Fluorinated Tetraarylethenes: Universal Tags for the Synthesis of Solid State Luminogens.

The aggregation-induced emission properties of tetraarylethenes (TAEs) have led to numerous applications in chemistry, biology, and materials science. Herein, we describe two fluorinated tetraarylethenes, which can be employed as universal tags for the synthesis of solid state luminogens. The tags are accessible in one or two steps from commercially available starting materials. Facile coupling reactions with ubiquitous substrates such as thiols, alcohols, amines, phosphines, aldehydes, and enamines allow preparing a wide range of TAE conjugates, including tagged amino acids, peptides, carbohydrates, steroids, and commercial polymers.

Unsymmetrical Trifluoromethyl Methoxyphenyl ß-Diketones: Effect of the Position of Methoxy Group and Coordination at Cu(II) on Biological Activity.

Copper(II) complexes with 1,1,1-trifluoro-4-(4-methoxyphenyl)butan-2,4-dione (HL1) were synthesized and characterized by elemental analysis, FT-IR spectroscopy, and single crystal X-ray diffraction. The biological properties of HL1 and cis-[Cu(L1)2(DMSO)] (3) were examined against Gram-positive and Gram-negative bacteria and opportunistic unicellular fungi. The cytotoxicity was estimated towards the HeLa and Vero cell lines. Complex 3 demonstrated antibacterial activity towards S. aureus comparable to that of streptomycin, lower antifungal activity than the ligand HL1 and moderate cytotoxicity. The bioactivity was compared with the activity of compounds of similar structures. The effect of changing the position of the methoxy group at the aromatic ring in the ligand moiety of the complexes on their antimicrobial and cytotoxic activity was explored. We propose that complex 3 has lower bioavailability and reduced bioactivity than expected due to strong intermolecular contacts. In addition, molecular docking studies provided theoretical information on the interactions of tested compounds with ribonucleotide reductase subunit R2, as well as the chaperones Hsp70 and Hsp90, which are important biomolecular targets for antitumor and antimicrobial drug search and design. The obtained results revealed that the complexes displayed enhanced affinity over organic ligands. Taken together, the copper(II) complexes with the trifluoromethyl methoxyphenyl-substituted ß-diketones could be considered as promising anticancer agents with antibacterial properties.

Azo-MICs: Redox-Active Mesoionic Carbene Ligands Derived from Azoimidazolium Dyes.

Azoimidazolium dyes were used as precursors for mesoionic carbene ligands (Azo-MICs). The properties of these ligands were examined by synthesizing RhI , AuI , and PdII complexes. Experimental (NMR, IR) and theoretical investigations show that Azo-MICs are potent σ-donor ligands. Yet, they feature a small singlet-triplet gap and very low-lying LUMO levels. The unique electronic properties of Azo-MICs allow for reversible one-electron reductions of the metal complexes, as evidenced by cyclic voltammetry.

Lewis Acid-Mediated One-Electron Reduction of Nitrous Oxide.

The one-electron reduction of nitrous oxide (N2 O) was achieved using strong Lewis acids E(C6 F5 )3 (E=B or Al) in combination with metallocenes. In the case of B(C6 F5 )3 , electron transfer to N2 O required a powerful reducing agent such as Cp*2 Co (Cp*=pentamethylcyclopentadienyl). In the presence of Al(C6 F5 )3 , on the other hand, the reactions could be performed with weaker reducing agents such as Cp*2 Fe or Cp2 Fe (Cp=cyclopentadienyl). The Lewis acid-mediated electron transfer from the metallocene to N2 O resulted in cleavage of the N-O bond, generating N2 and the oxyl radical anion [OE(C6 F5 )3 ]⋅- . The latter is highly reactive and engages in C-H activation reactions. It was possible to trap the radical by addition of the Gomberg dimer, which acts as a source of the trityl radical.

Trimethylsilyl-Induced N-O Bond Cleavage in Nitrous Oxide-Derived Aminodiazotates.

The chemical activation of nitrous oxide (N2O) typically results in O-atom transfer and the extrusion of N2 gas. In contrast, reactions of N-trimethylsilyl (TMS)-substituted amides with N2O give inorganic or organic azides, with concomitant formation of silanols or siloxanes. N-TMS-substituted amides are also able to induce N-O bond cleavage in N2O-derived dialkylaminodiazotates, generating tetrazene salts. These results indicate the potential of silyl groups in devising transformations, in which N2O acts as an N-atom donor.

The Intricate Structural Chemistry of MII2nLn-Type Assemblies.

