Mechanism of CO2hydrogenation to formates by homogeneous Ru-PNP pincer catalyst: From a theoretical description to performance optimization

Article abstract of DOI:10.1039/c4cy00568f

The reaction mechanism of CO₂hydrogenation by pyridine-based Ru-PNP catalyst in the presence of DBU base promoter was studied by means of density functional theory calculations. Three alternative reaction channels promoted by the complexes potentially present under the reaction conditions, namely the dearomatized complex 2 and the products of cooperative CO₂(3) and H₂(4) addition, were analysed. It is shown that the bis-hydrido Ru-PNP complex 4 provides the unique lowest-energy reaction path involving a direct effectively barrierless hydrogenolysis of the polarized complex 5?. The reaction rate in this case is controlled by the CO₂activation by Ru-H that proceeds with a very low barrier of ca. 20 kJ mol^-1. The catalytic reaction can be hampered by the formation of a stable formato-complex 5. In this case, the rate is controlled by the H₂insertion into the Ru-OCHO coordination bond, for which a barrier of 65 kJ mol^-1is predicted. The DFT calculations suggest that the preference for the particular route can be controlled by varying the partial pressure of H₂in the reaction mixture. Under H₂-rich conditions, the former more facile catalytic path should be preferred. Dedicated kinetic experiments verify these theoretical predictions. The apparent activation energies measured at different H₂/CO₂molar ratios are in a perfect agreement with the calculated values. Ru-PNP is a highly active CO₂hydrogenation catalyst allowing reaching turnover frequencies in the order of 10⁶h^-1at elevated temperatures. Moreover, a minor temperature dependency of the reaction rate attainable in excess H₂points to the possibility of efficient CO₂hydrogenation at near-ambient temperatures. This journal is

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Mechanism of CO2 hydrogenation to formates by
homogeneous Ru-PNP pincer catalyst: from a
theoretical description to performance
optimization†
Cite this: DOI: 10.1039/c4cy00568f
ab
Georgy A. Filonenko,ab Emiel J. M. Hensenab and Evgeny A. Pidko
*
The reaction mechanism of CO2 hydrogenation by pyridine-based Ru-PNP catalyst in the presence of
DBU base promoter was studied by means of density functional theory calculations. Three alternative
reaction channels promoted by the complexes potentially present under the reaction conditions, namely
the dearomatized complex 2 and the products of cooperative CO2 (3) and H2 (4) addition, were analysed.
It is shown that the bis-hydrido Ru-PNP complex 4 provides the unique lowest-energy reaction path
involving a direct effectively barrierless hydrogenolysis of the polarized complex 5*. The reaction rate
in this case is controlled by the CO2 activation by Ru–H that proceeds with a very low barrier of ca.
20 kJ mol−1. The catalytic reaction can be hampered by the formation of a stable formato-complex 5.
In this case, the rate is controlled by the H2 insertion into the Ru–OCHO coordination bond, for which
a barrier of 65 kJ mol−1 is predicted. The DFT calculations suggest that the preference for the particular
route can be controlled by varying the partial pressure of H2 in the reaction mixture. Under H2-rich
conditions, the former more facile catalytic path should be preferred. Dedicated kinetic experiments
verify these theoretical predictions. The apparent activation energies measured at different H2/CO2 molar
ratios are in a perfect agreement with the calculated values. Ru-PNP is a highly active CO2 hydrogenation
catalyst allowing reaching turnover frequencies in the order of 106 h−1 at elevated temperatures.
Moreover, a minor temperature dependency of the reaction rate attainable in excess H2 points to the
possibility of efficient CO2 hydrogenation at near-ambient temperatures.
Received 2nd May 2014,
Accepted 31st May 2014
DOI: 10.1039/c4cy00568f
www.rsc.org/catalysis
processes for H2 storage/release have so far only been demon-
strated for the CO2/FA pair.18–24
Introduction
Utilization of CO2 as a renewable C1 building block in chemi-
cal synthesis is recognized as a key strategy for developing
more sustainable chemical technologies.1–4 Considerable atten-
tion has been devoted to CO2 coupling reactions for the pro-
duction of cyclic carbonates or carboxylic acid derivatives.5–10
Alternatively, CO2 can be hydrogenated to C1 chemicals, such
as formic acid (FA)11,12 and, more challenging, methanol.13–15
In addition to being important chemical intermediates, these
compounds can be utilized as hydrogen storage agents as long
as the reverse dehydrogenation reaction can be made to pro-
duce only carbon dioxide as the byproduct.16–20 Fully reversible
The hydrogenation of carbon dioxide to formates has been
the subject of many experimental and theoretical studies.
The main focus has been on homogeneous catalysts, some of
them with very high activity for the formation of formates
and also their decomposition.25–29 Most of the homogeneous
systems make use of noble metals,30,31 although a substantial
progress has recently been made using first-row transition
metal, namely, Fe32,33 and Co,34 complexes. Despite the
apparent simplicity of the overall reaction, the mechanism of
the catalytic CO2 hydrogenation by homogeneous catalysts is
still under debate. One of the first examples of an active cata-
lyst for CO2 hydrogenation under supercritical conditions,
[Ru(H)2(PMe3)3],35 has been studied computationally by
Sakaki and co-workers.36,37 The authors identified CO2 inser-
tion into the Ru–H bond as the rate determining step (RDS)
under water free conditions, whilst the coordination of H2 to
Ru–formate species was shown to determine the reaction rate
in the presence of water. A subsequent detailed investigation
by Urakawa et al. revealed that CO2 insertion is a facile
a Inorganic Materials Chemistry, Department of Chemical Engineering, Eindhoven
University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands.
