A Proton-magnetic-resonance Spectroscopic Investigation of Intramolecular Hydrogen Bonding in +Substituted- 2-nitrodiphenylamine Disperse Dyes

BRIAN J . DALE and DERRY W. JONES

School of Chemistry University of Bradford Bradford BD7 I DP

phenylamine, provided that the two protons are held in the ring plane in a trans-trans arrangement; the maximum value [ 101 of the coupling constant, J N H , s H is 0.65-0.70 Hz. An association between J N H , ~ H and internal hydrogen bonding has been recognised [ 111 in a series of 2-nitroarylamines, including 2,4-dinitrodiphenylamines and 2-nitrodiphenyl- amines further substituted in the unnitrated ring.

ARNOLD T. PETERS

School of Colour Chemistry University of Bradford Bradford BD7 1 DP

Detection of' long-range coupling constants, JNH,Hs, in 100-MHz proton-magnetic-resonance spectra confirms the presence of' intramolecular N H . . . . ON hydrogen bonding in some 2-nitrodiphenylamine disperse dyes. Measurements in polar and non-polar solvents on 2-nitrodiphenylamine and on twelve 4-substituted derivatives indicate weakening of the bonding, which is related to light fastness, when the 4-substituent is a strong eltPctron acceptor.

Introduction The relative fastness to light of nitrodiphenylamine dyes on synthetic-polymer fibres, whether assessed visually [ 11 or by methods involving the spectral reflectance curves of faded materials [ 1, 2.1 , indicates that 2-nitrodiphenylamine is more stable than 4-nitrodiphenylamine. This has been attributed [ 1, 3-51 to stabilisdtion of the 2-nitro isomer by ground-state intramolecular hydrogen bonding (1) and to the greater stability of the ortho-quinonoid charge-transfer excited state (11) than the corresponding para-quinonoid excited state of the 4-nitro isomer, Owing to electronic influences on the contri- bution of 11, the polarity of the 4-substituent may be correlated [ S ] with the position of the long-wavelength absorption band in the electronic spectra of these dyes; similar effects occur in the spectra of arylarninonitropyridines [6] and nitroarylarninopyridines [7].

0 0-

The presence of intramolecular hydrogen bonding in 2-nitroarylamines may be indicated by infrared spectroscopy 18, 91 ; NH-stretching frequencies of 4-substituted-2-nitro- diphenylamines have been related to the Hammett a-constants of the substituent [ 5 ] . Proton-magnetic-resonance (p.m.r.) spectroscopy may enable stereospecific five-bond ('long- range') spin-spin coupling to be detected along the 'zig-zag' path between the NH proton and the 5H ring proton (111) in a

A6J " X

( 1 1 1 )

We report here p.m.r. spectroscopic measurements on 2-nitrodiphenylamines containing substituents in the nitrated ring. In view of the non-planar configuration [12] of diphenylamine, these substituents would be expected to affect hydrogen bonding more noticeably than substituents in the unnitrated ring; in the latter, polarity has less influence, as is evident from electronic spectral data [ 5 , 131 and from studies [ 141 on the ionisation of substituted diphenylamines.

Experimental The nitrodiphenylamine dyes were prepared as described previously [6, 151 . Solvents were: chloroform - d l (Prgchem Ltd); acetone - d6 (Stohler); dimethyl sulphoxide - d6 (Prochem Ltd); and nitromethane (BDH Ltd). The p.m.r. spectra of acetone - d6 and dimethyl sulphoxide - d6 showed the presence of water (about 0.1%), which was removed by drying the solvents for several days over molecular. sieve type 3A (BDH Ltd). Solutions in these two solvents were prepared in a dry box and filtered through small glass beads before the spectra were recorded. The n.m.r. tubes were sealed with Parafim (Gallenkamp); spectra recorded within one hour of preparation of the solutions showed no presence of water.

