Carbon dioxide insertion into uranium-activated dicarbonyl complexes

Article abstract of DOI:10.1002/anie.201104189

Quick on the uptake: Uranium(III) complexes bearing hexadentate triazacyclononane macrocyclic ligands L¹ engage di-and triketones in both one-and two-electron reduction pathways. The dinuclear enolate generated through one of these pathways reacts with carbon dioxide to generate a new C-C bond (see scheme). Copyright

Full text of DOI:10.1002/anie.201104189

Communications
DOI: 10.1002/anie.201104189
CO2 Insertion
Carbon Dioxide Insertion into Uranium-Activated Dicarbonyl
Complexes**
Stephan J. Zuend, Oanh P. Lam, Frank W. Heinemann, and Karsten Meyer*
Dedicated to Professor Karl Wieghardt
The metal-mediated synthesis of complex molecules directly
from CO2 is a potentially important strategy for the prepa-
ration of fine chemicals from renewable feedstocks.[1] The
direct oligomerization of CO2 is a conceptually simple route
[1a]
À
for C C bond formation that may also be relevant to the
prebiotic synthesis of organic compounds.[2] Whereas metal-
catalyzed reductive dimerization of CO2 to form oxalate has
been observed,[3] insertion of CO2 into oxalate or oxalate
equivalents, such as diketones, to form chains with three or
more carbon atoms is not known. In principle, this process
could proceed in two steps: metal-mediated two-electron
reduction of a dicarbonyl compound to generate an enolate,[4]
followed by nucleophilic addition of the enolate to CO2
(Scheme 1). Herein we present low-valent uranium(III)
complexes that engage organic diketone ligands in both
one- and two-electron reduction pathways and a dinuclear,
two-electron-reduced diketone that engages CO2 in a pro-
Scheme 2. Uranium(III) complexes used in this study.
example, the ketyl, bound to a formal UIV center. Two factors
are thought to be critical to this result: 1) The ability of UIII
ions in coordination complexes to undergo selective one-
electron oxidation to the corresponding UIV species; and
2) the large ionic radius of uranium that allows high
coordination numbers and thus the binding of additional
ligands without displacement of the stabilizing macrocyclic
ligand. On the basis of these results, we reasoned that 1, or
structurally related UIII complexes, might be capable of
engaging dicarbonyl compounds in multiple one-electron
reduction pathways (Scheme 3),[7] and would thus be well-
suited to test the viability of CO2-insertion pathways into the
resulting reduced dicarbonyl compounds.
À
ductive C C bond formation reaction.
We began our study by examining the reactions of various
1,2-dicarbonyl compounds with UIII complexes. Oxalate-
derived esters, such as dimethyl oxalate, were observed to
react with 1 at room temperature to provide a complex
mixture of products. Although reactions under cryogenic
conditions proceeded more cleanly, the resulting products
underwent decomposition to provide the corresponding
uranium alkoxides, precluding definitive characterization of
the initial reaction products.
Scheme 1. A pathway for the dimerization and trimerization of CO2
with metal complexes.
Uranium(III) (f3) ions bearing macrocyclic ligands[5] are
potent one-electron reducing agents that have been shown to
stabilize bound organic radicals by generating charge-sepa-
À
[6]
rated {UIV LC } species. In particular, UIII complex 1
(Scheme 2) reduces benzophenone and diazomethane deriv-
atives to generate stable charge-separated radical anions, for
À
[*] Dr. S. J. Zuend, Dr. O. P. Lam, Dr. F. W. Heinemann,
Prof. Dr. K. Meyer
Department of Chemistry and Pharmacy
University of Erlangen-Nuremberg, Inorganic Chemistry
Egerlandstrasse 1, 91058 Erlangen (Germany)
E-mail: karsten.meyer@chemie.uni-erlangen.de
[**] This work was supported by the German Science Foundation (DFG)
through the Collaborative Research Center SFB 583 and by fellow-
ship support to S.J.Z. from the Alexander von Humboldt Founda-
tion.
