A tetracoordinate Co(II) single molecule magnet based on triphenylphosphine and isothiocyanato group

Article abstract of DOI:10.1016/j.poly.2016.02.026

A simple Co(II) mononuclear complex [Co(PPh₃)₂(NCS)₂] has been studied in details. Its crystal and molecular structure is reported along with its IR a UV–Vis spectra. The DC magnetization studies confirm a sizable zero-field splitting D/hc?=??9.4?cm^?1that is also confirmed by quantum-chemical calculations. The AC susceptibility data shows that there is a slow magnetic relaxation with two relaxation branches. The low-frequency branch occurs at about f?=?1–10?Hz and it tends to disappear progressively on heating. The high-frequency branch exists at the frequency ranges above f?>?200?Hz and this is shifted above the hardware limit (1500?Hz) on heating. The characteristics of this field-induced single molecule magnet strongly depend upon the applied DC field (0.2–0.4?T).

Full text of DOI:10.1016/j.poly.2016.02.026

Accepted Manuscript
A tetracoordinate Co(II) single molecule magnet based on triphenylphosphine
and isothiocyanato group
Cyril Rajnák, Alena Packová, Ján Titiš, Jozef Miklovič, Ján Moncoľ, Roman
Boča
PII:
S0277-5387(16)00120-0
DOI:
http://dx.doi.org/10.1016/j.poly.2016.02.026
POLY 11842
Reference:
Polyhedron
To appear in:
Received Date:
Accepted Date:
28 January 2016
18 February 2016
Please cite this article as: C. Rajnák, A. Packová, J. Titiš, J. Miklovič, J. Moncoľ, R. Boča, A tetracoordinate Co(II)
single molecule magnet based on triphenylphosphine and isothiocyanato group, Polyhedron (2016), doi: http://
dx.doi.org/10.1016/j.poly.2016.02.026
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers
we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and
review of the resulting proof before it is published in its final form. Please note that during the production process
errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Ms. Ref. No.: POLY-D-16-00096
A tetracoordinate Co(II) single molecule magnet based on triphenylphosphine
and isothiocyanato group
Cyril Rajnáka,*, Alena Packováa, Ján Titiša, Jozef Mikloviča, Ján Moncoľb and Roman Bočaa
a Department of Chemistry, FPV, University of SS Cyril and Methodius, 91701 Trnava,
Slovakia, E-mail: cyril.rajnak@ucm.sk
b Institute of Inorganic Chemistry, Slovak University of Technology, 812 37 Bratislava,
Slovakia
Abstract
A simple Co(II) mononuclear complex [Co(PPh3)2(NCS)2] has been studied in details. Its
crystal and molecular structure is reported along with its IR a UV-Vis spectra. The DC
magnetization studies confirm a sizable zero-field splitting D/hc = –9.4 cm–1 that is also
confirmed by quantum-chemical calculations. The AC susceptibility data shows that there is a
slow magnetic relaxation with two relaxation branches. The low-frequency branch occurs at
about f = 1 – 10 Hz and it tends to disappear progressively on heating. The high-frequency
branch exists at the frequency ranges above f > 200 Hz and this is shifted above the hardware
limit (1500 Hz) on heating. The characteristics of this field-induced single molecule magnet
strongly depend upon the applied DC field (0.2 – 0.4 T).
Key words
crystal structure, single molecule magnet, AC susceptibility, cobalt(II) complex
1. Introduction
In the past three decades new magnetic materials – single-molecule magnets (SMMs)
appear in the centre of interest due to their promising application potential particularly in the
design of ultra-dense magnetic memories and quantum computers [1-6]. A lot of large and
ultra-large complexes based on d- and f-metal centres have been synthesized as SMMs [7-9].
Mononuclear complexes of the first-row transition metals, from the perspective of SMMs,
were considered merely as building blocks of high-spin polynuclear compounds. However, as
has been shown, these simple molecules conceal much greater potential, since they can act as
molecular magnets individually. Several SMMs based on single d-metal ion (so called single-
ion magnets, SIMs) have been recently reported, and strategies for manipulation of magnetic
anisotropy were suggested. The family of known SMMs based on mononuclear 3d complexes
is large, and still growing. These systems involve high-spin Mn(III), Fe(III), Fe(II), Fe(I),
Co(II), Ni(I), and also Ni(II) complexes [10-13]. Perhaps the highest attention is paid to the
class of Co(II) complexes for versatility of their coordination polyhedra and large magnetic
anisotropy expressed by the axial zero-field-splitting (ZFS) parameter. Despite the significant
progresses in this field, some important aspects of SMM behaviour still remain unresolved,
and therefore, more detailed research is needed. It is noteworthy that a lot of these SMMs are
structurally rather simple entities.
The reports concerning the single molecule magnetism in tetracoordinate complexes of
[Co(PPh3)2X2] type (X = Cl-, Br-, I-) appeared rather recently [14-16]. Herein, we are
reporting about the magnetic properties of a mononuclear tetracoordinate complex
[Co(PPh3)2(NCS)2] that spans this family of SIMs.

