High-nuclearity Cu hydride nanoclusters of Cu₂₂H₁₉ and Cu₂₀H₁₃ consolidated by an N-containing triphosphine ligand: synthesis, characterization and catalysis

Abstract

The synthesis of copper hydride nanoclusters and the determination of their atomically precise structures are beneficial for gaining an in-depth understanding of the structure–property relationship. N,P-containing ligands with a combination of hard and soft donors exhibit versatile coordination modes which are promising for mediating the structures of Cu hydride nanoclusters and their properties. Herein, two Cu hydride nanoclusters, [Cu₂₂H₁₉(NP₃)₄(CH₃CN)₄]·(ClO₄)₃ (1) and [Cu₂₀H₁₃(NP₃)₄(CH₃CN)₃Cl₄]·(ClO₄)₃ (2), based on the tripodal N-containing triphosphine ligand of tris(diphenylphosphino-ethyl)amine (NP₃) are synthesized, and their compositions and structures are determined by ¹H NMR, high-resolution ESI-MS and single-crystal X-ray diffraction. The location of hydrides is established based on the optimized model obtained by DFT calculations. In cluster 1, the skeleton of 22 Cu can be described as a fusion of two distorted Cu₁₃ centred icosahedra by sharing four Cu atoms, and each NP₃ ligand exhibits a tridentate mode (P, P, P) to bind one Cu₃ triangular plane. The skeleton of 20 Cu in cluster 2 can be described as a Cu₁₃ centred icosahedron bridging a cap-shaped Cu₇ moiety, and the ligands exhibit two binding modes, the tridentate (P, P, P) and the tetradentate (N, P, P, P) modes, to consolidate these Cu atoms. The synthesis of clusters 1 and 2 is similar. The involvement of Cl⁻ anions and the flexibility of NP₃ play a synergistic role in mediating the construction of cluster 2, resulting in a structure distinct from that of cluster 1. Their deuteride analogues, 1_D and 2_D, are also synthesized for comparison. Cluster 2 exhibits better catalytic activity than cluster 1 for the reduction of 4-nitrophenol to 4-aminophenol.

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Introduction

The study of monodisperse and atomically precise coinage metal (Au, Ag, Cu and their alloys) nanoclusters protected by organic ligands has attracted continuing attention, not only because of their aesthetically pleasing molecular structures and interesting properties, but also as it allows for the development of direct structure–property relationships of nanoclusters.^1–13 Through tremendous efforts over the past few decades, a large number of nanoclusters containing from a few tens to hundreds metal atoms have been synthesized and their structures atom-precisely determined.^14–22 The detailed structural anatomy of nanoclusters can seek to understand the kernel structures and the growth mode of kernels, the surface structures and the self-assembly of surface protecting ligands, as well as the interfacial bonding between the kernel and the surface structure.^23–27 The observed growth modes of kernels in nanoclusters include kernel fusion, interpenetration, cluster of cluster, shell by shell, and layer by layer.^28,29 The peripheral ligands and the assembled metal–ligand motifs (termed as staple motifs) govern a nanocluster's stability and geometric and electronic structures. For example, the surface ligands in Au₂₄₇ induce nanocluster hierarchical self-assembly into complex structures,²⁵ and the clockwise/anticlockwise configuration of Au_n(SR)_n+1 bonds in Au₃₈(2-PET)₂₄ (2-PET: 2-phenylethylthiolate) creates a pair of enantiomers.³⁰ M_n(SR)_n+1 (M = Au and Ag) bonds are commonly observed in nanoclusters, and their flexibility and diversity contribute to various nanoclusters.^31–33 Moreover, metal-alkynyl and mixed P–metal–S–metal–P bonds are also reported with various configurations to anchor the kernel of nanoclusters for structural stability.³⁴ As is well known, systematic studies of nanoclusters can establish the structure–property relations.

Though a large number of atom-precise Cu nanoclusters have been reported, the metastability of Cu nanoclusters resulting from their high susceptibility towards oxidation,^35–37 to a certain degree, prevents systematic comparative studies. Among the family of Cu clusters, Cu hydride nanoclusters have attracted special attention since they are key intermediates in numerous copper-catalyzed reactions and promising materials for hydrogen storage.^38–43 Some successful synthetic protocols have been established for the preparation of copper hydride nanoclusters based on stabilizing reagents such as thiolates, selenide, phosphines, dithiophosphate, dithiocarbamates, or their combinations. But only a handful of copper hydride nanoclusters with more than 20 Cu atoms have been reported with atomically precise structures, including Cu₂₀H₁₁,⁴⁴ Cu₂₃H₆,⁴⁵ Cu₂₅H₁₀,⁴⁶ Cu₂₅H₂₂,⁴⁷ Cu₂₆H₁₁,⁴⁸ Cu₂₈H₈,⁴⁹ Cu₂₈H₁₀,⁵⁰ Cu₂₈H₁₅,³³ Cu₂₈H₁₆,⁵¹ Cu₂₈H₂₀,⁵² Cu₂₉H₂₂,⁵³ Cu₃₀H₁₈,⁵⁴ Cu₃₁H₆,⁵⁵ Cu₃₂H₈,⁵⁶ Cu₃₂H₂₀,⁵⁷ Cu₃₆H₁₀,⁵⁸ Cu₄₀H₃₈,⁵⁹ Cu₄₁H₂₅,⁶⁰ Cu₅₃H₁₈,⁶¹ Cu₅₇H₂₀,⁶² Cu₅₈H₂₀,^63,64 Cu₆₁H₁₄,⁶⁵ Cu₇₀H₂₂ ⁶⁶ and Cu₈₁H₃₂.⁶⁷ Unlike thiolates and alkynes having variable coordination modes, neutral phosphine ligands just terminally coordinate to metal atoms and have, to a certain extent, predetermined steric configurations.^68–70 Therefore, a phosphine ligand with definite coordination mode can be an appropriate candidate to regulate the size and shape of nanoclusters depending on phosphine's steric and size constraint. But the stability of Cu hydride clusters just based on phosphine ligands is a big problem. So phosphine ligands usually perform as auxiliary ligands to construct high-nuclearity nanoclusters.^{9,21,45,56,58,71} Hexameric complexes are most common with monodentate phosphine ligands, such as Stryker's reagent, [(PPh₃)-CuH]₆, which is commercially available.⁷² To date, only six Cu hydride clusters with more than 10 Cu atoms have been reported just based on phosphine ligands, of which three clusters, [Cu₁₄H₁₂(phen)₆(PPh₃)₄]²⁺,⁷³ [Cu₁₈H₁₇(PPh₃)₁₀]⁺,⁴⁷ and [Cu₂₅H₂₂(PPh₃)₁₂]⁺,⁴⁷ are based on monodentate phosphine ligands and three examples, [Cu₁₆H₁₄(dpmppm)₄]²⁺,⁷⁴ [Cu₁₆H₁₄(DPPA)₆]²⁺,⁷⁶ and [Cu₁₈H₁₆(DPPE)₆]²⁺,⁷⁵ are based on polyphosphine ligands. Polyphosphine ligands can also perform as a bonding mode to stabilize metal clusters, similar to Au(Ag)_n(SR)_n+1. Moreover, polyphosphine ligands with predetermined steric constraints can aggregate metal atoms into clusters with metal–metal bonds. We selected a branched tetradentate ligand, tris(diphenylphosphino-methyl)amine (NP₃), containing three P(CH₂)N sites with a combination of hard and soft donors, to stabilize some Ag nanoclusters: [Ag₁₅Cl₄(NP₃)₄]³⁺,⁵ [Ag₁₂(NP₃)₂(Spz)₁₂] and [Ag₁₈(NP₃)₄(Spz)₁₂]⁶⁺ (Spz = pyrazine-2-thiolate).⁷⁶ In the Ag₁₅ cluster, each NP₃ ligand performs as a tetradentate mode (N, P, P, P) to participate in coordination, but the central N atom of NP₃ in the Ag₁₈ and Ag₁₂ clusters does not bond with metal ion where each NP₃ just performs as a tridentate mode (P, P, P). Guan et al. used HN(CH₂CH₂PR₂)₂ (RPNHP) to construct [Cu₆H₆(RPNHP)₃] and [Cu₄H₄(RPNHP)₂] clusters where the N atom is also uncoordinated, but a mononuclear copper hydride, (RPNHP)CuH, with the NH group bound to copper is proposed as a catalytically active species.⁴¹ Surely in these types of ligands, the hemilabile coordination of hard N atom to soft coinage metal atom will favor cluster transformation, shape/size adjustment and metal surface reaction.

