Cobalt Hydride Complex Research Articles

Synthesis of Dinuclear Cobalt Silylene Complexes and Their Catalytic Activity for Alkene Hydrosilylation Reactions.

A novel dinuclear silylene cobalt complex [((Me3P)2Co)(PMe2)(CoCl(PMe3))(Si(NCH2PPh2)2C6H4)] (1) supported by the [PSi(silylene)P] ligand was prepared through the reaction of N-heterocyclic [PSiP] pincer ligand L1 (HSiCl(NCH2PPh2)2C6H4) with Co(PMe3)4. Complex [((Me3P)2Co)2(Si(NCH2PPh2)2C6H4)] (2) was formed through the reaction of complex 1 with MeLi. To the best of our knowledge, complexes 1 and 2 are the first examples of dinuclear silylene cobalt complexes supported by the [PSi(silylene)P] ligand. A new preligand L2 (SiCl2(NCH2PPh2)2C6H4) was synthesized, and the reaction of preligand L2 with Co(PMe3)4 afforded silyl cobalt complex [((Me3P)2Co)(SiCl(NCH2PPh2)2C6H4)] (3). The reaction of 3 with CO delivered cobalt carbonyl complex [((Me3P)(CO)Co)(Si(NCH2PPh2)2C6H4)]2O (4). The catalytic activity of cobalt complexes 1-4 on the hydrosilylation of alkenes was explored. Among the four complexes, complex 1 has the best catalytic activity. The catalytic process could be promoted with NaBHEt3 as an additive, and a complete conversion with an excellent selectivity of 98:2 (b/l) could be reached at 120 °C within 8 min for aryl alkenes. A possible catalytic cycle was proposed on the basis of the experimental results and literature reports, with a cobalt hydride complex as an active intermediate. The molecular structure of complexes 1-4 was determined by single-crystal X-ray diffraction analysis.

Bifunctional diphosphine ligand-enabled cobalt catalyzed bis-alkoxycarbonylation of alkynes

Herein, we report a straightforward approach to access 1,4-dicarboxylic acid diesters via BiPyPhos-enabled cobalt-catalyzed bis-alkoxycarbonylation of alkynes. This strategy utilizes an earth-abundant and cost-efficient cobalt catalyst, free from acid additives or noble metal catalysts. The bifunctional ligand BiPyPhos plays a crucial character in both the rate-determining step and the stability of the cobalt catalytic system. The reaction delivers the product directly with an associative abstraction process. Mechanism studies coupled with NMR, FI-IR, HRMS, and single-crystal X-ray analysis indicate that the carbonyl cobalt hydride complex might be the catalytically active species. Utilizing CoCl2 for bis-alkoxycarbonylation of alkynes has hardly any precedent.

Mechanism of Alkene Hydrofunctionalization by Oxidative Cobalt(salen) Catalyzed Hydrogen Atom Transfer.

Oxidative MHAT hydrofunctionalization of alkenes provides a mild cobalt-catalyzed route to forming C-N and C-O bonds. Here, we characterize relevant salen-supported cobalt complexes and their reactions with alkenes, silanes, oxidant, and solvent. These stoichiometric investigations are complemented by kinetic studies of the catalytic reaction and catalyst speciation. We describe the solution characterization of an elusive cobalt(III) fluoride complex, which surprisingly is not the species that reacts with silane under catalytic conditions; rather, a cobalt(III) aquo complex is more active. Accordingly, the addition of water (0.15 M) speeds the catalytic reaction, and kinetic studies show that water addition enables catalytic product formation in 2 h at -50 °C in acetone. Under these conditions, cobalt(III) resting states can be observed by UV-vis spectrophotometry, including a cobalt(III)-alkyl complex. It comes from a transient cobalt(III) hydride complex that is formed in the turnover-limiting step of the catalytic cycle. This hydride readily degrades but not to H2; it releases H+ through a bimetallic pathway that explains the [Co]2 dependence of the off-cycle reaction. In contrast, the rate of the catalytic reaction follows the power law kobs[Co]1[silane]1. Because of the different [Co] dependence of the catalytic reaction and the degradation reaction, lower catalyst loading improves the yield of the catalytic reaction by reducing the relative rate of unproductive silane/oxidant consumption. These studies illuminate mechanistic details of oxidative MHAT hydrofunctionalization of alkenes and lay the groundwork for understanding other catalytic reactions mediated by cobalt hydride and cobalt alkyl complexes.

