Thermodynamics and high-pressure kinetics of a fast carbon dioxide fixation reaction by a (2,6-pyridinedicarboxamidato-hydroxo)nickel(ii) complex

Article abstract of DOI:10.1039/c3dt53004c

The previously reported carbon dioxide fixation reaction by the planar terminal hydroxide complex [Ni(pyN₂^Me2)(OH)]^1- in DMF has been further characterized by determination of the equilibrium constants K298eq = 2.4 ± 0.2 × 10⁵ M^-1 and K223eq = 1.3 ± 0.1 × 10⁷ M^-1, as well as the volume of activation for the CO₂ binding (ΔV≠223on = -21 ± 3 cm³ mol^-1) and back decarboxylation (ΔV≠223off = -13 ± 1 cm³ mol^-1) by high-pressure kinetics. The data are consistent with an earlier DFT computation, including the probable nature of the transition state, and support designating the reaction as one of the most completely investigated carbon dioxide fixation reactions of any type. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c3dt53004c

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Thermodynamics and high-pressure kinetics of
a fast carbon dioxide fixation reaction by a
(2,6-pyridinedicarboxamidato-hydroxo)nickel(II)
complex†
Cite this: Dalton Trans., 2014, 43,
5274
Received 24th October 2013,
Accepted 29th January 2014
DOI: 10.1039/c3dt53004c
O. Troeppner,a D. Huang,b R. H. Holmb and I. Ivanović-Burmazović*a
www.rsc.org/dalton
The previously reported carbon dioxide fixation reaction by the
planar terminal hydroxide complex [Ni(pyN2Me2)(OH)]1− in DMF
has been further characterized by determination of the equilibrium
298
223
constants K
= 2.4 0.2 × 105 M−1 and K
= 1.3 0.1 × 107 M−1
,
=
=
eq
eq
≠223
as well as the volume of activation for the CO2 binding (ΔV
on
≠223
−21
3 ) and back decarboxylation (ΔV
cm3 mol−1
off
−13 1 cm3 mol−1) by high-pressure kinetics. The data are consist-
ent with an earlier DFT computation, including the probable
nature of the transition state, and support designating the reaction
as one of the most completely investigated carbon dioxide fixation
reactions of any type.
Scheme 1 Reversible carbon dioxide fixation reaction mediated by
[Ni(pyN2R2)(OH)]1−
.
Fixation and utilization of CO2 is an increasingly prominent
problem1–4 not only because CO2 is a contributor to the green-
house effect, but also because of its importance as a poten-
tially cheap, nontoxic and abundant source of renewable
synthetic fuels.5 There are many general approaches that can
lead to fixation and utilization of CO2 to form energetically
more valuable molecules like carbon monoxide, methanol
or methane, ranging from photochemical6–18 to electro-
chemical1,5,19–34 to chemical reduction35–43 pathways. They
often involve a variety of transition metal centers as redox
catalysts, ranging from rather expensive and rare metals like
ruthenium21 and platinum44 to cheaper and more abundant
ones like nickel22,45 and copper.1 A number of metal com-
plexes have been synthesized and investigated as possible
mimics of naturally occurring carbon monoxide dehydrogen-
ase (CODH), an enzyme catalyzing the reversible redox reaction
CO2 + 2H+ + 2e− ⇌ CO + H2O and either containing Mo–Cu–S
or Ni–Fe–S active sites.46–49
Besides redox methods, carbon dioxide fixation can also be
achieved through non-redox processes.4 Of the latter, reactions
of a terminal MII/III–OH group with CO2 to afford bridging
carbonate and terminal or bridging bicarbonate products are
well documented.50–54 Nature utilizes such a reaction within
the catalytic cycle of carbonic anhydrases, where the terminal
hydroxo group at the four-coordinate ZnII center plays a
crucial role in the reversible hydration of CO2.55 Formation of
a unidentate bicarbonate complex seems to be critical for the
efficiency of that catalytic cycle.56 Some of us have recently
observed that members of a set of planar terminal hydroxo
complexes derived from N,N′-2,6-disubstituted phenyl-2,6-
dicarboxamide pincer ligands react rapidly and completely
with CO2 in DMF to form η1-bicarbonate products
(Scheme 1).3,57 Reactions are second-order with rate constants
(ca. 104–106 M−1 s−1) that tend to decrease as the steric size of
the phenyl R-substituents increases. The most notable feature
is the second-order rate constant kon = 9.5 × 105 M−1 s−1 for
reaction (1) at 298 K (Table 1), extrapolated from measure-
ments at lower temperatures.3 [Ni(pyNMe2)(OH)]1− carries the
smallest ortho-substituent in the set; the unsubstituted ligand
aDepartment of Chemistry and Pharmacy, University of Erlangen-Nürnberg,
Egerlandstr. 1, 91058 Erlangen, Germany.
