Hydride-doped coinage metal superatoms and their catalytic applications.

Superatomic constructs have been identified as a critical component of future technologies. The isolation of coinage metal superatoms relies on partially reducing metallic frameworks to accommodate the mixed valent state required to generate a superatom. Controlling this reduction requires careful consideration in reducing the agent, temperature, and the ligand that directs the self-assembly process. Hydride-based reducing agents dominate the synthetic wet chemical routes to coinage metal clusters. However, within this category, a unique subset of superatoms that retain a hydride/s within the nanocluster post-reduction have emerged. These stable constructs have only recently been characterized in the solid state and have highly unique structural features and properties. The difficulty in identifying the position of hydrides in electron-rich metallic constructs requires the combination and correlation of several analytical methods, including ESI-MS, NMR, SCXRD, and DFT. This text highlights the importance of NMR in detecting hydride environments in these superatomic systems. Added to the complexity of these systems is the dual nature of the hydride, which can act as metallic hydrogen in some cases, resulting in entirely different physical properties. This review includes all hydride-doped superatomic nanoclusters emphasizing synthesis, structure, and catalytic potential.

Doping effect on a two-electron silver nanocluster.

This study investigates the effects of metal addition and doping of a 2-electron silver superatom, [Ag10{S2P(OiPr)2}8] (Ag10). When Ag+ is added to Ag10 in THF solution, [Ag11{S2P(OiPr)2}8(OTf)] (Ag11) is rapidly formed almost quantitatively. When the same method is used with Cu+, a mixture of alloys, [CuxAg11-x{S2P(OiPr)2}8]+ (x = 1-3, CuxAg11-x), is obtained. In contrast, introducing Au+ to Ag10 leads to decomposition. The structural and compositional analysis of Ag11 was characterized by single-crystal X-ray diffraction (SCXRD), ESI-MS, NMR spectroscopy, and DFT calculations. While no crystal structure was obtained for CuxAg11-x, DFT calculations provide insights into potential sites for copper location. The absorption spectrum exhibits a notable blue shift in the low-energy band after copper doping, contrasting with that of the slight shift observed in 8-electron Cu-doped Ag nanoclusters. Ag11 and CuxAg11-x are strongly emissive at room temperature, and solvatochromism across different organic solvents is highlighted. This study underscores the profound influence of metal addition and doping on the structural and optical properties of silver nanoclusters, providing important contributions to understanding the nanoclusters and their photophysical behaviors.

A Silver Nanocluster Assembled by a Superatomic Building Unit.

A unique assembly of a two-electron superatom, [Ag10{S2P(OiPr)2}8], as a primary building unit in the construction of a supramolecule [Ag10{S2P(OiPr)2}8]2(µ-4,4'-bpy) through a 4,4'-bipyridine (4,4'-bpy) linker is reported. This approach is facilitated by an open site in the structure that allows for effective pairing. The assembled structure demonstrates a minimal solvatochromic shift across organic solvents with variable polarities, highlighting the influence of self-assembly on the photophysical properties of silver nanoclusters.

Hydride-Containing Pt-doped Cu-rich Nanoclusters: Synthesis, Structure, and Electrocatalytic Hydrogen Evolution.

A structurally precise hydride-containing Pt-doped Cu-rich nanocluster [PtH2 Cu14 {S2 P(Oi Pr)2 }6 (CCPh)6 ] (1) has been synthesized. It consists of a bicapped icosahedral Cu14 cage that encapsulates a linear PtH2 unit. Upon the addition of two equivalents of CF3 COOH to 1, two hydrido clusters are isolated. These clusters are [PtHCu11 {S2 P(Oi Pr)2 }6 (CCPh)4 ] (2), which is a vertex-missing Cu11 cuboctahedron encaging a PtH moiety, and [PtH2 Cu11 {S2 P(Oi Pr)2 }6 (CCPh)3 ] (3), a distorted 3,3,4,4,4-pentacapped trigonal prismatic Cu11 cage enclosing a PtH2 unit. The electronic structure of 2, analyzed by Density Functional Theory, is a 2e superatom. The electrocatalytic activities of 1-3 for hydrogen evolution reaction (HER) were compared. Notably, Cluster 2 exhibited an exceptionally excellent HER activity within metal nanoclusters, with an onset potential of -0.03 V (at 10 mA cm-2 ), a Tafel slope of 39 mV dec-1 , and consistent HER activity throughout 3000 cycles in 0.5 M H2 SO4 . Our study suggests that the accessible central Pt site plays a crucial role in the remarkable HER activity and may provide valuable insights for establishing correlations between catalyst structure and HER activity.

