Hydride & dihydrogen complexes of earth abundant metals: structure, reactivity, and applications to catalysis

Cooperative H2 activation at a nickel(0)-olefin centre

Catalytic olefin hydrogenation is ubiquitous in organic synthesis. In most proposed homogeneous catalytic cycles, reactive M-H bonds are generated either by oxidative addition of H2 to a metal centre or by deprotonation of a non-classical metal dihydrogen (M-H2) intermediate. Here we provide evidence for an alternative H2-activation mechanism that instead involves direct ligand-to-ligand hydrogen transfer (LLHT) from a metal-bound H2 molecule to a metal-coordinated olefin. An unusual pincer ligand that features two phosphine ligands and a central olefin supports the formation of a non-classical Ni-H2 complex and the Ni(alkyl)(hydrido) product of LLHT, in rapid equilibrium with dissolved H2. The usefulness of this cooperative H2-activation mechanism for catalysis is demonstrated in the semihydrogenation of diphenylacetylene. Experimental and computational mechanistic investigations support the central role of LLHT for H2 activation and catalytic semihydrogenation. The product distribution obtained is largely determined by the competition between (E)-(Z) isomerization and catalyst degradation by self-hydrogenation.

Exploring the Multifarious Role of the Ligand in Electrocatalytic Hydrogen Evolution Reaction Pathways

In recent years, researchers have shifted their focus towards investigating the redox properties of ancillary ligand backbones for small-molecule activation. Several metal complexes have been reported for the electrocatalytic H2 evolution reaction (HER), providing valuable mechanistic insights. This process involves efficient coupling of electrons and protons. Redox-active ligands stipulate internal electron transfer and promote effective orbital overlap between metal and ligand, thereby, enabling efficient proton-coupled electron transfer reactions. Understanding such catalytic mechanisms requires thorough spectroscopic and computational analyses. Herein, we summarize recent examples of molecular electrocatalysts based on 3d transition metals that have significantly influenced mechanistic pathways, thus, emphasizing the multifaceted role of metal-ligand cooperativity.

Closed Synthetic Cycle for Nickel-Based Dihydrogen Formation

Dihydrogen evolution was observed in a two-step protonation reaction starting from a Ni0 precursor with a tripodal N-heterocyclic carbene (NHC) ligand. Upon the first protonation, a NiII monohydride complex was formed, which was isolated and fully characterized. Subsequent protonation yields H2 via a transient intermediate (INT) and an isolable NiII acetonitrile complex. The latter can be reduced to regenerate its Ni0 precursor. The mechanism of H2 formation was investigated by using a deuterated acid and scrutinized by 1 H NMR spectroscopy and gas chromatography. Remarkably, the second protonation forms a rare nickel dihydrogen complex, which was detected and identified in solution and characterized by 1 H NMR spectroscopy. DFT-based computational analyses were employed to propose a reaction profile and a molecular structure of the Ni-H2 complex. Thus, a dihydrogen-evolving, closed-synthetic cycle is reported with a rare Ni-H2 species as a key intermediate.

Multimetallic Permethylpentalene Hydride Complexes

The synthesis and characterization of group 4 permethylpentalene (Pn* = C₈Me₆) hydride complexes are explored; in all cases, multimetallic hydride clusters were obtained. Group 4 lithium metal hydride clusters were obtained when reacting the metal dihalides with hydride transfer reagents such as LiAlH₄, and these species featured an unusual hexagonal bipyramidal structural motif. Only the zirconium analogue was found to undergo hydride exchange in the presence of deuterium. In contrast, a trimetallic titanium hydride cluster was isolated on reaction of the titanium dialkyl with hydrogen. This diamagnetic, mixed valence species was characterized in the solid state, as well as by solution electron paramagnetic resonance and nuclear magnetic resonance spectroscopy. The structure was further probed and corroborated by density functional theory calculations, which illustrated the formation of a metal-cluster bonding orbital responsible for the diamagnetism of the complex. These permethylpentalene hydride complexes have divergent structural motifs and reactivity in comparison with related classical cyclopentadienyl analogues.

