Cooperative Activation of CO2 and Epoxide by a Heterobinuclear Al-Fe Complex via Radical Pair Mechanisms

Fixation of CO2 with Epoxides Catalyzed by Pincer-Type Azo-Aromatic Complexes of Cobalt as Catalysts

Employing a series of azo-aromatic pincer-type cobalt(II) complexes, 1-5, and an imine-based cobalt complex, 6, a highly efficient catalytic protocol for the cycloaddition of CO2 with epoxides at low pressure of CO2 is reported. The electron-withdrawing group-substituted ligands containing complexes 2 and 4 were most efficient. The catalytic protocol with 2 involved a synergistic participation of an azo-aromatic catalyst (0.1 mol %) and tetra-butyl ammonium iodide (TBAI), a cocatalyst (0.2 mol %) at 90 °C temperature, and 1 bar CO2 pressure. A very good conversion, high turnover number, and reusability were observed. Complex 4 worked directly in the reaction, and its efficiency was similar to the efficiency of 2 and TBAI. As 2 was synthesized from a cheaper CoCl2, 2 showed to be more stable than 4; the combination of 2 and TBAI was used for a detailed study. The imine-based complex 6 was less efficient than the corresponding azo-aromatic complex 5. The catalytic protocol was versatile. It was also very effective for the full conversion of bis-epoxides to bis-carbonates at only 2 bar of CO2 pressure in 24 h. The reaction mechanism was investigated using various spectroscopic and computational studies.

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.

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.

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.

Heterodinuclear AuNi(CO)n- (n = 2-3) Complexes Featuring an Anionic Au- as a Donor Ligand for Ni(CO)n

The gas-phase heterodinuclear gold-nickel carbonyl AuNi(CO)n- (n = 2-3) anion complexes were mass-selected and studied by using photoelectron velocity-map imaging spectroscopy in combination with quantum-chemical calculations, which can establish both the geometries and electronic structures of these anions. These complexes are all confirmed to be singlet ground states with one gold atom bonded at the central nickel atom of the Ni(CO)n moieties. Further bonding analyses indicate that unlike the alkali-metals as covalently bonded ligands to form the electron-sharing alkali-metal-nickel bonding in the alkali-metal-nickel carbonyl anionic complexes, the Au atom in the AuNi(CO)n- complexes serves as a datively bound ligand for Ni(CO)n to form gold-to-nickel dative bonding.

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.

Mechanochemical synthesis of iron aluminyl complexes

A series of iron aluminyl complexes have been synthesised in good crystalline yields from reactions between bulky diamido aluminium iodide complexes and K[Fe(CO)2Cp] in the solid state. The series of metal-metal bonded complexes have been characterised by X-ray crystallography and were investigated using density functional theory to probe the effects of ligand substitution on the Al-Fe bond.

CO2 cleavage by tantalum/M (M = iridium, osmium) heterobimetallic complexes

A novel Ta/Os heterobimetallic complex, [Ta(CH2tBu)3(μ-H)3OsCp*], 2, is prepared by protonolysis of Ta(CHtBu)(CH2tBu)3 with Cp*OsH5. Treatment of 2 and its iridium analogue [Ta(CH2tBu)3(μ-H)2IrCp*], 1, with CO2 under mild conditions reveal the efficient cleavage of CO2, driven by the formation of a tantalum oxo species in conjunction with CO transfer to the osmium or iridium fragments, to form Cp*Ir(CO)H2 and Cp*Os(CO)H3, respectively. This bimetallic reactivity diverges from more classical CO2 insertion into metal-X (X = metal, hydride, alkyl) bonds.

