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

Abstract

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′4Ag3Br2][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′2Pd2Br2. While in the coordination polymer [L2Ag2]n[PF6]2n and macrocyclic complex [L′2Ag2][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″2Ag2Cl2 and a bridging [C,N,N] tridentate ligand in L″2Ag2X2 (X = BF4 or PF6), like L′ and L in L′2Pd2Br2 as well as [L2Ag2]n[PF6]2n and [L′2Ag2][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.