Reactions of transition metal dihydrogen complexes

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.

Thermodynamic Modulation of Dihydrogen Activation Through Rational Ligand Design in GeII-Ni0 Complexes

A family of chelating aryl-functionalized germylene ligands has been developed and employed in the synthesis of their corresponding 16-electron Ni0 complexes (PhiPDippGeAr·Ni·IPr; PhiPDipp = {[Ph2PCH2Si(iPr)2](Dipp)N}-; IPr = [{(H)CN(Dipp)}2C:]; Dipp = 2,6-iPr2C6H3). These complexes demonstrate the ability to cooperatively and reversibly activate dihydrogen at the germylene-nickel interface under mild conditions (1.5 atm H2, 298 K). We show that the thermodynamics of the dihydrogen activation process can be modulated by tuning the electronic nature of the germylene ligands, with an increase in the electron-withdrawing character displaying more exergonic ΔG298 values, as ascertained through NMR spectroscopic Van't Hoff analyses for all systems. This is also shown to correlate with experimental 31P NMR and UV/vis absorption data as well as with computationally derived parameters such as Ge-Ni bond order and Ni/Ge NPA charge, giving a thorough understanding of the modulating effect of ligand design on this reversible, cooperative bond activation reaction. Finally, the utility of this modulation was demonstrated in the catalytic dehydrocoupling of phenylsilane, whereby systems that disfavor dihydrogen activation are more efficient catalysts, aligning with H2-elimination being the rate-limiting step. A density functional theory analysis supports cooperative activation of the Si-H moiety in PhSiH3.

Exploring the Tl 2 H 2 potential energy surface: A comparative analysis with group 13 systems and experiment

Thallium chemistry is experiencing unprecedented importance. Therefore, it is valuable to characterize some of the simplest thallium compounds. Stationary points along the singlet and triplet Tl  2 H  2 potential energy surface have been characterized. Stationary point geometries were optimized with the CCSD(T)/aug-cc-pwCVQZ-PP method. Harmonic vibrational frequencies were computed at the same level of theory while anharmonic vibrational frequencies were computed at the CCSD(T)/aug-cc-pwCVTZ-PP level of theory. Final energetics were obtained with the CCSDT(Q) method. Basis sets up to augmented quintuple-zeta cardinality (aug-cc-pwCV5Z-PP) were employed to obtain energetics in order to extrapolate to the complete basis set limits using the focal point approach. Zero-point vibrational energy corrections were appended to the extrapolated energies in order to determine relative energies at 0 K. It was found that the planar dibridged isomer lies lowest in energy while the linear structure lies highest in energy. The results were compared to other group 13 M  2 H  2 (M = B, Al, Ga, In, and Tl) theoretical studies and some interesting variations are found. With respect to experiment, incompatibilities exist.

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.

Frustrated Lewis pairs on pentacoordinated Al3+-enriched Al2O3 promote heterolytic hydrogen activation and hydrogenation

As an emerging class of metal-free catalysts, frustrated Lewis pairs (FLPs) catalysts have been greatly constructed and applied in many fields. Homogeneous FLPs have witnessed significant development, while limited heterogeneous FLPs catalysts are available. Herein, we report that heterogeneous FLPs on pentacoordinated Al3+-enriched Al2O3 readily promote the heterolytic activation of H2 and thus hydrogenation catalysis. The defect-rich Al2O3 was prepared by simple calcination of a carboxylate-containing Al precursor. Combinatorial studies confirmed the presence of rich FLPs on the surface of the defective Al2O3. In contrast to conventional alumina (γ-Al2O3), the FLP-containing Al2O3 can activate H2 in the absence of any transition metal species. More importantly, H2 was activated by surface FLPs in a heterolytic pathway, leading to the hydrogenation of styrene in a stepwise process. This work paves the way for the exploration of more underlying heterogeneous FLPs catalysts and further understanding of accurate active sites and catalytic mechanisms of heterogeneous FLPs at the molecular level.

Unravelling strong temperature-dependence of JHD in transition metal hydrides: solvation and non-covalent interactions versus temperature-elastic H-H bonds

A number of transition metal hydrides reveal intriguing temperature-dependent JHD in their deuterated derivatives and possibly the temperature dependent hydrogen-hydrogen distance (r(H-H)) as well. Previously, theoretical studies rationalized JHD and r(H-H) changes in such compounds through a "temperature-elastic" structure model with a significant population of vibrational states in an anharmonic potential. Based on the first variable temperature neutron diffraction study of a relevant complex, (p-H-POCOP)IrH2, observation of its elusive counterpart with longer r(H-H), crystallized as an adduct with C6F5I, and thorough spectroscopic and computational study, we argue that the model involving isomeric species in solution at least in some cases is more relevant. The existence of such isomers is enabled or enhanced by solvation and weak non-covalent interactions with solvent, such as halogen or dihydrogen bonds. "Non-classical" hydrides with r(H-H) ≈ 1.0-1.6 Å are especially sensitive to the above-mentioned factors.

