A pair of enantiopure cubane-type Cu^II₄O₄ clusters was constructed from simple chiral ligands (R or S)-2-[(2-hydroxy-1-phenylethylimino)methyl]phenol (H₂L^{R or S}). Single-crystal X-ray diffraction studies demonstrated that complexes 1R and 1S are enantiomers. They consist of a Cu^II₄O₄ cubane core in which the four Cu^II centres are linked by a μ₃-oxo bridge. The four Cu^II ions in one cluster are all in a distorted square-pyramidal geometry. Circular dichroism (CD) spectroscopy also confirmed that complexes 1R and 1S are enantiomers and that the chirality was successfully transferred and amplified from the ligand to the coordination environment of the Cu^II ions. Magnetic susceptibility measurements show an overall antiferromagnetic interaction which could be fitted by using MAGPACK with a two parameter model (J₁ = –5.58 cm^–1, J₂ = 3.80 cm^–1).

We present three Mg–formate frameworks, incorporating three different ammoniums: [NH₄][Mg(HCOO)₃] (1), [CH₃CH₂NH₃][Mg(HCOO)₃] (2) and [NH₃(CH₂)₄NH₃][Mg₂(HCOO)₆] (3). They display structural phase transitions accompanied by prominent dielectric anomalies and anisotropic and negative thermal expansion. The temperature-dependent structures, covering the whole temperature region in which the phase transitions occur, reveal detailed structural changes, and structure–property relationships are established. Compound 1 is a chiral Mg–formate framework with the NH₄⁺ cations located in the channels. Above 255 K, the NH₄⁺ cation vibrates quickly between two positions of shallow energy minima. Below 255 K, the cations undergo two steps of freezing of their vibrations, caused by the different inner profiles of the channels, producing non-compensated antipolarization. These lead to significant negative thermal expansion and a relaxor-like dielectric response. In perovskite 2, the orthorhombic phase below 374 K possesses ordered CH₃CH₂NH₃⁺ cations in the cubic cavities of the Mg–formate framework. Above 374 K, the structure becomes trigonal, with trigonally disordered cations, and above 426 K, another phase transition occurs and the cation changes to a two-fold disordered state. The two transitions are accompanied by prominent dielectric anomalies and negative and positive thermal expansion, contributing to the large regulation of the framework coupled the order–disorder transition of CH₃CH₂NH₃⁺. For niccolite 3, the gradually enhanced flipping movement of the middle ethylene of [NH₃(CH₂)₄NH₃]²⁺ in the elongated framework cavity finally leads to the phase transition with a critical temperature of 412 K, and the trigonally disordered cations and relevant framework change, providing the basis for the very strong dielectric dispersion, high dielectric constant (comparable to inorganic oxides), and large negative thermal expansion. The spontaneous polarizations for the low-temperature polar phases are 1.15, 3.43 and 1.51 μC cm⁻² for 1, 2 and 3, respectively, as estimated by the shifts of the cations related to the anionic frameworks. Thermal and variable-temperature powder X-ray diffraction studies confirm the phase transitions, and the materials are all found to be thermally stable up to 470 K.

We present here the compound [NH₄][Cu(HCOO)₃], a new member of the [NH₄][M(HCOO)₃] family. The Jahn–Teller Cu²⁺ ion leads to a distorted 4⁹⋅6⁶ chiral Cu–formate framework. In the low-temperature (LT) orthorhombic phase, the Cu²⁺ is in an elongated octahedron, and the ions in the framework channel are off the channel axis. From 94 to 350 K the ion gradually approaches the channel axis and the related modulation of the framework and the hydrogen-bond system occurs. The LT phase is simple antiferroelectric (AFE). The material becomes hexagonal above 355 K. In the high-temperature (HT) phase, the Cu²⁺ octahedron is compressed, and the ions are arranged helically along the channel axis. Therefore, the phase transition is one from LT simple AFE to HT helical AFE. The temperature-dependent structure evolution is accompanied by significant thermal and dielectric anomalies and anisotropic thermal expansion, due to the different status of the ions and the framework modulations, and the structure–property relationship was established based on the extensive variable-temperature single-crystal structures. The material showed long range ordering of antiferromagnetism (AFM), with low dimensional character and a Néel temperature of 2.9 K. Therefore, within the material AFE and AFM orderings coexist in the low-temperature region.

