John R. Pilbrow

Monash University (Australia), Melbourne, Victoria, Australia

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Publications (148)212.64 Total impact

  • 01/2011;
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    ABSTRACT: A family of spin crossover cobalt(II) complexes of the type [Co(terpyRX)(2)](Y)(2) x nH(2)O (X = 4, 8, 12 and Y = BF(4)(-), ClO(4)(-), PF(6)(-), BPh(4)(-)) has been synthesized, whereby the alkyl chain length, RX, and counteranion, Y, have been systematically varied. The structural (single crystal X-ray diffraction) and electronic (magnetic susceptibility, electron paramagnetic resonance (EPR)) properties have been investigated within this family of compounds. Single crystal X-ray diffraction analysis of [Co(terpyR8)(2)](ClO(4))(2), [Co(terpyR8)(2)](BF(4))(2) x H(2)O, and [Co(terpyR4)(2)](PF(6))(2) x 3 H(2)O, at 123 K, revealed compressed octahedral low spin Co(II) environments and showed varying extents of disorder in the alkyl tail portions of the terpyRX ligands. The magnetic and EPR studies were focused on the BF(4)(-) family and, for polycrystalline solid samples, revealed that the spin transition onset temperature (from low to high spin) decreased as the alkyl chain lengthened. EPR studies of polycrystalline powder samples confirmed these results, showing signals only due to the low spin state at the temperatures seen in magnetic measurements. Further to this, simultaneous simulation of the EPR spectra of frozen solutions of [Co(terpyR8)(2)](BF(4))(2) x H(2)O, recorded at S-, X-, and Q-band frequencies, allowed accurate determination of the g and A values of the low spin ground state. The temperature dependence of the polycrystalline powder EPR spectra of this and the R4 and R12 complexes is explained in terms of Jahn-Teller effects using the warped Mexican hat potential energy surface model perturbed by the low symmetry of the ligands. While well recognized in Cu(II) systems, this is one of the few times this approach has been used for Co(II).
    Inorganic Chemistry 09/2009; 48(15):7033-47. · 4.59 Impact Factor
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    Applied Magnetic Resonance 01/2009; 36(2):131-132. · 0.83 Impact Factor
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    ABSTRACT: Continuous-wave and pulsed electron paramagnetic resonance have been applied to the study of the CuII site of the copper-resistance protein PcoC from Escherichia coli and certain variant forms. Electron spin echo envelope modulation (ESEEM) experiments confirm the presence of two histidine ligands, His1 and His92, at the CuII site of wild-type PcoC, consistent with the available X-ray crystallographic data for the homolog CopC (67% sequence identity) from Pseudomonas syringae pv. tomato. The variants H1F and H92F each lack one of the histidine residues close to the CuII site. The ESEEM data suggest that the surviving histidine residue remains as a ligand. The nA variant features an extra alanine residue at the N terminus, which demotes the His1 ligand to position 2. At least one of the two histidine residues is bound at the CuII site in this form. Simulation of the 14N superhyperfine structure in the continuous-wave spectra confirms the presence of at least three nitrogen-based ligands at the CuII sites of the wild-type, H92F and nA forms, while the H1F variant has two nitrogen ligands. The spectra of wild-type form can be fitted adequately with a 3N or a 4N model. The former is consistent with the crystal structure of the CopC homolog, where His1 acts as a bidentate ligand. The latter raises the possibility of an additional unidentified nitrogen ligand. The markedly different spectra of the H1F and nA forms compared with the wild-type and H92F proteins further highlight the integral role of the N-terminal histidine residue in the high-affinity CuII site of PcoC.
