Proton-Catalyzed Hydrogenation of a d 8 Ir(I) Complex Yields a trans Ir(III) Dihydride

Hydrogenation of the (PONOP)Ir(I)CH(3) complex [PONOP = 2,6-bis(di-tert-butylphosphinito)pyridine] yields the unexpected trans-dihydride species (PONOP)IrCH(3)(H)(2). Mechanistic investigations have revealed that this reaction proceeds via proton-catalyzed H(2) cleavage, a pathway that circumvents the intermediacy of the typically invoked cis-dihydride isomer. Protonation yields the cationic (PONOP)Ir(CH(3))(H)(+) complex, which is then trapped by H(2) to yield an eta(2)-H(2) complex. Deprotonation of this species yields the trans-dihydride. Intermediates in the proposed pathway have been confirmed by independent low-temperature syntheses and spectroscopic observations

Selective Iron-Catalyzed <i>N</i>‑Formylation of Amines using Dihydrogen and Carbon Dioxide

A family of iron­(II) carbonyl hydride species supported by PNP pincer ligands was identified as highly productive catalysts for the <i>N-</i>formylation of amines via CO₂ hydrogenation. Specifically, iron complexes supported by two different types of PNP ligands were examined for formamide production. Complexes containing a PNP ligand with a tertiary amine afforded superior turnover numbers in comparison to complexes containing a bifunctional PNP ligand with a secondary amine, indicating that bifunctional motifs are not required for catalysis. Systems incorporating a tertiary amine containing a PNP ligand were active for the <i>N-</i>formylation of a variety of amine substrates, achieving TONs up to 8900 and conversions as high as 92%. Mechanistic experiments suggest that <i>N-</i>formylation occurs via an initial, reversible reduction of CO₂ to ammonium formate followed by dehydration to produce formamide. Several intermediates relevant to this reaction pathway, as well as iron-containing deactivation species, were isolated and characterized

Intermolecular Methyl Group Exchange and Reversible P–Me Bond Cleavage at Cobalt(III) Dimethyl Halide Species

The cobalt­(III) dimethyl halide complexes <i>cis,mer</i>-(PMe₃)₃Co­(CH₃)₂X (X = Cl, I) were found to undergo a degenerate cobalt-to-cobalt transfer of the methyl ligands during isotopic labeling experiments. Extensive mechanistic studies exclude radical, methyl iodide elimination, and disproportionation/comproportionation pathways for exchange of the methyl groups between metals. A related cobalt­(III) dimethyl complex supported by the tridentate phosphine ligand MeP­(CH₂CH₂PMe₂)₂ showed dramatically slower methyl ligand transfer, indicative of a mechanism for intermetallic exchange with a requisite phosphine dissociation. Crossover experiments between cobalt­(III) dimethyl halide complexes supported by PMe₃ and MeP­(CH₂CH₂PMe₂)₂ are consistent with a dicobalt transition structure in which only one cobalt center requires phosphine dissociation prior to methyl transfer. An additional methyl group scrambling process between <i>cis,mer</i>-(PMe₃)₃Co­(CH₃)₂I and free PMe₃ was also identified during the investigation and originates from reversible P–CH₃ bond cleavage

C–CN Bond Activation of Acetonitrile using Cobalt(I)

A cobalt­(I) methyl species, (PMe₃)₄CoCH₃, was found to promote C–CN bond oxidative addition of acetonitrile at ambient temperature. The isolated product of acetonitrile activation, <i>cis,mer</i>-(PMe₃)₃Co­(CH₃)₂CN, was characterized by NMR, IR, and single-crystal X-ray diffraction studies and presents a higher valent metal in comparison to those previously observed for base-metal-mediated nitrile activations. A short-lived reaction intermediate was detected during nitrile cleavage and identified as <i>fac</i>-(PMe₃)₃Co­(CH₃)₂CN, the kinetic product of C–CN oxidative addition. Conversion of the kinetic product to <i>cis,mer</i>-(PMe₃)₃Co­(CH₃)₂CN proceeds with a rate constant of [1.0(1)] × 10^–3 s^–1 at 27 °C

