Redox noninnocent ligands present opportunities to access reactive metal-radical complexes, but competing redox pathways often hinder their selective formation. Here, a pentadentate arylhydroxylamine-containing (ArNHOH) ligand directs copper salts to form a single, well-defined CuII-arylnitrosyl (ArNO•-) radical complex via an unprecedented oxidative binding pathway. To the best of our knowledge, this is the first crystallographically characterized mononuclear κN-bonded CuII-(ArNO•-) complex. Magnetometry, NMR, and density functional theory (DFT) reveal a pronounced difference in copper-radical antiferromagnetic coupling (AFC) between the solid state and solution (-1290 vs -428 cm-1, respectively). This phase-dependent magnetism is associated with different geometries that impart different dσ(Cu)-π*(ArNO) overlaps of the magnetic orbitals. Quantitative L-edge X-ray absorption spectra (XAS) together with DFT shows that the CuII-(ArNO•-) interaction is strongly covalent, consistent with its robust antiferromagnetism. Mechanistic studies, including electrochemistry, establish conditions where oxidative binding is operative and show that product selectivity is governed by Cu oxidation state, base concentration, and the order and rate of addition of constituents. Taken together, this work introduces oxidative binding as a distinct route to access Cu-arylnitrosyl radical species and highlights how ligand design and geometric tuning dictate electronic structure and spin-state energetics in metal-radical complexes.

The mechanism of Cu/aminoxyl-catalyzed aerobic alcohol oxidation has been widely debated. Two pathways have been proposed: hydride transfer to an oxoammonium-like intermediate or hydrogen atom transfer (HAT) to the aminoxyl radical followed by single-electron transfer (SET). Using ab initio (CASSCF/MR-DDCI3) methods, we show that the reactive Cu intermediate is dominated by an open-shell singlet Cu(II)-aminoxyl configuration (|J| ≈ 3400 cm-1). Intrinsic bond orbital analysis reveals a stepwise HAT/SET mechanism, with the two electrons of the C-H bond traveling to different acceptors in succession. These findings reconcile all experimental observations, and provide a unified mechanistic basis for the observed redox cooperativity and selectivity across Cu/aminoxyl catalyst systems.

The mechanochemistry of metal-organic frameworks (MOFs) is a well-established field whose development has advanced the understanding and the design of both MOF materials and mechanochemical reactions. This review outlines the close and mutually beneficial interplay of these two fields over the past two decades, including the description of mechanochemical strategies to access MOFs as well as the response of these materials to mechanical treatment and/or stress. Furthermore, we highlight how the use of MOFs as model targets for mechanochemical synthesis simultaneously improves the accessibility and understanding of this class of materials and, conversely, advances the experimental and fundamental understanding of mechanochemical reactions. Similarly, we show the reciprocal benefits of comparing the mechanochemistry of organic molecular solids to that of MOFs. Finally, this review also portrays the rapid emergence of mechanochemistry of covalent-organic frameworks, a young area that promises to deliver new, rapid, efficient, solventless, and room-temperature access to these materials.

We report a rapid, room-temperature mechanochemical synthesis of 2- and 3-dimensional boroxine covalent organic frameworks (COFs), enabled by using trimethylboroxine as a dehydrating additive to overcome the hydrolytic sensitivity of boroxine-based COFs. The resulting COFs display high porosity and crystallinity, with COF-102 being the first example of a mechanochemically-prepared 3D COF, exhibiting a surface area exceeding 2,400 m2 g–1. Mechanochemistry enabled a >20-fold reduction in solvent use and ~100-fold reduction in reaction time compared with solvothermal methods, providing target COFs quantitatively with no additional work-up besides vacuum drying. Real-time Raman spectroscopy permitted the first quantitative kinetic analysis of COF mechanosynthesis, while transferring the reaction design to Resonant Acoustic Mixing (RAM) enabled synthesis of multi-gram amounts of the target COFs.

Resonant acoustic mixing (RAM) enables mechanoredox catalysis with BaTiO3 as the piezoelectric catalyst on model diazonium coupling reactions. RAM proceeds without formal grinding or impact media, is faster than the analogous ball-milling strategy, and is readily scalable. X-ray diffraction and spectroscopy indicate that reusability of BaTiO3 as a mechanoredox catalyst under ball-milling or RAM might be limited by boration.

Lanthanide upconversion luminescence in nanoparticles has prompted continuous breakthroughs in information storage, temperature sensing, and biomedical applications, among others. Achieving upconversion luminescence at the molecular scale is still a critical challenge in modern chemistry. In this work, we explored the upconversion luminescence of solution dispersions of co-crystals composed of discrete mononuclear Yb(DBM)(3)Bpy and Eu(DBM)(3)Bpy complexes (DBM: dibenzoylmethane, Bpy: 2,2 '-bipyridine). The 613 nm emission of Eu3+ was observed under excitation of Yb3+ at 980 nm. From the series of molecular assemblies studied, the most intense luminescence was obtained for a 1 : 1 molar ratio of Yb3+ : Eu3+, resulting in a high quantum yield of 0.67 % at 2.1 W cm(-2). The structure and energy transfer mechanism of the assemblies were fully characterized. This is the first example of an Eu3+-based upconverting system composed of two discrete mononuclear lanthanide complexes present as co-crystals in non-deuterated solution.