Scheme 3. The uranium(III)-mediated reduction of dicarbonyl com-
pounds: A) Benzil; B) UIV complex with benzil ketyl ligand; C) dinu-
clear UIV complex with benzil enolate ligand; D) mononuclear UIV
À
À
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201104189.
complex with benzil enolate ligand. Expected C C and C O bonds [ꢀ]
(from Ref. [7a]) are indicated.
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Angew. Chem. Int. Ed. 2011, 50, 10626 –10630

In contrast, experiments with 1,2-diketones led to the
formation of isolable products. Thus, treatment of a dark
brown solution of 1 with benzil (ꢀ 1 equiv; Scheme 4A) in
benzene or [D6]benzene resulted in the formation of a red–
violet solution containing a single new species as judged by
1H NMR spectroscopy. Slow diffusion of n-hexane into a
concentrated benzene solution provided red–violet needles in
58% yield. Single-crystal X-ray diffraction (XRD) analysis
revealed a 1:1 complex of cis-benzil and 1 bound through both
oxygen atoms of the diketone (3a; Figure 1 and Table 1). In
Wieghardtꢀs recent studies of first-row transition-metal dike-
À
À
tone-based complexes, the C C and C O bonds are shown to
be sensitive probes of the dicarbonyl oxidation state; for
À
example, C C bond distances of about 1.56, 1.46, and 1.39 ꢁ
correspond to the neutral, monoanionic, and dianionic forms
of the dicarbonyl ligand, respectively (Scheme 3A,B,D).[7] In
À
3a, the observed C C bond distance of 1.44 ꢁ is substantially
different from that expected for a neutral diketone (Table 1),
but corresponds closely to that expected for the singly
reduced radical form of the diketone ligand (Scheme 3B).
À
Similarly, the observed C O bond distances of 1.29–1.30 ꢁ
are most consistent with the ketyl formulation.
Scheme 4. Formation of A) a mononuclear ketyl, B) a dinuclear dien-
olate and its CO2-insertion product, and C) a mononuclear enolate.
(HMDS)n =uncharacterized by-product derived from hexamethyldisila-
zane.
Figure 1. Molecular structures of the monomeric one-electron reduced
form of benzil (3a), the dimeric two-electron reduced form (4a), and
the monomeric two-electron reduced form (5a) bound to uranium(IV).
U pink, C gray, O red, N blue. Ellipsoids are set at 50% probability;
hydrogen atoms are omitted for clarity.
In analogous experiments, treatment of 1 with approx-
imately 0.5 equiv of benzil (Scheme 4B) resulted in the
formation of a yellow–orange solution and a new species
1
distinct from either 1 or 3a, as judged by H NMR spectros-
copy.[8] Orange crystals suitable for XRD analysis were
obtained by slow diffusion of hexane into a saturated benzene
solution. The resulting molecular structure consists of trans-
benzil bridging two uranium centers, with a crystallographic
Table 1: Selected bond lengths in complexes 3a, 4a, and 5a.
[a]
[a]
[a]
À
À
À
Complex
C C [ꢀ]
C O [ꢀ]
U O [ꢀ]
À
3a
4a
5a
1.440(8)
1.36(3)
1.366(8)
1.286(7), 1.301(7)
1.393(12)
1.367(7), 1.369(7)
2.404(4), 2.463(4)
2.117(3)
2.182(4), 2.247(4)
inversion center at the central C C bond (4a; Figure 1 and
À
À
Table 1). The substantially shorter C C and longer C O bond
in this structure relative to 3a are consistent with an enolate
rather than a ketyl formulation (Table 1 and Scheme 3C).[9]
[a] Determined by X-ray diffraction analysis.
