Crystal structure of zwitterionic 2-[bis(2-methoxyphenyl)phosphaniumyl]-4-methylbenzenesulfonate monohydrate dichloromethane monosolvate

The phosphonium–sulfonate zwitterion has the acidic H atom located on the P atom rather than the sulfonate group. The PH+ group is not involved in intermolecular interactions.

In the title compound, C 21 H 21 O 5 PSÁH 2 OÁCH 2 Cl 2 , the phosphonium-sulfonate zwitterion has the acidic H atom located on the P atom rather than the sulfonate group. The S-O bond lengths [1.4453 (15)-1.4521 (14) Å ] are essentially equal. In the crystal, the water molecules bridge two zwitterions via O water -HÁ Á ÁO sulfonate hydrogen bonds into a centrosymmetric dimer. The dimers are further linked by weak C Aryl -HÁ Á ÁO sulfonate hydrogen bonds into chains extending along [100]. The PH + group is not involved in intermolecular interactions.

Structural commentary
Compound 1 crystallizes as the phosphonium-sulfonate zwitterion in which the acidic H atom is located on the P atom rather than the sulfonate group (Fig. 1). The S-O bond distances fall within the narrow range of 1.4453 (15) to 1.4521 (14) Å , and the P-C distances lie within the range of 1.7794 (18) to 1.7984 (18) Å . The P-H atom was located in a difference Fourier map and refined without additional restraints. The P-H bond length is 1.22 (2) Å . Compound 1 adopts an exo 3 conformation, i.e. the ortho methoxy and sulfonate groups point toward the PH + group (Feng et al., 2014). Tris(ortho-substituted aryl)phosphanes normally exhibit exo 3 conformations (Howell et al., 1999) because the ortho substituents cause less steric congestion when they point toward the P lone pair (exo) rather than toward the other aryl rings (endo). Addition of an H + at phosphorous should not add significant steric congestion and therefore it is not surprising that 1 also adopts the exo 3 conformation. The O methoxy Á Á ÁP distances, 2.7691 (14) and 2.7940 (14) Å , are shorter than the sum of the O and P van der Waals radii (3.35 Å ). The O3Á Á ÁH1(P1) distance is 2.44 (2) Å .

DFT calculations
The relative stability of the observed exo 3 conformation versus alternative exo 2 and exo 1 conformations was investigated by DFT calculations using the hybrid exchange-correlation functional PBE0 (Perdew et al., 1996(Perdew et al., , 1997 and the 6-311G(d,p) basis set for all atoms. The optimized structure is the exo 3 conformer, in which the methoxy and sulfonate groups point toward the PH + group. Geometry optimizations were also carried out on two conformers in which the SO 3 group was kept exo but one (exo 2 ) or two (exo 1 ) methoxy groups were rotated away from the PH + group. The exo 2 and exo 1 conformers were calculated to be 1.2 and 2.5 kcal mol À1 less stable than the exo 3 isomer, respectively. The HOMO of the exo 3 conformer is comprised of p orbitals of the sulfonate O atoms, while the LUMO is delocalized over the phenyl rings and P-C aromatic bonds (Fig. 2 The molecular structure of the title compound, showing the atom labelling. Displacement ellipsoids are drawn at the 50% probability level. The dashed line denotes a hydrogen bond. Table 1 Hydrogen-bond geometry (Å , ).

Database survey
A search of the Cambridge Structural Database (CSD, Version 5.36, last update May 2015; Groom & Allen, 2014) revealed structural reports for two analogues of 1 that contain 4-chloro-substituted methoxyphenyl (CSD refcode ODUNOS; Wucher et al., 2013) or 2,6-dimethoxyphenyl substituents at phosphorous (CSD refcode: LEXLEG; Liu et al., 2007). These compounds also crystallized as zwitterions in which the acidic proton is located on the P atom and feature close O methoxy Á Á ÁP contacts (2.764 to 2.927 Å ). The structure of the triethylammonium salt of 2-[bis(2-methoxyphenyl)phosphanyl]benzenesulfonate has also been reported (CSD refcode HAGKEH; Bettucci et al., 2008). In this case, the acidic H atom is located at triethylamine rather than on the P atom and the O methoxy Á Á ÁP distances are 2.877 and 2.903 Å .

Synthesis and crystallization
Compound 1 was synthesized by a modification of a previously reported procedure (Vela et al., 2007) comprising sequential reaction of PCl 3 with dilithiated p-toluenesulfonate and 1-lithio-2-methoxybenzene, followed by acidification of HCl, to afford 1 in 70-75% yield on a 3-4 g scale (Scheme 2). The product was purified by recrystallization (CH 2 Cl 2 /Et 2 O, volume ratio 1/3, layering at 273K). Crystals of 1ÁH 2 OÁCH 2 Cl 2 (I) suitable for the X-ray diffraction analysis were obtained by layering Et 2 O on a CH 2 Cl 2 solution of 1 at 277 K.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2 Dimer formation through O water -HÁ Á ÁO sulfonate hydrogen bonds (dashed lines).