New POCN-Type Pincer Complexes of Nickel(II) and Nickel(III)

Abstract

Unsymmetrical POC(H)N-type pincer ligands react with NiBr2(NCMe)x to give the complexes (POCN)NiIIBr (POCN= κP,κC,κN-{2-(i-Pr2PO),6-(NR2CH2)-C6H3}; NR2= 3-morpholino (3a), NMe2 (3b), and NEt2 (3c)). The presence of an added base such as NEt3 in these metalation reactions maximizes the yields of the target pincer complexes by suppressing the formation of side-products arising from protonation of the ligand by the in situ-generated HBr. The cyclic voltammograms of 3 exhibit a quasi-reversible, one-electron oxidation at ca. +1.0 V, in addition to an irreversible oxidation at higher potentials likely due to ligand oxidation. Reaction of the 16-electron, yellow complexes 3 with Br2, N-bromosuccinimide, or CBr4 gives black crystalline compounds identified as the 17-electron complexes (POCN)NiIIIBr2 (5). Complexes 3 and 5 adopt square-planar and distorted square-pyramidal geometries, respectively. The Ni−Br bond lengths in 5 are significantly longer for the Br atom occupying the axial position (2.43−2.46 vs 2.37 Å), consistent with the repulsive interactions expected between the bromide lone pair and the singly occupied dz2/dpz hybrid MO. In agreement with this picture, the g-values obtained from the EPR spectrum of 5a are gxx ≈ gyy ≈ 2.2, gzz ≈ 2.00, and strong Br hyperfine coupling is observed on the gzz component. Our preliminary studies indicate that the thermal stabilities of 5, both in the solid state (as probed by differential scanning calorimetry) and in solution (as probed by UV−vis spectroscopy), vary in the order 5a ≈ 5b > 5c; this order of stability is consistent with the relative steric demands of the N-moiety. Complexes 5a (or 3a) and 5b (or 3b) promote the Kharasch addition of CX4 (X = Cl, Br) to styrene at 80 °C, giving PhC(X)CH2CX3, the product of an anti-Markovnikov addition, in up to 50 catalytic turnovers