First Cobalt(II) Spin Crossover Compound with N4S2-Donorset

Spin-crossover cobalt(II) complexes exhibiting temperature- and concentration-dependent optical changes in solution

This work investigated the spin states of the cobalt(II) complexes [Co(L1)2](X)2 (1·X; L1 = 4'-(4-N,N'-diphenylaminophenyl)-2,2':6',2''-terpyridine, X = PF6, BPh4) and [Co(L2)2](X)2 (2·X; L2 = 4'-(4-N,N'-dimethylaminophenyl)-2,2':6',2''-terpyridine, X = PF6, BPh4) in the solid state and in solution. In the solid state, 1·PF6 and 2·PF6, both containing smaller PF6- counter anions, showed gradual spin-crossover. In contrast, 1·BPh4 and 2·BPh4 remained in the high-spin state over the temperature range of 5-400 K due to a lower degree of molecular cooperativity. Each of the cobalt(II) complexes exhibited effects of temperature and concentration on their absorption spectra that were related to the spin states in various organic solvents. This work provides new insights into the spectroscopic properties resulting from the spin states of cobalt(II) complexes in solution.

Spin Crossover and Fluorine-Specific Interactions in Metal Complexes of Terpyridines with Polyfluorocarbon Tails

In coordination chemistry and materials science, terpyridine ligands are of great interest, due to their ability to form stable complexes with a broad range of transition metal ions. We report three terpyridine ligands containing different perfluorocarbon (PFC) tails on the backbone and the corresponding FeII and CoII complexes. The CoII complexes display spin crossover close to ambient temperature, and the nature of this spin transition is influenced by the length of the PFC tail on the ligand backbone. The electrochemical properties of the metal complexes were investigated with cyclic voltammetry revealing one oxidation and several reduction processes. The fluorine-specific interactions were investigated by EPR measurements. Analysis of the EPR spectra of the complexes as microcrystalline powders and in solution reveals exchange-narrowed spectra without resolved hyperfine splittings arising from the 59 Co nucleus; this suggests complex aggregation in solution mediated by interactions of the PFC tails. Interestingly, addition of perfluoro-octanol in different ratios to the acetonitrile solution of the sample resulted in the disruption of the F ⋯ ${\cdots }$ F interactions of the tails. To the best of our knowledge, this is the first investigation of fluorine-specific interactions in metal complexes through EPR spectroscopy, as exemplified by exchange narrowing.

Field-Induced Single Molecule Magnetic Behavior of Mononuclear Cobalt(II) Schiff Base Complex Derived from 5-Bromo Vanillin

A mononuclear Co(II) complex of a Schiff base ligand derived from 5-Bromo-vanillin and 4-aminoantipyrine, that has a compressed tetragonal bipyramidal geometry and exhibiting field-induced slow magnetic relaxation, has been synthesized and characterized by single crystal X-ray diffraction, elemental analysis and molecular spectroscopy. In the crystal packing, a hydrogen-bonded dimer structural topology has been observed with two distinct metal centers having slightly different bond parameters. The complex has been further investigated for its magnetic nature on a SQUID magnetometer. The DC magnetic data confirm that the complex behaves as a typical S = 3/2 spin system with a sizable axial zero-field splitting parameter D/hc = 38 cm−1. The AC susceptibility data reveal that the relaxation time for the single-mode relaxation process is τ = 0.16(1) ms at T = 2.0 K and BDC = 0.12 T.

