i Jcs t 2011010406

8/3/2019 i Jcs t 2011010406

http://slidepdf.com/reader/full/i-jcs-t-2011010406 1/10

International Journal of Chemical Science and Technology (ISSN: 2248-9797)

Volume 1-Issue4, October 2011.pp.158-167

www.ijcst.net

158

Buttressing Effect and Angle of Twist in Alpha-Aryl Pyridines

P. TomasikChemistry Department, Agricultural University of Cracow, Al. Mickiewicza 21, 31-120 Kraków

K. M. Darwish*

Chemistry Department, Science Faculty, Garyounis University, Benghazi – Libya.

Corresponding Author E mail: [email protected]

ABSTRACT : The ortho effect is studied in pyridine systems in which the lone electron pair of the nitrogen

atom is expressed as the reaction site. The process obscured by the ortho effect is the interaction between metal

ions and this reaction site in 2-phenyl and 2- p- arylpyridines. The ortho effect is manifested by the twist of phenyl

or aryl group out of the plane of the pyridine ring. The angle of twist is calculated from the Braude-Sondheimer

equation1 that involves the molal extinctions of the 1La ← 1A absorption bands in the UV – spectra of the compounds

studied. Due to the ortho effect in these compounds the intensity of the 1La ← 1A band is reduced. The angle of

twist in these compounds depends on the substituents (hydrogen, nitro, amino and acetylamino groups) residing inthe aryl group. These substituents do not exhibit any remarkable buttressing effect.

1. INTRODUCTION

The ortho effect in presented arylpyridines is a result of several effects operating simultaneously

These mutually dependent effects include (1) Inductive (stronger than that transmitted from the

para position) (2) Resonance effect (weaker, due to possible twist of group out of the plane of

benzene ring) (3) Interaction through the space (4) Steric hindrance (5) Intramolecular hydrogen

bonding (6) Buttressing effect which affects all the previous effects composing the ortho effect.

The ortho effect with the involvement of the aza atom as the reaction site may be approached

on two manners. The first is dealing with the selection of appropriate reaction series with variable 2-X-substituents. In this series the reaction site will be attacked by the same reagent. The second

approach deals with studying the effect of variable reagent upon one 2-substituted pyridine

derivative. The latter case is our field of study in this paper. This model is suitable for studying an

intervention of one of components of overall ortho effect. This component is the so-called

buttressing effect. Buttressing effect is due to the resonance interaction between the reaction

site (the endocyclic ring nitrogen atom) and the substituent (the aryl group). It either prohibits

or favours the steric release by the twist of substituent out of the plane of aromatic ring.

X

N

. .

X = NO2 : e - withdrawing, larger twist for more favorable complex and ion-dipole pair formation

X = H : No twist, aromatic rings give planner and conjugated system in cyclohexane

X = NH2, NHAc : e - donating, more resonance coupling between rings, less twist

leading to obstructed metal-to-ligand coordination

XN.

metaln+

XN

+ metaln+

This case forces the lone electron pair orbital of the pyridine ring nitrogen atom to suffer

significant steric hindrance and as a result become available for any interacting metal cation

only by the possible twist of phenyl group. Buttressing effect was preliminarily investigated

via the interaction between various metal cations and the lone electron pair orbital of the aza

atom in 2-phenylpyridine. Thus 2-phenylpyridine in cyclohexane solution should possess bothrings in the same plane and the π– electron system of both of them should be conjugated.

mailto:[email protected]



8/3/2019 i Jcs t 2011010406




www.ijcst.net

159

This result formed a standard for further comparisons included in studying similar effect

with 2- ( p-nitrophenyl)pyridine, 2- ( p-acetylaminophenyl)pyridine and 2- ( p-aminophenyl)pyridine

applied as the ligands in the complexes and ion-dipole pairs supposingly formed. The groups

residing in the para-position of the phenyl ring were supposed to form a buttress. Previous

studies2

revealed that the resonance interaction between the pyridine nitrogen atom and any

electron withdrawing α- substituent is reduced to minimum. Since p-nitrophenyl group is an

electron withdrawing substituent the order of bond between both aromatic rings should be

closer to unity than that in the case of that bond in 2-phenylpyridine. This fact speaks of

the easy steric release from 2- p-nitrophenyl group and the formation of stronger complexes

or ion-dipole pairs with various metal cations. Following this pathway of deduction, the

twist of p-aminophenyl and p-acetylaminophenyl groups should be obstructed by the electron

donation from these substituents and in consequence the species formed should be less stable.

