Cooperative C-H Bond Activation by a Low-Spin d6 Iron-Aluminum Complex

Catalytic H/D exchange of (hetero)arenes with early-late polyhydride heterobimetallic complexes: impact of transition metal pairs

Metal-catalyzed hydrogen isotope exchange (HIE) has become a valuable method for incorporating deuterium and tritium into organic molecules, with applications in a wide range of scientific fields. This study explores the role of transition metal cooperativity in enhancing catalytic hydrogen/deuterium (H/D) exchange using early-late heterobimetallic polyhydride (ELHB) complexes. A series of four ELHB complexes, of general formula [M(CH2tBu)3(H)xM'Cp*], combining early transition metals (M = Hf, Ta) with late metals (M' = Ir, Os), were synthesized and evaluated for their catalytic activity in HIE of (hetero)arenes. Hafnium-iridium and hafnium-osmium complexes showed a clear improvement in catalytic efficiency and reaction rate over monometallic analogues, suggestive of metal-metal synergy. Conversely, the tantalum-based heterobimetallic complexes showed lower catalytic performance, revealing that not all metal combinations are equally effective. These results underline the importance of careful metal selection to optimize transition metal cooperativity, and open up new possibilities for the design of more efficient H/D exchange catalysts.

Synthesis, Structure, and Bonding of Actinide-Rhenium Polyhydrides

The synthesis of actinide tetrarhenate complexes completes a series of iridate, osmate, and rhenate polyhydrides, allowing for structural and bonding comparisons to be made. Computational studies examine the bonding interactions, particularly between metals, in these complexes. Several factors─including metal oxidation state, coordination number, and dispersion effects─affect metal-metal distances and covalency in these actinide tetrametallates. Related osmium and rhenium octametallic U2M6 clusters are synthesized and described, and subjected to similar structural and electronic analyses.

Bis-Silylene-Supported Aluminium Atoms with Aluminylene and Alane Character

The suitability of electron-rich bis-silylenes, specifically the neutral chelating [SiII(Xant)SiII] ligand (SiII=PhC(NtBu)2Si, Xant=9,9-dimethylxanthene) and the anionic [SiII(NAcrid)SiII)]- pincer ligand (NAcrid=2,7,9,9-tetramethylacridane), has been successfully probed to stabilize monovalent bis-silylene-supported aluminium complexes (aluminylenes). At first, the unprecedented aluminium(III) iodide precursors [SiII(Xant)SiII]AlI2 + I- 1 and [SiII(NAcrid)SiII)]AlI2 2 were synthesized using AlI3 and [SiII(Xant)SiII] or [SiII(NAcrid)SiII)]Li(OEt2)], respectively, and structurally characterized. While reduction of 1 with KC8 led merely to unidentified products, the dehalogenation of 2 afforded the dimer of the desired {[SiII(NAcrid)SiII)]Al:} aluminylene with a four-membered SiIV 2AlIII 2 ring. Remarkably, the proposed aluminylene intermediates [SiII(Xant)SiII]AlII and {[SiII(NAcrid)SiII)]Al:} could be produced through reaction of 1 and 2 with Collman's reagent, K2Fe(CO)4, and trapped as AlI:→Fe(CO)4 complexes 5 and 6, respectively. While 6 is stable in solution, 5 loses one CO ligand in solution to afford the silylene- and aluminylene-coordinated iron(0) complex 7 from an intramolecular substitution reaction. The electronic structures of the novel compounds were investigated by Density Functional Theory calculations.

