23

CHAPTER 2

COMPOUNDS OF INTEREST

2.1 ACRIDINES AND ACRIDINEDIONES

Acridine, a dibenzopyridine compound, is structurally related to

anthracene with one of the central CH groups replaced by nitrogen. Acridine,

a colourless solid known to have first isolated from coal tar, is an important

basic material used in the synthesis of dyes and many potential drugs.

Derivatives of acridine such as proflavine are successful antibacterial agents

and find principal use as local antiseptics for wound therapy (Acheson 1956).

Acridine and related derivatives are also expected to bind effectively to DNA

and RNA due to their abilities to intercalate. Many acridines and their

derivatives exhibit a wide range of biological activity such as antiprotozoal,

antiamoebic and antitumour properties (Karolak et al 1996; Krishnaprasad

et al 1984; Karle et al 1980). Acridinediones, the acridine derivatives having

two keto functional groups at the 1st and 8th positions are found to be good

antimalarial agents (Keston et al 1992; Raether et al 1989).

The present work consists of precise crystallographic investigations

carried out on some tetramethyl-substituted acridinediones leading to accurate

descriptions of their molecular structure and intermolecular aggregation

patterns. The chemical diagram of acridine, acridinedione and tetramethyl-

substituted acridinedione with crystallographic numbering scheme is given in

Figure 2.1.

24

N N

H

O O

NH

OO

12

3

45

6

7

8

91011

12

1314

15 116

17

12

Figure 2.1 Molecular diagrams of Acridine, Acridinedione and

Tetramethyl-substituted Acridinedione

2.2 IMPORTANCE OF ACRIDINEDIONES

Acridine derivatives, the earliest known antibiotics, are known to

display a variety of biological activities. It is interesting to note that

evaluation of a dihydroacridinedione derivative, floxacrine, demonstrated its

potential use as an anti-malarial agent (Schmidt 1979) and subsequently led to

a quinacrine-floxacrine analogue (WR 243251) that supposedly inhibits the

respiration pathways of Plasmodium falciparum, a protozoan parasite that

causes malaria in humans (Dorn et al 2001). Certain 1,8-(2H,5H)-

acridinediones were found to act as cell potassium channel openers in

mammals such as man in the treatment of urinary incontinence. Some

tetramethyl-acredinediones show good inhibition on the pathogen Vibrio

isolate-I (Josephrajan et al 2005), a human pathogenic bacteria considered to

be the most common and significant cause of severe infections in humans.

Some of the important biological activities of acridinediones are briefly dealt

with in the following sections.

25

2.2.1 DNA Intercalation

Acridinediones are known to bind to DNA by intercalation. This

intercalation is essential for biological activities (Sakore et al 1979; Reddy

et al 1979). Some acridinedione derivatives are not planar but are buckled.

Acridines having greater bucking between the outer rings are found to be

strong in DNA binding (Karle et al 1980; Sivaraman et al 1996a). Some

Reactive Oxygen Species (ROS) viz. ions or small molecules that include

oxygen ions, free radicals, inorganic and organic peroxides, etc. produced in

living cells as a by-product of respiration, react with DNA and abstract an

electron from the double helix, leading to long range electron transfer

reactions. Thus, the DNA of living cells may be in a continuous state of

electron transfer. Acridine-based anticancer or antimicrobial drugs, amsacrine

and N-[2-(dimethylamino)ethyl]acridine-4-carboxamide, which bind to DNA

by intercalation, act as an electron donor and an acceptor respectively. Such

reactions make important contributions to the antitumor activity of these

drugs (Baguley et al 2003).

2.2.2 Potassium Channel Openers

Potassium channel opening is a physiological mechanism by which

excitable cells exploit to maintain or restore their resting state. Potassium

channel openers have the potential to restrain or prevent contractile responses

of smooth muscle to excitatory stimuli. Because these derivatives of

acridinedione function to open cell potassium channels, they may be useful in

the treatment of diseases in which the action of a therapeutic agent is to open

potassium channels is desired or is known to provide amelioration. Such

conditions or diseases include hypertension, asthma, peripheral vascular

disease, right heart failure, angina, ischemic heart disease, cerebrovascular

disease, disorders associated with kidney stones, irritable bowel syndrome,

26

male pattern baldness, premature labor and peptic ulcers (Li et al 1996;

US Patent 5484792, 1996; US Patent 5258390, 1993).