We seek to exploit the bonding coordination mode and the steric confinement of an N-containing polyphosphine ligand to synthesize stable Cu hydride clusters and to regulate their geometric configuration, electronic structures and properties. Herein, the tripodal tetradentate ligand NP₃ is selected as a stabilizing agent for the construction of Cu hydride nanoclusters, due to its geometric flexibility and variable coordination modes. And two nanoclusters, [Cu₂₂H₁₉(NP₃)₄(CH₃CN)₄](ClO₄)₃ (1) and [Cu₂₀H₁₃(NP₃)₄(CH₃CN)₃Cl₄](ClO₄)₃ (2), are successfully synthesized and characterized by single-crystal X-ray diffraction, ¹H NMR, and high-resolution electrospray ionization mass spectrometry (HR ESI-MS). There are four NP₃ ligands involved in each of 1 and 2. However, no N atom from NP₃ ligands participates in coordination in cluster 1, while only the N atom in one NP₃ ligand does not participate in coordination in cluster 2.

Results

Synthesis and characterization

Synthetic methods for 1 and 2 are similar (see the experiment part in the SI). The reactions proceeded at 0 °C in a solution of CH₃OH and CH₃CN containing Cu powder, [Cu(CH₃CN)₄]ClO₄ and NaBH₄ (Scheme 1). The addition of CuCl₂ for the preparation of cluster 2 is just to provide a Cl⁻ source. If Ph₄PCl is used instead of CuCl₂, nanocluster 2 can also be afforded with a comparable yield. The hydrides originate from NaBH₄. Due to the different sizes of H⁻ and Cl⁻ anions, the introduction of Cl⁻ ions aims to modify the structure of the Cu hydride nanocluster. Interestingly, the presence of Cu powder is crucial for the formation of 1 and 2, which could be due to Cu powder inhibiting the oxidation of Cu⁺ in the reaction process. In the absence of Cu powder, the yields of resulting cluster crystals are much lower, even though the reactions are also carried out under the reducing condition provided by NaBH₄. Perhaps the fast reaction between Cu²⁺ ions and Cu powder ensures all Cu species exist as Cu⁺ ions in the reactive system, improving the yields of theses clusters. High-quality orange-yellow rhombic crystals of 1 and red cubic crystals of 2 are obtained from a solution with mixed solvents of MeCN/MeOH/Et₂O in a refrigerator at −20 °C (Fig. S1). The UV-visible absorption spectra of the as-obtained nanoclusters 1 and 2 have no dominant visible absorption peaks (Fig. S2), and their time-dependent optical absorption spectra show that they are stable in CH₃CN solvent (Fig. S3). The optical energy gaps were determined to be 2.28 eV for 1 and 2.79 eV for 2 by extrapolating the lowest-energy absorption peak to zero absorbance (see the insets in Fig. S2). The high-resolution XPS spectrum of Cu 2p shows two sharp peaks at binding energies of 932.3 and 952.1 eV for cluster 1, as well as at 932.8 and 952.7 eV for cluster 2, which originate from Cu 2p_3/2 and 2p_1/2, respectively, and are similar to those of Cu(I) (Fig. S4).^45,50 Moreover, there is no observable satellite signal at around 943 eV, which indicates the absence of Cu(II) in clusters 1 and 2. These two clusters are stable in DMSO at low temperature (−20 °C) for longer than three months (Fig. S5). The compositions of clusters 1 and 2 are determined by single-crystal X-ray diffraction, ¹H NMR, and HR ESI-MS, and supported by theoretical calculation. To investigate the presence of hydrides, their corresponding deuteride analogues, 1_D and 2_D, are also synthesized. The crystal data and structure refinement for 1 and 2 are given in Table S1.

Scheme 1 The synthetic routes to nanoclusters 1 and 2.

The crystal structure, ESI-MS and ¹H NMR for nanocluster 1

Single-crystal X-ray diffraction shows that cluster 1 crystallizes in the triclinic P1̄ space group. The asymmetric unit contains a pair of enantiomers of [Cu₂₂(NP₃)₄(CH₃CN)₄] moiety and six ClO₄⁻ anions, indicating that the electronic charge of cluster 1 would be +3. All NP₃ ligands in an enantiomer exhibit the same conformation, clockwise or anticlockwise (Fig. S6). Below we choose an enantiomer for structural analysis, due to these two enantiomeric pairs showing similarity in their coordination environments. The skeleton of 22 Cu in 1 can be viewed as two fused Cu₁₃ centered icosahedra which share four common atoms that construct a twisted quadrilateral, as shown in Fig. 1a. Each icosahedron associates with two NP₃ ligands and two coordinated MeCN molecules. Moreover, each NP₃ ligand adopts a tridentate mode (P, P, P) to bind one Cu₃ triangular plane. The steric effect of the large-sized NP₃ ligand can lead to icosahedra being distorted, resulting in two adjacent Cu₄ quadrilaterals formed at the surface of each distorted Cu₁₃ centered icosahedra. The two centred Cu atoms and four fused Cu atoms construct a twisted octahedral Cu₆ core which does not contact any NP₃ and solvent ligands (Fig. 1b). The fusion of icosahedral M₁₃ moieties is usually observed for the construction of nanoclusters, such as the vertex-fusion of Au₁₃ in [Au₃₇(PPh₃)₁₀(SR)₁₀Cl₂]⁺ (ref. 77) and (PtAg₁₂) in [Pt₃Ag₄₄{S₂P-(OPr)₂}₂₂] (ref. 78)⁷⁸ to construct linear structures, Au₁₃ in [Au₆₀Se₂-(PPh₃)₁₀(SePh)₁₅]⁺ (ref. 79)⁷⁹ and (Au/Ag)₁₃ in [(p-Tol₃P)₁₂Au₁₈-g₁₉Br₁₁]⁺ (ref. 80)⁸⁰ to construct cyclic structures, as well as the fusion of triangular faces of Au₁₃ in Au₃₈(SC₂H₄Ph)₂₄³⁰ and in the computational mode of [Au₃₃(Au₁₈Cl₃₀)]⁻,⁸¹ and the interpenetration fusion of Au₁₃ in Au₃₀S(S-^tBu)₁₈.⁸² The structure of 1 can be also described as that an octahedral Cu₆ core is pocketed by two roof-shaped [Cu₈(NP₃)₂(CH₃CN)₂] units in which eight Cu atoms construct four triangles and two quadrilaterals (Fig. 1b and S7).

Fig. 1 The crystal structure of cluster 1. (a) The two centred Cu₁₃ fused icosahedra; and (b) the centred octahedron capped by two roof-shaped [Cu₈(NP₃)₂(CH₃CN)₂] units. The centred and fused Cu atoms are labelled in yellow colour. All phenyl groups and hydrogen atoms are omitted for clarity.

The twisting of two fused icosahedra results in the overall conformation of 1 with internal helicity, exhibiting a chiral feature. Both chiral enantiomers crystallize as a racemic mixture. The Cu–Cu distances in 1 are in the range of 2.430(2)–2.883(1) Å, and the average bond lengths of Cu_center–Cu_icos, Cu_octa–Cu_shell and Cu_shell–Cu_shell are 2.595 Å (range: 2.452(3)–2.856(3) Å), 2.533 Å (range: 2.430(2)–2.782(4) Å) and 2.683 Å (range: 2.508(3)–2.883(1) Å), respectively, which are comparable to those found in other Cu hydride clusters.⁴⁷ Also, the Cu–P distances (av. 2.263 Å) are typical of those found in Cu(I) phosphine complexes.^41,47,74

To further determine the composition and charge state of 1, HR ESI-MS is performed (Fig. S8). The positive mode ESI-MS spectrum of 1 shows two prominent peaks centred at 1287.80 with a charge of +3 and 1981.17 with a charge of +2, which are assigned respectively to the formulas of [Cu₂₂H₁₉(NP₃)₄]³⁺ and [Cu₂₂H₁₉(NP₃)₄(ClO₄)]²⁺ (Fig. 2), while the ESI-MS spectrum of its deuteride analogue, 1_D, shows signals centred at 1293.51 with a charge of +3 and 1990.72 with a charge of +2, which correspond respectively to the formulas of [Cu₂₂D₁₆H₃(NP₃)₄]³⁺ and [Cu₂₂D₁₉(NP₃)₄(ClO₄)]²⁺ (Fig. S9). These formula assignments are validated by the good agreement between calculated and experimental isotopic patterns. Three hydrides in [Cu₂₂D₁₆H₃(NP₃)₄]³⁺ are due to the impurity of the borodeuteride source (the purity of NaBD₄ is 95%), which is unavoidable in polyhydrido copper clusters.⁶⁷ All these assigned species and the 19 mass difference between [Cu₂₂H₁₉(NP₃)₄(ClO₄)]²⁺ and [Cu₂₂D₁₉(NP₃)₄(ClO₄)]²⁺ confirm that cluster 1 has 19 hydride ligands (Fig. S8). Based on ESI-MS and SCXRD results, the total composition of 1 is proposed as [Cu₂₂H₁₉(NP₃)₄(CH₃CN)₄]·(ClO₄)₃, and all the copper atoms are in the +1 state. The formulas assigned by ESI-MS are derived from the parent cluster 1³⁺ with the loss of four MeCN molecules.

