ANION-BINDING WITHIN THE CAVITY OF PI-METALATED CALIXARENES

The synthesis of a range of bi- and tetrametalated macrocyclic complex es based upon calix[4]arene, p-tert-butylcalix[4]arene and related tet raalkoxy derivatives, of type [{M(L)}(2)(calixarene-nH)]X4-n and [{MCL )}(4)(calixarene-2H)]X-6 (M = Rh, Ir, Ru; L = Cp, p-MeC6H4CHMe2; n = 0, 1, 2; X = BF4, CF3SO3, HSO4, I, ReO4, H2PO4, etc., not all combinat ions) is reported. A related trimetallic calix[5]arene derivative )-C5 Me5)}(3)(p-tert-butylcalix[5]arene-H)][BF4](5) has also been synthesiz ed. In all cases, the presence of the transition metal centers results in a significant enhancement in the acidity of the hydroxyl functiona lities at the calixarene lower rim. For the tri- and tetrametalated de rivatives, the host-guest behavior of the calixarene is drastically al tered such that anionic guest species are included within the molecula r cavity. The function of the new tetrametallic hosts (eta(6)-p-MeC6H4 -CHMe2)}4(eta(6):eta(6):eta(6):eta 6-C28H22O4)](6+) (15) and eta(5)-C5 Me5)}(4)(eta(6):eta(6):eta(6):eta(6)C(28) H22O4)](6+) (16) as anion re ceptors has been confirmed by X-ray crystal structure investigations u pon BF4- (15a, 16a), HSO4- (16c), SO42- (15c), and I- (15d) derivative s, which clearly demonstrate a cooperative effect arising from the arr angement of four metal centers about a common, rigid binding pocket, r esulting in anion-host contacts as low as 2.85 Angstrom (BF4 ... C-cal ix). The anion binding properties of host 15 in aqueous solution have also been investigated by H-1 NMR titration, giving binding constants in the range 100-550 M-1 for nitrate and halide anions, with the bindi ng constant decreasing in the order Cl- > Br- > I-. Significantly incr eased binding is observed in nonaqueous media. This novel form of anio n complexation contrasts to the formation of weakly bound, van der Waa ls inclusion complexes commonly observed in calixarene chemistry.