Catalytic Conversion of Carbon Dioxide Using Binuclear Double-Stranded Helicates: Cyclic Carbonate from Epoxides and Diol

The construction of sophisticated molecular architectures from chemical subunits requires careful selection of the spacers, precise synthetic strategies, and substantial efforts. Here, we report a series of binuclear double-stranded helicates synthesized from different combinations of pyridyl hydrazone-based multidentate ligands (H₂ 1, H₂ 2, H₂ 3) by increasing the methylene spacer and transition metals (Co, Ni, and Zn). The ligands H₂ 1 (N'1,N'3-bis((E)-pyridin-2-ylmethylene)malonohydrazide), H₂ 2 (N'1,N'4-bis((E)-pyridin-2-ylmethylene)succinohydrazide), and H₂ 3 (N'1,N'5-bis((E)-pyridin-2-ylmethylene)glutarohydrazide) and their respective complexes with Co, Ni, and Zn were obtained. Single-crystal X-ray diffraction studies of these binuclear metallohelicates confirm the double-stranded helical structure of the complexes derived from H₂ 2. The set of helicates Co-1, Co-2, and Co-3; Ni-1, Ni-2, and Ni-3; and Zn-1, Zn-2, and Zn-3 were investigated for its catalytic activity in the cyclic carbonate formation reaction. Intriguingly, among the synthesized catalyst, Co-1 was found to be better in terms of conversions with the calculated TOF (turnover frequency) of 128/h. The catalytic performance was significantly improved by adding 0.2 mmol of tetrabutylammonium bromide by achieving 76% conversion in 30 min, with the observed TOF of 15,934 h^-1/molecule and 7967 h^-1/Co center. The results obtained herein show that the double-stranded helicates are effective catalysts for converting both terminal and non-terminal epoxides into their corresponding cyclic carbonates. The striking feature of this catalytic protocol lies in demonstrating the catalytic activity for the conversion of diol to cyclic carbonate, and the detailed kinetic experiments tempted us to propose a tentative reaction mechanism for this conversion.

Ni(II)-Aroylhydrazone Complexes as Catalyst Precursors Towards Efficient Solvent-Free Nitroaldol Condensation Reaction

The aroylhydrazone Schiff bases 2-hydroxy-(2-hydroxybenzylidene)benzohydrazide and (2,3-dihydroxybenzylidene)-2-hydroxybenzohydrazide have been used to synthesize the bi- and tri-nuclear Ni(II) complexes [Ni2(L1)2(MeOH)4] (1) and [Ni3(HL2)2(CH3OH)8]· (NO3)2 (2). Both complexes have been characterized by elemental analysis, spectroscopic techniques [IR spectroscopy and electrospray ionization-mass spectrometry (ESI-MS)], and single-crystal X-ray crystallography. The coordination behavior of the two ligands is different in the complexes: The ligand exhibits the keto form in 2, while coordination through enol form was found in 1. Herein, the catalytic activity of 1 and 2 has been compared with the nitroaldol condensation reaction under various conditions. Complex 2 exhibits the highest activity towards solvent-free conditions.

Enantiomeric Resolution of Asymmetric-Carbon-Free Binuclear Double-Stranded Cobalt(III) Helicates and Their Application as Catalysts in Asymmetric Reactions

A series of double-stranded binuclear helicates [Co₂(H1)₂]⁴⁺, [Co₂(H2)₂]⁴⁺, and [Co₂(H3)₂]⁴⁺, derived from monodeprotonated bis-pyridyl hydrazine-based ligands of H₂1, H₂2, and H₂3 with one, two, and three -CH₂ spacers, were obtained. These asymmetric-carbon-free racemic helicates were separated into their ΔΔ and ΛΛ enantiomers. The resolved helicates were examined for the first time as enantioselective catalysts in asymmetric benzoylation and nitroaldol reactions.

Synthesis and structural characterization of a novel dinuclear Cu(ii) complex: an efficient and recyclable bifunctional heterogeneous catalyst for the diastereoselective Henry reaction

We describe the synthesis and structural characterization of a novel dinuclear Cu(ii) complex containing a new flexible mixed pyridine-carboxylate ligand. Single crystal X-ray diffraction reveals its uniqueness with two coordinated water molecules and four uncoordinated pyridyl groups. Utilizing the bifunctional catalytic sites, it has been found to be an efficient, robust and recyclable heterogeneous catalyst for the diastereoselective Henry (nitroaldol) reaction of aldehydes with nitroethane.

Chiral copper-salen complex grafted over functionalized mesoporous silica as an efficient catalyst for asymmetric Henry reactions and synthesis of the potent drug (R)-isoproterenol

A new chiral Cu(ii)@AFS-1 catalyzed asymmetric Henry reaction (ee = 94%, yield 96%) and diastereoselective Henry reaction have been reported. Additionally, drug (R)-(−)-isoproterenol was synthesized using this protocol.