Reaction of Ag 2 O with the N-heterocyclic carbene (NHC) precursors 3,5-bis [3-(2,4,6-trimethylphenyl)imidazolium-1-ylmethyl]-1H-pyrazole bishexafluorophosphate (1a) and 3,5-bis[3-(2,6-diisopropylphenyl)imidazolium-1-ylmethyl]-1H-pyrazole bishexafluorophosphate (1b) yielded silver(I) complexes 2 and 3, respectively. X-ray crystallography revealed a tetranuclear dimeric solid state structure in the case of 2, whereas an unprecedented octanuclear tetrameric "double-crown"-like structure was found for 3, presumably due to the slightly bulkier ligand. Unlike 2, complex 3 is not stable in solution but in equilibrium with tetranuclear and hexanuclear complexes 4 and 5, respectively. The identity of those species could be elucidated by detailed NMR spectroscopic studies ( 1 H, 13 C, 15 N, 109 Ag, and DOSY experiments), which also provide valuable benchmark data for correlating 1 J AgN couplings and 109 Ag chemical shifts with structural information.

A set of compartmental pyrazole‐based ligands with appended NHC donors has been synthesized and isolated as [H4L]Cl3 and [H3L](PF6)2 salts. Dinuclear (allyl)palladium complexes of these ligands are conveniently accessible via the in situ prepared silver species. Three complexes [(allyl)2Pd2L]PF6 and one derivative [(methallyl)2Pd2L]PF6 have been characterized crystallographically, which revealed that the metal ions are positioned in close proximity [d(Pd···Pd) = 3.97–4.05 Å], with two possible mutual orientations of the (meth)allyl ligands within the dimetallic pocket. NMR spectroscopy shows slow interconversion of these isomers (k = 0.05–0.4 s–1), where the (meth)allyl ligands are detached trans to the carbene during this η3‐η1‐η3 dynamic process.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

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A family of new pyridazine‐bridged NHC/pyrazole ligand precursors HL1–5 were prepared and fully characterized including analysis by XRD {HL1 = 3‐[3‐(2,6‐diisopropylphenyl)‐3H‐imidazolium‐1‐yl]‐6‐(3‐pyridin‐2‐yl‐pyrazol‐1‐yl)‐pyridazine, HL2 = 3‐[3‐(2,4,6‐trimethylphenyl)‐3H‐imidazolium‐1‐yl]‐6‐(3‐pyridin‐2‐yl‐pyrazol‐1‐yl)‐pyridazine, HL3 = 3‐[3‐(2,4,6‐trimethylphenyl)‐3H‐imidazolium‐1‐yl]‐6‐(3,5‐dimethylpyrazol‐1‐yl)‐pyridazine, HL4 = 3‐(3‐tert‐butyl‐3H‐imidazolium‐1‐yl)‐6‐(3,5‐dimethylpyrazol‐1‐yl)‐pyridazine, HL5 = 3‐[3‐(2,4,6‐trimethylphenyl)‐3H‐imidazolium‐1‐yl]‐6‐(3‐methyl‐5‐phenylpyrazol‐1‐yl)‐pyridazine, X = PF6– or BF4–}. Reaction of the ligand precursors with Ag2O yielded various silver(I) complexes whose structures have been elucidated crystallographically. In complexes [(L3)2Ag](PF6) (4) and[(L4)2Ag](PF6) (4′) the single silver(I) ion is coordinated in a linear fashion by the NHC moieties of the two ligand strands. An NMR titration with AgBF4 reveals that 4 can bind two more silver(I) ions. The complex [(L5)2Ag2](PF6)2 (5′) features an additional silver(I) centre bound to the two pyrazole rings, whereas in [(L3)2Ag2](BF4)2 (6) ligand reshuffling has occurred to give antiparallel ligand strands with {CNHCNpyrazole} coordination of each metal ion. Secondary interactions with the pyridazine N‐atom are observed in some cases. An additional third silver(I) ion can be accommodated between the central pyridazine bridges, as shown in [(L3)2Ag3](PF6)2(BF4) (7). The sequence of binding events and the identity of the species in solution have been investigated by NMR spectroscopy and ESI mass spectrometry.