One-Dimensional Chains, Two-Dimensional Corrugated SheetsHaving a Cross-Linked Helix in Metal-Organic Frameworks:Exploring Hydrogen-Bond Capable Backbones and LigatingTopologies in Mixed Ligand Systems

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 bysingle-crystal X-ray diffraction.1a, 1b, 4, and5 display a grid architecture, whereas2, 3, and6 display a one-dimensional polymericchain.5 is an example of a chiral metal-organic framework (due to the presence of a right-handed helix) assembled from achiralcomponents. The supramolecularly recognizable backbone, namely, secondary amide of N-donor ligands does not show typicalself-recognition. This study represents one of the limited explorative studies on counterion free mixed ligand systems containing anN-donor exo-bidentate ligand with a supramolecularly recognizable backbone and O-donor carboxylate ligand.

Introduction

Crystal engineering1 in general and crystal engineering ofmetal-ligand coordination (MLC) polymers2 in particular haveemerged as highly challenging as well as rewarding areas ofresearch due to their various potential applications in designingfunctional 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 magneticmaterials.7 Intelligent ligand design and the proper choice of ametal center are the main keys to the design of intriguing anduseful coordination polymers.8 On the other hand, crystalengineering of organic solids9 are mainly governed by strongand directional hydrogen bonding.10 The combination of bothMLC and hydrogen bonding in designing various supramolecu-lar architectures should be considered as an attractive designstrategy because of the possibility of structural variations andguest entrapment induced by specific hydrogen-bonding interac-tions.

One of the major problems in generating porous openframeworks in coordination polymers is the presence of coun-terions that often occupy the pores, thereby reducing thepossibility of occluding the guests in the framework. By usinganionic ligands such as dicarboxylate anions along with pyridylN-donor exo-bidentate ligands, it is possible to create openframeworks devoid of counterions.11 A CSD (Version 5.27, Nov.2005) search with a search moiety containing any transitionmetal ion coordinated with a 4-substituted pyridyl fragment anda carboxylate fragment gave only 75 relevant hits (consideringonly bidentate N-donors and dicarboxylate ligands coordinatingthrough both ends) wherein 4,4′-bipyridine, 1,2-bis(4-pyridyl)-ethane, 1,2-bis(4-pyridyl)ethylene are the main N-donor biden-tate ligands used. A similar search with a search fragmentcontaining 3-substituted pyridyl fragments resulted in zerorelevant hits. Thus, such mixed ligand systems are much lessexplored, and in these studies, largely pyridyl N-donor exo-

bidentate ligands having a linear ligating topology and innocentbackbone have been used. The advantage of using a hydrogen-bond capable backbone of the ligand is in the inter-networksupramolecular recognition process, which may play a signifi-cant role in making the architecture more robust and allow theguest molecules to bind strongly through hydrogen bonding withthe backbone. Recently, we have shown12 along with others13

that hydrogen-bonding backbones of N-donor ligands indeedplay a significant role in inter-network hydrogen-bondingrecognition.

Examples of mixed ligand systems containing N-donorligands (having hydrogen-bonding capable backbones) andcarboxylate O-donor ligands are rare. To the best of ourknowledge, 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 supramolecularstructural diversity that could have been achieved by takingadvantage of topological variation in ligating sites have not beenexplored to a great extent presumably because of the difficultyin synthesizing the topological variants of 4,4′-bipyridine.

Thus, we are interested in exploring various supramolecularstructures that can be generated in a counterion free mixed ligandsystems containing pyridyl N-donors and carboxylate O-donorligands. For this purpose, we have chosen a series of pyridylN-donor bidentate ligands having various ligating topologies(linear and angular) and a 2°-amide backbone which is knownto self-assemble through complementary hydrogen bonding.15

The anionic ligands chosen are maleate and succinate. Whilemaleate represents a rigid ligand owing to the presence of anunsaturated backbone, succinate is a flexible ligand due to itssaturated backbone, thereby allowing a C-C bond rotation. Wehave chosen Zn2+, Co2+, and Cd2+ as metal centers since all ofthem are known to form mainly an octahedral coordinationgeometry in the presence of strongσ-donor ligands such asoxygen. An octahedral metal center is particularly important to

* To whom correspondence should be addressed. E-mail: parthod123@rediffmail.com; dastidar@csmcri.org (P.D.); amitava@csmcri.org (A.D.)