Fig. 2 HR ESI-MS spectra of dominant species [Cu₂₂H₁₉(NP₃)₄]³⁺ and [Cu₂₂H₁₉(NP₃)₄(ClO₄)]²⁺ along with simulated ones.

To further characterize the hydrides in 1, ¹H NMR measurements were performed. The ¹H NMR spectrum of 1 features three broad peaks at 0.45, 0.62, and 1.06 ppm, which integrate for 10 H, 6 H and 3 H, respectively, and are assignable to 19 hydrides in different chemical environments. These peaks are completely missing in the ¹H NMR spectrum of 1_D (Fig. 3 and Fig. S10), validating the assignment of hydrides. The peaks centred at 2.35 (4 H), 2.89 (8 H), 3.45 (6 H) and 3.60 (6 H) ppm in the spectrum of 1 are ascribed to 24 H of –CH₂– moieties, while the peaks at 6.2–8.3 ppm correspond to 120 H of phenyl groups in ligands. Furthermore, the comparison of ¹H NMR spectra shows that the peaks of –CH₂– and phenyl groups in 1 and 1_D are almost consistent. In CD₃CN, the ³¹P NMR spectrum of complex 1 exhibits three broad peaks at −3.44, −0.61, and 0.03 ppm, with an integration ratio of 1 : 1 : 1 (Fig. S11), suggesting that the twelve phosphine atoms in 1 are distributed into three distinct groups. Four phosphines are assigned to the terminal sites of the fused Cu₂₂ polyhedron, while the remaining eight phosphines, located at the waist of the Cu₂₂ framework, are further divided into two groups each of which is associated with one of the two Cu₁₃ icosahedra.

Fig. 3 ¹H NMR spectra of 1 and 1_D in CD₃CN. The marked peaks correspond to CH₃OH (@), water (&), CH₃CN (*), and impurities in the solvent.

The crystal structure, ESI-MS and ¹H NMR for cluster 2

Cluster 2 crystallizes in the trigonal R3̄c space group. X-ray diffraction study reveals that it consists of 20 Cu atoms, 4 NP₃, 4 coordinated Cl⁻ ions, 3 coordinated CH₃CN solvent molecules, and 3 ClO₄⁻ anions, indicating that the cation charge of cluster 2 would be +3 which is balanced by three ClO₄⁻ counterions. The skeleton of 20 Cu can be described as a centred Cu₁₃ icosahedron covered by a cap-like Cu₇ moiety which consists of a Cu₆ hexagon (Cu6, Cu6i, Cu6ii, Cu7, Cu7i and Cu7ii) with nonplanar chair configuration and a top Cu atom (Cu₈) that is bonded to three interval Cu atoms in the hexagon and a terminal Cl⁻ ion with Cu–Cu and Cu–Cl distances of 2.528(1) and 2.183(2) Å respectively (Fig. 4). The C3 axis passes through the top Cu atom in the Cu₇ moiety and the centred Cu atom in the icosahedron. Besides the Cu–Cu bonding between a Cu₃ triangle (Cu5, Cu5i and Cu5ii) in the Cu₁₃ icosahedron and the Cu₆ hexagon, three Cl⁻ ions perform as a μ₃-bridging mode to link the Cu₁₃ and Cu₆ moieties (the Cu–Cl bond length range is 2.298(2)–2.956(1) Å). Furthermore, three NP₃ ligands perform in a tetradentate mode (N, P, P, P) to bind four Cu atoms of which three Cu atoms belong to the Cu₆ hexagon and one to the icosahedron. The fourth NP₃ ligand performs in a tridentate (P, P, P) mode to bind a triangular Cu₃ plane (Cu1, Cu1i and Cu1ii) of this icosahedron. The remaining three Cu atoms on the surface of Cu₁₃ are coordinated by three MeCN solvent molecules. The structure of 2 can be seen as an evolution from 1 by losing two Cu⁺ ions. The skeleton of 20 Cu can be also viewed as an interpenetrating structure of a centred Cu₁₃ icosahedron and a twisted Cu₁₀ tetrahedron which share four Cu atoms, along with a top Cu atom capping a face of this tetrahedron (Fig. S12).

Fig. 4 The crystal structure of cluster 2 (a) and the skeleton of 20 Cu (b) in cluster 2. All phenyl groups and hydrogen atoms are omitted for clarity. Symmetry codes: i = 1 − y, x − y, z; ii = 1 − y − x, 1 − x, z.

The range of Cu–Cu distances in 2 is 2.455(4)–3.004(3) Å which is a little larger than that in cluster 1. But the average Cu_center–Cu_icos and Cu–P distances are 2.583 Å (range: 2.465(2)–2.682(4) Å) and 2.248 Å (range: 2.230(2)–2.267(3) Å), respectively, which are comparable with those observed in cluster 1 and other reported Cu hydride clusters.^47,73–75 The average distance of Cu–N is 2.065 Å and the Cu–μ₃–Cl distances are 2.298 and 2.570 Å.⁸⁴

By comparing the geometric parameters of NP₃ ligands, it is found that the bond angle of C–N–C is a significant mark for the (P, P, P) tridentate and the (N, P, P, P) tetradentate coordination modes (Scheme S1 and Table S2). The average bond angle of C–N–C is about 108° in the tetradentate coordination mode, but this value experiences a large range with the tridentate mode in different compounds. Moreover, the tridentate mode of NP₃ is appropriate to construct a triangular metal plane which exists in octahedral, icosahedral and tetrahedral structures, while the tetradentate coordination mode can match with complicated polyhedral structures.

To determine the number of hydrides in 2, HR ESI-MS analyses of 2 and 2_D are also performed (Fig. S13). The positive ESI-MS of 2 shows three characteristic signals at 1290.76 with a charge of +3 and 1953.67 and 1985.65 with a charge of +2, which match well respectively with the simulated isotopic patterns of [Cu₂₀H₁₃(NP₃)₄Cl₄]³⁺, [Cu₂₀H₁₃(NP₃)₄Cl₅]²⁺ and [Cu₂₀H₁₃(NP₃)₄Cl₄(ClO₄)]²⁺ (Fig. 5). While the ESI-MS peaks of its deuteride analogue, 2_D, at 1294.81 with a charge of +3 and 1959.70 and 1991.71 with a charge of +2 match well respectively with the simulated isotopic patterns of [Cu₂₀D₁₂H(NP₃)₄Cl₄]³⁺, [Cu₂₀D₁₂H(NP₃)₄Cl₅]²⁺ and [Cu₂₀D₁₂H(NP₃)₄Cl₄(ClO₄)]²⁺ (Fig. S14). These formula assignments are validated by the good agreement between simulated and experimental isotopic patterns. All the species reveal the incorporation of 13 hydrides in 2 (Fig. S13). So the composition of 2 is proposed as [Cu₂₀H₁₃Cl₄(CH₃CN)₃(NP₃)₄]·(ClO₄)₃. The formulas assigned by ESI-MS are derived from the loss of MeCN molecules from the parent cluster 2³⁺.

Fig. 5 HR ESI-MS spectra of characteristic species of [Cu₂₀H₁₃(NP₃)₄Cl₄]³⁺, [Cu₂₀H₁₃(NP₃)₄Cl₅]²⁺ and [Cu₂₀H₁₃(NP₃)₄Cl₄(ClO₄)]²⁺ along with simulated ones.

