Anion influence on the structure and magnetic properties of a series of multidimensional pyrimidine-2-carboxylato-bridged copper(II) complexes

Article abstract of DOI:10.1021/ic800625w

Seven new polynuclear copper(II) complexes of formula [Cu(μ-pymca) ₂] (1) (pymca^- = pyrimidine-2-carboxylato), [Cu(μ-pymca)Br] (2), [Cu(μ-pymca)CI] (3), [Cu(μ-pymca)(SCN)(H ₂O)]·4H₂O (4), [Cu(μ-pymca)N₃] (5), [Cu₂(μ_1,5-dca)₂(pymca)₂] (6) (dca = dicyanamide), and K{[μ-Au(CN)₂]₂[(Cu(NH ₃)₂)₂(μ-pymca)]}[Au(CN)₂] ₂ (7) have been synthesized by reactions of K-pymca with copper(II) ions in the presence of different counteranions. Compound 1 is a linear neutral chain with a carboxylato bridging ligand in a syn-anti coordination mode, whereas complexes 2 and 3 consist of cationic linear chains with cis and trans bis(chelating) pymca bridging ligands. Complex 4 adopts a helical pymca-bridged chain structure. In complex 5, zigzag pymca-bridged chains are connected by double end-on azide bridging ligands to afford a unique honeycomb layer structure. Complex 6 is a centrosymmetric dinuclear system with double μ_1,5-dicyanamide bridging ligands and pymca end-cap ligands. Complex 7 is made of pymca-bridged dinuclear [Cu(NH₃) ₂(μ-pymca)Cu(NH₃)₂]³⁺ units connected by [Au(CN)₂]^- anions to four other dinuclear units, giving rise to cationic (4,4) rectangular nets, which are linked by aurophilic interactions to afford a singular 3D network. Variable-temperature magnetic susceptibility measurements show that complex 1 exhibits a very weak antiferromagnetic coupling through the syn-anti (equatorial-axial) carboxylate bridge (J = -0.57 cm^-1), whereas complexes 2-4 and 7 exhibit weak to strong antiferromagnetic couplings through the bis(chelating) pymca bridging ligand (J = -17.5-276.1 cm^-1). Quantum Monte Carlo methods have been used to analyze the experimental magnetic data for 5, leading to an antiferromagnetic coupling (J = -34 cm^-1) through the pymca ligand and to a ferromagnetic coupling (J = 71 cm^-1) through the azide bridging ligands. Complex 6 exhibits a very weak antiferromagnetic coupling through the dicyanamide bridging ligands (J = -5.1 cm^-1). The magnitudes of the magnetic couplings in complexes 2-5 have been explained on the basis of the overlapping between magnetic orbitals and DFT theoretical calculations.

Full text of DOI:10.1021/ic800625w

Inorg. Chem. 2008, 47, 8143-8158
Anion Influence on the Structure and Magnetic Properties of a Series of
Multidimensional Pyrimidine-2-carboxylato-Bridged Copper(II) Complexes
Jose´ Sua´rez-Varela,† Antonio J. Mota,† Hakima Aouryaghal,‡ Joan Cano,*,§ A. Rodr´ıguez-Die´guez,†
Dominique Luneau,| and Enrique Colacio*,†
Departamento de Qu´ımica Inorga´nica, UniVersidad de Granada, AV. FuentenueVa S/N, 18071
Granada, Spain, Departement de Chimie, UniVersite´ Abdelmalek Essaadi, Faculte´ des Sciences,
P.O. 2121, Te´touan, Marocco, Departament de Qu´ımica Inorga`nica and Institut de Recerca de
Qu´ımica Teo`rica i Computacional (IQTC), UniVersitat de Barcelona, AV. Diagonal 647,
08027 Barcelona, Spain, and Laboratoire des Multimateriaux et Interfaces, (UMR 5615),
UniVersite´ ClaudeBernard Lyon-1, 69622 Villeurbanne cedex, France
Received April 11, 2008
Seven new polynuclear copper(II) complexes of formula [Cu(µ-pymca)2] (1) (pymca- ) pyrimidine-2-carboxylato),
[Cu(µ-pymca)Br] (2), [Cu(µ-pymca)Cl] (3), [Cu(µ-pymca)(SCN)(H2O)]·4H2O (4), [Cu(µ-pymca)N3] (5), [Cu2(µ1,5-
dca)2(pymca)2] (6) (dca ) dicyanamide), and K{[µ-Au(CN)2]2[(Cu(NH3)2)2(µ-pymca)]}[Au(CN)2]2 (7) have been
synthesized by reactions of K-pymca with copper(II) ions in the presence of different counteranions. Compound 1
is a linear neutral chain with a carboxylato bridging ligand in a syn-anti coordination mode, whereas complexes
2 and 3 consist of cationic linear chains with cis and trans bis(chelating) pymca bridging ligands. Complex 4 adopts
a helical pymca-bridged chain structure. In complex 5, zigzag pymca-bridged chains are connected by double
end-on azide bridging ligands to afford a unique honeycomb layer structure. Complex 6 is a centrosymmetric
dinuclear system with double µ1,5-dicyanamide bridging ligands and pymca end-cap ligands. Complex 7 is made
of pymca-bridged dinuclear [Cu(NH3)2(µ-pymca)Cu(NH3)2]3+ units connected by [Au(CN)2]- anions to four other
dinuclear units, giving rise to cationic (4,4) rectangular nets, which are linked by aurophilic interactions to afford a
singular 3D network. Variable-temperature magnetic susceptibility measurements show that complex 1 exhibits a
very weak antiferromagnetic coupling through the syn-anti (equatorial-axial) carboxylate bridge (J ) -0.57
cm-1), whereas complexes 2-4 and 7 exhibit weak to strong antiferromagnetic couplings through the bis(chelating)
pymca bridging ligand (J ) -17.5-276.1 cm-1). Quantum Monte Carlo methods have been used to analyze the
experimental magnetic data for 5, leading to an antiferromagnetic coupling (J ) -34 cm-1) through the pymca
ligand and to a ferromagnetic coupling (J ) 71 cm-1) through the azide bridging ligands. Complex 6 exhibits a
very weak antiferromagnetic coupling through the dicyanamide bridging ligands (J ) -5.1 cm-1). The magnitudes
of the magnetic couplings in complexes 2-5 have been explained on the basis of the overlapping between magnetic
orbitals and DFT theoretical calculations.
Introduction
netism, molecular absorption, catalysis, ion-exchange, and
so forth.1 Among them, molecule-based magnetic materials2
have been commonly prepared through a bottom-up ap-
proach, connecting paramagnetic transition metal ions with
appropriate bridging ligands. The information encoded in the
transition metal and organic ligands directs the self-assembly
of these building blocks in a programmed way. The metal
ions are the source of magnetic moments, whereas the
bridging ligands allow for the magnetic exchange coupling
between the magnetic centers. Only a few polyatomic
During the two past decades, crystal engineering tech-
niques have been widely used for the design and synthesis
of coordination framework materials with specific solid-state
properties, such as electrical conductivity, molecular mag-
* Authors to whom correspondence should be addressed. E-mail:
ecolacio@ugr.es (E.C.).
†
Universidad de Granada.
Universite´ Abdelmalek Essaadi.
Universitat de Barcelona.
Universite´ ClaudeBernard Lyon-1.
‡
§
|
10.1021/ic800625w CCC: $40.75  2008 American Chemical Society
Inorganic Chemistry, Vol. 47, No. 18, 2008 8143
Published on Web 08/13/2008