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10.1021/cg0600344 CCC: $33.50 © 2006 American Chemical SocietyPublished on Web 07/15/2006

facilitate a grid-type framework involving an N-donor, O-donorligand and a metal center in a 1:1:1 molar ratio (Scheme 1).

Results and Discussions

Crystal Structures. Crystallographic parameters of thecoordination polymers reported herein are listed in Table 1.Selected bond distances and angles of the coordination sphereof the metal centers and hydrogen-bonding parameters are listedin Tables S1 and S2, respectively (Supporting Information).

{Co(µL1)(µL2)(H2O)}n (1a). The crystals of1a belong tospace groupP21/c, and the coordination geometry of the metalcenter Co2+ is a slightly distorted octahedron, the equatorialsites of which are occupied by four oxygen atoms coming fromtwo maleate moieties and one water molecule. The axialpositions are occupied by both the 4-amino pyridyl and iso-nicotinic acid moieties of the N-donor exo-bidentate ligandL1.

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 anglesvary from 87.8(1) to 92.5(1)°. While one of the maleate moietiesis coordinated to the metal center in a chelate fashion, the otherone coordinates in a monodentate fashion. The crystal structureof 1acan be best described as a two-dimensional (2D) grid typecoordination polymer wherein the N-donor exo-bindentateligand, namely,N-(4-pyridyl)isonicotinamideL1, forms one-dimensional (1D) polymeric chains by coordinating the adjacentmetal centers. Such chains are further bridged by maleate ligandL2 resulting in the formation of a 2D grid polymer. The gridsize is approximately 8.60× 3.80 Å after taking the van derWaals radii into account. Because of the syn coordinationtopology of the maleate, the resultant framework displays acorrugated sheet type of architecture. The 2D sheets are furtherpacked on top of each other in an off-set fashion stabilizedthrough various hydrogen-bonding interactions; while thecoordinated water molecule of one network forms hydrogenbonds with the amide oxygen atom of the neighboring sheet[O‚‚‚O ) 2.734(3) Å; ∠O-H‚‚‚O ) 175(4)°], the amide

nitrogen atom interacts through hydrogen bonds with thecarobxylate 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 of1b which isa Zn analogue of1a is isostructural with that of1a displayingan identical space group and similar cell dimensions (Table 1).Thus, a supramolecular architecture and hydrogen-bondinginteractions are also identical with that of1a.

{[Zn(H 2O)4(µL1′)Zn(L2)2]‚H2O}n (2). Crystals of2 belongto the monoclinicC2/c space group. There are two types ofcoordination geometries of the metal centers in the crystalstructure. Both the metal centers are found to be hexacoordi-nated. Zn(1) is coordinated by two symmetry related ligands,namely,N-(3-pyridyl)isonicotinamideL1′, through its pyridylring of isonicotinic acid moiety approaching the metal centerin a cis fashion; the corresponding Zn-N distance is 2.078(1)Å; ∠N-Zn-N ) 90.6(1)°. The other four coordination sitesare occupied by four oxygen atoms coming from two symmetryrelated carboxylate moieties of maleate ligandL2. The car-boxylate moieties display chelating coordination modes withthe metal center showing one short [2.027(1) Å] and one long[2.443(1) Å] Zn-O coordination bond. Thus, the coordinationgeometry of the metal center Zn(1) cannot be categorized toany conventional coordination geometry such as octahedron.Whereas, the other metal center Zn(2) displays a perfectoctahedral coordination sphere. The metal center Zn(2) iscoordinated centrosymmetrically to two symmetry related ligandL1′ through its 3-pyridylamine moiety (axial coordination) andto four water molecules; the corresponding Zn-N and Zn-Odistances are 2.178(1) and 2.096(1) Å, respectively; all∠N-Zn-N and∠O- Zn-O display ideal 180.0(1)°. The MOF in2 can be best described as a 1D zigzag coordination polymerwherein the N-donor exo-bidentate ligandL1′ coordinates theadjacent metal centers and the anionic ligandL2 coordinatesthe metal center through one of its carboxylate moieties; theother one remains free from coordination (Figure 2a).

