A cuboctahedral supramolecular capsule by 4:4 self-assembly of Tris(Zn(II)-cyclen) and trianionic trithiocyanurate in aqueous solution at neutral pH (cyclen=1,4,7,10-tetraazacyclododecane).

Abstract

A 1:1 mixture of a tris(Zn(II)-cyclen) (1: [Zn(3)L(1)], L(1)=1,3,5-tris(1,4,7,10-tetraazacyclododecan-1-ylmethyl)benzene) and trithiocyanuric acid (TCA) yielded a 4:4 self-assembly complex [(Zn(3)L(1))(4)-(TCA(3-))(4)] (6) through the formation of Zn(II)-S(-) coordination bonds and hydrogen bonds between 1,3,5-triazine N and cyclen NH (cyclen=1,4,7,10-tetraazacyclododecane); the supramolecular capsule structure was revealed by X-ray crystal structure analysis. The capsule exterior represents a twisted cuboctahedral framework containing a nanoscale truncated tetrahedral cavity. The crystal data: formula C(144)H(308)N(72)O(58)S(12)Zn(12) (6[NO(3)](12) x 22 H(2)O), M(r)=5145.75, cubic, space group F432 (No. 209), a=39.182(1) A, V=60153(3) A (3), Z=8, R=0.100, R(w)=0.259. Lipophilic organic molecules with the matching sizes, for example, ([D(4)]-2,2,3,3)-3-(trimethylsilyl)propionic acid (TSP), 1-adamantanecarboxylic acid, 2,4-dinitrophenol (2,4-DNP), adamantane (ADM), or the tetra-n-propylammonium (TPA) cation, are encapsulated in the inner cavity, as revealed by remarkable upfield shifts of the (1)H NMR signals of these guest molecules. The encapsulation of ADM was confirmed by X-ray crystal structure analysis. Crystal data of the ADM-encapsulating complex: formula C(154)H(334)N(72)O(63)S(12)Zn(12) (6-ADM[NO(3)](12) x 27 H(2)O), M(r)=5372.06, cubic, space group F432 (No. 209), a=39.061(1) A, V=59599(3) A(3), Z=8, R=0.103, R(w)=0.263. The 4:4 self-assembly was stabilized by incorporation of one of these guest molecules. The apparent 4:4 self-assembly constants for 6 in the presence of an excess amount of a guest TPA, log K(app) (K(app)=[6-TPA]/[1](4)[TCA](4)) (M(-7))), were determined to be 34.0+/-2.0 and 35.5+/-3.0 by potentiometric pH and UV spectrophotometric titrations, respectively. An apparent encapsulation constant for 2,4-DNP, log K(enc) (K(enc)=[6-2,4-DNP]/[6][2,4-DNP] (M(-1))), was 6.0+/-0.1 at pH 7.0 (50 mM HEPES with I=0.1 (NaNO(3))), as determined by UV titrations. The lipophilicity of the inner cavity was close to that of 2-propanol, as a quantum yield (phi) of 0.24+/-0.1 for the fluorescent emission of 7-diethylaminocoumarin-1-carboxylic acid (20 microM) in the capsule was close to the phi of 0.22 found for 2-propanol. Encapsulation properties of the present Zn(II)-containing cage have been compared with those of cyclodextrins and Fujita's Pd(II)-containing supramolecular cage. The exterior chirality of the 4:4 complex was controlled from within by an encapsulated chiral guest molecule, 2,10-camphorsultam, as indicated by Cotton effects in the circular dichroism spectra.