Structural Variety of Isopropyl-bis(2-picolyl)amine Complexes with Zinc(II) and Copper(II)

Article abstract of DOI:10.1021/acs.cgd.9b01625

A variety of structurally different complexes of the isopropyl-bis(2-picolyl)amine (iPr-bpa) ligand were prepared with ZnA₂ and CuA₂ salts (A = Br^-, Br^-/PF₆ ^-, BF₄ ^-/F^-, ClO₄ ^-). The choice of different counterion affected the stoichiometry, coordination number, geometry, and formation of geometrical isomers. Crystal structures of four Zn(II) complexes, namely, two monomers (mer-[Zn(iPr-bpa)Br₂] and fac-[Zn(iPr-bpa)Br₂]), one F^--bridged dimer ([Zn₂(μ-F)₂(iPr-bpa)₂](BF₄)₂), and one ML₂ complex ([Zn(iPr-bpa)₂](ClO₄)₂) were determined, and their solution structures were studied by NMR spectroscopy. For the ML₂ complex, relative stabilities of geometrical isomers were determined using density functional theory calculations. For Cu(II) complexes, five crystal structures were determined, namely, two monomers ([Cu(iPr-bpa)Br₂] and [Cu(iPr-bpa)(ClO₄)₂(H₂O)]), a Br^--bridged dimer ([Cu₂(μ-Br)(Br)₂(iPr-bpa)₂](PF₆)), a F^--bridged coordination polymer ([Cu(μ-F)(iPr-bpa)]_n(BF₄)_n × nCH₃OH), and a cyclic, CO₃ ^2--bridged trimer ([Cu₃(tri-μ-CO₃)(ClO₄)₃(iPr-bpa)₃](ClO₄)). The different crystallographic structures of Cu(II) complexes are reflected in their different magnetic properties investigated by electron spin resonance spectroscopy and magnetic susceptibility measurements.

Full text of DOI:10.1021/acs.cgd.9b01625

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Article
Structural variety of isopropyl-bis(2-picolyl)amine
complexes with zinc(II) and copper(II)
Natalija Pantalon Juraj, Senada Muratovi#, Berislav Peric, Natasa Sijakovic
Vujicic, Robert Vianello, Dijana Žili#, Zvonko Jaglicic, and Sre#ko I. Kirin
Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.9b01625 • Publication Date (Web): 20 Feb 2020
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Page 1 of 16
Crystal Growth & Design
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Structural Variety of Isopropyl-bis(2-picolyl)amine Complexes with
Zinc(II) and Copper(II)
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Natalija Pantalon Juraj,a Senada Muratović,a Berislav Perić,a Nataša Šijaković Vujičić,a Robert
Vianello,a Dijana Žilić,a Zvonko Jagličićb and Srećko I. Kirina,*
aRuđer Bošković Institute, Bijenička c. 54, HR-10000 Zagreb, Croatia
b Institute of Mathematics, Physics and Mechanics & Faculty of Civil and Geodetic Engineering University of
Ljubljana, Jamova 2, 1000 Ljubljana, Slovenia
ABSTRACT: A variety of structurally different complexes of the isopropyl-bis(2-picolyl)amine (iPr-bpa) ligand were prepared
with ZnA2 and CuA2 salts (A = Br─, Br─/PF6─, BF4─/F─, ClO4─). The choice of different counterion affected the stoichiometry,
coordination number, geometry and formation of geometrical isomers. Crystal structures of four Zn(II) complexes; two mono-
mers (mer-[Zn(iPr-bpa)Br2] and fac-[Zn(iPr-bpa)Br2]), one F─ bridged dimer ([Zn2(μ-F)2(iPr-bpa)2](BF4)2) and one ML2
complex ([Zn(iPr-bpa)2](ClO4)2) were determined, and their solution structures were studied by NMR spectroscopy. For the
ML2 complex, relative stabilities of geometrical isomers were determined using DFT calculations. For Cu(II) complexes, five
crystal structures were determined; two monomers ([Cu(iPr-bpa)Br2] and [Cu(iPr-bpa)(ClO4)2(H2O)]), a Br─ bridged dimer
([Cu2(μ-Br)(Br)2(iPr-bpa)2](PF6) × CH3OH), a F─ bridged coordination polymer ([Cu(μ-F)(iPr-bpa)]n(BF4)n × nCH3OH), and a
cyclic, CO3 ─ bridged trimer ([Cu3(tri-μ-CO3)(ClO4)3(iPr-bpa)3](ClO4)). The different crystallographic structures of Cu(II) com-
2
plexes are reflected in their different magnetic properties investigated by ESR spectroscopy and magnetic susceptibility meas-
urements.
acetamide (imda) moieties. Interestingly, despite their simi-
larity in structure, the two ligand systems differ significantly
in their stereochemical preference for formation of ML2
Introduction
Transition metal complexes of the bis(2-picolyl)amine
(bpa) ligand are extensively studied in literature; the Zn(II)
and Cu(II) bpa complexes find use as enzyme mimics for
structural features or function,1–4 promising anticancer
agents,5,6 potent metal ion sensors,7–9 and selective
catalysts.10,11 A particular focus of recent research efforts are
various effects that govern the formation of complexes of
different stoichiometry, coordination numbers and coordina-
tion geometry, including geometrical isomers.12 The contrib-
uting effects include the electronic or steric properties of the
ligand,13 and the coordinating ability of the counterion.14
complexes. In particular, imda ligands form almost exclu-
sively trans-fac, while bpa ligands gave preferentially ML2
cis-fac isomers (Figure 1). Herein, we aim to prepare com-
plexes of ML2 stoichiometry and study the stability of their
different geometrical isomers. We focused on the effect of
the counterion, preparing complexes of ligand iPr-bpa (L)
with Zn(II) or Cu(II) and different counterions (Br─,
Br─/PF6─, BF4─/F─, ClO4─). Of the four crystal structures of
Zn(II) complexes, two of the structures were found to be
polymorphs (1aZn and 1bZn) and the stability of the two pol-
Transition metal complexes of the bpa ligand with ML2
stoichiometry are found as both trans- and cis-fac isomers,
apparently influenced by the coordinating ability of the
counterion and the substituent on the central amine nitro-
gen. All published ML2 complexes of R-bpa ligands with R ≠
H are cis-fac isomers in the solid state,15,16 while both cis- and
trans-fac isomers are reported for R-bpa (R = H) ligands, the
cis-fac predominantly forming when weakly coordinating
anions were used in the synthesis.17,18 In the Cambridge
Structural Database CSD, there is also a significant number
of bridged bpa metal complexes, mostly the bridging func-
tional groups are hydroxide or halogenide ions.19,20
Recently, we have reported on metal complexes of bpa and
iminodiacetamide (imda) ligands.21,22 Both bpa and imda
ligands are tridentate with a central amine nitrogen atom
substituted with two methylene pyridine (bpa) or methylene
Figure 1. Different stereochemical preference of ML2
complexes with bpa and imda ligands.
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Crystal Growth & Design
Page 2 of 16
ymorphs was studied by differential scanning calorimetry
Scheme 1. Synthesis of Zn(II) complexesa
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and DFT calculations. The solution structure of the Zn(II)
complexes was studied by nuclear magnetic resonance
(NMR) spectroscopy. Five crystal structures of Cu(II) com-
plexes were determined and their local magnetic properties
studied by electron spin/paramagnetic resonance ESR/EPR
spectroscopy and magnetic susceptibility measurements.
Additionally, the CO2 binding properties of 3bCu were stud-
ied by UV-Vis spectroscopy.
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Results and discussion
Synthesis and characterization of Zn(II) complexes
Four complexes were prepared using the ligand iPr-bpa
and several different Zn(II) salts; two polymorphs, 1aZn and
1bZn, a bridged dimer 2Zn and a ML2 complex 3Zn (Scheme 1).