E-mail: e.a.pidko@tue.nl
b Institute for Complex Molecular Systems, Eindhoven University of Technology,
P.O. Box 513, 5600 MB Eindhoven, The Netherlands
† Electronic supplementary information (ESI) available: Reaction free energy
diagrams for catalysis by 4 in DMF, results of small-scale CO2 hydrogenation
experiments with 1. See DOI: 10.1039/c4cy00568f
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process, whereas the H2 insertion in the Ru–formate complex
represents the rate determining step for [Ru(dmpe)2H2]-
catalysed CO2 hydrogenation.38 These findings were used to
rationalize the increased activity at elevated H2 partial pres-
sure, which represented a major inconsistency with the ear-
lier proposal on the RDS nature of the CO2 insertion step.
Substantial progress in the catalytic CO2 hydrogenation
was made when Ir-PNP pincer complexes were introduced as
catalysts by Nozaki39 and co-workers in 2009. The presence of
a non-innocent PNP pincer ligand, which can be directly
involved in chemical transformations in the course of the cat-
alytic reaction,40–42 increases the complexity with respect to
the mechanistic analysis. In the presence of a base, the PNP
ligands can be deprotonated resulting in formation of a basic
cooperative site on the side-arm of the dearomatized
PNP* ligand that can participate in substrate activation40,41
(see Scheme 1 for a related reaction for Ru-PNP complex,
1 → 2). As a result, two alternative pathways were proposed
for the hydrogenation of CO2 over Ir-PNP,43 the first of which
involves the deprotonative ligand dearomatization as the
key reaction step. The alternative mechanism involves the
OH−-assisted hydrogen cleavage in the H2 σ-complex, which
regenerates the initial state of the catalyst, as the RDS. The
latter mechanism was supported in a theoretical study by
Yang44 and Ahlquist45 who found that the direct base-
assisted H2 cleavage was more favourable than pathways
involving the ligand participation. A similar conclusion was
drawn for iron- and cobalt-based PNP catalysts.
the catalytic cycle. However, the possibility of ligand depro-
tonation with catalytically superior K2CO3 base has not been
confirmed yet.
Previously, we have demonstrated that a related Ru-PNP
catalyst 1 (ref. 50) (Scheme 1) shows remarkable catalytic per-
formance in reversible CO2 hydrogenation.22,47 In a previous
communication,47 we investigated the mechanism of the
catalytic reaction by a combination of kinetic experiments
and in situ NMR experiments supported by DFT calculations.
The results pointed to the inhibiting effect of the CO2 adduct
3 on catalytic performance. In agreement with previous
reports,38,43,44 bis-hydrido Ru species were postulated as the
active state. It was argued that the dearomatized Ru-PNP*
complex 2 did not contribute to the catalytic reaction. Never-
theless, the contribution of multiple reaction paths involving
different species (Scheme 2) cannot be omitted and careful
mechanistic analysis is required.
Herein, we present a systematic DFT study of the CO2
hydrogenation to formates by Ru-PNP complexes that can be
formed under reaction conditions. The catalytic cycles over
the bis-hydrido complex 4, the dearomatized species 2 and
the CO2-adduct 3 were considered (Scheme 2). This work is a
continuation of our previous mechanistic study.47 The main
focus will be on the analysis of the reaction networks under-
lying the catalytic process to identify the catalytic role of dif-
ferent intermediates present in the reaction mixture. The
theoretical insights are validated by dedicated experiments.
Recently, the application of ruthenium pincer catalysts in Computational and experimental
CO2 hydrogenation has been described.46,47 The correspond-
details
Density functional theory calculations
ing ruthenium pincer complexes bearing pyridine-based PNN
48,49
and PNP ligands are known to reversibly bind CO2
via a
metal–ligand cooperative mechanism (2 → 3, Scheme 1). The
resulting products of [1,3]-CO2 addition can contribute to the
overall performance of these catalysts in the hydrogenation
reaction. Huff and Sanford46 reported that Ru-PNN pincer50
catalyst can be used for hydrogenation of CO2 to formates
with a rate (turnover frequency, TOF) of 2200 h−1. A mecha-
nism involving the dearomatization of the PNN ligand has
been proposed. This proposal was confirmed in reactivity
studies, employing KOtBu to liberate HCOO− at the end of
Similar to our previous work,47 density functional theory (DFT)
calculations were performed with the PBE0 (also denoted as
PBE1PBE and PBEh)51 hybrid exchange-correlation functional
using Gaussian 09, revision D.01 program.52 The high accuracy
of this method has been demonstrated by previous benchmark
studies on a wide set of different chemical systems53,54 and by
our own accuracy tests employing different DFT methods for
modelling CO2 hydrogenation to formic acid.47 The full
electron 6-311G(d,p) basis set55,56 was used for all atoms except
ruthenium, for which the LanL2DZ basis set57,58 was employed.
