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NH stretching frequencies and the structures of I2 and IBr complexes with piperidine and piperazine
J. inorg, nucl. Chem., 1973. Vol. 35, pp. 101- t07.
Pergamon Press.
Printed in Great Britain
NH STRETCHING FREQUENCIES AND THE STRUCTURES O F 12 A N D I B r C O M P L E X E S WITH PIPERIDINE AND PIPERAZINE Y. OKISHI, Y. IMAI and K. A I D A * Department of Applied Science, Faculty of Engineering, Tohoku University, Sendai, Japan (Received 3 April 1972)
Abstract-Molecular addition compounds of 12 and IBr with piperidine, piperazine, N-methyl-, Nphenyl- and N-benzyl-piperazine have been prepared. Piperazine and N-methyl-piperazine form two series of complexes, i.e. 1 : 1 and 1 : 2 mole ratios in amines to halogens. The structures of these complexes have been discussed from their N H stretching frequencies. INTRODUCTION
IT IS KNOWN that piperidine and piperazine form charge-transfer complexes with sacrificial "art-type" acceptors. For example, Yada et al. [1] have observed a charge-transfer band for piperidine-I2 in n-heptane solution. The stoichiometries and the melting points have been reported for piperidine-ICl[2], piperazineBr2 [3] and -I2 [3]. Person et al. [4] have studied an i.r. spectrum for piperidine-lCN complex with main interest in the I-C stretching vibration. As there are two active sites in piperazine, it is of interest to see whether it can form 1 : 1 complexes as well as 1 : 2 with halogens. 1, 4-Dioxan, which has a similar skeleton with that of piperazine, forms a 1 : 1 complex with Br2. The X-ray study reveals that it contains "halogen bridges" [5]. On the other hand, 1 : 2 complexes are formed by 1, 4-dithian, which also has the same skeleton, with I2 and IBr. McCuUough et al. have reported the X-ray studies on these complexes [6, 7] and have shown that the dithian ring is in the chair conformation with each Is or IBr molecule attached to sulfur in equatorial position. No structural study has yet been reported on piperazine-halogen complexes. The aim of the present study is to obtain evidence relating to the structures of piperidine and piperazine complexes with Is and IBr from their N H stretching frequencies. EXPERIMENTAL Chemicals Piperidine, N-methyl- and N-phenyl-piperazine were obtained from Tokyo Kasei Co. They were
*To whom all correspondence should be sent. 1. H. Yada, J. Tanaka and S. Nagakura, Bull. chem. Soc., Japan 33, 1660 (1960). 2. R. D. Whitaker, J. R. Ambrose and C. W. Hickam, J. inorg, nucl. Chem. 17, 254 (1961). 3. N. P. Yavorskii and S. N. Garasevick, Ukr. Khim. Zh. 35, 941 (1969); Chem. Abstr. 72, 6666e (1970). 4. W, B. Person, R. E. Humphry and A. I. Popov, J. Am. chem. Soc. 81, 273 (1959). 5. O. Hassel and J. Hvoslef, Acta chem. scand. 8, 873 (1954). 6. G. Y. Chao andJ. D. McCullough,Acta Crystallorg. 13,727 (1960); ibid. 14, 940 (1961). 7. C. Knobler, C. Baker, H. Hope and J. D. McCullough, lnorg. Chem. 10, 697 (1971). 101
102
Y. OKISHI, Y. IMAI and K. A I D A
dried with metallic sodium and then distilled under reduced pressure. N-Ethyl- [8], N-propyl- [9] and N-benzyl-piperazine[10] were synthesized from N-formyl-piperazine and piperazine monohydrochloride, respectively, dried with metallic sodium and then distilled under reduced pressure. Carbon tetrachloride was dried with phosphorous pentoxide at its boiling temperature for several days and fractionated. Only constant boiling middle cuts of carbon tetrachloride and amines were collected. Piperazine was recrystallized from carbon tetrachloride for several times. Iodine was purified by sublimation with barium oxide and potassium iodide. Iodine monobromide was prepared and purified by the method of Cornog and Karges [11].
