Tuning Reactivity of Platinum(II) Complexes

ln(k 2/2 nd)/T)
-8.0
-8.2
-8.4
-8.6
-8.8
-9.0
-9.2
-9.4
-9.6
-9.8
-10.0
-10.2
-10.4
-10.6
SCN
TU
DMTU
TMTU
0.00325 0.00330 0.00335 0.00340 0.00345 0.00350
1/T, K -1
Figure 6.8: Plots of ln(k2(2 nd )/T) versus (1/T) for the second step reaction of Pt1 with a
6.4 Discussion
series of different nucleophiles at varying temperatures.
6.4.1 pKa Determination for the Diaqua Complexes
The thermodynamic data in Table 6.1 demonstrates that the pKa values of the
coordinated aqua moieties are dependent on the nature of the linker. The pKa1 values of
the studied dinuclear complexes Pt1, Pt2 and Pt3 (ranging from 4.86 ± 0.05 to 5.19 ±
0.02) are lower than that of the mononuclear analogue Pt4 (pKa = 5.63 ± 0.01) and those
of the Pt(II) amphiphiles recently published: 48 [Pt(H2O)(N,N-bis(2-pyridylmethyl)-
NCH2)n-CH3; NH] +2 n = 1, 2, 3, 4, 5, 9 (pKa = 5.45 ± 0.05 to 5.52 ± 0.02); signifying an
increase in acidities of the coordinated water molecules in these dinuclear complexes.
This is in agreement with what has been reported in other studies which have shown
that Pt atoms in dinuclear complexes are more acidic than the Pt atoms of the
mononuclear complexes. 24,25,48 A higher overall positive charge of +4 on the Pt atoms
coupled with higher electrophilicity is the reason for the lower pKa value observed in the
dinuclear complexes than that found for +2 charged Pt(II) centres of the mononuclear
complexes. Furthermore, a short distance between the Pt(II) centre of the dinuclear
complexes, results in single charge additions (controlled by in-space electrostatic forces
and/or H-bond) on the platinum atoms that increases the effective positive charge at the
metal centre. 25(b & c) This explains why with decreasing Pt---Pt separation distance the
24