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A hybrid silicotungstate based on tri-coordination copper complex and Keggin type cluster with reactive oxygen species catalytic ability
Journal of Molecular Structure 1206 (2020) 127714
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Journal of Molecular Structure journal homepage: http://www.elsevier.com/locate/molstruc
A hybrid silicotungstate based on tri-coordination copper complex and Keggin type cluster with reactive oxygen species catalytic ability Xiang Ma a, b, *, Qiang Zhao a, Bin Wang a, Danni Li a, Yujie Zhou a, Jiai Hua a, c, **, Pengtao Ma b, *** a b c
Chemistry and Chemical Engineering Department, Taiyuan Institute of Technology, Taiyuan, 030008, PR China Henan Key Laboratory of Polyoxometalate Chemistry, College of Chemistry and Chemical Engineering, Henan University, Kaifeng, Henan, 475004, PR China State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing, 210023, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 November 2019 Received in revised form 2 January 2020 Accepted 8 January 2020 Available online 14 January 2020
A hybrid polyoxometalate based on binuclear copper complex and a-Keggin-type silicotungstate [Cu2(4,40 -bpy) (Phen)2][SiW12O40] (abbreviated as CSW) (4,40 -bpy ¼ 4,40 -bipyridine; Phen ¼ 1,10phenanthroline) has been synthesized and structurally characterized by elemental analyses, X-ray powder diffraction (XRPD), IR spectrum, X-ray photoelectron spectrum (XPS) and single-crystal X-ray diffraction. Structural analysis shows that CSW is an ionic compound, in which the cation is a binuclear copper complex [Cu2(4,40 -bpy) (Phen)2]4þ and the anion is a a-Keggin-type silicotungstate [SiW12O40]4e cluster. Notably, the Cu ions in the [Cu2(4,40 -bpy) (Phen)2] exist in the form of tri-coordination, which the phenomenon is very rare in polyoxometalates. Moreover, CSW can catalyze the production of reactive oxygen species efficiently. © 2020 Elsevier B.V. All rights reserved.
Keywords: Silicotungstate Tri-coordination binuclear copper Reactive oxygen species Crystal structure analysis a-Keggin-type cluster Antiferromagnetism
1. Introduction Reactive oxygen species (ROS) are the most common intermediate and medium in bio-chemical reactions, in which it is present in almost all life processes and pathological process [1,2]. Hence, the researches of ROS catalyst have received more and more interest from laboratory to clinical researches [3e5]. The variable valence transition metal ions, such as Fe2þ/Fe3þ, Mn2þ/Mn4þ, Cuþ/ Cu2þ, etc., are believed in catalyzing the production of ROS, in which those ions are used as active centers to construct reactive proteins in living organisms, for example hemoglobin (Hb), myoglobin (Mb) and superoxide dismutase (SOD) [6e8]. Recently, we have successfully synthesized a series of artificial enzyme complexes that can catalyze the production of ROS [9,10] in which
* Corresponding author. Chemistry and Chemical Engineering Department, Taiyuan Institute of Technology, Taiyuan, 030008, PR China. ** Corresponding author. Chemistry and Chemical Engineering Department, Taiyuan Institute of Technology, Taiyuan, 030008, PR China. *** Corresponding author. E-mail addresses: [email protected] (X. Ma), [email protected] (J. Hua), [email protected] (P. Ma). https://doi.org/10.1016/j.molstruc.2020.127714 0022-2860/© 2020 Elsevier B.V. All rights reserved.
the performance of copper complexes is superior [9]. In view of their good biological activity, as a continuation of the above work, we will continue to focus on the synthesis of copper complexes with ROS catalytic activity. Polyoxometalates (POMs) are a fascinating class of metaloxygen clusters, which have multitudinous structures and fascinating properties in various fields such as catalysis, magnetism, fluorescence, medicine materials and nano-science [11e15]. POMs have several inherent irreplaceable advantages such as nano-size, nucleophilic oxygen-enriched surface and poly-bond-making sites, which endow the cluster with the ability to serve as bulky polydentate ligands to incorporate in multiple transition-metal ions with flexible coordination modes [16e25]. Moreover, recently, researches have demonstrated that the Keggin-type POMs possessed an excellent oxidation catalytic capacity [26e28]. These properties may be beneficial to catalyze the production of ROS with transition-metal ions [9]. Hence, the combination of Cu ions and POMs in one hybrid material may not only maintain the desirable properties of all precursor components, but also express them in a synergistic manner. Herein we report a newly designed POM, [Cu2(4,40 -bpy) (Phen)2][SiW12O40] (abbreviated as CSW) (4,40 -bpy ¼ 4,40 -
2
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bipyridine; Phen ¼ 1,10-phenanthroline), which was derived from pure inorganic structure Keggin-type silicotungstate anion [SiW12O40]4e and a dinuclear copper complex [Cu2(4,40 -bpy) (Phen)2]4þ. As expected, CSW can catalyze ROS generation efficiently. 2. Experimental 2.1. Materials and physical measurements Reagents used in this study were all of analytical grade, purchased from commercial suppliers and used as received unless otherwise stated. 