2.1 Hydrothermal synthesis and characterization

Na₂MoO₄·2H₂O (1.20 g, 4.96 mmol), MnSO₄ (0.38 g, 2.50 mmol), ethylenediamine (0.40 ml, 3.03 mmol), H₃PO₄ (0.5 ml) and H₂O (15 ml) were mixed and transferred into a 30-ml Teflon-lined stainless steel autoclave and heated at 160 °C for 6 days. Then, the autoclave was cooled to room temperature at 5 °C h⁻¹. The black crystals of compound 1 were filtered off, washed with water and dried in a desiccator at ambient temperature (yield 37% based on Mo). Infrared spectrum was measured on a Nicolet AVATAR 360 FT-IR spectrometer on KBr pellets in the 4000–400 cm⁻¹ region.

2.2 X-ray crystallography

A black single crystal with dimensions of 0.40 × 0.10 × 0.40 mm was mounted on a glass fibre. Data collection was performed on a Rigaku Weissenberg IP diffractometer with graphite-monochromated MoKα radiation (λ = 0.71073 Å). The structure was solved by the direct method using SHELXS-86 and refined by full-matrix least squares calculations using SHELXL-97 software. All non-hydrogen atoms were refined with anisotropic thermal parameters. Most of the hydrogen atoms were located in calculated positions and/or in the positions from difference Fourier map. The crystallographic data are given in Table 1. Selected bond lengths and angles are provided in Tables 2 and 3.

3.1 The crystal structure of complex 1

The single-crystal X-ray analysis revealed that compound 1 consists of {Mo₆P₄} hexamers, manganese cations and lattice water molecules. As shown in Figs. 1 and 2, in each {Mo₆P₄}, there is a six-membered ring of oxo-bridged Mo atoms with alternating Mo–Mo single bonds (2.5870(2)–2.6053(2) Å), and there are four phosphate groups. The phosphorous atoms have P–O bond lengths in the range 1.492(1)–1.586(9) Å, while the O–P–O bond angles are in the range 105.9(5)°–112.6(5)°. As illustrated in Fig. 3, the Mn(1) and Mn(2) atoms are located at the octahedral MnO₆ center, the Mn–O distances are ranging from 2.103(9) to 2.236(2)Å, and the O–Mn–O angles vary from 78.31(6)° to 180.0(1)°, while Mn(3) atoms adopt a five-coordinated geometry.

The structure of 1 was constructed from molybdenum hexamers bonded together with octahedral Mn²⁺ into a two-dimensional framework (see Fig. 3). There are three independent Mn atoms in the compound (see Fig. 3) which may be divided into two types: (i) Mn(1) and Mn(2), Mn(1) bridge two {Mo₆P₄} units via three μ₃-O atoms, which link the metal–metal-bonded molybdenum atoms together with Mn–O bonds at lengths of 2.214(8) Å × 2, 2.217(8) Å × 2, and 2.192(7) Å × 2 to produce a centrosymmetric [Mn(Mo₆P₄)₂] dimer. Mn(2) is similar to Mn(1). (ii) Mn(3) bridges four P–O groups from different {Mo₆P₄} (i.e. one {Mn(1)(Mo₆P₄)₂} unit and two {Mn(2)(Mo₆P₄)₂} units) and has a terminal coordinated water molecule O(1W) with the distance of Mn(3)–O(1W) 2.237(2) Å. Two-dimensional structure of 1 can be described in terms of crosslinking among different {Mn(Mo₆P₄)₂} units. Interestingly,{Mn(1)(Mo₆P₄)₂} unit and {Mn(2)(Mo₆P₄)₂} unit are linked by P(7)O₄ and Mn(3)O₅, while {Mn(2)(Mo₆P₄)₂} unit and {Mn(2)(Mo₆P₄)₂} unit are linked by the oxygen atom of two P(5)O₄, one P(2)O₄, one P(6)O₄ and two Mn(3)O₅ polyhedra.

Bond-valence calculations for 1 suggest that the bond-valence values for all the molybdenum and phosphorous atoms are in the ranges 5.15–5.22 and 4.89–5.05, respectively. This indicates that all Mo and P atoms exhibit a +5 oxidation state. Each Mo⁵⁺ cation in compound 1 has a distorted octahedral configuration with an apical Mo–O bond at a length of 1.682(8)–1.694(9) Å and five other Mo–O bonds at lengths ranging from 1.932(8) to 2.343(7) Å. The valence sums for three Mn atoms are 1.93, 1.94 and 1.79, respectively, the average value being 1.89.

3.2 IR

The IR spectrum of compound 1 exhibits a strong band at 946 cm⁻¹, characteristic of ν(Mo = O), and features at 727 and 1045 cm⁻¹ associated with ν(Mo–O–M) and ν(P–O), respectively. The feature at 1517 cm⁻¹ is ascribed to ethylenediamine, while the broad band at 3418 cm⁻¹ is ascribed to water molecules [10].