Electrical conductivity of some amino-hydroxypyridine complexes

May 1995

ELSEVIER

MaterialsLetters 23 ( 1995) 325-330

Electrical conductivity of some amino-hydroxypyridine

complexes

M.G. Abd El Wahed, K.A. El Manakhly, SM. Metwally, H.A. Hammad Fucuhy of Science, Zaguzig and Al Azhar Universities, Zogozig, Egypt Received20 November 1994; in final form 6 March 1995; accepted 7 March 1995

Abstract The electrical conductivity of 2-amino-3-hydroxypyridine, 2-amino-pyridine and their complexes with Mn(II), Fe(U), II), Ni( II) and Cu( II) was measured in the temperature range 293430 K. The conductivity is increased by complexation while the activation energy required for conduction is lowered. The conduction takes place according to the hopping theory. To characterize the composition and structure of the complexes, conductometric titration and IR spectra were used. Co(

1. Introduction A considerable amount of work has been concentrated on the electrical conductivity of organic compounds, mostly in the form of pressed powders [ 11. The T electrons of unsaturated organic compounds are able to move over the entire molecules causing a considerable conductivity. In several organic solids, conduction takes place by the hopping mechanism in which the variation of mobility of charge carriers with temperature is attributed to the jumping from one organic molecule to the next 12-41. An important group of organic solids leading to a higher conductivity are the charge-transfer complexes, in which one type of molecules acts as a donor while another type acts as an acceptor [ 5-71. The electrical properties of 2-amino-3-hydroxypyridine complexing with Mn(II), Co(II), Ni(II), Nd( II), Gd( III) and Yb( III), in molar ratio 1 : 1, were investigated. It was found that the conductivity is increased in complexation and the charge transport occurs by the hopping mechanism [ 81. The complexation of 2-amino-3-hydroxypyridine with cobalt, nickel and copper was characterized by Ir spectra [ 91. The pyridine molecule is N, N, 0 donor in the case of Co 0167-577x/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved .SSDIO167-577x(95)00044-5

and Ni complexes. In the Cu complex, the ligand is N, N bidentate donor. The purpose of the present work was to obtain more information about the electrical conductivity of divalent transition metal-pyridine complexes having molar ratios M : 2L and M, : M, : L. Conductometric titration and IR spectra were used to interpret the composition and structure of the complexes.

2. Experimental All chemicals used in this work were of AR grade. The composition of the complexes was characterized in the basis of conductometric titration using a digital conductivity meter 5800-05 solution analyzer (ColeParmer Instrument Co.). 30 ml of lop3 M of different transition metal cations, Mn( II), Fe( II), Co( II), Ni(II), Cu(II), Cu(II)/Mn(II), Cu(II)/Fe(II), Cu(II)/Co(II) and Cu(II)/Ni(II), were titrated against lo-’ M of amino-hydroxypyridine solution at room temperature. Solid complexes of stoichiometric ratios M : 2L and Mi : M, : L, where L stands for pyridine derivatives, M, is Cu( II) and M or M2 are different divalent transition

326

M.G. Abd El Wahed et al. /Materials

Letters 23 (1995) 325-330

metal cations, were prepared in an aqueous solution. The reaction mixtures were refluxed on a water bath for 5 h, then cooled at room temperature. Complexes were filtered off, recrystallized and dried over silica gel. To identify the structure of the complexes, inliared spectra (potassium bromide disks) were recorded on a Shimadzu IR-440 spectrometer (5000-300 cm-i). To determine the number of crystallization water molecules, a definite weight of a complex was heated in an oven for 5 h at 120°C. By determination of the weight loss, the percent of HZ0 molecules could be calculated. The results indicate that the complexes of type M : 2L have two water molecules, while those of type M, : M2 : L have three water molecules of crystallization/ Electrical measurements were made on solid discs having 13 mm diameter and 0.7 mm thickness. The discs were pressed under a pressure of 4 ton/cm’. The measurements were carried out as described previously

