Strong antiferromagnetic coupling in doubly N,O oximato-bridged dinuclear copper(II) complexes

Article abstract of DOI:10.1016/j.poly.2009.07.010

The use of di-2-pyridyl ketone oxime, (py)pkoH, and phenyl 2-pyridyl ketone oxime, ppkoH, in copper(II) hexafluoroacetylacetonate chemistry is reported. The reaction of CuCl₂·2H₂O with one and two equivalents of ppkoH and Na(hfac), respectively, in CH₂Cl₂ affords the dinuclear complex [Cu₂(hfac)₂(ppko)₂] (1) in excellent yield. The replacement of ppkoH by (py)pkoH gives the isostructural compound [Cu₂(hfac)₂{(py)pko}₂] (2) in good yield. The Cu^II atoms in both 1 and 2 are doubly bridged by the oximate groups of two η¹:η¹:η¹:μ₂ ppko^- and (py)pko^- ligands, respectively. The bridging Cu-(R-NO)-Cu′ units are not planar, with the torsion angles being 23.2° (1) and 20.3° (2). A bidentate chelating hfac^- ligand completes five-coordination at each square pyramidal metal ion. The hfac^--free reaction system CuCl₂·2H₂O/(py)pkoH/NEt₃ (1:2:1) gives instead the mononuclear complex [CuCl{(py)pko}{(py)pkoH}] (3) in very good yield. The Cu^II atom is coordinated by two N,N′-bidentate (py)pko^-/(py)pkoH chelates and a monodentate chloride anion resulting in a distorted square pyramidal geometry around the metal center. Variable-temperature, solid-state dc magnetic studies were carried out on the representative dinuclear complex 1 in the 2.0-300 K range. The data indicate a very strong antiferromagnetic exchange interaction and a resulting S = 0 ground state, which is well isolated from the S = 1 excited state. The J value of -720 cm^-1 was derived from the fitting of the experimental data using the Hamiltonian H = -J(S₁ · S₂).

Full text of DOI:10.1016/j.poly.2009.07.010

Polyhedron 29 (2010) 204–211
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Strong antiferromagnetic coupling in doubly N,O oximato-bridged dinuclear
copper(II) complexes
Evangelia S. Koumousi a, Catherine P. Raptopoulou b, Spyros P. Perlepes a, Albert Escuer c,
Theocharis C. Stamatatos a,
*
a Department of Chemistry, University of Patras, GR-265 04 Patras, Greece
b Institute of Materials Science, NCSR ‘‘Demokritos”, GR-153 10 Aghia Paraskevi Attikis, Athens, Greece
c Departament de Quimica Inorganica, Universitat de Barcelona, Diagonal 647, 08028 Barcelona, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 5 August 2009
The use of di-2-pyridyl ketone oxime, (py)pkoH, and phenyl 2-pyridyl ketone oxime, ppkoH, in copper(II)
hexafluoroacetylacetonate chemistry is reported. The reaction of CuCl2ꢀ2H2O with one and two equiva-
lents of ppkoH and Na(hfac), respectively, in CH2Cl2 affords the dinuclear complex [Cu2(hfac)2(ppko)2]
(1) in excellent yield. The replacement of ppkoH by (py)pkoH gives the isostructural compound [Cu2(h-
fac)2{(py)pko}2] (2) in good yield. The CuII atoms in both 1 and 2 are doubly bridged by the oximate
Keywords:
b-Diketonato ligands
Crystal structures
Di-2-pyridyl ketone oxime
Dinuclear copper(II) complexes
Magnetochemistry
groups of two g1 g1 g1 2 ppkoꢁ and (py)pkoꢁ ligands, respectively. The bridging Cu–(R–NO)–Cu0 units
: : :l
are not planar, with the torsion angles being 23.2° (1) and 20.3° (2). A bidentate chelating hfacꢁ ligand
completes five-coordination at each square pyramidal metal ion. The hfacꢁ-free reaction system
CuCl2ꢀ2H2O/(py)pkoH/NEt3 (1:2:1) gives instead the mononuclear complex [CuCl{(py)pko}{(py)pkoH}]
(3) in very good yield. The CuII atom is coordinated by two N,N0-bidentate (py)pkoꢁ/(py)pkoH chelates
and a monodentate chloride anion resulting in a distorted square pyramidal geometry around the metal
center. Variable-temperature, solid-state dc magnetic studies were carried out on the representative
dinuclear complex 1 in the 2.0–300 K range. The data indicate a very strong antiferromagnetic exchange
interaction and a resulting S = 0 ground state, which is well isolated from the S = 1 excited state. The J
value of ꢁ720 cmꢁ1 was derived from the fitting of the experimental data using the Hamiltonian
H = ꢁJ(S1 ꢀ S2).
Phenyl 2-pyridyl ketone oxime
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
group is also a 2-pyridyl group; the compound is thus a bis(2-pyr-
idyl) oxime.
There is currently a renewed interest in the coordination chem-
istry of oximes [1]. The research efforts are driven by a number of
considerations. These include the employment of oximate ligands
in the synthesis of homometallic [1,2] and heterometallic [1,2c,3]
polynuclear complexes (clusters) and coordination polymers [4]
with interesting magnetic properties, including single-molecule
magnetism [2b–d,5] and single-chain magnetism [4,6] behaviors.