forms octahedral [Ni(pyN2)2]2− 58
.
E-mail: ivana.ivanovic-burmazovic@fau.de
kon
bDepartment of Chemistry and Chemical Biology, Harvard University, Cambridge,
1ꢁ
½NiðpyN2 ÞðOHÞꢀ1ꢁ þ CO2 Ð ½NiðpyN2 ÞðOCO2HÞꢀ
ð1Þ
Me2
Me2
koff
Massachusetts 02138, USA. E-mail: holm@chemistry.harvard.edu
†Electronic supplementary information (ESI) available: Rate constants for the
CO2 uptake (kon) and decarboxylation (koff) at elevated pressures. See DOI:
10.1039/c3dt53004c
298
on
Comparison of k
for reaction (1) with second-order rate
constants for other systems including model reactions for
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Table 1 Observed equilibrium and kinetic parameters for the reaction
(1) of the R = CH3 complex in DMF
298
K
K
k
2.4 0.2 × 105 M−1 a
1.3 0.1 × 107 M−1 a
9.5 × 105 M−1 s−1 b
3.2 0.5 kcal mol−1 b
−20 3 cal mol−1 Kb
−21 3 cm3 mol−1 a
−13 1 cm3 mol−1 a
eq
223
eq
298
on
ΔH≠on
ΔS≠on
≠223
ΔV
ΔV
on
≠223
Fig. 1 UV/visible spectral changes upon addition of small aliquots of
CO2 in DMF to a 0.1 mM solution of [Ni(pyN2Me2)(OH)]1− in DMF at room
temperature (left). Plot of the concentration changes of the bicarbonate
reaction product ([ML], based on the absorbance change at 381 nm) as
dependent on [CO2]; the red line is a fit to eqn (2) (right).
off
a This work. b Ref. 3.
the Zn(II) enzyme carbonic anhydrase reveals an increase of
≥103 M−1 s−1. Further, under the approximation kon ≈ kcat/KM
the rate constant overlaps the enzyme range of ca. 105–108 M−1
prepared by bubbling CO2 gas at room temperature through
argon-saturated dry DMF in a 10 mL gas-tight Hamilton
syringe. Because the concentration of CO2 required for the
titration was not in a high excess over the complex concen-
tration, spectrophotometric data were analyzed with eqn (2) in
which [ML] is the concentration of the product as a function of
the concentration of CO2 ([L0]), [M0] is the total complex con-
centration and Keq is the equilibrium constant. The change in
the overall volume of a reaction mixture in the course of a titra-
tion is negligible because of the small amount of CO2 titrant
solution added. After the addition of CO2 and prior to an
absorbance measurement, the reaction mixture was allowed to
equilibrate for 20 s.
s−1 55
While precise comparisons cannot be made between
.
