Question

Write short notes on:

1. Preparation of elements by chemical and electrochemical routes.
2. Variable oxidation states in oxides and oxoanions of main group elements. ( chloride or nitrogen or
3. Use of metal complexes as anticancer agents.
4. Different structures and functions in some copper proteins.
5. The design and mechanism of the dye-sensitized solar cell.
6. Electrochemical synthesis of the elements.
7. Harnessing energy in natural and artificial photosynthesis.
8. Use of electrolysis in preparation of the elements.

Answer 1

1.A) The method of direct electrochemical synthesis consists of oxidizing a metal anode in a non-aqueous solution containing a ligand (or ligand precursor) to produce the appropriate inorganic or organometallic compound. In many cases, the product precipitates directly in the cell, making for easy isolation, so that the technique is both direct and simple, and in addition the product yields are very high. One advantage of the technique is that the products are often derivatives of a low oxidation state of the metal; \examples of this include chromium(III) bromide, tin(II) and lead(II) diolates and thiolates, hexahalogenodigallate(II) anions, thorium diiodide, copper(I) thiolate complexes, and indium(I) derivatives of thiols, dithiols, and diols. In some systems, the low oxidation state compound undergoes subsequent reaction; for example, in the synthesis of RInX2 the reaction sequence involves the oxidation of indium metal to give InX, which then reacts with RX to give RInX2

B) Chemists synthesize chemical compounds that occur in nature in order to gain a better understanding of their structures. Synthesis also enables chemists to produce compounds that do not form naturally for research purposes. In industry, synthesis is used to make products in large quantity.Chemical compounds are made up of atoms of different elements, joined together by chemical bonds. A chemical synthesis usually involves the breaking of existing bonds and the formation of new ones. Synthesis of a complex molecule may involve a considerable number of individual reactions leading in sequence from available starting materials to the desired end product. Each step usually involves reaction at only one chemical bond in the molecule.In planning the route of chemical synthesis, chemists usually visualize the end product and work backward toward increasingly simpler compounds. For many compounds, it is possible to establish alternative synthetic routes. The ones actually used depend on many factors, such as cost and availability of starting materials, the amount of energy needed to make the reaction proceed at a satisfactory rate, and the cost of separating and purifying the end products. Moreover, knowledge of the reaction mechanism and the function of the chemical structure (or behaviour of the functional groups) helps to accurately determine the most-favoured pathway that leads to the desired reaction product.A goal in planning a chemical synthesis is to find reactions that will affect only one part of the molecule, leaving other parts unchanged. Another goal is to produce high yields of the desired product in as short a time as possible.

3.Platinum complexes are widely used anticancer agents. Their use is primarily based on the pharmacological properties of cisplatin,73 which acts as a model for the design of other metal-based compounds for use in cancer therapyMetal ions are known to bind with nucleic acids and thereby alter their conformation and biological function. The metal ion-base interaction depends on the nature of both metal and bases; a certain site of coordination is preferred. One of the most notable successes for inorganic drugs has been the effectiveness of platinum complexes against cancer. These advances have spurred a surge of investigations to identify new inorganic agents for use in chemotherapy with improved specificity and decreased toxic side effects. Gold (I) and gold (III) complexes, the last isostructural and isoelectronic with platinum (II) complexes, are potentially attractive as anticancer agents. The design of an effective anticancer agent is a complicated game that must encompass not only the drug's inherent inhibitory properties but also its delivery, dosage, and residence time in vivo. Gold (I) and gold (III)complexes, the last isostructural and isoelectronic with platinum (II) complexes, are potentially attractive as anticancer agents. The design of an effective anticancer agent is a complicated game that must encompass not only the drug's inherent inhibitory properties but also its delivery, dosage, and residence time in vivo. Gold (I) and gold (III) complexes overcome some of these challenges by forming strong covalent attachments to targets. Au (III) isoelectronic with Pt (I1)-d8 system usually forms square planar complexes in solution. Since the square planar geometry of Pt (II) is important for its action as an anticancer drug, Au (III) compounds also can be used for the same purpose with the added advantage of decreased toxicity. This, together with the recent finding that certain transitional metal complexes like Au and Pt complexes have been found to be potentially useful in cancer chemotherapy, created a renewed interest in the study of the interactions of metal ions with respect to the site of binding and the structure and stability of the complexes. This work was motivated by the thought that information onthe variety of Au (III) complexes and their effects can be obtained by studying the properties of Au complexes with various ligands. Various studies in the past have shown that Au complexes are very attractive in view of their application as anticancer agents.

4) Copper proteins are proteins that contain one or more copper ions as prosthetic groups.

I) Type I copper centres (T1Cu) are characterized by a single copper atom coordinated by two histidine residues and a cysteine residue in a trigonal planar structure, and a variable axial ligand. In class I T1Cu proteins (e.g. amicyanin, plastocyanin and pseudoazurin) the axial ligand is the sulfur of methionine, whereas aminoacids other than methionine (e.g. glutamine) give rise to class II T1Cu copper proteins. Azurins contain the third type of T1Cu centres: besides a methionine in one axial position, they contain a second axial ligand (a carbonyl group of a glycine residue). T1Cu-containing proteins are usually called "cupredoxins", and show similar three-dimensional structures, relatively high reduction potentials (> 250 mV), and strong absorption near 600 nm (due to S→Cu charge transfer), which usually gives rise to a blue colour. Cupredoxins are therefore often called "blue copper proteins". This may be misleading, since some T1Cu centres also absorb around 460 nm and are therefore green. When studied by EPR spectroscopy, T1Cu centres show small hyperfine splittings in the parallel region of the spectrum (compared to common copper coordination compounds).

