[HRh(CO)₄]-catalyzed hydrogenation of Co: A systematic Ab initio quantum-chemical investigation of the reaction mechanism

The mechanism of the [HRh(CO)₄]-catalyzed hydrogenation of carbon monoxide has been studied with the help of ab initio quantum-chemical calculations at the MP2 level of theory. A quasirelativistic effective core potential was used at rhodium in combination with valence basis sets of double-zeta quality with additional polarization functions. A systematic and unbiased approach has been followed: In the first part of the investigation, for each intermediate of the postulated catalytic mechanism all stable conformations were determined and characterized as minima on the potential energy surface by calculation of the energy Hessian. In the geometry optimizations all theoretically reasonable structures were taken into account (up to 32 starting structures per intermediate) and not just those which fit well into the proposed mechanism. In the second part of the work, the calculated energy-minimum structures were integrated into the catalytic mechanism and the transition states for the individual reaction steps were determined and characterized by calculations of the intrinsic reaction coordinate (IRC) connecting them with the corresponding energy-minimum structures. In spite of this unbiased approach, a rather simple and very smooth reaction profile for the catalytic mechanism is found, without thermodynamic traps or insurmountable barriers. The first step of the catalysis, the formation of the formyl complex [(HCO)Rh(CO)₃] from the starting catalyst [HRh(CO)₄], is the rate-determined step of the whole reaction and thus responsible for the requirement of high temperature and pressure. The details of the calculated mechanism help to understand a number of experimental observations and answer several questions discussed in the literature. Furthermore, the presented results might serve as a basis for a rational improvement of the catalyst systems currently in use.