Organic Frameworks Based on Pyridyl Carboxylate ... - ACS Publications

A Series of Lead(II)-Organic Frameworks Based on Pyridyl Carboxylate Acid N-Oxide Derivatives: Syntheses, Structures, and Luminescent Properties Ya-Hui Zhao,†,§ Hong-Bin Xu,† Yao-Mei Fu,† Kui-Zhan Shao,† Shuang-Yang Yang,† Zhong-Min Su,*,† Xiang-Rong Hao,† Dong-Xia Zhu,† and En-Bo Wang*,‡

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 10 3566–3576

Institute of Functional Material Chemistry, Faculty of Chemistry, Northeast Normal UniVersity, Changchun 130024, People’s Republic of China, Institute of Polyoxometalate Chemistry, Faculty of Chemistry, Northeast Normal UniVersity, Changchun 130024, People’s Republic of China, and, College of Urban and EnVironment Technology, Northeast Normal UniVersity, Changchun 130024, People’s Republic of China ReceiVed December 13, 2007; ReVised Manuscript ReceiVed June 2, 2008

ABSTRACT: Six new lead coordination polymers, namely, [Pb(INO)2] · 3H2O (1), [Pb(INO)2 · NO3 · H2O] (2), [Pb(INO) · Cl] (3), Na1.5[Pb2(INO)5] · 0.5NO3 (4), [Pb3(NNO)4 · (NO3)2 · H2O] (5), and [Pb(NNO)2] (6), have been prepared by the reaction of Pb(NO3)2 with isonicotinic acid N-oxide (HINO), or nicotinic acid N-oxide (HNNO), and characterized by elemental analysis, IR, and singlecrystal X-ray diffraction. Compound 1, consisting of a one-dimensional (1D) infinite chain, is a three-dimensional (3D) supramolecular framework with a 1D rectanglar channel in which free water molecules locate. The structure of 2 consists of 1D chains built by two parallel INO ligands bridging a pair of rhombic-planar [Pb2O2] units, which is further extended into a 2D supramolecular layer via hypervalent interactions and interlayer π-π interactions. Compound 3 consists of a two-dimensional (2D) inorganic layer containing 16-membered rings, which are further linked through µ-INO to generate a unique 3D open framework. In compound 4, the selfassembly based on 2D motifs with side arms leads to the formation of a new type of polythreaded network, which contains 1D channels with guests molecules along the b-axis. In compound 5, NNO ligands in three kinds of coordination modes link to three unique lead centers to generate two kinds of Pb-O chains which are bridged by NNO to give a 2D network. Compound 6 is a 2D layer structure and in the intralayer regions parallel left- and right-handed helical chains exist. In addition, compounds 1, 2, 3, and 6 exhibit strong phosphorescent emissions in the solid state at room temperature. The results of theoretical calculations show that the absorptions of these complexes derive mainly from ligand to ligand charge transfer (LLCT) transitions. Introduction The current interest in the crystal engineering of coordination polymer frameworks is increasing not only because of their intriguing variety of architectures and topologies but also their fascinating potential applications in functional solid materials, ions exchange, catalysis, and the development of optical, electronic, and magnetic devices.1-4 A successful strategy in building such networks is to employ appropriate bridging ligands that can bind metal ions in different modes and provide a possible way to achieve more new materials with beautiful architectures and excellent physical properties.5,6 Organic aromatic polycarboxylate ligands containing multidentate Odonors, such as 1,4-benzenedicarboxylate, 1,3,5-benzenetricarboxylate, and 1,2,4,5-benzenetetracarboxylate, have been extensively employed in the construction of a rich variety of highdimensional structures.7,8 Pyridyl carboxylic acids and their N-oxide derivatives are also especially useful in this regard, but the exploitation of the ligands in the construction of open framework materials is its infancy.9 Pyridyl carboxylic acids containing N- and O-donors bond to metal centers by pyridyl N atoms or carboxylic oxygen atoms through the self-assembly between metal and ligands, while N-oxide of pyridyl carboxylic acid having an oxygen atom in place of the nitrogen donor site should increase the flexibility and enrich coordination modes compared with the original ligand, and at the same time enlarge the separation of the bridging sites. However, studies on such * To whom correspondence should be addressed. Tel: +86-431-85099108. E-mail: (Z.-M. S.) [email protected] and (E.-B.W.) [email protected]. † Institute of Functional Material Chemistry, Faculty of Chemistry. § Institute of Polyoxometalate Chemistry, Faculty of Chemistry. ‡ College of Urban and Environment Technology, Northeast Normal University.

ligand behavior toward coordination polymers are limited, and only a few transition metal-, lanthanide metal-, and main group metal-based coordination polymers incorporating pyridyl carboxylic acid N-oxide have been structurally examined.10,13 On the other hand, the intrinsic features of lead(II), the presence of a 6s2 outer electron configuration, inspire chemists’ extensive interests in coordination chemistry, photophysics, and photochemistry.11 The unique structural features not only lead to interesting topological arrangements, but also play important roles in the luminescence actions of the complexes. However, little attention has been focused on luminescent lead polymers even though there are many interesting structures reported.12 In previous studies, our interests mainly focused on building stable metal-organic frameworks (MOFs) with large pores by controlling the pH of the systems. As a result, we first obtained a series of three-dimensional (3D) MOFs with one-dimensional (1D) channels constructed from isonicotinic acid N-oxide (INO) ligand bridged dinuclear or tetranuclear clusters. The results suggest that the pH value can effectively influence the formation of crystal seeds of lead systems. As a continuation of systematic investigations as well as to deeply understand the inherent correlation of the coordination chemistry, crystallographical characterization, and applications of lead/pyridine carboxylic acid N-oxide hybrid systems,13 in this paper, we apply the same strategy to prepare novel MOFs structures with charming topologies, and further study the effect of different ions, the molar ratio of metal/ligand14-19 and secondary bonding on extending architecture dimensionality.20 Herein, six new compounds, [Pb(INO)2] · 3H2O (1), [Pb(INO)2 · NO3 · H2O] (2), [Pb(INO) · Cl] (3), Na1.5[Pb2(INO)5] · 0.5NO3 (4), [Pb3(NNO)4 · (NO3)2 · H2O] (5), and

