Transition metal phosphido complexes
A transition metal phosphido complex is a coordination complex containing a phosphido ligand (R2P, where R = H, organic substituent). With two lone pairs on phosphorus, the phosphido anion (R2P−) is comparable to an amido anion (R2N−), except that the M-P distances are longer and the phosphorus atom is more sterically accessible. For these reasons, phosphido is often a bridging ligand.[1]
Synthesis
Phosphido ligands are often installed by salt metathesis reactions. Sources of R2P+ and R2P− are provided by phosphorus halides and alkali metal phosphides respectively. Illustrative of the use of R2PCl-like reagents is the synthesis of a diiron diphosphide:[2]
- Na2Fe2(CO)8 + 2 Ph2PCl → Fe2(PPh2)2(CO)6 + 2 NaCl + 2 CO
The alternative salt metathesis route involves the reaction of alkali metal diorganophosphides with metal halides. A typical phosphide reagent is lithium diphenylphosphide.[3]
Alkali metal phosphides sometimes reduce the metal center.[4]
Another way to generate the transition-metal phosphido complexes are by direct activation of P-H bonds and is mostly seen in the late-transition-metal complexes. For example, the reaction of Vaska's complex analogs with the parent phosphine generate the following transition-metal phosphido complex.[5]
Structure
Complexes of the phosphide ligand can be classified into one of three classes:
- those where the phosphide is a terminal ligand and phosphorus is pyramidal,
- those where the phosphide is a terminal ligand and phosphorus is planar,
- those where the phosphide is a bridging ligand and phosphorus is tetrahedral.
Terminal phosphido ligands
In most complexes with terminal phosphido ligands, phosphorus is pyramidal, as expected with a stereochemically active lone pair of electrons. The M-P bond length in the pyramidal phosphide complex is longer than the M-P bond length in corresponding transition metal phosphine complexes. The pyramidal phosphido complex. In the complex, the Os-PHPh bond is 0.11 Å longer than the Os-PPh3 and the Os-P-C angle is 113o. The elongated Os-PHPh bond is often attributed to the electronic repulsion of the lone pair and nonbonding electrons on Os.[7] Also, in another ruthenium complex, the Ru-P(Me)Ph bond is 0.17 Å longer than Ru-PH(Me)Ph in the related phosphine ligand version of the complex, [(dmpe)2Ru(H)PH(Me)Ph]+.[8] Additionally electronic repulsion of the P-centered lone pair and metal-based electrons enhance the nucleophilicity of the phosphide ligand. This high basicity and high nucleophilicity leads to the dimerization reaction.
As implied in the resonance structures A and B, some terminal phosphido ligands engage in M-P multiple bonding. In the resonance structure A, the lone pair from the p-orbital on phosphorus donates to the vacant orbital on the metal to form a π-bond. Because of the π-bonding interaction in resonance structure A, it is planar at phosphorus and M-P bond-length is shorter and M-P-R bond-angle is larger. Planar phosphido complexes usually have shorter M-P bonds and larger M-P-R angles. In the tungsten complex, the W-PHPh bond is 0.26 Å shorter than W-PEt3 bond in the same complex, and the W-P-C angle is 140°.[9] Another example is a ruthenium complex. In those complex, the Ru-PCy2 bond is 0.11 Å shorter than Ru-PPh3 bond and the Ru-P-C angle is 127°.[10]
While the planar and pyramidal phosphides can be distinguished clearly, in one case, a pyramidal phosphide can be converted to planar phosphide by one-electron oxidation.[11]
The inversion of configuration at pyramidal terminal phosphides has been observed by 31P NMR spectroscopy.[12][13]
Bridging phosphido ligands
In most of its complexes, the phosphido ligand is a bridging ligand. No lone pairs remain on phosphorus. These complexes have the formula [M(μ-PR2)Ln]2. One example is [Fe(μ-PPh2)(CO)3]2.
Applications
Metal phosphido complexes are however intermediates the catalytic hydrophosphinations.
Some late metal hydrophosphination catalysts rely on oxidative addition of a P-H bond. For example, a Pt(0) catalyst that undergoes oxidative addition of a secondary phosphine to form the corresponding Pt(II) phosphido complex, which react with electrophilic alkenes such as acrylonitrile. This P-C bond forming step proceeds through an outer-sphere, Michael-type addition.[15] cAlkene insertion into the metal-hydrogen bond is also invoked in some hydrophosphinations.[16]
Metal phosphide have been used in the synthesis of P-stereogenic phosphines by exploiting the high nucleophilicity in the pyramidal phosphide complex.[17][1]
References
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- Collman, James P.; Rothrock, Richard K.; Finke, Richard G.; Rose-Munch, Francoise (1977). "Metal promoted alkyl migration in a bimetallic complex". Journal of the American Chemical Society. 99 (22): 7381–7383. doi:10.1021/ja00464a061.
- Hey-Hawkins, E. (September 1994). "Bis(cyclopentadienyl)zirconium(IV) or hafnium-(IV) Compounds with Si-, Ge-, Sn-, N-, P-, As-, Sb-, O-, S-, Se-, Te-, or Transition Metal-Centered Anionic Ligands". Chemical Reviews. 94 (6): 1661–1717. doi:10.1021/cr00030a009. ISSN 0009-2665.
