Influence of the bridging azine ligand on the rate of ligand substitution in a series of dinuclear platinum(II) complexes

Abstract

A series of azine-bridged dinuclear platinum(II) complexes of the type [{trans-Pt(NH 3 ) 2 (OH 2 )} 2 (μ-azn)](ClO 4 ) 4 (where azn = pyrazine (pzn, Pt1), 2,3-dimethylpyrazine (2,3-pzn, Pt2), and 2,5-dimethylpyrazine (2,5-pzn, Pt3)) were synthesized to investigate the influence of the bridging azine ligand on the reactivity of the platinum(II) centers. The pK a values of the complexes were determined via acid-base titration, and the rate of substitution of the aqua moiety by a series of neutral nucleophiles, viz. thiourea (TU), 1,3-dimethyl-2-thiourea (DMTU), and 1,1,3,3-tetramethyl-2-thiourea (TMTU), was determined under pseudo-first-order conditions as a function of concentration and temperature using standard spectrophotometric techniques. The introduction of the methyl groups to the bridging azine linker in Pt2 and Pt3 leads to a moderate increase in the pK a values obtained for the first and second deprotonation steps, respectively, as a result of the increased σ-donor capacity of the bridging azine ligand trans to the aqua moiety. A comparison of the rate constants, k 1 and k 2 , at 298 K, obtained for the substitution of the aqua moieties from Pt1, Pt2, and Pt3 by TU, shows that the introduction of the σ-donating methyl groups on the bridging azine ligand in Pt2 and Pt3 results in a corresponding decrease in the reactivity, by ca. five times for the first substitution step and ca. 10 times for the second substitution step. Density functional theory calculations at the B3LYP/LACVP** level of theory for the complexes demonstrate that the introduction of electron-donating methyl groups results in (i) increased steric hindrance over the metal centers and (ii) decreased the positive charge on the metal center and increases energy separation of the frontier molecular orbitals (E HOMO -E LUMO ) of the ground-state platinum(II) complexes, leading to a less-reactive metal center. C

A kinetic study of aqua ligand substitution in dinuclear Pt(II) complexes containing four non-coplanar pyridine ligands

“…It has been reported that the DNA binding properties of Pt(II) complexes differ dramatically when the ligand coordinated to the Pt centre is varied from NH 3 to pyridine [15]. The current study is an expansion of earlier work from our group on azine-bridged dinuclear Pt(II) complexes with NH 3 ligands cis or trans to each other [7,14] and is intended to resolve the thermodynamic and kinetic properties of the complexes as a function of pyridine ligands bonded to the Pt centre. We are particularly interested to know whether the pyridine rings will enter into electronic communication with the Pt(II) orbitals through p-backbonding and how will this affect the reactivity of the complexes, taking into account the steric effects of the methyl groups on the pyrazine linker.…”

“…Calculations were performed in order to gain an in-depth understanding of the structural as well as the electronic differences that exist in these complexes compared to those reported in previous studies [7,14]. Geometry-optimized structures and the HOMO/LUMO maps are shown in Table 1, while the selected metric data are presented in Table 2.…”

“…Kinetic studies of the ligand substitution of dinuclear Pt(II) complexes with flexible and rigid linkers have been reported [1][2][3][4][5][6][7][8][9][10][11][12]. The reactivity of the metal centres has been shown to depend on the length and nature of the bridging ligand [1][2][3][4][5][6][7][8][9][10][11][12].…”

“…The reactivity of the metal centres has been shown to depend on the length and nature of the bridging ligand [1][2][3][4][5][6][7][8][9][10][11][12]. For example, when a straight aliphatic chain is used in dinuclear complexes with monodentate ammine [8] and bis (2-pyridylmethyl)amine [1,[9][10][11][12] carrier ligands around the Pt centres, the reactivity increases with the increase in chain length until it reaches the point where it is independent of the chain length.…”

“…This causes accumulation of the electron density at the metal centre, making it less electrophilic [13]. On the other hand, when rigid and semi-rigid ligands are used to link Pt(II) centres, the reactivity of the complex depends on the ability of the linker to decrease the electron density at the metal centre through p-back bonding [5,7,14].…”