Activation of dihydrogen and coordination of molecular H2 on transition metals

A Lead-μ2-Tetrylide Complex with Osmium(IV) Terminal Components

A bare lead atom is a σ-donor ligand capable of linearly bonding and stabilizing two units of a classical polyhydride complex, with a high-valent metal center. As a proof of concept, we have prepared and characterized the μ2-tetrylide complex (PiPr3)2H4Os═Pb═OsH4(PiPr3)2 in the reaction of OsH6(PiPr3)2 with Pb{N(SiMe3)2}2. Although the Pb-Os bonds exhibit electrostatic interaction, the main orbital interactions result from two dative σ bonds from the lead atom to the osmium centers. The latter also provide much weaker π-backdonations.

Hydrogenolysis of Cationic Half-Sandwich Zinc Complexes Containing a Chelating Amine: Facile Cleavage of Zinc-Carbon Bond by Dihydrogen to Give Zinc Hydride Cations

Cationic half-sandwich zinc complexes containing chelating amines [Cp*Zn(Ln)][BAr4 F] (2 a, Cp*=η3-C5Me5, Ln=N,N,N',N'-tetramethylethylenediamine, TMEDA; 2 b, Ln=N,N,N',N'-tetraethylethylenediamine, TEEDA; 2 c, Cp*=η1-C5Me5, Ln=N,N,N',N'',N''-pentamethyldiethylenetriamine, PMDTA; Ar4 F=(3,5-(CF3)2C6H3)4) reacted with dihydrogen (ca. 2 bar) in THF at 80 °C to give molecular zinc hydride cations [(Ln)ZnH(thf)m][BAr4 F] (3 a,b, m=1; 3 c, m=0) previously reported along with Cp*H. Pseudo first-order kinetics with respect to the concentration of 2 b suggests heterolytic cleavage of dihydrogen by the Zn-Cp* bond, reminiscent of σ-bond metathesis. Hydrogenolysis of the zinc cation 2 b in the presence of benzophenone gave the zinc alkoxide [(TEEDA)Zn(OCHPh2)(thf)][BAr4 F] (5 b). Cation 2 b was shown to catalytically hydrogenate N-benzylideneaniline. The PMDTA complex 2 c underwent C-H bond activation in acetonitrile to give a dinuclear μ-κC,κN-cyanomethyl zinc complex [(PMDTA)Zn(CH2CN)]2[BAr4 F]2 (6 c).

Low oxidation state and hydrido group 2 complexes: synthesis and applications in the activation of gaseous substrates

Numerous industrial processes utilise gaseous chemical feedstocks to produce useful chemical products. Atmospheric and other small molecule gases, including anthropogenic waste products (e.g. carbon dioxide), can be viewed as sustainable building blocks to access value-added chemical commodities and materials. While transition metal complexes have been well documented in the reduction and transformation of these substrates, molecular complexes of the terrestrially abundant alkaline earth metals have also demonstrated promise with remarkable reactivity reported towards an array of industrially relevant gases over the past two decades. This review covers low oxidation state and hydrido group 2 complexes and their role in the reduction and transformation of a selection of important gaseous substrates towards value-added chemical products.

Dihydrogen Cleavage by a Zinc-Zinc Bond of a Heteroleptic Dizinc(I) Cation

Oxidative addition of dihydrogen across a metal-metal bond to form reactive metal hydrides in homogeneous catalysis is known for transition metals but not for zinc(I)-zinc(I) bond as found in Carmona's eponymous dizinconene [Zn2Cp*2] (Cp* = η5-C5Me5). Dihydrogen reacted with the heteroleptic zinc(I)-zinc(I) bonded cation [(L2)Zn-ZnCp*][BAr4F] (L2 = TMEDA, N,N,N',N'-tetramethylethylenediamine, TEEDA, N,N,N',N'-tetraethylethylenediamine; ArF = 3,5-(CF3)2C6H3) under 2 bar at 80 °C to give the zinc(II) hydride cation [(L2)ZnH(thf)][BAr4F] along with zinc metal and Cp*H derived from the intermediate [Cp*ZnH]. DFT calculations show that the cleavage of dihydrogen occurs through a highly unsymmetrical transition state. Mechanistic studies agree with a heterolytic cleavage of dihydrogen as a result of the cationic charge and unsymmetrical ligand coordination. To explore the existence of zinc(I) hydride, thermally unstable hydridotriphenylborate complexes of zinc(I) [(L2)Zn(HBPh3)-ZnCp*] (L2 = TMEDA, TEEDA; TMPDA, N,N,N',N'-tetramethyl-1,3-propylenediamine) have been prepared by salt metathesis and were shown to undergo fast exchange with both BPh3 and [HBPh3]-.

