CHEMIJA. 2019. Vol. 30. No. 2. P. 89–97 © Lietuvos mokslų akademija, 2019

Solvothermally derived Me2(H2O)4(5SSA)3·DMF and Me2(H2O)4(4NSA)2·DMF: Application as Biginelli reaction catalysts

Andrius Laurikėnas1,

Fatma Yalçin2,

Robertas Žilinskas1,

Ayse Uztetik Morkan2,

Albinas Žilinskas1,

Izzet Amour Morkan2,

Aivaras Kareiva1*

1 Institute of Chemistry, Vilnius University, 24 Naugarduko Street, 03225 Vilnius, Lithuania

2 Department of Chemistry, Institute of Natural Sciences, Bolu Abant Izzet Baysal University, Gölköy Yerleşkesi, 14030, Bolu, Turkey

The solvothermal synthesis between Me(NO3)x ∙ yH2O (Mex+ = Fe3+, Ni2+, Mn2+, Co2+, Cu2+) and respectively 4-nitrosalycilic (4NSA) and 5-sulfosalicylic (5SSA) acids produced hybrid organic-inorganic compounds composed of Mex+ ions and organic fragments which include three different functional groups, carboxylic, hydroxyl and sulfonic, each coordinated to the Mex+ ions. The phase and chemical composition, microstructure and properties of Me2(H2O)4(5SSA)3·DMF and Me2(H2O)4(4NSA)2·DMF (DMF – dimethylformamide) hybrid compounds were evaluated and discussed. The synthesised materials were characterised by X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM) and nuclear magnetic resonance (NMR) spectroscopy. The catalytic properties of the obtained compounds were also investigated.

Keywords: solvothermal synthesis, hybrid organic-inorganic compounds, Biginelli reaction, catalytic activity

INTRODUCTION

The assembly of multifunctional ligands with metal ions is currently of great interest due to their use in constructing two- and three-dimensional polymers with special properties, such as electrical conductivity, magnetism, host-guest chemistry, catalysis and luminescence [1–4]. Reactions of Mex+NO3 · yH2O (Mex+ = Fe3+, Ni2+, Mn2+, Co2+) with respectively 4-nitrosalycilic (4NSA) and 5-sulfosalicylic (5SSA) acids [5] produced hybrid organic-inorganic compounds. One of the most commonly used synthesis methods of hybrid organic-inorganic compounds is the solvothermal synthesis, which is less harmful to the environment. This method is ideal for the synthesis of solid-state compounds such as organic hybrid and inorganic hybrid materials. The solvothermal synthesis method is defined as a heterogeneous reaction in the presence of aqueous or organic solvents under high pressure and temperature to dissolve and crystallise materials which are insoluble under normal conditions [6–9].

The Biginelli reaction discovered by Pietro Biginelli in 1891 [10–12] is the acid-catalysed reaction between aldehyde, β-ketoester and urea constitutes yielding dihydropyrimidones. Originally, this reaction was conducted by refluxing a mixture of benzaldehyde, ethyl acetoacetate and urea in ethanol in the presence of HCl as a catalyst. These compounds, also called ‘Biginelli compounds’, are interesting materials with a potential for pharmaceutical application. The first step in the reaction mechanism (see Fig. 1) is believed to be the condensation between aldehyde and urea. The iminium intermediate generated acts as an electrophile for the nucleophilic addition of ketoester enol and the ketone carbonyl of the resulting adduct undergoes condensation with the urea -NH2 to give the cyclised product.

Me2(H2O)4(5SSA)3·DMF and Me2(H2O)4(4NSA)2· DMF compounds are possibly acting as Brønsted or Lewis [13] catalysts, donating protons. Proceeding along our research in this area we report here the solvothermal synthesis of hybrid organic-inorganic compounds of Fe3+, Ni2+, Mn2+, Co2+ and Cu2+ ions with 4-nitrosalycilic (4NSA) and 5-sulfosalicylic (5SSA) acids, and the efficient use of the obtained materials as catalysts in the Biginelli reaction.

EXPERIMENTAL

Metal nitrate salts, 5-sulphosalycilic acid (5SSA), 4-nitrosalycilic acid (4NSA) N,N-dimethylformamide and 2-isopropanol were purchased from Aldrich and Merck and used directly without further purification.

