Small Cobalt Nanoparticles Favor Reverse Water-Gas Shift Reaction Over Methanation Under CO2 Hydrogenation Conditions

Cobalt-based catalysts are well-known to convert syngas into a variety of Fischer-Tropsch (FTS) products depending on the various reaction parameters, in particular particle size. In contrast, the reactivity of these particles has been much less investigated in the context of CO2 hydrogenation. In that context, Surface organometallic chemistry (SOMC) was employed to synthesize highly dispersed cobalt nanoparticles (Co-NPs) with particle sizes ranging from 1.6 to 3.0 nm. These SOMC-derived Co-NPs display significantly different catalytic performances under CO2 hydrogenation conditions: while the smallest cobalt nanoparticles (1.6 nm) catalyze mainly the reverse water-gas shift (rWGS) reaction, the larger nanoparticles (2.1-3.0 nm) favor the expected methanation activity. Operando X-ray absorption spectroscopy shows that the smaller cobalt particles are fully oxidized under CO2 hydrogenation conditions, while the larger ones remain mostly metallic, paralleling the observed difference of catalytic performances. This fundamental shift of selectivity, away from methanation to reverse water-gas shift for the smaller nanoparticles is noteworthy and correlates with the formation of CoO under CO2 hydrogenation conditions.

Diphenylacetylene stabilised alkali-metal nickelates: synthesis, structure and catalytic applications

Whilst low-valent nickelates have recently been proposed as intermediates in Ni-catalysed reactions involving polar organometallics, their isolation and characterisation is often challenging due to their high sensitivity and reactivity. Advancing the synthetic, spectroscopic and structural insights of these heterobimetallic systems, here we report a new family of alkyne supported alkali-metal nickelates of the formula Li₄(solv)_n(Ar)₄Ni₂{μ₂:η²,η²-Ph-CC-Ph} (where solv = Et₂O, THF; Ar = Ph, o-Tol, naphthyl, 4-^tBu-C₆H₄) which can be accessed through the combination of Ni(COD)₂, Ph-CC-Ph and the relevant lithium aryl in a 2 : 1 : 4 ratio. Demonstrating the versatility of this approach, the sodium and potassium nickelates can also be accessed when using PhNa or via alkali-metal exchange with AMO^tBu (AM = Na, K). When employing bulky or structurally constrained aryl-lithiums, mononickel complexes of the formula Li₂(solv)_n(Ar)₂Ni{η²-Ph-CC-Ph} are instead obtained, highlighting the structural diversity of alkali-metal nickelates bearing alkyne ligands. Expanding the catalytic potential of these systems, their ability to promote the catalytic cyclotrimerisation of diphenylacetylene to hexaphenylbenzene was explored, with mononickel compounds bearing electron rich aryl-substituents displaying the best performance.

C-H Activation by Isolable Cationic Bis(phosphine) Cobalt(III) Metallacycles

Five- and six-coordinate cationic bis(phosphine) cobalt(III) metallacycle complexes were synthesized with the general structures, [(depe)Co(cycloneophyl)(L)(L')][BAr^F₄] (depe = 1,2-bis(diethylphosphino)ethane; cycloneophyl = [κ-C:C-(CH₂C(Me)₂)C₆H₄]; L/L' = pyridine, pivalonitrile, or the vacant site, BAr₄^F = B[(3,5-(CF₃)₂)C₆H₃]₄). Each of these compounds promoted facile directed C(sp²)-H activation with exclusive selectivity for ortho-alkylated products, consistent with the selectivity of reported cobalt-catalyzed arene-alkene-alkyne coupling reactions. The direct observation of C-H activation by cobalt(III) metallacycles provided experimental support for the intermediacy of these compounds in this class of catalytic C-H functionalization reaction. Deuterium labeling and kinetic studies provided insight into the nature of C-H bond cleavage and C-C bond reductive elimination from isolable cobalt(III) precursors.

Cobalt-N-Heterocyclic Carbene Complexes in Catalysis

Cobalt-NHC complexes have emerged as an attractive class of 3d transition metal catalysts for a broad range of chemical processes, including cross-coupling, hydrogenation, hydrofunctionalization and cycloaddition reactions. Herein, we present a comprehensive review of catalytic methods utilizing cobalt-NHC complexes with a focus on catalyst structure, the role of the NHC ligand, properties of the catalytic system, mechanism and synthetic utility. The survey clearly suggests that the recent emergence of well-defined cobalt-NHC catalysts may have a tremendous utility in the design and application of catalytic reactions using more abundant 3d transition metals.

Giant N-heterocyclic carbene-containing macrocycles for cobalt-catalysed hydroboration of alkynes

Giant N-heterocyclic carbene-containing organic macrocycles larger than “Texas-sized” molecular boxes have been synthesized and structurally characterized. The new macrocyles were employed for the Co-NHC promoted syn-selective hydroboration of alkynes with...

