2-{(E)-[1-(2-Hydroxyethyl)-3,3-dimethyl-3H-indol-1-ium-2-yl]vinyl}-6-hydroxymethyl-4-nitrophenolate dihydrate

The title merocyanine-type molecule, C21H22N2O5·2H2O, crystallizes in a zwitterionic form and has an E configuration at the styryl C=C bond. The styryl part of the molecule and the indolium ring are slightly twisted and form a dihedral angle of 13.4 (1)°. The 1.274 (3) Å C—O bond length in the phenolate fragment is the longest among similar molecules. Hydrogen bonds between solvent water molecules, two hydroxyl groups and the phenolate O atom dictate the packing arrangement of molecules in the crystal and join the molecules into a two-dimensional polymeric network which propagates parallel to (001). Four water molecules and four hydroxy groups form a centrosymmetric homodromic cyclic motif of O—H⋯O hydrogen bonds. Another cyclic centrosymmetric motif is generated by four water molecules and two phenolate O atoms.

The title merocyanine-type molecule, C 21 H 22 N 2 O 5 Á2H 2 O, crystallizes in a zwitterionic form and has an E configuration at the styryl C C bond. The styryl part of the molecule and the indolium ring are slightly twisted and form a dihedral angle of 13.4 (1) . The 1.274 (3) Å C-O bond length in the phenolate fragment is the longest among similar molecules. Hydrogen bonds between solvent water molecules, two hydroxyl groups and the phenolate O atom dictate the packing arrangement of molecules in the crystal and join the molecules into a two-dimensional polymeric network which propagates parallel to (001). Four water molecules and four hydroxy groups form a centrosymmetric homodromic cyclic motif of O-HÁ Á ÁO hydrogen bonds. Another cyclic centrosymmetric motif is generated by four water molecules and two phenolate O atoms.

Related literature
This structure is similar to the perviously reported trans-MEH compound, see: Raymo et al. (2003). For similar structures, see also: Aldoshin & Atovmyan (1985), Hobley et al. (1999), Zou et al. (2003). For the synthetic procedure, see: Raymo & Giordani (2001 Table 1 Hydrogen-bond geometry (Å , ). Symmetry codes: (i) Àx þ 1; Ày þ 1; Àz þ 1; (ii) x þ 1; y; z; (iii) x À 1; y; z; (iv) x þ 1; y þ 1; z. to the plane associated with the indole ring portion of the molecule. Thermal ellipsoids for most of the atoms are well defined. Only the O20 oxygen atom associated with one of two solvent water molecules shows some enlargement, and such enlargement is not unexpected. The title compound is similar to another merocyanine molecule (trans-MEH) as documented by Raymo & Giordani (2001) and Raymo et al. (2003), with the difference being that the title compound possesses an additional methanol group on the phenolate portion of the molecule. A review of similar structures which contain terminal alkoxy ligands (C-O -) shows C-O bond lengths in the range of 1.228 to 1.260 Angstroms; see Aldoshin & Atovmyan (1985), Hobley, et al. (1999, and Zou, et al. (2003). The C-Obond for the title compound falls outside this range at 1.274 (3) Angstroms. This elongation is likely a result of H-bonding interactions as discussed below. Figure 2 shows the packing arrangement and intermolecular interactions for the title compound. One can see the nearly planar nature of the molecule from this perspective. There are two cyclic motifs assocated with the solvent water molecules in the structure.

2-{(E)-[1-(2-Hydroxyethyl
The ethanol group attached to the indole portion of the molecule is linked to the hydroxy O2 atom via hydrogen bonding interactions of the O10 solvent water. In addition, the intermolecular linkage of the molecules occurs via the O20 solvent water which connects the hydroxy O2 with the coordinated O10 solvent water. In addition, there is a second (Larger) cyclic motif generated by solvent water and OH groups from the hydroxymethyl and hydroxyethyl groups of the molecule. These H-bond interactions generate a two-dimensional polymeric network along the a-b plane of the structure. All O-H···O lengths and angles for these interactions are typical for hydrogen bonds as listed in Table 1.

Refinement
H atoms present on the molecule were located in a straightforward manner using HFIX commands of SHELXL97 with attention to hybridization of the bound atom. The H atoms from water molecules were located in a difference Fourier map. They were refined using a riding-model approximation with C-H = 0.95-0.99 Å and O-H = 0.85-0.96 Å with U iso (H)=1.2U eq (C) except methyl group and water molecule, where U iso (H)=1.5U eq (C,O). Fig. 1. The molecular structure of the title compound, with labels and 50% probability displacement ellipsoids for non-H atoms.

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.
Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating Rfactors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.