CHAPTER ONE
1.0 INTRODUCTION
Transition metal complexes of the Schiff base ligands have attracted the attention of inorganic chemists for over two decades because of vast applications in pharmaceutical and industrial fields. Due to their simple synthesis and versatility, Schiff complexes continue to remain an important and a popular area of research as such complexes persistently play a very significant role in understanding the various aspect of coordination chemistry of transition metals. Schiff bases and its complexes of both transition and inner transition metals ions are extensively used in the study of structure and bonding in coordination complexes. Schiff bases also called imines are characterised by the azomethine (-C=N-) functional group and are usually formed by condensation reaction of aldehyde or ketone with a primary amine (Graham and Fryhle, 2004). This condensation reaction between a carbonyl compound and an amine leading to the formation of Schiff base is a facile reaction due to the good electrophilic characteristic properties of the carbonyl and the amine group (Petilet al, 2012).
Works on Schiff bases as ligand are rapidly developing because ligands are potentially capable of forming stable complexes (Xavier et al, 2012). Many Schiff bases and their complexes are widely used for industrial purposes and also exhibit a broad range of biological activities including; anti-fungal, anti-bacterial, anti-malaria, anti-proliferation, anti-inflammatory, anti-viral and anti-pyretic properties (Abu-Dief and Mohammed, 2015; Raj et al, 2013). In addition, some Schiff base complexes containing nitrogen and oxygen donor atoms are very effective in biocidal activities and other transformations in organic and inorganic chemistry (Mishra et al, 2012; Kotkar and Juneja, 2013). Schiff base formed from pyrazole moiety is gaining research
attention and are potential bioactive molecules in pharmaceutical industry (Salamaet al., 2015). Due to their structures, pyrazole derivatives play a vital role in medicinal, pesticide and coordination chemistry. UV-Visible spectroscopy is routinely used in analytical chemistry for the quantitative determination of different analytes, such as transition metal ions, highly conjugated organic compounds and biological macromolecules.
Keeping these facts in view, the present research work is focused on the synthesis of copper complex of Schiff base ligand derived from 5-a mino-3-methyl-1-phenylpyrazole and 2-hydroxy-1-naphthadehyde.
1.1 Coordination compounds
Complexes or coordination compounds are molecules that possess a metal centre that is bound to ligands (atoms, ions or molecules that donate electrons to the metal). These complexes can be neutral or charged. When the complex is charged, it is stabilised by neighbouring counter ions. Coordination compounds are also addition compounds which do not give all their constituents ions when dissolved in water or any other solvent. In these compounds, the individual properties of some constituents’ ions are lost. The central atom or ion in the coordination complex which is usually metallic is called coordination centre (IUPAC, 2006).
1.2 Ligands
Ligands are ions or neutral molecules that bind to a central metal atom or ion to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligands electron pairs. The nature of the metal ligand bonding can range from covalent to ionic. In General, ligands act as Lewis bases (electron pair donors), and
the central atom acts as Lewis acid (electron pair acceptor). This is because, the ligand and the central metal are bonded to one another and the ligand is providing both electrons to the bond instead of the metal. Bonding is often described using the molecular orbital theory. The Highest Occupied Molecular Orbital (HOMO) can be mainly of ligands or metal characters. Metal ions bonded to strong field ligands follow the Aufbau principle, whereas complexes bonded to weak field ligands follow the Hunds’s rule.
1.2.1 Classification of Ligands
Depending on the number of sites at which one molecule of the ligand is coordinated to the central metallic atoms, the ligands have been classified as:
Monodentate ligands.Monodentate ligands are ligands coordinatedto the central metal ion at one site or one metal ligand point. The ligands maybe neutral molecules or anions. Examples are chloride ions (referred to as chloro when it is a ligand), water (referred to aqua), ammonia (referred to amine) and hydroxide ions (referred to hydroxo).
Bidentate ligands.Bidentate ligands are ligands that have two donor atoms which allow them to bind to a central metal atom or ion at two points. Common examples are ethylenediamine (en), and the oxalate ion (ox).
Polydentate ligands.polydentate ligands are ligands that bind to the central metal atom through several points. Polydentate ligands maybe bidentate, tridentate, tetradentate, etc depending on the number of donor atoms present in one molecule of the ligand attached to one central atom. Example is EDTA, a hexadentate ligand. The coordination of a polydentate ligand to the metal ion leads to the formation of a chelate.
