2 ml) at ca. 50 "C. After 3 h at this temperature, the mixture was poured onto ice, neutralised with ammonium hydroxide solution, and the blue precipitate filtered off. Recrystal-lisation from benzene gave N1,N4-dimethyZ-2,B-dinitro-p-phenylenediamine (3c) as lustrous black plates (0.102 g, 89), m.p. 277 "C (dec.) (Found: C, 42.2; H, 4.5; N, 25.0.C8H,,N,O4 requires C, 42.5; euro;3, 4.4; N, 24.8), vmX. (KBr) 3 420 cm-l. (b) 2,5-Dinitro-N1,N4-bis(phenylsulphony1)-P-phenylene-diamine (0.1 g) was dissolved in concentrated sulphuric acid (10 ml), and after 30 h at 15 "C further sulphuric acid (3 ml) and methanolic formaldehyde solution (37; 1 ml) were added. The temperature was raised to 50 "C for 4.5 h, and the mixture poured over ice. The blue precipitate, after purification by column chromatography (alumina ; benzene) gave (3c) as black plates (0.034 g, 72), identical to the product from (a). N1,N4-Dimethyl-2,3-dinitro-p-phenylenediamine (2c) .-2,3- Dinitro-N1,N4-bis(phenylsulphonyl)-~-phenylenediamine (0.1 g) was hydrolysed in concentrated sulphuric (10 ml) at 15 "C for 30 h.Sulphuric acid (3 ml) and methanolic formaldehyde solution (3774, 1 ml) were added, and the mixture heated at 50 "C for 45 min. The mixture was H. H. Hodgsonand J. H. Crook, J. Chem. SOC.,1932, 2976. poured over ice and neutralised with amonium hydroxide solution to give a purple solution which was extracted with ether. Removal of the solvent from the ethereal extracts gave a dark solid, homogenous by t.1.c. Recrystallisation from benzene-petroleum (b.p. 80-100 "C) gave N1,N4-dimethyl-2,3-dinitro-p-phenylenediamine(2c) as black leaflets (0.028 g, 59y0), m.p. 171-172 "C (Found: C, 42.7; H, 4.4; N, 24.3. C,Hl,N404 requires C, 42.5; H, 4.4; N, 24.8). N4-Methyl-2,6-dinitro-p-Phenylenediamine (4b) .-2,6-Dini- tro-p-phenylenediamine (0.1 g) was methylated with formaldehyde-sulphuric acid as described for the prepar- ation of (3c). Recrystallisation of the crude product from benzene-petroleum (b.p. 80-100 "C) gave N4-methyl-2,6- J.C.S. Perkin I dinitro-p-phenylenediamine (4b) as black needles with a metallic lustre (0.085 g, 79y0), m.p. 200-201 "C (Found: C, 40.2; H, 4.0; N, 26.2. C,H,N404 requires C, 39.6; H, 3.8; N, 26.4y0), vmax. (KBr) 3 350, 3 395, and 3 465 cm-l. The same compound was formed in 56 yield by hydrolysis of 2,6-dinitro-N1,N4-bis(phenylsulphonyl)-p-phenylene-diamine and subsequent methylation in concentrated sulphuric acid as described for (3c). We thank the S.R.C. for provision of mass spectrometry services and the Association of Commonwealth Universities for the award of a Scholarship (to K. Y.C.). 7/2102 Received, 1st December, 19771
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