Decomposition of Methylnickel(II) Amido, Alkoxo, and Alkyl Complexes by β-Hydrogen Elimination: A Comparative Study

Abstract

Thermal decomposition of structurally related amido, alkoxo, and alkyl complexes of type [Ni(Me)(EC(H)RR′)(dippe)] (EC(H)RR′ = cyclo-NC4H8 (pyrrolidino), N(CH2Ph)2, OCH2Ph, OCH(Me)Ph, and CH2CH2Ph) takes place through formally analogous processes involving β-hydrogen elimination and reductive elimination of methane, cleanly affording corresponding Ni(0) imine, aldehyde, ketone, and olefin complexes, [Ni(η2-ECRR′)(dippe)]. Kinetic studies on these decomposition reactions show that, in spite of their similarity, they involve substantial mechanistic differences.

Nickel-Mediated Hydrogenolysis of C–O Bonds of Aryl Ethers: What Is the Source of the Hydrogen?

“…β-H elimination could occur via a five-coordinate intermediate, 32–34 although, in coordinatively saturated systems it has been proposed to occur via alkoxide dissociation 35–37 or in bimolecular fashion. 38,39 Given the strain of the chelates in complex 4 , phosphine dissociation may also be possible, 32,33,40,41 opening up a cis coordination site for β-H elimination. 42–44 Further studies are necessary to determine the mechanism in the present system.…”

β‐Hydrogen Elimination Reactions of Nickel and Palladium Methoxides Stabilised by PCP Pincer Ligands

“…The final stages of the decomposition of the Ni methoxide correspond to the transformation of the monomeric carbonyl intermediate into the observed polynuclear carbonyls [Ni(CO) 2 (m-iPr PCHP)] 2 and [Ni 4 (m-CO) 5 (m-iPr PCHP) 2 ]. Note that neither of these two compounds has the 1:1N i/CO stoichiometry of the monomeric carbonyl intermediate;t herefore, their formation requires somed egree of decomposition.…”

“…As discussed previously,the binuclear carbonyl precedes the tetranuclears pecies, which is also in accordance with the highert emperatures required to complete the formation of the latter.T his might suggest that the formation of the cluster could be favoured because its open structure is especially stable or,ino ther words, because this product represents a" thermodynamic sink" for the system.H owever, despite our efforts, we failed to assemble an alternative synthetic method for this compound. For example, inspired in Bochmann's cluster carbonyl syntheses, [27] we carriedo ut reactions of Ni(cod) 2 , iPr PCHP and CO either at room temperature or at 100 8C, but this afforded mixtures of different species, noneo fw hich showedt he characteristic 31 Ps ignatures of [Ni(CO) 2 (m-iPr PCHP)] 2 or [Ni 4 (m-CO) 5 (m-iPr PCHP) 2 ]. These were not observed when [( iPr PCP)Ni-H] wast reated with as toichiometric amount or excess of CO in an NMR tube.…”

“…Absolute energies and atomic coordinates for all optimised structures are provided in the Supporting Information. X-ray structure analysis for bicarbonate and carbonate complexes [Ni(CO) 2 (m-iPr PCHP)] 2 and [Ni 4 (m-CO) 5 (m-iPr PCHP) 2 ]…”

“…[4] In recent years, our group has devoted attention to the synthesis and the chemical reactivity of nickel and palladium hydroxide and alkoxide complexes, with the purpose of developing applications in homogeneous catalysis. [5] In particular,w ef ocusedo nt he strongly donor pincer ligand 2,6-bis(diisopropylphosphino)methylphen-yl ( iPr PCP) due to its excellent balance between adequate steric shielding of the metal centre and the ability to promote nucleophilic and/orb asic reactivity on the alkoxide or hydroxide ligands. [6] We recently reported that the palladium hydroxide complex [( iPr PCP)Pd-OH]i sr eadily reduced by methanol or isopropanol to aw ell-defined Pd 0 species through as equence of reactions that involve the decomposition of the intermediate methoxide or isopropoxide complexes [( iPr PCP)Pd-OR] (R = Me, iPr) by bhydrogen elimination (BHE).…”