2.2.3 Fluorescence and Laser Activities

Acridione-1,8-diones exhibit fluorescence and laser activities

(Thiagarajan and Ramamurthi 2007a). It has also been found that the

fluorescence quantum yield and the laser efficiency could be altered by

changing the substituents at the 9-position and on the nitrogen centre

(Murugan et al 1998; Srividya et al 1998). Various highly fluorescent

acridinediones have been synthesized by microwave and ultrasound

irradiations and by conventional thermal methods. It was found that most of

the acridinedione derivatives could be obtained in very high yields (80~90%)

in the microwave-assisted, solvent-free, one-pot synthesis process within

minutes (~2 min). These highly fluorescent acridinediones are potential new

materials for electroluminescence (EL) devices (Murugan et al 2004; Islam

et al 2003). Acridinedione derivatives connected with the thiourea receptor

are found to act as a fluorescent chemosensor that can convert a chemical

stimulus into some form of action (Thiagarajan and Ramamurthi 2007b).

It has been found that the fluorescence of acridinediones shifts to

shorter wavelengths upon polymerization of the medium and this qualifies

them as fluorescent molecular probes for monitoring the progress of

polymerization processes. This has been particularly suitable for the

monitoring of polymerization in the thin layers used as photo curable coatings

as well as for quality control of coating formulations. Acridinediones have

potential application in the coating industry (Popielarz et al 1997). The

acridinedione derivatives also show photophysical (Srividya et al 1998) and

electrochemical properties (Srividya et al 1996).

27

2.3 LITERATURE ON THE STRUCTURAL FEATURES OF RELATED ACRIDINEDIONE DERIVATIVES

A survey carried out in the Cambridge Strutural Database (Allen

2002) revealed about forty-nine crystal structures of tetramethyl substituted acridinedione derivatives. They have been classified as N1-substituted,

C9-substituted and both N1- and C9- substituted. The substituent groups

range from small to bulky such as simple methyl groups to larger further substituted phenyl groups.

2.3.1 Unsubstituted Tetramethyl-Acridinediones

In the crystal structure of the unsubstituted tetramethyl-acridinedione derivative, 1,2,3,4,5,6,7,8-Octahydro-3,3,6,6-tetramethyl

acridine-1,8-dione (Sankaranarayanan et al 1999), it is seen that the central

ring is almost planar with the other two rings adopting a half-chair conformation. Also it is seen that in stabilizing the crystal structure both the

keto oxygens participate in C–H…O hydrogen bonds. There is no N–H…O

hydrogen bond present in this structure as expected. Figure 2.2 shows the hydrogen-bonding pattern observed in the unsubstituted tetramethyl-

acridinedione.

Figure 2.2 Hydrogen bonding scheme observed in 1,2,3,4,5,6,7,8-octahydro -3,3,6,6-tetramethylacridine-1,8-dione

28

2.3.2 N1-substituted Tetramethyl-Acridinediones

In the molecular structures of 3,3,6,6-Tetramethyl-10-(4-

methylphenyl)- 3,4,6,7,9,10-hexahydro- 1,8(2H,5H)-acridinedione

(Sivaraman et al 1996b) with substitutions at N1, both keto oxygen atoms are

involved in C–H…O hydrogen bonds linking two different molecules,

whereas for the structure of 10-[2-(4-Hydroxyphenyl)ethyl]-3,3,6,6,9-

pentamethyl-3,4,6,7,9,10-hexahydroacridine-1,8(2H,5H)-dione (Seshadri et al

2002), only one of the keto oxygen atoms form O–H…O hydrogen bond with

the hydroxyl oxygen atom of the 2-(4-Hydroxyphenyl)ethyl substitution at N1.

The formation of C–H…O hydrogen bonds in 3,3,6,6-Tetramethyl-10-(4-

methylphenyl)- 3,4,6,7,9,10-hexahydro- 1,8(2H,5H)-acridinedione

(Sivaraman et al 1996) is shown in Figure 2.3.

