Iridium, ruthenium, and palladium complexes containing a mesoionic fused imidazolylidene ligand

Abstract

Imidazo[1,2-a]pyridine consisting of a pyridine fused to an imidazolium salt at the imidazolium N1-C2 bond and hence protected from forming normal imidazole-2-ylidene complexes undergoes selective activation of the C5-H bond with Ag 2 O, i.e. at the imidazolium carbon that is proximal to the pyridine nitrogen. While the silver carbene complex is unstable, transmetallation with [IrCp*Cl 2 ] 2 , [RuCl 2 (cym)] 2 , and [PdCl(allyl)] 2 afforded stable mesoionic carbene complexes. Two iridium(III) complexes containing one fused carbene ligand and one palladium(II) complex containing two carbene ligands at the metal center were structurally characterized. The absence of substituents adjacent to the carbene carbon prevents wingtip group activation, and it imparts a reduced stability of the complexes in particular under (mildly) acidic conditions.

N-Heterocyclic Carbenes Derived from Guanosine: Synthesis and Evidences of Their Antiproliferative Activity

“…The Pd− C-8 bond length is 1.981(8) Å, which correlates well with related N-heterocyclic carbene compounds of palladium. 21,22 The sugar adopts a puckered conformation, with the C2 turned out of the plane formed by C1′−O4′−C4′ by 109.8(5)°, following the trend found for purine nucleosides with bulky substituents at the C-8 (C2′ endo). 23−26 The torsion angle around the glycosidic bond (χ, defined by O4′−C1′−N9−C4) is 77.5(9)°, corresponding to a synclinal orientation.…”

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

“…243 Similarly, the methyl salt of imidazo[1,2-a]pyridine 241 affords ruthenium(II), iridium(III), and palladium(II) complexes 242−244 with a mesoionic imidazolylidene-type ligand via in situ formation of the silver carbene intermediate (Scheme 100). 244 Metalation takes place exclusively at the position proximal to the alkylated nitrogen. The limited stability of complexes 242−244, especially toward acids, may be a direct consequence of the absence of a protecting bulky substituents adjacent to the carbene site, which may be improved upon introduction of supporting functional groups as N-substituents.…”

“…In gold­(I) complexes, 4-imidazolylidene ligands lead to a longer Au–C bond than 2-imidazolylidenes, even though the mesoionic carbenes are predicted by DFT to form stronger bonds . In contrast, the Ir–C and Cu–C bond lengths are identical within esd’s in 2- and 4-imidazolylidene complexes, , while in cobalt­(III) complexes, mesoionic 4-imidazolylidene complexes feature a shorter M–C bond than the analogous 2-imidazolylidene complexes . Similarly, the bond between the imidazolylidene ligand and the group 14 metals (Ge, Si, Sn) is shorter when the imidazolylidene is mesoionic. , Hence, the M–C bond does not provide any trend that reflects the distinct differences of 2- vs mesoionic 4-imidazolylidene ligands.…”

Quantifying the Electronic and Steric Properties of 1,3-Imidazole-Based Mesoionic Carbenes (iMICs)

“…The C2-protection strategy, preferably with an aryl substituent, has been found to be the most successful . Several iMIC-metal complexes in which the C2 position is fused with an N-heterocycle have been also reported. , Isomerization of C2-carbenes into C4-carbenes has been also shown with sterically demanding main-group reagents , and transition-metal compounds . The preferential C4–H bond activation over the C2–H bond of 1,3-imidazolium salts assisted by a pendant N-donor ligand was the first methodology to synthesize iMIC-metal complexes .…”