Two Series of Isostructural Coordination Polymers ... - ACS Publications

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Crystal Growth & Design

Two

Series

of

Isostructural

Coordination

Polymers

with

Isomeric

Benzenedicarboxylates and Different Azine Based N,N′–donor Ligand: Syntheses, Characterization and Magnetic Properties Biswajit Bhattacharya†, Dilip Kumar Maity†, Rajarshi Mondal†, Enrique Colacio# and Debajyoti Ghoshal†* † Department of Chemistry, Jadavpur University, Jadavpur, Kolkata, 700 032, India #

Departamento de Química Inorgánica, Universidad de Granada, 18071–Granada, Spain

___________________________________________________________________________ ABSTRACT: Seven coordination polymers (CPs) with two types of framework structures, namely, {[M(4–bpdb)(1,3–bdc)].(4–bpdb)0.5}n [M = Mn (1), Fe (2) and Co (3)], and {[M(4– bpdh)(1,4–bdc)]}n (M = Mn (4), Fe (5), Co (6) and Cd (7)] have been synthesized through the slow diffusion technique using 1,3–bdc and 1,4–bdc ligands with two different azine based N,N′–donor linkers [1,3–bdc = benzene–1,3–dicarboxylate, 1,4–bdc = benzene–1,4– dicarboxylate, 4–bpdb = N, N′–bis–pyridin–4–ylmethylene–hydrazine and 4–bpdh = N, N′–bis– (1–pyridin–4–yl–ethylidene)–hydrazine]. Their structures have been determined by single– crystal X–ray diffraction analysis and further characterized by elemental analysis, IR spectra, and powder X–ray diffraction (PXRD) analysis. Compounds 1–3 are isostructural and feature a two– dimensional (2D) framework structure formed by pillaring the one–dimensional [M(1,3–bdc)]n double chains through 4–bpdb linkers. In all cases of compounds 1–3, free 4–bpdb linkers are present in the lattice and lattice 4–bpdb ligands are involved in π–π and C–H⋯π interactions with bridging 1,3–bdc and 4–bpdb ligands, to afford three–dimensional supramolecular structures. Compounds 4–7 are also isostructural and here the bridging of 1,4–bdc with the divalent metal centers forms a two–dimensional (2D) layer of [M(1,4–bdc)]n which is further pillared by 4–bpdh to form a 3D pillared–layer framework. Variable temperature magnetic measurements of two sets of isostructural complexes (1–3 and 4–7) have been carried out. Compounds 1–6 clearly indicate the existence of a weak antiferromagnetic interaction between the metal ions through the bridging 1,3–bdc and 1,4–bdc ligands. The magnetic properties depend strongly on the nature of the metal center. The luminescence spectra of the complexes have been measured which indicates a ligand based emission for all seven compounds. ______________________________________________________________________________

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INTRODUCTION: The prudent design of coordination polymers by connecting the metal ions as single point nodes or secondary building units (SBU) with a variety of organic ligands as linkers, has attracted an immense interest in the past few decades.1–5 This is not only due to their fascinating network structures and novel functionalities but also due to the ease of tuning such structures with the change of linkers and metals. It has been observed in the showcase of coordination polymers (CPs) there are promising functional molecules which are useful in gas storage and sequestration,6–12

ion

exchange,13–20

drug

delivery,21–24

electronics,25–27

catalysis,28–32

magnetism33–38 and sensing39–43; can be achieved by the careful control of their crystalline and periodic structure as well as stability. The designed syntheses of CPs having attractive magnetic properties are immensely important among the ever growing number of functional applications. Such class of compounds took up the interest for the purpose of designing magnetic materials because, magnetic coupling can easily be tuned and controlled by altering the linkers and nodes.44–47 As a result, there are large numbers of CPs exhibiting ferromagnetic,48–51 antiferromagnetic,52–56 metamagnetism,57–60 single chain magnetism,61–65 spin canting,66–69 and spin glass behavior70 have been synthesized and studied. Side by side it is also very important to understand magnetic exchange pathway in order to rationalize the exact design of magnetically functional CPs. The carboxylate containing bridging ligand is one of the very popular choices for the execution of magnetic CPs and thus the coordination architectures having carboxylate– donating linkers and paramagnetic metal ions got the contented space for contemporary curiosity in understanding the magnetic exchange through the OCO bridge because of versatile binding abilities of the carboxylate ligands.71–75 Although a great variety of CPs with diverse frameworks structure have been reported so far, however, till date, the synthesis of desired networks is still a far–reaching challenge in this field due to the fact that the self assembly process is often governed by many parameters, such as the coordination geometry preferred by the metal, chemical structures of the selected ligands, solvent systems, templates, pH values, counteranions, reaction temperature, and the reactant ratio. But a careful synthetic approach and the choice of suitable co–linkers in mixed ligand approach may fabricate desirable CPs with different functional properties. The mixed ligand assembly system has been extensively adopted for the construction of new coordination networks with versatile topologies and fascinating properties. Use of a mixture


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of carboxylic acids and N,N′–donor auxiliary ligands to extend the metal–carboxylate layers to a higher dimensional network can achieve a range of polymeric structures with attractive functional properties.76–80 In mixed ligands CPs, N,N′–donor spacer ligands play crucial role for the generation of overall frameworks, and even small changes can result in a remarkable diversity of both architectures and properties.81–85 In our earlier work, we have used the rigid benzene–1,4–dicarboxylate (1,4–bdc) with two bent azine based N,N′–donor linkers having pyridyl nitrogen at 3–position, 3–bpdb and 3–bpdh [3–bpdb = 1,4–bis–(3–pyridyl)–2,3–diaza– 1,3–butadiene and 3–bpdh = 2,5–bis–(3–pyridyl)–3,4–diaza–2,4–hexadiene] to construct a series of coordination polymers and we obtained different topologies and metal–ligand connectivity in the resultant compounds.86 In order to investigate the effect of position of pyridyl nitrogen on the structure and properties of the synthesized coordination polymers, here we have chosen two rigid linear azine based N,N′–donor linkers having pyridyl nitrogen at 4–position, N, N′–bis–pyridin– 4–ylmethylene–hydrazine (4–bpdb) and N, N′–bis–(1–pyridin–4–yl–ethylidene)–hydrazine (4– bpdh) as one of the components of mixed ligands, to construct coordination polymers along with benzene–1,3–dicarboxylate (1,3–bdc) and benzene–1,4–dicarboxylate (1,4–bdc) (Scheme 1).

