One-Dimensional Chains, Two-Dimensional Corrugated Sheets Having a Cross-Linked Helix in Metal-Organic Frameworks: Exploring Hydrogen-Bond Capable Backbones and Ligating Topologies in Mixed Ligand Systems
CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 8 1903-1909
D. Krishna Kumar, Amitava Das,* and Parthasarathi Dastidar* Analytical Science Discipline, Central Salt & Marine Chemicals Research Institute, G. B. Marg, BhaVnagar - 364 002, Gujarat, India ReceiVed January 19, 2006; ReVised Manuscript ReceiVed June 6, 2006
ABSTRACT: Six intriguing metal-organic coordination polymers, {Co(µL1)(µL2)(H2O)}n (1a), {Zn(µL1)(µL2)(H2O)}n (1b), {[Zn(H2O)4(µL1′)Zn(L2)2]‚H2O}n (2), {[Co(H2O)4(µL1′′)Co(L2)2(H2O)2]‚H2O}n (3), {Zn(µL1′′)(µL2)}n (4), {[Cd(H2O)(µL1′′)(µL2)‚ 2H2O]}n (5), and {[Co(H2O)4(µL1′′)Co(L3)2](H2O)2]‚1.3H2O]}n (6) (L1 ) N-(4-pyridyl)isonicotinamide, L1′ ) N-(3-pyridyl)isonicotinamide, L1′′ ) N-(4-pyridyl)nicotinamide, L2 ) maleate, L3 ) succinate), have been synthesized and characterized by single-crystal X-ray diffraction. 1a, 1b, 4, and 5 display a grid architecture, whereas 2, 3, and 6 display a one-dimensional polymeric chain. 5 is an example of a chiral metal-organic framework (due to the presence of a right-handed helix) assembled from achiral components. The supramolecularly recognizable backbone, namely, secondary amide of N-donor ligands does not show typical self-recognition. This study represents one of the limited explorative studies on counterion free mixed ligand systems containing an N-donor exo-bidentate ligand with a supramolecularly recognizable backbone and O-donor carboxylate ligand. Introduction engineering1
Crystal in general and crystal engineering of metal-ligand coordination (MLC) polymers2 in particular have emerged as highly challenging as well as rewarding areas of research due to their various potential applications in designing functional materials that can play a role in gas adsorption,3 chemical adsorption,4 selective guest exchange properties,5 heterogeneous catalysis,6 and design of molecular magnetic materials.7 Intelligent ligand design and the proper choice of a metal center are the main keys to the design of intriguing and useful coordination polymers.8 On the other hand, crystal engineering of organic solids9 are mainly governed by strong and directional hydrogen bonding.10 The combination of both MLC and hydrogen bonding in designing various supramolecular architectures should be considered as an attractive design strategy because of the possibility of structural variations and guest entrapment induced by specific hydrogen-bonding interactions. One of the major problems in generating porous open frameworks in coordination polymers is the presence of counterions that often occupy the pores, thereby reducing the possibility of occluding the guests in the framework. By using anionic ligands such as dicarboxylate anions along with pyridyl N-donor exo-bidentate ligands, it is possible to create open frameworks devoid of counterions.11 A CSD (Version 5.27, Nov. 2005) search with a search moiety containing any transition metal ion coordinated with a 4-substituted pyridyl fragment and a carboxylate fragment gave only 75 relevant hits (considering only bidentate N-donors and dicarboxylate ligands coordinating through both ends) wherein 4,4′-bipyridine, 1,2-bis(4-pyridyl)ethane, 1,2-bis(4-pyridyl)ethylene are the main N-donor bidentate ligands used. A similar search with a search fragment containing 3-substituted pyridyl fragments resulted in zero relevant hits. Thus, such mixed ligand systems are much less explored, and in these studies, largely pyridyl N-donor exo* To whom correspondence should be addressed. E-mail: parthod123@ rediffmail.com;
[email protected] (P.D.);
[email protected] (A.D.)
