Synthesis and crystal structures of [Al(H2O)6](SO4)NO3·2H2O and [Al(H2O)6](SO4)Cl·H2O

Common features in the crystal structures of [Al(H2O)6](SO4)NO3·2H2O and [Al(H2O)6](SO4)Cl·H2O are [Al(H2O)6]3+ octahedra and sulfate tetrahedra. These components, the remaining anion (NO3 − and Cl−, respectively) and lattice water molecules are separated from each other. Interactions are mediated via medium–strong hydrogen bonding.

and (2), respectively. Compound (1) further contains two unligated water molecules while compound (2) only contains one unligated water molecule. In the crystal structures, all components are spatially separated and interactions are mediated via medium-strong hydrogen bonding, compared to many other reported aluminium sulfates where corner-sharing of the building units is common. The two compounds represent rare cases where one aluminium(III) cation is charge-balanced by two different anions.

Chemical context
Aluminium is one of the most common elements in Earth's crust and is predominantly found in oxides and silicates. The far most common oxidation state for inorganic compounds is +III. Aluminium is found in many double salts with numerous other cations and sulfate, such as the industrially important alums MAl(SO 4 ) 2 Á12H 2 O (M = monovalent cation; Greenwood & Earnshaw, 1997). At low pH, aluminium mainly exists in solution as the [Al(H 2 O) 6 ] 3+ cation (Hay & Myneni, 2008).

Structural commentary
The crystal structure of (1) comprises an [Al(H 2 O) 6 ] 3+ cation charge-balanced by one sulfate and one nitrate anion as well as two unligated water molecules; all building units are separated from each other (Fig. 1). Bond lengths in the components are summarized in Table 1. The aqua ligands (O1-O6) of the complex cations serve as hydrogen-bonding donor groups. They connect through O-HÁ Á ÁO hydrogen bonds to the two types of anions and to the two unbound water molecules, forming a three-dimensional network (Fig. 2 In the crystal structure of compound (2), the chargebalancing nitrate anion of (1) is exchanged for a chloride anion, and the formula unit only contains one additional water molecule (Fig. 3). is a hard acid. The higher charge (2+) of the sulfate group compared to the nitrate group and chloride is a likely reason that the sulfate group is present in both structures while the two latter ones can be interchanged, possibly related to their relative abundance. The chloride ions in the reaction mixture of (1) might also have been bonded to the titanium(IV) and bismuth(III) cations, preventing the formation of (2). In Packing in the crystal structure of compound (1). Hydrogen bonding is indicated by dotted lines. Table 2 Hydrogen-bond geometry (Å , ) for (1). Symmetry codes: (i) Àx þ 2; Ày À 1; Àz þ 1; (ii) Àx þ 2; Ày; Àz þ 1; (iii) Àx þ 1; Ày À 1; Àz þ 1; (iv) Àx þ 1; Ày; Àz þ 1; (v) x; y À 1; z; (vi) x À 1; y; z; (vii) Àx þ 1; Ày þ 1; Àz; (viii) Àx þ 1; Ày; Àz.

Figure 1
The asymmetric unit of (1), representing the building units. Displacement ellipsoids are drawn at the 50% probability level. Table 1 Selected bond lengths (Å ) for (1).

Figure 3
The asymmetric unit of (2), representing the building units. Displacement ellipsoids are drawn at the 50% probability level.
particular Bi 3+ tends to form insoluble BiOCl. Furthermore, (1) contains two extra water molecules while (2) only contains one of them. The average Al-O bond lengths are 1.880 and 1.884 Å for (1) and (2), respectively, which is slightly shorter than the literature average distance of 1.90 Å (Hay & Myneni, 2008;Veillard, 1977). Structures of aluminium sulfate, Al 2 (SO 4 ) 3 , and derivatives thereof have been reported with different amounts of additional structural water and varying connectivities. Sabelli & Ferroni (1978) reported an aluminium sulfate structure (Al 2 (OH) 4 SO 4 Á7H 2 O) where six hydrated aluminum(III) ions are connected via edge-and face sharing. These aluminum 'hexamers' are linked via hydrogen bonding with unligated water and sulfate ions. In the crystal structure of Al 2 (SO 4 ) 3 Á8H 2 O, hydrated aluminum(III) ions are connected via corner sharing with sulfate groups and a rather extensive hydrogen-bond network between sulfate, aqua ligands, and unligated, structural water molecules (Fischer et al., 1996). In the Al(SO 4 )OH structure reported by Anderson et al. (2015), each sulfate group connects three different aluminium(III) ions via corner sharing. The structures of the two reported compounds herein are more open and the principal building units are only connected via hydrogen bonding, which may be due to the presence of another anion (NO 3 À /Cl À ).

Database survey
According to a database survey using the Inorganic Crystal Structure Database (ICSD), aluminium compounds with an additional cation charge-balanced by sulfate anions appear to be common [e.g. KAl(SO 4 ) 2 , FeAl(SO 4 ) 3 (Demartin et al., 2010), or CsAl(SO 4 ) 2 (Beattie et al., 1981)]. However, compounds with aluminium as the single cation but with two different anions were found to be much less common although examples include Al(H 2 PO 4 ) 2 F (Parnham & Morris, 2006) or Al(SO 4 )OH (Anderson et al., 2015).

Synthesis and crystallization
Compound (1) was obtained by mixing equimolar solutions of TiOSO 4 (Aldrich) and Bi(NO 3 ) 3 Á5H 2 O (Aldrich), both dissolved in 1 M nitric acid (Sigma-Aldrich), and two equivalents of AlCl 3 Á6H 2 O (Mallinckrodt Chemical Works) dissolved in 1 M hydrochloric acid (Sigma-Aldrich). Colorless needle-shaped crystals formed on a glass substrate after about a week of slow evaporation of the solvent at room temperature. Elemental analysis by energy-dispersive X-ray spectroscopy using a Hitachi TM-1000 scanning electron microscope with an Oxford Instruments EDS system revealed a molar Al:S ratio of 1.37 (expected 1:1). In an attempt to synthesize compound (1) by a direct route, aluminium(III) chloride was changed to aluminium(III) lactate to avoid chloride ions. This resulted in formation of crystals with very poor quality that were not suitable for X-ray diffraction.
Compound (2) was obtained by dissolving 1 M AlCl 3 Á6H 2 O in 1 ml of 1 M hydrochloric acid and adding one equivalent of 1 M sulfuric acid (Sigma-Aldrich), or making a 1 M AlCl 3 Á6H 2 O solution in 0.5 ml of 1 M H 2 SO 2 plus 0.5 ml of 1 M HNO 3 . The solution was poured into a Petri dish and left for slow evaporation. After a few days of evaporation of the solvent, colorless block-shaped crystals suitable for single X-ray crystal diffraction were obtained. The crystals were somewhat fragile. EDS analysis of (2) revealed an S:Al:Cl molar composition of 0.9:0.9:1.17 (expected 1:1:1).
For the data collection, both types of crystals were mounted on a glass needle and protected by a layer of paraffin oil.