Cobalt-Catalyzed Bis-Borylation of Quinolines: The Importance of the Cobalt Triplet State.

We report herein a mild stereo- and regioselective dearomatization of quinolines using the simple low valent HCo(N2 )(PPh3 )3 complex that exhibits labile ligands. Conditions to form selectively, at room temperature, high-valued 1,4-bis-borylated tetrahydroquinolines from simple starting heteroaromatic compounds have been developed. The efficient and selective functionalization of a large scope of quinolines bearing various electron-donating or electron-withdrawing substituents is presented, as well as the post-modification of the resulting C-B bond. NMR and labelling studies are consistent with a cascade mechanism pathway, starting from an in situ generated paramagnetic bis-quinoline cobalt(I) hydride complex. A first quinoline dearomatization followed by a cobalt(I)-catalyzed Markovnikov hydroboration of the remaining double bond allows the introduction of the boronic ester group only at C4 position. DFT calculations particularly highlight the importance of the cobalt triplet state throughout the reaction pathway, and bring some rationalization for the observed C4 selective borylation.

Structure and Thermodynamic Hydricity in Cobalt(triphosphine)(monophosphine) Hydrides.

The mononuclear cobalt hydride complex [HCo(triphos)(PMe3)], in which triphos = PhP(CH2CH2PPh2)2, was synthesized and characterized by X-ray crystallography and by 1H and 31P NMR spectroscopy. The geometry of the compound is a distorted trigonal bipyramid in which the axial positions are occupied by the hydride and the central phosphorus atom of the triphos ligand, while the PMe3 and terminal triphos donor atoms occupy the equatorial positions. Protonation of [HCo(triphos)(PMe3)] generates H2 and the Co(I) cation, [Co(triphos)(PMe3)]+, and this reaction is reversible under an atmosphere of H2 when the proton source is weakly acidic. The thermodynamic hydricity of HCo(triphos)(PMe3) was determined to be 40.3 kcal/mol in MeCN from measurements of these equilibria. The reactivity of the hydride is, therefore, well suited to CO2 hydrogenation catalysis. Density functional theory (DFT) calculations were performed to evaluate the structures and hydricities of a series of analogous cobalt(triphosphine)(monophosphine) hydrides where the phosphine substituents are systematically changed from Ph to Me. The calculated hydricities range from 38.5 to 47.7 kcal/mol. Surprisingly, the hydricities of the complexes are generally insensitive to substitution at the triphosphine ligand, as a result of competing structural and electronic trends. The DFT-calculated geometries of the [Co(triphos)(PMe3)]+ cations are more square planar when the triphosphine ligand possesses bulkier phenyl groups and more tetrahedrally distorted when the triphosphine ligand has smaller methyl substituents, reversing the trend observed for [M(diphosphine)2]+ cations. More distorted structures are associated with an increase in ΔGH-°, and this structural trend counteracts the electronic effect in which methyl substitution at the triphosphine is expected to yield smaller ΔGH-° values. However, the steric influence of the monophosphine follows the normal trend that phenyl substituents give more distorted structures and increased ΔGH-° values.

Hydrogenation of Olefins Catalyzed by a Cobalt(I) Hydride Complex with N‐Heterocyclic Silylene

AbstractHerein, we report the synthesis, characterization, and reactivity of the cobalt(I) complexes with an N‐heterocyclic imino substituted silylene. The cobalt(I) hydride complex exhibits high catalytic efficiency and chemoselectivity as well as good functional group compatibility in the hydrogenation of olefins.

