Enantiopure Cyclometalated Rh(III) and Ir(III) Complexes Displaying Rigid Configuration at Metal Center: Design, Structures, Chiroptical Properties and Role of the Iodide Ligand

Antoine Groué,Marion Jean,Jean-Philippe Tranchier,Geoffrey Gontard,Hani Amouri,Nicolas Vanthuyne

doi:10.3390/chemistry4010014

Abstract

Enantiopure N-heterocyclic carbene half-sandwich metal complexes of the general formula [Cp*M(C^C:)I] (M = Rh, Ir; C^C: = NI-NHC; NI-H = Naphthalimide; NHC = N-heterocyclic carbene) are reported. The rhodium compound was obtained as a single isomer displaying six membered metallacycle and was resolved on chiral column chromatography to the corresponding enantiomers (S)-[Cp*Rh(C^C:)I] (S)-2 and (R)-[Cp*Rh(C^C:)I] (R)-2. The iridium congener, however, furnishes a pair of regioisomers, which were resolved into (S)-[Cp*Ir(C^C:)I] (S)-3 and (R)-[Cp*Ir(C^C:)I] (R)-3 and (S)-[Cp*Ir(C^C:)I] (S)-4 and (R)-[Cp*Ir(C^C:)I] (R)-4. These regioisomers differ from each other, only by the size of the metallacycle; five-membered for 3 and six-membered for 4. The molecular structures of (S)-2 and (S)-4 are reported. Moreover, the chiroptical properties of these compounds are presented and discussed. These compounds display exceptional stable configurations at the metal center in solution with enantiomerization barrier ΔG≠ up to 124 kJ/mol. This is because the nature of the naphthalimide-NHC clamp ligand and the iodide ligand contribute to their configuration’s robustness. In contrast to related complexes reported in the literature, which are often labile in solution.

Highlights

Chirality is an ever-fascinating topic and occurs in many fields of science [1–4]
Half-sandwich rhodium and iridium complexes with piano stool geometry, displaying central chirality are labile in solution, as demonstrated by Brunner and co-workers and others [26–29]
We recently described the synthesis of some half-sandwich iridium complexes displaying stable configuration at the metal center [32]

Summary

Introduction

Chirality is an ever-fascinating topic and occurs in many fields of science [1–4]. In the area of transition metal complexes, Brunner [5,6], Gladysz [7,8], von Zelewsky [9], Meggers [10], Constable [11] and others [12,13] have made great contributions to the advancement and comprehension of the elements that control the chirality at metal centers at the molecular and supramolecular levels [11,14]. Octahedral iridium complexes displaying helical chirality (∆, Λ) show a stable configuration at the metal center [15–19]. Coordination and organometallic complexes with planar chirality show stable configuration [20–25]. Half-sandwich rhodium and iridium complexes with piano stool geometry, displaying central chirality are labile in solution, as demonstrated by Brunner and co-workers and others [26–29]. Efforts were devoted to using strongly coordinated N-heterocyclic carbene (NHC) ligands to increase the stability at the metal center; only a few examples were reported [30–32]

Methods

Discussion

Conclusion