Catalytic, Z-Selective, Semi-Hydrogenation of Alkynes with a Zinc-Anilide Complex

The reversible activation of dihydrogen with a molecular zinc anilide complex is reported. The mechanism of this reaction has been probed through stoichiometric experiments and density functional theory (DFT) calculations. The combined evidence suggests that H₂ activation occurs by addition across the Zn-N bond via a four-membered transition state in which the Zn and N atoms play a dual role of Lewis acid and Lewis base. The zinc hydride complex that results from H₂ addition has been shown to be remarkably effective for the hydrozincation of C═C bonds at modest temperatures. The scope of hydrozincation includes alkynes, alkenes, and a 1,3-butadiyne. For alkynes, the hydrozincation step is stereospecific leading exclusively to the syn-isomer. Competition experiments show that the hydrozincation of alkynes is faster than the equivalent alkene substrates. These new discoveries have been used to develop a catalytic system for the semi-hydrogenation of alkynes. The catalytic scope includes both aryl- and alkyl-substituted internal alkynes and proceeds with high alkene: alkane, Z:E ratios, and modest functional group tolerance. This work offers a first example of selective hydrogenation catalysis using zinc complexes.

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.

Advancing homogeneous catalysis for parahydrogen-derived hyperpolarisation and its NMR applications

Parahydrogen-induced polarisation (PHIP) is a nuclear spin hyperpolarisation technique employed to enhance NMR signals for a wide range of molecules. This is achieved by exploiting the chemical reactions of parahydrogen (para-H2), the spin-0 isomer of H2. These reactions break the molecular symmetry of para-H2 in a way that can produce dramatically enhanced NMR signals for reaction products, and are usually catalysed by a transition metal complex. In this review, we discuss recent advances in novel homogeneous catalysts that can produce hyperpolarised products upon reaction with para-H2. We also discuss hyperpolarisation attained in reversible reactions (termed signal amplification by reversible exchange, SABRE) and focus on catalyst developments in recent years that have allowed hyperpolarisation of a wider range of target molecules. In particular, recent examples of novel ruthenium catalysts for trans and geminal hydrogenation, metal-free catalysts, iridium sulfoxide-containing SABRE systems, and cobalt complexes for PHIP and SABRE are reviewed. Advances in this catalysis have expanded the types of molecules amenable to hyperpolarisation using PHIP and SABRE, and their applications in NMR reaction monitoring, mechanistic elucidation, biomedical imaging, and many other areas, are increasing.

Stabilization of CO2 as Zwitterionic Carbamate within a Coordination Polymer (CP): Synthesis, Structure and Anions Sensing Behaviour of Tb-CP composite

A new gadolinium (III) based coordination polymer (CP), [Gd(3,5-pydc)1.5(CO2)0.5(H2O)4].3H2O (where 3,5-pydc =3,5-pyridinedicarboxylate) , 1, has been successfully synthesized using slow diffusion method at room temperature. Single crystal X-ray diffraction study...

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.

Metal Effect Meets Volcano Plots: A DFT Study on Tris(phosphino)borane-Transition Metal Complexes Catalyzed H2 Activation

Bifunctional transition metal complexes are of particular interest in metal-ligand cooperative activation of small molecules. As a novel type of bifunctional catalyst, Lewis acid transition metal (LA-TM) complexes have attracted increasing interest in hydrogen activation and storage. To advance the catalyst design, herein the metal effect of LA-TM complexes on the hydrogen activation has been systematically studied with a series of tris(phosphino)borane (TPB) complexes with V, Cr, Mn, Fe, Co, and Ni as metal centers. The metal effect not only influences the mechanism of hydrogen activation, but also notably casts a volcano plot for the activity. TPB complexes of V, Cr, Mn, Fe, and Co tend to activate H₂ through a stepwise mechanism, while TPB-Ni prefers a synergetic mechanism for H₂ activation. More importantly, the metal effect significantly influences the activity of H₂ activation and the formation of the LA-H-TM bridging hydride. The trend of changes in the LA-H-TM structures, the second-order perturbation stabilization energies, and the Laplacian bond orders, along with different metals (from V to Ni), are all interestingly constitute volcano plots for the performance of TPB-TM complexes catalyzed H₂ activation. TPB-Mn and TPB-Fe are found to be the optimal catalysts among the discussed TPB-TM complexes. The volcano plots disclosed for the metal effects should be informative and instructive for homogeneous and heterogeneous LA-TM catalysts development.

