Crystal structures and magnetic properties of a set of dihalo-bridged oxalamidato copper(ii) dimers

Article abstract of DOI:10.1039/c4dt00925h

A set of four copper(ii) complexes, L¹-X and L²-X (X = Cl, Br; L¹ = N-(l-leucine methyl ester)-N′-((2-pyridin-2-yl) methyl)oxalamide and L² = N-benzyl-N′-((2-pyridin-2-yl)methyl) oxalamide), have been synthesized and characterized by X-ray structural analysis, electron paramagnetic resonance (EPR) spectroscopy on single crystals and by SQUID magnetization measurements. X-ray diffraction studies show one-dimensional hydrogen bonded networks of dimeric copper(ii)-complexes bridged by two halide ions and with the two metal centers 3.44-3.69 A apart. The geometry at each copper(ii) atom is ideal or near ideal square pyramidal. EPR and SQUID studies indicate that all complexes exhibit weak antiferromagnetic interactions between the Cu(ii) paramagnetic centers, with exchange parameter |J| ～ 1 cm^-1. Magneto-structural comparisons among similar dihalo-bridged Cu(ii) dinuclear complexes are also provided, and a possible correlation has been established. This journal is the Partner Organisations 2014.

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Crystal structures and magnetic properties of a set
of dihalo-bridged oxalamidato copper(II) dimers†
Cite this: Dalton Trans., 2014, 43,
11877
Dijana Žilić,a Boris Rakvin,a Dalibor Milić,‡b Damir Pajić,c Ivica Đilović,b
Massimo Camettid and Zoran Džolić*a
A set of four copper(II) complexes, L1–X and L2–X (X = Cl, Br; L1 = N-(L-leucine methyl ester)-N’-
((2-pyridin-2-yl)methyl)oxalamide and L2 = N-benzyl-N’-((2-pyridin-2-yl)methyl)oxalamide), have been
synthesized and characterized by X-ray structural analysis, electron paramagnetic resonance (EPR)
spectroscopy on single crystals and by SQUID magnetization measurements. X-ray diffraction studies
show one-dimensional hydrogen bonded networks of dimeric copper(II)-complexes bridged by two
halide ions and with the two metal centers 3.44–3.69 Å apart. The geometry at each copper(II) atom is
ideal or near ideal square pyramidal. EPR and SQUID studies indicate that all complexes exhibit weak anti-
ferromagnetic interactions between the Cu(II) paramagnetic centers, with exchange parameter |J| ∼
1 cm−1. Magneto-structural comparisons among similar dihalo-bridged Cu(II) dinuclear complexes are
also provided, and a possible correlation has been established.
Received 28th March 2014,
Accepted 30th May 2014
DOI: 10.1039/c4dt00925h
www.rsc.org/dalton
complexes, for example, the dihalo-bridged Cu(II) complexes,4,5
the situation improves a little. However, the structural variabil-
1. Introduction
Dinuclear copper(II) complexes have attracted considerable ity remains wide. Indeed, the metal centers can be four- or
interest over the last few decades, not only for the role played five-coordinated, depending on the nature of the ligands, and
in catalytic enzymatic reactions,1 but also because they possess this gives rise to a variety of different structures, ranging from
the essential structural features for the study of magnetic inter- square-pyramid to trigonal bipyramid.6
actions between two close magnetic centers.2,3 Not surpris-
Over the years, several theoretical analyses have been
ingly, the different chemical environment around the copper carried out to obtain an empirical relationship between an
ions may greatly influence the resulting magnetic behavior. exchange coupling constant and structural features of
Hence, investigations of the magnetic properties of such com- copper(II) complexes and to explain different magnetic beha-
plexes in light of their relation to structural features are often viors, from ferro- to antiferromagnetic interactions.7 Magneto-
attempted for the discovery of definite magneto-structural structural correlation in dihalo-bridged copper(II) complexes
relationships whose utility increases further, the larger their seems to be more complicated compared to dihydroxo-bridged
field of applicability is. In general, correlations of this kind are complexes.8–11 It should also be noted that among dihalo-
not easily obtained, especially due to a large structural variabil- bridged complexes, the vast majority of reports are related to
ity. Taking into consideration a smaller ensemble of Cu(II) dichloro species and considerably less information is available
on structural and magnetic properties of dibromo-bridged
copper(II) dimers.10,12
aRuder Bošković Institute, Bijenička cesta 54, 10000 Zagreb, Croatia.
E-mail: Zoran.Dzolic@irb.hr; Fax: +385 1 4680 195; Tel: +385 1 457 1210
N,N′-Disubstituted oxalamides have proved to be very useful
ligands for designing homo- and heterometallic complexes.13
Indeed, they provide a tunable molecular environment due to
cis–trans conformational freedom, different coordination geo-
metries and ligand charges availability (for example by NH
deprotonation). On the other hand, studies on the metal com-
plexes involving oxalyl retro-peptide ligands are very limited.
