Magneto-structural relationships in a series of dinuclear oxalato-bridged (diphenyldipyrazolylmethane)copper(II) complexes

Article abstract of DOI:10.1021/ic048229t

A series of oxalato-bridged dinuclear copper(II) complexes of the general formula [Cu₂(Pz₂CPh₂)₂(X) ₂(μ-C₂O₄)] (X = Cl^- (1), NO ₃^- (2), ClO₄^- (3); Pz ₂CPh₂ = diphenyldipyrazolylmethane) or [Cu ₂(Pz^3m₂CPh₂)₂(H ₂O)₂(μ-C₂O₄)]-(NO ₃)₂·H₂O (4) (Pz^3m ₂CPh₂ = diphenylbis(3-methylpyrazolyl)methane) was synthesized where the axial ligand was systematically varied to study its effect on structure and magnetic coupling. The structures of compounds 1, 2, and 4 have been elucidated by single-crystal X-ray diffraction. [Cu ₂(Pz₂CPh₂)₂(Cl)₂(μ- C₂O₄)] and [Cu₂(Pz₂CPh ₂)₂(NO₃)₂-(μ-C₂O ₄)] are isostructural and crystallize in the triclinic system, space group P1, Z= 2, with a = 8.6155(8) A, b = 10.1435(9) A, c = 11.3612(11) A, α = 95.535(2)°, β = 110.303(2)°, and γ = 106.111(2)° for 1 and with a = 8.863(7) A, b = 10.241(9) A, c = 11.425(10) A, α = 98.985(14)°, β = 110.449(13)°, and γ = 103.664(14)° for 2. [Cu₂(Pz ^3m₂CPh₂)₂(H₂O) ₂(μ-C₂O₄)]·NO₃· H₂O crystallizes in the monoclinic system, space group C2/c, Z = 4, with a = 23.4588(14) A, b = 8.8568(5) A, c = 21.7818(13) A, α = γ = 90°, and β = 100.8890(10)°. Variable-temperature magnetic susceptibility studies indicate that all four compounds are strongly antiferromagnetically coupled (2J/k = -364, -344 cm ^-1 (2), -424 cm^-1 (3), and -378 cm^-1 (4)). Magnetic and EPR results are discussed with respect to structural parameters to explore possible magneto-structural correlations.

Full text of DOI:10.1021/ic048229t

Inorg. Chem. 2005, 44, 5060−5067
Magneto−Structural Relationships in a Series of Dinuclear
Oxalato-Bridged (Diphenyldipyrazolylmethane)copper(II) Complexes
Janet L. Shaw,† Gordon T. Yee,‡ Guangbin Wang,‡ David E. Benson,§ Cagil Gokdemir,§ and
Christopher J. Ziegler*,†
Departments of Chemistry, UniVersity of Akron, Akron, Ohio 44325-3601, Virginia Polytechnic
Institute and State UniVersity, Blacksburg, Virginia 24061, and Wayne State UniVersity,
Detroit, Michigan 48202
Received December 16, 2004
A series of oxalato-bridged dinuclear copper(II) complexes of the general formula [Cu2(Pz2CPh2)2(X)2(
µ
-C2O4)]
-C2O4)]-
diphenylbis(3-methylpyrazolyl)methane) was synthesized where the axial ligand was
systematically varied to study its effect on structure and magnetic coupling. The structures of compounds 1, 2, and
4 have been elucidated by single-crystal X-ray diffraction. [Cu2(Pz2CPh2)2(Cl)2( -C2O4)] and [Cu2(Pz2CPh2)2(NO3)2-
-C2O4)] are isostructural and crystallize in the triclinic system, space group P1, Z 2, with a 8.6155(8) Å, b
10.1435(9) Å, c 11.3612(11) Å, R ) 95.535(2) â ) 110.303(2) , and γ ) 106.111(2) for 1 and with a
8.863(7) Å, b 10.241(9) Å, c 11.425(10) Å, R ) 98.985(14) â ) 110.449(13) , and γ ) 103.664(14)
-C2O4)] NO3 H2O crystallizes in the monoclinic system, space group C2/c, Z 4,
8.8568(5) Å, c 21.7818(13) Å, R ) γ ) 90 , and â ) 100.8890(10) . Variable-
-
-
(X
(NO3)2
)
Cl- (1), NO3 (2), ClO4 (3); Pz2CPh2
) µ
diphenyldipyrazolylmethane) or [Cu2(Pz3m2CPh2)2(H2O)2(
‚
H2O (4) (Pz3m2CPh2
)
µ
(
µ
h
)
)
)
)
)
°
,
°
°
)
)
°
,
°
°
for 2. [Cu2(Pz3m2CPh2)2(H2O)2(
µ
‚
‚
)
with a
)
23.4588(14) Å, b
)
)
°
°
temperature magnetic susceptibility studies indicate that all four compounds are strongly antiferromagnetically coupled
1
1
1
(2J/k ) −364,
−
344 cm- (2),
−
424 cm- (3), and
−
378 cm- (4)). Magnetic and EPR results are discussed with
respect to structural parameters to explore possible magneto−structural correlations.
