Preparation, Properties And Characterization Of Adeninium Salts Of Dichromate And Molybdate
1.1 TRANSITION METALS AND COORDINATION COMPOUND
A transition metal is an element whose atom has an incomplete d sub-shell, or which can give rise to cations with an incomplete d sub-shell. They are the d-block elements of the periodic table, which includes groups 3 to 12 on the periodic table. There are 38 elements. They serve as transitional links between the most and the least electropositive in a series of elements. The electronic structure of transition metal atoms can be written as [ ] ns2(n-1)d1-10 or ns1(n-1)d1-10. As with all metals, the transition elements are both ductile and malleable, and conduct electricity and heat.
(i) They have a partly filled d-shell either as the element or in their compounds.
(ii) They are all metals.
(iii) They are all shiny metals with the typical metallic grey / white color, except gold, which is gold colored, and copper, which is copper colored.
(iv) They are all good conductors of heat and electricity.
(v) They have high melting and boiling points.
(vi) Most transition metals form colored compounds.
(vii) They have several stable oxidation states or valencies.
(viii) Many are used as catalysts, either as the metal itself or as some of their compounds.
(ix) They form complex ions, with various co-ordination numbers and geometries.
(x) Many form compounds which are paramagnetic (have unpaired electrons).
Coordination compounds are the backbone of modern inorganic chemistry and bio-inorganic chemistry and chemical industry. A characteristic feature of the transition metals is their ability to form a group of compounds called coordination compounds, complex compounds, or sometimes simply complexes. Due to the influence of the incomplete inner d orbitals in the transition elements they form quite a large number of coordination compounds, consisting of a central atom or molecule connected to surrounding array of atoms or molecules which are known as ligand or complexing agents[1]. The number of ligands attached to the metal atom or ion is its coordination number. Ligands are generally bound to the central atom by a coordinate covalent bond. A central atom or ion and an array of molecules or anions, the ligands, around comprise the coordination sphere. The central atoms or ion and the donor atoms comprise the first coordination sphere.
In coordination chemistry, a structure is first described by its coordination number. Coordination numbers are normally between two and nine but large numbers of ligands are not uncommon for the lanthanides and actinides. The number of bond depends on the size, charge and ligands.
The chemistry of complexes is dominated by interactions between s and p molecular orbitals of the ligands and the d orbitals of the metal ions. Different ligand structural arrangements result from the coordination number. Most structure follow the points-on-a sphere pattern (or as if the central atom were in the middle of a polyhedron where the corners of that sphere are the location of the ligands), where orbital overlap and ligand-ligand repulsions tend to lead to certain regular geometries. The most observed geometries are listed below, but there are many cases that deviate from a regular geometry e.g., due to the use of ligands of different types due to size of ligands, or due to electronic effects.
· Linear for two coordination
· Trigonal planar for three coordination
· Tetrahedral or square planar for four coordination
· Trigonal bipramidal or square pyramidal for five coordination
· Octahedron or trigonal prismatic for six coordination
· Pentagonal bipyramidal for seven coordination
· Hexagonal bipyramidal for eight coordination
· Tri-capped trigonal prismatic for nine coordination
The higher coordination numbers of 7, 8 and 9 are unusual for d-block metals, although some examples can be found among the complexes of the early second- and third-row metals. In contrast of 8 or 9 are quite typical among the complexes formed by the f-block elements and coordination numbers up to 14 are known. The idealized regular coordination geometries of complexes with coordination numbers from 1 to 6 are summarized in Table 1.
Salts are ionic compounds that result from the neutralization reaction of an acid and base. They are composed of cations and anions so that the product is electrically neutral. These component ions can be inorganic such as chloride (Cl–) as well as organic such as acetate (CH3COO–) and monoatomic ions such as fluoride (F–), as well as polyatomic ions such as sulfate (SO42-). Any organic compound containing primary, secondary or tertiary amine group behaves like a Bronsted base and neutralize an acid with the formation of a salt like compound. The amine group, R abstracts a proton from the acid HX forming imium cation, HR+ and the X– anion.
Table 1. Idealized structures for coordination numbers 1-6.
| Coordination | Comments | |||||
| Number | Geometry | Polyhedron | ||||
| 2 | Linear | Uncommon: found mainly with d10 metal ions | ||||
| 3 | Trigonal planar | Rare: can be induced by use of sterically bulky ligands | ||||
| 4 | Square planar | Common for d8 metal ions otherwise unusual | ||||
| 4 | Tetrahedral | Fairly common, especially for d10 and some d5 ions | ||||
| 5 | Trigonal bipyramidal | Rare: | Examples are often
similar in structure and energy so may easily interconvert |
|||
| 5 | Square pyramid | Rare: | ||||
| 6 | Octahedral | Very common; usually the most favored energetically and gives the lowest ligand-ligand repulsions | ||||
| 6 | Trigonal prismatic | Rare, and requires some extra steric or electronic benefit to be faboured over octahedral | ||||
1.2 IMPORTANCE OF COORDINATION COMPLEXS AND METAL IONS
Metals play a vital role in an immense number of extensively differing biological processes. Some of these processes are quite specific in their metal ion requirements, in that only certain metal ions in specified oxidation states can accomplish the necessary catalytic structural requirement. Metal ion dependent processes are found throughout the life science and vary tremendously in their function and complexity.
