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

Reaction of the diphosphine ligand i-Pr2P(CH2)(5)P(i-Pr)(2) with NiBr2 at 110 degrees C gives a mixture of the 16-membered bimetallacycle {i-Pr2(P)(CH2)SP(i-Pr)(2)}(2)Ni2Br4, 1, and the pincer-type complex {(i-Pr2PCH2CH2)(2)CH}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)(2)CH}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 Ni 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 Ni-III species {(i-Pr2PCH2CH2)(2)CH}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.