Cavitand-Based Pd-Pyridyl Coordination Capsules: Guest-Induced Homo- or Heterocapsule Selection and Applications of Homocapsules to the Protection of a Photosensitive Guest and Chiral Capsule Formation.

A 2 : 4 mixture of tetrakis[4-(4-pyridyl)phenyl]cavitand (1) or tetrakis[4-(4-pyridyl)phenylethynyl]cavitand (2) and Pd(dppp)(OTf)2 self-assembles into a homocapsule {12 ⋅ [Pd(dppp)]4 }8+ ⋅ (TfO- )8 (C1) or {22 ⋅ [Pd(dppp)]4 }8+ ⋅ (TfO- )8 (C2), respectively, through Pd-Npy coordination bonds. A 1 : 1 : 4 mixture of 1, 2, and Pd(dppp)(OTf)2 produced a mixture of homocapsules C1, C2, and a heterocapsule {1 ⋅ 2 ⋅ [Pd(dppp)]4 }8+ ⋅ (TfO- )8 (C3) in a 1 : 1 : 0.98 mole ratio. Selective formation (self-sorting) of homocapsules C1 and C2 or heterocapsule C3 was controlled by guest-induced encapsulation under thermodynamic control. Applications of Pd-Npy coordination capsules with the use of 1 were demonstrated. Capsule C1 serves as a guard nanocontainer for trans-4,4'-diacetoxyazobenzene to protect against the trans-to-cis photoisomerization by encapsulation. A chiral capsule {12 ⋅ [Pd((R)-BINAP)]4 }8+ ⋅ (TfO- )8 (C5) was also constructed. Capsule C5 induces supramolecular chirality with respect to prochiral 2,2'-bis(alkoxycarbonyl)-4,4'-bis(1-propynyl)biphenyls by diastereomeric encapsulation through the asymmetric suppression of rotation around the axis of the prochiral biphenyl moiety.