Magnetic Anisotropy: Structural Correlation of a Series of Chromium(II)-Amidinate Complexes

Article abstract of DOI:10.1021/acs.inorgchem.0c02065

Systematic substituent variations on amidinate ligands bring delicate changes of CrN4 coordination in a family of chromium(II) complexes with the common formula of Cr(RNC(CH3)NR)2, where R = iPr (1), Cy (2), Dipp (Dipp = 2, 6-diisopropylphenyl) (3), and tBu (4). With the largest substituent group, 4 shows the largest distortion of the N4 coordination geometry from square-planar to seesaw shape, which leads to its field-induced single-molecule magnet (SMM) behavior. This is an indication that 4 has the strongest axial magnetic anisotropy and/or optimized magnetic relaxation process. Combined with high-frequency/field electron paramagnetic resonance (HF-EPR) experiments and ab initio calculations, we deduce that the smallest energy gap between ground 4ψ0 and the first excited 4ψ1 orbitals in 4 contributes the most to its strongest magnetic anisotropy. Moreover, the lower E value of 4 ensures its being a field-induced SMM. Specifically, the D and E values were found to be correlated to the dihedral angle between the ΔN1CrN2 and ΔN3CrN4 triangles, indicating that distortion from ideal square-planar geometry to the seesaw help increase axial magnetic anisotropy and suppress the transversal part. Thus, the study on this system not only expands the family of Cr(II)-based SMMs but also contributes to a deeper understanding of magneto-structural correlation in four-coordinate Cr(II) SMMs.

Full text of DOI:10.1021/acs.inorgchem.0c02065

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Magnetic Anisotropy: Structural Correlation of a Series of
Chromium(II)−Amidinate Complexes
Yuan-Qi Zhai, Ning Ge, Zi-Han Li, Wei-Peng Chen, Tian Han, Zhong-Wen Ouyang, Zhenxing Wang,*
and Yan-Zhen Zheng*
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ABSTRACT: Systematic substituent variations on amidinate ligands bring
delicate changes of CrN4 coordination in a family of chromium(II) complexes
with the common formula of Cr(RNC(CH3)NR)2, where R = iPr (1), Cy (2),
Dipp (Dipp = 2, 6-diisopropylphenyl) (3), and tBu (4). With the largest
substituent group, 4 shows the largest distortion of the N4 coordination
geometry from square-planar to seesaw shape, which leads to its field-induced
single-molecule magnet (SMM) behavior. This is an indication that 4 has the
strongest axial magnetic anisotropy and/or optimized magnetic relaxation
process. Combined with high-frequency/field electron paramagnetic reso-
nance (HF-EPR) experiments and ab initio calculations, we deduce that the
smallest energy gap between ground 4Ψ0 and the first excited 4Ψ1 orbitals in 4
contributes the most to its strongest magnetic anisotropy. Moreover, the lower
E value of 4 ensures its being a field-induced SMM. Specifically, the D and E
values were found to be correlated to the dihedral angle between the ΔN1CrN2 and ΔN3CrN4 triangles, indicating that distortion
from ideal square-planar geometry to the seesaw help increase axial magnetic anisotropy and suppress the transversal part. Thus, the
study on this system not only expands the family of Cr(II)-based SMMs but also contributes to a deeper understanding of magneto-
structural correlation in four-coordinate Cr(II) SMMs.
