I h z u o NAKAMOTO
1420
Vol. 64
ULTRAVIOLET SPECTRA AND STRUCTURES OF %$'-BIPYRIDINE AND %,%',%"-TERPYRIDINE IN AQUEOUS SOLUTION' BY KAZUO NAKAMOTO Department of Chemistry, Clark University, FVwcester, Mass. Received March I O . 1060
The ultraviolet spectra of 2,2'-bipyridine and 2,2',2''-terpyridine obtained a t various pH values in aqueous solutions are compared with those of the metal-chelate compounds and with those of the ligands in organic solvents. It is found that, in basic solution as well a s in organic solvents, the molecules are trans and trans-trans whereas, in acidic solutions, the cis and cis-cis forms predominate in 2,2'-bipyridine and 2,2',2"-terpyridine, respectively. The cis-trans form is also found for 2,2',2"-terpyridine a t an intermediate pH. The existence of small twists along the central carbon-carbon bonds is suggested for these species. No positive evidence is found for intramolecular N+-H. N hydrogen bonds in the protonated Epeaes.
..
Introduction Although many studies have been made on the ultraviolet spectra and ionization constants of 2,2'bipyridine anti its derivatives, the configuration of the molecule in solution has not yet been reported, The following trans and cis forms are probable in 2,2'-bipyridine. The result of X-ray
Q-0
trans-trans (IIa)
1 7
trans ((a)
Ncis (Ib)
analysis2 definitely indicates that the molecule has a trans-plana,r configuration in the crystalline state. I t was also shown by the measurement of dipole moment that the molecule in solution is \ g-\ trans although small twisting around the central carbon-carbon bond was suggested.3 The lack of cis-cis (IIc) information on the cis form may be due to the fact that the trans :form is more table.^ Although the Experimental cis form has not yet been observed, the chelatedMaterials .-2,2'-Bipyridine was purchased from Eastman and was recrpstal1lz:d cis form undoubtedly exists in the metal chelate Organic Chemicals, Rochester, S .IT., from ethanol solution; colorless crystal, m.p. 70". 2,2 ,compounds of 2,2'-bipyridine.6 was purchased from G. F. Smith Chemical The following three configurations are probable 2"-Terpyridine Co., Columbus, Ohio, and was recrystallized from ether in 2,2',2''-terpyridine. As in the case of 2,2'- solution; pale yellow crystal, m.p. 89". So far no complete descriptions of the hydrochlorides of bipyridine, the transtrans form (IIa) of minimum dipole moment is expected to be most stable. these compounds are available in the literature. In order to determine the combining ratios between these bases and In the metal-chelate compounds, however, the hydrochloric acid in the crystalline state, we have prepared molecule is planar cis-cis as is shown by the X-ray the hydrochlorides and determined their chemical composition by analysis. Besides the above two bases, 1,lOanalysis on [Z~~(trpy)Cl]Cl.~ The purpose of this work is to investigate the phenanthroline hydrochloride was also prepared for comparison. All t,he hydrochlorides were prepared by saturatstructures of tlhe species predominant a t various ing hydrogen chloride to ether solution of the base. pH values in aqueous solutions mainly based on They were dried for five days in a phosphorus pentoside the ultraviolet spectra. Although the ultraviolet desiccator. 2,2'-Bipyridine Dihydrochloride: colorless crystal, m.p. spectra cQ various isomers of phenylpyridines,'J 150-155". Anal. Calcd. for C1oH~,ClzNa: C, 52.43; biphenylpyridines18 bipyridines? and phenanthro- H,4.40; N, 12.22; C1, 30.95. Found: C, 52.64; H,4.68; lines7 have already been measured, no studies such N, 12.20; C1,30.09. as mentioned above have been attempted. 1,lO-Phenanthrolme Monohydrochloride Monohydrate: ~
(1) This iiirestigation ivas partly supported by a research grant, H-3246, f r o m t!ie Na.tiona1 Heart Institute, Public Health Servire. (2) L. L. Nerritt, Jr., and E. D. Sehroeder, Acta Crust., 9 , 801
(1956). (3) P. E. P'ie!ding and R. J. W. LeFevre. J . Chem. Soe.. 1811 (1951): the twisting snale w t s estimated t o be at most 28" based on the observed dipole moment, 0.9 D . This result is. however, difficult to understand since no serious hydrogen repulsion is seen in the trans-
planar form. (4) G. H~ l i t w v a T t and H. Eyring, J . Chem. Bduc., 35, 550 (1958). (5) H. J . Ilothie, I?. J. Llewellyn, W. Wardlan and A. J. E. Welch J . Chem. Soe., 426 (1'339). (6) D. E. C. Corbridge and E. G. Cox. ibid., 594 (1958). (7) P. Krurnhols, J . Am. Chem. Soe., 73,3487 (1951). (8) A. E. CiUam, D. 1-1. IIey and A. Lambert. J . Chem. SOC.,364 (1941).
