Design of a bifunctional alkoxy-NHC ligand for assembling tantalum-rhodium heterobimetallic molecular and silica-supported complexes

• Design von bifunktionalen Alkoxy-NHCs für die Synthese von molekularen und Silica-immobilisierten hetero-bimetallischen Tantal-Rhodium-Komplexen

Srivastava, Ravi; Leitner, Walter (Thesis advisor); Quadrelli, Elsje Alessandra (Thesis advisor)

Aachen (2020)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2020. - Dissertation, Université de Lyon, 2020

Abstract

Early-Late hetero bimetallic (ELHB) transition metal complexes offer unique perspectives in small molecules activation reactions due to the bifunctionality of the metal atoms involved. One identified strategy in order to direct and stabilize ELHB assemblies is to take profit of bifunctional bridging ligands featuring two distinct coordination motifs: one hard, able to form strong bonds with electrophilic early metal centers and one soft nucleophilic site featuring a strong affinity for electron rich late metal centers. In most examples of bifunctional ligands reported to date in the literature, the late-metal donor is a phosphine-derived moiety. N-Heterocyclic carbenes (NHCs) have evolved as a substitute to phosphine ligandsdue to their relatively easy synthesis and ability to form stable and strong metal complexes when compared to phosphines. Moreover, a versatile range of functionalized NHCs could be synthesized by incorporating wide variety of functional groups in the NHC backbone. Surprisingly, very few ELHB complexes supported by bifunctional NHC ligands have been reported to date. Hence, the aim of this PhD work is to report the synthesis of tantalum-rhodium ELHB complexes utilizing the bifunctional NHC ligands. The efficient, scalable, simple and versatile synthesis of a new unsymmetrical hydroxyl-tethered NHC has been developed. The exact structure of the freeligand platform has been investigated in detail: the analyses reveal that this ligand adopts a neutral hydroxyl-carbene form that features an unusual OH-carbene hydrogen-bonding interaction. This ligand platform was successfully used to yield rare examples of Ta-NHC complexes, as well as a series of Rh-NHC monometallic species. The potential of these derivatives for the preparation of Ta/Rh heterobimetallic assemblies was then explored through either i) the protonolysis reaction between the free hydroxyl pendant group in the monometallic Rh complexand the alkyl moiety from tantalum precursorsor ii) the incorporation of Rh into Ta-NHC complexes through carbene transmetallation from Ta to Rh. Mitigated results were obtained in the case of tantalum alkylidene derivatives, likely due to the high reactivity of this chemical motif. However, well defined NHC-based Ta/Rh hetero bimetallic entities were obtained in the case of Ta-imido or Ta-siloxy alkyl derivatives. This showcases the utility of this bifunctional alcoxy-carbene motif to promote the assembly of the two metals, due to the preferred coordination of the “soft” carbene ligand moiety to Rh while the “hard” alkoxy ligand group is selective to Ta. The insight obtained from the reactivity in solution with molecular silanols was utilized to develop rare examples of silica-supported tantalum-rhodium ELHB complexes. Finally,as a prospective exploration of a potential reaction of interest; computational study for nitrogen activation to NH3 was carried out using the homogeneous bimetallic complex. The first few steps of the catalytic cycle involve the transfer of hydride from the rhodium center to the tantalum bound ƞ2-N2 and are energetically favorable i.e., the transition states and the local minima energies are below 30 kcal.mol-1. This work opens attractive perspectives for small molecules activation by ELHB species, and future work will focus on the implementation of the N2 activation in laboratory using the silica-supported complexes.