Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 319–326
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Crystal structures and spectral properties of two polyoxometalate-based inorganic–organic compounds from silver–azine building blocks with bis-bidentate and tridentate ligands Bing An, Rui-Min Zhou, Li Sun, Yan Bai ⇑, Dong-Bin Dang ⇑ Henan Key Laboratory of Polyoxometalate Chemistry, Institute of Molecular and Crystal Engineering, School of Chemistry and Chemical Engineering, Henan University, Kaifeng 475004, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Two novel POM-based compounds
have been synthesized with silver– azines. The compounds have been characterized by IR, UV, XRPD and elemental analysis. X-ray single-crystal structure analyses and discussion for the compounds. The two compounds exhibit fluorescent emission in solid state, respectively.
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 22 November 2013 Received in revised form 15 February 2014 Accepted 21 February 2014 Available online 12 March 2014
Two polyoxometalate-based inorganic–organic hybrid compounds constructed from silver(I)-L species and saturated Keggin type polyoxoanion, [Ag2L12 ]2(SiMo12O40)1.5DMF0.5CH3OHH2O 1 and [{Ag4L22 (DMF)5}(SiMo12O40)] 2 (L1 = phenyl 2-pyridyl ketone azine, L2 = 3-phenyltriazolo[1,5-a]pyridine), have been synthesized and structurally characterized by IR, UV, elemental analysis, XRPD and complete single crystal structure analyses, where the ligands L1 and L2 are bis-bidentate and tridentate azines synthesized with the same materials under different conditions. The structure of 1 exhibits a dinuclear double-helicate with [SiMo12O40]4 anions as counter ions, where all of the Ag centers coordinate to bis-bidentate chelating ligands. Compound 2 displays a two-dimensional sheet formed by the Ag–organic infinite chains and the [SiMo12O40]4 alternately arranged in a ‘‘rail-like’’ fashion. The luminescent properties of 1 and 2 in the solid state were investigated. Ó 2014 Elsevier B.V. All rights reserved.
Keywords: Silver(I) Polyoxometalate Crystal structure
Introduction Inorganic–organic hybrid materials have attracted a lot of attention from chemists worldwide owing to their multiformity
⇑ Corresponding authors. Tel./fax: +86 378 3881589. E-mail addresses: (D.-B. Dang).
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(Y.
http://dx.doi.org/10.1016/j.saa.2014.02.154 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.
Bai),
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in species and value-adding properties such as redox activity, optics, magnetism and catalysis [1–4]. The utilization of functional inorganic clusters as building blocks can likely introduce the physical and/or chemical properties to the target inorganic–organic compounds [5–7]. Polyoxometalates (POMs) are a unique class of inorganic metal–oxygen aggregates with particular interest in the fields of catalysis, surface science, and materials science because their chemical properties including redox potentials,
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acidities, and solubility in various media, can be finely tuned by choosing the appropriate constituent elements and countercations [8–11]. One of the remarkable approaches in the construction of POM-based inorganic–organic hybrid materials is to combine the POMs with transition-metal compounds (TMCs) [12–15]. This strategy is anticipated to obtain new POM-based compounds with a variety of structural motifs and interesting properties, which should lead to new functional materials. The organic ligands play a significant role in forming structures with different dimensionalities and fantastic topologies [16–18]. Azines taking on a well-known class of organic ligands can be divided into several species, and one type derived from the condensation of an pyridyl-based aldehyde or ketone with hydrazine can possibly obtain different polydentate azines which are widely reported in coordination chemistry [19–21]. Symmetrical diazine Schiff base ligands with two pyridylimine sites connected by amine are excellent ligands allowing us to probe systematically the effect of modifications to the ligand backbone through which we are attempting to control the precise topography, or microarchitecture, of the arrays [22–24]. In the solid state, such ligands can be locked in a twist conformation to show a double helix resulting from the constraint of the rotation around ANANA fragments. In this kind of composite, phenyl 2-pyridyl ketone azine (L1) (Scheme 1), which is prepared by mixing hydrazine hydrate with phenyl 2-pyridyl ketone, is selected to construct double helical compounds in which the ligand acts as a bis-bidentate chelate agent employing all four nitrogen atoms to get two five-numbered chelate rings [25,26]. Whereas there are only a few literature on the crystal engineering of POM species with Schiff base ligands [27–29], despite the Schiff base ligands are commonly used in metal–organic coordination compounds. On the other hand, the strong oxidation process leads to the formation of 3-phenyltriazolo[1,5-a] pyridine (L2) caused by oxidative cyclization of the corresponding hydrazone in the synthesis [30,31]. Curiously however, this ring system has been ignored as a ligand in coordination chemistry. There have been only a few reports of compounds with L2 acting as a donor to metal ions and no POM-based inorganic–organic hybrid compound with this ligand was reported [21]. Continuing our studies on the construction of metal–azine species and POM-based compounds [11,32–35], in this paper we report the synthesis and characterization of two interesting polyoxometalate-based inorganic–organic hybrid compounds constructed from AgAL species and saturated Keggin type
[SiMo12O40]4 polyoxoanion, including a new POM-based hybrid compound [Ag2L12 ]2(SiMo12O40)1.5DMF0.5CH3OHH2O 1 with double helical coordination cation units and a 2D crystalline array [{Ag4L22 (DMF)5}(SiMo12O40)] 2, where L1 and L2 is phenyl 2-pyridyl ketone azine and 3-phenyltriazolo[1,5-a]pyridine, respectively. Experimental Materials and physical measurements All the chemicals were of reagent grade quality obtained from commercial sources and used without further purification. H4SiMo12O40xH2O was synthesized by the literature procedures [36]. Ligands L1, phenyl 2-pyridyl ketone azine and L2, 3-(2pyridyl)-triazolo[1,5-a]pyridine were prepared by refluxing di-2pyridylketone and hydrazine hydrate in methanol solution [21]. Elemental analyses (C, H and N) were carried out on a Perkin– Elmer 240C analytical instrument. IR spectra were recorded in KBr pellets with a Nicolet 170 SXFT-IR spectrophotometer in the 4000–400 cm–1 region. X-ray powder diffraction patterns were recorded on a D/max-c A rotating anode X-ray diffractometer with Cu sealed tube (k = 1.54178 Å). The UV–vis spectra were obtained on a Shimazu UV-250 spectrometer in the range of 400–190 nm in aqueous solution, and the luminescent spectra were performed on a Hitachi F-7000 fluorescence spectrophotometer. Preparation of the compound 1 Ligand L1 (0.040 g, 0.1 mmol) and AgNO3 (0.051 g, 0.3 mmol) were stirred in methanol solution (5 mL) for 0.5 h. The resulting solution was slowly layered onto a solution of H4SiMo12O40xH2O (0.250 g, 0.1 mmol) in DMF (4 mL). The solution was left for several days at room temperature and yellow block crystals 1 suitable for X-ray structure determination were obtained in 67% yield (based on H4SiMo12O40xH2O). Anal. calc. for C101H86.5O43N17.5Ag4Mo12Si (%): C, 31.56; H, 2.27; N, 6.38. Found (%): C, 31.63; H, 2.23; N, 6.27. IR (KBr pellet, cm 1): 3450(w), 3064(w), 1651(s), 1558(w), 1490(m), 1436(m), 1385(w), 1329(s), 1252(m), 1101(w), 987(w), 945(vs), 901(vs), 854(s), 802(vs), 789(vs), 701(w), 658(w). Preparation of the compound 2 H4SiMo12O40xH2O (0.250 g, 0.1 mmol) was dissolved in DMF (4 mL) with AgNO3 (0.068 g, 0.4 mmol) and then the solution was stirred for 0.5 h. Ligand L2 (0.013 g, 0.1 mmol) was stirred in methanol solution (5 mL), and the resulting solution was slowly layered onto the DMF solution of AgNO3 and H4SiMo12O40xH2O. The final solution was left for two weeks at room temperature in darkness to give X-ray quality yellow block crystals in 53% yield (based on H4SiMo12O40xH2O). Anal. calc. for C39H53O45N11Ag4Mo12Si (%): C, 15.58; H, 1.78; N, 5.12. Found (%): C, 15.68; H, 1.64; N, 5.07. IR (KBr pellet, cm 1): 2924(w), 1630(s), 1494(w), 1385(vs), 1328(m), 1255 (w), 1159(m), 1080(w), 997(w), 984(w), 954(vs), 901(vs), 851(s), 803(vs), 778(vs), 701(w). Crystallographic studies
Scheme 1. The syntheses of the two ligands.
