Magnetic, high-field EPR studies and catalytic activity of Schiff base tetranuclear CuII2FeIII2 complexes obtained by direct synthesis

Article abstract of DOI:10.1039/c3dt51800k

Two novel heterometallic complexes [Cu₂Fe₂(HL ¹)₂(H₂L¹)₂] ·10DMSO (1) and [Cu₂Fe₂(HL²) ₂(H₂L²)₂]·2DMF

Full text of DOI:10.1039/c3dt51800k

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Magnetic, high-field EPR studies and catalytic activity
of Schiff base tetranuclear CuII FeIII2 complexes
2
Cite this: DOI: 10.1039/c3dt51800k
obtained by direct synthesis†
Oksana V. Nesterova,a Eduard N. Chygorin,b Vladimir N. Kokozay,*b
Volodymyr V. Bon,c Irina V. Omelchenko,d Oleg V. Shishkin,d Ján Titiš,e Roman Boča,e
Armando J. L. Pombeiro*a and Andrew Ozarowskif
Two novel heterometallic complexes [Cu2Fe2(HL1)2(H2L1)2]·10DMSO (1) and [Cu2Fe2(HL2)2(H2L2)2]·2DMF
(2) have been prepared using the open-air reaction of copper powder, iron(II) chloride and DMSO (1) or
DMF (2) solutions of the polydentate Schiff base (H4L1, 1; H4L2, 2) formed in situ from salicylaldehyde (1)
or 5-bromo-salicylaldehyde (2) and tris(hydroxymethyl)aminomethane. Crystallographic analysis revealed
that both compounds are based on the centrosymmetric tetranuclear core {CuII2FeIII2(µ-O)6} where metal
centres are joined by µ-O bridges from the deprotonated ligands forming a nonlinear chain-like arrange-
ment. Variable-temperature (1.8–300 K) magnetic susceptibility measurements of 1 and 2 showed a
decrease of the effective magnetic moment value at low temperature, indicative of antiferromagnetic
coupling (JCu–Fe/hc = −10.2 cm−1, JFe–Fe/hc = −10.5 cm−1 in 1, JCu–Fe/hc = −10.5 cm−1, JFe–Fe/hc =
−8.93 cm−1 in 2) between the magnetic centres in both compounds. They reveal an exceptionally high
catalytic activity in the oxidation of cyclohexane with hydrogen peroxide under mild conditions, with the
best observed yield/TON combined values of 36%/596 and 44%/1.1 × 103 for 1 and 2, respectively.
Received 4th July 2013,
Accepted 9th September 2013
DOI: 10.1039/c3dt51800k
www.rsc.org/dalton
demonstrated, for example, in the large MnIII28MnII8NiII
4
Introduction
assembly with a high ground spin state value,1 a bell-shaped
In recent years, heterometallic polynuclear complexes have Mn11Gd2 aggregate2 or octanuclear Cr4Dy4 cluster3 with SMM
attracted much attention mainly due to their sophisticated behaviour, a calix[4]arene based Mn4Gd4 complex with the be-
crystal structures and possibility of possessing high-spin haviour of a magnetic refrigerant for low-temperature appli-
ground states as well as “single molecular magnet” (SMM) cations,4 etc. Obviously, by incorporating different spins within
behaviour. Such an amazing combination of complicated one molecule, it is possible to affect the spin ground state,
architectures and fascinating magnetic properties was magnetic anisotropy, and magnetic exchange interactions and
try to predetermine the desired magnetic properties. In spite
of considerable success in the investigation of structure–mag-
aCentro de Química Estrutural, Complexo I, Instituto Superior Técnico,
The University of Lisbon, Avenida Rovisco Pais, 1049-001 Lisboa, Portugal.
E-mail: pombeiro@ist.utl.pt
bDepartment of Inorganic Chemistry, Taras Shevchenko National University of Kyiv,
Volodymyrska str. 64/13, Kyiv 01601, Ukraine. E-mail: kokozay@univ.kiev.ua
cDepartment of Chemistry of Complex Compounds, V.I. Vernadsky Institute of
General and Inorganic Chemistry, National Academy of Sciences of Ukraine, 32/34
Palladin Ave., Kyiv 03680, Ukraine
dSTC “Institute for Single Crystals” National Academy of Sciences of Ukraine, 60,
Lenina Avenue, Kharkiv 61001, Ukraine
netism correlations, another important application field for
heterometallic coordination compounds with classical donor
ligands, catalysis, is still poorly explored.5 For instance, it was
shown that heterometallic precatalysts, oxo-centred M2Ni (M =
Fe, Cr) triangles and Cr7Ni wheels,6 reveal a high catalytic
activity in the vinyl/addition polymerization of norbornene.
Further, chiral heterometallic d/s, d/d and d/f complexes were
found to catalyze various asymmetric transformations.5 In the
field of alkane functionalization under mild conditions, the
eDepartment of Chemistry, FPV, University of SS Cyril and Methodius, Trnava,
Slovakia
fNational High Magnetic Field Laboratory, Florida State University, 1800 E. Paul
8
9
heterometallic CuCo3,7 Cu2Co2Fe2 and Co4Fe2 polynuclear
assemblies, as well as {Cu6Fe}n coordination polymers,10 were
recognized as highly efficient catalysts. Moreover, the catalytic
activity (as measured by the yields of products, TONs and
TOFs) of heterometallic Co/Fe complexes8,9 was found to be
among the highest ones, demonstrating the catalytic potential
of the synergistic effect of a few different metals.
