CO2-based hydrogen storage – hydrogen liberation from methanol/water mixtures and from anhydrous methanol

New strategies for the reforming of methanol under mild conditions on the basis of heterogeneous and molecular catalysts have raised the hopes and expectations on this fuel. This contribution will focus on the progress achieved in the production of hydrogen from aqueous and anhydrous methanol with molecular and heterogeneous catalysts. The report entails thermal approaches, as well as light-triggered dehydrogenation reactions. A comparison of the efficiency and mechanistic aspects will be made and principles of catalytic pathways operating in biological systems will be also addressed.

Mononuclear complexes of a tridentate redox-active ligand with sulfonamido groups: structure, properties, and reactivity

The design of molecular complexes of earth-abundant first-row transition metals that can catalyze multi-electron C-H bond activation processes is of interest for achieving efficient, low-cost syntheses of target molecules. To overcome the propensity of these metals to perform single-electron processes, redox-active ligands have been utilized to provide additional electron equivalents. Herein, we report the synthesis of a novel redox active ligand, [ibaps]3-, which binds to transition metals such as FeII and CoII in a meridional fashion through the three anionic nitrogen atoms and provides additional coordination sites for other ligands. In this study, the neutral bidentate ligand 2,2'-bipyridine (bpy) was used to complete the coordination spheres of the metal ions and form NEt4[MII(ibaps)bpy] (M = Fe (1) or Co (1-Co)) salts. The FeII salt exhibited rich electrochemical properties and could be chemically oxidized by 1 and 2 equiv. of ferrocenium to form singly and doubly oxidized species, respectively. The reactivity of 1 towards intramolecular C-H bond amination of aryl azides at benzylic and aliphatic carbon centers was explored, and moderate to good yields of the resulting indoline products were obtained.

Reaction of CO2 with a Vanadium(II) Aryl Oxide: Synergistic Activation of CO2 /Oxo Groups towards H-Atom Radical Abstraction

Treatment of divalent (ONNO)V(TMEDA) (1; ONNO=[2,4-Me₂ -2-(OH)C₆ H₂ CH₂ ]₂ N(CH₂ )₂ NMe₂ ) with CO₂ afforded [(ONNO)V]₂ (μ-OH)(μ-formate) (2). Whereas the bridging hydroxo and formate groups both originated from CO₂ , the H atoms present on the two residues were obtained through H-atom radical abstraction from the solvent. DFT calculations revealed an initially linear CO₂ bonding mode, followed by deoxygenation, and highlighted a synergistic effect between the so-formed oxo group and an additional bridging CO₂ residue in promoting radical behavior.

A Multifunctional Pincer Ligand for Cobalt-Promoted Oxidation by N2 O

The divalent cobalt complex of the diprotic pincer ligand bis-pyrazolylpyridine, (H₂ L)CoCl₂ , is dehydrohalogenated twice by LiN(SiMe₃ )₂ in the presence of PEt₃ to give monomeric S=1/2 LCo(PEt₃ )₂ (1), fully characterized in the solid-state and solution as a square pyramidal monomer with a long axial Co-P bond. This 17-electron species reacts in time of mixing with N₂ O to form L₂ Co₂ (μ-OPEt₃ ) (2)+3 OPEt₃ , the former the first example of phosphine oxide bridging two transition metals. The same products are formed from O₂ , and divalent cobalt persists even in the presence of excess oxidant. Species (2) catalyzes oxygen atom transfer (OAT) for generation of O=PEt₃ from PEt₃ from either N₂ O or O₂ . Bridging and terminal cobalt oxo intermediates are suggested, and the electron donor power, and potential redox activity of the dianionic pincer ligand is emphasized.

Reactivity of a dearomatised pincer CoIIBr complex with PNCNHC donors: alkylation and Si–H bond activation via metal–ligand cooperation

Whereas [Co(^CyP*N_aC^NHC)Br] (1) with dearomatised pincer ^CyP*N_aC^NHC affords the Co^II–alkyl complex 3, uncommon silane reduction yields the Co^I complex 4.

Dehydrohalogenation of proton responsive complexes: versatile aggregation via pyrazolate pincer ligand arms

The behavior of the complex (H₂L)CoCl₂, where H₂L is a bis-(pyrazol-3-yl)pyridine, towards Brønsted bases is studied, to evaluate peripheral NH deprotonation as a route to a dianionic pincer ligand on a d⁷ center.

