Zhao-Ping Ni

Symmetry-Supported Magnetic Blocking at 20 K in Pentagonal Bipyramidal Dy(III) Single-Ion Magnets

Single-molecule magnets (SMMs) that can be trapped in one of the bistable magnetic states separated by an energy barrier are among the most promising candidates for high-density information storage, quantum processing, and spintronics. To date, a considerable series of achievements have been made. However, the presence of fast quantum tunnelling of magnetization (QTM) in most SMMs, especially in single-ion magnets (SIMs), provides a rapid relaxation route and often sets up a limit for the relaxation time. Here, we pursue the pentagonal bipyramidal symmetry to suppress the QTM and present pentagonal bipyramidal Dy(III) SIMs [Dy(Cy3PO)2(H2O)5]Cl3·(Cy3PO)·H2O·EtOH (1) and [Dy(Cy3PO)2(H2O)5]Br3·2(Cy3PO)·2H2O·2EtOH (2), (Cy3PO = tricyclohexyl phosphine oxide). Magnetic characterizations reveal their fascinating SMM properties with high energy barriers as 472(7) K for 1 and 543(2) K for 2, along with a record magnetic hysteresis temperature up to 20 K for 2. These results, combined with the ab initio calculations, offer an illuminating insight into the vast possibility and potential of what the symmetry rules can achieve in molecular magnetism.

Tuning the Spin-Crossover Behaviour of a Hydrogen-Accepting Porous Coordination Polymer by Hydrogen-Donating Guests

A Hoffman-like coordination polymer with appreciable porosity and uncoordinated pyridyl groups, namely, [Fe(2,5-bpp){Au(CN)2}2]⋅x Solv (2,5-bpp = 2,5-bis(pyrid-4-yl)pyridine; Solv = solvent), was synthesised and characterised. A series of fascinating spin-crossover behaviours with abrupt, stepwise and hysteretic features were obtained by exchange with a range of protic solvents (ethanol, n-propanol, isopropyl alcohol, sec-butanol and isobutanol). Guest-host hydrogen-bonding interactions involving the H-accepting site of the framework are primarily responsible for the pronounced cooperativity of these spin-crossover behaviours. Meanwhile, the tunable critical temperatures over a range of about 130 K are presumably attributable to a certain degree of competition between internal pressure and local electronic influences of solvents.

Synergistic electrical bistability in a conductive spin crossover heterostructure

A flexible heterostructure is reported herein, which elastically couples the volumetric variation during SCO to the electrical conductivity of a strain-sensitive metallic conductor, thereby resulting in synergistic electrical bistability.

Enhanced spin-crossover behavior mediated by supramolecular cooperative interactions.

Three one-dimensional (1D) hetereobimetallic coordination polymers [Fe(II)(L)2(AgCN)2]·Solv (L = bpt(-), 1; L = Mebpt(-), Solv = 1.75EtOH, 2; L = bpzt(-), 3) with in situ generated AgCN species were synthesized by solvothermal reactions of Fe(II) salt, K[Ag(CN)2], and the corresponding ligands [bptH = 3,5-bis(pyridin-2-yl)-1,2,4-triazole, MebptH = 3-(3-methyl-2-pyridyl)-5-(2-pyridyl)-1,2,4-triazole, and bpztH = 3,5-bis(pyrazin-2-yl)-1,2,4-triazole]. They were further characterized by X-ray crystallography, magnetic and photomagnetic measurements, and differential scanning calorimetry. Single-crystal X-ray analyses show that they are isostructural with 1D zigzag chain structures with rhombus {Fe2Ag2} units, in which the substituted bpt(-) ligand connects the Fe(II) ion and AgCN species in a cis bridging mode. Then the zigzag chains are packed into three-dimensional supramolecular structures by π···π interactions. Most importantly, weak Ag···N interactions (2.750 Å at 150 K) between the π-stacked neighboring chains present in complex 3. Magnetic susceptibility measurements exhibit that complex 1 displays characteristic paramagnetic behavior in the temperature range investigated. Complex 2 undergoes a gradual spin-crossover (SCO) with critical temperatures T(1/2)↓ = 232 K and T(1/2)↑ = 235 K, whereas 3 exhibits an abrupt SCO with critical temperatures T(1/2)↓ = 286 K and T(1/2)↑ = 292 K. The magnetostructural relationships suggest that the magnetic behaviors can be modulated from paramagnetic behavior to abrupt and hysteretic SCO near room temperature through adjustment of the electronic substituent effect and intermolecular interactions.

Guest-effected spin-crossover in a novel three-dimensional self-penetrating coordination polymer with permanent porosity.

Porous and nonporous 3D heterobimetallic coordination polymers based on the 1,4-di(pyridin-4-yl)benzene ligand (dpb), [Fe(dpb){Ag(CN)2}{Ag2(CN)3}]·nSolv (1·nSolv; nSolv = DMF·EtOH, 2DMF·MeCN) and [Fe(dpb)2{Ag(CN)2}2] (2), have been synthesized by diffusion technique, respectively. Single-crystal X-ray analysis shows that 1·nSolv consists of a 3D self-penetrating network with in-situ-generated [Ag2(CN)3](-) species and displays one of the largest volume values of porosity (299 Å(3) per iron atom) after desolvation for the Hoffman-like porous SCO coordination polymers to date. In contrast, nonporous compound 2 is composed of two independent interpenetrated 3D nets with in-situ-generated [Ag(dpb)(CN)2](-) species. Their significant distinctions of structural architectures lead to dramatically different magnetic properties: 1·nSolv displays two-step guest-effected SCO with hysteresis, whereas 2 presents characteristic paramagnetic behavior.