Coordination compounds play a very significant role in our lives; the study of them has contributed to the highest degree of understanding the chemical bond in inorganic chemistry.
One of the principal themes of bio-inorganic chemistry is the synthesis of metal complexes that have the ability to mimic the functional properties of natural metallopropeins. Proteins, some vitamins and enzymes contain metal ions in their structure involving macromolecular ligands. The chemistry of metal complexes with multidentate ligands having delocalized ?-orbitals, such as Schiff bases or porphyrins has recently gained more attention because of their use as models in biological systems.
In biological systems, metal ion complexes find diverse applications:
(i) transport and storage of oxygen, and other essential elements,
(ii) electron transfer agents,
(iii) catalyst (enzymes), and
(iv) Drugs. Hemoglobin, oxyhemoglobin, myoglobin, chlorophyll, cytochromes, metallo-enzymes, hemocynin, metallo-porphyrins-tyrosinase etc. are all coordination compounds.
Fig. 1. The heme complex in which a Fe2+ ion is coordinated to four nitrogen atoms of a planar porphyrin ligand.
1.3 CHEMISTRY OF CHROMIUM AND MOLYBDENUM
Chromium: Chromium is a steely-gray, lustrous, hard metal (Fig. 2), a chemical element with symbol Cr. Its atomic weight is 51.9961, atomic number 24, melting point 1907oC, boiling point 2671oC. It is also odorless, tasteless, and malleable. Chromium compounds are found in the environment, due to erosion of chromium containing rocks and can be distributed by volcanic eruptions. It is the only elemental solid which shows antiferromagnetic ordering at room temperature (and below). Above 38oC, it transform into a paramagnetic state [2.
| Chromium exhibits a wide range of possible oxidation states, where the +3 state is most stable energetically; the +3 and +6 states are most commonly observed in chromium compounds, where the +1, +4, and +5 states are rare [3]. Chromium (VI) compounds are powerful oxidants at low or neutral pH. Most important are chromate anion (CrO42?) and dichromate (Cr2O72 ?) anions. | Fig. 2. Chromium metal. |
Chromium is an essential mineral that plays a role in how insulin helps the body regulate blood sugar levels, which involved in the control of cholesterol.
As chromium compounds were used in dyes and paints and the tanning of leather, these compounds are often found in soil and ground water. Water insoluble chromium (III) compounds and chromium metal are not considered a health hazard, while toxicity and carcinogenic properties of chromium (VI) have been known for a long time [4]. High concentrations of chromium (III) in the cell can lead to DNA damage [5]. The acute oral toxicity for chromium (VI) ranges between 50 and 150 µg ? kg [6].
Molybdenum: Pure molybdenum is a silvery-grey metal (Fig. 3) with a Mohs hardness of 5.5. It has a melting point of 2623oC and high boiling point 4639oC, chemical elements with the symbol Mo, atomic number 42, and standard atomic weight 95.96.
| Molybdenum does not occur naturally as a free metal on Earth, but rather in various oxidation states in minerals. Variable oxidation states of molybdenum includes ?2, 0, +1, +2, +3, +5, +6, among them most stable are +4 and +6. Molybdenum (VI) oxide is soluble in strong alkaline water, forming molybdates (MoO42?). Molybdates are weaker oxidants than chromates, but they show a similar tendency to form complex oxyanions by condensation at lower pH values, such as [Mo7O24]6– and [Mo8O26]4?. | Fig. 3. Molybdenum metal. |
Molybdenum is used in steel alloys for its high corrosion resistance and weldability. Molybdenum powder is used as a fertilizer for some plants, such as cauliflower [7]. Tungsten replaces the molybdenum anodes in certain low-voltage X-ray sources for specialized uses such as mammography [8].
Molybdenum dusts and fumes, which can be generated by mining or metal working, can be toxic, especially if ingested (including dust trapped in the sinuses and later swallowed) [9]. Low levels of prolonged exposure can cause irritation to the eyes and skin. Dietary molybdenum deficiency from low soil concentration of molybdenum has been associated with increased rates of esophageal cancer in a geographical band from northern china to Iran.
1.4 NUCLEIC ACIDS
A group of substances found in the chromosomes of living cells and viruses that play a central role in the storage and replication of hereditary information and in the expression of this information through protein synthesis. Nucleic acids are colorless, amorphous, high molecular-weight polymers made up of three units: bases, sugar and phosphoric acid. These are obtained by hydrolysis of nucleoproteins which is a class of conjugated proteins. Nucleic acid constitutes the prosthetic group of the nucleoproteins whereas the protein part consists of protamines and histones.
According to the nature of the sugar present, the nucleic acids are generally divided into two main groups:
(i) Deoxyribonucleic acid (DNA)
(ii) Ribonucleic acid (RNA)
All living cells and organelles contain both DNA and RNA, while viruses contain either DNA or RNA but usually not both. RNAs are mainly found in the cytoplasm of the cell, e.g., yeast, liver and pancreas ribonucleic acids; whereas DNAs are present chiefly in the nucleus of the cell such as thymus deoxyribonucleic acids.
