Bite angle
Overview
 
The bite angle is a geometric parameter used to classify chelating
Chelation
Chelation is the formation or presence of two or more separate coordinate bonds between apolydentate ligand and a single central atom....

 ligand
Ligand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...

s in inorganic and organometallic chemistry
Organometallic chemistry
Organometallic chemistry is the study of chemical compounds containing bonds between carbon and a metal. Since many compounds without such bonds are chemically similar, an alternative may be compounds containing metal-element bonds of a largely covalent character...

. Together with ligand cone angle
Ligand cone angle
The ligand cone angle is a measure of the size of a ligand. It is defined as the solid angle formed with the metal at the vertex and the hydrogen atoms at the perimeter of the cone . Tertiary phosphine ligands are commonly classified using this parameter, but the method can be applied to any...

, this parameter is relevant to diphosphine ligands, which are used in industrial processes such as hydroformylation
Hydroformylation
Hydroformylation, also known as oxo synthesis or oxo process, is an important industrial process for the production of aldehydes from alkenes. This chemical reaction entails the addition of a formyl group and a hydrogen atom to a carbon-carbon double bond...

 and hydrocyanation
Hydrocyanation
Hydrocyanation is, most fundamentally, the process whereby H+ and –CN ions are added to a molecular substrate. Usually the substrate is an alkene and the product is a nitrile. When –CN is a ligand in a transition metal complex, its basicity makes it difficult to dislodge, so, in this...

. Even subtle changes in these parameters can significantly influence the selectivity and rate of catalytic reactions.
One of the first applications of phosphine ligands in catalysis was the use of triphenylphosphine in “Reppe
Walter Reppe
Walter Julius Reppe was a German chemist. He is notable for his contributions to the chemistry of acetylene.-Education and career:...

” chemistry (1948), which included reactions of alkyne
Alkyne
Alkynes are hydrocarbons that have a triple bond between two carbon atoms, with the formula CnH2n-2. Alkynes are traditionally known as acetylenes, although the name acetylene also refers specifically to C2H2, known formally as ethyne using IUPAC nomenclature...

s, carbon monoxide
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...

, and alcohol
Alcohol
In chemistry, an alcohol is an organic compound in which the hydroxy functional group is bound to a carbon atom. In particular, this carbon center should be saturated, having single bonds to three other atoms....

s.
Encyclopedia
The bite angle is a geometric parameter used to classify chelating
Chelation
Chelation is the formation or presence of two or more separate coordinate bonds between apolydentate ligand and a single central atom....

 ligand
Ligand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...

s in inorganic and organometallic chemistry
Organometallic chemistry
Organometallic chemistry is the study of chemical compounds containing bonds between carbon and a metal. Since many compounds without such bonds are chemically similar, an alternative may be compounds containing metal-element bonds of a largely covalent character...

. Together with ligand cone angle
Ligand cone angle
The ligand cone angle is a measure of the size of a ligand. It is defined as the solid angle formed with the metal at the vertex and the hydrogen atoms at the perimeter of the cone . Tertiary phosphine ligands are commonly classified using this parameter, but the method can be applied to any...

, this parameter is relevant to diphosphine ligands, which are used in industrial processes such as hydroformylation
Hydroformylation
Hydroformylation, also known as oxo synthesis or oxo process, is an important industrial process for the production of aldehydes from alkenes. This chemical reaction entails the addition of a formyl group and a hydrogen atom to a carbon-carbon double bond...

 and hydrocyanation
Hydrocyanation
Hydrocyanation is, most fundamentally, the process whereby H+ and –CN ions are added to a molecular substrate. Usually the substrate is an alkene and the product is a nitrile. When –CN is a ligand in a transition metal complex, its basicity makes it difficult to dislodge, so, in this...

. Even subtle changes in these parameters can significantly influence the selectivity and rate of catalytic reactions.

Background

One of the first applications of phosphine ligands in catalysis was the use of triphenylphosphine in “Reppe
Walter Reppe
Walter Julius Reppe was a German chemist. He is notable for his contributions to the chemistry of acetylene.-Education and career:...