Metal- and ligand-substitution-induced changes in the kinetics and thermodynamics of hydrogen activation and hydricity in a dinuclear metal complex

Catalytic function in organometallic complexes is achieved by carefully selecting their central metals and ligands. In this study, the effects of a metal and a ligand on the kinetics and thermodynamics of hydrogen activation, hydricity degree of the hydride complex, and susceptibility to electronic oxidation in bioinspired NiFe complexes, [NiIIX FeII(Cl)(CO)Y]+ ([NiFe(Cl)(CO)]+; X = N,N'-diethyl-3,7-diazanonane-1,9-dithiolato and Y = 1,2-bis(diphenylphosphino)ethane), were investigated. The density functional theory calculations revealed that the following order thermodynamically favored hydrogen activation: [NiFe(CO)]2+ > [NiRu(CO)]2+ > [NiFe(CNMe)]2+ ∼ [PdRu(CO)]2+ ∼ [PdFe(CO)]2+ ≫ [NiFe(NCS)]+. Moreover, the reverse order thermodynamically favored the hydricity degree.

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.

Spectroscopic and Computational Evidence of Uranium Dihydrogen Complexes

Dihydrogen complexation, a phenomenon with robust precedent in the transition metal series, is spectroscopically detected for a uranium(III) complex and thereby extended for the first time to the 5f series. The vacant coordination site and low valence of (C5H4SiMe3)3U prove to be key to the reversible formation of (C5H4SiMe3)3U-H2 (complex 1), and the paramagnetism of the f3 center facilitates the detection of complex 1 by NMR spectroscopy. Density functional theory calculations reveal that the delocalization of the 5f electron density from (C5H4SiMe3)3U onto the side-on dihydrogen ligand is crucial to complex formation, an unusual bonding situation for an actinide acid-base complex. The spectroscopic and computational results are compared to those reported for lanthanide metallocenes to yield insight into the nature of─and future possibilities for─f-element dihydrogen complexation.

Spin-Crossing in the (Z)-Selective Alkyne Semihydrogenation Mechanism Catalyzed by Mo3S4 Clusters: A Density Functional Theory Exploration

Semihydrogenation of internal alkynes catalyzed by the air-stable imidazolyl amino [Mo3S4Cl3(ImNH2)3]+ cluster selectively affords the (Z)-alkene under soft conditions in excellent yields. Experimental results suggest a sulfur-based mechanism with the formation of a dithiolene adduct through interaction of the alkyne with the bridging sulfur atoms. However, computational studies indicate that this mechanism is unable to explain the experimental outcome: mild reaction conditions, excellent selectivity toward the (Z)-isomer, and complete deuteration of the vinylic positions in the presence of CD3OD and CH3OD. An alternative mechanism that explains the experimental results is proposed. The reaction begins with the hydrogenation of two of the Mo3(μ3-S)(μ-S)3 bridging sulfurs to yield a bis(hydrosulfide) intermediate that performs two sequential hydrogen atom transfers (HAT) from the S-H groups to the alkyne. The first HAT occurs with a spin change from singlet to triplet. After the second HAT, the singlet state is recovered. Although the dithiolene adduct is more stable than the hydrosulfide species, the large energy required for the subsequent H2 addition makes the system evolve via the second alternative pathway to selectively render the (Z)-alkene with a lower overall activation barrier.

