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Article

Synthesis, Structural Features, and Catalytic Activity of an Iron(II) 3D Coordination Polymer Driven by an Ether-Bridged Pyridine-Dicarboxylate

1
Guangdong Research Center for Special Building Materials and Its Green Preparation Technology, Foshan Research Center for Special Functional Building Materials and their Green Preparation Technology, Guangdong Industry Polytechnic, Guangzhou 510300, China
2
College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, China
3
Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisbon, Portugal
4
Research Institute of Chemistry, Peoples’ Friendship University of Russia (RUDN University), 6 Miklukho-Maklaya st., 117198 Moscow, Russia
*
Authors to whom correspondence should be addressed.
Crystals 2019, 9(7), 369; https://doi.org/10.3390/cryst9070369
Received: 9 June 2019 / Revised: 16 July 2019 / Accepted: 18 July 2019 / Published: 20 July 2019
(This article belongs to the Section Crystal Engineering)

Abstract

:
New iron(II) three-dimensional coordination polymer (3D CP), [Fe(µ3-Hcpna)2]n (1), was assembled under hydrothermal conditions from 5-(4’-carboxyphenoxy)nicotinic acid (H2cpna) as a trifunctional organic N,O-building block. This stable microcrystalline CP was characterized by standard methods for coordination compounds in the solid state (infrared spectroscopy, elemental analysis, thermogravimetric analysis, powder and single-crystal X-ray diffraction). Structure and topology of 1 were examined and permitted an identification of a 3,6-connected framework of the rtl topological type. In addition, compound 1 acts as effective catalyst precursor for oxidative functionalization of alkanes (propane and cyclic C5−C8 alkanes) under homogeneous catalysis conditions, namely for the oxidation of saturated hydrocarbons with H2O2/H+ system to produce ketones and alcohols, and for alkane carboxylation with CO/H2O/S2O82− system to obtain carboxylic acids. The influence of an acid promoter and substrate scope (propane and cyclic C5−C8 alkanes) were investigated.

Graphical Abstract

1. Introduction

Coordination polymers (CPs) are now very popular compounds because of their highly diverse and unusual structures and interesting applications in various areas, which range from separation and sorption of gases [1,2,3] to luminescent [4,5] and catalytic materials [6,7,8], as well as molecular magnets [9,10] and sensors [11,12]. CPs are typically built from metal nodes and organic ligands such as polycarboxylic acids that act as spacers and/or linkers. Assembly and functional properties of CPs are usually affected by a diversity of parameters such as conditions of the synthesis, coordination preferences of metal nodes, as well as types of organic linkers or spacers [13,14,15,16,17,18,19].
The use of polycarboxylic acid ligands that combine different functional groups represents an interesting research direction for designing new types of coordination polymers or metal-organic frameworks (MOFs) [20,21,22]. Following our continuous interest in exploring new or poorly studied multicarboxylic acids for the design of CPs [23,24,25,26,27,28,29,30], in this work we selected a trifunctional nicotinic acid derivative, 5-(4’-carboxyphenoxy)nicotinic acid (H2cpna) as a main building block. Selection of H2cpna can be justified by the following points. (1) This carboxylic acid can potentially act as a trifunctional ligand containing one phenyl and one pyridine ring which are connected by a gyrating O-ether functionality. (2) There are three different functional group classes (i.e., N-pyridyl, –COOH, O-ether) in H2cpna and six possible coordination sites. (3) H2cpna is still barely investigated for synthesis of CPs or MOFs. In fact, a search of the Cambridge Structural Database shows that there are only several examples of Zn, Cu, Ni, Co, Mn, Cd, and Pb coordination polymers assembled from H2cpna; some of them revealed interesting catalytic [8], sensing [11], gas sorption [21], luminescent [11,27], and magnetic [27] properties. Notable examples include a 2D Co(II) CP [Co(μ3-cpna)(phen)(H2O)]n·nH2O {phen, 1,10-phenanthroline} applied as a heterogeneous catalyst for alcohol oxidation [8], a Tb3+-doped 3D Cu(II) MOF [Cu(μ3-cpna)2]n used as a luminescent probe for sensing H2S [11], and a 2D porous framework [Cu(µ3-cpna)(Me2NH)]n·nDMF·nH2O {DMF, dimethylformamide} with remarkable CO2 sorption behavior in addition to single-crystal to single-crystal transformation features [21]. Despite these examples, iron derivatives of H2cpna remain virtually unknown.
Apart from this point, the selection of iron as a metal to construct a new coordination polymer is explained by its availability and high coordination versatility, as well as interesting redox chemistry that might be important for applications in catalysis. Thus, iron(II) sulfate (FeSO4·7H2O) was explored as a low-cost and water-soluble iron(II) precursor for the hydrothermal synthesis of an H2cpna-derived compound. On the other hand, the hydrothermal synthetic conditions were used to aid the crystallization of a product as single crystals in addition to other advantages such as use of water as green reaction medium and no need for work-up to purify and isolate the product [26].
In this study, we describe a hydrothermal generation, characterization, crystal structure, topological interpretation, thermal stability, as well as catalytic behavior of a new iron(II) coordination polymer assembled from 5-(4’-carboxyphenoxy)nicotinic acid. The obtained product [Fe(µ3-Hcpna)2]n (1) is the first Fe coordination compound assembled from H2cpna. Besides, it also acts an efficient catalyst precursor for the oxidative functionalization of different saturated hydrocarbons under homogeneous conditions.