bidentate ligands having a linear ligating topology and innocent backbone have been used. The advantage of using a hydrogenbond capable backbone of the ligand is in the inter-network supramolecular recognition process, which may play a significant role in making the architecture more robust and allow the guest molecules to bind strongly through hydrogen bonding with the backbone. Recently, we have shown12 along with others13 that hydrogen-bonding backbones of N-donor ligands indeed play a significant role in inter-network hydrogen-bonding recognition. Examples of mixed ligand systems containing N-donor ligands (having hydrogen-bonding capable backbones) and carboxylate O-donor ligands are rare. To the best of our knowledge, only one such example is known in the literature.14 It is also interesting to note that CSD (Version 5.27, Nov. 2005) documents only three hits for metal-organic frameworks (MOF) involving 3,4′-bipyridine and eight hits for MOFs involving 3,3′bipyridine, which are the topological variants of 4,4′-bipyridine, which gave 1156 hits. Thus, investigations on supramolecular structural diversity that could have been achieved by taking advantage of topological variation in ligating sites have not been explored to a great extent presumably because of the difficulty in synthesizing the topological variants of 4,4′-bipyridine. Thus, we are interested in exploring various supramolecular structures that can be generated in a counterion free mixed ligand systems containing pyridyl N-donors and carboxylate O-donor ligands. For this purpose, we have chosen a series of pyridyl N-donor bidentate ligands having various ligating topologies (linear and angular) and a 2°-amide backbone which is known to self-assemble through complementary hydrogen bonding.15 The anionic ligands chosen are maleate and succinate. While maleate represents a rigid ligand owing to the presence of an unsaturated backbone, succinate is a flexible ligand due to its saturated backbone, thereby allowing a C-C bond rotation. We have chosen Zn2+, Co2+, and Cd2+ as metal centers since all of them are known to form mainly an octahedral coordination geometry in the presence of strong σ-donor ligands such as oxygen. An octahedral metal center is particularly important to
10.1021/cg0600344 CCC: $33.50 © 2006 American Chemical Society Published on Web 07/15/2006
1904 Crystal Growth & Design, Vol. 6, No. 8, 2006 Scheme 1
facilitate a grid-type framework involving an N-donor, O-donor ligand and a metal center in a 1:1:1 molar ratio (Scheme 1). Results and Discussions Crystal Structures. Crystallographic parameters of the coordination polymers reported herein are listed in Table 1. Selected bond distances and angles of the coordination sphere of the metal centers and hydrogen-bonding parameters are listed in Tables S1 and S2, respectively (Supporting Information). {Co(µL1)(µL2)(H2O)}n (1a). The crystals of 1a belong to space group P21/c, and the coordination geometry of the metal center Co2+ is a slightly distorted octahedron, the equatorial sites of which are occupied by four oxygen atoms coming from two maleate moieties and one water molecule. The axial positions are occupied by both the 4-amino pyridyl and isonicotinic acid moieties of the N-donor exo-bidentate ligand L1. The Co-N distances range from 2.170(2) to 2.149(2) Å, whereas the Co-O distances vary from 2.033(2) to 2.127(2) Å. The angle N-Co-N is 175.9(1)°, and the O-Co-O angles vary from 87.8(1) to 92.5(1)°. While one of the maleate moieties is coordinated to the metal center in a chelate fashion, the other one coordinates in a monodentate fashion. The crystal structure of 1a can be best described as a two-dimensional (2D) grid type coordination polymer wherein the N-donor exo-bindentate ligand, namely, N-(4-pyridyl)isonicotinamide L1, forms onedimensional (1D) polymeric chains by coordinating the adjacent metal centers. Such chains are further bridged by maleate ligand L2 resulting in the formation of a 2D grid polymer. The grid size is approximately 8.60 × 3.80 Å after taking the van der Waals radii into account. Because of the syn coordination topology of the maleate, the resultant framework displays a corrugated sheet type of architecture. The 2D sheets are further packed on top of each other in an off-set fashion stabilized through various hydrogen-bonding interactions; while the coordinated water molecule of one network forms hydrogen bonds with the amide oxygen atom of the neighboring sheet [O‚‚‚O ) 2.734(3) Å; ∠O-H‚‚‚O ) 175(4)°], the amide