Sua´rez-Varela et al.
bridging ligands have been shown to be able to mediate
strong magnetic coupling between transition metal ions,2a
and consequently there still exists great interest in the search
for new bridging ligands which can generate magnetic
materials with structural and topological novelty and intrigu-
ing magnetic properties. Recently, we succeeded in obtaining
by the hydrothermal reaction of 2-cyanopyrimidine and either
CoCl2 · 6H2O or FeCl2 · 4H2O honeycomb-layered metal
complexes [M2(µ-pymca)3]OH · H2O containing a new bis-
bidentate ligand (pymca ) pyrimidine-2-carboxylato), which
was generated in situ from the hydrolysis of 2-cyanopyri-
midine.3 The pymca ligand, which can be considered as an
intermediate case between bipyrimidine and oxalato, is able
to transmit efficiently antiferromagnetic interactions between
metal ions. After this pioneering work and by using the same
synthetic strategy, Eddaoudi et al.4 and Gao et al.5 have
succeeded in preparing two zeolite-like metal-organic frame-
works constructed from pymca and Cd(II) ions, which exhibit
interesting adsorption properties. Because pymca offers donor
atom sets and charge-balance requirements other than those
of bipyrimidine and oxalate and because it possesses great
potential for the design of new functional magnetic materials
based on coordination polymer networks, we decided to
undertake the synthesis of pymca and to explore its coor-
dinative ability against metal ions using mild conditions. In
this paper, we report the syntheses, structures, and magnetic
properties of a series of Cu(II)-pymca complexes, which
show wide structural diversity: (a) dinuclear [Cu2(µ1,5-
dca)2(pymca)2], (b) linear [Cu(µ-pymca)2] and helical [Cu(µ-
pymca)(SCN)(H2O)] ·4H2O chains, (c) (4,4) rectangular grid
[Cu(µ-pymca)Cl] and [Cu(µ-pymca)Br], (d) (6,3) honeycomb
layer [[Cu(µ-pymca)N3], and (e) 3D network K{[µ-
Au(CN)2]2[(Cu(NH3)2)2(µ-pymca)]}[Au(CN)2]2. It is of inter-
est that the nature of the copper(II) counterions has an
important influence on the structure of the pymca-bridged
copper(II) assemblies 1-7. Nowadays, the study of the
influence of the anionic counterion on the final structure is
an area of great research interest and activity.6
Experimental Section
All analytical reagents were purchased from commercial sources
and used without further purification. 2-Cyanopyrimidine was
prepared according to a previously described procedure.7
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Withersby, M. A.; Schro¨der, M. Coord. Chem. ReV. 1999, 183, 117.
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Evans, R. C.; Douglas, P.; Winscom, C. J. Coord. Chem. ReV. 2006,
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Foxman, B. M.; Ward, M. D. MRS Bull. 2007, 32 (7). (q) Robin, A. Y.;
Fromm, K. M. Coord. Chem. ReV. 2006, 250, 2127.
Preparation of the Compounds. H-pymca. A suspension of
2-cyanopyrimidine (2 g, 19 mmol) in 50 mL of water was treated
with KOH (2,24 g, 40 mmol) and the mixture refluxed for 2 h.
The resulting solution was neutralized with HCl 2N, and then the
solvent was removed in a vacuum. The obtained crude product was
treated with CH2Cl2; the KCl was filtered off and the filtrate
1
concentrated to afford the H-pymca acid in a yield of 40%. H
NMR (δ): H3 and H5 (8.76, doublet), H4 (7.44, triplet). 13C NMR
(δ): C4 (122.7), C3 and C5 (157.8), C1 (160.7), C7OOH (170.4).
Cu(pymca)2 (1). The reaction in a water/methanol mixture (50:
50, 30 mL) of the K-pymca salt, generated in situ from H-pymca
(0.012 g, 0.1 mmol) and KOH (0.006 g, 0.1 mmol), and
Cu(NO3)2 ·4H2O (0.013 g, 0.05 mmol) in a 2:1 molar ratio led to
a blue solution, which kept at room temperature for several days
gave rise to blue crystals of complex 1, which were filtered off
and air-dried. Alternatively, compound 1 can be obtained from the
reaction between 2-cyanopyrimidine and Cu(Ac)2 ·H2O in MeOH
at reflux for 2 h. Yield: ca 60%. Anal. calcd for C10H6N4O4Cu: C,
38.78; H, 1.95; N, 18.09. Found: C, 38.90; H, 2.05; N, 17.97. IR
(KBr, cm-1): 3080, ν(CH); 1641, ν(COO)as; 1597, ν(CdC); 1398,
ν(COO)s.
(2) (a) Magnetism: Molecules to Materials; Miller, J. S.; Drillon, M., Eds.;
Wiley-VCH: Weinheim, Germany, 2005; Vol. I-V. (b) Khan, O. AdV.
Inorg. Chem. 1995, 43, 179. (c) Miller, J. S.; Epstein, A. MRS Bull.
2000, 25, 11. (d) Khan, O. Acc. Chem. Res. 2000, 33, 647. (e)
Verdaguer, M.; Bleuzen, A.; Marvaud, V.; Vaissermann, J.; Seuleiman,
M.; Desplanches, C.; Scuiller, A.; Train, C.; Garde, R.; Gelly, G.;
Lomenech, C.; Rosenman, I.; Veillet, P.; Cartier, C.; Villain, F. Coord.
Chem. ReV. 1999, 190, 1023. (f) Ohba, M.; Okawa, K. Coord. Chem.
ReV. 2000, 198, 313. (g) Batten, S. R.; Murray, K. S. Coord. Chem.
ReV. 2003, 246, 103. (h) Sorai, M.; Nakano, M.; Miyazaki, Y. Chem.
ReV. 2006, 106, 976. (i) Przychodsen; Korzeniak, T.; Podgajni, R.;
Sieklucka, B. Coord. Chem. ReV. 2006, 250, 2234. (j) Beltran, L. M.;
Long, J. R. Acc. Chem. Res. 2005, 38, 325. (k) Arom´ı, G.; Brechin,
E. K. Struct. Bonding (Berlin) 2006, 122, 1. (l) Rebilly, J.-N.; Rebilly,
T.; Mallah, Struct. Bonding (Berlin) 2006, 122, 103. (m) Cornia, A.;
Costantino, A. F.; Zobbi, L.; Caneshi, A.; Gatteschi, D.; Mannini, M.;
Sessoli, R. Struct. Bonding (Berlin) 2006, 122, 133. (n) Gatteschi,
D.; Sessoli, Villain, J. Molecular Nanomagnets; Oxford University
Press: Oxford, U.K., 2006. (o) Lescoue¨zec, R.; Toma, L. M.;
Vaissermann, J.; Verdaguer, M.; Delgado, F. S.; Ruiz-Pe´rez, C.; Lloret,
F.; Julve, M. Coord. Chem. ReV. 2005, 249, 2691. (p) Coulon, C.;
Miyasaka, H.; Cle´rac, R. Struct. Bonding (Berlin) 2006, 122, 163. (q)
Bernot, K.; Bogani, L.; Caneschi, A.; Gatteschi, D.; Sessoli, R. J. Am.
Chem. Soc. 2006, 128, 7947. (r) Real, J. A.; Gaspar, A. B.; Niel, V.;
Mun˜oz, M. C. Coord. Chem. ReV. 2003, 236, 121. (s) Garc´ıa, Y.;
Niel, V.; Mun˜oz, M. C.; Real, J. A. Top. Curr. Chem. 2004, 233,
229.
[Cu(µ-pymca)Br] (2). The reaction of an aqueous solution (20
mL) of the K-pymca salt, generated in situ from H-pymca (0.031
g, 0.25 mmol) and KOH (0.014 g, 0.25 mmol), and CuBr2 (0.167
g, 0.75 mmol) in a 1:3 molar ratio afforded a green-blue solution,
which kept at room temperature for a week led to green crystals of
2, which were filtered off and air-dried. Yield: ca 52%. Anal. calcd
(6) (a) Khlobysov, A. N.; Blake, A. J.; Champness, N. R.; Lemenovski,
D. A.; Majouga, A. G.; Zyk, N. V.; Schro¨eder, M. Coord. Chem. ReV.
2001, 222, 155. (b) Hannon, M. J.; Painting, C. L.; Plummer, E. A.;
Childs, L. J.; Alcock, N. W. Chem.sEur. J. 2002, 8, 2225. (c) D´ıaz,
P.; Benet-Buchholz, J.; Vilar, R.; White, A. P. J. Inorg. Chem. 2006,
45, 1617. (d) Bisiwas, C.; Mukherjee, P.; Drew, M. G. B.; Go´mez-
Garc´ıa, C. J.; Clemente-Juan, J. M.; Ghosh, A. Inorg. Chem. 2007,
46, 10771. (e) Mukherjee, P.; Drew, M. G. B.; Estrader, M.; D´ıaz,
C.; Gosh, A. Inorg. Chim. Acta 2008, 361, 161. (f) Custelcean, R.
Chem. Commum. 2006, 295.
(3) Rodr´ıguez-Die´guez, A.; Cano, J.; Kiveka¨s, R.; Debdoubi, A.; Colacio,
E. Inorg. Chem. 2007, 46, 2503.
(4) Sava, D. F.; Kravtsov, V. C.; Nouar, F.; Wojtas, L.; Eubank, J. F.;
Eddaoudi, M. J. Am. Chem. Soc. 2008, 130, 3768.
(5) Zhang, J.-Y.; Cheng, A.-L.; Yue, Q.; Sun, W. W.; Gao, E.-Q. Chem.
Commun. 2008, 847.
(7) Case, F. H.; Koft, E. J. Am. Chem. Soc. 1959, 81, 905.
8144 Inorganic Chemistry, Vol. 47, No. 18, 2008

