Trinuclear Copper(II) Endomacrocyclic Complex with a New

Trinuclear Copper(II) Endomacrocyclic Complex with a New Nonaazamacrocyclic Ligand Showing a Trapped Perchlorate Anion M. del Carmen Ferna´ndez-Ferna´ndez,‡ Rufina Bastida,*,‡ Alejandro Macı´as,‡ Paulo Pe´rez-Lourido,† and Laura Valencia*,†

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 11 3924–3926

Departamento de Quı´mica Inorga´nica, Facultad de Quı´mica, UniVersidad de Vigo, 36310 Vigo, PonteVedra, Spain, and Departamento de Quı´mica Inorga´nica, UniVersidad de Santiago de Compostela, AVda. de las Ciencias s/n., 15782 Santiago de Compostela, La Corun˜a, Spain ReceiVed July 15, 2008; ReVised Manuscript ReceiVed September 15, 2008

ABSTRACT: The first structurally characterized example of an aza Schiff base tricopper(II) endomacrocyclic complex has been obtained. The complex presents a calixarene type conformation with a large central hole occupied by a perchlorate ion. It remains occluded in the cavity as it is trapped by acetonitrile and water molecules coordinated to the metal ions. The formation of polynuclear transition metal complexes offers an important route for the construction of new types of magnetic materials,1 development of catalytic reagents,2 models of metalloenzymes,3 and the construction of metallosupramolecular architectures.4 In last few years, one of the main targets in the field of metallosupramolecular chemistry is anion recognition5 by artificial receptors for chemical transport and screening by membrane channels in biological systems.6 The stability of metal complexes with polydentade ligands depends on a range of factors such as the number and type of donor atoms present and their relative positions within the ligand, the nature of the ligand backbone, and the number and size of the chelate rings formed on complexation. In last few decades, much attention has been given to the synthesis of macrocyclic ligands not only because they are good for synthesis of polynuclear metal complexes but they are also an excellent basis for the study of molecular recognition phenomena, since their cavity size, shape, and components can be varied readily.7 One strategy for the synthesis of macrocyclic ligands is template synthesis. Metal ions can be used to promote template synthesis of macrocyclic ligands and their utility depends on several factors related to the ligand characteristics, as well as on the nature of the metal ion.8 Copper is one of the most important metals in biological systems, mainly due to the role it plays in the binding, transport, and activation of molecular oxygen.9 Some of these systems, for example, oxidases, are based on dinuclear or oligonuclear active centers.10 One approach to the study of such biological systems involves the use of macrocyclic ligands with a large number of donor atoms and cavities with appropriate shapes and dimensions that may be able to coordinate two or more metal ions in a similar way to the multinuclear metal arrays at the active sites of several metallo-enzymes.11 However, the synthesis of multinuclear metal complexes with a single macrocyclic ligand for this purpose has not been reported often, and the majority of these macrocyclic complexes contain macrocyclic compartmental ligands12 or macrocyclic ligands joined by different bridging groups,13 for example, oxamides14 or phenylene spacers.15 As a continuation of our work on azamacrocyclic complexes, we report here the template synthesis and characterization of a trinuclear Cu(II) perchlorate complex with a new nonaazamacrocyclic ligand containing pyridine and cyclohexyl groups, L (Scheme 1), which represents, to the best of our knowledge, the first structurally characterized example of a tricopper(II) endomacrocyclic complex without bridging groups.16 * Corresponding author. ‡ Universidad de Santiago de Compostela. † Universidad de Vigo.

Scheme 1. Nonaazamacrocyclic Ligand (L)

The complex was obtained through a metal-templated Schiff base macrocyclization between 2,6-diformylpyridine, 1,3-bis(aminomethyl)cyclohexane, and Cu(ClO4)2 · 6H2O in a 1:1:1 molar ratio in acetonitrile. The crystal structure of [Cu3L(CH3CN)5(H2O)2](ClO4)6 · CH3CN.H2O17 is shown in Figure 1 together with the atomic numbering scheme adopted. The structure contains the trinuclear cation [Cu3L(CH3CN)5(H2O)2]6+, six perchlorate ions, one water and one acetonitrile molecule in the asymmetric unit. The structure can be described as a trinuclear endomacrocyclic complex with the three copper(II) ions within the macrocyclic cavity in different

Figure 1. Crystal structure of the cation [Cu3L(CH3CN)5(H2O)2]6+ with displacement ellipsoids drawn at the 50% probability level.

