Harnessing a Pd<sub>4</sub> Water-Soluble Molecular\nCapsule as a Size-Selective Catalyst for Targeted Oxidation of Alkyl\nAromatics

Molecular hosts with\nfunctional cavities can emulate enzymatic\nbehavior through selective encapsulation of substrates, resulting\nin high chemo-, regio-, and stereoselective product formation. It\nis still challenging to synthesize enzyme-mimicking hosts that exhibit\na narrow substrate scope that relies upon the recognition of substrates\nbased on the molecular size. Herein, we introduce a Pd<sub>4</sub> self-assembled water-soluble molecular capsule [<b>M</b><sub>4</sub><b>L</b><sub>2</sub>] (<b>MC</b>) that was formed\nthrough the self-assembly of a ligand <b>L</b> (4′,4‴′-(1,4-phenylene)bis(1′,4′-dihydro-[4,2′:6′,4″-terpyridine]-3′,5′-dicarbonitrile))\nwith the acceptor <i>cis</i>-[(en)Pd(NO<sub>3</sub>)<sub>2</sub>] [en = ethane-1,2-diamine] (<b>M</b>). The molecular\ncapsule <b>MC</b> showed size-selective recognition towards\n xylene isomers. The redox property of <b>MC</b> was explored\nfor efficient and selective oxidation of one of the alkyl groups of <i>m</i>-xylene and <i>p</i>-xylene to their corresponding\ntoluic acids using molecular O<sub>2</sub> as an oxidant upon photoirradiation.\nEmploying host–guest chemistry, we demonstrate the homogeneous\ncatalysis of alkyl aromatics to the corresponding monocarboxylic acids\nin water under mild conditions. Despite homogeneous catalysis, the\nproducts were separated from the reaction mixtures by simple filtration/extraction,\nand the catalyst was reused. The larger analogues of the alkyl aromatics\nfailed to bind within the <b>MC</b>’s hydrophobic cavity,\nresulting in a lower/negligible reaction outcome. The present study\nrepresents a facile approach for selective photo-oxidation of xylene\nisomers to their corresponding toluic acids in an aqueous medium under\nmild conditions.