The 1D chains are self-assembled in a parallel fashionsustained by several hydrogen-bonding interactions involvingan amide nitrogen atom and a carboxylate oxygen atom(coordinated to the metal center) [N‚‚‚O ) 2.804(1) Å;∠N-H‚‚‚O ) 162(2)°] and carboxylate oxygen atoms (free fromcoordination) and coordinated water molecules [O‚‚‚O ) 2.628-(1)-2.724(1) Å;∠O-H‚‚‚O ) 168(2)-172(2)°] of the neigh-boring chain. Such assembly of 1D chain forms sheet-likestructures that are further packed on top of each other in anopposite fashion sustained by hydrogen bonding involving amideoxygen 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 spacedisplaying hydrogen-bonding interactions with carboxylatemoieties 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 of3 belong to a centrosymmetric triclinic space groupPı. In thecrystal structure, the N-donor ligand, namely,N-(4-pyridyl)-nicotinamideL1′′, coordinates to the adjacent metal centerthrough both the pyridyl nitrogen atoms resulting in a 1D zigzagpolymeric structure (Figure 3a). There are two types ofcoordination environments of the metal centers, both of whichdisplay ideal octahedral geometry. The axial sites of Co(1) areoccupied by the N-donor bidentate ligandL1′′ through its4-amino pyridyl moiety, and the equatorial sites are coordinatedby four water molecules. Whereas the axial sites of Co(2) iscoordinated byL1′′ through its nicotinic acid moiety, the

Scheme 1

1904 Crystal Growth & Design, Vol. 6, No. 8, 2006 Kumar et al.

equatorial positions are occupied by four oxygen atoms, twoof which come fromL2 and the other two are from watermolecules. The Co-N and Co-O distances are within the rangeof 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 latticein a parallel fashion, further stabilized by hydrogen bondinginvolving coordinated water molecules at Co(1) and carboxylateoxygen atoms at Co(2) [O‚‚‚O ) 2.695(7)-2.746(7) Å;∠O-H‚‚‚O ) 132.2-170.0(12)°]. A solvate water molecule is foundto be trapped in the groove of 1D chain hydrogen bonded withamide nitrogen, carboxylate oxygen, and oxygen of coordinatedwater [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 alsoforms a hydrogen bond with the carboxylate oxygen of theneighboring chain [O‚‚‚O ) 2.760(8) Å;∠O-H...O ) 156.0-(10)°] (Figure 3b).

{Zn(µL1′′)(µL2)}n (4). Crystals of4 belong to a centrosym-metric monoclinic space groupP21/n. The metal center Zn2+ iscoordinated to two different pyridyl nitrogen atoms coming from

the ligandN-(4-pyridyl)nicotinamideL1′′ and two oxygen atomsof two symmetry related anionic ligandsL2. The metal centerdisplays a slightly distorted tetrahedral geometry. The Zn-Nand 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 correspondingangles are 113.1(1) and 121.7(1)°. Both the ligandsL1′′ andL2 display propagating coordination mode by connecting theadjacent tetrahedral Zn2+ metal centers, thereby generating anintriguing 2D polymeric MOF. Angular ligating topology ofthe ligandL1′′ produces a zigzag 1D polymer through coordina-tion to adjacent metal centers, and such chains are furtherbridged by the carboxylate ligandL2 resulting in a zigzag tapehaving a grid type of architecture containing four metal centers.Because of the tetrahedral geometry of the metal center, suchtapes further propagate into a 2D corrugated framework (Figure4).