Scheme 1. Synthetic scheme of compounds 1−7 Herein, we are reporting the syntheses, structural characterization of complexes 1–7, the magnetic properties of 1–6 and the luminescence properties of 7. These compounds are 3d–



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metal based coordination polymers with two types of framework structures of divalent transition metal ions, having the general formula {[M(4–bpdb)(1,3–bdc)].(4–bpdb)0.5}n [M = Mn (1), Fe (2) and Co (3)], and {[M(4–bpdh)(1,4–bdc)]}n (M = Mn (4), Fe (5), Co (6) and Cd (7)]. Single crystal X–ray structures showed that compounds 1–3 constitute novel two–dimensional (2D) arrangements which are further stitched to form a supramolecular three–dimensional (3D) structure by π–π and C–H···π interactions. Another series of isostructural compounds 4–7 have (3,5)–connected 3D network structure. The variable temperature magnetic properties of complexes 1–6 indicate weak antiferromagnetic interaction between the metal ions through the syn–anti carboxylate–bridging group. EXPERIMENTAL SECTION Materials. N, N′–bis–pyridin–4–ylmethylene–hydrazine (4–bpdb) and N, N′–bis–(1–pyridin–4– yl–ethylidene)–hydrazine (4–bpdh), were synthesized by the procedures reported earlier.87 High purity manganese (II) chloride tetrahydrate, iron (II) perchlorate hydrate, cobalt (II) nitrate hexahydrate, nickel (II) nitrate hexahydrate, cadmium (II) nitrate tetrahydrate, 1,3– benzenedicarboxylic acid (H2–1,3–bdc) and 1,4–benzenedicarboxylic acid (H2–1,4–bdc) were purchased from Sigma–Aldrich Chemical Co. Inc. and used as received. Na2–1,3–bdc or Na2– 1,4–bdc were synthesized by the slow addition of solid NaHCO3 to H2–1,3–bdc or H2–1,4–bdc ligand in water in a 2:1 ratio and was allowed to evaporate until dryness. All other chemicals including solvents were of AR grade and used as received. Physical Measurements. Elemental analyses (carbon, hydrogen, and nitrogen) were performed using a Perkin–Elmer 240C elemental analyzer. Infrared spectra (4000–400 cm−1) were taken on KBr pellet, using Perkin–Elmer Spectrum BX–II IR spectrometer. Powder X–ray diffraction (PXRD) data were collected on a Bruker D8 Discover instrument with Cu–Kα radiation. Thermal analysis (TGA) was carried out on a METTLER TOLEDO TGA 850 thermal analyzer under nitrogen atmosphere (flow rate: 50 cm3 min−1), at a temperature range of 30–500 °C with a heating rate of 2 °C min−1. All the solid state fluorescence measurements were recorded on HORIBA Jobin Yvon (Fluoromax–3) instrument. Variable temperature (2–300K) magnetic susceptibility measurements on polycrystalline samples were carried out with a Quantum Design SQUID MPMS XL–5 device operating at different magnetic fields. The experimental susceptibilities were corrected for the diamagnetism of the constituent atoms by using Pascal’s tables.


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SYNTHESIS {[Mn(4–bpdb)(1,3–bdc)].(4–bpdb)0.5}n (1). A methanolic solution (20 mL) of N, N′–bis– pyridin–4–ylmethylene–hydrazine (4–bpdb) (1 mmol, 0.210 g) was mixed with an aqueous solution (20 mL) of disodium–1,3–benzenedicarboxylate (Na2–1,3–bdc) (1 mmol, 0.210 g) and the resulting solution was stirred for 15 min to mix well. MnCl2·4H2O (1 mmol, 0.198 g) was dissolved in 20 mL water in a separate beaker. 6 mL of this mixed ligand solution was slowly and carefully layered above 3 ml of metal solution using 2 mL buffer (1:1 of water and MeOH) in a glass tube. After 1 week, orange single crystals were obtained at the wall of the tube in 76%; they were washed with methanol–water (1:1) mixture and dried. IR spectra (KBr, cm–1): 1610 (s), 1573 (s), 1544(s), 1479(m), 1452(m), 1398(br), 1307(m), 1231(m), 1201(m), 1156(m), 1107(m), 1075(m), 1057(m), 1013(s), 957(m), 916(m), 821(s), 811(s), 742(s), 719(s), 691(s), 571(m), 513(s), 465(m), 431(m). Anal. Calc. for C26H19MnN6O4: C, 58.44; H, 3.58; N, 15.73. Found: C, 58.35; H, 3.51; N, 15.68. {[Fe(4–bpdb)(1,3–bdc)].(4–bpdb)0.5}n (2). Blocked shaped brown crystal of 2 was synthesized by the same procedure as that of 1 in 71% yield, but Fe(ClO4).xH2O (1 mmol, 0.254 g) was used instead of MnCl2·4H2O. IR spectra (KBr, cm–1): 1606 (s), 1575 (m), 1551(m), 1482(m), 1454(m), 1402(s), 1311(m), 1278(m), 1237(m), 1201(m), 1109(m), 1075(m), 957(m), 919(m), 822(m), 810(m), 739(s), 721(s), 691(m), 574(m), 515(s), 466(m), 435(m). Anal. Calc. for C26H19FeN6O4: C, 58.34; H, 3.58; N, 15.70. Found: C, 58.31; H, 3.51; N, 15.54. {[Co(4–bpdb)(1,3–bdc)].(4–bpdb)0.5}n (3). Blocked shaped red crystal of 3 was synthesized by the same procedure as that of 1 in 73% yield, except Co(NO3)2.6H2O (1 mmol, 0.281 g) was used instead of MnCl2·4H2O. IR spectra (KBr, cm–1): 1610 (s), 1575 (m), 1546(m), 1482(m), 1456(m), 1400(s), 1313(m), 1236(m), 1203(m), 1111(m), 1057(m), 1015(m), 959(m), 917(m), 822(m), 810(m), 740(s), 721(s), 693(m), 576(m), 516(m), 466(m), 435(m). Anal. Calc. for C26H19CoN6O4: C, 58.00; H, 3.56; N, 15.61. Found: C, 57.91; H, 3.47; N, 15.45. {[Mn(4–bpdh)(1,4–bdc)]}n (4). A methanolic solution (20ml) of N, N′–bis–(1–pyridin–4–yl– ethylidene)–hydrazine (4–bpdh) (1 mmol, 0.238 g) was mixed with an aqueous solution (20 ml) of disodium–1,4–benzenedicarboxylate (Na2–1,4–bdc) (1 mmol, 0.210 g) and the resulting solution was stirred for 15 min to mix well. MnCl2·4H2O (1 mmol, 0.198 g) was dissolved in 20 ml water in a separate beaker. 6 mL of this mixed ligand solution was slowly and carefully layered above 3 ml of metal solution using 2 mL buffer (1:1 of water and MeOH) in a glass tube.