Control over Selectivity in Alkene Hydrosilylation Catalyzed by Cobalt(III) Hydride Complexes.

Two new bisphosphine [PCP] pincer cobalt(III) hydrides, [(L1)Co(PMe3)(H)(Cl)] (L11, L1 = 2,6-((Ph2P)(Et)N)2C6H3) and [(L2)Co(PMe3)(H)(Cl)] (L21, L2 = 2,6-((iPr2P)(Et)N)2C6H3), as well as one new bissilylene [SiCSi] pincer cobalt(III) hydride, [(L3)Co(PMe3)(H)(Cl)] (L31, L3 = 1,3-((PhC(tBuN)2Si)(Et)N)2C6H3), were synthesized by reaction of the corresponding protic [PCP] or [SiCSi] pincer ligands L1H, L2H, and L3H with CoCl(PMe3)3. Despite the similarities in the ligand scaffolds, the three cobalt(III) hydrides show remarkably different performance as catalysts in alkene hydrosilylation. Among the PCP pincer complexes, L11 has higher catalytic activity than complex L21, and both catalysts afford anti-Markovnikov selectivity for both aliphatic and aromatic alkenes. In contrast, the catalytic activity for alkene hydrosilylation of silylene complex L31 is comparable to phosphine complex L11, but a dependence of regioselectivity on the substrates was observed: While aliphatic alkenes are converted in an anti-Markovnikov fashion, the hydrosilylation of aromatic alkenes affords Markovnikov products. The substrate scope was explored with 28 examples. Additional experiments were conducted to elucidate these mechanisms of hydrosilylation. The synthesis of cobalt(I) complex (L1)Co(PMe3)2 (L17) and its catalytic properties for alkene hydrosilylation allowed for the proposal of the mechanistic variations that occur in dependence of reaction conditions and substrates.

Light-Gated Cobalt(I) Hydride Catalysed Alkyne Hydroboration

We report a light-gated, catalytic protocol for hydroboration of terminal alkynes based on a bench-stable cobalt(I) hydride complex which undergoes ligand photodissociation with visible light. The products are obtained in good yields and with excellent E-selectivity.

Synthesis and Reactivity of Heterobimetallic Co-PAlP Pincer Complexes

Abstract A cobalt complex ligated with a PAlP pincer ligand was synthesized. The Lewis acidity of the aluminum atom on the cobalt complex was confirmed by coordination with 4-dimethylaminopyridine (DMAP). Reactions of the cobalt complex with H2O, EtOH, i-PrOH, and t-BuOH efficiently afforded the corresponding cobalt hydride complexes.

Photo-Initiated Cobalt-Catalyzed Radical Olefin Hydrogenation.

Outer‐sphere radical hydrogenation of olefins proceeds via stepwise hydrogen atom transfer (HAT) from transition metal hydride species to the substrate. Typical catalysts exhibit M−H bonds that are either too weak to efficiently activate H2 or too strong to reduce unactivated olefins. This contribution evaluates an alternative approach, that starts from a square‐planar cobalt(II) hydride complex. Photoactivation results in Co−H bond homolysis. The three‐coordinate cobalt(I) photoproduct binds H2 to give a dihydrogen complex, which is a strong hydrogen atom donor, enabling the stepwise hydrogenation of both styrenes and unactivated aliphatic olefins with H2 via HAT.

A Well-Defined Isocyano Analogue of HCo(CO)4. 3: Hydride Migration to Olefins, H-Atom Transfer and Reactivity toward Protic Sources

The tetraisocyano cobalt hydride complex HCo(CNArMes2)4 (ArMes2 = 2,6-(2,4,6-Me3C6H2)2C6H3) has previously been shown to serve as a well-defined structural and spectroscopic analogue to the classic...

