Mobilities of cation-macrocyclic ligand complexes

Formation constants for complexion between alkali metal cations and the crown ether 18C6 in the solvent 2-cyanopyridine at 3OoC were determined from conductance measurements. The method allows for precise determination of these formation constants up to values of K around 10" mol-' dm3. The method also f. permits accurate evaluation of association equilibrium constants between the complex species (M-18C6)' and the anion. The magnitudes of these ion association constants are related to the nature of the solvation of the cation and of the complexed cation. The mobilities of the complexes are also dependent, in part, upon solvation effects. In 2-cyanopyridine the single ion molar conductance of the (Li-18C6)' complex is greater than that of the uncomplexed cation. Similar behavior is observed for alkali metal complexes with 18C6 and the cryptand 222 in other solvents such as propylene carbonate and propylene carbonate mixtures with dichloromethane. INTRODUCTION Since the syntheses of crown ethers by Pedersen (ref. 1) and cryptands by Lehn (ref. 2), the application of macrocyclic ligand chemistry has taken many directions as attested to by the diversity of the subject matter of this symposium and a recent review by Izatt and co-workers (ref. 3) which cites close to 1200 references to publications up to early 1990. A fairly recent application of macrocyclic ligand chemistry has been to nonaqueous electrolyte solutions for use in high energy lithium battery development. Yamaki and Tobishimo (refs. 4 3 ) and Matsuda et al. (ref. 6) noted that additions of 12C4 and 15C5 not only increases the conductivity of propylene carbonate (PC) solutions containing lithium salts, but also increased the cycling efficiency of the lithium anode. Yamaki and Tobishimo also reported that the lithium ion transport number, f+, is increased upon the addition of 12C4 to PC. Additions of crown ethers and cryptands to solid state ionically conducting polymers have also been noted to change the transport properties of salts (refs. 7,8), and the nature of the macrocyclic ligand-ion interactions in solid state polymers is a subject of a plenary lecture in this symposium (ref. 9). The present paper focuses upon the interactions of cations with selected macrocyclic ligands and liquid organic electrolytes. New results for 18C6 with alkali metal salts in 2-cyanopyridine (2CNP) at 3OoC are reported and compared to previous results for alkali metal salts in PC and a mixed solvent containing 36 mass % PC in CH,Cl,. We have used audiofrequency conductivity measurements to determine the stability (formation) constants for the alkali metal cation-macrocyclic ligand interaction. This method also yields accurate values for the molar conductivities at infinite dilution and ion association constants for the cation-ligand complexes with various anions. With the exception of the studies by Boileau et al. on Na and K complexes with the cryptands [221] and [222] in tetrahydrofuran (ref. lo), previous studies involving conductance measurements have neglected this type of ion association. Our results suggest that a number of cation-macrocylic ligand complexes undergo ion association and that this phenomenon is highly dependent on the nature of both ion-solvent and ion-macrocyclic ligand interactions. 'Permanent address: School of Mathematical and Physical Sciences, Murdoch University, Murdoch, WA 61 50, Australia.