Introduction
(II) complexes has received much attention in the literature
due to its relevance to the spin frustration problem in copper-
based superconductors.4
A very popular multiatom bridge for use in the synthesis
of these compounds is the oxalate dianion (ox2-). It is well-
known that the oxalate ligand allows for magnetic com-
munication between metal ions that can be greater than 5 Å
apart.5 However, with the elasticity of the copper(II)
coordination environment and the multiple binding modes
Ligand-mediated coupling between paramagnetic metal
centers continues to be an active area of investigation. The
simplest of such systems occur when two d1 or d9 transition
metal centers are coupled by a short bridging ligand.1 Of
the compounds that fit this description, dinuclear copper(II)
compounds have been heavily examined, both synthetically
and spectroscopically. Such complexes are relevant to
dinuclear copper enzymes2 including hemocyanin, an oxygen
transport protein, and tyrosinase, an enzyme responsible for
oxidation of phenolic substrates in certain organisms.3 In
addition, the magneto-structural analysis of bridged copper-
(4) Seeber, G.; Ko¨gerler, P.; Kariuki, B. M.; Cronin, L. Chem. Commun.
2004, 14, 1580-1581.
(5) (a) Gleizes, A.; Julve, M.; Verdaguer, M.; Real, J. A.; Faus, J.; Solans,
X. J. Chem. Soc., Dalton Trans. 1992, 3209-3216. (b) Geiser, U.;
Ramakrishna, B. L.; Willett, R. D.; Hulsbergen, F. B.; Reedijk, J.
Inorg. Chem. 1987, 26, 3750-3756. (c) Akhriff, Y.; Server-Carrio´,
J.; Sancho, A.; Garc´ıa-Lozano, J.; Escriva´, E.; Folgado, J. V.; Soto,
L. Inorg. Chem. 1999, 38, 1174-1185. (d) Zhang, L.; Bu, W.; Yan,
S.; Jiang, Z.; Liao, D.; Wang, G. Polyhedron 2000, 1105-1110. (e)
Nu´n˜ez, H.; Timor, J.; Server-Carrio´, J.; Soto, L.; Escriva`, E. Inorg.
Chim. Acta 2001, 318, 8-14. (f) Sme´kal, Z.; Kamen´ıe`ek, J.; Klasova´,
P.; Wrzeszcz, G.; ×b3indla´ø, Z.; Kopel, P.; _a´k, Z. Polyhedron 2002,
21, 1203-1209. (g.) Du, M.; Guo, Y.; Bu, X. Inorg. Chim. Acta 2002,
335, 136-140. (h.) Tuna, F.; Pascu, G. I.; Sutter, J.; Andruh, M.;
Golhen, S.; Guillevic, J.; Pritzkow, H. Inorg. Chim. Acta 2003, 342,
131-138.
* Author to whom correspondence should be addressed. E-mail: ziegler@
uakron.edu.
† University of Akron.
‡ Virginia Polytechnic Institute and State University.
§ Wayne State University.
(1) Kahn, O.; Galy, J.; Journaux, Y.; Jaud, J.; Morgenstern-Badarau, I. J.
Am. Chem. Soc. 1982, 104, 2165-2176.
(2) Lippard, S. J.; Berg, J. M. Principles of Bioinorganic Chemistry;
University Science Books: Mill Valley, CA, 1994.
(3) Gamez, P.; Koval, I. A.; Reedijk, J. Dalton Trans. 2004, 4079-4088.
5060 Inorganic Chemistry, Vol. 44, No. 14, 2005
10.1021/ic048229t CCC: $30.25
© 2005 American Chemical Society
Published on Web 06/15/2005

Cu(II)-Oxalato Magneto-Structural Correlations
bidentate metal ion coordination. We have found it useful
for the synthesis of 1:1 metal to ligand complexes where
the coordination environment at the copper center can be
adjusted by modifying the peripheral pyrazole substitution.11
In this work we report the synthesis of four oxalato-bridged
compounds where we have varied the copper(II) starting salt
to study the effects of the axial ligand on geometry and
temperature-dependent magnetic susceptibility. The com-
plexes are of the general formula [Cu2(Pz2CPh2)2(X)2(µ-
-
-
C2O4)] (X ) Cl- (1), NO3 (2), ClO4 (3); Pz2CPh2 )
diphenyldipyrazolylmethane) or [Cu2(Pz3m2CPh2)2(H2O)2(µ-
C2O4)](NO3)2‚H2O (4) (Pz3m2CPh2 ) diphenylbis(3-meth-
ylpyrazolyl)methane), where the oxalate bridges in a sym-
metric bis(bidentate) fashion. Structural elucidation by single-
crystal X-ray diffraction was possible for compounds 1, 2,
and 4, but only an isotropic solution of compound 3 was
possible due to extensive twinning in the crystalline product.
We are not reporting the final structure of compound 3 here,
but the isotropic solution can be used to obtain geometric
parameters for this complex. The degree of antiferromagnetic
coupling is discussed in relation to several structural
parameters measured from the crystal structures.
Figure 1. Selected binding modes of the oxalate dianion in dinuclear
copper(II) compounds.6
Figure 2. Diphenyldipyrazolylmethane (DPDPM), where R ) H and Pz2-
CPh2 and R ) CH3 and Pz3m2CPh2.
of the oxalate dianion come a plethora of possible symmetric
and asymmetric geometries for resultant complexes. Figure
1 shows several of the observed binding modes of the oxalate
dianion in dinuclear copper(II) compounds.
Experimental Section
With regard to the degree of antiferromagnetic coupling
between copper(II) centers, energies can range from 0 to
-400 cm-1 depending on structural variations such as
terminal ligand, coordination environment at the copper(II)
ions, and bridging mode of the oxalate moiety. These
structural factors are important in orienting the magnetic
orbitals of the copper(II) ions for overlap with the oxalate
σ-orbitals, which mediate the superexchange. The magnitude
of antiferromagnetic coupling is directly related to the square
of the overlap between magnetic orbitals centered on each
copper(II) ion as mediated by the bridging oxalate.7 Tuning
magnetic coupling between the copper(II) ions continues to
be of interest but is compounded by the degree of difficulty
in controlling product formation and parametrizing structural
effects.8
Caution! Perchlorate salts of metal complexes with organic
ligands are potentially explosiVe. A small amount of material only
should be prepared and it should be handled with care.