2. Experimental

2.1. Materials and Physical Measurements

Analytical reagent grade chemicals were applied. H2cpna was acquired from a commercial supplier (Jinan Henghua Sci. & Tec. Co., Ltd, http://www.chemhh.com, catalogue code: 120511H-1B, purity 98%, CAS: 1777822-70-4). Elemental C, H, N analysis was run on an Elementar Vario EL elemental analyzer. Infrared (IR) spectra were obtained on a Bruker EQUINOX 55 spectrometer (KBr disks). TGA (thermogravimetric analysis) was carried out on a LINSEIS STA PT1600 thermal analyzer (N2 atmosphere, 10 °C/min heating rate). PXRD (powder X-ray diffraction) pattern was obtained on a Rigaku-Dmax 2400 diffractometer (Cu-Kα radiation; λ = 1.54060 Å). UV-Vis absorption spectra were determined on a Cary 5000 UV-vis-NIR spectrophotometer. For catalytic tests, GC (gas chromatography) analyses were carried out using an Agilent Technologies 7820A series gas chromatograph (FID: flame ionization detector; carrier gas: He; capillary column: BP20/SGE).

2.2. Synthesis of [Fe(µ3-Hcpna)2]n (1)

FeSO4·7H2O (0.15 mmol, 41.7 mg), H2cpna (0.3 mmol, 77.7 mg), NaOH (0.3 mmol, 12.0 mg), and water (10 mL) were combined in a Teflon-lined stainless-steel reactor (25 mL volume) and stirred for 15 min at ambient temperature. The reactor was then sealed, heated for 3 days at 160 °C, and gradually cooled (10 °C/h rate) to ambient temperature. Yellow crystals (blocks) were manually isolated, washed with bidistilled water, and then air-dried to furnish compound 1. Yield: 55% (on the bases of H2cpna). Calcd for C26H16FeN2O10: C 54.57, H 2.82, N 4.90%. Found: C 54.79, H 2.80, N 4.93%. IR (KBr, cm−1): 1646 s, 1605 m, 1579 m, 1511 w, 1455 w, 1398 s, 1304 m, 1273 s, 1242 w, 1206 m, 1164 m, 1112 w, 1050 w, 1014 w, 962 w, 895 w, 848 w, 785 w, 713 w, 693 w, 604 w, 547 w.

