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.
Trang 1⃝ T¨UB˙ITAK
doi:10.3906/kim-1303-39
h t t p : / / j o u r n a l s t u b i t a k g o v t r / c h e m /
Research Article
Synthesis, characterization, and luminescence of zinc(II) and cadmium(II) coordination complexes: [Zn(phen)2(CH3COO)](ClO4), [Zn(bpy)2(ClO4)](ClO4),
and [Cd(phen)2(NO3)2]
˙Ibrahim KAN˙I,∗Mehmet KURTC ¸ 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)2(CH3COO)](ClO4) (1), [Zn(bpy)2(ClO4) ](ClO4) (2), and [Cd(phen)2 (NO3)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 4 nitrogen atoms
from 2 phen molecules and 2 oxygen atoms from 1 acetato to form an octahedral configuration In 2, Zn(II) has pentacoordination geometry with chelating 2 bpy and 1 perchlorato ion In 3, phen and NO−3 serve as bidentate ligands coordinating to Cd(II) through their nitrogen and oxygen atoms to form 8 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
1 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 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 A considerable number of transition metal complexes using anionic O-donor ligands such as carboxylic acids21,22 and neutral N-donor ligands such as phenanthroline, bipyridines,23−26
pyrazine,27,28 and triazines29 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 metal-organic supramolecular architectures In our attempt to design and synthesize d10 metal-organic architectures
∗Correspondence: ibrahimkani@anadolu.edu.tr
Trang 2with these ligands, we have used Cd(II) and Zn(II) metals As a continuation of our studies, we herein report the crystal structure of 3 metal-organic coordination complexes, namely [Zn(phen)2(CH3COO)](ClO4) , [Zn(bpy)2(ClO4) ](ClO4) , and [Cd(phen)2(NO3)2], which are structurally characterized by single-crystal X-ray diffraction analyses and their photoluminescence properties are also investigated
2 Experimental
2.1 Materials and methods
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 K α 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 1 The structure solution was found with the SHELXS-9730 package using direct methods and was refined SHELXL-97
against F2 using first isotropic and later anisotropic thermal parameters for all nonhydrogen atoms Hydrogen atoms were added to the structure model on calculated positions
Table 1 Crystal data and structure refinement of complexes 1, 2, and 3.
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
b = 13.5224(9) = 100.96
c = 21.1220(14) = 90
Volume 2251.7(3) Å 3
Z, Calculated density 4, 1.701 Mg /m 3
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 min Trans 0.71 7 and 0.657
Refinement method Full-matrix least-squares on F 2
Data/restraints/param 5655/0/316
Goodness-of-fit on F 2 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
C28 H Cl N4 O2 Zn 526.15
296(2) K 0.71073 Å Monoclinic, P 21/n
a = 8.2622(7) = 90
b = 19.2389(16) = 95.361(6)
c = 15.7225(15) = 90 2488.2(4) Å 3
4, 1.405 Mg /m 3
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 F 2
6112/0/344 0.895 R1 = 0.0619, wR2 = 0.1425 R1 = 0.1731, wR2 = 0.1982 0.643 and –0.426 e Å–3
C24 H16 Cd N6 O6 596.83
100(2) K 0.71073 Å Monoclinic, C 2/c
a = 11.6827(4) = 90
b = 15.2359(4) = 105.883(2)
c = 13.4202(4) = 90 2297.55(12) Å 3
4, 1.725 Mg /m 3
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 F 2
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
Trang 32.3 Synthesis of [Zn(phen)2(CH3COO)] (ClO4) (1)
Zn(ClO4)26H2O (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 6 h at room temperature and, after solvent evaporation to
2 mL, carefully layered with 6 mL of diethyl ether Suitable crystals of compound 1 for X-ray analysis were
(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)2(ClO4) ](ClO4) (2)
Zn(ClO4)26H2O (100 mg, 0.27 mmol) and bpy ligand (94 mg, 0.60 mmol) were stirred in 20 mL of methanol for 6 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)2(NO3)2] (3)
Cd(NO3)24H2O (100 mg, 0.32 mmol) and phen ligand (117.1 mg, 0.65 mmol) were stirred in 20 mL of methanol for 6 h at room temperature, and, after solvent evaporation to 2 mL, carefully layered with 6 mL of diethyl
ether Suitable crystals of compound 3 for X-ray analysis were obtained in a week The yield was 162 mg (85%,
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
3 Results and discussion
3.1 Synthesis and characterization
The synthesis of 1, 2, and 3 is shown in Scheme Complexes 1 and 2 are cationic with a single perchlorate
as counterion and complex 3 is neutral 1 and 2 crystallize in monoclinic with space group P 21/n and 3
crystallizes in monoclinic with space group C 2/c For all 3 complexes, the crystal data, some selected angles, bond distances, and hydrogen bonds are given in Tables 1–3, respectively
3.1.1 [Zn(phen)2(CH3COO)](ClO4) , (1)
A perspective view of complex 1 with the numbering scheme is shown in Figure 1 The unit structure contains
an isolated [Zn(phen)2(CH3COO)]+ 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–
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
Trang 4Table 2 Bond lengths [˚A] and angles [◦] for complexes 1, 2, and 3.
