Mechanochemical Synthesis of Boroxine-linked Covalent Organic Frameworks.

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 of ca. 2,500 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 (tested up to 10 g).

Cooperative Sensitization Upconversion in Solution Dispersions of Co-Crystal Assemblies of Mononuclear Yb3+ and Eu3+ Complexes.

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.

Resonant acoustic mixing (RAM) for efficient mechanoredox catalysis without grinding or impact media.

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.

Programmed Synthesis of Tetra-Aryl Thiophenes with Stepwise, Ester-Controlled Regioselectivity.

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.

Intramolecular H-bond stabilization of a primary hydroxylamine in salen-type metal complexes.

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.

Five Nitrogen Oxidation States from Nitro to Amine: Stabilization and Reactivity of a Metastable Arylhydroxylamine Complex.

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.

Tuning Inner-Sphere Electron Transfer in a Series of Copper/Nitrosoarene Adducts.

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.

Synthesis of Highly Functionalized Triarylbismuthines by Functional Group Manipulation and Use in Palladium- and Copper-Catalyzed Arylation Reactions.

Organobismuthines are an attractive class of organometallic reagents that can be accessed from inexpensive and nontoxic bismuth salts. Triarylbismuthines are particularly interesting due to their air and moisture stability and high functional group tolerance. We report herein a detailed study on the preparation of highly functionalized triarylbismuth reagents by triple functional group manipulation and their use in palladium- and copper-catalyzed C-, N-, and O-arylation reactions.

A bio-inspired synthesis of oxindoles by catalytic aerobic dual C-H functionalization of phenols.

Nitrogen-containing heterocycles are fundamentally important to the function of pharmaceuticals, agrochemicals and materials. Herein, we report a bio-inspired approach to the synthesis of oxindoles, which couples the energetic requirements of dehydrogenative C-N bond formation to the reduction of molecular oxygen (O2). Our method is inspired by the biosynthesis of melanin pigments (melanogenesis), but diverges from the biosynthetic polymerization. Mechanistic analysis reveals the involvement of CuII-semiquinone radical intermediates, which enable dehydrogenative carbon-heteroatom bond formation that avoids a catechol/quinone redox couple. This mitagates the deleterious polarity reversal that results from phenolic dearomatization, and enables a high-yielding phenolic C-H functionalization under catalytic aerobic conditions. Our work highlights the broad synthetic utility and efficiency of forming C-N bonds via a catalytic aerobic dearomatization of phenols, which is currently an underdeveloped transformation.

Redox noninnocence of the bridge in copper(II) salophen and bis(oxamato) complexes.

Two square-planar copper(II) complexes of 1,2-bis(2-hydroxy-3,5-di-tert-butylbenzimino)-4,5-bis(dimethylamino)benzene (1) and N-[4,5-bis(dimethylamino)-2-(oxalylamino)benzene]oxamate (2(2-)) were prepared. The crystal structures of the proligands H2L(1) and Et2H2L(2), as well as the corresponding complexes, are reported. The proligands each display a one-electron-oxidation wave, which is assigned to oxidation of the bis(dimethylamino)benzene moiety into a π radical. Complexes 1 and 2(2-) exhibit reversible one-electron-oxidation waves in their cyclic voltammograms (E1/2(1) = 0.14 and E1/2(2) = 0.31 V for 1 and E1/2(1) = -0.47 V vs Fc(+)/Fc for 2(2-)). The first process corresponds to oxidation of the bis(dimethylamino)benzene central ring into a π radical, while the second process for 1 is ascribed to oxidation of the π radical into an α-diiminoquinone. The one-electron-oxidized species 1(+) and 2(-) exhibit intense visible-near-IR absorptions, which are diagnostic of π radicals. They display a triplet signal in their electron paramagnetic resonance spectra, which stem from magnetic coupling between the ligand-radical spin and the copper(II) spin. The zero-field-splitting parameters are larger for 2(-) than 1(+) because of greater delocalization of the spin density onto the coordinated amidato N atoms. Density functional theory calculations support a π-radical nature of the one-electron-oxidized complexes, as well as S = 1 ground spin states. The electrogenerated 1(2+) comprises a closed-shell diiminoquinone ligand coordinated to a copper(II) metal center. Both 1 and 2 catalyze the aerobic oxidation of benzyl alcohol, albeit with different yields.