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nitrogen atom interacts through hydrogen bonds with the carobxylate oxygen of the neighboring sheet [N‚‚‚O ) 2.776(3) Å; ∠N-H‚‚‚O ) 165.2°] (Figure 1). {Zn(µL1)(µL2)(H2O)}n (1b). The structure of 1b which is a Zn analogue of 1a is isostructural with that of 1a displaying an identical space group and similar cell dimensions (Table 1). Thus, a supramolecular architecture and hydrogen-bonding interactions are also identical with that of 1a. {[Zn(H2O)4(µL1′)Zn(L2)2]‚H2O}n (2). Crystals of 2 belong to the monoclinic C2/c space group. There are two types of coordination geometries of the metal centers in the crystal structure. Both the metal centers are found to be hexacoordinated. Zn(1) is coordinated by two symmetry related ligands, namely, N-(3-pyridyl)isonicotinamide L1′, through its pyridyl ring of isonicotinic acid moiety approaching the metal center in a cis fashion; the corresponding Zn-N distance is 2.078(1) Å; ∠N-Zn-N ) 90.6(1)°. The other four coordination sites are occupied by four oxygen atoms coming from two symmetry related carboxylate moieties of maleate ligand L2. The carboxylate moieties display chelating coordination modes with the metal center showing one short [2.027(1) Å] and one long [2.443(1) Å] Zn-O coordination bond. Thus, the coordination geometry of the metal center Zn(1) cannot be categorized to any conventional coordination geometry such as octahedron. Whereas, the other metal center Zn(2) displays a perfect octahedral coordination sphere. The metal center Zn(2) is coordinated centrosymmetrically to two symmetry related ligand L1′ through its 3-pyridylamine moiety (axial coordination) and to four water molecules; the corresponding Zn-N and Zn-O distances are 2.178(1) and 2.096(1) Å, respectively; all ∠NZn-N and ∠O- Zn-O display ideal 180.0(1)°. The MOF in 2 can be best described as a 1D zigzag coordination polymer wherein the N-donor exo-bidentate ligand L1′ coordinates the adjacent metal centers and the anionic ligand L2 coordinates the metal center through one of its carboxylate moieties; the other one remains free from coordination (Figure 2a). The 1D chains are self-assembled in a parallel fashion sustained by several hydrogen-bonding interactions involving an amide nitrogen atom and a carboxylate oxygen atom (coordinated to the metal center) [N‚‚‚O ) 2.804(1) Å; ∠NH‚‚‚O ) 162(2)°] and carboxylate oxygen atoms (free from coordination) and coordinated water molecules [O‚‚‚O ) 2.628(1)-2.724(1) Å; ∠O-H‚‚‚O ) 168(2)-172(2)°] of the neighboring chain. Such assembly of 1D chain forms sheet-like structures that are further packed on top of each other in an opposite fashion sustained by hydrogen bonding involving amide oxygen and coordinated water molecules of the interacting sheets [O‚‚‚O ) 2.859(1) Å; ∠O-H‚‚‚O ) 169(2)°] (Figure 2b). Solvate water molecules are located within the intersheet space displaying hydrogen-bonding interactions with carboxylate moieties of the interacting sheets [O-O ) 2.801(1)-2.884(1) Å; ∠O-H‚‚‚O ) 163(2)-173(3)°]. {[Co(H2O)4(µL1′′)Co(L2)2(H2O)2]‚H2O}n (3). Crystals of 3 belong to a centrosymmetric triclinic space group Pıˆ. In the crystal structure, the N-donor ligand, namely, N-(4-pyridyl)nicotinamide L1′′, coordinates to the adjacent metal center through both the pyridyl nitrogen atoms resulting in a 1D zigzag polymeric structure (Figure 3a). There are two types of coordination environments of the metal centers, both of which display ideal octahedral geometry. The axial sites of Co(1) are occupied by the N-donor bidentate ligand L1′′ through its 4-amino pyridyl moiety, and the equatorial sites are coordinated by four water molecules. Whereas the axial sites of Co(2) is coordinated by L1′′ through its nicotinic acid moiety, the
1D Chains, 2D Corrugated Sheets in MOFs
Crystal Growth & Design, Vol. 6, No. 8, 2006 1905 Table 1. Crystal Data for 1a, 1b, 2-6
crystal data empirical formula formula weight crystal size (mm3) crystal system space group a (Å) b (Å) c (Å) R (°) β (°) γ (°) volume (Å3) Z Dcalc (g/cm3) F(000) µ Mo KR (mm-1) temperature (K) range of h, k, l θ min/max reflections collected/unique/observed data/restraints/parameters goodness of fit on F2 final R indices [I > 2σ(I)] R indices (all data) crystal data empirical formula formula weight crystal size (mm3) crystal system space group a (Å) b (Å) c (Å) R (°) β (°) γ (°) volume (Å3) Z Dcalc (g/cm3) F(000) µ MoKR (mm-1) temperature (K) range of h, k, l θ min/max reflections collected/unique/observed data/restraints/parameters goodness of fit on F2 final R indices [I > 2σ(I)] R indices (all data)
1a C15H13CoN3O6 390.21 0.58 × 0.38 × 0.31 monoclinic P21/c 9.1790(7) 8.7296(7) 19.4811(15) 90.00 91.3150(10) 90.00 1560.6(2) 4 1.661 796 1.139 100(2) -10/11, -8/11, -25/25 2.09/28.22 8801/3589/3193 3589/0/274 1.112 R1 ) 0.0475 wR2 ) 0.1043 R1 ) 0.0542 wR2 ) 0.1073 4 C15H11N3O5Zn 378.64 0.55 × 0.46 × 0.33 monoclinic P21/n 7.2357(4) 19.1593(11) 10.7206(7) 90.00 104.7290(10) 90.00 1437.37(15) 4 1.750 768 1.742 100(2) -9/9, -25/25, -6/14 2.13/28.15 8490/3328/3049 3328/0/261 1.045 R1 ) 0.0272 wR2 ) 0.0698 R1 ) 0.0303 wR2 ) 0.0712
