Mixed-ligand hydroxocopper ( II ) / pyridazine clusters embedded into 3 D framework lattices †

Rational combination of pyridazine, hydroxo and carboxylate bridging ligands led to the assembly of three types of mixed-ligand polynuclear Cu(II) clusters (A: [Cu2(μ-OH)(μ-pdz)(μ-COO)]; B: [Cu4(μ3OH)2(μ-pdz)2]; C: [Cu5(μ-OH)2(μ-pdz)4(μ-COO)2(μ-H2O)2]) and their integration into 3D framework structures. Mixed-ligand complexes [Cu2(μ-OH){TMA}(L)(H2O)] (1), [Cu4(μ3-OH)2{ATC}2(L)2(H2O)2]·H2O (2) [Cu4(μ3-OH)2{TDC}3(L)2(H2O)2]·7H2O (3) (L = 1,3-bis(pyridazin-4-yl)adamantane; TMA = benzene1,3,5-tricarboxylate, ATC = adamantane-1,3,5-tricarboxylate, TDC = 2,5-thiophenedicarboxylate) and [Cu5(μ-OH)2{X}4(L)2(H2O)2]·nH2O (X = benzene-1,3-dicarboxylate, BDC, n = 5 (4) and 5-hydroxybenzene-1,3-dicarboxylate, HO-BDC, n = 6 (5)) are prepared under hydrothermal conditions. Trigonal bridges TMA and ATC generate planar Cu(II)/carboxylate subtopologies further pillared into 3D frameworks (1: binodal 3,5-coordinated, doubly interpenetrated tcj-3,5-Ccc2; 2: binodal 3,8-coordinated tfz-d) by bitopic pyridazine ligands. Unprecedented triple bridges in 1 (cluster of type A) support short Cu⋯Cu separations of 3.0746(6) Å. The framework in 3 is a primitive cubic net (pcu) with multiple bispyridazine and TDC links between the tetranuclear nodes of type B. Compounds 4 and 5 adopt uninodal ten-coordinated framework topologies (bct) embedding unprecedented centrosymmetric openchain pentanuclear clusters of type C with two kinds of multiple bridges, Cu(μ-OH)(μ-pdz)2Cu and Cu(μ-COO)(μ-H2O)Cu (Cu⋯Cu distances are 3.175 and 3.324 Å, respectively). Magnetic coupling phenomena were detected for every type of cluster by susceptibility measurements of 1, 3 and 4. For binuclear clusters A in 1, the intracluster antiferromagnetic exchange interactions lead to a diamagnetic ground state (J = −17.5 cm; g = 2.1). Strong antiferromagnetic coupling is relevant also for type B, which consequently results in a diamagnetic ground state (J1 = −110 cm; J2 = −228 cm, g = 2.07). For pentanuclear clusters of type C in 4, the exchange model is based on a strongly antiferromagnetically coupled central linear trinuclear Cu3 group (J1 = −125 cm) and two outer Cu centers weakly antiferromagnetically coupled to the terminal Cu ions of the triad (J2 = −12.5 cm).


