A synthetic strategy for switching the single ion anisotropy in tetrahedral Co(II) complexes

Article abstract of DOI:10.1039/c4cc08305a

Four novel mononuclear tetrahedral cobalt(ii) complexes containing exocyclic mesoionic ligands of molecular formulae [Co^II(L₁)(X)₂(MeCN)] X = Cl (1) or Br (2) and [Co^II(L₂)(X)₂(MeCN)], X = Cl (3) or Br (4) have been reported. It is found that simple substitution of L₁ (O donor in 1 and 2) by L₂ (S donor in 3 and 4) results in switching of the single ion magnetic anisotropy parameter (D) from positive to negative, with a significant change in magnitude.

Full text of DOI:10.1039/c4cc08305a

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Synthetic Strategy for Switching the Single Ion
Anisotropy in Tetrahedral Co(II) Complexes
Cite this: DOI: 10.1039/x0xx00000x
Shefali Vaidya, a Apoorva Upadhyay, a Saurabh Kumar Singh, a Tulika Gupta, a
Subrata Tewary,a Stuart K. Langley, b James P. S. Walsh, c Keith S. Murray, b
Gopalan Rajaraman*,a and Maheswaran Shanmugam*,a
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
with the complex needs to be increased drastically.9 While
Four novel mononuclear tetrahedral cobalt(II) complexes
containing exocylic mesoionic ligands of molecular formulae
significant experimental and theoretical efforts have been
undertaken, a rational approach to fine tune the axial zfs parameter,
D, of mononuclear complexes has yet to be achieved. With this in
mind, we set out to investigate possible means to modulate the
magnetic anisotropy in Co(II) tetrahedral complexes, using a
combined experimental and theoretical approach. From this work we
propose a novel synthetic strategy for the synthesis of new
generation SIMs.
[CoII(L1)(X)2(MeCN)]
X
=
Cl (1) or Br (2) and
[CoII(L2)(X)2(MeCN)], X = Cl (3) or Br (4) have been
reported. It is found that by simple substitution of L1 (O
donor in 1 and 2) by L2 (S donor in 3 and 4) results in
switching of the single ion magnetic anisotropy parameter (
from positive to negative, with a significant change in
magnitude
D)
For this study we focused our attention on unconventional
exocyclic mesoionic ligands (Scheme S1). Mesoionic ligands are
dipolar fiveꢀ or sixꢀ membered heterocyclic compounds whose
canonical resonance structures cannot be represented without any
additional charges in them.10 To the best of our knowledge, however,
their coordination chemistry has yet to be investigated. Such ligands
offer flexibility allowing us to selectively change the coordinating
substituents and thus we were able to probe the influence of the
ligand donor atoms on the magnetic anisotropy.
.
Following the discovery of singleꢀmolecule magnet (SMM)
behavior in the famous {Mn12} cluster, the number of polynuclear
transition metal complexes expanded rapidly to probe this
phenomenon.1 It was subsequently found, however, that problems
arise for multinuclear transition metal clusters as the magnitude of
the anisotropy D is found to be inversely proportional to S2, thus
hampering the effort to find high temperature SMMs. 2 In 2003, slow
magnetic relaxation was reported for a mononuclear lanthanide
complex which displayed superior magnetic relaxation behavior
compared to that of any transition metal complex.3 Following this
observation, record blocking temperatures and anisotropic barriers
were subsequently developed which were based on lanthanide single
ion magnets (SIMs).4 On the other hand transition metal based SIMs
are very scarce in the literature due to quenching of the orbital
angular momentum by the ligand field. Low coordinate mononuclear
transition metal complexes such as two coordinate Fe(I), Fe(II) and
Ni(I) SIM complexes, however, have revealed a promising new
approach towards SIM synthesis, where the maximum single ion
anisotropy is harvested due to the unrestricted orbital angular
momentum by limiting the coordination number of the metal ion.5
The reaction of Co(II)X2·6H2O (where X = Cl or Br) with L1
(where L1 = 2,3ꢀdiphenylꢀ1,2,3,4ꢀtetrazoliumꢀ5ꢀolate) or with L2
(where L2 = 2,3ꢀdiphenylꢀ1,2,3,4ꢀtetrazoliumꢀ5ꢀthiolate) in methanol
led to the isolation of blue (in case of L1) or green (in case of L2)
single crystals when crystallized from acetonitrile.
