1 
 
Dinuclear, tetranuclear and polymeric complexes in 1 
copper(II) perchlorate/pyridine-2,6-diamidoxime 2 
chemistry: synthetic, structural and magnetic studies† 3 
 4 
 5 
Albert Escuer,*a Gina Vlahopoulou,a Spyros P. Perlepes,*b Mercé Font-Bardiac and 6 
Teresa Calvetc 7 
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 12 
 13 
 14 
a Departament de Química Inorgànica and Institut de Nanociència i Nanotecnologia de la 15 
Universitat de Bracelona (IN2UB), Martí i Franqués 1-11, 08028, Barcelona, Spain. E-16 
mail: albert.escuer@ub.edu; Fax: +34 934907725 17 
b Department of Chemistry, University of Patras, 26504, Patras, Greece 18 
c Departament de Mineralogia, Cristalografia i Diposits Minerals, Universitat de 19 
Barcelona, Martí i Franqués s/n, 08028, Barcelona, Spain 20 
 21 
 22 
 23 
  24 
2 
 
Abstract 25 
The initial use of pyridine-2,6-diamidoxime (pdamoH2) in metal cluster and polymer 26 
chemistry is described. Depending on the reaction conditions employed, the 27 
Cu(ClO4)2·6H2O/pdamoH2 system has provided access to the dinuclear compound 28 
[Cu2(pdamoH)2(ClO4)2(MeOH)2] (1), the chain-like polymer [Cu2(pdamoH)2]n(ClO4)2n 29 
(2) and to the tetranuclear cluster [Cu4(pdamo)2(pdamoH)2](ClO4)2 (3). Single-crystal, X-30 
ray crystallography reveals different coordination modes for the pdamoH- ligand in each 31 
compound, providing the first evidence for the flexibility and versatility of the anionic forms 32 
of pdamoH2. Variable-temperature magnetic susceptibility studies indicate very strong 33 
antiferromagnetic coupling in the three complexes, attributable to the double oximato 34 
bridges which link the CuII spin carriers. 35 
 36 
 37 
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 39 
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 41 
 42 
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 45 
 46 
  47 
3 
 
1. Introduction 48 
Molecular metal complexes (clusters1) and coordination polymers of paramagnetic 49 
3d-metal ions continue to be a major research topic of many groups around the world because 50 
of the aesthetic beauty and complexity of their structures, their exciting physical properties 51 
and their potential applications. Among the diverse reasons for the interest for new clusters 52 
are the search for models of metal-containing biological sites,2 and new high-spin 53 
molecules3 and single-moleculemagnets.4 The intense interest in the synthesis and study of 54 
new coordination polymers (also known as metal–organic frameworks, metal–organic 55 
coordination networks or organic–inorganic hybrid coordination polymers in the case where 56 
the metal–organic connectivity is interrupted by “inorganic” bridges)5 stems from their 57 
potential application in fields and areas, such as catalysis, electrical conductivity, 58 
magnetism, luminescence, non-linear optics, molecular electronics, medicine, gas storage, 59 
anion separation, sensing and anion exchange. The ultimate goal is the transformation of 60 
some coordination polymers to functional materials. Thus, there continues to be a need for 61 
new synthetic methods to clusters and coordination polymers. 62 
By contrast with organic chemists who have established methods for making 63 
complicated molecules and organic polymers in a systematic and controlled manner, 64 
inorganic chemists have made little progress in discovering general approaches to 65 
synthesizing complexes containing large or infinite numbers of metal centres. The lack of 66 
control in transition-metal chemistry has led to the neologism “self-assembly”.6 In the last 67 
decade or so, several groups have been introducing elements of design into the assembly 68 
process by employing rigid ligands (e.g. the cyanido ion7 and various tailored derivatives of 69 
4,4’-bipyridine8) that have strong preferences for specific bonding modes and metal ions 70 
with preferred coordination geometries. This “designed assembly” approach has led to many 71 
beautiful clusters9 and coordination polymers.10 Other researchers, including our groups, 72 
use much less well-behaved bridging organic ligands (some with the ability to 73 
simultaneously form chelating rings) for the preparation of clusters11 and polymeric metal 74 
compounds.12 The flexibility of the polydentate ligands often allows the stabilization of 75 
many unpredictable structures, almost invariably incorporating further ligands, e.g., 76 
hydroxido, oxido, alkoxido, other inorganic anions or donor solvate molecules. The so-77 
4 
 
