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Lee Brammer

Publications and source records attributed to Lee Brammer.

9 recordsLinked to original sources
PubMed2006

Two-dimensional metal-organic frameworks containing linear dicarboxylates.

The solvothermal synthesis of four two-dimensional metal-organic frameworks containing linear dicarboxylic acids as ligands for Zn(II) centres is described. Zn(BDC)(DMF) [(1) where BDC = benzene-1,4-dicarboxylic acid; DMF = N,N-dimethylformamide] adopts a common paddlewheel motif leading to a 4(4) grid network, whereas Zn(3)(BDC)(3)(EtOH)(2) (2), Zn(3)(BDC)(3)(H(2)O)(2) * 4DMF (3) and Zn(3)(BPDC)(3)(DMF)(2) * 4DMF (4) each form networks with the relatively uncommon 3(6) topology based upon Zn(3)(O(2)CR)(6) secondary building units. All contain coordinated solvent molecules, namely DMF [(1) and (4)], ethanol (2) or H(2)O (3). Comparison of structures (2) and (3) illustrates a clay-like flexibility in interplanar spacing which sheds light on the ability of the Zn(3)(BDC)(3) framework to undergo desolvation and uptake of small solvent and gas molecules.

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PubMed2006

Reversible extrusion and uptake of HCl molecules by crystalline solids involving coordination bond cleavage and formation.

Yellow crystalline salts (3-XpyH)2[CuCl4] (3-XpyH = 3-halopyridinium, X = Cl, Br) lose HCl upon exposure to air in an open vessel, yielding quantitatively blue crystalline coordination compounds [CuCl2(3-Xpy)2]. The reaction is prevented if the vessel is sealed, but can be driven forward under such conditions by providing a trapping agent for HCl, such as an aqueous solution of AgNO3. The reaction requires cleavage of Cu-Cl and N-H bonds and formation of Cu-N bonds. The metal coordination geometry also changes from distorted tetrahedral to square planar. Remarkably, the reaction is fully reversible upon exposure of the blue coordination compound to vapor from a concentrated aqueous solution of HCl, and the initial yellow crystalline salt results. The structural changes occurring in these reactions have been followed by X-ray powder diffraction, including Rietveld refinement, of the crystal structures.

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PubMed2005

Supramolecular chemistry of halogens: complementary features of inorganic (M-X) and organic (C-X') halogens applied to M-X...X'-C halogen bond formation.

Electronic differences between inorganic (M-X) and organic (C-X) halogens in conjunction with the anisotropic charge distribution associated with terminal halogens have been exploited in supramolecular synthesis based upon intermolecular M-X...X'-C halogen bonds. The synthesis and crystal structures of a family of compounds trans-[MCl(2)(NC(5)H(4)X-3)(2)] (M = Pd(II), Pt(II); X = F, Cl, Br, I; NC(5)H(4)X-3 = 3-halopyridine) are reported. With the exception of the fluoropyridine compounds, network structures propagated by M-Cl...X-C halogen bonds are adopted and involve all M-Cl and all C-X groups. M-Cl...X-C interactions show Cl...X separations shorter than van der Waals values, shorter distances being observed for heavier halogens (X). Geometries with near linear Cl...X-C angles (155-172 degrees ) and markedly bent M-Cl...X angles (92-137 degrees ) are consistently observed. DFT calculations on the model dimers {trans-[MCl(2)(NH(3))(NC(5)H(4)X-3)]}(2) show association through M-Cl...X-C (X not equal F) interactions with geometries similar to experimental values. DFT calculations of the electrostatic potential distributions for the compounds trans-[PdCl(2)(NC(5)H(4)X-3)(2)] (X = F, Cl, Br, I) demonstrate the effectiveness of the strategy to activate C-X groups toward halogen bond formation by enhancing their electrophilicity, and explain the absence of M-Cl...F-C interactions. The M-Cl...X-C halogen bonds described here can be viewed unambiguously as nucleophile-electrophile interactions that involve an attractive electrostatic contribution. This contrasts with some types of halogen-halogen interactions previously described and suggests that M-Cl...X-C halogen bonds could provide a valuable new synthon for supramolecular chemists.

