Introduction
1.4 Carboxylate based coordination polymers
1.4.1 One dimensional coordination polymers
The main factors which influence the structural variety of the coordination polymers constructed from poly-carboxylato ligands are: (i) the number of the carboxylate groups, (ii) the spatial orientation of the carboxylato groups, (iii) the coordination modes of carboxylate groups, (iv) ancillary ligands in the coordination sphere of the metal ions. So, depending on these factors, coordination polymers may adopt different structures. Various examples of one dimensional coordination polymers are presented in the next subsections.
1.4.1.1 Linear chain:
Linear chain is the simplest and commonly observed one dimensional motif. For instance,one dimensional coordination polymer [Ni(cyclam)(2,2'-bipyridyl-4,4'-dicarboxylate) ]n.5nH2O (1.37) can be prepared by the reaction of 2,2'-bipyridyl-4,4'-dicarboxylic acid and macrocyclic [Ni(cyclam)]2+. This belongs to an important class of linear carboxylate bridged co-ordination polymer. In this coordination polymer the dicarboxylic acid is coordinated to [Ni(cyclam)]2+ (Figure 1.27A) and form a linear coordination polymer (Figure 1.27B). These polymeric chins are assembled by weak C−H····π interaction leading to triangular channels by overlapping triangular array of linear coordination polymer chains120. The packing generates 1D channels with honeycomb like porous and robust structures. The frameworks are retained after several cycles of dehydration and rehydration processes. Many other interesting examples of linear chain coordination polymers are reported in literature121-122.
N N
N N Ni
H
H H H
[Ni(cyclam)]2+
A B
Figure 1.27 A) Schematic presentation of [Ni(cyclam)]2+, B) Structure of 1-D coordination polymer (1.37) constructed through [Ni(cyclam)]2+ and 2,2'-bipyridyl-4,4'-dicarboxylic acid
Coordination polymer 1.38 is the combination of a linear dimetallic spacer, Cu2(O2CMe)4, and a linear bidentate ligand, 1,3-di-4-pyridylpropane (Figure 1.28). The combined use of a long, linear ligand with a dimetallic fragment possessing linearly arranged binding sites to produce a porous channel-containing coordination polymer framework capable of including different guest species like methanol, acetic acid and ethylene glycol123. Interestingly this type
of coordination polymer could also be prepared mechanochemically by grinding solid copper(II) acetate dihydrate with 1,3-di-4-pyridylpropane124. One dimensional linear chain of copper(II) with terephthalate dianions and ammonia has square-planar geometry around the copper atoms. The coordination sites are fulfilled by two ammonia and two terephthalate molecules. Terephthalate molecules are coordinated in monodentate fashion, leading to a trans-arrangement of the ligands and linear chains125.
Figure 1.28 Structure of one dimensional coordination polymer 1.38
The use of N-(4-carboxyphenyl)iminodiacetic acid with copper(II) leads to linear chain coordination polymer126. The one-dimensional chain formation seems to be simple, but they show interesting packing pattern with unique physical property.
1.4.1.2 Zig-zag chain:
The formation of zig–zag chains can be induced in coordination polymers by controlling the shape of the ligand molecules. The complex [Mn(fumarat)(2,2'-bipyridine)(H2O)]n (1.39) has a zigzag polymeric chain connected by the flexible fumarate ligands and 2,2'-bipyridine. In this polymer each manganese(II) ion is seven-coordinated comprising of 2,2'-bipyridine, two bidentate carboxylates and one aqua ligand. Pair of chains are linked by hydrogen bonds, involving aqua ligands on one chain and two oxygen atoms of fumaric acid ligands on the other chain127. Hydrothermal reactions of copper(II) chloride with 1,4-dicyanobenzene in the presence of 1,10-phenanthroline results in the hydrolysis of 1,4-dicyanobenzene to terephthalic acid. The dicarboxylic acid formed in situ leads to the formation of zig-zag polymeric complex, [Cu(terephthalate)(1,10-phenanthroline)]n (1.40). The metal environment can best be described as highly distorted square planar geometry with two nitrogen atoms from phenanthroline, two carboxylato-oxygen atoms from different terephthalic acid ligands and one oxygen atom from the coordinated water molecule. Similar types of complexes are also obtained in the case of cobalt and zinc (Figure 1.29)128.
