Crystal structures of alkali salts of tetrahaolgenosemiquinone anion radical acetone solvates and their solvent-free salts are determined. p-Semiquinone anion radical reveals enhanced aromaticity of the ring compared to the quinone. A pair of p-stacked radical anion (psemiquinone) rings occurs in crystal structures of potassium and rubidium salts of tetrachlorosemiquinone anion acetone solvates and their potassium tetrabromo analogue. The ring centroid separation distances are about 3.2 Å and carbon-carbon contacts between the contiguous rings are 0.3 Å shorter than the sum of van der Waals radii. The spin-coupling of the two unpaired electrons between the two anion radical rings (forming a stacked dimer) correlates with the diamagnetic property of the crystals. Magnetic properties of alkali salts of tetrahaolgenosemiquinone anion radical acetone solvates were examined by electron paramagnetic resonance spectroscopy. IntroductionQuinones and semiquinone radicals undergo easily reversible oxidation-reduction reactions and they are excellent electron carriers. Due to influence of the functional groups, quinones may have various values of the standard redox potential. The oxidation potential of the quinones with electron-donating groups such as -OH, is lower, and with electronwithdrawing groups such as -Cl and -NO2, the potential becomes higher. These unique electron properties of quinones are exploited for syntheses, both, in laboratory and by nature in vivo. 1,2 Substituents on the quinoid ring modify electron density affecting oxidation potential and stability of the semiquinone radical (Scheme 1). Perhalogenated benzoquinones are easily reduced and their radicals are rather stable; four electronegative substituents make electron density in the ring significantly lower. Sodium and potassium salts of tetrachlorosemiquinone anion radical were first prepared in 1912 by Torrey and Hunter 3 by reaction of alkali iodide and tetrachloroquinone in acetone. Green salts with formulae NaC 6 C l4 O 2 and KC 6 C l4 O 2 precipitated from cold acetone but quickly decomposed upon heating. During the last fifty years perhalogenosemiquinones were studied by various techniques: EPR, 4-6 UV/Vis 7 and IR/Raman spectroscopy [8][9][10][11] and computational methods. [12][13][14][15] Several charge-transfer systems involving tetrachloro-1,4-benzoquinone (Cl4Q) [16][17][18][19][20] and tetrabromo-1,4-benzoquinone (Br4Q), 21,22 where radical anions can be stabilised under appropriate conditions, have also been designed. A crystallographic study of tetrachlorosemiquinone radical anion-in its well-known potassium salt3-was attempted in a Rudjer Bo_skovi_c Institute, Bijeni_cka 54, HR-10000 Zagreb, Croatia. E-mail: kmolcano@irb.hr Molčanov, K., , "Stabilisation of tetrabromo-and tetrachlorosemiquinone (bromanil and chloranil) anion radicals in crystals", CrystEngComm, Vol.13, No.16, 1973 23 using a film method (Weissenberg camera). However, due to an extremely weak diffraction of poor crystals the authors could only determine unit cells of...
We have applied in situ monitoring of mechanochemical reactions by high-energy synchrotron powder X-ray diffraction to study the role of liquid additives on the mechanochemical synthesis of the archetypal metal-organic framework (MOF) HKUST-1, which was one of the first and is still among the most widely investigated MOF materials to be synthesized by solvent-free procedures. It is shown here how the kinetics and mechanisms of the mechanochemical synthesis of HKUST-1 can be influenced by milling conditions and additives, yielding on occasion two new and previously undetected intermediate phases containing a mononuclear copper core, and that finally rearrange to form the HKUST-1 architecture. On the basis of in situ data, we were able to tune and direct the milling reactions toward the formation of these intermediates, which were isolated and characterized by spectroscopic and structural means and their magnetic properties compared to those of HKUST-1. The results have shown that despite the relatively large breadth of analysis available for such widely investigated materials as HKUST-1, in situ monitoring of milling reactions can help in the detection and isolation of new materials and to establish efficient reaction conditions for the mechanochemical synthesis of porous MOFs.
