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Mononuclear discrete complexes and coordination polymers based on metal(II) chelidonate complexes with aromatic N,N-chelating ligands† Ana Belen Lago, Rosa Carballo,* Nuria Fern andez-Hermida and Ezequiel M. Vazquez-Lopez
Downloaded by Universidad de Burgos on 03 March 2011 Published on 25 October 2010 on http://pubs.rsc.org | doi:10.1039/C0CE00330A
Received 23rd June 2010, Accepted 15th September 2010 DOI: 10.1039/c0ce00330a The versatile coordination chemistry of the anion chelidonate (chelidonic acid, H2chel, 4-oxo-4Hpyran-2,6-dicarboxylic acid) allowed for the synthesis of different mixed-ligand complexes with the N,N0 -chelating ligands 2,20 -bipyridine and 1,10-phenanthroline. We describe here the synthesis and structural characterization of a series of compounds that illustrate the structural diversity of which 4-oxo-4H-pyran-2,6-dicarboxylate ligands are capable: the 2D coordination polymer [Cu(chel)(bipy)] (1), the 1D coordination polymer [Zn(chel)(bipy)(OH2)]$1/2H2O (2), the discrete neutral mononuclear compounds [Zn(chel)(phen)(OH2)2] (3) and [M(chel)(phen)(OH2)3]$nH2O, M ¼ Zn and n ¼ 1 (4), M ¼ Ni and n ¼ 2 (5), and the discrete mononuclear cationic complex [Zn(OH2)2(phen)2]chel$3.3H2O (6). The effects of the weak interactions on the crystal packing were also analyzed. The thermal behaviour of the hydrated compounds was investigated. The study of the magnetic properties of 1 revealed weak antiferromagnetic coupling between the copper(II) centres.
Introduction Our knowledge about the construction of metal–organic frameworks using polycarboxylate ligands and suitable metal salts shows that the carboxylate unit provides an excellent means for the design of novel materials by acting as a linker or by imparting unique structural features due to its functionalizability.1 In particular, the exploration of rigid polycarboxylate ligands to construct novel architectures is of great interest,2 and in this field the rigid dicarboxylate linkers have produced a variety of metal– organic frameworks with diverse topologies and interesting properties.3 Chelidonic acid (H2chel, 4-oxo-4H-pyran-2,6-dicarboxylic acid) is one of the constituents of the greater celandine Chelidonium majus, which exhibits a wide range of therapeutic properties.4 It is a biogenic compound and the pKa value of about 2.4 for both carboxyl groups makes it an effective coordinating agent at physiological pH.5 H2chel (chelidonate ion, chel2 in Scheme 1) has two carbon-carbon double bonds within its heterocyclic ring and also has several functional groups that are
available for coordination with metal ions and can produce metallosupramolecular arrangements through weak interactions. As a consequence, chelidonate ion could be an appropriate ligand for the construction of metallosupramolecular compounds. However, to the best of our knowledge, very few metal complexes with the chelidonate ion have been described in the literature: i.e. some coordination polymers6–9 and some discrete metal complexes.6,10 In these polymeric and discrete compounds, the chelidonate ligand adopts diverse coordination modes (Scheme 2), a fact that suggests great possibilities for the construction of new metallosupramolecular architectures. Moreover, the introduction of ancillary ligands that occupy some coordination sites can inhibit the expansion of the polymeric frameworks, induce new structural evolution, and give rise to low dimensional coordination polymers. N,N0 -chelating ligands such as 2,20 -bipyridine11 and 1,10-phenanthroline12 are regarded as excellent N-donor candidates to control the dimensionality of coordination polymers and construct supramolecular structures through weak interactions.
Scheme 1 Dianionic chelidonate ion.
Departamento de Quımica Inorg anica, Facultade de Quımica, Universidade de Vigo, E-36310 Vigo, Galicia, Spain. E-mail:
[email protected]; Fax: +34 986 812556; Tel: +34 986 812273 † Electronic supplementary information (ESI) available: Additional data. CCDC reference numbers 779941 (1), 779942 (2), 779943 (3), 779944 (4), 779945 (5) and 779946 (6). For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c0ce00330a
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Scheme 2 Coordination modes of the chelidonate ligand found in the literature.
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We report here the synthesis, characterization and supramolecular structures of several M(II) mixed-ligand systems based on chelidonate ion and the N,N0 -chelating ligands 2,20 -bipyridine and 1,10-phenanthroline. The versatile coordination behaviour of chelidonate ligand allowed the synthesis of the 2D coordination polymer [Cu(chel)(bipy)] (1), the 1D coordination polymer [Zn(chel)(bipy)(OH2)]$1/2H2O (2), the neutral mononuclear compounds [Zn(chel)(phen)(OH2)2] (3) and [M(chel)(phen)(OH2)3]$nH2O, M ¼ Zn and n ¼ 1 (4), M ¼ Ni and n ¼ 2 (5), and the mononuclear cationic complex with the chelidonate ion in the second coordination sphere, [Zn(OH2)2(phen)2]chel$3.3H2O (6).
[Zn(chel)(H2O)2]9 corresponds to a 1D coordination polymer with the chelidonate ligand showing a bis-monodentate bridging coordination behaviour and in [Ni(H2O)6]chel$H2O10a the chelidonate ion acts as a counterion. The mixing of metal(II) chelidonates and the N,N-chelating ligands in traditional solvents resulted in rapid precipitation, which made it difficult to obtain single crystals of these complexes. As a result, in this work we used the solvothermal method and a microwave-assisted reflux technique to promote the crystallization processes. For compounds 1 and 3, the powder X-ray diffractograms of the bulk materials obtained by conventional synthesis were compared with those simulated for the crystal structures, showing that it is representative of the bulk material. All of the compounds were stable in air and generally had very poor solubility in commonly used solvents.