10.1021/cg800758d CCC: $40.75  2008 American Chemical Society Published on Web 10/15/2008

Communications

Crystal Growth & Design, Vol. 8, No. 11, 2008 3925

Figure 2. Crystal structure of the cation [Cu3L(CH3CN)5(H2O)2]6+ with a perchlorate ion occluded within the macrocyclic cavity. Figure 4. Crystal packing of the ligand in the network showing the hexagonal structures.

Figure 3. Hydrogen bond interactions in the structure.

environments. The Cu(1) and Cu(2) ions present a similar {N5} coordination sphere, with each metal bonded by one pyridine nitrogen atom from the macrocyclic backbone, the two imine groups contiguous to that pyridine ring and two acetonitrile molecules, giving rise to a distorted square planar pyramidal geometry [τ ) 0.13 for Cu(1) and 0.15 for Cu(2)],18 with Cu(1) and Cu(2) 0.20 and 0.19 Å away from the best plane defined by N(1)-N(2)-N(9)N(2s) (rms ) 0.0405) and N(3)-N(4)-N(5)-N(3s) (rms ) 0.0430), respectively. The remaining copper ion Cu(3) has a {N4O2} coordination sphere in an octahedral geometry with a tetragonal distortion, being coordinated by one pyridinic and two imine nitrogen atoms, one acetonitrile and two water molecules. The distances between the metal centers are similar [Cu(1)sCu(2), 8.8925(13) Å, Cu(1)sCu(3), 8.7581(13) Å and Cu(2)sCu(3), 8.9035(11) Å], and it can be considered that they are arranged on the vertices of an equilateral triangle. The pyridine groups provide the shortest bond to the copper atoms [Cu(1)-N(1) 1.929(3), Cu(2)-N(4) 1.926(4) and Cu(3)-N(7) 1.929(4) Å]. The bond lengths of the imine groups vary from 2.057(3) Å for Cu(1)-N(9) to 2.082(4) Å for Cu(3)-N(6). The longest distances, as expected for a higher coordination number, correspond to the axial positions of the octahedral copper ion Cu(3)-N(5S), 2.352(4) Å, and Cu(3)-O(1w), 2.360(5) Å, due to the well-known Jahn-Teller effect for d9 systems. The macrocyclic ligand is folded in such a way that all the pyridine and cyclohexane groups fall on the same side of the plane defined by the macrocyclic hole, thus adopting a cyclodextrin-type conformation. The large central hole of the complex is occupied by a perchlorate ion (Figure 2), which remains occluded within the cavity as it is trapped by the acetonitrile and water molecules bonded to the three copper metal ions in the structure; hydrogen bonding interactions are observed between this perchlorate ion and one of the water

molecules bonded to the Cu(3) ion. Another five perchlorate ions are present surrounding the trinuclear cation. Although positions of the hydrogen atoms of the water molecules could not be determined, some possible hydrogen bond interactions (Figure 3) can be observed between the trapped perchlorate ion [O6P-O2W 2.948(8) Å], one free acetonitrile molecule [O2W-N15 2.761(15) Å], the coordinated and free water molecules [O3W-O1W 2.651(8) Å] and one perchlorate ion [O1P-O3W 3.106(8) Å]. The triangular [Cu3L(CH3CN)5(H2O)2]6+ moieties are arranged in a tessellated fashion, which gives rise to hexagonal forms (Figure 4) - although evidence for inter- or intramolecular π,π-stacking interactions was not observed between them. In conclusion, we have presented a new an interesting example of a trinuclear Cu(II) aza Schiff base complex obtained by template synthesis. The structure shows that the large central hole in the complex is occupied by a perchlorate ion. Several attempts to obtain the macrocyclic ligand by direct synthesis, as well as the complex using other Cu(II) salts, were unsuccessful. This suggests the perchlorate anion plays a basic role in the template process. Caution: Perchlorate salts are potentially explosive. Although problems were not encountered during this research, only small amount of material should be prepared and such complexes should be handled with care.

Acknowledgment. We thank the Xunta de Galicia (PGIDT07PXIB209039PR) for financial support. Supporting Information Available: Sample preparation, characterization, and crystallographic data (CIF) for complex is available free of charge via the Internet at http://pubs.acs.org.

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