1.2.2 Coordination Geometry
The coordination geometry of an atom is the geometrical pattern formed by atoms around the central atom. In coordination chemistry, a structure is first described by its coordination number and the number of ligands attached to the metal (more especially the number of donor atoms). The geometrical arrangement will vary according to the number and type of ligands bonded to the metal centre, and to the coordination preference of the central atom, typically a metal in a coordination complex. The geometrical pattern can be described as a polyhedron where the vertices of the polyhedron are the centers of the coordinating atoms in the ligands (Lima-de-Faria et al., 1990).
The number of bonds depends on the size, charge and electron configuration of the metal ion and the ligands. Metal ions may have more than one coordination number. One of the most common coordination geometry is octahedral, where six ligands are coordinated to the metal in a symmetrical distribution, leading to the formation of an octahedron if lines were drawn between the ligands. Other common geometries are tetrahedral and square planar.
1.2.3 Applications of Coordination Compounds
Coordination compounds are used as catalysts for many industrial processes and have many applications in analytical chemistry, metallurgy and in medicine.
In analytical chemistry, it is used in the following:
- In the qualitative methods of analysis, complex formation is of immense importance in the identification and separation of most inorganic ions. Example, it is used to detect cupric ions in salt, when copper sulphite solution is mixed with aqueous ammonia; a deep blue complex soluble in water is formed.
ii The hardness of water is estimated by titration with the sodium salt of EDTA. The calcium and magnesium ions in hard water form the stable complexes: magnesium EDTA and calcium EDTA.
In medicine, coordination compounds are used in the following:
- Used in the treatment of rheumatoid arthritis,
- Coordination compounds containing vanadium as the central metal can be used for the treatment of diabetes, and
- Manganese complexes are used for the treatment of cardiovascular diseases and therapeutic radiopharmaceutical containing α- and β-emitters.
In metallurgy, coordination complex is used as follows:
- In the purification of metals by the formation and subsequent decomposition of their coordination compounds, and
- Noble metals like silver and gold are extracted from their ores by the formation of cyanide complexes.
1.3 UV-Visible spectroscopy
This absorption spectroscopy uses electromagnetic radiations between 190nm to 800nm and is divided into the ultraviolet [UV, 190-400nm] and visible [Vis, 400-800] regions. Since the absorption of ultraviolent or visible radiation by a molecule leads transition among electronic energy levels of the molecule, it is also often called an electronic spectroscopy. The information provided by this spectroscopy when combined with the information provided NMR and IR spectral data leads to valuable structural proposals.
1.3.1 Principles of absorption spectroscopy; Beers and Lamberts law.
The greater the number of molecules that absorb light of a given wave length, the greater the extent of light absorption, and the higher the peak intensity in absorption spectrum. If there are only few molecules that absorb radiation, the total absorption of energy is less and consequently lower intensity peak is observed. This makes the basis of Beer-Lambert Law which states that the fraction of incident radiation absorbed is proportional to the number of absorbing molecules in its path. When the radiation passes through a solution, the amount of light absorbed or transmitted is an exponential function of the molecular concentration of the solute and also a function of length of the path of radiation through the sample as shown in Equation 1.
Log IO\I = εcl (1)
where
Io = intensity of the incident light
I = intensity of light transmitted through the sample solution
c = concentration of the solute in mol/L
l = path length of the sample in cm, and
ε = molar absorptivity or the molar extinction coefficient of the substance whose light absorption is under investigation.
The ratio I/loisknownas transmittance I and the logarithm of the inverse ratio lo/l is known as the absorbance A
or A= εcl (2)
For presenting the absorption characteristics of a spectrum, the position of peaks are reported as λmax (in nm) values and the absorptivity is expressed in parenthesis.
1.4 Chemistry of Copper
Copper is a member of the first transition series of elements, which belongs to Group 11, Period 4 of the periodic table along with Silver (Ag) and Gold (Au). The element has an atomic number of 29, an atomic mass of 63g (Meija et al., 2016).Copper has a symbol, Cu (from latin: cuprum) and the main two oxidation states are +1 and +2. Two naturally occurring isotopes are 63Cu and 65Cu. Copper is the 25th earth most abundant element with abundances of 69.17% and 30.83%, respectively. It occurs as a soft reddish metal that can be found native as large boulders weighing
several hundred tons or as sulphide ores. The latter are complex copper, iron and sulphur mixture in combination with other metals such as arsenic, zinc and silver. The copper concentration in such ores is typically between 0.5-2percent. The electronic configuration of copper is (Ar) 3d104s1 and it is diamagnetic (Lide, 2005).