Additive-Free Formic Acid Dehydrogenation Catalyzed by a Cobalt Complex

The reversible storage of hydrogen through the intermediate formation of formic acid (FA) is a promising solution to its safe transport and distribution. However, the common necessity of using bases or additives in the catalytic dehydrogenation of FA is a limitation. In this context, two new cobalt complexes (1 and 2) were synthesized with a pincer PP(NH)P ligand containing a phosphoramine moiety. Their reaction with an excess FA yields a cobalt(I) hydride complex (3). We report here the unprecedented catalytic activity of 3 in the dehydrogenation of FA, with a turnover frequency (TOF) of 67 min–1 measured in the first minute and a turnover number (TON) of 454, without the need for bases or additives. A mechanistic study reveals the existence of ligand–metal cooperativity with intermolecular hydrogen bonding, also influenced by the concentration of formic acid.

Reactivity of a T-shaped cobalt(I) pincer-complex.

The reactivity of a paramagnetic T-shaped cobalt(i) complex, [(iPrboxmi)Co], stabilised by a monoanionic bis(oxazolinylmethylidene)-isoindolate (boxmi) NNN pincer ligand is described. The exposure to carbon monoxide as an additional neutral ligand resulted in the square-planar species [(iPrboxmi)Co(CO)], accompanied by a change in the electronic spin state from S = 1 to S = 0. In contrast, upon treatment with trimethylphosphine the formation of the distorted tetrahedral complex [(iPrboxmi)Co(PMe3)] was observed (S = 1). Reacting [(iPrboxmi)Co] with iodine (I2), organic peroxides (tBu2O2, (SiMe3)2O2) and diphenyldisulphide (Ph2S2) yielded the tetracoordinated complexes [(iPrboxmi)CoI], [(iPrboxmi)Co(OtBu)], [(iPrboxmi)Co(OSiMe3)] and [(iPrboxmi)Co(SPh)], respectively, demonstrating the capability of the boxmi-supported cobalt(i) complex to homolytically cleave bonds and thus its distinct one-electron reactivity. Furthermore, a square-planar cobalt(ii) alkynyl complex [(iPrboxmi)Co(CCArF)] was identified as the main product in the reaction between [(iPrboxmi)Co] and a terminal alkyne, 4-fluoro-1-ethynylbenzene. Putting such species in the context of the previously investigated hydroboration catalysis, its stoichiometric reaction with pinacolborane revealed its potential conversion into a cobalt(ii) hydride complex, thus confirming its original attribution as off-cycle species.

Electrocatalytic Proton Reduction by a Cobalt(III) Hydride Complex with Phosphinopyridine PN Ligands

Cobalt complexes with 2-(diisopropylphosphinomethyl)pyridine (PN) ligands have been synthesized with the aim of demonstrating electrocatalytic proton reduction to dihydrogen with a well-defined hydride complex of an Earth-abundant metal. Reactions of simple cobalt precursors with 2-(diisopropylphosphino-methyl)pyridine (PN) yield [CoII(PN)2(MeCN)][BF4]2 1, [CoIII(PN)2(H)(MeCN)][PF6]2 2, and [CoIII(PN)2(H)(Cl)][PF6] 3. Complexes 1 and 3 have been characterized crystallographically. Unusually for a bidentate PN ligand, all three exhibit geometries with mutually trans phosphorus and nitrogen ligands. Complex 1 exhibits a distorted square-pyramidal geometry with an axial MeCN ligand in a low-spin electronic state. In complexes 2 and 3, the PN ligands lie in a plane leaving the hydride trans to MeCN or chloride, respectively. The redox behavior of the three complexes has been studied by cyclic voltammetry at variable scan rates and by spectroelectrochemistry. A catalytic wave is observed in the presence of trifluoroacetic acid (TFA) at an applied potential close to the Co(II/I) couple of 1. Bulk electrolysis of 1, 2, or 3 at a potential of ca. -1.4 V vs E(Fc+/Fc) in the presence of TFA yields H2 with Faradaic yields close to 100%. A catalytic mechanism is proposed in which the pyridine moiety of a PN ligand acts as a pendant proton donor following opening of the chelate ring. Additional mechanisms may also operate, especially in the presence of high acid concentration where speciation changes.