20 ,70 -dichlorofluorescin diacetate (DCFH-DA) was purchased from Sigma-Aldrich. CuCl2$2H2O, NaWO4$2H2O, Na2SiO3$9H2O, Phen, 4,40 -bpy and C2H2O4$2H2O were purchased from J & K. All the solutions were prepared with Milli-Q water and filtered through a 0.22 mm filter (Millipore). X-Ray powder diffraction (XRPD) measurements were obtained using a Philips X’pert-MPD instrument with Cu-Ka radiation (l ¼ 1.54056 Å) at 293 K. IR spectrum was obtained from a sample powder palletized with KBr or dissolved in chloroform on a Nicolet 170 SXFT-IR spectrophotometer over the range 4000e400 cm1. The single crystal data of CSW were collected on a Bruker CCD, Apex-II diffractometer with graphite monochromated Mo Ka (l ¼ 0.71073 Å) radiation at room temperature. Routine Lorentz and polarization corrections were applied and an absorption correction was performed using the SADABS program. The structure was solved by direct methods and refined using full-matrix least squares on F2. All calculations were performed using the SHELXL-97 program package. DCF fluorescence were conducted on a Thermo Scientific Varioskan Flash microplate reader. Elemental analysis was performed on a PQEXCeII ICP-MS. 2.2. Synthesis Two solutions were prepared separately. Solution A: Na2WO4$2H2O (3.30 g, 10.00 mmol) and Na2SiO3$9H2O (1.90 g, 6.70 mmol) were dissolved in water (50 mL) under stirring. Solution B: CuCl2$2H2O (1.71 g, 10.00 mmol), Phen (0.52 g, 2.5 mmol), 4,40 -bpy (0.4 g, 2.0 mmol) and oxalic acid (C2H2O4$2H2O, 0.25 g, 2.0 mmol) were added to water (50 mL) under stirring. The resulting mixture of B is added to solution A. The mixture was stirred for 10 min at room temperature and then the pH value was adjusted to 4.0 by adding 6 mol L1 HCl dropwise. The solution was sealed in a 25 mL Teflon-lined autoclave kept at 150 C for 5 days and then cooled to room temperature. Then the mazarine crystals of CSW were separated with a 36% yield (based on Na2WO4$2H2O). Elemental analysis (%) calcd for [Cu2(4,40 -bpy) (Phen)2][SiW12O40]: C 11.61, N 2.39, Si 0.80, Cu 3.61, W 62.71; found: C 11.67, N 2.42, Si 0.76, Cu 3.66, W 62.61. The results showed that the optimal pH range for CSW generation is 2.0e5.0, and the crystal morphology was best when the initial pH was 4.0. The CSW yield was the highest when the initial pH was 3.0e4.0. When pH was 5.5 and above, no CSW crystals were obtained. 2.3. X-ray data collection and structure refinement Intensity data of CSW was collected on a Bruker Apex-2 diffractometer with a CCD detector using graphite monochromatized Mo Ka radiation (l ¼ 0.71073 Å) at 296 K. Data integration was performed using SAINT [29]. Routine Lorentz and polarization corrections were applied. Multiscan absorption corrections were performed using SADABS [30]. The structure was solved by direct methods and refined using full-matrix least
squares on F2. The remaining atoms were found from successive full-matrix least-squares refinements on F2 and Fourier syntheses. All calculations were performed using the SHELXL-97 program package [31]. No hydrogen atoms associated with the water molecules were located from the difference Fourier map. Positions of the hydrogen atoms attached to the carbon and nitrogen atoms were geometrically placed. All hydrogen atoms were refined isotropically as a riding mode using the default SHELXTL parameters. A summary of crystal data and structure refinements for CSW is listed in Table 1. 3. Results and discussion 3.1. Crystal structure The structure of CSW was characterized by single-crystal X-ray diffraction analysis. The crystallographic data, selected bond lengths are summarized inTable S1. Detailed information has been deposited at the Cambridge Crystallographic Data Centre with a CCDC number of 1959063. As shown in Fig. 1, X-ray structural analysis reveals that the unit of CSW consists of two parts: the binuclear copper complex [Cu2(4,40 -bpy) (Phen)2]4þ and the [SiW12O40]4e cluster. As shown in Fig. 1A and C, the [Cu2(4,40 -bpy) (Phen)2]4þ in CSW with an inversion center located at the center of 4,40 -bpy ligand, which consists two Cu ions, two Phen and one 4,40 bpy ligand. Each Cu ion is coordinated by three N atoms from Phen and 4,40 -bpy ligand with CueN distances ranging from 1.882 to 2.016 Å. The sum of the different angles around the Cu ion is 360 , which is indicating a trigonal-planar geometry around the center metal ion. The roles of 4,40 -bpy and Phen ligand coordinating to Cu are different. The 4,40 -bpy serves as a bridging ligand linked with two Cu ions, while Phen acts as a chelating agent coordinated to Cu. Alternatively, 4,40 -bpy ligand joins two [Cu(Phen)]þ to form a binuclear Cu complex. In coordination chemistry, Cu2þ always has a tetra-, penta-, and hexa-coordination configuration; In contrast, Cuþ often shows a bicoordination configuration, which the tri-coordination copper ion
Table 1 Crystallographic data and structural refinements for CSW. Empirical formula Formula weight Crystal system Space group a/Å b/Å c/Å a/deg b/deg g/deg V/Å3 Z Dc/g cm3 m/mm1 T/K Limiting indices
Measured reflections Independent reflections Rint Data/restrains/parameters GOF on F2 Final R indices [I > 2s(I)] R indices (all data) Completeness
C34H24N6Si1W12O40Cu2 3517.83 Orthorhombic Ibam 13.222 (2) 25.116 (4) 26.768 (4) 90 90 90 8890 (3) 4 3.109 16.494 296 (2) 15 h 15 23 k 29 31 l 29 20265 4002 0.0418 4002/36/326 1.011 R1 ¼ 0.0352, wR2 ¼ 0.1084 R1 ¼ 0.0484 wR2 ¼ 0.1164 99.8%
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center of [SiW12O40]4e cluster and shares four oxygens with four {Mo3O13} triad. Those four {Mo3O13} clusters are connected to each other by sharing edge and Om, forming an a-Keggin type cluster. The bond valence sums (Ss) and the protonation level (SH) of oxygen atoms in CSW were further calculated according to the reported methods [34,35]. Briefly, the oxidation states of the oxygen atoms in CSW were calculated on the following formula, which could be used to estimate the protonation of oxygen atoms:
Vi ¼
Fig. 1. Ball-and-stick view of the dinuclear copper complex [Cu2(4,40 -bpy) (Phen)2]4þ (A) and the Keggin-type silicotungstate [SiW12O40]4e (B) in [Cu2(4,40 -bpy) (Phen)2] [SiW12O40] (CSW); (C) Structural diagram of [Cu2(4,40 -bpy) (Phen)2]4þ; (D) Polyhedral view of the [SiW12O40]4e unit.