Intermolecular Methyl Group Exchange and Reversible P–Me Bond Cleavage at Cobalt(III) Dimethyl Halide Species

The cobalt­(III) dimethyl halide complexes <i>cis,mer</i>-(PMe₃)₃Co­(CH₃)₂X (X = Cl, I) were found to undergo a degenerate cobalt-to-cobalt transfer of the methyl ligands during isotopic labeling experiments. Extensive mechanistic studies exclude radical, methyl iodide elimination, and disproportionation/comproportionation pathways for exchange of the methyl groups between metals. A related cobalt­(III) dimethyl complex supported by the tridentate phosphine ligand MeP­(CH₂CH₂PMe₂)₂ showed dramatically slower methyl ligand transfer, indicative of a mechanism for intermetallic exchange with a requisite phosphine dissociation. Crossover experiments between cobalt­(III) dimethyl halide complexes supported by PMe₃ and MeP­(CH₂CH₂PMe₂)₂ are consistent with a dicobalt transition structure in which only one cobalt center requires phosphine dissociation prior to methyl transfer. An additional methyl group scrambling process between <i>cis,mer</i>-(PMe₃)₃Co­(CH₃)₂I and free PMe₃ was also identified during the investigation and originates from reversible P–CH₃ bond cleavage

C–CN Bond Activation of Acetonitrile using Cobalt(I)

A cobalt­(I) methyl species, (PMe₃)₄CoCH₃, was found to promote C–CN bond oxidative addition of acetonitrile at ambient temperature. The isolated product of acetonitrile activation, <i>cis,mer</i>-(PMe₃)₃Co­(CH₃)₂CN, was characterized by NMR, IR, and single-crystal X-ray diffraction studies and presents a higher valent metal in comparison to those previously observed for base-metal-mediated nitrile activations. A short-lived reaction intermediate was detected during nitrile cleavage and identified as <i>fac</i>-(PMe₃)₃Co­(CH₃)₂CN, the kinetic product of C–CN oxidative addition. Conversion of the kinetic product to <i>cis,mer</i>-(PMe₃)₃Co­(CH₃)₂CN proceeds with a rate constant of [1.0(1)] × 10^–3 s^–1 at 27 °C

Synthesis and Characterization of Pincer-Molybdenum Precatalysts for CO₂ Hydrogenation

A family of low-valent molybdenum complexes supported by the pincer ligand PN^MeP (PN^MeP = MeN­(CH₂CH₂PPh₂)₂) was prepared and characterized, including (PN^MeP)­Mo­(C₂H₄)₂, which contains an agostic interaction between the metal and the <i>N-</i>methyl substituent. This β-agostic C–H bond was cleaved by molybdenum and produced a cyclometalated molybdenum formate complex, (κ⁴-PN^MeP)­Mo­(C₂H₄)­(κ²-O₂CH), upon exposure to CO₂. This species serves as a promotor of CO₂ hydrogenation to formate under basic conditions, a rare transformation for group VI metals. The performance of the precatalyst was enhanced with the addition of Lewis acid salts

Effective Pincer Cobalt Precatalysts for Lewis Acid Assisted CO₂ Hydrogenation

The pincer ligand MeN­[CH₂CH₂(P^<i>i</i>Pr₂)]₂ (^<i>i</i>PrPNP) was employed to support a series of cobalt­(I) complexes, which were crystallographically characterized. A cobalt monochloride species, (^<i>i</i>PrPNP)­CoCl, served as a precursor for the preparation of several cobalt precatalysts for CO₂ hydrogenation, including a cationic dicarbonyl cobalt complex, [(^<i>i</i>PrPNP)­Co­(CO)₂]⁺. When paired with the Lewis acid lithium triflate, [(^<i>i</i>PrPNP)­Co­(CO)₂]⁺ affords turnover numbers near 30 000 (at 1000 psi, 45 °C) for CO₂-to-formate hydrogenation, which is a notable increase in activity from previously reported homogeneous cobalt catalysts. Though mechanistic information regarding the function of the precatalysts remains limited, multiple experiments suggest the active species is a molecular, homogeneous [(^<i>i</i>PrPNP)­Co] complex