Analogues Copper Nanoclusters (Cu16/17) with Two Electron Superatomic and Mixed Valence Copper(II)/Copper(I) and Copper(I)/Copper(0) Characters†

The reported copper nanoclusters (Cu NCs) of either CuII or CuI or mixed valence (MV) CuII/CuI or CuI/Cu0 characters are seen to be stabilized with a discrete set of ligand donors; hence, the analogues Cu NCs with a common architecture supported by same or nearly same donor set that exhibits different MV states of Cu, such as CuII/CuI and CuI/Cu0, are unknown. Such a series of highest nuclearity copper clusters supported by aromatic thiol-S donor ligands, like [(L4)12CuI15CuII(4-S)](PF6)3 (1), [(MeL4)12CuI15Cu0(4-S)]ClO4•8C7H8 (2) and [(L4)12CuI15Cu02)(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 (Ss) bridged tetracopper, SsCu4 core inside a sphere shaped Cu12S12 truncated octahedron. As 1 and 2 has a non metal (chalogen or halogen) central atom ( here Ss) instead of a metallic Cu core inside the Cu12S12 shell, these are of inverse coordination complex (ICC) category, rather than superatomic with a core-shell (core is metal and shell is metal-ligand framework) structure. The NC 1, in presence of polar solvents converts to a two electron superatomic Cu17 NC, 3. The NC 3 features a trigonal pyramidal shaped Cu4 core inside the 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 shifting of a Cu atom of SsCu4 unit to the Cu12S12 shell, resulting 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 aromatic thiol-S donor ligand and (iii) the spontaneous ICC (i.e. 1)  superatomic NC (i.e. 3) conversion that does not require any reducing agent rather occurs in presence of dioxygen oxidant. The probable mechanisms for the reversible 13 conversions have been discussed. The presence of Ss in 1 and 2 unveils the first evidence of the benzene thiol C-S bond clavage, to the best of our knowledge. The spectroelectrochemical studies shed light onto the choice of CuII/CuI and CuI/Cu0 character of 1 and 2 respectively which are supported by the high resolution XPS and Cu LMM Auger spectroscopy.