Photo-Initiated Cobalt-Catalyzed Radical Olefin Hydrogenation

Outer‐sphere radical hydrogenation of olefins proceeds via stepwise hydrogen atom transfer (HAT) from transition metal hydride species to the substrate. Typical catalysts exhibit M−H bonds that are either too weak to efficiently activate H₂ or too strong to reduce unactivated olefins. This contribution evaluates an alternative approach, that starts from a square‐planar cobalt(II) hydride complex. Photoactivation results in Co−H bond homolysis. The three‐coordinate cobalt(I) photoproduct binds H₂ to give a dihydrogen complex, which is a strong hydrogen atom donor, enabling the stepwise hydrogenation of both styrenes and unactivated aliphatic olefins with H₂ via HAT.

Reversible hydrogen adsorption at room temperature using a molybdenum-dihydrogen complex in the solid state

Reversible H₂ storage under mild conditions is one of the most important targets in the field of materials chemistry. Dihydrogen complexes are attractive materials for this target because they possess moderate adsorption enthalpy as well as adsorption without cleavage of the H-H bond. In spite of these advantages, H₂ adsorption studies of dihydrogen complexes in the solid state are scarce. We herein present H₂ adsorption properties of the 16-electron precursor complex ([Mo(PCy₃)₂(CO)₃]) in the solid state synthesized by two procedures. One is the direct synthesis under an Ar atmosphere (1), and the other is removal of the N₂-adduct under vacuum (2). 2 showed ideal Langmuir type reversible ad/desorption of H₂ above room temperature, whereas 1 showed irreversible adsorption. The adsorption enthalpy of 2 was larger than that in THF solution. Using DFT calculation, this difference was explained by the absence of the agostic interaction in the solid state.

Structural and Electronic Influences on Rates of Tertpyridine-Amine CoIII -H Formation During Catalytic H2 Evolution in an Aqueous Environment

In this work, the differences in catalytic performance for a series of Co hydrogen evolution catalysts with different pentadentate polypyridyl ligands (L), have been rationalized by examining elementary steps of the catalytic cycle using a combination of electrochemical and transient pulse radiolysis (PR) studies in aqueous solution. Solvolysis of the [Co^II -Cl]⁺ species results in the formation of [Co^II (κ⁴ -L)(OH₂ )]²⁺ . Further reduction produces [Co^I (κ⁴ -L)(OH₂ )]⁺ , which undergoes a rate-limiting structural rearrangement to [Co^I (κ⁵ -L)]⁺ before being protonated to form [Co^III -H]²⁺ . The rate of [Co^III -H]²⁺ formation is similar for all complexes in the series. Using E_1/2 values of various Co species and pK_a values of [Co^III -H]²⁺ estimated from PR experiments, we found that while the protonation of [Co^III -H]²⁺ is unfavorable, [Co^II -H]⁺ reacts with protons to produce H₂ . The catalytic activity for H₂ evolution tracks the hydricity of the [Co^II -H]⁺ intermediate.

Selectivity Reverse of Hydrosilylation of Aryl Alkenes Realized by Pyridine N-Oxide with [PSiP] Pincer Cobalt(III) Hydride as Catalyst

Six silyl cobalt(III) hydrides 1-6 with [PSiP] pincer ligands having different substituents at the P and Si atoms ([(2-Ph₂PC₆H₄)₂MeSiCo(H)(Cl)(PMe₃)] (1), [(2-Ph₂PC₆H₄)₂HSiCo(H)(Cl)(PMe₃)] (2), [(2-Ph₂PC₆H₄)₂PhSiCo(H)(Cl)(PMe₃)] (3), [(2-ⁱPr₂PC₆H₄)₂HSiCo(H)(Cl)(PMe₃)] (4), [(2-ⁱPr₂PC₆H₄)₂MeSiCo(H)(Cl)(PMe₃)] (5), and [(2-ⁱPr₂PC₆H₄)₂PhSiCo(H)(Cl)(PMe₃)] (6)) were synthesized through the reactions of the ligands (L1-L6) with CoCl(PMe₃)₃ via Si-H bond cleavage. Compounds 1-6 have catalytic activity for alkene hydrosilylation, and among them, complex 3 is the best catalyst with excellent anti-Markovnikov regioselectivity. A silyl dihydrido cobalt(III) complex 7 from the reaction of 3 with Ph₂SiH₂ was isolated, and its catalytic activity is equivalent to that of complex 3. Complex 7 and its derivatives 10-12 could also be obtained through the reactions of complexes 3, 1, 4, and 5 with NaBHEt₃. The molecular structure of 7 was indirectly verified by the structures of 10-12. To our delight, the addition of pyridine N-oxide reversed the selectivity of the reaction, from anti-Markovnikov to Markovnikov addition. At the same time, the reaction temperature was reduced from 70 to 30 °C on the premise of high yield and excellent selectivity. However, this catalytic system is only applicable to aromatic alkenes. On the basis of the experimental information, two reaction mechanisms are proposed. The molecular structures of cobalt(III) complexes 3-6 and 10-12 were determined by single crystal X-ray diffraction analysis.

