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

Hydrothermal Assembly, Structural Multiplicity, and Catalytic Knoevenagel Condensation Reaction of a Series of Coordination Polymers Based on a Pyridine-Tricarboxylic Acid

A pyridine-tricarboxylic acid, 5-(3',5'-dicarboxylphenyl)nicotinic acid (H3dpna), was employed as a adjustable block to assemble a series of coordination polymers under hydrothermal conditions. The seven new coordination polymers were formulated as [Co(μ3-Hdpna)(μ-dpey)]n·nH2O (1), [Zn4.5(μ6-dpna)3(phen)3]n (2), [Co1.5(μ6-dpna)(2,2'-bipy)]n (3), [Zn1.5(μ6-dpna)(2,2'-bipy)]n (4), [Co3(μ3-dpna)2(4,4'-bipy)2(H2O)8]n·2nH2O (5),[Co(bpb)2(H2O)4]n[Co2(μ3-dpna)2(H2O)4]n·3nH2O (6), and [Mn1.5(μ6-dpna)(μ-dpea)]n (7), wherein 1,2-di(4-pyridyl)ethylene (dpey), 1,10-phenanthroline (phen), 2,2'-bipyridine(2,2'-bipy),4,4'-bipyridine(4,4'-bipy),1,4-bis(pyrid-4-yl)benzene (bpb), and 1,2-di(4-pyridyl)ethane (dpea) were employed as auxiliary ligands. The structural variation of polymers 1-7 spans the range from a 2D sheet (1-4, 6, and 7) to a 3D metal-organic framework (MOF, 5). Polymers 1-7 were investigated as heterogeneous catalysts in the Knoevenagel condensation reaction, leading to high condensation product yields (up to 100%) under optimized conditions. Various reaction conditions, substrate scope, and catalyst recycling were also researched. This work broadens the application of H3dpna as a versatile tricarboxylate block for the fabrication of functional coordination polymers.

Topologically Driven Pore/Surface Engineering in a Recyclable Microporous Metal-Organic Vessel Decorated with Hydrogen-Bond Acceptors for Solvent-Free Heterogeneous Catalysis

The use of metal-organic frameworks (MOFs) comprising custom-designed linkers/ligands as efficient and recyclable heterogeneous catalysts is on the rise. However, the topologically driven bifunctional porous MOFs for showcasing a synergistic effect of two distinct activation pathways of substrates (e.g., involving hydrogen bonding and a Lewis acid) in multicomponent organic transformations are very challenging. In particular, the novelty of such studies lies in the proper pore and/or surface engineering in MOFs for bringing the substrates in close proximity to understand the mechanistic aspects at the molecular level. This work represents the topological design, solid-state structural characterization, and catalytic behavior of an oxadiazole tetracarboxylate-based microporous three-dimensional (3D) metal-organic framework (MOF), {[Zn₂(oxdia)(4,4'-bpy)₂]·8.5H₂O}_n (1), where the tetrapodal (4-connected) 5,5'-(1,3,4-oxadiazole-2,5-diyl)diisophthalate (oxdia^4-), the tetrahedral metal vertex (Zn(II)), and a 2-connected pillar linker 4,4'-bipyridine (4,4'-bpy) are unique in their roles for the formation, stability, and function. As a proof of concept, the efficient utilization of both the oxadiazole moiety with an ability to provide H-bond acceptors and the coordinatively unsaturated Zn(II) centers in 1 is demonstrated for the catalytic process of the one-pot multicomponent Biginelli reaction under mild conditions and without a solvent. The key steps of substrate binding with the oxadiazole moiety are ascertained by a fluorescence experiment, demonstrating a decrease or increase in the emission intensity upon interaction with the substrates. Furthermore, the inherent polarizability of the oxadiazole moiety is exploited for CO₂ capture and its size-selective chemical fixation to cyclic carbonates at room temperature and under solvent-free conditions. For both catalytic processes, the chemical stability, structural integrity, heterogeneity, versatility in terms of substrate scope, and mechanistic insights are discussed. Interestingly, the first catalytic process occurs on the surface, while the second reaction occurs inside the pore. This study opens new ways to catalyze different organic transformation reactions by utilizing this docking strategy to bring the multiple components close together by a microporous MOF.

