Theoretical Investigation of Water Gas Shift Reaction Catalyzed by Iron Group Carbonyl Complexes M(CO)₅ (M = Fe, Ru, Os)

We have investigated the mechanism of M­(CO)₅ (M = Fe, Ru, Os) catalyzed water gas shift reaction (WGSR) by using density functional theory and ab initio calculations. Our calculation results indicate that the whole reaction cycle consists of six steps: <b>1</b> → <b>2</b> → <b>3</b> → <b>4</b> → <b>5</b> → <b>6</b> → <b>2</b>. In this stepwise mechanism the metals Fe, Ru, and Os behave generally in a similar way. However, crucial differences appear in steps <b>3</b> → <b>4</b> → <b>5</b> which involve dihydride M­(H)₂(CO)₃COOH^– (<b>4′</b>) and/or dihydrogen complex MH₂(CO)₃COOH^– (<b>4</b>). The stability of the dihydrogen complexes becomes weaker down the iron group. The dihydrogen complex <b>4_Fe</b> is only 11.1 kJ/mol less stable than its dihydride <b>4′_Fe</b> at the B3LYP/II­(f)++//B3LYP/II­(f) level. Due to very low energy barrier it is very easy to realize the transform from <b>4_Fe</b> to <b>4′_Fe</b> and vice versa, and thus for Fe there is no substantial difference to differentiate <b>4</b> and <b>4′</b> for the reaction cycle. The most possible key intermediate <b>4′_Ru</b> is 38.2 kJ/mol more stable than <b>4_Ru</b>. However, the barrier for the conversion <b>3_Ru</b> → <b>4′_Ru</b> is 23.8 kJ/mol higher than that for <b>3_Ru</b> → <b>4_Ru</b>. Additionally, <b>4′_Ru</b> has to go through <b>4_Ru</b> to complete dehydrogenation <b>4′_Ru</b> → <b>5_Ru</b>. The concerted mechanism <b>4′_Ru</b> → <b>6_Ru</b>, in which the CO group attacks ruthenium while H₂ dissociates, can be excluded. In contrast to Fe and Ru, the dihydrogen complex of Os is too unstable to exist at the level of theory. Moreover, we predict Fe and Ru species are more favorable than Os species for the WGSR, because the energy barriers for the <b>4</b> → <b>5</b> processes of Fe and Ru are only 38.9 and 16.2 kJ/mol, respectively, whereas 140.5 kJ/mol is calculated for the conversion <b>4′</b> → <b>5</b> of Os, which is significantly higher. In general, the calculations are in good agreement with available experimental data. We hope that our work will be beneficial to the development and design of the WGSR catalyst with high performance