Introduction
The study of metal-organic framework (MOF) structures adopted by linkage of polynuclear metal ion clusters is a special topic in coordination and materials chemistry and crystal engineering, and has attracted rapidly growing interest during the last decade. 1In view of framework topologies, 2 the design of such solids offers a particularly successful approach.It allows to avoid the common limitations for the node connections imposed by the typical coordination numbers, 3 and therefore a diversity of extremely highly-connected frameworks became accessible by propagation of coordination geometries established by polynuclear clusters. 4An even more important aspect considers the utility of the latter for functionalization of the MOFs towards specific applications in magnetism and catalysis, 5 while imprinting the inherent properties of the clusters into the extended lattices. 6Therefore, the development of polynucleating ligand systems, which combine the abilities for generation of polynuclear fragments 7 and their further integration into the framework, has received a special value. 8ifferent types of azole and azine 1,2-dinitrogen donors are applicable to assemble clusters of variable nuclearity and establish pathways for magnetic exchange between paramagnetic metal ions, cf.copper(II) and cobalt(II).In this series, the pyridazine linker is a particular paradigm, while offering a large magnetic superexchange interaction and thus mediating magnetic exchange most efficiently in comparison with corresponding phthalazine, 1,2,4-triazole, 1,2,4-triazolate and pyrazolate analogs. 9The ability of pyridazine to generate polynuclear complexes with tunable spin states of the metal ions is also known. 10The attractiveness of polypyridazinyl ligands, however, was limited until now because of their low chemical accessibility.Actually, the preparative chemistry involving pyridazine relies on only one general method which is applicable for facile functionalization of different substrates, namely a very simple click reaction 11 involving inverse electron demand cycloadditions of 1,2,4,5-tetrazine. 12Recent developments have made this key intermediate readily available, 12,13 thus offering flexible pathways towards pyridazine ligands. 13,144][15][16] It manifests a pronounced affinity towards d 10 Cu(I) and Ag(I) ions, commonly generating either single, double or triple bridges between the metallic centres involved in the desired and peculiar polynuclear and polymeric motifs. 14,17In the case of divalent d-metal ions, bridging coordination of this electron deficient and low basic ligand ( pK a = 2.24 for pyridazine vs. pK a = 5.25 for pyridine) is less characteristic and predictable, compared with a simple monodentate pyridine-like function. 15,18Nevertheless, double coordination of pyridazine may be stabilized using suitable complementary short-distance co-bridges, such as hydroxo, 10,13,19 halogenido, 20 nitrato, 15 isothiocyanato, 10,21 and saccharinato, 22 and actually all of the reports for pyridazine bridges between 3d-metal ions consider such a multiple heteroligand linkage, which is best illustrated by a Cu(II) series (Table 1).Thus a promising approach towards the development of nanosized molecular magnets may rely on a synergism of the ligands in multicomponent systems 24 based upon bifunctional pyridazines, which could be well suited for the synthesis of extended frameworks incorporating polynuclear cluster units.