Tetratopic porphyrin-based MOFs represent a particularly interesting subclass of zirconium MOFs due to the occurrence of several divergent topologies. The control over the target topology is a demanding task and reports often show products containing phase contamination. We demonstrate how mechanochemistry can be exploited for controlling the polymorphism in 12-coordinated porphyrinic zirconium MOFs, gaining pure hexagonal (shp) PCN-223 and cubic (ftw) MOF-525 phases in 20-60 minutes of milling. The reactions are mainly governed by the milling additives and the zirconium precursor. In situ monitoring by synchrotron powder X-ray diffraction (PXRD) revealed that specific reaction conditions resulted in the formation of MOF-525 as an intermediate, which rapidly converted to PCN-223 upon milling. Electron spin resonance (ESR) measurements revealed significant differences between the spectra of paramagnetic centers in two polymorphs, showing a potential of polymorphic Zr-MOFs as tunable supports in spintronics applications. Metal-organic frameworks (MOFs) received wide attention due to their potential for applications in gas storage 1-3 and separation, 4,5 catalysis, 6,7 drug delivery, 8 light-harvesting, 9,10 and destruction of harmful compounds such as chemical warfare agents. 11 Their superior performance stems from the existence of pores and channels enabling easy access of substrates to the active sites inside the MOF crystals. The use of MOFs as heterogeneous catalysts and catalysts supports is broadened after the introduction of Zr-MOFs based on zirconium [Zr6(OH)4O4] 12+ oxo-clusters and carboxylate linkers, 12 which provided a way to overcome challenges related to the robustness of MOFs under humid, acidic or basic media. 11,13 They also drew significant interest in an area of MOF-polymorphism. Zr-MOFs based on tetratopic tetrakis(4-carboxyphenyl) porphyrin (TCPP) linkers displayed unprecedented flexibility in topological ordering. They are known to exist in six different topologies, 14,15-22 12-connected cubic ftw (MOF-525) 21 and hexagonal shp (PCN-223), 16 8-connected sqc (PCN-225), 17 csq (PCN-222/MOF-545), 19,21 and scu (NU-902), 15 and 6-connected she (PCN-224). 20 Recent studies have been focused on establishing different reaction
A series of six novel mononuclear, binuclear and linear one-dimensional (1D) compounds of copper(II) with chloranilic acid (3,6-dichloro-2,5-dihydroxybenzoquinone, H 2 CA) is prepared and a design strategy for the preparation of such complexes is discussed. Four described compounds are linear 1D coordination polymers [Cu(CA)] n , whereas another two involve a binuclear and a mononuclear, Cu 2 (CA) 3 and Cu(CA) 2 , core unit. A linear polymer incorporating bulky aromatic imidazole has been synthesized as a result of investigation of the influence of pH on the reaction mixture. Two coordination modes of the chloranilate dianion are observed. The bridging (bis)bidentate mode generates linear 1D polymeric species. Among these one reveals square-pyramidal coordination of Cu 2+ , whereas the three polymers contain Cu 2+ in an octahedral arrangement. However, the combination of both, terminal bidentate (ortho-quinone) and bridging (bis)bidentate modes of coordination produces a binuclear complex anion, which comprises a square-pyramidal coordination of Cu 2+ complex anions forming a supramolecular honeycomb-like network encapsulating 4,4 0 -bipyridine cations. When the chloranilate dianion coordinates the Cu 2+ atom only in a terminal bidentate mode, a mononuclear complex with an octahedral environment of the metal centre is formed. The presence of the bulky ancillary ligand imidazole produces an unprecedented packing involving chiral (racemic) and achiral (meso-compound) coordination polymers in the same crystal. Electron spin resonance spectroscopy of polycrystalline samples determined g-tensor parameters of copper(II) ions in different coordination geometries and revealed weak exchange interactions (|J| < 1 cm À1 ) in linear metal-complex polymers and dimeric species.
