Alkyne Semihydrogenation with a Well-Defined Nonclassical Co–H₂ Catalyst: A H₂ Spin on Isomerization and <i>E</i>‑Selectivity

The reactivity of a Co^I–H₂ complex was extended toward the semihydrogenation of internal alkynes. Under ambient temperatures and moderate pressures of H₂, a broad scope of alkynes were semihydrogenated using a Co^I-N₂ precatalyst, resulting in the formation of <i>trans</i>-alkene products. Furthermore, mechanistic studies using ¹H, ²H, and <i>para</i>-hydrogen induced polarization (PHIP) transfer NMR spectroscopy revealed <i>cis</i>-hydrogenation of the alkyne occurs first. The Co-mediated alkene isomerization afforded the <i>E</i>-selective products from a broad group of alkynes with good yields and <i>E</i>/<i>Z</i> selectivity

Facile Nitrite Reduction in a Non-heme Iron System: Formation of an Iron(III)-Oxo

Reaction of tetrabutylammonium nitrite with [N­(afa^Cy)₃Fe­(OTf)]­(OTf) cleanly resulted in the formation of an iron­(III)-oxo species, [N­(afa^Cy)₃Fe­(O)]­(OTf), and NO­(g). Formation of NO­(g) as a byproduct was confirmed by reaction of the iron­(II) starting material with half an equivalent of nitrite, resulting in a mixture of two products, the iron-oxo and an iron-NO species, [N­(afa^Cy)₃Fe­(NO)]­(OTf)₂. Formation of the latter was confirmed through independent synthesis. The results of this study provide insight into the role of hydrogen bonding in the mechanism of nitrite reduction and the binding mode of nitrite in biological heme systems

Isolation of Iron(II) Aqua and Hydroxyl Complexes Featuring a Tripodal H-bond Donor and Acceptor Ligand

A tripodal ligand platform, tris­(5-cycloiminopyrrol-2-ylmethyl)­amine (H₃[N­(pi^Cy)₃]), that features a hydrogen bond-accepting secondary coordination sphere when bound anionically to an iron center is reported. Neutral coordination to iron affords ligand tautomerization, resulting in a hydrogen bond-donating secondary coordination sphere, and formation of the tris­(5-cyclohexyl-amineazafulvene-2-methyl)­amine, H₃[N­(afa^Cy)₃], scaffold. Both binding motifs result in formation of stable, high-spin iron­(II) complexes featuring ancillary water, triflate, or hydroxo ligands. Structural analysis reveals that these complexes exhibit distorted trigonal-bipyramidal geometries with coordination of the apical nitrogen to iron as well as three equatorial amine or imine nitrogens, depending on the axial ancillary ligand. Formation of the aqua complex K­[(N­(pi^Cy)₃)­Fe­(OH₂)] (<b>3</b>) illustrated the propensity of the ligand to be hydrogen bond-accepting, whereas the iron triflate species [N­(afa^Cy)₃­Fe]­(OTf)₂ (<b>4</b>) features a hydrogen bond-donating secondary coordination sphere. The ability of each of the three arms of the ligand to tautomerize independently was observed during the formation of the iron–hydroxyl species [N­(afa^Cy)₂­(pi^Cy)]­FeOH (<b>5</b>) and characterized by X-ray crystallography and IR spectroscopy. The combined data for the iron complexes established that each arm of the tripodal ligand can tautomerize independently and is likely dependent on the electronic needs of the iron center when binding various substrates

Well-Defined Cobalt(I) Dihydrogen Catalyst: Experimental Evidence for a Co(I)/Co(III) Redox Process in Olefin Hydrogenation

The synthesis of a cobalt dihydrogen Co^I-(H₂) complex prepared from a Co^I-(N₂) precursor supported by a monoanionic pincer bis­(carbene) ligand, ^MesCCC (^MesCCC = bis­(mesityl-benzimidazol-2-ylidene)­phenyl), is described. This species is capable of H₂/D₂ scrambling and hydrogenating alkenes at room temperature. Stoichiometric addition of HCl to the Co^I-(N₂) cleanly affords the Co^III hydridochloride complex, which, upon the addition of Cp₂ZrHCl, evolves hydrogen gas and regenerates the Co^I-(N₂) complex. Furthermore, the catalytic olefin hydrogenation activity of the Co^I species was studied by using multinuclear and parahydrogen (<i>p</i>-H₂) induced polarization (PHIP) transfer NMR studies to elucidate catalytically relevant intermediates, as well as to establish the role of the Co^I-(H₂) in the Co^I/Co^III redox cycle

