Judith A. K. Howard

Durham University, Durham, England, United Kingdom

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Publications (793)2648.21 Total impact

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    ABSTRACT: A crystallographic and solid state spectroscopic study of the spin crossover behaviour of [Fe(abpt)2(NCS)2] (abpt = 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole) polymorph A is reported. Structural features including crystallographic cell parameters, bond lengths and distortion parameters are monitored between 375 K and 30 K and crystal structures are reported at seven temperatures across the spin transition. In addition, the light induced excited spin state trapping (LIESST) metastable high spin structure, HS*, is reported at 30 K by continuous irradiation with a 670 nm, 5 mW CW laser during the data collection. Relaxation of the HS* state at 30 K with the laser switched off is found to occur within ~4000 s in accordance with the literature. High pressure single crystal datasets are also reported to examine the effect of pressure on the spin transition. Single crystal variable temperature UV-Vis spectroscopy and resonant ultrasound spectroscopy support the crystallographic evidence relating to the spin crossover transition presented herein. Strain analysis of the lattice parameters yields the temperature dependence of the spin order parameter, indicating strong spin-lattice coupling to give a volume strain of up to ~4% and a shear strain of up to ~1.5%. These, in turn, are responsible for changes in elastic constants by up to ~35%.
    No preview · Article · Jan 2016 · New Journal of Chemistry
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    ABSTRACT: The syntheses of novel Group 5 and Group 6 hydrosilylamido complexes of the type R(ArN[double bond, length as m-dash])M{N((t)Bu)SiMe2-H}X (M = Ta, R = Cp; M = Mo, R = ArN; X = Cl, H, OBn, Me) are described. The various substituents in the X position seem to play the key role in determining the extent of β-agostic interaction with the Si-H bond. The Mo agostic hydrido complex (ArN[double bond, length as m-dash])2Mo{η(3)-N((t)Bu)SiMe2-H}H is a pre-catalyst for the hydrosilylation of carbonyls. The stoichiometric reaction between benzaldehyde and (ArN[double bond, length as m-dash])2Mo{η(3)-N((t)Bu)SiMe2-H}H gives the benzoxy complex (ArN[double bond, length as m-dash])2Mo{N((t)Bu)SiMe2-H}(OBn), which showed a similar catalytic reactivity compared to the parent hydride. Mechanistic studies suggest that a non-hydride mechanism is operative.
    No preview · Article · Jan 2016 · Dalton Transactions
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    ABSTRACT: Seventeen compounds including the parent ortho-, meta- and para-carboranes and derivatives of ortho-carborane were investigated for luminescence in cyclohexane and dichloromethane solutions. Fifteen of these carboranes revealed very weak emissions in the 285-493 nm range. These carboranes may arguably be viewed as non-emissive in solutions at room temperatures. No emissions could be observed for 1,2-dimethyl-ortho-carborane and 2-methyl-1-phenyl-ortho-carborane. The carboranes with a 2′-pyridyl substituent at the cluster carbon atom gave unusual local and charge-transfer emissions indicating that different excited states are generated on photoexcitation. Of all the carboranes investigated, only 2-(diphenylphosphino)-1-phenyl-ortho-carborane, 1,2-diphenyl-ortho-carborane and 1-phenyl-2-(2′-pyridyl)-ortho-carborane are luminescent in the solid state with emissions at 476-612 nm and large Stokes shifts of 12000-13600 cm-1. The solid-state structures of 1,2-bis(2′-pyridyl)- and 1-phenyl-2-(2′-pyridyl)-ortho-carborane were determined by X-ray crystallography.
    No preview · Article · Jan 2016 · Berichte der deutschen chemischen Gesellschaft
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    ABSTRACT: Molecular self-assembly is an effective strategy for controlling the [2+2] photocycloaddition reaction of olefins. The geometrical properties of supramolecular assemblies are proven to have a critical effect on the efficiency and selectivity of this photoreaction both in the solid state and in solution, but the role of other factors remains poorly understood. Convenient supramolecular systems to study the structure-property relationships are pseudocyclic dimers spontaneously formed by styryl dyes containing a crown ether moiety and a remote ammonium group. New dyes of this type were synthesized to investigate the effects of structural and electronic factors on the quantitative characteristics of supramolecular dimerization and [2+2] photocycloaddition in solution. Variable structural parameters for the styryl dyes were the size and structure of macrocyclic moiety, the nature of heteroaromatic residue, and the length of the ammonioalkyl group attached to this residue. Quantum chemical calculations of the pseudocyclic dimers were performed in order to interpret the relationships between the structure of the ammonium dyes and the efficiency of the supramolecular photoreaction. One of the dimeric complexes was obtained in the crystalline state and studied by X-ray diffraction. The results obtained demonstrate that the photocycloaddition in the pseudocyclic dimers can be dramatically affected by the electronic structure of the styryl moieties, as dependent on the electron-donating ability of the substituents on the benzene ring, and by the conformational flexibility of the pseudocycle, which determines the mobility of the olefinic bonds. The significance of electronic factors is highlighted by the fact that the photocycloaddition quantum yield in geometrically similar dimeric structures varies from ≤ 10(-4) to 0.38. The latter value is unusually high for olefins in solution.