Preparation of the complexes About 2 × 10-2 mole/1, solution of halogen in carbon tetrachloride was added dropwise to an amine solution of about 4 x 10-2 mole/I, in the same solvent with constant stirring at 0°C. The solid product was collected by suction filtration, washed with solvent thoroughly and placed in a desiccator. NEthyl- and N-propyl-piperazine seem to react with halogens, giving no solid addition compound. For piperazine and its derivatives, the effect of the ratio of the reagents upon the stoichiometry was investigated. When the excess amount of amines was added than 1 : 1, complexes having a 1 : 1 composition were obtained except piperazine and N-benzyl-piperazine with IBr. In these two cases, complexes seem to be unstable to be isolated. On the other hand, 1 : 2 complexes were precipitated when the excess amount of halogens was added than 1 : 2, except N-phenyl- and N-benzyl-piperazine. Varying the molar ratio in these two cases had no effect on their stoichiometry, giving only 1 : 1 complexes. The analytical results for these complexes are listed below. All compounds have been previously unreported except piperazine-Iz [3]. Piperidine-lBr*. Yellow powder, d.p. 72-3°C. Found: H, 3.80; C, 20'57; N, 4"80. Calc. for CsHIIN. IBr: H, 3.82; C, 19.60; N, 4.53. Piperazine-12. Yellow powder, d.p. 83-5°C. Found: H, 8.51 ; C, 14.32; N, 3.09. Calc. for C4H10N2. Is: H, 8.24; C, 14-13; N, 2.97. Piperazine-212. Lemon-yellow powder, d.p. 90-2°C. Found: H, 1.70; C, 8.09; N, 4.72. Calc. for C4H10N2.2I~: H, 1.70; C, 8.56; N, 4.61. Piperazine-21Br. Yellow powder, d.p. 129-31°C. Found: H, 2.02; C, 9.61; N, 5.61. Calc. for C4H10N2.2IBr: H, 1.99; C, 10-05; N, 5.31. N-methyl-piperazine-lz. Lemon-yellow powder, d.p. 75-6°C. Found: H, 3.46; C, 17.42; N, 7.89. Calc. for CsH12N~.I2: H, 3.42; C, 16.97; N, 7.91. N-methybpiperazine-212. Lemon-yellow powder, d.p. 66-7°C. Found: H, 1.84; C, 10.00; N, 4.77. Calc. for CsHI2N~.2I~: H, 1.92; C, 9.88; N, 4-61. N-methyl-piperazine-IBr. Yellow powder, d.p. 79-80°C. Found: H, 4.10; C, 20.34; N, 9.00. Calc. for C~H12N~.IBr: H, 3.97; C, 19.56; N, 9.13. N-methyl-piperazine-21Br. Yellow powder, d.p. 80-1°C. Found: H, 2-33; C, 12.07; N, 5.64. Calc. for CsH12N~.2IBr: H, 2.35; C, 11.69; N, 5.45. N-phenyl-piperazine-Iz. Lemon-yellow powder, d.p. 71-2°C. Found: H, 3.31; C, 28.77; N, 6.77. Calc. for C10H14N2.I., H, 3.39; C, 28.87; N, 6.73. N-phenybpiperazine-lBr. Yellow powder, d.p. 81-2°C. Found: H, 3.80; C, 33.01; N, 7.71. Calc. for C10H14N~.IBr: H, 3"82; C, 32'55; N, 7.59. N-benzyl-piperazine-12. Lemon-yellow powder, d.p. 62-40(:. Found: H, 3.89; C, 30.98; N, 6.50. Calc. for C11H16N2.I2: H, 3.75; C, 30.72; N, 6.51.
Spectroscopy I.R. spectra were recorded with a Perkin-Elmer Model 337 Spectrophotometer in the region from 4000 to 400 cm -~ on Nujol and hexachlorobutadiene mulls. * Piperidine-I~ was not precipitated. 8. K. Fujii, K. Tomino and H. Watanabe, J. pharm. Soc., Japan 74, 1049 (1954). 9. A. L. Mndzhoyan, M. T. Grigoryan, Yu. N. Sheinker, R. A. Aleksanyan, S. S. Vasil'yan, A. A. Kaldrikyan and I. A. Dzhagtspanyan, Arm. Khim. Zh. 21, 603 (1968); Chem. Abstr. 70, 96754t (1969). 10. J. C. Craig and R. J. Young, Organic Syntheses, Vol. 42, p. 19. John Wiley, New York. (1962). 11. J. Cornog and R. A. Karges, Inorganic Syntheses, Vol. 1, p. 165. McGraw-Hill, New York (1939).
