Diphosphines

Diphosphines, sometimes called bisphosphanes are organophosphorus compounds that are 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

Synthesis

The most widely used diphosphine ligands have the general formula Ph₂P(CH₂)_nPPh₂. These compounds can be prepared from the reaction of X(CH₂)_nX (X=halogen) and YPPh₂ (Y=alkali metal):^[1]

Cl(CH₂)_nCl + 2 NaPPh₂ → Ph₂P(CH₂)_nPPh₂ + 2 NaCl

Diphosphine ligands can also be prepared by other methods, such as from dilithiated reagents and chlorophosphines or by the addition of secondary phosphines to vinylphosphines.

Chain length and coordinating properties

The short-chain diphosphine dppm tend to promote metal-metal interactions as illustrated by A-frame complexes. When the two phosphine substituents are linked by 2-4 carbon centres, the resulting ligands often chelate rings with a single metal. The most common diphosphine ligand is 1,2-Bis(diphenylphosphino)ethane, which forms a 5-membered chelate ring with most metals.

Some diphosphines, such as the extraordinary case of ^tBu₂P(CH₂)₁₀P^tBu₂, give macrocyclic complexes with as many as 72 atoms in a ring.^[2]

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.^[3]

Common ligands

Diphosphine ligands that are particularly common are shown in the table below:^[4]

Abbreviation	Common Name (from which abbreviation derived)	IUPAC Name (rarely used)
dppm	1,1-Bis(diphenylphosphino)methane	Methylenebis(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	dimer 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)

References

1. ^ 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
2. ^ 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
3. ^ 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. doi:10.1002/anie.200390330. PMID 12645065. 
4. ^ http://old.iupac.org/reports/provisional/abstract04/RB-prs310804/TableVII-3.04.pdf