pH-Driven Variation of the Outer-Sphere Binding Mode of cis-[Co(Ad)(en) 2 Cl]Cl (en-Ethylendiamine, Ad-Adeninate) with p-Sulfonatothiacalix[4]arene V. V. SKRIPACHEVA 1 , A. R. MUSTAFINA 1, *, V. A. BURILOV 1,2 , L. F. GALIULLINA 1,2 , SH. K. LATYPOV 1 , S. E. SOLOV’EVA 1 , I. S. ANTIPIN 1,2 and A. I. KONOVALOV 1,2 1 A.E. Arbuzov Institute of Organic and Physical Chemistry, Kazan Research Center of the Russian Academy of Sciences, Arbuzov str. 8, Kazan, 420088, Russia; 2 Chemistry Institute, Kazan State University, Kremlyovskaya Str., 18, 420008, Kazan, Russia (Received: 03 February 2006; Accepted in final form: 20 April 2006) Key words: Co (III) bis-ethylendiaminate-adeninate, inclusion complex, NMR 1 H spectroscopy, outer-sphere coordination, p-Sulfonatothiacalix[4]arene Abstract The non-symmetrical cobalt (III) complex cis-[Co(Ad)(en) 2 Cl]Cl (1Cl), possessing two flexible ethylendiaminate chelate rings (en) and monodentate N(9)-bound adeninate (Ad - ) was chosen as the guest of p-sulfonatothiaca- lix[4]arene (STCA) to study the inclusion complex formation at wide range of pH 2–10. It was shown by 1 H, NOE NMR methods and pH-metric procedure, that pH-driven variation of the inclusion mode of 1 + into calixarene cavity is the result of the protonation of 1 + via adeninate moiety. Introduction The design of various molecular devices is a top of cur- rent interest during recent decades [1]. Transition metal complexes are of particular importance from this point of view [2–13]. Metal ion complexes are known to underlay the development of molecular scale machines [2–4], logic gates [5, 6], devices for memory storage [7], switches [8], wires [9–12] and sensors [13]. A pH- dependent metal ion coordination [14, 15] or a change of coordination polyhedron as a result of potential change [2–4] are well-known examples of a molecular move- ment. The development of such systems requires mac- rocyclic compounds with appropriately preorganized donor groups to provide switchable inner-sphere coor- dination of metal ion. An outer-sphere coordination of transition metal ions by macrocycles is also known to underlay the design of switchable systems. For example redox driven switching of inclusion complex formation was applied to produce molecular movement [16]. The pH-driven outer-sphere coordination of anionic com- plex of Fe(III) into polyammonium macrocyclic recep- tor may be viewed as pH-controlled on/off switch [17]. The inclusion of ferrocene and cobaltocene into p- sulfonatocalix[6]arene cavity is also known to result in some redox changes, which can be detected by conven- tional electrochemistry [18–20]. We have recently found out that some tris-chelates of cobalt (III) form inclusion complexes with p-sulfonatothiacalix[4]arene [21, 22]. The inclusion of flexible ethylendiamine chelate rings of cobalt (III) tris-ethylendiaminate and bis-ethylendiami- nato-oxalate complexes into the cavity of p-sulfonato- thiacalix[4]arene results in definite spectral changes of cobalt complexes due to conformational shift of ethy- lendiaminate ring [21]. Tris-dipyridyl of cobalt (III) was also found to form inclusion type complex with p- sulfonatothiacalix[4]arene, which results in detectable changes of redox properties of the former [22]. Thus it seems rather promising to develop inclusion complex formation between p-sulfonatothiacalix[4]arene and non-symmetrical cobalt (III) complex, where an inclu- sion mode of cobalt complex can be changed by a varying of external conditions. The non-symmetrical cobalt (III) complex cis-[Co(Ad)(en) 2 Cl]Cl (1Cl), pos- sessing two flexible ethylendiaminate chelate rings (en) and monodentate N(9)-bound adeninate (Ad - ) (Fig- ure 1a) with two basic nitrogens (N(7) and N(1)), able to two-step protonation with pK 1 = 6.03 and pK 2 = 2.53 accordingly [23], was chosen as the guest of p-sulfona- tothiacalix[4]arene (STCA) (Figure 1b). The choice of the latter instead of its classical analogue is conditioned by the solubility in aqueous solutions of inclusion complexes with charged cobalt (III) complexes being more for thia-derivative. So, the main goal of the work presented is to study the inclusion complex formation between 1Cl and STCA at the wide range of pH 2–10. * Author for correspondence. E-mail: asiya@iopc.knc.ru Journal of Inclusion Phenomena and Macrocyclic Chemistry (2006) 56: 369–374 Ó Springer 2006 DOI 10.1007/s10847-006-9118-x