Locating Interstitial Hydrides in MH2@Cu14 (M = Cu, Ag) Clusters by Single-Crystal X-ray Diffraction.

Two structures, [Cu15H2(S2CNnBu2)6(C≡CPh)6][CuCl2] (1) and [AgH2Cu14{S2P(OiPr)2}6(C≡CPh)6][PF6] (2), are characterized by X-ray crystallography with high-quality single crystals. The position of interstitial hydrides can be accurately located. In addition, the refinement of the hydrides with anisotropic displacement parameters (ADPs) was successful. The distances between the central atom and copper atoms, as well as the distances within the metal cages surrounding the hydrides, are analyzed and compared with similar MH2@Cu14 (M = Cu, Ag, Pd) compounds. This work provides a thoughtful and accurate assessment of the considerations and challenges associated with anisotropic refinement for H atoms, particularly in X-ray data collection.

Hydride Doping Effects on the Structure and Properties of Eight-Electron Rh/Ag Superatoms: The [RhHx@Ag21-x{S2P(OnPr)2}12] (x = 0-2) Series.

Three hitherto unknown eight-electron rhodium/silver alloy nanoclusters, [RhAg21{S2P(OnPr)2}12] (1), [RhHAg20{S2P(OnPr)2}12] (2), and [RhH2Ag19{S2P(OnPr)2}12] (3), have been isolated and fully characterized. Cluster 1 contains a regular Rh@Ag12 icosahedral core, whereas 2 and 3 exhibit distorted RhH@Ag12 and RhH2@Ag12 icosahedral cores. The single-crystal neutron structure of 2 located the encapsulated hydride at the center of an enlarged RhAg3 tetrahedron. A similar position was found by neutron diffraction for one of the hydrides in 3, whereas the other hydride is trigonally coordinated to Rh and an elongated Ag-Ag edge. The solid-state structures of 1-3 possess C1 symmetry due to the asymmetric arrangement of the surrounding capping Ag atoms. Our investigation shows that the insertion of one hydride dopant provokes the elimination of one capping silver atom on the cluster surface, resulting in the general formula [RhHx@Ag21-x{S2P(OnPr)2}12] (x = 0-2), which maintains the same number of cluster electrons as well as neutral charge. Clusters 1-3 exhibit an intense emission band in the NIR region. Contrarily to their PdAg21 and PdHAg20 relatives, the 4d orbitals of the encapsulated heterometal are somewhat involved in the optical processes.

Galvanic replacement-induced introduction of a heteroligand into bimetallic and trimetallic nanoclusters.

Heteroleptic 8-electron silver-rich alloy nanoclusters, [Au@Au4Ag12(dtp)7(PPh3)4]2+ (1) and [Pt@Au4Ag11(dtp)7(PPh3)4] (2), were successfully synthesized via a galvanic replacement reaction of 1,1-dithiolate-protected M@Ag20 (M = Au and Pt) nanoclusters with Au(I)-phosphine salts, leading to the alteration of the cluster nuclearity and geometry of shell skeletons but retaining the same 8-electron count.

Superatom Pruning by Diphosphine Ligands as a Chemical Scissor.