2. Experimental
2.1. Chemicals and handling
All chemicals were purchased and used as received. The solids (KSCN, CoCl2,
triphenylphosphine) and solvents (ethanol, acetonitrile, and dichloromethane) were used
without any further purification. Co(SCN)2 was prepared via an exchange reaction of cobalt
chloride with KSCN. All manipulations were conducted on air. The products were filtered
over ashless paper.
2.2. Physical Measurements
Elemental analysis was carried out by Flash 2000 CHNSO apparatus (Thermo Scientific).
FT-IR spectra were measured in KBr pellets in the region of 4000 - 400 cm−1 using
IRAffinity-1 (Shimadzu) spectrophotometer. The solid KBr for FT-IR measurements was
kept against absorption of moisture in an oven at 60°C. Absorption spectra in the UV-Vis
region (9000 – 50000 cm-1) were recorded by UV-Vis-NIR spectrophotometer 50 Bio
(Varian) and Specord 250 Plus (Analytica Jena) with the DAD detector. Electrospray
ionization time-of-flight (ESI-TOF) mass spectra were measured on a micrOTOF-Q II
(Bruker) spectrometer in the positive mode of ion polarity using the sample dissolved in
CH2Cl2.
2.3. Preparation of [Co(PPh3)2(SCN)2] (1)
A mixture of potassium isothiocyanate (1.94 g, 0.02 mmol) and anhydrous cobalt(II)
chloride (1.29 g, 0.01 mmol) in absolute ethanol (30 cm3) was stirred at the room temperature
for 1 h to form a blue solution and white insoluble precipitate. The mixture was filtered off
and blue solution of the cobalt(II) thiocyanate was added to the volumetric flask and filled up
with an adequate amount of ethanol (50 cm3). To a 100 cm3 round bottom flask, the
triphenylphosphine (0.52 g, 2 mmol), acetonitrile (20 cm3) and ethanol solution of the
Co(SCN)2 (5 cm3, 1 mmol) was added. The mixture was heated on an oil bath at 70 °C
overnight. The solvent was removed by reduced pressure. Green crystalline plates were
obtained by dissolving in CH2Cl2 and slow evaporation of the solvent at the room temperature
in two days. Yield: 0.62 g, 93%. Melting point: 163 - 165°C. ESI-TOF MS (CH2Cl2): m/z =
641.09 M+. Anal. Calc. for C38H32P2S2CoN2 (699.67 g·mol-1): C, 65.04; H, 4.60; N, 3.99; S,
9.14. Found: C, 65.21; H, 4.47; N, 4.31; S, 9.10%. Selected IR bands (KBr) ν/cm-1: 3051,
3001, 2903, 2673, 2573, 2501, 2357, 2324, 2048, 2025, 1811, 1773, 1699, 1614, 1477 (m),
1433 (s), 1391, 1327, 1310, 1285, 1184, 1157, 1096 (s), 1070, 1024, 995, 970, 849, 743 (s),
692 (s), 617, 521 (m), 505 (m), 490, (s = strong, m = medium). UV/Vis (CH3CN) νmax/103 cm-
1 (ε, M-1 cm-1): 15.6 (639), 17.7 (257), 12.6 (4149), 38.8 (22269). UV/Vis (Nujol) νmax/103 cm-
1 (relat. absorb.): 10.4 (0.56), 13.7 (1.00), 15.4 (1.06), 16.6 (0.93).
2.4. X-ray crystal structure determination
Single-crystal X-ray diffraction experiments and data collection of 1 was conducted using
Xcalibur S CCD (Oxford Diffraction) at 293 K. Intensities was recorded with ω scans.
Corrections to Lorentz, polarization and multi-scan absorption effects were applied. Graphite-
monochromated Mo Kα radiation, λ = 0.71093 Å, was used. The structure was solved by
charge-flipping method and subsequent Fourier differences techniques, and refined

