Engineered π⋯π interactions favour supramolecular dimers X@[FeL3]2 (X = Cl, Br, I): solid state and solution structure.

Ditopic bis-pyrazolylpyridine ligands usually react with divalent metal ions (M2+) to produce dinuclear triple-stranded helicates [M2L3]4+ or, via π⋯π interactions, dimers of monoatomic complexes ([ML3]2)4+. The introduction of an additional benzene ring at each end of ligand L increases the number of aromatic contacts within the supramolecular aggregate by 40%, driving the self-recognition process in an irreversible manner. Consequently, the mixing of new bis-pyrazolylquinoline L2 with FeX2 salts leads to crystallization of the tripartite high-spin assemblies (X@[Fe(L2)3]2)3+ (X = Cl, Br or I). The aggregates exhibit exceptional stability, as confirmed by a combination of paramagnetic 1H NMR techniques, demonstrating their persistence in solution. Our investigations further reveal that the guests Br- and I- are retained inside the associate in solution but Cl- is immediately released, resulting in the formation of the empty supramolecular dimer ([Fe(L2)3]2)4+.

MIL-100(Fe)-Based Composite Films for Food Packaging.

A biocompatible metal-organic framework MIL-100(Fe) loaded with the active compounds of tea tree essential oil was used to produce composite films based on κ-carrageenan and hydroxypropyl methylcellulose with the uniform distribution of the particles of this filler. The composite films featured great UV-blocking properties, good water vapor permeability, and modest antibacterial activity against both Gram-negative and Gram-positive bacteria. The use of metal-organic frameworks as containers of hydrophobic molecules of natural active compounds makes the composites made from naturally occurring hydrocolloids attractive materials for active packaging of food products.

Bis(2,6-pyrazolyl)pyridines as a New Scaffold for Coordination Polymers.

Two coordination polymers, Fe(LOBF3)(CH3COO)(CH3CN)2]n•nCH3CN and [Fe(LO-)2AgNO3BF4•CH3OH]n•1.75nCH3OH•nH2O (LO- = 3,3'-(4-(4-cyanophenyl)pyridine-2,6-diyl)bis(1-(2,6-dichlorophenyl)-1H-pyrazol-5-olate)), were obtained via a PCET-assisted process that uses the hydroxy-pyrazolyl moiety of the ligand and the iron(II) ion as sources of proton and electron, respectively. Our attempts to produce heterometallic compounds under mild conditions of reactant diffusion resulted in the first coordination polymer of 2,6-bis(pyrazol-3-yl)pyridines to retain the core N3(L)MN3(L). Under harsh solvothermal conditions, a hydrogen atom transfer to the tetrafluoroborate anion caused the transformation of the hydroxyl groups into OBF3 in the third coordination polymer of 2,6-bis(pyrazol-3-yl)pyridines. This PCET-assisted approach may be applicable to produce coordination polymers and metal-organic frameworks with the SCO-active core N3(L)MN3(L) formed by pyrazolone- and other hydroxy-pyridine-based ligands.

Homoleptic Fe(III) and Fe(IV) Complexes of a Dianionic C3-Symmetric Scorpionate.

High-valent iron species have been implicated as key intermediates in catalytic oxidation reactions, both in biological and synthetic systems. Many heteroleptic Fe(IV) complexes have now been prepared and characterized, especially using strongly π-donating oxo, imido, or nitrido ligands. On the other hand, homoleptic examples are scarce. Herein, we investigate the redox chemistry of iron complexes of the dianonic tris-skatylmethylphosphonium (TSMP2-) scorpionate ligand. One-electron oxidation of the tetrahedral, bis-ligated [(TSMP)2FeII]2- leads to the octahedral [(TSMP)2FeIII]-. The latter undergoes thermal spin-cross-over both in the solid state and solution, which we characterize using superconducting quantum inference device (SQUID), Evans method, and paramagnetic nuclear magnetic resonance spectroscopy. Furthermore, [(TSMP)2FeIII]- can be reversibly oxidized to the stable high-valent [(TSMP)2FeIV]0 complex. We use a variety of electrochemical, spectroscopic, and computational techniques as well as SQUID magnetometry to establish a triplet (S = 1) ground state with a metal-centered oxidation and little spin delocalization on the ligand. The complex also has a fairly isotropic g-tensor (giso = 1.97) combined with a positive zero-field splitting (ZFS) parameter D (+19.1 cm-1) and very low rhombicity, in agreement with quantum chemical calculations. This thorough spectroscopic characterization contributes to a general understanding of octahedral Fe(IV) complexes.

