Carbene vs Olefin Products of C−H Activation on Ruthenium via Competing α- and β-H Elimination

Coordination-Induced Weakening of a C(sp3)-H Bond: Homolytic and Heterolytic Bond Strength of a CH-Ni Agostic Interaction

The scission of a C(sp³)-H bond to form a new metal-alkyl bond is a fundamental step in coordination chemistry and catalysis. However, the extent of C-H bond weakening when this moiety interacts with a transition metal is poorly understood and quantifying this phenomenon could provide insights into designing more efficient C-H functionalization catalysts. We present a nickel complex with a robust adamantyl reporter ligand that enables the measurement of C-H acidity (pK_a) and bond dissociation free energy (BDFE) for a C(sp³)-H agostic interaction, showing a decrease in pK_a by dozens of orders of magnitude and BDFE decrease of about 30 kcal/mol upon coordination. X-ray crystallographic data is provided for all molecules, including a distorted square planar Ni^III metalloradical and "doubly agostic" Ni^II(κ²-CH₂) complex.

η6-Coordinated ruthenabenzenes from three-component assembly on a diruthenium μ-allenyl scaffold

The room temperature reactions with internal alkynes, RCCR, of the μ-allenyl acetonitrile complex [Ru₂Cp₂(CO)₂(NCMe){μ-η¹:η²-C¹HC²C³Me₂}]BF₄ (1-NCMe), freshly prepared from the tricarbonyl precursor [Ru₂Cp₂(CO)₃{μ-η¹:η²-C¹HC²C³Me₂}]BF₄, 1, proceeded with alkyne insertion into ruthenium-allenyl bond and allenyl-CO coupling, affording compounds [Ru₂Cp₂(CO)₂{μ-η²:η⁵-C(R)C(R)C¹HC²(C³MeCH₂)C(OH)}]BF₄ (R = Ph, 2; R = CO₂Me, 3; R = CO₂Et, 4) in 83-94% yields. Deprotonation of 2-4 by triethylamine gave [Ru₂Cp₂(CO)₂{μ-η²:η⁵-C(R)C(R)CHC(CMeCH₂)C(O)}] (R = Ph, 5; R = CO₂Me, 6; R = CO₂Et, 7) in 75-88% yields, and 2-4 could be recovered upon HBF₄·Et₂O addition to 5-7. All the products, 2-7, were fully characterized by elemental analysis, IR and multinuclear NMR spectroscopy. The structure of 2 was ascertained by single crystal X-ray diffraction and investigated by DFT calculations, revealing a six-membered ruthenacycle with Shannon aromaticity index in line with related compounds. The formation of ruthenium-coordinated ruthenabenzenes from a preexistent diruthenium scaffold is a versatile but underdeveloped approach exploiting cooperative effects typical of a dimetallic core.

Carbocyclic pincer carbene complexes of ruthenium: syntheses and reversible hydrogenation

The ruthenium carbene pincer complex 2 was synthesized treating the benzo annulated cycloheptatriene bisphosphine 1 with RuCl₃. Addition of three equivalents of hydrogen to the carbocyclic carbene complex 2 was achieved in reaction of 2 with hydrogen at elevated temperatures. Hydrogenated complex 4, exhibiting a rigid chair conformation in solution, was dehydrogenated by heating a toluene solution of complex 4 to reflux for 5-7 d. In reaction with ethylene, complex 4 transfers one equivalent of hydrogen, forming ethane and alkyl complex 5.

