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Bioinorganic Chemistry and Applications
Volume 2010 (2010), Article ID 367128, 7 pages
Synthesis and Structural Characterization of a Metal Cluster and a Coordination Polymer Based on the Unit
Department of Chemistry, University of Cyprus, 1678 Nicosia, Cyprus
Received 24 February 2010; Accepted 9 March 2010
Academic Editor: Spyros Perlepes
Copyright © 2010 Eleni E. Moushi et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
A new 1-D coordination polymer [M(CMe(O]2.5O (12.5O and the cluster [M((CPh (py(MeCN)(O)]2MeCN (22MeCN) are reported. Both compounds were synthesized by room temperature reactions of [M(-O)(CR(L()] (R = Me, L = = py, (12.5O; R = Ph, L = py, = O, 22MeCN) in the presence of 3-hydroxymethylpyridine (3hmpH) in acetonitrile. The structures of these complexes are based on hexanuclear mixed-valent manganese carboxylate clusters containing the [(-O structural core. (12.5O consists of zigzag chain polymers constructed from [M(CMe(O] repeating units linked through acetate ligands, whereas 22MeCN comprises a discrete M-benzoate cluster.
The synthesis of Mn clusters has attracted significant interest due to their relevance to many areas including molecular magnetism, catalysis, and bioinorganic chemistry [1, 2]. In the bioinorganic area, extensive work has been carried out to model the structure and catalytic activity of a tetranuclear Mn cluster, which is present in the water oxidizing centre (WOC) of Photosystem II [3–7]. As a result, a number of oligonuclear high oxidation state Mn-carboxylate clusters have been prepared [3, 5], some of which have been studied for their ability to oxidize H2O to molecular O2 [3, 6, 7]. Furthermore, considerable effort has been expended in order to prepare structural and reactivity models of other Mn-containing enzymes, such as Mn catalases. These studies have resulted in a number of oligonuclear Mn complexes with oxo/alkoxo/hydroxo or carboxylate bridges, some of which have proven to be very efficient catalytic scavengers of H2O2 . The synthesis of oligonuclear Mn model compounds often involves preformed Mn carboxylate clusters and coordination polymers as starting materials, with the most popular ones being complexes based on the [Mn3O]6+/7+ and the [Mn6O2]10+ units [3, 9–11]. Since the various characteristics of the starting materials including their structural core, carboxylate bridges, and terminal ligation have a significant influence on the identity of the reaction product, there is always a need for new additions in the list of known metal precursor compounds.
Herein, we report the syntheses and the crystal structures of the 1D coordination polymer [Mn6O2(O2CMe)10(H2O)4]2.5H2O (12.5H2O)∞ and the discrete cluster [Mn6O2(O2CPh)10(py)2(MeCN)(H2O)]2MeCN (22MeCN), which both contain the [(-O)2]10+ structural core. Compound 22MeCN is a new addition in the family of structurally-characterized Mn6-benzoate clusters [12, 13], whereas (12.5H2O)∞ represents one of the few coordination polymers based on hexanuclear Mn clusters [14–17].
All manipulations were performed under aerobic conditions using materials (reagent grade) and solvents as received; water was distilled in-house. [Mn3O(O2CMe)6(py)3]py and [Mn3O(O2CPh)6(py)2(H2O)]0.5CH3CN were prepared as described elsewhere .
2.2. Compound Preparation
[Mn6O2(O2CMe)10 (H2O)4]2.5H2O (12.5H2O)∞: [Mn3O(O2CMe)6(py)3]·py (0.2 g, 0.24 mmol) was dissolved in MeCN (10 mL), and then 3 hmpH (0.05 g, 0.46 mmol) was added to the dark brown solution. The resulting red-brown solution was left under magnetic stirring for ~50 minutes, filtered off, and the filtrate was left undisturbed at room temperature. After a few weeks, brown crystals of (12.5H2O)∞ suitable for X-ray crystallography were formed. The crystals were collected by filtration, washed with MeCN (10 mL), and Et2O (2 × 10 mL) and dried in vacuo. The yield was ~27% based on total Mn content. Anal. Calc. for C20H43Mn6O28.5 (12.5H2O)∞: C, 22.47; H, 4.05. Found: C 22.29; H 4.25%. IR data (KBr pellet, cm−1): = 3404 (m,br), 1582 (s), 1421 (s), 1371 (w), 1028 (m), 667 (s), 619 (s).