10.1021/cg701224h CCC: $40.75  2008 American Chemical Society Published on Web 08/22/2008

Lead(II)-Organic Frameworks

Crystal Growth & Design, Vol. 8, No. 10, 2008 3567 Table 1. Crystal Data and Structure Refinement for 1-6

formula fw space group a [Å] b [Å] c [Å] R [°] β [°] γ [°] volume, [Å3] Z Fcalcd [g · cm-1] µ [mm-1] R1 [I > 2σ(I)]a wR2 (all data)b a

1

2

3

4

5

6

C12H14N2O9Pb 537.39 C2/c 28.601(7) 7.295(4) 18.789(7) 90.00 130.971(7) 90.00 2960(2) 8 2.385 11.453 0.0239 0.0560

C6H6N2O7Pb 425.32 Pbcn 7.1828(6) 8.3690(8) 9.7794(9) 110.925(1) 90.643(1) 112.354(1) 500.42(8) 2 2.823 16.882 0.0291 0.09abftr;b > 99

C6H4ClNO3Pb 380.74 P1j 13.0592(8) 7.1478(4) 17.391(1) 90.00 90.00 90.00 1623.4(2) 8 3.116 21.076 0.0222 0.0525

C60H40N11Na3O33Pb4 2340.76 P1j 9.582(5) 10.467(5) 17.920(9) 74.753(8) 77.643(8) 73.093(8) 1640.6(2) 1 2.369 10.359 0.0391 0.1245

C24H18N6O19Pb3 1316.00 P21/c 20.146(2) 7.2233(6) 21.360 (2) 90.00 99.231(2) 90.00 3068.0(4) 4 2.845 16.521 0.0507 0.1112

C12H8N2O6Pb 483.39 P21/c 11.133(2) 7.2349(1) 15.719(3) 90.00 102.466(3) 90.00 1236.2(4) 4 2.597 13.680 0.0409 0.0984

R1 ) ∑|F0| - |Fc|/∑|F0|. b wR2 ) ∑[w(F02 - Fc2)2]/∑[w(F02)2]1/2.

[Pb(NNO)2] (6) (isonicotinic acid N-oxide (HINO), or nicotinic acid N-oxide (HNNO)), have been successfully isolated and structurally characterized, of which compounds 1, 2, 3, and 6 exhibit phosphorescent properties at room temperature. All crystal structures have been determined by single-crystal X-ray diffraction, and the compounds are also characterized by IR and elemental analyses. Experimental Section Materials and Physical Measurements. HINO and HNNO were prepared from isonicotinic acid and nicotinic acid, respectively. All other chemicals were commercially purchased and used without further purification. Elemental analyses (C, H, and N) were performed on a Perkin-Elmer 2400CHN Elemental analyzer. FT-IR spectra were recorded in the range 400-4000 cm-1 on an Alpha Centaurt FT/IR spectrophotometer as KBr pellets. The emission/excitation spectra were recorded on a Varian Cary Eclipse spectrometer. Thermogravimetric analysis (TGA) experiments were performed from room temperature to 800 °C using a Perkin-Elmer TG-7 analyzer under nitrogen at a heating rate of 10 °C /min. Synthesis of Isonicotinic acid N-Oxide (HINO) and Nicotinic Acid N-Oxide (HNNO). The ligands HINO and HNNO were synthesized according to the method previously reported in the literature,21 and their characterizations are detailed in the Supporting Information. [Pb(INO)2] · 3H2O (1): A mixture of Pb(NO3)2 (0.099 g, 0.3 mmol), HINO (0.084 g, 0.6 mmol), and water (14 mL) was stirred for 30 min in air, and the pH value of solution was about 3.8. The mixture was transferred and sealed into a 23-mL Teflon-lined autoclave, which was heated at 150 °C for 72 h. After the mixture had been allowed to slowly cool to room temperature, yellow blocks of 1 were filtered off, washed with distilled water, and dried at ambient temperature (yield: 80% based on Pb). IR (KBr, cm-1): 3418br, 2362w, 1646w, 1583s, 1545s, 1518w, 1483w, 1379s, 1211s, 1174m, 1135m, 862m, 782w, 680m, 637s, 455w, 421w; elemental analysis calcd (%) for C12H14N2O9Pb (537.39): C 26.80, H 2.60, N 5.21; found: C 27.07, H 2.54, N 5.18. [Pb(INO) · NO3 · H2O] (2): A similar procedure with 1 was followed to prepare 2 except the mass of water (10 mL) was changed, and the pH value of solution was adjusted to about 5.0 using NaOH solid. Slightly yellow blocks of 2 were obtained (yield: 60% based on Pb). IR (KBr, cm-1): 3420br, 2362s, 1706w, 1693w, 1646m, 1615w, 1581m, 1546s, 1517s, 1484w, 1466w, 1384s, 1211m, 1173m, 1134m, 861m, 782w, 679m, 637s, 451m, 421m; elemental analysis calcd (%) for C6H6N2O7Pb (425.32): C 16.93, H 1.41, N 6.58; found: C 16.97, H 1.53, N 6.66. [Pb(INO) · Cl] (3): A similar procedure with 2 was followed to prepare 3 except the pH value of solution was adjusted to about 6.2 using NaOH solid and dilute HCl aqua. Slight yellow blocks of 3 were obtained (yield: 50% based on Pb). IR (KBr, cm-1): 3444br, 2362s, 1705s, 1692m, 1647m, 1580 m, 1537s, 1514s, 1384s, 1207m, 1113m, 862m, 778m, 678w, 639m, 523w, 446w, 423w; elemental analysis calcd (%) for C6H4ClNO3Pb (380.74): C 18.91, H 1.05, N 3.68; found: C 18.97, H 1.13, N 3.73.