To further characterize the hydrides, the ¹H NMR spectra of 2 and 2_D are compared in CD₃CN. The peaks centred at −0.50, −0.10 and 1.85 ppm for 2 are absent for 2_D, showing that these peaks ascribed to H⁻ ions in 2 are replaced by D⁻ ions in 2_D (Fig. 6). The peak integration of the ¹H NMR spectrum of 2 shows that 9 hydrides are equally distributed to three peaks at −0.50, −0.10 and 1.85 ppm, and four hydrides are overlapped with –CH₂– in NP₃ at 3.37–3.99 ppm with one hydride at 3.37–3.66 ppm and three hydrides at 3.67–3.99 ppm. And these four overlapped hydride signals are also absent in the spectrum of 2_D. To further determine quantitatively these hydrides in 2, an internal standard reagent of 1,3,5-trioxane is used to perform ¹H NMR in (CD₃)₂SO. As shown in Fig. S15, the quantitative result is consistent with that of the peak integration. However, the peaks of –CH₂– in (CD₃)₂SO are not well resolved relative to those in CD₃CN (Fig. S16). Complex 2 features four broad resonances in its ³¹P NMR spectrum in CD₃CN, centered respectively at −8.88, 0.15, 0.95 and 7.34 ppm with an integration ratio of 1 : 1 : 1 : 1 (Fig. S17), suggesting that the twelve phosphine atoms in 2 are divided into four groups. Three phosphines are ascribed to the NP₃ ligand with the (P, P, P) tridentate coordination mode, and another three phosphines from different NP₃ ligands with the (N, P, P, P) tetradentate mode attach to the icosahedron. The remaining six phosphines, distributed into two groups, coordinate to the Cu atoms at the cap-like Cu₇ moiety.

Fig. 6 ¹H NMR spectra of 2 and 2_D in CD₃CN. The marked peaks correspond to water (&), CH₃CN (*), and impurities in the solvent.

The optimized results by DFT calculation

To ascertain the position of hydrides, several structural models consisting of hydrides for 1 and 2 are given based on the information from the crystal structural analysis, the observed hydride NMR chemical shifts and previously reported structures of hydride-containing copper clusters. Density functional theory (DFT) calculations are carried out for optimization until the default convergence limit is reached. It is worth mentioning that without any additional hydrogen atoms, the convergence cannot be carried out and the structure is cracked. The final optimized structure is nearly consistent with the experimental structure. The lowest energy structures for the corresponding clusters 1 and 2 are shown in Fig. 7 and S17.

Fig. 7 Optimal hydride sites in 1_opa (left) and 2 (right) based on DFT calculations. Colors: orange, H; green and yellow, Cu; and bright green, Cl.

There are six convergently optimized structures for cluster 1 (1_opa, 1_opb, 1_opc, 1_opd, 1_ope and 1_opf), having nearly the same lowest energies and similar structures. To describe the positions of hydrides, the optimized structure of 1_opa is used as an example. In optimized 1_opa, one μ₆-hydride (H_a) as an interstitial locates at the center of the octahedral Cu₆ core in the fused icosahedra, and another 18 hydrides are distributed on the icosahedral surface. Among the 18 surface hydrides, two μ₄-hydrides (H_b) locate at the waist of the fused icosahedra to bridge respectively two fused Cu atoms and two differently icosahedral Cu atoms, each of 12 hydrides (H_c and H_e) performs as a μ₃ coordination mode capping a Cu₃ triangle, and each of the remaining four hydrides (H_d) locates above one irregular Cu₄ quadrangle and bridges three Cu atoms with a μ₃ coordination mode. The discrepancy among 1_opa–1_opf results from the drift of H_d positions as shown in Fig. S18. These drifts of H_d hydrides (also called “fluxional” hydrides) may result in the split of the ¹H NMR signal for H_d. So four H_d hydrides belong to two adjacent wide peaks at 0.25–0.85 ppm in the ¹H NMR spectrum of cluster 1. The peak centred at 0.45 ppm contains 8 H_c and 2 H_d, while the peak centred at 0.62 ppm contains 4 H_e and 2 H_d. The H_a and 2 H_b are ascribed to the peak centred at 1.05 ppm.

In the optimized cluster 2, seven hydrides adopt μ₄-H coordination modes and six hydrides adopt μ₃-H coordination modes. Each of three μ₄-H atoms (H_m) bridges the top Cu atom and three adjacent Cu atoms in the Cu₆ hexagon, while each of another three μ₄-H atoms (H_n) bridges one Cu atom from the Cu₆ hexagon and three Cu atoms of the icosahedron. The remaining one μ₄-H atom (H_j) bridges the central Cu atom in the icosahedron and three Cu atoms bound by the NP₃ ligand with a tridentate (P, P, P) mode. Moreover, each of the six μ₃-H atoms (H_k and H_o) bridges three triangular Cu atoms on the surface of the icosahedron. The hydride H_m is ascribed to the wide peak at −0.50 ppm, H_n at −0.10 ppm, H_k at 1.85 ppm, H_j at 3.37 to 3.66 ppm, and H_o at 3.67 to 3.99 ppm. The difference in the ¹H NMR peak locations between hydrides H_k and H_o may be ascribed to the anion effect in solution, though they have similar environments based on the crystal structure.

The Cu–Cu distances in X-ray and DFT structures for 1 and 2 are almost in good agreement and summarized in Table S3. And the Cu–H distances in 1 and 2 are comparable with those in previously reported Cu hydride clusters.^47,74

The calculated band gaps based on the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are 1.99 eV for 1 and 1.65 eV for 2, which are smaller than their optical band gaps estimated by the extrapolation of the absorbance to the baseline.^45,67 The electronic charge densities of the frontier orbitals show that the nanocluster cores (including Cu and H) in 1 and 2 primarily contribute to the HOMOs and the LUMOs (Fig. S19 and S20).

Catalysis

To estimate the catalytic performances of clusters 1 and 2 with different structures, the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) by NaBH₄ in water is chosen as a model reaction (Scheme 2; the details of the catalytic experiment are provided in the SI). Clusters 1 and 2 do not dissolve in water, revealing that their catalytic reactions are heterogeneous. At a reaction temperature of 0 °C, the complete conversion of 4-NP catalyzed by cluster 2 takes 160 minutes, which is shorter than that catalyzed by cluster 1 (400 min) (Fig. 8). When the reaction is carried out at 25 °C, the time required for complete conversion of 4-NP catalyzed by cluster 2 is almost the same as that at 0 °C, which is still shorter than that catalyzed by cluster 1 at 25 °C (Fig. S21), showing that the temperature is not a strict requirement for the catalytic reaction. The time-dependent UV-visible spectra of 4-NP in an aqueous NaBH₄ solution remains unchanged in the absence of the nanocluster catalyst (Fig. S22), confirming that the nanocluster catalyst is necessary for the reduction of 4-NP.

Scheme 2 A schematic diagram showing the catalytic reduction of 4-NP to 4-AP.

Fig. 8 UV -visible spectra showing the gradual reduction of 4-NP catalyzed by different catalysts: (a) cluster 2 and (b) cluster 1 in the presence of NaBH₄ at 0 °C. Catalytic experiment: 4-NP (9.45 mg, 2.27 × 10⁻² M), NaBH₄ (257 mg, 6.8 mmol), cluster catalyst (3 mg) and 3 mL of H₂O.

Based on the change of UV-visible absorption of 4-NP with time at 0 °C and a wavelength of 400 nm, the calculated rate constant for cluster 1 is k = 0.0063 min⁻¹, but for cluster 2 the rate constants at high and low concentration of 4-NP are k^h = 0.013 min⁻¹ and k^l = 0.046 min⁻¹, respectively (Fig. S23). These rate constants indicate that the catalytic reactions are first-order processes. Their rate constants are lower than those of reported copper hydride nanoclusters (0.84 min⁻¹ for [(TOP)CuH]₆,⁸³ 0.5 min⁻¹ for Cu₁₁(Tf-dpf)₆(OAc)₂H₃),⁸⁴ but are comparable to that of [Cu₁₂(Tf-dpf)₆(OAc)₂H₃] (0.007 min⁻¹).⁸⁴ The concentration-dependent rate constants for cluster 2 can be ascribed to the selective absorbance for 4-NP due to the non-equilibrium absorbing structure of cluster 2. For a low concentration of 4-NP, the more active sites on the surface of cluster 2 are left for the cleavage of B–H in BH₄⁻.