Iridium Complexes with Proton-Responsive Azole-Type Ligands as Effective Catalysts for CO2 Hydrogenation

Pentamethylcyclopentadienyl iridium (Cp*Ir) complexes with bidentate ligands consisting of a pyridine ring and an electron-rich diazole ring were prepared. Their catalytic activity toward CO₂ hydrogenation in 2.0 m KHCO₃ aqueous solutions (pH 8.5) at 50 °C, under 1.0 MPa CO₂ /H₂ (1:1) have been reported as an alternative to photo- and electrochemical CO₂ reduction. Bidentate ligands incorporating an electron-rich diazole ring improved the catalytic performance of the Ir complexes compared to the bipyridine ligand. Complexes 2, 4, and 6, possessing both a hydroxy group and an uncoordinated NH group, which are proton-responsive and capable of generating pendent bases in basic media, recorded high initial turnover frequency values of 1300, 1550, and 2000 h^-1 , respectively. Spectroscopic and computational investigations revealed that the reversible deprotonation changes the electronic properties of the complexes and causes interactions between pendent base and substrate and/or solvent water molecules, resulting in high catalytic performance in basic media.

Mechanistic Insight into Electrocatalytic H2 Production by [Fe2(CN){μ-CN(Me)2}(μ-CO)(CO)(Cp)2]: Effects of Dithiolate Replacement in [FeFe] Hydrogenase Models

DFT has been used to investigate viable mechanisms of the hydrogen evolution reaction (HER) electrocatalyzed by [Fe₂(CN){μ-CN(Me)₂}(μ-CO)(CO)(Cp)₂] (1) in AcOH. Molecular details underlying the proposed ECEC electrochemical sequence have been studied, and the key functionalities of CN^- and amino-carbyne ligands have been elucidated. After the first reduction, CN^- works as a relay for the first proton from AcOH to the carbyne, with this ligand serving as the main electron acceptor for both reduction steps. After the second reduction, a second protonation occurs at CN^- that forms a Fe(CNH) moiety: i.e., the acidic source for the H₂ generation. The hydride (formally 2e/H⁺), necessary to the heterocoupling with H⁺ is thus provided by the μ-CN(Me)₂ ligand and not by Fe centers, as occurs in typical L₆Fe₂S₂ derivatives modeling the hydrogenase active site. It is remarkable, in this regard, that CN^- plays a role more subtle than that previously expected (increasing electron density at Fe atoms). In addition, the role of AcOH in shuttling protons from CN^- to CN(Me)₂ is highlighted. The incompetence for the HER of the related species [Fe₂{μ-CN(Me)₂}(μ-CO)(CO)₂(Cp)₂]⁺ (2⁺) has been investigated and attributed to the loss of proton responsiveness caused by CN^- replacement with CO. In the context of hydrogenase mimicry, an implication of this study is that the dithiolate strap, normally present in all synthetic models, can be removed from the Fe₂ core without loss of HER, but the redox and acid-base processes underlying turnover switch from a metal-based to a ligand-based chemistry. The versatile nature of the carbyne, once incorporated in the Fe₂ scaffold, could be exploited to develop more active and robust catalysts for the HER.

C-C and C-N Couplings Following Hydride Addition on Isocyanide Cyclopolyenyl Dimolybdenum Complexes to Give Tethered Aldimine and Aminocarbene Derivatives

Reaction of [Mo₂ Cp₂ (μ-κ¹ :κ¹ ,η⁶ -PMes*)(CO)₂ ] with S or Se followed by protonation with [H(OEt₂ )₂ ](BAr'₄ ) gave the cationic derivatives [Mo₂ Cp₂ {μ-κ²_P,E :κ¹_P ,η⁵ -EP(C₆ H₃ tBu₃ )}(CNR)(CO)₂ ](BAr'₄ ) (E=S; R=tBu, iPr, Ph, 4-C₆ H₄ OMe, Xyl; or E=Se; R=tBu; Ar'=3,5-C₆ H₃ (CF₃ )₂ ). Reaction of the latter with K[BHsBu₃ ] yielded the aldimine complexes [Mo₂ Cp₂ {μ-κ²_P,E :κ²_P,N ,η⁴ -SP(C₆ H₃ tBu₃ (CHNR))}(CO)₂ ] and their aminocarbene isomers [Mo₂ Cp₂ {μ-κ²_P,E :κ²_P,C ,η⁴ -SP(C₆ H₃ tBu₃ (NRCH))}(CO)₂ ] (R ≠ Xyl), following C-C and C-N couplings, respectively. Monitoring of these reactions revealed that the initial H^- attack takes place at a Cp ligand to give cyclopentadiene intermediates [Mo₂ Cp{μ-κ²_P,S :κ¹_P ,η⁵ -SP(C₆ H₃ tBu₃ )}(η⁴ -C₅ H₆ )(CNR)(CO)₂ ], which then undergo C-H oxidative addition to give the hydride isomers [Mo₂ Cp₂ {μ-κ²_P,S :κ¹_P ,η³ -SP(C₆ H₃ tBu₃ )}(H)(CNR)(CO)₂ ]. In turn, the latter rearrange to give the aldimine and aminocarbene complexes. DFT calculations revealed that the hydride intermediates first undergo migratory insertion of the isocyanide ligand into the Mo-H bond to give unobservable formimidoyl intermediates, which then evolve either by nucleophilic attack of the N atom on the C₆ ring (C-N coupling) or by migratory insertion of the formimidoyl ligand into the C₆ ring (C-C coupling). Our data suggest that increasing the size of the substituent R at the isocyanide ligand destabilizes the aldimine isomer to a greater extent, thus favoring formation of the aminocarbene complex.

