A Heteroleptic Bis(tridentate) Ruthenium(II) Complex of a Click‐Derived Abnormal Carbene Pincer Ligand with Potential for Photosensitzer Application

Abstract

[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 btp [2,6-bis(1,2,3-triazol-4-yl)pyridine] binding motif: a new versatile terdentate ligand for supramolecular and coordination chemistry

“…Solvent choice has relatively little effect on yields, and it appears to be a matter of preference and, sometimes, solubility of starting materials that determines the choice of solvent systems. Alcohols like ethanol 87,95,97,98 and tert-butyl alcohol 93,94,96 mixed with water are most common in the literature, while DMSO (with 99 or without 100 water), THF, 60 CH 3 CN 101 and DMF 70,91 are also attested.…”

“…59,60,93,95,97,98,100,105,106,111,128,129 Flood and co-workers showed that Ru(II) forms stable coordination compounds with 3b. X-ray crystal crystallography showed a 1 : 2 metal : ligand stoichiometry, with the ligands adopting a distorted octahedral geometry, as demonstrated in Fig.…”

“…In recent years, Schubert and various co-workers have published prolifically in the field of Ru(II) coordination by btp-containing ligands, mostly with regard to the preparation of heteroleptic Ru(II)-complexes of terpy and btp ligands, 97,98,100 and Ru(II) complexes containing p-conjugated ditopic btp ligands. 95 Single crystals suitable for X-ray diffraction studies were obtained of [Ru(40a)(4-Br-f-terpy)](PF 6 ) 2 , with the structure shown in Fig.…”

Isolation and Structures of 1,2,3‐Triazole‐Derived Mesoionic Biscarb­enes with Bulky Aromatic Groups

“…[7] However, tridentate bisMICs have not been isolated, although a silver-stabilized carbene was reported as an intermediate in the synthesis of a series of luminescent ruthenium complexes by Schubert et al [8] Bertrand et al reported the structure of a potassium-stabilized tridentate MIC. [9] In this paper, we report the first isolation of free tridentate carbenes and an iron complex.…”

Computational investigations of click-derived 1,2,3-triazoles as keystone ligands for complexation with transition metals: a review

“…Click-derived 3,107 ligands have been introduced as analogs of polypyridyl ligands, in particular of terpyridine (tpy), 96,[108][109][110][111] and successfully used to produce tridentate cyclometalating polypyridyl ligands, namely 1,3-dipyridylbenzene. In this respect, aryl azides react with diethynylbenzene building blocks, in good yields using a standard click procedure.…”