colorless crystal, m.p. 175-180". Anal. Calcd. for CIZH i l X ~ C l 0 : C, 61.42; H, 4.72; K, 11.93; C1, 15.11. Found: C,61.08; H,4.74; N, 1'2.30; C1,15.15. 2,2',2''-Terpyridine dihydrochloride monohydrate : pale yellow crystal m.p. 180-185". Anal. Calcd. for CuHlbN3ClLO: C, 55.57; H , 4.66; N , 12.96; C1, 21.87. Found: C,55.17; H , 4 83; N, 13.05; C1,21.80. As to the hydrochloride of 2,2',2"-terpyridine, Morgan and Burstall9 previously reported the trihydrochloride of the composition C15H22N3C1d04 (Cl, 25.8%), which is colorless and decomposes at 280-285'. Thus their compound seems to be different from ours. Preparation was repeated several times by changing the conditions. However, we could not obtain the hydrochloride of such a high chlorine content as they found.
__
(9) G . Morgan and F. H. Buratall, ibzd.. 16-19 (1937).
Oct., 1060
SPECTRA A N D S T R U C T U R E O F
~,~’-BIPYRIDINE AND 2 , 2 ’ , 2 ” - T E R P Y I 2 I D I N E
1421
0.8
B 0.7
0.6
0.5
6
i
P
8 0.4
3 4
0.3
0.2
f
0.1
250
300
300
250 X
(md.
Fig. 1.-Absorption spectra of 2,2’-bipyridine in buffer solutions (5 X 10-6 niolc/l.): A, in acidic solutions; -, pH 1.80; - .--., 3.63; 4.02; ......, 4.40. B, in neutral and basic solutions; 4.40; - * * - * -, 4.78: ., 5.2:3,. _ - _ . _ - . , 5.80: -, 9.15 N 12.05.
-.....
- . . - . e ,
.-
~
The met:tl chelate compounds of 2,2‘-bipyridine and 2,2‘,2”-terpyricline were prepared according to the literature.0 Potentiometric Titration.-Approximately 2 X mole/l. solution of terpyridine hydrochloride in 0.1 AI IiCl solution was titrated with 0.1 N NaOH solution. A S a t 2 moles of base per mole of sharp inflertion W ~ observed ligand.10 pH memurements were made a t 35’ with a Beckman Model GS pH meter fitted with extension glass and calomel electrodes. The pH readings were calibrated with acetic acid buffer and with standard hydrochloric acid and sodium hydroxide solutions. Approximate pK values wew estimated from the titration curve using Schwarzenbach and Ackermann’s melhod.11 The pk’ values of the first and second ionization were 2.59 and 4.16, respectively. Spectral Measurements.-The ultraviolet spectra were measured z b t 20” with a Cary Model 14 spertrophotometw. A pair of 1 cm. q u x t a cells were used. With the exception of ethanol, organic ;solvents were of “spectro-grade” quality purchmed from Eastman Organic Chemicals. The buffer IICI; solutions were: pIi 0 , 1 N HC1; 1.0-2.2, €IC1
-
+
(10) Same result was obtnined by W. W. Brandt a n d S. P. Wright, THISJ O U R N ~ I 76, . , 3082 (1954). However, they reported only t h e I for this compound. geometrical irverage d u e of P K I and ~ K I (7.1) (11) G. Srhwarzenbach and H. Ackermann, Heln. Chim. Acta. SO, 1788 (1947).
