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Bio-Based Magnetic Metal-Organic Framework Nanocomposites
Published in Anish Khan, Mohammad Jawaid, Abdullah Mohammed Ahmed Asiri, Wei Ni, Mohammed Muzibur Rahman, Metal-Organic Framework Nanocomposites, 2020
Manickam Ramesh, Mayakrishnan Muthukrishnan
Spin crossover is one of the interesting phenomena found due to the movement of transition metal ions in magnetic MOFs with variable pore sizes. It is also called spin transition or spin equilibrium behavior which results in response to external stimuli by switching electronic properties of transition metal ions’ atomic orbitals from a high spin (HS) state to low spin (LS) state [38]. The HS or LS electron configuration is determined by the magnitude of the ligand field splitting in association with the pairing strength of the configuration complexes whereas a HS state occurs when the complex’s pairing energy is greater than the ligand splitting and is a favorable process for spin crossover [39]. Some of the external stimuli are magnetic field, light, temperature, pressure, soft X-ray, guest molecule inclusion, chemical environments’ electric field, etc., which result in light-induced excited spin-state trapping (LIESST), ligand-driven light-induced spin change (LD-LISC), charge transfer-induced spin transition (CTIST), soft X-ray-induced excited spin state trapping (SOXIESST), etc. Spin crossover eliminates the necessity of designing MOFs with the required interactions through interfaces between adjacent magnetic centers. Instead, the necessary ligand field can be achieved using appropriate first-row transition metals that respond to the required transition from HS to LS. The advantages are i) the length of the ligand field is not constrained and ii) no metal center connectivity. The SCO phenomenon associated with inorganic chemical materials provides wider applications in the research areas of memory devices, displays, sensors, and artificial muscle actuators. Similarly, multifunctional SCO material like cobalt (II) terpyridin-4′-yl nitroxide complex is one of the MOFs with successful magnetic exchange coupling interactions [40]. A dithio oxalato-bridged iron mixed-valence complex also reported for its affirmative FeII and FeIII sites for electron ions phase transfer [41]. For smart materials, octahedral Fe(II) systems are widely investigated. Fe(2-pytrz)2{Pt(CN)4}]_3H2O is proven to exhibit HS spin behavior at high temperature.
Electronic Properties of Perovskite Oxides
Published in Gibin George, Sivasankara Rao Ede, Zhiping Luo, Fundamentals of Perovskite Oxides, 2020
Gibin George, Sivasankara Rao Ede, Zhiping Luo
Further, there are some notable factors that govern the crystal field splitting energies in transition elements. 3d Elements tend to have high spin states because their small crystal field splitting energies correspond to the electron–electron repulsions (Harvey et al. 2003). In contrast, 4d/5d elements preferably found in low spin states because of their high crystal field splitting energies and negligible inter electronic repulsions. Another reason is that high cation charge of 4d/5d elements results in high crystal field splitting energies (Lee et al. 2003). The higher cation charge means that 4d/5d cation sites tend to have fewer electrons, and often they found in t2g orbitals, while in 3d the electrons often filled in eg orbitals owing to their lower oxidation states, and this occurrence may hinder electron transfer because of the symmetry. The low spin state is also possible with 3d elements, especially with cobalt, by the weakening of crystal field splitting of B′ cation through the decrease of electron density around it due to the strong covalency of B″–O bond (Taguchi 1996). For example, Ba2CoB″O6 with B″ = Nb or Ta and La2CoFeO6 are proposed to have low or intermediate spin Co3+ cation (Yoshii 2000). Comparing unit cell volume of a series of R2B′PtO6 (B′ = Mg, Co, Ni or Zn) compounds reveals that Co yields a low unit cell volume compared to other compounds, which confirms the low spin or intermediate spin state of Co (Vasala and Karppinen 2015). Spin orbital coupling (SOC) is another factor that regulates the electronic structure 4d/5d B-site cations in double perovskites (Chen et al. 2010). In the case of 3d B-site cations, SOC effect is negligible because it is much weaker compared to other energy parameters (Svoboda et al. 2017). For instance, in the case of Ir4+ and Ir5+ B-site cations, the SOC found to split the degenerated t2g orbitals (Narayanan 2010). Nevertheless, the crystal structure has prime importance in governing electronic structure by lifting the degeneracy of d-orbitals and orbital moment quenching through crystal structure distortions (Varignon et al. 2019).
