Capsule–Capsule Conversion by Guest Encapsulation

Abstract

Guest-induced M 18 L 6 -M 24 L 8 capsule-capsule conversion is reported. Both capsules are composed of Pd II ethylenediamine units (M) and 1,3,5-tris(3,5-pyrimidyl)pyrimidine (L), and form trigonal bipyramidal (M 18 L 6 )a nd octahedral (M 24 L 8 )c losed-shell structures with huge hydrophobic inner spaces.T he M 18 L 6 trigonal bipyramid is converted to the M 24 L 8 octahedron through encapsulation of large aromatic guests,w ith the latter capsule possessing ac avity volume three times larger than the former.Despite the dynamic properties of the capsule host, the encapsulated guests are difficult to extract and are thus isolated from the external environment.Molecular capsules show unique host properties that are different from those of other synthetic hosts in that they possess isolated nanocavities that allow the properties and reactivities of encapsulated guests to be maintained.[1] Recently,al ot of effort has been put into constructing capsular structures of ever-increasing size, [2] but relatively few of these capsules show distinct guest-binding properties in their large cavities,t hus demonstrating that simple cavity expansion does not automatically lead to large guest encapsulation. We have previously reported trigonal bipyramidal M 18 L 6 coordination capsule 1a,b ut, unlike similar M 6 L 4 open cages that bind various organic guests in their cavities, [3] capsule 1a did not show any encapsulation behavior toward organic guests in water.[2b] We assumed that, in addition to the open framework, the highly electron-deficient environment of the M 6 L 4 cages offered by the metal-coordinated tris(4-pyridyl)triazine ligand is essential for guest binding.W etherefore redesigned the panel ligand of capsule 1a to give am ore electrondeficient 1,3-pyrimidine-cored panel, and created the more electron-deficient M 18 L 6 capsule 1b.H erein, we report that capsule 1b does indeed encapsulate large guests and, unexpectedly and surprisingly,t he large guest encapsulation triggers the conversion of M 18 L 6 capsule 1b into cavityexpanded M 24 L 8 octahedral capsule 2b (Figure 1). This guesttriggered M 18 L 6 -M 24 L 8 switch presents ar are example of capsule-capsule conversion with concurrent dramatic cavity expansion. [4] By simply mixing [(en)Pd(ONO 2 ) 2 ]( 3;e n= ethylenediamine) and our new ligand 2,4,6-tris(3,5-pyrimidyl)pyrimidine (4)i nw ater, trigonal bipyramidal M 18 L 6 coordination capsule 1b was obtained. Amixture of 3 (34.9 mg, 120 mmol) and 4 (12.6 mg, 40 mmol) was stirred in D 2 Osolution at 70 8 8C for 24 h, and the 1 HNMR spectrum after the complexation clearly showed the formation of as ingle product with panel ligand 4 placed in a C 2v environment. Six aromatic signals were observed in a1:2:2:2:2:1 integral ratio (Figure 2a), thus suggesting that 3 and 4 were assembled into trigonal bipyramid 1b with D 3h symmetry and am irror plane (s h ) vertical to the main apical C 3 axis.T here are many possible isomers of this M 18 L 6 capsular structure because ligand 4 does not have a C 3 symmetr...

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