anisotropically by common least-squares methods. The programs SUPERFLIP, SHELXL
(ver. 2014/7), OLEX2 and MERCURY have been used for structure determination,
refinement and drawing [17-22]. The hydrogen atoms were refined with fixed distances from
the parent carbon atoms. Crystal data on 1 are presented in Table 1.
Table 1
Crystal data and structure refinement for 1.
Empirical formula
Formula weight /g mol-1
C38H30CoN2P2S2
699.63
Crystal system
triclinic
Space group
P-1
Temperature /K
293(2)
Crystal size /mm
0.50 × 0.50 × 0.04
Z
2
a / Å
9.5656(9)
b / Å
c / Å
10.5460(9)
18.500(2)
100.452(8)
α /°
β /°
101.679(8)
γ /°
99.215(8)
1759.3(3)
1.321
0.726
V /Å3
ρcalc /g cm-3
µ /mm-1
F(000)
Radiation
722.0
MoKα (λ = 0.71073)
8.328 to 52.734
-11 ≤ h ≤ 11, -13 ≤ k ≤ 13, -23 ≤ l ≤ 23
11250/0/407
1.086
R1 = 0.0648, wR2 = 0.1242
R1 = 0.1109, wR2 = 0.1428
1042411
2Θ range for data collection/°
Index ranges
Data/restraints/parameters
Goodness-of-fit on F2
Final R indexes [I>=2σ (I)]
R indices (all data)
CCDC No
2.5. Magnetic data collection
The magnetic measurements were conducted using a SQUID apparatus (MPMS-XL7,
Quantum Design) in the RSO mode of detection. The polycrystalline sample was
encapsulated in a diamagnetic gelatin-made sample holder. The susceptibility taken at B = 0.1
T has been corrected for the underlying diamagnetism and transformed to the effective
magnetic moment calculated as µeff/µB = 798(χ′T)1/2 (SI units). Isothermal magnetization
curves were measured until B = 7 T at two temperatures: T = 2.0 and T = 4.6 K. The AC
susceptibility data was taken with the same sample at the oscillating field of BAC = 0.38 mT
for 22 frequencies ranging between 0.1 and 1500 Hz under the external field BDC = 0.2, 0.3,
and 0.4 T.
2.6. Quantum-chemical calculations
Ab initio calculations were performed with ORCA 3.0.3 computational package at the
experimental geometries determined by the X-ray diffraction. The relativistic effects were

included in the calculations with zero order regular approximation (ZORA) together with the
scalar relativistic contracted version of def2-TZVP basis functions.
The calculations of ZFS parameters were based on state average complete active space
self-consistent field (SA-CASSCF) wave functions complemented by N-electron valence
second order perturbation theory (NEVPT2) [23]. The active space of the CASSCF
calculations comprised of seven electrons in five metal-based d-orbitals. The state averaged
approach was used, in which all ten quartet states and forty doublets states were equally
weighted. The calculations utilized the RI approximation with the decontracted auxiliary
def2-TZV/C basis set for correlation calculations and the chain-of-spheres (RIJCOSX)
approximation to exact exchange. Increased integration grids (Grid4) and tight SCF
convergence criteria were used. The ZFS parameters were calculated through quasi-
degenerate perturbation theory in which an approximation to the Breit-Pauli form of the spin-
orbit coupling operator (SOMF) and the effective Hamiltonian theory was utilized [24].
3. Results and discussion
3.1. Molecular and crystal structure
A perspective view of 1 with atom numbering scheme is shown in Fig 1. The coordination
environment consist two phosphorus atoms of two triphenylphosphine ligands and two
nitrogen atoms of isothiocyanato groups which surround a tetra-coordinate Co(II) core. The
structure of the chromophore {CoN2P2} can be described as a distorted tetrahedron (close to
C2v symmetry) with Co-P bond distances in average 2.413 Å and Co-N (isothiocyanato group)
at 1.917 Å. Selected bond distances and bond angles are given in Tables 2 and 3. Two of the
N-donor atoms N(1)-Co(1)-N(2) enclose larger angle with respect to similar P(2)-Co(1)-P(1)
by 0.61° (Fig. 2). In general, the bond angles N-Co(1)-P are smaller than angles between two
same N-, P- donor atoms N(1)-Co(1)-N(2) and P(2)-Co-P(1) at 6.4°. The phenyl rings created
edge-to-face CH/π interactions [25] (C32–H32···π [C15–C20]; C37–H37···π [C21–C26] and
C11–H11···π [C3–C8]) at distances 3.663, 3.647 and 3.722 Å, respectively.
Fig. 1. Molecular structure of tetracoordinate Co(II) complex 1 and numbering of atoms
within chromophore {CoP2N2}.