Well-Defined Cobalt(I) Dihydrogen Catalyst: Experimental Evidence for a Co(I)/Co(III) Redox Process in Olefin Hydrogenation

The synthesis of a cobalt dihydrogen Co^I-(H₂) complex prepared from a Co^I-(N₂) precursor supported by a monoanionic pincer bis­(carbene) ligand, ^MesCCC (^MesCCC = bis­(mesityl-benzimidazol-2-ylidene)­phenyl), is described. This species is capable of H₂/D₂ scrambling and hydrogenating alkenes at room temperature. Stoichiometric addition of HCl to the Co^I-(N₂) cleanly affords the Co^III hydridochloride complex, which, upon the addition of Cp₂ZrHCl, evolves hydrogen gas and regenerates the Co^I-(N₂) complex. Furthermore, the catalytic olefin hydrogenation activity of the Co^I species was studied by using multinuclear and parahydrogen (<i>p</i>-H₂) induced polarization (PHIP) transfer NMR studies to elucidate catalytically relevant intermediates, as well as to establish the role of the Co^I-(H₂) in the Co^I/Co^III redox cycle

A Highly Chemoselective Cobalt Catalyst for the Hydrosilylation of Alkenes using Tertiary Silanes and Hydrosiloxanes

The hydrosilylation of alkene substrates bearing additional functionalities is difficult to achieve using earth-abundant catalysts and has not been extensively realized with both earth-abundant transition metals and tertiary silanes or hydrosiloxanes. Reported herein is a well-defined bis­(carbene) cobalt­(I)-dinitrogen complex for the efficient, catalytic anti-Markovnikov hydrosilylation of terminal alkenes, featuring a broad substrate scope. Alkenes containing hydroxyl, amino, ester, epoxide, ketone, formyl, and nitrile groups are selectively hydrosilylated in this reaction sequence. Multinuclear NMR studies of reactive intermediates gave insights into the mechanism

Accessing Pincer Bis(carbene) Ni(IV) Complexes from Ni(II) via Halogen and Halogen Surrogates

This communication describes the two-electron oxidation of (^DIPPCCC)­NiX (^DIPPCCC = bis­(diisopropylphenyl-benzimidazol-2-ylidene)­phenyl); X = Cl or Br) with halogen and halogen surrogates to form (^DIPPCCC)­NiX₃. These complexes represent a rare oxidation state of nickel, as well as an unprecedented reaction pathway to access these species through Br₂ and halogen surrogate (benzyltrimethylammonium tribromide). The Ni^IV complexes have been characterized by a suite of spectroscopic techniques and can readily reduce to the Ni^II counterpart, allowing for cycling between the Ni^II/Ni^IV oxidation states

Accessing Pincer Bis(carbene) Ni(IV) Complexes from Ni(II) via Halogen and Halogen Surrogates

This communication describes the two-electron oxidation of (^DIPPCCC)­NiX (^DIPPCCC = bis­(diisopropylphenyl-benzimidazol-2-ylidene)­phenyl); X = Cl or Br) with halogen and halogen surrogates to form (^DIPPCCC)­NiX₃. These complexes represent a rare oxidation state of nickel, as well as an unprecedented reaction pathway to access these species through Br₂ and halogen surrogate (benzyltrimethylammonium tribromide). The Ni^IV complexes have been characterized by a suite of spectroscopic techniques and can readily reduce to the Ni^II counterpart, allowing for cycling between the Ni^II/Ni^IV oxidation states

Synthesis and Characterization of Bidentate NHC‑C_Aryl Nickel(II) Complexes: Isocyanide Insertion To Form NHC‑η²‑iminoacyl Complexes

A bidentate monoanionic NHC-C_Aryl ligand framework was synthesized, and a host of Ni­(II) complexes were prepared. Addition of isocyanides to these complexes led to the formation of NHC-η²-iminoacyl nickel complexes. These complexes were characterized by a suite of spectroscopic techniques, including X-ray crystallography. The η²-iminoacyl was shown to be displaced from the nickel center with oxidant and could then be reattached with reductant