Tunable cooperativity in a spin-crossover Hoffman-like metal–organic framework material by aromatic guests

A spin-crossover Hoffman-like metal–organic framework, [Fe(dpb){Au(CN)2}2]·nSolv (dpb = 1,4-di(pyridin-4-yl)benzene; nSolv = 1.5DMF·0.3EtOH·0.2C6H12), is prepared and presents appreciable porosity. By introducing different aromatic guest molecules (benzene, naphthalene, anthracene, and ferrocene) and carbon disulfide into this framework, the spin-crossover cooperativity is dramatically modulated. The thermal hysteresis width is tuned from 0 K to 73 K by replacing the original solvents with naphthalene in the 1D channel. DFT calculations are carried out to evaluate the optimized positions of guests and offer an insight into the relationship between spin-crossover cooperativity and host–guest interaction. As a result, significant π⋯π stacking interactions (or S⋯Au contact) between the host framework and aromatic guest molecules (or CS2) are responsible for the strong cooperativity.

Hysteretic Spin Crossover in Two-Dimensional (2D) Hofmann-Type Coordination Polymers.

Three new two-dimensional (2D) Hofmann-type coordination polymers with general formula [Fe(3-NH2py)2M(CN)4] (3-NH2py = 3-aminopyridine, M = Ni (1), Pd (2), Pt (3)) have been synthesized. Magnetic susceptibility measurements show that they exhibited cooperative spin crossover (SCO) with remarkable hysteretic behaviors. Their hysteresis widths are 25, 37, and 30 K for 1-3, respectively. The single-crystal structure of 1 suggest that the pseudo-octahedral Fe sites are equatorially bridged by [M(CN)4](2-) to form 2D grids and axially coordinated by 3-NH2py ligands. The intermolecular interactions between layers (the offset face-to-face π···π interactions, hydrogen bonds, and weak N(amino)···Ni(II) contacts) together with the covalent bonds bridged by [M(CN)4](2-) units are responsible to the significant cooperativity.

Spin-Crossover Phenomenon in a Pentanuclear Iron(II) Cluster Helicate

The first spin-crossover cluster helicate [{Fe(II)(μ-bpt)3}2Fe(II)3(μ3-O)][Fe(II)2(μ-Cl)(μ-bpt)(NCS)4(H2O)]·2H2O·C2H5OH [Hbpt = 3,5-bis(pyridin-2-yl)-1,2,4-triazole] was reported, in which one spin-crossover unit and one low-spin triple-stranded [Fe(II)(bpt)3](-) unit wrapped a rare planar [Fe(II)3(μ3-O)](4+) core to form a bis(triple-helical) cation with trigonal-bipyramidal topology. The origin of different magnetic behaviors of two [Fe(II)(bpt)3](-) units could be ascribed to their different intermolecular interactions.

Cyanide-bridged bimetallic 3D Hoffman-like coordination polymers with tunable magnetic behaviour

Three cyanide-bridged bimetallic coordination polymers [Fe(bpmp){Ag(CN)2}2]·0.5DMF·0.5EtOH·2H2O (1), [Fe(bpmp){Au(CN)2}2]·DMF·0.5EtOH·H2O (2) and [Fe(bpmp){Ni(CN)4}] (3) (bpmp = 1,4-bis(pyridin-4-ylmethyl)piperazine), have been synthesized and characterized. Single crystal X-ray analysis shows that 1 and 2 are isostructural with a 3D doubly interpenetrated Hoffman-like network without metallophilic interactions, whereas 3 shows a 3D pronouncedly distorted Hoffman framework. Magnetic susceptibility measurements exhibit two-step spin transition properties in 1 and 2, whereas a characteristic paramagnetic behaviour in 3, respectively.

High-Temperature Spin Crossover in Two Solvent-Free Coordination Polymers with Unusual High Thermal Stability

Two solvent-free two-dimensional (2D) coordination polymers, (2)∞[Fe(ptim)2] (1) and (2)∞[Fe(ptpy)2] (2) (Hptim = 2-(5-(4-(1H-imidazol-1-yl)phenyl)-4H-1,2,4-triazol-3-yl)pyridine; Hptpy = 2-(5-(4-(pyridin-3-yl)phenyl)-4H-1,2,4-triazol-3-yl)pyridine), have been successfully prepared by solvothermal reactions. Their iron atoms are bridged by the corresponding multidentate anionic ligands into dense neutral structures. The magnetic data reveal that complexes 1 and 2 are rare examples exhibiting reversible one-step high-temperature spin crossover behaviors with spin transition temperatures of 419 and 416 K, respectively. Moreover, these structures also display remarkable thermal stability up to 714 K (for 1) and 690 K (for 2), which are confirmed by thermogravimetric and variable-temperature powder X-ray diffraction analyses.