Substitution behaviour of water-soluble organophosphine complexes of palladium (II)

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, of these complexes in aqueous solution. Structure determinations were done by X-ray crystallography and two square planar complexes, trans-[Pd(SC~)2(pTA)2](SCN)2 and trans-[PdBr2(PTA)2] were reported. Both crystallize 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 both the PTA ligands are protonated at one of the N atoms, while two thiocyanate moieties acts as counterions and occupy the axial positions. Both the [PdCI(PT A)3t and cis-[PdCh(PT A)2] complexes hydrolyze and decompose when dissolved in water. However, in an excess of chloride hydrolysis is prevented to a large extent. 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 pH dependences are observed; i.e., pKal = 5.11(4) and pKa2 = 2.92(6), which are assumed to represent 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 in neutral solution (pH = 5.0-6.5), results in the rapid substitution of the chloride to form, as final product, trans-[PdY2(PT A)2]. Similarly, upon addition of a halide/pseudo-halide, Y, to cis-[PdCh(PT A)2], the trans product is formed. Larger equilibrium constants were obtained for 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 to study these complexes by time-resolved spectroscopy. Significant line-broadening on 35Cl Nl\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 smaller the rate of chloride exchange becomes. It is concluded that two species contributed to the exchange, the [PdCl(PT A)3t and a palladium-aqua species, probably [Pd(OH)2(PTA)3t The chloride exchange rate constant for the [PdCI(PTA)3f, at 22°C, was determined as 1.64(8)xl05 M-IS-I. Both the [PdCl(PT A)3t and cis-[PdCh(PT A)2 complexes showed complicated solution behaviour as indicated in the scheme included in the full text.