Pyrimidine-2-carboxylato-Bridged Cu(II) Complexes
Table 1. Crystallographic Data and Structural Refinement Details for Compounds 1-7
compound
1
2
3
4
5
6
7
chemical formula C10H6CuN4O4 C5H3BrCuN2O2 C5H3ClCuN2O2 C6H3CuN3O2S·3.33H2O C5H3CuN5O2 C7H5CuN5O3 C13H3Au4Cu2KN14O2
M/gmol-1
T (K)
λ/Å
cryst syst
space group
a/Å
b/Å
c/Å
R/deg
ꢁ/deg
γ/deg
V/Å3
309.73
293(2)
0.71073
monoclinic
P21/c
5.1422(5)
13.2586(11)
7.6747(7)
90.00
93.961(2)
90.00
522.00(8)
2
1.971
2.110
1.072
266.54
293(2)
222.08
298(2)
0.71073
orthorhombic
Pbcm
6.5488(6)
9.4281(8)
10.6417(9)
90.00
90.00
90.00
657.05(10)
4
2.245
304.71
293(2)
228.67
293(2)
270.70
293(2)
1341.35
293(2)
0.71073
monoclinic
C2/c
19.731(5)
15.198(4)
9.498(2)
90.00
95.615(4)
90.00
2834.6(12)
4
3.143
22.282
1.029
0.71073
orthorhombic
Pmc21
5.2929(8)
6.7384(10)
9.8478(14)
90.00
90.00
90.00
351.23(9)
2
2.520
0.71073
trigonal
0.71073
monoclinic
C2/c
9.7896(8)
11.6751(10)
13.2753(11)
90.00
100.925(10)
90.00
1489.8(2)
8
0.71073
triclinic
j
j
R3
P1
21.5359(10)
21.5359(10)
13.2232(13)
90.00
6.5203(13)
8.1470(16)
9.998(2)
84.71(3)
72.64(3)
67.20(3)
467.16(16)
2
90.00
120.00
5311.2(6)
18
1.677
2.039
Z
F (g cm-3
)
2.039
2.901
1.063
1.924
2.337
1.047
µ (mm-1
)
8.737
1.097
3.666
1.135
GOF on F2
1.036
R1 [I > 2σ(I)]
0.0258
wR2 [I > 2σ(I)] 0.0725
0.0270
0.0745
0.0249
0.0745
0.0584
0.1660
0.0341
0.0796
0.0239
0.0639
0.0476
0.1274
for C5H3N2O2CuBr: C, 22.53; H, 1.13; N, 10.51. Found: C, 22.78;
H, 1.32; N, 10.05. IR (KBr, cm-1): 3098, 3074, ν(CH); 1629,
ν(COO)as; 1605, ν(CdC); 1392, ν(COO)s.
[Cu(µ-pymca)Cl] (3). This compound was prepared as blue
crystals by following the same method as for 2, but using
CuCl2 ·2H2O (0.127 g, 0.75 mmol) instead of CuBr2. Yield: ca 57%.
Anal. calcd for C5H3N2O2CuCl: C, 27.04; H, 1.36; N, 12.61. Found:
C, 27.42; H, 1.62; N, 12.15. IR (KBr, cm-1): 3106, 3079, ν(CH);
1641, ν(COO)as; 1609, ν(CdC); 1368, ν(COO)s.
[Cu(µ-pymca)(SCN)(H2O)] ·4H2O (4). To the blue aqueous
solution (20 mL) obtained from K-pymca salt, generated in situ
from H-pymca (0.031 g, 0.25 mmol) and KOH (0.014 g, 0.25
mmol), and CuCl2 ·2H2O (0.127 g, 0.75 mmol) was added with
stirring a water solution (10 mL) of KSCN (0.072 g, 0.75 mmol).
The resulting blue solution was allowed to stand at room temper-
ature for several days, whereupon blue crystals of 3 formed, which
were filtered off and air-dried. Yield: ca 62%. Anal. calcd for
C6H11N3O6CuS: C, 22.75; H, 3.50; N, 13.26. Found: C, 22.81; H,
3.80; N, 12.95. IR (KBr, cm-1): 3416, ν(OH); 3098, 3074, ν(CH);
2104, ν(Ct N); 1669, ν(OH); 1654, ν(COO)as; 1598, ν(CdC); 1384,
ν(COO)s.
ν(NH); 3062, ν(CH); 2175, 2137, ν(Ct N); 1649, ν(COO)as; 1583,
ν(CdC); 1374, ν(COO)s.
Physical Measurements. Elemental analyses were carried out
at the “Centro de Instrumentacio´n Cient´ıfica” (University of
Granada) on a Fisons-Carlo Erba analyzer model EA 1108. The
IR spectra on powdered samples were recorded with a ThermoNi-
colet IR200FTIR by using KBr pellets. Magnetization and variable-
temperature (1.9-300 K) magnetic susceptibility measurements on
polycrystalline samples were carried out with a Quantum Design
SQUID operating at a field of 0.5 T. The experimental susceptibili-
ties were corrected for the diamagnetism of the constituent atoms
by using Pascal’s tables. It should be noted at this point that the
powder X-ray diffractograms of the bulk samples used for the
magnetic measurements match well those calculated from the crystal
structures (see Figure S1, Supporting Information).
X-Ray Crystallography. Single-crystal data collections for 1-7
were carried out on a Bruker Smart Apex diffractometer using
graphite monochromatized Mo KR radiation. The structures were
solved by direct methods and refined on F2 by the SHELXL97
program.8 Hydrogen atoms bonded to water molecules could not
be reliably positioned, but the rest of the hydrogen atoms were
treated as riding atoms using the SHELX97 default parameters. A
summary of the crystallographic data and structure refinements is
given in Table 1.
Compounds [Cu(µ-pymca)N3] (5) and [Cu2(µ-dca)2(pymca)2] (6)
were prepared by following the same method as for (4), but using
NaN3 (0.049 g, 0.75 mmol) and Na(dca) (0.066 g, 0.75 mmol)
instead of KSCN, respectively.
[Cu(µ-pymca)N3] (5). Yield: ca 37%. Anal. calcd for
C5H3N5O2Cu: C, 26.26; H, 1.32; N, 30.62. Found: C, 26.45; H,
1.52; N, 30.18. IR (KBr, cm-1): 3108, 3084, ν(CH); 2069, 2043,
ν(N3); 1656, ν(COO)as; 1592, ν(CdC); 1367, ν(COO)s.
[Cu2(µ-dca)2(pymca)2] (6). Yield: ca 46%. Anal. calcd for
C7H3N8O2Cu: C, 28.53; H, 1.03; N, 38.02. Found: C, 28.70; H,
1.22; N, 38.18. IR (KBr, cm-1): 3092, 3067, ν(CH); 2422, 2335,
2268, 2197, ν(Ct N); 1656, ν(COO)as; 1592, ν(CdC); 1367,
ν(COO)s.
K{[µ-(CN)2]2[(Cu(NH3)2)2(µ-pymca)]}[Au(CN)2]2 (7). To 20
mL of water were successively added pyrimidin-2-carboxylic acid
(0.0062 g, 0.05 mmol) and CuCl2 ·2H2O (0.017 g, 0.1 mmol). The
mixture was stirred for 5 min, and then 10 mL of concentrated
ammonia was added. A solution of K[Au(CN)2] (0.058 g, 0.2 mmol)
in 10 mL of water was then added, and the resulting solution was
filtered. After 24 h, blue crystals of 7 appeared, which were
collected by filtration and dried in the air. Yield: ca 54%. Anal.
calcd for C13H15N14O2Cu2Au4: C, 11.54; H, 1.11; N, 14.49. Found:
C, 11.82; H, 1.28; N, 14.35. IR (KBr, cm-1): 3360, 3276, 3173,
Computational Details. All theoretical calculations were carried
out with the hybrid B3LYP method,9 as implemented in the
Gaussian 03 program.10 A quadratic convergence method was
employed in the self-consistent field process.11 The triple-ꢀ-quality
basis set proposed by Ahlrichs and co-workers was used for all
atoms.12 The calculations were performed on the complexes built
from the experimental geometries. More detailed information about
the models can be found in the discussion of the results. The
electronic configurations used as starting points were created using
Jaguar 6.0 software.13 The approach that we used herein to
determine the exchange coupling constants for polynuclear com-
plexes has been described in detail elsewhere.14
(8) Sheldrick, G. M. SHELX 97; University of Go¨ttingen: Go¨ttingen,
Germany, 1997.
(9) (a) Becke, A. D. Phys. ReV. A: At., Mol., Opt. Phys. 1988, 38, 3098.
(b) Lee, C. T.; Yang, W. T.; Parr, R. G. Phys. ReV. B: Condens. Matter
Mater. Phys. 1988, 37, 785. (c) Becke, A. D. J. Chem. Phys. 1993,
98, 5648.
Inorganic Chemistry, Vol. 47, No. 18, 2008 8145

Sua´rez-Varela et al.
Figure 1. Perspective view of the chain structure of 1.
Results and Discussion
tonate the H-pymca ligand, and it coordinates temporarily
or permanently to copper(II) ions.
The reaction of K-pymca, generated in situ from 2-car-
boxypyrimidine and KOH, with copper(II) salts was shown
to be strongly dependent on the anion present in the reaction
solution and the metal-to-ligand molar ratio. When either
1:1 or 1:2 metal-to-ligand molar ratios were used, only the
neutral complex [Cu(pymca)2] (1) was obtained. However,
in the presence of an excess of copper salt (3:1 metal-to-
ligand molar ratio), the products were those of the type
Crystal Structures. The structure of 1 is made of planar
[Cu(pymca)2] units connected by weak Cu-O axial interac-
tions to afford linear chains parallel to the a axis (Figure 1).
Cu(II) ions, which lie on a center of symmetry, exhibit a
4 + 2 CuO4N2 coordination polyhedron with tetragonally
distorted octahedral geometry. The equatorial positions at
about 2 Å are occupied by two nitrogen atoms and two
oxygen atoms belonging to two pymca bridging ligands,
which adopt a trans-planar configuration. Two carboxylate
oxygen atoms, O2, are situated in axial positions at a longer
distance of 2.729(2) Å. Neighboring [Cu(pymca)2] units are
held together by a pair of complementary weak Cu-O2
interactions, which ultimately lead to the chain structure with
syn-anti carboxylate bridges. The Cu · · · Cu distance along
the chain is 5.142(1) Å, whereas the shortest Cu · · ·Cu
interchain distance is 7.660(1) Å.
[Cu(pymca)X] (X ) Br- (2), Cl- (3), SCN- (4), N3 (5),
-
and [N(CN)2]- (6)). Compounds 4, 5, and 6 were actually
prepared from the methatetical substitution in a solution of
2 of the chloride by the corresponding anion. Although there
is not any experimental evidence supporting the mechanism,
we tentatively propose that the formation of 7 might occur
in two steps. First, the reaction of H-pymca with an excess
of CuCl2 ·2H2O (2:1 metal-to-ligand molar ratio) in aqueous
ammonia would lead to the in situ generated dinuclear cation
[(Cu(NH3)2S2(µ-pymca)Cu(NH3)2S2]3+ (S ) NH3 or H2O).
Second, the peripheral S ligands would be replaced by
[Au(CN)2]- anions, leading to complex 7. The ammonia
plays an important role during the reaction; as it helps to
dissolve reagents and products, it acts as a base to depro-
The structure of 2 consists of cationic linear chains
[Cu(pymca)]nn+ running along the a axis and bromide anions
that weakly connect the chains to afford sheets parallel to
the ac plane (Figure 2).
Within each chain, copper(II) ions, which are bridged by
bisbidentate pymca ligands, exhibit a 4 + 2 CuN2O2Br2
chromophore with tetragonally distorted octahedral geometry.
Four short bonds of about 2 Å are formed with two nitrogen
atoms and two oxygen atoms belonging to two pymca
bridging ligands, which adopt a cis-planar configuration. The
axial positions are occupied by the bromine ligands at longer
distances of 2.788(1) and 2.792 Å (Figure 2, left). N-Cu-O
angles markedly differ from 90° as a consequence of the
small bite angle of the pymca ligand (N-Cu-O ) 83.69(10)
Å). It should be noted that each chain made of [Cu(pymca)]+
units has two planes of symmetry orthogonal to the plane of
the pymca ligands, one along the Cu-Br-Cu-Br line and
the other one along the CPh-CCOO bond, and therefore the
chains are strictly planar with the aromatic ring displaying
the same orientation with respect to the chain. Neighboring
chains are turned to each other by 52.8°, and consequently
sheets are not planar but corrugated. The bromide bridging
anions are at the peaks and valleys of the sheets (Figure 2,
right). Within each sheet, the Cu · · · ·Cu distances across the
pymca and bromide bridging ligands are 5.293(1) and 4.924
(1) Å, respectively, with a Cu-Br-Cu angle of 123.87(1).
Corrugated sheets running in the ac plane stack along the b
(10) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. J. A.; Vreven, T.; Kudin,
K.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone,
V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa,
J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene,
M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.;
Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev,
O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala,
P.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.;
Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas,
O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.;
Ortiz, J. V.; Cui, Q.; Baboul, A.; Clifford, S.; Cioslowski, J.; Stefanov,
B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, L.;
Fox, D. J.; Keith, T.:; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara,
A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong,
M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision C.02;
Gaussian, Inc.: Wallingford, CT, 2004.
(11) Bacskay, G. B. Chem. Phys. 1981, 61, 385.
(12) Scha¨fer, A.; Huber, C.; Ahlrichs, R. J. Chem. Phys. 1994, 100, 5829.
(13) Jaguar 6.0; Schro¨dinger, Inc.: Portland, 2005.
(14) (a) Ruiz, E.; Cano, J.; Alvarez, S.; Alemany, P. J. Comput. Chem.
1999, 20, 1391. (b) Ruiz, E.; Alvarez, S.; Rodr´ıguez-Fortea, A.;
Alemany, P.; Puoillon, Y.; Massobrio, C. In Magnetism: Molecules
to Materials; Miller, J. S., Drillon, M., Eds.; Wiley-VCH: Weinheim,
Germany, 2001; Vol. II, p 5572. (c) Ruiz, E.; Rodriguez-Fortea, A.;
Cano, J.; Alvarez, S.; Alemany, P. J. Comput. Chem. 2003, 24, 982.
(d) Ruiz, E.; Alvarez, S.; Cano, J.; Polo, V. J. Chem. Phys. 2005,
123, 164110.
8146 Inorganic Chemistry, Vol. 47, No. 18, 2008