Among transition metal ions, Co(II) based complexes are
particularly attractive towards blocking the magnetization as they
possess a nonꢀinteger spin ground state,6 which reduces the
possibility of quantum tunnelling of magnetization (QTM).7 The
majority of reported tetrahedral Co(II) complexes, however, still
require a bias dc magnetic field to observe SIM behaviour, except
for two complexes, where the reported easy axis anisotropy
(negative zeroꢀfield splitting; zfs) values are significantly large in
magnitude.8 This implies that the single ion anisotropy associated
Fig 1. A) A representative thermal ellipsoid plot for complexes
, where X = Cl ( ) and X = Br ( ), B) A representative thermal
ellipsoid plot for complexes and , where X = Cl ( ) and X = Br ( );
Hydrogen atoms are omitted for clarity.
1 and
2
1
2
3
4
3
4
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The molecular structures were determined from single crystal parameters {
Xꢀray diffraction, revealing mononuclear complexes of formula 2.34 ,
= +11.16 cmꢀ1} (for
[CoII(L1)(X)2(MeCN)], where X = Cl ( = ꢀ10.32 cmꢀ1} (for
) or Br ( ) and (for ) and { = 2.34 ,
[Co(L2)(X)2(MeCN)] where X = Cl ( ) or Br (
Figure S1). Structural descriptions and related crystallographic to the sign of
parameters of the four complexes, , are provided in the
supplementary information (See Table S1ꢀS4). Structurally
S=3/2, g = 2.34 , D 1), {g =
= +15.61 cmꢀ1} (for
D
2
), {g
= 2.34 ,
D
= ꢀ11.30 cmꢀ1}
4) (see Table 1,
1
2
3
g
D
3
4
) (Figure 1 and Table S5 and Figure S6 and the related discussion with respect
, in the supplementary information).
D
1
– 4
In order to gain insight into the origin of the magnetic anisotropy
there are no appreciable changes in the CoꢀX (X = Cl or Br) of complexes 1 and 2, we performed ab initio CASSCF+RASSI
and CoꢀN bond lengths (Table S3). Significant differences in calculations (Tables S6 and S7). This method has been widely
the CoꢀS/O bond length is however observed; with longer Sꢀ employed to compute the zfs parameters of several transition metal
atom bond distances of 2.3189 (3) and 2.3909 Å (4
), compared and lanthanide complexes.12 The computed D values for complexes
to 1.957 (
ligands.
1
) and 1.9475 Å (
2
) for the O donor mesoionic 1 and 2 are found to be +20.35 cmꢀ1 and +18.54 cmꢀ1 respectively
(Table 1, Table S8 and Figure S7 for computed orientation of D
Variable temperature (300 – 2 K) direct current (dc) tensor). Both the sign and the trend from the computed results agree
magnetic susceptibility data were collected on polycrystalline well with the experimental fits, the magnitude however is
samples of
(Figure 2 and Figures S2 – S5). Room temperature χMT values parameters reproduce both the experimental dc and magnetization
of 2.83 ( ), 2.82 ( ), 2.81 ( ) and 2.84 (
) cm3 K molꢀ1 are data, adding further confidence to the extracted zfs parameters
larger than the expected spinꢀonly value of 1.875 cm3 K molꢀ1 (Figures S2 and S3). Although the large positive D values
1ꢀ 4
, under an external magnetic field of 0.1 Tesla overestimated.13 The simulated plots derived from the computed
1
2
3
4
(
S
= 3/2,
g
= 2) for a single tetrahedral CoII ion.
determined for complexes 1 and 2 are encouraging, the sign is
counterproductive towards the design of effective SIMs.
Table 1. CASSCF+RASSI[b] computed D and E values, along with the gꢀ
4
values. The ꢀE (4A2 → T2) indicates the first excitation energy computed at
the spinꢀfree state of the CoII complexes.