Na1.5[Pb2(INO)5] · 0.5NO3 (4): A similar procedure with 2 was followed to prepare 4 except the pH value of solution was adjusted to about 8.0 using NaOH solid. The colorless blocks of 4 were obtained (yield: 45% based on Pb). IR (KBr, cm-1): 3420br, 2362w, 1688w, 1641m, 1579s, 1545s, 1516m, 1478w, 1457w, 1388s, 1213s, 1178m, 1134m, 862m, 678m, 637s, 452w, 420w; elemental analysis calcd (%) for C30H20N5.5Na1.5O16.5Pb2 (2340.76): C 30.76, H 1.71, N 6.58; found: C 30.87, H 1.73, N 6.66. [Pb3(NNO)4 · (NO3)2 · H2O] (5): Complex 5 was prepared in similar procedure with 2 by using HNNO (0.07 g, 0.5 mmol) instead of HINO, and the pH value of solution was adjusted to about 6.5 using NaOH solid. The yellow blocks of 5 were obtained (yield: 40% based on Pb). IR (KBr, cm-1): 3742br, 3443w, 3074w, 1617m, 1587m, 1557s, 1382s, 1287w, 1208m, 937m, 799s, 760m, 667w, 448w; elemental analysis calcd (%) for C24H18N6O19Pb3 (1316.00): C 21.88, H 1.37, N 6.38; found: C 21.95, H 1.29, N 6.43. [Pb(NNO)2] (6): A similar procedure with 5 was followed to prepare 6 except the mass of HNNO (0.084 g, 0.6 mmol) and the pH value of solution (about 5.5) were adjusted. The yellow blocks of 6 were obtained (yield: 60% based on Pb). IR (KBr, cm-1): 3743br, 3422m, 2362w, 1591m, 1551s, 1481w, 1388s, 1309m, 1214m, 1124w, 942w, 806m, 767m, 688w, 666w, 556w, 443w, 421w; elemental analysis calcd (%) for C12H8N2O6Pb (483.39): C 29.79, H 1.65, N 5.79; found: C 29.87, H 1.73, N 5.92. Crystal Structure Determination. Suitable single crystals of compounds 1-6 were carefully selected under an optical microscope and glued to thin glass fibers. Intensity data were collected on a Bruker Apex CCD diffractometer with graphite-monochromated Mo KR radiation (λ ) 0.71069Å) at 293 K. The structures were solved with the direct methods using SHELXS-9722 and refined with full-matrix least-squares techniques using the SHELXL-97 program23 within WINGX.24 Nonhydrogen atoms were refined anisotropically. The hydrogen atoms attached to carbons were generated geometrically; the aqua hydrogen atoms were located from difference Fourier maps and refined with isotropic displacement parameters. Analytical expressions of neutral-atom scattering factors were employed, and anomalous dispersion corrections were incorporated.25 The crystal data and structure refinement of compounds 1-6 are summarized in Table 1. Selected bond lengths and angles of 1-6 are listed in Table S2, Supporting Information. Computational Details for DFT Calculations. All calculations were carried out with the Turbomole program suite.26 The structures of the ground states were optimized using the DFT with the Becke’s Threeparameter hybrid functional (B3LYP) with the def-SV(P) basis set.27 The electron dipole moment has been calculated at the optimized ground states structures. The excited energies have been calculated and were performed with the TDDFT method at the crystal structure.

Results and Discussion Preparation of the Complexes. According to the HSAB principle,28,29 the coordination of the harder base Npy to PbII, a soft acid, is predicted to be unfavorable.29d Previous research on dipole moments of pyridine-1-oxide and substituted pyridine-