Pyrimidine-2-carboxylato-Bridged Cu(II) Complexes
Figure 2. Perspective view of the 2D layer structure of 2 in the ac plane (left). View of the layer down the a axis (right).
Figure 3. Perspective view of the 2D layer structure of 3 in the bc plane (left). View of the layers down the c axis (right).
axis as a result of the bifurcated C-H · · · · Br interactions
involving the bromide anion on a sheet and the C3-H groups
of the same Cu(pymca)2 unit on a neighboring sheet with a
C · · · Br separation of 3.615(2) and 3.696(2) Å. The pyrimi-
dine aromatic rings are oriented in an alternated manner
above and below each sheet, leading to alternated CH · · · Br
interlayer interactions, the shortest Cu · · ·Cu interlayer sepa-
ration being 6.738 Å.
the neighboring sheet, with a C-Cl distance of 3.521 Å.
These interactions alternate below and above each sheet,
leading to a 3D network.
The main differences between both structures are (a) the
disposition of the pymca ligands around the copper(II) ion,
which is cis in 2 (pymca ligands along the chain are related
by a plane of symmetry) and trans in 3 (pymca ligands along
the chain are related by a center of symmetry), and (b) in
spite of the greater size of the bromide anion compared to
the chloride anion, Cu-Cl and Cu-Br distances are similar,
thus indicating that the Cu-Br interaction is stronger than
the Cu-Cl one.
The structure of [Cu(µ-pymca)(SCN)(H2O)] · 4H2O, 4,
consists of neutral helical pymca-bridged copper(II) chains
(extended along a 3-fold screw axis) and water molecules,
which are held together by hydrogen bonds (Figure 4).
The copper atom has a distorted octahedral mer-CuN3O3
chromophore. Four short bonds of about 2 Å are formed in
the equatorial plane with two pyrimidyl nitrogen atoms
belonging to two bridging pymca ligands, one thiocyanate
nitrogen atom and one carboxylate oxygen atom also
belonging to one of the pymca ligands. The axial positions
at longer distances are occupied by the carboxylato oxygen
atom of the other pymca ligand and the oxygen atom of the
water molecule. The dihedral angle between the mean planes
The structure of 3 is similar to that of 2 and consists of
n+
cationic linear chains [Cu(pymca)]n running along the
c axis and chloride anions connecting the chains to afford
a corrugated 2D network in the bc plane (Figure 3).
As in 2, the mean planes containing the pymca and
Cu(II) ions on neighboring chains form a dihedral angle
of 51.4°, leading to sheet corrugation. Owing to the center
of symmetry located on each copper(II) ion, the pymca-
containing chains are strictly planar, with the aromatic
ring displaying a zigzag distribution along the linear chain.
Within each sheet, the Cu · · · · Cu distance across the
pymca and chloride bridging ligands are 5.321(1) and
4.714(1) Å, respectively, with a Cu-Cl-Cu angle of
126.01(1)°. Corrugated sheets running in the bc plane stack
along the a axis as a result of the bifurcated Cl · · · H-C
interactions involving the chloride anion of a sheet and
two C3-H groups of two different [Cu(pymca)2] units of
Inorganic Chemistry, Vol. 47, No. 18, 2008 8147

Sua´rez-Varela et al.
Figure 4. Zig-zag chain structure of 4 (top). View along the c axis of the chain of 4 (bottom).
of two neighboring ligands along the chain is 88.15(1)°, while
the Cu · · · Cu · · · Cu angle is 117.6(1)°. Chains are linked by
hydrogen-bond interactions involving coordinated and non-
coordinated water molecules, leading to a supramolecular
arrangement of six chains around a 3-fold inversion axis,
with channels running through the crystals along the c-axis
direction. The O4 atoms belonging to noncoordinated water
molecules lie in channels at the 3-fold axis and are involved
in hydrogen-bond interactions with the sulfur atoms (S · · ·O4
) 3.331(2) Å). Since neighboring chains are related by
centers of symmetry, they exhibit opposite helicity, and
therefore the whole structure is not chiral.
The strong coordination of the thiocyanate ligand to the
copper(II) ion (Cu-N ) 1.914(4) Å) leads to the zigzag
helical chain structure instead of the linear chain structure
observed for 2 and 3.
The crystal structure of [Cu(µ-pymca)N3], 5, is made of
zigzag pymca-bridged copper(II) chains connected by double
end-on azide bridging ligands to afford a distorted (6,3)
honeycomb network in the ab plane (Figure 5).
There are two crystallographically independent copper(II)
ions in the structure, namely, Cu and Cu1, which exhibit
distorted octahedral CuN4O2 chromophores. In the Cu
coordination environment, two pyrimidyl and two azide
nitrogen atoms are located in equatorial positions, whereas
the carboxylate oxygen atoms from the pymca ligands are
coordinated in axial positions at a longer distance. Cu1 is
also coordinated to two pyrimidyl and two azide nitrogen
atoms and to two carboxylate oxygen atoms from the pymca
ligands. In this case, however, the cis Cu-Nazide bond
distances of 2.222(3) Å are the longest ones, and the Cu-O
bonds are located in the cis position. It should be noted that,
on one side of the honeycomb layer, and within each
hexagonal ring, the pyrimidine rings of the ligands are
Figure 5. The (6,3) honeycomb layer structure of 5 in the ab plane.
alternatively tilted (∼56°) toward and away from the normal
to the ring. Consequently, on the other side of the layer,
carboxylate groups follow the opposite sequence. The
dihedral angle between the planes of neighboring ligands is
83.68(2)°, whereas the dihedral angle between the mean
planes of the ligand and the Cu(N3)2Cu dinuclear fragment
is 86.75(2)°. The end-on azide bridging ligands are tilted
48.5(2)° with respect to the mean plane of the Cu(N3)2Cu
dinuclear fragment. Neighboring sheets, which are related
8148 Inorganic Chemistry, Vol. 47, No. 18, 2008

Pyrimidine-2-carboxylato-Bridged Cu(II) Complexes
Figure 6. Perspective view of the crystal structure of 6.
by centers of symmetry, present π-π stacking interactions
with a separation between mean planes of the phenyl rings
of 3.528(2)° (see Figure S2, Supporting Information). These
interactions lead to an ABAB... repeating pattern of the layers
along the c direction.
Although several examples of honeycomb layered com-
pounds with alternating bis(chelating) 2,2′-bipyrimidine and
double end-on azide bridging ligands have been reported so
far, none of them contains copper(II) ions.15 Moreover, as
far as we know, this is the first report concerning a
honeycomb structure in which two bis(chelating) bridges
alternate with one double end-on azide bridge within the
hexagonal rings. Such a unique structure may be a conse-
quence of the different charge balance requirements for
pymca and 2,2′-bipyrimidine ligands.
Figure 7. Asymmetric unit of 7.
formation of planar sheets, the shortest Cu · · · Cu interlayer
distance being 5.139 Å.
The crystal structure of 6 is made of centrosymmetric
dinuclear units [Cu(pymca)(µ-dca)2Cu(pymca)] connected
through hydrogen bonds to form a 2D network in the bc
plane (Figure 6).
The structure of K{[µ-Au(CN)2]2[(Cu(NH3)2)2(µ-pymca)]}-
[Au(CN)2]2, 7, is very similar to that previously reported by us
for {[µ-Au(CN)2]2[(Cu(NH3)2)2(µ-bpm)]}[Au(CN)2]216 and con-
sists of [Cu(NH3)2(µ-pymca)Cu(NH3)2] dinuclear units con-
nected to four other dinuclear units by [Au(CN)2]- anions,
giving rise to a cationic 2D (4,4) rectangular-grid network. The
charge of the structure is balanced by two noncoordinated
[Au(CN)2]- anions and one K cation (Figure 7).
Within the dinuclear fragment, the CuII ions exhibit a
CuN5O distorted octahedral coordination environment. In this
description, one nitrogen and one oxygen atom belonging
to a pymca ligand and two nitrogen atoms belonging to two
different [Au(CN)2]- anions are coordinated to the CuII ions
in the plane of the pymca bridging ligand, whereas, below
and above this plane, two NH3 molecules are coordinated.
The Cu-N distances are similar to those observed in {[µ-
Au(CN)2]2[(Cu(NH3)2)2(µ-bpm)]}[Au(CN)2]2, the distortion
of the CuN5O coordination polyhedron being through a
compression, where the axial distances Cu-NH3 are con-
siderably shortened with respect to those in the equatorial
plane. Since NH3 is a much stronger σ donor than pymca,
the ligand field asymmetry is dominant, leading to an unusual
tetragonally compressed geometry.
Within the dinuclear unit, copper(II) atoms are bridged
by two dca anions in an end-to-end fashion. Each copper
atom exhibits a square-pyramidal CuN3O2 chromophore
(Addison’s parameter τ ) 0).16 The basal plane is formed
by two nitrogen atoms of the dca bridging ligands and
one oxygen atom and one nitrogen atom belonging to the
bidentate pymca ligand with bond distances of about 2
Å. These four atoms are nearly coplanar, with a deviation
of the copper atom from the mean plane of 0.15 Å. The
apical position is occupied by a water molecule at a longer
distance (2.222(2) Å). The Cu · · · Cu distance through the
dca-bridging group is 7.114 Å. The dinuclear molecules
are connected by two couples of complementary hydrogen
bonds involving the water molecule and the O2 and N6
atoms belonging to the pymca ligand with donor-aceptor
distances of 2.932(2) Å (N6 · · · O3) and 2.731(2) Å
(O2 · · · O3). These hydrogen-bond interactions lead to the
(15) (a) De Munno, G.; Poerio, T.; Viau, G.; Julve, M.; Lloret, F.; Journaux,
Y.; Riveiere, E. Chem. Commun. 1996, 2587. (b) De Munno, G.; Julve,
M.; Viau, G.; Lloret, F.; Faus, J.; Viterbo, D. Angew. Chem., Int. Ed.
1996, 35, 1807. (c) Cortes, R.; Lezama, L.; Pizarro, J. L.; Arriortua,
M. I.; Rojo, T. Angew. Chem., Int. Ed. 1996, 35, 1810.
Within the planar rectangular-grid layer, two of the corners
are occupied by two CuII ions, whereas the other two are
found at the pymca ligands (Figure 8). The Cu · · ·Cu distance
(16) Addison, A. W.; Rao, T. N.; Reedijk, J.; van Rijn, J.; Verschoor, G. C.
J. Chem. Soc., Dalton Trans. 1984, 1349.
Inorganic Chemistry, Vol. 47, No. 18, 2008 8149

