Metal-phosphine complex

A metal-phosphine complex is a coordination complex containing one or more phosphine ligands. Almost always, the phosphine is an organophosphine of the type R₃P (R = alkyl, aryl). Metal phosphine complexes are useful in homogeneous catalysis.^[1][2] Prominent examples of metal phosphine complexes include Wilkinson's catalyst (Rh(PPh₃)₃Cl), Grubbs' catalyst, and tetrakis(triphenylphosphine)palladium(0).^[3]

Wilkinson's catalyst, a popular catalyst for hydrogenation.

Preparation

Many metal phosphine complexes are prepared by reactions of metal halides with preformed phosphines. For example, treatment of a suspension of palladium chloride in ethanol with triphenylphosphine yields monomeric bis(triphenylphosphine)palladium(II) chloride units.^[4]

[PdCl₂]_n + 2n PPh₃ → n PdCl₂(PPh₃)₂

The first reported phosphine complexes were cis- and trans-PtCl₂(PEt₃)₂ reported by Cahours and Gal in 1870.^[5]

Often the phosphine serves both as a ligand and as a reductant. This property is illustrated by the synthesis of many platinum-metal complexes of triphenylphosphine:^[6]

RhCl₃(H₂O)₃ + 4 PPh₃ → RhCl(PPh₃)₃ + OPPh₃ + 2 HCl + 2 H₂O

M-PR3 bonding

See also: π-backbonding

Phosphines are L-type ligands. Unlike most metal ammine complexes, metal phosphine complexes tend to be lipophilic, displaying good solubility in organic solvents.

TEP for selected phosphines^[7] (A₁ mode of Ni(CO)₃L in CH₂Cl₂)

L	ν(CO) cm−1
P(t-Bu)3	2056.1
PMe3	2064.1
PPh3	2068.9
P(OEt)3	2076.3
PCl3	2097.0
PF3	2110.8

Phosphine ligands are also π-acceptors. Their π-acidity arises from overlap of P-C σ* anti-bonding orbitals with filled metal orbitals. Aryl- and fluorophosphines are stronger π-acceptors than alkylphosphines. Trifluorophosphine (PF₃) is a strong π-acid with bonding properties akin to those of the carbonyl ligand.^[8] In early work, phosphine ligands were thought to utilize 3d orbitals to form M-P pi-bonding, but it is now accepted that d-orbitals on phosphorus are not involved in bonding.^[9] The energy of the σ* orbitals is lower for phosphines with electronegative substituents, and for this reason phosphorus trifluoride is a particularly good π-acceptor.^[10]

• 
  R₃P–M σ bonding
• 
  R₃P–M π backbonding

Steric properties

Cone angle is a common and useful parameter for evaluating the steric properties of phosphine ligands.

In contrast to tertiary phosphines, tertiary amines, especially arylamine derivatives, are reluctant to bind to metals. The difference between the coordinating power of PR₃ and NR₃ reflects the greater steric crowding around the nitrogen atom, which is smaller.

By changes in one or more of the three organic substituents, the steric and electronic properties of phosphine ligands can be manipulated.^[11] The steric properties of phosphine ligands can be ranked by their Tolman cone angle^[7] or percent buried volume.^[12]

Spectroscopy

An important technique for the characterization of metal-PR₃ complexes is ³¹P NMR spectroscopy. Substantial shifts occur upon complexation. ³¹P-³¹P spin-spin coupling can provide insight into the structure of complexes containing multiple phosphine ligands.^[13][14]

Reactivity

Phosphine ligands are usually "spectator" rather than "actor" ligands. They generally do not participate in reactions, except to dissociate from the metal center. In certain high temperature hydroformylation reactions, the scission of P-C bonds is observed however.^[15] The thermal stability of phosphines ligands is enhanced when they are incorporated into pincer complexes.