Probing the redox-conversion of Co(II)-disulfide to Co(III)-thiolate complexes: the effect of ligand-field strength

The redox-conversion reaction of cobalt(II)-disulfide to cobalt(III)-thiolate complexes triggered by addition of the bidentate ligand 2,2'-bipyridine has been investigated. Reaction of the cobalt(II)-disulfide complex [Co2(L1SSL1)(X)4] (L1SSL1 = di-2-(bis(2-pyridylmethyl)amino)-ethyldisulfide; X = Cl or Br) [1X] with 2,2'-bipyridine (bpy) resulted in the formation of two different products, namely the cobalt(III)-thiolate complex [Co(L1S)(bpy)]X2 and the unexpected side product [Co2(L1SSL1)(bpy)2(X)2]X2. Crystals of [Co2(L1SSL1)(bpy)2(Cl)2](BPh4)2 [2Cl](BPh4)2 obtained after anion exchange showed the cobalt(II) ions to be in octahedral geometries with the nitrogen donors of the disulfide ligand arranged in a facial conformation and the chloride ion trans to the tertiary amine nitrogen. Remarkably, this side product cannot be converted to the cobalt(III)-thiolate compound [Co(L1S)(bpy)](SbF6)2 [3](SbF6)2 by removal of the chloride ion with use of a silver salt, as this causes scrambling of the ligands, resulting in the formation of [Co(bpy)3]n+. [Co(L1S)(bpy)](SbF6)2 was obtained in a pure form by addition of bpy to a solution in acetonitrile of the compound [Co(L1S)(MeCN)2]2+ [4]2+. Addition of NEt4Cl to [3](SbF6)2 regenerates the cobalt(II)-disulfide complex [1Cl] as confirmed spectroscopically. DFT studies revealed that the conversion from [1Cl] to [3]2+ most likely occurs via the hypothetical intermediate species [2Cl]2+mer, in which the nitrogen atoms of the disulfide ligand are arranged in a meridional conformation. Interestingly, the estimated d-orbital splitting energy of [3]2+ is lower than that of [4]2+, indicating that the ligand-field strength of bpy is lower than anticipated, which hampers clean conversion in the redox-conversion reaction. This study shows that the redox-conversion reaction between cobalt(II)-disulfide and cobalt(III)-thiolate complexes is intricate rather than straightforward.

Three Novel Thiazole-Arm Containing 1,3,4-Oxadiazole-Based [HS-HS] Fe(II) Dinuclear Complexes

Novel synthesis of 2,5-bis[(1,3-thiazol-2-ylmethyl)amino]methyl-1,3,4-oxadiazole (LThiazole) is reported, along with the preparation of three new dinuclear Fe(II) complexes with different counterions [FeII2(μ2-LThiazole)2](BF4)4·2CH3CN (1), [FeII2(μ2-LThiazole)2](ClO4)4 (2) and [FeII2(μ2-LThiazole)2](CF3SO3)4·2CH3CN (3). The obtained complexes were characterized by single-crystal X-ray crystallography, SQUID magnetometry and IR spectroscopy. The structure of the crystalline material was determined at 120 K for 1 and 3. The magnetic properties of all three complexes (1–3) were measured between 2–300 K and clearly show that all three complexes stay in the high-spin state over the measured temperature range.

Pronounced Magnetic Bistability in Highly Cooperative Mononuclear [Fe(Lnpdtz)2(NCX)2] Complexes

Molecular materials that exhibit stimuli-responsive bistability are promising candidates for the development of molecular switches and sensors. We herein report on the coexistence of a wide thermal hysteretic spin crossover (SCO) effect and a thermally inducible metastable high-spin state at low temperatures achieved with the two new complexes [Fe^II(L^npdtz)₂(NCX)₂] (X = S; Se), with L^npdtz being (2-naphthyl-5-pyridyl-1,2,4-thiadiazole) and X = S (1) and Se (2). Pronounced π-π-stacking of the aromatic side residues of the ligands enables strong intermolecular interactions, leading to abrupt SCO properties and broad magnetic hysteresis of 10 K for X = S and 58 K for X = Se. In this paper, we also present the pressure-induced spin-state switching around 0.8 GPa. A pronounced thermally induced excited spin state trapping (TIESST effect) is observed for the highly cooperative SCO compounds, which was experimentally followed by low-temperature single crystal structure analysis (20 K) and temperature-dependent Mössbauer spectroscopy.