New insights into H2 activation by intramolecular frustrated Lewis pairs based on aminoboranes: the local electrophilicity index of boron as a suitable indicator to tune the reversibility of the process

A large set of intramolecular aminoborane-based FLPs was studied employing density functional theory in the H2 activation process to analyze how the acidity and basicity of boron and nitrogen atoms, respectively, affect the reversibility of the process. Three different linkers were employed, keeping the C-C nature in the connection between both Lewis centers: -CH2-CH2-, -CH[double bond, length as m-dash]CH-, and -C6H4-. The results show that significant differences in the Gibbs free energy of the process are found by considering all the combinations of substituents. Of the 75 systems studied, only 9 showed the ability to carry out the process reversibly (ΔGH2 in the range of -3.5 to 2.0 kcal mol-1), where combinations of alkyl/aryl or aryl/alkyl in boron/nitrogen generate systems capable of reaching reversibility. If the alkyl/alkyl or aryl/aryl combination is employed, highly exergonic (non-reversible H2 activation) and endergonic (unfeasible H2 activation) reactions are found, respectively. No appreciable differences in the linker were found, allowing us to continue the analysis with the most entropically favorable linker, the -C6H4- linker. From this, 25 different FLP systems of type 2-[bis(X)boryl]-(Y)aniline (X: H, CF3, C6F5, PFtB, FMes and Y: H, CH3, t-but, Ph, Mes) can be formed. By analyzing the electronic properties of each system, we have found that the condensed-to-boron electrophilicity index ωB+ is inversely related to the ΔGH2. Interestingly, two relationships were found; the first is for alkyl groups (Y: CH3 and t-but) and the second for aryl groups (Y: H, Ph, and Mes), which is intimately related to the proton affinity of each aniline. In addition, it is quite interesting when the frustration degree, given by B⋯N distance dB-N, is brought together with ωB+, since the quotient has unit energy/length corresponding to unit force; concomitantly, a measure of the FLP strength in H-H bond activation can be defined. With this finding, a rational design of this kind of FLP can be performed by analyzing the acidity of boron through condensed-to-boron electrophilicity and knowing the nature of the substituent of nitrogen according to whether the Y is alkyl or aryl, optimizing the H2 reversible activation in a rational way, which is crucial to improve the catalytic performance.

H-atom site exchange in an iridium trans-dihydrogen/hydride complex, trans-[Ir(H)(η2-H2)(iPr)4(POCOP)(DMAP)]. Dalton Trans 2023;52:13858-13863. [PMID: 37743752 DOI: 10.1039/d3dt02366d] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Treatment of trans-[Ir(H)(N)2(iPr)4(POCOP)(DMAP)][BAr4f] (2) with H2 (1 bar) under ambient conditions (298 K) results in the formation of a trans-[Ir(H)(η2-H2)(iPr)4(POCOP)(DMAP)][BAr4f] (3) complex. Complex 3 exhibits H-atom site exchange between the bound H2 and the hydride ligands which are mutually trans to one another. A plausible mechanism of this exchange involves metal-ligand cooperativity as studied by computations.

Probing the binding and activation of small molecules by gas-phase transition metal clusters via IR spectroscopy

Isolated transition metal clusters have been established as useful models for extended metal surfaces or deposited metal particles, to improve the understanding of their surface chemistry and of catalytic reactions. For this objective, an important milestone has been the development of experimental methods for the size-specific structural characterization of clusters and cluster complexes in the gas phase. This review focusses on the characterization of molecular ligands, their binding and activation by small transition metal clusters, using cluster-size specific infrared action spectroscopy. A comprehensive overview and a critical discussion of the experimental data available to date is provided, reaching from the initial results obtained using line-tuneable CO₂ lasers to present-day studies applying infrared free electron lasers as well as other intense and broadly tuneable IR laser sources.

Microwave-assisted C-C bond formation of diarylacetylenes and aromatic hydrocarbons on carbon beads under continuous-flow conditions

The synthesis of polycyclic aromatic compounds generally requires stoichiometric oxidants or homogeneous metal catalysts, however, the risk of contamination of inorganic residues can affect their properties. Here we present a microwave (MW)-assisted platinum on beaded activated carbon (Pt/CB)-catalyzed C-C bond formation of diarylacetylenes and aromatic hydrocarbons under continuous-flow conditions. Various fused aromatic compounds were continuously synthesized via dehydrogenative C(sp²)-C(sp²) and C(sp²)-C(sp³) bond formation with yields of up to 87% without the use of oxidants and bases. An activated, local reaction site on Pt/CB in the flow reaction channel reaching temperatures of more than three hundred degrees Celsius was generated in the catalyst cartridge by selective microwave absorption in CB with an absorption efficiency of > 90%. Mechanistic experiments of the transformation reaction indicated that a constant hydrogen gas supply was essential for activating Pt. This is an ideal reaction with minimal input energy and no waste production.

