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Sarah A Barnett

Publications and source records attributed to Sarah A Barnett.

7 recordsLinked to original sources

Accurate molecular structures and hydrogen bonding in two polymorphs of ortho-acetamidobenzamide by single-crystal neutron diffraction.

The structures of both known forms of the polymorphic material ortho-acetamidobenzamide, C(9)H(10)N(2)O(2), have been determined by low-temperature neutron single-crystal diffraction. Neutron diffraction allows the full description of the H-atom positions in this molecular material, which is vital in benchmarking related crystal-structure predictions. Significant conformational differences are indicated by a number of the torsion angles involving H atoms when compared with previous X-ray studies. A comprehensive description of the hydrogen-bonding scheme in both polymorphs is given.

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Control of copper(I) iodide architectures by ligand design: angular versus linear bridging ligands.

A family of coordination polymers formed by the reaction of copper(I) iodide with a range of angular bidentate or tridentate N-donor ligands is reported. The framework polymers [CuI(dpt)](infinity) 1 [dpt = 2,4-bis(4-pyridyl)-1,3,5-triazine], [CuI(dpb)](infinity) 2 [dpb = 1,4-bis-(4-pyridyl)-benzene], [(CuI)3(dpypy)2](infinity) 3, [CuI(dpypy)](infinity) 4 [dpypy = 3,5-bis(4-pyridyl)-pyridine], and [Cu3I3(pypm)](infinity) 5 [pypm = 5-(4-pyridyl)pyrimidine] have been prepared and structurally characterized. It was found that the angular nature of the dpypy and dpt ligands favors the formation of discrete (CuI)2 dimeric subunits as observed in [CuI(dpt).MeCN](infinity) 1 and [(CuI)3(dpypy)2](infinity) 3. In contrast, reaction with the linear ligand dpb affords [CuI(dpb)](infinity) 2 which incorporates a one-dimensional (CuI)(infinity) chain structure. Moreover, the additional donor available on the central ring of the dpypy ligand generates a novel two-dimensional bilayer structure in 3, in contrast to the one-dimensional ribbon structure observed in the case of 1. Interestingly, the bilayer structure of 3 additionally exhibits 2-fold interpenetration. The reaction of CuI with dpypy produces not only 3 but a further product [CuI(dpypy)](infinity) 4 that has been characterized as a one-dimensional chain constructed from trigonal-planar Cu(I) centers bridged by bidentate dpypy ligands. Compound 5, [Cu3I3(pypm)](infinity), exhibits a highly unusual three-dimensional structure in which the pypm ligand bridges two-dimensional brick-wall (CuI)(infinity) sheets.

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5-Fluorouracil and thymine form a crystalline solid solution.

The crystal structure of a 5-fluorouracil-thymine [5-fluoropyrimidine-2,4(1H,3H)-dione-5-methylpyrimidine-2,4(1H,3H)-dione, C(4)H(3)FN(2)O(2)xC(5)H(6)N(2)O(2)] solid solution has been determined. Both of the crystallographically independent sites can accommodate either 5-fluorouracil or thymine molecules, leading to occupational disorder [C(5-x)H(6-3x)F(x)N(2)O(2)xC(5-y)H(6-3x)F(y)N(2)O(2), with x = 0.52 and y = 0.7 for determination (I), x = 0.55 and y = 0.69 for (II), and x = 0.67 and y = 0.76 for (III)]. The 5-fluorouracil-thymine ratio in the crystal structure is influenced by the 5-fluorouracil-thymine ratio in the crystallization solution, though it does not exactly mirror it. The crystal structure comprises interpenetrating hydrogen-bonded nets, containing four independent hydrogen bonds.

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Single-crystal X-ray and neutron powder diffraction investigation of the phase transition in tetrachlorobenzene.

The polymorphic phase transition of 1,2,4,5-tetrachlorobenzene (TCB) has been investigated using neutron powder diffraction and single-crystal X-ray diffraction. The diffraction experiments show a reversible phase change that occurs as a function of temperature with no apparent loss of sample quality on transition between the two phases. Neutron powder diffraction gives detailed information on the molecular structural changes and lattice parameters from 2 K to room temperature. The structure of the low-temperature form has been elucidated for the first time using single-crystal X-ray diffraction. Comparison of the alpha and beta structures show that they are both based on the same sheet motif, with the differences between the two being very subtle, except in terms of crystal symmetry. Detailed analysis of the structures revealed the changes required for inter-conversion. A computational polymorph search showed that these two sheet structures are more thermodynamically stable than alternative herringbone-type structures.

