The synthesis of metal-organic frameworks has attracted considerable interest due to their potential applications as functional materials in catalysis; 1,2 gas sorption, storage, and separation; 3,4 molecular magnetism5 and recognition; 6 and nonlinear optics. 7 While the synthesis of fascinating self-assembly metal-organic coordination polymers occurs via bridging ligand and also through weak noncovalent interactions, such as π − π stacking or hydrogen bonding, 8−20 the construction of metal-organic compounds mainly depends on the nature of the organic ligands and metal ions.
Turkish Journal of Chemistry http://journals.tubitak.gov.tr/chem/ Research Article Turk J Chem (2013) 37: 993 1006 ă ITAK c TUB ⃝ doi:10.3906/kim-1303-39 Synthesis, characterization, and luminescence of zinc(II) and cadmium(II) coordination complexes: [Zn(phen) (CH COO)](ClO ), [Zn(bpy) (ClO )](ClO ), and [Cd(phen) (NO )2 ] ˙ ˙ ∗ Mehmet KURTC Ibrahim KANI, ¸A Department of Chemistry, Faculty of Sciences, Anadolu University, Eski¸sehir, Turkey Received: 14.03.2013 • Accepted: 22.06.2013 • Published Online: 04.11.2013 • Printed: 29.11.2013 Abstract: Three metal complexes, [Zn(phen) (CH COO)](ClO ) (1), [Zn(bpy) (ClO ) ](ClO ) (2), and [Cd(phen) (NO )2 ] (3) (phen = 1,10-phenanthroline, bpy = 2,2{Abedini, 2005 #222} -bipyridine), were synthesized and their structures were determined by single-crystal X-ray diffraction analyses In 1, Zn(II) is coordinated by nitrogen atoms from phen molecules and oxygen atoms from acetato to form an octahedral configuration In 2, Zn(II) has pentacoordination geometry with chelating bpy and perchlorato ion In 3, phen and NO − serve as bidentate ligands coordinating to Cd(II) through their nitrogen and oxygen atoms to form coordination Three-dimensional frameworks of complexes 1–3 are produced by hydrogen bonding, and π − π and C-H · · ·π interactions Additionally, complexes 1–3 exhibit strong solid state fluorescent emission at room temperature Key words: Coordination complex, cadmium, zinc, 1,10-phenanthroline, photoluminescence, 2,2’-bipyridine Introduction The synthesis of metal-organic frameworks has attracted considerable interest due to their potential applications as functional materials in catalysis; 1,2 gas sorption, storage, and separation; 3,4 molecular magnetism and recognition; and nonlinear optics While the synthesis of fascinating self-assembly metal-organic coordination polymers occurs via bridging ligand and also through weak noncovalent interactions, such as π − π stacking or hydrogen bonding, 8−20 the construction of metal-organic compounds mainly depends on the nature of the organic ligands and metal ions A considerable number of transition metal complexes using anionic O-donor ligands such as carboxylic acids 21,22 and neutral N-donor ligands such as phenanthroline, bipyridines, 23−26 pyrazine, 27,28 and triazines 29 have been reported during the last decade In particular, 1,10-phenanthroline (phen) and 2,2’-bipyridine (bpy) ligands have been widely used to construct supramolecular architectures The chelating ability of these ligands shows the easy formation of mononuclear metal complexes and these complexes could be used as building blocks for the construction of polymeric compounds through weak nonclassical C/N– H · · ·X , C/N–H O, and C–H · · ·π hydrogen bonding In addition, phen