Carbonyl group and carbon dioxide activation by rare-earth-metal complexes

Multi-electron redox reactivity of a samarium(ii) hydrido complex

Well-defined low-valent molecular rare-earth metal hydrides are rare, and limited to Yb2+ and Eu2+ centers. Here, we report the first example of the divalent samarium(ii) hydrido complex [(CpAr5)SmII(μ-H)(DABCO)]2 (4) (CpAr5 = C5Ar5, Ar = 3,5-iPr2-C6H3; DABCO = 1,4-diazabicyclooctane) supported by a super-bulky penta-arylcyclopentadienyl ligand, resulting from the hydrogenolysis of the samarium(ii) alkyl complex [(CpAr5)SmII{CH(SiMe3)2}(DABCO)] (3). Complex 4 exhibits multi-electron redox reactivity toward a variety of substrates. Exposure of complex 4 to CO2 results in the formation of the trivalent samarium(iii) mixed-bis-formate/carbonate complex [(CpAr5)SmIII(μ-η2:η1-O2CH)(μ-η2:η2-CO3)(μ-η1:η1-O2CH)SmIII(CpAr5)(DABCO)] (8), mediated by hydride insertion and reductive disproportionation reactions. Complex 4 shows four-electron reduction toward four equivalents of CS2 to afford the trivalent samarium(iii) bis-trithiocarbonate complex [(CpAr5)SmIII(μ-η2:η2-CS3)(DABCO)]2 (9). A mechanistic study of the formation of complex 8 was carried out using DFT calculations.

Activation and functionalisation of carbon dioxide by bis-tris(pyrazolyl)borate-supported divalent samarium and trivalent lanthanide silylamide complexes

Synthesis and reactivity with carbon dioxide (CO2) of divalent samarium in the bis-tris(pyrazolyl)borate ligand environment has been reported. In addition, CO2 activation and functionalisation by lanthanide silylamides in the bis-tris(pyrazolyl)borate ligand environment was demonstrated. Reduction of the Sm(III) precursor [Sm(Tp)2(OTf)] (Tp = hydrotris(1-pyrazolyl)borate; OTf = triflate) with KC8 yielded the insoluble Sm(II) multi-metallic coordination polymer [{Sm(Tp)2}n] 1-Sm. Addition of 1,2-dimethoxyethane (DME) to 1-Sm enabled isolation of the monomeric complex [Sm(Tp)2(DME)] 1-Sm(DME). Complex 1-Sm(DME) reduced CO2 to yield the oxalate-bridged dimeric Sm(III) complex [{Sm(Tp)2}2(μ-η2:η2-O2CCO2)] 2-Sm. The reactions of heteroleptic Ln(III) silylamide complexes [Ln(Tp)2(N'')] (Ln = Y, Sm; N'' = N(SiMe3)2) with CO2 yielded monomeric Ln(III) silyloxides [Ln(Tp)2(OSiMe3)] 3-Ln and trimethylsilyl isocyanate (OCNSiMe3). Complexes 3-Ln are the first crystallographically characterised examples of Ln(III)-OSiMe3 bonds accessed via CO2 activation and functionalisation. Full characterisation data are presented for all complexes, including solid-state molecular structure determination by single-crystal X-ray diffraction.

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.

Samarium- and Ytterbium-Grafted Periodic Mesoporous Silica for Carbon Dioxide Capture and Conversion

