Intriguing I2 Reduction in the Iodide for Chloride Ligand Substitution at a Ru(II) Complex: Role of Mixed Trihalides in the Redox Mechanism

The compound [Ru­(CNtBu)4­(Cl)2], 1, reacts with I2, yielding the halogen-bonded (XB) 1D species {[Ru­(CNtBu)4­(I)2]·I2}n, (2·I2)n, whose building block contains I– ligands in place of Cl– ligands, even though no suitable redox agent is present in solution. Some isolated solid-state intermediates, such as {[Ru­(CNtBu)4­(Cl)2]·2I2}n, (1·2I2)n, and {[Ru­(CNtBu)4­(Cl)­(I)]·3I2}n, (3·3I2)n, indicate the stepwise substitution of the two trans-halide ligands in 1, showing that end-on-coordinated trihalides play a key role in the process. In particular, the formation of ClI2– triggers electron transfer, possibly followed by an inverted coordination of the triatomic species through the external iodine atom. This allows I–Cl separation, as corroborated by Raman spectra. The process through XB intermediates corresponds to reduction of one iodine atom combined with the oxidation of one coordinated chloride ligand to give the corresponding zerovalent atom of I–Cl. This redox process, explored by density functional theory calculations (B97D/6-31+G­(d,p)/SDD (for I and Ru atoms)), is apparently counterintuitive with respect to the known behavior of the corresponding free halogen systems, which favor iodide oxidation by Cl2. On the other hand, similar energy barriers are found for the metal-assisted process and require a supply of energy to be passed. In this respect, the control of the temperature is fundamental in combination with the favorable crystallizations of the various solid-state products. As an important conclusion, trihalogens, as XB adducts, are not static in nature but are able to undergo dynamic inner electron transfers consistently with implicit redox chemistry.