Directed Design of a Au^I Complex with a Reduced Mesoionic Carbene Radical Ligand: Insights from 1,2,3‐Triazolylidene Selenium Adducts and Extensive Electrochemical Investigations

Abstract

Carbene‐based radicals are important for both fundamental and applied chemical research. Herein, extensive electrochemical investigations of nine different 1,2,3‐triazolylidene selenium adducts are reported. It is found that the half‐wave potentials of the first reduction of the selones correlate with their calculated LUMO levels and the LUMO levels of the corresponding triazolylidene‐based mesoionic carbenes (MICs). Furthermore, unexpected quasi‐reversibility of the reduction of two triazoline selones, exhibiting comparable reduction potentials, was discovered. Through UV/Vis/NIR and EPR spectroelectrochemical investigations supported by DFT calculations, the radical anion was unambiguously assigned to be triazoline centered. This electrochemical behavior was transferred to a triazolylidene‐type MIC‐gold phenyl complex resulting in a MIC‐radical coordinated AuI species. Apart from UV‐Vis‐NIR and EPR spectroelectrochemical investigations of the reduction, the reduced gold‐coordinated MIC radical complex was also formed in situ in the bulk through chemical reduction. This is the first report of a monodentate triazolylidene‐based MIC ligand that can be reduced to its anion radical in a metal complex. The results presented here provide design principles for stabilizing radicals based on MICs.

Mono‐ and Di‐Mesoionic Carbene‐Boranes: Synthesis, Structures and Utility as Reducing Agents

“…[1] While most of these metal complexes were tested for their activity in homogeneous catalysis, recent works have also shown the utility of such complexes in photochemistry, [2,3] in electrocatalysis, [3,4] in redox-switchable catalysis, [5] in the construction of supramolecular assemblies [6] and as electro-active compounds. [7,8] In comparison to transition metal chemistry, there are extremely few reports of the use of such MICs in combination with main-group elements. [7,9,10,11] The better σdonor ability of these MICs in comparison to their N-heterocyclic carbene (NHC) counterparts as well as their tunable πacceptor ability, [12] might also help in generating unusual properties with main-group elements, a fact that has already been seen often in transition metal chemistry.…”

“…[7,8] In comparison to transition metal chemistry, there are extremely few reports of the use of such MICs in combination with main-group elements. [7,9,10,11] The better σdonor ability of these MICs in comparison to their N-heterocyclic carbene (NHC) counterparts as well as their tunable πacceptor ability, [12] might also help in generating unusual properties with main-group elements, a fact that has already been seen often in transition metal chemistry. We have recently reported on triazoline-selones, and have used those compound to gauge the π-acceptor strength of 1,2,3-triazolylidenes.…”

“…We have recently reported on triazoline-selones, and have used those compound to gauge the π-acceptor strength of 1,2,3-triazolylidenes. [7,12] In this report we turn our attention to the combination of 1,2,3triazolylidenes with the element boron. More specifically, we report on the reactions of three mono-1,2,3-triazol-5-ylidenes and one bi-1,2,3-triazol-5-ylidene, all of which are derived from 1,4-disubstituted-1,2,3-triazoles, with BH 3 (Figure 1).…”

“…1,2,3‐Triazol‐5‐ylidenes, which are examples of mesoionic carbenes (MICs), have been widely used as ligands in transition metal organometallic chemistry in the last years [1] . While most of these metal complexes were tested for their activity in homogeneous catalysis, recent works have also shown the utility of such complexes in photochemistry, [2,3] in electrocatalysis, [3,4] in redox‐switchable catalysis, [5] in the construction of supramolecular assemblies [6] and as electro‐active compounds [7,8] . In comparison to transition metal chemistry, there are extremely few reports of the use of such MICs in combination with main‐group elements [7,9,10,11] .…”

“…The better σ‐donor ability of these MICs in comparison to their N‐heterocyclic carbene (NHC) counterparts as well as their tunable π‐acceptor ability, [12] might also help in generating unusual properties with main‐group elements, a fact that has already been seen often in transition metal chemistry. We have recently reported on triazoline‐selones, and have used those compound to gauge the π‐acceptor strength of 1,2,3‐triazolylidenes [7,12] . In this report we turn our attention to the combination of 1,2,3‐triazolylidenes with the element boron.…”