    JBIC Journal of Biological Inorganic Chemistry 07/2008; 13(6):899-907. · 3.35 Impact Factor
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    ABSTRACT: A copper(II) complex containing the ligand 4′-octoxy-2,2′:6′,2″-terpyridine (terpyR8), [Cu(terpyR8)2](ClO4)2 (1) has been synthesized and characterized by X-ray crystallography and X-band EPR spectroscopy. Structural analysis at 123K revealed a compressed octahedral geometry, whereby the axial Cu–N bond lengths are significantly shorter than the equatorial ones. The mononuclear units in 1 are close packed such that there is no included solvent. This close packed nature is highlighted through a series of π–π interactions between mononuclear units and an array of hydrogen-bonding interactions between mononuclear units, the long C8 alkyl chains and ClO4- anions. Detailed EPR studies were carried out on both polycrystalline powder and frozen solution samples of 1, over a temperature range from 2.5K up to 200–260K. The polycrystalline powder exhibits a spectrum characteristic of a dx2-y2 ground configuration at temperatures below ∼10K. Above ∼100K, the spectrum had the appearance of originating from a dz2 ground configuration, but it is best interpreted in terms of fluxional disorder with a dx2-y2 ground state. The spectra of the frozen solutions are consistent with the dx2-y2 configuration (elongated tetragonal) at all temperatures, although the spectra show complex behaviour below ∼30K, due to microwave power saturation effects, and above ∼120K due to motional averaging effects. Apparent contradictions between the structural and EPR work of the kind seen here on the solid sample have recently been discussed elsewhere for related bis-tridentate Cu(II) complexes and are resolved here such that complex 1 has a Jahn–Teller elongated configuration masked by a structural disorder.
    Inorganica Chimica Acta. 01/2008; 361:3453-3461.
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    ABSTRACT: ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.
    ChemInform 07/2007; 38(29).
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    ABSTRACT: The synthesis, isolation, and structural characterization of the fully oxidized sulfite-based polyoxotungstate cluster (Pr4N)4{alpha-[W18O54(SO3)2]}.2CH3CN and the one-electron reduced form (Pr4N)5{alpha-[W18O54(SO3)2]}.2CH3CN has been achieved. alpha-[W18O54(SO3)2]5- was obtained as a Pr4N+ salt by reducing the "Trojan Horse" [W18O56(SO3)2(H2O)2]8- cluster via a template orientation transformation. Acetonitrile solutions of pure alpha-[W18O54(SO3)2]5- also were prepared electrochemically by one-electron bulk reductive electrolysis of alpha-[W18O54(SO3)2]4-. Cyclic voltammetry of alpha-[W18O54(SO3)2]4- and alpha-[W18O54(SO3)2]5- in CH3CN (0.1 M Hx4NClO4) produces evidence for an extensive series of reversible one-electron redox processes, that are associated with the tungsten-oxo framework of the polyoxometalate cluster. Hydrodynamic voltammograms in CH3CN exhibit the expected sign and magnitude of the steady-state limiting current values for the alpha-[W18O54(SO3)2]4-/5-/6- series and confirm the existence of a stable one-electron reduced species, alpha-[W18O54(SO3)2]5-. Employment of the Randles-Sevcik (cyclic voltammetry) and Levich (rotating disk electrode) equations at a glassy carbon electrode (d=3 mm) enable diffusion coefficient values of 3.7 and 3.8x10(-6) cm2 s-1 to be obtained for alpha-[W18O54(SO3)2]4- and alpha-[W18O54(SO3)2]5-, respectively. The tungsten polyoxometalates are highly photoactive, since measurable photocurrents and color changes are detected for both species upon irradiation with white light. EPR spectra obtained from both acetonitrile solution and solid samples, down to temperatures as low as 2.3 K, of the chemically and electrochemically prepared one-electron reduced species provided evidence that the unpaired electron in alpha-[W18O54(SO3)2]5- is delocalized over a number of atoms in the polyoxometalate structure, even at very low temperatures.