Given the interesting properties that the above mentioned
ligands might bring about, we have designed and synthesized
bDepartment of Chemistry, Faculty of Science, University of Zagreb, Horvatovac 102A,
10000 Zagreb, Croatia
cDepartment of Physics, Faculty of Science, University of Zagreb, Bijenička cesta 32,
10000 Zagreb, Croatia
dDepartment of Chemistry, Materials and Chemical Engineering “Giulio Natta”,
Politecnico di Milano, Via L. Mancinelli 7, 20131 Milano, Italy
†Electronic supplementary information (ESI) available: FT-IR spectra and XRPD
data of the complexes L1–Cl, L1–Br, L2–Cl and L2–Br; Additional crystallographic
information; Angular variation of g-values and the Wpp linewidths of EPR lines
for L2–Cl, L1–Br and L2–Br; Curie–Weiss magnetization fitting curves. CCDC
919441–919443. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c4dt00925h
four
dihalo-bridged
copper(II)
oxalamidato
dimers:
[CuL1(μ-Cl)]2·CH3OH
(L1–Cl),
[CuL2(μ-Cl)]2
(L2–Cl),
[CuL1(μ-Br)]2 (L1–Br) and [CuL2(μ-Br)]2 (L2–Br), where ligands
‡Present address: Paul Scherrer Institut, 5232 Villigen PSI, Switzerland.
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L2–Br: Anal. calcd for C30H40Br2Cu2N6O8: C, 43.76; H, 3.43;
N, 10.20; Cu, 15.43. Found C, 43.87; H, 3.29; N, 10.29; Cu,
15.44. IR (KBr): ν = 3211, 3182, 3143, 3058, 2935, 1663, 1617,
1435, 1411 cm−1
.
2.1. X-ray crystallographic study
Fig. 1 Schematic presentation of ligands N-(L-leucine methyl ester)-N’-
((2-pyridin-2-yl)methyl)oxalamide (L1) and N-benzyl-N’-((2-pyridin-2-yl)
methyl)oxalamide (L2).
We have already reported single-crystal X-ray structural analysis
of L1–Cl (CCDC 907166).14 X-ray diffraction data from single
crystals of L2–Cl, L1–Br and L2–Br were collected on an Oxford
Diffraction Xcalibur 3 CCD diffractometer with graphite-mono-
chromated MoKα radiation (λ = 0.71073 Å) and reduced using
the CrysAlis PRO software package.15 The solution (SHELXS),16
refinement (SHELXL-97),16 building and analysis of the struc-
tures were performed using Coot17 and the programs inte-
grated in the WinGX system.18 Data processing and refinement
statistics are given in Table 1. All non-hydrogen atoms were
refined anisotropically. H atoms attached to C atoms were
positioned geometrically and refined isotropically applying the
usual riding model [d(C–H) = 0.93–1.00 Å and Uiso (H) = 1.2 or
1.5Ueq (C)]. The H atom attached to N3 in L2–Cl was refined
isotropically with a restraint on the N–H distance (DFIX 0.86
0.02), while H atoms of the same type in L1–Br and L2–Br were
positioned geometrically and refined using the appropriate
riding model [AFIX 43, d(N–H) = 0.88 Å and Uiso (H) =
1.2Ueq (N)]. The N-benzyl moiety in L2–Cl is discretely dis-
ordered over two conformations with the relative population
parameter 0.840(5) for the major one. The disordered atoms in
L2–Cl were refined with restraints on their geometrical (SAME)
and displacement parameters (SIMU, DELU, and ISOR). The
final structural models were analyzed using PLATON.19 Mole-
cular graphics were prepared in Mercury,20 ORTEP-3,21 and
POV-Ray.22
L1 and L2 stand for N-(L-leucine methyl ester)-N′-((2-pyridin-2-
yl)methyl)oxalamide and N-benzyl-N′-((2-pyridin-2-yl)methyl)-
oxalamide, respectively (Fig. 1).
To the best of our knowledge, the described compounds
represent the first set of structures of oxalamido-complexes
which display discrete dinuclear dihalo-bridged units.
The magnetic characterization of the compounds was per-
formed by Electron Paramagnetic Resonance (EPR) spec-
troscopy and SQUID magnetization measurements. Single
crystal EPR spectroscopy performed on the dinuclear copper
complexes provided information about the coordination geo-
metry around copper(II) ions. Moreover, from the angular
dependence of linewidth, additional information about the
mechanism of interaction between copper(II) ions was
obtained. Temperature and field dependencies of magnetiza-
tion were used for the determination isotropic exchange inter-
action between copper ions. Finally, a relation between
magnetic behavior and molecular structure of the investigated
complexes has been presented and discussed in the framework
of existing correlations for dihalo-bridged copper(II)
complexes.
