Inspiration from old molecules: Field-induced slow magnetic relaxation in three air-stable tetrahedral cobalt(ii) compounds

Article abstract of DOI:10.1039/c3cc00277b

We have investigated the dynamics of the magnetization of three four-coordinate mononuclear cobalt(ii) compounds, which are synthesized conveniently and are air stable. Slow magnetic relaxation effects were observed for the compounds in the presence of a dc magnetic field. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3cc00277b

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Inspiration from old molecules: field-induced slow
magnetic relaxation in three air-stable tetrahedral
cobalt(II) compounds†
Fen Yang,a Qi Zhou,a Yiquan Zhang,b Guang Zeng,a Guanghua Li,a Zhan Shi,*a
Bingwu Wang*b and Shouhua Fenga
Cite this: Chem. Commun., 2013,
49, 5289
Received 12th January 2013,
Accepted 22nd April 2013
DOI: 10.1039/c3cc00277b
www.rsc.org/chemcomm
We have investigated the dynamics of the magnetization of three for mononuclear transition-metal compounds.6a Thereafter
four-coordinate mononuclear cobalt(II) compounds, which are intensive research interest and considerable effort has been
synthesized conveniently and are air stable. Slow magnetic relaxa- devoted to the design of SIMs based on transition-metal ions,
tion effects were observed for the compounds in the presence of a but to date, this still remains a challenge and the number of
dc magnetic field.
such species is still limited.6,7
Usually, when designing a mononuclear 3d metal com-
Single-molecule magnets (SMMs) displaying slow relaxation of pound with the expectation of a slow magnetic relaxation effect,
the magnetic moment upon removal of a magnetizing field a low coordination number is favourable. The reason lies in the
have aroused widespread interest, due to their potential appli- fact that for a 3d metal compound the first-order orbital
cations in quantum computing, spintronics, and high-density angular momentum is largely quenched by the ligand field so
storage devices.1 As more and more accurate control of the that the spin–orbit coupling may be compensated.6b,7c,8 A low
synthetic process is achieved, new systems with unprecedented coordination number can afford a relatively weak ligand field
and fascinating magnetic phenomena are being produced. The and prevent the quenching effect up to a certain point.
field of SMMs is experiencing very fast evolution. Scientists’ However, for a low coordinated compound the synthetic
main attention has shifted from the synthesis of giant clusters environment may be critical and the compound itself may be
with the highest possible ground-state spin (S) and negative air-sensitive. By accident, we turned our attention to the field
axial magnetic anisotropy (D) to the search for smaller and of late transition metal polymerization catalysis, and to our
smaller SMMs.2 Especially, the design and synthesis of single- surprise, there is a rich variety of such mononuclear 3d metal
ion magnets (SIMs), which refer to mononuclear compounds compounds with a low coordination number.9 We believe that
with a single slow relaxing metal centre, is a new emerging many of them could be used in probing slow magnetic relaxa-
trend in molecular magnetism. They could be considered as the tion behaviour or give us some inspiration at least. It will
smallest SMMs and the simplest models for fundamental certainly enlarge the scope of SIMs with first-row transition
research on magnetic relaxation. Since the first publication of metal ions.
slow magnetic relaxation in double-decker compound Tb(III)Pc2
Here we report the dynamics of the magnetization of three
by Ishikawa et al. in 2003,3 more and more mononuclear very simple molecules, [Co(PPh3)2Cl2] (1), [Co(DPEphos)Cl2]
compounds based on anthanides4 or actinides5 metal ions (2) and [Co(Xantphos)Cl2] (3) (where PPh3, DPEphos, and
exhibiting SIM behaviour have been reported. In 2010, Chang Xantphos represent triphenylphosphine, 2,20-bis(diphenyl-
and Long et al. proved that slow relaxation can also be possible phosphino) diphenyl ether, and 9,9-dimethyl-4,5-bis(diphenyl-
phosphino) xanthenes, respectively), which had been reported
as highly selective catalysts for the hydrovinylation of styrene.10,11
Compared to many other SIMs based on 3d metal reported
a State Key Laboratory of Inorganic Synthesis & Preparative Chemistry,
College of Chemistry, Jilin University, Changchun 130012, P. R. China.
previously, they are synthesized conveniently and are air-stable.
E-mail: zshi@mail.jlu.edu.cn; Fax: +86-431-85168624; Tel: +86-431-85168662
Their molecular structures are shown in Fig. 1. The Co2+
centers in the three compounds are coordinated by two P
atoms and two Cl anions, forming slightly distorted tetrahedral
geometry. There are subtle differences in their bond angles of
P–Co–P, P–Co–Cl, and Cl–Co–Cl (Table S2, ESI†). The mono-
nuclear units are well isolated from each other, the shortest
intermolecular Co–Co distance being 8.253 Å, 9.127 Å,
b Beijing National Laboratory of Molecular Science, College of Chemistry and
Molecular Engineering, State Key Laboratory of Rare Earth Materials Chemistry
and Applications, Peking University, Beijing, 100871, P. R. China.
E-mail: wangbw@pku.edu.cn
† Electronic supplementary information (ESI) available: Full experimental details
including additional crystallographic and magnetic data. CCDC 920314–920316.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c3cc00277b
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Fig. 1 Crystal structures of the three tetrahedral compounds. Blue, green, pink,
and gray spheres represent Co, Cl, P, and C atoms, respectively; H atoms are
omitted for clarity.
and 9.011 Å for 1, 2, and 3, respectively. No short intermolecular
exchange pathway is apparent in 2 and 3, while compound 1
