Cooperative Reactivity in Carbometalated Pincer-Type Complexes Possessing an Appended Functionality

Hydrogenation of CO2 by a Bifunctional PC(sp3 )P Iridium(III) Pincer Complex Equipped with Tertiary Amine as a Functional Group

Reversible hydrogen storage in the form of stable and mostly harmless chemical substances such as formic acid (FA) is a cornerstone of a fossil fuels-free economy. In the past, we have reported a primary amine-functionalized bifunctional iridium(III)-PC(sp3 )P pincer complex as a mild and chemoselective catalyst for the additive-free decomposition of neat formic acid. In this manuscript, we report on the successful application of a redesigned complex bearing tertiary amine functionality as a catalyst for mild hydrogenation of CO2 to formic acid. The catalyst demonstrates TON up to 6×104 and TOF up to 1.7×104  h-1 . In addition to the practical value of the catalyst, experimental and computational mechanistic studies provide the rationale for the design of improved next-generation catalysts.

Cooperation towards nobility: equipping first-row transition metals with an aluminium sword

The exploration for noble metals substitutes in catalysis has become a highly active area of research, driven by the pursuit of sustainable chemical processes. Although the utilization of base metals holds great potential as an alternative, their successful implementation in predictable catalytic processes necessitates the development of appropriate ligands. Such ligands must be capable of controlling their intricate redox chemistry and promote two-electron events, thus mimicking well-established organometallic processes in noble metal catalysis. While numerous approaches for infusing nobility to base metals have been explored, metal-ligand cooperation has garnered significant attention in recent years. Within this context, aluminium-based ligands offer interesting features to fine-tune the activity of metal centres, but their application in base metal catalysis remains largely unexplored. This perspective seeks to highlight the most recent breakthroughs in the reactivity of heterobimetallic aluminium-base-metal complexes, while also showcasing their potential to develop novel and predictable catalytic transformations. By turning the spotlight on such heterobimetallic species, we aim to inspire chemists to explore aluminium-base-metal species and expand the range of their applications as catalysts.

Synthesis and Magnetic Studies of Two Neutral, Bis-Ligand Fe(II) Complexes Containing Carbazole-Bis(tetrazole) Ligands

Previously reported carbazole-bis(tetrazole) (CzT^R) ligands (where R = iPr and CH₂-2,4,6-C₆H₂Me₃) were used to synthesize air-stable, six-coordinate, octahedral bis-ligand Fe(II) complexes (CzT^R)₂Fe. The synthesis and characterization of these complexes using ¹H nuclear magnetic resonance (NMR), X-ray crystallography, Mössbauer spectroscopy, and density functional theory (DFT) calculations are reported. Analysis of the magnetic properties revealed that the isopropyl derivative displays thermally induced spin crossover (SCO) over a temperature range of 150-350 K. This transition appears as an abrupt two-step transition in the solid state but simplifies to a smooth one-step transition in solution. The two-step transition in the solid state has been postulated to be due to lattice and solvation effects. In contrast, the slightly bulkier substituted CH₂-2,4,6-C₆H₂Me₃ (CH₂Mes) Fe complex displays dramatically different magnetic behavior with no SCO and magnetic data suggesting low-spin Fe(II) with a possible TIP contribution. DFT calculations support the postulate that the change in magnetic behavior is primarily due to the nature of the ligand substituents.

Evaluating the Catalytic Activities of PNCNP Pincer Group 10 Metal Hydride Complexes: Pd-Catalyzed Reduction of CO2 to the Formic Acid Level with NH3·BH3 and NaBH4 under Ambient Conditions

In order to develop efficient protocols for CO₂ reduction with less expensive and more convenient hydrogen sources, the catalytic reactivities of group 10 metal hydride complexes supported by a PNCNP pincer ligand, [2,6-(^tBu₂PNH)₂C₆H₃]MH (M = Ni, 1a; Pd, 1b; Pt, 1c), against the hydroboration of CO₂ with NH₃·BH₃ and NaBH₄ have been explored. Both 1a and 1b readily react with CO₂ at room temperature to form the corresponding formato complexes, [2,6-(^tBu₂PNH)₂C₆H₃]MOC(O)H (M = Ni, 2a; Pd, 2b), in nearly quantitative yields. Treatment of NH₃·BH₃ with CO₂ (1 atm) in 1,4-dioxane or THF at room temperature in the presence of 0.05-1.0 mol % of 1b followed by hydrolysis of the resulting mixtures produces formic acid in 105-186% yields, and initial turnover frequencies of up to 2000 h^-1 are observed. In the presence of 1.0 mol % of 1b, NaBH₄ reacts with CO₂ (1 atm) in THF at room temperature to form NaB[OC(O)H]₄ (3) in 87% isolated yield. In situ NMR spectroscopy indicates that the reactions proceed through the insertion of the C═O bond in CO₂ into the Pd-H bond in 1b to form 2b, which sequentially reacts with the hydrides in NH₃·BH₃ or NaBH₄ to produce boron formato species and regenerate 1b. This work represents one of the rare examples of catalytic transfer hydrogenation of CO₂ with NH₃·BH₃ to the formic acid level under very mild conditions without any additives and also the first example of 4 equiv of CO₂ uptake by NaBH₄ in a reaction.

