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Three-Dimensional (3-D) Ferromagnetic Network of Mn12 Single-Molecule Magnets: Subtle Environmental Effects and Switching to Antiferromagnetic
journal contribution
posted on 2017-08-22, 19:17 authored by Adeline D. Fournet, Kylie J. Mitchell, Wolfgang Wernsdorfer, Khalil A. Abboud, George ChristouA new member of the
Mn12 family of single-molecule magnets (SMMs) has been
prepared and found to be the first of this family to give a 3-D ferromagnetic
network. [Mn12O12(O2CC6H4-p-F)16(H2O)4] (2) was prepared by carboxylate substitution on the acetate
derivative with p-F-benzoic acid and crystallizes
as 2·8MeCN in space group I4̅2m with extensive formation of intermolecular C–H···F
hydrogen-bonding. The latter leads to a combination of ferromagnetic
(F) and antiferromagnetic (AF) interactions and an overall F network
that gives a χMT value at low T that is abnormally high for an S = 10
ground state. 2·8MeCN undergoes solvent loss under
vacuum to 2, with a decrease in unit-cell volume of 17%,
primarily due to a 13% decrease in the c-axis. The
χMT vs T plot for 2 indicates a switch to a net AF network. Exposure to air
causes hydration to 2·3H2O, a concomitant
increase in unit cell volume, and a switch back to a F network. The
same conversion of 2·8MeCN to 2·3H2O can also be accomplished in one step rather than two steps,
by leaving crystals of the former exposed to air at ambient temperature
and pressure for 10 days, giving the same magnetic plots. Interestingly,
the desolvation/solvation processes cause Jahn–Teller isomerism
to occur, but the ratio of the faster-relaxing isomer to the normal
slowly relaxing one does not change monotonically. Single-crystal
micro-SQUID studies on 2·8MeCN show the expected
magnetization hysteresis loops for a SMM and a small exchange-bias
from the intermolecular interactions that is unexpectedly AF. Since
the micro-SQUID study only identifies interactions along the easy-axis
(z-axis) of the crystal, this is readily rationalized
as due to the Jz components
of the intermolecular interactions in 2·8MeCN being
net AF. The combined results offer useful insights into the degree
of sensitivity of the magnetic properties to small environmental perturbations.