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.

The Reaction of Hydrogen Halides with Tetrahydroborate Anion and Hexahydro-closo-hexaborate Dianion

The mechanism of the consecutive halogenation of the tetrahydroborate anion [BH₄]⁻ by hydrogen halides (HX, X = F, Cl, Br) and hexahydro-closo-hexaborate dianion [B₆H₆]²⁻ by HCl via electrophile-induced nucleophilic substitution (EINS) was established by ab initio DFT calculations [M06/6-311++G(d,p) and wB97XD/6-311++G(d,p)] in acetonitrile (MeCN), taking into account non-specific solvent effects (SMD model). Successive substitution of H⁻ by X⁻ resulted in increased electron deficiency of borohydrides and changes in the character of boron atoms from nucleophilic to highly electrophilic. This, in turn, increased the tendency of the B–H bond to transfer a proton rather than a hydride ion. Thus, the regularities established suggested that it should be possible to carry out halogenation more selectively with the targeted synthesis of halogen derivatives with a low degree of substitution, by stabilization of H₂ complex, or by carrying out a nucleophilic substitution of B–H bonds activated by interaction with Lewis acids (BL₃).

Facile H/D Exchange at (Hetero)Aromatic Hydrocarbons Catalyzed by a Stable Trans-Dihydride N-Heterocyclic Carbene (NHC) Iron Complex

Earth-abundant metal pincer complexes have played an important role in homogeneous catalysis during the last ten years. Yet, despite intense research efforts, the synthesis of iron PC_carbeneP pincer complexes has so far remained elusive. Here we report the synthesis of the first PC_NHCP functionalized iron complex [(PC_NHCP)FeCl₂] (1) and the reactivity of the corresponding trans-dihydride iron(II) dinitrogen complex [(PC_NHCP)Fe(H)₂N₂)] (2). Complex 2 is stable under an atmosphere of N₂ and is highly active for hydrogen isotope exchange at (hetero)aromatic hydrocarbons under mild conditions (50 °C, N₂). With benzene-d₆ as the deuterium source, easily reducible functional groups such as esters and amides are well tolerated, contributing to the overall wide substrate scope (e.g., halides, ethers, and amines). DFT studies suggest a complex assisted σ-bond metathesis pathway for C(sp²)–H bond activation, which is further discussed in this study.

Substituent-regulated mechanism on reaction Cp2NbH3 (Cp = η5-C5H5) with RCCR (R = COOMe and Me)

The mechanisms and the regulating effect of substituents in alkynes of the title reaction have been investigated and compared.

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 diverse mechanisms for the oxidative addition of C-Br bonds to Pd(PR3) and Pd(PR3)2 complexes

The reaction between bromobenzene and palladium(0) complexes leading to a palladium(ii) complex containing bromide and phenyl ligands is studied computationally with DFT methods. Three different mechanisms are considered: concerted, nucleophilic substitution and radical. A systematic analysis is carried out on the effect on each of these mechanisms of a number of variables: the identity of the phosphine (PF₃, PH₃, PMe₃ or PPh₃), the nature of the solvent (vacuum, tetrahydrofuran, dimethylformamide or water) and the number of phosphine ligands (mono- or bis-phosphine). The concerted and nucleophilic substitution mechanisms are competitive in many cases, the identity of the preferred one depending on a combination of factors. Additional calculations with bromomethane, bromoethylene and bromoethane are carried out in selected cases for further clarification.

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

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.