Dirac Drive, Tallahassee, Florida 32310, USA
†Electronic supplementary information (ESI) available: Selected bond lengths
and angles for crystal structures of 1 and 2, plots of the hydrogen-bonding
schemes for 1 and 2, IR spectra of 1 and 2. CCDC 907644 for 1 and 908089 for 2.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c3dt51800k
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In pursuit of our research on the preparation and investi- red solutions were obtained at the end of both reactions. Red-
gation of heterometallic polynuclear complexes, we continued brown microcrystals of 1 and 2 that showed analytical data
to apply the direct synthesis method of coordination com- consistent with the Cu(II) : Fe(III) = 1 : 1 stoichiometry were
i
pounds.11 This strategy, based on spontaneous self-assembly of formed within two days after addition of PrOH (1) or Et2O (2).
free metal ions with commonly simple and flexible ligands The general reaction for the syntheses of both compounds can
without significant geometrical restrictions, already resulted in be written as follows (Solv = DMSO, DMF; n = 10, 2):
a great diversity of high-nuclearity assemblies with two12 or
Cu0 þ FeCl2ꢀ4H2O þ 2H4L þ 2Et3N þ 0:75O2 þ 0:5nSolv
even three13 different metals. In addition to sophisticated
crystal structures, the complexes synthesized in this way show
! 0:5½Cu2Fe2ðHLÞ2ðH2LÞ2ꢁꢀnSolv þ 2Et3NꢀHCl þ 5:5H2O
interesting magnetic, spectroscopic and catalytic properties,
Although the stoichiometry of both reactions could suggest
Cu : FeCl2·4H2O : H4L = 1 : 1 : 2 as the best initial ratio, the
experimental results pointed to the Cu : FeCl2·4H2O : H4L =
e.g. strong exchange interaction in the Cu2Zn2 complex
mediated by diamagnetic metal atoms,14 a single-ion contri-
bution to the zero field splitting parameters in the Zn2Cr2
2 : 1 : 4 system as the most appropriate one, which results not
tetramer determined by using high-field, high-frequency EPR
only in higher yields but also in more favorable conditions for
spectroscopy,15 and highly active and selective oxidation of
crystal growth in both cases.
cycloalkanes by Cu2Co2Fe2 and Co4Fe2 complexes.8,9 Working
The 1H-NMR spectra (in DMSO–CCl4) of yellow crystals of
with N,O-donor ligands, namely with aminoalcohols,16 under
H4L1 and H4L2, obtained in separate reactions in CH3OH
the direct synthesis conditions we tried to expand this direction
without addition of the metals, confirmed the in situ for-
and started to explore the possibilities of constructing novel
mation of the Schiff bases. The singlet peak observed at
high-nuclearity aggregates using polydentate Schiff base
8.55 ppm for H4L1 (1) and at 8.51 ppm for H4L2 (2) corre-
ligands.17 The tendency of Schiff bases to form oxo- and
sponds to the –CHvN– proton in the Schiff base.18
hydroxo-bridges between different metal centres is of great
The IR spectra of 1 and 2 in the 4000–400 cm−1 range
importance for magnetic materials design. Also, the synergistic
showed all the characteristic ligand peaks (Fig. S3 and S4†).
effect of dissimilar metals linked only by single bridging
The broad medium-intensity bands in the 3330–3470 cm−1
atoms, instead of bridging groups, could have a significant
region were assigned to ν(O–H) of the Schiff base ligands,
influence on the catalytic behaviour of coordination com-
pounds. Herein we report synthetic and structural features, as
while the very strong ones at 1618 (1) and 1621 cm−1 (2) were
assigned to ν(CvN). The presence of DMSO and DMF mole-
well as spectroscopic, magnetic and catalytic investigations of
cules was identified by the peaks at 1026 and 950 cm−1 (1) and
the two novel tetranuclear complexes [CuII2FeIII2(HL1)2-
the band at 1674 cm−1 (2), respectively.
(H2L1)2]·10DMSO (1) and [CuII2FeIII2(HL2)2(H2L2)2]·2DMF (2).
Crystal structures
Complexes 1 and 2 (Fig. 1 and 2) reveal similar structural con-
figurations with relatively small changes in bond lengths and
Results and discussion
Synthesis and spectroscopic analysis
angles (Tables S1 and S2†). Both compounds are centro-
symmetric and based on the tetranuclear core {Cu2Fe2(µ-O)6}
where the metals are joined by O bridges from the deproto-
nated Schiff base ligands forming a nonlinear CuII⋯FeIII⋯
FeIII⋯CuII chain-like arrangement (Fig. 3). Despite the
M4(µ-O)6 molecular structural type belonging to the wide-
spread family of M4(µ-X)6 tetranuclear complexes where the
metal centres are linked by two bridging atoms (ca. 250 hits
according to CSD19), complexes 1 and 2 represented the first
The synthetic procedures to obtain 1 and 2 are similar and can
be divided into two stages. In the first stage, the Schiff bases
H4L1 and H4L2 (Scheme 1) were obtained by condensation of
the appropriate aldehyde [salicylaldehyde (1) or 5-bromo-
salicylaldehyde (2)] and primary amine [tris(hydroxymethyl)-
aminomethane] in DMSO (1) or DMF (2) solution in a basic
(triethylamine) medium.
Then the copper powder and iron(II) chloride were added to
the hot yellow solutions of the ligands and the reactions were
brought to completion by heating and stirring until the total
dissolution of copper was observed (approximately 5 h). Dark-
Fig. 1 Left: molecular structure of 1 with the atom numbering (H atoms are
omitted for clarity, the non-hydrogen atoms are shown as 50% thermal ellip-
soids). Right: schematic representation of the molecular structure of 1.
