Theoretical Studies on the Influence of Metallic Cations on Ring Opening
of Propylene Oxide Catalyzed by Metal Salen Complexes
Abstract
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, the Fukui function
condensed on the OH- group. The nucleophilicity ordering in 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.