The research on 1,2,3-triazoles has been lively and ever-growing since its stimulation by the advent of click chemistry. The attractiveness of 1H-1,2,3-triazoles and their derivatives originates from their unique combination of facile accessibility via click chemistry and truly diverse supramolecular interactions, which enabled myriads of applications in supramolecular and coordination chemistry. The nitrogen-rich triazole features a highly polarized carbon atom allowing the complexation of anions by hydrogen and halogen bonding or, in the case of the triazolium salts, via charge-assisted hydrogen and halogen bonds. On the other hand, the triazole offers several N-coordination modes including coordination via anionic and cationic nitrogen donors of triazolate and triazolium ions, respectively. After CH-deprotonation of the triazole and the triazolium, powerful carbanionic and mesoionic carbene donors, respectively, are available. The latter coordination mode even features non-innocent ligand behavior. Moreover, these supramolecular interactions can be combined, e.g., in ion-pair recognition, preorganization by intramolecular hydrogen bond donation and acceptance, and in bimetallic complexes. Ultimately, by clicking two building blocks into place, the triazole emerges as a most versatile functional unit allowing very successful applications, e.g., in anion recognition, catalysis, and photochemistry, thus going far beyond the original purpose of click chemistry. It is the intention of this review to provide a detailed analysis of the various supramolecular interactions of triazoles in comparison to established functional units, which may serve as guidelines for further applications.

A series of heteroleptic bis(tridentate) ruthenium(II) complexes, each bearing a substituted 2,2':6',2″-terpyridine (terpy) ligand, is characterized by room temperature microsecond excited-state lifetimes. This observation is a consequence of the strongly σ-donating and weakly π-accepting tridentate carbene ligand, 2',6'-bis(1-mesityl-3-methyl-1,2,3-triazol-4-yl-5-idene)pyridine (C^N^C), adjacent to the terpy maintaining a large separation between the ligand field and metal-to-ligand charge transfer (MLCT) states while also preserving a large (3)MLCT energy. The observed lifetimes are the highest documented lifetimes for unimolecular ruthenium(II) complexes and are four orders in magnitude higher than that associated with [Ru(terpy)(2)](2+).

A systematic series of heteroleptic bis(tridentate)ruthenium(II) complexes of click-derived 1,3-bis(1,2,3-triazol-4-yl)benzene N^C^N-coordinating ligands was synthesized, analyzed by single crystal X-ray diffraction, investigated photophysically and electrochemically, and studied by computational methods. The presented comprehensive characterization allows a more detailed understanding of the radiationless deactivation mechanisms. Furthermore, we provide a fully optimized synthesis and systematic variations towards redox-matched, broadly and intensely absorbing, cyclometalated ruthenium(II) complexes. Most of them show a weak room-temperature emission and a prolonged excited-state lifetime. They display a broad absorption up to 700 nm and high molar extinction coefficients up to 20 000 M(-1)cm(-1) of the metal-to-ligand charge transfer bands, resulting in a black color. Thus, the complexes reveal great potential for dye-sensitized solar-cell applications.