Spectra of 6% wt/wt dye solutions, internally referenced with 1-274 vol./vol. of TMS, were recorded at 100 MHz and 298°K on a JEOL MH- 100 operating in the field-sweep mode with TMS as the internal lock. The instrumental resolution was maintained at 0.4-0.5 Hz, but a few samples gave resonances with a half-height width greater than 0.5 Hz; for some of these compounds in chloroform and nitromethane, improved resolu- tion, particularly of the 5H resonance, was attained by outgassing for several minutes with dry nitrogen. The presence of a long-range NH-5H coupling was confirmed by decoupling at chemical shift 6 N H and/or addition of a few drops of D, 0; coupling constants, J , were measured from a 54-Hz sweep width.

For the compounds in which the 4-substituent was H, Me, C1, and F, the 5H resonance was obscured in the normal absorption spectrum, so that decoupling at 6 3 H (to locate the 5H resonance) and subsequently 6 N H was inconclusive about J N H , ~ H. Consequently, the INDOR spectrum for these com-

JSDC March 1974 101

TABLE 1

P.m.r. Parameters for Some 4-Substituted-2-nitrodiphenylamines

4-Substituent and solvent

CN

CDC13 Acetone - d6 DMSO d,

SO2 NEt2 CDCl3 Acetone d6 CH3 NO2 DMSO d6

Br

Acetone - d6 DMSO - d6

COOEt

CDC13 CH3 NOz

CDCl3

Acetone - d6 DMSO - d6

Chemical shifts in p.p.m. downfield from TMS 6H 5H 3H NH CH3 CH2 J N H , H S

(HZ)

7.12 7.50 3.50 7.97 0.6-0.7 7.33 7.83 8.67 10.02 0.6-0.7 7.30 8.01 8.82 10.18 absent

7.18 7.66 8.63 9.74 1.16 3.22 0.6-0.7 7.39 7.92 8.69 9.96 1.18 3.29 0.6-0.7

7.40 8.03 8.66 10.12 1.12 3.25 absent 7.36 7.82 8.64 9.84 0.6-0.7

7.04 7.36 8.29 9.41 0.6-0.7 7.28 7.70 8.38 9.57 0.6-0.7 7.32 7.85 8.46 9.69 0.6-0.7

7.14 7.97 8.89 9.76 1.39 4.37 0.6-0.7 7.28 8.04 8.86 9.84 obs. obs. 0.6-0.7 7.30 8.09 8.87 9.92 1.36 4.37 0.6-0.7 7.37 8.19 8.90 10.14 1.37 4.43 absent

Aza

CDCl 6.93 8.24 9.26 Acetone ~ d6 7.06 8.36 9.24 DMSO - d6 7.03 8.49 9.36

NO2 CDCl3 7.17 8.17 9.15

CH3 NO2 7.31 8.26 9.13 DMSO - d 6 7.33 8.47 9.15

Acetone -d6 7.35 8.36 9.14

*u.c. denotes unresolved coupling

9.64 0.6-0.7 9.83 0.6-0.7 0.1 1 absent

9.95 0.6-0.7 0.24 absent

0.45 absent 0.08 0.6-0.7

pounds, in which [16] a single transition is monitored under non-saturating conditions by a frequency f, while the decoupling frequency f2 is swept, was recorded at 90 MHz on a Perkin Elmer R32 or at 100 MHz on a JEOL PS-100. Three types of signals can result, viz. zero, if f2 perturbs transitions with no energy level in common with the line monitored; and positive or negative if the connection of lines by a common energy level is ‘progressive’ or ‘regressive’ respectively.

Results and Discussion

COUPLING CONSTANTS Table 1 shows the p.m.r. parameters of some 4-substituted-2- nitrodiphenylamines in several solvents; protons of the unnitrated ring are omitted and will not be discussed. Except for 2-nitrodiphenylamine, all compounds have an ABX-type spectrum for the aromatic protons (with 3H at lowest field). Where the 5H peak was visible, it contained eight lines as a result of coupling to the amino proton as well as to the 3H and

4Substituent Chemical shifts in p.p.m. downfield from TMS and solvent 6H 5H 3H NH CH3 CH2 J N H , H ~