3568 Crystal Growth & Design, Vol. 8, No. 10, 2008

1-oxide30 inspired us to change the ligand from a hard base to a soft base ny adjusting the molecular dipole moment. Thus, we performed quantum chemical calculations on the molecular dipole moments of two pyridine carboxylate acids (isonicotinic acid and nicotinic acid) and their N-oxide derivatives (HINO and HNNO) using the Turbomole program suite.27 The result shows that when an oxygen atom is bonded to the nitrogen atom, the molecules dipole moments of two N-oxide derivatives are both increased compared with pyridine carboxylate acid respectively (see Table S1, Supporting Information). We predict that oxygen atom of the N-oxide would be softer and easier to bond with PbII than pyridine nitrogen atom. So, we synthesized HINO and HNNO ligands, and then six lead compounds with HINO and HNNO ligands were prepared. The aim of this study is to construct diverse MOFs by increasing the flexibility of ligands and their coordinating capacities to explore the influence of the change of molecular dipole moment on the ultimate structures. Meanwhile, further investigations on the effects of the pH values of systems on the final frameworks also have been done. Description of Crystal Structures. Single-crystal X-ray analysis reveals the asymmetric unit of 1 contains one PbII center, two INO ligands, and three disordered uncoordinated water molecules. Each PbII center is tetra-coordinated and exhibits a tetrahedral environment supplied by four oxygen atoms from the carboxylate groups and N-oxide moieties of two unique INO anions, as illustrated in Figure 1a. The four oxygen atoms are located on one side of the PbII ion which adopts a hemidirected structural categories and shows the presence of a stereochemically active lone electron pair.31 The Pb-Ocarboxyl bond lengths for the coordinated carboxylate ranging from 2.431(3) to 2.481(3) Å are obviously shorter than the PbON-oxide lengths of 2.621(3) and 2.641(3) Å. The coordination modes of INO in structurally characterized coordination polymers observed up to now are summarized in Scheme 2. In 1, all INO ligands adopting a bidentate-bridged coordination mode (Scheme 2a) through monodentate carboxylate moieties and pyridyl N-oxide groups link to a pair of lead centers to form [Pb2(INO)2] units with separations of Pb · · · Pb of 9.897 Å, which are expanded into a 1D infinite chain along the c-axis through corner-sharing lead polyhedrons (Figure 1b). The 1D chains are further extended into a 3D supramolecular framework through hypervalent interactions20 between PbII ions and O atoms and multipoint hydrogen bonds between water molecules and carboxylate oxygen atoms. Interestingly, when viewed along the b-axis the 3D network contains 1D rectangle channels (Figure 1c) which are filled by three crystallographically unique uncoordinated water molecules. When the pH value of the aqueous solution was adjusted to 5.3, compound 2 consisting of a 1D infinite chain was obtained. The asymmetric unit of 2 contains one PbII center, one INO ligand, one nitrate anion, and one coordinated water molecule. Each PbII center is penta-coordinated and exhibits a distorted squaral-pyramidal environment. Three oxygen atoms (Pb1-O1 ) 2.658(5), Pb1-O1a ) 2.521(5), Pb1-O2 ) 2.430(5) Å) from N-oxide moieties and carboxylate groups of the INO ligand and one oxygen atom (Pb1-O4 ) 2.630(7) Å) of the nitrate anion are ligated to the PbII center in the quasi-plane, with another one oxygen atom belonging to a water molecule (Pb1-O7 ) 2.436(7) Å) located in the axial position, as displayed in Figure 2a. In fact, each Pb atom in this structure with five normal bonds forms two “weak” Pb · · · O bonds (Pb1 · · · O3 ) 2.877(6), Pb1 · · · O5)2.879(6)Å).LongPb-Oweakbonds(2.876(9)-2.910(6) Å) have also been reported in other lead complexes.32

Zhao et al.

Figure 1. (a) ORTEP diagram of 1 with thermal ellipsoids at 50% probability and partial atom numbering scheme. Free water molecules have been omitted for clarity. (b) The packing arrangement of 1D chain containing [Pb2(INO)2] units in 1, viewed along the a-axis. (c) Perspective view of the 3D supramolecular network in 1, highlighting the rectangular channels along the b-axis. Free water molecules are located in the channels.

All INO ligands adopt an effective tridentate bridging coordination mode by µ-O atoms of N-oxide groups and monodentate carboxylate oxygen atoms. Two µ-O atoms (O1) from N-oxide moieties and two PbII atoms form a usual [Pb2O2] unit with separations of Pb · · · Pb of 4.373 Å comparable with


Crystal Growth & Design, Vol. 8, No. 10, 2008 3569

Scheme 1. Structures of the Pyridyl Carboxylic Acid and Pyridyl Carboxylic Acid N-Oxide Ligands in This Work

those observed in the literature,33a and then the unit is expanded through the two parallel INO ligands along the c-axis to an extended 1D double-chain polymer with strong intralayer π-π interactions (face-face distance ca. 3.281(3) Å) between the aromatic groups of two INO ligands. Similar [Pb2O2] structural units have also been observed in those reported structures.33b-g The edge of the Pb2O2 ring in 2 (2.658(5) Å) is shorter in length than those (2.734-3.37 Å) reported in the literature.33 This indicates the strong coordinating ability of the N-oxide with the lead atom. Hypervalent interactions between PbII ions and O atoms, interlayer π-π interactions (3.366(2) Å) between INO ligands of adjacent chains and van der Waals interactions extend 1D chains into a 2D supramolecular layer, as illustrated in Figure 2b. The remarkable structural features of 2 different from those of 1 derived from the coordination mode and the stericorientation of INO ligand. In 2, the N-oxide group bridges two metal atoms in a µ-O mode to form a planar [Pb2O2] unit, consequently, resulting in the formation of the final doublechain structure. Although INO ligands act as bi- or triconnectors and link to several lead centers, compounds 1 and 2 show lower dimensionality than our expectations. The results from compounds 1 and 2 show that when the number of the coordination sites for

Figure 2. (a) ORTEP drawing of 2 with thermal ellipsoids at 50% probability and partial atom numbering scheme. (b) The packing arrangement of compound 2, highlighting the formation of the 2D layer through hypervalent interactions between PbII ions and O atoms and interchains π-π interactions.

the lead centers decreases or when the coordination sites of central ions are located by terminal ligands, lower dimension

Scheme 2. Representations of the Observed Coordination Modes of Isonicotinic Acid N-Oxide (a-f) and of Nicotinic Acid N-Oxide (g-k)


Zhao et al.

