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

摘要

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 [L_2Ag][AgCl_2] and [L′ _4Ag_3Br_2][AgBr_2]. 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′2Pd2Br2. While in the coordination polymer [L_2Ag_2]_n[PF_6]_(2n) and macrocyclic complex [L′_2Ag_2][PF _6]_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″_2Ag_2Cl_2 and a bridging [C,N,N] tridentate ligand in L″_2Ag_2X_2 (X = BF_4 or PF_6), like L′ and L in L′ _2Pd_2Br_2 as well as [L_2Ag _2]_n[PF_6]_(2n) and [L′ _2Ag_2][PF_6]_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.