Spin-crossover Behavior Research Articles

Bulky anion supported a five-coordinate spin-crossover cobalt(II) complex with slow magnetic relaxation

In this work, using a bulky anion, a mononuclear five-coordinate Co(II) complex [Co(12-TMC)(CH3CN)][B(C6F5)4]2·CH2Cl2 (12-TMC ​= ​1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane) (1) has been constructed. X-ray single crystal diffraction shows that the central Co(II) ion of 1 binds with five N atoms, generating a distorted square pyramidal geometry. The direct-current (dc) magnetic data of 1 displayed a gradual and incomplete spin-crossover behaviour. Compared with reported complexes [Co(12-TMC)(CH3CN)](X)2 (X ​= ​BF4−, 2; and PF6−, 3), complex 1 exhibited much slower magnetic relaxation under an applied dc field of 1000 ​Oe, which originated from the low-spin state (S ​= ​1/2) of Co(II) ion.

Epitaxial Thin-Film vs Single Crystal Growth of 2D Hofmann-Type Iron(II) Materials: A Comparative Assessment of their Bi-Stable Spin Crossover Properties.

Integration of the ON-OFF cooperative spin crossover (SCO) properties of FeII coordination polymers as components of electronic and/or spintronic devices is currently an area of great interest for potential applications. This requires the selection and growth of thin films of the appropriate material onto selected substrates. In this context, two new series of cooperative SCO two-dimensional FeII coordination polymers of the Hofmann-type formulated {FeII(Pym)2[MII(CN)4]·xH2O}n and {FeII(Isoq)2[MII(CN)4]}n (Pym = pyrimidine, Isoq = isoquinoline; MII = Ni, Pd, Pt) have been synthesized, characterized, and the corresponding Pt derivatives selected for fabrication of thin films by liquid-phase epitaxy (LPE). At ambient pressure, variable-temperature single-crystal X-ray diffraction, magnetic, and calorimetric studies of the Pt and Pd microcrystalline materials of both series display strong cooperative thermal induced SCO properties. In contrast, this property is only observed for higher pressures in the Ni derivatives. The SCO behavior of the {FeII(L)2[PtII(CN)4]}n thin films (L = Pym, Isoq) were monitored by magnetization measurements in a SQUID magnetometer and compared with the homologous samples of the previously reported isostructural {FeII(Py)2[PtII(CN)4]}n (Py = pyridine). Application of the theory of regular solutions to the SCO of the three derivatives allowed us to evaluate the effect on the characteristic SCO temperatures and the hysteresis, as well as the associated thermodynamic parameters when moving from microcrystalline bulk solids to nanometric thin films.

Iron(II) Spin Crossover Complexes Based on a Redox Active Equatorial Schiff-Base-Like Ligand.

In this work, two iron(II) coordination compounds with a N2O2 coordinating Schiff base-like ligand bearing a redox active tetrathiafulvalene (TTF) unit and pyridine or trans-1,2-bis(4-pyridylethylene) as an axial ligand are synthesized. Crystals suitable for single X-ray structure analysis were obtained for the new ligand. The complexes were characterized by magnetic susceptibility measurements, T-dependent UV-vis spectroscopy, and cyclic voltammetry. Both complexes display spin transition behavior below room temperature with T1/2 values of 146 and 156 K. The mononuclear iron(II) complex [FeTTFL(py)2] is relatively stable up to 400 K compared to similar complexes, showing no loss of axial ligands upon heating. Temperature dependent Mössbauer spectroscopy was conducted for the coordination polymer {[FeTTFL(bpee)]}n to get more information regarding the origin of the stepwise spin crossover (SCO) behavior observed in the magnetic measurements. The change of the spin state is accompanied by a change of the optical properties, which can be monitored by VT-UV-vis spectroscopy for the mononuclear complex and has been analyzed in theoretical studies. The redox behavior of the iron(II) complexes reveals three reversible redox steps which are located at the iron center and at the TTF unit of the ligand. Oxidation of the TTF unit induces characteristic changes in the UV-vis spectrum that can be followed by spectroelectrochemical UV-vis spectroscopy. Addressing the potential of the iron-centered redox process results in similar changes in the UV-vis spectrum, which indicates an electronic coupling of the redox active unit with the metal center under certain circumstances.

Multiresponsive Spin Crossover Driven by Rotation of Tetraphenylborate Anion in an Iron(III) Complex

Dynamic molecular materials, which involve mechanical motions in crystals, usually exhibit tunable properties related to conformational polymorphs. Herein, we describe a solvent-free molecular crys...