    Inorganic Chemistry 04/2007; 46(9):3502-10. · 4.59 Impact Factor
  • Sushil K Misra, John R Pilbrow
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    ABSTRACT: Spin-lattice relaxation times (T1) of two borate glasses doped with different concentrations of Fe2O3 were measured using the Electron Spin-Echo (ESE) technique at X-band (9.630 GHz) in the temperature range 2-6K. In comparison with a previous investigation of Fe3+-doped silicate glasses, the relaxation rates were comparable and differed by no more than a factor of two. The data presented here extend those previously reported for borate glasses in the 10-250K range but measured using the amplitude-modulation technique. The T1 values were found to depend on temperature (T) as T(n) with n approximately 1 for the 1% and 0.1% Fe2O3-doped glass samples. These results are consistent with spin-lattice relaxation as effected by exchange interaction of a Fe3+ spin exchange-coupled to another Fe3+ spin in an amorphous material.
    Journal of Magnetic Resonance 04/2007; 185(1):38-41. · 2.30 Impact Factor
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    ABSTRACT: The potentially bis-terdentate diamide ligand N,N'-bis[2-(2-pyridyl)ethyl]pyrazine-2,3-dicarboxamide (H(2)L(Et)) was structurally characterised. Potentiometric titrations revealed rather low pK(a) values for the deprotonation of the first amide group of H(2)L(Et) (14.2) and N,N'-bis(2-pyridylmethyl)pyrazine-2,3-dicarboxamide (H(2)L(Me), 13.1). Two tetranuclear copper(ii) square complexes of H(2)L(Et) with a paddle-wheel appearance, in which each ligand strand acts as a linear N(3)-NO hybrid terdentate-bidentate chelate, have been isolated and structurally characterised. Complex [Cu(II)(4)(H(2)L(Et))(2)(HL(Et))(2)](BF(4))(6).3MeCN.0.5H(2)O (.3MeCN.0.5H(2)O), with two nondeprotonated zwitterionic ligand strands and two monodeprotonated ligand strands, is formed in the 1 : 1 reaction of H(2)L(Et) and Cu(BF(4))(2).4H(2)O. It has a polymeric chain structure of tetranuclear subunits connected by N-H[dot dot dot]N hydrogen bonds. The same reaction carried out with one equivalent of base gives the related compound [Cu(II)(4)(HL(Et))(4)](BF(4))(4) (), with all four ligand strands monodeprotonated. It consists of isolated tetranuclear units. In both .3MeCN.0.5 H(2)O and the copper(ii) ions are in five-coordinate N(4)O environments but the degree of trigonality (tau) differs [.3MeCN.0.5H(2)O 0.14 </=tau</= 0.26; tau = 0.45]. Under the same reaction conditions as for but using Ni(BF(4))(2).6H(2)O a tetranuclear [2 x 2] grid-type complex, [Ni(II)(4)(HL(Et))(4)](BF(4)).10H(2)O (.10H(2)O), is formed. The structure determination showed that the nickel(ii) ions have N(6) distorted octahedral coordination spheres and all four ligand strands are monodeprotonated and act as N(3)-N(3) bis-terdentate chelates. Magnetic susceptibility data show that the complexes .4H(2)O, and .10H(2)O exhibit very weak antiferromagnetic spin coupling. The energies and multiplicities of the spin states of .4H(2)O and were determined from the temperature dependence of the magnetic susceptibility and indicate that a singlet state is lowest and the quintet state highest. This is consistent with the X-band EPR spectra of polycrystalline powders of .4H(2)O and , measured down to 2.3 K, where the resonances observed at the lowest temperature are due to a triplet state. The g-values of the individual ions of are consistent with the expected d(x(2)-y(2)) ground state for five-coordinate copper(ii) in an approximately square pyramidal configuration.
    Dalton Transactions 02/2007; · 3.81 Impact Factor
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    ABSTRACT: ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.
    ChemInform 01/2007; 38(5).