The catalytic performance of cluster 2 is better than that of cluster 1, showing the existence of a structure effect. In addition to the different surface adsorbing ability of these two clusters, which influences their catalytic effect,^84,85 the possible transient dissociation of the hemilabile Cu–N bonds associating to NP₃ ligands in 2 could also produce active sites for effective catalytic reaction. However, the multidentate chelating ability of NP₃ still keeps the structure stable during the catalytic process. The recyclability of cluster 2 for the catalytic reduction of 4-NP is explored at 25 °C with a 160 min reaction time. After the twelfth cycle, the conversion of 4-NP is still 100% (Fig. S24), while the catalyst remains stable which is monitored by UV-visible spectroscopy of cluster 2 in MeCN (Fig. S25). Moreover, the reductions of 4-NP to 4-AP with NaBD₄ in place of NaBH₄ are also explored using clusters 1 and 2 as catalysts. After catalysis, the ESI-MS results show that no hydrides in 1 and 2 are replaced by D⁻ of BD₄⁻ (Fig. S26), which supports that the catalytic reaction happens on the surface of the clusters but the hydrides in the clusters are not involved in the catalysis.

Conclusions

In summary, we report the synthesis of two high-nuclearity Cu hydride nanoclusters of Cu₂₂H₁₉ and Cu₂₀H₁₃ and their deuteride analogues, Cu₂₂D₁₉ and Cu₂₀D₁₃, which are characterized by HR ESI-MS, ¹H NMR and single-crystal X-ray diffraction. DFT calculations support their compositions, structures and the sites of hydrides. The tetradentate NP₃ ligand acts to stabilize these structures with a tridentate mode (P, P, P) or a tetradentate mode (N, P, P, P). The small ionic ligands of H⁻ and Cl⁻ ions perform not only as bridging ligands but also as charge transport agents. However, the participation of Cl⁻ can also adjust the structure of Cu hydride nanoclusters due to the different sizes of H⁻ and Cl⁻ ions. Interestingly, the coordination of labile N atom can diversify structures and favor surface reactions. The structure of 2 can be seen as an evolution from 1 by losing two Cu⁺ ions and due to the synergistic mediation of Cl⁻ ions and the NP₃ ligand. The catalytic performance of cluster 2 is better than that of cluster 1 for the reduction of 4-NP to 4-AP. This could result from the transient dissociation of hemilabile Cu–N bonds in 2 producing additional active sites for effective catalytic reaction, in addition to the different surface adsorption catalysis. Furthermore, cluster 2 exhibits different adsorption performances toward high and low concentrations of 4-NP.

Author contributions

The manuscript was written through contributions from all authors. All authors have given approval to the final version of the manuscript.

Conflicts of interest

The authors declare no competing financial interests.

Data availability statements

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: experimental details and methods, including synthesis, physical measurements, X-ray diffraction analysis of crystal structures, catalytic measurements and DFT calculations; and additional figures relating to crystal structures, UV-vis, XPS, ¹H NMR, and HR ESI-MS. See DOI: https://doi.org/10.1039/d5dt02358k.

CCDC 2241274 (Cu₂₀H₁₃) and 2241275 (Cu₂₂H₁₉) contain the supplementary crystallographic data for this paper.^86a,b

Acknowledgements

We are grateful for financial support from the NSF of Guangxi Province in China (2023GXNSFDA026030 and 2019GXNSFGA245003) and the NSF of China (22165003 and 21861004). DFT calculations were provided by the High Performance Computing Center of Science and Engineering of Sichuan University of Science and Engineering.