Synergistic Catalysis in Heterobimetallic Complexes for Homogeneous Carbon Dioxide Hydrogenation

Two heterobimetallic Mo,M’ complexes (M’ = IrIII, RhIII) were synthesized and fully characterized. Their catalytic activity in homogeneous carbon dioxide hydrogenation to formate was studied. A pronounced synergistic effect between the two metals was found, most notably between Mo and Ir, leading to a fourfold increase in activity compared with a binary mixture of the two monometallic counterparts. This synergism can be attributed to spatial proximity of the two metals rather than electronic interactions. To further understand the nature of this interaction, the mechanism of the CO2 hydrogenation to formate by a monometallic IrIII catalyst was studied using computational and spectroscopic methods. The resting state of the reaction was found to be the metal-base adduct, whereas the rate-determining step is the inner-sphere hydride transfer to CO2. Based on these findings, the synergism in the heterobimetallic complex is beneficial in this key step, most likely by further activating the CO2.

Vibrational spectroscopy of Cu+(H2)4: about anharmonicity and fluxionality

The vibrational spectra of the copper(I) cation-dihydrogen complexes Cu⁺(H₂)₄, Cu⁺(D₂)₄ and Cu⁺(D₂)₃H₂ are studied using cryogenic ion trap vibrational spectroscopy in combination with quantum chemical calculations. The infrared photodissociation (IRPD) spectra (2500-7300 cm^-1) are assigned based on a comparison to IR spectra calculated using vibrational second-order perturbation theory (VPT2). The IRPD spectra exhibit ≈60 cm^-1 broad bands that lack rotational resolution, indicative of rather floppy complexes even at an ion trap temperature of 10 K. The observed vibrational features are assigned to the excitations of dihydrogen stretching fundamentals, combination bands of these fundamentals with low energy excitations as well as overtone excitations of a minimum-energy structure with C_s symmetry. The three distinct dihydrogen positions present in the structure can interconvert via pseudorotations with energy barriers less than 10 cm^-1, far below the zero-point vibrational energy. Ab initio Born-Oppenheimer molecular dynamics (BOMD) simulations confirm the fluxional behavior of these complexes and yield an upper limit for the timeframe of the pseudorotation on the order of 10 ps. For Cu⁺(D₂)₃H₂, the H₂ and D₂ loss channels yield different IRPD spectra indicating non-ergodic behavior.

An Anionic Dinuclear Ruthenium Dihydrogen Complex of Relevance for Alkyne gem ‐Hydrogenation

During an investigation into the fate of ruthenium precatalysts used for light‐driven alkyne gem‐hydrogenation reactions with formation of Grubbs‐type ruthenium catalysts, it was found that the reaction of [(IPr)(η⁶‐cymene)RuCl₂] with H₂ under UV‐irradiation affords an anionic dinuclear σ‐dihydrogen complex, which is thermally surprisingly robust. Not only are anionic σ‐complexes in general exceedingly rare, but the newly formed species seems to be the first example lacking any structural attributes able to counterbalance the negative charge and, in so doing, prevent oxidative insertion of the metal centers into the ligated H₂ from occurring.

Synthesis and Reactivity of Triangular Heterometallic Complexes Containing Zn-Zn Bond

This work provides a facile access to a series of triangular [Zn₂M] (M = group 10 and 11 metals) clusters. Treatment of Zn-Zn-bonded compounds [LZn-ZnL] (L = CH₃C(2,6-ⁱPr₂C₆H₃N)CHC(CH₃)(NCH₂CH₂PR₂); R = Ph, ⁱPr) with zero-valent transition-metal reagents selectively afforded the corresponding triangular clusters [Zn₂M], where M = Ni(0), Pd(0), and Pt(0). Notably, the isoelectronic triangular clusters [Zn₂M]⁺, where M = Ag(I) and Cu(I), could also be obtained by reactions of [LZn-ZnL] with AgOTf and CuOTf, respectively. The [Zn₂Ag]⁺ complex containing elusive Zn-Ag bonds was investigated by density functional theory analysis, showing a 3c-2e bonding feature in the metallic ring. The electrochemical behaviors of [Zn₂M] complexes were examined and revealed the donation of electron density from the Zn-Zn σ-bond to the metal centers. Reaction of the [Zn₂Ni] complex with isocyanide gave heterometallic species by coordination of isocyanide to the nickel center, keeping the trimetallic ring core structure intact. In contrast, the Zn-Zn bond was rapidly cleaved upon treatment of the [Zn₂Ni] complex with dihydrogen or phenyl acetylene, generating the hydride- or acetylide-bridged heterotrimetallic complex.

