Dimethylphenylphosphine Explained

Dimethylphenylphosphine is an organophosphorus compound with a formula P(C₆H₅)(CH₃)₂. The phosphorus is connected to a phenyl group and two methyl groups, making it the simplest aromatic alkylphosphine. It is colorless air sensitive liquid. It is a member of series (CH₃)_3-n(C₆H₅)₂P that also includes n = 0, n = 2, and n = 3 that are often employed as ligands in metal phosphine complexes.

Preparation

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

(C₆H₅)Cl₂P + 2CH₃MgBr → (C₆H₅)(CH₃)₂P + 2MgBrCl

The phosphine is purified by distillation under reduced pressure.^[1] 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.^[2]

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°.^[3]

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.^{[4] [5]}

In terms of basicity, dimethylphenylphosphine is intermediate between that of trialkyl- and triphenylphosphine:^{[6] [7]}

• [HPEt₃]⁺ = 8.7
• [HPMe₂Ph]⁺ = 6.8
• [HPPh₃]⁺ = 2.7

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.^[8]

Notes and References

1. Book: C. Frajerman. B. Meunier. Inorganic Syntheses. Preparation of Dimethylphenylphosphine. 1983. 22. 133–135. 10.1002/9780470132531. 9780470132531.
2. Laszlo T. Mika. Laszlo Orha. Norbert Farkas. Istvan T. Horváth. Efficient Synthesis of Water-Soluble Alkyl-bis(m-sulfonated-phenyl)- and Dialkyl-(m-sulfonated-phenyl)-phosphines and Their Evaluation in Rhodium-Catalyzed Hydrogenation of Maleic Acid in Water. 10.1021/om800570r. Organometallics. 2009. 28. 5. 1593.
3. Novikov, V. P.; Kolomeets, V. I., Syshchikov, Yu. N.; Vilkov, L. V.; Yarkov, A. V.; Tsvetkov, E. N.; Raevskii, O.A. "Investigation of structure of dimethylphenylphosphine by means of gas-phase electron diffraction and vibrational spectroscopy" Zh. Strukt. Khim. (J. Struc. Chem.) 1984, volume 25, No. 5, 688.
4. S. A. Cotton, Chemistry of Precious Metals., 1997, 152-157,,
5. A. R. Norris; J. A. V. Kessel, "Oxidative addition of 3,5-Dinitrobenzoyl Chloride to trans-Chlorocarbonylbis(dimethylphenylphosphine)iridium(I) Canadian Journal of Chemistry, 1973, volume 51, 4145-4151, .
6. Russell C. Bush. Robert Angelici. Robert J. Angelici. Inorg. Chem.. Phosphine Basicities As Determined by Enthalpies of Protonation. 1988. 27. 4. 681–686. 10.1021/ic00277a022.
7. Tianshu Li. Alan J. Lough. Robert H. Morris. Chemistry: A European Journal. An Acidity scale of Tetrafluoroborate Salts of Phosphonium and Iron Hydride compounds in [D2]Dichloromethane. 2007. 13. 13. 3796–3803. 10.1002/chem.200601484. 17245785. .
8. C. A. Tolman, Chem. Rev., Steric effects of Phosphorus Ligands in Organometallic Chemistry and Homogeneous Catalysis., 1977, volume 77, pages 313-348.