References

1. I. Chakraborty and T. Pradeep, Atomically Precise Clusters of Noble Metals: Emerging Link between Atoms and Nanoparticles, Chem. Rev., 2017, 117, 8208–8271.
2. X. Kang, Y. Li, M. Zhu and R. Jin, Atomically precise alloy nanoclusters: syntheses, structures, and properties, Chem. Soc. Rev., 2020, 49, 6443–6514.
3. N. A. Sakthivel and A. Dass, Aromatic Thiolate-Protected Series of Gold Nanomolecules and a Contrary Structural Trend in Size Evolution, Acc. Chem. Res., 2018, 51, 1774–1783.
4. Q. Tang, G. Hu, V. Fung and D.-e. Jiang, Insights into Interfaces, Stability, Electronic Properties, and Catalytic Activities of Atomically Precise Metal Nanoclusters from First Principles, Acc. Chem. Res., 2018, 51, 2793–2802.
5. V. K. Kulkarni, B. N. Khiarak, S. Takano, S. Malola, E. L. Albright, T. I. Levchenko, M. D. Aloisio, C.-T. Dinh, T. Tsukuda, H. Häkkinen and C. M. Crudden, N-Heterocyclic Carbene-Stabilized Hydrido Au₂₄ Nanoclusters: Synthesis, Structure, and Electrocatalytic Reduction of CO₂, J. Am. Chem. Soc., 2022, 144, 9000–9006.
6. Q.-Y. Wang, J. Wang, S. Wang, Z.-Y. Wang, M. Cao, C.-L. He, J.-Q. Yang, S.-Q. Zang and T. C. W. Mak, o-Carborane-Based and Atomically Precise Metal Clusters as Hypergolic Materials, J. Am. Chem. Soc., 2020, 142, 12010–12014.
7. Y. J. Kong, Z. P. Yan, S. Li, H. F. Su, K. Li, Y. X. Zheng and S. Q. Zang, Photoresponsive Propeller–like Chiral AIE Copper(I) Clusters, Angew. Chem., Int. Ed., 2020, 59, 5336–5340.
8. G. Soldan, M. A. Aljuhani, M. S. Bootharaju, L. G. AbdulHalim, M. R. Parida, A. H. Emwas, O. F. Mohammed and O. M. Bakr, Gold Doping of Silver Nanoclusters: A 26−Fold Enhancement in the Luminescence Quantum Yield, Angew. Chem., Int. Ed., 2016, 55, 5749–5753.
9. M. L. Fu, I. Issac, D. Fenske and O. Fuhr, Metal–Rich Copper Chalcogenide Clusters at the Border Between Molecule and Bulk Phase: The Structures of [Cu₉₃Se₄₂(SeC₆H₄SMe)₉(PPh₃)₁₈], [Cu₉₆Se₄₅(SeC₆H₄SMe)₆(PPh₃)₁₈], and [Cu₁₃₆S₅₆(SCH₂C₄H₃O)₂₄-(dpppt)₁₀], Angew. Chem., Int. Ed., 2010, 49, 6899–6903.
10. C.-L. Deng, B.-L. Han, Z.-Y. Liu, Z.-H. Pan, J. He, Y.-L. Li, Z.-L. Yang, G.-G. Luo, C.-H. Tung, D. Sun and L.-S. Zheng, Hierarchical Homochiral Assembly of Polyhedral Cage-Type Nanoclusters, CCS Chem., 2024, 6, 2537–2548.
11. B. Zhang, X. Tan, Z.-H. Zhang, C.-Y. Liu, F. Hu, H.-S. Hu, C. Chen and Q.-M. Wang, Atomically Precise Copper Nanoclusters with Cu-N Interfaces Toward Efficient CO₂-to-CH₄ Electrocatalysis, CCS Chem., 2025 DOI:10.31635/ccschem.025.202505606.
12. T. Jia, Z.-J. Guan, C. Zhang, X.-Z. Zhu, Y.-X. Chen, Q. Zhang, Y. Yang and D. Sun, Eight-Electron Superatomic Cu₃₁ Nanocluster with Chiral Kernel and NIR-II Emission, J. Am. Chem. Soc., 2023, 145, 10355–10363.
13. H. Zhao, C. Zhang, B. Han, Z. Wang, Y. Liu, Q. Xue, C.-H. Tung and D. Sun, Assembly of Air-stable Copper(I) Alkynide Nanoclusters Assisted by Tripodal Polydentate Phosphoramide Ligands, Nat. Synth., 2024, 3, 517–526.
14. N. A. Sakthivel, S. Theivendran, V. Ganeshraj, A. G. Oliver and A. Dass, Crystal Structure of Faradaurate-279: Au₂₇₉(SPh^tBu)₈₄ Plasmonic Nanocrystal Molecules, J. Am. Chem. Soc., 2017, 139, 15450–15459.
15. M.-X. Ma, X.-L. Ma, G.-M. Liang, X.-T. Shen, Q.-L. Ni, L.-C. Gui, X.-J. Wang, S.-Y. Huang and S.-M. Li, A Nanocluster [Ag₃₀₇Cl₆₂(SPh-^tBu)₁₁₀]: Chloride Intercalation, Specific Electronic State, and Superstability, J. Am. Chem. Soc., 2021, 143, 13731–13737.
16. H. Y. Yang, Y. Wang, X. Chen, X. J. Zhao, L. Gu, H. Q. Huang, J. Z. Yan, C. F. Xu, G. Li, J. C. Wu, A. J. Edwards, B. Dittrich, Z. C. Tang, D. D. Wang, L. Lehtovaara, H. Häkkinen and N. F. Zheng, Plasmonic Twinned Silver Nanoparticles with Molecular Precision, Nat. Commun., 2016, 7, 12809.
17. Y. Song, Y. Li, H. Li, F. Ke, J. Xiang, C. Zhou, P. Li, M. Zhu and R. Jin, Atomically Resolved Au₅₂Cu₇₂(SR)₅₅ Nanoalloy Reveals Marks Decahedron Truncation and Penrose Tiling Surface, Nat. Commun., 2020, 11, 478.
18. X.-T. Shen, X.-L. Ma, Q.-L. Ni, M.-X. Ma, L.-C. Gui, C. Hou, R.-B. Hou and X.-J. Wang, [Ag₁₅(N-triphos)₄(Cl₄)](NO₃)₃: a Stable Ag–P Superatom with Eight Electrons (N-triphos = tris((diphenyl-phosphino)methyl)amine), Nanoscale, 2018, 10, 515–519.
19. X. Ma, Y. Bai, Y. Song, Q. Li, Y. Lv, H. Zhang, H. Yu and M. Zhu, Rhombicuboctahedral Ag₁₀₀: Four–Layered Octahedral Silver Nanocluster Adopting the Russian Nesting Doll Model, Angew. Chem., Int. Ed., 2020, 59, 17234–17238.
20. F. Hu, J. J. Li, Z. J. Guan, S. F. Yuan and Q. M. Wang, Formation of an Alkynyl–Protected Ag₁₁₂ Silver Nanocluster as Promoted by Chloride Released In Situ from CH₂Cl₂, Angew. Chem., Int. Ed., 2020, 59, 5312–5315.
21. J. Y. Liu, F. Alkan, Z. Wang, Z. Y. Zhang, M. Kurmoo, Z. Yan, Q. Q. Zhao, C. M. Aikens, C. H. Tung and D. Sun, Different Silver Nanoparticles in One Crystal: Ag₂₁₀(ⁱPrPhS)₇₁(Ph₃P)₅Cl and Ag₂₁₁(ⁱPrPhS)₇₁(Ph₃P)₆Cl, Angew. Chem., Int. Ed., 2019, 58, 195–199.
22. M. Diecke, C. Schrenk and A. Schnepf, Synthesis and Characterization of the Highly Unstable Metalloid Cluster Ag₆₄(PⁿBu₃)₁₆Cl₆, Angew. Chem., Int. Ed., 2020, 59, 14418–14422.
23. T. Higaki, M. Zhou, K. J. Lambright, K. Kirschbaum, M. Y. Sfeir and R. Jin, Sharp Transition from Nonmetallic Au₂₄₆ to Metallic Au₂₇₉ with Nascent Surface Plasmon Resonance, J. Am. Chem. Soc., 2018, 140, 5691–5695.
24. Z. Gan, J. Chen, J. Wang, C. Wang, M.-B. Li, C. Yao, S. Zhuang, A. Xu, L. Li and Z. Wu, The Fourth Crystallographic Closest Packing Unveiled in the Gold Nanocluster Crystal, Nat. Commun., 2017, 8, 14739.
25. C. Zeng, Y. Chen, K. Kirschbaum, K. J. Lambright and R. Jin, Emergence of Hierarchical Structural Complexities in Nanoparticles and Their Assembly, Science, 2016, 354, 1580–1584.
26. H. Yoshida, M. Ehara, U. D. Priyakumar, T. Kawai and T. Nakashima, Enantioseparation and Chiral Induction in Ag₂₉ Nanoclusters with Intrinsic Chirality, Chem. Sci., 2020, 11, 2394–2400.
27. N. Xia, J. Xing, D. Peng, S. Ji, J. Zha, N. Yan, Y. Su, X. Jiang, Z. Zeng, J. Zhao and Z. Wu, Assembly-induced Spin Transfer and Distance-dependent Spin Coupling in Atomically Precise AgCu Nanoclusters, Nat. Commun., 2022, 13, 5934.
28. X. Kang and M. Zhu, Intra-cluster Growth Meets Inter-cluster Assembly: The Molecular and Supramolecular Chemistry of Atomically Precise Nanoclusters, Coord. Chem. Rev., 2019, 394, 1–38.
29. J.-Q. Wang, S. Shi, R.-L. He, S.-F. Yuan, G.-Y. Yang, G.-J. Liang and Q.-M. Wang, Total Structure Determination of the Largest Alkynyl-Protected fcc Gold Nanocluster Au₁₁₀ and the Study on Its Ultrafast Excited-State Dynamics, J. Am. Chem. Soc., 2020, 142, 18086–18092.
30. H. Qian, W. T. Eckenhoff, Y. Zhu, T. Pintauer and R. Jin, Total Structure Determination of Thiolate-Protected Au₃₈ Nanoparticles, J. Am. Chem. Soc., 2010, 132, 8280–8281.
