Reactivity of CuI and CuBr toward Et₂S: a Reinvestigation on the Self-Assembly of Luminescent Copper(I) Coordination Polymers

CuI reacts with SEt₂ in hexane to afford the known strongly luminescent 1D coordination polymer [(Et₂S)₃{Cu₄(μ₃-I)₄}]_n (1). Its X-ray structure has been redetermined at 115, 235, and 275 K in order to address the behavior of the cluster-centered emission and is built upon Cu₄(μ₃-I)₄ cubane-like clusters as secondary building units (SBUs), which are interconnected via bridging SEt₂ ligands. However, we could not reproduce the preparation of a coordination polymer with composition [(Et₂S)₃{Cu₄(μ₃-Br)₄}]_n as reported in Inorg. Chem. 1975, 14, 1667. In contrast, the autoassembly reaction of SEt₂ with CuBr results in the formation of a novel 1D coordination polymer of composition [(Cu₃Br₃)(SEt₂)₃]_n (2). The crystal structure of 2 has been solved at 115, 173, 195, and 235 K. The framework of the luminescent compound 2 consists of a corrugated array with alternating Cu(μ₂-Br)₂Cu rhomboids, which are connected through two bridging SEt₂ ligands to a tetranuclear open-cubane Cu₄Br₄ SBU, ligated on two external Cu atoms with one terminal SEt₂. The solid-state luminescence spectra of 1 and 2 exhibit intense halide-to-metal charge-transfer emissions centered at 565 and 550 nm, respectively, at 298 K. A correlation was also noted between the change in the full width at half-maximum of the emission band between 298 and 77 K and the relative flexibility of the bridging ligand. The emission properties of these materials are also rationalized by means of density functional theory (DFT) and time-dependent DFT calculations performed on 1.