Tuning Reactivity of Platinum(II) Complexes

compared to CH3PhPtCl. The “unusual” decrease in reactivity of CH3PhisoqPtCl is
ascribed to the decrease in delocalisation of π-electron density as a result of the poor π-
acceptor property of the isoquinoline ligand. The isoquinoline ligand is a net σ-donor,
which slows down substitution reactions. This is supported by the Electronic absorption
spectra and DFT-calculations which show that this system has the largest HOMO-LUMO
energy gap. This behaviour is contrary to the cis σ-effect reported in other studies. The
activation parameters obtained in the study supported an associative mode of
substitution mechanism in all cases.
In Chapter 4, a series of bridged dinuclear platinum(II) complexes of the type, [{cis–
Pt(H2O)(NH3)2}2–µ–pzn] 4+ {where pzn = pyrazine (pzn), 2,3-dimthylpyrazine (2,3pzn),
2,5-dimethylpyrazine (2,5pzn) and 2,6-dimethylpyrazine (2,6pzn) were synthesised to
investigate the influence of the rigid bridging ligand on the reactivity of the platinum(II)
centres in dinuclear complexes.
The higher pKa values for first and second deprotonation of 2,3pzn, 2,5pzn and 2,6pzn
compared to pzn are indicative of the σ-inductive effect of the methyl substituents on
the pyrazine moiety that increases the electron density at the Pt(II) centre. The effect of
increasing or decreasing electron density on the Pt(II) centre is also visible in the
reactivity of the complexes. The reactivity of pzn is higher in comparison to 2,3pzn,
2,5pzn and 2,6pzn. This is due to the moderate electrophilicity of the Pt(II) centre,
which also experiences less steric hindrance. In addition, The DFT calculated bond
angles show that the bridging pyrazine ligand lies almost perpendicular to the square-
planar Pt(II) centre enabling the methyl groups on the linker ligand to effectively block
the entry of the incoming nucleophile from above or below the metal centre ,slowing
down the substitution process. The introduction of the methyl groups to bridging ligand
also results in an increase in the separation energy of the frontier molecular orbitals
(ΔE), leading to a less reactive metal centre in the ground-state.
1 H and 195 Pt NMR spectroscopy was applied to demonstrate stepwise substitution of the
chloro ligands from the pyrazine bridged dichloro Pt(II) complex (pzn) by thiourea. A
third step was observed in all the complexes and was attributed to degradation of the
2