and bpy have extended conjugated planar π systems and can be used in model compounds to mimic the noncovalent interactions in biological processes Therefore, both ligands as good candidates have attracted our interest for construction of metalorganic supramolecular architectures In our attempt to design and synthesize d 10 metal-organic architectures ∗ Correspondence: ibrahimkani@anadolu.edu.tr 993 KANI˙ and KURTC ¸ A/Turk J Chem with these ligands, we have used Cd(II) and Zn(II) metals As a continuation of our studies, we herein report the crystal structure of metal-organic coordination complexes, namely [Zn(phen) (CH COO)](ClO ), [Zn(bpy) (ClO )](ClO ) , and [Cd(phen) (NO )2 ], which are structurally characterized by single-crystal X-ray diffraction analyses and their photoluminescence properties are also investigated Experimental 2.1 Materials and methods All reagents were purchased from commercial sources and used as received IR spectra were recorded on a PerkinElmer spectrophotometer within 400–4000 cm −1 using samples prepared as pellets with KBr The emission spectra were recorded on a PerkinElmer LS55 fluorescence spectrophotometer 2.2 X-ray crystallography Diffraction data for the complex were recorded with a Bruker SMART APEX CCD diffractometer equipped with a rotation anode at 296(2) K using graphite monochrometed Mo Kα radiation ( λ = 0.71073 ˚ A) Diffraction data were collected over the full sphere and were corrected for absorption The data reduction was performed with the Bruker SAINT software package For further crystal and data collection details see Table The structure solution was found with the SHELXS-97 30 package using direct methods and was refined SHELXL-97 against F using first isotropic and later anisotropic thermal parameters for all nonhydrogen atoms Hydrogen atoms were added to the structure model on calculated positions Table Crystal data and structure refinement of complexes 1, 2, and Empirical formula C20 H16 Cl2 N4 O8 Zn Formula weight 576.64 Temperature 100(2) K Wavelength 0.71073 Å Crys system, space group Monoclinic, P 21 /n Unit cell dimensions (Å, °) a = 8.0301(5) = 90 = 100.96 b = 13.5224(9) c = 21.1220(14) = 90 Volume 2251.7(3) Å Z, Calculated density 4, 1.701 Mg /m3 Absorption coefficient 1.385 mm –1 F(000) 1168 Crystal size 0.31 0.28 0.24 mm Theta range for data collc 1.80 to 28.53 deg Limiting indices –10 < = h < = 10, –17 < = k < = 17, –28 < = l < = 28 Reflec collected /unique 44,884 /5655 [R(int) = 0.0363] Completeness to theta 98.8% Absorption correction multi-scan Max and Trans 0.71 and 0.657 Refinement method Full-matrix least-squares on F2 Data/restraints/param 5655/0/316 Goodness-of-fit on F 0.806 Final R indices [I>2sigma(I)] R1 = 0.0468, wR2 = 0.1428 R indices (all data) R1 = 0.0678, wR2 = 0.1659 Largest diff peak and hole 1.667 and –0.739 e Å –3 994 C28 H Cl N4 O2 Zn 526.15 296(2) K 0.71073 Å Monoclinic, P 21 /n a = 8.2622(7) = 90 = 95.361(6) b = 19.2389(16) c = 15.7225(15) = 90 2488.2(4) Å 4, 1.405 Mg /m3 1.126 mm –1 1040 0.26 0.19 0.09 mm 1.68 to 28.28 deg –11 < = h < = 10, –25 < = k < = 25, –20 < = l < = 20 23,150/6112 [R(int) = 0.1000] 99.2% multi-scan 0.9055 and 0.7599 Full-matrix least-squares on F2 6112/0/344 0.895 R1 = 0.0619, wR2 = 0.1425 R1 = 0.1731, wR2 = 0.1982 –3 0.643 and –0.426 e Å C24 H16 Cd N6 O6 596.83 100(2) K 0.71073 Å Monoclinic, C 2/c