Crystal structure of hexakis(dmpu)-di-μ2-hydroxido-dialuminium tetraiodide dmpu tetrasolvate [dmpu is 1,3-dimethyltetrahydropyrimidin-2(1H)-one]: a centrosymmetric dinuclear aluminium complex containing AlO5 polyhedra

Compared to the corresponding hydrate, the space-demanding solvent ligand N,N′-dimethylpropyleneurea [dmpu; systematic name: 1,3-dimethyltetrahydropyrimidin-2(1H)-one] often lowers the coordination number of metal ions. For dmpu-solvated aluminium iodide, the resulting complex is a di-μ2-hydroxide dimer with AlO5 centres.


Chemical context
The solvent ligand N,N 0 -dimethylpropyleneurea (dmpu; IUPAC name: 1,3-dimethyltetrahydropyrimidin-2(1H)-one, C 6 H 12 N 2 O) is known to be space-demanding upon coordination. This has been shown for several different metal ions which have a lower coordination number than the corresponding hydrates (Lundberg, 2006;Lundberg et al., 2010). In the boron group (group 13), the trivalent metal ions have previously been studied in dmpu solution and the solid state, with reported crystal structures for trichloridobis(dmpu)thallium(III) (Carmalt et al., 1996) and tribromidobis(dmpu)indium(III) (Topel et al., 2010). In the case of dmpu-solvated gallium(III) bromide, the gallium cation was determined to be five-coordinate in solution but crystallization was not successful despite of repeated attempts (Topel et al., 2010). The title compound was prepared in an attempt to reveal the ISSN 2056-9890 dmpu coordination for the last remaining naturally occurring trivalent group 13 metal ion, aluminium(III). Since both chloride and bromide ions are more prone to form aluminium complexes, the iodide salt was chosen as a starting material.

Structural commentary
The asymmetric unit of the title structure comprises one Al(dmpu) 3 moiety, two dmpu solvent molecules and two iodide counter anions. The dinuclear cationic aluminium complex ( Fig. 1) is generated by inversion symmetry and contains two five-coordinate aluminium cations, in which each cation is coordinated by the oxygen atoms of three dmpu ligand molecules and two 2 -bridging hydroxide ions, completing an AlO 5 coordination sphere. The Al-O bond lengths in the Al 2 ( 2 -OH) 2 bridge are 1.804 (2) and 1.859 (2) Å , while the Al-O bonds to the dmpu ligand molecules are 1.789 (2), 1.792 (2), and 1.846 (2) Å , respectively. The two aluminium cations are separated by 2.883 (1) Å from each other. The Al-O-C angles for the coordinating dmpu ligand molecules lie in the range of 144.0 (2) to 154.7 (2) . The dmpu ligand molecules are all essentially flat with the exception of the middle propylene carbon atom which is bent out of the plane with a dihedral angle of ca 50 .

Supramolecular features
In the crystal packing, the complex cations are arranged in rods parallel to [001] with the counter-anions situated between the rods (Fig. 2). The hydroxide ion forms a medium-strength O-HÁ Á ÁO hydrogen bond of 2.625 (3) Å to one of the noncoordinating dmpu ligand molecules, with an HÁ Á ÁO-C angle for this interaction of 134.8 (17) . The other non-coordinating dmpu molecule is stabilized by a much weaker OÁ Á ÁH-C interaction of 3.190 (5) Å . Other OÁ Á ÁH-C interaction between the moieties range from 3.404 (5)-3.561 (4) Å . The remaining positive charges on the aluminium atoms in the complex are compensated by the presence of non-coordin-

Figure 2
The crystal packing of the title structure in a view along [001]. ating iodide anions, which interact with the cationic complex by weak IÁ Á ÁH-C hydrogen bonds in the range 3.932 (4)-4.070 (4) Å (Table 1).

Synthesis and crystallization
The title compound was prepared by dissolving anhydrous aluminium(III) iodide (Sigma-Aldrich) in distilled dmpu in a glass vial, and subsequently heated in an oil bath to approximately 323 K, and then allowed to cool while still in the oil bath. After cooling to room temperature, the sample was refrigerated (277 K) for several weeks to allow for crystal growth. The presence of hydroxide ions in the title compound was most likely caused during preparation of the mother liquor. It appears possible that with additional precautions, a hydroxide-free compound might be obtained. A part of the solid was photographed in detail at ambient room temperature (Fig. 3), whereas attempts to study smaller crystals failed, presumably due to the hygroscopicity of the material.

Refinement
Hydrogen atoms bonded to carbon atoms were placed in High-resolution photograph of another, partially crystalline sample of the title compound. Multiple exposures were stacked for an increased depth of field.  (methylene) and refined isotropically using a riding model with U iso (H) equal to 1.5U eq (C) or 1.2U eq (C) for methyl and methylene hydrogen atoms, respectively. The hydrogen atom of the hydroxide group was located in a difference map and its position and U iso value were freely refined. Crystal data, data collection and structure refinement details are summarized in Table 2.  (Agilent, 2014); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: DIAMOND (Crystal Impact, 2001); software used to prepare material for publication: SHELXL2014 (Sheldrick, 2015).