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Open AccessCCS ChemistryRESEARCH ARTICLES3 Jun 2024

Dramatic Difference Between Cu20H8 and Cu20H9 Clusters in Catalysis

    CCS Chem. 2024, 6, 1581–1590
    https://doi.org/10.31635/ccschem.023.202303448

    Sensitivity to structure and composition is very challenging to establish in nanocatalysis due to inadequate definition of structures that are very close in composition. We synthesized a pair of atomically precise copper clusters that are very close in composition, [Cu20H9(Tf-dpf)10] · BF4 ( Cu20H9) and [Cu20H8(Tf-dpf)10] · (BF4)2 ( Cu20H8), by using a pyridyl-functionalized flexible amidinate ligand, N,N′-di(5-trifluoromethyl-2-pyridyl)formamidinate. The one-hydride difference in their composition leads to significant variation in geometric and electronic structures and, consequently, distinctly different optical and catalytic properties. Cu20H8 exhibits 25 times higher catalytic activity than Cu20H9 (96.7% vs 3.7% in yield) in the selective hydrogenation of an α,β-unsaturated aldehyde (cinnamaldehyde) to saturated aldehyde (3-phenylpropanal). Electrospray ionization mass spectrometry combined with density functional theory calculations reveal that the greater ease of dissociation of one Tf-dpf ligand compared to Cu20H8 is the key to its higher activity. This work demonstrates a clear case of structure and composition sensitivity in nanocatalysis and that one hydride, out of ∼330 atoms in the nanoclusters, can make a huge difference in the catalytic activity. These insights will be useful in the design and synthesis of atomically precise nanocatalysts.

    Introduction

    Atomically precise metal clusters offer valuable opportunities to obtain fundamental insights into catalytic reactions.1–13 Copper hydride clusters are of great interest because of their potential applications in hydrogenation catalysis.14–20 Although a large number of atomically precise copper hydride clusters have been reported,21–33 their poor stability has limited their catalytic applications.34–36 Previously we have demonstrated that pyridyl-functionalized amidinates are promising candidates for the protection of stable copper hydride clusters.37–40 The flexible ligand containing four N-donors favors the structural transformation of copper hydride clusters, which is beneficial to their structural control in order to tune the catalytic activity of the clusters.

    In the course of our attempt to establish the relationships between structures and synthetic parameters of copper hydride clusters, we found that a very small change in composition makes a big difference in structure and catalytic performance and that even one hydride matters. Herein, we report a pair of copper hydride clusters, [Cu20H9(Tf-dpf)10] · BF4 ( Cu20H9) and [Cu20H8(Tf-dpf)10] · (BF4)2 ( Cu20H8), where Tf-dpf is N,N′-di(5-trifluoromethyl-2-pyridyl)formamidinate (HTf-dpf). Their formulas have only one hydride of difference, but their geometries and electronic structures are distinctly different; as a result, these two Cu20 clusters display quite different optical and catalytic properties. Metal clusters have been known to show different optical properties and catalytic activities as a result of the change of ligands41,42 or metal numbers,43,44 but to the best of our knowledge, such high sensitivity to the composition by one hydride difference has so far not been reported.

    Experimental Methods

    We followed the procedure in the literature for the HTf-dpf ligand.40 All of the chemicals were obtained from commercial sources and used as received without further purification.

    Synthesis of [Cu20H9(Tf-dpf)10] · BF4 (Cu20H9)

