supplementary materials

Water clusters, encapsulated as crystal hydrates in various solid networks, were recently introduced as a new field of considerable scientific relevance, both in theoretical and in experimental studies (Ludwig, 2001; Infantes & Motherwell, 2002; Mascal et al., 2006). The point at stake relates to the need for a comprehensive analysis of hydrogen-bonded water clusters, (H₂O)_n, where n > 1. Infantes and Motherwell (2002) have selected the cluster patterns and sorted their structures into five broad classes designated D, R, C, T and L.

We describe here compound (I) as a salt of the rare complex cation, [Co(H₂oxado)₃]³⁺ (Bekaroglu et al., 1978; Bélombé et al., 1993; Nenwa, 2004). It is a transition metal complex system that crystallizes in a nanochannelled lattice encapsulating infinite tapes of cyclic water hexamers, and it provides, therefore, another well documented example to be added to category T of the aforementioned classification.

The constituent parts of (I) are depicted in Fig. 1. The pseudo-octahedral coordination in the complex cation is similar to the chiral geometries, and the bond lengths and angles compare within experimental error with those reported previously (Nenwa, 2004; Bekaroglu et al., 1978; Bélombé et al., 1993). The host lattice of the structure is realised by the ionic partners which are linked together via a three dimensional network of O–H···O and N–H···O hydrogen bonds. The ions of each kind pile up in an eclipsed sequence to generate the corresponding charged stacks. The electrically neutral scaffold thus constructed is characterized by infinite channels (ca 6.4 Å wide) oriented parallel to [100], and encapsulating twelve water molecules of crystallization per unit cell.

The projection of a unit cell of (I) in Fig. 2 shows the positioning of the water molecules within the channels, reminiscent of the structure of a silver salt reported recently (Bélombé et al., 2007). The oxalate ion is centrosymmetric. Thus, only one oxalate ion is present in the unit cell, the other species being represented twice. A short segment of a water tape in (I) viewed down [010] is shown in Fig. 3. One distinguishes, from the ellipsoid size, two types of water molecules per asymmetric unit. Those containing atoms O1, O2 and O3, positioned close to the periphery of the channels – due to their involvement in H-bonding to the ionic building blocks of the host lattice – may be dubbed "peripheral" waters. The reduced ellipsoid size of these O atoms reveals that they are well ordered. The water molecules with the atoms O4, O5A and O6A may be referred to as "central" waters, as they are located around the central axis of the channel. The ellipsoid extent of these O atoms indicates their high disorder, probably due to their increased mobility since they are less strongly bonded than their "peripheral" congeners.

Table 2 summarizes the three categories of H-bonds that are effective in this structure. O–H···O and N–H···O bridges interconnect the ionic partners amongst themselves into the three dimensional host lattice. Note that these bridges represent the most efficient ones, with the shortest O···O separation of 2.584 (4)Å linking an oxamide dioxime to an oxalate O atom, viz. O15···O43^x (see Table 2 for symmetry code). Then, O–H···O and N–H···O bridges of medium efficiency interlink the "peripheral" water molecules to the ionic partners. Finally, O–H···O bridges weakly interconnect water molecules amongst themselves within the nanochannels.