Synthesis, X-ray structure and magnetic properties of the apically functionalized monocapped cobalt(II) tris-pyridineoximates possessing SMM behaviour.

The title cobalt(II) pseudoclathrochelate complexes possess an intermediate trigonal prismatic-trigonal antiprismatic geometry. As follows from PPMS data, they exhibit an SMM behaviour with Orbach relaxation barriers of approximately 90 K. Paramagnetic NMR experiments confirmed a persistence of these magnetic characteristics in solution. Therefore, a straightforward apical functionalization of this 3D molecular platform for its targeted delivery to a given biosystem can be performed without substantial changes.

Synthesis, crystal polymorphism and spin crossover behavior of adamantylboron-capped cobalt(II) hexachloroclathrochelate and its transformation into the CoIIICoIICoIII-bis-macrobicyclic derivative.

Fast crystallization of the monoclathrochelate cobalt(II) intracomplex [Co(Cl2Gm)3(BAd)2] (where Cl2Gm2- is a dichloroglyoxime dianion and BAd is an adamantylboron capping group, 1), initially obtained by the direct template condensation of the corresponding chelating α-dioximate and cross-linking ligand synthons on the Co2+ ion as a matrix, from benzene or dichloromethane afforded its structural triclinic and hexagonal polymorphs. Its prolonged recrystallization from dichloromethane under air atmosphere and sunlight irradiation unexpectedly gave the crystals of the CoIIICoIICoIII-trinuclear dodecachloro-bis-clathrochelate intracomplex [[CoIII(Cl2Gm)3(BAd)]2CoII] (2), the molecule of which consists of two macrobicyclic frameworks with encapsulated low-spin (LS) Co3+ ions, which are cross-linked by a µ3-bridging Co2+ ion as a bifunctional Lewis-acidic center. The most plausible pathway of such a 1 → 2 transformation is based on the photoinitiated radical oxidation of dichloromethane with air oxygen giving the reactive species. Cobalt(II) monoclathrochelate 1 was found to undergo a temperature-induced spin crossover (SCO) both in its solutions and in the solid state. In spite of the conformational rigidity of the corresponding quasiaromatic diboron-capped tris-α-dioximate framework, the main parameters of this SCO transition (i.e., its completeness and gradual character) are strongly affected by the nature of the used solvent (in the case of its solutions) and by the structural polymorphism of its crystals (in the solid state). In the latter case, the LS state (S = 1/2) of this complex is more thermally stable and, therefore, the cobalt(II)-centered 1/2 → 3/2 SCO is more gradual than that in solutions.

Selective Pathway toward Heteroleptic Spin-Crossover Iron(II) Complexes with Pyridine-Based N-Donor Ligands.

A new synthetic pathway is devised to selectively produce previously elusive heteroleptic iron(II) complexes of terpyridine and N,N'-disubstituted bis(pyrazol-3-yl)pyridines that stabilize the opposite spin states of the metal ion. Such a combination of the ligands in a series of the heteroleptic complexes induces the spin-crossover (SCO) not experienced by the homoleptic complexes of these ligands or shifts it to lower/higher temperatures respective to the SCO-active homoleptic complex. The midpoint temperatures of the resulting SCO span from ca. 200 K to the ambient temperature and beyond the highest temperature accessible by NMR spectroscopy and SQUID magnetometry. The proposed "one-pot" approach is applicable to other N-donor ligands to selectively produce heteroleptic complexes─including those inaccessible by alternative synthetic pathways─with highly tunable SCO behaviors for practical applications in sensing, switching, and multifunctional devices.

Universal Character of Breaking of Wormlike Surfactant Micelles by Additives of Different Hydrophobicity.

