Organometallics最新文献

Based on previously accumulated experimental material on the use of {nEt₂AlCl + Bu₂Mg} mixtures for the activation of Ti postmetallocene precatalysts for ethylene polymerization, an attempt is made to find the reason for the universal activating ability of such cocatalysts by analyzing the chemical processes occurring during the interaction of alkylaluminum chlorides and dibutylmagnesium. ¹H and ²⁷Al NMR spectroscopy data indicate that in nonsolvating media the formation of ionic products (with which earlier the unique activating ability of Al/Mg activators was associated) does not occur. The “true” Al/Mg cocatalysts are uncharged adducts of MgCl₂ with alkylaluminum chlorides and/or trialkylaluminum. It was shown that all used Al/Mg cocatalyst compositions are capable of activating a model titanium dichloride complex with a saligenin ligand with varying efficiencies. The product of the ethylene polymerization is an ultrahigh molecular weight polymer (M_v from 1.1 to 5.1 × 10⁶ Da); the latter is evidenced by the possibility of solid-phase processing of nascent polymer powders into high-strength, high-modulus tapes (breaking strength up to 2.4 GPa and elastic modulus up to 112 GPa).

Nickel(II) complexes bearing tetradentate NCCN ligands composed of optionally protic pyridine and N-heterocyclic carbene (NHC) donors have been synthesized and used as catalysts for carbon dioxide reduction. These complexes were synthesized bearing OMe, OBn, or OH substituents on the pyridine rings and were characterized by ¹H NMR, ¹³C NMR, UV–vis, IR, HR-MS, and single crystal X-ray diffraction. The OH substituent was partially deprotonated, as shown by the crystal structure. Electrochemical studies show that these nickel complexes undergo two electron reduction events prior to CO₂ reduction. Catalytic current enhancement under CO₂ relative to N₂ is not observed under dry conditions, but the addition of proton sources leads to modest current enhancement (i_cat/i_p < 2). Visible light driven photochemical CO₂ reduction with a photosensitizer (Ir(ppy)₃, where ppy = 2-phenylpyridine) and sacrificial electron and proton donors was studied, and formate is the major product with ∼10:1 formate to CO production. Electron donor groups (OMe, OBn, OH) do not enhance formate production (relative to the unsubstituted analogue), and CO production is only slightly enhanced. Overall with Ni(II), the tetradentate ligands are comparable to recently published pincer ligands for sensitized CO₂ reduction, but pincer ligands offer a clear advantage in self-sensitized catalysis.

A simplified one-pot procedure for the synthesis of a useful class of mixed chloro-/fluoroaryl boranes following the formula B(C₆Cl₅)_n(C₆F₅)_3–n is reported. Standard organolithium and Grignard reagents are utilized, and intermediate arylzinc or arylcopper reagents are not required. With regard to their applications to C–O cleavage reactions, the increased steric bulk of having ortho-chlorine enhances the catalyst tolerance of water, allowing for C–O deoxygenations under atmospheric conditions. As the degree of chlorination increases, the reactivity of the borane catalyst decreases with B(C₆Cl₅)(C₆F₅)₂ being the most active of the boranes studied. This mono-C₆Cl₅ borane was able to cleave or reduce the C–O bond of ethers, carbonyls, and sugars under benchtop (open air) conditions. B(C₆Cl₅)(C₆F₅)₂ thus demonstrates a favorable balance of water tolerance and preserved reactivity relative to that of BCF itself.

We used a computational density functional theory approach to study second-generation Hoveyda–Grubbs-type complexes bearing Tl/Ga/In(AmIm) ligands, focusing on their application as olefin metathesis catalysts. Of these three, the organothallium compound was assessed as the most potent candidate for an efficient catalyst, exhibiting low free energy barriers (14.51 kcal/mol for the rate-determining step of the simplest metathesis process), which suggested its activity, even at low temperatures. This species may potentially catalyze not only the formation of simple olefins but also more sterically hindered ones, including the synthesis of demanding tetra-substituted alkenes.

Ring size effects on geometries and electronic structures were investigated for the (C_nH_n)M(C_mH_m) (n = 4, 5, or 6; m = 8, 7, or 6; m + n = 12; M = Ti–Ni) systems using density functional theory. The lowest-energy C₁₂H₁₂M structures for the early transition metals titanium, vanadium, and chromium are the experimentally known singlet (η⁵-C₅H₅)Ti(η⁷-C₇H₇), doublet (η⁵-C₅H₅)V(η⁷-C₇H₇), and singlet (η⁶-C₆H₆)₂Cr, respectively. The likewise experimentally known singlet (η⁶-C₆H₆)₂Ti, doublet (η⁶-C₆H₆)₂V, and singlet (η⁵-C₅H₅)Cr(η⁷-C₇H₇) are the second-lowest-energy structures with only a small energy difference between the two vanadium structures. For the later transition metals, dibenzenemetal complexes are the lowest-energy C₁₂H₁₂M species with two fully bonded hexahapto benzene rings in the lowest-energy manganese and iron derivatives and one hexahapto and one dihapto benzene ring in the lowest-energy cobalt and nickel derivatives. The lowest-energy (C₅H₅)M(C₇H₇) structures for the later transition metals iron, cobalt, and nickel have partially bonded nonplanar C₇H₇ rings with one or two uncomplexed C═C bonds. The (C₄H₄)M(C₈H₈) (M = Ti–Ni) structures with the metal sandwiched between four- and eight-membered rings were found to be much higher in energy than their (C₅H₅)M(C₇H₇) and (C₆H₆)₂M isomers.

