Crystal structure and magnetic properties of catena-μ-(pyrazine-N,N′)bis[(p-nitrobenzoyl)-trifluoroacetonato-O, O′]copper(II)

Article abstract of DOI:10.1016/S0277-5387(01)00771-9

The X-ray crystal structure, variable-temperature magnetic susceptibilities and e.s.r. spectra are reported for the title complex. The complex crystallises as infinite chains in which the copper(II)-β-diketonate units are bridged axially by ambidentate pyrazine ligands with a Cu···Cu separation of 7.6700(2) A?. The magnetic susceptibility measurements in the range 5-325 K are consistent with the ferromagnetic intrachain exchange. The S = 1/2 Heisenberg ferromagnetic linear chain analysis results in magnetic parameters of g = 2.12 and J/k = 0.6 K.

Full text of DOI:10.1016/S0277-5387(01)00771-9

Polyhedron 20 (2001) 1097–1100
www.elsevier.nl/locate/poly
Crystal structure and magnetic properties of
catena-m-(pyrazine-N,N%)bis[(p-nitrobenzoyl)-
trifluoroacetonato-O,O%]copper(II)
Edmund Kwiatkowski a,*, Grzegorz Romanowski a, Waldemar Nowicki a,
Kinga Suwin´ska b
a Department of Chemistry, Uni6ersity of Gdan´sk, Sobieskiego 18, PL-80952 Gdan´sk, Poland
b Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, PL-01224 Warsaw, Poland
Received 21 November 2000; accepted 28 February 2001
Abstract
The X-ray crystal structure, variable-temperature magnetic susceptibilities and e.s.r. spectra are reported for the title complex.
The complex crystallises as infinite chains in which the copper(II)-b-diketonate units are bridged axially by ambidentate pyrazine
,
ligands with a Cu···Cu separation of 7.6700(2) A. The magnetic susceptibility measurements in the range 5–325 K are consistent
with the ferromagnetic intrachain exchange. The S=1/2 Heisenberg ferromagnetic linear chain analysis results in magnetic
parameters of g=2.12 and J/k=0.6 K. © 2001 Elsevier Science Ltd. All rights reserved.
Keywords: Crystal structures; Magnetic properties; Copper complexes
for [Cu(nbtfa)2diox]n by the X-ray structural analysis.
The absence of any rich splitting in the e.s.r. spectrum
of the related 1:1 pyrazine adduct in which the copper
atoms might be similarly separated as in [Cu(nbtfa)2
diox]n, and a particular interest in pyrazine-bridged
copper(II) complexes because of the potential for the
propagation of exchange interactions over extended
distances, prompted us to study the crystal structure
and magnetic properties of the title compound. A pe-
rusal of literature shows that only few examples exists
of structurally characterised pyrazine-bridged copper
complexes in which the Cu···Cu separation exceeds 7.2
1. Introduction
We reported earlier [1] that the 1:1 dioxane adduct of
copper(II) para-substituted benzoyltrifluoroacetate,
[Cu(nbtfa)2]n possesses a linear chain structure and
shows unusual, well resolved, nine line e.s.r. spectrum
as a result of a combination of magnetic dipole cou-
pling with the unpaired electron spins of the copper
ions bridged by dioxane molecules and the copper
hyperfine interaction. A simple point-dipole approxima-
tion used for the interpretation of the unusual splitting
allowed to estimate the optimal values for the inter-
dipole separation and the angle between the dipole–
dipole vector and the z molecular axis in reasonable
,
A. The extended Cu···Cu distances of 7.867(7), 7.490(4)
,
and 8.193 A were found in polymers comprising axially
,
bound pyrazine: Cu(pyz)(hfac)2 [2], Cu(pyz)(ttfa)2 [3]
and along the axial direction in the two-dimensional
polymer Cu(pyz)2(CH3SO3)2 [4], where pyz, hfac and
ttfa denote pyrazine, hexafluroacetylacetonate and
thenoilotrifluroacetonate anions, respectively. Magnetic
susceptibility measurements of copper(II) complexes
bridged axially by pyrazine molecules revealed either
antiferromagnetic [5] or no exchange coupling [2,6].
agreement with the values of 7.59 A and 7° suggested
* Corresponding author. Fax: +48-58-3410357.
E-mail address: kwiat@chemik.chem.univ.gda.pl (E. Kwiatkowski).
0277-5387/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0277-5387(01)00771-9

