Aminophosphine
# Aminophosphine

In organophosphorus chemistry, an aminophosphine is a compound with the formula $R_{3−n}P(NR_2)_n$ where R = H or an organic substituent, and n = 0, 1, 2. At one extreme, the parent $H_2PNH_2$ is lightly studied and fragile, but at the other extreme tris(dimethylamino)phosphine $(P(NMe_2)_3)$ is commonly available. Intermediate members are known, such as $Ph_2PN(H)Ph.$ These compounds are typically colorless and reactive toward oxygen. They have pyramidal geometry at phosphorus.

## Parent members

The fundamental aminophosphines have the formulae $PH_{2−n}(NH_2)_n$ (n = 1, 2, 3). These species cannot be isolated in a practical quantities, although they have been examined theoretically. $H_2NPH_2$ is predicted to be more stable than the P(V) tautomer $HN=PH_3.$

With secondary amines, the chemistry is more straightforward. Trisaminophosphines are prepared by treatment of phosphorus trichloride with secondary amines:

    $PCl_3$ + 6 $HNMe_2$ → $(Me_2N)_3P$ + 3 $[H_2NMe_2]Cl$

## Aminophosphine chlorides

The amination of phosphorus trihalides occur sequentially, with each amination proceeding more slowly than the preceding:

    $PCl_3$ + 2 $HNMe_2$ → $Me_2NPCl_2$ + $[H_2NMe_2]Cl$
    $Me_2NPCl_2$ + 2 $HNMe_2$ → $(Me_2N)_2PCl$ + $[H_2NMe_2]Cl$

With bulky amines like diisopropylamine, the selectivity for the monosubstitution improves. Commercially available aminophosphine chlorides include dimethylaminophosphorus dichloride and bis(dimethylamino)phosphorus chloride.

Related aminophosphine fluorides compounds are available from trifluorophosphine. The diphosphine $MeN(PF_2)_2$ is prepared from methylamine:

    2 $PF_3$ + 3 $MeNH_2$ → $MeN(PF_2)_2$ + 2 $[MeNH_3]F$

$Me(PF_2)_2$ is used as a bridging ligand in organometallic chemistry.

Substituted aminophosphines are generally prepared from organophosphorus chlorides and amines. The method is used to prepare ligands for homogeneous catalysis. Chlorodiphenylphosphine and diethylamine react to give an aminophosphine:

    $Ph_2PCl$ + 2 $HNEt_2$ → $Ph_2PNEt_2$ + $[H_2NEt_2]Cl$

Primary amines react with phosphorus(III) chlorides give aminophosphines with acidic α-NH centers:

    $Ph_2PCl$ + 2 $H_2NR$ → $Ph_2PN(H)R$ + $[H_3NR]Cl$

## Reactions

### Protonolysis

The P-N bond is susceptible to attack by protic reagents. Alcoholysis occurs readily:

    $Ph_2PNEt_2$ + ROH → $Ph_2POR$ + $HNEt_2$

The P-N bond reverts to the chloride upon treatment with anhydrous hydrogen chloride:

    $Ph_2PNEt_2$ + 2 HCl → $Ph_2PCl$ + $[H_2NEt_2]Cl$

Similarly, transamination is used in the conversion of one aminophosphine to another:

    $P(NMe_2)_3$ + $R_2NH$ ⇌ $P(NR_2)(NMe_2)_2$ + $HNMe_2$

With tris(dimethylamino)phosphine is a reactant, the equilibrium can be driven by evaporation of dimethylamine.

Since the $P-NR_2$ bond is not attacked by Grignard reagents, aminophosphine chlorides are useful reagents in the preparation of unsymmetrical tertiary phosphines. Illustrative is the conversion of dimethylaminophosphorus dichloride to chlorodimethylphosphine:

    2 MeMgBr + $Me_2NPCl_2$ → $Me_2NPMe_2$ + 2 MgBrCl
    $Me_2NPMe_2$ + 2 HCl → $ClPMe_2$ + $Me_2NH_2Cl$

Illustrative also is the chemistry of 1,2-bis(dichlorophosphino)benzene, a versatile precursor to diphosphine ligands, is prepared using aminophosphine reagents. It is prepared from 1,2-dibromobenzene via lithiation and treatment with $(Et_2N)_2PCl$ (Et = ethyl). This route gives $C_6H_4[P(NEt_2)_2]_2,$ which is treated with hydrogen chloride:

    $C_6H_4[P(NEt_2)_2]_2$ + 8 HCl → $C_6H_4(PCl_2)_2$ + 4 $Et_2NH_2Cl$

### Conversion to phosphenium salts

Diaminophosphorus chlorides and tris(dimethylamino)phosphine are precursors to phosphenium ions of the type $[(R_2N)_2P]^+:$

    $R_2PCl$ + $AlCl_3$ → $[R_2P^+]AlCl_4^−$
    $P(NMe_2)_3$ + 2 HOTf → $[P(NMe_2)_2]OTf$ + $[H_2NMe_2]OTf$

### Oxidation and quaternization

Typical aminophosphines undergo oxidation to the oxide. Alkylation, e.g. by methyl iodide, gives the phosphonium cation.

