Copper-hydride nanoclusters with enhanced stability by N- heterocyclic carbenes

Copper-hydrides have been intensively studied for a long time due to their utilization in a variety of technologically important chemical transformations. Nevertheless, poor stability of the species severely hinders its isolation, storage and operation, which is worse for nano-sized ones. We report here an unprecedented strategy to access to ultrastable copper-hydride nanoclusters (NCs), namely, using bidentate N-heterocyclic carbenes as stabilizing ligands in addition to thiolates. In this work, a simple synthetic protocol was developed to synthesize the first large copper-hydride nanoclusters (NCs) stabilized by N-heterocyclic carbenes (NHCs). The NC, with the formula of Cu31(RS)25(NHC)3H6 (NHC = 1,4-bis(1-benzyl-1H-benzimidazol-1-ium-3-yl) butane, RS = 4-fluorothiophenol), was fully characterized by high resolution Fourier transform ion cyclotron resonance mass spectrum, nuclear magnetic resonance, ultra-violet visible spectroscopy, density functional theory (DFT) calculations and single-crystal X-ray crystallography. Structurally, the title cluster exhibits unprecedented Cu4 tetrahedron-based vertex-sharing (TBVS) superstructure (fusion of six Cu4 tetrahedra). Moreover, the ultrahigh thermal stability renders the cluster a model system to highlight the power of NHCs (even other carbenes) in controlling geometrical, electronic and surface structure of polyhydrido copper clusters.

Interest for coordination chemistry of N-heterocyclic carbenes (NHCs) with coinage metal atoms has been increasing in materials research including complexes, nanoparticles and surfaces [21][22][23][24]. In recent reports, this class of ligands were successfully introduced in clusters [22]. Several works on gold NCs, such as Au3 [25,26], Au11 [27], Au13 [28,29], Au25 [30], and Au44 [31], demonstrated that NHCs could endow them with exciting stability, luminescence and reactivity. NHC-stabilized Ag and Cu clusters have also been prepared, in which no hydride was observed [32][33][34][35][36][37]. In our continuing interest in NHCstabilized nanomaterials, we wish to investigate: (1) Whether NHCs could be employed to stabilize large copper-hydride clusters, thus filling the gap of copper-hydride complexes with carbene ligands and copper nanoparticles; (2) whether Cu NCs stabilized by NHC would exhibit distinct structure, in comparison to those protected by other ligands (phosphines, thiols and alkynyls); (3) whether NHC-stabilized copper-hydride NCs would display unique properties, such as ultrahigh stability, thus could accelerate the understanding and functionalization of Cu nanomaterials at the molecular level.

Synthesis and characterizations of clusters Cu31H6 and Cu31D6
The synthesis of title cluster began with the preparation of bi-dentate NHC ligands. The benzimidazoles were reacted with benzyl chloride, followed by alkylation with a 1,4-dibromobutane to afford the bis-benzimidazolium salts 1b (Scheme 1, Figs. S1-S4 in the Electronic Supplementary Material (ESM)) [18]. The desired copper complex 1c was produced by the reaction of equivalent CuCl with 1b under basic condition (Figs. S5 and S6 in the ESM) [38]. The cluster was prepared in one-pot (see Experimental Section in the ESM for more details). In brief, complex 1c was reduced by excess NaBH4 in the presence of stoichiometric thiol to afford a clear orange solution (Figs. S7 and S8 in the ESM). After aging, purification, and further crystallization, bright orange crystals were obtained in the yield of 29.6% (based on Cu).
The sample was first characterized by FTICR-MS in positive mode. Cu31H6 features a signal at m/z 6,439.61 in its FTICR-MS spectra ( Fig. 1(a)), which corresponds to the [Cu31(NHC)3(RS)24H6] + ion (calcd m/z 6,439.63). It should be noted that a mass peak corresponding to [Cu31(RS)24(NHC)3H6(CH3CN)] + was also observed (Fig. S9 in the ESM). To further corroborate this composition, we prepared the deuteride analogue, namely, Cu31(RS)25(NHC)3D6 (hereafter abbreviated as Cu31D6), by using NaBD4 instead of NaBH4 as the reductant for the synthesis of the clusters. As shown in Fig. 1(a), there was an increase of exactly 6 Da for the [Cu31(RS)24(NHC)3H6] + peak as expected. The experimental UV/Vis spectrum of Cu31H6 in CH2Cl2 is shown in Fig. 1(b). We also recorded the 1 H and 13 C NMR spectra of Cu31H6 and Cu31D6 in CD2Cl2 as well as 2 H NMR spectrum of Cu31D6 in CH2Cl2 (Figs. S10-S19 in the ESM). The 6 deuterides display two distinct peaks at 2.68 and 3.94 ppm with intensity ratios of 1:1 ( Fig. 1(c)), corresponding to the signals at 2.51 and 3.80 ppm in the 1 H NMR spectrum of Cu31H6 (Fig. S10 in the ESM) in CD2Cl2. The appearance of two distinct peaks with intensity ratios of 1:1 strongly suggests the presence of 6 D atoms in the cluster, in two different environments.