    Excess Et3N (20 μL) was added to a 3 mL CH2Cl2/CH3OH (v:v = 2:1) solution containing HTf-dpf (0.1 mmol, 33.4 mg) and Cu(MeCN)4BF4 (63 mg, 0.2 mmol) under an ice bath. Then freshly prepared 2 mL MeOH solution of tBuNH2 · BH3 (0.2 mmol, 17.4 mg) was added dropwise to the above mixture. The color changed from yellow to orange and finally to red. The reaction continued for 4 h at 0 °C. Then the solvents were removed by rotary evaporation. The obtained product was washed with n-hexane (3 × 2 mL) and then dissolved in 3 mL CH2Cl2/CH3OH (v:v = 2:1). After centrifugation, the supernatant solution was divided into two parts, and each part was subjected to the diffusion of n-hexane at 4 °C to afford red crystals in a week with 57% yield (27.4 mg, based on Cu). Anal. UV–vis (λ, nm): 246; 262; 338. Electrospray ionization mass spectrometry (ESI-MS) (CH3CN): 4612.2 ([Cu20H9(Tf-dpf)10]+) and 2337.6 ([Cu21H9(Tf-dpf)10]2+).

    Synthesis of [Cu20H8(Tf-dpf)10] · (BF4)2 (Cu20H8)

    The synthesis of Cu20H8 was similar to that of Cu20H9. Excess Et3N (20 μL) was added to a 3 mL CH2Cl2/CH3OH (v:v = 2:1) solution containing HTf-dpf (0.1 mmol, 33.4 mg) and Cu(MeCN)4BF4 (47.5 mg, 0.15 mmol) at room temperature (30 °C). Then freshly prepared 2 mL MeOH solution of tBuNH2 · BH3 (0.08 mmol, 7.0 mg) was added dropwise to the above solution. The color changed from yellow to orange and finally to dark brown-green. The reaction continued for 4 h at room temperature (30 °C). The workup was also the same as that for Cu20H9. Block brown-green crystals were obtained at 4 °C within a week with a 78% yield (27.9 mg, based on Cu). Anal. UV–vis (λ, nm): 250; 271; 290; 570. ESI-MS (CH3CN): 2305.6 ([Cu20H8(Tf-dpf)10]2+).

    Experimental procedure for cluster transformation from Cu20H9 to Cu20H8

    The sample of Cu20H9 (5 mg) was dissolved in 2 mL of methanol. Then the solution was kept at 30 °C, under nitrogen, which was monitored by real-time UV–vis spectrometry.

    Catalytic reduction of α,β-unsaturated carbonyl compounds

    2 mL ethanol was added to 1 mg Cu20H9 (0.21 umol) or Cu20H8 (0.21 umol), cinnamaldehyde 13 μL (0.1 mmol), and Ph2SiH2 36 μL (0.2 mmol) in a 25 mL round-bottom flask at room temperature. Then the mixture was encapsulated well and stirred at 30 °C. After reaction, the mixture was evaporated and extracted with 0.5 mL CDCl3.

    Results and Discussion

    Synthesis and crystal structures

    Cu20H8 was prepared in 78% yield by reduction of Cu(MeCN)4BF4 and HTf-dpf /Et3N with tert-butylamine-borane in CH3OH/CH2Cl2. Cu20H9 was obtained in 57% yield by a similar synthetic procedure with a different reactant ratio. It was found that the reaction temperature is essential for the preparation. Cu20H9 was synthesized with an ice bath, while Cu20H8 was obtained under ambient conditions. Single crystals of Cu20H9 (red) and Cu20H8 (brown-green) were obtained by the diffusion of n-hexane into a CH2Cl2/ CH3OH solution of the as-obtained crystals ( Supporting Information Figure S1).

    The compositions of these two clusters have been studied with ESI-MS (Figure 1a,b). Cu20H9 shows a prominent peak at m/z = 4612.2, corresponding to the molecular ion [Cu20H9(Tf-dpf)10]+, and the weak peak at m/z = 2337.6 was from [Cu21H9(Tf-dpf)10]2+. The spectrum of Cu20H8 gives off the signal of [Cu20H8(Tf-dpf)10]2+ at m/z = 2305.6. These results indicate that both Cu20H9 and Cu20H8 are Cu(I) hydride clusters. The X-ray photoelectron spectroscopy of Cu20H9 and Cu20H8 shows the Cu 2p1/2 and 2p3/2 peaks at 952.4 and 932.7 eV ( Supporting Information Figure S2), respectively. The doublet separation was calculated to be 19.7 eV, suggesting the presence of Cu(I). Moreover, the Cu L-inner level-M-inner level-M-inner level electron transition (LMM) Auger spectra show a prominent peak at 915.7 eV for Cu20H9 and Cu20H8, respectively, confirming that all copper atoms are monovalent Cu(I).45

    Figure 1

    Figure 1 | ESI-MS of (a) Cu20H9 and (b) Cu20H8 in positive mode. Inset: the simulated (red) and experimental (black) isotropic patterns of Cu20H9 and Cu20H8.

