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 NiCl 2·6H2O 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 CO 2 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.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Organic Chemistry
- Inorganic Chemistry