Experimental and Theoretical Study of Ni II ‐ and Pd II ‐Promoted Double Geminal C(sp 3 )−H Bond Activation Providing Facile Access to NHC Pincer Complexes: Isolated Intermediates and Mechanism

We report the first examples of metal‐promoted double geminal activation of C(sp³)−H bonds of the N−CH₂−N moiety in an imidazole‐type heterocycle, leading to nickel and palladium N‐heterocyclic carbene complexes under mild conditions. Reaction of the new electron‐rich diphosphine 1,3‐bis((di‐tert‐butylphosphaneyl)methyl)‐2,3‐dihydro‐1H‐benzo[d]imidazole (1) with [PdCl₂(cod)] occurred in a stepwise fashion, first by single C−H bond activation yielding the alkyl pincer complex [PdCl(PCsp3^HP)] (3) with two trans phosphane donors and a covalent Pd−Csp3 bond. Activation of the C−H bond of the resulting α‐methine Csp3 H−M group occurred subsequently when 3 was treated with HCl to yield the NHC pincer complex [PdCl(PC^NHCP)]Cl (2). Treatment of 1 with [NiBr₂(dme)] also afforded a NHC pincer complex, [NiBr(PC^NHCP)]Br (6), but the reactions leading to the double geminal C−H bond activation of the N−CH₂−N group were too fast to allow identification or isolation of an intermediate analogous to 3. The determination of six crystal structures, the isolation of reaction intermediates and DFT calculations provided the basis for suggesting the mechanism of the stepwise transformation of a N−CH₂−N moiety in the N−C^NHC−N unit of NHC pincer complexes and explain the key differences observed between the Pd and Ni chemistries.

Using molten salts to probe outer-coordination sphere effects on lanthanide(III)/(II) electron-transfer reactions

Controlling structure and reactivity by manipulating the outer-coordination sphere around a given reagent represents a longstanding challenge in chemistry. Despite advances toward solving this problem, it remains difficult to experimentally interrogate and characterize outer-coordination sphere impact. This work describes an alternative approach that quantifies outer-coordination sphere effects. It shows how molten salt metal chlorides (MCl_n; M = K, Na, n = 1; M = Ca, n = 2) provided excellent platforms for experimentally characterizing the influence of the outer-coordination sphere cations (Mⁿ⁺) on redox reactions accessible to lanthanide ions; Ln³⁺ + e^1- → Ln²⁺ (Ln = Eu, Yb, Sm; e^1- = electron). As a representative example, X-ray absorption spectroscopy and cyclic voltammetry results showed that Eu²⁺ instantaneously formed when Eu³⁺ dissolved in molten chloride salts that had strongly polarizing cations (like Ca²⁺ from CaCl₂) via the Eu³⁺ + Cl^1- → Eu²⁺ + ½Cl₂ reaction. Conversely, molten salts with less polarizing outer-sphere M¹⁺ cations (e.g., K¹⁺ in KCl) stabilized Ln³⁺. For instance, the Eu³⁺/Eu²⁺ reduction potential was >0.5 V more positive in CaCl₂ than in KCl. In accordance with first-principle molecular dynamics (FPMD) simulations, we postulated that hard Mⁿ⁺ cations (high polarization power) inductively removed electron density from Lnⁿ⁺ across Ln-Cl⋯Mⁿ⁺ networks and stabilized electron-rich and low oxidation state Ln²⁺ ions. Conversely, less polarizing Mⁿ⁺ cations (like K¹⁺) left electron density on Lnⁿ⁺ and stabilized electron-deficient and high-oxidation state Ln³⁺ ions.

Cyclometallated complexes as catalysts for C-H activation and functionalization

The development of novel catalysts for C-H activation reactions with increased reactivity and improved selectivities has been attracting significant interest over the last two decades. More recently, promising results have been developed using tridentate pincer ligands, which form a stable C-M bond. Furthermore, based on mechanistic studies, the unique catalytic role of some metallacyclic intermediate species has been revealed. These experimental observations have subsequently translated into the rational design of advanced C-H activation catalysts in both Ru- and Ir-based systems. Recent breakthroughs in the field of C-H activation catalysed by metallacyclic intermediates are thus discussed.

Alternative Conceptual Approach to the Design of Bifunctional Catalysts: An Osmium Germylene System for the Dehydrogenation of Formic Acid

The reaction of the hexahydride OsH₆(PⁱPr₃)₂ with a P,Ge,P-germylene-diphosphine affords an osmium tetrahydride derivative bearing a Ge,P-chelate, which arises from the hydrogenolysis of a P–C(sp³) bond. This Os(IV)–Ge(II) compound is a pioneering example of a bifunctional catalyst based on the coordination of a σ-donor acid, which is active in the dehydrogenation of formic acid to H₂ and CO₂. The kinetics of the dehydrogenation, the characterization of the resting state of the catalysis, and DFT calculations point out that the hydrogen formation (the fast stage) exclusively occurs on the coordination sphere of the basic metal center, whereas both the metal center and the σ-donor Lewis acid cooperatively participate in the CO₂ release (the rate-determining step). During the process, the formate group pivots around the germanium to approach its hydrogen atom to the osmium center, which allows its transfer to the metal and the CO₂ release.

An alternative class of bifunctional catalysts can be assembled by coordination of σ-donor Lewis acids to platinum-group-metal basic fragments. In contrast to what happens with the previously reported bifunctional catalysts, this design allows enhancing the basicity of the base and the acidity of the acid. According to this, a bifunctional catalyst for the dehydrogenation of formic acid, based on an osmium(IV)-germylene cooperative system, has been prepared and the mechanism of the catalysis established.