Visible and PMR spectroscopies of 4-N,N-dimethylammonitrosobenzene and its methyl derivatives

Visibltttand PXE lapis of ~~,~~~~~~~~~~ and its methyl derivtttives GIN-ICTSU MATSUBA~A~ZIX, YASUO TAUYA and Tosnzo TANAKA Department of Chemistry, Faculty of Engiueeriug, O&a University Y&madakami, Suita, Osaka, Japan

AWraet--The tiible

absorption ape&m of 4-~,~-~ethyl~o~ (A), 2-m~hyl-4-~,~~methyl~o(B) aud 2,6-dimethyI-4-N,N-dimethylaminonitroso {C) heve been measnred. The solvent effects in ?z+ rr* end I -+ P* tr~ition bands of {A) were ex8miued. It was eonduded that the protic solvents and ~ethylt~ diubloxidein-t with the oxygen atom of (A), (B) and (G), and that protomxinter8etwith both the oxygen and the nitrogeu atoms of the %-O group. The PMR spectra of (A), (Bf and (G) were mwured. ‘Ike enisotropio effect of the N-lone p&r was e&mated.

INTRODUCTION IT is well known that the Ar-NO bond of 4-N,N-dimethylsminonitrosobenzene (A) has tl considerable double bond oharaoter due to the contribution of the resonance atrneture (II). The ?z.-+ TP and rr -+ V* transition bands of this compound are

observed in the visible region fl, 21. We have examined the solvent effect on the visible spectra, and the ~~r~ctio~ of (A) snd its methyl derivatives with protir, solvents, protons and ~methylt~ ~~~o~de. RewntlyS &JNI)BBRU f3] measured the PMR speotre of Z-~kyl~substi~~d nitrosobenxene and found that 2elkyl proton signs&3appem in an unusuehy lower Geld than those of the usual alkyls &ached to a+phenyl ring. It was considered as due to the paramagnetic anisotropy of the N-lone pair of nitroso group. On the other hand, in 4-N,N-dimethyl&minonitrosobenzene the 2-proton near the N-lone pair of nitroso group was assigned to & higher field than the other ((J-proton) [4,5]. This assignment is inconsistent with the above-mentioned one.

1852

GEN-ETSTJ MATSUBAYASHI,

YASUO

TAKAYA and

Tosmo TANAJU

We have measured the PMR spectra of the 2-methyl and 2,6dimethyl substituted 4-N,N-dimethylaminonitrosobenzene in dichloromethane and trifluoroacetic acid and clarified the paramagnetic effect of the N-lone pair of nitroso group. EXPERIMENTAL

Materials

4-N,N-Dimethylaminonitrosobenzene (A) [6], 2-methyl-4-N,N-dimethylaminonitrosobenzene (B) [7] and 2,6-dimethyl-4-N,N-dimethylaminonitrosobenzene (C) [8] were synthesized by nitrosation of N,N-dimethylaniline, N,N-dimethyl-3-toluidine and N,N-dimethyl-3,5-xylidine with sodium nitrite. Their melting points and analyticaldatawere; (A), m.p. 845-85@C (lit. 85%) [0]; Anal. Calc. for CsH,,N,O: C, 63.98; H, 6.71; N, 18-65. Found: C, 63.87; H, 6.92; N, 18.71. (B), m.p. 909-91.0% (lit. 92%) [7]; Anal. Calc. for C,H,&aO : C, 65.83; H, 7.37; N, 17.06. Found: C, 66.73; H, 7.32; N, 17.12. (C), m.p. 101-103°C (lit. 104’C) [8]; Anal. Calc. for C,,H,,N,O: C, 67.39; H, 7.92; N, 15.72. Found: C, 67.30; H, 7.73; N, 15.72. Table 1. Visible absorption data in 1,2-dichloroethrtne 7r+ R* transition ilm8X(mp) (9

Compound 4-N,N-dimethylsminonitrosobenzcne 2-methyl-4-N,N-dimethyleminonitrosobenzene 2,6-dimethyl-4-N,N-dimethylaminonitrosobenzenc

n + 7r* transition rlmsx (mp) (9

420

(33200)

