Ancillary Ligand Modification via Reductive Elimination at Nickel(II)

Abstract

The nickel­(II) complex [κ3-BnPPC]­NiBr (1), stabilized by an orthometalated-phosphine benzyl group and two bulky phosphine donors, results in facile reductive elimination with certain organometallic reagents that can attain the η3-bonding mode. Thus, the reaction of diphenylmethyllithium with 1 produces the Ni(0) derivative [κ2-BnPP­(η2-CHPh2)]Ni (4), which forms as a single diastereomer and displays an η2-phenyl moiety. Similarly, reaction of benzylpotassium with 1 results in the analogous nickel(0) complex [κ2-BnPP­(η2-CH2Ph)]­Ni, albeit less cleanly. Reaction of 1 with allylmagnesium chloride also results in reductive elimination to generate the Ni(0) species [κ2-BnPP­(η2-CH2CHCH2)]­Ni. In contrast, reaction with trimethylsilylmethyllithium generates the stable Ni­(II) derivative, [κ3-BnPPC]­Ni­(CH2SiMe3), which can only be induced to undergo clean reductive elimination by heating to 80°C in the presence of ethylene. Similarly, the bulky 2,6-di-iso-propyl-phenylamidolithium reacts with 1 to give the stable Ni­(II) species [κ3-BnPPC]­Ni­(NH-2,6-iso-Pr2C6H3), which upon thermolysis in the presence of ethylene results in a mixture of products. It is proposed that facile reductive elimination occurs only for those hydrocarbyl groups that can attain a transient η3-bonding mode that accelerates this process due to the formation of an 18-electron intermediate.

Density functional theory study on the reductive elimination of ancillary ligand at PPC nickel complexes

“…Table 1 lists the Gibbs free energy of activation, Δ G †, for the reactions at the Ni center in the PPC Ni complexes calculated at the B3PW91‐d3/LACVP** level with the continuum solvation model of tetrahydrofuran. The activation barrier for the reductive eliminations for all cases is much lower than that for the α‐abstraction, which is in good agreement with the experimental results 2,3 . The Gibbs free energy of activation for both reactions is not sensitive to the change in the substituents R 1 on P atoms, unlike other symmetric pincer complexes 1 .…”

“…The Gibbs free energy of activation for both reactions is not sensitive to the change in the substituents R 1 on P atoms, unlike other symmetric pincer complexes 1 . Gibbs free energy of activation for diphenylmethyl substrate is smaller than that for methyl substrate, which in good agreement with the experimental results 2,3 . For the reductive elimination reaction, C sp3 –C sp2 coupling (R 2 = Me) is slower than C sp2 –C sp2 coupling (R 2 = Ph), which is found for the nonpincer type complexes.…”

“…Tridentate ligands serve as a rigid framework to ensure the arrangements of the donor atoms around the central metal. Recently, novel PPC Ni pincer complexes were reported (Scheme 1), in which an asymmetric arrangement of donor atoms around the metal center was adopted 2,3 . The complexes were designed to study various organometallic reactions, such as reductive elimination, β‐H elimination or α‐abstraction.…”

“…1 Gibbs free energy of activation for diphenylmethyl substrate is smaller than that for methyl substrate, which in good agreement with the experimental results. 2,3 For the reductive elimination reaction, C sp3 -C sp2 coupling (R 2 = Me) is slower than Reductive elimination C sp2 -C sp2 coupling (R 2 = Ph), which is found for the nonpincer type complexes. The rate of C sp3 -C sp2 coupling is comparable to or faster than that of C sp2 -C sp2 coupling for symmetric tridentate pincer complexes.…”

“…The activation barrier for the reductive eliminations for all cases is much lower than that for the α-abstraction, which is in good agreement with the experimental results. 2,3 The Gibbs free energy of activation for both reactions is not sensitive to the change in the substituents R 1 on P atoms, unlike other symmetric pincer complexes. 1 Gibbs free energy of activation for diphenylmethyl substrate is smaller than that for methyl substrate, which in good agreement with the experimental results.…”