Figure 2.3 Formation of C–H…O hydrogen bonds in N-substituted

Tetramethyl-Acridinedione

29

2.3.3 C9-substituted Tetramethyl-Acridinediones

In the crystal structures with substitutions at central carbon atom

C9, the nitrogen atom N1 forms an N–H…O hydrogen bond with one of the

keto oxygen atoms in almost all the cases (Aravindan et al 2003; Tu et al

2001; Novoa de Armas et al 1999; Ganesh et al 1998; Gunasekaran et al

1997) with a few exceptions. The formation of N–H…O hydrogen bond

involving central nitrogen atom and the keto oxygen atom for the structure 9-

(4-Chlorophenyl)-3,3,6,6-tetramethyl-1,2,3,4,5,6,7,8,9,10-decahydroacridine-

1,8-dione (Tu et al 2001) is shown in the Figure 2.4.

Figure 2.4 Formation of N–H…O hydrogen bond in C-substituted

Tetramethyl-Acridinedione

The other keto oxygen is either C–H…O bonded in the structure of

3,3,6,6-Tetramethyl-9-(2-phenylethyl)- 3,4,6,7,9,10-hexahydro-1,8(2H,5H)-

acridinedione (Gunasekaran et al 1997) or short-contacted with halogen

atom present in the structure 9-(4-Chlorophenyl)-3,3,6,6-tetramethyl-

1,2,3,4,5,6,7,8,9,10-decahydroacridine-1,8-dione (Tu et al 2001) and 9-(3,4-

Dichlorophenyl)-3,3,6,6-tetramethyl- 1,2,3,4,5,6,7,8,9,10-decahydroacridine-

1,8-dione (Rong et al 2006). The N–H…O and C–H…O hydrogen bonds

involving keto oxygen atoms for the structure 3,3,6,6-Tetramethyl-9-(2-

30

phenylethyl)-3,4,6,7,9,10-hexahydro-1,8(2H,5H)-acridinedione (Gunasekaran

et al 1997) is shown in the following Figure 2.5.

Figure 2.5 Formation of N–H…O and C–H…O hydrogen bonds in

C-substituted Tetramethyl-Acridinedione

2.3.4 N1- and C9-substituted Tetramethyl-Acridinediones

More number of structures with both N1- and C9- substitutions has

been reported when compared to the N1-substituted and C9-substituted

structures. The molecular packing of 3,4,6,7,9,10-Hexahydro-3,3,6,6-

tetramethyl- 1,8(2H,5H)- dioxo-10- phenyl-9- acridinyl-methylacetate’

(Gunasekaran et al 1996), discloses that only one of the keto oxygen atoms

participate in a bifurcated C–H…O hydrogen bond and these hydrogen bond

patterns are shown in the Figure 2.6.

In the molecular packing of 1,2,3,4,5,6,7,8,9,10- Decahydro-

3,3,6,6-tetramethyl- 1,8-dioxo-10-vinylacridin-9-yl methyl Acetate

(Sankaranarayanan et al 1998) also, only one of the keto oxygen atoms acts as

a bifurcated C–H…O hydrogen bond acceptor. One of the keto oxygen atoms

and one oxygen atom of the acetyl substitution are C–H…O hydrogen bonded

to two different molecules in the case of 10-[2-(4-acetylpiperzain-1-yl)ethyl]-

9-(4-chlorophenyl)-3,3,6,6-tetramethyl-3,4,6,7,9,10- hexahydro-2H,5H-

acridine-1,8-dione (Seshadri et al 2003). In the structure of 9-(4-

31

Chlorophenyl)-3,3,6,6-tetramethyl- 10-(4-methylphenyl)- 1,2,3,4,5,6,7,8,9,10-

decahydroacridine -1,8-dione (Wang et al 2003), there is an intermolecular C–

H…O hydrogen bond involving one of the keto oxygen atoms in addition to

C–H…Cl short contact.

Figure 2.6 The formation of bifurcated C–H…O hydrogen bonds in the

case of 3,4,6,7,9,10-Hexahydro-3,3,6,6-tetramethyl-1,8-

(2H,5H)-dioxo-10-phenyl-9-acridinyl-methyl acetate

The structure with water molecules as solvent of crystallization,

3,3,6,6-Tetramethyl-N-hydroxy- 9-(4-fluorophenyl)- 1,2,3,4,5,6,7,8,9,10-

decahydro acridine -1,8-dione mono hydrate (Tu et al 2004a), has been

reported. In this structure, the keto oxygens make hydrogen bonds linking

other acridinedione molecules and these hydrogen bond patterns are shown in

Figure 2.7.

