The facile reduction of carbon dioxide to carbon monoxide with an amido-digermyne

Full text of DOI:10.1002/anie.201203607

Angewandte
Chemie
DOI: 10.1002/anie.201203607
Green Chemistry
The Facile Reduction of Carbon Dioxide to Carbon Monoxide with an
Amido-Digermyne**
Jiaye Li, Markus Hermann, Gernot Frenking,* and Cameron Jones*
The steady increase in the atmospheric concentration of the
greenhouse gas, CO2, since the industrial revolution is thought
to be the main cause of recent increases in global temper-
atures.[1] The future implications of this phenomenon are
driving significant current efforts to develop efficient methods
for the sequestration of CO2,[2] and for its use as a C1
feedstock in the formation of useful chemicals.[3] With
regard to the latter, the reduction of CO2 to CO is of
particular interest as carbon monoxide can be used as a fuel or
as a chemical feedstock in its own right. However, CO2
myne,
[LGe-GeL]
1
(L = N(Ar*)(SiMe3),
Ar* =
C6H2{C(H)Ph2}2Me-2,6,4).[14] Unlike all previously reported
ꢀ
bulky aryl-substituted digermynes which have Ge Ge multi-
ple bonds,[15,16] compound
1
has an extremely long
ꢀ
(2.7093(7) ꢀ) Ge Ge single bond, which is thought to be
the basis of its very narrow HOMO–LUMO energy gap
(0.62 eV, calculated using RI-BP86/def2-SVP; HOMO/
LUMO = highest occupied/lowest unoccupied molecular
orbital). The resultant high reactivity of 1 has been demon-
strated by the fact that it quantitatively and rapidly activates
dihydrogen, to give [LGeGe(H)2L], at temperatures as low as
ꢀ108C, both in solution and the solid state. It seemed
reasonable to us that 1 could participate in other small
molecule activations that have not been previously achieved
with germanium compounds, and which are normally thought
to be the realm of transition-metal systems. In this respect,
here, we show that the digermyne facilely and quantitatively
reduces CO2 to CO at temperatures as low as ꢀ408C. The
mechanism of this reduction has been explored using
spectroscopic and computational techniques, and the reduc-
tions of the CO2 analogs, CS2 and tBuNCO, are also described.
A toluene solution of 1 was exposed to an excess of dry
CO2 at ꢀ708C, and the reaction vessel was sealed. Upon
warming, the solution slowly lost the deep purple color of the
digermyne at about ꢀ608C and became orange-brown. From
about ꢀ40 to ꢀ308C the color of the solution changed to pale
yellow, and remained that color when warmed to room
temperature. At that point, an IR spectroscopic analysis of
the head space gas of the reaction vessel confirmed the
presence of CO (n˜ = 2143 cmꢀ1). Subsequent removal of
volatiles from the mixture afforded an essentially quantitative
yield of the novel bis(germylene) oxide 2 (Scheme 1). To
ascertain the yield of generated CO, the method of Baceiredo
reduction is problematic because of the considerable strength
ꢀ1 [4]
=
of its O C(O) bond (532 kJmol ), and for kinetic rea-
sons.[3] With that said, CO2 reduction has been achieved,
either stoichiometrically or catalytically, in nature (e.g. with
CO dehydrogenase/acetyl-coenzyme A synthase)[5] and in the
laboratory by use of photolytic,[6] electrochemical,[7] or metal-
based oxygen abstraction protocols.[8]
While the vast majority of methodologies for the reduc-
tion of CO2 to CO require d- or f-block metal-containing
materials to proceed, a handful of low oxidation state p-block
compounds have recently been shown to effect CO2 reduc-
tions at room temperature. These include three-coordinate,
intramolecularly donor-stabilized (D!) silylenes (R2(D!
!