Three-Coordinate Formal Cobalt(0), Iron(0), and Manganese(0) Complexes with Persistent Carbene and Alkene Ligation

Low-coordinate transition-metal species, i.e., metal species with coordination numbers of less than 4, represent a category of ubiquitous reactive intermediates in metal-catalyzed reactions that take place in solution, in metalloenzymes, on supported nanomaterials and single-atom catalysts, and so on. While reactive intermediates are usually transient and hard to isolate, which makes detailed investigation challenging, molecular representatives of low-coordinate transition-metal intermediates can be synthesized by the judicious use of supporting ligands, allowing detailed study of their inherent chemical and physical properties. By the use of bulky nitrogen- and oxygen-based anionic ligands, plenty of three- and two-coordinate group 4-10 metal complexes with the oxidation states of the metal centers being +3, + 2, and +1 have been prepared and subjected to extensive study. Much less known are low-coordinate zero-valent metal complexes, and knowledge about them had been restricted to group 10 metal complexes until very recently. In this Account, we summarize the studies of the synthesis, spectroscopic features, electronic structures, and reactivities of three-coordinate formal cobalt(0), iron(0), and manganese(0) complexes with persistent carbene and alkene ligation. The introduction of the π-accepting alkene ligands 1,3-divinyl-1,1,3,3-tetramethyldisiloxane (dvtms) and vinyltrimethylsilane (vtms) into the reduction reactions of MCl₂ (M = Co, Fe, Mn) with persistent carbenes and alkaline metals effectively suppresses ligand C-H bond activation reactions, leading to the successful preparation of three-coordinate formal cobalt(0), iron(0), and manganese(0) complexes LM(η²:η²-dvtms), LM(η²-vtms)₂, and L₂M(η²-vtms) (M = Co, Fe, Mn; L = N-heterocyclic carbene (NHC), cyclic (alkyl)(amino)carbene (cAAC)). These three-coordinate metal complexes feature pronounced back-donation from the filled metal 3d orbitals to the alkene π* orbital(s), resulting in electronic configurations of (d_xy+π_alkene^*)²(d_x²-y²+π'_alkene^*)²(d_z²,d_xz,d_yz)ⁿ (the coordination plane was chosen as the xy plane; n = 5, 4, and 3 for Co, Fe, and Mn, respectively) for the bis(alkene) complexes LM(η²:η²-dvtms) and LM(η²-vtms)₂. The alkene ligands in the low-coordinate formal zero-valent metal complexes are amenable to undergo ligand-exchange reactions with better π-accepting ligands. In reactions with organic azides, hydrosilanes, nitrosoarenes, alkynes, etc., the alkene ligands dissociate from the metal coordination sphere, and three-coordinate formal zero-valent metal complexes function as synthons of L_nM⁰ (L = NHC, cAAC; n = 1, 2; M = Co, Fe, Mn) to perform redox reactions with these substrates, affording divalent and tetravalent cobalt, iron, or manganese complexes. These electronic structure and reactivity features hint at the potential of low-coordinate zero-valent group 7-9 metal complexes for the development of new 3d metal catalysts and magnetic materials.

Pogo-Stick Iron and Cobalt Complexes: Synthesis, Structures, and Magnetic Properties

The synthesis, structures, and magnetic properties of monomeric half-sandwich iron and cobalt imidazolin-2-iminato complexes have been comprehensively investigated. Salt metathesis reactions of [Cp'M(μ-I)]₂ (1-M, M = Fe, Co; Cp' = η⁵-1,2,4-tri-tert-butylcyclopentadienyl) with [Im^DippNLi]₂ (Im^DippN = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-iminato) furnishes the terminal half-sandwich compounds [Cp'M(NIm^Dipp)] (2-M, M = Fe, Co), which can be regarded as models for elusive half-sandwich iron and cobalt imido complexes. X-ray diffraction analysis confirmed the structure motif of a one-legged piano stool. Complex 2-Co can also be prepared by an acid-base reaction between [Cp'Co{N(SiMe₃)₂}] (3-Co) and Im^DippNH. The electronic and magnetic properties of 2-M and 3-Co were probed by ⁵⁷Fe Mössbauer spectroscopy (M = Fe), X-band EPR spectroscopy (M = Co), and solid-state magnetic susceptibility measurements. In particular, the central metal atom adopts a high-spin (S = 2) state in 2-Fe, while the cobalt complex 2-Co represents a rare example of a Co(II) species with a coordination number different from six displaying a low-spin to high-spin spin-crossover (SCO) behavior. The experimental observations are complemented by DFT calculations.

Substrate Redox Non-innocence Inducing Stepwise Oxidative Addition Reaction: Nitrosoarene C-N Bond Cleavage on Low-Coordinate Cobalt(0) Species

The reactions of nitrosoarenes with transition-metal species are fundamentally important for their relevance to metal-catalyzed transformations of organo-nitrogen compounds in organic synthesis and also the metabolization of nitroarenes and anilines in biology. In addition to the well-known reactivity of metal-mediated N-O bond activation and cleavage of nitrosoarenes, we present herein the first observation of a nitrosoarene C-N bond oxidative addition reaction upon the interaction of a three-coordinate cobalt(0) species [(IPr)Co(vtms)₂] with 2,4,6-tri( tert-butyl)-1-nitroso-benzene (Ar*NO). The reaction produces a cobalt nitrosyl aryl complex, [(IPr)Co(Ar*)(NO)] (1), with a bis(nitrosoarene)cobalt complex, [(IPr)Co(η²-ONAr)(κ¹- O-ONAr)] (2), as an intermediate. Spectroscopic characterizations, DFT calculations, and kinetic studies revealed that the redox non-innocence of nitrosoarene induces a stepwise pathway for the C-N bond oxidative addition reaction.