π-Bonding of Group 11 Metals to a Tantalum Alkylidyne Alkyl Complex Promotes Unusual Tautomerism to Bis-alkylidene and CO2 to Ketenyl Transformation

A salt metathesis synthetic strategy is used to access rare tantalum/coinage metal (Cu, Ag, Au) heterobimetallic complexes. Specifically, complex [Li(THF)2][Ta(CtBu)(CH2tBu)3], 1, reacts with (IPr)MCl (M = Cu, Ag, Au, IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) to afford the alkylidyne-bridged species [Ta(CH2tBu)3(μ-CtBu)M(IPr)] 2-M. Interestingly, π-bonding of group 11 metals to the Ta─C moiety promotes a rare alkylidyne alkyl to bis-alkylidene tautomerism, in which compounds 2-M are in equilibrium with [Ta(CHtBu)(CH2tBu)2(μ-CHtBu)M(IPr)] 3-M. This equilibrium was studied in detail using NMR spectroscopy and computational studies. This reveals that the equilibrium position is strongly dependent on the nature of the coinage metal going down the group 11 triad, thus offering a new valuable avenue for controlling this phenomenon. Furthermore, we show that these uncommon bimetallic couples could open attractive opportunities for synergistic reactivity. We notably report an uncommon deoxygenative carbyne transfer to CO2 resulting in rare examples of coinage metal ketenyl species, (tBuCCO)M(IPr), 4-M (M = Cu, Ag, Au). In the case of the Ta/Li analogue 1, the bis(alkylidene) tautomer is not detected, and the reaction with CO2 does not cleanly yield ketenyl species, which highlights the pivotal role played by the coinage metal partner in controlling these unconventional reactions.

Ring-Opening Polymerization of Cyclohexene Oxide and Cycloaddition with CO2 Catalyzed by Amine Triphenolate Iron(III) Complexes

A series of novel amine triphenolate iron complexes were synthesized and characterized using UV, IR, elemental analysis, and high-resolution mass spectrometry. These complexes were applied to the ring-opening polymerization (ROP) of cyclohexene oxide (CHO), demonstrating excellent activity (TOF > 11050 h-1) in the absence of a co-catalyst. In addition, complex C1 maintained the dimer in the presence of the reaction substrate CHO, catalyzing the ring-opening polymerization of CHO to PCHO through bimetallic synergy. Furthermore, a two-component system consisting of iron complexes and TBAB displayed the ability to catalyze the reaction of CHO with CO2, resulting in the formation of cis-cyclic carbonate with high selectivity. Complex C4 exhibited the highest catalytic activity, achieving 80% conversion of CHO at a CHO/C4/TBAB molar ratio of 2000/1/8 and a CO2 pressure of 3 MPa for 16 h at 100 °C, while maintaining >99% selectivity of cis-cyclic carbonates, which demonstrated good conversion and selectivity.

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.

Synthesis and Modular Reactivity of Low Valent Al/Zn Heterobimetallics Supported by Common Monodentate Amides

Recent advances on low valent main group metal chemistry have shown the excellent potential of heterobimetallic complexes derived from Al(I) to promote cooperative small molecule activation processes. A signature feature of these complexes is the use of bulky chelating ligands which act as spectators providing kinetic stabilization to their highly reactive Al-M bonds. Here we report the synthesis of novel Al/Zn bimetallics prepared by the selective formal insertion of AlCp* into the Zn-N bond of the utility zinc amides ZnR2 (R=HMDS, hexamethyldisilazide; or TMP, 2,2,6,6-tetramethylpiperidide). By systematically assessing the reactivity of the new [(R)(Cp*)AlZn(R)] bimetallics towards carbodiimides, structural and mechanistic insights have been gained on their ability to undergo insertion in their Zn-Al bond. Disclosing a ligand effect, when R=TMP, an isomerization process can be induced giving [(TMP)2AlZn(Cp*)] which displays a special reactivity towards carbodiimides and carbon dioxide involving both its Al-N bonds, leaving its Al-Zn bond untouched.

Coordination-induced O-H/N-H bond weakening by a redox non-innocent, aluminum-containing radical

Several renewable energy schemes aim to use the chemical bonds in abundant molecules like water and ammonia as energy reservoirs. Because the O-H and N-H bonds are quite strong (>100 kcal/mol), it is necessary to identify substances that dramatically weaken these bonds to facilitate proton-coupled electron transfer processes required for energy conversion. Usually this is accomplished through coordination-induced bond weakening by redox-active metals. However, coordination-induced bond weakening is difficult with earth's most abundant metal, aluminum, because of its redox inertness under mild conditions. Here, we report a system that uses aluminum with a redox non-innocent ligand to achieve significant levels of coordination-induced bond weakening of O-H and N-H bonds. The multisite proton-coupled electron transfer manifold described here points to redox non-innocent ligands as a design element to open coordination-induced bond weakening chemistry to more elements in the periodic table.