axial zero-field splitting parameter, the D value, at a 14−15°
deviation from a tetrahedron.10
INTRODUCTION
■
Magneto-structure correlations which can guide the chemical
synthesis for specific magnetism of coordination complexes are
fundamentally important. As magnetic anisotropy is the
decisive factor in making high-performance single-ion magnets
(SIMs) or mononuclear single-molecule magnets (SMMs), the
correlation between the structure and magnetic anisotropy of
such complexes is crucial.1−3 However, unambiguous magneto-
structural correlation needs systematic study of a series of
relevant complexes, which posts a challenge for synthetic
chemists. So far, there are two geometries that are well
understood. The first one is the linear two-coordinate
geometry. These complexes, such as [(NHC)CoNDmp]
(NHC = siPr, Dmp = 2,6-dimesitylphenyl),4 [Ni-
(MDABCO)2Cl3]ClO4 (MDABCO+ = 1-methyl-4-aza-1
azoniabicyclo[2.2.2]octanium cation),5 {[Fe(C-
(SiMe3)3)2]0 or −1},6,7 FeL2 (L = N(SiMe3)(Dipp), N(H)Ar′,
N(H)Ar*),6 and [Co(C(SiMe2ONaph)3)2]8 (Naph is a
naphthyl group), show strong axial magnetic anisotropy,
which means this coordination mode is suitable to enhance
the axial magnetic anisotropy for metal ions such as iron(I or
II), cobalt(II), and nickel(II). The other one is the four-
coordinate geometry, but this is only for cobalt(II)
complexes.9,10 This geometry shows an intriguing maximized
In contrast to cobalt(II)8−12 and iron(I or II),6,7,13−15 other
3d transition-metal-based SIMs are less systematically studied.
Chromium, whose dominant isotopes have no nuclear spin, is
very attractive. Until now, only a limited number of Cr(II)
SMMs have been reported, including three multinuclear cluster
SMMs, namely, [Cr5(tpda)4Cl2] (where H2tpda = N2,N6-
di(pyridin-2-yl)pyridine-2,6-diamine),16 {SrCr2(1,10-phenan-
throline)2(C2O4)4}·H2O, and {SrCr2(2,2′-bipyridi-
ne)2(C2O4)4(H2O)}·5H2O,17 as well as two SIMs, namely,
[Cr(N(TMS)2)2(pyridine)2] (TMS = SiMe3), [Cr(N-
(TMS)2)2(tetrahydrofuran)2],18 and {[(η1-N-3,5-tBu2dp)4Cr]-
[(η5-(N,N,C,C,P))2-K(η1-O-THF)2]2} (dp− = 1,2-diaza-4-
phospholide).19 In this aspect, a study on Cr(II)-based SIMs
would be very interesting.
Herein, we report four mononuclear Cr(II) amidinate
complexes, namely, Cr(iPrNC(CH3)NiPr)2 (iPrNC(CH3)-
Received: July 13, 2020
Published: January 11, 2021
https://dx.doi.org/10.1021/acs.inorgchem.0c02065
© 2021 American Chemical Society
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NiPr = N,N′-diisopropylamidinate) 1, Cr(CyNC(CH3)NCy)2
(CyNC(CH3)NCy = N,N′-dicyclohexylamidinate) 2, Cr-
(DippNC(CH3)NDipp)2 (DippNC(CH3)NDipp = N,N′-bis-
(2,6-diisopropylphenyl)amidinate) 3, and Cr(tBuNC(CH3)-
NtBu)2 (tBuNC(CH3)NtBu = N,N′-ditert-butylamidinate) 4.
Each of the complexes feature a similar CrN4 coordination
environment. However, with the variation induced by different
substituents, complex 4 with the largest tBu group exhibits an
obvious field-induced slow magnetic relaxation behavior.
Further high-frequency/field electron paramagnetic resonance
(HF-EPR) experiments and ab initio calculations indicate the
largest axial magnetic anisotropy of 4 and reveal that the
distortion from ideal square-planar geometry to the seesaw
enhances the axial magnetic anisotropy in this series of Cr(II)
complexes. Hence, a magneto-structural correlation of the
magnetic anisotropy and the four coordinate Cr(II) with a N4
first coordination sphere is elaborated in this work.
