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    Iron Complexes of a Macrocyclic N-Heterocyclic Carbene/Pyridine Hybrid Ligand

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    Type
    Article
    Authors
    Klawitter, Iris
    Anneser, Markus R.
    Dechert, Sebastian
    Meyer, Steffen
    Demeshko, Serhiy
    Haslinger, Stefan
    Poethig, Alexander
    Kuehn, Fritz E.
    Meyer, Franc
    Date
    2015
    Permanent link to this record
    http://hdl.handle.net/10754/673016
    
    Metadata
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    Abstract
    The tetradentate ligand system L<sup>1</sup> combines two N-heterocyclic carbene (NHC) and two pyridine donor functions in a macrocyclic scaffold. Its iron(II) complex [FeL<sup>1</sup>(MeCN)<inf>2</inf>](PF<inf>6</inf>)<inf>2</inf> (1) has been synthesized and fully characterized. The macrocyclic ligand in 1 is puckered and shows a significant barrier for ring inversion (ΔH<sup>‡</sup> = 15.1 kcal mol<sup>-1</sup>, and ΔS<sup>‡</sup> = -4.7 cal mol<sup>-1</sup> K<sup>-1</sup>). Axial ligands in 1 can be readily substituted to give heteroleptic [FeL<sup>1</sup>(CO)(MeCN)](PF<inf>6</inf>)<inf>2</inf> (2) or neutral [FeL<sup>1</sup>(N<inf>3</inf>)<inf>2</inf>] (3). The strong ligand field of the NHC/pyridine hybrid ligand imparts low-spin states (S = 0) for all iron(II) complexes 1-3. Mössbauer data reflect the asymmetric electronic situation that results from the strongly covalent Fe-C<sup>NHC</sup> bonds in the basal plane constituted by the macrocyclic ligand L<sup>1</sup>. Oxidation of 1 has been monitored by UV-vis spectro-electro chemistry, and the resulting iron(III) complex [FeL<sup>1</sup>(MeCN)<inf>2</inf>](PF<inf>6</inf>)<inf>3</inf> (4) has been isolated after chemical oxidation. SQUID and Mössbauer data have shown an S = <sup>1</sup>/<inf>2</inf> ground state for 4, and X-ray crystallographic analyses of 1 and 4 revealed that structural parameters of the {FeL<sup>1</sup>} core are basically invariant with respect to changes in the metal ion's oxidation state. Density functional theory calculations support the experimental findings. The combined structural, spectroscopic, and electrochemical data for 1 with its {C<inf>2</inf>N<inf>2</inf>} hybrid ligand L<sup>1</sup> allowed for useful comparison with the related iron(II) complex that has a macrocyclic {C<inf>4</inf>} tetracarbene ligand. Chemical Equation Presented).
    Citation
    Klawitter, I., Anneser, M. R., Dechert, S., Meyer, S., Demeshko, S., Haslinger, S., … Meyer, F. (2015). Iron Complexes of a Macrocyclic N-Heterocyclic Carbene/Pyridine Hybrid Ligand. Organometallics, 34(12), 2819–2825. doi:10.1021/acs.organomet.5b00103
    Sponsors
    Financial support by the Georg-August-University Gottingen (F.M.) and the King Abdullah University of Science and Technology (ICAUST) (F.E.K.) is gratefully acknowledged. S.H. and M.R.A. are thankful for financial support by the TUM graduate school. We thank A. Bretschneider (Institute for Inorganic Chemistry, Georg-August-University Gottingen) for the preparation of Th<SUP>center dot+</SUP> ClO<INF>4</INF><SUP>-</SUP> and Dr. H. Frauendorf (Institute for Organic and Biomolecular Chemistry, Georg-AugustUniversity Gottingen) for collecting HRNIS spectra.
    Publisher
    AMER CHEMICAL SOC
    Journal
    ORGANOMETALLICS
    DOI
    10.1021/acs.organomet.5b00103
    Additional Links
    https://pubs.acs.org/doi/10.1021/acs.organomet.5b00103
    ae974a485f413a2113503eed53cd6c53
    10.1021/acs.organomet.5b00103
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