A Unique Diamondoid Network Resulting from the Convolution of π-π Stacking and Lipophilic Interactions Augustin M. Madalan,† Victor Ch. Kravtsov,‡ Yurii A. Simonov,‡ Violeta Voronkova,§ Ludmila Korobchenko,§ Narcis Avarvari,| and Marius Andruh*,† Inorganic Chemistry Laboratory, Faculty of Chemistry, University of Bucharest, Str. Dumbrava Rosie 23, 020464 Bucharest, Romania, X-ray Laboratory, Institute of Applied Physics, Academy of Sciences of Moldova, Str. Academiei nr. 5, 2028-Kishinev, Moldova, Laboratory of Molecular Photochemistry, Department of Chemical Physics, Zawoisky Physical-Technical Institute of the Russian Academy of Sciences, Sibirsky Trakt 10/7, Kazan, Russia, and Laboratoire Chimie, Inge´ nierie Mole´ culaire et Mate´ riaux (CIMMA) UMR 6200 CNRS-Universite´ d’Angers, UFR Sciences, Baˆ t. K, 2 bd. Lavoisier, 49045 Angers, France
CRYSTAL GROWTH & DESIGN 2005 VOL. 5, NO. 1 45-47
Received June 28, 2004
ABSTRACT: A novel supramolecular pseudo-tetrahedral synthon, which generates two independent, interpenetrating, diamondoid nets, results from the lipophilic interactions between the aliphatic chains of the spacers connecting the copper(II) ions within two binuclear [{Cu(acac)(phen)}2(µ-bpp)]2+ species [bpp ) 1,3-bis(4-pyridyl)propane]. The deliberate construction of diamondoid nets has played a key role in the development of crystal engineering.1,2 The archetype is diamond itself, which is built up from self-complementary synthons, i.e., the tetrahedral carbon atoms. Other classical diamondoid networks, based also upon building blocks exhibiting internal complementarity, are ice and potassium dihydrogenophosphate. More sophisticated self-complementary building blocks are tetrafunctionalized organic molecules with Td or close to Td symmetry: adamantane-1,3,5,7-tetracarboxylic acid,3 methanetetraacetic acid,4 and tetrakis-pyridones.5 Their self-assembly into supramolecular diamondoid architectures is based on well-known supramolecular synthons, namely, the dimeric motifs of the carboxylic acids and pyridones. An alternative approach in designing diamond-like lattices relies upon metal-ligand bonds, when metallic ions, adopting a tetrahedral stereochemistry (CuI and ZnII), are connected by linear exo-bidentate ligands, such as CN-,6 rigid bis(4-pyridyl) derivatives [4,4′-bipyridine, bis(4-pyridyl)ethane, bis(4-pyridyl)butadiyne, etc.],7 or pyrazine derivatives.8 An elegant strategy in designing supramolecular diamondoid networks is the modular approach developed by Zaworotko.9 This is a multicomponent assembling process involving mutually complementary nodes and linear spacers. As a node, Zaworotko used a tetrafunctional strong hydrogen bond donor, the [Mn(CO)3(µ3-OH)]4 heterocubane, with the four OH groups oriented toward the vertices of a tetrahedron. Various difunctional molecules have been used as spacers and hydrogen bond acceptors (ethylenediamine, 1,4-diaminobenzene, 4,4′-bipyridyl, etc.). In this paper, we report on a novel supramolecular diamondoid network resulting from the convolution of π-π stacking and lipophilic interactions. The building block is quite unusual: It is a binuclear copper(II) complex, [{Cu(acac)(phen)}2(µ-bpp)](ClO4)2‚6H2O (1), with the copper(II) ions connected by a long, flexible ligand, 1,3-bis(4-pyridyl)propane (bpp). The bpp molecule can adopt various conformations when it acts as a ligand.10 The binuclear complex has been obtained by reacting a mononuclear precursor, [Cu(acac)(phen)(H2O)](ClO4), with bis(4-pyridyl)propane.11 The crystal structure of 1 has been * To whom correspondence should be addressed. E-mail: marius.andruh@ dnt.ro. † University of Bucharest. ‡ Academy of Sciences of Moldova. § Zawoisky Physical-Technical Institute of the Russian Academy of Sciences. | CNRS-Universite´ d’Angers.