named “serendipitous assembly”13 in cluster chemistry and the less-designed (or “non-78 
programmed”) assembly in the chemistry of coordination polymers vastly increases the 79 
range of compounds available for study, while sometimes the unusual structures lead to 80 
interesting properties. However, luck is not enough in making clusters and polymers, and 81 
considerable forethought in the metal ions, ligands and reaction conditions is necessary for 82 
any significant progress to be made.14 Carboxylato,15 pyridonato,16 polyalcoholato,17 83 
pyridylalcoholato18 and oximato19 ligands are frequently used in this chemistry. The 84 
deprotonated oxygen atoms of these ligands are not coordinatively saturated by binding to 85 
one metal centre, and therefore act as bridges leading to the build-up of larger metal clusters 86 
or polymers. As the fields develop, the boundary between “designed” and “serendipitous” 87 
assembly becomes not clear: examples are the interesting compounds synthesized by 88 
Saalfrank’s group,20 the ground-state spin-switching of clusters by targeted structural 89 
distortion described by Brechin’s group21 and the switchingon of single-molecule 90 
magnetism properties in triangular MnIII complexes reported by Christou and co-workers.22 91 
We and others have been investigating a number of oxime-based ligands, and one 92 
broad family of these have been the 2-pyridyl oximes19,23 and 2,6-pyridyl dioximes24 93 
(Scheme 1).  94 
There is currently a renewed interest in the coordination chemistry of oximes, with the 95 
efforts of several groups being driven by a number of considerations.25 2-Pyridyl oximes 96 
and 2,6- pyridyl dioximes are popular molecules whose anions are versatile ligands for a 97 
variety of research objectives. Such ligands have been key “players” in several areas of 98 
single-molecule22,26 and singlechain magnetism. 27 The activation of 2-pyridyl oximes by 99 
3d-metal centres towards further reaction is also becoming an exciting area of research.28 100 
For many years, our groups have been engaged in the use of 2-pyridyl oximate ligands 101 
for cluster and coordination polymer synthesis. One step was the study of complexes of pao-102 
,29 mpko-,30 ppko-,31 i.e., with anionic ligands in which R contains no additional donor site 103 
(Scheme 1). In another step, we studied complexes with anionic ligands in which R contains 104 
a donor site, i.e., complexes of pyaoxH-/pyaox2- 32 and (py)pko- 29b,33 In a third step, we 105 
5 
 