Homogeneous catalysis with polyhydride complexes

This review analyzes the role of transition metal polyhydrides as homogeneous catalysts for organic reactions. Discussed reactions involve nearly every main organic functional group.

On the energetics of binding and hydride exchange in the RuH2(H2)2[P(C5H9)3)]2 complex as revealed by inelastic neutron scattering and DFT studies

Low temperature quantum rotation of dihydrogen in RuH2(H2)2[P(C5H9)3)]2 switched to a facile hydride exchange above 150 K.

Activation of D2 by Neodymium Cation (Nd+): Bond Energy of NdH+ and Mechanistic Insights through Experimental and Theoretical Studies

The kinetic-energy-dependent cross section for the reaction of Nd⁺ with D₂ was studied by using a guided ion beam tandem mass spectrometer. The formation of NdD⁺ is endothermic, and analysis of the reaction cross section gave an NdH⁺ 0 K bond dissociation energy (BDE) of 1.99 ± 0.06 eV. Theoretical calculations for the NdH⁺ BDE were performed for comparison with the experimental thermochemistry and generally gave accurate results. Additionally, relaxed potential energy surfaces for NdH₂⁺ were performed, and no strongly bound dihydride intermediate was located. The Nd⁺ + D₂ reactivity and BDE of NdH⁺ are compared with analogous results for the lanthanide cations La⁺, Ce⁺, Pr⁺, Sm⁺, Gd⁺, and Lu⁺ to establish periodic trends and insight into the role of the electronic configurations on this reactivity and the lanthanide hydride cation bond strengths.

Systematic Design of a Frustrated Lewis Pair Containing Methyleneborane and Carbene for Dinitrogen Activation

Activation of atmospherically abundant dinitrogen (N₂) by metal-free species under mild reaction conditions has been one of the most challenging areas in chemistry for decades. Very recent but limited progress in N₂ activation by boron species, including two-coordinated borylene and methyleneborane and three-coordinated borole and borane, has been made toward metal-free N₂ activation. Here, we systematically probe an experimentally viable frustrated Lewis pair (FLP) containing two moieties (methyleneborane and carbene) for N₂ activation via density functional theory (DFT) calculations, which has proven to be an efficient approach for N₂ activation in a thermodynamically and kinetically favorable manner. Aromaticity is found to play a crucial role in stabilization of the product. This study could be a valuable alternative for the development of metal-free N₂ activation chemistry, highlighting great potential of FLP for N₂ activation and functionalization.

Electronic Communication in Binuclear Osmium- and Iridium-Polyhydrides

Reactions of polyhydrides OsH₆(PⁱPr₃)₂ (1) and IrH₅(PⁱPr₃)₂ (2) with rollover cyclometalated hydride complexes have been investigated in order to explore the influence of a metal center on the MH_n unit of the other in mixed valence binuclear polyhydrides. Hexahydride 1 activates an ortho-CH bond of the heterocyclic moiety of the trihydride metal–ligand compounds OsH₃{κ²-C,N-[C₅RH₂N-py]}(PⁱPr₃)₂ (R = H (3), Me (4), Ph (5)). Reactions of 3 and 4 lead to the hexahydrides (PⁱPr₃)₂H₃Os{μ-[κ²-C,N-[C₅RH₂N-C₅H₃N]-N,C-κ²]}OsH₃(PⁱPr₃)₂ (R = H (6), Me (7)), whereas 5 gives the pentahydride (PⁱPr₃)₂H₃Os{μ-[κ²-C,N-[C₅H₃N-C₅(C₆H₄)H₂N]-C,N,C-κ³]}OsH₂(PⁱPr₃)₂ (8). Pentahydride 2 promotes C—H bond activation of 3 and the iridium-dihydride IrH₂{κ²-C,N-[C₅H₃N-py]}(PⁱPr₃)₂ (9) to afford the heterobinuclear pentahydride (PⁱPr₃)₂H₃Os{μ-[κ²-C,N-[C₅H₃N-C₅H₃N]-N,C-κ²]}IrH₂(PⁱPr₃)₂ (10) and the homobinuclear tetrahydride (PⁱPr₃)₂H₂Ir{μ-[κ²-C,N-[C₅H₃N-C₅H₃N]-N,C-κ²]}IrH₂(PⁱPr₃)₂ (11), respectively. Complexes 6–8 and 11 display HOMO delocalization throughout the metal–heterocycle-metal skeleton. Their sequential oxidation generates mono- and diradicals, which exhibit intervalence charge transfer transitions. This notable ability allows the tuning of the strength of the hydrogen–hydrogen and metal–hydrogen interactions within the MH_n units.

