In organophosphorus chemistry, aminophosphines are compounds with the formula R3−nP(NR2)n where R is a hydrogen or organic substituent, and n = 0, 1, or 2. At one extreme, the parents H2PNH2 and 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]
The fundamental aminophosphines have the formulae PH3−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 [H<sub>2</sub>NMe<sub>2</sub>]Cl
The amination of phosphorus trihalides occur sequentially, with each amination proceeding slower than before:[3]
PCl3 + 2 HNMe2 → Me2NPCl2 + [H<sub>2</sub>NMe<sub>2</sub>]Cl
Me2NPCl2 + 2 HNMe2 → (Me2N)2PCl + [H<sub>2</sub>NMe<sub>2</sub>]Cl
(Me2N)2PCl + 2 HNMe2 → (Me2N)3P + [H<sub>2</sub>NMe<sub>2</sub>]Cl
Monosubstitution selectivity improves with bulky amines such as diisopropylamine.[4] Commercially available aminophosphine chlorides include dimethylaminophosphorus dichloride and bis(dimethylamino)phosphorus chloride.
Related aminophosphine fluorides compounds are available from trifluorophosphine. Methylamine prepares Diphosphine MeN(PF2)2:
2 PF3 + 3 MeNH2 → MeN(PF2)2 + 2 [MeNH<sub>3</sub>]F
Me(PF2)2 is a bridging ligand in organometallic chemistry.
Substituted aminophosphines are generally made from organophosphorus chlorides and amines. The method prepares ligands for homogeneous catalysis.[5] Chlorodiphenylphosphine and diethylamine react to give an aminophosphine:[6]
Ph2PCl + 2 HNEt2 → Ph2PNEt2 + [H<sub>2</sub>NEt<sub>2</sub>]Cl
Primary amines react with phosphorus(III) chlorides to make aminophosphines with acidic α-NH centers:[7]
Ph2PCl + 2 H2NR → Ph2PN(H)R + [H<sub>3</sub>NR]Cl
Protic reagents attack the susceptible 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 chemistry of 1,2-bis(dichlorophosphino)benzene, a versatile diphosphine ligand precursor prepared using aminophosphine reagents. It is made from 1,2-dibromobenzene via lithiation and treatment with (Et2N)2PCl (Et = ethyl). This route gives C6H4[P(NEt<sub>2</sub>)<sub>2</sub>]2, which is treated with hydrogen chloride:[10]
C6H4[P(NEt<sub>2</sub>)<sub>2</sub>]2 + 8 HCl → C6H4(PCl2)2 + 4 Et2NH2Cl
Diaminophosphorus chlorides and tris(dimethylamino)phosphine are precursors to phosphenium ions of the type [(R<sub>2</sub>N)<sub>2</sub>P]+:[11] [12]
R2PCl + AlCl3 → [R<sub>2</sub>P<sup>+</sup>]AlCl4−
P(NMe2)3 + 2 HOTf → [P(NMe<sub>2</sub>)<sub>2</sub>]OTf + [H<sub>2</sub>NMe<sub>2</sub>]OTf
Typical aminophosphines oxidize. Alkylation, such as by methyl iodide, gives the phosphonium cation.