N H stretching frequencies
I 03
RESULTS AND DISCUSSION
The observed N H stretching frequencies for solid complexes are listed in Table 1. The spectra between 2000 and 400 cm -1 are fairly complicated and could not to be assigned to certain vibrations at present. As they are not of major importance to this work, the observed frequencies for only two cases are given later (Tables 2 and 3). It is seen from Table 1 that all N H stretching frequencies appear above 3000 cm -1, which indicates that these complexes are not salts because the N H ÷ stretching band lies below 3000 cm -1 [12]. Hendra and Powell have studied the i.r. spectra of piperazine and its metal complexes [13] and have shown that a 1:2 complex with C2H4.PtCI2 contains a piperazine molecule in a chair configuration, bridging two acceptor molecules. The i.r. spectra between 2000 and 400 cm -1 of piperazine-212 and -2IBr are quite similar to that of piperazine-2(C2H4.PtCl2), as is seen from Table 2. This suggests that the structure of piperazine-2IX(X = I, Br) is likely the one given in Fig. !, which is analogous to that of C2H4.PtCIz complex proposed by Hendra and Powell. N-Methyl-piperazine-2IX complexes probably have the same structure, for the N H stretching frequencies are observed at almost the same positions with those of piperazine-2IX. N-Phenyl- and N-benzyl-piperazine do not form 1:2 complexes with halogens, even though excess halogens than 1 : 2 were added. This is probably partly due to Table
1. Observed NH stretching frequencies (cm -1) Piperidine-IBr Piperazine-212
3118 3215 3274, 3050 3126 3200 3068 3147 3050 3128 3114 3064
-I2 -21Br
N-Me-piperazine-212 -12 -2IBr -IBr
N-Ph-piperazine-I2 -IBr
N-By*-piperazine-I2 * Benzyl. H
I
XI---N~
~ N---IX
I
H Fig. 1. Proposed structure for piperazine-2IX (apart from their conformations of N H
groups). 12. L. J. Bellamy, The Infrared Spectra of Complex Molecules, 2nd Edn., p. 260. John Wiley, New York (1958). 13. P. J. Hendra and D. B. Powell, J. chem. Soc. 5105 (1960); Spectrochim. Acta 18, 299 (1962).
104
Y. OKISHI, Y. IMAI and K. AIDA Table 2. Infrared spectra (2000-400 cm -1) for 1 : 2 complexes of piperazine (cm -~) Pr*-2(C2H4.PtCI~)T 1463(w) 1452(m) 1432(ms) 1368(m) 1346(w) 1250(ms)¢ l157(vs) 1073(m) 1010(s)~ 882(vs)
Pr*-212
Pr*-2IBr
1446(m) 1410(m) 1369(m) 1345(w) 1248(m)
1445(m) 1413(m) 1360(m) 1320(w) 1250(m)
1087 (m) 995 (m) 868 (s) 640 (m)
1078(m) 1017(m) 872 (s) 651 (m)
s, strong; m, medium; w, weak; v, very. *Piperazine. fRef.[13]. Bands due to C2H4 group are skipped. ~tHendra and Powell have assigned these two bands to C2H4 group. However, as these bands appear constantly in all metal halide complexes, it is more probable to think that these bands originate both from C2H4 and piperazine.
the steric hinderance of the substituents and partly to the lesser donor property of the nitrogen atom caused by the conjugation. In conclusion, 1:2 complexes of piperazine and N-methyl-piperazine with halogens possibly have a 1,4-dithian2IX type structure shown in Fig. 1. Taking this structure of 1 : 2 complexes into consideration, let us now consider the structure of 1 : 1 complexes. It is known that piperidine exists mostly in a chair form[14]. This is also the case for piperazine[13]. Hendra and Powell have reported the i.r. spectra for 1 : 1 complexes ofpiperazine with HgCI2 and CdC12 [13]. The frequencies for HgC12 complex are reproduced in Table 3. They are more complicated than those of piperazine-2(C2H4.PtCl~) given in Table 2. They have proposed a polymer chain structure for these HgClz and CdCI2 complexes. The i.r. spectrum of piperazine-I2 in many respects resembles that of HgC12 complex (Table 3), so it seems probable that the I2 complex has also a polymer chain structure. Then, we can expect two different types for its structure, besides the existence of the axial and equatorial conformers. These are a "halogen bridge" type (Fig. 2(a)), as is found in 1,4-dioxan-Br2 [5], and a "hydrogen bridge" type (Fig. 2(b)), in which one of the nitrogen atoms, N °, is involved both in the hydrogen bond formation and in the charge-transfer interaction. In the former "halogen bridge" type, two N H groups would be expected to be equivalent with each other and the situation around the N H groups is analogous 14. N. L. Allinger, J. G. D. Carpenter and F. M. Karkowski, J. A m . chem. Soc. 87, 1232 (1965).
