Photochemical Self-Assembly Reaction of β-[Mo 8 O 26 ] 4- to Mixed-Valence Cluster [Mo 37 O 112 ] 26- in Aqueous Media

Prolonged photolysis of β-[Mo 8 O 26 ] 4 - in the presence of electron-donative MeOH in aqueous solutions at pH 3.3 provides stepwise self-organization to paramagnetic reddish-brown mixed-valence species [H 14 Mo V 24 Mo VI 13 O 112 ] 12 - . The X-ray structural analysis of the reddish brown crystals, isolated as [NMe 4 ] + salt, shows that [H 14 Mo V 24 Mo VI 13 O 112 ] 12 - anion consists of a central T d [H 10 (Mo V 12 O 40 ) (Mo VI O 2 ) 3 ] 4 - core sandwiched by two [H 2 Mo V 6 Mo VI 5 O 33 ] 4 - ligands, with overall symmetry of D 2 d . The central {Mo V 12 O 40 } subcore (with six short Mo V - Mo V contacts, 2.56(1) and 2.625(7) Å) is a tetrahedral ε-Keggin structure with a cavity (in the size of ∼3.3 Å) accommodating four protons and is stabilized by capture of four {Mo VI O 2 } groups with 3/4 occupancy. The {Mo V 6 Mo VI 5 O 33 } framework as the ligand consists of two sets of a dinuclear {Mo V 2 O 4 } pair with a Mo V ···Mo V distance of 2.567(7) Å, two sets of the edge-shared {Mo VI 2 O 10 } octahedra, and three mononuclear groups of one Mo VI O and two Mo V O 2 . The Mo V atoms in the two Mo V O 2 groups in the ligand are separated with a distance of 6.31(1) Å, being ESR-active (〈 g 〉 = 1.909, g ⊥ = 1.894, and g ∥ = 1.939 at 77 K). The structure of the [H 14 Mo V 24 Mo VI 13 O 114 ] 12 - anion is slightly different from the diamagnetic species {[H 6 Mo V 12 O 40 (Mo V O 3 ) 4 ][H 3 Mo V 4 Mo VI 6 O 29 ][H 5 Mo V 6 Mo VI 5 O 31 ]} 1 4 - prepared by thermal reduction of [Mo 7 O 24 ] 6 - with N 2 H 4 ·H 2 SO 4, in both structure and mixed-valency of the ligand. The stepwise self-assembly processes from β-[Mo 8 O 26 ] 4 - to [H 14 Mo V 24 Mo VI 13 O 114 ] 12 - are discussed in terms of the photochemical multi- (six- and four-) electron reduction of β-[Mo 8 O 26 ] 4 -, which results in splitting into {Mo 4 }, {Mo 2 }, and {Mo 1 } fragments.