Synthesis, Crystal Structures, and Thermolysis Studies of Heteronuclear Transition Metal Aluminum Alcoholates

Article abstract of DOI:10.1002/zaac.202000246

Heteronuclear alcoholate complexes [M{Al(OiPr)₄}₂(bipy)] (2-M, M = Fe, Co, Ni, Cu, Zn) and [M{Al(OcHex)₄}₂(bipy)] (3-M, M = Fe, Co, Ni, Zn) are formed by adduct formation of [M{Al(OiPr)₄}₂] (1-M, M = Fe, Co, Ni, Cu, Zn) with 2,2'-bipyridine and transesterification reaction with cHexOAc. According to crystal structure analyses, in 2-M and 3-M the central transition metal ion M²⁺ is coordinated by two chelating Al(OR)₄^– moieties and one bipyridine ligand in an octahedral arrangement. Treating 1-Cu with 2,2'-bipyridine leads to a reduction process, whereat the intermediate [Cu{Al(OiPr)₄}(bipy)₂][Al(OiPr)₄] (4) could be structurally characterized. During conversion of the iso-propanolate ligands in 1-Cu to cyclohexanolate ligands, Cu²⁺ is reduced to Cu⁺ forming [Cu{Al(O^cHex)₄}(py)₂] (5). UV/Vis-spectra and results of thermolysis studies by TG/DTA-MS are reported.

Full text of DOI:10.1002/zaac.202000246

Journal of Inorganic and General Chemistry
DOI: 10.1002/zaac.202000246
ARTICLE
Zeitschrift für anorganische und allgemeine Chemie
Synthesis, Crystal Structures, and Thermolysis Studies of
Heteronuclear Transition Metal Aluminum Alcoholates
Sebastian Küsel[a] and Harald Krautscheid*[a]
Abstract. Heteronuclear alcoholate complexes [M{Al(OiPr)4}2(bipy)] idine ligand in an octahedral arrangement. Treating 1-Cu with 2,2Ј-
(2-M, M = Fe, Co, Ni, Cu, Zn) and [M{Al(OcHex)4}2(bipy)] (3-M,
Fe, Co, Ni, Zn) are formed by adduct formation of
bipyridine leads to a reduction process, whereat the intermediate
[Cu{Al(OiPr)4}(bipy)2][Al(OiPr)4] (4) could be structurally charac-
M
=
[M{Al(OiPr)4}2] (1-M, M = Fe, Co, Ni, Cu, Zn) with 2,2Ј-bipyridine terized. During conversion of the iso-propanolate ligands in 1-Cu to
and transesterification reaction with cHexOAc. According to crystal cyclohexanolate ligands, Cu2+ is reduced to Cu+ forming
structure analyses, in 2-M and 3-M the central transition metal ion
M2+ is coordinated by two chelating Al(OR)4– moieties and one bipyr- studies by TG/DTA-MS are reported.
[Cu{Al(OcHex)4}(py)2] (5). UV/Vis-spectra and results of thermolysis
ary alcohols [Equation (2)].[49] For shifting the equilibrium to
Introduction
the product side, the liberated isopropyl alcohol is distilled off.
During that reaction, the steric demand of the alcohol is cru-
cial. If the organic group of the alcohol becomes too de-
manding, e.g. tBuOH, the alkoxide ligands cannot be substi-
tuted quantitatively.[30] Most of such compounds investigated
by Mehrotra and co-workers appear as volatile viscous liquids,
except derivatives containing tertiary alkoxides, which form
solids. Another possibility to substitute iso-propanolate ligands
by other alkoxides is a transesterification reaction [Equation
(3)].[50] The advantage compared to the alcoholysis reaction is
the lower coordination ability of esters in contrast to alcohols,
so that usually no coordinated solvent molecules are present
within the product. Similar to the reaction type in Equation
(2), the liberated volatile isopropyl acetate (Tb = 89 °C) can be
distilled off to shift the equilibrium to the product side.
Increasing interest in alcoholate compounds since 1950 lead
to the synthesis of alcoholates of nearly every stable metal in
the periodic table, and their reactivity was investigated.[1]
These compounds, in principle, can act as single-source pre-
cursors (SSPs) to synthesize binary or multinary metal ox-
ides.[2–5] They can be applied in different synthesis strategies,
like sol-gel, hydrothermal, CVD (chemical vapor deposition)
or other thermolytic processes.[6–20] The properties of ternary
oxides can be modified in a broader range compared to binary
oxides, since a second metal ion establishes another opportu-
nity to modify the structural properties. Therefore, material
properties for applications, such as semi-conductivity, magnet-
ism or superconductivity, can be adjusted.[21–28] For that
reason, the interest in synthesis of heteronuclear alcoholates,
which contain different types of metal ions, is quite high.