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The yellow block–shaped crystals were obtained after three weeks. The crystals were separated and washed with methanol–water (1:1) mixture and dried. IR spectra (KBr, cm–1): 1609 (s), 1570 (s), 1503(m), 1397(s), 1331(m), 1301(m), 1223(m), 1064(m), 1011(m), 832(s), 753(s), 692(m), 639(m), 577(m), 525(m), 505(m). Anal. Calc. for C22H18MnN4O4: C, 57.78; H, 3.97; N, 12.25. Found: C, 57.35; H, 3.68; N, 12.14. {[Fe(4–bpdh)(1,4–bdc)]}n (5). Blocked shaped brown crystal of 5 was synthesized by the same procedure as that of 1 in 56% yield, except Fe(ClO4).xH2O (1 mmol, 0.254 g) was used instead of MnCl2·4H2O. IR spectra (KBr, cm–1): 1606 (s), 1571 (s), 1503(m), 1392(s), 1331(m), 1302(m), 1270(m), 1222(m), 1064(m), 1012(m), 877(m), 835(s), 694(m), 577(m), 526(m), 507(m). Anal. Calc. for C22H18FeN4O4: C, 57.66; H, 3.96; N, 12.23. Found: C, 57.52; H, 3.74; N, 12.14. {[Co(4–bpdh)(1,4–bdc)]}n (6). Blocked shaped red crystal of 6 was synthesized by the same procedure as that of 1 in 67% yield, except Co(NO3)2.6H2O (1 mmol, 0.281 g) was used instead of MnCl2·4H2O. IR spectra (KBr, cm–1): 1607 (s), 1575 (s), 1504(m), 1402(s), 1331(m), 1303(m), 1269(m), 1222(m), 1066(m), 1014(m), 877(m), 835(s), 754(s), 694(m), 577(m), 526(m), 507(m). Anal. Calc. for C22H18CoN4O4: C, 57.28; H, 3.93; N, 12.14. Found: C, 57.12; H, 3.56; N, 12.03. {[Cd(4–bpdh)(1,4–bdc)]}n (7). Blocked shaped colorless crystal of 7 was synthesized by the same procedure as that of 1 in 71% yield, except Cd(NO3)2.6H2O (1 mmol, 0.397 g) was used instead of MnCl2·4H2O. IR spectra (KBr, cm–1): 1610 (m), 1562 (s), 1500(m), 1403(s), 1325(m), 1290(m), 1219(m), 1063(m), 975(m), 882(m), 821(s), 744(s), 688(m), 639(m), 576(m), 507(m), 439(m). Anal. Calc. for C22H18CdN4O4: C, 51.33; H, 3.52; N, 10.88. Found: C, 51.14; H, 3.27; N, 10.66. IR spectra (in cm–1): ν(C=N), 1608; ν(C–O), 1290–1220; ν(CH–Ar), 3100–2900 and ν(C=C), 1600–1440. For the bulk synthesis of compounds 1–7, aqueous solution of corresponding metal was mixed with above mentioned mixed ligand solution. The above solution was stirred overnight and then the precipitate was filtered and air–dried, and phase purity was confirmed by elemental analysis, IR spectra powder (Figures S1–S2) and powder X–ray diffraction (Figures S3–S9). Crystallographic Data Collection and Refinement. The single crystals of compounds 1–7 were mounted on a thin glass fiber with commercially available super glue. X–ray single crystal data collection of all seven crystals were performed at room temperature using Bruker APEX II


