A Ni0‐NCN pincer complex featuring a six‐membered N‐heterocyclic carbene (NHC) central platform and amidine pendant arms was synthesized by deprotonation of its NiII precursor. It retained chloride in the square‐planar coordination sphere of nickel and was expected to be highly susceptible to oxidative addition reactions. The Ni0 complex rapidly activated ammonia at room temperature, in a ligand‐assisted process where the carbene carbon atom played the unprecedented role of proton acceptor. For the first time, the coordinated (ammine) and activated (amido) species were observed together in solution, in a solvent‐dependent equilibrium. A structural analysis of the Ni complexes provided insight into the highly unusual, non‐innocent behavior of the NHC ligand.
A Ni0‐NCN pincer complex featuring a six‐membered N‐heterocyclic carbene (NHC) central platform and amidine pendant arms was synthesized by deprotonation of its NiII precursor. It retained chloride in the square‐planar coordination sphere of nickel and was expected to be highly susceptible to oxidative addition reactions. The Ni0 complex rapidly activated ammonia at room temperature, in a ligand‐assisted process where the carbene carbon atom played the unprecedented role of proton acceptor. For the first time, the coordinated (ammine) and activated (amido) species were observed together in solution, in a solvent‐dependent equilibrium. A structural analysis of the Ni complexes provided insight into the highly unusual, non‐innocent behavior of the NHC ligand.
This work explores avenues toward PCP pincer complexes with acyclic diaminocarbene central moieties that are suitable for ligand-assisted reactivity involving the carbene carbon. For this purpose, diphosphine chelating ligands 1 and 2, with bis(imidazolyl)methyl and bis(pyrazolyl)methyl backbones, respectively, were prepared in two high-yield synthetic steps. Nickel(II) and platinum(II) dihalide complexes 3 and 6, incorporating 1, were prepared and converted into PCP pincer ligands 5 and 7 upon deprotonation with KHMDS. Chelated nickel(I) complex 4 was obtained as a byproduct to 3. Although the solid-state structures of 5 and 7 presented geometric strain at the diaminomethyl carbon, this moiety could not be converted into a diaminocarbene by proton or hydride abstraction. 7 could be converted into platinum(IV) analog 8 by oxidation with PhICl 2 . Ligand 2 proved more versatile than 1, or less dependable in its behavior, generating P,N-chelating cobalt(II) dibromide complex 9. Easily accessible ligands 1 and 2 provide a new and versatile PCP pincer platform.
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