Various heteronuclear aluminum alcoholates were investi-
gated by Mehrotra and co-workers in several studies.[29–48]
Starting from water-free metal chlorides, compounds like
[M{Al(OR)4}2] (M = bivalent transition metal ion, R = iPr,
tBu) were synthesized by salt metathesis reaction [Equation
(1)]. Derivatives with different organic groups are accessible
by alcoholysis reaction with other primary, secondary or terti-
MCl2 + 2 K[Al(OR)4] Ǟ [M{Al(OR)4}2] + 2 KCl
(M = divalent transition metal ion, R = iPr, tBu)
(1)
[M{Al(OiPr)4}2] + 8–2x RЈOHexc. Ǟ [M{Al(ORЈ)4–x(OiPr)x}2] +
8–2x iPrOH
(2)
[M = divalent transition metal ion, RЈ = Me, Et, nBu, iBu,
nPr, (x = 0) or tBu, CEtMe2 (x = 1)]
[M{Al(OiPr)4}2] + 8 RЈЈOAcexc. Ǟ [M{Al(ORЈЈ)4}2] + 8 iPrOAc (3)
(M = divalent transition metal ion, RЈЈ = nBu, iBu, nPr, tBu,
cHex)
* Prof. Dr. H. Krautscheid
E-Mail: krautscheid@rz.uni-leipzig.de
Although some of the heteronuclear aluminum alkoxides
have been used as SSPs in sol-gel synthesis, systematic ther-
molysis studies are still rare.[7–17] Indeed, the thermolysis
mechanism of [Zn{AlH2(OtBu)2}2] was clarified during iso-
tope tracing in mass spectrometry experiments and reported by
Veith and co-workers, but the investigation of the solid residues
depending on the included bivalent metal ion is still miss-
ing.[51] This work contributes to that topic as well as to the
influence of the 2,2Ј-bipyridine co-ligand on the thermolysis
properties.
[a] Institut für Anorganische Chemie
Universität Leipzig
Johannisallee 29
04103 Leipzig, Germany
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/zaac.202000246 or from the au-
thor.
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an open access article under the terms of the Creative Commons
Attribution-NonCommercial-NoDerivs License, which permits use
and distribution in any medium, provided the original work is prop-
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tations are made.
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Results and Discussion
Molecular Structures
The hereronuclear alcoholates 1-M are used as starting ma-
terials for compounds 2-M, 3-M, and 5. They are synthesized
according to literature procedures, purified by distillation and
appear as viscous liquids.[31,43,47] As shown in Scheme 1, com-
pounds 2-M could be crystallized after adding one equivalent
of 2,2’-bipyridine as co-ligand. Since the molecular structures
of 2-M show no major differences depending on the divalent
cation, compound 2-Co (Figure 1, left) represents a typical ex-
ample of 2-M. Both aluminum atoms are surrounded by four
iso-propanolate ligands forming a typical anionic [Al(OiPr)4]–
moiety. Two of these units coordinate to the central divalent
cation via two μ2-bridging alcoholate ligands each. The Al–O
bonds to terminal iso-propanolate ligands [Al1–O3 169.9(4)
pm; Al1–O4 170.0(4) pm] are shorter than to the bridging ones
[Al1–O1 177.3(3) pm; Al1–O2 178.1(2) pm] leading to a dis-
torted tetrahedral oxygen environment for the Al3+ ions. The
coordination sphere of the divalent cation is completed by one
2,2Ј-bipyridine molecule, which coordinates as chelating li-
gand, forming a slightly distorted octahedron. The distances of
Co2+ to the donor atoms vary from 207.8(2) pm (Co1–O2) to
214.8(3) pm (Co1–N1) and the angles from 72.96(9) to
102.51(10)° and 168.47(8) to 170.66(9)°. Typical bond lengths
and angles are shown in Table 1.
