Substitution behaviour of watersoluble organophosphine complexes of palladium (II)

[摘要] English: The aim of this study was to prepare complexes of the type [PdCI(PT A)3]Cl and cis-[PdCh(PTA)2] (pTA = 1,3,5- triaza-7-phosphaadamantane) and to investigate i) the stability,ii) protonation behaviour, and iii) the substitution behaviour with halides/pseudo-halides, ofthese complexes in aqueous solution.Structure determinations were done by X-ray crystallography and two square planarcomplexes, trans-[Pd(SC~)2(pTA)2](SCN)2 and trans-[PdBr2(PTA)2] were reported. Bothcrystallize in the monoclinic space group, P21/n, refined to final R values of 2.88 % and 4.15%, respectively. For the trans-[Pd(SCN)2(PTA)2](SCN)2 complex it was observed that boththe PTA ligands are protonated at one of the N atoms, while two thiocyanate moieties acts ascounterions and occupy the axial positions.Both the [PdCI(PT A)3t and cis-[PdCh(PT A)2] complexes hydrolyze and decompose whendissolved in water. However, in an excess of chloride hydrolysis is prevented to a largeextent. In solution, a dynamic equilibrium exists between [PdCI(PT A)3r and cis-[PdCh(PT A)2] and over a period of time, depending on the chloride concentration added,[PdCI(PT A)3r slowly converts to cis-[PdCh(PT A)2].In acidic medium [PdCI(PT A)3r rapidly converts to cis-[PdCh(PT A)2 and two pHdependences are observed; i.e., pKal = 5.11(4) and pKa2 = 2.92(6), which are assumed torepresent the protonation of the [PdCI(PTA)3t and [PdCh(PTA)2] complexes respectively.The addition of a halide/pseudo-halide, Y, (Y = Br, I, SCN and N3) to [PdCI(PTA)3t inneutral solution (pH = 5.0-6.5), results in the rapid substitution of the chloride to form, asfinal product, trans-[PdY2(PT A)2]. Similarly, upon addition of a halide/pseudo-halide, Y, tocis-[PdCh(PT A)2], the trans product is formed. Larger equilibrium constants were obtainedfor cis-[PdCh(PTA)2] and Y (K24), than for [PdCI(PTA)3t and Y (K14) (K24 = 3.0(3)x103,1.1(6)x102, 6.0(4)x10 and 4.1(3) Ml, for I', SCN, N3- and Br', respectively, and K14 =5.8(4)x102, 1.9(4)x10 and 3.0(3) Ml, for r, SeN and Br, respectively). High reactivity of all the complexes investigated was observed and it was not possible tostudy these complexes by time-resolved spectroscopy. Significant line-broadening on 35ClNl\1R was observed which enabled the study of the rapid chloride exchange for[PdCl(PT A)3t. It was observed that the higher the concentration of the chloride, the smallerthe rate of chloride exchange becomes. It is concluded that two species contributed to theexchange, the [PdCl(PT A)3t and a palladium-aqua species, probably [Pd(OH)2(PTA)3tThe chloride exchange rate constant for the [PdCI(PTA)3f, at 22°C, was determined as1.64(8)xl05 M-IS-I.Both the [PdCl(PT A)3t and cis-[PdCh(PT A)2 complexes showed complicated solutionbehaviour as indicated in the scheme included in the full text.

[发布机构] University of the Free State