The electron-donating properties of 2,2′-bipyridine. Charge-transfer studies

SpectrochimicaActa. Vol. 47A. No. 12, pp. 1727-1733. 1991 Printed in Great Britain

ow-8539/91 s3.00+0.00 0 1991 Pergamon Press plc

The electron-donating properties of 2,2’-bipyridine. Charge-transfer studies ABDULRAHMAN 0. ALYOUBI Chemistry Department, Facultyof Science, King Abdulaziz University, P.O. Box 9028, Jeddah 21413, Saudi Arabia (Received

12 March 1991; in final

form25July 1991; accepted 28 July 1991)

Ah&n&-Molecular charge-transfer complexes of 2,2’-bipyridine with iodine and tetracyanoethylene have been investigated. The formation constants of these complexes were determined at different temperatures and the thermodynamic functions, AH, and As, were calculated. The spectra and stability of the complexes were discussed and interpreted.

THE

electron-donating power of a molecule depends mainly on two factors: the basicity of the donating site and the steric effect, where the former enhances the donating power and the latter opposes it. The formation of electron-donor-acceptor (EDA) complexes (charge-transfer complexes) between two molecules involves the transfer of an electron from the highest occupied orbital (HOMO) of the donor to the lowest unoccupied orbital (LUMO) of the acceptor. A general theoretical description of such molecular complexes was provided by MULLIKEN’Swell-known theory [ 11. Several experimental techniques have been used for studying charge-transfer complexes; these include NMR, IR, EPR, polarography, calorimetry and spectrophotometric methods. Examples of this type of work can be found in several books and reviews [2-S]. A great deal of work has already been done on the charge-transfer complexes of pyridine compounds (as donors) with iodine (as acceptor) [9-131. 2,2’-Bipyridine and its derivatives have interesting properties ranging from excellent chelation ability [14] and herbicidal action [15] to their use as one-electron transfer agents in biological systems [16]. Recently, the physical and chemical properties of bipyridines have been reviewed by SUMMERS[17]. To throw more light on the donating properties of 2,2’-bipyridine, the present work is devoted to investigating the charge-transfer interaction of this important molecule with iodine and tetracyanoethylene, by using spectrophotometric methods.

EXPERIMENTAL Purum grade 2,2’-bipyridine (BDH) was recrystalized from ethanol. Iodine (purum, Fluka) was purified by sublimation. Tetracyanoethylene (purum, Fluka) was recrystallized from chlorobenzene. Spec quality cyclohexane and chloroform (BDH) were used as solvents.

Spectral.measurements were carried out using a 260-Shimadzu UV-visible equipped with a variable temperature controller.

spectrophotometer

RESUL-KSAND DISCUSSION

Equilibriumstudies

When a solution of iodine (A) is mixed with that of 2,2’-bipyridine (B), the following equilibrium is established, assuming that only a 1:l complex is formed; 1727

1728

ABDULRAHMAN~.

ALYOUM

Fig. 1. Absorption spectra of pure iodine (l), 5 X 10v4M and 2,2’-bipyridine-I, mixtures in cyclohexane at 18 “C. Concentrations of 2,2’-bipyridine in the mixtures are: (2) 0.02 M, (3) 0.035 M, (4) 0.065 M, (5) 0.08 M and (6) 0.095 M.

I2 + 2,2’-bipyridine *2,2’-bipyridine A+B=B.A.

I*, (1)

If C, = the initial concentration of Iz, Cb = the initial concentration of 2,2’-bipyridine, C,,, = the concentration of the charge-transfer (ct) complex, then the formation constant, K,,, for the complex is given by:

&= (Co -

cc, C,,)(C, - Cc,)’

(2)

Variation of the absorbance of the hypsochromically shifted iodine band or the et band, with the concentration of the donor, can be used to study the equilibrium in the system. Assuming that Beer’s law is obeyed and L) is the absorbance due to the complex only, Eqn (3) can be derived:

GCb

1

D -=KnE,+

co+

cc,

E,

Ecr’

cb

P-e

(3)

where E, is the molar extinction coefficient of the cc complex. When C, and C, are small, but greater than C,,, the last term of Eqn (3) becomes negligible. A plot of C,CJD versus C, + cb will give a straight line; from its slope and intercept the values of k,, and ecr can then be calculated. Equation (3) is Scorr’s form [18] of the original BENESI-HILDEBRAND relation [19]. Figure 1 shows the absorption spectra of solutions in cyclohexane of pure iodine (5 X 10P4 M) and 2,2’-bipyridine-I, mixtures at 18 “C using cyclohexane as a blank. The Iz concentration in the mixtures is kept constant at 5 x 10m4M, whereas 2,2’-bipyridine is varied from 0.02 M to 0.095 M. 2,2’-Bipyridine has no absorbance in the region scanned. The free iodine band appears at 520 nm and its absorbance decreases by the increase of

Charge-transferstudies

1729

0.625-

: 2 0.3128 9

0 300

400

500

600

700

Vnm Fig. 2. Absorbance spectrum of 2,2’-bipyridine-I, mixture in cyclohexane. Reference solution is bipyridine of the same concentration (0.07 M) as in the mixture.

the donor concentration. The complexed iodine band appears at 445 nm and its absorbance increases as 2,2’-bipyridine concentration increases. Figure 1 indicates a well-defined isobestic point, meaning that the system exhibits one equilibrium. The charge-transfer band of this complex appears in the same region in which 2,2’-bipyridine possesses strong absorbance. In order to obtain the ct band on its own, the spectrum of 2,2’-bipyridine-I2 mixture was scanned against a blank containing 2,2’-bipyridine at the same concentration as in the mixture (Fig. 2; A,,, of ct band ~312 nm). The ct band is very sensitive for error in adjusting the donor reference concentration; therefore, the other alternative is to use the hypsochromically shifted iodine band for the calculation of the complex formation constant. But this is restricted due to the fact that this band (A,, = 445 nm) falls at the short wavelength tail of the free iodine band (A,,, = 520 nm). T o overcome this difficulty, the spectra of 2,2’-bipyridine-I2 mixtures were scanned (and hence their absorbances at 445 nm were measured) against a blank containing I2 at the same concentration (5 x 10m4M) as in the mixtures (Fig. 3). The experiment has been carried out at four different temperatures (11 “C, 18 “C, 27 “C, 34 “C). The absorbance of the shifted iodine band decreases with the increase of the temperature, indicating the exothermic nature of the process of the complex formation. The ielation between C&,/D and C,+ C, at 445 nm and at different temperatures is plotted. The graph shows prominent straight lines, suggesting the formation of a 1:l complex. Values of & and E, calculated from the slopes and intercepts are given in Table 1. For comparative purposes, the pyridine-Iz complex was also investigated in the same solvent and under the same conditions. Figure 4 shows the absorption spectra of pyridine-I2 mixtures at 18 “C, using cyclohexane as a blank. Pyridine concentration is varied from 0.01 to 0.09 M. Features of the spectra are quite similar to those of 2,2’bipyridine-I, solutions. The complexed iodine band appears at 420 nm. The values of K,, and E, of the pyridine-I2 complex were obtained from the graphical plot of Eqn (3) and were found to be 175 dm3 mol-’ and 1337 dm3 mol-’ cm-‘, respectively. The absorption maximum for free iodine (A = 520 nm) is assigned as due to x*+=0* transition, and pyridines are considered as predominantly (n) donors [20]. The ct complex of an n-donor base with iodine has three important electronic states as follows: (i) the ground normal state (N), (ii) the locally excited state of iodine (B), and (iii) the CC excited state (E) [21]. The BtN transitions for the 2,2’-bipyridine-I, system and the pyridine-I2 system appear at A,,, 445 nm and A,,, 420 nm, respectively, meaning that the N state of the pyridine-I, system is more stabilized than that of 2,2’-bipyridine+ by