2.3. X-ray Crystallography

For 1, single-crystal X-ray data were collected using a Bruker APEX-II CCD diffractometer with a graphite-monochromated Mo Kα radiation (λ = 0.71073 Å, Bruker Corporation, Billerica, MA, USA). Semiempirical absorption corrections were performed with SADABS software (Bruker AXS, version 5.624, Madison, Wisconsin, USA). Structure of 1 was determined by direct methods, followed by a refinement (full-matrix least-squares on F2) with SHELXS-97/SHELXL-97 [31,32]. Apart from H atoms, all other types of atoms underwent an anisotropic refinement (full-matrix least-squares on F2). Hydrogen atoms (with an exception of COOH group) were positioned in calculated sites having fixed isotropic thermal parameters. These were considered in structure factor calculations at the final stage of full-matrix least-squares refinement. For COOH group, H atom of was found using difference maps, followed by its restriction to be at its parent oxygen atom. Crystal parameters for 1 are summarized in Table 1. Topological description of metal-organic net in 1 was performed following the concept of underlying (simplified) network [33]. This net was built [33] by reducing µ3-Hcpna blocks to respective centroids, while their connections with iron(II) nodes were maintained [34]. CCDC-1906984 for 1 contains the supplementary crystallographic data.

2.4. Catalytic Oxidation of Cyclic Alkanes

The reactions were carried out in glass reactors (50 mL volume, thermostated at 50 °C, and equipped with a reflux condenser) under aerobic conditions and constant magnetic stirring; acetonitrile was used as solvent and added up to 2.5 mL of the total volume of the reaction mixture. Typical reaction procedure: catalyst precursor 1 (0.005 mmol) was suspended in CH3CN and then an acid promoter (0.015–0.1 mmol) and a gas chromatography (GC) internal standard (CH3NO2, 250 μL) were introduced. The oxidation reaction began on addition of cycloalkane (1 mmol) and H2O2 (5 mmol, 50% in water) and its progress was followed by taking small aliquots of the reaction mixture at different periods of time. Before GC analysis (internal standard method), each aliquot was treated with a small amount of solid triphenylphosphine that is necessary to reduce cycloalkyl hydroperoxides (primary products in the oxidation of cycloalkanes) and remaining hydrogen peroxide; the product yields are based on the GC data after the treatment with PPh3. Blank tests were performed and showed that the oxidation of cycloalkanes does not occur in the absence of Fe catalyst.

2.5. Catalytic Oxidation of Propane

The following reagents were combined in a stainless-steel reactor (20.0 mL total volume): catalyst precursor 1 (0.005 mmol), solvent CH3CN (up to 2.5 mL of total volume of the reaction mixture), acid promoter (PCA, 0.015 mmol or TFA, 0.1 mmol), CH3NO2 (GC internal standard, 250 μL), and H2O2 (5 mmol, 50% in water). The reactor was then sealed, pressurized with C3H8 (1 atm) and heated at 50 °C for 4 h under magnetic stirring. After cooling the reactor, it was degassed and the samples of the reaction mixtures were analyzed by GC (internal standard method). Before the analysis, the samples were treated with PPh3. Blank tests disclosed that the oxidation of propane does not occur in the absence of Fe catalyst.

2.6. Catalytic Carboxylation of Alkanes

Typical procedure: catalyst precursor 1 (0.01 mmol), CH3CN (4.0 mL), H2O (2.0 mL), cycloalkane substrate (1.0 mmol), and K2S2O8 (1.50 mmol) were combined in a stainless-steel reactor (20.0 mL total volume). It was sealed and flushed three times with carbon monoxide to remove air and then pressurized with CO (20 atm). For carboxylation of C3H8, the reactor was first flushed and pressurized with the substrate (1 atm) and then CO was added. The reactor was heated at 50 °C for 4 h under magnetic stirring. After cooling the reactor, it was degassed and the reaction mixture was transferred to a glass flask. Diethyl ether (9.0 mL) and GC internal standard (cycloheptanone, 45 µL) were added (cyclohexanone was used as a GC standard in cycloheptane carboxylation). The obtained mixture was stirred for 10 min and then the aliquots were withdrawn from organic layer and analyzed by GC (internal standard method).