RESULTS AND DISCUSSION
■
Syntheses. The syntheses of the corresponding lithium
salts, namely, [iPrNC(CH3)NiPr]Li(Et2O), [CyNC(CH3)-
NCy]Li(Et2O), [DippNC(CH3)NDipp]Li(Et2O), and
[tBuNC(CH3)NtBu]Li(Et2O), are detailed in the Supporting
Information (Scheme 1). Compounds 1−4 were obtained by
Figure 1. (Left) Molecular structures of 1−4 (a−d). Hydrogen atoms
are not drawn for the sake of clarity. Color codes: Cr, pink; C, gray;
N, blue. (Right) The diagrammatic sketch of CrN4 geometry, and the
atom positions correspond to the left ones.
Scheme 1. Synthetic Route for Compounds 1−4
Complex 3 crystallizes in same space group as complex 1
(Figure 1c). In this case, the Cr(II) ion and four N atoms are
no longer coplanar, where the dihedral angle between the
ΔN1CrN2 and ΔN3CrN4 triangles is 18(1)°, indicating the
existence of a steric hindrance effect. The averaged bond
length of Cr−N bonds is 2.05(1) Å, while the acute N−Cr−N
angle is 64(1)°. The distance of the shortest intermolecular
Cr···Cr separation is 10.9 Å (Figure S1c).
Complex 4 crystallizes in monoclinic space group P2/n,
which is different from the above three complexes. Two
independent molecules were localized into one unit cell
(Figure 1d). For the first one, named by Cr1, owing to the
further increase of steric hindrance by the -tBu substituent on
the ligands, the dihedral angle between the ΔN1Cr1N2 and
ΔN1aCr1N2a triangles becomes larger, being 44(1)°. However,
the average Cr1−N bond length (2.09(1) Å) and the acute
N1−Cr1−N2 (N1a−Cr−N2a as well) angles (64(0)°) remain
similar to those of compound 1. For Cr2, the average Cr−N
bond length and the acute N−Cr−N angle are 2.06(1) Å and
64(0)°, respectively, while the dihedral angle between the
ΔN3Cr2N4 and ΔN3bCr2N4b triangles remain nearly un-
changed at 43(1)°. The distance of shortest intermolecular
Cr···Cr separation is 7.7 Å (Figure S1d).
Magnetic Properties. Direct current (dc) susceptibility
measurements were conducted on powder samples of
complexes 1−4 under a 1000 Oe dc field and between 2
and 300 K (Figure S2). The room temperature χMT values are
3.05, 2.87, 2.77, and 2.87 cm3 K mol−1 for 1, 2, 3, and 4,
respectively. The values for the four complexes were consistent
with the expected value 2.88 cm3 K mol−1 calculated from a
free high-spin Cr(II) ion with a g factor of 1.96. The χMT value
decreases continuously upon cooling from 300 to 20 K, while
below 20 K it meets a sharp drop. The low-temperature
magnetization data (inset of Figure S2) for 1−4 are similar,
where magnetization values continuously increase until 7 T
treatment of anhydrous CrCl2 with the corresponding lithium
salt (1:1) in THF, according to the literature-reported
synthetic procedure.20−22 All compounds were isolated as a
crystalline solid crystallized in pentane at −35 °C (for details,
see the Supporting Information).
Structures. Single-crystal X-ray diffraction analyses were
performed, revealing that complex 1 crystallizes in triclinic
̅
space group P1. The central Cr(II) ion displays a rectangular
configuration with four N atoms of two [iPrNC(CH3)NiPr]−
ligands coordinated in the equatorial plane (Figure 1a). The
four Cr−N bond lengths are almost the same (ca. 2.08(1) Å),
and the acute N−Cr−N angle averages at 63(1)°. The crystal
packing diagram (Figure S1a) indicates that the shortest
intermolecular separation among Cr(II) ions is 6.5 Å.
The crystal structure of complex 2 is similar to that of 1 but
the crystal space group is different (monoclinic, P21/c). Two
[CyNC(CH3)NCy]− ligands are coordinated to the central
Cr(II) ion, keeping four coordinated N atoms sharing the same
plane with the Cr(II) ion (Figure 1b). The higher symmetry
leads the Cr−N distances and the acute N−Cr−N angles to all
be exactly the same (2.09(4) Å for the bonds and 64(0)° for
the angles). The shortest Cr···Cr separation is 7.4 Å between
neighboring molecules (Figure S1b).