32

Figure 2.7 Formation of water molecule-mediated O–H…O hydrogen

bonds linking the molecules of acridinediones. Non-

participating atoms in hydrogen bond network are omitted

for clarity

2.4 SCHIFF BASES

A Schiff base is a functional group that contains a carbon-nitrogen

double bond with the nitrogen atom connected to an aryl or alkyl group.

Schiff bases, named after Hugo Schiff (1834-1915), and their transition metal

complexes continue to be of interest even after over a hundred years of study.

Schiff bases have a chelating structure and are in demand because they are

straightforward to prepare and are moderate electron donors with easily-

tunable electronic and steric effects thus being versatile.

33

Schiff bases are the condensation product of aldehyde or ketone

with the primary amine, which leads to the displacement of C=O group by

C=N–R group. The Schiff base ligands are considered as privileged ligand in

coordination chemistry due to their facile syntheses, easily tunable steric,

electronic properties and good solubility in common solvents (Shi et al 2004).

Schiff base ligands are extensively used in coordination chemistry, since they

can be applied in the construction of new frameworks with interesting

properties (Sun et al 2000). Transition metal complexes with oxygen and

nitrogen donor Schiff bases are of particular interest (You et al 2004a; You

et al 2004b) because of their ability to possess unusual configurations, be

structurally labile and their sensitivity to molecular environments (Golcu et al

2005).

Schiff base complexes can be classified in a number of ways, as

mononuclear, binuclear and polynuclear on the basis of the number of metal

atoms present and as monodentate, bidentate and polydentate on the basis of

donor atoms present in the back bones of the ligand. The presence of hard and

soft donor atoms in the backbone of Schiff base ligands enable them for

versatility.

Schiff bases derived from salicylaldehyde and dehydroacetic acids

have been synthesized and their crystal and molecular structures have been

elucidated and details presented in this thesis.

2.5 IMPORTANCE OF SCHIFF BASE DERIVATIVES

Schiff bases occupy an important position in metal coordination

chemistry even almost a century since their discovery. Also, due to the

simplicity in preparation, diverse properties, medicinal, biochemical and

industrial applications, a keen interest in the study of these compounds arose

34

in the recent years. Some of the important features of Schiff base derivatives

are discussed in brief, in this section.

2.5.1 Biological Significance

Schiff bases are class of important compounds in the field of

medicine and pharmaceutics. They exhibit biological activities such as

antibacterial (Sari et al 2003; Thangadurai et al 2002), antifungal (Parekh et al

2006; Ugras et al 2006), anticancer (Kuz’min et al 2000) and antimicrobial

(Raman et al 2008; Yilmaz and Cukurovali 2003) activities. Also, Schiff

bases seem to be important intermediates in many enzymatic reactions

involving interaction of the amino group of an enzyme with a carbonyl group

of the substrate. Stereochemical investigations carried out with the aid of

molecular models show that a charge transfer may occur between these

groups and the oxygen atoms of the Schiff bases.

The biosynthesis of porphyrins, a group of chemical compounds of

which many occur in green leaves and red blood cells, is another important

pathway involving the intermediate formation of Schiff base. Also, Schiff

base formation plays a role in the chemistry of vision and in many

transamination reactions. Thus, precise structural investigation on Schiff base

derivatives is expected to throw light on enzyme reaction models.

2.5.2 Photochromism and Thermochromism of Schiff base

Derivatives

Photochromism is defined as the reversible photocolouration of a

single chemical species between two states having distinguishably different

absorption spectra, which are brought about by the action of electromagnetic

radiation in at least one direction. Reversible change in colour of substances

with variation of the temperature is known as thermochromism and has

35

attracted much interest from chemists for a long time. Schiff bases of

salicylaldehyde with amines comprise a chemical system undergoing

hydrogen-atom tautomerism between enol and keto forms and show the

phenomena of solid state photochromism and thermochromism. It has been

shown that the photochromic property is a characteristic of the molecules but

their chromobehaviour is influenced by the crystal structure of the compounds

(Hadjoudis and Mavridis 2004). On the basis of some thermochromic and

photochromic Schiff base compounds, it was proposed that molecules

exhibiting thermochromism are planar, while those exhibiting photochromism

are non-planar (Hadjoudis et al 1983).