Four azide–copper coordination polymers, [Cu₂L¹(N₃)₄]_n, [Cu₂L²(N₃)₄]_n, [Cu₂L^3R(N₃)₄]_n, and [Cu₂L^3S(N₃)₄]_n, were synthesized by using pybox [pyridine-2,6-bis(oxazolines)] as coligands {L¹: 2,6-bis(4,5-dihydrooxazol-2-yl)pyridine; L²: 2,6-bis(5,6-dihydro-4H-1,3-oxazin-2-yl)pyridine; L^{3R or 3S}: 2,6-bis[(R or S)-4-benzyl-4,5-dihydrooxazol-2-yl]pyridine}. Compounds 1 and 2 possess similar 1D infinite azide–copper hexagonal tapes with three types of N₃ bridges (two single end-on N₃ bridges and one double end-on N₃ bridge). Compounds 3R and 3S possess an azide–copper 2D honeycomb layer with two types of N₃ bridges (a single end-to-end N₃ bridge and a double end-on N₃ bridge). The chirality of these enantiopure layered structures is controlled by the addition of the chiral pybox ligand in the synthesis, which is very rare for the reported azide–copper coordination polymers. The double end-on N₃ bridge transfers mainly ferromagnetic exchange coupling interactions in these four compounds. Owing to the steric hindrance of the pybox ligands, the interchain and interlayer separations are broadened, which weakens the magnetic interactions between them. Thus, no long-range ferromagnetic ordering was observed above 1.8 K. A magnetostructural correlation was also discussed in detail.

Two mononuclear spin-crossover compounds Fe(L¹)₂ (1) and Fe(L²)₂ (2) {HL¹ = 2-hydroxy-3-methyl-N′-[1-(pyridine-2-yl)butylidene]benzohydrazide, HL² = 2-hydroxy-N′-[1-(pyridine-2-yl)butylidene]benzohydrazide} were successfully synthesized. The magnetic measurements of the compounds revealed different thermal magnetic behavior. Compound 1 undergoes a gradual complete thermal spin crossover (SCO) with T_1/2/ = 243/240 K and a hysteresis loop of 3 K. Compound 2 shows a gradual incomplete SCO during the measurable temperature region with T_1/2 ≈ 330 K. Through the comparison of the magnetic and structural results for 1 and 2 with previously reported related compounds, the alkyl substitutions of the ligand were considered to affect the ligand-field strength and hydrogen-bonding interactions. In addition, photomagnetism results showed an 87 % photoconversion and 36 K of T(LIESST) (LIESST = light-induced excited spin-state trapping) for compound 1. The results enrich the knowledge on how to control the spin-crossover behavior in the new system.

Three coordination polymers, [Cd₂(pvba)₂(tbdc)(dmf)₂] (1), [Co₂(pvba)₂(tbdc)(dmf)₂(H₂O)₂] (2), and [Ni₂(pvba)₂(tbdc)(dmf)₂(H₂O)₂] (3) (H₂tbdc=2,3,5,6-tetrabromobenzenedicarboxylic acid, Hpvba=trans-2-(4′-pyridyl)vinylbenzoic acid), were synthesized by solvothermal methods. The solid-state structures of compounds 1 and 2 were determined by X-ray crystallography. In compounds 1 and 2, the bimetallic cores acted as secondary building units that connected the tbdc ligands in one direction and a pair of pvba ligands, which were aligned in a head-to-tail parallel manner, in the orthogonal direction to form sheet structures. The CC bonds in these pvba ligand pairs in all three compounds were well-aligned to undergo quantitative [2+2] cycloaddition reactions in the solid state under UV irradiation, thereby yielding their cyclobutane derivatives. This photochemical reaction appeared to facilitate structural transformations from one 2D structure into another in the solid state. The photoreactive Co^II- and Ni^II coordination polymers exhibited a reversible dehydration–rehydration reaction that was accompanied by color changes from pink to purple and green to yellow, respectively, owing to a change in coordination number from six to five. Magnetic studies showed that compound 2 was an antiferromagnet, which displayed a field-dependent transition with a critical field (H_c) of 40 kOe at 2 K; the antiferromagnetic interaction between the Co₂ units was strengthened and weakened by dehydration and UV irradiation, respectively. The cyclobutane ligand in the photodimerized products was cleaved on heating to yield a mixture of trans- and cis-isomers of pvba, as monitored by ¹H NMR spectroscopy. The Cd^II coordination polymer underwent quantitative cleavage of the cyclobutane ring whilst the other two underwent partial cleavage.