Figure 3. (a) ORTEP diagram of 3 with thermal ellipsoids at 50% probability and partial atom numbering scheme. Symmetry mode A: -x + 3/2, -y + 3/2, z + 1/2; B: x + 1/2, -y + 3/2, -z; C: -x + 3/2, y + 1/2, z. (b) View of the 2D inorganic layer with 18-membered O-Pb-Cl rings in 3. (c) Perspective of the structure of 3 in the [100] plane.

framework structures occur. As a result of the presence of a stereochemically active lone electron pair around the PbII ion, in compound 1 each lead center provides four coordination sites for oxygen atoms leading to the formation of 1D chain structure, whereas in compound 2, one NO3- anion and one terminal water molecule are involved in the coordination sphere of lead(II), consequently, leading to the formation of a 1D chain structure. When the pH value of aqueous solution was 6.2, compound 3 with a 3D framework structure was obtained. Compared with 1 and 2, the structure of 3 is significantly different from them because of the different coordination modes of ligands and halogen residing. Compound 3 is a unique 3D open framework containing a two-dimensional inorganic Pb-O-Cl layer. The asymmetric unit of 3 consists of one lead atom, one INO ligand, and one coordinated chlorine anion, as depicted in Figure 3a. Each PbII center is an irregular hepta-coordinated in hemidirected pentagonal-bipyramidal geometry supplied by two oxygen atoms (Pb1-O1 ) 2.517(3), Pb1-O1a ) 2.587(3) Å) from N-oxide moieties of two equivalent INO liangds, three oxygen atoms (Pb1-O2c ) 2.612(3), Pb1-O2b ) 2.729(4), Pb1-O3b ) 2.600(3) Å) from carboxylate groups of two INO ligands, and two chlorine anions (Pb1-Cl1 ) 2.845(2), Pb1-Cl1a ) 2.966(1) Å). The aforementioned Pb-Cl distances are similar to those observed in the literature.34 Each chlorine anion establishes a bridge between Pb centers to generate a Pb-Cl chain along the b-axis, imposing Pb · · · Pb separations of 4.307 Å and Cl · · · Cl separations of 4.356 Å; meanwhile the µ-O bridging of INO ligand also bridges the metal centers and contributes to the formation of the chain. The 1D chain structures are further linked by the chelating-bridging bidendate of

carboxylate groups of the INO ligand (Scheme 2f) into a 2D inorganic layer with 18-membered rings (Figure 3b). Each 18membered ring shows circularity-like conformation and is formed by six PbII centers, four chlorine anions or µ-O atoms, and two bridging carboxylate groups. Each PbII center is possessed by three rings; meanwhile, two neighboring rings share two PbII centers and one ring connects with other six rings through two sharing lead atoms to generate a 2D layer. The adjacent layers are further connected by INO bridges to result in a 3D coordination polymer with offset face-to-face π-π interactions (ca. 3.22 Å) between aromatic rings (Figure 3c). Interestingly, when the hydrothermal reaction of HINO and metal salts was carried out at about pH 8.0, a novel 2D framework of 4 was produced. In 4, the asymmetric unit contains two crystallographically unique PbII centers, five INO ligand molecules, half a nitrate anions, and three halves of sodium cations. Two unique PbII centers assume different coordination geometries. The Pb1 sits in an irregular hepta-coordinated environment (see Figure 4a) composed of four oxygen atoms from N-oxide moieties of three different INO ligands (Pb1-O1 ) 2.751(7), Pb1-O4 ) 2.632(6) Å, Pb1-O4a ) 2.630(6), Pb1-O13d ) 2.758(6) Å) and three carboxylate oxygen donors from two crystallographically equivalent INO ligands (Pb1-O3 ) 2.492(6), Pb1-O8c ) 2.746(7), Pb1-O9c ) 2.643(7) Å); the Pb2 is penta-coordinated in a distorted squaral-pyramidal environment supplied by one N-oxide oxygen atom from INO ligands (Pb2-O7 ) 2.559(6) Å) and four carboxylate oxygen atoms from three crystallographically inequivalent INO ligands (Pb2-O6 ) 2.561(6), Pb2-O12 ) 2.409(6), Pb2-O14 ) 2.643(7), Pb2-O15 ) 2.626(6) Å), as illustrated in Figure 4a.



Figure 4. (a) ORTEP diagram of 4 with thermal ellipsoids at 50% probability and partial atom numbering scheme. Solution molecules have been omitted for clarify. Symmetry mode A: 2 - x, -y, -z; B: x, -1 + y, -1 + z; C: x, -1 + y, -1 + z; d: x, -2 + y, z. (b) View of 1D chain showing the [Pb4(INO)4] unit. (c) view of the mutual polythreading of the 2D sheets in 4. (d) View of the 3D supramolecular network, where the parallelogram channels located guest NO3 f ions are created by stacking of 2D layer along the b-axis.

