Aminophosphine

In organophosphorus chemistry, aminophosphines are compounds with the formula R_3−nP(NR₂)_n where R is a hydrogen or organic substituent, and n = 0, 1, or 2. At one extreme, the parents H₂PNH₂ and P(NH₂)₃ are lightly studied and fragile. At the other extreme, tris(dimethylamino)phosphine (P(NMe₂)₃) is commonly available. Intermediate members are known, such as Ph₂PN(H)Ph. Aminophosphines are typically colorless and reactive to oxygen. Aminophosphines are pyramidal geometry at phosphorus.^[1]

Parent members

Structure of P(NMe₂)₃.

The aminophosphine called the Verkade base is a superbase.

The fundamental aminophosphines have the formulae PH_3−n(NH₂)_n (n = 1, 2, or 3). Fundamental aminophosphines can not be isolated in a practical quantities but have been examined theoretically. H₂NPH₂ is predicted to be more stable than the P(V) tautomer HN=PH₃.^[2]

Secondary amines are more straightforward. Trisaminophosphines are made by treating phosphorus trichloride with secondary amines:

PCl₃ + 6 HNMe₂ → (Me₂N)₃P + 3 [H₂NMe₂]Cl

where Me = methyl.

Aminophosphine chlorides

Structure of Me₂NPCl₂.

The amination of phosphorus trihalides occur sequentially, with each amination proceeding slower than before:^[3]

PCl₃ + 2 HNMe₂ → Me₂NPCl₂ + [H₂NMe₂]Cl

Me₂NPCl₂ + 2 HNMe₂ → (Me₂N)₂PCl + [H₂NMe₂]Cl

(Me₂N)₂PCl + 2 HNMe₂ → (Me₂N)₃P + [H₂NMe₂]Cl

Monosubstitution selectivity improves with bulky amines such as diisopropylamine.^[4] Commercially available aminophosphine chlorides include dimethylaminophosphorus dichloride and bis(dimethylamino)phosphorus chloride.

Methylamine and trifluorophosphine react to give MeN(PF₂)₂:

2 PF₃ + 3 MeNH₂ → MeN(PF₂)₂ + 2 [MeNH₃]F

MeN(PF₂)₂ is a bridging ligand in organometallic chemistry.

Aminophosphines can also made from organophosphorus chlorides and amines.^[5] Chlorodiphenylphosphine and diethylamine react to give an aminophosphine:^[1][6]

Ph₂PCl + 2 HNEt₂ → Ph₂PNEt₂ + [H₂NEt₂]Cl

Primary amines react with phosphorus(III) chlorides to give aminophosphines with acidic α-NH centers:^[7]

Ph₂PCl + 2 H₂NR → Ph₂PN(H)R + [H₃NR]Cl

Reactions

Protonolysis

Protic reagents attack the P-N bond. Alcoholysis readily occurs:

Ph₂PNEt₂ + ROH → Ph₂POR + HNEt₂

The P-N bond reverts to the chloride when treated with anhydrous hydrogen chloride:

Ph₂PNEt₂ + HCl → Ph₂PCl + HNEt₂

Transamination similarly converts one aminophosphine to another:

P(NMe₂)₃ + R₂NH ⇌ P(NR₂)(NMe₂)₂ + HNMe₂

With tris(dimethylamino)phosphine, dimethylamine evaporation can drive the equilibrium.^[8]

Since Grignard reagents do not attack P-NR₂ bond, aminophosphine chlorides are useful reagents in preparing unsymmetrical tertiary phosphines. Illustrative is converting dimethylaminophosphorus dichloride to chlorodimethylphosphine:^[9]

2 MeMgBr + Me₂NPCl₂ → Me₂NPMe₂ + 2 MgBrCl

Me₂NPMe₂ + 2 HCl → ClPMe₂ + Me₂NH₂Cl

Also, illustrative is the synthesis of 1,2-bis(dichlorophosphino)benzene using (Et₂N)₂PCl (Et = ethyl). This route gives C₆H₄[P(NEt₂)₂]₂, which is treated with hydrogen chloride:^[10]

C₆H₄[P(NEt₂)₂]₂ + 8 HCl → C₆H₄(PCl₂)₂ + 4 Et₂NH₂Cl

Conversion to phosphenium salts

Diaminophosphorus chlorides and tris(dimethylamino)phosphine are precursors to phosphenium ions of the type [(R₂N)₂P]⁺:^[11][12]

R₂PCl + AlCl₃ → [R₂P⁺]AlCl₄⁻

P(NMe₂)₃ + 2 HOTf → [P(NMe₂)₂]OTf + [H₂NMe₂]OTf

Oxidation and quaternization

Typical aminophosphines oxidize. Alkylation, such as by methyl iodide, gives the phosphonium cation.

Addition to carbonyls

In diazaphospholenes the polarity of the P-H bond is inverted compared to traditional secondary phosphines. They have some hydridic character. One manifestation of this polarity is their reactivity toward benzophenone in yet another way.^[13]

Diazaphospholene phosphine hydride