The phase purity of each sample was confirmed by X-ray
powder diffraction (XRPD) (see Fig. S3–S6 in ESI†). Patterns
used for qualitative PXRD analysis of the samples were col-
2. Materials and methods
The preparation and structural details of ligands L1 and L2 lected on a Philips PW 3710 diffractometer, CuKα radiation, a
have been previously described.14 Complexes L1–Cl, L2–Cl, flat plate sample on a zero background in Bragg–Brentano geo-
L1–Br and L2–Br were prepared by the reaction of equimolar metry, tension 40 kV, current 40 mA. The patterns were col-
quantities of ligands with CuCl2·2H2O or CuBr2 in methanol. lected in the angle region between 4° and 40° (2Θ) with a step
Single crystals suitable for X-ray diffraction were obtained by size of 0.02° and 1.0 s counting per step.
slow evaporation of corresponding methanol solutions.
2.2. EPR study
FT-IR spectra of all complexes (see Fig. S1 and S2 in ESI†)
were recorded at a resolution of 4 cm−1 on an ABB Bomem EPR measurements were performed on the single crystals and
MB102 single beam FT-IR spectrometer at room temperature.
on the powder forms of the investigated compounds. EPR
L1–Cl: Anal. calcd for C31H44Cl2Cu2N6O9: C, 44.19; H, 5.26; experiments were carried out with a Bruker 580 FT/CW X-band
N, 9.97; Cu, 15.08. Found C, 44.25; H, 5.33; N, 9.89; Cu, 15.06. spectrometer equipped with a standard Oxford Instruments
IR (KBr): ν = 3189, 3071, 2956, 1745, 1670, 1619, 1435, model DTC2 temperature controller. The microwave frequency
1416 cm−1
.
was ≈ 9.6 GHz with the magnetic field modulation amplitude
L2–Cl: Anal. calcd for C30H28Cl2Cu2N6O4: C, 49.05; H, 3.84; of 0.5 mT at 100 kHz. The crystals of chloride (bromide) com-
N, 11.44; Cu, 17.30. Found C, 48.97; H, 3.88; N, 11.69; Cu, pounds were elongated along the crystallographic b (a)-axis.
17.22. IR (KBr): ν = 3218, 3181, 3147, 3056, 1672, 1624, 1426, They were rotated around three mutually orthogonal axes: a
1412 cm−1
.
crystallographic b (a)-axis, an arbitrary chosen c* (b*)-axis per-
L1–Br: Anal. calcd for C30H40Br2Cu2N6O8: C, 40.06; H, 4.48; pendicular to b (a)-axis and a third a* (c*)-axis, perpendicular
N, 9.34; Cu, 14.13. Found C, 39.98; H, 4.58; N, 9.39; Cu, 14.15. to the previous two axes. EPR spectra were recorded at 5° steps
IR (KBr): ν = 3180, 3149, 3067, 2953, 1744, 1731, 1668, 1614, and the rotation was controlled by a home-made goniometer
1437, 1430, 1414 cm−1
.
with the accuracy of 1°. A larger uncertainty (2–3°) was related
11878 | Dalton Trans., 2014, 43, 11877–11887
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Table 1 X-ray crystallographic data for L2–Cl, L1–Br and L2–Br
Structure
L2–Cl
L1–Br
L2–Br
Chemical formula
Mr
C
30H28Cl2Cu2N6O4
C30H40Br2Cu2N6O8
899.58
C30H28Br2Cu2N6O4
823.48
734.56
Crystal color, habit
Blue, plate
0.01 × 0.20 × 0.75
Monoclinic
P21/c
7.7369(3)
9.8930(3)
21.9325(10)
90
101.955(4)
90
1642.33(11)
2
Blue, prism
0.15 × 0.40 × 0.50
Triclinic
P1
8.4814(3)
9.3314(4)
11.7613(3)
81.072(3)
78.321(3)
76.097(4)
879.28(6)
1
Blue-green, plate
0.05 × 0.10 × 0.10
Triclinic
Crystal size (mm3)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
α (°)
β (°)
ˉ
P1
8.0988(6)
9.6585(6)
10.1449(6)
93.172(5)
90.749(5)
94.721(5)
789.53(9)
1
γ (°)
V (Å3)
Z
T(K)
150(1)
1.485
1.501
150(1)
1.699
3.538
120(1)
1.732
3.923
Dcalc (g cm−3
)
μ (mm−1
)
Data total/unique
Rint
Observed data [I > 2σ(I)]
Restraints/parameters
R1 [I > σ(I)]
10 974/3231
0.042
2607
185/258
0.0366
0.0891
9239/6032
0.014
5715
3/433
0.0196
0.0508
1.04
9243/2771
0.084
1927
0/199
0.0465
0.0902
0.99
wR2 (all data)
S
1.04
Flack parameter
Min. and max. resd. dens. (e Å−3
CCDC number
n/a
−0.57, 0.51
919441
0.015(7)
−0.36, 0.32
919442
n/a
−0.50, 0.83
919443
)
to the optimal deposition of the crystals on the quartz holder.