Consequently, the order of influence on the stability of species should be as follows:

The angle of twist depend on size, charge, polarity and electrophilicity of the approaching

metal cation. At the same time the angle of twist can be influenced by any para-substituent

of the phenyl group (aryl group). Thus electron withdrawing groups ( for instance NO 2 group)

reduce the resonance coupling between both rings, and make the rotation of the aryl group

more favorable and in conclusion provide conditions for stronger metal to ligand interaction.

On the other hand electron donating groups (for instance NH2 group) increase the resonance

coupling between both rings and prohibit the twist of the aryl group and obscure the metal

to ligand coordination. The so-called buttressing effect can be overshadowed by the obvious

substituent effects. If it so, 2-( p-aminophenyl) pyridine should interact more strongly with metal

cations than 2-( p-nitrophenyl) pyridine. The UV spectroscopy seems to be a suitable tool for

studying these effects. In this method comparison can be made for the molal extinctions at

the maximum of the1La ←

1A band in the spectra of 2-( p-X-phenyl)pyridines (X = H, NO2,

NH2, NHCOCH3) run in aqueous solutions of some metal salts (εx) and in cyclohexane (εо)

and the angle of twist (Ө) can be calculated from the Braude - Sondheimer equation

1

:

⁄

Based on the identity criterion with the observation in the case of 2-phenylpyridine (taken as a

standard without steric effect being planner) the importance of buttressing effect can be deduced.

2. EXPERIMENTAL PART

All chemicals (solvents and salts) used were of analytical grade. Water was redistilled and

organic ligands were prepared according to the literature methods. 2-( p-Nitrophenyl)pyridine was

prepared by decomposition of the p-nitrophenylbenzenediazonium chloride in pyridine according

to Haworth et al.16 This free radical reaction gives the three (2-, 3-, and 4-) p-nitrophenylatedproducts. An aqueous solution of p-nitrobenzenediazonium chloride (from p-nitroaniline, 70 g)

was added dropwise during 2h to pyridine (500 cm3) stirred at 40 ºC; the heat of reaction

maintained the temperature. Nitrogen was freely evolved. The reaction was completed by

warming on the steam bath for 1h. The mixture cantaining some nitrophenylpyridine in

suspension was then poured into a large volume of water and the brown precipitate

obtained was filtered off, washed, and dried in a steam oven. The product (75 g) was dissolved

in boiling benzene (300 cm3) and filtered from some insoluble matter. When cold, a yellow solid

(0.5 g) separated, which after sublimation in vacuum and crystallization from nitrobenzene yielded

a compound, regarded as α, α'-di- p-nitrophenylpyridine in fibrous needles of m. p. 293 ºC.

Removal of benzene from the filterate left a yellow solid residue of nitrophenylpyridines (55 g,

m. p. 105 – 115 ºC). Separation into the constituent isomerides was effected by the method of

Forsyth and Pyman; 100 g of the purified mixed nitrophenylpyridines giving α- m. p. 130-131ºC,

β- m. p. 146-147 ºC, γ- m. p. 122-123 ºC. The crude bases (200 g) were dissolved in hot 5N-

8/3/2019 i Jcs t 2011010406




www.ijcst.net

160

hydrochloric acid (1.2 L) and solution was digested with charcoal, filtered and kept. The

hydrochlorides of 2- and 4-isomerides that crystallized were collected and further crops were

obtained by condensation of the mother liquors. The bases regenerated from the crystalline

hydrochlorides were crystallized fractionally from alcohol; the 2-isomeride was then obtained in

nearly pure state. Its purification was completed by further crystallization from 5N- hydrochloric

acid followed by crystallization of the base from alcohol. The fraction of the base most

soluble in alcohol gave on crystallization from 5N- hydrochloric acid the hydrochloride of the

4-isomeride. After recrystallization as hydrochloride and then as a base from acetone the 4-

isomeride was obtained in a pure state. The purity of the final product was controlled by thin layer

chromatography. Thus the ethanolic solution of the sample was put on the start of silica-gel

precoated aluminium foil (manufactured by merck) and developed subsequently by cyclohexane,

benzene, ether, ethanol. After each developing the plate was observed under the UV-light (254

nm). The pure compound left always single non-tailing spot on the plate.