Results and discussion Spectroscopic studies
Synthesis 0
Reactions of Cu(II) and Zn(II) chelidonates with 2,2 -bipyridine led to the coordination polymers [Cu(chel)(bipy)] (1) and [Zn(chel)(bipy)(OH2)]$1/2H2O (2). The reactions of Zn(II) and Ni(II) chelidonates with 1,10-phenanthroline produced three neutral mononuclear complexes—[Zn(chel)(phen)(OH2)2] (3), (4) and [Ni(chel)(phen)[Zn(chel)(phen)(OH2)3]$H2O (OH2)3]$2H2O (5)—and also the cationic mononuclear complex [Zn(OH2)2(phen)2]chel$3.3H2O (6) with the chelidonate anion in the second coordination sphere. The synthetic procedures are shown in Scheme 3. The three metal(II) chelidonate precursors were prepared prior to the complexation reactions. The structures of these precursors have been described previously in the literature: [Cu(chel)(H2O)5]$H2O10b is a discrete complex in which the chelidonate ligand is coordinated to the copper ion through the ketonic oxygen atom; the Zn(II) precursor
The IR spectra of complexes 2–6 show medium-intensity broad bands in the 3260–3440 cm1 region and these correspond to overlapping OH stretching vibrations of the coordinated and crystallization water molecules and the y(CH) of the aromatic rings. This band is more asymmetric and broad in the spectra of 2 and 4–6, suggesting the presence of different kinds of water molecules in their structures. The absence in all of the complexes of strong bands between 1690 and 1750 cm1 indicates that all the chelidonate species are deprotonated. The absorption bands in the region 1650–1510 cm1 are due to the overlap of yasym(OCO) and the y(C]C) and y(C]N) modes of the aromatic rings. Despite this overlap, the yasym(OCO) mode could be tentatively assigned in all cases to the strong band around 1600 cm1. The medium and strong bands found between 1390 and 1320 cm1 are attributable to ysym(OCO).
Scheme 3 Synthetic procedures used for the complexes in this work: MW: microwave synthesis; R: synthesis at reflux, Solv: solvothermal synthesis, RT: room temperature.
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The diffuse reflectance spectrum of the Cu(II) polymer 1 shows a band centred at 15770 cm1, which corresponds to a d-d transition in a square-pyramidal copper(II) species, and a higher energy charge transfer band at 38 500 cm1. The electronic spectrum of the Ni(II) compound 5 shows three distinct bands at 10 800 cm1 (3T2g ) 3A2g), 19 200 cm1 (3T1g ) 3A2g) and 27 700 cm1 (3T1g(P) ) 3A2g) and these are characteristic of octahedral Ni(II) species.13
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Thermal behaviour The thermal decomposition of the hydrated compounds (2, 4, 5, and 6) was studied by thermogravimetric analysis. Thermal decomposition of compounds started with the release of water molecules. In compound 2, an initial mass loss of 4.56% was observed between 85 and 136 C, which corresponds to the release of the lattice water molecule and the half aqua ligand (calcd 4.18%). In 4, a weight loss of 9.3% in two steps at 131 and 200 C corresponds to the release of 2.5 water molecules (calcd 9%). In compounds 5 and 6, the release of the lattice and coordinated water molecules takes place is one step between room temperature and 130 C in 6 (obsd 12.99%) and 146 C in 5 (obsd 17.20%), corresponding to the loss of five molecules in both compounds (calcd 12.79% in 6 and 17.6% in 5). These dehydration processes are followed by one or two steps corresponding to oxidation of the organic component, which takes place in the range 250–300 C. In all compounds a plateau is reached at approximately 300 C, suggesting that the corresponding anhydrous species are thermally stable. Structural studies Coordination polymers 1 and 2. The coordination environments of the metal ions in compounds 1 and 2 are represented in Fig. 1 and 2, respectively. The structures are polymeric and are based on the neutral complexes [Cu(chel)(bipy)] (1) and [Zn(chel)(bipy)(OH2)] (2). Furthermore, the zinc compound is monohydrated. In both compounds the chelidonate ligand uses both carboxylate groups in the coordination to the metal ion but shows a different coordinative behaviour: in 2 the chelidonate ion acts as bismonodentate bridging ligand (m-kOIII:kOV) to produce an onedimensional polymer and in 1 it acts as a triconnective bridging ligand (m3-1kOV:2 : 3kOIV) that allows the formation of a 2D coordination polymer. This is the first example of this coordinative mode observed for the chelidonate ligand. Both metal centers are five-coordinated but they have different coordination geometries. In 1 the copper ion adopts an almost ideal squarepyramidal geometry with an Addison’s s value14 of 0.05. Each copper(II) ion is coordinated in the basal plane of the square pyramid by two pyridine nitrogen atoms of one 2,20 -bipy molecule and by two oxygen atoms of two different chelidonate ligands. In the apex of the square pyramid is the bridging oxygen atom O6 (Fig. 1). In this way a Cu2O2 core is formed with a Cu/ Thus, the Cui (i ¼ 1 x, 2 y, 1 z) distance of 3.333(1) A. chelidonate ligand, which contains all atoms in the same plane, connects three metal centers, two by one oxygen atom (O6) of one carboxylate group and the third by one oxygen atom of another carboxylate group, producing Cu/Cu distances of This journal is ª The Royal Society of Chemistry 2011
Fig. 1 Top: view of the coordination environment of 1. Bottom: view of the 2D coordination polymer [Cu(chel)(bipy)] (1) together with the coordination environment of the copper ions, a schematic representation of the layer and the coordination mode of the chelidonate ligand (top left, for clarity the 2,20 -bipy molecules are represented as a ring chelate). Main and angles ( ): Cu–O4 1.908(3), Cu–O6i 1.940(3), Cu– bond lengths (A) N1 1.983(3), Cu–N2 1.992(4), Cu–O6ii 2.564(3); O4–Cu–O6i 97.46(12), O4–Cu–N1 91.91(13), O6i–Cu–N1 169.60(12), O4–Cu–N2 166.75(14), O6i–Cu–N2 90.44(13), N1–Cu–N2 81.23(14), O4–Cu–O6ii 96.02(12), O6i–Cu–O6ii 85.56(11), N1–Cu–O6ii 88.99(12), N2–Cu–O6ii 95.19(12). Symmetry code: i ¼ x, 3/2 y, z 1/2; ii ¼ 1 x, ½ + y, 1.5 z.