Wormlike surfactant micelles are widely used in various applications including fracturing technology in oil industry, template synthesis of different nanoobjects, micellar copolymerization of hydrophilic and hydrophobic monomers, and so forth. Most of those applications suggest the solubilization of different additives in the micelles. The present paper is aimed at the comparative study of the effect of the solubilization of hydrophobic (n-decane and 1-phenylhexane) and hydrophilic (N-isopropylacrylamide and acrylamide) substances on the rheological properties and structure of the micelles using several complementary techniques including rheometry, small angle neutron scattering, dynamic light scattering, and diffusion ordered NMR spectroscopy. For these studies, mixed micelles of potassium oleate and n-octyltrimethylammonium bromide containing the excess of either anionic or cationic surfactants were used. It was shown that hydrophobic additives are completely solubilized inside the micelles being localized deep in the core (n-decane, 1-phenylhexane) or near the core/corona interface (1-phenylhexane). At the same time, only a small fraction of hydrophilic additives (14% of N-isopropylacrylamide and 4% of acrylamide) penetrate the micelles being localized at the corona area. Despite different localization of the additives inside the micelles, all of them induce the breaking of wormlike micelles with the formation of either ellipsoidal microemulsion droplets (in the case of hydrophobic additives) or ellipsoidal surfactant micelles (in the case of hydrophilic additives). The breaking of micelles results in the drop of viscosity of the solution up to water value. The main result of this paper consists in the observation of the fact that for all the additives under study, the dependences of the viscosity on the volume fraction of additive lie on the same master curve being shifted along the volume fraction axis by a certain factor depending on the hydrophobicity of the added species. Those data are quite useful for various applications of wormlike surfactant micelles suggesting the solubilization of different additives inside them.

Fluoride Additive as a Simple Tool to Qualitatively Improve Performance of Nickel-Catalyzed Asymmetric Michael Addition of Malonates to Nitroolefins.

Nowadays, design of the new chiral ligands for organometallic catalysts is often based on the step-by-step increase in their complexity to improve efficiency. Herein we describe that simple in situ addition of the fluoride source to the asymmetric organometallic catalyst can improve not only activity but also enantioselectivity. Bromide-nickel diimine complexes were found to catalyze asymmetric Michael addition in low yields and ee, but activation with fluoride leads to a significant improvement in catalyst performance. The developed approach was applied to prepare several enantioenriched GABA analogues.

Molecular Compounds in Spintronic Devices: An Intricate Marriage of Chemistry and Physics.

Spintronics, a flourishing new field of microelectronics, uses the electron spin for reading and writing information in modern computers and other spintronic devices with a low power consumption and high reliability. In a quest to increase the productivity of such devices, the use of molecular materials as a spacer layer allowed them to perform equally well or even better than conventional all-inorganic heterostructures from metals, alloys, or inorganic semiconductors. In this review, we survey various classes of chemical compounds that have already been tested for this purpose─from organic compounds and coordination complexes to organic-inorganic hybrid materials─since the creation of the first molecule-based spintronic device in 2002. Although each class has its advantages, drawbacks, and applications in molecular spintronics, together they allowed nonchemists to gain insights into spin-related effects and to propose new concepts in the design and fabrication of highly efficient spintronic devices. Other molecular compounds that chemistry could offer in great numbers may soon emerge as suitable spacers or even electrodes in flexible magnetic field sensors, nonvolatile memories, and multifunctional spintronic devices.

Iron(IV) complexes with tetraazaadamantane-based ligands: synthesis, structure, applications in dioxygen activation and labeling of biomolecules.

4,6,10-Trihydroxy-1,4,6,10-tetraazaadamantane (TAAD) has been shown to form a stable Fe(IV) complex having a diamantane cage structure, in which the metal center is coordinated by three oxygen atoms of the deprotonated ligand. The complex was characterized by X-ray diffraction analysis, HRMS, NMR, FT-IR, Mössbauer spectroscopy and DFT calculations, which supported the d4 configuration of iron. The Fe(IV)-TAAD complex showed excellent performance in dioxygen activation under mild conditions serving as a mimetic of the thiol oxidase enzyme. The nucleophilicity of the bridgehead nitrogen atom in TAAD provides a straightforward way for the conjugation of Fe(IV)-TAAD complexes to various functional molecules. Using this approach, steroidal and peptide molecules having an iron(IV) label have been prepared for the first time. In addition, the Fe(IV)-TAAD complex was covalently bound to a polystyrene matrix and the resulting material was shown to serve as a heterogeneous catalyst for aerobic oxidation of thiols to disulfides.

Parahydrogen-Induced Hyperpolarization of Unsaturated Phosphoric Acid Derivatives.

