Dimethylphenylphosphine

Preparation

Dimethylphenylphosphine is prepared by the reaction of methylmagnesium halide with dichlorophenylphosphine.

The phosphine is purified by distillation under reduced pressure. A solution of (C₆H₅)(CH₃)₂P in CDCl₃ shows proton NMR signals at δ 7.0-7.5 and a doublet at δ 1.2. The phosphorus-31 NMR spectrum shows a singlet at -45.9 ppm in CDCl₃.

Structure and properties

Dimethylphenylphosphine is a pyramidal molecule where the phenyl group and two methyl groups are connected to the phosphorus. The bond length and angles are the following: P-C_Me: 1.844, P-C_Ph: 1.845 Å, C-C: 1.401 Å, C-H_Me: 1.090 Å, C-H_Ph: 1.067 Å, C-P-C: 96.9°, C-P-C (ring): 103.4°, P-C-H: 115.2°.

When attached to chiral metal centers, the P-methyl groups are diastereotopic, appearing as separate doublets in the ¹H NMR spectrum.

Comparisons with related phosphine ligands

The ν_CO of IrCl(CO)(PPh₃)₂ and IrCl(CO)(PMe₂Ph)₂ are both at 1960 cm⁻¹, whereas ν_CO for IrCl(CO)(PMe₃)₂ is at 1938 cm⁻¹.

In terms of basicity, dimethylphenylphosphine is intermediate between that of trialkyl- and triphenylphosphine:

The ligand cone angle (θ) is the apex angle of a cylindrical cone, which is centered 2.28 Å from the center of the P atom. However, the cone angle of an unsymmetrical ligand cannot be determined in the same. In order to determine an effective cone angle for an unsymmetrical ligand PX₁X₂X₃, the following equation is used:

Where θ_i represent the half angle.

The resulting angles for PMe₃, PMe₂Ph, PPh₃ are: PMe₃ = 118°, PMe₂Ph = 122°, PPh₃ = 145°. Thus, PMe₂Ph is intermediate in size relative to PMe₃ and PPh₃.