is usually the intermediate state in the REDOX process of Cu2þ-Cuþ [32]. To our knowledge, in the solid state, the stable tricoordination copper ion is very rare. Since the coordination configuration is between that of Cu2þ and Cuþ, Cu ions in [Cu2(4,40 bpy) (Phen)2]4þ complex may be easy to cycle between þ2 and þ 1; and this is also a necessary structural feature for catalytic production of ROS, which the phenomenon is similar with the Fe2þ-Fe3þ REDOX actions in heme [6]. Further investigation using X-ray photoelectron spectrum (XPS) has been carried out to detect the valence of Cu in CSW. As shown in Fig. 2, there are two broad peaks with a shoulder peak located at 954.0 and 933.8 eV, which were assigned to Cu 2p3/2 and Cu 2p1/2, respectively [33]. The fitted curves in Fig. 2 (dashed lines) suggest that Cu2þ (2p3/2, 934.0 eV; 2p1/2, 953.9 eV) and Cuþ (2p3/2, 932.9 eV; 2p1/2, 952.7 eV) are involved in the peaks of Cu 2p [33]. These results may indicate that Cu ions in the þ2 and þ 1 valence states coexist in the Cu complex of CSW, in which the content of Cuþ is very small according to the peak strength. As shown in Fig. 1B and D, tetrahedral {SiO4} is located in the
’ X X r rij sij ¼ exp 0 B j j
where r0’ represents the theoretical value of bond distance between two atoms, and rij represents the observed values of bond distance that are listed in Table S1; B was set to 0.37 [36]. The theoretical value of WeO, SieO and CueN from literatures, which the r0 (W6þeO) is 1.881 Å, r0 (Si4þeO) is 1.622 Å, r0 (Cu2þeN) is 1.504 Å [36]. As a results, the average valence state sums (Ss) of Cu ions in CSW is 1.864, which is basically consistent with the above XPS experimental results. Based on the observed value of bond distance of CSW, the Ss and SH of CSW can be calculated and the results are summarized in Table S2 and Fig. 3. Since the POMs fragment has high negative charges and rich basic surface oxygen atoms, they can easily been protonated [37]. The 40 oxygen atoms in CSW can be classified into terminal Ot, bridging m2-O, m3-O and m4-O. The O atoms with SH of 0.4e0.5 could act as H-donors owing to the delocalized protons on them, whereas the O atoms with SH of 0e0.1 possess dense electron cloud (see Table S2). In generally, the multiply protons usually assigned to be delocalized on the whole polyoxoanion, which the phenomenon is common in POM chemistry and has been reported in many leteratures, for example, [Ni(enMe)2]3 [H6Ni20P4W34(OH)4O136 (enMe)8(H2O)6]$12H2O [34], and [H3W12O40]5e [38].
̍ ̍
Fig. 2. X-ray photoelectron spectrum (XPS) and the fitted curves of Cu in [Cu2(4,40 bpy) (Phen)2][SiW12O40] (CSW).
̍
Fig. 3. Protonation of oxygen atoms in the Keggin unit of [Cu2(4,40 -bpy) (Phen)2] [SiW12O40] (CSW). The extent of protonation for each oxygen atom is indicated by different colors.
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These results may indicate that several counter positive charges in CSW are delocalized in the [SiW12O40]4e skeleton, which may help to absorb protons, and hence balance valence states when copper changes valence [39]. Therefore, the coexistence of Cu complexes and POM may enable them to catalyze the production of ROS in a synergistic manner. 3.2. XRPD and IR spectrum The phase purity of CSW was confirmed by a comparison of the experimental X-ray powder diffraction (XRPD) pattern with the simulated pattern from single-crystal X-ray diffraction (Fig. 4). Owing to the variation in the preferred orientation of the powder sample in the experimental XRPD, the intensities of the experimental and simulated XRPD patterns are different. As shown in Fig. 5 and Fig. S1, the IR spectrum of CSW is similar with that of the Keggin-type silicotungstate reported, which is indicative of retention of the [SiW12O40]4e cluster [40]. In the lowwavenumber region, four characteristic bands assigned to n(WeOt), n(WeOm2), n(WeOm3) and n(SieO) appear at 948, 828, 745 and 898 cm1, respectively. In comparison with that of [SiW12O40]4e cluster, the n(WeOt) vibrational bands for CSW split, which may indicate the structural distortion and the consequent lowering of the symmetry [40]. The IR spectra of 4,40 -bpy and Phen were also tested and shown in Figs. S2 and S3. Compared the IR spectra of Fig. S2 and S3 to S1, the peaks observed at 1616e1116 and 3300e3500 cm1 in Fig. S1 may attribute to the peaks of 4,40 -bpy and Phen and is consistent with literature report [41].
Fig. 5. IR spectra for [Cu2(4,40 -bpy) (Phen)2][SiW12O40] (CSW), 4,40 -bipyridine (4,40 bpy), 1,10-phenanthroline (Phen).
3.3. Magnetic property The solid state direct-current magnetic susceptibilities of CSW have been measured on its polycrystalline samples from 2 to 300 K in the 1 kOe field, since CSW incorporates isolated bi-Cu linked by non-magnetic ligand. The magnetic data for CSW are plotted in Fig. 6 in the form of cM and cMT versus T. The temperature dependence of cM shows a slight increase from 0.0022 to 0.0052 emu mol1 in the range of 300e120 K, and then exponentially reaches the maximum value of 0.0442 emu mol1 at 3 K. At room temperature the cMT values is 0.66, which are a little lower than expected for the spin-only value for two isolated Cu2þ with S ¼ 1/2 and g ¼ 2.00. The cMT value of CSW decreased with the
Fig. 6. Temperature dependence of the molar magnetic susceptibility cM and the cMT for [Cu2(4,40 -bpy) (Phen)2][SiW12O40] (CSW) between 2 and 300 K.
temperature going down gradually, which indicated the presence of antiferromagnetic interactions in binuclear entities [42]. As shown in inset of Fig. 6, in the [Cu2(4,40 -bpy) (Phen)2]4þ, the distance between two Cu ions is 10.80 Å, in which may be difficult to generate magnetic exchange phenomenon. The dependence of the reciprocal susceptibility data is well fitted by CurieeWeiss expression [cM ¼ C/(T e q) with C ¼ 0.695 emu K mol1, q ¼ 9.852 K], which consolidates the presence of overall antiferromagnetic coupling within the copper centers(see Fig. S4).