The Pb-O distances range from 2.409(6) to 2.758(6) Å, in the order of Pb1-Omean(N-oxide) > Pb1-Omean(carboxylate) > Pb2-Omean(N-oxide) ) Pb2-O(carboxylate), and the O-PbO bond angles vary from 48.61(19) to 151.87(19)°. Besides those in compounds 1, 2, and 3, the INO ligands in compound 4 exhibit two additional kinds of new coordination modes with Pb atoms: one acts as a diconnector to link with two Pb2 centers through a chelating carboxylate group and N-oxide O atom, the other being a terminal ligand coordinated with the Pb2 center through a monodentate carboxylate group like a dancing arm, as shown in Scheme 2a,b. Two Pb1 atoms are linked by two equivalent oxygen atoms (O4 and O4a) of N-oxide moieties, forming a planar four-membered ring Pb1-O4-Pb1a-O4a with O-Pb-O bond angles of 61.6(2)° and Pb-O-Pb bond angles of 118.4(2)°. As shown in Figure 4b, each [Pb2O2] unit is bridged to one Pb2 by two parallel INO ligands along the [001]-direction, and then is bridged to

the second Pb2 through another two parallel INO ligands from the opposite direction yielding [Pb4(INO)4] unit. Likewise, two INO ligands from the [010]- and [01j0]-directions connect with a [Pb4(INO)4] unit to give a 1D motif with very large rhombic windows (dimensions 9.99 × 7.53 Å2) along the c-axis, which is then further extended into a 2D layer by bridging INO ligands. The dancing monodentate INO ligands and Pb2 centers of each 2D layer are threaded into a 1D rhombic channel with each square penetrated oppositely by two INO lateral arms that belong to two different layers, as shown in Figure 4c. This results in a novel 3D polythreaded array (2D f 3D), originating from the entanglement of three adjacent polymeric units at a time. Unfortunately, the occurrence of such polythread drastically reduces the total effective pore size. To our knowledge, such examples, polythreaded species involving finite components built up exclusively from 2D motifs, have been rarely reported.35 It is noteworthy that one-dimensional rectangular channels also


Figure 5. Coordination environment of 5 with a partial atom numbering scheme. Symmetry mode A: -x + 1, y + 1/2, -z + 1/2; B: x, y - 1, z.

exist with the dimension of ca. 4.25 × 2.68 Å2 running along the b-axis direction, which are occupied by solvent anions (see Figure 4d). From the structural descriptions above, it can be seen that Pb-HINO or Pb-HNNO system might be sensitive to the pH value of the solution, which derives from the slight change of the pH value dirrectly affecting the coordination modes of the pyridyl carboxylate N-oxide and the coordination geometry of the central metal ion. As demonstrated by a comparison of compounds 3 and 4 with 1 and 2, the coordination mode of the pyridine carboxylate N-oxide exhibits diversity and complexity when the pH value is changed, which results in the change of the final structure from 1D to 2D, to 3D. In addition, the presence of some terminal or shorter bridging ligands, such as nitrate anions, chlorine anions and water molecules, also plays a crucial role in the construction of leadHINO complexes. In 2, the terminal coordinations of water molecules and nitrate anions occupy the coordinated sites of metal ions, and accordingly restrain the extending of the structural dimension. In 3 and 4, it is evident that inorganic ions, such as Cl- in the former and Na+ in the latter, contribute to the complexity and functionality of the stuctures and exert a synergistic influence on the structure determining. By inspection of the structures of compounds 1-4, it is believed that the variety of coordination abilities for INO is superior to those of isonicotinic acid, which is important for the formation of the higher dimensional structure. In compounds 1-4, the N-oxide group can link to two metal centers in µ-O bridging mode compared with that of pyridinyl N atom, which derives from the N-oxide O atom being a softer base than the pyridinyl N atom. The results further confirm that the employment of N-oxide could improve the connectivity and complexity of the polymeric structure. When the position of carboxyl was changed from the 4-position in HINO to the 3-position in HNNO, two 2D polymers, compounds 5 and 6, were obtained. Single-crystal X-ray analysis reveals that compound 5 is a 2D-layer built by two kinds of Pb-O chains and NNO ligands. In the 2D network, each crystallographically asymmetric unit contains three PbII ions, four NNO, one water molecule, and two nitrate anions (see Figure 5). Three independent PbII centers display different coordination geometries: Pb1 and Pb3 exhibit distorted pentagonal-bipyramidal geometries, whereas Pb2 exhibits distorted trigonal-bipyramidal geometry. Each Pb1 center bonds with four oxygen atoms (Pb1-O4 ) 2.55(1), Pb1-O4a ) 2.679(9), Pb1-O7 ) 2.67(1), Pb1-O7a ) 2.71(1) Å) from N-oxide moieties of four NNO ligands, two oxygen atoms (Pb1-O11 ) 2.58(1), Pb1-O12 ) 2.61(1) Å) from carboxylate group of one NNO ligand, and one oxygen atom of nitrate anion

Zhao et al.