Structure of the coordination compounds
All the compounds adopt 3D framework structures based upon multinuclear heteroligand clusters, which represent topological net nodes and thus provide a primary factor for the complicated connectivities.Three types of the observed clusters, such as binuclear in 1, tetranuclear in 2, 3 and pentanuclear units in 4 and 5 (Scheme 2), originate from a combination of three kinds of ligand bridges ( pyridazine, hydroxo and carboxylate), while preserving a common simple submotif in the form of Cu ions linked by a double pdz/OH bridge.This suggests perfect compatibility of μ-pdz and μ-OH linkers, which are commonly concomitant and act in a synergetic manner. 10,13,19In fact, the significance of the hydroxo bridges is most crucial for the present systems since the bidentate coordination of pyridazine itself is less applicable for the chemistry of transition metal dications, as stated above.7) Å (for two orientations of the disordered fragment).The environments of metal ions are complete with monodentate carboxylate and pyridazine groups and aqua ligands (Fig. 1, Table 3).As was indicated by the values of Addison τ parameters, 26 the geometries of the coordination polyhedra are intermediate between idealized trigonal-bipyramidal (τ = 1) and square-pyramidal (τ = 0) extremes: for orientation A, τ = 0.66 (Cu1) and 0.39 (Cu2A); for orientation B, τ = 0.53 (Cu1) and 0.32 (Cu2B).
The low nuclearity of the cluster is likely a consequence of topology limitations imposed by the rigid trigonal triplecharged TMA 3− connector: the planar Cu/carboxylate subtopology in the form of a hexagonal net (Fig. 1b) clearly implies a three-fold coordination of the hydroxocopper nodes and their complementary charge of +3 [e.g.Cu 2 (μ-OH)].The bipyridazine ligands are accommodated on both axial sides of this plane, and they connect pairs of clusters at a distance of 13.56 Å and expand the structure in a third dimension as pillars between the successive hexagonal layers.The result is a rare binodal three-and five-connected {6 3 }{6 9 •8} net (three-letter notation is  tcj-3,5-Ccc2), which appears to be very open and, therefore, two identical nets interpenetrate (Fig. 2).That is a class IIa interpenetration, Z = 2, with a full symmetry element 1 ˉ.27,28 A comparable morphology of the framework is observed for the aliphatic analog of trimesic acid, 1,3,5-adamantanetricarboxylate That is a first coordination polymer generated by this ligand and we introduce ATC 3− as a novel geometrically rigid tripodal linker, potentially complementing and expanding the chemistry of functional frameworks based upon TMA 3− (such as HKUST-1). 29Both metal ions adopt typical Jahn-Teller polyhedra in the form of square pyramids (Cu1; Addison parameter 26 τ = 0.18) or axially elongated octahedra (Cu2) (Table 4), with pyridazine-N donors positioned at the equatorial planes.In the metal-carboxylate subtopology, the net nodes exist as the above six-connected tetranuclear clusters [Cu 4 (μ 3 -OH) 2 ] and three-connected ATC 3− (in 1 : 2 proportion), giving rise to a 2D structure of the CdI 2 or Mg(OH) 2 type (Kagomé dual net, {4 3 } 2 {4 6 •6 6 •8 3 }, three-letter notation "kgd") (Fig. 3).Pairs of bipyridazine ligands, as double links, interconnect the clusters from the successive layers (separated at 12.83 Å), yielding a 3D eight-and three-coordinated framework, with a Schläfli point symbol {4 3 } 2 {4 6 •6 18 •8 4 } (tfz-d, topological type of UO 3 ) (Table 2).There are a limited number of structural precedents for such a linkage, most of which also incorporate polynuclear cluster nodes and trigonal TMA 3− links.Similar tetranuclear net nodes are also observed for . However, substitution of the carboxylate portion of the structure for dibasic thiophenedicarboxylate leads to simplification of the entire array.This concerns the elimination of three-connected nodes and an increased carboxylate/cluster ratio (3 : 1), which in effect increases the dimensionality of the carboxylate subconnectivity, existing in the form of a primitive cubic net ( pcu, Schläfli point symbol {4 12 •6 3 }).The bipyridazine bridges did not generate additional intercluster links, but rather repeat the existing links already established by the TDC 2− bridges.In this way, four out of six topological links are doubled as a result of the concerted action of both sorts of ligands (Fig. 4).Two Cu II ions adopt square-pyramidal environments (Addison parameters 26 τ are 0.23 for Cu1 and 0.22 for Cu2) (Table 4).