3. Results and Discussion

3.1. Synthesis and Characterization

The hydrothermal synthesis (at 160 °C for 3 days) using a mixture in water of iron(II) sulfate (FeSO4·7H2O), H2cpna (5-(4’-carboxyphenoxy)nicotinic acid) as a trifunctional organic block, and sodium hydroxide as a base resulted in a 3D coordination polymer formulated as [Fe(µ3-Hcpna)2]n (1). The molar ratio between FeSO4·7H2O, H2cpna, NaOH, and H2O was 1:2:2:3700, and the pH value of the initial reaction mixture was in the 4–5 range. The compound 1 was isolated in a good yield as a crystalline solid (including single crystals of X-ray quality) and characterized by conventional techniques, which also include a single-crystal X-ray diffraction. Product 1 is insoluble in any solvent and maintains its stability in e.g. CH3CN/H2O medium at 60 °C, as confirmed by powder X-ray diffraction (Figure S4, Supplementary Materials). In the IR spectrum of compound 1 (Figure S3, Supplementary Materials), the most characteristic, broad and intense bands of the carboxylate groups appear at 1605 and 1398 cm1 and correspond to νas(COO) and νs(COO) vibrations, respectively. Apart from these broad bands, there are also several minor shoulders or neighboring ν(COO/COOH) bands centered at 1455, 1398, and 1304 cm1. For H2cpna, the strongest vibrations of carboxylic acid groups are observed at 1676 and 1305 cm1 (Figure S2, Supplementary Materials). Different position of the main band maxima in 1 and H2cpna indicates the coordination of carboxylic acid ligand to iron(II) centers in 1, as further supported by elemental and thermogravimetric analyses along with the PXRD and single-crystal X-ray diffraction data.

3.2. Description of Structural and Topological Features

Product 1 possesses a 3D coordination polymer structure (Figure 1). Its asymmetric unit bears one Fe(II) atom (with a half occupancy) and one µ3-Hcpna block. The six-coordinate Fe1 atom displays the distorted {FeN2O4} octahedral environment (Figure 1a and Figure S1). It is completed by two nitrogen and four carboxylate oxygen donors coming from six µ3-Hcpna moieties. The Fe–O [2.075(3)–2.183(3) Å] and Fe–N [2.152(3) Å] distances are typical for a present type of Fe(II) coordination compounds [35,36]. The Hcpna ligand acts as a µ3-N,O2-spacer (Scheme 1) with the COOH and COO groups adopting monodentate modes. The Hcpna block is considerably bent showing a dihedral angle of 87.94° between benzene and pyridyl functionalities, while a C–Oether–C angle is 117.97°. The µ3-N,O2-Hcpna blocks multiply link neighboring Fe(II) nodes to form a 3D CP structure (Figure 1b,c). Compound 1 shows no porosity, as confirmed by calculating an effective free volume of the crystal volume by PLATON; its unit cell contains no residual solvent accessible voids.
To better understand such intricate 3D coordination polymer structures, topological classification was carried out. Topologically, the 3D underlying net in 1 (Figure 1d) is constructed from 6-connected Fe1 and 3-connected µ3-Hcpna nodes, thus disclosing a 3,6-connected binodal net of the rtl (rutile) topological type. This net can be described with a (4·62)2(42·610·83) point symbol, in which the (4·62) and (42·610·83) notations are those of µ3-Hcpna and Fe1 nodes, respectively.

3.3. TGA and PXRD

The stability and thermal behavior of CP 1 were investigated by running TGA (thermogravimetric analysis, Figure 2a) under nitrogen atmosphere in the 20–800 °C range of temperatures. CP 1 does not contain solvent of crystallization or H2O ligands and remains stable until 248 °C; further increase of temperature leads to the decomposition of the sample.
Besides, crystalline sample of CP 1 was studied by PXRD (powder X-ray diffraction analysis) and an experimental PXRD pattern of the bulk product is shown in Figure 2b. Comparison of the experimental pattern with that calculated from single-crystal X-ray diffraction confirms the presence of a single pure phase in the sample of 1. Moreover, elemental analysis corroborates an analytical purity of this coordination polymer.