Their molecular structures determined by single crystal X-ray
crystallography are shown in Figure 2 and selected bond
lengths are collected in Table 1.
aReaction conditions: (a) ZnBr2, methanol; (b) 2 ZnBr2, 4
NH4PF6, methanol, (c) Zn(BF4)2, methanol; (d) Zn(ClO4)2,
Attempting to prepare a complex of ML2 stoichiometry,
iPr-bpa and ZnBr2 were mixed in a 2:1 ratio. However, due to
the strong coordinating ability of the bromide anion, a com-
plex of ML stoichiometry was obtained, namely 1aZn.21 The
solid state structure of 1aZn was determined using single
crystal X-ray diffraction. Complex 1aZn contains two inde-
pendent molecules in the unit cell. Both independent mole-
cules of 1aZn are penta-coordinated, having distorted square
pyramidal [ZnBr2N3] geometry (τ1 = 0.34 and τ2 = 0.17, re-
spectively)23, where the ligand is coordinated meridionally
[angle N1─Zn─N3 = 152.36(10)° for Zn1 and 150.34(10)° for
Zn2, angles between pyridine planes are 12.19(15)° and
10.88(15)°, respectively]. The remaining two coordination
sites of the central zinc cation are occupied by bromide ani-
ons.
pounds, namely [M(R-bpa)X2] complexes containing any
transition metal M or any halogen ions X, the bpa fac-isomer
was found relatively rarely in the solid state, with only a
handful of structures in the literature.12,26–30
To study the conditions for the formation of the two pol-
ymorphs, 1aZn (mer) and 1bZn (fac), syntheses in different
conditions were performed (Table S8). The phase purity of
the obtained polycrystalline samples was analyzed with X-ray
powder diffraction. During numerous attempts of crystalliza-
tion, formation of the mer polymorph was observed more
frequently, while in crystallizations using diethyl-ether diffu-
sion the more frequently obtained polymorph was fac.
The stability of the two polymorphs was studied by differ-
ential scanning calorimetry and hot-stage microscopy, re-
vealing a solid-to-solid polymorph transition from the poly-
morph fac to the more stable polymorph mer (Figure 3 and
Figures S19-S22). The DSC heating curves of the mer poly-
morph showed only one endothermic transition with peak
maximum at 266,6 °C and enthalpy change of H = 6.1 kcal
mol-1 (Figure 3). The DSC thermograms of the fac polymorph
showed two endothermic transitions; the first transition with
peak maximum at 186,7 °C and enthalpy change of H = 1.6
kcal mol-1 and the second one at 264,7 °C and enthalpy
change of 6.2 kcal mol-1.
In the synthesis using iPr-bpa, ZnBr2 and NH4PF6 in a ra-
tio of 1:1:2, complex 1bZn was obtained. In the crystal struc-
ture of 1bZn, the PF6─ anion was not incorporated, but it was
a polymorph of 1aZn, with a penta-coordinated zinc atom and
ML stoichiometry. As opposed to 1aZn, in 1bZn the ligand is
coordinated facially [angle N1-Zn-N3 = 109.56(15)°, angle
between pyridine planes = 79.1(2)°]. The [ZnBr2N3] coordina-
tion polyhedron in 1bZn is at the border between a trigonal
bipyramid and square pyramid (τ = 0.50).
A survey of the CSD revealed that in the few known penta-
coordinated [Zn(R-bpa)Br2] complexes the bpa ligand is
bound to the central metal as mer-isomer.5,6,21,24,25 On the
other hand, even for a significantly broader class of com-
Having compared the DSC thermograms of polymorph fac
subjected to different heating-cooling procedures, we con-
3Zn
1aZn
1bZn
2Zn
Figure 2. Molecular structures of complexes 1aZn (mer-[Zn(iPr-bpa)Br2]), 1bZn (fac-[Zn(iPr-bpa)Br2]), 2Zn ([Zn2(μ-F)2(iPr-
bpa)2](BF4)2) and 3Zn ([Zn(iPr-bpa)2](ClO4)2). Displacement ellipsoids were drawn at the 30% probability level. The anions
tetrafluoroborate (in 2Zn) and perchlorate (in 3Zn) were omitted for clarity.
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Crystal Growth & Design
BF4─ ⇄ BF3 + F─, forming fluoride ions. Fluorinated counteri-
ons like BF4 are known to be potential sources of fluoride
─
1
ions.22,34,35 The counterion decomposition could be accelerat-
ed by the basicity of the iPr-bpa ligand and/or by the metal,
oxygen and water.20,36 The free fluoride ion, being a small ion
of localized charge, can easily bind to the metal center. The
crystal structure of 2Zn shows a complex of M2L2 stoichiome-
try, with two bridging fluoride anions. Complex 2Zn is centro-
symmetric with penta-coordinated Zn(II) atoms, [ZnN3(μ-
F)2ZnN3], revealing two trigonal bipyramidal coordination
polyhedrons (τ1 = 0.86) that share one edge. In 2Zn, the lig-
ands are coordinated facially, with an N1─Zn─N3 angle of
111.44(7)° and an angle between pyridine planes of 79.30(12)°.
In the literature, only one zinc bpa complex is known with a
double fluoride bridge, [(bpa)Zn(μ-F)2Zn(bpa)],37 but sever-
al similar structures are published with other transition met-
als including iron, cobalt, nickel and copper.20,38–42
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3
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Figure 3. DSC thermograms of a) heating cycles of 1aZn
(mer, red) and 1bZn (fac, blue); b) temperature-controlled
transition of fac to mer polymorph.
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cluded that the first endothermic transition corresponds to
the fac to mer conversion process. This was confirmed by X-
ray powder diffraction analysis of the sample of the fac pol-
ymorph heated to 220 °C and cooled to room temperature
that showed the pattern of the mer polymorph (Figure S21).
The enthalpy change of the second endothermic transition at
264 °C corresponds to melting of polymorph mer.
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To avoid decomposition of the anion, perchlorate anion
was used instead of tetrafluoroborate. The perchlorate anion
is also weakly coordinating, but it is more stable in the ap-
plied reaction conditions. Complex 3Zn was prepared using
iPr-bpa and Zn(ClO4)2 in a 2:1 ratio, yielding a complex of
ML2 stoichiometry as determined by single crystal X-ray
diffraction. The two ligands in 3Zn are coordinated facially,
with angles N1─Zn─N3 91.9(3)° and 93.5(3)°, and the angle
between pyridine planes = 81.1(5)° and 82.3(6)°, respectively.
The hexa-coordinated complex 3Zn forms a [ZnN6] distorted
octahedron of cis-fac geometry [angle N─Zn─N (amine)
102.9(2)°]. Our interest is to study effects that govern the
formation of different geometrical isomers in ML2 complex-
es21,22, therefore we described the relative stability of cis-fac,
mer and trans-fac isomers using DFT calculations. Our ap-
proach relied on optimizing molecular geometries with the
M05-2X/6-31+G(d)/LanL2DZ + ECP model both in gas-phase
and acetonitrile, the latter modeled through the implicit
SMD solvation, being in line with our earlier reports.21,22 The
quality of this methodology was evaluated by performing a
series of single-point electronic energy calculations using a
range of DFT functionals. The respective relative energies are
displayed in Table 2; selected bond lengths, angles and coor-
dination geometries of the calculated geometrical isomers of
[Zn(iPr-bpa)2]2+ and [Zn(Me-bpa)2]2+ are shown in Tables
S6 and S7 and compared to the experimentally determined
structures 3Zn and [Zn(bpa-CH2-CH2-CO2Me)2](BF4)2 (from
ref. 21). The results reveal a very good performance of the
M05-2X functional, which gives trends in a close agreement
with a majority of tested approaches, particularly with dis-
persion-corrected functionals such as ω-B97XD, B97D3 and
Computational analysis shows that mer 1aZn is by 1.1 kcal
mol–1 on the Gibbs free-energy scale, and by 1.5 kcal mol–1 on
the enthalpy scale, more stable than its fac 1bZn analogue,
placing this result in firm agreement with experiments, thus
confirming the higher thermodynamic stability and solution
prevalence of the former. In addition, the transition state
structure connecting these two isoforms lies only 7.4 kcal
mol–1 above the more stable 1aZn, confirming that the transi-
tion between polymorphs is feasible even at room tempera-
ture.
The lattice energies of conformational polymorphs are very
close and differences in ∆H between 0.05 and 2.4 kcal mol–1
have been reported for polymorphic transitions31–33. In this
particular case there is a noticeable difference in energy and
structure for the two polymorphs due to the different geome-
try of the complexes. In agreement with this conclusion, the
hot-stage polarizing optical microscopy images in Figure S22
clearly showed the melting transition of crystals at 270 °C,
and a small change during interconversion from the poly-
morph fac to polymorph mer.