Bond lengths, [Å] Bond angles, [°]
1 N(1)-Zn(1) 2.097(4) N(2)-Zn(1) 2.148(4) N(3)-Zn(1) 2.140(4) N(4)-Zn(1) 2.127(4) O(1)-Zn(1) 2.148(4) O(2)-Zn(1) 2.285(4) C(12)-N(1)-Zn(1) 128.2(4) C(6)-N(1)-Zn(1) 113.0(3) C(1)-N(2)-C(5) 118.1(5) C(1)-N(2)-Zn(1) 129.7(4) C(5)-N(2)-Zn(1) 111.6(3) C(19)-N(3)-C(18) 117.8(5) C(19)-N(3)-Zn(1) 128.5(4) C(18)-N(3)-Zn(1) 113.3(3) C(13)-N(4)-C(17) 118.6(5) C(13)-N(4)-Zn(1) 128.0(4) C(17)-N(4)-Zn(1) 113.1(3) C(26)-O(1)-Zn(1) 93.3(3) C(26)-O(2)-Zn(1) 87.4(4) N(1)-Zn(1)-N(4) 113.6(17)
Cl(1)-O(5) 1.338(6) Cl(1)-O(7) 1.398(5) Cl(1)-O(4) 1.402(5) Cl(1)-O(6) 1.414(5) C(1)-N(2) 1.331(6) C(1)-C(2) 1.392(8) N(4)-Zn(1)-N(3) 77.92(16) N(1)-Zn(1)-N(2) 78.74(16) N(4)-Zn(1)-N(2) 98.43(16) N(3)-Zn(1)-N(2) 175.03(17) N(1)-Zn(1)-O(1) 145.53(16) N(4)-Zn(1)-O(1) 100.49(16) N(3)-Zn(1)-O(1) 92.24(16) N(2)-Zn(1)-O(1) 91.75(16) N(1)-Zn(1)-O(2) 89.03(16) N(4)-Zn(1)-O(2) 156.56(16) N(3)-Zn(1)-O(2) 92.92(16) N(2)-Zn(1)-O(2) 91.72(15) O(1)-Zn(1)-O(2) 57.91(15) N(1)-Zn(1)-N(3) 99.53(17)
2 Zn(1)-N(3) 2.060(2) Zn(1)-N(1) 2.064(3) Zn(1)-N(2) 2.066(3) C(1)-N(1)-Zn(1) 126.5(2) C(5)-N(1)-Zn(1) 114.3(2) C(10)-N(2)-C(6) 119.0(3) C(10)-N(2)-Zn(1) 126.6(2) C(6)-N(2)-Zn(1) 114.3(2) C(16)-N(4)-C(20) 119.0(3) C(16)-N(4)-Zn(1) 114.4(2) C(20)-N(4)-Zn(1) 126.6(2) Cl(2)-O(9)-Zn(1) 136.51(19) C(11)-N(3)-Zn(1) 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)
3 Cd(1)-O(1) 2.588(3) Cd(1)-N(2) 2.332(2) Cd(1)-N(1) 2.335(2) Cd(1)-O(2) 2.570(2) O(1)-N(3) 1.246(4) N(2)#1-Cd(1)-N(1)#1 164.82(7) N(2)-Cd(1)-N(1) 110.22(6) N(2)-Cd(1)-O(1)#1 76.0(8) N(1)-Cd(1)-O(2)#1 119.51(8) N(2)-Cd(1)-O(1) 117.60(8) N(1)-Cd(1)-O(2) 84.22(8) N(2)-Cd(1)-O(2) 75.50(7) O(2)#1-Cd(1)-O(2) 152.06(8) O(1)#1-Cd(1)-O(2) 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: # 1 – x + 1, y, – z + 1/2
The metal-Nphen 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)
Trang 5A 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 3 The intermolecular hydrogen bonding geometry (˚A, ◦)
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 + 1 (iv) –x + 2,
–y, –z + 1 (v) –x + 2, –y, –z + 1
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
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
Trang 6Figure 1 The molecular structure of 1.
