NHC Versus Pyridine: How “Teeth” Change the Redox Behavior of Iron(II) Complexes

Abstract

A series of octahedral iron(II) complexes with tetradentate NHC/pyridine hybrid ligands containing up to three pyridyl units was designed to study the influence of NHC and pyridine donors on the electronic structure of the metal center. Structural analysis of the iron complexes by NMR spectroscopy and single-crystal X-ray diffraction reveals different coordination modes of the ligand depending on the linkage of the different donor moieties. The oxidation potentials of all complexes correlate linearly with the number of NHC moieties coordinated to iron, as shown by cyclic voltammetry. The influence, although minor, of structural properties on the oxidation potential and (in one case) the influence of the oxidation state of the coordination geometry of the hybrid ligand are also demonstrated. ■ INTRODUCTIONN-heterocyclic carbene (NHC) ligands have become a multifunctional tool in organometallic chemistry and homogeneous catalysis, as steric and electronic properties can be easily influenced by modification of the substituents. 1−6 During the past decade, iron NHC complexes have especially received increasing attention. 7−9 Examples of remarkable reactivity were obtained on the basis of the use of chelating, polydentate NHC ligands, 10−17 including oxidation catalysis with NHC/pyridine hybrid ligands. 18,19 Recently, it could be shown that replacing acetonitrile ligands of the catalytically active iron(II) NCCN complex has a strong influence on the electronic structure of the metal center. 20 However, the literature lacks comprehensive studies on the influence of NHC/pyridine hybrid ligands with different NHC/pyridyl ratios on the oxidation potential of the metal. Such manipulations would allow fine-tuning of the metal center without blocking labile binding sites.Several alkylene-bridged tetradentate NHC/pyridine hybrid ligands are known (Figure 1). Ligands of the general types A (NCCN) and B and C (both CNNC) as well as their arylated derivatives 21−27 count for open-chain structures, while E fits into the class of macrocyclic ligands. 28 Three types of alkylenebridged tetra-NHC ligands (CCCC) have been described so far: open-chain (D), 29 macrocyclic (F), 30−32 and mixed aryl-or heteroatom-bridged ligands. 33−38 The coordination chemistry of the aforementioned acyclic type A di-NHC ligand derivatives has been well investigated. Complexes are known for group 10 metals, 21,22,25,26,39−41 coinage metals, 22,42 cobalt, 41 ruthenium, 43 and iron. 41,44 Only one example of class B is known; it is a highly rigid ligand and does not permit mononuclear, chelated metal complexes, with only a silver complex being reported. 23 The more flexible skeletal structure of type C offers chelating abilities, 24,27,45 although the reported palladium complex is strongly distorted with N−Pd−N and N−Pd−C angles of 80°in comparison to C−Pd−C angles of 120°. 24 Acyclic tetra-NHC ligands (D) are highly flexible, and complexes are known for coinage metals, group 10 metals, and iron. 29,46 For macrocyclic ligands of type E and F a broad r...

Osmium(II) Complexes Bearing Chelating N-Heterocyclic Carbene and Pyrene-Modified Ligands: Surface Electrochemistry and Electron Transfer Mediation of Oxygen Reduction by Multicopper Enzymes

“…46 The 180 mV negative shift confirms that the bis-NHC ligand is a strong electron donor as compared to the bipyridine ligand. 47 The redox potential of 1 is therefore very close to the redox potential of ABTS (E = +0.50 V vs Ag/AgCl at pH 7), the mainly used redox mediator for oxygen reduction by multicopper oxidases. 48 Thus, complex 1 has a well-suited redox potential for mediating the high potential oxygen reduction by these enzymes.…”

Solar energy conversion using first row d-block metal coordination compound sensitizers and redox mediators

“…13) led to an excited 3 MLCT state lifetime of 13 ps. 266 Another highly relevant publication around this time came from Kuhn and coworkers 267 who demonstrated that the electronic structure of NHC iron(II) complexes is strongly influenced by the number of NHC donors. Cyclic voltammetric data confirmed a linear correlation between the oxidation potentials of the iron(II) complexes and the number of NHC donors.…”

Controlling the Coordination Modes of Pyridyl-Functionalized N-Heterocyclic Dicarbene Ligands

“…These ligands consist of two rigid pyridyl-NHC moieties that are linked by a flexible propylene or o -xylylene spacer. Related tetradentate ligands containing varying numbers of NHC versus pyridyl moieties have been reported and were found to influence the redox behavior of iron­(II) complexes . In our case, the inclusion of two opposing elements, i.e.…”

“…Related tetradentate ligands containing varying numbers of NHC versus pyridyl moieties have been reported and were found to influence the redox behavior of iron(II) complexes. 14 In our case, the inclusion of two opposing elements, i.e. coplanar C/N vs perpendicular diNHC chelation (Chart 1), should prevent simple in-plane coordination of the potentially tetradentate ligand, while the bromide counteranions are a source for competing coligands.…”