Cobalt(II) and copper(II) covalently and non-covalently dichlorido-bridged complexes of an unsymmetrical tripodal pyrazolyl-pyridyl amine ligand: Structures, magnetism and cytotoxicity

Article abstract of DOI:10.1016/j.ica.2016.06.030

The reaction of a methanolic solution containing M^IICl₂(M = Co or Cu) with bis(3,5-dimethyl-1H-pyrazol-1-yl-1-ethyl)(2-pyridylmethyl)amine (bedmpzp) in the presence of NH₄PF₆afforded the dinuclear doubly bridged-dichlorido complex [Co₂(bedmpzp)₂(μ-Cl)₂](PF₆)₂(1) and the mononuclear [Cu(bedmpzp)Cl]PF₆(2) one. The complexes were structurally and magnetically characterized. The weak ferromagnetic exchange and the axial type of magnetic anisotropy found in 1 is associated with slow relaxation of magnetization as revealed by AC susceptibility measurements. This finding puts 1 into a class of polynuclear single-molecule magnets based on 3d metals. X-ray structure of 2 revealed mononuclear nature of the complex, forming supramolecular dimers in the solid state. The non-covalent interactions of the Cu???Cl type present in its crystal structure induced a weak antiferromagnetic exchange. The results of magnetic analysis were also supported by DFT and CASSCF/NEVPT2 calculations. The in vitro cytotoxicity of the complexes against MCF7 and HeLa human cancer cell lines were also tested. The best cytotoxicity was achieved for complex 2 on HeLa, with IC₅₀= 2.5(0.9) μM.

Full text of DOI:10.1016/j.ica.2016.06.030

Accepted Manuscript
Research paper
Cobalt(II) and copper(II) covalently and non-covalently dichlorido-bridged
complexes of an unsymmetrical tripodal pyrazolyl-pyridyl amine ligand: struc-
tures, magnetism and cytotoxicity
Radovan Herchel, Zdeněk Dvoř ák, Zdeněk Trávní ček, Masahiro Mikuriya,
Febee R. Louka, Franz A. Mautner, Salah S. Massoud
PII:
S0020-1693(16)30340-1
http://dx.doi.org/10.1016/j.ica.2016.06.030
ICA 17121
DOI:
Reference:
Inorganica Chimica Acta
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Revised Date:
Accepted Date:
3 May 2016
20 June 2016
21 June 2016
Please cite this article as: R. Herchel, Z. Dvoř ák, Z. Trávní ček, M. Mikuriya, F.R. Louka, F.A. Mautner, S.S.
Massoud, Cobalt(II) and copper(II) covalently and non-covalently dichlorido-bridged complexes of an
unsymmetrical tripodal pyrazolyl-pyridyl amine ligand: structures, magnetism and cytotoxicity, Inorganica
Chimica Acta (2016), doi: http://dx.doi.org/10.1016/j.ica.2016.06.030
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Cobalt(II) and copper(II) covalently and non-covalently
dichlorido-bridged complexes of an unsymmetrical tripodal pyrazolyl-
pyridyl amine ligand: structures, magnetism and cytotoxicity
Radovan Herchela, Zdeněk Dvořáka, Zdeněk Trávníčeka,*, Masahiro Mikuriyab,
Febee R. Loukac, Franz A. Mautnerd,*, Salah S. Massoudc,*
aRegional Centre of Advanced Technologies and Materials, Division of Biologically Active
Complexes and Molecular Magnets, Faculty of Science, Palacký University, 17. listopadu
12, CZ-77146 Olomouc, Czech Republic
bDepartment of Chemistry and Research Center for Coordination Molecule-based Devices,
School of Sciences and Technology, Kwansei Gakuin University, 2-1 Gakuen, Sanda 669-
1337, Japan
c Department of Chemistry, University of Louisiana at Lafayette, P.O. Box 44370 Lafayette, LA,
USA, E-mail: ssmassoud@louisiana.edu
d Institut für Physikalische and Theoretische Chemie, Technische Universität Graz,
A-8010 Graz, Austria
Keywords: Cobalt, Copper; polynuclear Complexes; Crystal structure; Magnetic properties;
Cytotoxic activity
* Corresponding authors: E-mail, E-mail: zdenek.travnicek@upol.cz, Tel. +420 585-634-352, Fax:
+420 585-634-954 (Z. Trávníček); E-mail: mautner@tugraz.at; Tel. ++43-316-873-32270, Fax: ++43-
316-873-8225 (FAM) and E-mail: ssmassoud@louisiana.edu; Tel. +01 337-482-5672; fax: +01 337-
482-5676 (SSM).

ABSTRACT
The reaction of a methanolic solution containing MIICl2 (M = Co or Cu) with bis(3,5-dimethyl-
1H-pyrazol-1-yl-1-ethyl)(2-pyridylmethyl)amine (bedmpzp) in the presence of NH4PF6 afforded
the dinuclear doubly bridged-dichlorido complex [Co2(bedmpzp)2(µ-Cl)2](PF6)2 (1) and the
mononuclear [Cu(bedmpzp)Cl]PF6 (2) one. The complexes were structurally and magnetically
characterized. The weak ferromagnetic exchange and the axial type of magnetic anisotropy
found in 1 is associated with slow relaxation of magnetization as revealed by AC susceptibility
measurements. This finding puts 1 into a class of polynuclear single-molecule magnets based on
3d metals. X-ray structure of 2 revealed mononuclear nature of the complex, forming
supramolecular dimers in the solid state. The non-covalent interactions of the Cu⋅⋅⋅Cl type
present in its crystal structure induced a weak antiferromagnetic exchange. The results of
magnetic analysis were also supported by DFT and CASSCF/NEVPT2 calculations. The in vitro
cytotoxicity of the complexes against MCF7 and HeLa human cancer cell lines were also tested.
The best cytotoxicity was achieved for complex 2 on HeLa, with IC50 = 2.5(0.9) µM.
1. Introduction
The chemistry of tripodal tetradentate amines containing four N-donor atoms is very rich
due to the possibility synthetizing ligands with either aliphatic- or aromatic-based N-donor arms
as well as due to the feasibility of combining different arm groups resulting in unsymmetrical
tripod ligands [1]. Restricting our discussion to unsymmetrical tripod ligands comprising pyridyl
and pyrazolyl groups, we have found that only three such ligands were utilized in the transition
metal coordination chemistry according to Cambridge Crystallographic Database up to now [2].
Namely, (3,5-dimethyl-1H-pyrazol-1-yl)-N,N-bis(pyridin-2-ylmethyl)methanamine (pzpy2), (3,5-
dimethyl-1H-pyrazol-1-yl)-N-((3,5-dimethyl-1H-pyrazol-1-yl)methyl)-N-(pyridine-2-
ylmethyl)methanamine (pz2py), bis(1-pyrazolylmethyl)(2-pyridylmethyl)amine (bppa) – Scheme
1.
N
N
CH3
N
N
N
N
N
CH3
N
N
CH3
N
N
N
N
N
CH3
N
N
N
N
N
H3C
N
N
N
CH3
N
H3C
CH3
CH3
H3C
pzpy2
pz2py
bppa
bedmpzp
Scheme 1. Unsymmetrical tripod ligands comprising pyridyl and pyrazolyl groups.
2