    ESI-MS revealed that Cu20H9 and Cu20H8 have roughly identical compositions. They both have 20 Cu atoms and 10 Tf-dpf ligands, and the only difference is the number of hydrides, that is, 9 vs 8. Fortunately, their structures, including the number and positions of the hydrides, have been determined by single-crystal X-ray diffraction (SCXRD) measurements,30,35 which were further confirmed by a machine-learning approach46,47 and density functional theory (DFT) calculations. Nine hydrides for Cu20H9 and eight hydrides for Cu20H8 were confirmed.

    The anatomy of the two Cu20 structures is shown in Figure 2a–h. The middle Cu12 units of Cu20H9 and Cu20H8 have different configurations. The Cu12 unit in Cu20H9 comprises two parallel triangular prisms, while that in Cu20H8 consists of two octahedrons. The Cu···Cu distances of the Cu12 unit in Cu20H9 range from 2.504(1) to 2.851(1) Å with an average of 2.617 Å. For Cu20H8, these values are in the range of 2.416(1)–3.042(1) Å with an average of 2.650 Å. Each of the Cu12 units links two identical Cu4 units to form Cu20H9 and Cu20H8, respectively. These Cu4 units show a similar coordinating environment in two Cu20 clusters, and all Tf-dpf ligands are ligated to Cu4 in linear coordination mode. The average Cu···Cu bond lengths between these additional two Cu4 units and the Cu12 unit are 2.700 Å for Cu20H9 and 2.631 Å for Cu20H8. The Cu···Cu distances in the whole Cu20 kernel give an average of 2.636 Å for Cu20H9 and 2.625 Å for Cu20H8 ( Supporting Information Table S2). Overall, the entire structure of Cu20H9 has C2 symmetry. Ten Tf-dpf ligands in Cu20H9 are in five different coordination modes. On the contrary, Cu20H8 has D2 symmetry, and its ten Tf-dpf ligands are in three different coordination modes.

    Figure 2

    Figure 2 | Anatomy of two Cu20 clusters. The Cu12 kernel in Cu20H9 (a) and Cu20H8 (e); Cu12 + 2Cu4 in Cu20H9 (b) and Cu20H8 (f); the core + hydride structure in Cu20H9 (c) and Cu20H8 (g); molecular structures of Cu20H9 (d) and Cu20H8 (h). Color code: C, grey; F, bright green; hydride, red; N, dark blue; Cu, bright blue and dark green.

    The deep-learning prediction46,47 of hydride occupancies based on the heavy-atom positions (Figure 3a,b) coupled with DFT geometry optimization ( Supporting Information Figure S3) was able to verify the hydride positions.28,30,48 Given the refined SCXRD data and the deep-learning predictions, DFT-optimized structures are in good agreement, confirming the positions of hydrides ( Supporting Information Figure S4). Nine hydride ligands in [ Cu20H9]11+ are grouped into three types of coordination modes (Figure 3a), of which two are μ4-H ligands, six are μ5-H ligands. and one is in interstitial μ6-H location ( Supporting Information Figure S5a). The 1H nuclear magnetic resonance (NMR) of Cu20H9 shows peaks of μ4-H and μ6-H at 3.91 ppm and 10.69 ppm in 2:1 ratio, respectively. Unfortunately, the signals of six μ5-H ligands in 6.29–9.45 ppm overlapped with aromatic pyridine signals from Tf-dpf ligands, which are difficult to identify ( Supporting Information Figure S6). NMR simulation with the gage independent atomic orbital (GIAO) method on Cu20H9 displays similar 2:1 hydride signals at 4.11 ppm for two μ4-H and 11.03 ppm for one μ6-H (Figure 3c), while the six μ5-H ligands were calculated to be at 7.32, 6.80, and 7.98 ppm, respectively ( Supporting Information Figure S7).

