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1. Electrocatalytic Formate Oxidation by Cobalt-Phosphine Complexes

   https://doi.org/10.26434/chemrxiv-2023-tfn6t  

   Katipamula, Sriram; Cook, Andrew W; Niedzwiecki, Isabella; Emge, Thomas J; Waldie, Kate M (December 2023, ChemRxiv)

   We report a family of cobalt complexes based on bidentate phosphine ligands with two, one, or zero pendent amine groups in the ligand backbone. The pendent amine complexes are active electrocatalysts for the formate oxidation reaction, generating CO2 with near-quantitative faradaic efficiency at moderate overpotentials (0.45 – 0.57 V in acetonitrile). These homogeneous electrocatalysts are the first cobalt example and second first-row transition metal example for formate oxidation. Thermodynamic measurements reveal these complexes are energetically primed for formate oxidation via hydride transfer to the cobalt center, followed by deprotonation of the resulting cobalt-hydride by formate acting as a base. The complex with the strongest cobalt- hydride bond, given by its thermodynamic hydricity, is the fastest electrocatalyst in this series, with an observed rate constant for formate oxidation of 135 ± 8 h−1 at 25 °C. Electrocatalytic turnover is not observed for the complex with no pendent amine groups: decomposition of the complex structure is evident in the presence of high formate concentrations. 

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2. Activator-free single-component Co( i )-catalysts for regio- and enantioselective heterodimerization and hydroacylation reactions of 1,3-dienes. New reduction procedures for synthesis of [L]Co( i )-complexes and comparison to in situ generated catalysts

   https://doi.org/10.1039/d2dt01484j  

   Parsutkar, Mahesh M.; Moore, Curtis E.; RajanBabu, T. V. (June 2022, Dalton Transactions)

   Warren Piers (Ed.)

   Although cobalt( i ) bis-phosphine complexes have been implicated in many selective C–C bond-forming reactions, until recently relatively few of these compounds have been fully characterized or have been shown to be intermediates in catalytic reactions. In this paper we present a new practical method for the synthesis and isolation of several cobalt( i )-bis-phosphine complexes and their use in Co( i )-catalyzed reactions. We find that easily prepared ( in situ generated or isolated) bis-phosphine and (2,6- N -aryliminoethyl)pyridine (PDI) cobalt( ii ) halide complexes are readily reduced by 1,4-bis-trimethylsilyl-1,4-dihydropyrazine or commercially available lithium nitride (Li 3 N), leaving behind only innocuous volatile byproducts. Depending on the structures of the bis-phosphines, the cobalt( i ) complex crystallizes as a phosphine-bridged species [(P∼P)(X)Co I [μ-(P∼P)]Co I (X)(P∼P)] or a halide-bridged species [(P∼P)Co I [μ-(X)] 2 Co I (P∼P)]. Because the side-products are innocuous, these methods can be used for the in situ generation of catalytically competent Co( i ) complexes for a variety of low-valent cobalt-catalyzed reactions of even sensitive substrates. These complexes are also useful for the synthesis of rare cationic [(P∼P)Co I -η 4 -diene] + X − or [(P∼P)Co I -η 6 -arene] + X − complexes, which are shown to be excellent single-component catalysts for the following regioselective reactions of dienes: heterodimerizations with ethylene or methyl acrylate, hydroacylation and hydroboration. The reactivity of the single-component catalysts with the in situ generated species are also documented. 

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3. Cationic Bis(phosphine) Cobalt(I) Arene Complexes as Precatalysts for the Asymmetric Synthesis of Sitagliptin

   https://doi.org/10.1021/acscatal.2c01059  

   MacNeil, Connor S.; Zhong, Hongyu; Pabst, Tyler P.; Shevlin, Michael; Chirik, Paul J. (April 2022, ACS Catalysis)