2.6 LITERATURE ON THE STRUCTURAL FEATURES OF

RELATED SCHIFF BASE DERIVATIVES

Tautomerism refers to equilibrium between two different structures

of the same compound. Usually the tautomers differ in the point of attachment

of a hydrogen atom. One of the most common examples of a tautomeric

system is the equilibrium between a ketone and its enol form. In Schiff base

compounds; there exist two types of intramolecular hydrogen bonds viz.,

N–H…O type and O–H…N type (Garnovskii et al 1993). The intramolecular

hydrogen bond between oxygen atom of the ortho-hydroxyl group and

nitrogen atom in these systems plays a vital role in the formation of Schiff

base compounds in the solid state by proton transfer from hydroxyl oxygen

atom to the imine nitrogen atom. Extensive studies on Schiff bases have

disclosed that ortho-hydroxy Schiff bases exist as enol or keto or as enol/keto

mixtures. A Schiff base derived from aniline can be called an anil. Anils

always form the O–H…N type of hydrogen bonding regardless of

N–substituent (alkyl or aryl) (Gavranic et al 1996), while the Schiff bases of

dehydroacetic acid form N–H…O hydrogen bond (Gilli et al 2000).

36

Strong intramolecular N–H…O hydrogen bond between oxygen

atom of the ortho-hydroxyl group and nitrogen atom and intermolecular

O–H…O interactions influence the conformation of the molecules of 6-[N-(2-

Hydroxyphenyl)aminomethylene]cyclohexa-2,4-dien-1-one and its crystal

packing. The formation of intramolecular N–H…O and intermolecular

O–H…O hydrogen bonds is shown in Figure 2.8. Thus in this case, the

salicylaldimine takes the tautomeric keto form (Mukherjee et al 1999). This

N–H…O type of intramolecular hydrogen bond patterns is also exhibited by

other related structures (Odabasoglu et al 2003; Elmali et al 2001; Ondracek

et al 1993).

Figure 2.8 Formation of Intramolecular N–H…O and intermolecular

O–H…O hydrogen bonds in 6-[N-(2-Hydroxyphenyl)

aminomethylene] cyclohexa-2,4-dien-1-one

37

The four structures of N-(2-hydroxybenzylidene)aniline derivatives

show strong intramolecular O–H…N hydrogen bond. In all these structures,

the molecules adopt enol form rather than keto form (Burgess et al 1999). The

formation of O–H…N intramolecular hydrogen bond in the case of

N-(2-hydroxybenzylidene)-4-fluoroaniline is shown in Figure 2.9. The O–

H…N intramolecular hydrogen bond is also observed in other cases (Akkurt

et al 2006; Cheng et al 2005; Zheng et al 2005; Lin et al 2005; Arod et al

2005; Karakas et al 2004; Yuce et al 2004; Francis et al 2003; Karadayi et al

2003; Unver et al 2003; Wang et al 2003; Yeap et al 2003; Darensbourg et al

2001; Elmali et al 1997; Tenon et al 1995).

Figure 2.9 Formation of Intramolecular O–H…N hydrogen bond in

N-(2-hydroxybenzylidene)-4-fluoroaniline

2.7 DIHYDROGEN PHOSPHATE SALTS

A phosphate, an inorganic chemical, is a salt of phosphoric acid.

Inorganic phosphates are mined to obtain phosphorus for use in agriculture

and industry. In organic chemistry, a phosphate, or organophosphate, is an

ester of phosphoric acid. Organic phosphates are important in biochemistry

and biogeochemistry. Phosphate, an essential nutrient for all organisms, is

used in the biosynthesis of diverse cellular components such as nucleic acids,

proteins, lipids and sugars. Interestingly, phosphoric acid readily forms

complexes with many organic compounds and the inorganic-organic hybrid

38

materials are considered to be of great importance in the development of

materials with novel properties and these hybrid materials have numerous

uses in various fields such as liquid crystals, catalysts and fuel cells (Desiraju

1989; 1995). Some organic dihydrogen phosphate salts have been prepared

and determined the crystal structures in order to understand the ionic

interaction via hydrogen bonds and formation of supramolecular hydrogen

bond motifs.

