Solid state coordination chemistry of pyridinedicarboxylic acid isomers. I copper(II) disodium bis(pyridine-2,3-dicarboxylate) octahydrate and copper(n) pyridine-3,4-dicarboxylate 3-5 hydrate

Article abstract of DOI:10.1071/C98120

The crystal structures of Na₂Cu(2,3-pydc)2.8H₂O (1) and Cu(3,4-pydc).3.5H₂O (2) (pydc = pyridinedicarboxylato) are presented. Compound (1) is monoclinic; space group (72/c, with a 21.274(1), b 6.114(1), c 17.136(1) A, β 101.62(1)°, Z 4, D_c 1.776 g cm^-3; compound (2) is also monoclinic, space group P2₁/c, with a 7.272(2), b 11.319(3), c 25.453(9) A, β 90.58(1)°, Z 4, D_c 1.850 g cm^-3. Their stereochemistries are analysed, and compared with other copper(n) pyridine-2,6-dicarboxylates. Ligand constraints are shown to determine the crystal structures and the magnetic properties. Thermal dependence of magnetic susceptibility data and t.g., d.t.g. and d.t.a. measurements are also presented.

Full text of DOI:10.1071/C98120

C S I R O P U B L I S H I N G
Australian Journal
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Volume 52, 1999
© CSIRO Australia 1999
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Aust. J. Chem., 1999, 52, 205–212
.
Solid State Coordination Chemistry of
Pyridinedicarboxylic Acid Isomers. I
Copper(II) Disodium Bis(pyridine-2,3-
dicarboxylate) Octahydrate and Copper(II)
Pyridine-3,4-dicarboxylate 3 5 Hydrate
Elsa E. Sileo,A,B Daniel Vega,C Ricardo Baggio,C
Mar a T. Garland D and Miguel A. Blesa A,E
A
Departamento de Qu mica Inorganica, Anal tica y Qu mica F sica,
Instituto de Qu mica F sica de los Materiales, Medio Ambiente y Energ a,
Universidad de Buenos Aires, Ciudad Universitaria,
Pabellon II, Piso 3, 1428 Buenos Aires, Argentina.
B
Author to whom correspondence should be addressed.
Departamento de F sica, Comision Nacional de Energ a Atomica,
C
Avda del Libertador 8250, 1429 Buenos Aires, Argentina.
D
Departamento de F sica, Facultad de Ciencias F sicas y Matematicas,
Universidad de Chile, Avda Blanco Encalada 2008, Casilla 487-3, Santiago, Chile.
E
Unidad de Actividad Qu mica, Comision Nacional de Energ a Atomica,
Avda del Libertador 8250, 1429 Buenos Aires, Argentina.
The crystal structures of Na2Cu(2,3-pydc)2.8H2O (1) and Cu(3,4-pydc).3 5H2O (2) (pydc = pyridinedi-
carboxylato) are presented. Compound (1) is monoclinic, space group C 2/c, with a 21 274(1), b
6 114(1), c 17 136(1) A, 101 62(1) , Z 4, Dc 1 776 g cm 3; compound (2) is also monoclinic, space
3
group P 21/c, with a 7 272(2), b 11 319(3), c 25 453(9) A,
90 58(1) , Z 4, Dc 1 850 g cm
.
Their stereochemistries are analysed, and compared with other copper(II) pyridine-2,6-dicarboxylates.
Ligand constraints are shown to determine the crystal structures and the magnetic properties. Thermal
dependence of magnetic susceptibility data and t.g., d.t.g. and d.t.a. measurements are also presented.
Introduction
complexity, as illustrated in the present work. The
possibility of protonating the ligands or introducing
a second metal cation, such as Na+, permits the
alteration of both the stoichiometric ratio L : Cu and
the coordinative characteristic of the ligand.
A number of metal carboxylates in the series have
been the subject of crystallographic, magnetic and/or
thermal studies, in particular derivatives of pyridine-
2,6-dicarboxylic (or dipicolinic, hereafter H22,6-pydc)
acid (by far the most widely explored), pyridine-
2,3-dicarboxylic (or quinolinic, hereafter H22,3-pydc)
acid and pyridine-3,4-dicarboxylic (or cinchomeronic,
hereafter H23,4-pydc) acid. As the most relevant for
the forthcoming discussion, we can quote the following
works.
