Crystalline 1H‐1,2,3‐Triazol‐5‐ylidenes: New Stable Mesoionic Carbenes (MICs)

Abstract

Until recently, the availability of neutral carbon-based κ1C ligands was limited to carbon monoxide, isocyanides, and carbenes. Compared to phosphorus-based ligands, carbenes tend to bind more strongly to metal centers, avoiding the necessity for the use of excess ligand in catalytic reactions. The corresponding complexes are often less sensitive to air and moisture, and are remarkably resistant to oxidation.[1] As the robustness of carbene complexes is largely due to the presence of strong carbon–metal bonds, other types of carbon-based ligands are highly desirable. It is noteworthy that, although complexes between a carbene and a transition metal have been known for a long time,[2] the recent developments in their application in catalysis[3] have been greatly facilitated by the availability of carbenes that are stable enough to be bottled.[4,5] Moreover, carbenes, especially imidazol-2-ylidenes I[4c] and 1,2,4-triazol-5-ylidenes II,[4e] are also excellent organocatalysts (Scheme 1).[6]

Isolation and Structures of 1,2,3‐Triazole‐Derived Mesoionic Biscarb­enes with Bulky Aromatic Groups

“…The 2,6-diisopropylphenyl groups on the triazole rings are twisted perpendicularly from the three central rings, as suggested by the 1 H NMR spectrum. Similar to the reported triazolylidene, the five-membered ring has a conjugated structure: [5,11] the five bond lengths of one of the triazolylidene rings, N2-N3, N3-N4, N4-C6, C6-C7, and C7-N2 are 1.350(4), 1.329(4), 1.370(4), 1.411(4), and 1.368(4) Å, respectively. The carbene carbon atom angles, C6-C7-N2 and C8-C9-N5, are 99.6(3) and 99.7(3), respectively; these are smaller than those of the NHC analogue, which are 101.1(3).…”

“…The σ donation by a 1,2,3-triazole-derived carbene to metals is stronger than that by common NHCs. [5] Many metal com-tal results, the carbenes are partly reduced to anionic radical species in the presence of strong bases. This suggests that the pyridylene and triazolylidene moieties endow molecules with both electron-acceptor and -donor properties.…”

“…[4] Bertrand et al first reported a 1,2,3-triazole-derived MIC (triazolylidene) ligand. [5] MICs have been attracting increasing attention, because a click reaction, that is, the copper-catalyzed [2+3] Huisgen cycloaddition to form 1,2,3-triazoles, [6] makes this ligand easier to synthesize than other carbene ligands. The σ donation by a 1,2,3-triazole-derived carbene to metals is stronger than that by common NHCs.…”

Synthesis and properties of heteroaromatic carbenes of the imidazole and triazole series and their fused analogues

“…The starting 1,2,3-triazolіum salts were obtained by cycloaddition of 1,3-diaryltriazenium salts to acetylenes 235 or by quaternization of the corresponding 1,2,3-triazoles, which in turn were synthesized by click-cycloaddition of azides to acetylenes in the presence of copper(II) compounds. 243 The individual 1,2,3-triazolium-5-ides 52C were synthesized by deprotonation of the appropriate 1,2,3-triazolіum salts 52B with potassium tert-butoxide or KHMDS in THF or ether solutions (Scheme 52). 234 Similarly, the pyrazolіum-4-іde 52D was synthesized from a 3,5-diphenoxypyrazolium salt in which the distribution of electron density is more like that of a conjugated cyclic allene.…”

Transition Metal Complexes of Anionic N‐Heterocyclic Dicarbene Ligands

“…[12] A closely related, metal-free mesoionic carbene was also isolated by Bertrand and coworkers. [13] More recently, Robinson and co-workers reported the first example of an anionic N-heterocyclic dicarbene, that is, an NHC in which one of the sites of the imidazol-2-ylidene backbone has been deprotonated, giving rise to an NHC capable of coordinating through the C2 and C4 positions simultaneously (NHDC, C, Figure 1). [14] A related anionic NHC which can be rationalized as an adduct of a dicarbene and B(C 6 F 5 ) 3 was also recently reported.…”