{Synthesis and structural analysis of polynuclear silver(I) complexes with 4,7-phenanthroline} J. Serb. Chem. Soc. 84 (7) 689–699 (2019) UDC 547.677.5+546.571:543.42:548.0–77 JSCS–5219 Original scientific paper 689 Synthesis and structural analysis of polynuclear silver(I) complexes with 4,7-phenanthroline IVANA M. STANOJEVIĆ#, NADA D. SAVIĆ1*#, AURÉLIEN CROCHET2, KATHARINA M. FROMM2, MILOŠ I. DJURAN3# and BILJANA Đ. GLIŠIĆ1**# University of Niš, Faculty of Agriculture, Kosančićeva 4, 37000 Kruševac, Serbia, 1University of Kragujevac, Faculty of Science, Department of Chemistry, Radoja Domanovića 12, 34000 Kragujevac, Serbia, 2Department of Chemistry, University of Fribourg, Chemin du Musée 9, CH-1700 Fribourg, Switzerland and 3Serbian Academy of Sciences and Arts, Knez Mihailova 35, 11000 Belgrade, Serbia (Received 26 February, revised 8 March, accepted 18 March 2019) Abstract: New polynuclear silver(I) complexes, [Ag(CF3SO3)(4,7- -phen)(CH3CN)]n (1) and [Ag(PO2F2)(4,7-phen)]n (2), were synthesized by the reaction of 4,7-phenanthroline (4,7-phen) and the corresponding AgX salt (X = CF3SO3 - and PF6 -) in 1:2 mole ratio, respectively, in methanol/acetone (1:1 volume ratio) at room temperature. The characterization of the complexes was established on the basis of elemental microanalysis, IR and NMR (1H and 13C) spectroscopic techniques, while their crystal structures were determined by single-crystal X-ray diffraction analysis. The results of spectroscopic and crys- tallographic analyses revealed that in these complexes, 4,7-phen behaves as a bridging ligand between two metal ions, while the remaining coordination sites of the Ag(I) ions are occupied by the oxygen atom of CF3SO3 - and an acetonit- rile nitrogen atom in 1 or by two oxygen atoms from two PO2F2 -, formed after hydrolysis of PF6 -, in 2. In the solid state, both complexes are coordination polymers in which the geometry around the Ag(I) ions is distorted tetrahedral. Keywords: silver(I) complexes; 4,7-phenanthroline; polynuclear complexes; spectroscopy; X-ray crystallography. INTRODUCTION Silver(I) complexes with aromatic nitrogen-containing heterocycles (N-hete- rocycles) are of great importance in the field of medicinal chemistry and the coordination polymer design.1,2 Generally, silver(I) complexes with these ligands are known to possess significant antibacterial activity against a wide range of Gram-positive and Gram-negative bacteria and have shown remarkable activity against different Candida species.3–14 Moreover, they have manifested cytotoxic *,** Corresponding authors. E-mail: (*)nada.savic@kg.ac.rs; (**)biljana.glisic@pmf.kg.ac.rs # Serbian Chemical Society member. https://doi.org/10.2298/JSC190226024S 690 STANOJEVIĆ et al. activity against different human tumor cell line, being more active and less toxic to humans than the clinically used platinum(II) complexes.15 Their effectiveness is thought to be the consequence of the presence of a weak Ag−N bond, which can be easily cleaved in the interaction with thiol-containing proteins and DNA, the process that is a prerequisite for their action.3 Besides the significant antimicrobial and antiproliferative activities of sil- ver(I) complexes with aromatic N-heterocycles, the Ag(I) ions coordinated by this type of ligands are favorable building blocks for coordination polymers which may find interesting applications for the development of innovative mat- erials, such as liquid crystals.1,16,17 Coordination polymers based on Ag(I) ions are attracting great attention due to the flexibility of the coordination sphere of this metal ion.1,16,17 Thus, it can adopt coordination numbers between