a = 11.6827(4) = 90 = 105.883(2) b = 15.2359(4) c = 13.4202(4) = 90 2297.55(12) Å 4, 1.725 Mg /m3 1.006 mm –1 1192 0.30 0.28 0.24 mm 2.25 to 28.44 deg –14 < = h < = 15, –19 < = k < = 20, –17 < = l < = 17 10,327/2858 [R(int) = 0.0186] 98.3% multi-scan 0.7943 and 0.7523 Full-matrix least-squares on F2 2858/0/168 1.161 R1 = 0.0464, wR2 = 0.1494 R1 = 0.0501, wR2 = 0.1540 1.621 and –0.868 e Å –3 KANI˙ and KURTC ¸ A/Turk J Chem 2.3 Synthesis of [Zn(phen) (CH COO)] (ClO ) (1) Zn(ClO )2 6H O (100 mg, 0.27 mmol) and phen ligand (90.1 mg, 0.50 mmol) with 0.5 mL (1.5 × 10 −3 M) of acetic acid were stirred in 20 mL of methanol for h at room temperature and, after solvent evaporation to mL, carefully layered with mL of diethyl ether Suitable crystals of compound for X-ray analysis were obtained in a week The yield was 93 mg (60% based on Zn) for compound mp = 162 ◦ C IR (KBr) m (cm −1 ): 3430(m), 3060(m), 1719(w), 1624(m), 1604(s), 1587(s), 1554(s), 1518 (s) 1428(s), 1384(vs), 1317(s), 1144(m), 1104(m), 853(m), 727(s), 702(m), 668(w), 645(w) 2.4 Synthesis of [Zn(bpy) (ClO )](ClO ) (2) Zn(ClO )2 6H O (100 mg, 0.27 mmol) and bpy ligand (94 mg, 0.60 mmol) were stirred in 20 mL of methanol for h at room temperature Colorless single crystals were obtained at room temperature by slow evaporation of the filtrate over several days The yield was 112 mg (79%, based on Zn) mp = 170 ◦ C IR (KBr) m (cm −1 ): 3447(m), 1601(m), 1478(m), 1444(m), 1320(w), 1109(vs), 1090(vs), 1044(s), 768(m), 735(m), 637(m), 626(m) 2.5 Synthesis of [Cd(phen) (NO )2 ] (3) Cd(NO )2 4H O (100 mg, 0.32 mmol) and phen ligand (117.1 mg, 0.65 mmol) were stirred in 20 mL of methanol for h at room temperature, and, after solvent evaporation to mL, carefully layered with mL of diethyl ether Suitable crystals of compound for X-ray analysis were obtained in a week The yield was 162 mg (85%, based on Cd) mp = 110 ◦ C IR (KBr) m (cm −1 ): 3435(w), 3063(w), 1759(w), 1740 (w), 1620(w), 1579(s), 1515(s), 1429(s), 1410(s), 1385(s), 1311(s), 1225(s), 1154(m), 1103(w), 1038(w), 851 (s), 823(w), 780(s), 728 (s), 641(s) Caution: zinc, cadmium, and their compounds are toxic and perchlorate salts are potentially explosive Results and discussion 3.1 Synthesis and characterization The synthesis of 1, 2, and is shown in Scheme Complexes and are cationic with a single perchlorate as counterion and complex is neutral and crystallize in monoclinic with space group P 21/n and crystallizes in monoclinic with space group C 2/c For all complexes, the crystal data, some selected angles, bond distances, and hydrogen bonds are given in Tables 1–3, respectively 3.1.1 [Zn(phen) (CH COO)](ClO ), (1) A perspective view of complex with the numbering scheme is shown in Figure The unit structure contains an isolated [Zn(phen) (CH COO)] + cation and a perchlorate counterion to make the charge balance The Zn(I) ion displays a distorted octahedral coordination with the phen ligands in a N,N-bidentate fashion (bite angle of N3–Zn–N4 = 77.9 ◦ (1), N1–Zn–N2 = 78.7 ◦ (1)) and acetato ligand in a O,O-bidentate fashion (O1–Zn– O2 = 57.9 ◦ (1)) The phen ligands (N1–N2–C1–C12 and N3–N4–C13–C24) are planar; the largest deviations from mean planes with torsion angles are 2.70 ◦ for N2–C4–C5–C7 and 1.80 ◦ for N4–C17–C18–C22 atoms The mean plane angle between phen ligands is 59.54 ◦ The phen ligands show no