Synthesis and reactivity of a new oxidation-labile heterobimetallic Mn 6Zn2 carbamate cluster and precursor to nanosized magnetic oxide particles

Article abstract of DOI:10.1002/ejic.201100019

The synthesis and characterization of the new octanuclear mixed carbamate complex [Zn₂Mn₆(μ₄-O)₂(O ₂CN(iPr)₂)₁₂] is reported. This complex is readily oxidized by O₂ to give [Zn₂Mn₆(μ ₄-O)₂(μ₃-O)(O₂CN(iPr) ₂)₁₂] featuring a central O atom statistically distributed over two sites within the void of the inner Mn₆ cage of the carbamate complex. In relation to the situation before oxidation, the metal cage is expanded by a change of the carbamate bonding mode. The two carbamate complexes were used as precursors to magnetic oxide nanoparticles under mild conditions (210-270 °C depending on the heating rate). A mixture of crystalline ZnMn₂O₄ and MnO nanoparticles resulted. The experimental data are complemented by some quantum chemical (DF) calculations.

Full text of DOI:10.1002/ejic.201100019

FULL PAPER
DOI: 10.1002/ejic.201100019
Synthesis and Reactivity of a New Oxidation-Labile Heterobimetallic Mn6Zn2
Carbamate Cluster and Precursor to Nanosized Magnetic Oxide Particles
Dan Domide,[a] Olaf Hübner,[a] Silke Behrens,[a] Olaf Walter,[b] Hubert Wadepohl,[a]
Elisabeth Kaifer,[a] and Hans-Jörg Himmel*[a]
Keywords: Zinc / Manganese / Cluster compounds / Magnetic properties / Oxido ligands / Oligonuclear complexes /
Carbamates
The synthesis and characterization of the new octanuclear
mixed carbamate complex [Zn2Mn6(μ4-O)2(O2CN(iPr)2)12] is
reported. This complex is readily oxidized by O2 to give
[Zn2Mn6(μ4-O)2(μ3-O)(O2CN(iPr)2)12] featuring a central O
atom statistically distributed over two sites within the void of
the inner Mn6 cage of the carbamate complex. In relation to
the situation before oxidation, the metal cage is expanded by
a change of the carbamate bonding mode. The two carb-
amate complexes were used as precursors to magnetic oxide
nanoparticles under mild conditions (210–270 °C depending
on the heating rate). A mixture of crystalline ZnMn2O4 and
MnO nanoparticles resulted. The experimental data are com-
plemented by some quantum chemical (DF) calculations.
carbamato complex of CeIII, Ce4(O2CNiPr2)12, and O2
leading to the product Ce4(μ3-O)2(O2CNiPr2)12.[11] This is
up to now the only example in which partial oxidation of a
carbamate complex with O2 is reported. Examples of
hexanuclear carbamate complexes include [Co6(O2-
CNEt2)12][12] and MnnMg6–n(O2CNEt2)12 (n = 1–6),[13] and
examples for structurally characterized octanuclear carb-
amate complexes comprise [Zn8(μ4-O)2(O2CNiPr2)12],[14]
[Co8(μ4-O)2(O2CNiPr2)12],[15] [Fe8(μ4-O)2(O2CNiPr2)12],[16]
[Zn2Ni6(μ4-O)2(O2CNiPr2)12],[17] and [Zn8–nMgn(μ4-O)2-
(O2CNiPr2)12] (n = 0.62).[18]
Tetranuclear Zn carbamate complexes of the form
[ZnR(O2CNRЈ2)]4 (R, RЈ = alkyl) are the products of reac-
tion between RZnNRЈ2 (or ZnR2/HNRЈ2 mixtures) and
CO2.[3,19,20] We recently reported on the thermal decompo-
sition of the tetranuclear Zn carbamate complexes [ZnEt-
(O2CNR2)]4 (R = iPr or iBu) leading at relatively mild con-
ditions (200 °C) to ZnO nanoparticles.[3] It was also shown
that the tetrameric structure can be split by the action of
the N-base pyridine into dimeric units.[21,22] We reported on
the first monomeric Zn complex featuring κ1-coordinated
carbamato ligands, namely [(tmg)2Zn(O2CN(iBu)2)2] by re-
action between [ZnEt(O2CN(iBu)2)]4 and four or more
equivalents of tetramethylguanidine (tmg).[24] We also
showed that reaction of [ZnEt(O2CN(iPr)2)]4 with only two
equivalents of tmg leads to the binuclear complex [(tmg)-
ZnEt(O2CN(iPr)2)]2.[22] The product of the reaction be-
tween [ZnEt(O2CNR2)]4 and four equivalents of btmgn
(1,8-bis(tetramethylguanidino)naphthalene) turned out to
be the mononuclear complex [(btmgn)ZnEt(O2CN(iPr)2)],
again with a κ1-coordinated carbamato ligand. Finally, if
only two equivalents are used, the trinuclear complex
[(btmgn)Zn3Et3(O2CN(iPr)2)3] is formed.[25] These results
Introduction
Carbamates are of interest for a number of reasons.