Copper(II) forms solid carboxylates with a wide vari-
ety of coordination stereochemistries. The Jahn–Teller
e ect together with the ligand’s steric requirements
and chelate constraints prevent CuII from adopting
regular coordination geometries.1 Pyridinedicarboxylic
acids (H2pydc) may act as chelating agents for CuII
when the relative positions of the donor atoms are
adequate. Several 1 : 1 and 1 : 2 compounds that illus-
trate the versatility of copper(II) ion coordination and
the corresponding variety in magnetic interactions are
known. The whole series includes the pyridine-2,3-,
2,4-, 2,5-, 2,6-, 3,4- and 3,5-dicarboxylic acids, all of
which have several donor sites: the oxygen atoms of
the two carboxylic groups and the nitrogen atom in
the aromatic ring. In the solid state, the ve donor
atoms may bind to one, two or three CuII centres, thus
leading to molecular solids, as in some dipicolinates,
or to chains or three-dimensional structures of diverse
(i) X-Ray crystal structure determinations for the 1 : 1
and 1 : 2 copper dipicolinates2,3 and for 1 : 2 copper
bis(hydrogen quinolinate).4 The thermal behaviour
of the copper dipicolinates has also been explored.3
Manuscript received 20 July 1998
᭧ CSIRO 1999
10.1071/CH98120
0004-9425/99/030205$ 05.00

206
E. E. Sileo et al.
Cu(3,4-pydc).3 5H2O (2). To a hot solution of pyridine-3,4-
dicarboxylic acid (0 50 g, 3 mmol) and NaOH (0 12 g, 3 mmol)
in water (200 ml) was added, with stirring, CuSO4.5H2O (0 37 g,
1 5 mmol).
In both cases, the solids were obtained by slow evaporation
of the solutions at 298 K; the blue crystals formed were collected
by ltration and washed with bidistilled water (Found for (1):
C, 28 7; H, 3 7; Cu, 11 2; N, 4 9; Na, 7 9; H2O, 25 8.
Na2Cu(C14H6N2O8).8H2O requires C, 28 8; H, 3 7; Cu, 10 9;
N, 4 7; Na, 7 8; H2O, 24 7%. Found for (2): C, 28 8; H, 3 3;
Cu, 21 8; N, 4 8; H2O, 20 4. Cu(C7H3NO4).3 5H2O requires
C, 28 8; H, 3 4; Cu, 21 8; N, 4 8; H2O, 21 6%).
(ii) Crystal structure determinations for Ni(H2,3-
pydc)2.2H2O,5 Mn(H2,3-pydc)2.2H2O,5 Co(H2,3-
pydc)2.2H2O,6 and Ag(H2,3-pydc)2.2H2O.7 The
interest on quinolinic acid derivatives may well
rely on the neurological activity8 exhibited
by the acid and its behaviour like
a
neurotoxic tryptophan metabolite.9 Its content
in humans with immunode ciency virus infection
is currently being discussed.10
(iii) Magnetic susceptibility and single-crystal elec-
tron spin resonance determinations11 for the two
known polymorphic forms of Cu(2,6-pydc).2H2O,
monoclinic and triclinic.
X-Ray Structural Solution and Re nement
(1), C14H22CuN2Na2O16, M 583 33. Monoclinic, space
group C 2/c (C62h
17 136(1) A,
,
No. 15),
101 62(1) ,
1 776 g cm 3; F(000) 1196.
by 0 20 by 0 20 mm. N tot 3283, N ind 3180, N obs 1783.
29 < h < 29, 0 < k < 8, 24 < l < 0; 59 92 , R1
0 0547, wR2 0 1199 for 164 parameters. Largest di erence
a
21 274(1),
2183 3(4) A3.
11 26 cm 1; specimen 0 30
b
6 114(1),
Dc(Z = 4)
c
(iv) Thermal stability studies on CdII,12 and CuII,13
cinchomeronates and studies on the kinetics of
complexation14 of the NiII and CoII derivatives in
aqueous solution. Though no unambiguous struc-
tural data are currently available for the metal
cinchomeronates, CuII coordination on Cu(3,4-
pydc).2H2O has been postulated to be tetra-
hedral, from the interpretation of its electronic
spectrum.15
V
Mo
2
max
3
peak and hole 0 677 and 0 664 e A
.
(2), C14H20Cu2N2O15, M 583 40. Monoclinic, space group
P 21/c (C52h, No. 14), a 7 272(2), b 11 319(3), c 25 453(9) A,
3
90 58(1) , V 2095 0(11) A3. Dc(Z = 4) 1 850 g cm
;
F(000) 1184.