unusual features; the variations in the C–N and C–C lengths (Table 2) are closely parallel in phen and follow the pattern observed in other phenanthroline complexes 31,32 995 KANI˙ and KURTC ¸ A/Turk J Chem Table Bond lengths [˚ A] and angles [ ◦ ] for complexes 1, 2, and Bond lengths, [Å] Bond angles, [°] N(1)-Zn(1) N(2)-Zn(1) N(3)-Zn(1) N(4)-Zn(1) O(1)-Zn(1) O(2)-Zn(1) C(12)-N(1)-Zn(1) C(6)-N(1)-Zn(1) C(1)-N(2)-C(5) C(1)-N(2)-Zn(1) C(5)-N(2)-Zn(1) C(19)-N(3)-C(18) C(19)-N(3)-Zn(1) C(18)-N(3)-Zn(1) C(13)-N(4)-C(17) C(13)-N(4)-Zn(1) C(17)-N(4)-Zn(1) C(26)-O(1)-Zn(1) C(26)-O(2)-Zn(1) N(1)-Zn(1)-N(4) 2.097(4) 2.148(4) 2.140(4) 2.127(4) 2.148(4) 2.285(4) 128.2(4) 113.0(3) 118.1(5) 129.7(4) 111.6(3) 117.8(5) 128.5(4) 113.3(3) 118.6(5) 128.0(4) 113.1(3) 93.3(3) 87.4(4) 113.6(17) Cl(1)-O(5) Cl(1)-O(7) Cl(1)-O(4) Cl(1)-O(6) C(1)-N(2) C(1)-C(2) N(4)-Zn(1)-N(3) N(1)-Zn(1)-N(2) N(4)-Zn(1)-N(2) N(3)-Zn(1)-N(2) N(1)-Zn(1)-O(1) N(4)-Zn(1)-O(1) N(3)-Zn(1)-O(1) N(2)-Zn(1)-O(1) N(1)-Zn(1)-O(2) N(4)-Zn(1)-O(2) N(3)-Zn(1)-O(2) N(2)-Zn(1)-O(2) O(1)-Zn(1)-O(2) N(1)-Zn(1)-N(3) 1.338(6) 1.398(5) 1.402(5) 1.414(5) 1.331(6) 1.392(8) 77.92(16) 78.74(16) 98.43(16) 175.03(17) 145.53(16) 100.49(16) 92.24(16) 91.75(16) 89.03(16) 156.56(16) 92.92(16) 91.72(15) 57.91(15) 99.53(17) Zn(1)-N(3) Zn(1)-N(1) Zn(1)-N(2) C(1)-N(1)-Zn(1) C(5)-N(1)-Zn(1) C(10)-N(2)-C(6) C(10)-N(2)-Zn(1) C(6)-N(2)-Zn(1) C(16)-N(4)-C(20) C(16)-N(4)-Zn(1) C(20)-N(4)-Zn(1) Cl(2)-O(9)-Zn(1) C(11)-N(3)-Zn(1) 2.060(2) 2.064(3) 2.066(3) 126.5(2) 114.3(2) 119.0(3) 126.6(2) 114.3(2) 119.0(3) 114.4(2) 126.6(2) 136.51(19) 126.5(2) Zn(1)-N(4) 2.091(3) Zn(1)-O(9) 2.212(3) N(3)-Zn(1)-N(1) 173.44(11) N(3)-Zn(1)-N 106.70(10) N(1)-Zn(1)-N(2) 79.82(10) N(3)-Zn(1)-N(4) 79.63(10) N(1)-Zn(1)-N(4 99.69(11) N(2)-Zn(1) ) 104.36(11) N(3)-Zn(1)-O( 87.13(11) N(1)-Zn(1)-O(9) 86.66(11) N(2)-Zn(1)151.35(11) N(4)-Zn(1)-O( 02.74(11) C(15)-N(3)-Zn( 113.5(2) Cd(1)-O(1) Cd(1)-N(2) Cd(1)-N(1) Cd(1)-O(2) O(1)-N(3) N(2)#1-Cd(1)-N(1)#1 N(2)-Cd(1)-N(1) N(2)-Cd(1)-O(1)#1 N(1)-Cd(1)-O(2)#1 N(2)-Cd(1)-O(1) N(1)-Cd(1)-O(2) N(2)-Cd(1)-O(2) O(2)#1-Cd(1)-O(2) O(1)#1-Cd(1)-O(2) 2.588(3) 2.332(2) 2.335(2) 2.570(2) 1.246(4) 164.82(7) 110.22(6) 76.0(8) 119.51(8) 117.60(8) 84.22(8) 75.50(7) 152.06(8) 137.72(9) O(3)-N(3) 1.193(3) O(2)-N(3) 1.243(4) C(1)-N(1) 1.325(4) C(11)-N(2) 1.351(3) N(1)#1-Cd(1)-N(1) 71.48(7) O(1)-Cd(1)-N(1) 90.01(8) O(1)-Cd(1)-O(2) 47.64(9) O(1)-Cd(1)-N(2) 117.60(8) N(2)#1-Cd(1)-N(2) 72.32(6) N(1)#1-Cd(1)-O(1) 77.13(8 ) O(1)#1-Cd(1)-O(1) 64.24(9) O(2)#1-Cd(1)-O(1) 137.72(9) N(2)-Cd(1)-O(2)#1 81.98(7) Symmetry transformations used to generate equivalent atoms: # – x + 1, y, – z + 1/2 The metal-N phen bond distances are not equivalent The distances of Zn–N2 (2.148 (4) ˚ A) and Zn–N3 (2.140) (4) ˚ A, which are in trans position, are close to each other Other bond distances of Zn–N1 (2.097) (4) ˚ A and Zn–N4 (2.127) (4) ˚ A, which are in cis position, are shorter than trans bond lengths The coordination polyhedron is highly distorted The distortion is reflected on the cisoidal angles (57.93–78.73 (1) ◦ ) and transoid angles (174.98 ◦ , 145.57 ◦ , and 156.56 ◦ ) The Zn–O and Zn–N bond distances (Table 2) range from 2.127 (4) 996 KANI˙ and KURTC ¸ A/Turk J Chem ˚ A to 2.285 (4) ˚ A and from 2.127 (4) ˚ A to 2.148 (4) ˚ A, respectively, values that are within the range of those observed for other related