One-Pot Synthesis of Symmetric and Asymmetric <i>p</i>-Quinone Ligands and Unprecedented Substituent Induced Reactivity in Their Dinuclear Ruthenium Complexes

The compounds 2-[2-(trifluoromethyl)-anilino]-5-hydroxy-1,4-benzoquinone (<b>L</b><sup><b>1</b></sup>), 2,5-di-[2-(trifluoromethyl)-anilino]-1,4-benzoquinone (<b>L</b><sup><b>2</b></sup>), 2-[2-(methylthio)-anilino]-5-hydroxy-1,4-benzoquinone (<b>L</b><sup><b>3</b></sup>), and 2,5-di-[2-(methylthio)-anilino]-1,4-benzoquinone (<b>L</b><sup><b>4</b></sup>) were prepared in high yields by reacting 2,5-dihydroxy-1,4-benzoquinone with the corresponding amines in a one-pot synthesis in refluxing acetic acid. This straightforward and “green” synthesis delivers biologically relevant asymmetric <i>p</i>-quinones such as <b>L</b><sup><b>1</b></sup> and <b>L</b><sup><b>3</b></sup> in a rare, simple, one-step process. The proposed synthetic route is general and can be applied to generate a variety of such molecules with different substituents on the nitrogen atoms. Structural characterization of <b>L</b><sup><b>2</b></sup> and <b>L</b><sup><b>4</b></sup> shows electron delocalization across the “upper” and “lower” parts of the molecule, thus showing the importance of charge separated species in the proper description of such molecules. Reactions of these ligands with [Cl(η<sup>6</sup>-Cym)Ru(μ-Cl)<sub>2</sub>Ru(η<sup>6</sup>-Cym)Cl] (Cym = <i>p</i>-Cymene = 1-isopropyl-4-methyl-benzene) in the presence of a base result in the formation of complexes [{Cl(η<sup>6</sup>-Cym)Ru}<sub>2</sub>(μ-<b>L<sub>−2H</sub></b><sup><b>1</b></sup>)] (<b>1</b>), [{Cl(η<sup>6</sup>-Cym)Ru}<sub>2</sub>(μ-<b>L</b><b><sub>−2H</sub></b><sup><b>2</b></sup>)] (<b>2</b>), [{Cl(η<sup>6</sup>-Cym)Ru}<sub>2</sub>(μ-<b>L</b><b><sub>−2H</sub></b><sup><b>3</b></sup>)] (<b>3</b>), and [{Cl(η<sup>6</sup>-Cym)Ru}<sub>2</sub>(μ-<b>L</b><b><sub>−2H</sub></b><sup><b>4</b></sup>)] (<b>4</b>). Structural characterization of <b>2</b> and <b>4</b> shows a rare <i>syn</i>-coordination of the chloride atoms. The SMe groups in <b>3</b> and <b>4</b> are not coordinated to the ruthenium center, and the bridging ligands thus function in a bis-bidentate form. Abstraction of the chloride atoms in these complexes with AgClO<sub>4</sub> in CH<sub>3</sub>CN results in the expected formation of solvent substituted complexes [{(CH<sub>3</sub>CN)(η<sup>6</sup>-Cym)Ru}<sub>2</sub>(μ-<b>L</b><b><sub>−2H</sub></b><sup><b>1</b></sup>)][ClO<sub>4</sub>]<sub>2</sub> (<b>5</b>[ClO<sub>4</sub>]<sub>2</sub>) and [{(CH<sub>3</sub>CN)(η<sup>6</sup>-Cym)Ru}<sub>2</sub>(μ-<b>L</b><b><sub>−2H</sub></b><sup><b>2</b></sup>)][ClO<sub>4</sub>]<sub>2</sub> (<b>6</b>[ClO<sub>4</sub>]<sub>2</sub>) with the ligands where there are no additional donor atoms on the nitrogen substituents. The same chloride abstraction reaction in the cases of <b>3</b> and <b>4</b> leads to an unprecedented substituent induced release of the Cym ligand, resulting in complexes of the form [(CH<sub>3</sub>CN)(η<sup>6</sup>-Cym)Ru(μ-<b>L</b><b><sub>−2H</sub></b><sup><b>3</b></sup>)Ru(CH<sub>3</sub>CN)<sub>3</sub>][ClO<sub>4</sub>]<sub>2</sub> (<b>7</b>[ClO<sub>4</sub>]<sub>2</sub>) and [{(CH<sub>3</sub>CN)<sub>3</sub>Ru}<sub>2</sub>(μ-<b>L</b><b><sub>−2H</sub></b><sup><b>4</b></sup>)][ClO<sub>4</sub>]<sub>2</sub> (<b>8</b>[ClO<sub>4</sub>]<sub>2</sub>), where the SMe groups are now coordinated to the metal center. In the case of complex <b>3</b>, which contains an asymmetric bridging ligand, Cym release is observed only at the side that contains an additional SMe donor, thus proving the necessity of such donor substituents for the observed reactivity. The increase in Lewis acidity at the ruthenium center on chloride abstraction is made responsible for SMe coordination and the rigidity of the ligand systems, and their concomitant failure to coordinate in a “<i>fac</i>” manner as is required for a piano stool configuration results in the eventual Cym release. The bridging ligand which then coordinates in a bis-meridional fashion in <b>8</b>[ClO<sub>4</sub>]<sub>2</sub> results in a bis-pincer type of coordination. These observations were validated by a structural analysis of <b>8</b>[ClO<sub>4</sub>]<sub>2</sub>. The results show the potential hemilabile character of ligands such as <b>L</b><sup><b>3</b></sup> and <b>L</b><sup><b>4</b></sup>. Electrochemical and spectroscopic investigations are reported on <b>8</b>[ClO<sub>4</sub>]<sub>2</sub>, and substitution reactions of the CH<sub>3</sub>CN molecules are presented to show the use of <b>8</b>[ClO<sub>4</sub>]<sub>2</sub> as a versatile precursor for other reactions.