Browsing by Author "Walter, Robert R. M."

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Open Access
The best of both worlds : combining the power of MICs and WCAs to generate stable and crystalline CrI‐tetracarbonyl complexes with π‐accepting ligands
(2023) Bens, Tobias; Walter, Robert R. M.; Beerhues, Julia; Schmitt, Manuel; Krossing, Ingo; Sarkar, Biprajit
Here we present stable and crystalline chromium(I) tetracarbonyl complexes with pyridyl-MIC (MIC=mesoionic carbene) ligands and weakly coordinating anions (WCA=[Al(ORF)4]-, RF=C(CF3)3 and BArF=[B(ArF)4]-, ArF=3,5-(CF3)2C6H3). The complexes were fully characterized via crystallographic, spectroscopic and theoretical methods. The influence of counter anions on the IR and EPR spectroscopic properties of the CrI complexes was investigated, and the electronic innocence versus non-innocence of WCAs was probed. These are the first examples of stable and crystalline [Cr(CO)4]+ complexes with a chelating urn:x-wiley:09476539:media:chem202301205:chem202301205-math-0001 accepting ligand, and the data presented here are of relevance for both the photochemical and the electrochemical properties of these classes of compounds.
Open Access
Isolation, characterization and reactivity of key intermediates relevant to reductive (electro)catalysis with Cp*Rh complexes containing pyridyl‐MIC (MIC=mesoionic carbene) ligands
(2023) Bens, Tobias; Walter, Robert R. M.; Beerhues, Julia; Lücke, Clemens; Gabler, Julia; Sarkar, Biprajit
In recent years, metal complexes of pyridyl‐mesoionic carbene (MIC) ligands have been reported as excellent homogeneous and molecular electrocatalysts. In combination with group 9 metals, such ligands form highly active catalysts for hydrogenation/transfer hydrogenation/hydrosilylation catalysis and electrocatalysts for dihydrogen production. Despite such progress, very little is known about the structural/electrochemical/spectroscopic properties of crucial intermediates for such catalytic reactions with these ligands: solvato complexes, reduced complexes and hydridic species. We present here a comprehensive study involving the isolation, crystallographic characterization, electrochemical/spectroelectrochemical/theoretical investigations, and in‐situ reactivity studies of all the aforementioned crucial intermediates involving Cp*Rh and pyridyl‐MIC ligands. A detailed mechanistic study of the precatalytic activation of [RhCp*] complexes with pyridyl‐MIC ligands is presented. Intriguingly, amphiphilicity of the [RhCp*]‐hydride complexes was observed, displaying the substrate dependent transfer of H+, H or H-. To the best of our knowledge, this study is the first of its kind targeting intermediates and reactive species involving metal complexes of pyridyl‐MIC ligands and investigating the interconversion amongst them.
Open Access
Mesoionic imines (MIIs) : strong donors and versatile ligands for transition metals and main group substrates
(2022) Rudolf, Richard; Neuman, Nicolás I.; Walter, Robert R. M.; Ringenberg, Mark. R.; Sarkar, Biprajit
We report the synthesis and the reactivity of 1,2,3‐triazolin‐5‐imine type mesoionic imines (MIIs). The MIIs are accessible by a base‐mediated cycloaddition between a substituted acetonitrile and an aromatic azide, methylation by established routes and subsequent deprotonation. C=O‐stretching frequencies in MII-CO2 and -Rh(CO)2Cl complexes were used to determine the overall donor strength. The MIIs are stronger donors than the N‐heterocyclic imines (NHIs). MIIs are excellent ligands for main group elements and transition metals in which they display substituent‐induced fluorine‐specific interactions and undergo C-H activation. DFT calculations gave insights into the frontier orbitals of the MIIs. The calculations predict a relatively small HOMO-LUMO gap compared to other related ligands. MIIs are potentially able to act as both π‐donor and π‐acceptor ligands. This report highlights the potential of MIIs to display exciting properties with a huge potential for future development.