[a]
[b]
[b]
[b]
E[b]
Com
plex
Dfit
Dcal
|E/D|cal
gxx, gyy, gzz
ꢀ
(cmꢀ1)
15.61
11.16
ꢀ11.30
ꢀ10.32
(cmꢀ1)
20.35
18.54
ꢀ15.90
ꢀ16.61
(cmꢀ1)
(cmꢀ1)
1
2
3
4
0.177
0.238
0.197
0.163
2769.54
2644.26
2342.58
1966.356
2.47, 2.39, 2.21
2.49, 2.38, 2.23
2.28, 2.36, 2.50
2.31, 2.38, 2.54
[a] SH parameters extracted from PHI fit
A search of the Cambridge structural database (CSD) revealed a
number of Co(II) tetrahedral complexes with np orbital (n is
principal quantum number of group 15 and 16) containing ligands,
where n >2, which possess a negative zfs parameter (Table S9). In
order to verify whether the diffuse orbitals play a role in modulating
the sign of D, we replaced the oxygen atom in L1 with sulphur or
selenium atoms and proceeded with the structure optimization using
the B3LYP/TZV setup (see ESI for computational details). The
optimized structures and selected structural parameters are given in
Table S10. These structures were then used for ab initio calculations
to compute the SH parameters. Interestingly, it is predicted that the
sign of D for both models are negative, with values of ꢀ20.46 cmꢀ1
and ꢀ20.52 cmꢀ1 for S and Se donors, respectively. It was based on
these computational predictions that we synthesised complexes 3 and
4 which contain the sulphur (3p) donor ligand. (Table 1; See ESI for
synthetic procedure and structural description. Also see Figure S7
and Table S8 for the computed orientation of the D tensor).
Fig 2
sample of
the simulated χMT values using CASSCF+RASSI calculated parameters
respectively. B) Field dependant magnetization data of (symbols)
measured at the indicated temperatures. The solid lines represent the
simulated magnetization data using CASSCF+RASSI calculated
parameters. C) Temperature dependant outꢀofꢀphase alternating current
(ac) susceptibility measurement performed on polycrystalline sample of
.
A) Temperature dependant χMT data recorded on polycrystalline
3
open and filled stars represent the experimental data and
3
3
between 1.8 K to 7.0 K at the indicated optimum external magnetic
field. D) Arrhenius plot constructed from ac susceptibility measurement
of all the four complexes ( ).
1ꢀ4
Due to the presence of a significant magnetic anisotropy, and with
the prediction of an easy axis type anisotropy for 3 and 4, alternating
current (ac) susceptibility measurements were performed on 1 ꢀ 4, to
check for SIM behaviour (Figure S2ꢀS5). No outꢀofꢀphase (χM”)
susceptibility signals were observed using a 3.5 Oe oscillating ac
field, under a zero external magnetic field. Upon application of a
bias dc field of 0.25 T, 1 ꢀ 4 display frequency dependent χM”
signals, characteristic of a SIM. This implies unambiguously that
each complex possesses significant anisotropy and QTM is likely to
be the dominant mechanism for relaxation of the magnetization.14
From the frequency and temperature dependent data the effective
energy barriers (Ueff) for the reorientation of magnetization are
estimated to be 10.3 cmꢀ1 (τo = 7.68 x 10ꢀ7 s), 8.2 cmꢀ1 (τo = 8.39 x10ꢀ
7 s), 20.2 cmꢀ1 (τo = 1.49 x 10ꢀ9 s) and 13.8 cmꢀ1 (τo = 8.12 x 10ꢀ8) for
1 ꢀ 4 respectively (Figure 2 and Figures S2ꢀS5). Interestingly, slow
magnetic relaxation is observed for the Co(II) ions with nonꢀuniaxial
Each complex displays temperature dependent behaviour as
observed via a decrease in the χMT value as the temperature is
reduced from RT to 50 K. This observation suggests that
1 ꢀ 4
possess a significant magnetic anisotropy. The influence of the
anisotropy is even more pronounced below 50 K, resulting in a
steeper decline of the χMT product, however, intermolecular
antiferromagnetic interactions may also contribute to this
behaviour. Isothermal magnetization data collected on
polycrystalline samples of 1ꢀ 4 at several temperatures (Figure
S2–S5), reveal that the magnetization does not saturate at high
fields, reaching values of ~2.25 NµB at 2 K. Furthermore the
nonꢀoverlapping nature of the reduced magnetization plots
confirms the presence of anisotropy/zfs (Figures S2ꢀ S5). In
order to extract Spin Hamiltonian (SH) parameters, the χMT(T)
and
M(H) data were simultaneously fitted using the PHI
software package.11 Best fits of the data yielded the following
2 | J. Name., 2012, 00, 1-3
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anisotropy (1 and 2). A detailed rationale for this behaviour has It is then found that the splitting of the dxz, dyz and dxy orbitals differ
recently been documented, revealing this can only be observed in the drastically between complexes 1 and 3. The dxy orbital which
presence of a static magnetic field.15 The energy barrier observed for
interacts with the O/S atoms in a σ fashion is found to be
3 is one of the largest reported for a Co(II) tetrahedral SIM and the
field induced SIM behaviour exhibited by 3 and 4 are due to easy
axis anisotropy (see EPR analysis below). Each isostructural
complex reported displays significantly different energy barriers,
implying a substantial change in the SH parameters, a trend which is
clearly reflected in the computed values.