Hence they can be used as precursors for oxide thin films
or nanoparticles.[1,2] In addition carbamate complexes of
the elements zinc and magnesium were identified at the
active site of phosphotriesterase and rubisco enzymes. The
active site of rubisco consists of an octahedrally coordi-
nated Mg atom, one of the ligands being a presumably κ1-
coordinated carbamate.[3] In Zn-containing phosphotri-
esterase two Zn atoms are bridged by a carbamate ligand.[4]
A number of carbamate complexes of main group, transi-
tion metal and lanthanide elements have been synthesized
in the past. Of the many examples, we briefly present some
complexes with four or more metals. The tetranuclear Zn
carbamate complexes Zn4(μ4-O)(O2CNR2)6 (e.g., R = Et)[5–6]
could be regarded as a molecular model for ZnO.[7] They
were indeed used as single-source precursors for chemical
vapour deposition of thin ZnO films.[2] Further studies on
these benchmark systems showed that they are amenable to
transamination reactions [8] and exchange of the carboxyl
groups (as evidenced by 13CO2 uptake).[9] The complex
[Al4(μ3-O)2(O2CNiPr2)8] is an example for a tetranuclear
carbamate with two oxide ions in its center.[10] DellЈAmico
et al. reported on the reaction between a tetranuclear
[a] Anorganisch-Chemisches Institut, Ruprecht-Karls-Universität
Heidelberg,
Im Neuenheimer Feld 270, 69120 Heidelberg, Germany
Fax: +49-6221-545707
E-mail: hans-jorg.himmel@aci.uni-heidelberg.de
[b] Institut für Katalyseforschung und -technologie,
Karlsruher Institut für Technologie (KIT), Campus Nord,
Postfach 3640, 76021 Karlsruhe, Germany
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201100019.
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motivated us to find ways to break up the tetranuclear car-
bamato complex by an N-base which itself is attached to a
transition metal complex of a different late-transition metal
in the formal oxidation state +II. By this means it should
be possible to cleave the complex and at the same time in-
troduce a second metal. Herein we report on the synthesis
of a new octanuclear Mn6Zn2 carbamate complex using this
new synthetic approach to heterobimetallic carbamato
complexes. The complex exhibits a central empty void. Oxi-
dation with O2 leads to the filling of the void with one oxy-
gen. The carbamate complex will be shown to be a precur-
sor for the formation of Zn/Mn oxide nanoparticles under
mild conditions.
Figure 1. Section of the polymeric structure of [(mapy)MnCl2] in
the crystalline state. Vibrational ellipsoids are drawn at the 50%
probability level. Hydrogen atoms bound to carbon atoms were
omitted for sake of clarity. Colour code: Mn purple, N blue, C dark
grey.
zinc carbamate [ZnEt(O2CN(iPr)2)]4. In the case of the li-
gands py and maepy, the experiments gave no evidence for
the formation of a mixed metal carbamate. On the other
hand, reaction between [ZnEt(O2CN(iPr)2)]4 and [(mapy)-
MnCl2] resulted in a solid product which was recrystallized
from toluene to give colourless crystals of the octanuclear
mixed metal carbamate [Zn2Mn6(μ4-O)2(O2CN(iPr)2)12]
(toluene solvate). The molecular structure as derived from
an XRD analysis of the crystalline material is visualized in
Figure 2. The molecules possess crystallographic C2 sym-
metry. The six Mn atoms are arranged in the form of a
highly distorted trigonal antiprism. The trigonal faces are
capped by OZn units. Three carbamate ligands on each side
of the complex connect the ZnII with the MnII atoms in a
μ2 bonding mode. The two trigonal Mn3 units of the central
trigonal antiprism are connected by additional six carb-
amate ligands with a μ3 bonding mode. One of the carb-
amate oxygen atoms is bound to only one manganese ion,
while the other carbamate oxygen is bound to one manga-
nese ion of the same trigonal face and in addition to one
manganese ion from the other trigonal face of the Mn6
trigonal antiprism. The molecular structure thus resembles
the structures of the previously characterized octa-
nuclear carbamate complexes [Zn8(μ4-O)2(O2CN(iPr)2)12],[16]
Results and Discussion
Synthesis and Characterization of the New Mn6Zn2
Carbamate
As already mentioned, we studied recently the fragmen-
tation of the tetranuclear Zn carbamate complex [ZnEt-
(O2CNR2)]4 (R = iPr or iBu) by N-bases.[25] Motivated by
these studies, we tried to synthesize new mixed carbamates
by reaction of this tetramer with a transition metal complex
to which ligands with secondary amino groups are at-
tached. The base fulfils two tasks: first it should break up
the tetrameric unit, and secondly it should provide protons
for ethane abstraction. We therefore synthesized three MnII
dichloride complexes, containing as ligands the bases pyr-
idine (py), 2-(methylamino)pyridine (mapy), or 2-[2-(meth-
ylamino)ethyl]pyridine (maepy), see Scheme 1. The com-
plexes were generally used immediately after their prepara-
tion without isolation. However, in the case of py and
maepy, we obtained crystal structures. Of these, the struc-
ture of [(py)2MnCl2] in the solid state was already deter-
mined previously and shown to consist of linear chains of
octahedrally coordinated MnII centers, in which all chlo-
rides adopt bridging positions.[23] On the other hand, the
structure of the complex [(maepy)MnCl2] was previously
unknown. It was crystallized from methanol solutions, pro-
viding crystals suitable for an XRD analysis (see Figure 1).