Sua´rez-Varela et al.
torsion angle between adjacent [Au(CN)2]- units.18c,19 In
keeping with this and with the short Au-Au distances
observed for 7, adjacent [Au(CN)2]- anions are staggered
with a C-Au1-Au2-C torsion angle of 57.5°. Neighboring
chains are further connected by weaker aurophilic interactions
with Au1-Au1 distances of 3.447 Å, affording a honeycomb-
like 2D network of gold atoms with interlayer distances of
9.65 Å. The (4,4) rectangular sheets and (6,3) honeycomb
sheets share the Au2 atoms and make dihedral angles of
51.61°, leading to a unique 3D network (Figure 9). K cations,
which are disordered on two positions, are located between
layers near the carboxylate part of the pymca ligands, with
K-O distances of 2.560(1) Å.
Magnetic Properties. The temperature dependences of ꢂM
and ꢂMT for 1 (ꢂM is the molar magnetic susceptibility per
Cu(II) ion) in an applied magnetic field of 0.5 T are displayed
in Figure S3 (Supporting Information). At room temperature,
the ꢂMT product (0.435 cm3 mol-1 K) matches well with the
spin-only value expected for one Cu(II) ion with S ) 1/2
and g ) 2.15 and remains almost constant down to about
25 K. Below this temperature, the ꢂMT product decreases to
0.346 cm3 mol-1 K at 2 K. This behavior is indicative of
the presence of a weak antiferromagnetic interaction between
the Cu(II) ions through the pymca bridging ligand. Since 1
is a linear chain complex, the susceptibility data were fitted
using the equation derived from the Bonner-Fischer calcula-
tion for an antiferromagnetic chain of equally spaced
copper(II) ions, on the basis of the isotropic Hamiltonian H
) -J ∑i (Si Si+1).20 The best fit parameters were J )
-0.57(1) cm-1 and g ) 2.16(1). The low value of the
exchange coupling constant, J, can be related to the nature
of the bridge between neighboring copper(II) ions. The
magnetic coupling in this compound takes place through the
carboxylate group of the ligand in a syn-anti conformation,
which links basal (short) and axial (long) positions on the
neighboring copper(II) ions. This exchange pathway produces
always very small coupling (either antiferro- or ferromag-
netic) regardless of the structural parameters of the bridge.21
This is because (i) the axial Cu-O2 distance is long and (ii)
the magnetic orbital on the Cu(II) is mainly located in the
equatorial plane, and therefore the spin density on its axial
position is expected to be very small. Nevertheless, the fact
that the O-Cu-O axis of the CuN2O4 distorted octahedral
coordination polyhedron is tilted away from the normal to
Figure 8. Planar rectangular grid layers of 7. The noncoordinated
dicyanoaurate anions are omitted for clarity.
Figure 9. Honeycomb 2D network of gold atoms formed by aurophilic
interactions.
across the pymca ligand is 5.786(2) Å, whereas the M · · ·Au
distances across the cyanide ligand are 5.192(2) and 5.298(2)
Å. Layers are stacked in such a way that the ammonia-
coordinated molecules are interdigitated and aligned above
and below each sheet with cavities in neighboring sheets,
giving rise to an ABAB... repeating pattern of layers, the
interlayer distance being 3.69 Å. This relatively short
interlayer separation is clearly due to the presence of
aurophilic interactions.
Dicyanoaurate anions and K cations, which play a filling
space and charge compensating role in the structure, are
located between layers. Gold atoms of bridging and non-
bridging dicyanoaurate anions are involved in short auro-
philic interactions (Au1-Au2 distances are in the range
3.15-3.16 Å), leading to a chain of gold atoms running along
the c direction (Figure 9) with a Au2-Au1-Au2 angle of
97.54(3)°. The Au-Au distances are among the shortest
observed for [Au(CN)2]- anions involved in aurophilic
interactions.17-19
(18) (a) Leznoff, D. B.; Lefebvre, J. Gold Bull. 2005, 38, 47, and references
therein. (b) Zhou, H.-B.; Wang, S.-P.; Dong, W.; Liu, Z.-Q.; Wang,
Q.-L.; Liao, D.-Z.; Jiang, Z.-H.; Yan, S.-P.; Cheng, P. Inorg. Chem.
2004, 43, 4552. (c) Assefa, Z.; Omary, M. A.; McBurnett, B. G.;
Mohamed, A. A.; Patterson, H. H.; Staples, R. J., Jr. Inorg. Chem.
2002, 41, 6274. (d) Colacio, E.; Lloret, F.; Kiveka¨s, R.; Sua´rez-Varela,
J.; Sundberg, M. R.; Uggla, R. Inorg. Chem. 2003, 42, 560. (e)
Lefebvre, J.; Callaghan, F.; Katz, M. J.; Sonier, J. E.; Leznoff, D. B.
Chem.sEur. J. 2006, 12, 6248. (f) Lefebvre, J.; Chartrand, D.; Leznoff,
D. B. Polyhedron 2007, 26, 2189. (g) Katz, M. J.; Kaluarachchi, H.;
Batchelor, R. J.; Bokov, A. A.; Ye, Z.-G.; Leznoff, D. B. Angew.
Chem., Int. Ed. 2007, 46, 8804.
(19) Colacio, E.; Lloret, F.; Kiveka¨s, R.; Sua´rez-Varela, J.; Sundberg, M. R.
Chem. Commum. 2002, 592.
Theoretical studies and experimental results have shown
that the Au · · · Au distances decrease with an increase in the
(20) Bonner, J. C.; Fischer, M. E. Phys. ReV. A: At., Mol., Opt. Phys. 1964,
135, 640.
(21) (a) Colacio, E.; Ghazi, M.; Kiveka¨s, R.; Moreno, J. M. Inorg. Chem.
2000, 39, 2882. (b) Baldoma´, R.; Monfort, M.; Ribas, J.; Solans, X.;
Maestro, M. Inorg. Chem. 2006, 45, 8144.
(17) Sua´rez-Varela, J.; Sakiyama, H.; Cano, J.; Colacio, E. Dalton Trans.
2007, 249.
8150 Inorganic Chemistry, Vol. 47, No. 18, 2008

Pyrimidine-2-carboxylato-Bridged Cu(II) Complexes
Figure 10. Plots of ꢂM (left) and ꢂMT (right) for 2. The solid line corresponds to the best fit.
the equatorial plane could lead to a non-negligible spin
density on the axial positions and, consequently, favor a weak
antiferromagnetic interaction.
The temperature dependences of ꢂM and ꢂMT for 2 (ꢂM is
the molar magnetic susceptibility per Cu(II) ion) in an applied
field of 0.5 T are given in Figure 10.
The value of the ꢂMT product at room temperature (0.186
cm3 mol-1 K) is significantly lower than that expected for
an uncoupled Cu(II) ion with g ) 2 (0.375 cm3 mol-1 K),
which is probably due to the existence of medium to strong
antiferromagnetic interactions between the copper(II) ions
mediated by the bisbidentate pymca bridging ligand. The ꢂMT
Figure 11. Temperature dependence of ꢂM (left) and ꢂMT (right) for 3.
product shows a continuous decrease with decreasing tem-
perature to reach a value of 0.0019 cm3 mol-1 K at 2 K,
thus supporting the presence of antiferromagnetic interactions
between Cu(II) ions. In keeping with this, the thermal
variation of ꢂM shows a maximum near 252 K. Below this
temperature, ꢂM decreases to reach a minimum at 20 K and
then increases to a value of 0.0009 cm3 mol-1 K at 2 K.
This low-temperature tail is due to the presence of a small
amount of mononuclear impurity. As indicated elsewhere,
complex 2 can be viewed as formed by cationic chains of
trans-pymca-bridged copper(II) ions connected by bromide
anions. The interaction through the bromide bridging ligand
is expected to be very small (almost negligible), owing to
the long Cu-Br bond distances and the coordination of the
bromide ligand in axial position where the spin density of
the unpaired electron is, if at all, very small (the dx2-y2
magnetic orbital is directed to the equatorial nitrogen and
oxygen atoms of the trans-coordinated pymca ligands). As
a result, only the J parameter through the bisbidentate pymca
bridges needs to be considered. Taking this into account,
the magnetic data were fitted to the equation for an
antiferromagnetic chain of equally spaced copper(II) ions,20
in which a P parameter was included to account for the
mononuclear impurity. The best fit parameters were J )
-276.1(1) cm-1, g ) 2.122(1), and P ) 0.002. The strong
antiferromagnetic interaction occurring in 2 can be ascribed
to the fact that the magnetic orbitals centered on nearest
neighbor copper ions are located in the plane of the pymca
bridging ligand, and therefore, they are favorably oriented
The solid line is a fit to the experimental data using the values given in the
text.
to overlap on both sides of the pymca group. Recently, we
have shown through theoretical calculations and experimental
results that the pymca ligand is able to produce medium to
strong antiferromagnetic interactions that are intermediate
between those produced by oxalate and 2,2′-bipyrimidine.3
Polynuclear copper(II) complexes with bisbidentate oxalato
bridging ligands that connect equatorial positions on planar,
square-based pyramidal or 4 + 2 tetragonally elongated
distorted octahedral copper(II) ions exhibit -J values in the
range 290-400 cm-1,22 whereas the analogous bipyrimidine-
bridged complexes show -J values in the range 139-238
cm-1.23 The J value for 2 of -276.1 cm-1 is, as expected,
intermediate between the above indicated values for the
oxalate and bipyrimidine-bridged dinuclear planar copper(II)
complexes.
The magnetic properties of complex 3 are shown in Figure
11 in the form of ꢂM and ꢂMT versus T.
The room temperature ꢂMT value (0.333 cm3 mol-1 K) is
slightly lower than that expected for magnetically isolated
copper(II) ions, thus suggesting the existence of an antifer-
romagnetic interaction between copper(II) ions. Upon cooling
from room temperature, the ꢂMT product steadily decreases
(22) Julve, M.; Verdaguer, M.; Gleizes, A.; Philoche-Levisalles, M.; Khan,
O. Inorg. Chem. 1984, 23, 3808.
(23) De Munno, G.; Julve, M.; Lloret, F.; Cano, J.; Caneschi, A. Inorg.
Chem. 1995, 34, 2048.
Inorganic Chemistry, Vol. 47, No. 18, 2008 8151

