In situ formation of cocatalytic sites boosts single-atom catalysts for nitrogen oxide reduction.

Nitrogen oxide (NOx) pollution presents a severe threat to the environment and human health. Catalytic reduction of NOx with H2 using single-atom catalysts poses considerable potential in the remediation of air pollution; however, the unfavorable process of H2 dissociation limits its practical application. Herein, we report that the in situ formation of PtTi cocatalytic sites (which are stabilized by Pt-Ti bonds) over Pt1/TiO2 significantly increases NOx conversion by reducing the energy barrier of H2 activation. We demonstrate that two H atoms of H2 molecule are absorbed by adjacent Pt atoms in Pt-O and Pt-Ti, respectively, which can promote the cleave of H-H bonds. Besides, PtTi sites facilitate the adsorption of NO molecules and further lower the activation barrier of the whole de-NOx reaction. Extending the concept to Pt1/Nb2O5 and Pd1/TiO2 systems also sees enhanced catalytic activities, demonstrating that engineering the cocatalytic sites can be a general strategy for the design of high-efficiency catalysts that can benefit environmental sustainability.

CO2 and H2 Activation on Zinc-Doped Copper Clusters.

Here we systematically investigate the CO2 and H2 activation and dissociation on small Cun Zn0/+ (n=3-6) clusters using Density Functional Theory. We show that Cu6 Zn is a superatom, displaying an increased HOMO-LUMO gap and is inert towards CO2 or H2 activation or dissociation. While other neutral clusters weakly activate CO2 , the cationic clusters preferentially bind the CO2 in monodentate nonactivated way. Notably, Cu4 Zn allows for the dissociation of activated CO2 , whereas larger clusters destabilize all activated CO2 binding modes. Conversely, H2 dissociation is favored on all clusters examined, except for Cu6 Zn. Cu3 Zn+ and Cu4 Zn, favor the formation of formate through the H2 dissociation pathway rather than CO2 dissociation. These findings suggest the potential of these clusters as synthetic targets and underscore their significance in the realm of CO2 hydrogenation.

Unveiling the Promoting Mechanism of H2 Activation on CuFeOx Catalyst for Low-Temperature CO Oxidation.

The effect of H2 activation on the performance of CuFeOx catalyst for low-temperature CO oxidation was investigated. The characterizations of XRD, XPS, H2-TPR, O2-TPD, and in situ DRIFTS were employed to establish the relationship between physicochemical property and catalytic activity. The results showed that the CuFeOx catalyst activated with H2 at 100 °C displayed higher performance, which achieved 99.6% CO conversion at 175 °C. In addition, the H2 activation promoted the generation of Fe2+ species, and more oxygen vacancy could be formation with higher concentration of Oα species, which improved the migration rate of oxygen species in the reaction process. Furthermore, the reducibility of the catalyst was enhanced significantly, which increased the low-temperature activity. Moreover, the in situ DRIFTS experiments revealed that the reaction pathway of CO oxidation followed MvK mechanism at low temperature (<175 °C), and both MvK and L-H mechanism was involved at high temperature. The Cu+-CO and carbonate species were the main reactive intermediates, and the H2 activation increased the concentration of Cu+ species and accelerated the decomposition carbonate species, thus improving the catalytic performance effectively.

Hydrogenolysis of haloboranes: from synthesis of hydroboranes to formal hydroboration reactions.

Hydroboranes are versatile reagents in synthetic chemistry, but their synthesis relies on energy-intensive processes. Herein, we report a new method for the preparation of hydroboranes from hydrogen and the corresponding haloboranes. Triethylamine (NEt3) form with dialkylchloroboranes a Frustrated Lewis Pair (FLP) able to split H2 and afford the desired hydroborane with ammonium salts. Unreactive haloboranes were unlocked using a catalytic amount of Cy2BCl, enabling the synthesis of commonly used hydroboranes such as pinacolborane or catecholborane. The mechanisms of these reactions have been examined by DFT studies, highlighting the importance of the base selection. Finally, the system's robustness has been evaluated in one-pot B-Cl hydrogenolysis/hydroboration reactions of C=C unsaturated bonds.

Aromative Dephosphinidenation of a Bisphosphirane-Fused Anthracene toward E-H (E = H, Si, N and P) Bond Activation.

