Introduction
1.6 Porous carboxylate networks
1.5.3 Heterodinuclear metalloenzyme: Purple Acid Phosphatases
Purple acid Phosphatases catalyze hydrolysis of phosphomonoesters in vitro under acidic conditions (optimum pH of 4.9-6.0). They contain two irons (or one iron and another dipositive metal ion), and exhibit a characteristic purple color (λmax~550 nm) in their inactive oxidized form and a pink color (λmax~510 nm) in their active reduced form212-213. They are isolated from mammals, plants and fungi. The structures of several enzymes from different sources have been determined by X-ray crystallography.
Fe Zn
HO
O
N(His286) N(His323 O
(His325)N (Try167)O
OH2 OH
O Asp164
Asn201 NH2
O
135 Asp O
Figure 1.55 Active site structure of heterodinuclear metalloenzyme Purple acid Phosphatases
Two different coordination environments are present around the trivalent and the divalent metal ions. Iron (III) is coordinated by an aspartate, a histidine and a deprotonated tyrosine residue (Figure 1.55). The latter stabilizes the iron at +3 oxidation state and gives rise to the intense LMCT band around 560 nm. This is responsible for the colour of these enzymes. The two metal centers are bridged by a hydroxide and an aspartate residue. Other zinc (II) is further coordinated by two histidines and one asparagine. The octahedral coordination sites are occupied by one water and one hydroxide. There are also a few other non-coordinating amino acid residues in the active site that helps in binding and activation of the substrate by hydrogen bonding214-215.
molecules. The metal organic framework obtained by the reaction of zinc(II) salt with 1,4- benzenedicarboxylate has a cubic three-dimensional extended porous structure and able to adsorb hydrogen (Figure 1.56). These types of porous coordination polymers are applicable as hydrogen storage materials220-221. The Zn4O-based metal organic frameworks, which have high porosity and large surface area, are well known for their application to fuel gas storage.
The spherical pore size of the metal organic framework constructed by zinc and 1,4- benzenedicarboxylic acid is about 15Å in diameter.
Figure 1.56 3D metallo-organic framework of zinc with 1,4-benzenedicarboxylate
Some of the metal organic frameworks based on Zn4O network topology are shown in Figure 1.57. The structures are made up of oxo-centered Zn4O tetrahedron residing at the edges of the cube and bridged by six carboxylate units to give octahedron shaped secondary building units (SBUs). These SBUs are further linked by the bridging dicarboxylate groups to form the 3D cubic networks. The pore size can be controlled by using carboxylate linkers of varying lengths such as biphenyl, tetrahydropyrene, pyrene, and terphenyl moieties. Additionally, various functional groups such as –Br, –NH2, –OC3H7, –OC5H11, –C2H4 and –C4H4 can be incorporated into the frameworks that are orientated towards the pores. The resulting networks are highly porous with very low crystal densities218.
Figure 1.57 Metal organic frameworks obtained by using different dicarboxylate linkers
Different types of 3D metallo-organic frameworks of zinc(II) with 4,4'-biphenylcarboxylic acid are reported in literature222. The asymmetric unit of these complexes have three crystallographically unique zinc atoms bridged by four carboxylate groups, each from different biphenylcarboxylate units. The zinc ions are also bound to two hydroxy groups to give two tetrahedral and one octahedral zinc center arranged in coplanar fashion.The infinite Zn-O-C columns are stacked in parallel and connected in the [110] direction by the biphenyl links to give one-dimensional (1D) rhombic channels of 12.2 Å along an edge and 16.6 Å along the diagonal (Figure 1.58A). Similar type of motifs are also obtained in the zinc complexes of 2,6-naphthalenedicarboxylic acid.