   The cobalt-catalyzed asymmetric hydrogenation of dehydro-sitagliptin was studied and applied to the synthesis of sitagliptin (Januvia®). Catalyst discovery efforts were accelerated by the application of a general method for the synthesis of cationic bis(phosphine) cobalt η6-arene complexes. One-electron oxidation of bis(phosphine) cobalt(II) dialkyl complexes in the presence of arenes furnished the corre-sponding, bench stable cobalt precatalysts, [(P-P)Co(η6-C6H6)][BArF4]. Asymmetric hydrogenation utilized 0.5 mol% of the optimal catalyst, [(R,R)-(iPrDuPhos)Co(η6-C6H6)][BArF4] in THF solution and produced sitagliptin in >99% yield with 97% ee. Cobalt catalysts were compatible with a range of solvents and maintained excellent activity and selectivity after standing in air in the solid state for two weeks. Deuterium labeling studies support an enamine-imine tautomerization process resulting in reduction of the metalated imine. Notably, state-of-the-art neutral bis(phosphine) cobalt precatalysts were ineffective, emphasizing the utility of a class of cationic cobalt precatalyst. 

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4. Tellurorhodamine photocatalyzed aerobic oxidation of organo-silanes and phosphines by visible-light

   https://doi.org/10.1039/C9DT00487D  

   Rettig, Irving D.; Van, Jackson; Brauer, Jacob B.; Luo, Wentai; McCormick, Theresa M. (April 2019, Dalton Transactions)

   Tellurorhodamine, 9-mesityl-3,6-bis(dimethylamino)telluroxanthylium hexafluorophosphate ( 1 ), photocatalytically oxidizes aromatic and aliphatic silanes and triphenyl phosphine under mild aerobic conditions. Under irradiation with visible light, 1 can react with self-sensitized 1 O 2 to generate the active telluroxide oxidant ( 2 ). Silanes are oxidized to silanols and triphenyl phosphine is oxidized to triphenyl phoshine oxide either using 2 , or 1 with aerobic irradiation. Kinetic experiments coupled with a computational study elucidate possible mechanisms of oxidation for both silane and phosphine substrates. First-order rates were observed in the oxidation of triphenyl phosphine and methyldiphenyl silane, indicating a substitution like mechanism for substrate binding to the oxidized tellurium( iv ). Additionally, these reactions exhibited a rate-dependence on water. Oxidations were typically run in 50 : 50 water/methanol, however, the absence of water decreased the rates of silane oxidation to a greater degree than triphenyl phosphine oxidation. Parallel results were observed in solvent kinetic isotope experiments using D 2 O in the solvent mixture. The rates of oxidation were slowed to a greater degree in silane oxidation by 2 ( k H / k D = 17.30) than for phosphine ( k H / k D = 6.20). Various silanes and triphenyl phosphine were photocatalytically oxidized with 1 (5%) under irradiation with warm white LEDs using atmospheric oxygen as the terminal oxidant. 

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5. Heteroleptic Square Planar Cobalt(I/II) Complexes

   https://doi.org/10.1002/ejic.202200675  

   Gao, Yafei; Lee, Wei‐Tsung; Carta, Veronica; Chen, Chun‐Hsing; Telser, Joshua; Smith, Jeremy_M (January 2023, European Journal of Inorganic Chemistry)

   Abstract Reduction of the cobalt(II) chloride complex, Ph₂B(^tBuIm)₂Co(THF)Cl (1) in the presence of^tBuN≡C affords the diamagnetic, square planar cobalt(I) complex Ph₂B(^tBuIm)₂Co(C≡N^tBu)₂(2). This is a rare example of a 16‐electron cobalt(I) complex that is structurally related to square planar noble metal complexes. Accordingly, the electronic structure of2, as calculated by DFT, reveals that the HOMO is largely d_z²in character. Complex2is readily oxidized to its cobalt(II) congener [Ph₂B(^tBuIm)₂Co(C=N^tBu)₂]BPh₄(3‐BPh₄), whose EPR spectral parameters are characteristic of low‐spin d⁷with an unpaired electron in an orbital of d_z²parentage. This is also consistent with the results of DFT calculations. Despite its 16‐electron configuration and the d_z²parentage of the HOMO, the only tractable reactions of2involve one electron oxidation to afford3. 

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