All solvents and starting materials were obtained from com-
mercial sources and were used without further purification.
Syntheses of the ligands Pz2CPh2 and Pz3m2CPh2 were reported
previously.11 Mass spectra were recorded using an ES MS Bruker
Esquire-LC ion-trap mass spectrometer. The elemental analyses
were performed on a CE440 analyzer at the School of Chemical
Sciences Microanalysis Laboratory at the University of Illinois at
Urbana-Champaign. Temperature-dependent magnetic susceptibility
data were collected at Virginia Tech, and EPR data were obtained
at Wayne State University. UV-visible spectra were obtained using
a Hitachi U-3010 instrument.
Synthesis of [Cu2(Pz2CPh2)2(Cl)2 (µ-C2O4)] (1). CuCl2 (0.091
g, 0.7 mmol) dissolved in CH3OH/H2O (5/10 mL) was added to a
solution of Pz2CPh2 (0.201 g, 0.7 mmol) in CH3OH (20 mL). The
blue-green solution was allowed to stir for 30 min. To the reaction
mixture was added Na2C2O4 (0.045 g, 0.3 mmol). After the mixture
was stirred for 24 h, a blue-green precipitate was collected by
filtration and washed with H2O (54% yield). Dissolving the
precipitate in warm CH3OH followed by slow evaporation of the
We have begun to explore the synthesis of copper(II)
dinuclear oxalato-bridged complexes using the ligand diphen-
yldipyrazolylmethane (DPDPM) (Figure 2). Synthesized
previously for use in polymerization catalysis,9,10 this ligand
is neutral and contains two pyrazole moieties available for
solvent led to X-ray-quality crystals. IR (KBr): νasy(CO) 1640 cm-1
,
(6) (a) Chaudhuri, P.; Oder, K. J. Chem. Soc., Dalton Trans. 1990, 1597-
1605. (b) Sme´kal, Z.; Thornton, P.; ×b3indla´ø, Z.; Klie`ka, R.
Polyhedron 1998, 17, 1631-1635. (c) Go´mez-Saiz, P.; Garc´ıa-Tojal,
J.; Maestro, M.; Mah´ıa, J.; Lezama, L.; Rojo, T. Eur. J. Inorg. Chem.
2003, 2123-2132.
(7) Julve, M., Verdaguer, M.; Kahn, O.; Gleizes, A.; Philoche-Levisalles,
M. Inorg. Chem. 1983, 368-370.
(8) (a) Hay, P. J.; Thibeault, J. C.; Hoffman, R. J. Am. Chem. Soc. 1975,
97, 4884-4899. (b) Felthouse, T. R.; Laskowski, E. J.; Hendrickson,
D. N. Inorg. Chem. 1977, 16, 1077-1089. (c) Kahn, O. Inorg. Chim.
Acta 1982, 62, 3-14. (d) Julve, M.; Verdaguer, M.; Gleizes, A.;
Philoche-Levisalles, M.; Kahn, O. Inorg. Chem. 1984, 23, 3808-3818.
(e) Kahn, O. Angew. Chem., Int. Ed. Engl. 1985, 24, 834-850. (f)
Cabrero, J.; Amor, N. B.; de Graff, C.; Illas, F.; Caballol, R. J. Phys.
Chem. A 2000, 104, 9983-9989.
ν
sym(CO) 1312 cm-1, δ(O-C-O) 757 cm-1. UV-vis (CH3OH;
λ/nm): 647 (1.28 × 102 M-1 cm-1). ES MS (+ ion, CH3OH): m/z
851.1 [M - Cl]. Anal. Calcd for C40H32N8Cl2Cu2O4 (Mr
)
886.72): C, 54.18; H, 3.64; N, 12.63. Found: C, 53.29; H, 3.60;
N, 12.13.
Synthesis of [Cu2(Pz2CPh2)2(NO3)2(µ-C2O4)] (2). Cu(NO3)2
(0.633 g, 3.4 mmol) dissolved in H2O (30 mL) was added to a
solution of Pz2CPh2 (1.015 g, 3.4 mmol) in CH3OH (150 mL). The
blue solution was allowed to stir for 30 min. To the reaction was
added Na2C2O4 (0.227 g, 1.7 mmol) dissolved in H2O (20 mL).
The solution was allowed to stir overnight resulting in a blue
(9) Shiu, K.; Yeh, L.; Peng, S.; Cheng, M. J. Organomet. Chem. 1993,
460, 203-211.
(10) Tsuji, S.; Swenson, D. C.; Jordan, R. F. Organometallics 1999, 18,
4758-4764.
(11) Shaw, J. L.; Cardon, T. B.; Lorigan, G. A.; Ziegler, C. J. Eur. J. Inorg.
Chem. 2004, 1073-1080.
Inorganic Chemistry, Vol. 44, No. 14, 2005 5061

Shaw et al.
precipitate that was removed by filtration and washed with H2O
and hexane (48% yield). Slow evaporation from a solution of warm
CH3OH or DMF produced X-ray-quality crystals. IR (KBr):
ν
asy(CO) 1647 cm-1, νsym(CO) 1312 cm-1, δ(O-C-O) 755 cm-1
,
(NO3-) 1384 cm-1. UV-vis (CH3OH; λ/nm): 670 (1.39 × 102
M-1 cm-1). ES MS (+ ion, CH3OH; m/z): 878.2 [M - NO3], 407.9
[M - 2NO3]. Anal. Calcd for C40H32N10Cu2O10‚C3H7NO (Mr )
1012.82): C, 50.99; H, 3.89; N, 15.21. Found: C, 51.39; H, 3.77;
N, 15.36.
