Metal-organic frameworks (MOFs) have recently emerged as a promising platform for heterogeneous catalysts owing to their high surface area and tunability. Herein, we designed a MOF, flexMOF(CN) possessing free nitrile groups to anchor single-site catalyst. This flexMOF(CN) shows flexible behaviors such as gate-opening and breathing upon kinds and amounts of guest molecules, especially showing the structural transformation upon CO₂ pressure. With the active transition metals anchored to the MOF scaffold, we expected the synergistic effects of the flexible system on the catalytic reaction. Indeed, its flexibility resulted in the interesting catalytic performance with selectivity upon reaction conditions in gas-phase ethylene oligomerization reaction. More importantly, in the CO₂ atmosphere at the gate-opening pressure, the catalytic conversion was significantly improved and the selectivity was also changed. As a result, flexible MOFs can give an opportunity to control and enhance the catalytic performances by tuning specific reaction conditions.