1,5-Diaza-3,7-diphosphacyclooctanes

P2N2 ligand structure.

1,5-Diaza-3,7-diphosphacyclooctanes are organophosphorus compounds with the formula [R'NCH2P(R)CH2]2, often abbreviated PR2NR'2. They are air-sensitive, white solids that are soluble in organic solvents. Metal complexes of these bidentate ligands are widely studied because they catalyze the production and oxidation of hydrogen (H2).[1][2] The ligands exist as meso and d,l-diastereomers, but only the meso form is of current interest.

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(PR2NR'2)2]2+.

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

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

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.[3] From the meso-isomer, typical nickel complexes contain two such ligands, i.e. [Ni(PR2NR')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'.

References

  1. ↑ Yang, J. Y.; Chen, S.; Dougherty, W. G.; Kassel, W. S.; Bullock, R. M.; DuBois, D. L.; Raugei, S.; Rousseau, R.; Dupuis, M.; Rakowski DuBois, M. (2010). "Hydrogen oxidation catalysis by a nickel diphosphine complex with pendant tert-butyl amines". Chem. Commun.: 8618–8620. doi:10.1039/c0cc03246h.
  2. ↑ Bullock, R. M.; Helm, M. L. (2015). "Molecular Electrocatalysts for Oxidation of Hydrogen Using Earth-Abundant Metals: Shoving Protons Around with Proton Relays". Acc. Chem. Res. 48: 2017–2026. doi:10.1021/acs.accounts.5b00069.
  3. ↑ Karasik, A. A.; Balueva, A. S.; Moussina, E. I.; Naumov, R. N.; Dobrynin, A. B.; Krivolapov, D. B.; Litvinov, I. A.; Sinyashin, O. G. (2008). "1,3,6-azadiphosphacycloheptanes: a novel type of heterocyclic diphosphines". Heteroatom Chemistry. 19: 125–132. doi:10.1002/hc.20397.
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