Nucleic acids viz. ribonucleic acid (RNA) and deoxyribonucleic acid (DNA), contain three types of basic structural nucleotide units: (i) pentose sugar, (ii) nitrogenous base (pyrimidines/purines) and (iii) phosphate residue. They are long, thread-like polymers, consisting of a linear array of monomers called nucleotides. Nucleotides are the phosphate ester of nucleosides, which are the basic components of DNA. The stacks of DNA contain A, G, C, T while RNA contains A, G, C, U. The types of pentose also distinguish the nucleic acids 2-deoxyribose is found in DNA while it is ribose in RNA.
| Components of Nucleic Acids | ||||||||
| DNA only | DNA and RNA | RNA only | ||||||
| Nitrogen Bases | ||||||||
| Thymine (T) | Adenine (A) | Guanine (G) | Cytosine (C) | Uracil (U) | ||||
| Sugars and Phosphates | ||||||||
| 2-Deoxyribose | Phosphate | Ribose | ||||||
| DNA usually consists of two complementary polymeric chains twisted about each other in the form of a right-handed helix, making a complete turn every 34 Å (3.4 nm), with a diameter of 20 Å (2 nm) (Fig. 4). Since the distance between adjacent nucleotides is 3.4 Å, there must be 10 nucleotides per turn. The constant diameter of the helix can be explained if the bases in each chain face inward and are restricted so that a purine is always opposites to a pyrimidine avoiding organization of purine-purine (too tick) or pyrimidine-pyrimidine (too thin). Irrespective of the actual amounts of each base, the proportion of G is always the same as the proportion of C in DNA, and the proportion of A is always the same as that of T. The two chains of DNA are complementary to each other. | RNA DNA
Fig. 4. Structure of RNA and DNA. |
|||||||
RNA is a single-stranded molecule (Fig. 4) in most of its biological roles and has a much shorter chain of nucleotides. RNA is a polymer of ribonucleoside – phosphates. Its backbone is comprised of alternating ribose and phosphate groups.
| Nucleosides are glycosylamines consisting of a nucleobases (often referred to as simply base) bound to a ribose sugar via a beta-glycoside linkage. The names derived from the nucleobase names. The nucleosides commonly occurring in DNA and RNA include cytidine, uridine, adenosine, guanosine and thymidine. | ||
| Fig. 5. Structure of nucleotides. | ||
Nucleotides formed when a phosphate group is added to a nucleoside (by phosphorylated by a specific kinase enzyme), a nucleotide is produced. Nucleotides are the monomers of RNA and DNA, as well as forming the structural units of several important cofactor -Co A, flavin adenine dinucleotide, flavin mononucleotide, adenosine triphosphate and nicotinamide, adenine dinucleotide phosphate. In the cell nucleotides play important roles in metabolism and signaling.
1.5 ADENINE: IT’S CHEMISTRY AND PROPERTIES
Adenine is a purine base, which has the chemical structure:
| Fig. 6. Structure of adenine. | ||
is an integral part of DNA, RNA, and ATP. Some of the physical properties of adenine are given below:
| IUPAC name | 9 H-purine-6-amine |
| Synonyms | 6-aminopurine |
| Molecular formula | C5H5N5 |
| Appearance | White, crystalline |
| Molar mass | 135.13 g/mol |
| Melting point | 360-365oC |
| Solubility | Hot water |
| pKa | 4.15 |
Adenine was sometimes called vitamin B4[10]. It is no longer considered a true vitamin or part of the vitamin B complex.
According to Traube (1904) adenine [11] can be synthesized from thiourea.
1.6 SOME METAL COMPOUNDS OF ADENINE REPORTED IN THE LITERATURE
There are various compounds of adenine found in literature. Some of them are given below:
In 1967, E. Sletten[12] quoted the works on different metal-adenine complexes of several authors [13-20] and showed that the deep blue inner complexes Cu(C5H5N5)2.nH2O is dimeric with a structure closely related to that of cupric acetate monohydrate. Pairs of copper atoms are held 2.947 Å apart by four adenine bridges, coordinated via N (3) and N (9). A water molecule completes the coordination environment of each metal ion. He also estimated the value of n from crystal density measurement that is equal to 4.
In 1973, P. de Meester, and A. C. Skapski [21] reported the crystal structure of bis (adeninium) trans-bis (adenine) tetra-aquocobalt (II) bis (sulphate).hexahydrate. Crystals are monoclinic, with unit-cell dimensions: a = 13.971(2), b = 7.190(1), c = 19.900(3) Å, ? = 101.82(2) o, space group P21/n, Z = 2. The structure contains the centrosymmetric [Co (H2O)4(adenine)2]2+ ion, adeninium and sulphate ions, and molecules of water of solvation, held together by an intricate network of hydrogen bonds. In the complex cation the cobalt atom has octahedral co-ordination, and the coordinated adenine is unidentate, bonding via N (9). Co-N is 2.164 Å, and the two independent cobalt-water distances are 2.073 and 2.114 Å. The hydrogen-bond network contains the following types of linkage: O?H···O, N?H···O, O?H···N, N?H···N, and possibly C?H···O. A feature of the structure is the presence of adenine-adeninium pairs hydrogen-bonded together via N (1) ···H?N (10) and N (10) ?H···N (7).
