Synthesis of N-heterocyclic carbene silver and palladium complexes bearing bis(pyrazol-1-yl)methyl moieties

A series of N-heterocyclic carbene (NHC) silver and palladium complexes derived from N-[bis(pyrazol-1-yl)methyl]phenyl-N'-benzylimidazolium and N-[bis(pyrazol-1-yl) methyl]phenoxy- methyl-N'-benzylimidazolium salts have been synthesized and characterized by elemental analysis, NMR spectroscopy and single-crystal X-ray diffraction. The NHCs functionalized by bis(pyrazol-1-yl) methyl groups show three different coordination modes in these complexes. N-[Bis(pyrazol-1-yl)methyl]phenyl-N'-benzylimidazol-2-ylidene (L) and N-[bis(3,5-dimethylpyrazol-1-yl)methyl]phenyl-N'-benzylimidazol-2-ylidene (L') act as monodentate ligands through the carbene carbon in cationic complexes [L2Ag][AgCl2] and [L-4'Ag3Br2][AgBr2]. A bridging [C,N]bidentate coordination mode, through one pyrazolyl nitrogen and the carbene carbon to two palladium centers, was observed in macrocyclic complex L-2'Pd2Br2. While in the coordination polymer [L2Ag2](n)[PF6](2n) and macrocyclic complex [L-2'Ag-2][PF6](2), L and L' act as bridging [C,N,N] tridentate ligands through two pyrazolyl nitrogens and the carbene carbon. Although N-[bis(pyrazol-1-yl)methyl]phenoxymethyl-N'-benzylimidazol-2-ylidene (L '') also acts as a bridging [C,N] bidentate ligand in L-2 '' Ag2Cl2 and a bridging [C,N,N] tridentate ligand in L-2 '' Ag2X2 (X = BF4 or PF6), like L' and L in L-2'Pd2Br2 as well as [L2Ag2](n)[PF6](2n) and [L-2'Ag-2][PF6](2), respectively, an obvious difference among them is present. L '' prefers to coordinate to two silver atoms in a head (carbene)-to tail (nitrogen) coordination mode, while L and L' show head (carbene)-to head (carbene) and tail (nitrogen)-to tail (nitrogen) coordination modes to two silver atoms. Additionally, these silver complexes exhibit good catalytic activity in the three-component coupling reaction of alkyne, aldehyde and amine forming propargylamines. (C) 2013 Elsevier B. V. All rights reserved.