707

(70)

422

(32200)

727

(68)

407

(31300)

748

(80)

4-Methoxy- and 4bromo-N,N-dimethylaniline were synthesized by the N,Ndimethylation of p-anisidine and p-bromoaniline according to the literature [9]. They were identified from the PMR spectra. Trifluoroacetic acid, N,N-dimethylaniline and 4-methyl-N,N-dimethylaniline were purified by distillation of commercially available reagents. The purifkations of dimethyltin dichloride, diohloromethane and 1,2dichloroethane were described in the previous paper [lo]. Spectrometers

The visible spectra were measured using a Hitachi 124 apectrophotometer with 1 cm and 1 mm quartz cells at room temperature. The measurements of PMR spectra were previously described [ 111. RESULTS AND DISCUSSION

Visible spectra

Table 1 shows the visible absorption data of the nitroso compounds in 1,2dichloroethane. The introduction of the methyl group shifts the rr -+ 7r*band to a [0] G. M. BENNE~ and E. V. BELL, Orgakc Syntkw, Coll. Vol. II, p. 223. John Wiley (1963). [7] C. WURSTERctndC. RIEDEL,Chum. Ber. 12, 1797 (1879). md A. NOLD, Ch.wn.Ber. 81,657(1898). [S] H. V. PIG[Q] IL FUKUI, Y. INAICIOTO, S. TAEASEand H. KJ.TANO,Hogyo Kugaku Zamhi 62, 632 (1969). [IO] G. MATSUBAYASEI, T. TMAKA and R. OEAWARA,J. Itaorg. Nd. Chena. So, 1831 (1968). [II] G. MATSUBAYASEI and T. TANAKA,ibid. 31,IQ63 (1969).

PMR speotroscopiesof 4-N,Ndimethylaminonitrosobenzene and its methyl derivtLtives 1853

shorter wave length and the n + rr*band to a longer wave length. These shifts may be att~bu~ to the decreasing ~n~ibution of the resonance form (II) [l, 21, because of the deviation of the N--O group from the aromatic plane due to the steric hindrance of methyl groups. However, the deviation would be small considering that the intensities of the w -+ W*or n + n’* bands are almost equal. Table 2 indicates the rr -+ rr* and n -+ n* bands of LL-N,N-dimethylaminonitrosobenzene (A) in various solvents. In general, as the resonance becomes stronger, the TabIe 2. Data of w-+ #

and 71) -+ X* transition bands of 4-N,~-~methyl~onitrosobe~ene in various solvents rr+ X* transition

n --+ #

transition

;:;

2;

n-hexane Carbon tetrachloride Ethyl ether Acetone Acetonitrile

393 400 399 418 421

736 725 720 707 702

Chloroform

418 422 411 425 433 439 441 441

700 062 718 683 654 622 587 -563

Solvent

BdE&bXlO~

Dioxane Dioxane : water = 4 : 1f 3:2 2:3 1:4 Water

Dielectric oonstant (E) of a solvent I.88 2.17 4.34 20.7 37.6 440

32-6 2.10 10.72: 25.86 42.50 60.81 78.54

f In volume ratio. $ From Ref. [12].

n --t rr* band is shifted to a shorter wave length and rr --t n* band to a longer wave Iength[l, 131. As a solvent becomes polar, the v + 7r*band appears at a longer wave length and the n + 1~*band at a shorter wave length. Therefore, in a polar solvent the polar resonance form (II) of the solute would be stabilized. The dependence of the band position on the dielectric constant (8) of a solvent is illustrated by the continuous variation of Ein dioxane-water mixtures. In water the red shift of the v -+ IT*band and the blue shift of the n + rr* band are extremely large, which would be due to the suflicient stabilization of the polar form (II) through the hydrogen bonding of these solvent molecules to the oxygen atom of (A). The si~ificant contribution of hydrogen bonding is also observed in the rather larger shifting of both bands in chloroform and in methanol than in other solvents with the similar E values to those of chloroform and methanol. In Fig. 1 are indicated the 72+ # absorption spectra of (A) in the presence of varying amounts of dimethyltin dichloride in 1,2-~chloroethane. As the red shift [X2] J. T-S, The Phyeioo-cknical &?&8ti?& of Binmy h’y&wns, Vol. 4, p. 16. Interscience (1960). [13] L. GOODMAN and H. SHULL,J. Ohem. Ph?llp.92, 1138 (1954).

