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

Alfred Muller; Cedric W Holzapfel

doi:10.1107/S1600536812049045

. 2012 Dec 5;69(Pt 1):o20. doi: 10.1107/S1600536812049045

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

Alfred Muller ^a,^*, Cedric W Holzapfel ^a

PMCID: PMC3588282 PMID: 23476408

Abstract

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.

Related literature

For background to catalytic studies on palladacycles, see: Herrman et al. (2003 ▶); Beletskaya & Cheprakov (2004 ▶); Omondi et al. (2011 ▶); Williams et al. (2008 ▶); d’Orlye & Jutland (2005 ▶). For a description of the Cambridge Structural Database, see: Allen (2002 ▶).

Experimental

Crystal data

C₂₀H₂₆P⁺·CF₃O₃S⁻
M _r = 446.45
Tetragonal,
a = 12.1339 (10) Å
c = 30.057 (2) Å
V = 4425.4 (6) Å³
Z = 8
Mo Kα radiation
μ = 0.26 mm⁻¹
T = 293 K
0.4 × 0.26 × 0.2 mm

Data collection

Bruker SMART 1K CCD diffractometer
Absorption correction: multi-scan (SADABS; Bruker, 2008 ▶) T _min = 0.902, T _max = 0.949
25275 measured reflections
5502 independent reflections
3241 reflections with I > 2σ(I)
R _int = 0.101

Refinement

R[F ² > 2σ(F ²)] = 0.054
wR(F ²) = 0.117
S = 0.99
5502 reflections
268 parameters
H-atom parameters constrained
Δρ_max = 0.17 e Å⁻³
Δρ_min = −0.25 e Å⁻³
Absolute structure: Flack (1983 ▶), 2283 Friedel pairs
Flack parameter: 0.05 (11)

Data collection: SMART-NT (Bruker, 1998 ▶); cell refinement: SAINT-Plus (Bruker, 2008 ▶); data reduction: SAINT-Plus and XPREP (Bruker, 2008 ▶); program(s) used to solve structure: SIR97 (Altomare et al., 1999 ▶); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008 ▶); molecular graphics: DIAMOND (Brandenburg & Putz, 2005 ▶); software used to prepare material for publication: WinGX (Farrugia, 2012 ▶).

Supplementary Material

Click here for additional data file.^{(30.1KB, cif)}

Crystal structure: contains datablock(s) global, I. DOI: 10.1107/S1600536812049045/aa2079sup1.cif

e-69-00o20-sup1.cif^{(30.1KB, cif)}

Click here for additional data file.^{(264KB, hkl)}

Structure factors: contains datablock(s) I. DOI: 10.1107/S1600536812049045/aa2079Isup2.hkl

e-69-00o20-Isup2.hkl^{(264KB, hkl)}

Additional supplementary materials: crystallographic information; 3D view; checkCIF report

Table 1. Hydrogen-bond geometry (Å, °).

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
C2—H2⋯O1ⁱ	0.93	2.49	3.381 (4)	161
C19—H19A⋯O2ⁱⁱ	0.96	2.52	3.458 (4)	165
C11—H11⋯O3ⁱⁱⁱ	0.93	2.70	3.601 (4)	162
C15—H15B⋯O3ⁱⁱⁱ	0.96	2.69	3.470 (4)	138

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Symmetry codes: (i) Inline graphic ; (ii) ; (iii) .

Acknowledgments

The University of Witwatersrand is thanked for the use of their diffractometer. The research fund of the University of Johannesburg is gratefully acknowledged.

supplementary crystallographic information

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 Pd⁰ 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 and Williams 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 Pd⁰ 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 (acetato-κ²O,O')[2'-(di-tert- butylphosphanyl)-1,1'-biphenyl-κ²P,C²]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). The aqueous phase was extracted with dichloromethane (3 x 30 ml). The combined extract furnished crystalline (413 mg, 82%) on removal of the solvent in vacuo. Good crystals (mp. 157 – 159 °C) was obtained by diffusion of the vapours of ether into a solution in dichloromethane. Analytical data: ¹H-NMR: δ 1.45 (18H, d, J = 17 Hz), 7.68 (2H, dt, J = 2.5 and 7.5 Hz), 7.85 (2H, t, J = 7.8 Hz), 7.99 (2H, t, J = 7.8 Hz), and 8.64 (2H, dd, J = 2.5 and 7.8 Hz) ¹³C{H}-NMR: δ 26.74 (s), 36.42 (d, J = 42 Hz), 118.72 (d, J = 101 Hz), 123.58 (d, J = 12.3 Hz), 130.90 (d, J = 14.0 Hz), 131.90 (d, J = 14.0 Hz), 131.67 (d, J = 11.4 Hz), 135.95 (d, J = 2.7 Hz), 144.97 (d, J = 17.4 Hz) ³¹P-NMR: δ 51.24