In all cases, the X-ray determined structures of the com-
The EPR spectra were measured at two temperatures: T = 297 K plexes show penta-coordinated Cu(II) ions adopting a square
and T = 80 K.
pyramidal geometry. The square base coordination sites are
occupied by a deprotonated ligand, acting as NNO tridentate,
and by one of the bridging halo-ions; the apical position of the
pyramid is instead occupied by the other bridging halo-ion, as
shown in Fig. 3. The coordination polyhedron around the
copper(II) ion could be best described as an ideal or near ideal
square pyramid, with τ = 0.00–0.17 (where τ = 0 implies the
ideal square-pyramidal geometry and τ = 1 an ideal trigonal
bipyramid;28 Table 2). As said, the two Cu(II) centers are
bridged by two halo-ions in such a way that the two square pyr-
amids share one base-to-apex edge while having their basal
planes parallel (Fig. 3). Rodríguez et al. have designated this kind
of configurations for copper complex containing the Cu–(μ-X)2–
Cu core as a type II pyramidal arrangement.29 Although the
dimeric molecules with achiral ligand L2 are positioned on the
crystallographic inversion centers, N-benzyl groups in L2–Cl are
discretely disordered over two conformations (Fig. 2).
2.3. Magnetization study
Magnetic measurements of the investigated compounds in the
form of powders were performed using a commercial MPMS5
SQUID magnetometer. Temperature dependent magnetization
M(T) for 2 K < T < 300 K was measured in a constant magnetic
field of 1000 Oe and, after correction against the sample
holder, the temperature independent contributions of inner
electrons were also subtracted. The field dependence of mag-
netization M(H) was measured at a lowest temperature of 2 K
in field up to 5 T.
3. Results and discussion
3.1. Description of structures
Bond distances and angles relevant to the coordination of
copper(II) ions are given in Table 2. The Cu2X2 unit is planar in
all the four dimers with the bridging Cu–X–Cu′ angles [86.53
(3)–91.50(3)°] being close to a right angle and the Cu⋯Cu′
intra-dimeric distances ranging from 3.4408(4) to 3.6852(6) Å.
Similarly as in other dihalo-bridged Cu(II) complexes with
square-pyramidal coordination,30,31 the axial Cu–X bonds
[2.6674(8)–2.7582(6) Å in the Cl-complexes and 2.7475(9)–
2.8282(5) Å in the Br-complexes] are significantly longer than
the corresponding basal Cu–X bonds [2.2221(6)–2.2443(8) Å in
the Cl-complexes and 2.3910(5)–2.4056(9) Å in the Br-com-
Oxalyl retro-peptide compounds represent a versatile class of
molecules which has been previously employed, with consider-
able success, in the study of gel formation.23 There are only a
few examples of structurally characterized halo-bridged oxal-
amidato metal complexes.24–26 Mixing equimolecular amounts
of CuCl2·2H2O or CuBr2 and the asymmetrical N,N′-disubsti-
tuted oxalamide ligands L1 and L2 in MeOH did not result in
oxalamidato-bridged complexes, as in the case of copper(II)
nitrate or perchlorate and the symmetrical N,N′-bis-(2-methyl-
pyridyl)-oxalamide.27 Instead, the dihalo-bridged dinuclear
complexes L1– and L2–X (X = Cl and Br) were obtained (Fig. 2).
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Fig. 2 The molecular structures of complexes (a) L1–Cl, (b) L2–Cl, (c) L1–Br and (d) L2–Br. Displacement ellipsoids are drawn at the 30% probability
level. H atoms are shown as spheres of an arbitrary radius. Dashed lines represent the O–H⋯Cl hydrogen bond in L1–Cl and the apical Cu–X bonds
(X = Cl or Br) in all the complexes. Atoms labeled with “i” in L2–Cl and L2–Br are centrosymmetrically related to those in the other half of a molecule.
The less populated conformation [the relative population parameter 0.160(5)] of the disordered N-benzyl group in L2–Cl (yellow) is shown only for a
crystallographically independent half of the dinuclear complex. Although modeled, H atoms in this conformation are not depicted for clarity.
(Fig. 4; Table 5 in the ESI†) and form infinite chains of di-
nuclear units. Such a supramolecular arrangement resembles
the structure of the only linear-chain Cu(II) compound
reported so far which displays alternating dichloro- and oxala-
midato-bridges.24 Stacking interactions between pyridyl and
metalloaromatic chelate rings32 (Cu–O–C–C–N in our case),
which we have observed for L1–Cl,14 are also present in crystal
structures of the bromo-complexes (Table 6 and Fig. S10 and
S11 in ESI†), but – surprisingly – not in L2–Cl. In addition, we
Fig. 3 For all the investigated compounds, the coordination environ-
ment around Cu(II) is an ideal or near ideal square-pyramidal. Cu(II) ions
are penta-coordinated by an oxalamidato O and N, pyridyl N atom and
also found weak hydrogen bond C–H⋯A and C–H⋯π inter-
actions with either a chelate33 or a phenyl ring (as detailed in
two halo-ions (X−; X = Cl or Br). The halo-ion that is farther from the
Cu(II) occupies the apical position of the square pyramid. Two pyramids
share one base-to-apex edge with the parallel basal plane.
