Nuclear Magnetic Resonance Study of the Stoichiometry and Stability of Several Li⁺‐Crown Ether Complexes in Various Acetonitrile‐Nitrobenzene Mixtures

Abstract

Lith ium-7 NMR spec trom e try was used to study the complexation re ac tion be tween lith ium ions and several 12-, 15-and 18-membered crown ethers in a num ber of bi nary acetonitrile-nitrobenzene mix tures. Forma tion con stants of the re sult ing 1:1 com plexes in dif fer ent sol vent mix tures were de ter mined by com puter fit ting of the chem i cal shift-mole ra tio data. There is an in verse re la tion ship be tween the com plex sta bil ity and the amount of acetonitrile in the mixed sol vent. Among dif fer ent sized crown ethers used, 15-crowns were found to form the most sta ble Li + com plexes in the se ries. The in flu ence of sub sti tu tion on the macrocyclic rings on the sta bil ity of the re sult ing com plexes is dis cussed. IN TRO DUC TIONSince the first syn the sis of macrocyclic polyether (crown) com pounds that are ca pa ble of form ing se lec tive com plexes with the al kali ions, 1 ex ten sive re search has been car ried out on the sta bil ity and se lec tiv ity of the re sult ing com plexes. [2][3][4][5] Due to their pro nounced se lec tiv ity and ex traction ef fi ciency, crown ether de riv a tives have been ex tensively used as suit able neu tral ion car ri ers for se lec tive transport through liq uid mem branes 6,7 and for con struct ing polymeric mem brane sen sors for dif fer ent metal ions. 8,9 In for mation about the se lec tiv ity and sta bil ity of metal ion com plexes with macrocyclic lig ands are of crit i cal im por tance in de velop ment of new meth ods for their re moval, con cen tra tion, puri fi ca tion and de ter mi na tion from their mix tures. Among dif fer ent meth ods em ployed for the complexation of metal ion-macrocycle com plexes in so lu tion, 2-5 nuclear mag netic res o nance is well known as a pow er ful technique for study ing ther mo dy nam ics and ki net ics of complexation re ac tions. 10,11In re cent years, we have em ployed al kali metal and proton NMR tech niques to study the ex change ki net ics 12-14 and ther mo dy nam ics 15-17 of a num ber of metal ion-macrocycle com plexes in nonaqueous and mixed sol vents. In this pa per we used lith ium-7 NMR to de ter mine the stoichiometry and sta bil ity of Li + ion com plexes with 12-crown-4 (12C4), 15-crown-5 (15C5), benzo-15-crown-5 (B15C5), 18-crown-6 (18C6), dicyclohexyl-18-crown-6 (DC18C6) and dibenzo-18-crown-6 (DB18C6) in bi nary acetonitrile-nitrobenzene mix tures at 25.0 0.1 C. Struc tures of the crown ethers used are given in Fig. 1. EX PER I MEN TALCrown ethers B15C5, 18C6, DC18C6 and DB18C6 (all from Merck) were pu ri fied and dried as de scribed elsewhere. 20,21 Re agent grade 12C4 and 15C5 (both from Merck)were used as re ceived. Re agent grade lith ium per chlor ate (Merck) was pu ri fied and dried us ing a pre vi ously re ported method. 22 Spec tro scopic grade acetonitrile (AN, Merck) and ex tra pure nitrobenzene (NB, Merck) were used to pre pare the sol vent mix tures by weight.