Extensive hydrogen-bonding network and an unusual cation conformation in [tris(hydroxymethyl)methyl]ammonium tetraoxidorhenate(VII)

The title compound, (C4H12NO3)[ReO4], contains two cations and two anions in the asymmetric unit, related by a non-crystallographic centre of symmetry. The crystal structure is stabilized by an extensive hydrogen-bonding network with the formation of puckered layers perpendicular to [001]. In the tris(hydroxymethyl)ammonium cations, intramolecular O—H⋯O hydrogen bonds are present with the formation of an S 1 1(6) graph-set motif. The crystal structure is further consolidated by N—H⋯O hydrogen bonds.

The title compound, (C 4 H 12 NO 3 )[ReO 4 ], contains two cations and two anions in the asymmetric unit, related by a noncrystallographic centre of symmetry. The crystal structure is stabilized by an extensive hydrogen-bonding network with the formation of puckered layers perpendicular to [001]. In the tris(hydroxymethyl)ammonium cations, intramolecular O-HÁ Á ÁO hydrogen bonds are present with the formation of an S 1 1 (6) graph-set motif. The crystal structure is further consolidated by N-HÁ Á ÁO hydrogen bonds.

Comment
The title compound was obtained as starting material for other syntheses (e.g. reaction with acethyl chloride -Hołyńska & Lis, 2008). It was chosen as the tris(hydroxymethyl)methylammonium cation gives rise to an extensive hydrogen bonding network, which allows for selective crystallization of products containing Re, reducing the risk of cocrystallization of impurities and crystal structure disorder. Moreover, rhenates(VII) with organic ammonium cations crystallizing in noncentrosymmetric space groups are promising materials with respect to their dielectric properties. For example, the previously discovered ferroelectric with a Curie temperature above room temperature is pyridinium rhenate(VII) (Czarnecki & Małuszyńska, 2000).
There are two symmetry-independent rhenate(VII) anions (containing atoms Re1 and Re2, respectively) with the expected (see e.g. Hołyńska & Lis, 2004 for example of rhenate(VII) anions in low symmetry environment) slightly distorted tetrahedral geometry. The Re-O bond lengths are listed in Table 1. Their values are consistent with those for other rhenates(VII), e.g. 1.723 (4) Å for potassium rhenate(VII) reported by Lock & Turner (1975). These bond lengths are not much affected by the presence of hydrogen bonds, as all rhenate(VII) O atoms participate in these interactions as acceptors (Table 2).
It is interesting to note that both symmetry-independent TRISH + cations are of unusual conformation. Usually the cation symmetry is close to C 3 (e.g. Rudman et al., 1983) or even exactly threefold (as in [TRISH]Cl appearing in a preliminary report by Rudman et al., 1979) with no intramolecular hydrogen bonds. In (1) both cations exhibit the presence of such intramolecular hydrogen bond (Table 2) with the formation of a S 1 1 (6) graph-set motif (Etter et al., 1990). The relevant N-C-C-O torsion angles are given in Table 1. Bond lengths characterizing the cations, among them the C-N bond length (which is longer than in the TRIS molecule -1.477 (3) Å as reported for the neutral TRIS molecule by Eilerman & Rudman, 1980) are in accordance with the values reported for other structures (e.g. Castellari & Ottani, 1997). TRIS is a constituent of buffers used in biochemical studies in the pH range of 7-9 (Castellari & Ottani, 1997). Upon protonation it forms salts with biologically relevant anions (e.g. tris(hydroxymethyl)methylammonium deoxycholate reported by Tusvik et al., 1999), also a report on its interaction with nucleotides in the crystalline state is available (Shakked et al., 1980). All cation ammonium and hydroxyl groups are donors in N-H···O or O-H···O hydrogen bonds, both to other TRISH + cations or to rhenate(VII) anions. The shortest Re···Re distance is 4.210 (2) Å. Thus, puckered hydrogen-bonded layers perpendicular to [001] are formed (Fig. 2). The hydrogen bonding scheme stabilizing an individual layer is illustrated in The title compound was obtained in the reaction of 0.19 g of tris(hydroxymethyl)methylamine (TRIS) with an excess of rhenic(VII) acid in aqueous solution, with slow evaporation leading to colourless crystals. The reaction was carried out in a quartz beaker. Rhenic(VII) acid was obtained according to the literature procedure (Johnson et al., 1967) in reaction of 0.3 g of metallic Re with an excess of a 30% aqueous hydrogen peroxide solution.

Refinement
The structure was solved by direct methods in the space group P1, and the present solution was obtained by switching to a higher symmetry. It was possible to end up in a false minimum in the Pca2 1 space group (e.g. with the following approximate coordinates for the Re atoms: 0.57, 0.024, 0.97 for Re1; 0.69, 0.51, 1.01 for Re2; see Marsh et al., 1998, for a review of some pitfalls connected with the Pca2 1 space group). All H atoms were generated geometrically and refined with U eq =nU eq (parent atom), where n = 1.5 for hydroxyl H atoms, and n = 1.2 for the remaining H atoms. During the refinement, extinction was also taken into account. Furthermore, it was found that the structure is a racemic twin (with a refined BASF parameter value of 0.375 (6)). On the final difference Fourier map the highest peak of 2.00 e/Å 3 was found at 0.62 Å from atom Re2. The crystal structure contains a pseudosymmetry centre at approximately (0.37, 0.73, 0.38). Fig. 1. The symmetry-independent part of (1). Thermal ellipsoids are drawn at 30% probability level. Hydrogen bonds are denoted with dashed lines.   Table 2.