)Si:),[9,10] a disilyne (R(D!)SiSi( D)R),[10] and N-hetero-
cyclic carbenes.[11] Moreover, a small number of normal
oxidation state p-block systems, for example, Al/P-based
“frustrated Lewis pairs”, have been reported to reduce CO2 to
CO.[12] The reduction reactions involving these compounds
exemplify the rapidly emerging interest in the “transition-
metal-like” reactivity of main group compounds.[13] We have
become involved in this area with, for example, the prepara-
tion of the bulky amido-substituted two-coordinate diger-
[*] J. Li, Prof. C. Jones
School of Chemistry, Monash University
PO Box 23, Melbourne, VIC, 3800 (Australia)
E-mail: cameron.jones@monash.edu
M. Hermann, Prof. Dr. G. Frenking
Fachbereich Chemie, Philipps-Universitꢀt Marburg
35032 Marburg (Germany)
E-mail: frenking@chemie.uni-marburg.de
[**] We thank the Australian Research Council (C.J.), The U.S. Air Force
Asian Office of Aerospace Research and Development (grant
FA2386-11-1-4110 to C.J.), and the Deutsche Forschungsgemein-
schaft. The EPSRC is also thanked for access to the UK National
Mass Spectrometry Facility.
Supporting information for this article, including full synthetic and
spectroscopic details for compounds 2–5, crystallographic data for
2–4, and full details and references for the DFT calculations, is
available on the WWW under http://dx.doi.org/10.1002/anie.
201203607.
Scheme 1. Synthesis of compounds 2–5.
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and co-workers was employed.[10] That is, the reaction was
repeated in a vessel that was connected through a gas bridge
to another vessel containing half an equivalent of [RhCl-
(COD)(IPrMe)]
(COD = cycloocta-1,5-diene;
IPrMe =
:C{N(iPr)C(Me)}2) in a [D6]benzene solution. The sealed
two-component vessel was allowed to stand for 48 h at 208C,
1
after which time an H NMR spectroscopic analysis of the
[D6]benzene solution revealed the quantitative conversion of
[RhCl(COD)(IPrMe)] to [RhCl(CO)2(IPrMe)] and free
COD (see the Supporting Information for further details).
Accordingly, the reduction of CO2 to CO by 1 was confirmed
to be essentially quantitative.[17]
A series of related reactions were subsequently inves-
tigated for the purpose of comparison (Scheme 1). First,
compound 2 was found to be alternatively accessible in high
yield by treatment of 1 with excess N2O. Interestingly, the
germanium(II) centers of 2 are not further oxidized by N2O.
This outcome contrasts with the reaction of Powerꢁs multiply
bonded digermyne, Ar’GeGeAr’ (Ar’ = C6H3(C6H3iPr2-2,6)2-
2,6), with N2O, which yields a cyclic germanium(IV) peroxo-
species.[18] The reaction of 1 with an excess of CS2 proceeded
rapidly at ꢀ708C and afforded a high yield of the pale yellow
bis(germylene) sulfide 3 after subsequent warming to ambient
temperature. Treatment of 1 with the isocyanate, tBuNCO,
gave a mixture of products which included significant
quantities of 2, but no observable tBuNC. It was thought
that if tBuNC was generated in this reaction it could compete
with tBuNCO for reaction with 1. This proposal was assessed
by reacting 1 directly with tBuNC, which gave a good yield of
the green reductively coupled product 4. Compound 4 was
subsequently found to be a component of the tBuNCO/
digermyne reaction mixture (as determined by NMR spec-
troscopy). Therefore, it seems likely that tBuNC is generated
in that reaction, but is rapidly consumed by reduction with 1.
While reductive coupling reactions of isonitriles with low
oxidation state main group complexes are not uncommon,[19]
the reaction of tBuNC with Powerꢁs digermyne, Ar’GeGeAr’,
yielded only the 1:1 adduct, [Ar’GeGe(Ar’)(CNtBu)].[18] This
could indicate that 1 is both more electrophilic and more
reducing towards isonitriles than Ar’GeGeAr’.