In the next attempt of preparing a complex of ML2 stoichi-
ometry, iPr-bpa and Zn(BF4)2 were mixed in a 2:1 ratio. The
tetrafluoroborate counterion was chosen as a weakly coordi-
nating anion that would not interfere with binding of the
second ligand to the metal. However, in these conditions the
tetrafluoroborate ion undergoes a decomposition reaction,
Table 1. Selected bond lengths (Å) for Zn(II) complexes.
Complex
Zn─N1 (Å)
Zn─N2 (Å)
Zn─N3 (Å)
Zn─anion (Å)
2.177(2)
2.142(2)
2.073(4)
2.0371(17)
2.229(9)
2.195(9)
2.215(2)
2.270(2)
2.338(4)
2.1717(16)
2.287(7)
2.417(7)
2.162(2)
2.143(2)
2.101(4)
2.4061(5) (Br11); 2.4075(5) (Br21)
2.4188(5) (Br12); 2.4165(4) (Br22)
2.5079(7) (Br1); 2.4247(7) (Br2)
2.0462(12) (F1); 1.9313(11) (F1i)
1aZn
1bZn
2Zn
2.0269(17)
2.100(7)
2.155(8)
─
─
3Zn
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Table 2. Relative Gibbs free energies of different structural isomers involving the Zn2+ cation and ligands iPr-
1
2
bpa or Me-bpa in the 1:2 ratio.a
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4
5
GAS-PHASE
Complex cation
Isomer
M05–2X B3LYP PBE0 BLYP M06L ω-B97XD B97D3 B3LYP-D3
6
7
8
9
cis-fac
mer
0.0
6.1
0.0
4.4
4.7
1.5
0.0
4.9
5.0
1.2
0.0
3.7
4.1
0.0
3.7
5.2
0.0
1.4
0.0
5.1
0.0
4.1
0.0
4.4
5.6
0.0
0.6
3.5
[Zn(iPr-bpa)2]2+
trans-fac
cis-fac
mer
6.7
0.0
0.7
3.1
6.3
0.0
0.1
5.3
0.0
0.7
3.3
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1.5
[Zn(Me-bpa)2]2+
Complex cation
[Zn(iPr-bpa)2]2+
0.0
3.7
0.0
3.4
0.0
3.5
trans-fac
3.7
3.5
ACETONITRILE SOLUTION
M05–2X B3LYP PBE0 BLYP M06L ω-B97XD B97D3 B3LYP-D3
Isomer
cis-fac
mer
0.0
4.1
0.6
0.0
4.3
3.3
0.0
1.3
2.2
0.0
5.2
3.4
0.0
3.0
0.0
1.4
0.0
2.3
5.3
0.6
0.0
2.5
0.0
0.1
0.0
1.0
trans-fac
cis-fac
mer
5.9
0.9
0.0
2.7
4.2
2.5
0.0
2.7
3.7
1.3
4.0
0.2
0.0
1.9
4.5
0.4
0.0
2.2
[Zn(Me-bpa)2]2+
0.0
3.1
0.0
2.6
trans-fac
aAll values are in kcal mol–1. The Gibbs free energies are obtained on the geometries optimized at the M05-2X/6-
31+G(d)/LanL2DZ + ECP level of theory, with electronic energies attained as single-point calculations using the indicated DFT
functional and the same basis sets. Solvent effects are modeled through the implicit SMD solvation.
B3LYP-D3. On the other hand, pure BLYP functional and its
hybrid B3LYP analogue lean to give wrong trends, especially
for [Zn(Me-bpa)2]2+ in the gas-phase and [Zn(iPr-bpa)2]2+ in
acetonitrile. Therefore, in what follows we will discuss the
results pertaining the M05-2X functional, which is beneficial
because it allows direct comparison with our earlier data.21,22
The inspection of the calculated structures reveals that com-
plexes of the iPr-bpa derivative have a more distorted geom-
etry than those of Me-bpa (Tables S6 and S7). In addition,
the calculations show that the most stable geometrical iso-
mer of [Zn(iPr-bpa)2]2+ is cis-fac in both the gas phase and
acetonitrile solution, which is in accordance with the ob-
tained crystal structure of 3Zn. The prevalence of this isomer
is retained for [Zn(Me-bpa)2]2+ in the gas-phase, yet in ace-
tonitrile mer becomes the most stable, which is consistently
predicted with all tested DFT functionals. Nevertheless, for
Me-bpa, the calculated energy differences among isomers
were rather small, Table 2. In the literature, a number of
[Zn(R-bpa)2]2+ have been studied by single crystal X-ray
diffraction. Interestingly, cis-fac isomers were found even for
R-bpa complexes with a large substituent R on the amine
nitrogen atom, where trans-fac would be expected due to
steric crowding. This trend is also observed here, as the pre-
dominance of mer with a smaller R = Me changes to cis-fac
with a larger R = iPr. Additionally, cis-fac single crystal struc-
tures are known for derivatives with bulky R groups, includ-
ing methyl ethanoate, p-chlorobenzyl and anthranyl,43 phe-
nyl-triazole,44 ferrocenyl,16 methyl propanoate21 and phat-
limide.45
The complexes of diamagnetic Zn(II) were studied by NMR
spectroscopy to give information about the formation of
complexes in solution. The NMR spectra were recorded ei-
ther of complexes prepared in situ in deuterated solvent or of
complexes that were previously prepared in methanol, isolat-
ed and dissolved in deuterated solvent. As expected, the
NMR spectra of previously isolated polymorphs 1aZn and 1bZn
show presence of identical ML species 1Zn at the NMR time-
1
scale. Compared to the free ligand, the H NMR spectrum of
1Zn in CD3CN shows shifted peaks and broadening of the α-
CH2 peak. The broad peak indicates a weak Zn-N bond at
room temperature, which enables nitrogen inversion and
equivalence of the α-CH2 protons. Strengthening of the
1
Figure 4. H NMR (CD3CN) titration of iPr-bpa with
Zn(ClO4)2.
NMR study of Zn(II) complexes
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Crystal Growth & Design
Scheme 2. Synthesis of Cu(II) complexesa
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aReaction conditions: (a) CuBr2, methanol; (b) 2 CuBr2, 4 NH4PF6, methanol, (c) Cu(BF4)2, methanol; Cu(ClO4)2, (d) methanol,
chloroform.
Zn─N bond was assumed to occur at lower temperatures and
complex more stable than ML2. Similar cases are described in
literature22,44 The CD3CN solution of iPr-bpa and Zn(ClO4)2
at a 2:1 ratio was cooled to ─40°C in order to slow down the
exchange. Sharpening of the signals was observed, but the
many signals suggest a mixture of several species (Figure
S27).
1
the H NMR spectrum recorded at ─40°C (Figure S24) shows
splitting of the α-CH2 peak into two doublets (4.36 and 4.12
ppm) with geminal coupling (J = 16 Hz), indicating a stronger
bond and inhibited nitrogen inversion.21,22
1
The H NMR spectrum of previously isolated 2Zn shows a
single species, possibly the bridged complex corresponding
to the crystal structure. The NMR spectrum of the in situ
prepared complex in a 2:1 ratio showed broad peaks (Figure
S25), indicating an intermediate ligand exchange, similar as
for complex 3Zn at a 2:1 ratio (described later, Figure 4) These
Synthesis and characterization of Cu(II) complexes
Five complexes of iPr-bpa and Cu(II) were prepared using
four different counterions (Scheme 2). Single crystal struc-
tures of two monomers 1aCu and 3aCu, a bridged dimer 1bCu, a
coordination polymer 2Cu and a cyclic trimer 3bCu were de-
termined, their molecular structures are shown on Figures 5
and 6. Selected bond lengths are collected in Table 3.
results show that in the previously isolated complex 2Zn
,
there was enough fluoride ion to coordinate the Zn(II) and
form a (bridged) ML complex, while in the in situ experi-
ment, fluoride had not yet formed and it is likely a mixture of
complexes with intermediate ligand exchange. When com-
paring the previously isolated 2Zn and the in situ iPr-bpa and
Zn(BF4)2 in a 1:1 ratio, the spectra show slightly different
chemical shifts. We assume that both complexes have ML
stoichiometry, but the remaining coordination sites are oc-
cupied by F─ anions in the isolated complex, and by acetoni-
trile molecules in the in situ experiment.