A suitable sample of size 0.26 mm 0.17 mm 0.15 mm for 1 and 0.20 mm 0.18 mm 0.16 mm for 2 was chosen for the crystallographic study and then mounted on a BRUKER SMART APEX CCD diffractometer with x and u scan mode in the range of 1.21° < h < 25.00° for 1 and 1.79° < h < 25.00° for 2, respectively. All diffraction measurements were performed at room temperature using graphite monochromatized Mo Ka radiation (k = 0.71073 Å). A total of 60329 (21155 independent, Rint = 0.0815)
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reflections for 1 and a total of 11973 (6617 independent, Rint = 0.2328) for 2 were measured. The structures were solved by direct methods and refined by full-matrix least-squares on F2 using SHELXL 97 program [37]. All non-hydrogen atoms were refined anisotropically by full-matrix least-squares techniques and all hydrogen atoms were geometrically fixed to allow riding on the parent atoms to which they are attached. CCDC reference numbers: 967605 (1) and 967606 (2). Space group, lattice parameters and other relevant information are listed in Table 1. The relevant bond lengths and bond angles are listed in Table 2 for 1 and Table 3 for 2.
Table 2 Selected bond distances (Å) and bond angles (°) for compound 1.
Results and discussion Synthesis L1 and L2 were previously prepared by refluxing di-2-pyridylketone and hydrazine hydrate in a methanol solution. If the reaction was carried out in the absence or lack of oxygen, L2 was not obtained (Scheme 1). According to the reports, L1 can decompose to L2 undergoing a copper catalyzed, which was most found in the synthesis of compounds with copper nitrate. By contrast, using silver nitrate replaced copper nitrate, silver nitrate cannot achieve the decomposition. In our previous work of Keggin-POM system, when copper ion acting as metal center, L1 is the main coordination ligand and L2 did not appear in the compounds [34]. Therefore, it can conjecture the decomposition occurs under some conditions. Not in all cases can L1 transfer to L2 with copper catalyzed at least in Keggin-POM-based system. IR and UV spectra The IR spectra of compounds 1 and 2 exhibit the characteristic bands of a-Keggin anion. The band at 901 cm 1 for both 1 and 2 shows the characteristic of stretching frequency of SiAO. The bands at 945, 854 and 789 cm 1 for 1 and 954, 851 and 778 cm 1 for 2 should be attributed to m(MoAOt) (terminal oxygen), m(MoAObAMo) (bridging oxygen) and m(MoAOcAMo) (central oxygen), respectively. Compared with the IR spectrum of a-H4SiMo12O40xH2O, the MoAOcAMo vibrational band is blueshifted from 770 to 789 or 778 cm 1, and the vibrational bands of the MoAOt and MoAObAMo are red-shifted from 957 and Table 1 Crystallographic data and structure refinement parameters for compounds 1 and 2.
Bond length (Å) Si1AO38 Si1AO39 Ag1AN5 Ag1AN6 Ag2AN8 Ag2AN3
1.614(8) 1.620(8) 2.199(12) 2.448(11) 2.196(11) 2.517(11)
Si1AO40 Si1AO37 Ag1AN1 Ag1AN2 Ag2AN4 Ag2AN7
1.621(8) 1.625(8) 2.205(11) 2.626(12) 2.233(11) 2.568(11)
Bond angle (°) O38ASi1AO39 O39ASi1AO40 O39ASi1AO37 N5AAg1AN1 N1AAg1AN6 N1AAg1AN2 N8AAg2AN4 N4AAg2AN3 N4AAg2AN7 N13AAg3AN9 N9AAg3AN10 N9AAg3AN14 N12AAg4AN16 N16AAg4AN15 N16AAg4AN11
109.6(5) 108.5(5) 109.5(5) 162.2(4) 128.0(4) 69.2(4) 162.3(4) 69.7(4) 111.3(4) 164.3(4) 69.6(4) 121.3(4) 161.0(4) 69.6(4) 107.4(4)
O38ASi1AO40 O38ASi1AO37 O40ASi1AO37 N5AAg1AN6 N5AAg1AN2 N6AAg1AN2 N8AAg2AN3 N8AAg2AN7 N3AAg2AN7 N13AAg3AN10 N13AAg3AN14 N10AAg3AN14 N12AAg4AN15 N12AAg4AN11 N15AAg4AN11
110.0(4) 110.3(5) 109.0(4) 69.7(4) 116.2(4) 83.1(4) 127.2(4) 70.0(4) 82.7(4) 125.1(4) 68.9(4) 85.5(4) 125.8(4) 67.5(4) 83.3(4)
855 cm 1 to 945 and 854 cm 1 for 1 and 954 and 851 cm 1 for 2, respectively. The strong peaks at 1651 cm 1 for 1 and 1630 cm 1 for 2, respectively, are attributable to the m(C@N) vibration. The relatively weak absorption bands at around 3064 cm 1 for 1 and 2924 cm 1 for 2 are due to the CAH modes involving the aromatic ring hydrogen atoms. The IR spectrum of 1 gives clear evidence of m(OAH) by the existence of a broad absorption band centered at 3450 cm 1. These results were finally confirmed by X-ray crystallography. As shown in Fig. 1, the UV spectrum of compound 1 in aqueous solution displays two absorption peaks at 205 nm and 308 nm, respectively. The higher energy spectral band can be assigned to the charge transfer transition of Ot ? Mo, whereas the lower energy spectral band can be attributed to the Ob,c ? Mo charge transition. Owing to the bad solubility of compound 2, the UV spectrum was not investigated. Polyoxometalates are commonly sensitive to the pH value of the studied media. The influences of the pH value on the stabilities of 1 have been elaborately probed by means of UV spectra in aqueous solution. Scrutinizing the variable processes adjusted using NaOH (1 mol L 1) solution allows us to ascertain that compound 1 is stable under the pH about 11.3, which basically matches
Crystal data Formula FW Crystal system Space group a (Å) b (Å) c (Å) a (°) b (°) c (°) Volume (Å3) Z Dc (Mg m 3) Crystal size (mm) Radiation (Å) Theta min–max (°) Tot., uniq. Data, R(int) Observed data [I > 2.0 sigma(I)] Nref, Npar R, wR2, S Min. and max. resd. dens. (e/Å3)
C101H86.5Ag4Mo12N17.5O43Si 3844.23 Monoclinic P2(1)/c 22.935(3) 27.216(3) 20.705(2) 90.00 111.524(2) 90.00 12022(2) 4 2.124 0.26 0.17 0.15 Mo Ka 0.71073 2.24 21.77 60,329, 21,155, 0.0815
C39H53Ag4Mo12N11O45Si 3006.77 Triclinic P1 13.14370(10) 13.3759(2) 13.62870(10) 70.6970(10) 70.5930(10) 62.1990(10) 1954.28(4) 1 2.555 0.20 0.18 0.16 Mo Ka 0.71073 2.63 27.08 11,973, 6617, 0.2328
11350
3813
21,155, 1639 0.0778, 0.1930, 1.067 1.336, 3.465
6617, 528 0.0783, 0.1495, 1.048 0.959, 1.176
Table 3 Selected bond distances (Å) and bond angles (°) for compound 2.a.