Deciphering supramolecular isomerization in coordination polymers: connected molecular squares vs. fused hexagons

The self-assembly of Mn(ii), bis(tridentate) ligands and bent dicarboxylate linkers under ambient conditions has been exploited to generate a series of 1D coordination polymers in good yields. For a set of seven compounds, structural isomerization of these architectures is demonstrated through the variation of length and nature of the spacer between the tridentate capping sites of the bis(tridentate) ligands, such as tpbn (N,N',N'',N'''-tetrakis-(2-pyridylmethyl)-1,4-diaminobutane), tphxn (N,N',N'',N'''-tetrakis-(2-pyridylmethyl)-1,6-diaminohexane), and tpxn (N,N',N'',N'''-tetrakis-(2-pyridylmethyl)-xylylamine) or by varying the bent dicarboxylate linker 4,4'-(dimethylsilanediyl)bis-benzoic acid (H2L1) or 4,4'-oxybis-benzoic acid (H2L2). These compounds have been structurally characterized by single-crystal and powder X-ray diffraction, FTIR, and thermogravimetric and elemental analyses. This study reveals that the supramolecular structural variation can be precisely controlled either by a judicious selection of reaction conditions or linker/ligand combinations. For example, the self-assembly of Mn(ii), tpbn and H2L1 in DMF/EtOH/water affords a mixture of products (1 and 1a) while changing the solvent combination to EtOH/water results in the generation of a single isomer (1a) in a highly selective manner. On the other hand, for the Mn(ii)-tphxn system, different structural isomers have been isolated by varying the dicarboxylates, H2L1 and H2L2 (2vs.5). Similarly, for the Mn(ii)-H2L2 system, a variation in the spacer chain length of bis(tridentate) ligands, tpbn and tphxn resulted in the formation of different structural isomers (4vs.5).

Synthesis and Catalytic Application of Two Mononuclear Complexes Bearing Diethylenetriamine Derivative Ligand

Two mononuclear zero-dimensional Ni(II) and Zn(II) complexes bearing diethylenetriamine derivative ligand, namely [NiL(CH₃COO)₂(H₂O)] (1) and [ZnL(CH₃COO)₂] (2) [L = N, N'-bis(2-hydroxybenzyl)diethylenetriamine], were synthesized under reflux conditions. The molecular composition and structure of the complexes were identified by IR, PXRD, elemental analyses, and single crystal X-ray diffraction. Complex 1 belongs to a monoclinic crystal system with the P2₁/n space group, and Complex 2 belongs to a monoclinic crystal system with the C2/c space group. The Henry reaction of nitromethane with aromatic aldehydes was explored with Complexes 1 and 2 as the catalyst. Results from the catalytic reaction revealed that the complexes displayed excellent catalytic activities under the optimized conditions and that the substrate scope of aromatic aldehydes could be extended to a certain extent. In addition, the possible catalytic mechanism of the Henry reaction was also deduced.

Highly Efficient Nanosized Mesoporous CuMgAl Ternary Oxide Catalyst for Nitro-Alcohol Synthesis: Ultrasound-Assisted Sustainable Green Perspective for the Henry Reaction

Nanosized mesoporous CuMgAl ternary oxide catalysts were prepared by thermal decomposition of CuMgAl-layered double hydroxides at 500 °C with nominal Cu/Mg/Al ratios of 1:1:1 (Cu-LDH-I), 1.5:0.5:1 (Cu-LDH-II), and 2:0:1 (Cu-LDH-III). The synthesized catalysts were characterized by inductively coupled plasma atomic emission spectroscopy (ICP-AES), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), Fourier transform infrared (FTIR) spectroscopy, CO2-TPD, and N2 physisorption analysis techniques. The catalytic activity of the synthesized materials was investigated for the Henry reaction between nitromethane and numerous aldehyde derivatives under ultrasonic irradiation. The three CuMgAl ternary oxide catalysts exhibited a high catalytic activity, forming nitro alcohol products with 100% atom economy. The CuMgAl-I catalyst derived from Cu-LDH-I offered high turnover frequencies (TOFs in the synthesis of all of the nitro alcohols in shorter reaction times). The superior catalytic activity of the CuMgAl-I sample is attributed to the synergistic effect between the physicochemical properties of the catalysts and ultrasonic irradiation. The HRTEM analysis of the used CuMgAl-I catalyst revealed the evidence for the cavitation collapse, which causes localized deformation and surface erosion. Moreover, the synthesized catalysts also exhibited robust sustainable activity that resisted deactivation over repeated usage. The present example of ultrasonic-assisted catalyzed organic synthesis represents a novel strategy for the solvent-free green synthesis of nitro-alcohols by the Henry reaction with 100% atom economy.

Metal ion exchange in Prussian blue analogues: Cu(ii)-exchanged Zn–Co PBAs as highly selective catalysts for A3 coupling

The occurrence of metal ion exchange in Zn₃[Co(CN)₆]₂ and Cu₃[Co(CN)₆]₂ Prussian blue analogues (Zn–Co and Cu–Co PBAs) was demonstrated for the first time.