In: Chemistry
Discuss Tolman’s Cone Angle (TCA) method to quantify the (i) steric effects and the electronic effects of P-donor ligands.
8(b) Discuss the application of the TCA concept in transition metal catalysis.
8(c) Why does PMe3 have a larger cone angle than P(OMe)3?
(a) Tolman first applied the cone angle concept to phosphines. Tertiary phosphines are commonly classified using this concept.
For e.g. The cone angle in R3P = 118o (R = CH3)
= 132o (R = CH2CH3)
= 160o (R = CH(CH3)2)
= 182o (R = C(CH3)3)
= 145o (R = C6H5)
= 184o (R = C6F5)
The first four angles explain that as the sterics around phosphorus increases, the cone opens up, i.e. cone angle increases.
The last two angles explain that as the lone pair of electrons present on phosphorus involves in delocalization of the substituent aryl group, the cone opens up, i.e. cone angle increases.
(b) Example: Hydroformylation of alkenes using CO/H2 by rhodium catalyst(syn gas
If you take internal alkene for the hydroformylation by using the tertiary phosphine with high cone angle, you will end up with linear aldehyde because of the high steric around phosphorus.
If you take internal alkene for the hydroformylation by using the tertiary phosphine with low cone angle, you will end up with branched aldehyde because of the low steric around phosphorus.
(c) PMe3 is a σ-donor ligand, whereas P(OMe)3 is a σ-donor as well as a π-acceptor ligand, as a result the electron density on metal is higher in case of PMe3 binding compared to P(OMe)3 binding. Hence, the cone angle opens up more in case of PMe3.