a b c
Bond length (Å) Si1AO21 Si1AO22 Ag1AN3 Ag2AN2 Ag3AO24 Ag3AO25
1.57(2) 1.682(18) 2.174(14) 2.231(13) 2.511(14) 2.28(3)
Si1AO20 Si1AO19 Ag2AO23 Ag2AAg3 Ag3AO24b Ag3AO25b
1.69(2) 1.741(18) 2.191(13) 3.276(8) 2.390(16) 2.28(2)
Bond angle (°) O21ASi1AO22c O22ASi1AO20 O22ASi1AO19 N3aAAg1AN3 O23AAg2AAg3 O25bAAg3AO24b O25AAg3AO24 O24bAAg3AO24 O25AAg3AAg2 O24bAAg3AAg2
111.7(10) 107.3(9) 108.2(8) 180.000(1) 122.7(4) 92.3(7) 89.3(7) 154.9(3) 133.5(6) 141.2(5)
O21ASi1AO20c O21ASi1AO19c O20ASi1AO19 O23AAg2AN2 O25bAAg3AO25 O25AAg3AO24b O25bAAg3AO24 Ag3bAAg3AAg2 O25bAAg3AAg2 O24AAg3AAg2
113.7(10) 111.2(9) 104.4(10) 167.2(5) 152.9(4) 84.8(7) 82.0(7) 107.9(8) 57.1(7) 52.4(4)
Symmetry code: 1 Symmetry code: 2 Symmetry code: 1
x, x, x,
y, 2 z. 1 y, 2 y, 1 z.
z.
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5 4.1 4.8 5.5 6.6 7.5 8.5 9.3 11.3
Absorbance
4 3 2 1 0 200
250
300
350
400
Wavelength (nm) Fig. 1. The influence of the pH value on the stability of compound 1 in the aqueous solution.
with the pH value on the stability of [SiMo12O40]4 . Therefore, it can indicate that the structure of polyoxoanion is destroyed with high pH value (pH P 11.3) in the POM-based compounds [38]. Description of the crystal structure of compound 1 Single-crystal structural analysis for 1 has unequivocally confirmed that it is constructed from two double-helical architectures [Ag2L12 ]2+, one [SiMo12O40]4 anion, one and a half DMF molecules, one half CH3OH molecule and one H2O molecule (Fig. 2). The polyoxoanion [SiMo12O40]4 is a well-known a-Keggin
structure composed of 12 corner- or edge-sharing MoO6 octahedra with the central silicon ordered and coordinated to four oxygen atoms in a tetrahedral fashion with average SiAO distances of 1.62 Å and OASiAO angles in the range of 108.5(5)–110.3(5)°. These data indicate that the SiO4 tetrahedron of the cluster has a small distorted in the coordination compound. According to the kind of oxygen atoms bonded to the molybdenum atoms, the distances of MoAO bonds are divided in three sets: 1.657(8)–1.697(8) Å for terminal oxygen atoms (MoAOt), 1.795(9)–2.067(9) Å for bridging oxygen atoms (MoAOb) and 2.322(8)–2.385(8) Å for center oxygen atoms (MoAOc). There are two double helices [Ag2L12 ]2+ occurring as a racemic mixture in which the silver centers exhibit DD [Ag(1), Ag(2)] and KK [Ag(3), Ag(4)] absolute configuration, whereby the equivalent fragments are interrelated by the inversion center of the silicon atom (Fig. 3). In other words, there are one P helicate and one M helicate in the current asymmetric unit. For each helical cation, the two silver centers are coordinated by two coupled ligands with Ag Ag separations of 4.94 Å for Ag(1) Ag(2) and 4.85 Å for Ag(3)Ag(4). Each silver ion is coordinated to two imine and two pyridyl nitrogen atoms, forming a distorted seesaw geometry (s = 0.5). There are weak coordination interactions between Ag(3) and O(1 W) of aqueous molecule and Ag(4) and O(36A) of the [SiMo12O40]4 anion with AgO distances of 2.77 Å and 2.85 Å, respectively (Symmetry code A: x, 1 y, z). If considering the weak interactions, both silver atoms exhibit square pyramidal geometries. Owing to the rigidity of L1, the formation of helicates with the close proximity of the two silver centers should have been unfavorable. But the nonplanar bridging mode in double helicates with two pyridylimine units linked by a single ANANA might be essential for
Fig. 2. ORTEP drawing of the coordination environment for Ag(I) atoms in complex 1 with the atomic labeling scheme as 30% probability thermal ellipsoids. The hydrogen atoms and partial solvent molecules are omitted for clarity.
Fig. 3. Polyhedral and space-filling packing representation of the double-helicate unit in 1.
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dihedral angles of two pyridine rings are in the ranges of 77–86°, which are similar to those in related helical compounds [35,25]. Therefore, the twisted ligands can relieve the steric interactions to make helicates steady with low energy. Analysis of the crystal packing of 1 suggests that adjacent [Ag2L12 ]2+ helical building blocks generate a 2D layer through four types of CAH p interactions (Fig. 4). The CAH p interactions are characterized by the C p separations of 3.51 Å for C(17) and [C(73)AC(77)N(13)], 3.59 Å for C(21) and [C(62)AC(67)], 3.64 Å for C(65) and [C(14)AC(19)] and 3.55 Å for C(89) and [C(1)AC(5)N(1)] and the CAH p angles of 135°, 131°, 132° and 138°, respectively. Additionally, the other two CAH p are the interactions of solvents and aromatic rings to demonstrate that solvents also play a vital role both in molecular structure and molecular packing. Otherwise, multiform hydrogen bonds are found in the network to give rise to a 3D supramolecular framework.