1730

ABDULRAHMANO.ALYOUBI

0.166 eV. Moreover, the formation constant, Kc,, for pyridine-I, is almost 15 times as high as that of 2,2’-bipyridine. The difference in basicity between pyridine (~K=5.2) [22] and 2,2’-bipyridine (pK= 4.25) [17] could not wholly account for this low stability of 2,2’-bipyridine-I*. We think that this low stability is mainly due to the steric hindrance between the or&o-hydrogens, resulting from the adoption of a c&conformation of the 2,2’-bipyridine during complex formation. Adoption of a &conformation may lead to appreciable repulsive interaction between the nitrogen lone pairs; an effect which also retards the donation of these electrons to the iodine molecule. To obtain better insight into the donating strength of 2,2’-bipyridine, the equilibrium properties of its molecular complex with tetracyanoethylene (TCNE; a n-acceptor) have been investigated and compared with the corresponding ones of the 2,2’-bipyridine-I, complex. TCNE is not very soluble in cyclohexane; hence the investigation was carried out in chloroform. Following the same procedure adopted when I2 was the acceptor, the spectra of solutions containing varying 2,2’-bipyridine concentration (0.03 to 0.1 M) and a constant TCNE concentration (2 x 10m3M) were scanned against chloroform as a blank. The 2,2’-bipyridine-TCNE system has two charge-transfer bands, one at 400 nm and the other at 415 nm (Fig. 5). The values of K, and E,~were obtained from a Sco-rr-type [18] plot (Eqn 3) and are given in Table 1. The experiments were carried out at different temperatures, and the exothermic nature of the process is seen from the values of Kct. It is evident that K,, values for the 2,2’-bipyridine-I, system are higher than Kc, for 2,2’-bipyridine-TCNE as expected. The 2,2’-bipyridine-I, system involves n+o* transition whereas 2,2’-bipyridine-TCNE involves n-m*. Were 2,2’-bipyridine a x donor, then the Kc, value of TCNE complexes would be higher than that of the iodine (zr+~* complexes are much more stable than n-+u* ones). This result supports our previous explanation, that adoption of &s-conformation is the most likely cause for the low stability observed for the 2,2’-bipyridine-I, complex. The phenomena of observing two ct transitions for 2,2’-bipyridine-TCNE can be attributed to the fact that the transferred electron might originate from the two levels n,

X/nm

Fig. 3. Absorption spectra of 2,2’-bipyridine-I, mixtures in cyclohexane at 18°C. Reference, solution is I2 of the same concentration (5 x 10m4M) as in the mixtures. 2,2’-Bipyridine concentrations are: (1) 0.01 M, (2) 0.02 M, (3) 0.03 M, (4) 0.04 M, (5) 0.05 M, (6) 0.06 M, (7) 0.07 M and (8) 0.085 M.

1731

Charge-transfer studies Table 1. Equilibrium and thermodynamic characteristics of the charge-transfer complexes studied

Complex 2,2’-Bipyridine-I2

2,2’-Bipyridine-TCNE

Temperature (“C)

(dm3>ol- ‘)

% (dm3 mol-’ cm-‘)

I1 18 27 34 I1 18 27 34

13.58 12.03 10.06 8.72 2.13 2.03 1.90 1.80

866 757 757 740 595 584 572 540

-AH, (kJ mol-I)

- AS, (Jmol-‘)

14.6

29.5

5.3

12.4

and n_, resulting from the symmetric and antisymmetric combinations of nitrogen lone pairs, respectively. The energy difference of the ct transitions (0.11 eV) is about the same as the difference in ionization potentials (0.1 eV) [23], when the c&-conformer is considered. Thermodynamics of complex formation The values of the formation constant Kc, at different temperatures were used to calculate the change in Gibb’s free energy change, AG,, for the formation of the molecular com$lexes studied. Estimates of the numerical values of enthalpy (A&) and entropy (AS,> of the ‘complexes fo~ation have been obtained from Van’t Hoff plots of the variation of In K,, with l/T. The results are given in Table 1. The reaction of 2,2’-bipyridine with I2 is more exothermic than its reaction with TCNE. Also, A,!$for the first reaction is more negative than that of the second. According to MULLIKEN [ 11, the wavefunction of the ground state YN of the molecular complex BA can be written as YN=

aq+,(B,A) + bY,(B+ -A-),

Fig. 4. Absorption spectra of pyridine-Ir mixtures in cyclohexane at 18°C. I2 concentration is 5 x 10e4 M in all mixtures. Pyridine concentrations are: (1) O.01 M, (2) 0.02 M, (3) 0.03 M, (4) 0.04 M, (5) 0.07 M and (6) 0.08 M.