Ligands containing one oxime group and one pyridyl group, with-
out other donor atoms, are popular in coordination chemistry.
Most of these ligands contain a 2-pyridyl group and thus are
named 2-pyridyl oximes, (py)C(R)NOH (Scheme 1). The anionic
forms, (py)C(R)NOꢁ, of these molecules are versatile ligands for a
variety of research objectives, including l2 and l3 behavior
[1a,2c]. Di-2-pyridyl ketone oxime [(py)pkoH, Scheme 1] occupies
a special position amongst the 2-pyridyl oximes because the R
We have been exploring ‘‘ligand blend” reactions involving
mainly carboxylates and the anions of 2-pyridyl oximes (Scheme
1) as a means to high-nuclearity 3d-metal species [2c,7,8–12].
Our results with Cr [7b,7c], Mn [7a,8], Fe [9], Co [10], Ni [11] and
Cu [12] have been very encouraging. For example, the use of
methyl 2-pyridyl ketone oxime (Scheme 1, R = Me) in Mn carbox-
ylate chemistry has yielded
a new family of triangular
7þ
fMnIII3ð
l
3-OÞg core-containing products; the products are very
unusual in being ferromagnetically coupled with a resultant S = 6
ground-state spin and are the first triangular single-molecule mag-
nets [8e,8f]. Furthermore, the reactions of various CuII carboxylate
sources with (py)pkoH have provided access to the family of trian-
gular complexes [Cu3(OH)(O2CR)2{(py)pko}3] (R = various) con-
taining the {Cu3(l
3-OH)}5+ core and possessing the extremely
rare inverse 9-MC-3 motif [12a]. Other structurally characterized
compounds derived from the general CuII/(py)pkoH reaction
system are the dinuclear complex [Cu2{(py)pko}4] [13a], the fasci-
nating 18-MC-6 cluster [Cu6(ClO4){(py)pko}6(MeCN)6][Cu6(ClO4)3-
{(py)pko}6(MeCN)4](ClO4)8 [13b,13c], compounds [Cu3(OH)Cl-
* Corresponding author. Tel.: +30 2610 996020; fax: +30 2610 997118.
E-mail address: thstama@chemistry.upatras.gr (T.C. Stamatatos).
0277-5387/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2009.07.010

E.S. Koumousi et al. / Polyhedron 29 (2010) 204–211
205
Et2O/n-hexane (1:1 v/v, 20 mL). After 3 days, X-ray quality, green
prismatic crystals of 1 were collected by filtration, washed with
CH2Cl2 (2 ꢂ 2 mL) and Et2O (2 ꢂ 3 mL), and dried in air. Yield:
80%. Anal. Calc. for C34H20Cu2F12N4O6: C, 43.65; H, 2.15; N, 5.99.
Found: C, 43.51; H, 2.03; N, 6.04%. IR data (KBr pellet, cmꢁ1):
1652s, 1598m, 1548w, 1524m, 1488m, 1470m, 1442w, 1256s,
1218s, 1138vs, 1088w, 1030w, 984w, 792m, 748m, 712m, 666m,
582 m, 454m.
2.2.2. [Cu2(hfac)2{(py)pko}2] (2) and
[CuCl{(py)pko}{(py)pkoH}]ꢀCH2Cl2 (3ꢀCH2Cl2) in a mixture
To a stirred, colorless solution of (py)pkoH (0.20 g, 1.00 mmol)
and Na(hfac) (0.23 g, 1.00 mmol) in CH2Cl2 (30 mL) was added so-
lid CuCl2ꢀ2H2O (0.17 g, 1.00 mmol). The resulting dark green slurry
was stirred for 40 min, filtered and the filtrate was left undisturbed
in a closed flask at ambient temperature. After 5 days, X-ray qual-
ity, green plate-like crystals of 2 and green rod-like crystals of
3ꢀCH2Cl2 were collected by filtration, washed with CH2Cl2
(2 ꢂ 2 mL) and Et2O (2 ꢂ 3 mL), and dried in air. The two products
were separated manually and individually identified as complexes
2 and 3ꢀCH2Cl2, respectively, by single-crystal, X-ray crystallogra-
phy. Typical yields were ꢃ10% (2) and ꢃ40% (3).
Scheme 1. General structural formulae and abbreviations of the 2-pyridyl oximes
[(py)C(R)NOH] that are used in our laboratories, including phenyl 2-pyridyl ketone
oxime (ppkoH) and di-2-pyridyl ketone oxime [(py)pkoH] employed in this work.
{(py)pko}3]2(Ph4B)2 and [Cu5{(py)pko}7](ClO4)3 [13c], the unique
complex [Cu2Cl4{(py)pkoH2}2(H2O)2]Cl2 that contains the monoca-
tion of (py)pkoH as ligand [13d], and the CuI complexes [Cu(NCS)
{(py)pkoH}]n and [Cu2Cl2{(py)pkoH}2] [13e].