Scheme 1
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H-bonded to six DMSO molecules [O4–H4A⋯O5S, D–A = 2.739(4)
Å, D–H⋯A = 178.44(14)°; O6–H6A⋯O2S, D–A = 2.685(4) Å,
D–H⋯A = 158.78(14)°; O3–H3A⋯O4SA, D–A = 2.828(48) Å,
D–H⋯A = 165.92(18)°]. In 2 (Fig. S2†), only two DMF molecules
are involved in hydrogen bonding with the complex molecule
by strong O–H⋯O interaction [O7–H7A⋯O9, D–A = 2.666(7) Å,
D–H⋯A = 168.69(31)°]. Although the compounds have rather
different amounts of solvent molecules, uncoordinated DMSO
and DMF provide steric hindrance around the tetranuclear
aggregates that prevents their joining into polymeric supra-
molecular architectures.
Fig. 2 Left: molecular structure of 2 with the atom numbering (H atoms are
omitted for clarity, the non-hydrogen atoms are shown as 40% thermal ellip-
soids). Right: schematic representation of the molecular structure of 2.
Thermal analysis
The thermal analysis curves of 1 and 2 revealed two main
decomposition steps over the 40–700 °C temperature range
(Fig. 4). For both compounds, the first step involved the elimi-
nation of the solvated molecules, DMSO in 1 (40–254 °C, calcu-
lated and observed mass residues are 84.8 and 85.1%,
respectively) and DMF in 2 (40–155 °C, calculated and
observed mass residues are 90.8 and 90.7%, respectively). In
the case of compound 2, no weight loss occurred in the
155–272 °C region, revealing that the [Cu2Fe2(HL2)2(H2L2)2]
molecule is stable below 272 °C. A broad weight loss in the
254–390 °C (1) and 272–462 °C (2) ranges revealed a decompo-
sition of the organic ligands, resulting in an oxide material
2CuO + Fe2O3 (calculated and observed mass residues for 1 are
23.9 and 26.4%, respectively, and calculated and observed
mass residues for 2 are 20.1 and 20.6%, respectively). In
general, the thermal behaviour of 1 and 2 exhibits an oxidative
character being typical for other copper-containing complexes
with similar ligands.8,20
Fig. 3 The ball-and-stick (a) and polyhedral (b) representations of the tetra-
nuclear {Cu2Fe2(µ-O)6} core in 1 and 2. Color codes: Cu, cyan; Fe, olive; O, red;
N, blue.
examples of heterometallic CuIIFeIII compounds with such a
molecular core.
Magnetic properties
The Schiff bases in 1 and 2 have quite similar structural for-
mulae and differ only by the presence of the Br substituent in
the ligand of 2. In both compounds, two types of Schiff bases
exist: a doubly deprotonated, of the tridentate (N,O,O) fashion,
and a triply deprotonated, of the tetradentate (N,O,O,O)
fashion. The coordination environments around all FeIII and
CuII metal atoms in 1 and 2 are formed exclusively by the
Schiff base ligand. Moreover, the deprotonated ligands play a
role of the charge compensators. Each Fe atom, in both com-
plexes, adopts a distorted octahedral geometry formed by the
O and N atoms of the ligands with the Fe–O(N) distances
varying from 1.926(2) to 2.127(6) Å. The cis and trans O–Fe–
O(N) bond angles range from 76.84(16) to 102.46(17)° and
from 163.37(17) to 171.30(7)°, respectively. All Cu atoms, in
1 and 2, are four coordinated and have NO3 donor sets. The
Cu–O(N) bonds in 1 and 2 lie in the range from 1.873(5) to
1.934(5) Å. The cis and trans O–Cu–O(N) bond angles vary from
79.55(17) to 100.00(17)° and from 164.32(19) to 177.40(6)°,
respectively.
Thermal variations of the effective magnetic moment in com-
plexes 1 and 2 are very similar and are displayed in Fig. 5 and
6, respectively. It can be seen that the room-temperature value
In spite of the similarity of the crystal structures of 1 and 2
their molecular packings are quite different, which could be
caused by the presence of different solvent molecules in the
crystal lattice. In 1 (Fig. S1†), each tetranuclear aggregate is
Fig. 4 Thermogravimetric plot showing the weight loss with the temperature
increase for complexes 1 (blue) and 2 (red) at 10° min−1 heating speed.
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Fig. 6 Magnetic functions for 2. Left
–
temperature dependence of the
Fig. 5 Magnetic functions for 1. Left
– temperature dependence of the
effective magnetic moment, right – field dependence of the magnetization,
inset – temperature dependence of the molar magnetic susceptibility. Open
circles – experimental data, lines – fitted.
effective magnetic moment, right – field dependence of the magnetization,
inset – temperature dependence of the molar magnetic susceptibility. Open
circles – experimental data, lines – fitted.
corresponds to µeff = 8.1µB and this is still increasing. The
theoretical value for the uncoupled {CuII–FeIII–FeIII–CuII}
system is
μeff ¼ ½2gFe sFeðsFe þ 1Þ þ ½2gCu sCuðsCu þ 1Þꢁꢁ1=2μB
ð1Þ
2
2
which yields an estimate µeff = 8.7µB when uniform g-factors
(g = 2) are assumed. On cooling the effective magnetic
moment decreases gradually and this is a fingerprint of the
dominating antiferromagnetic interaction. This conclusion is
confirmed by the magnetization data: the magnetization for T
= 2.0 K is lower than for T = 4.6 K. With antiferromagnetic
Fig. 7 Energy levels for 1 and 2 sorted by the total spin: one S = 6 state, 3 S =
5 states, 4 S = 4, 4 S = 3, 4 S = 2, 4 S = 1, and 2 S = 0 states.
interaction, the susceptibility should pass through
a
maximum, which is seen at Tmax = 47 K for 1 and Tmax = 44 K
for 2.
magnetization are R( χ) = 0.050 and R(M) = 0.121, respectively.