[a] Ruthenium(II) polypyridyl complexes have received particular interest with respect to photosensitizer applications, because they are stable and inert complexes that show a defined metal-to-ligand charge transfer (MLCT).[1] A central dilemma is that trisbidentate complexes (e.g., of 2,2'-bipyridine, bpy) show long excited-state lifetimes, whereas bis(tridentate) complexes (e.g., of 2,2':6',2''-terpyridine, tpy) allow the isomer-free construction of linear assemblies for vectorial electron-transfer processes.[2] The quest of diminishing the fast radiationless deactivation of the 3 MLCT state through the triplet metal-centered state ( 3 MC) of bis(tridentate) ruthenium(II) polypyridyl complexes [3] has motivated numerous approaches [4][5][6] that aim at 3 MLCT lowering or 3 MC raising or both. Ideally, electronic manipulations are realized by direct incorporation of stronger donors, that is, by cyclometalation [7] or coordination through anionic N-heterocycles [8] and N-heterocyclic carbenes (NHCs).[9] Thereby, strong s and p donation by coordination through anionic carbon or nitrogen donors lead to a destabilized ground state and, thus, a lowered 3 MLCT, resulting in a radiationless deactivation governed by the energy-gap law [10] and a low driving force for the potential electron-transfer processes. In contrast, classical NHC ligands are strong, charge-neutral s donors and p acceptors, thus causing a favorable 3 MC destabilization, but also undesirably blue-shifted MLCT transitions. Alternatively, the expansion to six-membered ring chelators [6] leads to excellent excited-state lifetimes by a more favorable bite angle, but can also cause the formation of isomers (fac, mer) that show very different properties and that are hard to separate.In this regard, abnormal or mesoionic carbene ligands [11] provide superior s-donating and only moderate p-accepting properties that ideally would lead to strongly destabilized 3 MC states and a maintained 3 MLCT energy. 1,2,3-Triazolylidenes match these demands and are readily accessible by modular click chemistry. Herein we present a heteroleptic bis(tridentate) ruthenium(II) complex (RuCNC) of the new 2',6'-bis(1-mesityl-3-methyl-1,2,3-triazol-4-yl-5-idene)pyridine (CNC) ligand and the parent tpy. A heteroleptic complex with tpy is particularly interesting, because it preserves the elaborated terpyridine chemistry, including a variety of ruthenium precursors, allows for asymmetric functionalization, and includes a reference ligand. The electronic and optical properties of RuCNC were investigated by experimental and theoretical studies.The synthesis of RuCNC was achieved under mild reaction conditions with a high selectivity and reasonable yield (Scheme 1). For the preparation of 2',6'-bis(1-mesityl-3-methyl-1,2,3-triazolium-4-yl)pyridine tetrafluoroborate (H 2 CNC), the parent click-derived 2',6'-bis(1-mesityl-1,2,3-triazol-4-yl)pyridine (tripy) [12] could be methylated selectively with Meerweins salt [13] as evidenced by single-crystal Xray diffraction (Figure ...

The synthesis of a series of heteroleptic ruthenium(ii)-complexes containing both, 2,2':6',2''-terpyridine and 2,6-bis(1H-1,2,3-triazol-4-yl)pyridine, is reported for the first time. The provided complexes feature photophysical and electrochemical properties in between those known for the respective homoleptic complexes. The flexibility with respect to lateral functional groups to be introduced into the complexes underlines the high potential for further functionalization steps.

By utilizing click chemistry and methylation, the triazolium motif was employed to design tridentate "ligands" that bind by electron acception instead of electron donation. As electronically inverted ligands they are able to complex sulfate ions by hydrogen bonding and electrostatic interactions. The formation of mono- or bis-tridentate complexes could be achieved by controlling the degree of methylation with the appropriate reagents and was proven by NMR spectroscopy and computational methods.

A series of heteroleptic bis(tridentate) ruthenium(II) complexes bearing ligands featuring 1,2,3-triazolide and 1,2,3-triazolylidene units are presented. The synthesis of the C^N^N-coordinated ruthenium(II) triazolide complex is achieved by direct C-H activation, which is enabled by the use of a 1,5-disubstituted triazole. By postcomplexation alkylation, the ruthenium(II) 1,2,3-triazolide complex can be converted to the corresponding 1,2,3-triazolylidene complex. Additionally, a ruthenium(II) complex featuring a C^N^C-coordinating bis(1,2,3-triazolylidene)pyridine ligand is prepared via transmetalation from a silver(I) triazolylidene precursor. The electronic consequences of the carbanion and mesoionic carbene donors are studied both experimentally and computationally. The presented complexes exhibit a broad absorption in the visible region as well as long lifetimes of the charge-separated excited state suggesting their application in photoredox catalysis and photovoltaics. Testing of the dyes in a conventional dye-sensitized solar cell (DSSC) generates, however, only modest power conversion efficiencies (PCEs).

A systematic series of anion receptors based on bidentate halogen bonding by halo-triazoles and -triazoliums is presented. The influence of the halogen bond donor atom, the electron-withdrawing group, and the linker group that bridges the two donor moieties is investigated. Additionally, a comparison with hydrogen bond-based analogues is provided. A new, efficient synthetic approach to introduce different halogens into the heterocycles is established using silver(I)-triazolylidenes, which are converted to the corresponding halo-1,2,3-triazoliums with different halogens. Comprehensive nuclear magnetic resonance binding studies supported by isothermal titration calorimetry studies were performed with different halides and oxo-anions to evaluate the influence of key parameters of the halogen bond donor, namely, polarization of the halogen and the bond angle to the anion. The results show a larger anion affinity in the case of more charge-dense halides as well as a general preference of the receptors to bind oxo-anions, in particular sulfate, over halides.