Such 2D corrugated sheets are further packed in a parallelfashion and stacked perpendicular to the a-c plane. It isinteresting to note that there is no solvate molecule in the crystal

Table 1. Crystal Data for 1a, 1b, 2-6

crystal data 1a 1b 2 3empirical formula C15H13CoN3O6 C15H13N3O6Zn C15H17N3O8Zn C15H19CoN3O9

formula weight 390.21 396.65 432.69 444.26crystal size (mm3) 0.58× 0.38× 0.31 0.63× 0.56× 0.43 0.78× 0.38× 0.27 0.67× 0.34× 0.22crystal system monoclinic monoclinic monoclinic triclinicspace group P21/c P21/c C2/c P1ha (Å) 9.1790(7) 9.1935(9) 28.4154(16) 7.8921(11)b (Å) 8.7296(7) 8.8044(9) 7.3477(4) 10.2130(14)c (Å) 19.4811(15) 19.555(2) 17.0939(10) 11.6733(15)R (°) 90.00 90.00 90.00 104.831(2)â (°) 91.3150(10) 91.111(2) 108.4690(10) 95.276(3)γ (°) 90.00 90.00 90.00 90.161(3)volume (Å3) 1560.6(2) 1582.5(3) 3385.2(3) 905.4(2)Z 4 4 8 2Dcalc (g/cm3) 1.661 1.665 1.698 1.630F(000) 796 808 1776 458µ Mo KR (mm-1) 1.139 1.591 1.503 1.004temperature (K) 100(2) 100(2) 100(2) 100(2)range ofh, k, l -10/11,-8/11,-25/25 -12/9,-11/10,-25/26 -27/37,-9/9,-22/22 -3/10,-12/13,-15/14θ min/max 2.09/28.22 2.08/28.23 2.49/28.27 1.81/28.29reflections collected/unique/observed 8801/3589/3193 9170/3651/3432 9823/3913/3720 5123/3845/2374data/restraints/parameters 3589/0/274 3651/0/278 3913/0/314 3845/0/278goodness of fit onF2 1.112 1.037 1.072 1.065final R indices [I > 2σ(I)] R1 ) 0.0475 R1 ) 0.0265 R1 ) 0.0240 R1 ) 0.0710

wR2 ) 0.1043 wR2 ) 0.0672 wR2 ) 0.0662 wR2 ) 0.1620R indices (all data) R1 ) 0.0542 R1 ) 0.0283 R1 ) 0.0253 R1 ) 0.1169

wR2 ) 0.1073 wR2 ) 0.0682 wR2 ) 0.0670 wR2 ) 0.2042crystal data 4 5 6empirical formula C15H11N3O5Zn C15H15CdN3O8 C15H21CoN3O9.28

formula weight 378.64 477.70 450.76crystal size (mm3) 0.55× 0.46× 0.33 0.42× 0.24× 0.15 0.68× 0.54× 0.50crystal system monoclinic monoclinic triclinicspace group P21/n C2 P1ha (Å) 7.2357(4) 24.139(2) 8.0039(5)b (Å) 19.1593(11) 9.1383(8) 10.7289(7)c (Å) 10.7206(7) 8.2704(8) 11.3359(8)R (°) 90.00 90.00 78.3630(10)â (°) 104.7290(10) 107.591(3) 87.1160(10)γ (°) 90.00 90.00 79.8900(10)volume (Å3) 1437.37(15) 1739.0(3) 938.52(11)Z 4 4 2Dcalc(g/cm3) 1.750 1.825 1.595F(000) 768 952 466µ MoKR (mm-1) 1.742 1.307 0.971temperature (K) 100(2) 100(2) 100(2)range ofh, k, l -9/9,-25/25,-6/14 -31/26,-11/11,-10/8 -10/9,-13/14,-12/14θ min/max 2.13/28.15 1.77/28.24 1.83/28.20reflections collected/unique/observed 8490/3328/3049 5214/3706/3374 5618/4114/3778data/restraints/parameters 3328/0/261 3706/1/257 4114/0/344goodness of fit onF2 1.045 1.096 1.111final R indices [I > 2σ(I)] R1 ) 0.0272 R1 ) 0.0461 R1 ) 0.0282

wR2 ) 0.0698 wR2 ) 0.1237 wR2 ) 0.0767R indices (all data) R1 ) 0.0303 R1 ) 0.0522 R1 ) 0.0308

wR2 ) 0.0712 wR2 ) 0.1295 wR2 ) 0.0861

1D Chains, 2D Corrugated Sheets in MOFs Crystal Growth & Design, Vol. 6, No. 8, 20061905

structure; the amide oxygen atom is free from hydrogen bonding,and the amide nitrogen atom is involved in an intra-networkhydrogen bonding with one of the carboxylate oxygens of themaleate ligand [N‚‚‚O ) 2.984(2) Å;∠N-H‚‚‚O ) 160(2)°].