A bisphosphirane-fused anthracene (5) was prepared by treatment of a sterically encumbered amino phosphorus dichloride (3) with MgA•3THF (A = anthracene). X-ray diffraction analysis revealed a pentacyclic framework consisting of 5 with two phosphirane rings fused to the anthracene in a trans-fashion. Compound 5 has been shown to be an efficient phosphinidene synthon, readily liberating two transient phosphinidene units for subsequent downstream bond activation via the reductive elimination of anthracene under mild conditions. The formal oxidative addition of H2 and E-H (E = Si, N, P) bonds by the liberated phosphinidene provided diphosphine and substituted phosphines. Furthermore, phosphinidene transfer to alkenes and alkynes smoothly yielded the corresponding phosphiranes and phosphirenes. The mechanism of the H2 activation by 5 was investigated by density functional theory (DFT) calculations.

Reversible Dihydrogen Activation and Catalytic H/D Exchange with Group 10 Heterometallic Complexes.

Reaction of a hexagonal planar palladium complex featuring a [PdMg3 H3 ] core with H2 is reversible and leads to the formation of a new [PdMg2 H4 ] tetrahydride species alongside an equivalent of a magnesium hydride co-product [MgH]. While the reversibility of this process prevented isolation of [PdMg2 H4 ], analogous [PtMg2 H4 ] and [PtZn2 H4 ] complexes could be isolated and characterised through independent syntheses. Computational analysis (DFT, AIM, NCIPlot) of the bonding in a series of heterometallic tetrahydride compounds (Ni-Pt; Mg and Zn) suggests that these complexes are best described as square planar with marginal metal-metal interactions; the strength of which increases slightly as group 10 is descended and increases from Mg to Zn. DFT calculations support a mechanism for H2 activation involving a ligand-assisted oxidative addition to Pd. These findings were exploited to develop a catalytic protocol for H/D exchange into magnesium hydride and zinc hydride bonds.

Molybdenum(VI) Bis(imido) Complexes: From Frustrated Lewis Pairs to Weakly Coordinating Cations.

Molybdenum(VI) bis(imido) complexes [Mo(NtBu)2 (LR )2 ] (R=H 1 a; R=CF3 1 b) combined with B(C6 F5 )3 (1 a/B(C6 F5 )3 , 1 b/B(C6 F5 )3 ) exhibit a frustrated Lewis pair (FLP) character that can heterolytically split H-H, Si-H and O-H bonds. Cleavage of H2 and Et3 SiH affords ion pairs [Mo(NtBu)(NHtBu)(LR )2 ][HB(C6 F5 )3 ] (R=H 2 a; R=CF3 2 b) composed of a Mo(VI) amido imido cation and a hydridoborate anion, while reaction with H2 O leads to [Mo(NtBu)(NHtBu)(LR )2 ][(HO)B(C6 F5 )3 ] (R=H 3 a; R=CF3 3 b). Ion pairs 2 a and 2 b are catalysts for the hydrosilylation of aldehydes with triethylsilane, with 2 b being more active than 2 a. Mechanistic elucidation revealed insertion of the aldehyde into the B-H bond of [HB(C6 F5 )3 ]- . We were able to isolate and fully characterize, including by single-crystal X-ray diffraction analysis, the inserted products Mo(NtBu)(NHtBu)(LR )2 ][{PhCH2 O}B(C6 F5 )3 ] (R=H 4 a; R=CF3 4 b). Catalysis occurs at [HB(C6 F5 )3 ]- while [Mo(NtBu)(NHtBu)(LR )2 ]+ (R=H or CF3 ) act as the cationic counterions. However, the striking difference in reactivity gives ample evidence that molybdenum cations behave as weakly coordinating cations (WCC).

Synthesis of an N-Heterocylic Boryl-Stabilized Disilyne and Its Application to the Activation of Dihydrogen and C-H Bonds.

The synthesis of low-valent silicon compounds that enable the activation of small molecules has been of great current interest. Reduction of N-heterocyclic boryltribromosilane (NHB)SiBr3 (2, NHB=[ArN(CMe)2 NAr]B, Ar=2,6-iPr2 C6 H3 ) with three equiv. of lithium in diethyl ether yielded the NHB-stabilized disilyne (NHB)Si≡Si(NHB) (3). Disilyne 3 slowly reacted with toluene, leading to the activation of one benzylic C-H bond and one C=C double bond with the formation of 4. Treatment of 3 with dihydrogen under 1 atm at room temperature resulted in the exclusive formation of the first boryl-stabilized 1,2-dihydrodisilene. Compounds 3-5 have been characterized by single-crystal X-ray diffraction studies, which indicated the co-planarity of the B-Si-Si-B plane with the NHB rings in compounds 3 and 5. DFT calculations indicated the significant π electron delocalization of the Si-Si multiple bonds to the B-N bonds in NHB rings.

Metal-Free Catalytic Hydrogenolysis of Silyl Triflates and Halides into Hydrosilanes.