In 1985, C. M. Mikulshi et al.[22] reported that, upon refluxing 2:1 mixtures of adenine (adH) and divalent 3d metal chloride hydrates in a 7:3 (v/v) mixture of ethanol-triethyl orthoformate for several days, partial substitution of ad? for Cl? ligands occurs, and solid complexes of the M(ad)Cl.2H2O (M = Mn, Zn), Fe2(ad)(adH)2Cl3.2H2O, M(ad)(adH)Cl. H2O (M = Co, Cu) and Ni2(ad)3Cl.6H2O types are isolated. Characterization studies suggest that the complexes are linear chainlike polymeric species, involving single adenine bridges between adjacent M2+ ions. Terminal chloro, adenine and aqua ligands complete the coordination around each metal ion. The Ni2+ complexis hexacoordinated, whilst the rest of the complexes are pentacoordinated. Most likely binding sites are considered to be N(9) for terminal unidentate and N(7), N(9) for bridging bidentate adenine.
In 1996, R. Ilavarsi, M. N. S. Rao and M. R. Udupa[23] reported on the complexes [Cu(CH3COO)(ade-H)(H2O)] H2O (I), [Cu(CH3COO)(ade-H)(0.5 ade)(0.5 H2O)] 2H2O(Ia), [Cu(CH3COO)(ade-H)(ade)].2H2O (Ib), [Cu(ClCH2COO)(ade-H)(H2O)].H2O(II), [Cu(CNCH2COO)(ade-H)(H2O)].H2O(III), where ade = adenine (C5H5N5), have been prepared. I, II, and III on interaction with aminoacids such as glycine, alanine or glutamic acid give the complexes of the type [Cu(XCH2COO)(aa)(ade)(2H2O)], where X = H, Cl or CN and Haa = amino acid, which are characterized by elemental and thermal analysis, electronic, infrared and EPR spectroscopic studies and variable temperature magnetic susceptibility measurements. Simple adenine complexes are found to be dimeric, involving the bridging of the anionic adenine through the imidazole N (9) and the pyrimidine N (3) nitrogens and the anionic and the acetate group through the carboxylate oxygens, while mixed adenine-aminoacidato complexes are monomeric, involving the bonding of the N(7) of adenine, the oxygen of acetate and chelation of the aminoacid, in addition to the coordination of water molecules.
In 2002, S. Rajender et al.[24] reported on mixed-ligand complexes of Co(II), Ni(II).Cu(II), Zn(II) and Cd(II) with adenine as primary ligand and 5-bromouracil as secondary ligand were prepared in aqueous ethanol solution at pH of about 7 and subjected to a screening system of Dalton’s Lymphoma (DL) tumer cells in vitro for the evalution of viability, 3H-thymine incorporation and induction of apoptosis in tumer cells. The isolated complexes were characterized by various physic-chemical methods. Solution studies were performed pH-metrically for the interaction of the above metal ions with adenine and 5-bromouracil separately (binary) and in the presence of each other (ternary) at 25? 0.1oC temperature and a constant ionic strength of 0.1 M NaNO3 in aqueous solution.
In 2007, M. P. B. Balco et al.[25] studied on the complex, [Cu(C4H6N2O3)(C5H5N5)(H2O)], in which the Cu(II) atom is five-coordinated in a square-pyramidal geometry by a tridentate glycylglycinate ligand (glygly), an N atom from an adenine ligand (Hade) and a water molecule in the apical position. The adenine coordination is reinforced by an intramolecular hydrogen-bonding interaction. A much lower precision structure has already been determined using intensities collected by the film method.
| In 2008, L. Naher[26] synthesized Adenine compounds of Co(II), Cu(II) and Zn(II) and also characterized by their IR and UV-Visible spectra, magnetic moment, thermal analysis, conductance and cyclic voltammetric measurements. | (Adenine)trisaquadichlorocopper(II).monohydrate |
1.7 OBJECTIVES OF THE PRESENT WORK
Adenine (A) is one of the four bases that make up nucleic acids. It is a purine base that complementarily binds to Thymine (T) in DNA and Uracil (U) in RNA. This bond is formed by two hydrogen bonds, which help stabilize the nucleic acid structures. As a result the chemical and biological importance of adenine is enormous.
Since micromolar levels of loosely bound metal ions like Cu(II), Co(II), Zn(II) etc. are present in our body system, it is expected that these metal ions will interact with the DNA/RNA especially with the nucleobases viz. adenine, guanine, uracil, etc. under in-vivo conditions. Thus the interaction of metal ions with adenine has been an active area of research at the interface of chemistry and biology.
Moreover, chromium (VI) is a poison at any level. However, the exact mechanism of its poisoning in the animal body is still not known. It is therefore necessary to investigate the interaction that occurs between dichromate compound(s) with adenine.
