A kinetic and mechanistic study into the substition behaviour of Plantinum (II) Polypyridyl complexes with a series of Azole Ligands.

The work presented in this thesis is being carried out at the Faculty of Chemistry, University of KwaZulu-Natal, Pietermaritzburg under the supervision of Professor Deogratius Jaganyi, Ph.D. Where the work of others has been used, this is duly accredited in the text.

Given in the diagram are the NBO charges for the nitrogen atoms and the potential energy of the electron density with respect to the electron distribution. 5 Geometry-optimized structures of the investigated platinum complexes and distribution of the electron density on the platinum complexes.

Introduction to Platinum Chemistry 1.1. Overview

The Anticancer Activity of Platinum Complexes

• The Mechanism of Action of Cisplatin
• Reactivity of Cisplatin with DNA
• Cisplatin Resistance

The nature of the amine group also affects the solubility of the drug in the lipid medium. Aquation: The binding of cisplatin to DNA starts with the formation of the monoaqueous species.

Figure 1.3 Suggested reaction pathway for cisplatin in the cell and binding to DNA. [5, 9, 49]

Current Findings on Anticancer Platinum(II) Drugs

• Mononuclear Platinum(II) Complexes

Multinuclear platinum complexes are an important class of platinum compounds that have gained considerable research interest. The dinuclear platinum complexes (21) (n = 4 - 9) were found to have desirable anticancer activities against human tumor cells.[45] Another type of dinuclear complex with a rigid linking group (22) showed higher intrastrand cross-linking with DNA, causing changes in the double helix.

Figure 1.7 Some platinum(II) complexes which deviate from the proposed structural-activity relation, yet have shown antitumor activity in line with cisplatin

Current Research on Platinum(II) Terpy and Polypyridine Complexes

• Biological Importance
• DNA Intercalation
• Substitution of Platinum(II) Terpy with Biologically Active Nucleophiles
• Current Study on Substitution Reactions on Platinum(II) Polypyridyl and Platinum(II) Terpy complexes

11 Some of the platinum(II) terp complexes and biologically active nucleophiles studied by Bugarčić et al. This increases the electrophilicity of the platinum center which in turn increases the reactivity of the metal center.

Figure 1.9 Structure of some of the well studied platinum(II) terpyridine complexes which have shown cytotoxicity and DNA intercalation against tumor cells (anions are omitted for simplicity)

Aims of This Study

The substitution rate of the [Pt{4'-(o-CH3-phenyl)-terpy}Cl]+ is lower compared to that of [Pt{4'-(o-CF3-phenyl)-terpy}Cl]+ as due to the presence of an electron-withdrawing group at the ortho position of the phenyl ring which increases the π-back-bonding ability of the terpier ring system. The opposite is true for the former complex where CH3 on the ortho-phenyl group is an electron donor.

Substitution Reactions

Introduction to substitution reactions

Mechanisms of Substitution Reactions

• Associative Mechanism
• Associatively Activated Interchange Mechanism

The formation of the product will therefore take place while the leaving group is in the immediate vicinity of the intermediate. The leaving group bond weakens before the entering group binds tightly to the metal center.

Figure 2. 2 Energy profiles for the A mechanism for the substitution, showing the relationship between the intermediate and the bond-breaking transition states: (a) the bond breaking transiting state at higher energy (b) the

Substitution Reactions of Square Planar Complexes

• Mechanism of Substitution Reaction Kinetics

In this mechanism, the reaction rate is more dependent on the nature of the incoming group as bond formation occurs between the incoming group and the reactive center in the transition state. However, most substitution reactions of square, planar platinum(II) systems often involve direct replacement of a ligand by the incoming nucleophile.

Figure 2. 3 Schematic representation of the energy profile and possible steric changes during an associative substitution of leaving group, X by the entering group, Y of a square planar complex: energies at 2, 4, 6, and 8 represent t

Factors Influencing the Rate of Substitution Reactions

• The Spectator Ligand
• The trans Effect
• The Molecular Orbital Theory
• The Effect of the Entering Nucleophile
• The Steric Effect
• The Effect of Solvent

Therefore, the transstabilizing effect of the ligands follows the order NO2 > Cl > NH3. The Tran effect was associated with the ability to π-accept the ligand from the metal center in the trigonal bipyramidal intermediate complex. For a square planar complex undergoing an associative mechanism, increasing the size of the input ligand would decrease the rate of the substitution reaction.

The influence of the steric bulk on the reaction rate can either be due to the steric bulk in the spectator ligand or in the incoming nucleophile.

Figure 2. 6 Schematic representation of the R 3 P—Pt double bond. If ligands PR 3 and X are in the xy plane, then the d orbitals shown are either d xz or d yz

Dissociative Mechanisms of Square Planar Complexes

Dissociative mechanisms also have very low solvent effects.[21] With very weak nucleophiles, the solvolytic pathway dominates if the solvent is strongly coordinating.[2] This is one of the shortcomings of this weak nucleophile mechanism.