Again octahedral coordination is achieved, and both the
pyridine N as well as the amino N atoms are bound to the
MnII centers. Linear chains result as depicted in Figure 1.
The Mn–Cl distances cover the range 253–261 pm, and the
closest Mn···Mn separations amount to 379.6(3) pm. In the
case of mapy, no crystal formation was monitored.
Scheme 1.
Figure 2. Molecular structure of the octanuclear carbamate com-
plex [Zn2Mn6(μ4-O)2(O2CN(iPr)2)12]. Vibrational ellipsoids are
drawn at the 50% probability level. The amino groups (NiPr2) of
the carbamato ligands were omitted for sake of clarity. Colour
THF solutions of each of the three Mn complexes,
formed from manganese dichloride and the ligand, were di-
rectly brought to reaction with a toluene solution of the code: Mn purple, Zn cyan, N blue, C dark grey.
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New Heterobimetallic Mn6Zn2 Carbamate Cluster
[Co8(μ4-O)2(O2CN(iPr)2)12],[17] [Fe8(μ4-O)2(O2CN(iPr)2)12],[18] already initially coordinated to the manganese dichloride,
[Zn2Ni6(μ4-O)2(O2CN(iPr)2)12],[19] and [Zn8–nMgn(μ4-O2)- which was used in the overall formula MnCl2·0.22H2O. TG
(O2CN(iPr)2)2]12 (n = 0.62).[20] The UV/Vis spectrum of measurements confirmed this MnCl2/H2O ratio (see Sup-
[Zn2Mn6(μ4-O)2(O2CN(iPr)2)12] in a CHCl3 solution is porting Information). The coordinated water can also be
shown in Figure 3 (a). It contains relatively weak absorp- clearly seen in the IR spectra (see also Supporting Infor-
tions centered near 238 and 301 nm. The magnetic curve mation).[24] The amount of water is critical, and a larger
derived from SQUID measurements is presented in Fig- excess of it reduces the yield. Hence when we increased the
ure 3 (b). At 300 K, the χT value was found to be MnCl2/H2O ratio further to 1:2 by addition of extra water
15.6 Kcm3 mol–1. In the absence of magnetic coupling be- to a mixture of the manganese dichloride, mapy, and
tween the MnII centers and high-spin configuration of each [ZnEt(O2CN(iPr)2)]4 (see Exp. Section), we only isolated
MnII (S = 5/2), the maximum: χT value for the 6ϫ5 spins crystals of the salt [(mapy)H]+[(mapy)ZnCl3]– (see Support-
can be estimated to be 26.26 Kcm3 mol–1 (if g = 2 and spin- ing Information) from the reaction mixture.
orbit coupling can be neglected). As anticipated, antiferro-
magnetic coupling between the spin centers reduces the χT
value considerably. The behaviour at lower temperatures is
in line with antiferromagnetic coupling resulting in a singlet
Oxygen Uptake
ground electronic state. EPR spectra of the complex are
When a solution of the carbamate [Zn2Mn6(μ4-O)2-
provided in the Supporting Information. They show a rela- (O2CN(iPr)2)12] was stirred under air, a colouring of the
tively broad signal with no fine structure, and are similar solution signals (partial) MnII oxidation. First information
to the spectra reported for the Mn carbamate [Mn6(O2- about the product was derived from mass spectrometric
CNEt2)12].[15]
data. Figure 4 (a) shows the M+ peak region of the mass
spectrum recorded after 15 min air expose of a [Zn2Mn6-
(μ4-O)2(O2CN(iPr)2)12] solution. The isotopic pattern fits
nicely to the simulated pattern. In addition, a peak appears
which can be explained by the uptake of one oxygen atom.
After completion of this oxidation process, the peak due to
[Zn2Mn6(μ4-O)2(O2CN(iPr)2)12] almost completely disap-
peared (see Figure 4, b). The spectrum provided in Figure
S3 of the Supporting Information also indicates that at this
stage no product with two or more additional oxygen atoms
is formed. It proved possible to crystallize the oxidation
product as a toluene solvate, and Figure 5 shows its molecu-
lar structure. The molecules are centrosymmetric. The void
of the carbamate is now filled with one oxygen atom, lead-
ing to the formula [Zn2Mn6(μ4-O)2(μ3-O)(O2CN(iPr)2)12].
The oxygen atom occupies two different sites (separated by
80.1(1) pm) in this void. Three of the six Mn atoms become
sixfold coordinated by oxygen [distorted octahedral coordi-
nation mode, with Mn–O distances of 208.1(9), 226(1) and
239(1) pm]. The three remaining Mn···O separations
amount to 247(1), 261(1) and 271(2) pm. The six Mn atoms
are now clearly arranged in the form of a trigonal anti-
prism. The six carbamate ligands bridging the two trigonal
faces still adopt a μ3-bonding mode. However, one of the
carbamate oxygen atoms is now bound to two Mn centers
of the same trigonal face of this antiprism, while the other
oxygen atom is bound to only one Mn ion of the other
trigonal face. This change in the carbamate bonding mode
relative to the situation before oxidation increases the size
of the cage within the complex. Figure 5 also illustrates the
packing of the molecules in the crystalline state, showing
Figure 3. a) UV/Vis spectra in CHCl3 solutions; and b) χT vs. T
plots from SQUID measurements (at 500 Oe) of the solid com-
pound as recorded for the carbamate complexes Zn2Mn6(μ4-O)2-
(O2CN(iPr)2)12] and [Zn2Mn6(μ4-O)2(μ3-O)(O2CN(iPr)2)12].
