Macrocyclic Platforms for the Construction of Tetranuclear Oxo and Hydroxo Zinc Clusters

The design of ligands that can act as platforms for the controlled; bottom-up synthesis of transition-metal clusters is a promising approach to accessing enzymatic mimics and new small-molecule reaction chemistry. This approach is exemplified here through the coordination chemistry of two compartmental Schiff-base calixpyrroles (H4L) that usually act as dinucleating ligands for transition metals. While reactions between H4L and Zn{N(SiMe3)(2)}(2) form the expected dinuclear Zn Pacman complexes Zn-2(L), reactions with ZnEt2 result in the tetranuclear Zn alkyl complexes Zn4Et4(THF)(4)(L), in which open, bowl-shaped structures are adopted due to the flexibility of the macrocyclic platform: The outcome of hydrolysis reactions of these tetranuclear complexes is found to depend on the macrocyclic Cavity size, with the smaller macrocycle favoring oxo formation in Zn-4(mu(4)-O)Et-2(L) and the larger macrocycle favoring complete hydrolysis to form the hydroxide-bridged cluster Zn-4(mu(2)-OH)(4)(L). This latter complex reacts with carbon dioxide at elevated temperature, reforming the free macrocycle H4L and eliminating ZnCO3.