A Biomimetic Mechanism for the Copper-Catalyzed Aerobic Oxygenation of 4-tert-Butylphenol.

Controlling product selectivity during the catalytic aerobic oxidation of phenols remains a significant challenge that hinders reaction development. This work provides a mechanistic picture of a Cu-catalyzed, aerobic functionalization of phenols that is selective for phenoxy-coupled ortho-quinones. We show that the immediate product of the reaction is a Cu(II)-semiquinone radical complex and reveal that ortho-oxygenation precedes oxidative coupling. This complex is the resting state of the Cu catalyst during turnover at room temperature. A mechanistic study of the formation of this complex at low temperatures demonstrates that the oxygenation pathway mimics the dinuclear Cu enzyme tyrosinase by involving a dinuclear side-on peroxodicopper(II) oxidant. Unlike the enzyme, however, the rate-limiting step of the ortho-oxygenation reaction is the self-assembly of the oxidant from Cu(I) and O2. We provide details for all steps in the cycle and demonstrate that turnover is contingent upon proton-transfer events that are mediated by a slight excess of ligand. Finally, our knowledge of the reaction mechanism can be leveraged to diversify the reaction outcome. Thus, uncoupled ortho-quinones are favored in polar, coordinating media, highlighting unusually high levels of chemoselectivity for a catalytic aerobic oxidation of a phenol.

Controlled nitrene transfer from a tyrosinase-like arylnitroso-copper complex.

The reaction between p-nitrosonitrobenzene and the tetramethylpropylenediamine-copper(i) complex yields a dinuclear complex that is structurally and electronically similar to side-on peroxo species known in Cu/O2 chemistry. The complex reacts with di-tert-butylphenolate via nitrene transfer, as observed through an intermediate and the aminophenol product obtained upon reductive work-up.

Asymmetric synthesis of fortucine and reassignment of its absolute configuration.

A convergent and enantioselective synthesis of fortucine was achieved from the starting materials tyrosine methyl ester and 3-hydroxy-4-methoxybenzaldehyde. The synthesis is based on two key steps mediated by a hypervalent iodine reagent. This work has enabled us to reassign the absolute configuration of the natural product reported in the literature.

Reversible double oxidation and protonation of the non-innocent bridge in a nickel(II) salophen complex.

Substitution on the aromatic bridge of a nickel(II) salophen complex with electron-donating dimethylamino substituents creates a ligand with three stable, easily and reversibly accessible oxidation states. The one-electron-oxidized product is characterized as a nickel(II) radical complex with the radical bore by the central substituted aromatic ring, in contrast to other nickel(II) salen or salophen complexes that oxidize on the phenolate moieties. The doubly oxidized product, a singlet species, is best described as having an iminobenzoquinone bridge with a vinylogous distribution of bond lengths between the dimethylamino substituents. Protonation of the dimethylamino substituents inhibits these redox processes on the time scale of cyclovoltammetry, but electrolysis and chemical oxidation are consistent with deprotonation occurring concomitantly with electron transfer to yield the mono- and dioxidized species described above.

Charge-assisted hydrogen bond-directed self-assembly of an amphiphilic zwitterionic quinonemonoimine at the liquid-solid interface.

Charge-assisted hydrogen bond-directed self-assembly of a zwitterionic quinonemonoimine was investigated at the liquid/solid interface using scanning tunnelling microscopy. Factors governing morphology, chirality and multilayer formation are discussed, presenting an important foundation for understanding the properties of a large family of related molecules with interesting potential in supramolecular design.

The two spin states of an end-on copper(II)-superoxide mimic.

The reaction of nitrosobenzene with copper(I) complexes of a tetradentate ligand led to two novel species that are best described as copper(II) complexes of an O-bonded nitrosobenzyl radical anion, in either the singlet or the triplet spin-state. Both states were characterized by crystal structures, magnetic measurements and DFT calculations.

Electrocatalytic O2 reduction by covalently immobilized mononuclear copper(I) complexes: evidence for a binuclear Cu2O2 intermediate.