Figure 2.14 Representation of the non-stereospecific substitution of dissociation mechanism of a platinum(II) square planar complex due to the intermolecular rearrangements

Kinetic Theory and Associated Techniques 3.1. Introduction

Rate Laws

The reaction rate can be expressed as the change in the rate of reactant or product per unit time. [1, 8] The rate of reaction expressed in this way is independent of the size of the sample under consideration. Rate (3.2) where the negative sign indicates that the concentration of the reactant decreases with time. Since the rate equation has the right signs, the rate will always be a positive number.

Integrated Rate Expressions

• Irreversible First-Order Reactions
• Reversible First-Order Reactions
• Consecutive First-Order Reactions
• Irreversible Second-Order Reactions
• Reversible Second-Order Reactions

Since the initial concentration is much greater than the concentration of the reactants at time t, we can write Equation 3.50 as Assuming that the reaction order with respect to A is one, then the rate law in Equation 3.45 becomes,. This produces a number of kobs values ​​for [B]0. Thus, from equation 3.52, kobs can be written as:.

The equilibrium reaction can be represented as follows. 3.54) It can be seen from equation 3.54 that the forward reaction is of the second order, and the reverse reaction is of the first order.

Activation Parameters

• The Arrhenius Equation

The Transition-State Theory, originally called 'the absolute reaction rate theory', was developed in 1935 for a dissociation process in the gas phase[10] by Henry Eyring and Michael Polanyi. This theory was based on the assumption that many reactions occur via the formation of a pre-reaction. -equilibrium between the reactants and the activated transition state complex.[10, 19] According to this theory, a reaction can be described as follows[10]. By referring to Equation 3.70, the second-order experimental rate constant kexpt can be written as. Thus, activation parameters, enthalpy of activation (H) and entropy of activation (S) can be determined from a graph of.

Techniques Associated with the Study of Chemical Kinetics

• UV/ Visible Spectrophotometry

UV/visible spectrophotometry is one of the most powerful[16] and commonly used techniques involved in the kinetic studies.[8] It is a sensitive technique that can detect the sample concentrations ranging from 10-4 to 10-6 M.[21] The key components of a spectrophotometer include the monochromator, light source, detector and the data processor. A log (3.83) By applying Beer's law (Equation 3.84), the concentration of the sample can be determined from its absorbance. Although this makes the analysis of the product more difficult, kinetic analysis can be done on overlapping absorption spectra [19].

This technique is designed to study the reaction of two substances, where one of the reactants is inserted into syringe A and the other into syringe B.

Figure 3. 1 A summary of reaction techniques and their corresponding time scales. [20]

Reddy, PhD thesis, Tuning the reactivity of platinum(II) complexes, University of Natal, Pietermaritzburg, South Africa, 2009, p.

Materials and Methodology

Synthesis

• Synthesis of the Ligand Precursors and the Polypyridyl Ligand
• Synthesis of 3-acetylisoquinoline
• Synthesis of Dichloro(1,5-cyclooctadiene) Platinum(II) [10]
• Synthesis of [Pt{4´-(o-tolyl)terpy}MIm](CF 3 SO 3 ) 2

After decanting the phases, the benzene layer was washed with water and the combined aqueous layers were extracted with ether (3 x 30 mL). The product was washed on the frit with large amounts of diethyl ether and then smaller amounts of cold acetonitrile. The crystals were filtered using 0.45 μm nylon filter membrane on millipore filtration unit, washed with abundant amount of water, ethanol, and diethyl ether and dried in the oven at 100 oC for 1 hour.

The crystals were filtered using a 0.45 μm nylon filter membrane in a millipore filtration unit, washed with diethyl ether and air dried.

Single Crystal X-ray Diffraction Studies

• Crystal Structure of [Pt{4 ´ -(o-tolyl)terpy}MIm](CF 3 SO 3 ) 2

The N2 torsion angle is quite large at 9.5o indicating an out-of-plane twist of the outer pyridine ring. The direction of the out-of-plane bends of the o-tolyl and imidazole groups is the same, as reflected in a dihedral angle between the planes. Presumably this is due to crystal packing effects, as considering the isolated dedication would not necessarily lead to the same result.

Finally, note that the dication is chiral: although it has no chiral center as such, the out-of-plane twists of the o-tolyl and imidazole groups ensure its chirality.

