Radical Initiators in the Nanoconfinement Paradise of a Molecular Prism

A new Pd6L′6L3-type water-soluble prismatic nanocage has been exclusively prepared by combining cis-Pd(tmeda)(NO3)2 with the tetra-pyridyl ligand 1,3,6,8-tetra(pyridyn-3-yl)pyrene in 6:3 ratio in the absence of any externally added template. The nanocage has been characterized by NMR spectroscopy, ESI-MS, and single-crystal XRD techniques. The confined nanospace of the cage has been utilized to encapsulate nonpolar/polar/anionic-aromatic guest molecules in water. The cage shows size selectivity toward aromatic guest molecules (as studied for nonpolar guests) having length and width in the limits of ∼9.5 and ∼5.5 Å, respectively. The cavity binds guests of compatible sizes, where the relative binding preference for guests is nonpolar < polar < anionic. Radical initiators tend to decompose when placed under UV light, and these can be protected in a supramolecular way by storing in the cavity of a suitable host molecule. The structural design of our cage made it possible to bind and stabilize radical initiators such as 2-hydroxy-2-methylpropiophenone (HMPP) and azobisisobutyronitrile (AIBN). The half-lives of encapsulated radical initiators are found to be significantly increased (238 times for HMPP and 305 times for AIBN) compared to the half-lives of the same molecules in the absence of a host. Performing a Wohl-Ziegler bromination reaction using a radical initiator that was freshly released from the cage cavity demonstrates secure storage and on-demand release of the active initiator molecule.