Experimental and Theoretical Study of Ni - and Pd -Promoted Double Geminal C(sp )-H Bond Activation Providing Facile Access to NHC Pincer Complexes: Isolated Intermediates and Mechanism.

We report the first examples of metal-promoted double geminal activation of C(sp )-H bonds of the N-CH -N moiety in an imidazole-type heterocycle, leading to nickel and palladium N-heterocyclic carbene complexes under mild conditions. Reaction of the new electron-rich diphosphine 1,3-bis((di-tert-butylphosphaneyl)methyl)-2,3-dihydro-1H-benzo[d]imidazole (1) with [PdCl (cod)] occurred in a stepwise fashion, first by single C-H bond activation yielding the alkyl pincer complex [PdCl(PC P)] (3) with two trans phosphane donors and a covalent Pd-C bond. Activation of the C-H bond of the resulting α-methine C H-M group occurred subsequently when 3 was treated with HCl to yield the NHC pincer complex [PdCl(PC P)]Cl (2). Treatment of 1 with [NiBr (dme)] also afforded a NHC pincer complex, [NiBr(PC P)]Br (6), but the reactions leading to the double geminal C-H bond activation of the N-CH -N group were too fast to allow identification or isolation of an intermediate analogous to 3. The determination of six crystal structures, the isolation of reaction intermediates and DFT calculations provided the basis for suggesting the mechanism of the stepwise transformation of a N-CH -N moiety in the N-C -N unit of NHC pincer complexes and explain the key differences observed between the Pd and Ni chemistries.