Generally, the μ4-O atoms in carbamate complexes arise that all complexes are lined up. With 885.8(2) pm, the short-
from water which is added after carbamate formation to the est intermolecular Zn···Zn separations are shorter than the
solution. For example, [Zn4(μ4-O)(O2CNEt2)6] was pre- intramolecular Zn···Zn separation of 889.0(2) pm. The UV/
pared by addition of H2O to a reaction mixture of ZnEt2, Vis spectra recorded for [Zn2Mn6(μ4-O)2(μ3-O)(O2CN-
HNEt2 and CO2.[16] In our case, we did not add any extra (iPr)2)12] is compared to that of the initial [Zn2Mn6-
water at the end of the synthesis of [Zn2Mn6(μ4-O)2- (μ4-O)2(O2CN(iPr)2)12] complex in Figure 3 (a). The main
(O2CN(iPr)2)12]. The water needed to form the complex is difference between the spectra before and after oxidation is
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the increase of the extinction coefficient upon oxidation.
The wavenumbers of the absorption maxima (294 and
232 nm) are virtually unchanged. The χT–T curve for
[Zn2Mn6(μ4-O)2(μ3-O)(O2CN(iPr)2)12] as derived from
SQUID measurements is presented in part b of Figure 3.
As anticipated (simply on the basis of the reduced number
of unpaired electrons), the χT value at room temperature
(9.7 Kcm3 mol–1) is smaller than that of the complex before
oxidation (15.6 Kcm3 mol–1).
It should be noted that the carbamate [Zn2Mn6(μ4-O)2-
(μ3-O)(O2CN(iPr)2)12] is not formed if pure O2 is bubbled
(for 20 min) through a toluene solution of [Zn2Mn6-
(μ4-O)2(O2CN(iPr)2)12]. Instead, we observed in this case
decomposition and further oxidation of manganese to give
MnO2. The [Zn2Mn6(μ4-O)2(μ3-O)(O2CN(iPr)2)12] complex
also is oxidized further to give MnO2 if pure O2 is used as
reactant. Thus [Zn2Mn6(μ4-O)2(μ3-O)(O2CN(iPr)2)12] most
likely represents only an intermediate of the oxidation pro-
cess.
Thermal Stability and Decomposition to Magnetic Oxides
Figure 4. Mass spectrometry of the carbamate complexes. a) mix-
ture of non-oxidized and oxidized carbamate obtained upon expo-
sure of Zn2Mn6(μ4-O)2(O2CN(iPr)2)12] to air for a period of
15 min. b) The oxidized carbamate [Zn2Mn6(μ4-O)2(μ3-O)-
(O2CN(iPr)2)12].
In line with other carbamates,[3] decomposition of
[Zn2Mn6(μ4-O)2(O2CN(iPr)2)12] occurs already at mild con-
ditions. Figure 6 shows the TG curves for three different
heating rates (2, 5 and 10 °C min–1). For all heating rates,
the complex witnesses a similar loss of ca. 73% of its initial
mass. This fits nicely to the theoretical mass loss of 74%
for decomposition to give an oxide of the overall formula
ZnMn3O4. The decomposition is completed at 210 °C for a
heating rate of 2 °C min–1, but requires temperatures of
more than 270 °C if the heating rate is increased to 10 °C
min–1. This behaviour indicates kinetic control of the de-
composition for high heating rates implying that decompo-
sition is a relatively slow process. A similar behaviour was
observed for decomposition of [ZnEt(O2CN(iPr)2)]4 to give
ZnO.[3] The decomposition product appears almost black.
For comparison, ZnO doped with Mn (as often the case in
the naturally occurring mineral zincite)[25] is red-coloured.
The potential to use Mn doped ZnO as red pigment was
recently evaluated in detail.[26] In this study, the Zn1–xMnxO
samples (xՅ0.17) were synthesized from ZnO and MnO in
Figure 5. Molecular structure of the octanuclear carbamate com-
plex [Zn2Mn6(μ4-O)2(μ3-O)(O2CN(iPr)2)12]. Vibrational ellipsoids
are drawn at the 50% probability level. The amino groups (NiPr2)
of the carbamato ligands were omitted for sake of clarity. The μ3-
O (pale colour) is distributed equally over two sites within the cen-
tral cage. The packing of the molecules in the crystalline material
is also shown. Colour code: Mn purple, Zn cyan, N blue, C dark
grey.
Figure 6. TG curves recorded for [Zn2Mn6(μ4-O)2(O2CN(iPr)2)12]
at three different heating rates (2, 5 and 10 °C min–1).
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New Heterobimetallic Mn6Zn2 Carbamate Cluster
the presence of NH4Cl at 850 °C. In our case, the dark col- Quantum Chemical Calculations
our of the product points to formation of a different oxide,
in line with the high Mn content in the precursor.
