COMPLEXATION THERMODYNAMICS OF IMINE-TYPE BIS(BENZOCROWN ETHER)S .1. SANDWICH COMPLEXATION OF K-PHENYLENE BRIDGED CARBONYLHYDRAZONES OF 4'-FORMYLBENZO-15-CROWN-5(, RB+, CS+ AND TL+ WITH POLYMETHYLENE AND M)

The complexation thermodynamics of polymethylene and m-phenylene bridg ed carbonylhydrazones of 4'-formylbenzo-15-crown-5 with K+, Rb+, Cs+ a nd Tl+ was investigated by spectrophotometric titrations in methanol a t 283-313 K. The carbonylhydrazone sequences in the bridge of the bis( benzocrown ether)s are optical sensors for the sandwich complexation o f metal ions. Within the homologous series of polymethylene bridged ca rbonylhydrazones a stability peak was found for the sandwich complexes of the 'propylene bridged' compound with K+, Rb+ and Tl+. The potassi um complex of this ligand shows the highest stability constant compare d to all other known bis(benzo-15-crown-5) complexes with K+ in methan ol. In terms of thermodynamics the formation of intramolecular sandwic h complexes is driven by a gain of enthalpy. The unusual high gains of enthalpy and losses of entropy on formation of the most stable sandwi ch complexes allow us to propose an additional stabilization of the sa ndwich arrangement by intramolecular hydrogen bridge bonds. The DELTAH -degrees - TDELTAS-degrees diagram gave an excellent straight line all owing discussion of the enthalpy-entropy compensation effect at these complexation reactions.