Most widely used ligands are pz2py and pzpy2 for which mononuclear penta-coordinate
[Co(pz2py)Cl]BF4⋅½CH3OH [3], [Co(pz2py)(CH3COO)]PF6 [4], and hexa-coordinate
[Fe(pz2py)Cl2] [5], and [Fe(pzpy2)(NCS)2] [6] complexes were prepared, and their cytotoxic
activities [3], catalytical properties [5] were investigated, or pressure-driven thermal SCO
behavior was observed [6]. Moreover, terminal pz2py ligand was introduced in the preparation of
cyanato-bridged [{Ni(pz2py)(µ-NCO)}2](PF6)2, [{Cu(pz2py)(µ-NCO)}2](PF6)2 [7], or azido-
bridged [{Ni(pz2py)(µ-N3)}2](ClO4)2⋅2EtOH, [{Cu(pz2py)(µ-N3)}2](ClO4)2 [8]. Dinuclear
complexes and magnetic exchange within these compounds were studied. Interestingly,
removing the bulky methyl substituents from pyrazolyl groups resulted in the bppa ligand
(Scheme 1) for which bridging mode was observed in polymeric Cu(I) compound [Cu2(µ-
bppa)(µ-I)2]n [9].
Herein, we report the synthesis of a novel unsymmetrical tripodal ligand bis(3,5-
dimethyl-1H-pyrazol-1-yl-1-ethyl)(2-pyridylmethyl)amine (bedmpzp) shown in Scheme 1, in
which two pyrazolyl groups are attached to tertiary nitrogen atom by longer ethyl groups. The
first two structurally characterized covalently and non-covalently doubly bridged dichlorido
[Co2(bedmpzp)2(µ-Cl)2](PF6)2 (1) and [Cu(bedmpzp)Cl]PF6 (2) complexes, respectively, were
synthesized. Their static and dynamic magnetic properties and in vitro cytotoxicity were studied.
Furthermore, the DFT and CASSCF calculations were used to rationalize their magnetic
behavior.
2. Experimental
2.1. Materials and physical measurements
Bis(2-chloroethyl)amine
hydrochloride,
3,5-dimethylpyrazole
and
(2-
chloromethyl)pyridine hydrochloride were purchased from Aldrich Chem. Company. All other
chemicals were commercially available and used without further purification. Infrared spectra of
compounds were recorded on a JASCO FTIR-480 plus spectrometer as KBr pellets. Electronic
spectra were recorded using an Agilent 8453 HP diode array and Perkin-Elmer Lambda35 UV-
1
Vis spectrophotometers. H and 13C NMR spectra were obtained at room temperature on a
Varian 400 NMR spectrometer operating at 400 MHz (1H) and 100 MHz (13C). 1H and 13C NMR
chemical shifts (δ) are reported in ppm and were referenced internally to residual solvent
resonances (DMSO-d6: δH = 2.49, δC = 39.4 ppm). ESI-MS spectrum of bis(3,5-dimethyl-1H-
pyrazol-1-yl-1-ethyl)(2-pyridylmethyl)amine (bedmpzp) was measured on an LC-MS Varian
Saturn 2200 spectrometer. The conductivity measurements were performed using a Mettler
3

