Syntheses, characterization, and supramolecular architectures of two lead(II) complexes of 8-quinolinol

Syntheses and characterization of two novel tetranuclear lead(II) clusters self-assembled by hydrogen bonded interactions

Background

The usage of polynuclear metal clusters as secondary building units (SBU’s) in designing of metal organic frameworks (MOF’s) is a field of current interest. These metal clusters have attracted a great deal of attention not only due to their interesting structural topologies but also due to promising physical and chemical properties. In this regard various d,f block (transition and lanthanide) metal clusters have been widely investigated so far. Less attention is paid to construction of heavy p-block Pb(II) clusters.

Results

Two mixed ligand Pb(II) clusters have been synthesized with bipy(2,2’-Bipyridine), phen(1,10-Phenanthroline), quin (8-Hydroxy quinolinate) and 5-tpc (5-chloro thiophene 2-carboxylate). They have been characterized by elemental analysis, IR, TGA and X-ray crystallography. X-ray diffraction analysis reveals that the complexes [Pb₄(quin)₄(bipy)₂(5-tpc)₄] (1) and [Pb₄(quin) ₄(phen) ₂(5-tpc)₄] (2) are tetranuclear. The complexes show a slight variation in unit cell parameters, due to the replacement of bipy and the phen ligands. Both complexes contain two types of Pb(II) ions which differ in the coordination geometry around the Pb(II) ion.

Conclusions

In both complexes the four lead ions Pb1,Pb2, Pb1ⁱ and Pb2ⁱ lie on the same plane bridged by the 5-tpc anions. Pb1 and Pb2 of both complexes contain a 5-tpc and quin coordinated in a bidentate chelating bridging fashion. In addition the Pb2 and Pb2ⁱ ions alone contain a bipy and phen in a bidentate chelating fashion in (1) and (2) respectively. An additional notable feature in both of these complexes are the bridging ability of the quin oxygen which forms a network of coordination bonds in between the four Pb(II) ions. In both complexes the individual units are self-assembled by C-H---Cl/C-H---S hydrogen bonding interactions to generate 2-D aggregates.

A three-dimensional Pb(II) coordination framework: poly[[μ4-(E)-2,2'-(diazene-1,2-diyl)dibenzoato]dimethanollead(II)]

In the title coordination polymer, [Pb(C14H8N2O4)(CH3OH)2]n, the asymmetric unit contains half of a Pb(II) cation, half of a 2,2'-(diazene-1,2-diyl)dibenzoate dianionic ligand (denoted L(2-)) and one methanol ligand. Each Pb(II) centre is eight-coordinated by six O atoms of chelating/bridging carboxylate groups from four L(2-) ligands and two O atoms from two terminal methanol ligands, forming a distorted dodecahedron. The [PbL2(MeOH)2] subunits are interlinked via the sharing of two carboxylate O atoms to form a one-dimensional [PbL2(MeOH)2]n chain. Adjacent chains are further connected by L(2-) ligands, giving rise to a two-dimensional layer, and these layers are bridged by L(2-) linkers to afford a three-dimensional framework with a 4(12)6(3) topology.

Design of co-crystals/salts of some Nitrogenous bases and some derivatives of thiophene carboxylic acids through a combination of hydrogen and halogen bonds

Background

The utility of N-heterocyclic bases to obtain molecular complexes with carboxylic acids is well studied. Depending on the solid state interaction between the N-heterocyclic base and a carboxylic acid a variety of neutral or ionic synthons are observed. Meanwhile, pyridines and pyrimidines have been frequently chosen in the area of crystal engineering for their multipurpose functionality. HT (hetero trimers) and LHT (linear heterotetramers) are the well known synthons that are formed in the presence of pyrimidines and carboxylic acids.