Hydrogenolysis of Dinuclear PCNR Ligated PdII μ-Hydroxides and Their Mononuclear PdII Hydroxide Analogues

The hydrogenolysis of mono- and dinuclear Pd^II hydroxides was investigated both experimentally and computationally. It was found that the dinuclear μ-hydroxide complexes {[(PCN^R )Pd]₂ (μ-OH)}(OTf) (PCN^H =1-[3-[(di-tert-butylphosphino)methyl]phenyl]-1H-pyrazole; PCN^Me =1-[3-[(di-tert-butylphosphino)methyl]phenyl]-5-methyl-1H-pyrazole) react with H₂ to form the analogous dinuclear hydride species {[(PCN^R )Pd]₂ (μ-H)}(OTf). The dinuclear μ-hydride complexes were fully characterized, and are rare examples of structurally characterized unsupported singly bridged μ-H Pd^II dimers. The {[(PCN^Me )Pd]₂ (μ-OH)}(OTf) hydrogenolysis mechanism was investigated through experiments and computations. The hydrogenolysis of the mononuclear complex (PCN^H )Pd-OH resulted in a mixed ligand dinuclear species [(PCN^H )Pd](μ-H)[(PCC)Pd] (PCC=a dianionic version of PCN^H bound through phosphorus P, aryl C, and pyrazole C atoms) generated from initial ligand "rollover" C-H activation. Further exposure to H₂ yields the bisphosphine Pd⁰ complex Pd[(H)PCN^H ]₂ . When the ligand was protected at the pyrazole 5-position in the (PCN^Me )Pd-OH complex, no hydride formed under the same conditions; the reaction proceeded directly to the bisphosphine Pd⁰ complex Pd[(H)PCN^Me ]₂ . Reaction mechanisms for the hydrogenolysis of the monomeric and dimeric hydroxides are proposed.

H2 addition to (Me4PCP)Ir(CO): studies of the isomerization mechanism

Reduced steric demand of the Me4PCP pincer ligand (PCP = κ3-C6H4-1,3-[CH2PR2]2, R = Me), allows for a more open metal center. This is evident through structure and reactivity comparisons between (Me4PCP)Ir derivatives and other (R4PCP)Ir complexes (R = tBu, iPr, CF3). In particular, isomerization from cis-(R4PCP)Ir(H)2(CO) to trans-(R4PCP)Ir(H)2(CO) is more facile when R = Me than when R = iPr. Deuterium incorporation in the hydride ligands from solvent C6D6 was observed during this isomerization when R = Me. This deuterium exchange has not been observed for other analogous R4PCP iridium complexes. A kinetic study of the cis/trans isomerization combined with computational studies suggests that the cis/trans isomerization proceeds through a migratory-insertion pathway involving a formyl intermediate.

DFT Provides Insight into the Additive-Free Conversion of Aqueous Methanol to Dihydrogen Catalyzed by [Ru(trop2dad)]: Importance of the (Electronic) Flexibility of the Diazadiene Moiety

The mechanism for complete dehydrogenation of aqueous methanol to CO₂ and three equivalents of H₂ catalyzed by [Ru(trop₂dad)] was investigated with DFT (trop₂dad = 1,4-bis(5H-dibenzo[a,d]cyclohepten-5-yl)-1,4-diazabuta-1,3-diene). To date, this is the only catalyst that promotes the acceptorless dehydrogenation of aqueous methanol in homogeneous phase under mild conditions without the addition of an additive (base, acid, or a secondary catalyst). A detailed understanding of the mechanism of this transformation may therefore be of significant importance for the conversion of liquid organic fuels. Previous computational studies using simplified models of the catalyst suggested entirely ligand-centered reaction pathways with rather high-energy barriers for complete dehydrogenation of aqueous methanol. These are, however, not consistent with the experimental data. In the present paper, we reveal a different reaction mechanism for aqueous methanol dehydrogenation that involves metal–ligand cooperativity involving the diazadiene (dad) ligand and has substantially lower barriers, in good agreement with the experimental data. The dad moiety of the ligand actively participates in the alcohol activation mechanism. In the first step of the reaction, the dad ligand rearranges from a σ- to a π-bound coordination mode. This adjusts the electronic structure of both the metal and the ligand, leading to an enhanced Brønsted basicity of the nitrogen centers and higher Lewis acidity of the ruthenium center. As a result, concerted proton-hydride transfer to/from metal-hydride and N-protonated dad-ligand moieties becomes possible, leading to low-barrier metal–ligand cooperative elementary steps for alcohol activation and H₂ elimination.