{[Cd(H2O)(µL1′′)(µL2)‚2H2O]}n (5). Crystals of5 belongto the noncentrosymmetric monoclinic space groupC2. Themetal center Cd2+ is hepta-coordinated; two pyridyl nitrogenatoms of the ligandL1′′, four oxygen atoms of maleate ligandL2, and one oxygen atom of a water molecule occupy the metalcoordination sphere; the Cd-N and Cd-O distances are withinthe 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 theL1′′ ligands are approaching the metal centerin a cis fashion, whereas the angle involving the carboxylate Catoms and metal center is 141.6(1)°, indicating that thecarboxylate 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 thecrystal structure, the anionic maleate ligandL2 forms a right-handed helical chain by coordinating adjacent metal centers,which runs approximately along theb-axis. Such helical chainsare cross-linked by the topologically angular ligandL1′′,resulting into the formation of a 2D framework (Figure 5). Ahydrogen-bonded water dimer [O‚‚‚O ) 2.842(11) Å;∠O-H‚‚‚O ) 155(8)°] is located within the 2D framework. The waterdimer is further hydrogen bonded with the oxygen atoms comingfrom 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 intra-network hydrogen bonding with one of the carboxylate oxygenatoms [N-H‚‚‚O ) 2.928(8) Å; ∠N-H‚‚‚O ) 158.0°]. 2Dframeworks are further packed in parallel fashion perpendicularto thea-c plane.

{[Co(H2O)4(µL1′′)Co(L3)2](H2O)2]‚1.3H2O]}n (6). Com-pound6 crystallizes in a centrosymmetric triclinic space groupPı. In the crystal structure, the N-donor ligand, namely,N-(4-pyridyl)nicotinamideL1′′, coordinates to the adjacent metalcenter through both the pyridyl nitrogen atoms resulting in a1D zigzag polymeric structure (Figure 6a).

There are two types of coordination environment of the metalcenters, both of which display ideal octahedral geometry. Theaxial sites of Co(1) are occupied by the N-donor bidentate ligandL1′′ through its 4-amino pyridyl moiety, and the equatorial sitesare coordinated by four water molecules. On the other hand,the axial sites of Co(2) are coordinated byL1′′ through itsnicotinic acid moiety; the equatorial positions are occupied byfour oxygen atoms, two of which come from the anionic ligand

Figure 1. Crystal structure of1a displaying two interacting grids(orange and purple); metal centers as well as atoms involved inhydrogen bonding are shown as solid balls; red) O; blue ) N;hydrogen atoms are not shown for clarity; dotted lines representhydrogen bonding.

Figure 2. Illustration of the crystal structure of2; (a) 1D zigzagpolymeric chain displaying two differently coordinated metal centers(solid ball); (b) self-assembly of 1D chains and their packing in oppositefashion; packed chains are shown in two different models (space filling/ball and stick) and two different colors (purple and orange); solvatewater molecules, hydrogen atoms are not shown for clarity; dotted linesrepresent hydrogen bonding.

Figure 3. Crystal structure of3; (a) 1D zigzag polymeric chain showingtwo differently coordinated metal centers (solid ball); (b) aggregationof 1D chain in parallel fashion through hydrogen bonding; interactingchains are shown in two different colors (purple and orange); solvateoxygen atoms are shown as solid red balls; hydrogen atoms are notshown for clarity; dotted lines represent hydrogen bonding.

Figure 4. Illustration of the crystal structure of4 displaying corrugatedsheet architecture; metal centers are shown as solid balls; N-donor andO-donor ligands are shown in purple and orange, respectively; hydrogenatoms are not shown for clarity.

