Compounds containing divalent carbon centers have sparked the interest of organic, inorganic, and theoretical chemists like no other single class of molecules in chemistry. This is probably due to their fascinating molecular and electronic structures, challenging syntheses, and versatile properties. The study of carbene compounds has proven to be rewarding for material scientists as well as preparative chemists and has resulted in promising materials, such as single molecule magnets, liquid crystals, and a new generation of catalysts for organic synthesis. The latter is especially true for metal complexes of imidazol-2-ylidenes. These N-heterocyclic carbenes (NHC) are important ligands in organometallic chemistry. Chelating NHC ligands of the pincer-type reportedly yield especially stable complexes, and thus, have many advantages over conventional catalysts. The increased stability of these compounds is likely due to the chelating effect of the bidentate ligand, yet, only very few chelating biscarbene ligands are reported in the literature; known examples include linkage by a pyridine or simple CH2-unit.

The synthesis of chelating poly-dentate carbene ligands and their corresponding metal complexes should make available the many valuable properties of carbene based ligand systems. Our preparative chemistry is routinely supplemented by a large variety of spectroscopic and computational methods, such as SQUID magnetization, electronic absorption, multi-nuclear NMR and EPR spectroscopy as well as density functional theory calculations.

Much of our research focuses on the synthesis and coordination chemistry of tridentate percarbene ligands anchored to either a tri-functionalized arene moiety (Figure 1, center) or single atoms such as the non-coordinating carbon (Figure 1, top) or the stabilizing nitrogen atom (Figure 1, bottom).

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Xile Hu & Karsten Meyer
Copper Complexes of Nitrogen-Anchored Tripodal N-Heterocyclic Carbene Ligands
J. Am. Chem. Soc. 2003 , 125, 12237-12245. get it here

Recently, we reported an unprecedented tripodal N-heterocyclic carbene ligand system with a carbon anchor and important silver- and thallium precursors that give access to synthetic routes for the isolation of a new generation of potentially catalytically active metal-carbene complexes. Traditionally, NHC ligands almost exclusively are referred to as pure sigma-donors when coordinated to metal ions. Only few theoretical studies on transition metal NHC complexes have been reported. In some cases, the existence of metal to ligand pi-back bonding was suggested but the magnitudes of such interactions were reported to be minimal. We computed the electronic structure of our newly synthesized, highly symmetrical group 11 carbene complexes. The analysis of the molecular orbitals reveals interesting and significant pi-back bonding features in the linear, co-planar carbene-metal-carbene entities (see Figure, below).

The silver complex of the tripodal N-heterocyclic carbene ligand TIMEMe, [(TIMEMe)2Ag3](PF6)3 , reacts with copper(I) bromide and (dimethylsulfide)gold(I) chloride to yield the corresponding D3-symmetrical copper(I) and gold(I) complexes, [(TIMEMe)2Cu3](PF6)3 and [(TIMEMe)2Au3](PF6)3. Single-crystal X-ray diffraction, spectroscopic, and computational studies of this series of metal NHC complexes are studied. The group 11 metal complexes of TIMEME ligand exhibit iso-structural geometries, with three metal ions bridging two of the TIMEMe ligands. Each metal ion is linearly coordinated to two carbene centers, with each of the carbenoid carbons stemming from a different ligand. Overall, the molecules possess D3 symmetry. The electronic structure of these newly synthesized compounds was elucidated with the aid of DFT calculations. In contrast to the common assumption that NHCs are pure sigma donor ligands, our calculations reveal the existence of both pi- and sigma-type interactions between the metal ions and the carbenoid carbons. It was found that pi-backbonding interactions in electron-rich diaminocarbene metal complexes typically contribute to approximate 15 - 30% of the complexes' stability.