Tables 5 and 7 in ESI†). Intriguingly, the crystal packing of
L2–Cl molecules allows discrete conformational disorder of
their N-benzyl groups (Fig. S9 in ESI†), while preserving C–
H⋯π interactions for both observed conformations (Table 7 in
ESI†). An aryl π–π stacking interaction exists only between two
phenyl rings in the crystal structure of L2–Br (Fig. S11 and
Table 6 in ESI†). A more detailed description of crystal packing
is provided in ESI.†
plexes], due to the Jahn–Teller effect. However, the coordi-
nation to a negatively charged ligand (at the oxalamidato
N atoms N2 or N5) shortens the observed Cu–N(oxalamide)
bonds in all complexes [1.896(4)–1.917(4) Å] with respect to
the usual Cu–N(pyridyl) bonds [1.996(4)–2.034(4) Å], observed
in similar copper(II) complexes, for example those made of
N,N′-bis-(2-methylpyridyl)-oxalamide.27
3.2. EPR study
The single crystal EPR spectrum of the complex L1–Cl shows a
single, fairly Lorentzian, line in every direction of the magnetic
field. Similar spectra are also observed for the dibromo-
bridged complexes L1–Br and L2–Br. Differently, the complex
Within the crystal, in all cases, the complexes are linked to
each other by hydrogen bonds between oxalamide units
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Table 2 Molecular geometry (Å, °, ° Å−1) of the analyzed complexes
L1–Cla 14
L2–Cl
L1–Bra
L2–Br
Cu–Xb (basal)
Cu–X (axial) (R)
Cu–O (oxalamide)
Cu–N (pyridyl)
Cu–N (oxalamide)
τc
2.2221(6)
2.7582(6)
2.064(2)
2.030(2)
1.899(2)
0.17
2.2552(6)
2.7049(6)
2.052(2)
2.014(2)
1.906(2)
0.15
2.2443(8)
2.6674(8)
2.010(2)
2.003(2)
1.910(2)
0.02
0.1990(3)
3.4408(4)
91.50(3)
34.30
2.4001(5)
2.8282(5)
2.013(3)
1.996(4)
1.917(4)
0.04
2.3910(5)
2.8071(5)
2.055(3)
2.034(4)
1.910(4)
0.00
2.4056(9)
2.7475(9)
2.026(3)
2.007(4)
1.896(4)
0.10
0.2492(7)
3.5405(9)
86.53(3)
31.49
d(Cu)d
0.1310(2)
0.1828(2)
0.2282(4)
0.2362(5)
Cu⋯Cu′
Cu–X–Cu′ (α)
α/R
3.5239(3)
3.6852(6)
90.78(2)
32.91
88.73(2)
32.80
89.74(2)
31.73
89.43(2)
31.86
a Data are given for each of the two crystallographically independent parts of a dinuclear complex. b X = Cl or Br. c Reedijk’s trigonal distortion τ
(ideally, τ = 0 for a square-pyramidal and τ = 1 for a trigonal-bipyramidal geometry).14 d Displacement of Cu atoms from the mean basal plane.
copper ions. Hence, the observation of one line – instead of
two lines – in the EPR spectra of the latter compounds points
to the existence of exchange interaction with 2J > ΔgβH, where
Δg is the difference between g-factors of the copper ions and
other symbols have their usual meaning.34 Hyperfine inter-
actions or any half-field transitions associated with a ΔMs =
2
were not detected from room temperature down to 80 K.
Angular dependencies of g-factor and peak-to-peak line-
widths, Wpp, recorded at room temperature, are presented in
Fig. 5, as well as in Fig. S12–S14 in ESI.† The dependencies
obtained at T = 80 K were approximately the same as those at
room temperature and therefore they are omitted.
The elements of a (gTg)-tensor were determined using the
following equation:35
g 2 ¼ ðgTgÞaa sin2 θ cos2 ϕ þ ðgTgÞab sin2 θ sin2ϕ
þ ðgTgÞbb sin2 θ sin2 ϕ þ ðgTgÞac sin 2θ cosϕ
þ ðgTgÞbc sin 2θ sin ϕ þ ðgTgÞcc cos2 θ
ð1Þ
where θ and ϕ are the polar and azimuthal angles of the mag-
netic field vector in the a*–b–c* (a–b*–c*) coordinate system,
respectively. The calculated g-tensors are presented in Fig. 5
and in Fig. S12–S13 in ESI.† Some EPR lines for the compound
L2–Br were too weak and/or too broad to be detected; therefore
the calculation of g-tensor for this complex was not performed.
The principal values of the g-tensors, obtained by diagonaliza-
tion of the gTg-matrix at room temperature, are shown in
Fig. 4 Infinite molecular chains in the crystal structures of (a) L1–Cl,
(b) L2–Cl, (c) L1–Br and (d) L2–Br formed by hydrogen-bonding (dotted
lines) between oxalamide units. The minor conformation of the dis-
ordered N-benzyl moiety in L2–Cl and the H atoms attached to C atoms
are omitted for clarity.