The softness of copperpartly explains its high electrical conductivity (59.6 x 106 s/m) and high thermal conductivity, second highest (second only silver) among pure metals at room temperature (Hammond, 2004). Copper is one of a few metallic elements with a natural colour other than gray or silver (Chambers et al., 1884). Pure copper is orange-red and acquires a reddish tarnish when exposed to air. The characteristic colour of copper results from the electronic transitions between the filled 3d and half-empty 4s atomic shells. The energy difference between these shells corresponds to orange light. As with other metals, if copper is put in contact with another metal, galvanic corrosion will occur. In spite of a similarity in electronic structure of copper, there are few resemblances between the chemistry of the three elements in Group 11, although certain complexes of Cu2+ and Ag2+are isomorphous (Cotton and Wilkinson, 1972).
1.4.1 Copper (II) Complex
Copper (II) ion complex have electronic configuration 3p6 3d9on its valence orbital and usually adopts the coordination numbers of four and five and most commonly six. It prefers complex coordination with ligands that have oxygen, sulphur and nitrogen as binding sites or donor atom. During bonding which is covalent, Cu (II) ion usually accepts electron. This indicates the acidic nature of Cu (II) complexes in Lewis sense. Copper complexes mostlyexist in its +2 oxidation state. The d9Cu2+ is characterised by a high degree of distortions bringing about unequal bond
Lengths and inter-bond angles, hence, their electronic spectra transitions cannot be assigned for certain to justify the structure. The +1 and +2 oxidation states are common and more stable than +3, which is uncommon. Electronic spectrum of an octahedral copper (II) complex has an absorption band which is due to 2Eg → 2T2g transition. Copper (II) complex is electrolytic in nature.
1.5 Schiff Bases
Schiff bases named after Hugo Schiff are compounds formed when any primary amine reacts with an aldehyde or ketone under specific conditions. Structurally, a Schiff base (also known as imine or azomethine) is a nitrogen analoque of an aldehyde or ketone in which the carbonyl group (C=O) has been replaced by an imine or azomethine group. Schiff bases are some of the most widely used organic compounds. They are used as pigments and dyes, catalysis intermediates in organic synthesis and as polymer stabilisers (Dhar, 1998). Schiff bases have also been shown to exhibit a broad range of biological activities including antifungal, antibacterial, antimalarial, anti-proliferative, anti- inflammatory, anti- viral and anti-pyretic properties. Schiff bases are gaining special interest due to their preparative accessibility and structural variety.
Both aldehydes and ketones form Schiff base, however, the formation takes place less readily with ketones than with aldehydes. Schiff bases derived from aliphatic aldehydes are unstable and are readily polymerisable. The presence of a dehydrating agent normally favours the formation of Schiff base. The common Schiff bases are crystalline and feebly basic in nature. They are characterised by the presence of an azomethine group –RC=NR; where R and R1 are alkyl, cycloalkyl, aryl or heterocyclic group. Often they are referred to as anils, imines or azomethines.
Schiff bases also played an important role in the development of inorganic chemistry and interestingly important in applications such as biochemical, analytical, anti- microbial, anti-cancer, anti- tubercular, hypertensive and hypothermic reagent.
Figure 1.1: Structure of Schiff base
1.5.1 Synthesis of Schiff Base
Schiff bases can be synthesised from an aliphatic or aromatic amine and a carbonyl compound by nucleophilic addition forming a hemiaminal, followed by a dehydration to generate an imine. Schiff bases are common ligands in coordination chemistry. The imine nitrogen is basic and exhibits pi- acceptor properties. The ligands are typically derived from alkyl diamines and aromatic aldehydes.
1.5.2 Applications of Schiff Bases in Medicine and Pharmacy
Imine complexes have a broad range of biological properties: anti-tumor, anti-viral, anti- fungal, and anti- bacterial. They are also used in the treatment of diabetes and AIDS. As biological models, they help in understanding the structure of biomolecules and biological processes occurring in living organisms. They participate inter alia, in photosynthesis and oxygen transport in organisms. They are involved in the treatment of cancer drug resistance and often tested as anti-malarial. It also could be used for the immobilisation of enzymes. Other properties include:
i Antiviral Properties
The use of vaccines may lead to the eradication of pathogens known as viruses, such as smallpox, poliomyelitis (polio), whether rubella. Although there are many therapeutic ways to work against viral infections, currently available antiviral agents are not fully effective, which is likely to cause a high rate of mutation of viruses and the possibility of side effects. Salicylaldehyde Schiff bases derived from 1-amino-3-hydroxyguanidine tosylate are good material for the design of new antiviral agents. Isatin Schiff base ligands are marked by antiviral activity and this fact is very useful in the treatment of HIV. In addition, it was also found that these compounds have anticonvulsant activity and may be included in the anti-epileptic drugs. Gossypol derivatives also present high antiviral activity. Increasingly, gossypol often used in medical therapy is replaced by its derivatives, because of their much lower toxicity. Schiff bases have obtained acceptable results for cucumber mosaic virus, whose effectiveness was estimated at 74.7 %. (Kumar, Dhar et al., 2009).