0 05 mm. N tot 4008, N ind 3767, N obs 1910.
0 < k < 13, 0 < l < 30; 2 50 30 , R1 0 0513, wR2 0 0888
21 12 cm 1; specimen 0 30 by 0 10 by
Mo
8 < h < 8,
In the present work, and as a part of a general
study of metal(II) pyridinedicarboxylates, we present
the synthesis, single-crystal X-ray di raction study,
magnetic measurements and thermal behaviour of
Na2Cu(2,3-pydc)2.8H2O (1) and Cu(3,4-pydc).3 5H2O
(2).
max
for 338 parameters. Largest di erence peak and hole 0 471
3
and 0 406 e A
Details in common. Di ractometer: Siemens R3m; radia-
tion used was Mo K , 0 71074 A; scan mode ω/2 ; temp.
.
293 K; re nement method used was full-matrix least-squares
on F2; programs used were SHELXS,17 SHELXL,18 and PARST.19
H atoms attached to C atoms were placed at their idealized
positions and allowed to ride. Those bonded to O atoms
were located in a di erence Fourier synthesis, and subsequently
Experimental
The ligands pyridine-2,3- and -3,4-dicarboxylic acids (Sigma
Chemical Company) as well as all the other reagents (analytical-
grade purity) were used as provided. Water was puri ed by
a Millipore Milli-Q system yielding 18 M cm. Elemental
analyses (C, H, N) were performed in a Carlo Erba EA 1108
microanalyser at Instituto de Qu mica F sica de los Materiales,
Medio Ambiente y Energ a. The metal contents of the solids
were obtained at the Departamento de Qu mica Inorganica,
Universidad de Buenos Aires, with a Varian Techtrom A-A5R
atomic absorption spectrometer. Thermogravimetric data were
obtained in a Mettler Recording Thermoanalyzer 1 with pow-
re ned with restrained O–H and H
H distances. In all cases
isotropic thermal parameters were used for H atoms, 1 3 times
larger than the equivalent isotropic displacement factors of the
host atom. Non-hydrogen atoms were re ned anisotropically.
Material deposited comprises hydrogen coordinates, structure
factors, bond lengths and angles, torsion angles and anisotropic
displacement parameters.
Results and Discussion
Table 1 shows the fractional atomic positions and
equivalent isotropic displacement factors for compound
(1) while Table 2 presents a selection of relevant bond
lengths and angles. The displacement ellipsoids plot
in Fig. 1 shows the numbering scheme used.
dered samples of about 30 mg under an inert (N2) atmosphere
1
(temperature range 298–693 K) at a heating rate of 6 K s
.
Water content was calculated from t.g. determinations. Variable-
temperature magnetic susceptibility studies were performed on
powdered samples in the range 5–70 K by using a SQUID SHE
905 magnetometer on a QUANTUM Design Model MPMS,
with the eld set at 0 5 T. Experimental susceptibilities were
corrected for diamagnetic contributions according to Pascal’s
constants16 and correction for temperature-independent para-
The crystal structure can be described as being
formed by chains running along the crystallographic
b axis, their elemental units being centrosymmetric
Cu2(pydc)2 dimers centred at crystallographic sites d
and joined together through the sharing of the common
copper centres at sites c (Fig. 2). The double-link
chain thus formed is transversally connected along a
through double bridges, –Na–(H2O)2–Na–, to end up
with a two-dimensional network parallel to (0 0 1).
The copper atom lies on a symmetry centre, and
it is bonded to four symmetry-related quinolinate
groups: two of them coordinate equatorially in a
bidentate fashion through their nitrogen atom and
6
1
magnetism was estimated to be 60 10
cm3 mol
. Elec-
tronic spectra were recorded in the range 250–800 nm in a
Shimadzu u.v.-3101 PC scanning spectrophotometer equipped
with an integrating sphere attachment.
Synthesis of the Solids
Na2Cu(2,3-pydc)2.8H2O (1). A CuSO4 solution (1 2 ml,
0 1 M) was added to an aqueous solution (200 ml) containing
pyridine-2,3-dicarboxylic acid (0 133 g, 0 8 mmol) and NaOH
(0 064 g, 1 6 mmol). The mixture was stirred for 30 min at
80 C.
Copies are available (until 31 December 2004) from the Australian Journal of Chemistry, P.O. Box 1139, Collingwood, Vic. 3066.