+
3.6-5.4, CH&OOH CH3COOSa; 6.0-8.0, NaOH 4KH2POn; 8.9-11.4, NaHCO, KazC03; 12.0-14.0 NaOH. The ionic strength was maintained approximately at 0.2 for most of the solutions. The infrared spectra were obtained by a Perkin-Elmer Model 21 infrared spectrophotometer equipped with a sodium chloride prism. The potassium bromide disk method was employed t,o obtain the spectra in the crystalline state.
+
Results and Discussion I. 2,2 -Bipyridine.-Figure 1 indicates the ultraviolet spectra of 2,2-bipyridine a t pH values ranging between 1.8 and 12. In basic solution, two bands appear a t 279 and 232 mp. As is seen in Fig. 2, the spectrum in basic solution is very similar to those in organic solvents. Since the molecule is proved to be trans in organic these two bands can be attributed to the characteristic absorption of the trans form. In acidic solution, two bands are observed a t 301 and 240 mp. It was shown by Krumholz6 and Westheimer and BenfeyI2that the mono-cation predominates in ordinary acidic niediuni (for
KAZUONAKAMOTO
1422
Vol. 64
tion of the mono-cation. As is seen in Fig. 2, the spectrum in acidic solution is similar to that of the metal chelate compounds such as [ N i ( b i p ~ ) ~ ] C1d3 except for the fact that the fine structure of the longer wave length band seen in the latter is blurred in the former. The following two forms are probable for the mono-cation ,N= -\
Q-d G./+\;;TI.> H H trans (IC)
cis (Id)
Both of these two forms are expected to be slightly twisted along the central carbon-carbon bond because of steric hindrance between two ortho hydrogen~.’~From the standpoint of electrostatic energy, Id is expected to be more stable than IC because less negative nitrogen ( K+-H) is more closely located to the more negative nitrogen ( 3 N) in Id than in IC. The similarity of the spectra between the mono-cation and the metal-chelate compounds also favors the cis form. Thus we conclude that the mono-cation is a slightly twisted
>
cis. The spectral change observed in Fig. 1 is then interpreted by the equilibrium between Id (301, 240 mp) and Ia (279, 232 mp). Hereafter these two bands observed in each species are called 7r1 and 7rz-bands, respectively, from the longer wave length. By plotting pH against the absorbancies of the 7rl-bands (Fig. 3), the pK values for the equilibrium was estimated as 4.45. This value is in good accord with 4.44 obtained by potentiometric titration.’ As is seen above, 2,2’-bipyridine exhibits two 250 300 bands in the ultraviolet region. This is an indi(md. cation that a twist along the central bond is small, Fig. 2 . --Compi~rison of the spectra in 2,2’-bipyridine: -, acidic solution (5 X 10-6 mole/l.); ----, basic since it is shown both empirically6 and theroretimole/l.); -.-, [Ni(bipp)8]C12.7H20 cally15 that, for a large twist, only one band usually is solution ( 5 X (10-5molej1.): . . . . . ., ethanol solution (6 x 10-5 mole 11.). observed in this region. The splitting of the TI-band in the metal chelate compounds was shown to be due to the vibrational fine structures.16 If so, blurring of the fine structures in I d and Ia may imply that they are not completely planar. li I n the metal chelate compounds, however, a perfect planarity is maintained because the resonance structures such as shown below stabilize the planar form.
0.1 o.2
I 0
~
(13) T h e band6 a t 308, 296 and 246 nip observed in [NiibipyhlCla are undoubtedly dire to the “ligand absorption” and not d u e t,o the
1
2
3
4
5
ti
7
8
9 1 0
PH. Fig. 3. -A hsorhniicy of ‘2,2‘-bipyridine a? R function of pH.
example, 1N IIC1). Therefore these two bands are reasonably attributed t o the characteristic absorp(12) F. E. Westtieimer a n d 0. T. Benfey. J . Am. Chem. Soc., 78,
5309 (1956)
metal.
