KINETICS

Kinetics



Kinetics

Introduction

The effect of the deprotonation of an amine ligand coordinated to a transition metal ion on the substitutional lability of the other ligands has been extensively studied in the case of octahedral Co(III) complexes. Rate enhancements by factors ranging from 105 to 1013 in favour of the amido species have been observed . As far as square planar d8 substrates are concerned, investigations on this subject are very limited. Baddley and Basolo studied chloride substitution by bromide on the system [Au(dien)Cl]2+/[Au(dienH-1)Cl]+ (dien=1,5-diamino-3-azapentane) and found that the reactivity was halved in the amido complex. On the contrary, in the solvolysis reactions of the system [Au(Et4dien)Cl]2+/[Au(Et4dienH-1)Cl]+ a factor of 60 in favour of the amido species was found . A similar factor (30) has been reported for the hydrolysis of the [Pd(Et4dien)Cl]+/[Pd(Et4dienH-1)Cl] system . Romeo et al. studied the displacement of dimethylsulfoxide (dmso) from [Pt(en)(dmso)2]2+ , [Pt(dien)(dmso)]2+ , [PtMe(dpa)(dmso)]+ (en=ethylendiamine, DPA=bis(2-pyridyl)amine) and the corresponding amido species. In these cases the differences in reactivity between the amino and the amido species were small and not always in the same direction.

We have recently reported the synthesis of the cationic species [Pt(bpma)Cl]+ [bpma=bis(2-pyridylmethyl)amine] and studied the chloride substitutions with nucleophiles . In aqueous basic solution the complex undergoes deprotonation of the secondary amine to give the conjugate amido species [Pt(bpmaH-1)Cl]. Following on these observations and considering the paucity of kinetic data on amine deprotonation of Pt(II) systems and the effect of amine deprotonation on associatively activated substitution reactions, we thought of interest to carry out a kinetic study on the base hydrolysis of [Pt(bpma)Cl]+. An investigation on the acid-base behaviour of the aqua species [Pt(bpma)(OH2)]2+ has also been performed in order to elucidate the nature and the distribution of the species formed in solution. The crystal structures of [Pt(bpma)Cl]Cl•H2O and [Pt(bpma)(OH2](ClO4)2•2H2O are also included.

Discussion

The asymmetric unit of compound 1 consists of a square-planar [Pt(bpma)Cl]+ cation, a Cl- anion and a water molecule. The bpma molecule binds the central metal with its three N atoms; the coordination around Pt is completed by a Cl atom in trans position with respect to N2 atom, which is found to be disordered over two positions, N2 and N2', below and above the mean coordination plane with deviations of -0.45(2) and +0.34(5) Å, respectively. Pt---N and Pt---Cl distances (Table 4) compare well with the mean values of 2.01 and 2.29 Å, respectively, obtained from a systematic analysis of several PtCl2L2 complexes (L=N-donor ligand) . Due to the strain induced by the small bite of the chelate ligand both the N---Pt---N(cis) angles have a value of 83.3°, rather smaller than 90°.

The Cl- counterion acts as a bridge between the cation and the water molecule being involved in two hydrogen bonds with O1 and N2 atoms (Table 5). The square-planar cations are assembled in dimeric units, through a crystallographic centre of symmetry, by means of PtPt interactions of 3.7467(4) Å and N2---HCl1 hydrogen bonds. This arrangement is shown in Fig. 3 and the intra- and intermolecular H-bonds are listed in ...