Complex 1aCu was prepared using iPr-bpa and CuBr2 in a
2:1 ratio. However, the crystal structure of 1aCu shows that
due to coordination of bromide anions, a complex of ML
stoichiometry was obtained, similarly as for previously de-
scribed 1aZn / 1bZn. 1aCu is a penta-coordinated [CuBr2N3]
complex of distorted square pyramidal geometry (τ = 0.33),
where the ligand is bound meridionally [angle N1─Cu─N3
163.81(12)°, angle between pyridine planes = 8.56(18)°]. In the
literature, a large number of penta-coordinated complexes
mer-[CuX2(R-bpa)] are known.10,14,46,47
1
To describe the solution structure of 3Zn, a H NMR titra-
tion was performed (Figure 4), by adding Zn(ClO4)2 to a
solution of iPr-bpa. With the addition of 0.5 eq Zn(II), a ML2
complex was expected to form, that would correspond to the
solid state structure 3Zn. However, the NMR spectrum
showed broad signals, indicating an intermediate ligand
exchange compared to the NMR timescale. By further adding
Zn(II), a complex of ML stoichiometry is formed and remains
the only species in solution for higher equivalents of Zn(II)
(above 1 eq). Sharper signals of the ML complex indicate that
the exchange is slower than the NMR timescale and the ML
In the synthesis using iPr-bpa, CuBr2 and NH4PF6 in a ra-
tio of 1:1:2, complex 1bCu was prepared. Unlike the Zn(II)
complex 1bZn prepared in the same way, that was an ML
monomer, the crystal structure of 1bCu reveals a complex of
M2L2 stoichiometry, where two pentacoordinated Cu(II)
atoms are bridged by one bromide ion, [CuBrN3(μ-
Br)CuBrN3]. Both Cu(II) have square pyramidal geometry (τ1
= 0.08, τ2 = 0.28). The ligand is coordinated meridionally
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Page 6 of 16
3bCu, the carbonate ion connects three Cu(II) centers, form-
1
2
3
4
5
6
7
8
ing a cyclic trimer of C3 molecular symmetry. Each Cu(II) is
hexacoordinated, with a distorted octahedral geometry due
to the bidentate binding of the carbonate ion. Three coordi-
nation sites in 3bCu are occupied by the ligand, two coordi-
nated to the carbonate ion and one bound perchlorate anion,
[Cu3(tri-μ-O)O3N9]. The ligand is a mer isomer [angles
N1─Cu─N3 166.4(3)°, angles between pyridine planes =
6.6(5)°].
1aCu
1bCu
The synthesis of 3bCu was repeated with bubbling CO2
through the methanol solution of iPr-bpa and Cu(ClO4)2 in a
2:1 ratio; phase purity of the obtained product was confirmed
by X-ray powder diffraction (Figure S52). The attempts to
prepare a higher yield of 3aCu in inert conditions, excluding
water and/or air, were not successful.
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Binding of CO2 in the iPr-bpa / Cu(ClO4)2 system was
studied by UV-Vis spectroscopy. In basic solutions, CO2 is
present in the form of a carbonate ion so we assumed that
the excess of ligand in the solution acted as a base. Two
equivalents of iPr-bpa were added to Cu(ClO4)2 in an ace-
tonitrile solution, in an atmosphere of N2, then CO2 was
bubbled through the solution. For a 1:1 ligand to metal ratio,
no significant change was observed. However, for the 2:1
ratio, binding of the carbonate ion caused a bathochromic
shift, from 619 to 637 nm, corresponding to a similar car-
bonate bridged complex from literature (λmax = 639 nm).55
These results indicate that an excess of ligand is necessary for
the formation of the carbonate ion and carbonate bridged
complex. It is interesting to note that in literature, similar
Zn(II) complexes are studied as mimics of the enzyme car-
bonic anhydrase for binding of CO2.57–59 In our case, CO2
binding was observed only for the Cu(II) complex.
3aCu
2Cu
Figure 5. Molecular structures of complexes 1aCu ([Cu(iPr-
bpa)Br2]), 1bCu ([Cu2(μ-Br)(Br)2(iPr-bpa)2](PF6) × CH3OH),
2Cu ([Cu(μ-F)(iPr-bpa)]n(BF4)n
× nCH3OH) and 3aCu
([Cu(iPr-bpa)(ClO4)2(H2O)]).
[angles N1─Cu─N3 164.17(15)° and 164.56(17)°, angle between
pyridine planes = 8.3(2)°, 8.7(3)°, respectively]. Similar
bridged structures obtained by using the combination of
─
halogen and PF6 anions are described in literature.48
In the synthesis of 2Cu, iPr-bpa and Cu(BF4)2 were used in
a 2:1 ratio. Similar as discussed for 2Zn, decomposition of the
BF4─ anion leads to the formation of fluoride ion. The crystal
structure of 2Cu showed that the F─ ion acted as a bridging
ligand, connecting the Cu(II) centers into a coordination
polymer (ML)n. In 2Cu, the Cu(II) center is penta-coordinated
[Cu(μ-F)2N3]n, with an almost ideal square pyramidal geome-
try (τ = 0.03). The ligand in 2Cu is bound meridionally [angle
N1─Cu─N3 167.0(2)°, and angle between pyridine planes =
1.1(3)°]. In the CSD database, only three halogenide bridged
copper(II) coordination polymers with ligands structurally
similar to bpa are published.49–51
In the synthesis using iPr-bpa and Cu(ClO4)2 in a 2:1 ratio,
two products with different crystal habitus were obtained.
The main product was hexagonal complex 3bCu, with a small
amount of needle-like 3aCu crystals. The crystal structure of
complex 3aCu shows a monomer of ML stoichiometry. The
hexa-coordinated [CuO3N3] coordination polyhedron reveals
a distorted octahedral geometry. Due to Jahn-Teller distor-
sion, the axial bonds between Cu(II) and the perchlorate
anions are elongated [2.5711(16) and 2.6875(14) Å] compared
to the equatorial bonds [1.9652(18)-2.0429(14) Å]. The ligand
is meridionally bound [angles N1─Cu─N3 165.67(7)°, angle
between pyridine planes = 7.23(10)°], and the remaining three
coordination sites are occupied by two perhlorate anions and
one water molecule. In the literature, two structures with a
similar [Cu(R-bpa)(ClO4)2(H2O)] coordination sphere are
known, namely with bpa ligands bearing anthranyl or
guanidyl side chains.52
Figure 6. Molecular structure of complex 3bCu ([Cu3(tri-μ-
CO3)(ClO4)3(iPr-bpa)3](ClO4)) viewed from two different
perspectives; side view (left) and top view (right). Dis-
placement ellipsoids were drawn at the 30% probability
level. Perchlorate anions bound to Cu(II) ions were trun-
cated from the structure (left) for better visibility of the
ligand. In both views, the anions not bound to the metal
are omitted.
Magnetic susceptibility of Cu(II) complexes
Magnetic susceptibility was measured for Cu(II) complexes
1aCu, 1bCu, 2Cu and 3bCu.The susceptibility  in Figure 7 mon-
otonically increases with decreasing temperature from 300 K
down to 2 K for all investigated copper complexes. The prod-
ucts T at room temperature (inset in Figure 7) are 1.26 emu
K /mol (3bCu), 0.85 emu K/mol (1bCu), and 0.42 emu K/mol
(2Cu and 1aCu). Taking into account different number of cop-
per ions in a molecule (three in 3bCu, two in 1bCu, and one in
2Cu and 1aCu), the effective magnetic moment
is 1.8 B for all four samples,60
Surprisingly, complex 3bCu was found to capture atmos-
pheric CO2. Due to the presence of water in the solvent as
well as in the Cu(ClO4)2 × 6H2O precursor and the basicity of
the ligand, CO2 was present in 3bCu in the form of carbonate
ion. Several similar systems are described in literature.53–56 In
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Crystal Growth & Design
In 3bCu the magnetic moments compose equilateral trian-
1
2
3
gles with -CO3- unit as a bridge in the centre. Due to large
distances between triangles, we assume only intramolecular
interactions with an interaction Hamiltonian:
4
.
(2)
5
6
7
8
An excellent agreement between the experimental data
and the susceptibility calculated from (1) using the PHI soft-
ware64 was obtained (full line in Figure 7 inset). The g-factor
was set to 2.1 as the average value obtained from ESR, while
the interaction parameter J = 19 cm-1 was obtained as a result
of the fitting procedure.
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12
13
14
15
Figure 7. Temperature dependence of the susceptibility 
and the product T (inset). The full orange lines are fits as
described in the main text.
