Fixation of carbon dioxide by oxalic amidinato magnesium complexes: Structures and reactions of trimetallic magnesium carbamato and related complexes

Article abstract of DOI:10.1002/(sici)1099-0682(200005)2000:5<1055::aid-ejic1055>3.0.co;2-m

The reaction of oxalic amidines R¹-N=C(NHR²)-C(NHR²)=N-R¹ with CH₃MgX followed by uptake of CO₂ results in the formation of the trimeric carbamato complexes [R¹-N=C(NR²-COO)-C(NR²-COO)-C(NR²COO)=N- R¹]₃Mg₃(THF)₆ (2a: R¹ = R² = Ph; 2b: R¹ = R² = p-tolyl) as the thermodynamically stable final products of the reaction. Their X-ray crystal structures show that the three metal centres are in a linear arrangement. The central magnesium ion is octahedrally surrounded by six O-donor atoms of the μ₂-carbamato bridges, while both peripheral magnesium ions are facially coordinated by three O-donor atoms of the carbamato groups and three THF molecules. This coordination sphere can be considered as a structural model for the active centre in the ribulose-1,5-bisphosphate carboxylase/oxygenase enzyme. Compound 2a reacts with ZnCl₂ or CoBr₂, with CO₂ elimination, to form dimeric complexes of the type [X₂M(oxalamidinato)MX₂][Mg(DMF)₆] (M = Zn, Co; X = Cl, Br). X-ray crystal structure analyses show that the d-metals are tetrahedrally coordinated. The magnesium-bromide-containing intermediates in the formation of 2a and 2b are able to transfer CO₂ to acetophenone, thus simulating the CO₂ activation step in enzymatic biotin-dependent carboxylation reactions.

Full text of DOI:10.1002/(sici)1099-0682(200005)2000:5<1055::aid-ejic1055>3.0.co;2-m

FULL PAPER
Fixation of Carbon Dioxide by Oxalic Amidinato Magnesium Complexes:
Structures and Reactions of Trimetallic Magnesium Carbamato and Related
Complexes
[c]
[a]
[a]
[a]
[b]
´
Mario Ruben, Dirk Walther,* Rene Knake, Helmar Görls, and Rainer Beckert
Dedicated to Professor Heinrich Vahrenkamp on the occasion of his 60th birthday
Keywords: Carbon dioxide fixation / Magnesium / N ligands / C–C coupling
The reaction of oxalic amidines R1–N=C(NHR2)–C(NHR2)=
N–R1 with CH3MgX followed by uptake of CO2 results in
the formation of the trimeric carbamato complexes [R1–N=
molecules. This coordination sphere can be considered as a
structural model for the active centre in the ribulose-1,5-bi-
sphosphate carboxylase/oxygenase enzyme. Compound 2a
reacts with ZnCl2 or CoBr2, with CO2 elimination, to form
dimeric complexes of the type [X2M(oxalamidina-
to)MX2][Mg(DMF)6] (M = Zn, Co; X = Cl, Br). X-ray crystal
C(NR2–COO)–C(NR2COO)=N–R1]3Mg3(THF)6 (2a: R1 = R2
=
Ph; 2b: R1 = R2 = p-tolyl) as the thermodynamically stable
final products of the reaction. Their X-ray crystal structures
show that the three metal centres are in a linear arrange- structure analyses show that the d-metals are tetrahedrally
ment. The central magnesium ion is octahedrally surrounded
by six O-donor atoms of the µ2-carbamato bridges, while
both peripheral magnesium ions are facially coordinated by
three O-donor atoms of the carbamato groups and three THF
coordinated. The magnesium-bromide-containing interme-
diates in the formation of 2a and 2b are able to transfer CO2
to acetophenone, thus simulating the CO2 activation step in
enzymatic biotin-dependent carboxylation reactions.
Introduction
octahedrally surrounded by a monodentate carbamato
group, two carboxylato ligands and three water molec-
ules.[12]
Biological transformation reactions of carbon dioxide
into organic material by C–C linkage are either activated
by biotin-depending enzymes (Acetyl CoA carboxylase) in
the fatty acids synthesis, or catalysed by Rubisco in the dark
reaction of natural photosynthesis. In both enzymes carba-
mato magnesium complexes seems to play an essential role.
In biotin-containing systems the carbamato magnesium
complexes are carriers of ‘‘active CO2’’ which can be trans-
ferred to C–H bonds to form the C–COOH group. How-
ever, neither the structure of the carbamato complex nor
the mechanism of the CO2 activation reaction have yet been
established.[1–8]
In contrast, in Rubisco the carbamato group does not act
as a CO2 source, but is proposed to serve as proton relay
which abstracts the C3 proton from ribulose-1,5-bisphosph-
ate. The resulting enol reacts with carbon dioxide in the
coordination sphere of the magnesium centre to form a new
C–C bond (for reviews see refs.[9–11]). The structure of vari-
ous forms of Rubisco have been elucidated by X-ray crystal-
lography.[9] In the substrate-free complex the MgII centre is
It is noteworthy that, so far, the chemistry of abiotic car-
bamato magnesium complexes is poorly developed, espe-
cially with respect to the understanding of their reactivity
as CO2-transfer reagents.[13–19] Furthermore, structurally
elucidated complexes which might act as structural or func-
tional models for the above-mentioned enzymes are very
rare. Only one organometallic mixed Al–Mg alkyl com-
pound has been structurally characterised.[20a] Recently the
X-ray structure of an oligomer consisting of six diethylcar-
bamato–Mg units has also been reported.[20b]
We describe here the synthesis, structure, and reactivity
of a variety of new magnesium carbamato complexes which
may act as models for the above-mentioned CO2-trans-
forming systems. The complexes were synthesised from
magnesium oxalic amidinates or from the di(isopropyl)am-
ido magnesium complex and carbon dioxide.