The resulting corrugated sheet (Fig. 1, bottom 8.2 and 8.7 A. right), which is based on [Cu2(chel)2(bipy)2] units, forms 32membered macrocycles (Cu4O12C16) (Fig. 1, top left) and is reinforced by p–p interactions between the chelidonate rings and the interplanar with a centroid-centroid distance of 3.7 A dihedral angle a of 0.0 indicating that the rings are parallel. The structure of the 1D coordination polymer 2 (Fig. 2) has two five-coordinated crystallographically independent zinc atoms with similar coordination environments. Both coordination polyhedra are close to trigonal bipyramids, with Addison’s s values14 of 0.51 for Zn1 and 0.60 for Zn2. Two carboxylate oxygen atoms belonging to two chel2 molecules and one nitrogen atom of one 2,20 -bipy molecule are located on the equatorial plane and the second nitrogen atom of the 2,20 -bipy molecule and one coordinated water molecule are in the axial positions. The Zn–Owater distances are slightly longer than those observed in the related 1D coordination polymer [Zn(chel)(H2O)2],9 which has a tetrahedral geometry around the zinc ion. In the chain of 2 there are two different chelidonate CrystEngComm, 2011, 13, 941–951 | 943
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addition, p–p interactions between the 2,20 -bipy rings (centroid a ¼ 1.9 and angles b and g ¼ 24.9 ) centroid distance ¼ 3.8 A, contribute to the formation of the 3D network (Fig. 3). The packing of the metal–organic layers of 1 is organized by CH/O hydrogen bonds between the 2,20 -bipyridine and, in particular, the ketonic oxygen atom of chelidonate ligand as an acceptor (ESI, Table S3).† The formation of the resulting 3D network (Fig. 3) is also supported by a p–p interaction between and a ¼ the 2,20 -bipy rings (centroid–centroid distance ¼ 3.7 A 6.6 ).
Fig. 2 View of the 1D coordination polymer [Zn(chel)(bipy)(OH2)]$1/ 2H2O (2), the coordination environment of zinc ions and the 2D supra and angles ( ): Zn1)–O6 molecular association. Main bond lengths (A) 1.987(5), Zn1–O14 2.011(5), Zn1–O1W 2.089(5), Zn1–N3 2.100(6), Zn1– N4 2.122(7), Zn2–O16 2.007(5), Zn2–O4i 2.023(5), Zn2–N2 2.093(6), Zn2–O2W 2.093(5), Zn2–N1 2.118(6); O6–Zn1–O14 97.3(2), O6–Zn1– O1W 99.7(2), O14–Zn1–O1W 86.4(2), O6–Zn1–N3 126.8(2), O14–Zn1– N3 135.0(2), O1W–Zn1–N3 94.0(2), O6–Zn1–N4 95.5(2), O14–Zn1–N4 92.4(2), O1W–Zn1–N4 164.8(2), N3–Zn1–N4 76.2(2), O16–Zn2–Oi 100.7(2), O16–Zn2–N2 132.0(2), O4i–Zn2–N2 126.4(2), O16–Zn2–O2W 87.5(2), Oi–Zn2–O2W 97.9(2), N2–Zn2–O2W 94.2(2), O16–Zn2–N1 92.8(2), O4i–Zn2–N1 93.5(2), N2–Zn2–N1 77.0(3), O2W–Zn2–N1 168.4(2). Symmetry code: i ¼ x 1, y, z 1.
ligands alternating along the chain. All the atoms of one chel2 ligand are in the same plane as in 1, producing a separation The other chel2 ligand has between the two zinc ions of 10.1 A. two oxygen atoms (one from each carboxylate group) 0.3 and below the main plane and the other two oxygen atoms of 0.8 A above the the two carboxylate groups are located 0.5 and 0.6 A main plane. This latter disposition produces a shorter separation between the bridged metal centers. Compound 2 also of 9.9 A contains isolated water crystallization molecules that are hydrogen bonded to the chain, one involving as hydrogen acceptors the two oxygen atoms (O4 and O16) coordinated to Zn2 and the other the water molecule (O2W) coordinated to Zn2. These water molecules of crystallization do not interact with the environment of Zn1 and do not contribute to the supramolecular association of the chains. The chains of 2 are organized into layers (Fig. 2, bottom) by means of hydrogen bonds involving the coordinated water molecules O1w and O2w as donors and the ketonic oxygen atoms of the two chelidonate ligands and one uncoordinated oxygen atom belonging to one carboxylate group as acceptors (ESI, Table S3).† The formation of this 2D association is also reinforced by p–p interactions between the chelidonate rings, with an interplanar dihedral a centroid-centroid distance of 3.9 A, angle a of 0.03 (indicating that the rings are in a parallel disposition) and slipping angles b and g of 31.5 (revealing the slippage of the stacked chelidonate rings). The layers of 2 are further associated into a 3D supramolecular network (Fig. 3) by numerous CH/O hydrogen bonds (ESI, Table S3).† In 944 | CrystEngComm, 2011, 13, 941–951
