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.

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Treatment of the chloride bridged species [{M(L)Cl(μ-Cl)}2] (M = Ru, L = 4-MeC6H4CHMe2, C6H6, or C6Me6; M = Ir, L = C5Me5) with silver salts AgX (X = BF4, CF3SO3, CF3CO2 etc.) followed by reflux with the bowl-shaped macrocycle cyclotriveratrylene (CTV) results in the clean formation of mono-, di-, and trimetallic CTV complexes [{M(L)} n (CTV)]X2 n (n = 1, 2, 3). Further salts (X = ReO4 -, I-) may be generated by anion metathesis. All three types of complex display novel host−guest properties. In the case of the monometallic hosts the disruption of the characteristic columnar packing mode of the CTV, a result of the presence of the metal center, leads to the inclusion of neutral and anionic guest species (NO2Me, Et2O, H{CF3CO2}2 -, etc.) within the CTV cavity. For complexes where n = 2 or 3 the inclusion of anionic guests is invariably observed. The extent of anion binding has been established by means of X-ray crystal structure determinations upon various di- and trimetallic species containing BF4 -, CF3SO3 -, or ReO4 -, by radiotracer analysis in solution using 99mTcO4 - and 188ReO4 -, and by cyclic voltammetry. The trimetallic complex [{Ir(Cp*)}3(CTV)][BF4]6 (9a) in particular exhibits F···CCTV contacts as short as 2.78(3) Å. The dimetallic host [{Ru(η6-4-MeC6H4CHMe2)}2(η6:η6-CTV)]4+ (4) is shown to have a specific affinity for large tetrahedral anions and will selectively extract both 99TcO4 - and ReO4 - from aqueous solution, even in the presence of a large excess of Cl-, CF3SO3 -, NO3 -, SO4 2-, and to some extent ClO4 -.