Table 2
Selected bond lengths (Å) and angles (°) in 1
Co(1)-P(1)
Co(1)-P(2)
Co(1)-N(1)
Co(1)-N(2)
P(2)-Co(1)-P(1)
2.4122(16)
2.4135(14)
1.915(5)
1.919(5)
113.39(5)
N(1)-Co(1)-P(1)
N(1)-Co(1)-P(2)
N(1)-Co(1)-N(2)
N(2)-Co(1)-P(1)
N(2)-Co(1)-P(2)
105.03(15)
107.66(15)
114.3(2)
108.60(15)
107.98(13)
Fig. 2. Characteristic angles for 1.
Table 3
Summary of structural characteristics for chromophores of [Co(PPh3)2X2] complexes.
Chromophore
dCo-P
dCo-N
α
β
τ1, τ1´
τ2, τ2´
this work
[26]
{CoN2P2}
2.412
2.414
2.384
2.384
2.385
2.385
1.918
1.915
2.212
2.212
2.350
2.350
113.39
114.26
108.61
107.66
104.85
104.85
105.03
105.03
105.01
107.99
107.24
107.24
107.27
107.27
{CoCl2P2}
{CoBr2P2}
{CoI2P2}
115.87
117.45
108.25
117.27
115.23
111.44
[26]
[16]
2.390
2.413
2.543
2.558
105.39
109.84
112.71
109.04
3.2. Spectroscopic characterization
The room temperature solid state IR spectra of the complex 1 revealed a strong absorption
band at the 2025 cm-1, pertaining the asymmetric stretching frequency νas of NCS- group,
-1
≡N) with typical frequency 2260
which excluded the band of ν(-S-C
-2240 cm .
The UV-VIS spectra in the Nujol suspension display relative absorption for four bands at
10.4, 13.7, 15.4 and 16.6 × 103 cm-1. The bands for Co(II) in Td symmetry are typically spin-
allowed and followed by a charge transfer band. The electronic transitions can be assigned as
4T2 ← 4A2 (in near-IR region), 4T1(F) ← 4A2, 4T1(P) ← 4A2 and they are further split owing to
symmetry lowering (Table 4).

Table 4
Electronic transitions in related complexes (idealized to Td symmetry)
Complex
Chromo-
phore
Electronic transition (E/hc) / 103 cm-1
Ref.
4T1(4F)← 4A2(4F)
-, -, 10.4
4T1(4P) ←4A2(4F)
13.7, 15.4, 16.6
13.6, 15.8, 16.7
13.1, 14.9, 15.6
13.5, 14.9, 15.9
12.8, 13.8, 14.7
[Co(PPh3)2(NCS)2]
[Co(PPh3)2Cl2]
[Co(PPh3)2Br2]
{CoN2P2}
{CoCl2P2}
{CoBr2P2}
this work
[27]
[28]
[27]
[27]
6.4, 8.0, 10.7
6.2, 7.7, 10.2
6.0, 7.6, 9.7
5.8, 7.1, 9.2
[Co(PPh3)2I2]
{CoI2P2}
3.3. DC magnetic data
The effective magnetic moment for 1 adopts a room-temperature value of µeff = 4.30 µB
and it slightly decreases (owing to the temperature-independent paramagnetism) in a linear
manner down to 50 K. (If susceptibility in the high-temperature limit is temperature-
independent, then χT product or µeff ~ (χT)1/2 possesses a positive slope.) Below 50 K it drops
down rapidly to µeff = 3.28 µB at T = 1.9 K. This behaviour is characteristic for an S = 3/2
paramagnet showing a zero-field splitting (ZFS). The magnetization per formula unit adopts a
value of M1 = Mmol/NA = 2.22 µB at T = 2.0 and B = 7 T. This subnormal value relative to
spin-only magnetization (M1 = 3.0 µB for S = 3/2 system) confirms a sizable zero-field
splitting (Fig. 3).
3
2
1
0
5
4
3
2
1
0
B = 0.1 T
T = 2.0 K
µ
10
5
µ
T = 4.6 K
µ
χ
0
0
10
150
T/K
20
200
30
40
0
1
2
3
4
5 6 7
0
50
100
250
300
B/T
Fig. 3. DC susceptibility and magnetization data for 1. Lines – fitted (see main text).
The DC magnetic data was fitted by a standard model appropriate to a ZFS system that
accounts to the axial zero-field splitting parameter D, and the anisotropic Zeeman term that is
distributed uniformly over 120 knots over one hemisphere. Both, the susceptibility and the
magnetization data was fitted simultaneously using a joint error functional F = R(χ)× R(M )
consisting of the relative errors of susceptibility and magnetization, respectively. To this end
the final set of magnetic parameters reads: D/hc = –9.44 cm-1, gx = 2.125, gz = 2.250, the

