ORIGINAL PAPER

Quantum Chemical Determination of Stable Intermediateson CO2 Adsorption Onto Metal(Salen) Complexes

Maria C. Curet-Arana • Paul Meza •

Radames Irizarry • Rafael Soler

Published online: 21 April 2012

� Springer Science+Business Media, LLC 2012

Abstract Coupling reactions of CO2 and epoxides to

produce either cyclic carbonates or polycarbonates are

environmentally friendly reactions that allow the use of an

inexpensive and renewable feedstock while achieving

carbon efficiency. In this study, density functional theory

calculations were used to understand the role the

metal(salen) catalyst on CO2 adsorption. We have per-

formed a systematic analysis of the plausible interactions

of CO2 with metal(salen) catalysts and ethylene oxide/

metal(salen) complexes. Adsorption reactions were ana-

lyzed on six metal(salen) complexes: Co, Cr, Mn, Fe, Zn,

and Al, using the unrestricted OPBE functional. Geometry

optimizations were carried out beginning with a variety of

different conformations and frequency calculations were

used to verify that structures lie in an energy minimum.

Our results demonstrate that CO2 does not bind to the metal

atom of the bare metal(salen). The adsorption of CO2 onto

metal(salen) complexes is an endothermic reaction and the

lowest energy adsorbed complex involves the interaction of

CO2 with the adsorbed opened-epoxide.

Keywords CO2 adsorption � Coupling reaction �Quantum mechanical calculations � Metal(salen)

complexes � Al � Fe � Mn � Co � Cr � Zn

1 Introduction

Since petroleum resources are predicted to be depleted in

the near future, there is a growing effort to develop new

chemical processes using biorenewable resources, such as

CO2. A chemical process that uses CO2 for the production

of valuable chemicals would benefit from using an inex-

pensive, nontoxic, abundant, and renewable feedstock.

However, the thermodynamic stability of CO2 has ham-

pered its use as a reagent for chemical synthesis.

Inoue and coworkers demonstrated in 1969 that it was

possible to catalyze the reaction of CO2 with propylene

oxide in the presence of a zinc based catalyst [1]. The

coupling of CO2 with epoxides could yield cyclic carbon-

ates or polycarbonates via two competing reactions.

OO O

O

CO2catalyst+

1

O

O O

OCO2

n

catalyst+

2

The products from Reactions 1 and 2 are widely used in

industry. Cyclic carbonates are used as precursors in fine

chemical and pharmaceutical synthesis due to their cyclic

chiral nature. Cyclic carbonates are also used as aprotic

solvents, as electrolytes in high-energy density batteries,

and as raw materials for the synthesis of polymers [2].

Polycarbonate, on the other hand, is one of the most widely

used plastics, with an annual worldwide production of 2

million tons, and this number increases between 5 and

Electronic supplementary material The online version of thisarticle (doi:10.1007/s11244-012-9802-6) contains supplementarymaterial, which is available to authorized users.

M. C. Curet-Arana (&) � P. Meza � R. Irizarry � R. Soler

Department of Chemical Engineering, University of Puerto Rico,

Mayaguez, PR 00680, USA

e-mail: maria.curetarana@upr.edu

123

Top Catal (2012) 55:260–266

DOI 10.1007/s11244-012-9802-6

10 % every year [3]. The production of polycarbonates

involves the use of toxic reactants, such as phosgene, and

the formation of side products, such as hydrogen chloride.

Therefore, using an alternate route to produce polycar-

bonates based on CO2 and epoxides would greatly decrease

the environmental concerns associated with this process.

Numerous catalysts have been tested for Reactions 1

and 2, but metal(salen) complexes exhibit some advantages

over other catalytic materials for these systems [4–8]. A

schematic diagram of a metal(salen) complex is shown in

Fig. 1. Metal(Salen) complexes are cheap and easy to

synthesize, they can be easily modified, tailored, and

immobilized on a solid support, and they have excellent

thermal and chemical stability. Nevertheless, while Reac-

tions 1 and 2 are exothermic (DHrxn for Reaction 1 is

-56.9 kJ/mol), typical experimental conditions for

metal(salen) catalyzed reactions involve high pressures

(20–150 atm) and temperatures (80–130 �C) [9–12].

