Diphosphines

Skeletal formula of a generic diphosphine ligand. R represents a side chain.

Diphosphines, sometimes called bisphosphanes, are organophosphorus compounds used as ligands in inorganic and organometallic chemistry. They are identified by the presence of two phosphino groups linked by a backbone, and are usually chelating.[1]

Synthesis

Many widely used diphosphine ligands have the general formula Ar2P(CH2)nPAr2. These compounds can be prepared from the reaction of X(CH2)nX (X=halogen) and MPPh2 (M = alkali metal):[2]

Cl(CH2)nCl + 2 NaPPh2 → Ph2P(CH2)nPPh2 + 2 NaCl

Diphosphine ligands can also be prepared from dilithiated reagents and chlorophosphines:[3]

XLi2 + 2 ClPAr2 → X(PAr2)2 + 2 LiCl (X = hydrocarbon backbone)

This approach is suitable for installing two dialkylphosphino groups, using reagents such as chlorodiisopropylphosphine.

Another popular method, suitable for preparing unsymmetrical diphosphines, involves the addition of secondary phosphines to vinylphosphines:

Ph2PH + 2 CH2=CHPAr2 → Ph2PCH2-CH2PAr2

(2-Lithiophenyl)diphenylphosphine can be used also to give unsymmetrical diphosphines. The lithiated reagent is available from (2-bromophenyl)diphenylphosphine:

Ph2P(C6H4Br) + BuLi → Ph2P(C6H4Li) + BuBr
Ph2P(C6H4Li) + R2PCli → Ph2P(C6H4PR2) + LiCl

Chain length and coordinating properties

The short-chain diphosphine dppm tends to promote metal-metal interactions as illustrated by A-frame complexes. When the two phosphine substituents are linked by two to four carbon centres, the resulting ligands often chelate rings with a single metal. A common diphosphine ligand is dppe, which forms a five-membered chelate ring with most metals.

Some diphosphines, such as the extraordinary case of tBu2P(CH2)10PtBu2, give macrocyclic complexes with as many as 72 atoms in a ring.[4]

To position phosphine donor groups trans on a coordination sphere, several atoms are required to link the donor centres and long-chain diphosphines are typically floppy and do not chelate well. This challenge has been resolved by the long but rigid diphosphine SPANphos.[5] The bite angle of the diphosphine influences the reactivity of the metal center.[6]

Some examples of non-chelating diphosphine also exist. Due to steric effect, these phosphorus atoms can not react with anything except a proton. [7]

Representative ligands

Particularly common diphosphine ligands are shown in the table below:[8]

Abbreviation Common name
(from which abbreviation derived)		 IUPAC Name (rarely used)
dppm 1,1-Bis(diphenylphosphino)methane Methylene bis(diphenylphosphane)
dmpe 1,2-Bis(dimethylphosphino)ethane Ethane-1,2-diylbis(dimethylphosphane)
dippe 1,2-Bis(diisopropylphosphino)ethane Ethane-1,2-diylbis(di(propan-2-yl)phosphane)
dppe 1,2-Bis(diphenylphosphino)ethane Ethane-1,2-diylbis(diphenylphosphane)
DIPAMP derivative of phenylanisylmethylphosphine Ethane-1,2-diylbis[(2-methoxyphenyl)phenylphosphane]
dppp 1,3-Bis(diphenylphosphino)propane Propane-1,3-diylbis(diphenylphosphane)
dppb 1,4-Bis(diphenylphosphino)butane Butane-1,4-diylbis(diphenylphosphane)
Chiraphos 2,3-Bis(diphenylphosphino)butane Butane-2,3-diylbis(diphenylphosphane)
BINAP 2,2'-Bis(diphenylphosphino)-1,1'-binaphthyl 1,1'-Binaphthalene-2,2'-diylbis(diphenylphosphane)
Xantphos 4,5-Bis(diphenylphosphino)-9,9-dimethylxanthene 9,9-Dimethyl-9H-xanthene-4,5-diylbis(diphenylphosphane)
SPANphos 4,4,4',4',6,6'-Hexamethyl-2,2'-spirobichromane-8,8'-diylbis(diphenylphosphane)
SEGPHOS 4,4'-Bi-1,3-benzodioxole-5,5'-diylbis(diphenylphosphane)
dppf 1,1'-Bis(diphenylphosphino)ferrocene
Me-DuPhos 1,2-Bis(2,5-dimethylphospholano)benzene 1,2-Phenylenebis(2,5-dimethylphospholane)
Josiphos (Diphenylphosphino)ferrocenyl-ethyldicyclohexylphosphine 1,5-Diaza-3,7-diphosphacyclooctanes
P2N2 1,5-Diaza-3,7-diphosphacyclooctanes

References

  1. ↑ Hartwig, J. F. Organotransition Metal Chemistry, from Bonding to Catalysis; University Science Books: New York, 2010. ISBN 189138953X
  2. ↑ Wilkinson, G.; Gillard, R.; McCleverty, J. Comprehensive Coordination Chemistry: The synthesis, reactions, properties & applications of coordination compounds, vol.2.; Pergamon Press: Oxford, UK, 1987; p. 993. ISBN 0-08-035945-0
  3. ↑ Gareth J. Rowlands "Planar Chiral Phosphines Derived from [2.2]Paracyclophane" Israel Journal of Chemistry 2012, Volume 52, Issue 1-2, pages 60–75.doi:10.1002/ijch.201100098
  4. ↑ Cotton, F.A.; Wilkinson, G. Advanced Inorganic Chemistry: A Comprehensive Text, 4th ed.; Wiley-Interscience Publications: New York, NY, 1980; p.246. ISBN 0-471-02775-8
  5. ↑ Z. Freixa, M. S. Beentjes, G. D. Batema, C. B. Dieleman, G. P. F. v. Strijdonck, J. N. H. Reek, P. C. J. Kamer, J. Fraanje, K. Goubitz and P. W. N. M. Van Leeuwen (2003). "SPANphos: A C2-Symmetric trans-Coordinating Diphosphane Ligand". Angewandte Chemie. 42 (11): 1322–1325. PMID 12645065. doi:10.1002/anie.200390330.
  6. ↑ Birkholz, M.-N., Freixa, Z., van Leeuwen, P. W. N. M., "Bite angle effects of diphosphines in C-C and C-X bond forming cross coupling reactions", Chem. Soc. Rev. 2009, vol. 38, 1099.
  7. ↑ Zong, J., J. T. Mague, C. M. Kraml, and R. A. Pascal, Jr., A Congested in,in-Diphosphine, Org. Lett. 2013, 15, 2179-2181.
  8. ↑ http://old.iupac.org/reports/provisional/abstract04/RB-prs310804/TableVII-3.04.pdf
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