Diiron Dithiolate Hydrides Complemented with Proton-Responsive Phosphine-Amine Ligands

The reaction of Fe₂(pdt)(CO)₆ with two equivalents of Ph₂PC₆H₄NH₂ (PNH₂) affords the amido hydride HFe₂(pdt)(CO)₂(PNH₂)(PNH) {[H1H]⁰, pdt^2- = CH₂(CH₂S^-)₂}. Isolated intermediates in this conversion include Fe₂(pdt)(CO)₅-(κ¹-PNH₂) and Fe₂(pdt)(CO)₄(κ²-PNH₂). X-ray crystallographic analysis of [H1H]⁰ shows that the chelating amino/amido-phosphine ligands occupy trans-dibasal positions. The ³¹P NMR spectrum indicates that [H1H]⁰ undergoes rapid proton exchange between the amido and amine centers. No exchange was observed for the hydride. Protonation of [H1H]⁰ gives [HFe₂(pdt)(CO)₂(PNH₂)₂]⁺ ([H₂1H]⁺), which contains two equivalent amino-phosphine ligands. Single-crystal X-ray crystallographic analysis of [H₂1H]⁺ also reveals hydrogen bonds between the exo amine protons with a THF molecule and BF₄. Deprotonation of [H1H]⁰ with potassium tert-butoxide gave [HFe₂(pdt)(CO)₂(PNH)₂]^- ([1H]^-), which was characterized spectroscopically. The complex has time-averaged C₂ symmetry with two amido-phosphine ligands. FTIR spectroscopic measurements show that υ_CO shifts by approximately 20 cm^-1 in the series [1H]^-, [H1H]⁰, and [H₂1H]⁺. These shifts are comparable to those seen for the S-protonation of the (NC)₂(CO)Fe-(μ-Scys)₂Ni(Scys)₂ site in the [NiFe]-hydrogenases.^[1].

Control of Ligand pKa Values Tunes the Electrocatalytic Dihydrogen Evolution Mechanism in a Redox-Active Aluminum(III) Complex

Redox-active ligands bring electron- and proton-transfer reactions to main-group coordination chemistry. In this Forum Article, we demonstrate how ligand pK_a values can be used in the design of a reaction mechanism for a ligand-based electron- and proton-transfer pathway, where the ligand retains a negative charge and enables dihydrogen evolution. A bis(pyrazolyl)pyridine ligand, ^iPrPz₂P, reacts with 2 equiv of AlCl₃ to afford [(^iPrPz₂P)AlCl₂(THF)][AlCl₄] (1). A reaction involving two-electron reduction and single-ligand protonation of 1 affords [(^iPrHPz₂P^-)AlCl₂] (2), where each of the electron- and proton-transfer events is ligand-centered. Protonation of 2 would formally close a catalytic cycle for dihydrogen production. At -1.26 V versus SCE, in a 0.3 M Bu₄NPF₆/tetrahydrofuran solution with salicylic acid or (HNEt₃)⁺ as the source of H⁺, 1 produced dihydrogen electrocatalytically, according to cyclic voltammetry and controlled potential electrolysis experiments. The mechanism for the reaction is most likely two electron-transfer steps followed by two chemical steps based on the available reactivity information. A comparison of this work with our previously reported aluminum complexes of the phenyl-substituted bis(imino)pyridine system (^PhI₂P) reveals that the pK_a values of the N-donor atoms in ^iPrPz₂P are lower, which facilitates reduction before ligand protonation. In contrast, the ^PhI₂P ligand complexes of aluminum are protonated twice before reduction liberates dihydrogen.

