Theoretical Study of Gas-Phase Reactions of Fe(CO)₅ with OH^- and Their Relevance for the Water Gas Shift Reaction

Abstract

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 SN2-type reaction is explored for the last stages of this chemical process as a mechanistic alternative to regenerate the starting catalyst.

Ligand Site Preference in Iron Tetracarbonyl Complexes Fe(CO) 4 L (L = CO, CS, N 2 , NO + , CN – , NC – , η 2 ‐C 2 H 4 , η 2 ‐C 2 H 2

“…The bond dissociation energy D 0 of the equatorial Fe(CO) 4 (g 2 -C 2 H 4 ) isomer 8 b is 39.2 kcal mol ±1 and close to the experimental value of 37.2 kcal mol ±1 [56] and other theoretical data [19]. The calculated Fe±C ethylene bond lengths of 2.145 A Ê resembles the experimental values of 2.116(14)±2.118 (14) A Ê based on microwave spectra quite well [57].…”

“…Experimental evidence and qualitative molecular orbital considerations suggest that strong p-accepting ligands prefer the equatorial position of trigonal bipyramidal complexes containing d 8 number of quantum chemical studies of complexes Fe(CO) 4 L (L = N 2 [18], H 2 [19], PR 3 [20], C 2 H 4 [21] and C 2 H 2 [22]). Experimental evidence and qualitative molecular orbital considerations suggest that strong p-accepting ligands prefer the equatorial position of trigonal bipyramidal complexes containing d 8 number of quantum chemical studies of complexes Fe(CO) 4 L (L = N 2 [18], H 2 [19], PR 3 [20], C 2 H 4 [21] and C 2 H 2 [22]).…”

Catalytic Semi‐Water–Gas Shift Reaction: A Simple Green Path to Formic Acid Fuel

“…As imilar WGSR mechanism is proposed for the catalysis of CO/H 2 Ot oF Aw hen using transition-metal carbonyls involving Fe or Ru atoms in basic aqueous solutions in which the basic speciesc arbonate, hydroxide, and formiate ions are involved. [31,[65][66][67][68][69] The formed carbonate ion (CO 3 À )r eacts with H 2 Ot of orm OH À and CO 2 . [70] The formed OH À ion reacts with CO to generatet he formiate ion.…”

Harnessing the Power of the Water‐Gas Shift Reaction for Organic Synthesis

“…[35] DFT studies also support this catalytic cycle, although alternative proposals have been formulated. [36] The mechanism appears to be more complex with Ru, Os, Rh and Ir carbonyl complexes due to the formation of metal clusters. [37] A different pathway has been suggested for Group 6 (Cr, Mo, W) metal–carbonyl complexes.…”