Magnetic properties
Compounds 1, 3 and 4, representing the cluster types A, B and C, respectively, were selected for an investigation of their magnetic properties.The χ m T (T ) plot of a polycrystalline sample of [Cu 2 (μ-OH){TMA}(L)(H 2 O)] (1) (scaled per Cu 2 unit) exhibits decreasing χ m T values on cooling, starting from 0.91 cm 3 K mol −1 at 300 K and approaching a value of 0.05 cm 3 K mol −1 at 1.9 K (Fig. 6).This plot is typical of intracluster antiferromagnetic exchange interactions leading to a diamagnetic ground state.This behaviour may be compared with simpler binuclear systems sustained by the mixed hydroxo + syn-syn carboxylate bridge between the Cu ions, which exhibit a strong ferromagnetic coupling. 35The small low temperature χ m T value originates from a minor paramagnetic phase; this is reflected in the Curie tail as seen in the χ m (T ) plot (Fig. S24 †).
The inverse magnetic susceptibility curve (Fig. S25 †) shows a linear behavior in the temperature range of 50 to 300 K, which results in a Weiss constant θ = −37 K, in good agreement with the negative slope of the χ m T versus T curve.We can use the following isotropic Hamiltonian to describe the intracluster magnetic exchange interactions: The magnetic susceptibility data were least-squares fit to the Bleaney-Bowers equation 36 for isotropic exchange in the Cu(II) pair.A good simulation of the data is achieved with J = −17.5 cm −1 (g = 2.1).The fact that compound 1 shows a very complex bonding pattern with a heteroligand pyridazine/OH/ carboxylate triple bridge connecting the two Cu(II) ions prevents a detailed structure-property correlation, and thus the experimentally determined J parameter comprises all possible exchange pathways between the spin centers.
The χ m T (T ) plot of a polycrystalline sample of [Cu 4 (μ 3 -OH) 2 {TDC} 3 (L) 2 (H 2 O) 2 ]•7H 2 O (3) (scaled per Cu 4 unit) exhibits strongly decreasing χ m T values on cooling, starting from 0.34 cm 3 K mol −1 at 300 K and approaching a zero value around 90 K (Fig. 7).The high temperature χ m T values are much smaller than the spin-only value of 1.5 cm 3 K mol −1 for Fig. 6 Thermal variation of χ m T for 1 (the solid line is drawn based on the Bleaney-Bowers equation).
Table 5 Selected bond distances (Å) and angles (°) for pentanuclear complexes 4 and 5 a Fig. 7 Thermal variation of χ m T for 3 (solid line is a fit according to eqn (1)).
four non-interacting spins with S = 1/2 (g = 2.0) which is due to strong intracluster antiferromagnetic exchange interactions.The low temperature χ m T values indicate a diamagnetic ground state of the tetranuclear complex.
Given the butterfly-type arrangement of the tetranuclear cluster fragment (Scheme 3) and leaving aside the asymmetry of the bonding pattern on the wings, we can use it as an approximation which is justified below.The following isotropic Hamiltonian describes the intracluster magnetic exchange interactions [eqn (1)]: This model approximates the [Cu 4 ] core with a rhombic symmetry while differentiating two magnetic exchange pathways, namely Cu2-Cu2 i ( J 1 ) vs. Cu1-Cu2, Cu1-Cu2 i , Cu2-Cu1 i , Cu2 i -Cu1 i ( J 2 ).Consequently, this Hamiltonian gives rise to six spin states comprising the total spin values (S T ) of 2, 1, 0 with the corresponding energy levels in terms of the magnetic coupling constants as given below: Applying these energy values to the van Vleck equation gives the following analytical expression [eqn (2)]: To reiterate at this stage, the linking pattern at the wing sides of the {Cu 4 (μ 3 -OH) 2 } core differs since on two sides a chelating pyridazine is involved, a fact which would principally ask for two different exchange parameters, say J 2 and J 3 .This would result in six energy levels in the function of three J parameters.25b However, in view of the rather smooth curve of the χ m T (T ) plot one can understand that any trials to fit the data to a three parameter model are not conclusive due to overparametrization. Next, it clearly turned out best to fix the J 1 parameter to a reasonable value of −110 cm −1 ; this coupling strength results from an analogous core structure 25a and then using eqn (2), the experimental data were fitted satisfactorily in the temperature range 300-30 K with J 2 = −228 cm −1 (g = 2.07).Both values of the coupling constants express the fairly strong antiferromagnetic intracluster coupling which consequently results in a diamagnetic ground state.One has to bear in mind, however, that the J 2 parameter now represents an average coupling value on the wing sides.
The χ m T (T ) and χ m (T ) plots of a polycrystalline sample of 4) (scaled per Cu 5 unit) are shown in Fig. 8.The high-temperature χ m T value of 1.75 cm 3 K mol −1 is smaller than the spin-only value of 2.06 cm 3 K mol −1 for five non-interacting spins with S = 1/2 (g = 2.2) and they decrease to a value approaching zero at 2 K.Both are indicative of strong antiferromagnetic exchange interactions within the pentameric unit.
A structural analysis reveals that the central Cu 3 subunit involves Cu1 bound to two equivalent inversion related Cu2 via two bridges, hydroxo and pyridazine.The pyridazine bridges can be considered to be orthogonal connections, because of the short-long bonds to Cu2 and Cu2′ within each pair.Cu-O connections to the bridging OH − groups are short and since the Cu-OH-Cu angle (114.3°) is large, one would anticipate strong antiferromagnetic exchange between Cu1 and the two Cu2 atoms through these bridges (equatorialequatorial connections).The Cu2-Cu3 connection is really just a 1,3-carboxylate with short contacts to both copper centers (the aqua O2 donor is just bonded to Cu2 with a short contact, whereas the secondary Cu3-O2 interactions are only very distal and weak, Table 6).Therefore both Cu2 atoms are going to be Scheme 3 {Cu 4 (μ 3 -OH) 2 } core fragment of 3 and the corresponding magnetic coupling scheme.antiferromagnetically coupled to Cu3, but with a much smaller J value.The corresponding magnetic coupling scheme is given as and the isotropic Hamiltonian is The exchange model is therefore based on a strongly antiferromagnetically coupled central linear trinuclear Cu 3 group, with the Cu3 centers weakly antiferromagnetically coupled to the terminal coppers (Cu2) of the triad.
In the data fitting J 1 and J 2 were represented as a ratio ( J 1 /J 2 ) and after varying the ratio, fitting the data, and observing and minimizing the fitting coefficient, a satisfactory fit was obtained using MAGMUN4.11. 37This program calculates the total spin states and their energies based on the exchange Hamiltonian, and determines the fitted parameters internally through weighted non-linear least squares procedures.The optimum ratio was found to be J 1 /J 2 = 10, with the best fit parameters at J 1 = −125(3) cm −1 , J 2 = −12.5(3)cm −1 and g = 2.18, providing the eminently sensible model based on the regression statistics and the structure.

