Theoretical studies on the influence of metallic cations on ring opening of propylene oxide catalyzed by metal‐salen complexes

We studied the ring opening of propylene oxide (PO) by salen‐M coordinated OH− group [M = Al(III), Sc(III), Cr(III), Mn(III), Fe(III), Co(II), Co(III), Ni(II), Cu(II), Zn(II), Ru(III) and Rh(III)]. The results show that the ring‐opening energy barriers for M(II) complexes are much lower than those with M(III) complexes in the gas phase, and the barriers correlate linearly with the negative charges on the OH− group and the Fukui function condensed on the OH− group. The nucleophilicity ordering in the gas phase can be rationalized by the ratio of formal positive charges/radius of M cations. Solvent effect greatly increases the barriers of M(II) complexes but slightly changes the results of M(III) ones, making the barriers similar. Analysis indicates that the reaction heats are linearly proportional to the reverse reaction barriers. The relationships established here can be used to estimate the ring‐opening barriers and to screen epoxide ring‐opening catalysts. Ring opening of propylene oxide catalyzed by a series of metal‐salen complexes was investigated in both gas phase and solution using DFT methods. It is revealed that the nucleophilicity of the metal‐salen coordinated OH− group dominates the activation barriers of the ring opening. We proposed several relationships characterizing the activation barriers for fast screening and better hydrolytic kinetic resolution catalysts of metal‐salen complexes.