destabilized in 1 and stabilized in 3 compared to the dxz/dyz orbitals,
which are interacting with the chloride ligands. This is consistent
with the Xꢀray structural parameters, where the CoꢀO bond length
(1.957 Å) is found to be significantly shorter than the CoꢀS bond
length (2.319 Å). Thus the lowest energy transition for complex 1 is
2
predicted to be dx ꢀy2ꢀdyz, which predominantly contributes to the
DXX and DYY terms, leading to a positive D value. On the other hand
We performed preliminary EPR spectroscopy studies on all four
complexes in order to experimentally observe the change of sign
from easy plane anisotropy (+D) to easy axis anisotropy (ꢀD)
(Figure 3 and Figure S8). The spectra of 1 and 2 are similar, as are
those of 3 and 4, but both sets are markedly different from each
other, implying a significant difference in the underlying electronic
structure. The spectra of the oxo analogues (1 and 2) are
straightforward, and are amenable to modelling only when D is
constrained to be large (much larger than the energy of the
microwave radiation being used) and, more importantly, positive:
giso = 2.4, D >>1.1 cm–1, E/D = 0.20 (1); giso = 2.4, D >> 1.1 cm–1,
E/D = 0.27 (2). The spectra of 3 and 4 are not as straightforward,
with not one but three features appearing at lower field. The most
dominant of these features can be modelled with parameters
consistent with a large negative Dꢀvalue: giso = 2.4, D << 1.1 cm–1,
E/D = 0.28 (3); giso = 2.4, D << 1.1 cm–1, E/D = 0.28 (4). In all
cases, the simulations allowed for a small degree of orientation in the
field. Although the large effective gꢀvalues observed in the thio
complexes are consistent only with a negative D,16 the origin of the
additional structure is not immediately apparent.
2
in complex 3 the dx ꢀy2ꢀ dxy is the lowest energy excitation which
primarily contributes to the DZZ component (same ml transitions) and
this leads to a negative D parameter for this complex. The observed
difference in the orbital ordering between 1 and 3 arises due to the
difference in the metalꢀligand interaction and the hard/soft nature of
the ligands, with the CoꢀS bond being more covalent than CoꢀO
bond.
Fig 4. DFT computed Eigen value plots of the five dꢀorbitals for complex 3
(left panel) and 1 (right panel).
This is also reflected in the computed charges and the spin density
values (Table S13). On a similar note the CoꢀSe model predicts a
negative D value due to the soft nature of the ligand.18
The results presented in this article show that combined
experimental, spectroscopic and computational methods reveal how
to predict/switch the sign of D in Co(II) tetrahedral complexes. In
order to generalize our prediction further, we have looked at
different donor atoms, such as N and P (2p vs. 3p valence orbitals)
containing ligands. The calculations were performed on crystal
structures of reported complexes [CoBr2(phenanthroline)] and
[CoCl2(PPh3)2], where the D values have been quantitatively
determined by EPR spectroscopy.8c, 19 The calculated values are in
agreement with their experimental findings [11.7 cmꢀ1 (experimental)
vs 16.28 cmꢀ1 (computed)] and [ꢀ16.2 (experimental) cmꢀ1 vs ꢀ21.38
cmꢀ1 (computed)] respectively13 and follow the trend described in
this article (Table S14). Further calculations performed on model
complexes of [CoCl2(PPh3)(MeCN)] and [CoCl2(py)(MeCN)]
predict a negative (ꢀ41.13 cmꢀ1 ) and positive (+22.13 cmꢀ1) D value
respectively (Table S15).
