The chemistry of N-heterocyclic carbenes (NHCs) is dominated by N,N'-dialkylated or -diarylated derivatives. Such NHC ligands are normally obtained by C2-deprotonation of azolium cations or by reductive elimination from azol-2thiones. A simple one-step procedure is described that leads to complexes with NH,NH-functionalized NHC ligands by the oxidative addition of 2-halogenoazoles to complexes of zerovalent transition metals.
Metallosupramolecular poly‐NHC‐metal assemblies were prepared from trigonal hexakis (H6‐1 a(PF6)6 and H6‐1 b(PF6)6) or nonakis (H9‐3(BF4)9) imidazolium salts and Ag2O. Complexes [Ag6(1 a)2](PF6)6 and [Ag6(1 b)2](PF6)6 are built from six Ag+ ions sandwiched between two trigonal hexacarbene ligands with an inner and an outer NHC donor in each of the three ligand arms. The metal atoms are arranged in two triangles. The hexakis‐NHC ligands bear cinnamic ester groups at the outlying NHC donors, used in postsynthetic [2+2] cycloaddition reactions linking two hexakis‐NHC ligands by three cyclobutane units to give complexes [Ag6(2 a)](PF6)6 and [Ag6(2 b)](PF6)6 bearing a dodecacarbene ligand. From the related nonakisimidazolium salt H9‐3(BF4)9, complex [Ag9(4)](BF4)9 bearing an octadecacarbene ligand was obtained. Removal of the template metals yielded very large, stable, polyimidazolium cations with 12 or 18 internal imidazolium groups.
A versatile, one-pot synthesis for the preparation of transition metal complexes bearing protic NH,NH-NHC ligands is disclosed. The reaction of unsubstituted 2-halogenoazoles with zerovalent metal complexes of the type [M(PPh 3 ) 4 ] (M = Pd, Pt) in the presence of NH 4 BF 4 proceeds by oxidative addition of the C2−X (X = Cl, Br, I) bond to the transition metal followed by protonation at the ring nitrogen atom, leading to complexes of the type trans-[MX(PPh 3 ) 2 (NH,NH-NHC)] (M = Pd II , Pt II ; NHC = benzimidazolylidene or imidazolylidene). The trans-complexes have been obtained in all cases. The time needed to complete the reaction depends on the halogen present in the azole, and the rate of the oxidative addition follows the order C2−I > C2−Br > C2−Cl. The N−H groups of the coordinated NH,NH-NHC have been deprotonated followed by reaction with MeI to give the NMe,NMe-substituted classical NHC ligands. The N−H groups of the protic NHCs are hydrogen bond donors. The formation of N−H•••O hydrogen bonds has been observed via 1 H NMR spectroscopy upon titration of the complexes bearing protic NHCs with DMPU.
The synthesis of dinuclear ruthenium alkenyl complexes with {Ru(CO)(P i Pr 3 ) 2 (L)} entities (L = Cl À in complexes Ru 2 -3 and Ru 2 -7; L = acetylacetonate (acac À ) in complexes Ru 2 -4 and Ru 2 -8) and with π-conjugated 2,7-divinylphenanthrenediyl (Ru 2 -3, Ru 2 -4) or 5,8-divinylquinoxalinediyl (Ru 2 -7, Ru 2 -8) as bridging ligands are reported. The bridging ligands are laterally π-extended by anellating a pyrene (Ru 2 -7, Ru 2 -8) or a 6,7-benzoquinoxaline (Ru 2 -3, Ru 2 -4) π-perimeter. This was done with the hope that the open π-faces of the electron-rich complexes will foster association with planar electron acceptors via π-stacking. The dinuclear complexes were subjected to cyclic and square-wave voltammetry and were characterized in all accessible redox states by IR, UV/Vis/ NIR and, where applicable, by EPR spectroscopy. These studies signified the one-electron oxidized forms of divinylphenylene-bridged complexes Ru 2 -7, Ru 2 -8 as intrinsically delocalized mixed-valent species, and those of complexes Ru 2 -3 and Ru 2 -4 with the longer divinylphenanthrenediyl linker as partially localized on the IR, yet delocalized on the EPR timescale. The more electron-rich acac À congeners formed non-conductive 1 : 1 charge-transfer (CT) salts on treatment with the F 4 TCNQ electron acceptor. All spectroscopic techniques confirmed the presence of pairs of complex radical cations and F 4 TCNQ *À radical anions in these CT salts, but produced no firm evidence for the relevance of πstacking to their formation and properties.
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