[Mn6O2(O2CPh)10(py)2(MeCN)(H2O)]2MeCN (22MeCN): [Mn3O(O2CPh)6(py)2(H2O)]0.5CH3CN (0.27 g, 0.24 mmol) was dissolved in MeCN (10 mL) and then, 3 hmpH (0.05 g, 0.46 mmol) was added to the dark brown solution. The resulting red-brown solution was left under magnetic stirring for ~45 minutes, filtered off and the filtrate was left undisturbed at room temperature. After a few weeks, brown crystals of (22MeCN) suitable for X-ray crystallography were formed. The crystals were collected by filtration, washed with MeCN (10 mL) and Et2O (2 10 mL) and dried in vacuo. The yield was ~20% based on total Mn content. Anal. Calc. for C86H71Mn6N5O23 (22MeCN): C, 55.17; H, 3.82; N, 3.74. Found: C 54.98; H 3.91; N, 3.53%. IR data (KBr pellet, cm−1): = 3398 (m,br), 1607 (s), 1570 (s), 1430 (s) 720 (s), 691 (m), 676 (m), 614 (m).
2.3. X-Ray Crystallography
Data were collected on an Oxford-Diffraction Xcalibur diffractometer, equipped with a CCD area detector and a graphite monochromator utilizing Mo-Kα radiation ( = 0.71073 Å). Suitable crystals were attached to glass fibers using paratone-N oil and transferred to a goniostat where they were cooled for data collection. Unit cell dimensions were determined and refined by using 4714 () and 23078 () reflections for (12.5H2O)∞ and 22MeCN, respectively. Empirical absorption corrections (multiscan based on symmetry-related measurements) were applied using CrysAlis RED software . The structures were solved by direct methods using SIR92  and refined on using full-matrix least squares with SHELXL97 . Software packages used: CrysAlis CCD  for data collection, CrysAlis RED  for cell refinement and data reduction, WINGX for geometric calculations , and DIAMOND  and MERCURY  for molecular graphics. The non-H atoms were treated anisotropically, whereas the aromatic and methyl-hydrogen atoms were placed in calculated, ideal positions and refined as riding on their respective carbon atoms. The H atoms of water molecules could not be located. Unit cell data and structure refinement details are listed in Table 1.
2.4. Physical Measurements
Elemental analyses (C, H, N) were performed by the in-house facilities of the University of Cyprus, Chemistry Department. IR spectra were recorded on KBr pellets in the 4000–400 cm-1 range using a Shimadzu Prestige-21 spectrometer.