Results and Discussion
[a]
Institut für Anorganische und Analytische Chemie der
Synthesis and Structure of Carbamato Mg Complexes
Universität Jena,
August-Bebel-Straße 2, 07743 Jena, Germany
[b]
The reaction of tetraphenyloxalic amidine H2A
(2.56 mmol, Figure 1) with two equivalents of CH3MgBr in
50 mL of THF resulted in the deprotonation of the ligand
and formation of methane. Subsequent reaction with CO2
at –20 °C afforded a white precipitate (1a, 60%). According
to the elemental analyses it was contaminated with ca. 10%
of complex 2a which was formed by elimination of MgBr2
Institut für Organische Chemie und Makromolekulare Chemie
der Universität Jena,
Humboldtstraße, 07743 Jena, Germany
[c]
Institut für Anorganische und Analytische Chemie der
Universität Jena,
August-Bebel-Straße 2, 07743 Jena, Germany;
´
On leave from the Laboratoire de Chimie Supramoleculaire,
ISIS/ULP Strasbourg, 4 rue Blaise Pascal, 67000 Strasbourg,
France
Eur. J. Inorg. Chem. 2000, 1055Ϫ1060
WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2000
1434Ϫ1948/00/0505Ϫ1055 $ 17.50ϩ.50/0
1055

M. Ruben, D. Walther, R. Knake, H. Görls, R. Beckert
FULL PAPER
Figure 1. Pathway of the formation of 2a and 2b
from 1a. Colourless crystals of pure 2a could be obtained
from the filtrate in 30% yield.
When the synthesis was carried out in dilute solution
(90 mL of THF) only pure 2a was isolated in 80% yield. In
contrast, the product of the analogous reaction with tetra-
p-tolyloxalic amidine was analytically pure 1b (Figure 1).
The IR spectrum of 2a exhibits two bands in the region
of the CϭO and CϭN valence frequencies at 1630 (less in-
tense) and 1656 cm–1 (more intense). This suggests that
both CϭN and carbamato groups are present in the molec-
ule.
Figure 3. Coordination of the three carbamato strains in 2a around
the Mg centres
Elemental analysis of 2a showed that there was no brom-
ide in the CO2 insertion product and pointed to the analyt-
ical composition of [Mg(CO2)2(A)(THF)2]n. A spectroscop-
ically identical product was obtained by using dried air for
the CO2 insertion.
molecule and Figure 3 the inner part of 2a. Relevant bond
lengths and angles are also summarised in Figure 2.
The shape of the molecule resembles a straight prism
˚
(h ϭ 6.4 A) with two eclipsed (by 17°) triangles (3 ϫ 3 ϫ 3
˚
A) (Figure 3). The three magnesium atoms are exactly lin-
Single crystals of 2a were grown from a dilute THF solu-
tion. The X-ray crystal structure revealed a trimeric struc-
ture of the product in which three magnesium centres are
connected by six µ2-carbamato bridges according to the
above-mentioned analytical composition [Mg(CO2)2(A)-
(THF)2]3. Figure 2 shows the molecular structure of the
ear (the Mg–Mg–Mg angle is 180°) and lie within a distance
of dMg ϭ 4.211(6) A. The central magnesium ion is sur-
˚
rounded by six O-donor atoms of the µ2-carbamato bridges
in an octahedral fashion, while the two peripheral magnes-
ium ions are facially coordinated by three O-donor atoms
of the carbamato groups and three THF molecules. The
six negatively charged carbamato groups in 2a have almost
˚
identical C–O distances [1.25(1) A] and O–C–O angles
[127.0(4) –127.6(4)°]. Three strongly twisted oxalamidine
groups are located above the edges of the prism, with the
1,2-diimine units directed outwards, with a torsion angle for
the NϭC–CϭN groups of 122.3°.
Thus, the molecule exhibits three soft Lewis base centres
(the uncoordinated 1,2-diimine units) above the edges of the
prism and two (potentially hard) Lewis centres (the peri-
pheral magnesium atoms) above the basic triangles of the
same prism. Because of the energetically unfavourable
‘‘hard-soft’’ combination the Lewis acid and base centres
do not proceed to polymers. However, the free 1,2-diimine
units can react with further metal centres (vide infra).
The 13C-NMR spectrum of 2a in [D6]DMSO has only
twelve resonances which correspond to the carboxylated ox-
alic amidinato ligands. The very strong carbamate signal
appears at δ ϭ 159.5 and the signals for THF at δ ϭ 67.9
Figure 2. View of the inner sphere of complex 2a with selected and 26.3. This simple pattern clearly shows that the highly
˚
bond lengths [A] and bond angles [°]: Mg1–O1A 2.021(6), Mg1–O4
symmetrical trimeric structure found by X-ray diffraction
2.026(5), Mg1–O6 2.016(6), Mg1–O7 2.169(6), Mg1–O8 2.144(6),
remains intact in solution.
