Field effects to slow magnetic relaxation in a mononuclear Ni(II) complex

Article abstract of DOI:10.1039/c7cc03275g

A mononuclear Ni(ii) complex [Ni(NCS)₂(nqu)₂(H₂O)₂]·2nqu (nqu-5-nitroquinoline) shows a field induced slow magnetic relaxation with three relaxation domains. The relaxation time for the low-frequency mode is as slow as τ = 0.3 s at T = 1.9 K and B_DC = 0.4 T.

Full text of DOI:10.1039/c7cc03275g

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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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Field effect to slow magnetic relaxation in a mononuclear Ni(II)
complex
D. Lomjanský,a J. Moncoľb, C. Rajnáka, J. Titiša and R. Boča*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
A mononuclear Ni(II) comlex [Ni(NCS)2(nqu)2(H2O)2]·2nqu (nqu - 5- In searching for the slow magnetic relaxation in mononuclear
nitroquinoline) shows a field induced slow magnetic relaxation Ni(II)
complexes
we
prepared
the
compound
with three relaxation domains. The relaxation time for the low- [Ni(NCS)2(nqu)2(H2O)2]·2nqu (hereafter
1
)
where nqu
frequency mode is as slow as
τ
= 0.3 s at T = 1.9 K and BDC = 0.4 T.
abbreviates 5-nitroquinoline. Details about synthesis, analysis
and molecular spectra are forwarded to Electronic
supplementary information (ESI).
While data about slow magnetic relaxation in mononuclear
Co(II) complexes grew rapidly in the recent period,1 little is
known about single molecule magnetism (SMM) in
mononuclear Ni(II) complexes. The first SMM report on Ni(II)
complexes refers to the [Ni(pydc)(pydm)]·H2O complex
showing a field supported slow magnetic relaxation with two
relaxation
channels.2
Another
report
dealt
with
[Ni(mdabco)2Cl3]ClO4 complex.3 Next neighbour – the Cu(II)
complex was discriminated for a long time by the claim that
owing to absence of the axial zero-field splitting parameter the
magnetic relaxation should be fast. However, the
[CuII(pydc)(pydm)]2·H2O complex shows slow magnetic
relaxation with two relaxation channels as well.4 There is data
about a slow magnetic relaxation in dinuclear {CuII-EuIII} and
{CuII-LaIII} complexes where the slow magnetic relaxation is an
Figure 1 Molecular structure of 1 at 100 K.
inherent property of the Cu(II) centre.5 The family of single The complex
1 crystallizes in the triclinic system with the space
molecule ions (SIMs) with low spin S = 1/2 is enriched also by group P–1. Its crystal structure consists of complex molecules
low-spin Ni(I), Mn(IV) and V(IV) complexes.6 The existence of a [Ni(NCS)2(nqu)2(H2O)2] and uncoordinated 5-nitroquinoline
slow magnetic relaxation in low-spin systems is a task molecules (Figure 1).‡ The nickel(II) central atom lies in centre
provoking revision of the generally accepted theories that of symmetry, and is hexacoordinated by two nitrogen atoms of
SMMs can be found in systems with high spin possessing coordinated 5-nitroquinoline ligands [Ni1–N1 = 2.031(1) Å],
negative zero-field splitting parameter D that cause high- two water oxygen atoms [Ni1–O1W = 2.078(1) Å], and two
enough barrier to spin reversal
for the Kramers ions.
ꢀ
= |D|S2 or
ꢀ
= |D|(S2 – 1/4) thiocyanato nitrogen atoms [Ni1–N2 = 2.214(1) Å]. The
complex molecules and uncoordinated 5-nitroquinoline
molecules are connected through O–H···N hydrogen bonds
between coordinated water molecules and nitrogen atoms of
uncoordinated 5-nitroquinoline molecules [d(O1W···N4)
=
2.770(2) Å] (see ESI Fig S3, Table S3). The complex molecules
are joined through O–H···S hydrogen bonds between
coordinated water molecules and sulphur atoms of
thiocyanato ligands of neighbouring complex molecules
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[d(O1W···S1) = 3.213(2) Å] into iron-chain like supramolecular is this response also at BDC = 0.1 T. However, with increasing
chains (Fig. S3, Table S3). The coordinated and uncoordinated external field the
χ
5-nitroquinoline molecules are stacked (Fig S4). The π–π different frequency.
