1,5-Diaza-3,7-diphosphacyclooctanes

1,5-Diaza-3,7-diphosphacyclooctanes are organophosphorus compounds with the formula [R'NCH₂P(R)CH₂]₂, often abbreviated P^R₂N^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.^[1][2]

P2N2 ligand structure

Some metal-P^R₂N^R'₂ complexes catalyze the hydrogen evolution reaction as well as the oxidation of hydrogen (H₂). The catalytic mechanism involves the interaction of substrate with the amines in the second coordination sphere.^[3][4]

Synthesis and reactions

The ligands are prepared by the condensation of a primary phosphine, formaldehyde, and a primary amine:^[5]

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^R₂N^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.

Conformation of chelate rings in P2N2-M and P2N-M complexes

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.^[6] From the meso-isomer, typical nickel complexes contain two such ligands, i.e. [Ni(P^R₂NR')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 (R₂PCH₂)NR'.