Tuning Reactivity of Platinum(II) Complexes

5.2.6 Spectrophotometric pKa Titrations
The UV-Vis spectra were recorded on a Varian Cary 100 Bio spectrophotometer
equipped with a thermostated cell holder. The pH of the aqueous solutions was
measured at 25°C, using a Jenway 4330 Conductivity/pH meter equipped with a Micro
4.5 diameter glass electrode. The electrode was calibrated using standard buffer
solutions at pH 4.0, 7.0 and 10.0, purchased from Saarchem chemicals. The pH electrode
was filled with 3 M NaCl electrolyte to prevent precipitation of KClO4.
Spectrophotometric pH titrations were carried out using NaOH as the base. In order to
avoid absorbance corrections due to dilution, a large volume (200ml) of each of the
complex solution was used during the titration. Subsequent pH changes were obtained
by stepwise additions of crushed solid NaOH pellets in the pH range 2-3, micropipette
dropwise addition of saturated, 1.0 and 0.1 M NaOH or conc. HClO4 (for reversibility of
the pH) to the complex solution, prior to withdrawal of 2 ml aliquots of the solution for
pH determination. After each of the measurements, the aliquots were discarded to avoid
contamination by chloride ions coming from the pH electrode.
5.2.7 Kinetic Measurements
Spectral changes resulting from mixing separately each of the complexes pzn, pmn,
pdn, qzn and pht with ligand solutions were recorded over the wavelength range 200-
600 nm to establish a suitable wavelength at which the kinetic measurement would be
performed on a Varian Cary 100 Bio spectrophotometer. The kinetics of the lability of
coordinated water was followed spectrophotometrically by monitoring the change in
absorbance at suitable wavelengths. An Applied Photophysics SX 18MV (v4.33)
stopped-flow system coupled to an online data acquisition system was used for fast
substitution reactions. The UV-Vis spectrophotometer was used for subsequent slow
reaction steps. The reactions were studied under pseudo first-order conditions with at
least 10-fold excess of nucleophile concentration per Pt-atom. The temperature of both
instruments was controlled to within ± 0.1° C. Analyses of the pH- and time-dependent
spectra were graphically performed using Origin 7.5 ®37 software package.
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