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diffractometer, equipped with a normal focus, sealed tube X–ray source with graphite monochromated Mo–Kα radiation (λ= 0.71073Å). The data were integrated using SAINT88 program and the absorption corrections were made with SADABS.89 All the structures were solved by SHELXS 9790 using Patterson method and followed by successive Fourier and difference Fourier synthesis. Full matrix least–squares refinements were performed on F2 using SHELXL–9790 with anisotropic displacement parameters for all non–hydrogen atoms. All the hydrogen atoms were fixed geometrically by HFIX command and placed in ideal positions in case of all structures. Calculations were carried out using SHELXS 97, SHELXL 97, PLATON v1.15,91 ORTEP–3v2,92 WinGX system Ver–1.8093 and TOPOS.94–95 The coordinates, anisotropic displacement parameters, and torsion angles for all seven compounds are submitted as supporting information in CIF format. Data collection and structure refinement parameters along with crystallographic data for all compounds are given in Table 1. The selected bond lengths and angles are given in Table S1–S10. RESULT AND DISCUSSION Structural Description of {[M(4–bpdb)(1,3–bdc)].(4–bpdb)0.5}n [M = Mn (1), Fe (2) and Co (3)]. Compounds 1–3 are isostructural differing only with respect to the metal ion and these compounds crystallize in the triclinic crystal system with the Pī space group. Single crystal X– ray structure analysis reveals that these compounds originate a 2D pillared structure connected by the bridging 1,3–benzenedicarboxylate (1,3–bdc) and linear N,N′–donor organic linker (4– bpdb). The asymmetric unit of these three complexes contains one metal atom, one 1,3–bdc ligand, and one bridging 4–bpdb linker and half 4–bpdb linker. In the 3D framework, each hexa– coordinated M(II) ion with the MO4N2 coordination environment (M = Mn for 1, Fe for 2 and Co for 3) shows distorted octahedral geometry (Figure 1 for 1, Figure S10 for 2 and Figure S15 for 3). The degree of distortion from the ideal octahedral geometry is reflected in cisoid angles and transoid angles around M(II) (Table S1–S3). Here, each metal centers is ligated to four oxygen atoms, O1, O2, O3a and O4a of three different 1,3–bdc, which create the basal plane and two pyridyl nitrogens, N1 and N4c of two different 4–bpdb linkers occupy the apical position. Two of this MO4 linked with each other via carboxylate oxygens to form a M2(CO2)4N4 SBU (Figure 2 for 1, Figure S11 for 2 and Figure S16 for 3). In compounds 1–3, the M(II)–O bond length varies from 2.013(3)–2.266(3) Å (Mn–O, for 1), 2.0451(15)–2.2468(16) Å (Fe–O, for 2) and 2.0146(15)–2.1956(16) Å (Co–O, for 3), respectively, and the corresponding M(II)–N bond



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lengths for 1–3 are 2.264(3)–2.292(3) Å (Mn–N, for 1), 2.1840(16)–2.2004(17) Å (Fe–N, for 2)and 2.1454(16)–2.1642(16) Å (Co–N, for 3), respectively (Table S1–S3). The other selected bond lengths and bond angles for compounds 1–3 are reported in Table S1–S3, respectively. These two hexadentate M(II) centers are propagated along c–direction with the bridging 1,3–bdc to form a 1D ladder like structures (Figure 3a for 1, Figure S12a for 2 and Figure S17a for 3). In 1D ladder structures, the 1,3–bdc ligand bridge two M(II)centers, chelating a M(II) center at one end and binding through a single carboxylate oxygen atom at the other end. The second carboxylate group acts as a bridge between the two parallel rows. Thus each M(II) is linked to three 1,3–bdc ligands and each 1,3–bdc ligand is bound to three M(II) centers; the M(II) atoms and the atoms of 1,3–bdc all lie close to the mean plane of this [M(1,3–bdc)]n double chain. In the case of 1–3, the distances between the adjacent M(II) centers in binuclear core of the ladder network are 4.097 Å, 4.079 Å and 4.142 Å, respectively and the intra ladder M–M distances are 7.573 Å, 10.217 Å; 7.483 Å, 10.104 Å and 7.407 Å, 10.094 Å, respectively. This 1D ladder is pillared with the 4–bpdb ligands perpendicularly forming a 2D–grid like structure (Figure 3b for 1, Figure S12b for 2 and Figure S17b for 3). The M…M distances in 1–3 are 15.877 Å, 15.725 Å and 15.645 Å, respectively, based on 4–bpdb ligand. In order to better understand the whole structure, we can simplify the intricate structure as node–and–connecting nets. The 1,3–bdc anions can be assigned to three–connectors, while the M(II) ions can be considered as five– connectors, and the 4–bpdb ligands can be viewed as linkers. Therefore, the whole structure can thus be represented as a (3,5)–connected net topology (Figure 4 for 1, Figure S13 for 2 and Figure S18 for 3) with the Schläfli symbol {42.67.8}{42.6}.94–95 In addition there is an uncoordinated lattice 4–bpdb molecule which is present in the crystal structure. In the crystal packing, the lattice 4–bpdb are locked by π–π interaction with coordinated 4–bpdb of 2D grids. The 1,3–bdc are also locked by π–π interaction with adjacent ladder and C–H...π interaction with lattice 4–bpdb. These π–π and C–H...π interactions fabricate a 3D suprmolecular arrangement (Figure 5 for 1, Figure S14 for 2 and Figure S19 for 3) by locking the 2D grids (Table S4–S6). Structural Description of {[M(4–bpdh)(1,4–bdc)]}n [M = Mn (4), Fe (5), Co (6) and Cd (7)]. Compounds 4–7 are also isostructural and crystallize in the monoclinic crystal system with the P21/c space group. X–ray structure determination of 4–7 reveals a three–dimensional (3D) coordination framework constituted by 1,4–benzenedicarboxylate (1,4–bdc) and N,N′–donor linker (4–bpdh). The asymmetric unit of these four complexes comprises one metal atom, one