Encapsulation and confinement of fullerene guests in metal-organic frameworks (MOFs) lead to a novel class of crystalline fulleretic materials with unique physicochemical properties and a broad field of potential applications. The control over the amount of target guests confined in the MOF structure remains a significant challenge, which is particularly pronounced in the confinement of hardly accessible fullerene derivatives. The main strategies used in constructing fulleretic composites are limited by the solubility of components used and solvent versus guest competition for inhabitation of the framework voids. As mechanochemical procedures often overcome these issues, we developed here solvent-free processing by ball milling to gain control over the encapsulation of bulky and rigid C 60 -fullerene into a sodalite MOF with large cages and narrow cage-apertures. A rapid, green, efficient, and stoichiometry-controlled mechanochemical processing afforded four model C 60 @zeolitic-imidazolate framework 8 (ZIF-8) crystalline materials containing target 15, 30, 60, and 100 mol % of fullerene entrapped in the accessible cages of the model sodalite zeolitic-imidazolate framework 8 (ZIF-8), in stark contrast to the solution-based strategies that resulted in almost no loading. Varying the fullerene content affects the framework's vibrational properties, color and luminescence of the composites, and the electron-dose radiation stability. The computational and spectroscopic studies show that the fullerene is accommodated in the cage's center and that the cage-to-cage transport is a hardly feasible and energetically unfavored process. However, the fast release of C 60 molecules from ZIF-8 can be effectively controlled by the pH. The entrapment of fullerene molecules in ZIF-8 resulted in their effective isolation even in higher loadings, paving the way to other tunable porous fulleretics containing single-molecule magnets or nanoprobes available on low scales.
We present an experimental study of macroscopic and microscopic magnetic anisotropy of a spin tetramer system SeCuO 3 using torque magnetometry and ESR spectroscopy. Large rotation of macroscopic magnetic axes with temperature observed from torque magnetometry agrees reasonably well with the rotation of the g tensor above T 50 K. Below 50 K, the g tensor is temperature independent, while macroscopic magnetic axes continue to rotate. Additionally, the susceptibility anisotropy has a temperature dependence which cannot be reconciled with the isotropic Heisenberg model of interactions between spins. ESR linewidth analysis shows that anisotropic exchange interaction must be present in SeCuO 3 . These findings strongly support the presence of anisotropic exchange interactions in the Hamiltonian of the studied system. Below T N = 8 K, the system enters a long -range antiferromagnetically ordered state with easy axis along the <101 > * direction. Small but significant rotation of magnetic axes is also observed in the antiferromagnetically ordered state suggesting strong spin-lattice coupling in this system.
Copper-catalyzed mechanochemical click reactions using Cu(II), Cu(I) and Cu(0) catalysts have been successfully implemented to provide novel 6-phenyl-2-(trifluoromethyl)quinolines with a phenyl-1,2,3-triazole moiety at O-4 of the quinoline core. Milling procedures proved to be significantly more efficient than the corresponding solution reactions, with up to a 15-fold gain in yield. Efficiency of both solution and milling procedures depended on the p-substituent in the azide reactant, resulting in H < Cl < Br < I reactivity bias. Solid-state catalysis using Cu(II) and Cu(I) catalysts entailed the direct involvement of the copper species in the reaction and generation of highly luminescent compounds which hindered in situ monitoring by Raman spectroscopy. However, in situ monitoring of the milling processes was enabled by using Cu(0) catalysts in the form of brass milling media which offered a direct insight into the reaction pathway of mechanochemical CuAAC reactions, indicating that the catalysis is most likely conducted on the surface of milling balls. Electron spin resonance spectroscopy was used to determine the oxidation and spin states of the respective copper catalysts in bulk products obtained by milling procedures.
Magnetic properties of single crystals of the heterometallic complex [Cu(bpy) 3 ] 2 [Cr(C 2 O 4 ) 3 ]NO 3 ·9H 2 O (bpy = 2,2'-bipyridine) have been investigated. From the recorded EPR spectra, the spin-Hamiltonian parameters have been determined. The magnetization measurements have shown magnetic anisotropy at low temperatures, which has been analysed as a result of the zero-field splitting of the Cr III ion. By fitting the exactly derived magnetization expression to the measured magnetization data, the axial zero-field splitting parameter, D, has been calculated. Comparing to the EPR measurements, it has been confirmed that D can be determined from the measurements of the macroscopic magnetization on the single crystals.
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