A Cobalt(I) Pincer Complex with an η(2) -C(aryl)-H Agostic Bond: Facile C-H Bond Cleavage through Deprotonation, Radical Abstraction, and Oxidative Addition

Formation and reactivity of a unique M⋯C-H interaction stabilized by carborane cages

Broadening carborane applications has consistently been the goal of chemists in this field. Herein, compared to alkyl or aryl groups, a carborane cage demonstrates an advantage in stabilizing a unique bonding interaction: M⋯C-H interaction. Experimental results and theoretical calculations have revealed the characteristic of this two-center, two-electron bonding interaction, in which the carbon atom in the arene ring provides two electrons to the metal center. The reduced aromaticity of the benzene moiety, long distance between the metal and carbon atom in arene, and the upfield shift of the signal of M⋯C-H in the nuclear magnetic resonance spectrum distinguished this interaction from metal⋯C π interaction and metal-C(H) σ bonds. Control experiments demonstrate the unique electronic effects of carborane in stabilizing the M⋯C-H bonding interaction in organometallic chemistry. Furthermore, the M⋯C-H interaction can convert into C-H bond metallization under acidic conditions or via treatment with t-butyl isocyanide. These findings deepen our understanding regarding the interactions between metal centers and carbon atoms and provide new opportunities for the use of carboranes.

Base metal complexes featuring a new pyrazole-derived PCP pincer ligand

A new pyrazole-derived PCP pincer ligand featuring a 1-methylpyrazole backbone tethered to two di(isopropyl)phosphine moieties via phenylene spacers (P(CH)P-iPr) was prepared. When reacting the ligand with group six carbonyl complexes [M(CO)6] (M = Cr, Mo, W) at 130 °C, complexes of the type [M(κ2PN-PCP-iPr)(CO)4] were obtained featuring a κ2P,N-bound ligand with a pendant phosphine arm. Upon an increase of the reaction temperature to 150 °C, in the case of molybdenum, the formation of the complex [Mo(κ3PCP-PCP-iPr)(CO)3] was observed featuring a weak Mo-C bond. DFT calculations reveal that there is no agostic η2-C-H interaction. Treatment of [Mn2(CO)10], [Fe2(CO)9], [Co2(CO)8] and [Ni(COD)2] afforded complexes [Mn(κ3PCP-PCP-iPr)(CO)3], [Fe(κ3PCP-PCP-iPr)(H)(CO)2], [Co(κ3PCP-PCP-iPr)(CO)2] and [Ni(κ3PCP-PCP-iPr)(H)], respectively, where the PCP ligand is coordinated in the typical meridional κ3-fashion. Postfunctionalization of the anionic PCP pincer ligand was possible via N-methylation of the second nitrogen atom of the pyrazole unit with the oxonium salt [Me3O]BF4. Treatment of [Mn(κ3PCP-PCP-iPr)(CO)3] and [Fe(κ3PCP-PCP-iPr)(H)(CO)2] with [Me3O]BF4 resulted in the formation of the cationic complexes [Mn(κ3PCP-PCPMe-iPr)(CO)3]+ and [Fe(κ3PCP-PCPMe-iPr)(Cl)(CO)2]+. In the case of the latter, the chloride ligand seems to originate from the solvent CH2Cl2 undergoing a hydride chloride exchange. All complexes were characterized by means of 1H, 13C{1H}, and 31P{1H} NMR spectroscopy, IR spectroscopy and HR-MS. In addition, the structures of representative complexes were determined by X-ray crystallography.

γ-Agostic interactions in (MesCCC)Fe-Mes(L) complexes

Agostic interactions were observed in the bound mesityl group in a series of iron compounds bearing a bis(NHC) pincer CCC ligand. The L-type ligand on [(CCC)Fe^IIMes(L)] complexes influences the strength of the agostic interaction and is manifested in the upfield shift of the ¹H NMR resonance for the mesityl methyl resonances. The nature of the interaction was further investigated by density functional theory calculations, allowing rationalization of some unexpected trends and proving to be a powerful predictive tool.

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.