Synthesis of [Cu2(Pz2CPh2)2(ClO4)2 (µ-C2O4)] (3). Cu(ClO4)2‚
6H2O (1.204 g, 3.2 mmol) was dissolved in H2O (25 mL) and added
to a solution of Pz2CPh2 (0.984 g, 3.3 mmol) in CH3OH (100 mL).
The solution turned deep blue and was allowed to stir for 30 min.
To the reaction a solution of Na2C2O4 (0.219 g, 1.6 mmol) in H2O
(25 mL) was added. After the mixture was stirred for 1 h, a blue
precipitate formed, which was removed by filtration and washed
with H2O and hexanes (74% yield). Crystals can be grown from
warm acetonitrile layered with H2O, but the quality of the crystals
is poor due to extensive twinning. All attempts to grow suitable
crystals for X-ray structural elucidation resulted in twinned mor-
phologies that were not suitable for anisotropic solutions. IR
(KBr): νasy(CO) 1654 cm-1, νsym(CO) 1320 cm-1, δ(O-C-O) 749
cm-1, (ClO4-) 1120, 1071, 1056 cm-1. UV-vis (CH3OH; λ/nm):
679 (1.16 × 102 M-1 cm-1). ES MS (+ ion, CH3OH; m/z): 915.1
[M - ClO4], 407.9 [M - 2ClO4]. Anal. Calcd for C40H32N8Cl2-
Cu2O12 (Mr ) 1014.82): C, 47.34; H 3.18; N, 11.04. Found: C,
47.31; H, 3.09; N, 11.09.
Figure 3. Structure of compound 1 with 50% thermal ellipsoids and
hydrogen atoms omitted for clarity.
Synthesis of [Cu2(Pz3m2CPh2)2(H2O)2 (µ-C2O4)](NO3)2‚H2O
11
(4). Cu(Pz3m2CPh2)(NO3)2 (0.766 g, 1.5 mmol) in H2O (30 mL)
was combined with a solution of Na2C2O4 (0.099 g, 0.7 mmol) in
H2O (10 mL). A blue precipitate formed immediately, which was
removed by filtration and washed with additional H2O (51% yield).
Dissolving the precipitate in warm CH3OH followed by slow
evaporation of the solvent led to X-ray-quality crystals. IR (KBr):
ν
asy(CO) 1654 cm-1, νsym(CO) 1309 cm-1, δ(O-C-O) 763 cm-1
.
UV-vis (CH3OH; λ/nm): 702 (1.24 × 102 M-1 cm-1). ES MS (+
ion, CH3OH; m/z): 934.2 [M - 2H2O + NO3], 435.9 [M - 2H2O].
Anal. Calcd for C44H44N8Cu2O6‚2NO3‚H2O (Mr ) 1049.99): C,
47.34; H 3.18; N, 11.04. Found: C, 47.31; H, 3.09; N, 11.09.
X-ray Crystallography. The X-ray intensity data for compounds
1-4 were measured at 100 K (Bruker KRYO-FLEX) on a Bruker
SMART APEX CCD-based X-ray diffractometer system equipped
with a Mo-target X-ray tube (λ ) 0.710 73 Å) operated at 2000 W
power. Crystals were mounted on a cryoloop using Paratone
N-Exxon oil and placed under a stream of nitrogen. The detector
was placed at a distance of 5.009 cm from the crystals. Frames
were collected with a scan width of 0.3° in ω. Analyses of the
data sets showed negligible decay during data collection. The data
were corrected for absorption with the SADABS program. The
structures were refined using the Bruker SHELXTL software
package (version 6.1) and were solved using direct methods until
the final anisotropic full-matrix, least-squares refinement of F2
converged.12 Experimental details for all of the structures are
provided in Table 3.
Figure 4. Structure of compound 2 with 50% thermal ellipsoids and
hydrogen atoms omitted for clarity.
Magnetic Susceptibility. Magnetic susceptibility measurements
were performed on a 7 T Quantum Design MPMS SQUID
magnetometer. Measurements of magnetization as a function of
temperature were performed from 1.8 to 300 K and in a 5000 G
field. Samples were prepared by crushing single crystals to a fine
powder. Approximately 15 mg samples were packed between cotton
plugs, placed into gelatin capsules, cooled in zero applied field,
and measured upon warming. Diamagnetic corrections were applied
on the basis of Pascal’s constants. Data of øT vs T were modeled
by nonlinear least-squares fitting to the Bleaney-Bowers expression
for two Cu2+ centers, including the usual term for the presence of
uncoupled Cu2+ and temperature-independent paramagnetism where
necessary. The data were corrected for the diamagnetism of the
gel cap and the cotton plug.
EPR Spectroscopy. EPR spectra were recorded on a Bruker
ESP-300 X-band EPR spectrometer. Analyses were performed in
the solid state using 0.01% (w/w) silica mixtures. All spectra were
collected at 105 K using the following parameters: microwave
power, 50 mW; microwave frequency, 9.43 GHz (confirmed by
TEMPO standard13); modulation frequency, 100 kHz; modulation
amplitude, 5 G.
Results and Discussion
The perspective views of compounds 1, 2, and 4 are shown
in Figures 3-5, crystallographic data are located in Table
1, and selected bond distances and angles are provided in
(13) Forrester, A. R.; Neugebauer, F. A. In Landolt-Borstein: Magnetic
properties of free radicals; Fischer, H., Hellwege, K. H., Eds.; Springer
Verlag: Berlin, 1979; Part C1.