ꢁ
Since the RCO2 ions are structure-determining components in
the [Cu3(OH)(O2CR)2{(py)pko}3] complexes [12a], we anticipated
that the absence of RCO2 from the reactions would give distinctly
2.2.3. [Cu2(hfac)2{(py)pko}2] (2)
ꢁ
To a stirred, colorless solution of (py)pkoH (0.20 g, 1.00 mmol)
and Na(hfac) (0.46 g, 2.00 mmol) in CH2Cl2 (30 mL) was added so-
lid CuCl2ꢀ2H2O (0.17 g, 1.00 mmol). The resulting dark green slurry
was stirred for 40 min, filtered and the filtrate was layered with
Et2O/n-hexane (1:1 v/v, 30 mL). After 4 days, X-ray quality, green
plate-like crystals of 2 were collected by filtration, washed with
CH2Cl2 (2 ꢂ 2 mL) and Et2O (2 ꢂ 3 mL), and dried in air. Yield:
60%. Anal. Calc. for C32H18Cu2F12N6O6: C, 40.99; H, 1.94; N, 8.96.
Found: C, 41.12; H, 2.06; N, 8.89%. IR data (KBr pellet, cmꢁ1):
1650s, 1600m, 1574w, 1554m, 1528s, 1484s, 1440m, 1342m,
1258s, 1198vs, 1140s, 1086m, 1062w, 1030m, 1018m, 952m,
904w, 798s, 760m, 694w, 668s, 586m, 528w, 478w.
different new products, and we have therefore explored this possi-
bility. We have investigated the reactions between the 2-pyridyl
oximes ppkoH and (py)pkoH and a CuII b-diketonate starting mate-
rial, namely copper(II) hexafluoroacetylacetonate (hfacꢁ). Note
that b-diketonate ligands have been found to exhibit a dual role
in the 3d-metal/organic ligand(s) chemistry: first, they can en-
hance the deprotonation of neutral organic ligands that bear ioniz-
able hydrogen(s) acting as bases, and second they are excellent
ancillary chelating/bridging groups favoring the formation of ther-
modynamically stable products. We herein report that the CuII/
hfacꢁ/ppkoH and CuII/hfacꢁ/(py)pkoH reaction schemes have suc-
cessfully led to two dinuclear, doubly N,O oximate-bridged com-
plexes and to one interesting mononuclear, hfacꢁ-free compound
which incorporates both the neutral and anionic forms of (py)p-
koH. The syntheses, structures and magnetochemical characteriza-
tion (for one representative Cu2 complex) of these compounds are
described in this paper.
2.2.4. [CuCl{(py)pko}{(py)pkoH}]ꢀCH2Cl2 (3ꢀCH2Cl2)
To a stirred, colorless solution of (py)pkoH (0.40 g, 2.00 mmol)
and NEt3 (0.14 mL, 1.00 mmol) in CH2Cl2 (30 mL) was added
CuCl2ꢀ2H2O (0.17 g, 1.00 mmol). The resulting dark green solution
was stirred for 30 min, filtered and layered with Et2O/n-hexane
(1:1 v/v, 60 mL). After 4–5 days, X-ray quality, green rod-like crys-
tals of 3ꢀCH2Cl2 were collected by filtration, washed with CH2Cl2
(2 ꢂ 2 mL) and Et2O (2 ꢂ 3 mL), and dried in air. Yield: 70%. The
dried sample analyzed as CH2Cl2-free. Anal. Calc. for C22H17CuCl-
N6O2: C, 53.23; H, 3.45; N, 16.93. Found: C, 53.16; H, 3.36; N,
17.04%. IR data (KBr pellet, cmꢁ1): 3448mb, 3046m, 1586s,
1566w, 1528w, 1470vs, 1438m, 1306w, 1286w, 1218m, 1152sh,
1136m, 1108m, 1099sh, 1054w, 1020m, 994m, 980w, 834sh,
800m, 786m, 748m, 728s, 700s, 650m, 608m, 534w, 470w.
2. Experimental
2.1. General and physical measurements
All manipulations were performed under aerobic conditions
using materials (reagent grade) and solvents as received.
Microanalyses (C, H, N) were performed by the University of
Ioannina (Greece) Microanalytical Laboratory using an EA 1108
Carlo Erba analyzer. IR spectra (4000–450 cmꢁ1) were recorded
on Perkin–Elmer 16 PC FT spectrometer with samples prepared
as KBr pellets. Variable-temperature magnetic studies for complex
1 were performed using a Quantum Design SQUID magnetometer
at the Magnetochemistry Service of the University of Barcelona.
Diamagnetic corrections were applied to the observed paramag-
netic susceptibilities using Pascal’s constants.