Thermal stability
The thermal behavior of compounds 1-5 was examined by complementary TG/DTA-MS and temperature-dependent powder X-ray diffractometry (TD PXRD) techniques.With the exception of complex 4, the stages of dehydration and further weight losses due to the thermal destructions are not separated and proceed above 150 °C, with the crystallization of an inorganic product (CuO) observed above 350 °C.Compound 1 is somewhat more stable.It does not show any weight loss until a temperature of 260 °C.In the range 260-340 °C, with a DTG peak maximum at 300 °C, it decomposes (−29.7%) with dehydration (m/z 18) and release of CO 2 (m/z 44) due to decarboxylation.This is accompanied by a loss of crystallinity at 260 °C, as evidenced by PXRD patterns.The stability of the aliphatic analog 2 is comparable.Dehydration starts at 160 °C, and above 220 °C it was accompanied by decarboxylation which results in one unresolved stage of 12.3% weight loss in the range of 160-270 °C (maximum at 250 °C).The PXRD patterns are also indicative of the dehydration process (160-170 °C) since the interlayer spacing is sensitive to elimination of the guest molecules located between the coordination layers.Disintegration of the structure was observed at 230 °C.Complex 3 experiences partial dehydration above 60 °C, with the release of 5 water molecules in the temperature range of 60-170 °C (5.60% observed; 5.82% calculated).This results in a phase transition at 170 °C, and at 195 °C the compound gets amorphous due to decomposition with the release of CO 2 .For the closely related 4 and 5, disintegration of the frameworks occurs at an identical temperature of 235 °C.However, due to a more hydrophobic nature of the isophthalate framework in 4, vs. the 5-hydroxyisophthalate analog 5, the initial dehydration in the first case proceeds more readily.The TG curve indicates two insufficiently separated stages at 110-200 °C and 200-250 °C, with a total weight loss of 7.3% corresponding to the release of coordinated and outer sphere water molecules (calculated 7.32%).Further thermal decomposition proceeds at 270-340 °C.In the case of 5, the dehydration begins at 160 °C.Progressive release of water molecules (m/z 18) coincides with beginning of decarboxylation (m/z 44) at 265 °C.The weight loss of 8.5% in the temperature range 160-260 °C corresponds to the elimination of 8 water molecules (calculated 8.0%).

