Diphosphine-Protected IrAu12 Superatom with Open Site(s): Synthesis and Programmed Stepwise Assembly.

One or two phenylacetylide (PA) ligand(s) were successfully removed from the IrAu12 superatomic core of [IrAu12(dppe)5(PA)2]+ (dppe=1,2-bis(diphenylphosphino)ethane) by reaction with controlled amounts of tetrafluoroboric acid. Optical and nuclear magnetic resonance spectroscopies and density functional theory calculations revealed the formation of open Au site(s) on the IrAu12 core of [IrAu12(dppe)5(PA)1]2+ and [IrAu12(dppe)5]3+ with the remaining structure intact. Isocyanide was efficiently trapped at the open electrophilic site on [IrAu12(dppe)5(PA)1]2+, whereas a dimer or trimer of the IrAu12 superatoms was formed using diisocyanide as a linker. These results open the door to designed assembly of chemically modified metal superatoms.

Doping-Mediated Energy-Level Engineering of M@Au12 Superatoms (M=Pd, Pt, Rh, Ir) for Efficient Photoluminescence and Photocatalysis.

We synthesized a series of MAu12 (dppe)5 Cl2 (MAu12 ; M=Au, Pd, Pt, Rh, or Ir; dppe=1,2-bis(diphenylphosphino)ethane), which have icosahedral M@Au12 superatomic cores, and systematically investigated their electronic structures, photoluminescence (PL) and photocatalytic properties. The energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) was expanded when doping an M element positioned at the lower left of the periodic table. The PL quantum yield was enhanced with an increase in the HOMO-LUMO gap and reached 0.46-0.67 for MAu12 (M=Pt, Rh, or Ir) under deaerated conditions. The bright PLs from MAu12 (M=Pt, Rh, or Ir) were assigned to phosphorescence based on quenching by O2 . MAu12 (M=Pt, Rh, or Ir) acted as a more efficient and stable photocatalyst than Au13 for intramolecular [2+2] cycloaddition of bisenone via the oxidative quenching cycle. This study provides rational guides for designing photoluminescent and photocatalytic gold superatoms by the doping of heterometal elements.

Photoluminescence of Doped Superatoms M@Au12 (M = Ru, Rh, Ir) Homoleptically Capped by (Ph2)PCH2P(Ph2): Efficient Room-Temperature Phosphorescence from Ru@Au12.

A series of doped gold superatoms M@Au12 (M = Ru, Rh, Ir) was synthesized by capping with the bidentate ligand (Ph2)PCH2P(Ph2). A single-crystal X-ray diffraction analysis showed that all the M@Au12 superatoms had icosahedral motifs with a significantly higher symmetry than that of the pure Au13 counterpart due to different coordination geometries. The Ru@Au12 superatom exhibited a room-temperature phosphorescence with the highest quantum yield of 0.37 in deaerated dichloromethane. Density functional theory calculations suggested that the efficient phosphorescence is ascribed to a rapid intersystem crossing due to the similarity between the singlet and triplet excited states in terms of structure and energy.

Understanding Doping Effects on Electronic Structures of Gold Superatoms: A Case Study of Diphosphine-Protected M@Au12 (M = Au, Pt, Ir).

Dopants into ligand-protected Au superatoms have been hitherto limited to group X-XII elements (Pt, Pd, Ag, Cu, Hg, and Cd). To expand the scope of the dopants to the group IX elements, we synthesized unprecedented [IrAu12(dppe)5Cl2]+ [IrAu12; dppe = 1,2-bis(diphenylphosphino)ethane] and [PtAu12(dppe)5Cl2]2+ (PtAu12) and compared their electronic structures with that of [Au13(dppe)5Cl2]3+ (Au13). Single-crystal X-ray diffractometry, 31P{1H} NMR, and Ir L3-edge extended X-ray absorption fine structure analysis of IrAu12 revealed that the single Ir atom is located at the center of the icosahedral IrAu12 core. Electrochemical analysis demonstrated that the energy levels of the highest occupied molecular orbitals are upshifted in the order of Au13 < PtAu12 < IrAu12. This trend was qualitatively explained in such a manner that the jellium core potential at the central position becomes shallower by replacing Au+ with Pt0 and further with Ir-. IrAu12 underwent reversible redox reactions between the charge states of 1+ and 2+. The gradual increase of the energy gap between the highest occupied molecular orbital and lowest unoccupied molecular orbital in the order of Au13 < PtAu12 < IrAu12 was observed by electrochemical measurement and optical spectroscopy. This study provides a simple guiding principle to tune the electronic structures of heterometal-doped superatoms.