Toledo Seven Easy conductivity meter and the cell constant was determined by the aid of 1413
µS/cm conductivity standard. The molar conductivity of the complexes were determined from
ΛM = (1.0x103 κ)/M, where κ = cell constant and M is the molar concentration of the complex.
Magnetic measurements were performed with SQUID MPMS magnetometer for 1 (T = 1.9–300
K at B = 0.1 T; B = 0–5 T at T = 2 and 5 K) and with PPMS Dynacool VSM magnetometer for 2
(T = 1.9–300 K at B = 1 T; B = 0–9 T at T = 2 and 5 K). The magnetic data were corrected for
sample holder signal and for diamagnetic susceptibility. Elemental analyses were carried out by
the Atlantic Microlaboratory, Norcross, Georgia U.S.A.
2.2. Syntheses
2.2.1. Bis(3,5-dimethyl-1H-pyrazol-1-yl-1-ethyl)(2-pyridylmethyl)amine (bedmpzp)
A mixture of bis(3,5-dimethyl-1H-pyrazol-1-yl-1-ethyl)amine [10] (1.31 g, 5 mmol), 2-
(chloromethyl)pyridine hydrochloride (0.82 g, 5 mmol) and anhydrous K2CO3 (1.73 g, 12.5
mmol) was suspended in dry CH3CN. The resulting mixture was stirred and refluxed gently
under N2 gas for 3 days, during which color turns yellowish-brown. This was cooled in ice,
filtered off KCl and unreacted K2CO3 and solvent was removed by rotary evaporator under
reduced pressure. The resulting residue was extracted with CH2Cl2 (3 x 25 mL) and 10% NaOH
followed by washing with H2O (3 x 25 mL), and addition of anhydrous MgSO4. The solution
was then filtered and charged on a column containing silica gel. The desired product was
collected upon elution with 5% MeOH/CH2Cl2 (by volume). The collected organic layer was
further treated with anhydrous MgSO4, filtration and evaporating solvent resulted in the
formation of the compound as a pure orange-yellow liquid (yield: 1.20 g, 68%). Characterization
of bedmpzp: IR (KBr disc, cm-1): 3054 (vw) (C-H, aromatic); 2949 (m), 2924 (m), 2932(vw) (C-
H, aliphatic); 1664 (m), 1591 (s), 1570 (m), 1552 (vs), 1461 (vs), 1435 (vs), 1387 (s) (pyridyl
1
and pyrazolyl groups). ESI-MS (MeOH): m/z = 353.245 (Calcd for [M+H]+ = 353.49). H NMR
(DMSO-d6, 400 MHz, δ in ppm): δ = 2.01, 2.04, 2.11, 2.13 (s, each corresponds to 3H, CH3);
2.90, 3.93 (t, each corresponds to 2H, NCH2CH2-pz); 3.75 (s, 2H, CH2-py); 5.75 (s, 2H, H-pz);
7.12, 7.22, 7.39, 7.65 (py-protons). 13C NMR: (DMSO-d6, 100 MHz) δ = 10.46, 13.30 (CH3-pz);
46.09, 53.69, 59.69 (CH2-pz and CH2-py carbons); 104.36, 104.60, 122.46, 122.60 (pz-carbons);
136.4, 138.7, 145.7, 147.6, 148.6 (py-carbons).
2.2.2. [Co2(bedmpzp)2(µ-Cl)2](PF6)2 (1)
To the bedmpzp ligand (0.088 g, 0.25 mmol) dissolved in MeOH (20 mL), CoCl2·6H2O
(0.060 g, 0.25 mmol) was added and the reaction mixture was heated for 10 min, followed the
4

addition of NH4PF6 (0.065 g, 0.40 mmol). The resulting solution was filtered through celite and
then allowed to stand at room temperature. After two days, the dark magenta crystals which
were separated were collected by filtration, washed with propan-2-ol, ether and dried in air
(overall yield: 0.120 g, 81%): Characterization: Anal. Calcd for (C20H28ClCoF6N6P)2 (MM =
1183.668 g/mol): C, 40.59; H, 4.77; N, 14.20%. Found: C, 40.63; H, 4.89; N, 14.08%. Selected
IR bands (cm-1): 2969 (vw), 2932(vw) (C-H, aliphatic); 1606 (m), 1555 (m) 1463 (m),
-
1445)1392 (m) (pyridyl and pyrazolyl groups); 842 (vs) ν(P-F) (PF6 ). UV-VIS in CH3CN: λmax
in nm (εmax/Co atom, M-1cm-1): 479 (74, sh), 536 (87), 560 (74, sh), 617 (80), 799 (15, b). ΛM
(CH3CN) = 283 Ω-1cm2mol-1.
2.2.3. [Cu(bedmpzp)Cl]PF6 (2)
This complex was synthesized using a procedure similar to that described for complex 1,
but CuCl2·2H2O was used instead of the corresponding CoCl2·6H2O. Crystallization of the
precipitate from CH3CN and further re-crystallization from MeOH afforded bluish-green single
crystals (overall yield: 67%): Characterization: Anal. Calcd for C20H28ClCuF6N6P (MM = 596.45
g/mol): C, 40.28; H, 4.73; N, 14.09%. Found: C, 40.29; H, 4.68; N, 14.13.99%. Selected IR
-
bands (cm-1): 2932 (vw) (C-H, aliphatic); 1611 (m), 1577 (m); 841 (vs) ν(P-F) (PF6 ). UV-VIS in
CH3CN: λmax in nm (εmax M-1cm-1): 370 (s, sh), 674 (260, b). ΛM (CH3CN) = 139 Ω-1cm2mol-1.
2.3. X-Ray crystal structure analysis
The X-ray single-crystal data of the two compounds were collected on a Bruker-AXS
SMART APEX II CCD diffractometer at 100(2) K. The crystallographic data, conditions
retained for the intensity data collection and some features of the structure refinements are listed
in Table S1. The intensities were collected with Mo-Kα radiation (λ= 0.71073 Å). Data
processing, Lorentz-polarization and absorption corrections were performed using SAINT,
APEX and the SADABS computer programs [11]. The structures were solved by direct methods
and refined by full-matrix least-squares methods on F2, using the SHELXTL program package
[12]. All non-hydrogen atoms were refined anisotropically. The hydrogen atoms were located
from difference Fourier maps, assigned with isotropic displacement factors and included in the
final refinement cycles by use of geometrical constraints. Molecular plots were performed with
the Mercury program [13].
5