1101. 250

3. Results and discussion The conductance of an electrolytic solution at any temperature depends on the ions present and their concentration. The addition of a solution to another one will affect the conductance according to whether or not ionic reactions occur. Thus, by the addition of a base to an acidic solution, the conductance decreases owing to the replacement of the hydrogen ion of high conductivity by another cation of lower conductivity [ 111. Fig. 1 shows the relation between the conductance measured for various cations and the volume of 2amino-3-hydroxypyridine added. A common behavior of the conductograms is the continuous decrease in conductance with addition of pyridine (basic solution). This is attributed to the complex formation. The breaks of the curves demonstrate the stoichiometry of the complexes studied. It is clear that amino-hydroxypyridine reacts with the various transition metal cations in stoichiometric ratios 2 : 1, 1: 1 and 1: 2, respectively. The 2-amino-3-hydroxypyridine molecule has three active centres, namely, the nitrogen of the NH2 group, the oxygen of the OH group and the nitrogen of the pyridine ring. Good evidence for the structure of its complexes can be attained from IR spectra. Fig. 2 shows the IR spectra of amino-hydroxypyridine

t . 123L5670 ml

added

of

ligand

Fig. 1. Conductometric titration of amino-hydroxypyridine complex.

(ligand I) and its complexes. All complexes display a strong and broad band at 3100 cm-’ which is assigned to vou of coordinated water molecules [ 121. A medium and sharp band at 3300 cm- ‘, which is attributed to N-H stretching, disappeared in complexation [ 131. Hydrogen bonding lowers and broadens NH stretching frequency to a lesser extent than in the case of the OH group. The intensity of N-H absorption is usually less than that of OH absorption. This is supported by the disappearance of the strong and broad absorption band at 2300 cm- ’ corresponding to the intramolecular H-bonded OH [ 131. On the other hand, the shift of the medium band observed at 1620 cm- ‘, which is assigned to the bending of NH,, to 1600 cm ~ ’ confirms that the nitrogen atom of amino group is one of the coordination sites, for both types of complexes, M:2L and M1 :M*:L. The spectrum of free amino-hydroxypyridine shows a medium absorption band at 1560 cm-’ due to vc,,,.

M.G. Abd El Wahed et al. /Materials

Letters 23 (1995) 325-330

321

M, : M2 : L, the amino-hydroxypyridine molecule is N, N, 0 donor. To ascertain the structure of amino-hydroxypyridine complexes, the infrared spectra of aminopyridine and hydroxypyridine complexes were also studied. Fig. 3 shows the spectra of 2aminopyridine (ligand II) and its complexes as well as 3-hydroxypyridine (ligand III) and its copper complex. On comparison of the infrared spectra of aminopyridine and amino-hydroxypyridine complexes, it is inferred that they are virtually identical. However, the spectra of aminopyridine complexes show an absorption band around 380 cm-’ which is due to v~_~. It is clear that the aminopyridine molecule is N, N donor. On the other hand, the spectrum of the hydroxypyridine complex demonstrates an absorption band at 500 cm- * corresponding to G+. A strong

LOO0

2000

1500

1000

Fig. 2. IR spectra of amino-hydroxypyridine

600

cm-’

and its complexes.

This band is weakened and shifted to a higher frequency after complexation. According to Gill et al. [ 141, coordination of the pyridine nucleus to the divalent metal ions results in a pronounced shift of the band located at 1560 cm-’ in the spectra of uncoordinated pyridine to higher frequency with significant lowering in intensity. Such spectral behavior suggests coordination of the pyridine nitrogen atom. In the spectra of binuclear complexes, M, : Mz : L, a new band is observed at 500 cm-’ which is attributed to VM_o[ 151. The above observations indicate that the aminohydroxypyridine molecule is N, N donor in the case of M: 2L complexes. For binuclear complexes of type

L

ual

2000

1500

1000

600

cm-’

Fig. 3. IR spectra of aminopyridine and its complexes hydroxypyridine and its copper complex.

as well as

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M.G. Abd El Wahed et al. /Materials

Letters 23 (1995) 325-330

M : 2L and Mr : M, : L as well as the complexes of aminopyridine of type M : 2L. The linear behavior of the temperature dependence of conductivity is like in an intrinsic semiconductor. The plots obtained consist of two linear parts indicating that a phase transition takes place during the increase of temperature, It is known that at low temperatures, an electron tunnels slowly through the crystal as if in a band of large effective mass, but at high temperatures it moves from site to site by thermally activated hopping [ 171. Therefore, according to the Holstein model of small polaron [ 181, the phase transition observed can be attributed to a change from electronic to ionic conduction, i.e. from band to hopping modes. The electrical data for pyridine complexes are summarized in Tables 1 and 2. It is apparent that the conductivity of pyridine derivative is increased in complexation with transition metal cations. At the same time, the activation energy between the valence state and the next allowed energy state is lowered after complexation. The obtained conductivity values at 294 K follow the order Mn
2.6 conductivity