Cooperative small molecule activation by apolar and weakly polar bonds through the lens of a suitable computational protocol

Small molecule activation processes are central in chemical research and cooperativity is a valuable tool for the fine-tuning of the efficiency of these reactions. In this contribution, we discuss recent and remarkable examples in which activation processes are mediated by bimetallic compounds featuring apolar or weakly polar metal-metal bonds. Relevant experimental breakthroughs are thoroughly analyzed from a computational perspective. We highlight how the rational and non-trivial application of selected computational approaches not only allows rationalization of the observed reactivities but also inferring of general principles applicable to activation processes, such as the breakdown of the structure-reactivity relationship in carbon dioxide activation in a cooperative framework. We finally provide a simple yet unbiased computational protocol to study these reactions, which can support experimental advances aimed at expanding the range of applications of apolar and weakly polar bonds as catalysts for small molecule activation.

Photochemical H2 activation by an Zn-Fe heterometallic: a mechanistic investigation

Addition of H2 to a Zn-Fe complex was observed to occur under photochemical conditions (390 or 428 nm LED) and leads to the formation of a heterometallic dihydride complex. The reaction does not occur under thermal conditions and DFT calculations suggest this is an endergonic, light driven process. Through a combined experimental and computational approach, the plausible mechanisms for H2 activation were investigated. Inhibition experiments, double-label cross-over experiments, radical trapping experiments, EPR spectroscopy and DFT calculations were used to gain insight into this system. The combined data are consistent with two plausible mechanisms, the first involving ligand dissociation followed by oxidative addition of H2 at the Fe centre, the second involving homolytic fragmentation of the Zn-Fe heterometallic and formation of radical intermediates.

Two active species from a single metal halide precursor: a case study of highly productive Mn-catalyzed dehydrogenation of amine-boranes via intermolecular bimetallic cooperation

Metal-metal cooperation for inert bond activation is a ubiquitous concept in coordination chemistry and catalysis. While the great majority of such transformations proceed via intramolecular mode in binuclear complexes, to date only a few examples of intermolecular small molecule activation using usually bimetallic frustrated Lewis pairs (Mδ+⋯M'δ-) have been reported. We introduce herein an alternative approach for the intermolecular bimetallic cooperativity observed in the catalytic dehydrogenation of amine-boranes, in which the concomitant activation of N-H and B-H bonds of the substrate via the synergetic action of Lewis acidic (M+) and basic hydride (M-H) metal species derived from the same mononuclear complex (M-Br). It was also demonstrated that this system generated in situ from the air-stable Mn(i) complex fac-[(CO)3(bis(NHC))MnBr] and NaBPh4 shows high activity for H2 production from several substrates (Me2NHBH3, tBuNH2BH3, MeNH2BH3, NH3BH3) at low catalyst loading (0.1% to 50 ppm), providing outstanding efficiency for Me2NHBH3 (TON up to 18 200) that is largely superior to all known 3d-, s-, p-, f-block metal derivatives and frustrated Lewis pairs (FLPs). These results represent a step forward towards more extensive use of intermolecular bimetallic cooperation concepts in modern homogeneous catalysis.

Controlling the Activation at NiII -CO2 2- Moieties through Lewis Acid Interactions in the Second Coordination Sphere

Nickel complexes with a two-electron reduced CO2 ligand (CO2 2- , "carbonite") are investigated with regard to the influence alkali metal (AM) ions have as Lewis acids on the activation of the CO2 entity. For this purpose complexes with NiII (CO2 )AM (AM=Li, Na, K) moieties were accessed via deprotonation of nickel-formate compounds with (AM)N(i Pr)2 . It was found that not only the nature of the AM ions in vicinity to CO2 affect the activation, but also the number and the ligation of a given AM. To this end the effects of added (AM)N(R)2 , THF, open and closed polyethers as well as cryptands were systematically studied. In 14 cases the products were characterized by X-ray diffraction and correlations with the situation in solution were made. The more the AM ions get detached from the carbonite ligand, the lower is the degree of aggregation. At the same time the extent of CO2 activation is decreased as indicated by the structural and spectroscopic analysis and reactivity studies. Accompanying DFT studies showed that the coordinating AM Lewis acidic fragment withdraws only a small amount of charge from the carbonite moiety, but it also affects the internal charge equilibration between the LtBu Ni and carbonite moieties.

