Rhodium(iii) and ruthenium(ii) complexes of redox-active, chelating N-heterocyclic carbene/thioether ligands

Abstract

Dedicated to Professor Didier Astruc on the occasion of his 65th birthday.Half-sandwich rhodium(III) and ruthenium(II) complexes bearing a new redox-active ferrocenyl NHCthioether ligand have been prepared. The synthesis of ferrocenyl thioether-imidazolium salts 3a and 3b was carried out via intermediate 2 using an improved procedure. Rhodium(III) complex 4 and ruthenium(II) complex 5 were obtained in good yields and were fully characterised by NMR 10 spectroscopy, X-ray diffraction analysis and electrochemistry. Complex 4 shows a complex ABCD system by 1 H NMR, which denotes conformational rigidity due to the presence of several bulky groups.Electrochemical analysis by cyclic voltammetry reveals reversible redox behaviour about the iron centre in 4 and 5, and indicates electronic communication between iron and rhodium or ruthenium. Introduction 15N-Heterocyclic carbenes (NHCs) have become widely used ligands in many areas from transition metal catalysis, 1 organocatalysis, 2 to biochemistry and medicine. 3 They are generally described as organophosphine analogues, although their -donor character is more pronounced. Unlike phosphines, they 20 generally lead to air-stable, robust complexes which do not easily undergo ligand dissociation. However, the M-C bond stability could be detrimental to the catalytic activity. Functionalized Nheterocyclic carbene ligands combine the robust nature of the resulting metal-NHC bond with the flexibility of the secondary 25 donor function (hemilability, stereoelectronic control on the active site) and have been successfully used in many catalytic reactions. [4][5][6] However, any alteration of steric or electronic properties of a ligand implies structural modifications and thus more synthetic work. 30Fine tuning of the electronic properties can also be achieved by introduction of redox-active moieties in the ligands. 7 This has been demonstrated in the field of small molecule sensing 8,9 and in catalysis. 10 In the latter domain, transition metal complexes containing either substitutionally inert 11 or hemilabile 12 redox-35 active ligands have shown charge-dependent behaviour. Our group has a strong interest in the chemistry and catalytic activity of functionalised N-heterocyclic carbene complexes. We have shown that NHCs bearing thioether or phosphine donors give very efficient and robust catalysts. 13,14 The work reported 40 here describes the preparation of redox-active NHC-thioether ligands incorporating the ubiquitous ferrocene group and the study of their coordination chemistry and electrochemical behaviour. Results and discussion 45 SynthesisScheme 1 Synthesis of imidazolium salts 3a and 3b.The synthesis of the imidazolium salt 3b, i.e. the NHC-thioether precursor, was achieved in three steps from bromoferrocene 1. 50Although the preparation of intermediate 2 and its 1,1'-disubstituted analogue have been described by Mirkin et al., 15,16 we decided to follow a different synthetic route. Mirkin's reported procedures involved the monolithiation of ferrocene and afforded th...

Application of ferrocenylimidazolium salts as catalysts for the transfer hydrogenation of ketones

“…Coordination of potassium to the deprotonated reactive carbene species was confirmed by the appearance of a downfield signal at 152.46 ppm which corresponded to a carbene–K bond. This is similar to values recorded for related metal–NHC bonds . An important work by Arnold et al .…”

Synthesis, Structural Characterization and Antiproliferative Activity of Gold(I) and Gold(III) Complexes Bearing Thioether‐Functionalized N‐Heterocyclic Carbenes

“…In this regard, N‐heterocyclic carbenes bearing a thioether pendant function (Figure 1) are also an interesting class of ligands, that has found application both in homogeneous catalysis and bioinorganic chemistry. Particularly, these S‐functionalized NHCs may show a chelating ability which results in the formation of highly stable complexes with a strong σ‐donor function (the NHC moieties) and a weaker one (the sulfur atom) [12–17] . The hemilabile equilibrium of sulfur chelating ligand can be of high importance for the development of efficient catalysts: a momentary dissociation of the heteroatom from the metal during a catalytic cycle releases a coordination site and allows the coordination of the substrate, while preserving the stability of the metal complex with the coordination of the NHC ligand.…”

A rhodium(I) dicarbonyl complex with a redox-active ferrocenyl phosphine-NHC ligand: Enhanced reactivity of the metal centre through ferrocene oxidation

“…Our group has a strong interest in the chemistry and catalytic activity of functionalised Nheterocyclic carbene complexes [17][18][19][20][21]. We have recently investigated the behaviour of a 1,5-cyclooctadiene (COD) rhodium(I) complex bearing a ferrocenyl phosphine-NHC ligand, 1, towards oxidation (figure 1) [21].…”