In the present work, our studies are associated with the following areas of interest:
(i) Synthesis of dichromate and molybdate derivatives of adenine,
(ii) Studies on their various physical properties such as melting point, solubility, etc.,
(iii) Elemental analysis of the complexes with a view to determine their empirical composition,
(iv) Characterization of the compounds with the help of infrared (IR) spectral analysis to elucidate their micro-structural features of bonding,
(v) UV-Visible spectral analysis of the compounds to know the arrangement of electrons in various molecular orbital and finally the geometry of the compounds,
(vi) Thermal analysis of the compounds to obtain the idea about the coordinated and crystalline water and the structure of the compounds.
2. EXPERIMENTAL
2.1 MATERIALS, METHODS AND EQUIPMENTS
The materials, methods and equipments which were used to perform the experimental work of this project are reported in this section.
2.1.1 CHEMICALS
Adenine was produced from LOBA Chemicals in India. Potassium dichromate, ammonium molybdate, sodium bicarbonate, hydrochloric acid, sodium thiosulphate, and organic solvents etc. used in all synthetic and analytical work were analytical grade, either Aldrich (U.S.A) or E.merck (Germany).
2.1.2 MELTING POINT
Melting points of the compounds were recorded in a Thermometer Stuart Melting Point, SMP11, made in UK.
2.1.3 SOLUBILITY
The solubility of the compounds was determined using various solvents in the usual manner. The solvents used for this purpose are water, methanol, ethanol, and acetone, n-hexane at cold and hot condition.
2.1.4 ELEMENTAL ANALYSIS
Chromium determination: The chromium content of the compound was determined iodometrically using Na2S2O3 solution as the titrating agent. A stock solution was first prepared in a 100 mL volumetric flask. A known amount of compound was taken into a volumetric flask, dissolved it in water and made the flask up to the mark with distilled water. The Na2S2O3 was a secondary standard, was standardized first with a standard solution of potassium dichromate. Starch indicator was added to determine the end point, changes from light yellow to bottle green. Calculation of metal content has been done from the equivalence relationship.
| Cr2O72- + 6I– + 14H+ ? 2Cr3+ + 3I2 + 7H2O |
| 2S2O32- + I2 ? S4O62- + 2I– |
Molybdenum determination: The molybdenum content was determined by heating the compound in a furnace at 700oC. A porcelain crucible was washed and dried. Weight of empty porcelain crucible was recorded in a balance and a known amount of the studied compound was taken in it and weighed. The difference between these two weights gave the weight of the compound. The porcelain crucible with its content was put into the furnace and heated it about 2 hours at 700oC. It was then cooled at room temperature, weighed and identified as MoO3 by its color and X-ray powder diffraction analysis. After that its molybdenum content was calculated.
2.1.5 INFRARED SPECTRA
Fourier transformed Infrared spectra (FTIR) of the compounds were recorded with Fourier transformed Infrared Spectrometer Shimadzu (Japan) of model FTIR-8400 S in the range of 400 – 4000 cm-1 using KBr pellets. Pellets were made with about 100 mg mixture of KBr and sample (in the ratio of 100:0.3 by weight).
2.1.6 ULTRAVIOLET-VISIBLE SPECTRA
The UV-visible spectra (electronic spectra) of the compounds were recorded using ultraviolet recording spectrometer, Model UV-1600 A Shimadzu (Japan), in the wavelength range, 200-800 nm using water as a solvent.
2.1.7 THERMAL ANALYSIS
The thermogravimetric analysis (TGA) of the compounds was carried out with TGA-50 Shimadzu analyzer (Japan). The analysis of both Cr(VI) and Mo(VI) compounds was recorded using alluminium pan as the sample support and in nitrogen atmosphere.
In order to run differential scanning calorimetry (DSC), DSC-60, Shimadzu analyzer was used. Samples were sealed in aluminum pans for analysis. The DSC thermograms were recorded from 30oC to 500oC at a heating rate of 10oC/min. An empty pan was used as a reference. A nitrogen flow rate of 20 mL/min was used.
2.2 SYNTHESIS AND CHEMICAL COMPOSITION OF ADENINE –CHROMIUM AND ADENINE – MOLYBDENUM COMPOUNDS
2.2.1 Adeninium dichromate, (C5H6N5)2Cr2O7
Aqueous solutions of adenine (0.1367 g in 20 mL) and K2Cr2O7 (0.2958 g in 10 mL) were mixed together in a beaker. Before mixing both solutions were filtered. Dilute HCl (7-8 drops) was added to the mixture. An orange red crystalline product was obtained after 24 hours. Dissolved the product in distilled water (about 15-16 mL) with stirring and reduced the volume of the solution on hot water bath. The solution was then kept at room temperature for overnight while orange red needle crystals were obtained. The crystals were then washed with ethanol and dried in air.