Visible-Light Controlled Divergent Catalysis Using a Bench-Stable Cobalt(I) Hydride Complex.

While the use of visible light in conjunction with transition metal catalysis offers powerful opportunities to switch between on/‐off states of catalytic activity, the next frontier would be the ability to switch the actual function of the catalyst and resulting products. Here we report such an example of multi‐dimensional catalysis. Featuring an easily prepared, bench‐stable cobalt(I) hydride complex in conjunction with pinacolborane, we can switch the reaction outcome between two widely employed transformations, olefin migration and hydroboration, with visible light as the trigger.

Additive-Free Isomerization of Allylic Alcohols to Ketones with a Cobalt PNP Pincer Catalyst.

Catalytic isomerization of allylic alcohols in ethanol as a green solvent was achieved by using air and moisture stable cobalt (II) complexes in the absence of any additives. Under mild conditions, the cobalt PNP pincer complex substituted with phenyl groups on the phosphorus atoms appeared to be the most active. High rates were obtained at 120 °C, even though the addition of one equivalent of base increases the speed of the reaction drastically. Although some evidence was obtained supporting a dehydrogenation-hydrogenation mechanism, it was proven that this is not the major mechanism. Instead, the cobalt hydride complex formed by dehydrogenation of ethanol is capable of double-bond isomerization through alkene insertion-elimination.

Potential energy-driven spin manipulation via a controllable hydrogen ligand.

Spin-bearing molecules can be stabilized on surfaces and in junctions with desirable properties, such as a net spin that can be adjusted by external stimuli. Using scanning probes, initial and final spin states can be deduced from topographic or spectroscopic data, but how the system transitions between these states is largely unknown. We address this question by manipulating the total spin of magnetic cobalt hydride complexes on a corrugated boron nitride surface with a hydrogen-functionalized scanning probe tip by simultaneously tracking force and conductance. When the additional hydrogen ligand is brought close to the cobalt monohydride, switching between a correlated S = 1/2 Kondo state, where host electrons screen the magnetic moment, and an S = 1 state with magnetocrystalline anisotropy is observed. We show that the total spin changes when the system is transferred onto a new potential energy surface that is defined by the position of the hydrogen in the junction. These results show how and why chemically functionalized tips are an effective tool to manipulate adatoms and molecules and a promising new method to selectively tune spin systems.

Reaction Parameters Influencing Cobalt Hydride Formation Kinetics: Implications for Benchmarking H(2)-Evolution Catalysts.

The need for benchmarking hydrogen evolution catalysts has increasingly been recognized. The influence of acid choice on activity is often reduced to the overpotential for catalysis. Through the study of a stable cobalt hydride complex, we demonstrate the influence of acid choice, beyond pKa, on the kinetics of hydride formation. A linear free energy relationship between acid pKa and second-order rate constants is observed for weaker acids. For stronger acids, however, further increases in pKa do not correlate to increases in rate constants. Further, steric bulk around the acidic proton is shown to influence rate constants dramatically. Together, these observations reveal the complex factors dictating catalyst performance.

