Abstract
We report a new class of stable mesoionic N‐heterocyclic olefins, featuring a highly polarized (strongly ylidic) double bond. The ground‐state structure cannot be described through an uncharged mesomeric Lewis‐structure, thereby structurally distinguishing them from traditional N‐heterocyclic olefins (NHOs). mNHOs can easily be obtained through deprotonation of the corresponding methylated N,N′‐diaryl‐1,2,3‐triazolium and N,N′‐diaryl‐imidazolium salts, respectively. In their reactivity, they represent strong σ‐donor ligands as shown by their coordination complexes of rhodium and boron. Their calculated proton affinities, their experimentally derived basicities (competition experiments), as well as donor abilities (Tolman electronic parameter; TEP) exceed the so far reported class of NHOs.
Highlights
Novel bonding modes of carbon, especially strong carbonbased donor ligands, such as carbenes, have received great interest throughout the last decades and continue to have large impact on the chemical community and beyond
We report a new class of stable mesoionic Nheterocyclic olefins, featuring a highly polarized double bond
The ground-state structure cannot be described through an uncharged mesomeric Lewis-structure, thereby structurally distinguishing them from traditional Nheterocyclic olefins (NHOs). mesoionic N-heterocyclic olefins (mNHOs) can be obtained through deprotonation of the corresponding methylated N,N’diaryl-1,2,3-triazolium and N,N’-diaryl-imidazolium salts, respectively
Summary
In contrast to I–IV, such “olefins” would be mesoionic,[15] in analogy to MICs, as they cannot be represented by a canonical Lewis resonance structure without charge separation (V– VIII). Such structural motive is rare, as in general, mesoionic compounds typically feature chalcogen atoms and not carbon in the exocyclic position.[15] Such an overlooked class of mesoionic N-heterocyclic olefins (mNHOs) should exhibit strong donor abilities, considering a correlation with the parent carbene entities (see below)
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