Dyotropic Rearrangement of an Iron-Aluminium Complex

Ligand exchange processes at metal complexes underpin their reactivity and catalytic applications. While mechanisms of ligand exchange at single site complexes are well established, occurring through textbook associative, dissociative and interchange mechanisms, those involving heterometallic complexes are less well developed. Here we report the reactions of a well-defined Fe-Al dihydride complex with exogenous ligands (CO and CNR, R=Me, tBu, Xyl=2,6-Me2C6H3). Based on DFT calculations we suggest that these reactions occur through a dyotropic rearrangement, this involves initial coordination of the exogenous ligand at Al followed by migration to Fe, with simultaneous migration of a hydride ligand from Fe to Al. Such processes are rare for heterometallic complexes. We study the bonding and mechanism of the dyotropic rearrangement through in-depth computational analysis (NBO, IBOs, CLMO analysis, QTAIM, NCIplot, IGMH), shedding new light on how the electronic structure of the heterometallic core responds to the migration of ligands between metal sites. The dyotropic rearrangement fundamentally changes the nature of the hydride ligands, exposing new nucleophilic reactivity as evidenced by insertion reactions with CO2, isocyanates, as well as isocyanides.

Frustrated Al/M Heterobimetallic Complexes (M = Cr, Mo, W) That Exhibit Both Lewis and Radical Pair Behavior

Exploration of new heterobinuclear Al/M combinations is relevant to contemporary strategies for cooperative bond activation. Here, we report the synthesis and characterization of six new Al/M heterobimetallic complexes (M = Cr, Mo, W) that exhibit end-on "isocarbonyl"-type Al─O═C═M bridges with metalloketene character rather than featuring Al─M─C≡O motifs with metal-metal bonding. The new compounds were characterized experimentally by nuclear magnetic resonance and infrared spectroscopies and theoretically using density functional theory, natural bond orbital, and quantum theory of atoms in molecules calculations. Factors influencing Al─O═C═M vs Al─M─C≡O isomerism were probed both experimentally and computationally. Crossover experiments between different group VI Al/M derivatives and regioselective epoxide ring opening indicate that the Al/M complexes act as masked frustrated Lewis pairs in solution under certain conditions. However, crossover experiments between group VI Al/M complexes and a previously studied Al-Fe complex, as well as computational modeling, imply that the same complexes can also reasonably act as masked frustrated radical pairs (FRPs). FRP reactivity with the group VI Al/M complexes was achieved under photochemical conditions, producing unsaturated metal-carbonyl dimers [(CpCr)2(CO)3]2- and [Mn2(CO)8]2-, which would otherwise be unstable under standard conditions but that are isolable here due to Al(III) coordination. The metal-metal bonding in these unsaturated metal-carbonyl dimers was also analyzed theoretically.

Transition Metal Parent Alumylene Complexes: Synthesis, Structures, and XPS Characterization of Aluminum Oxidation State

The first isolation and characterization of transition metal complexes with the parent Al(I)-H unit were achieved in base-stabilized forms. W and Fe complexes, Cp*(CO)n(H)M←:AlH(NHC)2 (NHC = N-heterocyclic carbene, n = 1 or 2), were synthesized in 43-63% yields by the one-step reaction of Cp*M(CO)n(py)Me with H3Al·NHC. The characterization included 1H and 27Al nuclear magnetic resonance (NMR), and infrared (IR) spectroscopic analysis, as well as DFT calculations, which revealed the extremely strong σ-donating ability of the :AlH(NHC)2 ligand, and the highly polarized M(δ-)←:Al(δ+) coordination bonds. The monovalent oxidation state of the Al center of these complexes was confirmed by X-ray photoelectron spectroscopy (XPS). The hydroalumination of carbodiimide and the reduction of CO2 to CO were also demonstrated.

Synthesis and characterization of NiAl-hydride heterometallics: perturbing electron density within Al-H-Ni subunits

Heterometallic hydride complexes are of growing interest due to their potential to contribute to highly active insertion-based catalysis; however, methods to modulate electron density within this class of molecules are underexplored. Addition of ancillary ligands to heterotrimetallic NiAl2H2 species (1) results in the formation of heterobimetallic NiAl-hydride complexes with varying phosphine donors (2-(L)2). Incorporation of sigma donating ancillary ligands of increasing strength led to contractions of the Ni-Al distances correlated to a strengthening of a back donation interaction to the Al-H sigma antibonding orbital, most prominently present in 2-(PMe3)2. Demethylation of the aryl ether from 2-(PMe3)2 provides access to a novel anionic nickel-aluminum complex (3) with a maintained bridged hydride moiety between Ni and Al. Increased negative charge in complex 3 results in an elongation of the Ni-Al interaction. Combined crystallographic, spectroscopic, and computational studies support a 3-center interaction within the Al-H-Ni subunits and were used to map the degree of Ni-H character of the series within the Al-H-Ni bonding continuum.