Figure 1. Molecular structures of 2-Co (top) and 3-Ni (bottom) as
examples for compounds 2-M and 3-M. Hydrogen atoms, disordered
atoms, and solvent molecules are omitted for clarity; thermal ellipsoids
are drawn at the 50% probability level. Symmetry code: (Ј) 1–x, y,
0.5–z.
can be described as a square planar environment (created by
two N and two O atoms with similar distances to Cu2+) and
two longer Cu–O bonds. The second longest distances vary
between 241.7(3) pm (Cu2–O9) and 247.6(3) pm (Cu3–O22),
the longest distances vary from 258.2(3) pm (Cu3–O18) to
Scheme 1. Preparation of alcoholates 1-M, 2-M, 3-M, 4, and 5 (M =
Fe, Co, Ni, Cu, Zn).
All compounds 2-M crystallize in the monoclinic crystal 264.5(3) pm (Cu4–O30).
system, except triclinic 2-Cu (see structure data in the Support-
All compounds 2-M could be isolated as pure phases as
ing Information). Within the group of monoclinic structures 2- confirmed by PXRD measurements, except 2-Zn. The latter
Fe, 2-Co and 2-Ni are isomorphous with space group C2/c. compound appears polymorphous with structures in space
Four symmetry equivalent, C2-symmetric molecules with dif- groups C2, Cc and C2/c (unit cell data obtained from Rietveld
ferent orientations of the molecules are present in the unit cell. analyses are reported in Table S1 and Figure S1, Supporting
2-Zn crystallizes in the non-centrosymmetric space group C2 Information). Corresponding packing diagrams of all observed
with a smaller angle β and also four different orientations of structures are presented in Figure 2. The structures with space
the molecules in the unit cell. In contrast to the former com- group C2 and C2/c show a similar packing which deviates
pounds, complex 2-Cu crystallizes in the triclinic space group from that in Cc. However, elemental analysis and NMR spec-
¯
P1 with four slightly different molecules in the asymmetric troscopy indicate the isolation of pure 2-Zn.
unit and eight molecules per unit cell. The independent mol-
During synthesis of 2-Cu, the blue color changed to brown-
ecules differ in the length of the two longest Cu–O bonds due ish-yellow while 2,2Ј-bipyridine was added. 2-Cu as well as 4
to the Jahn–Teller effect of Cu2+ ions. The coordination mode could be crystallized from this solution. 4 is an ionic com-
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Table 1. Selected distances /pm and angles /° in 2-Co, for comparison in 2-Cu, where a Jahn–Teller distortion is observed, and in 3-Ni.
2-Co
2-Cu
3-Ni
Co1···Al1
Co1–O1
Co1–N1
Al1–O1
Al1–O3
294.4(1)
213.9(2)
214.8(3)
177.4(3)
169.9(4)
168.47(8)
170.66(9)
Cu1···Al1
Cu1–O1
Cu1–O2
Cu1–O5
Cu1–O6
Cu1–N1
Cu1–N2
O1–Cu1–N1
O2–Cu1–O6
O5–Cu1–N2
304.7(1)
200.0(2)
243.6(3)
196.2(2)
263.8(3)
202.8(3)
200.9(3)
165.7(1)
168.7(1)
165.6(1)
Ni1···Al1
Ni1–O1
Ni1–N1
Al1–O1
292.1(1)
208.0(2)
208.9(2)
177.4(2)
172.3(2)
167.57(8)
169.81(8)
168.52(8)
88.79(9)
109.3(1)
Al1–O4
O1–Co1–O1’
O2–Co1–N1’
O1–Ni1–O5
O2–Ni1–N2
O6–Ni1–N1
O1–Al1–O2
O1–Al1–O4
O1–Al1–O2
O1–Al1–O3
89.7(1)
113.8(2)
Figure 2. Comparison of packing diagrams of the different structures observed in the powder diffraction pattern of 2-Zn. For clarity, just the
metal-donor-atom skeleton is shown.
pound consisting of a cationic [Cu{Al(OiPr)4}(bipy)2]+ unit
and an [Al(OiPr)4]– anion (Figure 3). Compound 4 crystallizes
in the tetragonal space group P4122 with four molecules per
unit cell. Each of the two aluminum atoms in the asymmetric
units are surrounded by four alcoholate ligands resulting in a
distorted tetrahedral environment. One of them is connected
by two μ2-bridging alcoholate oxygen atoms to the Cu2+ ion
whose octahedral coordination is completed by two chelating
2,2Ј-bipyridine molecules. The organic ligands tend to appear
highly disordered (except the μ2-bridging ones). Therefore,
metal-donor atom distances cannot be discussed accurately.
