The title cluster was synthesized by reducing a yellow mixture of Cu(CH3COO)2•2H2O, 2,4-dimethylbenzenethiol (RSH), PPh3 and Ph4PBPh4 in CH2Cl2 with aqueous NaBH4 (see detailed synthetic procedure in the SI). The as-synthesized bright yellow product was then purified by washing with methanol and water. And cuboid yellow crystals suitable for single-crystal X-ray diffraction (SCXRD) were grown from dichloromethane solution after about 7 days.
The title cluster crystallized in a triclinic unit cell with space group P\(a\stackrel{-}{3}\) (CCDC 2265075) with eight formula units of [Cu14H10(PPh3)8(SPhMe2)3]+. The cationic cluster is intrinsically chiral and exists as racemic pairs in the unit cell. As shown in Fig. 1&2, the cationic cluster consists of a fcc Cu14 core passivated by eight PPh3 and three thiolate ligands. Figure 3 is a layer-by-layer representation, as viewed normal to the C3 axis. Here Cua(1), Cub(3), Cuc(3), Cud(1) constitute the cube and Cue(3) and Cuf(3) are the face centers, as shown in Fig. 2a&3. The eight PPh3 are coordinated to the eight cubic Cu vertices (one each). Interestingly, one unique Cu atom, denoted as Cua, is discretely protruded outwards. As a result, the Oh symmetry is lowered to (idealized) C3 (Figs. 2 and 3). Note that each of the three µ3-S atoms not only are coordinated to the protruded vertex Cua, but also to one adjacent vertex, Cub, and one face center, Cue (see also Fig. 2b and c).
The Cu-Cu bonds distances are listed in Table S1, ranging from 2.42 Å to 2.97 Å (ave. 2.61 Å), longer than that of 2.56 Å in copper metal, indicating weak bonding interactions at best.32 A twist of the three thiolates by 26 degrees about the idealized C3, in either clockwise or counter-clockwise directions, endows the cluster with intrinsic chirality (see also Fig. S1, S2&S3), giving rise to racemic pairs of R and S enantiomers (Fig. S3a&b) in unit cell. The average Cu-P and Cu-S distances are 2.23, 2.34 Å respectively, which are comparable with cluster 1.
Comparing Cu fcc structures of clusters [CuHPPh3]6, 1, 2 and Zang’ works, it shows a progressive, gradual interactions of the outer Cu8 cube to the inner Cu6 core, shown as in Fig. S4. This gradually change highly indicates the formation process of the fcc cell: when steric 2-diphenyl-2-hydroxylmethylpyrrolidine-1-propyne was used in the synthesis, the 14-Cu skeleton were stretched as discrete core-shell26; while with Ph3P and monodentate thiol in this work, the core-shell compacted closer but left only one vertex discrete, and Ph3P encapsulated all the vertices; and finally, when suitable bidentate thiol was utilized in the synthesis, the standard fcc 14-Cu cell were obtained with acetonitrile coordinated to the cube vertices27. Further comparison with clusters 1 and 2, as shown in Fig. S5, it was found that cluster 2 showed weak intracluster C − H‧‧‧π interactions between PPh3 and thiolates around the protruded vertex, while cluster 1 showed severe steric hindrance with short C-H‧‧‧H-C distances leaving no space for 8th PPh3 added to the thiolated vertex Cu. It can be concluded that when suitably steric monodentate thiol and Ph3P were applied, all the vertices were compacted to form the closest fcc packing, and this explains the absence of one PPh3 in cluster 1 in comparison with cluster 2 (this work).Thus, the growth of the fcc 14-Cu core-shell formation in fcc clusters can be understood, at least qualitatively, although the underlying mechanism of the effects requires further investigation.
ESI-MS was carried out to determine the number of hydrides. The result, depicted in Fig. 4, suggested that this cluster was a mono-cationic species containing 10H, where a distinct peak in the positive ionization mode at m/z 3408.95 attributed to [Cu14H10(PPh3)8(SPhMe2)3]+. Following the strategy for investigation of the hydride locations in metal clusters33–36, we also synthesized the deuterated analogue of the cluster by using NaBD4 as the reducing agent. ESI-MS result of the deuteride cluster shows a 10-shift, which was caused by 10 D, giving the m/z 3419.09 attributed to [Cu14D10(PPh3)8(SPhMe2)3]+, denoted as Cu14D10 (abbrev. as 2D), (see Fig. S6).
To determine the locations of 10 Hs, analysis of bonds evaluation, 2H-NMR spectroscopy and single crystal XRD at low temperature were carried out33–35. The 2H-NMR spectrum (Fig. 5) of the cluster was recorded by an 850 MHz spectrometer, which clearly showed the signals different from CD2Cl2 with four distinct peaks with intensity ratios of 1:3:3:3. This strongly suggested the presence of ten D atoms in the cluster with four different environments, at least on the NMR time scale. These Hs are disposed in a three-fold symmetric arrangement: one on the C3 symmetry axis and three other groups (3 Hs for each group respectively), as shown in Fig. 5.
Taking the consideration of shorter Cu-Cu bonds evaluation (as shown in Fig. S7), symmetry and validation against the peaks obtained in the difference electron density map of the crystallographic data collected at low temperature, the locations of the 10 H were determined as follows. They are separated into four groups: the centered one (as Ho) adopts a µ6- coordination connected to the six face-center copper atoms (see Fig. S8a); three µ3-H (as Ha) facially coordinates one facial and two vertex Cu opposite to the thiolated cubic vertex, as shown in Fig. S8b; three µ4-H (as Hb) on the edge share two Cu with thiolates (see Fig. S8c); three µ4-H (as Hc) on the edge share one vertex Cu with thiolates (see Fig. S8d). In view of the fact that each fcc unit cell contains one octahedral hole and eight tetrahedral holes, the hydride locations may also be described as follows: Ho(1) is in the octahedral hole; Hb(3) and Hc(3) reside in six “butterfly” (µ4’) holes (which correspond to six of the eight tetrahedral holes); and Ha(3) are face caps. These assignments are fully consistent with the three recently reported empirical rules34–36, viz., the 2H-NMR peaks at 11.2, 5.2, 3.6, and 2.8 ppm can be assigned to µ6-Ho, µ4’-Hb, µ4’-Hc, and µ3-Ha, respectively. Here the decreasing downfield shifts correlate well with the decreasing degree of coordination. The Cu-H bonds distances are listed in Table S2.
In general, copper hydrides are active for hydrogenation catalysis37–40, we carried out hydrogenation catalytic reactions with different substances, unfortunately, cluster 1 irreversibly released Hs and transformed into amorphous nanoparticles. For example, when heated to 50 ℃, H2 was detected by gas chromatography.