Effect of Mechanochemical Recrystallization on the Thermal Hysteresis of 1D FeII-triazole Spin Crossover Polymers.

The thermal hysteresis in the cooperative spin crossover (SCO) polymer [Fe(trz)(Htrz)2]n[BF4]n (1) has been tuned by a simple ball milling grinding process. Mechanical treatment affects the size and morphology of the crystallite domains, as confirmed by multiple complementary techniques, including ESEM, DLS, and PXRD data. Upon milling, the regular cubic shape particles recrystallize with slightly different unit cell parameters and preferential orientation. This macroscopic change significantly modifies the thermally induced SCO behavior, studied by temperature-dependent magnetic susceptibility, X-ray diffraction, and DSC analysis. Transition temperatures downshift, closer to room temperature, while hysteresis widens, when particle sizes are actually decreasing. We relate this counterintuitive observation to subtle modifications in the unit cell, offering new alternatives to tune and enhance SCO properties in this class of 1D-cooperative polymers.

Tuning of crystallization method and ligand conformation to give a mononuclear compound or two-dimensional SCO coordination polymer based on a new semi-rigid V-shaped bis-pyridyl bis-amide ligand.

With the new semi-rigid V-shaped bidentate pyridyl amide compound 5-methyl-N,N'-bis(pyridin-4-yl)benzene-1,3-dicarboxamide (L) as an auxiliary ligand and the FeII ion as the metal centre, one mononuclear complex, bis(methanol-κO)bis[5-methyl-N,N'-bis(pyridin-4-yl)benzene-1,3-dicarboxamide-κN]bis(thiocyanato-κN)iron(II), [Fe(SCN)2(C19H16N4O2)2(CH3OH)2] (1), and one two-dimensional coordination polymer, catena-poly[[[bis(thiocyanato-κN)iron(II)]-bis[μ-5-methyl-N,N'-bis(pyridin-4-yl)benzene-1,3-dicarboxamide-κ2N:N']] methanol disolvate dihydrate], {[Fe(SCN)2(C19H16N4O2)2]·2CH3OH·2H2O}n (2), were prepared by slow evaporation and H-tube diffusion methods, respectively, indicating the effect of the method of crystallization on the structure type of the target product. Both complexes have been structurally characterized by elemental analysis, IR spectroscopy and single-crystal X-ray crystallography. The single-crystal X-ray diffraction analysis shows that L functions as a monodentate ligand in mononuclear 1, while it coordinates in a bidentate manner to two independent Fe(SCN)2 units in complex 2, with a different conformation from that in 1 and the ligands point in two almost orthogonal directions, therefore leading to a two-dimensional grid-like network. Investigation of the magnetic properties reveals the always high-spin state of the FeII centre over the whole temperature range in 1 and a gradual thermally-induced incomplete spin crossover (SCO) behaviour below 150 K in 2, demonstrating the influence of the different coordination fields on the spin properties of the metal ions. The current results provide useful information for the rational design of functional complexes with different structure dimensionalities by employing different conformations of the ligand and different crystallization methods.

From a mononuclear FeL2 complex to a Fe4L4 molecular square: Designed assembly and spin-crossover property

By introduction of a new Fe(L1)2 spin-crossover (SCO) unit into the polynuclear system, a nano-scale Fe4(L2)4 molecular square architecture is designed through coordination-directed self-assembly strategy. Both the mononuclear Fe(L1)2 and tetranuclear Fe4(L2)4 complexes have been structurally confirmed by 1H nuclear magnetic resonance (NMR), electrospray ionization time-of-flight mass spectrometry (ESI-TOF-MS), and temperature-dependent single crystal X-ray diffraction studies. Variable-temperature magnetic susceptibility measurements reveal the presence of an abrupt SCO behavior with a thermal hysteresis width of 4 K for Fe(L1)2. By clear contrast, Fe4(L2)4 undergoes a gradual spin transition behavior with enlarged thermal hysteresis width and higher spin transition temperature.

Symmetry breakings in a metal organic framework with a confined guest

The MOF $[{\text{Fe(tvp)}}_{2}{(\text{NCS})}_{2}]\ifmmode\cdot\else\textperiodcentered\fi{}2\text{BzCHO}$ is demonstrated to undergo a complex sequence of phase transitions and spin-crossover behavior of its constitutive ${\text{Fe}}^{\text{II}}$ ions upon adsorption of benzaldehyde guest molecules. Our study, combining Raman and synchrotron x-ray diffraction measurements on a single crystal, reveals that the conversion from the pure high-spin to the pure low-spin phases implies a rich sequence of intermediate phases, with symmetry breaking forming at least three different space groups. These different symmetries involve spin-state ordering, ligand ordering, and guest ordering, interpreted by combining magnetic susceptibility, Raman measurements, density functional theory, and force field molecular dynamics calculations.