NH stretching frequencies
105
Table 3. Infrared spectra (2000-400 cm -1) for 1 : 1 complexes of piperazine (cm-') Pr*-HgC12t
Pr*-I2
1447 (m) 1418 (ms) 1377 (m) 1347 (w) 1314(w) 1256 (w) 1174 (vw) 1130 (vw) 1110 (m) 1072 (vs) 1040 (vw) 1017 (m) 1000 (m) 966 (vw) 890 (w) 879 (ms) 858 (vs) 690 (wm) 673 (m) 656 (vw) 458 (vw)
1434 (m) 1429 (m) 1380 (m) 1351 (m) 1312(m) 1212 (m) 1181 (m) 1128 (s) 1109 (w) 1081 (s) 1058 (m) 1029 (s) 922 (m) 890 (w) 866 (s) 850 (vs) 720 (w) 626 (s) 468 (m) 435 (m)
s, strong; m, medium; w, weak; v, very. *Piperazine. tRef. [13]. H
(O)
H
2
la " ~ N ""H -- N ~-..~ b
a
(0,
12
kS_
Fig. 2. Proposed structures for piperazine-I2 (apart from their conformations of NH groups): (a) "halogen bridge" type; (b) "hydrogen bridge" type. to that of the 1 : 2 complex. Therefore, only one NH stretching band (or closely s e p a r a t e d d o u b l e t if t h e i n t e r m o l e c u l a r c o u p l i n g is s t r o n g ) n e a r t h e s a m e p o s i t i o n w i t h t h a t o f t h e 1 : 2 c o m p l e x , will b e e x p e c t e d . O n t h e o t h e r h a n d , t w o N H g r o u p s
106
Y. OKISHI, Y. IMAI and K. A I D A
in the "hydrogen bridge" type have different natures. One of them, Nail a is expected to be weak as this group takes part both in the hydrogen bond formation and in the charge-transfer interaction, resulting in a bathochromic shift for its stretching frequency from that of the 1 : 2 complex. This in turn will make another group, NbH b, to be slightly free, resulting in a hypsochromic shift for its vibration from that of the 1:2 complex. The N H stretching region for piperazine-I2 is compared with that ofpiperazine212 in Fig. 3. It is seen that two N H stretching absorptions separated by about 200 cm -1, one is lower and the other is higher (and sharper) than that of the 1 : 2 complex, are appeared in the case of the 1 : 1 complex, suggesting that the "hydrogen bridge" type structure for this 1 : 1 complex. The N H stretching frequencies for N-methyl-piperazine-IX are observed at almost the same position with the lower band found in piperazine-I2 (Table 1). This is in accordance with the expectation on the basis of the "hydrogen bridge" structure, for the H b atom has now been replaced by the methyl group, leaving Nail a group only in these complexes. In order to ascertain this further, the N H stretching frequencies for N-phenylpiperazine-IX were measured. A phenyl group reduces the donor property of the N b atom, resulting in a weaker hydrogen bond, Nb...H~--N ~. This in turn should shift the N~H a stretching frequency to a shorter wavelength side. This is exactly the case, as is seen from Table 1. That the shift is not caused by the steric effect, is apparent from the result found in the benzyl derivative. There remains one problem concerning to the configurations of the N H groups
tO
7~ ttl t-~
I
I
I
I
I
3200
3000
Wovenumber,
c m -I
3400
Fig. 3. N H stretching region for piperazine-212 (upper) and piperazine-I2 (lower).
NH stretching frequencies
107
in these complexes. Unfortunately, nothing can be said about their conformations from this study. Attempts to obtain the deuterated complexes were unsuccessful. Partly deuterated compounds gave very complicated spectra and no additional evidence relating to the structures was deduced.