(12) Sheldrick, G. M. SHELXTL, Crystallographic Software Package,
version 6.10; Bruker-AXS: Madison, WI, 2000.
5062 Inorganic Chemistry, Vol. 44, No. 14, 2005

Cu(II)-Oxalato Magneto-Structural Correlations
Table 1. Crystallographic Data for Compounds 1, 2, and 4
param
mol formula
1
2
4
Cu2Cl2O4C40H32N8
Cu2O10C40H32N10
939.84
Cu2O6C44H44N8(N2O6)‚H2O
fw
886.74
triclinic
P1h
1049.99
monoclinic
C2/c
cryst system
space group
cell constants
a (Å)
b (Å)
c (Å)
triclinic
P1h
8.6155(8)
10.1435(9)
11.3612(11)
95.535(2)
110.303(2)
106.111(2)
2
8.863(7)
10.241(9)
11.425(10)
98.985(14)
110.449(13)
103.664(14)
2
23.4588(14)
8.8568(5)
21.7818(13)
90
R (deg)
â (deg)
100.8890(10)
90
γ (deg)
Z
4
V (Å3)
873.90(14)
1.428
911.1(13)
1.247
4444.1(5)
1.036
1.569
0.0320
0.0823
1.063
abs coeff µcalc (mm-1
)
δ
calc (Mg/m3)
1.685
0.0365
0.0934
1.064
1.713
0.0555
0.1168
1.045
R(F)
Rw(F2)
GOF
Table 2. Selected Bond Lengths (Å) and Angles (deg) for Compounds
1, 2, and 4
Table 2. Structures of all three complexes are very similar
and will be discussed together to avoid redundancy.
The compounds consist of centrosymmetric [Cu2(Pz2-
CPh2)2(X)2(µ-C2O4)] (X ) Cl (1) NO3 (2)) neutral entities
or [Cu2(Pz3m2CPh2)2(H2O)2(µ-C2O4)]2+ (4) cationic entities.
In compounds 1-3 the axial ligand is a counteranion, but
in 4 a water molecule is coordinated in the apical site and
two noncoordinating nitrates exist in the crystal lattice for
charge balance. Each copper(II) ion is in a 4 + 1 distorted
square pyramidal coordination environment with two nitro-
gen atoms from the terminal ligand and two oxygen atoms
from the bridging oxalate in the basal plane. The Cu-Oox
bond lengths are in the range 1.957(3)-2.011(3) Å with the
two extreme lengths of this range both arising in compound
2. The Cu-N bond lengths are in the range 1.959(3)-
2.002(2) Å, and the Cu-Cu distances are in the range
5.130-5.212 Å (Table 2) which are typical for this class of
compounds.5
[Cu2(Pz2CPh2)2(Cl)2(µ-C2O4)] (1)
Cu-O1
Cu-O2
Cu-N2
Cu-N4
Cu-Cl
O1-C20
C20-C20A
Cu-Cu
1.9921(17)
2.0009(17)
1.991(2)
2.002(2)
2.3961(7)
1.257(3)
1.535(5)
5.212
N2-Cu-O1
N2-Cu-O2
O1-Cu-O2
N2-Cu-N4
O1-Cu-N4
O2-Cu-N4
N2-Cu-Cl
O1-Cu-Cl
O2-Cu-Cl
N4-Cu-Cl
162.21(8)
90.25(7)
83.78(7)
85.39(8)
92.15(7)
152.39(8)
100.14(6)
97.64(5)
108.27(5)
99.33(6)
[Cu2(Pz2CPh2)2(NO3)2(µ-C2O4)] (2)
Cu-O4
Cu-O5
Cu-N2
Cu-N4
Cu-O1
O5-C20
C20-C20A
Cu-Cu
2.011(3)
1.957(3)
1.982(3)
1.959(3)
2.192(3)
1.259(4)
1.540(7)
5.161
N4-Cu-O5
N4-Cu-O4
O4-Cu-O5
N2-Cu-N4
O4-Cu-N2
O5-Cu-N2
N4-Cu-O1
O4-Cu-O1
O5-Cu-O1
N2-Cu-O1
168.53(11)
91.32(12)
84.55(11)
86.46(12)
151.01(12)
91.97(11)
91.24(12)
88.55(12)
99.33(12)
120.36(12)
The phenyl group of the DPDPM ligand partially shields
the copper(II) ions preventing coordination of a sixth ligand
to the metal centers and enforcing the square pyramidal
[Cu2(Pz3m2CPh2)2(H2O)2(µ-C2O4)](NO3)2‚H2O (4)
Cu-O4
Cu-O5
Cu-N2
Cu-N4
Cu-O6
O5-C22
C22-C22A
Cu-Cu
1.9996(12)
1.9799(11)
1.9955(14)
1.9699(13)
2.1719(13)
1.254(2)
N4-Cu-O5
N4-Cu-O4
O4-Cu-O5
N2-Cu-N4
O4-Cu-N2
O5-Cu-N2
N4-Cu-O6
O5-Cu-O6
O4-Cu-O6
N2-Cu-O6
162.84(5)
89.08(5)
84.06(5)
90.18(6)
157.66(5)
90.24(5)
94.32(6)
102.54(5)
103.01(5)
99.32(5)
1.540(3)
5.178
geometry. The shortest Cu-Cphenyl distances are in the range
3.031-3.163 Å for compounds 1, 2, and 4. It has been shown
in molybdenum complexes with DPDPM9 that π-bonding
interactions can occur between the phenyl ring and the metal
center, but here there are no such observed interactions in
our compounds.
In compounds 1-4 the oxalate bridges the two copper
atoms in the very common symmetric bis(bidentate) fashion.