two and six and form complexes of various geometries such as linear, bent, trigonal, T-shaped, tetrahedral, trigonal pyramidal and octahedral. Besides that, weak contacts such as Ag⋅⋅⋅Ag, Ag⋅⋅⋅π and Ag⋅⋅⋅solvent/counterion interactions significantly affect the geometry and topology of the silver(I)-based coordination polymers in the solid state.1,18 In the design of silver(I) coordination polymers having different possible applications, various bridging and chelating N-heterocyclic ligands have been used. Among them, phenanthrolines have been investigated due to their coordin- ation diversity which allowed the tuning of the nuclearity and biological acti- vities in a series of silver(I) complexes.5,13,14 Very recently, five silver(I) com- plexes with 4,7-phenanthroline ligand (4,7-phen), [Ag(NO3)(4,7-phen)]n, [Ag(ClO4)(4,7-phen)]n, [Ag(CF3COO)(4,7-phen)]n, [Ag2(H2O)0.58(4,7-phen)3] (SbF6)2 and {[Ag2(H2O)(4,7-phen)2](BF4)2}n, have been synthesized and biolo- gically evaluated, showing higher selectivity towards Candida spp. in com- parison to bacteria, while being only moderately cytotoxic against healthy human fibroblasts.14 Moreover, [Ag(NO3)(4,7-phen)]n and [Ag(CF3COO)(4,7-phen)]n complexes were effective in vivo rescuing zebrafish embryos from lethal C. albi- cans infection and reducing the fungal burden by preventing fungal filament- ation.14 In the present study, 4,7-phen reacted with two AgX salts (X = CF3SO3– and PF6–), yielding new polynuclear silver(I) complexes, [Ag(CF3SO3)(4,7- -phen)(CH3CN)]n (1) and [Ag(PO2F2)(4,7-phen)]n (2). These complexes were fully characterized by spectroscopy (IR, 1H- and 13C-NMR) and single-crystal X-ray crystallography. EXPERIMENTAL Reagents Silver(I) salts (AgCF3SO3 and AgPF6), 4,7-phenanthroline (4,7-phen), methanol, ace- tone, acetonitrile, dimethyl sulfoxide (DMSO) and deuterated dimethyl sulfoxide (DMSO-d6) SILVER(I) COMPLEXES WITH 4,7-PHENANTHROLINE 691 were purchased from the Sigma–Aldrich Chemical Co. All the employed chemicals were of analytical reagent grade and used without further purification. Measurements Elemental microanalysis of the silver(I) complexes for carbon, hydrogen and nitrogen was performed at the Adolphe Merkle Institute of the University of Fribourg. The NMR spectra were recorded at 25 °C on a Varian Gemini 2000 spectrometer (1H at 200 MHz, 13C at 50 MHz). 5.0 mg of 4,7-phen and the corresponding silver(I) complex was dissolved in 0.6 mL of DMSO-d6 and transferred into a 5 mm NMR tube. Chemical shifts are expressed in ppm (δ / ppm) and scalar couplings are reported in Hertz (J / Hz). Chemical shifts were cal- ibrated relative to those of the solvent. The IR spectra were recorded as KBr pellets on a Per- kin–Elmer Spectrum One FT-IR spectrometer over the wavenumber range of 4000–450 cm-1. Analytical and spectral data of the compounds are given in Supplementary material to this paper. Synthesis of complexes 1 and 2 10.0 mL of methanolic solution of the corresponding silver(I) salt (0.50 mmol, 128.4 mg of AgCF3SO3 for 1 and 126.4 mg of AgPF6 for 2) was added dropwise to a solution of 4,7- -phen (0.25 mmol, 45.0 mg) in 10.0 mL of warm acetone with stirring at room temperature. The stirring was continued for 3 h in the dark at room temperature, the white precipitate was filtered off and dissolved in acetonitrile. The obtained solution was left to evaporate slowly at room temperature. After 3–5 days, colorless crystals of 1 and 2 suitable for single crystal X-ray analysis were formed. These crystals were filtered off and dried in the dark at room temperature. Yield (calculated on the basis of the N-heterocyclic ligand): 73 % (87.3 mg) for 