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1,4–bdc ligand, and one 4–bpdh linker. In the 3D framework, each hexa–coordinated M(II) ion with the MO4N2 coordination environment (M = Mn for 4, Fe for 5, Co for 6 and Cd for 7) shows distorted octahedral geometry (Figure 6 for 4, Figure S20 for 5, Figure S24 for 6 and Figure S28 for 7). Deviation from ideal octahedral geometry can be realized from the cisoid and transoid angle (Table S7–S10). Each metal atom is connected to four oxygen atoms (O1, O2b, O3c and O4c) of three different bridging 1,4–bdc ligands that occupy the equatorial positions, whereas two pyridyl nitrogen atoms (N1 and N4a) of 4–bpdh occupy the axial positions. Two metal ions are octahedrally coordinated by eight carboxylate oxygen atoms of 1,4–bdc ligands and four nitrogen from bridging 4–bpdh ligands creating a M2(CO2)4N4 SBU (Figure 7 for 4, Figure S21 for 5, Figure S25 for 6 and Figure S29 for 7). In compounds 4–7, the M(II)–O bond length varies from 2.1141(13)–2.3244(14) Å (Mn–O, for 4), 2.0365(14)–2.2851(16) Å (Fe–O, for 5), 2.0225(15)–2.2287(16) Å (Co–O, for 6) and 2.2475(14)–2.4295(14) Å (Cd–O, for 7), respectively, and the corresponding M(II)–N bond lengths for 4–7 are 2.2851(15)–2.3300(15) Å (Mn–O, for 4), 2.2274(17)–2.2460(16) Å (Fe–O, for 5), 2.1805(18)–2.1934(17) Å (Co–O, for 6) and 2.3155(15)–2.3844(16) Å (Cd–O, for 7), respectively (Table S7–S10). The other selected bond lengths and bond angles for compounds 4–7 are reported in Table S7–S10, respectively. Here, each 1,4–bdc connects three M(II) ions through chelating and bridging bidentate fashion forming M2(CO2)2 unit and this binuclear units are connected by 1,4–bdc ligands forming a two– dimensional [M2(1,4–bdc)2]n sheet lying in the crystallographic bc plane (Figure 8a for 4, Figure S22a for 5, Figure S26a for 6 and Figure S30a for 7). For 4–7, in each dinuclear unit, the M(II) centers are separated by 4.540 Å, 4.505 Å, 4.546 Å and 4.545 Å, respectively and the intrasheet M–M distances are 9.043 Å, 10.969 Å; 8.988 Å, 10.889 Å; 8.945 Å, 10.842 Å and 9.134 Å, 11.164 Å, respectively. The 2D sheet formed by the 1,4–bdc is further connected by 4–bpdh forming three–dimensional (3D) pillared layer framework (Figure 8b for 4, Figure S22b for 5, Figure S26b for 6 and Figure S30b for 7). The M…M distances in 4–7 are 15.687 Å, 15.556 Å, 15.467 Å and 15.744 Å, respectively, based on 4–bpdh ligand. Better insight into such intricate frameworks can be accessed by reducing multidimensional structures to a simple node and connecting nets via TOPOS analysis software.94–95 Topologically, the M(II) ions can be defined as 5–connected nodes, 1,4–bdc dianions can be considered as 3–connected nodes and 4–bpdh ligands can be regarded as linkers. Thus, the structure can be described by TOPOS94–95 software,



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as a (3,5)–connected net with Schläfli symbol {4.64.83.102}{4.82}net (Figure 9 for 4, Figure S23 for 5, Figure S27 for 6 and Figure S31 for 7). MAGNETIC PROPERTIES To examine the effect of bridging ligand on the magnetic properties of the complexes with paramagnetic metal centers, variable temperature magnetic measurements were performed. The temperature dependence of the χMT product for compounds 1–6 (χMT being the molar magnetic susceptibility per M2 unit) is displayed in Figure 10, whereas the χMT (both experimental and theoretical) at 300K and 2K are summarized in Table 2 along with their magnetization data at 2K. The room temperature χMT values for the Mn2+ complexes (1 and 4) are close to the spin only calculated values for isolated Mn2+ ions (Table 2), whereas those for the Fe2+ (2 and 5) and Co2+ (3 and 6) complexes are higher than the spin–only values, which is indicative of the unquenched orbital contribution of theses ions in distorted octahedral geometry. On cooling, the χMT of Mn2+ complexes remains almost constant until ~ 75 K and then decreases sharply down to 2 K. Since Mn2+ ions present no first order spin–orbit coupling, the decrease of the χMT product at low temperature is mainly due to the presence of a weak antiferromagnetic interaction between the Mn2+ ions through the syn–anti carboxylate–bridging groups and probably due to the presence of very weak ZFS effects in their ground state. In the case of the Fe2+ complexes (2 and 5) the severe decrease at low temperature is mainly due to the combined action of weak magnetic exchange interactions between the Fe2+ ions through syn–anti carboxylate–bridging groups and significant ZFS effects in their ground state. The drop off for the Co2+ complexes (3 and 6) is most likely due to spin–orbit coupling (SOC) effects and weak magnetic exchange interactions between the Co2+ ions through syn–anti carboxylate–bridging groups. The absence of a clear maximum in the χM vs T plot (Figure S32) for complexes 1–6 indicates that the magnetic exchange through these long dicarboxylate linkers is pretty weak. Generally, the magnetic exchange interactions transmitted through the benzene ring of 1,4–bdc and 1,3–bdc ligands are moderately weak whereas the other pyridyl based linkers 4–bpdb and 4– bpdh are very weak in terms of magnetic exchange, creating long separations between the metal ions. Therefore, from the magnetic point of view, all the complexes can be considered as double syn–anti carboxylate–bridged M2+ systems with a six–membered M(OCO)2M fragment in a chair conformation. Considering this model, the data concerning the Mn2+ complexes 1 and 4 were analyzed with the following Hamiltonian for a dinuclear system:


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r H = − JSˆMn SˆMn + g i β H ∑ Sî

(1)

The best fitting parameters using the PHI program96 were J = –1.50 cm–1 and g = 2.03 for 1 and J = –1.20 cm–1 and g = 2.030 for 4. The values are similar to those previously reported for other similar syn–anti carboxylato–bridged Mn2+ complexes.97–99 The field dependence of the molar magnetization at 2 K for compounds 1 and 4 (Figure 11) shows that the magnetization increases slowly at low field and then more abruptly at higher field and is well below to the Brillouin function for two Mn2+ ions and does not achieve saturation at 5T in both the cases. This behavior corroborates the presence of antiferromagnetic interactions between the Mn2+ ions in 1 and 4. For the Fe2+ complexes, considering their significant ZFS effects, the magnetic properties of compounds 2 and 5 have been analyzed by using the following Hamiltonian: 2 r 2 H = − JSˆFe SˆFe + ∑ DFe SˆFe − g i β H ∑ Sî