A Cu(I) complex of 3-ethynyl-phenanthroline covalently immobilized onto an azide-modified glassy carbon surface is an active electrocatalyst for the four-electron (4-e) reduction of O(2) to H(2)O. The rate of O(2) reduction is second-order in Cu coverage at moderate overpotential, suggesting that two Cu(I) species are necessary for efficient 4-e reduction of O(2). Mechanisms for O(2) reduction are proposed that are consistent with the observations for this covalently immobilized system and previously reported results for a similar physisorbed Cu(I) system.

Three-coordinate terminal imidoiron(III) complexes: structure, spectroscopy, and mechanism of formation.

Reaction of 1-adamantyl azide with iron(I) diketiminate precursors gives metastable but isolable imidoiron(III) complexes LFe=NAd (L = bulky beta-diketiminate ligand; Ad = 1-adamantyl). This paper addresses (1) the spectroscopic and structural characterization of the Fe=N multiple bond in these interesting three-coordinate iron imido complexes, and (2) the mechanism through which the imido complexes form. The iron(III) imido complexes have been examined by (1)H NMR and electron paramagnetic resonance (EPR) spectroscopies and temperature-dependent magnetic susceptibility (SQUID), and structurally characterized by crystallography and/or extended X-ray absorption fine structure (EXAFS) measurements. These data show that the imido complexes have quartet ground states and short (1.68 +/- 0.01 A) iron-nitrogen bonds. The formation of the imido complexes proceeds through unobserved iron-N(3)R intermediates, which are indicated by QM/MM computations to be best described as iron(II) with an N(3)R radical anion. The radical character on the organoazide bends its NNN linkage to enable easy N(2) loss and imido complex formation. The product distribution between imidoiron(III) products and hexazene-bridged diiron(II) products is solvent-dependent, and the solvent dependence can be explained by coordination of certain solvents to the iron(I) precursor prior to interaction with the organoazide.

Oxygen-atom transfer to a nucleophilic molybdenum complex.

The reactivity of the imido-Mo(IV) species [(eta(5)-Cp)(2)Mo(N(t)Bu)], 1, towards the iodine-based oxidant PhIO was investigated and yielded the micro-oxodimolybdenum(V) dimer [{(eta(5)-Cp)(eta(1)-Cp)Mo(N(t)Bu)}(2)O], 2. X-Ray crystallography and (1)H-NMR of 2 indicate slipping of one of the eta(5)-Cp ring on each Mo center to eta(1)-Cp as the result of the coordination of the oxygen atom. DFT investigation of the OAT reaction to complex 1 revealed a transient Mo(IV)-oxo species as the most likely intermediate.

Ferromagnetic coupling by spin polarization in a trinuclear copper(II) metallacyclophane with a triangular cage-like structure.

A series of trinuclear copper(II) complexes of general formula A(6)[Cu(3)L(2)] x nH(2)O [L = benzene-1,3,5-tris(oxamate); A = Li(+) (n = 8), 1a; Na(+) (n = 11.5), 1b; and K(+) (n = 8.5), 1c] have been synthesized, and they have been structurally and magnetically characterized. X-ray diffraction on single crystals of 1c shows the presence of three square-planar copper(II)-bis(oxamato) moieties which are connected by a double benzene-1,3,5-triyl skeleton to give a unique metallacyclophane-type triangular cage. The copper basal planes are virtually orthogonal to the two benzene rings, which adopt an almost perfect face-to-face alignment. Complexes 1a-c exhibit a quartet (S = 3/2) ground spin state resulting from the moderate ferromagnetic coupling (J values in the range of +7.3 to +16.5 cm(-1)) between the three Cu(II) ions across the two benzene-1,3,5-tris(amidate) bridges [H = -J(S(1) x S(2) + S(2) x S(3) + S(3) x S(1)) with S(1) = S(2) = S(3) = S(Cu) = 1/2]. Density functional theory calculations on the S = 3/2 Cu(II)(3) ground spin state of 1c support the occurrence of a spin polarization mechanism for the propagation of the exchange interaction, as evidenced by the sign alternation of the spin density in the 1,3,5-substituted benzene spacers.