Other coordination polymers, based on bis- and tris-bidentate carboxylate linkers are copolymerized with different transition metals to construct stable porous materials. The sorption processes of these compounds are well studied affording efficient and robust materials for gas storage or liquid separation216, 223-226. The hydrothermal reaction of copper nitrate with 3,3',5,5'-biphenyl tetracarboxylic acid leads to 3D metallorganic frameworks. The Cu2(CO2)4 core is a secondary building unit, which is connected through biphenyl rings to construct 3D framework (Figure 1.58B).
A B
Figure 1.58 A) 3D metallo-organic frameworks of zinc(II) with 4,4'-biphenylcarboxylic acid, B) Structure of 3D metallorganic frameworks of copper with biphenyl tetracarboxylate
The carboxylate functionalities of the biphenyl tetracarboxylate ligand are nearly coplanar with the biphenyl rings. This metallo-organic framework has two kinds of pores, and posses permanent porosity with high capacity for hydrogen adsorption227. Similar type of copper motifs with 4,4',4''-benzene-1,3,5-triyl-tribenzoicacid is also reported228. Many porous network solids with very large pore size and surface areas are known229-231. Fourfold interpenetrated 3D networks, of zinc with 6,6'-dichloro-2,2'-diethoxy-1,1'-binaphthyl-4,4'-
dibenzoic acid and 6,6'-dichloro-2,2'-dibenzyloxy-1,1'-binaphthyl-4,4'-dibenzoic acid generates 3D channels and exhibit permanent porosity. These motifs are good for hydrogen uptake at room temperature232.
1.6.1 Gas separation by carboxylate frameworks
The carboxylate MOF can be designed to distinguish multiple gases and also for potential gas absorption. Octanuclear nickel(II) cluster are useful for the separation of different gases. It is prepared by using tert-butyl-1,3-benzenedicarboxylic acid to form a trilayer, in which a hydrophilic cluster layer is sandwiched by two hydrophobic tert-butyl- 1,3,- benzenedicarboxylate layers (Figure 1.59). The network is flexible, and can separate commercially relevant gases such as H2/N2, H2/CO, N2/O2, N2/CH4, CH4/C2H4, and C2H4/C3H6. The gas separation capability of the network comes from the hydrophobic tert- butyl-1,3-benzenedicarboxylate gate in the hydrophilic/hydrophobic interface, which opens at higher temperatures due to thermal vibration of hydrophobic tert-butyl-1,3- benzenedicarboxylate groups233.
Figure 1.59 Octanuclear nickel(II) cluster with tert-butyl-1,3-benzenedicarboxylic acid
Microporous complexes of copper with 4,4'-(hexafluoroisopropylidene)-bis(benzoic acid), are able to separate hydrocarbons234. The complex consists of [Cu2(COO)4] paddle-wheel in the equatorial plane and 4,4'-(hexafluoroisopropylidene)-bis(benzoic acid) ligand in the axial sites to construct a 3D network. Another example of a doubly interpenetrated framework of zinc with 1,4-benzenedicarboxylic acid and 4,4'-bipyridine has 1D channels (channel size, 4.0 Å x 4.0 Å)235. This compound is able to separate n-pentane from n-hexane, branched 2- methylbutane from n-pentane, and 2-methylpentane, 2,2-dimethylbutane from n-hexane, respectively. In addition, the mixture of 2-methylbutane, n-pentane, 2,2-dimethylbutane, 2-
methylpentane, and n-hexane can be easily separated with different retention times. The selective separation of alkanes is attributed to their different van der Waals interactions with the host. Mixed alkyl aromatic compounds can be separated by using a MOF having composition [VIII(OH)(O2C–C6H4–CO2)2·0.75 (HO2C–C6H4–CO2H)]236. This compound is able to separate p-xylene, m-xylene, o-xylene, and ethylbenzene.
1.6.2 Selective guest binding in carboxylate networks
The 3D network {[Ni(L)(H2O)2]3[BTC]2}n·24nH2O (1.66) (where BTC= benzene tricarboxylate, L = 1,4,8,11-tetraazacyclotetradecane), selectively binds D-glucose into the channels over maltose. It has 1D channels of honeycomb aperture whose effective window size is 10.3 Å in diameter237 (Figure 1.60A).