Molecular structure of Cu31H6 and its hydride positions
The structure of the title cluster Cu31H6 was determined by Scheme 1 Preparation of NHC ligand 1b, Cu complex 1c and Cu31H6. single-crystal X-ray diffraction analysis at 100 K (see Table S1 in the ESM for crystallographic data). It crystalizes in the triclinic space group P1 _ . This revealed that each unit cell contains two independent clusters (Figs. S20-S22 in the ESM), each of which consists of 31 Cu atoms, 25 thiolates, 3 bidentate NHC and 6 hydrides, giving rise to the expected composition of Cu31(RS)25(NHC)3H6. Based on peaks in the difference electron density map of the crystallographic data, the positions of the 6 H (or D) atoms were determined and their coordinates were successfully least-squares refined [6,39].
The structure of individual Cu31H6 is shown in Fig. 2. It has an oblate shape (cf. Figs. 2(a) and 2(d)), in sharp contrast to most other Cu hydride clusters. Interestingly, the cluster exhibits a triangular architecture (see Figs. 2(b) and 2(e)). The metal framework conforms to an idealized C3 symmetry, with one unique Cu atom residing on the C3 axis ( Fig. S23 in the ESM). An anatomy of the inner layer reveals a concaved frequencytwo Cu18 triangle (cf. Figs. 2(c) and 2(f)), assembled from six tetrahedral Cu4 units via vertex-sharing. The six tetrahedral units can be divided into two types: Three of them are assembled into a Cu9 triangular superstructure (Figs. 2(c) and 2(f), rose colored), and remaining three inlay on each edge of the triangle (Figs. 2(c) and 2(f), lime colored). It is worthy noting that for the first time TBVS growth mode was observed in ligated Cu NCs, even the isolated Cu4 tetrahedral kernel has been rarely reported [40]. The discovery of molecular complexity in this cluster indicates certain generality of the supermolecular complexity could also exist in copper clusters [41]. The 12 Cu atoms in the outer layer can also be divided into two categories, according to the coordinating ligands (cf. Figs. 2(b) and 2(e)). They form two sets, three each, of dimers, bridged by thiolate and NHC ligands. As stated above, two kinds of Cu4 tetrahedral units are present in Cu31, with average Cu-Cu bond lengths of 2.835 Å (Figs. 2(c) and 2(f), rose colored) and 2.847 Å (Figs. 2(c) and 2(f), lime colored), respectively. These values are comparable to those reported for other related copper hydride clusters, suggesting significant Cu-Cu interactions within the tetrahedral units [42]. In contrast, the interactions between the otherlayer Cu atoms and those in the concave triangle are much weaker, as indicated by their long bond distance (av = 3.203 Å). The average Cu-Cu bond distances bridged by thiolate (pink colored) and NHC ligands (green colored) are measured to be 3.265 and 2.642 Å, respectively. All facial Cu atoms in the triangles were capped by thiolates, with two types of binding modes (μ3 and μ2). The Cu atoms bind with NHC ligands tightly, as suggested by the short Cu-C bond length (av = 1.910 Å).
As shown in Figs. 2(c) and 2(f), and Fig. S24 in the ESM, 6 equivalent hydride ligands are clustered into two groups. Each of these hydride ligands exhibit asymmetric μ4-coordination in Cu4 tetrahedra. In the central three tetrahedra, the hydrides are centered in the tetrahedral holes, with short Cu-H bond lengths (average 1.757 Å). The remaining three hydrides were encapsulated in other three tetrahedra with an average Cu-H bond length of 1.755 Å. Overall, this 1:1 arrangement of hydrides is fully consistent with the NMR spectral data.