2( p-Aminophenyl)pyridine was prepared by reduction of 2( p-Nitrophenyl)pyridine by two ways:

Accoding to Forsyth and Pyman17

: 2( p-Nitrophenyl)pyridine (20 g) was reduced by tin foil

(40 g) and concentrated hydrochloric acid (120 cm

3

). The solution was diluted with waterand after removal of tin by conc. NaOH (formation of soluble Na2SnO2), then the 2-( p-

aminophenyl)pyridine separates. It crystallizes from ether (as a free base) or from alcohol.

The purity of the compound was checked by the thin layer chromatography as described

above. M. p. 97- 98 ºC.

According to Tomasik 18

: Reduced iron powder (20 g) was suspended in an aqueous

solution (20 mL) of glacial acetic acid (2 mL) followed by portionwise addition of

2( p-Nitrophenyl)pyridine (5 g). After addition of each portion of the nitro-compound the

content of the beaker is stirred with glass rod. If the reaction does not appear to be

exothermic the reaction vessel is immersed into the steam bath. When the reduction is

completed the reaction mixture is left in the steam bath for 30 minutes followed by

addition of anhydrous sodium carbonate (10 g) which was thoroughly homogenized with the

reaction mixture. The resultant alkalized solid was extracted with hot chloroform. The extractwas evaporized to dryness and the left dry residue was crystallized from ethanol. The

purity of the product was controlled by thin layer chromatography as above. M. p. 97-

98ºC.

2( p-Acetylaminophenyl)pyridine was prepared by heating 2( p-Aminophenyl)pyridine with excess

amount of acetic anhydride on a steam bath. After 5 h the reaction mixture was left to stand

for 24 h followed by evaporation on the steam bath. The white or yellowish crystals which

precipitated were crystallized from hot water. The yield of product was quantitative. M. p. 85 ºC.

2-Phenylpyridine was prepared according to Forsyth and Pyman17 by decomposition of benzene-

diazonium chloride in pyridine. Similarly as in the case of 2( p-Nitrophenyl) pyridine three isomers are

formed. They were isolated by ethanol and acetone crystallization of their picrates. Picrates were then

decomposed by 40 % aqueos NaOH. The liberated 2-phenylpyridine was extracted by ether andafter removal of ether the residue was distilled under reduced pressure. The purity of the

compound was checked in the usual manner using thin layer chromatography.

The UV- spectra were recorded using the double-beam Pye - Unicam 1800 spectrophotometer

equipped with 1 X10-3

m quartz cells. The solutions of pyridine derivatives in salts were prepared

in such a manner that the concentration of salt exceeded that of the ligand by four orders.

3. RESULTS AND DISCUSSION

The twist of any substituent results in partial deterioration of the conjugated π-electron system

and in consequence reduces the delocalization of electrons. This is manifested by lowered

propabilities of the1

La ←1

A electronic transition in the molecule measure of which is theintensity of a relevant spectral band. In this manner always εx < εо if any possible twist of