IR spectra
The IR spectra of complexes 1-5 exhibit strong and broad absorption bands in the region of 3230-3500 cm −1 , which are attributed to the ν(OH) vibrations of the aqua and hydroxo ligands.Bands at 2850-2933 cm −1 correspond to ν(CH) vibrations of adamantane and aromatic moieties.The absorptions for the carbonyl group, ν(CO), appear as very strong bands at 1590-1627 cm −1 (see the Experimental section).The absence of bands at 1730-1690 cm −1 , where ν(CO) of COOH is expected to appear, confirms full deprotonation of the polycarboxylate ligands in 1-5. 38Strong absorption bands at 1358-1382 cm −1 are characteristic of all the compounds; they could be assigned to ν(CN) of the pyridazine rings. 39

Conclusions
The present results are important for providing innovative strategies for the construction of extended coordination lattices incorporating polynuclear metal-organic clusters.A combination of pyridazine, carboxylate and hydroxo ligands is especially well suited for sustaining coordination patterns of different nuclearities and connectivities: this kind of bridges reveals a perfect compatibility and they readily complement each other and act in a synergistic manner.In cooperation with μ-carboxylate and, especially μ-hydroxo groups, pyridazine typically behaves as a short-distance diatomic bridge.Therefore, the present heteroligand system unites and extends the structural and functional potential of such common types of organic and inorganic bridges for the generation of discrete polynuclear arrangements of metal ions.At the same time, a multiplication of the ligand functionality, as it occurs for diand tricarboxylate and bipyridazine ligands, allows the integration of the clusters into polymeric arrays, which could be anticipated for a broad range of transition metal ions and different kinds of organic linkers.Our study suggests also a new approach and attractive preparative sequence towards bridge-head heteroaryl C-functionalization of adamantane, which could find wider applications for the development of multivalent geometrically rigid molecular building blocks incorporating "nanodiamond scaffolds". 40Moreover, the study of magnetic properties of coordination network compounds is a very topical issue in the field of molecular magnetism. 41In the present case, the magnetic susceptibility data for clusters 1, 3 and 4 reveal substantial antiferromagnetic coupling strengths, however, with varying ratios of the coupling parameters.In particular, common to all three structure types A, B and C is the quite complex nature of bonding patterns including heteroligand multiple bridges connecting the spin centers, which prevents a discussion of more detailed structure-property correlations.

Preparation of the coordination compounds
The complexes were prepared under hydrothermal conditions as follows.A mixture of the starting compounds and distilled water were placed in a 20 mL Teflon-lined stainless steel autoclave, stirred for 10-30 min, and heated at 140 °C for 40-70 h in an oven, with further cooling to room temperature.In each case, the excess of bipyridazine ligand was essential for partial hydrolysis of Cu(II) ions and generation of the desired hydroxobridged species.Under these conditions and absence of di-or tricarboxylate components, the reactions of Cu(OAc) 2 •H 2 O and the bipyridazine ligand did not afford insoluble products.
Synthesis of [Cu 2 (μ-OH){TMA}(L)(H 2 O)] (1).A mixture of 6.8 mg (0.034 mmol) Cu(OAc) 2 •H 2 O, 3.1 mg (0.015 mmol) trimesic acid and 10.0 mg (0.034 mmol) ligand in a 1 : 0.44 : 1 molar ratio with 4 mL water was stirred for 30 min in a Teflon vessel, and then it was heated at 140 °C for 70 h.Slow cooling to r.t.over a period of 48 h (cooling rate 2.5 °C h −1 ) afforded a pure product as green prisms, which were washed with 3 mL of water and dried in air for 1 h (yield: 7.9 mg, 80% Thermogravimetric/differential thermal analysis mass spectrometry (TG/DTA-MS) was performed on a Netzsch F1 Jupiter device connected to an Aeolos mass spectrometer.The sample was heated at a rate of 10°min −1 .The temperature-dependent X-ray measurements were carried out on a Stoe STADIP with a high-temperature attachment and a image-plate detector system.PXRD was carried out on a Stoe STADIP (Cu K α1 ) using a linear PSD detector and on a Shimadzu XRD-6000 (Cu Kα radiation).Elemental analysis was carried out with a Vario EL-Heraeus microanalyzer.IR spectra (400-4000 cm −1 , KBr disks) were collected using a Perkin-Elmer FTIR spectrometer.
Magnetic susceptibility measurements were made on a Quantum Design MPMS SQUID-XL magnetometer under an applied magnetic field of 10 3 Oe between 300 and 1.9 K.The samples were prepared in a gelatine capsule.Diamagnetic corrections were made for the samples using the approximation −0.45 × molecular weight × 10 −6 cm 3 mol −1 and the sample holder was corrected for by measuring directly the susceptibility of the empty capsule.

