Tuning Reactivity of Platinum(II) Complexes

Abstract
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 square-
planar Pt(II) complexes, viz. (i) mononuclear Pt(II) complexes with tridentate nitrogen-
donor 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