The influence of a metal center on the MH_n unit of the other in binuclear osmium- and iridium-polyhydride complexes bearing rollover cyclometalated bipyridines as a bridging ligand has been studied.

High-Frequency Fe-H and Fe-H2 Modes in a trans-Fe(η2-H2)(H) Complex: A Speed Record for Nuclear Resonance Vibrational Spectroscopy

Nuclear resonance vibrational spectroscopy (NRVS) and density functional theory (DFT) are complementary tools for studying the vibrational and geometric structures of specific isotopically labeled molecular systems. Here we apply NRVS and DFT to characterize the trans-[57Fe(η2-H2)(H)(dppe)2][BPh4] [dppe = 1,2-bis(diphenylphosphino)ethane] complex. Heretofore, most NRVS observations have centered on the spectral region below 1000 cm-1, where the 57Fe signal is strongest. In this work, we show that state-of-the-art synchrotron facilities can extend the observable region to 2000 cm-1 and likely beyond, in measurements that require less than 1 day. The 57Fe-H stretch was revealed at 1915 cm-1, along with the asymmetric 57Fe-H2 stretch at 1774 cm-1. For a small fraction of the H2-dissociated product, the 57Fe-H stretch was detected at 1956 cm-1. The unique sensitivity to 57Fe motion and the isolated nature of the Fe-H/H2 stretching modes enabled NRVS to quantitatively analyze the sample composition.

Cerium Cation (Ce+) Reactions with H2, D2, and HD: CeH+ Bond Energy and Mechanistic Insights from Guided Ion Beam and Theoretical Studies

Reactions of the atomic lanthanide cerium cation (Ce⁺) with H₂, D₂, and HD were studied by using guided ion beam tandem mass spectrometry. Analysis of the kinetic-energy-dependent endothermic reactions to form CeH⁺ (CeD⁺) led to a 0 K bond dissociation energy (BDE) for CeH⁺ of 2.19 ± 0.09 eV. Theoretical calculations for CeH⁺ were performed at the B3LYP, BHLYP, and PBE0 levels of theory and overestimate the experimental BDE. In contrast, extrapolation to the complete basis set limit using coupled-cluster with single, double, and perturbative triple excitations, CCSD(T), gave a value (2.33 eV) in reasonable agreement with the experimental BDE. The branching ratio of the CeH⁺ and CeD⁺ products in the HD reaction suggests that the reaction occurs via a statistical mechanism involving a long-lived intermediate. Relaxed potential energy surfaces for CeH₂⁺ were computed and are consistent with the availability of such an intermediate, but the crossing point between quartet and doublet surfaces helps explain the inefficiency of the association reaction observed in the literature. The reactivity and CeH⁺ BDE are compared with previous results for group 4 transition metal cations (Ti⁺, Zr⁺, and Hf⁺), other lanthanides (La⁺, Sm⁺, Gd⁺, and Lu⁺), and the isovalent actinide Th⁺. Periodic trends and insight into the role of the electronic configuration on metal-hydride bond strength are discussed.

H2 activation by zirconaziridinium ions: σ-bond metathesis versus frustrated Lewis pair reactivity

Zirconaziridinium ions [Cp₂Zr(η²-CH₂NR₂)]⁺ can potentially activate H₂ by two routes: σ-bond metathesis or FLP reactivity. We show here that Zr-C hydrogenolysis by σ-bond metathesis precedes and enables subsequent heterolytic H₂ cleavage through FLP pathways. DFT calculations show the involvement of transition states with approximately linear NHH and bent ZrHH arrangements without any direct Zr-amine interaction.

Catalysis, kinetics and mechanisms of organo-iridium enantioselective hydrogenation-reduction

The synthesis of chiral molecules is of great importance to the pharmaceutical, agrochemical, flavour and fragrance industries.

