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.

Mechanistic investigations of the Fe(ii) mediated synthesis of squaraines

The scission and homologation of CO is a fundamental process in the Fischer-Tropsch reaction. However, given the heterogeneous nature of the catalyst and forcing reaction conditions, it is difficult to determine the intermediates of this reaction. Here we report detailed mechanistic insight into the scission/homologation of CO by two-coordinate iron terphenyl complexes. Mechanistic investigations, conducted using in situ monitoring and reaction sampling techniques (IR, NMR, EPR and Mössbauer spectroscopy) and structural characterisation of isolable species, identify a number of proposed intermediates. Crystallographic and IR spectroscopic data reveal a series of migratory insertion reactions from 1Mes to 4Mes. Further studies past the formation of 4Mes suggest that ketene complexes are formed en route to squaraine 2Mes and iron carboxylate 3Mes, with a number of ketene containing structures being isolated, in addition to the formation of unbound, protonated ketene (8). The synthetic and mechanistic studies are supported by DFT calculations.

Divalent metallocenes of the lanthanides - a guideline to properties and reactivity

Since the discovery in the early 1980s, the soluble divalent metallocenes of lanthanides have become a steadily growing field in organometallic chemistry. The predominant part of the investigation has been performed with samarium, europium, and ytterbium, whereas only a few reports dealing with other rare earth elements were disclosed. Reactions of these metallocenes can be divided into two major categories: (1) formation of Lewis acid-base complexes, in which the oxidation state remains +II; and (2) single electron transfer (SET) reductions with the ultimate formation of Ln(III) complexes. Due to the increasing reducing character from Eu(II) over Yb(II) to Sm(II), the plethora of literature concerning redox reactions revolves around the metallocenes of Sm and Yb. In addition, a few reactivity studies on Nd(II), Dy(II) and mainly Tm(II) metallocenes were published. These compounds are even stronger reducing agents but significantly more difficult to handle. In most cases, the metals are ligated by the versatile pentamethylcyclopentadienyl ligand: (C₅Me₅). Other cyclopentadienyl ligands are fully covered but only discussed in detail, if the ligand causes differences in synthesis or reactivity. Thus, the focus lays on three compounds: [(C₅Me₅)₂Sm], [(C₅Me₅)₂Eu] and [(C₅Me₅)₂Yb] and their solvates. We discuss the synthesis and physical properties of divalent lanthanide metallocenes first, followed by an overview of the reactivity rendering the full potential of these versatile reactants.

CO reductive oligomerization by a divalent thulium complex and CO2-induced functionalization

The divalent thulium complex [Tm(Cpttt)2] (Cpttt = 1,2,4-tris(tert-butyl)cyclopentadienyl) reacts with CO to afford selective CO reductive dimerization and trimerization into ethynediolate (C2) and ketenecarboxylate (C3) complexes, respectively. DFT calculations were performed to shed light on the elementary steps of CO homologation and support a stepwise chain growth. The attempted decoordination of the ethynediolate fragment by treatment with Me3SiI led to dimerization and rearrangement into a 3,4-dihydroxyfuran-2-one complex. Investigation of the reactivity of the C2 and C3 complexes towards other electrophiles led to unusual functionalization reactions: while the reaction of the ketenecarboxylate C3 complex with electrophiles yielded new multicarbon oxygenated complexes, the addition of CO2 to the ethynediolate C2 complex resulted in the formation of a very reactive intermediate, allowing C-H activation of aromatic solvents. This original intermolecular reactivity corresponds to an unprecedented functionalization of CO-derived ligands, which is induced by CO2.