Stable group 8 metal porphyrin mono- and bis(dialkylcarbene) complexes: synthesis, characterization, and catalytic activity

Alkyl-substituted carbene (CHR or CR2, R = alkyl) complexes have been extensively studied for alkylcarbene (CHR) ligands coordinated with high-valent early transition metal ions (a.k.a. Schrock carbenes or alkylidenes), yet dialkylcarbene (CR2) complexes remain less developed with bis(dialkylcarbene) species being little (if at all) explored. Herein, several group 8 metal porphyrin dialkylcarbene complexes, including Fe- and Ru-mono(dialkylcarbene) complexes [M(Por)(Ad)] (1a,b, M = Fe, Por = porphyrinato dianion, Ad = 2-adamantylidene; 2a,b, M = Ru) and Os-bis(dialkylcarbene) complexes [Os(Por)(Ad)2] (3a-c), are synthesized and crystallographically characterized. Detailed investigations into their electronic structures reveal that these complexes are formally low-valent M(ii)-carbene in nature. These complexes display remarkable thermal stability and chemical inertness, which are rationalized by a synergistic effect of strong metal-carbene covalency, hyperconjugation, and a rigid diamondoid carbene skeleton. Various spectroscopic techniques and DFT calculations suggest that the dialkylcarbene Ad ligand is unique compared to other common carbene ligands as it acts as both a potent σ-donor and π-acceptor; its unique electronic and structural features, together with the steric effect of the porphyrin macrocycle, make its Fe porphyrin complex 1a an active and robust catalyst for intermolecular diarylcarbene transfer reactions including cyclopropanation (up to 90% yield) and X-H (X = S, N, O, C) insertion (up to 99% yield) reactions.

Conformation controlled stepwise hydride shuffling from the metal to the ligand backbone

The protonation of the benzo annulated cycloheptatriene PCP pincer ruthenium hydrido dicarbonyl complex using the superacid [H(Et₂O)₂][BArF₂₄] launches a cascade of hydride shifts which lead to the hydrogenation of the ligand backbone.

Ruthenium-Catalyzed Reductive Cleavage of Unstrained Aryl-Aryl Bonds: Reaction Development and Mechanistic Study

Cleavage of carbon-carbon bonds has been found in some important industrial processes, for example, petroleum cracking, and has inspired development of numerous synthetic methods. However, nonpolar unstrained C(aryl)-C(aryl) bonds remain one of the toughest bonds to be activated. As a detailed study of a fundamental reaction mode, here a full story is described about our development of a Ru-catalyzed reductive cleavage of unstrained C(aryl)-C(aryl) bonds. A wide range of biaryl compounds that contain directing groups (DGs) at 2,2' positions can serve as effective substrates. Various heterocycles, such as pyridine, quinoline, pyrimidine, and pyrazole, can be employed as DGs. Besides hydrogen gas, other reagents, such as Hantzsch ester, silanes, and alcohols, can be employed as terminal reductants. The reaction is pH neutral and free of oxidants; thus a number of functional groups are tolerated. Notably, a one-pot C-C activation/C-C coupling has been realized. Computational and experimental mechanistic studies indicate that the reaction involves a ruthenium(II) monohydride-mediated C(aryl)-C(aryl) activation and the resting state of the catalyst is a η4-coordinated ruthenium(II) dichloride complex, which could inspire development of other transformations based on this reaction mode.

Ethyl Lactate: A Green Solvent for Olefin Metathesis

Compatibility of selected, commercially available ruthenium olefin metathesis catalysts with ethyl lactate as solvent was evaluated using a range of substrates and conditions. In addition, the preparation of a metathesis catalyst in simplified manner by using the advantages of ethyl lactate was accomplished. The application of ethyl lactate facilitates product isolation (also allowing for lower ruthenium contamination in crude metathesis products) and improves the overall green angle of olefin metathesis.

Cooperative bond activation reactions with carbene complexes

This review highlights the recent advances in the application of carbene complexes in bond activation reactions via metal–ligand cooperation.

