Benjamin Schulze

Beyond click chemistry – supramolecular interactions of 1,2,3-triazoles

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.

Ruthenium(II) photosensitizers of tridentate click-derived cyclometalating ligands: a joint experimental and computational study.

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.

Anion Receptors Based on Halogen Bonding with Halo-1,2,3-triazoliums

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.

Physicochemical Analysis of Ruthenium(II) Sensitizers of 1,2,3-Triazole-Derived Mesoionic Carbene and Cyclometalating Ligands

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).

Ruthenium(II) Metallo-Supramolecular Polymers of Click-Derived Tridentate Ditopic Ligands

New ditopic 2,6-bis(1,2,3-triazol-4-yl)pyridine ligands featuring a π-conjugated spacer and clicked-on solubilizing groups were employed in the synthesis of Ru(II) metallo-supramolecular polymers that exhibit an intense metal-to-ligand charge transfer absorption in the visible light region. The coordination polymers obtained were studied in solution by means of size exclusion chromatography and analytical ultracentrifugation, revealing a comparably high molar mass and moderate rigidity. Investigations in the solid state by atomic force and transmission electron microscopy confirmed the formation of rod-like polymers. Furthermore, film preparation by drop-casting showed good film-forming properties. Thus, the solution-processable, photoredoxactive polymers might be applicable in solar cells.

Preorganization in a Cleft-Type Anion Receptor Featuring Iodo-1,2,3-Triazoles As Halogen Bond Donors

Preorganization via intramolecular hydrogen bonds was applied in a cleft-type receptor by exploiting the excellent halogen bond donor ability as well as hydrogen bond acceptor function of iodo-1,2,3-triazoles. As investigated by isothermal calorimetric titrations, the restriction of conformational freedom causes an enhanced entropic contribution resulting in a strongly increased binding affinity. This efficient way to improve the binding strength of 5-halo-1,2,3-triazoles paves the way for applications of new charge-neutral halogen bond donors in solution.

Designing cyclometalated ruthenium(II) complexes for anodic electropolymerization.

The anodic electropolymerization of thiophene-functionalized cyclometalated ruthenium(II) complexes is shown for the first time. Oxidative decomposition reactions can be overcome by modification of the involved redox potentials through the introduction of electron-withdrawing substituents, namely nitro groups, at the cyclometalating phenyl ring. The generated functionalized ruthenium(II) complexes allow the electrochemical preparation of thin polymer films, which show a broad UV/Vis absorption as well as reversible redox switchability. The presented complexes are promising candidates for future photovoltaic applications based on photo-redox-active films.