Phosphine oxides are phosphorus compounds with the formula OPX
3. When X =
alkyl
In organic chemistry, an alkyl group is an alkane missing one hydrogen.
The term ''alkyl'' is intentionally unspecific to include many possible substitutions.
An acyclic alkyl has the general formula of . A cycloalkyl is derived from a cycloalk ...
or
aryl
In organic chemistry, an aryl is any functional group or substituent derived from an aromatic ring, usually an aromatic hydrocarbon, such as phenyl and naphthyl. "Aryl" is used for the sake of abbreviation or generalization, and "Ar" is used as ...
, these are organophosphine oxides.
Triphenylphosphine oxide
Triphenylphosphine oxide (often abbreviated TPPO) is the organophosphorus compound with the formula OP(C6H5)3, also written as Ph3PO or PPh3O (Ph = phenyl, C6H5). This colourless crystalline compound is a common but potentially useful waste prod ...
is an example. An inorganic phosphine oxide is
phosphoryl chloride
Phosphoryl chloride (commonly called phosphorus oxychloride) is a colourless liquid with the formula . It hydrolyses in moist air releasing phosphoric acid and fumes of hydrogen chloride. It is manufactured industrially on a large scale from phosp ...
(POCl
3).
Structure and bonding
Tertiary phosphine oxides
Tertiary phosphine oxides are the most commonly encountered phosphine oxides. With the formula R
3PO, they are tetrahedral compounds. They are usually prepared by oxidation of tertiary phosphines. The P-O bond is short and polar. According to
molecular orbital theory
In chemistry, molecular orbital theory (MO theory or MOT) is a method for describing the electronic structure of molecules using quantum mechanics. It was proposed early in the 20th century.
In molecular orbital theory, electrons in a molecule ...
, the short P–O bond is attributed to the donation of the lone pair electrons from oxygen p-orbitals to the antibonding phosphorus-carbon bonds. The nature of the P–O bond was once hotly debated. Some discussions invoked a role for phosphorus-centered d-orbitals in bonding, but this analysis is not supported by computational analyses. In terms of simple
Lewis structure
Lewis structures, also known as Lewis dot formulas, Lewis dot structures, electron dot structures, or Lewis electron dot structures (LEDS), are diagrams that show the chemical bonding, bonding between atoms of a molecule, as well as the lone pairs ...
, the bond is more accurately represented as a
dative bond
In coordination chemistry, a coordinate covalent bond, also known as a dative bond, dipolar bond, or coordinate bond is a kind of two-center, two-electron covalent bond in which the two electrons derive from the same atom. The bonding of metal io ...
, as is currently used to depict an
amine oxide
In chemistry, an amine oxide, also known as an amine ''N''-oxide or simply ''N''-oxide, is a chemical compound that contains the functional group , a nitrogen-oxygen coordinate covalent bond with three additional hydrogen and/or substituent-grou ...
.
Secondary phosphine oxides
Secondary phosphine oxides (SPOs), formally derived from secondary phosphines (R
2PH), are again tetrahedral at phosphorus. One commercially available example of a secondary phosphine oxide is
diphenylphosphine oxide
Diphenylphosphine oxide is an organophosphorus compound with the formula (C6H5)2P(O)H. It is a white solid that soluble in polar organic solvents.
Synthesis
Diphenylphosphine oxide can be prepared by the reaction of phosphonic esters, such as d ...
. SPOs are used in the formulation of catalysts for
cross coupling reaction
In organic chemistry, a cross-coupling reaction is a reaction where two fragments are joined together with the aid of a metal catalyst. In one important reaction type, a main group organometallic compound of the type R-M (R = organic fragment, M = ...
s.
Unlike tertiary phosphine oxides, SPOs often undergo further oxidation, which enriches their chemistry:
:R
2P(O)H + H
2O
2 → R
2P(O)OH + H
2O
These reactions are preceded by tautomerization to the
phosphinous acid (R
2POH):
:R
2P(O)H R
2POH
Primary phosphine oxides
Primary phosphine oxides, formally oxidized derivatives of primary phosphines, are again tetrahedral at phosphorus. With four different substituents (O, OH, H, R) they are chiral. The primary phosphine oxides subject to tautomerization, which leads to racemization, and further oxidation, analogous to the behavior of SPOs. Additionally, primary phosphine oxides are susceptible to disproportionation to the
phosphinic acid
Hypophosphorous acid (HPA), or phosphinic acid, is a phosphorus oxyacid and a powerful reducing agent with molecular formula H3PO2. It is a colorless low-melting compound, which is soluble in water, dioxane
and alcohols. The formula for this ...
and the primary phosphine:
:2RP(O)H
2 → RP(O)(H)OH + 2RPH
2
Syntheses
Phosphine oxide are typically produced by oxidation of
organophosphine Organophosphines are organophosphorus compounds with the formula PR''n''H3−''n'', where R is an organic substituent. These compounds can be classified according to the value of ''n'': primary phosphines (''n'' = 1), secondary phosphines ...