8/3/2019 i Jcs t 2011010406




www.ijcst.net

161

Table. 1

UV-spectral characteristics of 2-phenyl pyridine

Solventa

λ (nm) εmax* Cos2 Ө

Cyclohexane 211, 216, 250*, 270, 298 13200, 8800,13800*, 8000, 2000 0.000 00.00

Ethanol 250*, 268 13700*, 11600 0.993 4.88

H2O 250*, 270 13000*, 10000 0.942 13.93HCl (0.1M) 210, 246*, 294 13000, 5500*, 14900 0.399 50.85

LiCl 248*, 268 11900*, 9500 0.862 21.78

LiNO3 254*, 272 12000*, 12100 0.870 21.17

NaCl 249*, 270 10700*, 9700 0.775 28.29

NaNO3 252*, 270 10300*, 9600 0.746 30.24

KCl 248*, 274 9600*, 8700 0.696 33.48

KNO3 252*, 270 11100*, 9700 0.804 26.25

CsCl 248*, 270 12100*, 10100 0.877 20.55

CsNO3 251*, 270 10500*, 9700 0.761 29.28

MgCl2 247*, 272 11500*, 10700 0.833 24.09

Mg(NO3)2 254*, 270 11600*, 10300 0.841 23.53

CaCl2 249*, 271 11000*, 9500 0.797 26.77

Ca(NO3)2 254*, 270 10900*, 10600 0.790 27.28

SrCl2 249

*

, 270 11000

*

, 9300 0.797 26.77Sr(NO3)2 250*, 270 9100*, 6500 0.659 35.70

BaCl2 249*, 271 11900*, 10300 0.862 21.78

Ba(NO3)2 249*, 270 8200*, 9700 0.594 39.57

Pb(NO3)2 255*, 288 5000*, 12100 0.362 52.99

CuCl2 249*, 273 8900*, 8700 0.645 36.58

Cu(NO3)2 258*, 275 10000*, 11400 0.725 31.65

AgNO3 250*, 11000* 0.797 26.77

ZnCl2 251*, 271 11300*, 11300 0.819 25.19

Zn(NO3)2 254*, 277 8500*, 11300 0.616 38.29

CdCl2 210*, 247, 288 12900*, 7500, 12700 0.543 42.53

Cd(NO3)2 255*, 274 10900*, 12300 0.790 27.28

La(NO3)3 255*, 277 9300*, 11600 0.674 34.82

CrCl3 253*, 275 10000*, 12000 0.725 31.65

Cr(NO3)3 255*, 291 8100*, 12500 0.587 39.99

CoCl2 250*, 272 11300*, 11100 0.819 25.19Co(NO3)2 255*, 272 9300*, 9800 0.674 34.82

MnCl2 249*, 272 11000*, 9500 0.797 26.77

NiCl2 254*, 270 11900*, 12600 0.862 21.78

Ni(NO3)2 255*, 271 9400*, 13900 0.681 34.40

the substituent takes place. The set of εx values was measured for all four pyridine derivatives,

dissolved in aqueous solutions of either nitrates or chlorides of the following metals: Li, Na, K,

Cs, Mg, Ca, Sr, Ba, Pb (II) (only nitrate), Cu (II), Ag (I) (only nitrate), Zn, Cd, La (only nitrate),

Mn (II) (only chloride), Cr (III), Co (II), and Ni.

The concentration of the relevant metal cations was by ~ 4 orders higher (10-1

M) than that of the

pyridine derivatives (5 x 10

-5

M) to shift of the equilibrium towards complex or ion-dipole pair:Simultaneously, based on the experience of many authors3

the species studied are believed to

have anion of the salt beyond the inner coordination sphere of eventual complex. Thus they

have the general formula MLn+

. Moreover the εо values introduced into the Braude-Sondheimer

equation are taken from cyclohexane spectra of the four pyridine derivatives. Only in solutions with

solvent-solute interactions limited to the van der Waals forces and without solvation of the lone

electron pair of the aza-atom both rings may be believed to be planner. Comparison of intensities

of the1La ←

1A band in cyclohexane, aqueous and ethanolic spectra allows to realize that due to

hydration and solvation by alcohol the aryl groups are twisted by the angles given in tables 1- 4.

In these spectra the most intensive band is the1La ←

1A band (actually ICT – intramolecular

charge transfer band in the case of the nitro derivative, which can be subject to correlation,

unlike that in the spectra of 2-phenylpyridine). In the 2-phenylpyridine spectra the most intensiveband at 250 nm is the