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.

Donor-Acceptor Activation of Carbon Dioxide

The activation and functionalization of carbon dioxide entails great interest related to its abundance, low toxicity and associated environmental problems. However, the inertness of CO2 has posed a challenge towards its efficient conversion to added-value products. In this review we discuss one of the strategies that have been widely used to capture and activate carbon dioxide, namely the use of donor-acceptor interactions by partnering a Lewis acidic and a Lewis basic fragment. This type of CO2 activation resembles that found in metalloenzymes, whose outstanding performance in catalytically transforming carbon dioxide encourages further bioinspired research. We have divided this review into three general sections based on the nature of the active sites: metal-free examples (mainly formed by frustrated Lewis pairs), main group-transition metal combinations, and transition metal heterobimetallic complexes. Overall, we discuss one hundred compounds that cooperatively activate carbon dioxide by donor-acceptor interactions, revealing a wide range of structural motifs.

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.

Activation of robust bonds by carbonyl complexes of Mn, Fe and Co

Metal carbonyl complexes possess among the most storied histories of any compound class in organometallic chemistry. Nonetheless, these old dogs continue to be taught new tricks. In this Feature, we review the historic discoveries and recent advances in cleaving robust bonds (e.g., C-H, C-O, C-F) using carbonyl complexes of three metals: Mn, Fe, and Co. The use of Mn, Fe, and Co carbonyl catalysts in controlling selectivity during hydrofunctionalization reactions is also discussed. The chemistry of these earth-abundant metals in the field of robust bond functionalization is particularly relevant in the context of sustainability. We expect that an up-to-date perspective on these seemingly simple organometallic species will emphasize the wellspring of reactivity that continues to be available for discovery.

Frustrated Radical Pairs in Organic Synthesis

Frustrated radical pairs (FRPs) describe the phenomenon that two distinct radicals─which would otherwise annihilate each other to form a closed-shell covalent adduct─can coexist in solution, owing to steric repulsion or weak bonding association. FRPs are typically formed via spontaneous single-electron transfer between two sterically encumbered precursors─an oxidant and a reductant─under ambient conditions. The two components of a FRP exhibit orthogonal chemical properties and can often act in cooperativity to achieve interesting radical reactivities. Initially observed in the study of traditional frustrated Lewis pairs, FRPs have recently been shown to be capable of homolytically activating various chemical bonds. In this Perspective, we will discuss the discovery of FRPs, their fundamental reactivity in chemical bond activation, and recent developments of their use in synthetic organic chemistry, including in C-H bond functionalization. We anticipate that FRPs will provide new reaction strategies for solving challenging problems in modern organic synthesis.

Clustering-Evolved Frontier Orbital for Low-Temperature CO2 Dissociation

In this study, single Ni2 clusters (two Ni atoms bridged by a lattice oxygen) are successfully synthesized on monolayered CuO. They exhibit a remarkable activity toward low-temperature CO2 thermal dissociation, in contrast to cationic Ni atoms that nondissociatively adsorb CO2 and metallic Ni ones that are chemically inert for CO2 adsorption. Density functional theory calculations reveal that the Ni2 clusters can significantly alter the spatial symmetry of their unoccupied frontier orbitals to match the occupied counterpart of the CO2 molecule and enable its low-temperature dissociation. This study may help advance single-cluster catalysis and exploit the unexcavated mechanism for low-temperature CO2 activation.

