Mapping the Assembly of Neutral Tetrahedral Cages Tethered by Oximido Linkers and Their Guest Encapsulation Studies

Abstract

A primary criterion for the design of polyhedral metal–organic cages is the requirement of geometrically matched pairs of metal ions and ligand moieties. However, understanding the pathway it takes to reach the final polyhedral structure can provide more insights into the self-assembly process and improved design strategies. In this regard, we report two neutral tetrahedral cages with the formulas {[Pd3(NiPr)3PO]4(L1)6} (1-TD) and {[Pd3(NiPr)3PO]4(L2)6} (2-TD) starting from the acetate-bridged cluster {[Pd3(NiPr)3PO]2(OAc)2(OH)}2·2(CH3)2SO (HEXA-Pd) and the respective oxamide precursors (L1H2: [C2(NH2)2O2]) and (L2H2: (C2(NHMe)2O2]). When subtle variations in the reaction conditions were made, two new tetrameric Pd12 assemblies, {[Pd3(NiPr)3PO]4(L1)2(OAc)4(OMe)4} (1-TM) and {[Pd3(NiPr)3PO]4(L2)2(OAc)4(OMe)4} (2-TM), were obtained from the same precursors. Detailed investigations using NMR, mass spectrometry, X-ray crystallography, and computational studies indicate that the macrocyclic complexes 1-TM and 2-TM are the reaction intermediates involved in the formation of the tetrahedral cages 1-TD and 2-TD, respectively. Moreover, the tetrahedral cages 1-TD and 2-TD exhibited intrinsic cavities of volume ∼85 Å3. Guest encapsulation studies revealed that the cage 1-TD can encapsulate a wide range of guest molecules such as CH2Cl2, CHCl3, CCl4, C6H6, and C6H5F. Interestingly, 1-TD was shown to exhibit a preferential binding of C6H5F and C6H6 over other halogenated guest molecules, as determined from NMR titrations and computational studies