The Tyranny of Arm-Wrestling Methyls on Iron(II) Spin State in Pseudo-Octahedral [Fe(didentate)3] Complexes

The connection of a sterically constrained 3-methyl-pyrazine ring to a N-methyl-benzimidazole unit to give the unsymmetrical α,α’-diimine ligand L5 has been programmed for the design of pseudo-octahedral spin-crossover [Fe(L5)3]2+ units, the transition temperature (T1/2) of which occurs in between those reported for related facial tris-didentate iron chromophores fitted with 3-methyl-pyridine-benzimidazole in a LaFe helicate (T1/2 ~ 50 K) and with 5-methyl-pyrazine-benzimidazole L2 ligands (T1/2 ~350 K). A thorough crystallographic analysis of [Fe(L5)3](ClO4)2 (I), [Ni(L5)3](ClO4)2 (II), [Ni(L5)3](BF4)2∙H2O (III), [Zn(L5)3](ClO4)2 (IV), [Ni(L5)3](BF4)2∙1.75CH3CN (V), and [Zn(L5)3](BF4)2∙1.5CH3CN (VI) shows the selective formation of pure facial [M(L5)3]2+ cations in the solvated crystals of the tetrafluoroborate salts and alternative meridional isomers in the perchlorate salts. Except for a slightly larger intra-strand interannular twist between the aromatic heterocycles in L5, the metric parameters measured in [Zn(L5)3]2+ are comparable to those reported for [Zn(L2)3]2+, where L2 is the related unconstrained ligand. This similitude is reinforced by comparable ligand-field strengths (∆oct) and nephelauxetic effects (as measured by the Racah parameters B and C) extracted from the electronic absorption spectra recorded for [Ni(L5)3]2+ and [Ni(L2)3]2+. In this context, the strictly high-spin behavior observed for [Fe(L5)3]2+ within the 5–300 K range contrasts with the close to room-temperature spin-crossover behavior of [Fe(L2)3]2+ (T1/2 = 349(5) K in acetonitrile). This can be unambiguously assigned to an intraligand arm wrestling match operating in bound L5, which prevents the contraction of the coordination sphere required for accommodating low-spin FeII. Since the analogous 3-methyl-pyridine ring in [Fe(L3)3]2+ derivatives are sometimes compatible with spin-crossover properties, the consequences of repulsive intra-strand methyl–methyl interactions are found to be amplified in [Fe(L5)3]2+ because of the much lower basicity of the 3-methyl-pyrazine ring and the resulting weaker thermodynamic compensation. The decrease of the stability constants by five orders of magnitude observed in going from [M(L2)3]2+ to [M(L5)3]2+ (M = NiII and ZnII) is diagnostic for the operation of this effect, which had been not foreseen by the authors.

Influence of CF3 Substituents on the Spin Crossover Behavior of Iron(II) Coordination Polymers with Schiff Base‐like Ligands

A Schiff base‐like ligand bearing CF3 substituents was synthesized and converted to iron(II) coordination polymers [{FeL(Lax)}n] using five different bridging ligands Lax. The structure of the coordination polymers was investigated using powder X‐ray diffraction and single‐crystal X‐ray diffraction in the case of [{FeL(bipy)}n]. The later revealed an untypical ABAB pattern of alternating equatorial ligands rotated by 180° with regard to each other along the chain. The temperature‐dependent magnetic behavior was investigated with a SQUID magnetometer and the spin states at room temperature were confirmed by 57Fe‐Mössbauer spectroscopy. Three out of five coordination polymers show spin crossover behavior in the temperature range between 50 and 400 K with different kind of curve progressions (abrupt, gradual, step‐wise). The other two coordination polymers are either fully highspin or fully low spin.