Distinctive Reaction Pathways at Base Metals in High-Spin Organometallic Catalysts

Inexpensive "base" metals are more affordable and sustainable than precious metals and also offer opportunities to discover new mechanisms for selective catalytic reactions. Base metal complexes can have high-spin electronic configurations that are rare in precious metal complexes. This Account describes some concepts relevant to high-spin organometallic complexes, focusing on our recent work with β-diketiminate complexes of iron and cobalt. Even though high-spin organometallic complexes have some unfamiliar spectroscopic properties, they can be studied using NMR spectroscopy as well as techniques that focus on the magnetism brought about by the unpaired electrons. Understanding the mechanisms of reactions using these complexes can be complicated, because complexes with a high-spin electronic configuration may need to change spin states to avoid high barriers for reaction. These spin-state changes can be rapid, and the ability of an excited spin state to "cut through" the barrier for a reaction can lead to spin acceleration. These concepts, originally developed by Poli, Shaik, Schwarz, and Harvey, are applied here to the fundamental organometallic reaction of β-hydride elimination (BHE). Experimentally validated density-functional calculations show spin acceleration in BHE using three-coordinate iron(II) and cobalt(II) complexes. A square-planar transition state is particularly beneficial for accelerating BHE when a high-spin iron(II) complex goes from an S = 2 ground state to an S = 1 transition state or when a high-spin cobalt(II) complex goes from an S = 3/2 ground state to an S = 1/2 transition state. The relative energies of the spin states can be controlled with the choice of the supporting ligand. Using an appropriate ligand, isomerization of 1-alkenes to their Z-2 isomers can be catalyzed in high yields using the cobalt(II) alkyl complexes as catalysts. Though an earlier paper attributed the regioselectivity and stereoselectivity to the preferred geometry of the BHE step, the results of isotope labeling experiments suggest that the selectivity may actually come from the alkene exchange step (again with spin acceleration). In general, the use of multiple intersecting spin states is envisioned as a profitable strategy for bringing about low reaction barriers and high selectivity in catalytic reactions. This effort requires high-accuracy computational models as well as ligand design that gives nearby spin states with appropriate geometries.

Ruthenium(ii)-catalyzed switchable C3-alkylation versus alkenylation with acrylates of 2-pyridylbenzofurans via C–H bond activation

We documented an interesting observation of ruthenium(ii)-catalyzed benzofuran C–H activation and subsequent functionalization with acrylates that reveals that a simple base can switch the process from alkylation to alkenylation.

Square-planar ruthenium(II) complexes: control of spin state by pincer ligand functionalization

Functionalization of the PNP pincer ligand backbone allows for a comparison of the dialkyl amido, vinyl alkyl amido, and divinyl amido ruthenium(II) pincer complex series [RuCl{N(CH2 CH2 PtBu2 )2 }], [RuCl{N(CHCHPtBu2 )(CH2 CH2 PtBu2 )}], and [RuCl{N(CHCHPtBu2 )2 }], in which the ruthenium(II) ions are in the extremely rare square-planar coordination geometry. Whereas the dialkylamido complex adopts an electronic singlet (S=0) ground state and energetically low-lying triplet (S=1) state, the vinyl alkyl amido and the divinyl amido complexes exhibit unusual triplet (S=1) ground states as confirmed by experimental and computational examination. However, essentially non-magnetic ground states arise for the two intermediate-spin complexes owing to unusually large zero-field splitting (D>+200 cm(-1) ). The change in ground state electronic configuration is attributed to tailored pincer ligand-to-metal π-donation within the PNP ligand series.

A new PC(sp3)P ligand and its coordination chemistry with low-valent iron, cobalt and nickel complexes

A new PC(sp³)P ligand and its pincer complexes with Fe and Co are reported.

Decomposition pathways of Z-selective ruthenium metathesis catalysts

The decomposition of a Z-selective ruthenium metathesis catalyst and structurally similar analogues has been investigated utilizing X-ray crystallography and density functional theory. Isolated X-ray crystal structures suggest that recently reported C-H activated catalysts undergo decomposition via insertion of the alkylidene moiety into the chelating ruthenium-carbon bond followed by hydride elimination, which is supported by theoretical calculations. The resulting ruthenium hydride intermediates have been implicated in previously observed olefin migration, and thus lead to unwanted byproducts in cross metathesis reactions. Preventing these decomposition modes will be essential in the design of more active and selective Z-selective catalysts.