Reduction of CO2 to CO using low-coordinate iron: Formation of a four-coordinate iron dicarbonyl complex and a bridging carbonate complex

Article abstract of DOI:10.1021/ic701914m

A reduced diiron(I) complex reacts with CO₂ to give two iron-containing products. One product has a carbonate bridge, which isomerizes rapidly at -70°C and may be derived from an oxodiiron intermediate. The formation of this product releases free CO, which leads to a four-coordinate iron dicarbonyl complex. This product is the first crystallographically characterized example of a four-coordinate iron dicarbonyl species, a moiety that may be present in the active site of Hmd ( iron-sulfur cluster free ) hydrogenase.

Full text of DOI:10.1021/ic701914m

Inorg. Chem. 2008, 47, 784−786
Reduction of CO2 to CO Using Low-Coordinate Iron: Formation of a
Four-Coordinate Iron Dicarbonyl Complex and a Bridging Carbonate
Complex
Azwana R. Sadique, William W. Brennessel, and Patrick L. Holland*
Department of Chemistry, UniVersity of Rochester, Rochester, New York 14627
Received September 27, 2007
A
reduced diiron(I) complex reacts with CO2 to give two
iron-containing products. One product has a carbonate bridge,
which isomerizes rapidly at 70 C and may be derived from an
as the major product a diiron(II) complex with a Fe(µ-CO)-
(µ-O)Fe core. Herein, we report the reaction of CO2 with
another iron(I) system, which gives a different outcome but
may use a similar mechanism.
We have described the iron-dinitrogen complex
LtBuFeNNFeLtBu, where LtBu ) 2,2,6,6-tetramethyl-3,5-bis-
[(2,6-diisopropylphenyl)imino]hept-4-yl.8 Spectroscopic and
−
°
oxodiiron intermediate. The formation of this product releases free
CO, which leads to a four-coordinate iron dicarbonyl complex. This
product is the first crystallographically characterized example of a
four-coordinate iron dicarbonyl species, a moiety that may be
present in the active site of Hmd (“iron
hydrogenase.
magnetic studies are consistent with the assignment of its
−sulfur cluster free”)
2- 9
iron atoms as FeII and the dinitrogen ligand as N2
.
However, in its reactivity, it generally behaves as a source
of the iron(I) fragment LtBuFe.8b,10 For example, it reacts with
neutral ligands C6H6 and PR3 to give monomeric iron(I)
complexes, and it reductively couples ketones and aldehydes
to give diiron(II) pinacolate complexes.8b
Carbon dioxide plays an important role in synthetic and
biological chemistry as a one-carbon building block.1 Despite
the large bond enthalpy of the CdO double bond in CO2
(532 kJ/mol),2 nature is capable of using first-row late
transition metal ions to reduce CO2 to CO in the enzymes
acetyl coenzyme A synthase/CO dehydrogenase3 and nitro-
genase.4 This is also the reverse of the famous water-gas
shift reaction.5 Interest in the reduction of CO2 to CO has
led to the investigation of numerous schemes for achieving
the reduction of CO2.1 Transition metal promoted reduction
The treatment of LtBuFeNNFeLtBu with 2 equiv of dry CO2
in pentane, benzene, or diethyl ether affords a mixture of
the dicarbonyliron(I) compound LtBuFe(CO)2 (1) and the
bridging carbonatodiiron(II) compound LtBuFe(µ-OCO2)-
FeLtBu (2). The 1H NMR spectrum of the crude mixture from
the reaction of LtBuFeNNFeLtBu and CO2 shows the formation
(5) (a) King, R. B.; King, A. D.; Yang, D. B. ACS Symp. Ser. 1981, 152,
123-132. (b) Ford, P. C. Electrochem. Electrocatal. React. Carbon
Dioxide 1993, 68-93. (c) Kochloefl, K. Water Gas Shift and COS
Removal. In Handbook of Heterogeneous Catalysis; Ertl, G. et al.,
Eds.; Vol. 4; VCH: Weinheim, Germany, 1997; pp 1831-1843.
