Mechanism of Iron Carbonyl-Catalyzed Hydrogenation of Ethylene. 1. Theoretical Exploration of Molecular Pathways

Abstract

The hydrogenation of alkenes catalyzed by metal carbonyls is an intricate process involving reactions of various isomers of labile π- and σ-complexes, hydrides, dihydrides, and their radicals. Two general mechanisms have been suggested in the literature regarding the catalytic hydrogenation of simple alkenes by photochemically activated iron pentacarbonyl: a molecular mechanism that involves the sequential replacement of two carbonyl ligands by hydrogen and unsaturated ligands, and a radical mechanism involving the EPR-identified iron carbonyl hydride radicals. Even though significant results were obtained in numerous experiments and few theoretical studies, these mechanisms remain phenomenological, without detailed information regarding the potential energy surfaces (PES) and the elementary processes. Several major issues also remain open. It is still unclear, for instance, whether the ethane can be formed via a monomolecular reaction of ethyl hydride isomer intermediates HFe(CO)3(C2H5) or the only way to produce C2H6 is a bimolecular reaction assisted by a second ethylene. It is also uncertain if a dihydride or a dihydrogen complex is operating in olefin hydrogenation. To gain insight into these processes, a detailed theoretical examination of various PES for gas-phase reactions of ethylene with potential metallocomplex reagents to primary and secondary products (both singlet and triplet electronic states) was performed using DFT and ab initio methods. Calculations have been carried out for a set of reactions of ethylene with all possible isomers of tricarbonyliron hydrogenates, viz., dihydrides of trans-, cis-, and gauche-configurations (isomers with respect to the two hydridic atoms), and two nonclassical singlet and two triplet dihydrogen complexes, some of them being identified for the first time. The hydrogenation pathways (both molecular and radical) are shown to be strongly stereoselective and dependent on the spin configurations of the initial reagents. The combination of various dihydride isomers with C2H4 as separate reaction channels allowed us to explore relevant PES cross sections and to identify corresponding stereoregulated elementary processes. The reaction channels can alternatively start from the association of ethylene with dihydrogen tricarbonyliron complexes and may involve intersystem crossings with triplet pathways, followed by that of the corresponding singlet PES. Various interconversion and isomerization processes involving single-olefin adducts were found to precede the major ethane-elimination reactions (through both monomolecular and second-ethylene-assisted pathways). Monomolecular processes are suggested to occur under appropriate conditions. The stereospecific mechanistic results and thermochemical parameters constitute a basis for developing detailed kinetic models for iron-carbonyl catalysis.

Revisiting the polytopal rearrangements in penta-coordinate d7-metallocomplexes: modified Berry pseudorotation, octahedral switch, and butterfly isomerization

“… 56 Its performance has been extensively studied for various properties and systems, including cases in the same problem domain as the work presented here. 15 , 36 , 51 , 57 – 59 , 64 For instance, the barrier height of the iconic BPR rearrangement in Fe(CO) 5 was computed by Harris et al at the BP86 GGA level to be 2.1 kcal mol –1 , 36 which is in remarkably good agreement to the experimental activation energy of 1.6 ± 0.3 kcal mol –1 obtained via temperature-dependent two-dimensional infrared spectroscopy. 36 While no experimental data is available for the geometries of the HFe˙(CO) 4 isomers to benchmark the employed geometry optimization approach, it is known that DFT successfully predicts the geometries of the precursor H 2 Fe(CO) 4 ( ref.…”

“… 36 While no experimental data is available for the geometries of the HFe˙(CO) 4 isomers to benchmark the employed geometry optimization approach, it is known that DFT successfully predicts the geometries of the precursor H 2 Fe(CO) 4 ( ref. 58 and 59 a ) and various open-shell derivatives of the HFe˙(CO) 4 radical (a review can be found in ref. 15 ).…”

“… 13 The resulting conformational flexibility affects the (stereo-)selectivity and mechanism of reactions and it thus plays a key role in the stability, reactivity, and catalytic activity of both metallocomplexes and solid state surfaces. 2 – 6 , 9 – 11 , 13 – 15 The structural theory of polytopal processes has been developed on a conceptual level using the valence shell electron pair repulsion (VSEPR) theory 16 and related points-on-a-sphere models. 9 , 10 , 17 , 18 A more quantitative, first-principles understanding has been afforded by computational quantum chemistry.…”

“… 50 Our work was motivated by the observation of several elementary reactions involving polytopal rearrangements during a study of radical pathways in the photochemical and thermal hydrogenation of ethylene by Fe(CO) 5 -derived catalysts. (The study of the corresponding molecular pathways was reported in ref. 15 .)…”

Computational insights into the mechanism of iron carbonyl-catalyzed ethylene hydrosilylation or dehydrogenative silylation

“…Here, the Glaser-Tilley type mechanism is excluded because the model silane SiR 3 H (R -H) is not possible to form the metal silylene species. Very recently, Asatryan and Ruckenstein [38] have investigated the detailed mechanism of iron carbonyl-catalyzed hydrogenation of ethylene using DFT methods. The water gas shift reaction catalyzed by M(CO) 5 (M = Fe, Ru, Os) has been studied by density functional theory and ab initio calculations [39].…”