2.4. Theoretical calculations
All ab initio calculations were performed with ORCA 3.0.3 computational package [14]
on the molecular fragments derived from X-ray structures of 1 and 2. Polarized triple-zeta basis
set def2-TZVP(-f) were used for all atoms except carbon and hydrogen atoms for which de2-
SVP basis set was utilized [15]. The DFT calculations were based on B3LYP functional [16] and
utilized the RI approximation with the decontracted auxiliary def2-TZV/J or def2-SVP/J
Coulomb fitting basis set and the chain-of-spheres (RIJCOSX) approximation to exact exchange
[17]. Increased integration grids (Grid5 and GridX5in ORCA convention) and tight SCF
convergence criteria were also used. The isotropic exchange J values were calculated by Ruiz’s
formula [18] and also by the more general Yamaguchi’s formula [19].
The calculations of zero-field splitting (ZFS) and g tensors were based on state average
complete active space self-consistent field (SA-CASSCF) [20] wave functions complemented by
N-electron valence second order perturbation theory (NEVPT2) [21]. The active space of the
CASSCF calculations comprised of seven electrons in five metal-based d-orbitals (CAS(7,5)).
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 and def2-SVP/C Coulomb fitting basis sets and the chain-of-spheres
(RIJCOSX) approximation to exact exchange. Increased integration grids (Grid5 in ORCA
convention) and tight SCF convergence criteria were used. The ZFS parameters, based on
−orbit coupling contributions from excited states, were calculated through quasi
dominant spin
-
degenerate perturbation theory (QDPT) [22] in which an approximation to the Breit-Pauli form
of the spin-orbit coupling operator (SOMF approximation) [23] and the effective Hamiltonian
theory were utilized [24].
2.5. The analysis of the magnetic data
The experimental magnetic data were fitted with program POLYMAGNET [25], in which both
temperature and field dependent magnetization data were treated simultaneously using this error
functional
calc.
eff,i
obs.
eff,i
calc.
eff,i
obs.
eff,i
N1
N2
µ
− µ
obs.
µ
− µ
obs.
F = w
/ N + 1− w
2
/ N2
(
)
∑
∑
1
1
µ
µ
eff,i
i=1
i=1
eff,i
6

where the calculated and observed effective magnetic moments are used. The N1 and N2 are
numbers of temperature, and field dependent experimental magnetic data, respectively. The
weights were set to w1 = 0.65 and w2 = 0.35.
-1
The standard deviations of the fitted parameters were calculated as σi = (Pii ·S/(N–k))-1/2, where
calc.
Pij = Σ(δµeff,ncalc/δai·δµeff,ncalc/δaj) and S = Σ(µeff,n
–µeff,nobs.)2 with n = 1 to N. ai and aj are
fitted parameters, N is number of experimental points (sum of temperature and field dependent
calc
data), µeff,n
and µeff,nobs. are the calculated and observed effective magnetic moments for given
temperature and magnetic field. The σi was then multiplied by Student’s t95% to provide
confidence limits with 95% probabilities listed in text.
2.6. Cytotoxicity
In vitro cytotoxicity of complexes (1) and (2), and cisplatin as a standard, was studied by
the MTT assay (MTT = 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) on
human breast adenocarcinoma (MCF7; ECACC No. 86012803) and human cervix epithelioid
carcinoma (HeLa; ECACC No. 93021013) cell lines. The human cancer cell lines were
purchased from European Collection of Cell Cultures (ECACC). The cells were cultivated
according to the manufacturer´s manuals and maintained in an atmosphere containing 5% CO2 in
a humidified incubator at 37 °C. The experimental procedure was performed according to the
literature [26]. All the experiments were conducted in triplicate. The results are expressed as IC50
values with their standard deviations (SD). The significance of the differences between the
compared groups of results was assessed by the ANOVA analysis, with p < 0.05 considered to
be significant (QC Expert 3.2, Statistical software, TriloByte Ltd.) [27].
3. Results and discussion
3.1. Synthesis and spectroscopic characterization of the complexes
The
tripod
tetraamine
ligand
bis(3,5-dimethyl-1H-pyrazol-1-yl-1-ethyl)(2-
pyridylmethyl)amine (bedmpzp) was obtained in 68% in a pure state following the reactions
outlined in Scheme 2 (see Experimental section for more details).
7

Cl
H3C
N
NaH
DMF
H3C
CH3
+
N
N
HN
N
H
Cl
CH3
NH
N
N
N
Cl
N
N
N
CH3
H3C
N
N
N
ACN
H3C
CH3
CH3 H3C
bedmpzp
Scheme 2. The reaction pathway to tripod ligand bis(3,5-dimethyl-1H-pyrazol-1-yl-1-ethyl)(2-
pyridylmethyl)amine (bedmpzp).
Reaction of bedmpzp in a methanolic solution containing CoCl2 or CuCl2 in a 1:1 molar ratio,
followed by the addition of NH4PF6 afforded the dinuclear [Co2(bedmpzp)2(µ-Cl)2](PF6)2 (1) and
mononuclear [Cu(bedmpzp)Cl]PF6 (2), complexes in reasonable yields. The isolated complexes
were characterized by elemental microanalyses, molar conductivity, IR and UV-VIS
spectroscopy, and single crystal X-ray crystallography. The molar conductivity measurements in
CH3CN as 283 Ω-1cm2mol-1 is consistent with the 1:2 electrolytic nature of complex (1), whereas
a value of 139 Ω-1cm2mol-1 for the Cu(II) complex (2) is typical for 1:1 electrolytic behavior.
The IR spectra of the complexes display a very strong band at 841 cm-1 which is assigned to
stretching vibration of ν(P-F) of the hexafluorophosphate counter ions.
The UV-visible spectra of the two complexes were recorded in CH3CN and in the solid
state for compound 1. The Co(II) complex (1) exhibits absorption bands at 477, 529, 570 with
shoulder at 610, broad weaker band at 720 nm and another band above 1100 nm in the solid state
(Fig. S1). The measurement in CH3CN revealed three absorption bands at 799, 617 and 536 nm
with shoulders around 479 and 560 nm. Such richness of the absorption bands attributable to d-d
transitions is the result of lowering the symmetry of the chromophore (Oh → C2v), thus three
4
4
4
4
4
4T1(F) spin allowed in the
←
T1(F) and A2(P)
←
←
electronic transitions T2(F)
T1(F), T1(F)
octahedral symmetry are split into six transitions in the idealized C2v symmetry of cis-[CoN4Cl2]
chromophore of 1. The corresponding Cu(II) complex (2) displayed a broad single absorption
maximum at 674 nm with no indication of absorption maximum beyond λ > 800 nm. This
8