Pyridinedicarboxylic Acid Isomers. I
207
one oxygen, O(3), from the 2-carboxylate group. The
axial sites are provided by the other two quinolinates,
through the oxygen atoms O(2) from the remaining
3-carboxylate group. Each pydc group thus binds to
two di erent copper centres (Fig. 2). As typical in
Jahn–Teller deformed octahedra, apical bonds are elon-
gated (2 660(4) A) and equatorial ones, from the two
symmetry-related bidentate bites, are shorter (mean
value 1 949(3) A). Due to strong hydrogen-bonding
interactions the apical bond Cu–O(2) is bent c. 14
out of the normal plane, in the most conspicuous
deformation shown by the coordination polyhedron.
The 2-carboxylate group is coplanar with the pyri-
dine ring (O(3)–C(8)–C(2)–N 0 8(6) ) whilst the one
at position 3 is twisted out (O(1)–C(7)–C(3)–C(2)
83 6(6) ), allowing O(2) to coordinate apically to
a vicinal Cu. Both carboxylates di er in their inter-
atomic distances: while the latter shows the same
C–O bond lengths (1 242(6) A) the former presents a
marked di erence (C(8)–O(3) 1 279(5) and C(8)–O(4)
1 228(5) A) attributable to the pronounced double-
bond character of the C=O group bound to Na.
The Na ions appear in pairs of centrosymmetrically-
related octahedral polyhedra (de ned by one carboxylate
O atom (O(4)) and ve hydration water molecules),
and are linked through the sharing of a common edge
bisected by a symmetry centre. Similar con gurations
have been found in the literature.20 Na2(H2O)8 dimers
bind neighboring copper chains through the carboxylato
oxygen atom. Distances in the Na octahedron reveal
that the interaction to O(4) (Na–O(4) 2 333(4) A)
is stronger than those to the water groups (mean
Na–Owater 2 48(1) A).
All water molecules are involved in a fairly com-
plex hydrogen-bonding scheme stabilizing the structure
through a four-step cascade process starting at O(2w),
and which involves ve di erent quinolinate groups
interacting via non-ketonic atoms. Details of the
hydrogen-bonding geometry are presented in Table 3.
The structural details found in (1) are not unusual
in compounds containing pydc: a similar double-link
chain structure has been observed in Cu(H2,3-pydc)2,4
and the coordination around CuII is analogous to that
shown by AgII, also a d9 ion, in the related complex
Ag(2,3-pydc)2.2H2O.7 However, it may be contrasted
with the structures of Cu(2,6-pydc)(H22,6-pydc).H2O3
and Cu(2,6-pydc)(H22,6-pydc).3H2O,2,3 where a dian-
ion and a diprotonated acid ligand coexist.
Comparison of the CuII environments for Cu(2,6-
pydc)(H22,6-pydc).H2O and compound (1) proves to
be illuminating: while the coordination polyhedron in
(1) is rather regular, with interatomic angles di ering
from expected values by less than 10% (Table 3),
Table 1. Atomic coordinates and equivalent isotropic displace-
ment parameters for compound (1)
U eq is de ned as one-third of the trace of the orthogonalized
U ij tensor
Atom
x
y
z
Ueq (A2)
Cu
N
0 2500
0 2500
0 0000
0 033(1)
0 027(1)
0 024(1)
0 028(1)
0 033(1)
0 033(1)
0 033(1)
0 031(1)
0 041(1)
0 043(1)
0 025(1)
0 032(1)
0 038(1)
0 050(1)
0 039(1)
0 058(1)
0 070(2)
0 053(1)
0 2670(2)
0 2180(2)
0 2172(2)
0 2702(2)
0 3205(2)
0 3173(2)
0 1616(2)
0 1172(2)
0 1670(2)
0 1655(2)
0 1755(2)
0 1185(2)
0 0173(1)
0 0894(2)
0 0354(2)
0 0017(2)
0 0679(2)
0 0116(6)
0 1308(7)
0 3005(7)
0 3223(8)
0 1789(8)
0 0118(8)
0 4565(7)
0 3916(6)
0 6408(6)
0 0878(7)
0 0732(5)
0 2089(6)
0 1220(4)
0 0406(6)
0 2512(8)
0 4688(8)
0 0795(7)
0 0769(2)
0 0698(3)
0 1232(3)
0 1842(3)
0 1917(3)
0 1372(3)
0 1190(3)
0 1504(2)
0 0905(2)
0 0023(3)
0 0458(2)
0 0155(2)
0 0902(1)
0 1594(2)
0 0127(3)
0 1585(3)
0 1807(2)
C(2)
C(3)
C(4)
C(5)
C(6)
C(7)
O(1)
O(2)
C(8)
O(3)
O(4)
Na
O(1w)
O(2w)
O(3w)
O(4w)
Table 2. Selected bond lengths (A) and angles (deg) for
compound (1)
Symmetry transformations used to generate equivalent atoms:
a
b
x, y+1, z;
x, y,
z
Bond distances
Cu–O(3)
Cu–N
1 948(3)
Na–O(3w)
Na–O(4w)
Na–O(2w)b
O(2w)–Nab
2 413(6)
2 400(5)
2 405(5)
2 405(5)
3 638(5)
1 950(4)
2 660(4)
2 333(4)
2 392(4)
2 629(5)
Cu–O(2)a
Na–O(4)
Na–O(1w)
Na–O(2w)
Na
Nab
Bond angles
Fig. 1. CuII and Na coordination polyhedra in Na2Cu(2,3-
pydc)2.8H2O (1). Displacement ellipsoids are shown at a 50%
level.