Organo-phosphoric complexes have attracted wide attention of

researchers due to their many practical and potential applications in various

fields such as biomolecular sciences. The present thesis deals with three

organic phosphate salts viz. complexes of phosphoric acid with piperazine,

2-chloroaniline and 4-chloroaniline. These compounds may be termed ‘proton

transfer’ complexes as a ‘proton’ from phosphoric acid is transferred to the

organic moiety such that the phosphate remains in the anionic state and the

organic moiety in the cationic state. These compounds, also called ‘hybrid

materials’, have attracted wide attention from crystallographers, chemists,

physicists, technologists, etc. and remain a focus area in materials science.

Such compounds exhibit interesting structural features and find potential

application in the field of new materials science such as non-linear optical

(NLO) materials, ion-exchange, adsorption, molecular recognition, catalysis,

etc.

Piperazine, a strong basic amine, readily forms the dication

[H2N(CH2CH2)2NH2]2+ in which all four N–H bonds lying in a common plane

generally participate actively in hydrogen bond formation (Coupar et al 1996;

Ferguson et al 1998). Structurally, a difucntional amine, piperazine may be

used as a potential building block to assemble supramolecular networks. Its

ability to form hydrogen bonds with guest molecules or ligands these

compounds often leads to novel hydrogen bonding networks in the crystalline

state and hence would throw light in the study, design and understanding of

39

supramolecular networks. Also, it is well-known that many piperazine

derivatives are useful drugs.

Aniline, one of the most important aromatic amines consisting of an

amino group and a phenyl group, is used as a precursor to more complex

chemicals and manufacture of many drugs such as paracetamol. Its main

application is in the manufacture of polyurethane. Chloroaniline, a chlorinated

derivative of aniline, is used as an intermediate in the production of several

urea herbicides and insecticides, azo dyes and pigments, and pharmaceutical

and cosmetic products. The 4-chloroaniline based azo dyes and pigments are

especially used for the dyeing and printing of textiles. Like aniline,

substituted anilines also react readily with acids to form an anilinium cation.

The crystal structures of salts of 2-chloroaniline and 3-Methylaniline with

picric acid (Muthamilzhchelvan et al 2005a; 2005b) and 4-Fluoroaniline and

4-Methylaniline with picric acid (Saminathan 2007) have already been

reported.

2.8 LITERATURE ON THE STRUCTURAL FEATURES OF

RELATED DIHYDROGEN PHOSPHATE SALTS

Phosphate anion is known to exhibit novel hydrogen bonding

patterns. Of the variety of hydrogen bonded patterns exist in crystals, the

hydrogen bonded dimer is considered to be the simplest and most important

of all. In crystal engineering, moieties that can form dimers are often used as

building blocks of supramolecular networks. These building blocks are

expected to provide insights into the understanding of the elusive problem of

predicting crystal structures, which still remains as intense a problem as

predicting protein folding. For instance, in the crystal structure of the 3-

Chloro-2-methylanilinium dihydrogen phosphate, 2-carboxyanilinium

dihydrogen phosphate and 2-(Hydroxymethyl) pyridinium dihydrogen

phosphate, the phosphate anions form centrosymmetric O–H…O hydrogen

40

bonded dimers (Khemiri et al 2008; Benali-Cherif et al 2007; Demir et al

2003). In the case of 2-(Hydroxymethyl)pyridinium dihydrogen phosphate,

these dimers form an infinite chain as shown in the Figure 2.10.

Figure 2.10 Formation of O–H…O hydrogen bonded dimers

A unique feature observed in the crystal structures of dihydrogen

phosphate salts is that the respective cations are linked to the infinitely

extending O–H …O hydrogen bonded dimers as side chains. As an example,

the linkage of 3-amino-2-chlropyridinium cations as side chains in the crystal

structure of 3-Amino-2-chloropyridinium dihydrogen phosphate (Hamed et al

2007) is shown in Figure 2.11.

Figure 2.11 Linkage of 3-Amino-2-chloropyridinium cations as side

chains to O–H ...O hydrogen bonded phosphate dimers

41

Other aggregation mechanisms observed in these crystal structures

include cation mediated phosphate chains in which there are no direct

interaction between phosphate anions except for the O–H…O dimer

formation and these dimers being linked through respective cations. Also,

there are crystal structures in which phosphate anions link the respective

cations without being able to form dimers between themselves.

