Metallodendrimers for catalytic epoxidation - theoretical insights into structure of Mo(VI) complexes of poly(propylene imine) dendrimers

The present theoretical study is focused on detailed structural description of Mo(VI) complex of a second generation poly(propylene imine) dendrimer DAB-G(2)-PPI-(NH2)(8) that has previously been synthesized and evaluated as catalyst for alkenes epoxidation with organic hydroperoxides. In order to verify the suggested geometry with five-coordinate metal centers, which is rare case for Mo(VI), we performed structural description of possible complexes by quantum chemical (DFT) calculations. This was achieved through modeling and geometry optimization of the MoO22+ complex with the smallest triamine fragment of the dendrimer. Different compositions of model complexes were taken into account and numerous combinations of DFT functionals (B3LYP, B2LYP, O3LYP, M05 and M06) and basis sets were used for optimizations. The M06/6-31G(d, p)-(LanL2DZ; Mo) calculations gave the best agreement with available crystallographic data for similar cis-dioxo Mo(VI) complexes. Therefore, this method was used to optimize the structure of the tetrameric Mo(VI) complex of DAB-G2-PPI-(NH2)(8). The results pointed out that M06/6-31G(d, p) optimized structure of the five-coordinate cis-dioxo Mo(VI) complex with the tridentate dendrimer fragments is possible and confirms the feasibility of the experimentally suggested coordination mode of DAB-G(2)-PPI-(NH2)(8). Calculations on a complex with additionally coordinated water molecule indicated that the five-coordinate Mo(VI) complexes of PPI dendrimer preserve the potential to coordinate one O-donor solvent molecule. These structural characteristics can explain the ability of the Mo-centers of the modeled metallodendrimer to coordinate also hydroperoxides, used as oxygen sources for the catalytic epoxidation of alkenes, and thus, ensure the realization of a crucial catalytic step in the olefin epoxidation reactions.