ii Anti-malarial Properties
Malaria is a disease which when it is neglected can cause serious health problems. Human malaria is largely caused by four species of the genus plasmodium (P. Falciparium, P. Vivax, P. Ovaleand P. malariae). The search for new drugs, vaccines and insecticides for the prevention or treatment of this disease is a priority. Schiff bases are interesting compounds which could be part of anti-malarial drugs. For example, the compound with such effect is Ancistrocladidine, which is a secondary metabolite produced by plants of the family Ancistrocladaceae and Dioncophyllaceae, and presenting an imine group in molecular chain. Cryptolepine, a valid indolchinpline alkaloid, isolated from African plant CryptolepisSanguinolenta and also used in the treatment of malaria, is the product of multi-stage reaction in which Schiff base is involved.
1.6 Pyrazole Derivatives
Pyrazoles are five membered heterocycles that constitute a class of compounds particularly useful in organics synthesis. Pyrazole contain two adjacent nitrogen in the 5-membered ring system. Among the two nitrogen atoms, one is basic and the other is neutral in nature. These are aromatic molecules due to their planar conjugated ring structures with six delocalized π-electrons. The aromatic nature arises from the four π-electrons and the unshared pair of electrons on the –NH nitrogen. The partially reduced forms of pyrazole are named as pyrazolines, while completely reduced form is pyrazolidine. Pyrazoles are one the most studied groups of compounds among the azole family. It is a tautomeric substance but the existence of tautomerism cannot be demonstrated in pyrazole itself. It can be inferred by the consideration of pyrazole derivatives.
Pyrazoles and its derivatives, a class of well known nitrogen heterocycles, occupy a prime position in medicinal and pesticide chemistry for their diverse biological activities. Pyrazole
analogues have found use as building blocks in organic synthesis for designing pharmaceutical, agrochemicals and as bifunctional ligands for metal catalysis. They have been known to exhibit anti-microbial, analgesic, anti-cancer, anti-tuberculosis, anti-inflammatory, anti-depressant, anti-convulsant, anti-hyperglycemic, anti-pyretic, anti-helmintic, anti-oxidant and herbicidalproperties(Abu-Dief, 2015). The pyrazole ring is present as the core in a variety of leading drugs such as Celebrex, Sildenafil (Viagra), Ionazlac, Rimonabant as anti-obesity, Difenamizole and so forth. German chemist Ludwig Knorr in 1883 was the first to discover anti-pyretic action of pyrazole derivatives in man and to name the compound anti-pyrine. Anti-pyrine (2, 3-dimethyl-1-phenyl-1, 3-pyrazoline-5-one) which has analgesic, anti-pyretic and anti-rheumatic activity was accidentally obtained in an attempt to synthesisequinoline derivatives with anti-pyretic activity, and this stimulated interest in pyrazole chemistry.
1.7 Aim and Objectives of the Research
1.7.1 Aim of the Research
The aim of this study is to synthesise and characterise copper (II) complex of Schiff base ligand derived from 5-amino-3-methyl-1-phenylpyrazole and 2-hydroxy-1-naphthaldehyde and to carry out UV-Visible spectroscopic analysis on the synthesised Schiff base ligand and its copper (II) complex.
1.7.2 Objectives of the Research
In order to achieve the aim of this work, the following objectives shall be carried out:
(i) 1-{(Z)-[3-methyl-1-phenyl-1H-pyrazol-5-yl) imino] methyl} naphthalen-2-ol shall be synthesise by the reaction of 5-amino-3-methyl-1-phenylpyrazole and 2- hydroxy-1-naphthaldehyde,
(ii) Copper (II) complex shall be synthesised by the reaction of the ligand and Cu2+ ion,
(iii) Solubility tests shall be conducted on the Schiff base ligand and copper (II) complex,
(iv) Melting point of the Schiff base ligand and copper (II) complex shall be determined, and
(v) UV-Visible studies of the Schiff base ligand and copper (II) complex shall be carried out
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