Electrocatalytic hydrogen production by dinuclear cobalt(ii) compounds containing redox-active diamidate ligands: a combined experimental and theoretical study

The chiral dicobalt(ii) complex [Co^II₂(μ₂-L)₂] (1) (H₂L = N²,N⁶-di(quinolin-8-yl)pyridine-2,6-dicarboxamide) and its tert-butyl analogue [Co^II₂(μ₂-LBu)₂] (2) were structurally characterized and their catalytic evolution of H₂ was investigated.

Configurational landscape of chiral iron(ii) bis(phosphane) complexes

An [FeH(η²-H₂){Me-DuPhos}₂]⁺ complex reacts with ethers and halides to give cis- and trans-dihydrogen substituted isomers and [FeX{Me-DuPhos}₂]⁺ (X = Cl, I) complexes.

Thermodynamic and kinetic studies of H2 and N2 binding to bimetallic nickel-group 13 complexes and neutron structure of a Ni(η2-H2) adduct

Binding energies for H₂ and N₂ at nickel become more exergonic for the larger group 13 sigma-accepting supports.

Understanding H₂ binding and activation is important in the context of designing transition metal catalysts for many processes, including hydrogenation and the interconversion of H₂ with protons and electrons. This work reports the first thermodynamic and kinetic H₂ binding studies for an isostructural series of first-row metal complexes: NiML, where M = Al (1), Ga (2), and In (3), and L = [N(o-(NCH₂PⁱPr₂)C₆H₄)₃]^3–. Thermodynamic free energies (ΔG°) and free energies of activation (ΔG^‡) for binding equilibria were obtained via variable-temperature ³¹P NMR studies and lineshape analysis. The supporting metal exerts a large influence on the thermodynamic favorability of both H₂ and N₂ binding to Ni, with ΔG° values for H₂ binding found to span nearly the entire range of previous reports. The non-classical H₂ adduct, (η²-H₂)NiInL (3-H₂), was structurally characterized by single-crystal neutron diffraction—the first such study for a Ni(η²-H₂) complex or any d¹⁰ M(η²-H₂) complex. UV-Vis studies and TD-DFT calculations identified specific electronic structure perturbations of the supporting metal which poise NiML complexes for small-molecule binding. ETS-NOCV calculations indicate that H₂ binding primarily occurs via H–H σ-donation to the Ni 4p_z-based LUMO, which is proposed to become energetically accessible as the Ni(0)→M(iii) dative interaction increases for the larger M(iii) ions. Linear free-energy relationships are discussed, with the activation barrier for H₂ binding (ΔG^‡) found to decrease proportionally for more thermodynamically favorable equilibria. The ΔG° values for H₂ and N₂ binding to NiML complexes were also found to be more exergonic for the larger M(iii) ions.

Recent progress in ligand-centered homogeneous electrocatalysts for hydrogen evolution reaction

Recent advances in metal and metal-free ligand-centred electrocatalytic H₂ evolution have been reviewed.

Mechanisms of catalytic reduction of CO2 with heme and nonheme metal complexes

The catalytic conversion of CO2 into valuable chemicals and fuels has attracted increasing attention, providing a promising route for mitigating the greenhouse effect of CO2 and also meeting the global energy demand. Among many homogeneous and heterogeneous catalysts for CO2 reduction, this mini-review is focused on heme and nonheme metal complexes that act as effective catalysts for the electrocatalytic and photocatalytic reduction of CO2. Because metalloporphyrinoids show strong absorption in the visible region, which is sensitive to the oxidation states of the metals and ligands, they are suited for the detection of reactive intermediates in the catalytic CO2 reduction cycle by electronic absorption spectroscopy. The first part of this review deals with the catalytic mechanism for the one-electron reduction of CO2 to oxalic acid with heme and nonheme metal complexes, with an emphasis on how the formation of highly energetic CO2˙ is avoided. Then, the catalytic mechanism of two-electron reduction of CO2 to produce CO and H2O is compared with that to produce HCOOH. The effect of metals and ligands of the heme and nonheme complexes on the CO or HCOOH product selectivity is also discussed. The catalytic mechanisms of multi-electron reduction of CO2 to methanol (six-electron reduced product) and methane (eight-electron reduced product) are also discussed for both electrocatalytic and photocatalytic systems.