Zn(II) complexes with oxygen or nitrogen donors (Table 2) 33−38 Table The intermolecular hydrogen bonding geometry (˚ A, ◦ ) D-H…A Complex D-H H…A D…A D-H…A i C3-H3…O1 0.93 2.71 3.471 (6) 139.2 C9-H9…O1i 0.93 2.31 3.175 (6) 154.5 C12-H12…O6 0.93 2.36 3.386 (7) 152.1 C2-H2…O4ii 0.93 2.68 3.425 (6) 137.9 C10-H10…O2 iii 0.93 2.48 3.098 (6) 154.0 C8-H8…O5iv 0.93 2.68 3.525 (3) 152.0 C20-H20…O3 v 0.93 2.63 3.373 (3) 137.4 Symmetry codes: (i) x + 1/2, –y + 1/2, + z + 1/2 (ii) x – 1/2, –y + 1/2, + z + 1/2 (iii) –x + 1, –y, –z + (iv) –x + 2, –y, –z + (v) –x + 2, –y, –z + Complex C7-H7…O2i 0.95 2.47 3.406 (5) 166.3 C10-H10…O7 ii 0.95 2.56 3.259 (8) 130.6 C8-H8…O3iii 0.95 2.60 3.361 (5) 137.5 C14-H14…O4 iv 0.95 2.38 3.458 (4) 172.4 C13-H13…O8 v 0.95 2.41 3.246 (8) 147.1 C3-H3…O9vi 0.95 2.70 3.368 (8) 128.0 Symmetry codes: (i) x – 1, +y, +z (ii) –x + 1/2, +y + 1/2, –z + 1/2 (iii) –x, –y + 1, –z (iv) x + 1/2, –y + 1/2, +z + 1/2 (v) –x + 1/2 + 1, +y + 1/2, –z + 1/2 (vi) –x, –y, –z Complex C1-H1…O2i 0.95 2.34 3.231 (3) 154.9 C3-H3…O3ii 0.95 2.51 3.430 (5) 162.1 C9-H9…O1iii 0.95 2.36 3.208 (4) 148.1 Symmetry codes: (i) –x + 1, –y + 1, –z (ii) x + 1/2, + y – 1/2, + z (iii) x – 1/2, + y + 1/2, + z Scheme No classical hydrogen bonding was found in the crystal structure (Table 3) The host perchlorate anion stabilizes the structure via hydrogen-bond patterns The perchlorate group acts as a 4-hydrogen-bond acceptor 997 KANI˙ and KURTC ¸ A/Turk J Chem Figure The molecular structure of Figure The intermolecular C–H O perchlorate interactions in for hydrogen of phen (Figure 2) Nonclassical C bpy –H· · ·O perchlorate hydrogen bonds (average C–O distance = 3.280 (9) ˚ A) link adjacent columns to form the resulting 3-dimensional network (Figure 3(a)) The closest intermolecular contacts are observed between carbon atoms (C9, C12) of phen and oxygen atoms of acetate ˚ The rings of the phen (O1) and perchlorate ion (O6) Other nonhydrogen contacts are longer than 3.6 A ligands interact in an offset or parallel displaced mode The observed C–H· · ·π interactions are 3.139 ˚ A (C18– ˚ ˚ C21N3· · · H3), 3.750 A (C18–C21N3 · · · H2), and 3.792 A (C13–C17N4 · · · H8) (Figure 3(a)) The shortest 998 KANI˙ and KURTC ¸ A/Turk J Chem centroid–centroid distance of parallel phen ligands is 4.131 ˚ A The existence of these stacking interactions confirms the formation of a 3D molecular network (Figure 3(b)) 3.1.2 [Zn(bpy) (ClO )](ClO ), (2) The coordination environment of the Zn(II) ion in is shown in Figure Each Zn(II) ion is coordinated to a oxygen atom (O9) of perchlorate ion and nitrogen atoms of bpy ligand in a pentacoordinated fashion One uncoordinated perchlorate ion is also present in the lattice to complete the charge balance and extensively involved in hydrogen bonding The Zn(II) center adopts a square pyramidal geometry The basal plane in consists of N1, N3, N4, and O1, while the apical position is occupied with bpy N2 atom The degrees of distortion from ideal square pyramidal geometry are reflected in cisoid (57.9 (1)–78.7(1) ◦ ) and transoid (145.6 (1)–175.0 (1) ◦ In compound 2, each bpy ligand adopts a chelating mode The bite distances are 2.679 (5) and 2.691 (5) ˚ A for N · · · N and 2.150 (5) for O O, while the bite angles are 77.9 (1) (N3Zn1N4), 78.7 (1) (N1Zn1N2), and 57.9 (1) (O1Zn1O2) There is a difference between the bond lengths