Investigation of easy-plane magnetic anisotropy in P-ligand square-pyramidal CoII single ion magnets

Article abstract of DOI:10.1039/c7cc02584j

In this work we report two pentacoordinated Co^II-P₄X₁ Single Ion Magnets (SIMs) based on P-donor ligands. The tetradentate ligand tris[2-(diphenylphosphino)ethyl]phosphine allows the obtention of the isostructural square pyramidal [Co(PP₃)Cl]·ClO₄ (1) and [Co(PP₃)Br]·ClO₄ (2) complexes. Consistent theoretical and experimental studies indicate that these complexes have a high spin (S = 3/2) ground state and suggest that the relaxation dynamics is governed by ground state quantum tunneling, whereas its temperature dependence is directed by optical or acoustic Raman processes.

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
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Investigation of easy-plane magnetic anisotropy in P-ligand
square-pyramidal CoII single ion magnets
Amit Kumar Mondal,a Jesús Jover,b Eliseo Ruiz*b and Sanjit Konar*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
In this work we report two pentacoordinated CoII-P4X1 Single Ion
Magnets (SIMs) based on P-donor ligand. The tetradentate ligand
contribution of some excited states, can be tuned via variation of
the ligand field around the metal center. The most remarkable
tris[2-(diphenylphosphino)ethyl]phosphine allows the obtention feature of SIMs lies in the design and possible estimation of their
of the isostructural square pyramidal [Co(PP3)Cl]·ClO4 (1) and
[Co(PP3)Br]·ClO4 (2) complexes. Consistent theoretical and
experimental studies indicate that these complexes have a high
spin (S=3/2) ground state and suggest that the relaxation
dynamics is governed by ground state quantum tunneling,
whereas its temperature dependence is directed by optical or
acoustic Raman processes.
magnetic anisotropy based on the ligand field theory. Among 3d-
SIMs, CoII complexes are mostly important due to the presence of
non-integer spin ground state, that decreases the probability of
quantum tunnelling of magnetization (QTM).9 The first CoII based
SIM was reported for
a
pentacoordinated system with
bis(imino)pyridine and thiocyanido ligands.10 Although many other
CoII-based SIMs are reported later with different geometry and
coordination environments,8 number of pentacoordinated
complexes are still scarce and most of them are based on the
different combinations of multidentate N-donor and monodentate
halido/pseudohalido ligands. In contrast, despite the similarity
between nitrogen and phosphorus, the usage of P-donor ligands to
produce CoII pentacoordinate SIMs has not been reported yet. This
work reports the first example of pentacoordinate CoII SIMs based
on P-donor ligand having general formula CoII-P4X1: [Co(PP3)Cl]·ClO4
(1) and [Co(PP3)Br]·ClO4 (2) where PP3 = tris[2-(diphenylphosphino)
ethyl]phosphine. The investigation also reveals the influence of
metal coordination geometry on the magnetic anisotropy of the
metal complexes. The single-crystal X-ray analysis of 1 and 2 reveals
that both complexes are isostructural and crystallize in the triclinic
P-1 space group (Fig. 1 and Table S1). The tripodal PP3 ligand
coordinates in a tetradentate fashion, with one of the P atom is
occupying the axial position, while the fifth position is taken by the
halide ion (X = Cl (1) and Br (2)). The geometry at the CoII centre is
Single-molecule magnets (SMMs) are molecules that exhibit a slow
relaxation of their magnetization from a pure molecular origin.
When one of these molecules contains only one metal ion they are
referred as single-ion magnets (SIMs). The study of these systems
has experienced a huge increase in recent times due to their
promising properties in molecular spintronics,1 high-density
information storage,2 and qubits for quantum computation.3 In
SMMs and SIMs the thermal energy barrier for the reversal of the
magnetization (U) depends on the total spin (S) and the easy axis
anisotropy parameter (D). U can be quantified as |D|S2 and |D|(S2
− 1/4) for integer and half-integer spins, respectively.4 A substantial
effort has been devoted to the preparation of new molecules to
better understand different phenomena that effect slow relaxation
behaviour. The early examples of SMMs were based on polynuclear
transition metal complexes5 but recently low-nuclearity lanthanide
complexes6 and even mononuclear lanthanide7 and transition-
metal complexes8 have been reported to show SMM behaviour.
