Synthesis and Applications of New Spin Crossover Compounds

The spin crossover (SCO) between multi-stable states in transition metal material is one of the attractive molecular switching phenomena which is responsive to various external stimuli such as temperature, pressure, light, electromagnetic field, radiation, nuclear decay, soft-X-ray, guest molecule inclusion, chemical environments and so forth [...]

complex series [8] and SCO Fe(II) imidazolylimine complexes [9] were reported in supramolecular crystal systems with delicate and subtle host-guest interactions. The synthesis, crystal chemistry, and photomagnetic properties of the SCO complexes with [Fe(bpp) 2 ] 2+ were researched.in a 3D supramolecular architecture, including hydrogen bonds between iron(II) complexes, nicotinate anions, and water molecules [10]. The 1,2,3-triazole-containing polydentate ligand iron(II) SCO family into a linear pentadentate ligand system was reported. This was shown in an abrupt and incomplete HT SCO at approximately 400 K while the SCO transition was irreversible due to the crystal-to-amorphous transformation in association with the loss of the lattice MeCN solvent molecules [11]. A series of SCO Fe(II) complexes based on dipyridyl-N-alkylamine and thiocyanate ligands were investigated, and the higher SCO transition temperature explained the more pronounced linearity of the Fe-N-C angles in the crystal recently indicated by experimental and theoretical magneto-structural research [12].
A particularly successful and potentially developing synthetic kingdom for SCO iorn(II) polymeric complexes with valuable and sophisticated functional crystal properties are the SCO Hofmann-type coordination polymers. The first compound of this type Fe(pyridine) 2 Ni(CN) 4 reported in 1996 [13], opened various roads to a number of Hofmann-like SCO compounds with a large display of functional properties. The special issue contains 4 original research articles which are devoted to the synthesis and characterizations of various Hofmann-like polymeric systems. The optical microscopy technique to investigate the thermal and the spatio-temporal properties of the Hofmann-related SCO single crystal [Fe(2-pytrz) 2 {Pt(CN) 4 }]_3H 2 O was described to show a first-order SCO behavior from a full high-spin (HS) state at high temperatures to intermediate, high-spin low-spin (HS-LS) states [14]. The precise crystallographic investigation on the polymeric SCO Hofmann-like compound Fe(3,4-dimethyl-pyridine) 2 [Ag(CN) 2 ] 2 was reported, and its temperature dependence was followed by the means of a single-crystal and powder X-ray diffraction [15]. These very important article reported in the special issue demonstrate a soft X-ray-induced excited spin state trapping (SOXEISST) effect in Hofmann-like SCO coordination polymers of Fe II (4-methylpyrimidine) 2 [Au(CN) 2 ] 2 and Fe II (pyridine) 2 [Ni(CN) 4 ] [16]. The emission Mossbauer spectra of 57 Co-labelled Co(pyridine) 2 Ni(CN) 4 indicated that 57 Fe atoms were assumed to be trapped in the excited electronic state ( 5 T) by the nuclear decay induced excited spin state trapping (NIESST) effect [17]. The two SCO coordination polymers built up by the Hofmann-like frameworks combining Fe II octahedral ions, 4-cyanopyridine and [Au(CN) 2 ] − liner units were described exhibiting ferromagnetic interaction [18]. A single crystal X-ray structural analysis showed that polymeric [[Fe(NCS) 2 (bpa) 2 ]·biphenyl] n and [[Co(NCS) 2 (bpa) 2 ]·biphenyl] n had a chiral propeller structure of pyridines around the central metal, which was associated with crystal chemistry and their SCO phenomena for the [[Fe(NCX) 2 (bpa) 2 ]_(guest)] n family [19].
The Fe(III) SCO compounds are also important and attractive compounds as smart materials with multifunctional properties. The influence of geometry and counterion effects in determining the spin states in an iron (III) complex [Fe(5F-sal 2 333)]X was investigated using a crystal analysis, UV-Vis spectroscopy, SQUID and EPR spectroscopy. The R-sal 2 333 ligands promoting SCO in Fe(III) sites both in the solid state and in solution was established [20]. The hybrid ion-pair crystals containing hexadentate [Fe(III)(3-OMesal 2 -trien)] + SCO cationic coordination units and anionic gold complex units [Au(dmit) 2 ] − and [Au(dddt) 2 ] − were investigated by a single-crystal X-ray diffraction method, P-XRD, and SQUID measurements [21]. Fe(III) SCO compounds from qsal ligand (Hqsal = N-(8-quinolyl)salicylaldimine) were described. The optical conductivity spectra were calculated from the single-crystal reflection spectra, which were, to the best of their knowledge, the first optical conductivity spectra of SCO complexes [22] Finally, the contribution of all the authors for sending their works is greatly appreciated. The Special Issue, "Synthesis and Applications of New Spin Crossover Compounds" presents a comprehensive report on the current work on SCO materials and will be interesting for the readers. The author is also deeply grateful to all the anonymous reviewers for their valuable suggestions and very dedicated evaluations, which have been very helpful for improving the quality of the Special Issue. The author thanks the editorial staff for their valuable efforts in the planning, review processes and publication of this Special Issue.
In addition, the readers' submission of their valuable papers to the Special Issue "Synthesis and Applications of New Spin Crossover Compounds (Volume II)" would be further appreciated.

Conflicts of Interest:
The author declares no conflict of interest.