METAL-METAL MULTIPLE BONDS IN EARLY/LATE HETEROBIMETALLIC COMPLEXES: APPLICATIONS TOWARD SMALL MOLECULE ACTIVATION AND CATALYSIS

Heteromultimetallic Platform for Enhanced C-H Bond Activation: Aluminum-Incorporated Dicopper Complex Mimicking Cu-ZSM-5 Structure and Oxidative Reactivity

Bimetallic complexes have sparked interest across various chemical disciplines, driving advancements in research. Recent advancements in this field have shed light on complex reactions in metalloenzymes and unveiled new chemical transformations. Two primary types of bimetallic platforms have emerged: (1) systems where both metals actively participate in reactivity, and (2) systems where one metal mediates the reaction while the other regulates reactivity. This study introduces a novel multinucleating ligand platform capable of integrating both types of bimetallic systems. To demonstrate the significance of this platform, we synthesized a unique dicopper complex incorporating aluminum in its coordination environment. This complex serves as the first structural model for the active site in copper-based zeolites, highlighting the role of aluminum in hydrogen atom abstraction reactivity. Comparative studies of oxidative C-H bond activation revealed that the inclusion of aluminum significantly alters the thermodynamic driving force (by -11 kcal mol-1) for bond activation and modifies the proton-coupled electron-transfer reaction mechanism, resulting in a 14-fold rate increase. Both computational and experimental data support the high modularity of this multinucleating ligand platform, offering a new approach to fine-tune the reactivity of bimetallic complexes.

Polarized metal-metal multiple bonding and reactivity of phosphinoamide-bridged heterobimetallic group IV/cobalt compounds

Heterobimetallic complexes are studied for their ability to mimic biological systems as well as active sites in heterogeneous catalysts. While specific interest in early/late heterobimetallic systems has fluctuated, they serve as important models to fundamentally understand metal-metal bonding. Specifically, the polarized metal-metal multiple bonds formed in highly reduced early/late heterobimetallic complexes exemplify how each metal modulates the electronic environment and reactivity of the complex as a whole. In this Perspective, we chronicle the development of phosphinoamide-supported group IV/cobalt heterobimetallic complexes. This combination of metals allows access to a low valent Co-I center, which performs a rich variety of bond activation reactions when coupled with the pendent Lewis acidic metal center. Conversely, the low valent late transition metal is also observed to act as an electron reservoir, allowing for redox processes to occur at the d0 group IV metal site. Most of the bond activation reactions carried out by phosphinoamide-bridged M/Co-I (M = Ti, Zr, Hf) complexes are facilitated by cleavage of metal-metal multiple bonds, which serve as readily accessible electron reservoirs. Comparative studies in which both the number of buttressing ligands as well as the identity of the early metal were varied to give a library of heterobimetallic complexes are summarized, providing a thorough understanding of the reactivity of M/Co-I heterobimetallic systems.

Tuning Photoredox Catalysis of Ruthenium with Palladium: Synthesis of Heterobimetallic Ru-Pd Complexes That Enable Efficient Photochemical Reduction of CO2

New Ru-Pd heterobimetallic complexes were synthesized and structurally characterized utilizing 6,6″-bis(phosphino)-2,2':6',2″-terpyridine as a scaffold for the metal-metal bond. The dicationic Ru-Pd complex was found to exhibit high catalytic activity as a photocatalyst for photochemical reduction of CO2 to CO under visible light irradiation. This study established a new design of transition metal catalysts that tune photoredox catalysis with metalloligands.

Cooperative dihydrogen activation with unsupported uranium-metal bonds and characterization of a terminal U(iv) hydride