temperature-independent magnetism χTIM = 2.4 × 10-9 m3 mol-1, and the molecular-field
correction (zj)/hc = –0.033 cm-1; the discrepancy factors R(χ) = 0.016 and R(M) = 0.020.
Verification of these findings, as well as specification of the electronic origin of the ZFS
parameters has been made by contemporary quantum-chemical calculations [29].
CASSCF/NEVPT2/QDPT method gave the D-parameter in a good agreement with that
obtained by fitting the magnetometric data: Dcalc/hc = −12.18 cm-1 and E/D = 0.17. The
individual contributions to the overall D value are listed in Table S1 (see ESI). A somewhat
larger discrepancy can be found between the experimental and calculated giso values; giso(calc)
= 2.219 (gx = 2.163, gy = 2.190, gz = 2.302). This inconsistency can be explained by the
forming of supramolecular structures in the solid sample.
3.4. AC magnetic data
The field dependence of the AC susceptibility components at the fixed temperature T =
2.0 K is shown in Fig. 4. The out-of-phase susceptibility displays a complex behaviour: it
passes through a maximum at B = 0.3 T for f = 1116 Hz but at B = 0.15 T for f = 111 Hz. On
the contrary, there is no maximum for low frequencies f = 11 and 1.1 Hz, respectively and the
′′
χ component still increases with increasing field until B = 0.5 T.
3
2
1
0
10
1.1 Hz
11 Hz
111 Hz
1116 Hz
5
χ
χ
0
0.0
0.1
0.2
0.3
0.4
0.5
0.0
0.1
0.2
0.3
0.4
0.5
B/T
B/T
′
′′
Fig. 4. The in-phase χ and out-of-phase χ molar susceptibility (SI units) for 1 in
dependence upon external magnetic field BDC at T = 2.0 K. Lines serve as a guide for eyes
(two independent data takings, each with five scans were applied).
The frequency dependence of the AC susceptibility components for a set of temperatures
is presented in Fig. 5 for B = 0.2, 0.3, and 0.4 T, respectively. The principal observation is that
′′
there are two relaxation branches: the low-frequency (LF) peaks at
appear between f = 1 –
χ
10 Hz and they tend to disappear progressively on heating; the high-frequency (HF) peaks lie
above 200 Hz and their maxima move to higher frequencies with temperature. On passing
from BDC = 0.2 to 0.3 and 0.4 T the LF peaks are better developed whereas the HF peaks are
shifted above the hardware limit (1500 Hz).
The AC susceptibility components were fitted by a two-component Debye model.
′
′′
Actually the error function F = R(χ )×R(χ ) has been minimized where R are the relative
errors for the in-phase and the out-of-phase susceptibility, respectively. The data fitting was
restricted to a rather limited range of temperatures because of an unfortunate situation: either
the LF peak is oddly resolved above T > 2.6 K or the HF peak lies above the limit of 1500 Hz
and thus it is extrapolated with high uncertainty. The quality of the fit is expressed by the

discrepancy factors R(χ’) and R(χ’’) along with the standard deviations for each optimized
parameter; these are listed in ESI.
The resulting parameters (see ESI) were used in generating interpolation/extrapolation
lines that are drawn in Fig. 5 as solid curves. These lines were rearranged into the Argand
′′
(Cole-Cole) diagrams as shown in Fig. 6 – left. The position of the maximum at χ refers to
′′
fmax
the characteristic frequency
that enters the Arrhenius-like plot for the relaxation time
′′
lnτ = ln(1/ 2π fmax ) as shown in Fig. 6 – right. Unfortunately, due to obstacles mentioned
above the SMM characteristics, such as the barrier to spin reversal U, and the extrapolated
relaxation time τ0 is impossible to extract from the given temperature window.
For comparison with analogous systems the value of τ @ 1.9 or 2.0 K can be used (Table
5). It can be seen that the complex 1 behaves analogously to [Co(PPh3)2Br2] where two
relaxation branches were identified.

10
3
2
1
0
BDC = 0.2 T
T = 1.9 K
T = 2.1 K
T = 2.3 K
T = 2.5 K
5
χ
χ
χ
χ
χ
χ
0
10-1
100
101
102
102
102
103
104
10-1
10-1
10-1
100
100
100
101
102
102
102
103
103
103
104
104
104
f/Hz
f/Hz
10
3
2
1
0
T = 2.0 K
T = 2.2 K
T = 2.4 K
T = 2.6 K
T = 2.8 K
T = 3.0 K
T = 3.2 K
BDC = 0.3 T
5
0
10-1
100
101
103
104
101
f/Hz
f/Hz
10
3
2
1
0
BDC = 0.4 T
T = 1.9 K
T = 2.1 K
T = 2.3 K
T = 2.5 K
T = 2.7 K
5
0
10-1
100
101
103
104
101
f/Hz
f/Hz
Fig. 5. Frequency dependence of the AC susceptibility components for fixed temperature.
Lines – fitted with the two-component Debye model.