spectral feature is consistent with a distorted square pyramidal geometry (SP) around the central
Cu(II)atom. The strong intense absorption shoulder observed around 370 nm is most probably
→
attributed to CT (L M) transition. The geometrical assignments in acetonitrile solution were in
full agreements with those obtained from X-ray structural determinations (see next section).
3.2. Description of X-ray structures of [Co2(bedmpzmpy)2(µ-Cl)2](PF6)2 (1) and
[Cu(bedmpzmpy)Cl]PF6 (2)
Single crystal structure determination revealed that Co(II) compound (1) forms dinuclear
[Co2(bedmpzp)2(µ-Cl)2]2+ complex cations, whereas Cu(II) compound (2) forms mononuclear
-
[Cu(bedmpzp)Cl]+ units and both compounds co-crystallize with PF6 counter anions.
Perspective views together with the atom numbering scheme are presented in Figures 1 and 2,
respectively, while the selected interatomic parameters are given in Table 1. Each Co(II) center
of the dinuclear complex cation is octahedrally coordinated by four N-donor atoms of the
bedmpzp ligand [Co(1)-N from 2.0807(14) to 2.2446(14) Å] and two bridging chlorido ligands
[Co(1)-Cl(1) = 2.4036(5) and Co(1)-Cl(1’) = 2.7503(5) Å]. The centrosymmetric dinuclear
cations show a Co(1)···Co(1’) intra-dimer distance of 4.0061(6) Å, whereas the shortest inter-
dimer Co···Co separation is 8.6540(10) Å. The Co(1)-Cl(1)-Co(1’) and Cl(1)-Co(1)-Cl(1’) bond
angles are 101.82(2), and 78.18(2)°, respectively.
Fig. 1. Perspective view of the dinuclear unit [Co2(bedmpzp)2(µ-Cl)2]2+ of (1) with the atom
numbering scheme.
9

Table 1. Selected bond lengths (Å) and angles (°) for compounds 1 and 2.
Compound 1
Co(1)-N(1)
Co(1)-N(2)
Co(1)-N(3)
N(3)-Co(1)-N(1)
N(5)-Co(1)-Cl(1’)
2.1065(14)
2.2446(14)
2.0807(14)
166.28(6)
171.70(4)
Co(1)-N(5)
Co(1)-Cl(1)
Co(1)-Cl(1’)
N(2)-Co(1)-Cl(1) 165.37(4)
Cl(1)-Co(1)-Cl(1’) 78.18(2)
2.2087(15)
2.4036(5)
2.7503(5)
Co(1)-Cl(1)-Co(1’) 101.82(2)
Compound 2
Cu(1)-N(1)
Cu(1)-N(2)
2.005(4)
2.147(4)
Cu(1)-N(3)
Cu(1)-N(5)
1.991(4)
2.291(4)
Cu(1)-Cl(1)
N(1)-Cu(1)-N(3)
Cl(1)-Cu(1)-N(5)
2.2868(12)
166.17(15)
103.27(10)
N(2)-Cu(1)-Cl(1) 161.64(11)
Symmetry code: (‘): -x,-y,2-z.
The CuN4Cl chromophore of the monomeric [Cu(bedmpzp)Cl]+ complex cation of (2) with a
distorted SP geometry (τ = 0.076) [28] is achieved by the four N-donor atoms of the bedmpzp
ligand and one terminal chlorido ligand. The basal sites are occupied by Cl(1) [Cu(1)-Cl(1) =
2.2826(12) Å], N(1), N(2) and N(3) donor atoms [Cu(1)-N from 1.991(4) to 2.147(4) Å],
whereas the apical site is occupied by N(5) donor atom [Cu(1)-N(5) = 2.291(4) Å]. The Cu(1)
center deviates by 0.056(4) Å from its basal N3Cl plane. The shortest Cu···Cu separation is
4.6307(10) Å, and the Cu(1)···Cl(1) [2-x,1-y,-z] separation is 3.5712(14) Å, thus forming
supramolecular dimer is observed (Fig. 2).
10

Fig. 2. Perspective view of the supramolecular dimer [{Cu(bedmpzp)Cl}]2+ of 2 with the atom
numbering scheme. The non-covalent Cu···Cl interactions are shown with magenta dashed lines.
3.3. Theoretical calculations
The DFT broken-symmetry (BS) approach with the B3LYP functional and def2-TZVP(-
f) basis set was employed to estimate the isotropic exchange coupling J between paramagnetic
metal(II) atoms in the two compounds. The computational software ORCA 3.0 was used and we
followed the same procedure as described in details in ref. 29 to calculate J-values for the
dinuclear spin Hamiltonian defined as
ꢀ
ꢀ
ˆ
(1)
H = −J (SA ⋅ SB )
by both Ruiz’s (JRuiz) and Yamaguchi’s (JYam) formulas. The results of DFT calculations are
summarized in Table 2. The spin densities for broken-symmetry spin states (BS) states are
visualized in Fig. 3 and the corresponding orbitals of BS spin states with the highest overlap
(Sαβ) are shown in Fig. S2 (Supplementary Material). The DFT calculations predict weak
antiferromagnetic coupling for both compounds with spin delocalization to be more pronounced
in the copper(II) compound (2).
Table 2 The DFT-calculated net Mulliken spin densities (ρ), expected values <S2>, overlap Sαβ
between the corresponding orbitals and isotropic exchange parameters (J) from high-spin (HS)
and broken symmetry spin (BS) states of the dinuclear molecular fragments based on X-ray
structural structures of (1) and (2).
(1)
(2)
ρHS(M1)/ ρHS(M2)
ρBS(M1)/ ρBS(M2)
2.71/2.71
-2.71/2.71
12.01
0.59/0.59
-0.59/0.59
2.01
1.01
0.01215
<S2
<S2
Sαβ
>
>
HS
3.01
BS
0.04654
0.03524
0.00180
-27.306
-4.56
-6.07
+2.82
101.82
4.006
εBS-εHS/cm-1
-2.213
-2.21
-4.43
-4.45
102.32
J
J
J
Ruiz/cm-1
Yam/cm-1
mag/cm-1
<(M-Cl-M)/°
d(M-M)/10-10
m
4.631
11

