Aqueous substitution behaviour of the chlorotris (1,3,5-triaza-7-phosphaadamantane) platinum(II) ion

English: The aim of this study was to prepare, characterise and investigate the interesting reactivity and substitution behaviour of the platinum(ll) analogue of the Wilkinson catalyst, [PtCI(PTA)3t, in aqueous medium. The water-soluble ligand 1,3,5-triaza-7- phosphaadamantane (PTA) was employed in the synthesis of the various complexes in this study. This title complex was reacted with various halides and pseudo-halides like SC~, Br", N3- to form products, which could be characterised, and also to investigate the aqueous substitution behaviour of these. Characterisation of the complexes that were synthesised was done by the use of multi-nuclear NMR, IR, UV-Vis techniques and X-ray crystallography. X-ray crystallography was utilised to study the crystal structure of {[Pt(NCS)(PTA)3]NCS}3-5H20 which crystallises in a triclinic space group, Pi; and was refmed to a final R value of3.67%. The structure of this compound showed that the Pt(II) centre displays a well defined square planar geometry but it crystallised with three platinum complexes, three thiocyanate counter-ions and five water solvent molecules in the asymmetric unit. The three platinum complexes are chemically equivalent, but are crystallographically slightly different due to small variations in their packing modes. The above-mentioned substitution reactions were investigated by means of UV-Vis spectrophotometry and stopped-flow techniques. Since the [PtCI(PTA)3t complex was not stable in aqueous medium, (represented in the scheme below), an excess of the chloride ion concentration (as NaCI) was added in all investigations done with the complex. Diagram included on full text. The equilibrium studies of [PtCI(PT A)3t were done by the addition of halides and pseudo-halides, Y (Br", N3-, SCN") which resulted in the rapid substitution of the chloride to form [PtY(PTA)3f as the final product. The equilibrium constants obtained were 10(1), 7(1) and 2.8(5) M-I for SCN", N3- and Br" respectively. The protonation behaviour of the [PtCI(PT Abt complex was also investigated and it was found that it subsequently converts to the cis-[PtCh(PTA)2]. Two pKa values, pKal = 2.1(1) and pKa2= 3.3(8) which are assumed to represent the protonation of the [PtCI(PTA)31CI and cis-[PtCbCPTA)21 complexes respectively, were determined. The rates of substitution reactions were too fast to monitor by time-resolved spectroscopy, which enabled the study of the rapid chloride exchange process by 35CI NMR since significant line broadening was observed. From the experiments done it was noted that increased concentration of the chloride resulted in slower rates of the chloride exchange, The chloride exchange rate constant for the [PtCI(PT A)3]+ at 22 °C was determined to be 2(1) x103 M-1 S-1. The complex aqueous solution behaviour of the [PtCI(PTA)3t complex can be represented in the scheme below, where the charges of the complexes have been omitted for simplicity, PTAH+ = protonated PTA ligand, and iso = isomerisation. Illustration included in full text.