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Communications
À
À
We sought to prepare a complex containing a monome-
been identified previously in {UIV LC } systems,[6b–d] and have
been ascribed to magnetic contributions arising from the one
unpaired electron residing on the reduced ligand. Conse-
quently, the variable-temperature magnetic behavior of 3a is
related to complexes possessing open-shell radical anionic
ligands. The SQUID data for complexes 3a and 5a are thus
consistent with both the computational analysis and with
magnetization data collected on other UIV complexes with
coordinated (open-shell) radical anionic ligands.
tallic two-electron-reduced diketone, both to determine
accurately the bond lengths in such a species and to allow
comparison with 3a and 4a in its reaction chemistry. To
accomplish this, pentadentate triazacyclononane-derived
ligands were prepared (analogues of the hexadentate (ArO)3-
(tacn)3À ligand, as in 1) with the general structure
(ArO)2(R)tacn2À.[10] The uranium complex derived from the
ligand in which R = CH2Ph (2) emerged as a promising
candidate for further study: treatment of 2 with benzil
(1 equiv; Scheme 4C) yielded a dark orange solution con-
taining a mixture of compounds, as judged by 1H NMR
spectroscopic analysis.[11] The major species could be isolated
as a yellow–orange powder in 44% yield by selective
crystallization.[12] Crystals suitable for XRD analysis were
grown by slow diffusion of hexane into a concentrated
benzene solution, revealing the monometallic complex 5a
To determine their reactivity properties and to test the
viability of the proposed C C bond formation pathway,
À
complexes 3a, 4a, and 5a were each exposed to CO2 (1 atm)
under a variety of conditions. Benzil ketyl complex 3a proved
unreactive towards CO2, as might be expected because of the
reluctance of the CO2 molecule to engage in one-electron
reduction pathways.[6b] In contrast, dinuclear enolate 4a
underwent complete reaction with CO2 to form a new product
1
À
À
(Figure 1 and Table 1). The observed C C (1.37 ꢁ) and C O
(1.37 ꢁ) bonds are consistent with a doubly reduced enolate
(Scheme 3D) rather than a singly reduced ketyl complex.
Interestingly, the most pronounced effect of the diketone
within 3 h at room temperature (Scheme 4B). The H NMR
spectrum was consistent with that of a dinuclear species of low
symmetry. Definitive characterization of the reaction product
was achieved using an analogous complex: a solution of 1 in
benzene was treated with di-tert-butyl diketone[14] followed
immediately by CO2. Single crystals suitable for XRD
analysis could be grown by slow evaporation of a concen-
trated solution of the reaction product from n-hexane, and
revealed a highly unsymmetrical dinuclear complex in which a
CO2 molecule had inserted into the enolate and formed a new
À
oxidation state on the structure is observed in the U O
distances, which are about 0.2 ꢁ shorter in enolate 5a
compared with ketyl 3a; this effect can be ascribed to the
greater negative charge density on oxygen in 5a. Thus,
À
comparison of the U O bonds in 4a and 5a indicates that the
diketone ligand in dinuclear complex 4a also has a somewhat
greater negative charge density than in its mononuclear
counterpart, indicating a cooperative role of the two uranium
ions in ligand reduction.
C C bond (Figure 2).[15] The molecular structure reveals that
À
the CO2 unit is coordinated in an h2 fashion, with U OCO
À
2
À
bond lengths of 2.36 and 2.52 ꢁ. Accordingly, the two C O
bond lengths corresponding to the diketone show distinct
single- and double-bond character (1.22 and 1.39 ꢁ), whereas
We used a combination of experimental (SQUID magnet-
ization) and DFT-based computational analyses to assess the
validity of the structural assignments and to help probe the
electronic structure of the prepared species. Structures of
model complexes were fully optimized using the B3LYP level
of theory, with the Stuttgart–Dresden 60-electron pseudopo-
tential used on the uranium atom (Supporting Informa-
tion).[13] Computed mononuclear complexes (models of 3a
and 5a) have structures that are almost identical to those of
their experimental counterparts. The triplet state correspond-
ing to 5a, having two unpaired f electrons, is substantially
more stable than the corresponding singlet state, as has
generally been observed with analogous UIV complexes. In
contrast, the doublet and quartet states corresponding to 3a,
corresponding to species in which the spin of the ligand-
centered electron is orthogonal and aligned with those of the
f electrons, respectively, are isoenergetic. This result indicates
that the electronic system of the one-electron reduced
diketone ligand and the uranium metal are essentially
completely decoupled from each other.
