Metallacryptate Single-Molecule Magnets: Effect of Lower Molecular Symmetry on Blocking Temperature

The structural characterization of complexes [MnII4MnIII22(pdol)12(OCH3)12(O)16(N3)6] (1) and [MnII4MnIII22(pdol)12(OCH3)12(O)16(OH)2(H2O)(OCH3)3]·ClO4·5CH3OH (2), where pdol2- is di-2-pyridyl methanediol,
reveals that each has a metallacryptand shell that encapsulates a manganese oxide core. Variable-temperature direct current magnetic susceptibility measurements on 2 indicate a paramagnetic ground
state that results from an overall antiferromagnetic interaction in the cluster, with χT values decreasing
from 300 K (51.2 cm3 K mol-1) to 2 K (19.8 cm3 K mol-1). Variable-temperature alternating current magnetic
susceptibility measurements imply that both 1 and 2 behave as single-molecule magnets. Fitting the
frequency-dependent out-of-phase magnetic susceptibility to the Arrhenius equation yields an effective
energy barrier, Ueff, to magnetization relaxation of 16.5 ± 0.7 K (11.5 ± 0.5 cm-1) for 1 and 36.2 ± 2.0 K
(25.1 ± 1.4 cm-1) for 2. The larger value for 2 is in agreement with the lower molecular symmetry, larger
magnetoanisotropy, and higher ground spin state of 2 compared to those of 1. This observation suggests
a new strategy for increasing the blocking temperatures in high-nuclearity manganese clusters.