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without saturation (the maximum values are 3.72, 3.43, 3.32,
and 3.45 Nβ for 1−4, respectively), suggesting strong magnetic
anisotropy for these complexes.
was no improvement for 1−3 under the magnetic field range of
0−5000 Oe (Figure S4). While an optimum field of 3000 Oe
was found for 4 to significantly suppress the QTM effect
(Figure 2). The generalized Debye model was applicable for
the fitting of Cole−Cole plots for 4, which reveal the
distribution of relaxation times (τ). The ln (τ) vs 1/T plot
shows a nice linear relationship, and thus, only Orbach process
́
To gain the Lande factor (g), axial zero-field splitting (ZFS)
parameter (D), and rhombic ZFS parameter (E) defined in the
following spin Hamiltonian, we performed simulations on both
magnetic susceptibility and magnetization plots using the
program PHI.23
was considered in the fitting. Using the equation τ−1 = τ0
−1
exp(−Ueff/kBT) the effective energy barrier (Ueff) and the
prefactor τ0 were then determined as 8(2) K and 9(4) × 10−6
s, respectively.
i
j
j
2
z
1
3
y
z
z
2
x
2
̂
̂
̂
̂
̂
̂
y
̂
H = D S − S(S + 1) + E(S − S ) + μ gS·B
j
z
B
(1)
k
{
HF-EPR Studies. Multifrequency HF-EPR experiments at
4.2 K were performed on samples 1−4 (details are in the
Supporting Information). Figure S5 shows the raw HF-EPR
spectra of polycrystalline powder samples of 1−4. The HF-
EPR spectra in the frequency limited up to 448 GHz were
observed with strong resonances though some shaky waves
were present in the plots due to the small amount of samples.
The obtained variable-frequency resonance results were
plotted in Figure 3 (left) as resonant field (H) vs frequency
(v) 2D plots. To investigate the ZFS parameters from the
above-mentioned spin Hamiltonian, we first need to
distinguish the allowed (|ΔMs| = 1) transitions from the
nominally forbidden (|ΔMs| > 1) ones.25 Such 2D map was
previously obtained in other d4 systems.26
Then, a least-squares fit was used for simulating to the in-
phase HF-EPR resonances, with the resulted parameters listed
in Table 2.
Moreover, the sign of D values can be confirmed by
performing the simulating of HF-EPR spectra at selected
frequency for each complex (Figure 3 (right)). It is found that,
in all cases, negative D values reproduced better simulation
curves to the experimental plots than positive ones.
To further assign the location of spin states and state
transitions between them, we plot the Zeeman splitting
diagrams for the complexes 1−4 (Figure 4) in three
orthogonal directions where the z direction represents the
principal ZFS axis. Along the z direction, a strong EPR signal
was observed at 2.77 T (for selected 230 GHz frequency), 0.22
T (at 170 GHz frequency), 3.38 T (at 250 GHz frequency),
and for 1−4, respectively. Such an obvious signal was
corresponding to the energy transition of |2, −2⟩ → |2, −1⟩
states (where the state represents |S, Ms⟩). Besides this, a very
weak EPR signal was found for 4 at 10.32 T owing to a |2, 0⟩
→ |2, 1⟩ transition between excited states but with relatively
low occupancy. Along the x direction, more than one
resonance was found for all complexes. For 1, the subsequent
resonances at 7.09 and 10.79 T can be attributed to the energy
transition of |2, −1⟩ → |2, + 1⟩ and |2, −2⟩ → |2, + 2⟩. For 2,
when B||x, four resonances were observed almost in the same
resonant magnetic field (1.87, 4.79, 6.63, and 9.34 T), referring
to |2, −1⟩ → |2, 0⟩, |2, −1⟩ → |2, + 1⟩, |2, + 2⟩ → |2, −1⟩, and |
2, −2⟩ → |2, + 2⟩, respectively. For 3, when B||x, three
resonances (6.86, 8.92, and 11.96 T) were observed referring
to |2, −2⟩ → |2, + 2⟩, |2, + 2⟩ → |2, −1⟩, and |2, −1⟩ → |2, +
1⟩. For 4, when B||x, two resonances (9.25 and 12.09 T) were
observed referring to |2, −2⟩ → |2, + 2⟩ and |2, + 2⟩ → |2, −1⟩.