X-Ray crystallography
The diffraction data were collected with graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) (Table 6).Measurements for 1 at 213 K and for 4 (4a: 296 K, 4b: 105 K) were made using a Stoe Image Plate Diffraction System, φ oscillation scans (numerical absorption correction using X-RED and X-SHAPE). 44Measurements for 2, 3 and 5 were performed at 173 K on a Bruker APEXII CCD area-detector diffractometer (ω scans).The data were corrected for Lorentz-polarization effects and for the effects of absorption (multi-scans method).The structures were solved by direct methods and refined by full-matrix least-squares on F 2 using the SHELX-97 package. 45he CH-hydrogen atoms were added geometrically, with U iso = 1.2U eq (C) and OH-hydrogen atoms were located and included with fixed d (O-H) = 0.85 Å and U iso = 1.5U eq (O).Part of the solvate water molecules in 2-4 is disordered.They were refined anisotropically and the hydrogen atoms were not added.In 1, the binuclear cluster is equally disordered over two positions (Cu ion, μ-OH and aqua ligand; one of the pyridazine cycles is also disordered adopting two orientations).Attempted refinements in space groups of lower symmetry did not afford an ordered model.This fragment was freely refined anisotropically and the hydrogen atoms were added as stated above with partial contributions of 0.5.In 3, one of the TDC 2− ligands is equally disordered over two positions across a center of inversion.The disorder was resolved without restraints in geometry, but with restrained parameters for thermal motion of the carbon atoms.The topological analysis was performed using TOPOS 4.0 46 and Graphical visualization of the structures was made using the program Diamond 2.1e. 47

Scheme 2
Scheme 2 Three kinds of polynuclear Cu/OH/pyridazine clusters, representing the structures of the reported complexes: Abinuclear motif in 1; Btetranuclear clusters in 2 and 3; Cpentanuclear units in 4 and 5, with a central trinuclear hydroxo/pyridazine core.

Fig. 1
Fig. 1 (a) Binuclear cluster in the environment of carboxylate and pyridazine ligands in the structure of 1; (b) 2D Cu/carboxylate subtopology in the form of hexagonal net, with the pyridazine-N donors accommodated at two sides of the plane.Only one orientation of the disordered cluster is shown.

Fig. 2
Fig. 2 Interpenetration of two inversion-related 3D frameworks (marked with blue and grey bonds) in the structure of 1.The hydroxocopper-carboxylate layers are orthogonal to the drawing plane.

Fig. 4
Fig. 4 (a) Primitive cubic framework of 3 viewed down the b direction showing interconnection of heteroligand carboxylate/pyridazine planes by additional carboxylate linkers.(b) The heteroligand planes with double organic bridges between the tetranuclear clusters constituting the framework nodes.

Fig. 8
Fig. 8 Thermal variation of χ m and χ m T for 4 (the solid lines are a fit to the data).

Table 1
Bridging coordination of non-chelating pyridazines towards Cu(II) ions a

Table 2
Highly-connected topologies of MOFs 1-5 a Dimensionality of the Cu/carboxylate subtopology and the overall dimensionality of the resulting heteroligand frameworks. 30