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    ABSTRACT: The synthesis, isolation and structural characterization of the sulfite polyoxomolybdate clusters alpha-(D(3h))(C(20)H(44)N)(4){alpha-[Mo(18)O(54)(SO(3))(2)]}CH(3)CN and beta-(D(3d))(C(20)H(44)N)(4){beta-[Mo(18)O(54)(SO(3))(2)]}CH(3)CN is presented. Voltammetric studies in acetonitrile (0.1 M Hx(4)NClO(4), Hx(4)N=tetra-n-hexylammonium) reveal the presence of an extensive series of six one-electron reduction processes for both isomers. Under conditions of bulk electrolysis, the initial [Mo(18)O(54)(SO(3))(2)](4-/5-) and [Mo(18)O(54)(SO(3))(2)](5-/6-) processes produce stable [Mo(18)O(54)(SO(3))(2)](5-) and [Mo(18)O(54)(SO(3))(2)](6-) species, respectively, and the same reduced species may be produced by photochemical reduction. Spectroelectrochemical data imply that retention of structural form results upon reduction, so that both alpha and beta isomers are available at each of the 4-, 5-, and 6-redox levels. However, the alpha isomer is the thermodynamically favored species in both the one- and two-electron-reduced states, with beta-->alpha isomerization being detected in both cases on long time scales (days). EPR spectra also imply that increasing localization of the unpaired electron occurs over the alpha- and beta-[Mo(18)O(54)(SO(3))(2)](5-) frameworks as the temperature approaches 2 K where the EPR spectra show orthorhombic symmetry with different g and hyperfine values for the alpha and beta isomers. Theoretical studies support the observation that it is easier to reduce the alpha cluster than the beta form and also provide insight into the driving force for beta-->alpha isomerization in the reduced state. Data are compared with that obtained for the well studied alpha-[Mo(18)O(54)(SO(4))(2))](4-) sulfate cluster.
    Chemistry 11/2006; 12(33):8472-83. · 5.83 Impact Factor
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    ABSTRACT: Amyloid-beta peptide (Abeta) is pivotal to the pathogenesis of Alzheimer disease. Here we report the formation of a toxic Abeta-Cu2+ complex formed via a histidine-bridged dimer, as observed at Cu2+/peptide ratios of >0.6:1 by EPR spectroscopy. The toxicity of the Abeta-Cu2+ complex to cultured primary cortical neurons was attenuated when either the pi -or tau-nitrogen of the imidazole side chains of His were methylated, thereby inhibiting formation of the His bridge. Toxicity did not correlate with the ability to form amyloid or perturb the acyl-chain region of a lipid membrane as measured by diphenyl-1,3,5-hexatriene anisotropy, but did correlate with lipid peroxidation and dityrosine formation. 31P magic angle spinning solid-state NMR showed that Abeta and Abeta-Cu2+ complexes interacted at the surface of a lipid membrane. These findings indicate that the generation of the Abeta toxic species is modulated by the Cu2+ concentration and the ability to form an intermolecular His bridge.
    Journal of Biological Chemistry 06/2006; 281(22):15145-54. · 4.65 Impact Factor
  • Article: P3-357
    Alzheimers & Dementia - ALZHEIMERS DEMENT. 01/2006; 2(3).
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    ABSTRACT: The synthesis and structural determination of the tripodal triethanolamine (teaH3) based octanuclear cluster [FeIII8O3(O2CCH2CH3)6(tea)(teaH)3(F)3]·0.5MeOH·0.5H2O (1) is reported. The octanuclear core in 1 has only been observed once before in the analogous teaH3 based [Fe8O3(O2CPh)9(tea)(teaH)3]·MeCN cluster and, as observed in that case, magnetic susceptibility measurements on 1 indicate a ground spin state of S = 0. Reaction of the trinuclear complex [FeIII3O(O2CCMe3)6(H2O)3](O2CCMe3) with the tripodal ligands 1-[N,N-bis(2-hydroxyethyl)-amino]-2-propanol (bheapH3) and triethanolamine (teaH3) gives [FeIII7O3(O2CCMe3)9(bheapH)3(H2O)3] (2) and [FeIII7O3(O2CCMe3)9(teaH)3(H2O)3] (3), respectively. The cores in 2 and 3 are dome-like in topology, an architecture unseen in any heptanuclear clusters reported to date. Magnetisation and susceptibility measurements show that clusters 2 and 3 exhibit ground spin states of S = 5/2. Low temperature EPR measurements confirm these ground spin states and yield zero-field splitting parameters of D = 0.28 cm−1 and E/D of 0.21 (for 3).