Variable-temperature SQUID magnetization data of 5a
reveal magnetic moments of 3.00 B.M. at 300 K and 0.47 B.M.
at 3.5 K, consistent with a UIV f2 ion with a singlet ground state
in the [{(RArO)3tacn}U(L)] system in which L is an axially
coordinated closed-shell ligand (Supporting Information). In
contrast, complex 3a exhibits a similar magnetic moment of
3.19 B.M. at 300 K; however, the magnetic moment at 2 K of
1.59 B.M. is significantly higher than that of 5a (0.47 B.M. at
5 K). Increased low-temperature magnetic moments have
À
the two C O bond lengths within the CO2 unit are essentially
identical, suggesting almost complete charge delocalization
(1.25–1.26 ꢁ). Finally, in contrast to the dinuclear enolate,
mononuclear enolate 5a proved to be unreactive towards CO2
under identical reaction conditions. These results are in
accord with the analysis described above, which suggested a
somewhat greater negative charge density than on the enolate
in 4a compared with 5a.[16]
To extend the process depicted in Scheme 1 to generate
extended carbon chains, the reduction and chain extension of
tricarbonyl (and greater) compounds must be achieved. In
efforts to test the viability of such processes, 1 was treated
with tBuCOCOCOtBu,[17] resulting in the formation of a
dinuclear complex 8b, in which the triketone has undergone
two-electron reduction, as judged by x-ray crystallography
À
(Figure 2). The C C bonds of the tricarbonyl moiety of this
complex measure 1.45 and 1.39 ꢁ, indicating that the two C
À
C bonds are inequivalent and that one of the two has almost
complete double-bond character. Reactivity studies of 8b
(generated in situ) with CO2 demonstrate that this complex is
reactive: for example, under conditions similar to those used
for the synthesis of 7b, a new product of low symmetry is
1
formed within about 1 h, as revealed by H NMR spectros-
copy. However, this initial product appears to be unstable and
it reacts further to generate a complex mixture of products, as
judged by 1H NMR spectroscopy. In one case, we were able to
characterize the connectivity of one of these species by XRD
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Angew. Chem. Int. Ed. 2011, 50, 10626 –10630

Keywords: CO2 insertion · coordination complexes · enolates ·
.
uranium · X-ray crystallography
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Figure 2. Molecular structure of CO2-insertion product (7b) and
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À
CO2 unit had undergone insertion into one of the U O bonds
of the enolate.[18] Although it has also not been possible to
isolate the initially formed species observed by 1H NMR
spectroscopy, our experiments demonstrate that CO2 inser-
tion into the triketone enolate is a viable process that is
accompanied by complex reorganization under all conditions
examined
This work demonstrates that metal-mediated CO2 inser-
tion into di- and triketone-derived enolates (Scheme 1) is a
productive process in the context of diketones, and initial data
indicate that this process is available to triketones as well.
Substrate activation by a two-metal, two-electron mechanism
activation appears to play a role in this process, as neither the
corresponding singly or doubly reduced mononuclear species
are observed to form stable CO2 insertion products. The
insertion reaction does not require a net change in metal
oxidation state or cleavage of a metal–carbon bond. We thus
expect that this work will lead to the development of
productive reaction sequences for the reductive coupling of
multiple CO2 units to generate small organic molecules.
À
[9] Although a C C distance of about 1.39 ꢁ is suggested to be
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[11] This complex was prepared from U(HMDS)3 and the corre-
sponding macrocyclic ligand in 62% yield on a 0.5 g scale.
Details are provided in the Supporting Information.
[12] 1H NMR spectroscopic analysis of the crude reaction mixture
reveals that 4a is formed in 60–70% yield. It has not been
possible to isolate or characterize the minor species formed in
this reaction.
Received: June 17, 2011
Published online: September 20, 2011
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Communications
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[14] Complex 4b, which is presumably formed initially, appears to
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XRD analysis at ambient or low temperatures resulted in the
isolation of decomposition products, including U2-m-O species or
complexes in which the macrocyclic ligand has undergone
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[15] 1H NMR spectroscopic analysis of the reaction mixture indicates
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À
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