Masked Reactivity of Hydrated Clusters of Monovalent Manganese Ions: Water Insertion versus Nitrous Oxide Activation–A Density Functional Theory Investigation

Previous mass spectrometric (MS) studies demonstrated that singly charged hydration clusters of manganese ions [Mn(H₂O)_n]⁺ were, on one hand, highly reactive toward intracluster water insertion but, on the other hand, inert toward nitrous oxide activation. This contrast in reactivity has been rationalized by our present theoretical investigation for the interconversion between the pristine Mn(I) monovalent form as a monatomic ion in [Mn^I(H₂O)_n]⁺ and the oxidized Mn(III) trivalent form as a hydride–hydroxide in [HMnIIIOH(H₂O)_n−1], as well as their reactivity toward nitrous oxide activation. Our theoretical interpretations are supported with quantum chemical calculations based on density functional theory (DFT), performed systematically for the cluster-size range of n = 1 – 12. Our DFT results show that water insertion is kinetically and thermodynamically favorable for n ≥ 8, suggesting [HMn^IIIOH(H₂O)_n−1]⁺ is the predominant form, as observed in previous MS experiments. While [Mn^I(H₂O)_n]⁺ is capable of N₂O reduction, the process of which is highly exothermic, similar reactions are unfavorable with [HMn^IIIOH(H₂O)_n−1]⁺, which can only form weakly bound adducts with N₂O. This work demonstrates the masking effect of water molecules over the high reactivity of the hydrated Mn(I) center and sheds light on the potential roles of water in transition metal systems. © 2024 American Society for Mass Spectrometry.