The polarisable continuum model (PCM) with standard param-
eters for THF and DMF solvents, as implemented in the Gauss-
ian 09 rev. D.01 program package, was used during the
geometry optimization and frequency analysis to account for
bulk solvent effects. Because the differences in reaction free
energies computed with PCM model of THF and DMF solvents
do not exceed 5 kJ mol−1 (see ESI†), only the results obtained
for the THF solvent are discussed here. The accuracy of this
computational method was tested by calculating energetics of
selected elementary reaction steps (2 + H2 → 4 (ref. 47) and
4 + CO2 → 5*, Scheme 1) using a larger triple-zeta + polariza-
tion quality basis set combination employing Def2-TZVPP59
basis set for the Ru centre and 6-311+G(d,p) for the light
atoms. The resulting reaction and activation energies agreed
Scheme 1 Activation of a Ru-PNP precursor 1 by deprotonation with
a strong base and the subsequent metal–ligand cooperative activation
of CO2 and H2.
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frequencies were found for the optimized structures,
confirming that these correspond to local minima on the
potential energy surface. All transition states exhibited a sin-
gle imaginary frequency, corresponding to the eigenvector
along the reaction path. The assignment of the transition
state structure to a particular reaction path was tested
by perturbing the structure along the reaction path eigenvec-
tor in the directions of the product and the reagent followed
by geometry optimization. For catalytic cycles I, II and III
starting from the activated species 3°–H2 IRC calculations
were performed to additionally confirm the assignment of
the transition states. The reaction (ΔE) and activation ener-
gies (E‡) reported in the manuscript were corrected for zero
point (EZPE) energy contribution computed using the results
of the normal-mode analysis. Free energy values (ΔG°) were
computed using the results of the normal-mode analysis
within the ideal gas approximation at a pressure of 1 atm
and temperatures of 298 K.
Catalytic CO2 hydrogenation
All manipulations unless otherwise stated were performed
using Schlenk techniques. Argon was dried with a Sicapent
column. Air sensitive compounds were stored in a MBraun
glovebox under an atmosphere of dry nitrogen or argon.
Solvents were dispensed from MBraun solvent purification
system. 1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU) was pur-
chased from Fluorochem and vacuum distilled from calcium
hydride. Catalyst 1 was prepared according to original litera-
ture procedure.60
Small scale CO2 hydrogenation experiments were performed
in A96 parallel reactor at 70 °C under 40 bar of equimolar
H2–CO2 mixture. In a typical experiment 3 mL THF or DMF,
and appropriate amount of base DBU (3.3 mmol) or KOtBu
(0.33 mmol) were mixed with 0.1 μmol of catalyst. The reac-
tion was quenched after 2 hours by addition of water–ethanol
mixture and immediately analyzed. Concentrations of formic
acid were analyzed using Shimatzu HPLC setup with 25 mM
phosphate buffer of pH = 2 as mobile phase using Prevail
Organic Acid column. GC measurements, where appropriate,
were performed using Shimatzu GC-17A instrument.
Kinetic measurements were carried out in Top Industrie
100 mL stainless steel autoclave. The vessel was evacuated
overnight at 150 °C, purged several times with Ar, and the
reaction medium was introduced by cannulae transfer. The
autoclave was flushed with H2–CO2 mixture or hydrogen,
preheated to reaction temperature and filled with H2–CO2
mixture up to operating pressure of 40 bar. The catalyst was
then introduced via a dosage device and the reaction started.
Constant pressure was maintained by a compensation device
fitted with Bronkhorst EL-FLOW MFC unit and digital pres-
sure meter with equimolar H2–CO2 mixture. Samples were
withdrawn via dip-tube installation (dead volume 4 μL, sam-
pling volume 110 μL), diluted to 1 mL and immediately
analysed by HPLC and GC-FID. In a typical experiment 30 mL
solvent, 5 mL DBU (33.4 mmol), 1 mL toluene or THF (used
Scheme 2 Possible catalytic cycles for CO2 hydrogenation to formates
by Ru-PNP pincer complexes (tBu substituents of the ligand are omit-
ted for clarity).
within 5 kJ mol−1 with those obtained using the standard methodol-
ogy (see ESI† and ref. 47). Note that the expansion of the
basis set with diffuse functions has a negligible effect on
the computed energetics, while it resulted in a much slower
SCF convergence (when PCM model was used to account for
solvent effects).
The nature of the stationary points was evaluated from
the analytically computed harmonic modes. No imaginary
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as an internal standard) and appropriate amount of catalysts
dispensed from the stock solution were used. The kinetic
traces represent single run results.
The cooperative [1,3]-addition of CO2 (2–CO2 → TS2–3 → 3)
is exothermic by −51 kJ mol−1 and proceeds with a low activa-
tion barrier of 34 kJ mol−1. When corrected for entropic
effects, the reaction and activation Gibbs free energies are,
respectively, equal to −31 and 50 kJ mol−1. This evidences a
pronounced entropy loss due to the decrease in the degrees of
freedom upon the chemical binding of the non-specifically
coordinated CO2. The TS2–3 is an early transition state that
features a distorted CO2 molecule that forms rather elongated
bonds with the basic C1 site of the ligand (r(C1–C2) = 2.543 Å)
and the Ru center (r(Ru–O1) = 2.537 Å). The structural proper-
ties of TS2–3 suggest that the dominant destabilizing contribu-
tion to the activation energy in this case is associated with the
bending of the linear CO2 molecule necessary for the attack by
the basic C1 centre at the PNP* pincer arm. The promoting
effect of the concomitant coordination to Ru is the polariza-
tion of the CO2 molecule and stabilization of the negative
charge at the O1 atom (Mulliken atomic charge in TS2–3 is
−0.280 e−) in the course of the [1,3]-addition reaction towards
3. The optimized structure of 3 agrees well with the single-
crystal X-ray diffraction data reported by Milstein and co-
workers.49 The accuracy of the current computational
method is supported by a good match between the calcu-
lated E‡ and G‡ (85 and 81 kJ mol−1, respectively) and the
experimental values (94 and 83 kJ mol−1)49 determined for the
reverse 3 → 2 + CO2 transformation.