1 and 66 % (64.2 mg) for 2. Crystallographic data collection and refinement of the structure Crystal data and details of the structure determinations are listed in Table I. A suitable crystal was mounted on a mylar loop in oil on a STOE IPDS 2 diffractometer. The crystals were kept at 250(2) K for 1 and 293(2) K for 2 during data collection. Using Olex2,19 the structures were solved with the ShelXT20 structure solution program using Intrinsic Phasing and refined with the ShelXL21 refinement package using Least Squares minimization. Drawings were prepared with Mercury computer graphics program.22 TABLE I. Details of the crystal structure determination of [Ag(CF3SO3)(4,7-phen)(CH3CN)]n (1) and [Ag(PO2F2)(4,7-phen)]n (2) complexes Property 1 2 Empirical formula C15H11AgF3N3O3S C12H8AgF2N2O2P Formula weight 478.20 389.04 Crystal system, space group monoclinic, P21/c monoclinic, P21/c a / Å 13.6834(11) 10.3939(4) b / Å 8.0480(5) 14.7587(5) c / Å 15.5345(13) 8.0940(5) α / ° – – β / ° 103.199(6) 97.683(4) γ / ° – – V / Å3 1665.5(2) 1230.48(10) F000 944 760 Ζ 4 4 692 STANOJEVIĆ et al. TABLE I. Continued Property 1 2 X-radiation, λ / Å Mo-Kα 0.71073 Mo-Kα 0.71073 Data collect. temperature, K 250(2) 293(2) Calculated density, Mg m-3 1.907 2.100 Absorption coefficient, mm-1 1.389 1.794 Crystal size, mm3 0.270 × 0.190 × 0.110 0.260 × 0.153 × 0.040 2θ range, ° 5.3 to 50.3 3.9 to 50.3 index ranges h, k, l -16 ... 16, -9 ... 9, -18 ... 18 -12 ... 12, -17 ... 17, -9 ... 9 No. of collected & indep. reflections 20622, 2962 15604, 2192 Rint 0.0842 0.0315 Data / restraints / parameters 2962 / 34 / 237 2192 / 0 / 181 Goodness-on-fit on F2 1.066 1.047 Final R indices [I ≥ 2σ(I)] 0.0597, 0.1595 0.0269, 0.0687 Final R indices (all data) 0.0679, 0.1660 0.0325, 0.0711 Difference density: max, min / e Å-3 0.94, –1.86 0.67, –0.49 CCDC number 1899714 1899713 RESULTS AND DISCUSSION Synthesis and structural features of complexes 1 and 2 The silver(I) complexes with 4,7-phenanthroline (4,7-phen) were synthe- sized according to the route presented in Scheme 1. The reactions between 4,7- -phen and AgX (X = CF3SO3– and PF6–) in 1:2 mole ratio were performed in methanol/acetone (1:1 volume ratio) at room temperature yielding the polynuc- lear [Ag(CF3SO3)(4,7-phen)(CH3CN)]n (1) and [Ag(PO2F2)(4,7-phen)]n (2) com- plexes. The composition and structural formula of both silver(I) complexes were consistent with the elemental analysis, IR and solution NMR (1H and 13C) spectroscopic results and were also supported by single-crystal X-ray diffraction analyses. In these complexes, 4,7-phen acts as a bridging ligand between two metal ions, being in accordance with its coordination mode in the previously synthesized silver(I) complexes with this N-heterocycle.14 In complex 1, the CF3SO3– is monodentately coordinated to the Ag(I) ion, while the fouth coordin- ation site is occupied by the acetonitile nitrogen atom. Instead of the expected PF6– in 2, this complex contains two bridging PO2F2– that are monodentately coordinated to Ag(I) ion via oxygen atoms. It can be assumed that PO2F2– was formed by the hydrolysis of PF6–, a process previously described in the litera- ture:23,24 PF6– + H2O → POF4– + 2HF (1) POF4– + H2O → PO2F2– + 2HF (2) Description of the single crystal structure The molecular structures of silver(I) complexes 1 and 2 with the anisotropic displacement ellipsoids and the atom numbering scheme are shown in Fig. 1, SILVER(I) COMPLEXES WITH 4,7-PHENANTHROLINE 693 while the selected bond distances, Å and angles, °, with the estimated standard deviations are given in Table II. Scheme 1. Schematic presentation of the synthesis of [Ag(CF3SO3)(4,7-phen)(CH3CN)]n (1) and [Ag(PO2F2)(4,7-phen)]n (2) complexes.The solid products of these reactions were recrystallized in acetonitrile. The numbering scheme of carbon atoms in 4,7-phen is in agreement with IUPAC recommendations for fused ring systems and does not match the one applied in the X-ray analysis of silver(I) complexes. a b Fig. 1. Molecular structures of: a) [Ag(CF3SO3)(4,7-phen)(CH3CN)]n (1) and b) [Ag(PO2F2)(4,7-phen)]n (2) complexes. Displacement ellipsoids are drawn at 50 % probability level and H atoms are represented by spheres of arbitrary size. Symmetry codes for complex 1: #1: 1–x, 1/2+y, 3/2–z and for 2: #1: 1–x, -1/2+y, 1/2–z, #2: x, 3/2–y, 1/2+z. 694 STANOJEVIĆ et al. TABLE II. Selected bond distances and valence angles (°) in silver(I) complexes 1 and 2; Symmetry code: (#2) x, –y+3/2, z–1/2 1 2 Bond Bond distance, Å Bond Bond distance, Å Ag1—N1 2.305(5) Ag1—N1 2.240(3) Ag1—N2 2.251(5) Ag1—N2 2.252(3) Ag1—N3 2.284(7) Ag1—O1 2.500(3) Ag1—O1 2.505(6) Ag1—O2 2.499(3) Valence angles, ° Valence angles, ° N1—Ag1—N2 130.94(17) N1—Ag1—N2 143.77(10) N2—Ag1—N3 128.3(2) N1—Ag1—O1 108.67(10) N1—Ag1—N3 96.5(2) N1—Ag1—O2 115.62(9) N1—Ag1—O1 98.1(2) N2—Ag1—O1 90.84(10) N2—Ag1—O1 94.7(2) N2—Ag1—O2 95.47(10) N3—Ag1—O1 97.8(3) O1—Ag1—O2 85.80(11) C1—N1—Ag1 120.9(4) C1—N1—Ag1 121.2(2) C5—N1—Ag1 121.6(4) C5—N1—Ag1 121.2(2) C12—N2—Ag1 121.2(4) C12—N2—Ag1 117.0(2) C8—N2—Ag1 121.8(4) C8—N2—Ag1 123.7(2) C14—N3—Ag1 162.2(9) P1—O1—Ag1 122.28(16) S1—O1—Ag1 145.9(4) P1#2—O2—Ag1 151.58(19) In the solid state, both complexes 1 and 2 are coordination polymers. In 1, each Ag(I) ion is surrounded by two 4,7-phen, one acetonitrile and one mono- dentately coordinated trifluoromethanesulfonate (triflate) anion (Fig. 1a). An extended view of polynuclear silver(I) complex 1 is shown in Fig. 2a. This complex has a distorted tetrahedral geometry, what can be concluded from the value of τ4 parameter25 of 0.71, τ4 = [360° – (β + α)]/141°, where β and α are the largest angles around the Ag(I) ion (β = N1—Ag1—N2 = 130.9(2)° and α = N2—Ag1—N3 = 128.3(2)°). The Ag1–N1/N2(4,7-phen) bond distances in 1 (Table I) adopt values of 2.305(5) and 2.251(5) Å, respectively, and are compar- able with those observed in the other pseudo tetrahedral silver(I) complexes with aromatic N-heterocycles.8–14 The Ag1–N3 (acetonitrile) bond distance of 2.284(7) Å falls in the normal range of 2.18–2.33 Å.26 On the other hand, the Ag1–O1 bond distance of 2.505(6) Å is much longer than usual covalent silver(I)–oxygen bonds of approximately 2.3 Å.8 In complex 1, two Ag(I) ions are connected by one 4,7-phen, which behaves as a bridging ligand. The intramolecular Ag⋅⋅⋅Ag interaction is not observed in this complex, considering the fact that Ag∙∙∙Ag distance of 7.689 Å is much longer than the commonly Ag∙∙∙Ag bond range of 2.853–3.290 Å.27 Similar to 1, complex 2 has a distorted tetrahedral geometry (τ4 = 0,71; Fig. 1b and Table II). In this coordination polymer, each Ag(I) ion is coordinated by two 4,7-phen and two PO2F2– (Fig. 1). Two Ag(I) ions are bridged by one 4,7- -phen and one PO2F2–, with d(Ag–N) = 2.240(3) and 2.252(3) Å and d(Ag–O) = SILVER(I) COMPLEXES WITH 4,7-PHENANTHROLINE 695 2.500(3) and 2.499(3) Å (an extended view of 2 is presented in Fig. 2b). The Ag– –N(4,7-phen) bond lengths in 2 are slightly shorter than in 1, while the Ag–O bond distances are comparable in these two silver(I) complexes (Table II). a b Fig. 2. An extended view of polynuclear silver(I) complexes: a) 1 and b) 2. The coordination mode of 4,7-phenanthroline in silver(I) complexes 1 and 2 is the same as in the previously characterized silver(I) complexes with