CH3 CDC13 Acetone - d, DMSO - d6

c1 CDC13 Acetone - d6

F CDC13 Acetone - d b DMSO - d6

SO2 NH2

Acetone - d6 CH3 NO2 DMSO - d,

SO2 CH3

CDC13 Acetone - d,

CF3

Acetone - d6 DMSO - d6

H

CDClj Acetone - d6

DMSO - d6

CDC13

DMSO - d6

7.6 7.6 7.96 9.32 8.07 9.39 8.17 9.49

7.12 7.28 8.16 9.39 8.25 9.58

7.92 9.33 8.00 9.41 8.16 9.52

7.35 7.97 8.74 9.88 7.42 7.95 8.78 9.90 7.44 8.07 8.80 10.06

CH3 7.33 7.93 8.92 9.98 7.39 8.01 8.78 10.02 3.19 7.42 8.15 8.83 10.21 3.30

7.30 7.82 8.61 9.82 7.53 7.96 8.70 10.07 7.42 7.98 8.61 10.06

4H

7.5 8.20 9.46 6.72 8.25 9.53 6.93

7.7 8.38 9.65 7.09

(i-12)

Obscured Obscured 0 b scu red

0.6-0.7 Obscured

Obscured Obscured 0 b scu red

Absent

Absent 0.6-0.7

0.6-0.7 0.6-0.7 u.c.*

0.6- 0.7 Absent Absent

0.5-0.6

U.C.

6H. The J N H , S H coupling of 0.6-0.7 Hz is in agreement with the results of Karabatsos and Vane [I71 and of Gale and Wilshire [ 1 1 1 . For 2-nitrodiphenylamine in CDC13 complexity of the aromatic spectrum obscures the 5H peak; however, if a line of the 4H resonance is monitored, the INDOR spectrum (Figure 1) clearly reveals a coupling of 0.5-0.6 Hz between the 5H and NH protons. Although the long-range coupling was identified for 2-nitro-4-chlorodiphenylamine in CDCI3 by the INDOR method, INDOR spectra of the 4-methyl and 4-fluoro derivatives were inconclusive about J N H , S H for the fluoro compound; coupling between 5H and F complicates the spectrum. For the methyl derivative, there is a long-range coupling (approximately 1 Hz) between 5H and CH3 and also the small internal chemical shift between 5H and 6H at 90 MHz results in a deceptively simple ABX spectrum.

J N H , ~ H is present in CDC13 and CH3N02 for all corn- pounds in which 5H was assigned. However, in acetone, the coupling and so, presumably, the N-H . . *O bond are absent when the 4-position is occupied by certain (e.g. CF3, SO2 NH2

102 J SDC March 1974

Figure I ~ Normal (lower) and INDOR (upper) 90-MHz spectra oj" 2-nitrodiphenylarnine in CDC1,; INDOR spectrum was obtained by monitoring line a. Chemical shifts are in p.p.m. from TMS

and NO2), but not all, electron-withdrawing groups. In dimethyl sulphoxide (DMSO), the compounds exhibit a long-range coupling only when the 4-substituent is Br, H, or SOzMe. In all cases the coupling is less pronounced than, but of similar magnitude to, that in CDCI3. The 5H peak could be sharpened by either decoupling at ~ N H or by addition of a few drops of D,O (deuterium replaces the amino proton). The presence of an intramolecular hydrogen bond for 2-nitro- diphenylamine in DMSO is in agreement with the observation of the int ramolecularly hydrogen-bonded NH infrared stretch- ing frequency in DMSO [ 181 .

Most of the ortho and meta coupling constants of the substituted ring, J5 ,6 and J3 ,5 , are about 9.5 Hz and 2.4 Hz, respectively. However, for 2,4dinitrodiphenylamine in acetone, J5,h and J3,5 are 10.2 Hz and 2.8 Hz, respectively, and 4-aza-2-nitrodiphenylamine has J5 ,6 about 6 Hz (J3 ,5 was undetected). The coupling constants are insensitive to a change in solvent.