Figure 1. Molecular structures of compounds a) 2 and b) 3 (25%
ellipsoids; hydrogen atoms are omitted). Relevant bond lengths (ꢁ)
and angles (8). 2: Ge(1)-O(1) 1.8088(16), Ge(1)-N(1) 1.8784(19),
Ge(2)-O(1) 1.8154(15), Ge(2)-N(2) 1.8749(19), O(1)-Ge(1)-N(1)
100.95(8), O(1)-Ge(2)-N(2) 97.50(8), Ge(1)-O(1)-Ge(2) 122.30(9). 3:
Ge(1)-N(1) 1.868(2), Ge(1)-S(1) 2.2854(8), Ge(2)-N(2) 1.877(2),
Ge(2)-S(1) 2.2869(8), N(1)-Ge(1)-S(1) 99.41(7), N(2)-Ge(2)-S(1)
99.73(7), Ge(1)-S(1)-Ge(2) 98.21(3).
bis(silylene) oxide complex, [(COD)Ni{Si(amid)}2O], amid =
(tBuN)2CPh).[22] The molecular structure of 4 (Figure 2)
reveals the compound to be a dimeric digermabicycle with
pyramidal germanium(II) centers, the lone pairs of which are
The spectroscopic data for 2–4 are fully consistent with
their solid-state structures (see the Supporting Information
for details). Compounds 2 and 3 represent the first crystallo-
graphically characterized examples of two-coordinate, amido-
substituted bis(germylene) chalcogenides (Figure 1), though
two related three-coordinate examples, [{(amid’)Ge}2E] (E =
O or S, amid’ = {(C6H3iPr2-2,6)N}2CtBu), and a two-coordi-
nate aryl-substituted complex, [Ar’GeOGeAr’], have been
reported.[20] The two compounds in the current study are
broadly isostructural, though it is noteworthy that they exhibit
markedly different Ge-E-Ge angles (E = O: 122.30(9)8; S:
98.21(3)8), in line with the lesser propensity of sulfur, relative
to oxygen, to undergo hybridization. The Ge–E distances in
both compounds are in the normal range for single-bond
interactions,[21] while their Ge–N distances are close to those
in 1 (1.872(2) ꢀ). It is interesting that the germanium lone
pairs in 2 and 3 adopt a cis disposition relative to each other,
and therefore the compounds have the potential to act as
chelating Ge-donor ligands (compare the three-coordinate
Figure 2. Molecular structure of compound 4 (25% ellipsoids; hydro-
gen atoms are omitted). Relevant bond lengths (ꢁ) and angles (8).
Ge(1)-N(1) 1.887(3), Ge(1)-C(73) 2.087(11), Ge(1)-N(4) 2.371(7),
Ge(2)-N(2) 1.885(3), Ge(2)-C(74) 2.071(11), Ge(2)-N(3) 2.358(6),
N(3)-C(73) 1.279(12), N(4)-C(74) 1.286(12), C(73)-C(74) 1.476(17),
C(73)-Ge(1)-N(4) 60.1(4), N(1)-Ge(1)-N(4) 107.42(15), N(1)-Ge(1)-
C(73) 105.3(3), C(74)-Ge(2)-N(3) 60.0(3), N(2)-Ge(2)-N(3) 108.34(15),
N(2)-Ge(2)-C(74) 109.3(2), N(3)-C(73)-C(74) 107.6(12), N(4)-C(74)-
C(73) 108.6(11).
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in cis positions relative to each other, as in 2 and 3. The
geometry of the central coupled isonitrile unit is indicative of
an essentially localized 1,4-diazabutadiene-2,3-diyl fragment,
that is (N = C-)2, the N centers of which have markedly longer
dative interactions with the Ge atoms than the exocyclic
amide nitrogen atoms.
