Electronic Tuning of the Lability of Pt(II) Complexes through π-Acceptor Effects. Correlations between Thermodynamic, Kinetic, and Theoretical Parameters

Abstract

pi-Acceptor effects are often used to account for the unusual high lability of [Pt(terpy)L]((2)(-)(n)+) (terpy = 2,2':6',2' '-terpyridine) complexes. To gain further insight into this phenomenon, the pi-acceptor effect was varied systematically by studying the lability of [Pt(diethylenetriamine)OH(2)](2+) (aaa), [Pt(2,6-bis-aminomethylpyridine)OH(2)](2+) (apa), [Pt(N-(pyridyl-2-methyl)-1,2-diamino-ethane)OH(2)](2+) (aap), [Pt(bis(2-pyridylmethyl)amine)OH(2)](2+) (pap), [Pt(2,2'-bipyridine)(NH(3))(OH(2))](2+) (app), and [Pt(terpy)OH(2)](2+) (ppp). The crystal structure of the apa precursor [Pt(2,6-bis-aminomethylpyridine)Cl]Cl.H(2)O was determined. The substitution of water by a series of nucleophiles, viz. thiourea, N,N-dimethylthiourea, N,N,N',N'-tetramethylthiourea, I(-), and SCN(-), was studied under pseudo-first-order conditions as a function of concentration, pH, temperature, and pressure, using stopped-flow techniques. The data enable an overall comparison of the substitution behavior of these complexes, emphasizing the role played by the kinetic cis and trans pi-acceptor effects. The results indicate that the cis pi-acceptor effect is larger than the trans pi-acceptor effect, and that the pi-acceptor effects are multiplicative. DFT calculations at the B3LYP/LACVP level of theory show that, by the addition of pi-acceptor ligands to the metal, the positive charge on the metal center increases, and the energy separation of the frontier molecular orbitals (E(LUMO) - E(HOMO)) of the ground state Pt(II) complexes decreases. The calculations collectively support the experimentally observed additional increase in reactivity when two pi-accepting rings are adjacent to each other (app and ppp), which is ascribed to "electronic communication" between the pyridine rings. The results furthermore indicate that the pK(a) value of the platinum bound water molecule is controlled by the pi-accepting nature of the chelate system and reflects the electron density around the metal center. This in turn controls the rate of the associative substitution reaction and was analyzed using the Hammett equation.

Influence of Solvent on Ligand‐Substitution Reactions of PtII Complexes as Function of the ‐Acceptor Properties of the Spectator Chelate

“…More information on the crystal structure of the complexes is available from the literature. [7] Solutions of the complexes were prepared by dissolving known amounts of the chloro complexes in the selected solvents in the presence of 0.002 m LiCl in order to prevent spontaneous solvolysis reactions. The solubility of the complexes differed significantly and UV/Vis spectra were recorded to monitor the solvolysis reactions.…”

“…[7] Kotowski et al determined rate and activation parameters for the reaction of trans-[Pt(py) 2 (NO 2 )Cl] with pyridine in nitromethane, methanol, ethanol and dichloromethane. [8,9] The solvent dependence of the activation volume, ΔV ϶ , could be used to separate intrinsic and solvational volume contributions which result from changes in bond lengths and angles, and changes in polarity, dipole interactions and electrostriction during formation of the transition state of the reaction, respectively.…”

“…In addition, formation of the five-coordinate transition state may involve changes in dipole moment which will affect the activation parameters, especially the entropy and volume of activation. [7] and adopted abbrevi- ations are summarized in Scheme 1. A series of solvents of different polarity were selected and are summarized in Table 1.…”

Interactions of the Platinum(II) Complexes with Nitrogen- and Sulfur-Bonding Bio-Molecules in Chronic Lymphocytic Leukemia

“…In fact, [Pt(terpy)Cl] + complex is much more reactive than complex with dien ligand. (Hofmann et al, 2003) The obtained values for lengths of chemical bonds between the platinum(II) and three nitrogen donor atoms of terpyridine system show that the shortest connection is to the secondary nitrogen atom. This feature is certainly reflected on its reactivity in the processes of substitution.…”

“…In addition, the presence of electrostatic interactions between the pyridine units and metal ions has been observed in crystal structures of various Pt-terpy complexes. (Bailey et al, 1995) Although the high reactivity of the complex depend on the electronic interactions between the terpyridine system and Pt(II), (Hofmann et al, 2003) bulkiness of the terpy ligand has great influence on the characteristics of this complex. The substitution reactions of the [Pt(terpy)Cl] + complexes with different thioethers has been confirmed that there is no reaction, (Petrović B. et al, 1999;Bugarčić et al, 1997) which can be attributed to the strong steric effect.…”