31. D.-e. Jiang, M. L. Tiago, W. Luo and S. Dai, The “Staple” Motif: A Key to Stability of Thiolate-Protected Gold Nanoclusters, J. Am. Chem. Soc., 2008, 130, 2777–2779.
32. S. Zhuang, L. Liao, Y. Zhao, J. Yuan, C. Yao, X. Liu, J. Li, H. Deng, J. Yang and Z. Wu, Is the Kernel–staples Match a Key–lock Match?, Chem. Sci., 2018, 9, 2437–2442.
33. A. J. Edwards, R. S. Dhayal, P. K. Liao, J. H. Liao, M. H. Chiang, R. O. Piltz, S. Kahlal, J. Y. Saillard and C. W. Liu, Chinese Puzzle Molecule: A 15 Hydride, 28Copper Atom Nanoball, Angew. Chem., Int. Ed., 2014, 53, 7214–7218.
34. F. Tian and R. Chen, Ag₁₈(μ₈-S)(p-TBBT)₁₆(PPh₃)₈: Symmetry Breaking Induced by the Core to Generate Chirality, Chem. Commun., 2020, 56, 2719–2722.
35. A. Baghdasaryan and T. Bürgi, Copper Nanoclusters: Designed Synthesis, Structural Diversity, and Multiplatform Applications, Nanoscale, 2021, 13, 6283–6340.
36. A. J. Jordan, G. Lalic and J. P. Sadighi, Coinage Metal Hydrides: Synthesis, Characterization, and Reactivity, Chem. Rev., 2016, 116, 8318–8372.
37. S. Shahsavari, S. Hadian-Ghazvini, F. Hooriabad Saboor, I. Menbari Oskouie, M. Hasany, A. Simchi and A. L. Rogach, Ligand Functionalized Copper Nanoclusters for Versatile Applications in Catalysis, Sensing, Bioimaging, and Optoelectronics, Mater. Chem. Front., 2019, 3, 2326–2356.
38. R. S. Dhayal, W. E. van Zyl and C. W. Liu, Copper Hydride Clusters in Energy Storage and Conversion, Dalton Trans., 2019, 48, 3531–3538.
39. R. S. Dhayal, W. E. van Zyl and C. W. Liu, Polyhydrido Copper Clusters: Synthetic Advances, Structural Diversity, and Nanocluster-to-Nanoparticle Conversion, Acc. Chem. Res., 2015, 49, 86–95.
40. C. Sun, B. K. Teo, C. Deng, J. Lin, G.-G. Luo, C.-H. Tung and D. Sun, Hydrido-coinage-metal Clusters: Rational Design, Synthetic Protocols and Structural Characteristics, Coord. Chem. Rev., 2021, 427, 213576.
41. D. A. Ekanayake, A. Chakraborty, J. A. Krause and H. Guan, Hydrogenation Reactions Catalyzed by HN(CH₂CH₂PR₂)₂-ligated Copper Complexes, Inorg. Chem. Front., 2021, 8, 4634–4649.
42. R. S. Dhayal, J.-H. Liao, Y.-R. Lin, P.-K. Liao, S. Kahlal, J.-Y. Saillard and C. W. Liu, A Nanospheric Polyhydrido Copper Cluster of Elongated Triangular Orthobicupola Array: Liberation of H₂ from Solar Energy, J. Am. Chem. Soc., 2013, 135, 4704–4707.
43. R. P. B. Silalahi, Q. Wang, J. H. Liao, T. H. Chiu, Y. Y. Wu, X. Wang, S. Kahlal, J. Y. Saillard and C. W. Liu, Reactivities of Interstitial Hydrides in a Cu₁₁ Template: En Route to Bimetallic Clusters, Angew. Chem., Int. Ed., 2022, 61, e202113266.
44. R. S. Dhayal, J. H. Liao, X. Wang, Y. C. Liu, M. H. Chiang, S. Kahlal, J. Y. Saillard and C. W. Liu, Diselenophosphate-Induced Conversion of an Achiral [Cu₂₀H₁₁{S₂P(OiPr)₂}₉] into a Chiral [Cu₂₀H₁₁{Se₂P(OiPr)₂}₉] Polyhydrido Nanocluster, Angew. Chem., Int. Ed., 2015, 54, 13604–13608.
45. R.-W. Huang, J. Yin, C. Dong, P. Maity, M. N. Hedhili, S. Nematulloev, B. Alamer, A. Ghosh, O. F. Mohammed and O. M. Bakr, [Cu₂₃(PhSe)₁₆(Ph₃P)₈(H)₆]·BF₄: Atomic-Level Insights into Cuboidal Polyhydrido Copper Nanoclusters and Their Quasi-simple Cubic Self-Assembly, ACS Mater. Lett., 2021, 3, 90–99.
46. C. Sun, N. Mammen, S. Kaappa, P. Yuan, G. Deng, C. Zhao, J. Yan, S. Malola, K. Honkala, H. Häkkinen, B. K. Teo and N. Zheng, Atomically Precise, Thiolated Copper–Hydride Nanoclusters as Single-Site Hydrogenation Catalysts for Ketones in Mild Conditions, ACS Nano, 2019, 13, 5975–5986.
47. T.-A. D. Nguyen, Z. R. Jones, B. R. Goldsmith, W. R. Buratto, G. Wu, S. L. Scott and T. W. Hayton, A Cu₂₅ Nanocluster with Partial Cu(0) Character, J. Am. Chem. Soc., 2015, 137, 13319–13324.
48. S. Li, X. Yan, J. Tang, D. Cao, X. Sun, G. Tian, X. Tang, H. Guo, Q. Wu, J. Sun, J. He and H. Shen, Cu₂₆ Nanoclusters with Quintuple Ligand Shells for CO₂ Electrocatalytic Reduction, Chem. Mater., 2023, 35, 6123–6132.
49. Y. Bao, X. Wu, B. Yin, X. Kang, Z. Lin, H. Deng, H. Yu, S. Jin, S. Chen and M. Zhu, Structured Copper-hydride Nanoclusters Provide Insight into the Surface-vacancy-defect to Non-defect Structural Evolution, Chem. Sci., 2022, 13, 14357–14365.
50. S. Nematulloev, A. Sagadevan, B. Alamer, A. Shkurenko, R. Huang, J. Yin, C. Dong, P. Yuan, K. E. Yorov, A. A. Karluk, W. J. Mir, B. E. Hasanov, M. Nejib Hedhili, N. M. Halappa, M. Eddaoudi, O. F. Mohammed, M. Rueping and O. M. Bakr, Atomically Precise Defective Copper Nanocluster Catalysts for Highly Selective C−C Cross–Coupling Reactions, Angew. Chem., Int. Ed., 2023, 62, e202303572.
51. Q.-L. Guo, B.-L. Han, C.-F. Sun, Z. Wang, Y. Tao, J.-Q. Lin, G.-G. Luo, C.-H. Tung and D. Sun, Observation of a Bcc-like Framework in Polyhydrido Copper Nanoclusters, Nanoscale, 2021, 13, 19642–19649.
52. X. Liu, H. Shen, Y. Gao, G. Deng, H. Deng, Y.-Z. Han, B. K. Teo and N. Zheng, Cu₂₈H₂₀: a Peculiar Chiral Nanocluster with an Exposed Cu Atom and 13 Surface Hydrides, Chem. Commun., 2022, 58, 7670–7673.
53. T.-A. D. Nguyen, Z. R. Jones, D. F. Leto, G. Wu, S. L. Scott and T. W. Hayton, Ligand-Exchange-Induced Growth of an Atomically Precise Cu₂₉ Nanocluster from a Smaller Cluster, Chem. Mater., 2016, 28, 8385–8390.
54. S. K. Barik, S. C. Huo, C. Y. Wu, T. H. Chiu, J. H. Liao, X. Wang, S. Kahlal, J. Y. Saillard and C. W. Liu, Polyhydrido Copper Nanoclusters with a Hollow Icosahedral Core: [Cu₃₀H₁₈{E₂P(OR)₂}₁₂] (E=S or Se; R=nPr, iPr or iBu), Chem. – Eur. J., 2020, 26, 10471–10479.
55. H. Shen, L. Wang, O. López-Estrada, C. Hu, Q. Wu, D. Cao, S. Malola, B. K. Teo, H. Häkkinen and N. Zheng, Copper-hydride Nanoclusters with Enhanced Stability by N-heterocyclic Carbenes, Nano Res., 2021, 14, 3303–3308.
56. S. Lee, M. S. Bootharaju, G. Deng, S. Malola, W. Baek, H. Häkkinen, N. Zheng and T. Hyeon, [Cu₃₂(PET)₂₄H₈Cl₂](PPh₄)₂: A Copper Hydride Nanocluster with a Bisquare Antiprismatic Core, J. Am. Chem. Soc., 2020, 142, 13974–13981.
57. R. S. Dhayal, J. H. Liao, S. Kahlal, X. Wang, Y. C. Liu, M. H. Chiang, W. E. van Zyl, J. Y. Saillard and C. W. Liu, [Cu₃₂(H)₂₀{S₂P(OiPr)₂}₁₂]: The Largest Number of Hydrides Recorded in a Molecular Nanocluster by Neutron Diffraction, Chem. – Eur. J., 2015, 21, 8369–8374.
58. C. Dong, R.-W. Huang, C. Chen, J. Chen, S. Nematulloev, X. Guo, A. Ghosh, B. Alamer, M. N. Hedhili, T. T. Isimjan, Y. Han, O. F. Mohammed and O. M. Bakr, [Cu₃₆H₁₀(PET)₂₄(PPh₃)₆Cl₂] Reveals Surface Vacancy Defects in Ligand-Stabilized Metal Nanoclusters, J. Am. Chem. Soc., 2021, 143, 11026–11035.
59. J. Zhou, W.-W. Zhan, Y. Li, X.-R. Gao, C. Zhang, D. Sun and Y. Yang, Hydride-Richest Molecular Complex: Ligand-Length Facilitated Cu₄₀H₃₈ Nanoclusters Exhibiting High Catalytic Performance for Selective Hydrogenation, Angew. Chem., Int. Ed., 2025, 64, e202504671.