Mononuclear transition metal complexes are significant because
the relaxation process concerning the ground state, or the
best described as a distorted square pyramidal; the calculated
τ
values for 1 and 2 are 0.331 and 0.354, respectively (Table S2).11
Additional SHAPE12 analysis confirms that both the complexes can
be better described as square pyramidal (1.14 and 1.41 for 1 and 2)
than as trigonal bipyramidal (3.31 and 3.29 for 1 and 2, Table S3). In
both complexes, substantial π⋯π interactions and intermolecular
hydrogen-bonding are found, favoring the formation of
a
supramolecular two dimensional arrangement (Fig. S1-S2 and Table
S4-S5).
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Table 1. ORCA and MOLCAS CASSCF+RASSI computed D, |E/D| and g-values for
complexes 1 and 2. ꢀE indicates the first excitation energy computed in the spin-free
state of the CoII complexes
.
Dfit
Complex
Dcalc
(cm-1)
40.1
45.6
38.3
36.9
ꢀE
|E/D|calc
gxx, gyy, gzz
(cm-1)
(cm-1)
1a
1b
2a
2b
46.4
46.4
40.7
40.7
0.24
0.24
0.24
0.24
1568.7
1638.3
1559.9
1635.2
2.03, 2,27, 2,55
2.04, 2,29, 2,56
2.03, 2.28, 2.55
2.04, 2,30, 2,56
a ORCA. b MOLCAS
.
Fig. 1 View of the molecular structures for complexes 1 and 2, where X = Cl (1)
and X = Br (2); hydrogen atoms are omitted for clarity; χMT vs. T plot measured at
0.1 T for complex
1
(left). The red line is the best fit.
The purity of the as-synthesized products has been confirmed by
the good agreement of experimental and simulated bulk phase
powder X-ray diffraction patterns based on the single crystal
structure data (Fig. S3). DC (direct current) magnetic susceptibility
measurements have been performed under an applied field of 0.1
T. At room temperature, χMT values (χM
= molar magnetic
susceptibility) of 2.90 and 2.84 cm3 K mol−1 have been obtained for
1 and 2, respectively. These values are larger than the spin-only
value of 1.87 cm3 mol−1 K for a high-spin CoII ion but, nevertheless,
they fall within the usual range of 2.1-3.4 cm3 mol−1 K found for
highly anisotropic CoII ions with a significant orbital contribution.13
Upon cooling from 300 K, the χMT values of 1 and 2 remain constant
down to 100 K, below which they decrease, reaching a value of 1.81
1
and 1.78 cm3 mol− K at 2 K, respectively (Fig. 1 and S5). The
decrease of the χMT curves at low temperature is mainly due to the
intrinsic magnetic anisotropy of the CoII center. Reduced
magnetization data (M/NμB vs. H) attain the highest values of 2.42
and 2.35 NμB for 1 and 2 at 2 K and 7 T (Fig. S4-S5). These values are
Fig. 2 CoII core and computed d-orbitals for complexes
1 (left) and 2 (right).
Orbital relative energies are given in cm-1. Color code: Co = pink C = gray, P =
orange, Cl or Br = green; outer C and H atoms have been omitted for clarity. The
curvy orange arrows indicate the lowest energy transitions.
well below the theoretical saturation for an S = 3/2 system (Msat
=
because those cannot be distinguished) and the first semioccupied
orbital is dxy for both complexes. Since these orbitals have a
different |ml| value, the contribution to the D value should be
positive18 in complete agreement with the experimental fit. The d-
orbital splitting allows the rationalization of the difference in the
magnitude of D between both complexes. The presence of a
chloride ligand produces a 1746.1 cm-1 energy gap while with
bromide the gap slightly increases to 1806.1 cm-1; since the energy
gap is smaller for complex 1 it should have a higher D value, as
observed both in experiments and calculations.
