Frustrated Lewis Pair-Type Reactivity of Intermolecular Rare-Earth Aryloxide and N-Heterocyclic Carbene/Olefin Combinations

This work reports the cooperative reactivity of rare-earth aryloxide complexes with N-heterocyclic carbene (NHC) or N-heterocyclic olefin (NHO), showcasing their synergistic effect on the activation of H2 and diverse organic substrates. Reactions of RE(OAr)3 (RE=La, Sm, and Y; Ar=2,6-tBu2-C6H3) with unsaturated NHC ItBu (:C[N(R)CH]2, R=tBu) isolated abnormally bound RE metal NHC complexes RE/aNHC. In contrast, no metal-NHO adducts were formed when RE(OAr)3 were treated with NHO (R2C=C[N(R)C(R)]2, R=CH3). Both RE/aNHC and RE/NHO Lewis pairs enabled cooperative H2 activation. Furthermore, RE(OAr)3 were found to catalyze the hydrogenation of the exocyclic C=C double bond of NHO under mild conditions. Moreover, treatment of the La/aNHC complex with benzaldehyde produced a La/C4 1,2-addition product. The La/NHO Lewis pair could react with (trimethylsilyl)diazomethane and α, β-conjugated imine, affording an isocyanotrimethylsilyl lanthanum amide complex and a La/C 1,4-addition product, respectively.

Unraveling differences in aluminyl and carbene coordination chemistry: bonding in gold complexes and reactivity with carbon dioxide

The electronic properties of aluminyl anions have been reported to be strictly related to those of carbenes, which are well-known to be easily tunable via selected structural modifications imposed on their backbone. Since peculiar reactivity of gold-aluminyl complexes towards carbon dioxide has been reported, leading to insertion of CO2 into the Au-Al bond, in this work the electronic structure and reactivity of Au-Al complexes with different aluminyl scaffolds have been systematically studied and compared to carbene analogues. The analyses reveal that, instead, aluminyls and carbenes display a very different behavior when bound to gold, with the aluminyls forming an electron-sharing and weakly polarized Au-Al bond, which turns out to be poorly modulated by structural modifications of the ligand. The reactivity of gold-aluminyl complexes towards CO2 shows, both qualitatively and quantitatively, similar reaction mechanisms, reflecting the scarce tunability of their electronic structure and bond nature. This work provides further insights and perspectives on the properties of the aluminyl anions and their behavior as coordination ligands.

Aromaticity-promoted CS2 activation by heterocycle-bridged P/N-FLPs: a comparative DFT study with CO2 capture

Carbon dioxide (CO₂) capture has attracted considerable attention from both experimental and theoretical chemists. In comparison, carbon disulfide (CS₂) activation is less developed. Here, we carry out a thorough comparative density functional theory study to examine the reaction mechanisms of CS₂ activation by five-membered heterocycle-bridged P/N frustrated Lewis pairs (FLPs). Calculations suggest that despite a weaker carbon-sulfur bond, all the CS₂ activations have higher reaction barriers than the CO₂ capture, which could be attributed to electrostatic repulsion between FLPs and CS₂ caused by the reversed polarity of CS in CS₂ rather than the electrostatic attraction in CO₂ capture. In addition, aromaticity is found to play an important role in CS₂ capture as it stabilizes both the transition states and products in heterocycle-bridged FLPs. All these findings could be useful for experimentalists to realize small molecule activations by FLPs.

Recent Advances in Catalysis Involving Bidentate N-Heterocyclic Carbene Ligands

Since the discovery of persistent carbenes by the isolation of 1,3-di-l-adamantylimidazol-2-ylidene by Arduengo and coworkers, we witnessed a fast growth in the design and applications of this class of ligands and their metal complexes. Modular synthesis and ease of electronic and steric adjustability made this class of sigma donors highly popular among chemists. While the nature of the metal-carbon bond in transition metal complexes bearing N-heterocyclic carbenes (NHCs) is predominantly considered to be neutral sigma or dative bonds, the strength of the bond is highly dependent on the energy match between the highest occupied molecular orbital (HOMO) of the NHC ligand and that of the metal ion. Because of their versatility, the coordination chemistry of NHC ligands with was explored with almost all transition metal ions. Other than the transition metals, NHCs are also capable of establishing a chemical bond with the main group elements. The advances in the catalytic applications of the NHC ligands linked with a second tether are discussed. For clarity, more frequently targeted catalytic reactions are considered first. Carbon-carbon coupling reactions, transfer hydrogenation of alkenes and carbonyl compounds, ketone hydrosilylation, and chiral catalysis are among highly popular reactions. Areas where the efficacy of the NHC based catalytic systems were explored to a lesser extent include CO2 reduction, C-H borylation, alkyl amination, and hydroamination reactions. Furthermore, the synthesis and applications of transition metal complexes are covered.