3. Results and Discussion
The goal of the described research is the synthesis of multidimensional coordination polymers composed of polynuclear Mn carboxylate clusters with the use of hydroxymethyl-pyridine derivatives [e.g., 4-hydroxymethyl-pyridine (4hmpH), 3-hydroxymethyl-pyridine (3hmpH)] as bridging ligands. The initial result from these investigations was a new hexanuclear Mn complex [Mn6O2(O2CPh)10(4hmpH)3(MeCN)], which contains the [Mn6O2]10+ structural core and terminal 4 hmpH ligands . This compound was prepared from a reaction of [Mn(O2CPh)2]2H2O with 4hmpH in MeCN. Various modifications of this reaction system that were performed involved the use of preformed Mn clusters as precursor compounds together with 3hmpH. Thus, the reaction of [Mn3(μ3-O)(O2CR)6(L)2()] (R = Me, L = = py, (12.5H2O)∞;R = Ph, L = py, = H2O, 22MeCN) with 3hmpH in acetonitrile resulted in the isolation of compounds (12.5H2O)∞ and 22MeCN, which however did not contain the 3hmpH ligands. The formation of (1)∞ and 2 is summarized in equations 1 and 2, respectively: As it will be discussed in detail below, the structures of (12.5H2O)∞ and 22MeCN are very similar with one major difference between them being the fact that (12.5H2O)∞ is a coordination polymer, whereas 22MeCN is a discrete metal cluster. A possible explanation for this is that the bulky groups that are present in 22MeCN prevent the polymerization of the Mn6 clusters, whereas in (12.5H2O)∞ there are only acetate ligands that are more flexible and thus can easily bridge Mn6 units leading to a polymeric species. We also note that the average oxidation state of the final products () of the two reactions is lower than that of the starting materials (). Such a reduction could be explained assuming that a disproportionation reaction of the Mn3 starting materials takes place upon their dissolution in MeCN in the presence of 3hmpH. Then, the reduced species are aggregated to form (12.5H2O)∞ or 22MeCN and the products with Mn ions in higher oxidation states remain in the solution. Similar reactions as those leading to the isolation of (12.5H2O)∞ or 22MeCN were performed using 4hmpH or pyridine instead of 3hmpH in the reaction mixtures. These reactions resulted in the isolation of microcrystalline products that have not been completely characterized so far, but seem to be different than compounds (12.5H2O)∞ and 22MeCN (by comparisons of infrared spectra). Reactions were also carried out by us in the past, where no other reagent (e.g., pyridine or triethylamine) was used besides the [Mn3O(O2CMe)6(py)3] precursor compound and the solvent. In that case, an 1D coordination polymer based on Mn3-carboxylate cluster linked by Mn2+ ions was isolated . Therefore, 3hmpH seems to play an important role in the formation of compounds (12.5H2O)∞ and 22MeCN, since different compounds are isolated in the absence of 3hmpH. However, the exact role of 3 hmpH in the assembly of these compounds is yet unidentified.
3.2. Crystal Structures
The structure of the repeating unit of (12.5H2O)∞ is very similar to that of compound 22MeCN (with the main differences between the two compounds being the terminal ligation and the type of carboxylate ligands) and thus, only the first one will be discussed in detail. Selected interatomic distances for (12.5H2O)∞ and 22MeCN are given in Tables 2 and 3, respectively.
Compound (12.5H2O)∞ crystallizes in the orthorhombic space group Pbca. Its repeating unit comprises the hexanuclear cluster [Mn6O2(O2CMe)10(H2O)4] (Figure 1) and totally 2.5H2O molecules of crystallization. Charge considerations, bond valence sum calculations (Table 4) and inspection of metric parameters indicate that the cluster is mixed-valent containing four and two ions. The [()2]10+ core of 1 has appeared several times in the literature as will be discussed in detail below and can be described as consisting of two edge-sharing ()Mn4 tetrahedra. Such units are defined as anti-T1 tetrahedra (T1 is a structural unit having a cation at the center and four anions at the apices of the tetrahedron) . The common edge of the two anti-T1 tetrahedra is formed by the two ions, whereas the four ions occupy the corners of the [()2]10+ core. The peripheral ligation of the Mn atoms is completed by 4 terminal H2O molecules (ligated to the four atoms) and 10 acetate ligands.
All Mn atoms are in distorted octahedral geometries. Five of the intra-cluster acetate groups are with each of their carboxylate oxygen atoms acting as terminal ligand for a Mn center. Four acetate ligands are coordinated in : : fashion. The remaining carboxylate ligand bridges two atoms (MnMn distance = 4.7914() Å) of adjacent Mn6 clusters, thus resulting in the formation of a zigzag chain structure (Figure 2). The chains are interacting through hydrogen bonds (OO distances 2.7−2.9 Å) involving the coordinated water molecules and carboxylate O atoms. Thus, a two-dimensional hydrogen-bonded polymer with a 4-connected topology is formed (Figure 3). The hydrogen bonds involving the lattice water molecules cannot be identified with accuracy due to the positional disorder of these molecules and thus, are not discussed here.
