Synthesis of Complexes Containing an Anionic NHC Ligand with an Unsubstituted Ring‐Nitrogen Atom

Abstract

Oxidative addition: complexes containing an anionic NHC ligand with an unsubstituted nitrogen atom within the heterocycle have been prepared by oxidative addition of 2-chloro-N-methylbenzimidazole to Ni(0) and Pt(0). The "naked" nitrogen atom of the NHC ligand reacts as a strong nucleophile.

Complexes Bearing Protic N-Heterocyclic Carbenes: Synthesis and Applications

“…40 Metric parameters in the dinuclear complex [30] do not differ significantly from equivalent parameters found in the NH,NMe-NHC complexes of type trans-[21]BF 4 . The molecular structure of complex [31], however, shows different metric parameters when compared to its Nprotonated congener trans-[22]BF 4 .…”

“…44 The doubly (C 2 and C 5 ) iodated neutral benzimidazole 27 was subsequently used to identify the CI bond most activated for an oxidative addition reaction. 40 Reaction of 27 with equimolar amounts of [M(PPh 3 ) 4 ] (M = Pd and Pt) in the presence of the proton source NH 4 BF 4 (Scheme 12, bottom) gave exclusively complexes trans- [28] and trans- [29]. 40 This outcome provides clear evidence for the higher reactivity of the C 2 I bond compared to the C 5 I bond.…”

“…40 Reaction of 27 with equimolar amounts of [M(PPh 3 ) 4 ] (M = Pd and Pt) in the presence of the proton source NH 4 BF 4 (Scheme 12, bottom) gave exclusively complexes trans- [28] and trans- [29]. 40 This outcome provides clear evidence for the higher reactivity of the C 2 I bond compared to the C 5 I bond. Based on the results available, a reaction mechanism for the oxidative addition of the C 2 X (X = Cl, Br, and I) bond of 2-halogeno-N-methylbenzimidazoles to complexes of lowvalent transition metals was proposed (Scheme 13).…”

“…While oxidative addition to Pd 0 gave exclusively the dinuclear complex [30] in 42% yield, 40 the reaction with Pt 0 led to the formation of a mixture of mononuclear complex [31] and dinuclear complex [32] in a ratio of 4:1. 45 The dinuclear complexes [30] and [32] are obtained by the intermolecular attack of the unsubstituted ring-nitrogen atom of intermediate I at the metal atom of a second complex with the loss of a phosphine ligand. This observation confirms that the oxidative addition of the C 2 X bond is the first step in the reaction sequence.…”

“…DFT studies have shown that the heterocycle in [31] is best described as an anionic ylidene with the negative charge residing on the unsubstituted ring-nitrogen atom. 45 This negative charge leads to the shortening of the C NHC N bond length in comparison to the C NHC NMe bond length via electrostatic interactions.…”