Mononuclear discrete complexes 3–6. The coordination environments of the metal ions and the supramolecular organization for the neutral mononuclear complexes 3, 4 and 5 are represented in Fig. 4, 5 and 6, respectively. The molecular structure of the cationic complex 6 is represented in Fig. 7 and the supramolecular assembly is depicted in Fig. 7 and 8. Compound 3 is anhydrous but compounds 4, 5 and 6 are hydrates. The structures of 3, 4 and 5 are based on mononuclear neutral complexes [Zn(chel)(phen)(OH2)2] (3, Fig. 4) and [M(chel)(phen)(OH2)3] (M ¼ Zn in 4, Fig. 5, and M ¼ Ni in 5, Fig. 6), where the chelidonate ion acts as a monodentate ligand. In all three compounds the coordination environment around the metal center is different. In 3 (Fig. 4) the coordination geometry around the zinc ion can be described as trigonal bipyramidal (Addison’s value14 of 0.75) with the nitrogen atoms of phenanthroline and the carboxylate oxygen atom of chelidonate ligand in equatorial positions and the two coordinated water molecules in the axial positions. In 4 (Fig. 5) the zinc ion is in a distorted octahedral geometry with the two Nphen atoms, one Ochel and one coordinated water molecule in the equatorial plane and the other two coordinated water molecules in axial positions. An interesting aspect to note in these discrete complexes is the different orientation of the phenanthroline plane relative to that defined by the chelidonate ligand in each case. For example, in 3 all the chelidonate and phenanthroline atoms are in the same plane but this situation is very different for 4 and 5. In 5 and 4 the angle between the planes defined by the chelidonate ligand and phenanthroline are 22.1 and 60.2 , respectively. Furthermore, in 3 and 5 the atoms of the corresponding chelidonate ligands are in the same plane but in 4 the oxygen atoms of the uncoordinated below and 0.6 A above the carboxylate group are located 0.5 A main plane. A consequence of this different relative orientation of the phenanthroline and chelidonate planes can be seen in the intramolecular hydrogen bond involving one coordinated water molecule as the donor and the free oxygen atom of the coordinated carboxylate group as the acceptor. This hydrogen bond, which does not exist in 3, involves one of the water molecules in axial positions (O3w) in 4 whereas in 5 the water molecule involved is in the equatorial position (O1w). The structure of 6 (Fig. 7) is based on the cationic octahedral complex [Zn(OH2)2(phen)2]2+, a dianionic chelidonate ion and four crystallization water molecules in the outer coordination sphere. This cation is also present in other cationic discrete complexes16 and these have similar values for the interatomic distances and angles. The chelidonate anion is planar and shows interatomic distances and angles comparable to those observed in the complexes [Be(H2O)4]chel6 and [Ni(H2O)6]chel$H2O.10a This journal is ª The Royal Society of Chemistry 2011
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Fig. 3 View of the 3D metallosupramolecular networks of compounds 1 and 2.
and angles ( ): Zn–O1w 2.004(3), Fig. 4 Molecular structure and supramolecular organization of [Zn(chel)(phen)(OH2)2] (3). Main bond lengths (A) Zn–O2w 1.992/3), Zn–O11 2.032(2), Zn–N1 2.177(3), Zn–N2 2.092(3); O2w–Zn–O1w 117.60(13), O11–Zn–O2w 93.73(11), O1w–Zn–O11 96.96(11), O2w–Zn–N2 121.22(13), O1w–Zn–N2 120.34(12), O11–Zn–N2 88.59(10), O2w–Zn–N1 90.39(12), O1w–Zn–N1 92.79(11), N1–Zn–O11 166.17(10), N2– Zn–N1 78.00(10).
Crystal packing in compounds 3–6. The supramolecular association of the molecules in 3–6 is essentially based on hydrogen bonds between the water molecules as donors and the diverse oxygen atoms of chelidonate ion as acceptors. We present below a brief analysis of the supramolecular organization in each case. This journal is ª The Royal Society of Chemistry 2011
The molecules of the anhydrous compound 3 are organized into layers (Fig. 4) by means of hydrogen bonding interactions between the coordinated water molecules, which are at the two sides of the plane defined by phen and chel, and the uncoordinated oxygen atoms of the carboxylate groups of chelidonate CrystEngComm, 2011, 13, 941–951 | 945
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Fig. 5 Molecular structure and supramolecular organization of and angles ( ): [Zn(chel)(phen)(OH2)3]$H2O (4). Main bond lengths (A) Zn–O1W 2.033(2), Zn–O11 2.045(2), Zn–O2W 2.077(2), Zn–N2 2.154(2), Zn–N1 2.194(3), Zn–O3W 2.229(2); O1W–Zn–O11 97.31(10), O1W–Zn– O2W 88.28(10), O11–Zn–O2W 87.49(9), O1W–Zn–N2 94.48(10), O11– Zn–N2 167.89(9), O2W–Zn–N2 95.64(10), O1W–Zn–N1 171.12(10), O11–Zn–N1 91.53(9), O2W–Zn–N1 91.35(10), N2–Zn–N1 76.73(10), O1W–Zn–O3W 91.34(11), O11–Zn–O3W 88.56(9), O2W–Zn–O3W 175.95(9), N2–Zn–O3W 88.41(9), N1–Zn–O3W 89.64(11).
Fig. 6 Molecular structure and supramolecular organization of and angles ( ): [Ni(chel)(phen)(OH2)3]$2H2O (5). Main bond lengths (A) Ni–O11 2.028(5), Ni–O3W 2.055(6), Ni–N1 2.059(6), Ni–O1W 2.090(5), Ni–N(2 2.096(6), Ni–O2W 2.096(6); O11–Ni–O3W 90.9(2), O11–Ni–N1 90.7(2), O3W–Ni–N1 86.9(2), O11–Ni–O1W 94.8(2), O3W–Ni–O1W 88.4(2), N1–Ni–O1W 172.8(2), O11–Ni–N2 170.5(2), O3W–Ni–N2 90.8(2), N1–Ni–N2 80.1(2), O1W–Ni–N2 94.5(2), O11–Ni–O2W 85.1(2), O3W–Ni–O2W 175.8(2), N1–Ni–O2W 94.6(2), O1W–Ni–O2W 90.5(2), N2–Ni–O2W 93.3(2).