this aro- matic N-heterocycle.14 Almost all of the synthesized silver(I) complexes with 4,7-phen are polynuclear species, with the exception of [Ag2(H2O)0.58(4,7- -phen)3](SbF6)2 complex, which is obtained in the reaction of AgSbF6 with an equimolar amount of 4,7-phen in the ethanolic solution.14 Contrary to 4,7-phen, its structural isomer 1,7-phenanthroline affords mononuclear silver(I) complexes, namely [Ag(NO3)(1,7-phen)2] and [Ag(1,7-phen)2]X (X = ClO4–, CF3SO3–, BF4– and SbF6–), in which it is monodentately coordinated via the less sterically hin- dered N7 nitrogen atom.12 696 STANOJEVIĆ et al. Spectroscopic characterization The IR and NMR (1H and 13C) spectroscopic data for silver(I) complexes 1 and 2 are given in Supplementary material. The IR spectra of the complexes show the bands which can be attributed to the coordinated 4,7-phen, CF3SO3– and PO2F2– ligands. Thus, in the IR spectrum of 1 with coordinated triflate, a number of strong absorptions in the 1300–1000 cm–1 region can be observed. The bands at 1263, 1255 and 1031 cm–1 can be assigned to the asymmetric and symmetric stretching modes of the –SO3 group of the triflate anion.28,29 The splitting of the band corresponding to asymmetric stretching vibration of –SO3 group is an indi- cation of the monodentate coordination of the triflate in 1.29 The two bands at 1247 and 1168 cm–1 are due to the symmetric and asymmetric stretching modes of –CF3 group, respectively. Additionally, the medium intensity bands at 635 and 517 cm–1, and at 754 and 594 cm–1 can be ascribed to the symmetric and asym- metric deformations of –SO3 and –CF3 groups in CF3SO3–, respectively.29 The hydrolysis of PF6– and the coordination of PO2F2– to Ag(I) in 2 can be also con- firmed from the IR spectrum of this complex. Beside the very strong band at 838 cm–1 which is attributed to the ν(PF) mode, two bands at 1305 and 1149 cm–1 due to the ν(PO) indicate that the hydrolysis of PF6– to PO2F2– occurred in the investigated reaction.23 Solution state 1H- and 13C-NMR spectra were recorded in deuterated DMSO with the aim to confirm the coordination of 4,7-phen to the Ag(I) ion. The spectra of the complexes 1 and 2 were compared with those for the free ligand. In the aro- matic region, 1H-NMR spectra of the complexes contain the same number of sig- nals as that of the 4,7-phen, indicating that symmetric species are present in sol- ution and that the ligand is coordinated to the Ag(I) ion via both donor nitrogen atoms (N4 and N7). However, the resonances for these protons are only slightly shifted downfield compared to those of the free 4,7-phen (up to +0.05 for H5/H6 in 2). Only small shifts of the resonances of the silver(I) complexes with respect to those for the corresponding ligand seem to be characteristic spectroscopic feature of the silver(I) complexes in solution and was assumed to be the conse- quence of the fast ligand exchange on the NMR timescale.6 In addition, a singlet at 2.07 ppm, attributed to the acetonitrile protons, is observed in the spectrum of 1. The 13C-NMR spectra were found to be almost identical for the both silver(I) complexes, excluding resonances assigned to the carbon atoms of the coordinated acetonitrile in 1. Similar to the proton resonances, upon 4,7-phen coordination to Ag(I) ion, the signals of its carbon atoms remain almost unaffected. CONCLUSION We have shown that the reactions between 4,7-phen and AgX salts (X = CF3SO3– and PF6–) in 1:2 mole ratio, respectively, in methanol/acetone lead to the formation of polynuclear silver(I) complexes, [Ag(CF3SO3) (4,7-phen)(CH3CN)]n SILVER(I) COMPLEXES WITH 4,7-PHENANTHROLINE 697 and [Ag(PO2F2)(4,7-phen)]n. The