recently initiated a study of the metal cluster and polymer chemistry of 2,6-diacetylpyridine 106 
dioxime (dapdoH2; Scheme 1) and reportedMnII3, (MnII3)n, MnII2MnIII4, MnII6 MnIII2, 107 
NiII2, NiII3 and NiII4 complexes.34–36 Our studies in metal/dapdoH2 chemistry joined 108 
previous literature reports24,37 on the employment of dapdoH- and dapdo2- for the 109 
preparation of MnII2 MnIII4,24a,b MnII2 MnIII6,24a GdIII2 MnIV,24c CuII2 CrIII2,37a and 110 
FeII FeIII237b clusters, as well as dinuclear CuII2 complexes.37c A natural extension of such 111 
studies is the investigation of 2,6-pyridyl dioximes in which the non-donor Me groups are 112 
replaced by potentially donor groups. If R is the amino group, the resulting ligand is 113 
pyridine-2,6-diamidoxime or pyridine-2,6-dicarboxamide oxime (pdamoH2; Scheme 1), 114 
which belongs to the class of amidoximes. The presence of the two amine functionalities is 115 
expected-due to their coordination capability, potential for deprotonation32,38 and hydrogen 116 
bonding effects-to alter the coordination behaviour of this ligand (and hence the identity of 117 
the resultant metal complexes) in comparison with that of the dapdoH2 ligand. 118 
In the present work, we report the initial use of the anionic pdamoH- and pdamo2- 119 
ligands in metal cluster and polymer chemistry by describing the products from the reaction 120 
of Cu(ClO4)2·6H2O and pdamoH2 under various conditions. There are no literature reports 121 
of anymetal clusters or coordination polymers of singly or doubly deprotonated pdamoH2, 122 
but its pyridyl, oxime and amino functionalities suggested a rich potential for the formation 123 
of such compounds. The use of neutral pdamoH2 in metal chemistry had previously given 124 
[Ni(pdamoH2)2](SO4)·5H2O and {[Cu(pdamoH2)(m-SO4)]·2H2O}n,39a while Salonen and 125 
coworkers reported recently the crystal structure of the mononuclear complex 126 
[Ni(pdamoH)2]·4H2O39b that contains the monoanionic form of the ligand. 127 
 128 
 129 
 130 
  131 
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2. Experimental 132 
Syntheses 133 
[Cu2(pdamoH)2(ClO4)2(MeOH)2]·4MeOH (1·4MeOH). A colourless solution of 134 
39 mg (0.2 mmol) of pdamoH2 and 55 mg (0.4 mmol) of triethylamine in 15 mL of methanol 135 
was mixed with a solution of 74 mg (0.2 mmol) of Cu(ClO4)2·6H2O in 10 mL of methanol. 136 
For about one hour the resulting dark green solution was stirred under heating at 50 C and 137 
left to crystallize at room temperature in a closed vial. After four days, well formed dark 138 
green cubic crystals of compound 1 were obtained in a 65% yield. Analytical data for 139 
C20H40Cl2Cu2N10O18 (906.59): calcd. C 26.50, H 4.45, N 15.45; found: C 26.0, H 4.2, N 140 
15.8. IR data (cm-1): 3440 (m), 3353 (sb), 3310 (m), 1669 (s), 1627 (s), 1578 (s), 1410 (m), 141 
1378 (w), 1137 (sh), 1091 (s), 1063 (sh), 927 (w), 819 (m), 732 (w), 681 (m), 626 (s), 543 142 
(m), 465 (m). 143 
[Cu2(pdamoH)2]n(ClO4)2n·2nMeCN (2·2MeCN). A colourless solution of 39 mg 144 
(0.2 mmol) of pdamoH2 in 10 mL of methanol wasmixed with a solution of 74mg (0.2 mmol) 145 
ofCu(ClO4)2·6H2O in 20 mL of acetonitrile and stirred under heating at 50 C for 30 min. 146 
Upon layering the resulting dark green solution with diethyl ether, well-formed dark green 147 
crystals of compound 2 were obtained in three days in a 55% yield. Analytical data for 148 
C18H22Cl2Cu2N12O12 (796.44): calcd. C 27.14, H 2.78, N 21.10; found: C 27.3, H 2.5, N 149 
21.0. IR data (cm-1): 3435 (m), 3339 (sb), 3150 (mb), 1659 (s), 1631 (s), 1576 (s), 1559 (m), 150 
1411 (w), 1348 (w), 1108 (vs), 919 (w), 812 (m), 730 (w), 625 (s), 545 (m), 461 (m). 151 
[Cu4(pdamo)2(pdamoH)2](ClO4)2·2.5H2O (3·2.5H2O). A colourless solution of 78 152 
mg (0.4 mmol) of pdamoH2 and 46 mg (0.8 mmol) of sodium methoxide in 15 mL of 153 
methanol was mixed with a solution of 296 mg (0.8 mmol) of Cu(ClO4)2·6H2O in 10 mL 154 
of methanol and stirred under heating at 50 C for one hour. Slow evaporation at room 155 
temperature gave well-formed dark green crystals of compound 3 in 1–2 days in a 75% yield. 156 
Analytical data for C28H35Cl2Cu4N20O18.5 (1273.79): calcd. C 26.42, H 2.77, N 22.01; 157 
found: C 26.8, H 2.6, N 22.2. IR data (cm-1): 3425 (mb), 3307 (sb), 3150 (mb), 1631 (vs), 158 
7 
 