The metal-free catalytic hydrogenolysis of silyl triflates and halides (I, Br) to hydrosilanes is unlocked by using arylborane Lewis acids as catalysts. In the presence of a nitrogen base, the catalyst acts as a Frustrated Lewis Pair (FLP) able to split H2 and generate a boron hydride intermediate capable of reducing (pseudo)halosilanes. This metal-free organocatalytic system is competitive with metal-based catalysts and enables the formation of a variety of hydrosilanes at room temperature in high yields (>85 %) under a low pressure of H2 (≤10 bar).

Rational design of the nickel-borane complex for efficient hydrogenation of styrene.

The Ni-B complex 1BCF with a facilely accessible monophosphine (Pt Bu3 ) unit was theoretically designed, which was found to be more active than that with an ambiphilic ligand for hydrogenation of styrene. Substituting Pt Bu3 with a stronger electron donating ligand N-heterocyclic carbene largely improves the activity of the Ni-B complex.

Contrasting the Mechanism of H2 Activation by Monomeric and Potassium-Stabilized Dimeric AlI Complexes: Do Potassium Atoms Exert any Cooperative Effect?

Aluminyl anions are low-valent, anionic, and carbenoid aluminum species commonly found stabilized with potassium cations from the reaction of Al-halogen precursors and alkali compounds. These systems are very reactive toward the activation of σ-bonds and in reactions with electrophiles. Various research groups have detected that the potassium atoms play a stabilization role via electrostatic and cation ⋯ π interactions with nearby (aromatic)-carbocyclic rings from both the ligand and from the reaction with unsaturated substrates. Since stabilizing K⋯H bonds are witnessed in the activation of this class of molecules, we aim to unveil the role of these metals in the activation of the smaller and less polarizable H2 molecule, together with a comprehensive characterization of the reaction mechanism. In this work, the activation of H2 utilizing a NON-xanthene-Al dimer, [K{Al(NON)}]2 (D) and monomeric, [Al(NON)]- (M) complexes are studied using density functional theory and high-level coupled-cluster theory to reveal the potential role of K+ atoms during the activation of this gas. Furthermore, we aim to reveal whether D is more reactive than M (or vice versa), or if complicity between the two monomer units exits within the D complex toward the activation of H2 . The results suggest that activation energies using the dimeric and monomeric complexes were found to be very close (around 33 kcal mol-1 ). However, a partition of activation energies unveiled that the nature of the energy barriers for the monomeric and dimeric complexes are inherently different. The former is dominated by a more substantial distortion of the reactants (and increased interaction energies between them). Interestingly, during the oxidative addition, the distortion of the Al complex is minimal, while H2 distorts the most, usually over 0.77 Δ E d i s t ≠ . Overall, it is found here that electrostatic and induction energies between the complexes and H2 are the main stabilizing components up to the respective transition states. The results suggest that the K+ atoms act as stabilizers of the dimeric structure, and their cooperative role on the reaction mechanism may be negligible, acting as mere spectators in the activation of H2 . Cooperation between the two monomers in D is lacking, and therefore the subsequent activation of H2 is wholly disengaged.

C-H Arylation of Benzene with Aryl Halides using H2 and a Water-Soluble Rh-Based Electron Storage Catalyst.

This paper reports the first example of C-H arylation of benzene under mild conditions, using H2 as an electron source {turnover numbers (TONs)=0.7-2.0 for 24 h}. The reaction depends on a Rh-based electron storage catalyst, and proceeds at room temperature and in aqueous solution. Furthermore, the H2 is inactive during the radical transfer step, greatly reducing unwanted side reactions.

Effects of Ionic Liquids on the Thermodynamics of Hydrogen Activation by Frustrated Lewis Pairs: A Density Functional Theory Study*.

Nowadays, hydrogen activation by frustrated Lewis pairs (FLPs) and their applications are one of the emerging research topics in the field of catalysis. Previous studies have shown that the thermodynamics of this reaction is determined by electronic structures of FLPs and solvents. Herein, we investigated systems consisting of typical FLPs and ionic liquids (ILs), which are well known by their large number of types and excellent solvent effects. The density functional theory (DFT) calculations were performed to study the thermodynamics for H2 activation by both inter- and intra-molecular FLPs, as well as the individual components. The results show that the computed overall Gibbs free energies in ILs are more negative than that computed in toluene. Through the thermodynamics partitioning, we find that ILs favor the H-H cleavage elemental step over the elemental steps of proton attachment, hydride attachment and zwitterionic stabilization. Moreover, the results show that these effects are strongly dependent on the type of FLPs, where intra-molecular FLPs are more affected compared to the inter-molecular FLPs.

Physical Separation of H2 Activation from Hydrogenation Chemistry Reveals the Specific Role of Secondary Metal Catalysts.

