Bite angle

Chelate bite angle is a geometric parameter used to classify chelating ligands in inorganic and organometallic chemistry. Together with ligand cone angle, this parameter is relevant to diphosphine ligands, which are used in industrial processes such as hydroformylation and hydrocyanation. Even subtle changes in these parameters can significantly influence the selectivity and rate of catalytic reactions.

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Background

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.^[1] 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).2 The success achieved with monodentate phosphine ligands led to the use of diphosphine ligands.3 Today, diphosphines are most commonly known for their industrial application in catalytic processes such as hydroformylation, hydrogenation, and hydrocyanation.^[2]

Diphosphines

Main article: 1,2-Bis(diphenylphosphino)ethane

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, and DPEphos (Figure 1, 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 effects.

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.^[3] 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.^[4] 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 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.^[4] 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.^[5] 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 models. Octahedral complexes prefer angles near 90º and tetrahedral complexes near 110°. Since catalyts often interconvert between various geometries, the rigidity of the chelate ring can be decisive.^[5] Bidentate phosphine with a natural bite angle of 120º preferentially occupy diequatorial sites in a trigonal bipyramidal complex whereas a bidentate phosphine with a natural bite angle of 90º preferentially occupy apical-equatorial positions.^[2] Diphosphine ligands with bite angles of ≥120º are obtained using a bulky, stiff diphosphine backbones.^[5] Diphosphines of wide bite angles are used in some industrial processes.

A practical application: hydroformylation

Main article: Hydroformylation

The hydroformylation of alkenes to give aldehydes is important : almost 6 million tons of aldehydes are produced by this method annually (Figure 4).^[6] The ratio of linear to branched aldehydes depends on the structure of the catalyst.^[2] The mechanism for rhodium-phosphine catalyzed hydroformylation was first suggested by Breslow and Heck (Figure 5).^[7]

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 6).^[2]  

Diphosphine ligands such as dppe, which has a bite angle of about 90º, occupy the equatorial and apical positions, whereas diphosphine ligands with larger bite angles (>120º) 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 7) has been investigated.

  BISBI has a bite angle of approximately 113º and therefore it selectively occupies the equatorial plane of the trigonal bipyrimidal intermediate complex (Figure 8).^[5]  

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.^[5]

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 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.^[5] The electronic effect of this increase in reaction rate is uncertain since it mainly depends on the bonding between the alkene and rhodium.^[2] 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.

References