A two-electron silver superatom, [Ag6{S2P(OiPr)2}4(dppm)2] (1), was synthesized by adding dppm (bis(diphenylphosphino)methane) into [Ag20{S2P(OiPr)2}12] (8e). It was characterized by single-crystal crystallography, multinuclear NMR spectroscopy, electrospray ionization-mass spectrometry, density functional theory (DFT), and time-dependent DFT calculations. The added dppm ligands, which carry out the nanocluster-to-nanocluster transformation, act as a chemical scissor to prune the nanocluster geometrically from an icosahedron-based Ag20 nanocluster (NC) to an octahedral Ag6 NC and electronically from eight-electron to two-electron. Eventually, dppm was involved in the protective shell to form a new heteroleptic NC. The temperature-dependent NMR spectroscopy confirms its fluxional behavior, showing the fast atomic movement at ambient temperature. Compound 1 exhibits a bright yellow emission under UV irradiation at ambient temperature with a quantum yield of 16.3%. This work demonstrates a new methodology to achieve nanocluster-to-nanocluster transformation via stepwise synthesis.

Hydride-containing 2-Electron Pd/Cu Superatoms as Catalysts for Efficient Electrochemical Hydrogen Evolution.

The first hydride-containing 2-electron palladium/copper alloys, [PdHCu11 {S2 P(Oi Pr)2 }6 (C≡CPh)4 ] (PdHCu11 ) and [PdHCu12 {S2 P(Oi Pr)2 }5 {S2 PO(Oi Pr)} (C≡CPh)4 ] (PdHCu12 ), are synthesized from the reaction of [PdH2 Cu14 {S2 P(Oi Pr)2 }6 (C≡CPh)6 ] (PdH2 Cu14 ) with trifluoroacetic acid (TFA). X-ray diffraction reveals that the PdHCu11 and PdHCu12 kernels consist of a central PdH unit encapsulated within a vertex-missing Cu11 cuboctahedron and complete Cu12 cuboctahedron, respectively. DFT calculations indicate that both PdHCu11 and PdHCu12 can be considered as axially-distorted 2-electron superatoms. PdHCu11 shows excellent HER activity, unprecedented within metal nanoclusters, with an onset potential of -0.05 V (at 10 mA cm-2 ), a Tafel slope of 40 mV dec-1 , and consistent HER activity during 1000 cycles in 0.5 M H2 SO4 . Our study suggests that the accessible central Pd site is the key to HER activity and may provide guidelines for correlating catalyst structures and HER activity.

Unusual core engineering on a copper hydride nanoball.

A neutral polyhydrido copper cluster, [Cu27H15{S2CNnBu2}12] (abbreviated as [Cu27H15]), was prepared by the reaction of dithiocarbamates (dtc), Cu(I) salts and NaBH4. The isolated cluster provides insights into core engineering, demonstrating its novel ability to reversibly add or remove one copper atom from the cluster core. Single-crystal X-ray analysis reveals that the new core-shell structure exhibits a Cu24 rhombicuboctahedral outer cage and an inner Cu3 triangular kernel. The two core-shell clusters, [Cu27H15{S2CNnBu2}12] and previously published [Cu28H15(S2CNnBu2)12]+ (abbreviated as [Cu28H15]+), are only differentiated by one copper atom in their inner core. Importantly, we demonstrate core engineering with the controllable reversible transition between an irregular Cu4 tetrahedron and a Cu3 triangle, whilst maintaining their outer Cu24 shell intact. The 15 hydride atoms in [Cu27H15], coordinated in three different modes, are co-incident with the hydride positions in [Cu28H15]+. The degradation of [Cu27H15] in solution or the addition of one eq. of Cu(I) ions leads to the conversion of [Cu27H15] into [Cu28H15]+, while the reverse transformation can be achieved by the addition of either formic acid or a reducing agent to [Cu28H15]+. A dicationic species was observed in the ESI mass spectrum, and the composition is formulated as [Cu56H30(S2CNnBu2)24]2+, a dimer of [Cu27H15(S2CNnBu2)12 + Cu+]22+. The dimeric species was further explored by DFT calculations, suggesting that the lowest energy structure consists of a [Cu28H15]+ and a [Cu27H15] cluster connected through one Cu+ atom bridge. As a result, [Cu27H15] is considered an intermediate species in the formation of the more stable [Cu28H15]+ nanoball.