3.4. Catalytic Functionalization of Alkanes

Following our continuous interest in developing different metal-complex-catalysts and protocols for oxidative functionalization of saturated hydrocarbons under mild conditions [37,38,39,40], we explored a catalytic potential of 1 in oxidation of cyclic C5−C8 alkanes and propane to give the respective alcohol and ketone products. Cyclohexane was investigated as a model substrate and the reactions were performed at 50 °C with hydrogen peroxide as oxidant (50% in water) in CH3CN/H2O medium and using a slight amount of acid as a promoter.
The use of CH3CN as a solvent is important for solubilization of an alkane and catalyst since the oxidation reactions practically do not undergo in only H2O as a solvent. Selection of acetonitrile can be justified by the following factors: (i) miscibility of CH3CN with H2O and aqueous H2O2, (ii) sufficient solubility of alkanes in CH3CN, (iii) good stability of acetonitrile in the reaction medium, and (iv) good prior results for oxidative functionalization of alkanes obtained when using CH3CN as a solvent of choice [37,38,39,40,41,42,43,44].
It should be mentioned that the CP 1 is not intact during catalytic experiments and acts as a precursor of homogeneous catalytically active species. These form upon dissolution of 1 in the presence of oxidant and acid promoter. Cyclohexane oxidation to cyclohexanone and cyclohexanol does not occur without catalyst 1 (Figure 3), either in the absence or in the presence of acid promoter. In the presence of 1, the oxidation of C6H12 undergoes very slowly unless an acid promoter is added (Figure 3). However, an addition of TFA (trifluoroacetic acid; commonly used for promoting the activity of various Cu and Fe based catalytic systems [37,38,39]) causes a drastic acceleration of reaction rate and higher quantity of products generated (8% yield after 90 min of the reaction). The presence of PCA (2-pyrazinecarboxylic acid) in a very low amount (molar ratio PCA:1 = 3:1) results in higher catalytic activity (15% total product yield after 90 min, Figure 3). It should be mentioned that PCA is a recognized and powerful promoter in different metal-catalyzed oxidations of hydrocarbons [41].
Other cycloalkanes also undergo oxidation with H2O2 in the presence of catalyst 1 and PCA promoter (Figure 4). In particular, the oxidation of cycloheptane (to cycloheptanol and cycloheptanone) and cyclooctane (to cyclooctanol and cyclooctanone) proceeds more efficiently (total product yield up to 22%) than that of cyclohexane and cyclopentane.
Upon addition of PCA, compound 1 also efficiently catalyses the oxidation of a light gaseous alkane such as propane (Table 2) to give a mixture of i-propanol, acetone, n-propanol, and propanal with a total product yield of 22% (based on C3H8). However, if TFA is used as a promoter, the reaction is less efficient (total product yield of 7.3%). It should be highlighted that the product yield of up to 22% (based on propane) obtained herein is remarkable, especially if taking into account a very high inertness of this gaseous hydrocarbon along with rather mild conditions applied for running the oxidation reaction [37,38,39,40,41].
In addition, compound 1 was evaluated as catalyst precursor in the carboxylation of cyclic Cn (n = 5−8) alkanes and propane in the presence of CO (carbonyl source), H2O (hydroxyl source) and K2S2O8 (oxidant and radical initiator) [42,43,44]. These carboxylations undergo in H2O/CH3CN at 60 °C and result in generation of the respective Cn+1 carboxylic acids (main products, Table 3). The oxidation Cn products (e.g., cyclic ketones and alcohols) are also formed due to competing oxidation reactions, especially when using cyloheptane and cyclooctane as substrates.
Among the cycloalkane substrates tested, more elevated yields of carboxylic acid products are obtained in cyclohexane carboxylation (21% of C6H11COOH) and cyclopentane carboxylation (14% C5H9COOH). Interestingly, the transformations of cycloheptane and cyclooctane result in lower yields of carboxylic acids (5–9%) but higher yields of ketones and alcohols, while the total yields of products in all systems are in the 18–23% range. The carboxylation of propane generates i-butyric acid (13.6% yield; main product) and n-butyric acid (3.8% yield; minor product) owing to two types of C atoms in C3H8 molecule.
Taking into account the mechanistic considerations from the related oxidation and carboxylation processes catalysed by various coordination compounds [37,38,39,40,41,42,43,44], we can propose the free radical pathways for the reactions studied in the present work (Figure 5). In both alkane oxidation and carboxylation reactions, compound 1 acts as a precursor of homogeneous Fe-cpna species when reacting with oxidant and/or acid promoter; these species are formed upon partial protonation of some carboxylate groups and ligand decoordination. In fact, the catalyst precursor facilitates the formation of active oxidizing species (i.e., hydroxyl radicals from H2O2 in alkane oxidation or sulfate radical anions from K2S2O8 in alkane carboxylation) and eventually participates in other mechanistic steps (Figure 5).
Hence, in alkane oxidation, the substrate molecules (RH) are activated by HO to produce alkyl radicals (R). These are coupled with dioxygen (present in air or generated from hydrogen peroxide) to furnish alkyl peroxo radicals (ROO) that are further converted, via ROO, to alkyl hydroperoxides (ROOH) as intermediate products. In fact, the formation of ROOH was corroborated by performing the duplicate GC tests for selected reaction mixtures (Shul’pin’s method), namely by analyzing them after and before treating with PPh3 [45,46]. ROOH are not very stable under the reaction conditions and undergo decomposition (can also be iron-catalyzed) to generate ROH and R’=O as final alkane oxidation products [37,42].
In carboxylation of saturated hydrocarbons, RH react with SO4 as active oxidizing species to form R. Alkyl radicals are carboxylated by CO to give acyl radicals (RCO) and then acyl cations (RCO+). These are prone to undergo hydrolysis by H2O to generate carboxylic acids (RCOOH) as principal products (Figure 5). Competitive oxidation of R to form ROH and R’=O as by-products is also observed in the present type of carboxylation processes [42,43,44].

