Abstract:
Computational studies of a model of a recently synthesized Ir_4 cluster with phosphine-calixarene ligands have been made to better understand how the catalytic properties can be controlled by creating a selective nanoscale environment. The calculations show that the binding energy of C_2H_4 on the apical site is lower than on the basal-plane site by 4 – 9 kcal/mol, and does not depend on the size of the phosphine. Electronic effects dominate in controlling the selective binding. The energetic low-lying structures of Irx(PH3)y(CO)z were optimized using density functional theory (DFT). The energies of small clusters were calculated using DFT and coupled cluster theory (CCSD(T)) was used to benchmark the DFT calculations. The best exchange-correlation functional, ωB97X-D, was used to predict the energies of the Ir_4 clusters. The calculations predict as carbonyls are replaced by PH_3 that the ligands dissociation energies (LDEs) of CO increase due to stronger π-back-bonding. The LDEs of PH3 decrease for the smaller clusters, and exhibit no discernable trend for the Ir4 clusters. The structures and LDEs of Irx(CO)y(NHC)z have been calculated using the same approach as for the Irx(PH_3)y(CO)z clusters, except that the CAM-B3LYP functional was found to be better and it was used to predict the energies of the Ir_4 clusters. The results were compared to experiment and the Irx(PH_3)y(CO)z results. The NHC ligands act as stronger σ-donors and have larger LDEs than CO’s. The trend for how the LDEs change is consistent with the trend for Irx(PH3)y(CO)z results. The Ir4 cluster with phosphine-calixarene ligands was treated oxidatively, leading to an increase in the rate of hydrogenation of C_2H_4. The Ir_4L_3 clusters (L = PMe_3 and PMePh_2) before and after oxidative treatment and the transition states for the hydrogen atom transfer to form C_2H_6 have been studied. The DFT calculation predicted that the reductive elimination reaction is more exothermic after oxidation, and that the oxidation decreases the barrier of the reactions. New site-isolated Os complexes in various oxidation states have been studied for different models ofh Os on an MgO lattice. The calculated bond lengths and C-O frequencies were compared with experiments to determine the possible structures.
