Crystal structure of BaMnB2O5 containing structurally isolated manganese oxide sheets

The structure of BaMnB2O5 is characterized by infinite sheets of B2O5 units and Mn2O8 dimers of edge-sharing MnO5 square pyramids while Ba2+ cations interleave successive sheets.

In an attempt to search for mixed alkaline-earth and transition metal pyroborates, the title compound, barium manganese(II) pyroborate, has been synthesized by employing a flux method. The structure of BaMnB 2 O 5 is composed of MnO 5 square pyramids that form Mn 2 O 8 dimers by edge-sharing and of pyroborate units ([B 2 O 5 ] 4À ) that are composed of two corner-sharing trigonal-planar BO 3 units. These building blocks share corners to form 1 2 [MnB 2 O 5 ] 2À layers extending parallel to (100). The Ba 2+ cations reside in the gap between two manganese pyroborate slabs with a coordination number of nine. The title compound forms an interesting spiral framework propagating along the 2 1 screw axis. The structure is characterized by two alternating layers, which is relatively rare among known transition-metal-based pyroborate compounds.

Chemical context
Numerous borates with various crystal structures and compositions have been widely investigated over the last few decades (Heller et al., 1986). Pyroborates containing the (B 2 O 5 ) 4À anion were first structurally characterized in 1950 (Berger, 1950). Pyroborates can be divided into two subclasses such as alkaline-earth-based pyroborates with general formula A 2 B 2 O 5 (A = alkaline earth metal) and transition-metal-based pyroborates with general formula MM 0 B 2 O 5 . If M = M 0 , the pyroborate is considered to be homo-metallic, otherwise it is hetero-metallic.
Alkaline-earth-based pyroborates adopt different structure types. During the investigation of the BaO/B 2 O 3 system, Hubner revealed Ba 2 B 2 O 5 crystallizing in space group P2/m (Hubner, 1969). The other alkaline-earth-based A 2 B 2 O 5 pyroborates (A = Mg, Ca, Sr) have been synthesized by hightemperature solid-state reactions. Mg 2 B 2 O 5 (Guo et al., 1995b) crystallizes in space group P2 1 /c. Ca 2 B 2 O 5 (Lin et al., 1999a) and Sr 2 B 2 O 5 (Lin et al., 1999b) are isotypic and crystallize in the same space group type as Mg 2 B 2 O 5 but have a different structure from the latter. Additionally, there exists a triclinic magnesium pyroborate (P1; Guo et al., 1995a). The existence of mixed alkaline-earth-based pyroborates (AA 0 B 2 O 5 ) has been proven by the study of naturally occurring minerals. The crystal structures of two polymorphs of CaMgB 2 O 5 , kurchatovite and clinokurchatovite, have been originally determined in space group types Pc2 1 b (Yakubovich et al., 1976) and P2 1 /c (Simonov et al., 1980). However, the crystal structures of both minerals have been re-examined and refined in different space group types (Callegari et al., 2003). Based on these models, kurchatovite crystallizes in space group type Pbca whilst clinokurchatovite crystallizes in space group type P2 1 /c.
Investigations of the BaO/CuO/B 2 O 3 phase diagram has resulted in the isolation of a non-centrosymmetric pyroborate, BaCuB 2 O 5 (Smith & Keszler, 1997) with a unique structure type in space group type C2. As part of an effort to isolate new mixed alkaline earth and transition metal pyroborates, we have investigated the BaO/MnO/B 2 O 3 phase diagram. In this study, we have grown single crystals of BaMnB 2 O 5 and analyzed its crystal structure.