In compounds 1 and 2 the oxalate bridge is essentially planar;
however, the Cu-ox-Cu unit is not perfectly planar with
copper deviations of (0.121 and (0.101 Å with respect to
the mean plane of the oxalate dianion. The oxalate bite angles
at copper for these two compounds are very similar
Figure 5. Structure of compound 4 with 50% thermal ellipsoids and
hydrogen atoms omitted for clarity.
Inorganic Chemistry, Vol. 44, No. 14, 2005 5063

Shaw et al.
Figure 6. Common geometries and the magnetic orbitals used to describe bridged dinuclear copper(II) oxalate compounds: NNOO, left; NNLO, middle;
TBP, right.14
(84.55(11)° (1) and 83.78(7)° (2)). In compound 4 the oxalate
bridge is significantly distorted from planarity with carbon
deviations of (0.273 Å. In addition, the Cu-ox-Cu entity
is not planar with copper deviations of (0.266 Å. In
compound 4 the bite angle of oxalate at copper is similar to
that of compounds 1 and 2 with a value of 84.06(5)°. The
planarity of this Cu-ox-Cu unit may affect the degree of
overlap between the magnetic orbitals on copper(II) and the
σ-orbitals of the oxalate dianion, thus affecting antiferro-
magnetic coupling.
To explain the degree of antiferromagnetic coupling in
oxalato-bridged dinuclear compounds several structural
parameters have been invoked in the literature. Variation in
the degree of coupling has been primarily attributed the
orientation of the semioccupied magnetic orbitals (SOMOs).
SOMOs are defined as the half-filled highest occupied
molecular orbitals (HOMOs) of the copper(II) d9 metal ions.14
For a five coordinate copper(II) center, the two possible
geometries are square pyramidal and trigonal bipyramidal.
For an N2LO2 coordination environment where N2 is the
DPDPM ligand, O2 is the oxalate dianion, and L is the axial
ligand, three SOMO orientations can be envisioned. Figure
6 shows the ideal geometries and the magnetic orbitals used
to describe these coordination modes. These arrangements
are named on the basis of their basal planes and can be called
NNOO, NNLO, and TBP, respectively.
can quantify variations in geometry from ideal tetragonal to
ideal trigonal bipyramidal for five-coordinate copper(II)
complexes. The formula used is τ ) (â - R)/60, where â
and R are the largest and second largest angles of the type
L-Cu-L, surrounding the metal center (τ ) 0 for tetragonal
geometry and τ ) 1 for trigonal bipyramidal geometry). Any
deviations from ideal tetragonal geometry should decrease
the overlap of magnetic orbitals with the σ-oxalate orbitals,
thus decreasing the observed antiferromagnetic coupling. In
addition, the degree of displacement of the copper(II) atom
from the basal coordination plane (hM),14,16 and the dihedral
angle formed between the basal plane and the oxalate
bridging plane (γ) can also be considered.14,16 A density
functional calculation paper by Cano et al. proposes a
correlation between these distortion parameters and magnetic
exchange.14 The apical ligand was chosen for study here
because it is partially responsible for both of these distortions.
Tabulated below are the values for τ, hM, and γ for
compounds 1-4 and for several structurally similar com-
pounds taken from the literature for comparison (Table 3).
Values for τ vary significantly across the series with τ )
0.16 (1), 0.29 (2), 0.039 (3) (parameters for compound 3
are based on an isotropic solution of the twinned single-
crystal data), and 0.086 (4). On the basis of this distortion
from ideal tetragonal geometry, one would predict the
antiferromagnetic exchange to increase on the order 2 < 1
< 4 < 3. The parameter hM follows the series 1 (0.39), 2
(0.35), 3 (0.26), and 4 (0.34). Although these numbers are
very similar across the series, on the basis of this distortion,
one would predict the magnetic exchange to increase in the
order 1 < 2 ∼ 4 < 3. Finally, the angle represented by γ is
significant in compounds 1, 2, and 4 (18.7-23.6°) and much
smaller in compound 3 (2.6°). This difference in γ for the
series must result from the variation of the axial ligand and
its interactions with the copper(II) ions. On the basis of γ,
the coupling values should increase in the order 4 < 1 < 2
< 3. All three of these predictions agree in the placement of
compound 3 as the strongest antiferromagnetically coupled,
but they differ in the order of the remaining three compounds.
Experimentally these predictions for the degree of anti-
ferromagnetic coupling based on structural parameters can
Compounds 1-4 fit the NNOO category with the magnetic
2
2
orbitals of the copper(II) ions best described as dx -y . In the
literature, this configuration typically leads to the highest
degree of antiferromagnetic coupling. The oxalate bridges
the metal centers symmetrically, and the magnetic orbitals
of the copper(II) ions are ideally situated for optimal overlap
with the in-plane oxalate σ-orbitals. Therefore, compounds
1-4 should all exhibit strong antiferromagnetic coupling on
the basis of the orientation of their SOMOs.
Simple parameters have been used in the literature to
correlate the degree of antiferromagnetic coupling to the
structures of oxalato-bridged dinuclear copper(II) com-
pounds. The first of these, τ, developed by Addison et al.15
(14) Cano, J.; Alemany, P.; Alvarez, S.; Verdaguer, M.; Ruiz, E. Chem.s
Eur. J. 1998, 4, 476-484.
(15) Addison, A. W.; Rao, T. N.; Reedijk, J.; van Rijn, J.; Verschoor, G.
C. J. Chem. Soc., Dalton Trans. 1984, 1349-1356.
(16) Alvarez, S.; Julve, M.; Verdaguer, M. Inorg. Chem. 1990, 29, 4500-
4507.