(Pb1-O16 ) 2.71(1) Å). The Pb2 center is defined by two N-oxide groups (Pb2-O1 ) 2.57(1), Pb2-O10 ) 2.47(1) Å) from two inequivalent NNO ligands, and three carboxylate oxygen atoms (Pb2-O2 ) 2.50(9), Pb2-O3 ) 2.60(1), Pb2-O6 ) 2.56(1) Å) from two NNO ligands. The Pb3 is coordinated by two N-oxide groups (Pb3-O1 ) 2.52(1), Pb3-O10b ) 2.66(1) Å) from two inequivalent NNO liangds, and three carboxylate oxygen atoms (Pb3-O8 ) 2.66(1), Pb3-O9 ) 2.61(1), Pb3-O2b ) 2.75(1) Å) from two NNO ligands, one oxygen atom of nitrate anion (Pb3-O13 ) 2.58(1) Å), and one oxygen atom from coordinated water molecule (Pb3-O19 ) 2.64(1) Å). Four crystallographically unique NNO ligands in 5 adopt three different types of coordination modes, as shown in Scheme 2. The first NNO ligand in compound 5 is a tetra-connector and bridges four PbII ions. The N-oxide moiety bridges Pb2 and Pb3 (µ-O), whereas the carboxylate group forms three bonds, a chelate to one Pb2 atom and a bridge between the Pb2 and Pb3 atoms (Scheme 2k). The second NNO ligand is triconnected, whose N-oxide moiety bridges two Pb1 through a µ-O atom, and the carboxylate group links one Pb2 atom in monodetate mode (Scheme 2h). Another two NNO ligands adopt the same coordination mode, in which NNO acts as a triconnector and bridges three PbII ions (link to Pb1, Pb1, Pb3, or Pb2, Pb3, Pb1, respectively) through µ-O atom and chelated carboxylate group (Scheme 2j). It is noticed that the interconnection of two Pb1 atoms by the N-oxide moieties of NNO ligands results in the formation of one 1D Pb-O chain (I) along the a-axis with a Pb1 · · · Pb1 separation of 4.028 Å; the interconnection of Pb2, Pb3 and two µ-O atoms results in the formation of the other Pb-O chain (II) with a Pb2 · · · Pb3 separation of 3.979 Å (see Figure 6c). As described in Figure 6b, two adjacent chains II are linked together via two parallel NNO ligands in a tetra-connector fashion to form two parallel Pb-O chains, which are further linked to two chains I through triconnector NNO liangds forming a 2D layer (Figure 6a). The asymmetric unit of 6 contains one PbII center, and two NNO ligands. Each PbII center is penta-coordinated and exhibits a distorted squaral-pyramidal environment. Two oxygen atoms (Pb1-O1 ) 2.658(5), Pb1-O4 ) 2.521(5) Å) from N-oxide moieties and two carboxylate O atoms (Pb1-O5 ) 2.630(7), Pb1-O6b ) 2.430(5) Å) from two equivalent NNO ligands are ligated to the PbII center in the quasi-plane, with another oxygen atom belonging to the third carboxylate group (Pb1-O3a ) 2.436(7) Å) located in the axial position (Figure 7). Different from that of 5, NNO ligands adopt two kinds of new coordination modes in 6 (shown in Scheme 2g-i): one acts as a diconnector to link to two lead centers through monodentate carboxylate oxygen atom and N-oxide, and the other is a triconnector to bridge three PbII ions by bidentate carboxylate oxygen atoms and N-oxide. Two kinds of NNO ligands bridge four lead-oxygen polyhedrons and result in the formation of a 2D layer, as depicted in Figure 8a. The most interesting structural feature of compound 6 is that in the interlayer regions there exist parallel left- and right-handed helical chains, which are formed by NNO ligands bridging lead atoms along the crystallographic 21-axis with the same pitches of 7.235 Å (see Figure 8d). The adjacent helical chains are linked by NNO ligands to generate a 2D layer running parallel to the [100] plane direction, which are alternately arranged in a right- and left-handed sequence, so that the whole structure does not show chirality (Figure 8b,c).



Figure 6. View of the 2D layer containing two kinds of Pb-O chains, (a) in the ab-plane; (b) viewed down the b-axis, showing the array of two kinds; (c) Pb-O chains I and II in 5.

Figure 7. Coordination environment of 6 with thermal ellipsoids at 50% probability and partial atom numbering scheme. Symmetry mode A: x, -y + 3/2, z - 1/2; B: -x, y + 1/2, -z + 3/2.

From the above results, it could be found that the reason why the extended structures of compounds 5 and 6 are considerably different from those of compounds 1-4arises from the difference of the carboxyl position in INO and NNO. In the series of compounds, the coordination geometries and the orientations of metal ions, which are determined by the lone pair of electrons of PbII, have an important influence on the formation of the final structure. Therefore, when the carboxyl of ligand is the 3-position, NNO ligand takes on a V-shape and probably links to two metal ions to form dinuclear units [Pb2(NNO)2], which further extended to a 2D layer (e.g., compounds 5 and 6), where the interpenetration of the structure is prohibited by the stereoconfiguration of ligand and the

Figure 8. View of the two-dimensional network in 6, highlighting the helical chains within the intralayer.

coordination orientation of PbII; the INO ligand, 4-position carboxyl one, is straight line-shaped and decreases the effects of orientation of metal ions, which facilitates the formation of a compound with higher dimension. Thus, for a rationally


Zhao et al.

Figure 9. Solid-state excitation and emission spectra of complexes at room temperature: (a) 1, (b) 2, (c) 3, (d) 6.

designed MOF with an expected structure, it is instructive to investigate the coordination geometries of metal centers and the steric configurations of ligands. In addition, it is well-known that secondary bonds play pivotal roles in the stabilization of molecular solids, which may offer new opportunities for the preparation of novel solids. In compounds 1 and 2, polymer chains are extended into a supramolecular network through hypervalent interactions and hydrogen bonds, which exhibit two novel examples in the structural chemistry of a class of lead compounds based on secondary interactions. Compositional Stability of the New MOFs. Thermogravimetric analysis (TGA) experiments were conducted to explore the compositional stability of 1-6, since it is an important parameter for MOF materials (see Figure S1-S6 in Supporting Information). The TGA curve of 1 indicates that the first major weight loss of the lattice water occurred between 40 and 175 °C by 9.87% weight. This framework was stable up to 315 °C; the framework started to decompose above 315 °C, which is consistent with the removal of organic ligands. The TGA curve for 2 shows two weight-loss stages: the first starts at 40 °C up to 100 °C, giving a weight loss of about 4.67%, corresponding to the loss of aqua ligands (calcd, 4.23%), following by a stage until 302 °C. Complete decomposition of 2 was finished at above 800 °C. The TGA curve of 3 indicates that only one weight loss stage exists in the region of 268-778 °C. The weight loss at this stage is observed to be 42.23%, corresponding to the decomposition of INO ligand and nitrate anions (calculated 41.41%). For 4, a slight decrease in mass about 2.83% at 104-118 °C marks the expulsion of water molecules of crystallization. Elimination of the remaining components in 4