Structural commentary
The crystal structure of BaMnB 2 O 5 defines a new structure type that can be described as being composed of manganese pyroborate slabs with composition 1 2 [MnB 2 O 5 ] 2À that extend parallel to (100). Fig. 1a shows a perspective drawing of the BaMnB 2 O 5 structure with the quasi-two-dimensional lattice characterized by [MnB 2 O 5 ] 2À slabs. The barium cations reside between the parallel slabs and maintain the interslab connectivity through coordination to nine oxygen atoms (Fig. 2c).
Two non-equivalent boron atoms are present in the structure; both are surrounded by three oxygen atoms to form almost regular trigonal-planar units. As depicted in Fig. 2b, the isolated [B 2 O 5 ] 4À pyroborate groups are composed of two corner-sharing trigonal-planar BO 3 units. In the reported pyroborate structures (Thompson et al., 1991), the terminal BO 2 planes pivot about the torsion angles to afford deviations from coplanarity that can range from 0 to 76.8 where the B-O-B angle ranges from 112 to 180 . In BaMnB 2 O 5 , the pyroborate groups show closely related geometric features as previously noted (Thompson et al., 1991) Zobetz, 1982). Fig. 1c shows the arrangement of isolated pyroborate units, appearing as two parallel pseudoone-dimensional chains spiraling around the 2 1 axis.
There is one crystallographically independent Mn atom which is coordinated by five oxygen atoms to form a square pyramid with four longer equatorial Mn-O bonds and one short apical Mn-O bond. Fig. 2a shows two Mn1O 5 square pyramids sharing a common edge, O2-O2(Àx + 1, Ày, Àz), to form an Mn 2 O 8 unit. As shown in Fig. 3a, Mn atoms are connected to each other via oxygen atoms with a Mn1Á Á ÁMn1 separation of 3.317 (2) Å and an Mn1-O2-Mn1 angle of 101.23 (16) . The neighboring Mn 2 O 8 dimers share vertices through oxygen atom O3. The oxygen atom O1 in the Mn 2 O 8 dimer is only corner-shared by the pyroborate group. The only unshared oxygen, O4, of the pyroborate group is pointing into the free space towards the neighboring slabs to form a bond with the barium atom. As shown in Fig. 3b, with respect to the pyroborate group, the B2O 3 unit shares two corners with neighboring MnO 5 square pyramids through O1 and O2 while the B1O 3 unit corner-shares a common oxygen atom, O3, with two other MnO 5 square pyramids. This arrangement facilitates the observed curvature which is necessary for the spiral framework found in the extended lattice (Fig. 1b). The unique arrangement of B 2 O 5 groups around the 2 1 screw axis provides an essential element allowing the spiral chain to propagate along the b axis. It is well known that the interplanar angle of the B 2 O 5 group is primarily dictated by packing effects and the nature of the associated cations in the given structure (Thompson et al., 1991). In addition to that, as previously noted, the greater deviations from coplanarity are observed in the arrangement of the B 2 O 5 groups due to variation of the sizes of alkali metals in alkali metal Nb and Ta oxide pyroborates (Akella & Keszler, 1995). Accordingly, the interplanar angle of the B 2 O 5 group is likely to be determined by the associate coordination environment of the barium cations in the title compound. It should be noted that the connectivity of the One of the interesting features of the title compound is that the structure can be alternatively viewed as a 'porous' framework as shown in Fig. 4a. The B 2 O 5 units together with interconnected Mn 2 O 8 dimers extend along the b axis in a standing wave fashion, creating oval shape windows which also arrange in a zigzag fashion along the same direction. It is intriguing to notice that the two B 2 O 5 groups along with the two Mn 2 O 8 dimers and two MnO 5 square pyramids create an empty cage (Fig. 4c,d). The polyhedral and ball-and-stick drawing (Fig. 4b) clearly shows the three-dimensional framework bearing large cavities. This unusual structural arrangement is conceivably attributed to the limitation of the size of the pyroborate unit that simultaneously tends to interconnect with barium cations and neighboring Mn 2 O 8 dimers in a corner-shared fashion (Fig. 3b). As shown in Fig. 5a, the layered nature of the title compound is characterized by parallel [MnB 2 O 5 ] 2À slabs outlined by a dotted rectangle viewed along [010]. Fig. 5b shows the ball-and-stick drawing of a portion of the layered manganese oxide network. Each Mn 2 O 8 dimer shares vertices with four other MnO 5 square pyramids through oxygen atoms to form these sheets. Within the extended Mn-O sheet, the MnO 5 square pyramids which share edges are separated from each other by 3.317 (2) Å (Mn1Á Á ÁMn1 distance) whereas those which share corners are separated by 3.435 (1) Å . The distance between the Mn atoms of the adjacent sheets is 8.287 (2) Å . Given the description of the local configuration of the manganese oxide polyhedra, their connectivity along the sheet, and the structural isolation of neighboring Mn-O sheets from each other, one would suspect that the title compound offers opportunities for the study of spin exchange in a confined Mn-O lattice. In theory, a periodic array of well-defined transition metal oxide lattices could provide a useful model for experimental and theoretical developments of magnetic and electronic interactions in transition metal oxides because of their simplified structures (Snyder et al., 2001).
The structure of BaMnB 2 O 5 is somewhat related to that of triclinic M 2 B 2 O 5 (M = Mn, Sarrat et al., 2005;Fe, Neumair & Huppertz, 2009;Co, Rowsell et al., 2003) (001) while the boron atoms hold the ribbons together forming B 2 O 5 groups. These extended ribbons are parallel to each other and therefore these M 2 B 2 O 5 phases have a quasi-one-dimensional structure in contrast to the two-dimensional Mn-O lattice in the title compound. Magnetic properties have been widely studied for Co 2 B 2 O 5 (Kawano et al., 2010), Fe 2 B 2 O 5 (Kawano et al., 2009), and Mn 2 B 2 O 5 (Fernandes et al., 2003) to understand the low-dimensional interactions derived from the ribbon-like substructures in these compounds. The spin configuration based on the electron-density distribution has been proposed (Sarrat et al., 2005) for Mn 2 B 2 O 5 in which the distance between manganese atoms of adjacent ribbons are 4.526-6.272 Å and electron-density distributions were indicated in the regions between the ribbons. According to their model, all coplanar ribbons of Mn 2 B 2 O 5 are ferromagnetic; their antiferromagnetic behavior is derived from antiparallel magnetic orientations between adjacent ribbons. In the title compound, the distance between manganese atoms within the sheets and adjacent sheets are 3.317 (2)-3.435 (1) Å and 8.287 (2) Å , respectively. It is important to note that the Ba 2+ cations reside in the gap between the two Mn-O sheets. This, together with the greater separation between manganese atoms of adjacent sheets, leads us to believe that magnetic interactions that occur between Mn-O sheets can be extremely weak and the dominant magnetic exchange should be between Mn 2+ ions within the Mn-O sheet. Judging from the interesting magnetic properties reported for M 2 B 2 O 5 (M = Mn, Fe, Co) compounds, we expect interesting magnetic phenomena from a systematic investigation of the magnetic susceptibility of BaMnB 2 O 5 .