Search

Visible-light photoredox catalysis has become a practical tool in the last years for driving energy-demanding chemical reactions. Owing to their exceptional photoelectrochemical properties, classical octahedral ruthenium and iridium complexes still dominate the field of photoredox catalysis despite their drawbacks, such as sustainability and costs. Luminescent Cu (I) complexes are considered one of the most plausible candidates to replace such traditional Ru- and Ir photoredox catalysts in the near future. This review covers the development of Cu(I) complexes for photoredox catalysis with a special focus on collecting and categorizing the available photophysical and electrochemical information. The principal characteristics, advantages, drawbacks and examples of applications of the distinct classes (homoleptic, heteroleptic, in-situ generated tetra/tri/di coordinated, etc.) of Cu(I) complexes used as photoredox catalysts are presented and discussed in a systematic manner. The strategies that have been implemented to extend the lifetimes and to control the photo/electrochemical properties of Cu(I) complexes are detailed with particular emphasis on the role and potential of using dative carbon contact atoms in different ligand scaffolds. Especially, the incorporation of carbon-donor ligands such as isonitriles and carbenes has been proven to be a promising approach for controlling the photoelectrochemical properties and stability of photoredox catalysts.

Two hitherto unknown organometallic compounds with antitumor activity, [Ru(η6-2-(1-propenyl)anisole)(en)(Cl)]PF6 (3) and [Ru(η6-2-(1-propenyl)anisole)(en)(l)]PF6 (4), where en is ethylenediamine, were synthesized and completely characterized using standard techniques (1H and 13C NMR, high-resolution MS and elemental analysis). The lipophilicity and hydrolysis rate kinetics were assessed and compared to the previously reported [Ru(η6-4-(1-propenyl)anisole)(en)(halogen)]PF6 derivatives (4-(1-propenyl)anisole or anethole), where the halogen is Cl (1) or I (2). Based on the obtained rate constants, the coordination of (1-propenyl)anisole to the Ru(en) moiety yielded organometallic compounds that are as active as compounds that bind p-cymene as the arene ligand. Consistent with previously reported kinetic data, our density functional theory-based computational study revealed that an associative interchange mechanism predominates in the hydrolysis of this type of compound, and only small variations (∼1 kcal mol−1) were observed between the stabilities of the transition states corresponding to different derivatives. In vitro analyses of the anti-proliferative activity revealed that compounds 1 to 3 generally exhibit better cytotoxicity and selectivity (tumor versus non tumor cells) toward the gastric tumor cell lines AGS and SNU-1, compared to the parent [Ru(η6-p-cymene)(en)X]PF6 (X: Cl and I) systems. Compound 3 showed similar cytotoxicity to compound 1 toward the AGS cell line, indicating that the change in the substitution pattern of the coordinated arene from 4-(1-propenyl)anisole to 2-(1-propenyl)anisole did not prominently affect the biological behavior. Compound 2 remained the most promising candidate to treat gastric cancer.

The nature and origin of ion-π and ion-σ interactions has been systematically investigated using dispersion-corrected density functional theory and the recently developed noncovalent interaction (NCI) method. A detailed analysis of these interactions is performed with the aim to identify the requirements that have to be fulfilled by the molecular system for strong ion-ligand interactions. Interestingly, our results indicate that aliphatic systems, such as cyclohexane, can interact as strong as aromatic ones with both cations and anions, despite of having a negligible quadrupole moment. In fact, cyclohexane binds anions stronger than benzene itself but slightly weaker that hexafluorobenzene. The NCI method reveals that the interaction between the ions and three C–H bonds of the saturated fragment are responsible for the surprisingly strong ion-σ interaction. A weakening of the ion-σ interactions is observed in the order: Li + > F − > Na + > Cl − > Br − ≈ K + . In addition, a complete Ziegler–Rauk type energy decomposition analysis has been carried out in order to reveal the origins of the thermodynamic driving force for complex formations. The electron density deformation upon complex formation has been scrutinized with a complementary NOCV analysis allowing the identification of molecular orbital interaction contributions to the stabilization. Based on these analysis, it is shown that the formally anion-π interaction is rather an anion-σ ∗ interaction.