ic6b02720_si_003.cif (19.17 kB)
Exploring the Influence of Diamagnetic Ions on the Mechanism of Magnetization Relaxation in {CoIII2LnIII2} (Ln = Dy, Tb, Ho) “Butterfly” Complexes
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posted on 2017-02-17, 15:53 authored by Kuduva R. Vignesh, Stuart K. Langley, Keith S. Murray, Gopalan RajaramanThe synthesis and
magnetic and theoretical studies of three isostructural heterometallic
[CoIII2LnIII2(μ3-OH)2(o-tol)4(mdea)2(NO3)2] (Ln = Dy (1), Tb
(2), Ho (3)) “butterfly” complexes
are reported (o-tol = o-toluate,
(mdea)2– = doubly deprotonated N-methyldiethanolamine). The CoIII ions are diamagnetic
in these complexes. Analysis of the dc magnetic susceptibility measurements
reveal antiferromagnetic exchange coupling between the two LnIII ions for all three complexes. ac magnetic susceptibility
measurements reveal single-molecule magnet (SMM) behavior for complex 1, in the absence of an external magnetic field, with an anisotropy
barrier Ueff of 81.2 cm–1, while complexes 2 and 3 exhibit field
induced SMM behavior, with a Ueff value
of 34.2 cm–1 for 2. The barrier height
for 3 could not be quantified. To understand the experimental
observations, we performed DFT and ab initio CASSCF+RASSI-SO calculations
to probe the single-ion properties and the nature and magnitude of
the LnIII–LnIII magnetic coupling and
to develop an understanding of the role the diamagnetic CoIII ion plays in the magnetization relaxation. The calculations were
able to rationalize the experimental relaxation data for all complexes
and strongly suggest that the CoIII ion is integral to
the observation of SMM behavior in these systems. Thus, we explored
further the effect that the diamagnetic CoIII ions have
on the magnetization blocking of 1. We did this by modeling
a dinuclear {DyIII2} complex (1a), with the removal of the diamagnetic ions, and three complexes
of the types {KI2DyIII2} (1b), {ZnII2DyIII2} (1c), and {TiIV2DyIII2} (1d), each containing
a different diamagnetic ion. We found that the presence of the diamagnetic
ions results in larger negative charges on the bridging hydroxides
(1b > 1c > 1 > 1d), in comparison to 1a (no diamagnetic ion),
which reduces quantum tunneling of magnetization effects, allowing
for more desirable SMM characteristics. The results indicate very
strong dependence of diamagnetic ions in the magnetization blocking
and the magnitude of the energy barriers. Here we propose a synthetic
strategy to enhance the energy barrier in lanthanide-based SMMs by
incorporating s- and d-block diamagnetic ions. The presented strategy
is likely to have implications beyond the single-molecule magnets
studied here.
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Zn II 2 Dy III 2OHanisotropy barrier U effCo III ionCASSCFdiamagnetic Co III ionsLn III ionsdiamagnetic Co III ionDy III 2Ti IV 2 Dy III 2U eff valuediamagnetic ions results2 Dy III 2Co III 2 Ln III 2DFTCo III ionsSMM behaviorcomplexsusceptibility measurements3 exhibit fieldd-block diamagnetic ionsdiamagnetic ions
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