1098
E. Kwiatkowski et al. / Polyhedron 20 (2001) 1097–1100
2. Experimental
2.1. Syntheses
constants and temperature independent paramagnetism
(TIP) equal to 60×10−6 cm3 mol−1. E.s.r. spectra
were obtained using a JES-ME-3X spectrometer work-
ing at 9 GHz.
The title compound was prepared by mixing of the
copper(II) bis(p-nitrobenzoyl-trifluoroacetonate) (584
mg, 0.001 mmol) dissolved in boiling benzene (25 ml)
with an equimolar solution of pyrazine (70 mg,
0.001mol) in the same solvent (20 ml). The solid adduct
which separated after cooling was recrystallised from
benzene to yield dark green crystals of X-ray quality.
Yield 68%. Anal. Found: C, 43.2; H, 2.0; N, 8.2. Calc.
for C24H14F6N4O8Cu: C, 43.4; H, 2.1; N, 8.4%.
2.3. Crystal data for the title compound
A crystal of Cu(nbtfa)2pyz with dimensions 0.45×
0.15×0.10 mm3 was selected for the studies. Chemical
formula for the monomeric unit is C24H24F6N4O8. Cell
parameters are a=7.6700(2), b=10.6700(3), c=
3
,
,
15.9490(8) A, i=95.443(1)°, V=1299.36(7) A . Crys-
tal system is monoclinic, space group P21/n, number of
formulae per unit cell Z=2. All the intensity data were
collected on a Kappa CCD diffractometer using
graphite monochromatised Mo Ka radiation of wave-
2.2. Instruments
,
Microanalyses were performed with a Carlo Erba
MOD 1106 elemental analyser. Magnetic susceptibility
data were recorded on powdered samples with the use
of a Faraday type magnetometer fitted with a helium
continuous flow cryostat. All data were corrected of
diamagnetism of the ligands estimated from Pascal
length u=0.71073 A, with a scan mode ꢀ–2q. Reflec-
tions (5325) were collected in the range
3.07BqB27.51, 2939 of which were unique (Rint
=
0.24). Coverage of the unique set up to q=27.51° is in
98.8% complete. The crystal structure was solved by
direct methods and refined by full matrix least squares
technique with program systems SHELXS-97 [7] and
SHELXL-97 [8], respectively. Non-hydrogen atoms were
treated anisotropically. Hydrogen atoms were intro-
duced at calculated positions and allowed to ride with
isotropic displacement factors 1.2 times larger than
those of their hosts. The refinement of F2 was carried
out against all (2939) reflections. The number of refined
parameters is 225. The agreement factors defined as
R=[ꢀ(ꢁFoꢁ−ꢁFcꢁ)/ꢀꢁFoꢁ] and Rw=[ꢀw(ꢁFoꢁ−ꢁFcꢁ)2/
Table 1
,
Selected bond length (A) and angles (°) for [Cu(nbtfa)2pyz]n
Bond lengths
CuꢀO(1)
CuꢀO(2)
1.953(1) CuꢀN(1’)
1.950(1)
2.445(2)
Bond angles
O(2)c1ꢀCuꢀO(1)
88.17(6) C(2)ꢀO(1)ꢀCu
91.83(6) C(4)ꢀO(2)ꢀCu
122.0(1)
O(2)ꢀCuꢀO(1)
125.6(1)
129.5(2)
123.6(2)
−43.8(2)
ꢀwꢁFoꢁ ] where w=1/[|2(Fo)2+(0.0529P)2+0.4165P]
2
O(1)ꢀCuꢀN(1%)c1 92.24(6) O(1)ꢀC(2)ꢀC(3)
and P=(Fo2+2Fc2)/3 were as follows: R=0.0516 and
Rw=0.1048 for all (2939) reflections, but R=0.0388
and Rw=0.0989 for 2409 reflections with I\2|(I).
Selected geometric parameters are listed in Table 1.
O(2)c1ꢀCuꢀN(1%) 95.25(6) O(2)ꢀC(4)ꢀC(3)
O(2)ꢀCuꢀN(1%)
O(1)ꢀCuꢀN(1%)
84.75(6) O(2)ꢀCuꢀN(1%)ꢀC(1%)
87.76(6) O(1)ꢀCuꢀN(1%)ꢀC(1%) −135.9(2)
3. Results and discussion
X-ray diffraction studies have shown that the title
compound forms a linear coordination polymer with
Cu(nbtfa)2 moieties bridged by pyrazine (pyz)
molecules. The view of a part of the chain polymeric
structure with the numbering scheme is shown in Fig. 1.
The copper(II) atom is hexacoordinate where the
sphere about any copper atom includes four short (1.95
,
A) in plane bonds to the oxygen atoms of the b-diketo-
,
nate ligands and two longer (2.445(2) A) out of plane
bonds to the pyrazine nitrogens and moreover a range
of angles around the metal centre (from 84.75(6) to
95.25(6)°) giving rise to the distorted tetragonal 4+2
coordination. Copper atoms are disposited in crystallo-
graphic inversion centres, their relative positions being
(0,1/2,0). Pyrazine molecules bridge the copper atoms
to give a polymeric chain propagating in the crystallo-
Fig. 1. An ORTEP drawing of the crystal structure of [Cu(nbtfa)2pyz]n
illustrating copper environment with the atomic numbering scheme.