depicted in Figure 3. We located two transition states, TS1-
2a and TS1-2b, for the initial stage of the reaction, which
involve a side-on approach of the CO2 molecule to one or
both Ge centers. While these transition states lead to the same
ꢀ
intermediate, IM2, through the insertion of CO2 into the Ge
Ge bond, the energetically lowest lying entrance channel
(activation barrier of DG° = 17.0 kcalmolꢀ1) is through TS1-
2a. Intermediate, IM2, then rearranges through a low-energy
transition state (TS2-3, DG° = 4.1 kcalmolꢀ1) to give the
trans-germacarboxylato germanium(II) amide complex, IM3,
which is 5.0 kcalmolꢀ1 lower in energy than IM2. The
elimination of CO from this to give 2, then proceeds with
an energy barrier (DG° = 16.3 kcalmolꢀ1) similar to that for
the entrance channel. The overall CO2 reduction reaction is
exergonic by only ꢀ17.7 kcalmolꢀ1. Because of the relatively
low energy of intermediate IM3, and the calculated barrier to
its elimination of CO, we conclude that the unsymmetrical,
spectroscopically observed reaction intermediate resembles
that compound (i.e. 5 in Scheme 1). Indeed, insertions of CO2
into metal–metal bonds to give metalla-carboxylate com-
plexes (compare 5) are not uncommon,[3] and the low-field
13C NMR resonance for 5 is in the expected region for such
complexes. This conclusion is supported by calculation of the
13C NMR chemical shift for a model compound of 5, that is
IM4, in which the amido groups (NMe2) have methyl
substituents. The calculated value at GIAO/MP2/TZVPP of
d = 216.3 ppm is in good agreement with the experimental
value of d = 222.8 ppm.
To shed light on the mechanism of the reduction of CO2 by
1, and the temperature-dependent color changes of the
reaction mixture, the reaction was followed by 1H and
13C NMR spectroscopy ([D8]toluene solution) from ꢀ70 to
308C (see the Supporting Information for NMR spectra). No
reaction occurred at ꢀ708C, but upon warming to ꢀ608C
signals for a new, unsymmetrical compound with chemically
inequivalent amide ligands began to appear. After warming to
ꢀ508C the conversion of 1 to this compound was largely
complete. Although no signal was evident for free CO in the
13C NMR spectrum of the mixture at this temperature, a low-
field resonance (d = 222.8 ppm) had appeared. The unsym-
metrical compound was stable until ꢀ408C, whereupon it
began converting to symmetrical 2. This conversion became
more rapid at ꢀ308C, and was complete by ꢀ208C. There was
essentially no further change in the NMR spectra up to 308C.
At this temperature, the signal at d = 222.8 ppm in the
13C NMR spectrum had vanished, and one for free CO (d =
184.5 ppm) had appeared. The results of this NMR experi-
ment strongly suggest that the reduction of CO2 proceeds
through an unsymmetrical, metastable intermediate.
Unfortunately, all attempts to isolate crystalline samples of
this intermediate at low temperatures proved fruitless.
So as to elucidate the nature of this intermediate, the free-
energy profile of the reaction of 1 with CO2 was calculated
using density functional theory with inclusion of dispersion
interactions (BP86-D3/def2-TZVPP//BP86/SVP), and is
In summary, the quantitative reduction of CO2 to CO by
a digermyne at temperatures as low as ꢀ408C has been
achieved. Moreover, strong evidence for the mechanism of
the reaction has been gained from a combination of spectro-
scopic and computational studies. There is little precedent for
Figure 3. Free-energy profile of the reaction of 1 with CO2 (BP86-D3/def2-TZVPP//BP86/def2-SVP). Selected interatomic distances (ꢁ), bond
angles (8), and NGeGeN torsion angles, q (8), are given with the molecular structures.
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[11] L. Gu, Y. Zhang, J. Am. Chem. Soc. 2010, 132, 914 – 915. N. B.
The validity of the reported generation of CO from the reaction
of CO2 with an NHC was subsequently questioned, see P.-C.
Chiang, J. W. Bode, Org. Lett. 2011, 13, 2422 – 2425.
the generation of CO from CO2 using p-block compounds,
and, as far as we are aware, none for germanium systems.[23]
We are currently exploring the possibility of rendering the
reduction reaction described above, catalytic; in addition to
examining the activation of other small molecules using 1 and
related compounds.