1864

GEN-ETSU

MATSIJBAYASEI,

TAKAYA and TOSEIO TANAKA

YASUO

0 700t 0600 0500 E x= 0.400 3 8 0300 0200 0100 0 340

360

380

400

420 Wave

length,

440

460

460

500

mp

Fig. 1. The absorption spectra of 4-N,N-dimethylaminonitrosobenzene (2.07 x 10”’ mole/l.) in 1,2dichloroethane containing varying amounts of dimethyltin dichloride: (1) 0; (2) 3.53 x 10A2; (3) Q-81 X 10ea; (4) l-59 X 10-l mole/l.

of this band is as large as in water, the interaction of the solute with dimethyltin dichloride would be similar to that between the solute and water. Furthermore, the red shift suggests the nitroso oxygen atom-tin bond formation. This is consistent with the blue shift of the n + # band by the addition of dimethyltin dichloride (see Fig. 2). On the other hand, in the presence of protons the rr + 7r* band gives the blue shift snd the n 3 rr*band disappears ss is shown in Figs. 3 and 4, respectively, which indicate the spectral changes in the presence of varying amounts of trifluoroecetic

:: 5 0600.a $ B Q 0.400-

0.200-

0

' 500

I 600

I 700 Wave

length,

I 000

rnp

Fig. 2. The absorption spectra of 4-N,N-dimethylaminonitrosobenzene (7.04 x 10” mole/l.) in 1,2dichloroethane containing varying amounts of dimethyltin dichloride: (1) 0; (2) 1.72 x 10F2; (3) 3.47 x lOma; (4) 6-84 x 10d2; (6) 1.37 x 10-l mole/l.

PMR spectroscopiee of 4-N,N-dimethylaminonitrosobenzene

Wave

length,

and its methyl derivatives

1855

mp

Fig. 3. Spectra SB in Fig. 1 but with varying amounts of ktiuoroacetic acid: (1)

0; (2) tF88 x I@;

(3) I.72 x 10”; (4) 3.44 x 10-a; (5) 8.60 x IO-*; (6) 1.72 x 10w2; (7) 8.62 x 10S8 mole/l.

700

600 Wave length,

800

mp

Fig, 4. The absorption spectra of 4-N,N-dimethylaminonitrosobenzene (8.95 x lo-8 mole/l.) in 1,2-dichloroethane containing varying amounts of trifluoroeoetic a&d: (1) 0; (2) 4.37 x 10G; (3) 8.74 x lOa; (4) 1.75 X IO-“; (5) 3.50 x lVs mole[l.

acid. These phenomena are interpreted as N-protonation in addition to O-protonation.* The above-mentioned spectral ahanges (Figs. l-4) in (A) are also observed

in the cmes of (B) and (C). It is worthy to note that in the hydrogen bonding and the co-ordination of dimethyltin dichloride only the oxygen-interaction occurs, while in the case of protons both the oxygen- and the ~~ogen-~~ra~tion occur simultaneously, which would be due to the smaI1 size of the proton. * The protonation of the N(CH&, group would be neglected from the PMR re+mlts, which will be described later.