Results

Fourteen crystals involving various substituted thiophene carboxylic acid derivatives and nitrogenous bases were prepared and characterized by using single crystal X-ray diffraction. The 14 crystals can further be divided into two groups [1a-7a], [8b-14b] based on the nature of the nitrogenous base. Carboxylic acid to pyridine proton transfer has occurred in 3 compounds of each group. In addition to the commonly occurring hydrogen bond based pyridine/carboxylic acid and pyrimidine/carboxylic acid synthons which is the reason for assembly of primary motifs, various other interactions like Cl…Cl, Cl…O, C–H…Cl, C-H…S add additional support in organizing these supermolecules into extended architectures. It is also interesting to note that in all the compounds π-π stacking occurs between the pyrimidine-pyrimidine or pyridine-pyridine or acid-acid moieties rather than acid-pyrimidine/pyridine.

Conclusions

In all the compounds (1a-14b) either neutral O–H…N_{pyridyl/pyrimidine} or charge-assisted N_pyridinium-H…O_carboxylate hydrogen bonds are present. The HT (hetero trimers) and LHT (linear heterotetramers) are dominant in the crystal structures of the adducts containing N-heterocyclic bases with two proton acceptors (1a-7a). Similar type supramolecular ladders are observed in 5TPC44BIPY (8b), TPC44BIPY (9b), TPC44TMBP (11b). Among the seven compounds [8b-14b] the extended ligands are linear in all except for the TMBP (10b, 11b, 12b). The structure of each compound depends on the dihedral angle between the carboxyl group and the nitrogenous base. All these compounds indicate three main synthons that regularly occur, namely linear heterodimer (HD), heterotrimer (HT) and heterotetramer (LHT).

Supramolecular architectures and structural diversity in a series of lead (II) Chelates involving 5-Chloro/Bromo thiophene-2-carboxylate and N,N'-donor ligands

Background

Lead is a heavy toxic metal element in biological systems and is one of the major pollutants as a result of its widespread use in industries. In spite of its negative roles the coordination chemistry of Pb(II) complexes is a matter of interest. The N,N’-bidentate aromatic bases such as BPY,4-BPY and PHEN (BPY = 2,2′bipyridine, 4-BPY = 4,4′-dimethyl-2,2′-bipyridine, PHEN = 1,10-Phenanthroline) are widely used to build supramolecular architectures because of their excellent coordinating ability and large conjugated system that can easily form π-π interactions among their aromatic moieties. A series of novel Pb(II) complexes in concert with 5-CTPC, 5-BTPC (5-CTPC = 5-chlorothiophen-2-carboxylate, 5-BTPC = 5-bromothiophen-2-carboxylate) and corresponding bidentate chelating N.N′ ligands have been synthesized and characterized.

Results

Five new Pb (II) complexes [Pb(BPY)(5-CTPC)₂] (1), [Pb(4-BPY)(5-CTPC)₂] (2), [Pb₂(PHEN)₂(5-CTPC)₄] (3), [Pb(4-BPY)(5-BTPC)₂] (4) and [Pb₂(PHEN)₂(5-BTPC)₂(ACE)₂] (5) have been synthesized. Even though in all these complexes the molar ratio of Pb, carboxylate, N,N-chelating ligand are the same (1:2:1), there is a significant structural diversity. These complexes have been characterised and investigated by elemental analysis, IR, ¹H-NMR,¹³C-NMR, TGA, and photoluminescence studies. Single crystal X-ray diffraction studies reveal that complexes (1, 2) and (4) are mononuclear while (3 and 5) are dinuclear in nature which may result from the chelating nature of the ligands, various coordination modes of the carboxylates, and the coordination geometry of the Pb(II) ions.

Conclusions

The observation of structures 2,4 and 3,5 show the structural changes made just chloro/bromo substituent of the thiophene ring. A detailed packing analysis has been undertaken to delineate the role of valuable non covalent interactions like X…π, H…X, (X = Cl/Br). A quadruple hydrogen bond linking the monomeric units and generating a supramolecular architecture is observed in (1). The metal bite unit comprised of PbN₂C₂ (i.e. Pb-N-C-C-N-Pb) is the repeating unit in all the five complexes and they have almost same geometrical parameters. This metal bite has been identified as the self assembly unit in complexes.