(9H-Fluoren-9-yl)(phenyl)phosphinic acid

The crystal structure of the title compound, C19H15O2P, features pairs of molecules joined by O—H⋯O hydrogen bonds across crystallographic inversion centers. In addition, π–π interactions, with a centroid–centroid distance of 3.6273 (9) Å between the fluorene ring systems, connect the dimers into chains along [01-1]. The three rings make dihedral angles of 1.34 (9), 1.52 (10) and 1.51 (7)° with each other.

The crystal structure of the title compound, C 19 H 15 O 2 P, features pairs of molecules joined by O-HÁ Á ÁO hydrogen bonds across crystallographic inversion centers. In addition, interactions, with a centroid-centroid distance of 3.6273 (9) Å between the fluorene ring systems, connect the dimers into chains along [011]. The three rings make dihedral angles of 1.34 (9), 1.52 (10) and 1.51 (7) with each other.
Data collection: APEX2 (Bruker, 2012); cell refinement: SAINT (Bruker, 2012); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 2012); software used to prepare material for publication: PLATON (Spek, 2009) and publCIF (Westrip, 2010 (Burrow et al., 2000;Burrow & Siqueira da Silva, 2011a,b;Burrow & Siqueira da Silva, 2012). This tendency, due to the strong P-O dipole moment, aids in the formation of chain like structures in coordination polymers (Vioux et al., 2004;Siqueira et al., 2006). Smaller organyl groups, such as the methyl group, bound to the P atom can direct the the structure to form lamellar structures (Burrow & Siqueira da Silva, 2011b). Larger groups such as fluorenyl in the title compound have the ability to do the opposite, reducing the structure to dimeric structures by making chain formation an unfavorable process due to steric interactions between the bulky groups. Here we report the synthesis and crystal structure of the title compound which demonstrates the steric effects of the organyl groups in the solid state.
The molecular structure of the title compound is shown in Fig. 1. The geometry of the molecule shows no usual features. An analysis by Mogul (Bruno et al., 2004) using the Cambridge Strucrtural Database (Version 5.32, May, 2012 update;Allen, 2002) reports no |z-score| greater than 1 for the bond lengths. The highest |z-scores| for bond angles are 1.743 and 1.661 for the C31-C32-C33 and C22-C23-C24 angles, respectively, which are significantly smaller than the 120 ° expected for a benzene ring. This deviation is due to the strained, planar 5-membered C21/C22/C27/C28/C33 ring. The three ring systems of the fluorenyl group is almost planar with an r.m.s. deviation of 0.026 Å of the thirteen atoms from a least squares fitted plane through the atoms. Considering each ring systems individually, central fivemembered ring and the two benzene rings of the fluorenyl moiety are all each essentially planar with r.m.s. deviations of 0.0099 Å, 0.0022 Å and 0.0032 Å for the rings C21/C22/C27/C28/C33 (A), C22/C23/C24/C25/C26/C27 (B) and C28/C29/C30/C31/C32/C33 (C) rings, respectively. The dihedral angles between the planes (A) and (B), (A) and (C), and (B) and (C) are 1.34 (9) °, 1.52 (10) °, and 1.51 (7) °, respectively.
In the crystal, molecules are joined into dimeric units by pairs of O-H···O═P hydrogen bonds across crystallographic inversion centers (Fig. 2). The O···O distance at 2.5107 (15) Å is reasonably short indicating moderately-strong hydrogen bonding (Jeffrey, 1997). The dimeric units are joined into continuous chains along the the crystallographic [011] direction by π-π interactions between the fluorenyl ring systems across crystallographic inversion centers. An analysis by PLATON (Spek, 2009) shows a ring-to-ring centroid distance of 3.6273 (9)Å for the 5-membered rings (C21/C22/C27/C28/C33) which are co-planar and have a ring slippage distance of 0.825 Å. The perpendicular distances between the benzene rings are 3.4831 (6) and 3.4608 (6) Å.

Experimental
The procedure of Boyd & Regan (1994) was followed. To a solution of phenylphosphinic acid (2.0 g, 14.1 mmol) in dichloromethane (30 ml), diisopropylethylamine (5.16 ml, 29.6 mmol) and trimethylsilyl chloride (3.74 ml, 29.6 mmol) were separately added at 273K under argon. The reaction mixture was stirred at room temperature for 2-3 h, cooled to 273K and 9H-9-bromofluorene (3.46 g, 14.1 mmol) was added. After further stirring at room temperature for 3 days, the solvent was removed under vacuum. The residue was suspended in hydrochloric acid (2 M, 20 ml) and filtered on a glass frit. The white solid was washed with water and dried giving a yield of 2.30 g (53%) of pure product. IR: 3064 (w), 1592 Crystals suitable for single-crystal X-ray analysis were grown from an acetone solution of the title compound in a desiccator with silical gel.

Refinement
The H atom on O1 was found in the difference Fourier map and its position was allowed to refine freely while its isotropic displacement factor was set to 1.5 times that of O1. The H atoms attached to C atoms were positioned geometrically and allowed to ride on their parent atoms, with C-H bond lengths of 0.95 Å (aromatic CH) and 1.00 Å (sp 3 C), and isotropic displacement parameters equal to 1.2 times U eq of the parent atom.

Figure 2
The H bonding interactions, dashed red lines, and π-π interactions, dashed gray lines, which join the molecules of the title compound into continuous chains along the crystallographic [011] direction. Non-essential H atoms are omitted and C atoms are shown as sticks for clarity. Symmetry codes: (i) 1-x, 1-y, 1-z; (ii) 1-x, 2-y, 1-z; (iii) x, -1 + y, 1 + z.  The packing diagram of the title compound in the crystallographic bc direction with the crystallographic a axis pointing down. Non-essential H atoms are omitted and C atoms are shown as sticks for clarity.  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.