4. Conclusions

This work described a facile hydrothermal generation and full characterization of the new Fe(II) 3D coordination polymer derived from 5-(4’-carboxyphenoxy)nicotinic acid (H2cpna) as a multifunctional organic building block. The obtained product 1 reveals the first structurally characterized Fe(II) compound assembled from H2cpna [47].
The structure and topology of this iron 3D CP were established and discussed, revealing a 3,6-connected underlying network of the rtl topological type. Compound 1 also functions as active catalyst precursor for the homogeneous oxidation and carboxylation of propane and cycloalkanes under mild conditions, leading to up to 23% yields of products (based on alkane substrate). These values of yields are excellent considering a particular inertness of saturated hydrocarbons (especially propane) and mild conditions of the reactions studied [48,49,50]. For example, a related industrial process of cyclohexane oxidation to cyclohexanone and cyclohexanol (DuPont process with a homogeneous Co naphthenate catalyst) shows only 5–10% substrate conversions [51,52] and proceeds under harsher reaction conditions (10–15 atm pressure, 150 °C, air oxidant).
Although the present homogeneous catalytic systems derived from 1 still cannot be recycled, we believe future research aiming at trapping 1 or its soluble derivatives on solid supports might result in the development of reusable heterogeneous catalysts for alkane functionalization. Furthermore, further studies on exploring H2cpna and related multifunctional building blocks for the hydrothermal design of new metal-organic architectures will be continued in our laboratories.

Supplementary Materials

The following are available online at https://www.mdpi.com/2073-4352/9/7/369/s1, Figure S1: ellipsoid plot of 1, Figures S2 and S3: FT-IR spectra, Figure S4: additional PXRD patterns and discussion, Figures S5 and S6: UV-vis spectra, Tables S1 and S2: selected bonding and H-bonding parameters for 1.

Author Contributions

Conceptualization, J.G. and A.M.K.; data curation, N.Z., Y.L., and M.V.K.; funding acquisition, Y.L., J.G., and A.M.K.; investigation, N.Z., Y.L., J.G., and M.V.K.; methodology, M.V.K.; visualization, J.G. and A.M.K.; writing—original draft, Y.L., J.G, M.V.K., and A.M.K.; writing—review and editing, A.M.K.

Acknowledgments

This work was supported by Guangdong Province Higher Vocational Colleges & Schools Pearl River Scholar Funded Scheme (2015, 2018), Innovation Team Project on University of Guangdong Province of P. R. China (2017GKCXTD001), Science and Technology Planning Project of Guangzhou (201904010381), the Foundation for Science and Technology (FCT) and Portugal 2020 (CEECIND/03708/2017, LISBOA-01-0145-FEDER-029697). The publication was also prepared with the support of the RUDN University Program 5-100.

Conflicts of Interest

The authors declare no conflict of interest.

