The synthesis of copper, gold, nickel, palladium, platinum, and iron complexes with open chain tetra-N-heterocyclic carbene (NHC) ligands via transmetalation using silver NHC complexes is presented. The obtained complexes show differing coordination geometries depending on both ligand structure and metal. While the complexes of the coinage metals form di- or tetranuclear structures, the group 10 metal complexes exhibit a distorted square planar coordination geometry at the metal centers. In the case of iron an enhanced flexibility of the ligand - caused by a longer alkyl bridge - leads to octahedral complexes with a sawhorse-type coordination by the tetracarbene ligand and two cis acetonitrile ligands. To the best of our knowledge, this is the first known example of a tetracarbene ligand in sawhorse-type coordination within an octahedral coordination sphere. The remaining cis-labile sites are prone to exchange reactions as shown by addition of trimethylphosphine.

A tetranuclear silver(I) N-heterocyclic carbene (NHC) complex bearing a macrocyclic, exclusively methylene-bridged, tetracarbene ligand was synthesized and employed as transmetalation agent for the synthesis of nickel(II), palladium(II), platinum(II), and gold(I) derivatives. The transition metal complexes exhibit different coordination geometries, the coinage metals being bound in a linear fashion forming molecular box-type complexes, whereas the group 10 metals adapt an almost ideal square planar coordination geometry within the ligand's cavity, resulting in saddle-shaped complexes. Both the Ag(I) and the Au(I) complexes show ligand-induced metal-metal contacts, causing photoluminescence in the blue region for the gold complex. Distinct metal-dependent differences of the coordination behavior between the group 10 transition metals were elucidated by low-temperature NMR spectroscopy and DFT calculations.

The synthesis of 3-((1H-imidazol-1-yl)methyl)-1-methyl-1H-imidazol-3-ium iodide 1 for the synthesis of multidentate tetra-N-heterocyclic carbene (NHC) structures is described. Two acyclic open chain tetra(NHC) precursors are synthesized together with their corresponding silver complexes. On the basis of nuclear magnetic resonance (NMR) spectroscopy and single crystal X-ray diffraction (XRD) data, the coordination geometry of the silver complexes is discussed. Dependent on the length of the alkyl bridge of the tetradentate ligand either a double helix structure with four linear-coordinated silver cations or a twisted geometry with two linear-coordinated silver cations is obtained.

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...

The synthesis of the first ruthenium(II) complexes bearing open-chain tetra-N-heterocyclic carbene (tetra-NHC) ligands via in situ transmetalation is described. The ruthenium complexes show differing coordination geometries depending on the length of the alkyl linker. Replacement of the two cis acetonitrile ligands in the methylene bridged ruthenium complex by nucleophiles also influences the coordination geometry. Both structural motifs were evaluated in transfer hydrogenation (TH) of acetophenone, and in particular the sawhorse-type coordinated system exhibits remarkable activity with turnover frequencies of more than 100 000 h(-1).

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