3.4. Catalytic property
Fig. 4. Comparison of the simulated and experimental XRPD patterns of [Cu2(4,40 -bpy) (Phen)2][SiW12O40] (CSW).
ROS is considered to be a major species accounting for the every aspects of the process of life, especially in immunity, inflammation, enzyme activity, antibacterial, anti-cancer and neurodegenerative diseases [1,32,43,44]. Recently, it reported that Cu or Zn ions substituted phosphomolybdate possess the ability of catalytic oxidation [45]. Hence, if the likely synergize with silicotungstate to
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[1,49,50]. Since Hþ can be produced in Fenton-like system reaction, adsorption of Hþ to POM may enhance the catalytic capacity of Cu complex [39].
Cu2þ þ H2 O2 /Cuþ þ $O2 H þ Hþ
(1)
Cuþ þ H2 O2 /Cu2þ þ $OH þ OH
(2)
4. Conclusion
Fig. 7. Fluorescence intensity of DCF (lex ¼ 485 nm, lem ¼ 650 nm) by [Cu2(4,40 -bpy) (Phen)2][SiW12O40] (CSW), Na4 [SiW12O40] (C1), Cu2þ, Cu2þ þ GSH, GSH, purified water and control group from 0 to 2600 min.
further enhance catalytic activity. The effect of CSW on the mediated ROS generation was thus investigated by a dichlorofluorecin (DCF) assay. DCF is a fluorescent marker derived from the reaction of non-fluorescent 20 ,70 -dichlorofluorecin (DCFH) with ROS in the presence of horseradish peroxidase (HRP), which can indicate the total output of ROS from the system [46]. As shown in Fig. 7, the fluorescence intensity of the group of CSW are obviously higher than that of control, indicating that the production of ROS with CSW are more than that without them. In addition, Na4 [SiW12O40] (abbreviated as C1) also has catalytic effect to some extent. However, the fluorescence intensity of CSW is ~6 times higher than that of C1, which is verified that the Cu in collaboration with [SiW12O40]4e of CSW has stronger catalytic capacity than the single cluster [SiW12O40]4e of C1. Interestingly, as shown in Fig. 7, the experimental group with Cu2þ produced significantly less ROS. This phenomenon may be attributed to the interaction between Cu2þ and HRP enzyme, which deactivates it. In fact, in microenvironment, no matter cells or bacteria can secrete a considerable number of proteins, polypeptides and chelators, which may coordinate with Cu2þ and make them lose their catalytic position [47]. We also used GSH as a simulated enzyme to verify the above phenomenon. As shown in Fig. 7, the catalytic ROS capacity of Cu2þ þ GSH is only higher than that of GSH, which is also a significant decline compared with Cu etc. Therefore, the combination of Keggin-type fragment and Cu ions not only contributes to the synergistic production of ROS, but also protects the catalytic centers from the interference of external proteins. It is known that, in organisms, there are many metalloprotein that can catalyze the production of ROS, for example, the FeS cluster in mitochondria, which can produce ROS by catalytic substrates of Fe2þ/Fe3þ [48]. In pathology, Cuþ/Cu2þ can also act as a catalytic center to produce ROS, for example, ROS produced by Ab þ Cu2þ/ Cuþ cause irreversible damage to neuron cells, and the correlation between b-sheet formation and ROS generation has been well described by Faller P. and Kepp, K. P.. [1,49]. In brief, Cu2þ induces Ab to form misfolding structures, amino acid residues coordinate with Cu2þ to form the Cu-Ab coordination complexes. The Cu2þ centers are easy to be oxidized or reduced in dynamic equilibrium of structural changes, and thus catalyze the generation of ROS. As a dinuclear-copper coordination compound, CSW may catalyze Fenton-like system reactions, as shown in equations (1) and (2)
In this paper, a hybrid polyoxometalate (POM) based on binuclear copper complex and a-Keggin-type silicotungstate [Cu2(4,40 bpy) (Phen)2][SiW12O40] (abbreviated as compound CSW) (4,40 bpy ¼ 4,40 -bipyridine; Phen ¼ 1,10-phenanthroline) has been designed and synthesized successfully. The novelty of the structure is that the Cu ions in the [Cu2(4,40 -bpy) (Phen)2] exist in the form of tri-coordination in solid state, in which the phenomenon is rare in POMs. And the synthetic method may serve as a good example for subsequent synthesis and formation of such POMs clusters [51]. As expected, CSW can catalyze the production of reactive oxygen species (ROS) efficiently, which may be due to the synergy effect of Cu ions and Keggin-structure. The novelty structure and interesting properties may make CSW a broad application prospects in the biochemistry and inorganic chemistry researches of ROS catalyst. Acknowledgments This work was supported by the Natural Science Foundation of China (Grants 21573056), Shanxi Province Science Foundation for Youths (Grants 201901D211453), the Research Foundation for Advanced Talents of Shanxi Province (1800008001), the Research Foundation of Taiyuan Institute of Technology (03000352), and the Research Foundation of the Chinese State Key Laboratory of Coordination Chemistry (SKLCC1912). Appendix B. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.molstruc.2020.127714. Appendix A. Supplementary material Crystallographic data for the structural analysis have been deposited with the Cambridge Crystallographic Data Center, CCDC reference number: 1959063 for CSW. This data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associated with this article can be found in the online version, at Author contributions X. Ma and P.T. Ma conceived the idea of the research. X. Ma and J.A. Hua designed the molecule, and J.A. Hua and Q. Zhao synthesized the molecule. P.T. Ma performed the single-crystal X-ray diffraction and magnetic measurement. X. Ma, B. Wang, D.N. Li and Y.J. Zhou designed and conducted the experiments. J.A. Hua and P.T. Ma analyzed the data. All authors wrote and reviewed the manuscript. References [1] K.P. Kepp, Chem. Rev. 112 (2012) 5193e5239.
