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

[摘要] English: The aim of this study was to prepare, characterise and investigate the interestingreactivity and substitution behaviour of the platinum(ll) analogue of the Wilkinsoncatalyst, [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 complexesin this study. This title complex was reacted with various halides and pseudo-halideslike SC~, Br, N3- to form products, which could be characterised, and also toinvestigate the aqueous substitution behaviour of these. Characterisation of thecomplexes 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; andwas refmed to a final R value of3.67%. The structure of this compound showed thatthe Pt(II) centre displays a well defined square planar geometry but it crystallisedwith three platinum complexes, three thiocyanate counter-ions and five water solventmolecules in the asymmetric unit. The three platinum complexes are chemicallyequivalent, but are crystallographically slightly different due to small variations intheir packing modes.The above-mentioned substitution reactions were investigated by means of UV-Visspectrophotometry and stopped-flow techniques. Since the [PtCI(PTA)3t complexwas not stable in aqueous medium, (represented in the scheme below), an excess ofthe chloride ion concentration (as NaCI) was added in all investigations done withthe complex. Diagram included on full text. The equilibrium studies of [PtCI(PT A)3t were done by the addition of halides andpseudo-halides, Y (Br, N3-, SCN) which resulted in the rapid substitution of thechloride to form [PtY(PTA)3f as the final product. The equilibrium constantsobtained 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 itwas 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-resolvedspectroscopy, which enabled the study of the rapid chloride exchange process by 35CINMR since significant line broadening was observed. From the experiments done itwas noted that increased concentration of the chloride resulted in slower rates of thechloride 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 berepresented in the scheme below, where the charges of the complexes have beenomitted for simplicity, PTAH+ = protonated PTA ligand, and iso = isomerisation.Illustration included in full text.

[发布机构] University of the Free State