1La ←

1A band. It partly hides more longwavelengths band of lower

8/3/2019 i Jcs t 2011010406




www.ijcst.net

162

Table. 2

UV-spectral characteristics of 2-( p-nitrophenyl) pyridine

Solventa

λ (nm) εmax*

Cos2 Ө

Cyclohexane 300* 18600* 0.000 00.00

Ethanol 302* 16800* 0.903 18.12

H2O 302* 18000* 0.968 10.35HCl (0.1M) 297* 14800* 0.796 26.87

LiCl 303* 15200* 0.817 25.31

LiNO3 303* 16200* 0.871 21.05

NaCl 302* 14500* 0.780 28.00

NaNO3 302* 13000* 0.700 33.20

KCl 300* 13000* 0.699 33.28

KNO3 303* 13200* 0.710 32.60

CsCl 303* 14000* 0.753 29.82

CsNO3 302* 13500* 0.726 31.58

MgCl2 303* 16000* 0.860 21.95

Mg(NO3)2 302* 15400* 0.828 24.51

CaCl2 305* 13700* 0.736 30.88

Ca(NO3)2 305* 12800* 0.688 33.95

SrCl2 305

*

14200

*

0.763 29.10Sr(NO3)2 310* 10300* 0.553 41.92

BaCl2 304* 16000* 0.860 21.95

Ba(NO3)2 314* 10000* 0.538 42.84

Pb(NO3)2 302* 13000* 0.699 33.28

CuCl2 304* 13200* 0.710 32.60

Cu(NO3)2 301* 12000* 0.645 36.56

AgNO3 300* 14600* 0.785 27.63

ZnCl2 302* 14200* 0.763 29.10

Zn(NO3)2 302* 12000* 0.645 36.56

CdCl2 301* 16000* 0.860 21.95

Cd(NO3)2 305* 14600* 0.785 27.63

La(NO3)3 302* 14800* 0.796 26.87

CrCl3 301* 13500* 0.726 31.58

Cr(NO3)3 305* 14600* 0.785 27.63

CoCl2 302* 15800* 0.849 22.83Co(NO3)2 304* 12600* 0.677 34.61

MnCl2 304* 15200* 0.817 25.31

NiCl2 302* 16400* 0.882 20.12

Ni(NO3)2 303* 13900* 0.745 30.33

intensity which is the1Lb ←

1A . Since the shape of spectrum suggests the type of interaction

between pyridine derivative and the metal ion, the spectra run in aqueous solutions of salts

can be divided into four types (see reproduced examples of plots in Figures 1- 4). The first

type (Fig 1) resembles the cyclohexane spectrum, i. e.1La ←

1A band.

To this type belong spectra measured in cyclohexane, ethanol, water and aqueous solutions

of both salts of Li, Na, K, Cs, Ca, Sr, Ba and Mn. Formation of the spectra of this type should

be understood as an evidence for weak interactions between cation and lone electron pair of

the aza atom. These interactions are too weak to form any complex but strong enough to cause

the twist of the phenyl ring. The atoms causing this type are known as weakly coordinating

atoms. The number of their complexes with pyridine ligands is almost negligible.

The second type (Fig 2) exhibits two bands of approximately equal intensity.

To this type belong spectra for Li (nitrate), Mg, Cu (nitrate) and Zn (chloride). The second

group of spectra presents an intermediary case between the first and third type of spectra. The

coordination ability of atoms responsible for the second type of spectra is unquestionable.3

8/3/2019 i Jcs t 2011010406




www.ijcst.net

163

Table. 3

UV-spectral characteristics of 2-( p-aminophenyl) pyridine

Solventa

λ (nm) εmax*

Cos2 Ө

Cyclohexane 300* 23400* 0.000 00.00

Ethanol 307*, 350 20600*, 2000 0.880 20.23

H2O 296*, 320 19400*, 14000 0.830 24.42HCl (0.1M) 239, 289* 9400, 14000* 0.598 39.35

LiCl 297* 16500* 0.705 32.88

LiNO3 296* 14500* 0.619 38.05

NaCl 297* 17000* 0.726 31.53

NaNO3 295* 13500* 0.577 40.58

KCl 296* 15000* 0.641 36.80

KNO3 270*, 298 10300*, 15800 0.675 34.74

CsCl 296* 17300* 0.739 30.70

CsNO3 295* 15900* 0.679 34.48

MgCl2 297* 17300* 0.739 30.70

Mg(NO3)2 297* 18400* 0.786 27.53

CaCl2 285, 296*, 330 14000, 17800*, 3000 0.761 29.28

Ca(NO3)2 285, 296*, 328 15000, 15100*, 1000 0.645 36.55

SrCl2 296

*

17400

*

0.743 30.42Sr(NO3)2 295* 10500* 0.449 47.94

BaCl2 296* 20200* 0.863 21.70

Ba(NO3)2 299* 13600* 0.581 40.32

Pb(NO3)2 280*, 356 6200*, 15600 0.667 35.26

CuCl2 296*, 356 9000*, 15000 0.641 36.81

Cu(NO3)2 253* 13900* 0.594 39.58

AgNO3 279*, 292 10200*, 11800 0.504 44.76

ZnCl2 284, 294*, 330 15600, 18000*, 4000 0.769 28.71

Zn(NO3)2 296* 12900* 0.551 42.07

CdCl2 294* 10800* 0.462 47.21

Cd(NO3)2 332* 16900* 0.722 31.81

La(NO3)3 320* 13400* 0.573 40.82

CrCl3 354* 14800* 0.632 37.32

Cr(NO3)3 290, 353* 9900, 12100* 0.517 44.02

CoCl2 295* 15200* 0.735 31.00Co(NO3)2 355* 15100* 0.650 36.30

MnCl2 296* 17200* 0.645 36.50

NiCl2 296* 19800* 0.846 23.09

Ni(NO3)2 294* 13300* 0.568 41.07

The third type (Fig 3) have the1La ←

1A band less intensive than the

1Lb ←

1A band. To this

type belong spectra for Cu, Ag, Zn (nitrate), Cd (nitrate), La, Cr (chloride), Co and Ni. This group

of spectra suggests much stronger interactions between 2-phenylpyridine and the metal cation.

