The dehydrated copper silicate Na2[Cu2Si4O11]: a three-dimensional microporous framework with a linear Si—O—Si linkage

The structure of the title dehydrated copper silicate, disodium dicopper undecaoxide tetrasilicate, Na2(Cu2O11Si4), was determined by single-crystal X-ray diffraction from a non-merohedral twin. It exhibits an effective three-dimensional microporous framework with the major channels, in which the Na+ cations are placed, running along the a-axis direction and smaller channels observed along the b-axis direction. The structure is unusual in that it contains a symmetry-constrained Si—O—Si angle of 180°. The Cu centre is coordinated to five O atoms, exhibiting a slightly distorted square-pyramidal coordination geometry. The Na cation is interacting with five neighbouring O atoms, exhibiting an uncharacteristic coordination environment.

The structure of the title dehydrated copper silicate, disodium dicopper undecaoxide tetrasilicate, Na 2 (Cu 2 O 11 Si 4 ), was determined by single-crystal X-ray diffraction from a nonmerohedral twin. It exhibits an effective three-dimensional microporous framework with the major channels, in which the Na + cations are placed, running along the a-axis direction and smaller channels observed along the b-axis direction. The structure is unusual in that it contains a symmetry-constrained Si-O-Si angle of 180 . The Cu centre is coordinated to five O atoms, exhibiting a slightly distorted square-pyramidal coordination geometry. The Na cation is interacting with five neighbouring O atoms, exhibiting an uncharacteristic coordination environment.
The dehydrated copper silicate Na 2 [Cu 2 Si 4 O 11 ]: a three-dimensional microporous framework with a linear Si-O-Si linkage L. Cunha-Silva, P. Brandão, J. Rocha and F. A. Almeida Paz Comment Molecular sieves containing metal cations with a range of coordination geometries have been extensively studied due to their novel topologies, interesting chemical properties and potential aplications in optoelectronics, batteries, magnetic materials and sensors (besides the traditional applications of zeolites) (Rocha & Anderson, 2000;Rocha & Lin, 2005). In the last decade, we have been interested in the synthesis and structural characterization of novel open-frameworks containing Si and metal cations (such as Ti, V, Cr, Nb, Zr and Sn) in tetrahedral and (more commonly) octahedral coordination environments, and lanthanide silicates exhibiting interesting photoluminescence properties (Anderson et al., 1994;Ananias et al., 2001;Ferreira et al., 2003;Ananias et al., 2006). As part of this research line, we prepared and characterized the hydrated copper silicate Na 2 (Cu 2 Si 4 O 11 )·2H 2 O (Brandão et al., 2005). This compound was dehydrated and the magnetic properties of both hydrated and dehydrated forms were investigated (Santos et al., 2005), however the crystalline structure of the dehydrated compound was not reported. Here we describe the structure of the dehydrated microporous copper silicate, Na 2 (Cu 2 Si 4 O 11 ) (I).
The asymmetric unit of the copper silicate (I) comprises one Cu(II) cation, two corner-shared SiO 4 groups and one Na + counter-cation ( Figure 1). The crystallographic unique Cu(II) metal centre is coordinated to five O-atoms from five distinct SiO 4 tetrahedral moieties (four basal SiO 4 and one apical SiO 4 ), in a geometry resembling a distorted square pyramid for which the apical Cu-O bond is longer than the basal ones ( Figure 2a and Table 1). Adjacent SiO 4 tetrahedral moieties are linked along the a direction by corner-shared oxygen atoms (O3 and O4 are shared alternately) leading to the formation of zigzag metallic anionic chains, [(Cu 2 Si 4 O 11 ) ∞ ] 2− , in which the Cu···Cu distances alternate between 2.9921 (8) Å (via bridging basal SiO 4 , green bonds in Fig. 2 b) and 3.1031 (10) Å (via the apical SiO 4 tetrahedron, yellow bonds in Fig. 2 b). [(Cu 2 Si 4 O 11 ) ∞ ] 2− chains are interconnected via corner-sharing SiO 4 tetrahedra through linear interactions Si1-O1-Si iv [angle is 180.0°; symmetry code: (iv) 2 − x, −y, 1 − z] to form infinite layers (Fig. 2c). This linear Si-O-Si interaction is very rare and represents a remarkable structural feature of the copper silicate (I) framework. We note that such occurrence was also recently reported in the lanthanide silicate K 3 (NdSi 7 O 17 ) (Haile & Wuensch, 2000). From the evaluation of the structures of several hundred silicates it was concluded that the average of an unstrained Si-O-Si bond angle is ca 139° and that truly linear bonds are energetically unfavorable (Liebau, 1985).
In fact, the crystallographically determined values of 180° are more likely to represent a time average rather than the actual value of the bond angle. The bond, at any instant in time, should have an O-atom displaced from its average position such that the instantaneous value of Si-O-Si is less than 180° (Haile & Wuensch, 2000). This structural feature is ultimately reflected in the anisotropic displacement parameters associated with this bridging O-atom. Indeed, the thermal parameters associated with this atom are unusually large, with the greatest displacement occurring in the plane perpendicular to the Si1···Si1 iv vector (Figure 2c). supplementary materials sup-2 As observed for the chains, adjacent layers are also interconnected via corner-sharing SiO 4 tetrahedra generating a threedimensional microporous framework with the major channels running along the a direction, formed by eight-membered rings and having a cross-section of ca 7.5 × 4.3 Å (Figure 3a). Interestingly, the Na + cations are located within the channels but are remarkably close to the previously described layers, creating an effective porous copper framework (Figure 3a). In addition, remarkably large channels are also observed along the b direction, which are formed by six-membered rings and display a cros-section of ca 5.2 × 4.6 Å (Figure 3 b).

Experimental
Chemicals were purchased from commercial sources and used without further purification. An alkaline solution was prepared by mixing 13.86 g of a sodium silicate solution (Na 2 O 8 wt%, SiO 2 27 wt%), 16.13 g H 2 O and 4.11 g NaOH, and a second solution was prepared by mixing 17.87 g H 2 O with 7.60 g of Cu(SO 4 ).15H 2 O. These two solutions were combined, stirred thoroughly during 2 h and the resulting gel, with a molar composition of CuO: 3.1SiO 2 : 1.4Na 2 O: 94.5H 2 O, was autoclaved for 10 days at 503 K. A crystalline material was obtained [Na 2 (Cu 2 Si 4 O 11 )·2H 2 O], filtered and treated thermally at 573 K for six hours leads to the removal of the crystallization water molecules.

Refinement
Even though crystals of the title compound could be indexed with the unit-cell parameters summarized in Table 1, a visual inspection of the centered reflections using RLATT showed the presence of a rotational twin (non-merohedral). A full sphere of reflections was collected and a partial data set was then deconvoluted using CELL_NOW (Sheldrick 2004) into a two-component twin. Data integration was performed by assuming that the second twin domain was identical to the first. The final structural model exhibits a large average U(i,j) tensor, most likely due to the applied twinning correction which ultimately seems to lead to large U3/U1 ratios. Fig. 1. Fragment of the crystal structure of the title compound with the atoms represented as thermal displacement ellipsoids drawn at the 50% probability level [Symmetry codes: (i) 2 − x, -y, 2 − z; (ii) x, -1 + y, z; (iii) 1 − x, -y, 2 − z; (iv) 2 − x, 1 − y, 2 − z; (v) x, y, 1 + z; (vi) 1 + x, y, z].   Geometric parameters (Å, °)