Mg1–O9 2.110(7), Mg2–O2 2.075(4), Mg2–O3 2.089(6), Mg2–O5
2.076(6), C42–O5 1.239(8), C42–O6 1.263(9), N6–C29 1.377(10),
N6–C42 1.420(10), N5–C29 1.271(10), C29–C29A 1.54(2); O1A–
Mg1–O4 97.0(2), O1A–Mg1–O6 98.7(3), O1A–Mg1–O7 89.5(2),
O1A–Mg1–O8 85.7(3), O1A–Mg1–O9 168.4(3), O2–Mg2–O2A
170.6(3), O2–Mg2–O3 84.9(2), O2–Mg2–O3A 88.8(2), O2–Mg2–
O5 96.3(2), O2–Mg2–O5A 90.6(2), O5–C42–O6 127.6(7), N5–C29–
N6 120.3(7); symmetry transformation used to generate equivalent
atoms A: –x – 1, –y, z
The coordination of the peripheral magnesium atoms re-
sembles the coordination of the magnesium ion in the sub-
strate-free Rubisco enzyme[4] in which the magnesium
centre also has an octahedral environment formed by three
facially coordinated COO groups (one from a carbamato
ligand and two from carboxylato groups) and three coord-
1056
Eur. J. Inorg. Chem. 2000, 1055Ϫ1060

Fixation of Carbon Dioxide by Oxalic Amidinato Magnesium Complexes
inated solvent molecules (water). However, the Mg–O(car-
FULL PAPER
˚
bamato) distances in 2a [dMg–O ϭ 2.016(6)–2.169(6) A] are
significantly shorter than in the enzyme crystal (dMg–O
2.31 A).
ϭ
˚
In contrast to the above-described reaction with tetra-
phenyloxalic amidine, the reaction of tetra-p-tolyloxalic am-
idine H2B with CH3MgBr, followed by treatment with car-
bon dioxide, resulted in the pure bromo magnesium com-
plex 1b with the analytical composition [(MgBr)2(CO2)2-
(B)(THF)4]n which was isolated as a microcrystalline solid.
In this reaction the subsequent elimination of MgBr2 pro-
ceeded very slowly (Figure 1). Nevertheless, some crystals
of bromide-free carbamato complex 2b could be obtained
from the reaction mixture after some weeks. Compound 2b
was found to be isostructural with 2a by an X-ray struc-
tural analysis.
The 13C-NMR investigations of 1b pointed to a molecule
with only low symmetry; the signal of the carbamato group
was found at δ ϭ 161.7. The IR spectrum showed a strong
band at 1639 cm–1 which may be assigned to the carbonyl
group of the carbamato ligand. Since single crystals of 1b
could not be isolated, the structure of this compound re-
mains unknown.
To gain further insight into the coordination sphere in
MgBr-containing carbamato complexes, we have also syn-
thesised complex 3 by deprotonation of (isopropyl)2NH
with one equivalent MeMgBr and subsequent reaction with
CO2 to form a white, crystalline product with the analytical
composition [(iPr)2–N–CO2–MgBr(THF)2]2 in high yield
(95%) (Equation 1).
Figure 4. Molecular structure of the dimeric complex 3 with se-
˚
lected bond lengths [A] and bond angles [°]: Mg–Br 2.572(1), Mg–
O1 1.963(2), Mg–O2A 2.042(2), Mg–O1A 2.358(2), Mg–O3
2.117(2), Mg–O4 2.119(2), C1–O1 1.286(3), C1–O2 1.262(4), C1–
N1 1.348(4); Br–Mg–O1 108.88(7), Br–Mg–O1A 168.95(7), Br–
Mg–O2A 110.06(7), Br–Mg–O3 89.92(7), Br–Mg–O4 91.49(7), O1–
C1–O2 118.8(3), N1–C1–O1 120.5(3), N1–C1–O2 120.7(3); sym-
metry transformation used to generate equivalent atoms A:
–x ϩ 2, –y ϩ 2, –z ϩ 2
Reactions of the Carbamato Complexes With Metal
Halides
The reaction of 2a with ZnCl2 in DMF gave the yellow,
sparingly soluble complex [Cl2Zn(A)ZnCl2][Mg(DMF)6] (4)
in 80% yield (with respect to ZnCl2) even when the
starting materials were reacted in the molar ratio 1:1. Using
an excess of ZnCl2 (6:1) resulted in higher yields of 4
(Figure 5)
2 (iPr)2NH ϩ 2 CH3MgBr ϩ n THF Ǟ
2 (iPr)2N–MgBr(THF)n ϩ 2 CH4
(1)
2 (iPr)2N–MgBr(THF)n ϩ 2 CO2 Ǟ
[(iPr)2N–CO2)(MgBr(THF)2]2 ϩ n – 2 THF
1
The H-NMR spectrum of 3 ([D6]DMSO, 20 °C) shows
only the resonances for the protons of the isopropyl group
(δ ϭ 1.17, 1.20, and 3.28) and the protons of THF (δ ϭ
1.73, 3.57). However, no signal for the N–H proton was
found. Furthermore, the 13C-NMR spectrum exhibits only
one, weak carbamate signal at δ ϭ 166.39 in [D7]DMF. In
addition, the IR spectrum showed no band for the N–H
stretching frequency in the 3300 cm–1 region. An X-ray
Figure 5. Formation reaction of the dimeric complexes 4 and 5
An X-ray crystal structure determination (Figure 6) and
crystal structure determination showed the dimeric struc- FAB-MS showed a complex anion with two tetrahedrally
ture of 3, which is shown in Figure 4 along with relevant coordinated zinc ions bridged by the deprotonated oxalami-
bond lengths and angles. The coordination sphere around dine ligand. The electrostatic charge is counterbalanced by
the magnesium ions consists of three oxygen donor atoms a magnesium ion which is surrounded by six DMF molec-
of the bridging carbamato groups, two axially coordinated ules. No CO2 is present in the complex. This structure
THF molecules and one monodentate bromide anion, and seems to indicate that coordination of metal fragments at
can be described as a distorted octahedron. Two of the car- the peripheral 1,2-diimine units of the trimeric complex 2a
˚
bamato oxygens are bonded at a distance of 1.963(2) A, causes steric strain in the inner part of the trimer leading
while the third one is more weakly bound with a bond to a complete rearrangement of the molecule with elimina-
˚
length of 2.358(2) A. Both carbamato groups act as trident- tion of CO2 and formation of the dimer 4.