Applications to homogeneous catalysis

One of the first applications of phosphine ligands in catalysis was the use of triphenylphosphine in "Reppe" chemistry (1948), which included reactions of alkynes, carbon monoxide, and alcohols.^[16] In his studies, Reppe discovered that this reaction more efficiently produced acrylic esters using NiBr₂(PPh₃)₂ as a catalyst instead of NiBr₂. Shell developed cobalt-based catalysts modified with trialkylphosphine ligands for hydroformylation (now a rhodium catalyst is more commonly used for this process).^[17] The success achieved by Reppe and his contemporaries led to many industrial applications.^[18]

Illustrative PPh₃ complexes

• Tetrakis(triphenylphosphine)palladium(0) is widely used to catalyse C-C coupling reactions in organic synthesis, see Heck reaction.
• Wilkinson's catalyst, RhCl(PPh₃)₃ is a square planar Rh(I) complex of historical significance used to catalyze the hydrogenation of alkenes.
• Vaska's complex, trans-IrCl(CO)(PPh₃)₂, is also historically significant; it was used to establish the scope of oxidative addition reactions. This early work provided the insights that led to the flowering of the area of homogeneous catalysis.
• NiCl₂(PPh₃)₂ is a tetrahedral (spin triplet) complex of Ni(II). In contrast PdCl₂(PPh₃)₂ is square planar.
• Stryker's reagent, [(PPh₃)CuH]₆, PPh₃-stabilized transition metal hydride cluster that used as a reagent for "conjugate reductions".
• (Triphenylphosphine)iron tetracarbonyl (Fe(CO)₄(PPh₃)) and bis(triphenylphosphine)iron tricarbonyl (Fe(CO)₃(PPh₃)₂).

3,3′,3″-Phosphanetriyltris(benzenesulfonic acid) trisodium salt forms water-soluble complexes.^[19]

Complexes of other organophosphorus ligands

The popularity and usefulness of phosphine complexes has led to the popularization of complexes of many related organophosphorus ligands.^[5] Complexes of arsines have also been widely investigated, but are avoided in practical applications because of concerns about toxicity.

Complexes of primary and secondary phosphines

Most work focuses on complexes of triorganophosphines, but primary and secondary phosphines, respectively RPH₂ and R₂PH, also function as ligands. Such ligands are less basic and have small cone angles. These complexes are susceptible to deprotonation leading to phosphido-bridged dimers and oligomers:

2 L_nM(PR₂H)Cl → [L_nM(μ-PR₂)]₂ + 2 HCl

Complexes of PR_x(OR')_3−x

Nickel(0) complexes of phosphites, e.g., Ni[P(OEt)₃]₄ are useful catalysts for hydrocyanation of alkenes. Related complexes are known for phosphinites (R₂P(OR')) and phosphonites (RP(OR')₂).

Diphosphine complexes

Main article: Diphosphines

Due to the chelate effect, ligands with two phosphine groups bind more tightly to metal centers than do two monodentate phosphines. The conformational properties of diphosphines makes them especially useful in asymmetric catalysis, e.g. Noyori asymmetric hydrogenation. Several diphosphines have been developed, prominent examples include 1,2-bis(diphenylphosphino)ethane (dppe) and 1,1'-Bis(diphenylphosphino)ferrocene, the trans spanning xantphos and spanphos. The complex dichloro(1,3-bis(diphenylphosphino)propane)nickel is useful in Kumada coupling.