(2)

i =1

where J is the magnetic exchange pathway through the double syn–anti carboxylate bridges and DFe is the axial single ion zero field splitting parameter, which is assumed to be the same for both Fe2+ ions. The fit of the experimental susceptibility data with the above Hamiltonian using the full–matrix diagonalization PHI program96 afforded the following set of parameters: J = –1.37 cm–1, g = 2.19 and D = 6.5 cm–1 for 2 and J = –0.26 cm–1, g = 2.08 and D = 3.6 cm–1 for 5. The DFe values are lower but still in agreement with the single–ion values reported in the literature and the positive values of D are usually found for Fe2+ high spin species.100–101 Since J and DFe are strongly correlated and provoke the same result at low temperature it is difficult to determine their independent contributions accurately from the fit of the magnetic data. Therefore, we have fixed to zero either J or D. If J fixed to zero, the fit of the experimental susceptibility data with the above Hamiltonian using the full–matrix diagonalization PHI program96 afforded the following set of parameters: D = 24.7 cm–1 and g = 2.18 for 2 and D = 6.6 cm–1 and g = 2.08 for 5. For fixing D to zero, the following parameters were extracted: J = 1.77 cm–1 and g = 2.20 for 2 and J = –0.48 cm–1 and g = 2.09 for 5. It should be noted that the DFe values obtained with J = 0 and the J values obtained with DFe = 0 can be considered as the limiting values for these parameters.The field dependence of the magnetization for complexes 2 and 5 (Figure 11) shows that the magnetization steadily increases with the field but without reaching saturation at 5T which is due to the magnetic anisotropy and the antiferromagnetic exchange coupling between the Fe2+ ions in 2 and 5.



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Finally, the magnetic susceptibility data for Co2+ complexes 3 and 6 were analyzed by introducing spin orbit coupling (SOC) effects which is quite common in case of octahedral Co2+ systems. The axial distortion of the octahedral geometry and the exchange between the magnetic moment of Co2+ has been taken into account, following the treatment of Lines,102 which considers only the coupling between true spins (S = 3/2). In addition, the effect of the magnetic field on the various magnetic moments resulting from these interactions (Zeeman effect) was included. The Hamiltonian used is as follows:

[

]

r H = −σλ∑Lî Sî + ∆∑ Lˆ2z − Lˆ(Lˆ + 1) / 3 − JSˆ1Sˆ2 + βH·(−σ ∑Lî + gi ∑Sî )

(3)

where λ is the spin–orbit coupling parameter, σ is a combined orbital reduction factor, and ∆ is the axial splitting of the T1 term. In order to avoid over–parametrization we have fixed λ and σ to reasonable values of –175 cm–1 and –1.5 cm–1, respectively. Afterwards, using the PHI package,96 the best–fit was found with the values J = –0.39 cm–1, ∆ = 771 cm–1 and g = 2.11 for 3 and J = –0.30 cm–1, ∆ = 750 cm–1 and g = 2.07 for 6. When ∆ is large enough and positive, as in these cases, only the two lowest Kramers doublets arising from the 4A2 ground term (the 4T1 ground term in Oh symmetry splits in 4A2 and 4E as terms a result of the axial distortion), are thermally populated and the energy gap between them can be considered as an axial zero–field splitting (ZFS) within the quartet state. The magnetic properties can then be also analyzed by using the Hamiltonian:

[

]

r 2 ˆ ˆ ˆ ˆ ˆ H = −D∑ Sz − S(S +1) / 3 − JS1S2 + βHgi ∑ Sî

(4)

where S is the spin ground state, D is the axial magnetic anisotropy, β is the Bohr magneton and H is the applied magnetic field. 2D accounts for the energy separation between ±1/2 and ±3/2 doublets arising from second order SOC from the quartet ground state of the distorted octahedral Co(II) ion (Figure S33). The best fit of the susceptibility data led to the following parameters: D = 27.4 cm–1 , J = –0.44 cm–1 and g = 2.37 for 3 D= 27.6 cm–1 , J = –0.32 cm–1 and g =2.33 for 6. The large g values show the spin–orbit coupling effects. It is interesting to note that the J values obtained here are in well concurrence with those extracted from the full Hamiltonian (equation 3). The field dependence of the magnetization for complexes 2 and 5 (Figure 11) undergoes a progressive increase with the field and reaches its maximum in the applied field of 5T. The saturation values agree well with those expected if the ground spin–orbit doublet (Seff = 1/2) is the only populated state at 2 K of approximately 2.1 µB per Co2+ ion.


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It is well known that syn–anti carboxylate bridges transmit weak ferromagnetic or antiferromagnetic interactions between the metal ions. The ability of this type of bridging group to propagate magnetic coupling mainly depends on two vital factors: (i) the planarity of the M– O–C–O–M bridging fragment and (ii) the C–O–M angle.103 Thus, as the metal atoms depart from the O–C–O plane and the M–O–C angle deviates from 120° the contributions from the 2p orbitals of the carboxylate oxygen atoms belonging to the magnetic orbitals of the metal ions are less favorably oriented to overlap and therefore the antiferromagnetic interaction between the metal ions decreases (JAF contribution to J depends on the square of the overlap integral). For complexes 1–3, containing 1,3–bdc bridging ligands, the M–O–C–O torsional angles are approximately 79° and 1°, whereas those for complexes 4–6 bearing 1,4–bdc bridging ligands are approximately 113° and 27°, which indicates less planarity of the M–O–C–O–M bridging fragment for the later group of compounds. Moreover, the C–O–M angles for complexes 1–3 are smaller than those observed for complexes 4–6. Therefore, in view of the above considerations, complexes 4–6 are expected to exhibit weaker antiferromagnetic interactions rather than those for complexes 1–3, which are in well agreement with the experimental results. It should be noted that for isostructural complexes the magnetic exchange coupling generally follows the order: JCoCo > JFeFe > JMnMn.99 However, both series of isostructural complexes, 1–3 and 4–6, do not appear to obey this trend, which could be due to the fact that the J values for Fe2+ (2 and 5) and Co2+(3 and 6) are not accurately determined because of the simultaneous presence of ZFS and antiferromagnetic interactions, and both are acting in declining the χMT product at low temperature. This may also because of considering a simplified model for analysis, where the magnetic coupling through the benzene ring of 1,3–bdc and 1,4–bdc ligands and the presence of 4–bpdb and 4–bpdh are ignored. LUMINESCENCE PROPERTIES: The photoluminescent spectra of the complexes 1–7 and the free ligands (4–bpdb and 4–bpdh) in the solid state were recorded at room temperature, which are shown in Figure 12. The complexes and ligands exhibit similar kind of emission spectra. The photoluminescent spectra of N,N′– donor ligands, 4–bpdb and 4–bpdh show the main emission peak at 395 nm (λex = 250 nm) and 401 (λex = 260 nm) nm, respectively. These emissions can be attributable to the π* → n or π* → π transitions. The emission spectra for 4–bpdb–containing compounds (1–3) exhibit emission peaks at 398 nm (λex = 253 nm) for 1, 406 nm (λex = 265 nm) for 2 and 403 nm (λex = 263 nm)



