Turning reactivity of platinum(II) complexes : a kinetic and mechanistic investigation into substitution behaviour of mono-and Dinuclear platinum(II) complexes.
Systematic kinetic and thermodynamic analyses of the substitution reactions of different Pt(II) complexes with a series of bio-relevant nucleophiles have been investigated as a function of concentration and temperature, using standard stopped-flow and UV–Vis Spectrophotometric methods. For this purpose, five different systems involving squareplanar Pt(II) complexes, viz. (i) mononuclear Pt(II) complexes with tridentate nitrogendonor ligands of varying degree of π-conjugation, and (ii) polynuclear Pt(II) complexes with azine, pyridyl units separated by S, S-S and CH2CH2 spacer groups, and α,ω- alkanediamine bridging ligands were synthesised and characterised by various spectroscopic methods. All substitution reactions of the Pt(II) chlorido complexes of the type [Pt(terpy)Cl]+ were studied in the presence of 10 mM LiCl to prevent spontaneous parallel reaction due to hydrolysis or solvolysis. The substitution reactions of the coordinated water molecules in the dinuclear Pt(II) complexes by thiourea nucleophiles of varying steric hindrance were studied under acidic conditions. The concentration of the nucleophile solution was prepared in 0.1 M NaClO4, at pH 2.0 and always at least 10- fold excess to provide pseudo first-order conditions. The pKa values of the coordinated aqua ligands of the dinuclear Pt(II) complexes were determined by Spectrophotometric acid-base titrations. DFT calculations were also performed in an effort to account for the observed reactivity of homologous analogues in each series of complexes, in terms of NBO charges and energies of frontier molecular orbitals. Substitution reactions of the Mononuclear Pt(II) complexes with tridentate ligands showed reactivity of the complexes is controlled by the π–acceptor characteristics of the chelate ligands. The fused rigid pyridyl system allows electronic interaction between the platinum centre and the pyridyl ligands, because of the extended conjugated π-system. This effect is controlled by how the fused ring system around the terpy moiety is structured. The isoquinoline moiety was found to reduce the effective π-backbonding and the lability of CH3PhisoqPtCl complex compared to 1,10-phenanthroline and terpyridine systems, indicating that isoquinoline ligand is a net σ-donor. The results obtained for the substitution reactions of the diaqua Pt(II) complexes with the thiourea and ionic (Br-, I- SCN-) nucleophiles demonstrate that reactivity increases with decreasing pKa values as well as decreasing distance between the Pt(II)centres. An increase in steric crowding at the Pt(II) centre imposed by the methyl groups on the azine linker decelerates the lability of the aqua ligands. The 1H and 195Pt NMR spectroscopic results confirmed degradation of the aromatic-based bridging ligand from the metal centre. The final cleavage of the complex linkers was only achieved after addition of excessive amounts of thiourea and other strong nucleophiles. The negative activation entropies and second-order kinetics for all the substitution reactions support the Associative mode of substitution mechanism.