A representation of the structure of 22MeCN is given in Figure 4. The structure of 22MeCN is very similar to that of (12.5H2O)∞ with the main differences between them being (i) the type of terminal ligands [4H2O for (12.5H2O)∞; 2 py, one MeCN and one H2O for 22MeCN] (ii) the type of carboxylate groups (acetate for (12.5H2O)∞; benzoate for 22MeCN) and (iii) their dimensionality [(12.5H2O)∞ is a coordination polymer, whereas 22MeCN is a discrete metal cluster]. Regarding point (iii) we note that examination of the packing of 22MeCN revealed the existence of intermolecular hydrogen bonding interactions (OO distances 2.792() and 2.809() Å) involving the terminal H2O molecule and two atoms of two neighboring Mn6 molecules resulting in the formation of a dimeric (22MeCN)2 aggregate.
The Mn6 unit that appears in (12.5H2O)∞ and 22MeCN, that is, the cluster [Mn6O2(O2CR)10(L)2() ()] (R = Me, L = = = H2O, 1; R = Ph, L = py, = H2O, = MeCN, 2), has a structural motif found in several hexanuclear Mn clusters and coordination polymers [9, 12–16]. For example, we have recently reported the discrete cluster [Mn6O2(O2CPh)10(4hmpH)3(MeCN)] containing the [()2]10+ core and also three terminal 4hmpH groups linked through their atom and a MeCN molecule . In addition, compound (12.5H2O)∞ is closely related to compound [Mn6O2(O2CEt)10(H2O)4]2EtCO2H (32EtCO2H)∞, recently published . The main structural differences between them lie in the type of carboxylate ligands in these compounds, being acetate groups in (12.5H2O)∞ and propionate ligands in (32EtCO2H)∞ and also in the type of the crystallization solvent molecules. Other related examples to (12.5H2O)∞ comprise the chain polymers [Mn6O2(O2CCMe3)10(thf)2(NIT-Me)][Mn6O2(O2CCMe3)10(thf)(CH2Cl2)(NIT-Me)] (thf = tetrahydrofuran, NIT-Me = 4,5-dihydro-1H-imidazolyl-3-oxide-1-oxyl)  and [Mn6O2(O2CCMe3)10(HO2CCMe3)2(bpy)] (bpy = 4,-bipyridine) .
We reported the syntheses and the crystal structures of compounds (12.5H2O)∞ and 22MeCN, which are based on the well-known [Mn6()2]10+ structural core. Both compounds were prepared serendipitously in our attempt to prepare polymeric species consisting of polynuclear Mn clusters linked through 3 hmpH. Compound (12.5H2O)∞ features a zigzag chain structure formed by [Mn6()2(O2CMe)10(H2O)4] clusters linked via bridging acetate ligands. This compound joins a family of coordination polymers based on the [Mn6()2]10+ unit, which numbers only a few members. Furthermore, compound 22MeCN represents a new addition in the growing family of Mn6-benzoate clusters. Further work may involve replacement of the terminal solvent molecules in (12.5H2O)∞ or 22MeCN by various bridging polytopic ligands, in order to isolate higher dimensionality (2D, 3D) polymers. Multidimensional coordination polymers consisting of oligonuclear Mn clusters would be potential candidates for various applications including gas storage and catalysis.
The authors thank the Cyprus Research Promotion Foundation (Grant: ΔIΔAKTP/ΔIΣEK/0308/22), for financial support of this research. Crystallographic data in CIF format have been deposited at the Cambridge Crystallographic Data Centre with CCDC no. 767225 [(12.5H2O)∞] and 767226 (22MeCN). Copies of this information can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: +44-1223-336033; e-mail: firstname.lastname@example.org). This work is dedicated to Professor Nick Hadjiliadis in recognition of his great contribution for the advancement of Bioinorganic and Inorganic Chemistry, both in Greece and internationally, and for his retirement
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