Mesoionic and Related Less Heteroatom-Stabilized N-Heterocyclic Carbene Complexes: Synthesis, Catalysis, and Other Applications

Abstract

Mesoionic carbenes are a subclass of the family of N-heterocyclic carbenes that generally feature less heteroatom stabilization of the carbenic carbon and hence impart specific donor properties and reactivity schemes when coordinated to a transition metal. Therefore, mesoionic carbenes and their complexes have attracted considerable attention both from a fundamental point of view as well as for application in catalysis and beyond. As a follow-up of an earlier Chemical Reviews overview from 2009, the organometallic chemistry of N-heterocyclic carbenes with reduced heteroatom stabilization is compiled for the 2008-2017 period, including specifically the chemistry of complexes containing 1,2,3-triazolylidenes, 4-imidazolylidenes, and related 5-membered N-heterocyclic carbenes with reduced heteratom stabilization such as (is)oxazolylidenes, pyrrazolylidenes, and thiazolylidenes, as well as pyridylidenes as 6-membered N-heterocyclic carbenes with reduced heteroatom stabilization. For each ligand subclass, metalation strategies, electronic and steric properties, and applications, in particular, in metal-mediated catalysis, are compiled. Mesoionic carbenes demonstrate particularly high activity in (water) oxidation, hydrogen transfer reactions, and cyclization reactions. Unique features of these ligands are identified such as their dipolar structure, their specific donor properties, as well as stability aspects of the ligand and the complexes, which provides opportunities for further research.

Luminescent Platinum(IV) Complexes Bearing Cyclometalated 1,2,3‐Triazolylidene and Bi‐ or Terdentate 2,6‐Diarylpyridine Ligands

“…The complexes[ Pt 2 (m-Cl) 2 (dtpyH) 2 ]( 1)a nd [Pt(dtpy)(DMSO)] (2), bearing mono-o rd icyclometalated2 ,6-di(p-tolyl)pyridine ligands (dtpyH or dtpy,r espectively), were synthesized following the procedures described fort he analogous 2,6-diphenylpyridine complexes [60] and employed as starting materials.I ntroductiono ft he aryl-NHC ligand was then attempted by transmetalation from the silver carbenei ntermediate "AgI(trzH)" (trzH = 4-butyl-3-methyl-1-phenyl-1H-1,2,3-triazol-5-ylidene), which can be generated by reacting the triazolium salt with Ag 2 Oa nd used in situ. [42,43] Reaction of the dichloro-bridged dimer 1 with "AgI(trzH)" in a1:2 molarratio led to the selective formationo ftrans-C,C*-[PtCl(dtpyH)(trzH)] (trans-C,C*-3;9 1%), in which the triazolylidene ligand coordinates trans to the metalated tolyl group (Scheme 1). Thee xclusive formation of this isomer is likely ac onsequence of the higherk inetic lability of the PtÀCl bond trans to the metalated tolyl group in 1 compared to that trans to the pyridine moiety.…”

“…The chosen aryl-NHC ligand belongs to the 1,2,3-triazolylidene subclass, presenting ar educed heteroatom stabilization and stronger s-donating ability compared to normalA rduengo-type NHCs. [42,43] We anticipated that the donor properties of the triazolylidene moiety would benefit the emission efficiencies of cyclometalatedP t IV complexes by increasing the energy of deactivating excited states of LMCT character to ag reater extent than in the case of 2-arylpyridines. Cyclometalating aryl-NHC ligands have previously been employed for the synthesis of luminescentc omplexes of Ir III , [44][45][46][47][48][49][50][51][52] Pt II , [53][54][55][56] and Au III [57] with enhanced emission properties, often as non-chromophorics upporting ligands.…”

Relevance of Chemical vs. Electrochemical Oxidation of Tunable Carbene Iridium Complexes for Catalytic Water Oxidation

“…Subsequent coppercatalyzed "click" cycloaddition formed the triazoles 2-4, which were alkylated selectively at the N3 position in high yields by using MeOTf, thus affording the pyridyl-triazolium salts 5-7 as carbene ligand precursors. [53][54][55][56][57][58] A variety of functional groups were incorporated to probe their effect on catalytic water oxidation. Specifically, different electron donating groups were installed onto the pyridyl ring in order to stabilize the critical high-valent iridium transition states, [33] including -OMe, -tBu or -NMe 2 groups in 4-position, and a -OMe group in 5-position (5b-e).…”

Study on the interaction between [Pt(μ‐Cl)(isopropyl eugenoxyacetate‐1H)]2 and 1,3‐diisopropylbenzimidazolium bromide

“…In the past two decades, transition-metal complexes bearing N-heterocyclic carbenes (NHCs) have enticed chemists by their promising applications in homogeneous catalysis and medicine. [1][2][3][4] For example, various of Pd(II) or Pt(II) complexes containing NHC have displayed the potential catalytic activities for C-C coupling reactions and atomic optimization reactions such as hydroamination, hydrosilylation. [1,2,4] In organoplatinum(II) chemistry, olefin and olefin-functionalized benzenes such as propenylbenzenes were used as very common ligands.…”

“…[1][2][3][4] For example, various of Pd(II) or Pt(II) complexes containing NHC have displayed the potential catalytic activities for C-C coupling reactions and atomic optimization reactions such as hydroamination, hydrosilylation. [1,2,4] In organoplatinum(II) chemistry, olefin and olefin-functionalized benzenes such as propenylbenzenes were used as very common ligands. [5][6][7][8] Many mono and dinuclear complexes of platinum(II) with ditopic / chelators derived from natural sources such as eugenol, methyl eugenol and methyl/ethyl/isopropyl eugenoxyacetate have been synthesized and studied their properties.…”