II) Type II copper centres (T2Cu) exhibit a square planar coordination by N or N/O ligands. They exhibit an axial EPR spectrum with copper hyperfine splitting in the parallel region similar to that observed in regular copper coordination compounds. Since no sulfur ligation is present, the optical spectra of these centres lack distinctive features. T2Cu centres occur in enzymes, where they assist in oxidations or oxygenations

III) Type III copper centres (T3Cu) consist of a pair of copper centres, each coordinated by three histidine residues. These proteins exhibit no EPR signal due to strong antiferromagnetic coupling (i.e. spin pairing) between the two S = 1/2 metal ions due to their covalent overlap with a bridging ligand. These centres are present in some oxidases and oxygen-transporting proteins (e.g. hemocyanin and tyrosinase).

IV) Binuclear Copper A centres (CuA) are found in cytochrome c oxidase and nitrous-oxide reductase . The two copper atoms are coordinated by two histidines, one methionine, a protein backbone carbonyl oxygen, and two bridging cysteine residues.

V) Copper B centres (CuB) are found in cytochrome c oxidase. The copper atom is coordinated by three histidines in trigonal pyramidal geometry

VI) A tetranuclear Copper Z centre (CuZ) is found in nitrous-oxide reductase. The four copper atoms are coordinated by seven histidine residues and bridged by a sulfur atom.

#Blue Copper Proteins, a class of Type 1 copper proteins, are small proteins containing a cupredoxin fold and a single Type I copper ion coordinated by two histidine N-donors, a cysteine thiolate S-donor and a methionine thioether S-donor.

2)

5) A dye-sensitized solar cell (DSSC, DSC, DYSC[1] or Grätzel cell) is a low-cost solar cell belonging to the group of thin film solar cells. It is based on a semiconductor formed between a photo-sensitized anode and an electrolyte, a photoelectrochemical system.

Design-In the case of the original Grätzel and O'Regan design, the cell has 3 primary parts. On top is a transparent anode made of fluoride-doped tin dioxide (SnO2:F) deposited on the back of a (typically glass) plate. On the back of this conductive plate is a thin layer of titanium dioxide (TiO2), which forms into a highly porous structure with an extremely high surface area. The (TiO2) is chemically bound by a process called sintering. TiO2 only absorbs a small fraction of the solar photons (those in the UV).The plate is then immersed in a mixture of a photosensitive ruthenium-polypyridine dye (also called molecular sensitizers) and a solvent. After soaking the film in the dye solution, a thin layer of the dye is left covalently bonded to the surface of the TiO2. The bond is either an ester, chelating, or bidentate bridging linkage.

A separate plate is then made with a thin layer of the iodide electrolyte spread over a conductive sheet, typically platinum metal. The two plates are then joined and sealed together to prevent the electrolyte from leaking. The construction is simple enough that there are hobby kits available to hand-construct them. Although they use a number of "advanced" materials, these are inexpensive compared to the silicon needed for normal cells because they require no expensive manufacturing steps. TiO2, for instance, is already widely used as a paint base.

One of the efficient DSSCs devices uses ruthenium-based molecular dye, e.g. [Ru(4,4'-dicarboxy-2,2'-bipyridine)2(NCS)2] (N3), that is bound to a photoanode via carboxylate moieties. The photoanode consists of 12 μm thick film of transparent 10–20 nm diameter TiO2 nanoparticles covered with a 4 μm thick film of much larger (400 nm diameter) particles that scatter photons back into the transparent film. The excited dye rapidly injects an electron into the TiO2 after light absorption. The injected electron diffuses through the sintered particle network to be collected at the front side transparent conducting oxide (TCO) electrode, while the dye is regenerated via reduction by a redox shuttle, I3−/I−, dissolved in a solution. Diffusion of the oxidized form of the shuttle to the counter electrode completes the circuit.

Mechanism of DSSc-

The following steps convert in a DSSC photons (light) to current:

1.The incident photon is absorbed by Ru complex photosensitizers adsorbed on the TiO2 surface.

2.The photosensitizers are excited from the ground state (S) to the excited state (S∗). The excited electrons are injected into the conduction band of the TiO2 electrode. This results in the oxidation of the photosensitizer (S+).

S + hν → S∗

3.The injected electrons in the conduction band of TiO2 are transported between TiO2 nanoparticles with diffusion toward the back contact (TCO). And the electrons finally reach the counter electrode through the circuit.

4) The oxidized photosensitizer (S+) accepts electrons from the I− ion redox mediator leading to regeneration of the ground state (S), and two I−-Ions are oxidized to elementary Iodine which reacts with I− to the oxidized state, I3−.

S+ + e− → S

5) The oxidized redox mediator, I3−, diffuses toward the counter electrode and then it is reduced to I− ions.

I3− + 2 e− → 3 I−

The efficiency of a DSSC depends on four energy levels of the component: the excited state (approximately LUMO) and the ground state (HOMO) of the photosensitizer, the Fermi level of the TiO2 electrode and the redox potential of the mediator (I−/I3−) in the electrolyte.

8) By electrolysis, common salt, sodium chloride, NaCl, can be broken down into its elements, sodium and chlorine. This is an important method for the production of sodium. It is used also for producing other alkali metals and alkaline earth metals from their salts.