Results and discussion
Catalyst activation
Hydrogenation activity of Ru-PNP pincer is usually initiated
by reacting a catalyst precursor 1 with a strong base to form
a dearomatized Ru-PNP* species 2 (Scheme 1). Both sub-
strates of the catalytic CO2 hydrogenation reaction, namely,
CO2 and H2 can then undergo a metal–ligand cooperative
addition to 2 resulting in rearomatized Ru-PNP complexes
349 and 4,50,61 respectively. Fig. 1 shows the optimized struc-
tures of the involved reaction intermediates and transition
states together with the computed energetics of the elemen-
tary reaction steps.47 The reaction starts with the formation
of molecular complexes of 2 with the substrate molecules.
Despite very similar thermodynamics of complexation
with CO2 and H2, the nature of the formed species is quite
different. Whereas no specific interaction between CO2
and Ru-PNP* is observed in 2–CO2, 2–H2 represents a classi-
cal example of a σ-H2 complex62–66 featuring a highly sym-
metric η2-coordination of dihydrogen with short Ru–H
distances and a considerably elongated H–H bond (r(H–H) =
0.821 Å vs. 0.747 Å for the free molecule).
Dissociation of H2 over 2 gives a bis-hydrido Ru-PNP com-
plex 4 (2–H2 → TS2–4 → 4, Fig. 1) and proceeds with an activa-
tion barrier (EZPE = 75 kJ mol−1) substantially higher than
‡
that computed for the reaction with CO2. Because H2 is effec-
tively immobilized within the σ-complex 2–H2, the entropic
contribution to the reaction and activation energy in this case
is negligible. As a result the overall reaction 2 + H2 → 4
(ΔG° = −40 kJ mol−1) is more thermodynamically favourable
than the reaction with CO2 (2 + CO2 → 3, ΔG° = −8 kJ mol−1).
In the TS2–4, the polarized H2 molecule undergoes a hetero-
lytic cleavage over a Ru⋯C1 acid–base pair. The calculated
Mulliken charges on the H1 and H2 atoms in TS2–4 are
−0.056 and 0.187 e−, respectively. The high activation barrier
in this case is most likely due to the relatively large distance
between the acid and the base sites in the dearomatized
Ru-PNP* (2: r(C1⋯Ru) = 3.191 Å) that hampers the efficient
stabilization of both the H− and H+ species formed in the
transition state.
An insight into the origin of the different coordination
behaviour and reactivity of 2 towards CO2 and H2 can be
obtained from the frontier orbital analysis (Fig. 3). An unoc-
cupied dz2 orbital on Ru and an occupied formally pz orbital
on basic C1 contribute mostly to the LUMO and HOMO of 2.
HOMO-1 is dominated by an occupied Ru dyz orbital. These
molecular orbitals (MOs) cannot form a positive overlap with
the π* LUMO and n HOMO orbitals of linear non-perturbed
CO2. Their linear combination leads to a non-bonding inter-
action. This explains the non-specific coordination of carbon
dioxide in 2–CO2. The change of hybridization of the C atom
in CO2 upon bending (CO2*) make a positive orbital overlap
Fig. 1 Optimized structures of reaction intermediates and transition
states involved in the metal–ligand cooperative activation of H2 and CO2
by 2 (in the graphical representation tBu substituents at the phosphine
moieties are simplified for clarity). ZPE-corrected reaction (ΔEZPE) and
activation energies (EZPE‡), reaction and activation Gibbs free energies
(ΔG° and G‡) are given in kJ mol−1 (for individual elementary steps).47
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between LUMO of CO2* and HOMO of 2 possible. The rela-
tively small energy gap between these orbitals enhances the
acid–base interaction resulting in a very low barrier of the
[1,3]-addition of CO2 to 2.
HOMO σ and LUMO σ* orbitals of H2 can be involved in
donation and back-donation interactions with LUMO and
HOMO − 1 orbitals of 2, respectively, in 2–H2 σ-complex.
However, because of the large energy difference between the
interacting orbitals, binding within the molecular complex is
weak (Fig. 1). The specific coordination of H2 to 2 promotes
the formation of 4. In support of the above proposition, the
high energy barrier for H2 dissociation is due to the distant
location of the base site. Indeed, the complexation with H2
does not change the properties of the HOMO.
These results suggest that the preference towards the for-
mation of complexes 3 and 4 during the catalytic CO2 hydro-
genation can be altered by varying the reaction conditions.
