Methylphosphonic acid, CH3PO(OH)2

The asymmetric unit of the title compound, CH5O3P, contains two independent molecules with nearly identical bond lengths and angles. In the crystal, each of the molecules acts as acceptor (P=O) and donor (P—OH) of four hydrogen bonds to three adjacent molecules, resulting in the formation of two different bilayers (one for each molecule) stacked perpendicular to the a axis in the crystal.


Synthesis and crystallization
Single crystals of methyl phosphonic acid (Merck-Schuchardt) were obtained as side products from several experiments when we tried to growth singles crystals of diorganotin(IV) methyl phosphonates by solvent evaporation. A suitable single crystal was selected under a polarization microscope and mounted on a 50 µm MicroMesh MiTeGen Micromount TM using FROMBLIN Y perfluoropolyether (LVAC 16/6, Aldrich).

Refinement
All hydrogen atoms could be localized in difference Fourier synthesis. Those of the methyl groups were idealized and refined at calculated positions riding on the carbon atoms with C-H distance of 0.98 Å. Those of the hydroxyl groups were refined with respect to a common O-H distance of 0.96 Å before they were fixed and allowed to ride on the corresponding oxygen atoms. For the hydrogen atoms of each methyl group a common isotropic displacement parameter was refined as well as one common isotropic displacement parameter for the hydrogen atoms of all hydroxyl groups.

Results and discussion
Methylphosphonic acid, CH 3 PO(OH) 2 , is a well-established reagent in inorganic as well as in organometallic chemistry forming numerous salts respectively coordination compounds with a lot of different metals and organometallic fragments.
For an example, from tin(II) the synthesis and structural characterization of the mixed methylphosphonate oxalate Sn 2 (MePO 3 )(C 2 O 4 ) was described by Adair et al. (1998) whereas diorganotin(IV) moieties will give rise to the formation of dimeric species like [Bu 2 Sn(MePO(OH)) 2 ] 2 as was shown by Ribot et al. (2001). During a systematic study on the complex formation of methylphosphonic acid towards other organotin(IV) compounds we became aware, that the crystal structure of the title compound was never determined in detail. Only Kodolov et al. (1977) referred in a brief communication to the cell parameters of a monoclinic unit cell but no atomic coordinates were given.
During our studies, we never were able to confirm these cell parameters, although we also found a monoclinic unit cell.
Moreover, we observed a strong broadening and multiplication of the reflections on cooling down the crystal to 100 K.
For this reason, we present the results of a measurement at T = 200 K, before reflection broadening started.
The asymmetric unit of the title compound consists of two crystallographic independent molecules ( Fig. 1) with nearly identical geometrical parameters (Tab. 1). These values correspond very well with those of phenyl phosphonic acid that was determined by Mahmoudkhani et al. (2002)  Each of the two molecules in the asymmetric unit is involved into four hydrogen bonds with molecules of the same kind giving rise to two different bilayers stacked perpendicular to the crystallographic a-axis (Fig. 2). Within each bilayer, each molecule of the upper layer is hydrogen bonded to three molecules of the lower layer and vice versa. In summary, the arrangement of the phosphorous atoms in each bilayer corresponds to the arrangement of the arsenic atoms in the doublelayer structure of grey arsenic consisting of an extended network of fused, six-membered rings of arsenic atoms with chair conformation.
Two of the four hydrogen bonds of each molecule result from centrosymmetric dimers (Fig. 3) in which one hydroxyl group (O22-H22) acts as donor and the double-bonded oxygen atom (O22) as acceptor for almost linear hydrogen bonds with a bond angle H···O-P of about 120° at the double bonded oxygen atom. These dimers are very similar those found in carbonic acids like propionic acid (Allan et al. 2000) or benzoic acid (Bruno & Randaccio, 1980). The double bonded oxygen atoms, additionally, are involved into a second pair of intermolecular hydrogen bonds to the second hydroxyl group. These hydrogen bonds, also being less linear as the former one, are of similar length (Tab. 2). program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 2006) and Mercury (Macrae et al., 2008); software used to prepare material for publication: SHELXTL (Sheldrick, 2008).

Figure 1
Ball-and-stick model of the two molecules in the asymmetric unit of the title compound with the atomic numbering scheme used; with exception of the hydrogen atoms, which are shown as spheres with a common isotropic radius, all other atoms are represented as thermal displacement ellipsoids at the 50% probability level; hydrogen bonds are indicated by broken sticks (red).    Hydrogen bonding system in the bilayer of MePO(OH) 2 resulting from molecule 2; with exception of the hydrogen atoms, which are shown as spheres with a common isotropic radius, all other atoms are represented as thermal displacement ellipsoids at the 50% probability level; methyl groups are omitted for clarity; hydrogen bonds are indicated by broken sticks (red); small black balls = center of symmetry; symmetry transformations used to generate equivalent atoms: 1) -x, 2 -y, 1 -z; 2) -x, 1/2 + y, 3/2 -z; 3) -x, -1/2 + y, 3/2 -z.

Methylphosphonic acid
Crystal data