ESR study of Cu(II) complexes
The ESR spectra of the investigated Cu(II) complexes, ob-
tained at selected temperatures, are shown in Figure 8. The
simulations were performed using EasySpin software.65
where the constant μB is the Bohr magneton. This value is in
agreement with the expected value for Cu(II) ion with spin
S=1/2.61
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Considering the susceptibility results, for the simulation of
the ESR spectra of complexes 1aCu and 1bCu, the following
reduced form of spin-Hamiltonian was assumed:
With decreasing temperature, the product T remains
more or less constant down to the lowest temperature for
1aCu and 1bCu. The result means there is no considerable
magnetic interaction between magnetic moments in these
two complexes. On the other hand, for 2Cu and 3bCu the
product T starts to increase below approximately 100 K
indicating ferromagnetic interactions between magnetic
moments. Considering the structures of these two complexes
we have estimated the interaction parameter J in the com-
plexes.
(3)
For the polymer complex 2Cu, the additional interaction
term (1) was counted to (3). In Equation (3), is the external
magnetic field, is the -tensor and is the electron spin
operator for the unpaired copper spin (S = 1/2). The simulat-
ed spectra are shown in Figure 8 while the parameters used
for the simulations are given in Table 4. The spectra were
simulated using the same spin-Hamiltonian parameters in
the whole temperature range, taking into consideration only
the change of line-width of the assumed Lorentzian lines. All
simulations were performed without using strain parameters.
The spectra of complexes 1aCu, 1bCu and 2Cu were simulated
considering a rhombic -tensor with three different g-values.
No hyperfine lines, due to the interaction between electron
(S = 1/2) and nuclear spins (I = 3/2), were detected. The simu-
lations reproduce the experimentally observed spectra well
(Figure 8).
In the polymer complex 2Cu copper S=1/2 ions form a quasi
one dimensional structure and the interaction Hamiltonian
can be written as a sum:
.
(1)
For J > 0 (a ferromagnetic interaction as in our case) no
analytical equation has been proposed for (T) calculated
from the Hamiltonian (2).62 A high-temperature series ex-
pansion was proposed in references 62 and 63. The fit of the
experimental data above 50 K with the proposed rationale
function from the reference62 and g-factor 2.1, gave us the
interaction parameter J = 12 cm-1 (the full orange line in Fig-
ure 7 inset).
The spectra of trimer complex 3Cu were analyzed using the
spin-Hamiltonian (3) expanded with an electron-electron
interaction term:
(4)
Table 3. Selected bond lengths (Å) for Cu(II) complexes.
Complex
Cu─N1 (Å)
Cu─N2 (Å)
Cu─N3 (Å)
Cu─anion or solvent (Å)
1aCu
2.004(3)
2.106(3)
1.979(3)
2.6254(6) (Br1); 2.4261(6) (Br2)
2.015(4)
1.992(4)
1.972(5)
2.059(4)
2.075(4)
2.043(5)
1.996(4)
1.981(4)
1.982(5)
2.7341(7) (Br11); 2.4037(8) (Br21)
2.7341(7) (Br12); 2.3978(8) (Br22)
2.282(3) (F1); 1.893(3) (F1ii)
1bCu
2Cu
2.5711(16) (O1p1); 2.6875(14) (O1p2)
1.9777(13) (O1w, H2O)
3aCu
1.9758(17)
2.0429(14)
1.9652(18)
-
1.967(5) (O1, CO3 );
-
3bCu
1.956(5)
2.040(6)
1.963(5)
2.623(4) (O1iii, CO3 )
-
2.353(7) (O2, ClO4 )
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Table 4: The values of spin-Hamiltonian parameters obtained from simulations. The calculated R parameter,70
Addison structural parameters  23 and copper coordination geometries with RMSD value are also presented.69
1
2
3
4
5
6
7
8
9
R 70
Coordination Geometry, RMSD 69
trigonal bipyramid, 0.485
 23
Compound
S
g1
g2
g3
D
1aCu
1/2
2.075
2.210
2.235
-
5.400
0.33
(monomer)
0.28,
0.08
distorted trigonal bipyramid, 0.540
square pyramid, 0.435
1bCu
(dimer)
1/2
2.050
2.136
2.174
-
2.263
2Cu
(polymer)
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1/2
2.060
2.05
2.135
2.10
2.175
2.19
-
1.875
0.03
-
square pyramid, 0.286
3bCu
(trimer)
3/2
1.18 GHz
0.556
distorted octahedra, 0.635
Square pyramid: 0 < τ < 0.5, trigonal bipyramid 0.5 < τ < 1.
Contrary to Equation (2) where J describes the parameter
of isotropic exchange interaction, here tensor
values g2 ≈ g3 > g1. These shapes are characteristic for trigonal
bipyramidal complexes, in agreement with the observed
coordination geometry, as could be seen from Table 4. The
value of parameter R = 5.4 reveals that the unpaired electron
presents
total interaction between three copper spins that includes
the isotropic, antisymmetric (Dzyaloshinskii-Moriya) and
symmetric interactions.66,67 However, due the large number
of parameters that have to be varied in the simulations, it is
not straightforward to use Hamiltonian (4). Therefore, using
the susceptibility study result that shows that copper ions in
the triangular topology are coupled ferromagnetically with J
= 19 cm-1, ESR simulations were performed assuming the total
spin of the copper triangle S = 3/2 and magnetic anisotropy
described68 by tensor D:
2
is predominantly in the dz orbital. The nearest distance
between two copper ions, determined from the crystal struc-
ture is 7.321 Å, resulting in weak electron-electron interaction
that leads to non-observance of hyperfine interaction in the
spectra.4,47
Complex 1bCu with copper dimers shows one ESR line at all
temperatures. The angle Cu1─Br2─Cu2 shown in Figure S39
is 131.40 and the shortest distance between copper ions in
the dimer is 4.954 Å. This copper arrangement is not favora-
ble for the strong superexchange intra-dimer interaction, in
line with the susceptibility results, confirmed also by the
absence of half-field ESR transition.20 However, the presence
of weak exchange interaction between the copper ions could
clearly be seen in the absence of two ESR signals from two
non-equivalent copper ions in the unit cell but only one
exchange narrowed line.19
(5)
From the obtained ESR simulation parameters given in
Table 4, it is possible to correlate the copper coordination
geometry23,69 with the location of the unpaired electron.70
A
useful parameter is the value R = (g2─g1)/(g3─g2). For param-
eter R larger than 1, the ground state of the unpaired electron
is predominantly dz while for R smaller than 1, the ground
2
2
2 70
state is predominantly dx
.
─
y
In complex 2Cu, the copper ions are arranged in a 1D zig-
zag chain with the shortest distance between the copper ions
The spectra of the monomeric 1aCu complex have a so
called "inverse" shape70 and they can be simulated using g-
Figure 8. Experimental and simulated ESR spectra of the investigated complexes (from left to right, 1aCu, 1bCu, 2Cu, 3bCu) at the
indicated temperatures.
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Crystal Growth & Design
complex 3Zn, the cyclic M3L3 complex 3bCu and a small quan-
4.136 Å and Cu-F-Cu angle 164.22. The ESR spectra are simi-
1
2
3
4
5
6
7
8
tity of the ML complex 3aCu were prepared. It is particularly
interesting to mention that 3bCu was found to bind CO2 from
the atmosphere in the form of carbonate, in a slightly basic
solution due to the excess of free ligand.
lar to the spectra of monomeric complex 1aCu. Adding the
intra-chain exchange interaction term (1) to the spin-
Hamiltonian (3) with the value of parameter J = 12 cm-1 ob-
tained from the susceptibility study does not influence the
ESR patterns but only the line intensity. The simulations
were tested for a different number of copper ions in the
chain (N < 10). The presence of spin-spin interaction is indi-
cated by the absence of a hyperfine pattern in the ESR spec-
tra.71 The obtained value R = 1.875 shows that the ground
state of the unpaired electron is a combination of the dz2 and
The stability of polymorphs 1aZn and 1bZn was investigated
by differential scanning calorimetry, hot-stage microscopy
and X-ray powder diffraction. The diamagnetic zinc com-
plexes have been studied in solution by NMR spectroscopy,
for isolated complexes 1aZn/1bZn, 2Zn and 3Zn as well as for
complexes prepared in situ using different zinc / ligand rati-
os. The geometry and stability of different isomers of the
[Zn(iPr-bpa)2]2+ cation (cis-fac, trans-fac and mer) have
been investigated by DFT calculations and compared to the
experimentally obtained cis-fac 3Zn. Magnetic susceptibility
was measured and ESR spectra were recorded, simulated and
analyzed for four copper complexes: the monomer 1aCu, the
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dx 2 orbitals.70
2
─
y
From the spectral simulation of complex 3bCu, it could be
concluded that using approximation (5) was justified. ESR
study also confirms that the coupling in the triangle has to
be ferromagnetic, in line with the magnetization study of
similar tri-nuclear copper complexes.55,72,73 Here, instead of
using three copper spins with S = 1/2 that interact through
three equivalent superexchange patways,55,73 and Hamiltoni-
an (4) the simulations were simplified using effective total
spin of the triangle S = 3/2 with small magnetic anisotropy
described by zero-field splitting term D = 1.18 GHz = 0.04 cm-
1. In this rough approximation, the agreement between ex-
perimental and simulated spectra for complex 3bCu is satis-
factory.
dimer 1bCu, the trimer 3bCu and coordination polymer 2Cu
.