Indeed, whereas the reaction of 2 with H2 to the bis-hydrido
complex 4 is more thermodynamically favourable, the alter-
native path towards the CO2 adduct 3 proceeds with a much
lower activation barrier. This implies that by increasing par-
tial pressure of H2 in the reaction mixture, the formation of 4
can be promoted. However, in the presence of excess base
necessary to promote catalytic CO2 hydrogenation, one can-
not exclude the transient formation of the dearomatized spe-
cies 2, which can also contribute to the catalytic activity of
the Ru-PNP catalyst. To understand the behaviour of this sys-
tem, analysis of reaction paths over these three alternative
potentially active species is necessary.
Fig. 2 Frontier orbitals of the dearomatized complex 2 optimized H2
and CO2 molecules (H2(opt) and CO2(opt)) as well as those of the bent
CO2 moiety from the TS2–3 structure (CO2(TS2–3)). Bu at the phosphine
t
moieties in
2 are omitted for clarity, while the respective MO
contributions are visualized. Dotted lines depict possible orbital
interactions following simplified symmetry considerations and solid
grey lines represent the qualitative energetics of the resulting MOs in
complexes 3 and 2–H2 with the energy values indicated for the
orbitals, for which the contribution of the interacting molecular
fragments could be unambiguously defined.
charge-compensated by HCOO− hydrogen bonded with
the CH2 moieties of the ligand (5* + H2 → 5–H2). Subsequent
heterolytic dissociation of H2 over an acid–base pair composed
of the Ru centre and the adjacent formate anion results in a
molecular complex of formic acid with 4 (4–FA). Despite a lower
nucleophilicity of the HCOO− moiety (EHOMO(5**) = −6.10 eV,
Fig. 5) compared to the C1 site in 2 (EHOMO(2) = −4.55 eV, Fig. 3),
the direct availability of a proton-accepting species in the imme-
diate vicinity of the dissociating H2 molecule leads to a less
strained TS5–4 transition state structure and, accordingly, to an
extremely low activation barrier for the reaction. Indeed, the
reactive moiety Ru⋯H–H in TS5–4 is characterized by substan-
tially shorter interatomic distances (r(Ru⋯H) = 1.785 Å and
r(H⋯H) = 0.920 Å, Fig. 3) compared to the respective param-
eters in TS2–4 (r(Ru⋯H) = 1.881 Å and r(H⋯H) = 0.989 Å, Fig. 3).
Reaction of 4–FA with the DBU base at the next step releases
DBU–FA product and regenerates the initial complex 4.
Alternatively, 5* can rearrange to a stable complex 5
(ref. 67) (5* → 5, ΔEZPE = −39 kJ mol−1, Fig. 3) featuring a
direct Ru–O coordination (r(Ru–O) = 2.261 Å). Previously, this
species has been proposed to be a resting state in the cata-
lytic cycle by 4.47 To proceed with the catalytic cycle (path Ia),
the ionization of 5 (i.e. the formation of an ion pair 5*) and
the replacement of HCOO− with H2 has to take place. This
reaction shows an activation barrier of 65 kJ mol−1.
CO2 hydrogenation over 4
The calculated reaction energy diagram for the catalytic
cycle I (Scheme 2) of CO2 hydrogenation by bis-hydrido
Ru-PNP complex 447 is shown in Fig. 3. The starting
point of the reaction is an energy-neutral binding of CO2 to
4 resulting in 4–CO2. Weak intermolecular contacts between
the O atoms of CO2 and acidic CH2 protons of the PNP pin-
cer arms in 4–CO2 (r(O⋯H) = 2.584 Å) direct the CO2 coordi-
nation towards the Ru–H moiety. This allows a facile attack
of CO2 by Ru-bound hydride (Mulliken charge on the
Ru-bound H atom is −0.152 e−) resulting in a formate
anion (4–CO2 → TS4–5 → 5*). The reaction is exothermic
by −13 kJ mol−1 and shows a very low activation barrier of
23 kJ mol−1. Similar to [1,3]-CO2 addition to 2, the reaction
is triggered by the increased acidity of the sp2-like C atom
in bent CO2 moiety formed in TS4–5 that ensures an efficient
overlap between the s orbital of the H− ligand being one of
the main contributors of HOMO (4) (Fig. 4) and LUMO of
CO2* (Fig. 2). Note that HOMO in complex 4 lies 1 eV lower
than that in 2 indicating a lower nucleophilicity of the H−
ligand compared to the basic C1 centre in complexes 4 and 2,
respectively. This is in line with the much higher exothermi-
city of the cooperative CO2 activation by 2.
At the next step Ru⋯H-coordinated HCOO− anion is
replaced by H2 yielding a cationic σ-H2 Ru-PNP complex
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Fig. 3 DFT computed reaction energy diagram and optimized structures of the intermediate and transition states (CH3 groups at the tBu
substituents of the PNP ligand are omitted for clarity) for the hydrogenation of CO2 over 4.47
Fig. 4 Frontier orbitals of complexes 4 and 7.
