Role of water in molecular oxygen activation in hydrated chromium(I) cluster ions: A theoretical insight

Abstract

The monovalent Cr(I) ion is unstable and may only be formed transiently in aqueous solution. By contrast, the bare Cr+ and hydrated clusters [Cr(H2O)m]+ can be isolated in the gas phase. Many experimental works on their reactions toward molecular O2 in the gas phase have been performed aiming to understand their intrinsic redox properties in the absence of extensive solvation in bulk solution. In this work, the microsolvation structures of [Cr(H2O)m]+ and [CrO2(H2O)n]+ have been examined theoretically using density functional theory at the M06/6-31++G(d,p) level. The structures of [Cr(H2O)m]+ contain a linear, strongly bound di-coordination ionic core [Cr(H2O)2]+, which could be rationalized with the typical sd hybridization. Together with an additional of two to four weaker bound water molecules in the first solvation shell forms overall tetra-, penta- and hexa-coordination geometries, which are fluxional and close in energy. Mechanistic studies on the reaction of [Cr(H2O)m]+ toward O2 have revealed different activated forms of O2, including the dioxygen peroxo complex [Cr(2η-O2)(H2O)n]+ and hydroperoxo complex [Cr(2η-OOH)(OH)(H2O)n-1]+, preferring penta-coordination, and the OO bond cleaved oxo complex [Cr(O)(OH)2(H2O)n-1]+, preferring hexa-coordination. This reaction can proceed either on the quartet surface or via the spin crossing between the quartet and doublet surfaces, both facing similar energy barriers. Increase of cluster size n raises these barriers as hydration stabilizes the dioxygen complex more than the OO bond cleaved complex. As a consequence, while water loss is the dominant process at large n, as clusters shrink the barrier to the OO bond cleavage and the water evaporation energy can become comparable. Hence, [Cr(O)(OH)2(H2O)n-2]+ can only be formed at small n, in agreement with previous experimental observations. This work suggests that the proton transfer in [Cr(2η-O2)(H2O)n]+ to form [Cr(OOH)(OH)(H2O)n-1]+ and its subsequent OOH bond cleavage are retarded by a more completed second solvation shell at the initial [Cr(2η-O2)(H2O)4]+ core, demonstrating the subtle role of water in molecular oxygen activation by the low-valent Cr+.