Core-Size Conversion of Au 38 (SCH 2 CH 2 Ph) 24 to Au 30 (S−tBu) 18 Nanomolecules Milan Rambukwella, † Luca Sementa, ‡ Alessandro Fortunelli, ‡ and Amala Dass* ,† † Department of Chemistry and Biochemistry, University of Mississippi, Oxford, Mississippi 38677, United States ‡ CNR-ICCOM, Consiglio Nazionale delle Ricerche, via Giuseppe Moruzzi 1, 56124 Pisa, Italy * S Supporting Information ABSTRACT: Au 38 (SCH 2 CH 2 Ph) 24 nanomolecules upon etching with tert-butylthiol undergo core-size conversion to green-gold Au 30 (S − t Bu) 18 via Au 36 (SCH 2 CH 2 Ph) 24‑x (S−tBu) x intermediate. The structural transformation from Au 38 to Au 30 indicates a strong steric effect due to the tert-butyl group of the exchanging ligand in contrast to electronic effect by ligands such as thiophenol, which transforms the Au 38 (SCH 2 CH 2 Ph) 24 to Au 36 (S−tBu) 24 . In this work, fast reaction kinetics were observed and thermodynamically stable Au 30 (S−tBu) 18 was obtained in molecular purity. ■ INTRODUCTION Gold nanomolecules 1,2 have become a topic of great interest in chemistry due to their atomic monodispersity (±0 atom variation), molecule-like properties, 3,4 and stability arising from geometric 5 and electronic shell 6 closings. Obtaining molecular pure product is challenging by typical synthetic protocol. In order to achieve high monodispersity, chromatographic techniques, solvent fractionation, crystallization, and core-size conversion methods are being employed. 7−9 Core-size conversion reactions are known to form a new core-size or core sizes that are different from the starting nanomolecules or nanoclusters. 8,10 This is being achieved upon reacting the starting nanomolecules or the nanoclusters with physiochemically different thiols. The reaction proceeds with ligand exchange, core-size conversion, or both occurring simultaneously. In our previous work, we have reported the aromatic ligand induced core-size conversion of Au 67 (SR) 35 and Au 103−105 (SR) 44−46 nanocluster mixture to Au 36 (SPh) 24 , with its X-ray crystal structure reported subsequently, 11, 12 and Au 38 (SCH 2 CH 2 Ph) 24 core-size conversion to Au 36 (SPh) 24 in the presence of thiophenol (HSPh) ligand, where the aromatic nature of the −SPh ligand is inducing the core-size conversion to Au 36 . 9 A subsequent work by Jin and co-workers confirmed this core-size conversion by providing evidence for the aromatic tert-butylbenzene thiol (TBBT) ligand induced core-size conversion of Au 38 (SCH 2 CH 2 Ph) 24 starting material to Au 36 (TBBT) 24 , in the presence of TBBT, at elevated temperature (80 °C). 13 Aromatic ligand effect of TBBT was also evident by the report of core-size conversion of Au 144 (SCH 2 CH 2 Ph) 60 to Au 133 (SPh-tBu) 52 . 7 It is interesting to note that TBBT ligand has a bulky p-tBu group that gives rise to steric effect opposed to the electronic effect that is arising from the phenyl aromatic ring. Thorough theoretical studies on this reaction reveal that core-size conversion is an electronic and kinetic effect due to the aromatic nature of the new ligand, TBBT, and the purely steric effect due to p-tBu is rather small. 7 Aromatic TBBT ligand introduces both aromatics and bulkiness to the ligand shell; thus, it is not possible to attribute the Au 38 (SCH 2 CH 2 Ph) 24 core-size conversion to Au 36 (TBBT) 24 to either electronic or steric effects. However, the report on Au 38 (SCH 2 CH 2 Ph) 24 core-size conversion to Au 36 (SPh) 24 suggests that electronic effect due to extended conjugation by −SPh ligand is sufficient to induce the core-size conversion. 11 Green-gold Au 30 (S−tBu) 18 was first reported in 2013 by our group, followed by its crystal structure. 9,14,15 Two other groups reported the synthesis and characterization of Au 30 (S−tBu) 18 , suggesting the reproducible nature of the synthesis. 16,17 Zhang and co-workers have successfully synthesized Au 30 S(S−tBu) 18 nanomolecules upon deliberately introducing NaS 2 into the reaction to incorporate S 2− onto the surface of the nano- molecules. 15 Unlike analogous nanomolecules such as Au 36 (SR) 24 and Au 38 (SR) 24 , which have aromatic ligand and aliphatic-like ligand effects on core stability, respectively, Au 30 (S−tBu) 18 nanomolecules have a sterically crowded thiolate ligand shell. Also, other core-sizes Au 21 , Au 23 , and Au 30 have been reported with bulky thiolate ligands. Existing method of Au 30 (S−tBu) 18 synthesis involves chromatographic purification of the final product. 9,15 Workentin and co-workers have utilized tert-butylthiol in one step to synthesize Au 23 (S− Received: May 4, 2017 Revised: June 15, 2017 Published: July 5, 2017 Article pubs.acs.org/JPCC © 2017 American Chemical Society 14929 DOI: 10.1021/acs.jpcc.7b04201 J. Phys. Chem. C 2017, 121, 14929−14935