Facile Transformations of a Binuclear Cp*Co(II) Diamidonaphthalene Complex to Mixed-Valent Co(II)Co(III), Co(III)(μ-H)Co(III), and Co(III)(μ-OH)Co(III) Derivatives

A diamido-bridged dicobalt complex supported by a diamidonaphthalene ligand, Cp*₂Co₂(μ-1,8-C₁₀H₈(NH)₂) (1), was synthesized, and the reactivity relevant to redox transformations of the Co₂N₂ core was investigated. It was found that the Co(II)-Co(II) bond allows for protonation by [HPPh₃][BF₄] resulting in a bridging hydride, [1H]⁺, with pK_a ∼ 7.6 in CH₂Cl₂. The diamidonaphthalene ligand can stabilize the binuclear system in the Co(II)Co(III) mixed-valent state (1⁺), which is capable of binding CO to afford [1-CO]⁺. Surprisingly, the mixed-valent complex also activates H₂O to furnish a Co(III)Co(III) hydroxy complex [1-OH]⁺ accompanied by release of H₂. The hydroxy ligand in [1-OH]⁺ is exchangeable, as demonstrated by ¹⁸O-labeling experiments on [1-OH]⁺ with H₂¹⁸O that led to the heavier isotopolog [1-¹⁸OH]⁺.

Manganese and iron PCP pincer complexes - the influence of sterics on structure and reactivity

The syntheses of various manganese and iron PCP pincer complexes via a solvothermal oxidative addition methodology is described. Upon reacting [Mn₂(CO)₁₀] with the ligands (P(C-Br)P^CH₂-iPr) (1a) and (P(C-Br)P^O-iPr) (1b), Mn(I) PCP pincer complexes [Mn(PCP^CH₂-iPr)(CO)₃] (2a) and [Mn(-PCP^O-iPr)(CO)₃] (2b) were obtained. Protonation of 2a with HBF₄·Et₂O led to the formation of [Mn(κ³P,CH,P-P(CH)P^CH₂-iPr)(CO)₃]BF₄ (3) featuring an η²-C_aryl-H agostic bond. The solvothermal reaction of 1a with [Fe₂(CO)₉] afforded the Fe(II) PCP pincer complex [Fe(PCP^CH₂-iPr)(CO)₂Br] (4). Treatment of 4 with Li[HBEt₃] afforded the Fe(I) complex [Fe(PCP^CH₂-iPr)(CO)₂] (5a). When using the sterically more demanding ligands (P(C-Br)P^CH₂-tBu) (1c) and (P(C-Br)P^O-tBu)(1d) striking differences in reactivity were observed. While neither 1c nor 1d showed any reactivity towards [Mn₂(CO)₁₀], the reaction with [Fe₂(CO)₉] and [Fe(CO)₅] led to the formation of the Fe(I) complexes [Fe(PCP^CH₂-tBu)(CO)₂] (5b) and [Fe(PCP^O-tBu)(CO)₂] (5c). X-ray structures of representative complexes are provided.

Synthesis, Characterization, and Catalytic Reactivity of {CoNO}8 PCP Pincer Complexes

The reaction of coordinatively unsaturated Co(II) PCP pincer complexes with nitric oxide leads to the formation of new, air-stable, diamagnetic mono nitrosyl compounds. The synthesis and characterization of five- and four-coordinate Co(III) and Co(I) nitrosyl pincer complexes based on three different ligand scaffolds is described. Passing NO through a solution of [Co(PCP^NMe-iPr)Cl], [Co(PCP^O-iPr)Cl] or [Co(PCP^CH2-iPr)Br] led to the formation of the low-spin complex [Co(PCP-iPr)(NO)X] with a strongly bent NO ligand. Treatment of the latter species with (X = Cl, Br) AgBF₄ led to chloride abstraction and formation of cationic square-planar Co(I) complexes of the type [Co(PCP-iPr)(NO)]⁺ featuring a linear NO group. This reaction could be viewed as a formal two electron reduction of the metal center by the NO radical from Co(III) to Co(I), if NO is counted as NO⁺. Hence, these systems can be described as {CoNO}⁸ according to the Enemark–Feltham convention. X-ray structures, spectroscopic and electrochemical data of all nitrosyl complexes are presented. Preliminary studies show that [Co(PCP^NMe-iPr)(NO)]⁺ catalyzes efficiently the reductive hydroboration of nitriles with pinacolborane (HBpin) forming an intermediate {CoNO}⁸ hydride species.

A Structurally Characterized Cobalt(I) σ-Alkane Complex

A cobalt σ-alkane complex, [Co(Cy2 P(CH2 )4 PCy2 )(norbornane)][BArF 4 ], was synthesized by a single-crystal to single-crystal solid/gas hydrogenation from a norbornadiene precursor, and its structure was determined by X-ray crystallography. Magnetic data show this complex to be a triplet. Periodic DFT and electronic structure analyses revealed weak C-H→Co σ-interactions, augmented by dispersive stabilization between the alkane ligand and the anion microenvironment. The calculations are most consistent with a η1 :η1 -alkane binding mode.