Recently, we have reported on the synthesis and character-
ization of [M(imda)2]2+ complexes,22 that show a strong
preference for trans-fac ML2 complexes. Surprisingly, the
rather similar bpa ligand presented herein showed (a) high
coordination versatility (Table 5) and (b) if [M(bpa)2]2+ com-
plexes are formed, the preferred isomer is cis-fac. Therefore,
careful design is necessary for the incorporation of bpa metal
complexes in functional systems.
Conclusions
This work describes late transition metal complexes of the
iPr-bpa ligand featuring different coordination numbers,
coordination geometry and stoichiometry depending on the
metal (M = Zn2+ or Cu2+) and anion (A = Br─, PF6─, BF4─, F─
and/or ClO4─), see Table 5.
Experimental
General remarks
Reactions were carried out in ordinary glassware and
chemicals were used as purchased from commercial suppli-
ers without further purification. Synthesis of the ligand was
carried out in a microwave reactor (CEM Discover), moni-
tored by TLC on Aluminium oxide 60 F254 neutral plates and
detected with UV lamp (254 nm). Mass spectra were record-
ed on a HPLC-MS system (Agilent Technologies 1200) cou-
pled with a 6410 Triple-Quadrupole mass spectrometer,
operating in a positive ESI mode. The high-resolution mass
spectra were obtained with a MALDI TOF/TOF instrument
with α-cyano-4-hydroxycinnamic acid (CHCA) as the matrix.
NMR spectra were obtained on a Bruker Avance 300 or 600
Single crystal structures have been determined for all nine
reported metal complexes. First, complexes [M(bpa)Br2]
with ML stoichiometry were obtained in the reaction of iPr-
bpa with MBr2 salts, namely 1aZn, 1bZn and 1aCu. An attempt
─
to incorporate the PF6 instead of the Br─ anion gave com-
plex 1bCu, a single bridged M2L2 dimer containing both Br─
─
and PF6─. Second, in the syntheses using the BF4 anion,
fluoride ions were formed that coordinated to the metal. In
the M2L2 dimer 2Zn two F─ counterions act as a double
bridge, while the (ML)n polymer 2Cu contains a single fluo-
ride bridge. And third, using the perchlorate anion the ML2
Table 5. Properties of Zn(II) and Cu(II) complexes of iPr-bpa.
property
anion
1aZn
Br─
1bZn
Br─, PF6
2Zn
3Zn
1aCu
Br─
1bCu
Br─, PF6
2Cu
BF4─ / F─
3aCu
3bCu
BF4─ / F─ ClO4
ClO4
ClO4
─
─
─
─
─
[Zn2(μ-
[Cu2(μ-
Br)Br2N6]
[Cu(μ-
F)2N3]n
[Cu3(tri-μ-
O)O3N9]
coordination [ZnBr2N3] [ZnBr2N3]
[ZnN6] [CuBr2N3]
[CuO3N3]
F)2N6]
M2L2
5
stoichiometry
CN
ML
5
ML
5
ML2
6
ML
5
M2L2
5
(ML)n
5
ML
6
M3L3
6
geometry
isomer
SP
TB / SP
fac
TB
O
SP
SP
SP
O
O
mer
fac
cis-fac
mer
mer
mer
mer
mer
Geometry classified according to the τ parameter23 and FindGeo program.69 SP, TB and O are sqare pyramid, trigonal bipyra-
mid and octahedron, respectively.
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Page 10 of 16
1
spectrometer, operating at 300 or 600 MHz for H and 75 or
150 MHz for 13C. The spectra are recorded at room tempera-
ture. Chemical shifts, δ (ppm), indicate a downfield shift
from the residual solvent signal (δH: 1.94 ppm, δC: 118.26 ppm,
CD3CN). Coupling constants, J, are given in Hz. Infrared
spectra were recorded using KBr pellets with a Bruker Alpha
FT-IR spectrometer, in the 4000–350 cm−1 region. The UV-Vis
spectra were recorded on a Varian Cary 50 spectrophotome-
ter using a 1 cm Suprasil quartz cell. The powder diffracto-
grams were measured on a PANalytical Aeris instrument;
conditions: Bragg-Brentano geometry (θ-2θ), source Cu-Kα
(λ=1.5418 Å), measurement from 5° to 70° (2θ), with 5.2°/min
(0.0216° step and 0.25 s/step). ESR study was performed
using a Bruker Elexsys 580 FT/CW X-band spectrometer
(microwave frequency 9.7 GHz). The spectra were recorded
in the range from the room down to liquid helium tempera-
ture. Magnetic field modulation amplitude was 0.5 mT and
modulation frequency was 100 kHz. The measurements were
obtained on powder samples i.e. polycrystalline samples were
grinded into powder. Dc susceptibility was measured with a
Quantum Design MPMS-XL-5 magnetometer in a constant
Method A. Saturated methanol solutions of the ligand (1 or
1
2
3
4
5
6
7
8
2 eq) and metal salt (1 eq) were heated and boiled shortly in
separate beakers until completely dissolved. The metal salt
solution was added to the ligand solution and the mixture
was cooled to room temperature and left partially covered for
slow evaporation until crystals appeared (2 days to 2
months). The solvent was decanted and the crystals washed
with diethyl ether (3 x 2 mL) and air-dried. Complexes that
did not crystallize by method of slow evaporation were
placed in a container with diethyl ether for slow diffusion.
9
Method B. To a chloroform solution (0.5 mL) of the ligand
(2 eq) a solution of metal salt in methanol (2 mL) was slowly
added. The bilayer solution was kept at 4°C and later at room
temperature, partly covered for slow evaporation.
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mer-[Zn(iPr-bpa)Br2], 1aZn. iPr-bpa (17.26 mg, 0.072
mmol), ZnBr2 (8.05 mg, 0.036 mmol). Method A, the vial was
placed in a screw capped container filled with diethyl-ether.
After 2 weeks, crystals suitable for X-ray single crystal analy-
sis were formed. Yield: 12.8 mg (0.027 mmol, 76%) of color-
─
1
less cubic crystals. IR (KBr) /cm : 3442, 2966, 2928, 1605,
1592, 1477, 1441, 1165, 1048, 1019, 783, 770, 642, 620, 420.
magnetic field of
between 2 K and 300 K. All the pre-
fac-[Zn(iPr-bpa)Br2], 1bZn
. iPr-bpa (23.0 mg, 0.095
sented data were corrected for the contribution of the sam-
ple holder and the diamagnetic susceptibility of inner shell
electrons.74 Differential Scanning Calorimetry (DSC) was
carried out on a Perkin-Elmer Diamond. The sample cup was
placed in the DSC apparatus together with an empty sample
cup as a reference. Heating and cooling scans were recorded
at a scan rate of 10 °C min-1. The phase transition tempera-
tures were taken at the maximum of DSC transition peaks.
mmol), ZnBr2 (21.4 mg, 0.095 mmol), NH4PF6 (31.0 mg, 0.190
mmol). Method A, the vial was placed in a screw capped
container filled with diethyl-ether. After 5 days, crystals
suitable for X-ray single crystal analysis were formed. Yield:
26.9 mg (0.058 mmol, 61%) of colorless plate-like crystals
─
1
1bZn. IR (KBr) /cm : 3453, 3065, 2975, 1607, 1422, 1088, 778,
647, 418.