A single-chain magnet (SCM) was constructed from manganese(III) 5,10,15-tris(pentafluorophenyl)corrole complex [Mn^III(tpfc)] through supramolecular π–π stacking without bridging ligands. In the crystal structures, [Mn(tpfc)] molecules crystallized from different solvents, such as methanol, ethyl acetate, and ethanol, exhibit different molecular orientations and intermolecular π–π interaction or weak Mn⋅⋅⋅O interaction to form a supramolecular one-dimensional motif or dimer. These three complexes show very different magnetic behaviors at low temperature. Methanol solvate 1 shows obvious frequency dependence of out-of-phase alternating-current magnetic susceptibility below 2 K and a magnetization hysteresis loop with a coercive field of 400 Oe at 0.5 K. It is the first example of spin-canted supramolecular single-chain magnet due to weak π–π stacking interaction. By fitting the susceptibility data χ_MT (20–300 K) of 1 with the spin Hamiltonian expression =−2J_AiAi+1+D_iZ², the intrachain magnetic coupling parameter transmitted by π–π interaction of −0.31 cm⁻¹ and zero field splitting parameter D of −2.59 cm⁻¹ are obtained. Ethyl acetate solvate 2 behaves as an antiferromagnetic chain without ordering or slow magnetic relaxation down to 0.5 K. The magnetic susceptibility data χ_MT (20–300 K) of 2 was fitted by assuming the spin Hamiltonian =−2J_AiAi+1, and the intrachain antiferromagnetic coupling constant of −0.07 cm⁻¹ is much weaker than that of 1. Ethanol solvate 3 with a dimer motif shows field-induced single-molecule magnet like behavior below 2.5 K. The exchange coupling constant J within the dimer propagated by π–π interaction is −0.14 cm⁻¹ by fitting the susceptibility data χ_MT (20–300 K) with the spin Hamiltonian =−2J_AB+β(_Ag_A+_Bg_B)H. The present studies open a new way to construct SCMs from anisotropic magnetic single-ion units through weak intermolecular interactions in the absence of bridging ligands.

A family of linear Dy₃ and Tb₃ clusters have been facilely synthesized from the reactions of DyCl₃, the polydentate 3-methyloxysalicylaldoxime (MeOsaloxH₂) ligand with auxiliary monoanionic ligands, such as trichloroacetate, NO₃⁻, OH⁻, and Cl⁻. Complexes 1–5 contain a nearly linear Ln₃ core, with similar Ln⋅⋅⋅Ln distances (3.6901(4)–3.7304(3) Å for the Dy₃ species, and 3.7273(3)–3.7485(5) Å for the Tb₃ species) and Ln⋅⋅⋅Ln⋅⋅⋅Ln angles of 157.036(8)–159.026(15)° for the Dy₃ species and 157.156(8)–160.926(15)° for the Tb₃ species. All three Ln centers are bridged by the two doubly-deprotonated [MeOsalox]²⁻ ligands and two of the four [MeOsaloxH]⁻ ligands through the N,O-η²-oximato groups and the phenoxo oxygen atoms (Dy-O-Dy angles=102.28(16)–106.85(13)°; Tb-O-Tb angles=102.00(11)–106.62(11)°). The remaining two [MeOsaloxH]⁻ ligands each chelate an outer Ln^III center through their phenoxo oxygen and oxime nitrogen atoms. Magnetic studies reveal that both Dy₃ and Tb₃ clusters exhibit significant ferromagnetic interactions and that the Dy₃ species behave as single-molecule magnets, expanding upon the recent reports of the pure 4f type SMMs.

Four cyano-bridged 1D bimetallic polymers have been prepared by using the paramagnetic building block trans-[Ru(acac)₂(CN)₂]⁻ (Hacac=acetylacetone): {[{Ni(tren)}{Ru(acac)₂(CN)₂}][ClO₄]⋅CH₃OH}_n (1) (tren=tris(2-aminoethyl)amine), {[{Ni(cyclen)}{Ru(acac)₂(CN)₂}][ClO₄]⋅ CH₃OH}_n (2) (cyclen=1,4,7,10-tetraazacyclododecane), {[{Fe(salen)}{Ru(acac)₂(CN)₂}]}_n (3) (salen²⁻=N,N′-bis(salicylidene)-o-ethyldiamine dianion) and [{Mn(5,5′-Me₂salen)}₂{Ru(acac)₂(CN)₂}][Ru(acac)₂(CN)₂]⋅ 2 CH₃OH (4) (5,5′-Me₂salen=N,N′-bis(5,5′-dimethylsalicylidene)-o-ethylenediimine). Compounds 1 and 2 are 1D, zigzagged NiRu chains that exhibit ferromagnetic coupling between Ni^II and Ru^III ions through cyano bridges with J=+1.92 cm⁻¹, z J′=−1.37 cm⁻¹, g=2.20 for 1 and J=+0.85 cm⁻¹, z J′=−0.16 cm⁻¹, g=2.24 for 2. Compound 3 has a 1D linear chain structure that exhibits intrachain ferromagnetic coupling (J=+0.62 cm⁻¹, z J′=−0.09 cm⁻¹, g=2.08), but antiferromagnetic coupling occurs between FeRu chains, leading to metamagnetic behavior with T_N=2.6 K. In compound 4, two Mn^III ions are coordinated to trans-[Ru(acac)₂(CN)₂]⁻ to form trinuclear Mn₂Ru units, which are linked together by π–π stacking and weak Mn⋅⋅⋅O* interactions to form a 1D chain. Compound 4 shows slow magnetic relaxation below 3.0 K with ϕ=0.25, characteristic of superparamagnetic behavior. The Mn^III⋅⋅⋅Ru^III coupling constant (through cyano bridges) and the Mn^III⋅⋅⋅Mn^III coupling constant (between the trimers) are +0.87 and +0.24 cm⁻¹, respectively. Compound 4 is a novel single-chain magnet built from Mn₂Ru trimers through noncovalent interactions. Density functional theory (DFT) combined with the broken symmetry state method was used to calculate the molecular magnetic orbitals and the magnetic exchange interactions between Ru^III and M (M=Ni^II, Fe^III, and Mn^III) ions. To explain the somewhat unexpected ferromagnetic coupling between low-spin Ru^III and high-spin Fe^III and Mn^III ions in compounds 3 and 4, respectively, it is proposed that apart from the relative symmetries, the relative energies of the magnetic orbitals may also be important in determining the overall magnetic coupling in these bimetallic assemblies.

