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

Oxidative Atom-Transfer to a Trimanganese Complex To Form Mn 6 (μ 6 -E) (E = O, N) Clusters Featuring Interstitial Oxide and Nitride Functionalities

Utilizing a hexadentate ligand platform, a trinuclear manganese complex of the type (HL)Mn3(thf)3 was synthesized and characterized ([HL]6– = [MeC(CH2N(C6H4-o-NH))3]6–). The pale-orange, formally divalent trimanganese complex rapidly reacts with O-atom transfer reagents to afford the μ6-oxo complex (HL)2Mn6(μ6-O)(NCMe)4, where two trinuclear subunits bind the central O-atom and the (HL) ligands cooperatively bind both trinuclear subunits. The trimanganese complex (HL)Mn3(thf)3 rapidly consumes inorganic azide ([N3]NBu4) to afford a dianionic hexanuclear nitride complex [(HL)2Mn6(μ6-N)](NBu4)2, which subsequently can be oxidized with elemental iodine to (HL)2Mn6(μ6-N)(NCMe)4. EPR and alkylation of the interstitial light atom substituent were used to distinguish the nitride from the oxo complex. The oxo and oxidized nitride complexes give rise to well-defined Mn(II) and Mn(III) sites, determined by bond valence summation, while the dianionic nitride shows a more symmetric complex, giving rise to indistinguishable ion oxidation states based on crystal structure bond metrics.Chemistry and Chemical Biolog

Oxidative Group Transfer to a Triiron Complex to Form a Nucleophilic μ3-Nitride, [Fe3(μ3-N)]−

Utilizing a hexadentate ligand platform, a high-spin trinuclear iron complex of the type (tbsL)Fe3(thf) was synthesized and characterized ([tbsL]6− = [1,3,5-C6H9(NPh-o-NSitBuMe2)3]6−). The silyl-amide groups only permit ligation of one solvent molecule to the tri-iron core, resulting in an asymmetric core wherein each iron ion exhibits a distinct local coordination environment. The triiron complex (tbsL)Fe3(thf) rapidly consumes inorganic azide ([N3]NBu4) to afford an anionic, trinuclear nitride complex [(tbsL)Fe3(μ3-N)]NBu4. The nearly C3-symmetric complex exhibits a highly pyramidalized nitride ligand that resides 1.205(3) Å above the mean triiron plane with short Fe–N (1.871(3) Å) distances and Fe–Fe separation (2.480(1) Å). The nucleophilic nitride can be readily alkylated via reaction with methyl iodide to afford the neutral, trinuclear methylimide complex (tbsL)Fe3(μ3-NCH3). Alkylation of the nitride maintains the approximate C3-symmetry in the imide complex, where the imide ligand resides 1.265(9) Å above the mean triiron plane featuring lengthened Fe–Nimide bond distances (1.892(3) Å) with nearly equal Fe–Fe separation (2.483(1) Å).Chemistry and Chemical Biolog

Insights into a Chemoselective Cobalt Catalyst for the Hydroboration of Alkenes and Nitriles

A chemoselective hydroboration protocol with terminal alkene substrates is reported using an electron-rich, low-valent cobalt pincer compound. The process is catalytic and leads to exclusive formation of anti-Markovnikov products, tolerating amino groups, esters, epoxides, ketones, and other functionalities. The protocol was successfully extended toward the hydroboration of nitriles, generating the corresponding amines in moderate to good yields. Labeling studies with deuterated pinacolborane gave insights into the mechanism, establishing the intermediacy of a cobalt hydride, as well as an insertion, β-hydride elimination, and alkene isomerization pathway. These insights provide a rationale for the observed regioselectivity and allow us to propose a catalytic mechanism

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

γ-Agostic interactions in (MesCCC)Fe–Mes(L) complexes

Producción CientíficaAgostic interactions were observed in the bound mesityl group in a series of iron compounds bearing a bis(NHC) pincer CCC ligand. The L-type ligand on [(CCC)FeIIMes(L)] complexes influences the strength of the agostic interaction and is manifested in the upfield shift of the 1H NMR resonance for the mesityl methyl resonances. The nature of the interaction was further investigated by density functional theory calculations, allowing rationalization of some unexpected trends and proving to be a powerful predictive tool.Universidad de Valladolid. Margarita Salas Postdoctoral Fellowship (CONVREC-2021-221

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