Inclusion of a Cu 2+ Ion by a Large-Cavity Crown Ether Dibenzo-24-Crown-8 through Supramolecular Interactions † Vaddypally Shivaiah and Samar K. Das* School of Chemistry, UniVersity of Hyderabad, Hyderabad 500046, India Received July 13, 2005 A supramolecular copper-aqua-crown ether complex, [Cu II (H 2 O) 4 - (dibenzo-24-crown-8] 2+ (1c) is stabilized with a Lindqvist-type polyoxometalate anion, [Mo VI 6 O 19 ] 2- (1a), in an ion-pair compound [Cu II (H 2 O) 4 (dibenzo-24-crown-8][Mo VI 6 O 19 ] ≡ [1c][1a] ≡ 1. In the crystal, 1c and 1a assemble to a chainlike structure in which each polyoxoanion 1a is sandwiched by two 1c cations. 1c is a structurally characterized dibenzo-24-crown-8 (a larger-cavity crown ether) supramolecular complex that shows encapsulation of a small cation at the center of its internal cavity, and compound 1 represents a unique example of a first-row transition metal-crown ether inclusion complex that interacts with a polyoxometalate anion. Crown ethers have been extensively used in supramolecu- lar chemistry because they have potential applications in molecular recognition, transportation, and catalysis. 1 The most striking characteristics of crown ethers is their selective complexation ability with the cationic species. 2 These macrocyclic polyethers (crown ethers), which have “hard” donor atoms, do not readily form complexes with first-row transition metals in their low oxidation states because such metal ions provide only “soft” coordination (acceptor) sites. Only a small number of first-row transition metal-crown ether complexes had been structurally characterized, in which the direct bond formation between the transition metal and the crown ether oxygens became possible (in complexes with the smaller ring crown ethers, e.g., 15-crown-5 and 12- crown-4 ethers). 3 In the case of (comparatively) larger ring crown ethers, the linkage between the metal center and the crown ether is usually provided by one or more water molecules that are directly coordinated to the metal ion and hydrogen-bonded to the oxygens of the crown ether. In such cases, the crown ethers act as second-sphere ligands. 4 The macrocyclic polyether dibenzo-24-crown-8, originally syn- thesized by Pedersen, 2a has been shown to form complexes with alkaline and alkaline-earth metal ions 4f and a ScCl 2 + ion (as a first-row transition-metal complex ion). 4a Most of these dibenzo-24-crown-8 complexes include both direct linkage and second-sphere complex formation via hydrogen- bonding interactions. Because dibenzo-24-crown-8 is a relatively larger cavity crown ether, it accommodates more than one K + in its cavity using direct covalent bonds 5 and it hosts one Ba 2+ ion 6 (a bigger cation) or one ScCl 2 + ion through both direct coordination and second-sphere com- plexation but in an “off-center” position. 4a,6 The encapsulation of any cation at the center of the cavity of dibenzo-24- crown-8 has not yet been achieved probably because of its larger cavity size and flexible nature. We report here the inclusion of a Cu 2+ ion (a cation, smaller than a Na + ion) by introducing a polyoxomolybdate anion [Mo 6 O 19 ] 2- that stabilizes the Cu 2+ ion at the center of the “large-cavity crown ether” dibenzo-24-crown-8 via supramolecular hydrogen- bonding interactions. The coexistence of polyoxoanion and crown ether has attracted special attention and opened a new field in the area of polyoxometalate (POM) chemistry. 7 The crown ethers, supported by polyoxoanions, can be used to separate specific metal ions (depending on the cavity size of the crown ether) from aqueous solutions by precipitation methods. 8 To date, the reported crown ether-POM com- pounds contain mostly alkali and alkaline-earth metals 7a-i and sometimes lanthanides 7j and hydronium ions 7g as cations. * To whom correspondence should be addressed. E-mail: skdsc@ uohyd.ernet.in. † Dedicated to Professor Animesh Chakravorty on the occasion of his 70th birthday. (1) Lehn, J.-M. Angew. Chem., Int. Ed. 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