Calculations were carried out using the BP functional in
Further information was obtained from X-ray powder combination with a def2-SV(P) basis set. The carbamate
diffraction data (for a heating rate of 5 K min–1). Figure 7 [Zn2Mn6(μ4-O)2(O2CN(iPr)2)12] features six MnII centers
displays a typical diffraction pattern, which cannot be ex- each with five unpaired electrons, yielding in total 30 un-
plained by a single phase, but with respect to the results paired electrons. The oxidized carbamate [Zn2Mn6(μ4-O)2-
from the EDX analyses rather points to formation of (μ3-O)(O2CN(iPr)2)12] contains four MnII and two MnIII
ZnMn2O4 and MnO nanoparticles. For comparison, the centers yielding 28 unpaired electrons. Owing to magnetic
characteristic lines of these two lattices are included in Fig- coupling between the different centers a large number of
ure 7. No additional amorphous particles are formed. A electronic states with small energy separations is expected.
Debye–Scherer analysis to estimate the particle sizes proved For simplification we only considered high-spin terms and
impossible due to peak overlapping. However, from the also replaced the iPr groups by hydrogen atoms. Thus the
powder pattern it can be derived that the particles have to calculations assume a 31A term for [Zn2Mn6(μ4-O)2-
be small as otherwise sharper peaks would have been ob- (O2CNH2)12], and a 29A term for [Zn2Mn6(μ4-O)2(μ3-
served in the diffractogramms. These findings are supported O)(O2CNH2)12]. They especially addressed the following
by the results of a TEM analyses (see Figure 7 and Support- points: 1) Search for other possible energy minima and the
ing Information) showing that the particle size is Յ23 nm activation barrier for conversions from one minimum to the
(mean particle size of 9 nm). EDX measurements at dif- other for oxygen in the void. 2) The thermodynamic proper-
ferent places of the sample (see Supporting Information) ties for O2 uptake.
show slight variations in the Mn/Zn ratio, which in average
amounts to ca. 3:1, in line with the other analytical data.
The minimum energy structure of [Zn2Mn6(μ4-O)2-
(O2CNH2)12] turned out to exhibit D3 symmetry. In the case
of the oxidized complex [Zn2Mn6(μ4-O)2(μ3-O)(O2-
CNH2)12], two different kinds of C1 symmetric structures
resulted, in which the additional O atom within the Mn6
void is bound to three Mn atoms (with Mn–O distances of
about 200 pm) and exhibits longer O···Mn separations to
the three remaining Mn atoms of ca. 300 pm. For each of
the two kinds of minima, there exist six identical structures
on the hypersurface that result from binding the oxygen
atom to six different but symmetry equivalent sites within
the D3 symmetric structure of the [Zn2Mn6(μ4-O)2-
(O2CNH2)12] complex. The two different kinds of minima
differ in the location of the MnIII centers, given the O atom
is bound to the same three Mn atoms, see Min1Ј and Min2
in Figure 8. (The short Mn–O distances indicate the MnIII
centers.) The energy difference between the structures
amounts to 0.3 kJmol–1 only, Min1Ј being the lower one.
Furthermore, a transition structure was calculated that
separates two minimum structures of different kind with the
oxygen at adjacent sites. Those two structures share the lo-
cation of the MnIII centers, i.e. within both structures, the
MnIII oxidation state is attributed to the same Mn atoms
and the motion corresponds to a simple oxygen shift.
Within this transition structure (C1 symmetry), the central
O atom is bound to two Mn atoms with relatively short
distances (189.2 and 192.3 pm), and to two further Mn
atoms with much longer distances of 240.2 and 275.2 (see
Figure 8). According to the calculations, the energy of the
Figure 7. a) Powder diffractogramm, and b) TEM image with par-
ticle size distribution (mean particle diameter 8.8Ϯ4.3 nm) ob-
tained after thermal decomposition of [Zn2Mn6(μ4-O)2(O2CN-
(iPr)2)12] (end temperature 300 °C, 5 °C min–1 heating rate).
The TG curves recorded for [Zn2Mn6(μ4-O)2(μ3-O)- transition state lies only 0.8 kJmol–1 higher than that of the
(O2CN(iPr)2)12] are similar to those of [Zn2Mn6(μ4-O)2- lower minimum structure. Thus
a barrier less than
(O2CN(iPr)2)12], showing the same dependence of the final 1 kJmol–1 separates the two minima.
temperature for complete decomposition on the heating
For a free relocation of the oxygen within the cage via
rate. Again, the XRD data (see Supporting Information) the calculated transition structure two steps are necessary:
showed the presence of crystalline ZnMn2O4 and MnO (a) the transfer of the O atom to an adjacent place and (b)
nanoparticles. An analysis of the TEM pictures (see Sup- the interchange of the oxidation states of a MnII/MnIII pair.
porting Information) might indicate a slightly narrower Unfortunately, the transition structure for the second step is
particle size distribution.
hard to obtain, because different electronic configurations
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tion between [Zn2Mn6(μ4-O)2(O2CNH2)12] and H2O to give
[Zn2Mn6(μ4-O)2(μ3-O)(O2CNH2)12] and H2, the reaction
enthalpy ΔRH (0 K) is –86.3 kJmol–1.