The reaction of cis-blocked, square-planar MII complexes with tetratopic N-donor ligands is known to give metallasupramolecular assemblies of the formula M2nLn. These assemblies typically adopt barrel-like structures, with the ligands paneling the sides of the barrels. However, alternative structures are possible, as demonstrated by the recent discovery of a Pt8L4 cage with unusual gyrobifastigium-like geometry. To date, the factors that govern the assembly of MII2nLn complexes are not well understood. Herein, we provide a geometric analysis of M2nLn complexes, and we discuss how size and geometry of the ligand is expected to influence the self-assembly process. The theoretical analysis is complemented by experimental studies using different cis-blocked PtII complexes and metalloligands with four divergent pyridyl groups. Mononuclear metalloligands gave mainly assemblies of type Pt8L4, which adopt barrel- or gyrobifastigium-like structures. Larger assemblies can also form, as evidenced by the crystallographic characterization of a Pt10L5 complex and a Pt16L8 complex. The former adopts a pentagonal barrel structure, whereas the latter displays a barrel structure with a distorted square orthobicupola geometry. The Pt16L8 complex has a molecular weight of more than 23 kDa and a diameter of 4.5 nm, making it the largest, structurally characterized M2nLn complex described to date. A dinuclear metalloligand was employed for the targeted synthesis of pentagonal Pt10L5 barrels, which are formed in nearly quantitative yields.

Heptacoordinate CoII Complex: A New Architecture for Photochemical Hydrogen Production.

The first heptacoordinate cobalt catalyst for light-driven hydrogen production in water has been synthesized and characterized. Photochemical experiments using [Ru(bpy)3 ]2+ as photosensitizer gave a turnover number (TON) of 16300 mol H2 (mol cat.)-1 achieved in 2 hours of irradiation with visible (475 nm) light. This promising result provides a path forward in the development of new structures to improve the efficiency of the catalysis.

Ligand Aspect Ratio as a Decisive Factor for the Self-Assembly of Coordination Cages.

It is possible to control the geometry and the composition of metallasupramolecular assemblies via the aspect ratio of their ligands. This point is demonstrated for a series of iron- and palladium-based coordination cages. Functionalized clathrochelate complexes with variable aspect ratios were used as rod-like metalloligands. A cubic Fe(II)8L12 cage was obtained from a metalloligand with an intermediate aspect ratio. By increasing the length or by decreasing the width of the ligand, the self-assembly process resulted in the clean formation of tetrahedral Fe(II)4L6 cages instead of cubic cages. In a related fashion, it was possible to control the geometry of Pd(II)-based coordination cages. A metalloligand with a large aspect ratio gave an entropically favored tetrahedral Pd(II)4L8 assembly, whereas an octahedral Pd(II)6L12 cage was formed with a ligand of the same length but with an increased width. The aspect ratio can also be used to control the composition of dynamic mixtures of Pd(II) cages. Out of two metalloligands with only marginally different aspect ratios, one gave rise to a self-sorted collection of Pd(II)4L8 and Pd(II)6L12 cages, whereas the other did not.

Neutral Aminyl Radicals Derived from Azoimidazolium Dyes.

The synthesis and characterization of a new class of neutral aminyl radicals is reported. Monoradicals were obtained by reduction of azoimidazolium dyes with potassium. Structural, spectroscopic, and computational data suggest that the spin density is centered on one of the nitrogen atoms of the former azo group. The reduction of a dimeric dye with an octamethylbiphenylene bridge between the azo groups resulted in the formation of a biradical with largely independent unpaired electrons. Both the monoradicals and the biradical were found to display high stability in solution as well as in the solid state.

Carboxylic Acid Functionalized Clathrochelate Complexes: Large, Robust, and Easy-to-Access Metalloligands.

Polycarboxylate ligands are among the most important building blocks for the synthesis of metal-organic frameworks (MOFs). The ability to access these ligands in an efficient way is of key importance for future applications of MOFs. Here, we demonstrate that mono- and dinuclear clathrochelate complexes are versatile scaffolds for the preparation of polytopic carboxylate ligands. The largely inert clathrochelate complexes have a trigonal-bipyramidal shape. The synthesis of functionalized clathrochelates with two, three, four, or five carboxylic acid groups in the ligand periphery can be achieved in a few steps from simple starting materials. Apart from being easily accessible, the metalloligands display interesting characteristics for applications in metallasupramolecular chemistry and materials science: they are rigid, large (up to 2.2 nm), and robust and they can show additional functions (e.g., fluorescence or extra charge) depending on the metal ion that is present in the clathrochelate core. The utility of these new metalloligands in MOF chemistry is demonstrated by the synthesis of zinc- and zirconium-based coordination polymers. The combination of Zn(NO3)2 with clathrochelates having two or three carboxylic acid groups gives MOFs in which the clathrochelate ligands are connected by Zn4O clusters or zinc paddlewheel links. The structures of the resulting two- and three-dimensional networks could be established by single-crystal X-ray crystallography. The reaction of carboxylic acid functionalized clathrochelates with ZrCl4 gives amorphous powders that display permanent porosity after solvent removal.

