Xin-Hua Zhao

Cation-Dependent Magnetic Ordering and Room-Temperature Bistability in Azido-Bridged Perovskite-Type Compounds

A series of end-to-end azido-bridged perovskite-type compounds [(CH3)nNH4-n][Mn(N3)3] (n = 1-4) were synthesized and characterized. Structural phase transitions indicating the general lattice flexibility were observed and confirmed by the crystal structures of different phases. These materials show cation-dependent magnetic ordering at up to 92 K and magnetic bistability near room temperature.

Spin Crossover in [Fe(2-Picolylamine)3]2+ Adjusted by Organosulfonate Anions

Three mononuclear spin crossover (SCO) compounds [Fe(2-pic)3]·A2·Solv (A = m-ABS(-), Solv = MeOH, 1; A = p-ABS(-), 2; A = OTf(-), 3) were prepared and characterized magnetically and structurally (2-pic = 2-picolylamine, m-HABS = m-aminobenzenesulfonic acid, p-HABS = p-aminobenzenesulfonic acid, HOTf = trifluoromethanesulfonic acid). Single-crystal X-ray analyses show that they are constructed from the charge-assisted hydrogen bonds between the 2-pic donors and the organosulfonate acceptors, forming the hydrogen-bonded three-dimensional networks for 1 and 2 and one-dimensional columns for 3. While the [Fe(2-pic)3](2+) cations in compounds 1 and 2 are in the meridional (mer-) configuration, it has a facial (fac-) configuration in complex 3. Magnetic susceptibility measurements revealed the SCO transitions and the SCO properties in all three complexes are quite different. Compound 1 undergoes an abrupt SCO with critical temperatures T1/2↓ = 100 K and T1/2↑ = 103 K, while compound 2 exhibits a gradual SCO with T1/2 = 218 K. Compound 3, with the fac-configuration, has an abrupt SCO transition accompanied by the structural phase transition with critical temperatures T1/2↓ = 333 K and T1/2↑ = 343 K. The SCO transitions were further confirmed by the detailed structural analyses of the coordination environments of the Fe(II) centers in both spin states and also by differential scanning calorimetry. Compared to the famous [Fe(2-pic)3]·A2·Solv compounds in the literature, compound 2 has the highest transition temperature for the mer-[Fe(2-pic)3](2+)-containing compounds, while compound 3 represents the first example of the structurally characterized compound of the fac-[Fe(2-pic)3](2+) motif showing SCO behavior. These results show that the organosulfonate anions are very promising to adjust the hydrogen-bonded structures of the SCO compounds and improve the SCO properties of those structures.

Syntheses, structures, and magnetic properties of three new chain compounds based on a pentagonal bipyramidal Co(II) building block

Starting from the pentagonal bipyramidal single-ion magnet [Co(tdmmb)(H2O)2][BF4]2 and a series of linear bridging ligands of different lengths, three new cobalt(II) one-dimensional (1D) coordination polymers, [Co(tdmmb)(pyz)][BF4]2·CH3CN·H2O (1), [Co(tdmmb)(apy)][BF4]2·2CH3CN (2), and [Co(tdmmb)(bpe)][BF4]2·3CH3CN (3) (tdmmb = 1,3,10,12-tetramethyl-1,2,11,12-tetra-aza[3](2,6)pyridino[3](2,9)-1,10-phenanthrolinophan-2,10-diene) (pyz = pyrazine) (apy = 4-aminopyridine) (bpe = 1,2-di(4-pyridyl)ethane)), have been synthesized and characterized structurally and magnetically. Symmetric ligands pyz and bpe result in the nearly linear chains in 1 and 3, while the asymmetric ligand apy leads to the zig-zag chain in 2. As the length of the ligands increases, the intrachain Co⋯Co distance also increases, i.e. 7.56, 7.70, and 13.85 A for 1–3, respectively. Magnetic investigation revealed the gradual decrease in the magnetic interactions between the Co2+ ions from 1 to 3, along with the increase in the intrachain Co–Co distances. Due to the existence of the weak antiferromagnetic (AF) interaction, the SIM behavior of the starting material is surpressed in compounds 1 and 2, and they show a simple paramagnetic behavior. While for 3, field-induced slow magnetic relaxation was observed at a low temperature with an energy barrier of Ueff = 19 K. This observation suggests that the magnetic interaction between the Co2+ centers in 3 is negligible, consistent with the very long Co–Co distance in 3. These 1D compounds are rare chain compounds composed of metal centers with a nearly D5h symmetry. Notably, compound 3 is the second example of a 1D Co(II) chain compound displaying a SIM behaviour. These studies illustrated that the magnetic interaction between the SIMs can be used to tune slow magnetic relaxation.