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Journal of Molecular Structure journal homepage: http://www.elsevier.com/locate/molstruc
A hybrid silicotungstate based on tri-coordination copper complex and Keggin type cluster with reactive oxygen species catalytic ability Xiang Ma a, b, *, Qiang Zhao a, Bin Wang a, Danni Li a, Yujie Zhou a, Jiai Hua a, c, **, Pengtao Ma b, *** a b c
Chemistry and Chemical Engineering Department, Taiyuan Institute of Technology, Taiyuan, 030008, PR China Henan Key Laboratory of Polyoxometalate Chemistry, College of Chemistry and Chemical Engineering, Henan University, Kaifeng, Henan, 475004, PR China State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing, 210023, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 November 2019 Received in revised form 2 January 2020 Accepted 8 January 2020 Available online 14 January 2020
A hybrid polyoxometalate based on binuclear copper complex and a-Keggin-type silicotungstate [Cu2(4,40 -bpy) (Phen)2][SiW12O40] (abbreviated as CSW) (4,40 -bpy ¼ 4,40 -bipyridine; Phen ¼ 1,10phenanthroline) has been synthesized and structurally characterized by elemental analyses, X-ray powder diffraction (XRPD), IR spectrum, X-ray photoelectron spectrum (XPS) and single-crystal X-ray diffraction. Structural analysis shows that CSW is an ionic compound, in which the cation is a binuclear copper complex [Cu2(4,40 -bpy) (Phen)2]4þ and the anion is a a-Keggin-type silicotungstate [SiW12O40]4e cluster. Notably, the Cu ions in the [Cu2(4,40 -bpy) (Phen)2] exist in the form of tri-coordination, which the phenomenon is very rare in polyoxometalates. Moreover, CSW can catalyze the production of reactive oxygen species efficiently. © 2020 Elsevier B.V. All rights reserved.
Keywords: Silicotungstate Tri-coordination binuclear copper Reactive oxygen species Crystal structure analysis a-Keggin-type cluster Antiferromagnetism
1. Introduction Reactive oxygen species (ROS) are the most common intermediate and medium in bio-chemical reactions, in which it is present in almost all life processes and pathological process [1,2]. Hence, the researches of ROS catalyst have received more and more interest from laboratory to clinical researches [3e5]. The variable valence transition metal ions, such as Fe2þ/Fe3þ, Mn2þ/Mn4þ, Cuþ/ Cu2þ, etc., are believed in catalyzing the production of ROS, in which those ions are used as active centers to construct reactive proteins in living organisms, for example hemoglobin (Hb), myoglobin (Mb) and superoxide dismutase (SOD) [6e8]. Recently, we have successfully synthesized a series of artificial enzyme complexes that can catalyze the production of ROS [9,10] in which
* Corresponding author. Chemistry and Chemical Engineering Department, Taiyuan Institute of Technology, Taiyuan, 030008, PR China. ** Corresponding author. Chemistry and Chemical Engineering Department, Taiyuan Institute of Technology, Taiyuan, 030008, PR China. *** Corresponding author. E-mail addresses: [email protected] (X. Ma), [email protected] (J. Hua), [email protected] (P. Ma). https://doi.org/10.1016/j.molstruc.2020.127714 0022-2860/© 2020 Elsevier B.V. All rights reserved.
the performance of copper complexes is superior [9]. In view of their good biological activity, as a continuation of the above work, we will continue to focus on the synthesis of copper complexes with ROS catalytic activity. Polyoxometalates (POMs) are a fascinating class of metaloxygen clusters, which have multitudinous structures and fascinating properties in various fields such as catalysis, magnetism, fluorescence, medicine materials and nano-science [11e15]. POMs have several inherent irreplaceable advantages such as nano-size, nucleophilic oxygen-enriched surface and poly-bond-making sites, which endow the cluster with the ability to serve as bulky polydentate ligands to incorporate in multiple transition-metal ions with flexible coordination modes [16e25]. Moreover, recently, researches have demonstrated that the Keggin-type POMs possessed an excellent oxidation catalytic capacity [26e28]. These properties may be beneficial to catalyze the production of ROS with transition-metal ions [9]. Hence, the combination of Cu ions and POMs in one hybrid material may not only maintain the desirable properties of all precursor components, but also express them in a synergistic manner. Herein we report a newly designed POM, [Cu2(4,40 -bpy) (Phen)2][SiW12O40] (abbreviated as CSW) (4,40 -bpy ¼ 4,40 -
2
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bipyridine; Phen ¼ 1,10-phenanthroline), which was derived from pure inorganic structure Keggin-type silicotungstate anion [SiW12O40]4e and a dinuclear copper complex [Cu2(4,40 -bpy) (Phen)2]4þ. As expected, CSW can catalyze ROS generation efficiently. 2. Experimental 2.1. Materials and physical measurements Reagents used in this study were all of analytical grade, purchased from commercial suppliers and used as received unless otherwise stated. 