Pergamon Press.
Printed in Great Britain
NH STRETCHING FREQUENCIES AND THE STRUCTURES O F 12 A N D I B r C O M P L E X E S WITH PIPERIDINE AND PIPERAZINE Y. OKISHI, Y. IMAI and K. A I D A * Department of Applied Science, Faculty of Engineering, Tohoku University, Sendai, Japan (Received 3 April 1972)
Abstract-Molecular addition compounds of 12 and IBr with piperidine, piperazine, N-methyl-, Nphenyl- and N-benzyl-piperazine have been prepared. Piperazine and N-methyl-piperazine form two series of complexes, i.e. 1 : 1 and 1 : 2 mole ratios in amines to halogens. The structures of these complexes have been discussed from their N H stretching frequencies. INTRODUCTION
IT IS KNOWN that piperidine and piperazine form charge-transfer complexes with sacrificial "art-type" acceptors. For example, Yada et al. [1] have observed a charge-transfer band for piperidine-I2 in n-heptane solution. The stoichiometries and the melting points have been reported for piperidine-ICl[2], piperazineBr2 [3] and -I2 [3]. Person et al. [4] have studied an i.r. spectrum for piperidine-lCN complex with main interest in the I-C stretching vibration. As there are two active sites in piperazine, it is of interest to see whether it can form 1 : 1 complexes as well as 1 : 2 with halogens. 1, 4-Dioxan, which has a similar skeleton with that of piperazine, forms a 1 : 1 complex with Br2. The X-ray study reveals that it contains "halogen bridges" [5]. On the other hand, 1 : 2 complexes are formed by 1, 4-dithian, which also has the same skeleton, with I2 and IBr. McCuUough et al. have reported the X-ray studies on these complexes [6, 7] and have shown that the dithian ring is in the chair conformation with each Is or IBr molecule attached to sulfur in equatorial position. No structural study has yet been reported on piperazine-halogen complexes. The aim of the present study is to obtain evidence relating to the structures of piperidine and piperazine complexes with Is and IBr from their N H stretching frequencies. EXPERIMENTAL Chemicals Piperidine, N-methyl- and N-phenyl-piperazine were obtained from Tokyo Kasei Co. They were
*To whom all correspondence should be sent. 1. H. Yada, J. Tanaka and S. Nagakura, Bull. chem. Soc., Japan 33, 1660 (1960). 2. R. D. Whitaker, J. R. Ambrose and C. W. Hickam, J. inorg, nucl. Chem. 17, 254 (1961). 3. N. P. Yavorskii and S. N. Garasevick, Ukr. Khim. Zh. 35, 941 (1969); Chem. Abstr. 72, 6666e (1970). 4. W, B. Person, R. E. Humphry and A. I. Popov, J. Am. chem. Soc. 81, 273 (1959). 5. O. Hassel and J. Hvoslef, Acta chem. scand. 8, 873 (1954). 6. G. Y. Chao andJ. D. McCullough,Acta Crystallorg. 13,727 (1960); ibid. 14, 940 (1961). 7. C. Knobler, C. Baker, H. Hope and J. D. McCullough, lnorg. Chem. 10, 697 (1971). 101
102
Y. OKISHI, Y. IMAI and K. A I D A
dried with metallic sodium and then distilled under reduced pressure. N-Ethyl- [8], N-propyl- [9] and N-benzyl-piperazine[10] were synthesized from N-formyl-piperazine and piperazine monohydrochloride, respectively, dried with metallic sodium and then distilled under reduced pressure. Carbon tetrachloride was dried with phosphorous pentoxide at its boiling temperature for several days and fractionated. Only constant boiling middle cuts of carbon tetrachloride and amines were collected. Piperazine was recrystallized from carbon tetrachloride for several times. Iodine was purified by sublimation with barium oxide and potassium iodide. Iodine monobromide was prepared and purified by the method of Cornog and Karges [11].