We report the synthesis, crystal structures, and spectral, thermal, and magnetic properties of a family of metal–organic perovskite ABX₃, [C(NH₂)₃][M^II(HCOO)₃], in which A=C(NH₂)₃ is guanidinium, B=M is a divalent metal ion (Mn, Fe, Co, Ni, Cu, or Zn), and X is the formate HCOO⁻. The compounds could be synthesized by either diffusion or hydrothermal methods from water or water-rich solutions depending on the metal. The five members (Mn, Fe, Co, Ni, and Zn) are isostructural and crystallize in the orthorhombic space group Pnna, while the Cu member in Pna2₁. In the perovskite structures, the octahedrally coordinated metal ions are connected by the anti–anti formate bridges, thus forming the anionic NaCl-type [M(HCOO)₃]⁻ frameworks, with the guanidinium in the nearly cubic cavities of the frameworks. The Jahn–Teller effect of Cu²⁺ results in a distorted anionic Cu–formate framework that can be regarded as Cu–formate chains through short basal CuO bonds linked by the long axial CuO bonds. These materials show higher thermal stability than other metal–organic perovskite series of [AmineH][M(HCOO)₃] templated by the organic monoammonium cations (AmineH⁺) as a result of the stronger hydrogen bonding between guanidinium and the formate of the framework. A magnetic study revealed that the five magnetic members (except Zn) display spin-canted antiferromagnetism, with a Néel temperature of 8.8 (Mn), 10.0 (Fe), 14.2 (Co), 34.2 (Ni), and 4.6 K (Cu). In addition to the general spin-canted antiferromagnetism, the Fe compound shows two isothermal transformations (a spin-flop and a spin-flip to the paramagnetic phase) within 50 kOe. The Co member possesses quite a large canting angle. The Cu member is a magnetic system with low dimensional character and shows slow magnetic relaxation that probably results from the domain dynamics.

Four types of cobalt–lanthanide heterometallic compounds based on metalloligand Co(2,5-pydc)₃³⁻ (2,5-H₂pydc=pyridine-2,5-dicarboxylate acid), [Ln₂Co₂(2,5-pydc)₆(H₂O)₄]_n⋅2n H₂O (1) (Ln=Tb, Dy for 1 a, 1 b respectively), [Tb₂Co₂(2,5-pydc)₆(H₂O)₄]_n⋅3n H₂O (2), [Tb₂Co₂(2,5-pydc)₆(H₂O)₉]_n⋅4n H₂O (3), and [LaCo(2,5-pydc)₃(H₂O)₂]_n⋅2n H₂O (4) have been synthesized. Compound 1 has a layer structure with well-isolated carboxylate-bridged Ln³⁺ chains, compound 2 is a three-dimensional (3D) porous network with Tb³⁺ chains that are also well isolated and carboxylate bridged, 3 is a layer structure based on dinuclear units, and 4 is a 3D network with boron nitride (BN) topology. DC magnetic studies reveal ferromagnetic coupling in all the carboxylate-bridged Ln³⁺ chains in 1 a, 1 b, and 2. Compared to the silence of the out-of-phase ac susceptibility of 2, above 1.9 K the magnetic relaxation behavior of both 1 a and 1 b is slow like that of a single-chain magnet.