s. The oxygen in air is often sufficiently oxidizing to fully convert trialkylphosphines to their oxides at room temperature:
:R
3P + 1/2 O
2 → R
3PO
This conversion is usually undesirable. In order to suppress this reaction,
air-free technique Air-free techniques refer to a range of manipulations in the chemistry laboratory for the handling of compounds that are air-sensitive. These techniques prevent the compounds from reacting with components of air, usually water and oxygen; less comm ...
s are often employed when handling say,
trimethylphosphine
Trimethylphosphine is a neutral organophosphorus compound with the formula P(CH3)3, commonly abbreviated as PMe3. This colorless liquid has a strongly unpleasant odor, characteristic of alkylphosphines. The compound is a common ligand in coordin ...
.
Less basic phosphines, such as
methyldiphenylphosphine
Methyldiphenylphosphine is the organophosphine with the formula CH3(C6H5)2P, often abbreviated PMePh2. It is a colorless, viscous liquid. It is a member of series (CH3)3-n(C6H5)2P that also includes n = 0, n = 1, and n = 3 that are ofte ...
are converted to their oxides by treatment with hydrogen peroxide:
: PMePh
2 + H
2O
2 → OPMePh
2 + H
2O
Phosphine oxides are generated as a by-product of the
Wittig reaction
The Wittig reaction or Wittig olefination is a chemical reaction of an aldehyde or ketone with a triphenyl phosphonium ylide called a Wittig reagent. Wittig reactions are most commonly used to convert aldehydes and ketones to alkenes. Most o ...
:
:R
3PCR'
2 + R"
2CO → R
3PO + R'
2C=CR"
2
Another albeit unconventional route to phosphine oxides is the
thermolysis
Thermal decomposition, or thermolysis, is a chemical decomposition caused by heat. The decomposition temperature of a substance is the temperature at which the substance chemically decomposes. The reaction is usually endothermic as heat is req ...
of phosphonium hydroxides:
:
4">Ph4l + NaOH → Ph
3PO + NaCl + PhH
The hydrolysis of phosphorus(V) dihalides also affords the oxide:
:R
3PCl
2 + H
2O → R
3PO + 2 HCl
A special nonoxidative route is applicable secondary phosphine oxides, which arise by the hydrolysis of the chlorophosphine. An example is the hydrolysis of
chlorodiphenylphosphine
Chlorodiphenylphosphine is an organophosphorus compound with the formula (C6H5)2PCl, abbreviated Ph2PCl. It is a colourless oily liquid with a pungent odor that is often described as being garlic-like and detectable even in the ppb range. It is u ...
to give
diphenylphosphine oxide
Diphenylphosphine oxide is an organophosphorus compound with the formula (C6H5)2P(O)H. It is a white solid that soluble in polar organic solvents.
Synthesis
Diphenylphosphine oxide can be prepared by the reaction of phosphonic esters, such as d ...
:
:Ph
2PCl + H
2O → Ph
2P(O)H + HCl
Deoxygenation
The deoxygenation of phosphine oxides has been extensively developed because many useful stoichiometric reactions convert tertiary phosphines to the corresponding oxides. Regeneration of the tertiary phosphine requires cheap oxophilic reagents, which are usually silicon-based. These deoxygenation reactions can be subdivided into stoichiometric and catalytic processes.
Stoichiometric processes
Use of
trichlorosilane
Trichlorosilane is an inorganic compound with the formula HCl3Si. It is a colourless, volatile liquid. Purified trichlorosilane is the principal precursor to ultrapure silicon in the semiconductor industry. In water, it rapidly decomposes to pr ...
is a standard laboratory method. Industrial routes use
phosgene
Phosgene is the organic chemical compound with the formula COCl2. It is a toxic, colorless gas; in low concentrations, its musty odor resembles that of freshly cut hay or grass. Phosgene is a valued and important industrial building block, espe ...
or equivalent reagents, which produce chlorotriphenylphosphonium chloride, which is separately reduced.
For chiral phosphine oxides, deoxygenation can proceed with retention or inversion of configuration. Classically, inversion is favored by a combination of trichlorosilane and triethylamine, whereas in the absence of the Lewis base, the reaction proceeds with retention.
:HSiCl
3 + Et
3N ⇋ SiCl
3− + Et
3NH
+
:R
3PO + Et
3NH
+ ⇋ R
3POH
+ + Et
3N
:SiCl
3− + R
3POH
+ → PR
3 + HOSiCl
3
The popularity of this method is partly attributable to the availability of inexpensive trichlorosilane. Instead of HSiCl
3, other perchloropolysilanes, e.g.
hexachlorodisilane
Hexachlorodisilane is the inorganic compound with the chemical formula Si2Cl6.Simmler, W. "Silicon Compounds, Inorganic", ''Ullmann's Encyclopedia of Industrial Chemistry'', Weinheim: Wiley-VCH. It is a colourless liquid that fumes in moist air. ...