synthetic or enzyme systems and reaction (1) owing to solvent
differences (aqueous vs. DMF), it is nonetheless evident that
this reaction is an unusually rapid CO2 fixation process. Such a
rapid CO2 fixation appears to be related to several notable fea-
tures of [Ni(pyN2Me2)(X)]−: presence of the uncommon term-
inal NiII–OH group in the reactant and unidentate η1-OCO2H
bicarbonate in the product complex, despite the four-coordi-
nate ligand environment around the NiII center.3
Although the reversible character of this process has been
qualitatively demonstrated,57 quantification of this equili-
brium and the back decarboxylation reaction has not yet been
reported. Since the efficiency of a CO2 uptake is related to a
CO2 binding constant, the equilibrium constant Keq of reaction
(1) was thermodynamically and kinetically determined in this
study at 298 K and 223 K, respectively, contributing to under-
standing of the overall solution thermodynamics. Moreover,
there are no literature data for binding constants of CO2 to
Ni–OH species. The only binding constant reported is for a
Zn-hydroxo complex at 217 K.53 Kinetic characterization of the
reaction so far has disclosed a low enthalpic barrier and a
large negative entropy of activation consistent with a second-
order reaction (Table 1). However, to fully comprehend the
nature of this rather unique low activation pathway for CO2
insertion, here we sought to determine the pressure effect on
its kinetics, applying unique high-pressure cryo-temperature
stopped-flow measurements at 223 K and 0.5–70 MPa. This is
important not only with respect to the determination of the
volume of activation (ΔV≠) and thus visualization of a reaction
transition state, but also considering an increase in solubility
of CO2 under low temperature and high pressure conditions,
which can have an impact on the efficiency of CO2 sequester-
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
À
Á
2
2
1 þ Keqð½M0ꢀ þ ½L0ꢀÞ ꢁ
1 þ Keqð½M0ꢀ þ ½L0ꢀÞ ꢁ4Keq ½M0ꢀ½L0ꢀ
½MLꢀ ¼
2Keq
ð2Þ
The UV/visible spectra obtained in the course of the titra-
tion are depicted in Fig. 1(left). Observed absorbance changes
and tight isosbestic points at 279 and 395 nm are consistent
with previous results3 describing the expected single-step reac-
tion; the concentration of the product vs. [CO2] is shown in
Fig. 1(right). Analysis of the global spectra data with eqn (2)
leads to K2eq98 = 2.4 0.2 × 105 M−1
.
Using Keq = kon/koff, we estimate the first order rate constant
koff for the back decarboxylation reaction to be 4 s−1 at 298 K.
We have previously demonstrated the reversibility of the
binding reaction by its reversal upon passing dinitrogen
through a solution of [Ni(pyN2Me2)(OCO2H)]1− 3,57
.
The only other quantified equilibrium for CO2 fixation
is bicarbonate formation by CO2 insertion in a Zn–OH bond
(Keq = 6 2 × 103 M−1 in dichloromethane at 217 K, corres-
ponding to ΔG = −3.8 0.2 kcal mol−1).53 Under comparable
low temperature conditions in DMF, reaction (1) is three to five
orders of magnitude more efficient with estimated binding
constants in the 4 × 107 to 8 × 108 M−1 range. The values were
estimated from our Keq value and reaction enthalpies
previously computed with BP86 and B3LYP functionals,
respectively.3
ing on a larger scale.
The compounds (Et4N)[Ni(pyN2Me2)X] (X = OH−, HCO3
)
−
were prepared as described.46‡ To elucidate the reversible
nature of reaction (1) and to determine its equilibrium con-
stant Keq, a spectrophotometric titration was undertaken in
which small aliquots of solutions of CO2 in DMF were added
to a 0.1 mM solution of [Ni(pyN2Me2)(OH)]1− in DMF at 298 K
in a 1 cm path-length septum-capped and gas-tight UV/visible
cuvette. Saturated solutions of CO2 in DMF (0.2 M)59,60 were
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Fig. 4 Plot of ln(koff
)
vs. pressure for the reaction of 0.08 mM
Fig. 2 Spectral changes upon the reaction of 1 mM CO2 and 0.1 mM
[Ni(pyN2Me2)(OH)]1− in DMF at room temperature (left). The kinetics
trace (black) is overlaid with the one-exponential fit to the data (red) for
the same reaction system at 223 K and 0.5 MPa (right).
[Ni(pyN2Me2)(OH)]1− with CO2 in DMF at 223 K (left). Corresponding time
resolved UV/visible spectra in the course of the back decarboxylation
reaction; 100 s observation time (right).
Fig. 3 Plots of the observed rate constants (kobs) as a function of CO2
concentration at different pressures: 0.1 mM [Ni(pyN2Me2)(OH)]1− in DMF
at 223 K (left). Corresponding plot of ln(kon) vs. pressure (right).
Fig. 5 Volume profile for CO2 fixation in reaction (1).