The coordination of the Cu2+ ion can be described as com-
pressed octahedron, which is quiet unusual in terms of Jahn–
Teller distortion. However, we cannot exclude that this is the
result of a superposition of axially elongated octahedra in the
crystal structure caused by disorder. After storage of the
mother liquor for three weeks, yellow crystals can be observed
which belong neither to 2-Cu nor to 4. Due to the color which
is typical for Cu+ ions coordinated by pyridine derivatives and
due to its high air sensitivity, we assume that a reduction of
Cu2+ to Cu+ occurred. However, the structure could not be
clarified so far. Thus, 4 seems to be an intermediate appearing
during the reduction of Cu2+ to Cu+ induced by 2,2Ј-bipyr-
idine.
For the synthesis of 3-M the iso-propanolate ligands in 1-
M are converted to cyclohexanolate ligands by a transesterifi-
cation reaction with cyclohexylacetate. At 130 °C the formed
isopropylacetate is distilled off whereby the equilibrium is
shifted towards the product side. Adding 2,2Ј-bipyridine leads
to crystallization of 3-M. All compounds 3-M are isomorphous level.
Figure 3. Molecular structure (top) and packing diagram (bottom) of
the ionic compound 4; hydrogen atoms and disordered atoms are omit-
ted for clarity; thermal ellipsoids are drawn at the 50% probability
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and crystallize in the monoclinic system with space group
P21/n and four molecules per unit cell. The molecular structure
of 3-Ni as a representative compound for 3-M (Figure 1, right)
shows the same connectivity between metal ions and ligands
as 2-M (typical bond lengths and angles are given in Table 1).
Four complex molecules as well as four non-coordinating Et2O
molecules are present in the unit cell. Evacuation of the crys-
tals during the isolation process leads to the loss of solvent
molecules, which causes a different XRD powder pattern than
expected from single crystal data. The Et2O molecules can also
be extracted from the crystal structure by treating 3-Co with
n-pentane whereby the crystal structure changes to the triclinic
¯
modification with space group P1. In this single-crystal-to-sin-
gle-crystal transformation the packing of four molecules in the
unit cell remains roughly the same; the unit cell angles change
by up to 6° compared to monoclinic 3-Co (Figure 4 and struc-
ture data in Table S3, Supporting Information).
Figure 5. Molecular structure (top) and packing diagram (bottom) of
5; hydrogen atoms and disordered atoms are omitted for clarity; ther-
mal ellipsoids are drawn at the 50% probability level.
Figure 4. Distortion of the unit cell of 3-Co due to the loss of solvent
molecules.
Table 2. Selected distances /pm and angles /° in 5.
5
Cu1···Al1
Cu1–O1
Cu1–O2
Cu1–N1
Cu1–N2
Al1–O1
Al1–O3
308.5(1)
233.5(3)
227.4(3)
194.3(3)
193.9(3)
174.9(3)
171.3(4)
N1–Cu1–N2
N1–Cu1–O1
O1–Al1–O2
O1–Al1–O3
147.2(1)
105.6(1)
94.8(1)
Using 1-Cu in the procedure for the synthesis of 3-M, re-
duction of Cu2+ to Cu+ takes place during heating in cyclohex-
ylacetate, which is indicated by a fading color of the solution.
The resulting colorless precipitate could be dissolved in THF,
addition of pyridine leads to crystallization of 5 as pale yellow
needles. The molecular structure (Figure 5) is similar to that
of the compounds [Cu{Al(OR)4}(py)2] (R = iPr, tBu) pub-
lished by Veith and co-workers.[52] The asymmetric unit con-
sists of three independent molecules, one of which is shown
in Figure 5. Compound 5 crystallizes in the monoclinic space
group P21/c. All Al–O bonds range from 171.4(4) pm to
176.0(3) pm to form an AlO4 tetrahedron. This fragment is
connected to the Cu+ ion by two μ2-bridging cyclohexanolate
ligands, which show long Cu–O bond lengths. The variation
of this Cu–O bond length is the only significant difference
between the three molecules within the asymmetric unit. They
vary from 220.6(2) pm to 250.5(3) pm. Each [2+2] coordina-
tion of the Cu+ ions is completed by two pyridine molecules
with very similar Cu–N bond length of around 194 pm and N–
Cu–N angles between 144.7(1)° and 147.5(1)° (for selected
bond lengths and angles see Table 2). The unit cell (Figure 5)
contains twelve molecules.