5064 Inorganic Chemistry, Vol. 44, No. 14, 2005

Cu(II)-Oxalato Magneto-Structural Correlations
Table 3. Geometric Parameters for Compounds 1-4 and Several Selected Complexes with Similar Geometry Taken from the Literature for
Comparison
hM
0.39
0.35
0.26
0.34
0.15/0.18
0.24
0.15
0.18
γ (deg)
τ
ref
[Cu2(Pz2CPh2)2(Cl)2(µ-C2O4)] (1)
20.3
18.7
2.6
0.16
0.29
0.039
0.086
[Cu2(Pz2CPh2)2(NO3)2(µ-C2O4)] (2)
[Cu2(Pz2CPh2)2(ClO4)2(µ-C2O4)]a (3)
[Cu2(Pz3m2CPh2)2(H2O)2(µ-C2O4)](NO3)2‚H2O (4)
[Cu2(tmen)2(H2O)2(C2O4)](ClO4)2‚1.25H2Ob
[Cu2(mpz)2(C2O4)(H2O)2](PF6)2‚mpz‚3H2Oc
23.6
0.14/0.11
0.13
0.09
0.18
0.16
NA
17
5
5
5
18
d
[Cu2(bpy)2(H2O)2C2O4][Cu(bpy)C2O4](NO3)2
[Cu2(bpy)2(H2O)2C2O4][Cu(bpy)C2O4](ClO4)2
[Cu2(bpy)2(H2O)2C2O4][Cu(bpy)C2O4](BF4)2
[Cu2(bpy)2(H 2O)2(NO3)2C 2O4]
0.16
0.11
a Parameters based on an isotropic solution of single-crystal X-ray diffraction data. b tmen ) N,N,N′,N′-tetramethylethylenediamine. c mpz ) 4-methoxy-
2-(5-methoxy-3-methylpyrazol-l-yl)-6-methylpyrimidine. d bpy ) 2,2′-bipyridyl.
On the basis of the calculated values for 2J, it is tempting
to conclude that the structural parameter τ correctly predicts
the ordering of magnetic exchange owing to the shift from
tetragonal to trigonal bipyramidal geometry. The parameter
hM correlates fairly well with observed antiferromagnetic
coupling, but the overall trend predicted by γ is not in
agreement with the experimental data. However, the similar-
ity of the 2J values for compounds 1, 2, and 4 precludes
any concrete correlations between these structural parameters
and magnetic exchange. For compound 3, where the 2J value
is considerably higher than the others, these parameters
correlate well with the observed coupling. The high degree
of planarity at the copper(II) center indicated by the shortest
hM length, the smallest bending angle γ, and the lowest τ
value should encourage increased antiferromagnetic coupling,
Figure 7. Plot of øT vs T and the fit to the Bleaney-Bowers expression
for compound 4.
and they do. Clearly the perchlorate anion is weakly bound,
and this leads to one of the strongest instances of anti-
ferromagnetic coupling for dinuclear oxalato-bridged
copper(II) complexes that can be found in the literature.
be tested using variable-temperature magnetic susceptibility
measurements. A representative plot of the product of molar
susceptibility and temperature (ømT) versus temperature (T)
for compound 4 is given in Figure 7. As was expected on
In addition to temperature-dependent magnetic susceptibil-
ity, EPR spectroscopy can also provide an indication of the
degree of communication between metal centers via hyper-
fine and superhyperfine coupling interactions, although such
coupling does not always manifest. X-band EPR spectra of
polycrystalline samples diluted with silica were collected at
105 K for all four compounds (Figure 8). In each case the
signal is weak which therefore hinders interpretation.
2
2
the basis of the NNOO dx -y -type SOMO orientation, all
four compounds exhibit behavior characteristic of strongly
antiferromagnetically coupled copper(II) ions. It should be
noted that these samples were difficult to measure because
they are essentially diamagnetic at low temperatures. The
data were successfully fit using a modified Bleaney-Bowers
equation for coupled dinuclear copper(II) complexes starting
with the Heisenberg Hamiltonian H ) -2JS1S2.19 The form
of the expression used includes terms for uncoupled copper-
(II) impurities and temperature-independent paramagnetism
(TIP). The terms N, â, k, g, and T have their usual meanings,
and F is the mole percent of paramagnetic impurity. A
summary of the magnetic susceptibility parameters for all
four compounds is tabulated below along with several
structurally similar complexes selected from the literature
for comparison (Table 4).
Compound 2 has g| ) 2.24 and g ) 2.08 values and
shows unresolved hyperfine splitting. The trend of g| > g
2
2
is indicative of a dx -y -type orbital for the unpaired electron
in a CuN2O2 chromophore. Compounds 1 and 4 exhibit four
line hyperfine splitting patterns due to the Cu 3/2 nuclear
spin. Values for g| and g were not distinguishable due to
the broadness of the signals, but gave ) 2.09 and 2.16 for 1
and 4, respectively. In addition, the spectrum of compound
4 may also exhibit features of the triplet state along with
g-anisotropy. The most interesting spectrum is observed for
compound 3 which has a nine line splitting pattern. Hen-
drickson et al. report 7-14 line coupling patters in the EPR
spectrum of the compound [Cu2(Me5dien)2(C2O4)](BPh4)2
(where Me5dien ) 1,1,4,7,7-pentamethyldiethylenetriamine)
which the authors believe result from interdimer magnetic
2Ng2â2
Ng2â2
2k
øT )
(1 - F) +
F + (TIP)T (1)
2J
kT
k 3 + exp -
[
(
)
]
(17) Soto, L.; Garcia, J.; Escriva, E.; Legros, J. P.; Tuchagues, J. P.; Dahan,
F.; Fuertes, A. Inorg. Chem. 1989, 28, 3378-3386.