E. Kwiatkowski et al. / Polyhedron 20 (2001) 1097–1100
1099
Fig. 2. Temperature dependence of the molar susceptibility (ꢀ) (the product of gram susceptibility and molecular weight of the monomeric unit)
corrected for diamagnetism and TIP, and of the effective magnetic moment (ꢁ) for [Cu(nbtfa)2pyz]n. Calculated dependences are shown as solid
lines.
in CuꢀO(1) and CuꢀO(2) bond lengths is conspicuous
graphic direction [100]. The plane of pyrazine ring is
oriented at nearly right angle (92.4°) to the equatorial
plane defined by Cu, O(1), O(2), O(1)c1, and O(2)c1
atoms and twisted with respect to the CuꢀO(1) and
CuꢀO(2) bonds what can be seen from the torsion
,
in comparison with the apparent differences of 0.080 A
,
and 0.026 A found in [Cu(hfacac)2pyz]n and [Cu-
(ttfa)2pyz]n, respectively. The CuꢀN bond length of
,
,
2.445(6) A is apparently shorter than that of 2.529(9) A
in Cu(hfacac)2pyz]n and significantly longer than that of
angles
−43.8(2)°
[O(2)ꢀCuꢀN(1%)ꢀC(1%)]
and
,
2.337(3) A in [Cu(ttaca)2pyz]n. The bonds and angles of
−135.9(2)° [O(1)ꢀCuꢀN(1%)ꢀC(1%)]. The projection of
the normal to the pyrazine plane on the CuO4 plane is
at 43.27(9) and 48.63(8)° to the CuꢀO(1) and CuꢀO(2)
bonds, respectively. These angles are much different
from the 8.5° angle at which the pyrazine ring is
oriented with respect to the CuꢀO bond in [Cu-
(hfac)2pyz]. The conjugated C3O2 fragments of b-diket-
onate fragments are each closely coplanar (maximum
deviation from the least squares plane defined by these
the pyrazine systems agree with the observed values in
[Cu(hfac)2pyz]n and in [Cu(pyz)2(ClO4)2]n [9].
Magnetic susceptibilities of Cu(nbtfa)2pyz were mea-
sured in the temperature range 5–325 K. The  and
m
veff versus T plots for the compound are shown in Fig.
2.
The compound has the 325 K temperature effective
magnetic moment of 1.84 mBM. However, when the
temperature is lowered the veff value increases and
reaches a maximum of 2.00 mBM at 10 K and then
decreases. The variation of magnetic susceptibility with
temperature in the 10–325 K range is consistent with a
weak ferromagnetic coupling between copper atoms
within any chain and could satisfactorily be fitted fol-
lowing the Baker et al. [10] equation:
,
atoms is 0.0130(2) A) and are more canted to the basal
plane (dihedral angle 16.46(7)°) than these fragments in
[Cu(nbtfa)2diox]n (dihedral angle 10.9(4)°). There is a
considerable asymmetry of the six-membered chelate
ring incorporating this fragment reflected by the exter-
nal angles at O(1), O(2), C(2) and C(4) atoms. The C6
skeleton of the aromatic ring is oriented at 17.13(6)° to
the basal plane, while the nitro group has a dihedral
angle of 10.9(4)° for its associated C6 aromatic plane.
The trifluoromethyl groups are disordered and each
occurs at two different orientations. High thermal
parameters for fluorine atoms are consistent with a
rapid interconversion between these orientations. The
CuꢀO(1) and CuꢀO(2) bond lengths of 1.953(2) and
m=(Ni2g2/4kT)[(1+Ax+Bx2+Cx3+Dx4+Ex5)/
(1+Fx+Gx2+Hx3+Ix4)]2/3
obtained from an expansion for the S=1/2 Heisenberg
ferromagnetic linear chain model, where x=2J/kT and
A=5.7979916, B=16.902653, C=29.376885, D=
29.832959,
E=14.036918,
F=2.7979916,
G=
7.0086780, H=8.6538644, I=4.5743114, and the
ferromagnetic interaction is represented by positive J
value.
The best fit to the ferromagnetic S=1/2 Heisenberg
regular chain model was obtained for g=2.12 and
J/k=0.60. The decrease of veff below 10 K is most
likely due to an intrachain antiferromagnetic interac-
,
1.950(2) A, respectively differ only marginally from the
average values of CuꢀO bond lengths found in
analogous linear polymers comprising axially coordi-
,
nated bifunctional ligands being 1.949(4)
A
in
,
[Cu(nbtfa)2diox]n, 1.964(7) A in [Cu(hfac)2pyz]n and
,
1.956(4) A in [Cu(ttfa)2pyz]. The insignificant difference