Parahydrogen-induced nuclear polarization offers a significant increase in the sensitivity of NMR spectroscopy to create new probes for medical diagnostics by magnetic resonance imaging. As precursors of the biocompatible hyperpolarized probes, unsaturated derivatives of phosphoric acid, propargyl and allyl phosphates, are proposed. The polarization transfer to 1H and 31P nuclei of the products of their hydrogenation by parahydrogen under the ALTADENA and PASADENA conditions, and by the PH-ECHO-INEPT+ pulse sequence of NMR spectroscopy, resulted in a very high signal amplification, which is among the largest for parahydrogen-induced nuclear polarization transfer to the 31P nucleus.

Unravelling of a [High Spin-Low Spin] ↔ [Low Spin-High Spin] Equilibrium in Spin-Crossover Iron(II) Dinuclear Helicates Using Paramagnetic NMR Spectroscopy.

Spin-crossover between high-spin (HS) and low-spin (LS) states of selected transition metal ions in polynuclear and polymeric compounds is behind their use as multistep switchable materials in breakthrough electronic and spintronic devices. We report the first successful attempt to observe the dynamics of a rarely found broken-symmetry spin state in binuclear complexes, which mixes the states [HS-LS] and [LS-HS] on a millisecond timescale. The slow exchange between these two states, which was identified by paramagnetic NMR spectroscopy in solutions of two spin-crossover iron(II) binuclear helicates that are amenable to molecular design, opens a path to double quantum dot cellular automata for information storage and processing.

Analysis of reduced paramagnetic shifts as an effective tool in NMR spectroscopy.

A recently introduced concept of reduced paramagnetic shifts (RPS) in NMR spectroscopy is applied here to a series of paramagnetic complexes with different metal ions, such as iron(II), iron(III) and cobalt(II), in different coordination environments of N-donor ligands, including a unique trigonal-prismatic geometry that is behind some record single-molecule magnet behaviours. A simple, almost visual analysis of the chemical shifts as a function of temperature, which is at the core of this approach, allows for a correct signal assignment and evaluation of the anisotropy of the magnetic susceptibility, the key indicator of a good single molecule magnet, that often cannot be done using traditional techniques rooted in quantum chemistry and NMR spectroscopy. The proposed approach thus emerged as a powerful alternative in deciphering the NMR spectra of paramagnetic compounds for applications in data processing and storage, magnetic resonance imaging and structural biology.

Room-Temperature Spin Crossover in a Solution of Iron(II) Complexes with N,N'-Disubstituted Bis(pyrazol-3-yl)pyridines.

Here, we report a combined study of the effects of two chemical modifications to an N,N'-disubstituted bis(pyrazol-3-yl)pyridine (3-bpp) and of different solvents on the spin-crossover (SCO) behavior in otherwise high-spin iron(II) complexes by solution NMR spectroscopy. The observed stabilization of the low-spin state by electron-withdrawing substituents in the two positions of the ligand that induce opposite electronic effects in SCO-active iron(II) complexes of isomeric bis(pyrazol-1-yl)pyridines (1-bpp) was previously hidden by NH functionalities in 3-bpp precluding the molecular design of SCO compounds with this family of ligands. With the recent SCO-assisting substituent design, the uncovered trends converged toward the first iron(II) complex of N,N'-disubstituted 3-bpp to undergo an almost complete SCO centered at room temperature in a less polar solvent of a high hydrogen-bond acceptor ability.

3D-Printed Porous Magnetic Carbon Materials Derived from Metal-Organic Frameworks.

Here we report new porous carbon materials obtained by 3D printing from photopolymer compositions with zinc- and nickel-based metal-organic frameworks, ZIF-8 and Ni-BTC, followed by high-temperature pyrolysis. The pyrolyzed materials that retain the shapes of complex objects contain pores, which were produced by boiling zinc and magnetic nickel particles. The two thus provided functionalities-large specific surface area and ferromagnetism-that pave the way towards creating heterogenous catalysts that can be easily removed from reaction mixtures in industrial catalytic processes.

Transformations of wormlike surfactant micelles induced by a water-soluble monomer.

