scholarly journals Molecular S = 2 High-Spin, S = 0 Low-Spin and S = 0 ⇄ 2 Spin-Transition/-Crossover Nickel(II)-Bis(nitroxide) Coordination Compounds

Inorganics ◽  
2021 ◽  
Vol 9 (2) ◽  
pp. 10
Author(s):  
Takayuki Ishida ◽  
Saki Ito ◽  
Yuta Homma ◽  
Yukiya Kyoden

Heterospin systems have a great advantage in frontier orbital engineering since they utilize a wide diversity of paramagnetic chromophores and almost infinite combinations and mutual geometries. Strong exchange couplings are expected in 3d–2p heterospin compounds, where the nitroxide (aminoxyl) oxygen atom has a direct coordination bond with a nickel(II) ion. Complex formation of nickel(II) salts and tert-butyl 2-pyridyl nitroxides afforded a discrete 2p–3d–2p triad. Ferromagnetic coupling is favored when the magnetic orbitals, nickel(II) dσ and radical π*, are arranged in a strictly orthogonal fashion, namely, a planar coordination structure is characterized. In contrast, a severe twist around the coordination bond gives an orbital overlap, resulting in antiferromagnetic coupling. Non-chelatable nitroxide ligands are available for highly twisted and practically diamagnetic complexes. Here, the Ni–O–N–Csp2 torsion (dihedral) angle is supposed to be a useful metric to describe the nickel ion dislocated out of the radical π* nodal plane. Spin-transition complexes exhibited a planar coordination structure in a high-temperature phase and a nonplanar structure in a low-temperature phase. The gradual spin transition is described as a spin equilibrium obeying the van’t Hoff law. Density functional theory calculation indicates that the energy level crossing of the high- and low-spin states. The optimized structures of diamagnetic and high-spin states well agreed with the experimental large and small torsions, respectively. The novel mechanism of the present spin transition lies in the ferro-/antiferromagnetic coupling switch. The entropy-driven mechanism is plausible after combining the results of the related copper(II)-nitroxide compounds. Attention must be paid to the coupling parameter J as a variable of temperature in the magnetic analysis of such spin-transition materials. For future work, the exchange coupling may be tuned by chemical modification and external stimulus, because it has been clarified that the parameter is sensitive to the coordination structure and actually varies from 2J/kB = +400 K to −1400 K.

Symmetry ◽  
2021 ◽  
Vol 13 (5) ◽  
pp. 828
Author(s):  
Mamadou Ndiaye ◽  
Nour El Islam Belmouri ◽  
Jorge Linares ◽  
Kamel Boukheddaden

The jungle of experimental behaviors of spin-crossover materials contains a tremendous number of unexpected behaviors, among which, the unsymmetrical hysteresis loops having different shapes on heating and cooling, that we often encounter in literature. Excluding an extra effect of crystallographic phase transitions, we study here these phenomena from the point of view of elastic modeling and we demonstrate that a simple model accounting for the bond lengths misfits between the high-spin and low-spin states is sufficient to describe the situation of unsymmetrical hysteresis showing plateaus at the transition only on cooling or on heating branches. The idea behind this effect relates to the existence of a discriminant elastic frustration in the lattice, which expresses only along the high-spin to low-spin transition or in the opposite side. The obtained two-step transitions showed characteristics of self-organization of the spin states under the form of stripes, which we explain as an emergence process of antagonist directional elastic interactions inside the lattice. The analysis of the spin state transformation inside the plateau on cooling in terms of two sublattices demonstrated that the elastic-driven self-organization of the spin states is accompanied with a symmetry breaking.


