Aliphatic versus Aromatic C−H Activation in the Formation of Abnormal Carbenes with Iridium:  A Combined Experimental and Theoretical Study

Abstract

The metalation of a series of C2-Me-substituted monoimidazolium and bisimidazolium salts to [Cp*IrCl2]2 is described. The reaction of the monoimidazolium salt provides the species Cp*Ir(aNHC)Cl2, in which the NHC shows an abnormal coordination mode. The use of the bisimidazolium salt provides different reaction patterns depending on the linker length between the two azolium rings. For the methylene-linked bisimidazolium salt, the only compound obtained shows an unusual type of coordination in which the chelating ligand is coordinated through an abnormal NHC and a methylene group resulting from the CH activation of the C2-Me group. For the ethylene-linked bisimidazolium salt, a similar product is obtained, together with the chelating bis-abnormal-NHC species. All compounds have been fully characterized by usual spectroscopic techniques, and X-ray molecular structures are described. The formation of the reaction products, in the case of the methylene linker, has been rationalized by means of DFT calculations with inclusion of solvent effects (PCM). The calculations could not discriminate the nature of the first metalation between direct deprotonation of the ligand by the base and metalation through C−H activation at Ir. However both cases point to a kinetic preference for first metalation at the C2-Me group. The second metalation process is the result of kinetically preferred C−H activation at the C5 position.

Beyond ConventionalN-Heterocyclic Carbenes: Abnormal, Remote, and Other Classes of NHC Ligands with Reduced Heteroatom Stabilization

“…33 This C(sp 3 )-H activation pathway is similar to that observed with Ag 2 O (Vide supra). It was partially suppressed by extending the linker between the two imidazolium moieties from a methylene unit to an ethylene unit.…”

“…33,34 While yields are generally low, product formation at room temperature indicates that the stability of the Ag complex may be sufficiently high to provide access to the desired product.…”

“…33 In the presence of NaOAc, smooth formation of the chelate 36 was observed (see Scheme 13). No activation of the exocyclic C-H bond was evidenced, suggesting that the formation of complexes such as 33 might be a consequence of the initial carbene bonding to the iridium center in the abnormal bonding mode.…”

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

“…The Ir-C bond length is identical within esd's in both complexes, 2.034(5) Å and 2.036(2) Å, respectively (Table 1). This distance is similar to related iridium NHC complexes [30,[60][61][62][63][64]. The piano-stool geometry is slightly flatter in the iodide complex 2b, as indicated by the consistently wider angles between the three "legs", while most of the angles involving the Cp* centroid are more acute than in the chloride analogue 2a.…”

C4-bound imidazolylidenes: from curiosities to high-impact carbene ligands

“…28 Such C(sp 3 )-H bond activation and formation of complexes similar to 4 has been suppressed, at least in part, by connecting the two imidazolium sites via an ethylene rather than a methylene linker, thus increasing the flexibility and the distance between the two donor moieties.…”