Reduction of the incomplete cuboidal molybdenum(IV) aqua ion [Mo3(µ3-O)(µ-O)3(OH2)9]4+ by Eu2+. Characterisation of the trinuclear molybdenum(III) and mixed-valence molybdenum(III,III,IV) aqua ions by oxygen-17 nuclear magnetic resonance spectroscopy

Abstract

Reduction of ¹⁷O-enriched samples of [Mo₃(µ₃-O)(µ-O)₃(OH₂)₉]⁴⁺ by Eu²⁺ in toluene-p-sulphonic acid (Hpts) solution has allowed an evaluation of the solution molecular structures of the trinuclear molybdenum(III) and the mixed-valence molybdenum(III,III,IV) reduction products by¹⁷O n.m.r. spectroscopy. An analysis of chemical shifts and peak integrations indicates the structure [Mo₃(µ-OH)₄(OH₂)₁₀]⁶⁺ for Mo^III,III,IV delocalised on the n.m.r. time-scale at 298 K and two structures [Mo₃(µ₃-OH)(µ-OH)₃(OH₂)₉]⁵⁺(II) and [Mo₃(µ-OH)₄(OH₂)₁₀]⁵⁺[Mo₃(µ-OH)₄(OH₂)₁₀]⁵⁺(III) for trinuclear Mo^III in an [H⁺]-dependent equilibrium, (III) being favoured at high acidity. Shifts in the energy of the band maximum of Mo^III,III,IV at 1 050 nm in media of various dielectric constants together with a successful analysis of the band profile by the method of Hush support its assignment to an intervalence charge-transfer transition. Earlier electrochemical data on [Mo₃O₄]⁴⁺ and trinuclear Mo^III have been re-evaluated with respect to these solution structures. Redox interconversions between the various structures are summarised.