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

Abstract:

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 (H<inf>4</inf>L) that usually act as dinucleating ligands for transition metals. While reactions between H<inf>4</inf>L and Zn{N(SiMe<inf>3</inf>)<inf>2</inf>}<inf>2</inf> form the expected dinuclear Zn "Pacman" complexes Zn<inf>2</inf>(L), reactions with ZnEt<inf>2</inf> result in the tetranuclear Zn alkyl complexes Zn<inf>4</inf>Et<inf>4</inf>(THF)<inf>4</inf>(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<inf>4</inf>(μ<inf>4</inf>-O)Et<inf>2</inf>(L) and the larger macrocycle favoring complete hydrolysis to form the hydroxide-bridged cluster Zn<inf>4</inf>(μ<inf>2</inf>-OH)<inf>4</inf>(L). This latter complex reacts with carbon dioxide at elevated temperature, re-forming the free macrocycle H<inf>4</inf>L and eliminating ZnCO<inf>3</inf>. (Chemical Equation Presented).