Metal–Metal Bonds in Catalysis

From Pincer to Paddlewheel: C-H and C-S Bond Activation at Bis(2-pyridylthio)methane by Palladium(II)

The bis(2-pyridylthio)methanidopalladium(II) pincer complex (1), containing a Pd-C bond, was obtained from the reaction of bis(2-pyridylthio)methane (H₂L) with palladium(II) acetate in toluene under reflux. When palladium(II) trifluoroacetate was used, H₂L reacted to generate the tetrakis(pyridine-2-thiol)palladium(II) complex (2). Complex 2 was converted to a heterobimetallic palladium(II)-iron(II) paddlewheel complex (3) upon treatment with iron(II) triflate in the presence of a base in acetonitrile at room temperature.

Bimetallic nickel-cobalt hydrides in H2 activation and catalytic proton reduction

The synergism of the electronic properties of nickel and cobalt enables bimetallic NiCo complexes to process H2. The nickel-cobalt hydride [(dppe)Ni(pdt)(H)CoCp*]+ ([1H]+ ) arising from protonation of the reduced state 1 was found to be an efficient electrocatalyst for H2 evolution with Cl2CHCOOH, and the oxidized [Ni(ii)Co(iii)]2+ form is capable of activating H2 to produce [1H]+ . The features of stereodynamics, acid-base properties, redox chemistry and reactivity of these bimetallic NiCo complexes in processing H2 are potentially related to the active site of [NiFe]-H2ases.

Reactions of dicobalt octacarbonyl with dinucleating and mononucleating bis(imino)pyridine ligands

This work focuses on the application of dicobalt octacarbonyl (Co₂(CO)₈) as a metal precursor in the chemistry of formally low-valent cobalt with redox-active bis(imino)pyridine [NNN] ligands. The reactions of both mononucleating mesityl-substituted bis(aldimino)pyridine (L¹) and dinucleating macrocyclic xanthene-bridged di(bis(aldimino)pyridine) (L²) with Co₂(CO)₈ were investigated. Independent of the metal-to-ligand ratio (1 : 1 or 1 : 2 ligand to Co₂(CO)₈), the reaction of the dinucleating ligand L² with Co₂(CO)₈ produces a tetranuclear complex [Co₄(L²)(CO)₁₀] featuring two discrete [Co₂[NNN](CO)₅] units. In contrast, a related mononucleating bis(aldimino)pyridine ligand, L¹, produces different species at different ligand to Co₂(CO)₈ ratios, including dinuclear [Co₂(CO)₅(L¹)] and zwitterionic [Co(L¹)₂][Co(CO)₄]. Interestingly, [Co₄(L²)(CO)₁₀] features metal-metal bonds, and no bridging carbonyls, whereas [Co₂(CO)₅(L¹)] contains cobalt centers bridged by one or two carbonyl ligands. In either case, treatment with excess acetonitrile leads to disproportionation to the zwitterionic [Co[NNN](NCMe)₂][Co(CO)₄] units. The electronic structures of the complexes described above were studied with density functional theory. All the obtained bis(imino)pyridine complexes serve as catalysts for cyclotrimerization of methyl propiolate, albeit their reactivity is inferior compared with Co₂(CO)₈.

Methane Activation by Gas Phase Atomic Clusters

The increasing supply of natural gas has created a strong demand for developing efficient catalytic processes to upgrade methane, the most stable alkane molecule, into value-added chemicals. Currently, methane conversion in laboratory and industry is mostly performed under high-temperature conditions. A lot of effort has been devoted to exploring chemical entities that are able to activate the C-H bond of methane at lower temperatures, preferably room temperature. Gas phase atomic clusters with limited numbers of atoms are ideal models of active sites on heterogeneous catalysts. The cluster systems are being actively studied to activate methane under room-temperature conditions. State-of-the-art mass spectrometry, photoelectron imaging spectroscopy, and quantum chemistry calculations have been combined in our laboratory to reveal the molecular-level mechanisms of methane activation by atomic clusters. In this Account, we summarize our recent progress on thermal methane activation by metal oxide clusters doped with noble-metal atoms (Au, Pt, and Rh) as well as by oxygen-free species including carbides and borides of base metals (V, Ta, Mo, and Fe). In contrast to the generations of CH₃^• free radicals in many of the previously reported cluster reactions with methane, the generations of stable products such as formaldehyde, acetylene, and syngas as well as closed-shell species AuCH₃ and B₃CH₃ have been identified for the cluster reaction systems herein. Besides the well recognized mechanisms of methane activation by the O^-• radicals through hydrogen atom abstraction and by metal atoms through oxidative addition, the new mechanisms of synergistic methane activation by Lewis acid-base pairs (such as Au^δ+-O^δ- and B^δ+-B^δ-) and by dinuclear metal centers (such as Ta-Ta) have been recently revealed. In the reactions between methane and oxide clusters doped with noble-metal atoms, the oxide cluster "supports" can accept the H atoms and the CH _x species delivered through the noble-metal atoms and then transform methane into stable oxygenated compounds. The product selectivity (such as formaldehyde versus syngas) can be controlled by different noble-metal atoms (such as Pt versus Rh). The electronic structures of base metal centers can be engineered through carburization so that the low-spin states can be accessible to reduce the C-H bond of methane. Such active base metal centers in low-spin states resemble related noble-metal atoms in methane activation. The boron clusters (such as B₃ in VB₃⁺) can be polarized by the metal cations to form the Lewis acid-base pair B^δ+-B^δ- to cleave the C-H bond of methane very easily. These molecular-level mechanisms may well be operative in related heterogeneous catalysis and can be a fundamental basis to design efficient catalysts for activation and conversion of methane under mild conditions.