1100
E. Kwiatkowski et al. / Polyhedron 20 (2001) 1097–1100
tion. The magnetic properties of the title complex
reflect a ferromagnetic exchange coupling between the
copper ions not encountered in unsubstituted pyrazine-
bridged copper complexes. A different magnetic be-
haviour of [Cu(nbtfa)2pyz]n to that of [Cu(nbtfa)2diox]n
in spite of similarities in the shortest Cu···Cu intrachain
interactions on various dynamical processes involving
magnetic exchange and dipole–dipole interactions
within the chain and throughout the solid.
4. Supplementary material
,
(7.6700(2) vs 7.587(2) A) and interchain (10.0478(2) vs
,
11.262(3) A) separations suggests that the exchange of
Crystallographic data for the structural analysis have
been deposited with the Cambridge Crystallographic
Data Centre, CCDC No. 152154 for compound catena-
m - (pyrazine - N,N%)bis[(p - nitrobenzoyl)trifluoroaceto-
nato-O,O%]copper(II). Copies of this information may
be obtained free of charge from The Director, CCDC,
12 Union Road, Cambridge, CB2 1EZ, UK (fax: +44-
1223-336033; e-mail: deposit@ccdc.cam.ac.uk or www:
http://www.ccdc.cam.ac.uk).
the unpaired electron spin states in the title compound
occurs via the p-electron system of pyrazine and results
in from an d–p overlap. Richardson et al. [11] pointed
out that for an axially bridged copper(II) complex, in
which the site symmetry around the copper atom is C2h,
the only symmetry allowed overlap with the pyrazine
p-system arises from dxy–p (b1g) overlap and that under
that symmetry some of the unpaired spin density may
be incorporated in the dxy orbital. Some structural
features of the title compound like a more favourable
orientation of the pyrazine ring with respect to the dxy
orbital and a shorter axial Cu–N bond than in [Cu-
(hfac)2pyz]n can result in an efficient dxy–p overlap and
a dxy–p–dxy superexchange not observed in [Cu-
(hfac)2pyz]n. The axial bridging of pyrazine nitrogens, a
long Cu–N bond and parallelism of pyrazine rings
form a different bridging arrangement than that in
Cu(pzdc)HCl [12], the only reported so far ferromag-
netic pyrazine-bridged copper(II) complex in which ni-
trogen atoms of disubstituted pyrazine ligands (pzdc
denotes the 2,3-pyrazinedicarboxylate anion) are coor-
Acknowledgements
This research was supported by the Grant from the
Polish Scientific Research Council (KBN No. 7 T09A
132 20).
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