Stabilization of Ni2+ dimers in hexacyano Mo6 cluster-based Prussian blue derivatives: experimental and theoretical investigations of magnetic properties

Abstract

Herein, two new octahedral molybdenum cyanide cluster compounds, namely [{Ni(NH₃)₆}₄{Ni₂(NH₃)₈}₁][Mo₆Br₆Q₂(CN)₆]₃·12H₂O, Q = S (1) and Se (2), have been synthesized as single crystals by slow diffusion of a solution of nickel chloride into aqueous ammonia solutions of a K₂Cs₂[Mo₆Br₆Q₂(CN)₆] molybdenum cyanide cluster-based compound. Both 1 and 2 were structurally characterized by single-crystal X-ray diffraction. They are isostructural and crystallize in the cubic system (Imm (no. 229); Z = 2, a = 18.147(1) Å, and V = 5976(1) ų and a = 18.188(2) Å and V = 6016(2) ų for 1 and 2, respectively). 1 and 2 are based on the association of [Mo₆Brⁱ₆Qⁱ₂(CN)^a₆]⁴⁻ (Q = S, Se) cluster anions with Ni²⁺ dimer-based cubic [Ni₂(NH₃)₈]⁴⁺ and octahedral [Ni(NH₃)₆]²⁺ cations. The structure is based on 2-fold interpenetrated [{Ni(NH₃)₆}₄{Ni₂(NH₃)₈}₁][Mo₆Br₆Q₂(CN)₆]₃ frameworks related to each other by [½, ½, ½] translation. The unit cell is based on a body-centered cubic framework of cubic [Ni₂(NH₃)₈]⁴⁺. The [Mo₆Brⁱ₆Qⁱ₂(CN)^a₆]⁴⁻ (Q = S, Se) cluster units are located in the middle of the edges and at the center of the faces of the cell. The [{Ni(NH₃)₆}]²⁺ cations are located at the center of the cubes of the a/2 edge. The dimers [Ni₂(NH₃)₈]⁴⁺ are stabilized by hydrogen bonds between the cyanide ligands of the cluster unit and the hydrogen atoms of the ammonia molecules. Both compounds exhibit a weak antiferromagnetic coupling within the [Ni₂(NH₃)₈]⁴⁺ dimer entities at low temperatures together with a paramagnetic behavior originating from the cations of the octahedral [{Ni(NH₃)₆}]²⁺ complexes.