Herein, we report a modular synthetic route to access tetra-arylated thiophene compounds with four different substituents with programmed chemical control provided by an ester activating/directing group. This method enables the functionalization of individual positions of thiophene sequentially via regioselective halogenations and cross-coupling reactions. The reaction sequence described provides tetra-arylated thiophenes in higher yields than previous routes and employs practical reaction protocols, simple catalytic systems, and short reaction times.

Over the past 15 years, mechanochemistry has developed into a powerful tool for the synthesis of molecules and materials while avoiding the consumption of bulk solvents. With tailored instruments and well-understood milling/grinding techniques, experimentalists are now tackling synthetic problems of increasing complexity, such as implementing the use of a catalyst to enable specific chemical pathways and control reaction selectivity. This Opinion provides a snapshot of the current state of affairs in the field, highlighting recent examples that employ metal-based catalysts under mechanochemical conditions, as well as the context and specific features of the underlying methods.

Primary hydroxylamines, RNHOH, decompose readily in the presence of transition metal ions. We show that this reactivity can be arrested by ligand design via an intramolecular hydrogen bond. Six metal complexes with an intact NHOH group were synthesized and crystallographically characterized. The Cu-hydroxylamine complexes can catalyze the aerobic oxidation of benzylic alcohols.

Redox noninnocent ligands enhance the reactivity of the metal they complex, a strategy used by metalloenzymes and in catalysis. Herein, we report a series of copper complexes with the same ligand framework, but with a pendant nitrogen group that spans five different redox states between nitro and amine. Of particular interest is the synthesis of a unprecedented copper(I)-arylhydroxylamine complex. While hydroxylamines typically disproportionate or decompose in the presence of transition metal ions, the reactivity of this metastable species is arrested by the presence of an intramolecular hydrogen bond. Two-electron oxidation yields a copper(II)-(arylnitrosyl radical) complex that can dissociate to a copper(I) species with uncoordinated arylnitroso. This combination of ligand redox noninnocence and hemilability provides opportunities in catalysis for two-electron chemistry via a one-electron copper(I/II) shuttle, as exemplified with an aerobic alcohol oxidation.

A series of copper/nitrosoarene complexes was created that mimics several steps in biomimetic O2 activation by copper(I). The reaction of the copper(I) complex of N,N,N',N'-tetramethypropylenediamine with a series of para-substituted nitrosobenzene derivatives leads to adducts in which the nitrosoarene (ArNO) is reduced by zero, one, or two electrons, akin to the isovalent species dioxygen, superoxide, and peroxide, respectively. The geometric and electronic structures of these adducts were characterized by means of X-ray diffraction, vibrational analysis, ultraviolet-visible spectroscopy, NMR, electrochemistry, and density functional theory (DFT) calculations. The bonding mode of the NO moiety depends on the oxidation state of the ArNO moiety: κN for ArNO, mononuclear η2-NO and dinuclear μ-η2:η1 for ArNO•-, and dinuclear μ-η2:η2 for ArNO2-. 15N isotopic labeling confirms the reduction state by measuring the NO stretching frequency (1392 cm-1 for κN-ArNO, 1226 cm-1 for η2-ArNO•-, 1133 cm-1 for dinuclear μ-η2:η1-ArNO•-, and 875 cm-1 for dinuclear μ-η2:η2 for ArNO2-). The 15N NMR signal disappears for the ArNO•- species, establishing a unique diagnostic for the radical state. Electrochemical studies indicate reduction waves that are consistent with one-electron reduction of the adducts and are compared with studies performed on Cu-O2 analogues. DFT calculations were undertaken to confirm our experimental findings, notably to establish the nature of the charge-transfer transitions responsible for the intense green color of the complexes. In fine, this family of complexes is unique in that it walks through three redox states of the ArNO moiety while keeping the metal and its supporting ligand the same. This work provides snapshots of the reactivity of the toxic nitrosoarene molecules with the biologically relevant Cu(I) ion.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

The simultaneous synthesis of 5-hydroxymethyl-2-furoic acid and 2,5-hydroxymethylfuran from biomass-derived 5-hydroxymethyl furan was developed using a solvent-free mechanochemical approach.

The topology and degree of electron transfer in self-assembled redox reactions between copper(i) species and nitrosoarenes are controlled by ligand properties.

We develop 3,5-di-tert­butylphenol as a strategic substrate for the evaluation of biomimetic Cu2–O2 complexes intended to mimic the activity of tyrosinase. We describe a practical and scalable synthesis and validate its use in an aerobic ortho-oxygenation catalyzed by N,N′-di-tert-butylethylenediamine and [Cu(CH3CN)4]PF6.

Three copper complexes with an N-methylated cryptand, LTEA, bearing a tris(2-aminoethylamine moiety have been synthesized and compared. Two copper(II)-chloride complexes, [LTEACuCl](SbF6)(MeOH) and [LTEACuCl(2)](MeCN) were characterized in solution and solid state by UV/Vis spectroscopy and X-ray crystallography. Both had square-based geometries with C-1-symmetry and no encapsulation of the chloride ion. A copper(II)-fluoride complex, in which the ligand is protonated, [LTEAHCuF](B-4)(2)(MeCN)(0.5), adopted C-3-symmetry with complete encapsulation of the coordination sphere as characterized by UV/Vis, EPR and X-ray crystallography. Reactivity of the complexes with H2O2/Et3N was explored using UV/Vis and CSI-MS. Only the fluoride complex was found to form a Cu(II)-hydroperoxo intermediate. (C) 2017 Elsevier B.V. All rights reserved.