Synthesis, characterization, and structural studies of two heteroleptic Mn(II) complexes with tridentate N,N,N-pincer type ligand
Published in Journal of Coordination Chemistry, 2018
Saied M. Soliman, Ayman El-Faham
Moreover, pincer ligands comprising s-triazine attached with two pyrazole rings are considered powerful N,N,N-chelators. One of the advantages of the s-triazine pincer ligands is the ease of preparation from cyanuric chloride. In addition, they are considered as electron-deficient ligands, which can offer donor N-atoms to form high spin complexes with interesting magnetic properties. The presence of the aromatic s-triazine allows the formation of interesting coordination compounds in the field of supramolecular chemistry, in which the molecular packing involves weak intermolecular forces such as π–π and anion–π interactions [20–22]. Hence, these polydentate ligands have a crucial role in coordination and supramolecular chemistry. In this work, two new octahedral Mn(II) complexes with N,N,N-tridentate pincer ligand were synthesized and characterized. The effect of the anion on their molecular and supramolecular structure is discussed. The nature and strength of the coordinate bonds are explained in the light of the atoms in molecules (AIM) theory.
Iron(III) chloride and its coordination chemistry
Published in Journal of Coordination Chemistry, 2018
Because of its d5 electron configuration, the high-spin Fe3+ ion, present in nearly all the complexes examined here, has no ligand-field effects driving the adoption of a particular coordination number or geometry. The d0 Sc3+ ion also forms a number of hydrated chloride complexes, again a case where no ligand-field factors enter in. ScCl3·7H2O contains [Sc(H2O)7]3+ ions [155], ScCl3·6H2O has the structure trans-[Sc(H2O)4Cl2]Cl·2H2O [156], analogous to the corresponding compounds of iron and chromium, and mer-[ScCl3(H2O)3] is trapped by a cryptate ligand [157]. The Sc–OH2 distances in mer-[ScCl3(H2O)3] range from 2.078 to 2.155 Å, with a mean of 2.122 Å; this moiety also occurs in mer-[ScCl3(H2O)3]·18-crown-6 [158], where the Sc–O distances of 2.142(1)–2.177(2) Å are again on the long side for six-coordinate scandium, and may reflect congestion introduced by the chloride ions. This isomerism phenomenon is not confined to d0 and d5 cases, though, as the d7 Co2+ ion illustrates. Crystalline CoCl2·6H2O is trans-[CoCl2(H2O)4]·2H2O, whilst solid CoCl2·4H2O contains cis-[CoCl2(H2O)4] molecules, as stronger hydrogen-bonding between chlorides and the hydrogen in water in the adjacent CoCl2(H2O)4 molecule in the lattice make the cis- structure more favorable [159].
Phase behaviour and magnetocaloric effect of poly(propylene imine) Iron(III) dendromesogen of the third generation
Published in Liquid Crystals, 2021
V.V. Korolev, M.S. Gruzdev, A.G. Ramazanova, O.V. Balmasova, U.V. Chervonova
The spin state of Fe(III) ions was determined based on EPR data and Mossbauer spectroscopy [33]. It was established that the Fe(III) ion is in a high-spin state with 6S5/2 (3d5) electron configuration at temperatures below 100 K. However, a spin transition occurs between the low spin and high-spin centres in the complex when the temperature increases (above 100 K). Based on the presented data, we studied the magnetocaloric effect of the complex for the first time by the calorimetric method in magnetic fields from 0 to 1.0 T and in the temperature range of 263–358 K.