O(3)–Cu–N
O(3)b–Cu–N
83 28(14)
96 72(14)
O(3)–Cu–O(2)a
N–Cu–O(2)a
78 01(13)
81 27(14)

208
E. E. Sileo et al.
Fig. 2. Packing view of (1) showing interlinked chains.
Table 3. Hydrogen-bonding geometry for compound (1)
a
b
c
d
e
Symmetry codes:
x,
y
1, z;
x, y, z;
x, y,
z
1/2; x, y+1, z; x, y 1, z
D–H
A
D–H (A)
D
A (A)
H
A (A)
D–H
A (deg)
O(1w)–H(1wA)
O(1w)–H(1wB)
O(2w)–H(2wA)
O(2w)–H(2wB)
O(3w)–H(3wA)
O(3w)–H(3wB)
O(4w)–H(4wA)
O(4w)–H(4wB)
O(2)a
O(1)b
O(2)d
0 744(4)
0 750(3)
0 759(4)
0 750(4)
0 743(4)
0 900(5)
0 747(4)
0 749(4)
2 950(5)
2 720(5)
3 066(5)
2 996(6)
2 701(6)
3 158(6)
2 959(5)
2 907(4)
2 209(4)
1 985(3)
2 329(3)
2 379(5)
2 019(4)
2 421(4)
2 243(4)
2 243(3)
174 2(2)
166 6(3)
163 9(3)
140 5(3)
152 6(3)
139 2(3)
160 7(3)
148 3(3)
O(3w)d
O(1)a
O(4w)e
O(1w)c
O(3)
the one in Cu(2,6-pydc)(H22,6-pydc).H2O appears as
more distorted3 from the ideal D4h symmetry, and
with a smaller axial ratio. This diversity in geometry
may be explained through the ligand characteristics:
in dipicolinic ligands, carboxylate groups are located
in positions 2 and 6 determining a at or planar
ligand that strongly coordinates the metal ion in three
short equatorial bonds.2,3 Octahedral coordination is
completed with a neutral dipicolinic acid molecule that
perpendicularly chelates the copper(II) ion. Longer
axial Cu–O distances, as required by the Jahn–Teller
e ect, are thus associated with a higher basicity of
axial carboxylic groups, at longer distances from the
metal centre. Ligand constraints and Jahn–Teller
requirements determine a nal highly distorted octa-
hedral coordination; protonation of the ligands is thus
de ned by the coordination chemistry of copper(II).
Quinolinate ligands, instead, present carboxylates in
the 2- and 3-positions that do not permit a tridentate
chelation to the same metal centre. Thus, a bidentate
chelate forms a strong planar bond in the equatorial
plane and the twisted, out-of-the-plane, carboxylate

Pyridinedicarboxylic Acid Isomers. I
209
at position 3 makes a longer bond to one CuII from
an adjacent link. Again, metal con guration (d9) and
ligand hindrance determine the crystal characteristics.
