Organometallics最新文献

Replacing wasteful metal-based reducing agents with H₂ is an important goal for green chemistry. For this reason, we outline our design principles for building catalysts that use electrons from hydrogen to activate organohalides for reaction. These designs rely on an electron-withdrawing ligand to support low-valent metal centers, an electron-donating ligand to support oxidative addition, and the capacity for vacant sites to allow substrate docking. We begin by outlining our previous work in this field before describing a new rhodium complex that activates a particularly stubborn organohalide, 2,2-dibromopropane, using electrons from hydrogen. We then react this activated organohalide with styrene to generate a synthetically useful fragment of murraol in an efficient manner.

Herein, a library of ferrocenyl–thioether derivatives of acrylaldehyde and acrylonitrile is developed via a direct C–S bond formation reaction under mild conditions. Various aromatic and aliphatic thiols were successfully coupled with ferrocenyl acrylaldehyde/acrylonitrile in the presence of a chalcogen-stabilized iron–carbonyl cluster (Fe₃Se₂(CO)₉). All the reactions were carried out in water under aerobic conditions, and the transformation of a wide range of ferrocenyl–thioether derivatives in good yields were obtained. Furthermore, cytotoxicity studies of some selected ferrocenyl–thioethers were performed against the prostate cancer cell line (PC-3) and normal human embryonic kidney cell line (HEK). 3-Ferrocenyl-3-(4-trifluoromethyl)-phenylsulfanyl was found to be significantly active. It showed an IC₅₀ of 5.5 μM toward prostate cancer cell lines. Moreover, it also showed activity comparable to that of standard anticancer drugs including axitinib, nelfinavir, thymitaq, and (±) thioridazine. The anticancer activity was further supported by density functional theory calculations including the HOMO–LUMO energy gap, cyclic voltammetry, UV–vis studies, molecular docking, and reactive oxygen species analysis. All compounds synthesized in this report are new, and they may serve as milestones in the futuristic research of anticancer drugs.

When all substituents of bis(cyclopentadienyl)titanacyclopentadienes 5 are primary alkyl groups, coupling of a Cp ligand and the diene moiety in 5 gave dihydroindenyl titanium complexes 2. The intermediate complexes 1 were not obtained because complexes 2 are more thermodynamically stable than complexes 1 as previously reported. To prevent this further transformation, the influence of the bulkiness of the substituents was investigated. Pinacolboryl (Bpin) groups as substituents R¹ were found to be a key factor in disturbing the further transformation of complex 1 to 2. Complex 1 bearing Bpin substituents as R¹ was confirmed by NMR. Its dihydroindenyl moiety was removed from the titanium and converted to a TCNE-adduct to verify the structure by X-ray diffraction.

An N-heterocyclic carbene (NHC) ligand supports a stable [Ni₂H]⁺ core, formally dinickel(I). This diamagnetic cation complex features a bent hydride bridge and a Ni···Ni distance, 2.9926(5) Å, larger than two covalent radii. The cation displays weakly protic character, undergoing deprotonation by strong base to form the corresponding (NHC)nickel(0) dimer. Its reaction with aliphatic nitriles results in C–CN bond cleavage. The organic products of this reaction suggest that this bond-breaking step involves reactive nickel alkyl intermediates and occurs reversibly.

An N-heterocyclic carbene (NHC) ligand supports a stable [Ni₂H]⁺ core, formally dinickel(I). This diamagnetic cation complex features a bent hydride bridge and a Ni···Ni distance, 2.9926(5) Å, larger than two covalent radii. The cation displays weakly protic character, undergoing deprotonation by strong base to form the corresponding (NHC)nickel(0) dimer. Its reaction with aliphatic nitriles results in C-CN bond cleavage. The organic products of this reaction suggest that this bond-breaking step involves reactive nickel alkyl intermediates and occurs reversibly.