For the synthesis of Me2(H2O)4(5SSA)3·DMF, 0.015 mol of metal (Fe3+, Ni2+, Mn2+, Co2+, Cu2+) nitrate was added to 60 ml of DMF/H2O (1/1 v/v) solution and mixed for 30 min at 50°C. 0.0075 mol of 5SSA added afterwards continues mixing for 15 min at the same temperature. The resulted solution was poured into an autoclave vessel and heated for 24 h at 120°C. The synthesis scheme of Me2(H2O)4(5SSA)3·DMF hybrid organic-inorganic compound is presented in Fig. 2. The reaction solution was filtered and shiny crystals (Ni2+, Co2+, Cu2+) or fine powders (Fe3+, Mn2+) were obtained. Crystals or powders were immersed in 2-pro-panol and the residual of DMF/H2O solvent was removed.

Fig. 1. The proposed Biginelli reaction mechanism [7]
Fig. 2. The synthesis scheme of Me2(H2O)4(5SSA)3·DMF organic-inorganic hybrid

A similar procedure as for Me2(H2O)4(5SSA)3· DMF was used for the synthesis of Me2(H2O)4(4NSA)2·DMF. Instead of 5SSA ligand 0.0075 mol of 4NSA was used. The synthesis scheme of Me2(H2O)4(4NSA)2·DMF hybrid organic-inorganic compound is presented in Fig. 3.

For the synthesis of ethyl 6-methyl-2-oxo-4-phenyl-3,4-dihydro-1H-pyrimidine-5-carbox-ylate, ethyl acetoacetate (10 mmol) (1), benzal-dehyde (10 mmol) (2), urea (10 mmol) (3) and Me2(H2O)4(5SSA)3·DMF or Me2(H2O)4(4NSA)2·DMF (3 mol%) were mixed by stirring the mixture for 24 h at 80°C. The synthesis scheme of ethyl 6-methyl- 2-oxo-4-phenyl-3,4-dihydro-1H-pyrimidine- 5-carboxylate is shown in Fig. 4. The ratio of reagents 1:2:3 was 10:10:10 (mmol). The experiment was performed at reflux conditions in the presence of 2-propanol (12 mL). After the completion of the reaction (monitored by thin layer chromatography), the catalyst was separated from the reaction mixture by filtration and then the liquid part was poured into ice cold water. The obtained solid product was filtered and dried. The crude product was purified by recrystallisation in methanol.

X-ray diffraction (XRD) data were collected in the range of 20 ≤ 2θ ≤ 70 using Ni-filtered Cu Kα radiation on a Rigaku MiniFlex II diffractometer working in Bragg–Brentano (θ/2θ) geometry. The step width and integration time were 0.02 and 1 s, respectively. The morphology of the final powders was characterised by scanning electron microscopy performed with Hitachi Tabletop Microscope TM3000 scanning electronic microscopes (SEM). 1H NMR and 13C spectra were recorded on a Bruker Ascend 400 spectrometer operating at 9.4 Tesla, corresponding to the resonance frequency of 400 MHz for the 1H nucleus, equipped with a direct detection four nuclei probe head and field gradients on the z axis. The samples were analysed in 5 mm NMR tubes.

Fig. 3. The synthesis scheme of Me2(H2O)4(4NSA)2·DMF organic-inorganic hybrid
Fig. 4. The synthesis scheme of ethyl 6-methyl-2-oxo-4-phenyl-3,4-dihydro-1H-pyrimidine-5-carboxylate

RESULTS AND DISCUSSION

The solvothermal synthesis of hybrid organic-inorganic compounds yielded small crystals or fine powders insoluble in non-polar solvents [14,15]. Slightly acidic or basic solutions tend to destroy hybrid structure yielding metal salts. As was expected, methanol and ethanol tend to dissolve hybrid organic-inorganic compounds without destruction of the crystalline structure [16]. The powder X-ray diffraction analysis (see Fig. 5) of the synthesised samples indicates the formation of single phase materials and a good crystallinity with the exception of formation of partially amorphous Fe2(H2O)4(5SSA)3·DMF. As one can see, the hybrid organic-inorganic compounds obtained with 4NSA are high crystalline materials. The hybrid organic-inorganic compounds with 5SSA also display a moderate crystallinity with the exception of partially amorphous Fe2(H2O)4(5SSA)3·DMF. The formation of amorphous intermediates or the end products is rather unusual in the common synthesis of such type compounds [17–19].

