Cuprospinel

Cuprospinel, like many other spinels has the general formula AB₂O₄. Yet, cuprospinel is an inverse spinel in that its A element, in this case copper (Cu²⁺), only occupies octahedral sites in the structure and the B element, iron (Fe²⁺ and Fe³⁺), is split between the octahedral and tetrahedral sites in the structure.[10][11] The Fe²⁺ species will occupy some of the octahedral sites and there will only be Fe³⁺ at the tetrahedral sites.[10][11] Cuprospinel adopts both cubic and tetragonal phases at room temperature, yet as temperature is elevated the cubic form is most stable.[4][11]

CuFe₂O₄ nanoparticles have been characterized as a superparamagnetic material with saturated magnetization of Ms = 49 emu g⁻¹,[12] remnant magnetization (M_r = 11.66 emu g⁻¹) and coercivity (H_c = 63.1 mT).[13] The magnetic properties of CuFe₂O₄ are correlated with the size of particles. Particularly, the decreasing in saturated magnetization and remanence correspond to the decreasing in the size of CuFe₂O₄ particles, whereas the coercivity increases.[14]

Spinel CuFe₂O₄ can be synthesized by solid phase synthesis at high temperature. In a particular procedure for this type of synthesis, the stoichiometric mixture of Cu(CH₃COO)₂· and FeC₂O₂ is ground together and stirred in a solvent. After evaporation of the solvent, the resulting powder is heated in a furnace at constant temperature around 900 °C in normal air-atmosphere environment. Then the resulting product is slowly cooled to room temperature in order to obtain the desired stable spinel structure.[14]

A method combining a first precipitation step at room temperature in triethylene glycol (TEG), a viscous and highly hygroscopic liquid with an elevated boiling point, 285 °C (545 °F; 558 K), followed by a thermal treatment at elevated temperature is an effective way to synthesize spinel oxide, especially copper iron oxide. Typically, NaOH is first added dropwise to a solution of Fe³⁺ (Fe(NO₃)₃ or Fe(acac)₃) and Cu²⁺ (Cu(NO₃)₂ or CuCl₂) in triethylene glycol at room temperature with constant stirring until a reddish-black precipitate completely form. The resulting viscous suspension is then placed in an ultrasonic bath to be properly dispersed, followed by heating in a furnace at high temperature. The final product is then washed in diethyl ether, ethyl acetate, ethanol and deionized water, and then dried under vacuum to obtain oxide particles.[15][16][17]

Cuprospinel is used in various industrial processes as a catalyst. An example is the water–gas shift reaction:[11]

H₂O_(v) + CO_(g) → CO_2(g) + H_2(g)

This reaction is particularly important for hydrogen production and enrichment.

The interest of cuprospinel arises in that magnetite is a widely used catalyst for many industrial chemical reactions, such as the Fischer–Tropsch process, the Haber–Bosch process and the water-gas shift reaction. It has been shown that doping magnetite with other elements gives it different chemical and physical properties; these different properties sometimes allow the catalyst to work more efficiently. As such, cuprospinel is essentially magnetite doped with copper and this enhances magnetite's water gas shift properties as a heterogeneous catalyst.[18][19]

Recent years, various research towards the heterogeneous catalytic ability of CuFe₂O₄ in organic synthesis have been published ranging from traditional reactions to modern organometallic transformation.[20][21] By taking advantages of magnetic nature, the catalyst can be separated simply by external magnetism, which can overcome the difficulty to separate nano-scaled metal catalyst from the reaction mixture. Particularly, only by applying magnetic bar at the outer vessel, the catalyst can easily be held at the edge of container while removing solution and washing particles.[12] The obtained particles can be readily used for the next catalyst cycles. Moreover, the catalytic site can be exploited in either cooper or iron center because of the large-surface area of nanoparticles, leading to wide scope to apply this material in various types of reactions.[16][20]

Catalyst for multi-component reaction (MCR)

Nano CuFe₂O₄ can be utilized as a catalyst in a one-pot synthesis of fluorine containing spirohexahydropyrimidine derivatives. It has also been observed that the catalyst can be reused five times without significant loss in catalytic activity after each runs. In the reaction, iron plays a vital role in the coordination with the carbonyl group in order to increase the electrophilic property, which can facilitate the reaction conditions and increase the reaction rate.[16]

One-pot synthesis of fluorine containing spirohexahydropyrimidine derivatives. Adapted from Dandia, Jain & Sharma 2013.[16]

Another example for MCR utilizing CuFe₂O₄ was published in a research towards the A3 coupling of aldehydes, amine with phenylacetylene to give the corresponding propargylamines. The catalyst can be reused three times without remarkable reduce in reaction yield.[22]

A3 coupling of aldehydes, amine with phenylacetylene. Adapted from Tamaddon & Amirpoor 2013.[23]

Other reactions

CuFe₂O₄ can also be applied for C-C cleavage α-arylation between acetylacetone with iodobenzene. The phenylacetone product was obtained with excellent yield at 99% and 95% selectivity observed for principal product compared to 3-phenyl-2,4-pentanedione as the byproduct. The XRD results were observed that crystal structure of catalyst remained unchanged after the sixth run while catalytic activity slightly decreases at 97% conversion in the final run. In this reaction, the mechanistic study showed the catalytic cycle started from Cu^II to Cu^I and then oxidized to Cu^II by aryl iodine.[12]

Arylation between acetylacetone with iodobenzene. Adapted from Nguyen et al. 2014.[12]

The role of copper has been further emphasized in the coupling reaction of ortho-arylated phenols and dialkylformamides. It was observed that there was a single-electron oxidative addition of copper^II to copper^III through a radical step, then transformed back to copperI by reductive elimination in the presence of either oxygen or peroxide. Catalyst can be reused 9 times without significant loss in catalytic activities.[26]

Coupling reaction of ortho-arylated phenols and dialkylformamides. Adapted from Nguyen et al. 2017.[26]

Synergistic effect of catalytic activity

Notably, synergistic effect was demonstrated for the case of CuFe₂O₄ in Sonogashira reaction. Both Fe and Cu center contribute to catalytic activity of the transformation between aryl halide and substituted alkynes. The product was obtained with 70% yield in the presence of Nano CuFe₂O₄, while only 25% yield and <1% yield observed when using CuO and Fe₃O₄ respectively.[27]

Transformation between aryl halide and substituted alkynes