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Scheme 1. The coordination mode of Hcpna block in 1.
Scheme 1. The coordination mode of Hcpna block in 1.
Crystals 09 00369 sch001
Figure 1. Different structural representations of 1. (a) Connectivity and coordination sphere of the Fe1 center; hydrogen atoms are not shown with an exception of H in COOH. Symmetry operators: i = –x + 1, –y + 1, –z; ii = x, y, z – 1; iii = –x + 1, –y +1, –z + 1; iv = –x + 1/2, y + 1/2, –z + 1/2; v = x + 1/2, –y + 1/2, z – 1/2. (b,c) Three-dimensional metal-organic network along the a and c axis, respectively. (d) Topological view of a 3D coordination polymer displaying a 3,6-connected binodal underlying net of the rtl (rutile) topological type. View along the c axis; colors: centroids of 3-connected µ3-Hcpna nodes (gray), 6-connected Fe1 nodes (orange balls).
Figure 1. Different structural representations of 1. (a) Connectivity and coordination sphere of the Fe1 center; hydrogen atoms are not shown with an exception of H in COOH. Symmetry operators: i = –x + 1, –y + 1, –z; ii = x, y, z – 1; iii = –x + 1, –y +1, –z + 1; iv = –x + 1/2, y + 1/2, –z + 1/2; v = x + 1/2, –y + 1/2, z – 1/2. (b,c) Three-dimensional metal-organic network along the a and c axis, respectively. (d) Topological view of a 3D coordination polymer displaying a 3,6-connected binodal underlying net of the rtl (rutile) topological type. View along the c axis; colors: centroids of 3-connected µ3-Hcpna nodes (gray), 6-connected Fe1 nodes (orange balls).
Crystals 09 00369 g001aCrystals 09 00369 g001b
Figure 2. Thermogravimetric analysis (TGA) curve (a) and powder X-ray diffraction (PXRD) patterns (b) of 1.
Figure 2. Thermogravimetric analysis (TGA) curve (a) and powder X-ray diffraction (PXRD) patterns (b) of 1.
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Figure 3. Influence of acid promoter on C6H12 oxidation by hydrogen peroxide to a mixture of cyclohexanone and cyclohexanol (total product yield vs. reaction time) catalysed by 1. Conditions: 1 (0.005 mmol), C6H12 (1.0 mmol), H2O2 (5.0 mmol, 50% in H2O), 2-pyrazinecarboxylic acid (PCA, 0.015 mmol) or trifluoroacetic acid (TFA, 0.05–0.10 mmol), CH3CN (solvent added up to 2.5 mL of total volume of the reaction), 50 °C. Blank tests without catalyst precursor 1 (curve: no catalyst; in the presence of TFA, 0.1 mmol) or acid promoter (curve: no acid, in the presence of 1; 0.005 mmol) are given for comparison.
Figure 3. Influence of acid promoter on C6H12 oxidation by hydrogen peroxide to a mixture of cyclohexanone and cyclohexanol (total product yield vs. reaction time) catalysed by 1. Conditions: 1 (0.005 mmol), C6H12 (1.0 mmol), H2O2 (5.0 mmol, 50% in H2O), 2-pyrazinecarboxylic acid (PCA, 0.015 mmol) or trifluoroacetic acid (TFA, 0.05–0.10 mmol), CH3CN (solvent added up to 2.5 mL of total volume of the reaction), 50 °C. Blank tests without catalyst precursor 1 (curve: no catalyst; in the presence of TFA, 0.1 mmol) or acid promoter (curve: no acid, in the presence of 1; 0.005 mmol) are given for comparison.
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Figure 4. Substrate scope in oxidation of cyclic C5–C8 alkanes with H2O2 to corresponding ketones and alcohols (total product yield vs. reaction time) catalyzed by 1. Conditions: 1 (0.005 mmol), C5H10–C8H16 (1.0 mmol), H2O2 (50% in H2O, 5.0 mmol), PCA (0.015 mmol), CH3CN (solvent added up to 2.5 mL of total volume of the reaction), 50 °C.