Cobalt-Pincer Complexes in Catalysis

Non-noble metal catalysts based on pincer type compounds are of special interest for organometallic chemistry and organic synthesis. Next to iron and manganese, currently cobalt-pincer type complexes are successfully applied in various catalytic reactions. In this review the recent progress in (de)hydrogenation, transfer hydrogenation, hydroboration and hydrosilylation as well as dehydrogenative coupling reactions using cobalt-pincer complexes is summarised.

Formation of Agostic Structures Driven by London Dispersion

Agostic interactions between a C-H bond and a transition metal are commonly crucial in catalytic polymerization processes. Herein, a quantitative study of the nature of β-agostic interactions in a series of systems of importance in C-H bond activation reactions is reported. The analysis, characterized by the use of a coupled-cluster-based energy decomposition scheme, demonstrates that short-range London dispersion between the agostic C-H bond and the metal center plays a fundamental role in affecting the structural stability of these systems, contrary to a widely held view. These results are used to rationalize a series of previously published experimental findings.

Cobalt-Catalyzed Alkylation of Secondary Alcohols with Primary Alcohols via Borrowing Hydrogen/Hydrogen Autotransfer

Alcohols are promising sustainable starting materials because they can be obtained from abundant and indigestible biomass. The substitution of expensive noble metals in catalysis by earth abundant 3d metals, such as Mn, Fe, or Co, (nonprecious or base metals) is a related key concept with respect to sustainability. Here, we report on the first cobalt-catalyzed alkylation of secondary alcohols with primary alcohols. Easy-to-synthesize and easy-to-activate PN₅ P-pincer-ligand-stabilized Co complexes developed in our laboratory mediate the reaction most efficiently. The catalysis is applicable to a broad substrate scope and proceeds under relatively mild conditions. We have even demonstrated the coupling of a variety of purely aliphatic alcohols with a base or nonprecious metal catalyst. Mechanistic studies indicate that the reaction follows the borrowing hydrogen or hydrogen autotransfer concept.

Chasing the agostic interaction in ligand assisted cyclometallation reactions of palladium(ii)

Computations show that a possible aromatic ring agostic intermediate seen by NMR spectroscopy in a cyclometallation reaction by palladium(ii) can involve CCπ electron density close to the agostic carbon being donated to the metal.

New complexity for aromatic ring agostic interactions

Aromatic ring agostic interactions can be augmented by previously unrecognised CCπ to metal donation which has significant implication for ligand-assisted C–H bond functionalization.

General and Mild Cobalt-Catalyzed C-Alkylation of Unactivated Amides and Esters with Alcohols

The borrowing hydrogen or hydrogen autotransfer methodology is an elegant and sustainable or green concept to construct carbon-carbon bonds. In this concept, alcohols, which can be obtained from barely used and indigestible biomass, such as lignocellulose, are employed as alkylating reagents. An especially challenging alkylation is that of unactivated esters and amides. Only noble metal catalysts based on iridium and ruthenium have been used to accomplish these reactions. Herein, we report on the first base metal-catalyzed α-alkylation of unactivated amides and esters by alcohols. Cobalt complexes stabilized with pincer ligands, recently developed in our laboratory, catalyze these reactions very efficiently. The precatalysts can be synthesized easily from commercially available starting materials on a multigram scale and are self-activating under the basic reaction conditions. This Co catalyst class is also able to mediate alkylation reactions of both esters and amides. In addition, we apply the methodology to synthesize ketones and to convert alcohols into aldehydes elongated by two carbon atoms.

Selective Catalytic Hydrogenations of Nitriles, Ketones, and Aldehydes by Well-Defined Manganese Pincer Complexes

Hydrogenations constitute fundamental processes in organic chemistry and allow for atom-efficient and clean functional group transformations. In fact, the selective reduction of nitriles, ketones, and aldehydes with molecular hydrogen permits access to a green synthesis of valuable amines and alcohols. Despite more than a century of developments in homogeneous and heterogeneous catalysis, efforts toward the creation of new useful and broadly applicable catalyst systems are ongoing. Recently, Earth-abundant metals have attracted significant interest in this area. In the present study, we describe for the first time specific molecular-defined manganese complexes that allow for the hydrogenation of various polar functional groups. Under optimal conditions, we achieve good functional group tolerance, and industrially important substrates, e.g., for the flavor and fragrance industry, are selectively reduced.