Final atomic parameters for compound (2) and a
selection of bond lengths and angles involving copper
Table 4. Atomic coordinates and equivalent isotropic displace-
ment parameters for compound (2)
U eq is de ned as one-third of the trace of the orthogonalized
U ij tensor
Atom
x
y
z
Ueq (A2)
Cu(1)
Cu(2)
Cu(3)
O(1)
O(2)
O(3)
O(4)
N(1)
C(1)
C(2)
C(3)
C(4)
C(5)
C(6)
C(7)
O(5)
O(6)
O(7)
0 0000
0 7587(1)
0 5000
0 0000
0 2620(1)
0 5000
0 0000
0 2482(1)
0 0000
0 018(1)
0 022(1)
0 032(1)
0 028(1)
0 041(2)
0 021(1)
0 036(1)
0 016(1)
0 019(2)
0 014(2)
0 015(2)
0 022(2)
0 023(2)
0 018(2)
0 020(2)
0 035(1)
0 036(1)
0 030(1)
0 020(1)
0 016(1)
0 023(2)
0 016(2)
0 014(2)
0 017(2)
0 020(2)
0 024(2)
0 019(2)
0 055(2)
0 053(2)
0 037(2)
0 096(3)
0 066(2)
0 051(2)
0 105(3)
can be found in Tables 4 and 5, respectively.
A
molecular plot is shown in Fig. 3, with the atom
numbering and displacement ellipsoids at a 50% level.
Three independent copper centres are found in the
structure, each one presenting a di erent coordina-
tion: Cu(1) is situated in an inversion centre; the
octahedral environment is generated by the equatorial
0 0758(6)
0 1805(6)
0 5545(6)
0 5652(7)
0 1523(7)
0 0811(8)
0 1834(8)
0 3731(9)
0 4478(9)
0 3378(9)
0 0909(9)
0 5059(9)
0 5113(7)
0 2229(7)
0 0233(6)
0 1725(6)
0 5801(7)
0 5484(9)
0 4098(8)
0 2969(8)
0 3334(9)
0 4745(9)
0 3774(10)
0 1342(9)
0 0399(9)
0 7289(9)
0 6950(7)
0 9514(8)
0 8477(10)
0 2151(10)
1 2351(10)
0 1988(5)
0 2980(5)
0 1792(4)
0 3478(5)
0 0902(5)
0 1364(6)
0 1872(6)
0 1921(6)
0 1464(6)
0 0961(7)
0 2329(6)
0 2460(7)
0 3768(5)
0 3085(5)
0 1403(5)
0 0493(5)
0 2079(5)
0 2669(7)
0 2401(6)
0 1456(6)
0 0783(6)
0 1144(7)
0 3134(7)
0 1106(6)
0 5404(6)
0 3847(7)
0 5776(5)
0 1876(10)
0 4472(6)
0 1706(5)
0 4531(8)
0 1933(2)
0 2163(2)
0 2095(2)
0 1646(2)
0 0541(2)
0 0975(3)
0 1383(2)
0 1320(2)
0 0869(2)
0 0491(2)
0 1862(2)
0 1728(2)
0 0524(2)
0 0466(2)
0 1091(2)
0 0537(2)
0 1939(2)
0 1496(2)
0 1154(2)
0 1271(2)
0 1714(4)
0 2034(2)
0 0677(3)
0 0935(2)
0 0935(2)
0 0593(2)
0 0397(2)
0 2091(2)
0 2046(3)
0 0117(2)
0 2108(3)
Table 5. Selected bond lengths (A) and angles (deg) for
compound (2)
Symmetry transformations used to generate equivalent atoms:
c
d
x, y 1/2, z+1/2;
x
1, y 1/2, z+1/2
Bond distances
1 947(4) Cu(2)–N(2)
O(8)
N(2)
C(8)
C(9)
Cu(1)–O(8)
Cu(1)–N(1)
Cu(1)–O(6w)
Cu(2)–O(4w)
Cu(2)–O(3)c
Cu(2)–O(1)d
2 002(5)
2 456(7)
1 932(5)
1 958(6)
2 590(7)
2 033(5)
2 501(6)
1 907(6)
1 944(4)
1 974(4)
Cu(2)–O(5w)
Cu(3)–O(5)
Cu(3)–O(3w)
Cu(3)–O(2w)
C(10)
C(11)
C(12)
C(13)
C(14)
O(1w)
O(2w)
O(3w)
O(4w)
O(5w)
O(6w)
O(7w)
Bond angles
O(8)–Cu(1)–N(1)
O(8)–Cu(1)–O(6w)
N(1)–Cu(1)–O(6w)
O(4w)–Cu(2)–O(1)d
O(4w)–Cu(2)–N(2)
91 1(2)
95 8(2)
88 4(2)
92 5(2)
89 0(2)
O(1)d–Cu(2)–O(5w) 89 4(2)
O(3)c–Cu(2)–O(1)d 88 5(2)
O(3)c–Cu(2)–N(2)
89 4(2)
O(3)c–Cu(2)–O(5w) 98 9(2)
N(2)–Cu(2)–O(5w)
O(5)–Cu(3)–O(3w)
O(5)–Cu(3)–O(2w)
96 8(2)
86 0(2)
90 4(2)
O(4w)–Cu(2)–O(5w) 87 2(4)
O(4w)–Cu(2)–O(3)c 173 8(4)
O(1)d–Cu(2)–N(2)
173 7(2)
O(3w)–Cu(3)–O(2w) 93 6(2)
Fig. 3. Molecular structure of Cu(3,4-pydc).3 5H2O (2). Displacement ellipsoids are shown at a 50% level.