Immobilized coordination compounds of Lewis acidic metals are powerful catalytic components of systems for the cycloaddition of CO2 to epoxides that do not require sophisticated coordination frameworks to harness the metal center and modulate its activity. Surface organometallic chemistry (SOMC) is a valuable methodology to prepare well-defined and site-isolated surface complexes and coordination compounds on metal oxides, with ligand environments easily adjustable to a targeted catalytic reaction. In this work, the SOMC methodology is applied to prepare SmII, YbII, and SmIII alkoxide surface complexes on periodic mesoporous (organo)silica of distinct pore symmetry/size for application in the CO2 cycloaddition reaction. The surface complexes are readily accessible by the grafting of the bis(trimethylsilyl)amide precursors LnII[N(SiMe3)2]2(THF)2 (Ln = Sm, Yb) and SmIII[N(SiMe3)2]3, followed by ligand exchange with alcohols (ethanol and neopentanol). The use of periodic mesoporous supports led to hybrid materials with relatively high surface areas and pore sizes, affording good performance in CO2 capture and in the cycloaddition of CO2 to epoxides under mild conditions (60-80 °C, 1-10 bar). In terms of catalytic performance, recyclability, and low amount of added nucleophile TBAX (X = Br, I), the most active materials prepared in this work compare well to a variety of previously reported SOMC-derived surface complexes and to other heterogeneous Lewis acids displaying more elaborate ligand environments.

Highly Diastereoselective Multicomponent Synthesis of Spirocyclopropyl Oxindoles Enabled by Rare-Earth Metal Salts

The synthesis of polysubstituted spirocyclopropyl oxindoles using a series of rare-earth metal (REM) salts is reported. REMs, in particular Sc(OTf)₃, allowed access to the target compounds by a multicomponent reaction with high diastereoselectivity (≤94:6:0:0). Density functional theory calculations on the model reaction are consistent with the observed selectivity and revealed that the special coordinating capabilities and the oxophilicity of the metal are key factors in inducing the formation of one main diastereoisomer.

Direct coupling of CO2 with epoxides catalyzed by lanthanum(III) supported on magnetic mesoporous organosilica nanoparticles

Lanthanum(III) supported on the magnetic mesoporous organosilica nanoparticle (La@MON) has been described as an efficient, simple, and durable heterogeneous catalyst for the synthesis of 5-membered cyclic carbonates from carbon dioxide (CO₂) and epoxides. Under optimized reaction conditions, various terminal epoxides have been converted to the corresponding carbonates in the presence of 0.3 mol% La@MON and 0.5 mol% tetrabutylammonium iodide (TBAI) as co-catalyst at relatively mild reaction conditions. It was also found that La@MON catalysts had significantly higher catalytic activity than some selected reference catalysts, which can be explained by the abundance of lanthanum(III) species acting as Lewis acidic sites for activating both carbon dioxide and epoxide molecules, along with the fact that the catalyst channels are short and provided facile mass transfer. The catalyst showed good reusability for at least five reaction cycles while the magnetic core of the catalyst helps the easy separation of the catalyst by just using an external magnet.

Recent Advances in Carbon Dioxide Adsorption, Activation and Hydrogenation to Methanol using Transition Metal Carbides

The inevitable emission of carbon dioxide (CO₂ ) due to the burning of a substantial amount of fossil fuels has led to serious energy and environmental challenges. Metal-based catalytic CO₂ transformations into commodity chemicals are a favorable approach in the CO₂ mitigation strategy. Among these transformations, selective hydrogenation of CO₂ to methanol is the most promising process that not only fulfils the energy demands but also re-balances the carbon cycle. The investigation of CO₂ adsorption on the surface of heterogeneous catalyst is highly important because the formation of various intermediates which determines the selectivity of product. Transition metal carbides (TMCs) have received considerable attention in recent years because of their noble metal-like reactivity, ceramic-like properties, high chemical and thermal stability. These features make them excellent catalytic materials for a variety of transformations such as CO₂ adsorption and its conversion into value-added chemicals. Herein, the catalytic properties of TMCs are summarize along with synthetic methods, CO₂ binding modes, mechanistic studies, effects of dopant on CO₂ adsorption, and carbon/metal ratio in the CO₂ hydrogenation reaction to methanol using computational as well as experimental studies. Additionally, this Review provides an outline of the challenges and opportunities for the development of potential TMCs in CO₂ hydrogenation reactions.

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.