Fig. 3. The calculated isodensity surfaces of the broken symmetry spin states using B3LYP/def2-
TZVP(-f) for [Co2(bedmpzp)2(µ-Cl)2](PF6)2 (1) (left) and {[Cu(bedmpzp)Cl]PF6}2 of (2) (right).
The positive and negative spin densities are represented by blue and red surfaces, respectively,
with the cutoff values of 0.005 e·boh-3. Hydrogen atoms are omitted for clarity.
We also calculated ZFS D and g tensors for the Co(II) atom in (1) with ORCA using state
average complete active space self-consistent field (SA-CASSCF) wave functions complemented
by N-electron valence second order perturbation theory (NEVPT2) with the active space defined
as CAS (7,5). The calculations were done for the dinuclear fragment [CoZn(bedmpzmpy)2(µ-
Cl)2]2+, in which one cobalt atom in 1 was replaced by zinc. The resulting values of ZFS
parameters were as follows: D = –111.9 cm-1 and E/D = 0.270, where individual contributions to
ZFS parameters from excited states are listed in Table S2 (Supplementary Material). In addition,
g-tensor values were found as: g1 = 1.839, g2 = 2.244, g3 = 3.150 resulting in giso = 2.411.
However, these parameters must be treated with caution because according to these calculations,
there are low-lying excited states, which means that ground spin state is not well separated and
spin Hamiltonian formula may not hold (Table S3). As can be seen in Fig. 4 both the g- and D-
tensors coincide. Therefore, from the mutual relationship we can conclude that gx = g2, gy = g1,
and gz = g3. The DZ axis can be identified with N-Co-N bonds within the chromophore, where
the N atoms belong to the pyrazole and pyridine rings, while other D-tensor axes do not coincide
with the donor-acceptor bonds tightly. Moreover, CASSCF/NEVPT2 calculations of ligand field
terms were used to interpret d-d transitions for 1 at 409, 441, 511, 582, 1028, 1205 nm (Fig. S1)
which is an agreement with the experimental solid state electronic spectrum.
12

Fig. 4. Molecular fragment of [CoZn(bedmpzp)2(µ-Cl)2]2+ overlaid with g-tensor and D-tensor
axes derived from CASSCF/NEVPT2 calculations.
3.4. Magnetic properties of complexes
The experimental magnetic data of complexes 1 and 2 are depicted in Figures 5, and 6,
respectively. The room temperature values of the effective magnetic moment, µeff are equal to
6.8 µB for 1 and 1.9 µB for 2. These values are higher than spin only values (5.58 µB and 1.73 µB
for 1 and 2, respectively, where the first value corresponds to two non-interacting spins Si = 3/2
and g = 2.0, while the second one is for one spin S = 1/2 and g = 2.0) due to the significant
contribution of angular momentum of excited states to the ground spin states [30]. In the case of
cobalt(II) complex (1), there is shallow maximum around 260 K and then the µeff drops reaching
a minimum value of 6.3 µB at T = 21 K, afterwards it starts to rise to a maximal value of 6.8 µB at
T = 2.3 K. This behavior is typical for octahedral Co(II) complexes with non-zero orbital angular
momentum in the ground state [30] and the very low temperature data are affected by weak
ferromagnetic isotropic exchange (J). On contrast, the copper(II) complex (2) shows almost
constant µeff down to ≈ 20 K and then decreases to a value of 0.81 µB at T = 1.9 K which is also
accompanied by a maximum on the mean susceptibility located at T = 3.6 K (Mmol vs. T curve,
see inset in Fig. 6). This feature is a fingerprint for antiferromagnetically coupled homospin
dimer.
13

Fig. 5 Magnetic data for (1). Left: the temperature dependence of the effective magnetic moment
and molar magnetization measured at B = 0.1 T. Right: the reduced magnetization data measured
at T = 2 and 5 K. Open circles: experimental data and red solid lines: calculated data using the
equation 4, with J = +2.8(2) cm-1, λ = -136(4) cm-1, α = 1.27(1), ∆ = -383(18) cm-1.
Fig. 6 Magnetic data for (2). Left: the temperature dependence of the effective magnetic moment
and molar magnetization measured at B = 1 T. Right: the reduced magnetization data measured
at T = 2 and 5 K. Open circles: experimental data and red solid lines: calculated data using the
equation 2, with J = –4.45(7) cm-1, g = 2.12(1), χTIP = 4.8(7) x10-9 m3mol-1. Data are scaled per
one Cu(II) ion.
14

In order to quantitatively analyze the experimental magnetic data of Cu(II) compound
(2), the following spin Hamiltonian for dinuclear system was postulated
ꢀ
ꢀ
B
ˆ
ˆ
ˆ
H = −J(S ⋅S )+ µ Bg Sz,A + Sz,B
(2)
(
)
A
B
where the isotropic exchange parameter J determines the energy gap between the singlet (S = 0)
and triplet states (S = 1), resulting from the coupling of two local spins SA = SB = 1/2. The, simple
formula for the molar magnetization exists (x = µBgB) [30]
Mmol = µ gN exp (J + x) / kT −exp (J − x) / kT  /
(
)
(
)
B
A


1+ exp (J + x) / kT + exp J / kT + exp (J − x) / kT 
(
)
(
)
(
)