Synthesis of azoimidazolium dyes with nitrous oxide.

A new method for the synthesis of industrially important azoimidazolium dyes is presented. The procedure is based on a reagent which is rarely used in the context of synthetic organic chemistry: nitrous oxide ("laughing gas"). N2O is first coupled to N-heterocyclic carbenes. Subsequent reaction with aromatic compounds through an AlCl3-induced C-H activation process provides azoimidazolium dyes in good yields.

Adducts of nitrous oxide and N-heterocyclic carbenes: syntheses, structures, and reactivity.

N-Heterocyclic carbenes (NHCs) react at ambient conditions with nitrous oxide to give covalent adducts. In the crystal, all compounds show a bent N2O group connected via the N-atom to the former carbene carbon atom. Most adducts are stable at room temperature, but heating induces decomposition into the corresponding ureas. Kinetic experiments show that the thermal stability of the NHC-N2O adducts depends on steric as well as electronic effects. The coordination of N2O to NHCs weakens the N-N bond substantially, and facile N-N bond rupture was observed in reactions with acid or acetyl chloride. On the other hand, reaction with tritylium tetrafluoroborate resulted in a covalent modification of the terminal O-atom, and cleavage of the C-N2O bond was observed in a reaction with thionyl chloride. The coordination chemistry of IMes-N2O (IMes = 1,3-dimesitylimidazol-2-ylidene) was explored in reactions with the complexes CuOTf, Fe(OTf)2, PhSnCl3, CuCl2, and Zn(C6F5)2. Structural analyses show that IMes-N2O is able to act as a N-donor, as an O-donor, or as a chelating N,O-donor. The different coordination modes go along with pronounced electronic changes as evidenced by a bond length analysis.

Insertion of zerovalent nickel into the N-N bond of N-heterocyclic-carbene-activated N2O.

Metal-mediated cleavage of the N-N bond is a rarely observed phenomenon in the chemistry of nitrous oxide (N2O). We demonstrate that, upon activation of N2O with N-heterocyclic carbenes, zerovalent nickel is able to insert into the N-N bond to give nitrosyl complexes.

Electro- and photochemical H2 generation by Co(ii) polypyridyl-based catalysts bearing ortho-substituted pyridines.

Cobalt(ii) complexes featuring hexadentate amino-pyridyl ligands have been recently discovered as highly active catalysts for the Hydrogen Evolution Reaction (HER), whose high performance arises from the possibility of assisting proton transfer processes via intramolecular routes involving detached pyridine units. With the aim of gaining insights into such catalytic routes, three new proton reduction catalysts based on amino-polypyridyl ligands are reported, focusing on substitution of the pyridine ortho-position. Specifically, a carboxylate (C2) and two hydroxyl substituted pyridyl moieties (C3, C4) are introduced with the aim of promoting intramolecular proton transfer which possibly enhances the efficiency of the catalysts. Foot-of-the-wave and catalytic Tafel plot analyses have been utilized to benchmark the catalytic performances under electrochemical conditions in acetonitrile using trifluoroacetic acid as the proton source. In this respect, the cobalt complex C3 turns out to be the fastest catalyst in the series, with a maximum turnover frequency (TOF) of 1.6 (±0.5) × 105 s-1, but at the expense of large overpotentials. Mechanistic investigations by means of Density Functional Theory (DFT) suggest a typical ECEC mechanism (i.e. a sequence of reduction - E - and protonation - C - events) for all the catalysts, as previously envisioned for the parent unsubstituted complex C1. Interestingly, in the case of complex C2, the catalytic route is triggered by initial protonation of the carboxylate group resulting in a less common (C)ECEC mechanism. The pivotal role of the hexadentate chelating ligand in providing internal proton relays to assist hydrogen elimination is further confirmed within this novel class of molecular catalysts, thus highlighting the relevance of a flexible polypyridine ligand in the design of efficient cobalt complexes for the HER. Photochemical studies in aqueous solution using [Ru(bpy)3]2+ (where bpy = 2,2'-bipyridine) as the sensitizer and ascorbate as the sacrificial electron donor support the superior performance of C3.

Sequential N-O and N-N bond cleavage of N-heterocyclic carbene-activated nitrous oxide with a vanadium complex.

