Pressure Effect Studies on the Spin Transition of Microporous 3D Polymer [Fe(pz)Pt(CN)₄]

Pressure effects on the spin transition of the three-dimensional (3D) porous coordination polymer {Fe­(pz)­[Pt­(CN)₄]} have been investigated in the interval 10⁵ Pa–1.0 GPa through variable-temperature (10–320 K) magnetic susceptibility measurements and spectroscopic studies in the visible region at room temperature. These studies have disclosed a different behavior of the compound under pressure. In the magnetic experiments, a temperature independent paramagnetic behavior has been observed under 0.4 GPa. In contrast, at room temperature and at 0.8 GPa, a complete HS-to-LS transition has been evidenced. The differences in the magnetic behavior are strongly related with the porous structure of the compound and its capability to adsorb the oil used as pressure transmission media in the magnetic experiments

Novel Iron(II) Microporous Spin-Crossover Coordination Polymers with Enhanced Pore Size

In this Communication, we report the synthesis and characterization of novel Hofmann-like spin-crossover porous coordination polymers of composition {Fe­(L)­[M­(CN)₄]}·G [L = 1,4-bis­(4-pyridylethynyl)­benzene and M^II = Ni, Pd, and Pt]. The spin-crossover properties of the framework are closely related to the number and nature of the guest molecules included in the pores

Novel Iron(II) Microporous Spin-Crossover Coordination Polymers with Enhanced Pore Size

In this Communication, we report the synthesis and characterization of novel Hofmann-like spin-crossover porous coordination polymers of composition {Fe­(L)­[M­(CN)₄]}·G [L = 1,4-bis­(4-pyridylethynyl)­benzene and M^II = Ni, Pd, and Pt]. The spin-crossover properties of the framework are closely related to the number and nature of the guest molecules included in the pores

Strong Cooperative Spin Crossover in 2D and 3D Fe^II–M^I,II Hofmann-Like Coordination Polymers Based on 2‑Fluoropyrazine

Self-assembling iron­(II), 2-fluoropyrazine (Fpz), and [M^II(CN)₄]^2– (M^II = Ni, Pd, Pt) or [Au^I(CN)₂]⁻ building blocks have afforded a new series of two- (2D) and three-dimensional (3D) Hofmann-like spin crossover (SCO) coordination polymers with strong cooperative magnetic, calorimetric, and optical properties. The iron­(II) ions, lying on inversion centers, define elongated octahedrons equatorially surrounded by four equivalent centrosymmetric μ₄-[M^II(CN)₄]^2– groups. The axial positions are occupied by two terminal Fpz ligands affording significantly corrugated 2D layers {Fe­(Fpz)₂([M^II(CN)₄]}. The Pt and Pd derivatives undergo thermal- and light-induced SCO characterized by <i>T</i>_1/2 temperatures centered at 155.5 and 116 K and hysteresis loops 22 K wide, while the Ni derivative is high spin at all temperatures, even at pressures of 0.7 GPa. The great stability of the high-spin state in the Ni derivative has tentatively been ascribed to the tight packing of the layers, which contrasts with that of Pt and Pd derivatives in the high- and low-spin states. The synthesis and structure of the 3D frameworks formulated {Fe­(Fpz)­[Pt­(CN)₄]}·1/2H₂O and {Fe­(Fpz)­[Au­(CN)₂]₂}, where Fpz acts as bridging ligand, which is also discussed. The former is high spin at all temperatures, while the latter displays very strong cooperative SCO centered at 243 K accompanied by a hysteresis loop 42.5 K wide. The crystal structures and SCO properties are compared with those of related complexes derived from pyrazine, 3-fluoropyridine, and pyridine

Homoleptic Iron(II) Complexes with the Ionogenic Ligand 6,6′-Bis(1<i>H</i>‑tetrazol-5-yl)-2,2′-bipyridine: Spin Crossover Behavior in a Singular 2D Spin Crossover Coordination Polymer

Deprotonation of the ionogenic tetradentate ligand 6,6′-bis­(1<i>H</i>-tetrazol-5-yl)-2,2′-bipyridine [H₂bipy­(ttr)₂] in the presence of Fe^II in solution has afforded an anionic mononuclear complex and a neutral two-dimensional coordination polymer formulated as, respectively, NEt₃H­{Fe­[bipy­(ttr)₂]­[Hbipy­(ttr)₂]}·3MeOH (<b>1</b>) and {Fe­[bipy­(ttr)₂]}<i>_n</i> (<b>2</b>). The anions [Hbipy­(ttr)₂]⁻ and [bipy­(ttr)₂]^2– embrace the Fe^II centers defining discrete molecular units <b>1</b> with the Fe^II ion lying in a distorted bisdisphenoid dodecahedron, a rare example of octacoordination in the coordination environment of this cation. The magnetic behavior of <b>1</b> shows that the Fe^II is high-spin, and its Mössbauer spectrum is characterized by a relatively large average quadrupole splitting, Δ<i>E</i>_Q = 3.42 mm s^–1. Compound <b>2</b> defines a strongly distorted octahedral environment for Fe^II in which one [bipy­(ttr)₂]⁻ anion coordinates the equatorial positions of the Fe^II center, while the axial positions are occupied by peripheral <i>N</i>-tetrazole atoms of two adjacent {Fe­[bipy­(ttr)₂]}⁰ moieties thereby generating an infinite double-layer sheet. Compound <b>2</b> undergoes an almost complete spin crossover transition between the high-spin and low-spin states centered at about 221 K characterized by an average variation of enthalpy and entropy Δ<i>H</i>^av = 8.27 kJ mol^–1, Δ<i>S</i>^av = 37.5 J K^–1 mol^–1, obtained from calorimetric DSC measurements. Photomagnetic measurements of <b>2</b> at 10 K show an almost complete light-induced spin state trapping (LIESST) effect which denotes occurrence of antiferromagnetic coupling between the excited high-spin species and <i>T</i>_LIESST = 52 K. The crystal structure of <b>2</b> has been investigated in detail at various temperatures and discussed

