Spin and valence isomerism in cyanide-bridged {FeMII} (M = Fe and Co) clusters

Electron transfer coupled spin transition in cyano-bridged mixed-valence {FeIII2FeII2} molecular squares

The research on electron transfer coupled spin transition regulating the valence state and spin state transition of metal ions is promising and challenging. Herein, we report a cyano-bridged {FeIII2FeII2} molecular square complex, {[Fe(Tp)(CN)3]2 [Fe(bnbpen)]2}(ClO4)2·8CH3OH (1·8CH3OH, bnbpen = N,N'-bis-(2-naphthylmethyl)-N,N'-bis(2-picolayl)-ethylenediamine), and its free of solvents form (1). Combined single-crystal X-ray diffraction, temperature-dependent infrared (IR) spectra, magnetic measurements, and Mössbauer spectra reveal that 1·8CH3OH and 1 exhibit reversible one-step and two-step electron transfer coupled spin transition (ETCST) with temperature change, between the low-temperature state {FeII,LS(μ-CN)FeIII,LS}2 (LS = low spin, HS = high spin) and the high-temperature state {FeIII,LS(μ-CN)FeII,HS}2, respectively.

Observation of protonation-induced intra-molecular metal-to-metal charge transfer in cyano-bridged [Fe-CN-Mn/Ni] dinuclear complexes

A key challenge in the design of magnetic molecules with intramolecular charge transfer behavior is to obtain reversible magnetic bistability triggered by external stimuli. Here, we show that two dinuclear metal complexes, [(bbp)Fe(CN)3Mn(Py5Me2)]·2.5CH3OH (4) and [(bbp)Fe(CN)3Ni(Py5Me2)]·2.5CH3OH (5) (Py5Me2 = 2,6-bis(1,1-di(pyridine-2-yl)ethyl)pyridine, H2bbp = 2,6-bis(benzimidazole-2-yl)pyridine), were self-assembly synthesized by (Bu4N)2[(bbp)FeIII(CN)3] and [Mn(Py5Me2)(OH2)](ClO4)2 or [Ni(Py5Me2)(OH2)](ClO4)2, respectively. Complexes 4 and 5 exhibited intramolecular metal-to-metal charge transfer with the addition of acids or bases in solution by UV-visible spectrophotometric measurements and electrochemistry studies, and concomitant switching of the {FeIII(μ-CN)MnII/NiII} state to the {FeII(μ-CN)MnIII/NiIII} state.

Thermo- and photo-induced electron transfer in a series of [Fe2Co2] capsules

Recently, a family of [Fe₂Co₂] molecular capsules that display tunable electron transfer-coupled spin transition (ETCST) behavior were reported via a smart approach through Schiff-base condensation of aldehyde-functionalized 2,2-bipyridines (bpy^CHO) and 1,7-heptanediamine (H₂N(CH₂)₇NH₂). Here, three more capsule complexes {[(Tp^R)Fe(CN)₃]₂[Co(bpy^{CN(CH₂)_nNC}bpy)]₂[ClO₄]₂}·n(solvent) (1, Tp^R = Tp*, n = 5, sol = 8DMF; 2, Tp^R = Tp^Me, n = 9, sol = 5MeCN; and 3, Tp^R = Tp*, n = 11, sol = 5MeCN), where Tp* = hydridotris(3,5-dimethylpyrazol-1-yl)borate and Tp^Me = hydridotris(3-methylpyrazol-1-yl)borate are reported, demonstrating a successful extension of such an approach with other alkyldiamines of different lengths. Combined X-ray crystallographic, infrared spectroscopic and magnetic studies reveal incomplete electron transfer with either changing temperature or upon light exposure.