Ab initio accelerated molecular dynamics study of the hydride ligands in the ruthenium complex: Ru(H2)2H2(P(C5H9)3)2

The dihydrogen complex Ru(H₂)₂H₂(P(C₅H₉)₃)₂ has been investigated, via ab initio accelerated molecular dynamics, to elucidate the H ligands dynamics and possible reaction paths for H₂/H exchange. We have characterized the free energy landscape associated with the H atoms positional exchange around the Ru centre. From the free energy landscape, we have been able to estimate a barrier of 6 kcal mol^-1 for the H₂/H exchange process. We have also observed a trihydrogen intermediate as a passing state along some of the possible reaction pathways.

The Role of Proton Shuttles in the Reversible Activation of Hydrogen via Metal-Ligand Cooperation

The reversible activation of H₂ via a pathway involving metal-ligand cooperation (MLC) is proposed to be important in many transition metal catalyzed hydrogenation and dehydrogenation reactions. Nevertheless, there is a paucity of experimental information probing the mechanism of this transformation. Here, we present an in-depth kinetic study of the 1,2-addition of H₂ via an MLC pathway to the widely used iron catalyst [(^iPrPNP)FeH(CO)] (1) (^iPrPNP = N(CH₂CH₂PⁱPr₂)₂^-). We report one of the first experimental demonstrations of an enhancement in rate for the activation of H₂ using protic additives, which operate as "proton shuttles". Our results indicate that proton shuttles need to be able to both simultaneously donate and accept a proton, and the best shuttles are molecules that are strong hydrogen bond donors but sufficiently weak acids to avoid deleterious protonation of the transition metal complex. Additionally, comparison of the rate of H₂ activation via an MLC pathway between 1 and two widely used ruthenium catalysts enables more general conclusions about the role of the metal, ancillary ligand, and proton shuttles in H₂ activation. The results of this study provide guidance about the design of catalysts and additives to promote H₂ activation via an MLC pathway.

σ-H-H, σ-C-H, and σ-Si-H Bond Activation Catalyzed by Metal Nanoparticles

Activation of H-H, Si-H, and C-H bonds through σ-bond coordination has grown in the past 30 years from a scientific curiosity to an important tool in the functionalization of hydrocarbons. Several mechanisms were discovered via which the initially σ-bonded substrate could be converted: oxidative addition, heterolytic cleavage, σ-bond metathesis, electrophilic attack, etc. The use of metal nanoparticles (NPs) in this area is a more recent development, but obviously nanoparticles offer a much richer basis than classical homogeneous and heterogeneous catalysts for tuning reactivity for such a demanding process as C-H functionalization. Here, we will review the surface chemistry of nanoparticles and catalytic reactions occurring in the liquid phase, catalyzed by either colloidal or supported metal NPs. We consider nanoparticles prepared in solution, which are stabilized and tuned by polymers, ligands, and supports. The question we have addressed concerns the differences and similarities between molecular complexes and metal NPs in their reactivity toward σ-bond activation and functionalization.

Formation of Zn-Zn and Zn-Pd Bonded Complexes by Reactions of Terminal Zinc Hydrides with Pd(II) Species

Divalent palladium-induced homocoupling of terminal zinc hydrides to zinc-zinc bonded complexes was achieved herein. Reactions of zinc hydrides [LZnH] (L = CH₃C(2,6-ⁱPr₂C₆H₃N)CHC(CH₃)(N(CH₂)_nCH₂PPh₂); 1a: n = 1; 1b: n = 2) with 0.5 equiv of allyl(cyclopentadienyl)palladium(II) afforded heterotrinuclear [Zn₂Pd] complexes 3 containing direct Zn-Zn and Zn-Pd bonds, with concomitant elimination of propylene and cyclopentadiene. Complexes 3 were also accessed by the reactions of zinc hydrides 1 with allylpalladium(II) chloride with release of propylene and hydrogen chloride. Treatment of zinc hydrides 1 with 1 equiv of allyl(cyclopentadienyl)palladium(II) gave Zn-Pd bonded complex 5 by elimination of propylene, which can be transformed into heterotrinuclear complex 3 by further reaction with one additional molar equivalent of zinc hydrides. Heterobimetallic Zn-Pd complex 5b was found to be an effective catalyst in the hydrosilylation of benzaldehyde and its derivatives. Reaction of 5b with silane reagent Ph₂SiH₂ produced [Pd₂Si₂H₂] complex 8 with cleavage of the Pd-Zn bond, which served as an initiating species in the catalytic reaction. Complexes 4b, 5, and 8 in this study were characterized by X-ray diffraction.