Cooperative chemistry between two or more metal centres can show enhanced reactivity compared to the monometallic fragments. Given the paucity of actinide-metal bonds, especially those with group 13, we targeted uranium(iii)-aluminum(i) and -gallium(i) complexes as we envisioned the low-valent oxidation state of both metals would lead to novel, cooperative reactivity. Herein, we report the molecular structure of [(C5Me5)2(MesO)U-E(C5Me5)], E = Al, Ga, Mes = 2,4,6-Me3C6H2, and their reactivity with dihydrogen. The reaction of H2 with the U(iii)-Al(i) complex affords a trihydroaluminate complex, [(C5Me5)2(MesO)U(μ2-(H)3)-Al(C5Me5)] through a formal three-electron metal-based reduction, with concomitant formation of a terminal U(iv) hydride, [(C5Me5)2(MesO)U(H)]. Noteworthy is that neither U(iii) complexes nor [(C5Me5)Al]4 are capable of reducing dihydrogen on their own. To make the terminal hydride in higher yields, the reaction of [(C5Me5)2(MesO)U(THF)] with half an equivalent of diethylzinc generates [(C5Me5)2(MesO)U(CH2CH3)] or treatment of [(C5Me5)2U(i)(Me)] with KOMes forms [(C5Me5)2(MesO)U(CH3)], which followed by hydrogenation with either complex cleanly affords [(C5Me5)2(MesO)U(H)]. All complexes have been characterized by spectroscopic and structural methods and are rare examples of cooperative chemistry in f element chemistry, dihydrogen activation, and stable, terminal ethyl and hydride compounds with an f element.

Heterometallic Clusters with Cerium-Transition-Metal Bonding Supported by Nitrogen-Phosphorus Ligands

Ligands are known to play a crucial role in the construction of complexes with metal-metal bonds. Compared with metal-metal bonds involving d-block transition metals, knowledge of the metal-metal bonds involving f-block rare-earth metals still lags far behind. Herein, we report a series of complexes with cerium-transition-metal bonds, which are supported by two kinds of nitrogen-phosphorus ligands N[CH2CH2NHPiPr2]3 (VI) and PyNHCH2PPh2 (VII). The reactions of zerovalent group 10 metal precursors, Pd(PPh3)4 and Pt(PPh3)4, with the cerium complex supported by VI generate heterometallic clusters [N{CH2CH2NPiPr2}3Ce(μ-M)]2 (M = Pd, 2 and M = Pt, 3) featuring four Ce-M bonds; meanwhile, the bimetallic species [(PyNCH2PPh2)3Ce-M] (M = Ni, 5; M = Pd, 6; and M = Pt, 7) with a single Ce-M bond were isolated from the reactions of the cerium precursor 4 supported by VII with Ni(COD)2, Pd(PPh3)4, or Pt(PPh3)4, respectively. These complexes represent the first example of species with an RE-M bond between Ce and group 10 metals, and 2 and 3 contain the largest number of RE-M donor/acceptor interactions ever to have been observed in a molecule.

Combining [MoVIO3] and [M0(CO)3] (M = Mo, Cr) Fragments within the Same Complex: Synthesis and Reactivity of the Single Oxo-Bridged Heterobimetallics Supported by Xanthene-Based Heterodinucleating Ligands

A functional model of Mo-Cu carbon monoxide dehydrogenase (CODH) enzyme requires the presence of an oxidant (metal-oxo) and a metal-bound carbonyl in close proximity. In this work, we report the synthesis, characterization, and reactivity of a heterobimetallic complex combining Mo(VI) trioxo with Mo(0) tricarbonyl. The formation of the heterobimetallic complex is facilitated by the xanthene-bridged heterodinucleating ligand containing a hard catecholate chelate and a soft iminopyridine chelate. A catechol-coordinated square-pyramidal [MoVIO3] fragment interacts directly with the iminopyridine-bound [Mo0(CO)3] fragment via a single (oxo) bridge, with the overall disposition being related to the proposed first step in the CODH mechanism, where square-pyramidal [MoVIO2S] interacts with the [Cu-CO] via a single sulfido bridge. Our attempt to obtain a sulfido-bridged analogue (using [MoO3S]2- precursor) led to a mixture of products possibly containing different (oxo and sulfido) bridges. Despite a direct interaction between Mo(VI) and Mo(0) segments, no internal redox is observed, with the high lying occupied MOs being mostly d-π orbitals at Mo0(CO)3 and the low lying unoccupied MOs being d-π orbitals at MoVIO3. Due to the overall rigid structure, the heterobimetallic complex was found to be stable up to 100 °C in DMF-d7 (based on 1H NMR). The decomposition of the complex above this temperature does not produce CO2 (based on gas chromatography), dissociating stable Mo(CO)3(DMF)3 instead (based on IR). We also synthesized and studied the reactivity of the Mo(VI)/Cr(0) analogue. While this complex demonstrated more facile decomposition, no CO2 production was observed. Density functional theory calculations suggest that the formation of [CO2]2- and its subsequent reductive elimination is endergonic in the present system, likely due to the stability of fac-Mo0(CO)3 and the relative nucleophilic character of the carbonyl carbon engendered by back donation from Mo(0). The calculations also indicate that the replacement of one oxo by sulfido (both terminal and bridging), replacement of catechol with dithiolene, and replacement of Mo(0) with Cr(0) does not affect significantly the energetics of the process, likely requiring the use a less stable and less π-basic CO anchor.