Crystallization Kinetics of a Liquid-Forming 2D Coordination Polymer

We investigated a mechanism of crystal melting and crystallization behavior of a two-dimensional coordination polymer [Ag₂(L1)(CF₃SO₃)₂] (1, L1 = 4,4'-biphenyldicarbonitrile) upon heating-cooling processes. The crystal showed melting at 282 °C, and the following gentle cooling induced the abrupt crystallization at 242 °C confirmed by DSC. A temperature-dependent structural change has been discussed through calorimetric, spectroscopic, and mechanical measurements. They indicated that the coordination-bond networks are partially retained in the melt state, but the melt showed a significantly low viscosity of 9.8 × 10^-2 Pa·s at T_m which is six orders lower than that of ZIF-62 at T_m (435 °C). Rheological studies provided an understanding of the fast relaxation dynamics for the recrystallization process, along with that the high T_m provides enough thermal energy to crossover the activation energy barrier for the nucleation. The isothermal crystallization kinetics through calorimetric measurements with applying the Avrami equation identified the nature of the nuclei and its crystal growth mechanism.

Tuning the electron transfer events in a series of cyanide-bridged [Fe2Co2] squares according to different electron donors

It has been recognized that both the ligand fields and intermolecular interactions may greatly impact the electron-transfer-coupled spin transition (ETCST) events in switchable magnetic materials; however, the engineering of these factors within a given system is still challenging. In this article, we chose the 4,4'-substituent 2,2'-bipyridine derivatives as chelating ligands according to their increasing electron-donating strength and incremental potential for forming hydrogen bonds (bpy^CHO,CH₃(L1) < bpy^{CH₂OH,CH₃} (L2) < bpy^{CH₂OH,CH₂OH} (L3)), and prepared three new [Fe₂Co₂] complexes, {[(Tp*)Fe(CN)₃Co(L)₂]₂[ClO₄]₂}·Sol (1, L = L1, Sol = 4MeCN·2H₂O; 2, L = L2, Sol = 3MeCN; 3, L = L3, Sol = 4MeOH; Tp* = hydrotris(3,5-dimethylpyrazol-1-yl)borate). X-ray crystallography studies revealed that all the complexes share similar cyanide-bridged [Fe₂Co₂] square compositions except for the different substituted groups of L ligands, which led to the clearly evidenced intercluster hydrogen bonds between the neighbouring hydroxyl groups in 2 and 3. As a result, 1 remained in the paramagnetic [FeIII,LS2CoII,HS2] state over the whole temperature range, while 2 and 3 showed complete ETCST behaviour with the transition temperatures (T_1/2) being 221 and 294 K, respectively.

[AuI(CN)2]-Armed [FeIII2FeII2] Square Complex Showing Unusual Spin-Crossover Behavior Due to a Symmetry-Breaking Phase Transition

The incorporation of two different cyanide building blocks of [(Tp^R)Fe^III(CN)₃]^- and [Au^I(CN)₂]^- into one molecule afforded a novel hexanuclear [Fe^III₂Fe^II₂Au^I₂] complex (1·2Et₂O), in which the cyanide-bridged [Fe^III₂Fe^II₂] square was further grafted by two [Au^I(CN)₂]^- fragments as long arms in syn orientations. Complex 1·2Et₂O undergoes a gradual spin crossover (SCO) ffrom low-spin (LS) to high-spin (HS) state for the Fe(II) centers upon desolvation. Remarkably, its desolvated phase (1) exhibits a reversible but atypical two-step (sharp-gradual) SCO behavior with considerable hysteresis (21 K). Variable-temperature single-crystal X-ray structural studies reveal that the hysteretic spin transition takes place synchronously with the concerted displacive motions of the molecules, representing another rare example including multistep and hysteretic spin transitions due to the synergetic SCO and structural phase transition.

Heterobimetallic cyanide-bridged FeIII(μ-CN)MII complexes (M = Mn and Cu): synthesis, structure and magnetism

Systematic magneto-structural studies reveal ‘[FeIII(Tp)(CN)2(μ-CN)MnIICl(L1/L2)]′ (both antiferromagnetic) and [FeIII(Tp)(CN)2(μ-CN)CuII(L1/L2)]+′(L1 ferro- and L2 antiferromagnetic).

Manipulating the spin crossover behaviour in a series of cyanide-bridged {FeIII2FeII2} molecular squares through NCE- co-ligands

Manipulating the transition temperature (T1/2) of spin-crossover (SCO) complexes capable of fulfilling practical criteria through different synthetic strategies is one of the main focuses in the field of molecular magnetism....