P,P′-Diphenylethylenediphosphinic acid dihydrate

The title compound, C14H16O4P2·2H2O, possesses a crystallographic inversion center where two –P(=O)(OH)(C6H5) groups are joined together via two –CH2 groups. In the crystal, the acid molecules are linked by the water molecules via O—H⋯O hydrogen bonds, leading to the formation of a two-dimensional network lying parallel to (101).


Comment
In a recent publication, we showed that complexes of the type trans-Fe(P 2 ) 2 Cl 2 (P 2 = a bidentate phosphine) will react with dinitrogen at high pressure to form trans-[Fe(P 2 ) 2 (N 2 )Cl] + (Miller et al., 2002). This reaction is potentially useful as a way to scrub dinitrogen from natural gas contaminated with dinitrogen. Unfortunately, the phosphine ligands in these dinitrogen-scrubbing complexes slowly dissociate in aqueous solution, leading to degradation of the complexes. This prevents a practical pressure-swing process from being developed. One potential method to obtain complexes that are more robust is to use a phosphine macrocycle in place of the two bidentate ligands. (The "macrocycle effect" predicts that the binding constant for a macrocyclic ligand is orders of magnitude higher than the binding constant for two bidentate ligands (Melson, 1979)).
In addition to their usefulness in the N 2 -scrubbing scheme described above, macrocyclic phosphine compounds are sought after in general as ligands for transition metal complexes because of their strong binding properties. However, the synthesis of phosphine macrocycles is a relatively underdeveloped area. One approach to macrocyclic phosphines is a template synthesis in which two secondary bidentate phosphines are coordinated to a common metal center and then covalently linked. The title molecule is both a precursor in the synthesis of the secondary bidentate phosphine 1,2-bis-(phenylphosphino)ethane (MPPE, used in our laboratory for subsequent conversion into a macrocyclic phosphine ligand) and the oxidation product of MPPE. The X-ray structure of the title molecule recrystallized from ethanol has been reported (Costantino et al., 2008). As might be expected, the structure has an extensive hydrogen bonding network involving oxygen atoms (in the P=O and -OH groups) and H atoms (in the O-H groups). In contrast to the method used in this previous report, the structure reported here was recrystallized from water, which resulted in a different structure due to solvent water molecules.
The title compound has a centrosymmetrical structure where two -P(=O)(OH)(C 6 H 5 ) groups are joined together via two -CH 2 groups. The terminal -OH group forms a very strong O (2)

Experimental
The title molecule was prepared serendipitously while attempting to synthesize a phosphine macrocycle using a Cu(I) template. 1,2-Bis(phenylphosphino)ethane (MPPE) (2 equiv.) was reacted with Cu(MeCN) 4 PF 6 (1 equiv.) in acetonitrile to yield the corresponding Cu(MPPE) 2 PF 6 complex. A similar complex (with trifluoromethanesulfonate counter anion) was reported to be relatively air-stable for several months (Lambert & Desreux, 2000). However, after several weeks of exposure to air, the Cu(MPPE) 2 PF 6 complex decomposed and the phosphine ligands were fully oxidized, yielding the title compound. The crude oxidized phosphine was recrystallized from water, yielding crystals of the title molecule. Note that the title compound can be reduced back to the starting secondary bis-phosphine.

Refinement
The structure was solved using direct methods and refined with anisotropic thermal parameters for non-H atoms. H atoms in the main molecule were positioned geometrically and refined in a rigid group model, C-H = 1.2U eq (C) for -CH 2 and -CH groups. H atoms in the terminal -OH group and in a solvent water molecule involved in intermolecular H-bonds were found from the residual density and refined with isotropic thermal parameters. There are some alongations of thermal parameters of the carbon atoms in the phenyl rings indicating that the phenyl rings in the structure are flexible.

Computing details
Data collection: SMART (Bruker, 2000); cell refinement: SAINT (Bruker, 2000); data reduction: SAINT (Bruker, 2000); program(s) used to solve structure: SHELXTL (Sheldrick, 2008); program(s) used to refine structure: SHELXTL (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXTL (Sheldrick, 2008). 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 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.