Magnetic Properties of Fe(II) Complexes of Cyclam Derivative with One p-Aminobenzyl Pendant Arm

In order to prepare an Fe(II) spin crossover (SCO) complex that could be consequently modified to a bimetallic coordination compound that possesses another magnetic property of interest, a specially designed ligand L-NH2 (1-(4-aminobenzyl)-4,11-bis(pyridine-2-ylmethyl)- 1,4,8,11-tetraazacyclotetradecane) was prepared. This ligand consists of a macrocyclic cyclam part containing two 2-pyridylmethyl pendant arms (expecting SCO upon Fe(II) complexation) and one p-aminobenzyl pendant arm with an NH2 group. The presence of this group enables the consequent transformation to various functional groups for the selective complexation of other transition metals or lanthanides (providing the second property of interest). Furthermore, the performed theoretical calculations (TPSSh/def2-TZVP) predicted SCO behavior for the Fe(II) complex of L-NH2. Thus, Fe(II) complexes [Fe(L-NH2)](ClO4)2 (1) and [Fe(L-NH2)]Cl2·6H2O (2) were synthesized and thoroughly characterized. Based on the crystal structure of an isostructural analogous Ni(II) complex [Ni(L-NH2)]Cl2·6H2O (3), the coordination number six was confirmed with an octahedral coordination sphere and a cis-arrangement of the pyridine pendant arms. The measured magnetic data confirmed the high-spin behavior of both compounds with large magnetic anisotropy (D = 17.8 for 1 and 20.9 cm−1 for 2 complemented in both cases also with large rhombicity), though unfortunately without any indication of the SCO behavior with decreasing temperature. The lack of SCO can be ascribed to the crystal packing and/or the non-covalent intermolecular interactions stabilizing the high-spin state in the solid state.

Density Functional Theory Prediction of the Electrocatalytic Mechanism of Proton Reduction by a Dicobalt Tetrakis(Schiff Base) Macrocycle.

A dicobalt tetrakis(Schiff base) macrocycle has recently been reported to electrochemically catalyze the reduction of H+ to H2 in an acetonitrile solution. Density functional theory (DFT) calculations using the ωB97X-D functional are shown to produce structural and thermodynamic results in good agreement with the experimental data. A mechanistic model based on thermodynamics is developed that incorporates electrochemical and magnetic details of the complex, accounting for electron-spin reorganization of the metal center after redox steps. The model is validated through a comparison of the predicted electrochemical potentials with the irreversible cyclic voltammogram of [Co2LAc]+, which shows redox-coupled spin-crossover (RCSCO) behavior for the CoII/III transitions. Using our model, we predict the thermodynamically favored mechanism of H2 evolution by [Co2L]2+ to be one of heterolytic proton attack on a [CoII2L(μ-H)]+ species. Understanding the electronic details and thermodynamically preferred mechanism of this catalyst will aid in improving its efficiency and the future design of bimetallic Co-based H+ electrocatalysts. Also, this work will assist in the future DFT modeling of bimetallic RCSCO complexes.

Controlling Spin Switching with Anionic Supramolecular Frameworks

Encasing the classic spin-crossover (SCO) complex cation [Co(terpy)2]2+ in different anionic supramolecular frameworks modulates its SCO behavior, tuning both its T1/2 (shift of up to 200 K) and th...

Effect of Guest Molecules on Spin Transition Temperature in Loaded Hofmann‐Like Clathrates with Improved Porosity

The synthesis, crystal structure, magnetic and calorimetric studies of a new clathrate compound of the Hofmann‐type spin crossover (SCO) metal‐organic framework (MOF) {Fe(bpb)[MII(CN)4]}·xGuest (bpb = bis(4‐pyridyl)butadiyne, and MII = Ni, Pt) with characteristic fsc topology is reported. The framework {Fe(bpb)[MII(CN)4]} can host up to 1.5 guest molecules of (trifluoromethyl)benzene and display complete one‐step cooperative SCO behavior. Our systematic study on {Fe(bpb)[Pt(CN)4]}·xGuest shows a general reciprocal correlation between the SCO temperature with the volume of the guest molecules.

Spin Transition of an Iron(II) Organoborate Complex in Different Polymorphs and in Vacuum-Deposited Thin Films: Influence of Cooperativity.

Two polymorphic modifications (1-I and 1-II) of the new spin crossover (SCO) complex [Fe{H2B(pz)(pypz)}2] (pz = pyrazole, pypz = pyridylpyrazole; 1) were prepared and investigated by differential scanning calorimetry (DSC), magnetic measurements, Mößbauer, vibrational, and absorption spectroscopy as well as single-crystal and X-ray powder diffraction. DSC measurements reveal that upon heating the thermodynamically metastable form 1-II to ∼178 °C it transforms into 1-I in an exothermic reaction, which proves that these modifications are related by monotropism. Both forms show thermal SCO with T1/2 values of 390 K (1-II) and 270 K (1-I). An analysis of the crystal structures of 1-II and the corresponding Zn(II) (2) and Co(II) (3) complexes that are isotypic with 1-I reveals that form II consists of dimers coupled by strong intramolecular π···π interactions, which is not the case for 1-I. In agreement with these findings, investigations of thin films of 1, where significant π···π interactions should be absent, reveal SCO behavior similar to that of 1-I. These results underscore the importance of cooperativity for the spin-transition behavior of this class of complexes.