The morphological data of Me2(H2O)4(5SSA)3· DMF and Me2(H2O)4(4NSA)2·DMF samples gathered from SEM micrographs reveal differences in particle size and shape (see Fig. 6 and Table 1). The microstructure of Ni2(H2O)4(4NSA)2·DMF and Fe2(H2O)4(4NSA)2·DMF samples is very similar. The solids are composed of different size (from 5 to 40 μm) plate-like crystals. However, almost regular cubes were formed in the case of Mn and Co analogues [20]. In the case of Cu2(H2O)4(4NSA)2·DMF, the hexagonally shaped solids in size similar to Mn2(H2O)4(4NSA)2·DMF and Co2(H2O)4(4NSA)2·DMF cubes were formed [21]. When 5-sulfosalicylic (5SSA) acid was used as a ligand, similar regular cubes were formed in the case of Mn2(H2O)4(5SSA)3·DMF. Cu2(H2O)4(5SSA)3·DMF was also composed of cubic crystallites, however, much smaller in size (5–10 μm). The plate-like crystals, however, were not formed when 5SSA was used. The Fe2(H2O)4(5SSA)3·DMF solids are composed of homogeneously-distributed nanosized (less than 10 nm) spherically-shaped crystallites. A similar microstructure was observed also for the Co2(H2O)4(5SSA)3·DMF sample. In this case, however, these nanoparticles show a tendency to form agglomerates. Even larger agglomerates were formed during the synthesis of Ni2(H2O)4(5SSA)3·DMF specimens. Interestingly, the morphology of these Ni2(H2O)4(5SSA)3·DMF agglomerates could be on the transformation to a hexagonic microstructure stage [22]. The morphological features of the synthesised hybrid materials are summarised in Table 1. Interestingly, only the manganese containing hybrids synthesised with different ligands (5SSA and 4NSA) show almost identical morphology and are composed of cubic crystallites in both of the cases.

Fig. 5. The XRD patterns of Me2(H2O)4(5SSA)3·DMF (left) and Me2(H2O)4(4NSA)2·DMF (right) samples
Fig. 6. SEM micrographs of Me2(H2O)4(4NSA)2·DMF; Mex+ = Fe3+ (A), Ni2+ (B), Mn2+ (C), Co2+ (D), Cu2+ (E); and Me2(H2O)4(5SSA)3·DMF; Mex+ = Fe3+ (F), Ni2+ (G), Mn2+ (H), Co2+ (I), Cu2+ (J)
Table 1. The morphological features of synthesised Me2(H2O)4(4NSA)2·DMF and Me2(H2O)4(5SSA)3·DMF hybrid materials

The 1H and 13C NMR spectra of the synthesised Biginelli compounds are presented in Fig. 7. The NMR analysis data clearly revealed that the pure product (ethyl 6-methyl-2-oxo-4-phenyl-3,4-dihydro-1H-pyrimidine-5-carboxylate) could be synthesised from benzaldehyde, ethyl acetoacetate and urea using the coordination polymers as reaction catalysts. The spectral data of the obtained Biginelli product (ethyl 6-methyl-2-oxo-4-phenyl-3,4-dihydro-1H-pyrimidine-5-carboxylate) are the following: 1H NMR (400 MHz, CDCl3): δ (ppm) = 8.26 (s, 1H), 7.37–7.28 (m, 5H), 5.83 (s, 1H), 5.44–5.39 (m, 1H), 4.09 (qd, J = 7.1, 2.1 Hz, 2H), 2.36 (d, J = 0.9 Hz, 3H), 1.18 (t, J = 7.1 Hz, 3H). 13C NMR (100 MHz, CDCl3): δ (ppm) = 165.65, 146.35, 143.74, 128.71, 127.95, 126.61, 101.34, 77.36, 77.04, 76.72, 60.02, 55.74, 30.95, 18.67, 14.15. FTIR: cm–1 3241, 3113, 1727, 1711, 1656.

The yields of the products of heterogeneous catalysis using the synthesised hybrid organic-inorganic compounds are displayed in Table 2. As seen, the Ni2(H2O)4(5SSA)3·DMF and Cu2(H2O)4(4NSA)2·DMF catalysts yielded a higher conversion ratio than Mn, Fe and Co analogues. Surprisingly, the most effective catalysts Cu2(H2O)4(4NSA)2·DMF and Ni2(H2O)4(5SSA)3· DMF for the Biginelli reaction have showed a slightly different surface microstructure from other studied hybrid materials. Cu2(H2O)4(4NSA)2·DMF was composed of hexagonic crystallites and Ni2(H2O)4(5SSA)3·DMF agglomerates showed an evident tendency of transformation to hexagonic crystallites. Therefore, we can conclude that the catalytic activity of the synthesised hybrid organic-inorganic compounds was very dependent on the surface microstructure of these hybrid materials. The possible defects in the structure also could create open sites and increase catalytic activity [23]. Since the catalysts are heterogeneous, they could be recycled for at least several times [24].

