The incorporation of spin crossover (SCO) properties into metal–organic frameworks (MOFs) has led to an appealing subclass of multifunctional MOFs. Because temperature is the most common way to manipulate the SCO phenomenon, the spin state of relevant complexes is conveniently monitored by variable‐temperature magnetic susceptibility (χMT) measurements. Other physical and spectroscopic techniques may also monitor the SCO behavior. In this context, the present work focuses on the use of variable‐temperature micro‐Raman spectroscopy. More specifically, the SCO behavior of the two‐dimensional Hofmann‐type MOF [FeII(2‐mpz)2Ni(CN)4], (2‐mpz = 2‐methylpyrazine), in the form of both bulk microcrystalline powder and nanoparticles is in depth analyzed via temperature‐dependent Raman spectroscopy. Magnetic susceptibility measurements were used in order to benchmark the vibrational spectroscopic ones. A brief introduction on the basic synthetic features of the samples and the morphological characteristics of representative samples are described, while the detailed syntheses of the MOF compounds in the microscale and the nanoscale were recently comprehensively addressed. It is unambiguously revealed for the samples in the different particle sizes that the SCO characteristics, such as Tc values, hysteretic behavior, and high spin (HS) population curves as a function of temperature, derived from Raman measurements are in close comparison with those of the magnetic susceptibility. In addition, spectroscopic analysis in the high‐ and low‐wavenumber regions suggests the 2D network structure of the specific compound and reveals the alterations at molecular level associated with the HS and low spin (LS) states. The significance of Raman spectroscopy for the study of SCO materials is demonstrated, because the technique offers combined structural characterization and detailed study of the transition.

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Nucleophilic attack by the carbanion :CHCOCH at the carbonyl group of di-2-pyridyl ketone, (py)CO, in the presence of Cu under moderately basic conditions has yielded the cationic mononuclear complex [Cu{(py)C(CHCOCH)(OH)}](NO)·2HO (1·2HO) in ∼40% yield, where (py)C(CHCOCH)(OH) is the ligand bis(2-pyridine-2-yl)butane-1-ol-3-one. The Cu atom of the cation sits on a crystallographically imposed inversion center. The neutral molecule is coordinated to the metal ion as a tridentate fac chelating ligand through the hydroxyl oxygen atom and two 2-pyridyl nitrogen atoms. The pyridyl nitrogens are strongly coordinated to the metal ion, while the hydroxyl oxygen atoms form weak bonds with Cu. The coordination geometry at the Cu center is elongated octahedral. Various interactions build the crystal structure of the complex and Hirshfeld surface analysis was applied to evaluate the magnitude of interactions between the different chemical species in the crystal of 1·2HO. IR, Raman and UV/VIS data of the solid complex are discussed in terms of the coordination mode of (py)C(CHCOCH)(OH), the ionic nature of nitrates and the stereochemistry at copper(ii). The complex was studied in a frozen solution (MeOH-toluene, 1 : 1 v/v) by CW-EPR spectroscopy and advanced EPR methods such as ENDOR and HYSCORE. The results show that the low symmetry of the cation is retained in solution, with the four nitrogen atoms arranged in a square planar configuration and the unpaired electron residing in an orbital pointing towards them. The bonding parameters in the first coordination sphere and the spin density distribution have been fully analyzed based on the ligand hyperfine coupling constants.

The spin-crossover (SCO) polymorph B (complex 1) of the known compound [FeII{N­(CN)2}2(abpt)2], where abpt is 4-amino-3,5-bis­(pyridin-2-yl)-1,2,4-triazole, has been prepared in three different particle sizes averaging ∼300 (sample 1a), ∼80 (sample 1b), and ∼20 nm (sample 1c). Two independent octahedral molecules possessing Fe1 and Fe2 were found to be present in the crystal of B. Magnetostructural relationships had established that at room temperature both FeII sites are in the high-spin state (HS-HS), whereas a decrease in the temperature to 90 K induces the complete high-spin to low-spin conversion of the Fe1 site, with Fe2 remaining in the high-spin state (LS-HS). The three samples have been characterized by elemental analyses, ATR spectra, solution UV/vis spectra (to exclude resonance Raman effects) and powder X-ray diffraction patterns, while their morphological characteristics have been examined by scanning electron microscopy (SEM). The SCO behavior of the originally prepared sample 1a has been monitored in detail by variable-temperature Raman studies in the 300–80 K range using mainly low-frequency ν­(Fe–N) and δ­(NFeN) modes and the ν­(CN) mode of the axial dicyanamido groups as spin-sensitive vibrations. The new peaks that appear in the low-temperature Raman spectra of the LS-HS form of the complex are reproduced in the calculated spectrum of the LS state of [FeII{N­(CN)2}2(abpt)2]. The influence of the average particle size on the SCO properties of 1 has also been studied by variable-temperature Raman spectra. The studies indicate that, during the HS-HS → LS-HS transition, the latter form of the complex appears at higher temperatures for the smaller particles; the T 1/2 shift accomplished by manipulating the particle size within a range of roughly 1 order of magnitude (300–20 nm) may be as high as ∼30 K. The SCO features of 1, as deduced from the Raman study, are in excellent agreement with those derived from a traditional variable-temperature magnetic susceptibility study, indicating the utility of the former.