(6) (a) Fachinetti, G.; Floriani, C.; Chiesi-Villa, A.; Guastini, C. J. Am.
Chem. Soc. 1979, 101, 1767-1775. (b) Lee, G. R.; Maher, J. M.;
Cooper, N. J. J. Am. Chem. Soc. 1987, 109, 2956-2962. (c) Alvarez,
R.; Atwood, J. L.; Carmona, E.; Perez, P. J.; Poveda, M. L.; Rogers,
R. D. Inorg. Chem. 1991, 30, 1493-1499.
(7) Lu, C. C.; Saouma, C. T.; Day, M. W.; Peters, J. C. J. Am. Chem.
Soc. 2007, 129, 4-5. This paper reports an oxalate product as well,
but in the work presented here, there is no sign of oxalate formation.
(8) (a) Smith, J. M.; Lachicotte, R. J.; Pittard, K. A.; Cundari, T. R.; Lukat-
Rodgers, G.; Rodgers, K. R.; Holland, P. L. J. Am. Chem. Soc. 2001,
123, 9222-9223. (b) Smith, J. M.; Sadique, A. R.; Cundari, T. R.;
Rodgers, K. R.; Lukat-Rodgers, G.; Lachicotte, R. J.; Flaschenriem,
C. J.; Vela, J.; Holland, P. L. J. Am. Chem. Soc. 2006, 128, 756-
769.
2-
of CO2 to give CO and CO3 has been observed in many
systems, and this type of transformation (eq 1) is known as
reductive disproportionation.6
2-
2CO2 + 2e- f CO + CO3
(1)
Recently, Peters reported the reductive cleavage of CO2
by a low-coordinate iron(I) system.7 This reaction afforded
* To whom correspondence should be addressed. E-mail: holland@
chem.rochester.edu.
(1) Reviews: (a) Arakawa, H.; et al. Chem. ReV. 2001, 101, 953-996.
(b) Louie, J. Curr. Org. Chem. 2005, 9, 605-623. (c) Omae, I. Catal.
Today 2006, 115, 33-52. (d) Darensbourg, D. J. Chem. ReV. 2007,
107, 2388-2410. (e) Aresta, M.; Dibenedetto, A. Dalton Trans. 2007,
2975-2992.
(9) Stoian, S. A.; Vela, J.; Smith, J. M.; Sadique, A. R.; Holland, P. L.;
Mu¨nck, E.; Bominaar, E. L. J. Am. Chem. Soc. 2006, 128, 10181-
10192.
(2) Heats of formation taken from: CRC Handbook, 87th ed.; http://
www.hbcpnetbase.com/.
(3) (a) Ragsdale, S. W.; Kumar, M. Chem. ReV. 1996, 96, 2515-2539.
(b) Drennan, C. L.; Doukov, T. I.; Ragsdale, S. W. J. Biol. Inorg.
Chem. 2004, 9, 511-515. (c) Hegg, E. L. Acc. Chem. Res. 2004, 37,
775-783.
(10) (a) Vela, J.; Stoian, S.; Flaschenriem, C. J.; Mu¨nck, E.; Holland, P.
L. J. Am. Chem. Soc. 2004, 126, 4522-4523. (b) Yu, Y.; Smith, J.
M.; Flaschenriem, C. J.; Holland, P. L. Inorg. Chem. 2006, 45, 5742-
5751. (c) Sadique, A. R.; Gregory, E. A.; Brennessel, W. W.; Holland,
P. L. J. Am. Chem. Soc. 2007, 129, 8112-8121.
(4) Rasche, M. E.; Seefeldt, L. C. Biochemistry 1997, 36, 8574-8585.
