Mechanochemical Synthesis of 3d Transition-Metal-1,2,4-Triazole Complexes as Precursors for Microwave-Assisted and Thermal Conversion to Coordination Polymers with a High Influence on the Dielectric Properties

Synthesis and crystal structure of an iron triazole complex resulting from the unexpected ligand cleavage of a triazolium carbene precursor

Typically reactions of N-heterocyclic carbenes with transition metals are straightforward and require a carbene salt, a base strong enough to deprotonate such a salt and a metal. Yet when carbene precursors are in the form of triazolium salts, reaction may not proceed as easily as expected. In our work, we intended to obtain a triazolylidene complex of iron(II) chloride, but due to the presence of small amounts of water in the tetrahydrofuran solvent used, bis(acetonitrile)tetrakis(1-benzyl-1H-1,2,4-triazole-κN4)iron(II) μ-oxido-bis[trichloridoferrate(III)] acetonitrile disolvate, [Fe(C9H9N3)4(CH3CN)2][Fe2Cl6O]·2CH3CN - an interesting anion with a linear geometry of the O atom - was formed instead of the iron carbene complex. Reaction proceeded via cleavage of the alkyl N-substituent of the triazolium salt. The formation of the product was confirmed by X-ray crystallography. The crystal structure and possible reaction pathways are discussed.

Fe-Triazole coordination compound-derived Fe2O3 nanoparticles anchored on Fe-MOF/N-doped carbon nanosheets for efficient electrocatalytic oxygen evolution reaction

Using FeCl₃·6H₂O and 1,2,4-triazole (Htrz) as starting materials, an Fe coordination compound (CC), [FeCl₃(Htrz)₃]·H₂O, was synthesized at room temperature. Fe-CC can be partially transformed into an Fe metal-organic framework (MOF), [FeCl₂(Htrz)], via low-temperature annealing. After sulfurization at 250, 300, and 400 °C, S-doped Fe₂O₃/N-doped carbon (denoted as NC)/Fe-MOF, FeS₂/NC/Fe-MOF, and FeS₂/NC were obtained, respectively. S-doped Fe₂O₃/NC/Fe-MOF shows the best oxygen evolution reaction (OER) catalytic activity in 1 M KOH solution, with overpotentials (η) of 185, 232, and 258 mV required to reach current densities of 10, 30, and 50 mA cm^-2, respectively, outperforming commercial RuO₂ and most transition-metal oxides reported to date; this high performance is associated with the Fe₂O₃ nanoparticles (NPs) anchored on the Fe-MOF/NC nanosheets. The Fe-MOF/NC matrix can act as a support to prevent the agglomeration of Fe₂O₃ NPs. In addition, S-doped Fe₂O₃/NC/Fe-MOF exhibits long-term OER activity at 20 mA cm^-2, which is related to the partial transformation of Fe₂O₃/Fe-MOF into FeOOH. In addition, density functional theory (DFT) calculations show that the rate-determining step of the OER process at the Fe sites of both Fe₂O₃ and FeS₂ is the formation of Fe*-OH, and the Fe₂O₃ sites display a lower Gibbs free energy (ΔG_max) of 1.674 eV and a smaller η value of ∼0.444 V. Bader charge, differential charge density mapping, and density of states (DOS) analysis all reveal more charge accumulation at the Fe sites of FeS₂ than at the Fe sites of Fe₂O₃, which is due to the lower electronegativity of S than of O. As a result, the Fe sites of FeS₂ show weaker affinity for -OH intermediates, giving rise to inferior OER performance compared with Fe₂O₃.

Chiral Fluorescent Metal-Organic Framework with a Pentanuclear Copper Cluster as an Efficient Luminescent Probe for Dy3+ Ion and Cyano Compounds

Luminescent probes have been used for the detection of various heavy metals and toxic compounds. A novel sensor with excellent sensitivity and selectivity is in high demand. Herein, we designed and synthesized a three-dimensional copper-organic framework of "pcu" α-Po primitive cubic topology with a Schläfli symbol of {4.4.4.4.4.4.4.4.4.4.4.4.*.*.*}. By taking advantage of metal clusters and a triazole ligand as the metal-organic framework (MOF) components, the newly obtained MOF is stable in various environments and can be potentially used as the sensor. Remarkably, this MOF-based sensor shows high sensitivity and selectivity toward a dysprosium ion (Dy³⁺) in a multiple-lanthanide mixed solution. Besides, it exhibits luminescent quenching toward various cyano compounds. This chiral cluster-based network provides a potential luminescent probe for various inorganic and organic compounds with high sensitivity and selectivity.