210
E. E. Sileo et al.
Table 6. Hydrogen-bonding geometry for compound (2)
b
d
f
g
h
i
Symmetry codes:
x, y, z; x, y+1, z; x+1, y, z;
k
x
1, y, z; x+1, y+1, z;
1, y 1/2, z+1/2
x
1,
j
l
y, z;
x
1,
y
1, z;
x
1, y 1, z;
x
D–H
A
D–H (A)
D
A (A)
H
A (A)
D–H
A (deg)
O(1w)–H(1wA)
O(1w)–H(1wB)
O(2w)–H(2wA)
O(2w)–H(2wB)
O(3w)–H(3wA)
O(3w)–H(3wB)
O(4w)–H(4wA)
O(4w)–H(4wB)
O(5w)–H(5wB)
O(6w)–H(6wA)
O(6)d
O(5w)h
O(1w)i
O(4)b
O(1w)k
O(6)j
0 78(9)
0 78(4)
0 80(7)
0 80(6)
0 80(6)
0 80(6)
0 79(7)
0 79(6)
0 78(6)
0 80(6)
2 82(1)
2 06(8)
170(8)
167(4)
173(6)
165(5)
166(6)
159(6)
150(7)
138(5)
139(5)
166(6)
2 94(1)
2 99(1)
2 97(1)
2 73(1)
2 61(1)
2 55(1)
2 60(1)
2 82(1)
3 06(1)
2 18(5)
2 20(7)
2 20(6)
1 94(6)
1 86(6)
1 84(8)
1 97(7)
2 19(5)
2 28(6)
O(2)l
O(7)g
O(7w)
O(2w)f
bonds of a nitrogen atom and a 4-carboxylate oxygen
atom, and the axial coordination of a water molecule.
The angular geometry of the elongated octahedron
is almost undistorted (maximum O–Cu–O 95 8(2) ).
Cu(2) is pentacoordinated in a slightly distorted square
pyramidal structure; the basal plane is formed by two
oxygen atoms from carboxylates in positions 3 and 4
belonging to di erent pydc molecules, a nitrogen and
a water ligand; the apical position is occupied by a
second water ligand. Finally, Cu(3) is at an inversion
centre, its octahedral environment being de ned by
the equatorial bonds to a 3-carboxylate and a water
molecule, and an apical one to a second water oxygen.
Distortions from a regular symmetry are also small.
Water molecules O(2w)–O(6w) are involved in coor-
dination to copper centres, and are accordingly very
well de ned with rather small displacement factors.
The same can be said about O(1w), which, in spite
of its hydration character, is strongly anchored to the
structure through four di erent hydrogen bonds. Only
O(7w) is weakly bonded to the rest, as evidenced
from its large displacement factor and the fact that
no hydrogen atom could be con dently found in the
di erence Fourier. As in (1), hydrogen bonding plays
a fundamental role in the crystal stabilization of (2).
Details of these interactions can be found in Table 6.