    Figure 3

    Figure 3 | Probability distribution of hydride occupation in possible locations as predicted by convolutional neural network: (a) Cu20H9; (b) Cu20H8. Insets show the probabilities of the top-ranked hydride locations in the clusters. Color legend: Cu, blue; hydride, white. The 1H NMR spectrum for Cu20H9 (c) and Cu20H8 (d) in acetonitrile-d3. The signals of hydrides are marked according to different coordination modes: μ4-H, green; μ5-H, yellow; and μ6-H, red.

    In the [Cu20H8]12+ core of Cu20H8, eight hydrides are also grouped into three types, of which four are interfacial μ4-H ligands, two are μ5-H ligands and the other two are in interstitial μ6-H locations ( Supporting Information Figure S5b). The 1H NMR of Cu20H8 has three hydride peaks 4.38, 5.01, and 5.36 ppm in 2:1:1 ratio for the μ4-H, μ5-H and μ6-H, respectively (Figure 3d and Supporting Information Figure S8). NMR simulation with the GIAO method on Cu20H8 shows similar hydride signals at 4.53–5.33 ppm ( Supporting Information Figure S9).

    Optical properties and time-dependent DFT simulations

    The absorption spectra of these two clusters are shown in Figure 4a. Significantly different in their spectra, they demonstrate that a small change in the composition makes a big difference in electronic structures.49,50 Cu20H9 and Cu20H8 show similar absorption bands below 400 nm, whereas Cu20H8 displays a prominent peak at 570 nm, and no corresponding absorption in Cu20H9 is observed. Moreover, these two clusters are different in color: Cu20H8 is green, and Cu20H9 is orange in MeOH or CH2Cl2. The energy-scale absorption spectra of Cu20H9 and Cu20H8 are shown in Supporting Information Figure S10. The optical gap for Cu20H8 is determined to be 1.65 eV by extrapolating the absorbance to zero, which is much smaller than 2.68 eV of Cu20H9.

    Figure 4

    Figure 4 | (a) UV–visible absorption spectra of Cu20H9 (black) and Cu20H8 (red) in MeOH. Insets are the photo of the MeOH solution of Cu20H9 and Cu20H8, and enlarged absorption spectra in the range from 500 to 700 nm of Cu20H9 and Cu20H8; (b) TD-DFT-simulated UV–vis spectrum of the [Cu20H8(dpf)10]2+ model cluster in comparison with the experiment for Cu20H8. Inset: orbitals involved in the α transition of Cu20H8; Kohn-Sham molecular orbital energy levels diagram, the associated populations of atomic orbitals in each KS molecular orbital, and frontier orbitals of HOMO and LUMO states for [Cu20H9(dpf)10]+ (c) and [Cu20H8(dpf)10]2+ (d).

    To understand the origin of the absorption spectra of these two Cu20 clusters, we performed time-dependent density functional theory (TD-DFT) calculations ( Supporting Information Figure S11). The simulated spectra agree well with the experiment for both Cu20H8 (Figure 4b) and Cu20H9 ( Supporting Information Figure S12). For Cu20H8, the peak (α) at 1.80 eV corresponds to highest occupied molecular orbital (HOMO)→lowest unoccupied molecular orbital (LUMO) and HOMO-1→LUMO transitions (Figure 4b, inset), and the higher energy absorption band at 2.91 eV (β) mainly comprises HOMO-10/HOMO-3→LUMO+2 and HOMO-5→LUMO+1 transitions (Figure 4d; Supporting Information Table S3). For Cu20H9, the main peak at 3.03 eV (∼400 nm) arises from multiple transitions (Figure 4c and Supporting Information Table S4). One can see from the frontier Kohn–Sham orbitals and their atomic orbital contributions that the key difference between Cu20H8 and Cu20H9 is the lower lying LUMO in Cu20H8 primarily contributed by Cu atoms (Figure 4d), which is responsible for the low-energy absorption band around 570 nm (the α peak in Figure 4b), whereas the LUMO of Cu20H9 is mainly from ligand contributions (Figure 4c).