Many research groups have postulated reaction mecha-

nisms for Reactions 1 and 2 based on kinetic experiments

[11, 13–21]. While most of the debate in the literature

centers on the step involving the opening of the epoxide

ring, most of the postulated reaction mechanisms assume

the same intermediate for CO2 adsorption, shown as the

product in Fig. 2. The coupling reaction of epoxides with

CO2 catalyzed by metal(salen) complexes involve the

interaction of both reactants with the catalyst. While the

adsorption of epoxides on metal(salen) complexes has been

extensively studied computationally [22–24], the interac-

tion of CO2 with metal(salen)/epoxide complexes is not

fully understood. To fill this void, we used density func-

tional theory (DFT) calculations to investigate proposed

intermediates for the adsorption of CO2 onto six

metal(salen) catalysts: Mn, Fe, Co, Cr, Zn, and Al. The

main focus of the calculations was the stability and

reactivity of the proposed CO2/salen intermediates. Dif-

ferent spin multiplicities were examined, as well as the

effect of a surrounding solvent. We also calculated the

geometry and energetics of the bare salen catalysts in order

to calculate reaction energies.

2 Methodology

Spin-unrestricted DFT calculations were performed to

obtain optimized geometries for metal(salen) complexes

and stable intermediates for the adsorption of CO2. We

have performed a systematic study on the adsorption of

CO2 on six different metal(salen) complexes: chromium,

manganese, iron, cobalt, zinc and aluminum. The specific

metal(salen) complexes analyzed are shown in Fig. 1. Two

models of the salen catalyst (labeled m1 and m2 in Fig. 1)

were used in the calculations. Model-1 consists of 14 heavy

atoms, while model-2 consists of 22. These models are a

mimic of the metal(salen) complexes that have been

studied in previous experiments for the coupling of CO2

and epoxides[13, 14]. Unchlorinated Zn(salen) was used to

analyze the adsorption in these complexes.

The OPTX-based exchange functional [25, 26] and the

Perdew–Becke –Ernzerhof (PBE) correlation functional

[27, 28] were used to analyze this system. The selection for

both functionals was based on the work published by

Ghosh and coworkers, where they assessed the perfor-

mance of different functionals for the electronic structure

of transition metal salens [29]. Among the functionals that

they analyzed, they report that O-PBE exchange–correla-

tion functional accurately captures the electronic structure

of the transition metals in the salens. The effective core

potential LANL2DZ was used as the basis set [30–33].

This is a double-f quality Dunning basis set that replaces

core electrons with a potential field to save computational

cost. This basis set has been shown by many research

groups to yield accurate energies of reaction with great

computational efficiency on these systems, and additional

test calculations using a larger basis set on similar systems

provide similar results [23, 34, 35].

Geometries and energies were obtained by performing

full geometry optimization with no symmetry constraints.

Optimizations were performed on the three lowest multi-

plicities for each structure. The optimization of stable

model 1 (m1) model 2 (m2)

Fig. 1 Metal(salen) complexes used in the DFT calculations.

M = metal atom, such as Co, Cr, Mn, Fe, Zn, Al

...

X XXX

Fig. 2 Elementary steps

involving coupling reaction of

ethylene oxide and CO2

catalyzed by a metal(salen)

complex

Top Catal (2012) 55:260–266 261

123

species was verified with frequency calculations to ensure

that the structures lied in a minimum energy configuration.

Standard statistical mechanics was used to calculate the

translational, vibrational, rotational, and electronic contri-

butions to the partition function. The rigid-rotor, harmonic-

oscillator approximation was used to obtain the vibrational

frequencies. Wavefunction stability was confirmed using

procedure described by Seeger and Pople, and Bauernsch-

mitt and Ahlrichs [36, 37]. The \S2[ values in all of the

calculations were verified to ensure that the results do not

have spin contamination. Natural bond orbital population

analysis was performed for all intermediates to extract

electronic structure information [38, 39]. A polarizable

continuum model (PCM) was used to study solvent effects

on the energies of the stable intermediates [40]. For these

calculations, dichloromethane was used as the solvent,

which has a dielectric constant of 8.93. All calculations were

carried out with the Gaussian 09 program package [41].

3 Results

Table 1 describes the systems that are discussed in this

section. This section first presents an analysis of CO2

adsorption onto the bare metal(salen) catalysts. Next,

results for CO2 adsorption onto epoxide/metal(salen)

complexes are presented followed by the interaction of

CO2 with the opened epoxide ring adsorbed onto the cat-

alysts. All systems were analyzed for six metal(salen)

complexes: Cr, Mn, Fe, Co, Zn, and Al. Geometry opti-

mizations were performed beginning with several config-

urations using model 1 (m1). Adsorbed stable

intermediates found with m1 were then used as initial

configurations in optimizations with model 2 (m2).

