Crystallographic and magnetic studies of the 2-pyridone/copper halide system
Graphical abstract
Copper(II) bromide and chloride complexes of 2-pyridone of the formula CuX2(2-pyridone)n(H2O)m (X = Cl, Br; n = 1, 2,3; m = 0, 2) have been prepared and studied magnetically and crystallographically. Three of the complexes form dimers linked either via bichloride bridges, or via bridging 2-pyridone O-atoms, while one forms an isolated species and one forms a chloride bibridged coordination polymer. The complexes exhibit a mixture of ferromagnetic and antiferromagnetic interactions.
Introduction
The study of coordination complexes as molecular magnets has the potential to allow for the construction of materials that have designed physical and magnetic properties depending upon the structure of the compound. Through choice of metal ion and ligand, the complex can potentially be modified to have different opacities, densities, and pathways of magnetic exchange. Studying the mechanism through which magnetic exchange occurs and how the structure of the transition metal complexes affects the magnitude and sign of the exchange will allow for the design of materials with specific magnetic properties.
The focus of our research has been to create a catalog of metal–organic copper halide complexes and salts through which crystal packing and magnetic exchange can be classified based on specific properties such as size, electronic nature of substituents, and hydrogen bonding properties of the organic material used. In recent years, much work has been done related to complexes of substituted 2-aminopyridine compounds with copper halides [1]. By altering the substituents on the 2-aminopyridine ring, a variety of structural motifs and therefore a variety of magnetic properties were obtained.
It is important to note the hydrogen bonding properties of the 2-aminopyridine/2-aminopyridinium moieties in these complexes. Hydrogen bond donation via both the amino group and pyridinium ion were significant in controlling the structure of the crystal [1]. If such a ligand, when substituted with different functional groups, is able to form transition metal complexes with a wide variety of structural and magnetic properties, what additional effects could be observed by replacing the hydrogen bond donating amino group with a substituent which could serve as both a hydrogen bond donor and acceptor? Although the amino substituent is in principle a hydrogen-bond acceptor, in practice the conjugation of the lone pair on the amino-N atom to the aromatic ring makes it a poor acceptor at best. However, 2-hydroxypyridine can serve as both a hydrogen bond donor (at the O atom) and a hydrogen bond acceptor (at both N and O atoms), either of which could aid in the packing and stabilization of complexes. It readily tautomerizes to the 2-pyridone form (Scheme 1) which is a hydrogen bond donor only at the N atom while the O atom may now serve only as a hydrogen bond acceptor.
Both the X-ray [2] and neutron [3] scattering studies indicate that the preferred form for the compound in the solid state is that of the 2-pyridone tautomer. A number of coordination complexes have been isolated showing coordination to the O-atom of 2-pyridone to Cu(II) [4], as well as Fe(II) [5], Mn(II) [6], second and third row transition metals [7], and lanthanide ions [8] and a significant number of Cu(II), Co(II), and Ni(II) complexes of the 6-chloro- and 6-methyl-2-pyridone compounds (many heterobimetallics incorporating lanthanide ions) have been reported [9]. Far fewer structures have been reported with coordination to the N-atom of the 2-hydroxypyridine tautomer, and all are of second and third row transition metals [10]. To date, only two 2-pyridone copper chloride compounds have been reported in the literature [11], [12], including the room temperature crystal structure of [(C5H5NO)2CuCl2]2, but magnetic studies had not been completed on these compounds. Here, we report the synthesis and characterization of structural and magnetic properties of five complexes of 2-pyridone with copper halides: [(C5H5NO)2CuBr2]2 (1) [(C5H5NO)2CuCl2]2 (2), [(C5H5NO)CuCl2]2 (3), [(C5H5NO)3CuCl2] (4) and [(C5H5NO)2][CuCl2·2H2O]2 (5).
Section snippets
Experimental
2-Hydroxypyridine was purchased from the Aldrich Chemical Company and used without further purification. Copper chloride and copper bromide were obtained from Baker and used without further purification. IR spectra were recorded on a Paragon 500 spectrometer. Elemental analyses were carried out by the Marine Science Institute, University of California, Santa Barbara, CA. Repeated combustion analyses consistently showed a low percent composition of hydrogen in compounds 1–3. However, powder
Results
Reaction of CuBr2 in acidic THF/H2O yielded crystals of 1, [CuBr2(2-pyridone)2]2 in good yield. Although the yield varied slightly, complex 1 was obtained as the product under a variety of conditions, included changes of solvent and pH. However, the chemistry of the CuCl2/2-pyridone system is substantially more complex. Although previous reports of the synthesis of 2, [CuCl2(2-pyridone)2]2, [12] were unsuccessful in our hands, direct combination of copper(II) chloride and 2-pyridone in
Discussion
Reaction of 2-hydroxypyridine with CuBr2 in water, acetonitrile, and ethanol at a variety of temperatures consistently produced [CuBr2(2-pyridone)2]2 (1), with yields near 80%. Reaction of 2-hydroxypyridine with CuCl2 yielded a variety of products depending upon the reaction conditions. When the reaction takes place in water at room temperature, [CuCl2(H2O)2]·2(2-pyridone) (5) is consistently the major product. However, when the reaction occurs in acetonitrile, the products are temperature
Acknowledgments
Financial assistance from the NSF (IMR-0314773), the Kresge Foundation, and PCISynthesis Inc. toward the purchase of the MPMS SQUID magnetometer and powder diffractometer are gratefully acknowledged. K.C.S. is grateful for a James and Ada Bickman Summer Science Internship. The authors thank M.Sc. T. González (IVIC-Venezuela) for technical support with the structure of 5.
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