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1. Crystal structure of bulky-ligand-protected Au ₂₄ (S-C ₄ H ₉ ) ₁₆

   https://doi.org/10.1107/s2053229622006738  

   Wijesinghe, Kalpani Hirunika; Oliver, Allen G.; Dass, Amala (August 2022, Acta Crystallographica Section C Structural Chemistry)

   Atomically precise thiolate-protected gold nanomolecules have attracted interest due to their distinct electronic and chemical properties. The structure of these nanomolecules is important for understanding their peculiar properties. Here, we report the X-ray crystal structure of a 24-atom gold nanomolecule protected by 16 tert -butylthiolate ligands. The composition of Au 24 (S-C 4 H 9 ) 16 {poly[hexadecakis(μ- tert -butylthiolato)tetracosagold]} was confirmed by X-ray crystallography and electrospray ionization mass spectrometry (ESI–MS). The nanomolecule was synthesized in a one-phase synthesis and crystallized from a hexane–ethanol layered solution. The X-ray structure confirms the 16-atom core protected by two monomeric and two trimeric staples with four bridging ligands. The Au 24 (S-C 4 H 9 ) 16 cluster follows the shell-closing magic number of 8. 

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2. Ion mobility–tandem mass spectrometry of bulky tert ‐butyl thiol ligated gold nanoparticles

   https://doi.org/10.1002/jms.4998  

   Wijesinghe, Kalpani_H; Hood, Christopher; Mattern, Daniell; Angel, Laurence_A; Dass, Amala (January 2024, Journal of Mass Spectrometry)

   Abstract Gold nanoparticles (AuNPs) synthesized in the 1–3 nm range have a specific number of gold core atoms and outer protecting ligands. They have become one of the “hot topics” in recent decades because of their interesting physical and chemical properties. The characterization of their structures is usually achieved by crystal X‐ray diffraction although the structures of some AuNPs remain unknown because they have not been successfully crystallized. An alternative method for studying the structure of AuNPs is electrospray ionization–ion mobility–tandem mass spectrometry (ESI‐IM‐MSMS). This research evaluated how effectively ESI‐IM‐MSMS using the commercially available Waters Synapt XS instrument yielded useful structural information from two AuNPs; Au₂₃(S‐tBu)₁₆and Au₃₀(S‐tBu)₁₈. The study used the maximum range of available collision energies along with ion mobility separation to measure the energy‐dependence of the product ions and their drift times which is a measure of their spatial size. For Au₂₃(S‐tBu)₁₆, the dissociation gave the masses of the outer protecting monomeric [RS–Au–SR] and trimeric [SR–Au–SR–Au–SR–Au–SR] staples where R = tBu, and complete dissociation of the outer layer Au andtBu groups to reveal the Au₁₅S₈core. For Au₃₀(S‐tBu)₁₈, the dissociation products was primarily through the loss of the partial ligands S‐tBu andtBu from the outer protecting layer and the loss of single Au₄(S‐tBu)₄unit. These results showed the that ESI‐IM‐MSMS analysis of the smaller Au₂₃(S‐tBu)₁₆gave information on all it major structural components whereas for Au₃₀(S‐tBu)₁₈, the overall structural information was limited to the ligands of the outer layer. 

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3. Active Site Characterization of a Campylobacter jejuni Nitrate Reductase Variant Provides Insight into the Enzyme Mechanism

   https://doi.org/10.1021/acs.inorgchem.4c01991  

   Yang, Jing; Mintmier, Breeanna; KC, Khadanand; Metzger, Mikayla C; Radhakrishnan, Manohar; McGarry, Jennifer; Wilcoxen, Jarett; Basu, Partha; Kirk, Martin L (July 2024, Inorganic Chemistry)