Slow Magnetic Relaxation in One-Dimensional Azido-Bridged CoII Complexes

Crystal structures and magnetic properties of three one-dimensional (1D) azido-bridged cobalt(II) chains with different amide ligands (L), [Co2(N3)4(DMF)3] (1), [Co4(N3)8(DEF)5] (2), and [Co2(N3)4(DIPF)2] (3) (DMF = N,N-dimethylformamide, DEF = N,N-diethylformamide, and DIPF = N,N-diisopropylformamide), are reported to investigate the influence of L on their structures and magnetic properties. Single-crystal X-ray crystallographic analysis revealed that, although 1-3 all consist of cobalt chains bridged by end-on (EO) azides, the coordination geometry of the CoII ions and the repeating units of the 1D structures are quite different. As the size of L increases, the ratio of L to CoII decreases from 6:4 in 1 to 5:4 and 4:4 in 2 and 3, respectively. In 1, two [CoN5O1] and two [CoN4O2] distorted octahedra form the {[CoN5O1][CoN4O2]2[CoN5O1]} tetramers (denoted as Co4A), which are linked to each other by sharing the N-N edge to form the chain. Similarly, the chain structure of 3 is constructed from a similar tetramer unit {[CoN5][CoN4O2]2[CoN5]} (denoted as Co4B), where half of the CoII centers are in the [CoN5] trigonal bipyramid because of the larger steric effect of the DIPF ligand, while, for compound 2 of the medium-sized amide, it has the transition structure between those of 1 and 3. The chain is composed of two different repeating units: Co4A unit similar to that in 1 and Co4B unit similar to that in 3. Because of their similar structures, compounds 1-3 exhibit analogical magnetic properties. Direct-current magnetic measurements demonstrated that all compounds show intrachain ferromagnetic coupling through the EO azides and interchain anti-ferromagnetic interactions. Alternating-current data revealed the slow magnetic relaxation in the anti-ferromagnetic ordered phases. While compound 1 exhibits spin glass behavior, compounds 2 and 3 behave as the single-chain magnets. This difference might come from the interference of the anti-ferromagnetic ordering on the magnetic dynamic of the magnetic chain.

Structural and magnetic tuning from a field-induced single-ion magnet to a single-chain magnet by anions

We herein report the syntheses, crystal structures, and magnetic properties of two complexes based on the anisotropic pentagonal bipyramidal FeII starting material [Fe(LN3O2)]2+, namely [Fe(LN3O2)(H2O)2][MQ]2·H2O (1) and [Fe(LN3O2)(CN)][ABSA]·3H2O (2) (LN3O2 = 2,13-dimethyl-6,9-dioxa-3,12,18-triazabicyclo[12.3.1]octadeca-1(18),2,12,14,16-pentaene; MQ− = methyl orange anion; ABSA− = 4-aminoazobenzene-4′-sulfonic anion). Compound 1 is a mononuclear material where the [Fe(LN3O2)(H2O)2]2+ cations form a one-dimensional (1D) chain by the hydrogen bonds between the bulky MQ− anions and the coordinated water molecules. With a slightly different anion, ABSA−, a cyano-bridged FeII chain is formed for compound 2. This chain has a similar structure as that in our recently reported compound [Fe(LN5)(CN)][BF4] (Chem. Commun., 2015, 51, 4360). However, compared with the reported result where the chains interact with each other through the π⋯π interactions, the chains in 2 are well isolated by the bulky ABSA− anions with the shortest interchain Fe⋯Fe distances around 12.0 A. Magnetic investigation on 1 reveals the easy-axis magnetic anisotropy of the mononuclear FeII centre (zero-field splitting parameter D = −3.7 cm−1), which leads to the field-induced slow magnetic relaxation. For compound 2, the Fe2+ spins are antiferromagnetically coupled through the cyano bridges with a coupling constant of J = −4.13(2) cm−1 with the Hamiltonian H = −J∑Si·Si+1. AC magnetic measurements revealed the pure single-chain magnetic (SCM) behaviour of these isolated chains with an effective energy barrier of 26.1(5) K. This system represents a good example showing that the structures and magnetic properties, such as field-induced single-ion magnets, SCMs, and SCM-based magnets, can be selectively prepared by anion modification.