We designed and explored novel dimeric alumatranes with tricyclic five-membered rings. The obtained alumatranes were all dimeric in the solid state, solution phase, and the gas phase. According to single-crystal X-ray analysis, the first structurally characterized dimeric alumatranes abnormally had their sterically bulky side arms with dimethyl substituents in the bridging sites of the tetradentate ligand, which was also determined by DFT calculations. The new alumatranes were used as catalysts for the trimethylsilylcyanation reaction of aldehydes under extremely mild conditions of room temperature, less than 0.5 mol% catalyst loading, and a short reaction time of 1 h. The new catalytic systems showed high catalytic activities regardless of the aldehyde type, which included electron-rich, neutral, and deficient aryl aldehydes, heterocyclic aldehydes, and alkyl aldehydes. Figure 1. Computed free energy diagram for possible alumatrane isomers. ACKNOWLEDGMENT: This work was supported by the National Research Foundation of Korea (NRF), the Korean Ministry of Education (MOE) through the Creative Human Resource Training Project for Regional Innovation (grant number 2014H1C1A1066874).