Tuning Reactivity of Platinum(II) Complexes

4.5 Discussion
By introducing the methyl groups at the 2,3-, 2,5-,and 2,6-positions of the pyrazine
bridge, it was possible to investigate their role as electron-donating groups in
controlling the extent of σ-electron density at the metal centre and also the influence of
steric hindrance on reactivity at the Pt(II) centres. As the number of methyl groups is
increased on the pyrazine moiety, the σ-electron donating ability of the linker
improves. 31, 32 Pyrazine is the smallest and most rigid linear aromatic linker, which due
to the short distance between the two Pt(II) centres and the resonance effect of pyrazine
induces “metal-metal electronic communication” by electrostatic charge transfer from
the metal centre into the lowest unoccupied molecular orbitals (LUMO) of the bridging
ligand, 42 and as expected will result in a more electron-deficient Pt(II) centre.
Comparing the reactivity of the aqua complexes in Table 4.3, the ratio of the substitution
of the first aqua ligand by TU is: 1: 1: 2: 10, respectively for 2,5pzn, 2,3pzn, 2,6pzn and
pzn. This means that the rate of substitution of the aqua ligand by the S-donor
nucleophiles increases in the order of 2,5pzn ≈ 2,3pzn < 2,6pzn < pzn for the first step.
The high reactivity of pzn is attributed to a more positively charged metal centres that
also experiences less steric hindrance. This is in line with the determined lower pKa
values (Table 4.2), higher values of DFT calculated positive NBO charges on the Pt(II)
ions and narrow HOMO-LUMO energy gap (Table 4.1). These findings support higher
electrophilicity and hence, enhanced reactivity at the Pt(II) centre. 18-35,43 On the
contrary, due to lack of appropriate overlapping of orbitals between the metal centred
HOMO mapped on the dz 2 orbitals and the linker lying perpendicular to the plane of the
Pt(II) centre (Figure 2), π-back bonding between the metal and the pyrazine ligand 42,43
is missing.
To understand further the role of the linker σ-donor effect on the rate of substitution of
the cis-aqua ligand for the first step, the reactivity of pzn and 2,6pzn are compared. The
difference between them is on the second Pt2(II) centre of 2,6pzn that is surrounded by
methyl groups. From the kinetic data in Table 4.3, the lability of the aqua ligand in
2,6pzn (k1 = 8.13 M
29
-1 s-1) is reduced by a factor of 5 compared to pzn (k1 = 42.02 M-1 s-1).