ate bridging ligands and form, together with the two Mg
The same reaction was observed when CoBr2 was treated
ions, a planar eight-membered ring with a Mg–Mg distance with 2a (Figure 5). Single crystals of the resulting green Co
˚
of 3.277(2) A.
complex 5 were obtained from THF. According to the X-
Eur. J. Inorg. Chem. 2000, 1055Ϫ1060
1057

M. Ruben, D. Walther, R. Knake, H. Görls, R. Beckert
FULL PAPER
followed by hydrolysis, resulted in the formation of the
carboxylation product benzoylic acid. The free carboxylic
acid could either be determined by NMR spectroscopy or
by HPLC determination. Although the yields of this C–C-
linkage reaction were low (10%), the reaction may be con-
sidered as a model step for the CO2 transfer reaction of
carboxybiotin in biological reactions which proceed with
acetylCoA to form the carboxylated product HOOC–CH2–
CO–CoA. In contrast, 2a, 2b, and 3 do not contain ‘‘activ-
ated’’ CO2. This suggests that the coordination environment
of the metal centre essentially influences the reactivity of
the carbamato–Mg group.
Conclusions
˚
Figure 6. Molecular structure of 4 with selected bond lengths [A]
and bond angles [°] (the cation [Mg(DMF)6]2ϩ is omitted for clar-
ity): Zn–Cl1 2.296(1), Zn–Cl2 2.278(1), Zn–N1 2.016(5), Zn–N2
2.020(5), N1–C1A 1.338(7), N2–C1 1.320(7), C1–C1A 1.565(11);
Cl1–Zn–Cl2 113.10(7), Cl1–Zn–N1 114.7(1), Cl1–Zn–N2 111.1(1),
N1–C1A–N2A 131.9(5), N2–C1–C1A 114.4(6); symmetry trans-
formation used to generate equivalent atoms A: –x ϩ 2, –y ϩ 2,
–z ϩ 1
In the present paper we have shown that carbamato mag-
nesium complexes can be synthesised by treating oxalic am-
idines or di(isopropyl)amine with CH3MgBr, followed by
uptake of CO2. The complexes 2a, 2b, and 3 represent struc-
turally elucidated carbamato magnesium complexes which
are of interest with respect to the magnesium containing
active centres of biotin-dependent enzymes and Rubisco.
The reaction of ZnCl2 or CoBr2 with the free diimine
groups of 2a results in the complete elimination of CO2
and a rearrangement of the system to form the dinuclear
complexes 4 and 5.
ray structural analysis, compound 5 exhibits the same di-
meric structure as found for the Zn complex 4. The bond
lengths and angles in the bridging oxalic amidinate, and
between the tetrahedral CoII centres and the nitrogen donor
atoms, are also similar (Figure 7).
Whereas 2a, 2b, and 3 are not able to transfer CO2 to
compounds with C–H-acidic bonds, both the carbamato
complexes 1b and 1a (containing ca. 10% of the inactive
trimer 2a) react with acetophenone to form benzoylic acid
upon hydrolysis, simulating the C–C linking step in biotin-
dependent CO2 transfer reactions.
Experimental Section
General: The oxalic amidine ligands were prepared according to
literature procedures.[21] CH3MgBr and CH3MgCl (3  in diethyl
ether), purchased from Aldrich, and pure CO2 (4.8) from Linde
AG, were used without additional purification.
All reactions were carried out under argon with standard Schlenk
techniques and all solvents dried and distilled before use.
