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

Iris Klawitter, Markus R. Anneser, Sebastian Dechert, Steffen Meyer, Serhiy Demeshko, Stefan Haslinger, Alexander Poethig, Fritz E. Kuehn, Franc Meyer

Research output: Contribution to journalArticlepeer-review

40 Scopus citations


The tetradentate ligand system L1 combines two N-heterocyclic carbene (NHC) and two pyridine donor functions in a macrocyclic scaffold. Its iron(II) complex [FeL1(MeCN)2](PF6)2 (1) has been synthesized and fully characterized. The macrocyclic ligand in 1 is puckered and shows a significant barrier for ring inversion (ΔH‡ = 15.1 kcal mol-1, and ΔS‡ = -4.7 cal mol-1 K-1). Axial ligands in 1 can be readily substituted to give heteroleptic [FeL1(CO)(MeCN)](PF6)2 (2) or neutral [FeL1(N3)2] (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-CNHC bonds in the basal plane constituted by the macrocyclic ligand L1. Oxidation of 1 has been monitored by UV-vis spectro-electro chemistry, and the resulting iron(III) complex [FeL1(MeCN)2](PF6)3 (4) has been isolated after chemical oxidation. SQUID and Mössbauer data have shown an S = 1/2 ground state for 4, and X-ray crystallographic analyses of 1 and 4 revealed that structural parameters of the {FeL1} 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 {C2N2} hybrid ligand L1 allowed for useful comparison with the related iron(II) complex that has a macrocyclic {C4} tetracarbene ligand. Chemical Equation Presented).
Original languageEnglish (US)
Pages (from-to)2819-2825
Number of pages7
Issue number12
StatePublished - 2015
Externally publishedYes

Bibliographical note

KAUST Repository Item: Exported on 2021-11-01
Acknowledgements: 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 Thcenter dot+ ClO4- and Dr. H. Frauendorf (Institute for Organic and Biomolecular Chemistry, Georg-AugustUniversity Gottingen) for collecting HRNIS spectra.
This publication acknowledges KAUST support, but has no KAUST affiliated authors.

ASJC Scopus subject areas

  • Organic Chemistry
  • Physical and Theoretical Chemistry
  • Inorganic Chemistry


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