Metal-Controlled Magnetoresistance at Room Temperature in Single‑Molecule Devices

The appropriate choice of the transition metal complex and metal surface electronic structure opens the possibility to control the spin of the charge carriers through the resulting hybrid molecule/metal <i>spinterface</i> in a single-molecule electrical contact at room temperature. The single-molecule conductance of a Au/molecule/Ni junction can be switched by flipping the magnetization direction of the ferromagnetic electrode. The requirements of the molecule include not just the presence of unpaired electrons: the electronic configuration of the metal center has to provide occupied or empty orbitals that strongly interact with the junction metal electrodes and that are close in energy to their Fermi levels for one of the electronic spins only. The key ingredient for the metal surface is to provide an efficient <i>spin texture</i> induced by the spin–orbit coupling in the topological surface states that results in an efficient spin-dependent interaction with the orbitals of the molecule. The strong magnetoresistance effect found in this kind of single-molecule wire opens a new approach for the design of room-temperature nanoscale devices based on spin-polarized currents controlled at molecular level

Fast Detection of Water and Organic Molecules by a Change of Color in an Iron(II) Microporous Spin-Crossover Coordination Polymer

Here we present a novel three-dimensional iron­(II) spin-crossover porous coordination polymer based on the bis­(1,2,4-triazol-4-yl)­adamantane (tr₂ad) ligand and the [Au­(CN)₂]⁻ metalloligand anions with the formula {Fe₃(tr₂ad)₄[Au­(CN)₂)]₂}­[Au­(CN)₂]₄·G. The sorption/desorption of guest molecules, water, and five/six-membered-ring organic molecules is easily detectable because the guest-free and -loaded frameworks present drastically distinct coloration and spin-state configurations

Fast Detection of Water and Organic Molecules by a Change of Color in an Iron(II) Microporous Spin-Crossover Coordination Polymer

Here we present a novel three-dimensional iron­(II) spin-crossover porous coordination polymer based on the bis­(1,2,4-triazol-4-yl)­adamantane (tr₂ad) ligand and the [Au­(CN)₂]⁻ metalloligand anions with the formula {Fe₃(tr₂ad)₄[Au­(CN)₂)]₂}­[Au­(CN)₂]₄·G. The sorption/desorption of guest molecules, water, and five/six-membered-ring organic molecules is easily detectable because the guest-free and -loaded frameworks present drastically distinct coloration and spin-state configurations

Metal-Controlled Magnetoresistance at Room Temperature in Single‑Molecule Devices

The appropriate choice of the transition metal complex and metal surface electronic structure opens the possibility to control the spin of the charge carriers through the resulting hybrid molecule/metal <i>spinterface</i> in a single-molecule electrical contact at room temperature. The single-molecule conductance of a Au/molecule/Ni junction can be switched by flipping the magnetization direction of the ferromagnetic electrode. The requirements of the molecule include not just the presence of unpaired electrons: the electronic configuration of the metal center has to provide occupied or empty orbitals that strongly interact with the junction metal electrodes and that are close in energy to their Fermi levels for one of the electronic spins only. The key ingredient for the metal surface is to provide an efficient <i>spin texture</i> induced by the spin–orbit coupling in the topological surface states that results in an efficient spin-dependent interaction with the orbitals of the molecule. The strong magnetoresistance effect found in this kind of single-molecule wire opens a new approach for the design of room-temperature nanoscale devices based on spin-polarized currents controlled at molecular level

Large Conductance Switching in a Single-Molecule Device through Room Temperature Spin-Dependent Transport

Controlling the spin of electrons in nanoscale electronic devices is one of the most promising topics aiming at developing devices with rapid and high density information storage capabilities. The interface magnetism or <i>spinterface</i> resulting from the interaction between a magnetic molecule and a metal surface, or <i>vice versa</i>, has become a key ingredient in creating nanoscale molecular devices with novel functionalities. Here, we present a single-molecule wire that displays large (>10000%) conductance switching by controlling the spin-dependent transport under ambient conditions (room temperature in a liquid cell). The molecular wire is built by trapping individual spin crossover Fe^II complexes between one Au electrode and one ferromagnetic Ni electrode in an organic liquid medium. Large changes in the single-molecule conductance (>100-fold) are measured when the electrons flow from the Au electrode to either an α-up or a β-down spin-polarized Ni electrode. Our calculations show that the current flowing through such an interface appears to be strongly spin-polarized, thus resulting in the observed switching of the single-molecule wire conductance. The observation of such a high spin-dependent conductance switching in a single-molecule wire opens up a new door for the design and control of spin-polarized transport in nanoscale molecular devices at room temperature