1866

GEN-ETSU MATSUBAYA~EI, YASUO TAKAYA and TOSHIO T~AXA

Aa is observed in Fig. 5, the Z-methyl proton signal of 2-methyl-4-N,N-~methylaminonitrosobe~ene (B) appears at an extremely low field in dichloromethane. In

I

2-CH,

NKH,),

6.00

I

I

700

I

(a)

I

I

I

I a 8.00

wm Fig. 6;. The N-CH, rurd 2-CH, PMR-signals of 2.methyl-4-N,N-dimethykninonitrosobenzene in (a) dichloromethane and (b) trifluoroacetic acid at 23%.

trifluoroacetic acid, the 2-methyl signal causes the high-field shift. These phenomena are interpreted by assuming that (B) would have the configuration (XV) in dichloromethane, owing to the steric hindrance of the methyl group with the oxygen atom. A similar co&gun&ion has already been suggested in 2-methyl-nitrosobenzene by STJ~DBERU [3]. It is to be noted that (B) does not contain dimers in solution &though 2-methyl-~t~be~ene contains a smsll amount of dimeric species [3]. Because of the par&magnetic anisotropy effect of the N-lone pair [3, 14, 151, the 2-methyl signal would appear at a lower field. In the presence of protons, the N-lone pair is attacked and its anisotropic effect disappears. This is consistent with the abovementioned result of visible spectra

[Ia] H. SA.ITO and K. NUKADA, J. MO.!. Spectrfl 18, 1 (1965). [16] J. U. VEIWAND and TE. J. DE BOER, Xpeotroc&m. A&a !%A, 376 (1969).

PMR spectroscopiesof 4-N,N-dimethyl~minonitrosobenzeneand its methyl derivatives 1867

In the PMR spectrum of (A), MaoNicol et al. sssigned the lower-field signal to the &proton rather than the 2-proton in (I) [4,5]. However, the paramagnetic anisotropy effect of the N-lone pair should shift the 2-proton to a lower field rather than the 6-proton. We have estimated the chemical shift differences of 2- and 6,- 3- and 5and 7- and &protons assuming that they are affected mainly by the paramagnetic anisotropy effect of the N-lone pair, and compared those with the observed values. According to the procedures of SAITO and NUKADA [14], the effects of the anisotropy of the N-lone pair have been calculated in the co-ordinate system (V), in which the z axis is taken along the spa hybridized N-lone pair orbital and the z axis is perpendicular to the aromatic ring plane.

The magnetic shielding constant for a proton is expressed as CT= fyd’a+ @=J where bdi” is the diamagnetic screening constant depending on the electron density around the proton and u?~ is due to the neighbouring paramagnetic anisotropy effect. As long as we take $,z = a, - Us, ASS8= us - u, and Aa,? = u8 - u,, it is sufficient to consider only a term, u = up*. It is because that the electron density on each carbon atom (6 and 2,5 and 3, or 8 and 7-carbons) is almost equal [16], and the contribution of Bi” is considered to be cancelled each other. and uz =

-

kH

(3 cos* e, -

1) p*

uy =

-

&

(3 co82 6, -

1) /A,

6, =

-

3&H

(3 cost 8, -

1) p*

where EI is the magnitude of the external magnetic field, R is the distance from the nitrogen atom to the proton in question, and O,,, OrIIis the angle between the cc,3 or [l&j H. LAB-T

and G. WAUNJBRE,Helv. Cl&n. Aota 48, 1314 (1963).

1888

DEN-ETSU MATSUBAYAE(IP, YASUO

TAKAYA md

TCNDIHITANAKA

z axis and the vector directed from the nitrogen atom to the proton. Three components of the paramagnetic moment are given by ,u~= 0, ,u, = A . H and p, = B . H, where A and B are constants. As the effect of pu, gives the same contribution to the above-mentioned i, j protons, At,, depends only on the difference of pv. Therefore, Ai,r would be given by Ai,j = K i, j = $2;

53;

8,7

where K is constant and the values in parentheses are called the structure parameter. Substituting the values of the structure parameter by making use of the X-ray crystal analysis data of the 4-iodonitrosobenzene Cl?], the Ai,, are estimated as follows. The agreement between the calculated and the observed values are satisfactory. (ppm)

Obs. (ppm)

1.81 0.40 O-08

1+at 0.32-t 0*04f

CdC.* A6.2 A6.3 Aa.7

* The caloulated value of Ass2ie taken aa the same as the observed value. t These values are obtained from Ref. [4] after the assignment-alternationbetween 2- and &protons or 3- and S-protons. $ From the present study.