Preparation of the complexes About 2 × 10-2 mole/1, solution of halogen in carbon tetrachloride was added dropwise to an amine solution of about 4 x 10-2 mole/I, in the same solvent with constant stirring at 0°C. The solid product was collected by suction filtration, washed with solvent thoroughly and placed in a desiccator. NEthyl- and N-propyl-piperazine seem to react with halogens, giving no solid addition compound. For piperazine and its derivatives, the effect of the ratio of the reagents upon the stoichiometry was investigated. When the excess amount of amines was added than 1 : 1, complexes having a 1 : 1 composition were obtained except piperazine and N-benzyl-piperazine with IBr. In these two cases, complexes seem to be unstable to be isolated. On the other hand, 1 : 2 complexes were precipitated when the excess amount of halogens was added than 1 : 2, except N-phenyl- and N-benzyl-piperazine. Varying the molar ratio in these two cases had no effect on their stoichiometry, giving only 1 : 1 complexes. The analytical results for these complexes are listed below. All compounds have been previously unreported except piperazine-Iz [3]. Piperidine-lBr*. Yellow powder, d.p. 72-3°C. Found: H, 3.80; C, 20'57; N, 4"80. Calc. for CsHIIN. IBr: H, 3.82; C, 19.60; N, 4.53. Piperazine-12. Yellow powder, d.p. 83-5°C. Found: H, 8.51 ; C, 14.32; N, 3.09. Calc. for C4H10N2. Is: H, 8.24; C, 14-13; N, 2.97. Piperazine-212. Lemon-yellow powder, d.p. 90-2°C. Found: H, 1.70; C, 8.09; N, 4.72. Calc. for C4H10N2.2I~: H, 1.70; C, 8.56; N, 4.61. Piperazine-21Br. Yellow powder, d.p. 129-31°C. Found: H, 2.02; C, 9.61; N, 5.61. Calc. for C4H10N2.2IBr: H, 1.99; C, 10-05; N, 5.31. N-methyl-piperazine-lz. Lemon-yellow powder, d.p. 75-6°C. Found: H, 3.46; C, 17.42; N, 7.89. Calc. for CsH12N~.I2: H, 3.42; C, 16.97; N, 7.91. N-methybpiperazine-212. Lemon-yellow powder, d.p. 66-7°C. Found: H, 1.84; C, 10.00; N, 4.77. Calc. for CsHI2N~.2I~: H, 1.92; C, 9.88; N, 4-61. N-methyl-piperazine-IBr. Yellow powder, d.p. 79-80°C. Found: H, 4.10; C, 20.34; N, 9.00. Calc. for C~H12N~.IBr: H, 3.97; C, 19.56; N, 9.13. N-methyl-piperazine-21Br. Yellow powder, d.p. 80-1°C. Found: H, 2-33; C, 12.07; N, 5.64. Calc. for CsH12N~.2IBr: H, 2.35; C, 11.69; N, 5.45. N-phenyl-piperazine-Iz. Lemon-yellow powder, d.p. 71-2°C. Found: H, 3.31; C, 28.77; N, 6.77. Calc. for C10H14N2.I., H, 3.39; C, 28.87; N, 6.73. N-phenybpiperazine-lBr. Yellow powder, d.p. 81-2°C. Found: H, 3.80; C, 33.01; N, 7.71. Calc. for C10H14N~.IBr: H, 3"82; C, 32'55; N, 7.59. N-benzyl-piperazine-12. Lemon-yellow powder, d.p. 62-40(:. Found: H, 3.89; C, 30.98; N, 6.50. Calc. for C11H16N2.I2: H, 3.75; C, 30.72; N, 6.51.
Spectroscopy I.R. spectra were recorded with a Perkin-Elmer Model 337 Spectrophotometer in the region from 4000 to 400 cm -~ on Nujol and hexachlorobutadiene mulls. * Piperidine-I~ was not precipitated. 8. K. Fujii, K. Tomino and H. Watanabe, J. pharm. Soc., Japan 74, 1049 (1954). 9. A. L. Mndzhoyan, M. T. Grigoryan, Yu. N. Sheinker, R. A. Aleksanyan, S. S. Vasil'yan, A. A. Kaldrikyan and I. A. Dzhagtspanyan, Arm. Khim. Zh. 21, 603 (1968); Chem. Abstr. 70, 96754t (1969). 10. J. C. Craig and R. J. Young, Organic Syntheses, Vol. 42, p. 19. John Wiley, New York. (1962). 11. J. Cornog and R. A. Karges, Inorganic Syntheses, Vol. 1, p. 165. McGraw-Hill, New York (1939).