Bimetallic Sites for Catalysis: From Binuclear Metal Sites to Bimetallic Nanoclusters and Nanoparticles

Heterogeneous bimetallic catalysts have broad applications in industrial processes, but achieving a fundamental understanding on the nature of the active sites in bimetallic catalysts at the atomic and molecular level is very challenging due to the structural complexity of the bimetallic catalysts. Comparing the structural features and the catalytic performances of different bimetallic entities will favor the formation of a unified understanding of the structure-reactivity relationships in heterogeneous bimetallic catalysts and thereby facilitate the upgrading of the current bimetallic catalysts. In this review, we will discuss the geometric and electronic structures of three representative types of bimetallic catalysts (bimetallic binuclear sites, bimetallic nanoclusters, and nanoparticles) and then summarize the synthesis methodologies and characterization techniques for different bimetallic entities, with emphasis on the recent progress made in the past decade. The catalytic applications of supported bimetallic binuclear sites, bimetallic nanoclusters, and nanoparticles for a series of important reactions are discussed. Finally, we will discuss the future research directions of catalysis based on supported bimetallic catalysts and, more generally, the prospective developments of heterogeneous catalysis in both fundamental research and practical applications.

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.

C-H bond activations by the HO˙/(Salophent-Bu)Co(II) radical pair generated via homolysis of a terminal Co(III)-OH bond

The reactive HO˙/(Salophen^t-Bu)Co(II) radical pair was observed to be generated via homolysis of the terminal Co(III)-OH bond in transient (Salophen^t-Bu)(L)Co(III)(OH) (L = Py, MeOH) complexes as indicated by UV-Vis and EPR measurements. Based on this elementary process, C-H bond activations in acetone, 2-butanone, acetonitrile and benzene were achieved under ambient conditions. For the reactions of the first three substrates, the alkylcobalt(III) complexes were formed as the products.

An Aluminum Telluride with a Terminal Al=Te Bond and its Conversion to an Aluminum Tellurocarbonate by CO2 Reduction

Facile access to dimeric heavier aluminum chalcogenides [(NHC)Al(Tipp)-μ-Ch]₂ (NHC=IiPr (1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene, IMe₄ (1,3,4,5-tetramethylimidazol-2-ylidene); Tipp=2,4,6-iPr₃ C₆ H₂ ; Ch=Se, Te) by treatment of NHC-stabilized aluminum dihydrides with elemental Se and Te is reported. The higher affinity of IMe₄ in comparison with IiPr toward the Al center in [(NHC)Al(Tipp)-μ-Ch]₂ can be used for ligand exchange. Additionally, the presence of excess IMe₄ allows for cleavage of the dimers to form a rare example of a neutral multiply bonded heavier aluminum chalcogenide in the form of a tetracoordinate aluminum complex, (IMe₄ )₂ (Tipp)Al=Te. This species reacts with three equivalents of CO₂ across two Al-C^NHC and the Al=Te bond affording a pentacoordinate aluminum complex containing a dianionic tellurocarbonate ligand [CO₂ Te]^2- , which is the first example of tellurium analogue of a carbonate [CO₃ ]^2- .

Widening the Landscape of Small Molecule Activation with Gold-Aluminyl Complexes: A Systematic Study of E-H (E=O, N) Bonds, SO2 and N2 O Activation

The electronic features of gold-aluminyl complexes have been thoroughly explored. Their similarity with Group 14 dimetallenes and other metal-aluminyl complexes suggests that their reactivity with small molecules beyond carbon dioxide could be accessed. In this work, the reactivity of the [^t Bu₃ PAuAl(NON)] (NON=4,5-bis(2,6 diisopropylanilido)-2,7-ditert-butyl-9,9-dimethylxanthene) complex towards water, ammonia, sulfur dioxide and nitrous oxide is computationally explored. The reaction mechanisms computed for each substrate strongly suggest that all activation processes are in principle experimentally feasible. Electronic structure analysis highlights that, in all cases, the reactivity is driven by the presence of the poorly polarized electron-sharing gold-aluminyl bond, which induces a radical-like reactivity of the complex towards all the substrates. A flat topology of the potential energy surface (PES) has been found for the reaction with N₂ O, where two almost isoenergetic transition states can be located along the same reaction coordinate with different geometries, suggesting that the N₂ O binding mode may not be a good indicator of the nature of N₂ O activation in a cooperative bimetallic reactivity. In addition, the catalytic potentialities of these complexes have been explored in the framework of nitrous oxide reduction. The study reveals that the [^t Bu₃ PAuAl(NON)] complex might be an efficient catalyst towards oxidation of phosphines (and boranes) via N₂ O reduction. These findings underline recurring trends in the novel chemistry of gold-aluminyl complexes and call for experimental feedbacks.