Conclusions
Herein we presented a new method for the preparation
of heterobimetallic carbamates, which we applied exemplar-
ily for the formation of Mn/Zn carbamates. It starts with
an alkylzinc carbamate [ZnR(O2CNRЈ2)]4 (R = Et, RЈ =
iPr), which is brought to reaction with a manganese dichlo-
ride complex featuring a pyridine ligand to which a second-
ary amine unit is attached. This amine unit fulfils two tasks:
it splits up the tetrameric Zn carbamate and provides pro-
tons for alkane elimination. In this way we synthesized the
octanuclear carbamate complex [Zn2Mn6(μ4-O)2(O2CN-
(iPr)2)12]. We are currently evaluating the synthesis of Co/
Zn carbamates using a similar route, and preliminary re-
sults show that indeed new heterobimetallic carbamates can
be formed also in this case.[27] These heterobimetallic carba-
mates can be used for the synthesis of mixed metal oxide
nanoparticles by decomposition of the carbamates under
mild conditions (210–270 °C). A mixture of ZnMn2O4 and
MnO nanoparticles is produced. Nanocrystalline ZnMn2O4
(spinel structure) is of considerable interest for several ap-
plications, e.g. as a novel lithium-storage material.[28] Its
Figure 8. Calculated structure of two different minima (Min1 and
Min2) and a relating transition structure of [Zn2Mn6(μ4-O)2(μ3-
O)(O2CNH2)12]. In addition, a further minimum Min1Ј equivalent
to Min1 is shown. Min2 and Min1Ј can be interconverted by single
electron transfer between two M atoms. The calculations relied on fabrication by polymer-pyrolysis methods needs much
the BP functional and a SV(P) basis set.
higher temperatures (pyrolysis step at 600 °C) than the pro-
cedure described herein.
The [Zn2Mn6(μ4-O)2(O2CN(iPr)2)12] complex turned out
to be extremely oxidation sensitive. Hence reaction with O2
yields the new carbamate complex [Zn2Mn6(μ4-O)2(μ3-
O)(O2CN(iPr)2)12] (as first isolated intermediate on the way
associated with the different oxidation states are involved,
and therefore the determination of the barrier for oxidation
state interchange demands a multireference treatment. For
to MnO2 formation). In this complex, the empty void
the given system the effort would be to large to get results
within the carbamate complex is filled with one oxygen
in a reasonable time scale, and therefore a value for the
atom, which in the crystalline material is equally distributed
effective barrier of the oxygen movement cannot be pro-
over two sites. The void is increased relative to the situation
vided.
before oxidation by a change in the carbamate bonding
In any case, since the movement is accompanied by a
change in the coordination sphere and since packing effects
mode.
are likely to reduce the flexibility of the complex, the free
movement is probably restricted within the crystalline
Experimental Section
phase. Apparently, the shape of the complex allows the sta-
tistical packing of complexes with two orientations. The cal-
culated dipole moment for [Zn2Mn6(μ4-O)2(μ3-O)(O2CN-
(iPr)2)12] amounts to ca. 0.83 debye. On the basis of these
calculations and the favourable packing of the molecules in
General: All reactions were carried out using standard Schlenk
technique under an atmosphere of N2. All solvents were dried prior
to their use. UV/Vis spectra were taken with a Varian CARY 5000
spectrometer. For the SQUID direct current (dc) measurements, a
the crystal (see Figure 5), it might be possible to orientate Quantum Design MPMS-XL 5 machine was used. EPR measure-
ments relied on a Bruker Elexsys E500 spectrometer. Positive field
desorption (FD+) mass spectrometric measurements were taken on
a JeolJMS mass spectrometer. Elemental analyses were carried out
at the Microanalytical Laboratory of the University of Heidelberg.
IR spectra were recorded on a Biorad Excalibur FTS3000 spec-
trometer. Thermogravimetric (TG) measurements were carried out
on a Mettler TC15 under Ar atmosphere in a temperature range
30–600 °C. The heating rate was varied between 2 and 10 K/min.
TEM experiments were performed with a FEI Tecnai F20 TEM
[equipped with an energy dispersive X-ray (EDX) spectrometer]
the individual dipole moments by an external electric field
(ferroelectric behaviour).
For both the oxidized and the reduced form of the com-
plex, vibrational spectra have been calculated and they turn
out to be very similar (see Supporting Information), which
is also the case for the observed spectra. The calculations
estimate the reaction of [Zn2Mn6(μ4-O)2(O2CNH2)12] with
1/2 O2 to give [Zn2Mn6(μ4-O)2(μ3-O)(O2CNH2)12] to be ac-
companied by a reaction enthalpy ΔRH (0 K) (only elec-
tronic contribution) of –454.1 kJmol–1. In the case of reac- with field emission gun at 200 kV on carbon-coated 400 mesh cop-
1392 Eur. J. Inorg. Chem. 2011, 1387–1394
www.eurjic.org
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

New Heterobimetallic Mn6Zn2 Carbamate Cluster
per grids. The XRD measurements were performed on a STADI P
machine from STOE and a Siemens D500 diffractometer using Cu
radiation. The data were evaluated with EVA (DIFFRACplus,
EVA 9.0, XRD evaluation programme, Bruker AXS, 2003).
