Theoretical study of gas-phase reactions of Fe(CO)₅ with OH^- and their relevance for the water gas shift reaction

Revision of the homogeneously Fe(CO)₅-catalyzed water gas shift reaction in the gas phase has been performed by means of quantum chemical calculations using gradient-corrected density functional theory (B3LYP) and ab initio methods at the CCSD(T) level. The classically assumed reaction path has been scrutinized step by step, and enlarged with novel mechanistic proposals. Our calculations lend additional credit to some of the previously accepted steps in the catalytic cycle, such as the initial attack of OH^- to Fe(CO)₅ and also to the recently accepted decarboxylation of (CO)₄FeCOOH^- (via a concerted mechanism involving a four-centered transition state), as well as to the acidification of the metal hydride (CO)₄FeH^- with water to yield the dihydride (CO)₄FeH₂. The present investigation also examines in terms of energies and activation barriers the existence/participation of new intermediates (in particular, a metalloformate species, a water-hydride adduct, and a dihydrogen complex), not mentioned in prior studies. Finally, a transition-metal-containing S_N2-type reaction is explored for the last stages of this chemical process as a mechanistic alternative to regenerate the starting catalyst.