1732

ABDULRAHMAN~.

ALYOUBI

0

\

1

0.0;

2 a

J;L , 350

450

5x)

Vnm

Fig. 5. Absorption

spectrum of 2,2’-bipyridine-TCNE mixture in chloroform at 18°C. [2,2’bipyridine] = 0.05 M; [I*] = 5 x lo-’ M.

Yc refers to the non-bond wavefunction and YI corresponds to the dative-bond wave function. For a weak complex, the ratio between the coefficient of the dative bond (ct structure) to the non-bond wave functions is given by [24,25]:

where

where hv,, is the energy of the ct band. For 2,2’-bipyridine complexes with iodine and TCNE, this ratio was found to be 0.038 and 0.018, respectively; a result which follows the order of their stabilities.

REFERENCES

[l] [2] [3] [4] [5] [6] [7] [8] [9]

[lo] [ll] [12] [13] [14] [15] [16] [17] [18] [19]

R. S. Mulliken, J. Am. Chem. Sot. 72,600 (1950); 74,811 (1952). H. Ratajczak and W. J. Orville-Thomas (eds), Molecular Interactions,Vol. 1. Wiley, New York (1980). R. Foster (ed), Molecular Association, Vol. 2. Academic Press, New York (1979). J. Yarwood (ed), Spectroscopy and Structure ofMoleculur Complexes. Plenum Press, New York (1973). C. N. R. Rao, S. N. Bhat and P. C. Dwivedi, Appl. Spectrosc. Reo. 5.1 (1971). R. Foster, Organic Charge-Transfer Complexes, Academic Press, New York (E&9). R. S. Mulliken and W. B. Person, Molecular Complexes: A Lecture and Reprint Volume. Wiley, New York (1969). L. J. Andrews and R. M. Keefer, Molecular Complexes in Organic Chemistry. Holden-Day. San Francisco (1964). K. 0. Cheun, K. J. Burum and C. K. Joon, Taehun Hwahahhoe Chi 26,363 (1982) (in Korean) [Gem. Abstr. 142862e, 98 (1983)]. I. Uruska and H. Inerowicz, J. Sol. Chem. 9,97 (1980); I. Uruska and H. Karaczewska, 1. Sol. Chem. 8, 105 (1979). A. J. Sonnessa and J. M. Daisey, Spectrochim. Acfa 32A, 465 (1976). R. F. Lake and H. W. Thompson, Proc. R. Sot. Lond. A297,440 (1%7). J. N. Chaudhuri and S. Basu, Trans. Faraday Sot. 55, 898 (1955). For example see: R. D. Gillard, Coord. Chem. Rev. 16,67 (1975); C. Creutz, Commebts Inorg. Chem. 1, 293 (1982). L. A. Summers, The Bipyridinium Herbicides. Academic Press, New York (1980). K. V. Thiman and S. Satler, Proc. Nutn. Acad. Sci. U.S.A. 76.2770 (1979). L. A. Summers, in Aduances in Heterocyclic Chemistry (edited by A. R. Katritzky), Vol. 35. Academic Press, New York (1984). R. L. Scott, Rec. Truu. Chem. 75, 787 (1956). H. A. Benesi and J. H. Hildebrand, J. Am. Chem. Sot. 71,2703 (1949).

Charge-transfer studies [20] G. G. Aloisi, G. Beggiato and U. Mazzucato, Truns. Faraday Sot. 66.3075 (1970). (211 V. G. Krishna and B. B. Bhowmik. 1. Am. Chem. Sot. 90,170O (1968). [22] S. F. Mason, X C/tern. Sot. 1240 (1959). [23] V. Barone, P. L. Cristinziano, F. Lelj and A. Pastore, Gazz. Wm. Ital. 112, 195 (1982). [24] G. Briegleb. Electronen hnator-Akeptor Komplexe. Springer, Berlin (l%l). [Z] J. A. A. Ketalaar. 1. Phys. Rathn 15, 197 (1954).

1733