Cu dimer anchored on C2N monolayer: low-cost and efficient Bi-atom catalyst for CO oxidation

By means of density functional theory (DFT) computations, we systemically investigated CO/O2 adsorption and CO oxidation pathways on a bi-atom catalyst, namely, a copper dimer anchored on a C2N monolayer (Cu2@C2N), and we compared it with its monometallic counterpart Cu1@C2N. The Cu dimer could be stably embedded into the porous C2N monolayer. The reactions between the adsorbed O2 and CO via both bi-molecular and tri-molecular Langmuir-Hinshelwood (L-H) and Eley-Rideal (E-R) mechanisms were comparably studied, and we found that the bi-atom catalyst Cu2@C2N possessed superior performance toward CO oxidation as compared to the single-atom catalyst Cu1@C2N. Our comparative study suggested that the newly predicted bi-atom catalyst, i.e., a copper dimer anchored on a suitable support is highly active for CO oxidation, which can provide a useful guideline for further developing highly effective and low-cost green nanocatalysts.

Preparation and characterisation of heterobimetallic copper-tungsten hydride complexes

The preparation and structural characterisation of three new heterobimetallic hydride complexes containing 3-centre,2-electron W-H-Cu bonds is reported. These complexes have been characterised by single crystal X-ray crystallography and multinuclear NMR spectroscopy. The bonding in these complexes has been analysed by DFT calculations.

Reactivity of a trans-[H-Mo[quadruple bond, length as m-dash]Mo-H] unit towards alkenes and alkynes: bimetallic migratory insertion, H-elimination and other reactions

Complex [Mo2(H)2{μ-HC(NDipp)2}2(THF)2], (1·THF), reacts with C2H4 and PhCH[double bond, length as m-dash]CH2 to afford hydrido-hydrocarbyl and bis(hydrocarbyl) derivatives of the Mo[quadruple bond, length as m-dash]Mo bond. Reversible migratory insertion and β-hydrogen elimination, as well as reductive elimination and other reactions, have been uncovered. PhC[triple bond, length as m-dash]CH behaves instead as a Brönsted-Lowry acid towards the strongly basic Mo-H bonds of 1·THF.

Iron-based trinuclear metal-organic nanostructures on a surface with local charge accumulation

Coordination chemistry relies on harnessing active metal sites within organic matrices. Polynuclear complexes-where organic ligands bind to several metal atoms-are relevant due to their electronic/magnetic properties and potential for functional reactivity pathways. However, their synthesis remains challenging; few geometries and configurations have been achieved. Here, we synthesise-via supramolecular chemistry on a noble metal surface-one-dimensional metal-organic nanostructures composed of terpyridine (tpy)-based molecules coordinated with well-defined polynuclear iron clusters. Combining low-temperature scanning probe microscopy and density functional theory, we demonstrate that the coordination motif consists of coplanar tpy's linked via a quasi-linear tri-iron node in a mixed (positive-)valence metal-metal bond configuration. This unusual linkage is stabilised by local accumulation of electrons between cations, ligand and surface. The latter, enabled by bottom-up on-surface synthesis, yields an electronic structure that hints at a chemically active polynuclear metal centre, paving the way for nanomaterials with novel catalytic/magnetic functionalities.