” chemistry (1948), which included reactions of alkyne
Alkyne
Alkynes are hydrocarbons that have a triple bond between two carbon atoms, with the formula CnH2n-2. Alkynes are traditionally known as acetylenes, although the name acetylene also refers specifically to C2H2, known formally as ethyne using IUPAC nomenclature...

s, carbon monoxide
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...

, and alcohol
Alcohol
In chemistry, an alcohol is an organic compound in which the hydroxy functional group is bound to a carbon atom. In particular, this carbon center should be saturated, having single bonds to three other atoms....

s. In his studies, Reppe discovered that this reaction more efficiently produced acrylic esters using NiBr2(PPh3
Triphenylphosphine
Triphenylphosphine is a common organophosphorus compound with the formula P3 - often abbreviated to PPh3 or Ph3P. It is widely used in the synthesis of organic and organometallic compounds. PPh3 exists as relatively air stable, colorless crystals at room temperature...

)2 as a catalyst instead of NiBr2
Nickel(II) bromide
Nickel bromide, NiBr2, is the nickel salt of hydrobromic acid. It can be made by reacting nickel, nickel oxide, nickel carbonate, or nickel hydroxide with hydrobromic acid. It can also be made by reacting nickel with bromine. It is a weak reducing agent.It is yellow-brown, rhombohedral,...

. Shell developed cobalt-based catalysts modified with trialkylphosphine ligands for hydroformylation (now a rhodium catalyst is more commonly used for this process). The success achieved with monodentate phosphine ligands led to the use of diphosphine ligands. Today, diphosphines are most commonly known for their industrial application in catalytic processes such as hydroformylation, hydrogenation, and hydrocyanation.

Diphosphines


Diphosphines are a class of chelating ligands that contain two phosphine groups connected to each other by a bridge (also referred to as a backbone). The bridge, for instance, might consist of one or more methylene groups or multiple aromatic rings with heteroatoms attached. Examples of common diphosphines are dppe, dcpm (Figure 1), and DPEphos (Figure 2).

The diphosphine forms two to the metal. The structure of the backbone and the substituents attached to the phosphorus atoms influence the chemical reactivity of the diphosphine ligand in metal complexes through steric and electronic effect
Electronic effect
An electronic effect influences the structure, reactivity, or properties of molecule but is neither a traditional bond nor a steric effect. In organic chemistry, the term stereoelectronic effect is also used to emphasize the relation between the electronic structure and the geometry of a...

s.

Examples

Steric characteristics of the diphosphine ligand that influence the regioselectivity and rate of catalysis include the pocket angle, solid angle, repulsive energy, and accessible molecular surface. Also of importance is the cone angle, which in diphosphines is defined as the average of the cone angle for the two substituents attached to the phosphorus atoms, the bisector of the P-M-P angle, and the angle between each M-P bond. Larger cone angles usually result in faster dissociation of phosphine ligands because of steric crowding.

The natural bite angle

The natural bite angle (βn) of diphosphines, obtained using molecular mechanics
Molecular mechanics
Molecular mechanics uses Newtonian mechanics to model molecular systems. The potential energy of all systems in molecular mechanics is calculated using force fields...

 calculations, is defined as the selective chelation angle (P-M-P bond angle) that is determined by the diphosphine ligand backbone (Figure 3).
Both steric bite angle effect and the electronic bite angle effects are recognized. The steric bite angle effect involves the steric interactions between ligands or between a ligand and a substrate. The electronic bite angle effect, on the other hand, relates to the electronic changes that occur when the bite angle is modified. This effect is sensitive to the hybridization of metal orbitals. This flexibility range accounts for the different conformations of the ligand with energies slightly above the strain energy of the natural bite angle.

The bite angle of a diphosphine ligand also indicates the distortion from the ideal geometry of a complex based on VSEPR
VSEPR theory
Valence shell electron pair repulsion theory is a model in chemistry used to predict the shape of individual molecules based upon the extent of electron-pair electrostatic repulsion. It is also named Gillespie–Nyholm theory after its two main developers...

 models. Octahedral complexes prefer angles near 90° and tetrahedral complexes near 110°. Since catalyts often interconvert between various geometries, the rigidity of the chelate ring can be decisive. Bidentate phosphine with a natural bite angle of 120° preferentially occupy diequatorial sites in a trigonal bipyramidal complex whereas a bidentate phosphine with a natural bite angle of 90° preferentially occupy apical-equatorial positions. Diphosphine ligands with bite angles of ≥120° are obtained using a bulky, stiff diphosphine backbones. Diphosphines of wide bite angles are used in some industrial processes.