B12-TiO2 Hybrid Catalyst for Light-Driven Hydrogen Production and Hydrogenation of Carbon-Carbon Multiple Bonds

Naturally-occurring B12(cobalamin)-dependent enzymes catalyze various molecular transformations that are of particular interest from the viewpoint of biological chemistry as well as synthetic organic chemistry and catalytic chemistry.1 For example, the B12-dependent enzyme catalyzes the rearrangement reactions and the methylation reaction as in the synthesis of methionine. All of these reactions are mediated by the cobalt alkylated complex which is generally formed by the reaction of the Co(I) state of the B12 with various electrophiles in vitro. Recently, we have reported the unique catalysis of the B12-titanium oxide (TiO2) hybrid catalyst in which the B12 complex, cyanoaquacobyrnic acid, is immobilized on the surface of TiO2 and the B12 complex is reductively activated to form the Co(I) species by electron transfer from TiO2 under UV-light irradiation.2 The hybrid catalyst mediated the dehalogenation of various organic halides and was applied to the radical-mediated organic reaction via an alkylated complex as a catalytic intermediate. The great advantage of the catalyst is the facile and efficent formation of Co(I) species by only UV light irradiation. This property prompted us to investigate further applications of the B12-TiO2 catalyst utilizing the high reactivity of the Co(I) species of the B12 complex. We now report the new catalysis of the B12-TiO2 for H2O reduction to form hydrogen. Cobalt complexes have been studied as excellent catalysts for hydrogen production, and the cobalt hydride complex is thought to be an intermediate for the reaction which could be formed by the reaction of Co(I) and a proton. As the metal hydride complexes are widely used for the reduction of unsaturated compounds, such as alkenes, an application of the B12-TiO2catalyst for the hydrogenation of C-C mutiple bonds was also investigated. [1] a) K. ó Proinsias, M. Giedyk, D. Gryko, Chem. Soc. Rev. 2013, 42, 6605-6619; b) K. Gruber, B. Puffer, B. Kräutler, Chem. Soc. Rev. 2011, 40, 4346-4363; c) W. Buckel, B. T. Golding, Annun. Rev. Microbiol. 2006, 60, 27-49; d) K. L. Brown, Chem. Rev. 2005, 105, 2075-2149; e) T. Toraya, Chem. Rev. 2003, 103, 2095-2127, f) R. Banerjee, S. W. Ragsdale, Annu. Rev. Biochem. 2003, 72, 209-247. [2] a) H. Shimakoshi, Y. Hisaeda, Angew. Chem. Int. Ed. 2015, in press; b) H. Shimakoshi, Y. Hisaeda, ChemPlusChem 2014, 79, 1250-1253; c) H. Shimakoshi, M. Abiru, S. Izumi, Y. Hisaeda, Chem. Commun. 2011, 6427-6429; d) S. Izumi, H. Shimakoshi, M. Abe, Y. Hisaeda, Dalton Trans. 2010, 39, 3302-3307; e) H. Shimakoshi, M. Abiru, K. Kuroiwa, N. Kimizuka, M. Watanabe, Y. Hisaeda, Bull. Chem. Soc. Jpn. 2010, 83, 170-172; f) H. Shimakoshi, E. Sakumori, K. Kaneko, Y. Hisaeda, Chem. Lett. 2009, 38, 468-469. Figure 1

A Two-Coordinate Cobalt(II) Imido Complex with NHC Ligation: Synthesis, Structure, and Reactivity.

The synthesis, structural characterization, and reactivity of the first two-coordinate cobalt complex featuring a metal-element multiple bond [(IPr)Co(NDmp)] (4; IPr=1,3-bis(2',6'-diisopropylphenyl)imidazole-2-ylidene; Dmp=2,6-dimesitylphenyl) is reported. Complex 4 was prepared from the reaction of [(IPr)Co(η(2) -vtms)2 ] (vtms=vinyltrimethylsilane) with DmpN3 . An X-ray diffraction study revealed its linear C-Co-N core and a short Co-N distance (1.691(6) Å). Spectroscopic characterization and calculation studies indicated the high-spin nature of 4 and the multiple-bond character of the Co-N bond. Complex 4 effected group-transfer reactions to CO and ethylene to form isocyanide and imine, respectively. It also facilitated E-H (E=C, Si) σ-bond activation of terminal alkyne and hydrosilanes to produce the corresponding cobalt(II) alkynyl and cobalt(II) hydride complexes as 1,2-addition products.