    Transformation

    Interestingly, we found that Cu20H9 can be readily converted to Cu20H8 as triggered by protic solvents such as methanol, which we monitored with optical absorption spectroscopy.44,51,52 The absorption spectrum of Cu20H9 in methanol begins to change after only 10 min: the absorptions below 400 nm decrease, and the absorption at 570 nm increases gradually (Figure 5a–c). After 5 h, the solution color changed from orange to brown-green with the absorption spectrum exhibiting a very similar profile to that of Cu20H8. These changes indicate that Cu20H9 has been gradually converted into Cu20H8. ESI–MS further supports this transformation ( Supporting Information Figure S13). Because of the structural similarity of these two structures, in the conversion from Cu20H9 to Cu20H8, the cluster needs only to slightly adjust its structure to release a hydride. The releasing hydride then combines with H+ from solvent to form H2 just as what happened in other reported copper clusters.35 One more BF4− is included in Cu20H8 for the sake of charge balance. However, the reverse conversion was not successful under various investigated conditions, due to the lack of an additional hydride. Moreover, there is no observable change in the UV absorption spectra of Cu20H8 in methanol and dichloromethane within a week, which indicates that Cu20H8 is quite stable under ambient conditions ( Supporting Information Figure S14).

    Figure 5

    Figure 5 | (a) Schematic diagram of the conversion from Cu20H9 to Cu20H8; (b) time-dependent UV–vis absorption spectra of Cu20H9 in MeOH at 30 °C under N2; (c) photographs of the Cu20H9 before and after the transformation in methanol.

    Catalytic properties

    Since these two closely related Cu20 clusters show distinctly different electronic structures and stability, we were interested in exploring their structure-property correlation in terms of catalytic hydrogenation reactions. We explored the catalytic activity of these two Cu20 clusters in the conjugate reduction of cinnamaldehyde, which have been used extensively to evaluate copper hydride catalysts.53,54

    To our surprise, Cu20H8 exhibits extremely higher catalytic activity than Cu20H9. Cinnamaldehyde can be reduced to 3-phenylpropanal in 96.7% yield with 0.2 mol % Cu20H8 catalyst within 4 h, whereas only 3.7% yield was observed under the same conditions when Cu20H9 was used as the catalyst. Prolonging the reaction time (12 h) or increasing the amount of Cu20H9 catalyst (1 mol %, 5 mg) hardly improved the yield. Control experiments with 1 mg Cu(MeCN)4BF4 Cu11H3(Tf-dpf)6(OAc)2 or [Cu12H3(Tf-dpf)6(OAc)2] · OAc40 as the catalyst gave only limited yield (Figure 6a).

    Figure 6

    Figure 6 | (a) Selective catalytic reduction of cinnamaldehyde. aThe selectivity and conversion were determined by NMR analysis; (b) the ESI-MS of used Cu20H8 in positive mode. Inset: the simulated (red) and experimental (black) and isotropic patterns of after-catalytic Cu20H8; DFT structures and energetics of one Tf-dpf− ligand (highlighted in blue) dissociating from Cu20H9 (c) or Cu20H8 (d).

    To investigate the cause of the different catalytic performance, we separated the cluster species from the solution after catalysis. The reaction mixture was evaporated to dry, and organic species were washed away by dichloromethane (DCM) to obtain the solid of used cluster catalysts due to their poor solubility in DCM. It should be noted that Cu20H9 survives in the catalytic environment because of the reduction environment presented by Ph2SiH2. The optical absorption spectra of both Cu20 clusters in methanol show no change after catalytic reactions ( Supporting Information Figure S15), indicating that the frameworks of these two clusters remain intact under the catalytic reaction conditions.