784 Inorganic Chemistry, Vol. 47, No. 3, 2008
10.1021/ic701914m CCC: $40.75
© 2008 American Chemical Society
Published on Web 01/03/2008

COMMUNICATION
Compound 1 is interesting because it is the first example
of a four-coordinate dicarbonyl complex of iron. This
structural feature has been postulated to be present in the
active site of the “Fe-S cluster free” hydrogenase Hmd, on
the basis of IR, Mo¨ssbauer, and X-ray absorption data.12 IR
spectra of the enzyme showed two bands of identical
intensity, leading to an estimate of the C-Fe-C angle of
86-94° based on the literature relationship tan θ ) Iasymm
/
12a,13
Isymm
.
However, 1 shows virtually identical intensities
for the two CO stretching vibrations despite the C-Fe-C
angle of 81.44(8)° in the X-ray crystal structure. On this
basis, we suggest that a broader range of CO angles should
be considered as possible in the enzyme.
The use of 1 for further comparison to the enzyme is
limited because the Fe atom in Hmd hydrogenase is
diamagnetic, either low-spin FeII or low-spin Fe0, in contrast
to the FeI oxidation state in 1. So, although 1 is not an
electronic match for the active site of Hmd, its structure is
the best mimic yet known. The redox chemistry of 1 is clearly
of interest and will be reported in the future.
Figure 1. Crystal structure of 1. Thermal ellipsoids are shown at 50%
probability. H atoms have been omitted for clarity. Fe1-C1 ) 1.784(2) Å,
Fe1-C2 ) 1.785(2) Å, Fe1-N11 ) 1.967(1) Å, C1-O1 ) 1.151(2) Å,
C2-O2 ) 1.149(2) Å, N11-Fe1-N21 ) 93.43(5)°, C1-Fe1-C2 )
81.44(8)°, O1-C1-Fe1 ) 172.8(1)°, O2-C1-Fe1 ) 172.9(2)°.
of 2 as 70 ( 5% of the total iron (using an internal integra-
tion standard). The theoretical stoichiometry of this reaction
is shown in eq 2, and the observed spectroscopic yield
of 2 agrees fairly well with the 80% expected. Because 1
is not NMR-active (see below), its presence was deduced
by the observation of CO stretching vibrations at 1996 and
1917 cm-1 in the solid-state IR spectrum of the crude
product.
The other product of CO2 reduction, LtBuFe(µ-OCO2)FeLtBu
(2), could be isolated in pure form by performing the reaction
of LtBuFeNNFeLtBu and CO2 in the presence of Pd/C to trap
the CO. The crude product of this reaction shows no carbonyl
stretching bands in the IR spectrum. The ability to divert
the reaction shows that free CO is involved in the formation
of complex 1.
5LtBu FeNNFeLtBu + 8CO2 f
t
t
t
2LBu Fe(CO)2 4LBu Fe(CO3)FeLBu
+
(2)
1
2
1
Complex 2 has been characterized by H NMR and IR
In this reaction, eight Fe atoms are oxidized from FeI to
FeII, concomitant with the reduction of eight molecules of
CO2. This creates four carbonate anions that bind to FeII, as
well as four molecules of CO that are trapped by FeI.
Compound 1 can be synthesized independently by reacting
LtBuFeNNFeLtBu with 4 equiv of CO in diethyl ether.
Using excess CO results in a mixture of LtBuFe(CO)2 and
LtBuFe(CO)3, as described in a recent publication.8b In that
work, we were frustrated by the cocrystallization of these
dicarbonyl and tricarbonyl complexes, and an X-ray crystal
structure showed a superposition of these two molecules.