Ligand-protected gold/silver superatoms: current status and emerging trends.

Monolayer-protected gold/silver clusters have attracted much interest as nano-scale building units for novel functional materials owing to their nonbulk-like structures and size-specific properties. They can be viewed as ligand-protected superatoms because their magic stabilities and fundamental properties are well explained in the framework of the jellium model. In the last decade, the number of ligand-protected superatoms with atomically-defined structures has been increasing rapidly thanks to the well-established synthesis and structural determination by X-ray crystallography. This perspective summarizes the current status and emerging trends in synthesis and characterization of superatoms. The topics related to synthesis include (1) development of targeted synthesis based on transformation, (2) enhancement of robustness and synthetic yield for practical applications, and (3) development of controlled fusion and assembly of well-defined superatoms to create new properties. New characterization approaches are also introduced such as (1) mass spectrometry and laser spectroscopies in the gas phase, (2) determination of static and dynamic structures, and (3) computational analysis by machine learning. Finally, future challenges and prospects are discussed for further promotion and development of materials science of superatoms.

Synthesis of Trimetallic (HPd@M2Au8)3+ Superatoms (M = Ag, Cu) via Hydride-Mediated Regioselective Doping to (Pd@Au8)2.

We have recently reported that hydride (H-) doped superatom (HPd@Au8)+ protected by eight PPh3 ligands selectively grew into (HPd@Au10)3+ by the nucleophilic addition of two Au(I)Cl units. In the present study, (HPd@Au8)+ was successfully converted to unprecedented trimetallic (HPd@M2Au8)3+ superatoms (M = Ag, Cu) by controlled doping of two Ag(I)Cl or Cu(I)Cl units, respectively. Single-crystal X-ray diffraction analysis demonstrated that two Ag(I) or Cu(I) ions were regioselectively incorporated. Theoretical calculations suggested that hydrogens in (HPd@M2Au8)3+ (M = Au, Ag, Cu) occupy the same bridging site between the central Pd atom and the surface Au atom. (HPd@Ag2Au8)3+ exhibited photoluminescence at 775 nm, with the enhanced quantum yield of 0.09%, although it is structurally and electronically equivalent with (HPd@Au10)3+. This study demonstrates that hydride-mediated growth process is a promising atomically-precise bottom-up synthetic method of new multimetallic superatoms.

Hydride-Mediated Controlled Growth of a Bimetallic (Pd@Au8)2+ Superatom to a Hydride-Doped (HPd@Au10)3+ Superatom.

A hydride (H-)-doped bimetallic superatom (HPdAu8)+ was produced by reacting BH4- with an oblate (PdAu8)2+ superatom protected by PPh3. The H atom in (HPdAu8)+ survived during the sequential addition of Au(I)Cl to form an (HPdAu10)3+ superatom, in sharp contrast to the proton release from a H--doped pure gold superatom (HAu9)2+ in the growth process to (Au11)3+. Single-crystal X-ray diffraction analysis and density functional theory calculations on (HPdAu10)3+ showed that the interstitially doped H atom induced a notable deformation of the core.

Hydride-Doped Gold Superatom (Au9H)2+: Synthesis, Structure, and Transformation.

Doping of a hydride (H-) into an oblate-shaped gold cluster [Au9(PPh3)8]3+ was observed for the first time by mass spectrometry and NMR spectroscopy. Density functional theory calculations for the product [Au9H(PPh3)8]2+ demonstrated that the (Au9H)2+ core can be viewed as a nearly spherical superatom with a closed electronic shell. The hydride-doped superatom (Au9H)2+ was successfully converted to the well-known superatom Au113+, providing a new atomically precise synthesis of Au clusters via a bottom-up approach.