2.3. Single-crystal X-ray crystallography
Crystallographic data and structure refinement details for the
three complexes are summarized in Table 1. Selected crystals of
2 (0.50 ꢂ 0.22 ꢂ 0.06 mm) and 3ꢀCH2Cl2 (0.70 ꢂ 0.30 ꢂ 0.20 mm)
were mounted in air, whereas
a
selected crystal of
1
(0.45 ꢂ 0.25 ꢂ 0.20 mm) was mounted in capillary filled with
drops of mother liquor. Crystallographic data for complex 1 were
collected on a P21 Nicolet diffractometer upgraded by Crystal Logic
using graphite-monochromated Cu radiation. Diffraction measure-
ments for complexes 2 and 3ꢀCH2Cl2 were made on a Crystal Logic
Dual Goniometer diffractometer using graphite-monochromated
Mo radiation. Unit cell dimensions were determined and refined
by using the angular settings of 25 automatically centered reflec-
2.2. Compound preparation
2.2.1. [Cu2(hfac)2(ppko)2] (1)
To a stirred, colorless solution of ppkoH (0.20 g, 1.00 mmol) and
Na(hfac) (0.46 g, 2.00 mmol) in CH2Cl2 (30 mL) was added solid
CuCl2ꢀ2H2O (0.17 g, 1.00 mmol). The resulting dark green slurry
was stirred for 40 min, filtered and the filtrate was layered with

206
E.S. Koumousi et al. / Polyhedron 29 (2010) 204–211
Table 1
Crystallographic data for complexes 1, 2 and 3ꢀCH2Cl2.
Parameter
1
2
3ꢀCH2Cl2
Formula
C34H20Cu2F12N4O6
935.62
C32H18Cu2F12N6O6
937.60
monoclinic
P21/a
C23H19CuCl3N6O2
581.33
Formula weight
Crystal system
Space group
triclinic
triclinic
ꢀ
ꢀ
P1
P1
a
b (Å)
c (Å)
a
(Å)
9.921(4)
13.203(6)
7.755(3)
83.78(2)
70.25(2)
69.82(2)
897.3(6)
1
8.628(5)
15.122(9)
13.833(8)
90
100.70(2)
90
1773.4(18)
2
1.756
8.872(4)
11.523(6)
12.878(7)
110.71(2)
94.82(2)
91.33(2)
1225.1(11)
2
(°)
b (°)
c
(°)
V (Å3)
Z
qcalc (g cmꢁ3
)
1.731
1.576
Radiation, k (Å)
Cu K
298
2.508
a, 1.54180
Mo K
298
1.316
a
, 0.71073
Mo K
298
1.253
a, 0.71073
Temperature (K)
l
(mmꢁ1
)
Data collected/unique (Rint
Data with I > 2 (I)
R1 (I > 2
(I))a
wR2 (I > 2
(I))b
)
2445/2318 (0.0431)
2192
0.0498
3263/3130 (0.0424)
2360
0.0617
4835/4321 (0.0105)
3954
0.0361
r
r
r
0.1351
0.1524
0.0994
P
P
a
R1
=
(|Fo| ꢁ |Fc|)/ (|Fo|).
P
P
wR2 = { [w(F2o ꢁ (F2c )2]/ [w((F2o)2]}1/2
.
b
tions in the range 22o < 2h < 54o (for 1) and 11o < 2h < 22o (for 2
and 3ꢀCH2Cl2). Intensity data were recorded using a h–2h scan to
a maximum 2h value of 118o (for 1) and 50o (for 2 and 3ꢀCH2Cl2).
Note that the crystals of 1, despite their sufficient size, had poor
diffraction ability and the data were therefore collected in increas-
ing 2h shells; the data collection was terminated when almost half
of the collected shell data were unobserved. Three standard reflec-
tions monitored every 97 reflections showed less than 3% variation
and no decay. Lorentz, polarization corrections were applied using
Crystal Logic software. All three structures were solved by direct
methods using SHELXS-97 [14a] and refined on F2 by full-matrix
least-squares techniques with SHELXL-97 [14b]. All H atoms were lo-
cated by Fourier difference maps and refined isotropically. All non-
H atoms were refined anisotropically. For complex 1, the fluorine
atoms of the hexafluoroacetylacetonato (hfacꢁ) ligands were found
disordered and refined over two positions with occupation factors
fixed at 0.50. As a result of the disordered positions, the fluorine
atoms present large thermal motion.
CuCl2ꢀ2H2O/(py)pkoH/Na(hfac) reaction mixture in CH2Cl2; both
types of crystals were of X-ray quality.
With the identities of 2 and 3 established by crystallographic
studies, a conventional high-yield synthesis of 3 was easily estab-
lished by avoiding the presence of hfacꢁ in the reaction. The reac-
tion of CuCl2ꢀ2H2O with (py)pkoH and NEt3 in an 1:2:1 ratio in
CH2Cl2 gave a dark green solution and the subsequent isolation
of 3 (ꢃ65%). Further increase of the amount of base should be
avoided. Ratios of (py)pkoH to NEt3 between 1:0.5 and 1:1 lead
to a mixture of green crystals of 3 and green-black crystals of the
known [13a,c] complex [Cu2{(py)pko}4], while ratios between 1:1
and 1:1.5 give exclusively the latter compound.
In order to prepare exclusively pure 2 in a rational manner, we
had to deprotonate all the available quantity of (py)pkoH in solu-
tion; we thus increased the hfacꢁ:(py)pkoH ratio in the 1:1
CuCl2ꢀ2H2O/(py)pkoH reaction mixture. The 1:1:2 reaction be-
tween CuCl2ꢀ2H2O, (py)pkoH and Na(hfac) in CH2Cl2 gave a slurry,
which was filtered to remove insoluble NaCl. Layering of the resul-
tant dark green solution with Et2O/n-hexane gave 2 in good yield
(ꢃ60%). As expected 3 is easily transformed into 2 by treatment
with one equivalent of Na(hfac).
