Anion Binding within the Cavity of π-Metalated Calixarenes

Abstract

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.

Water-Soluble Calix[4]arene Derivatives: Binding Stoichiometry and Spectroscopic Evaluation of the Host-Guest Recognition Mechanism

“…A series of π-metalated calix[n]arenes were synthesised, among which compounds 29 are water-soluble due to the presence of six positive charges. 50 The calixarenes cavities are therefore electron-poor and able to complex anions both in the solid state and in water. The X-ray crystal structures of compounds 29a, 29c and 29d showed that a BF 4 -, SO 4 2-, and Ianion is complexed in the calixarene cavity, respectively, tetrafluoroborate being the most deeply included one.…”

Anion‐π‐Wechselwirkungen ins rechte Licht gerückt

“…[100][101][102] Diese zeigten mit Rçntgenkristallographie und 1 H-NMR-Titration, dass das Wirt-Gast-Verhalten von Calixarenen und Cyclotriveratrylenen durch Komplexierung der Arenringe mit Übergangsmetallen (Ru, Ir, Rh) umgekehrt werden kann, sodass anionische Gastspezies (anstelle von kationischen) in der molekularen Hçhlung eingeschlossen werden kçnnen. Ein Beispiel dafür ist in Abbildung 5 (oben) zu sehen.…”

“…Ferner wird eine wesentliche VerkleiAbbildung 5. Rçntgenkristallstrukturen der Übergangsmetall-Makrocyclus-Komplexe NAYRIO [102] und RAYFED [104] mit Anion-p-Kation-Bindungen.…”

Stepwise Construction of a Re–Pd Heterodinuclear Core Inside the Cavity of p‐tBu‐Calix[4]arene

“…[2,3] The majority of the metallocalixarenes so far developed are oxophilic group 4-6 transition-metal derivatives; few examples have been reported for late-transitionmetal complexes of unmodified calixarenes. [3][4][5][6] However, recent studies have revealed that late transition metals can also form stable aryloxo and alkoxo complexes that have intriguing reactivities.…”

“…[2,3] The majority of the metallocalixarenes so far developed are oxophilic group 4-6 transition-metal derivatives; few examples have been reported for late-transitionmetal complexes of unmodified calixarenes. [3][4][5][6] However, recent studies have revealed that late transition metals can also form stable aryloxo and alkoxo complexes that have intriguing reactivities. [7] This background prompted us to investigate the synthesis and properties of late-transitionmetal derivatives of calixarenes, and we have recently found that calix[4]arenes undergo site-selective and stepwise complexation with two {M(cod) + } fragments (M = Rh, Ir; cod = 1,5-cyclooctadiene), where the first metal center is coordinated to the arene ring and the second is coordinated to the phenolic oxygen atoms at the narrow rim.…”