In this study, we synthesized and characterized a
series of cobalt(II) complexes bearing linear tetradentate N4
ligands. These Co(II)−N4 complexes proved to be efficient
catalysts for the cycloaddition reaction between carbon dioxide and
epoxides even at room temperature and 1 bar pressure of carbon
dioxide without the need for solvents or cocatalysts. Furthermore,
when combined with (triphenylphosphoranylidene)ammonium
chloride (PPNCl) as a cocatalyst, the Co−N4 catalysts exhibited
an impressive turnover frequency of up to 41,000 h−1 for coupling
of epichlorohydrin/CO2. These Co(II)−N4 catalysts were found
to have excellent stability and reusability, retaining their catalytic activity after they were recycled seven times. Density functional
theory (DFT) calculations provided a comprehensive mechanism for the cycloaddition reaction, indicating that the rate-determining
step is the epoxide ring opening, in both the presence and absence of PPNCl. Further kinetic studies allow us to determine the
activation parameters (ΔH‡, ΔS‡, and ΔG‡ at 25 °C) of the coupling reaction using the Eyring equation. The Gibbs free activation
energy obtained from the kinetic studies was in close agreement with that of the DFT calculations. The substituent effect on the
cycloaddition reaction of CO2 with various substituted styrene oxides was also examined for the first time.
■ INTRODUCTION
Growing environmental concerns about the elevated levels of
atmospheric CO2, largely attributable to the combustion of
fossil fuels, have inspired interest in the transformation of CO2
into value-added products.1−3 CO2 is inherently stable, and its
conversion generally necessitates appropriate catalysts and the
input of energy. Over the past several decades, significant
advances have been made in the chemical conversion of carbon
dioxide.4−6 For instance, the electrocatalytic conversion of
CO2 into value-added chemicals and fuels powered by
renewable energy sources is an encouraging avenue.7−9
Notably, a range of well-defined metal−porphyrin complexes
have been identified as potent molecular electrocatalysts for
such transformations.10−12 Moreover, alternative to electrocatalytic methods for CO2 utilization, the synthesis of fivemembered cyclic carbonates via the cycloaddition of epoxides
and CO2 offers a viable and efficient synthesis route, featuring
a reaction that is 100% atom-economical, as shown in Figure
1a.13 These cyclic carbonates, with their diverse functionalization potential, have applications as eco-friendly solvents,14,15
fuel additives, and electrolytes in lithium-ion batteries.16,17
They also serve as precursors for synthesizing polymers18,19
and fine chemicals,20,21 thereby fascinating widespread efforts
toward more sustainable chemical processes and products.
Although the cycloaddition of CO2 with epoxides is a highly
exothermic reaction, the free energy barrier for the uncatalyzed
reaction remains relatively high. Based on DFT calculations
found in prior studies, the barriers for the reaction between
propylene oxide (PO) and CO2, in the absence of a catalyst,
are 59.7 and 55.1 kcal/mol for the two possible isomeric
transition states.22 Thus, a suitable catalytic system is essential
for the coupling of the CO2 and epoxides. In terms of the
activation mode for this transformation, the common feature
for a catalytic system involves the activation of epoxide with a
Lewis acid center followed by a nucleophilic attack, subsequent
ring opening of epoxide and insertion of CO2 to form the
desired cyclic carbonates.13 To date, various kinds of catalysts,
such as organocatalysts23−29 (e.g., quaternary ammonium salts,
ionic liquids, and organoboron catalysts) and metal complexes30,31 (e.g., salen−metal complexes, metalloporphyrins
and their analogues), have been developed to mediate the
formation of cyclic carbonates. Most successful examples of
binary/bifunctional catalytic systems consist of two active
centers in which metal complex serves as Lewis acid metal
center in combination with cocatalyst as a nucleophilic site