ion. In this way each molecule is connected by hydrogen bonds to five neighbouring molecules, with the shortest Zn–Zn distance The layers are further conbetween two molecules being 5.3 A. nected by CHphen/O hydrogen bonds involving the ketonic oxygen atoms of chelidonate ion as acceptors and these are oriented towards the outside layer. In compounds 4 and 5 the 946 | CrystEngComm, 2011, 13, 941–951
Fig. 7 Molecular structure of [Zn(OH2)2(phen)2]chel$3.3H2O (6). 2D supramolecular association of the second coordination sphere based on chelidonate anions and crystallization water molecules. Main bond and angles ( ): Zn–O1W 2.095(4), Zn–O2W 2.094(4), Zn–N12 lengths (A) 2.130(4), Zn–N1 2.152(4), Zn–N2 2.189(4), Zn–N11 2.201(4); O1W–Zn– O2W 93.64(15), O1W–Zn–N12 91.62(15), O2W–Zn–N12 94.21(16), O1W–Zn–N1 96.09(14), O2W–Zn–N1 92.64(15), N12–Zn–N1 169.33(16), O1W–Zn–N2 92.12(14), O2W–Zn–N2 168.74(15), N12–Zn– N2 95.29(16), N1–Zn–N2 77.12(15), O1W–Zn–N11 168.40(15), O2W– Zn–N11 88.48(15), N12–Zn–N11 76.84(17), N1–Zn–N11 95.20(16), N2–Zn–N11 87.81(16).
Fig. 8 Supramolecular organization of the cationic units in [Zn(OH2)2(phen)2]chel$4H2O (6) showing its insertion into the holes of the 2D chelidonate ion/water association.
lattice water molecules associate the complex molecules into dimers (Fig. 5 and 6). In 4 (Fig. 5) the lattice water molecule is hydrogen bonded to the ketonic oxygen of one molecule of complex and to one oxygen atom of the uncoordinated carboxylate group of another, (Fig. 5, bottom right). The uncoordinated part of chelidonate ligand is only involved in the formation of the dimer and an elongated association with a distance between the two farthest is observed. Consequently, the points in the dimer of 27.6 A This journal is ª The Royal Society of Chemistry 2011
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phenanthroline rings are appropriately disposed to establish efficient aromatic interactions (Fig. 5). The degree of these interactions may be evaluated by the centroid to centroid and the corresponding a dihedral distances of 3.6 and 3.8 A angles of 4.7 and 0 , respectively. In 5 (Fig. 6) one of the two lattice water (O6w) produces a dimeric association through hydrogen bonding with two oxygen atoms of both the coordinated and uncoordinated carboxylate groups and one water ligand of a neighbouring molecule of complex. In this case the distance between the two and the availability farthest points in the dimer is around 15.9 A of the phenanthroline rings to establish aromatic interactions is less than in 4. Only the interaction between the chelidonate ring and one of the phenanthroline rings, with a centroid to (a ¼ 19.6 ), may be described as centroid distance of 3.8 A aromatic. The dimers of 4 are organized into a 3D supramolecular association by several hydrogen bonds (ESI, Table S3)† involving as donors the two water ligands not involved in intramolecular hydrogen bonding and as acceptors all of the chelidonate oxygen atoms except the coordinated one. This 3D net is reinforced by the contribution of a C–H/O hydrogen bond and the aromatic phen–phen interactions described previously. In 5 the water ligand O2w that is not involved in intramolecular hydrogen bonding or in the formation of the dimer is responsible for the 2D supramolecular arrangement of the dimers, with the uncoordinated carboxylate groups involved as acceptors. Also the second lattice water (O5w) contributes to the organization of the dimers into layers (ESI, Table S3).† The further 3D organization is achieved by the contribution of the weak chel–phen aromatic interaction described above and one CHphen/O hydrogen bond with the ketonic oxygen atom of chelidonate ligand. It is interesting to note that the monohydrate 4 and the dihydrate 5 are similar compounds but with differences in their crystal packing. When the crystallization water molecules were omitted from the calculation, we found solvent area volumes of 3 (4.5% of the cell unit) and a K. P. I.15 of 70.1% for 4 and 979 A 3 (12.7% of the cell unit) and 61% for 5. These data 2145.3 A suggest that the different degrees of hydration in 4 and 5 are determined by the different crystal packing in each case. In summary, the solid state structures of 4 and 5 are clear examples of the key role in metallosupramolecular organization of factors such as the different relative orientation of two ligand planes (phen and chel) in one complex molecule. The supramolecular organization of compound 6 is shown in Fig. 7 and 8 and the main hydrogen bond distances are listed in the ESI, Table S4.† The lattice water molecules of 6 are associated by hydrogen bonds into a discrete tetrameric cluster (Fig. 7). Taking into account the participation of the coordinated water molecule in the association with the lattice water molecules, the water cluster can be considered as a pentamer. Each water cluster is connected to three chelidonate anions to produce a 2D association through Ow/O hydrogen bonds involving all the water molecules of the cluster and, as acceptors, some of the carboxylate groups and one ketonic oxygen of a chelidonate molecule. In this way the second coordination sphere forms a supramolecular layer that contains holes (Fig. 7) in which the cationic molecules are located (Fig. 8). This journal is ª The Royal Society of Chemistry 2011