3
2
1
0
3
2
1
-2
-4
1.9 K
2.1 K
2.3 K
2.5 K
2.7 K
2.9 K
3.1 K
3.3 K
3.5 K
3.7 K
4.1 K
4.5 K
4.9 K
LF
BDC = 0.2 T
HF
-6
τ
HF
-8
χ
-10
-12
LF
0
5
10
0.3
0.4
0.5
0.6
χ'
mol/10-6 m3 mol-1
-1
(1/T) /K
-2
-4
2.0 K
2.2 K
2.4 K
2.6 K
2.8 K
3.0 K
3.2 K
BDC = 0.3 T
LF
HF
-6
τ
HF
-8
χ
-10
-12
LF
0
3
0
5
10
0.3
0.4
0.5
0.6
χ'
mol/10-6 m3 mol-1
-1
(1/T) /K
-2
-4
1.9 K
2.1 K
2.3 K
2.5 K
2.7 K
2.9 K
3.1 K
3.3 K
3.5 K
3.7 K
4.1 K
4.5 K
4.9 K
LF
BDC = 0.4 T
HF
2
1
0
-6
τ
-8
HF
χ
-10
-12
LF
0
5
10
0.3
0.4
0.5
0.6
χ'
mol/10-6 m3 mol-1
-1
(1/T) /K
Fig. 6. Argand (Cole-Cole) diagram for 1 (left); Arrhenius-like plot (right) with dashed curves
serving as a guide for eyes.

Table 5
SMM characteristics for related tetracoordinate Co(II) complexes. #
Chromo-
Complex
(D/hc) BDC (U/kB) pτ0
Ref.
tw
τ0
/ s
-
τ / s @ 1.9
or 2.0 K
/ cm-1
/ T
/ K
-
phore
[Co(PPh3)2(NCS)2]
LF 34 × 10-3
HF 425 × 10-6
LF 45 × 10-3
HF 196 × 10-6
LF 54 × 10-3
HF 81 × 10-6
{CoN2P2}
-9.4
0.2
-
-
-
{CoN2P2}
{CoN2P2}
-9.4
-9.4
0.3
0.4
-
-
-
-
tw
tw
[14]
[15]
[15]
[Co(PPh3)2Cl2]
[Co(PPh3)2Br2]
{CoCl2P2}
{CoBr2P2}
-14.8 e 0.1 37.1
8.92
1.2 × 10-9
9.44 × 10-11
5.98 × 10-11
-12.5
-12.5
0.1 37.3
0.2 40.0
10.03
10.22
HF 693 × 10-6
LF 13 × 10-3
HF 83 × 10-6
[Co(PPh3)2I2]
[Co(AsPh3)2I2]
[Co(biq)Cl2]
{CoI2P2}
-36.9
-74.7
10.5
0.1 30.6
0.1 32.1
0.2 42.6 a
9.34
7.82
9.72
4.65 × 10-10
1.5 × 10-8
1.9 × 10-10
[16]
[16]
[30]
{CoI2As2}
{CoCl2N2}
LF 47 × 10-3
HF 1644 × 10-6
LF 71 × 10-3
HF 859 × 10-6
LF 32 × 10-3
HF 830 × 10-6
LF 46 × 10-3
HF 467 × 10-6
LF 45 × 10-3
HF 201 × 10-6
LF 52 × 10-3
HF 110 × 10-6
10.5
0.3 41.2 a
0.2 39.6 a
0.3 42.3 a
0.2 57.0 a
0.3 38.7 a
9.52
9.92
3.0 × 10-10
1.2 × 10-10
[Co(biq)Br2]
{CoBr2N2} 12.5
12.5
[30]
[30]
10.30 5.0 × 10-11
12.49 3.2 × 10-13
10.00 9.9 × 10-11
[Co(biq)I2]
{CoI2N2}
10.3
10.3
[Co(DPEphos)Cl2]
[Co(Xantphos)Cl2]
[Co(dmph)Br2]
{CoCl2P2}
{CoCl2P2}
-14.4 c 0.1 35.0
-15.4 c 0.1 29.9
9.68
8.22
9.46
2.1 × 10-10
6.0 × 10-9
3.7 × 10-10
[14]
[14]
[31]
{CoBr2N2} 11.7 e
0.1 22.8
# pτ0 = -log(τ0/s), c – calculated, e – high-field electron magnetic resonance, a – extended Arrhenius equation; tw
– this work.