Synthesis, structural studies, and photophysical properties of heteroleptic inverse-coordination clusters.

Two series of hyper-coordinated halide-centered M12 cuboctahedral clusters, [M12(µ12-X){S2P(OnPr)2}6{CCPh}4](PF6), 1a-c and 2a-c (where M = Cu, 1; Ag, 2; X = Cl, a; Br, b; I, c), were synthesized and fully characterized by ESI-MS, multi-NMR spectroscopy, IR and UV-Vis spectroscopy, photoluminescence analysis, and single-crystal X-ray crystallography. Structures 1c, 2b, and 2c show a twelve-coordinated halide encapsulated in the M12 cage, which is stabilized by six dithiophosphate and four alkynyl ligands. Compound 2b is the first Ag(I) cluster containing a twelve-coordinated bromide. The structural features of all six clusters are highly similar, providing a comparison basis of the inverse coordination for halides. Besides, the detailed structural analysis illustrates how the inverse coordination of a halide has influenced the size of the cuboctahedral M12 framework.

Oxocarbon Anions Templated in Silver Clusters.

Three types of oxocarbon anions as templates were used to synthesize high-nuclear silver clusters, [Ag16(C2O4){S2P(OEt)2}12]2(PF6)4 (1), [Ag16(C4O4){S2P(OEt)2}12]2(PF6)4 (2), and [Ag32(S)2(C5O5)2{S2P(OEt)2}22](PF6)2 (3), and characterized by multi-NMR spectroscopy and X-ray crystallography. As the template size increases, the shape and size of the clusters change accordingly. The template effect in high-nuclear silver clusters has been investigated in this work.

Hydride-Containing Eight-Electron Pt/Ag Superatoms: Structure, Bonding, and Multi-NMR Studies.

Recent reports on hydride-doped noble metal nanoclusters strongly suggest that the encapsulated hydride is a part of the superatom core, but no accurate location of the hydride could be experimentally proved, so far. We report herein a hydride-doped eight-electron platinum/silver alloy nanocluster in which the position of four-coordinated hydride was determined by neutron diffraction for the first time. X-ray structures of [PtHAg19(dtp/desp)12] (dtp = S2P(OnPr)2, 1; dsep = Se2P(OiPr)2, 2) describe a central platinum hydride (PtH) unit encapsulated within a distorted Ag12 icosahedron, the resulting (PtH)@Ag12 core being stabilized by an outer sphere made up of 7 capping silver atoms and 12 dichalcogenolates. Solid-state structures of 1 and 2 differ somewhat in the spatial configuration of their outer spheres, resulting in overall different symmetries, C1 and C3, respectively. Whereas the multi-NMR spectra of 2 in solution at 173 K reveal that the structure of C3 symmetry is the predominant one, 1H and 195Pt NMR spectra of 1 at the same temperature disclose the presence of isomers of both C1 and C3 symmetry. DFT calculations found both isomers to be very close in energy, supporting the fact that they co-exist in solution. They also show that the [PtH@Ag12]5+ kernel can be viewed as a closed-shell superatomic core, the µ4-hydride electron contributing to its eight-electron count. On the other hand, the 1s(H) orbital contributes only moderately to the superatomic orbitals, being mainly involved in the building of a Pt-H bonding electron pair with the 5dz2(Pt) orbital.

Homoleptic Silver-Rich Trimetallic M20 Nanocluster.

Two silver-rich M20 alloy nanoclusters (NCs), [Cu3.5Ag16.5{S2P(OnPr)2}12] (1) and [Cu2.5AuAg16.5{S2P(OnPr)2}12] (2), were synthesized and fully characterized by electrospray ionization mass spectrometry, NMR spectroscopy, and X-ray crystallography. Cluster 2, the first structurally characterized trimetallic M20 NC, was produced by doping one Au atom into a bimetallic M20 NC. Structural analyses showed the preferred positions of Group 11 metals in the yielded M20 NCs. Their antioxidation ability has been investigated, and the time-dependent UV-vis spectrum shows that the presence of CuI atoms in structures 1 and 2 can improve the antioxidant ability.

