Synthesis and Structural Characterization of a Metal Cluster and a Coordination Polymer Based on the [Mn6(μ 4-O)2]10+ Unit

A new 1-D coordination polymer {[Mn6O2(O2CMe)10(H2O)4]·2.5H2O}∞ (1·2.5H2O)∞ and the cluster [Mn6O2(O2(O2CPh)10 (py)2(MeCN)(H2O)]·2MeCN (2·2MeCN) are reported. Both compounds were synthesized by room temperature reactions of [Mn3(μ 3-O)(O2CR)6(L)2(L′)] (R = Me, L = L′ = py, (1·2.5H2O)∞; R = Ph, L = py, L′ = H2O, 2·2MeCN) 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 [Mn4 IIMn2 III(μ 4-O)2]10+ structural core. (1·2.5H2O)∞ consists of zigzag chain polymers constructed from [Mn6O2(O2CMe)10(H2O)4] repeating units linked through acetate ligands, whereas 2·2MeCN comprises a discrete Mn6-benzoate cluster.


Introduction
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][4][5][6][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 H 2 O to molecular O 2 [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 H 2 O 2 [8]. 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 [Mn 3 O] 6+/7+ and the [Mn 6 O 2 ] 10+ units [3,[9][10][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

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 (λ = 0.71073Å). 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 (3.14 ≤ θ ≤ 30.42 • ) and 23078 (3.07 ≤ θ ≤ 31.25 • ) reflections for (1·2.5H 2 O) ∞ and 2·2MeCN, respectively. Empirical absorption corrections (multiscan based on symmetry-related measurements) were applied using CrysAlis RED software [19]. The structures were solved by direct methods using SIR92 [20] and refined on F 2 using full-matrix least squares with SHELXL97 [21]. Software packages used: CrysAlis CCD [19] for data collection, CrysAlis RED [19] for cell refinement and data reduction, WINGX for geometric calculations [22], and DIAMOND [23] and MERCURY [24] 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.

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 , and m and n are constants.
on KBr pellets in the 4000-400 cm −1 range using a Shimadzu Prestige-21 spectrometer.

Results and Discussion
As it will be discussed in detail below, the structures of (1·2.5H 2 O) ∞ and 2·2MeCN are very similar with one major difference between them being the fact that (1·2.5H 2 O) ∞ is a coordination polymer, whereas 2·2MeCN is a discrete metal cluster. A possible explanation for this is that the bulky PhCO 2 − groups that are present in 2·2MeCN prevent the polymerization of the Mn 6 clusters, whereas in (1·2.5H 2 O) ∞ there are only acetate ligands that are more flexible and thus can easily bridge Mn 6 units leading to a polymeric species. We also note that the average oxidation state of the final products (2.33) of the two reactions is lower than that of the starting materials (2.66). Such a reduction could be explained assuming that a disproportionation reaction of the Mn 3 starting materials takes place upon their dissolution in MeCN in the presence of 3hmpH. Then, the reduced species are aggregated to form (1·2.5H 2 O) ∞ or 2·2MeCN and the products with Mn ions in higher oxidation states remain in the solution. Similar reactions as those leading to the isolation of (1·2.5H 2 O) ∞ or 2·2MeCN 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 (1·2.5H 2 O) ∞ and 2·2MeCN (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 [Mn 3 O(O 2 CMe) 6 (py) 3 ] precursor compound and the solvent. In that case, an 1D coordination polymer based on Mn 3 -carboxylate cluster linked by Mn 2+ ions was isolated [25]. Therefore, 3hmpH seems to play an important role in the formation of compounds (1·2.5H 2 O) ∞ and 2·2MeCN, 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.

Crystal Structures.
The structure of the repeating unit of (1·2.5H 2 O) ∞ is very similar to that of compound 2·2MeCN (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 (1·2.5H 2 O) ∞ and 2·2MeCN are given in Tables 2 and 3, respectively. Compound (1·2.5H 2 O) ∞ crystallizes in the orthorhombic space group Pbca. Its repeating unit comprises the    considerations, bond valence sum calculations ( Table 4) and inspection of metric parameters indicate that the cluster is mixed-valent containing four Mn II and two Mn III ions. The [Mn 4 II Mn 2 III (μ 4 -O) 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 (μ 4 -O)Mn 4 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) [26]. The common edge of the two anti-T1 tetrahedra is formed by the two Mn III ions, whereas the four Mn II ions occupy the corners of the [Mn 4 II Mn 2 III (μ 4 -O) 2 ] 10+ core. The peripheral ligation of the Mn atoms is completed by 4 terminal H 2 O molecules (ligated to the four Mn II atoms) and 10 acetate ligands.
All Mn atoms are in distorted octahedral geometries. Five of the intra-cluster acetate groups are μ 2 with each of their carboxylate oxygen atoms acting as terminal ligand for a Mn center. Four acetate ligands are coordinated in η 1 : η 2 : μ 3 fashion. The remaining carboxylate ligand bridges two Mn II atoms (Mn· · · Mn distance = 4.7914(2)Å) of adjacent Mn 6 clusters, thus resulting in the formation of a zigzag chain structure ( Figure 2). The chains are interacting through hydrogen bonds (O· · · O distances 2.7−2.9Å) 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.

Conclusions
We reported the syntheses and the crystal structures of compounds (1·2.5H 2 O) ∞ and 2·2MeCN, which are based on the well-known [Mn 6 (μ 4 -O) 2 ] 10+ structural core. Both compounds were prepared serendipitously in our     4 ] clusters linked via bridging acetate ligands. This compound joins a family of coordination polymers based on the [Mn 6 (μ 4 -O) 2 ] 10+ unit, which numbers only a few members. Furthermore, compound 2·2MeCN represents a new addition in the growing family of Mn 6 -benzoate clusters. Further work may involve replacement of the terminal solvent molecules in (1·2.5H 2 O) ∞ or 2·2MeCN 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.