Lessons from recent theoretical treatments of Al-M bonds (M = Fe, Cu, Ag, Au) that capture CO2

Complexes with Al-M bonds (M = transition metal) have emerged as platforms for discovering new reaction chemistry either through cooperative bond activation behaviour of the heterobinuclear unit or by modifying the properties of the M site through its interaction with the Al centre. Therefore, elucidating the nature of Al-M bonding is critical to advancing this research area and typically involves careful theoretical modelling. This Frontier article reviews selected recent case studies that included theoretical treatments of Al-M bonds, specifically highlighting complexes capable of cooperative CO2 activation and focusing on extracting lessons particular to the Al-M sub-field that will inform future studies with theoretical/computational components.

Bimetallic H2 Addition and Intramolecular Caryl-H Activation Mediated by an Iron-Zinc Hydride

Heteronuclear Fe(μ-H)Zn hydride Cp*Fe(1,2-Cy2PC6H4)HZnEt (3) undergoes reversible intramolecular Caryl-H reductive elimination through coupling of the cyclometalated phosphinoaryl ligand and the hydride, giving rise to a formal Fe(0)-Zn(II) species. Addition of CO intercepts this equilibrium, affording Cp*(Cy2PPh)(CO)Fe-ZnEt that features a dative Fe-Zn bond. Significantly, this system achieves bimetallic H2 addition, as demonstrated by the transformation of the monohydride Fe(μ-H)Zn to a deuterated dihydride Fe-(μ-D)2-Zn upon reaction with D2.

Accessing metal-specific orbital interactions in C-H activation with resonant inelastic X-ray scattering

Photochemically prepared transition-metal complexes are known to be effective at cleaving the strong C-H bonds of organic molecules in room temperature solutions. There is also ample theoretical evidence that the two-way, metal to ligand (MLCT) and ligand to metal (LMCT), charge-transfer between an incoming alkane C-H group and the transition metal is the decisive interaction in the C-H activation reaction. What is missing, however, are experimental methods to directly probe these interactions in order to reveal what determines reactivity of intermediates and the rate of the reaction. Here, using quantum chemical simulations we predict and propose future time-resolved valence-to-core resonant inelastic X-ray scattering (VtC-RIXS) experiments at the transition metal L-edge as a method to provide a full account of the evolution of metal-alkane interactions during transition-metal mediated C-H activation reactions. For the model system cyclopentadienyl rhodium dicarbonyl (CpRh(CO)2), we demonstrate, by simulating the VtC-RIXS signatures of key intermediates in the C-H activation pathway, how the Rh-centered valence-excited states accessible through VtC-RIXS directly reflect changes in donation and back-donation between the alkane C-H group and the transition metal as the reaction proceeds via those intermediates. We benchmark and validate our quantum chemical simulations against experimental steady-state measurements of CpRh(CO)2 and Rh(acac)(CO)2 (where acac is acetylacetonate). Our study constitutes the first step towards establishing VtC-RIXS as a new experimental observable for probing reactivity of C-H activation reactions. More generally, the study further motivates the use of time-resolved VtC-RIXS to follow the valence electronic structure evolution along photochemical, photoinitiated and photocatalytic reactions with transition metal complexes.