Reactivities of Interstitial Hydrides in a Cu11 Template: En Route to Bimetallic Clusters.

In sharp contrast to surface hydrides, reactivities of interstitial hydrides are difficult to explore. When treated with a metal ion (Cu+ , Ag+ , and Au+ ), the stable CuI dihydride template [Cu11 H2 {S2 P(Oi Pr)2 }6 (C≡CPh)3 ] (H2 Cu11 ) generates surprisingly three very different compounds, namely [CuH2 Cu11 {S2 P(Oi Pr)2 }6 (C≡CPh)3 ]+ (1), [AgH2 Cu14 {S2 P(Oi Pr)2 }6 ((C≡CPh)6 ]+ (2), and [AuCu11 {S2 P(Oi Pr)2 }6 (C≡CPh)3 Cl] (3). Compounds 1 and 2 are both MI species and maintain the same number of hydride ligands as their H2 Cu11 precursor. Neutron diffraction revealed the first time a trigonal-pyramidal hydride coordination mode in the AgCu3 environment of 2. 3 has no hydride and exhibits a mixed-valent [AuCu11 ]10+ metal core, making it a two-electron superatom.

Alloying dichalcogenolate-protected Ag21 eight-electron nanoclusters: a DFT investigation.

The isoelectronic doping of dichalcogenolato nanoclusters of the type [Ag21{E2P(OR)2}12]+ (E = S, Se) by any heteroatom belonging to groups 9-12 was systematically investigated using DFT calculations. Although they can differ in their global structure, all of these species have the same M@M12-centered icosahedral core. In any case, the different structure types are all very close in energy. In all of them, three different alloying sites can be identified (central, icosahedral, peripheral) and calculations allowed the trends in heteroatom site occupation preference across the group 9-12 family to be revealed. These trends are supported by complementary experimental results. They were rationalized on the basis of electronegativity, potential involvement in the bonding of valence d-orbitals and atom size. TD-DFT calculations showed that the effect of doping on optical properties is sizable and this should stimulate research on the modulation of luminescence properties in the dithiolato and diseleno families of complexes.

A New Synthetic Methodology in the Preparation of Bimetallic Chalcogenide Clusters via Cluster-to-Cluster Transformations.

A decanuclear silver chalcogenide cluster, [Ag10(Se){Se2P(OiPr)2}8] (2) was isolated from a hydride-encapsulated silver diisopropyl diselenophosphates, [Ag7(H){Se2P(OiPr)2}6], under thermal condition. The time-dependent NMR spectroscopy showed that 2 was generated at the first three hours and the hydrido silver cluster was completely consumed after thirty-six hours. This method illustrated as cluster-to-cluster transformations can be applied to prepare selenide-centered decanuclear bimetallic clusters, [CuxAg10-x(Se){Se2P(OiPr)2}8] (x = 0-7, 3), via heating [CuxAg7-x(H){Se2P(OiPr)2}6] (x = 1-6) at 60 °C. Compositions of 3 were accurately confirmed by the ESI mass spectrometry. While the crystal 2 revealed two un-identical [Ag10(Se){Se2P(OiPr)2}8] structures in the asymmetric unit, a co-crystal of [Cu3Ag7(Se){Se2P(OiPr)2}8]0.6[Cu4Ag6(Se){Se2P(OiPr)2}8]0.4 ([3a]0.6[3b]0.4) was eventually characterized by single-crystal X-ray diffraction. Even though compositions of 2, [3a]0.6[3b]0.4 and the previous published [Ag10(Se){Se2P(OEt)2}8] (1) are quite similar (10 metals, 1 Se2-, 8 ligands), their metal core arrangements are completely different. These results show that different synthetic methods by using different starting reagents can affect the structure of the resulting products, leading to polymorphism.