A practical application: hydroformylation

The hydroformylation of alkenes to give aldehydes is important : almost 6 million tons of aldehydes are produced by this method annually.

The ratio of linear to branched aldehydes depends on the structure of the catalyst. The mechanism for rhodium-phosphine catalyzed hydroformylation was first suggested by Breslow and Heck
Richard F. Heck
Richard Fred Heck is an American chemist noted for the discovery and development of the Heck reaction, which uses palladium to catalyze organic chemical reactions that couple aryl halides with alkenes....

.

One intermediate, RhH(alkene)(CO)L2, exists in two different structures: the equatorial-apical (ea) and the equatorial-equatorial (ee) isomers, depending on the position of phosphine ligands (Figure 4).
Diphosphine ligands such as dppe, which has a bite angle of about 90°, occupy the equatorial and apical positions, whereas diphosphine ligands with larger bite angles (>120°) preferentially occupy the equatorial positions. It is believed that the ee isomer leads to the formation of more of the linear aldehyde, the desired product. In an effort to create rhodium complexes in which the phosphine ligands preferentially occupy the equatorial positions, the use of diphosphine ligands with wide bite angles such as BISBI (Figure 5) has been investigated.
BISBI has a bite angle of approximately 113° and therefore it selectively occupies the equatorial plane of the trigonal bipyrimidal intermediate complex (Figure 6).
The RhH(diphosphine)(CO)2 catalyst, however, is not the species involved in the step that determines the regioselectivity of this reaction. The formation of the linear vs. branched aldehydes is determined instead during the coordination of the alkene to the intermediate RhH(diphosphine)CO complex and during the hydride migration step that follows. During the coordination of the alkene to the intermediate complex, the effect of bite angle on the regioselective formation of the linear product is explained by the steric crowding at the Rh atom that results from the interactions of the bulky backbone of the ligand with substrate molecules. The large bite angle that results from the bulky backbone causes the five-coordinate RhH(diphosphine)CO(alkene) intermediate to adopt a structure that relieves steric hindrance. Thus, BISBI occupies the equatorial positions, where it would have the most “space.” This preference of a transition state that relieves steric hindrance pushes the reaction toward the formation of the linear aldehyde. The regioselectivity is also controlled by the hydride migration, which is usually irreversible in the formation of linear aldehydes.

A large bite angle stabilizes the intermediate in this process; however, after the addition of the alkene occurs considerable steric congestion causes the phosphine ligand backbone to conform in such manner that the substituent attached to the alkene is moved towards the hydride. This results in the formation of more of the linear product than the branched product.

Furthermore, studies using Xantphos
Xantphos
Xantphos is an organophosphorus compound derived from the heterocycle xanthene. It is used as a bidentate ligand and is noteworthy for having a particularly wide bite angle. Such ligands are useful in the hydroformylation of alkenes. Illustrative of its wide bite angle, it forms both cis and...

ligands (ligands with bulky backbones) in hydroformylation have indicated an increase in the rate of catalysis in metal complexes that contain diphosphine ligands with larger bite angles. The electronic effect of this increase in reaction rate is uncertain since it mainly depends on the bonding between the alkene and rhodium. Large bite angles promote alkene to rhodium electron donation, which would result in an accumulation of electron density on the rhodium atom. This increased electron density would be available for pi donation into the antibonding orbitals of other ligands, which could potentially weaken other bonds within the catalytic complex, leading to faster reactivity.

The application of catalysts containing phosphine ligands is not limited to the process of hydroformylation. Hydrocyanation and hydrogenation reactions also implement phosphine-mediated catalysts. The use of diphosphine ligands in catalysis has allowed the optimization of industrial processes and has led to the development of new catalytic systems.
The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
x
OK