Description of the crystal structure of compound 2
Fig. 4. (a) The 2D sheet based on the double-helicate cation [Ag2L12 ]2+ units in 1 showing C H p interactions in dashed linesand (b) a view of the crystal packing in 1 with the 2D layer constructed by two double helical chains.
such kind of ligands to encode metal ions in the conformation of helicates. In the structure of 1, the torsion angles about the central N N bond are 154.3° for C(6)@N(2)AN(3)@C(13), 120.1° for C(30)@N(6) N(7)@C(37), 127.9° for C(54)@N(10)AN(11)@C(61) and 157.0° for C(78)@N(14)AN(15)@C(85), respectively. The
Single-crystal X-ray diffraction analysis revealed that the structure of [{Ag4L22 (DMF)5}(SiMo12O40)] 2 exhibits a new crystalline 2D coordination polymer constructed through the coordination interaction between the saturated Keggin polyoxoanion [SiMo12O40]4 and silver-L2 cations. The asymmetry unit consists of one half [SiMo12O40]4 anion, two Ag(I) centers, one polydentate ligand L2 and 2.5 coordinated DMF molecules (Fig. 5). The polyoxoanion [SiMo12O40]4 is a well-known a-Keggin structure. The center Si atom is located on a center of inversion, which is incompatible with the tetrahedral point symmetry of the Keggin structure. Thus, the polyoxoanion was modeled as disordered over two orientations and the Si atom is surrounded by a cube of eight O atoms, each with half-occupancy. The SiAO distances range from 1.57(2) to 1.741(18) Å, and the OASiAO angles are in the range of 104.4(10)–113.7(10)°. The Mo O bonds can be grouped into three sets, according to the kind of oxygen atoms bound to the molybdenum atoms, MoAOt of 1.634(13)–1.688(10) Å, MoAOb of 1.762(14)–2.048(10) Å and MoAOc of 2.26(2)–2.459(17) Å, respectively. A significant aspect of compound 2 is that, there are three crystallographically independent silver(I) centers and such multi-silver atoms with polytropic coordinating configurations construct an intriguing structure. Ag(1) lies on an inversion center with the occupancy rate of 0.5 and is coordinated by two terminal oxygen
Fig. 5. Molecular structure with atomic numbering of 2. The thermal ellipsoids are drawn at the 30% probability level. All H atoms are omitted for clarity.
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Fig. 6. A view of the 1D structure in 2.
Fig. 7. The 2D layer structure with a rail-like fashion.
Fig. 8. A view of the crystal packing in 2.
Intensity
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325
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375
400
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Wavelength (nm)
(a)
325
As shown in Fig. 6, the neighboring silver centers are linked together by either bis-dentate position of ligands or DMF molecules to yield a 1D coordination polymer chain. The silver-organic polymer chains are further connected to form a ‘‘rail-like’’ 2D layer by the terminal oxo groups of the quadridentate ligand Keggin clusters [SiMo12O40]4 linking four silver ions [Ag(1), Ag(1C), Ag(2) and Ag(2C)] (Symmetry code C: 1 x, y, 1 z). In the 2D layer, the polyanions play the role of middle rails (Fig. 7). Compared with the reported compounds by L2, it acts as either monodentate ligands or bidentate chelate ligands to engender isolated structure, while it is a bis-dentate bridged ligand in compound 2 which makes contribution to dimensionality [21,31,41]. On the other hand, the metal centers of all reported compounds adopt monotonous octahedral geometry. Different from those compounds, compound 2 is crystallized by AgAL2–POM species with multiple coordination geometries of Ag centers to construct a 2D layer. Otherwise, the 2D layers are stacked in a parallel fashion to form a 3D structure through supramolecular interactions to stabilize the structure (Fig. 8).
Intensity
Luminescence properties
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325
350
375
400
425
450
Wavelength (nm)
(b) Fig. 9. Emission spectra of complexes 1 (a) and 2 (b) in the solid state at room temperature.
atoms O(5) and O(5A) of [SiMo12O40]4 anions and two nitrogen atoms N(3) and N(3A) from symmetric L2 molecules, exhibiting a perfect square planar geometry (Symmetry code A: 1 x, y, 2 z). The coordination sphere of Ag(2), defined by three oxygen atoms of one polyoxoanion and two DMF molecules and one nitrogen atom of L2, corresponds to a seesaw environment with the topological parameter s of 0.64 (Symmetry code B: 2 x, 1 y, 2 z) [39]. There is a weak coordination interaction between Ag(2) and O(25B) with a Ag O distance of 2.80 Å. Adjacent Ag centers are bridged by one twisted ligand with a separation of 3.56 Å for Ag(1) Ag(2). Ag(3) is rotationally disordered over two orientations in the refined ratio 0.5: 0.5, which is bound to O(24), O(24B), O(25) and O(25B) of DMF molecules and shows a quadrilateral planar. The average bond distance of AgAO is 2.36 Å. Among them, O(24) is the bridge of Ag(2) and Ag(3). Importantly, a [Ag2] dimer is self-organized by argentophilic metal-metal interaction with the separation of 3.276(8) Å which is shorter than 3.44 Å (the sum of the van der Waals radii of two silver atoms) and only 0.378 Å longer than metallic silver bond (in open-shell the AgAAg bond 2.889 Å and 2.996 Å for closed-shell bond distance). Since a short distance does not necessarily mean that a bond is formed, molecular orbital calculations have been carried out to see what light they might throw on this point [40]. In addition, there is a weak interaction between the silver atom and the O(23) atom of a DMF molecule with the Ag(3) O(23) distance of 2.80 Å. The presence of the weak interaction was important influence the coordination geometry of Ag(3) to be a square pyramidal geometry.
The luminescent properties of some POM-based metal-organic compounds assembled from azines ligands have been investigated previously [11,34,35,42–45]. To enrich the fluorescent database of this type, the luminescent emissions of compounds 1 and 2 were investigated in the solid state, respectively (Fig. 9). Compound 1 shows a mainly emission maximum at 340 nm and three shoulder peaks at 355, 373 and 394 nm (excited wavelength: 220 nm). Compared to those of two structures based on silver-L1 helicate, [Ag2L12 ]6(PMo12O40)45DMF3H2O and [Ag2L12 ](NO3)2, both compound 1 and two reported structures have similar solid-state emission spectra, which should be attributed to the intraligand p–p charge transfer [35,42]. Compound 2 exhibits the emission maximum at 350 nm and shoulder peaks at 307 and 414 nm upon excitation at 230 nm. Therefore, the emission bands of compounds 1 and 2 are comparable to those of the corresponding ligands. These results have been commonly observed in other azines compounds [32,46].
Conclusion In summary, we presented two novel POM-based compounds [Ag2L12 ]2(SiMo12O40)1.5DMF0.5CH3OHH2O 1 and [{Ag4L22 (DMF)5} (SiMo12O40)] 2 displaying luminescent emission at room temperature have been synthesized based on silver(I)-azine species and saturated Keggin type polyoxoanion, where azines are phenyl 2pyridyl ketone azine L1 and 3-phenyltriazolo[1,5-a]pyridine L2. The incorporation of symmetrical or asymmetrical azines (L1, L2) with multidentate nitrogen donors into the POM species leads to silver(I) helicates with the [SiMo12O40]4 anions as counter ions and silver-organic ‘‘railways’’ structures with the [SiMo12O40]4 anions as bridging ligands, respectively. Studies on the assembly of other types of metal-azines systems and polyoxometalates are currently under way.
Acknowledgements This work was supported by the National Natural Science Foundation of China, Innovation Scientists and Technicians Troop Construction Projects of Henan Province, the Natural Science Foundation of Henan Province and the Foundation Co-established by the Province and the Ministry of Henan University.