1b C15H13N3O6Zn 396.65 0.63 × 0.56 × 0.43 monoclinic P21/c 9.1935(9) 8.8044(9) 19.555(2) 90.00 91.111(2) 90.00 1582.5(3) 4 1.665 808 1.591 100(2) -12/9, -11/10, -25/26 2.08/28.23 9170/3651/3432 3651/0/278 1.037 R1 ) 0.0265 wR2 ) 0.0672 R1 ) 0.0283 wR2 ) 0.0682 5 C15H15CdN3O8 477.70 0.42 × 0.24 × 0.15 monoclinic C2 24.139(2) 9.1383(8) 8.2704(8) 90.00 107.591(3) 90.00 1739.0(3) 4 1.825 952 1.307 100(2) -31/26, -11/11, -10/8 1.77/28.24 5214/3706/3374 3706/1/257 1.096 R1 ) 0.0461 wR2 ) 0.1237 R1 ) 0.0522 wR2 ) 0.1295
equatorial positions are occupied by four oxygen atoms, two of which come from L2 and the other two are from water molecules. The Co-N and Co-O distances are within the range of 2.204(6)-2.142(5) Å and 2.066(5)-2.121(4) Å, respectively. The 1D zigzag chains are further packed in the crystal lattice in a parallel fashion, further stabilized by hydrogen bonding involving coordinated water molecules at Co(1) and carboxylate oxygen atoms at Co(2) [O‚‚‚O ) 2.695(7)-2.746(7) Å; ∠OH‚‚‚O ) 132.2-170.0(12)°]. A solvate water molecule is found to be trapped in the groove of 1D chain hydrogen bonded with amide nitrogen, carboxylate oxygen, and oxygen of coordinated water [N‚‚‚O ) 2.937(8) Å; ∠N-H‚‚‚O ) 166.2°; O‚‚‚O ) 2.799(8)-2.785(9) Å; ∠O-H‚‚‚O ) 161.0(8)-167.8°]. It also forms a hydrogen bond with the carboxylate oxygen of the neighboring chain [O‚‚‚O ) 2.760(8) Å; ∠O-H...O ) 156.0(10)°] (Figure 3b). {Zn(µL1′′)(µL2)}n (4). Crystals of 4 belong to a centrosymmetric monoclinic space group P21/n. The metal center Zn2+ is coordinated to two different pyridyl nitrogen atoms coming from
2 C15H17N3O8Zn 432.69 0.78 × 0.38 × 0.27 monoclinic C2/c 28.4154(16) 7.3477(4) 17.0939(10) 90.00 108.4690(10) 90.00 3385.2(3) 8 1.698 1776 1.503 100(2) -27/37, -9/9, -22/22 2.49/28.27 9823/3913/3720 3913/0/314 1.072 R1 ) 0.0240 wR2 ) 0.0662 R1 ) 0.0253 wR2 ) 0.0670 6 C15H21CoN3O9.28 450.76 0.68 × 0.54 × 0.50 triclinic P1h 8.0039(5) 10.7289(7) 11.3359(8) 78.3630(10) 87.1160(10) 79.8900(10) 938.52(11) 2 1.595 466 0.971 100(2) -10/9, -13/14, -12/14 1.83/28.20 5618/4114/3778 4114/0/344 1.111 R1 ) 0.0282 wR2 ) 0.0767 R1 ) 0.0308 wR2 ) 0.0861
3 C15H19CoN3O9 444.26 0.67 × 0.34 × 0.22 triclinic P1h 7.8921(11) 10.2130(14) 11.6733(15) 104.831(2) 95.276(3) 90.161(3) 905.4(2) 2 1.630 458 1.004 100(2) -3/10, -12/13, -15/14 1.81/28.29 5123/3845/2374 3845/0/278 1.065 R1 ) 0.0710 wR2 ) 0.1620 R1 ) 0.1169 wR2 ) 0.2042
the ligand N-(4-pyridyl)nicotinamide L1′′ and two oxygen atoms of two symmetry related anionic ligands L2. The metal center displays a slightly distorted tetrahedral geometry. The Zn-N and Zn-O distances are within the range of 2.025(1)-2.026(1) Å and 1.935(2)-1.945(2) Å, respectively; the corresponding angles are 113.1(1) and 121.7(1)°. Both the ligands L1′′ and L2 display propagating coordination mode by connecting the adjacent tetrahedral Zn2+ metal centers, thereby generating an intriguing 2D polymeric MOF. Angular ligating topology of the ligand L1′′ produces a zigzag 1D polymer through coordination to adjacent metal centers, and such chains are further bridged by the carboxylate ligand L2 resulting in a zigzag tape having a grid type of architecture containing four metal centers. Because of the tetrahedral geometry of the metal center, such tapes further propagate into a 2D corrugated framework (Figure 4). Such 2D corrugated sheets are further packed in a parallel fashion and stacked perpendicular to the a-c plane. It is interesting to note that there is no solvate molecule in the crystal
1906 Crystal Growth & Design, Vol. 6, No. 8, 2006
Figure 1. Crystal structure of 1a displaying two interacting grids (orange and purple); metal centers as well as atoms involved in hydrogen bonding are shown as solid balls; red ) O; blue ) N; hydrogen atoms are not shown for clarity; dotted lines represent hydrogen bonding.
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Figure 3. Crystal structure of 3; (a) 1D zigzag polymeric chain showing two differently coordinated metal centers (solid ball); (b) aggregation of 1D chain in parallel fashion through hydrogen bonding; interacting chains are shown in two different colors (purple and orange); solvate oxygen atoms are shown as solid red balls; hydrogen atoms are not shown for clarity; dotted lines represent hydrogen bonding.
Figure 4. Illustration of the crystal structure of 4 displaying corrugated sheet architecture; metal centers are shown as solid balls; N-donor and O-donor ligands are shown in purple and orange, respectively; hydrogen atoms are not shown for clarity.
