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CarBoRu - Anionic Carbenes and Borylanions: Tuning the properties of ruthenium complexes in olefin metathesis

Subject Area Inorganic Molecular Chemistry - Synthesis and Characterisation
Organic Molecular Chemistry - Synthesis and Characterisation
Term from 2017 to 2023
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 380280088
 
Final Report Year 2023

Final Report Abstract

In accordance with the specialized expertise of both applicants anionic N-heterocyclic carbenes with weakly coordinating anionic borate entity (WCA-NHCs, Prof. Dr. Matthias Tamm) and N-heterocyclic carbene stabilized boryl anions (NHC-BAs, Dr. René Frank) were proposed as strong ligands in the synthesis of ruthenium complexes suitable for olefin metathesis. While WCA-NHCs were well-known at the commencement of this project, this was not the case for NHC-BA species, a general access for which was successfully developed throughout this project. Reactions of both anionic species (WCA-NHC and NHC-BA) with a broad scope of ruthenium precursors as demonstrated in the original proposal did not afford the attempted complexes and only gave protonated neutral species of both anion types. Since transmetallation reactions are known to be milder with respect to ligand transfer, the anionic species (WCA-NHC and NHC-BA) were employed in the synthesis of copper and silver complexes. By way of example the isolated WCA-NHC complexes of copper and silver were reacted with representative ruthenium complexes, e.g. [Ru(para-cymene)Cl2]2, which are commonly used as precursors in olefin metathesis protocols. Interestingly, the formation of dinuclear heterobimetallic (Ag-Ru or Cu-Ru) complexes was observed. The expected salt elimination (AgCl, CuCl) with formation of WCA-NHC ruthenium complexes did not occur. Similar attempts in the context of boryl anions NHC-BAs afforded functionalized NHC-stabilized boranes. In particular, the rare class of NHC-stabilized benzyl boranes is now readily accessible from this project and underwent interesting peculiar chemistry. In the context of this project a new dihydroxy functionalized carbene ligand cyclo-C{N[C(CF3)2OH]CH}2 could be developed. The deprotonation of both inherent alcoholic hydroxyl groups gave rise to a ruthenium Grubbstype complexes, in which the dianionic carbene ligand is bound in a tridentate κ3-C,O,O´ type fashion. The complex in its parent form was found inactive in olefin metathesis. However, activation could be demonstrated upon treatment with hydrogen chloride with complete protonation of the chelating alkoxide anchors giving rise to a the neutral carbene form. The latter was an efficient catalyst in ring closing and ring-opening metathesis polymerization reactions (RCM, ROMP). Approaches toward a new ligand class of neutral N-heterocyclic borylenes (NHB) stronger than common NHC-analogues gave rise to amido-imidazolin-2-imines (AmIm−) as new monoanionic N,N´-chelating ligands for boron centres. The isolation of N-heterocyclic borylenes of type (AmIm)B: with the formal +1 oxidation state at boron proved to be difficult, but the new AmIm ligand system gave rise to a complete series of boraketones (AmIm)B=E (E = O, S, Se, Te) for the first time and also allowed for the preparation of parent iminoboranes of type (AmIm)B=NH. The stabilization of these terminally double bound boron species is interesting since B=E bonds (E = O, S, Se, Te, NH) usually undergo dimerization / oligomerization and are hard to isolate.

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