Ruthenium(II) Thiacrown Complexes Incorporating Noninnocent Redox Active Ligands: Synthesis, Electrochemical Properties, and Theoretical Studies

Abstract

The synthesis and characterization of a series of nine new complexes incorporating [Ru II Cl([n]aneS 3 )] (n = 12,14, 16) metal centers coordinated to redox active catechol ligands is reported. The solid-state structure of one of these complexes has been determined by X-ray crystallography. The redox properties of these complexes have been probed experimentally through absorption spectroscopy, cyclic voltammetry, and spectroelectrochemistry, as well as computationally through density functional theory calculations. These studies reveal that, whereas the tetrachlorocatechol-based complexes are isolated with the dioxolene unit in the catechol form, the rest of the complexes are isolated in the semiquinone oxidation state. It was also found that the Ru III/II -based couple for the complexes is dependent on the nature of the thiacrown ligand coordinated to the metal center. A combination of optical and theoretical studies revealed that the absorption spectra of the complexes contain contributions from a variety of charge transfer processes; in the case of the tetrachlorocatechol complexes these transitions include catechol-to-thiacrown ligand-to-ligand charge transfer. ■ INTRODUCTIONIn metal complexes, noninnocent redox behavior occurs when the oxidation states of the metal and a coordinated ligand cannot be defined without ambiguity. 1 Noninnocence arises when the frontier orbitals of the metal and the ligand are close enough in energy for mixing to occur, and is therefore dependent on both the ligand and the metal. This phenomenon has been observed in the active site of metallo-enzymes 2 and is also exploited in the construction of novel redox-active catalysts. 3 Although this phenomenon was first studied in detail in dithiolene complexes, 1,4 transition metal complexes of another redox active ligand, catechol, have also attracted much interest. 5−15 The redox properties of catechol (1,2-dihydroxybenzene) are well-known: deprotonation of both hydroxy groups yields a dianion, which can then be oxidized by two electrons to obenzoquinone. The oxidation from catecholate (CAT) to quinone (Q) is fully reversible and takes place via two consecutive one-electron processes; the intermediate between CAT and Q states is the radical anion semiquinone (SQ), Scheme 1.Early work on the redox behavior of these coordinated ligands concerned the properties of dioxolene complexes of chromium, molybdenum, and tungsten. 5,6 Following initial work by Balch and Sohn using Ru 0 centers, 7 the Lever group carried out related studies on complexes containing Ru II -based moieties coordinated to a variety of catechol derivatives. 8 Electrochemical studies on these systems revealed that although the redox couples for the CAT-SQ and SQ-Q processes were all at more negative potentials than the Ru III/II oxidation, the exact position of the ligand-centered processes varied depending on the ligand used. The same group has also studied the properties of similar complexes containing coordinated catechol derivatives with a range of O-, N-, and S-do...

Increasing the Cytotoxicity of Ru(II) Polypyridyl Complexes by Tuning the Electronic Structure of Dioxo Ligands

“…Over the years, many studies have been performed focusing on the non-innocent character of different metal complexes both from an experimental and theoretical point of view. [34][35][36][37][38][39][40][41] In 2006, Wada and co-workers demonstrated how the oxidation state of the dioxo ligand in a given metal-coligand environment depends on the nature of its substituents. 42 More specifically, the authors investigated a series of [Ru(OAc)(dioxolene)(terpy)] complexes with dioxo ligands carrying electron withdrawing (EWG) or electron donating (EDG) groups.…”

“…42 compounds were investigated in depth. As described in the literature, [34][35][36][37][38][39][40][41] the variation of the electron density on the dioxo ligand leads to the modification of its oxidation state when bound as a ligand. The different oxidation states of the dioxo ligands have a strong bearing on the physical properties of the complexes, affecting, among other parameters, their electronic structures and hence the charge state.…”

“…Driven by the promising properties unveiled for Ru-sq, we undertook a structure–activity relationship (SAR) study, keeping the same Ru­(II) polypyridyl core (i.e., Ru­(DIP) 2 ) but substituting the catechol-type dioxo ligand. Over the years, many studies have been performed that focus on the noninnocent character of different metal complexes from both experimental and theoretical points of view. − In 2006, Wada and co-workers demonstrated how the oxidation state of the dioxo ligand in a given metal-coligand environment depends on the nature of its substituents . More specifically, the authors investigated a series of [Ru­(OAc)­(dioxolene)­(terpy)] complexes with dioxo ligands carrying electron-withdrawing (EWG) or electron-donating (EDG) groups .…”

Macrocyclic coordination chemistry

“…A range of ruthenium(II) thiacrown complexes based on L 64 have been synthesised which incorporate noninnocent redox active ligands and their electrochemical properties have been probed experimentally and computationally. 106 Variation was observed in the ruthenium(III/II) couple that is dependent on the thioether macrocycle.…”

Increasing the Cytotoxicity of Ru(II) Polypyridyl Complexes by Tuning the Electronic Structure of Dioxo Ligands

“…The physico-chemical and biological properties of these compounds were investigated in depth. As described in the literature, [34][35][36][37][38][39][40][41] the variation of the electron density on the dioxo ligand leads to the modification of its oxidation state when bound as a ligand. The different oxidation states of the dioxo ligands have a strong bearing on the physical properties of the complexes, affecting, among other parameters, their electronic structures and hence the charge state.…”