Tuning Reactivity of Platinum(II) Complexes
Tuning Reactivity of Platinum(II) Complexes Tuning Reactivity of Platinum(II) Complexes
For the associative mechanism (A), the rate determining step is the making of the bond between the metal centre and the incoming group (Y). Hence, in a non-coordinating medium in the presence of excess of the concentration of Y, the rate of substitution reaction is strongly dependent on the nature of the incoming ligand, given that it participates in the early section of the transition state. Also, the stereochemistry of the complex is retained during the course of the substitution process. 15,16 In an associative mode of substitution mechanism, all the ligands involved can influence the stability and activation energy of the five- coordinate activated complex (see Scheme2.1). As a result, all the groups (i.e. entering, leaving and co-ligands) will affect the magnitude of rate obtained from the substitution reaction of a d 8 square-planar Pt(II) complex. Because of this many studies have systematically been carried out by varying the character of the ligands. 1719 In previous studies, 20,21,22,23 Pt(II) complexes of the type cis-[Pt(Me)2R2] and cis- [Pt(Ph)2R2] (where R= DMSO or thioether) revealed that the two strong cis σ-donor atoms, i.e. carbene in the organo-ligands, S or P, display a strong preference for dissociative mechanism. As a result of the high trans-influence of the two strong σ-donor bonding ligands, the Pt-S bond of the thioether leaving groups are weakened by the increase in electron density at the metal centre. This prevents the facile attack by the incoming nucleophile, but there is stabilisation of the 14-electron intermediate by the two strong σ-bonded ligands. On the other hand, when one of the thioethers is replaced by a strong π-acceptor ligand such as CO or CN ¯ , an associative mechanism is favoured. This is because the stronger π-accepting CO or CN ¯ ligand removes the electron density from the metal centre, hence increasing its electrophilicity. This allows delocalization of the electron density-received from the incoming nucleophile-away from the Pt(II) centre in a five-coordinate transition-state stabilisation interaction. Therefore, introduction of σ-donor and/or π-acceptor ligands into the Pt(II) complexes provides an electronic torque through the molecule that is a determinant on the type of reaction mechanism. 6
2.2.3 The Interchange Mechanism (I) Between the limiting dissociation (D) and the limiting association (A) mechanisms there exists a continuum of mechanisms that are characterised by a single activated complex in which bond-making and bond-breaking between the metal centre and the entering-plus-the leaving groups are synchronised. If the bond between the metal and the leaving group is weakened before the incoming group tightly binds to the metal centre, then the probability of the solvent attaching to the reactive metal centre is higher leading to a dissociation mechanism. 7 This permits the solvent, which is normally in a higher molar excess to dominate the substitution process, leading to a dissociative- activated interchange mechanism (ID). If the reaction rate is more dependent on the nature of the incoming group and the leaving group leaves the metal centre only once the incoming group is fully bound to the reactive centre, the mode of activation changes from ID to associatively-activated interchange mechanism (IA). 5 2.3 Measurements of Integrated Rate Constants 2.3.1 Reversible Second-order Reactions Often, ligand substitution reactions in square-planar complexes do not go to completion, but have a tendency to attain a state of equilibrium and can be written as: M L 3 X + Y A + B k2 k-2 C or k 2 k -2 M L 3 Y + X w here, A = M etal com plex (M L 3 X), X is the leaving group B = incom ing nucleophile, Y 7 (2.4) The forward reaction step is thus second-order, while the reverse reaction step is first- order, which results in mixed order dependence. Because of the complex nature of the reaction, the reaction can be studied by selecting pseudo first-order conditions, in which
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For the associative mechanism (A), the rate determining step is the making <strong>of</strong> the bond<br />
between the metal centre and the incoming group (Y). Hence, in a non-coordinating<br />
medium in the presence <strong>of</strong> excess <strong>of</strong> the concentration <strong>of</strong> Y, the rate <strong>of</strong> substitution<br />
reaction is strongly dependent on the nature <strong>of</strong> the incoming ligand, given that it<br />
participates in the early section <strong>of</strong> the transition state. Also, the stereochemistry <strong>of</strong> the<br />
complex is retained during the course <strong>of</strong> the substitution process. 15,16<br />
In an associative mode <strong>of</strong> substitution mechanism, all the ligands involved can influence<br />
the stability and activation energy <strong>of</strong> the five- coordinate activated complex (see<br />
Scheme2.1). As a result, all the groups (i.e. entering, leaving and co-ligands) will affect<br />
the magnitude <strong>of</strong> rate obtained from the substitution reaction <strong>of</strong> a d 8 square-planar<br />
Pt(<strong>II</strong>) complex. Because <strong>of</strong> this many studies have systematically been carried out by<br />
varying the character <strong>of</strong> the ligands. 1719<br />
In previous studies, 20,21,22,23 Pt(<strong>II</strong>) complexes <strong>of</strong> the type cis-[Pt(Me)2R2] and cis-<br />
[Pt(Ph)2R2] (where R= DMSO or thioether) revealed that the two strong cis σ-donor<br />
atoms, i.e. carbene in the organo-ligands, S or P, display a strong preference for<br />
dissociative mechanism. As a result <strong>of</strong> the high trans-influence <strong>of</strong> the two strong σ-donor<br />
bonding ligands, the Pt-S bond <strong>of</strong> the thioether leaving groups are weakened by the<br />
increase in electron density at the metal centre. This prevents the facile attack by the<br />
incoming nucleophile, but there is stabilisation <strong>of</strong> the 14-electron intermediate by the<br />
two strong σ-bonded ligands. On the other hand, when one <strong>of</strong> the thioethers is replaced<br />
by a strong π-acceptor ligand such as CO or CN ¯ , an associative mechanism is favoured.<br />
This is because the stronger π-accepting CO or CN ¯ ligand removes the electron density<br />
from the metal centre, hence increasing its electrophilicity. This allows delocalization <strong>of</strong><br />
the electron density-received from the incoming nucleophile-away from the Pt(<strong>II</strong>) centre<br />
in a five-coordinate transition-state stabilisation interaction. Therefore, introduction <strong>of</strong><br />
σ-donor and/or π-acceptor ligands into the Pt(<strong>II</strong>) complexes provides an electronic<br />
torque through the molecule that is a determinant on the type <strong>of</strong> reaction mechanism.<br />
6
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