Construction Of Coordination Polymers With Betain Derivatives

The simple association of a pair of starting components through metal coordination and/or non-covalent interactions can be used to assemble new coordination polymers showing such properties as electronic, magnetic, optical, absorbent and catalytic, which are not displayed by either of the components alone. Generally the architecture of coordination polymers can be reasonably well predicated rests upon implicit understanding those influent factors including the coordination geometry of metal ions, the conformational preference of the ligands, counter ions, solvent, the metal/ligand ratio and hydrogen bonding. In present studies, two novel double betaine ligands, namely 2-(4-carboxy-pyridinium)ether (L) and 1, 6-bis–(4-carboxypyridinium)hexane (L′), have been synthesized and allowed to interact with various metal salts to investigate their coordination behavior and the ability of constructing coordination supramolecules. The single crystals of 10 coordination compounds with these two ligands were obtained, and their molecular and crystal structures have been determined by X–ray single crystal structure analysis. The compounds include [MnL2(H2O)4]·2Cl2·4H2O (1) , [MnL2 (H2O)4]·2NO3·2H2O (2), [Mn1.5L3(H2O)6]·3ClO4·2H2O (3), [Cd1.5L3 (H2O)6]·3ClO4·H2O (4), {[Ni1.5L3(H2O)6] ·ClO4·H2O (5), [CoL2(H2O)4] ·2ClO4·H2O (6) , [Ag2L2] ·2CF3SO3 (7), [Ag7L6H2O] ·7ClO4 (8), [Cu2L2(H2O)4Cl2] ·2H2O (9) and [Co2L′(H2O)5] ·2ClO4. The results show that: (1) In all the compounds other than (8), in which one of the carboxylate groups of L acts in a monodentate mode and the other a syn-syn bridging mode, the carboxylate group involved in ligation of L acts in a monodentate coordination mode towards metal ion. (2) Because of the existence of a ? O? unit, ligand L exhibits two different configurations, that is, cis-in 3 –9 and trans-in 1 and 2. The configuration of L plays a thumb role in determining the result structures, e.g. in the case of cis-configuration, hydrogen-bonded spirocyclic (in 3 –6) or large cyclic coordination structures (in 7 –9) were formed, however, in the case of trans-configuration, a polyrotaxane topology is preferred. (3) The nature of anions has great influence on the configuration of L as revealed in the structures of 1, 2 and 3. In all the three compounds the cations are Mn(II), and the anions are Cl? , NO3? and ClO?4 , respectively. The configuration of L in 1 and 2 is trans-, but it is cis-in 3. (4) Repeating the routes for preparing 3 –6 under the same conditions, but with different metal/ligand ratio, leads to the formation of the same compounds, which indicates that the formation of spirocyclic structure is thermodynamically controlled. (5) It is expected that, with a relatively long alkyl chain ( )? CH2 6? , the ligand L′will be more flexible, but the structure of 10 illustrates that the chain preferes to stretch out other than to coil. In conclusion, double betain ligands with ? O? linkage are more flexible than that with only ? CH2? linkage, and their variability of configurations enable them to construct supramolecules of various structures. In addition, it is the flexibility of the ligand that makes them be suitable to investigate the influence of different cations, anions etc. on the topology of the result network.