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Crystal structures and spectral properties of two polyoxometalate-based inorganic–organic compounds from silver–azine building blocks with bis-bidentate and tridentate ligands Bing An, Rui-Min Zhou, Li Sun, Yan Bai ⇑, Dong-Bin Dang ⇑ Henan Key Laboratory of Polyoxometalate Chemistry, Institute of Molecular and Crystal Engineering, School of Chemistry and Chemical Engineering, Henan University, Kaifeng 475004, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Two novel POM-based compounds
have been synthesized with silver– azines. The compounds have been characterized by IR, UV, XRPD and elemental analysis. X-ray single-crystal structure analyses and discussion for the compounds. The two compounds exhibit fluorescent emission in solid state, respectively.
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 22 November 2013 Received in revised form 15 February 2014 Accepted 21 February 2014 Available online 12 March 2014
Two polyoxometalate-based inorganic–organic hybrid compounds constructed from silver(I)-L species and saturated Keggin type polyoxoanion, [Ag2L12 ]2(SiMo12O40)1.5DMF0.5CH3OHH2O 1 and [{Ag4L22 (DMF)5}(SiMo12O40)] 2 (L1 = phenyl 2-pyridyl ketone azine, L2 = 3-phenyltriazolo[1,5-a]pyridine), have been synthesized and structurally characterized by IR, UV, elemental analysis, XRPD and complete single crystal structure analyses, where the ligands L1 and L2 are bis-bidentate and tridentate azines synthesized with the same materials under different conditions. The structure of 1 exhibits a dinuclear double-helicate with [SiMo12O40]4 anions as counter ions, where all of the Ag centers coordinate to bis-bidentate chelating ligands. Compound 2 displays a two-dimensional sheet formed by the Ag–organic infinite chains and the [SiMo12O40]4 alternately arranged in a ‘‘rail-like’’ fashion. The luminescent properties of 1 and 2 in the solid state were investigated. Ó 2014 Elsevier B.V. All rights reserved.
Keywords: Silver(I) Polyoxometalate Crystal structure
Introduction Inorganic–organic hybrid materials have attracted a lot of attention from chemists worldwide owing to their multiformity
⇑ Corresponding authors. Tel./fax: +86 378 3881589. E-mail addresses: (D.-B. Dang).
[email protected]
(Y.
http://dx.doi.org/10.1016/j.saa.2014.02.154 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.
Bai),
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in species and value-adding properties such as redox activity, optics, magnetism and catalysis [1–4]. The utilization of functional inorganic clusters as building blocks can likely introduce the physical and/or chemical properties to the target inorganic–organic compounds [5–7]. Polyoxometalates (POMs) are a unique class of inorganic metal–oxygen aggregates with particular interest in the fields of catalysis, surface science, and materials science because their chemical properties including redox potentials,
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acidities, and solubility in various media, can be finely tuned by choosing the appropriate constituent elements and countercations [8–11]. One of the remarkable approaches in the construction of POM-based inorganic–organic hybrid materials is to combine the POMs with transition-metal compounds (TMCs) [12–15]. This strategy is anticipated to obtain new POM-based compounds with a variety of structural motifs and interesting properties, which should lead to new functional materials. The organic ligands play a significant role in forming structures with different dimensionalities and fantastic topologies [16–18]. Azines taking on a well-known class of organic ligands can be divided into several species, and one type derived from the condensation of an pyridyl-based aldehyde or ketone with hydrazine can possibly obtain different polydentate azines which are widely reported in coordination chemistry [19–21]. Symmetrical diazine Schiff base ligands with two pyridylimine sites connected by amine are excellent ligands allowing us to probe systematically the effect of modifications to the ligand backbone through which we are attempting to control the precise topography, or microarchitecture, of the arrays [22–24]. In the solid state, such ligands can be locked in a twist conformation to show a double helix resulting from the constraint of the rotation around ANANA fragments. In this kind of composite, phenyl 2-pyridyl ketone azine (L1) (Scheme 1), which is prepared by mixing hydrazine hydrate with phenyl 2-pyridyl ketone, is selected to construct double helical compounds in which the ligand acts as a bis-bidentate chelate agent employing all four nitrogen atoms to get two five-numbered chelate rings [25,26]. Whereas there are only a few literature on the crystal engineering of POM species with Schiff base ligands [27–29], despite the Schiff base ligands are commonly used in metal–organic coordination compounds. On the other hand, the strong oxidation process leads to the formation of 3-phenyltriazolo[1,5-a] pyridine (L2) caused by oxidative cyclization of the corresponding hydrazone in the synthesis [30,31]. Curiously however, this ring system has been ignored as a ligand in coordination chemistry. There have been only a few reports of compounds with L2 acting as a donor to metal ions and no POM-based inorganic–organic hybrid compound with this ligand was reported [21]. Continuing our studies on the construction of metal–azine species and POM-based compounds [11,32–35], in this paper we report the synthesis and characterization of two interesting polyoxometalate-based inorganic–organic hybrid compounds constructed from AgAL species and saturated Keggin type
[SiMo12O40]4 polyoxoanion, including a new POM-based hybrid compound [Ag2L12 ]2(SiMo12O40)1.5DMF0.5CH3OHH2O 1 with double helical coordination cation units and a 2D crystalline array [{Ag4L22 (DMF)5}(SiMo12O40)] 2, where L1 and L2 is phenyl 2-pyridyl ketone azine and 3-phenyltriazolo[1,5-a]pyridine, respectively. Experimental Materials and physical measurements All the chemicals were of reagent grade quality obtained from commercial sources and used without further purification. H4SiMo12O40xH2O was synthesized by the literature procedures [36]. Ligands L1, phenyl 2-pyridyl ketone azine and L2, 3-(2pyridyl)-triazolo[1,5-a]pyridine were prepared by refluxing di-2pyridylketone and hydrazine hydrate in methanol solution [21]. Elemental analyses (C, H and N) were carried out on a Perkin– Elmer 240C analytical instrument. IR spectra were recorded in KBr pellets with a Nicolet 170 SXFT-IR spectrophotometer in the 4000–400 cm–1 region. X-ray powder diffraction patterns were recorded on a D/max-c A rotating anode X-ray diffractometer with Cu sealed tube (k = 1.54178 Å). The UV–vis spectra were obtained on a Shimazu UV-250 spectrometer in the range of 400–190 nm in aqueous solution, and the luminescent spectra were performed on a Hitachi F-7000 fluorescence spectrophotometer. Preparation of the compound 1 Ligand L1 (0.040 g, 0.1 mmol) and AgNO3 (0.051 g, 0.3 mmol) were stirred in methanol solution (5 mL) for 0.5 h. The resulting solution was slowly layered onto a solution of H4SiMo12O40xH2O (0.250 g, 0.1 mmol) in DMF (4 mL). The solution was left for several days at room temperature and yellow block crystals 1 suitable for X-ray structure determination were obtained in 67% yield (based on H4SiMo12O40xH2O). Anal. calc. for C101H86.5O43N17.5Ag4Mo12Si (%): C, 31.56; H, 2.27; N, 6.38. Found (%): C, 31.63; H, 2.23; N, 6.27. IR (KBr pellet, cm 1): 3450(w), 3064(w), 1651(s), 1558(w), 1490(m), 1436(m), 1385(w), 1329(s), 1252(m), 1101(w), 987(w), 945(vs), 901(vs), 854(s), 802(vs), 789(vs), 701(w), 658(w). Preparation of the compound 2 H4SiMo12O40xH2O (0.250 g, 0.1 mmol) was dissolved in DMF (4 mL) with AgNO3 (0.068 g, 0.4 mmol) and then the solution was stirred for 0.5 h. Ligand L2 (0.013 g, 0.1 mmol) was stirred in methanol solution (5 mL), and the resulting solution was slowly layered onto the DMF solution of AgNO3 and H4SiMo12O40xH2O. The final solution was left for two weeks at room temperature in darkness to give X-ray quality yellow block crystals in 53% yield (based on H4SiMo12O40xH2O). Anal. calc. for C39H53O45N11Ag4Mo12Si (%): C, 15.58; H, 1.78; N, 5.12. Found (%): C, 15.68; H, 1.64; N, 5.07. IR (KBr pellet, cm 1): 2924(w), 1630(s), 1494(w), 1385(vs), 1328(m), 1255 (w), 1159(m), 1080(w), 997(w), 984(w), 954(vs), 901(vs), 851(s), 803(vs), 778(vs), 701(w). Crystallographic studies
Scheme 1. The syntheses of the two ligands.