The [Zn(OH2)2(phen)2]2+ cations are associated into dimers by C–H/p interactions (Fig. 8) that show a C.centroid distance and a C–H.centroid angle of 171.4 . These cationic of 3.8 A dimers are linked to the chelidonate ion/water layer by several kinds of interactions: Ow/O hydrogen bonds involving as donors the coordinated water molecules, C–H/O hydrogen bonds between phenanthroline and carboxylate groups of chelidonate ion, and p–p interactions between the chelidonate and phenanthroline rings with a centroid-centroid distance of and an a angle of 1.7 . In addition, an unusual contact 3.597(3) A between the carboxylate C]O group of one chelidonate ion and the p system of each phenanthroline molecule is observed. Indeed, the carboxylate oxygen atom O32 (which is also involved in hydrogen bonding, see ESI Table S4)† is in contact with the phenanthroline rings N12/C18/C19/C22/C23/C24 [C311–O32/ and angle ¼ 75.7 ; symmetry centroid: distance ¼ 3.476(6) A operation ¼ 1 x, 2 y, 1 z] and N2/C6/C7/C10/C11/C12 and angle ¼ 75.2 ; [C311–O32/centroid: distance ¼ 3.4 A symmetry operation ¼ 1/2 + x, 3/2 y, 1/2 + z]. Egli and Sarkhel16 showed that dihedral angles between the C]O bond and the plane of the aromatic ring in the range 0 to 24 are characteristic of a significant carbonyl-aromatic interaction. Here, the dihedral angles for first and the second C]O/p interactions are 14.3 and 10.9 , respectively. As a result of these interactions the cationic dimer is surrounded by six chelidonate anions and two water clusters. Magnetic properties of compound 1. The magnetic properties of [Cu(chel)(bipy)] (1) as represented by cMT and cM versus T plots [cM being the molar susceptibility per copper(II) ion] are shown in Fig. 9. The cMT value at room temperature is 0.80 cm3 mol1 K and this is consistent with two magnetically isolated Cu2+ ions. The cMT values remain almost constant down to 20 K and then smoothly decrease to 0.15 cm3 mol1 K at 1.9 K, indicating that there is a very weak antiferromagnetic coupling between the copper(II) ions. Taking into account the crystal structure of this compound, the susceptibility data were fitted to the Bleaney–Bowers equation18 derived from the Heisenberg spin Hamiltonian (H ¼ 2JS1$S2) for two coupled S ¼ ½ ions, where J is the magnetic coupling constant and N, g, b and k have their usual meaning. ! 2Nb2 g2 1 kT !# cM ¼ " J 3 þ exp kT
The best-fit parameters using a non-linear regression analysis are as follows: J ¼ 4.0(3) cm1, g ¼ 2.10(2) and R ¼ 1.35 105 P (R is the agreement factor defined as [(cMT)obs (cMT)calc]2/ P [(cMT)obs]2. The calculated curve matches the experimental data well over the whole temperature range (Fig. 9). The exchange pathway through the double m-oxo bridge is most likely responsible for this coupling between copper atoms. Previous magneto-structural studies have shown that the nature and magnitude of the magnetic coupling between the parallel magnetic orbitals depends on the values of the angle at the bridge 4 and the out-of-plane Cu/O bond of 97.5 and R0 ¼ 2.565 A CrystEngComm, 2011, 13, 941–951 | 947
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Fig. 9 Thermal dependence of the cMT product and cM for complex 1. (o) experimental data, (–) best fit curve.
allow a weak antiferromagnetic interaction through this bridge to be predicted. Thus, the small J value (–4.0 cm1) observed is a consequence of the disposition of the copper ions in this system, with their magnetic orbitals (mainly dx y ) located in practically parallel planes and the interaction transmitted through the O6 atom. Numerous examples of this kind of compound are known and from both a structural and magnetic point of view it is evident that the J value is strongly correlated to the Cu–O–Cu bridge,20 with J values between 1.51 and 2.15 cm1 for Cu–O– Cu angles of 95.4 and 101.0 , respectively. However, a correlation between the exchange parameters with respect to the Cu–Oaxial distance is more difficult to deduce.21 2
2
fact that slight deviations with respect to the main plane can be achieved by the oxygen atoms of the two carboxylate groups. The chelidonate ligand also proved to be an interesting tool in supramolecular coordination chemistry due to the presence of carboxylate and ketonic oxygen atoms, which can act as acceptors in classical and non-classical hydrogen bonds, and the ability to establish aromatic interactions such as p–p, CH/p and the less common C]O/p. The contribution of several of these interactions produces efficient packing and gives rise to thermally stable systems.
Experimental Conclusions
Materials and physical measurements
Six Cu(II), Zn(II) and Ni(II) mixed-ligand complexes have been synthesized by reaction of the corresponding M(II) chelidonate with two aromatic N,N0 -chelating ligands. The use of 2,20 bipyridine led to the isolation of coordination polymers 1 and 2, whereas the use of 1,10-phenanthroline produced the discrete mononuclear complexes 3–6. All of the discrete complexes were neutral except for 6, which was a cationic complex with the chelidonate ion acting as a dianionic counterion. The synthetic methods employed to facilitate the crystallization of all the complexes were the solvothermal and the microwave-assisted reflux approaches. In these complexes, the chelidonate ion has proven to be a versatile ligand as it showing shows three different coordination modes, one of which (the triconnective m31kOV;2:3kOIV) has been observed here for the first time. In each case the coordination mode observed seems to be controlled by the N,N0 -auxiliary ligand used. The chelidonate anion acts as a planar and rigid ligand but some flexibility is possible due the