(3)
which was subsequently used to fit both temperature and field dependent data of (2). The best fit
parameters were found as J = –4.45(7) cm-1, g = 2.12(1), χTIP = 4.8(7) x10-9 m3mol-1, where χTIP
is the temperature-independent paramagnetism (Fig. 6). The fitted J-value (J = –4.45(7) cm-1) is
in good agreement with value derived from a simple formula for dinuclear homospin species,
|J|/kTmax = 1.599 [30], which yields J = –4.0 cm-1. Also, the fitted J-value is close to JYam = –4.43
cm-1 derived from DFT calculations (Table 2).
It is well known that the six-coordinated octahedral Co(II) complexes may have more
complex magnetic behavior due to the contribution of the orbital angular momentum (L) as also
suggested by CASSCF/NEVPT2 calculations. Concisely, the lowest-lying atomic term 4F is split
in octahedral ligand field symmetry to three crystal-field terms 4T1, 4T2 and 4A2, from which only
the first term is thermally populated and has non-zero orbital angular momentum. On lowering
the symmetry, the 4T1 term is further split into lower crystal field terms separated by ∆ [31]. Due
to the heteroleptic distorted octahedral coordination sphere of Co(II) in 1, such energy splitting is
expected and accordingly the following Hamiltonian was used to describe the magnetic
properties of 1:
ˆ
H = −J S ⋅S −αλ S ⋅L + S ⋅L
(
)
(
)
1
2
1
1
2
2
+ ∆ L2 − L (L +1) / 3+ L2 − L (L +1) / 3
(4)
(
+ µ B g S + g S −αL −αL
)
z,1
1
1
z,2
2
2
(
)
B
e
1
e
2
1
2
where α is orbital reduction factor, λ is spin-orbit coupling and ge = 2.0023. The Hamiltonian is
ML,1,MS,1
ML,2,MS,2
applied to
functions with ML,k = 0, 1 and MS,k = 1/2, 3/2. The angular
orbital momentum L is considered as fictitious angular momentum, L = 1, with the effective
Lande g-factor, gL = -α, due to T1-P isomorphism [32]. Then, there are 144 magnetic levels,
which are calculated and used in partition function Z to calculate molar magnetization as
15

d ln Z
dBa
Ma = NAkT
(5)
for given direction of magnetic field Ba = B·(sinθcosφ, sinθsinφ, cosθ). Finally, the integral
(orientational) average of molar magnetization is calculated by Eq. 6 in order to properly
simulate experimental powder magnetization data.
2π
π
Mmol =1/ 4π
Ma sinθdθdϕ
(6)
∫0 ∫0
The orbital reduction factor comprises of two parameters, α = Aκ, where A varies from 1 to 3/2
and results from admixture of the excited terms reflecting the ligand field strength, and κ
describes the lowering orbital contribution due to covalency of the metal-ligand bond and it
usually holds 0.70 < κ < 1. Moreover, the spin-orbit coupling parameter λ can also be reduced
compared to its free-ion value λ0 = -180 cm-1 as a result of the covalent character of the donor-
acceptor bond. By employing this advanced model, we obtained the following parameters: J =
+2.8(2) cm-1, λ = -136(4) cm-1, α = 1.27(1), ∆ = -383(18) cm-1, again by simultaneously fitting
both temperature and field dependent data (Fig. 5). The negative value of ∆ means that there is
an easy axis type of magnetic anisotropy in contrast to the positive ∆ found in another
ferromagnetically coupled Co(II) complex with easy plane anisotropy [33]. The easy axis type of
magnetic anisotropy is clearly demonstrated and visualized in 3D plot of molar magnetization in
Fig. S2.
It is evident from analysis of the experimental magnetic data of 1 that there is a weak
ferromagnetic exchange between Co(II) ions. This contradicts DFT calculations predicting
antiferromagnetic exchange, which may be surprising, because we used this method several
times for complexes with related ligands and always good agreements were found between the
calculated and fitted J-parameters [29]. However, in this peculiar case, we can speculate that this
discrepancy is caused by unquenched orbital momentum in the ground state and therefore the
broken-symmetry procedure is not suitable to treat this system.
Unfortunately, there are not many information available about hexa-coordinated di-(µ-Cl)
Co(II) complexes as manifested in Table 3. Magnetic exchange seems to be weak and either
ferro- or anti-ferromagnetic, and its proper determination was often hampered either by large
ZFS or magnetic ordering. Therefore, more data are needed in order to properly correlate the
magneto-structural properties in this class of Co(II) complexes.
Table 3. Magneto-structural data for hexa-coordinate µ-dichlorido-bridged Co(II) compounds.a
16

Complex
Chromophore d(Co-Cl) < (Co-Cl-Co) d(Co-Co)
J
Ref.
(Å)
(º)
(Å)
(cm-1)
[Co2(L1)2Cl2][CoCl4]
CoN4Cl2
CoN2Cl4
2.480
2.481
2.487
2.456
2.503
2.404
2.750
94.97
95.24
93.11
95.52
93.14
101.82
3.657
|J| << 1b 29
[Co(4,4`-bpy)Cl2]n
{(Me3NH)[CoCl3(H2O)2]}n CoO2Cl4
3.611
3.637
J > 0 b,c 30
+5.4 c
31
CoN4Cl2
4.006
This work
1
+2.8(2)
a
Ligand abbreviations: L1 = 1,1'-[(5-methyl-1,3-phenylene)bis(methyleneoxy)]bis[3-methyl-N-[(2-
pyridinyl-κN)methylene]-2-butanamine-κN]
Experimental magnetic data were not quantitatively analyzed.
b
c
Magnetic ordering at low temperatures.
Dynamic magnetic properties of (1). The observed axial magnetic anisotropy and weak
ferromagnetic exchange motivated us to measure the AC susceptibility for (1). At zero static
magnetic field, there was no out-of-phase susceptibility signal, which also confirms that there is
no long-range ferromagnetic ordering. However, the field dependent measurement performed at
T = 1.9 K revealed non-zero imaginary susceptibility (Fig. S3). Therefore, the AC susceptibilities
were measured at non-zero static field, Bdc = 0.5 T at low temperatures. The susceptibility data,
depicted in Fig. 7 showed a pattern that is characteristic for slow relaxation of the magnetization
and similar to that observed in single-molecule magnets (SMM) [34]. Unfortunately, the curves
did not show the clear maxima for out-of-phase susceptibility up to the lowest available
temperature. This trend prohibits us from getting detailed analysis (relaxation times and spin
reversal barrier). So far, there are many reports of the mononuclear Co(II) SMM compounds
[35,36,37], and there is also a large group of polynuclear Co(II) SMMs [38], but to the best of
our knowledge, only one paper on dinuclear Co(II) complexes showing slow relaxation of
magnetization was reported up to now [39]. The borderline between mononuclear and dinuclear
Co(II) SMM is found in penta-coordinate Co(II) complex [40], where a weak ferromagnetic
exchange coupling was mediated through π-π stacking interactions. Therefore, herein presented
dinuclear Co(II) complex (1) seems to be rare example of dinuclear Co(II) complex exhibiting
slow relaxation of the magnetization.
17