60. H. Zhou, T. Duan, Z. Lin, T. Yang, H. Deng, S. Jin, Y. Pei and M. Zhu, Total Structure, Structural Transformation and Catalytic Hydrogenation of [Cu₄₁(SC₆H₃F₂)₁₅Cl₃(P(PhF)₃)₆(H)₂₅]₂− Constructed from Twisted Cu₁₃ Units, Adv. Sci., 2023, 11, 2307085.
61. P. Yuan, R. Chen, X. Zhang, F. Chen, J. Yan, C. Sun, D. Ou, J. Peng, S. Lin, Z. Tang, B. K. Teo, L. S. Zheng and N. Zheng, Ether–Soluble Cu₅₃ Nanoclusters as an Effective Precursor of High–Quality CuI Films for Optoelectronic Applications, Angew. Chem., Int. Ed., 2019, 58, 835–839.
62. G. G. Luo, Z. H. Pan, B. L. Han, G. L. Dong, C. L. Deng, M. Azam, Y. W. Tao, J. He, C. F. Sun and D. Sun, Total Structure, Electronic Structure and Catalytic Hydrogenation Activity of Metal–Deficient Chiral Polyhydride Cu₅₇ Nanoclusters, Angew. Chem., Int. Ed., 2023, 62, e202306849.
63. C. Dong, R. W. Huang, A. Sagadevan, P. Yuan, L. Gutiérrez–Arzaluz, A. Ghosh, S. Nematulloev, B. Alamer, O. F. Mohammed, I. Hussain, M. Rueping and O. M. Bakr, Isostructural Nanocluster Manipulation Reveals Pivotal Role of One Surface Atom in Click Chemistry, Angew. Chem., Int. Ed., 2023, 62, e202307140.
64. S. Biswas, S. Hossian, T. Kosaka, J. Sakai, D. Arima, Y. Niihori, M. Mitsui, D.-e. Jiang, S. Das, S. Wang and Y. Negishi, Nested Keplerian Architecture of [Cu₅₈H₂₀(SPr)₃₆(PPh₃)₈]²⁺ Nanoclusters, Chem. Commun., 2023, 59, 9336–9339.
65. A. Ghosh, R.-W. Huang, B. Alamer, E. Abou-Hamad, M. N. Hedhili, O. F. Mohammed and O. M. Bakr, [Cu₆₁(StBu)₂₆S₆Cl₆H₁₄]⁺: A Core–Shell Superatom Nanocluster with a Quasi-J36 Cu₁₉ Core and an “18-Crown-6” Metal-Sulfide-like Stabilizing Belt, ACS Mater. Lett., 2019, 1, 297–302.
66. H.-N. Qin, M.-W. He, J. Wang, H.-Y. Li, Z.-Y. Wang, S.-Q. Zang and T. C. W. Mak, Thiacalix[4]arene Etching of an Anisotropic Cu₇₀H₂₂ Intermediate for Accessing Robust Modularly Assembled Copper Nanoclusters, J. Am. Chem. Soc., 2024, 146, 3545–3552.
67. R.-W. Huang, J. Yin, C. Dong, A. Ghosh, M. J. Alhilaly, X. Dong, M. N. Hedhili, E. Abou-Hamad, B. Alamer, S. Nematulloev, Y. Han, O. F. Mohammed and O. M. Bakr, [Cu₈₁(PhS)₄₆(tBuNH₂)₁₀(H)₃₂]³⁺ Reveals the Coexistence of Large Planar Cores and Hemispherical Shells in High-Nuclearity Copper Nanoclusters, J. Am. Chem. Soc., 2020, 142, 8696–8705.
68. T. Tanase, K. Nakamae, Y. Ura and T. Nakajima, Fine Tunable Metal Assemblies Constrained by Multidentate Phosphine Ligands, Coord. Chem. Rev., 2022, 466, 214581.
69. T. Nakajima, K. Nakamae, Y. Ura and T. Tanase, Multinuclear Copper Hydride Complexes Supported by Polyphosphine Ligands, Eur. J. Inorg. Chem., 2020, 2020, 2211–2226.
70. C. M. Zall, J. C. Linehan and A. M. Appel, Triphosphine-Ligated Copper Hydrides for CO₂ Hydrogenation: Structure, Reactivity, and Thermodynamic Studies, J. Am. Chem. Soc., 2016, 138, 9968–9977.
71. S. Nematulloev, R. W. Huang, J. Yin, A. Shkurenko, C. Dong, A. Ghosh, B. Alamer, R. Naphade, M. N. Hedhili, P. Maity, M. Eddaoudi, O. F. Mohammed and O. M. Bakr, [Cu₁₅(PPh₃)₆(PET)₁₃]²⁺: a Copper Nanocluster with Crystallization Enhanced Photoluminescence, Small, 2021, 17, 2006839.
72. M. R. Churchill, S. A. Bezman, J. A. Osborn and J. Wormald, Synthesis and Molecular Geometry of Hexameric Triphenyl-phosphinocopper(I) Hydride and the Crystal Structure of H₆Cu₆(PPh₃)₆. HCONMe₂ [Hexameric Triphenylphosphino Copper(I) Hydride Dimethylformamide], Inorg. Chem., 1972, 11, 1818–1825.
73. T. A. D. Nguyen, B. R. Goldsmith, H. T. Zaman, G. Wu, B. Peters and T. W. Hayton, Synthesis and Characterization of a Cu₁₄ Hydride Cluster Supported by Neutral Donor Ligands, Chem. – Eur. J., 2015, 21, 5341–5344.
74. K. Nakamae, T. Nakajima, Y. Ura, Y. Kitagawa and T. Tanase, Facially Dispersed Polyhydride Cu₉ and Cu₁₆ Clusters Comprising Apex–Truncated Supertetrahedral and Square–Face–Capped Cuboctahedral Copper Frameworks, Angew. Chem., Int. Ed., 2020, 59, 2262–2267.
75. J. Li, H. Z. Ma, G. E. Reid, A. J. Edwards, Y. Hong, J. M. White, R. J. Mulder and R. A. J. O'Hair, Synthesis and X–Ray Crystallographic Characterisation of Frustum–Shaped Ligated [Cu₁₈H₁₆(DPPE)₆]²⁺ and [Cu₁₆H₁₄(DPPA)₆]²⁺ Nanoclusters and Studies on Their H₂ Evolution Reactions, Chem. – Eur. J., 2018, 24, 2070–2074.
76. Q. L. Ni, F. Zhang, T. H. Huang, X. P. Wu, X. J. Wang, G. M. Liang and K. G. Yang, Octahedral and Cyclic [Ag₆] Cluster Cores Stabilized by {Ag₃(Spz)₃(N-triphos)₃} Motifs [HSpz = Pyrazine-2-thiolate, N-triphos = Tris((diphenylphosphanyl)methyl)-amine]: Ag⋯Ag Interactions, Z. Anorg. Allg. Chem., 2015, 641, 2664–2669.
77. R. Jin, C. Liu, S. Zhao, A. Das, H. Xing, C. Gayathri, Y. Xing, N. L. Rosi and R. Jin, Tri-icosahedral Gold Nanocluster [Au₃₇(PPh₃)₁₀(SC₂H₄Ph)₁₀X₂]⁺: Linear Assembly of Icosahedral Building Blocks, ACS Nano, 2015, 9, 8530–8536.
78. T.-H. Chiu, J.-H. Liao, F. Gam, I. Chantrenne, S. Kahlal, J.-Y. Saillard and C. W. Liu, Homoleptic Platinum/Silver Superatoms Protected by Dithiolates: Linear Assemblies of Two and Three Centered Icosahedra Isolobal to Ne₂ and I₃⁻, J. Am. Chem. Soc., 2019, 141, 12957–12961.
79. Y. Song, F. Fu, J. Zhang, J. Chai, X. Kang, P. Li, S. Li, H. Zhou and M. Zhu, The Magic Au60 Nanocluster: A New Cluster–Assembled Material with Five Au₁₃ Building Blocks, Angew. Chem., Int. Ed., 2015, 54, 8430–8434.
80. B. K. Teo, H. Zhang and X. Shi, a Modular Approach to Large Metal Clusters. Structural Characterization of a 38-atom Cluster [(p-Tol₃P)₁₂Au₁₈Ag₂₀Cl₁₄] Based on Vertex-sharing Triicosahedra, J. Am. Chem. Soc., 1990, 112, 8552–8562.
81. C. Xu, Y. Zhou, J. Yi, D. Li, L. Shi and L. Cheng, Tri- and Tetra-superatomic Molecules in Ligand-Protected Face-Fused Icosahedral (M@Au₁₂)n (M = Au, Pt, Ir, and Os, and n = 3 and 4) Clusters, J. Phys. Chem. Lett., 2022, 13, 1931–1939.
82. H. Yang, Y. Wang, A. J. Edwards, J. Yan and N. Zheng, High-yield Synthesis and Crystal Structure of a Green Au₃₀cluster Co-capped by Thiolate and Sulfide, Chem. Commun., 2014, 50, 14325–14327.
83. X. Zheng, Y. Liu, W. Ma, Y. Su and Y. Wang, The structure–activity Relationship of Copper Hydride Nanoclusters in Hydrogenation and Reduction Reactions, Nanoscale Adv., 2024, 6, 1374–1379.
84. C.-Y. Liu, S.-F. Yuan, S. Wang, Z.-J. Guan, D.-e. Jiang and Q.-M. Wang, Structural Transformation and Catalytic Hydrogenation Activity of Amidinate-protected Copper Hydride Clusters, Nat. Commun., 2022, 13, 2082.
85. S. M. Ansar and C. L. Kitchens, Impact of Gold Nanoparticle Stabilizing Ligands on the Colloidal Catalytic Reduction of 4-Nitrophenol, ACS Catal., 2016, 6, 5553–5560.
86. (a) CCDC 2241274: Experimental Crystal Structure Determination, 2025,  DOI:10.5517/ccdc.csd.cc2f7753; (b) CCDC 2241275: Experimental Crystal Structure Determination, 2025,  DOI:10.5517/ccdc.csd.cc2f7764.

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Footnote

† These authors contributed equally.

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