Showing papers by "Cameron J. Kepert published in 2005"

Negative thermal expansion and low-frequency modes in cyanide-bridged framework materials

Abstract: We analyze the intrinsic geometric flexibility of framework structures incorporating linear metal–cyanide– metal sM–CN–M8d linkages using a reciprocal-space dynamical matrix approach. We find that this structural motif is capable of imparting a significant negative thermal expansion sNTEd effect upon such materials. In particular, we show that the topologies of a number of simple cyanide-containing framework materials support a very large number of low-energy rigid-unit phonon modes, all of which give rise to NTE behavior. We support our analysis by presenting experimental verification of this behavior in the family of compounds ZnxCd1−xsCNd2, which we show to exhibit a NTE effect over the temperature range 25–375 K more than double that of materials such as ZrW2O8.

Guest-Dependent Negative Thermal Expansion in Nanoporous Prussian Blue Analogues MIIPtIV(CN)6·x{H2O} (0 ≤ x ≤ 2; M = Zn, Cd)

Abstract: The guest-dependent thermal expansion behavior of the nanoporous Prussian Blue analogues MIIPtIV(CN)6.x{H2O} (0

Direct Observation of a Transverse Vibrational Mechanism for Negative Thermal Expansion in Zn(CN) 2 : An Atomic Pair Distribution Function Analysis

Abstract: The instantaneous structure of the cyanide-bridged negative thermal expansion (NTE) material Zn(CN)2 has been probed using atomic pair distribution function (PDF) analysis of high energy X-ray scattering data (100−400 K) The temperature dependence of the atomic separations extracted from the PDFs indicates an increase of the average transverse displacement of the cyanide bridge from the line connecting the ZnII centers with increasing temperature This allows the contraction of non-nearest-neighbor Zn···Zn‘ and Zn···C/N distances despite the observed expansion of the individual direct Zn−C/N and C−N bonds Thus, this analysis provides definitive structural confirmation that an increase in the average displacement of bridging atoms is the origin of the NTE behavior The lattice parameters reveal a slight reduction in the NTE behavior at high temperature from a minimum coefficient of thermal expansion (α = dl/ldT) of −198 × 10-6 K-1 below 180 K, which is attributed to interaction between the doubly interp

In situ single-crystal X-ray diffraction studies of desorption and sorption in a flexible nanoporous molecular framework material.

Abstract: The subtle flexibility of the framework material Co(bpy)1.5(NO3)2·(guest) (bpy = 4,4‘-bipyridine) (1·(guest)) is demonstrated quantitatively through in situ single-crystal X-ray diffraction measurements of guest desorption and sorption processes. Variable temperature unit cell determinations were employed to monitor the uptake and release of guest species, and full structural determinations have been carried out for the as-grown ethanol-loaded framework (1·(EtOH)), for the empty host framework (1), and for each of the five introduced guests (methanol: 1·(MeOH), acetone: 1·(ACN), acetonitrile: 1·(MeCN), tetrahydrofuran: 1·(THF), dichloromethane: 1·(DCM)). The framework consists of interdigitated two-dimensional bilayers of cobalt(II) centers bridged by bpy ligands, with one-dimensional pores that account for approximately 20% of the total volume. The sorption of guest species of varying size and shape has revealed the framework's ability to adapt to different guests through a range of different framewor...

Selective Recovery of Dynamic Guest Structure in a Nanoporous Prussian Blue through in Situ X-ray Diffraction: A Differential Pair Distribution Function Analysis

Abstract: We use in situ X-ray diffraction to obtain data suitable for differential PDF analysis, providing unique insight into the structure of weakly bound, dynamic N2 molecules in the Prussian blue system Mn3[Co(CN)6]2·x{N2}. The differential PDF shows a distributed orientation of N2 molecules constrained to sites centered about the (1/4,1/4,1/4) positions within the pores. The results show a subtle response of the framework to guest loading which corresponds principally to the perturbation of the Mn ion coordination.

New cobalt(II) and zinc(II) coordination frameworks incorporating a pyridyl–pyrazole ditopic ligand

Abstract: The metal-directed assembly of new molecular frameworks incorporating 4-(4-pyridyl)pyrazole (L), containing non-linear coordination vectors, is presented. Three metallo-arrays of types [Co(LH)2(NO3)4], [Co(LH)2(H2O)4][NO3]4·H2O and [Zn2(L–H)2Cl2]·2EtOH are reported. The cobalt(II) in [Co(LH)2(NO3)4] displays distorted octahedral geometry, with the two protonated pyridyl–pyrazole ligands coordinated through their pyrazole nitrogen atoms in a trans-orientation; the remaining four coordination sites are occupied by nitrate anions. Two internal hydrogen bonds occur between each pyrazole NH and the oxygens of adjacent coordinated nitrato ligands. Short intermolecular hydrogen bonds also occur between the two pyridinium hydrogens and bound nitrate ligands on different molecules to yield a two-dimensional hydrogen-bonded array. Two of these arrays interpenetrate to form an extended two dimensional layer; such layers stack throughout the crystal structure. A second product of type [Co(LH)2(H2O)4][NO3]4·H2O exists as two crystallographically independent, but chemically similar, forms. In each form, the two protonated pyridyl–pyrazole ligands occupy trans positions about the cobalt, with the remaining four coordination sites being filled by water molecules to yield a distorted octahedral coordination geometry. Intramolecular hydrogen-bonding is observed between the two non-coordinated pyrazoyl nitrogen atoms and bound water oxygen atoms. The third complex, [Zn2(L–H)2Cl2]·2EtOH, contains dimer units consisting of two zinc(II) ions bridged by two pyrazoylate groups in which the coordination geometry of each zinc approximates a tetrahedron. Each zinc is bound to two deprotonated pyridine–pyrazole ligands (L–H), one pyridyl group (from a different dimeric unit) and one chloro ligand. Each pyridyl nitrogen thus connects each of these zinc dimers to an adjacent dimer unit, forming a three-dimensional network containing small voids. The latter are occupied by ethanol molecules which form hydrogen bonds to the chloro ligands.