An electrocatalytic palladium membrane reactor (ePMR) uses electricity and water to drive hydrogenation without H2 gas. The device contains a palladium membrane to physically separate the formation of reactive hydrogen atoms from hydrogenation of the unsaturated organic substrate. This separation provides an opportunity to independently measure the hydrogenation reaction at a surface without any competing H2 activation or proton reduction chemistry. We took advantage of this feature to test how different metal catalysts coated on the palladium membrane affect the rates of hydrogenation of C=O and C=C bonds. Hydrogenation occurs at the secondary metal catalyst and not the underlying palladium membrane. These secondary catalysts also serve to accelerate the reaction and draw a higher flux of hydrogen through the membrane. These results reveal insights into hydrogenation chemistry that would be challenging using thermal or electrochemical hydrogenation experiments.

Crystalline Boragermenes.

Boragermene 3 featuring a double bond between the Ge and dicoordinate B atoms has been synthesized for the first time by reacting the cyclic (alkyl)(boryl)germylene-PMe3 adduct 1 with Cl2 BN(SiMe3 )2 followed by reductive dehalogenation with KC8 . Addition of a Lewis base (Me NHC) to 3 leads to the formation of the corresponding adduct 4, which shows double bond character between the Ge and tricoordinate B atoms. Compound 3 undergoes hydrogenation with H2 concomitant with a complete scission of the Ge=B bond.

Dihydrogen Adduct (Co-H2 ) Complexes Displaying H-Atom and Hydride Transfer.

The prototypical reactivity profiles of transition metal dihydrogen complexes (M-H2 ) are well-characterized with respect to oxidative addition (to afford dihydrides, M(H)2 ) and as acids, heterolytically delivering H+ to a base and H- to the metal. In the course of this study we explored plausible alternative pathways for H2 activation, namely direct activation through H-atom or hydride transfer from the σ-H2 adducts. To this end, we describe herein the reactivity of an isostructural pair of a neutral S= 1 / 2 and an anionic S=0 Co-H2 adduct, both supported by a trisphosphine borane ligand (P3 B ). The thermally stable metalloradical, (P3 B )Co(H2 ), serves as a competent precursor for hydrogen atom transfer to t Bu3 ArO⋅ . What is more, its anionic derivative, the dihydrogen complex [(P3 B )Co(H2 )]1- , is a competent precursor for hydride transfer to BEt3 , establishing its remarkable hydricity. The latter finding is essentially without precedent among the vast number of M-H2 complexes known.

Mechanistic Insights into Hydrogen Activation by Frustrated N/Sn Lewis Pairs.

The mechanism of H2 activation by recently reported N/Sn Lewis pairs is unravelled using the representative iPr3 SnOTf/DABCO combination. Computations provide evidence for weak intermolecular associations between Lewis acid and Lewis base (LA/LB) in which the counteranion to cationic LA fragment plays a critical role. Two frustrated Lewis pairs (FLPs) are observed; an unprecedented counteranion-mediated noncovalent LA/LB association is characterised along with the usual FLP structure. Both the FLPs are shown to be capable of H2 activation through cooperative electron transfer processes involving the LA/LB centres. Overall, computed results are in good agreement with the experimental findings and account for the observed reactivity. Insights obtained in this study are fundamentally important for the rational design of Sn-based alternative FLP LAs. The present findings could also provide a general mechanistic framework for H2 activation by FLPs having an ion pair LA component.

Hydrido Complexes of Calcium: A New Family of Molecular Alkaline-Earth-Metal Compounds.

The application of solid calcium hydride CaH2 has been mostly confined to its use as a desiccant, although its catalytic activity has long been known. Since the first isolation of a well-defined molecular calcium hydride in 2006, the past decade has witnessed a gradual emergence of this new family of compounds. Although the detrimental Schlenk equilibrium has kept the number of examples low, the novelty of their reactivity, especially in small-molecule activation, holds great promise. This Minireview gives an overview of the molecular calcium hydrides reported to date, highlighting their synthesis, structure, and reactivity.

Alkali Metal Species in the Reversible Activation of H2.

MP(tBu)2 (M=Li, Na, K), KH and KN(SiMe3 )2 are shown to activate HD reversibly. In the case of MP(tBu)2 this leads to isotopic scrambling and the formation of H2 , D2 , H(D)P(tBu)2 and MH(D) in C6 D6 . In toluene, KP(tBu)2 reacts with H2 but also leads to isotopic scrambling into the methyl groups of the solvent toluene. DFT calculations reveal that these systems effect H2 activation via cooperative interactions with the Lewis acidic alkali metal and the basic phosphorus, carbanion, or hydride centres, mimicking frustrated Lewis pair (FLP) behaviour.