[12] G. Mꢃnard, D. W. Stephan, Angew. Chem. 2011, 123, 8546 – 8549;
Angew. Chem. Int. Ed. 2011, 50, 8396 – 8399. N. B. The low yield
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[13] Selected recent reviews: a) P. P. Power, Nature 2010, 463, 171 –
177; b) P. P. Power, Acc. Chem. Res. 2011, 44, 627 – 637; c) M.
Asay, C. Jones, M. Driess, Chem. Rev. 2011, 111, 354 – 396; d) S.
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[14] a) J. Li, C. Schenk, C. Goedecke, G. Frenking, C. Jones, J. Am.
Chem. Soc. 2011, 133, 18622 – 18625; b) J. Li, A. Stasch, C.
Schenk, C. Jones, Dalton Trans. 2011, 40, 10448 – 10456.
[15] a) P. P. Power, Organometallics 2007, 26, 4362 – 4372; b) E.
Rivard, P. P. Power, Inorg. Chem. 2007, 46, 10047 – 10064; c) Y.
Peng, R. C. Fischer, W. A. Merrill, J. Fischer, L. Pu, B. D. Ellis,
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[16] As 1 possesses a Ge–Ge single bond it could be regarded as a 1,2-
bis(germylene). We use the term digermyne to describe the
compound in line with nomenclature established for both single
and multiple bonded heavier analogs of alkynes, that is the
ditetrelynes, REER, E = Si, Ge, Sn, or Pb. See Refs. [13] and
[14].
[17] In a separate experiment, compound 1 was found to be
unreactive towards CO at ambient temperature.
[18] C. Cui, M. M. Olmstead, J. C. Fettinger, G. H. Spikes, P. P.
Power, J. Am. Chem. Soc. 2005, 127, 17530 – 17541.
[19] See for example: a) X. Li, X. Cheng, H. Song, C. Cui, Organo-
metallics 2007, 26, 1039 – 1043; b) W. Uhl, U. Schꢂtz, W. Hiller,
M. Heckel, Chem. Ber. 1994, 127, 1587 – 1592; c) W. Uhl, I.
Hahn, U. Schꢂtz, S. Pohl, W. Saak, J. Martens, J. Manikowski,
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[20] a) S.-H. Zhang, C.-W. So, Organometallics 2011, 30, 2059 – 2062;
b) O. T. Summerscales, M. M. Olmstead, P. P. Power, Organo-
metallics 2011, 30, 3468 – 3471.
[21] As determined from a survey of the Cambridge Crystallographic
Database, May, 2012.
[22] W. Wang, S. Inoue, S. Yao, M. Driess, J. Am. Chem. Soc. 2010,
132, 15890 – 15892.
[23] The reduction of CO2 by germanium(II) hydride complexes, to
give hydrogermylation products, has been previously reported;
a) S. L. Choong, W. D. Woodul, C. Schenk, A. Stasch, A. F.
Richards, C. Jones, Organometallics 2011, 30, 5543 – 5550; b) A.
Jana, D. Ghoshal, H. W. Roesky, I. Objartel, G. Schwab, D.
Stalke, J. Am. Chem. Soc. 2009, 131, 1288 – 1293.
Received: May 9, 2012
Revised: June 21, 2012
Published online: && &&, &&&&
Keywords: carbon dioxide · density functional calculations ·
.
germanium · green chemistry · reduction
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Green Chemistry
Taking the fizz out: A digermyne com-
ꢀ
pound with a Ge Ge single bond has
J. Li, M. Hermann, G. Frenking,*
been shown to quantitatively reduce CO2
to CO at temperatures as low as ꢀ408C.
The mechanism of this unprecedented
reaction has been probed by spectro-
scopic and computational techniques
and involves a metastable intermediate
(see picture, Ar*=C6H2{C(H)Ph2}2Me-
2,6,4).
C. Jones*
&&&& — &&&&
The Facile Reduction of Carbon Dioxide to
Carbon Monoxide with an Amido-
Digermyne
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