’’ component increases but differently for
stacking interactions7 are observed between benzene rings
0.2
8
[C4–C9] and benzene rings [C14–C19]; between benzene rings
1, T = 1.9 K
1.1 Hz
11 Hz
111 Hz
1116 Hz
[C4–C9] and pyridine rings [C11–C14/C19/N4]; between
6
pyridine rings [C1–C4/C9/N1] and benzene rings [C14–C19];
and between [C1–C4/C9/N1] and pyridine rings [C11–
C14/C19/N4] with centroid–centroid distances7 of 3.79, 3.50,
0.1
4
χ
χ
2
3.52 and 3.74 Å, respectively, and shift distances of 1.64, 0.90,
0.56 and 1.63 Å, respectively.
4
0.0
0
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
1, B = 0.1 T
B/T
B/T
T = 2.0 K
2
1
0
3
2
1
0
Figure 3. Field dependence of the AC susceptibility components (SI units). Lines serve as
a guide for eyes. Left – the in-phase component; right – the out-of-phase component.
ꢁ
6
4
6
5
4
ꢁ
T = 4.6 K
ꢁ
Frequency dependence of the AC susceptibility components
for a set of DC magnetic fields between BDC = 0.4 and 1.4 T is
displayed in Figure 4. In all cases three relaxation domains are
visible. The low-frequency channel (LF) occurs at f < 1 Hz which
causes that the relaxation time is τLF ~ 0.3 s (T = 1.9 K). At f ~
100 Hz an intermediate-frequency (IF) relaxation mode is
visible whereas around 1000 Hz an onset of the high-frequency
1.5
2.0
2.5
2
χ
0
0
10
20
30
40
300
0
50
100
150
200
250
0 1 2 3 4 5 6 7
T/K
B/T
Figure 2. DC magnetic functions for 1. Left – temperature dependence of the effective
magnetic moment; inset – molar magnetic susceptibility, right – magnetization per (HF) channel is recognized. Two or three relaxation channels
formula unit. Circles – experimental data; lines – a simultaneous fit of the susceptibility
are not rare: they were identified in a number of Dy(III), Co(II)
and also in Ni(II) and Cu(II) complexes.2,4,5,10 Owing to the
and magnetization data using the standard zero-field splitting model.
strong field dependence and also doping experiments the low-
The DC magnetic data was taken with the SQUID
frequency relaxation mode has been assigned to the
magnetometer (MPMS-XL7, Quantum Design) between 1.9
intermolecular interactions that create aggregates of various
and 300 K at the applied field of BDC = 0.1 T for the
size;11 on heating these oligomers are disintegrated to
susceptibility. Raw magnetic data was corrected for the
mononuclear entities belonging to the HF mode.
underlying diamagnetism. Temperature dependence of the
effective magnetic moment confirms a Curie-like behaviour
7
6
5
4
3
2
1
0
0.4
0.2
0.0
1, T = 1.9 K
BDC = 0.4 T
BDC = 0.6 T
BDC = 0.8 T
BDC = 1.0 T
BDC = 1.2 T
BDC = 1.4 T
between 300 – 20 K with almost constant ꢁeff = 3.24 ꢁB; on
further cooling it drops down to a limit of ꢁeff = 2.40 ꢁB at T =
1.9 K (Figure 2). This fingerprint of the zero-field splitting (zfs)
with
measurements since at T = 2.0 K and B = 7.0 T the
magnetization per formula unit is only Mmol/NA = 1.93 ꢁB
Fitting of the DC magnetic data (both, susceptibility and
magnetization) by the standard zfs model (see ESI) gave giso
2.180(27), D/hc = –5.86(69) cm-1, molecular field correction
zj/hc
0.020 cm-1, and the temperature-independent
D < 0 is confirmed also by the magnetization
.