-
(14) Because of hydrogen rPpulaion, biphenyl is twisted by 4 5 ” in t h e gaseous stat.e a n d 20 26’ in solution (seereference 1.5). T h e twisting angle in ICa n d Id is expected t o be somewhat smaller t h a n t h a t of biphenyl, since only two orfho-hydrogens are sterically hindered. (15) H. Suzuki, Bull. Chrm. Soc. Japan, 82, 1340 (1959). (16) K. Sone, P. Krumholz a n d H. Statnmreich, J . Am. Chem. Soc., 7 7 , 777 (1955).
(17) See, for example, M. 8. Newman, “Steric Effects in Organic Chemistry,” John Wiley a n d Sons. Inc.. New York, N. Y., 1956, p . 500.
Oct., 1960
SPECTHA AND STRUCTURE OF
2,2'-BIPYRIDINE
TABLE I THEULTRAVIOLET SPECTRA O F 2,2'-BIPYRIDINE
ANI)
2,2'-2''-TE;RPYRIDINE
I N VARIOUS SOLVENTS (mp) 71
Compound
2,T-Bipyridine
[Ni,(bipy):;]Clt
Solvent
iT?
Protonated
or chelated cis
trans
Cyclohexane
283(15.0)"
Chloroforni
284( 14.5)"
Ethanol
283(10.2)"
Water (pH 12j Water (pH 1.8) Water
1423
.... . . .
281 (0.69)"
301(0.78)*
.......
245 (1I . 3)" 237(12.1)" 245(9.9)" 240( 10.3)"
244(6.6)" 23q7.7)" 233(0.53'1h 239(0. 37)b
308(42.6)" .7Hz0 ZgG(45.2)" ...... . 246(34.0)" The number denotes the molecular extinction coefficient ( X 10-3). b The number indicates the absorbancy of 5 X 1 0 F
mole/l. solution in which two species are in equilibrium.
These structures are also responsible for the red- This is due to the lack of resonance between tbe shift of the nl-band of the metal chelate compounds bipyridine portion of 2,2',2''-terpyridine aiid the relative to that of the ligand, although the degree third pyridine attached to the meta-position of t8he of their contribution is slightly different from one former. The ultraviolet spectra of meta-polymetal to mother. phenyls show similar trends.19 Figure 5 indicates that the spectrum in acidic Similar red-shift of the nl-band of the cation (Id) relative to that of the base (1%) can be ex- solution is similar to that of the metal chelate coniplained by the following resonance contribution. pounds such as [Zn(trpy)Cl]Cl in which the ligand is definitely c i s c i s (IIc). The result of potentiometric titration clearly indicates that only two ++ -" \N= Nprotons combine with one base. Furthermore H H no appreciable change of the spectrum was obThus the resonance caused by salt formation is served even in extremely strong acid (5 M HzSOd. responsible for the observed red-shift of the nl- Considering charge distribution together with t'he band. Although conversion of I a into Id involves above mentioned observations, we conclude that a change of configuration from trans to cis, this the most probable structure for the di-cation is effect may be much smaller compared with the c i s c i s shown below effect of salt formation or even act in the opposite direction to th.e effect of salt formation since cisisomers usually absorb at shorter wave length H than the trans-isomers.'* The di-cation of 2,2'HN ? y , bipyridine which exists only in extremely strong \ d acid absorbs a t 290 mp.12 From the viewpoint of C ~ S - C ~ S(IId) charge distribution, a complete planar trans AS is seen in Figs. 4 and 5, the spectra of acidic form is mostfavorable for the di-cation. Because of hydrogen repulsion, however, it may be slightly solution and the metal chelate compound exhibits 3251 285 and 230 m p ~ twisted. Since no resonance contributions such as three main bands a t though the h e structures in the latter are blurred H in the former. The interval between 325 and 285 mp bands is too large to assign them to the vibra-N tional structures belonging to the same electronic H transition. We have measured the ultraviolet seen in the m.ono-cation are expected for the di- spectra of the metal chelate compounds of 2,2',2"cation, the blue-shift of the al-band of the di- terpyridine with various metals. All of these cation relative to that of the mono-cation is un- compounds exhibit three bands around 340320, 285-270 and 235-220 mp although blurring derstood on this basis. n. 2,,2',2''..Terpyridine.