112.8(2)
Thermal Analyses
In order to study compounds 2-M and 3-M as precursors
for ternary oxides, their thermolytic properties were investi-
gated using TG/DTA-MS measurements. The collected data
are summarized in Table 3, typical TG/DTA-MS results are
visualized in Figure 6. The observed mass loss of compounds
1-M and 2-M is higher than the expected values, because even
at ambient pressure these compounds are volatile. In Figure 7
the deviations of the observed mass loss compared to the ex-
pected values are shown. Fe2+ containing compounds show the
highest volatility rates. Due to the co-ligand 2,2Ј-bipyridine
compounds 2-M are less volatile. In case of 3-M with the big-
ger cyclohexanolate ligands the volatility is suppressed com-
pletely. Thermolytic properties of complexes 1-M were inves-
tigated under (a) an inert argon atmosphere and (b) an oxidiz-
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Table 3. Data from TG/DTA experiments and detected phases in X-ray powder diffraction patterns of the residues.
a)
Compound
Atmosphere
Tonset /°C
Tend /°C
Δmcalcd /%
Δmobs /%
Phases in PXRD
FeAl2O4
Fe2O3, Al2O3, FeAlO3
γ-Al2O3, Fe, FeAl2O4
FeAl2O4, Fe
Co, γ-Al2O3
CoAl2O4
CoAl2O4
CoAl2O4
NiAl2O4, Ni, γ-Al2O3
NiAl2O4, NiO
γ-Al2O3, NiAl2O4, Ni, NiO
γ-Al2O3, NiO, NiAl2O4
CuAl2O4, CuAlO2, CuO,
Cu2O
1-Fe
argon
dry air
argon
argon
argon
dry air
dry air
dry air
argon
dry air
dry air
dry air
argon
259
242
273
284
227
257
254
262
247
235
267
262
185
323
317
321
492
328
320
429
515
269
276
565
568
257
70.2
95.7
74.3
89.8
83.8
92.0
85.6
85.8
83.3
82.1
79.9
81.5
83.7
86.8
2-Fe
3-Fe
1-Co
76.5
83.6
69.8
2-Co
3-Co
1-Ni
76.2
83.3
69.8
2-Ni
3-Ni
1-Cu
76.2
83.4
69.2
dry air
dry air
argon
153
157
113
230
250
249
233
304
259
367
164
254
294
303
524
547
72.3
81.7
80.7
CuAl2O4, CuO
CuAl2O4, CuO
Cu
2-Cu
5
75.7
82.3
b)
1-Zn
argon
69.0
82.4
84.9
80.7
83.2
ZnAl2O4
ZnAl2O4
ZnAl2O4, ZnO
ZnAl2O4
dry air
dry air
dry air
2-Zn
3-Zn
75.5
82.8
a) Calculated for the formation of MAl2O4. b) Calculated for the formation of CuAlO2.
ing atmosphere of dry air. The comparison of both experiments
shows the general trend of higher volatility of 1-M under argon
atmosphere due to the absence of oxidative decomposition.
During thermolytic experiments of 2-M an endothermic DTA
peak can be assigned to the melting of the substance (Figure 6
and Figures S3–S21, Supporting Information). These melting
temperatures are summarized in Table 4.
Figure 7. Deviation of the observed values of mass loss during TG
measurements compared to the expected values.
Table 4. Melting temperatures (peak temperature) of 2-M observed
during DTA measurements.
2-Fe
2-Co
2-Ni
2-Cu
2-Zn
DTA peak /°C 157
164
173
161
143
Figure 6. Idealized thermolysis equation (top) and TG/DTA-MS data
(bottom) of the thermolysis of 2-Co under dry air. TG – solid black
line, DTA – blue dashed line, MS ion current with m/e = 15, 43, and
45 – blue, yellow and magenta lines, respectively.
FeϮ0 can be observed under oxygen-free conditions. During
thermolysis studies of all cobalt compounds in dry air, phase
The solid decomposition residues were analyzed by PXRD. pure CoAl2O4 is formed as a residue, but the reduction of Co2+
In the residue of 1-Fe only FeAl2O4 is found as crystalline to CoϮ0 can be observed when 1-Co is heated under inert at-
phase after thermolysis in an argon atmosphere. In contrast, mosphere. To understand the reduction process, 2-Fe and 1-
thermolysis in air leads to oxidation of Fe2+ to Fe3+, the forma- Co were decomposed in larger scale in a nitrogen atmosphere
tion of binary oxides, Al2O3 and Fe2O3, and FeAlO3. In the and the formed thermolysis gases were trapped in an NMR
presence of the bipyridine ligand a partial reduction of Fe2+ to tube. In the NMR spectra propene, iso-propanol and, in ad-
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Figure 8. Temperature dependent X-ray powder diffraction pattern (Guinier-Simon diagram, HT-PXRD) of 2-Zn.
dition, acetone could be detected. These products indicate that PXRD). During the heating process, 2-Zn shows a phase tran-
the iso-propanolate ligands act as reducing agents. Presence of sition between 65 and 75 °C forming [Zn{Al(OiPr)4}2(bipy)]
the bipy co-ligand favors the reduction of Fe2+ to FeϮ0 so that in space group C2/c (Figure 8, diffraction pattern B), which
a partial reduction takes place.
was identified by comparison with the PXRD pattern of 2-Ni.