Mesoionic Carbenes in Low- to High-Valent Vanadium Chemistry

We report the synthesis of vanadium(V) oxo complex 1 with a pincer-type dianionic mesoionic carbene (MIC) ligand L1 and the general formula [VOCl(L1)]. A comparison of the structural (SC-XRD), electronic (UV-vis), and electrochemical (cyclic voltammetry) properties of 1 with the benzimidazolinylidene congener 2 (general formula [VOCl(L2)]) shows that the MIC is a stronger donor also for early transition metals with low d-electron population. Since electrochemical studies revealed both complexes to be reversibly reduced, the stronger donor character of MICs was not only demonstrated for the vanadium(V) but also for the vanadium(IV) oxidation state by isolating the reduced vanadium(IV) complexes [Co(Cp*)2][1] and [Co(Cp*)2][2] ([Co(Cp*)2] = decamethylcobaltocenium). The electronic structures of the compounds were investigated by computational methods. Complex 1 was found to be a moderate precursor for salt metathesis reactions, showing selective reactivity toward phenolates or secondary amides, but not toward primary amides and phosphides, thiophenols, or aryls/alkyls donors. Deoxygenation with electron-rich phosphines failed to give the desired vanadium(III) complex. However, treatment of the deprotonated ligand precursor with vanadium(III) trichloride resulted in the clean formation of the corresponding MIC vanadium(III) complex 6, which undergoes a clean two-electron oxidation with organic azides yielding the corresponding imido complexes. The reaction with TMS-N3 did not afford a nitrido complex, but instead the imido complex 10. This study reveals that, contrary to popular belief, MICs are capable of supporting early transition-metal complexes in a variety of oxidation states, thus making them promising candidates for the activation of small molecules and redox catalysis.

Cerium Pyrazolates Grafted onto Mesoporous Silica SBA-15: Reversible CO2 Uptake and Catalytic Cycloaddition of Epoxides and Carbon Dioxide

The activation and catalytic conversion of CO₂ is a current topic relating to molecular chemistry and materials science alike. As a transdisciplinary field of research, surface organometallic chemistry (SOMC) might be applicable to perform synergistically, thus striking a new path in sustainable chemistry. Both ceric and cerous rare-earth-metal pyrazolates, which were recently shown to reversibly insert CO₂ and to promote the catalytic cycloaddition of epoxides and carbon dioxide, were grafted onto large-pore mesoporous silica SBA-15₅₀₀, thermally pretreated at 500 °C. The obtained hybrid materials [Ce(Me₂pz)₄]₂@SBA-15₅₀₀, Ce(Me₂pz)₄(thf)@SBA-15₅₀₀, Ce₄(Me₂pz)₁₂@SBA-15₅₀₀, and [Ce(Me₂pz)₃(thf)]₂@SBA-15₅₀₀ (Me₂pz = 3,5-dimethylpyrazolato) were characterized by DRIFTS (diffuse reflectance infrared Fourier transform spectroscopy), solid-state ¹H/¹³C NMR spectroscopy, elemental analysis, ICP/OES, and N₂ physisorption. The lanthanum(III)-based material [La(Me₂pz)₃(thf)]₂@SBA-15₅₀₀ was synthesized for better assessment of the cerous materials being highly sensitive to oxidation. To mimic ceric surface species, Ce[OSi(OtBu)₃]₃Cl was treated with 1 equiv of K(Me₂pz), generating the mixed pyrazolyl/siloxy complex KCe[OSi(OtBu)₃]₄(Me₂pz) featuring a cerium(IV)-bonded terminal pyrazolato ligand. All hybrid materials show efficient and reversible carbon dioxide uptake of maximum 20 wt % in the solid state. When combined with tetra-n-butylammonium bromide (TBAB), the hybrid materials catalyze the cycloaddition of CO₂ and epoxides, displaying good conversion of various epoxides and reusability.