-Figure 4 indicates the of the satellite bands depends 011 the kind of the ultraviolet spectra of 2,2',2' '-terpyridine a t various metal. 2o AS stated before, the conjugation between pH values in t)uffer solutions, As is shown in Fig. pyridine rings is insulated in meta-polypyridinrs. 5, the spectrum of basic solution ( p 12) ~ is very Then the spectrum of IId can be interpreted based similar to those in organic solvents. Therefore, on the interaction between two bipyridines (300 it is reasonable to conclude that the nlolecule is mp) both of which share the Pentral pyridine riiig. trans-trans (I:[a) of minimum dipole moment in Thus the interval between 325 and 285 mp 1,nlltI these solvents. The11 the bands at 285 and 235 may indicate the magnitude of such a pertiirbatiOli. mp are attributable to the nl and nz-bands of IIa. In this sense, we call these two bands sia iilttl It is noted that IIa does not show appreciable r l h l respectively. Other satellite bands ck:irl)' red-shift compared with trans-2,2'-bipyridine (Ia). observed in the metal chelate compounds w(' assigned t o their vibrational structures. (18) For example, see: A. E. Gillam a n d E. S. Stern, "Electronic
r)=(c>=(-)+
Absorption Spectroscopy." E. Arnold Publishers Ltd.. London, 1954, p.
233.
(19) A. E. Gillam e n d D. H. Hey, J . Chem. Soc., 1170 (1939). ( 2 0 ) K. Nakamoto, unpublished.
KAZUO NAKAMOTO
1424
Vol. 64
0.5
0.4
0.1
0 250
300
350
250
300
350
(md.
Fig. 4.-.4beorption spectra of 2,2’,2’’-terpyridine in buffer solutions ( 2 x 10-6 rnole/l.): A, in acidic solutions: -, p H 180: -..-.., 3.00: ,3.65; -,4.05. B, in neutral and basic solutions; * 4.05; 4.40; 4.87; .-. . ] 5.25; -.---, 5.83; -I 12.0.
-. .
.
.- . -
-..-..
e,
---e,
-. .-. .
dominant a t an intermediate pH. Thc following two cis-trans structures are most probable for the mono-cation. The cis-cis and trans-trans forms can be ruled out from consideration of charge distribution and spectra. Of these two, IIe is more favorable than IIf in explaining the process of protonation from the mono-cation to the dication (IId). Similar to the case of IId, t.he splitting of the Irl-band into 320 and 279 my band is interpreted as a perturbation between Ia and Id which share the central pyridine ring. Finally blurring of vibrational fine structures both in IId and IIe can be accounted for by a small twist around the central carbon-carbon bond. Thus the spectral change in Fig. 4 is interpreted by the equilibria between IId, IIe and IIa. It is rather difficult, however, to determine the individual ionization constants of the above two step dis250 300 350 200 sociation from the spectroscopic data since over(md. Fig. 5..-(33mparison of thc spectra in 2,2’,2‘’-tcfpa.ridir1~: lapping of the bands Of each species is serious. Thus the individual ionization Constants were -, acidic sojution (2 x 10-6 mole/i.) : . . . . . ., basic solution (2 x 10-5 mole/l.): -.--. , [Zn(trpy)Cl]Cl (10-6 mole/l.); determined from the analysis of the titration curve. , ethanol solut,ion (10-6 mole/l.). The vK values were estimated to be 2.59 and 4.16. resp&tively. If the small difference between these The spectrum at an intermediate pH is different two ionization constants is considered, the complex either from that of basic or acidic solution. As is feature of the ultraviolet spectra a t the intermeseen in Fig. 4, three broad bands appear at 320,279 diate pH range is reasonably understood. and 232 my. The mono-cation is expected to be pre111. Possibility for Intramolecular Hydrogen Bonding.-In the foregoing, we have concluded that the cations of 2,2‘-bipyridine and 2,2’,2’’-terpyridine have slightly twisted cis configurations. I1 \ L Y Intramolecular hydrogen bonded sti-uctures of thcsc spccics such as shown below have advantagcs IIC
c;/x= (--> , : q
7
Lx2---3&> IIf
%>
in the following facts. (1) The di-cation of 2,fl’-
TABLE I1 ~,~',~''-TERPYRIDINE I N VARIOUS SOLVENTS (m@)
-
'rHE ULTRAVIOLET SPECTRA O F
Cornpoiind
Solvent
2,2 ',l"-Terpyridine
Cyclohexane Chloroform Ethanol Water (pH 12) Water (pH 4) Water (pH 1.8)
Protonated or chelated cis-cis
............
