Structural Characterization of Tridentate N-Heterocyclic Carbene Titanium(IV) Benzyloxide, Silyloxide, Acetate, and Azide Complexes and Assessment of Their Efficacies for Catalyzing the Copolymerization of Cyclohexene Oxide with CO2

Bridging Titanium Nitrido Complexes Containing A Linear Ti-N-Ti Core with A Two-Coordinate Nitrido Ligand

A series of titanium μ2-nitrido complexes supported by the triamidoamine ligand Xy-N3N (Xy-N3N={(3,5-Me2C6H3)NCH2CH2}3N3-) is reported. The titanium azido complex [(Xy-N3N)TiN3] (1-N3), prepared by salt metathesis of the chloride complex [(Xy-N3N)TiCl] (1-Cl) with NaN3, reacted with lithium metal or with alkali metal naphthalenides (alkali metal M=Na, K, and Rb) in THF to give the corresponding dinuclear μ2-nitrido complexes M[(Xy-N3N)Ti=N-Ti(Xy-N3N)] (2-M; M=Li, Na, K, Rb). Single crystal X-ray diffraction studies of 2-Li, 2-Na, and 2-K revealed alkali metal dependent structures in the solid state. While 2-Li and 2-K contain a μ2-nitrido ligand with a linear Ti-N-Ti core, 2-Na includes a μ3-nitrido ligand as part of a T-shape Ti2NaN fragment with the sodium cation weekly coordinated to the nitrido nitrogen atom. When the synthesis of the nitrido complexes was carried out in the presence of excess alkali metals, decomposition of the nitrido complexes was observed affording some intractable titanium species along with the trialkali metal salts [M3(Xy-N3N)] (3-M) (M=Li, Na, K, and Rb). These salts were also prepared by deprotonation of (Xy-N3N)H3 with the corresponding alkali metal hexamethyldisilazide and characterized by multinuclear NMR spectroscopy as well as single crystal X-ray diffraction.

Ti(III) Catalysts for CO2/Epoxide Copolymerization at Unusual Ambient Pressure Conditions

Titanium compounds in low oxidation states are highly reducing species and hence powerful tools for the functionalization of small molecules. However, their potential has not yet been fully realized because harnessing these highly reactive complexes for productive reactivity is generally challenging. Advancing this field, herein we provide a detailed route for the formation of titanium(III) orthophenylendiamido (PDA) species using [LiBHEt3] as a reducing agent. Initially, the corresponding lithium PDA compounds [Li2(ArPDA)(thf)3] (Ar = 2,4,6-trimethylphenyl (MesPDA), 2,6-diisopropylphenyl (iPrPDA)) are combined with [TiCl4(thf)2] to form the heterobimetallic complexes [{TiCl(ArPDA)}(μ-ArPDA){Li(thf)n}] (n = 1, Ar = iPr 3 and n = 2, Ar = Mes 4). Compound 4 evolves to species [Ti(MesPDA)2] (6) via thermal treatment. In contrast, the transformation of 3 into [Ti(iPrPDA)2] (5) only occurs in the presence of [LiNMe2], through a lithium-assisted process, as revealed by density functional theory (DFT). Finally, the Ti(IV) compounds 3-6 react with [LiBHEt3] to give rise to the Ti(III) species [Li(thf)4][Ti(ArPDA)2] (Ar = iPr 8, Mes 9). These low-valent compounds in combination with [PPN]Cl (PPN = bis(triphenylphosphine)iminium) are proved to be highly selective catalysts for the copolymerization of CO2 and cyclohexene epoxide. Reactions occur at 1 bar pressure with activity/selectivity levels similar to Salen-Cr(III) compounds.

Titanium(IV) Surface Complexes Bearing Chelating Catecholato Ligands for Enhanced Band-Gap Reduction

