Tritopic NHC Precursors: Unusual Nickel Reactivity and Ethylene Insertion into a C(sp³)–H Bond

Abstract

The imidazolium chloride [C3H3N(C3H6NMe2)N{C(Me)(=NDipp)}]Cl (1; Dipp=2,6‐diisopropyl phenyl), a potential precursor to a tritopic NimineCNHCNamine pincer‐type ligand, reacted with [Ni(cod)2] to give the NiI‐NiI complex 2, which contains a rare cod‐derived η3‐allyl‐type bridging ligand. The implied intermediate formation of a nickel hydride through oxidative addition of the imidazolium C−H bond did not occur with the symmetrical imidazolium chloride [C3H3N2{C(Me)(=NDipp)}2]Cl (3). Instead, a Ni−C(sp3) bond was formed, leading to the neutral NimineCHNimine pincer‐type complex Ni[C3H3N2{C(Me)(=NDipp)}2]Cl (4). Theoretical studies showed that this highly unusual feature in nickel NHC chemistry is due to steric constraints induced by the N substituents, which prevent Ni−H bond formation. Remarkably, ethylene inserted into the C(sp3)−H bond of 4 without nickel hydride formation, thus suggesting new pathways for the alkylation of non‐activated C−H bonds.

Nickel(II) Complexes with Tritopic NimineCNHCNamine Pincer Ligands

“…Our investigations on the oxidative‐addition reactions of N imine C NHC N′ (N′=N amine or N ammonium ) pincer ligands to [Ni(cod) 2 ] have revealed diverse coordination behaviors that are strongly dependent on the length of the spacer connecting the heterocycle to the amine or protonated amine groups. The use of the N imine C NHC N amine ligand with the C 2 spacer afforded the unexpected dinuclear Ni II NHC complex 2 with a partially fragmented ligand, whereas formation of a dinuclear Ni I −Ni I NHC complex containing a bridging η 3 ‐allyl‐type ligand has been previously observed with the C 3 spacer . Interestingly, in compound 2 , the imine arm C(Me)=NDipp liberated from the N imine C NHC N amine ligand by breaking of the N imid −C imine bond becomes a bridging ligand, together with the remaining imidazole.…”

“…The imidazolium, ammonium salts [(ImH){C(Me)=NDipp}(C 2 NHMe 2 )]Cl 2 ( 4 ) and [(ImH){C(Me)=NDipp}(C 3 NHMe 2 )]Cl 2 ( 5 ) [Eq. ], synthesized from their amine precursors with a C 2 or C 3 spacer, respectively, were chosen to facilitate a one‐pot access to NHC Ni II pincer complexes.…”

“…‐ Although the donor groups in the side arms of the pincer architecture may not always bind firmly to the metal center because of hemilability, they can affect the course of reactions and provide anchimeric assistance. The diverse structures and reactivity of complexes resulting from the use of tritopic N imine C NHC N′ ligands was recently illustrated in a study in which only one E function was varied, the other being kept constant ( ECE′ system) . When N′=N amine with a C 3 spacer between the heterocycle and the amine group [Scheme , Eq.…”

“…When N′=N amine with a C 3 spacer between the heterocycle and the amine group [Scheme , Eq. (1)], a rare example of a nickel(I) complex containing a cod‐derived bridging η 3 ‐allyl‐type ligand was isolated and structurally characterized from the oxidative addition reaction of the corresponding imidazolium salts across [Ni(cod) 2 ] (cod=1,5‐cyclooctadiene) . This complex displays a monodentate C NHC ligand with two dangling arms.…”

“…However, when using ligands where both donor groups E were imines instead of one amine and one imine, a similar reaction led to a very unusual mononuclear imidazolyl nickel(II) pincer complex with a Ni−C sp3 bond [N=N′=N imine , Eq. (3)], in which the central imidazolium proton (−NC H N−) was retained …”