of Zn1–O9 (2.215 (3) ˚ A) and Zn–N distances (Zn1–N1 = 2.064 (3), Zn1–N2 = 2.066 (3), Zn1–N3 = 2.062 (2), Zn1–N4 = 2.091 (3) ˚) The 5-membered chelating rings of bpy are considerably planar; the N1–C5–C6–N2 and N4–C15–C16–N3 A torsion angles are 1.04 (4) ◦ and 5.1 (3) ◦ , respectively The bpy ligands and perchlorato anion are in cis position and the mean plane angle between bpy ligands is 75.43 ◦ Figure a) 3D perspective view of b) C–H π and π π interactions Experimental investigations showed that electron withdrawing substituents or heteroatoms lead to the strongest π − π interaction by decreasing the π -electron density in the rings and subsequently increasing the 999 KANI˙ and KURTC ¸ A/Turk J Chem Figure The molecular structure of Figure The intermolecular C–H O perchlorate interactions in π -electron repulsion 39 The face-to-face π stacking of aromatic moieties shows increased stability when both partners are electron-poor, whereas electron-donating substituents disfavored a π −π interaction 40,41 Pyridine, bipyridines, phen, and other aromatic nitrogen heterocycles are known as electron-poor ring systems Moreover, a metal that is coordinated to a nitrogen donor heterocycle will further enhance the electron-withdrawing effect through its positive charge Hence, aromatic nitrogen heterocycles should in principle be well suited for π − π interactions because of their low π -electron density In this respect, complex showed that π − π stacking is 1000 KANI˙ and KURTC ¸ A/Turk J Chem ˚ (C6–C10N2), 3.797 ˚ an offset or slipped facial arrangement of the bpy rings: 3.178 A A (C6–C10N2), and 3.809 ˚ A (C16–C20N4) (Figure (a)) The mean plane angle between the rings is 62.86 ◦ Such a parallel-displaced structure has a contribution from π − σ attraction, the more so with increasing offset Then the interaction is probably more of a C–H· · ·π type and driven by the known π − σ attraction 39 The closest face-to-face distance ˚ between the bpy ligands of adjacent chains is 3.937 A In addition, C–H · · · O hydrogen bonds are observed between oxygen atoms of coordinated (O7, O8, O9) and uncoordinated (O2, O3, O4) perchlorate ions with hydrogen atoms of bpy ligands, which brings further ˚ (Figure 5(b)), Table 3) stability for the network The closest H· · · O distance (C14–H14· · · O4) is 2.38 A Both the hydrogen-bonding and π − π stacking interactions connect adjacent polymers and extend them into a 3-dimensional network (Figure 6) 3.1.3 [Cd(phen) (NO )2 ] (3) The symmetric unit of compound is shown in Figure The structure of is similar to that of previously reported complexes: one has a slight difference in the coordination mode of nitrate ions 42 and the other has the same coordination environment with Cd(II) with a different symmetry group 43 The coordination geometry around the Cd(II) center is dodecahedron The 8-coordinate cadmium center is chelated by N-donors (N1, N2, N3, and N4) from phen and O atoms (O1, O2, O3, and O4) of bidentate nitrato ions The bond ˚) are close to lengths of Cd1–N2 (2.332(2)), Cd1–N1 (2.335(2)), Cd1–O1 (2.588(3)), and Cd1–O2 (2.570(2) A those reported for similar Cd(II) complexes 44−51 The phen ligands are coordinated trans to each other, while the nitrato ions occupy the other positions in trans fashion Figure a) 3D packing diagram of interactions b) C–H π Figure The symmetric molecular