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Paul Kögerler

Publications and source records attributed to Paul Kögerler.

At least 19 recordsLinked to original sources
PubMed2006

Experimental and theoretical investigations of the sulfite-based polyoxometalate cluster redox series: alpha- and beta-[Mo(18)O(54)(SO(3))(2)](4-/5-/6-).

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.

Journal Article↗
PubMed2006

The true nature of the Di-iron(III) gamma-Keggin structure in water: catalytic aerobic oxidation and chemistry of an unsymmetrical trimer.

The complex [gamma(1,2)-SiW(10){Fe(OH(2))}(2)O(38)](6)(-) (1) has been reported to catalyze the much sought reductant-free selective O(2)-based epoxidation of alkenes (Nishiyama, Y.; Nakagawa, Y.; Mizuno, N. Angew. Chem. Int. Ed. 2001, 40, 3639-3641) in chlorocarbon-acetonitrile solution. The challenge of reproducing catalysis by 1 led us to examine this chemistry in detail. In H(2)O, a desirable solvent for catalysis, 1, does not exist in the proposed organic-medium form in which the two iron atoms are in the binding pocket defined by the equatorial oxygens and, importantly, by two oxygens bound to the central Si heteroatom. Instead, 1 in H(2)O initially forms an unusual trimer [{Fe(2)(OH)(3)(H(2)O)(2)}(3)(gamma-SiW(10)O(36))(3)](15)(-) (2). The X-ray structure of 2 shows that the Fe-O(Si) bonds are cleaved and new bonds (mu-hydroxo bridges) form between these Fe centers and those of the neighboring [gamma(1,2)-SiW(10)Fe(2)] units. Structural, physical, and computational evidence indicate that if the bonds between the d-electron center, M (Fe in the case of 1 and 2), and the terminal ligands on M are stronger than the M-O(x)() bonds, then the out-of-pocket form is more stable and is the one observed. Significantly, 2 in H(2)O forms an intermediate that catalyzes the effective aerobic oxidation of sulfur compounds (mercaptoethanol is oxidized to the corresponding disulfide by O(2) at ambient pressure and temperature). All experimental findings are consistent with dissociation of a gamma-SiW(10) Keggin unit from the trimer, 2, to form the catalytically active species.

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PubMed2006

A nanoring-nanosphere molecule, {Mo214V30}: pushing the boundaries of controllable inorganic structural organization at the molecular level.

A controlled, Raman-monitored chemical reduction of a molybdate and vanadate mixture affords a new type of molybdenum-oxide-based cluster showing an unprecedented level of inorganic structural organization. The cluster incorporates two nanosized substructures (a ring and a sphere) in an open clam-like assembly. Multiple methods indicate that the nanoring contains delocalized electrons and the nanosphere contains localized but interacting electrons.

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PubMed2006

Controlling aggregation of copper(II)-based coordination compounds: From mononuclear to dinuclear, tetranuclear, and polymeric copper complexes.

The use of a strategy combining ligand design and changes of reaction conditions has been investigated with the goal of directing the assembly of mononuclear, dinuclear, tetranuclear, and polymeric copper(II) complexes. As a result, closely related copper monomers, alkoxo dimers, and hydroxo cubanes, along with a carbonate-bridged polymeric species, have been synthesized using the rigid, aliphatic amino ligands cis-3,5-diamino-trans-hydroxycyclohexane (DAHC), cis-3,5-diamino-trans-methoxycyclohexane (DAMC), and the glutaryl-linked derivative glutaric acid bis-(cis-3,5-diaminocyclohexyl) ester (GADACE). The composition of the monomeric complex has been determined by X-ray crystallography as [Cu(DAHC)2](ClO4)2 (1), the two dimers as [{Cu(DAHC)(OMe)}2](ClO4)2.MeOH (2) and [{Cu(DAMC)(OMe)(ClO4)}2] (3), the three Cu4O4 cubanes as [{Cu(DAHC)(OH)}4](ClO4)(4).2.5MeOH (4), [{Cu(DAMC)(OH)}4](ClO4)4.H2O (5), and [{Cu2(OH)2(GADACE)}2]Cl4.2MeOH.6H2O (6), and an infinite-chain structure as [{Cu(DAHC)(CO3)}n] (7). Furthermore, the cubane structures 4 and 5 have been investigated magnetically. Our studies indicate that formation of the monomeric, dimeric, and tetranuclear DAHC and DAMC complexes can be controlled by small changes in reaction conditions and that further preorganization of the ligand moiety by linking the DAHC cores (GADACE) allows more effective direction of the self-assembly of the Cu4O4 cubane core.

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PubMed2005

A new structural form for a decanuclear copper(II) assembly.

The synthesis and structure of a novel decanuclear copper(II) cage is reported. The assembly of the cage is facilitated by the cumulative coordinative interaction of tert-butyl phosphonate, 2-pyridylpyrazole and hydroxide ligands with copper(II) ions. Magnetic studies of this decanuclear copper(II) cage indicate complex antiferromagnetic behaviour.

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PubMed2005

Asymmetric terminal ligation on substituted sites in a disorder-free Keggin anion, [beta-SiFe2W10O36(OH)2(H2O)Cl]5-.