............ ............
............ ............ (A)
{ 322(0.36)b 333(0.30)*
~ - - -
*I
Protonated cis-trans
nz
trans-trans
........... ........... ........... ........... (A) 320(0.28)' (B) 279(0. 29)b
...........
278(18.8)" 280 (17.9) 279(18.6)" 290(0.32)*
.......
235(90.4)" 240(17.2)" 234 (19 4)" 227(0.39Ib 232 (0.38)
.......
231(0.36)'
I
i{ i
288(0.41)'
(B)
280(0.33)' 270(0.22Ib
331 (17.3)" ........... ....... 232(20.4)" 318(17.2)" 283(14.8) (B) 274(12.0)" 264(9.7)" 5 The number denotes the molecular extinction coefficient ( X 10-3). a The number indicates the absorbancy of 2 X mole/l. solution in which three species are in equilibria. [Zn(trypy)Cl]C1
Water
(A)
bipyridine does not exist in ordinary acid, although the second proton is forced to combine with another nitrogen in extremely strong acid.12 (2) The tri-cation of 2,2',2"-terpyridine cannot be found either in the crystalline state or in strong acid. (3) Similarity of the spectra between the cations and the metal chelate compounds are well explained. It should be noted, however, t,hat (1) and (2) can be accounted for on the basis of unstable charge distribution caused by close location of ammonium nitrogens and (3) does not necessarily require the chelated structure. The estimated N - . .H distance from the reported structural data,22.60 A., seems to be too long compared with that of other N. -H distances so far reported.21 Also, similarity of the second ionization constants of vsirious isomers of bipyridinesa do not afford evidence for such a hydrogen-bonded structure. Westheimer and Benfey12 arrived a t the same conclusion b,y comparing the ratio of the first and second ionization constants in many di-basic acids.
-.
(21) J. M. Robertson, "Organic Crystals and Molecules, ' Cornel1 University Presa, Ithaca, N. Y.. 1953. p. 245.
The infrared spectra of the hydrochlorides of pyridine, 2,2'-bipyridine and 2,2',2''-terpyridine exhibit their N+-H stretching bands at 2760, 2550 and 2680 cm.-l, respectively, in the crystalline state. Although the bands are shifted to lower frequencies in the latter two compounds compared with the former, these shifts are not necessarily attributable to the effect of intramolecular hydrogen bonds since intermolecular hydrogen .O and K+-H. . . .C1- types bonds of N+-H. coexist in the crystalline state. Also an attempt has been made to study the proton magnetic resonance spectra of these compounds in solution, which was not successful, however, possibly because nuclear quadrupole relaxation of N14smeared out the N+-H proton signal. Thus no positive evidence was found for the N+H- . . .N hydrogen bonding. It is hoped that more detailed studies on the structures of these compounds will be carried out in the near future.
..
Acknowledgment.-The author wishes to express his sincere thanks to Professor Arthur E. 3Iartell for criticism of this manuscript.