| dil. HCl
C5H5N5 + K2Cr2O7 ? (C5H6N5)2Cr2O7 |
Yield = 0.1611 g
2.2.2 Triammonium-trisadeninium molybdate. tetrahydrate,
(NH4)3(C5H6N5)3Mo7O24 .4H2O
Aqueous solutions of adenine (0.1360 g in 20 mL) and (NH4)6Mo7O24 .4H2O (0.2069 g in 20 mL) were mixed together in a beaker. Solutions were filtered before mixing. To it dilute HCl (7-8 drops) was added. A milky white powdery product was formed immediately. It was filtered through sintered glass crucible, washed with water, dried in 100oC in oven.
| dil. HCl
C5H5N5 + (NH4)6Mo7O24 .4H2O ? (NH4)3(C5H6N5)3 Mo7O24 .4H2O |
Yield = 0.2444 g
2.3 PHYSICAL PROPERTIES AND CHARACTERIZATION
2.3.1 MELTING POINT
The melting point of a solid is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exists in equilibrium. Pure crystalline substances have a sharply defined melting point. The temperature at which, melting occurs depends on the strength of the interaction between the atoms or ions. During the melting process, all of the energy added to a substance is consumed as heat of fusion, and the temperature remains constant. Melting point may provide the idea about the nature of the compound basically ionic or nonionic. The melting point of the studied compounds is given below:
Table 2. Melting point of the studied compounds.
| Compounds | Melting point, oC |
| Adeninium dichromate | >350 |
| Triammonium-trisadeninium molybdate. tetrahydrate | >350 |
The m.p. of both compounds are very high suggests the compounds are essentially ionic in character.
2.3.2 SOLUBILITY
Solubility is the property of a solid, liquid, or gaseous chemical substance called solute to dissolve in a solid, liquid, or gaseous solvent to form a homogeneous solution of the solute in the solvent. The solubility of a substance fundamentally depends on the used solvent as well as on temperature and pressure. The solubility of one substance in another is determined by the balance of intermolecular forces between the solvent and solute, and the entropy change that accompanies the solvation.
Polar substance is soluble in polar solvent and non-polar substance soluble in non-polar solvent. Thus the solubility of a substance in a particular solvent provides information about whether a substance is polar or non-polar. The solubility of the studied complexes is given below (Table 3):
Table 3. The solubility of the studied compounds.
| Solvent | Condition | Adeninium dichromate | Triammonium-trisadeninium molybdate. Tetrahydrate |
| Water | Cold | Soluble | Insoluble |
| Hot | Soluble | Insoluble | |
| Methanol | Cold | Insoluble | Insoluble |
| Hot | Slightly soluble | Insoluble | |
| Ethanol | Cold | Insoluble | Insoluble |
| Hot | Slightly soluble | Insoluble | |
| Acetone | Cold | Insoluble | Insoluble |
| Hot | Slightly soluble | Insoluble | |
| n-Hexane | Cold | Insoluble | Insoluble |
| Hot | Insoluble | Insoluble | |
| Dil HCl | Cold | Soluble | Soluble |
| Hot | Soluble | Soluble |
2.3.3 ELEMENTAL ANALYSIS
Elemental analysis is the first step in characterizing any chemical compound. Determination of different elements present in any unknown sample yields from which the empirical formula can easily be deduced. The contents of the compounds under study were determined and the results are presented below:
Table 4. Data for metal contents of the studied compounds.
| Compounds | Methods | Metal content, % | |
| Found | Calculated | ||
| (C5H5N5)2Cr2O7 | Iodometric | 10.93 | 10.65 |
| (NH4)3(C5H6N5)3Mo7O24.4H2O | Gravimetric | 6.03 | 9.39 |
2.3.4 INFRARED SPECTRA
Infrared spectroscopy utilizes the fact that molecules absorb specific frequencies that are characteristic of their structure. These absorption are resonant frequencies, i.e, the frequency of the absorbed radiation matches the frequency of the bond or group that vibrates. The important observation of IR spectrum of a compound consists of a number of characteristic group frequencies, which are highly useful in identifying the functional group present in the complex. This is achieved by comparing the numbers, positions and intensities of various adsorption bands observed with those reported in group frequency charts. The abbreviations and symbols are used in the Table 5-6 to present the relative intensity of various absorption frequencies and their associated mode of vibrations is the following:
| v = very | ? = stretching | sym = symmetric |
| w = weak | ? = bending | asym = asymmetric |
| s = strong | ?? = deformation | aro = aromatic |
| m = medium | b = broad | d = doublet |
The IR bands assignments have been done or the basis of literature survey [27-30].
2.3.4.1 IR SPECTRUM OF ADENINE (C5H5N5)
Fig. 7 shows the IR spectrum of the adenine. The peaks in the spectrum with relative intensities and tentative assignments are listed in Table 5.
Adenine shows a two peaks in the region 3358-3294 cm-1 due to (N?H) stretching vibrations. The peak at higher frequency, 3358.13 cm-1 is an indication that adenine contains primary amine functional group. A broad peak of strong intensity at 3117.02 cm-1 is due to aromatic (C?H) stretching vibration. The characteristic patterns of aromatic ring vibrations in the region 1603.84-1308.72 cm-1 are associated with the interaction of the C=C and C=N vibrations. The C-NH2 stretching vibration observed at 1252.79 cm-1.