Here, use of the appropriate stoichiometry of starting
materials gives the pure dicarbonyliron(I) complex, with CO
stretching vibrations at 1994 and 1915 cm-1 in the solid-
spectroscopies, X-ray diffraction, and elemental analysis. The
IR spectrum of 2 shows two strong bands at 1493 and 1481
cm-1 that are characteristic of a bridging carbonate ligand.14
The solution magnetic moment of 6.0(3) µB/molecule in C6D6
suggests that the high-spin FeII centers in 2 have very weak
magnetic coupling. The solid-state structure of 2 has two
different molecules in the unit cell (Figure 2). In one
molecule (A), the bridging carbonate is coordinated in a µ-η2:
η1 mode, and in the other (B), it is coordinated in a µ-η2:η2
mode to the metal centers. The bond lengths and bond angles
of these two molecules are compared in Table 1. Structural
type A is unprecedented for a carbonate bridge on iron but
has been seen with other metals.15 Structural type B has
precedent in a tris(pyrazolyl)boratoiron(II) complex, which
was derived from a metal hydroxide and CO2.16
1
state IR spectrum.11 Complex 1 has a low-spin d7 (S ) /2)
electronic configuration, as shown by electron paramagnetic
resonance (EPR) spectroscopy in the solution and solid
state.8b,11 The crystal structure of 1 is illustrated in Figure
1. The Fe atom has a square-planar geometry [sum of the
angles around Fe ) 361.8(1)°]. The C-O bond lengths are
1.149(2) and 1.151(2) Å, and the Fe-C-O units are slightly
bent [172.8(1)° and 172.9(2)°].
1
The H NMR spectrum of 2 in C6D6 shows seven peaks,
which is characteristic of the protons on â-diketiminate
ligands in local C2V symmetry. There is no decoalescence in
the temperature range of +70 to -70 °C. This implies that
there are at least two rapid dynamic processes. First, the
(12) (a) Lyon, E. J.; Shima, S.; Boecher, R.; Thauer, R. K.; Grevels,
F.-W.; Bill, E.; Roseboom, W.; Albracht, S. P. J. J. Am. Chem. Soc.
2004, 126, 14239-14248. (b) Shima, S.; Lyon, E. J.; Thauer, R. K.;
Mienert, B.; Bill, E. J. Am. Chem. Soc. 2005, 127, 10430-10435.
(13) Beck, W.; Melnikoff, A.; Stahl, R. Angew. Chem., Int. Ed. 1965, 4,
692-693.
(14) Palmer, D. A.; Van Eldik, R. Chem. ReV. 1983, 83, 651-731.
(15) For example, see: Chatt, J.; Kubota, M.; Leigh, G. J.; March, F. C.;
Mason, R.; Yarrow, D. J. J. Chem. Soc., Chem. Commun. 1974, 1033-
1034.
(11) In ref 8b, we reported bands at 2036, 2000, 1967, 1953, and 1922
cm-1 in a pentane solution of LtBuFe(CO)n (n ) 2 and 3) and at 2042,
1971, and 1960 in a pentane solution of LMeFe(CO)3. This implies
that the 2000 and 1922 cm-1 solution bands are from LtBuFe(CO)2,
consistent with the solid-state bands at 1994 and 1915 cm-1 in solid
1 as isolated here. The IR spectrum of a solution of 1 in pentane shows
that, in solution, 1 undergoes partial disproportionation to form some
LtBuFe(CO)3. This disproportionation in solution is also evident in
X-band EPR spectra at 77 K. See the Supporting Information for more
detail.
(16) Kitajima, N.; Hikichi, S.; Tanaka, M.; Moro-oka, Y. J. Am. Chem.
Soc. 1993, 115, 5496-5508.