The key role of R-NHC coupling (R = C, H, heteroatom) and M-NHC bond cleavage in the evolution of M/NHC complexes and formation of catalytically active species

Complexes of metals with N-heterocyclic carbene ligands (M/NHC) are typically considered the systems of choice in homogeneous catalysis due to their stable metal-ligand framework. However, it becomes obvious that even metal species with a strong M-NHC bond can undergo evolution in catalytic systems, and processes of M-NHC bond cleavage are common for different metals and NHC ligands. This review is focused on the main types of the M-NHC bond cleavage reactions and their impact on activity and stability of M/NHC catalytic systems. For the first time, we consider these processes in terms of NHC-connected and NHC-disconnected active species derived from M/NHC precatalysts and classify them as fundamentally different types of catalysts. Problems of rational catalyst design and sustainability issues are discussed in the context of the two different types of M/NHC catalysis mechanisms.

Triple Frustrated Lewis Pair-Type Reactivity on a Single Rare-Earth Metal Center

Rare-earth metal cations have been used rarely as Lewis-acidic components in the chemistry of frustrated Lewis pairs (FLPs). Herein, we report the first cerium/phosphorus system (2) employing a heptadentate N₄ P₃ ligand, which exhibits triple FLP-type reactivity towards a series of organic substrates, including isocyanates, isothiocyanates, diazomethane, and azides on a single rare-earth Lewis acidic Ce center. This result shows that the Ce center and three P atoms in 2 could simultaneously activate three equivalents of small molecules under mild conditions. This study broadens the diversity of FLPs and demonstrates that rare earth based FLP exhibit unique properties compared with other FLP systems.

Effective and Reversible Carbon Dioxide Insertion into Cerium Pyrazolates

The homoleptic pyrazolate complexes [CeIII 4 (Me2 pz)12 ] and [CeIV (Me2 pz)4 ]2 quantitatively insert CO2 to give [CeIII 4 (Me2 pz⋅CO2 )12 ] and [CeIV (Me2 pz⋅CO2 )4 ], respectively (Me2 pz=3,5-dimethylpyrazolato). This process is reversible for both complexes, as observed by in situ IR and NMR spectroscopy in solution and by TGA in the solid state. By adjusting the molar ratio, one molecule of CO2 per [CeIV (Me2 pz)4 ] complex could be inserted to give trimetallic [Ce3 (Me2 pz)9 (Me2 pz⋅CO2 )3 (thf)]. Both the cerous and ceric insertion products catalyze the formation of cyclic carbonates from epoxides and CO2 under mild conditions. In the absence of epoxide, the ceric catalyst is prone to reduction by the co-catalyst tetra-n-butylammonium bromide (TBAB).

Recent Advances in Rare Earth Complexes Containing N-Heterocyclic Carbenes: Synthesis, Reactivity, and Applications in Polymerization

N-heterocyclic carbenes (NHCs) are ubiquitous ancillary ligands employed in metal-catalyzed homogeneous reactions and polymerization reactions. Of significance is the use of NHCs as the supporting ligand in second- and third-generation Grubbs catalysts for their application in olefin metathesis and ring-opening metathesis polymerization. While the applications of transition metal catalysts ligated with NHCs in polymerization chemistry are well-documented, the use of analogous rare earth (Ln = Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) catalysts in this area remains under-developed, despite the unique role of rare earth elements in regio- and stereo-specific (co)polymerization reactions. By using hetero-atom-tethered chelating NHCs and, more recently, the employment of other structurally related NHCs, NHC-ligated Ln complexes have proven to be promising and fruitful catalysts for selective polymerization reactions. This review summarizes the recent developments in the coordination chemistry of Ln complexes containing NHCs and their catalytic performance in polymerization.