(18) Castillo, O.; Muga, I.; Luque, A.; Gutie´rrez-Zorrilla, J. M.; Sertucha,
J.; Vitoria, P.; Roma´n, P. Polyhedron 1999, 18, 1235-1245.
(19) Kahn, O. Molecular Magnetism; VCH: New York, 1993; p 107.
Inorganic Chemistry, Vol. 44, No. 14, 2005 5065

Shaw et al.
Table 4. Summary of Magnetic Susceptibility Parameters for Compounds 1-4 and Coupling Values for Several Selected Compounds Taken from the
Literature for Comparison
2Ja (cm-1
)
gb
r
TIP
µ
eff(rt) (µB)
ref
[Cu2(Pz2CPh2)2(Cl)2(µ-C2O4) ] (1)
-364
-344
-424
-378
-385
-402
-386
-376
-378
-382
2.24
2.16
2.12
2.23
2.16
2.18
2.217
2.23
2.20
2.10
0.031
0.013
0.032
0.013
0.0509
0.005
c
1.90
1.65
1.38
1.53
[Cu2(Pz2CPh2)2(NO3)2(µ-C2 O4)] (2)
1 × 10-3
2 × 10-4
2 × 10-4
[Cu2(Pz2CPh2)2(ClO4)2(µ-C2 O4)] (3)
[Cu2(Pz3m2CPh2)2(H2O)2(µ-C2O4)](NO3)2‚H2O (4)
[Cu2(tmen)2(H2O)2(C2O4)](C lO4)2‚1.25 H2Od
[Cu2(mpz)2(C2O4)(H2O)2](PF 6)2‚mpz‚3H2Oe
8d
17
5a
5a
5a
18
f
[Cu2(bpy)2(H2O)2C2O4][Cu(bpy)C2O4](NO3)2
[Cu2(bpy)2(H2O)2C2O4][Cu(bpy)C2O4](ClO4)2
[Cu2(bpy)2(H2O)2C2O4][Cu(bpy)C2O4](BF4)2
[Cu2(bpy)2(H2O)2(NO3)2C2O4]
0.018
a All values have a standard deviation of (2, and the correlation coefficient for each of the fits was 0.9999. b Values have a standard deviation of (0.01.
c Inclusion of the TIP term was not necessary to fit data for compound 1. d tmen ) N,N,N′,N′-tetramethylethylenediamine. e mpz ) 4-methoxy-2-(5-methoxy-
3-methylpyrazol-l-yl)-6-methylpyrimidine. f bpy ) 2,2′-bipyridyl.
Figure 8. Solid-state X-band EPR spectra of [Cu2(Pz2CPh2)2(Cl)2(µ-C2O4)] (1) (top, left), [Cu2(Pz2CPh2)2(NO3)2(µ-C2O4)] (2) (top, right), [Cu2(Pz2CPh2)2-
(ClO4)2(µ-C2O4)] (3) (bottom, left), and [Cu2(Pz3m2CPh2)2(H2O)2(µ-C2O4)](NO3)2‚H2O (4) (bottom, right) at 105 K.
communiction.20 However, the coupling constants observed
for compound 3 are significantly smaller than those in
Hendrickson’s compound. The splitting in 3 most likely
results from superhyperfine interactions between the four
pyrazole nitrogen atoms across the oxalate bridge, resulting
in a nine line pattern. This explanation is consistent with
the high degree of planarity in compound 3 which places
the nitrogen atoms close to the magnetic orbitals. Both
interpretations are indicative of a highly planar, highly
coupled dimer complex. Once again, the broadness and
possible overlap of the signal inhibited obtaining individual
g| and g values, but gave ) 2.12. The data were examined
for the forbidden dimer transition at half-field, but the signal
intensity and the background of the silica dilutent hindered
its detection.
Conclusion
We have begun to study the ligand diphenyldipyrazolyl-
methane and its coordination chemistry with copper(II) in
oxalate-bridged dinuclear compounds to probe ligand-metal
coupling interactions. By varying the donor ability of the
axial ligand, we have slightly tuned the antiferromagnetic
coupling between metal centers. All four compounds are
strongly antiferromagnetically coupled. Structural parameters
taken from the single-crystal X-ray data indicate that the
parameter τ somewhat correlates structure with the degree
of antiferromagnetic coupling. However, even in this small
set of very structurally similar compounds, these simple
parameters fail to provide any concrete predictive qualities.
Future work will include the use of noncoordinating anions
(i.e. triflate) to probe magneto-structural correlations.
Acknowledgment. J.L.S. acknowledges the GAANN
fellowship for financial support. C.J.Z. acknowledges the
(20) Felthouse, T. R.; Laskowski, E.; Hendrickson, D. N. Inorg. Chem.
1977, 16, 1077-1089.
5066 Inorganic Chemistry, Vol. 44, No. 14, 2005

Cu(II)-Oxalato Magneto-Structural Correlations
Supporting Information Available: Magnetic susceptibility for
compounds 1-3, isotropic crystallographic refinement data tables
for 3, and crystallographic data in CIF format for compounds 1, 2,
and 4. This material is available free of charge via the Internet at
http://pubs.acs.org.
University of Akron for a faculty research grant (FRG-1524).
G.T.Y. acknowledges the National Science Foundation
(Grant CHE-023488) and the Virginia Tech ASPIRES
program for each providing partial support for the purchase
of the SQUID magnetometer. D.E.B and C.G. acknowledge
Wayne State University.
IC048229T
Inorganic Chemistry, Vol. 44, No. 14, 2005 5067