1582 (s), 1558 (m), 1490 (w), 1407 (w), 1300 (vw), 1121 (sh), 1089 (vs), 1052 (s), 1020 (m), 159 
810 (w), 737 (w), 681 (vw), 636 (vw), 625 (m), 536 (vw), 467 (vw). 160 
 161 
X-Ray structure determination 162 
Prismatic crystals were selected and mounted on a MAR345 diffractometer with an 163 
image plate detector (1 and 2)/Enraf-Nonius CAD4 four-circle diffractometer (3). 164 
Wavelength 0.71073 Å was used for 1–3. Lorentz-polarization (for 1–3) and absorption (for 165 
1 and 3) corrections were made. The structures were solved by direct methods using the 166 
SHELXS computer program40 and refined by full-matrix least-squares method with 167 
SHELX9741 The function minimized was ∑w ||Fo|2 - |Fc|2|2, where w = [σ2(I) + (0.0672P)2 168 
+ 0.3349P]-1 for 1, w = [σ2(I) + (0.1246P)2 + 0.2506P]-1 for 2, w = [σ2(I) + (0.1304P)2]-1 169 
for 3 and P = (|Fo|2 +2|Fc|2)/3. f, f’ and f’’ were taken from International Tables of X-Ray 170 
Crystallography.42 The non-hydrogen atoms were refined anisotropically, whereas 171 
hydrogen atoms were treated by a mixture of restrained and constrained refinement. 172 
For compound 1·4MeOH, the hydrogen atoms of the hydroxyl group of the tree MeOH 173 
molecules have been localized by difference Fourier maps and refined by fixing the bond 174 
lengths and angles. Other 17 hydrogen atoms were placed at calculated positions and refined 175 
using a riding model; the isotropic temperature factors have been restrained to a value 1.2 176 
times that of the corresponding atom which are linked. The bond lengths of the perchlorate 177 
molecule were fixed. 178 
For compound 2·2MeCN and 3·2.5H2O, all the hydrogen atoms were computed and 179 
refined using a riding model, with an isotropic temperature factor equal to 1.2 time that of 180 
the corresponding atom which are linked. 181 
Itmust be emphasized that, in the above mentioned compounds, the hydrogen atoms 182 
of solvent molecules could not be calculated. Details of the data collection and refinement 183 
and bond parameters are summarized in Table 1. 184 
CCDC-770935 (for 1), 770937 (for 2) and 770936 (for 3) contain the supplementary 185 
crystallographic data for this paper.† 186 
 187 
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Physical measurements 188 
Magnetic susceptibility measurements were carried out on polycrystalline samples 189 
with a Quantum Design susceptometer working in the range 2–300 K under magnetic fields 190 
of 0.3 T. Diamagnetic corrections were estimated from Pascal Tables. Infrared spectra 191 
(4000–400 cm-1) were recorded from KBr pellets. 192 
 193 
 194 
  195 
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3. Results and discussion 196 
Synthetic aspects 197 
The formation of 1–3 can be represented by eqn (1)–(3). 198 
2Cu(ClO4)2 · 6H2O + 2pdamoH2 + 2Et3N + 2MeOH  199 
[Cu2 (pdamoH)2 (ClO4)2 (MeOH)2] +2(Et3NH)ClO4 + 12H2O 200 
 201 
2n Cu(ClO4)2 · 6H2O + 2n pdamoH2 202 
[Cu2 (pdamoH)2]n (ClO4)2n + 2n HClO4 + 12n H2O 203 
 204 
4Cu(ClO4)2 · 6H2O + 4pdamoH2 + 6NaOME  205 
[Cu4 (pdamo)2 (pdamoH)2]n (ClO4)2 + 6NaClO4 + 24H2O 206 
The nature of the solvent and base, as well as the crystallization method affect the 207 
product identity. Complexes 1 and 3 can be prepared only in MeOH in the presence of 208 
different bases. Et3N singly deprotonates the pdamoH2 ligand leading to the dinuclear 209 
complex 1, whereas double deprotonation of the half amount of the dioxime ligand is 210 
achieved through the use of the stronger MeO- base leading to the tetranuclear cluster 3. An 211 
excess of the base (base : pdamoH2 = 2 : 1 in both cases) is necessary for the isolation of 212 
clean products. Somewhat to our surprise, complex 3 is prepared only in the presence of an 213 
excess of CuII, because otherwise the product is contaminated with variable amounts of 1. 214 
There are probably several solution species in equilibrium and it is likely that factors such 215 
as pH, relative solubilities, lattice energy, crystallization kinetics and nature of the cation of 216 
the base, amongst others, determine the identity of the isolated product. The 1D complex 2 217 
can be isolated from the 1 : 1 Cu(ClO4)2 ·6H2O/pdamoH2 reaction mixture in MeCN or 218 
MeOH in the absence of base; addition of base causes rapid precipitation of non-crystalline 219 
solids. Addition of Et2O into the reaction solution improves the yield, but it is not necessary 220 
for the precipitation of the complex under the concentration employed. 221 
MeOH 
(1) 
(2) 
MeCN;MeOH 
(3) 
MeOH 
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Description of structures 222 
Selected interatomic distances and angles for the three complexes are listed in Tables 223 
2 and 3. The three compounds contain the planar {Cu(m-ON)2Cu}2+ subunit. The donor 224 
atoms at the axial/apical coordination sites determine the final nuclearity or dimensionality 225 
(1, dinuclear; 2, polymeric; 3, tetranuclear) of the products. 226 
The coordination modes found for the anionic forms of pdamoH2 ligand in compounds 227 
1–3 are plotted in Scheme 2, following the rules of Harris notation.43  228 
Complex 1 crystallizes in the triclinic space group P 1� . Its structure consists of 229 
dinuclear [Cu2(pdamoH)2(ClO4)2(MeOH)2] molecules (Fig. 1) and solvate MeOH 230 
molecules. There is a crystallographic inversion centre at the midpoint of the CuII · · · CuII 231 
axis. The CuII atoms are doubly bridged by the deprotonated diatomic oximate groups of 232 
two 2.1011100 pdamoH- ligands (Scheme 2). Each ligand chelates one CuII atom forming 233 
two five-membered CuNCCN chelating rings, while its deprotonated oximate oxygen atom 234 
is terminally bound to the other metal center. The neutral oxime oxygen atom and both amino 235 
groups of pdamoH- remain uncoordinated. A terminal MeOH molecule and a monodentate 236 
perchlorate group complete an elongated octahedral (4+2) coordination at each metal; these 237 
two ligands define the Jahn–Teller axis of the CuII atom [Cu(1) – O(6) = 2.699(4)Å , Cu(1)–238 
O(7) = 2.630(3) Å ]. 239 
The equatorial bond angles deviate from the ideal 90° value due to the restrictions 240 
imposed from the chelating rings, with the N(1)–Cu(1)–N(3) and N(3)–Cu(1)–N(4) angles 241 
being 80.40(8)° and 77.69(8)°, respectively. The torsion Cu(1)N(1)O(1)Cu(1’) angle is 0° 242 
and the dinuclear unit is fully planar. 243 
The oxygen atoms of the neutral oxime groups, coordinated and lattice MeOH 244 
molecules and the perchlorato ligands, as well as the nitrogen atoms of the uncoordinated 245 
amino groups are involved in a set of intra- and intermolecular hydrogen bonds that 246 
contribute to the stabilization of the dinuclear molecules and create a 3D network. Each 247 
coordinated MeOH molecule is intramolecularly hydrogen-bonded to an uncoordinated 248 
perchlorate oxygen atom, the O(7) · · · O(4’) distance being 2.946 Å . Atom O(9) from a 249 
lattice MeOH molecule acts as acceptor in two hydrogen bonds; the donors for these 250 
11 
 