In terms of the mechanism of reaction (1), one can envision
Analysis of the data leads to ΔV≠on = –21 3 cm3 mol−1 for
a transition state that is less or more compact than the reaction (1). This value is entirely consistent with the negative
product η1-OCO2H bicarbonate complex. The former will speak value of ΔS≠ for this reaction (Table 1) and supports an associ-
in favour of a four-coordinate transition state (hydroxo attack ative mechanism where bond breaking does not have a signifi-
on CO2 followed by a proton transfer),61 whereas the latter sup- cant role in the rate-determining step. Unlike many classical
ports a five-coordinate transition state (insertion of a carbon substitution reactions, there is neither a leaving ligand nor a
dioxide C–O bond into the Ni–OH bond).3 This ambiguity is change in charge. Thus, the observed ΔV ≠ corresponds to the
possible to overcome by determining the volume of activation, intrinsic volume change and directly reflects the transition
ΔV ≠, which will offer a clearer picture of the transition state in state at the molecular level. Activation volumes for the
the solution. Given the information potentially forthcoming same type of reaction are reported only for CO2 uptake by
from the effects of applied pressure on reaction rates,62–64 the [M(NH3)5(OH)]2+ complexes in aqueous solution (M = CoIII,
high-pressure kinetics of reaction (1) was investigated. To –10.1 0.6; RhIII, –4.7 0.8; IrIII, –4 1 cm3
M
−1).61 These
determine the activation volume ΔV≠, pressure-dependent values are significantly less negative than that of reaction (1),
stopped-flow spectrophotometry at low temperature was suggesting a more compact transition state in that reaction. By
employed.§ The decrease in absorbance of the NiII complex at way of comparison, the partial molar volume of CO2 in DMF is
450 nm and constant temperature, during the course of CO2 ca. 43 cm3 M−1 65
.
By a dilution experiment (stopped-flow
binding, was monitored within a pressure range of 0.5–70 MPa mixing of [Ni(pyN2Me2)(OCO2H)]1− with degassed DMF at
(Fig. 2).
223 K) at variable pressures we have directly determined the
Reactions were performed under pseudo-first-order con- activation volume for the back decarboxylation reaction ΔVo≠ff
=
ditions, with CO2 concentrations ranging between 0.5 and –13 1 cm3 mol−1 (Fig. 4, Table S1†). This enabled us to con-
1.5 mM and a NiII concentration of 0.1 mM. The temperature struct a volume profile (Fig. 5), which clearly indicates that the
of the reaction cell and the solutions was maintained at {[Ni(pyN2Me2)(OH)]1−⋯CO2}§ transition state is more compact
223 0.1 K. Table S1† contains the resulting second order rate than the product bicarbonate complex. In addition, based
constants (kon) derived from the slopes of plots of the observed on the kinetic parameters at 223 K (Table S1†) the corres-
223
rate constants as a function of CO2 concentration at variable ponding binding constant could be obtained as K
= kon/
eq
pressures (Fig. 3(left)). At least five kinetic runs were per- koff = 1.3 0.1 × 107 M−1, which is in accord with the esti-
formed per data point up to 70 MPa at 223 K. Under these con- mations based on the thermodynamic parameters (vide supra).
ditions, the volume of activation is independent of pressure
Such a volume profile supports a five-coordinate transition
state with bidentate bicarbonate coordination on the way to
the four-coordinate product.