3.3 for g = 2.2). A spin Hamiltonian of eqn (1) is used to describe the
magnetic anisotropy qualitatively:
2
H = gμBS × B + D[Sz2 − S(S + 1)/3] + E(Sx2 – Sy )
(1)
where the D and E terms represent the single-ion axial and rhombic
ZFS parameters. The PHI code14 has been employed to quantify the
anisotropy parameters of the CoII centres by fitting of the χMT vs. T
plots. The best fits of the magnetic susceptibility data give D = 46.4
cm−1, E = 10.1 cm−1, and g = 2.31 for 1; D = 40.7 cm−1, E = 9.3 cm−1
,
and g = 2.28 for 2. Similar values are found for the anisotropy
parameters with electronic structure CASSCF calculations, carried
out with either ORCA15 and MOLCAS16 (Table 1). In all cases the
calculations indicate a quadruplet (3/2) ground state. The quasi-
degenerate perturbation theory (QDPT) implemented in ORCA
produces very similar results to those obtained by the fit of
experimental data although the computed D values seem to be
slightly underestimated for both complexes 1 and 2. The SO-RASSI
approach included in the MOLCAS package produces also quite
similar E and D values for both complexes, again in good agreement
with those obtained from the experimental fit. Other useful
computational information, such as the computed low-lying spin-
orbit energy states and the orientation of the g- and D-tensors, can
be found in the ESI (Table S6-S9 and Fig. S6). The relative energy
order of the 3d orbitals has been extracted from the ORCA
calculation using the ab initio ligand field theory (AILF) method.17
The final d-orbital splitting for complexes 1 and 2, which allows the
prediction of the lowest energy transitions, is shown in Fig. 2. As
may be observed the last doubly occupied orbital is dyz (or dxz,
As mentioned above complexes 1 and 2 constitute a new class of
CoII SIMs and therefore the impact of the P-donor atoms on the
magnetic properties is difficult to assess. We have carried out an
exploration of the Cambridge Structural Database (CSD) looking for
other pentacoordinate CoII complexes containing P4 ligands (either
mono or polydentate) and one halide, which could be compared to
complexes 1 and 2. For those complexes, 7 in total, we have carried
out a CASSCF calculation equivalent to those of 1 and 2, and we
have also computed their SHAPE analysis regarding the typical 5
vertexes polyhedra (Table 2, entries 1-7). The computed D value for
all these CoII-P4X1 complexes is positive independent of the ligand
arrangement or nature, suggesting that the presence of a P-donor
ligand tends to produce easy-plane complexes. CSD refcode DIZQAF
(Entry 1 in Table 2) has a negative D value but other parameters
derived from the calculations indicate it should be positive (see ESI).
Another interesting feature consists of checking whether the
magnetic behavior of complexes 1 and 2 can be modulated by
2 | J. Name., 2012, 00, 1-3
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Table 2. SHAPE analysis and computed D values for the studied Co(II)-L4X1 complexes.
Entry
1
Structure
DIZQAF
vOC-5
6.77
0.74
3.76
4.62
6.51
0.77
0.72
2.08
2.01
2.73
2.76
2.93
TBPY-5
0.74
5.76
1.04
2.01
1.01
4.29
5.91
5.50
4.54
3.46
2.40
5.76
SPY-5
4.89
1.10
2.49
3.69
5.11
1.50
1.37
1.39
0.30
1.07
1.18
0.63
D (cm-1)
-32.8a
53.3
E/D
0.28
0.12
0.12
0.28
0.08
0.10
0.15
0.29
0.15
0.10
0.31
0.30
2
JIPCER01
NIZDOP
RUTSUU
UFUCUV
VAKFAR
VAKFEV
FAWYUX
NADTUH
NUQMAP
RUJSOE
XOBFEZ
3
52.4
4
86.4
5
53.0
6
36.6
Fig.
susceptibility plots for complex
3
Frequency dependency of the out-of-phase (χM″) (left) AC magnetic
1 at 2000 Oe; Cole-Cole plots for complex 1
7
46.8
107.0a
113.6
104.0
-71.9a
-65.9a
(right). Solid lines represent the best fit.
8
9
Kramer’s doublets (KDs) responsible for the relaxation process (Fig.
S12). The spin relaxation mechanisms for both complexes show a
plausible pathway via a direct quantum tunneling (QTM) in the
ground state. The relaxation through a thermally-assisted QTM via
the first excited states seems also possible since the second KDs are
found at relatively low energies of 83.8 and 79.9 cm-1. On the other
10
11
12
vOC-5: vacant octahedron, TBPY-5: trigonal bipyramid, SPY-5: spherical square
pyramid. a See full explanation in the ESI.
replacing the P-donor ligand by an equivalent structure containing hand, the third KD lies much higher in energy (aprox. 1700 cm-1) for
N-donor atoms i.e. CoII-N4X1 complexes. This has been already both complexes, and thus those states are not expected to
achieved by replacing a (NN3Me) with an equivalent (NS3iPr) ligand in participate in the spin relaxation process. The computed spin
mononuclear trigonal bipyramidal CoII complexes.19 In that case, the relaxation mechanisms also indicate that the Orbach processes
decrease of the sigma donor ability in the equatorial plane cannot be completely ruled out and can play a significant role under
produces an increase in absolute value in the magnetic anisotropy. certain conditions.