- Schäufer, H.; Binder, D. (March 1987). "Übergangsmetallphosphidokomplexe. XI. Diphosphenkomplexe des Typs (R3P)2Ni[η2-(PR′)2] und phosphidoverbrückte Nickel(I)-Komplexe des Typs [R3PNiP(SiMe3)2]2 (Ni–Ni)". Zeitschrift für anorganische und allgemeine Chemie (in German). 546 (3): 55–78. doi:10.1002/zaac.19875460307. ISSN 0044-2313.
- Ebsworth, E. A. V.; Gould, Robert O.; Mayo, Richard A.; Walkinshaw, Malcolm (1987). "Reactions of phosphine, arsine, and stibine with carbonylbis(triethylphosphine)iridium( I ) halides. Part 1. Reactions in toluene; X-ray crystal structures of [Ir(CO)ClH(PEt 3 ) 2 (AsH 2 )] and [Ir(CO)XH(PEt 3 ) 2 (µ-ZH 2 )RuCl 2 (η 6 -MeC 6 H 4 CHMe 2 -p)](X = Br, Z = P; X = Cl, Z = As)". J. Chem. Soc., Dalton Trans. (11): 2831–2838. doi:10.1039/DT9870002831. ISSN 0300-9246.
- Jones, Richard A.; Lasch, Jon G.; Norman, Nicholas C.; Whittlesey, Bruce R.; Wright, Thomas C. (1983). "Synthesis and x-ray crystal structure of Mo2(μ-t-Bu2P)2(t-Bu2P)2(Mo-Mo); the first structurally characterized binary transition-metal phosphide". Journal of the American Chemical Society. 105 (19): 6184–6185. doi:10.1021/ja00357a054.
- Bohle, D. Scott.; Jones, Tony C.; Rickard, Clifton E. F.; Roper, Warren R. (August 1986). "Terminal phosphido complexes of ruthenium(II) and osmium(II): synthesis, reactivity, and crystal structures of Os(PHPh)Cl(CO)2(PPh3)2 and Os{PH(OMe)Ph}(CO)2(PPh3)2". Organometallics. 5 (8): 1612–1619. doi:10.1021/om00139a017. ISSN 0276-7333.
- Chan, Vincent S.; Stewart, Ian C.; Bergman, Robert G.; Toste, F. Dean (March 2006). "Asymmetric Catalytic Synthesis of P-Stereogenic Phosphines via a Nucleophilic Ruthenium Phosphido Complex". Journal of the American Chemical Society. 128: 2786–2787. doi:10.1021/ja058100y. ISSN 0002-7863. PMID 16506742.
- Rocklage, Scott M.; Schrock, Richard R.; Churchill, Melvyn Rowen; Wasserman, Harvey J. (October 1982). "Multiple Metal Carbon Bonds. Part 29. Facile Conversion of Tungsten(VI) Neopentylidyne Complexes into Oxo and Imido Neopentylidene Complexes and the Crystal Structure of W(CCMe3)(PHPh)(PEt3)2Cl2". Organometallics. 1 (10): 1332–1338. doi:10.1021/om00070a015. ISSN 0276-7333.
- Derrah, Eric J.; Pantazis, Dimitrios A.; McDonald, Robert; Rosenberg, Lisa. "A Highly Reactive Ruthenium Phosphido Complex Exhibiting Ru−P π-Bonding". Organometallics. 26: 1473–1482. doi:10.1021/om0700056. ISSN 0276-7333.
- Melenkivitz, Rory; Mindiola, Daniel J.; Hillhouse, Gregory L. (2002). "Monomeric Phosphido and Phosphinidene Complexes of Nickel". Journal of the American Chemical Society. 124: 3846–3847. doi:10.1021/ja017787t. ISSN 0002-7863. PMID 11942818.
- Baker, R. T.; Krusic, P. J.; Tulip, T. H.; Calabrese, J. C.; Wreford, S. S. (October 1983). "Synthesis and molecular structures of homoleptic dicyclohexylphosphide complexes of the early transition metals". Journal of the American Chemical Society. 105 (22): 6763–6765. doi:10.1021/ja00360a061. ISSN 0002-7863.
- Baker, R. T.; Whitney, J. F.; Wreford, S. S. (August 1983). "Characterization and interconversion of metal-phosphorus single and double bonds: bis(cyclopentadienyl)zirconium and -hafnium bis(diorganophosphide) complexes". Organometallics. 2 (8): 1049–1051. doi:10.1021/om50002a022. ISSN 0276-7333.
- Ballinas-López, María Gabriela; Padilla-Martínez, Itzia I.; Martínez-Martínez, Francisco J.; Höpfl, Herbert; García-Báez, Efrén V. (2005). "Di-μ-diphenylphosphido-bis[tricarbonyliron(II)] dichloromethane solvate". Acta Crystallographica Section E. 61 (8): m1475–m1477. doi:10.1107/S1600536805020982.
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- Shulyupin, M. O.; Kazankova, M. A.; Beletskaya, I. P. Org. Lett. 2002, 4, 761.Shulyupin, M. O.; Kazankova, M. A.; Beletskaya, I. P. (2002). "Catalytic Hydrophosphination of Styrenes". Organic Letters. 4 (5): 761. doi:10.1021/ol017238s.
- Chan, Vincent S.; Stewart, Ian C.; Bergman, Robert G.; Toste, F. Dean (March 2006). "Asymmetric Catalytic Synthesis of P -Stereogenic Phosphines via a Nucleophilic Ruthenium Phosphido Complex". Journal of the American Chemical Society. 128 (9): 2786–2787. doi:10.1021/ja058100y. ISSN 0002-7863.