and can be evaluated from the slope of the linear plot (ln(kon
)
vs. pressure)62 in Fig. 3(right).35
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The four-coordinate structure of the starting complex and Based on the determined equilibrium constant of 2.4 0.2 ×
the binding of the dinegative pincer ligand enhance the 105 M−1 at 298 K and 1.3 0.1 × 107 M−1 at 223 K, it seems
nucleophilicity of a bound hydroxide. This favors an initial that the thermodynamics of this process is also highly
nucleophilic attack of a hydroxide and a cycloaddition pathway efficient, though there is limited literature information for
that should promote fast kinetics. The analogous reactions for comparisons.53
the [M(NH3)5(OH)]2+ systems (M = CoIII, RhIII or IrIII)61 have
Our high-pressure kinetic studies revealed a large negative
three orders of magnitude smaller rate constants than that in activation volume for both the CO2 uptake and the back
this study. This parallels a significantly lower nucleophilicity decarboxylation reaction. These results support a very compact
of OH− coordinated to the +3 metal centers within the posi- transition state, which can be visualized as a five-coordinate
tively charged [M(NH3)5(OH)]2+ complexes in comparison to species with a bicarbonate bound in a bidentate fashion. The
the nucleophilicity of OH− within negatively charged [Ni- observed character of the transition state, in accord with com-
(pyN2Me2)(OH)]1−. The steric protection of the activated hydrox- putational results,3 explains a very low activation enthalpy and
ide and the increased electron density caused by the pincer a negative activation entropy consistent with rapid CO2 fix-
ligand control the reaction thermodynamics and leads to a ation. Reaction (1) is currently the most thoroughly character-
stable η1-bound product. These collective features render CO2 ized carbon dioxide fixation reaction by a metal hydroxide
fixation by [Ni(pyN2Me2)(OH)]1− kinetically and thermodynami- complex, and among the more thoroughly examined carbon
cally favorable. In addition to reaction (1),3 the enthalpic reac- dioxide fixation reactions of any type.
tion coordinate has also been computed by DFT methods
for the slower reaction of CO2 with [Ni(pyN2iPr2)(OH)]1− (kon
=
5.1 × 104 M−1 s−1, ΔS≠ = −27 cal mol−1 K−1 in DMF).57 Results
for both systems indicate a mechanism proceeding by a
nucleophilic attack at the carbon atom of CO2 by a hydroxide,
formation of a cyclic transition state as depicted in Fig. 5,
rupture of one Ni–O bond, and reorientation of the resulting
monodentate bicarbonate by a Ni–O–C pivot motion. The
predicted final perpendicular orientation of the bicarbonate
relative to the NiN3 plane is observed in crystalline salt of the
reaction products.46 The herewith visualized mechanism
differs from that reported for [M(NH3)5(OH)]2+ (M = CoIII, RhIII
or IrIII),61 where the rate-determining step in decarboxylation
proceeds according to a dissociative mechanism that involves
breakage of the oxygen–carbon bond on the carbonate ligand.
In contrast, our back reaction has a clearly associative charac-
ter, involving a Ni–O bond formation and closure of a chelate
bicarbonate ring. We should point out that the reactions
between CO2 and [M(NH3)5(OH)]2+ operate in aqueous solu-
tions, where a hydrogen-bonding network assists in proton
transfer from the bound hydroxide to one of the CO2 oxygen
atoms. On the other hand, our reaction operates in DMF,
within a hydrophobic environment of the sterically bulky
ligand, and there is no possibility that the CO2 oxygen atoms
can be involved in some prominent hydrogen-bonding inter-
actions. This makes the CO2 oxygen atoms even more nucleo-
philic and prone to bind to the NiII center, resulting in a five-
coordinate transition state. This may also explain the fact that
our interpretation of the overall volume profile is consistent
with, and may be said to support, the computed reaction
pathway, although these computational models do not address
hydrogen bonding.3
Acknowledgements
I.I.-B. and O.T. gratefully acknowledge support from the “Solar
Technologies Go Hybrid” initiative of the State of Bavaria.
Research at Harvard University was supported by NIH grant
GM-28856.
Notes and references
‡All manipulations and measurements were performed under a pure argon
atmosphere. All reagents and solvents were purchased from commercial sources
(Sigma-Aldrich, Acros Organics) and were of p.a. grade and used without purifi-
cation. DMF was purchased as an extra dry solvent and was stored in a sure-seal
bottle under argon and molecular sieves. All solutions were prepared in dry DMF
in an MBraun inert atmosphere glove box.
§Measurements were performed using a Hi-Tech Scientific HPSF 56 stopped-
flow instrument externally coupled to a Huber CC905 thermostat. UV/visible
spectra were recorded using an Energetiq LDLS ENQ EQ-99 FC laser-driven light
source coupled to an Applied Photo Physics photomultiplier. Solutions in DMF
were prepared immediately before measurement. Lower concentrations of CO2
were obtained by dilution of a saturated solution in graduated gas-tight syringes.
Neither the solution of [Ni(pyN2Me2)(OH)]1− nor the CO2 solution was exposed to
the atmosphere.
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