A tetravalent water-soluble cluster of 29 silver atoms and 12 lipoate acid ligands [Ag-29] has potential as a potent antimicrobial agent and biomarker on account of its amenability to surface chemical modification and intriguing luminescent properties. Ag29 may be prepared abundantly as summarized in Scheme 1. Yet, to this date, its total structure remains a mystery. The challenges of crystallizing Ag29 are its apparent tendency to aggregate upon purification, its light sensitivity, and waxy consistency imparted by its twelve lipoate (fatty) ligands. In this work we propose to crystallize Ag29 from nonaqueous solvents by: 1) Control of its polarity via reversible protonation of lipoate terminus; in this manner the cluster can be transferred into a non-aqueous solvent such as DCM. 2) Introducing bulky lipophilic counter ions (i.e. tetraphenyl phosphonium cation) via ion exchange reactions to maximize cluster-cluster distance and minimize the random tangling of lipoate ligands between clusters, 3) Adding triphenyl phosphine (Figure 1) as a weakly coordinating ligand to satisfy the cluster's tetravalency requirements and to induce a tetrahedral packing motif (following Bakr’s approach for the nonaqueous homolog of Ag29). Once modified as suggested above, our working hypothesis is that the cluster will crystallize from saturated solutions in DCM/methanol mixtures. 29AgNO3 + 12LA + 4NaBH4 +32NaOH -&gt; Na3(Ag29LA12) + 18H2O + 4NaB(OH)4 +29NaNO3 Scheme 1. To obtain clusters, reduce the silver salt by sodium borohydride in the presence of Lipoate (LA) ligand under alkaline conditions. LA and reducing agent are added in 2.5:1 molar excess relative to silver.

Crystallizing Ag29

ACS Spring 2022

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