A detailed inspection to Table 5 reveals that the values of the D-parameter for relater
tetracoordinate Co(II) complexes vary from –40 to +13 cm-1. Thus the negative sign of the D-
parameter is not a prerequisite of the SMM behaviour any longer. Some authors claim that the
orthorhombic zero-field splitting parameter E hinders the fast relaxation when D > 0. Most of
the Co(II) based SIMs show slow magnetic relaxation under the applied DC field (typically
BDC = 0.1 – 0.2 T), though there are some Co(II) based SIMs at the zero field, such as
[Co(PPh3)2Br2], [Co(Me6tren)Cl]ClO4 and (Ph4P)2[Co(SPh)4] [15, 32, 33]. The magnetic field
splits the energies of the Kramers doublets and causes that the magnetic tunnelling is
prevented. However, there is another factor in the play: the electron-nuclear hyperfine
coupling. Owing to the nuclear spin I(Co) = 7/2 a set of hyperfine levels occurs
S*, I,MS*, MI
and certain transitions among them are phonon allowed [34].
Parameters that characterize the SIM behaviour listed in Table 5 show a mutual
relationship: with increasing barrier to spin reversal U the extrapolated relaxation time
decreases and the pτ0 vs U function develops approximately along a straight line (see Fig. S5
in ESI). This fact points to an importance not only to increase the U-value but also to increase
the τ0 value in order to get a material with promising utilization.
4. Conclusions
The mononuclear complex [Co(PPh3)2(NCS)2] possesses quasi-tetrahedral {CoP2N2}
chromophore and a negative zero-field splitting parameter D/hc = –9.4 cm-1. This prerequisite
of the single-molecule magnetism manifests itself in the slow magnetic relaxation as
confirmed by the AC susceptibility measurement in an external magnetic field of BDC = 0.2,
0.3, and 0.4 T. There are two relaxation branches. The low-frequency branch occurs at about f
= 1 – 10 Hz and it tends to disappear on the heating. The high-frequency branch exists at the
frequency ranges above f > 200 Hz and this is shifted above the hardware limit (1500 Hz) on
heating. The characteristics of this field-induced single molecule magnet depend upon the
applied DC field and they span a typical range for analogous Co(II) complexes of the
[CoL2X2] type.
Acknowledgments
Slovak grant agencies (APVV-14-0078, APVV-14-0073 and VEGA 1/0534/16) are
acknowledged for the financial support.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in the online version, at
http://dx.doi.org/. CCDC 1042411 contains the supplementary crystallographic data for
[Co(PPh3)2(NCS)2].
These
data
can be
obtained
free of charge
via
http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail:
deposit@ccdc.cam.ac.uk.