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PubMed2004

Developments in inorganic crystal engineering.

The design and synthesis of crystalline materials through the self-assembly of molecular building blocks and the pursuit of functional materials based upon this approach are usually classified under the banner Crystal Engineering. The field is interdisciplinary in nature involving synthetic, materials, structural and theoretical chemists. There are strong ties to modern crystallography which can offer rapid and accurate structure determination and, in particular, insight into molecular and intermolecular geometries. Illustrative examples that chart the development field and provide an assessment of the current state of the art will be reviewed with an emphasis on inorganic chemistry. Broadly speaking, two classes of compounds will be discussed: those based upon molecules or ions linked into networks via noncovalent interactions and those (coordination polymers) in which metal centres are linked using coordination bonds through bridging ligands into extended networks.

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PubMed2004

Water molecules insert into N-H...Cl-M hydrogen bonds while M-Cl...X-C halogen bonds remain intact in dihydrates of halopyridinium hexachloroplatinates.

The crystal structures of the 3-halopyridinium hexachloroplatinate(IV) dihydrates (HPyX-3)(2)[PtCl(6)]x2H(2)O [(1), X = Br; (2a), (2b), X = I] comprise networks in which the molecular components are linked via N-H...O and O-H...Cl-Pt hydrogen bonds and Pt-Cl...X-C halogen bonds (X = Br, I). The iodo derivative has been isolated in two polymorphic forms. Of particular interest to the understanding of the utility of the hydrogen bonds and M-X...X'-C halogen bonds that propagate the networks in anhydrous salts of this type is that the water molecules insert exclusively into the putative N-H...Cl-Pt hydrogen bonds, while the Pt-Cl...X'-C halogen bonds remain undisrupted by the presence of water molecules.

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PubMed2002

Hydrogen bonding and perhalometallate ions: a supramolecular synthetic strategy for new inorganic materials.

A synthetic strategy for constructing ionic hydrogen-bonded materials by combining perhalometallate anions with cations able to serve as hydrogen bond donors is presented. The approach is based on identification of well defined hydrogen bond acceptor sites on the anions by a combination of experimental and theoretical approaches. Selective population of these sites by hydrogen bond donors has the potential to afford organized crystalline arrays in one, two, or three dimensions. The approach is applicable to a wide range of metal centers.

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PubMed1998

Unexpectedly Lengthened N-H.Co Hydrogen Bonds?

Low-temperature crystal structures of QuinH(+)Co(CO)(4)(-), 1 (QuinH(+) = quinuclidinium), (DABCO)H(+)Co(CO)(3)P(p-tolyl)(3)(-), 2, and (DABCO)H(+)Co(CO)(3)PPh(2)(p-tolyl)(-), 3 (DABCO = 1,4-diazabicyclooctane), 2 and 3 as their acetonitrile solvates, demonstrate that these salts exhibit intermolecular N-H.Co hydrogen bonding between the cation and anion components. NMR and IR data demonstrate the persistence of these interactions in toluene solution. Such solution-state data, which examine solvated ion pairs, suggest little difference between these salts and the corresponding previously reported salts (DABCO)H(+)Co(CO)(3)L(-) (4, L = CO; 5, L = PPh(3)). However, in the solid state, the N-H.Co hydrogen bonds in 1-3 are some 0.1-0.15 Å longer than would be predicted from consideration of the structures of 4 and 5 and the aforementioned similarity to 4 and 5 in solution. In previous reports we have shown that major steric or electronic changes to the anion or cation have resulted in substantial changes (0.15-0.3 Å) in the N.Co [H.Co] separation for N-H.Co hydrogen bonds in related R(3)NH(+)Co(CO)(3)L(-) (L = CO, PR(3)) salts. In this report, we present examples in which small changes are made to the anion or cation remote from the N-H.Co hydrogen bond. In the solid state, the effect of these small changes on this hydrogen bond is subsumed by the effect of changes in the supramolecular structure. This clearly indicates the sensitivity of the geometry of these hydrogen bonds to the overall balance of intermolecular interactions in the solid state and as such is pertinent to current interest in weak (intermolecular) interactions for which characterization by X-ray crystallography is important.

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