Heterometallic bond activation enabled by unsymmetrical ligand scaffolds: bridging the opposites

Heterobi- and multimetallic complexes providing close proximity between several metal centers serve as active species in artificial and enzymatic catalysis, and in model systems, showing unique modes of metal-metal cooperative bond activation. Through the rational design of well-defined, unsymmetrical ligand scaffolds, we create a convenient approach to support the assembly of heterometallic species in a well-defined and site-specific manner, preventing them from scrambling and dissociation. In this perspective, we will outline general strategies for the design of unsymmetrical ligands to support heterobi- and multimetallic complexes that show reactivity in various types of heterometallic cooperative bond activation.

Recent advances in heterometallic clusters with f-block metal-metal bonds: synthesis, reactivity and applications

Due to the heterometallic synergistic effects from different metals, heterometallic clusters are of great importance in small-molecule activation and catalysis. For example, both biological nitrogen fixation and photosynthetic splitting of water into oxygen are thought to involve multimetallic catalytic sites with d-block transition metals. Benefitting from the larger coordination numbers of f-block metals (rare-earth metals and actinide elements), heterometallic clusters containing f-block metal-metal bonds have long attracted the interest of both experimental and theoretical chemists. Therefore, a series of effective strategies or platforms have been developed in recent years for the construction of heterometallic clusters with f-block metal-metal bonds. More importantly, synergistic effects between f-block metals and transition metals have been observed in small-molecule activation and catalysis. This tutorial review highlights the recent advances in the construction of heterometallic molecular clusters with f-block metal-metal bonds and also their reactivities and applications. It is hoped that this tutorial review will persuade chemists to develop more efficient strategies to construct clusters with f-block metal-metal bonds and also further expand their applications with heterometallic synergistic effects.

Recent developments on the transformation of CO2 utilising ligand cooperation and related strategies

A portfolio of value-added chemicals, fuels and building block compounds can be envisioned from CO₂ on an industrial scale. The high kinetic and thermodynamic stabilities of CO₂, however, present a significant barrier to its utilisation as a C1 source. In this context, metal-ligand cooperation methodologies have emerged as one of the most dominant strategies for the transformation of the CO₂ molecule over the last decade or so. This review focuses on the advancements in CO₂ transformation using these cooperative methodologies. Different and well-studied ligand cooperation methodologies, such as dearomatisation-aromatisation type cooperation, bimetallic cooperation (M⋯M'; M' = main group or transition metal) and other related strategies are also discussed. Furthermore, the cooperative bond activations are subdivided based on the number of atoms connecting the reactive centre in the ligand framework (spacer/linker length) and the transition metal. Several similarities across these seemingly distinct cooperative methodologies are emphasised. Finally, this review brings out the challenges ahead in developing catalytic systems from these CO₂ transformations.

Bimetallic Ru-Pd and Trimetallic Ru-Pd-Cu Assemblies on the Carborane Cluster Surface

The synthesis of well-defined heterometallic complexes remains a frontier challenge in inorganic chemistry. We report an approach that relies on the sequential insertion of electrophilic metal fragments into electron-rich Ru-B bonds of the η²-BB-carboryne complex (POBBOP)Ru(CO)₂ [POBBOP = 1,7-OP(ⁱPr)₂-m-2,6-dehydrocarborane]. Utilizing this synthetic strategy, bimetallic (POBBOP)(Ru)(CO)₂[Pd(P^tBu₃)] and trimetallic (POBBOP)(Ru)(CO)₂[Pd(P^tBu₃)](CuBr) complexes were selectively prepared. Structural and theoretical analysis of the features of chemical bonding within Ru-B-B-Cu and Ru-B-B-Pd fragments is presented.

