Masked Reactivity of Hydration Clusters of Monovalent Manganese Ion: Water Insertion versus Nitrous Oxide Activation ― A DFT Investigation

Introduction: Previous studies performed with Fourier transform ion cyclotron resonance (FTICR) mass spectrometry (MS) demonstrated that singly-charged hydration clusters of manganese ion [Mn, (H2O)n]+ were, on one hand, highly reactive toward intra-cluster water insertion but, on the other hand, inert toward nitrous oxide activation.

Methods: 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 [MnI(H2O)n]+ and the oxidized Mn(III) trivalent form as a hydride–hydroxide in [HMnIIIOH(H2O)n-1]+ as well as their reactivity towards nitrous oxide activation.

Preliminary data: 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 [HMnIIIOH(H2O)n-1]+ being the predominant form as observed in previous FTICR-MS experiments. While [MnI(H2O)n]+ is capable of N2O reduction of which the process is highly exothermic, similar reactions are unfavorable with [HMnIIIOH(H2O)n-1]+, which can only form weakly bound adducts with N2O.

Novel aspect: The masking effect of H2O over the intrinsically quite high reactivity of Mn(I) reveals its potential roles in transition-metal catalysis.