Protonolysis reactions between dimethylamido titanium(IV) catecholate [Ti(CAT)(NMe₂)₂]₂ and neopentanol or tris(tert-butoxy)silanol gave catecholato-bridged dimers [(Ti(CAT)(OCH₂tBu)₂)(HNMe₂)]₂ and [Ti(CAT){OSi(OtBu)₃}₂(HNMe₂)₂]₂, respectively. Analogous reactions using the dimeric dimethylamido titanium(IV) (3,6-di-tert-butyl)catecholate [Ti(CATtBu₂-3,6)(NMe₂)₂]₂ yielded the monomeric Ti(CATtBu₂-3,6)(OCH₂tBu)₂(HNMe₂)₂ and Ti(CATtBu₂-3,6)[OSi(OtBu)₃]₂(HNMe₂)₂. The neopentoxide complex Ti(CATtBu₂-3,6)(OCH₂tBu)₂(HNMe₂)₂ engaged in further protonolysis reactions with Si-OH groups and was consequentially used for grafting onto mesoporous silica KIT-6. Upon immobilization, the surface complex [Ti(CATtBu₂-3,6)(OCH₂tBu)₂(HNMe₂)₂]@[KIT-6] retained the bidentate chelating geometry of the catecholato ligand. This convergent grafting strategy was compared with a sequential and an aqueous approach, which gave either a mixture of bidentate chelating species with a bipodally anchored Ti(IV) center along with other physisorbed surface species or not clearly identifiable surface species. Extension of the convergent and aqueous approaches to anatase mesoporous titania (m-TiO₂) enabled optical and electronic investigations of the corresponding surface species, revealing that the band-gap reduction is more pronounced for the bidentate chelating species (convergent approach) than for that obtained via the aqueous approach. The applied methods include X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and solid-state UV/vis spectroscopy. The energy-level alignment for the surface species from the aqueous approach, calculated from experimental data, accounts for the well-known type II excitation mechanism, whereas the findings indicate a distinct excitation mechanism for the bidentate chelating surface species of the material [Ti(CATtBu₂-3,6)(OCH₂tBu)₂(HNMe₂)₂]@[m-TiO₂].

Reactivity and Structure of a Bis-phenolate Niobium NHC Complex

We report the facile synthesis of a rare niobium(V) imido NHC complex with a dianionic OCO-pincer benzimidazolylidene ligand (L ¹ ) with the general formula [NbL ¹ (N ^t Bu)PyCl] 1-Py. We achieved this by in situ deprotonation of the corresponding azolium salt [H ₃ L ¹ ][Cl] and subsequent reaction with [Nb(N ^t Bu)Py ₂ Cl ₃ ]. The pyridine ligand in 1-Py can be removed by the addition of B(C₆F₅)₃ as a strong Lewis acid leading to the formation of the pyridine-free complex 1. In contrast to similar vanadium(V) complexes, complex 1-Py was found to be a good precursor for various salt metathesis reactions, yielding a series of chalcogenido and pnictogenido complexes with the general formula [ NbL ¹ (N ^t Bu)Py(EMes)] (E = O (2), S (3), NH (4), and PH (5)). Furthermore, complex 1-Py can be converted to alkyl complex (6) with 1 equiv of neosilyl lithium as a transmetallation agent. Addition of a second equivalent yields a new trianionic supporting ligand on the niobium center (7) in which the benzimidazolylidene ligand is alkylated at the former carbene carbon atom. The latter is an interesting chemically "noninnocent" feature of the benzimidazolylidene ligand potentially useful in catalysis and atom transfer reactions. Addition of mesityl lithium to 1-Py gives the pyridine-free aryl complex 8, which is stable toward "overarylation" by an additional equivalent of mesityl lithium. Electrochemical investigation revealed that complexes 1-Py and 1 are inert toward reduction in dichloromethane but show two irreversible reduction processes in tetrahydrofuran as a solvent. However, using standard reduction agents, e.g., KC₈, K-mirror, and Na/Napht, no reduced products could be isolated. All complexes have been thoroughly studied by various techniques, including ¹H-, ¹³C{¹H}-, and ¹H-¹⁵N HMBC NMR spectroscopy, IR spectroscopy, and X-ray diffraction analysis.

Metallocene catalysts for the ring-opening co-polymerisation of epoxides and cyclic anhydrides

The ring-opening co-polymerization (ROCOP) of epoxides and cyclic anhydrides is a versatile route to new polyesters. The vast number of monomers that are readily available means that an effectively limitless...

Synthesis, Crystal Structures, and Spectroscopic Properties of Novel Gadolinium and Erbium Triphenylsiloxide Coordination Entities

In alkali metal and lanthanide coordination chemistry, triphenylsiloxides seem to be unduly underappreciated ligands. This is as surprising as that such substituents play a crucial role, among others, in stabilizing rare oxidation states of lanthanide ions, taking a part of intramolecular and molecular interactions stabilizing metal-oxygen cores and many others. This paper reports the synthesis and characterization of new lithium [Li₄(OSiPh₃)₄(THF)₂] (1), and sodium [Na₄(OSiPh₃)₄] (2) species, which were later used in obtaining novel gadolinium [Gd(OSiPh₃)₃(THF)₃]·THF (3), and erbium [Er(OSiPh₃)₃(THF)₃]·THF (4) configuration, it can result in res were determined for all 1-4 compounds, and in addition, IR, Raman, absorption spectroscopy studies were conducted for 3 and 4 lanthanide compounds. Furthermore, direct current (dc) variable-temperature magnetic susceptibility measurements on polycrystalline samples of 3 and 4 were carried out in the temperature range 1.8-300 K. The 3 shows behavior characteristics for the paramagnetism of the Gd³⁺ ion. In contrast, the magnetic properties of 4 are dominated by the crystal field effect on the Er³⁺ ion, masking the magnetic interaction between magnetic centers of neighboring molecules.