Both polymorphs 1aZn and 1bZn have the same properties in
solution. The dissolved complex is referred to as 1Zn. 1H NMR
(300 MHz, CD3CN) δ/ppm: 9.10 – 9.00 (m, 2H, HPy-6), 8.05 –
7.91 (m, 2H, HPy-4), 7.61 – 7.51 (m, 2H, HPy-5), 7.51-7.42 (m, 2H,
HPy-3), 4.28 (s, 4H, Hα), 2.92 (hept, J = 6.7 Hz, 1H, HCH), 0.89
(d, J = 6.7 Hz, 6H, HCH3). 13C NMR (75 MHz, CD3CN) δ/ppm:
155.99, 148.69, 141.19, 125.20, 124.20, 55.76, 53.12, 18.31. MALDI-
HRMS (m/z): calcd 384.0048 [M─Br─], found 384,0062.
Synthesis of N,N-Bis(pyrid-2-ylmethyl)-iso-propylamine (iPr-
bpa).21 A mixture of isopropylamine (389 μL, 4.6 mmol), 2-
picolyl chloride hydrochloride (1.5 g, 9.1 mmol), K2CO3 (6.3 g,
45.7 mmol) and KI (758.6 mg, 4.6 mmol) in acetonitrile (10
mL) was stirred in a round bottom flask equipped with a
condenser in a microwave reactor (50 W, reflux) for 1 hour.
The solvent was evaporated in a vacuum, the residue sus-
pended in ethyl acetate and filtered. The filtrate was washed
with saturated NaHCO3 and brine, the organic layer dried
over anhydrous sodium sulfate, filtered and evaporated in a
vacuum. The crude product was filtered under vacuum
through a Büchner funnel, washed with water and dried in a
vacuum. No further purification was required. Yield: 675.1 mg
(2.8 mmol, 61%), brown powder, Rf = 0.38, neutral alumini-
[Zn2(μ-F)2(iPr-bpa)2](BF4)2, 2Zn. iPr-bpa (40.2 mg, 0.166
mmol), Zn(BF4)2 × H2O (19.9 mg, 0.083 mmol). Method A,
the vial was partly covered and left in the fume hood for slow
evaporation for 2 days, then the vial was placed in a contain-
er with diethyl ether (10 mL) for diffusion for 8 days. Yield:
21.2 mg (0.03 mmol, 62%), yellow, needle-like crystals, suita-
1
1
um oxide, ethyl acetate:hexane = 3:7. H NMR (300 MHz,
ble for X-ray single crystal analysis. H NMR (600 MHz,
CD3CN) δ/ppm: 8.47 – 8.37 (m, 2H, HPy-6), 7.72-7.61 (m, 2H,
HPy-4), 7.59 – 7.51 (m, 2H, HPy-3), 7.19 – 7.08 (m, 2H, HPy-5),
3.75 (s, 4H, Hα), 2.91 (hept, J = 6.6 Hz, 1H, HCH), 1.09 (d, J =
6.6 Hz, 6H, HCH3). 13C NMR (75 MHz, CD3CN) δ/ppm: 162.16,
149.58, 137.17, 123.33, 122.64, 56.71, 51.21, 18.17. Mr(C15H19N3) =
241.34. ESI-MS (m/z): 264.1 (M+Na+, 100%), 242.1 (M+H+,
35%). MALDI-HRMS (m/z): calcd 264.1471 [C15H19N3 + Na+],
CD3CN) δ/ppm: 8.62 – 8.54 (m, 4H, HPy-6), 8.10 – 8.02 (m, 4H,
HPy-4), 7.58 (d, J = 7.9 Hz, 4H, HPy-3), 7.53 – 7.46 (m, 4H, HPy-
5), 4.39 (d, J = 16.8 Hz, 4H, Hα), 3.98 (d, J = 16.8 Hz, 4H, 4H,
Hα’), 3.56 – 3.46 (m, 2H, HCH), 1.19 (d, J = 6.6 Hz, 12H, HCH3).
13C NMR (151 MHz, CD3CN) δ/ppm: 157.46, 148.89, 142.75,
125.95, 125.38, 57.37, 55.90, 18.76. MALDI-HRMS (m/z): calcd
─
324.0849 [M─2BF4 ─F──Zn2+─C15H19N3], found 324,0867. IR
─
1
─
1
found 264,1467. IR (KBr) /cm : 3452, 2964, 2876, 2835, 1591,
1567, 1474, 1435, 1180, 1048, 891, 770, 621, 550.
(KBr) /cm : 3442, 2977, 1610, 1446, 1309, 1161, 1057, 770, 655,
522, 420.
[Zn(iPr-bpa)2](ClO4)2, 3Zn. iPr-bpa (20.2 mg, 0.084
mmol), Zn(ClO4)2 × 6H2O (15.6 mg, 0.042 mmol). Method B,
after 2 months, colorless plate-like crystals suitable for X-ray
single crystal analysis were obtained. Yield: 8.41 mg (0.01
mmol, 27%). 1H NMR spectrum of 3Zn shows broad peaks and
the peaks were therefore not assigned. MALDI-HRMS (m/z):
calcd 404.0350 [M─ClO4── C15H19N3], found 404,0353. IR
Synthesis of metal complexes
Caution! Perchlorate salts of metal complexes with organic
ligands are known to be explosive and should be handled
with great caution and in small quantities. Therefore, ther-
mal measurements were not performed for perchlorate com-
plexes 2Zn, 3aCu and 3bCu
.
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Page 11 of 16
Crystal Growth & Design
─
processed with CrysalisPro program75 (unit cell determina-
tion and data reduction). The structures were solved by di-
rect methods with SIR2011 program76 and refined against F2
on all data by a full-matrix least squares procedure with
SHELXL-97 program.77 The monocrystalline samples dif-
fracted up to ≈76o for all compounds except 3Zn for which the
diffraction was observed up to ≈62o. This structure was
solved by direct methods in the space group Pna21 with
recognition of all non-hydrogen atoms. The central Zn atom
from the cation and chlorine atoms from the perchlorate
anions had positions in the vicinity of crystallographic glide
planes of type a from space group Pna21 (normal to the b
axis). However, the significant residual density peaks were
always present at the positions related to these glide planes
imitating mirror symmetry with respect to the atoms from
the staring model obtained by direct methods. Actually,
during refinement in the space group Pna21 the significant
number of systematic absence violations (66) related with
this glide plane symmetry was observed. The model was tried
to be refined with assumption of the twinning law (mirror
symmetry perpendicular to the b axis), but without any im-
provement. Therefore, disordered models of complex cation
and perchlorate anions were used in the final model for the
refinement, where all atoms from individual disordered parts
shared common occupancies. Perchlorate anions were mod-
eled as rigid groups with oxygen atoms riding on the chlorine
atoms having Uiso = 1.5 × Ueq(Cl). Part of the complex cation
with lower occupancy was restrained to have identical dis-
tances and angles as the part having higher occupancy
(SAME instruction) and similar thermal parameters (SIMU
and DELU instructions). All atoms with lower occupancy
parts were refined in isotropic displacement parameter mod-
el. All hydrogen atoms were modeled as riding on carbon
atoms on which they were bonded, without refinement of
torsion angles. Due to lack of centers of inversion in space
group Pna21, the final model also assumed racemic twinning
as suggested by SHELXL program.77 Such a model produced
stabile least-square refinements with a satisfying R value
(6.89 %) and residual density at the level of the noise. Re-
fined values for occupancies were: 0.887(3) and 0.113(3) for
the complex cation; 0.587(12) and 0.413(12) for the first per-
chlorate anion in addition to 0.583(8) and 0.417(8) for the
second perchlorate anion. Compound 3bCu was solved in
trigonal space group P-3c1 with recognition of all non-
hydrogen atoms from the Cu trinuclear complex cation
[(C15H19ClCuN3O5)3C]+ lying on the three-fold symmetry axis.
