Two tris(3,5-disubstituted phenyl)phosphines and their isostructural PV oxides

The crystal structures of two bulky triarylphosphines of emerging interest in coordination chemistry and catalysis have been determined, and the syntheses and crystal structures of their corresponding oxides are reported. Each oxide is isostructural to its corresponding phosphine.


Chemical context
The two bulky triarylphosphines (I) and (III) are of considerable interest in coordination chemistry and catalysis (Kakizoe et al., 2017;Lian et al., 2017;Ogiwara et al., 2017;Nishikawa et al., 2016;Naruto et al., 2015;Jover et al., 2010;Romain et al., 2000) and have been investigated for frustrated Lewis-pair activity (Wang & Stephan, 2014;Ullrich et al., 2010). The synthesis of (I) was first mentioned in the nonpatent literature by Hengartner et al. (1979) and in more detail twelve years later (Culcasi et al., 1991) and is now commercially available from several sources, but its crystal structure has not been reported. The preparation of (III) was reported by Romain et al. (2000) some 11 years after it appeared in the patent literature. These authors reported a crystal structure, Cambridge Structural Database (CSD, Version 5.39, with updates to November 2017; Groom et al., 2016) refcode: FOQNOO. However, as this determination used molybdenum radiation and a serial diffractometer, we have repeated it here under the same conditions as the other three compounds to improve comparability. Phosphine oxide (II) was first mentioned for its use as an additive that enhances the enantiomeric excess in stoichiometric asymmetric epoxidation of E-methylstyrene (Kerrigan et al., 2002) and a schematic synthesis was reported a year later (Henschke et al., 2003) but the characterization details are not found in the open literature. Similarly, phosphine oxide (IV) is mentioned only in the patent literature. Here we report the crystal structures of (I), (II) and (IV) and full details for synthesis and characterization of (II) and (IV), for the first time, and the redetermination of (III).

Structural commentary
Phosphine (I) crystallizes in P2 1 /c with one molecule in the asymmetric unit that is distinctly pyramidal (Fig. 1). It has a sum of angles around the central phosphorus atom, the pyramidality index (see Boeré & Zhang, 2013), P (C-P-C) = 305. 35 (16) . This is a smaller value than that in PPh 3 , P (C-P-C) = 308.3 (2) (Boeré & Zhang, 2005), indicating a more pyramidal structure, despite the potential steric interference of the three endo-oriented methyl substituents at C3, C13, and C23. Similarly, (III) crystallizes in Pbca also with Z 0 = 1 and P (C-P-C) = 307.2 (4) . By contrast, phosphines with 2,6-disubstitution patterns have greatly reduced pyramidality. For example, P (C-P-C) = 335.6 (3) in Dipp 3 P, (Boeré et al., 2008) 334.4 (3) in Tripp 3 P, (Sasaki et al., 2002) and 329.1 (5) in Mes 3 P, (Blount et al., 1994). Oxidation or protonation of Ar 3 P always leads to some flattening at the phosphorus atom. Thus, although (II) is isostructural with (I), P (C-P-C) = 317.23 (15) differs by some 12 , while (IV), which is isostructural with (II), has P (C-P-C) = 318.67 (18) (Fig. 2). In sixteen independent structure determinations of Ph 3 PO reported in the CSD, the average value with s.u. of P (C-P-C) is 319.3 (3) . Thus, for both the title phosphines and their oxides, the pyramidality index for the title compounds is lower than in the corresponding Ph 3 P or Ph 3 PO. That all these 3,5-dimethyl-substituted compounds should be more pyramidal than corresponding C 6 H 5 -derivatives is at first surprising. A plausible explanation for this is that the substitution induces greater intramolecular dispersion interactions, i.e. between the methyl groups and the -clouds of adjacent rings. To find evidence for this, hybrid density functional theory (DFT) calculations [with Becke's non-local three parameter exchange and the Lee-Yang-Parr correlation functional (B3LYP) and also incorporating Grimme's D3 empirical dispersion corrections] with the 6-31G(2d,p) basis set, as implemented in the Gaussian16 program package (Frisch et al., 2016), were undertaken. The optimized geometries by DFT are characterized by common P (C-P-C) = 304.8 for both (I) and (III) and 317.4 for both (II) and (IV). This supports dispersion as an origin for the observed increased pyramidality caused by 3,5-dimethyl group substi- Stacking interactions (-and 'T' type) linking centrosymmetric pairs of (a) phosphine (I) and (b) phosphine oxide (II), which is a likely cause of the conformations adopted by the C1 rings. [Symmetry code: (i) Àx, 1 À y, Àz].
as seen by ring-tilt dihedral angles of 35.6 (1), 8.3 (1) and 58.1 (1) in (I) and 29.4 (1), 9.1 (1) and 61.2 (1) in (II). In each of these structures, the C1 aryl rings are almost parallel to the molecular threefold axes, a geometry that was defined as the transition state for Mislow's 'one-ring flip' mechanism for racemization of propeller-shaped molecules (Gust & Mislow, 1973). As shown in Fig. 3a, the molecules in (I) are centrosymmetrically related to one another and there are short intermolecular contacts between the C1 rings on adjacent molecules (C2 and C1 to methyl hydrogen H7C i of 2.84 and 2.90 Å and H4 to C14 i of 2.87 Å . It is likely that this packing preference is responsible for the non-helical arrangement of the rings in this structure. Similarly, in (II) short contacts link C14 with H4 i at 2.88 Å and C16 with methyl hydrogen H7B i at 2.68 Å (Fig. 3b) [Symmetry code: (i) Àx, 1 À y, Àz]. There are some short intermolecular C-HÁ Á ÁO interactions in structures (II)-(IV), as listed in Tables 1-3.

Preparation of (II)
Tris(3,5-dimethylphenyl)phosphine oxide [381212-20-0], (II), was prepared by dissolving 0.10 g (I), 0.29 mmol, in 15 ml of acetone (thin-layer chromatography, TLC, monitoring: R f = 0.32 in 1:9 ethyl acetate/hexanes), heating to the boil, and adding 3.0 mL of 4% aqueous H 2 O 2 dropwise. After gentle reflux for 1.5 h, the mixture was checked again by TLC (R f = 0) indicating reaction completion. Removal of all volatiles, dissolving in 10 ml CH 2 Cl 2 and drying overnight with Na 2 SO 4 , filtering and evaporating, left a dry solid. Recrystallization from mixed solvents of 5 ml heptane and 2 ml CH 2 Cl 2 at the boil produced colourless blocks on cooling, recovered by slow evaporation to afford 0.06 g (II), 0.17 mmol, 57% yield. Identity was established by X-ray crystallography and very high purity by nuclear magnetic resonance (NMR) spectroscopy (atom numbers are those from the C1 ring in Fig. 1b).

Refinement details
Crystal data, data collection and structure refinement details are summarized in Table 4. H atoms attached to C atoms were treated as riding, with C-H = 0.98 Å and U iso (H) = 1.5U eq (C) for methyl and C-H = 0.95 Å and U iso (H) = 1.2U eq (C) for aromatic H atoms.

Tris(4-methoxy-3,5-dimethylphenyl)phosphane (III)
Crystal data where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 0.54 e Å −3 Δρ min = −0.67 e Å −3 Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq P1 0.79578 (