Deciphering Cryptic Behavior in Bimetallic Transition-Metal Complexes with Machine Learning

We demonstrate an alternative, data-driven approach to uncovering structure-property relationships for the rational design of heterobimetallic transition-metal complexes that exhibit metal-metal bonding. We tailor graph-based representations of the metal-local environment for these complexes for use in multiple linear regression and kernel ridge regression (KRR) models. We curate a set of 28 experimentally characterized complexes to develop a multiple linear regression model for oxidation potentials. We achieve good accuracy (mean absolute error of 0.25 V) and preserve transferability to unseen experimental data with a new ligand structure. We also train a KRR model on a subset of 330 structurally characterized heterobimetallics to predict the degree of metal-metal bonding. This KRR model predicts relative metal-metal bond lengths in the test set to within 5%, and analysis of key features reveals the fundamental atomic contributions (e.g., the valence electron configuration) that most strongly influence the behavior of these complexes. Our work provides guidance for rational bimetallic design, suggesting that properties, including the formal shortness ratio, should be transferable from one period to another.

Taming the Antiferromagnetic Beast: Computational Design of Ultrashort Mn-Mn Bonds Stabilized by N-Heterocyclic Carbenes

The development of complexes featuring low-valent, multiply bonded metal centers is an exciting field with several potential applications. In this work, we describe the design principles and extensive computational investigation of new organometallic platforms featuring the elusive manganese-manganese bond stabilized by experimentally realized N-heterocyclic carbenes (NHCs). By using DFT computations benchmarked against multireference calculations, as well as MO- and VB-based bonding analyses, we could disentangle the various electronic and structural effects contributing to the thermodynamic and kinetic stability, as well as the experimental feasibility, of the systems. In particular, we explored the nature of the metal-carbene interaction and the role of the ancillary η6 coordination to the generation of Mn2 systems featuring ultrashort metal-metal bonds, closed-shell singlet multiplicities, and positive adiabatic singlet-triplet gaps. Our analysis identifies two distinct classes of viable synthetic targets, whose electrostructural properties are thoroughly investigated.

An IrVO4+ Cluster Catalytically Oxidizes Four CO Molecules: Importance of Ir-V Multiple Bonding

The generation and characterization of multiple metal-metal (M-M) bonds between early and late transition metals is vital to correlate the nature of multiple M-M bonds with the related reactivity in catalysis, while the examples with multiple M-M bonds have been rarely reported. Herein, we identified that the quadruple bonding interactions were formed in a gas-phase ion IrV⁺ with a dramatically short Ir-V bond. Oxidation of four CO molecules by IrVO₄⁺ is a highly exothermic process driven by the generation of stable products IrV⁺ and CO₂, and then IrV⁺ can be oxidized by N₂O to regenerate IrVO₄⁺. This finding overturns the general impression that vanadium oxide clusters are unwilling to oxidize multiple CO molecules because of the strong V-O bond and that at most two oxygen atoms can be supplied from a single V-containing cluster in CO oxidation. This study emphasizes the potential importance of heterobimetallic multiple M-M bonds in related heterogeneous catalysis.

Cationic Heterobimetallic Mg(Zn)/Al(Ga) Combinations for Cooperative C-F Bond Cleavage

Low-valent (Me BDI)Al and (Me BDI)Ga and highly Lewis acidic cations in [(tBu BDI)M+ ⋅C6 H6 ][(B(C6 F5 )4 - ] (M=Mg or Zn, Me BDI=HC[C(Me)N-DIPP]2 , tBu BDI=HC[C(tBu)N-DIPP]2 , DIPP=2,6-diisopropylphenyl) react to heterobimetallic cations [(tBu BDI)Mg-Al(Me BDI)+ ], [(tBu BDI)Mg-Ga(Me BDI)+ ] and [(tBu BDI)Zn-Ga(Me BDI)+ ]. These cations feature long Mg-Al (or Ga) bonds while the Zn-Ga bond is short. The [(tBu BDI)Zn-Al(Me BDI)+ ] cation was not formed. Combined AIM and charge calculations suggest that the metal-metal bonds to Zn are considerably more covalent, whereas those to Mg should be described as weak AlI (or GaI )→Mg2+ donor bonds. Failure to isolate the Zn-Al combination originates from cleavage of the C-F bond in the solvent fluorobenzene to give (tBu BDI)ZnPh and (Me BDI)AlF+ which is extremely Lewis acidic and was not observed, but (Me BDI)Al(F)-(μ-F)-(F)Al(Me BDI)+ was verified by X-ray diffraction. DFT calculations show that the remarkably facile C-F bond cleavage follows a dearomatization/rearomatization route.