Spin-state control of cobalt(II) and iron(II) complexes with click-derived tripodal ligands through non-covalent and fluorine-specific interactions

The fine-tuning of intermolecular or intramolecular non-covalent interactions (NCIs) and thus the precise synthesis of metal complexes in which the spin states can be controlled by NCIs remains challenging, even though several such complexes have been intensively studied. In this regard, we present mononuclear cobalt(II) and iron(II) complexes with "click"-derived tripodal ligands that contain fluorinated benzyl substituents in the secondary coordination sphere. The complexes were co-crystallized with different solvent molecules to decipher the effect of the crystallized solvents on NCIs, and on the spin state of the metal ion. Additionally, the fluorine-specific interactions in the secondary coordination sphere were examined. We present a first structure-property correlation between the nature of interaction of the (per)fluorinated aromatic substituents on the ligand periphery, and the spin state of the metal complexes. In particular, the TF₅TA containing ligand show interesting stacking motifs depending on the used solvent, and these interactions have an influence on the spin state of the cobalt(II) complexes. Furthermore, the iron(II) complex thereof, Fe(TF₅TA)₂(BF₄)₂·2EtOH displays spin crossover (SCO).

Spin Crossover and High-Spin State in Fe(II) Anionic Polymorphs Based on Tripodal Ligands

Two new mononuclear Fe(II) polymorphs, [(C₂H₅)₄N]₂[Fe(py₃C-OEt)(NCS)₃]₂ (1) and [(C₂H₅)₄N][Fe(py₃C-OEt)(NCS)₃] (2) (py₃C-OEt = tris(pyridin-2-yl)ethoxymethane), have been synthesized and characterized by single-crystal X-ray diffraction, by magnetic and photomagnetic measurements, and by detailed variable-temperature infrared spectroscopy. The molecular structure, in both complexes, is composed of the same anionic [Fe(py₃C-OEt)(NCS)₃]^- complex (two units for 1 and one unit for 2) generated by coordination to the Fe(II) metal center of one tridentate py₃C-OEt tripodal ligand and three terminal κN-SCN coligands. Magnetic studies revealed that polymorph 2 displays a high-spin (HS) state over the entire studied temperature range (300-10 K), while complex 1 exhibits an abrupt and complete spin crossover (SCO) transition at ca. 132.3 K, the structural characterizations of which, performed at 295 and 100 K, show a strong modification, resulting from the thermal evolutions of the Fe-N bond lengths and of the distortion parameters (∑ and Θ) of the FeN₆ coordination sphere, in agreement with the presence of HS and low-spin (LS) states at 295 and 100 K, respectively. This thermal transition has been also confirmed by the thermal evolution of the maximum absorbance for ν(NCS) vibrational bands recorded in the temperature range 200-10 K. In 1 the signature of a metastable photoinduced HS state has been observed using photomagnetic and photoinfrared spectroscopy, leading to a similar T(LIESST) relaxation temperature (LIESST = light-induced excited spin-state trapping) of 70 K.

Interplay of Jahn-Teller Ordering and Spin Crossover in Co(II) Compounds

The spin crossover phenomena in Co(II) compounds are in the focus of the present paper. A microscopic theoretical approach for the description of spin transitions in mononuclear Co(II) compounds is suggested. Within the framework of this approach there are taken into account two types of interionic interactions that may be operative in the problem such as the electron-deformational interaction and the cooperative Jahn-Teller interaction arising from the coupling of the low-spin state of the Co(II) ion with the tetragonal vibrations of the nearest surrounding. The different role of these interactions in the spin transformation is demonstrated and discussed. On the basis of developed approach a qualitative and quantitative explanation of the experimental data on the temperature dependence of the magnetic susceptibility for the [Co(pyterpy)2](PF6)2, [Co(pyterpy)2](TCNQ)2⋅DMF⋅MeOH and [Co(pyterpy)2](TCNQ)2⋅MeCN⋅MeOH compounds is given.