On the ineffectiveness of grubbs-type iron olefin metathesis catalysts: Role of spin-state isomerism and cyclopropanation

Abstract

Unlike Grubbs-type Ru(II) catalysts, the corresponding Fe(II) catalysts are ineffective for olefin metathesis. A DFT study on the mechanistic pathways of Fe(II) analogues of the Grubbs first and second generation catalysts has been done on the basis of 16-electron dissociative (16eD) and 18-electron associative (18eA) pathways to unravel the reasons behind the ineffectiveness of the catalysts to perform CC bond metathesis. Both the singlet and triplet spin states of the catalysts and the intermediates have been considered for the evaluation of the reaction profile. The [2 + 2] cycloaddition step to form the singlet ironacyclobutane, FeCB is very facile for Fe(II) systems and the behavior is very similar to the Grubbs catalysts. Though FeCB systems showed activated agostic-type CC bonds, metathesis is prevented due to the spin crossover transition during the cycloaddition to significantly more stable 3FeCB. Formation of 3FeCB is exergonic with respect to the olefin coordinated Fe(II) complex. Also the CC bonds of 3FeCB show normal inactivated single bond character suggesting high energy barrier for the cycloreversion to produce a new olefin. Moreover, cyclopropane formation from 3FeCB is very facile, exergonic and leads to the degradation of the catalyst.