Smart N-Heterocyclic Carbene Ligands in Catalysis

It is well-recognized that N-heterocyclic carbene (NHC) ligands have provided a new dimension to the design of homogeneous catalysts. Part of the success of this type of ligands resides in the limitless access to a variety of topologies with tuned electronic properties, but also in the ability of a family of NHCs that are able to adapt their properties to the specific requirements of individual catalytic transformations. The term "smart" is used here to refer to switchable, multifunctional, adaptable, or tunable ligands and, in general, to all those ligands that are able to modify their steric or electronic properties to fulfill the requirements of a defined catalytic reaction. The purpose of this review is to comprehensively describe all types of smart NHC ligands by focusing attention on the catalytically relevant ligand-based reactivity.

A Redox-Switchable Germylene and its Ligating Properties in Selected Transition Metal Complexes

The synthesis, structure, and full characterization of a redox-switchable germylene based on a [3]ferrocenophane ligand arrangement, [Fc(NMes)₂ Ge] (4), is presented. The mesityl (Mes)-substituted title compound is readily available from Fc(NHMes)₂ (2) and Ge{N(SiMe₃ )₂ }₂ , or from the dilithiated, highly air- and moisture-sensitive compound Fc(NLiMes)₂ ⋅3 Et₂ O (3) and GeCl₂ . Cyclic voltammetry studies are provided for 4, confirming the above-mentioned view of a redox-switchable germylene metalloligand. Although several 1:1 Rh^I and Ir^I complexes of 4 (5-7) are cleanly formed in solution, all attempts to isolate them in pure form failed due to stability problems. However, crystalline solids of [Mo(κ¹ Ge-4)₂ (CO)₄ ] (8) and [W(κ¹ Ge-4)₂ (CO)₄ ] (9) were isolated and fully characterized by common spectroscopic techniques (8 by X-ray diffraction). DFT calculations were performed on a series of model compounds to elucidate a conceivable interplay between the metal atoms in neutral and cationic bimetallic complexes of the type [Rh(κ¹ E-qE)(CO)₂ Cl]^0/+ (qE=[Fc(NPh)₂ E] with E=C, Si, Ge). The bonding characteristics of the coordinated Fc-based metalloligands (qE/qE⁺ ) are strongly affected upon in silico oxidation of the calculated complexes. The calculated Tolman electronic parameter (TEP) significantly increases by approximately 20 cm^-1 (E=C) to 25 cm^-1 (E=Si, Ge) upon oxidation. The change in the ligand-donating abilities upon oxidation can mainly be attributed to Coulombic effects, whereas an orbital-based interaction appears to have only a minor influence.

Phosphine-imine and -enamido ligands for acceptorless dehydrogenation catalysis

A highly tunable phosphine-imine ligand family is introduced. Following metallation with ruthenium, deprotonation of the ligand affords a phosphine-enamido species. Complexes with the ligand in both the imine and enamido forms are active toward acceptorless dehydrogenation reactions.

Palladium complexes of ferrocene-based phosphine ligands as redox-switchable catalysts in Buchwald–Hartwig cross-coupling reactions

Redox-switchable catalysis: Palladium(ii) complexes of two differently substituted [1]phosphaferrocenophanes FcPR (R = Mes, biaryl) and of diphenylferrocenyl phosphine Ph₂PFc were applied in redox-switchable Buchwald–Hartwig cross-coupling reactions.

Azobenzene-based ruthenium(ii) catalysts for light-controlled hydrogen generation

Photo-controlled hydrogen generation catalysts were developed based on ruthenium(ii) azobenzene-containing half-sandwich complexes.

Non-symmetrical, potentially redox non-innocent imino NHC pyridine ‘pincers’ via a zinc ion template-assisted synthesis

A Zn^II-promoted modular synthesis allows access to new non-symmetrical, redox-active imino NHC pyridine pincer ligands. Radical anionic and dianionic redox states of the ligand are involved in its Fe^II complexes obtained from FeBr₂/KC₈.

Synthesis of a sterically bulky diphosphine synthon and Ru(ii) complexes of a cooperative tridentate enamide-diphosphine ligand platform

The preparation of the bulky diphosphine synthon 1 is described and used to generate a tridentate enamido diphosphine ligand platform that can be installed on Ru(ii).

Coordination chemistry and applications of versatile 4,5-diazafluorene derivatives

This review article highlights the versatile nature of 4,5-diazafluorene derivatives as ligands, and details some recent advances made using this ligand family.

Polyaromatic N-heterocyclic carbene ligands and π-stacking. Catalytic consequences

This article highlights how π-stacking interactions have an important influence on the catalytic properties of transition metal complexes decorated with rigid polyaromatic ligands.