hydrogen bonds are an amino group [O(9) · · · N(2’) = 2.961 Å ] and a neutral oxime group 251 
[O(9) · · · O(2) = 2.699 Å ]. 252 
Complex 2·2MeCN crystallizes in the monoclinic space group P21/n. Its crystal 253 
structure consists of cationic polymeric chains based on planar (Cu–O–N–Cu torsion angles 254 
are lower than 1°), centro symetric dinuclear {Cu2(pdamoH)2}2+ units (Fig. 2), perchlorate 255 
counterions and solvate MeCN molecules in an 1 : 2 : 2 ratio; the latter two will not be further 256 
discussed. The CuII atoms in the dinuclear unit are doubly bridged by the deprotonated 257 
oximate groups of two symmetry-related pdamoH- ligands. Two nitrogen atoms (one from 258 
the neutral oxime group and one from the pyridyl group) and a neutral oxime oxygen atom 259 
from a neighbouring dinuclear unit complete five-coordination at each metal centre. The 260 
coordination geometry about each metal ion is well described as square pyramidal, in which 261 
the oxygen atom from the neutral oxime group occupies the apical position. 262 
Thus, the cationic chains have a ladder-like appearance (Fig. 3) and the pdamoH- 263 
ligand adopts the 3.1111100 coordination mode (Scheme 2). Both neutral amino groups of 264 
the pdamoH- ligand remain uncoordinated. Analysis of the shape-determining angles by 265 
using the approach of Reedijk, Addison and co-workers 44 yield a value for the trigonality 266 
index, τ, of 0.26 for Cu(1) [and Cu(1’)]. As expected, the axial bond [Cu(1)–O(2b) = 267 
2.869(5)Å ] is the longest; if we consider this weakly bonding interaction as nonbonding, 268 
complex 2·2MeCN can be considered as dinuclear with a distorted square planar 269 
coordination around the CuII atoms. Main hydrogen bonding in the complex involves the 270 
amino nitrogen and neutral oxime oxygen atoms as donors, and ClO4- oxygen and MeCN 271 
nitrogen atoms as acceptors. 272 
Complex 3·2.5H2O crystallizes in the monoclinic space group C2. Its structure (Figs. 273 
4 and 5) consists of tetranuclear [Cu4(pdamo)2(pdamoH)2]2+ cations, ClO4- counterions 274 
and solvate H2O molecules. The tetranuclear cation contains two dinuclear 275 
{Cu2(pdamo)(pdamoH)}+ subunits (A and B, Fig. 4) linked through four oximate µ-O- 276 
atoms; two of them [O(1b), O(3a)] belong to two different pdamo2- ligands and the other 277 
two [O(1a), O(3b)] to the remaining pdamoH- ligands. In each dinuclear subunit the CuII 278 
12 
 
