Substitution Reaction Kinetics
2.5 Factors Influencing the Reactivity of Square Planar Platinum(II) Complexes Complexes
2.5.5 Non-Participating Ligand
2.5.5.2 Cis- Effect
36 ii π-trans effect
Good trans directing groups such as CO abd C2H4 stabilize the complex by strong π- back donation from the filled Pt(II) 5dxz or 5dyz to the empty π* (px or py) orbital of the ligand.14a In the trigonal plane, the four orbitals are shared in π-bonding with the three ligands, L, X and Y. This stabilises the trigonal bipyramidal transition state if trans ligand, L, can form bonds with the π* orbitals which results in the transfer of electron density from the platinum centre to the π-acceptor ligands thereby lowering the energy of the system.14a Thus, a good trans ligand lowers the activation energy of the reaction.9b,14a This theory rates the trans ligands according to the following orders:14a,17 C2H4 , CO > CNˉ > SCNˉ > Iˉ > Brˉ > Clˉ > NH3 > OHˉ
Recent studies reported36a,b that trans π-accepting ligands can cause an increase in the substitution reaction of Pt(II) complexes by increasing the electrophilicity of the Pt(II) centre.
37
discrimination of the (NNC) complex since the cis σ-donor lowers the electrophilicity of the Pt(II) metal centre due to their electron donating effect.47
Additionally, Jaganyi et al.62 also reported the Pt―C cis σ-donor effect on the substitution reaction of Pt(II) terpyridine type complexes by thiourea nucleophiles. The authors reported a decrease in the reactivity due to the accumulation of electron density at the Pt(II) metal centre of a Pt(II) (NNC) thereby decreasing its electrophilicity. This further prevents the approach of the nucleophiles due to a destabilization of the transition state. Furthermore, in different studies Jaganyi et al.21a,63 reported the σ-donor ability of an isoquinoline ligand which is cis to the leaving group effectively decreases the electrophilicity of the Pt(II) metal centre thereby decreasing the substitution reactivity.
So far the factors that control the reactivity are studied mostly for mononuclear complexes with square planar Pt(II) geometry. However, not much studies have been reported for multinuclear and heterometallic square planar complexes with bridging linkers. Nevertheless, it is known from literature28,36d that the bridging linker confers special structural properties such as flexibility and rigidity which then influences the substitution reactivity of the complex. Thus, an understanding of how the subtle changes in the structural feature of the complex affects the substitution reactivity at the Pt(II) centre is important. In the following chapters, the influence of some of these factors on the rate of substitution kinetics of square planar Pt(II) complexes will be reported based on the experimental findings.
38 2.6 References
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i Table Contents- 3
List of Figures ... ii List of Tables ... iii List of Schemes ... iii Chapter Three ... 1 Mixed-metal Ruthenium(II)-Platinum(II) and Cobalt(II)-Platinum(II) Complexes of Tetra-2-pyridyl-1,4-pyrazine Bridging Ligand. A Kinetic, Mechanistic and Computational Investigation ... 1
3.0 Abstract ... 1 3.1 Introduction ... 2 3.2 Experimental ... 5 3.2.1 Chemicals ... 5 3.2.2 Characterizations and Instrumentations ... 5 3.2.3 Synthesis of Ruthenium Precursors and Intermediate Complexes ... 5 3.2.4 Synthesis of Platinum(II) Complexes ... 6 3.2.5 Preparation of Nucleophile Solutions for Kinetic Measurements ... 8 3.2.6 Kinetic Measurements ... 9 3.2.7 Computational Modelling ... 9 3.3 Results and Discussion ... 12 3.3.1 Synthesis and Characterization ... 12 3.3.2 Computation Calculations ... 12 3.3.3 Kinetic Analyses ... 13 3.4 Conclusion ... 31 3.5 References ... 33 3.6 Supporting Information ... 40
ii List of Figures
Figure 3.1 Optimized molecular structure of CoPt, showing the torsion angles of the pyridyl groups. ... 13 Figure 3.2 1H NMR spectra of PtRuPt (6.48 mM) with TU in acetonitrile at 298 K showing the dechelation of the coordinated platinum complex to form the (Ru(tppz)2) unit. The spetra also indicates the formation of other intermediate products, in which some of their chemical shifts merges making it difficult to assign them exactly. The numbering system used to monitor the reaction progress is shown on the structure of PtRuPt (inset). ... 15 Figure 3.3 195Pt NMR spectra for the reaction of RuPt (6.28 mM) with TU, showing the changes in the chemical shift of the Pt before adding the TU nucleophile and the degradation after addition of TU for the new complex [Pt(TU)4]2+. ... 16 Figure 3.4 (a) Typical two well-resolved kinetic traces at 382 nm for the two-steps reaction between RuPt (2.0 x 10-5 M) by TU (6.00 x 10-4 M) followed on stopped-flow spectrophotometer at 298 K. (b) A typical plot showing the changes in absorbance between 250 – 750 nm wavelength range for the degradation of the chelate ligand in RuPt (2.00 x 10 -5 M) by TU (6.00 x 10-4 M) at 298 K. Inset is the kinetic trace followed at 382 nm. I = 0.02 M (adjusted with LiCF3SO3 and LiCl). ... 18 Figure 3.5 Dependence of the pseudo first-order rate constants (kobs) on the concentrations of the nucleophiles (a) for the simultaneous displacement of chloride ligands in kobs.(1st), s-1, (b) for the dechelation of the ligands in kobs.(2nd), s-1, from PtRuPt in methanol solution at 298 K and I = 0.02 M (adjusted with LiCF3SO3 and LiCl). ... 20 Figure 3.6 Eyring plots obtained for (a) RuPt with the nucleophiles for the substitution of chloride ligand, (b) Plots of ln(k-2/T) against 1/T for the reactions of RuPt with the nucleophiles for the dechelation of the linker at various temperatures in the range 15 - 35 °C. ... 22 Figure 3.7 (a) UV/visible spectra of Pttpy, RuPt, PtRuPt, PtRuRuPt and CoPt in methanol (0.01 mM). (b) Energy of highest absorption wavelength peak of band against the number of tppz units in the complexes. CoPt deviates from the straight line. ... 26
iii List of Tables
Table 3.1 Selected bond lengths (Å), bond angles (°), natural bond orbital (NBO) charges, HOMO and LUMO energies and other computational data obtained for the complexes Pttpy, RuPt, PtRuPt, PtRuRuPt and CoPt obtained from the computational studies. Data for Pttpy is included for reference. ... 10 Table 3.2 Density functional theoretical (DFT) calculated minimum energy structures, HOMO and LUMO frontier molecular orbitals for the complexes investigated. The planarity of the molecules is viewed along the propagation axis showing the different planes. ... 11 Table 3.3 Summary of the rate constants and activation parameters for the displacement of the chloride ligand(s) by the nucleophiles studied and the kinetic data for the dechelation of the tppz units by thiourea nucleophiles. Data for Pttpy except with MTU is obtained from references and is included for comparisons. ... 23
List of Schemes
Scheme 3.1 Structural formulae of investigated complexes. The numbering schemes used for DFT calculations and the other references are shown on the structure of PtRuRuPt. ... 4 Scheme 3.2 Proposed reaction mechanism for the reactions between the complexes, RuPt, PtRuPt, PtRuRuPt and CoPt with thiourea nucleophiles. The full reaction mechanism holds for RuPt, PtRuPt, PtRuRuPt and CoPt with thiourea nucleophiles only. For the ionic nucleophiles studied, Iˉ and SCNˉ, the reaction mechanism holds only for the first step. The charges on the complexes are omitted for clarity. ... 17
1