A suitable sample of size 0.26 mm 0.17 mm 0.15 mm for 1 and 0.20 mm 0.18 mm 0.16 mm for 2 was chosen for the crystallographic study and then mounted on a BRUKER SMART APEX CCD diffractometer with x and u scan mode in the range of 1.21° < h < 25.00° for 1 and 1.79° < h < 25.00° for 2, respectively. All diffraction measurements were performed at room temperature using graphite monochromatized Mo Ka radiation (k = 0.71073 Å). A total of 60329 (21155 independent, Rint = 0.0815)
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reflections for 1 and a total of 11973 (6617 independent, Rint = 0.2328) for 2 were measured. The structures were solved by direct methods and refined by full-matrix least-squares on F2 using SHELXL 97 program [37]. All non-hydrogen atoms were refined anisotropically by full-matrix least-squares techniques and all hydrogen atoms were geometrically fixed to allow riding on the parent atoms to which they are attached. CCDC reference numbers: 967605 (1) and 967606 (2). Space group, lattice parameters and other relevant information are listed in Table 1. The relevant bond lengths and bond angles are listed in Table 2 for 1 and Table 3 for 2.
Table 2 Selected bond distances (Å) and bond angles (°) for compound 1.
Results and discussion Synthesis L1 and L2 were previously prepared by refluxing di-2-pyridylketone and hydrazine hydrate in a methanol solution. If the reaction was carried out in the absence or lack of oxygen, L2 was not obtained (Scheme 1). According to the reports, L1 can decompose to L2 undergoing a copper catalyzed, which was most found in the synthesis of compounds with copper nitrate. By contrast, using silver nitrate replaced copper nitrate, silver nitrate cannot achieve the decomposition. In our previous work of Keggin-POM system, when copper ion acting as metal center, L1 is the main coordination ligand and L2 did not appear in the compounds [34]. Therefore, it can conjecture the decomposition occurs under some conditions. Not in all cases can L1 transfer to L2 with copper catalyzed at least in Keggin-POM-based system. IR and UV spectra The IR spectra of compounds 1 and 2 exhibit the characteristic bands of a-Keggin anion. The band at 901 cm 1 for both 1 and 2 shows the characteristic of stretching frequency of SiAO. The bands at 945, 854 and 789 cm 1 for 1 and 954, 851 and 778 cm 1 for 2 should be attributed to m(MoAOt) (terminal oxygen), m(MoAObAMo) (bridging oxygen) and m(MoAOcAMo) (central oxygen), respectively. Compared with the IR spectrum of a-H4SiMo12O40xH2O, the MoAOcAMo vibrational band is blueshifted from 770 to 789 or 778 cm 1, and the vibrational bands of the MoAOt and MoAObAMo are red-shifted from 957 and Table 1 Crystallographic data and structure refinement parameters for compounds 1 and 2.
Bond length (Å) Si1AO38 Si1AO39 Ag1AN5 Ag1AN6 Ag2AN8 Ag2AN3
1.614(8) 1.620(8) 2.199(12) 2.448(11) 2.196(11) 2.517(11)
Si1AO40 Si1AO37 Ag1AN1 Ag1AN2 Ag2AN4 Ag2AN7
1.621(8) 1.625(8) 2.205(11) 2.626(12) 2.233(11) 2.568(11)
Bond angle (°) O38ASi1AO39 O39ASi1AO40 O39ASi1AO37 N5AAg1AN1 N1AAg1AN6 N1AAg1AN2 N8AAg2AN4 N4AAg2AN3 N4AAg2AN7 N13AAg3AN9 N9AAg3AN10 N9AAg3AN14 N12AAg4AN16 N16AAg4AN15 N16AAg4AN11
109.6(5) 108.5(5) 109.5(5) 162.2(4) 128.0(4) 69.2(4) 162.3(4) 69.7(4) 111.3(4) 164.3(4) 69.6(4) 121.3(4) 161.0(4) 69.6(4) 107.4(4)
O38ASi1AO40 O38ASi1AO37 O40ASi1AO37 N5AAg1AN6 N5AAg1AN2 N6AAg1AN2 N8AAg2AN3 N8AAg2AN7 N3AAg2AN7 N13AAg3AN10 N13AAg3AN14 N10AAg3AN14 N12AAg4AN15 N12AAg4AN11 N15AAg4AN11
110.0(4) 110.3(5) 109.0(4) 69.7(4) 116.2(4) 83.1(4) 127.2(4) 70.0(4) 82.7(4) 125.1(4) 68.9(4) 85.5(4) 125.8(4) 67.5(4) 83.3(4)
855 cm 1 to 945 and 854 cm 1 for 1 and 954 and 851 cm 1 for 2, respectively. The strong peaks at 1651 cm 1 for 1 and 1630 cm 1 for 2, respectively, are attributable to the m(C@N) vibration. The relatively weak absorption bands at around 3064 cm 1 for 1 and 2924 cm 1 for 2 are due to the CAH modes involving the aromatic ring hydrogen atoms. The IR spectrum of 1 gives clear evidence of m(OAH) by the existence of a broad absorption band centered at 3450 cm 1. These results were finally confirmed by X-ray crystallography. As shown in Fig. 1, the UV spectrum of compound 1 in aqueous solution displays two absorption peaks at 205 nm and 308 nm, respectively. The higher energy spectral band can be assigned to the charge transfer transition of Ot ? Mo, whereas the lower energy spectral band can be attributed to the Ob,c ? Mo charge transition. Owing to the bad solubility of compound 2, the UV spectrum was not investigated. Polyoxometalates are commonly sensitive to the pH value of the studied media. The influences of the pH value on the stabilities of 1 have been elaborately probed by means of UV spectra in aqueous solution. Scrutinizing the variable processes adjusted using NaOH (1 mol L 1) solution allows us to ascertain that compound 1 is stable under the pH about 11.3, which basically matches
Crystal data Formula FW Crystal system Space group a (Å) b (Å) c (Å) a (°) b (°) c (°) Volume (Å3) Z Dc (Mg m 3) Crystal size (mm) Radiation (Å) Theta min–max (°) Tot., uniq. Data, R(int) Observed data [I > 2.0 sigma(I)] Nref, Npar R, wR2, S Min. and max. resd. dens. (e/Å3)
C101H86.5Ag4Mo12N17.5O43Si 3844.23 Monoclinic P2(1)/c 22.935(3) 27.216(3) 20.705(2) 90.00 111.524(2) 90.00 12022(2) 4 2.124 0.26 0.17 0.15 Mo Ka 0.71073 2.24 21.77 60,329, 21,155, 0.0815
C39H53Ag4Mo12N11O45Si 3006.77 Triclinic P1 13.14370(10) 13.3759(2) 13.62870(10) 70.6970(10) 70.5930(10) 62.1990(10) 1954.28(4) 1 2.555 0.20 0.18 0.16 Mo Ka 0.71073 2.63 27.08 11,973, 6617, 0.2328
11350
3813
21,155, 1639 0.0778, 0.1930, 1.067 1.336, 3.465
6617, 528 0.0783, 0.1495, 1.048 0.959, 1.176
Table 3 Selected bond distances (Å) and bond angles (°) for compound 2.a.