Metal–ligand cooperative CO2 hydrogenation by 2
The dearomatized pyridine-based Ru pincer complexes have
been suggested to play key role in catalytic CO2 hydrogenation.46
The catalytic cycle II over 2 largely overlaps with cycle I
discussed above. The two cycles differ in the mechanism of
product formation via the transformations of the formato-
complex 5* (Scheme 2), which in cycle II involves a direct
Fig. 5 Frontier orbitals of complexes 3° and 5**.
deprotonation of the PNP ligand resulting in a one-step FA
formation. The DFT computed reaction energy diagram for
CO2 hydrogenation over 2 is shown in Fig. 6. Heterolytic dis-
sociation of H2 over 2 yielding 4 is the first and the most
energy demanding step of the cycle (E‡ = 76 kJ mol−1). It is
followed by a facile CO2 activation by the bis-hydrido species
4 resulting in 5*. The weakly bound HCOO− in 5* plays then
a role of a base that attacks the acidic CH2 moiety at the
PNP pincer arm resulting in its deprotonation and the
formation of FA molecule hydrogen-bonded with a basic
C1 site at Ru-PNP* (2–FA). The reaction and activation
energies are very similar for this step (ΔEZPE = 46 kJ mol−1 and
E‡ = 47 kJ mol−1). Because of the very high basicity of the
deprotonated pincer arm in 2, the reverse FA dissociation
reaction is effectively barrierless. Therefore, to promote the
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Fig. 6 DFT computed reaction energy diagram and optimized structures of the intermediate and transition states (CH3 groups at the tBu
substituents of the PNP ligand are omitted for clarity) for the direct hydrogenation of CO2 over 2 (cycle II).47
catalytic cycle FA has to be eliminated from the complex by a
strongly exothermic (ΔEZPE = −51 kJ mol−1) reaction with DBU
base. The DBU–FA product is formed at this step and the origi-
nal catalytic species 2 is regenerated.
carboxylate moiety on the pincer arm in 3°. The reaction in this
case is less energetically favorable and proceeds with a higher
barrier (3°–H2 → TS3–7 → 7, ΔE = 9 kJ mol−1, E‡ = 15 kJ mol−1
)
than the respective step in cycle I (5–H2 → TS5–4 → 4–FA,
ΔE = −9 kJ mol−1, E‡ = 2 kJ mol−1). This is in line with the differ-
ences in energies of the frontier orbitals (Fig. 5) and, accord-
ingly, the acid–base properties, of the reactive moieties in these
complexes. Reaction of 7 with CO2 yields a formato-complex
8 that resembles 5* in cycle I. This step is slightly endothermic
CO2 hydrogenation by 3
Potentially, the CO2 adduct 3 can also act as catalytic species
for CO2 hydrogenation. In a very recent study by Huff and
Sanford on the mechanism of CO2 hydrogenation by a related
Ru-PNN catalyst, the role of an Ru-PNN CO2 adduct analo-
gous to 3 has been discussed.46 Although the mechanism for
the catalytic reaction over such species was proposed, the
authors concluded that it most likely represents a minor
pathway in the overall catalytic process. The catalytic activity
of the CO2 adduct at elevated temperatures predominantly
stems from the reversible binding of CO2 that allows its
transformations to a more reactive dearomatized Ru-PNN*
complex under the catalytic conditions. Similarly, in the pres-
ence of H2 or H2–CO2 mixture, 3 transforms directly to 5,
from which cycle Ia can in principle be initiated. The catalytic
activities of these complexes are very different47 suggesting
the principle possibility of the 3-catalyzed hydrogenation
of CO2.
The DFT-computed reaction energy diagram for the 3-catalyzed
hydrogenation of CO2 is shown in Fig. 7. To initiate the cycle III,
complex 3 has to be activated via a rather unfavourable reac-
tion (ΔE = 33 kJ mol−1, E‡ = 69 kJ mol−1) with H2 that results in
an opening of the Ru–O coordination and the formation of a
3°–H2. The coordinated dihydrogen molecule undergoes then a
heterolytic dissociation over a cationic Ru center and the basic
(ΔE = 4 kJ mol−1) and shows an activation barrier of 45 kJ mol−1
.
The lower reactivity of 7 towards CO2 compared to that of cata-
lyst 4 stems from the decreased hydricity of the Ru–H moiety
interacting with the carboxylic acid group at the pincer arm of
7 (Fig. 4). Subsequent barrierless proton transfer from the
ligand-bound –COOH moiety to the HCOO− anion in 8 results
in FA hydrogen-bonded to the activated complex 3° (FA–3°).
The removal of FA by the reaction with DBU regenerates the
initial CO2-adduct 3.
Implications for catalysis
The computational results presented above suggest that all
candidate Ru-PNP complexes 2–4 can be formed under the
CO2 hydrogenation conditions and may contribute to the
overall catalytic reaction. To directly compare the above three
alternative mechanism for CO2 hydrogenation by Ru-PNP, we
further analysed reaction Gibbs free energy diagrams for
these three catalytic cycles (Fig. 8).
The bis-hydrido complex 4 provides the lowest free energy
reaction path for the conversion of CO2 to FA–DBU along the
cycle I (Fig. 8). The reaction in this case does not involve the
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Fig. 7 DFT computed reaction energy diagram and optimized structures of the intermediate and transition states (CH3 groups at the tBu
substituents of the PNP ligand are omitted for clarity) for the direct hydrogenation of CO2 over 3 (cycle III).
metal–ligand cooperation in Ru-PNP and proceeds via the
direct hydrogenolysis of transient species 5* containing a
non-coordinated formate anion (5* + H2 → 5–H2 → 4–FA,
Fig. 3 and 8). DFT calculations predict that the reaction along
this path shows a very low apparent activation energy68 of
24 kJ mol−1 (4 + CO2 → 5*, Fig. 3) associated with the initial
CO2 activation step. The free energy barrier ΔG‡ for this
transformation is 67 kJ mol−1 (Fig. 8). The subsequent facile
Fig. 8 A comparison of Gibbs free energy diagrams for catalytic cycles I, II and III plotted relative to the dearomatized Ru-PNP* species 2.