[Mn6Zn2(μ4-O)2(μ3-O)(O2CN(iPr)2)12] (0.012 g, 0.005 mmol) was
filled in a platinum crucible. The compound was heated with a
heating rate of 5 °C min–1 (for the TG analysis also heating rates
of 2 and 10 °C min–1 were used) to an end temperature of 300 °C
under a nitrogen atmosphere with a flow rate of 150 mLmin–1.
XRD measurements indicated formation of the oxides ZnMn2O4
and MnO. The EDX analysis is in line with an overall Mn/Zn ratio
of, 3:1. TEM data of 210 particles argue for a mean particle size
of 8.4Ϯ3.1 nm (minimum: 2.5 nm, maximum: 18.2 nm).
[Mn6Zn2(μ4-O)2(O2CN(iPr)2)12]: 2-(Methylamino)pyridine (mapy)
(0.440 g, 4.06 mmol) was added to a THF (10 mL) suspension of
MnCl2·0.22H2O (0.255 g, 1.96 mmol). The clear reaction mixture
was stirred at room temp. for a period of 1 h. Upon addition of a
toluene solution (10 mL) of [EtZn(O2CN(iPr)2)]4 (1 g, 1.02 mmol),
the reaction mixture turned to a yellow colour. It was stirred at
room temp. for 72 h and then evaporated to dryness. Recrystalli-
zation of the solid product from toluene at room temp. yielded
colourless crystals of the product (0.21 g, 0.094 mmol, 27.8%).
C84H168Mn6N12O26Zn2·C7H8(toluene) (2313.81): calcd. C 47.19, H
7.62, N 7.26; found C 47.33, H 7.62, N 7.46. IR (CsI): 1566, 1450
X-ray Crystallographic Study: Suitable crystals were taken directly
out of the mother liquor, immersed in perfluorinated polyether oil,
and fixed on top of a glass capillary.
Intensity data were collected at low temperature (100 K) with Non-
ius Kappa CCD and Bruker AXS Smart 1000 CCD diffractometers
(Mo-Kα radiation, graphite monochromator, λ = 0.71073 Å). The
data collected were processed using the standard Nonius[29] and
Bruker AXS software.[30] The structures were solved by conven-
tional direct methods [31] ([(maepy)MnCl2] and [(mapy)H]+[(mapy)-
ZnCl3]– (see Supporting Information)) or by the charge flip pro-
cedure,[32,33] and refined by full-matrix least-squares methods based
on F2 against all unique reflections.[34] All non-hydrogen atoms
were given anisotropic displacement parameters. Hydrogen atoms
were put at calculated positions and refined with a riding model.
Similarity restraints were used to achieve acceptable geometry of
the twelve isopropyl groups (some of which were disordered) in
[Zn2Mn6(μ4-O)2(μ3-O)(O2CN(iPr)2)12]. Due to severe disorder, elec-
tron density attributed to the solvent of crystallization (toluene)
was removed from the structure (and the corresponding Fobs) of
this complex with the BYPASS procedure,[35] as implemented in
PLATON (SQUEEZE).[36] The occupancy of the two symmetry
equivalent (μ3-O) positions refined to 0.54(1). In view of the short
distance between the two sites their occupancy was constrained to
0.5. Graphical handling of the structural data during solution
and refinement was performed with XPMA.[37] Crystal
[ν(CO)] cm–1. UV/Vis (CHCl3,
c
=
3.10ϫ10–3 mol·L–1):
33112 cm–1 (301 nm, ε = 32 dm3 mol–1 cm–1), 42016 cm–1 (238 nm,
ε = 110 dm3 mol–1 cm–1). UV/Vis (CH3CN, c = 2.6ϫ10–3 mol·L–1):
33112 cm–1 (301 nm, ε = 54 dm3 mol–1 cm–1), 41322 cm–1 (238 nm,
ε = 113 dm3 mol–1 cm–1). UV (solid): λmax = 314, 238 nm. MS(FD+)
calcd. 2222.70; found 2222.72. Crystal data for [Mn6Zn2(μ4-
O)2(O2CN(iPr)2)12]·toluene, C91H175Mn6N12O26Zn2, Mr = 2313.81,
0.22ϫ0.18ϫ0.16 mm3, orthorhombic, space group Pbcn, a =
22.466(11) Å,
b = 23.113(12) Å, c = 23.385(11) Å, V =
12143(11) Å3, Z = 4, dcalcd. = 1.266 Mgm–3, Mo-Kα radiation
(graphite-monochromated, λ = 0.71073 Å), T = 100 K, θrange 1.74
to 30.03°. Reflections measured 289662, independent 17779, Rint
0.0661. Final R indices [IϾ2σ(I)]: R1 = 0.0516, wR2 = 0.1308.
=
[Mn6Zn2(μ4-O)2(μ3-O)(O2CN(iPr)2)12]: A toluene solution (5 mL)
of [Zn2Mn6(μ4-O)2(O2CN(iPr)2)12] (0.0231 g, 0.01 mmol) was
stirred for 48 h at room temp. (allowing a few air contacts) during
which time the reaction mixture changes the colour from colourless
to brown. The solution was then evaporated to dryness. The solid
product was recrystallized from toluene/hexane, 4:1 at room tem-
perature to give brown crystals (0.0224 g, 0.0097 mmol, 97%).