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Structural diversity in two-dimensional coordination polymers constructed from simple building-blocks; a rare example of coordination polymer polymorphs structurally characterised from multiple crystals.

A family of two-dimensional coordination polymers formed from the reaction of Cd(NO(3))(2) with pyrazine or pyrimidine is reported, including rare examples of polymorphic coordination polymers which crystallise as multiple crystals. Six coordination polymers have been structurally characterised, four for pyrazine and two for pyrimidine-based systems, all of which form two-dimensional arrays utilising pyrazine/pyrimidine bridging, in some instances in combination with nitrate bridging. The compounds form either 4(4) grids (1,3,4,5), or in one instance, a 6(3) herringbone sheet structure (2). In the case of 3, two polymorphs have been identified, 3a and 3b, in which the three-dimensional arrangements of the coordination polymers differ only in the relative ordering of adjacent two-dimensional sheets. It was found that these two polymorphs crystallise in a simultaneous fashion such that each crystal studied was found to contain regions of both polymorphs and was believed to be a multiple crystal. Assessment of the phase purity of the product from the reaction of Cd(NO(3))(2) with either pyrazine or pyrimidine indicates that compounds 1and 5 are not formed when the products are formed by rapid precipitation but only when using slow-diffusion methods. It is also apparent that in almost all instances more than one product is formed from a given reaction thereby illustrating the complexity of coordination polymer formation even when using simple building-blocks. For the crystal engineer this complexity is perhaps best illustrated by the simultaneous formation of 3a and 3b where no chemical interactions differentiate the two polymorphs, presenting a seemingly insurmountable complexity in the engineering of these systems.

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Carbon-carbon bond formation by radical addition-fragmentation reactions of O-alkylated enols.

Alpha-tert-butoxystyrene [H2C=C(OBut)Ph] reacts with alpha-bromocarbonyl or alpha-bromosulfonyl compounds [R1R2C(Br)EWG; EWG =-C(O)X or -S(O2)X] to bring about replacement of the bromine atom by the phenacyl group and give R1R2C(EWG)CH2C(O)Ph. These reactions take place in refluxing benzene or cyclohexane with dilauroyl peroxide or azobis(isobutyronitrile) as initiator and proceed by a radical-chain mechanism that involves addition of the relatively electrophilic radical R1R2(EWG)C* to the styrene. This is followed by beta-scission of the derived alpha-tert-butoxybenzylic adduct radical to give But*, which then abstracts bromine from the organic halide to complete the chain. Alpha-1-adamantoxystyrene reacts similarly with R1R2C(Br)EWG, at higher temperature in refluxing octane using di-tert-amyl peroxide as initiator, and gives phenacylation products in generally higher yields than are obtained using alpha-tert-butoxystyrene. Simple iodoalkanes, which afford relatively nucleophilic alkyl radicals, can also be successfully phenacylated using alpha-1-adamantoxystyrene. O-Alkyl O-(tert-butyldimethylsilyl) ketene acetals H2C=C(OR)OTBS, in which R is a secondary or tertiary alkyl group, react in an analogous fashion with organic halides of the type R1R2C(Br)EWG to give the carboxymethylation products R1R2C(EWG)CH2CO2Me, after conversion of the first-formed silyl ester to the corresponding methyl ester. The silyl ketene acetals also undergo radical-chain reactions with electron-poor alkenes to bring about alkylation-carboxymethylation of the latter. For example, phenyl vinyl sulfone reacts with H2C=C(OBut)OTBS to afford ButCH2CH(SO2Ph)CH2CO2Me via an initial silyl ester. In a more complex chain reaction, involving rapid ring opening of the cyclopropyldimethylcarbinyl radical, the ketene acetal H2C=C(OCMe2C3H5-cyclo)OTBS reacts with two molecules of N-methyl- or N-phenyl-maleimide to bring about [3 + 2] annulation of one molecule of the maleimide, and then to link the bicyclic moiety thus formed to the second molecule of the maleimide via an alkylation-carboxymethylation reaction.

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Structural isomerism in CuSCN coordination polymers.

CuSCN reacts with the angular ligand 2,4-bis(4-pyridyl)-1,3,5-triazine (dpt) to afford rare examples of coordination polymer structural isomers including a non-centrosymmetric three-dimensional framework with Cd(SO4) topology constructed from tetrahedral metal cations.

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