Table 5. Infrared band assignment of adenine (C5H5N5).
| Wave number, cm-1 | Relative intensity | Tentative band assignment |
| 3358.13 | w | ?(N?H) |
| 3294.47 | m | |
| 3117.02 | s, b | ?(C?H) |
| 2977.18 | w, b | |
| 2795.87 | w, b | |
| 2693.64 | w | |
| 2599.12 | w | |
| 1672.31 | s | ?(C=NH+) |
| 1603.84 | v, s | ?(C=C)/?(C=N) |
| 1450.49 | w | |
| 1419.63 | m, s | |
| 1308.72 | s | |
| 1252.79 | m | ?(C?NH2) |
| 1125.48 | m | ??(C?H)(in plane) |
| 1024.22 | w | |
| 939.35 | s | ?(N?H)(out of plane) |
| 912.35 | w | |
| 843.87 | w | |
| 796.61 | w | ?(C?H)(out of plane) |
| 723.32 | s | |
| 654.84 | w, b | |
| 542.97 | m |
Fig. 7. IR spectrum of adenine, (C5H5N5).
2.3.4.2 IR SPECTRUM OF (C5H6N5)2Cr2O7
Fig. 8 shows the IR spectrum of the above compound. The peaks in the spectrum with relative intensities and tentative assignments are listed in Table 6.
The above compound shows a broad band of medium intensity centered at 3461 cm-1 due to (N?H) stretching vibration. A broad peak appears at 3074 cm-1 due to (C?H) stretching vibration of the heteroaromatic ring of the adeninium ion. The free adenine molecule displays two strong peaks at 1603 and 1419 cm-1 due to the coupling of the ring stretching vibration with the N-H bending motion. However, in the compound these vibrations appear at higher frequencies 1691 and 1599 cm-1 because of the involvement of ring nitrogen in protonation as N+H. A strong peak at 1412 cm-1 arises due to u(C = C). A peak of medium intensity at 1313 cm-1 and a strong peak at 1250 are attributed to u(C-NH2) vibration. The in plane and out of plane ?'(C-H) deformation peaks coupled with ? (N?H)(out of plane) appear as several peaks of medium/weak intensities at 960-660 cm-1
A strong peak appears in the spectrum of the complex at 538 cm-1, may be assigned to u(Cr=O) vibration.
Table 6. Infrared band assignment of (C5H6N5)2Cr2O7.
|
Fig. 8. IR spectrum of (C5H6N5) Cr2O7.
2.3.4.3 IR SPECTRUM OF (NH4)3(C5H6N5)3Mo7O24.4H2O
Fig. 9 shows the IR spectrum of the above compound. The peaks in the spectrum with relative intensities and tentative assignments are listed in Table 7.
Peaks of medium intensity in the region 3431-3324 cm-1 arises due to N-H stretching vibration. The aromatic (C-H) of the adenine molecule present in the compound vibrates at 3131 cm-1. Like free adenine molecule this spectrum also shows several characteristic ring vibrations in the region 1696-1452 cm-1. A medium peak at 1220 cm-1 is assigned to u(C-NH2). There are three strong peaks and two medium intensity peaks ranging from 946-736 cm-1 are assigned to out of plane ?'(N?H) and out of plane ?'(C-H).
A weak peak appears at 466 cm-1 is assigned to u(Mo=O) vibration.
Table 7. Infrared band assignment of (NH4)3(C5H6N5)3Mo7O24.4H2O.
| Wave number, cm-1 | Relative intensity | Tentative band assignment |
| 3431.42 | w, b | ?(N?H) |
| 3324.37 | m | |
| 3131.49 | w, | ?(C?H) |
| 1696.42 | w | ?(C=NH+) |
| 1656.88 | s, b | ?(C=C)/?(C=N) |
| 1576.83 | s | |
| 1452.42 | s | |
| 1400.34 | s | |
| 1329.94 | ||
| 1220.00 | s | ?(C?NH2) |
| 1178.53 | m | ??(C?H)(in plane) |
| 1111.98 | m | |
| 946.10 | s, d | ?(N?H)(out of plane) |
| 922.95 | s, d | |
| 794.69 | w |
?(C?H)(out of plane) |
| 771.54 | w | |
| 736.82 | w | |
| 570.78 | b | |
| 466.78 | m | ?(Mo=O) |
Fig. 9. IR spectrum of (NH4)3(C5H6N5)3Mo7O24.4H2O.
2.3.5 ULTRAVIOLET-VISIBLE SPECTRA
Ultraviolet-visible spectroscopy refers to absorption in the ultraviolet-visible spectral region. When an atom or molecule absorbs energy, electrons are promoted from their ground state to an excited state. In a molecule, the atoms can rotate and vibrate with respect to each other. These vibrations and rotations also have discrete energy levels, which can be considered as being packed on top of each electronic level.
