Synthesis of Silver(I) and Gold(I) Complexes with Cyclic Tetra‐ and Hexacarbene Ligands

Abstract

Cyclic polyimidazolium salts have been investigated with respect to their function as anion receptors and cyclophanelike tetraimidazolium tetracations have been shown to bind anions inside the ring cavity. Cyclic polyimidazolium cations can also serve as precursors for N-heterocyclic carbenes (NHCs) and their complexes. Complexes of cyclic di-NHC ligands have been generated from bisazolium cyclophanes like A. However, such dicarbene ligands coordinate in a cisfashion to the metal center. They act as classical bidentate ligands rather than encapsulating the metal in a manner typical for macrocyclic ligands. Only one cyclic tetradentate dicarbene ligand is known to form a saddle-shaped macrocyclic complex B with nickel(II). We have described the first complex with a macrocyclic tetracarbene ligand C. The rhenium complex of a tridentate, facially coordinated [11]ane-P2C NHC macrocycle has also been reported. Since the preparation of complex C in a metal-template controlled domino-reaction is cumbersome we became interested in the preparation of complexes with macrocyclic poly-NHC ligands from cyclic polyimidazolium cations. This approach has previously been applied successfully for the preparation of some mono (Pd) and dinuclear (Cu, Ag) carbene complexes from the cyclic tetraimidazolium salt D. We became particularly interested in the lutidine-bridged tetraimidazolium salt H4-1 ACHTUNGTRENNUNG(Br4)[8] which upon C deprotonation would yield a potentially hexadentate ligand possessing two C-N-C pincer-type binding sites. Here we describe the mono and dinuclear silver(I) complexes with the carbene ligands derived from H4-1(Br)4 and the transfer of the carbene ligand to yield the mono and dinuclear gold(I) complexes (Scheme 1). A slight modification in the synthesis protocol employed for the preparation of H4-1(Br)4 yields the cyclic hexaimidazolium salt H6-6Br ACHTUNGTRENNUNG(BPh4)5 (Scheme 2). The preparation and molecular structure of the hexanuclear silver(I)-NHC complex derived from this ligand is also ACHTUNGTRENNUNGdescribed. The tetraimidazolium salt H4-1(Br)4 is soluble in water. [8]