Tuning the π‐Accepting Properties of Mesoionic Carbenes: A Combined Computational and Experimental Study

Abstract

Mesoionic imidazolylidenes are recognized as excellent electron‐donating ligands in organometallic and main group chemistry. However, these carbene ligands typically show poor π‐accepting properties. A computational analysis of 71 mesoionic imidazolylidenes that bear different aryl or heteroaryl substituents in C2 position was performed. The study has revealed that a diphenyltriazinyl (Dpt) substituent renders the corresponding carbene particularly π‐acidic. The computational results could be corroborated experimentally. A mesoionic imidazolylidene with a Dpt substituent was found to be a better σ‐donor and a better π‐acceptor compared to an Arduengo‐type N‐heterocyclic carbene. To demonstrate the utility of the new carbene, the ligand was used to stabilize a low‐valent paramagnetic tin compound.

Mesoionic Carbenes in Low- to High-Valent Vanadium Chemistry

“…Almost two decades after the first report of an abnormal 5-imidazolinylidene carbene complex, mesoionic carbenes have been developed into a distinguished ligand class. , Among them, 1,2,3-triazole derived mesoionic carbenes, namely 1,2,3-triazolinylidenes, stand out by their modular synthesis via the copper-catalyzed [3 + 2] cycloaddition between azides and alkynes. − After their initial reporting by Albrecht et al , they quickly became prominent synthetic targets for (electro-)catalysis, − supramolecular chemistry, − magnetism, and photochemistry due to their versatile synthesis and comparatively straightforward handling. − Throughout these studies, a great effort has been made to decipher their electronic structure. Mesoionic carbenes are commonly believed to be strong σ-donor ligands paralleling heteroaryls; however, recent reports emphasize their π-accepting properties − as demonstrated by the isolation of a reduced triazolinylidene ligand . Nevertheless, most studies targeted hitherto late transition metals or main group elements, while early transition-metal complexes with mesoionic carbenes have been rarely explored. − Arguably, this can be attributed to the relatively weak bond between N -heterocyclic carbenes and early transition metals .…”

A Mesoionic Diselenolene Anion and the Corresponding Radical Dianion

“…We recently reported that deprotonation of imidazolium salt 1 (Scheme 1) with a diphenyltriazinyl (Dpt) substituent at C2‐position gave a π‐acidic mesoionic carbene ligand [8] . In the course of our investigations, we found that 1 could be used as precursor to prepare the mesoionic diselenolene ligand 2 .…”

“…We recently reported that deprotonation of imidazolium salt 1 (Scheme 1) with a diphenyltriazinyl (Dpt) substituent at C2position gave a π-acidic mesoionic carbene ligand. [8] In the course of our investigations, we found that 1 could be used as precursor to prepare the mesoionic diselenolene ligand 2. The reaction of two equivalents of potassium tert-butoxide (tBuOK) with imidazolium salt 1 in THF at À 40 °C, followed by addition of excess elemental selenium, [9] resulted in the formation of 2, which could be isolated in the form of a purple solid in 90 % yield (Scheme 1).…”

“…These features distinguish the new ligand from standard dichalcogenolenes, where the dianionic form is diamagnetic and the monoanionic form is paramagnetic. The presence of the Dpt substituent is a crucial design element, because its electron‐withdrawing nature is expected to facilitate the reduction to the radical state [8] . The importance of the Dpt substituent is substantiated by the fact that the spin density of the radical dianion is largely localized on this group.…”

“…The presence of the Dpt substituent is a crucial design element, because its electron-withdrawing nature is expected to facilitate the reduction to the radical state. [8] The importance of the Dpt substituent is substantiated by the fact that the spin density of the radical dianion is largely localized on this group.…”