In thermolysis experiments of 1-Ni and 2-Ni, NiAl2O4, γ- At 115 °C 2-Zn melts (diffraction pattern C) and re-crystallizes
Al2O3, and Ni-containing compounds as by-products are again after cooling down to 50 °C in space group C2/c (diffrac-
formed. In inert or oxidizing atmosphere nickel metal or NiO, tion pattern D). This indicates that the thermodynamically fa-
respectively, show up in the diffraction patterns. In addition, vored structure is in the centrosymmetric space group. The
NiϮ0 can be observed in the residue of 2-Ni, which emphasizes varying intensities in diffraction pattern D are caused by tex-
the role of the bipyridine ligand in the reduction process even ture effects due to a preferred orientation within the sample
in dry air. With copper compounds (1-Cu, 2-Cu, and 3-Cu) containing capillary. In another experiment it is proven by
similar results can be observed. The residue after thermolysis Rietveld analysis, that 2-Zn crystallizes in the C2/c structure
of 1-Cu under argon consists of binary (CuO, Cu2O) and ter- from its melt (Figure S2, Supporting Information). For 2-Fe, 2-
nary oxides (CuAl2O4, CuAlO2) with Cu2+ and Cu+ ions. In Co and 2-Ni, also melting can be observed, however, without
contrast, under oxidizing conditions just CuAl2O4 and CuO additional phase transitions. In contrast, 2-Cu does not recrys-
can be detected in XRD powder patterns, also formed Al2O3 tallize after cooling below the melting temperature, but a
remains amorphous. Independent of the gas atmosphere, ther- change in color from blue (2-Cu) to brownish-yellow can be
molysis of 1-Zn leads to ZnAl2O4. The same result is observed noticed, indicating that a reduction process takes place.
for 3-Zn, whereas the thermolysis residue of 2-Zn contains a
very small amount of ZnO (Ͻ 1 wght-%) which may be caused
UV/Vis Spectroscopy
by a byproduct in the synthesis of 2-Zn.
The thermolysis of the Cu+ compound 5 in argon proceeds
Electronic spectra of compounds 1-M, 2-M, 3-M and 5
(M = Fe, Co, Ni, Cu; solution in MTBE) are shown in Fig-
ure 9. The d–d transition bands at 297 nm, 1015 nm, and
in two steps of mass loss. The first step (Tonset = 113 °C, Δm
= –25.6%) indicates the loss of pyridine molecules, afterwards,
1312 nm for 1-Fe are characteristic for octahedral Fe2+ envi-
at Tonset = 230 °C (Δm = –55.1%) the cyclohexanolate ligands
decompose. The residue contains copper metal and amorphous
Al2O3, the alcoholate ligands may act as reducing agent.
ronment, establishing that the [Al(OiPr)4]– fragment acts in
solution as tridentate ligand.[53] The spectra of 2-Fe and 3-Fe
show additional charge-transfer bands at 561 nm and 571 nm,
respectively.[54] 1-Co also shows characteristic d–d transitions
HT-PXRD
The thermal behavior of complexes 2-M was also investi- for octahedral Co2+ ions at 499 nm and 551 nm. In addition, a
gated by temperature dependent PXRD measurements (HT- shoulder at 642 nm can be observed, which indicates Co2+ in
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Figure 9. UV/Vis spectra of compounds 1-M, 2-M, 3-M, and 5 measured in MTBE as solvent: (a) M = Fe, (b) M = Co, (c) M = Ni, (d) M =
Cu.