References
[1] E. Coronado, D. Gatteschi, J. Mater. Chem. 16 (2006) 2513.
[2] M. Cavallini, M. Facchini, C. Albonetti, F. Biscarini, Phys. Chem. Chem. Phys. 10
(2008) 784.
[3] M. L. Perrin, E. Burzurí, H. S. J. van der Zant, Chem. Soc. Rev. 44 (2015), 902.
[4] L. Sun, Y. A. Diaz-Fernandez,T. A. Gschneidtner, F. Westerlund, S. Lara-Avilab, K.
Moth-Poulsen. Chem. Soc. Rev. 43 (2014) 7378.
[5] K. S. Pedersen, J. Bendix, R. Clérac, Chem. Commun. 50 (2014) 4396.
[6] Yan-Zhen Zheng, Guo-Jun Zhou, Z. Zheng, R. E. P. Winpenny, Chem. Soc. Rev. 43
(2014) 1462.
[7] H. L. C. Feltham, S. Brooker, Coord. Chem. Rev. 276 (2014) 1.
[8] M. Murrie, Chem. Soc. Rev. 39 (2010) 1986.
[9] R. Bagai, G. Christou, Chem. Soc. Rev. 38 (2009) 1011.
[10] G. A. Craig, M. Murrie, Chem. Soc. Rev. 44 (2015) 2135.
[11] J. Miklovič, D. Valigura, R. Boča, J. Titiš, Dalton Trans. 44 (2015) 12484.
[12] K. E. R. Marriott, L. Bhaskaran, C. Wilson, M. Medarde, S. T. Ochsenbein, S. Hill, M.
Murrie, Chem. Sci. 6 (2015) 6823.
[13] S.-D. Jiang, D. Maganas, N. Levesanos, E. Ferentinos, S. Haas, K. Thirunavukkuarasu,
J. Krzystek, M. Dressel, L. Bogani, F. Neese, P. Kyritsis, J. Am. Chem. Soc. 137 (2015)
12923.
[14] F.Yang, Q. Zhou, Y. Zhang, G. Zeng, G. Li, Z. Shi, B. Wang, S. Feng, Chem. Commun.
49 (2013) 5289.
[15] R. Boča, J. Miklovič, J. Titiš, Inorg. Chem. 53 (2014) 2367.
[16] M. R. Saber, K. R. Dunbar, Chem. Commun. 50 (2014) 12266.
[17] L. Palatinus, G. Chapuis, J. Appl. Cryst. 40 (2007) 786.
[18] L. Palatinus, A. van der Lee, J. Appl. Cryst. 41 (2008) 975.
[19] L. Palatinus, S. J. Prathapa, S. van Smaalen, J. Appl. Cryst. 45 (2012) 575.
[20] G.M. Sheldrick, Acta Cryst. C71,(2015) 3.
[21] O.V. Dolomanov, L.J. Bourhis, R. J Gildea, J.A.K. Howard, H. Puschmann, J. Appl.
Cryst. 42 (2009) 339.
[22] C. F. Macrae, I. J. Bruno, J. A. Chisholm, P. R. Edgington, P. McCabe, E. Pidcock, L.
Rodriquez-Monge, R. Taylor, J. van de Streek, P. A. Wood, J. Appl. Cryst. 41 (2008)
466.
[23] (a) Neese, F. ORCA – An Ab Initio, Density Functional and Semi-empirical Program
Package, Version 3.0.3. (b) Atanasov, M.; Ganyushin, D.; Pantazis, D. A.; Sivalingam,
K.; Neese, F. Inorg. Chem. 50 (2011) 7460. (c) Angeli, C.; Borini, S.; Cestari, M.;
Cimiraglia, R. J. Chem. Phys. 121 (2004) 4043. (d) Angeli, C.; Cimiraglia, R.;
Evangelisti, S.; Leininger, T.; Malrieu, J.–P. J. Chem. Phys. 114 (2001) 10252. (e)
Angeli, C.; Cimiraglia, R.; Malrieu, J.–P. J. Chem. Phys. 117 (2002) 9138.
[24] (a) Neese, F. J. Chem. Phys. 122 (2005) 34107. (b) Ganyushin, D.; Neese, F. J. Chem.
Phys. 125 (2006) 24103. (c) Neese, F. J. Chem. Phys. 127 (2007) 164112.
[25] H. Suezawa, T. Yoshida, Y. Umezawa, S. Tsuboyama, M. Nishio. Eur. J. Inorg. Chem.
(2002) 3148.
[26] R.L.Carlin, R.D.Chirico, E.Sinn, G.Mennenga, L.J.de Jongh, Inorg.Chem. 21 (1982)
2218.
[27] A.A.G. Tomlinson, C. Bellitto, O. Piovesana, C. Furlani, J. Chem Soc., Dalton (1972)
350.
[28] F.A. Cotton, O.D. Faut, D.M.L. Goodgame, R.H. Holm, J. Am. Chem. Soc. 83 (1961)
1780.

[29] F. Neese, The ORCA program system, Wiley Interdiscip. Rev.: Comput. Mol. Sci., 2
(2012) 73.
[30] L. Smolko, J. Černák, M. Dušek, J. Miklovič, J. Titiš, R. Boča, Dalton Trans. 44 (2015)
17565.
[31] W. Huang., T. Liu, D. Wu, J. Cheng, Z. W. Ouyang, C. Duan, Dalton Trans. 42 (2013)
15326.
[32] A. Packová, J. Miklovič, R. Boča, Polyhedron 102 (2015) 88.
[33] J.M. Zadrozny, J.R. Long, J. Am. Chem. Soc. 133 (2011) 20732.
[34] S. Gómez-Coca, A. Urtizberea, E. Cremades, J.P. Alonso, A. Camón, E. Ruiz, L. Ruiz,
Nature Commun. 5 (2014) 4300.

3
2
1
0
BDC = 0.3 T
T = 2.0 K
T = 2.2 K
T = 2.4 K
T = 2.6 K
T = 2.8 K
T = 3.0 K
χ
10-1
100
101
102
103
104
f/Hz
Synopsis
Mononuclear complex [Co(PPh3)2(NCS)2] possesses a sizable zero-field splitting D/hc = –9.4
cm–1. It exhibits a slow magnetic relaxation with two relaxation branches: the low-frequency
at f = 1 – 10 Hz and the high-frequency branch above f > 200 Hz. The characteristics of this
field-induced single molecule magnet strongly depend upon the applied external field BDC
=
0.2 – 0.4 T.