Inorganic Chemistry
ARTICLE
relaxation could be measured to determine the values of
quantum tunneling time τQ relating to the transverse aniso-
tropic parameter, E.1,21 The obtained τQ values for Co(X)2-
(C1py)4 depended on the axial ligands and increased in the
order X = NCSꢀ, Clꢀ, and NCOꢀ. The τQ values were
estimated to be 6 ꢁ 102, 4 ꢁ 103, and 2 ꢁ 105 s for Co(X)2-
(C1py)4, X = NCSꢀ, Clꢀ, and NCOꢀ, respectively. The τQ
must be considered as a factor affecting Ueff; it reduces the U
value to produce the Ueff value (U > Ueff). The Ueff values were
89, 91, and 130 K for Co(X)2(C1py)4, X = NCSꢀ, Clꢀ, and
NCOꢀ, respectively. Although the values for Co(NCS)2-
(C1py)4 and Co(Cl)2(C1py)4 were comparable, that for Co-
(NCO)2(C1py)4 was larger. Furthermore, the τQ also affected
the hysteresis loop in the M versus H plot and the coercive
force, Hc in the hysteresis loop increased with increasing τQ.
Actually, the Hc values of 3.5, 7.0, and 20 kOe were obtained for
Co(X)2(C1py)4, X = NCSꢀ, Clꢀ, and NCOꢀ, respectively, and
the order was in agreement with that for τQ.
In the cobalt-aminoxyl system, the change of Ueff depending
on the axial ligand was also observed under similar conditions.8c
(The Ueff values for Co(X)2(4NOpy)4, X = Brꢀ, NCSꢀ, and
NCOꢀ, were 20, 31, and 50 K, respectively). The difference of
the obtained Ueff values in Co(X)2(4NOpy)4 was considered to
result from the change of the |D| value by the axial ligands. In
Co(X)2(4NOpy)4, on the other hand, the DC magnetic relaxa-
tions for the τQ values could not be followed by the using DC
decay measurement and thus were fast. The τQ values for
Co(X)2(4NOpy)4 were therefore considered to be smaller than
those for the corresponding Co(X)2(C1py)4. The observed
difference in τQ between the cobalt-carbene and the cobalt-
aminoxyl systems suggest three important pieces of information
for the construction of heterospin SMMs as follows: (1) the
τQ was affected by not only the axial ligand but also the organic
spin at the peripheral position in the 1:4 complex; (2) the
magnetic interaction of carbene with the cobalt ion, which is
larger than that of aminoxyl with the cobalt ion,7 effectively
increases the τQ value; and (3) the elongation of τQ leads to an
increase in the Ueff value.4a,c,d,22
acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*Phone: 81 92 642 6590. Fax 81 92 642 6590. E-mail: koga@fc.
phar.kyushu-u.ac.jp.
’ ACKNOWLEDGMENT
This work was partially supported by “Nanotechnology Support
Project” of the Ministry of Education, Culture, Sports, Science and
Technology (MEXT), Japan.
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istic SMM values of Ueff = 130 K with τQ = 2 ꢁ 105
s
corresponding to t1/2 of 14 h and showed a hysteresis loop with
the Hc value of 20 kOe below 2.0 K. These values are fairly large
compared with those for the metal clusters2ꢀ4 reported in the
literature.
’ ASSOCIATED CONTENT
S
Supporting Information. CIF files (CCDC-809474 and
b
-809469 for [Co(Cl)2(D1py0)4] and [Co(NCO)2(D1py)4],
respectively), the Vis. spectral change of D1py on photoly-
sis (Figure S1), the plots of χmolT vs T (Figure S2), Mmol vs
H (Figure S3), χ0molT and χ00mol vs T (Figure S4), M/M0 vs time
(Figure S6), and ln τ vs Tꢀ1 (Figure S7) for [Co(Cl)2(C1py0)4],
and the plot of M vs time (Figure S5) and the values of τ and
B (Table S1) for Co(X)2(C1py)4; X = Cl, NCS, and NCO. This
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dx.doi.org/10.1021/ic200482y |Inorg. Chem. 2011, 50, 5186–5195