Journal of Inclusion Phenomena and Macrocyclic Chemistry 31: 109–118, 1998. 109 c 1998 Kluwer Academic Publishers. Printed in the Netherlands. 13-Membered Azocrown Ether. Structure of the Lithium Bromide Complex and Membrane Properties E. LUBOCH and J. F. BIERNAT Technical University of Gda´ nsk, Department of Chemical Technology, 80-952 Gda´ nsk, Poland V. CH. KRAVTSOV and YU. A. SIMONOV Institute of Applied Physics, Academy of Sciences of Moldova, Kishinev, Moldova Received: 13 March 1997; accepted: 27 June 1997 Abstract. The lithium bromide hydrate complex of the 13-membered dibenzoazocrown ether of composition [LiL13(H2O)]Br 2 has been synthesized. Its structure has been determined by the X-ray method. The crystals are monoclinic, space group C2/c, = 12.720(6), = 19.969(9), = 13.963(6) Å; = 104.32(2) . Selectivities of membrane electrodes based on 13-membered azocrown ethers are discussed in terms of complex stabilities and metal cation extractabilities as related to the structures of the respective complexes. Key words: azocrown ether, lithium complex, X-ray structure, membrane electrodes Supplementary Data related to this article have been deposited with the British Library as Supple- mentary Publication No. SUP82232 (5 pages). 1. Introduction The 13-membered azocrown ether L13 obtained using different methods [1,2] presents an interesting compound capable of forming complexes with alkali metal cations. The stability constants of the sodium and potassium complexes of L13 are very small, whereas they are much higher for the lithium complex (log LLi = 4.10; acetonitrile) [1]. On the other hand the crown ether and its lipophilic derivatives used as ionophores in ion-selective membrane electrodes show high sodium selectivity in the presence of lithium, potassium and other cations [2]. The sodium iodide complex of L13 is ionic and the complex cation consists of one sodium cation and two ligand molecules [3]. Inspection of the X-ray structures of crown ether complexes gives valuable information on the ligand’s behavior as ionophores in ion-selective membrane electrodes. Their selectivities can correlate with some structural features of the respective complexes [4].