    No preview · Article · Dec 2015 · The Journal of Physical Chemistry A
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    ABSTRACT: To produce a novel class of structurally ordered poly-β-prolines, an emergent method for synthesizing chiral β-peptide molecular frameworks was developed based on 1,3-dipolar cycloaddition chemistry of azomethine ylides. Functionalized short β-peptides with up to six monomeric residues were efficiently synthesized in homochiral forms using a cycloadditive oligomerization approach. X-ray, NMR, and CD structural analyses of the novel β-peptides revealed secondary structure features that were generated primarily by Z/E-β-peptide bond isomerism. Anticancer in cellulo activity of the new β-peptides toward hormone-refractory prostate cancer cells was observed and was dependent on the absolute configuration of the stereogenic centers and the chain length of the β-proline oligomers.
    No preview · Article · Dec 2015 · Organic Letters
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    ABSTRACT: Reaction of complex [CpRu(pyr)3][PF6] () with the NHC carbene IPr (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) results in the NHC complex [Cp(IPr)Ru(pyr)2][PF6] (), which was studied by NMR specroscopy and X-ray diffraction analysis. Reaction of [Cp(IPr)Ru(pyr)2][PF6] () with LiAlH4 leads to the trihydride Cp(IPr)RuH3 () characterised by spectroscopic methods. Heating compound with hydrosilanes gives the dihydrido silyl derivatives Cp(IPr)RuH2(SiR3) (). Systematic X-ray diffraction studies suggest that complexes have stronger interligand SiH interactions than the isolobal phosphine complexes Cp(Pr3P)RuH2(SiR3).
    No preview · Article · Nov 2015 · Dalton Transactions
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    ABSTRACT: A series of cross-conjugated compounds based on an (E)-4,4'-(hexa-3-en-1,5-diyne-3,4-diyl)bis(N,N-bis(4-methoxyphenyl)aniline) skeleton (1-6) have been synthesized. The linear optical absorption properties can be tuned by modification of the substituents at the 1 and 5 positions of the hexa-3-en-1,5-diynyl backbone (1: Si(CH(CH3)2)3, 2: C6H4C CSi(CH3)3, 3: C6H4COOCH3, 4: C6H4CF3, 5: C6H4C N, 6: C6H4C CC5H4N), although attempts to introduce electron donating (C6H4CH3, C6H4OCH3, C6H4Si(CH3)3) substituents at these positions were hampered by the ensuing decreased stability of the compounds. Spectroelectrochemical investigations of selected examples, supported by DFT based computational studies, have shown that one- and two-electron oxidation of the 1,2-bis(triarylamine)ethene fragment also results in electronic changes to the perpendicular pi-system in the hexa-3-en-1,5-diynyl branch of the molecule. These properties suggest that (E)-hexa-3-en-1,5-diynyl based compounds could have applications in molecular sensing and molecular electronics.
    No preview · Article · Oct 2015 · The Journal of Organic Chemistry
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    ABSTRACT: Reactions of the vinylidene complexes [M([double bond, length as m-dash]C[double bond, length as m-dash]CH2)(PP)Cp']PF6 with NH3 readily afford the acyclic amino carbene complexes [M{C(Me)NH2}(PP)Cp']PF6 [M(PP)Cp' = Fe(dppe)Cp, Ru(dppe)Cp, Ru(PPh3)2Cp, Ru(dppe)Cp*]. Crystallographic, spectroscopic and computational data support a description of these complexes in terms of a M-{C(Me)[double bond, length as m-dash]N(+)H2} mesomeric form, and simple σ-coordination of the singlet acyclic amino carbene.
