posted on 2022-09-21, 18:44authored byArron
C. Deacy, Andreas Phanopoulos, Wouter Lindeboom, Antoine Buchard, Charlotte K. Williams
A combined computational
and experimental investigation
into the
catalytic cycle of carbon dioxide and propylene oxide ring-opening
copolymerization is presented using a Co(III)K(I) heterodinuclear
complex (Deacy, A. C.Co(III)/Alkali-Metal(I) Heterodinuclear
Catalysts for the Ring-Opening Copolymerization of CO2 and
Propylene Oxide. J. Am. Chem. Soc.2020, 142(45), 19150−19160). The complex
is a rare example of a dinuclear catalyst, which is active for the
copolymerization of CO2 and propylene oxide, a large-scale
commercial product. Understanding the mechanisms for both product
and byproduct formation is essential for rational catalyst improvements,
but there are very few other mechanistic studies using these monomers.
The investigation suggests that cobalt serves both to activate propylene
oxide and to stabilize the catalytic intermediates, while potassium
provides a transient carbonate nucleophile that ring-opens the activated
propylene oxide. Density functional theory (DFT) calculations indicate
that reverse roles for the metals have inaccessibly high energy barriers
and are unlikely to occur under experimental conditions. The rate-determining
step is calculated as the ring opening of the propylene oxide (ΔGcalc† = +22.2 kcal mol–1); consistent with experimental measurements (ΔGexp† = +22.1 kcal mol–1, 50 °C). The calculated barrier to the selectivity
limiting step, i.e., backbiting from the alkoxide intermediate to
form propylene carbonate (ΔGcalc† = +21.4 kcal mol–1), is competitive
with the barrier to epoxide ring opening (ΔGcalc† = +22.2 kcal mol–1) implicating an equilibrium between alkoxide and carbonate intermediates.
This idea is tested experimentally and is controlled by carbon dioxide
pressure or temperature to moderate selectivity. The catalytic mechanism,
supported by theoretical and experimental investigations, should help
to guide future catalyst design and optimization.