3. Results and discussion
The presence of a neutral oxime group in 3 is manifested by a
medium intensity, broad band at ꢃ3450 cmꢁ1 assigned to
m(OH);
3.1. Brief synthetic comments and IR spectra
its broadness and relatively low wavenumber are both indicative
of a hydrogen bonding [11c]. The in-plane deformation band of
the 2-pyridyl ring of free ppkoH at 622 cmꢁ1 shifts upwards in 1
(666 cmꢁ1), confirming the involvement of the ring N-atom in
coordination [15]. The corresponding band in the spectrum of free
(py)pkoH appears at 600 cmꢁ1. The appearance of two bands in
this region in the spectra of 2 and 3, one at approximately the same
wavenumber (586 cmꢁ1 in 2, 608 cmꢁ1 in 3) and the other at a
higher wavenumber (668 cmꢁ1 in 2, 650 cmꢁ1 in 3), reflects the
presence of both coordinated and uncoordinated 2-pyridyl groups
(vide infra) in the two complexes. The medium band at 1094 cmꢁ1
The 1:1:2 reaction between CuCl2ꢀ2H2O, ppkoH and Na(hfac) in
CH2Cl2 gave a slurry, which was filtered to remove insoluble NaCl.
Crystallization of the dark green solution gave green prisms of 1 in
high yield (ꢃ80%). The same complex is isolated from the 1:1 Cu
(hfac)2/ppkoH in CH2Cl2 (this procedure is not reported in the
experimental section); however, the yield was lower (ꢃ50%).
We then decided to seek the (py)pkoꢁ version of complex 1. Our
main goal was to determine whether the (py)pkoꢁ product would
have a similar molecular structure. Due to the presence of an extra
donor atom in the ligand, we might obtain more than one products
from the use of the hfacꢁ/(py)pkoH ligand combination (‘‘blend”)
in CuII chemistry; one dinuclear with a structure similar to that
of 1 and possibly a high-nuclearity cluster in which the (py)pkoꢁ
ligand would be tetradentate. Our expectations proved to be both
correct and incorrect; we did obtain two products, but the second
compound is a hfacꢁ-free, mononuclear complex. Green plate-like
crystals of 2 and green rod-like crystals of 3ꢀCH2Cl2, in an approx-
imate 1:1 visual ratio, were simultaneously isolated from the 1:1:1
for free ppkoH is assigned to the m(NO)oxime mode [15], which in-
creases to 1138 cmꢁ1 in 1. This shift to higher wavenumbers has
been discussed [11c] and is in accord with thee fact that upon
deprotonation and oximate O-coordination, there is a higher con-
tribution of N@O to the electronic structure of the oximate group;
consequently, the
relative to that for ppkoH. The
oximato group appears at 1140 cmꢁ1 in 2 [11c]. The 1108 cmꢁ1
m
(NO) vibration shifts to a higher wavenumber
m
(NO) vibration of the coordinated

E.S. Koumousi et al. / Polyhedron 29 (2010) 204–211
207
ꢀ
Complex 1 crystallizes in the triclinic space group P1. Its struc-
ture consists of isolated dinuclear [Cu2(hfac)2(ppko)2] molecules
(Fig. 1). There is a crystallographic inversion center at the mid-
point of the CuII. . .CuII axis. The CuII atoms are doubly bridged by
the diatomic oximate groups of two g1 g1 g1 l2 ppkoꢁ ligands
: : :
(Scheme 2); each ligand chelates one CuII atom forming a five-
membered CuNCCN chelating ring, while its oximate oxygen atom
is terminally bound to the other metal center. The bridging
CuN(2)O(1)Cu0 unit is not planar; the torsion angle is 23.2°. The
deviations of atoms Cu, O(1), N(2) and Cu0 from the CuN(2)O(1)Cu0
best mean plane are 0.035, 0.100 Å above, and 0.099, 0.034 Å below
the plane, respectively. A bidentate chelating (g
2) hfacꢁ ligand
(Scheme 2) completes five-coordination at each metal. Analysis
of the shape-determining bond angles using the approach of Reed-
ijk and co-workers [18] yields a value for the trigonality index,
s, of
0.11 for the metal ions ( = 0 and 1 for perfect square pyramidal
s
and trigonal bipyramidal geometries, respectively). Thus, the
geometry about each CuII center can be described as slightly dis-
torted square pyramidal (sp), with the apical position of Cu occu-
pied by an oxygen atom [O(12)] of a chelating hfacꢁ ligand. One
2-pyridyl [N(1)] and an oximate [N(2)] nitrogen atom arising from
the same bridging ppkoꢁ ligand,ꢁas well as the oximate oxygen
atom [O(10)] from the other ppko ligand and the remaining oxy-
gen atom [O(11)] of a hfacꢁ group form the basal plane for Cu.
As expected, the Cu–O(12) bond distance is the longest. Cu lies
0.147 Å out of the basal plane towards O(12). The Cuꢀ ꢀ ꢀCu0 distance
is 3.699(1) Å. There are no significant intermolecular interactions,
Fig. 1. Partially labeled PovRay representation of complex 1, with H atoms omitted
for clarity. Primes are used for symmetry-related atoms. Color scheme: CuII, green;
O, red; N, blue; F, yellow; C, gray. (For interpretation of the references to color in
this figure legend, the reader is referred to the web version of this article.)
either hydrogen bonds or p–p stacking interactions.
