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

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• 作者：Matthias Vogt ; Orestes Rivada-Wheelaghan ; Mark A. Iron ; Gregory Leitus ; Yael Diskin-Posner ; Linda J. W. Shimon ; Yehoshoa Ben-David ; David Milstein
• 刊名：Organometallics
• 出版年：2013
• 出版时间：January 14, 2013
• 年：2013
• 卷：32
• 期：1
• 页码：300-308
• 全文大小：412K
• 年卷期：v.32,no.1(January 14, 2013)
• ISSN：1520-6041

文摘

The aromatization鈥揹earomatization reaction of pincer-type complexes prompted by protonation鈥揹eprotonation of the pincer 鈥渁rm鈥?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 = ⁱPr, ^tBu), which can be readily transformed into the doubly deprotonated anionic species. The complexes [Ni(PNP^R)Cl]Cl (3, R = ⁱPr; 4, R = ^tBu) are readily prepared from the reaction of NiCl₂路6H₂O 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 = 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(PNP^iPr**)Me] (9-DME, DME = 1,2-dimethoxyethane) and [Li(Et₂O)₂][Ni(PNP^tBu**)(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 蟺-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(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.