Tuning Reactivity of Platinum(II) Complexes

The pKa values obtained are summarized in Table 6.1. Included for comparison purpose
in Table 6.1 is the pKa value of the mononuclear analogue, cis-[Pt(NH3)2H2O(4-
methylpyridine)] +2 , Pt4. 21(a)
Table 6.1: Summary of the pKa values obtained for the deprotonation of platinumbound
water of the different complexes.
Complex Pt1 Pt2 Pt3 *Pt4
pKa1 4.86 ± 0.05 5.19 ± 0.02 5.04 ± 0.15 5.63 ± 0.01
pKa2 5.53 ± 0.03 6.42 ± 0.04 5.45 ± 0.17 N/A
* The pKa value was extracted from reference 21(a).
6.3.4 Computation Calculations
Representative geometry-optimized structures for the complexes are shown in Figure
6.2 while key geometrical data including HOMO-LUMO energy gap are summarized in
Table 6.2.
Figure 6.2 shows the minimum energy structures of the studied diaqua complexes
calculated at DFT (B3LYP/LACVP**) level of theory. The geometry about the Pt atoms is
slightly distorted square-planar as the bond angles show in Table 6.2. It is also noted
that the angle labelled c is smaller than all the other angles around the Pt(II) centre an
indication of a possible hydrogen bonding interaction (average separation distance =
2.484 Å) between the H2O/NH3 moieties.
The DFT calculated data from Table 6.2 shows that the NBO charges on the platinum
atoms, to a large extent, remain almost constant across the series of the investigated
complexes. This is not unexpected. X-rays structures of other Pt(II) complexes with such
aromatic bridging ligands (e.g. 4,4’-bis(pyridine)ethane, 4,4’-bis(pyridine)sulphide, 4,4’-
bis(pyridine)disulphide) indicate that the aromatic ring lies almost perpendicular to the
plane of the Pt(II) centre. 23,45 Therefore, the interaction between the metal centre and
the π-electrons of the pyridine ring is expected to be extremely small, if it exists.
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