Along the y direction, more than one resonance was found for
all complexes. Along the y direction, situations were similar to
the x direction. For 1, transitions at 7.04, 8.98, and 11.51 T
were viewed as energy transitions of |2, −1⟩ → |2, + 1⟩, |2, + 2⟩
→ |2, −1⟩, and |2, −2⟩ → |2, + 2⟩, respectively. For 2, when B||
y, four resonances were found at the similar positions as those
The best simulations were yielded in Figure S2, with
parameters listed in Table 1.
Table 1. Spin Hamiltonian Parameters from the Fitting of
Magnetic Properties
a
D (cm−1
)
E (cm−1
)
giso
χTIP (cm3 mol−1
)
1
2
3
4
−2.1(6)
−2.2(5)
−2.2(2)
−2.7(1)
0.1(4)
0.1(4)
0.0(1)
0.0(1)
1.99(7)
1.96(5)
1.91(1)
1.89(1)
1.1 × 10−3
3.6 × 10−4
4.5 × 10−4
6.3 × 10−4
a
χTIP represents temperature-independent paramagnetic component.
The D values for 1−3 are much smaller than that of 4,
whereas the E value of 4 is the smallest in the whole system.
Here, the obtained ZFS parameters are close to our previously
reported square-planar Cr(II)-based single-ion magnets
(SIMs).18 Meanwhile, the attempt of using positive D values
in fitting was unsuccessful.
Ac magnetic susceptibility measurements were performed on
investigating the dynamic magnetic relaxation of 1−4 (Figures
2, S3, and S4). In the absence of a dc applied field, no signals
of χ″ ac susceptibility appear for all the four complexes,
indicating a high quantum tunnelling of magnetization (QTM)
rate.24 Then frequency-dependent ac measurements were
carried out under various magnetic fields. However, there
Figure 2. Temperature (a) and frequency (b) dependence of the in-
phase (χ′) and out-of-phase (χ″) ac susceptibility and Cole−Cole
plots (c) under a 3000 Oe applied dc field for 4. The solid lines are
guides for the eyes. (d) The corresponding relaxation times for 4.
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Computational Studies. The ab initio calculations with
CASSCF/NEVPT2 protocol implemented in Orca 4.0 were
performed to study the magnetic anisotropy of transition metal
ions on 1−4.27 The X-ray crystallographically determined
structures were directly used for calculations. The ZFS
parameters have been calculated, showing negative D values
for 1−4 (Table 3), which agree well with the experimental
results.28
To determine the D values, the following equation was used:
Ä
É
Å
Ñ
Å
Å
Ñ
Ñ
4
9
1
1
2
Å
Å
Å
Ñ
Ñ
Ñ
D = ζeff
−
4
4
4
4
Å
Å
Å
Ñ
Ñ
Ñ
Δ( Ψ − Ψ )
Δ( Ψ − Ψ )
(2)
0
2
1
0
Å
Ç
Ñ
Ö
Equation 2 indicates that the magnetic anisotropy is arising
from the energy gap between the low-lying first excited state
(4Ψ1) and the second excited state (4Ψ2). For 1, the Ψ1 and
4
4Ψ2 states lie at 16359.7 and 17108.5 cm−1 above the ground
state ones. The contribution to the negative D values that
comes from the Ψ1 state is −0.78 cm−1, while from the Ψ2
state is −0.70 cm−1, indicating that the energy gap between the
ground state and two low-lying excited states determine the
axial magnetic anisotropy in 1.