For 2 the magnetic parameters were JCu–Fe/hc = −10.5 cm−1
,
The magnetic data were fitted by considering the following
spin Hamiltonian
JFe–Fe/hc = −8.93 cm−1, gCu = 2.00 (gFe(III) = 2.0– fixed); R( χ) =
0.075 and R(M) = 0.133. The energy spectrum sorted according
to the spin value is displayed in Fig. 7.
ꢂ2
ꢂ
~
~
~
~
ˆ
H ¼ ꢂ JCuꢂFe½ðSCu1 ꢀSFe1Þ þ ðSFe2 ꢀSCu2Þꢁh
ꢂ2
~
~
When magnetic data of 1 and 2 are analyzed, the following
can be concluded. The negative value of the Cu(II)–Fe(III)
exchange coupling constant matches the high Cu–O–Fe angle
of the superexchange path. The Fe(III)–Fe(III) coupling constant
is also negative. Some discrepancies seen at the highest fields
of the magnetization can be ascribed to the simplifications in
the model: no g-factor anisotropy, an average of the magnetiza-
tion over three Cartesian components. The improvement of
the model by the single-ion anisotropy at the Fe(III) centres is
also possible; however, with antiferromagnetic exchange the
D-values are fixed with high uncertainty and also the sign of
the D-parameter is problematic when |D| is small.
~
ꢂ
þ μBBꢀ½gCuðSCu1 þ SCu2Þ
~
~
ꢂ JFe1ꢂFe2ðSFe1 ꢀSFe2Þh
ꢂ1
ꢂ
~
~
þ gFe1ðSFe1 þ SFe2Þꢁh
ð2Þ
where the isotropic exchange for adjacent centres is considered
along with the spin-Zeeman term. The g-factor asymmetry was
omitted in order to avoid overparametrization. The Hamil-
tonian produced a matrix of 144 × 144 dimension, which after
diagonalization gave the energy levels for three field values;
then the partition function and its derivatives were obtained
and inserted in the thermodynamic formulae for the magneti-
zation and magnetic susceptibility, respectively.21 The fitting
procedure was applied simultaneously to the susceptibility and
magnetization data sets by minimizing the error functional
EPR spectra
"
#
"
#
N
M ꢀ
ꢀ
ꢀ
X
X
ꢀ
ꢀ
ꢀ
ꢀ
A coupled Fe2Cu2 system consists of 144 energy levels, grouped
into one S = 6 state, 3 S = 5 states, 4 S = 4, 4 S = 3, 4 S = 2, 4 S =
1 and 2 S = 0 states (see Fig. 7). These states are very closely
ꢀ
ꢀ
c
o
o
F ¼
χi ꢂ χi =χi
ꢃ
Mc ꢂ Mo =Mo ! min
ð3Þ
ꢀ
j
j
j
i
j
The final set of magnetic parameters for 1 reads JCu–Fe/hc = spaced because of weak exchange interactions as revealed by
−10.2 cm−1, JFe–Fe/hc = −10.5 cm−1, gCu = 2.00 (gFe(III) = 2.0– the magnetic susceptibility data. The 144 states are eigenfunc-
fixed). The discrepancy factors for the susceptibility and tions of the isotropic HDVV Hamiltonian, as well as of the
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Fig. 8 216 GHz EPR spectra of 2 at 5 K and at 10 K showing increasing
thermal population of the S = 2 spin state lying above the lowest triplet.
Fig. 9 Comparison of the 10 K, 216 GHz spectra of 1 and 2. Resonances corres-
ponding to the “Z” molecular orientation and to the “half-field” transitions in
the triplet state are indicated.
total spin-squared operator of the system,
2
2
ŜT ¼ ðŜFe1 þ ŜFe2 þ ŜCu1 þ ŜCu2
Þ
They, however, are mixed under the Zeeman interaction
and the zero-field splitting interactions. Accordingly, the
“giant spin” approximation (that is treating each of the total
spin states separately), which would be appropriate in the case
of larger isotropic exchange interactions, is not strictly appli-
cable in the present systems. Zero-field splitting on individual
Fe(III) ions is expected to provide the main contribution to the
zfs of the coupled system, but both the magnitude of the ani-
sotropic Cu–Fe and Fe–Fe interactions (not included in the
spin Hamiltonian (eqn (2))) and the orientation of the Cu–Fe
zfs tensor versus the Fe–Fe zfs tensor must affect the EPR
spectra. The number of variable parameters combined with
the size of the spin Hamiltonian matrix indicates that attempts
to simulate a powder spectrum would not be realistic and only
a limited interpretation of the EPR spectra is possible.
Fig. 10 Mössbauer spectra at room temperature. Circles: experimental, solid
lines: calculated with the isomer shift and quadrupole splitting as given. Top:
1 (IS = 0.391(1) mm s−1, QS = 0.614(1) mm s−1); bottom: 2 (IS = 0.383(1) mm s−1
,
QS = 0.505(1) mm s−1).
With the exchange coupling constants of the order of J =
−10 cm−1, as found from the magnetic susceptibility for 1, the
lowest spin state is a singlet (S = 0), followed by a triplet (S =
1), a quintet (S = 2) and two other triplet states. The same
holds true for 2. Triplet state EPR spectra were indeed recog-
nized at very low temperatures (Fig. 8 and 9). Attempts to
simulate them using the “giant spin” model with a standard
spin Hamiltonian
Catalytic properties
Complexes 1 and 2 were investigated as catalytic precursors for
the oxidation of cyclohexane to cyclohexanol and cyclohexa-
none by aqueous hydrogen peroxide under mild conditions.
