Three new MnII–[MoIII(CN)7]4− molecular magnets constructed from chiral bidentate chelating ligands

Abstract

Three new cyanide-bridged compounds {[Mn((S,S)-Dpen)]₃[Mn((S,S)-Dpen)(H₂O)][Mo(CN)₇]₂·4H₂O·4C₂H₃N}_n (1-SS), {[Mn((R,R)-Dpen)]₃[Mn((R,R)-Dpen)(H₂O)][Mo(CN)₇]₂·4.5H₂O·4C₂H₃N}_n (1-RR), and {[Mn(Chxn)][Mn(Chxn)(H₂O)_0.8][Mo(CN)₇]·H₂O·4C₂H₃N}_n (2) (SS/RR-Dpen = (S,S)/(R,R)-1,2-diphenylethylenediamine and Chxn = 1,2-cyclohexanediamine) have been successfully synthesized from the self-assembly reaction of the [Mo^III(CN)₇]⁴⁻ unit, the Mn^II ions, and two chiral bidentate chelating ligands. Single-crystal structure determinations show that compounds 1-SS and 1-RR containing ligands SS/RR-Dpen are enantiomers and crystallize in the chiral space group P2₁. On the other hand, compound 2 crystallizes in the achiral centrosymmetric space group P due to the racemization of the SS/RR-Chxn ligands during the growth of the crystals. Despite their different space groups and ligands, all three compounds exhibit similar framework structures consisting of cyano-bridged Mn^II–Mo^III two-dimensional layers separated by the bidentate ligands. The circular dichroism (CD) spectra have further demonstrated the enantiopure character of compounds 1-SS and 1-RR. Magnetic measurements revealed that all three compounds display ferrimagnetic ordering with similar critical temperatures of about 40 K. The chiral enantiomers 1-SS and 1-RR exhibit the magnetic hysteresis loop with a coercive field of about 8000 Oe at 2 K, which is by far the highest for all known Mn^II–[Mo^III(CN)₇]⁴⁻ magnets. Analyses of their structures and magnetic properties indicated that their magnetic properties depend on the anisotropic magnetic interactions between the Mn^II and Mo^III centers, which are closely related to the C–N–M bond angles.