N,N-dimethylaniline, 4-methyl, 4-methoxy- and 4-bromo-N,N-dimethylaniline give two N-CH, signals in trifluoroacetic acid, which is considered to be due to the spin-spin coupling (5.3 Hz) by a proton attached to the nitrogen atom of the dimethylamino groups. On the other hand, the compounds (A), (B) and (C) in trifluoroacetic acid show only one N-CH, proton signal, suggesting that protonation does not take place at the nitrogen atom of the dimethylamino group. Figure 6 shows the PMR spectra of 2,6-dimethyl-4-N,N-dimethylaminonitrosobenzene (C) at varying temperatures. Raising the temperature, the coalescence of N-methyl signals is observed, followed by the coalescence of 3,5-ring proton signals and finally that of 2,6dimethyl signals is observed. At room temperature (C) is in the rapid cotigurational equilibrium between (VI-a) and (VI-b). From the result of its visible spectrum, the N-O of (C) is situated near the aromatic plane. The PMR spectrum of (C) in trifluoroacetic acid shows two 2,6-dimethyl signals at higher fields(see Fig. 7). In this case, two effects would be important; one is the high-field shift due to the disappearance of the paramagnetic anisotropy of N-lone pair attacked by protons, and the other is the low-field shift due to the electron transfer from the aromatic ring to the proton. The former effect is fairly large. The assignment [l?] M. S. WEBSTXR, J. Chem. Sot. 2841 (1960).

PMB speotromopiea of 4-N,N-dimethylaminonitrosobenzene

and its methyl derivatives

1869

at 23%

ii.

--k

at -43Y

~

--b-

at -.52*c

7-M -CH,

~

8-H-CH,

~-CH~

I

I

I

I

3.00

4.00

5.00

6.00

1\““s

1 7.00

‘t

I

-65>

I

800

9.00

I 10.00

pm

Fig. 6, The PMR spectra of 2,6-dimethyl-4-N,N-dimethylwninonitrosobenzene diohloromethane at various temper&urea.

in

/6’HS

E&6,

N

(VI-a)

H3c\

N

/CH3

(VI-b)

of the 2,6dimethyl signals shown in Fig. 7 is made by reference to the 2-methyl signal of {B) in trifluoroacetic acid (see Fig. 5). In T&ble 3, the coalescence ~rn~r&t~es of methyl and ring proton signals of (A), (B) and (C) are irxlkted. The coalescence temperature, of the 2,6-(CH,), proton signals of(C) is lower than the 2,6-H, signals of (A). This is reasonable from the fact that the N-O group deviates slightly from the tcromstic ring-plane according to the visible spectral results and would be highly mobile between (VI-a) and (VI-b).

1860

GEN-ETSU

MATSUBAYASEI,

TAKAYA and TOSEIOTANAXA

YA+WO

N(CH,),

I

6-CH, E-CH,

TMS

3,5-.H, I 2.00

1

I

I 4.00

,

I

t

1, I 6.00

i-L;-T-ic I 8.00

10*00

pm

Fig. 7. The PMR spectrum of 2,0-dimethyl-4-N,N-dimethylaminonitrosobenzene in trifluoroaceticacid at 23%. Table 3. The coalescencetemper&ureaof methyl and ring proton signalsin dichloromethane(“C) Compound (‘=#C,H,NO (A) (CH,)W,H&H,)NO (CH,)W,H,(CH,),NO

2,6-H,

2,6-(CH,),

3,5-H,

NWH,),

-43

-57 -61 -60

20* (W (C)

* The value of (CH,)&C,H&NO(3,5-&)

-40

in deuterochloroformin Ref. [18].

Furthermore, it is notable that the anisotropic effect of N-O the N-CH, groups.

group rtffects even to

[lS] P. K. KOBVER,P. J. VAN DIR HAAE and TH. J. DE BOER, Tetrahedron22, 3157 (1966).