DFT calculations for the electronic structure, hydride NMR shifts and optical spectra
We used DFT calculations to explain the electronic and optical properties and confirm the stability of the measured structure of Cu31H6. The analysis for the electronic ground state and optical spectra were done with GPAW software using a real-space grid (details given in the Experimental Section in the ESM). Relaxation of the structures did not affect the overall symmetry of the cluster. The metal core expanded slightly, which is typical for the used DFT approximation (the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional). Based on the superatom electron count [43], the cluster should not possess any clear superatomic nature, if assuming hydrides (H − ) in the core. This was confirmed by the electronic structure analysis shown in Fig. 3(a) that shows no occupied states localized into the metal core close to Fermi energy. The highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) energy gap of the cluster is 0.95 eV. The two highest occupied orbitals are localized mainly in the ligand layer as shown in Fig. 3(b). The LUMO state is the only one having weight in the metal core.
Calculated Bader charges of atoms are shown in Table 1 for the ligands, Cu-atoms and hydrides. The charges confirm the nature of hydrides inside the core as a negatively charged species (−0.288 |e|). Furthermore, the thiolates are clearly with-drawing electrons (−0.418 |e|) whereas NHC-ligands are positively charged (+0.347 |e|). Remarkably, the reaction energy to remove the hydrides from the core is +2.78 eV endothermic. This highlights the importance of the hydrides in stabilizing the structure energetically. The reaction energies were determined with respect to H2 molecular phase and relaxed cluster structures with and without hydrides based on the experimental starting structure.
We were also able to assign the measured hydride 1 H NMR shifts (2.51 and 3.80 ppm, Fig. S9 in the ESM) to the symmetry environments in the atomic structure (Figs. 2(c) and 2(f)). We calculated the shifts at two levels of theory using the program deMon2k (details in the ESM). The results show consistently (Table 2 below) that the "A-type" hydrides inside the purple  Cu4 tetrahedra in Figs. 2(c) and 2(f) have a smaller shift (are better magnetically shielded) as compared to "B-type" hydrides inside the green Cu4 tetrahedra in Figs. 2(c) and 2(f).
To analyse further the experimental observations, the optical absorption spectra were calculated for the relaxed hydride and non-hydride clusters using linear response time-dependent density functional theory (lr-TDDFT) as implemented in GPAW. The calculated optical absorption spectrum of the hydride cluster is in good agreement with the experimental spectrum as shown in Fig. 4. The first increase in the intensity of the spectrum of Cu31H6 is seen around 700 nm in the calculated spectrum which can be correlated with the similar turning point at 600 nm in the experimental spectrum ( Fig. 1(b)). The second change in the slope of the calculated spectrum is seen around 460 nm, below which there is a rapid increase in the intensity. A similar behavior is seen in the experimental spectrum below 400 nm. The couple of faint features observed in the calculated spectrum go well along with these intensity changes. Figure 4 also shows that the model cluster without hydrides yields a computed spectrum that extends well beyond 1,100 nm, which is not in agreement with the observed data. This again confirms the existence of the hydrides in the experiment.
Finally, molecular dynamics simulations were done for the cluster with and without hydrides to see whether the structure remain stable also at increasing temperatures, see Movies ESM1 and ESM2. Interestingly, neither of the structures show any major deformations at the room temperature during the 4.3-4.5 ps runs including heating. One plausible explanation is that the metal core is oblate which confirms with the optimal shape of a stable 6e superatom (hypothetical non-hydride Cu31 cluster). Based on calculations, it can be concluded that the observed structural symmetry of Cu31H6 is unique for dynamical stability both with and without the hydride atoms in the core. However, hydrides are needed to make the cluster energetically more stable and to reproduce the experimentally observed optical properties.

Stability of Cu31 in solution
The instability of copper-hydride NCs has limited their practical applications, which probably results from active nature of hydride species [5]. To our surprise, Cu31 is unusually stable. As shown in Fig. S26 in the ESM, there is no obvious color change upon heating solution of Cu31H6 at 50 °C in air for 6 h. 2 H NMR track of Cu31D6 showed that the two peaks of Cu31D6 exhibits no decomposition and no new peak formed in the whole process, indicating that the hydrides in Cu31 were strongly stabilized ( Fig. 5(a)). The high robustness of Cu31D6 in CHCl3 up to at least 12 h was suggested by 2 H NMR measurements (Fig. S27 in the ESM). Moreover, UV/Vis profile of Cu31H6 was practically unchanged during the heating process ( Fig. 5(b)), and the relative intensity of the peak at 377 nm as a function of heating time (Fig. 5(e), black curve) showed almost no degradation in the thermal measurement, again proving the ultra-high stability of Cu31. As comparison, allthiolate-stabilized [Cu61(S t Bu)26S6Cl6H14] + (Cu61) [44] deteriorates progressively in the heating period (Fig. 5(c)), and the relative absorbance intensity decreases gradually (Fig. 5(e), red curve). Furthermore, all-phosphine-protected Cu25H22((p-FPh)3P)12 (Cu25) [45] exhibited much worse stability (Fig. 5(c) and pink curve in Fig. 5(d)). We attribute the ultra-high stability of Cu31 to be the same reason as in NHC-protected gold NCs, namely, unusually strong metal-carbene bonds [28].

Conclusions
In conclusion, a one-pot synthetic strategy was developed to access to the first N-heterocyclic carbene-stabilized copperhydride nanoclusters. Metal framework of the cluster displays unprecedented "Cu4 tetrahedron-based vertex-sharing" growth mode, suggesting the generality of molecular superstructure in ligated copper NCs. DFT calculations confirm the role of hydrides to stabilize the structure energetically and to produce the measured optical absorption. We were also able to assign the measured hydride NMR shifts to symmetry environments in the observed structure. Moreover, we confirmed that the dynamical stability of the cluster is unique but is not strongly dependent on existence of hydrides. We hope that the ultrahigh thermal and oxidation stability reported in this article will trigger more interests in the synthesis, structure, and properties of high-nuclearity copper hydride nanoclusters involving carbenes. More work, including extending the synthetic method to more underlying ones, and systematic studying their reactivity is under investigation. Research Center (Xiamen University) for the help in the NMR studies. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made.
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