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

The rare-earth elements (Ln = Sc, Y, La-Lu) are widely used in stoichiometric and catalytic carbonyl group transformations. Sufficient availability, non-toxicity, high oxophilicity, tunable ion size/Lewis acidity and enhanced ligand exchangeability have been major driving factors for their successful implementation. Routinely employed reagents for stoichiometric carbonyl group transformations are divalent ytterbium and samarium compounds (e.g., ketone reduction), bimetallic CeCl3/LiR (C-C coupling), or ceric ammonium nitrate CAN (cyclic ketone oxidation). Rare-earth-metal triflates, and in particular Sc(OTf)3, are prominent examples of Lewis acid catalysts for versatile use in organic synthesis (e.g., Aldol and Michael reactions). Moreover, Ln(ii) and Ln(iii) complexes efficiently catalyze the (co)polymerization of carbonyl group-containing monomers including lactones, lactides, acrylates, and carbon dioxide. Featuring the most notorious greenhouse gas, CO2 is currently assessed as a cheap, abundant, and non-toxic C1 building block. Ln(iii) complexes are not only capable of efficient CO2 capture via reversible insertion but also of CO2 activation for catalytic conversions (copolymerization/cycloaddition with epoxides). This perspective focuses on structurally elucidated Ln complexes resulting from ketone or carbonyl derivative activation/insertion as well as carbon dioxide insertion products. The respective compounds will be sorted by structural motifs and, if applicable, details on reactivity and feasibility of catalytic reactions are presented. The article is subdivided in three parts: (i) donor and insertion products of ketones and aldehydes, (ii) redox-enhanced activation of carbonyl derivatives, and (iii) CO2 insertion/redox products and homogeneous catalytic conversion.

Anionic hafnium species: an active catalytic intermediate for the coupling of epoxides with CO2?

A series of hafnium complexes were structurally identified showing high activity (up to 500 h^-1) in the selective alternated copolymerization of epoxides with CO₂ under low pressure.

Dihydrogen activation by intermolecular rare-earth aryloxide/N-heterocyclic carbene Lewis pairs

Cooperative H₂ activation can be readily achieved by combination of homoleptic rare-earth aryloxides with N-heterocyclic carbenes under mild conditions.

Scandium complexes containing β-diketiminato ligands with pendant phosphanyl groups: competition between Sc/γ-C [4 + 2] cycloaddition and Sc/P frustrated Lewis pair reactions

Scandium complexes based on β-diketimine bearing a phosphanyl group show divergent reaction pathways toward phenyl isocyanate, namely Sc/γ-C [4 + 2] cycloaddition and Sc/P FLP reactions.

FLP behaviour of cationic titanium complexes with tridentate Cp,O,N-ligands: highly efficient syntheses and activation reactions of C–X bonds (X = Cl, F)

Titanium FLPs! Solvent- and catalyst-free Aza-Michael reaction provides β-amino ketones which were employed as ligand precursors for the convenient synthesis of novel cationic titanium complexes with tridentate Cp,O,N-ligand frameworks, which activate C–X bonds in a FLP-like manner.

Access to divalent lanthanide NHC complexes by redox-transmetallation from silver and CO2 insertion reactions

Through a redox-transmetallation procedure, divalent NHC-LnII (NHC = N-heterocyclic carbene; Ln = Eu, Yb) complexes were obtained from the corresponding NHC-AgI. The lability of the NHC-LnII bond was investigated and treatment with CO2 led to insertion reactions without oxidation of the metal centre. The EuII complex [EuI2(IMes)(THF)3] (IMes = 1,3-dimesitylimidazol-2-ylidene) exhibits photoluminescence with a quantum yield reaching 53%.

Selective and catalytic carbon dioxide and heteroallene activation mediated by cerium N-heterocyclic carbene complexes

A series of rare earth complexes of the form Ln(L^R)₃ supported by bidentate ortho-aryloxide-NHC ligands are reported (L^R = 2-O-3,5-^tBu₂-C₆H₂(1-C{N(CH)₂N(R)})); R = ⁱPr, ^tBu, Mes; Ln = Ce, Sm, Eu). The cerium complexes cleanly and quantitatively insert carbon dioxide exclusively into all three cerium carbene bonds, forming Ce(L^R·CO₂)₃. The insertion is reversible only for the mesityl-substituted complex Ce(L^Mes)₃. Analysis of the capacity of Ce(L^R)₃ to insert a range of heteroallenes that are isoelectronic with CO₂ reveals the solvent and ligand size dependence of the selectivity. This is important because only the complexes capable of reversible CO₂-insertion are competent catalysts for catalytic conversions of CO₂. Preliminary studies show that only Ce(L^Mes·CO₂)₃ catalyses the formation of propylene carbonate from propylene oxide under 1 atm of CO₂ pressure. The mono-ligand complexes can be isolated from reactions using LiCe(NⁱPr₂)₄ as a starting material; LiBr adducts [Ce(L^R)(NⁱPr₂)Br·LiBr(THF)]₂ (R = Me, ⁱPr) are reported, along with a hexanuclear N-heterocyclic dicarbene [Li₂Ce₃(OArC^Me-H)₃(NⁱPr₂)₅(THF)₂]₂ by-product. The analogous para-aryloxide-NHC proligand (p-L^Mes = 4-O-2,6-^tBu₂-C₆H₂(1-C{N(CH)₂NMes}))) has been made for comparison, but the rare earth tris-ligand complexes Ln(p-L^Mes)₃(THF)₂ (Ln = Y, Ce) are too reactive for straightforward Lewis pair separated chemistry to be usefully carried out.

