Thermodynamic and kinetic analysis of the ligand substitution reactions of different
heterometallic Ru(II)-Pt(II) complexes with a series of bio-relevant thiourea nucleophiles
of different steric demands and ionic nucleophiles have been investigated as a function of
concentration and temperature using UV/visible and stopped-flow spectrophotometric
techniques. To achieve this, five different sets of complexes involving mono di and
multinuclear homo and heterometallic complexes with tridentate N-donor ligands of
different linker ligands were synthesized and characterized by various spectroscopic
methods. The substitution reactions of the chloride complexes were studied in methanol
in the presence of 0.02 M LiCf3SO3 adjusted with LiCl to prevent possible solvolysis. The
aqua complexes were studied in acidic aqueous medium at pH 2.0. All reactions were
investigated under pseudo first-order conditions. Density functional theory (DFT)
calculations were used to aid further interpretations and understandings of the
experimental results.
Substitution reactivity of heterometallic Ru(II)-Pt(II) and Co(II)-Pt(II) complexes bridged
by tetra-2-pyridyl-1,4-pyrazine (tppz) ligand was investigated for the first time. The
reactions proceeded via two steps. The pseudo first-order rate constants, kobs(1st and 2nd)
for
the substitution of the chloride ligand(s) from the Pt(II) complexes and subsequent
displacement of the linker. The dechelation step was confirmed by 1H NMR and
195Pt NMR studies. Incorporation of Ru(tppz) moiety increases the substitution reactivity
and is ascribed to the increased π-back donation from the tppz ligand which increases
the electrophilicity of the metal centre, overall charge and the global electrophilicity index
of the complex. However, when changed the second metal centre from a Ru(II) to a
Co(II), the rate of substitution decreased by a factor of four due to the weaker π-
backbonding from Co(II).
The substitution reactivity of another set of heterometallic Ru(II)-Pt(II) complexes with
a semi-rigid linker, 4’-pyridyl-2,2’:6’,2”-terpyridine (qpy) showed that replacing the cis
pyridyl group by a (tpy)Ru(qpy) moiety lowers the energy of anti-bonding LUMO (π*)
orbitals and increases the metal-metal interactions and electronic transition within the
complex whereby enhancing the reactivity of Pt(II) centre. However, when two Pt(II)
moieties are linked to a (qpy)Ru(qpy), the orthogonal geometry at the Ru(II) metal
centre prevents the extended π-electron density to flow through the three metal centres.
The kinetic results obtained were supported by pKa and 195Pt NMR studies.
Substitution reactions of the mononuclear Pt(II) complexes revealed that the
polyethylene glycoxy pendent units act as a σ-donors including the lone pair electrons on
the first oxygen atom thereby decreasing the reactivity of the parent Pt(II) terpyridine
complex. However, this σ-donation towards the terpyridine moiety was found to be
effective only up to one unit of the ethylene glycoxy pendant, beyond which the
reactivity was sterically controlled. The dinuclear Pt(II) complexes bridged by
polyehtyleneglycol ether units show that the reactivity of the complexes depend on the
Pt···Pt distance and the steric hindrance at the Pt(II) centre. The substitution reactivity
of heterometallic Ru(II)-Pt(II) complexes bridged by the same polyehtyleneglycol ether
units indicate that the presence of Ru(tpy)2 moiety influences the structural geometry of
the complex system which in turn controls the reactivity of the Pt(II) centre. This is
further driven by the entrapment effect of the nucleophile due to the V-shape geometry
adopted by the heterometallic complexes. In all cases the reactivity was also controlled by
steric and electronic effects. However, when two metal centres are bridged by a flexible
non-aromatic linker, the electronic transitions and the metal-metal interactions were
found to be minor, especially for the longer linkers.
The 1H and 195Pt NMR spectroscopic techniques were used to further understand the
observed substitution kinetics and to confirm the degradation of the bridging ligand
from the metal centre(s). In all cases, the negative activation entropies obtained support
the associative mode of substitution This investigation reveals that the length and the
nature of the bridging linker plays an important role in controlling the reactivity of the
heterometallic complexes. It is envisaged that the findings of this project would offer a
significant contribution to the pharmacological design of effective anticancer drugs. / Ph.D. University of KwaZulu-Natal, Pietermaritzburg 2013.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ukzn/oai:http://researchspace.ukzn.ac.za:10413/11314 |
| Date | 17 October 2014 |
| Creators | Shaira, Aishath. |
| Contributors | Jaganyi, Deogratius. |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
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