tetrahedral environment.[43] Here, the [Al(OiPr)4]– group acts be identified. In addition, a thermally induced reduction of 1-
as tridentate and bidentate ligand. This behavior can also be Cu can be observed during transesterification reaction leading
observed for 1-Ni, where the d–d bands for tetrahedrally sur- to the Cu+ compound [Cu{Al(OcHex)4}(py)2] (5). In thermol-
rounded Ni2+ ions (527 nm and 578 nm) are clearly visible as ysis studies under argon atmosphere a reduction of M2+ to MϮ0
well as the absorptions for octahedral Ni2+ environment takes place depending on the type of M2+ cation. Fe/Fe2+ rep-
(426 nm and 784 nm).[44,55] The complexes 2-Co, 2-Ni, 3-Co resents a limiting redox pair for the reduction process, whereat
and 3-Ni exhibit just characteristic d–d bands for sixfold coor- in presence of 2,2Ј-bipyridine reduction occurs more easily.
dinated M2+ ions. The copper compounds 1-Cu and 2-Cu dis- The alcoholate ligands act as reducing agents, as evident by
play similar spectra with a broad band at 699 nm and 718 nm, the detection of acetone as a volatile thermolysis product. De-
respectively. These belong to d–d transitions of octahedral pending on the gas atmosphere, phase pure FeAl2O4, CoAl2O4,
Cu2+ ions with Jahn–Teller distortion.[33,42,46] In the UV/Vis and ZnAl2O4 could be synthesized under mild conditions, as
spectrum of Cu+ containing compound 5 just π–π* transitions confirmed by powder X-ray diffraction. Thus, the required re-
of pyridine molecules are observed as well as a charge-transfer action temperatures are significantly lower than typical solid-
band at 335 nm.[56]
state reactions. 2-M and 3-M might be useful in processes like
CVD, sol-gel synthesis or liquid phase assisted techniques due
to their high solubility in organic solvents.
Conclusions
The
new
heteronuclear
alcoholate
complexes
Experimental Section
[M{Al(OiPr)4}2(bipy)] (2-M, M = Fe, Co, Ni, Cu, Zn) and
[M{Al(OcHex)4}2(bipy)] (3-M, M = Fe, Co, Ni, Zn) were suc-
cessfully synthesized from [M{Al(OiPr)4}2] (1-M) and 2,2Ј-
bipyridine, and in case of 3-M, by a transesterification reaction
prior to the adduct formation. The crystal structures of 2-M
and 3-M were determined. During synthesis of 2-Cu a bipy
ligand induced reduction process occurred, where the structure
Materials and General Procedures: All manipulations were carried
out in a nitrogen atmosphere using standard Schlenk technique or an
MBRAUN UniLab glovebox. Solvents were dried with sodium or
CaH2 and distilled prior to use. Potassium, NiCl2 and 2,2Ј-bipyridine
were purchased from Acros Organics, ABCR and TCI, respectively,
and used without further purification. Cyclohexylacetate (TCI) was
distilled from CaH2, and Al(OiPr)3 (Sigma Alrich) was recrystallized
of intermediate [Cu{Al(OiPr)4}(bipy)2][Al(OiPr)4] (4) could from n-pentane. Unhydrous CuCl2 was obtained by heating
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CuCl2·2H2O in vacuo. CoCl2 and ZnCl2 were generated by drying
CoCl2·6H2O and ZnCl2·4H2O with SOCl2.[57] Water-free FeCl2 was
thermal parameters. Hydrogen atoms were included on idealized posi-
tions. The structures were visualized using Diamond 3.2k.[61] The crys-
prepared by dissolving iron in methanol by addition of concentrated tallographic data are summarized in the Supporting Information
hydrochloric acid. The obtained FeCl2·xMeOH was decomposed in (Tables S2–S4, Supporting Information).
vacuo at 180 °C to form FeCl2.[58]
Crystallographic data (excluding structure factors) for the structures in
this paper have been deposited with the Cambridge Crystallographic
Data Centre, CCDC, 12 Union Road, Cambridge CB21EZ, UK. Copies
of the data can be obtained free of charge on quoting the depository
numbers CCDC-2007715, CCDC-2007716, CCDC-2007717, CCDC-
2007718, CCDC-2007719 for 2-M, CCDC-2007720, CCDC-2007721,
CCDC-2007722, CCDC-2007723, CCDC-2007724 for 3-M, CCDC-
2007962 for 4, and CCDC-2007725 for 5 (Fax: +44-1223-336-033;
E-Mail: structures@ccdc.cam.ac.uk, https://www.ccdc.cam.ac.uk).