Sua´rez-Varela et al.
Figure 12. Experimental temperature dependence of ꢂM (left) and ꢂMT
(right) for 4. The solid line represents the best fitting.
Figure 13. Thermal dependence of ꢂM (per copper ion) in 5. The circles
and lines represent, respectively, the experimental data and the simulation
using the parameters obtained in the fit process.
and reaches a value of 0.01 cm3 mol-1 K. The temperature
dependence of ꢂM shows a maximum near 65 K, thus
confirming the existence of a weak to medium antiferro-
magnetic interaction between the copper(II) ions, which is
mainly mediated by the pymca-bridged ligands. The best fit
of the susceptibility data to the equation for an antiferro-
magnetic chain of equally spaced copper(II) ions,20 with the
inclusion of a P parameter to account for the mononuclear
impurity, leads to the following magnetic parameters: J )
-72.5(5) cm-1, g ) 2.125, and P ) 0.015. In spite of the
structural similarity between 2 and 3 (4 + 2 elongated
octahedral copper coordination environment and planar
Cu-pymca-Cu-pymca fragment), the J value for the latter
is significantly lower than for the former. In order to try to
justify the low J value observed for 3, we have performed
DFT theoretical calculations (see below in the DFT calcula-
tions section).
Figure 14. Topology interaction observed in 5 matching with an alternating
2D honeycomb.
temperature, however, ꢂMT gradually decreases down to 2
K, whereas the ꢂM versus T plot exhibits a maximum near
35 K (Figure 13).
As indicated elsewhere, compound 5 exhibits a honeycomb
layer structure, in which two pymca bridges alternate with
one double end-on azide bridge within the hexagonal rings
(Figure 14). Because an analytical law is not known for this
complicated network topology with local quantum S ) 1/2
spin moments, we have used quantum Monte Carlo (QMC)
methods to interpret the experimental magnetic data. The
QMC method used for this purpose was that called the
decoupled cell method, proposed by Homma et al., where
the full system is split into several subsystems (cells) that
have a size small enough to obtain the probability of each
configuration by diagonalization of the energy matrix.24,25
These probabilities are used to calculate the spin flip
probabilities, that is, the probability to reverse a local spin
moment in a certain spin configuration of the full system.
Thus, the applicability of this method will be restricted to
the temperature region where the spin correlation length is
shorter than the used cell. As a consequence, using larger
subsystems allow us to correctly apply this method at lower
temperatures.
The temperature dependences of ꢂM and ꢂMT (ꢂM being
the magnetic susceptibility per copper atom) for 4 are shown
in Figure 12.
The value of ꢂMT at room temperature (0.384 cm3mol-1K)
is characteristic of a magnetically isolated spin doublet. The
appearance of a maximum in the ꢂM versus T plot near 28
K and the decreasing of ꢂMT upon cooling suggest dominant
antiferromagnetic interactions. In keeping with the uniform
chain structure of 4, the susceptibility data were fitted to the
equation for an antiferromagnetic chain of equally spaced
copper(II) ions,20 leading to J ) -31.4(2) cm-1, g )
2.070(6), and P ) 0.01. As indicated above, the coordination
environment of the copper(II) ion in 4 adopts an elongated
rhombic geometry with three short Cu-N, one short Cu-O,
and two long Cu-O distances. The magnetic orbital, which
points toward the four nearest neighbors of the copper atom,
is alternatively located along the chain in the plane of the
pymca bridging ligand for a copper atom and in the plane
perpendicular to the pymca ligand for the neighbor copper
atom. Now, only the Cu-pyrimidine-Cu pathway is opera-
tive, and therefore the interaction along the chain decreases
significantly with regard to those observed for 2 and 3, where
both the pyrimidine and carboxylato pathways were opera-
tive.
In the MC methods, from the spin flip probabilities and
using a metropolis algorithm, we can generate a sampling
where the states more present are those having a more
important contribution in the partition function. This sam-
pling allows us to calculate the average magnetization at a
(24) Homma, S.; Sano, K.; Matsuda, H.; Ogita, N. Prog. Theor. Phys. Suppl.
1986, 127.
The ꢂMT values for 5 do not show significant change
(25) Homma, S.; Matsuda, H.; Ogita, N. Prog. Theor. Phys. Suppl. 1986,
75, 1058.
(about 0.38 cm3 mol-1 K) above 150 K. Below this
8152 Inorganic Chemistry, Vol. 47, No. 18, 2008

Pyrimidine-2-carboxylato-Bridged Cu(II) Complexes
Table 2. Coefficients in the Empirical Law That Connect the ꢂMT
Product with the Exchange Coupling Constants (Ja and Jb) and the
Temperature for an S ) 1/2 Ferro-Antiferro Alternating Honeycomb
Network with a Predominant Ferromagnetic Couplinga
i
j
Ri,j
ꢁi,j
i
j
Ri,j
ꢁi,j
0
0
1
1
2
2
0
1
0
1
0
1
0
0
0.0302978
1.0966
-0.333124
-73.0225
-24.5132
996.144
2
3
3
3
4
4
2
0
1
2
0
1
168.706
645.985
-893.562
-105.264
998.9025
484.125
177.991
979.162
-1168.74
315.186
2663.74
1291.0
0.389577
2.89813
-31.6167
-95.0719
Figure 15. Decomposition in small cells of a honeycomb network which
are used in the MC simulations. Broken lines refer to the global exchange
topology, whereas the bold ones represent the cell obtained in the
decomposition process. While the darkened circle is the central site in a
given cell, the white circles are the remaining sites in the same cell.
a This law is expressed as in eq 2. The parameters ꢁ and R are defined
as T/|Jb| and |Ja|/|Jb|, respectively. The coefficients that have been considered
zero do not appear in the table.
given temperature. The molar magnetic susceptibility can
be obtained from the fluctuations in the magnetization
through eq 1, where 〈M〉 and 〈M〉2 are the mean values of
the magnetization and its square, and N, ꢁ, and k have their
usual meaning.
Table 3. Coefficients in the Empirical Law That Connect the ꢂMT
Product with the Exchange Coupling Constants (Ja and Jb) and the
Temperature for an S ) 1/2 Ferro-Antiferro Alternating Honeycomb
Network with a Predominant Antiferromagnetic Couplinga
i
j
Ri,j
ꢁi,j
i
j
Ri,j
ꢁi,j
0
0
1
1
2
0
1
0
1
0
0
0
0.403422
-0.0362409
2
3
3
4
4
1
0
1
0
1
-0.123109
2.73691
1.21881
1.0447875
-0.072878625 -0.194343
-0.926531
8.66979
2.52666
2.7861
ꢂMT ) Nꢁ2 ⁄ k(〈M2〉 - 〈M〉2)
(1)
0.0154042 0.931194
0.149732
-0.484058 3.30334
In all simulations, the number of MC steps for each
temperature is 5 × 106/T (T in K). Thus, we included more
steps in the sampling at low temperatures where the correct
equilibrium requires more recorded data. A total of 10% of
the MC steps are employed for the thermalization of the
system; thus, we stocked the physical properties when the
equilibrium was reached. A model of 30 × 30 sites was used
in the simulations. Periodic boundary conditions were
introduced in all simulations. Deeper details on the MC
simulations can be found in ref 26.
2.24815
a This law is expressed as in eq 2. The parameters ꢁ and R are defined
as T/|Ja| and |Jb|/|Ja|, respectively. The coefficients that have been considered
zero do not appear in the table.
In our simulations, we have broken down the network in
small cells that include the central site that undergoes a spin-
flip and the first and second neighbors (see Figure 15).
Several ꢂMT versus T/|Ji| simulations have been done for
different values of the ratio |Jj|/|Ji| (R), where R takes values
that are comprised between 0 and 1. Thus, in all cases, the
value of |Ji| is larger than that presented by |Jj|. Because in
our spin network there are two exchange couplings, one
ferromagnetic and one antiferromagnetic, two sets of simula-
tions have been done, one of them being the strongest in
each set. Analytical laws for each case have been extrapo-
lated by fitting the data obtained from the Monte Carlo
simulations to a quotient between two polynomial functions
that depends on the temperature, allowing us to fix a constant
value at higher temperatures that corresponds with that
expected for an isolated copper(II) ion. These laws take the
form of eq 2. The values of its coefficients are shown in
Tables 2 and 3 for an S ) 1/2 alternating honeycomb network
with a ferro- and an antiferromagnetic coupling as the
predominant interaction, respectively.
Figure 16. Temperature dependence of ꢂM (left) and ꢂMT (right) for 6.
The solid line is a fit to the experimental data using the values given in the
text.
fact that the weakest coupling is antiferromagnetic, its effect
is prevailing in the ꢂMT versus T curve due to a larger
presence of it. The least-squares fits of the experimental data
give exchange coupling constant values of +71 ( 11 and
-34 ( 7 cm-1 for Ja and Jb, respectively. The g-factor value
found for the copper(II) ions in 5 is 2.038 ( 0.009. The
agreement factor, defined as F ) ∑[(ꢂMT)exp - (ꢂMT)calcd]2/
∑[(ꢂMT)exp ]2, is 2 × 10-5. These values for the exchange
coupling constants are weaker than those expected from the
experimental data in this and previous works.3,27 A detailed
analysis of these results has been done from DFT calculations
(see below).
4
1
∑ ∑ ai,j · ꢁi · Rj
g2Cu i)0j)0
ꢂMT )
(2)
4
1
The magnetic properties of complex 6 in the form of ꢂM
and ꢂMT versus T (ꢂM being the molar susceptibility per
dinuclear unit) are shown in Figure 16.
4
∑ ∑ bi,j · ꢁi · Rj
i)0j)0
Two conclusions were reached when both expressions
were used to reproduce the experimental data: (i) it was
necessary to include a paramagnetic impurity, which was
removed in the final data, and (ii) a predominant ferromag-
netic interaction is suggested in both cases. In spite of the
(26) Cano, J.; Journaux, J. In Magnetism: Molecules to Materials; Miller,
J. S., Drillon, M., Eds.; VCH: Weinheim, Germany, 2005.
(27) Ruiz, E.; Cano, J.; Alvarez, S.; Alemany, P. J. Am. Chem. Soc. 1998,
120, 11122.
Inorganic Chemistry, Vol. 47, No. 18, 2008 8153

