Preparation and Reactivities of PCP-Type Pincer Complexes of Nickel. Impact of Different Ligand Skeletons and Phosphine Substituents

Abstract

Reaction of the diphosphine ligand i-Pr2P(CH2)5P(i-Pr)2 with NiBr2 at 110 °C gives a mixture of the 16-membered bimetallacycle {i-Pr2P(CH2)5P(i-Pr)2}2Ni2Br4, 1, and the pincer-type complex {(i-Pr2PCH2CH2)2CH}NiBr, 2; the latter can also be obtained by heating 1 in the presence of 4-(dimethylamino)pyridine or under vacuum. Complex 2 undergoes transmetalation with RMgCl or RLi to give {(i-Pr2PCH2CH2)2CH}NiR (R = Me (4), CCMe (5), Ph (6), n-Bu (7), and CCPh (8)); the phenylacetylide derivative 8 was also obtained from the reaction of PhCCH with 4, 5, or 7. The Ni−Me derivative 4 reacts with PhX (X = I, Br, Cl) to give the product of C−C coupling reaction (Ph−Me) and the Ni−X analogues of 2, and tests have shown that 2 is a competent catalytic precursor for the coupling of PhCl and MeMgCl. The available evidence suggests that this catalytic process does not involve Ni0 intermediates. Indeed, cyclic voltammetry measurements show that 2 can undergo two consecutive oxidations, and attempts to generate isolable high-valent species yielded the five-coordinate, 17-electron NiIII species {(i-Pr2PCH2CH2)2CH}NiX2, 9, from the reaction of 2 with FeX3 (X = Cl, Br). Characterization of all new complexes, including X-ray diffraction studies on single crystals of complexes 1−6 and 9, is described.

Synthesis, Characterization, and Catalytic Activity of Nickel(II) Alkyl Complexes Supported by Pyrrole–Diphosphine Ligands

“…Although uncommon, Ni(III) species have been invoked as intermediates in C-C coupling reactions. 11,17,34–38 …”

Nickel-catalyzed cross-coupling of primary alkyl halides with phenylethynyl- and trimethylsilyethynyllithium reagents

“…Zargarian proved that a closely related PCP-Ni Ⅱ -alkynyl complex could be prepared by reacting {(i-Pr 2 PCH 2 CH 2 ) 2 CH}NiBr with LiC^CPh [39], while it could not be isolated in pure form. Oxidative addition of alkyl halides delivers a formal nickel(IV) intermediate 3, which gives rise to the coupling product 4 via reductive elimination.…”

Nickel‐Catalyzed Cross‐Coupling with Pincer Ligands

“…The authors indicated that there is a direct correlation between the catalytic efficiency of these PCP systems and the respective electron-richness of their Ni centers (27 Ͼ 28 Ͼ 29) on the basis of this activity order and the cyclic voltammetry measurements of the complexes. [25] Scheme 7. PCP-and PCN-pincer-nickel complexes 27-31.…”