4
4
For 2 and 3, the situations are similar, where energies (in
cm−1) of 4Ψ1 and 4Ψ2 states are 19142.6 and 19701.6 for 2 and
are 18906.4 and 19100.6, for 3, respectively. The contributions
of D values from the former one are −0.74 and −0.76 cm−1
while they are −0.70 and −0.68 cm−1 from the latter one, for 2
and 3, respectively. For 4, the results for two individual Cr1
and Cr2 sites are similar due to similar coordination
geometries. The magnitude of the axial ZFS parameter of
Cr1 (Cr2) is much larger than that for the other three
complexes, which is mainly due to the stronger contribution
from the first excited quintet state with −0.97 (−1.04) cm−1.
Meanwhile, the 4Ψ1 state is much lower than the others, which
contributes to the stronger magnetic anisotropy to 4 according
to eq 2. Compared to 1−3, the most relevant factor is the
coordination geometry of the central Cr(II) ion. In this case,
4
Figure 3. (Left) H vs v plots at EPR transitions for 1 (a), 2 (c), 3 (e),
and 4 (g) at 4.2 K. The squares stand for experimental data with error
bars. Black, red, and blue curves are simulation lines to the
experimentally determined EPR transition points along the magnetic
field B following the x, y, and z directions, respectively. The green
curves represent resonances from off-axis transitions. (Right)
Experimental spectrum (black traces) of 1 (b), 2 (d), 3 (f), and 4
(h) at selected frequencies. Red and blue curves are simulated plots
using positive and negative D values, respectively.
the obvious lower-lying Ψ1 state in 4 is owed to the more
dramatic structure distortion from square-planar to seesaw
type. Such a magneto-structural correlation proposed here is
somehow similar to a previously reported CoN4 system.9
As presented in Figure 5, the five active orbitals of 4 are
mostly constructed by 3d-orbitals of the central Cr(II) ion
together with slight contributions from coordinated ligands.
The situations are similar for other three complexes (Figures
S7−S9). To further analyze the ab initio wave functions, we
deduce a clear connection between the spin excitation among
3d-orbitals and the energy gaps of states, which is suitable for
Table 2. Spin Hamiltonian Parameters from the Fitting of
HF-EPR Spectra
4
3d transition-ion-based SIM systems. In the case of Ψ0 and
D (cm−1
)
E (cm−1
)
gx
gy
gz
4Ψ1, the dx −y → dyz transition is responsible for the energy
difference of such states and contributes to the negative D
2
2
1
2
3
4
−1.74(13)
−1.82(17)
−1.71(11)
−1.94(10)
0.07(4)
0.09(3)
0.06(3)
0.03(5)
1.98(2)
1.97(2)
1.98(3)
1.98(2)
1.96(3)
1.97(2)
1.97(2)
1.96(1)
2.00(1)
2.00(1)
2.00(1)
2.00(1)
2
values. In case of the dz → dyz transition, on the contrary, it
contributes to the positive D values. Taken together, we can
4
4
see that the energy gap between Ψ0 and Ψ1 states offers a
negative D value, resulting in the axial magnetic anisotropy of
1−4.29
in B||x, referring to |2, −1⟩ → |2, 0⟩, |2, −1⟩ → |2, + 1⟩, |2, + 2⟩
→ |2, −1⟩, and |2, −2⟩ → |2, + 2⟩, respectively. For 3, only one
resonance at a magnetic field of 11.00 T for B||y belonged to |2,
−2⟩ → |2, + 2⟩. For 4, the transition for B||y at 11.35 T
belonged to |2, −2⟩ → |2, + 2⟩.
The negative D values of 1−4 indicated their uniaxial type
magnetic anisotropy, and 4 possessed the largest magnitude.
Here, we can rationalize a much lower E value for 4.
