Synthesis of tri-pyridinium and bis-imidazolium salts for the preparation of the related N-Heterocyclic carbene Au(I) complexes

Abstract

The main purpose of this research project is to develop new multimetallic complexes, and to explore the relationship between the structural features and the catalytic behavior. This general aim can be divided into the following: Synthesis of a star-shaped tris-pyridinium and its coordination capability. Design of a family of mono- and di-imidazolylidene ligand precursors bearing either a polyaromatic group (pyrene) or a methyl group as N-substituents, and study of their coordination abilities. Evaluation of the catalytic activities of the resulting metal complexes. Carbenes are a family of organic molecules containing a neutral divalent carbon atom with six electrons and two substituents (:CR2). Two extreme types of coordinated carbenes can be distinguished; the Fischer and the Schrock type (Figure 1). Carbenes, Ln=CR2, have Fischer[1] character in complexes containing late transition metals with low oxidation states; π-acceptor ligands (L) and π-donor R substituents (-OMe). This kind of carbene is electrophilic as a consequence of the partial σ + charge on the carbene carbon. Schrock[2] carbenes are bound to a higher oxidation state, early transition metals, with non-π- acceptor L ligands and non-π-donor R groups. In this case, the carbene behaves as a nucleophile, due to the partial σ charge on the carbon atom. The charge on the carbene carbon atom is determined by the relative energies of the HOMO-LUMO orbitals of the metal and the carbene. In general, late transition metals, known to be more electronegative, have more stable M(dπ) orbitals than early transition metals, which are more electropositive. Many of the properties of NHC-based complexes may be ascribed to the particular inertness of the M-CNHC bond. It is thus interesting to further examine the interaction between the metal center and the carbene donor in their coordination compounds. The difference between Fischer and N-heterocyclic carbenes is based on the different capability of the carbene to accept π-backdonation from the metal center. In fact, the backdonation in NHCs is reduced and, in most cases can be considered negligible. The peculiar binding properties of NHCs are easily understandable, since the energy of the vacant pπorbital at the carbene center is considerably increased by the strong N → C π-donation; this orbital is too high in energy to receive electron density from the metal center. As a result, NHCs are strong σ-donor ligands but poor π-acceptors. Indeed, NHC ligands tend to be more -donor than other classical two electron donor ligands like ethers, amines or phosphines, and their organometallic complexes are often characterized by their higher stability. Along with their chemical stability, the wide coordination versatility of NHC ligands has helped its great development. Additionally, the relative easy preparation of NHC-precursors has allowed an almost infinite access to new organometallic topologies. Figure 3 depicts some NHC frameworks.[11]