20 ,70 -dichlorofluorescin diacetate (DCFH-DA) was purchased from Sigma-Aldrich. CuCl2$2H2O, NaWO4$2H2O, Na2SiO3$9H2O, Phen, 4,40 -bpy and C2H2O4$2H2O were purchased from J & K. All the solutions were prepared with Milli-Q water and filtered through a 0.22 mm filter (Millipore). X-Ray powder diffraction (XRPD) measurements were obtained using a Philips X’pert-MPD instrument with Cu-Ka radiation (l ¼ 1.54056 Å) at 293 K. IR spectrum was obtained from a sample powder palletized with KBr or dissolved in chloroform on a Nicolet 170 SXFT-IR spectrophotometer over the range 4000e400 cm1. The single crystal data of CSW were collected on a Bruker CCD, Apex-II diffractometer with graphite monochromated Mo Ka (l ¼ 0.71073 Å) radiation at room temperature. Routine Lorentz and polarization corrections were applied and an absorption correction was performed using the SADABS program. The structure was solved by direct methods and refined using full-matrix least squares on F2. All calculations were performed using the SHELXL-97 program package. DCF fluorescence were conducted on a Thermo Scientific Varioskan Flash microplate reader. Elemental analysis was performed on a PQEXCeII ICP-MS. 2.2. Synthesis Two solutions were prepared separately. Solution A: Na2WO4$2H2O (3.30 g, 10.00 mmol) and Na2SiO3$9H2O (1.90 g, 6.70 mmol) were dissolved in water (50 mL) under stirring. Solution B: CuCl2$2H2O (1.71 g, 10.00 mmol), Phen (0.52 g, 2.5 mmol), 4,40 -bpy (0.4 g, 2.0 mmol) and oxalic acid (C2H2O4$2H2O, 0.25 g, 2.0 mmol) were added to water (50 mL) under stirring. The resulting mixture of B is added to solution A. The mixture was stirred for 10 min at room temperature and then the pH value was adjusted to 4.0 by adding 6 mol L1 HCl dropwise. The solution was sealed in a 25 mL Teflon-lined autoclave kept at 150 C for 5 days and then cooled to room temperature. Then the mazarine crystals of CSW were separated with a 36% yield (based on Na2WO4$2H2O). Elemental analysis (%) calcd for [Cu2(4,40 -bpy) (Phen)2][SiW12O40]: C 11.61, N 2.39, Si 0.80, Cu 3.61, W 62.71; found: C 11.67, N 2.42, Si 0.76, Cu 3.66, W 62.61. The results showed that the optimal pH range for CSW generation is 2.0e5.0, and the crystal morphology was best when the initial pH was 4.0. The CSW yield was the highest when the initial pH was 3.0e4.0. When pH was 5.5 and above, no CSW crystals were obtained. 2.3. X-ray data collection and structure refinement Intensity data of CSW was collected on a Bruker Apex-2 diffractometer with a CCD detector using graphite monochromatized Mo Ka radiation (l ¼ 0.71073 Å) at 296 K. Data integration was performed using SAINT [29]. Routine Lorentz and polarization corrections were applied. Multiscan absorption corrections were performed using SADABS [30]. The structure was solved by direct methods and refined using full-matrix least
squares on F2. The remaining atoms were found from successive full-matrix least-squares refinements on F2 and Fourier syntheses. All calculations were performed using the SHELXL-97 program package [31]. No hydrogen atoms associated with the water molecules were located from the difference Fourier map. Positions of the hydrogen atoms attached to the carbon and nitrogen atoms were geometrically placed. All hydrogen atoms were refined isotropically as a riding mode using the default SHELXTL parameters. A summary of crystal data and structure refinements for CSW is listed in Table 1. 3. Results and discussion 3.1. Crystal structure The structure of CSW was characterized by single-crystal X-ray diffraction analysis. The crystallographic data, selected bond lengths are summarized inTable S1. Detailed information has been deposited at the Cambridge Crystallographic Data Centre with a CCDC number of 1959063. As shown in Fig. 1, X-ray structural analysis reveals that the unit of CSW consists of two parts: the binuclear copper complex [Cu2(4,40 -bpy) (Phen)2]4þ and the [SiW12O40]4e cluster. As shown in Fig. 1A and C, the [Cu2(4,40 -bpy) (Phen)2]4þ in CSW with an inversion center located at the center of 4,40 -bpy ligand, which consists two Cu ions, two Phen and one 4,40 bpy ligand. Each Cu ion is coordinated by three N atoms from Phen and 4,40 -bpy ligand with CueN distances ranging from 1.882 to 2.016 Å. The sum of the different angles around the Cu ion is 360 , which is indicating a trigonal-planar geometry around the center metal ion. The roles of 4,40 -bpy and Phen ligand coordinating to Cu are different. The 4,40 -bpy serves as a bridging ligand linked with two Cu ions, while Phen acts as a chelating agent coordinated to Cu. Alternatively, 4,40 -bpy ligand joins two [Cu(Phen)]þ to form a binuclear Cu complex. In coordination chemistry, Cu2þ always has a tetra-, penta-, and hexa-coordination configuration; In contrast, Cuþ often shows a bicoordination configuration, which the tri-coordination copper ion
Table 1 Crystallographic data and structural refinements for CSW. Empirical formula Formula weight Crystal system Space group a/Å b/Å c/Å a/deg b/deg g/deg V/Å3 Z Dc/g cm3 m/mm1 T/K Limiting indices
Measured reflections Independent reflections Rint Data/restrains/parameters GOF on F2 Final R indices [I > 2s(I)] R indices (all data) Completeness
C34H24N6Si1W12O40Cu2 3517.83 Orthorhombic Ibam 13.222 (2) 25.116 (4) 26.768 (4) 90 90 90 8890 (3) 4 3.109 16.494 296 (2) 15 h 15 23 k 29 31 l 29 20265 4002 0.0418 4002/36/326 1.011 R1 ¼ 0.0352, wR2 ¼ 0.1084 R1 ¼ 0.0484 wR2 ¼ 0.1164 99.8%
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center of [SiW12O40]4e cluster and shares four oxygens with four {Mo3O13} triad. Those four {Mo3O13} clusters are connected to each other by sharing edge and Om, forming an a-Keggin type cluster. The bond valence sums (Ss) and the protonation level (SH) of oxygen atoms in CSW were further calculated according to the reported methods [34,35]. Briefly, the oxidation states of the oxygen atoms in CSW were calculated on the following formula, which could be used to estimate the protonation of oxygen atoms:
Vi ¼
Fig. 1. Ball-and-stick view of the dinuclear copper complex [Cu2(4,40 -bpy) (Phen)2]4þ (A) and the Keggin-type silicotungstate [SiW12O40]4e (B) in [Cu2(4,40 -bpy) (Phen)2] [SiW12O40] (CSW); (C) Structural diagram of [Cu2(4,40 -bpy) (Phen)2]4þ; (D) Polyhedral view of the [SiW12O40]4e unit.
