Enhanced Catalysis Activity in a Coordinatively Unsaturated Cobalt-MOF Generated via Single-Crystal-to-Single-Crystal Dehydration

Hydrothermal reaction of Co­(NO3)2 and terphenyl-3,2″,5″,3′-tetracarboxyate
(H4tpta) generated Co3(OH)2 chains
based 3D coordination framework Co3(OH)2­(tpta)­(H2O)4 (1) that suffered from single-crystal-to-single-crystal
dehydration by heating at 160 °C and was transformed into dehydrated
Co3(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. Heterogenous catalytic experiments on allylic oxidation of cyclohexene
show that dehydrated 1a 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 1 catalysts,
which clearly suggested that coordinatively unsaturated CoII sites in 1a 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 1a 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.