A new mixed-metal MnRh coordination polymer assembled from Mn-containing molecular building blocks and Rh2(OAc)4 dimers
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Introduction
The design of polymeric organic-inorganic materials with novel topologies and structural motifs is of current interest in the field of coordination chemistry [1], [2], [3], [4], [5], [6], [7], [8]. Although the precise assembly of such extended structures cannot be fully controlled, some fundamental aspects of coordination chemistry (the identity, oxidation state and coordination preference of the metal being some of the most important factors) can nonetheless be utilized to direct the overall product architecture. While a large variety of mono-metallic frameworks have been reported [1], [2], [3], [4], [5], [6], [7], [8], bi-metallic polymers are much less common. They are, however, of considerable interest because of the possibility of ligand-mediated communication between two different metal centers, which can potentially give rise to more complex physical properties, including electrical conductivity and magnetic ordering [9], [10], [11], [12], [13], [14]. We have developed a synthetic route to prepare mixed-metal and/or mixed-valent coordination polymers by utilizing a metal containing building block constructed from 2-pyrazinecarboxylic acid or 2-methylpyrazine-5-carboxylic acid (hereafter pyzcaH or MepyzcaH). These organic components were chosen because of their known chemical versatility [15], [16], [17], [18] and complex binding capabilities.
In M(pyzca) and M(Mepyzca) complexes, two coordination sites (neutral and/or charged) chelate the metal center while two donor sites remain free and can further participate in the assembly of a polymeric structure by an exo-binding mode to other metals or by supramolecular interactions with nearby H-bonding donors. In this context, Cu(pyzca)2 and Cu(Mepyzca)2 complexes have been successfully incorporated into several novel copper containing mixed-valent and mixed-metal frameworks [19], [20], [21]. We now report an extension of this chemistry to manganese-based bi-metallic coordination polymers using the manganese analogue, Mn(MePyzca)2(MeOH)2, as one of the building blocks. The Mn starting material was reacted with Rh2(OAc)4 to yield the first manganese-rhodium mixed-metal coordination polymer.
Coordination polymers and oligomers containing dimetal clusters have not been explored as much as other coordination polymers, although numerous bridgded dimetal units containing copper, rhodium, molybdenum and ruthenium are known [22], [23], [24], [25], [26], [27], [28] and recently, two reviews describing examples of dimetal tetracarboxylate unit containing one-dimensional polymers appeared [29], [30]. Tetrakis(carboxylato)rhodium compounds were first reported in 1960 [31], but it was not until 1981 that the first polymeric species containing this building block was reported by Cotton [32]. Rhodium dimers have been investigated extensively by Cotton [30], where the main focus was on modifications to the bridging groups and their cross-linking into polymeric structures, rather than the use of the open trans-coordination sites for polymer formation. The bridged dimetal clusters represent a useful linear ligand having two trans lewis acid binding sites that readily bond to lewis bases, such as are found in N,N′-bipyridine type ligands.
Our goal has been to use the open coordination site on the rhodium dimer as the attachment point to incorporate it into polymeric chain structures by connecting the dimers with N,N′-bipyridine type ligands. We and others have been very successful with this approach and several polymeric structures have been obtained [22], [23], [24], [25], [26], [27], [28], [33], [34], [35], [36], [37], [38]. An extension of this approach, the use of metal containing building block with free donor sites, enables us to tether the rhodium dimer, and prepare novel mixed metal systems. Herein we report the synthesis, crystal structure, and magnetic properties of this novel zigzag chain type mixed metal coordination polymer, {[Mn(Mepyzca)2(MeOH)2][Rh2(OAc)4]}·2MeOH (2).
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Materials and methods
MnCl2·6H2O (Aldrich, 99.99%), Rh2(OAc)4 (Strem Chemicals, 99%) and 2-methylpyrazine-5-carboxylic acid (Avocado, 98%) were used without further purification. Elemental analyses were performed by Desert Analytics Laboratory. The magnetic susceptibility of 2 was measured as a function of temperature using a Quantum Design MPMS XL SQUID magnetometer in an applied field of 10 kOe. A clear gelatin capsule was used as the sample container.
Results and discussion
The starting material Mn(Mepyzca)2(H2O)2, 1, was synthesized by reacting 2-methylpyrazine-5-carboxylic acid with MnCl2·6H2O in a basic solution. 1 was reacted further with Rh2(OAc)4 by layering the two reactants in methanol and ethanol, respectively. The reaction yielded small red block crystals. Single crystal X-ray diffraction revealed the red crystals to be a new mixed-metal manganese/rhodium coordination polymer (2).
Single crystal X-ray analysis of the molecular compound 1 showed the
Supplemental crystallographic data
Crystallographic data (excluding structure factors) for the structures in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication nos. CCDC 186396 & 186397. Copies of the data can be obtained, free of charge, on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK, (fax: +44 1223 336033 or e-mail: [email protected]).
Acknowledgements
Acknowledgement is made to the Donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support through grant PRF#36822 and to the National Science Foundation for partial support through Grant DMR:0134156. We also thank the NSF REU program for support of C.T.C.
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