    No preview · Article · Jun 2015 · Dalton Transactions
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    ABSTRACT: A new styryl dye of the 2-benzothiazole series which contains three methoxy groups and an iodide anion was synthesized. Three triclinic crystal forms of this dye were investigated by single crystal X-ray diffraction. All the modifications were shown to contain centrosymmetrically related stacks of cations with the syn-“head-to-tail” mutual arrangement. Stacks of cations are arranged as dimeric pairs with short interatomic distances between the ethylene group carbon atoms, d1 = 3.54–3.75 Å and significantly longer analogous distances between the adjacent pairs, d2 = 4.61–4.68 Å. The main difference of the packing consists in various orientations of N-ethyl substituents with respect to the centre of the dimer. The solid phase [2+2] photocycloaddition (PCA) reaction resulting in rctt isomer of the cyclobutane derivative, only proceeds in pairs where N-substituents are oriented outward from the centre of the dimeric pair. PCA is accomplished as a “single crystal-to-single crystal” transformation.
    Preview · Article · May 2015 · CrystEngComm
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    ABSTRACT: In the title compound, C11H7Br3ClN, the quinoline ring system is approximately planar (r.m.s. = 0.011 Å). In the crystal, mol­ecules are linked by C—H⋯Br inter­actions forming chains along [10-1]. The chains are linked by C—H⋯π and π-π inter­actions involving inversion-related pyridine rings [inter­centroid distance = 3.608 (4) Å], forming sheets parallel to (10-1). Within the sheets, there are two significant short inter­actions involving a Br⋯Cl contact of 3.4904 (18) Å and a Br⋯N contact of 3.187 (6) Å, both of which are significantly shorter than the sum of their van der Waals radii.
    Preview · Article · May 2015
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    ABSTRACT: This report deals with the synthesis, characterization, and crystal structure of a heteropentanuclear CuII3TlI2 compound [(CuIIL)3TlI2](NO3)2 (1), where H2L=N,N′-ethylenebis(3-ethoxysalicylaldimine). This compound crystallizes in the monoclinic crystal system within space group C2/c. Each of the two symmetry related thallium(I) centers is located between a terminal and a common, central [CuIIL] by forming bonds with four phenoxo and three ethoxy oxygens. The three [CuIIL] moieties are parallel and hence 1 is a triple-decker system. Neighboring triple-decker moieties are interlinked by π…π stacking interaction and weak hydrogen bonds to generate 3-D self-assembly in 1. Salient features in the composition and structure of the title compound are discussed; the title compound is the first example of a thallium(I) system in imino-phenolate Schiff base family.
    No preview · Article · Dec 2014 · Journal of Coordination Chemistry
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    ABSTRACT: The synthesis of new styryl dyes derived from 4-pyridine and 4-quinoline and having an ammonioalkyl N-substituent and a benzocrown-ether moiety of different size and with different sets of heteroatoms was developed. Spontaneous "head-to-tail" dimerization of these dyes via the formation of numerous hydrogen bonds between the terminal NH3+ groups and crown-ether moieties was detected in MeCN solutions. The stability constants of the dimeric complexes having pseudocyclic structure were studied by 1H NMR titration. The most stable complexes (logKd up to 8.2) were found in the case of dyes with the (aza)-18-crown-6-ether moiety, which is most complementary for binding a primary ammonium group. Stacking interaction of the conjugated systems in the dimeric complexes contributes to their stability to a much lesser extent. In dimeric complexes, the ethylene bonds of the dyes are pre-organized for stereospecific [2+2] photocycloaddition (PCA) induced by visible light. The PCA yields only rctt isomers of bis-crown-containing cyclobutane derivatives. The dyes were studied by X-ray diffraction; it was found that the dimeric arrangement is also retained in the crystalline state. The possibility of topochemical PCA of the dyes in single crystals without their destruction was demonstrated. The possibility of retro-PCA of the obtained cyclobutane derivatives to give the starting dyes was shown. The elucidated regularities of PCA can be used to fabricate optical data recording systems based on ammonioalkyl derivatives of crown-ether styryl dyes.