1H-NMR spectra were obtained with a Bruker AC 200 MHz spec-
trometer and all spectra were referenced to TMS or deuterated
solvent as an internal standard. FAB mass spectra were obtained
with a Finnigan MAT SSQ 710 spectrometer (2,4-dimethoxybenzyl
alcohol as matrix). IR measurements were carried out with a Per-
kin–Elmer System 2000 FT-IR. Analytical HPLC experiments were
carried out using a water HPLC system, consisting of a Knauer
HPLC pump 64, a Rheodyne 7125 5 µL injector loop, a Merck Li
Chrophor RP 18 (5µm) column and a Knauer variable wavelength
detector at λ ϭ 295 nm. The system was controlled by Knauer
HPLC 2.21a software. The mobile phase used was 50:50 CH3CN/
H2O containing 0.01  KH2PO4 and adjusted to pH ϭ 2. Ele-
mental analyses were carried out at the Microanalytical Laboratory
of the Friedrich Schiller University, Jena. Metals were determined
by complexometric titration of an aqueous solution, obtained by
˚
Figure 7. Molecular structure of 5 with selected bond lengths [A]
and bond angles [°] (the cation [Mg(DMF)6]2ϩ is omitted for clar-
ity): Co–Br1 2.375(1), Co–Br2 2.375(1), Co–N1 1.986(6), Co–N2
1.999(6), N1–C1 1.340(9), N2–C1A 1.328(9), C1–C1A 1.555(13);
Br1–Co–Br2 112.30(6), Br1–Co–N1 115.6(1), Br1–Co–N2 115.0(1),
N1–C1–N2A 132.7(6), N1–C1–C1A 113.2(8); symmetry trans-
formation used to generate equivalent atoms A: –x ϩ 1, –y ϩ 1,
–z ϩ 2
Carboxylation of Acetophenone
Acetophenone was used as the substrate for CO2 transfer
reactions because it contains the CH3CO–R group which
is carboxylated in biotion-dependent reactions as well. The
reaction of acetophenone with the carbamato complexes 1b
or 1a (contaminated with ca. 10% of the inactive trimer 2a)
either in THF at 70 °C or in DMF at room temperature, reaction of the complex with dilute H2SO4, followed by extraction
1058 Eur. J. Inorg. Chem. 2000, 1055Ϫ1060

Fixation of Carbon Dioxide by Oxalic Amidinato Magnesium Complexes
FULL PAPER
of the organic products with diethyl ether and subsequent addition
of ammonia. Halogen was determined by titration with silver ni-
trate.
31.2 10–5 mol), dissolved in 1 mL of DMF, was added. After a few
minutes of stirring, a yellow solid precipitated. The solid was fil-
tered off, washed with THF and dried in vacuo. Single crystals were
grown from the mother liquor at room temperature. Yield: 152 mg
(87%). – C44H62Cl2MgN10O6Zn2 (1123.9): calcd. C 47.0, H 5.5, Cl
12.6, N 12.5; found C 42.7, H 5.1, Cl 12.6, N 11.2. – 1H NMR
([D6]DMSO, 20 °C) δ ϭ 2.72 (s, CH3, 18 H, DMF), 2.88 (s, CH3,
18 H, DMF), 6.55–6.82 (m, 20 H, arom.), 7.94 (s, 6 H, CH,
DMF). – IR (nujol): ν˜ ϭ 1591 (CϭCarom), 1659 (CϭO, DMF) cm–1.
– FAB-MS [DMBA]: m/z ϭ 945 [Mϩ – 2 DMF – Cl ϩ 2 H], 622
[Mϩ – 6 DMF – Mg – Cl ϩ 2 H].
[Mg(CO2)2(A)(THF)2]3 (2a): One equivalent (per N–H group) of
CH3MgBr (3  in diethyl ether) was added slowly to a stirred solu-
tion of ligand H2A (1 g, 2.56 mmol) in 50 mL of anhydrous THF
until the gas evolution was finished. Then the yellowish solution
was cooled to –20 °C and CO2 was bubbled through the solution
for some minutes. The resulting solution was shaken at room tem-
perature for 1 h. After 24 h, a white precipitate of product 1a (con-
taining ca. 10% 2a as calculated from the elemental analysis) was
isolated by filtration, washed three times with diethyl ether and
subsequently dried in vacuo. Yield: 60%. Additionally, pure com-
plex 2a crystallised from the mother liquor after standing over-
night. Yield: 30% of pure 2a.
[Mg(DMF)6][Co2(A)Br4] (5): This compound was prepared in a
similar manner to 4. Single crystals of the green compound were
grown from a solution of the redissolved product in DMF at –20
°C. IR (nujol): ν˜ ϭ 1659 (CϭO, DMF), 1591 (CϭCarom) cm–1. –
FAB-MS [DMBA]: m/z ϭ 1288 [Mϩ], 838 [Mϩ – 2 DMF – Cl ϩ
2 H – 2 DMF ϩ 2 Hϩ], 602 [ACoBr(DMF) ϩ 2 Hϩ], 528 [ACoBr
ϩ 2 Hϩ].
When the reaction was carried out in 90 mL of THF as solvent
and a CH3MgBr solution in toluene was used, only pure crystalline
2a was isolated in 80% yield.
Carboxylation of Acetophenone: Compound 1b (1.35 mol), dis-
solved in 70 mL, of anhydrous DMF was treated with 1.2 mL of
acetophenone and the resulting solution was stirred for 12 h at
room temperature. The resulting benzoyl acetic acid was deter-
mined either by aqueous work up or by HPLC.
Single crystals of 2a were obtained by reducing the remaining THF
solution to half volume and allowing to crystallise at room temper-
ature, or by recrystallisation of the isolated crude product 1a from
THF. Longer reaction times and higher temperatures resulted in
the formation of 2a in higher yields by accomplished MgBr2 extru-
sion.
By Wet Workup: After removal of the DMF in vacuo, the re-
maining residue was hydrolysed with 50 mL of 25% HCl and the
water phase extracted three times with diethyl ether. The organic
phase was washed with a 10% solution of Na2CO3 and the separ-
ated water phase was acidified carefully until pH ϭ 5 and sub-
sequently extracted with diethyl ether. The diethyl ether phase was
dried with solid Na2SO4 and the solvent removed in vacuo to give
benzoyl acetic acid as a white solid. Yield 5–10%. – 1H NMR
([D6]acetone, 20 °C): δ ϭ 4.02 (–CH2–).