Complex 2 crystallizes in the monoclinic space group P21/a. Its
structure (Fig. 2) is very similar to that of 1, with the only differ-
ence being the nature of the bridging oximate ligands, i.e., the
two CuII atoms in centrosymmetric complex 2 are doubly bridged
by the diatomic oximate groups of two g1
:
g1
:
g1
:
l2 (py)pkoꢁ
groups (Scheme 2) instead of two g1
:
g1 g1 l2 ppko- groups that
: :
are present in 1. The bridging CuN(2)O(1)Cu0 unit is again not pla-
nar with the torsion angle being 20.3°, slightly less than the corre-
sponding value for complex 1. The deviations of atoms Cu, O(1),
N(2) and Cu0 from the CuN(2)O(1)Cu0 best mean plane are 0.029,
0.085 Å above, and 0.083, 0.030 Å below the plane, respectively.
A bidentate chelating (
tion at each metal. The metal coordination geometry is described
as almost perfect square pyramidal ( = 0.03 [18]), with the apical
g
2) hfacꢁ ligand completes five-coordina-
s
position of Cu occupied by an oxygen atom [O(12)] of the chelating
hfacꢁ ligand. Cu lies 0.220 Å out of the basal plane towards O(12),
while the Cuꢀ ꢀ ꢀCu0 distance is 3.772(2) Å, slightly longer than that
in 1.
It is of interest to note that the crystal structure of 2 is further
stabilized by four symmetry-equivalent, intermolecular Fꢀ ꢀ ꢀF Van
der Waals contacts [F(2)ꢀ ꢀ ꢀF(5) = 2.92(2) Å] which serve to link
neighboring Cu2 dimers into one-dimensional double chains along
the a axis (Fig. 3, top). These interactions create vacant (from
cocrystallized solvent molecules) channels (Fig. 3, bottom).
Complex 2 is structurally similar to compound [Cu2{(py)p-
ko}4]ꢀ2H2O [13a,c]; in the latter the capping terminal ligands are
provided by two g2 (py)pkoꢁ groups (instead of the two hfacꢁ li-
gands that are present in the former).
Fig. 2. Partially labeled PovRay representation of complex 2. H atoms have been
omitted for clarity. Primes are used for symmetry-related atoms. Color scheme: CuII,
green; O, red; N, blue; F, yellow; C, gray. (For interpretation of the references to
color in this figure legend, the reader is referred to the web version of this article.)
band in the spectrum of 3 is a serious candidate for the
m
(NOꢀ ꢀ ꢀH)
vibration.
The strong band at 1652 and 1650 cmꢁ1 in the spectra of 1 and
0
. . .
m(C O) mode of the O,O -chelat-
•
ꢀ
2, respectively, is assigned to the
Complex 3ꢀCH2Cl2 crystallizes in the triclinic space group P1. Its
ing hfacꢁ ligand [16,17]. This band appears at almost the same
wavenumber (1644 cmꢁ1) for complex [Cu(hfac)2], in which the
hfacꢁ ion is chelated to the metal [16].
structure consists of well-separated [CuCl{(py)pko}{(py)pkoH}]
molecules (Fig. 4) and solvate CH2Cl2 molecules in an 1:1 ratio;
the latter will not be further discussed. The CuII center is coordi-
nated by a monodentate Clꢁ group and two N,N0-bidentate chelat-
ing (g
2) (py)pkoꢁ and (py)pkoH ligands (Scheme 2). There is a
3.2. Description of structures
hydrogen atom [HO(11)] located at practically the middle of the
oximate oxygen atoms’ distance [O(1)–HO(11) 1.20(5)/O(11)–
HO(11) 1.26(5) Å]; this prevents any clear assignment on the exact
position of HO(11) and consequently a precise attribution of the
The molecular structures of complexes 1, 2 and 3 are depicted
in Figs. 1, 2 and 4, respectively. Selected interatomic distances
and angles are listed in Tables 2–4.

208
E.S. Koumousi et al. / Polyhedron 29 (2010) 204–211
Fig. 3. Top: A partially labeled section of the 1D double chains of 1 illustrating the inter-dimer Fꢀ ꢀ ꢀF linkages through the hfacꢁ groups along the a axis. Bottom: Space-filling
representation viewed along the a axis, emphasizing the large channels that are present in the crystal structure. Color scheme: CuII, green; O, red; N, blue; F, yellow; C, gray.
(For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Table 3
Selected interatomic distances (Å) and angles (°) for complex 2.a
Cu–O(10)
Cu–O(11)
Cu–O(12)
1.908(4)
1.984(4)
2.243(5)
Cu–N(1)
Cu–N(2)
Cuꢀ ꢀ ꢀCu0
1.990(5)
1.974(5)
3.772(2)
O(10)–Cu–O(11)
O(10)–Cu–O(12)
O(10)–Cu–N(1)
O(10)–Cu–N(2)
O(11)–Cu–O(12)
84.8(2)
88.9(2)
164.8(2)
101.3(2)
86.6(2)
O(11)–Cu–N(1)
O(11)–Cu–N(2)
O(12)–Cu–N(1)
O(12)–Cu–N(2)
N(1)–Cu–N(2)
90.0(2)
166.6(2)
105.1(2)
105.2(2)
81.1(2)
a
Primed atoms are related to the unprimed ones by the symmetry operation ꢁx,
2 ꢁ y, 1 ꢁ z.