Selective Synthesis of a Series of Isostructural MII CuI Heterobimetallic Complexes Spontaneously Assembled by an Unsymmetrical Naphthyridine-Based Ligand

Metal-metal cooperation is integral to the function of many enzymes and materials, and model complexes hold enormous potential for providing insights into the capabilities of analogous multimetallic cores. However, the selective synthesis of heterobimetallic complexes still presents a significant challenge, especially for systems that hold the metals in close proximity and feature open or reactive coordination sites for both metals. To address this issue, a rigid, naphthyridine-based dinucleating ligand featuring distinct binding environments was synthesized. This ligand enables the selective synthesis of a series of M^II Cu^I bimetallic complexes (M=Mn, Fe, Co, Ni, Cu, Zn), in which each metal center exclusively occupies its preferred binding pocket, from simple chloride salts. The precision of this selectivity is evident from cyclic voltammetry, ESI-MS and anomalous X-ray diffraction measurements.

Expansion of the (BB) 〉metallacycle with coinage metal cations: formation of B-M-Ru-B (M = Cu, Ag, Au) dimetalacyclodiboryls

In this work, we introduce a novel approach for the selective assembly of heterometallic complexes by unprecedented coordination of coinage metal cations to strained single ruthenium-boron bonds on a surface of icosahedral boron clusters. M(i) cations (M = Cu, Ag, and Au) insert into B-Ru bonds of the (BB)-carboryne complex of ruthenium with the formation of four-membered B-M-Ru-B metalacycles. Results of theoretical calculations suggest that bonding within these metalacycles can be best described as unusual three-center-two-electron B-M···Ru interactions that are isolobal to B-H···Ru borane coordination for M = Cu and Ag, or the pairs of two-center-two electron B-Au and Au-Ru interactions for M = Au. These transformations comprise the first synthetic route to exohedral coinage metal boryl complexes of icosahedral closo-{C2B10} clusters, which feature short Cu-B (2.029(2) Å) and Ag-B (2.182(3) Å) bonds and the shortest Au-B bond (2.027(2) Å) reported to date. The reported heterometallic complexes contain Cu(i) and Au(i) centers in uncharacteristic square-planar coordination environments. These findings pave the way to rational construction of a broader class of multimetallic architectures featuring M-B bonds.

Nickel(I) Aryl Species: Synthesis, Properties, and Catalytic Activity

In this work, Ni(I) aryl species that are directly relevant to cross-coupling have been synthesized. Transmetalation of (dppf)Ni^IX (dppf = 1,1'-bis(diphenylphosphino)-ferrocene, X = Cl, Br) with aryl Grignard reagents or aryl boronic acids in the presence of base produces Ni(I) aryl species of the form (dppf)Ni^I(Ar) (Ar = Ph, o-tolyl, 2,6-xylyl, 2,4,6-mesityl, 2,4,6-ⁱPr₃C₆H₂). The stability of the Ni(I) aryl species is inversely correlated to the steric bulk on the aryl ligand. The most unstable Ni(I) aryl species are the most active precatalysts for Suzuki-Miyaura reactions because they rapidly decompose to generate the active Ni(0) catalyst. This study shows that Ni(I) aryl species are initially formed in the activation of Ni(I) halide precatalysts for Suzuki-Miyaura reactions and establishes their stoichiometric and catalytic reactivity profile.

Synthesis and Characterization of Heterobimetallic Iridium-Aluminum and Rhodium-Aluminum Complexes

We demonstrate the synthesis and characterization of a new class of late-transition-metal-aluminum heterobimetallic complexes via a novel synthetic pathway. Complexes of this type are exceedingly rare. Joint experimental and theoretical data sheds light on the electronic effect of ligands containing aluminum moieties on late-transition-metal complexes.

Buchwald–Hartwig amination using Pd(i) dimer precatalysts supported by biaryl phosphine ligands

We report the synthesis of air-stable Pd(i) dimer complexes featuring biaryl phosphine ligands.

Diverse bimetallic mechanisms emerging from transition metal Lewis acid/base pairs: development of co-catalysis with metal carbenes and metal carbonyl anions

The rational development of catalytic reactions involving cooperative behavior between two catalytic reactive sites represents a frontier area of research from which novel reactivity and selectivity patterns emerge.

U- to Z-shape isomerization in a Pt2Ag2 framework containing pyridyl-NHC ligands

A highly twisted U-shaped Pt₂Ag₂ complex bearing pyridyl–NHC ligands showed isomerization to Z-shaped conformations via an intermolecular process. ¹H NMR experiments revealed that the isomerization reached U : Z = 2 : 1 ratio in a thermodynamic equilibrium state.