Figure 2. Illustration of the crystal structure of 2; (a) 1D zigzag polymeric chain displaying two differently coordinated metal centers (solid ball); (b) self-assembly of 1D chains and their packing in opposite fashion; packed chains are shown in two different models (space filling/ ball and stick) and two different colors (purple and orange); solvate water molecules, hydrogen atoms are not shown for clarity; dotted lines represent hydrogen bonding.
structure; the amide oxygen atom is free from hydrogen bonding, and the amide nitrogen atom is involved in an intra-network hydrogen bonding with one of the carboxylate oxygens of the maleate ligand [N‚‚‚O ) 2.984(2) Å; ∠N-H‚‚‚O ) 160(2)°]. {[Cd(H2O)(µL1′′)(µL2)‚2H2O]}n (5). Crystals of 5 belong to the noncentrosymmetric monoclinic space group C2. The metal center Cd2+ is hepta-coordinated; two pyridyl nitrogen atoms of the ligand L1′′, four oxygen atoms of maleate ligand L2, and one oxygen atom of a water molecule occupy the metal coordination sphere; the Cd-N and Cd-O distances are within the range of 2.304(6)-2.341(5) Å and 2.278(6)-2.683(5) Å, respectively. The corresponding angle ∠N-Cd-N is 89.2(2)°, indicating that the L1′′ ligands are approaching the metal center in a cis fashion, whereas the angle involving the carboxylate C atoms and metal center is 141.6(1)°, indicating that the carboxylate ligands approach the metal center in a trans fashion. The water molecule occupys the seventh coordination site, which
is opposite to one of the coordinating nitrogen atoms. In the crystal structure, the anionic maleate ligand L2 forms a righthanded helical chain by coordinating adjacent metal centers, which runs approximately along the b-axis. Such helical chains are cross-linked by the topologically angular ligand L1′′, resulting into the formation of a 2D framework (Figure 5). A hydrogen-bonded water dimer [O‚‚‚O ) 2.842(11) Å; ∠O-H‚ ‚‚O ) 155(8)°] is located within the 2D framework. The water dimer is further hydrogen bonded with the oxygen atoms coming from amide metal bound water and carboxylate moieties [O‚‚‚O ) 2.664(9)-2.792(6) Å; ∠O-H‚‚‚O ) 168.0(10)174.0(12)°]. The amide nitrogen is found to form an intranetwork hydrogen bonding with one of the carboxylate oxygen atoms [N-H‚‚‚O ) 2.928(8) Å; ∠N-H‚‚‚O ) 158.0°]. 2D frameworks are further packed in parallel fashion perpendicular to the a-c plane. {[Co(H2O)4(µL1′′)Co(L3)2](H2O)2]‚1.3H2O]}n (6). Compound 6 crystallizes in a centrosymmetric triclinic space group Pıˆ. In the crystal structure, the N-donor ligand, namely, N-(4pyridyl)nicotinamide L1′′, coordinates to the adjacent metal center through both the pyridyl nitrogen atoms resulting in a 1D zigzag polymeric structure (Figure 6a). There are two types of coordination environment of the metal centers, both of which display ideal octahedral geometry. The axial sites of Co(1) are occupied by the N-donor bidentate ligand L1′′ through its 4-amino pyridyl moiety, and the equatorial sites are coordinated by four water molecules. On the other hand, the axial sites of Co(2) are coordinated by L1′′ through its nicotinic acid moiety; the equatorial positions are occupied by four oxygen atoms, two of which come from the anionic ligand
1D Chains, 2D Corrugated Sheets in MOFs
Figure 5. Illustration of the crystal structure of 5 displaying 2D corrugated sheet architecture arising from cross-linking of a righthanded helical network of metal-carboxylate with N-donor ligands (shown in red and purple). Solvate water molecules (red balls) are shown to form various hydrogen-bonding interactions (dotted lines).
Figure 6. Crystal structure of 6; (a) 1D zigzag chain; (b) selfassociation of 1D chains via several hydrogen-bonding interactions displaying a channel space occupied by solvate water molecules (red balls); hydrogen atoms are not shown for clarity; dotted lines represent hydrogen bonding.
succinate L3 and the other two are from water molecules. The Co-N and Co-O distances are within the range of 2.200(1)2.144(1) Å and 2.025(1)-2.135(1) Å, respectively. The 1D zigzag chains are packed in the crystal lattice in a parallel fashion, further stabilized by hydrogen bonding involving coordinated water molecules at Co(1) and carboxylate oxygen atoms at Co(2) [O‚‚‚O ) 2.595(2)-2.875(2) Å; ∠OH‚‚‚O ) 168.0(3)-176.0(3)°]. In the crystal structure, one fully occupied solvate water molecule is found to be hydrogen bonded with amide nitrogen, amide oxygen, carboxylate oxygen, and coordinated water molecules of two adjacent 1D polymeric chains [N‚‚‚O ) 2.980(2) Å; ∠N-H‚‚‚O ) 169.0(2)°; O‚‚‚O ) 2.741(2)-3.003(2) Å; ∠O-H‚‚‚O ) 152.0(3)-175.0(3)°]. Crystal packing reveals that both the ordered and disordered water oxygen atoms are located in a channel space along the a-axis generated due to the parallel packing of the 1D polymeric chains (Figure 6b). The N-donor exo-bindentate ligands L1, L′, and L1′′ are structural isomers. While L1 has a linear ligating topology, the other two, namely, L′ and L1′′, display angular topology. On the other hand, the carboxylate ligands maleate L2 and succinate L3 can behave as extended bidentate ligands capable of