Fig. 4. Labelled PovRay representation of 3, with some H atoms omitted for clarity.
The dashed lines are H-bonds. Color scheme: CuII, green; O, red; N, blue; Cl, purple;
C, gray; H, sky blue. (For interpretation of the references to color in this figure
legend, the reader is referred to the web version of this article.)
Table 4
Selected bond lengths (Å) and angles (°) for complex 3ꢀCH2Cl2.
Cu–N(1)
Cu–N(2)
Cu–N(11)
2.052(2)
1.991(2)
2.045(2)
Cu–N(12)
Cu–Cl(1)
2.008(2)
2.407(1)
Table 2
Selected interatomic distances (Å) and angles (°) for complex 1.a
N(1)–Cu–N(2)
N(1)–Cu–N(11)
N(1)–Cu–N(12)
N(2)–Cu–N(11)
N(2)–Cu–N(12)
78.63(9)
103.52(9)
151.60(9)
165.29(9)
92.29(9)
N(11)–Cu–N(12)
Cl(1)–Cu–N(1)
Cl(1)–Cu–N(2)
Cl(1)–Cu–N(11)
Cl(1)–Cu–N(12)
78.94(9)
102.30(7)
96.96(8)
96.77(7)
105.50(7)
Cu–O(10)
Cu–O(11)
Cu–O(12)
1.914(2)
1.993(2)
2.230(3)
Cu–N(1)
Cu–N(2)
Cuꢀ ꢀ ꢀCu0
1.993(3)
1.978(3)
3.699(1)
O(10)–Cu–O(11)
O(10)–Cu–O(12)
O(10)–Cu–N(1)
O(10)–Cu–N(2)
O(11)–Cu–O(12)
84.5(1)
93.0(1)
164.7(1)
103.4(1)
87.9(1)
O(11)–Cu–N(1)
O(11)–Cu–N(2)
O(12)–Cu–N(1)
O(12)–Cu–N(2)
N(1)–Cu–N(2)
90.0(1)
171.2(1)
101.0(1)
95.5(1)
81.4(1)
gen atoms, with its existence rationalizing the high thermody-
namic stability of 3; the dimensions are: O(1)ꢀ ꢀ ꢀO(11) 2.442(3) Å
and O(1)–HO(11)ꢀ ꢀ ꢀO(11) 169.1(5)o. Alternatively, the two di-2-
pyridyl ketone oximate/oxime groups can be considered as one tet-
radentate N4-chelating {(py)pkoꢀ ꢀ ꢀHꢀ ꢀ ꢀ(py)pko} ligand. The _CN–
Oꢀ ꢀ ꢀHꢀ ꢀ ꢀO–NCff motif is a valuable synthon in crystal engineering
[19].
a
Primed atoms are related to the unprimed ones by the symmetry operation ꢁx,
2 ꢁ y, ꢁz.
negative charge on one of the two ligands. Undoubtedly, this is a
very strong intramolecular H-bond between the two oximate oxy-

E.S. Koumousi et al. / Polyhedron 29 (2010) 204–211
209
pounds with the neutral and anionic derivatives of ppkoH and
(py)pkoH [2c,12,13,21].
N
3.3. Magnetochemistry
N
N
Magnetostructural studies on polynuclear metal complexes are
of continuing interest for coordination chemists since they can pro-
vide the understanding of fundamental factors governing their
magnetic properties. The bridging diatomic @N–Oꢁ group is known
to be very efficient in mediating a medium-to-strong antiferro-
magnetic interaction, which is provided by an orbital exchange
N
N
Cu
O
Cu
O
Cu
Cu
η1:η1:η1:µ2
η1:η1:η1:µ2
N
F
F
pathway of
r symmetry, giving values of the coupling constants,
F
F
H
C
Jij, typically greater than ꢁ500 cmꢁ1. Thus, for instance, CuII com-
plexes with double oximato bridges usually exhibit complete or
nearly complete spin coupling even at room temperature [22].
Variable-temperature dc magnetic susceptibility data in an
1.0 T field and in the 2.0–300 K range were collected on a pow-
dered microcrystalline sample of the representative dinuclear
compound 1 restrained in eicosane to prevent torquing. The ob-
C
C
F
F
C
O
C
N
O
N
Cu
Cu
O(H)
η2
η2
tained data are plotted as
vMT versus T in Fig. 6. The vMT value
Scheme 2. The crystallographically established coordination modes of the ligands
discussed in the text.
at 300 K is only 0.082 cm3 molꢁ1 K. This value is much lower than
that expected for a molecule comprising two non-interacting CuII
ions (0.75 cm3 molꢁ1 K with g = 2). The
vMT product rapidly de-
creases with decreasing temperature, reaching a constant value
very close to 0 cm3 molꢁ1 K at ꢃ100 K. This behavior is indicative
of very strong antiferromagnetic interactions between the metal
centers with the low temperature value suggesting a well-isolated
singlet ground-state spin (S = 0). Thus, complex 1 is essentially dia-
magnetic even at room temperature, in a way that at the whole
temperature range (2.0–300 K) the S = 0 state is mainly populated.