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.

A bromo-capped diruthenium(i,i) N-heterocyclic carbene compound for in situ bromine generation with NBS: catalytic olefin aziridination reactions

A bromo-capped diruthenium(i,i) complex activates NBS to produce bromine in situ, and thus catalyses bromine-mediated olefin aziridination reactions.

Cu(i) coordination polymers (CPs) as tandem catalysts for three-component sequential click/alkynylation cycloaddition reaction with regiocontrol

Two synthesized Cu(i)-based CPs could catalyze the three-component sequential click/alkynylation cycloaddition reaction with high regioselectivity.

Heterobimetallic Complexes Featuring Fe(CO)5 as a Ligand on Gold

Iron(0) pentacarbonyl complexes of gold(I), [Mes₃ PAu-Fe(CO)₅ ][SbF₆ ] (1) and [(IPr*)Au-Fe(CO)₅ ][SbF₆ ] (2) (Mes=2,4,6-trimethylphenyl; IPr*=1,3-bis(2,6-bis(diphenylmethyl)-4-methylphenyl)imidazol-2-ylidene) have been synthesized using Mes₃ PAuCl and (IPr*)AuCl as the gold(I) precursor, AgSbF₆ halide ion abstractor, and the Lewis base Fe(CO)₅ . The Au-Fe bond lengths of these metal-only Lewis pair complexes are significantly shorter than the sum of the experimentally derived covalent radii of Au and Fe. The v̄(CO) bands of the molecules show a notable blueshift relative to those observed for free Fe(CO)₅ , indicating a substantial reduction in Fe→CO backbonding upon its coordination to gold(I) with either Mes₃ P or IPr* supporting ligands (L). The analysis of the electronic structure with quantum chemical method suggests that the Au-Fe bond consists mainly of [LAu]⁺ ←Fe(CO)₅ σ-donation and weaker [LAu]⁺ →Fe(CO)₅ π-backdonation. The donor strength of Fe(CO)₅ is similar to that of CO.

Reversible Cleavage/Formation of the Chromium-Chromium Quintuple Bond in the Highly Regioselective Alkyne Cyclotrimerization

Herein we report the employment of the quintuply bonded dichromium amidinates [Cr{κ² -HC(N-2,6-ⁱ Pr₂ C₆ H₃ )(N-2,6-R₂ C₆ H₃ )}]₂ (R=iPr (1), Me (7)) as catalysts to mediate the [2+2+2] cyclotrimerization of terminal alkynes giving 1,3,5-trisubstituted benzenes. During the catalysis, the ultrashort Cr-Cr quintuple bond underwent reversible cleavage/formation, corroborated by the characterization of two inverted arene sandwich dichromium complexes (μ-η⁶ :η⁶ -1,3,5-(Me₃ Si)₃ C₆ H₃ )[Cr{κ² -HC(N-2,6-ⁱ Pr₂ C₆ H₃ )(N-2,6-R₂ C₆ H₃ )}]₂ (R=ⁱ Pr (5), Me (8)). In the presence of σ donors, such as THF and 2,4,6-Me₃ C₆ H₂ CN, the bridging arene 1,3,5-(Me₃ Si)₃ C₆ H₃ in 5 and 8 was extruded and 1 and 7 were regenerated. Theoretical calculations were employed to disclose the reaction pathways of these highly regioselective [2+2+2] cylcotrimerization reactions of terminal alkynes.

Isolation and characterization of a tetranuclear Pt-FeFe-Pt intermediate en route to the trinuclear Pt-Fe-Pt cluster

