A series of cyanide bridged Fe-Co molecular squares, [Co(2)Fe(2)(CN)(6)(tp*)(2)(dtbbpy)(4)](PF(6))(2)·2MeOH (1), [Co(2)Fe(2)(CN)(6)(tp*)(2)(bpy)(4)](PF(6))(2)·2MeOH (2), and [Co(2)Fe(2)(CN)(6)(tp)(2)(dtbbpy)(4)](PF(6))(2)·4H(2)O (3) (tp = hydrotris(pyrazol-1-yl)borate, tp* = hydrotris(3,5-dimethylpyrazol-1-yl)borate, bpy =2,2'-bipyridine, dtbbpy =4,4'-di-tert-butyl-2,2'-bipyridine), were prepared by the reactions of [Fe(CN)(3)(L)](-) (L = tp or tp*) with Co(2+) and bidentate ligands (bpy or dtbbpy) in MeOH. In the molecular squares, Fe and Co ions are alternately bridged by cyanide ions, forming macrocyclic tetranuclear cores. Variable temperature X-ray structural analyses and magnetic susceptibility measurements confirmed that 1 exhibits two-step charge-transfer induced spin transitions (CTIST) centered at T(1/2) = 275 and 310 K in the solid state. The Fe and Co ions in 1 are the low-spin (LS) Fe(III) and high-spin (HS) Co(II) ions, described here in the high-temperature (HT) phase ([Fe(III)(LS2)Co(II)(HS2)]) at 330 K, while a low-temperature (LT) phase ([Fe(II)(LS2)Co(III)(LS2)]) with LS Fe(II) and Co(III) ions was dominant below 260 K. X-ray structural analysis revealed that in the intermediate (IM) phase at 298 K 1 exhibits positional ordering of [Fe(III)(LS2)Co(II)(HS2)] and [Fe(II)(LS2)Co(III)(LS2)] species with the 2:2 ratio. In photomagnetic experiments on 1, light-induced CTIST from the LT to the HT phase was observed by excitation of Fe(II) → Co(III) intervalence charge transfer (IVCT) band at 5 K and the trapped HT phase thermally relaxed to the LT phase in a two-step fashion. On the other hand, 2 and 3 are in the HT and LT phases, respectively, throughout the entire temperature range measured, and no CTIST was observed. UV-vis-NIR absorption spectral measurements and cyclic voltammetry in solution revealed that the different electronic states in 1-3 are ascribable to the destabilization of iron and cobalt ion d-orbitals by the introduction of methyl and tert-butyl groups to the ligands tp and bpy, respectively. Temperature dependence of UV-vis-NIR spectra confirmed that 1 exhibited a one-step CTIST in butyronitrile, of which T(1/2) varied from 227 to 280 K upon the addition of trifluoroacetic acid.

Co-crystallization of a cyanide-bridged tetranuclear complex [Co Fe ] with 4-cyanophenol (CP) gave a hydrogen bonding donor-acceptor system, [Co Fe (bpy*) (CN) (tp*) ](PF ) ⋅2 CP⋅8 BN (1). 1 exhibited a three-step phase transition between HT, IM1, IM2, and LT phases upon temperature variation. Variable temperature magnetic measurements and structural analyses revealed that the three-step spin transition is caused by electron-transfer-coupled spin transitions (ETCSTs) accompanied with alteration of the hydrogen bonding interactions.

A novel tetraoxolene-bridged Fe two-dimensional honeycomb layered compound, (NPr ) [Fe (Cl An) ] ⋅2 (acetone)⋅H O (1), where Cl An =2,5-dichloro-3,6-dihydroxy-1,4-benzoquinonate and NPr =tetrapropylammonium cation, has been synthesized. 1 revealed a thermally induced valence tautomeric transition at T =236 K (cooling)/237 K (heating) between Fe (m=2 or 3) and Cl An (n=2 or 3) that induced valence modulations between [Fe Fe (Cl An ) (Cl An )] at T>T and [Fe Fe (Cl An )(Cl An ) ] at T

Discrete cyanide-bridged Co−Fe multinuclear complexes can be considered as functional units of bulk Co− Fe Prussian blue analogues, and they have been recognized as a new class of switching molecules in the last decade. The switching property of the cyanide-bridged Co−Fe complexes is based on intramolecular electron transfers between Co and Fe ions, and we herein refer to this phenomenon as an electron transfer-coupled spin transition (ETCST). Although there have been numerous reports on the complexes exhibiting ETCST behavior, the systematic study of the substituent effects on the thermal ETCST equilibrium in solution has not been reported yet, and the rational control of the equilibrium temperature remains challenging. We report here the syntheses and thermal ETCST behavior both in the solid state and solution for a series of tetranuclear [Co 2 Fe 2 ] complexes, [Co 2 Fe 2 (CN) 6 (L1) 2 (L2) 4 ]X 2 (L1 and L2: tri-and bidentate capping ligands for Fe and Co ions, X: counteranions). All complexes showed thermal ETCST equilibrium between high-spin ([(hs-Co II ) 2 (ls-Fe III ) 2 ]) and low-spin ([(ls-Co III ) 2 (ls-Fe II ) 2 ]) states in butyronitrile, and the equilibrium temperatures (T 1/2 ) showed systematic shifts by chemical modifications and chemical stimuli. The T 1/2 values were correlated with the redox potential differences (ΔE) of the Fe and Co ions in the constituent units, and the larger ΔE values led to the lower T 1/2 . The present result suggests that the thermal ETCST behavior in solution can be rationally designed by considering the redox potentials of the constituent molecules.

Single-molecule magnets: A cyanide-bridged hexanuclear complex showed a thermal electron-transfer-coupled spin transition centered at 220 K. Light irradiation at low temperature (LT; HT = high temperature) generated a metastable state showing slow magnetic relaxation in measurements of the alternating-current magnetic susceptibility (χ(m); see picture).