Searching the CSD reveals the presence of limited no of CoII-N4X1 In order to acquire a deeper understanding of the magnetic
complexes (5) having a square pyramid shape similar to 1 and 2 relaxation behaviour, the relaxation times have been reassessed
(Table 2, entries 8-12). Among those, only one contains a neutral N4 considering the different relaxation processes. The two terms in
tetradentate ligand and a chloride where one of the nitrogen atoms equation (3) represent the strongly field dependent (related to
takes the axial position (CSD refcode FAWYUX, entry 8 in Table 2, QTM) and weakly field dependent (Raman, Orbach, etc.) processes
Fig. S7). The computed D value for this complex is positive (107.0 (kept as constant, C, in equation (3)).22
cm-1), indicating that the square pyramid arrangement shown in τ−1 = B1 / (1+B2H2) + C
(3)
complexes 1 and 2 seems to produce also easy plane CoII-N4X1 The relaxation time has been studied starting from its field
complexes, although more examples should be needed to extract a dependence at 2 K (Fig. S13 (left)) and is well defined by this
general conclusion. As may be observed, two complexes have method. Since the
negative D values (Table 2, entries 11-12); nevertheless, the E/D (Fig. S13 (left)), the direct term has not been taken into
value is in the limit (0.3), and thus the sign of D for those complexes consideration.
QTM has been calculated to be 1.36 × 10-4 s and 1.18
is not well defined and may well be positive, as hinted by other × 10-4 s for 1 and 2, respectively, using equation (3). These values
computed features (see ESI). suggest the presence of substantial contribution of ground state
τ parameter becomes larger for stronger fields
τ
To probe the magnetic relaxation dynamics of both complexes, AC quantum tunneling in the relaxation process, as observed in the
magnetic susceptibility measurements have been performed in the electronic structure calculations. The temperature dependence of
temperature range of 1.8-10 K at a 3.5 Oe ac field. No out-of-phase the relaxation time at 0.2 T was studied (Fig. S13 (right)), including
ac susceptibility (χ
M") signal was observed under a zero dc field. the thermally active Orbach and Raman processes.22 The relaxation
Nevertheless, upon application of a 2000 Oe dc field, both the time is well defined by the method with a single power law
complexes show temperature and frequency-dependent ac signals, (equation (4)).
typically observed for field-induced 3d-SIM species (Fig. 3 and S8- τ−1
=
τQTM-1 + bTn
(4)
S10). Furthermore, the Cole-Cole plots (Fig. 3 and S10) have been Values of n = 5.8 and 5.3 are obtained for complexes 1 and 2,
constructed from the frequency-dependent ac susceptibility data. respectively (note that the QTM values have been kept fixed at 1.36
The fit of the
M" vs χM′ data at each temperature, using the × 10-4 s and 1.18 × 10-4 s for 1 and 2, respectively), which are close
τ
χ
generalized Debye model,20 produces values of α within the ranges to the value reported by Colacio et al.23 The Orbach relaxation
0.03-0.22 (1) and 0.05-0.25 (2), showing a narrow distribution of the pathway is not applicable for the studied complexes because the
relaxation time. The effective energy barrier (Ueff) and relaxation energy barriers obtained from ac susceptibility measurements are
times (
ln(1/ ) = ln(1/
The best fit of equation (2) using the available data produces Ueff
τ
0) have been determined using the Arrhenius equation (2):21 lower than the energy gap between the MS = ± 1/2 and MS = ± 3/2
τ
τ0) - Ueff/kT
(2)
doublets. Hence, the combined study of field and temperature
=
dependence of the relaxation time suggests that quantum tunneling
37.8 K and
τ
0 = 8.2 × 10−6 s for 1, and Ueff = 34.5 K and
τ
0 = 6.7 × 10−6 is the leading process to relax the magnetization at low
s for 2 (Fig. S11).
temperature. Nevertheless, this relaxation mechanism is directed
Slightly higher energy barriers are found with the electronic by the optical or acoustic Raman processes which elucidate the
structure calculations, which allow the location of the lowest
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thermal dependence of the relaxation time.
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In summary, this work describes the magnetic anisotropy of the first
two pentacoordinate CoII SIMs based on P-donor ligand. The
experimental fit of the magnetic data indicates that the square
pyramid arrangement of the ligand around the metal center
produces positive D values, as confirmed by the electronic structure
calculations. Both experimental and computational approaches
indicate that the main relaxation process of the magnetization of
these compounds is the quantum tunneling through the ground
state.
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4 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C7CC02584J
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Table of Contents
Field induced slow magnetic relaxation behavior has been
studied for the first time for two P-donor ligand-based square-
pyramidal CoII complexes with an easy-plane magnetic
anisotropy.
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