Metal complexes with oxygen-functionalized NHC ligands: synthesis and applications

Ligand design has met with considerable success with both categories of hybrid ligands, which are characterized by chemically different donor groups, and of N-heterocyclic carbenes (NHCs). Their spectacular development and diversity are attracting worldwide interest and offers almost unlimited diversity and potential in e.g. coordination/organometallic main group and transition metal chemistry, catalysis, medicinal chemistry and materials science. This review aims at providing a comprehensive update on a specific class of ligands that has enjoyed much attention in the past few years, at the intersection between the two categories mentioned above, that of hybrid NHC ligands in which the functionality associated with the carbene donor is of the oxygen-donor type. For each type of oxygen-donor present in such chelating (Section 1) or bridging (Section 2) hybrid ligands, we will examine the synthesis, structures and reactivity of their metal complexes and their applications, with a special focus on homogeneous catalysis (Section 3). Thus, hydrogenation, C-H bond activation, C-C, C-N, C-O bond formation, hydrolysis of silanes, oligomerization, polymerization, metathesis, hydrosilylation, C-C bond cleavage, acceptorless dehydrogenation, dehalogenation/hydrogen transfer, oxidation and reduction reactions will be successively presented in a tabular manner, to facilitate an overview and a rapid identification of the relevant publications describing which metals associated with a given oxygen functionality are most suitable. The literature coverage includes the year 2015.

Bis(oxazoline)-derived N-heterocyclic carbene ligated rare-earth metal complexes: synthesis, structure, and polymerization performance

Bis(oxazoline)-derived NHC supported rare-earth complexes have been synthesized for the first time and showed good activity for 1-hexene (co)polymerization.

Frustrated Lewis pair behavior of a neutral scandium complex

A neutral scandium complex featuring a Sc/P combination underwent typical FLP-type addition reactions to yield Sc/Rh heterobimetallic and scandium phosphazine complexes, respectively.

Homo- and heteroleptic alkoxycarbene f-element complexes and their reactivity towards acidic N-H and C-H bonds

The reactivity of a series of organometallic rare earth and actinide complexes with hemilabile NHC-ligands towards substrates with acidic C-H and N-H bonds is described. The synthesis, characterisation and X-ray structures of the new heteroleptic mono- and bis(NHC) cyclopentadienyl complexes LnCp2(L) 1 (Ln = Sc, Y, Ce; L = alkoxy-tethered carbene [OCMe2CH2(1-C{NCHCHN(i)Pr})]), LnCp(L)2 (Ln = Y) , and the homoleptic tetrakis(NHC) complex Th(L)4 4 are described. The reactivity of these complexes, and of the homoleptic complexes Ln(L)3 (Ln = Sc 3, Ce), with E-H substrates is described, where EH = pyrrole C4H4NH, indole C8H6NH, diphenylacetone Ph2CC(O)Me, terminal alkynes RC≡CH (R = Me3Si, Ph), and cyclopentadiene C5H6. Complex 1-Y heterolytically cleaves and adds pyrrole and indole N-H across the metal carbene bond, whereas 1-Ce does not, although 3 and 4 form H-bonded adducts. Complexes 1-Y and 1-Sc form adducts with CpH without cleaving the acidic C-H bond, 1-Ce cleaves the Cp-H bond, but 2 reacts to form the very rare H(+)-[C5H5](-)-H(+) motif. Complex 1-Ce cleaves alkyne C-H bonds but the products rearrange upon formation, while complex 1-Y cleaves the C-H bond in diphenylacetone forming a product which rearranges to the Y-O bonded enolate product.

Bis(phenolate) N-heterocyclic carbene rare earth metal complexes: synthesis, characterization and applications in the polymerization of n-hexyl isocyanate

Polyhexyl isocyanantes catalyzed by N-heterocyclic carbene rare earth metal complexes show high molecular weight with narrow molecular weight distribution.