Fig. 7. In-phase χreal and out-of-phase χimag molar susceptibilities for 1 measured at the applied
external field Bdc = 0.5 T and various frequencies showed in legend. Lines serve as guides for the
eyes.
3.5. Cytotoxic activity
A number of pyrazole-containing metal complexes were demonstrated to be effective
against some cancer cells [41], therefore we made an effort to explore the biological potential of
complexes 1 and 2. The compounds were evaluated by means of the MTT assay for their in vitro
cytotoxicity against human breast adenocarcinoma (MCF7) and human cervix epithelioid
carcinoma (HeLa) cell lines. The results showed that the cobalt(II) complex 1 is inactive up to
the tested concentration of 50 µM. On the other hand, complex 2 exceeded the cytotoxicity of the
reference drug cisplatin significantly in the case of HeLa [IC50 = 2.5(0.9) µM as compared to
IC50 of 25.4(7.3) µM for cisplatin], while the complex was found to be less cytotoxic against
MCF7 [IC50 = 20.1(0.1) µM] as compared to cisplatin [IC50 = 11.9(2.5) µM]. Thus, the
copper(II) complex 2 has been revealed to be a promising compound for further and deeper
biological study.
4. Conclusions
The [Co2(bedmpzp)2(µ-Cl)2](PF6)2 (1) and [Cu(bedmpzp)Cl]PF6 (2) complexes, where bis(3,5-
dimethyl-1H-pyrazol-1-yl-1-ethyl)(2-pyridylmethyl)amine (bedmpzp) is a tripod tetradenate N-
donor ligand, were synthesized and structurally characterized. It is worth mentioning that
increased flexibility of the pyrazolyl groups in bedmpzp in comparison with pz2py resulted in
change of coordination number and nuclearity in Co(II) complexes with similar composition,
which is apparent from comparison of molecular structures of [Co(pz2py)Cl]BF4⋅1/2CH3OH [3],
and compound (1). Magnetic data of (1) revealed a weak ferromagnetic exchange (J = +2.8(2)
cm-1) and the axial type of magnetic anisotropy. Moreover, the AC susceptibility measurements
18

disclosed slow relaxation of magnetization in non-zero static magnetic field, thus setting (1) into
a class of field-induced single-molecule magnets. The X-ray structural analysis of (2) confirmed
the formation of supramolecular dimers of {[Cu(bedmpzp)Cl]PF6}2 connected through non-
covalent contacts of the Cu⋅⋅⋅Cl type with the separation distance of 3.5712(14) Å. This
arrangement is then associated with the observation of a weak antiferromagnetic exchange (J = –
4.45(7) cm-1), which was also supported by DFT calculations. Finally, the screening of in vitro
cytotoxicity revealed that complex 2 is a promising compound for forthcoming biological study
owing to its significant cytotoxicity on the HeLa cell line, with IC50 = 2.5(0.9) µM. To conclude,
the herein prepared unsymmetrical tripod ligand bedmpzp seems to be perspective for the
synthesis of magnetically and biologically interesting coordination compounds, thus
investigation of the small alternations of anionic ligands aiming at improving the physical
properties is underway.
Acknowledgments
This research was financially supported by the Department of Chemistry-University of Louisiana
at Lafayette. FAM thanks NAWI Graz for partial financial support and Dr. Baumgartner (TU
Graz) for assistance. RH, ZD and ZT acknowledge financial support by the Ministry of
Education, Youth and Sports of the Czech Republic (a project no. LO1305).
Appendix A and Supplementary Material
CCDC-1452109 and -1452110 contain the crystallographic data in CIF format for 1 and 2,
respectively. These data can be obtained free of charge from The Cambridge Crystallographic
Data Centre via www. ccdc.cam.ac.uk/data_request/cif. The crystallographic data for 1 and 2 are
in Table S1. Individual contributions to D-tensor for molecular fragment
[CoZn(bedmpzmpy)2(µ-Cl)2]2+ of 1 calculated by CASSCF/NEVPT2 and its energy levels of
ligand field multiplets in zero magnetic field derived from CASSCF/NEVPT2 calculations are
summarized in Tables S2 and S3, respectively. Figure S1 shows comparison of UV-VIS spectra
for 1, Figure S2 shows selected magnetic orbitals for 1 and 2, Figures S3-S4 are showing the
magnetic properties of the complexes. Supplementary data associated with this article can be
found, in the online version at http://dx.doi.org/??????????????????????
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Graphical Abstract (Pictogram)

Graphical Abstract (Synopsis)
The dinuclear doubly bridged-dichlorido complex [Co2(bedmpzp)2(μ-Cl)2](PF6)2 (1) and the
mononuclear [Cu(bedmpzp)Cl]PF6 (2) were structurally and magnetically characterized. Complex 1
revealed weak ferromagnetic exchange with slow relaxation of magnetization, whereas 2 showed
weak antiferromagnetic exchange. The in vitro cytotoxicity of the complexes against MCF7 and
HeLa human cancer cell lines were also tested.

Highlights
•
•
•
Dinuclear Co(II) and mononuclear Cu(II) complexes were prepared using a tripodal ligand
Bis(3,5-dimethyl-1H-pyrazol-1-yl-methyl)(2-pyridylmethyl)amine was utilized as the ligand
Co(II) complex revealed weak ferromagnetic exchange, axial anisotropy and slow relaxation of
magnetization
•
The in vivo cytotoxicity against MCF7 and HeLa human cancer cell lines were also examined.