Molybdenum-Mediated Coupling of Carbon Monoxide to a C3 Product on a Single Metal Site

The synthesis and characterization of a series of naphthalenediyl-diphosphine molybdenum complexes are reported. A novel dicarbonyl-Mo complex (3) converts to a bis(siloxy)acetylene complex (5) upon reduction and treatment with a silyl electrophile, Me₃SiCl. This process shows exclusive C-C coupling distinct from the previously reported phenylene-linked analogue that undergoes C-O cleavage. Further CO catenation can be engendered from 5 under mild conditions providing metallacyclobutenone complex 6, with a C₃O₃ organic motif derived from CO. Differences in reactivity are assigned to the nature of the arene linker, where the naphthalenediyl fragment shows a propensity for η⁴ binding previously not observed for phenylene. Consistent with this hypothesis, a Mo precursor with a 1,3-cyclohexadienediyl-based linker was prepared which also showed exclusive formation of a bis(siloxy)acetylene complex and subsequent coupling of a third CO molecule.

Molecular Catalysts for the Reductive Homocoupling of CO2 towards C2+ Compounds

The conversion of CO2 into multicarbon (C2+ ) compounds by reductive homocoupling offers the possibility to transform renewable energy into chemical energy carriers and thereby create "carbon-neutral" fuels or other valuable products. Most available studies have employed heterogeneous metallic catalysts, but the use of molecular catalysts is still underexplored. However, several studies have already demonstrated the great potential of the molecular approach, namely, the possibility to gain a deep mechanistic understanding and a more precise control of the product selectivity. This Minireview summarizes recent progress in both the thermo- and electrochemical reductive homocoupling of CO2 toward C2+ products mediated by molecular catalysts. In addition, reductive CO homocoupling is discussed as a model for the further conversion of intermediates obtained from CO2 reduction, which may serve as a source of inspiration for developing novel molecular catalysts in the future.

Reactivity of a Sterical Flexible Pentabenzylcyclopentadienyl Samarocene

Reactivity studies of the classical divalent lanthanide compound [CpBz52Sm] (CpBz5 = pentabenzylcyclopentadienyl-anion) towards diphenyl dichalcogenides and d-element carbonyl complexes led to remarkable results. In the compounds obtained, a different number of Sm-C(phenyl) interactions and differently oriented benzyl groups were observed, suggesting—despite the preference of these interactions in [CpBz52Sm] described in previous studies—a flexible orientation of the benzyl groups and thus a variable steric shielding of the metal center by the ligand. The obtained compounds are either present as monometallic complexes (reduction of the dichalcogenides) or tetrametallic bridged compounds in the case of the d/f-element carbonyl complexes.

Reactions of aluminium(i) with transition metal carbonyls: scope, mechanism and selectivity of CO homologation

Over the past few decades, numerous model systems have been discovered that create carbon–carbon bonds from CO. These reactions are of potential relevance to the Fischer–Tropsch process, a technology that converts syngas (H₂/CO) into mixtures of hydrocarbons. In this paper, a homogeneous model system that constructs carbon chains from CO is reported. The system exploits the cooperative effect of a transition metal complex and main group reductant. An entire reaction sequence from C₁ → C₂ → C₃ → C₄ has been synthetically verified. The scope of reactivity is broad and includes a variety of transition metals (M = Cr, Mo, W, Mn, Re, Co), including those found in industrial heterogeneous Fischer–Tropsch catalysts. Variation of the transition metal fragment impacts the relative rate of the steps of chain growth, allowing isolation and structural characterisation of a rare C₂ intermediate. The selectivity of carbon chain growth is also impacted by this variable; two distinct isomers of the C₃ carbon chain were observed to form in different ratios with different transition metal reagents. Based on a combination of experiments (isotope labelling studies, study of intermediates) and calculations (DFT, NBO, ETS-NOCV) we propose a complete mechanism for chain growth that involves defined reactivity at both transition metal and main group centres.

A homogeneous model system that constructs carbon chains from CO is reported. The system exploits the cooperative effect of a transition metal complex and main group reductant. An entire reaction sequence from C₁ → C₂ → C₃ → C₄ has been synthetically verified.