unit of 1001 KANI˙ and KURTC ¸ A/Turk J Chem No classical hydrogen bonding was found in the crystal structure The oxygen atoms of the coordinated nitrato ion and hydrogen atoms of the phen ligand from adjacent chains form hydrogen bonds (C1–H1· · · O2, ˚), which combine the 1-D chains along the a direction (Figure 8(a)) Moreover, the other O atom of the 2.34 A nitrato ion is engaged in C–H O hydrogen bonds (C9–H9· · · O1, 2.36 ˚ A) with phen C–H from the neighboring chains to form 2D layer along the b direction (Figure 8(b), Table 3) The rings of the phen ligand interact in an ˚ (N3C11C21N3C11C12 · · ·H3), offset or parallel displaced mode The observed C–H· · ·π interactions are 3.067 A 4.06 ˚ A (N1C1–C5 · · · H3), and 4.296 ˚ A (N3C9–C11 · · · H6) (Figure 9(a)) There are also weak aromatic π π stacking interactions between neighboring phen ligands (N1C1–C5· · · N3C8–C11, 4.152 ˚ A) Thus, the C–H · · · O hydrogen bonds along with aromatic π − π interactions further stabilize the crystal packing and extend the 3-D supramolecular framework (Figure 9(b), Table 3) 3.2 Photoluminescence studies Metal-organic frameworks, especially constructed from d 10 -metal centers (Zn(II), Cd(II), Cu(I), Ag(I), Hg(II)) and conjugated organic linkers, are promising candidates for photoactive materials 52−54 π − π stacking is an increasingly noted feature in the structural description of metal-ligand networks with multidentate ligands (i.e pyridine groups or nitrogen hetero-cycles) Moreover, it is not just a structural phenomenon but is also correlated with the solid-state luminescence properties of some metal complexes Therefore, the emission spectra of Zn(II) and Cd(II) complexes were measured in the solid state at room temperature and exhibit strong fluorescence at room temperature (Figures 10 and 11) To understand the nature of these emission bands, the emission spectra of the compounds and the free organic ligands were compared As shown in Figure 10, the free phen ·H O displays fluorescent emission bands at 361, 381, and 416 nm (λ exc 284 nm), which are attributable to the π −π * transition The emission band of 454 nm ( λ exc 387 nm), compared with its ligand, is red-shifted due to metal perturbed intra-ligand π − π * transitions of phen ligands (Figure 10) Complex displays relatively weak luminescences with respect to phen ligand at 366 and 385 nm (ex 284 nm), which may be attributed to intra-ligand emission from the phen (Figure 10) The emissions for and are neither LMCT (ligand-to-metal charge transfer) nor MLCT (metal-to-ligand charge transfer) in nature, 55−57 since Zn(II) and Cd(II) ions have d 10 configuration and so are difficult to oxidize or reduce 58−−60 Although the same phen ligand coordinated around the metal center, the fluorescence efficiency for is higher than that for 3, which may be due to fluorescence quenching of the carboxyl group of CH COO − (a strong electron-withdrawing group) Complex displays blue-shift fluorescence with the maximum emission observed around 432 nm upon excitation at 383 nm and stronger intensity compared with the bpy ligand under similar experimental conditions (Figure 11) The free bpy molecule displays a weak luminescence at ca 510 nm (ex 284 nm) in solid state at room temperature 51 It is hard to propose a correct mechanistic conclusion for their luminescence based only on