Sua´rez-Varela et al.
was included in the equation. The best fit parameters were
J ) -17.50(1), g ) 2.147(1), and P ) 0.04(2). The relatively
low J value for 7 can be explained by the unusual axial
compressed geometry of the copper(II) ion. In a simple way,
we can predict that an axial compression causes a decrease
and an increase of the energies of x2-y2 and z2 orbitals,
respectively, which could lead to a change in the nature of
the magnetic. Since only two-thirds of the electronic density
of the z2 orbital is placed in the equatorial plane and not
placed only in the direction of the metal-ligand bond, there
is a decrease in the overlapping between this orbital and
the ligand orbitals, similar to a case where the x2-y2 is the
magnetic orbital. As a consequence, a weakening of the
antiferromagnetic exchange coupling is expected. Recently,
theoretical calculations and experimental results have con-
firmed this prediction in the analogous bipyrimidine-bridg-
ed compound {[µ-Au(CN)2]2[(Cu(NH3)2)2(µ-bpm)]}[Au-
(CN)2]2.17Although pymca is a better mediator of the
magnetic exchange interaction than bpm, the exchange
coupling constant for this latter compound (J ) -20 cm-1)
is higher than that observed for 7. This fact might be due to
small structural differences affecting bond distances and
angles in the copper coordination polyhedra. A summary of
the magnetic properties of the seven compounds is gathered
in Table 4.
DFT Calculations. In order to explain the unexpected
magnitudes of the magnetic exchange couplings in some of
the compounds presented in this paper (3 and 5), we have
carried out a theoretical study based on density functional
theory. The first question to answer deals with the very
different strength of the magnetic exchange coupling between
copper(II) ions in 2 and 3. From a chemical point of view,
these compounds are very similar, displaying an axial
Jahn-Teller effect that places the equatorial plane of the
metal ion in the same plane of the pymca ligand, thus
favoring a one-dimensional arrangement and a large overlap-
ping between the copper(II) magnetic orbitals. Therefore, as
indicated elsewhere, a strong antiferromagnetic coupling is
expected for both compounds, which must be intermediate
between those observed for oxalate and 2,2′-bipyrimidine-
bridged copper(II) complexes, which (as in 2 and 3) connect
equatorial positions on the coordination sphere of the
copper(II) ions.22,23 In principle, the fact that the J value
for 3 is significantly lower than that for 2 might be only due
to the main differences observed between both structures,
which, as indicated elsewhere, are as follows: (i) the axial
positions of the metal ion are occupied by different halides
(X ) Br- or Cl-) in 2 and 3. Since Cu-X distances are
very long, the Cu-X-Cu pathway must not have an
important influence on the magnetic exchange coupling, and
(ii) there are different dispositions of the pymca ligands
around the copper(II) ion along the chain (cis in 2 and trans
in 3).
Figure 17. Plots of ꢂM (left) and ꢂMT (right) for 7. The solid line
corresponds to the best fit.
In this case, the appearance of a maximum near 5 K and
the sudden decrease of the ꢂMT value below 40 K indicate
the existence of a very weak antiferromagnetic coupling
between copper centers through the dicyanamide bridges.
As 6 is a dinuclear complex, the susceptibility data were
analyzed using the Bleaney-Bowers expression for two
magnetically interacting spin doublets derived from the
isotropic Hamiltonian, H ) sJS1S2 + gꢁ(S1S2), with the
inclusion of a P parameter to account for the presence of
paramagnetic impurity. Least-squares best-fit results were J
) -5.1(1) cm-1, g ) 2.00(4), and P ) 0.03(1). The J value
for 6 is similar but slightly higher than those found for other
µ-1,5-dicyanamide-bridged dinuclear complexes with dicy-
anamide ligands linking two equatorial positions on neigh-
boring copper atoms, which are found between -3.3 and
-4.4 cm-1.28 This fact may be due to an overestimation of
the antiferromagnetic interaction, as the interactions through
the hydrogen bond pathways were not taken into account.
Anyway, the results confirm that the µ-1,5-dicyanamide
bridge is a very poor mediator of the magnetic exchange
interaction.
The temperature dependences of ꢂM and ꢂMT (ꢂM is the
molar magnetic susceptibility per dinuclear unit) in an applied
magnetic field of 0.5 T for 7 are shown in Figure 17.
The room-temperature ꢂMT value of 0.834 cm3 mol-1
K
for 7 agrees well with that expected for two isolated Cu(II)
(S ) 1/2) ions. The ꢂM curve increases as the temperature is
lowered until a maximum is reached at 14 K and then
decrease very sharply. At very low temperatures (<10 K),
the ꢂM curves increase due to the presence of a small amount
of paramagnetic impurity. The occurrence of this maximum
in ꢂM is indicative of the existence of an antiferromagnetic
coupling between the metal ions. In keeping with this, the
ꢂMT curve exhibits a continuous decrease upon cooling down
to a value very close to zero. Because the magnetic exchange
interaction through the dicyanoaurate bridging groups, if any
exists, is negligible, compound 7 could be considered from
the magnetic point of view as a dinuclear system. The
magnetic data were fitted to the Bleaney-Bowers expression
derived from the isotropic Hamiltonian: H ) -JS1S2 +
gꢁ(S1S2). A term accounting for the paramagnetic impurity
To carry out this study, we have built two dinuclear model
complexes from the experimental crystal structures (see
outlined models in Figure 18). In both cases, to avoid highly
negatively charged complexes, square-planar copper(II) ions
have been considered by removing the halides located on
(28) Van Albada, G. A.; van der Horst, M. G.; Mutikainen, I.; Turpeinen,
U.; Reedijk, J. Inorg. Chem. Comum. 2007, 10, 1014.
8154 Inorganic Chemistry, Vol. 47, No. 18, 2008

Pyrimidine-2-carboxylato-Bridged Cu(II) Complexes
Table 4. Summary of Magnetic Properties for Compounds 1-7
compound
-J/cm-1
ligands bridging mode
magnetic orbitals
1
-0.57(1)
syn-anti-carboxylate pymca
O-equatorial/O-axial
dx2-y2/dx2-y2
2
3
4
-276.1(1)
-72.5(5)
cis-bis(chelating) pymca
(N,O-equatorial)/(N,O-equatorial)
trans-bis(chelating) pymca
(N,O-equatorial)/(N,O-equatorial)
bis(chelating) pymca
(N,O-equatorial.)/(N-equatorial-O-axial)
bis(chelating) pymca
dx2-y2/dx2-y2
dx2-y2/dx2-y2
dx2-y2/dx2-y2
-31.4(2)
-34(7)(pymca)
(N-equatorial-O-axial)/(O-equatorial-N-axial)
dx2-y2/dz2
dx2-y2/dz2
dx2-y2/dx2-y2
dz2/dz2
5
71(11)(azide)
-5.1(1)
double end-on azide
N-equatorial./N-equatorial
double 1,5-dicyanamide
N-equatorial/N-equatorial
bis(chelating) pymca
6
7
-17.50(1)
N,O-equatorial/N,O-equatorial
is not possible. Despite the absence of any discontinuity in
the magnetic data, we think that the only reasonable
explanation for the relatively low J value observed for 3
would be the existence of a structural change. This geo-
metrical alteration could induce a new spatial disposition of
the magnetic orbitals leading to a small overlapping between
them. The more convincing geometrical modification would
be that in which the Jahn-Teller axis is pointing at the
pymca ligand. In this case, a reduction factor for the
predominant antiferromagnetic contribution to J equal to 1/4
can be envisaged, leading to an exchange coupling constant
similar to that extracted from the experimental data (-72.5
cm-1).20,31 This topology of the magnetic orbitals is also
observed in 4; however, whereas in the proposed case for 3
the Jahn-Teller axis is placed on the less efficient magnetic
exchange pathway (oxygen atom), in 4 this axis lies on the
more efficient pathway, reducing the magnitude of the
magnetic interaction. We are now trying to crystallize the
Figure 18. Outlined d-p hybridation of the metal magnetic orbitals in
models I and II, which have been built from experimental geometries of 3
and 2, respectively.
-
-
complexes [Cu(pymca)X] (X ) NO3 and ClO4 ), which
exhibit the same magnetic behavior of complex 3, but until
now, all attempts in this direction were unsuccessful. The
structures of these complexes may help us to clarify the
origin of the low J value observed for 3.
the axial positions. In this situation, it can be presumed that
differences in the exchange coupling constants are only due
to the different effect of the peripheral ligand;29 that is, the
nitrogen and oxygen donor atoms of the pymca ligand lead
to an unequal hybridation of the copper dx2-y2 magnetic
orbital just as it is displayed (assuming a major effect of the
oxygen donor atom) in Figure 18. Qualitatively, when the
pymca ligands are in a trans arrangement, because the donor
atoms in opposite positions are equivalents, the net hybri-
dation is null, and the magnetic orbital of the metal ion can
be considered as a pure dx2-y2 orbital (Figure 18b). This is
not the case in the cis arrangement (Figure 18a). However,
since the nitrogen and oxygen atoms of the pyrimidine and
carboxylate groups provide similar crystal field ligand
strengths, a small difference in the J constants for models I
and II should be expected, the antiferromagnetic coupling
being stronger for model II than for model I. In agreement
with this, the values of the exchange coupling constants
estimated by means of DFT calculations are -128.3 and
-132.0 cm-1 for models I and II, respectively. Therefore,
an explanation based on changes on the electronic structure
On the other hand, as observed for 2, a strong antiferro-
magnetic exchange coupling (J1 ) -276.1 cm-1) is expected
for 5 when the pymca acts as a bridging ligand with the
magnetic orbitals centered on nearest neighbor copper ions
lying in the plane of the pymca bridge. Additionally, two
azido groups connect the metal ions from a µ-1,1 coordina-
tion (J2), expanding the interaction topology to a honeycomb
network. From a known theoretical magneto-structural cor-
relation published by one of us, a strong ferromagnetic
interaction is suspected for J2 () +130 cm-1).32 However,
the values of the J constants obtained from the experimental
data are less than those assumed from the data found in the
literature. The understanding of this new discrepancy is the
second problem to solve in this section. A detailed analysis
(30) Cano, J.; Alemany, P.; Alvarez, S.; Verdaguer, M.; Ruiz, E.
Chem.sEur. 1998, 4, 476.
(31) Julve, M.; Verdaguer, M.; Gleizes, A.; Philochelevisalles, M.; Kahn,
O. Inorg. Chem. 1984, 23, 3808.
(29) Roman, P.; Guzma´n-Miralles, C.; Luque, A.; Beitia, J. I.; Cano, J.;
(32) Ruiz, E.; Cano, J.; Alvarez, S.; Alemany, P. J. Am. Chem. Soc. 1998,
120, 11122.
Lloret, F.; Julve, M.; Alvarez, S. Inorg. Chem. 1996, 35, 3741.
Inorganic Chemistry, Vol. 47, No. 18, 2008 8155