Vinylic C-H Activation of Styrenes by an Iron-Aluminum Complex

The oxidative addition of sp2 C-H bonds of alkenes to single-site transition-metal complexes is complicated by the competing π-coordination of the C═C double bond, limiting the examples of this type of reactivity and onward applications. Here, we report the C-H activation of styrenes by a well-defined bimetallic Fe-Al complex. These reactions are highly selective, resulting in the (E)-β-metalation of the alkene. For this bimetallic system, alkene binding appears to be essential for the reaction to occur. Experimental and computational insights suggest an unusual reaction pathway in which a (2 + 2) cycloaddition intermediate is directly converted into the hydrido vinyl product via an intramolecular sp2 C-H bond activation across the two metals. The key C-H cleavage step proceeds through a highly asynchronous transition state near the boundary between a concerted and a stepwise mechanism influenced by the resonance stabilization ability of the aryl substituent. The metalated alkenes can be further functionalized, which has been demonstrated by the (E)-selective phosphination of the employed styrenes.

Cooperative activation of carbon-hydrogen bonds by heterobimetallic systems

The direct activation of C-H bonds has been a rich and active field of organometallic chemistry for many years. Recently, incredible progress has been made and important mechanistic insights have accelerated research. In particular, the use of heterobimetallic complexes to heterolytically activate C-H bonds across the two metal centers has seen a recent surge in interest. This perspective article aims to orient the reader in this fast moving field, highlight recent progress, give design considerations for further research and provide an optimistic outlook on the future of catalytic C-H functionalization with heterobimetallic complexes.

Cooperation towards nobility: equipping first-row transition metals with an aluminium sword

The exploration for noble metals substitutes in catalysis has become a highly active area of research, driven by the pursuit of sustainable chemical processes. Although the utilization of base metals holds great potential as an alternative, their successful implementation in predictable catalytic processes necessitates the development of appropriate ligands. Such ligands must be capable of controlling their intricate redox chemistry and promote two-electron events, thus mimicking well-established organometallic processes in noble metal catalysis. While numerous approaches for infusing nobility to base metals have been explored, metal-ligand cooperation has garnered significant attention in recent years. Within this context, aluminium-based ligands offer interesting features to fine-tune the activity of metal centres, but their application in base metal catalysis remains largely unexplored. This perspective seeks to highlight the most recent breakthroughs in the reactivity of heterobimetallic aluminium-base-metal complexes, while also showcasing their potential to develop novel and predictable catalytic transformations. By turning the spotlight on such heterobimetallic species, we aim to inspire chemists to explore aluminium-base-metal species and expand the range of their applications as catalysts.

Double Deprotonation of CH3 CN by an Iron-Aluminium Complex

Herein we present the first double deprotonation of acetonitrile (CH3 CN) using two equivalents of a bimetallic iron-aluminium complex. The products of this reaction contain an exceeding simple yet rare [CHCN]2- dianion moiety that bridges two metal fragments. DFT calculations suggest that the bonding to the metal centres occurs through heavily polarised covalent interactions. Mechanistic studies reveal the intermediacy of a monomeric [CH2 CN]- complex, which has been characterised in situ. Our findings provide an important example in which a bimetallic metal complex achieves a new type of reactivity not previously encountered with monometallic counterparts.[1, 2] The isolation of a [CHCN]2- dianion through simple deprotonation of CH3 CN also offers the possibility of establishing a broader chemistry of this motif.

Double Deprotonation of CH3CN by an Iron-Aluminium Complex

Herein we present the first double deprotonation of acetonitrile (CH3CN) using two equivalents of a bimetallic iron-aluminium complex. The products of this reaction contain an exceeding simple yet rare [CHCN]2- dianion moiety that bridges two metal fragments. DFT calculations suggest that the bonding to the metal centres occurs through heavily polarised covalent interactions. Mechanistic studies reveal the intermediacy of a monomeric [CH2CN]- complex, which has been characterised in situ. Our findings provide an important example in which a bimetallic metal complex achieves a new type of reactivity not previously encountered with monometallic counterparts.[1, 2] The isolation of a [CHCN]2- dianion through simple deprotonation of CH3CN also offers the possibility of establishing a broader chemistry of this motif.