Figure 1.29 Zig-zag structure of one dimensional coordination polymer (1.40) of copper
The [Cu(5-hydroxyisophthalate)(NEt3)(H2O)]n (1.41), has an infinite zig-zag chain constructed by the copper-trimethyl amine node with 5-hydroxy isophthalic as spacer129. Similar type of zig-zag coordination polymer are also obtained by the reaction of 2,5- dihydroxy terephthalic acid and 5-nitro isophthalic acid instead of 5-hydroxy-isophthalic acid with copper. In all cases one dimensional zig-zag coordination polymer, with a distorted octahedral geometry or a strongly distorted square-pyramidal geometry of copper (II) centers, are observed (Figure 1.30).
Figure 1.30 Crystal structure of [Cu(5-hydroxy-isophthalate)(NEt3)(H2O)]n (1.41)
Reaction of zinc salt with imidazole and 1,2-benzenedicarboxylic acid leads to a one dimensional zig-zag type coordination polymer. The carboxylate groups of the phthalate dianion are coordinated to zinc centers in a bridging fashion and form one dimensional zig- zag chain like structure130. The presence of the bipyridine and phenanthroline ligands lowers the tendency of metal complexes to form high-dimensional carboxylate bridged complexes.
The reaction of [Zn(MeCO2)2]·H2O with 1,4-benzenedicarboxylic acid in the presence of 2,2'- bipyridine leads to one dimensional infinite zig-zag chain. The one dimensional chains are connected by weak C−H····O hydrogen bonds and aromatic π····π stacking interactions, which makes them to assemble into a 3D network. The structural features shows the presence of nano-sized channels, capable of clathrating free 2,2'-bipyridine molecules. When the guest molecules are removed from the channels by heating, the evacuated framework is stable up to 340ºC131.
Figure 1.31 One dimensional infinite zig-zag chain of coordination polymer 1.42
Another example of one dimensional zig-zag coordination polymer is, [Zn(2,2'- bipyridine)(1,2-phenyldiacetato)2(H2O)]n, (1.42) obtained from zinc(II) nitrate and 1,2- phenyldiacetic acid in the presence of 2,2'-bipyridine. The zinc(II) ion in the polymer has a distorted octahedron coordination environment. Strong π····π stacking interactions (face to face distance is 3.49 Å) exist between the bipyridine ligands and phenyl rings of phenyldiacetic acid on adjacent chains (Figure 1.31)132.
1.4.1.3 Double chain:
Double chain type carboxylate polymers are formed by different metal ions, in which the ligands have an appropriate geometry to form a chain like structure by occupying axial and equatorial positions of distorted octahedral geometry. Double chain coordination polymers of 1,2,7,8-benzenetetracarboxylic acid with manganese(II) ions in presence of 4,4'-bipyridine are structurally characterised. In this polymer the acid molecules act as tridentate ligands to manganese(II) ion, the octahedral coordination sphere of manganese(II) ion is completed by two water molecules and one protonated 4,4'-bipyridine molecule133.