Iridium-based hydride transfer catalysts: from hydrogen storage to fine chemicals

Selective hydrogen transfer remains a central research focus in catalysis: hydrogenation and dehydrogenation have central roles, both historical and contemporary, in all aspects of fuel, agricultural, pharmaceutical, and fine chemical synthesis. Our lab has been involved in this area by designing homogeneous catalysts for dehydrogenation and hydrogen transfer that fill needs ranging from on-demand hydrogen storage to fine chemical synthesis. A keen eye toward mechanism has enabled us to develop systems with excellent selectivity and longevity and demonstrate these in a diversity of high-value applications. Here we describe recent work from our lab in these areas that are linked by a central mechanistic trichotomy of catalyst initiation pathways that lead highly analogous precursors to a diversity of useful applications.

Reaction mechanism of hydrogen evolution catalysed by Co and Fe complexes containing a tetra-dentate phosphine ligand - a DFT study

The reaction mechanism of the electro-catalytic proton reduction in neutral phosphate buffer enabled by mononuclear cobalt and iron complexes containing a tetra-dentate phosphine ligand (MP₄N₂, M = Fe, Co) has been elucidated by density functional calculations. The phosphate from the buffer was found to play a crucial role by coordinating to the metal and delivering a proton to the metal hydride in the H-H bond formation. For the more efficient cobalt catalyst, the starting species is a Co^II complex with a hydrogen phosphate and a water molecule ligated at the two vacant coordination sites. Two sequential proton-coupled electron transfer reductions lead to the formation of a Co^II-H intermediate with a dihydrogen phosphate ligand, and the reduction potentials for these two steps were calculated to be -0.58 V and -0.72 V, respectively. Subsequently, the H-H bond formation takes place via coupling of the Co^II-H and the proton from the dihydrogen phosphate ligand. The total barrier was calculated to be 18.2 kcal mol^-1 with an applied potential of -0.5 V, which can further decrease to only 11.2 kcal mol^-1 with an applied potential of -0.8 V. When the phosphate is displaced by a water molecule, the total barrier for the dihydrogen formation increases by 7.3 kcal mol^-1. For the iron catalyst, the overall mechanism is essentially the same; however, the first reduction (Fe^II/Fe^I, potential of -1.13 V) is likely the rate-limiting step. The calculated results are in good agreement with the experimental data, which showed an onset potential of -0.50 V for the cobalt complex and -1.03 V for the iron complex.

Cobalt-catalyzed acceptorless dehydrogenative coupling of aminoalcohols with alcohols: direct access to pyrrole, pyridine and pyrazine derivatives

A molecularly defined SNS-cobalt(ii) catalyst for the acceptorless dehydrogenative coupling (ADC) of unprotected amino alcohols with secondary alcohols leading to pyrrole and pyridine derivatives is reported.

DFT methods applied to answer the question: how accurate is the ligand acidity constant method for estimating the pKa of transition metal hydride complexes MHXL4 when X is varied?

Additive ligand acidity constants A_L of anionic ligands are calculated for neutral hydrides of iron(ii), ruthenium(ii) and osmium(ii) with phosphine and carbonyl co-ligands; constant A_L in green, more variable A_L in red.

Catalytic (de)hydrogenation promoted by non-precious metals – Co, Fe and Mn: recent advances in an emerging field

This review is aimed at introducing the remarkable progress made in the last three years in the development of base metal catalysts for hydrogenations and dehydrogenative transformations.

A mechanism study on the hydrogen evolution reaction catalyzed by molybdenum disulfide complexes

Water-mediated intermolecular H⁺/H⁻ coupling between two- or three-electron reduced sulfur hydride complexes with a hydrated proton is preferred to produce H₂ rather than intramolecular couplings between sulfur hydride and metal hydride complexes.