In this study, a simple, fast, one-pot approach for the isolation of Nanowires (NWs) in coordination chemistry is reported. Molecular compounds exhibiting the spin-crossover (SCO) behavior are motivating materials for...

The use of 2-pyridinealdoxime (paoH), methyl 2-pyridyl ketone oxime (mepaoH), phenyl 2-pyridyl ketone oxime (phpaoH) and pyridine-2-amidoxime (NHpaoH) for the synthesis of dinuclear Co/Dy complexes is described in the absence or presence of an external base. Complexes [CoDy(pao)(NO)] (1), [CoDy(mepao)(NO)] (2), [CoDy(phpao)(NO)] (3) and [CoDy(NHpao)(NO)]·3MeOH (4·3MeOH) have been isolated and their structures have been determined by single-crystal X-ray crystallography. The complexes crystallize in non-centrosymmetric (2, 3) or centrosymmetric (1, 4·3MeOH) trigonal space groups and form a family of triply-oximate bridged dinuclear Co(iii)-Dy(iii) complexes. The crystals of 1, 3 and 4·3MeOH contain mixtures of Δ and Λ enantiomers, whereas complex 2 is enantiomerically pure (Λ). A 3-fold crystallographic axis (C) passes through two metal ions in all complexes. The low-spin Co and Dy ions are bridged by three oximate groups belonging to the η:η:η:μ 2-pyridyloximate ligands. The Co centre is octahedrally coordinated by the six nitrogen atoms of the deprotonated organic ligands in a facial arrangement. The Dy centre is bound to an O set of donor atoms, its coordination sphere being completed by three bidentate chelating nitrato groups. The coordination polyhedron around Dy in 1 is best described as the Johnson tricapped trigonal prism, while the coordination geometries of the Dy centres in 2, 3 and 4·3MeOH are best described as consisting of spherical tricapped trigonal prismatic coordination polyhedra. The spectroscopic data of the complexes are also reported and discussed in the infra-red region in terms of the coordination modes of the ligands involved. The magnetic properties of these complexes were studied between 300 and 1.8 K revealing mainly the depopulation of the Dym sublevels of the ground H state. The intrinsic magnetic anisotropy of the Dy centers is clearly observed by the non-superimposed magnetization (M) versus H/T data, but single-molecule magnet (SMM) properties were detected only for the mepao-containing complex 2. The origin of these properties in 2 is critically discussed and supported by computational studies.

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The reactions of copper(ii) carboxylate sources with acetylacetone dioxime (acacdoH) in MeCO have been studied and a novel, metal ion-assisted ligand transformation has been discovered. The reaction of [Cu(diba)(dibaH)] and acacdoH (1 : 1.5) in MeCO has provided access to the complex {[Cu(diba)(qunx)]} (1) in low yield (25-30%), where dibaH is 3,3-dimethylbutyric acid and qunx is quinoxaline. The [Cu(piv)(pivH)]/acacdoH (1 : 1.5) reaction system in warm MeCO, where pivH is pivalic acid, gave the analogous complex {[Cu(piv)(qunx)]} (2) in moderate yield (∼50%). Complexes 1 and 2 can be easily prepared by the direct 1 : 1 reactions between the corresponding copper(ii) carboxylate starting materials and qunx in MeCO and MeOH, respectively. The formation of coordinated qunx in 1 and 2 is Cu-promoted (assisted) as suggested by the failure to synthesize the free qunx by a variety of reactions of acacdoH and MeCO under aerobic conditions in the absence or even the presence of dibaH and pivH, respectively. The observed acacdoH → qunx transformation is catalytic and new in the chemistry of the dioximes of β-diketones, and a mechanism has been proposed based on well-established reactions of organic chemistry. The mechanism is based on a double Beckmann rearrangement-type transformation and the overall scheme is represented by the 1 : 1 : 1 reaction between acacdoH, MeCO and O. Complexes 1 and 2 have similar molecular structures consisting of paddle-wheel {Cu(η:η:μ-OCR)} units bridged by qunx ligands in a zigzag 1D chain arrangement. The geometry of the Cu ions is square pyramidal with a quinoxaline nitrogen atom occupying the apical position at each metal ion. Weak H bonds are present within the chains, the donors being qunx carbon atoms and the acceptors being coordinated carboxylate oxygen atoms. Neighbouring chains interact through C-Hπ interactions between diba/piv methyl groups and the "pyrazine" part of qunx forming layers which are stacked along the b (1) or a (2) axis through weak van der Waals interactions. The packing of the layers is different in the two structures, due to the different nature of the carboxylate ligands. Hirshfeld surface analysis of the two structures reveals the similarity of the interchain (intralayer) interactions. The IR and Raman data of 1 and 2 are discussed in terms of the coordination mode of the carboxylate groups and permit assignments of some characteristic bands/peaks of coordinated qunx. Dc magnetic susceptibility studies in the 1.8-310 K range reveal very strong antiferromagnetic CuCu exchange interactions within the carboxylate-bridged Cu units (J = -479 K for 1 and -532 K for 2 using the H = - J∑S·S spin Hamiltonian) and weaker antiferromagnetic interactions between the Cu units via the qunx superexchange pathways, with the latter being ∼10% in strength compared to the former. A critical discussion of the acacdoH → qunx transformation in 1 and 2 is provided in the light of other impressive, recently discovered Cu-assisted transformations of acacdoH, pointing out the key role of ...