Temperature Dependence of the Ring Opening of Cyclopropene Imines on Thorium Metallocenes

The reactions of two highly strained cyclopropenimine ligands L1H and L2H (L1H = N-1,N-1,N-2,N-2-tetraisopropyl-3-iminocycloprop-1-ene-1,2-diamine, L2H = N-1,N-1,N-2,N-2-tetracyclohexyl-3-iminocycloprop-1-ene-1,2-diamine) with three thorium precursors Cp*2ThCl2, Cp*Th-2(Cl)(CH3), and Cp*Th-2(CH3)(2) were studied. At -20 degrees C, L1H and L2H react with Cp*2ThCl2 to form Th1 (Th1 = Cp*2ThCl2(L1H)) and Th2 (Th2 = Cp*2ThCl2(L2H)), respectively, where the neutral ligand coordinates to the thorium metal center. Coordination of the ligand to the thorium metal center introduces aromaticity at the cyclopropene ring of the ligand. Reaction at room temperature results in the ring opening of the ligand to form Th3 (Th3 = Cp*2ThCl2((Z)-2,3-bis(diisopropylamino)acrylonitrile) and Th4 (Th4 = Cp*2ThCl2((Z)-2,3-bis(dicyclohexylamino)acrylonitrile), where the cyclopropenimine converts into a nitrile and coordinates to the thorium metal center. Reaction of L1H and L2H with Cp*Th-2(Cl)(CH3) and/or Cp*Th-2(CH3)(2) at -20 degrees C results in a rapid methanolysis reaction and forms Cp*Th-2(L1/L2)(CH3/Cl)-type complexes Th5 (Th5 = Cp*Th-2(L1)(CH3)), Th6 (Th6 = Cp*Th-2(L2)(CH3), Th7 (Th7 = Cp*Th-2(L1)(Cl), and Th8 (Th8 = Cp*Th-2(L2)(Cl). On the other hand, at room temperature, these reactions result in a ring opening of the ligand. Room-temperature reaction of L1H and L2H with Cp*Th-2(CH3)(2) results in Th9 (Th9 = Cp*Th-2(CH3)((Z)-3-imino-N-1,N-1,N-2,N-2-tetraisopropylbut-1-ene-1,2-diamine) and Th10 (Th10 = Cp*Th-2(CH3)((Z)-3-imino-N-1,N-1,N-2,N-2-tetracyclohexylbut-1-ene-1,2-diamine). Similarly, at room temperature, L1H and L2H react with Cp*Th-2(Cl)(CH3) to form Th11 (Th11 = Cp*Th-2(Cl)((Z)-3-imino- N-1,N-1,N-2,N-2-tetraisopropylbut-1-ene-1,2-diamine) and Th12 (Th12 = Cp*Th-2(Cl)((Z)-3-imino-N-1,N-1,N-2,N-2-tetracyclohexylbut-1-ene-1,2-diamine). The ring-opening reaction is assisted by the nucleophilic attack of the thorium-coordinated methyl group to the highly strained cyclopropene imine carbon.