The substitution behaviour of terpy polyglycoxyl and ruthenium-platinum complexes with biologically significant nucleophiles. A kinetic and mechanistic study.
The kinetics of two sets of complexes was investigated under pseudo first order conditions using UV-Visible spectrophotometry as a function of concentration and temperature. The first set consisting 4’-substituted terpy polyglycoxyl complexes reacted with biological relevant N-donor nucleophiles showed that the oxygen on the polyglycoxyl group donates electrons to the Pt-metal resulting in a decrease in reactivity from Pt to Pt(eg). The first carbon-oxygen pendant bond plays a crucial role in regulating the electron density donated because there is insignificant change in the reactivity as the chain length is increased to Pt(deg) and then Pt(teg). The trend in the reactivity slightly increases in general from Pt(eg) to Pt(teg) due to reduction in steric hindrance imposed by the ethylene glycoxy unit on one side of the complex. The reactivity of the nucleophiles is pKa and steric dependent. It can be concluded that reactivity of the metal complexes is mostly electronically controlled while that of the nucleophiles is dependent on the basicity and steric bulk. The obtained kinetic data is supported by the DFT calculations that reveal a less electrophilic Pt(II) metal centre for complexes bearing the 4’-substituent. The temperature dependent studies support an associative mode of activation. The second set of complexes studied included Ru(III)-Pt(II) complexes with a semi-rigid linker 4’-pyridyl-2,2’:6’,2’’terpyridine (qpy) whose results indicated that increase in the overall charges of the respective complexes is the key reason for the observed increase in the reactivity. Additionally, replacing the cis pyridyl group in Pt1 by Ru(III) polypyridyl to give Pt2 and Pt3 respectively lowers the energy of the LUMO (ԉ*) orbitals. The two qpy groups in the tri-nuclear complex Pt3 only slightly increases the reactivity from that of Pt2 because the qpy groups are in orthogonal positions preventing π-electron communication hence the two Pt(II) centres act independently of each other. The observed activation parameters for both sets of complexes support an associative mode of substitution. The results of this project elucidate intrinsic, electronic and steric properties of the complexes that might be exploited for medicinal, photophysical or other applications.