Crystal Growth & Design, Vol. 6, No. 8, 2006 1907
displaying both linear and angular ligating topology; they can also show chelating topology. The coordination polymers studied here belong to two categories: (i) one that displays an extended 2D framework structure wherein both the ligands coordinate to the metal center through both of their ligating sites, and (ii) one that displays a 1D extended framework wherein one of the ligands (carboxylate) does not propagate through coordination. Thus, structures of 1a, 1b, 4, and 5 belong to category (i), and 2, 3, and 6 display features that match category (ii). 1a and 1b are isostructural, and it is quite expected since the only difference between these two polymers is in the metal center (Co2+ for 1a and Zn2+ for 1b). Because of the linear ligating topology of L1, the possibility of a linear ligating topology of L2 and frequently occurring octahedral metal center of both the metal ions, a grid type of extended 2D network is formed. It is interesting to note that the wavy (corrugated) 2D networks are densely packed on top of each other via hydrogen bonding involving the hydrogen-bond capable backbone (2° amide) of L1 and carboxylate oxygen and coordinated water oxygen of the neighboring framework instead of displaying amide-amide complementary hydrogen bonding.15 This could be because of the fact that the inter-framework amide functionalities cannot approach each other closely enough to form a complementary amide-amide hydrogen bond due to steric reasons arising because of the corrugated topology of the framework. It is also interesting to note that there are no solvate guest molecules in these structures. On the other hand, the N-donor ligand L1′′ used in 4 and 5 display angular ligating topology. Thus, it is remarkable that these two structures show an extended 2D framework. In 4, the anionic ligand L2 coordinates the tetrahedral Zn2+ in such a manner that a zigzag 1D chain is formed. Such chains are further bridged by the N-donor ligand L1′′ resulting in a 2D framework. On the other hand, the anionic ligand L2 and the metal center Cd2+ in 5 display extended right-handed helical chains which are bridged by the N-donor ligand L1′′. In category (ii), however, the N-donor ligands are L1′ and L1′′, both of which display angular ligating topology. Thus, formation of a 1D extended polymeric framework in the assemblies of these ligands, L2 and L3 (both may show linear ligating topology) and octahedral metal centers as observed in the corresponding crystal structures of 2, 3, and 6 is quite expected. It may be noted that all these three structures display two types of coordination spheres of the metal centers. It is also interesting to note that 3 and 6 can be considered as isostructural, displaying the same space group (Pıˆ) and similar cell axes dimensions. The overlay of these two structures further confirms this fact (Figure S1, Supporting Information). Thermal Analysis. Thermogravimetric (TG) data for compounds 1a-6 are recorded in Table 2, and the corresponding TG curves are given in the Supporting Information. It may be noted that the complete collapse of the framework in all the polymers studied here takes place within the temperature range of 401.0-582.7 °C except in 4 wherein the O-donor ligand is not released from the framework until 600 °C. All the water molecules, coordinated and uncoordinated, are released within the temperature range of 128.4-191.9 °C. In all cases, the O-donor ligand is released from the framework at a lower temperature compared to that of the N-donor ligand except in the case of 1a wherein the reverse is observed. The isostructural coordination polymers 1a and 1b show different thermal behaviors. This could be explained in the following manner. The ionic radius of Co(II) is 0.735 Å, while that for
1908 Crystal Growth & Design, Vol. 6, No. 8, 2006
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Table 2. TG Data for 1a-6 wt loss/% obsd
calcd
peak temperatures/°C
5.08 30.2 45.0
4.6 29.2 51.0
Compound 1a 191.9 306.0 401.0
loss of 1 H2O 1 L2 1 L1
5.7 55.2 24.2
4.5 50.2 28.7
Compound 1b 171.6 363.7 507.9
1 H2O 1 L1 1 L2
13.0 47.7 27.5
12.5 46.0 26.4
Compound 2 133.3 268.0, 353.0 530.8, 582.7
3 H2O 1 L1′ 1 L2
16.4 44.4 23.3
16.2 44.8 25.6
Compound 3 131.3 263.1, 310.8, 386.7 423.3
4 H2O 1 L1′′ 1 L2
50.6
52.6
Compound 4 356.9
1 L1′′
11.1 66.4