All reagents and solvents were obtained commercially and were used as supplied. Elemental analyses (C, H, N) were carried out with a Fisons EA-1108 microanalyser. IR spectra were recorded from KBr discs (4000–400 cm1) on a Bruker Vector 22 spectrophotometer. A Shimadzu UV-3101PC spectrophotometer was used to obtain the electronic spectra (diffuse reflectance) in the region 250–900 nm. Magnetic susceptibility measurements at room temperature were performed using a Johnson Matthey Alfa MSB-MK1 Gouy balance. Magnetic susceptibility measurements on a crystalline sample of 1 were performed in the temperature range 1.9–300 K with a Quantum Design SQUID magnetometer. Diamagnetic corrections of the constituent atoms were estimated from Pascal’s constants.22 Experimental susceptibilities were also corrected for the temperature-independent paramagnetism [60 106 cm3 mol1 per copper(II) ion] and the magnetization of the sample holder. TGA analysis was carried out using a TA Instruments Hi-Res TGA2950 Thermobalance
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coupled to Bruker Tensor 27 FT-IR apparatus. X-Ray powder diffraction (XRPD) characterization was performed using a Siemens D-5000 diffractometer with Cu-Ka radiation (l ¼ over the range 5.0–60.0 in steps of 0.20 (2q) with 1.5418 A) a count time per step of 5.0 s. Microwave-assisted syntheses were performed in a domestic oven modified as described by Ardon et al.23
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Synthesis of the precursors [Cu(chel)(H2O)5]$H2O10b was obtained by reaction of CuCO3, Cu(OH)2 and chelidonic acid in a 1 : 2 molar ratio in MeOH. The resulting suspension was stirred for 1 week. The precipitate was filtered off, washed with MeOH and dried under vacuum. [Zn(chel)(H2O)2] was obtained by reaction of ZnCO3 and chelidonic acid in a 1 : 1 molar ratio in MeOH. The resulting suspension was heated under reflux for 2 h, left to cool to room temperature and stirred for 1 d. The white product was filtered off, washed with MeOH and dried under vacuum. This compound was also obtained hydrothermally and was recently characterized in the solid state.9 [Ni(H2O)6]chel$H2O10a was obtained by reaction in a 1 : 1 molar ratio of Ni(Ac)2$4H2O and chelidonic acid in H2O– MeOH. The resulting solution was heated under reflux for 8 h, left to cool to room temperature and stirred for 5 d. The green solution was left to slowly evaporate in air and after 8 d green crystals of [Ni(H2O)6]chel$H2O were isolated. Synthesis and crystallization of the complexes [Cu(chel)(bipy)] (1). A mixture of [Cu(chel)(H2O)5]$H2O (215 mg, 0.6 mmol) and 2,20 -bipyridine (187 mg, 1.2 mmol) in EtOH (15 mL) was heated under reflux for 2 h. The blue solid was filtered off, washed with water and dried under vacuum. Single crystals of 1 were obtained after the slow cooling of the solution resulting from the microwave-assisted reflux (irradiation for 60 s at 350 W) of a mixture (1 : 1 molar ratio) of [Cu(chel)(H2O)5]$H2O and 2,20 -bipyridine in H2O–dmf. The powder X-ray diffractogram of the bulk material obtained by conventional synthesis was compared with that simulated for the crystal structure, showing that it is representative of the bulk material. Data for (1): Yield: (conventional synthesis), 50%; (microwave synthesis), 25%. m.p. >250 C. Anal. calcd for C17H10N2O6Cu: C 50.8, H 2.5, N 7.0%; Found: C 50.5, H 2.8, N 6.9%. IR (KBr, cm1): 3433 m; 1648 s; 1600 sh; 1374 m; 1303 m; 1028 w; 903 w; 785 m; 713 w. UV-vis (lmax, cm1): 38 500, 15 770. [Zn(chel)(bipy)(OH2)]$1/2H2O (2). [Zn(chel)(H2O)2] (71 mg, 0.25 mmol), 2,20 -bipyridine (39 mg, 0.25 mmol), dmf (4 mL) and H2O (2 mL) were added to a 25 mL round-bottomed flask, which was placed in a microwave oven. The mixture was irradiated for 60 s at 350 W. Slow evaporation (8 d) of the resulting solution afforded single crystals of 2. Data for 2: Yield: 70%. Anal. calcd for C17H13O7.5N2Zn: C 47.4, H 3.0, N 6.5%; Found: C 46.9, H 2.9, N 6.6%. IR (KBr, cm1): 3423 m, b; 1636 s; 1610 sh; 1470 sh; 1350 m; 1021 w; 922 w; 800 m; 771 m; 732 m. This journal is ª The Royal Society of Chemistry 2011
[Zn(chel)(phen)(OH2)2] (3). Compound 3 was synthesized using a procedure similar to that for 2 except that 1,10-phenanthroline was used instead of 2,20 -bipyridine. Data for 3: Yield: 50%. m.p. >250 C. Anal. calcd for C19H14O8N2Zn: C 49.2, H 3.0, N 6.0%; Found: C 49.5, H 2.9, N 6.1%. IR (KBr, cm1): 3438 m, b; 3061 m; 1640 s; 1600 sh; 1513 w; 1408 m; 1354 s; 1097 w; 914 m; 848 w; 804 w; 725 m. (4). A mixture of [Zn(chel)(phen)(OH2)3]$H2O [Zn(chel)(H2O)2] (213 mg, 0.75 mmol) and 1,10-phenanthroline (297 mg, 1.5 mmol) in H2O (6 mL) and ethanol (6 mL) was sealed in a Teflon-lined stainless steel autoclave and heated at 90 C for 3 h under autogenous pressure and then cooled to room temperature at a rate of 5 C h1. Colourless single crystals of compound 4 were formed during the cooling process. After