Coupling of CO2 and epoxides catalysed by novel N-fused mesoionic carbene complexes of nickel(II)

We report the syntheses of two rigid mesoionic carbene (MIC) ligands with a carbazole backbone via an intramolecular Finkelstein-cyclisation cascade and investigate their coordination behavior towards nickel(II) acetate. Despite the nickel(II) carbene complexes 4a,b showing only minor differences in their chemical composition, they display curious differences in their chemical properties, e.g. solubility. Furthermore, the potential of these novel MIC complexes in the coupling of carbon dioxide and epoxides as well as the differences in reactivity compared to classical NHC-derived complexes are evaluated.

Ekeli J, Gillis-D’Hamers F, Törnroos KW, Jensen VR, Le Roux E. Unsaturated and Benzannulated N-Heterocyclic Carbene Complexes of Titanium and Hafnium: Impact on Catalysts Structure and Performance in Copolymerization of Cyclohexene Oxide with CO2

Tridentate, bis-phenolate N-heterocyclic carbenes (NHCs) are among the ligands giving the most selective and active group 4-based catalysts for the copolymerization of cyclohexene oxide (CHO) with CO2. In particular, ligands based on imidazolidin-2-ylidene (saturated NHC) moieties have given catalysts which exclusively form polycarbonate in moderate-to-high yields even under low CO2 pressure and at low copolymerization temperatures. Here, to evaluate the influence of the NHC moiety on the molecular structure of the catalyst and its performance in copolymerization, we extend this chemistry by synthesizing and characterizing titanium complexes bearing tridentate bis-phenolate imidazol-2-ylidene (unsaturated NHC) and benzimidazol-2-ylidene (benzannulated NHC) ligands. The electronic properties of the ligands and the nature of their bonds to titanium are studied using density functional theory (DFT) and natural bond orbital (NBO) analysis. The metal-NHC bond distances and bond strengths are governed by ligand-to-metal σ- and π-donation, whereas back-donation directly from the metal to the NHC ligand seems to be less important. The NHC π-acceptor orbitals are still involved in bonding, as they interact with THF and isopropoxide oxygen lone-pair donor orbitals. The new complexes are, when combined with [PPN]Cl co-catalyst, selective in polycarbonate formation. The highest activity, albeit lower than that of the previously reported Ti catalysts based on saturated NHC, was obtained with the benzannulated NHC-Ti catalyst. Attempts to synthesize unsaturated and benzannulated NHC analogues based on Hf invariably led, as in earlier work with Zr, to a mixture of products that include zwitterionic and homoleptic complexes. However, the benzannulated NHC-Hf complexes were obtained as the major products, allowing for isolation. Although these complexes selectively form polycarbonate, their catalytic performance is inferior to that of analogues based on saturated NHC.

Ti(IV)-Tris(phenolate) Catalyst Systems for the Ring-Opening Copolymerization of Cyclohexene Oxide and Carbon Dioxide

Titanium(IV) complexes of amino-tris(phenolate) ligands (LTiX, X = chloride, isopropoxide) together with bis(triphenylphosphine)iminium chloride (PPNCl) are active catalyst systems for the ring-opening copolymerization of carbon dioxide and cyclohexene oxide. They show moderate activity, with turnover frequency values of ∼60 h^–1 (0.02 mol % of catalyst, 80 °C, 40 bar of CO₂) and high selectivity (carbonate linkages >90%), but their absolute performances are lower than those of the most active Ti(IV) catalyst systems. The reactions proceed with linear evolution of polycarbonate (PCHC) molar mass with epoxide conversion, consistent with controlled polymerizations, and evolve bimodal molar mass distributions of PCHC (up to M_n = 42 kg mol^–1). The stoichiometric reaction between [LTiOⁱPr] and tetraphenylphosphonium chloride, PPh₄Cl, allows isolation of the putative catalytic intermediate [LTi(OⁱPr)Cl]⁻, which is characterized using single-crystal X-ray diffraction techniques. The anionic titanium complex [LTi(OR)Cl]⁻ is proposed as a model for the propagating alkoxide intermediates in the catalytic cycle.

Update and Challenges in Carbon Dioxide-Based Polycarbonate Synthesis

The utilization of carbon dioxide as a comonomer to produce polycarbonates has attracted a great deal of attention from both industrial and academic communities because it promises to replace petroleum-derived plastics and supports a sustainable environment. Significant progress in the copolymerization of cyclic ethers (e.g., epoxide, oxetane) and carbon dioxide has been made in recent decades, owing to the rapid development of catalysts. In this Review, the focus is to summarize and discuss recent advances in the development of homogeneous catalysts, including metal- and organo-based complexes, as well as the preparation of carbon dioxide-based block copolymer and functional polycarbonates.

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.