Cyclohexane has been used as a recognized substrate model
for C–H bond activation investigations. No alkane oxidation
products (or only traces) were obtained in the absence of cata-
lyst or hydrogen peroxide (Scheme 2). The absence of a nitric
acid promoter results in much lower yields for both the 1 and
2 complexes. The final concentrations of the ketone and
alcohol were measured after the addition of PPh3 in accord
with the method developed earlier by Shul’pin.23 Cyclohexanol
2
2
2
Ĥ ¼ μBBꢀfgSgꢀŜ þ DSfŜz ꢂ SðS þ 1Þ=3g þ ESðŜx ꢂ Ŝy
Þ
ð4Þ
resulted in DS=1 = 3.1 cm−1 and ES=1 = 1.0 cm−1 for 1; analo-
gously, DS=1 = 2.7 cm−1 and ES=1 = 0.4 cm−1 for 2. Fig. 8 shows
an increase in the thermal population of the S = 2 spin state
lying above the lowest triplet caused by heating from 5 to 10 K.
Mössbauer spectra
A small isomer shift (IS) and quadrupole splitting (QS)
observed in the room-temperature Mössbauer spectra (Fig. 10)
confirm the presence of alkoxo-bridges (rather than oxo)
between the Fe3+ ions.22
Scheme 2
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Fig. 12 Effect of the cyclohexane (CyH) concentration on the total yield of
cyclohexanol and cyclohexanone (circle symbols) and total TONs (square
symbols) in the oxidation of cyclohexane with H2O2 (1.0 M) catalyzed by com-
plexes 1 and 2 (above and below, respectively; [1]0 = [2]0 = 7.6 × 10−5 M) in the
presence of HNO3 (0.04 M) in acetonitrile at room temperature, 5 h reaction
time. The total initial volume of the reaction solution was 5 mL. For the tests
with [CyH]0 > 0.4 M complete dissolution of cyclohexane was observed in ca.
30 min.
Fig. 11 Effect of the catalyst concentration on the total yield of cyclohexanol
and cyclohexanone (circle symbols, determined by GC after reduction with PPh3)
and total TONs (square symbols) in the oxidation of cyclohexane with H2O2 (1.0
M) catalyzed by complexes 1 (above) and 2 (below) in the presence of HNO3
(0.04 M) in acetonitrile (the total volume of the reaction solution was 5 mL),
room temperature, 5 h reaction time.
and cyclohexanone were essentially the only reaction products
detected by GC-MS studies.
stable in the [CyH]0 = 0.2–0.4 region,9 the catalytic systems
The dependence of the product yield (sum of cyclohexanol
and cyclohexanone) on catalyst concentration after 5 h is based on 1 and 2 show a gradual yield decay with [CyH]0
depicted in Fig. 11. The yield curve for 1 rises up to 36% in the increase (Fig. 12), while the TON is stable in the [CyH]0
=
0 < [1]0 < 2 × 10−4 M region, giving a maximum TON value of
0.2–0.5 M region.
596. Further increase of the concentration of 1 does not lead
to a yield increase (Fig. 11). This could be associated with the oxide) amount on the overall yield (Fig. 13) reveals that the
optimal concentration of [H2O2]0 ca. 0.6 M is surprisingly low,
The study of the influence of the oxidant (hydrogen per-
overoxidation effect as well as the catalase activity of 1. Com-
pound 2 exhibits, in general, similar behaviour (Fig. 11). In the with an [H2O2]/[CyH] ratio of 2.9. In accord, the relatively high
lowest concentration range, [2]0 < 2 × 10−4 M, the maximum yield of 10.3% based on hydrogen peroxide is achieved. Typi-
yield of 44% was observed for [2]0 = 1.38 × 10−4 M, supported
cally, complexes of copper and iron under similar conditions
by a TON of 1060. As for 1, after reaching a maximum, the exhibit the best yields based on cycloalkane in the [H2O2]/
yield undergoes a slight decay with the increase of [2]0. The [CyH] = 5–10 region with the yield based on hydrogen peroxide
lower than 5%.7,24
points [1]0 = [2]0 = 7.6 × 10−5 M were chosen for all further
experiments because they concern the “half yield” of ca. 20%,
The yield versus [H2O]0 dependencies for 1 and 2, studied
where both promotion (increase up to 40%) and suppression for [H2O2]0 = 0.6 M (highest yield based on cyclohexane) and
0.4 M (for comparative purpose), are shown in Fig. 14. Water
(decrease down to 0%) effects could be detected.
Recently, it was observed for the heterometallic complex typically has been considered an unwanted component of the
[Co4Fe2O(Sae)8] (H2Sae = salicylidene-2-ethanolamine) that the catalytic systems, although for some cases it was found to have
no influence until [H2O]0 = 10 M7 or even to show promoting
initial concentration increase of cyclohexane up to 0.6 M could
improve the TON values significantly.9 However, the depen- activity by catalyzing H+-transfer steps from coordinated H2O2
dence of the overall TON on the substrate concentration for 1 towards the formation of hydroxyl radicals.25 As can be seen,
in both the [H2O2]0 = 0.6 M and 0.4 M cases, an increase of
and 2 shows that the initial [CyH]0 = 0.2 M is optimal (see also
below the yield dependence) for the present systems (Fig. 12). water concentration leads to a yield decay (Fig. 14). This indi-
In contrast to [Co4Fe2O(Sae)8], where the yield was found to be cates a suppressing effect of the water in the concentration
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Fig. 13 Effect of the oxidant (H2O2) concentration on the total yield of cyclo-
hexanol and cyclohexanone on the basis of cyclohexane (circle symbols) and of
the total H2O2 (square symbols) in the oxidation of cyclohexane (0.2 M) with
H2O2 catalyzed by complexes 1 and 2 (above and below, respectively; [1]0 = [2]0
= 7.6 × 10−5 M) in the presence of HNO3 (0.04 M) in acetonitrile at room temp-
erature, 5 h reaction time. The total volume of the reaction solution was 5 mL.
