(Biphenyl-2,2′-diyl)di-tert-butylphosphonium trifluoromethanesulfonate

To aid in the elucidation of catalytic reaction mechanism of palladacycles, we found that reaction of trifluoromethanesulfonic acid with a phosphapalladacycle resulted in elimination of the palladium and formation of the title phospholium salt, C20H26P+·CF3SO3 −. Selected geometrical parameters include P—biphenyl (av.) = 1.801 (3) Å and P—t-Bu (av.) = 1.858 (3) Å, and significant distortion of the tetrahedral P-atom environment with biphenyl—P—biphenyl = 93.93 (13)° and t-Bu—P—t-Bu = 118.82 (14)°. In the crystal, weak C—H⋯O interactions lead to channels along the c axis that are occupied by CF3SO3 − anions.


Comment
The introduction of a cyclopalladated compound as a robust catalyst (Herrman et al., 2003) for Heck and cross-coupling reactions resulted in the design of structurally related palladacycles, phosphapalladacycles in particular (Beletskaya & Cheprakov, 2004). However, growing evidence suggests that palladacycles are disassembled during the pre-activation stage to yield low ligated Pd 0 species as the actual catalyst (d′Orlye & Jutland, 2005). This is further exemplified by our finding that the use of palladacycle (II in Fig. 1), an effective amination catalyst (Beletskaya & Cheprakov, 2004), as a source of palladium together with triphenylphosphine and a strong acid provided an extremely active catalytic system for the hydromethoxylation of alkenes (Omondi et al., 2011 andWilliams et al., 2008). In the reaction medium compound II ( Fig. 1) was rapidly converted into tetrakistriphenylphosphinepalladium(0) which acted as the actual catalyst. The facile formation of Pd 0 resulted from acid catalyzed elimination from the palladacycle. This treatment of II with ten equivalents of trifluoromethanesulfonic acid at room temperature resulted in the formation of colloidal palladium and the title compound (I in Fig. 1), the structure of which was confirmed by single-crystal X-ray crystallography.
The title compound I ( Fig. 2 and Scheme 1) is a salt consisting of phospholium cations and trifluoromethanesulfonate anions. All ions lie on general positions in the unit cell with no discernible differences in the bond distances of the coordination polyhedron of the phosphorus environment. The bond angles at the phosphorus center shows significant deviations from the expected 109.5° for the tetrahedral shape with biphenyl-P-biphenyl = 93.93 (13)° and t-Bu-P-t-Bu = 118.82 (14)°. These deviations can be ascribed to the somewhat strained 5-membered cyclisation of the dibenzo fragment to form the phospholium ring. This pinching effect in turn allows for more space for the bulky tertiary butyl groups positioned above and below the plane formed by the tricyclic phospholium conjugate and hence the observed t-Bu -P-t-Bu angle. The tricyclic phospholium conjugate marginally deviates from planarity (C1-C6-C7-C12 = 1.9 (4)°), and would have been the primary route to alleviate stress from the pinching effect. Data extracted from the Cambridge Structural Database (Allen, 2002) for the torsion angle between the planes shows a mean value of 1.72° (126 observations). The general trend seems to be that substituents opposite the phospholium cycle forces it to be planar. The preferred orientation of the tertiary butyls are due to several weak C-H···O interactions observed between ions (see Table 1).

Experimental
Trifluorometanesulfonic acid (150 mg, 1 mmol) in 5 ml me thanol was added dropwise to a stirred solution of (acetatoκ 2 O,O′)[2′-(di-tert-butylphosphanyl)-1,1′-biphenyl-κ 2 P,C 2 ]palladium (462 mg, 1 mmol) in dichloromethane (30 ml) at room temperature under argon. The solution changed from colourless to deep purple and then to dark brown over a period of 10 min. After several hours at room temperature a fine precipitate of palladium black started to form. After 24 h the reaction mixture was filtered through celite to remove the palladium. The solvent was removed in vacuo and the residue distributed between water (15 ml) and ether (15 ml

Refinement
All hydrogen atoms for methyl and aromatic H atoms were positioned in geometrically idealized positions with C-H = 0.96 Å and 0.93 Å respectively. Aromatic hydrogen atoms were allowed to ride on their parent atoms with U iso (H) = 1.2U eq , and for methyl hydrogen atoms U iso (H) = 1.5U eq was utilized. The initial positions of methyl hydrogen atoms were located from a Fourier difference map and refined as fixed rotor. The Flack parameter refined to 0.05 (11).   View of title compound showing displacement ellipsoids (drawn at a 30% probability level) and numbering scheme.

Computing details
Hydrogen atoms have been omitted for clarity.

Special details
Experimental. The intensity data was collected on a Bruker SMART 1 K CCD diffractometer using an exposure time of 20 s/frame. A total of 984 frames were collected with a frame width of 0.3° covering up to θ = 28.37° with 99.3% completeness accomplished. Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s 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 > 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.