ORCID Profile
0000-0002-9249-5199
Current Organisation
West Virginia University
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Publisher: American Chemical Society (ACS)
Date: 04-04-2011
DOI: 10.1021/IC102588H
Abstract: The reaction of [Fe(II)(BF(4))(2)]·6H(2)O with the nitroxide radical, 4,4-dimethyl-2,2-di(2-pyridyl) oxazolidine-N-oxide (L(•)), produces the mononuclear transition metal complex [Fe(II)(L(•))(2)](BF(4))(2) (1) which has been investigated using temperature dependent susceptibility, Mössbauer spectroscopy, electrochemistry, density functional theory (DFT) calculations, and X-ray structure analysis. Single crystal X-ray diffraction analysis and Mössbauer measurements reveal an octahedral low spin Fe(2+) environment where the pyridyl donors from L(•) coordinate equatorially while the oxygen containing the radical from L(•) coordinates axially forming a linear O(•)··Fe(II)··O(•) arrangement. Magnetic susceptibility measurements show a strong radical-radical intramolecular antiferromagnetic interaction mediated by the diamagnetic Fe(2+) center. This is supported by DFT calculations which show a mutual spatial overlap of 0.24 and a spin density population analysis which highlights the antiparallel spin alignment between the two ligands. Similarly the monocationic complex [Fe(III)(L(-))(2)](BPh(4))·0.5H(2)O (2) has been fully characterized with Fe-ligand and N-O bond length changes in the X-ray structure analysis, magnetic measurements revealing a Curie-like S = 1/2 ground state, electron paramagnetic resonance (EPR) spectra, DFT calculations, and electrochemistry measurements all consistent with assignment of Fe in the (III) state and both ligands in the L(-) form. 2 is formed by a rare, reductively induced oxidation of the Fe center, and all physical data are self-consistent. The electrochemical studies were undertaken for both 1 and 2, thus allowing common Fe-ligand redox intermediates to be identified and the results interpreted in terms of square reaction schemes.
Publisher: American Chemical Society (ACS)
Date: 17-05-2006
DOI: 10.1021/IC0603611
Abstract: Transition-metal complexes with redox-active catecholato ligands are of interest as models of bioinorganic systems and as potential molecular materials. This work expands our recent X-ray absorption spectroscopic (XAS) studies of Cr(V/IV/III) triscatecholato complexes (Levina, A. Foran, G. J. Pattison, D. I. Lay, P. A. Angew. Chem., Int. Ed. 2004, 43, 462-465) to a Cr(III) monocatecholato complex, [Cr(tren)(cat)]+ (tren = tris(2-aminoethyl)amine, cat = catecholato2-), and its oxidized analogue, as well as to a series of V(V/IV/III) triscatecholato complexes ([VL3]n-, where L = cat, 3,5-di-tert-butylcatecholato2-, or tetrachlorocatecholato2-, and n = 1-3). Various oxidation states of these complexes in solutions were generated by bulk electrolysis directly in the XAS cell. Increases in the edge energies and pre-edge absorbance intensities in XANES spectra, as well as decreases in the average M-O bond lengths (M = Cr or V) revealed by XAFS data analyses, are consistent with predominantly metal-based oxidations in both the Cr(V/IV/III) and V(V/IV/III) triscatecholato series, but the degree of electron delocalization between the metal ion and the ligands was higher in the case of Cr complexes. By contrast, oxidation of [Cr(III)(tren)(cat)]+ was mainly ligand-based and led to [Cr(III)(tren)(sq)]2+ (sq = semiquinonato-), as shown by the absence of significant changes in the pre-edge and edge features and by an increase in the average Cr-O bond length. The observed differences in electron-density distribution in various oxidation states of Cr and V mono- and triscatecholato complexes have been discussed on the basis of the results of density functional calculations. A crystal and molecular structure of (Et3NH)2[V(IV)(cat)3] has been determined at 25 K and the same complex with an acetonitrile of crystallization at 150 K.
Publisher: Wiley
Date: 10-12-2012
Abstract: The structural characteristics and physical properties of the 3,5‐bis(2‐pyridyl)‐1,2,4‐triazolate ( L 1 ) bridged dinuclear iron(II) spin‐crossover complex [{Fe(NCBH 3 )(py)} 2 (μ‐ L 1 ) 2 ] ( 1 ) in both powder ( 1p ) and single crystal ( 1c ) forms have been investigated. Both forms of [{Fe(NCBH 3 )(py)} 2 (μ‐ L 1 ) 2 ] display a thermally induced spin transition however, the transitions have different T 1/2 values and different degrees of spin conversion. Both forms display the photomagnetic light‐induced excited spin‐state trapping (LIESST) effect as well as reverse LIESST and have been compared by Raman spectral and powder X‐ray diffraction methods, which indicate that they are polymorphs. The single crystal form 1c shows a “half” spin transition and has been further characterised at temperatures above and below the spin transition by low temperature crystallographic methods including single crystal LIESST experiments (at 40 K) and by Mössbauer spectroscopy thus, the nature of the [HS‐LS] form and the different spin isomers were revealed. To complement the experimental results, compound 1 and several other related Fe II dinuclear spin‐crossover compounds have been evaluated by quantum‐chemical DFT calculations. Additionally, the susceptibilities for the powder form 1p , which displays a complete two‐step spin‐crossover, were also fitted to a phenomenological model for dinuclear spin‐crossover complexes.
No related grants have been discovered for Carsten Milsmann.