C84H168Mn6N12O27Zn2·C7H8(toluene) (2329.81): calcd. C 46.33, H
7.61, N 7.21; found C 47.07, H 7.71, N 7.46. IR (CsI): 1566, 1466
data for [(maepy)MnCl2], C16H24Cl4Mn2N4, Mr
= 524.07,
0.30ϫ0.20ϫ0.15 mm3, monoclinic, space group P21/n,
a
=
10.971(2) Å, b = 14.929(3) Å, c = 12.844(3) Å, β = 92.39(3)°, V =
2984(3) Å3, Z = 4, dcalcd. = 1.656 Mgm–3, Mo-Kα radiation (graph-
ite-monochromated, λ = 0.71073 Å), T = 100 K, θrange 2.09 to
[ν(CO)] cm–1. UV/Vis (CHCl3,
c
=
3.10ϫ10–3 mol·L–1):
34013 cm–1 (294 nm, ε = 114 dm3 mol–1 cm–1), 43103 cm–1 (232 nm,
ε = 376 dm3 mol–1 cm–1). UV (solid): λmax = 500, 288, 244 nm.
MS(FD+) calcd. 2238.69; found 2238.53. Crystal data for
30.03°. Reflections measured 11840, independent 6131, Rint
=
0.0174. Final R indices [IϾ2σ(I)]: R1 = 0.0292, wR2 = 0.0695.
Crystal data for [(mapy)H]+[(mapy)ZnCl3]–, C12H17Cl3N4Zn, Mr =
389.02, 0.20ϫ0.12ϫ0.12 mm3, monoclinic, space group P21/n, a
= 7.4320(15) Å, b = 14.229(3) Å, c = 15.378(3) Å, β = 90.88(3)°, V
= 1626.0(6) Å3, Z = 4, dcalcd. = 1.589 Mgm–3, Mo-Kα radiation
(graphite-monochromated, λ = 0.71073 Å), T = 100 K, θrange 1.95
[Mn6Zn2(μ4-O)2(μ3-O)(O2CN(iPr)2)12]·toluene,
C91H175Mn6-
N12O27Zn2, Mr = 2329.81, 0.12ϫ0.12ϫ0.06 mm3, triclinic, space
¯
group P1, a = 14.776(7) Å, b = 14.852(8) Å, c = 17.114(8) Å, α =
74.003(13)°, β = 66.337(10)°, γ = 60.644(8)°, V = 2984(3) Å3, Z =
1, dcalcd. = 1.297 Mgm–3, Mo-Kα radiation (graphite-monochro-
mated, λ = 0.71073 Å), T = 100 K, θrange 1.71 to 25.03°. Reflections
measured 50334, independent 10543, Rint = 0.076. Final R indices
[IϾ2σ(I)]: R1 = 0.0567, wR2 = 0.1341.
to 30.06°. Reflections measured 8934, independent 4739, Rint
0.0417. Final R indices [IϾ2σ(I)]: R1 = 0.0412, wR2 = 0.0876.
=
CCDC-793885 (for [Mn6Zn2(μ4-O)2(O2CN(iPr)2)12]), -793886 (for
[Zn2Mn6(μ4-O)2(μ3-O)(O2CN(iPr)2)12]), -793486 (for [(maepy)-
MnCl2]) and -793487 (for [(mapy)H]+[(mapy)ZnCl3]–) contain the
supplementary crystallographic data for this paper. These data can
be obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Preparation of Zn/Mn Heterobimetallic Nanoparticles Starting with
[Mn6Zn2(μ4-O)2(O2CN(iPr)2)12]:
The
carbamate
precursor
[Mn6Zn2(μ4-O)2(O2CN(iPr)2)12] (0.012 g, 0.005 mmol) was filled in
a platinum crucible. The compound was heated with a heating rate
of 5 °C min–1 (for the TG analysis also heatig rates of 2 and 10 °C
min–1 were used) to an end temperature of 300 °C under a nitrogen
atmosphere with a flow rate of 150 mLmin–1. XRD measurements
indicated formation of the oxides ZnMn2O4 and MnO. The EDX
analysis is in line with an overall Mn/Zn ratio of, 3:1. TEM data
of 105 particles argue for a mean particle size of 8.8Ϯ4.3 nm (mini-
mum: 1.9 nm, maximum: 23.0 nm).
Calculational Details: Density functional calculations were per-
formed with the program package TURBOMOLE[38] using the BP
functional[39] in combination with the def2-SV(P) basis set.[40] For
the numerical integration grid “m4“ was used. The two-electron
integrals were calculated with the RI approximation.[41] The appro-
priate auxiliary basis sets were employed.[42]
Preparation of Zn/Mn Heterobimetallic Nanoparticles Starting with
Supporting Information (see also the footnote on the first page of
[Mn6Zn2(μ4-O)2(μ3-O)(O2CN(iPr)2)12]: The carbamate precursor this article): TG and IR data for the MnCl2·0.22H2O sample used
Eur. J. Inorg. Chem. 2011, 1387–1394
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
1393

H.-J. Himmel et al.
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Received: January 7, 2011
Published Online: February 15, 2011
1394
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© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2011, 1387–1394