    No preview · Article · Nov 2014 · The Journal of Organic Chemistry
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    ABSTRACT: Diatrizoic acid (DTA), a clinically used X-ray contrast agent, crystallises in two hydrated, three anhydrous and nine solvated solid forms, all of which have been characterised by X-ray crystallography. Single-crystal neutron structures of DTA dihydrate and monosodium DTA tetrahydrate have been determined. All of the solid-state structures have been analysed using partial atomic charges and hardness algorithm (PACHA) calculations. Even though in general all DTA crystal forms reveal similar intermolecular interactions, the overall crystal packing differs considerably from form to form. The water of the dihydrate is encapsulated between a pair of host molecules, which calculations reveal to be an extraordinarily stable motif. DTA presents functionalities that enable hydrogen and halogen bonding, and whilst an extended hydrogen-bonding network is realised in all crystal forms, halogen bonding is not present in the hydrated crystal forms. This is due to the formation of a hydrogen-bonding network based on individual enclosed water squares, which is not amenable to the concomitant formation of halogen bonds. The main interaction in the solvates involves the carboxylic acid, which corroborates the hypothesis that this strong interaction is the last one to be broken during the crystal desolvation and nucleation process.
    Preview · Article · Nov 2014 · Chemistry - A European Journal
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    ABSTRACT: The reaction of trans-RuCl(C≡CC6H4R1-4)(dppe)2 (2: R1 = Me (a), C5H11 (b), OMe (c), CO2Me (d), NO2 (e), C≡CSiMe3 (f), C≡CBut (g), NH2 (h)), prepared in situ from reactions of [RuCl(dppe)2]OTf ([1]OTf) with terminal alkynes in CH2Cl2 solutions containing 1,8-diazabicycloundec-7-ene (DBU) and TlBF4, provides a convenient and rapid route to bis(acetylide) complexes trans-Ru(C≡CC6H4R1-4)2(dppe)2 (3a–h) and trans-Ru(C≡CC6H4R1-4)(C≡CC6H4R2-4)(dppe)2 (4, R1 = C≡CSiMe3, R2 = NH2; 5, R1 = CO2Me, R2 = NH2; 6, R1 = CO2Me, R2 = OMe). However, even in the absence of the chloride abstracting reagent, more strongly electron donating substituents (e.g., R1 = OMe (2c), NH2 (2h)) promote sufficient ionization of the Ru–Cl bond in trans-RuCl(C≡CC6H4R1-4)(dppe)2 to lead to slow conversion to bis(alkynyl) complexes 3c,h in the presence of excess alkyne and DBU. Desilylation of 2f and 3f affords 2i and 3i (R1 = C≡CH), respectively. The molecular structures of 3a–d,f–i have been determined and are reported together with the structures of the monoalkynyl complexes 2f,g,i and compared with related compounds from the literature. Complexes 3a–i and 4–6 undergo one reversible electrochemical oxidation process, which can be attributed to depopulation of an orbital with significant alkynyl ligand character. The one-electron-oxidation products [3f]•+, [3h]•+, [4]•+, and [5]•+, chosen to serve as representative examples of this family of complexes, each exhibit a series of NIR absorptions between 15000 and 5000 cm–1 which on the basis of TDDFT calculations cannot be attributed to a single, static lowest energy molecular structure. Rather, the transitions that are responsible for the absorption band envelope have varying degrees of LMCT and inter-alkynyl ligand IVCT or MLCT character that depend not only on the nature of the Rn groups but also on the ensemble of thermally populated molecular conformers in solution with various relative orientations of the metal fragment and arylethynyl moieties.
    Preview · Article · Sep 2014 · Organometallics
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    ABSTRACT: Reactions between the C,C-dicopper(l) derivative of ortho-carborane and ortho-, meta- and para-diiodobenzene are reported. The reaction with 1,2-C6H4I2 unexpectedly afforded 2,2 '-bis(1 '-ortho-carboranyl)biphenyl, HCB10H10CC6H4]2 2, whereas reactions with 1,3- or 1,4-C6H4I2 provided alternative routes to 1,3-bis(1 '-ortho-carboranyl)benzene 3 and 1,4-bis(1 '-ortho-carboranyl)benzene 4, respectively. The crystal structure of the biphenyl derivative 2 revealed significant distortions in the biphenylene framework attributable to the proximity of the two bulky carborane cages. UV absorption spectra and electrochemical data on 2 and 3 showed little electronic communication between the two carborane cages in either, and negligible pi-conjugation between the two ortho-phenylene rings in 2. However, substantial evidence was found of electronic communication between the carborane cages via the paraphenylene bridge in 4. B3LYP/6-31G* computations have been carried out on compounds 2 and 4, on 4,4 '-bis(ortho-carboranyl)biphenyl 6 and on 1,2-bis(1 '-ortho-carboranyl)benzene 7. Those on 2, 4 and 6 show the computed geometries to be in very good agreement with the experimental geometries: those on 7 allowed the reported molecular geometry of this compound to be revised and revealed a long cage C C bond of 1.725(3) angstrom.