Fig. 14 Effect of the water concentration on the total yield of cyclohexanol
and cyclohexanone on the basis of cyclohexane in the oxidation of cyclohexane
(0.2 M) with H2O2 (0.6 M or 0.4 M for circle and triangle symbols, respectively)
catalyzed by complexes 1 and 2 (above and below, respectively; [1]0 = [2]0
=
7.6 × 10−5 M) in the presence of HNO3 (0.04 M) in acetonitrile at room temp-
erature, 5 h reaction time. The total volume of the reaction solution was 5 mL.
ranges studied, although one cannot exclude an eventual pro-
To confirm this assumption, the selectivity parameters on
the oxidation of normal octane were studied (Tables 1 and 2).
moting effect of water25 at lower concentrations.
On the basis of previous studies, one may expect a free It was found that the catalytic systems based on 1 and 2
radical mechanism for the catalytic systems based on 1 and 2, possess low regioselectivity, close to that found for the catalytic
with the hydroxyl radicals (generated from H2O2) as the main systems where participation of hydroxyl radicals was
reacting species with the alkane.7,9,23–25 This assumption is detected.9,23,27
partially confirmed by the typical maximum yield of no higher
The stereoselectivity of the catalytic systems based on 1 and
than 40%,26 demonstrated by the yield versus [Cat] dependence 2 was studied under conditions that could suppress the free
(Fig. 11) and by the higher alcohol/ketone ratio (Cy–OH : CyvO radical mechanism:28 low concentrations of substrate, cis-1,2-
up to 10 : 1) observed after reduction of the reaction mixture by dimethylcyclohexane (cis-DMCH, 0.09 M), and oxidizer (H2O2,
PPh3, in comparison with the ratio obtained prior to the phos- 0.1 M in total), at 0 °C (hydrogen peroxide was added
phine addition.23
gradually for 10 min). The amounts of trans- and
Table 1 Selectivity parameters in oxidation of n-octane catalyzed by complex 1a
Products (concentrations (mM))
Time (min)
one-2
one-3
one-4
ol-1
ol-2
3.09
5.44
5.67
5.60
10.32
9.64
ol-3
3.05
5.81
5.50
5.83
10.73
8.62
ol-4
2.88
5.45
5.27
5.41
9.59
8.18
21.68
C(1) : C(2) : C(3) : C(4)b
30
60
90
120
150
180
210
1.02
2.01
2.81
3.35
3.61
3.97
2.88
0.77
1.66
2.49
3.05
3.54
3.96
3.37
0.62
1.35
2.08
2.55
2.93
3.33
2.69
0.98
1.56
1.65
1.80
2.56
2.31
4.01
1 : 6.3 : 5.8 : 5.3
1 : 7.2 : 7.2 : 6.5
1 : 7.7 : 7.3 : 6.7
1 : 7.4 : 7.4 : 6.6
1 : 8.1 : 8.4 : 7.3
1 : 8.9 : 8.2 : 7.5
1 : 9.1 : 10 : 9.1
21.45
23.36
a Reaction conditions: [1]0 = 4.7 × 10−4 M; [H2O2]0 = 1 M; [HNO3]0 = 0.04 M; [n-octane]0 = 0.25 M; MeCN up to 5 mL. b Normalized by taking into
account the numbers of H atoms at each of the C atoms.
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Table 2 Selectivity parameters in oxidation of n-octane catalyzed by complex 2a
Products (concentrations (mM))
Time (min)
one-2
one-3
one-4
ol-1
ol-2
2.69
2.54
7.52
2.70
8.41
ol-3
2.57
2.55
7.60
1.99
8.64
ol-4
2.26
2.28
6.81
1.86
7.99
C(1) : C(2) : C(3) : C(4)b
30
60
90
120
150
210
0.42
1.61
1.22
2.88
2.22
2.86
0.34
1.39
1.01
2.99
2.10
2.70
0.27
1.11
0.88
2.54
1.77
2.33
0.63
0.65
1.62
0.66
1.83
2.51
1 : 7.4 : 6.9 : 5.9
1 : 9.7 : 9.2 : 7.9
1 : 8.1 : 7.9 : 7.1
1 : 12.6 : 11.3 : 10.0
1 : 8.8 : 8.8 : 8
11.30
11.88
10.72
1 : 8.5 : 8.7 : 7.8
a Reaction conditions: [2]0 = 4.8 × 10−4 M; [H2O2]0 = 1 M; [HNO3]0 = 0.04 M; [n-octane]0 = 0.25 M; MeCN up to 5 mL. b Normalized by taking into
account the numbers of H atoms at each of the C atoms.
cis-dimethylcyclohexanol (after addition of PPh3 upon cooling) were recorded using a BX-FT IR “Perkin Elmer” instrument in
were found to be similar both for 1 and 2 after a short accumu- KBr pellets.
lation time (15 min), as well as after 5 h. The absence of stereo-
Synthesis of [Cu2Fe2(HL1)2(H2L1)2]·10DMSO (1). Salicylalde-
selectivity, as well as the low regioselectivity, clearly confirms hyde (0.53 mL, 5 mmol), tris(hydroxymethyl)aminomethane
the proposed free radical mechanism for the catalytic systems (0.61 g, 5 mmol) and triethylamine (0.7 mL, 5 mmol) were dis-
with complexes 1 and 2 as catalysts.
solved in DMSO (25 mL) in this order, forming a yellow solu-
tion which was magnetically stirred at 50–60 °C (10 min).
