A redetermination of the crystal structure of the mannitol complex NH4[Mo2O5(C6H11O6)]·H2O: hydrogen-bonding scheme and Hirshfeld surface analysis

The redetermined structure of the title compound, [H4N][C6H11Mo2O11]· H2O, was obtained from an attempt to prepare a glutamic acid complex from the [Co2Mo10H4O38]6− anion.

Over the past few years, there has been considerable interest in derivatives of polyoxo-and heteropolyxometallates for both biological and materials applications, particularly where chirality may be conferred by the attachment of chiral ligands (Arefian et al., 2017;Proust et al., 2012;Mirzaei et al., 2014;An et al., 2006). Recently our group prepared the aspartate complex [Co 2 (C 4 H 6 NO 4 ) 2 (-Mo 8 O 26 )(H 2 O) 10 ]Á4H 2 O from (NH 4 ) 6 [Co 2 Mo 10 H 4 O 38 ], and l-aspartic acid (Tahmasebi et al., 2019) and have now proceeded to explore the generality of this reaction with other chiral amino acids. We report here on the reaction of the heteropolyoxometallate with l-glutamic acid from which a mannitol complex of molybdenum was obtained as a result of the unexpected presence of a substantial impurity of mannitol in the glutamic acid sample used. ISSN 2056-9890

Structural commentary
Instead of the expected complex containing glutamate ligands, the crystals obtained were found to have a unit cell essentially identical to that reported previously for a compound formulated as NH 4 [Mo 2 O 5 (C 6 H 12 O 6 )]ÁH 2 O (Godfrey & Waters, 1975) and the structure obtained indicates that it is the same complex. Subsequent to the identification of the product as a mannitol complex, the original sample of glutamic acid was checked by 1 H and 13 C NMR spectroscopy and found to contain a significant amount of mannitol as an impurity, thus explaining the formation of the title complex. A comparison of the geometry of the {Mo 2 O 9 } skeleton found in the present study with that in the previous report (Table 1) indicates the two to be essentially identical, although the present structure, using low-temperature data and more modern instrumentation and software, is of improved precision. A particular feature is that all hydrogen atoms could be located in a difference map and those attached to the oxygen atoms of the mannitol ligand could be refined (although we ultimately chose to fix them in idealized positions because of the presence of heavy metal atoms), making it abundantly clear that three hydroxyl groups on the ligand are deprotonated and also providing a more complete description of the intermolecular hydrogen-bonding scheme. The terminal Mo O distances (Table 1 and Fig. 1) are short, indicating a degree of multiple bonding while those to O6 and O9 are longer and consistent with single bonds. For the bridging oxygen atoms, O5, O8 and O7, the Mo-O distances for O7 are about the same as for those to O6 and O9, consistent with this atom being a bridging oxide ion. Those to O8 are somewhat longer, as expected for a bridging alkoxide ion, while those to O7 are considerably longer. The previous authors (Godfrey & Waters, 1975) attributed this 'at least in part to stereochemical strain' but there is no indication from the relevant bond angles that this is the case. Having located all of the hydrogen atoms, we see that O7 is a hydroxyl group and so would be expected to be less strongly bound to the metal than the anionic oxygen atoms. The Mo1Á Á ÁMo2 separation is 3.1579 (7) Å .

Figure 1
The asymmetric unit with the atom-labeling scheme and 50% probability ellipsoids. The hydrogen bonds from the cation to the anion and from the anion to the water molecule of crystallization are shown by dashed lines.
H10Á Á ÁO8 iv hydrogen bond. Two C-HÁ Á ÁO hydrogen bonds, one relatively strong and the other weak (Table 2) Hedman, 1977). From Table 1, the geometries of the {Mo 2 O 9 } core in all three structures are quite comparable. The packing in MANMOL10 is also quite similar to that seen in the present work, particularly when viewed along the b-axis direction although the channel (Fig. 2) between anions contains sodium cations in place of ammonium cations so there are different hydrogenbonding interactions.