11.2 65.3
Compound 5 126.9 269.5, 332.7
3 H2O 1 L1′′, 1 L2
17.0 43.5 24.5
17.1 44.1 25.2
Compound 6 128.4 316.1, 349.5 434.8
4.3 H2O L1′′ L3
Zn(II) is 0.60 Å. Thus, the effective nuclear charge for Zn(II) is understandably higher than that of Co(II). Therefore, Zn(II) is a harder acid as compared to Co(II) and is expected to bind a hard base like carboxylate more strongly. On the other hand, the 2+ oxidation state of Co is known to be stabilized by the π acceptor ligands such as pyridine, cyanide, etc. Thus, the Co(II)-N (pyridine) coordinate bond is expected to be stronger than the corresponding bond with Zn(II). Therefore, in the TG analysis, the carboxylate ligand L2 in 1a is released at a lower temperature than that in 1b. However, in other Co(II)-based coordination polymers, namely, 3 and 6, the carboxylate ligands are released at a higher temperature compared to that of N-donor ligands. Critical examination of the crystal structures of 1a, 3, and 6 revealed that the carboxylate ligands in 3 and 6 are involved in much more hydrogen-bonding interactions (see crystal structure description) with the coordinated and uncoordinated water molecules compared to that of 1a. Therefore, the carboxylate ligands in 3 and 6 are released at a higher temperature. In 2, 3, 5 and 6, the ligand release process appears to be multistage displaying two or more peaks in the differential of the TG curves. Satisfactory correspondence between the observed and the calculated weight loss data indicates a good agreement between single crystal and TG data. Conclusions We studied a series of coordination polymers derived from N-donor exo-bidentate ligands (having a hydrogen-bonding capable backbone and different ligating topology) and O-donor anionic carboxylate ligands - a mixed ligand system not explored to a great extent in MOF research. As expected, all the polymers are counterion free due to the presence of an anionic carboxylate ligand. Out of six polymers studied herein, four (1a, 1b, 4, and 5) display extended grid type architecture, which is generally expected from ligands having linear ligating topology. However, it is remarkable that ligand L1′′, which has angular ligating topology, is able to form a grid type framework in 4 and 5. Especially in 5, it is interesting to note how L1′′
cross-links right-handed helical chains of metal-anion to form the grid architecture. Because of the presence of a right-handed helix, a chiral (noncentric space group C2) assembly is generated from achiral components in 5. It may be noted that in 2, 3, and 6, the anionic carboxylate ligands fail from extended coordination. Thus, a 1D polymeric chain arising from extended coordination between N-donor ligands and adjacent metal centers is observed in these structures. The hydrogen-bonding capable backbone (2°-amide) did not participate in complementary hydrogen bonding;15 instead, it forms hydrogen bonding with a carboxylate moiety, coordinated water, and solvate water molecules. In two cases, namely, in 3 and 4, the oxygen atom of the amide moiety is interestingly free from any hydrogenbonding interactions. Various other mixed ligand systems using similar N-donor and O-donor carboxylate ligands are currently under investigation in our laboratory. Experimental Section Materials and Measurements. Syntheses and characterization of ligands L1, L1′, and L1′′ were previously reported by our group.16 All other chemicals were commercially available (Aldrich) and used without further purification. Microanalyses were performed on a Perkin-Elmer elemental analyzer 2400 Series II. FT-IR spectra were recorded using Perkin-Elmer Spectrum GX, and TGA analyses were performed on a Mettler Toledo TGA/SDTA851e. Powder X-ray patterns were recorded on an XPERT Philips (CuKR radiation, λ ) 1.5418 Å) diffractometer. Syntheses. {Co(µL1)(µL2)(H2O)}n (1a). A ethanolic solution (15 mL) of N-(4-pyridyl)isonicotinamide (L1) (99 mg, 0.5 mmol) was added to an aqueous solution (15 mL) of Co(NO3)2‚6H2O (145 mg, 0.5 mmol) in a 50 mL R.B. flask. The solution thus obtained was stirred at 60 °C for 30 min in a Carousel 6 Place Workstation. A 10 mL solution of disodium maleate (L2) (80 mg, 0.5 mmol) in a water/ethanol (1:1 v/v) mixture was added to the above solution. The mixture was stirred at 70 °C for 4 h and cooled to room temperature followed by filtration. The filtrate was evaporated to a volume of ∼25 mL over a water bath. Slow evaporation of this solution at room temperature affords X-ray quality single crystals. Yield: 66.6% (130 mg, 0.33 mmol). Anal. Calc. for C15H13CoN3O6: C, 46.17; H, 3.36; N, 10.77 Found C, 45.85; H, 2.93; N, 10.28. FT-IR (cm-1): 3448b, 3312b, 3159w, 3066s, 2981w, 2893m, 2819w, 1971w, 1669vs, 1602s, 1567m, 1530b, 1428s, 1395vs, 1344vs, 1323s, 1311m, 1215s, 1130m, 1108m, 1073s, 1018s, 994m, 972m, 897m, 841vs, 768s, 709w, 692s, 644vs, 591s, 544vs, 508s {Zn(µL1)(µL2)(H2O)}n (1b). 