Based on the above results, we are able to explain the reason
why complex 4 is a field-induced SIM while others are not.
The most obvious difference on 4 is that E value, which is
much smaller than those of 1−3, leading to lower strength of
the QTM, as proven by both ab initio calculation and EPR
simulations. The average Cr−N bond lengths for 1−4 range
from 2.05−2.09 Å while the N1−Cr−N2 angles range from
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Figure 4. Magnetic field-dependent energy levels for 1−4 (a−d) in the xyz directions where the z orientation represents the principal ZFS axis.
Arrows stand for EPR transitions along x (red), y (blue), and z (black) in a selected frequency.
Table 3. Parameters of ORCA CASSCF+NEVPT2
Calculations for 1−4
plotted the correlation in Figure 6. When ϕ is zero, the
magnitude of D value is the smallest and that of E value is the
largest. Upon increasing ϕ from 0 to 42°, i.e., varying the
coordination geometry from square to seesaw, D values
increase steadily by 13% and E values decrease sharply by
66%. Hence, we deduced that four-coordinate Cr(II)
complexes with the seesaw geometry can dramatically lift the
magnitude of D value while reducing the E value, resulting in a
good SMM candidate. The tendency is obvious, but we cannot
obtain quantitative correlations.
1
2
3
4(Cr1) 4(Cr2)
D (cm−1) E/D
S = 2
−1.50
0.02
1.98
1.99
1.99
−1.47
0.02
1.98
1.99
1.99
−1.48
0.02
1.98
1.99
1.99
−1.68
0.00
1.93
1.95
2.00
−1.71
0.00
1.93
1.95
2.00
gx
gy
gz
63−64°, which do not exhibit much difference. However, the
dihedral angles, defined by the angle between ΔN1CrN2 and
ΔN1aCrN2a triangles (ϕ), show obvious distinction from zero
for 1 and 2 to 42° for 4. Thus, we developed a magneto-
structure correlation between this dihedral angle and ZFS
parameters (D and E values from HF-EPR simulations) and
CONCLUSIONS
■
By using four amidinate ligands we systematically studied the
peripheral substituents on magneto-structure correlations of a
series of Cr(II) complexes with a similar CrN4 coordination
environment but different degree of distortion away from the
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free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
■
Yan-Zhen Zheng − Xi’an Jiaotong University Shenzhen
Research School, Frontier Institute of Science and Technology
(FIST), State Key Laboratory for Mechanical Behavior of
Materials, MOE Key Laboratory for Nonequilibrium
Synthesis of Condensed Matter, Xi’an Key Laboratory of
Sustainable Energy and Materials Chemistry and School of
Chemistry, Xi’an Jiaotong University, Xi’an 710054, China;
orcid.org/0000-0003-4056-097X;
Email: zheng.yanzhen@xjtu.edu.cn
Zhenxing Wang − Wuhan National High Magnetic Field
Centre & School of Physics, Huazhong University of Science
and Technology, Wuhan 430074, China; orcid.org/0000-
0003-2199-4684; Email: zxwang@hust.edu.cn
Figure 5. NEVPT2 computed active orbital energy orderings for the
lowest-lying quintet state for 4.