Another important aspect of intermolecular interactions is the

halogen-halogen contact observed in crystal structures. In this context, the

packing modes displayed by crystal structures of halogen-substituted simple

organic molecules such as chloroaniline are of immense interest. The earliest

work on the significance of Cl…Cl interactions is that of Sakurai,

Sundaralingam and Jeffrey (Sakurai et al 1963) on the crystal structure of

2,5-dichloroaniline in which a compilation of short Cl…Cl distances between

3.27 Å and 3.49 Å were analyzed and reported. Subsequently, the importance

of halogen-halogen contacts in stabilizing crystal structures was emphasized

in the description of the crystal structures of halogen-substituted benzoic acids

and anhydrides (Miller et al 1974). Further, the emphasis that the role of

C–H…Cl in crystal structures is hydrogen bond-like was suggested (Taylor

and Kennard 1982). With the emergence of Cambridge Structural Database

Interactions such as C–H … Cl, Cl … Cl, C–H … , – gained importance

and many exhaustive studies on halogen-halogen contacts have provided

valuable information on the geometry of these interactions once thought were

non-existing. As an example, a dimeric pattern involving C–H …Cl and

N–H …Cl hydrogen bond in 2,6-Dichloroaniline (Dou et al 1993) is shown in

Figure 2.12.

42

Figure 2.12 Formation of C–H …Cl and N–H …Cl hydrogen bonded

dimers

2.9 PRESENT STUDY

Our study is aimed at the extent of buckling of the acridinedione

moiety, the possible conformational flexibility of the outer rings, the

involvement of the two keto oxygen atoms in the hydrogen bond formation

with respect to the substitutions at N1 and C9 and their orientation.

Importance is given to the conformational features, hydrogen bond motifs and

packing modes with the related structures. Intramolecular hydrogen transfer is

the major aspiration of our study on the Schiff base derivatives that is

expected to provide information on the formation of keto or enol form of the

compounds under study. Also, it is intended at the determination of precise

molecular structure, intra and intermolecular hydrogen bonding nature, crystal

structure stability and packing modes. In addition, preparation and structural

elucidation of organic dihydrogen phosphate salts were carried out.

In order to understand the molecular aggregation and packing

modes of acridinedione derivatives, a number of acridinedione derivatives

were prepared and crystal structures were elucidated for those which could be

crystallized in our laboratory and the results of the same is presented. Crystal

structures of some compounds of Schiff base derivatives are presented. Also,

with a view to ascertain proton transfer, interaction between molecular ions

and other inter-ionic interactions, three dihydrogen phosphate salts of certain

43

interesting organic moieties such as chloroaniline and piperazine were

prepared, crystallized and their crystal structures were determined and

presented in this thesis. The following list gives the names of the compounds

and their codes for which precise molecular and crystal structures are

presented in this thesis:

Acridinedione derivatives

1) 3,3,6,6-Tetramethyl-9-phenyl-10-[2-(dimethylamino)ethyl]-

3,4,6,7,9,10 -hexahydro-1,8(2H,5H)-acridinedione [ACDN-1]

2) 3,3,6,6-Tetramethyl-9-(4-methoxyphenyl)-10-[2-

dimethylamino)ethyl]- -3,4,6,7,9,10-hexahydro-1,8(2H,5H)-

acridinedione [ACDN-2]

3) 3,3,6,6,10-Pentamethyl-9-(2-nitrophenyl)-3,4,6,7,9,10-

hexahydro-1,8 (2H,5H)-acridinedione ethanol solvate

[ACDN-3]

4) 3,3,6,6,10-Pentamethyl-9-{4-[(2-Hydroxy-benzylidene)-

amino]-phenyl}-3,4,6,7,9,10-hexahydro-1,8(2H,5H)-

acridinedione [ACDN-4]

Schiff base derivatives of salicylaldehyde

5) 4-Chloro-2[(2,6-diisopropylphenyl)iminomethyl]phenol

[CDP]

6) 4-Bromo-2-[(2,6-diisopropylphenyl)iminomethyl]phenol

[BDP]

7) 4-Iodo-2-[(2,6-diisopropylphenyl)iminomethyl]phenol [IDP]

44

Schiff base derivative of dehydroacetic acid

8) 3-[1-(4-Bromo-phenylamino)-ethylidene]-6-methyl-pyran-2,4-

dione [BPMP]

Dihydrogen phosphate salts

9) 2-Chloroanilinium dihydrogen phosphate [2CADP]

10) 4-Chloroanilinium dihydrogen phosphate [4CADP]

11) Piperazinium Bis(dihydrogen phosphate) [PBDP].