Inorganic Chemistry, Vol. 47, No. 3, 2008 785

COMMUNICATION
Scheme 1. Proposed Mechanism for the Formation of 1 and 2
reductive cleavage of CO2 to give CO and O2-. While this
work was in progress, Peters reported an iron(I) complex
that reacts with 0.5 equiv of CO2 to give a diiron(II) complex
with an Fe(µ-CO)(µ-O)Fe core.7 This outcome corresponds
to direct reductive cleavage of the CdO bond. Earlier low-
valent iron complexes also give the overall reductive cleavage
of CO2.17 In our system, we have reported the isolation of
the oxodiiron(II) complex [LtBuFe]2O, which would be the
analogous oxo intermediate (Scheme 1).18 This hindered oxo
complex binds a number of Lewis bases reversibly, so it is
reasonable to assume that it could lose CO, at least tran-
siently. Although we have no direct spectroscopic evidence
that [LtBuFe]2O is formed during the course of the reaction
of CO2 and LtBuFeNNFeLtBu, the treatment of isolated
[LtBuFe]2O with CO2 in C6D6 rapidly yields 2, as shown by
1H NMR spectroscopy. Therefore, the oxo complex is
kinetically competent to be an intermediate in the for-
mation of 2. As shown above, CO reacts rapidly with
LtBuFeNNFeLtBu to give 1 in independent experiments; fur-
ther, the formation of 1 can be suppressed by trapping CO
with Pd/C. Therefore, Scheme 1 represents a reasonable
mechanism for this interesting new CO2 reduction reaction.
In conclusion, CO2 can be reduced cleanly to CO and car-
Figure 2. Crystal structure of 2, showing molecules A and B with different
bridging modes. Thermal ellipsoids are shown at 50% probability. H atoms
and isopropyl groups have been omitted for clarity. See Table 1 for metrical
parameters.
Table 1. Bond Lengths (Å) and Bond Angles (deg) of Fe(µ-CO3)Fe
Cores in the Two Independent Molecules Found in the X-ray Crystal
Structure of 2
A
B
A
B
Fe1-O1
Fe1-O2
Fe2-O3
Fe2-O2
2.079(1) 2.079(1) C1-O1
2.034(1) 2.107(1) C1-O2
1.881(1) 1.982(1) C1-O3
3.156(1) 2.358(1)
1.270(2) 1.256(2)
1.291(2) 1.323(2)
1.293(2) 1.275(2)
O1-Fe1-O2 64.52(4) 63.53(4) O1-C1-O3 120.9(1) 126.2(1)
2-
O2-Fe2-O3
60.45(4) O2-C1-O3 121.0(1) 116.4(1)
bonate at a diiron site. The CO (1) and CO3 (2) con-
O1-C1-O2 118.1(1) 117.4(1)
taining products are notable because of a structural similarity
between 1 and a newly discovered hydrogenase enzyme and
an interesting dynamic process in 2. The formation of 2 may
proceed through the intermediacy of an oxodiiron(II) com-
plex.
bridging carbonate ligand must exchange between binding
modes A and B rapidly, making the two metals equivalent
on the NMR time scale. Second, the equivalence of the two
faces of the â-diketiminate ligand indicates that there is rapid
rotation of the Fe atom around the Fe-O bond, probably in
the Fe ion with η1-carbonate binding.
Acknowledgment. This work was supported by the
National Institutes of Health (Grant GM065313).
There are multiple possibilities for the mechanism through
which 1 and 2 are formed. One is the reductive dispropor-
tionation of CO2, a series of one-electron reductions that is
well-established in electrochemical and other reactions with
reduced transition-metal complexes.1 In other systems,
reductive disproportionation is thought to proceed through
the intermediacy of the radical [OdC-O-C(dO)-O]-, for
which we have no evidence here. Another possibility is the
Supporting Information Available: Synthetic, spectroscopic,
and crystallographic details. This material is available free of charge
via the Internet at http://pubs.acs.org.
IC701914M
(17) Karsch, H. H. Chem. Ber. 1977, 110, 2213-2221.
(18) Eckert, N. A.; Stoian, S.; Smith, J. M.; Bominaar, E. L.; Mu¨nck, E.;
Holland, P. L. J. Am. Chem. Soc. 2005, 127, 9344-9345.
786 Inorganic Chemistry, Vol. 47, No. 3, 2008