χ
χ
=
10-1
100
101
102
103
10-1
100
101
102
103
f/Hz
f/Hz
=
magnetism χTIM = 0.99 × 10-9 m3 mol-1. Analogous predictions
offer the ab initio calculations using the ORCA package:8 gx =
2.220, gy = 2.224, gz = 2.277 (giso = 2.240), D/hc = –7.22 and
E/hc = 0.30 cm-1.#
Figure 4. Frequency dependence of the AC susceptibility components (SI units). Lines –
fitted with the three-set Debye model.
The
χ’ and χ’’ data (46 points) has been fitted by a threeꢁset
After corrections to the heterogeneous donor set,
Debye model
N
ꢀi = (di − di) , the structural distortion parameter is Dstr
ꢀx y)/ 2 = –16 pm and it matches predictions of the
magnetostructural D-correlation.9
= ꢀz –
χk
− χk −1
(1)
χ(ω) = χS +
∑
1−αk
(
+ ꢀ
k =1 1+ (iωτk
)
The AC susceptibility data was acquired at the amplitude of the
oscillating field BAC = 0.38 mT.§ The first scan refers to the field
dependence of the AC susceptibility components for four
representative frequencies (Figure 3). It can be seen that at
the zero field the out-of phase susceptibility component is
silent, pointing to a fast magnetic relaxation. Almost negligible
(N = 3) containing 10 free parameters: three isothermal
susceptibilities Tk, distribution parameters k, and relaxation
times τk along with the common adiabatic susceptibility χS
While the characteristics of the LF relaxation channel are
caught reliably, the parameters for IF and HF mode are less
reliable mostly owing to the lack of HF data passing through a
χ
α
.
2 | J. Name., 2012, 00, 1-3
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maximum (expected position is at f > 104 Hz); also the
adiabatic susceptibility χS in the high-frequency limit is not
fixed satisfactorily.
Notes and references
‡ Crystallographic data: single-crystal of 1 was mounted on Stoe
StadiVari diffractometer possessing PILATUS3R 300K detector and
microfocus source Xenocs FOX3D Cu HD (λ = 1.54186 Ǻ) at 100K.
The structure of 1 was solved by SHEXTL and refined by SHELXL (ver.
2016/6). Nitro groups of 5-nitrochinoline molecules are disordered
in two positions. The structure was drawn using OLEX2 program.12
Crystal data for 1: C38H28N10NiO10S2, triclinic P-1, a = 8.0112(3), b =
8.0734(3), c = 14.9981(5) Å, α = 82.284(3), β = 81.302(3), γ =
76.188(3) deg, V = 926.28(6) Å3, Z = 1, Dc = 1.627 g cm-3, μ = 2.473
mm-1, F(000) = 466, T = 100(1) K, 2θmax = 142.68o (-9 ≤ h ≤ 7, -9 ≤ k
≤ 9, -18 ≤ l ≤16). Final results (313 parameters and 3 restraints): R1
= 0.0314 and wR2 = 0.0883 [I > 2σ(I)], and R1 = 0.0351, wR2 =
0.0899 and S = 1.034 for all 22071 reflections. CCDC reference
number 1540217.
The difference in the height of the LF peak is expressive when
passing from BDC = 0.4 to 0.6 and 0.8 T. Next increase of the
field to 1.0 and 1.2 T brings only a little effect but at BDC = 1.4 T
a decrease of the height of the LF peak is registered.
With increasing temperature the AC susceptibility data are
more scattered and thus they are hardly fitted in order be
suitable for Argand and/or Arrhenius like diagrams.