Table 3, with the estimated error 0.0001. Powder averaged
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2
2
2
values gav are calculated as: gav
¼
ð1=3Þðgx þ gy þ gz Þ. The
principal axis gz is directed along the Cu–X bond (X ion at
apical position), while gx and gy lay in the basal plane of the
L2–Cl shows a single line in only one rotation plane and square pyramid. The obtained gz > gxgy values point out that
double lines in the other two (except for directions close to the dx2−y2 is the highest energy half-occupied orbital,36 which is in
crystal axes). The observed number of EPR lines can be simply agreement with the square-pyramidal coordination around
correlated to the number of molecules (dimers), Z, found in Cu(II) ions, as could be seen from Table 2.
the unit cell. In the L1–Cl, L1–Br and L2–Br complexes Z = 1,
The powder EPR spectra of the investigated compounds
while Z = 2 for L2–Cl complex. Moreover, while dimeric units recorded at T = 297 K and T = 80 K are shown in Fig. 6. The
for L2–complexes are crystallographically centrosymmetric spectra can be simulated using only g-tensor parameters
and, thus, the two bridged copper centers are magnetically obtained from the single crystal measurements given in
equivalent, different is the case of L1–complexes whose Table 3 while hyperfine A-tensors were taken to be zero. The
dimeric units are constituted by magnetically nonequivalent powder spectra for the complex L2–Br are simulated assuming
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Fig. 5 Angular variation of the g-values (black squares) and the Wpp
linewidths (red circles) of EPR lines for the single crystal of compound
L1–Cl, at room temperature, in three mutually perpendicular planes.
Solid lines represent the fitted g-values with parameters given in Table 2
and the Wpp linewidths with parameters given in the figure, according to
eqn (1) and (2), respectively.
Fig. 6 Experimental (solid lines) and simulated (dotted lines) X-band
EPR spectra of powdered samples of the compounds at the indicated
temperatures.
Table 3 Principal and average values of the g-tensors of the analyzed
compounds at room temperature
Compounds
gx
gy
gz
gav
the following parameters: gx = gy = 2.13 and gz = 2.39. The
spectra were simulated by EasySpin software37 using Lorent-
zian lineshapes, with different linewidths at different
temperatures.
L1–Cl
L2–Cl
L1–Br
L2–Br
2.0559
2.0504
2.0459
—
2.0662
2.0628
2.0847
—
2.2335
2.2352
2.2525
—
2.1201
2.1178
2.1296
—
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The observed EPR linewidth data show a strong angular
dependence, as could be seen in Fig. 5 and in Fig. S12–S14 in
ESI.† Linewidth minima are observed for some angles θ
between 0° and 90°. A similar linewidth anisotropy has been
observed for layered compounds. This behavior is in contrast
to the 3D situation, where the linewidths show dependence
(1 + cos2 θ). The minimum lies close to the magic angle θ = 55°
that is characteristic for low-dimensional systems and it corre-
sponds to the secular part of the dipolar interaction (3 cos2
θ − 1)2.38 However, this term was not enough to explain the
observed linewidth data and we have considered additional
sources of EPR line broadening and narrowing. The data were
fitted adequately, presented by solid lines in Fig. 5 and
Fig. S12–S13 in ESI†, by using the general expression:39
2
ΔWpp ¼ A þ Bð3 cos2 θ ꢀ 1Þ þ C cos2 θ
ð2Þ
Fig. 7 Temperature dependence of magnetization, measured in a field
of 0.1 T. Inset: temperature dependence of the T·M product. Lines are
fitting curves.
The A-term represents the isotropic contribution to the line-
width, the B-term describes the previously mentioned dipolar
interaction, while the C-term could be related to anisotropic
spin–spin interaction. The values of parameters A, B and C are
given in Fig. 5 and in Fig. S12–S13 in ESI.† Hyperfine terms
and contributions arising on the non-equivalence of the
copper sites for compounds L1–Cl and L1–Br were neglected in
eqn (2).
The obtained results, the absence of hyperfine interactions
(with the copper as well as two nitrogen nuclei), the unifying
line effect for magnetically non-equivalent copper centers and
Lorentzian-like EPR lines, reveal the presence of an exchange
interaction in the compounds. However, the absence of the
half-field EPR line shows that this interaction is not strong. By
using linewidth analysis and the method of moments:40
plexes. The interaction is weak, as could be clearly seen by
looking at the inset of Fig. 7, where the product M·T(T) curve
is shown. It deviates from the horizontal (paramagnetic) line
below a few Kelvins only and therefore the molar magnetic sus-
ceptibility cannot be modelled using the Bleaney–Bowers
expression for interacting spins in dimers. Instead, since J is
comparable to gβH, a more general approach, developed by
Friedberg, should be used:44,45
Ngβ sinhðgβH=kTÞ
M ¼
ð4Þ
expðj ꢀ 2Jj=kTÞ þ 2 coshðgβH=kTÞ þ 1
N, β and k are the well known constants, H is the applied
field, the variable T is temperature, g is the effective g-factor
and J is the isotropic exchange interaction parameter within
dimer, defined through the energy term −2JS1·S2. Besides the g
and J fitting parameters, a small correction to the temperature
independent core electron contribution is added. Fits of
eqn (4) go very well through the measured magnetization
points, as could be seen in Fig. 7. The fitting was performed in
both M(T) and T·M(T) forms giving approximately the same
results. The obtained values are presented in Table 4 and stan-
dard deviation is included together with fitting error in
parentheses.
