Comparison of the Magnetic Anisotropy and Spin Relaxation Phenomenon of Dinuclear Terbium(III) Phthalocyaninato Single-Molecule Magnets Using the Geometric Spin Arrangement

Herein
we report the synthesis and characterization of a dinuclear
TbIII single-molecule magnet (SMM) with two [TbPc2]0 units connected via a fused-phthalocyaninato ligand.
The stable and robust complex [(obPc)­Tb­(Fused-Pc)­Tb­(obPc)] (1) was characterized by using synchrotron radiation measurements
and other spectroscopic techniques (ESI-MS, FT-IR, UV). The magnetic
couplings between the TbIII ions and the two π radicals
present in 1 were explored by means of density functional
theory (DFT). Direct and alternating current magnetic susceptibility
measurements were conducted on magnetically diluted and nondiluted
samples of 1, indicating this compound to be an SMM with
improved properties compared to those of the well-known [TbPc2]−/0/+ and the axially symmetric dinuclear
TbIII phthalocyaninato triple-decker complex (Tb2(obPc)3). Assuming that the probability of quantum tunneling
of the magnetization (QTM) occurring in one TbPc2 unit
is PQTM, the probability of QTM simultaneously
occurring in 1 is PQTM2, meaning that QTM is effectively suppressed. Furthermore,
nondiluted samples of 1 underwent slow magnetic relaxation
times (τ ≈ 1000 s at 0.1 K), and the blocking temperature
(TB) was determined to be ca. 16 K with
an energy barrier for spin reversal (Ueff) of 588 cm–1 (847 K) due to D4d geometry and weak inter- and intramolecular
magnetic interactions as an exchange bias (Hbias), reducing QTM. Four hyperfine steps were observed by
micro-SQUID measurement. Furthermore, solution NMR measurements (one-dimensional,
two-dimensional, and dynamic) were done on 1, which led
to the determination of the high rotation barrier (83 ± 10 kJ/mol)
of the obPc ligand. A comparison with previously reported TbIII triple-decker compounds shows that ambient temperature NMR measurements
can indicate improvements in the design of coordination environments
for SMMs. A large Ueff causes strong uniaxial
magnetic anisotropy in 1, leading to a χax value (1.39 × 10–30 m3) that is
larger than that for Tb2(obPc)3 (0.86 ×
10–30 m3). Controlling the coordination
environment and spin arrangement is an effective technique for suppressing
QTM in TbPc2-based SMMs.