1b was synthesized by the same procedure adopted for 1a using Zn(ClO4)2‚6H2O. Yield: 60.6% (120 mg, 0.30 mmol) Anal. Calc. for C15H13N3.O6Zn: C, 45.42; H, 3.30; N, 10.59 Found C, 45.12; H, 3.11; N 9.92. FT-IR (cm-1): 3450b, 3323b, 3157m, 3066s, 2964w, 2893m, 2820w, 2363m, 1972b, 1670vs, 1531b, 1430s, 1396vs, 1345vs, 1315s, 1216vs, 1129m, 1109s, 1073s, 1018vs, 995vs, 971s, 895b, 842vs, 768vs, 709w, 691vs, 642vs, 591s, 574w, 544vs, 507s {[Zn(H2O)4(µL1′)Zn(L2)2]‚H2O}n (2). 2 was synthesized by the same procedure adopted for 1a using N-(3-pyridyl)isonicotinamide (L1′) and Zn(ClO4)2‚6H2O. Yield: 46.2% (100 mg, 0.23 mmol) Anal. Calc. for C15H17N3O8Zn: C, 41.64; H, 3.96; N, 9.71; Found C, 40.96; H, 3.12; N 9.26. FT-IR (cm-1): 3542vs, 3439m, 3199b, 3043m, 2359s, 1679vs, 1648s, 1549s, 1486vs, 1424s, 1341m, 1310vs, 1246m, 1223w, 1182s, 1136w, 1105m, 1065m, 1025s, 980s, 846vs, 810m, 765m, 701vs, 645s, 604s, 534s, 421s {[Co(H2O)4(µL1′′)Co(L2)2(H2O)2]‚H2O}n (3). 3 was synthesized by the same procedure adopted for 1a using N-(4-pyridyl)nicotinamide (L1′′). Yield: 63.0% (90 mg, 0.31 mmol). Anal. Calc. for C15H19CoN3O9: C, 40.55; H, 4.31; N, 9.46, Found C, 40.03; H, 3.73, N, 9.11. FT-IR (cm-1): 3259b, 2446b, 2000w, 1971w, 1945w, 1694vs, 1600s, 1551b, 1436s, 1401s, 1335vs, 1302vs, 1274m, 1214vs, 1119vs, 1053s, 1034m, 1012s, 984m, 935w, 896m, 854w, 836s, 809m, 763m, 716s, 694w, 647m, 621s, 591m, 537vs, 471m, 441s. {Zn(µL1′′)(µL2)}n (4). 4 was synthesized by the same procedure adopted for 1a using N-(4-pyridyl)nicotinamide (L1′′) and Zn(ClO4)2‚ 6H2O. Yield: 47.6% (90 mg, 0.24 mmol) Anal. Calc. for C15H11N3O5Zn: C, 47.58; H, 2.93; N, 11.10, Found C, 47.28; H, 2.60; N 11.01. FT-IR (cm-1): 3753w, 3273b, 3077m, 2348m, 2010w, 1701vs, 1656s,
1D Chains, 2D Corrugated Sheets in MOFs 1589s, 1510vs, 1420s, 1383vs, 1332vs, 1298vs, 1208vs, 1171s, 1112vs, 1066m, 1029s, 976w, 897b, 841vs, 703s, 653m, 626vs, 601s, 541s, 485m, 426s. {[Cd(H2O)(µL1′′)(µL2)‚2H2O]}n (5). 5 was synthesized by the same procedure adopted for 1a using N-(4-pyridyl)nicotinamide (L1′′) and Cd(NO3)2‚6H2O. Yield: 50.0% (120 mg, 0.25 mmol) Anal. Calc. for C15H15CdN3O7: C, 39.02; H, 3.27; N, 9.10. Found C, 40.62, H, 2.74, N, 9.88. FT-IR (cm-1): 3140b, 3250w, 3173w, 3075m, 3012w, 1948w, 1682vs, 1646w, 1601vs, 1561vs, 1517s, 1428vs, 1335vs, 1305vs, 1212vs, 1180s, 1138w, 1123m, 1102w, 1067w, 1046s, 1018s, 981m, 949w, 898m, 860s, 831vs, 738m, 702s, 671w, 641s, 596m, 538s, 471w, 420m {[Co(H2O)4(µL1′′)Co(L3)2](H2O)2]‚1.3H2O]}n (6). 6 was synthesized by the same procedure adopted for 3 using disodium succinate (L3). Yield: 53.3% (120 mg, 0.26 mmol) Anal. Calc. for C15H21CoN3 O9.28:C, 39.97; H, 4.81; N, 9.32; Found C, 40.08; H, 4.74; N 9.43. FT-IR (cm-1): 3080b, 2360vs, 2340s, 1685vs, 1597s, 1538b, 1422vs, 1334vs, 1297vs, 1211vs, 1185s, 1123s, 1053s, 1034m, 1012s, 963m, 837s, 718s, 681b, 597m, 539m, 433s. Single-Crystal X-ray Diffraction. X-ray single-crystal data were collected using MoKR (λ ) 0.7107 Å) radiation on a SMART APEX diffractometer equipped with CCD area detector. Data collection, data reduction,17 structure solution/refinement18-19 were carried out using the software package of SMART APEX. All structures were solved by direct methods and refined in a routine manner. In all cases, non-hydrogen atoms were treated anisotropically. Whenever possible, the hydrogen atoms were located on a difference Fourier map and refined. In other cases, the hydrogen atoms were geometrically fixed. In 6, the extra electron densities left at the final stage of refinement were assigned as an oxygen atom of disordered water molecules. The disordered oxygen atom was treated as follows. The SOF of this peak was refined by keeping the positional parameters and temperature factor fixed (at 0.05). After refinement of SOF, the thermal parameter of the disordered oxygen atoms was refined by keeping both SOF fixed at their refined values and positional parameters. At the final state of refinement, both positional and isotropic thermal parameters were refined.
Acknowledgment. Department of Science & Technology, New Delhi, India, is thankfully acknowledged for financial support. D.K.K. acknowledges CSIR, New Delhi, India, for a SRF. Supporting Information Available: X-ray crystallographic files in CIF format, selected bond distances and angles involving metal centers (Table S1), hydrogen-bonding parameters (Table S2), structure overlay of 3 and 6 (Figure S1), TG curves, and X-ray powder diffraction patterns. This material is available free of charge via the Internet at http://pubs.acs.org.
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