    The compositions of two used cluster catalysts have been studied with ESI-MS. The used Cu20H8 shows a prominent peak at m/z = 2184.6 (Figure 6b), corresponding to the [Cu20H8(Tf-dpf)9(EtO)(EtOH)]2+, which is derived from Cu20H8 via releasing a Tf-dpf− ligand and adding a EtO− anion and an ethyl alcohol (EtOH) molecule. The weak peak at m/z = 4456.2 is attributed to [Cu20H9(Tf-dpf)9(EtO)(C9H8O)]+, corresponding to a species with the leaving of a Tf-dpf− ligand and the adding of a EtO− anion, a cinnamaldehyde molecule, and a hydride in Cu20H8. Ph2SiH2 releases a hydride and then takes an EtO− anion to form the Ph2SiH(EtO) species.55 Moreover, a prominent peak at m/z = 333.1 corresponding to the leaving Tf-dpf− ligand was found in the negative mode ESI-MS ( Supporting Information Figure S16).

    These results indicate that a ligand dissociation process occurred in the reduction reaction catalyzed by Cu20H8. In contrast, the ESI-MS of Cu20H9 after catalysis showed no new peak in addition to the expected peak at m/z = 4612.2 belonging to the parental ion, indicating that no ligand dissociation occurs with Cu20H9 as the catalyst. Therefore, the ligand dissociation is the key factor of their different catalytic activities toward the reduction of cinnamaldehyde. To confirm this finding, DFT calculations were performed to compare the dissociation energetics of a Tf-dpf− ligand in the two Cu20 clusters (Figure 6c,d). By comparing and determining the first or easiest ligand to dissociate (highlighted in blue in Figure 6c,d), we found that it is 0.3 eV harder to dissociate the ligand from Cu20H9 than from Cu20H8, supporting the ESI-MS observation that the relative ease of ligand dissociation on Cu20H8 allows it to expose the surface Cu sites for the hydrogenation catalysis. The combined UV–vis, ESI-MS, and DFT results show that Cu20H9 and Cu20H8 have only one hydride difference in compositions but show remarkably different catalytic performance because the ligand-metal binding strength is changed.

    Conclusion

    In summary, we have isolated a pair of copper hydride clusters, Cu20H9 and Cu20H8. Both clusters have twenty copper atoms and ten amidinate ligands, but the number of hydrides are 9 and 8, respectively. This one hydride difference leads to quite different geometric and electronic structures, resulting in their different optical and catalytic properties. Cu20H8 shows 25 times higher catalytic activity than Cu20H9 in the conjugate reduction of cinnamaldehyde, which is due to the easier dissociation process of a Tf-dpf− ligand in Cu20H8, as confirmed by ESI-MS and DFT calculations. The fact that these two closely related clusters show distinctly different properties demonstrates that even one hydride matters. The structure–property correlation in this work provides insightful guidance in terms of design and synthesis of high-performance copper hydride catalysts.

    Supporting Information

    Supporting Information is available and includes the experimental instruments and computational details, DFT calculation results, crystallographic information, and characterizations. The X-ray crystallographic coordinates for structures reported in this article (see Supporting Information Table S1) have been deposited at the Cambridge Crystallographic Data Centre (CCDC) under deposition numbers 2169810 and 2169811. These data can be obtained free of charge from the CCDC via http://www.ccdc.cam.ac.uk/data_request/cif

    Conflict of Interest

    There is no conflict of interest to report.

    Funding Information

    This work was financially supported by the National Natural Science Foundation of China (grant nos. 91961201 and 21973116). F.H. acknowledges financial support from the Beijing Natural Science Foundation (grant no. 2234087) and the China Postdoctoral Science Foundation (grant nos. 2023T160357 and 2022M721797).

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