On the structures and bonding of copper boride nanoclusters, Cu2Bx– (x = 5–7)

Borophenes, a new class of synthetic 2D nanomaterials, are usually grown on coinage metal surfaces because of the weak boron-metal bonding. Coinage metal and boron binary clusters provide ideal systems to examine the bonding and interactions between coinage metals and boron, which are important to understand and control the growth of borophenes. Here we report a photoelectron spectroscopy and quantum chemical investigation on a series of di-copper boride clusters, Cu2Bx− (x = 5–7). Vibrationally resolved photoelectron spectra are measured for the binary cupper-boron clusters and are combined with quantum calculations to probe their structures and bonding. We find that all three clusters exhibit elongated ribbon-like structures consisting of a double-chain Bx motif and two terminal Cu–B covalent bonds, similar to the corresponding di-hydride (H2Bx−) and di-auride (Au2Bx−) clusters. The surprising covalent bonds between Cu and boron in these clusters are shown to be dictated by the high stability of the double-chain Bx motifs in this size range, whereas ionic bonding between Cu and boron is expected to take place in larger boron motifs.