In region of the electromagnetic spectrum, molecules undergo electronic transitions. Molecules containing ?-electrons or nonbonding electrons (n-electrons) can absorb the energy in the form of ultraviolet-visible light. The more easily excited the electrons, (i.e., lower energy gap between the HOMO and the LUMO), the longer wavelength of light it can absorb. There are three types of electronic transition which can be considered:
· Transition involving ?, ? and n electrons
· Transition involving charge-transfer electrons
· Transition involving d and f electrons
Possible electronic transitions for a complex compound or any compound containing ligand molecule(s) involving ?, ? and n electrons are:
In addition to these electronic transitions, compounds containing transition metal ions also exhibits transitions of two principal types:
d-d transitions: An electron jumps from one d-orbital to another. In complexes of the transition metals, the d-orbitals do not all have the same energy. According to the crystal field theory, the pattern of splitting of the d-orbital mostly depends on the metal type, its oxidation state and the nature of the ligands.
Charge-transfer transition: Most charge transfer complexes involve electron transfer between metal atoms and ligands. The charge-transfer bands in transition metal complexes result from shift of charge density between molecular orbitals (MO) that are predominantly metal in character and those that are predominantly ligand in character. An electron may jump from ligand orbital to a metal orbital giving rise to a ligand-to-metal charge transfer (LMCT) transition. These can most easily occur when the metal is in a high oxidation state. A metal to ligand charge transfer (MLCT) transition will be most likely when the metal is in a low oxidation state and the ligand is easily reduced.
It is extremely common for the coordination compounds to exhibit strong charge-transfer absorptions, typically in the ultraviolet and/or visible portions of the spectrum transitions are intense. The spectrum of adeninium dichromate is shown in Fig. 10. UV-Visible spectrum of pure potassium dichromate (Fig. 11) is also added below for making comparison.
Fig. 10. UV-Visible spectrum of (C5H6N5)2Cr2O7.
Fig. 11. UV-Visible spectrum of K2Cr2O7.
Table 8. Electronic spectral data of potassium dichromate and adeninium dichromate compounds.
| Compound | ? max (nm) |
| K2Cr2O7 | 256, 351 |
| (C5H6N5)2Cr2O7 | 206, 260, 349 |
The bands in the spectrum of K2Cr2O7 at about 256 nm possibly arises due to ???* and n??* electronic transitions. Chromium (VI) is a d0 ion and supposed to have no d?d transition(s) i.e., possesses no color. However, when it bounds to oxygen, a charge transfer transition occurs from the O2- to Cr(VI) responsible for the orange red appearance of Cr2O72- ion.
Very similar appearance is observed in case of the spectrum of adeninium dichromate. This compound shows an intense absorption band at nearly 206.0 nm because of ???* and n??* electronic transitions. The weak intensity band at weak field suggests that this is not a simple ligand field d-d transition. It may be a charge transfer (CT) transition. No band is found in the visible region.
Metal orbitals Tetrahedral Complex Ligand ? orbitals
Fig. 12. Charge transfer from ligand to metal (LMCT).
2.3.6 THERMAL ANALYSIS
Thermal analysis is defined as a group of methods by which the physical or chemical properties of a substance, a mixture or a reactant are measured as a function of temperature or time whilst the sample is subjected to a controlled temperature program. The program may involve heating or cooling (dynamic), or holding the temperature constant (isothermal), or any sequence of these. Thermal stability of a compound is very much dependent on the bond strength of constituent parts. Thermal decomposition studies therefore provide important information with respect to the nature of bonding of various atoms of the compound especially when the compound is nonvolatile. Thermal methods are multi-component techniques and include thermogravimetry, differential thermal analysis and differential scanning calorimetry.
The thermal analytical techniques are studied in this present work are:
Thermogravimetric Analysis (TGA): Thermogravimetric analysis is a type of testing performed on samples that determines changes in weight in relation to a temperature, program in a controlled atmosphere. Such analysis relies on a high degree of precision in the measurements: weight, temperature, and temperature change.
Differential Scanning Calorimetry (DSC): Differential scanning calorimetry is a thermoanalytical technique in which the difference in the amount of heat required to increase the temperature of a sample and reference is measured as a function of temperature or time. The basic principle underlying this technique is that when the sample undergoes a physical transformation such as phase transitions, more or less heat will need to flow to it than the reference to maintain both at the same temperature. Whether loss or more heat must flow to the sample depends on whether the process is exothermic or endothermic. Examples of endothermic process are fusion, boiling, sublimation, vaporization, desolvation, solid-solid transition and chemical degradation. Crystallization and degradation are usually exothermic process.
2.3.6.1 THERMAL ANALYSIS OF (C5H6N5)2Cr2O7
Fig. 13 represents TGA results of the above compound in nitrogen atmosphere. The compound exhibits thermal stability up to 205oC. Above this temperature rapid decomposition starts and up to 222oC about 38.69% weight losses is observed. This weight loss corresponds to the loss of an adenine molecule (calculated weight loss 44.24%). The remainder remains stable up to 311oC. Above this temperature it undergoes a steady decomposition up to 450oC where it attains a constant weight. The end product is about 23.94% of the original weight. The end product may be CrO. The percentage of CrO is 23.94% (observed) while the percentage calculated for the conversion of (C5H6N5)2Cr2O7 to CrO