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for 3. The emission spectra for 4–bpdh–containing compounds (4–7) display emission peaks at 399 nm (λex = 260 nm) for 4, 399 nm (λex = 260 nm) for 5, 401 nm (λex = 260 nm) for 6, and 400 nm (λex = 260 nm) for 7. The emissions of these compounds can be assigned to ligand–centered electronic transition (intraligand π*–π transition) that are tuned by the metal–ligand connections. CONCLUSIONS In conclusion, we have successfully synthesized and characterized seven new coordination polymers with two different types of framework structures by the self–assembly of 1,3–bdc and 1,4–bdc ligands, two different azine based N,N′–donor ligands (4–bpdb and 4–bpdh), and various divalent transition metal ions. In case of isostructural compounds 1–3, V–shaped 1,3–bdc ligands connect metal centres forming one–dimensional metal–carboxylate moieties which are further pillared by 4–bpdb ligands to form 2D framework structure containing lattice 4–bpdb ligands in their void spaces by π–π and C–H⋯π interactions. Whereas, in case of compounds 4– 7, linear 1,4–bdc ligands link with metal centres create 2D metal–carboxylate sheets. These sheets are further linked by 4–bpdh linkers to form a 3D framework structure. Furthermore, variable temperature magnetic studies show that all the complexes exhibit very weak antiferromagnetic properties and the magnetic exchange interactions occur between the M2+ ions through syn–anti carboxylate–bridging groups. Magnetically, the isostructural CPs displays different magnetic properties because of different spins of central ions. In molecule–based magnetism, magnetic exchange interactions between the paramagnetic transition metal ions through carboxylate bridges are very common, but interaction only via syn–anti bridge model is rare. Compound 7 was found to show a ligand based photoluminescence spectrum with the λmax at 362 nm. Following the above result, we have concluded that the synthesis of desired coordination polymers by judicious choice of bridging ligands is an attractive way to construct functional materials. Such types of isostructural coordination polymers with different paramagnetic metal ions are under exploration with both polycarboxylate and N,N′–donor ligands in our laboratory. ACKNOWLEDGEMENTS Dr. K. K. Rajak and Mr. A. Maity of JU are gratefully acknowledged for the photoluminescence study. Authors gratefully acknowledge the financial assistance given by DST [No. SB/S1/IC– 06/2014, grant to DG] and CSIR [No.01(2729)/13/EMR–II, grant to DG], Govt. of India. BB and


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DKM acknowledges UGC for the research fellowship. The X–ray diffractometer facility under the DST–FIST program of Department of Chemistry (JU) is also gratefully acknowledged. EC thanks Ministerio de Economíay Competitividad (MINECO) for Projects CTQ–2011–24478, CTQ2014–56312–P, the Junta de Andalucía (FQM–195 and the Project of excellence P11– FQM–7756), University of Granada financial support. ASSOCIATED CONTENT Supporting Information X–ray crystallographic data in CIF format for the structures reported in this paper and different figures and tables related to the crystal structure, FT–IR data, PXRD and magnetic data of the compounds; are available in free of charge via the internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author Debajyoti Ghoshal, e–mail: [email protected], FAX: +9133 2414 6223 Note: The authors declare no competing financial interest. REFERENCES (1) Yaghi, O. M.; O’Keeffe, M.; Ockwig, N. W.; Chae, H. K.; Eddaoudi, Kim, M. J. Nature 2003, 423, 705–714. (2) Férey, G.; Mellot–Draznieks, C.; Serre, C.; Millange, F.; Dutour, J.; Surblé, S.; Margiolaki, I. Science 2005, 309, 2040–2042. (3) Caskey, S. R.; Wong–Foy, A. G.; Matzger, A. J. J. Am. Chem. Soc. 2008, 130, 10870–10871. (4) Farha, O. K.; Yazaydın, A. Ö.; Eryazici, I.; Malliakas, C. D.; Hauser, B. G.; Kanatzidis, M. G.; Nguyen, S. T.; Snurr, R. Q.; Hupp, Joseph T. Nat. Chem. 2010, 2, 944–948. (5) Foo, M. L.; Horike, S.; Inubushi, Y.; Kitagawa, S. Angew. Chem. Int. Ed. 2012, 51, 6107– 6111. (6) Ma, S.; Sun, D.; Ambrogio, M.; Fillinger, J. A.; Parkin, S.; Zhou, H.–C. J. Am. Chem. Soc. 2007, 129, 1858–1859. (7) Vaidhyanathan, R.; Iremonger, S. S.; Dawson, K. W.; Shimizu, G. K. H. Chem. Commun. 2009, 5230–5232. (8) Dey, R.; Haldar, R.; Maji, T. K.; Ghoshal, D. Cryst. Growth Des. 2011, 11, 3905–3911.



