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

Abstract

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 PNP R type ligands (PNP = 2,6-bis[(dialkylphosphino)methyl]-pyridine with R = i Pr, t Bu), which can be readily transformed into the doubly deprotonated anionic species. The complexes [Ni(PNP R )Cl]Cl (3, R = i Pr; 4, R = t Bu) are readily prepared from the reaction of NiCl 2 ·6H 2 O and the PNP R ligand in THF. Treatment of the cationic chloro complexes 3 and 4 with 2 equiv of MeLi gives the nickel(II) methyl complexes [Ni(PNP R *)Me] (7, R = i Pr; 8, R = t Bu), 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(PNP iPr **)Me] (9-DME, DME = 1,2-dimethoxyethane) and [Li(Et 2 O) 2 ][Ni(PNP tBu **)(Me)] (10-Et 2 O), 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(PNP iPr *-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.

Synthesis and Characterization of Dimethylbis(2-pyridyl)borate Nickel(II) Complexes: Unimolecular Square-Planar to Square-Planar Rotation around Nickel(II)

“…3 In line with the latter, we have recently observed that ammonia–borane dehydrogenation catalysts 1 and 2 (Figure 1A), 4 each bearing an anionic bidentate borate ligand, are capable of CO 2 reduction concurrent with ammonia–borane dehydrogenation. We therefore became interested in investigating the reactivity of other nickel complexes of the anionic bidentate ligand dimethylbis(2-pyridyl)borate ( 3 ; Figure 1B).…”

A Multifunctional Pincer Ligand Supports Unsaturated Cobalt: Five Functionalities in One Pincer

“…In particular, coordinated phosphines should be compared to phosphonium substituents (P V ), that is, highly stabilizing towards the conjugate base carbanion, with considerable precedent. Both benzylic arms have been singly deprotonated in a PNP pincer complex of Ni II , whereby the buildup of negative charge on the dianionic ligand was determined, yet the electrophile CO 2 attacked the benzylic carbon atom, with no bond formation between CO 2 and nickel . The singly benzyl deprotonated product failed to react with CO 2 .…”

Benzo annulated cycloheptatriene PCP pincer iridium complexes

“…In this way the scope of pincer complex chemistry has been expanded. This has been demonstrated in a number of stoichiometric reactions 13,14 and catalytic transformations. 15,16 In these cases the metal ligand cooperation acts via dearomatisation and aromatisation steps of the ligand backbone, 17 which is facilitated by the acidic protons of the ring methylene groups and the ability of nitrogen to offer different binding modes to the metal centre.…”