Crystal structures of two decavanadates(V) with pentaaquamanganese(II) pendant groups: (NMe4)2[V10O28{Mn(H2O)5}2]·5H2O and [NH3C(CH2OH)3]2[V10O28{Mn(H2O)5}2]·2H2O

Two heterometallic decavanadate(V) hydrated salts with tetramethylammonium and [tris(hydroxymethyl)methyl]ammonium counter-cations have been synthesized under mild reaction conditions in an aqueous medium. Both polyanions present two [Mn(OH2)5]2+ complex units bound to the decavanadate cluster through oxide bridges.

Polyoxidovanadates containing manganese cations have been synthesized as ionic pairs (Shan & Huang, 1999;Lin et ISSN 2056-9890 al., 2011) or as heterometallic aggregates in which the oxidovanadate cluster acts as a metalloligand to the manganese complex (Inami et al., 2009;Klištincová et al., 2009). Recent interest in this kind of compound lies in a possible synergistic effect (involving the two metal elements) for the enhancement of the catalytic activity towards oxidation of organic substrates, such as in the photocatalytic degradation of dyes (Wu et al., 2012).
While the synthesis of decavanadates with different organic cations as building blocks for supramolecular assemblies is largely explored (da Silva et al., 2003), a systematic procedure for their functionalization with transition metal complexes has not been well established. Our research group is currently involved in the synthesis of heterometallic polyoxidovanadates containing manganese(II) because of their potential activity as catalysts of olefin epoxidation. In this context, the reaction between NH 4 VO 3 and mannitol to give A was carried out in aqueous solution in the presence of tetramethylammonium chloride (molar proportion 2:1:2), following a procedure described earlier by our group to produce the mixed-valence polyoxidovanadate (Me 4 N) 6 [V 15 O 36 (Cl)] (Nunes et al., 2012). The dark-green solution obtained after reflux for 24 h received one molar equivalent of Mn(OAc) 2 Á4H 2 O and was kept under reflux for 24 more hours. A mixture of dark-green crystals of (Me 4 N) 6 [V 15 O 36 (Cl)] and yellow prisms of (NMe 4 ) 2 [V 10 O 28 -{Mn(H 2 O) 5 } 2 ]Á5H 2 O (A) was isolated after four weeks at room temperature, the latter in 9% yield. Product A contains two tetramethylammonium cations and the [V 10 O 28 ] 6unit is covalently bound to two [Mn(OH 2 ) 5 ] 2+ complexes by terminal oxido bridges.

Structural commentary
The anionic heteropolyanions are essentially identical in the two complexes. However, in A, the molecule lies about the centre of the cell which is a point of 2/m symmetry, so that the unique part of the anionic cluster is one quarter of that heteropolyanion. The anion lies about a mirror plane which View of the components of (NMe 4 ) 2 [V 10 O 28 {Mn(H 2 O) 5 } 2 ]Á5H 2 O, A, indicating the atom-numbering scheme. No H atoms were identified on the disordered solvent water molecules. Displacement ellipsoids are drawn at the 50% probability level. [Symmetry codes: (1) 1 À x, y, 1 À z; (2) 1 À x, 1 À y, 1 À z; (3) x, 1 À y, z; (4) 1 À x, Ày, Àz; (5) 1 À x, y, Àz.] passes through the V2, V4 and manganese atoms, and there is a twofold symmetry axis which is perpendicular to the mirror plane and passes through V3 and the centre of the cell, Fig. 1.
The V 10 O 28 moiety in the structure of compound B lies about a twofold symmetry axis which passes through the vanadium atoms V6 and V7, Fig. 2. This is the only crystallographic symmetry in this ion which, nevertheless, shows a very similar structure to that found in the ion in compound A; views showing this pseudo-symmetry are presented in Figs. 3, 4 and 5. The unique part here is one half of the anion. The previously reported analysis of this anion [with a 2-(2-hydroxyethyl)pyridinium cation] showed the cluster to be lying about an inversion centre (Klištincová et al., 2009 (Klištincová et al., 2009). In both our compounds, there is a wide range of V-O bond lengths. The vanadium atoms on the outer shell of the heteropolyanions, e.g. V4 and V5 in A, and V2-V5 in B, are five-coordinate with a squarepyramidal pattern; there is a sixth oxygen atom in the direction of an octahedral site but, at ca 2.3 Å from the vanadium atom, rather longer than the normal coordination distance. Of the five bonded oxygen atoms, the apical site (opposite the distant, sixth, site) has the shortest V-O distance, ca 1.6 Å , corresponding to a vanadyl group. The more 'internal' vanadium atoms in each structure, viz V3 in A, and V6 and V7 in B, have more uniform V-O distances in more regular octahedral patterns. The anion of compound B viewed approximately down the b axis of the V 10 O 28 moiety. [Symmetry code: (1) 1 À x, y, 1 2 À z.]

