PMDTA (''N'',''N'',''N′'',''N′′'',''N′′''-pentamethyldiethylenetriamine) is an organic compound with the formula CH₃)₂NCH₂CH₂sub>2NCH₃. PMDTA is a basic, bulky, and flexible,

tridentate ligand A tridentate ligand (or terdentate ligand) is a ligand that has three atoms that can function as acceptor atoms in a coordination complex. Well-known tridentate ligands include diethylenetriamine with three nitrogen donor atoms, and the iminodia ...

that is a used in organolithium chemistry. It is a colorless liquid, although impure samples appear yellowish.

Synthesis

PMDTA is prepared from

diethylenetriamine Diethylenetriamine (abbreviated and also known as 2,2’-Iminodi(ethylamine)) is an organic compound with the formula HN(CH2CH2NH2)2. This colourless hygroscopic liquid is soluble in water and polar organic solvents, but not simple hydrocarbons. ...

by the Eschweiler-Clarke reaction, involving the use of formaldehyde and

formic acid Formic acid (), systematically named methanoic acid, is the simplest carboxylic acid, and has the chemical formula HCOOH and structure . It is an important intermediate in chemical synthesis and occurs naturally, most notably in some ants. Es ...

. :(H₂N H₂sub>2)₂NH + 5 CH₂O + 5 HCO₂H → (Me₂N H₂sub>2)₂NMe + 5 CO₂ + 5 H₂O

Comparison with diethylenetriamine

Unlike diethylenetriamine, all three amines in PMDTA are tertiary. Both PMDTA and diethylenetriamine are tridentate ligands that form two five-membered chelate rings. The σ-donating properties of the amino groups of diethylenetriamine are greater than that of PMDTA in copper(II) complexes. Both ligands can coordinate metal complexes in arrangements where the three nitrogen centers are co-planar or mutually ''cis''.

Organolithium compounds and PMDTA

PMDTA is used to modify the reactivity of organolithium compounds, which deaggregate in the presence of Lewis bases to enhance their reactivity. Commonly, the ''di''tertiary amine TMEDA is used in these applications; it binds to the lithium center as a bidentate ligand. PMDTA behaves analogously, but since it is tridentate, it binds more strongly to lithium. In contrast to TMEDA, PMDTA forms monomeric complexes with organolithium compounds. Both amines affect the regiochemistry of metalation. In the PMDTA/''n''-BuLi adducts, the Li-C bonds are highly polarized, thus increasing the basicity of the butyl group. The effect of PMDTA on lithium anilide is illustrative of PMDTA's complexing power. The complex, sub>3·2PMDTA is trinuclear, featuring approximately colinear Li⁺ centers that are three-, four-, and five-coordinate. The central three-coordinate lithium atom is not bonded to PMDTA. One of the terminal Li centers is pseudo-tetrahedral in an N₄ coordination sphere. The other terminal lithium atom is five-coordinate and binds to two anilino N centers and the PMDTA.

Transition metal and aluminium complexes

PMDTA often forms five-coordinate complexes due to steric bulk of the methyl groups. PMDTA stabilize unusual cations. The first cationic derivative of alane, ₂Al(PMDTA)sup>+ lH₄sup>− was prepared by treating H₃AlNMe₃ with PMDTA.

References

{{reflist Ethyleneamines Tertiary amines Tridentate ligands