A comparison of the SIM parameters of
1 with two so far
reported Ni(II) SIMs, the haxacoordinate complex
[Ni(pydc)(pydm)]·H2O
[Ni(mdabco)2Cl3]ClO4 (
(
2
)
and pentacoordinate complex
3
), is complicated by the fact that they
# The ab initio calculations have been performed for the whole
complex in its experimental geometry using complete active space
self-consistent field (CASSCF) method improved by the second-
order N-electron valence perturbation theory (NEVPT2). An active
space in which eight electrons are distributed into the five nickel d-
orbitals (CAS(8,5)) was employed along with the TZVP basis set for
depend not only upon temperature but also on the external
DC field.2,3 Usually the adiabatic susceptibility χS (in the high-
frequency limit f
with the applied BDC
There are two relaxation modes in
ꢀ ¶) decreases with temperature and also
.
2
with the relaxation times
s, respectively, at T = 1.9 K and BDC
mole fraction evaluated as
χS = 0.31. The height of the LF peak
(given by the thermal susceptibility χT(LF) on f 0) decreases
progressively with temperature confirming that the mole
fraction xLF decreases in favour of xHF
For the AC susceptibility data was reported only for
all elements. In CASSCF procedure, the orbitals were optimized for
τ
LF = 82 ms and
0.2 T, and
xLF = (χT(LF)
τ
HF = 527
ꢁ
=
3
the average of 10 triplet (3F and P terms of the free Ni(II)) and 15
the
singlet (1G, 1D and 1S terms) roots. The D-values were calculated
through quasi-degenerate perturbation theory (QDPT) in which the
spin-orbit coupling (SOC) operator (in SOMF approximation) is
diagonalized in the basis of the non-relativistic SA-CASSCF/NEVPT2
eigenfunctions.
−
χ
S )/(χT(HF)
−
)
ꢀ
.
3
§ Technical note: Some AC susceptibility data are uncommonly
scattered owing to a weak signal arising from the S = 1 spin system
and small amplitude of the AC field. Also fluctuations of the DC
magnetic field raise the possible scattering. Each experimental
point is an average of ten scans (each averaging four blocks) with
omission of data outside 0.7σ interval. Data taking below f < 1 Hz is
tedious: time to average is as long as 79 s for f = 0.1 s.
frequencies f > 100 Hz so that one cannot conclude about a
possible LF relaxation channel below this window. However,
the deviations from nice semi-circles suggest that a variety of
processes are contributing to the relaxation (SIM parameters
are not displayed).
A maximum of the out-of phase
susceptibility component at f = 3000 Hz implies
at T = 2.0 K and BDC = 0.2 T.
τ(HF) = 53 ꢁs
1
(a) G. A. Craig and A. Murrie, Chem. Soc. Rev., 2015, 44,
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1
the position of the peak at the out-of phase susceptibility
(LF) = 275(23) ms at T =
component lies at f = 0.6 Hz and thus τ
1.9 K and BDC = 0.4 T. This is the longest relaxation time among
M. Harriman and M. Murugesu, Chem. Sci., 2016,
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336(37) ms. Notice, this system possesses only a moderate
zero-field splitting parameter D = –6 cm-1 unlike to D = –12 and
–311 cm-1 reported previously for
2 and 3. Thus it is less
favoured for the SIM behaviour. Contrary to this handicap, the
system possesses a very long relaxation time.
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Acknowledgments
Slovak grant agencies (APVV-14-0078, APVV-16-0039 and
VEGA 1/0534/16) are acknowledged for the financial support.
This article was created with the support of the Research and
Development Operational Programme for the project
"University Science Park of STU Bratislava" (ITMS project no.
26240220084) co-funded by the European Regional
Development Fund.
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TOC
A mononuclear Ni(II) comlex [Ni(NCS)2(nqu)2(H2O)2]·2nqu (nqu ꢀ 5ꢀnitroquinoline) shows a
field induced slow magnetic relaxation with three relaxation domains. The relaxation time for
the lowꢀfrequency mode is as slow as τ = 0.3 s at T = 1.9 K and BDC = 0.4 T.