Such atoms like Li and Mg and others accounted to this group are known as those giving

weak but isolable solvates.

It is well noticed that any transfer from any spectrum of the first type into that of the third is

due to the decrease of intensity of the1La ←

1A band but also an increase of intensity of

the1Lb ←

1A band. Therefore one may carefully assume the formation of real coordination

bonds between the metal and ligand.

The fourth type (Fig 4) resembles that of the 2-phenylpyridinium chloride ( i. e. spectrum of

2- phenylpyridine in 0.1 M HCl). To this type belong those salts which underwent hydrolysis in

their aqueous solutions due to protonation by hydrochloric acid. The latter type is particularly

pronounced in solutions with more basic 2-( p-aminophenyl)pyridine.

8/3/2019 i Jcs t 2011010406




www.ijcst.net

164

Table. 4.

UV-spectral characteristics of 2-( p-acetylaminophenyl) pyridine

Solventa

λ (nm) εmax*

Cos2 Ө

Cyclohexane 212, 218, 274, 296* 14800, 14800, 15800, 22600* 0.000 00.00

Ethanol 276, 293* 12600, 17500* 0.774 28.36H2O 270, 287* 12800, 16600* 0.735 31.01

HCl (0.1M) 265, 320* 9000, 14300* 0.633 37.30

LiCl 287* 13600* 0.600 39.13

LiNO3 287* 16600* 0.734 31.01

NaCl 294* 15000* 0.663 35.48

NaNO3 266*, 285 13500*, 14000 0.597 39.39

KCl 270, 287* 12500, 15300* 0.667 34.63

KNO3 268*, 285 10500*, 15400 0.681 34.36

CsCl 272*, 288 12100*, 15500 0.686 34.09

CsNO3 286* 14600* 0.646 36.57

MgCl2 286* 14800* 0.655 35.98

Mg(NO3)2 272* 11000* 0.486 33.76

CaCl2 272*, 287 13200*, 13400 0.593 39.64

Ca(NO3)2 288* 16000* 0.708 32.71

SrCl2 287* 15400* 0.681 34.36

Sr(NO3)2 265*, 322 7500*, 13000 0.575 40.67

BaCl2 272*, 288 12200*, 15000 0.664 35.44

Ba(NO3)2 275*, 290 12000*, 14800 0.531 43.22

Pb(NO3)2 319* 9600* 0.425 49.32

CuCl2 305* 11000* 0.486 45.76

Cu(NO3)2 265, 287* 4000, 6700* 0.296 57.01

AgNO3 262, 285* 16500, 16400* 0.726 31.59

ZnCl2 287* 13600* 0.602 39.13

Zn(NO3)2 288* 11400* 0.504 44.74

CdCl2 287* 14800* 0.654 35.97

Cd(NO3)2 286* 13700* 0.606 38.88

La(NO3)3 285*

13200*

0.584 40.16CrCl3 275, 320* 6000, 12200* 0.540 42.72

Cr(NO3)3 275, 323* 1100, 12700* 0.562 41.44

CoCl2 287* 14800* 0.721 31.87

Co(NO3)2 287* 14300* 0.655 35.98

MnCl2 270, 287* 12900, 16300* 0.632 37.30

NiCl2 287* 14800* 0.655 35.98

Ni(NO3)2 287*

13600*

0.601 39.12 a : The salt should be understood as its 10 -1 M aqueous solution.* : ε max (at its relevant λ) chosen for calculation of Cos 2 Ө from the relation ε max / ε cyclohexane

In addition, the type of anion (chloride or nitrate) influences the phenomenon studied. For

instance the spectra of 2-phenylpyridine in solution of the chlorides and nitrates of the same

metal differ from one another in the intensity of spectral bands and sometimes even in thetype of spectra as simply discussed above. There seems to be no regularity of the influence of

anion on the observation.

According to Pearson theory of hard and soft acids and bases,4, 5

pyridine is a borderline base.

