Kinetic Theory and Associated Techniques 3.1. Introduction
4.5. Kinetic Analyses
All kinetic measurements were performed under pseudo-first-order conditions using at least 10-fold excess of the nucleophile. The wavelengths chosen for the kinetic investigations were pre-determined using UV/Visible absorption spectra obtained from Varian Cary 100 Bio UV/Visible spectrophotometer. Substitution reactions which took less than sixteen minutes to go to completion were studied on an Applied Photophysics SX.18MV (v4.33) stopped-flow system coupled with an online data acquisition system, otherwise UV/Visible spectrophotometer was used. All measurements were carried out in a thermostated environment to within ± 0.1 oC. All data were graphically analysed using a graphical analysis software package, Origin 5.0®.[33]
The electrolyte for the nucleophiles and the platinum complexes were prepared by dissolving a known amount of lithium trifluoromethanesulfonate (LiCF3SO3, 7.02 g, 0.045 mol) and NaCl (0.29 g, 0.005 mol) in freshly distilled methanol or HPLC grade methanol (500 ml) to afford a solution of concentration 0.10 M (0.09 M LiCF3SO3 and 0.01 M NaCl). Lithium trifluoromethanesulfonate was used as the anion since the triflate anion does not coordinate to platinum complexes.[34] Sodium chloride was used to prevent spontaneous solvolysis.
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4.5.1. Preparation of Platinum Complexes for Kinetic Analyses
Solutions of the metal complexes were prepared by dissolving a known amount of the platinum complex in methanol solution of constant ionic strength of 0.10 M (I = 0.1 M, LiCF3SO3 + NaCl). Solution of (Pt3) was prepared by dissolving a known amount of platinum complex in methanol solution to give a platinum concentration of 5.00 x 10-5 M for Pz, Im, MIm, DMIm and Tz before mixing. The concentration of (Pt4) was 5.00 x 10-5 M for the reactions of Pz, MIm, DMIm and Tz and 4.95 x 10-5 for the reactions of Im before mixing. The concentrations of Pt1 and Pt2 solutions for the substitution reactions were 3.01 x 10-5 M before mixing.
4.5.2. Preparation of Nucleophile Solutions for Kinetic Analysis
The solutions of nucleophiles, viz. Pz, Im, MIm, DMIm and Tz were prepared by dissolving a known amount of the required nucleophile in 100 ml methanol solution of fixed ionic strength (I = 0.1 M, (0.09 M LiCF3SO3 + 0.01 M NaCl)) to afford a concentration of ca. 50 times greater than that of the metal complex. The other nucleophile solutions were prepared by subsequent dilutions of the same stock solution to afford a series of standards of 10, 20, 30 and 40 times that of the platinum complex.
4.5.3. Preliminary Kinetic Investigations
The reaction time and the wavelength for the kinetic studies was determined using UV/Visible spectroscopy. The instrument lamps, i.e. UV (D2) lamp and a visible lamp
(tungsten halogen) were allowed to warm up for an hour prior to use. The two compartments of tandem quartz cuvettes were filled with equal volumes of nucleophile and platinum complex solutions with caution taken to prevent premature mixing of the solutions.
The solutions were allowed to equilibrate at 298.15 K for about ten minutes. After an initial scanning over a range of 200-800 nm, the solutions were mixed thoroughly and the second spectrum was obtained at subsequent scans recorded at three minute intervals for the first 20 minutes followed by every five minutes until no further change in absorption was observed in the spectral range. Kinetic investigations were done using the information from
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the UV scanning. The wavelength at which the greatest absorbance change with no steep tangent was used for the subsequent kinetic analyses. Reactions which take less than sixteen minutes to complete were studied using stopped-flow technique. Reactions of Pt2 with MIm were done on stopped-flow except at 288.15 K and both 10 and 20 fold at 293.15 K and 298.15 K. All the reactions of Pt1, Pt3 and Pt4 were studied on the UV/Visible spectrophotometer.
4.5.4. Kinetic Measurements
All reactions were done under pseudo first-order conditions with at least 10 fold excess of nucleophiles. As mentioned before, the lamps of the UV/Visible spectrophotometer were first warmed up and allowed to stabilise at the required temperature prior to kinetic measurements. The instrument was then zeroed at the required wavelength using the methanol solution (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. After an initial absorbance measurement the two solutions were quickly mixed and the substitution reaction was analysed by measuring the change in absorbance over time until no change in absorbance was observed. All kinetic substitutions investigated on UV/Visible spectrophotometry were done in duplicate.
The reactions which took less than sixteen minutes to go to completion were studied using stopped-flow technique (substitution reactions of Pt2 with MIm). The Xenon Arc Lamp (150 W) of the instrument was first allowed to warmed up for about an hour prior to use. The reaction chamber of the instrument was thoroughly rinsed with ultra pure water and then with methanol solution (I = 0.1 M, LiCF3SO3 + NaCl). After rinsing, the instrument was equilibrated at 298.15 K for 10 minutes and then zeroed using the methanol solution at the selected wavelength for the kinetic analysis. The sample syringes were then rinsed with a small amount of the relevant solutions (either the platinum complex solution or the nucleophile solution) and filled with the respective solutions of either the metal complex or the nucleophile. The solutions were then equilibrated to the required temperature for about ten minutes. Equal volumes of the two solutions were then injected into the reaction chamber under nitrogen pressure of 800 kPa. The change in absorbance for the nucleophile substitution reaction was then monitored spectroscopically. After determining the reaction time, the consecutive measurements were then carried out automatically. Each kobs value
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obtained from the stopped-flow represent an average of seven to eight values (kobs values given in Appendix B).
All the kinetic substitution reactions (studied both on UV/Visible spetrophotometry and stopped-flow technique) were fitted to first-order exponential decay function to generate the observed pseudo-first-order rate constants, (kobs), using Equation 4.1[35] at all concentrations and the temperatures.
) t k ( ) A A ( A
At 0 0 expt obs (4.1) where A0, At and A∞ represent the absorbance of the reaction mixture initially, at the time, t and at the end of the reaction respectively.
The observed rate constant, kobs, for the nucleophiles of different concentrations were determined by the same manner. The observed rate constants are given in the Appendix B (Table B.2- Table B.9). The second-order rate constant, k2,and k-2 for the reactions of the platinum complexes with the nucleophiles were obtained from the slopes and the intercepts of the graphs of kobs versus the concentration of the nucleophile (Equation 3.66)[35] drawn by using the graphical analysis software package, Origin 5.0®.[33]
2 0
2[ ]
k Nu k
kobs (3.66) The temperature dependent studies were conducted in a similar manner with all the concentrations of the nucleophiles. The temperature dependent studies were done at five different temperatures within the range 15 - 40 oC in 5 oC intervals since the concentration dependence studies showed a back reaction except for Pt1, Pt3 and Pt4 with DMIm, and Pz and Tz with Pt1 which gave linear fits with no meaningful intercepts for the plots of kobs
versus [Nu] at 298.15 K. Graphs of ln(k2/T) versus 1/T were then drawn using Origin 5.0®.[33]
Activation parameters, entropy of activation (∆S≠) and enthalpy of activation (∆H≠) were then obtained by applying the slopes and the y-intercepts to the Eyring equation (Equation 3.81).
R S T
R H T
k 1 23.8
ln 2
(3.81)
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