2010 ◽  
Vol 9 (3) ◽  
pp. 432-436
Author(s):  
Yusthinus T. Male ◽  
Djulia Onggo ◽  
Muhamad A. Martoprawiro ◽  
Ismunandar Ismunandar

Quantum chemical studies have been carried out on the Fe(en)2(NCS)2 (en = ethylenediamine) complex both in low and high spin states (S = 0 and S = 2) using hybrid exchange-correlation functional (B3LYP) and non-hybrid method (BLYP). Calculations were performed in vacuum and in methanol to study the effect of cis-trans geometry on the structure and energy difference between low-spin (LS) and high-spin (HS) states of iron (II) complexes. Full geometry optimizations of the complexes show that hybrid method consistently gives higher energy difference between LS and HS states than the nonhybrid methods. Calculations with reparameterized density functional theory that showed more reasonable electronic energy splittings in previous research was also carried out. In addition, the computational study of Fe(en)2(NCS)2 in vacuum and methanol with PCM method showed that the complexes tend to adopt cis geometry. This geometry showed much less charge transfer in the substitutions of NCS- ligands compare to trans geometry.   Keywords: Electronic structure, spin states, density functional, frontiers orbitals


Author(s):  
Raphael Johann Friedrich Berger ◽  
Georgiy V. Girichev ◽  
Angelika A. Petrova ◽  
Valery V. Sliznev ◽  
Nataliya V. Tverdova ◽  
...  

Quantum chemical calculations of the geometric structure, force fields and harmonic vibration frequencies of the molecule Mn(acac)3 for electronic states with multiplicities M = 1, 3 and 5 were performed using the GAUSSIAN 09 program in the framework of density functional theory (DFT/UB3LYP) with correlation-consistent valence three-exponential basis functions cc-pVTZ. The structure with high-spin state S=2 (symmetry of electronic state 5B) possesses the lowest energy and it is characterized by C2 symmetry. The coordination polyhedron MnO6 possesses the shape of “elongated octahedron”. The high-spin state 5A is characterized by structure of compressed octahedron. The distortion of octahedral structure of coordination polyhedron in the states 5A and 5B is significant, and this fact testifies to the strong Jahn-Teller effect, or vibronic effect, in 5E electronic state. The calculations for low-spin state S=0 are notable for some specifics. The optimization resulted in C2 symmetry of molecule having the symmetry of electronic state 1B. The bond distances Mn-O within 0.001 Å were equal to values obtained for structure with D3 symmetry with S=1. This result corresponds to the situation if two electrons occupy different 1e orbitals possessing opposite spins. The spin states 3A2 and 1B lie higher than the high-spin state by 5.2 and 17.3 kcal/mol, respectively. The structural features are explained well in a framework of simple crystal field theory indicating that d-orbitals of Mn3+ ion undergo the significant influence of ligand field.Forcitation:Berger R.J.F., Girichev G.V., Giricheva N.I., Petrova A.A., Sliznev V.V., Tverdova N.V. Molecular structure of manganese tris-acetylacetonate in different spin states. Izv. Vyssh. Uchebn. Zaved. Khim. Khim. Tekhnol. 2017. V. 60. N 4. P. 47-53.


2000 ◽  
Vol 80 (2) ◽  
pp. 155-163 ◽  
Author(s):  
S. Odin, F. Baudelet, E. Dartyge, J. P

1980 ◽  
Vol 41 (C10) ◽  
pp. C10-143-C10-154 ◽  
Author(s):  
A. Faessler

CrystEngComm ◽  
2021 ◽  
Vol 23 (15) ◽  
pp. 2854-2861
Author(s):  
Kristian Handoyo Sugiyarto ◽  
Djulia Onggo ◽  
Hiroki Akutsu ◽  
Varimalla Raghavendra Reddy ◽  
Hari Sutrisno ◽  
...  

Mononuclear complex [Fe(3-bpp)2](CF3COO)2 exhibits a thermal (HS + HS) ⇋ (HS + LS) transition at ∼226 K which is not associated with any crystallographic transition.


2021 ◽  
Vol 103 (10) ◽  
Author(s):  
Tharathep Plienbumrung ◽  
Maria Daghofer ◽  
Andrzej M. Oleś

2021 ◽  
Vol 103 (2) ◽  
Author(s):  
Junwei Tong ◽  
Feifei Luo ◽  
Liuxia Ruan ◽  
Guohuai Liu ◽  
Lianqun Zhou ◽  
...  

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