Then, copper powder (0.16 g, 2.5 mmol) and FeCl2·4H2O
(0.25 g, 1.25 mmol) were added to the hot yellow solution of
the ligand and magnetically stirred until the total dissolution
of copper powder was observed (5 h). Red-brown crystals suit-
able for X-ray crystallographic studies were formed in two days
Conclusions
Using the “direct synthesis” approach, we have synthesized
two novel heterometallic coordination compounds containing
the chain-like {Cu2Fe2(µ-O)6} core supported by Schiff base
ligands. Polynuclear heterometallic complexes are a class of
compounds that deserves special attention pertaining to the
development of novel magnetic and catalytic materials. The
magnetic studies of complexes 1 and 2 revealed the antiferro-
magnetic nature of the couplings between the paramagnetic
metal centres. These studies are supported by high-field EPR
investigations.
Both compounds were shown to act as catalyst precursors
in the mild oxidation of cyclohexane with hydrogen peroxide.
The catalytic activities were found to be among the highest
ones for this type of reaction, presumably due to a synergistic
effect of two different metals present in 1 and 2. The results
reported here demonstrate the power of the direct synthesis
method to generate polynuclear heterometallic assemblies,
possessing sophisticated structures. These heterometallic com-
plexes are novel catalytic materials, potentially of bio-mimick-
ing significance. These studies are going to be expanded to a
wider range of catalytic reactions, namely towards an under-
standing of the synergistic catalytic effect associated with the
presence of a few different metals in a molecule.
i
after addition of PrOH into the resulting dark-red solution.
Yield: 0.73 g, 61% (per iron). The crystals are not stable in air
and after several days they have lost a part of DMSO molecules
and finally, according to the elemental analysis, the complex
formula can be written as [Cu2Fe2(HL1)2(H2L1)2]·2.6DMSO.
Magnetic and catalytic investigations were made using the fol-
lowing data of elemental analysis. Anal. calc. for C49.2H65.6Cu2-
Fe2N4O18.6S2.6 (M = 1332.78): C, 44.34; N, 4.21; H, 4.97;
S, 6.25%. Found: C, 44.0; N, 4.2; H, 4.9; S, 5.9%. The compound
is sparingly soluble in DMSO and DMF and insoluble in water.
Synthesis of [Cu2Fe2(HL2)2(H2L2)2]·2DMF (2). This complex
was prepared in a way similar to that of 1, but in DMF solution
and using 5-bromo-salicylaldehyde (1.01 g, 5 mmol). Red-
brown crystals suitable for X-ray analysis grew within two days
after addition of diethyl ether into the resulting dark-red solu-
tion. Yield: 0.54 g, 54% (per iron). Anal. calc. for C50H58Br4Cu2-
Fe2N6O18 (Mr = 1589.44): C, 37.78; N, 5.29; H, 3.68%. Found:
C, 37.4; N, 5.2; H, 3.9%. The compound is sparingly soluble in
DMSO and DMF and insoluble in water.
Crystallography
The X-ray diffraction study of 1 was performed on an “Xcalibur
3” diffractometer (graphite-monochromated MoKα radiation,
CCD detector, ω-scans). Empirical correction for absorption
was provided with a multi-scan method using spherical har-
monics, implemented in the SCALE3 ABSPACK scaling algo-
Experimental section
General
All chemicals were of reagent grade and used as received. All rithm of the CrysAlis Red program package.29 The structure
experiments were carried out in air. Elemental analyses for was solved by direct methods and refined against F2 within
CHNS were provided by the Microanalytical Service of the anisotropic approximation for all non-hydrogen atoms using
Instituto Superior Técnico. Infrared spectra (4000–400 cm−1
)
the SHELXTL package.30 All H atoms were placed in idealized
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positions and constrained to ride on their parent atoms with detection. The susceptibility taken at B = 0.1 T was corrected
Uiso = nUeq (n = 1.5 for CH3 and OH groups and n = 1.2 for all for the underlying diamagnetism and converted to the
the other H atoms). Two solvent molecules of DMSO were dis- effective magnetic moment. The magnetization was measured
ordered, each one over two sites, with refined occupancy at two temperatures: T = 2.0 and T = 4.6 K.
factors 0.77(1)/0.23(1) for S(4A)–O(4SA)/S(4B)–O(4SB), and 0.94
High-field and high-frequency EPR spectroscopy
(1)/0.06(1) for S(5A)/S(5B). Anisotropic displacement para-
meters of non-hydrogen atoms of these molecules were
restrained within “rigid bond” approximation. Additionally,
S–O bonds in each disordered molecule were restrained to be
of the same lengths within an effective standard deviation of
0.01 Å.
High-frequency EPR spectra were recorded using a home-built
spectrometer at the EMR facility of NHMFL.34 The instrument
is a transmission-type device in which waves are propagated in
cylindrical lightpipes. The microwaves were generated by a
phase-locked oscillator (Virginia Diodes) operating at a fre-
quency of 13 1 GHz and generating its harmonics, of which
the 4th, 8th, 16th, 24th and 32nd were available. A supercon-
ducting magnet (Oxford Instruments) capable of reaching a
field of 17 T was employed.
A suitable single crystal of 2 was chosen for the X-ray diffr-
action experiment. Image frames were collected using a Bruker
SMART APEX2 diffractometer, equipped with MoKα graphite
monochromated (λ = 0.71073 Å) radiation, in the nitrogen
stream at 173 K. The cell parameters were determined using
436 reflections from 3 short runs. Image frames were inte-
grated using the SAINT v7.68a program.31 The intensities were
corrected for Lorentz and polarization effects as well as semi-
empirical (multiscan) adsorption correction using SADABS.32
The structure was solved by direct methods and refined by the
full matrix least-squares method on F2 using SHELXTL soft-
ware.30 All non-hydrogen atoms were refined anisotropically.
