Anion Binding within the Cavity of π-Metalated Calixarenes

The synthesis of a range of bi- and tetrametalated macrocyclic complexes based upon calix[4]arene, p-tert-butylcalix[4]arene and related tetraalkoxy derivatives, of type [{M(L)}2(calixarene-nH)]X4 - n and [{M(L)}4(calixarene-2H)]X6 (M = Rh, Ir, Ru; L = Cp*, p-MeC6H4CHMe2; n = 0, 1, 2; X = BF4, CF3SO3, HSO4, I, ReO4, H2PO4, etc., not all combinations) is reported. A related trimetallic calix[5]arene derivative [{Ir(η5-C5Me5)}3(p-tert-butylcalix[5]arene-H)][BF4]5 has also been synthesized. In all cases, the presence of the transition metal centers results in a significant enhancement in the acidity of the hydroxyl functionalities at the calixarene lower rim. For the tri- and tetrametalated derivatives, the host−guest behavior of the calixarene is drastically altered such that anionic guest species are included within the molecular cavity. The function of the new tetrametallic hosts [{Ru(η6-p-MeC6H4CHMe2)}4(η6:η6:η6:η6-C28H22O4)]6+ (15) and [{Ir(η5-C5Me5)}4(η6:η6:η6:η6-C28H22O4)]6+ (16) as anion receptors has been confirmed by X-ray crystal structure investigations upon BF4 - (15a, 16a), HSO4 - (16c), SO4 2- (15c), and I- (15d) derivatives, which clearly demonstrate a cooperative effect arising from the arrangement of four metal centers about a common, rigid binding pocket, resulting in anion−host contacts as low as 2.85 Å (BF4···Ccalix). The anion binding properties of host 15 in aqueous solution have also been investigated by 1H NMR titration, giving binding constants in the range 100−550 M-1 for nitrate and halide anions, with the binding constant decreasing in the order Cl- > Br- > I-. Significantly increased binding is observed in nonaqueous media. This novel form of anion complexation contrasts to the formation of weakly bound, van der Waals inclusion complexes commonly observed in calixarene chemistry.