Hysteretic three-step spin crosover in a thermo-and photochromic 3D pillared hofmann-type metal-organic framework
Abstract not provided
Sciortino, N F↗
Engineering topics
Chapman, K W
Publications and source records attributed to Chapman, K W.
Importance of halogen---halogen contacts for the structural and magnetic properties of CuX{<_2}(pyrazine-N,N'-dioxide)(H{<_2}O){<_2} (x = Cl & Br).
The structural and magnetic properties of the newly crystallized CuX{sub 2}(pyzO)(H{sub 2}O){sub 2} (X = Cl, Br; pyzO = pyrazine-N,N{prime}-dioxide) coordination polymers are reported. These isostructural compounds crystallize in the monoclinic space group C2/c with, at 150 K, a = 17.0515(7) {angstrom}, b = 5.5560(2) {angstrom}, c = 10.4254(5) {angstrom}, {beta} = 115.400(2){sup o}, and V = 892.21(7) {angstrom}{sup 3} for X = Cl and a = 17.3457(8) {angstrom}, b = 5.6766(3) {angstrom}, c = 10.6979(5) {angstrom}, {beta} = 115.593(2){sup o}, and V = 950.01(8) {angstrom}{sup 3} for X = Br. Their crystal structure is characterized by one-dimensional chains of Cu{sup 2+} ions linked through bidentate pyzO ligands. These chains are joined together through OH {hor_ellipsis} O hydrogen bonds between the water ligands and pyzO oxygen atoms and Cu-X {hor_ellipsis} X-Cu contacts. Bulk magnetic susceptibility measurements at ambient pressure show a broad maximum at 7 (Cl) and 28 K (Br) that is indicative of short-range magnetic correlations. The dominant spin exchange is the Cu-X {hor_ellipsis} X-Cu supersuperexchange because the magnetic orbital of the Cu{sup 2+} ion is contained in the CuX{sub 2}(H{sub 2}O){sub 2} plane and the X {hor_ellipsis} X contact distances are short. The magnetic data were fitted to a Heisenberg 1D uniform antiferromagnetic chain model with J1D/kB = -11.1(1) (Cl) and -45.9(1) K (Br). Magnetization saturates at fields of 16.1(3) (Cl) and 66.7(5) T (Br), from which J1D is determined to be -11.5(2) (Cl) and -46.4(5) K (Br). For the Br analog the pressure dependence of the magnetic susceptibility indicates a gradual increase in the magnitude of J1D/kB up to -51.2 K at 0.84 GPa, suggesting a shortening of the Br {hor_ellipsis} Br contact distance under pressure. At higher pressure X-ray powder diffraction data indicates a structural phase transition at 3.5 GPa. Muon-spin relaxation measurements indicate that CuCl{sub 2}(pyzO)(H{sub 2}O){sub 2} is magnetically ordered with TN = 1.06(1) K, while the signature for long-range magnetic order in CuBr{sub 2}(pyzO)(H{sub 2}O){sub 2} was much less definitive down to 0.26 K. The results for the CuX{sub 2}(pyzO)(H{sub 2}O){sub 2} complexes are compared to the related CuX{sub 2}(pyrazine) materials.
08 HYDROGEN↗
Pair distribution function-derived mechanism of a single-crystal to disordered to single-crystal transformation in a hemilabile metal-organic framework
Flexible metal-organic frameworks (MOFs) are materials of great current interest. A small class of MOFs show flexibility driven by reversible bonding rearrangements that lead directly to unusual properties. Cu-SIP-3 is a MOF based on the 5-sulfoisophthalate ligand, where the strong copper-carboxylate bonds ensure that the three-dimensional integrity of the structure is retained while allowing bonding changes to occur at the more weakly bonding sulfonate group leading to unusual properties such as the ultra-selective adsorption of only certain gases. While the integrity of the framework remains intact during bonding changes, crystalline order is not retained at all times during the transformations. X-Ray diffraction reveals that highly crystalline single crystals lose order during the transformation before regaining crystallinity once it is complete. Here we show how X-ray pair distribution function analysis can be used to reveal the mechanism of the transformations in Cu-SIP-3, identifying the sequence of atomic displacements that occur in the disordered phase. A similar approach reveals the underlying mechanism of Cu-SIP-3's ultra-selective gas adsorption.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Combining pair distribution function and computational methods to understand lithium insertion in brookite (TiO{sub 2}).
X-ray pair distribution function (PDF) methods and first-principles calculations have been combined to probe the structure of electrochemically lithiated TiO{sub 2} Brookite. Traditional powder diffraction studies suggest that Brookite amorphizes upon lithium insertion, with the Bragg reflections disappearing. However, PDF analysis indicates that the TiO{sub 2} framework connectivity is maintained throughout lithium intercalation, with expansions along the a and b axes. The Li{sup +} ions within the framework are poorly observed in the X-ray PDF, which is dominated by contributions from the more strongly scattering Ti and O atoms. First-principles calculations were used to identify energetically favorable Li{sup +} sites within the Brookite lattice and to develop a complete structural model of the lithiated material. This model replicates the local structure and decreased intermediate range order observed in the PDF data. The analysis suggests that local structural distortions of the TiO{sub 2} lattice accommodate lithium in five-coordinate sites. This structural model is consistent with the observed electrochemical behavior.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