Core-dependent properties of copper nanoclusters: valence-pure nanoclusters as NIR TADF emitters and mixed-valence ones as semiconductors

While valence-pure copper alkynyl nanoclusters show near-infrared TADF, the mixed-valence ones exhibit semiconductivity.

We report herein that copper alkynyl nanoclusters show metal-core dependent properties via a charge-transfer mechanism, which enables new understanding of their structure–property relationship. Initially, nanoclusters 1 and 2 bearing respective Cu(i)₁₅ (C1) and Cu(i)₂₈ (C2) cores were prepared and revealed to display near-infrared (NIR) photoluminescence mainly from the mixed alkynyl → Cu(i) ligand-to-metal charge transfer (LMCT) and cluster-centered transition, and they further exhibit thermally activated delayed fluorescence (TADF). Subsequently, a vanadate-induced oxidative approach to in situ generate a nucleating Cu(ii) cation led to assembly of 3 and 4 featuring respective [Cu(ii)O₆]@Cu(i)₄₇ (C3) and {[Cu(ii)O₄]·[VO₄]₂}@Cu(i)₄₆ (C4) cores. While interstitial occupancy of Cu(ii) triggers inter-valence charge-transfer (IVCT) from Cu(i) to Cu(ii) to quench the photoluminescence of 3 and 4, such a process facilitates charge mobility to render them semiconductive. Overall, metal-core modification results in an interplay between charge-transfer processes to switch TADF to semiconductivity, which underpins an unusual structure–property correlation for designed synthesis of metal nanoclusters with unique properties and functions.

Stoichiometric control: 8- and 10-coordinate Ln(hfac)3(bpy) and Ln(hfac)3(bpy)2 complexes of the early lanthanides La-Sm

The coordination sphere of early lanthanide(iii) ions is highly versatile, exhibiting the ability to form 8-, 9-, and 10-coordinate complexes with the same ligand set. The ability to isolate 10-coordinate complexes decreases across the period, and the late lanthanides typically cannot support a coordination number higher than eight. Using two common, commercially available ligands, hfac (1,1,1,5,5,5-hexafluoroacetylacetonato-) and bpy (2,2'-bipyridine), the 8- and 10-coordinate series Ln(hfac)3(bpy) and Ln(hfac)3(bpy)2 (Ln = La-Sm) are compiled in a single investigation, demonstrating that the desired coordination number can be targeted through stoichiometry. Solvent-free syntheses of Ln(hfac)3(bpy) and Ln(hfac)3(bpy)2 complexes from Ln(hfac)3(H2O)3 precursors are investigated using a mechanochemical approach. Structural and spectroscopic properties as well as melting point trends are reported for the series.

Mechanochemical and thermal formation of 1H-benzotriazole coordination polymers and complexes of 3d-transition metals with intriguing dielectric properties

Liquid-assisted grinding (LAG) reactions have been successfully applied to achieve a series of complexes and coordination polymers based on divalent 3d-transition metal chlorides (TM chlorides) and the aromatic ligand 1H-benzotriazole (BtzH). The obtained substances were investigated via single crystal X-ray, powder X-ray determination and simultaneous DTA/TG analysis as model compounds for structural and chemical influences on their dielectric properties. Depending on the synthesis method, different constitutions and structures are observed. Two polymorphous forms of the 1D polymer [MnCl2(BtzH)2] (1 and 2) as well as the complexes [ZnCl2(BtzH)2]·BtzH (3) and [CoCl2(BtzH)2]·BtzH (4) have been obtained as phase-pure bulk substances via the mechanochemical LAG route, and even single crystals are available. For comparison, thermal reactions were also carried out and have led to the formation of the neutral complexes: [CoCl2(BtzH)2] (5) and [CoCl2(BtzH)4]·4BtzH (6), [ZnCl2(BtzH)2] (7) and the anionic complex BtzH2[CoCl3BtzH] (8). In addition, thermal treatment of 3 yields the benzotriazolium salt {(BtzH)2H}Cl (9). The transition metal compounds were additionally analysed regarding their dielectric properties by frequency-dependent as well as temperature-dependent permittivity investigations. It is intriguing that compounds 1 and 3 show remarkably low dielectric constants and loss factors up to 50 °C highlighting them as potential "low-k materials".