No abstract

A unique hydrogen-bonded organic–inorganic framework (HOIF) constructed from a mononuclear cobalt­(II) complex, [Co­(MCA)2·(H2O)2] (HMCA = 4-imidazole­carboxylic acid), via multiple hydrogen-bonding interactions was synthesized and structurally characterized. The Co­(II) center in the HOIF features a highly distorted octahedral coordination environment. Remarkably, the CoII HOIF showed permanent porosity with superior stability as established by combined thermogravimetric analysis (TGA), variable-temperature infrared spectra (IR), variable-temperature powder X-ray diffraction data (PXRD), and a CO2 isotherm. Structural studies reveal that short multiple hydrogen bonds should be responsible for the superior thermal and chemical stability of a HIOF. Magnetic investigations reveal the large easy-plane magnetic anisotropy of the Co2+ ions with the fitted D values being 22.1 (magnetic susceptibility and magnetization data) and 29.1 cm–1 (reduced magnetization data). In addition, the HOIF exhibits field-induced slow magnetic relaxation at low temperature with an effective energy barrier of U eff = 45.2 cm–1, indicative of a hydrogen-bonded framework single-ion magnet of the compound. The origin of the significant magnetic anisotropy of the complex was also understood from computational studies. In addition, BS-DFT calculations indicate that the superexchange interactions between the neighboring CoII ions are non-negligible anti­ferro­magnetism with J Co–Co = −0.5 cm–1. The foregoing results provide not only a carboxylate–imidazole ligand approach toward a stable HOIF but also a promising way to build a robust single-ion magnet via hydrogen-bond interactions.

A novel Zn-MOF-based photochromic complex with tunable photophysical behavior has been developed. The control of luminescent emission and color can be achieved in a single compound by utilizing suitable molecular self-assembly of luminophore and photochromic component.

Magnetism in iron at high temperature is investigated by calculating the total electronic band-structure energy for four types of spin arrangements. A slow smooth spatial variation of spin direction costs relatively little energy and the atomic moment m is reduced only ~10%. More rapid variations have considerably higher energy, which may explain the high degree of short-range order and small 6m observed at T £T C . Other aspects are also discussed.PACS numbers: 75.50.Bb, 75.10.DgThe magnetic properties of metallic iron at high temperature are still not fully understood. A large body of opinion 1 " 8 believes that magnetic disorder sets in as a result of the rotation of atomic moments m^. This leads to a Curie temperature T c~v 2 /W an order of magnitude lower than in simple Stoner 9 theory where T c (Stoner) ~ v where W is the width of the electron energy bands and 2v their exchange splitting. Above T c iron 10 (and nickel 11 ) appears to retain a remarkable amount of short-range order, 1~4 » 10 well defined spin-wave excitations having been measured to 1AT C . Quantitative understanding of this situation in terms of metallic electronic structure is still lacking though there have been significant recent theoretical advances. 1 "* 8 We have calculated the electronic structure and total energy U of Fe with various spin arrangements to investigate (a) to what degree the atomic magnetic moment is retained 12 when spins rotate; (b) the reason for the large short-range order 10 above T c and its form; and (c) the magnitudes of the first-, second-, etc., neighbor couplings, among other aspects. We believe that these are the first ab initio calculations to throw light on the magnetic interactions in Fe beyond a nearestneighbor picture and on the short-range order above T c . Our tight-binding model has five d orbitals with or without an s orbital to represent somewhat crudely the sp band. The hopping parameters are taken from Eqs. (9) and (10) of Pettifor 13 with W d = 0A6 Ry. As in Ref. 5, the exchange potential 9 V ex = ±Vi on atom i acts on each d orbital with down (up) spin, with the ± sign defined with respect to the local 1 direction m { of the magnetic moment fn i on atom i. The directions rki are imposed in calculating the electronic structure for any desired spin arrangement. The magnitudes vi and corresponding m { have to be determined self-consistently 9 by 2v i = Im { in terms of the Stoner intra-atomic exchange interaction 7 = 0.07 Ry taken from Gunnarsson. 14 The time variation of the spin arrangement is ignored as slow compared with electronic motion, and the temperature T~T C assumed sufficiently below T c (Stoner) for single-particle excitations 9 to be neglected. The total energy U is U 1 (the sum of occupied one-electron energies) plus the correction Yj% v i 2 / I for double-counting exchange. The recursion method 15 applied to a large cluster of 700 atoms (8000 spin orbitals) allows us to calculate the local density of states near the cluster center and hence U for an arbitrary arrangement of spin d...