Browsing by Author "Overgaard, J"

Now showing 1 - 3 of 3
  • No Thumbnail Available
    Item
    Experimental and theoretical charge density distribution in two ternary cobalt(III) complexes of aromatic amino acids
    (American Chemical Society, 2007-10-11) Overgaard, J; Waller, MP; Piltz, RO; Platts, JA; Emseis, P; Leverett, P; Williams, PA; Hibbs, DE
    The experimental charge density distributions in two optically active isomers of a Co complex have been determined. The complexes are Δ-α-[Co(R,R-picchxn)(R-trp)](ClO4)(2)center dot H2O) (1) and Λ-β(1)-[Co(R,R-picchxn)(R-trp)](CF3SO3)(2)) (2), where picchxn is N,N '-bis(2-picolyl-1,2-diaminocyclohexane) and R-trp is the R-tryptophane anion. The molecular geometries of 1 and 2 are distinguished by the presence in complex 1 of intramolecular pi center dot center dot center dot pi stacking interactions and the presence in complex 2 of intramolecular hydrogen bonding. This pair of isomers therefore serves as an excellent model for studying noncovalent interactions and their effects on structure and electron density and the transferability of electron density properties between closely related molecules. For complex 2, a combination of X-ray and neutron diffraction data created the basis for a X-N charge density refinement. A topological analysis of the resulting density distribution using the atoms in molecules methodology is presented along with,d-orbital populations, showing that the metal-ligand bonds are relatively unaltered by the geometry changes between 1 and 2. The experimental density has been supplemented by quantum chemical calculations on the cobalt complex cations: close agreement between theory and experiment is found in all cases. The energetics of the weak interactions are analyzed using both theory and experiment showing excellent quantitative agreement. In particular it is found that both methods correctly predict the stability of 2 over 1. The transferability between isomers of the charge density and derived parameters is investigated and found to be invalid for these structurally related systems. © 2007, American Chemical Society
  • No Thumbnail Available
    Item
    Metal distribution and disorder in the crystal structure of [NH2Et2][Cr7MF8(tBuCO2)16] wheel molecules for M = Mn, Fe, Co, Ni, Cu, Zn and Cd
    (International Union of Crystallography, 2014-12) Larsen, FK; Overgaard, J; Christensen, M; McIntyre, GJ; Timco, GA; Winpenny, REP
    The homometallic wheel compound [Cr8F8(O2CCMe3)16] formed with fluorine and pivalic acid ligands can be modified by introducing in the synthesis process a divalent cation M capable of octahedral coordination instead of one of the trivalent Cr centres in the ring. Heterometallic mono-anionic species [Cr7MF8(O2CCMe3)16]− can form diethylammonium salts and be crystallized from ethylacetate solution as compounds with the general formula [NH2Et2][Cr7MF8(tBuCO2)16][C4H8O2]0.5 for M = Mn, Fe, Co, Ni, Cu, Zn and Cd. Their structures are isomorphous, belonging to the space group P21/c. The study has determined the degree of order for the individual M heterometal over the possible metal positions of the ring in the crystal structure by modelling based on X-ray diffraction data. The model took into account disorder in tert-butyl groups of the pivalate ligands and in the position and orientation of the ethylacetate solvent molecule. The heterometal turned out to be partly ordered in the crystal structure. © 2014, International Union of Crystallography.
  • No Thumbnail Available
    Item
    Relationships between electron density and magnetic properties in water-bridged dimetal complexes
    (ACS Publications, 2014) Overgaard, J; Walsh, JPS; Hathwar, VR; Jørgensen, MRV; Hoffman, C; Platts, JA; Piltz, RO; Winpenny, REP
    The experimental and theoretical electron density distributions in two structurally similar transition metal dimers (M = Ni, Co; see Figure) were analyzed using the atoms-in-molecules (AIM) approach, and selected properties related to the chemical bonding are compared to measured intramolecular magnetic exchange interaction parameters.The electron densities in two analogous dimetallic transition metal compounds, namely, [M2(μ-OH2)(tBuCOO)4(tBuCOOH)2(C5H5N)2] (M = Co(1), Ni(2)), were determined from combined X-ray and neutron single-crystal diffraction at 100 K. Excellent correspondence between the thermal parameters from X- and N-derived atomic displacement parameters is found, indicating high-quality X-ray data and a successful separation of thermal and electronic effects. Topological analysis of electron densities derived from high-resolution X-ray diffraction, as well as density functional theory calculations, shows no direct metal–metal bonding in either compound, while the total energy density at the bond critical points suggests stronger metal–oxygen interactions for the Ni system, in correspondence with its shorter bond distances. The analysis also allows for estimation of the relative strength of binding of terminal and bridging ligands to the metals, showing that the bridging water molecule is more strongly bound than terminal carboxylic acid, but less so than bridging carboxylates. Recently, modeling of magnetic and spectroscopic data in both of these systems has shown weak ferromagnetic interactions between the metal atoms. Factors related to large zero-field splitting effects complicate the magnetic analysis in both compounds, albeit to a much greater degree in 1. The current results support the conclusion drawn from previous magnetic and spectroscopic measurements that there is no appreciable direct communication between metal centers. © 2014, American Chemical Society.