Iron(II) Spin Crossover Complexes with 4,4'-Dipyridylethyne-Crystal Structures and Spin Crossover with Hysteresis.

Three new iron(II) 1D coordination polymers with cooperative spin crossover behavior showing thermal hysteresis loops were synthesized using N2O2 Schiff base-like equatorial ligands and 4,4′-dipyridylethyne as a bridging, rigid axial linker. One of those iron(II) 1D coordination polymers showed a 73 K wide hysteresis below room temperature, which, upon solvent loss, decreased to a still remarkable 30 K wide hysteresis. Single crystal X-ray structures of two iron(II) coordination polymers and T-dependent powder XRD patterns are discussed to obtain insight into the structure property relationship of those materials.

Iron(II) Complexes Having Dinuclear Mesocate or Octanuclear Bicapped Trigonal Prism Structures Dependent on the Rigidity of Bis(bidentate) Schiff Base Ligands Containing Imidazole Groups

Abstract Dinuclear complex, [Fe2(H2L1,Me)3](ClO4)4 (1Me, H2L1,Me = N,N′-(1,3-phenylene)bis(1-(5-methyl-1H-imidazol-4-yl)methanimine)), and octanuclear complexes, [Fe8(H2L2,H)12](ClO4)16 (2HClO4: H2L2,H = N,N′-(1,3-phenylenebis(methylene))bis(1-(1H-imidazol-4-yl)methanimine) and [Fe8(H2L2,Me)12](X)16 (2MeX: H2L2,Me = N,N′-(1,3-phenylenebis(methylene))bis(1-(5-methyl-1H-imidazol-4-yl)methanimine), X = ClO4, BF4), were synthesized. It was revealed by X-ray analysis that 1Me has a dinuclear mesocate structure. On the other hand, 2HClO4 and 2MeX have novel octanuclear bicapped trigonal prism structures with six iron(II) sites having the meridional configuration on vertexes and two iron(II) sites having the facial one on the centers of each triangular base. Magnetic susceptibility studies indicated that these dinuclear and octanuclear complexes show gradual spin-crossover (SCO) behavior.

Temperature-dependent hysteretic two-step spin crossover in two-dimensional Hofmann-type compounds.

Hofmann-type spin-crossover (SCO) frameworks attract much attention due to their predictable structures and diverse SCO behaviours. In this paper, we report two isostructural two-dimensional (2D) Hofmann-type SCO frameworks [FeII(ppe)2MII(CN)4]·3H2O (ppe = 1-(2-pyridyl)-2-(4-pyridyl)ethylene and M = Pd for 1 and Pt for 2). X-ray crystallographic studies, magnetic measurements and Mössbauer spectra jointly reveal that both compounds undergo complete hysteretic two-step SCO, i.e. HS1.0 ⇌ HS0.6-0.5LS0.4-0.5 ⇌ LS1.0 conversion with thermal hysteresis loops of 14 K and 28 K wide for 1 and 13 K and 26 K wide for 2. The dehydrated samples, 1-dehydrated and 2-dehydrated, display different SCO behaviours, unveiling that the SCO behaviours are affected by guest water molecules.

Solid state mononuclear divalent nickel spin crossover complexes.

Spin crossover complexes containing 3d4-3d7 transition metal ions with tunable electronic configurations in appropriate ligand field environments have been extensively investigated. In contrast, the development of 3d8 divalent nickel complexes displaying such a spin crossover behavior is far behind. The increasing number of X-ray single crystal structures along with magnetic evidence and thermodynamic equilibrium indicate that bistable divalent nickel complexes are gradually recognized to be a formal member of the "spin crossover family". Unfortunately, the rarity of nickel spin crossover complexes is occasionally mentioned. This Perspective article highlights examples of mononuclear 3d8 nickel spin crossover complexes in dynamic rearrangements with characterized solid state structures from the viewpoint of types of ligands utilized.

Extrinsicvs.intrinsic luminescence and their interplay with spin crossover in 3D Hofmann-type coordination polymers

Two series of coordination polymers have been synthesized to exhibit extrinsic or intrinsic luminescence modulated by spin crossover behavior. This study provides further insight into the features governing the synergy between both properties.