Table 2. The results obtained for the Biginelli reactions of benzaldehyde with ethyl acetoacetate and urea
Fig. 7. The NMR spectra of synthesised Biginelli compounds: 1H NMR (top) and 13C (bottom)

CONCLUSIONS

Hybrid organic-inorganic compounds Me2(H2O)4 (5SSA)3·DMF and Me2(H2O)4(4NSA)2· DMF were prepared by solvothermal synthesis using reactions between metal (Fe3+, Ni2+, Mn2+, Co2+ and Cu2+) nitrates and 4-nitrosalycilic (4NSA) and 5-sulfosalicylic (5SSA) ligands. The XRD analysis revealed that all samples displayed a good crystallinity with the exception of Fe2(H2O)4(5SSA)3·DMF which was partially amorphous. The SEM micrographs revealed differences in the particle size and crystallite shape of the obtained hybrid materials. The Ni2(H2O)4(4NSA)2·DMF and Fe2(H2O)4(4NSA)2·DMF samples were composed of different size (from 5 to 40 μm) plate-like crystals. However, almost regular cubes were formed in the case of the Mn2(H2O)4(4NSA)2·DMF and Co2(H2O)4(4NSA)2·DMF samples analogues. The hexagonally shaped solids of Cu2(H2O)4(4NSA)2· DMF, however, were formed. In the case of manganese, similar regular cubes were formed when 5-sulfosalicylic (5SSA) acid was used as a ligand. Cu2(H2O)4(5SSA)3·DMF MOFs were also composed of cubic crystallites, however, much smaller in size (5–10 μm). The Fe2(H2O)4(5SSA)3·DMF, Co2(H2O)4(5SSA)3·DMF and Ni2(H2O)4(5SSA)3· DMF solids were composed of homogeneously-distributed nanosized (less than 10 nm) spherically-shaped crystallites with a different tendency to form agglomerates. The Ni2(H2O)4(5SSA)3·DMF agglomerates showed an evident tendency of transformation to hexagonic crystallites. The solvotermally derived Me2(H2O)4(5SSA)3·DMF and Me2(H2O)4(4NSA)2·DMF hybrid organic-inorganic compounds were investigated as catalysts in the Biginelli reaction. It was demonstrated that Ni2(H2O)4(5SSA)3·DMF and Cu2(H2O)4(4NSA)2·DMF catalysts yielded a higher conversion ration than Mn, Fe and Co analogues. In conclusion, the catalytic activity of the synthesised hybrid organic-inorganic compounds was influenced by the surface microstructure of these hybrid materials.

Received 25 February 2019

Accepted 1 March 2019

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* Corresponding author. Email: aivaras.kareiva@chgf.vu.lt

Andrius Laurikėnas, Fatma Yalçin, Robertas Žilinskas, Ayse Uztetik Morkan, Albinas Žilinskas, Izzet Amour Morkan, Aivaras Kareiva

SOLVOTERMIŠKAI SUSINTETINTI Me2(H2O)4(5SSA)3·DMF IR Me2(H2O)4(4NSA)2·DMF: PANAUDOJIMAS BIGINELLI REAKCIJOS KATALIZATORIAIS

S a n t r a u k a
Solvoterminiu sintezės metu tarp Me(NO3)x ∙ yH2O (Mex+ = Fe3+, Ni2+, Mn2+, Co2+, Cu2+) ir atitinkamai 4-nitrosalicilo (4NSA) ir 5-sulfosalicilo (5SSA) rūgščių susintetinti hibridiniai organiniai–neorganiniai junginiai, sudaryti iš Mex+ jonų ir organinių fragmentų, turinčių tris skirtingas funkcines grupes: karboksilo, hidroksilo ir sulfono. Ištirta susintetintų Me2(H2O)4(5SSA)3·DMF ir Me2(H2O)4(4NSA)2·DMF (DMF-dimetilformamidas) hibridinių junginių fazinis grynumas ir mikrostruktūra. Sintetintos medžiagos buvo apibūdintos rentgeno spindulių difrakcijos (XRD) analize, skenuojančia elektronine mikroskopija (SEM) ir branduolinio magnetinio rezonanso (NMR) spektroskopija. Taip pat buvo tiriamos gautų junginių katalitinės savybės.