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hydrogenolysis of 5* is competing with its rearrangement to
a formato-complex 5. 5 is the most thermodynamically stable
species among the Ru-PNP intermediates considered here.
This finding is in line with the results of experimental 1H
NMR reactivity studies evidencing the exclusive formation of
5 under near-catalytic conditions.47 The polarization of 5
followed by H2 insertion (5 + H2 → 5–H2, Fig. 3) is the rate-
determining step (RDS) of the catalytic mechanism involving
be promoted in the presence of an excess H2 that would ensure
the rapid substitution of the non-coordinated HCOO− and its
replacement with H2 towards 5–H2.
To verify this hypothesis, we investigated the kinetics of
CO2 hydrogenation by Ru-PNP complex 1 (Fig. 9) in the pres-
ence of DBU with varying H2/CO2 molar ratio (ptotal = 40 bar).
At a H2/CO2 molar ratio of 3/1, the reaction showed an appar-
ent activation energy (Eapp‡) of 57 kJ mol−1 that is in very good
agreement with the computed value (Eapp‡,DFT) of 65 kJ mol−1
for cycle Ia (Fig. 3).
the formation of 5 (cycle Ia). This route is characterized by
‡,DFT
Eapp
of 65 kJ mol−1 that is comparable to the activation
energies predicted for the elementary reaction steps involved
in the alternative catalytic cycles II and III promoted by,
respectively, complexes 2 and 3.
The data in Fig. 6 and 8 suggest that the cooperative H2
activation by 2 represents the RDS of the MLC mechanism
along the cycle II. Since the formation of an 2–H2 σ-complex
The rate enhancement previously observed upon a slight
increase of the partial pressure of H2 (ref. 22) is in line
with the proposition on the rate-determining nature of the
5 + H2 → 5–H2 step in this case. As a result of the high activa-
tion energy, a strong temperature dependency of the reaction
rate is observed. Whereas the reaction at 132 °C shows an ini-
tial turnover frequency (TOF) of 1 892 000 h−1, the initial TOF
at 90 °C is only 266 000 h−1. When the reaction is carried out
in the presence of a large excess of H2 (H2/CO2 = 37/3), the
apparent activation energy drops to only 20 kJ mol−1, which
is in perfect agreement with the value of 24 mol−1 predicted
for the direct hydrogenolysis path I (Fig. 3). At a temperature
of 129 °C, the reaction shows a TOF of 1 099 000 h−1. In line
with the proposal on the RDS nature of the CO2 activation
step in cycle I, the lower reaction rate for a high temperature
is thermodynamically unfavourable, this route should pro-
‡,DFT
ceed with a moderate Eapp
of 64 kJ mol−1 (2 + H2 → 4,
Fig. 6). In free energy terms this pathway shows a rather high
free energy barrier of 95 kJ mol−1 (2 + H2 → 4, Fig. 8). The
catalysis on the CO2 adduct 3 (cycle III) proceeds via a
sequence of thermodynamically unfavourable steps (Fig. 8).
Although the initial coordination of H2 (3 + H2 → 3°–H2,
Fig. 7) shows an activation energy (E‡) of only 69 kJ mol−1
,
the overall barrier in this case is represented by the energy
difference between the initial state 3 and the high energy
TS7–8 for the CO2 activation (Eapp
ΔGapp
= 80 kJ mol−1 Fig. 7,
‡,DFT
= 149 kJ mol−1 Fig. 8). In line with the experimen-
‡,DFT
tal findings,47 this points to a lower catalytic activity of 3
compared to that of 2 and 4. Under the catalytic conditions,
the contribution of 2 is limited due to the low thermody-
namic stability of the reaction intermediates and high activa-
tion free energy barriers along the respective MLC path II.
Catalytic CO2 hydrogenation experiments with Ru-PNP
catalyst precursor 1 and a strong KOtBu support this proposi-
tion. We propose that KOtBu can promote the dearomatization
of the PNP pincer ligand60 in the stable intermediates formed
in the course of the reaction towards Ru-PNP* complex 2 and
therefore ensure the high steady-state concentration of this
activated complex under the catalytic conditions. Independent
of the reaction medium, a very low activity was observed in this
case. Turnover numbers after 2 hour reaction (TON(2 h)) were
only 728 and 649 in THF and DMF solvents, respectively. On
contrary, when a non-nucleophilic DBU base, which cannot
promote the ligand dearomatization,47 was used as a promoter,
much higher TON(2 h) values of 12 829 and 38 642 in THF and
DMF solvents, respectively, were obtained (Table S1, ESI†).
The catalytic CO2 hydrogenation by Ru-PNP complexes is
dominated by the competing reaction paths I and Ia realized by
the bis-hydrido Ru-PNP complex 4. These paths are differenti-
ated by the mechanism of the transformation of the polarized
formate complex 5*. The competing rearrangement (5* → 5,
path Ia) and hydrogenolysis (5* + H2 + DBU → 4 + FA–DBU,
path I) routes show similar reaction free energy changes
(Fig. 8) and involve effectively barrierless transformations
(Fig. 3). We therefore speculate that the latter mechanism can
Fig.
