Anionic Nickel(II) Complexes with Doubly Deprotonated PNP Pincer-Type Ligands and Their Reactivity toward CO2

The aromatization–dearomatization reaction of pincer-type complexes prompted by protonation–deprotonation of the pincer “arm” is a key step in bond activation chemistry and atom-economic catalytic transformations. However, the possibility of double deprotonation of ancillary pincer ligands is rarely discussed in the literature. Here we report on square-planar cationic nickel­(II) complexes of PNPR type ligands (PNP = 2,6-bis­[(dialkylphosphino)­methyl]­pyridine with R = iPr, tBu), which can be readily transformed into the doubly deprotonated anionic species. The complexes [Ni­(PNPR)­Cl]Cl (3, R = iPr; 4, R = tBu) are readily prepared from the reaction of NiCl2·6H2O and the PNPR ligand in THF. Treatment of the cationic chloro complexes 3 and 4 with 2 equiv of MeLi gives the nickel­(II) methyl complexes [Ni­(PNPR*)­Me] (7, R = iPr; 8, R = tBu), the asterisk indicates the deprotonated pincer arm). Reaction of 7 and 8 with an additional 1 equiv of MeLi gives the anionic complexes [Li­(DME)3]­[Ni­(PNPiPr**)­Me] (9-DME, DME = 1,2-dimethoxyethane) and [Li­(Et2O)2]­[Ni­(PNPtBu**)­(Me)] (10-Et2O), respectively. Single-crystal X-ray diffraction studies exhibit doubly deprotonated PNP-pincer ligands coordinated to a nickel­(II) center. DFT calculations, as well as multinuclear NMR spectroscopy and the X-ray structures, suggest a conjugated π-system with delocalization of the negative charge throughout the carbon backbone of the pincer ligand. The electrophilic attack of complex 9 by CO2 and tautomerization gives [Li]­[Ni­(PNPiPr*-COO)­(Me)] (11). The dearomatized complex that is formed contains an exocyclic methylene carbon atom and a carboxylate moiety adjacent to the second pincer arm.