Figure 1.32 Double chain structure of [Cu(2,3-pyridine dicaboxylate)2]n (1.43)
Double chain motif [Cu(2,3-pyridine dicaboxylate)2]n (1.43) of copper(II) with 2,3-pyridine dicarboxylic acid can be synthesised by high-dilution synthesis134. The repeating units of this coordination polymer are metallacycles, two ligands bridges two copper ions in the chain
(Figure 1.32). Each copper(II) centers has a distorted octahedral coordination geometry. The apical positions of the octahedron are occupied by the oxygen atoms of the non-deprotonated 3-carboxyl groups and the equatorial one are occupied by two nitrogen atoms and two oxygen atoms of the deprotonated 2-carboxyl groups. Similar type of double chain structure of zinc(II) with 2,3-pyridine dicaboxylic acid is reported in literature. Reaction of zinc(II) acetate with 2,3-pyridine dicarboxylic anhydride in presence of 4,4'-bipyridine leads to hydrolysis of the anhydride into 2,3-pyridine dicarboxylic acid and forms coordination polymer with double-chain structure135.
1.4.1.4 Ladder:
Great advances have been achieved for construction of infinite ladder like coordination polymers through the metal-directed self assembly method. Few examples of ladder like coordination polymers are described in this section. The reaction of cobalt(II) nitrate with nicotinic acid in the presence of sodium salt of dicynamide leads to a co-ordination polymer {[Co3(dicynamide)2(nicotinate)4(H2O)8].2H2O}n, (1.44) with ladder type structure. The one dimensional polymer contains bridging dicynamide anions, bridging and terminal nicotinate anions and two interstitial water molecules (Figure 1.33).
Figure 1.33 Ladder-like one dimensional coordination polymer (1.44) of cobalt
Due to the angular nature of the bridging nicotinic acid ligands the ladders are not flat, they have a ‘Z’-shaped structure. The interior of the layers are hydrophilic and contain extensive hydrogen bonding interactions which connect the ladders together136. Different types of ladder like architecture of copper(II) are also reported in literature. The reaction of copper (II) nitrate with maleic acid in presence of 2,2'-bipyridine leads to ladder-like one-dimensional coordination polymer. The two carboxylate groups of the dianion of maleic acid exhibit two different coordination modes in the complex. One acts as a bridge in syn–anti conformation,
while the other exhibits monodentate coordination mode137. Similar types of molecular ladders are also formed by the reaction of trans-1,2,3-propenetricarboxylic acid with copper (II) nitrate138. Hydrothermal reactions of metal acetates such as copper(II) acetate and zinc(II) acetate with 4,4'-bipyridine leads to 1D molecular ladder of the respective metals. The copper complex, {[Cu(4,4'-bipyridine)(OOCCH3)2].2.5H2O}n, (1.45) consists of a symmetric copper(II) dimer with bridging between copper centres furnished by mono-atomic acetate.
Further coordination about copper is provided by two trans 4,4'-bipyridine ligands and a terminally bound mono-dentate acetate, the resulting complex has square pyramidal geometry around metal centers139 (Figure 1.34).
Figure 1.34 Crystal structure of the complex 1.45
The zinc complex, [Zn(4,4'-bipyridine)(OOCCH3)2]n, (1.46) consists of a dimeric core. A quite irregular six coordinate geometry around zinc(II) is formed by a bidentate acetate which forms a four membered chelate ring139. Two trans 4,4'-bipyridine rigid ligands are situated above and below the core, and two syn–syn bridging acetate groups complete the coordination sphere (Figure 1.35).
Figure 1.35 Ladder type coordination polymer (1.46) of zinc
Zinc complex with diphenic acid [Zn(diphenate)(H2O)]n, (1.47) is a one-dimensional coordination polymer that consists of parallel ladder-like chains140. One carboxylate group of the diphenic acid coordinate with two zinc atoms and forms a dinuclear unit which composes
the steps of the ladder. The other carboxylate is connected to a zinc ion in the next step of the ladder. The fourth coordination site on the distorted tetrahedrally coordinated zinc ions is occupied by water molecule (Figure 1.36).