2a: C108H108Mg3N12O18 (1935.03): calcd. C 67.0, H 5.6, Mg 3.8, N
1
8.7; found C 66.3, H 5.8, Mg 3.7, N 8.7. – H NMR ([D6]DMSO,
20 °C): δ ϭ 6.63–7.28 (m, arom.). – 13C NMR ([D6]DMSO, 20 °C):
δ ϭ 25.2, 67.1 (THF), 120.4, 121.6, 122.0, 124.4, 127.2, 128.2,
(arom., phenyl), 143.3 (ipso-C-amine), 149.3 (NϭC–N), 150.4
(ipso-C-imine), 159.5 (carbamate). – IR (nujol): ν˜ ϭ 1592 (Cϭ
Carom), 1630, 1656, (N–COO, CϭN) cm–1.
Mixture 1a/2a: (90:10) {1a: [Mg2Br2(CO2)2(A)(THF)4]n}: calcd. C
56.6, H 4.9, Br 13.4 Mg 5.0, N 6.3; found C 55.4, H 5.3, Br 13.4,
Mg 5.0, N 5.8.
By HPLC: Benzoyl acetic acid was determined by comparison with
an authentic sample of benzoylic acid. Yield: 6–10%.
The reaction product 1a/2a (90:10) reacts analogously to form 6–
10% benzoylic acid.
[Mg2Br2(CO2)2(B)(THF)4]n (1b): The reaction was carried out as
described above for 2a. The main product in this reaction was 1b.
Yield 92%. – C48H60Br2Mg2N4O8 (1029.5): calcd. C 56.0, H 5.9,
Br 15.4, Mg 4,7, N 5.4; found C 55.0, H 6.1, Br 14.1, Mg 4.5, N
5.3. – 1H NMR ([D6]DMSO, 20 °C): δ ϭ 1.73 (dd, 16 H, CH2),
2.13, 2.26 (2 ϫ s, 12 H, CH3), 3.57 (dd, 16 H, CH2), 7.27–6.62 (m,
20 H, arom.). – 13C NMR ([D6]DMSO, 20 °C): δ ϭ 20.8, 20.9,
(CH3), 25.2, 67.0 (CH2, THF), 120.0, 121.7, 122.3, 128.8, 129.2,
130.8, 131.7, 134.9 (arom. tolyl), 141.6 (ipso-C-amine), 148.1 (Nϭ
Crystal Structure Determinations: The intensity data for the com-
pounds 2a and 3 were collected with a Nonius CAD4 diffracto-
meter and for the other compounds with a Nonius Kappa CCD
diffractometer, using graphite-monochromated Mo-Kα radiation.
Data were corrected for Lorentz and polarization effects. Only the
compounds 3 and 5 were corrected for absorption effects.[22,23]
C–N), 151.0 (ipso-C-imine), 161.7 (carbamate). – IR (Nujol): ν˜ ϭ The structures were solved by direct methods (SHELXS[24]) and
2
1592 (CϭCarom), 1630, 1639 (N–COO, CϭN) cm–1.
refined by full-matrix least-squares techniques against Fo
(SHELXL-97[25]). For compound 4 the hydrogen atoms were loc-
ated by difference Fourier synthesis and refined isotropically; the
hydrogen atoms of the other compounds were included at calcu-
lated positions with fixed thermal parameters. All non-hydrogen
atoms were refined anisotropically.[25] XP (SIEMENS Analytical
X-ray Instruments, Inc.) was used for structure representations.
Single crystals of 2b were isolated from the mother liquor after
storage at –20 °C for some weeks. Yield: 5%. Characterization by
X-ray analysis.
[(iPr)2NCO2MgBr(THF)2]2 (3): The reaction was analogously car-
ried out as described for 2a. Yield 95%. – C30H60Br2Mg2N2O8
(785.2): calcd. C 45.9, H 7.7, Br 20.4, N 3.6; found: C 44.6, H 8.2,
Crystal Data for 2a:[26] C108H108Mg3N12O18 1.5 C4H8O, Mr
ϭ
1
Br 24.1, N 3.6. – H NMR ([D6]DMSO, 20 °C): δ ϭ 1.17 (s, 6 H,
2043.15 gmol–1, colourless prism, size 0.40 ϫ 0.38 ϫ 0.36 mm, or-
CH3), 1.20 (s, 6 H, CH3), 1.73 (dd, 8 H, CH2, THF), 3.28 (m, 2 H,
CH), 3.57 (dd, 8 H, O–CH2, THF). – 13C NMR ([D6]DMSO, 20
°C): δ ϭ 19.9 (CH3), 28.8 (CH2, THF), 34.2 (C–H), 67.8 (O–CH2,
THF), 166.3 (carbamate). – IR (Nujol): ν˜ ϭ 1578 cm–1 (carba-
mate).
thorhombic, space group Aba2, crystal is a racemic twin, a ϭ
3
˚
˚
17.978(2), b ϭ 23.753(3), c ϭ 28.248(3) A, V ϭ 12063 (1) A , T ϭ –
90 °C, Z ϭ 4, ρcalcd. ϭ 1.125 gcm–3, µ (Mo-Kα) ϭ 0.91 cm–1,
F(000) ϭ 4320, 6269 reflections in h(0/22), k(0/29), l(0/35), meas-
ured in the range 2.27° Յ Θ Յ 26.33°, 6269 independent reflections,
3659 reflections with Fo Ͼ 4 σ(Fo), 620 parameters, 1 restraint,
R1obs ϭ 0.086, wR2obs ϭ 0.218, R1all ϭ 0.177, wR2all ϭ 0.274,
[Mg(DMF)6][Zn2(A)Cl4] (4): Compound 2a (100 mg, 5.2 10–5 mol)
was dissolved in 1 mL of anhydrous DMF and ZnCl2 (42 mg,
Eur. J. Inorg. Chem. 2000, 1055Ϫ1060
1059

M. Ruben, D. Walther, R. Knake, H. Görls, R. Beckert
FULL PAPER
GOOF ϭ 1.765, largest difference peak and hole: 0.862/–0.332 e
A .
ischen Industrie is gratefully acknowledged. M. R. wishes to thank
the Studienstiftung des Deutschen Volkes for a grant.