Iron(ii) Molecular Framework Materials with 4,4′-Azopyridine

Abstract: Three new iron(ii) molecular-framework materials incorporating the bridging ligand 4,4′-azopyridine (azpy) have been synthesized and structurally characterized: Fe2(azpy)4(NCS)4·(azpy) (A), Fe(azpy)(NCSe)2(EtOH)2·(azpy) (B), and Fe(azpy)2(NCSe)2·2(MeCN) (C). A and C consist of non-interpenetrating (4,4) grids of iron(ii) centres bridged by azpy ligands with non-coordinating azpy ligands or acetonitrile molecules occupying the spaces within and between the layers. For B, hydrogen-bonding interactions between coordinated ethanol molecules and non-coordinated azpy ligands link linear Fe–azpy chains to give a two-dimensional framework.

A highly distorted (10,3)-a coordination framework constructed from alternating T-shaped and trigonal nodes

Abstract: The isostructural 3-D coordination frameworks M2(tpt)2(NCS)4(BzOH)2·4(BzOH)·(H2O) (A: M = Fe(II), B: M = Co(II), tpt = 2,4,6-tris(4′-pyridyl)-1,3,5-triazine, BzOH = benzyl alcohol) consist of doubly interpenetrated (10,3)-a nets distorted by alternating T-shaped (M) and trigonal (tpt) three-connecting nodes.

Synthesis and characterization of subcell-supercell related ethylenediamine-pillared zinc hydroxysulfates

Abstract: Two ethylenediamine-pillared zinc hydroxysulfates related as subcell, Zn4SO4(OH)6(C2N2H8)0.5·3H2O, and supercell, [Zn4SO4(OH)6(C2N2H8)0.5]7·18H2O, the zinc analogues of Co4SO4(OH)6(C2N2H8)0.5·3H2O, were prepared and structurally characterized by single-crystal X-ray diffraction. The subcell−supercell relation is described. The analysis of hydrogen bonds and cavities were conducted, suggesting the existence of subcell−supercell to be due to a strong correlation between the organic pillars, the interlayer guests, and groups decorating the inorganic layers. Reversible dehydration−rehydration behavior on the subcell is demonstrated via thermogravimetric/differential thermal analysis and single-crystal X-ray diffraction.

New Cadmium(II) and Iron(II) Coordination Frameworks Incorporating a Di(4‐pyridyl)isoindoline Ligand

Abstract: The metal-directed assembly of molecular frameworks incorporating the ditopic heterocyclic ligand 1-(4-pyridyl)imino-2-(4-pyridyl)isoindoline (L) associated with nonlinear coordination vectors, is presented. Two infinite metallo-arrays of empirical formula [CdL(NO3)2(EtOH)]n·(1.4 H2O)n and [FeL2(NCS)2]n·{(2 CHClCCl2)·(1.4 MeOH)·(2.9 H2O)}n have been characterised. The X-ray structures of both arrays are reported. The cadmium(II) framework consists of co-aligned 1D coordinate polymer chains while the iron(II) framework consists of rhombic grids with 2D polymeric connectivity. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

A three‐dimensional hydrogen‐bonded coordination framework: catena‐poly[[[tetraaqua­iron(II)]‐μ‐4,4′‐bipyridine] benzene‐1,4‐dicarboxyl­ate]

Abstract: The title compound, {[Fe(C10H8N2)(H2O)4](C8H4O4)}n or [Fe(bpy)(H2O)4](bdc) (bpy is 4,4′-bipyridine and bdc is benzene-1,4-dicarboxyl­ate), contains one-dimensional chains of FeII centres bridged by bpy ligands along the b axis, with the equatorial coordination of four water molecules completing the octahedral coordination. Non-coordinated bdc anions balance the charge of the FeII centres and bridge adjacent one-dimensional chains through an extensive hydrogen-bonding network. The compound crystallizes in the centrosymmetric space group P2/c, which defines twofold axes along both the Fe–bpy chains and the bdc anions.

2,4,6-Tris(4-pyridylmethyl­sulfan­yl)-1,3,5-triazine monohydrate

Abstract: In the crystal structure of the title compound, C21H18N6S3·H2O, the ligands are held together through π–π stacking inter­actions and O—H⋯N hydrogen bonds involving the water mol­ecule.

4‐(4‐Pyridylamino)pyridinium isophthalate

Abstract: In the title salt, C10H10N3+·C8H5O4−, the protonated 4,4′-dipyridylamine and deprotonated isophthalic acid mol­ecules are bound together in the crystal structure through π–π stacking, O⋯H—N, O⋯H—C and N⋯H—O inter­actions.