may favor an intermolecular Co–Cl. . .Cl–Co superexchange
path (Fig. S1–S3, ESI†).12
Variable temperature dc susceptibility measurements were
collected on powdered crystalline samples of the compounds at
a field of 5000 Oe over the 2–300 K temperature range as shown
in Fig. S7 (ESI†). At 300 K, the wMT value is 2.50 emu K molꢀ1
,
2.55 emu K molꢀ1, and 2.54 emu K molꢀ1 for compounds 1, 2,
and 3, respectively. These values, while larger than the anti-
cipated spin-only value for S = 3/2 of 1.88 emu K molꢀ1, fall well
in the range of 2.1–3.4 emu K molꢀ1 for experimentally
observed highly anisotropic Co2+ centers.13 The wMT values
remain roughly constant at 100 K, 75 K, and 25 K for 1, 2,
and 3, respectively, before decreasing upon further cooling. The
phenomenon is consistent with Curie-type behaviour of iso-
lated Co2+ centers. The decrease at low temperature is possibly
due to intrinsic magnetic anisotropy of the Co2+ ions, and for 1
the weak intermolecular antiferromagnetic interaction could
make a certain contribution.
The field dependence of the magnetization for the com-
pounds at 1.8 K is shown in Fig. S8 (ESI†). The magnetization at
50 kOe reaches 2.0Nb, 2.3Nb, and 2.5Nb for 1, 2 and 3,
respectively. The lack of saturation for the compounds also
suggests the presence of magnetic anisotropy. Low-temperature
magnetization data at various applied dc fields for the compounds
were also collected. The resulting plots of reduced magnetization,
depicted in Fig. 2, Fig. S9 and S10 (ESI†), exhibit significant
separation between the isofield curves, confirming the existence
of magnetic anisotropy. Several attempts to fit the reduced mag-
netization data were unsuccessful. To estimate the nature of
magnetic anisotropy of the ground state, the zero-field-splitting
parameters were calculated using CASSCF/CASPT2/RASSI method.
Fig. 3 (a) Temperature dependence of the out-of-phase (w00) ac susceptibility
component at different ac frequency for 2 (Hac = 3 Oe and Hdc = 1000 Oe). (b)
Frequency dependence of out-of-phase ac susceptibility from 1.8 to 3.0 K under
1000 dc field for 2.
The calculation afforded D = ꢀ16.2 cmꢀ1 and E = 0.9 cmꢀ1 for 1,
D = ꢀ14.4 cmꢀ1 and E = 1.7 cmꢀ1 for 2, D = ꢀ15.4 cmꢀ1 and
E = 1.3 cmꢀ1 for 3, respectively. The negative sign of D and their
large magnitude indicate the large uniaxial anisotropy of the
compounds and therefore the possibility of observing slow
magnetic relaxation.
To investigate the potential for slow magnetic relaxation
in the compounds, temperature dependence and frequency
dependence of the ac susceptibility in the temperature range
1.8–4.0 K were determined. Under a zero dc field and a 3 Oe ac
field oscillating at frequencies between 1 and 834 Hz, no out-of-
phase ac susceptibility (w00) signal was observed. However, when
a 1000 Oe static dc field was applied, all the compounds
displayed a temperature and frequency dependent signal as
shown in Fig. 3 and Fig. S11–S18 (ESI†). These phenomena have
been observed in most of the mononuclear transition metal
SMMs reported previously. The absence of slow relaxation in
the compounds under zero applied field can most likely be
attributed to quantum tunneling of the magnetization (QTM)
through the spin-reversal barrier.
Here for these three half integer spin systems, the mixing of
the ground ꢁ3/2 levels by transverse zero-field-splitting (E) is
forbidden according to Kramer’s theorem.14 The observed QTM
effect may stem from the mixing of ꢁ3/2 levels via hyperfine
interaction and dipolar interaction,15 the relaxation processes
mediated by which could be suppressed when a proper dc field
is applied.
The magnetization relaxation time (t) is derived from the
frequency-dependence measurements and is plotted as a func-
tion of 1/T in Fig. 4; Fig. S14 and S18 (ESI†). The t of each
compound shows a strong temperature dependence and
fits well with the Arrhenius equation, t = t0exp(Ea/kBT),
Fig. 2 Low-temperature magnetization data for
2 collected under various
applied dc fields. The solid lines represent fits to the data.
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Program of Higher Education (No. 20110061110015) and
National High Technology Research and Development Program
(863 program) of China (No. 2013AA031702).
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Fig. 4 Cole–Cole plot of 2 under 1000 Oe at temperatures between 1.8 K (red)
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only one relaxation process for each compound and no quan-
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field of 1000 Oe is appropriate to suppress any QTM effect
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process. The calculated values of the pre-exponential factor
t0 = 1.2 ꢂ 10ꢀ9 s and the activation energy Ea = 37.1 K for 1,
t0 = 2.1 ꢂ 10ꢀ10 s and Ea = 35.0 K for 2, t0 = 6.0 ꢂ 10ꢀ9 s and
Ea = 29.9 K for 3 are consistent with those reported previously
for the very few other Co2+-based SIMs.
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the present relaxation process can be considered with a single
relaxation time for each compound.
´
In conclusion, we have probed SMM behaviour in three old
mononuclear Co2+ compounds, which have been reported as
highly selective catalysts for the hydrovinylation of styrene.
They are air stable while the Co2+ centers are in a low four-
coordinated environment. All of the compounds exhibit
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behaviour has been observed under an applied field. The subtle
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This work was supported by the Foundation of the National
Natural Science Foundation of China (No. 20971054, 90922034
and 21131002), Specialized Research Fund for the Doctoral
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Chem. Commun., 2013, 49, 5289--5291 5291