–3
˚
Crystal Data for 2b: C146H180Mg3N12O24, Mr ϭ 2559.95 gmol–1,
colourless prism, size 0.40 ϫ 0.35 ϫ 0.30 mm, monoclinic, space
group P21/c, a ϭ 24.085(2), b ϭ 18.1640(10), c ϭ 30.297(3) A, β ϭ
[1]
S. D. Taylor, R. Kluger, J. Am. Chem. Soc. 1993, 115, 867–871.
[2]
D. J. Kuo, I. A. Rose, J. Am. Chem. Soc. 1993, 115, 387–390.
˚
[3]
H. Martini, J. Retey, Angew. Chem. 1993, 105, 287–289; Angew.
3
˚
Chem. Int. Ed. Engl. 1993, 32, 278–280.
91.274(4) °, V ϭ 13251.1(19) A , T ϭ –90 °C, Z ϭ 4, ρcalcd.
ϭ
[4]
1.283 gcm–3, µ (Mo-Kα) ϭ 1 cm–1, F(000) ϭ 5472, 30541 reflections
in h(–25/25), k(0/20), l(–30/32), measured in the range 2.44° Յ Θ Յ
23.28°, completeness Θmax ϭ 97.2%, 17884 independent reflections,
F. Lynen, J. Knappe, E. Lorch, G. Jütting, E. Ringelmann, An-
gew. Chem.1959, 71, 481–486.
[5]
F. Lynen, J. Knappe, E. Lorch, G. Jütting, E. Ringelmann, J.
P. Lachance, Biochem. Z. 1961, 335, 123–167.
[6]
R
int ϭ 0.099, 10749 reflections with Fo Ͼ 4 σ(Fo), 1438 parameters,
S. Berger, A. Braune, W. Buckel, U. Härtel, M.-L. Lee, Angew.
0 restraints, R1obs ϭ 0.225, wR2obs ϭ 0.542, R1all ϭ 0.281, wR2all
ϭ
Chem. 1996, 108, 2259–2261; Angew. Chem. Int. Ed. Engl. 1996,
35, 2132–2133.
0.580, GOOF ϭ 1.255, largest difference peak and hole: 1.063/
[7]
Y. Musashi, T. Hamada, S. Sakaki, J. Am. Chem. Soc. 1995,
117, 11320–11326.
T. J. Andrews, G. H. Lorimer, N. E. Tolbert, Biochem. 1973,
12, 11–17.
–3
˚
–0.694 e A .
[8]
Crystal Data for 3:[26] C30H60Br2Mg2N2O8, Mr ϭ 785.24 gmol–1,
colourless prism, size 0.40 ϫ 0.35 ϫ 0.35 mm, monoclinic, space
[9]
W. W. Cleland, T. J. Andrews, S. Gutteridge, F. C. Hartman,
˚
group P21/n, a ϭ 11.062(3), b ϭ 11.743(2), c ϭ 15.184(3) A, β ϭ
G. H. Lorimer, Chem. Rev. 1998, 98, 549–561.
3
[10]
˚
101.38(2)°, V ϭ 1933.6(7) A , T ϭ –90 °C, Z ϭ 2, ρcalcd. ϭ 1.349
W. A. King, J. E. Gready, T. J. Andrews, Biochem. 1998, 37,
gcm–3, µ (Mo-Kα) ϭ 21.73 cm–1, ψ scan, transmin.: 0.117, transmax.
:
15414–15422.
[11]
I. Anderson, S. Knight, Y. Lindqvist, T. Lundqvist, C.-I.
0.279, F(000) ϭ 824, 3762 reflections in h(0/13), k(–14/0), l(–18/
17), measured in the range 2.21° Յ Θ Յ 26.30°, 3579 independent
reflections, Rint ϭ 0.028, 2440 reflections with Fo Ͼ 4 σ(Fo), 319
Bränden, G. H. Lorimer, Nature 1989, 337, 229–234.
[12]
T. C. Taylor, I. Anderson, Nat. Struct. Biol. 1996, 3, 95–101.
[13]
A. S. Mildvan, D. C. Fry, E. H. Serpersu, in: Enzymatic and
parameters, 0 restraints, R1obs ϭ 0.035, wR2obs ϭ 0.075, R1all
ϭ
Model Carboxylation and Reduction Reactions for Carbon Di-
oxide Utilization (Eds.: M. Aresta, J. V. Schloss), Kluwer Aca-
demic Publishers, Dordrecht, The Netherlands, 1990, p. 321–
345.
0.084, wR2all ϭ 0.091, GOOF ϭ 1.080, largest difference peak and
–3
˚
hole: 0.305/–0.504 e A .