Fig 3. Qꢀband (34 GHz) EPR spectra of polycrystalline samples of 1 (top)
and 3 (bottom) measured at 5 K. Red traces are simulations using the
parameters given in the main text.
The crystal structure parameters of complex 3 agree well with the
corresponding optimized model systems and the calculation
performed on the Xꢀray structures of 3 and 4 yielded similar
negative D values, in agreement with experimental evidence (Figure
2 and 3, Table 1, Tables S11 and S12). Since the sign of the zfs
between O vs. S/Se donor atoms differ simply by replacing L1 (in 1)
for L2 (in 3), we probed the reason for this switch. The magnetic
anisotropy of tetrahedral Co(II) complexes which have no firstꢀorder
orbital angular momentum is determined by a second order
interaction between the electronic ground state and anisotropic
excited states. This interaction is responsible for whether or not the
sign of D is positive or negative. In general the transition between dꢀ
orbitals having the same |ml| values contribute to the DZZ component,
leading to negative D values, while transitions between different |ml|
values contribute to the DXX and DYY component of the anisotropy
and thus enhances the positive value of D.17 To analyse these
features in detail, the Eigenꢀvalue diagram for complexes 1 and 3 are
In summary, we have reported the single crystal Xꢀray structures of
four isostructural Co(II) tetrahedral complexes (1 ꢀ 4) utilizing, for
the first time, exocyclic mesoionic ligands. Detailed dc and ac
magnetic susceptibility measurements reveal a significant anisotropy
is present, with field induced SIM behaviour observed for all four
complexes. Interestingly two of the four complexes (3 and 4) were
synthesized based on computational predictions that they possess
easyꢀaxis type anisotropy. The switching of the magnetic anisotropy
from easy plane to easy axis, by changing L1 (hard donor) with L2
(soft donor), was confirmed by EPR spectroscopy. To the best of our
knowledge this is first time where predictions have been made and
verified by parallel synthesis towards fine tuning the magnetic
anisotropy. This works details a significant step forward towards the
2
2
2
plotted (see Figure 4). It is found that the dx ꢀy and dz orbitals are
found to be the lowest lying doubly occupied orbitals in both cases.
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design and synthesis of future SIMs. Further characterization of
these complexes, and efforts to isolate other analogues are currently
underway.
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Notes and references
6
a
Department of Chemistry, Indian Institute of Technology Bombay,
Powai, Mumbai, Maharashtra, Indiaꢀ400076.
b
School of Chemistry, Monash University, Clayton, Victoriaꢀ3800,
Australia.
c
School of Chemistry, The University of Manchester, Oxford Road,
7
8
Manchester, UKꢀM13 9PL.
Corresponding Author
eswar@chem.iitb.ac.in (MS)
rajaraman@chem.iitb.ac.in (GR)
MS and GR acknowledges Department of Science and Technology, DST
(SR/S1/ICꢀ32/2011,
SR/S1/ICꢀ41/2010)
Nanomission
(SR/NM/NSꢀ
9
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L. McInnes and the National EPR research facility and service at the
University of Manchester, UK for EPR measurements. Also MS likes to
extend his sincere thanks to Prof. Stephen Hill, National high field magnetic
laboratory, Florida for his initial efforts for the EPR measurements on these
complexes. KSM and GR thanks the Australian Research Council and the
AustraliaꢀIndia AISRF for support.
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are given in ESI. In addition the spin density plots and other theoretical
calculations supporting evidences are also listed in ESI. CCDC numbers:
999464ꢀ999467. See DOI: 10.1039/c000000x/
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