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 PR2NR'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-PR2NR'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:

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 enhanced 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.[5] 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. Erika Bálint; Adám Tajti; Anna Tripolszky; György Keglevich (2018). "Synthesis of platinum, palladium and rhodium complexes of α-aminophosphine ligands". Dalton Trans. 47 (14): 4755–4778. doi:10.1039/C8DT00178B. PMID 29565437.
  2. Wiedner, Eric S.; Appel, Aaron M.; Raugei, Simone; Shaw, Wendy J.; Bullock, R. Morris (2022). "Molecular Catalysts with Diphosphine Ligands Containing Pendant Amines". Chemical Reviews. 122 (14): 12427–12474. doi:10.1021/acs.chemrev.1c01001. PMID 35640056. S2CID 249237539.
  3. 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. 46 (45): 8618–8620. doi:10.1039/c0cc03246h. PMID 20938535. S2CID 34168725.
  4. 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 (7): 2017–2026. doi:10.1021/acs.accounts.5b00069. OSTI 1582563. PMID 26079983.
  5. 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 (2): 125–132. doi:10.1002/hc.20397.
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