Low-temperature phase of hexaguanidinium heptamolybdate monohydrate

The crystal structure of the title compound, [C(NH2)3]6[Mo7O24]·H2O, previously determined at room temperature in the monoclinic space group C2/c from Weissenberg techniques [Don & Weakley (1981 ▶). Acta Cryst. B37, 451–453], has been redetermined from low-temperature single-crystal data in the monoclinic space group P21/c. The asymmetric unit contains one heptamolybdate anion, six guanidinium cations and one water molecule of hydration. The anions and cations are linked by an extensive network of N—H⋯O hydrogen bonds.

The crystal structure of the title compound, [C(NH 2 ) 3 ] 6 -[Mo 7 O 24 ]ÁH 2 O, previously determined at room temperature in the monoclinic space group C2/c from Weissenberg techniques [Don & Weakley (1981). Acta Cryst. B37, 451-453], has been redetermined from low-temperature single-crystal data in the monoclinic space group P2 1 /c. The asymmetric unit contains one heptamolybdate anion, six guanidinium cations and one water molecule of hydration. The anions and cations are linked by an extensive network of N-HÁ Á ÁO hydrogen bonds.

Related literature
For the previous determination of the title compound in the monoclinic space group C2/c, see: Don & Weakley (1981). For an example of a structurally characterized [Mo 7 O 24 ] 6À anion, see: Kortz & Pope (1995). For more information about isopolymolybdates and polyoxometalates in general, see: Pope (1983).
Data collection: APEX2 (Bruker, 2005); cell refinement: APEX2; data reduction: APEX2; program(s) used to solve structure: Routine examination of (I) at low temperature resulted in the assignment of the monoclinic space group P2 1 /c, with the C-centered reflections being systematically weak. However, a previous room-temperature study using Weissenberg techniques reported the structure of (I) in the monoclinic space group C2/c (Don & Weakley, 1981). Therefore, data were collected on a second crystal at room temperature, confirming the original assignment of C2/c; data were then collected on this same crystal cooled to 173 K, confirming the low-temperature assignment of P2 1 /c. The value of R int for the low-temperature data was 15.0%. We attribute this higher than usual value to the phase change; the value of R int for the room-temperature data was less than 10%. The low-temperature structure consists of one [Mo 7 O 24 ] 6anion, six [C(NH 2 ) 3 ] + cations, and one molecule of water of hydration in the asymmetric unit (Fig. 1). The [Mo 7 O 24 ] 6anion shows the well known bent arrangement of seven edge-sharing MoO 6 distorted octahedra (Kortz & Pope, 1995), which can be formally derived from the parent {M 10 O 28 } decametalate framework by removal of three octahedra from the central level (Pope, 1983). The anions and the cations are linked by an extensive and intricate three-dimensional network of N-H···O hydrogen bonds involving all guanidinium -NH 2 groups and all heptamolybdate terminal and bridging O atoms, with the exception of the terminal O7B and the bridging µ 4 -O atoms (O1M4 and O2M4).

Experimental
To an aqueous solution (30 ml) of Na 2 MoO 4 .2H 2 O (0.726 g) and Na 2 HPO 4 (0.071 g) was added solid (CH 3 )SnCl 2 (0.220 g). After stirring for 30 min at room temperature, few drops of aqueous 1M [C(NH 2 ) 3 ]Cl were added, and the resulting solution was left to slowly evaporate at room temperature, colorless block-like single crystals of the title compound forming in a few days.

Refinement
The H atoms of the guanidinium cations were positioned geometrically and refined as riding. The positional parameters of the H atoms of the water molecule were calculated using the CALCOH tool of WinGX (Farrugia, 1999) and fixed during the refinment. All H atoms were refined with isotropic displacement parameters fixed at 1.2 times U eq of their parent atoms. Fig. 1. ORTEP plot of (I) showing 50% probability displacement ellipsoids.