Amido Pincer Complexes of Nickel(II):  Synthesis, Structure, and Reactivity

Abstract

A series of diamagnetic divalent nickel complexes supported by a tridentate amido diphosphine ligand [N(o-C6H4PR2)2]- ([R-PNP]-; R = Ph, i Pr, Cy) have been prepared and characterized. Deprotonation of H[Ph-PNP], H[ i Pr-PNP], and H[Cy-PNP] with n-BuLi in ethereal solutions at −35 °C produced the lithium complexes [Ph-PNP]Li(THF)2, [ i Pr-PNP]Li(solv) (solv = THF, Et2O), and [Cy-PNP]Li(solv) (solv = THF, Et2O), respectively. The reactions of [R-PNP]Li(solv) n with NiCl2(DME) in THF at −35 °C generated [R-PNP]NiCl, which was then reacted with a variety of Grignard reagents to afford the corresponding hydrocarbyl complexes [R-PNP]NiR‘ (R = Ph, R‘ = Me, Et, n-Bu, i-Bu, n-hexyl, CH2SiMe3, Ph; R = i Pr, R‘ = Me, Et, n-Bu; R = Cy, R‘ = Me, Et, n-Bu). Of particular interest among the compounds isolated are alkyl complexes that contain β-hydrogen atoms. Treatment of the hydrocarbyl complexes with halogenated hydrocarbons such as dichloromethane, benzyl bromide, and phenyl iodide produced the corresponding nickel halide derivatives. The chloride complexes [Ph-PNP]NiCl, [ i Pr-PNP]NiCl, and [Cy-PNP]NiCl are all active catalyst precursors for Kumada coupling reactions, including those of alkyls containing β-hydrogen atoms. In addition to spectroscopic data for all new compounds, X-ray structures of [Cy-PNP]Li(OEt2), [Ph-PNP]NiCl, [Ph-PNP]NiBr, [Ph-PNP]NiMe, [Ph-PNP]Ni(n-Bu), [Ph-PNP]NiCH2SiMe3, [ i Pr-PNP]NiCl, [ i Pr-PNP]NiMe, [ i Pr-PNP]Ni(n-Bu), and [Cy-PNP]NiMe are presented.

Nickel Complexes of a Pincer Amidobis(amine) Ligand: Synthesis, Structure, and Activity in Stoichiometric and Catalytic CC Bond‐Forming Reactions of Alkyl Halides

“…Similar stability of Ni II alkyl species was reported by Liang et al on isoelectronic amidobis(phosphine) (pnp) complexes. [25] Salt metathesis reactions of 1 produced the corresponding Ni II acetate (6) and triflate (7) Figure S1 in the Supporting Information. There is a significant difference between the hydrocarbyl complexes (2-4) and the other derivatives (1 and 6-8) in the visible spectra: the hydrocarbyl complexes do not absorb between 600 and 800 nm, whereas the spectra of the other compounds exhibit an absorption maximum with a molar absorption coefficient of 100-200 L cm À1 mol…”

“…The coordination chemistry of Me NN 2 on Ni II is comparable to that of the analogous pincer amidobis(phosphine) (pnp) and arylbisA C H T U N G T R E N N U N G (amine) (ncn) ligands, whereas its electron-donating ability is similar to that of alkyl pnp. [25] A Ni bisA C H T U N G T R E N N U N G (alkyl) species with a formal oxidation state of + III or + IV is the presumed intermediate in such a reaction. Furthermore, compared to pnp complexes, intramolecular metal-to-ligand p backbonding is diminished in NN 2 complexes.…”

“…The acetate ligand in 6 is coordinated in an h 1 fashion, a common coordination mode for mononuclear Ni II acetate complexes. [32] The Ni-O distance is slightly shorter in 6 (1.890(3) ) than in 7 (1.946 (2) 2.1703(6) ), [21] [{N(o-C 6 H 4 PPh 2 ) 2 }NiCl] (11, 2.1636(11) ), [25] and [{2,6-bis({dimethylamino}methyl)phenyl}NiCl] (12, 2.2388 (5) ).…”

“…[25] Reactions (2) reacted with alkyl halides according to Equation (1). The rate of the reaction followed the orders: RI > RBr > RCl and benzyl > octyl > cyclohexyl.…”

“…The electron-donating property of the Me NN 2 ligand could be estimated by the oxidation potential of its metal complex, for example, 1 and 8. Thus, the Me NN 2 ligand is as good an electron donor as the electron-rich amidobis(alkylphosphine) ligand, which gave rise to a [(pnp)NiCl] complex with E o' = À0.06 V. [25] The cyclic voltammogram of the methyl and phenyl Ni complexes showed an irreversible oxidation at 0.13 and 0.27 V in THF, respectively ( Figure S2 in …”