is usually the intermediate state in the REDOX process of Cu2þ-Cuþ [32]. To our knowledge, in the solid state, the stable tricoordination copper ion is very rare. Since the coordination configuration is between that of Cu2þ and Cuþ, Cu ions in [Cu2(4,40 bpy) (Phen)2]4þ complex may be easy to cycle between þ2 and þ 1; and this is also a necessary structural feature for catalytic production of ROS, which the phenomenon is similar with the Fe2þ-Fe3þ REDOX actions in heme [6]. Further investigation using X-ray photoelectron spectrum (XPS) has been carried out to detect the valence of Cu in CSW. As shown in Fig. 2, there are two broad peaks with a shoulder peak located at 954.0 and 933.8 eV, which were assigned to Cu 2p3/2 and Cu 2p1/2, respectively [33]. The fitted curves in Fig. 2 (dashed lines) suggest that Cu2þ (2p3/2, 934.0 eV; 2p1/2, 953.9 eV) and Cuþ (2p3/2, 932.9 eV; 2p1/2, 952.7 eV) are involved in the peaks of Cu 2p [33]. These results may indicate that Cu ions in the þ2 and þ 1 valence states coexist in the Cu complex of CSW, in which the content of Cuþ is very small according to the peak strength. As shown in Fig. 1B and D, tetrahedral {SiO4} is located in the
’ X X r rij sij ¼ exp 0 B j j
where r0’ represents the theoretical value of bond distance between two atoms, and rij represents the observed values of bond distance that are listed in Table S1; B was set to 0.37 [36]. The theoretical value of WeO, SieO and CueN from literatures, which the r0 (W6þeO) is 1.881 Å, r0 (Si4þeO) is 1.622 Å, r0 (Cu2þeN) is 1.504 Å [36]. As a results, the average valence state sums (Ss) of Cu ions in CSW is 1.864, which is basically consistent with the above XPS experimental results. Based on the observed value of bond distance of CSW, the Ss and SH of CSW can be calculated and the results are summarized in Table S2 and Fig. 3. Since the POMs fragment has high negative charges and rich basic surface oxygen atoms, they can easily been protonated [37]. The 40 oxygen atoms in CSW can be classified into terminal Ot, bridging m2-O, m3-O and m4-O. The O atoms with SH of 0.4e0.5 could act as H-donors owing to the delocalized protons on them, whereas the O atoms with SH of 0e0.1 possess dense electron cloud (see Table S2). In generally, the multiply protons usually assigned to be delocalized on the whole polyoxoanion, which the phenomenon is common in POM chemistry and has been reported in many leteratures, for example, [Ni(enMe)2]3 [H6Ni20P4W34(OH)4O136 (enMe)8(H2O)6]$12H2O [34], and [H3W12O40]5e [38].
̍ ̍
Fig. 2. X-ray photoelectron spectrum (XPS) and the fitted curves of Cu in [Cu2(4,40 bpy) (Phen)2][SiW12O40] (CSW).
̍
Fig. 3. Protonation of oxygen atoms in the Keggin unit of [Cu2(4,40 -bpy) (Phen)2] [SiW12O40] (CSW). The extent of protonation for each oxygen atom is indicated by different colors.
4
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These results may indicate that several counter positive charges in CSW are delocalized in the [SiW12O40]4e skeleton, which may help to absorb protons, and hence balance valence states when copper changes valence [39]. Therefore, the coexistence of Cu complexes and POM may enable them to catalyze the production of ROS in a synergistic manner. 3.2. XRPD and IR spectrum The phase purity of CSW was confirmed by a comparison of the experimental X-ray powder diffraction (XRPD) pattern with the simulated pattern from single-crystal X-ray diffraction (Fig. 4). Owing to the variation in the preferred orientation of the powder sample in the experimental XRPD, the intensities of the experimental and simulated XRPD patterns are different. As shown in Fig. 5 and Fig. S1, the IR spectrum of CSW is similar with that of the Keggin-type silicotungstate reported, which is indicative of retention of the [SiW12O40]4e cluster [40]. In the lowwavenumber region, four characteristic bands assigned to n(WeOt), n(WeOm2), n(WeOm3) and n(SieO) appear at 948, 828, 745 and 898 cm1, respectively. In comparison with that of [SiW12O40]4e cluster, the n(WeOt) vibrational bands for CSW split, which may indicate the structural distortion and the consequent lowering of the symmetry [40]. The IR spectra of 4,40 -bpy and Phen were also tested and shown in Figs. S2 and S3. Compared the IR spectra of Fig. S2 and S3 to S1, the peaks observed at 1616e1116 and 3300e3500 cm1 in Fig. S1 may attribute to the peaks of 4,40 -bpy and Phen and is consistent with literature report [41].
Fig. 5. IR spectra for [Cu2(4,40 -bpy) (Phen)2][SiW12O40] (CSW), 4,40 -bipyridine (4,40 bpy), 1,10-phenanthroline (Phen).
3.3. Magnetic property The solid state direct-current magnetic susceptibilities of CSW have been measured on its polycrystalline samples from 2 to 300 K in the 1 kOe field, since CSW incorporates isolated bi-Cu linked by non-magnetic ligand. The magnetic data for CSW are plotted in Fig. 6 in the form of cM and cMT versus T. The temperature dependence of cM shows a slight increase from 0.0022 to 0.0052 emu mol1 in the range of 300e120 K, and then exponentially reaches the maximum value of 0.0442 emu mol1 at 3 K. At room temperature the cMT values is 0.66, which are a little lower than expected for the spin-only value for two isolated Cu2þ with S ¼ 1/2 and g ¼ 2.00. The cMT value of CSW decreased with the
Fig. 6. Temperature dependence of the molar magnetic susceptibility cM and the cMT for [Cu2(4,40 -bpy) (Phen)2][SiW12O40] (CSW) between 2 and 300 K.
temperature going down gradually, which indicated the presence of antiferromagnetic interactions in binuclear entities [42]. As shown in inset of Fig. 6, in the [Cu2(4,40 -bpy) (Phen)2]4þ, the distance between two Cu ions is 10.80 Å, in which may be difficult to generate magnetic exchange phenomenon. The dependence of the reciprocal susceptibility data is well fitted by CurieeWeiss expression [cM ¼ C/(T e q) with C ¼ 0.695 emu K mol1, q ¼ 9.852 K], which consolidates the presence of overall antiferromagnetic coupling within the copper centers(see Fig. S4).
