Asymmetric Cyclometalated RuII Polypyridyl-Type Complexes with π-Extended Carbanionic Donor Sets

Group VIII coordination complexes of bidentate P^N ligands bearing π-extended quinoline or phenanthridine N-heterocycles

Group 8 complexes of π-extended N-heterocyclic ligands present some unexpected results. Benzannulation shifts the lowest energy excitation to the blue by impacting the character of the transition rather than affecting frontier orbital energies.

High-Yielding Syntheses of Multifunctionalized RuII Polypyridyl-Type Sensitizer: Experimental and Computational Insights into Coordination

Ru^II complexes based on functionalized 2,6-di(quinolin-8-yl)pyridine (dqp) ligands feature excellent photophysical and geometrical properties, thus suggesting dqp ligands as ideal surrogates for 2,2'-bipyridine (bpy) or 2,2':6',2″-terpyridine (tpy). However, the synthesis of multifunctionalized [Ru(dqp)₂]²⁺-based complexes is often low-yielding, which has hampered their practical value to date. In this study, a universal high-yielding route was explored and corroborated by a mechanistic investigation based on ¹H NMR, MS, and density functional theory. With application of high-boiling but less-coordinating solvents (i.e., DMF) during the coordination of dqp by the precursor [Ru(dqp)(MeCN)₃]²⁺, the required reaction temperature is lowered considerably (by 30 °C). In comparison to tpy, the reaction rate for dqp is further reduced which is assigned to the higher steric demand upon the coordination process. Namely, the onset of coordination of a tpy derivative at 60 °C and of dqp at 90 °C is significantly milder than in previous protocols. The versatility of the procedure is demonstrated by the high-yielding syntheses of multifunctionalized Ru^II complexes reaching up to 90%, whereby the presence of hydroxyl groups and losses during purification may lower the isolated yields substantially. In addition, the same strategy of high-boiling but less-coordinating solvents enabled a milder one-pot protocol to prepare [Ru(dqp)₂]²⁺ from a [Ru(MeCN)₆]²⁺ source, i.e., without the need for in situ reduction or halide abstraction as typical for Ru^IIICl₃ hydrate. Hence, the developed protocol benefits from an improved thermal tolerance of sensitive functional groups, which may be applicable also to related polypyridyl-type ligands.

A ruthenium bisoxazoline complex as a photoredox catalyst for nitro compound reduction under visible light

An unreported ruthenium(ii) complex containing bisoxazoline ligands has been synthesized and characterized. To test the catalytic ability of the ruthenium complex, the synthesis of anilines from nitro compounds in the presence of a mild reducing agent sodium borohydride and visible light has been developed. Mechanistic studies involving the experiment and DFT calculations suggest that the reaction could involve a radical pathway with the assistance of a photoredox catalyst.

"Chemistry-on-the-complex": functional RuII polypyridyl-type sensitizers as divergent building blocks

Ruthenium polypyridyl type complexes are potent photoactive compounds, and have found - among others - a broad range of important applications in the fields of biomedical diagnosis and phototherapy, energy conversion schemes such as dye-sensitized solar cells (DSSCs) and molecular assemblies for tailored photo-initiated processes. In this regard, the linkage of RuII polypyridyl-type complexes with specific functional moieties is highly desirable to enhance their inherent photophysical properties, e.g., with a targeting function to achieve cell selectivity, or with a dye or redox-active subunits for energy- and electron-transfer. However, the classical approach of performing ligand syntheses first and the formation of Ru complexes in the last steps imposes synthetic limitations with regard to tolerating functional groups or moieties as well as requiring lengthy convergent routes. Alternatively, the diversification of Ru complexes after coordination (termed "chemistry-on-the-complex") provides an elegant complementary approach. In addition to the Click chemistry concept, the rapidly developing synthesis and purification methodologies permit the preparation of Ru conjugates via amidation, alkylation and cross-coupling reactions. In this regard, recent developments in chromatography shifted the limits of purification, e.g., by using new commercialized surface-modified silica gels and automated instrumentation. This review provides detailed insights into applying the "chemistry-on-the-complex" concept, which is believed to stimulate the modular preparation of unpreceded molecular assemblies as well as functional materials based on Ru-based building blocks, including combinatorial approaches.