In accordance with this, the polycrystalline EPR spectrum of the
complex shows no signal in this temperature range. On the other
hand, the molar magnetic susceptibility vM shows a continuous
decrease upon cooling down to a broad minimum centered at
ꢃ130 K, but below this temperature it rapidly increases due to
the presence of a small amount of paramagnetic, possibly mono-
meric, impurity. The maximum of vM should be located at a tem-
perature much larger than 300 K in agreement with the very strong
The donor atoms are the oxime/oximate nitrogens [N(2),
N(12)] and two pyridyl nitrogens [N(1), N(11)], each from a
different ligand. One terminal chloro ligand [Cl(1)] completes
five-coordination at the metal center. The Cu–N bond lengths
are normal for this class of compound [12,13], whereas the Cu–
Cl(1) distance [2.407(1)Å] indicates a rather weak, but not abnor-
mally weak [20], copper(II)–chloro bond. The metal coordination
geometry is well described as square pyramidal with the chloro
ligand occupying the apical position. Analysis of the shape-deter-
mining angles [18] yields a value for the trigonality index, s, of
0.23, indicating that the geometry about copper is significantly
distorted. The four nitrogen atoms N(1), N(2), N(11) and N(12)
define the basal plane, deviating from this plane by an average
of 0.12 Å [maximum deviation by N(2) and N(12), 0.131 Å]; the
metal ion lies 0.363 Å out of the best, least-squares basal plane,
towards Cl(1).
antiferromagnetic interaction suggested by the
vMT data.
A fit of the experimental data of 1 to the expression derived
The mononuclear molecules seem to be stabilized in the crystal
from the simplified spin Hamiltonian, H = ꢁJ(S1ꢀS2), and introducing
by a strong intermolecular
p–p stacking interaction between
(py)pkoꢁ/(py)pkoH ligands of adjacent molecules. The interaction
involves the uncoordinated 2-pyridyl rings that possess N(3) and
N(30) (Fig. 5); the intercentroid distance (Cgꢀ ꢀ ꢀC0g) is 3.275 Å and
the rings are perfectly planar to each other. Complexes 1–3 join a
very small family of structurally characterized copper(II) com-
a
q term to evaluate the monomeric paramagnetic impurity, gives
the parameters J = ꢁ720(10) cmꢁ1, g = 2.15(6) and
q = 0.13(5)%,
which lead to a quasi perfectly isolated S = 0 ground state with
the S = 1 excited state (triplet state) being 1440 cmꢁ1 above the
ground state. This magnetic behavior is not unexpected in view of
Fig. 5. A partially labeled representation of the intermolecular
p–p stacking interaction (yellow dashed line) found in 3, which serves to link adjacent monomers in the
crystal. Color scheme: CuII, green; O, red; N, blue; Cl, purple; C, gray. (For interpretation of the references to color in this figure legend, the reader is referred to the web version
of this article.)

210
E.S. Koumousi et al. / Polyhedron 29 (2010) 204–211
currently exploring the reactions of 2-pyridyl oximes with cop-
0.10
0.08
0.06
0.04
0.02
0.00
per(II) acetylacetonate in order to test the differences in the mag-
netic coupling induced by various b-diketonate ligands. Work also
is in progress in our groups to synthesize high-nuclearity cop-
per(II)/b-diketonate/2-pyridyl oximate clusters.
Supplementary data
CCDC 730888, 730889 and 730890 contain the supplementary
crystallographic data for 1, 2 and 3. These data can be obtained free
of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or
from the Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail:
deposit@ccdc.cam.ac.uk.
χ
T / K
300
0
50
100
150
200
250
Acknowledgements
Fig. 6. Plot of
vVT vs. T for 1 in a 10 kG field. The solid lines are the best fit of the
experimental data; see the text for the fit parameters.
This work was supported by EPEAEK II (Program PYTHAGORAS
I, Grant b.365.037 to S.P.P.) and MEC (Grant CTQ2006-01759 to
A.E.). We thank K.A. Kounavi for experimental assistance.
the remarkable ability of the oximato bridges to mediate strong
antiferromagnetic exchange interactions between paramagnetic
centers, either in syn, anti, or O-monoatomic coordination [1a,22].
Extended-Huckel MO calculations previously reported by some of
us [22h] and others [22f] on the Cu–(R@N–O)2–Cu core (R = various
substituted groups) have indicated that planar Cu–(R@N–O)2–Cu
‘‘rings” favor the strongest magnetic coupling, but other factors
such as the electronic properties of the R-substituted oximato
groups and/or the ligands that complete the coordination sphere
of the CuII ions (i.e., hfacꢁ in 1 and 2) play an important role modu-
lating the magnitude of the coupling [22h]. Complex 1 exhibits a
very strong antiferromagnetic coupling (J = ꢁ720 cmꢁ1) typical for
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