1906 Crystal Growth & Design, Vol. 6, No. 8, 2006 Kumar et al.

succinateL3 and the other two are from water molecules. TheCo-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 aparallel fashion, further stabilized by hydrogen bonding involv-ing coordinated water molecules at Co(1) and carboxylateoxygen atoms at Co(2) [O‚‚‚O ) 2.595(2)-2.875(2) Å;∠O-H‚‚‚O ) 168.0(3)-176.0(3)°]. In the crystal structure, one fullyoccupied solvate water molecule is found to be hydrogen bondedwith amide nitrogen, amide oxygen, carboxylate oxygen, andcoordinated water molecules of two adjacent 1D polymericchains [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 disorderedwater oxygen atoms are located in a channel space along thea-axis generated due to the parallel packing of the 1D polymericchains (Figure 6b).

The N-donor exo-bindentate ligandsL1, L ′, and L1′′ arestructural isomers. WhileL1 has a linear ligating topology, theother two, namely,L ′ andL1′′, display angular topology. Onthe other hand, the carboxylate ligands maleateL2 and succinateL3 can behave as extended bidentate ligands capable of

displaying both linear and angular ligating topology; they canalso show chelating topology.

The coordination polymers studied here belong to twocategories: (i) one that displays an extended 2D frameworkstructure wherein both the ligands coordinate to the metal centerthrough both of their ligating sites, and (ii) one that displays a1D extended framework wherein one of the ligands (carboxy-late) does not propagate through coordination. Thus, structuresof 1a, 1b, 4, and5 belong to category (i), and2, 3, and6 displayfeatures that match category (ii).

1a and1b are isostructural, and it is quite expected since theonly difference between these two polymers is in the metalcenter (Co2+ for 1a and Zn2+ for 1b). Because of the linearligating topology of L1, the possibility of a linear ligatingtopology ofL2 and frequently occurring octahedral metal centerof both the metal ions, a grid type of extended 2D network isformed. It is interesting to note that the wavy (corrugated) 2Dnetworks are densely packed on top of each other via hydrogenbonding involving the hydrogen-bond capable backbone (2°amide) ofL1 and carboxylate oxygen and coordinated wateroxygen of the neighboring framework instead of displayingamide-amide complementary hydrogen bonding.15 This couldbe because of the fact that the inter-framework amide func-tionalities cannot approach each other closely enough to forma complementary amide-amide hydrogen bond due to stericreasons arising because of the corrugated topology of theframework. It is also interesting to note that there are no solvateguest molecules in these structures.

On the other hand, the N-donor ligandL1′′ used in4 and5display angular ligating topology. Thus, it is remarkable thatthese two structures show an extended 2D framework. In4, theanionic ligandL2 coordinates the tetrahedral Zn2+ in such amanner that a zigzag 1D chain is formed. Such chains are furtherbridged by the N-donor ligandL1′′ resulting in a 2D framework.On the other hand, the anionic ligandL2 and the metal centerCd2+ in 5 display extended right-handed helical chains whichare bridged by the N-donor ligandL1′′.

In category (ii), however, the N-donor ligands areL1′ andL1′′, both of which display angular ligating topology. Thus,formation of a 1D extended polymeric framework in theassemblies of these ligands,L2 andL3 (both may show linearligating topology) and octahedral metal centers as observed inthe corresponding crystal structures of2, 3, and 6 is quiteexpected. It may be noted that all these three structures displaytwo types of coordination spheres of the metal centers. It isalso interesting to note that3 and 6 can be considered asisostructural, displaying the same space group (Pı) and similarcell axes dimensions. The overlay of these two structures furtherconfirms this fact (Figure S1, Supporting Information).

Thermal Analysis. Thermogravimetric (TG) data for com-pounds1a-6 are recorded in Table 2, and the correspondingTG curves are given in the Supporting Information.