a b c
Bond length (Å) Si1AO21 Si1AO22 Ag1AN3 Ag2AN2 Ag3AO24 Ag3AO25
1.57(2) 1.682(18) 2.174(14) 2.231(13) 2.511(14) 2.28(3)
Si1AO20 Si1AO19 Ag2AO23 Ag2AAg3 Ag3AO24b Ag3AO25b
1.69(2) 1.741(18) 2.191(13) 3.276(8) 2.390(16) 2.28(2)
Bond angle (°) O21ASi1AO22c O22ASi1AO20 O22ASi1AO19 N3aAAg1AN3 O23AAg2AAg3 O25bAAg3AO24b O25AAg3AO24 O24bAAg3AO24 O25AAg3AAg2 O24bAAg3AAg2
111.7(10) 107.3(9) 108.2(8) 180.000(1) 122.7(4) 92.3(7) 89.3(7) 154.9(3) 133.5(6) 141.2(5)
O21ASi1AO20c O21ASi1AO19c O20ASi1AO19 O23AAg2AN2 O25bAAg3AO25 O25AAg3AO24b O25bAAg3AO24 Ag3bAAg3AAg2 O25bAAg3AAg2 O24AAg3AAg2
113.7(10) 111.2(9) 104.4(10) 167.2(5) 152.9(4) 84.8(7) 82.0(7) 107.9(8) 57.1(7) 52.4(4)
Symmetry code: 1 Symmetry code: 2 Symmetry code: 1
x, x, x,
y, 2 z. 1 y, 2 y, 1 z.
z.
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5 4.1 4.8 5.5 6.6 7.5 8.5 9.3 11.3
Absorbance
4 3 2 1 0 200
250
300
350
400
Wavelength (nm) Fig. 1. The influence of the pH value on the stability of compound 1 in the aqueous solution.
with the pH value on the stability of [SiMo12O40]4 . Therefore, it can indicate that the structure of polyoxoanion is destroyed with high pH value (pH P 11.3) in the POM-based compounds [38]. Description of the crystal structure of compound 1 Single-crystal structural analysis for 1 has unequivocally confirmed that it is constructed from two double-helical architectures [Ag2L12 ]2+, one [SiMo12O40]4 anion, one and a half DMF molecules, one half CH3OH molecule and one H2O molecule (Fig. 2). The polyoxoanion [SiMo12O40]4 is a well-known a-Keggin
structure composed of 12 corner- or edge-sharing MoO6 octahedra with the central silicon ordered and coordinated to four oxygen atoms in a tetrahedral fashion with average SiAO distances of 1.62 Å and OASiAO angles in the range of 108.5(5)–110.3(5)°. These data indicate that the SiO4 tetrahedron of the cluster has a small distorted in the coordination compound. According to the kind of oxygen atoms bonded to the molybdenum atoms, the distances of MoAO bonds are divided in three sets: 1.657(8)–1.697(8) Å for terminal oxygen atoms (MoAOt), 1.795(9)–2.067(9) Å for bridging oxygen atoms (MoAOb) and 2.322(8)–2.385(8) Å for center oxygen atoms (MoAOc). There are two double helices [Ag2L12 ]2+ occurring as a racemic mixture in which the silver centers exhibit DD [Ag(1), Ag(2)] and KK [Ag(3), Ag(4)] absolute configuration, whereby the equivalent fragments are interrelated by the inversion center of the silicon atom (Fig. 3). In other words, there are one P helicate and one M helicate in the current asymmetric unit. For each helical cation, the two silver centers are coordinated by two coupled ligands with Ag Ag separations of 4.94 Å for Ag(1) Ag(2) and 4.85 Å for Ag(3)Ag(4). Each silver ion is coordinated to two imine and two pyridyl nitrogen atoms, forming a distorted seesaw geometry (s = 0.5). There are weak coordination interactions between Ag(3) and O(1 W) of aqueous molecule and Ag(4) and O(36A) of the [SiMo12O40]4 anion with AgO distances of 2.77 Å and 2.85 Å, respectively (Symmetry code A: x, 1 y, z). If considering the weak interactions, both silver atoms exhibit square pyramidal geometries. Owing to the rigidity of L1, the formation of helicates with the close proximity of the two silver centers should have been unfavorable. But the nonplanar bridging mode in double helicates with two pyridylimine units linked by a single ANANA might be essential for
Fig. 2. ORTEP drawing of the coordination environment for Ag(I) atoms in complex 1 with the atomic labeling scheme as 30% probability thermal ellipsoids. The hydrogen atoms and partial solvent molecules are omitted for clarity.
Fig. 3. Polyhedral and space-filling packing representation of the double-helicate unit in 1.
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dihedral angles of two pyridine rings are in the ranges of 77–86°, which are similar to those in related helical compounds [35,25]. Therefore, the twisted ligands can relieve the steric interactions to make helicates steady with low energy. Analysis of the crystal packing of 1 suggests that adjacent [Ag2L12 ]2+ helical building blocks generate a 2D layer through four types of CAH p interactions (Fig. 4). The CAH p interactions are characterized by the C p separations of 3.51 Å for C(17) and [C(73)AC(77)N(13)], 3.59 Å for C(21) and [C(62)AC(67)], 3.64 Å for C(65) and [C(14)AC(19)] and 3.55 Å for C(89) and [C(1)AC(5)N(1)] and the CAH p angles of 135°, 131°, 132° and 138°, respectively. Additionally, the other two CAH p are the interactions of solvents and aromatic rings to demonstrate that solvents also play a vital role both in molecular structure and molecular packing. Otherwise, multiform hydrogen bonds are found in the network to give rise to a 3D supramolecular framework.