Chemically induced bond cleavage of nitrous oxide typically proceeds by rupture of the N-O bond with concomitant O-atom transfer and liberation of dinitrogen. On a few occasions, N-N bond scission has been observed instead. We report a reaction sequence involving an N-heterocyclic carbene and a vanadium complex that results in cleavage of both the N-O bond and the N-N bond.

Multicomponent assembly of macrocycles and polymers by coordination of pyridyl ligands to 1,4-bis(benzodioxaborole)benzene.

Multicomponent reactions between 1,4-benzenediboronic acid, catechol, and different pyridyl ligands are reported. The condensation of 1,4-benzenediboronic acid with catechol gives 1,4-bis(benzodioxaborole)benzene. Upon crystallization, the ester aggregates with the N-donor ligands through dative B-N bonds. Depending on the nature of the pyridyl ligand, molecularly defined macrocycles or polymeric structures are obtained. 1D polymers are formed with 4,4-bipyridine and 1,2-di(4-pyridyl)ethylene, whereas a 2D network is obtained with the tetradentate ligand tetra(4-pyridylphenyl)ethylene. These results highlight the utility of dative B-N bonds in structural supramolecular chemistry and crystal engineering.

Novel cationic 1,2,4-selenadiazoles: synthesis via addition of 2-pyridylselenyl halides to unactivated nitriles, structures and four-center SeN contacts.

2-Pyridylselenyl halides undergo facile coupling with a triple CN bond of unactivated nitriles. Unprecedented heterocyclization allowed the preparation of a novel class of cationic 1,2,4-selenadiazoles in remarkably high yields. Cationic 1,2,4-selenadiazoles form supramolecular dimers in the crystal via SeN chalcogen bonding, which was studied theoretically.

Correction: Novel cationic 1,2,4-selenadiazoles: synthesis via addition of 2-pyridylselenyl halides to unactivated nitriles, structures and four-center Se⋯N contacts.

Correction for 'Novel cationic 1,2,4-selenadiazoles: synthesis via addition of 2-pyridylselenyl halides to unactivated nitriles, structures and four-center Se⋯N contacts' by Victor N. Khrustalev et al., Dalton Trans., 2021, 50, 10689-10691, DOI: 10.1039/D1DT01322J.

Rationalizing Photo-Triggered Hydrogen Evolution Using Polypyridine Cobalt Complexes: Substituent Effects on Hexadentate Chelating Ligands.

Four novel polypyridine cobalt(II) complexes were developed based on a hexadentate ligand scaffold bearing either electron-withdrawing (-CF3 ) or electron-donating (-OCH3 ) groups in different positions of the ligand. Experiments and theoretical calculations were combined to perform a systematic investigation of the effect of the ligand modification on the hydrogen evolution reaction. The results indicated that the position, rather than the type of substituent, was the dominating factor in promoting catalysis. The best performances were observed upon introduction of substituents on the pyridine moiety of the hexadentate ligand, which promoted the formation of the Co(II)H intermediate via intramolecular proton transfer reactions with low activation energy. Quantum yields of 11.3 and 10.1 %, maximum turnover frequencies of 86.1 and 76.6 min-1 , and maximum turnover numbers of 5520 and 4043 were obtained, respectively, with a -OCH3 and a -CF3 substituent.

Reactions of alkynes with [RuCl(cyclopentadienyl)] complexes: the important first steps.

Cyclopentadienyl-ruthenium half-sandwich complexes with eta(2)-bound alkyne ligands have been suggested as catalytic intermediates in the early stages of Ru-catalyzed reactions with alkynes. We show that electronically unsaturated complexes of the formula [RuCl(Cp--)(eta(2)-RC[triple bond]CR')] can be stabilized and crystallized by using the sterically demanding cyclopentadienyl ligand Cp-- (Cp--=eta(5)-1-methoxy-2,4-tert-butyl-3-neopentyl-cyclopentadienyl). Furthermore we demonstrate that [RuCl(2)(Cp==)](2) is an active and regioselective catalyst for the [2+2+2] cyclotrimerization of alkynes. The first elementary steps of the reaction of mono(eta(2)-alkyne) complexes containing {RuCl(Cp*)} (Cp*=eta(5)-C(5)Me(5)) and {RuCl(Cp--)} fragments with alkynes were investigated by DFT calculations at the M06/6-31G* level in combination with a continuum solvent model. Theoretical results are able to rationalize and complement the experimental findings. The presence of the sterically demanding Cp-- ligand increases the activation energy required for the formation of the corresponding di(eta(2)-alkyne) complexes, enhancing the initial regioselectivity, but avoiding the evolution of the system towards the expected cyclotrimerization product when bulky substituents are present. Theoretical results also show that the electronic structure and stability of a metallacyclic intermediate is strongly dependent on the nature of the substituents present in the alkyne.