Synthesis of [M{Al(OiPr)4}2] (1-M, M = Fe, Co, Ni, Cu, Zn): This
reaction was performed according to literature procedures.[31,43,47] In
a typical synthesis 1.57 g (40 mmol, 2 equiv.) of potassium metal was
dissolved in 20 mL isopropyl alcohol and then added to a suspension
of 8.19 g (40 mmol, 2 equiv.) Al(OiPr)3 in 35 mL iPrOH and stirred
overnight. The resulting solution was mixed with a stirred suspension
of 1 equiv. of the anhydrous metal chloride in 15 mL iPrOH and re-
fluxed for 24 h. The precipitated KCl was separated and the solvent
removed under reduced pressure. The crude product was purified by
distillation in vacuo. In case of 1-Ni the crude product was dissolved
in toluene. The solvent was distilled off prior to vacuum distillation to
remove coordinated iPrOH molecules. 1-M appears as a viscous liquid
in 38–67% yield. Experimental details are given in the Supporting
Information.
Powder X-ray Diffraction: Powder XRD patterns were measured
with a STOE STADI-P diffractometer system equipped with a sealed
Cu X-ray tube, a germanium (111) monochromator crystal [λ(Cu-Kα1
)
= 154.060 pm] and a Dectris Mythen-1 K detector. The alcoholate
complexes were measured in Debye–Scherrer setup in glass capillaries
(Hilgenberg, 0.5 mm), thermolysis products were measured as flat
samples on polymer films in transmission mode at room temperature.
For temperature dependent measurements sample-filled capillaries
were rotated in a high temperature device (Type 2416 by EURO-
THERM). Data processing was carried out with STOE WinXPOW.[62]
Rietveld analysis was performed with Bruker TOPAS 5 using the fun-
damental parameter approach.[63] The crystallographic data as refer-
ences for qualitative and quantitative analysis of the powder diffraction
patterns are taken from the literature.[64–77]
Synthesis of [M{Al(OiPr)4}2(bipy)] (2-M, M = Fe, Co, Ni, Cu, Zn):
In a typical synthesis, 156 mg (1 mmol, 1 equiv.) of 2,2Ј-bipyridine
was added to a solution of an equimolar amount of 1-M (1 mmol,
1 equiv.) in n-pentane. The mixture was kept stirring for 1 h at room
temperature. Crystals of 2-M could be isolated after cooling the solu-
tion to –25 °C with 25–70% yield. After several days, crystals of 4
could be observed from the copper containing reaction mixture. Fur-
ther details are given in the Supporting Information.
Supporting Information (see footnote on the first page of this article):
Experimental details, crystallographic data, TG/DTA-MS data and
Rietveld analyses of the solid residues, HT-PXRD measurements of
2-M, UV/vis spectrum of 5, NMR spectra of 1-Zn, 2-Zn, 3-Zn and
of trapped volatile thermolysis products are provided as supporting
information.
Synthesis of [M{Al(OcHex)4}2(bipy)] (3-M, M = Fe, Co, Ni, Zn): A
solution of 2 mmol (1 equiv.) 1-M in 10 mL cyclohexylacetate was
heated to 130 °C for 4–6 h. After removing the solvent, the residue
was dissolved in a n-pentane/Et2O mixture (1:2 V/V), and 312 mg
(2 mmol, 1 equiv.) 2,2Ј-bipyridine was added. Crystals of 3-M·Et2O
can be isolated by cooling the mother liquor to –25 °C with 48–67%
yield. Details are given in the Supporting Information.
Acknowledgements
Synthesis of [Cu{Al(OcHex)4}(py)2] (5): A solution of 0.75 g
(1.3 mmol, 1 equiv.) 1-Cu in 11 mL cyclohexylacetate was heated to
140 °C for 3 h. After cooling the reaction mixture to room temperature,
the colorless precipitate was separated and dispersed in 10 mL THF.
Addition of 0.5 mL pyridine lead to crystallization of 5 (yield: 239 mg
(56%)) by cooling the mother liquor to 2 °C. Details are given in the
Supporting Information.
We thank O. Erhart for TG/DTA-MS measurements and M. Roßberg
for CHN analyses. We also gratefully acknowledge Dr. L. Hennig,
Dr. M. Icker and their team for NMR measurements as well as Uni-
versität Leipzig for financial support. Open access funding enabled
and organized by Projekt DEAL.
Keywords: Heteronuclear alcoholates; Coordination chemis-
try; Structure elucidation; Thermolysis properties
Characterization: TG-DTA analysis was performed in an argon or
dry-air atmosphere with a Netzsch STA 449 F1 thermobalance (heating
rate 10 K·min–1) connected to an Aeolos QMS 403 D mass spectrome-
ter. Elemental analyses were measured with a Vario El by Heraeus,
UV/Vis spectra with a V-670 spectrometer by Jasco. NMR spectra
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