H2 and carbon-heteroatom bond activation mediated by polarized heterobimetallic complexes

The field of heterobimetallic chemistry has rapidly expanded over the last decade. In addition to their interesting structural features, heterobimetallic structures have been found to facilitate a range of stoichiometric bond activations and catalytic processes. The accompanying review summarizes advances in this area since January of 2010. The review encompasses well-characterized heterobimetallic complexes, with a particular focus on mechanistic details surrounding their reactivity applications.

Synthesis and characterization of heterometallic complexes involving coinage metals and isoelectronic Fe(CO)5, [Mn(CO)5]- and [Fe(CO)4CN]- ligands

The chemistry of coinage metal ions with Fe(CO)₅, [Mn(CO)₅]^- and [Fe(CO)₄CN]^- has been explored using Mes₃P and N-heterocyclic carbene supporting ligands. A comparison of [(SIPr)Au-Fe(CO)₅][SbF₆], [(^Et2CAAC)Au-Fe(CO)₅][SbF₆] and [(Mes₃P)Au-Fe(CO)₅][SbF₆] shows that the ligand donor strength towards Au(i) follows the order Mes₃P > ^Et2CAAC > SIPr. These Fe(CO)₅ complexes show significant blue shifts in [small nu, Greek, macron]_CO bands relative to those observed for free Fe(CO)₅ as a result of it serving as a net electron donor to Au(i). Au(i) is a much stronger acceptor in (SIPr)Au-Mn(CO)₅ compared to Ag(i) in (SIPr)Ag-Mn(CO)₅. The structural details of Mes₃PAu-Mn(CO)₅ are also presented. [Fe(CO)₄CN]^- afforded CN bridged coinage metal complexes with (IPr*)Au⁺, (SIPr)Ag⁺ and (SIPr)Cu⁺ moieties, rather than molecules with direct Fe/coinage metal bonds. The computed total interaction energies indicate that both [Mn(CO)₅]^- and [Fe(CO)₄CN]^- are stronger donors toward Au(i) than Fe(CO)₅. A detailed analysis of the bonding interactions between the coinage metal ions and Fe(CO)₅, [Mn(CO)₅]^- and [Fe(CO)₄CN]^- suggests that the largest contribution comes from electrostatic attraction, while the covalent component follows the Dewar-Chatt-Duncanson model. The σ-donor interactions of these organometallic ligands with coinage metal ions are considerably stronger than the π-backbonding from the coinage metal ions.

Paramagnetic Metal-Metal Bonded Heterometallic Complexes

Significant progress has been made in the past 10-15 years on the design, synthesis, and properties of multimetallic coordination complexes with heterometallic metal-metal bonds that are paramagnetic. Several general classes have been explored including heterobimetallic compounds, heterotrimetallic compounds of either linear or triangular geometry, discrete molecular compounds containing a linear array of more than three metal atoms, and coordination polymers with a heterometallic metal-metal bonded backbone. We focus in this Review on the synthetic methods employed to access these compounds, their structural features, magnetic properties, and electronic structure. Regarding the metal-metal bond distances, we make use of the formal shortness ratio (FSR) for comparison of bond distances between a broad range of metal atoms of different sizes. The magnetic properties of these compounds can be described using an extension of the Goodenough-Kanamori rules to cases where two magnetic ions interact via a third metal atom. In describing the electronic structure, we focus on the ability (or not) of electrons to be delocalized across heterometallic bonds, allowing for rationalizations and predictions of single-molecule conductance measurements in paramagnetic heterometallic molecular wires.

