Analogous copper nanoclusters (Cu16/17) with two electron superatomic and mixed valence copper(ii)/copper(i) and copper(i)/copper(0) characters

Abstract

The reported copper nanoclusters (Cu NCs) of either Cu^II or Cu^I or mixed valence (MV) Cu^II/Cu^I or Cu^I/Cu⁰ characters are found to be stabilized with a discrete set of ligand donors; hence, analogous Cu NCs with a common architecture supported by the same or nearly the same donor set that exhibit different MV states of Cu, such as Cu^II/Cu^I and Cu^I/Cu⁰, are unknown. Such a series of highest nuclearity copper clusters supported by aromatic thiol-S donor ligands, namely [(L4)₁₂Cu^I₁₅Cu^II(μ₄-S)](PF₆)₃ (1), [(^MeL4)₁₂Cu^I₁₅Cu⁰(μ₄-S)]ClO₄·8C₇H₈ (2) and [(L4)₁₂Cu^I₁₅Cu⁰₂(DMF)](PF₆)₃·C₂H₅OH·2C₇H₈ (3), where ^XL₄ = 2-((3-X-thiophen)-(2-yl-methylene)amino)-4-(trifluoromethyl)benzenethiol (X = H/Me), have been synthesized and their electronic structural properties have been examined and reported herein. The Cu₁₆ NCs, 1 and 2, feature a central sulfido-S (S^s) bridged tetracopper S^sCu₄ core inside a sphere-shaped Cu₁₂S₁₂ truncated octahedron. As 1 and 2 have a non-metal (chalcogen or halogen) central atom (here S^s) instead of a metallic Cu core inside the Cu₁₂S₁₂ shell, they are of the inverse coordination complex (ICC) category, rather than superatomic with a core–shell (the core is a metal and the shell is a metal–ligand framework) structure. The NC 1, in the presence of polar solvents, converts to a two electron superatomic Cu₁₇ NC, 3. The NC 3 features a trigonal pyramidal-shaped Cu₄ core inside a modified Cu₁₂S₁₂, i.e. Cu₁₃S₁₂ shell. The transformation of 1 to 3 may be visualized as the replacement of the central sulfido-S by an extra Cu atom (generated from decomposed molecules of 1) and the shifting of a Cu atom of the S^sCu₄ unit to the Cu₁₂S₁₂ shell, resulting in a Cu₁₃S₁₂ shell. The present work offers the first example of (i) an ICC that has Cu⁰ character (i.e.2), (ii) a superatomic Cu NC (i.e.3) stabilized by an aromatic thiol-S donor ligand and (iii) spontaneous ICC (i.e.1) → superatomic NC (i.e.3) conversion that does not require any reducing agent, but rather occurs in the presence of a dioxygen oxidant. The probable mechanisms for the reversible 1 ↔ 3 conversions have been discussed. The presence of S^s in 1 and 2 unveils the first evidence of benzene thiol C–S bond cleavage, to the best of our knowledge. The spectroelectrochemical studies shed light on the choice of Cu^II/Cu^I and Cu^I/Cu⁰ character of 1 and 2, respectively, which is supported by high resolution XPS and Cu LMM Auger spectroscopy.