Abstract:
Quantum mechanically derived molecular electrostatic potential (MESP) based descriptors have been proposed for the assessment of both the steric and electronic effects of phosphines in several first-generation Grubbs olefin metathesis catalysts. The MESP at the P nucleus of the active form of the catalyst Cl-2(PR3)Ru=CH2 (1) and its ethylene-coordinated complex (2) are determined. Further, frozen structures corresponding to 1 and 2 are located by replacing the P-R bonds with P H bonds. The MESP at the P of a frozen geometry is free from the electronic effect of R but is influenced by steric effects due to the structural restrictions imposed in the geometry. The difference between the MESP values at the P nucleus of Cl-2(PH3)Ru=CH2 and Cl-2(PR3)Ru=CH2 is taken as a measure of the combined steric and electronic effects of PR3 (V-SE1) in 1. Similarly, the combined steric and electronic effect of PR3 in 2 (V-SE2) is also calculated. The frozen structures allowed calculation of the steric-only effects for 1 (V-S1) as well as 2 (V-S2). Thus, the electronic effect of PR3, V-E1, in 1 is V-SE1 - V-S1 and that of PR3, V-E2,in 2 is V-SE2 - V-S2. Both V-S1 and V-S2 showed impressive linear correlations with popular geometric steric parameters: viz. the Tolman cone angle (theta) and the symmetric deformation coordinate (S4'). Moreover, V-E1 and V-S1 showed linear relationships to the binding energy of ethylene (E-1), suggesting that the steric effect is 1.88 times more dominant than the electronic effect in the olefin binding process. Similarly, both V-E2 and V-S2 showed linear correlation with the activation energy (E-2) for the formation of metallacyclobutane. In both the olefin binding process and the transition state formation leading to C-C bond coupling, a drastic reduction in E-1 as well as in E-2 is observed with an increase in the steric bulkiness of,PR3, while only a moderate decrease in energy parameters is observed with an increase in the electron-rich character of PR3. The stereoelectronic correlation studies presented herein demonstrate that the success of the first-generation Grubbs catalysts is Primarily due to the choice of the right mix of steric (bulky R substituents on P) and electronic (electron-donating R substituents on P) effects of the PR3 ligand.
