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The mixed 3d–4f
pentanuclear complex (Bu<sub>4</sub>N)[Mn<sup>III</sup><sub>4</sub>Y<sup>III</sup>(shi)<sub>4</sub>(OAc)<sub>4</sub>(CH<sub>3</sub>OH)<sub>4</sub>]·CH<sub>3</sub>OH·H<sub>2</sub>O (<b>1</b>) (H<sub>3</sub>shi = salicylhydroxamic acid)
was synthesized by the direct reaction of Y(NO<sub>3</sub>)<sub>3</sub>·6H<sub>2</sub>O, Mn(OAc)<sub>2</sub>·4H<sub>2</sub>O,
and H<sub>3</sub>shi. When an additional ligand, (NHBu<sub>3</sub>)<sub>3</sub>[W(CN)<sub>8</sub>]·2H<sub>2</sub>O, was added,
the mixed 3d–4f–5d hexanuclear complex (Et<sub>4</sub>N)<sub>5</sub>[Mn<sup>III</sup><sub>4</sub>Y<sup>III</sup>(shi)<sub>4</sub>(OAc)<sub>4</sub>W<sup>V</sup>(CN)<sub>8</sub>](WO<sub>4</sub>)<sub>0.5</sub> (<b>2</b>) was obtained. X-ray crystallographic
analysis shows that the 3d–4f complex <b>1</b> represents
a 12-metallacrown-4 (12-MC-4) structure, in which the metallacrown
ring [Mn–N–O]<sub>4</sub> connection captures one Y<sup>III</sup> ion with four bridging acetate anions, completing the eight-coordinated
environment around Y<sup>III</sup> ion, while four methanol molecules
each coordinate to the Mn<sup>III</sup> ions on the other side of
the Y<sup>III</sup> ion. After octacyanotungstate is introduced, the
[W<sup>V</sup>(CN)<sub>8</sub>] group substitutes for four methanol
molecules of <b>1</b> to form complex <b>2</b>. Magnetic
studies indicate the overall antiferromagnetic coupling present within
the MC ring of complex <b>1</b>. However, interestingly, the
dominant ferromagnetic coupling between Mn<sup>III</sup> ions was
observed in complex <b>2</b>. 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 Mn<sup>III</sup> ions and the W<sup>V</sup> ion. Complex <b>1</b> [Mn<sup>III</sup><sub>4</sub>Y<sup>III</sup>] retains an <i>S</i> = 0 ground state, and complex <b>2</b> [Mn<sup>III</sup><sub>4</sub>Y<sup>III</sup>W<sup>V</sup>] shows obvious single-molecule magnet (SMM) behavior with an <i>S</i><sub>T</sub> = 11/2 ground state, respectively, before
and after introduction of the octacyanotungstate group. The spin frustration
geometrical structure constructed by four Mn<sup>III</sup> ions and
one W<sup>V</sup> ion was considered as the key factor for switching
on the SMM properties of the 12-MC-4 system
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