3.1 CO2 Interaction with Bare Metal(salen) Complexes

Several geometry optimizations were performed in order to

elucidate the possible interactions between CO2 and the

bare metal(salen) complexes. Geometry optimizations

performed on the interaction of CO2 with m1 yielded four

stable adsorbed complexes, with no imaginary frequencies,

and they are labeled as 1-m1, 2-m1, 3-m1 and 4-m1 in

Table 2. Complex 4-m1, in which the CO2 interacts

through the oxygen atom with the metal atom of m1 was

only obtained for Aluminum-m1. As it can be observed

from this table, in complexes 1-m1 and 2-m1, the carbon

and the oxygen atoms of CO2 interacts with the aromatic

ring of the salen complex. The optimized geometries

obtained for 1-m1 and 2-m1 do not vary significantly

among the different metals. In 1-m1, the CO2 interacts

through the nitrogen and the carbon atoms of the salen ring.

For the six metal atoms considered in this model, the dis-

tance between the CO2 and the interacting atoms in 1-m1 is

1.55 ± 0.01 A, and the CO2 is bent with an angle of

126.7 ± 0.2�. In complex 2-m1, the interaction of CO2

with the salen model is through two of the carbon atoms of

the aromatic ring, and the distance between the CO2 and

the interacting atoms of the salen model, m1, is

1.61 ± 0.05 A. The angle of CO2 in this intermediate is

130.5 ± 0.2�. Similar results were obtained when model 2

was used in the calculations. Optimized geometries for the

cobalt intermediate in 1-m2 and 2-m2 are shown in

Table 3. The geometries of these intermediates are very

similar to the results obtained in 1-m1 and 2-m1. The

distance between CO2 and the interacting atoms of the

salen model-2 is 1.55 ± 0.02 A in 1-m2, and

1.64 ± 0.05 A. As it can be observed in the figures of

Table 3, the salen ring in m2 is bent downwards upon

adsorption of CO2.

Table 3 illustrates the only configurations that resulted

in adsorbed CO2 with the bigger salen model, m2. As

shown in this table, when expanding the salen model into

m2, CO2 remained adsorbed only in complexes in which

CO2 interacts with the aromatic ring of the salen complex.

Configurations in which CO2 interacts with the metal atom

of the salen complex, such as in 3-m1 and 4-m1, did not

yield an adsorbed complex using m2.

Table 1 Systems considered

for the adsorption of CO2CO2 / bare metal(salen) CO2 / adsorbed epoxide onto

metal(salen)CO2 / opened-epoxidering adsorbed onto metal(salen)

Cl

Cl

ClCl

Cl

Cl

+

ClCl

Cl

ClClCl

262 Top Catal (2012) 55:260–266

123

It is interesting to notice that the spin ground state of the

salen complex is conserved upon adsorption of CO2 to

form either 1-m1 and 2-m1. This is illustrated in Table 4,

where the relative energies of the spin states are tabulated

for the six metal complexes studied. A natural bond orbital

population analysis in 1-m2 and 2-m2 reveals that the

natural charge of the carbon atom of CO2 decreases from

0.947 (on the isolated CO2 molecule) to 0.779 and 0.801 on

1-m2 and 2-m2, respectively (See Supplementary Infor-

mation). Even when the CO2 does not bind to the metal

atom in 1-m2 and 2-m2, the charges of the metal atoms

change upon adsorption, and they slightly increase on both

1-m2 and 2-m2.

Energies for the adsorbed complexes illustrated in

Tables 2 and 3 relative to the isolated CO2 and salen

models, m1 or m2, are illustrated in Fig. 3 for the six

metals considered in this study. Two interesting features

can be observed in Fig 3: (1) energies of reaction for the

adsorption of CO2 in all the configurations presented in

Tables 2 and 3 are positive, and (2) for the complexes in

which CO2 interacts with the aromatic ring of the salen

models, the energies of reaction are independent of the

metal. It is interesting also to notice the different reaction

energies obtained when expanding the salen model for

complex 2-m1.

A PCM was used to study solvent effects on the energies

of stable complexes: metal-m1, 1-m1 and 3-m1. For these

calculations, dichloromethane was used as the solvent.