Phosphine-free cobalt pincer complex catalyzed Z-selective semi-hydrogenation of unbiased alkynes

A molecularly defined NNN-type cobalt pincer complex catalyzed semi-hydrogenation of unbiased alkynes to Z-selective alkenes is reported. The reaction operates at a low temperature (50–80 °C), phosphine ligand-free, and base-free conditions with no additive required.

Preparation and reactivity of a square-planar PNP cobalt(ii)–hydrido complex: isolation of the first {Co–NO}8–hydride

The synthesis of a square-planar cobalt(ii) hydrido complex supported by a pyrrole-based PNP ligand has been reinvestigated and its reactivity with various small molecules examined.

Elucidating metal hydride reactivity using late transition metal boryl and borane hydrides: 2c–2e terminal hydride, 3c–2e bridging hydride, and 3c–4e bridging hydride

A general trend for the hydrogenation reactivity of metal hydride(s): 3c–4e bridging hydride > 2c–2e terminal hydride > 3c–2e bridging hydride.

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.

Pairwise H2/D2 Exchange and H2 Substitution at a Bimetallic Dinickel(II) Complex Featuring Two Terminal Hydrides

A compartmental ligand scaffold HL with two β-diketiminato binding sites spanned by a pyrazolate bridge gave a series of dinuclear nickel(II) dihydride complexes M[LNi₂(H)₂], M = Na (Na·2) and K (K·2), which were isolated after reacting the precursor complex [LNi₂(μ-Br)] (1) with MHBEt₃ (M = Na and K). Crystallographic characterization showed the two hydride ligands to be directed into the bimetallic pocket, closely interacting with the alkali metal cation. Treatment of K·2 with dibenzo(18-crown-6) led to the separated ion pair [LNi₂(H)₂][K(DB18C6)] (2[K(DB18C6)]). Reaction of Na·2 or K·2 with D₂ was investigated by a suite of ¹H and ²H NMR experiments, revealing an unusual pairwise H₂/D₂ exchange process that synchronously involves both Ni-H moieties without H/D scrambling. A mechanistic picture was provided by DFT calculations which suggested facile recombination of the two terminal hydrides within the bimetallic cleft, with a moderate enthalpic barrier of ∼62 kJ/mol, to give H₂ and an antiferromagnetically coupled [LNi^I₂]^- species. This was confirmed by SQUID monitoring during H₂ release from solid 2[K(DB18C6)]. Interaction with the Lewis acid cation (Na⁺ or K⁺) significantly stabilizes the dihydride core. Kinetic data for the M[L(Ni-H)₂] → H₂ transition derived from 2D ¹H EXSY spectra confirmed first-order dependence of H₂ release on M·2 concentration and a strong effect of the alkali metal cation M⁺. Treating [LNi₂(D)₂]^- with phenylacetylene led to D₂ and dinickel(II) complex 3^- with a twice reduced styrene-1,2-diyl bridging unit in the bimetallic pocket. Complexes [LNi^II₂(H)₂]^- having two adjacent terminal hydrides thus represent a masked version of a highly reactive dinickel(I) core. Storing two reducing equivalents in adjacent metal hydrides that evolve H₂ upon substrate binding is reminiscent of the proposed N₂ binding step at the FeMo cofactor of nitrogenase, suggesting the use of the present bimetallic scaffold for reductive bioinspired activation of a range of inert small molecules.

Stable, Yet Highly Reactive Nonclassical Iron(II) Polyhydride Pincer Complexes: Z-Selective Dimerization and Hydroboration of Terminal Alkynes

The synthesis, characterization, and catalytic activity of nonclassical iron(II) polyhydride complexes containing tridentate PNP pincer-type ligands is described. These compounds of the general formula [Fe(PNP)(H)₂(η²-H₂)] exhibit remarkable reactivity toward terminal alkynes. They efficiently promote the catalytic dimerization of aryl acetylenes giving the corresponding conjugated 1,3-enynes in excellent yields with low catalyst loadings. When the reaction is carried out in the presence of pinacolborane, vinyl boronates are obtained. Both reactions take place under mild conditions and are highly chemo-, regio-, and stereoselective with up to 99% Z-selectivity.