Enhanced Catalysis Activity In A Coordinatively Unsaturated Cobalt-Mof Generated Via Single-Crystal-To-Single-Crystal Dehydration

Hydrothermal readion of Co(NO3)(2) and terpheny1-3,2",5",3'-tetracarboxyate (H(4)tpta) generated Co-3(O)(2) chains based 3D coordination framework Co-3(OH)(2)-(tpta)(H2O)(4) (1) that suffered from single-crystal-to-single-crystal dehydration by heating at 160 degrees C and was transformed into dehydrated Co-3(OH)(2)(tpta) (1a). During the dehydration course, the local coordination environment of part of the Co atoms was transformed from :saturated octahedron to coordinatively unsaturated tetrahedron. Heterogenons catalytic experiments on allylic oxidation of cyclohexene show that dehydrated la has 6 times enhanced catalytic activity than as-synthesized 1 by using tert-butyl hydroperoxide (t-BuOOH) as oxidant. The activation energy for the oxidation of cylcohexene with 1a catalyst was 67.3 kJ/mol, far below the value with I catalysts, which clearly suggested that coordinatively unsaturated Co-II sites in la have played a significant role in decreasing the activation energy. It is interestingly found that heterogeneous catalytic oxidation of cyclohexene in 1a not only gives the higher conversion of 73.6% but also shows very high selectivity toward 2-cyclohexene-1-one (ca. 64.9%), as evidenced in high turnover numbers (ca. 161) based on the open Co(II) sites of la catalyst. Further experiments with a radical trap indicate a radical chain mechanism. This work demonstrates that creativity of coordinatively unsaturated metal sites in MOFs could Significantly enhance heterogeneous catalytic activity and selectivity.