   Mo K-edge X-ray absorption spectroscopy (XAS) is used to probe the structure of wild-type Campylobacter jejuni nitrate reductase NapA and the C176A variant. The results of extended X-ray absorption fine structure (EXAFS) experiments on wt NapA support an oxidized Mo(VI) hexacoordinate active site coordinated by a single terminal oxo donor, four sulfur atoms from two separate pyranopterin dithiolene ligands, and an additional S atom from a conserved cysteine amino acid residue. We found no evidence of a terminal sulfido ligand in wt NapA. EXAFS analysis shows the C176A active site to be a 6-coordinate structure, and this is supported by EPR studies on C176A and small molecule analogs of Mo(V) enzyme forms. The SCys is replaced by a hydroxide or water ligand in C176A, and we find no evidence of a coordinated sulfhydryl (SH) ligand. Kinetic studies show that this variant has completely lost its catalytic activity toward nitrate. Taken together, the results support a critical role for the conserved C176 in catalysis and an oxygen atom transfer mechanism for the catalytic reduction of nitrate to nitrite that does not employ a terminal sulfido ligand in the catalytic cycle. 

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4. Digestive ripening yields atomically precise Au nanomolecules

   https://doi.org/10.1039/d1nj04042a  

   Eswaramoorthy, Senthil Kumar; Dass, Amala (November 2021, New Journal of Chemistry)

   Digestive ripening (DR) is a synthetic method where a polydisperse colloid of metal nanoparticles upon refluxing with a free ligand in a high boiling point solvent gives monodisperse nanoparticles. Brust synthesis is known to form atomically monodisperse thiolate protected gold nanoparticles also known as gold nanomolecules (Au NMs). Unlike the Brust method which gives smaller (1–3 nm) atomically precise nanomolecules, DR has been used only for the synthesis of large nanoparticles (>5 nm) with good monodispersity. In thiolate protected gold nanoparticle Brust synthesis, the yellow colored phase transferred Au( iii ) solution is converted to a colorless Au( i ) mixture after the addition of thiol by forming Au–SR, which is then reduced to form black colored Au NMs. However, in DR, by using the same primary chemicals, the two steps were reversed: the mixture was reduced before the addition of thiol. Here we show that in DR, adding thiol after 2 minutes of reduction gives larger particles (5 nm) as reported, whereas adding thiol 30 seconds after reduction results in smaller particles (<2 nm). In this work, for the first time, DR yields atomically precise Au 25 (SR) 18 and Au 144 (SR) 60 NMs. This is reported using two aliphatic thiols – hexanethiol and dodecanethiol – as the protecting ligands. DR was also repeated using an aromatic thiol, 4- tert -butyl benzene thiol (TBBT), which yields Au 279 (SR) 84 NMs consistent with the Brust method, thereby establishing that both DR and Brust methods lead to the formation of atomically precise Au NMs, regardless of the order of thiol addition and reduction steps. 

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5. Atomic-precision engineering of metal nanoclusters

   https://doi.org/10.1039/d0dt01853h  

   Du, Xiangsha; Jin, Rongchao (August 2020, Dalton Transactions)

   null (Ed.)

   Ultrasmall metal nanoparticles (below 2.2 nm core diameter) start to show discrete electronic energy levels due to strong quantum confinement effects and thus behave much like molecules. The size and structure dependent quantization induces a plethora of new phenomena, including multi-band optical absorption, enhanced luminescence, single-electron magnetism, and catalytic reactivity. The exploration of such new properties is largely built on the success in unveiling the crystallographic structures of atomically precise nanoclusters (typically protected by ligands, formulated as M n L m q , where M = metal, L = Ligand, and q = charge). Correlation between the atomic structures of nanoclusters and their properties has further enabled atomic-precision engineering toward materials design. In this frontier article, we illustrate several aspects of the precise engineering of gold nanoclusters, such as the single-atom size augmenting, single-atom dislodging and doping, precise surface modification, and single-electron control for magnetism. Such precise engineering involves the nanocluster's geometric structure, surface chemistry, and electronic properties, and future endeavors will lead to new materials design rules for structure–function correlations and largely boost the applications of metal nanoclusters in optics, catalysis, magnetism, and other fields. Following the illustrations of atomic-precision engineering, we have also put forth some perspectives. We hope this frontier article will stimulate research interest in atomic-level engineering of nanoclusters. 

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