Figure 1.36 One-dimensional coordination polymer 1.47 of zinc with diphenic acid that consists of parallel ladder-like chains
1.4.1.5 Helix:
Among different first row transition metal complexes having helical structure the complex [Co(C14H8O5)(C12H8N2)(H2O)]n (1.48), is an example of a neutral double-strand helix141. The V-shaped 4,4'-oxybis(benzenecarboxylic acid) ligand acts as a bridge to connect two adjacent cobalt ions. The helical nature depends upon the capacity to form V-shaped configuration of the bridging dicarboxylate ligands. The cobalt polymer has two adjacent chains that interacts through hydrogen bonds between the non-coordinating carboxylate oxygen atoms and the aqua ligand to generate a double-stranded chain, as shown in Figure 1.37A. The phenanthroline ligands are attached to one side of the double stranded chain. Additionally, supramolecular interactions viz. strong aromatic π−π stacking between the phenanthroline ligands and hydrogen bonds between the aqua molecules from two adjacent chains, makes the double-stranded chains to remain in layers. Helical, multinuclear assemblies mediated by metal–ligand coordination are well documented in the literature142.The double stranded helix of copper (II) with 4,4'-oxybis(benzenecarboxylic acid) and phenanthroline is also reported in literature143.
Helical coordination polymer of cobalt with 1,4-cyclohexanedicarboxylic acid and phenanthroline [Co(1,4-cyclohexanedicarboxylate)2(1,10-phenanthroline)2(H2O)2]n (1.49), is structurally characterised144. Each metal center of this complex is coordinated by one phenanthroline ligand, also with three oxygen atoms from two 1,4-cyclohexanedicarboxylic acid ligands and one aqua ligand. The repeated units propagate along the crystallographic 21-
screw axis (Figure 1.37B). The right-handed and left-handed helices are alternately packed through further interlinking via hydrogen bonds between the carboxylate and aqua ligands into a centrosymmetric 2D network.
A B
Figure 1.37 A) Neutral double-stranded helix of [Co(C14H8O5)(C12H8N2)(H2O)]n (1.48), B) Structure of helical coordination polymer 1.49 of cobalt with 1,4-cyclohexanedicarboxylic acid and 1,10-phenanthroline
The self-assembly of cobalt (II) ions with 2,2'-bipyridyl-3,3'-dicarboxylic acid in aqueous solution leads to one-dimensional (1D) helical coordination polymers 1.50. Crystal structure of the compound shows that each metal center displays a distorted octahedral coordination geometry including three water oxygen atoms; one oxygen atom of the carboxylate of a 2,2'- bipyridyl-3,3'-dicarboxylic acid belongs to the adjacent metal ion and two nitrogen atoms from the 2,2'-bipyridyl-3,3'-dicarboxylic acid act as a chelating ligand.
Figure 1.38 Structure of 1D helical coordination polymers 1.50
One carboxylate oxygen atom of coordinated 2,2'-bipyridyl-3,3'-dicarboxylic acid binds to the neighbouring metal ions, which give rise to 1D helical coordination polymers (Figure 1.38).
The helical chains are linked by the hydrogen bonding interactions between the oxygen atoms of carboxylate and the water molecule of the adjacent helical chain, which leads to 2D networks145. Similar type of one dimensional helical coordination polymer of nickel with 2,2'-
bipyridyl-3,3'-dicarboxylic acid is also reported in literature146. The reaction of copper (II) salt with 1,3-benzenedicarboxylic acid in the presence of 2,2'-bipyridine leads to a helical one dimensional coordination polymer, {[Cu(1,3-benzenedicarboxylate)( 2,2'-bipyridine)].2H2O}n
(1.51). In this complex each copper(II) ion is coordinated by two oxygen atoms from two bis monodentate carboxylate ligands and two nitrogen atoms from 2,2'-bipyridine. A distorted square-planar geometry around copper is observed. The helical chains are assembled through π−π interaction between the bipyridine molecules and a zipper-like arrangement is attend in the lattice143. Similar types of helical one dimensional chains of different metals with different dicarboxylic acids in presence of 2,2'-bipyridine are also reported147-148.