N H stretching frequencies
I 03
RESULTS AND DISCUSSION
The observed N H stretching frequencies for solid complexes are listed in Table 1. The spectra between 2000 and 400 cm -1 are fairly complicated and could not to be assigned to certain vibrations at present. As they are not of major importance to this work, the observed frequencies for only two cases are given later (Tables 2 and 3). It is seen from Table 1 that all N H stretching frequencies appear above 3000 cm -1, which indicates that these complexes are not salts because the N H ÷ stretching band lies below 3000 cm -1 [12]. Hendra and Powell have studied the i.r. spectra of piperazine and its metal complexes [13] and have shown that a 1:2 complex with C2H4.PtCI2 contains a piperazine molecule in a chair configuration, bridging two acceptor molecules. The i.r. spectra between 2000 and 400 cm -1 of piperazine-212 and -2IBr are quite similar to that of piperazine-2(C2H4.PtCl2), as is seen from Table 2. This suggests that the structure of piperazine-2IX(X = I, Br) is likely the one given in Fig. !, which is analogous to that of C2H4.PtCIz complex proposed by Hendra and Powell. N-Methyl-piperazine-2IX complexes probably have the same structure, for the N H stretching frequencies are observed at almost the same positions with those of piperazine-2IX. N-Phenyl- and N-benzyl-piperazine do not form 1:2 complexes with halogens, even though excess halogens than 1 : 2 were added. This is probably partly due to Table
1. Observed NH stretching frequencies (cm -1) Piperidine-IBr Piperazine-212
3118 3215 3274, 3050 3126 3200 3068 3147 3050 3128 3114 3064
-I2 -21Br
N-Me-piperazine-212 -12 -2IBr -IBr
N-Ph-piperazine-I2 -IBr
N-By*-piperazine-I2 * Benzyl. H
I
XI---N~
~ N---IX
I
H Fig. 1. Proposed structure for piperazine-2IX (apart from their conformations of N H
groups). 12. L. J. Bellamy, The Infrared Spectra of Complex Molecules, 2nd Edn., p. 260. John Wiley, New York (1958). 13. P. J. Hendra and D. B. Powell, J. chem. Soc. 5105 (1960); Spectrochim. Acta 18, 299 (1962).
104
Y. OKISHI, Y. IMAI and K. AIDA Table 2. Infrared spectra (2000-400 cm -1) for 1 : 2 complexes of piperazine (cm -~) Pr*-2(C2H4.PtCI~)T 1463(w) 1452(m) 1432(ms) 1368(m) 1346(w) 1250(ms)¢ l157(vs) 1073(m) 1010(s)~ 882(vs)
Pr*-212
Pr*-2IBr
1446(m) 1410(m) 1369(m) 1345(w) 1248(m)
1445(m) 1413(m) 1360(m) 1320(w) 1250(m)
1087 (m) 995 (m) 868 (s) 640 (m)
1078(m) 1017(m) 872 (s) 651 (m)
s, strong; m, medium; w, weak; v, very. *Piperazine. fRef.[13]. Bands due to C2H4 group are skipped. ~tHendra and Powell have assigned these two bands to C2H4 group. However, as these bands appear constantly in all metal halide complexes, it is more probable to think that these bands originate both from C2H4 and piperazine.
the steric hinderance of the substituents and partly to the lesser donor property of the nitrogen atom caused by the conjugation. In conclusion, 1:2 complexes of piperazine and N-methyl-piperazine with halogens possibly have a 1,4-dithian2IX type structure shown in Fig. 1. Taking this structure of 1 : 2 complexes into consideration, let us now consider the structure of 1 : 1 complexes. It is known that piperidine exists mostly in a chair form[14]. This is also the case for piperazine[13]. Hendra and Powell have reported the i.r. spectra for 1 : 1 complexes ofpiperazine with HgCI2 and CdC12 [13]. The frequencies for HgC12 complex are reproduced in Table 3. They are more complicated than those of piperazine-2(C2H4.PtCl~) given in Table 2. They have proposed a polymer chain structure for these HgClz and CdCI2 complexes. The i.r. spectrum of piperazine-I2 in many respects resembles that of HgC12 complex (Table 3), so it seems probable that the I2 complex has also a polymer chain structure. Then, we can expect two different types for its structure, besides the existence of the axial and equatorial conformers. These are a "halogen bridge" type (Fig. 2(a)), as is found in 1,4-dioxan-Br2 [5], and a "hydrogen bridge" type (Fig. 2(b)), in which one of the nitrogen atoms, N °, is involved both in the hydrogen bond formation and in the charge-transfer interaction. In the former "halogen bridge" type, two N H groups would be expected to be equivalent with each other and the situation around the N H groups is analogous 14. N. L. Allinger, J. G. D. Carpenter and F. M. Karkowski, J. A m . chem. Soc. 87, 1232 (1965).