atoms are doubly bridged by two deprotonated oximate groups. The subunits A and B are 279 
not planar as reflected on the Cu–O–N–Cu torsion angles of 17.1°/26.5° for A and 280 
23.2°/31.1° for B. The pdamo2- and pdamoH- ligands adopt the 3.2011100 coordination 281 
mode (Scheme 2). The coordination geometry about the four CuII centers is well described 282 
as square pyramidal; the apical position of each metal ion in unit A is occupied by an oximate 283 
µ-O- atom that belongs to unit B and vice versa. The core of the cation consists of a 284 
tetrahedron of CuII atoms linked together by four µ3-oximate groups to form a distorted 285 
{Cu4(NO)4}4+ ‘cube’ (we avoid the term cubane since it implies the existence of only 286 
monoatomic bridges between the metal ions) comprising alternate single (O) and double (N–287 
O) atom edges (Fig. 5, bottom). The very strong, intramolecular pdamoH- · · · pdamo2- 288 
hydrogen bonds stabilize further the linkage between the dinuclear subunits A and B (Fig. 289 
5, top). 290 
The donors are the neutral oxime oxygen atoms of the pdamoH- ligands and the 291 
acceptors are the uncoordinated, deprotonated oximate oxygen atoms of the pdamo2- 292 
ligands. The O(2a) · · · O(2b) and O(4b) · · · O(4a) distances are 2.495Å and 2.436 Å , 293 
respectively. Due to the short O · · · O distances the tetranuclear cation of 3·2.5H2O can be 294 
formulated as [Cu4(pdamoHpdamo)2]2+. This means that one pdamoH- and its opposite 295 
pdamo2- group can be considered as one 4.22001111110000 trianionic polydentate ligand 296 
(Scheme 3). The ≥ CNO–H ·· · · ONC≤ motif is a valuable synthon in Crystal 297 
Engineering.45 The crystal structure of 3·2.5H2O is stabilized by a hydrogen-bonding 298 
network involving the amino groups and solvate H2O molecules as donors, and perchlorate 299 
oxygen atoms as acceptors.  300 
Complexes 1–3 join a very small family, currently comprising three members,39 of 301 
structurally characterized metal complexes with the neutral or monoanionic forms of 302 
pyridine -2 , 6- diamidoxime as ligands. Two of them, namely [Ni (pdamoH2)2] 303 
(SO4)·5H2O39a and [Ni (pdamoH)2]·4H2O39b are mononuclear. The third complex, 304 
i.e.,{[Cu (pdamoH2) (SO4)2]·2H2O}n, is a coordination polymer, but the bridging ligand is 305 
13 
 