In CDC13 solution, the coupling constants of 2-nitro- diphenylamine, extracted by first-order analysis, are: J4,6 1.9 Hz, J4,5 6.5 Hz, and J3,4 8.5 Hz. In DMSO they are: J4,6 1.6 Hz, J 3 , 5 1.7 Hz, J5,6 8.5 Hz, and J4,5 6.8 Hz. Inview of the correlation [ 191 between n-bond-order and Jorlho, the larger J 3 , 4 and J5,6, than J4,5 are indicative of a significant contribution of the ortho-quinonoid form (I) to the ground state of 2-nitrodiphenylamine.

CHEMICAL SHIFTS It was hoped to use the NH chemical shifts (Table 2) as additional information to J N H - 5 H and as an indication of the presence of intramolecular hydrogen bonding for compounds in which the 5H absorption was obscured. ~ N H is significantly affected by formation of an intramolecular hydrogen bond such that, in CDC13, values of 5.62, 6.30, and 9.46 p.p.m. are observed for diphenylamine, 4-nitrodiphenylamine, and 2-nitrodiphenylamine, respectively. In addition, for diphenyl- amine, intermolecular hydrogen bonding between the amino proton and solvent, particularly DMSO, causes large downfield shifts for ~ N H ; the values in CDC13 and DMSO are 5.62 and 8.35 p.p.m., respectively.

The general downfield movement of most chemical shifts (Table 1) as the solvent changes from CDC13 to acetone to

DMSO does not appear to be characteristic of intramolecular or intermolecular hydrogen bonding, but rather to result primarily from solvent shifts of the aromatic protons and TMS (after correction for bulk susceptibility differences, 8 ~ M s in acetone is about 0.15 p.p.m. to high field of 8 ~ ~ s in CDC13). This interpretation contrasts with a previous suggestion [20] for 2,4dinitroaniline that rupture of the intramolecular hydrogen bond causes a concomitant upfield shift of 6 3 H .

TABLE 2

Chemical Shifts, ~ N H , of some Diphenylamines (p.p.m. Downfield from TMS)

4-Nitrodiphenylarnine 8 N H

in CDC13 6.38

Dipheny larnine

in CDC13 5.62 in CH3 NO2 6.4 in DMSO - d6 8.35

Conclusions The presence of an NH. . . O=N intramolecular hydrogen bond is confirmed in a range of substituted diphenylamines. In the parent 2-nitrodiphenylamine, persistence of the J l r j ~ , ~ 5 coupling in the highly polar dimethyl sulphoxide solution is evidence of the strength of the hydrogen bond, expected from the high photochemical stability of this compound.

For 2,4-dinitrodiphenylamine, on the other hand, in which J N H , H ~ coupling is not detected in acetone and DMSO, the readier disruption of hydrogen bonding is in accord with the lower photochemical stability of 2,4-dinitrodiphenylamine than 2-nitrodiphenylamine.

All the 4-substituted-2-nitrodiphenylamines in Table 1 are very fast to light on synthetic-polymer fibres; differences in stability associated with polarity of the 4-substituent are so small that they are not readily distinguished by visual methods of assessing light fastness. In that the strength of a bond can be gauged from its persistence in polar solvents, however, p.m.r. shows that the 4-substituent can influence hydrogen bonding. Thus, although all dyes in Table I have a perceptible J N H , H ~ in deuterochloroform, progressively few retain it as the solvent polarity increases to acetone and to DMSO; admittedly, in some cases, the aromatic peaks were so condensed at 100 MHz that INDOR experiments could not resolve any coupling involving the obscured 5H resonance.

P.m.r. evidence for the weakening of the intramolecular hydrogen bond, when, as in most of these dyes, the 4-substi- tuent X is a strong electron acceptor, confirms indications from shifts of the long-wavelength absorption bands in the elctronic spectra [5]. Finally, it should be emphasised that relations between light fastness and estimates of hydrogen- bond strength derived from retention of the J N H , ~ H in polar solvents are at best only qualitative.

* * *

We thank Dr R. S. Spragg (Perkin-Elmer Ltd) and Dr J. H.

JSDC March 1974 103

Beynon (Joel Ltd), for assistance with INDOR spectra and Wira for financial support to B.J.D.

(MS. received 24 July I9 73)

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