The presence of 2-phenyl group which is weakly electron withdrawing group eventually locate

this ligand closer to the hard bases but still in the area of borderline bases. Hence, such base

is able to form complexes or at least interact with Lewis acids from all these soft, hard and

borderline areas. According to the same theory all H+, Li

+, Na

+, K

+, Mg

2+, Ca

2+, Sr

2+, Mn

2+,

La3+

and Cr3+

are hard acids, Ag+, and Cd

2+are soft acids, whereas Co

2+, Ni

2+, Cu

2+, Zn

2+

and Pb2+

are borderline acids. On the other hand, there is even no qualitative correlation

between type of spectra and softness or hardness of cations.

8/3/2019 i Jcs t 2011010406




www.ijcst.net

165

Fig: 1 UV Spectra of 2-phenylpyridine in Fig: 2 UV Spectra of 2-phenylpyridine in

aq.soln. of SrCl2 aq.soln. of ZnCl2

Fig: 3 UV Spectra of 2-phenylpyridine in Fig: 4 UV Spectra of 2-phenylpyridine inaq.soln. of COCl2 aq.soln. of CdCl2

Fig 5: Identity Criterion For the Angles Fig 6: Identity Criterion For the Angles

of Twist in the Case of Chlorides of Twist in the Case of Nitrates

These visually prepared results in Fig 5 and Fig 6 differ to a certain extent from those statistically computed (Tab 5).

8/3/2019 i Jcs t 2011010406




www.ijcst.net

166

Table. 5

The Identity Criterion (Y = aX + b) for interaction of cations with 2-arylpyridines*

Series Anion α a b s r

2-( p-nitrophenyl)NO3 45.76 1.026 1.182 3.289 0.951

Cl 43.59 0.952 1.341 2.861 0.956

2-( p-aminophenyl)NO3 48.87 1.145 1.733 3.280 0.961Cl 45.79 1.028 3.697 3.977 0.911

2-( p-acetylaminophenyl)NO3 46.23 1.044 4.964 3.191 0.956

Cl 47.75 1.101 7.361 4.883 0.887*

Interactions with 2-phenylpyridine are taken as a standard

Table. 6

Correlation of the Effect of Anion upon the Angle of Twist in (Chlorides: Nitrates)

Spectral Series Slope of Correlation 2-( p-nitrophenyl)pyridine 0.928

2-( p-aminophenyl)pyridine 0.898

2-( p-acetylaminophenyl)pyridine 1.156

2-phenylpyridine 1.077

The diagrams (in fig 5 and fig 6) present the criteria of identity for the series of chlorides

and nitrates respectively. They are constructed in such a way that all straight lines leave the

origin (0.00). According to these diagrams, the slopes in both cases are as follows:

instead of the preliminary assumed

All slopes apart from one are above unity (α = 45º). In majority of cases the differences are

almost within the experimental error. In the spectral series composed of 14 and more

experimental points the correlation coefficient > 0.900 can be acceptable.6 In other cases a

strong tendency to the linearity is noticed. Imperfect correlations may be due to double cation

pyridine base interactions, i. e. in which the aza atom and substituent on the phenyl ring are

involved. The electronic effect of the p-substituents in the pyridine moiety upon the basicity

and nucleophilicity (softness) of the ligand is important. Based on the electronic properties of

these substituents expressed by σp-constant of Hammett7

these properties should increase in

the following order of p-substituents:

( ) ( ) ( )

It means that the stability of any metal complex with above ligands should increase in this

order. Indeed the UV – spectral investigation carried out thoroughly did not show any complex

formed between 2-( p-nitrophenyl)pyridine and any of the metal cations used. On the otherhand, literature reported the following metal complexes with 2-phenylpyridine (denoted by L):

LAlBr 3,8

LTeCl 4, 9

LCu 2+

,10,11

L 2CuCl 2 ,12

LZn 2+

,10 LUO 2 acetylacetone,

13 LNi

2+ ,

10 L 2 NiCl 2 .2H 2O,

12L 4 NiCl 2 .2H 2O,

12LRhCl(CO) 2 ,

14 and L 2 PdCl 2.

15

Investigations carried out in our present studies revealed that 2-( p-aminophenyl)pyridine is

also capable of forming metal complexes. Isospestic points could be observed as the metal to

ligand ratio was changed. Such points could be observed in case of 2-( p-aminophenyl)pyridine

with CoCl2 and with CuCl2 and in the case of 2-( p-acetylaminophenyl)pyridine with CuCl2

metal salts. Stability constants could not be calculated as the temperature of measurements was

not precisely stabilized. The differences in the case of straight lines in the series of metal

chlorides may eventually speak for the intervention of a buttressing effect ( lower twist of

p-aminophenyl group than of p-acetylaminophenyl group). The observation of that effect in

8/3/2019 i Jcs t 2011010406




www.ijcst.net

167

the case of nitrates is perhaps obscured by the effect of the nitrate anion. It can play a role

of additional ligand which by its bidentate character more readily than chloride undergoes

coordination into either inner or at least outer coordination sphere. The effect of anion can

easily be evaluated by similar identity criterion as above using data from table 5. The data in

table 6 present slopes of correlations of effects of anions upon the angle of twist within the

same spectral series, i. e. containing the same ligand.