Table 4.1. Plots were obtained with the program ORTEP-3 for Windows Version 2.02 (L

Instrumentation and Physical Measurements

• Characterisation

Computational modeling for complexes (Pt1), (Pt2), (Pt3), and (Pt4) were done as cations for the chloro complexes and as cations for the azole-substituted complexes using the Spartan® '04 software package for Windows®. 25, 26] using B3LYP[27], density functional method (DFT)[28, 29] and LACVP+** (Los Alamos Base Valence Potentials)[30]. The LACVP basis set uses effective core potentials for K-Cu, Pb-Ag, Cs-La and Hf-Au, while Pople's 6- 31G** basis set describes the s- and p-block elements of the second and third rows. [31. 32].

Kinetic Analyses

• Preparation of Platinum Complexes for Kinetic Analyses
• Preparation of Nucleophile Solutions for Kinetic Analysis
• Preliminary Kinetic Investigations
• Kinetic Measurements

Solutions of the metal complexes were prepared by dissolving a known amount of the platinum complex in a methanol solution with a constant ionic strength of 0.10 M (I = 0.1 M, LiCF3SO3 + NaCl). The tandem cuvettes were filled with the respective solutions of the platinum complex and the nucleophile and equilibrated at the required temperature for ten minutes. The reaction chamber of the instrument was thoroughly rinsed with ultrapure water and then with a methanol solution (I = 0.1 M, LiCF3SO3 + NaCl).

Equal volumes of the two solutions were then injected into the reaction chamber under nitrogen pressure of 800 kPa.

Results and Discussion

Synthesis and Characterisation of the Complexes

Successful synthesis of the enone was confirmed by the HNMR data showing a characteristic vicinal coupling constant, JHH 13.49 Hz, for the trans-alkene. The spectral data obtained are in good agreement with the structural formula of the ligand. Furthermore, the X-ray crystal structure obtained for the ligand indicates that the synthesis of the ligand was successful.

From the IR spectrum, a small shift towards a lower wavenumber (ca. < 15 cm-1) of υ(C=C) (aromatic) and υ(C=N) bands for the metal complex compared to that of the free ligand indicates to coordination via the nitrogen atoms of the ligand.

Figure 5.1 Structures of platinum(II) complexes and the nucleophiles used in kinetic investigations

Kinetic Results

Data for substitution reactions of Pt1 with Pz and Im from the literature[9] are included for comparison. Nu k2 and k-2 parameters in M-1 s-1, respectively, for the forward and reverse reactions of the studied complexes. The plots obtained for Pt2 for the forward reactions using the Eyring equation are shown in Figure 5.6.

Figure 5.7 shows a typical Eyring plot obtained for the reverse reaction of Pt3 with nucleophiles.

Figure 5.2 Absorbance change for the reactions of Pt3 ((2.50 x 10 -5 M) with triazole 10 x 2.50 x 10 -5 M and 50 x 2.50 x 10 -5 M) in methanol solution (I = 0.1 M, (0.09 M

Computational Analysis

Included in this figure is a nucleophilic substituted complex for the kinetic reaction product of Pt3 with MIm, ([Pt{4´-(o-tolyl)terpy}MIm](CF3SO3)2) (Pt3-MIm). The NBO charges on the donor nitrogen atoms as shown in Table 5.10 suggest that the platinum metal is surrounded by a ring of negative charge. Computational work also included the calculation of bond lengths (Table 5.11) and NBO charges (Table 5.12) for complexes substituted by N donors.

HOMO-LUMO for the substituted platinum complexes are tabulated in Table 5.13 and shown graphically in Figure 5.10.

Table 5. 5 Geometry-optimised structures of the platinum complexes investigated and distribution of the electron density on the platinum complexes

Discussion

• Kinetic Study of the Displacement of Chloride by Nitrogen Donors
• Accounting for the Reverse Reactions

This trend in the Pt-Cl bond length is due to the ground state p-accepting ability of the terpy part of the complexes. As extended π-conjugation increases in the cis/trans position, the π-acceptor ability of the ligand increases. However, it is arguable that increasing the base of the leaving group would always decrease reactivity.

This σ-inductive effect decreases with increasing distance from the ionizable proton of the azole to the metal center.[46].

Figure 5.11 Absorption spectrum of Pt3, Pt4 and the ligand of Pt4, (4-(o-tolyl)-6-(3˝-isoquinoyl)-2,2´- (4-(o-tolyl)-6-(3˝-isoquinoyl)-2,2´-bipyridine) in acetonitrile solution

Conclusions and Future Work 6.1 Conclusion

Future Work

In the current study, the effect of increasing π-conjugation in the cis and cis/trans positions on the substitution of chloride ligands was investigated. First, the complexes can be studied over a pH range, particularly around physiological pH and temperature. Second, since platinum(II) terpy, [6-13] polypyridyl and 1,10-phenanthroline [7, 14] complexes are biologically active, the kinetic behavior and selective binding of the complexes to DNA can be studied to understand the pharmaceutical significance of the complexes.

Furthermore, the increased water solubility of platinum(II) anticancer drugs is one of the main research interests in the pharmaceutical industry.

Supporting Information