All-selenolate-protected eight-electron platinum/silver nanoclusters.

The first atomically and structurally precise platinum/silver superatoms protected by Se-donor ligands were synthesized in high yield by adopting ligand replacements on [PtAg20{S2P(OnPr)2}12] (3) with 12 equiv. of di-alkyl diselenophosph(in)ates. Structures of [PtAg20{Se2P(OR)2}12] (R = nPr (1a), iPr (1b)) and [PtAg20{Se2P(CH2CH2Ph)2}12] (2) were accurately determined by single-crystal X-ray diffraction to reveal an eight-electron [Pt@Ag12]4+ icosahedral core embedded within a cube of eight silver(i) atoms and wrapped into a shell of 12 diselenophosph(in)ates. While the lowest energy absorption band of the Se derivatives is red-shifted to longer wavelengths in comparison with the S analogue, it is blue-shifted in the emission spectra. Density functional theory (DFT) and TD-DFT calculations rationalize the electronic structures as those of eight-electron superatoms, with their HOMO and LUMO being the 1P and 1D levels, respectively. The two UV-visible lowest bands are associated with 1P → 1D metal to metal charge transfer (MMCT) transitions. The blue shift observed for the S analogue results from a larger HOMO-LUMO gap in the case of dithiolate ligands.

Doping effect on the structure and properties of eight-electron silver nanoclusters.

The bimetallic M20 and M21 compounds, {[Cu3Ag17{S2P(OiPr)2}12]0.5 [Cu4Ag16{S2P(OiPr)2}12]0.5} ({[1a]0.5[1b]0.5}) and [Cu4Ag17{S2P(OiPr)2}12](PF6) (2), have been structurally characterized, in which the Cu(I) ions are randomly distributed on the eight outer positions capping the eight-electron [Ag13]5+ core. DFT calculations show that the statistical disorder results from the nearly neutral preference of copper to occupy any of the eight outer positions. Surprisingly, the UV-Vis absorption spectra of the M20 and M21 bimetallic nanoclusters display an almost identical absorption profile as that of their homometallic [Ag20{S2P(OiPr)2}12] and [Ag21{S2P(OiPr)2}12]+ relatives. This is rationalized by TD-DFT calculations, which show that the frontier orbitals of such eight-electron alloys are largely independent from the nature of the capping metal ions. A blue-shifted absorption is observed upon replacing by Au the central Ag atom in 2, forming the trimetallic compound [Cu4AuAg16{S2P(OiPr)2}12](PF6) (3).

[Cu{SC(O)OiPr}]96: A Giant Self-Assembled Copper(I) Supramolecular Wheel Exhibiting Photoluminescence Tuning and Correlations with Dynamic Solvation and Solventless Synthesis.

The hierarchical self-organization of structurally complex high-nuclearity metal clusters with metallosupramolecular wheel architectures that are obtained from the self-assembly of smaller solvated cluster units is rare and unique. Here, we use the potentially heteroditopic monothiocarbonate ligand and demonstrate for the first time the synthesis and structure of a solvated non-cyclic hexadecanuclear cluster [Cu{SC(O)OiPr}]16·2THF (1) that can simultaneously desolvate and self-assemble in solution and subsequently form a giant metallaring, [Cu{SC(O)OiPr}]96 (2). We also demonstrate a luminescent precursor to cluster (2) can be achieved through a solventless and rapid mechanochemical synthesis. Cluster (2) is the highest nuclearity copper(I) wheel and the largest metal cluster containing a heterodichalcogen (O, S) ligand reported to date. Cluster (2) also exhibits solid-state luminescence with relatively long emission lifetimes at 4.1, 13.9 (µs). The synthetic strategy described here opens new research avenues by replacing solvent molecules in stable {Cu16} clusters with designed building units that can form new hybrid and multifunctional finite supramolecular materials. This finding may lead to the development of novel high-nuclearity materials self-assembled in a facile manner with tunable optical properties.