Figure 4. Substrate scope in oxidation of cyclic C5–C8 alkanes with H2O2 to corresponding ketones and alcohols (total product yield vs. reaction time) catalyzed by 1. Conditions: 1 (0.005 mmol), C5H10–C8H16 (1.0 mmol), H2O2 (50% in H2O, 5.0 mmol), PCA (0.015 mmol), CH3CN (solvent added up to 2.5 mL of total volume of the reaction), 50 °C.
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Figure 5. Simplified mechanism for oxidation and carboxylation of alkanes.
Figure 5. Simplified mechanism for oxidation and carboxylation of alkanes.
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Table 1. Crystal structure parameters for coordination polymer 1.
Table 1. Crystal structure parameters for coordination polymer 1.
Compound1
Chemical formulaC26H16FeN2O10
Molecular weight572.26
Crystal systemMonoclinic
Space groupP21/n
a9.4673(7)
b9.2170(6)
c14.0358(9)
α/ (°)90
β/(°)108.079(8)
γ/ (°)90
V31164.30(15)
Z2
F(000)584
Crystal size/mm0.27 × 0.25 × 0.23
θ Range for data collection3.770–25.049
Limiting indices−9 ≤ h ≤ 11, −10 ≤ k ≤ 10, −16 ≤ l ≤ 10
Reflections collected/unique (Rint)4273/2053 (0.0558)
Dc/ (Mg·cm−3)1.632
μ/mm−10.715
Data/restraints/parameters2053/0/179
Goodness-of-fit on F21.029
Final R indices [(I ≥ 2σ(I))] R1, wR20.0502, 0.0890
R indices (all data) R1, wR20.0895, 0.1084
Largest diff. peak and hole/(e·Å−3)0.329 and −0.376
Table 2. Mild oxidation of propane with H2O2 catalyzed by 1 a.
Table 2. Mild oxidation of propane with H2O2 catalyzed by 1 a.
CatalystProduct Yield (%)b
i-PropanolAcetonen-PropanolPropanalTotal
1/PCA6.111.33.01.622.0
1/TFA2.12.31.51.47.3
a Conditions: 1 (0.005 mmol), PCA (0.015 mmol) or TFA (0.1 mmol), C3H8 (1 atm, 0.7 mmol), H2O2 (50% in H2O, 5.0 mmol), CH3CN (solvent added up to 2.5 mL of total volume of the reaction), 50 °C, 4 h in a stainless-steel reactor (20 mL capacity). b.Yields based on C3H8 substrate: (moles of products/moles of C3H8) × 100%.
Table 3. Single-pot carboxylation of cycloalkanes catalyzed by 1 a.
Table 3. Single-pot carboxylation of cycloalkanes catalyzed by 1 a.
SubstrateYield (%)b
Cycloalkane-Carboxylic AcidCyclic KetoneCyclic alcoholTotal
Cyclopentane13.93.41.719.0
Cyclohexane20.91.10.622.6
Cycloheptane9.26.52.518.2
Cyclooctane4.87.85.918.5
a Conditions: 1 (0.01 mmol), C5H10−C8H16 (1 mmol), CO (20 atm), K2S2O8 (1.5 mmol), CH3CN (4 mL)/H2O (2 mL), 60 °C, 4 h in a stainless-steel reactor (20 mL capacity). b Yields based on cycloalkane substrate: (moles of products/moles of cycloalkane) × 100%.

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Zhao, N.; Li, Y.; Gu, J.; Kirillova, M.V.; Kirillov, A.M. Synthesis, Structural Features, and Catalytic Activity of an Iron(II) 3D Coordination Polymer Driven by an Ether-Bridged Pyridine-Dicarboxylate. Crystals 2019, 9, 369. https://doi.org/10.3390/cryst9070369

AMA Style

Zhao N, Li Y, Gu J, Kirillova MV, Kirillov AM. Synthesis, Structural Features, and Catalytic Activity of an Iron(II) 3D Coordination Polymer Driven by an Ether-Bridged Pyridine-Dicarboxylate. Crystals. 2019; 9(7):369. https://doi.org/10.3390/cryst9070369

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Zhao, Na, Yu Li, Jinzhong Gu, Marina V. Kirillova, and Alexander M. Kirillov. 2019. "Synthesis, Structural Features, and Catalytic Activity of an Iron(II) 3D Coordination Polymer Driven by an Ether-Bridged Pyridine-Dicarboxylate" Crystals 9, no. 7: 369. https://doi.org/10.3390/cryst9070369

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