HYPOTHESIS: Wormlike surfactant micelles (WLMs) are prospective as nanoreactors for micellar copolymerization of hydrophilic and hydrophobic monomers. Hydrophilic monomers can destroy WLMs. Large size and cylindrical shape of micelles can be preserved by high salt content favoring closer packing of surfactant heads. EXPERIMENTS: The effect of a water-soluble monomer (acrylamide) on the structure and rheological properties of giant WLMs of an anionic surfactant potassium oleate at different salt content was investigated by combined experimental (SANS, rheometry, fluorescence and NMR spectroscopy, tensiometry) and molecular dynamics simulations studies. FINDINGS: At low salt content, when WLMs are linear, acrylamide induces their shortening and transformation into spherical micelles as a result of its incorporation into the micellar corona, leading to the drop of viscosity. At high salt content providing branched WLMs, monomer first triggers their transition to long linear chains, which enhances the viscoelasticity, and then to rods. This is the first report showing that the effect of monomer on the rheological properties is quite different for linear and branched micelles. Using branched micelles allows preserving large WLMs at high water-soluble monomer content, which is favorable for their use as nanoreactors for synthesis of copolymers with high degree of blockiness, which give mechanically tough polymer gels.

Revealing the Structure of Transition Metal Complexes of Formaldoxime.

Aerobic reactions of iron(III), nickel(II), and manganese(II) chlorides with formaldoxime cyclotrimer (tfoH3) and 1,4,7-triazacyclononane (tacn) produce indefinitely stable complexes of general formula [M(tacn)(tfo)]Cl. Although the formation of formaldoxime complexes has been known since the end of 19th century and applied in spectrophotometric determination of d-metals (formaldoxime method), the structure of these coordination compounds remained elusive until now. According to the X-ray analysis, [M(tacn)(tfo)]+ cation has a distorted adamantane-like structure with the metal ion being coordinated by three oxygen atoms of deprotonated tfoH3 ligand. The metal has a formal +4 oxidation state, which is atypical for organic complexes of iron and nickel. Electronic structure of [M(tacn)(tfo)]+ cations was studied by XPS, NMR, cyclic (CV) and differential pulse (DPV) voltammetries, Mössbauer spectroscopy, and DFT calculations. Unusual stabilization of high-valent metal ion by tfo3- ligand was explained by the donation of electron density from the nitrogen atom to the antibonding orbital of the metal-oxygen bond via hyperconjugation as confirmed by the NBO analysis. All complexes [M(tacn)(tfo)]Cl exhibited high catalytic activity in the aerobic dehydrogenative dimerization of p-thiocresol under ambient conditions.

A Synergy and Struggle of EPR, Magnetometry and NMR: A Case Study of Magnetic Interaction Parameters in a Six-Coordinate Cobalt(II) Complex.

Herein, we combine for the first time SQUID magnetometry, cw-EPR, THz-EPR, and paramagnetic NMR spectroscopies to study the magnetic properties of a high-spin cobalt(II) heteroscorpionate complex. Complementary information provided by these methods allowed precise determination of the magnetic interaction parameters, thereby removing the ambiguity inherit to single-method studies. We systematically investigate the extent to which information about the magnetic interaction parameters can be deduced from reduced data sets. The detailed study revealed significant different magnetic properties in solid state and solution. To further exploit the information content of the solution NMR experimental results, we introduce the new concept of reduced paramagnetic shift. It allows for the determination of the magnetic axes and, subsequently, full NMR signal assignment. It is shown that even in complicated cases, in which common NMR analytics (integral intensities, relaxation factors, etc.) fail, it yields robust results.

In Situ NMR Search for Spin-Crossover in Heteroleptic Cobalt(II) Complexes.

Here we report the first successful attempt to identify spin-crossover compounds in solutions of metal complexes produced by mixing different ligands and an appropriate metal salt by variable-temperature nuclear magnetic resonance (NMR) spectroscopy. Screening the spin state of a cobalt(II) ion in a series of thus obtained homoleptic and heteroleptic compounds of terpyridines (terpy) and 2,6-bis(pyrazol-3-yl)pyridines (3-bpp) by using this NMR-based approach, which only relies on the temperature behavior of chemical shifts, revealed the first cobalt(II) complexes with a 3-bpp ligand to undergo a thermally induced spin-crossover. A simple analysis of NMR spectra collected from mixtures of different compounds without their isolation or purification required by the current method of choice, the Evans technique, thus emerges as a powerful tool in a search for new spin-crossover compounds and their molecular design boosted by wide possibilities for chemical modifications in heteroleptic complexes.