1,5-Diaza-3,7-diphosphacyclooctanes

1,5-Diaza-3,7-diphosphacyclooctanes are organophosphorus compounds with the formula [R'NCH$2$P(R)CH$2$]$2$, often abbreviated P$R$$2$N$R'$$2$. They are air-sensitive white solids that are soluble in organic solvents. The ligands exist as meso and d,l-diastereomers, but only the meso forms function as bidentate ligands.

Some metal-P$R$$2$N$R'$$2$ complexes catalyze the hydrogen evolution reaction as well as the oxidation of hydrogen (H$2$). The catalytic mechanism involves the interaction of substrate with the amines in the second coordination sphere.

Synthesis and reactions
The ligands are prepared by the condensation of a primary phosphine, formaldehyde, and a primary amine:
 * 2 RPH2 + 4 CH2O + 2 RNH2 → [RNCH2P(R')CH2]2 + 4 H2O

Diazadiphosphacyclooctanes function as chelating diphosphine ligands. Typical nickel complexes contain two such ligands are give the formula [Ni(P$R$$2$N$R'$$2$)$2$]$2+$.

Cationic complexes of these P2N2 and related ligands often exhibit enhanced reactivity toward H2. These complexes serve as electrocatalysts for H2 evolution.

Related ligands
Azadiphosphacycloheptanes are a related family of diphosphines, but containing only one amine. They are prepared by condensation of 1,2-bis(phenylphosphino)ethane, formaldehyde, and a primary amine. From the meso-isomer, typical nickel complexes contain two such ligands, i.e. [Ni(P$R$$2$NR')$2$]$2+$. When bound to metals, these ligands adopt a conformation similar to that of 1,4-diazacycloheptanes. Acyclic phosphine-amine ligands have the formula (R$2$PCH$2$)NR'.