Anomalous Stoichiometry and Antiferromagnetic Ordering for the Extended Hydroxymanganese(II) Cubes/Hexacyanometalate-Based 3D-Structured [MnII 4 (OH)4 ][MnII (CN)6 ](OH2 )6 ⋅H2 O

Abstract

The reaction of Mn^II (O₂ CMe)₂ and NaCN or LiCN in water forms a light green insoluble material. Structural solution and Rietveld refinement of high-resolution synchrotron powder diffraction data for this unprecedented, complicated compound of previously unknown composition revealed a new alkali-free ordered structural motif with [Mn^II ₄ (μ₃ -OH)₄ ]⁴⁺ cubes and octahedral [Mn^II (CN)₆ ]^4- ions interconnected in 3D by Mn^II -N≡C-Mn^II linkages. The composition is {[Mn^II (OH₂ )₃ ][Mn^II (OH₂ )]₃ }(μ₃ -OH)₄ ][Mn^II (μ-CN)₂ (CN)₄ ]⋅H₂ O=[Mn^II ₄ (μ₃ -OH)₄ (OH₂ )₆ ][Mn^II (μ-CN)₂ (CN)₄ ]⋅H₂ O, which is further simplified to [Mn₄ (OH)₄ ][Mn(CN)₆ ](OH₂ )₇ (1). 1 has four high-spin (S=5/2) Mn^II sites that are antiferromagnetically coupled within the cube and are antiferromagnetically coupled to six low-spin (S=1/2) octahedral [Mn^II (CN)₆ ]^4- ions. Above 40 K the magnetic susceptibility, χ(T), can be fitted to the Curie-Weiss expression, χ ∝(T-θ)^-1 , with θ=-13.4 K, indicative of significant antiferromagnetic coupling and 1 orders as an antiferromagnet at T_c =7.8 K.

Keywords: Mn; antiferromagnet; crystal structure; magnetic properties.