Roles of Hydrogen Bonding in Proton Transfer to κP,κN,κP-N(CH2CH2P i Pr2)2-Ligated Nickel Pincer Complexes

The nickel PNP pincer complex ( ^{i Pr}PNP)NiPh ( ^{i Pr}PNP = κ^P,κ^N,κ^P-N(CH₂CH₂P ⁱ Pr₂)₂) was prepared by reacting ( ^{i Pr}PNP)NiBr with PhMgCl or deprotonating [( ^{i Pr}PN^HP)NiPh]Y ( ^{i Pr}PN^HP = κ^P,κ^N,κ^P-HN(CH₂CH₂P ⁱ Pr₂)₂; Y = Br, PF₆) with KO ^t Bu. The byproducts of the PhMgCl reaction were identified as [( ^{i Pr}PN^HP)NiPh]Br and ( ^{i Pr}PNP')NiPh ( ^{i Pr}PNP' = κ^P,κ^N,κ^P-N(CH=CHP ⁱ Pr₂)(CH₂CH₂P ⁱ Pr₂)). The methyl analog ( ^{i Pr}PNP)NiMe was synthesized from the reaction of ( ^{i Pr}PNP)NiBr with MeLi, although it was contaminated with ( ^{i Pr}PNP')NiMe due to ligand oxidation. Protonation of ( ^{i Pr}PNP)NiX (X = Br, Ph, Me) with various acids, such as HCl, water, and MeOH, was studied in C₆D₆. Nitrogen protonation was shown to be the most favorable process, producing a cationic species [( ^{i Pr}PN^HP)NiX]⁺ with the NH moiety hydrogen-bonded to the conjugate base (i.e., Cl^-, HO^-, or MeO^-). Protonation of the Ni-C bond was observed at room temperature with ( ^{i Pr}PNP)NiMe, whereas at 70 °C with ( ^{i Pr}PNP)NiPh, both resulting in [( ^{i Pr}PN^HP)NiCl]Cl as the final product. Protonation of ( ^{i Pr}PNP)NiBr was complicated by site exchange between Br^- and the conjugate base and by the degradation of the pincer complexes. Indene, which lacks hydrogen-bonding capability, was unable to protonate ( ^{i Pr}PNP)NiPh and ( ^{i Pr}PNP)NiMe, despite being more acidic than water and MeOH. Neutral and cationic nickel pincer complexes involved in this study, including ( ^{i Pr}PNP')NiBr, ( ^{i Pr}PNP)NiPh, ( ^{i Pr}PNP')NiPh, ( ^{i Pr}PNP)NiMe, [( ^{i Pr}PN^HP)NiPh]Y (Y = Br, PF₆, BPh₄), [( ^{i Pr}PN^HP)NiPh]₂[NiCl₄], [( ^{i Pr}PN^HP)NiMe]Y (Y = Cl, Br, BPh₄), [( ^{i Pr}PN^HP)NiBr]Br, and [( ^{i Pr}PN^HP)NiCl]Cl, were characterized by X-ray crystallography.

Polarization transfer efficiency in PHIP experiments

Parahydrogen induced polarization (PHIP) is a hyperpolarization method for NMR signal enhancement with applications in spectroscopy and imaging. Although parahydrogen can be easily enriched up to nearly 95%, the polarization detected on the hydrogenated substrate is substantially lower, where numerous loss mechanisms between the start of the hydrogenation reaction and detection affect polarization levels. The quality of PHIP systems is commonly determined by stating either the polarization degree or the enhancement factor of the product at the time of detection. In this study, we present a method that allows the distinction of polarization loss due to both the catalytic cycle and T₁ relaxation of the formed product prior to detection. We determine the influence of homogeneous catalysts and define a rigorous measure of the polarization transfer efficiency (PTE). Our results show that the PTE strongly depends on the concentration of all components and the chemical structure of the catalyst as well as on the magnetic field of detection.

Mechanisms of the H2- and transfer hydrogenation of polar bonds catalyzed by iron group hydrides

This Perspective describes the mechanism-based development of iron-group catalysts for the asymmetric hydrogenation of ketones and imines.

Reactant or reagent? Oxidation of H2 at electronically distinct nickel-thiolate sites [Ni2(μ-SR)2]+ and [Ni–SR]+

The chemical bond between a Lewis-acidic metal and a Brønsted/Lewis-basic sulphur donor provides M–S structures with functional properties that are relevant for a variety of processes such as the heterolytic cleavage of H₂.

Synthesis, structures, and reactivity studies of cyclometalated N-heterocyclic carbene complexes of ruthenium

Dehydrochlorination of Cp*Ru(IPr)Cl leads to an unusual C–C bond activation, yielding a cyclometalated Ru complex bearing an NHC-C(sp²) ligand. Reactivity studies of cyclometalated Ru complexes were explored.

DFT study of CO2 hydrogenation catalyzed by a cobalt-based system: an unexpected formate anion-assisted deprotonation mechanism

An unexpected formate anion-assisted deprotonation mechanism is unfolded by a DFT study of CO₂ hydrogenation catalyzed by a cobalt-based system.