Description of the crystal structure of compound 2
Fig. 4. (a) The 2D sheet based on the double-helicate cation [Ag2L12 ]2+ units in 1 showing C H p interactions in dashed linesand (b) a view of the crystal packing in 1 with the 2D layer constructed by two double helical chains.
such kind of ligands to encode metal ions in the conformation of helicates. In the structure of 1, the torsion angles about the central N N bond are 154.3° for C(6)@N(2)AN(3)@C(13), 120.1° for C(30)@N(6) N(7)@C(37), 127.9° for C(54)@N(10)AN(11)@C(61) and 157.0° for C(78)@N(14)AN(15)@C(85), respectively. The
Single-crystal X-ray diffraction analysis revealed that the structure of [{Ag4L22 (DMF)5}(SiMo12O40)] 2 exhibits a new crystalline 2D coordination polymer constructed through the coordination interaction between the saturated Keggin polyoxoanion [SiMo12O40]4 and silver-L2 cations. The asymmetry unit consists of one half [SiMo12O40]4 anion, two Ag(I) centers, one polydentate ligand L2 and 2.5 coordinated DMF molecules (Fig. 5). The polyoxoanion [SiMo12O40]4 is a well-known a-Keggin structure. The center Si atom is located on a center of inversion, which is incompatible with the tetrahedral point symmetry of the Keggin structure. Thus, the polyoxoanion was modeled as disordered over two orientations and the Si atom is surrounded by a cube of eight O atoms, each with half-occupancy. The SiAO distances range from 1.57(2) to 1.741(18) Å, and the OASiAO angles are in the range of 104.4(10)–113.7(10)°. The Mo O bonds can be grouped into three sets, according to the kind of oxygen atoms bound to the molybdenum atoms, MoAOt of 1.634(13)–1.688(10) Å, MoAOb of 1.762(14)–2.048(10) Å and MoAOc of 2.26(2)–2.459(17) Å, respectively. A significant aspect of compound 2 is that, there are three crystallographically independent silver(I) centers and such multi-silver atoms with polytropic coordinating configurations construct an intriguing structure. Ag(1) lies on an inversion center with the occupancy rate of 0.5 and is coordinated by two terminal oxygen
Fig. 5. Molecular structure with atomic numbering of 2. The thermal ellipsoids are drawn at the 30% probability level. All H atoms are omitted for clarity.
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Fig. 6. A view of the 1D structure in 2.
Fig. 7. The 2D layer structure with a rail-like fashion.
Fig. 8. A view of the crystal packing in 2.
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As shown in Fig. 6, the neighboring silver centers are linked together by either bis-dentate position of ligands or DMF molecules to yield a 1D coordination polymer chain. The silver-organic polymer chains are further connected to form a ‘‘rail-like’’ 2D layer by the terminal oxo groups of the quadridentate ligand Keggin clusters [SiMo12O40]4 linking four silver ions [Ag(1), Ag(1C), Ag(2) and Ag(2C)] (Symmetry code C: 1 x, y, 1 z). In the 2D layer, the polyanions play the role of middle rails (Fig. 7). Compared with the reported compounds by L2, it acts as either monodentate ligands or bidentate chelate ligands to engender isolated structure, while it is a bis-dentate bridged ligand in compound 2 which makes contribution to dimensionality [21,31,41]. On the other hand, the metal centers of all reported compounds adopt monotonous octahedral geometry. Different from those compounds, compound 2 is crystallized by AgAL2–POM species with multiple coordination geometries of Ag centers to construct a 2D layer. Otherwise, the 2D layers are stacked in a parallel fashion to form a 3D structure through supramolecular interactions to stabilize the structure (Fig. 8).
Intensity
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(b) Fig. 9. Emission spectra of complexes 1 (a) and 2 (b) in the solid state at room temperature.
atoms O(5) and O(5A) of [SiMo12O40]4 anions and two nitrogen atoms N(3) and N(3A) from symmetric L2 molecules, exhibiting a perfect square planar geometry (Symmetry code A: 1 x, y, 2 z). The coordination sphere of Ag(2), defined by three oxygen atoms of one polyoxoanion and two DMF molecules and one nitrogen atom of L2, corresponds to a seesaw environment with the topological parameter s of 0.64 (Symmetry code B: 2 x, 1 y, 2 z) [39]. There is a weak coordination interaction between Ag(2) and O(25B) with a Ag O distance of 2.80 Å. Adjacent Ag centers are bridged by one twisted ligand with a separation of 3.56 Å for Ag(1) Ag(2). Ag(3) is rotationally disordered over two orientations in the refined ratio 0.5: 0.5, which is bound to O(24), O(24B), O(25) and O(25B) of DMF molecules and shows a quadrilateral planar. The average bond distance of AgAO is 2.36 Å. Among them, O(24) is the bridge of Ag(2) and Ag(3). Importantly, a [Ag2] dimer is self-organized by argentophilic metal-metal interaction with the separation of 3.276(8) Å which is shorter than 3.44 Å (the sum of the van der Waals radii of two silver atoms) and only 0.378 Å longer than metallic silver bond (in open-shell the AgAAg bond 2.889 Å and 2.996 Å for closed-shell bond distance). Since a short distance does not necessarily mean that a bond is formed, molecular orbital calculations have been carried out to see what light they might throw on this point [40]. In addition, there is a weak interaction between the silver atom and the O(23) atom of a DMF molecule with the Ag(3) O(23) distance of 2.80 Å. The presence of the weak interaction was important influence the coordination geometry of Ag(3) to be a square pyramidal geometry.
The luminescent properties of some POM-based metal-organic compounds assembled from azines ligands have been investigated previously [11,34,35,42–45]. To enrich the fluorescent database of this type, the luminescent emissions of compounds 1 and 2 were investigated in the solid state, respectively (Fig. 9). Compound 1 shows a mainly emission maximum at 340 nm and three shoulder peaks at 355, 373 and 394 nm (excited wavelength: 220 nm). Compared to those of two structures based on silver-L1 helicate, [Ag2L12 ]6(PMo12O40)45DMF3H2O and [Ag2L12 ](NO3)2, both compound 1 and two reported structures have similar solid-state emission spectra, which should be attributed to the intraligand p–p charge transfer [35,42]. Compound 2 exhibits the emission maximum at 350 nm and shoulder peaks at 307 and 414 nm upon excitation at 230 nm. Therefore, the emission bands of compounds 1 and 2 are comparable to those of the corresponding ligands. These results have been commonly observed in other azines compounds [32,46].
Conclusion In summary, we presented two novel POM-based compounds [Ag2L12 ]2(SiMo12O40)1.5DMF0.5CH3OHH2O 1 and [{Ag4L22 (DMF)5} (SiMo12O40)] 2 displaying luminescent emission at room temperature have been synthesized based on silver(I)-azine species and saturated Keggin type polyoxoanion, where azines are phenyl 2pyridyl ketone azine L1 and 3-phenyltriazolo[1,5-a]pyridine L2. The incorporation of symmetrical or asymmetrical azines (L1, L2) with multidentate nitrogen donors into the POM species leads to silver(I) helicates with the [SiMo12O40]4 anions as counter ions and silver-organic ‘‘railways’’ structures with the [SiMo12O40]4 anions as bridging ligands, respectively. Studies on the assembly of other types of metal-azines systems and polyoxometalates are currently under way.
Acknowledgements This work was supported by the National Natural Science Foundation of China, Innovation Scientists and Technicians Troop Construction Projects of Henan Province, the Natural Science Foundation of Henan Province and the Foundation Co-established by the Province and the Ministry of Henan University.
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