Structure and Reactivity of Polynuclear Divalent Lanthanide Disiloxanediolate Complexes

Trinuclear molecular complexes of europium (II) and ytterbium(II) [Ln₃{(Ph₂SiO)₂O}₃(THF)₆], 1-Ln₃L₃ (Ln = Eu and Yb), supported by the dianionic tetraphenyl disiloxanediolate ligand, were synthesized via protonolysis of the [Ln{N(SiMe₃)₂}₂(THF)₂] complexes. In contrast, the reaction of [Sm{N(SiMe₃)₂}₂(THF)₂] with the (Ph₂SiOH)₂O ligand led to the isolation of the mixed-valent Sm(II)/Sm(III) complex [Sm₃{(Ph₂SiO)₂O}₃{N(SiMe₃)₂}(THF)₄], 2-Sm₃L₃, which was crystallographically characterized. The Eu(II) complex 1-Eu₃L₃ displays weak ferromagnetic coupling between the Eu(II) metal centers (J = 0.1035 cm^-1). The addition of 3 equiv of (Ph₂SiOK)₂O to 1-Eu₃L₃ resulted in the formation of the polynuclear Eu(II) dimer of dimers [K₄Eu₂{(Ph₂SiO)₂O}₄(Et₂O)₂]₂, 3-Eu₂L₄. Complexes 1-Ln₃L₃ (Ln = Eu and Yb) are stable in solution at room temperature, while 3-Eu₂L₄ shows higher reactivity and rapidly decomposes to give the mixed-valent Eu(II)/Eu(III) species [K₃Eu₂{(Ph₂SiO)₂O}₄], 4-Eu₂L₄. Complex 1-Yb₃L₃ affects the slow reductive disproportionation of carbon dioxide, but 1-Eu₃L₃ does not display any reactivity toward CO₂. However, the presence of one additional (Ph₂SiO^-)₂O per Eu(II) metal center in 3-Eu₂L₄ increases dramatically the reductive ability of the Eu(II) metal centers, affording the first example of carbon dioxide activation by an isolated divalent europium complex. The reduction of CO₂ by 3-Eu₂L₄ is immediate, and carbonate is formed selectively after the addition of a stoichiometric amount of CO_2.

CO, CO2 and CS2 activation by divalent ytterbium hydrido complexes

Treatment of a divalent ytterbium hydride complex [(Tp^Ad,iPr)Yb(H)(THF)] (Tp^Ad,iPr = hydrotris(3-adamantyl-5-isopropyl-pyrazolyl)borate) (1) with CO, CO₂ and CS₂ resulted in the formation of a divalent ytterbium ethenediolate complex [(Tp^Ad,iPr)Yb]₂(cis-OCHCHO) (2), a formate complex [(Tp^Ad,iPr)Yb(κ²-O₂CH)(THF)] (3), and a trivalent ytterbium ethenetetrathiolate complex [(Tp^Ad,iPr)Yb^III]₂(C₂S₄) (4), respectively. DFT calculations were carried out to elucidate the reaction profiles of complexes 3 and 4.

Non-redox reactivity of V(ii) and Fe(ii) formamidinates towards CO2 resulting in the formation of novel M(ii) carbamates

Multiple CO2 insertions into the M–N bonds of V(ii) and Fe(ii) bis(formamidinates) led to the isolation of three novel carbamates. The CO2 insertion effectivity depended on the solvent used and the metal centre's coordination sphere geometry.

Yttrium tris(trimethylsilylmethyl) complexes grafted onto MCM-48 mesoporous silica nanoparticles

A series of tris(trimethylsilylmethyl) yttrium donor adduct complexes was synthesized and fully characterized by X-ray diffraction, ¹H/¹³C/²⁹Si/³¹P/⁸⁹Y heteronuclear NMR and FTIR spectroscopies as well as elemental analyses. Treatment of Y(CH₂SiMe₃)₃(thf)_x with various donors Do led to complete (Do = TMEDA, DMAP) and partial displacement of THF (Do = NHC^iPr, DMPE). Exceptionally large ⁸⁹Y NMR shifts to low field were observed for the new complexes. Complexes Y(CH₂SiMe₃)₃(tmeda) and Y(CH₂SiMe₃)₃(dmpe)(thf) were chosen to perform surface organometallic chemistry, due to a comparatively higher thermal stability and the availability of the ³¹P nucleus as a spectroscopic probe, respectively. Mesoporous nanoparticles of the MCM-48-type were synthesized and used as a 3^rd generation silica support. The parent and hybrid materials were characterized using X-ray powder diffraction, solid-state-NMR spectroscopy, DRIFTS, elemental analyses, N₂-physisorption, and scanning electron microscopy (SEM). The presence of surface-bound yttrium alkyl moieties was further proven by the reaction with carbon dioxide. Quantification of the surface silanol population by means of HN(SiHMe₂)₂-promoted surface silylation is shown to be superior to titration with lithium alkyl LiCH₂SiMe₃.

