Crystal structure of triphenyl(vinyl)phosphonium tetraphenylborate

The title ionic salt, C21H20P+·C24H20B−, crystallized with two independent vinyltriphenylphosphonium cations and two independent tetraphenylborate anions per asymmetric unit. These four independent moieties contain nearly perfect tetrahedral symmetry about their respective central C atoms. In the crystal, there are no π-stacking or other intermolecular interactions present.


S1. Chemical Context
In recent years, phosphorus-carbon bond-forming reactions have been the object of interest in the modern organophosphorus chemistry due to the importance of these products in a variety of fields ranging from material science to the synthesis of biologically active molecules. Phosphonium salts have been known since the 1980s; however, they were rarely employed until the 1990s, when phosphines were used as precursors on the large scale, see: Bellina et al. (2012).
Phosphonium-based ionic liquids emerged as a very promising alternative to imidazolium analogs due to their superior thermal stability and inertness in the basic environment. The presence of an acidic hydrogen at the C2 position of imidazolium rings in many ionic liquids, leads to the thermal instability that limits the applications of these interesting organic fluids, see: Chowdhury et al. (2007).
In this study, we found that the vinyltriphenylphosphonium bromide could be synthesized via the nucleophilic attack of triphenylphosphine to allyl bromide to form the corresponding betaine 1, followed by the intra-hydrogen shift reaction ( Figure 2). In the next step, metathesis reaction of vinyltriphenyphosphonium bromide 2 with sodium tetraphenylborate led to the formation vinyltriphenyphosphonium tetraphenylborane 3 (T m = 72-74° C) as the final product in high yield (78%).

S2. Experimental
Triphenylphosphine (0.5 mmol) and allyl bromide (0.7 mmol) were dissolved in toluene (2.0 ml) and the mixture refluxed for 48 h. The solvent was removed in a vacuum and the crude product was dissolved in 4.0 ml of H 2 O. Then, a solution of sodium tetraphenylborate (0.6 mmol) in 2.0 ml of H 2 O was added to the original solution of crude product and was stirred for 24 h. The final product was separated via filtration and washed with H 2 O (3 x 5 ml) to yield white crystals of the title complex.

S3. Refinement
H-atoms were placed in calculated positions and allowed to ride during subsequent refinement with U iso (H) = 1.2U eq (C) and C-H distances of 0.93 Å, except for the methyl H atoms which had U iso (H) = 1.5U eq (C) and C-H distances of 0.96 Å.

Figure 1
A ball-and-stick representaion of the structure of I. Only one of the two cation/anion pairs are shown. Hydrogen atoms on the aromatic rings have been removed for clarity.

Figure 2
Reaction scheme for the synthesis of I. where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.002 Δρ max = 0.35 e Å −3 Δρ min = −0.23 e Å −3 Absolute structure: Flack (1983) Absolute structure parameter: 0.02 (7) Special details 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 > 2σ(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.