This compound also illustrates the versatility of CuII
coordination capability. The metal ion is found in two
di erent octahedral and one pyramidal environments,
all of them slightly distorted; the postulated tetrahe-
dral coordination15 is not con rmed in this case. The
di use re ectance spectra of the compound show a
broad peak centred at 20449 cm 1. The broadness of
the band precludes any attempt to distinguish between
the three di erent coordinative environments of the
CuII ions. The Jahn–Teller e ect is re ected in the
axial occupation by non-sterically constrained water
molecules. The organic ligand behaves as tridentate,
but in contrast with Cu(2,6-pydc).2H2O (monoclinic
and triclinic phases) and Cu(2,6-pydc).3H2O,2,3 where
the copper ion is trichelated by the 2,3-pydc2 lig-
and, coordination is established to three di erent
copper centres; bidentate and tridentate chelation is
precluded by the relative position of N and CO2
donors. Accordingly, a highly interconnected and com-
plex three-dimensional structure results that may be
attributed to constrained twisted carboxylates (dihe-
dral angles O(1)–C(6)–C(2)–C(1) 16(1), O(3)–C(7)–
C(3)–C(2) 85(1), O(5)–C(13)–C(9)–C(8) 20(1) and
O(8)–C(14)–C(10)–C(9) 82(1) ).
Thermal Properties
Fig. 4 shows the thermograms obtained for (1) and
(2). The one for compound (1) displays two dehydra-
tion steps: the rst one at 368 K (weight loss 18 53%)
accounts for six water molecules, the second at 436 K
is attributable to two other H2O groups. Decom-
position of the aromatic moiety together with redox
phenomena involving the metal centre are detected in
one endothermic and one exothermic process (d.t.a.
peaks at 525 and 535 K). At 693 K the nal products
are C, Cu0 and small quantities of Na2CO3, the last
two being characterized by X-ray di raction.
T.g., d.t.g. and d.t.a. measurements for compound
(2) reveal only one peak at 411 K corresponding to
dehydration (weight loss 20 35%); decomposition is
accomplished through two exothermic peaks at 520
and 526 K. The nal residue at 693 K comprises C
and Cu0.
The thermal behaviour is typical of CuII car-
boxylates,21 the decomposition of which is triggered
by internal electron transfer (O–Cu homolytic bond
breakage) followed by CO2 release and subsequent
decomposition of the organic radicals.
Magnetic Susceptibility
Magnetic susceptibility data for compound (1) col-
lected in the range 5–70 K were found to t the
Curie–Weiss law. When the temperature is lowered
the product
decreases steadily from an initial
T
1
value of 0 438 emu mol
to a value of 0 417 emu
mol at 5 K. This behaviour is described by a Curie
constant of 0 440 and a value of 0 281(0 007)
1
K (g = 2 167(1)). The results indicate a very weak
antiferromagnetic coupling between copper centres con-
nected by ligands placed in the equatorial plane of
one copper polyhedron and in an axial position of a
neighbouring copper atom.
Compound (2) is also paramagnetic and follows a
Curie behaviour in the range 16–70 K (C = 0 470 emu
1
K mol
and g = 2 238(2)). When the temperature
increases
is lowered from 16 to 5 K, the product
T

Fig. 4. T.g., d.t.g. and d.t.a. traces of: (a) (1); (b) (2).

212
E. E. Sileo et al.
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Fig. 5.
against T plot between 5 and 20 K for (2). Solid line
T
shows calculated values for the molecular eld approximation.
1
9
steadily from a value of 0 468 emu K mol
value of 0 525 emu K mol
features together with calculated values obtained from
the molecular eld treatment of the Curie equation:
to a
1
.
Fig. 5 shows these
10
11
12
13
14
15
16
Brondino, C., Sileo, E. E., Casado, N., Martino, D., and
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Ng2 2S(S + 1)
3kBT 2S(S + 1)zJ
B
=
where J is the exchange constant and z is the number
of interacting nearest neighbors (S = 1/2). The best
t parameters are g = 2 184 and zJ/k = 1 496.
These values and the appearance of the
curve are indicative of a weak ferromagnetic coupling
between non-equivalent copper ions.
In spite of the variety of coordination geometries of
the di erent Cu–pydc complexes, the coupling between
metal centres is weak in all cases studied until now
(including Cu(2,3-pydc).2H2O, monoclinic and triclinic
phases).11 In future work, the magnetic properties
of copper pyrazinedicarboxylates shall be studied and
compared with those of the Cu–pydc complexes reported
here.
against T
T
17
18
19
20
Acknowledgments
21
The authors acknowledge J. F. Jacquot (DRFMC/
CEA/Grenoble, France) for magnetic measurements.
This work was supported by a grant from Universidad
de Buenos Aires (Ex-036). M.A.B. is a member of
CONICET.