Tuning Reactivity of Platinum(II) Complexes
Tuning Reactivity of Platinum(II) Complexes Tuning Reactivity of Platinum(II) Complexes
ln([ML] t ) 4.0 3.5 3.0 2.5 2.0 1.5 110000 120000 130000 140000 150000 160000 170000 180000 Time (s) Figure 4.10: First order plot for the reaction of cis-[{PtCl(NH3)2}2-μ-pzn] +2 (0.221 mM) with excess TU (1.323 mM) (in the molar ratio complex: ligand = 1:6) in D2O at 30° C, using 1H NMR spectroscopy. The equation used in calculation is: ln[ML]t = -kobs1 + ln[ML]0, where [ML]0 = [free pyrazine ligand] = [L]infinite time, and [ML]t = [L]infinite–[L]t using the area of the signals for released pyrazine ligand. Looking at Figure 8 & 9, at t = 0, a signal due to the starting complex [{cis–PtCl(NH3)2}2– μ–pzn](ClO4)2 (δ 195Pt = -2302.2ppm or δ 1 H = 9.04 ppm) is observed. The 1H NMR spectra shows that after 48 h only two main products had been formed. These are identified as uncoordinated pyrazine ligand (L), which is expected to appear as a singlet at δ 1 H = 8.64 ppm, 39,40 and a singlet at δ 195 Pt = -4146.6 ppm 17 corresponding to Pt(TU)4, which is supported by the 195 Pt NMR spectra. The 1 H and 195 Pt NMR spectra reveal the formation of intermediate species, [PtCl–L–Pt(TU)] +3, that exhibits two chemical shifts of Pt(N3Cl) as B at δ 195 Pt = -2962.0 ppm and Pt(N3TU) as C at δ 195 Pt = - 3125.0 ppm (Figure 4.9). Pyrazine (L) protons appear as pairs of doublets at δ 1H = 8.96 as residual coordinated ligand and 8.68 ppm as free ligand seen in Figure 4.8. 39,40 Thus, the coordinated aqua molecules are sequentially substituted from Pt(II) centres in the first two steps. The 195 Pt NMR signals at -3125.0 ppm (marked as C) and -3276.0 ppm (marked as D) in Figure 4.9, are both typical of PtN3S coordinating sphere 41 and -4146.6 24
ppm shown as E corresponds to [Pt(TU)4] +2 , the end-product when the linker is replaced by thiourea in the third step. The disappearance of the signal at δ = 9.04 ppm for the coordinated pyrazine in dinuclear complex, and the appearance of the corresponding signal for the free pyrazine at δ = 8.68 ppm in the 1H NMR spectra together with 195 Pt NMR results all support the proposed reaction given in Scheme 4.2. Based on the NMR study it is reasonable to conclude that the presence of strong labilising thiourea nucleophiles at the Pt(II) centre enhances stepwise cleavage of the linker and the ammine groups attached to the metal centre. This conclusion contrasts previous studies by Farrell and his group 20 who reported that the integrity of the flexible diamine linker remains intact for the reactions of cis-1,1/c,c compound and related novel dinuclear platinum (II) complexes using S-donor nucleophiles. The results however, are consistent with the induced ring opening of bis-(2- pyridylmethethyl)amine chelate by strong labilising thiourea nucleophiles as reported in recent studies by van Eldik and co-workers. 17, 18 4.4.5 Thermodynamic Parameters The temperature dependence study of the second order rate constants, k1/2/3 was investigated over the temperature range of 15 to 35 °C at intervals of 5 °C. The thermodynamic activation parameters, enthalpy of activation (ΔH ≠) and entropy of activation (ΔS ≠ ), were calculated from the Eyring plots which are shown in Figures4.10, 4.11 and 4.12 for the 2,5pzn complex (see Figures. S4.8-S4.10, S4.14-S4.16 and S4.20- S4.22 for the corresponding complexes pzn, 2,6pzn and 2,3pzn, appendix), and the data is recorded in Table 4.4 as ∆Hi ≠ and ∆Si ≠ for the individual substitutions. 25
- Page 120 and 121: The geometry-optimised structures i
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- Page 160 and 161: H2O-Pt-L-Pt-OH2 H2O-Pt-L-Pt-OH2 H2O
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- Page 182 and 183: References 1 T. Storr, K. H.Thomson
- Page 184 and 185: 36 D. Jaganyi, D. Reddy, J.A. Gerte
- Page 186 and 187: Appendix 4 THE INFLUENCE OF THE PYR
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ppm shown as E corresponds to [Pt(TU)4] +2 , the end-product when the linker is<br />
replaced by thiourea in the third step. The disappearance <strong>of</strong> the signal at δ = 9.04 ppm<br />
for the coordinated pyrazine in dinuclear complex, and the appearance <strong>of</strong> the<br />
corresponding signal for the free pyrazine at δ = 8.68 ppm in the 1H NMR spectra<br />
together with 195 Pt NMR results all support the proposed reaction given in Scheme 4.2.<br />
Based on the NMR study it is reasonable to conclude that the presence <strong>of</strong> strong<br />
labilising thiourea nucleophiles at the Pt(<strong>II</strong>) centre enhances stepwise cleavage <strong>of</strong> the<br />
linker and the ammine groups attached to the metal centre. This conclusion contrasts<br />
previous studies by Farrell and his group 20 who reported that the integrity <strong>of</strong> the<br />
flexible diamine linker remains intact for the reactions <strong>of</strong> cis-1,1/c,c compound and<br />
related novel dinuclear platinum (<strong>II</strong>) complexes using S-donor nucleophiles. The results<br />
however, are consistent with the induced ring opening <strong>of</strong> bis-(2-<br />
pyridylmethethyl)amine chelate by strong labilising thiourea nucleophiles as reported<br />
in recent studies by van Eldik and co-workers. 17, 18<br />
4.4.5 Thermodynamic Parameters<br />
The temperature dependence study <strong>of</strong> the second order rate constants, k1/2/3 was<br />
investigated over the temperature range <strong>of</strong> 15 to 35 °C at intervals <strong>of</strong> 5 °C. The<br />
thermodynamic activation parameters, enthalpy <strong>of</strong> activation (ΔH ≠) and entropy <strong>of</strong><br />
activation (ΔS ≠ ), were calculated from the Eyring plots which are shown in Figures4.10,<br />
4.11 and 4.12 for the 2,5pzn complex (see Figures. S4.8-S4.10, S4.14-S4.16 and S4.20-<br />
S4.22 for the corresponding complexes pzn, 2,6pzn and 2,3pzn, appendix), and the<br />
data is recorded in Table 4.4 as ∆Hi ≠ and ∆Si ≠ for the individual substitutions.<br />
25
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