CeCl3 /n-BuLi: Unraveling Imamoto's Organocerium Reagent

CeCl₃(thf) reacts at low temperatures with MeLi, t‐BuLi, and n‐BuLi to isolable organocerium complexes. Solvent‐dependent extensive n‐BuLi dissociation is revealed by ⁷Li NMR spectroscopy, suggesting “Ce(n‐Bu)₃(thf)_x” or solvent‐separated ion pairs like “[Li(thf)₄][Ce(n‐Bu)₄(thf)_y]” as the dominant species of the Imamoto reagent. The stability of complexes Li₃Ln(n‐Bu)₆(thf)₄ increases markedly with decreasing Ln^III size. Closer inspection of the solution behavior of crystalline Li₃Lu(n‐Bu)₆(thf)₄ and mixtures of LuCl₃(thf)₂/n‐BuLi in THF indicates occurring n‐BuLi dissociation only at molar ratios of <1:3. n‐BuLi‐depleted complex LiLu(n‐Bu)₃Cl(tmeda)₂ was obtained by treatment of Li₂Lu(n‐Bu)₅(tmeda)₂ with ClSiMe₃, at the expense of LiCl incorporation. Imamoto's ketone/tertiary alcohol transformation was examined with 1,3‐diphenylpropan‐2‐one, affording 99 % of alcohol.

Yttrium germole dianion complexes with Y-Ge bonds

The reactions of dipotassium 3,4-dimethyl-2,5-bis(trimethylsilyl)-germole dianion K2[1] with YCl3 and Cp*YCl2 (Cp* = cyclopentadienyl) in THF at room temperature afforded the dianion salt [(K-cryptand-222)2][1-YCl3] (K2[2]) and the dimeric complex [1-Y-Cp*]2 (3), respectively. While the polymeric complex {[(1)2-Y-K(toluene)]2}n (4) was obtained from the reaction of K2[1] and half molar equivalent of YCl3(THF)3.5 in toluene at 80 °C. The germole dianions in complexes 3 and 4 feature η5/η1 coordination interactions with the yttrium atoms. They represent the first examples of rare earth (RE) complexes containing RE-Ge bonds other than the RE-GeR3 structural type.

Effect of Substituents of Cerium Pyrazolates and Pyrrolates on Carbon Dioxide Activation

Homoleptic ceric pyrazolates (pz) Ce(RR’pz)₄ (R = R’ = tBu; R = R’ = Ph; R = tBu, R’ = Me) were synthesized by the protonolysis reaction of Ce[N(SiHMe₂)₂]₄ with the corresponding pyrazole derivative. The resulting complexes were investigated in their reactivity toward CO₂, revealing a significant influence of the bulkiness of the substituents on the pyrazolato ligands. The efficiency of the CO₂ insertion was found to increase in the order of tBu₂pz < Ph₂pz < tBuMepz < Me₂pz. For comparison, the pyrrole-based ate complexes [Ce₂(pyr)₆(µ-pyr)₂(thf)₂][Li(thf)₄]₂ (pyr = pyrrolato) and [Ce(cbz)₄(thf)₂][Li(thf)₄] (cbz = carbazolato) were obtained via protonolysis of the cerous ate complex Ce[N(SiHMe₂)₂]₄Li(thf) with pyrrole and carbazole, respectively. Treatment of the pyrrolate/carbazolate complexes with CO₂ seemed promising, but any reversibility could not be observed.