Tuning Reactivity of Platinum(II) Complexes
Tuning Reactivity of Platinum(II) Complexes Tuning Reactivity of Platinum(II) Complexes
spectra were measured in and referenced to the solvent (D2O) signal at 30 °C, on a Bruker Avance DPX 500 spectrometer ( 1H, 500 MHz) and ( 195Pt, 107.5 MHz), respectively, to confirm the purity and identity of the complexes as well as to follow the reaction kinetics of the dinuclear complex pzn containing two Pt–Cl bonds with TU. The 195 Pt chemical shifts were referenced externally to K2[PtCl6]. Both 1 H and 195 Pt NMR are effective methods for the determination of the coordination details at the metal centre because their chemical shifts are usually influenced by the solvent, temperature and the coordinated donor atoms. 27 UV-Visible absorption spectra for the determination of pKa values and kinetic measurements of slow reactions were recorded on a Varian Cary 100 Bio UV-Visible spectrophotometer equipped with a thermostated cell holder. The pH of the aqueous solutions of the complexes were measured using a Jenway 4330 Conductivity/pH meter equipped with a Micro 4.5 diameter glass electrode. The electrode was calibrated at 25 °C using standard buffer solutions of pH 4.0, 7.0 and 10.0 (Merck). The pH electrode was filled with 3.0 M NaCl instead of KCl to prevent precipitation of KClO4 during use. An Applied Photophysics SX 18 MV (v4.33) stopped- flow reaction analyzer coupled to an online data acquisition system was used for fast reactions. The temperature of the instruments was controlled to within ± 0.1°C for all the measurements. Mass spectra were obtained on Hewlett Packard LC-MS using electron impact (EI) ionization. Analyses of the pH and time-dependent kinetic spectral data were graphically performed using Origin 5.0 ® 28software package. 4.2.6 Computational details DFT calculations were performed on all the complexes so as to provide an insight into their observed kinetic results. Ground-state electronic structures were optimized using density functional theory (DFT) method on Spartan `04 for Windows ® quantum chemical package. The functional used throughout this study was the B3LYP, 29 a non- local hybrid exchange functional defined by Becke`s three parameter equation, utilizing LACVP†** (Los Alamos Core Valence Potentials) 30 pseudo potential basis set. The ground-state geometries of the complexes were modelled as cations of total charge of +4 in the gas phase. 8
4.2.7 Determination of pKa values of the diaqua complexes Spectrophotometric pH titrations were performed using NaOH as the base, in pH range 2-10. A large volume (200.0 mL) of each of the complex solutions was used during the titration to avoid absorbance corrections due to dilution. Solid NaOH pellets was added stepwise in the pH range 2-3, while micropipette dropwise additions of saturated, 1.0 M and 0.1 M NaOH or conc. HClO4 (for reversibility of the pH) were added to the solution until the desired pH was attained prior to withdrawal of 2.0 mL aliquots from the solution for pH measurement. The 2.0 mL aliquots were discarded after each measurement to avoid in situ contamination of the complex solution by the chloride released from the pH electrode. The concentrations of the Pt(II) complexes used during these titrations ranged from 3.52 x 10 -5 to 1.06 x 10 -4 M. 4.2.8 Kinetic measurements All the kinetic measurements were performed under pseudo first-order conditions in order to force the reactions to go to completion. The desired concentration of the nucleophile was achieved by using at least a 20-fold (1:2 complex formation) excess of the nucleophile over the dinuclear metal complex. This afforded at least a 10-fold excess of the nucleophile at each centre. The kinetic reactions were studied at pH ca. 2.0 to guarantee the presence of only the diaqua form of the complexes. The suitable wavelengths at which the kinetic measurements could be followed were established spectrophotometrically by monitoring the change in absorbance of the mixture of the metal complex and the nucleophile as a function of time and are summarised in Table S4.1, (appendix). The temperature dependence was studied at five different temperatures ranging from 15 to 35 °C at intervals of 5 °C. 4.3 Results The role of the pyrazine as a bridging ligand and the influence of steric and electronic effects of the methyl substituents attached to it, on the kinetics and thermodynamic properties of each of the Pt(II) centres of the dinuclear platinum(II) complexes were investigated using the four complexes shown in Figure 4.1. 9
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- Page 142 and 143: Table S3.8: DFT calculated electros
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spectra were measured in and referenced to the solvent (D2O) signal at 30 °C, on a<br />
Bruker Avance DPX 500 spectrometer ( 1H, 500 MHz) and ( 195Pt, 107.5 MHz),<br />
respectively, to confirm the purity and identity <strong>of</strong> the complexes as well as to follow the<br />
reaction kinetics <strong>of</strong> the dinuclear complex pzn containing two Pt–Cl bonds with TU. The<br />
195 Pt chemical shifts were referenced externally to K2[PtCl6]. Both 1 H and 195 Pt NMR are<br />
effective methods for the determination <strong>of</strong> the coordination details at the metal centre<br />
because their chemical shifts are usually influenced by the solvent, temperature and the<br />
coordinated donor atoms. 27 UV-Visible absorption spectra for the determination <strong>of</strong> pKa<br />
values and kinetic measurements <strong>of</strong> slow reactions were recorded on a Varian Cary 100<br />
Bio UV-Visible spectrophotometer equipped with a thermostated cell holder. The pH <strong>of</strong><br />
the aqueous solutions <strong>of</strong> the complexes were measured using a Jenway 4330<br />
Conductivity/pH meter equipped with a Micro 4.5 diameter glass electrode. The<br />
electrode was calibrated at 25 °C using standard buffer solutions <strong>of</strong> pH 4.0, 7.0 and 10.0<br />
(Merck). The pH electrode was filled with 3.0 M NaCl instead <strong>of</strong> KCl to prevent<br />
precipitation <strong>of</strong> KClO4 during use. An Applied Photophysics SX 18 MV (v4.33) stopped-<br />
flow reaction analyzer coupled to an online data acquisition system was used for fast<br />
reactions. The temperature <strong>of</strong> the instruments was controlled to within ± 0.1°C for all<br />
the measurements. Mass spectra were obtained on Hewlett Packard LC-MS using<br />
electron impact (EI) ionization. Analyses <strong>of</strong> the pH and time-dependent kinetic spectral<br />
data were graphically performed using Origin 5.0 ® 28s<strong>of</strong>tware package.<br />
4.2.6 Computational details<br />
DFT calculations were performed on all the complexes so as to provide an insight into<br />
their observed kinetic results. Ground-state electronic structures were optimized using<br />
density functional theory (DFT) method on Spartan `04 for Windows ® quantum<br />
chemical package. The functional used throughout this study was the B3LYP, 29 a non-<br />
local hybrid exchange functional defined by Becke`s three parameter equation, utilizing<br />
LACVP†** (Los Alamos Core Valence Potentials) 30 pseudo potential basis set. The<br />
ground-state geometries <strong>of</strong> the complexes were modelled as cations <strong>of</strong> total charge <strong>of</strong> +4<br />
in the gas phase.<br />
8
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