Sua´rez-Varela et al.
Figure 19. Four possible arrangements (b-e) of the magnetic orbitals in the [Cu2(pymca)] core of 5. The usual and ideal arrangement is also displayed (a).
The experimental (Jexp), estimated (Jest), and calculated (Jcalcd) J constants are given in cm-1. The contractions bpm, ox, and pymca refer to the 2,2′-
bipyrimidine, oxalate and, pyrimidine-2-carboxylato ligands, respectively.
of the molecular geometry of the metal coordination sphere
in 5 indicates that (i) the metal-ligand distances are longer
than those presumed for a 4 + 2 tetragonally elongated
distorted octahedral copper(II) ion, (ii) the geometry of the
metal sphere coordination cannot be assigned to either a
compressed or an elongated octahedral geometry, and (iii)
the data seem to suggest an intermediate situation, which
could be induced by a dynamic Jahn-Teller effect on the
copper(II) ion. This phenomenon, which is rather unusual,
has already been observed in some copper(II) complexes
where the molecular geometries and electronic structure are
strongly dependent on the temperature.33-40 These static or
dynamic situations prevent the discovery of a pure magnetic
orbital (usually dx2-y2 in copper(II) complexes), leading to
a mixture of the two possible magnetic orbitals for axial
elongated and compressed geometries, dx2-y2 and dz2,
respectively. The degree of combination of these two orbitals
to form the magnetic orbital will strongly rely on the
geometrical parameters and, therefore, be changeable by the
temperature when the effect is dynamic. In a similar
copper(II) compound with 2,2′-bipyrimidine and oxalate as
bridging ligands organized in a honeycomb network, an
intermediate geometry for the coordination sphere of the
copper(II) ion was also observed.41 We think, in these
analogues systems, when the one-dimensional system evolves
to bidimensional honeycomb network, the axial Jahn-Teller
of the metal ion is lost to try a stronger bond with all bridging
ligands.
Usually, in real complexes, particularly when different
ligands are coordinated to the metal ion, there is no easy
way to know this degree of the mixture. That is why DFT
calculations can be useful to correctly describe the magnetic
properties of these kinds of complexes. In Figures 19 and
20 are shown the four contributions to the exchange coupling
when the magnetic orbitals of the copper(II) ions are
described as a mixture between the dx2-y2 and dz2 orbitals.
The orientation of the orbitals has been taken in the same
way that is found in DFT calculations. When the pymca
ligand is responsible for transmitting the magnetic coupling,
and following the simple ideas set out by Julve et al. in a
previous work, we can estimate from the electronic densities
of the d orbitals involved in the process a qualitative
magnitude for each contribution.31 Thus, due to a null
overlapping between magnetic orbitals in case b, the anti-
ferromagnetic contribution is negligible, and only a weak
ferromagnetic coupling is expected. Considering the wave
function that describes the different spatial distribution of
the electronic densities placed on the axial and equatorial
lobes of the dz2 orbitals, we can assumed that cases d and e
are similar and provide an antiferromagnetic contribution that
is reduced to 1/3 of the ideal case, where the pymca ligand
occupies the equatorial positions of the copper(II) ions and
dx2-y2 is the magnetic orbital (Figure 19a). The exchange
pathways through the pymca ligand can be connected to those
present in complexes with oxalate and 2,2′-bipyrimidine
(33) Bacci, M. New J. Chem. 1993, 17, 67.
(34) Fil, D. V.; Tokar, O. I.; Shelankov, A. L.; Weber, W. Phys. ReV. B:
Condens. Matter. Mater. Phys. 1992, 45, 5633.
(35) Hitchman, M. A.; Maaskant, W.; van der Plas, J.; Simmons, C. J.;
Stratemeier, H. J. Am. Chem. Soc. 1999, 121, 1488.
(36) Markiewicz, R. S. J. Phys. Chem. Sol. 1995, 56, 1637.
(37) O’Brien, M. C. M. Proc. R. Soc. London, Ser. A 1964, 281, 323.
(38) Riley, M. J.; Hitchman, M. A.; Mohammed, A. W. J. Chem. Phys.
1987, 87, 3766.
(39) Riley, M. J.; Hitchman, M. A.; Reinen, D. Chem. Phys. 1986, 102,
11.
(41) DeMunno, G.; Julve, M.; Nicolo, F.; Lloret, F.; Faus, J.; Ruiz, R.;
(40) Simmons, C. J. New J. Chem. 1993, 17, 77.
Sinn, E. Angew. Chem., Int. Ed. 1993, 32, 613.
8156 Inorganic Chemistry, Vol. 47, No. 18, 2008

Pyrimidine-2-carboxylato-Bridged Cu(II) Complexes
Figure 20. Four possible arrangements (a-d) of the magnetic orbitals in the [Cu2(µ1,1-azido)2] core of 5. The usual and ideal arrangement coincides with
that displayed in a. The experimental (Jexp), estimated (Jest), and calculated (Jcalcd) J constants are given in cm-1. The contraction refers to the azido group.
ligands. Taking this into account, the exchange coupling in
case d is assumed to be larger than in case e. On the other
hand, an antiferromagnetic coupling equivalent to 4/9 of the
ideal case is expected for arrangement c. This coupling can
also be expressed as a function of the J constants in
bipyrimidine (bpm) and oxalate (ox) complexes as follows:
2/9(Jbpm + Jox). It should be noted that these estimations do
not consider the changes in the metal-ligand distances. In
conclusion, when the ideal orbital topology is lost, a decrease
in the magnetic coupling must be observed, independently
of whether an axial compressed or an intermediate picture
was reached. The DFT calculations show that case e
summarizes perfectly the J1 coupling in 5, and an intermedi-
ate Jahn-Teller distortion cannot be presumed. From these
simple concepts, we have obtained an estimated value (-50
cm-1) that is in a good agreement with the value obtained
from the experimental data (-34 cm-1). However, a weak
ferromagnetic coupling (+9.5 cm-1) is found from DFT
calculations due to an underestimation of the antiferromag-
netic contributions induced probably by the high negative
charge of the used model.
calculations can lead to problems in the convergence process
and to shifts in the values of the J constants. Thus, we think
that an underestimation of the antiferromagnetic contributions
in J2 is caused by this fact.
Conclusions
The results reported in this paper clearly show that
pyrimidine-2-carboxylato (pymca) is a versatile ligand that
is able to afford a rich variety of polynuclear copper(II)
complexes with antiferromagnetic magnetic coupling be-
tween copper(II) ions. The structure and magnetic properties
of the polynuclear copper(II) complexes containing bis-
(chelating) pymca bridging ligands are strongly influenced
by the anion X present in the reaction solution and by the
strength of the Cu-X bond. If the X anion lies on the
Jahn-Teller distortion axis, as in the case of 2 and 3, the
Cu-X bond is weak, and linear chains with planar
pymca-Cu-pymca-Cu fragments are obtained. If X does
not act as a bridge and the Cu-X bond is strong, as in the
case of 4, a helical chain is formed, while if the Cu-X bond
is strong and the X acts as a bridge, as in the cases of 5 and
7, the reaction leads to 2D networks.
A similar scheme has been done for the J2 coupling, where
uniquely two azido ligands act as bridging ligands (Figure
20). In Figure 20a is shown the ideal case that favors the
strong magnetic coupling. As has been demonstrated in a
previous work, the magnitude and the nature of this coupling
depend on the Cu-N-Cu angle.31 Taking this into account,
a value of +130 cm-1 is expected for J2 in compound 5.
For the case shown in Figure 20b, a net ferromagnetic
coupling could be expected, but since the dz2 orbital shows
a lower spin density in the equatorial ring, a decrease in the
magnitude of the magnetic coupling must be observed. For
the remaining cases, a strict orthogonality takes place and a
ferromagnetic coupling is assumed, but its magnitude cannot
be easily estimated. One of these latter cases corresponds
with J2 in 5. DFT calculations are in agreement with the
conclusions reached in this section. Thus, even if, just like
observed for J1, there was a shift of the magnitude in our
calculations, a moderate ferromagnetic coupling is found
(+59.6 cm-1), in agreement with the experimental data (+71
cm-1). The high negative charge in the models used in DFT
The bis(chelating) pymca bridging ligand is very effective
in transmitting magnetic coupling between copper(II) ions.
Thus, if the magnetic orbitals centered on next neighboring
copper(II) atoms lie on the plane of the pymca ligand, as in
the case of 2, a strong antiferromagnetic coupling is observed
(-276.1 cm-1). It is interesting to note that, despite the
structural similarity of complexes 2 and 3, the J value for
the latter is unexpectedly low (-72.5 cm-1). DFT calcula-
tions show that the relatively low J value observed for 3
cannot be due to the different configuration of the pymca
ligands around the copper(II) ions (cis and trans for 2 and
3, respectively). The only reasonable explanation for the
reduction in the magnetic coupling on passing from 2 to 3
would be the existence of a structural change that places the
Jahn-Teller axis pointing at the pymca ligand. Complex 5
represents the first example of a copper(II) complex exhibit-
ing a honeycomb layer in which two pymca bridges alternate
with one double end-on azide bridge within the hexagonal
rings. Quantum Monte Carlo methods can be successfully
Inorganic Chemistry, Vol. 47, No. 18, 2008 8157

Sua´rez-Varela et al.
applied to calculate the exchange parameters for the com-
plicated topology observed in 5 with ferromagnetic coupling
through the double end-on azide bridges and antiferromag-
netic coupling through the pymca bridges. The magnitude
of the magnetic coupling in complexes 4-7 can be explained
on the basis of the overlapping between magnetic orbitals
and DFT theoretical calculations. Work is in progress in our
laboratory to obtain other homo- and heterometallic pymca-
bridged complexes with interesting structures and magnetic
properties.
02/BQU), the Junta de Andaluc´ıa (FQM-195), the University
of Granada (grant to A.J.M.), and the Catalan Government
(2005SGR-00036).
Supporting Information Available: X-ray crystallographic data
in CIF format. Calculated and experimental X-ray powder diffrac-
tograms for compounds 1-7 (Figure S1), π-π stacking interaction
in compound 5 (Figure S2), magnetic properties of compound 1
(Figure S3), and the theoretical equation used to fit the magnetic
data of compounds 1-4. This material is available free of charge
via the Internet at http://pubs.acs.org.
Acknowledgment. This work was supported by the MEC
(Spain; Projects CTQ2005/0935 and CTQ2005-08123-C02-
IC800625W
8158 Inorganic Chemistry, Vol. 47, No. 18, 2008