I

I

2.8

3.0

s

3.2

of amino-hydroxypyridine

Lo

-

1000/T

and its

band located at 1400 cm- ’ owing to OH in-plane bending vibration in the spectrum of free hydroxypyridine, disappeared after complex formation [ 161. The band at 1580cm-’ , due to vc = N, is still present in the copper complex, i.e. nitrogen atom of the pyridine ring does not take part in the complexation process of hydroxypyridine. Generally, the conductivity is defined as the reciprocal of resistivity and it is related exponentially to the temperature through the relation, U= a, exp( - EIkT)

,

where E is the activation energy of the conduction process. The electrical conductivity of the complexes under investigation was followed in the temperature range 293430 K. All complexes demonstrate an increase in conductivity with rising temperature. Figs. 4-6 show the relation between log aand 1ITfor aminohydroxypyridine and its complexes having molar ratios

-lo/

/ 2 .i

2.6

Fig. 5. Electrical conductivity complexes.

2.8 of binuclear

3.0

3.2 1ooo/K

amino-hydroxypyridine

M.G. Abd El Wahed et al. /Materials

-5

Letters 23 (1995) 325-330

329

It is obvious that the increase of complex stability, i.e. strong interaction between metal and ligand, will increase the number of dislocated electrons on pyridine molecule which leads to an increase in conductivity

I

1201. The increase in conductivity

-6

i

w

2.L

2.6

2.8

3.0

3.2

1000/T

Fig. 6. Electrical conductivity of aminopyridine complexes.

According to Irving and Williams [ 19 1, the stability of transition metal complexes with amines increases as the size of the metal ion decreases or the value of the ration charge/radii increases, i.e. the stability increases towards the copper-amino-hydroxypyridine complex.

of amino-hydroxypyridine ligand by complexing with a transition metal cation is attributed to the inclusion of the metal into the v electron delocalization of the ligand [ 201. Pyridine is u-donor and n-acceptor ligand; o-donor by using the lone pair of electrons on nitrogen and n-acceptor by using a delocalized orbital on the ring [ 2 1] . The interaction of the d-orbitals of the metals with the T-orbitals of pyridine derivatives gives rise to the formation of new orbitals which are delocalized over the entire molecular complex leading to an increase in conductivity. This is supported by comparing the conductivity values obtained for complexes having molar ratio M: 2L and those of type Mi :M,:L. The later type of complexes shows a strong interaction between the dorbitals of metals and rr system of the ligand. Therefore, the conductivity value of a binuclear complex is higher than that of the M : 2L complex as given in Table 1. The electron affinity and the degree of resonance of aminopyridine are higher than those of amino-hydroxypyridine. Consequently, the conductivity of aminopyridine complexes is higher than in the other complexes. In order to elucidate the nature of conduction in pyridine complexes, it is necessary to determine the mobility of charge carriers, p, using the conductivity equation u=nep,

Table 1 Electrical data for amino-hydroxypyridine complexes at 294 K Compound

(T (W’

ligand Mn:2L Fe:2L Co:2L Ni:2L Cu:2L Cu:Mn:L Cu:Fe:L Cu:Co:L Cu:Ni:L Cu:Cu:L

2.82~ 7.94x 1.00x 1.50x 2.00x 2.37~ 1.05x 5.31 x 2.11 x 2.75 X 3.98 x

cm-‘) lo-” 10-l’ lo-‘0 lo-‘0 lo-I0 lo-” lo-lo lo-‘O lo-‘0 IO-” IO- I0

E (eV)

W (W

n (cme3)

p (cm*/V s)

0.63 0.24 0.45 0.10 0.16 0.15 0.32 0.23 0.12 0.13 0.21

0.68 0.27 0.49 0.14 0.20 0.18 0.35 0.27 0.15 0.17 0.25

3.72X lOI 1.84x 10” 4.58 x 10” 4.66 x lo= 4.35 x ld2 6.46 x 10” 7.81 x 1019 2.74 x lo*’ 2.12x lo*3 l.43X10Z3 6.03 X 10”

4.74x 2.70X 1.36X 2.01 x 2.87~ 2.29 x 8.40X 1.21 x 6.22X 1.20x 4.13x

lo-’ IO-l3 1O-9 lo-‘5 lo-l4 lo-l4 lo-‘* 1o-‘2 lo-” lo-l4 lo-‘3

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M.G. Abd El Wahed et al. /Materials