Coordination and Dehydrogenation of Diphosphine-Borane Ph2PCH2PPh2·BH3 at a Heterometallic MoRe Center to Give an Agostic Boryl-Bridged Derivative

The coordination chemistry of the title diphosphine-borane adduct at heterometallic MoRe centers was examined through its reactions with the hydride complex [MoReCp(μ-H)(μ-PCy₂)(CO)₅(NCMe)] (Cp = η⁵-C₅H₅). The latter reacted rapidly with stoichiometric amounts of dppm·BH₃ (dppm = Ph₂PCH₂PPh₂) in refluxing toluene solution, with displacement of the nitrile ligand, to give [MoReCp(μ-H)(μ-PCy₂)(CO)₅(κ¹_P-dppm·BH₃)], with a P-bound diphosphine-borane ligand arranged trans to the PCy₂ group. Decarbonylation of the latter complex was accomplished rapidly upon irradiation with visible-UV light in toluene solution at 263 K, to give the agostic derivative [MoReCp(μ-H)(μ-PCy₂)(CO)₄(κ¹_P,η²-dppm·BH₃)] as major product (Mo-Re = 3.2075(5) Å), along with small amounts of the diphosphine-bridged complex [MoReCp(μ-H)(μ-PCy₂)(CO)₄(μ-dppm)]. Extended photolysis of the agostic complex at 288 K promoted an unprecedented dehydrogenation process involving the borane group and the hydride ligand, to give the diphosphine-boryl complex [MoReCp(μ-η²:κ²_P,B-H₂B·dppm)(μ-PCy₂)(CO)₄] (Mo-Re = 3.075(1) Å). The latter displayed a boryl ligand in a novel bridging coordination mode, it being σ-bound to one of the metal atoms (B-Re = 2.38(2) Å) while interacting with the second metal atom via a strong side-on tricentric B-H-M interaction (B-Mo = 2.31(1); H-Mo = 1.9(1) Å). The overall dehydrogenation process was endergonic by 43 kJ/mol, according to density functional theory calculations.

Synthesis and structural characterization of arsinoamides - early transition metal (Zr and Hf) and main group metal (Al, In, Sn, and Pb) complexes

By reaction of MCl4 (M = Zr, Hf) with 2 equiv. of [(Mes2AsNPh){Li(OEt2)2}], the first group 4 metal arsinoamide complexes [(Mes2AsNPh)2MCl2(THF)] were synthesized. They feature two weak M-As interactions. After formally replacing the chloride atoms by the amido ligands [NMe2]-, a more diverse M-As interaction arises in [(Mes2AsNPh)2M(NMe2)2]: only one M-As contact can be observed with a substantially shorter distance. This type of interaction may originate from the steric effect of the substituents on the metal center. In addition, the coordination behavior of arsinoamide in main group metal chemistry was investigated. Thus, arsinoamide complexes of Al(iii), In(iii), Sn(ii) and Pb(ii) are reported. In contrast to the group 4 complexes, no M-As interaction can be detected in these four complexes.

Preparation of Heterobimetallic Ketimido-Actinide-Molybdenum Complexes

During our attempts to prepare paddlewheel actinide-molybdenum complexes of the type [(X)An(MesNPR₂)₃Mo(CO)₃] (Mes= 2,4,6-trimethylphenyl; X = Cl or I; An = U or Th; R = ⁱPr or Ph) we have found that under certain conditions acetonitrile insertion reactions occur to give the heterobimetallic bridging ketimido species [ClAn(μ-MesNPⁱPr₂)₂(μ-MesNPⁱPr₂{μ-NCMe})Mo(CO)₃] (An = U, 1; Th, 2), [ClAn(μ-MesNPPh₂)₂(μ-MesNPPh₂{μ-NCMe})Mo(CO)₃] (An = U, 3; An = Th, 4), and [IAn(η²-MesNPⁱPr₂)(μ-MesNPⁱPr₂){μ-NC(Me)N(Mes)PⁱPr₂}Mo(CO)₃] (An = U, 5; Th, 6). Structural and spectroscopic data confirm the assignment of a ketimido ligand bridging An(IV) and Mo(0) centers. The isolation of 1-6 is in contrast to our previously reported preparations of [(X)An(MesNPPh₂)₃Mo(CO)₃] (An = U or Th; X= Cl or I; Chem. Commun. 2018, 54, 13515-13518) with the difference in reactivity being attributable to a combination of ancillary phosphino-amide, reaction solvent, and temperature variation. Complexes 1-5 represent the first examples of structurally characterized ketimido-bridged actinide-transition metal linkages and demonstrate the profound differences in reaction outcomes that can occur from relatively minor experimental changes.