It may be noted that the complete collapse of the frameworkin all the polymers studied here takes place within thetemperature range of 401.0-582.7°C except in4 wherein theO-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 frameworkat a lower temperature compared to that of the N-donor ligandexcept in the case of1a wherein the reverse is observed. Theisostructural coordination polymers1a and1b show differentthermal behaviors. This could be explained in the followingmanner. The ionic radius of Co(II) is 0.735 Å, while that for

Figure 5. Illustration of the crystal structure of5 displaying 2Dcorrugated sheet architecture arising from cross-linking of a right-handed helical network of metal-carboxylate with N-donor ligands(shown in red and purple). Solvate water molecules (red balls) areshown to form various hydrogen-bonding interactions (dotted lines).

Figure 6. Crystal structure of6; (a) 1D zigzag chain; (b) self-association of 1D chains via several hydrogen-bonding interactionsdisplaying a channel space occupied by solvate water molecules (redballs); hydrogen atoms are not shown for clarity; dotted lines representhydrogen bonding.

1D Chains, 2D Corrugated Sheets in MOFs Crystal Growth & Design, Vol. 6, No. 8, 20061907

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 binda hard base like carboxylate more strongly. On the other hand,the 2+ oxidation state of Co is known to be stabilized by theπ acceptor ligands such as pyridine, cyanide, etc. Thus, the Co-(II)-N (pyridine) coordinate bond is expected to be strongerthan the corresponding bond with Zn(II). Therefore, in the TGanalysis, the carboxylate ligandL2 in 1a is released at a lowertemperature than that in1b. However, in other Co(II)-basedcoordination polymers, namely,3 and6, the carboxylate ligandsare released at a higher temperature compared to that of N-donorligands. Critical examination of the crystal structures of1a, 3,and 6 revealed that the carboxylate ligands in3 and 6 areinvolved in much more hydrogen-bonding interactions (seecrystal structure description) with the coordinated and uncoor-dinated water molecules compared to that of1a. Therefore, thecarboxylate ligands in3 and 6 are released at a highertemperature. In2, 3, 5 and6, the ligand release process appearsto be multistage displaying two or more peaks in the differentialof the TG curves. Satisfactory correspondence between theobserved and the calculated weight loss data indicates a goodagreement between single crystal and TG data.

Conclusions

We studied a series of coordination polymers derived fromN-donor exo-bidentate ligands (having a hydrogen-bondingcapable backbone and different ligating topology) and O-donoranionic carboxylate ligands- a mixed ligand system notexplored to a great extent in MOF research. As expected, allthe polymers are counterion free due to the presence of ananionic carboxylate ligand. Out of six polymers studied herein,four (1a, 1b, 4, and5) display extended grid type architecture,which is generally expected from ligands having linear ligatingtopology. However, it is remarkable that ligandL1′′, which hasangular ligating topology, is able to form a grid type frameworkin 4 and 5. Especially in5, it is interesting to note howL1′′

cross-links right-handed helical chains of metal-anion to formthe grid architecture. Because of the presence of a right-handedhelix, a chiral (noncentric space group C2) assembly is generatedfrom achiral components in5. It may be noted that in2, 3, and6, the anionic carboxylate ligands fail from extended coordina-tion. Thus, a 1D polymeric chain arising from extendedcoordination between N-donor ligands and adjacent metalcenters is observed in these structures. The hydrogen-bondingcapable backbone (2°-amide) did not participate in comple-mentary hydrogen bonding;15 instead, it forms hydrogen bondingwith a carboxylate moiety, coordinated water, and solvate watermolecules. In two cases, namely, in3 and4, the oxygen atomof the amide moiety is interestingly free from any hydrogen-bonding interactions. Various other mixed ligand systems usingsimilar N-donor and O-donor carboxylate ligands are currentlyunder investigation in our laboratory.

Experimental Section

Materials and Measurements.Syntheses and characterization ofligandsL1, L1′, andL1′′ were previously reported by our group.16 Allother chemicals were commercially available (Aldrich) and used withoutfurther purification. Microanalyses were performed on a Perkin-Elmerelemental analyzer 2400 Series II. FT-IR spectra were recorded usingPerkin-Elmer Spectrum GX, and TGA analyses were performed on aMettler Toledo TGA/SDTA851e. Powder X-ray patterns were recordedon an XPERT Philips (CuKR radiation,λ ) 1.5418 Å) diffractometer.