Ring size effects on geometries and electronic structures were investigated for the (C _n H _n )M(C _m H _m ) (n = 4, 5, or 6; m = 8, 7, or 6; m + n = 12; M = Ti-Ni) systems using density functional theory. The lowest-energy C₁₂H₁₂M structures for the early transition metals titanium, vanadium, and chromium are the experimentally known singlet (η⁵-C₅H₅)Ti(η⁷-C₇H₇), doublet (η⁵-C₅H₅)V(η⁷-C₇H₇), and singlet (η⁶-C₆H₆)₂Cr, respectively. The likewise experimentally known singlet (η⁶-C₆H₆)₂Ti, doublet (η⁶-C₆H₆)₂V, and singlet (η⁵-C₅H₅)Cr(η⁷-C₇H₇) are the second-lowest-energy structures with only a small energy difference between the two vanadium structures. For the later transition metals, dibenzenemetal complexes are the lowest-energy C₁₂H₁₂M species with two fully bonded hexahapto benzene rings in the lowest-energy manganese and iron derivatives and one hexahapto and one dihapto benzene ring in the lowest-energy cobalt and nickel derivatives. The lowest-energy (C₅H₅)M(C₇H₇) structures for the later transition metals iron, cobalt, and nickel have partially bonded nonplanar C₇H₇ rings with one or two uncomplexed C=C bonds. The (C₄H₄)M(C₈H₈) (M = Ti-Ni) structures with the metal sandwiched between four- and eight-membered rings were found to be much higher in energy than their (C₅H₅)M(C₇H₇) and (C₆H₆)₂M isomers.

Industrial polylactide (PLA) production typically uses stannous octoate as a catalyst for lactide (LA) polymerization, which has drawback that PLA degradation caused by tin residual. This study explores the synthesis and application of tetradentate nitrogen-coordinated Ni(II) complexes (L¹Ni–L⁴Ni) as alternative catalysts for the ring-opening polymerization of unpurified l-LA. The robust Ni(II) complexes were synthesized and characterized, and their catalytic performance was evaluated. Results indicated that in the presence of benzyl alcohol, the Ni(II) complexes demonstrated relatively good catalytic activity, achieving over 90% conversion of l-LA within 4 h. The polymerization reaction even can be operated in air without affecting the catalytic effect. The molecular weight of the resulting PLA showed a linear relationship with the [LA]/[Ni] ratio, indicating characteristics of living polymerization. The polymerization mechanism was proposed based on MALDI–TOF, NMR, FT-IR and DFT calculation.

Four distinct organotin(IV) compounds, [R₂Sn(L¹)] [R = Ph (1) and n-Bu (2)] and [R₂Sn(L²)] [R = Ph (3) and n-Bu (4)], have been synthesized from two polydentate proligands such as 2-((2-hydroxy-3-methoxybenzylidene)amino)-4-methyl phenol (H₂L¹) and 4-(tert-butyl)-2-(((2-hydroxy-5-methylphenyl)imino)methyl)phenol (H₂L²). The organotin(IV) compounds were synthesized by the reaction of organotin(IV) halides such as Ph₂SnCl₂ and (n-Bu)₂SnCl₂ with both proligands using Et₃N as a base. All compounds were fully characterized using FT-IR spectroscopy; ¹H, ¹³C{¹H}, and ¹¹⁹Sn NMR spectroscopy; HRMS spectrometry; and single-crystal X-ray diffraction analysis. The Lewis acidity of all compounds was determined by the Gutmann–Beckett method. The catalytic activities of all of the compounds were investigated for the synthesis of naphthofurans from trans-β-nitrostyrene derivatives and β-naphthol or α-naphthol under solvent-free conditions. The maximum yield of naphthofurans is up to 95%.

A series of mononuclear square-planar Rh{κ²N,O-BHetA}(η²-coe)(NHC) (BHetA = Bis-Heteroatomic Acidato) complexes have been prepared. Modifications of the pyridonato BHetA-type ligand architecture include 4-Me, 5-Me, 6-Me, 3-Br, 4-Br, 4-OMe, and 5-NO₂ substitutions as well as pyrimidonato, succinimidato, and 2-piperidonato catalysts. Two structural isomers have been observed for the complexes, depending on the stereoelectronic properties of the ligand. The structure–activity relationship has been studied for gem-specific alkyne dimerization via a cooperative ligand-assisted proton shuttle mechanism. Density functional theory calculations have revealed a mechanistic pathway involving the hemilabile coordination of the BHetA ligand, CMD deprotonation, π-alkyne protonation, and reductive elimination. The increase in oxygen basicity imparted by the substituent in the pyridonato ligand is key, the 4-methyl derivative being the most active catalyst. However, a favored iminol–amide tautomerization precludes an increase in catalytic activity for the more basic saturated piperidonato catalyst.

Phosphine ligands bearing amidine groups were designed for the synthesis of complexes in which a metal center and ligands cooperate. Several palladium amidinate complexes were synthesized from these ligands. The reaction of these complexes with various organic molecules containing an acidic OH group gave the palladium amidine complexes. In the reaction, the O–H bond was activated by metal–ligand cooperation (MLC).