Figure 5
The anion of compound B viewed approximately down the c axis of the V 10 O 28 moiety. [Symmetry code: (1) 1 À x, y, 1 2 À z.]

Figure 3
The anion of compound B viewed approximately down the a axis of the V 10 O 28 moiety. [Symmetry code: (1) 1 À x, y, 1 2 À z.]

Supramolecular features
In both compounds, O-HÁ Á ÁO hydrogen bonds from all the coordinating water molecules link the anions with neighbouring anions, either directly, through both the cluster O atoms and the coordinating water molecules, or indirectly through the solvent water molecules (Tables 1 and 2). In compound B, additional hydroxyl groups are available in the 'tris' cation, and these add further links in the extensive hydrogen bonding scheme. Additional C-HÁ Á ÁO interactions are observed in the structures of both compounds.

Synthesis and crystallization
General All reactions were performed in air with purified (Milli-Q 1 ) water. Commercial reagents were used without purification. The starting materials NH 4 VO 3 , MnCl 2 Á4H 2 O and Mn(OAc) 2 Á4H 2 O were supplied by Aldrich, while mannitol [C 6 H 8 (OH) 6 ] and (Me 4 N)Cl were purchased from USB and Merck, respectively. Infrared (FTIR) spectra were recorded on a BIORAD FTS-3500GX spectrophotometer from KBr pellets in the 400-4000 cm À1 region.
Synthesis of (NMe 4 ) 2 [V 10 O 28 {Mn(H 2 O) 5 } 2 ]Á5H 2 O (A) Solid NH 4 VO 3 (0.500 g, 4.27 mmol) and [(CH 3 ) 4 N]Cl (0.468 g, 4.27 mmol) were added to a solution of mannitol (0.366 g, 2.13 mmol) in 60 mL of water to produce a suspension that turned into a deep blue-greenish solution after one hour under reflux. After 24 more hours, a solution of Mn(OAc) 2 Á4H 2 O (1.04 g, 4.27 mmol) in 10 mL of water was added to this reaction mixture, which remained under reflux for one more day. The solution was concentrated to one third of its initial volume and, after four weeks at room temperature, a mixture of deep-green crystals of (Me 4 N) 6 [V 15 O 36 (Cl)] (Nunes et al., 2012) and yellow prisms of A was obtained, the latter in 9% yield based on vanadium (56 mg A solution containing tris(hydroxymethyl)methylamine (0.720 g, 6.0 mmol) in 20 mL of water was added to a solution of NH 4 VO 3 (1.17 g, 10.0 mmol) in the same volume of solvent. This reaction mixture was then refluxed until it became a clear solution, after which its pH was adjusted to 3 with aqueous HCl. A solution of MnCl 2 Á4H 2 O (0.394 g, 2.0 mmol) in 10 mL of water was then added as a layer on top of the reaction mixture and, after two weeks at room temperature, yellow crystals of B were obtained (180 mg) in 12% yield based on vanadium. The FTIR spectrum of B shows characteristic bands of the trisH + cation at 3188,2927,2856,1743,1637,1417,1161 and 1112 cm À1 and of the inorganic anion at 941, 842 and 684 cm À1 .

Refinement details
Crystal data, data collection and structure refinement details for the two structures are summarized in Table 3.
Hydrogen atoms on the cation were included in idealized positions (with methyl and methylene group C-H distances set at 0.96 and 0.97 Å , N-H at 0.89 Å and O-H at 0.82 Å ) and their U iso values were set to ride on the U eq values of the parent atoms. Hydrogen atoms in the anions (on coordinating water molecules) were located in difference maps and were refined freely.
There are two independent solvent water molecules, one of which is disordered over two sites close to a centre of symmetry, in compound A. No hydrogen atoms were identified in these water molecules.