    Journal of Materials Chemistry 01/2006; 16(26). · 5.97 Impact Factor
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    ABSTRACT: Reaction of 1-propylamino-4-acetato-1,4,7-triazacyclononane (L1), 1-benzyl-4-acetato-1,4,7-triazacyclononane (L2) and 1-benzyl-4-propylamino-1,4,7-triazacyclononane (L3) with a copper(II) salt gave Na2[CuL1](ClO4)3(1a), [CuL2]Cl (2) and [Cu2L32](ClO4)4.5H2O (3), respectively. [CuL4]ClO4 (4) was formed by reacting 1-formyl-4-ethylacetato-1,4,7-triazacyclononane with cupric chloride in aqueous solution. The X-ray crystal structures of the complexes reveal that the ligands generate distorted square pyramidal or square planar coordination environments about the Cu(II) centre, but in three complexes (1b, 3 and 4) weak interactions to an oxygen atom from a perchlorate anion and, in the case of 4, also to an amide nitrogen leading to tetragonally elongated octahedral Cu(II) geometries. In 4, the formyl group is found to reduce the coordinating ability of the macrocyclic nitrogen to which it is attached, as evidenced by the weak CuN interaction. The formation of five-membered chelate rings on coordination of the ligands further contributes to the distortion from the ideal geometries. The crystal lattices contain a number of novel supramolecular features. 1a contains a negatively charged sodium perchlorate chain of composition [Na2(ClO4)3]x(x-), with a complex series of Na-O-Na bridges flanked by [CuL1]+ units, while 3 contains highly complex hydrogen bonded sheets approximately 20 A thick that stack through van der Waals interactions. One-dimensional chains comprised of copper complexes are found in 2 and 4, and are held together by hydrogen bonds in 2 and acetate bridges between the copper cations in 4. The solution EPR spectra indicate that the copper(II) centres exist in isolated distorted square pyramidal (possibly square planar for 4) environments, while in the solid state there is evidence for the existence of weak exchange and dipole-dipole coupling for some complexes.
    Dalton Transactions 06/2005; · 3.81 Impact Factor
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    ABSTRACT: Detailed electrochemical studies in dichloromethane (0.1 M Bu4NPF6) on the oxidation of the half-lantern [Pt2(kappa2As,C-C6H3-5-Me-2-AsPh2)2(mu-kappaAs,kappaC-C6H3-5-Me-2-AsPh2)2] (1) and full-lantern [Pt2(mu-kappaAs,kappaC-C6H3-5-Me-2-AsPh2)4] (2) complexes reveal the presence of an exceptionally stable dinuclear Pt cation 2+. Thus, oxidation of 1 occurs on the voltammetric time scale via a ladder-square scheme to give 2+, whereas 2 is directly converted to 2+. Electrochemically informed chemical synthesis enabled the isolation of solid [2+][BF4-] to be achieved. Single-crystal X-ray structural analysis showed that 2+ also has a lantern structure but with a shorter separation between the Pt centers [2.7069(3) A (2+), 2.8955(4) A (2)]. EPR spectra of 2+ provide unequivocal evidence for axial symmetry of the complex and are noteworthy because of an exceptionally large, nearly isotropic hyperfine coupling constant of about 0.1 cm(-1). Spectroscopic data support the conclusion that the unpaired electron in the 2+ cation is distributed equally between the two Pt nuclei and imply that oxidation of 2 to 2+ leads to the establishment of the metal-to-metal hemibond. Results of extended Huckel molecular orbital and density functional calculations on 2 and 2+ lead to the conclusions that s, p, dz2 mixing of orbitals contributes to the large EPR Pt hyperfine coupling and also that the structural adjustments that occur upon removal of an electron from 2 are driven by the metal-metal bonding character present in 2+.