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Tables Table 1. Crystallographic and Structural Refinement Parameters for 1–7. 1 2 3 4 Formula C26H19MnN6O4 C26H19FeN6O4 C26H19CoN6O4 C22H18MnN4O4 Formula Weight 534.41 535.32 538.40 457.34 Crystal System Triclinic Triclinic Triclinic Monoclinic Space group P–1 P–1 P–1 P21/c a/ Å 8.3865(5) 8.3880(2) 8.3602(2) 7.6059(2) b/Å 10.2168(6) 10.1041(2) 10.0943(2) 13.7196(3) c/ Å 15.8771(11) 15.7252(4) 15.6447(4) 19.6777(4) α/° 89.949(4) 90.586(1) 91.023(1) 90 β/° 74.926(4) 104.374(1) 104.254(1) 91.954(1) γ/° 78.451(4) 101.294(1) 101.386(1) 90 V/ Å3 1285.11(14) 1263.61(5) 1251.36(5) 2052.17(8) Z 2 2 2 4 Dc/ g cm–3 1.381 1.407 1.429 1.480 µ /mm–1 0.557 0.641 0.730 0.681 F(000) 548 550 552 940 θ range/° 2.0‒ 27.5 2.1‒27.5 1.4‒27.5 1.8‒27.6 Reflections collected 20026 20930 20535 31074 Unique reflections 5864 5766 5707 4681 Reflections I>2σ(I) 4113 4409 4545 3957 Rint 0.052 0.038 0.034 0.035 2 goodness–of–fit (F ) 1.04 1.01 1.04 1.07 R1 (I > 2σ(I)) [a] 0.0568 0.0396 0.0384 0.0356 [a] wR2(I > 2σ(I)) 0.1765 0.0907 0.0948 0.0926 3 −0.83,1.16 −0.35, 0.48 −0.29, 0.45 −0.22, 0.49 ∆ρ max/min/e Å 2 2 2 2 2 ½ [a] R1 = ΣFo–Fc/ΣFo, wR2 = [Σ (w (Fo – Fc ) )/ Σw (Fo ) ]


5 C22H18FeN4O4 458.25 Monoclinic P21/c 7.6123(3) 13.5665(5) 19.6056(8) 90 91.749(2) 90 2023.77(14) 4 1.504 0.783 944 1.8‒27.5 32933 4660 3621 0.076 1.02 0.0380 0.1011 −0.39, 0.34

6 C22H18CoN4O4 461.33 Monoclinic P21/c 7.6154(2) 13.4625(3) 19.6118(4) 90 91.013(1) 90 2010.33(8) 4 1.524 0.892 948 1.8‒25.7 28585 3837 3253 0.036 1.05 0.0328 0.0845 −0.29, 0.30

7 C22H18CdN4O4 514.81 Monoclinic P21/c 7.6373(2) 13.7679(4) 19.9752(6) 90 92.039(2) 90 2099.05(10) 4 1.629 1.077 1032 1.8–28.8 35793 5421 4684 0.030 1.02 0.0255 0.0612 –0.27, 0.39


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Table 2. Magnetic data for 1–6. Compound χMT theoretical/ χMT experimental (cm3Kmol–1) at 300 K 8.75/8.85 1 6.0/7.09 2 3.75/5.22 3 8.75/9.04 4 6.0/6.71 5 3.75/5.10 6 a

χMT experimental at at 2K (cm3Kmol–1)

Calculated saturation valuea / M at 2 K and 5 T (NµB)

0.77 0.80 2.04 1.18 2.97 2.21

10/5.33 8/4.79 6/4.51 10/6.66 8/4.89 6/4.24

Spin only values

Figures

Figure 1. View of coordination environment of hexa coordinated Mn(II) with atom labeling scheme of 1.


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Figure 2. (a) Polyhedral representation of 1 showing the coordination environment around Mn(II). (b) Octahedral Mn2(CO2)4N4 SBUs.

Figure 3. (a) 1D arrangement in 1 constructed by metal–1,3–benzenedicarboxylate moiety (4– bpdb has been omitted for clarity). (b) 2D–grid structure in 1.



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Figure 4. 3,5–connected 2D grid of compound 1.

Figure 5. Supramolecular 3D arrangement in 1 (π–π interactions: pink dotted lines, C– H…πinteractions: Cyan dotted).


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Figure 6. View of coordination environment around hexacoordinated Mn(II) with atom labeling scheme of 4.

Figure 7. (a) Polyhedral representation of 4 showing the coordination environment around Mn(II). (b) Octahedral Mn2(CO2)4N4 SBUs.



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Figure 8. (a) 2D arrangement in 4 constructed by metal–1,4–benzenedicarboxylate moiety in (4–bpdh has been omitted for clarity). (b) View of the pillared–layer 3D structure of 4 along crystallographic b–axis.

Figure 9. (3,5)–connected 3D net of compound 4.


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Figure 10. Temperature dependence of the χMT product for compounds 1–6.

Figure 11. Field dependence of the magnetization for complexes 1–6.



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Figure 12. Emission spectra of the free ligands (4–bpdb and 4–bpdh) and compounds 1–7 in the solid state at room temperature.


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For Table of Content Use Only Two

Series

of

Isostructural

Coordination

Polymers

with

Isomeric

Benzenedicarboxylates and Different Azine Based N,N′–donor Ligand: Syntheses, Characterization and Magnetic Properties Biswajit Bhattacharya, Dilip Kumar Maity, Rajarshi Mondal, Enrique Colacio and Debajyoti Ghoshal

Two series of isostructural coordination polymers of transition metals have been synthesized using isomeric benzene dicarboxylates and different N,N′–donor ligand. Isostructural 1–3 shows a 2D structure and exhibit supramolecular 3D structures through π–interactions. On the other hand isostructural 4–7 exhibits a 3D pillared–layer framework structure. Variable temperature magnetic study of 1–6 shows weak antiferromagnetic interaction between the metal centers.


Two Series of Isostructural Coordination Polymers ... - ACS Publications

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