Hydrogen atoms, connected to correspondent carbon parent
atoms, were refined in geometrically calculated positions
using the “riding” model. Hydrogens connected to oxygen
atoms were found from the difference Fourier map and refined
with AFIX 83 instruction. During the structure refinement, one
DMF lattice molecule per asymmetric unit was modeled from
the difference Fourier map. Further lattice solvent molecules,
supposedly diethyl ether and/or water molecules, were found
to be highly disordered. This may be connected with very high
flexibility of the molecule combined with the absence of
strong intermolecular interactions. On this ground, it was
impossible to model it with acceptable geometrical and
thermal parameters. The SQUEEZE procedure was used to
modify the reflection intensities, related to disordered lattice
solvent molecules.33 It resulted in 377 electrons, squeezed
from a 796 Å3 void with the centre in the 0.033 1/2 1/2
position.
Thermogravimetric measurements
A Perkin-Elmer STA-6000 model thermogravimetric analyzer
was used for determination of the thermal stability of com-
plexes 1 and 2. Samples weighing 5–30 mg were heated from
30 to 1000 °C at a heating rate of 1–50 °C min−1 in open air.
Catalytic activity studies
The reaction mixtures were prepared in two ways. (I) To
0.6–17 µmol of catalyst weighed into the reaction flask, 4.5 mL
CH3CN, 0.2 mmol HNO3 (65%, aqueous), 1.0 mmol of
cycloalkane and 4.00–20.0 mmol (typically 5.00 mmol) H2O2
(30% aqueous) were added in this order. (II) The catalyst pre-
cursor and the co-catalyst HNO3 were used in the form of stock
solutions in acetonitrile. To aliquots of these solutions,
1.0 mmol of cycloalkane and 4.00–20.0 mmol (typically
5.00 mmol) H2O2 (30% aqueous) were added, giving the total
volume of the reaction solution of 5 mL. (CAUTION: the com-
bination of air or molecular oxygen and H2O2 with organic
compounds at elevated temperatures may be explosive!). The
oxidation reactions were typically carried out in air in thermo-
stated Pyrex cylindrical vessels with vigorous stirring at room
temperature and atmospheric pressure. At the end of the reac-
tion (typically 5 h), 5 mL of diethyl ether and 90 μL of cyclo-
heptanone (as GC internal standard) were added and the
system was refluxed for 5 min. Then, an aliquot of ca. 0.5 mL
was taken and transferred, upon cooling, into a vial containing
an excess (ca. 150 mg) of solid PPh3. A Perkin-Elmer Clarus
500 gas chromatograph with a BP-20 capillary column (SGE,
30 m × 0.32 mm × 25 μm) and a Perkin-Elmer Clarus 600 gas
chromatograph, equipped with a Perkin-Elmer Clarus 600 C
mass-spectrometer (electron impact), with a BPX5 capillary
column (SGE, the same dimensions) and helium carrier gas
were used for quantitative analyses of the reaction mixtures.
Crystal data for 1: C64H110Cu2Fe2N4O26S10, M = 1910.94, tri-
ˉ
clinic, P1, a = 12.8911(4) Å, b = 13.6193(4) Å, c = 13.7806(4) Å,
α = 70.429(3)°, β = 65.864(3)°, γ = 85.283(2)°, V = 2076.14(11)
Å3, T = 100(2) K, Z = 1, 24 097 reflections measured, 11 889
independent reflections (Rint = 0.0565), R1 = 0.0408 (I > 2σ(I)),
wR(F 2) = 0.0533 (all data), GoF = 0.763.
Crystal data for 2: C50H58Br4Cu2Fe2N6O18, M = 1589.44, tri-
ˉ
clinic, P1, a = 12.7243(5) Å, b = 12.8239(6) Å, c = 13.7739(6) Å,
α = 105.861(2)°, β = 92.703(2)°, γ = 103.364(2)°, V = 2088.78(16)
Å3, T = 173(2) K, Z = 1, 25 610 reflections measured, 8393 inde-
pendent reflections (Rint = 0.0979), R1 = 0.0549 (I > 2σ(I)),
wR(F 2) = 0.1263 (all data), GoF = 0.807.
Acknowledgements
Magnetic measurements
This work was partially supported by the Foundation for
The magnetic data were taken with the SQUID apparatus Science and Technology (FCT), Portugal (projects PTDC/
(MPMS-XL7, Quantum Design) using the RSO mode of QUI-QUI/102150/2008, PTDC/QUI-QUI/121526/2010 and PEst-
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OE/QUI/UI0100/2013; fellowship SFRH/BPD/63710/2009) and 15 V. V. Semenaka, O. V. Nesterova, V. N. Kokozay,
the State Fund for Fundamental Research, Ukraine (project
54.3/005). Slovak grant agency VEGA (projects 1/0052/11 and
V. V. Dyakonenko, R. I. Zubatyuk, O. V. Shishkin, R. Boča,
J. Jezierska and A. Ozarowski, Inorg. Chem., 2010, 49, 5460.
1/0233/12) is also acknowledged for the financial support. The 16 V. V. Semenaka, O. V. Nesterova, V. N. Kokozay,
EPR and Mössbauer spectra were recorded at the NHMFL
which is funded by the NSF through Cooperative Agreement
no. DMR-1157490, the State of Florida and the DOE. The
Mössbauer spectrometer was funded by UCGP grant 5064 (AO).
R. I. Zybatyuk, O. V. Shishkin, R. Boča, C. J. Gomez-Garcia,
J. M. Clemente-Juan and J. Jezierska, Polyhedron, 2010, 29,
1326; V. V. Semenaka, O. V. Nesterova, V. N. Kokozay,
R. I. Zybatyuk, O. V. Shishkin, R. Boča, D. V. Shevchenko,
P. Huang and S. Styring, Dalton Trans., 2010, 2344;
V. V. Semenaka, O. V. Nesterova, V. N. Kokozay,
V. V. Dyakonenko, O. V. Shishkin, R. Boča and J. Jezierska,
Dalton Trans., 2010, 1734.
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