Linear Hydrocarbon Chain Growth from a Molecular Diruthenium Carbide Platform

The formation of linear hydrocarbon chains by sequential coupling of C₁ units on the metal surface is the central part of the Fischer-Tropsch (F-T) synthesis. Organometallic complexes have provided numerous models of relevant individual C-C coupling events but have failed to reproduce the complete chain lengthening sequence that transforms a linear C_n hydrocarbon chain into its C_n+1 homologue in an iterative fashion. In this work, we demonstrate stepwise growth of linear C_n hydrocarbon chains and their conversion to their C_n+1 homologues via consecutive addition of CH₂ units on a molecular diruthenium carbide platform. The chain growth sequence is initiated by the formation of a μ-η¹:η¹-C═CH₂ ligand from a C + CH₂ coupling between the μ-carbido complex [(Cp*Ru)₂(η-NPh)(μ-C)] (1; Cp* = η⁵-C₅Me₅) and Ph₂SCH₂. Then, the chain propagates via a general C═CHR + CH₂ coupling and subsequent hydrogen-assisted isomerization of the resulting allene ligand μ-η¹:η³-H₂C═C═CHR to a higher vinylidene homologue μ-η¹:η¹-C═CH(CH₂)R. By repeating this reaction sequence, up to C₆ chains have been synthesized in a stepwise fashion. The key step of this chain homologation sequence is the selective hydrogenation of the μ-η¹:η³-allene unit to the corresponding μ-alkylidene ligand. Isotope labeling and computational studies indicate that this transformation proceeds via the hydrogenation of the allene ligand to a terminal alkene form and its isomerization to the μ-alkylidene ligand facilitated by the coordinatively unsaturated diruthenium platform.

Systematic reductive oligomerization of isocyanides with a vanadium(ii) complex

Using a V(ii) complex as a reducing reagent, we demonstrate controlled reduction of isocyanides, resulting in decyanation of alkyl isocyanides to form a V(iii) cyanide complex or oligomerization of aryl isocyanides to form trimers and a tetramer. The pathways leading to the trimers involve a divanadium ynediamido intermediate, which further reacts with the third isocyanide molecule to selectively produce a tri(imino)deltate or an indolenine complex, by altering the temperature and stoichiometry.

Nonanuclear zinc-gold [Zn3Au6] heterobimetallic complexes

Nonanuclear zinc-gold heterobimetallic complexes were synthesized in a two-step process. Commercially available carboxy-functionalized phosphine ligands were used for selective binding to Zn and Au centers. In the first step, bipyridine coordinated Zn-metalloligands with free phosphine moieties were prepared. Reaction of Zn-metalloligands with [AuCl(tht)] (tht = tetrahydrothiophene) resulted in the formation of nonanuclear Zn-Au heterobimetallic complexes. The flexibility of the carboxy-functionalized phosphine ligands was shown to be crucial for the formation of aurophilic interactions. Further, the photoluminescence of the Zn-metalloligands and one Zn-Au complex was investigated at room temperature as well as 77 K. The emission spectra showed clear difference between the Zn-metalloligands and the Zn-Au complex.

Structure and small molecule activation reactivity of a metallasilsesquioxane of divalent ytterbium

The first metallasilsesquioxane of a divalent lanthanide was synthetized and structurally characterized. The dinuclear Yb(ii) complex effects the two electrons reduction of azobenzene, and the selective CO₂ reduction to CO and carbonate.

3d–4f heterometallic complexes by the reduction of transition metal carbonyls with bulky LnII amidinates

Heterometallic lanthanide-transition metal carbonyl complexes [Sm₂–Co₂], [Yb–Co], and [Sm₂–Fe₃] have been synthesized by redox reactions between bulky amidinate stabilized divalent Ln and TM carbonyl complexes.

Cooperative strategies for CO homologation

Recent approaches in which at least two metal or main-group centres are involved in the homologation of CO are reviewed.