Single point calculations were performed with PCM using

OPBE/LANL2DZ on optimized structures obtained in

Table 2 Minimum energy

configurations for adsorption of

CO2 onto a bare salen catalysts

with model 1

Table 3 Minimum energy configurations for adsorption of CO2 onto

a bare salen catalysts with model 2

Table 4 Relative energy between spin states for adsorbed CO2

intermediates onto bare salen catalysts with model m1

Esinglet Etriplet EquintetMn-m1 141 80 01-Mn-m1 - - 02-Mn-m1 - 83 03-Mn-m1 - - 0Co-m1 16 0 371-Co-m1 19 0 -2-Co-m1 25 0 -3-Co-m1 0 - -Zn-m1 0 243 4931-Zn-m1 0 - -2-Zn-m1 0 - -3-Zn-m1 - 0 -Al-m1 0 238 4971-Al-m1 0 - -2-Al-m1 0 243 -3-Al-m1 - 0 -4-Al-m1 - 0 246

Edoublet Equartet EsextetCr-m1 91 0 1551-Cr-m1 - 0 -2-Cr-m1 - 0 -3-Cr-m1 - 0 -Fe-m1 57 18 01-Fe-m1 - 30 02-Fe-m1 70 31 03-Fe-m1 0 - -

Top Catal (2012) 55:260–266 263

123

vacuum. When the solvent is taken into account, the dif-

ferences in energy among the complexes remain very

similar as in vacuum, and the topology of the energy dia-

gram does not change, as illustrated in Fig. 4. No signifi-

cant differences in atomic charges or spin densities were

obtained when the solvent was taken into account.

3.2 CO2 Interaction with Adsorbed-Epoxide-

Metal(Salen) Complexes

The interaction between CO2 and the adsorbed epoxide,

ethylene oxide, was analyzed for the six metal atoms in

models m1 and m2. An epoxide can adsorb on a salen catalyst

as either a concerted or as a radical intermediate. The two

minimum energy configurations for each type of interme-

diate are illustrated in Table 5. Concerted intermediates, c1

and c2, are equivalent in energy, and in these intermediates,

the epoxide is tilted and is oriented towards the oxygen atoms

of the salen ring. The main difference between structures c1

and c2 is the dihedral angle of the adsorbed epoxide relative

to the salen. Similarly, the radical intermediates r1 and r2 are

equivalent in energy, and the difference between these two

minimum energy structures is the orientation of the carbon

atoms of the epoxide relative to the salen ring.

In this study, we analyzed the adsorption of CO2 onto

concerted and radical intermediates c1 and r1, with ethyl-

ene oxide as the epoxide. Minimum energy structures for

the adsorption of CO2 and ethylene oxide onto metal salen

models m1 and m2 are illustrated in Table 6. In complexes

5-m1 and 5-m2, CO2 adsorbs onto the aromatic ring of the

salen with a configuration similar to 1-m1 and 1-m2,

respectively: the distance between CO2 and the interacting

atoms of the salen models are 1.55 ± 0.01 A, and the angle

of the adsorbed CO2 in these complexes is also 126 ± 0.2�for both models. The ground state for this configuration is

singlet for the cobalt, zinc and aluminum. The quartet and

sextet spin states are the lowest energy configurations for

chromium and iron, respectively. The configuration 5-m1

for manganese was obtained on the quintet spin state.

However, albeit numerous attempts to obtain intermediate

5-m2 for manganese, this configuration did not converge

with CO2 adsorbed on this model.

In the minimum energy structures 6-m1 and 6-m2, CO2

binds to the radical intermediate through the carbon atom

0

50

100

150

200

250

300

350E

lect

ron

ic E

ner

gy

(kJ/

mo

l) Co-Salen

Cr-Salen

Mn-Salen

Fe-Salen

Zn-Salen

Al-Salen

isolated CO2 +m1 or m2

1-m1 2-m1 3-m1 4-m1 1-m2 2-m2

Fig. 3 Relative energies for adsorbed CO2 onto metal salen models

m1 and m2

-150

-100

-50

0

50

100

150

200

250

300

350

Ele

ctro

nic

En

erg

y (k

J/m

ol) Co-Salen

Cr-Salen

Mn-Salen

Fe-Salen

Zn-Salen

Al-Salen

isolated

CO2 +m1 or m2

3-m11-m1

Fig. 4 Relative energies for adsorbed CO2 complexes 1-m1 and

3-m1 with PCM. Zero energy corresponds to isolated species in

vacuum

Table 5 Minimum energy configurations for adsorbed ethylene oxide onto metal salen catalysts

264 Top Catal (2012) 55:260–266

123

of the adsorbed epoxide withholding the unpaired electron.

