A highly-flexible cyclic-decavanadate ligand for interconversion of dinuclear- and trinuclear-cobalt(ii) and manganese(ii) cores

Abstract

The structure transformation of multinuclear-metal-cores can change catalytic, optical, and magnetic properties. Cyclic decavanadate ligands exhibit versatility in the number and the direction of the coordination sites by changing the conformation to stabilize various multinuclear-metal-cores, while organic multinucleating ligands require specific design for each of the multimetal complexes due to their structure directing ability. The flexibility of cyclic decavanadate ligands is demonstrated here to achieve accommodation of dinuclear or trinuclear units by using the same ligand. The reaction of a dinuclear-cobalt-core-containing decavanadate [Co₂(H₂O)₂V₁₀O₃₀]⁶⁻ (Co2) with 1 equiv. of Co(OAc)₂ (OAc = acetate) gave a trinuclear-cobalt-core-containing decavanadate [Co₃(H₂O)(OAc)V₁₀O₃₀]⁵⁻ (Co3) in high yield. The central cobalt core exhibited an incomplete-cubane-type structure. The decavanadate ring contracts to accommodate a smaller dinuclear unit by taking a wavy conformation and expands to accommodate the larger trinuclear unit. The reverse reaction quantitatively proceeded by the addition of 5 equiv. of [VO₃]⁻ with respect to Co3. Although a trinuclear-manganese-core-containing decavanadate [Mn₃(H₂O)(OAc)V₁₀O₃₀]⁵⁻ (Mn3) possesses the same structure as that of Co3, the addition of 5 equiv. of [VO₃]⁻ yielded a different structure of a dinuclear-manganese-core-containing decavanadate [Mn₂V₁₀O₃₀]⁶⁻ (Mn2) with two cyclic pentavanadate ligands sandwiching the manganese core. Thus, the conformations of the cyclic decavanadates are rearranged to respond to the central metal core structures. EXAFS study suggests both manganese complexes maintain the molecular structure in solution. The simultaneous analyses of the magnetic susceptibility data and the magnetization data revealed the switch of magnetic interaction modes from ferromagnetic in dinuclear complexes to mixed ferromagnetic and antiferromagnetic interactions in trinuclear complexes: the ferromagnetic interaction in dinuclear units of Co2 (J = 8.05 cm⁻¹) and Mn2 (J = 0.76 cm⁻¹) (H_ex = −JS₁S₂), and the ferromagnetic and antiferromagnetic interactions in Co3 (J = −1.59 cm⁻¹ and J′ = 13.6 cm⁻¹) and Mn3 (J = −2.20 cm⁻¹ and J′ = 0.07 cm⁻¹) (H_ex = −JS_A1S_A2 − J′[S_A1S_B + S_A2S_B]) were studied.