the sulfato group.39a In these three complexes, the pdamoH2 and pdamoH- groups behave 306 
as (Npyridyl) (N’oxime) (N’’oxime/oximate)-tridentate chelating ligands. Compounds 1–3 307 
are thus the first complexes of any metal in which the monoanionic pdamoH- ligand is 308 
bridging, while 3 is the first metal complex with a formally dianionic pdamo2- ligand. In 309 
addition, complex 3 is the first metal cluster containing any form of pdamoH2 as ligand. 310 
 311 
Magnetic studies 312 
Solid state dc magnetic susceptibility measurements were performed on 313 
polycrystalline samples of complexes 1–3 in the temperature range 2.0–300 K. The obtained 314 
data are plotted as χMT vs. T in Fig. 6. The χMT values at room temperature for Cu2 unit are 315 
0.07, 0.28 and 0.23 cm3 Kmol-1 for 1, 2 and 3, respectively. These values are much lower 316 
than the expected ones for systems comprising two (1, 2) and four (3) non-interacting CuII 317 
centres. The χMT products rapidly decrease with decreasing temperature, reaching a constant 318 
value very close to zero below ~100 K. The maxima in the χM vs. T plots should be located 319 
above room temperature in the three cases. The χM value shows a continuous decrease upon 320 
cooling and the curve shows a broad minimum at ~130 K, but below this temperature it 321 
slightly increases due to the presence of a small amount of paramagnetic, possibly 322 
monomeric impurity. The just described behaviour is indicative of very strong 323 
antiferromagnetic interactions between the CuII centres indicating zero spin ground states.  324 
Close inspection of the molecular structures of 1–3 reveals that the main exchange 325 
interaction should be through the double, diatomic oximato bridge; thus, the weak basal-326 
apical interactions can be neglected in the fitting procedures for compounds 2 and 3. On this 327 
basis, the fitting of the experimental data was performed by using the conventional analytical 328 
equation derived from the spin Hamiltonian of eqns (4) and (5) and introducing a ρ term to 329 
evaluate the monomeric paramagnetic impurities (the same g value was assumed for the 330 
monomeric impurity). 331 
 332 
 333 
14 
 
H = -J(S1S2) 334 
H = -J(S1S2 + S3S4)  335 
for compounds 1–2 and 3 respectively. 336 
Best fit parameters are J = -812 (6) cm-1, g = 2.13 (3), ρ = 0.4% for 1, J = -446 (2) cm-337 
1, g = 2.086 (8), ρ = 1.4% for 2, and J = -513(2) cm-1, g = 2.124(8), ρ = 2.0% for 3. 338 
Magnetic reports for similar systems are limited. However, the strong coupling for 1–339 
3, as evidenced by the large J values, was not unexpected. A very strong antiferromagnetic 340 
coupling (even diamagnetism at room temperature in some cases) has been also reported for 341 
other doubly N, O oximato-bridged CuII 2 units.46 The remarkable ability of the oximato 342 
bridges to mediate strong antiferromagnetic coupling between paramagnetic centres has 343 
been studied by some of us 46h and others46f by means of Extended-Huckel MO calculations 344 
on the Cu–(R N–O)2–Cu core (R = various groups). The conclusion was that the good orbital 345 
overlap in Cu–(R N–O)2–Cu “rings” favours strong magnetic coupling with a poor 346 
dependence of moderate geometric distortions (mainly planarity of the bridging region) on 347 
the strength of the interaction; in contrast, the electronic properties of the R-substituted 348 
oximate groups and/or the ligands that complete the coordination sphere of the CuII ions 349 
play an important role modulating the magnitude of the coupling. In this context, the reported 350 
compounds provides a nice example of the influence of these factors: the isolated and fully 351 
planar compound 1 gives a classical very strong coupling evaluated as greater than -800 cm-352 
1, whereas planar compound 2 and slightly distorted compound 3 exhibit a lower and similar 353 
coupling with values around -500 cm-1. This feature only can be attributed to electronic 354 
differences derived of the additional coordination of O-atoms of the oximato groups to the 355 
axial sites of neighbour CuII atoms. This conclusion is reinforced by the magnetic behaviour 356 
of the previously reported [Cu2(dapdoH)2(H2O)2](BF4)2 complex47 which consist of 357 
isolated dinuclear units that can be compared with compound 1. In spite that the Cu–(R ═ 358 
N–O)2–Cu core for this dapdoH- derivative is not fully planar (torsion angles of 6.0° and 359 
10.3°), room temperature χMT value was reported as 0.09 cm3 K mol-1 for dinuclear unit 360 
(4) 
(5) 
15 
 
which agree with the magnitude of the coupling found for 1 with a χMT room temperature 361 
value of 0.07 cm3 K mol-1. 362 
 363 
 364 
  365 
16 
 