Reaction of an arsinoamide with chloro tetrylenes: substitution and As-N bond insertion

Reaction of the arsinoamide [(Mes₂AsNPh){Li(OEt₂)₂}] with the low-valent group 14 compounds, [{PhC(^tBuN)₂}ECl] (E = Si, Ge) and GeCl₂·dioxane, resulted in two different reaction pathways: simple substitution or substitution accompanied by an insertion step. As a result, either insertion products with an As-Si[double bond, length as m-dash]N unit and an As-Ge bond, or substitution products, in which the intact arsinoamide binds to the group 14 elements via the N atom, were obtained.

O2 Activation by a Heterobimetallic Zr/Co Complex

Oxygen reduction is a critical half reaction in renewable fuel cell development and a key step in the development of aerobic oxidation reactions. Herein, we report rapid two-electron O₂ reduction by a d⁰ Zr^IV center with an appended redox-active Co^-I site serving as an electron reservoir. The early/late heterobimetallic Zr/Co complex (THF)Zr(MesNP ⁱPr₂)₃CoCN ^tBu (1) reacts readily with O₂ and O atom transfer reagents to generate reactive oxygenated species including terminal peroxo and oxo complexes, (O₂)Zr(MesNP ⁱPr₂)₃CoCN ^tBu (2) and O≡Zr(MesNP ⁱPr₂)₃CoCN ^tBu (3). The bimetallic Zr/Co complex provides a new cooperative synthetic pathway to promote multielectron redox processes such as oxygen reduction, with each metal playing a distinct role as a substrate binding site or redox mediator.

Bimetallic nickel-lutetium complexes: tuning the properties and catalytic hydrogenation activity of the Ni site by varying the Lu coordination environment

We present three heterobimetallic complexes containing an isostructural nickel center and a lutetium ion in varying coordination environments. The bidentate ⁱPr₂PCH₂NHPh and nonadentate (ⁱPr₂PCH₂NHAr)₃tacn ligands were used to prepare the Lu metalloligands, Lu(ⁱPr₂PCH₂NPh)₃ (1) and Lu{(ⁱPr₂PCH₂NAr)₃tacn} (2), respectively. Reaction of Ni(COD)₂ (where COD is 1,5-cyclooctadiene) and 1 afforded NiLu(ⁱPr₂PCH₂NPh)₃ (3), with a Lu coordination number (CN) of 4 and a Ni-Lu distance, d(Ni-Lu), of 2.4644(2) Å. Complex 3 can further bind THF to form 3-THF, increasing both the Lu CN to 5 and d(Ni-Lu) to 2.5989(4) Å. On the other hand, incorporation of Ni(0) into 2 provides NiLu{(ⁱPr₂PCH₂NAr)₃tacn} (4), in which the Lu coordination environment is more saturated (CN = 6), and the d(Ni-Lu) is substantially elongated at 2.9771(5) Å. Cyclic voltammetry of the three Ni-Lu complexes shows an overall ∼410 mV shift in the Ni(0/I) redox couple, suggesting tunability of the Ni electronics across the series. Computational studies reveal polarized bonding interactions between the Ni 3d _{z ²} (major) and the Lu 5d _{z ²} (minor) orbitals, where the percentage of Lu character increases in the order: 4 (6.0% Lu 5d _{z ²} ) < 3-THF (8.5%) < 3 (9.3%). All three Ni-Lu complexes bind H₂ at low temperatures (-30 to -80 °C) and are competent catalysts for styrene hydrogenation. Complex 3 outperforms 4 with a four-fold faster rate. Additionally, adding increasing THF equivalents to 3, which would favor build-up of 3-THF, decreases the rate. We propose that altering the coordination sphere of the Lu support can influence the resulting properties and catalytic activity of the active Ni(0) metal center.