NH stretching frequencies
105
Table 3. Infrared spectra (2000-400 cm -1) for 1 : 1 complexes of piperazine (cm-') Pr*-HgC12t
Pr*-I2
1447 (m) 1418 (ms) 1377 (m) 1347 (w) 1314(w) 1256 (w) 1174 (vw) 1130 (vw) 1110 (m) 1072 (vs) 1040 (vw) 1017 (m) 1000 (m) 966 (vw) 890 (w) 879 (ms) 858 (vs) 690 (wm) 673 (m) 656 (vw) 458 (vw)
1434 (m) 1429 (m) 1380 (m) 1351 (m) 1312(m) 1212 (m) 1181 (m) 1128 (s) 1109 (w) 1081 (s) 1058 (m) 1029 (s) 922 (m) 890 (w) 866 (s) 850 (vs) 720 (w) 626 (s) 468 (m) 435 (m)
s, strong; m, medium; w, weak; v, very. *Piperazine. tRef. [13]. H
(O)
H
2
la " ~ N ""H -- N ~-..~ b
a
(0,
12
kS_
Fig. 2. Proposed structures for piperazine-I2 (apart from their conformations of NH groups): (a) "halogen bridge" type; (b) "hydrogen bridge" type. to that of the 1 : 2 complex. Therefore, only one NH stretching band (or closely s e p a r a t e d d o u b l e t if t h e i n t e r m o l e c u l a r c o u p l i n g is s t r o n g ) n e a r t h e s a m e p o s i t i o n w i t h t h a t o f t h e 1 : 2 c o m p l e x , will b e e x p e c t e d . O n t h e o t h e r h a n d , t w o N H g r o u p s
106
Y. OKISHI, Y. IMAI and K. A I D A
in the "hydrogen bridge" type have different natures. One of them, Nail a is expected to be weak as this group takes part both in the hydrogen bond formation and in the charge-transfer interaction, resulting in a bathochromic shift for its stretching frequency from that of the 1 : 2 complex. This in turn will make another group, NbH b, to be slightly free, resulting in a hypsochromic shift for its vibration from that of the 1:2 complex. The N H stretching region for piperazine-I2 is compared with that ofpiperazine212 in Fig. 3. It is seen that two N H stretching absorptions separated by about 200 cm -1, one is lower and the other is higher (and sharper) than that of the 1 : 2 complex, are appeared in the case of the 1 : 1 complex, suggesting that the "hydrogen bridge" type structure for this 1 : 1 complex. The N H stretching frequencies for N-methyl-piperazine-IX are observed at almost the same position with the lower band found in piperazine-I2 (Table 1). This is in accordance with the expectation on the basis of the "hydrogen bridge" structure, for the H b atom has now been replaced by the methyl group, leaving Nail a group only in these complexes. In order to ascertain this further, the N H stretching frequencies for N-phenylpiperazine-IX were measured. A phenyl group reduces the donor property of the N b atom, resulting in a weaker hydrogen bond, Nb...H~--N ~. This in turn should shift the N~H a stretching frequency to a shorter wavelength side. This is exactly the case, as is seen from Table 1. That the shift is not caused by the steric effect, is apparent from the result found in the benzyl derivative. There remains one problem concerning to the configurations of the N H groups
tO
7~ ttl t-~
I
I
I
I
I
3200
3000
Wovenumber,
c m -I
3400
Fig. 3. N H stretching region for piperazine-212 (upper) and piperazine-I2 (lower).
NH stretching frequencies
107
in these complexes. Unfortunately, nothing can be said about their conformations from this study. Attempts to obtain the deuterated complexes were unsuccessful. Partly deuterated compounds gave very complicated spectra and no additional evidence relating to the structures was deduced.