It is interesting that the spin state of the radical interme-

diate, r1, is conserved upon CO2 adsorption. The lowest

energy states for r1, 6-m1 and 6-m2 are triplet for Co, Al

and Zn; quartet for Cr and Fe; and quintet for Mn. Upon

CO2 adsorption, the spin densities of the atoms corre-

sponding to CO2 add up to 1, suggesting that there is a

delocalized electron on those atoms. The O–C bond length

between the carbon atom of the adsorbed epoxide and the

oxygen atom of CO2 is 1.49 A, and the adsorbed CO2 is

bent with an angle of 126.9 ± 0.1�.

In the adsorbed complex 7-m1 and 7-m2, CO2 is

inserted between the metal atom the salen model and the

opened epoxide ring. This intermediate has been suggested

by many research groups in proposed reaction mechanisms

for the coupling of CO2 and epoxides. Natural bond orbital

analysis on these complexes reveal that the carbon atom

bonded to the three oxygen atoms is a carbocation with a

charge of ?1.01 ± 0.01 for all metal complexes. The

carbon atom withholding the unpaired electron in the rad-

ical intermediate, r1, remains as a radical upon CO2

adsorption, as the spin density for this carbon atom is

1.11 ± 0.02 for all metal complexes.

The energies for adsorbed complexes 5, 6 and 7 relative

to the isolated species are plotted in Fig. 5. The adsorption

of CO2 onto epoxide-metal(salen) complexes are endo-

thermic reactions. The lowest reaction energies are

obtained for complex 7 for all the metals considered either

when model 1 or 2 are used to describe the salen ring.

Similar results were obtained when PCM was used to

account for the solvent effects. The relative energies for

complexes 5-m2, 6-m2 and 7-m2 when dichloromethane is

used as the solvent are plotted in Fig. 6. Lowest energies

were obtained for 7-m2. Mulliken spin densities and atomic

charges do not change significantly when the solvent is

included.

4 Conclusions

Two metal(salen) models were used in order to elucidate

the plausible interactions between CO2 and metal(salen)

Table 6 Minimum energy configurations for the adsorption of CO2 and ethylene oxide onto metal salen catalysts with models m1 and m2

0

50

100

150

200

250

300

Ele

ctro

nic

En

erg

y (k

J/m

ol)

Co-Salen

Cr-Salen

Mn-Salen

Fe-Salen

Zn-Salen

Al-Salen

isolated epoxide+CO2 +m1 or m2

5-m1 6-m1 7-m15-m2 6-m2 7-m2

Fig. 5 Relative energies for adsorbed CO2 and ethylene oxide onto

metal salen models m1 and m2

-150

-100

-50

0

50

100

150

200

Ele

ctro

nic

En

erg

y (k

J/m

ol)

Co-Salen

Cr-Salen

Fe-Salen

Zn-Salen

Al-Salen

Mn-Salen

isolated epoxide +CO2 +m1 or m2

5-m2 6-m2 7-m2

Fig. 6 Relative energies for adsorbed CO2 and ethylene oxide onto

metal salen models m2 in dichloromethane as solvent. Zero energy

corresponds to the energy of the isolated species in vacuum

Top Catal (2012) 55:260–266 265

123

complexes with six different metals. Our calculations with

the bigger metal(salen) model demonstrate that CO2 does

not bind to the metal atom of the bare metal(salen). The

only possible interactions between CO2 and the bare

meta(salen) catalysts are through the aromatic rings of the

salen. However, the energy of creating such adsorbed

complexes are in the order of 200 kJ/mol even when eth-

ylene oxide is adsorbed onto the metal atom of the

metal(salen) catalyst. Moreover, energies of reaction do not

change significantly when a PCM is used to take into

account the solvent effect on the energies. Interactions of

CO2 with a radical intermediate formed with an adsorbed-

opened-epoxide onto the metal(salen) complex can yield in

two possible stable configurations: adsorption through the

radical atom or ‘‘insertion’’ between the metal and the

oxygen atom of the epoxide. The latter complex, which is a

carbo-cation-radical intermediate, is the lowest energy

configuration, but it is still above 85 kJ/mol relative to the

isolated species.

Acknowledgments Support from the National Science Foundation

(DMR-0934115) and the Puerto Rico Institute for Functional Mate-

rials under the National Science Foundation Award No. EPS-1002410

is gratefully acknowledged. PM, RS and RI have been partially

supported by NSF (HDR-0833112). The authors thank IFN for pro-

viding computational resources on the High Performance Computing

Facility. MCCA thanks Prof. Randall Snurr for helpful discussions.

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