Actinide-transition metal bonding in heterobimetallic uranium- and thorium-molybdenum paddlewheel complexes

We report the preparation of four heterobimetallic uranium- and thorium-molybdenum paddlewheel complexes. The characterisation data suggest the presence of Mo → An σ-interactions in all cases. These complexes represent unprecedented actinide-group 6 metal-metal bonds, where before heterobimetallic uranium-metal bonds were restricted to group 7-11 metals.

Trapping of an Heterometallic Unsaturated Hydride: Structure and Properties of the Ammonia Complex [MoMnCp(μ-H)(μ-PPh2)(CO)5(NH3)]

Complexes displaying multiple bonds between different metal atoms have considerable synthetic potential because of the combination of the high electronic and coordinative unsaturation associated to multiple bonds with the intrinsic polarity of heterometallic bonds but their number is scarce and its chemistry has been relatively little explored. In a preliminary study, our attempted synthesis of the unsaturated hydrides [MoMCp(μ-H)(μ-PR2)(CO)5] from anions [MoMCp(μ-PR2)(CO)5]− and (NH4)PF6 yielded instead the ammonia complexes [MoMCp(μ-H)(μ-PR2)(CO)5(NH3)] (M = Mn, R = Ph; M = Re, R = Cy). We have now examined the structure and behaviour of the MoMn complex (Mo–Mn = 3.087(3) Å) and found that it easily dissociates NH3 (this requiring some 40 kJ/mol, according to DFT calculations), to yield the undetectable unsaturated hydride [MoMnCp(μ-H)(μ-PPh2)(CO)5] (computed Mo–Mn = 2.796 Å), the latter readily adding simple donors L such as CNR (R = Xyl, p-C6H4OMe) and P(OMe)3, to give the corresponding electron-precise derivatives [MoMnCp(μ-H)(μ-PPh2)(CO)5(L)]. Thus the ammonia complex eventually behaves as a synthetic equivalent of the unsaturated hydride [MoMnCp(μ-H)(μ-PPh2)(CO)5]. The isocyanide derivatives retained the stereochemistry of the parent complex (Mo–Mn = 3.0770(4) Å when R = Xyl) but a carbonyl rearrangement takes place in the reaction with phosphite to leave the entering ligand trans to the PPh2 group, a position more favoured on steric grounds.

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.

Synthesis and structural characterization of alkali metal arsinoamides

The aminoarsane Mes2AsN(H)Ph was prepared from Mes2AsCl and aniline in good yields. Deprotonation of Mes2AsN(H)Ph with suitable alkali metal bases resulted in the corresponding alkali metal derivatives. Thus, reaction of Mes2AsN(H)Ph with nBuLi, NaN(SiMe3)2, or KH gave the metal complexes [(Mes2AsNPh){Li(OEt2)2}], [(Mes2AsNPh){Na(OEt2)}]2, and [(Mes2AsNPh){K(THF)}]2. These are the first metal complexes ligated by an arsinoamide. All solid-state structures were established by single crystal X-ray diffraction. The lithium compounds form a monomer in the solid-state, whereas the sodium and the potassium derivatives are dimers. In the dimeric compounds intra- and intermolecular π-interaction of the aromatic rings with the metal atoms is observed.

Acetonitrile Adduct [MoReCp(μ-H)(μ-PCy2)(CO)5(NCMe)]: A Surrogate of an Unsaturated Heterometallic Hydride Complex

The title compound was prepared upon irradiation of acetonitrile solutions of the readily available hexacarbonyl [MoReCp(μ-H)(μ-PCy₂)(CO)₆]. The acetonitrile ligand in this compound could be replaced easily by donor molecules or displaced upon two-electron reduction. In most cases, the substitution step was followed by additional processes such as insertion into the M-H bonds, E-H bond cleavage, H₂ elimination, and other transformations.

Cooperative bimetallic reactivity of a heterodinuclear molybdenum–copper model of Mo–Cu CODH

Modeling the reactivity of Mo–Cu CODH: Cu(i) brings the substrate close to Mo–oxo and develops electrophilic character in CO carbon.