9 Kinetic traces for CO2 hydrogenation by 1 at different
temperatures and H2–CO2 ratio of (a) 3/1 and (b) 37/3 (ptotal = 40 bar).
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reaction in this case is most likely due to the decreased par-
tial pressure of CO2. An important consequence of the low
activation barrier for CO2 hydrogenation at a high H2/CO2
molar ratio is the possibility to achieve high rates of the cata-
lytic reaction at lower temperatures (Fig. 9(b)). An initial rate
of 721 000 h−1 was obtained at 102 °C. From the Arrhenius
plot one can estimate TOF values above 100 000 h−1 to be
reachable at ambient temperature. These findings render the
Ru-PNP complex 1 in combination with non-nucleophilic
DBU base one of the most active CO2 hydrogenation catalytic
system reported to date.
thermodynamically and their transformations proceed with
rather high free energy barriers. This suggests only minor
contribution of these reaction paths under common CO2
hydrogenation conditions.
DFT calculations predict that the bis-hydrido Ru-PNP com-
plex 4 is the most active one. CO2 hydrogenation by 4 can fol-
low two mechanisms (cycle I and Ia) that do not involve
metal–ligand cooperative steps. A facile CO2 activation by
Ru–H moiety resulting in a non-coordinated HCOO− and a
5-coordinated cationic Ru-PNP (5*) represents the initial step
in both catalytic cycle. The cycles are distinguished by the
mechanism of the subsequent transformations of 5* that, in
turn, depends on the partial pressure of H2. Under excess H2,
the substitution of the non-coordinated HCOO− anion in 5*
with H2 resulting in a σ-complex 5–H2 initiates an almost
barrierless sequence of elementary steps towards the comple-
tion of the catalytic cycle and the formation of the FA–DBU
product. The competing route Ia involves the rearrangement
of 5* towards a stable formato-complex 5 that has previously
been proposed to be the resting state of the catalytic reaction.
The polarization of 5 followed by H2 insertion to yield 5*
determines the overall reaction rate for the Ia mechanism and
shows an activation barrier of 65 kJ mol−1. The initial CO2
activation is the rate-determining step in cycle I, for which an
apparent activation energy of only 24 kJ mol−1 is predicted.
The analysis of experimental reaction kinetics for CO2
hydrogenation by Ru-PNP confirms these theoretical predic-
tions. By increasing the molar H2–CO2 ratio in the catalytic
experiment from 3/1 to 37/3, the apparent activation energy
decreases from 57 to 20 kJ mol−1, respectively, in a perfect
agreement with the computed values. A relatively small tem-
perature dependency of the reaction rate in the latter case
points to the possibility of achieving very high catalytic per-
formance in CO2 hydrogenation by Ru-PNP under near-
ambient temperatures.
Conclusions
The mechanism of CO2 hydrogenation to DBU formate salt in
the presence of homogeneous pyridine-based Ru-PNP pincer
catalysts was investigated by means of density functional
theory calculations. The focus was on unravelling the com-
plexity of the underlying reaction mechanisms and determin-
ing the routes for the optimization of the performance of this
catalytic system. Dedicated catalytic tests and kinetic study
were carried out to verify the theoretical predictions.
Three major interconnected reaction paths catalysed by
dearomatised Ru-PNP* complex 2 or by its cooperative adducts
with CO2 and H2, Ru-PNP complexes 3 and 4, respectively, were
considered. On the basis of the theoretical results it is pro-
posed that depending on the reaction conditions, the relative
concentration of these reactive species and their contribution
to the overall catalytic performance varies. Whereas formation
of 4 upon the reaction of 2 with H2 is thermodynamically pre-
ferred, the competing [1,3]-CO2 addition towards 3 is favoured
kinetically. We propose that the latter route will dominate in
excess strong base needed to ensure a high steady-state concen-
tration of the activated precursor 2 and a high partial pressure
of CO2. At increased H2 partial pressure, the reaction will be
driven towards the most stable dihydrido-Ru-PNP complex.
The hydrogenation of CO2 catalysed by 3 proceeds via a
sequence of highly endothermic elementary steps showing
also rather high activation barriers. As a result, the highest
apparent activation barrier of 80 kJ mol−1 is predicted for the
respective catalytic cycle III. This cycle is also characterized
by a prohibitively high overall Gibbs free energy barrier of
149 kJ mol−1. In the case of the catalysis by the dearomatised
species 2, metal–ligand cooperation plays an important role
in the reaction mechanism. The initial heterolytic H2 dissoci-
ation over an acid–base pair formed by the 5-coordinated Ru
Acknowledgements
E.A.P. gratefully acknowledges the Technology Foundation
STW and the Netherlands Organization for Scientific Research
(NWO) for his personal VENI grant. SurfSARA and NWO are
acknowledged for providing access to supercomputer resources
(grant SH-231). The authors thank Dr. L. Leffort (DSM) for the
possibility to carry out high-throughput experiments.
centre and the deprotonated CH basic site at the PNP* pincer
Notes and references
= 64 kJ mol−1
,
‡,DFT
arm is the rate-determining step (Eapp
‡,DFT
ΔGapp
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68 Eapp
is defined as the difference in ZPE-corrected energy
of the species corresponding to the highest and the lowest
energy point in the reaction Gibbs free energy diagram for a
particular proposed catalytic cycle (Fig. 8).
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