began at 220 °C corresponding to the decomposition of the INO ligand. The TGA curve for 5 shows two weight-loss stages: the first starts at 40 °C up to 100 °C giving a weight loss of about 1.52% corresponding to the loss of aqua ligands (calcd. 1.35%), This framework was stable up to 208 °C; the framework started to decompose above 208 °C, which is consistent with the removal of organic ligands. The TGA curve of 6 indicates that only one weight loss stage exists in the region of about 252-800 °C. The weight loss at this stage is observed to be 55.68%, corresponding to the decomposition of NNO ligand (calculated 53.8%). Photoluminescent Properties. Complexes of heavy metals with s2 electron configuration, which may reduce the radiative lifetime of triplets by increased spin-orbit coupling and promote emission from the triplet state under ambient conditions, have attracted much recent attention.12 The luminescent properties of lead(II) complexes have been extensively investigated;11,36 however, studies concerning room temperature phosphorescence of lead coordination polymers are limited.12c,36 In our work, photoluminescent properties of frameworks 1, 2, 3, and 6 were investigated in the powdered solid state at room temperature. To understand the nature of the emission band, the photoluminescence properties of HINO and HNNO ligands were analyzed. Free ligands HINO and HNNO show weak emission with maxima at 412 and 377 nm (see Figure S7 in the Supporting Information). The solid-state emission spectra of compounds 1, 2, 3, and 6 at room temperature are depicted in Figure 9. It can be observed that they show maximum emission from 511 to 623 nm for these four compounds. Compounds 2 and 6 were found to show a slightly higher energy emission relative to compounds 1 and 3 at room temperature. For compound 4, the



can be derived from relatively simple building blocks under appropriate conditions, as well as the key role that the special electronic structure of Pb(II) can play in the construction of 0-, 1-, 2-, and 3-D framwork structures. Moreover, a subtle change of the pH value of the system can effectively influence the formation of crystal seeds of the family. Significant phosphorescent emissions are observed in several complexes, which may have some potential applications as organic light emitting diode materials. Figure 10. View of frontier orbitals of compound 1.

[Pb2O2] dinuclear units cannot be neglected compared with compound 1,35 the emission of 4 may be assigned partly as derived from a ligand to metal-metal charge transfer (LMMCT) origin, in which a shift in the emission energy to the blue about 100 nm was observed. While for compound 6, the blue shift of emission relative to compounds 1, 2 and 3 should be assigned the changing the ligand from isonicotinic acid N-oxide to nicotinic acid N-oxide, which is also in line with the higher π* orbital energy of nicotinic acid N-oxide than isonicotinic acid N-oxide unit. It is note that the presence of [Pb2Cl2] dinuclear units in compound 3 results in the difference of emission with that of compound 1. In this cluster, charge transfer states and states delocalized over the chlorine-bridged lead ions become important.11h With reference to our experimental results and the previous work on lead(II) systems,12,36 the origin of the emissions of 1, 2, 3, and 6 may be assigned to mainly derive from intraligand (IL) phosphorescent emission, while the presence of a heavy metal lead(II) facilitates intersystem crossing to successfully compete transitions in the triplet state with radiationless deactivations. Electronic Structure Analysis. In order to obtain insight into the photophysical properties of the series of compounds, we performed quantum chemical calculations on a minor system selected, [Pb(INO)2] · 3H2O (1). The calculated energies of the lowest ground states of 1 (376 nm) is similar in energy to that obtained experimentally (peak wavelengths of 365 nm). As depicted in Figure 10, the main contribution of the corresponding maximal absorption corresponds to the promotion of one electron from the highest-occupied orbital (HOMO)-3 to the lowest-unoccupied orbital (LUMO)+1 and (HOMO)-4 to (LUMO)+1. The electron densities of the (HOMO)-3 of groundstate are located largely on the carboxylate moiety of one INO ligand, while the electron densities of the (LUMO)+1 are mainly distributed on the other whole INO ligand, indicating that the lowest transition involves an n(carboxylate group of INO) f π/(INO) LLCT. As supported by theoretical calculations, the dominant absorption bands in the 365-470 nm region in compound 1 were ascribed to INO-to-INO interligand n f π/ transitions (LLCT). In the case of other compounds, though no quantum chemical calculations were provided due to their comparatively large system, the absorption types might be similar to compound 1 because of the same chemical component and similar structural character. The result further verified our supposition about the type of energy transfer, which is derived from the n-π/ transitions of ligands, and metal only acts as a bridge of electron transition and has a little contribution to absorption spectra. Conclusion In summary, we have synthesized a series of lead complexes based on two pyridyl carboxylate N-oxide ligands. The work presented here demonstrates the rich structural chemistry that

Acknowledgment. The work is financially supported by the Program for Changjiang Scholars and Innovative Research Team in University, National Natural Science Foundation of China (No. 20573016) and the Science Foundation for Young Teachers of Northeast Normal University (No. 20080502). Supporting Information Available: An additional plot, tables, experimental details, thermogravimetric analysis, and X-ray data files (CIF) are available free of charge via the Internet at http://pubs.acs.org.

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