4. CONCLUSIONS:

2-Phenyl- and 2-arylpyridines interact with metal cations in aqueous solutions to form

either complexes, ion-dipole pairs or pyridinium salts depending on the salt. In the case

of unsubstituted 2-phenylpyridines the following type of species could be recognized based

on the shape of the UV spectra in aqueous solutions of the salts.

Ion-dipole pairs are formed between 2-phenylpyridine and all LiCl, NaCl, NaNO3, KCl,

KNO3, CsCl, MgCl2, CaCl2, SrCl2, Sr(NO3)2, BaCl2, Ba(NO3)2, MnCl2, and ZnCl2.

Complexes are formed between 2-phenylpyridine and all LiNO3, Ca(NO3)2, Cu(NO3)2, CuCl2,

AgNO3, Zn(NO3)2, Cd(NO3)2, La(NO3)3, CrCl3, CoCl2, Co(NO3)2, NiCl2, and Ni(NO3)2.Pyridinium salts are formed in aqueous solutions of all CdCl2, Pb(NO3)2, Cr(NO3)3, due to

the hydrolysis of these salts.

The interactions are manifested by the twist of the phenyl (aryl) group and this twist

depends on the electronic effect of the group which resides in the aryl moiety.

The electronic effect of substituent on the basicity (nucleophilicity) of the pyridine moiety

seems to be a major factor governing observed interactions.

It can be stated that buttressing effect in the compounds under study operates as a factor of

minor importance in comparison with the effects listed under p. 1.

The interactions observed are also influenced by intervention coming from anion. The

nitrate anion may enter the inner coordination sphere as a bidentate ligand.

Authors’ Statement

Competing InterestsThe authors declare no conflict of interest.

5. REFERENCES:

[1] E. A. Braude, F. Sondheimer, J. Chem. Soc., 3754 (1955).

[2] P. Tomasik, R. Zalewski, chem.. Zvesti, 31, 246 (1977).

[3] P. Tomasik, Z. Ratajewioz, "Pyridine Metal Complexes", ed. B. R. Newkome, L. Strekowski), J. Wiley-

Interscience, New York, 1985.

[4] R. G. Pearson, J. Chem. Educ., 45, 581, 643 (1968).

[5] R. G. Pearson, "Adv. LFER", Plenum Press, New York, 1976, Ch. 6.

[6] A. Yingst, D. H. McDaniel, J. Inorg. Nucl. Chem., 28, 2919 (1966).

[7] J. Shorter, "Correlation Analysis in Organic Chemistry," Oxford Chemistry Series, Clarendon Press. Oxford, 1973.

[8] H. H. Perkampus, U. Krueger, Ber. Bunsenges. Phys. Chem., 71 , 439 (1967).

[9] L. Reichel, E. Kirschbaum, Chem. Ber., 76B, 1105 (1943).

[10] K. Kahman, H. Sigel, H. Erlenmeyer, Helv. Chim. Acta, 47 , 1754 (1964).

[11] H. Sigel, H. Wynberg, T. Y. Van Bergen, K. Kahman, Helv. Chim. Acta, 55, 610 (1972).

[12] J. Plognin, Compt. Rend ., 233,162 (1951).

[13] Y. M. Haigh, D. A. Thornton, J. Mol. Struct., 8, 351 (1971).

[14] M. J. Bruce, B. L. Goodall, J. Matsuda, Austr. J. Chem., 28, 1259 (1975).

[15] B. N. Cockburn, D. V. Howe, T. Keating, B. F. G. Johnson, J. Lewis, J. Chem. Soc., Dalton Trans., 404 (1973).

[16] J.W. Haworth, J. M. Heilborn, H. D. Hey, J. Chem. Soc., 349 (1940).

[17] R. Forsyth, E. L. Pyman, J. Chem. Soc., 2917 (1926).

[18] P. Tomasik, Roozniki Chem., 41, 509 (1970).

Documents

i Jcs t 2011010406