emission spectra According to the literature and by considering the fact that the molecular orbital calculations suggested the photoemission of previously reported Zn(II) metal complexes to be mainly π − π * transitions, the emission band of may be tentatively assigned to ligand-centered π − π * fluorescent emission as the chelation of the ligand to metal center increases the rigidity of the ligand, and thus reduces the loss of energy by thermal vibrational decay 51,55,60,61 1002 KANI˙ and KURTC ¸ A/Turk J Chem Figure One- a) and 2-dimensional b) chain of involving C–H O interactions along a and along b direction, respectively Figure a) 3D packing diagram of b) C–H π and π π interactions 1003 KANI˙ and KURTC ¸ A/Turk J Chem Figure 10 Solid-state emission spectra of phen and complex and at room temperature 3.3 IR spectra of the complexes The IR spectra of all complexes show absorption bands resulting from the skeletal vibrations of aromatic rings in the 1400–1615 cm −1 range On monodentate coordination to a metal ion, the perchlorate symmetry is reduced to C3v and the degenerate absorption peak present with ionic perchlorate splits into well-defined bands with maxima between 1200 and 1000 cm −1 when the perchlorate is coordinated in a monodentate fashion 62−64 Complexes and show this feature with well-resolved bands at 1104 and 1144 cm −1 and 1090 and 1109 cm −1 , respectively The IR spectrum of complex 3, the region associated with the NO stretches, shows many bands, and the most intense appear at approximately 1429, 1385, 1311, and 1225 cm −1 , clearly identifying these species as containing coordinate nitrate groups 65,66 In addition, bands measured in the 1800–1600 cm −1 region, assigned to combination bands (bending and asymmetrical vibrations), allowed us to determine the coordination mode 67 of the NO − This complex shows bands at 1759 and 1740 cm −1 (∆ν = 19), consistent with the anion bidentate behavior of the nitrate group The IR spectrum of complex displays the characteristic bands of the acetate anions at 1554 cm −1 for asym(COO) and 1384 cm −1 for sym(COO) The difference between asymmetric and symmetric vibrations is 170 cm −1 , indicating that the acetate group is coordinated to the metal ion in bidentate mode 68 Conclusion In summary, we have synthesized and crystallographically characterized coordination complexes containing Zn(II) and Cd(II) with phen and bpy under mild conditions Complexes 1–3 form 1D or 2D polymeric structures and further extend into higher dimensionality through hydrogen bonding and π − π interactions In addition, these complexes also exhibit strong emission in the solid state at room temperature; they may be good candidates as luminescent material Supplementary material CCDC 854863, 854864, and 854741 contain the supplementary crystallographic data for this paper These data can be obtained free of charge from The Cambridge 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Figure The structure of is similar to that of previously reported complexes: one has a slight difference in the coordination mode of nitrate ions 42 and the other has the same coordination environment... fluorescence at room temperature (Figures 10 and 11) To understand the nature of these emission bands, the emission spectra of the compounds and the free organic ligands were compared As shown in Figure