Figure 2 The intermolecular C–H Operchlorate interactions in 1.
for hydrogen of phen (Figure 2) Nonclassical Cbpy–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 (O1) and perchlorate ion (O6) Other nonhydrogen contacts are longer than 3.6 ˚A The rings of the phen 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
Trang 7centroid–centroid distance of 2 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)2(ClO4) ](ClO4) , (2)
The coordination environment of the Zn(II) ion in 2 is shown in Figure 4 Each Zn(II) ion is coordinated to
a oxygen atom (O9) of perchlorate ion and 4 nitrogen atoms of 2 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
2 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)
˚
A) The 5-membered chelating rings of bpy are considerably planar; the N1–C5–C6–N2 and N4–C15–C16–N3 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 3 a) 3D perspective view of 1 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
Trang 8Figure 4 The molecular structure of 2.
Figure 5 The intermolecular C–H Operchlorate interactions in 2.
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 2 showed that π − π stacking is
Trang 9an offset or slipped facial arrangement of the bpy rings: 3.178 ˚A (C6–C10N2), 3.797 ˚A (C6–C10N2), and 3.809
˚
A (C16–C20N4) (Figure 5 (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
is 3.937 ˚A between the bpy ligands of adjacent chains
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 stability for the network The closest H· · ·O distance (C14–H14· · ·O4) is 2.38 ˚A (Figure 5(b)), Table 3)
a 3-dimensional network (Figure 6)
3.1.3 [Cd(phen)2(NO3)2] (3)
The symmetric unit of compound 3 is shown in Figure 7 The structure of 3 is similar to that of 2 previously
reported complexes: one has a slight difference in the coordination mode of nitrate ions42 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 4 N-donors (N1, N2, N3, and N4) from 2 phen and 4 O atoms (O1, O2, O3, and O4) of 2 bidentate nitrato ions The bond lengths of Cd1–N2 (2.332(2)), Cd1–N1 (2.335(2)), Cd1–O1 (2.588(3)), and Cd1–O2 (2.570(2) ˚A) are close to those reported for similar Cd(II) complexes.44−51 The 2 phen ligands are coordinated trans to each other, while
the 2 nitrato ions occupy the other 2 positions in trans fashion.
Figure 6 a) 3D packing diagram of 2. b) C–H π
interactions
Figure 7 The symmetric molecular unit of 3.
Trang 10No 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,
2.34 ˚A), which combine the 1-D chains along the a direction (Figure 8(a)) Moreover, the other O atom of the
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
offset or parallel displaced mode The observed C–H···π interactions are 3.067 ˚A (N3C11C21N3C11C12···H3),
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 d10-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 1 and 2 exhibit strong
fluorescence at room temperature (Figures 10 and 11) To understand the nature of these emission bands, the emission spectra of the 3 compounds and the free organic ligands were compared As shown in Figure 10, the free phen ·H2O 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 1 454 nm (λexc 387 nm), compared with its ligand,
is red-shifted due to metal perturbed intra-ligand π − π * transitions of phen ligands (Figure 10) Complex 3
displays 2 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 1 and 3 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 d10 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 1 is higher than that for 3, which may
be due to fluorescence quenching of the carboxyl group of CH3COO− (a strong electron-withdrawing group).
Complex 2 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 2 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