(Si
2Cl
6), can also be used. In comparison, using the reaction of the corresponding phosphine oxides with perchloropolysilanes such as Si
2Cl
6 or Si
3Cl
8 in benzene or chloroform, phosphines can be prepared in higher yields.
:R
3PO + Si
2Cl
6 → R
3P + Si
2OCl
6
:2 R
3PO + Si
3Cl
8 → 2 R
3P + Si
3O
2Cl
8
Deoxygenation has been effected with boranes and alanes.
[
]
Catalytic processes
Phosphoric acids ((RO)2PO2H) catalyze the deoxygenation of phosphine oxides by hydrosilanes.
Use
Phosphine oxides are ligands in various applications of homogeneous catalysis
In chemistry, homogeneous catalysis is catalysis by a soluble catalyst in a solution. Homogeneous catalysis refers to reactions where the catalyst is in the same phase as the reactants, principally in solution. In contrast, heterogeneous catalysis ...
. In coordination chemistry
A coordination complex consists of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of bound molecules or ions, that are in turn known as ''ligands'' or complexing agents. Many ...
, they are known to have labilizing effects to CO ligands cis to it in organometallic reactions. The cis effect In inorganic chemistry, the cis effect is defined as the labilization (or destabilization) of CO ligands that are ''cis'' to other ligands. CO is a well-known strong pi-accepting ligand in organometallic chemistry that will labilize in the ''cis' ...
describes this process.
Parent compound
The parent compound phosphine oxide (H3PO) is unstable. It has been detected with mass spectrometry
Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a ''mass spectrum'', a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is use ...
as a reaction product of oxygen and phosphine
Phosphine (IUPAC name: phosphane) is a colorless, flammable, highly toxic compound with the chemical formula , classed as a pnictogen hydride. Pure phosphine is odorless, but technical grade samples have a highly unpleasant odor like rotting ...
, by means of FT-IR
Fourier-transform infrared spectroscopy (FTIR) is a technique used to obtain an infrared Electromagnetic spectrum, spectrum of Absorption (electromagnetic radiation), absorption or Emission (electromagnetic radiation), emission of a solid, liquid, ...
in a phosphine-ozone
Ozone (), or trioxygen, is an inorganic molecule with the chemical formula . It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic allotrope , breaking down in the lo ...
reaction and in matrix isolation
Matrix isolation is an experimental technique used in chemistry and physics. It generally involves a material being trapped within an unreactive matrix. A ''host'' matrix is a continuous solid phase in which ''guest'' particles (atoms, molecules, i ...
with a reaction of phosphine, vanadium oxytrichloride
Vanadium oxytrichloride is the inorganic compound with the formula VOCl3. This yellow distillable liquid hydrolyzes readily in air. It is an oxidizing agent. It is used as a reagent in organic synthesis. Samples often appear red or orange o ...
and chromyl chloride
Chromyl chloride is the inorganic compound with the formula CrO2Cl2. It is a reddish brown compound that is a volatile liquid at room temperature, which is unusual for transition metal complexes.
Preparation
Chromyl chloride can be prepared by th ...
. It has also been reported relatively stable in a water-ethanol solution by electrochemical oxidation of white phosphorus
Elemental phosphorus can exist in several allotropes, the most common of which are white and red solids. Solid violet and black allotropes are also known. Gaseous phosphorus exists as diphosphorus and atomic phosphorus.
White phosphorus
White ...
, where it slowly disproportionates
In chemistry, disproportionation, sometimes called dismutation, is a redox reaction in which one compound of intermediate oxidation state converts to two compounds, one of higher and one of lower oxidation states. More generally, the term can ...
into phosphine and hypophosphorous acid
Hypophosphorous acid (HPA), or phosphinic acid, is a phosphorus oxyacid and a powerful reducing agent with molecular formula H3PO2. It is a colorless low-melting compound, which is soluble in water, dioxane
and alcohols. The formula for this ...
. Secondary phosphine oxides (R2P(O)H) are tautomer
Tautomers () are structural isomers (constitutional isomers) of chemical compounds that readily interconvert.
The chemical reaction interconverting the two is called tautomerization. This conversion commonly results from the relocation of a hydr ...
s of phosphinous acids Phosphinous acids are usually organophosphorus compounds with the formula R2POH. They are pyramidal in structure. Phosphorus is in the oxidation state III. Most phosphinous acids rapidly convert to the corresponding phosphine oxide, which are tet ...
(R2POH).
Phosphine oxide is reported as an intermediate in the room-temperature polymerization of phosphine and nitric oxide
Nitric oxide (nitrogen oxide or nitrogen monoxide) is a colorless gas with the formula . It is one of the principal oxides of nitrogen. Nitric oxide is a free radical: it has an unpaired electron, which is sometimes denoted by a dot in its che ...
to solid PxHy.
References
{{Reflist, 30em
Functional groups
*
*