A new monomeric diiron beta-Keggin derivative, [(CH3)2NH2]5[beta-SiFe2W10O36(OH)2(H2O)Cl].7H2O (1) with non-adjacent substituted sites has been obtained in good yield by reaction of Fe(III) cations with [gamma-SiW10O36]8- in aqueous solution. The use of hydrogen bonding counter-cations produced a disorder-free polyanion. This situation facilitates unequivocal identification of the terminal ligands on Fe atoms and interpretation of the magnetic properties, which are also addressed by DFT calculations. Electrochemical studies establish that 1 can be step-wise reduced by two electrons and then reversibly oxidized.

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PubMed2005

Reactions of a {Mo16}-type polyoxometalate cluster with electrophiles: a synthetic, theoretical and magnetic investigation.

A medium-nuclearity mixed-valence polyoxomolybdate [H2Mo16O52]10-={Mo16}(1a) was synthesized using an approach that employed protonated hexamethylenetetramine (HMTAH+) as counter ion and yielded (HMTAH)10 1a.34 H2O (1). The {Mo16} cluster anion exhibits significant nucleophilicity and traps electrophiles such as divalent transition metal ions, resulting in a family of isostructural compounds based on {Mo16M2}-type anions [M(H2O)8H2Mo16O52]6- (M=FeII (2), MnII (3), CoII (4)). The highly reactive nature of the {Mo16} system is also revealed by rearrangement and decomposition reactions of to either slowly form a sodium-bridged heptamolybdate-based chain compound (5) when left in the reaction solution or, in the presence of very high concentrations of electrophiles, to heptamolybdate-based cluster compounds [M2(H2O)9Mo7O24]2- of the {M2Mo7}-type (M=FeII (6), MnII (7)). Compounds were characterised by single crystal X-ray diffraction, elemental analysis, IR spectroscopy, magnetic susceptibility measurements, and density functional theory calculations.

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PubMed2005

Controllable growth of chains and grids from polyoxomolybdate building blocks linked by silver(I) dimers.

Molecular growth processes utilizing a beta-octamolybdate synthon and {Ag2} dimers are described and the directing influence of "encapsulating" cations and coordinating solvent is also demonstrated. The growth of two 1D chains, (nBu4N)2n[Ag2Mo8O26]n (1) and (nBu4N)2n[Ag2Mo8O26(CH3CN)2]n (2), is achieved when nBu4N+ ions are used, and the diameter of the chains can be expanded by the coordination of CH3CN solvent (2). The formation of a type of gridlike structure in which 1D chains are crossed-over each other in alternatively packed layers is achieved in DMSO as the solvent; DMSO acts as a linking group to give (nBu4N)2n[Ag2Mo8O26(dmso)2]n (3), which, similar to 1 and 2, still incorporates the Bu4N+ ions that exert an "encapsulating" influence. However, in (HDMF)n[Ag3(Mo8O26)(dmf)4]n (4) the relatively bulky Bu4N+ ions are exchanged for protonated DMF cations, thereby allowing the chains to condense to a 2D array. The building block concept is further enforced by the isolation of a "monomeric" unit (Ph4P)2[Ag2Mo8O26(dmso)4] (5), which is isolated when the Ph4P+ ions are so "encapsulating" as to prevent aggregation of the {Ag-Mo8-Ag} building blocks. The nature of the AgAg dimers in each of the compounds 1-4 is examined by DFT calculations and the interplay between these Ag-Ag interactions and the structure types is described.

Acetonitriles↗
PubMed2005

Competing spin phases in geometrically frustrated magnetic molecules.

We identify a class of zero-dimensional classical and quantum Heisenberg spin systems exhibiting anomalous behavior in an external magnetic field B similar to that found for the geometrically frustrated kagome lattice of classical spins. Our calculations for the isotropic Heisenberg model show the emergence of a pronounced minimum in the differential susceptibility dM/dB at B(sat)/3 as the temperature T is raised from 0 K for structures based on corner-sharing triangles, specifically the octahedron, cuboctahedron, and icosidodecahedron. As the first experimental evidence we note that the giant Keplerate magnetic molecule {Mo(72)Fe(30)} (Fe(3+) ions on the 30 vertices of an icosidodecahedron) exhibits this behavior. For low T when B approximately B(sat)/3 two competing families of spin configurations exist of which one behaves magnetically "stiff" leading to a reduction of dM/dB.

Journal Article↗
PubMed2004

A high-nuclearity "Celtic-ring" isopolyoxotungstate, [H12W36O120]12-, that captures trace potassium ions.

A high-nuclearity {W36} isopolyoxotungstate cluster, [H12W36O120]12-, with a triangular shape has been isolated by using an encapsulating organic cation, protonated triethanolamine. In analogy to an 18-crown-6 crown ether, the cluster captures a potassium ion, present in trace amounts in the reaction system, within its cavity. DFT calculations suggest that the distribution of partial atomic charges over the oxygen framework shows maxima at certain "hotspot" sites, which imply possible routes for further cluster growth based on the {W36} system.

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