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= κ P ,κ C ,κ N -{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 d z 2 /dpz hybrid MO. In agreement with this picture, the g-values obtained from the EPR spectrum of 5a are g xx ≈ g yy ≈ 2.2, g zz ≈ 2.00, and strong Br hyperfine coupling is observed on the g zz 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.

Connecting Neutral and Cationic Pathways in Nickel-Catalyzed Insertion of Benzaldehyde into a C–H Bond of Acetonitrile

“…The bromide complex (κ 3 -15c5 NCOP iPr )Ni(Br) (1-15c5) was isolated in high yield using this procedure adapted from Zargarian, who reported the first nickel NCOP complexes using related ligands featuring simple secondary or tertiary amine donors. 33,34 Full synthetic details are provided in the Experimental Section; accompanying spectra can be found in the Supporting Information (SI).…”

Aromatic Methoxylation and Hydroxylation by Organometallic High-Valent Nickel Complexes

“…8 More than three decades ago, van Koten et al have shown that the pincer NCN -ligand can support organometallic Ni III complexes that are surprisingly stable, 9 as reported also later for a related pincer system. 10 Moreover, Hillhouse et al 11 and recently Zargarian et al 12 have reported oxidatively-induced reductive elimination of ethers from metallacyclic Ni(alkoxo)(alkyl) and Ni(alkoxo)(aryl) complexes, respectively, however no high-valent Ni species were detected in these cases. Recently, our group has employed the tetradentate ligand N,N'-di-tert-butyl-2,11-diaza[3.3](2,6)-pyridinophane ( tBu N4) to isolate organometallic Ni III complexes that undergo C-halide and C-C bond formation reactions and are active catalysts for Kumada and Negishi coupling reactions.…”

A Novel Ferrocene‐Backboned Unsymmetrical Pincer‐Type Proligand and Its Organotin Derivatives

“…Since our previous report on the phosphorus-containing non-symmetric O,C,S-coordinating pincer-type ligand {3-tBu-5-[P(S)Ph 2 ]C 6 H 3 }P(O)(OiPr) 2 [1] a number of novel such pincer-type ligands have been published and randomly selected examples are shown in Scheme 1. [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] The classic pincer-type ligands are based on aromatic backbones to which the donor functions are linked by spacers of different lengths. The steric and electronic properties as well as the reactivity of these pincer-type ligands can be controlled by variation of the donor atoms, the substituents attached to the latter, and by functionalization of the aromatic backbones.…”

“…[23] Among the pincer-type ligands there are only a few representatives with a redox-active ferrocene backbone (Scheme 2). In 2002, van Koten und Brown presented the first potentially P,C,P-coordinating elemental iodine was observed, providing the ammonium salt [η 5 -CpFeη 5 -C 5 H 2 -1-SnPh 2 I-2-P(O)Ph 2 -5-CH 2 NHMe 2 ]I (8). The compounds are characterized by elemental analyses, 1 H, 13 C, 31 P, 119 Sn NMR spectroscopy, electrospray ionization mass spectrometry, and single-crystal X-ray diffraction analyses (3)(4)(5)(6)(7)(8).…”