2
Γexp ꢁ γðΓdÞ =ωexch
ð3Þ
where Γexp is the experimental linewidth, γ is the gyromag-
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
netic ratio, ωexch ꢁ J SðS þ 1Þ and Γd is the dipolar linewidth
due to the contribution of the nearest copper ions (Γd ∼ B
from eqn (2)), calculated as previously described,41 an
exchange interaction parameter between copper(II) ions of the
order of |J| ∼ 1 cm−1 could be obtained.
A plausible reason for the relatively weak exchange inter-
action observed is given by the fact that the unpaired electron
in dx2−y2 orbital is localized in the basal plane of square
pyramid and the orbital is pointing toward the four nearest
neighbors of Cu(II) ions (N, N, O and X ions), as could be seen
in Fig. 3. Therefore, the two orbitals with unpaired electrons
are situated in parallel planes, separated by ∼3.5 Å, a particu-
larly unfavorable arrangement for strong exchange interactions
to occur.42
The field dependence of magnetization M(H) measured at
temperature 2 K is presented in Fig. 8. An advantage of the
Friedberg approach is the description of M(H) curves up to
Table 4 Exchange interaction parameters J and g-factors obtained
from magnetization study
3.3. Magnetization study
From M(T)
J (cm−1
From M(H)
J (cm−1
Temperature dependence of magnetization M(T) of the investi-
gated complexes is presented in Fig. 7.43 The observed magne-
tization, lower than expected for two independent copper(II)
spins at low temperature, indicates antiferromagnetic inter-
Compound
)
g
)
g
L1–Cl
L2–Cl
L1–Br
−0.59(1)
−1.19(3)
−0.07(3)
−1.18(1)
2.274(4)
2.06(2)
2.149(9)
2.140(3)
−0.64(1)
−1.10(4)
−0.13(3)
−1.18(1)
2.283(4)
2.00(2)
2.17(9)
2.167(3)
action between spins in structural dimers for all four com- L2–Br
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ferromagnetic and for the values α/R < 31 or α/R > 34.5 the
exchange interaction is antiferromagnetic.52 This is in agree-
ment with the fact that |J|, always composed of ferro- and anti-
ferromagnetic components, should have a minimal value.45,48
Ferromagnetic contributions are usually small but antiferro-
magnetic contributions are proportional to the square of the
overlap integral between orbitals. Therefore, the resulting sign
depends on the amplitude of that overlap.53 The above corre-
lation rule is valid only for complexes with square-pyramidal
arrangement of type II (pyramids share one base-to-apex edge
with parallel basal planes).29
For complexes of type III (pyramids sharing a basal edge
with coplanar basal planes), a linear dependence in the 2J vs.
α/R graph is found. In other words, magneto-structural corre-
lations of dichloro-bridged copper(II) complexes must take into
account the relative orientation of square pyramids to each
other, viz. coplanar, parallel and perpendicular. This is also in
accordance with molecular orbital calculations that indicate
different types of orbitals involved in each case.5 In this work,
Fig. 8 Field dependence of magnetization. Lines are fitting curves.
arbitrary field H at every temperature T. The results of fitting
eqn (4) to the M(H) data are presented also in Table 4.
The exchange parameters J obtained from fitting M(T) and
M(H) curves are mutually similar and they are also in agree-
ment with values obtained from EPR spectra, thus confirming
the consistency of two experimental methods. Small discrepan-
cies between the obtained g-factors and the average EPR values
given in Table 3 could derive from the uncertainty of absolute
magnetization due to sample mass measurements. Addition-
ally, the high degree of purity of the samples is confirmed
through this analysis. Hence, from a magnetic point of view,
the compounds can be considered as isolated halo-bridged
copper(II) dimers. This result is in agreement with the pre-
viously crystallographically described infinite chain of dimeric
units linked by H-bonds via oxalamide groups. Due to the
large size of the ligands, it can be seen that no magnetic inter-
action can take place through the organic bridge between
dimers.
the experimental values 2J = −1.2 cm−1 (with α/R = 32.9° Å−1
)
and 2J = −2.4 cm−1 (with α/R = 34.3° Å−1) determined for
dichloro-bridged complexes L1–Cl and L2–Cl respectively are in
contrast with the correlation rule for type II compounds pre-
viously described52 and add to other similar cases of inconsis-
tency.11,54 However, the obtained results are quantitatively in
agreement with other type II values, where the exchange inter-
action within dimer is generally −10 cm−1 < J < 10 cm−1 4
.