crystallographic results revealed that these complexes have a distorted tetrahedral geometry with 4,7-phen being bridging ligand between two Ag(I) ions. The present results are in accordance with those previously reported for the reactions of the same N-heterocyclic ligand with dif- ferent AgX salts (X = NO3–, CF3COO–, BF4–, ClO4– and SbF6–), most of them leading to the formation of polynuclear silver(I) species with different geometry of Ag(I) metal ion.14 The only exception was a dinuclear [Ag2(H2O)0.58(4,7- -phen)3](SbF6)2 complex, obtained in the reaction of AgSbF6 with 4,7-phen in ethanol.14 This study confirms that the nuclearity and geometry of the silver(I) complexes with aromatic N-heterocyclic ligands strongly depend on the reaction conditions, such as starting silver(I) compound, the nature of N-heterocyclic ligand and reaction solvent. All these together should be carefully considered during preparation of new silver(I) complexes for different applications in medi- cinal and supramolecular chemistry. SUPPLEMENTARY MATERIAL Analytical and spectral data are available electronically from http://www.shd.org.rs/ /JSCS/, or from the corresponding author upon request. Acknowledgements. This research has been financially supported by the Ministry of Education, Science and Technological Development of the Republic of Serbia, under Grant No. 172036, the SupraMedChem@Balkans.Net SCOPES Institutional Partnership (Project No. IZ74Z0_160515) and the Serbian Academy of Sciences and Arts (Project No. F128). И З В О Д СИНТЕЗА И СТРУКТУРНА АНАЛИЗА ПОЛИНУКЛЕАРНИХ КОМПЛЕКСА СРЕБРА(I) СА 4,7-ФЕНАНТРОЛИНОМ ИВАНА М. СТАНОЈЕВИЋ, НАДА Д. САВИЋ1, AURÉLIEN CROCHET2, KATHARINA M. FROMM2, МИЛОШ И. ЂУРАН3 и БИЉАНА Ђ. ГЛИШИЋ1 Универзитет у Нишу, Пољопривредни факултет, Косанчићева 4, 37000 Крушевац, 1Универзитет у Крагујевцу, Природно–математички факултет, Институт за хемију, Радоја Домановића 12, 34000 Крагујевац, 2Department of Chemistry, University of Fribourg, Chemin du Musée 9, CH-1700 Fribourg, Switzerland и 3Српска академија наука и уметности, Кнез Михаилова 35, 11000 Београд Полинуклеарни комплекси сребра(I), [Ag(CF3SO3)(4,7-phen)(CH3CN)]n (1) и [Ag(PO2F2)(4,7-phen)]n (2), синтетисани су у реакцијама између 4,7-фенантролина (4,7- -phen) и одговарајућих AgX соли (X = CF3SO3 - и PF6 - ) у молском односу 1:2 у смеши метанол/ацетон (запремински однос 1:1) на собној температури. Ови комплекси су окарактерисани применом елементалне микроанализе, IR и NMR ( 1 H и 13 C) спектро- скопије, док је њихова кристална структура одређена применом методе дифракције X-зрака са монокристала. Резултати спектроскопских и кристалографских испитивања показују да је у овим комплексима, 4,7-phen мостни лиганд између два јона метала, док се за преостала два координациона места координује атом кисеоника из CF3SO3 - и атом азота из ацетонитрила у комплексу 1, односно атоми кисеоника из два PO2F2 - , добијена хидролизом PF6 - , у комплексу 2. У чврстом стању, комплекси сребра(I) су координа- циони полимери, у којима је геометрија сребро(I) јона дисторгована тетраедарска. (Примљено 26. фебруара, ревидирано 8. марта, прихваћено 18. марта 2019) 698 STANOJEVIĆ et al. REFERENCES 1. A. N. Khlobystov, A. J. Blake, N. R. Champness, D. A. Lemenovskii, A. G. Majouga, N. V. Zyk, M. Schröder, Coord. Chem. Rev. 222 (2001) 155 (https://doi.org/10.1016/S0010- 8545(01)00370-8) 2. P. Smoleński, S. W. Jaros, C. Pettinari, G. Lupidi, L. Quassinti, M. Bramucci, L. A. Vitali, D. Petrelli, A. Kochel, A. M. Kirillov, Dalton Trans. 42 (2013) 6572 (https://doi.org/10.1039/C3DT33026E) 3. K. Nomiya, S. 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