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.

Origins of Stereospecificity and Divergent Reactivity of Pd-Catalyzed Cross Coupling with α,α-Disubstituted Alkenyl Hydrazones

This article presents an exploration of stereospecificity and divergent reactivity of Pd-catalyzed α,α-disubstituted alkenyl hydrazones to synthesize 1,4-dienes in the Z configuration and vinylcyclopropane. We calculated the energy profiles of four α,α-disubstituted alkenyl hydrazones. The results show that the energy profiles of the whole catalytic cycle are basically the same before the syn-carbopalladation step. Subsequent syn-β-C elimination yields skipping dienes, or direct β-H elimination yields vinylcyclopropane. Current theoretical calculations reveal that the stereospecificity and the divergent reactivity of reactions result from the competition between syn-β-C elimination and β-H elimination. The C-C bond rotation and subsequent syn-β-C elimination step control the stereospecificity of the reaction by changing the olefin stereostructure from E to Z configuration. The steric factor of α-substituted groups mediates the transformation between syn-β-C elimination and β-H elimination. The results are of great significance for the scientific design of substrates to achieve accurate synthesis of target products.

Divergent CO2 Activation by Tuning the Lewis Acid in Iron-Based Bimetallic Systems

Bimetallic motifs mediate the selective activation and functionalization of CO2 in metalloenzymes and some recent synthetic systems. In this work, we build on the nascent concept of bimetallic frustrated Lewis pairs (FLPs) to investigate the activation and reduction of CO2 . Using the Fe0 fragment [(depe)2 Fe] (depe=1,2-bis(diethylphosphino)ethane) as base, we modify the nature of the partner Lewis acid to accomplish a divergent and highly chemoselective reactivity towards CO2 . [Au(PMe2 Ar)]+ irreversibly dissociates CO2 , Zn(C6 F5 )2 and B(C6 F5 )3 yield different CO2 adducts stabilized by push-pull interactions, while Al(C6 F5 )3 leads to a rare heterobimetallic C-O bond cleavage, and thus to contrasting reduced products after exposure to dihydrogen. Computational investigations provide a rationale for the divergent reactivity, while Energy Decomposition Analysis-Natural Orbital for Chemical Valence (EDA-NOCV) method substantiates the heterobimetallic bonding situation.

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.

Reactivity of Unsupported Transition Metal-Aluminyl Complexes: A Nucleophilic TM-Al Bond

Despite the long history of research in transition metal (TM) complexes, the study of TM-aluminyl complexes is still in its early stage of development. It is expected that the presence of an electropositive Al donor atom would open up new possibilities in TM complex reactivity, and indeed TM-aluminyl has shown an early sign of success in small-molecule activation. On the other hand, the existing reports on TM-aluminyl reactivity are often explained to readers with different understanding on individual cases, and a general picture of TM-aluminyl reactivity is still not available. In this work, we have attempted to provide a systematic picture to explain some early explorations in this field, specifically a series of recently reported heteroallene insertion reactions involving unsupported TM-aluminyl complexes. Through density functional theory calculations of a number of TM-aluminyl complexes, covering both Au and Cu centers, we found that their reactivity against heteroallenes (including CO₂ and carbodiimides) is mostly based on the strong nucleophilicity of the TM-Al σ-bond.

Recent advances in cooperative activation of CO2 and N2O by bimetallic coordination complexes or binuclear reaction pathways

The gaseous small molecules, CO₂ and N₂O, play important roles in climate change and ozone layer depletion, and they hold promise as underutilized reagents and chemical feedstocks. However, productive transformations of these heteroallenes are difficult to achieve because of their inertness. In nature, these gases are cycled through ecological systems by metalloenzymes featuring multimetallic active sites that employ cooperative mechanisms. Thus, cooperative bimetallic chemistry is an important strategy for synthetic systems, as well. In this Perspective, recent advances (since 2010) in cooperative activation of CO₂ and N₂O are reviewed, including examples involving s-block, p-block, d-block, and f-block metals and different combinations thereof.