    Full-text · Article · Aug 2014 · Tetrahedron
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    ABSTRACT: A series of disubstituted urea, thiourea and dansyl amide calix[4]arene based anion receptors have been prepared and characterised by X-ray crystallography. The structures show a fine balance of intramolecular urea alpha-tape, and urea…calixarene phenolic oxygen atom hydrogen bonding patterns. Both motifs are in competition with the anion binding behaviour of the compounds in solution.
    Preview · Article · Jul 2014 · CrystEngComm
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    ABSTRACT: A family of styryl dyes of the 4-pyridine series, Et–Py+–CHCH–C6H3(OMe)2 X−, with different anions X−, was synthesized. Their ability to undergo [2 + 2] photocycloaddition (PCA) in the polycrystalline state with the formation of 1,2,3,4-tetrasubstituted rctt cyclobutanes under irradiation with visible light was investigated by 1H NMR spectroscopy. Crystal packing of the dyes and feasibility for the PCA reaction in single crystals were investigated by X-ray structural analysis. Small inorganic anions, such as ClO4−, BF4−, PF6− that do not form secondary interactions with the organic cation, aid the formation of cation stacking motifs of the “head-to-tail” type. In these stacks, any adjacent cations are arranged in an antiparallel fashion and approach one another at distances shorter than 4.2 Å, which corresponds to the Schmidt's rule for feasibility of PCA reaction in the solid state. As a result of PCA, rctt isomers of cyclobutane derivatives are formed. Success of the PCA reaction without single crystal degradation was only found for the dye containing BF4− anion. Upon irradiation with UV light of the cyclobutane product formed in the crystal, back phototransformation without single crystal degradation was observed. A number of weak interactions I−H–C in the dye with I− anion prevent significant atomic displacements that accompany the PCA transformation from being achieved. Occurrence of the bulky tosylate anion forming a set of hydrogen bonds with solvate acetonitrile and water molecules aids the formation of “head-to-head” type stacks, with a large separation between the adjacent ethylene bonds. In the dye containing bulky BPh4− anion, the main structure-forming role belongs to anions, which suppress the cation trend to form a stacking packing motif. The dye with picrate anion forms two types of crystals. Crystal packing of one of them excludes any PCA transformation, whereas, the other one permits it, but with single crystal degradation.
    No preview · Article · Jun 2014 · CrystEngComm
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    ABSTRACT: A short series of fluorotetrahydroquinolines was synthesised in two steps from diethyl fluoromalonate and appropriate ortho-nitrobenzyl bromide precursors.
    No preview · Article · Jun 2014 · CHIMIA International Journal for Chemistry
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    Full-text · Dataset · May 2014
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    Full-text · Dataset · May 2014

Publication Stats

19k Citations
2,648.21 Total Impact Points


  • 1992-2016
    • Durham University
      • Department of Chemistry
      Durham, England, United Kingdom
    • Baylor University
      • Department of Chemistry and Biochemistry
      Waco, Texas, United States
  • 2010
    • University of Glasgow
      • School of Chemistry
      Glasgow, Scotland, United Kingdom
  • 2002-2010
    • Georg-August-Universität Göttingen
      • Institute of Inorganic Chemistry
      Göttingen, Lower Saxony, Germany
    • University of Cambridge
      • Department of Chemistry
      Cambridge, England, United Kingdom
  • 1974-2010
    • University of Bristol
      • School of Chemistry
      Bristol, England, United Kingdom
  • 2005
    • The University of York
      • Department of Chemistry
      York, England, United Kingdom
  • 2004
    • Kiev Slavonic University
      Kievo, Kyiv City, Ukraine
  • 1981-2002
    • Institut Laue-Langevin
      Grenoble, Rhône-Alpes, France
  • 1999
    • Università degli Studi di Torino
      Torino, Piedmont, Italy
  • 1998
    • University of Waterloo
      • Department of Chemistry
      Waterloo, Ontario, Canada
  • 1995
    • Odense Zoo
      Odense, South Denmark, Denmark
  • 1994
    • University of Zaragoza
      Caesaraugusta, Aragon, Spain
  • 1989
    • University of Bahrain
      • Department of Chemistry
      Al Manāmah, Manama, Bahrain
  • 1987
    • University of Salford
      Salford, England, United Kingdom