Such cation already binds three perchlorate units while one
more perchlorate anion was located at two crystallographic
positions of high symmetry (Wyckoff positions a and b).78
Multiplicities of these Wyckoff positions are two times lower
than multiplicities of Wyckoff positions of centers of Cu
three-nuclear cations (Wyckoff positions d),78 in accordance
with stoichiometry of one perchlorate anion per one cation
in the formula unit. High symmetries of perchlorate anion
positions are not compatible with tetrahedral symmetry of
the perchlorate anion, thus orientation disorder of perchlo-
rate units is observed. Analogously with refinements of struc-
ture 3Zn, in refinements of 3bCu, perchlorate units were mod-
eled as rigid group with oxygen atoms riding on the chlorine
atoms with Uiso = 1.5 × Ueq(Cl). All hydrogen atoms were
modeled as riding on carbon atoms on which they were
bonded, without refinement of torsion angles. Also, by in-
spection of intensity data, the possibility of 2-fold twinning
1
(KBr) /cm : 3433, 3075, 2983, 1608, 1574, 1490, 1447, 1432,
1099, 797, 767, 623, 418.
1
2
3
4
5
6
7
8
[Cu(iPr-bpa)Br2], 1aCu. iPr-bpa (25.0 mg, 0.103 mmol),
CuBr2 (11.6 mg, 0.052 mmol). Method A, the vial was placed
in a screw capped container filled with diethyl-ether. After 2
weeks, crystals suitable for X-ray single crystal analysis were
formed. Yield: 9.5 mg (0.02 mmol, 39 %) of green cubic crys-
tals. MALDI-HRMS (m/z): calcd 383.0053 [M─Br─], found
─
1
383,0051. IR (KBr) /cm : 3454, 2969, 1611, 1487, 1452, 1289,
1159, 1048, 781, 768, 655, 427.
9
[Cu2(μ-Br)(Br)2(iPr-bpa)2](PF6) × CH3OH, 1bCu. iPr-bpa
(22.4 mg, 0.093 mmol), CuBr2 (20.7 mg, 0.093 mmol),
NH4PF6 (30.3 mg, 0.186 mmol). Method A, the vial was
placed in a screw capped container filled with diethyl-ether.
After 2 days, crystals suitable for X-ray single crystal analysis
were formed. Yield: 30.7 mg (0.03 mmol, 64%) of blue-green
needle-like crystals. MALDI-HRMS (m/z): calcd 383.0053
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[M─2Br──Cu2+─C15H19N3─PF6─], found 383,0060. IR (KBr)
/cm : 3444, 2975, 1611, 1451, 1288, 1154, 849, 749, 558.
─
1
[Cu(μ-F)(iPr-bpa)]n(BF4)n × nCH3OH, 2Cu. iPr-bpa (28.37
mg, 0.118 mmol), Cu(BF4)2 × H2O (13.9 mg, 0.059 mmol).
Method A, the vial was partly covered and left in the fume
hood for slow evaporation for 2 days, then the vial was placed
in a container with diethyl ether (10 mL) for diffusion for 2
days. Yield: 3.8 mg (0.01 mmol, 15%) blue, needle-like crys-
tals, suitable for X-ray single crystal analysis. MALDI-HRMS
(m/z): calcd 304.0864 [M─2F──BF4─], found 304,0863. IR
─
1
(KBr) /cm : 3450, 2971, 2932, 1610, 1445, 1289, 1126, 1029,
746, 483.
[Cu(iPr-bpa)(ClO4)2(H2O)], 3aCu. iPr-bpa (18.1 mg, 0.075
mmol), Cu(ClO4)2 × 6H2O (13.9 mg, 0.037 mmol). Method B,
after 1 week, a small amount of blue needle-like ([Cu(iPr-
bpa)(ClO4)2(H2O)]) 3aCu and large amount of hexagonal
([Cu3(tri-μ-CO3)(ClO4)3(iPr-bpa)3]ClO4) 3bCu crystals, suita-
ble for X-ray single crystal analysis were obtained.
[Cu3(tri-μ-CO3)(ClO4)3(iPr-bpa)3](ClO4), 3bCu. Method B,
CO2 was bubbled through the solution for 45 min, then the
solution was left partly covered for slow evaporation. After 1
week, crystals suitable for X-ray single crystal analysis were
formed. Yield: 7.1 mg (0.005 mmol, 36%) of blue hexagonal
crystals.
MALDI-HRMS
(m/z):
calcd
403.0355
─
─
[M─3ClO4 ─CO32 ─2Cu2+─2 C15H19N3], found 403,0359. IR
─
1
(KBr) /cm : 3441, 2975, 1610, 1445, 1121, 779, 765, 624, 425.
Spectroscopic procedures
NMR measurements (in situ). Complexes were prepared
by dissolving the ligand (0.5, 1 or 2 eq) and metal salt (1 eq) in
approximately 0.6 mL CD3CN.
NMR titration. iPr-bpa (5.6 mg) was dissolved in 2 mL
CD3CN (c(iPr-bpa)= 11.6 mM). Zn(ClO4)2 × 6H2O (31.1 mg)
was dissolved in 1 mL of the solution of iPr-bpa in CD3CN
(c(Zn2+) = 83.6 mM), to avoid dilution. 600 μL of the ligand
solution was placed in an NMR tube and the spectrum of the
free ligand was acquired. For each following measurement,
an aliquot of the Zn2+ in iPr-bpa solution was added until
the ratio of Zn2+: iPr-bpa was 2:1.
X-ray crystallography. The X-ray intensity data were col-
lected on an Oxford diffraction Xcalibur CCD diffractometer
using monochromatic Cu-Kα (λ = 1.54184 Å) radiation. For
temperature conditions see Tables S1 and S2. The data were
ACS Paragon Plus Environment

Crystal Growth & Design
Page 12 of 16
around b axis was observed. Addition of this twinning law Supporting Information is available free of charge on the ACS
1
2
3
4
5
6
7
8
significantly improved the R value. The precision of the de-
termined C─C bonds was found to be somewhat lower than
usual, presumably due to the twinning of the crystal and
observed disorder of perchlorate anions. Racemic twinning
Publications website.
AUTHOR INFORMATION
Corresponding Author
*E-mail: Srecko.Kirin@irb.hr
was taken into account during refinements of compound 2Cu
,
as suggested by program SHELXL.77 This structure crystalliz-
es in non-centrosymmetric space group P212121 although non-
chiral precursors were used in synthesis (crystallization reso-
ORCID
─
lution – conglomerates).79,80 Counter anions like ClO4─, BF4
9
Natalija Pantalon Juraj: 0000-0002-0357-5817
Senada Muratović: 0000-0001-5027-9191
Berislav Perić: 0000-0002-2397-2836
Nataša Šijaković Vujičić: 0000-0002-1455-9484
Robert Vianello: 0000-0003-1779-4524
Dijana Žilić: 0000-0003-2387-0853
─
or PF6 usually have large displacement parameters due to
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their high rotational freedom81 and this is observed in the
crystal structures of 3Zn, 1bCu, 2Cu and 3bCu. In the structure
of 1bCu, two solvent accessible voids of volume ≈ 142 Å3 were
observed with two peaks in the residual electron density. As
the centers of these voids were located on crystallographic
centers of inversion, the interpretation for these residual
peaks as solvent molecules leaded to disorder because the
symmetry related peaks were too close. Finally, the electron
density from these regions was treated by SQUEZZE proce-
dure in program PLATON82,83 and final refinement model
was performed without electron density contribution from
this solvent accessible area. In all structures hydrogen atoms
were positioned on calculated positions and refined within
the riding model, having thermal parameters associated to
carbon atoms on which they are bonded [Uiso = 1.5 × Ueq(C)
for methyl groups and Uiso = 1.2 × Ueq(C) for all other hydro-
gen atoms]. Except for structures of 3Zn and 3bCu, additional
refinements for torsional movement of methyl groups were
included. Details for the X-ray diffraction studies are collect-
ed in Tables S1 and S2.
Zvonko Jagličić: 0000-0002-4118-9677
Srećko I. Kirin: 0000-0002-0500-2422
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by the Croatian Science Founda-
tion (CSF, grants no. IP-2014-09-1461 and IP-2018-01-3168)
and partially supported by the Slovenian Research Agency
(Grant no. P2-0348).
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model, known to be successful in reproducing geometries,
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Structural Variety of Isopropyl-bis(2-picolyl)amine Complexes with Zinc(II) and Copper(II)
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Natalija Pantalon Juraj,a Senada Muratović,a Berislav Perić,a Nataša Šijaković Vujičić,a Robert Vianel-
lo,a Dijana Žilić,a Zvonko Jagličićb and Srećko I. Kirina,*
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The formation of different Zn(II) and Cu(II) complexes of the iPr-bpa ligand was affected by counteri-
ons of different coordinating ability, forming monomers, dimers, a cyclic trimer and a coordination pol-
ymer.
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