Syntheses.{Co(µL1)(µL2)(H2O)}n (1a). A ethanolic solution (15mL) of N-(4-pyridyl)isonicotinamide (L1) (99 mg, 0.5 mmol) was addedto 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°Cfor 30 min in a Carousel 6 Place Workstation. A 10 mL solution ofdisodium 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 at70 °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-rayquality 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 sameprocedure adopted for1a using Zn(ClO4)2‚6H2O. Yield: 60.6% (120mg, 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(H 2O)4(µL1′)Zn(L2)2]‚H2O}n (2). 2 was synthesized by thesame procedure adopted for1ausingN-(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 synthesizedby the same procedure adopted for1ausingN-(4-pyridyl)nicotinamide(L1′′). Yield: 63.0% (90 mg, 0.31 mmol). Anal. Calc. for C15H19-CoN3O9: 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 procedureadopted for1a usingN-(4-pyridyl)nicotinamide (L1′′) and Zn(ClO4)2‚6H2O. Yield: 47.6% (90 mg, 0.24 mmol) Anal. Calc. for C15H11N3O5-Zn: 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,

Table 2. TG Data for 1a-6

wt loss/%

obsd calcd peak temperatures/°C loss of

Compound1a5.08 4.6 191.9 1 H2O

30.2 29.2 306.0 1L245.0 51.0 401.0 1L1

Compound1b5.7 4.5 171.6 1 H2O

55.2 50.2 363.7 1L124.2 28.7 507.9 1L2

Compound213.0 12.5 133.3 3 H2O47.7 46.0 268.0, 353.0 1L1′27.5 26.4 530.8, 582.7 1L2

Compound316.4 16.2 131.3 4 H2O44.4 44.8 263.1, 310.8, 386.7 1L1′′23.3 25.6 423.3 1L2

Compound450.6 52.6 356.9 1L1′′

Compound511.1 11.2 126.9 3 H2O66.4 65.3 269.5, 332.7 1L1′′, 1 L2

Compound617.0 17.1 128.4 4.3 H2O43.5 44.1 316.1, 349.5 L1′′24.5 25.2 434.8 L3

1908 Crystal Growth & Design, Vol. 6, No. 8, 2006 Kumar et al.

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). 5was synthesized by the sameprocedure adopted for1a usingN-(4-pyridyl)nicotinamide (L1′′) andCd(NO3)2‚6H2O. Yield: 50.0% (120 mg, 0.25 mmol) Anal. Calc. forC15H15CdN3O7: 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 synthe-sized by the same procedure adopted for3 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 werecollected using MoKR (λ ) 0.7107 Å) radiation on a SMART APEXdiffractometer equipped with CCD area detector. Data collection, datareduction,17 structure solution/refinement18-19 were carried out usingthe software package of SMART APEX.

All structures were solved by direct methods and refined in a routinemanner. In all cases, non-hydrogen atoms were treated anisotropically.Whenever possible, the hydrogen atoms were located on a differenceFourier map and refined. In other cases, the hydrogen atoms weregeometrically fixed.

In 6, the extra electron densities left at the final stage of refinementwere assigned as an oxygen atom of disordered water molecules. Thedisordered oxygen atom was treated as follows. The SOF of this peakwas refined by keeping the positional parameters and temperature factorfixed (at 0.05). After refinement of SOF, the thermal parameter of thedisordered oxygen atoms was refined by keeping both SOF fixed attheir refined values and positional parameters. At the final state ofrefinement, both positional and isotropic thermal parameters wererefined.

Acknowledgment. Department of Science & Technology,New Delhi, India, is thankfully acknowledged for financialsupport. D.K.K. acknowledges CSIR, New Delhi, India, for aSRF.

Supporting Information Available: X-ray crystallographic filesin CIF format, selected bond distances and angles involving metalcenters (Table S1), hydrogen-bonding parameters (Table S2), structureoverlay of3 and6 (Figure S1), TG curves, and X-ray powder diffractionpatterns. This material is available free of charge via the Internet athttp://pubs.acs.org.

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1D Chains, 2D Corrugated Sheets in MOFs Crystal Growth & Design, Vol. 6, No. 8, 20061909