Authors
Yuan-Qi Zhai − Xi’an Jiaotong University Shenzhen Research
School, Frontier Institute of Science and Technology (FIST),
State Key Laboratory for Mechanical Behavior of Materials,
MOE Key Laboratory for Nonequilibrium Synthesis of
Condensed Matter, Xi’an Key Laboratory of Sustainable
Energy and Materials Chemistry and School of Chemistry,
Xi’an Jiaotong University, Xi’an 710054, China
Ning Ge − Xi’an Jiaotong University Shenzhen Research
School, Frontier Institute of Science and Technology (FIST),
State Key Laboratory for Mechanical Behavior of Materials,
MOE Key Laboratory for Nonequilibrium Synthesis of
Condensed Matter, Xi’an Key Laboratory of Sustainable
Energy and Materials Chemistry and School of Chemistry,
Xi’an Jiaotong University, Xi’an 710054, China
Zi-Han Li − Xi’an Jiaotong University Shenzhen Research
School, Frontier Institute of Science and Technology (FIST),
State Key Laboratory for Mechanical Behavior of Materials,
MOE Key Laboratory for Nonequilibrium Synthesis of
Condensed Matter, Xi’an Key Laboratory of Sustainable
Energy and Materials Chemistry and School of Chemistry,
Xi’an Jiaotong University, Xi’an 710054, China
Wei-Peng Chen − Xi’an Jiaotong University Shenzhen
Research School, Frontier Institute of Science and Technology
(FIST), State Key Laboratory for Mechanical Behavior of
Materials, MOE Key Laboratory for Nonequilibrium
Synthesis of Condensed Matter, Xi’an Key Laboratory of
Sustainable Energy and Materials Chemistry and School of
Chemistry, Xi’an Jiaotong University, Xi’an 710054, China
Tian Han − Xi’an Jiaotong University Shenzhen Research
School, Frontier Institute of Science and Technology (FIST),
State Key Laboratory for Mechanical Behavior of Materials,
MOE Key Laboratory for Nonequilibrium Synthesis of
Condensed Matter, Xi’an Key Laboratory of Sustainable
Energy and Materials Chemistry and School of Chemistry,
Xi’an Jiaotong University, Xi’an 710054, China;
Figure 6. Correlation of dihedral angle (ϕ) between the ΔN1CrN2
and ΔN3CrN4 triangles with D (left) and E (right) values from HF-
EPR simulations (black dots). The parameters for 4 are averaged from
those of Cr(1) and Cr(2).
rectangular geometry. Magnetic measurements show that only
the largest distorted complex, namely, 4, shows field-depend-
ent SIM behaviors. The HF-EPR study shows a relatively lower
transversal contribution of ZFS parameter for 4. Combined
with ab initio calculation results, we find that the smaller energy
4
gap between 4Ψ0 and Ψ1 orbitals leads to the largest negative
D value for 4, while the smaller E value enhances its field-
induced SIM behavior. Finally, the qualitative magneto-
structure correlations between E(ϕ) and ZFS have been
elaborated, indicating that the geometry distortion from square
to seesaw tends to diminish the transversal magnetic
anisotropy of the Cr(II) ion.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c02065.
■
sı
Crystallographic data (Table S1), bond distances and
angles (Table S2), magnetic relaxation data (Table S3),
ab initio calculation data (Tables S4−S6); crystallo-
graphic packing plots (Figure S1), static magnetic data
(Figure S2), dynamic magnetic data (Figures S3 and
S4), HF-EPR spectra (Figures S5 and S6), and NEVPT2
computed active orbitals (Figures S7−S9) (PDF)
orcid.org/0000-0002-8753-7999
Zhong-Wen Ouyang − Wuhan National High Magnetic Field
Centre & School of Physics, Huazhong University of Science
and Technology, Wuhan 430074, China
Accession Codes
CCDC 2011865−2011868 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c02065
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https://dx.doi.org/10.1021/acs.inorgchem.0c02065
Inorg. Chem. 2021, 60, 1344−1351

Inorganic Chemistry
pubs.acs.org/IC
Article
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
This work was supported by the Shenzhen Science and
Technology Program (JCYJ20180306170859634), the Na-
tional Natural Science Foundation of China (21701046,
21773130, 21801202, and 21971203), the Shaanxi National
Science Foundation (2019JQ-016), the China Postdoctoral
Science Foundation (2019T120891 and 2018M643615), the
Key Laboratory Construction Program of Xi’an Municipal
Bureau of Science and Technology (201805056ZD7CG40),
the Key Scientific and Technological Innovation Team of
Shaanxi Province (2020TD-001), the Cyrus Chung Ying Tang
Foundation, and the Fundamental Research Funds for Central
Universities.
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