References

1. Hartwig, J. F. Organotransition Metal Chemistry, from Bonding to Catalysis; University Science Books: New York, 2010. ISBN 1-891389-53-X
2. Paul C. J. Kamer, Piet W. N. M. van Leeuwen, ed. (2012). Phosphorus(III)Ligands in Homogeneous Catalysis: Design and Synthesis. New York: Wiley. ISBN 978-0-470-66627-2.
3. Iaroshenko, Viktor (4 January 2019). "Phosphines and Related Tervalent Phosphorus Systems". In Van Leeuwen, Piet W. N. M. (ed.). Organophosphorus Chemistry: From Molecules to Applications. pp. 1–58. doi:10.1002/9783527672240.ch1. ISBN 9783527672240.
4. Miyaura, Norio; Suzuki, Akira (1993). "Palladium-Catalyzed Reaction of 1-Alkenylboronates with Vinylic Halides: (1Z,3E)-1-Phenyl-1,3-octadiene". Org. Synth. 68: 130. doi:10.15227/orgsyn.068.0130.
5. C. A. McAuliffe, ed. (1973). Transition Metal Complexes of Phosphorus, Arsenic, and Antimony Ligands. J. Wiley. ISBN 0-470-58117-4.
6. Osborn, J. A.; Wilkinson, G. (1967). "Tris(triphenylphosphine)halorhodium(I)". Inorganic Syntheses. Vol. 10. p. 67. doi:10.1002/9780470132418.ch12. ISBN 9780470131695.
7. Tolman, C. A. (1977). "Steric effects of Phosphorus Ligands in Organometallic Chemistry and Homogeneous Catalysis". Chemical Reviews. 77 (3): 313–348. doi:10.1021/cr60307a002.
8. Orpen, A. G.; Connelly, N. G. (1990). "Structural Systematics: the Role of P-A σ* Orbitals in Metal-Phosphorus π-Bonding in Redox-Related Pairs of M-PA₃ Complexes (A = R, Ar, OR; R = alkyl)". Organometallics. 9 (4): 1206–1210. doi:10.1021/om00118a048.
9. Gilheany, D. G. (1994). "No d Orbitals but Walsh Diagrams and Maybe Banana Bonds: Chemical Bonding in Phosphines, Phosphine Oxides, and Phosphonium Ylides". Chem. Rev. 94 (5): 1339–1374. doi:10.1021/cr00029a008. PMID 27704785.
10. Crabtree, Robert H. (2009). The Organometallic Chemistry of the Transition Metals (5th ed.). Wiley. pp. 99–100. ISBN 978-0-470-25762-3.
11. R. H. Crabtree (2005). "4. Carbonyls, Phosphine Complexes, and Ligand Substitution Reactions". The Organometallic Chemistry of the Transition Metals (4th ed.). Wiley. ISBN 0-471-66256-9.
12. Newman-Stonebraker, Samuel H.; Smith, Sleight R.; Borowski, Julia E.; Peters, Ellyn; Gensch, Tobias; Johnson, Heather C.; Sigman, Matthew S.; Doyle, Abigail G. (2021). "Univariate classification of phosphine ligation state and reactivity in cross-coupling catalysis". Science. 374 (6565): 301–308. Bibcode:2021Sci...374..301N. doi:10.1126/science.abj4213. PMID 34648340. S2CID 238991361.
13. Nelson, John H. (2003). Nuclear Magnetic Resonance Spectroscopy. Prentice Hall. ISBN 978-0130334510.
14. Paul S. Pregosin, Roland W. Kunz (2012). ³¹P and ¹³C NMR of Transition Metal Phosphine Complexes. Berlin: Springer. ISBN 9783642488306.
15. Garrou, Philip E. (1985). "Transition-Metal-Mediated Phosphorus-Carbon Bond Cleavage and Its Relevance to Homogeneous Catalyst Deactivation". Chem. Rev. 85 (3): 171–185. doi:10.1021/cr00067a001.
16. Reppe, W.; Schweckendiek, W. J. (31 July 1948). "Cyclisierende Polymerisation von Acetylen. III Benzol, Benzolderivate und hydroaromatische Verbindungen". Justus Liebigs Annalen der Chemie. 560 (1): 104–116. doi:10.1002/jlac.19485600104.
17. Slaugh, L; Mullineaux, R. (1968). "Novel Hydroformylation catalysts". J. Organomet. Chem. 13 (2): 469. doi:10.1016/S0022-328X(00)82775-8.
18. P. W.N.M. van Leeuwen "Homogeneous Catalysis: Understanding the Art, 2004 Kluwer, Dordrecht. ISBN 1-4020-2000-7
19. Herrmann, W. A.; Kohlpaintner, C. W. (1998). "Syntheses of Water-Soluble Phosphines and their Transition Metal Complexes". Inorganic Syntheses. Vol. 32. pp. 8–25. doi:10.1002/9780470132630.ch2. ISBN 9780471249214. {{cite book}}: |journal= ignored (help)