Thermal Activation of CH4 and H2 as Mediated by the Ruthenium Oxide Cluster Ions [RuOx ]+ (x=1-3): On the Influence of Oxidation States

Thermal gas-phase reactions of the ruthenium-oxide clusters [RuO_x ]⁺ (x=1-3) with methane and dihydrogen have been explored by using FT-ICR mass spectrometry complemented by high-level quantum chemical calculations. For methane activation, as compared to the previously studied [RuO]⁺ /CH₄ couple, the higher oxidized Ru systems give rise to completely different product distributions. [RuO₂ ]⁺ brings about the generations of [Ru,O,C,H₂ ]⁺ /H₂ O, [Ru,O,C]⁺ /H₂ /H₂ O, and [Ru,O,H₂ ]⁺ /CH₂ O, whereas [RuO₃ ]⁺ exhibits a higher selectivity and efficiency in producing formaldehyde and syngas (CO+H₂ ). Regarding the reactions with H₂ , as compared to CH₄ , both [RuO]⁺ and [RuO₂ ]⁺ react similarly inefficiently with oxygen-atom transfer being the main reaction channel; in contrast, [RuO₃ ]⁺ is inert toward dihydrogen. Theoretical analysis reveals that the reduction of the metal center drives the overall oxidation of methane, whereas the back-bonding orbital interactions between the cluster ions and dihydrogen control the H-H bond activation. Furthermore, the reactivity patterns of [RuO_x ]⁺ (x=1-3) with CH₄ and H₂ have been compared with the previously reported results of Group 8 analogues [OsO_x ]⁺ /CH₄ /H₂ (x=1-3) and the [FeO]⁺ /H₂ system. The electronic origins for their distinctly different reaction behaviors have been addressed.

Small molecule activation by boron-containing heterocycles

Most of the chemical and biological processes involving the fixation and transformation of small molecules have long been exclusive for metal complexes. Meanwhile, the last decades have seen a significant advance in main group chemistry that mimics transition-metal complexes, among which various boron-containing systems have been successful in mediating the small molecule activation. In this review, we focus on boron-containing heterocycles enabling the activation of σ- and π-bonds in small molecules, in conjunction with the proposed mechanisms.

Transition metal-induced dehydrogenative coupling of zinc hydrides

Transition metal-induced dehydrogenative homocoupling of zinc(ii) hydrides to a zinc–zinc bonded complex has been achieved.

Samarium cation (Sm+) reactions with H2, D2, and HD: SmH+ bond energy and mechanistic insights from guided ion beam and theoretical studies

Guided ion beam tandem mass spectrometry is used to study the reaction of the lanthanide samarium cation (Sm⁺) with H₂ and its isotopologues (HD and D₂) as a function of collision energy. Modeling the resulting energy dependent product ion cross sections from these endothermic reactions yields 2.03 ± 0.06 eV (two standard deviations) for the 0 K bond dissociation energy of SmH⁺. Quantum chemical calculations are performed to determine stabilities of the ground and low-energy states of SmH⁺ for comparison with the experimentally measured thermochemistry. The calculations generally overestimate the SmH⁺ bond energy, but a better agreement between experiment and theory is achieved after correcting for spin-orbit energy contributions, with coupled-cluster with single, double and perturbative triple excitations/complete basis set [CCSD(T)/CBS] results reproducing the experiment well. In the HD reaction, the SmH⁺ product is observed to be favored over the SmD⁺ by about a factor of three, indicating that the reaction proceeds via a direct mechanism with short-lived intermediates. This is consistent with quantum chemical calculations of relaxed potential energy surface scans of SmH₂ ⁺, which show that there is no strongly bound dihydride intermediate. The reactivity and hydride bond energy of Sm⁺, which has a valence electron configuration typical of most lanthanides, are compared with previous results for the lanthanide cations La⁺, Gd⁺, and Lu⁺, which exhibit configurations more closely related to the group 3 metal cations, Sc⁺ and Y⁺. Periodic trends across the lanthanide series and insights into the role of the electronic configurations on hydride bond strength and reactivity with H₂ are discussed.