Investigating the Formation of Giant {Pd72}Prop and {Pd84}Gly Macrocycles Using NMR, HPLC, and Mass Spectrometry

Abstract

The formation of giant polyoxometalate (POM) species is relatively underexplored, as their self-assembly process is complex due to the rapid kinetics. Polyoxopalladates (POPds) are a class of POMs based on Pd, the largest of which is the {Pd₈₄}^Ac wheel, and its slower kinetics mean the system is more amenable to systematic study. Here, we show that it is possible to follow the assembly of two types of Pd wheels, {Pd₈₄}^Gly and the smaller {Pd₇₂}^Prop, formed using glycolate and propionate ligands, respectively. We analyzed the formation of {Pd₇₂}^Prop and {Pd₈₄}^Gly using mass spectrometry (SEC-HPLC-MS and preparative desalting followed by MS). This was accompanied by studies that followed the chemical shift differences between the outer/inner ligands and the free ligand in solution for the {Pd₈₄}^Ac, {Pd₇₂}^Prop, and {Pd₈₄}^Gly species using NMR, which showed it was possible to track the formation of the wheels. Our findings confirm that the macrocycles assemble from smaller building blocks that react together to form the larger species over a period of days. These findings open the way for further structural derivatives and exploration of their host-guest chemistry.