[14]
D. Walther, U. Ritter, R. Kempe, J. Sieler, B. Undeutsch, Chem.
Crystal Data for 4:[26] [C18H42MgN6O6]ϩ[C26H20Cl4N4Zn2]–, Mr ϭ
1123.9 gmol–1, colourless prism, size 0.32 ϫ 0.30 ϫ 0.28 mm, tri-
clinic, space group P–1, a ϭ 9.6879(4), b ϭ 10.9384(4), c ϭ
Ber. 1992, 125, 1529–1536.
[15]
R. E. Tirpak, R. S. Olsen, M. W. Rathke, J. Org. Chem. 1985,
50, 4877–4879.
[16]
˚
N. Matsamura, N. Asai, S. Yoneda, J. Chem. Soc., Chem. Com-
13.9082(4) A, α ϭ 78.401(2), β ϭ 71.791(2), γ ϭ 78.934(2) °, V ϭ
3
1358.19(8) A , T ϭ –90 °C, Z ϭ 1, ρcalcd. ϭ 1.367 gcm–3, µ (Mo-
˚
mun. 1983, 1487–1488.
[17]
N. Matsamura, T. Ohba, S. Yoneda, Chem. Lett. 1983, 317–
Kα) ϭ 11.43 cm–1, F(000) ϭ 578, 12317 reflections in h(0/12), k(–13/
318.
13), l(–16/17), measured in the range 3.12° Յ Θ Յ 26.35°, com-
[18]
N. Matsamura, T. Ohba, H. Inoue, Bull. Chem. Soc. 1982, 55,
pleteness Θmax ϭ 99.2%, 5496 independent reflections, Rint
0.036, 4374 reflections with Fo Ͼ 4σ(Fo), 304 parameters, 0 re-
straints, R1obs ϭ 0.083, wR2obs ϭ 0.210, R1all ϭ 0.104, wR2all
ϭ
3949–3950.
[19]
D. Walther, M. Ruben, S. Rau, Coord. Chem. Rev. 1999, 182,
68–100.
ϭ
[20] [20a]
C. C. Chang, B. Srinivas, M. L. Wu, W. H. Chiang, M. Y.
0.228, GOOF ϭ 1.004, largest difference peak and hole: 1.419/
Chiang, C. S. Hsiung, Organometallics 1995, 14, 5150–5159. –
[20b] C. C. Chang, M. S. Ameerunisha, Coord. Chem. Rev. 1999,
189, 199–278 and references therein.
–3
˚
–1.350 e A .
Crystal Data for 5:[26] [C18H42MgN6O6]ϩ[C26H20Co2Br4N4]–, Mr ϭ
1282.80 gmol–1, colourless prism, size 0.28 ϫ 0.22 ϫ 0.11 mm, tri-
clinic, space group P–1, a ϭ 9.8104(6), b ϭ 11.0742(5), c ϭ
[21]
A. Bauer, Ber. Dtsch. Chem. Ges. 1907, 40, 2650–2662.
[22]
MOLEN, An Interactive Structure Solution Procedure, Enraf–
Nonius, Delft, The Netherlands, 1990.
[23]
Z. Otwinowski, W. Minor, ‘‘Processing of X-ray Diffraction
˚
14.0020(9) A, α ϭ 76.997(3), β ϭ 71.602(2), γ ϭ 78.400(3) °, V ϭ
Data Collected in Oscillation Mode’’, in: Methods in Enzymo-
logy, vol. 276, ‘‘Macromolecular Crystallography’’, Part A
(Eds.: C.W. Carter, R.M. Sweet), Academic Press, 1997, pp.
307–326.
3
1392.46(14) A , T ϭ –90 °C, Z ϭ 1, ρcalcd. ϭ 1.530 gcm–3, µ (Mo-
˚
Kα) ϭ 35.27 cm–1, semi-empirical, transmin.: 0.432, transmax.: 0.776,
F(000) ϭ 644, 9237 reflections in h(0/12), k(–13/13), l(–16/17),
[24]
G. M. Sheldrick, Acta Crystallogr. Sect. A 1990, 46, 467–473.
measured in the range 3.04° Յ Θ Յ 26.49°, completeness Θmax
ϭ
[25]
G. M. Sheldrick, SHELXL-97, University of Göttingen, Ger-
98.5%, 5693 independent reflections, Rint ϭ 0.085, 3271 reflections
with Fo Ͼ 4σ(Fo), 304 parameters, 0 restraints, R1obs ϭ 0.078, wR
many, 1993.
[26]
Crystallographic data (excluding structure factors) for the
2
obs ϭ 0.178, R1all ϭ 0.1527, wR2all ϭ 0.2164, GOOF ϭ 0.986, largest
structures included in this paper have been deposited with the
Cambridge Crystallographic Data Centre as supplementary
publication no. CCDC-407983 (2a), -135156 (2b), -135157 (3),
-135158 (4), -135159 (5). Copies of the data can be obtained
free of charge on application to CCDC, 12 Union Road, Cam-
bridge CB2 1EZ, UK [Fax: (internat.) ϩ 44-1223/336-033; E-
mail: deposit@ccdc.cam.ac.uk].
–3
˚
difference peak and hole: 0.706/–1.020 e A .
Acknowledgments
Financial support for this work from the Deutsche Forschungsge-
meinschaft (SFB 436), the VW-Stiftung and the Fonds der Chem-
Received July 13, 1999
[I99373]
1060
Eur. J. Inorg. Chem. 2000, 1055Ϫ1060