An intermediate compound, [{PtFe(piam)₂(NH₃)₂(OCH₃)}₂(μ-OCH₃)₂](ClO₄)₂ (1, piam = pivalamidate), in the synthetic process to form [Pt₂Fe(piam)₄(NH₃)₄](ClO₄)₃ (2) by mixing cis-[Pt(piam)₂(NH₃)₂]·2H₂O and iron sources was successfully isolated and characterized by single-crystal X-ray analysis. In 1, the platinum and iron atoms are bridged by two piam ligands to afford a dinuclear Pt-Fe structure and are further linked to each other by methoxide bridges at the equatorial positions of iron atoms to form a tetranuclear Pt-FeFe-Pt complex. The Pt-Fe distances in 1 are 3.0010(16) and 2.9883(17) Å, which are significantly longer than those in 2 (2.5566(15) and 2.5718(15) Å). X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR), and magnetic susceptibility measurements revealed that the oxidation states are Pt(+2)-Fe(+3)Fe(+3)-Pt(+2) (1) and Pt(+2)-Fe(+3)-Pt(+2) (2) with high-spin (S = 5/2) configurations in iron atoms. The magnetic susceptibility of 1 has a χT value of 5.83 cm³ mol^-1 K per Pt(+2)-Fe(+3)Fe(+3)-Pt(+2) unit at 300 K, which decreases down to 0.04 cm³ mol^-1 K at 7 K due to antiferromagnetic coupling (J = -28 cm^-1) of the two Fe(+3) centers. Compound 2 maintains its trinuclear structure in MeCN, exhibiting reversible one-electron reduction and oxidation, Pt(+2)-Fe(+2)-Pt(+2) ↔ Pt(+2)-Fe(+3)-Pt(+2), at E_1/2 = -0.19 V (vs. Fc/Fc⁺). However, in MeOH, compound 2 is decomposed into a dinuclear structure of Pt-Fe involving an equilibrium between 1 and 2.

Divergent Reactivity of a Dinuclear (NHC)Nickel(I) Catalyst versus Nickel(0) Enables Chemoselective Trifluoromethylselenolation

We herein showcase the ability of NHC-coordinated dinuclear NiI -NiI complexes to override fundamental reactivity limits of mononuclear (NHC)Ni0 catalysts in cross-couplings. This is demonstrated with the development of a chemoselective trifluoromethylselenolation of aryl iodides catalyzed by a NiI dimer. A novel SeCF3 -bridged NiI dimer was isolated and shown to selectively react with Ar-I bonds. Our computational and experimental reactivity data suggest dinuclear NiI catalysis to be operative. The corresponding Ni0 species, on the other hand, suffers from preferred reaction with the product, ArSeCF3 , over productive cross-coupling and is hence inactive.

Heterobimetallic Silver-Iron Complexes Involving Fe(CO)5 Ligands

Iron(0) pentacarbonyl is an organometallic compound with a long history. It undergoes carbonyl displacement chemistry with various donors (L), leading to molecules of the type Fe(CO)_x(L)_5-x. The work reported here illustrates that Fe(CO)₅ can also act as a ligand. The reaction between Fe(CO)₅ with the silver salts AgSbF₆ and Ag[B{3,5-(CF₃)₂C₆H₃}₄] under appropriate conditions resulted in the formation of [(μ-H₂O)AgFe(CO)₅]₂[SbF₆]₂ and [B{3,5-(CF₃)₂C₆H₃}₄]AgFe(CO)₅, respectively, featuring heterobimetallic {Ag-Fe(CO)₅}⁺ fragments. The treatment of [B{3,5-(CF₃)₂C₆H₃}₄]AgFe(CO)₅ with 4,4'-dimethyl-2,2'-bipyridine (Me₂Bipy) and Fe(CO)₅ afforded a heterobimetallic [(Me₂Bipy)AgFe(CO)₅][B{3,5-(CF₃)₂C₆H₃}₄] species with a Ag-Fe(CO)₅ bond and a heterotrimetallic [{Fe(CO)₅}₂(μ-Ag)][B{3,5-(CF₃)₂C₆H₃}₄] with a (CO)₅Fe-Ag-Fe(CO)₅ core, respectively, illustrating that it is possible to manipulate the coordination sphere at silver while keeping the Ag-Fe bond intact. The chemistry of [B{3,5-(CF₃)₂C₆H₃}₄]AgFe(CO)₅ with Et₂O and PMes₃ (Mes = 2,4,6-trimethylphenyl) has also been investigated, which led to [(Et₂O)₃Ag][B{3,5-(CF₃)₂C₆H₃}₄] and [(Mes₃P)₂Ag][B{3,5-(CF₃)₂C₆H₃}₄] with the displacement of the Fe(CO)₅ ligand. X-ray structural and spectroscopic data of new molecules as well as results of computational analyses are presented. The Fe-Ag bond distances of these metal-only Lewis pairs range from 2.5833(4) to 2.6219(5) Å. These Ag-Fe bonds are of primarily an ionic/electrostatic nature with a modest amount of charge transfer between Ag⁺ and Fe(CO)₅. The ν̅(CO) bands of the molecules with Ag-Fe(CO)₅ bonds show a notable blue shift relative to those observed for free Fe(CO)₅, indicating a significant reduction in Fe→CO back-bonding upon its coordination to silver(I).