Ni^II-Pd^II as a More Solution-Stable Paddlewheel Alternative to Cu^II-Cu^II in Metal-Organic Cages

The dinuclear Cu^II-carboxylate paddlewheel, a linchpin of metal-organic cages (MOCs), is highly labile in solution. Herein, more kinetically stable heterometallic (NiPd)₂L₄ lantern-type helical cages have been assembled from preformed [NiPd(OAc)₄] precursors. Ligand exchange processes (monitored by HPLC) revealed the half-lives of the (NiPd)₂L₄ cages in solution to be 10³ times greater than those of the Cu₄L₄ analogues. The hydrophobic guest p-chloranil was encapsulated and reacted inside the unique noncoordinating, charge-neutral Pd^II···Pd^II cavity of a (NiPd)₂L₄ cage but not in the analogous Cu₄L₄ cage. The Ni^II-Pd^II paddlewheel system provides a kinetically stable yet synthetically accessible alternative to labile Cu^II-based metallosupramolecular cages that are more long-lived in solution and have unique internal environments.