Table 2 Electrical data for aminopyridine

complexes

Compound

cr (a-’

Co:2L Ni:2L Cu:2L

3.63~ lo-” 5.37 x lo- I0 8.71 X lo-‘”

Letters 23 (1995) 325-330

at 294 K

cm-‘)

E (eV)

W (eV)

n (cm-3)

p (cm’/V

1.14 0.16 0.20

1.18 0.20 0.24

6.55 x lo5 4.35 x 1022 8.96X 10”

3.46~ IO3 7.72~ lo-“’ 6.08x10-‘3

s)

where n is the concentration of charge carriers and can be obtained from the relation

I21 M.V. Anasuya, T.S. Natarajam, G. Rangarajan, S. Venkatechalam and P.T. Manoharan, Solid State Commun. 77 (1991) 651.

n=2(2Tm

131 H.K. Shim, SK. Kim, J.I. Jin,K.H. Kimand KoreanChem.Soc. 11 (1990) 11.

‘kT/h2)

3’2 exp( -Elki’J

,

where m + is the effective mass of the electron. The mobility value of amino-hydroxypyridine, 1O-7 cm2/ V s, is lowered in complexation reaching the value of lo-r5 cm/V s. The width of an energy band reflects the degree of the orbitals’ overlap of adjacent molecules, i.e. the force of interaction between orbitals. Then, for a weak overlap, the energy band is narrow and the charge carriers’ mobility is low. The very low value of mobility obtained suggests that the conduction in the investigated compounds takes place by the hopping mechanism operating between localized sites

[221. On the basis of hopping theory, the conductivity given by the relation,

is

141 V.M. Yartsev and A. Graja, Mater. Sci. 14 ( 1988) 95.

ISI K. Nakasuji, Kagaku Gijutsushi Mol. 28 (1990) 110. 161 M.M. Ayad, Z. Phys. Chem. (Munich) 166 (1990) 223. 171 K. Imaeda, T. Enoki, T. Mori, H. Inokuchi, M. Sasaki, K. Nakasuji and I. Murata, Bull. Chem. Sot. Japan 62 (1989) 372. 181 M.G. Abd El Wahed, A.M. Hassan and S.A. Hassan, J. Mater. Sci. Letters 12 (1993) 453. 191 M. Bala and AI. Sinha, Asian J. Chem. (India) 1 ( 1989) 392.

[IO] M.G. Abd El Wahed and SM. Metwally, Mater. Letters 20 ( 1994) 231.

I J. Bassett,

R.C. Denney, G.H. Jeffery and J. Mendham, Volgel’s textbook of quantitative inorganic analysis (Longman, New York, 1978) p. 615.

!I K. Nakamoto, Infrared and Raman spectra of inorganic and coordination

g= (Const./T)

exp( - WlkT)

,

in which W is the activation energy for the hopping process and can be defined as [ 231: w=w,+;w,.

compounds

(Wiley, New York, 1978) p. 226.

‘I D.H. Williams and I. Fleming, Spectroscopic organic chemistry 49.

(McGraw-Hill,

methods in New York, 1980) pp. 51,

1141 H.S.Gill,R.H.Nuttoll,D.E.ScuifeandA.W.D.Sharp,J.Inorg. Nucl. Chem. 18 (1961) 79.

[ 151 J. Kincaid and K. Nakamoto, Spcctrochim. Here W, is the polaron hopping energy and W, is the disorder activation energy. The difference between both activation energies Wand E gives the activation energy of charge carrier mobility. the experimental values of activation energy of mobility of pyridine complexes range from 0.03 to 0.05 eV as reported in Tables 1 and 2.

[ 1] M.G. Abd El Wahed, in: Handbook

of advanced materials testing, eds. N.P. Cheremisinoff and P.N. Cheremisinoff (Dekker, New York, 1995) p. 269.

Acta 32 (1976)

277.

I 161 R.M. Silverstein and G.C. Bassler, Spectrometric identification of organic compounds

(Wiley, New York, 1968) p. 85.

[ 17 I C. Kittel, Introduction to solid state physics (Wiley, New York, 1976) p. 313.

[ 181 T. Holstein, Ann. Phys. 8 ( 1959) 343. 1191 H. Irving and R.J.P. Williams, J. Chem. Sot. 3192 (1953). 1201 H. Meier, Organic semiconductors,

dark and photoconductivity

of organic solids (Verlag Chemie, Weinheim,

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