A mixed metal framework of Cu(II) and Hg(II) constructed from [(Cu(2- pyrazinecarboxylato)2] building block and HgI2 as linkers (Figure 1.60B). In this framework each copper(II) centers have distorted octahedral coordination geometry. It has cuboidal framework structure, possesses rectangular channels with dimension 7.24 x 7.24 Å.
Interestingly uncoordinated linear HgI2 molecules are encapsulated in the voids238.
A B
Figure 1.60 A) 3D network of {[Ni(L)(H2O)2]3[BTC]2}n·24nH2O (1.66), B) Three dimensional network of Cu(II) and Hg(II) which encapsulate HgI2 molecules in the voids
A microporous coordination polymer, of copper with pyrazine-2,3-dicarboxylic acid and 4,4'- bipyridine shows a reversible structural change on sorption/desorption of benzene239. The framework undergoes a deformation so that the channel cavities suit benzene molecules very well. This results in an appreciable difference in the channel shape with and without benzene.
The channel without benzene has nearly a rectangular shape of dimensions of 5.6Å x7.2 Å;
whereas that with a benzene has a ‘‘Z’’ shaped shown in Figure 1.61. In the absence of benzene, the geometry around the copper ion is square pyramidal, while that with benzene shows a square planar geometry. Eventually, the deformation produces a large contact area to the benzene plane.
Figure 1.61
A chromium (III) complex with 1,4-benzenedicarboxylic acid exhibits a 3D framework with a 1D pore channel system239-241. The transition between the hydrated form and anhydrous solid is reversible and followed by a high breathing effect, the pores being clipped in the presence of water molecules and re-opened when the channels are empty. No acetone or ethanol could be incorporated instead of water, whereas dimethyl formamide is incorporated into the pore instead of H2O. This selectivity is attributed to the higher capability of dimethyl formamide toward the formation of strong hydrogen bonds with the hydroxyl groups of the framework (Figure 1.62).
Figure 1.62 Structural transformation of chromium (III) complex after guest encapsulation
Microporous coordination polymers are one of the most reasonable candidates for the formation of specific molecular arrays because of their highly designable nature and pore homogeneity. Example of such type of framework is [Cu2(pzdc)2(pyz)] (1.67), where pzdc is pyrazine-2,3-dicarboxylate and pyz is pyrazine. The alignment of oxygen molecules in the channels of this complex is observed242. The confinement effect and the restricted geometry of 1D nanochannels leads to a 1D ladder-like structure of oxygen dimer (Figure 1.63). These types of metallo-organic frameworks are able to absorb CO2 and acetylene molecules.
Figure 1.63 1D ladder-like structure of oxygen dimer encapsulated into the voids of 3D network of copper (1.67) with pyrazine-2,3-dicarboxylate
1.6.3 Carboxylate frameworks for enantioselective transformations
The first example of asymmetric catalysis using a homochiral metal-organic network is achieved by a zinc complex of a enantiopure bridging ligand with both carboxylic and pyridyl functional groups. In this complex three zinc ions are held together by six carboxylate groups of the chiral ligand and bridging oxo oxygen, to form a trinuclear unit. The complex contains chiral channels with edge lengths of about 13 Å as shown in Figure 1.64. The porous structure is stable in the presence of solvents. The trans-esterification reactions are carried out in the presence of size selectivity. With a racemic mixture of a chiral alcohol (1-phenyl-2-propanol) a slight enantiomeric excess (8% ee) was observed in the product ester243.
O O O
HO NH
O
N
A B
Figure 1.64 A) Enantiopure bridging ligand with both carboxylic acid and pyridyl functional groups, B) Homochiral metal-organic network of zinc that catalyzes trans-esterification reactions