Synthesis and Characterization of an Iron Complex Bearing a Cyclic Tetra-N-heterocyclic Carbene Ligand: An Artifical Heme Analogue?

Abstract

An iron(II) complex with a cyclic tetradentate ligand containing four N-heterocyclic carbenes was synthesized and characterized by means of NMR and IR spectroscopies, as well as by single-crystal X-ray structure analysis. The iron center exhibits an octahedral coordination geometry with two acetonitrile ligands in axial positions, showing structural analogies with porphyrine-ligated iron complexes. The acetonitrile ligands can readily be substituted by other ligands, for instance, dimethyl sulfoxide, carbon monoxide, and nitric oxide. Cyclic voltammetry was used to examine the electronic properties of the synthesized compounds.

A Chiral Macrocyclic Tetra‐N‐Heterocyclic Carbene Yields an “All Carbene” Iron Alkylidene Complex

“…Allp reviousd iamagnetic macrocyclic tetra-NHC complexes have only shown one resonance in solution implyingt hat they are all equivalent on NMR time scale. [30,[37][38][39][40] The split resonances of both the 1 Ha nd 13 CNMR for 5 is ac ommont rait of constrained geometry C 2symmetric bidentate NHC complexes, but for 5 the difference between the 13 Cr esonances are more pronounced. [27,41,42] The solid state structure for 5 confirms this absolute C 2 symmetry and confirms that as ingle enantiomer is formed (Figure 2, top).…”

Molecular iron complexes as catalysts for selective C–H bond oxygenation reactions

“…147 Our group has reported the synthesis and characterisation of a series of iron complexes supported by cyclic and acyclic tetradentate N-heterocyclic carbene ligands 71-74 ( Figure 12). [148][149][150][151] The octahedral coordination geometry of iron is completed by coordination of labile acetonitrile solvent molecules. For ligands 71a, 71b, 73, and 74 the iron complexes exhibit trans-labile sites, for 71c and 72b cis-labile sites as a consequence of the coordination mode of the respective tetradentate ligand.…”

Structural diversity of late transition metal complexes with flexible tetra-NHC ligands

“…Depending on the linkage of the NHC subunits, macrocyclic tetra-NHC ligands are able to chelate metal ions with coordination numbers of four or higher. 19,[21][22][23][24][25][26][27][28][29] In case of rigid and bulky linkers like arene units, only coinage metal complexes are known. [30][31][32] Acyclic tetra-NHC ligands were limited to rigid, substituted arene entities, which are unable to chelate single metal ions.…”

“…[33][34][35][36] Therefore, reports on mononuclear metal complexes with tetra-NHC ligands are limited to macrocyclic systems. [19][20][21][22][23][24][25][26][27][28][29]37 In addition, the accessible coordination modes for tetra-NHC ligands are limited, as they coordinate generally in an equatorial fashion, with the only exceptions being a tetrahedral Co II complex and a trigonal prismatic Fe IV tetrazene complex. 20,23 To overcome the structural rigidity that is responsible for this limitation, two open chain tetra-NHC ligands L1 and L2 with alkyl linkers of different lengths have been reported by our group together with the respective silver(I) complexes 1 and 2 (Fig.…”