Ruthenium based Grubbs metathesis has become a commonplace reaction for synthetic chemists. Development of new generations of catalysts evolving from Grubbs I (GI) have led to greater stability, functional group compatibility, and superior reactivities. However, these advancements lead to increased costs. To this end, we report here how the addition of the commercially available tris(pentafluorophenyl)borane Lewis acid, which has become a common place catalyst in its own right, leads to enhanced reactivity of GI. Moreover, the increased reactivity arises via halide abstraction rather than traditional phosphine dissociation, providing ring-opening metathesis polymerization products that are divergent from those synthesized without the Lewis acid cocatalyst.

Reactions between (E)-N-(2,6-diisopropylphenyl)-2-methyl-6,7-dihydroquinolin-8(5H)-imine nickel(II) dihalides and organoaluminum compounds AlMe₃, AlⁱBu₃, MAO, and MMAO were monitored using EPR spectroscopy in situ. It was found that rapid one-electron reduction of the Ni(II) center results in formation of a variety of monovalent nickel species which are stable at room temperature in an oxygen-free atmosphere. On the basis of the EPR data obtained, these compounds were assigned to the heterobinuclear nickel-aluminum complexes of the type LNi^I(μ-R)₂AlX₂, where L = N,N′-donor ligand, R = Me, ⁱBu, H, Br, or Cl bridge, and X = Me, ⁱBu, Br or, Cl, depending on conditions. Formation of the dihydride-bridged LNi^I(μ-H)₂AlX₂ complex was confirmed using deuterium-labeled compounds. In the presence of an excess of ethylene or 1-hexene, LNi^I(μ-R)₂AlX₂ eliminates RAlX₂ to afford LNi^IR(C₂H₄) or LNi^IR(C₆H₁₂) species, respectively. Structural assignments of the Ni^I species are supported by DFT calculations of their g-tensor values.

We report two new Ni–group 13 pincer-type systems (Ni–Al and Ni–Ga) and describe the influence of Lewis acidity, covalency, and halide mobility on the oxidative addition reactivity toward unsubstituted aryl halides. Despite similar Lewis acidities of the Al- and Ga-based metalloligands, the Ni–Ga pairing exhibits a significantly reduced effective Lewis acidity toward triethylphosphine oxide, supported by the greater Ni–Ga covalency as determined by density functional theory. The higher effective Lewis acidity of Al in the Ni–Al complex and halide mobility are required to construct the key three-membered Ni–X–Al poised-metallacycle, an intermediate in the oxidative addition of unsubstituted aryl halides. The Al center further stabilizes the resulting Ni(II)–aryl product via a bridging halide, thereby eschewing common decomposition pathways to Ni(I) byproducts.

Herein, we demonstrate the significant impact of tetradentate-ligand-coordinated metal complexes, which have not yet been exploited for the direct catalytic hydrogenation of carboxylic acids (CAs). Our previously developed cationic iridium complex coordinated with a PNNP-ligand [(PNNP)Ir] is effective for hydrogenating esters and carboxylic anhydrides generated in situ from CAs but unsuitable for the direct hydrogenation of CAs. In sharp contrast, the corresponding neutral iridium complex with a PNCP-ligand [(PNCP)Ir] developed in this study facilitates the direct hydrogenation of CAs, including biorelevant and pharmaceutical compounds, under not more than 1 MPa of H₂. Quantum-chemical calculations indicated that (PNCP)Ir is kinetically a far more competent catalyst than (PNNP)Ir, particularly for the C–H bond formation via hydride transfer from Ir–H to the carbonyl carbon of CA, which was identified as the rate-determining step. While Ir-carboxylates are in resting states throughout the catalytic cycle, CA itself barely interacts with the Ir center during the hydride transfer process.

Our interest in the design of ambiphilic ligands and their coordination to gold has led us to synthesize an indazol-3-ylidene gold chloride complex functionalized at the 4-position of the indazole backbone by a stibine functionality. The antimony center of this new complex cleanly reacts with o-chloranil to afford the corresponding stiborane derivative. Structural analysis indicates that the stiborane coordination environment is best described as a distorted square pyramid whose open face is oriented toward the gold center, allowing for the formation of a long donor-acceptor, or pnictogen, Au → Sb bonding interaction. The presence of this interaction, which has been probed computationally, is also manifested in the enhanced catalytic activity of this complex in the cyclization of N-propargyl-4-fluorobenzamide.