Searching for stable copper borozene complexes in CuB₇^- and CuB₈

Copper has been shown to be an important substrate for the growth of borophenes. Copper-boron binary clusters are ideal platforms to study the interactions between copper and boron, which may provide insight about the underlying growth mechanisms of borophene on copper substrates. Here we report a joint photoelectron spectroscopy and theoretical study on two copper-doped boron clusters, CuB₇^- and CuB₈^-. Well resolved photoelectron spectra are obtained for the two clusters at different wavelengths and are used to understand the structures and bonding properties of the two CuB^- clusters. We find that CuB₈^- is a highly stable borozene complex, which possesses a half-sandwich structure with a Cu⁺ species interacting with the doubly aromatic η⁸-B₈^2- borozene. The CuB₇^- cluster is found to consist of a terminal copper atom bonded to a double-chain B₇ motif, but it has a low-lying isomer composed of a half-sandwich structure with a Cu⁺ species interacting with an open-shell η⁷-B₇^2- borozene. Both ionic and covalent interactions are found to be possible in the binary Cu-B clusters, resulting in different structures.