solved and consists of binuclear cationic species, [{Cu(acac)(phen)}2(µ-bpp)]2+, perchlorate ions, and disordered water molecules. The copper(II) ions exhibit a square pyramidal geometry, with the oxygen atoms from the acetylacetonato ligand [Cu(1)-O(1) ) 1.916(3) Å; Cu(1)-O(2) ) 1.913(3) Å] and the phenanthroline nitrogen atoms [Cu(1)-N(1) ) 2.017(3) Å; Cu(1)-N(2) ) 2.014(4) Å] forming the basal plane and the pyridyl nitrogen atom coordinated in the apical position [Cu(1)-N(3) ) 2.283(3) Å] (Figure 1a). The distance between the copper ions within the binuclear entity is 12.79 Å. The -CH2-CH2-CH2- chain from the bpp spacer can be involved in lipophilic interactions with the similar chain from another spacer. Moreover, the phenanthroline molecule, coordinated to the copper ion, can play an important role in sustaining the supramolecular architecture, through its ability to be involved in π-π stacking interactions. Indeed, the analysis of the packing diagram of 1 reveals the occurrence of both intermolecular interactions. Figure 1a shows the interaction between the aliphatic chains from two bpp spacers, which leads to a pseudo-catenane motif (Figure 1b). The closest contact between the central carbon atoms of two bpp spacers within the supramolecular dimer is 3.49 Å. The bpp spacer within each binuclear entity exhibits the antianti conformation. The four {Cu(acac)(phen)} moieties are oriented toward the vertices of a distorted tetrahedron. The two embracing aliphatic chains form a pseudo-tetrahedral supramolecular synthon, which is able to generate, through stacking interactions involving the chelating ligands attached to the copper ions, a three-dimensional diamandoid network (Figure 2). Each phenanthroline ligand from a copper ion interacts with an acetylacetonato ligand from another one, generating a second type of supramolecular synthon (Figure 2, insert). The distances associated with these π-π contacts range between 3.35 and 3.40 Å. The intermolecular Cu‚‚‚Cu distance is 4.810(1) Å. Extended Hu¨ckel calculations of the HOMO‚‚‚HOMO overlap integrals, β,12 gave values of 51 and 7.5 meV for the pseudocatenane and π-π interactions, respectively, showing that lipophilic interactions are stronger than the π-π ones. Two independent networks interpenetrate (Figure 3). The empty space is filled by disordered perchlorate anions and water molecules (Figure 4). As a part of our research program on the exchange interactions mediated by π-π contacts, we carried out variable temperature electron paramagnetic resonance
10.1021/cg049793h CCC: $30.25 © 2005 American Chemical Society Published on Web 09/01/2004
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Figure 3. Packing diagram showing the two interpenetrating nets. For clarity, the carbon atoms from the pyridyl rings as well as from the acac and phen ligands have been removed.
Figure 1. Supramolecular interactions of two [{Cu(acac)(phen)}2(µ-bpp)]+ species. (a) Lipophilic interactions between the aliphatic chains. (b) The supramolecular pseudotetrahedral synthon; for the sake of clarity, the carbon atoms from the pyridyl rings have been removed. Color code: Cu, green; N, blue; O, red; and C, gray.
Figure 4. Packing diagram showing the channels running along the crystallographic c-axis.
Figure 2. Perspective view of an adamantane-like unit in the structure of 1. For clarity, the carbon atoms from the pyridyl rings as well as those from the acac and phen ligands have been removed. Color code: Cu, green; N, blue; O, red; and C, gray.
Figure 5. (a) Experimental spectrum of the polycrystalline sample of 1 in the Q-band at T ) 293 K, g| ) 2.220, and g⊥ ) 2.055. (b) The simulated spectra for J ) 0 (dotted line), and |J| ) 0.03 cm-1 (solid line). See text.
(EPR) measurements on 1, to detect the intermolecular exchange interactions between the copper(II) ions belonging to adjacent {Cu(acac)(phen)} moieties. The EPR spectra of the polycrystalline sample of 1 were measured in the X-band in the temperature range of 300-4.2 K and in the Q-band at T ) 293 and 4.2 K. The Q-band spectrum (Figure
5a) can be interpreted by a spin Hamiltonian with the effective spin S ) 1/2 and an axial anisotropy of the {g} tensor. This anisotropy is not resolved in the X-band, but a half-field transition, which indicates the presence of the interaction in the system, is detected. The Q-band spectrum has been simulated by means of a model, which consists
Communications of equivalent Cu‚‚‚Cu fragments (dCu‚‚‚Cu ) 4.808 Å), by taking into account the exchange and dipole-dipole interactions between the copper ions within a fragment, the hyperfine structure of the copper ions, and the intermolecular exchange between the Cu‚‚‚Cu fragments (Figure 5b). The value of the exchange parameter, which averages the fine structure of the spectrum, was estimated as being |J| ∼ 0.03 cm-1. The detailed discussion of the EPR spectra together with the magnetic susceptibility data for 1 and related compounds will be presented separately. The crystal structure of compound 1 has particular relevance in crystal engineering, as it represents a novel example to illustrate the versatility of the bis(4-pyridyl)propane molecule in constructing a rich variety of extended structures. This paper enriches the library of supramolecular syntons13 with an unprecedented one. The preparation of 1 is as follows. A solution containing 0.2 mmol of [Cu(acac)(phen)(H2O)](ClO4) dissolved in 15 mL of acetonitrile was reacted with a solution of bpp (0.1 mmol dissolved in 10 mL of ethanol). The slow evaporation of the resulting mixture led to blue crystals of compound 1. The [Cu(acac)(phen)(H2O)](ClO4) precursor was obtained as follows: two solutions containing the ligands (the first one was obtained by dissolving 2 mmol of phen in 20 mL of ethanol, and the second one was obtained by reacting 2 mmol of acetylacetone, Hacac, with the stoichiometric amount on LiOH in water) were added rapidly over 25 mL of aqueous solution containing 2 mmol of Cu(ClO4)2. The slow evaporation of the resulting mixture gave blue crystals of compound [Cu(acac)(phen)(H2O)](ClO4). Acknowledgment. Financial support from the INTAS Program (Project 2000-00375) is gratefully acknowledged. Supporting Information Available: X-ray crystallographic file in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org.
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