Analysis of potential molecular catalysts for the hydroamination of ethylene with ammonia: A DFT study with [Ir(PCP)] and [Ir(PSiP)] complexes

摘要

Very few cases of oxidative addition of NH_3 to transition-metal complexes forming terminal amide hydrides have been experimentally observed. Here, two examples with the iridium pincer complexes [Ir(PCP)(NH_3)] A1 with PCP=[κ~3-(tBu_2P-C_2H _4)_2CH]- and [Ir(PSiP)(NH_3)] B1 with PSiP=[κ~3-(2-Cy_2P-C_6H_4) _2SiMe]- were investigated by DFT calculations applying the M06L density functional to successfully reproduce the trend of the experimentally observed thermochemical stabilities. According to the calculations, the corresponding hydrido-amido complexes A_2 and B2 are more stable than the corresponding ammine complexes by ΔG=-2.8 and -2.6kcal mol~(-1), respectively. Complexes such as A_2 and B2 are ideally suited entry points to catalytic cycles for the hydroamination of ethylene with ammonia. Therefore, the relevant stationary points of the potentially available cycles were studied computationally to verify if these complexes can catalyze the hydroamination. As a result, complex A_2 will clearly not catalyze the hydroamination as all energy spans calculated range close to 40kcal mol -1 or higher. The energy spans obtained with B2 are significantly lower in some cases and range around 35kcal mol~(-1), further indicating that no turnover can be expected. By systematically varying the structure of B2, the energy span could be reduced to 28.8kcal mol~(-1) corresponding to a TOF of 17h~(-1) at a reaction temperature of 140 °C. A reoptimization of relevant structures under the inclusion of cyclohexane as a typical solvent reduces the calculated TOF to 6.0h~(-1). A careful computational design is necessary to understand the strict energetic limitations of the hydroamination of ethylene with ammonia, with rhodium and iridium pincer catalysts. According to DFT computations, the iridium catalyst (see figure) with a finely tuned pincer ligand enables this challenging reaction