For dibromo-bridged copper dimers, the situation is even
more complicated. There is less information available for com-
plexes bridged by Br compared to Cl and magneto-structural
correlations are less studied for Br-bridged copper dimers.10 It
has been found that the previously mentioned correlation
2J vs. α/R ratio is not valid for dibromo-bridged copper com-
plexes.8,12 However, Landee and Greeney observed that the
magnetic interaction strength can be correlated to the degree
of non-planarity within the Cu basal plane (J vs. trans Br–Cu–L
bridging angle).8 A similar correlation was presented by Rojo
and coworkers who associated J with the extent of distortion
3.4. Magneto-structural correlation
The relationship between structural and magnetic properties within the Cu basal plane and with the Cu–Br (apical) dis-
in copper(II) dimer complexes has been intensively studied tance, including also different types of geometry such as
since 1970. It has been shown that exchange interaction is regular square pyramids, trigonal distorted square pyramids
affected by several structural parameters such as the identity of and tetrahedral distorted square pyramids.50 Additionally,
the bridging atoms (X), the Cu–Cu′ distances, the bridging Romero and coworkers found a difference between two groups
angles Cu–X–Cu′ (α), the dihedral angles containing Cu ions of dibromo-bridged copper complexes following the Rodríguez
and the coordination geometries around copper ions.5,9,46–50 classification.29,55
For planar dihydroxo-bridged copper complexes, a simple,
Here, a simple correlation between singlet–triplet splitting
linear, correlation between the singlet–triplet separation 2J 2J and Reedijk’s parameter τ of trigonal distortion is pre-
and the angle α was reported.46–48 However, in the case of sented. The selected structural and magnetic data for com-
dihalo-bridged copper complexes, the wider variety of geo- plexes reported in the literature56–62 are shown in Table 8 in
metries available to such systems and the possibility for rela- ESI,† while the related graph of 2J vs. parameter τ is shown in
tively low-lying halogens’ d-orbitals to interact with copper Fig. 9.
orbitals make the overall picture more complicated.51 Gener-
It should be noted that trigonal bipyramid could be con-
ally, a strong correlation (parabolic curve with a maximum of sidered as the limit case of distortion of square pyramid via
ca. 33° Å−1) has been found between 2J and the quotient α/R, the Berry mechanism.55 It could be seen that only one complex
where R is the longer (axial) Cu–Cl distance.52 It is pointed out is ferromagnetically coupled while most complexes are weakly
that for the values 31 < α/R < 34.5, the exchange interaction is antiferromagnetically coupled with |J| < 10 cm−1, including
11884 | Dalton Trans., 2014, 43, 11877–11887
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ferromagnetic interactions between the copper ions. By consid-
ering the data available in the literature for similar dihalo-
bridged copper(II) complexes, it is evident that more studies
and data analyses are required in order to obtain better and
more widely applicable magneto-structural correlations,
especially for the less common Br-derivatives. Finally, the one-
dimensional hydrogen bonded polynuclear arrangements
observed in the solid state suggest the potential application of
such ligands as building blocks for the self-assembly of mole-
cule-based magnetic materials. Work along these lines is in
progress and will be reported in due course.
Acknowledgements
Fig. 9 Magneto-structural correlations for dibromo-bridged copper(II)
complexes: plot of the singlet–triplet splitting 2J (cm−1) vs. Reedijk’s tri-
gonal distortion parameter τ. Black squares present literature data given
in Table 8 in ESI,† while red circles present values obtained for L1–Br
and L2–Br complexes in this work. SP = square pyramid; TBP = trigonal
bipyramid.
This research was supported by the Ministry of Science,
Education and Sports of the Republic of Croatia (projects 098-
0982915-2939, 098-0982904-2912, 119-1193079-1084 and 119-
1191458-1017) and the Programma per Giovani Ricercatori “Rita
Levi Montalcini” 2009.
the bromide complexes of this work. This is in agreement with
Kahn’s observation that 95% of copper(II) dinuclear com-
pounds have antiferromagnetic interaction.2 The strongest
couplings are shown by trigonal bipyramid complexes. The
observed behavior is relatively easy to be explained. For
example, for type II complexes, the exchange pathway takes
place through an interaction between copper dx2−y2 situated in
the basal plane and the apical p bromo-orbital. Therefore, for
small τ values, the density of magnetic orbital out of the plane
is small and superexchange coupling would be weak or slightly
ferromagnetic, while for an ideal square-pyramid geometry
(τ = 0), zero coupling is expected. For distorted geometry
around copper ions, magnetic orbitals are more mixed with
bromo orbitals and exchange interaction would be stronger.
For trigonal-bipyramid geometry (τ = 1), magnetic orbitals are
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