Fe-Catalyzed Difunctionalization of Aryl Titanates Enabled by Fe/Ti Synergism

Fe-catalyzed difunctionalization of aryl titanates via double C-H activation has been developed, where aryl titanates were arylated via ortho C-H activation, followed by ipso electrophilic trapping of the C-Ti bond. The ortho C-H arylation should be promoted by a 1,2-Fe/Ti synergistic heterobimetallic arylene intermediate and represents an ortho C-H ferration directed by a readily transformable C-Ti group. Common benzamides, esters, and nitriles function as arylating reagents, which involves another ortho C-H activation directed by these functionalities.

Heavy metalla vinyl-cations show metal-Lewis acid cooperativity in reaction with small molecules (NH3, N2H4, H2O, H2)

Halide abstraction from tetrylidene complexes [TbbE(Br)IrH(PMe3)3] [E = Ge (1), Sn (2)] and [Ar*E(Cl)IrH(PMe3)3] gives the salts [TbbEIrH(PMe3)3][BArF 4] [E = Ge (3), Sn (4)] and [Ar*EIrH(PMe3)3][BArF 4] [E = Ge (3'), E = Sn (4')] (Tbb = 2,6-[CH(SiMe3)2]2-4-(t-Bu)C6H2, Ar* = 2,6-Trip2C6H3, Trip = 2,4,6-triisopropylphenyl). Bonding analysis suggests their most suitable description as metalla-tetrela vinyl cations with an Ir[double bond, length as m-dash]E double bond and a near linear coordination at the Ge/Sn atoms. Cationic complexes 3 and 4 oxidatively add NH3, N2H4, H2O, HCl, and H2 selectively to give: [TbbGe(NH2)IrH2(PMe3)3][BArF 4] (5), [TbbE(NHNH2)IrH2(PMe3)3][BArF 4] [E = Ge (7), Sn (8)], [TbbE(OH)IrH2(PMe3)3][BArF 4] [E = Ge (9), Sn (10)], [TbbE(Cl)IrH2(PMe3)3][BArF 4] [E = Ge (11a), Sn (12a)], [TbbGe(H)IrH2(PMe3)3][BArF 4] (13), [TbbSn(μ-H3)Ir(PMe3)3][BArF 4] (14), and [TbbSn(H)IrH2(PMe3)3][BArF 4] (15). 14 isomerizes to give 15via an 1,2-H shift reaction. Hydride addition to cation 3 gives a mixture of products [TbbGeHIrH(PMe3)3] (16) and [TbbGeIrH2(PMe3)3] (17) and a reversible 1,2-H shift between 16 and 17 was studied. In the tin case 4 the dihydride [TbbSnIrH2(PMe3)3] (18) was isolated exclusively. The PMe3 and PEt3 derivatives, 18 and [TbbSnIrH2(PEt3)3] (19), respectively, could also be synthesized in reaction of [TbbSnH2]- with the respective chloride [(R3P) n IrCl] (R = Me, n = 4; R = Et, n = 3). Reaction of complex 19 with CO gives the substitution product [TbbSnIrH2(CO)(PEt3)2] (20). Further reaction with CO results in hydrogen transfer from the iridium to the tin atom to give [TbbSnH2Ir(CO)2(PEt3)2] (21). The reversibility of this ligand induced reductive elimination transferring 20 to 21 is shown.

Diverse reactivity of an iron-aluminium complex with substituted pyridines

The reaction of an Fe-Al complex with an array of substituted pyridines is reported. Depending on the substitution pattern of the substrate site-selective sp2 or sp3 C-H bond activation is observed. A series of reaction products are observed based on (i) C-Al bond formation, (ii) C-C bond formation by nucleophilic addition or (iii) deprotonation of the β-diketiminate ligand. A divergent set of mechanisms involving a common intermediate is proposed.