Ferromagnetic Polarization: The Quantum Picture of Switching On/Off Single-Molecule Magnetism

The mixed 3d–4f
pentanuclear complex (Bu4N)­[MnIII4YIII(shi)4(OAc)4(CH3OH)4]·CH3OH·H2O (1) (H3shi = salicylhydroxamic acid)
was synthesized by the direct reaction of Y­(NO3)3·6H2O, Mn­(OAc)2·4H2O,
and H3shi. When an additional ligand, (NHBu3)3[W­(CN)8]·2H2O, was added,
the mixed 3d–4f–5d hexanuclear complex (Et4N)5[MnIII4YIII(shi)4(OAc)4WV(CN)8]­(WO4)0.5 (2) was obtained. X-ray crystallographic
analysis shows that the 3d–4f complex 1 represents
a 12-metallacrown-4 (12-MC-4) structure, in which the metallacrown
ring [Mn–N–O]4 connection captures one YIII ion with four bridging acetate anions, completing the eight-coordinated
environment around YIII ion, while four methanol molecules
each coordinate to the MnIII ions on the other side of
the YIII ion. After octacyanotungstate is introduced, the
[WV(CN)8] group substitutes for four methanol
molecules of 1 to form complex 2. Magnetic
studies indicate the overall antiferromagnetic coupling present within
the MC ring of complex 1. However, interestingly, the
dominant ferromagnetic coupling between MnIII ions was
observed in complex 2. A susceptibility analysis shows
that the natural spin alignments in 12-MC-4 metallacrowns are tuned
from overall antiferromagnetic to dominant ferromagnetic fashions
by magnetic coupling between MnIII ions and the WV ion. Complex 1 [MnIII4YIII] retains an S = 0 ground state, and complex 2 [MnIII4YIIIWV] shows obvious single-molecule magnet (SMM) behavior with an ST = 11/2 ground state, respectively, before
and after introduction of the octacyanotungstate group. The spin frustration
geometrical structure constructed by four MnIII ions and
one WV ion was considered as the key factor for switching
on the SMM properties of the 12-MC-4 system.