the isolation of these first crystals, the resulting solution was left to evaporate in air. After 1 d, single crystals of compound 6 were isolated. Data for 4: Yield: 30%. Anal. calcd for C19H18O10N2Zn: C 45.7, H 3.6, N 5.6%; Found: C 45.1, H 3.9, N 6.1%. IR (KBr, cm1): 3265 m, b; 3070 m; 1639 vs; 1326 s; 1600 sh; 1406 sh; 915 w; 854 w; 792 w; 723 m. [Ni(chel)(phen)(OH2)3]$2H2O (5). A mixture of [Ni(H2O)6]chel$H2O (215 mg, 0.58 mmol), 1,10-phenanthroline (116 mg, 0.58 mmol), H2O (6 mL) and dmf (2 mL) was sealed in a Teflonlined stainless steel autoclave and heated at 100 C for 24 h under autogenous pressure and then cooled to room temperature at a rate of 1 C h1. Slow evaporation (2 months) of the resulting green solution afforded single crystals of 5. Data for 5: Yield: 45%. Anal. calcd for C19H20O11N2Ni: C 44.7, H 3.9, N 5.5%; Found: C 44.7, H 3.9, N 5.4%. IR (KBr, cm1): 3383 s, b; 1625 s; 1571 m; 1406 m; 1361 s; 964 w; 852 w; 727 m. UV-vis (lmax, cm1): 27 700, 19 230, 10 800. [Zn(OH2)2(phen)2]chel$3.3H2O (6). Compound 6 was obtained as single crystals in the same solvothermal reaction used to obtain 4. Yield: 20%. 6 was also obtained by reaction at room temperature of a suspension of [Zn(chel)(H2O)2] (213 mg, 0.75 mmol) in H2O (10 mL) and a solution of 1,10-phenanthroline (148 mg, 0.75 mmol) in ethanol (10 mL). The mixture was stirred for 24 h. A precipitate, which was identified by powder X-ray diffraction as [Zn(chel)(phen)(OH2)2] (3), was filtered off. Single crystals of 6 were obtained from the resulting solution by slow evaporation (1 month) at room temperature. Data for 6: Yield: 20%. Anal. calcd for C31H28.6O11.3N4Zn: C 52.9, H 4.1, N 8.0%; Found: C 52.7, H 4.3, N 8.1%. IR (KBr, cm1): 3380 m, b; 1631 vs; 1600 sh; 1517 m; 1427 m; 1389 s; 1333 m; 1102 w; 860 w; 840 w; 807 w; 726 m. Crystallography. Crystallographic data were collected on a Bruker Smart 1000 CCD diffractometer at 293 K using graphite monochromated Mo-Ka00 radiation (l ¼ 0.71073 A) and were corrected for Lorentz and polarization effects. The frames were integrated with the Bruker SAINT24 software package and the data were corrected for absorption using the program SADABS.25 The structures were solved by direct methods using the program SHELXS-97.26 Reflections with CrystEngComm, 2011, 13, 941–951 | 949
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Table 1 Crystal and structure refinement data Compound
1
2
3
4
5
6
Empirical formula Formula weight T/K Crystal system Space group a/A b/A c/A a/ b/ g/ 3 V/A Z rc/g cm3 m/mm1 F(000) Crystal size/mm q range/ Reflections collected Independent reflections (Rint) Max/min transmission Goodness-of-fit on F2 Final R indices [I > 2s(I)]
C17H10O6N2Cu 401.81 293(2) Monoclinic P21/c 9.6342(14) 12.1379(17) 13.3493(19) — 104.034(2) — 1514.5(4) 4 1.762 1.483 812 0.30 0.28 0.11 2.18–28.07 9331
C34H26O15N4Zn2 861.33 293(2) Triclinic P1 8.872(3) 14.004(5) 15.346(5) 71.070(6) 73.666(7) 76.130(7) 1707.4(10) 2 1.675 1.487 876 0.21 0.20 0.10 1.81–25.03 7615
C19H14O8N2Zn 463.69 293(2) Triclinic P1 8.5318(14) 10.7273(17) 11.1772(18) 71.720(3) 73.501(3) 70.674(2) 897.9(3) 2 1.715 1.423 472 0.40 0.35 0.25 1.96–27.98 5782
C19H18O10N2Zn 499.72 293(2) Triclinic P1 7.6821(8) 9.8561(10) 13.5031(14) 84.562(2) 80.973(2) 76.465(2) 979.89(17) 2 1.694 1.317 512 0.33 0.20 0.11 1.53–28.03 6300
C19H20O11N2Ni 511.08 293(2) Monoclinic P21/c 10.9267(15) 23.869(4) 8.9977(13) — 113.911(3) — 2145.3(5) 4 1.582 0.969 1056 0.18 0.18 0.12 2.04–24.71 3627
C31H28.6O11.3N4Zn 703.35 293(2) Monoclinic P21/n 12.634(2) 12.944(2) 19.357(3) — 102.815(3) — 3086.6(9) 4 1.514 0.866 1452 0.40 0.22 0.17 1.76–28.03 19 126
3569 (0.0652)
5041 (0.0448)
4061 (0.0312)
4429 (0.0241)
3627 (0.0822)
7285 (0.0820)
1.0000/0.6074
1.0000/0.7799
1.0000/0.7126
1.0000/0.8484
1.0000/0.7969
1.0000/0.8267
0.873
1.148
1.014
1.020
1.046
0.991
R1 ¼ 0.0494 wR2 ¼ 0.1310
R1 ¼ 0.0627 wR2 ¼ 0.1415
R1 ¼ 0.0466 wR2 ¼ 0.1057
R1 ¼ 0.0434 wR2 ¼ 0.1038
R1 ¼ 0.0829 wR2 ¼ 0.2139
R1 ¼ 0.0726 wR2 ¼ 0.1822
in compound 5 were omitted due to the resolution value < 0.85 A high value of the Rint. All non-hydrogen atoms were refined with anisotropic thermal parameters by full-matrix least-squares calculations on F2 using the program SHELXL-97.27 Hydrogen atoms were inserted at calculated positions and constrained with isotropic thermal parameters. The hydrogen atoms of the water molecules in 3 were located from a difference Fourier map and refined with isotropic parameters but the hydrogen atoms for the water molecules in 2, 5 and 6 and the hydrogen atoms for the crystallization water molecule in 4 could not be located. Occupancy factors of O3w and O4w in compound 2 and, O6w in compound 6 were refined but fixed in the last stage of refinement at 0.5, 0.5 (2) and 0.3 (6). Drawings were produced with MERCURY28 and special computations for the crystal structure discussions were carried out with PLATON.29 Crystal data and structure refinement data are listed in Table 1.
2
3
Acknowledgements Financial support from ERDF (EU), MEC (Spain) and Xunta de Galicia (Spain) (research projects CTQ2006-05642/BQU and PGIDIT06-PXIB314373PR) are gratefully acknowledged. A. B. L thanks the Xunta de Galicia for a postdoctoral contract under the ‘‘Angeles Alvari~ no’’ Program.
4 5
6
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