Hirshfeld surface analysis
The  Fig. 4a and Fig. 4b, which include the entities making the closest contacts as listed in Table 2. The O-HÁ Á ÁO and N-HÁ Á ÁO hydrogen bonds to and within the asymmetric unit are clearly shown by the dark-red circles while the light-red ones indicate weak C-HÁ Á ÁO interactions: these are consistent with the extensive hydrogen-bonding network depicted in Figs. 2 and 3. The Hirshfeld surface mapped over shape index (Fig. 5a) and curvedness ( Fig. 5b) indicate, as expected from the X-ray structure, that the anion is compact with relatively little flat surface exposed to its neighboring ions. Fig. 6a shows the overall fingerprint plot while Fig. 6b  Packing viewed along the b-axis direction with intermolecular hydrogen bonds depicted as in Fig. 2.

Figure 4
Two views of the Hirshfeld surface for the anion mapped over d norm over the range À0.779 to +1.091 arbitrary units with the nearest hydrogenbonded neighbors added.

Figure 5
The Hirshfeld surface for the asymmetric unit mapped over (a) the shapeindex property and (b) the curvedness property.  Fig. 6c are the two spikes at d e + d i = 1.56 Å , which is over 1.3 Å less than the sum of the van der Waals radii and consistent with the prevalence of these two types of hydrogen bonding.

Synthesis and crystallization
(NH 4 ) 6 [Co 2 Mo 10 H 4 O 38 ]Á7H 2 O (0.29 g, 0.15 mmol) was dissolved in 8 ml of water and 4 ml of ethanol were added, giving a solution pH above 4. Then, 8 ml of an aqueous solution of supposed l-glutamic acid, C 5 H 9 NO 4 (0.13 g, 0.9 mmol), was added leading to a solution pH of 3.2. The solution was stirred for 2 h and then transferred to a Teflonlined autoclave (30 ml) and kept at 383 K for 72 h. After the mixture had been cooled slowly to room temperature, it was filtered and with slow evaporation of the solution at room temperature, flat colorless crystals of the title compound were obtained in 73% yield (based on Mo). Subsequent to the identification of the crystals as a mannitol complex, the original sample of glutamic acid was examined by 1 H and 13 C NMR and these spectra clearly showed the glutamic acid to be contaminated by a substantial quantity of mannitol.

Refinement details
Crystal data, data collection and structure refinement details are summarized in Table 3. H atoms attached to carbon were placed in calculated positions (C-H = 0.99-1.00 Å ) while those attached to oxygen and to nitrogen were placed in locations derived from a difference map, refined for a few cycles to ensure that reasonable displacement parameters could be achieved, and then their coordinates were adjusted to give O-H = 0.87 and N-H = 0.88 Å . All were included as riding contributions with isotropic displacement parameters 1.2-1.5 times those of the parent atoms. Computer programs: APEX3 and SAINT (Bruker, 2016), SHELXT (Sheldrick, 2015a), SHELXL2018/1 (Sheldrick, 2015b), DIAMOND (Brandenburg & Putz, 2012) and SHELXTL (Sheldrick, 2008).

Special details
Experimental. The diffraction data were obtained from 3 sets of 400 frames, each of width 0.5° in ω, colllected at φ = 0.00, 90.00 and 180.00° and 2 sets of 800 frames, each of width 0.45° in φ, collected at ω = -30.00 and 210.00°. The scan time was 10 sec/frame. 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 > 2sigma(F 2 ) is used only for calculating R-factors(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. Hatoms attached to carbon were placed in calculated positions (C-H = 0.99 -1.00 Å) while those attached to nitrogen and oxygen were placed in locations derived from a difference map and their coordinates adjusted to give N-H = 0.88 and O -H = 0.87 %A. All were included as riding contributions with isotropic displacement parameters 1.2 -1.5 times those of the attached atoms.