4. Conclusions 366 
The use of pyridine-2,6-diamidoxime (pdamoH2) in reactions with Cu(ClO4)2·6H2O 367 
has led to the first dinuclear, cluster and polymeric complexes of any metal containing the 368 
anionic forms (pdamoH-, pdamo2-) of the ligand. The obtained products are novel in 369 
multiple ways, as described, but they also provide rare examples of CuII complexes with 370 
extremely strong antiferromagnetic coupling between the metal ions. Four different ligation 371 
modes have already been observed for pdamoH- and pdamo2- (Scheme 2) in only three CuII 372 
complexes, indicating the great coordination potential of this ligand systems. Our initial 373 
results described in this work suggest that reactions of pdamoH2 with CuII in the presence 374 
of coligands with a strong coordination ability (e.g. carboxylates and b-diketonates) and with 375 
other paramagnetic 3d-metal ions promise to deliver many new and exciting clusters and 376 
coordination polymers. 377 
 378 
 379 
 380 
 381 
  382 
17 
 
Acknowledgements 383 
G.V. thanks the Ministerio de Ciencia y Tecnología (Spain), project CTQ2006-01759 384 
for the research contract. A.E. thanks project CTQ2009-07264 and excellence in research 385 
ICREA-Academia Award for financial support. 386 
 387 
 388 
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  397 
18 
 
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551 
23 
 
Table 1. Crystallographic data for complexes 1·4MeOH, 2·2MeCN and 3·5H2O 552 
 553 
 554 
555 
24 
 
Table 2. Selected interatomic distances [Å ] and angles [°] for compounds 1 and 2 556 
 557 
 558 
 559 
560 
25 
 
Table 3. Selected interatomic distances [Å ] and angles [°] for compound 3 561 
 562 
  563 
26 
 
Figures Captions 564 
Scheme 1. Structural formulae and abbreviations of the 2-pyridyl oximes and 2,6-pyridyl 565 
dioximes discussed in the text. 566 
Scheme 2. The crystallographically established coordination modes of pdamoH- and 567 
pdamo2- in 1–3, and the Harris notation43 that describes these modes. 568 
Figure 1. Partially labeled ORTEP plot (ellipsoids at 20% probability) of complex 1·4MeOH. 569 
Dotted lines represent the main intramolecular hydrogen bonds. 570 
Figure. 2. Partially labeled ORTEP plot (ellipsoids at 20% probability) of the dinuclear 571 
{Cu2(pdamoH)2}2+ unit that is present in the 1D compound 2·2MeCN; one ClO4- 572 
counterion is also shown. The oxygen atoms of the neutral oxime groups [O(2) and its 573 
symmetry-partner] occupy the apical sites of CuII atoms (not shown) from two neighbouring 574 
dinuclear units. Dotted lines represent hydrogen bonds. 575 
Figure 3. A part of the cationic, ladder-like chain of complex 2·2MeCN; the solvate MeCN 576 
molecules are also shown. 577 
Figure 4. Partially labeled ORTEP plots (ellipsoid at 20% probability) for the dinuclear 578 
subunitsA(top) and B (bottom) that are present in 3·2.5H2O. The intramolecular linkages 579 
between the two subunits are not shown. 580 
Figure 5. View of the tetranuclear [Cu4(pdamo)2(pdamoH)2]2+ cation of 3·2.5H2O 581 
incorporating the two intramolecular pdamoH- · · · pdamo2- hydrogen bonds (top) and its 582 
{Cu4(NO)4}4+ core (bottom). 583 
Scheme 3. The 4.22001111110000 pdamoHpdamo3- ligand system in 3·2.5H2O. 584 
Figure 6. Plot of χMT vs.T for Cu2 unit of compounds 1 (circles) 2 (squares) and 3 (solid 585 
diamonds). The full lines represent the best theoretical fit (see text). 586 
 587 
 588 
  589 
27 
 
Scheme 1. 590 
 591 
 592 
 593 
 594 
 595 
596 
28 
 
Scheme 2. 597 
 598 
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29 
 
Figure 1 604 
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Figure 2 609 
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31 
 
Figure 3 621 
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Figure 4 627 
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33 
 
Figure 5 635 
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Scheme 3 643 
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35 
 
Figure 6 651 
 652 
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