    Inorganic Chemistry 05/2005; 44(7):2472-82. · 4.59 Impact Factor
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    ABSTRACT: Detailed electrochemical studies in dichloromethane (0.1 M Bu 4 NPF 6) on the oxidation of the half-lantern [Pt 2 -(κ 2 As,C-C 6 H 3 -5-Me-2-AsPh 2) 2 (µ-κAs,κC-C 6 H 3 -5-Me-2-AsPh 2) 2 ] (1) and full-lantern [Pt 2 (µ-κAs,κC-C 6 H 3 -5-Me-2-AsPh 2) 4 ] (2) complexes reveal the presence of an exceptionally stable dinuclear Pt cation 2 + . Thus, oxidation of 1 occurs on the voltammetric time scale via a ladder-square scheme to give 2 + , whereas 2 is directly converted to 2 + . Electrochemically informed chemical synthesis enabled the isolation of solid [2 + ][BF 4 -] to be achieved. Single-crystal X-ray structural analysis showed that 2 + also has a lantern structure but with a shorter separation between the Pt centers [2.7069(3) Å (2 +), 2.8955(4) Å (2)]. EPR spectra of 2 + provide unequivocal evidence for axial symmetry of the complex and are noteworthy because of an exceptionally large, nearly isotropic hyperfine coupling constant of about 0.1 cm -1 . Spectroscopic data support the conclusion that the unpaired electron in the 2 + cation is distributed equally between the two Pt nuclei and imply that oxidation of 2 to 2 + leads to the establishment of the metal-to-metal hemibond. Results of extended Hückel molecular orbital and density functional calculations on 2 and 2 + lead to the conclusions that s, p, d z 2 mixing of orbitals contributes to the large EPR Pt hyperfine coupling and also that the structural adjustments that occur upon removal of an electron from 2 are driven by the metal−metal bonding character present in 2 + .
    01/2005;
  • Australian Journal of Chemistry. 01/2004; 57(10):1011-1019.
  • Sushil K. Misra, John R. Pilbrow
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    ABSTRACT: Spin-lattice relaxation times (T1) of several Pilkington glasses doped with different concentrations of Fe3+ ions were measured using the electron spin-echo technique at X-band (9.630 GHz) in the temperature range 2–6 K in all but one case, where data were collected up to 8 K. The temperature dependence of the inverse relaxation times (relaxation rates) was found to depend on temperature (T) as Tn with n∼1, consistent with that affected by the strong spin-lattice relaxation of an Fe3+ spin exchange-coupled to another Fe3+ spin in an amorphous material.
    Physical Review B 01/2004; 69(21). · 3.66 Impact Factor
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    ABSTRACT: Electron spin transient nutation (ESTN) experiments show that the spin multiplicity of the ground state of C603- in frozen solution is a doublet with S = 1/2. In purified samples, there is no evidence for excited states or other species with higher multiplicity. In the anions of C120On- (n = 2, 3, 4), where the CW EPR experiments have shown that a mixture of species is present, ESTN experiments confirm that a doublet with S = 1/2 is associated with the 3− anion and triplets with S = 1 are associated with the 2− and 4− anions. A weak nutation peak attributable to ms = −1/2 ↔ 1/2 transitions within a quartet state may arise from association of anions with spins of 1/2 and 1 in solute aggregates.
    09/2003;

Publication Stats

418 Citations
212.64 Total Impact Points

Institutions

  • 1966–2009
    • Monash University (Australia)
      • School of Chemistry, Clayton
      Melbourne, Victoria, Australia
  • 2007
    • Concordia University Montreal
      • Department of Physics
      Montréal, Quebec, Canada
  • 2002
    • University of Melbourne
      • School of Chemistry
      Melbourne, Victoria, Australia