3.4. Catalytic property
Fig. 4. Comparison of the simulated and experimental XRPD patterns of [Cu2(4,40 -bpy) (Phen)2][SiW12O40] (CSW).
ROS is considered to be a major species accounting for the every aspects of the process of life, especially in immunity, inflammation, enzyme activity, antibacterial, anti-cancer and neurodegenerative diseases [1,32,43,44]. Recently, it reported that Cu or Zn ions substituted phosphomolybdate possess the ability of catalytic oxidation [45]. Hence, if the likely synergize with silicotungstate to
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[1,49,50]. Since Hþ can be produced in Fenton-like system reaction, adsorption of Hþ to POM may enhance the catalytic capacity of Cu complex [39].
Cu2þ þ H2 O2 /Cuþ þ $O2 H þ Hþ
(1)
Cuþ þ H2 O2 /Cu2þ þ $OH þ OH
(2)
4. Conclusion
Fig. 7. Fluorescence intensity of DCF (lex ¼ 485 nm, lem ¼ 650 nm) by [Cu2(4,40 -bpy) (Phen)2][SiW12O40] (CSW), Na4 [SiW12O40] (C1), Cu2þ, Cu2þ þ GSH, GSH, purified water and control group from 0 to 2600 min.
further enhance catalytic activity. The effect of CSW on the mediated ROS generation was thus investigated by a dichlorofluorecin (DCF) assay. DCF is a fluorescent marker derived from the reaction of non-fluorescent 20 ,70 -dichlorofluorecin (DCFH) with ROS in the presence of horseradish peroxidase (HRP), which can indicate the total output of ROS from the system [46]. As shown in Fig. 7, the fluorescence intensity of the group of CSW are obviously higher than that of control, indicating that the production of ROS with CSW are more than that without them. In addition, Na4 [SiW12O40] (abbreviated as C1) also has catalytic effect to some extent. However, the fluorescence intensity of CSW is ~6 times higher than that of C1, which is verified that the Cu in collaboration with [SiW12O40]4e of CSW has stronger catalytic capacity than the single cluster [SiW12O40]4e of C1. Interestingly, as shown in Fig. 7, the experimental group with Cu2þ produced significantly less ROS. This phenomenon may be attributed to the interaction between Cu2þ and HRP enzyme, which deactivates it. In fact, in microenvironment, no matter cells or bacteria can secrete a considerable number of proteins, polypeptides and chelators, which may coordinate with Cu2þ and make them lose their catalytic position [47]. We also used GSH as a simulated enzyme to verify the above phenomenon. As shown in Fig. 7, the catalytic ROS capacity of Cu2þ þ GSH is only higher than that of GSH, which is also a significant decline compared with Cu etc. Therefore, the combination of Keggin-type fragment and Cu ions not only contributes to the synergistic production of ROS, but also protects the catalytic centers from the interference of external proteins. It is known that, in organisms, there are many metalloprotein that can catalyze the production of ROS, for example, the FeS cluster in mitochondria, which can produce ROS by catalytic substrates of Fe2þ/Fe3þ [48]. In pathology, Cuþ/Cu2þ can also act as a catalytic center to produce ROS, for example, ROS produced by Ab þ Cu2þ/ Cuþ cause irreversible damage to neuron cells, and the correlation between b-sheet formation and ROS generation has been well described by Faller P. and Kepp, K. P.. [1,49]. In brief, Cu2þ induces Ab to form misfolding structures, amino acid residues coordinate with Cu2þ to form the Cu-Ab coordination complexes. The Cu2þ centers are easy to be oxidized or reduced in dynamic equilibrium of structural changes, and thus catalyze the generation of ROS. As a dinuclear-copper coordination compound, CSW may catalyze Fenton-like system reactions, as shown in equations (1) and (2)
In this paper, a hybrid polyoxometalate (POM) based on binuclear copper complex and a-Keggin-type silicotungstate [Cu2(4,40 bpy) (Phen)2][SiW12O40] (abbreviated as compound CSW) (4,40 bpy ¼ 4,40 -bipyridine; Phen ¼ 1,10-phenanthroline) has been designed and synthesized successfully. The novelty of the structure is that the Cu ions in the [Cu2(4,40 -bpy) (Phen)2] exist in the form of tri-coordination in solid state, in which the phenomenon is rare in POMs. And the synthetic method may serve as a good example for subsequent synthesis and formation of such POMs clusters [51]. As expected, CSW can catalyze the production of reactive oxygen species (ROS) efficiently, which may be due to the synergy effect of Cu ions and Keggin-structure. The novelty structure and interesting properties may make CSW a broad application prospects in the biochemistry and inorganic chemistry researches of ROS catalyst. Acknowledgments This work was supported by the Natural Science Foundation of China (Grants 21573056), Shanxi Province Science Foundation for Youths (Grants 201901D211453), the Research Foundation for Advanced Talents of Shanxi Province (1800008001), the Research Foundation of Taiyuan Institute of Technology (03000352), and the Research Foundation of the Chinese State Key Laboratory of Coordination Chemistry (SKLCC1912). Appendix B. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.molstruc.2020.127714. Appendix A. Supplementary material Crystallographic data for the structural analysis have been deposited with the Cambridge Crystallographic Data Center, CCDC reference number: 1959063 for CSW. This data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associated with this article can be found in the online version, at Author contributions X. Ma and P.T. Ma conceived the idea of the research. X. Ma and J.A. Hua designed the molecule, and J.A. Hua and Q. Zhao synthesized the molecule. P.T. Ma performed the single-crystal X-ray diffraction and magnetic measurement. X. Ma, B. Wang, D.N. Li and Y.J. Zhou designed and conducted the experiments. J.A. Hua and P.T. Ma analyzed the data. All authors wrote and reviewed the manuscript. References [1] K.P. Kepp, Chem. Rev. 112 (2012) 5193e5239.
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