Heterometallic CuIIFeIII and CuIIMnIII alkoxo-bridged complexes revealing a rare hexanuclear M6(μ-X)7(μ3-X)2 molecular core

Article abstract of DOI:10.1039/c8dt02290a

The novel hexanuclear complexes [Cu₄Fe₂(OH)(Piv)₄(tBuDea)₄Cl]·0.5CH₃CN (1) and [Cu₄Mn₂(OH)(Piv)₄(tBuDea)₄Cl] (2) were prepared through one-pot self-assembly

Full text of DOI:10.1039/c8dt02290a

Dalton
Transactions
View Article Online
View Journal
PAPER
Heterometallic CuIIFeIII and CuIIMnIII alkoxo-
bridged complexes revealing a rare hexanuclear
Cite this: DOI: 10.1039/c8dt02290a
M6(µ-X)7(µ3-X)2 molecular core†
Oksana V. Nesterova, *a Dmytro S. Nesterov, *a Beáta Vranovičová,b
a
Roman Bočab and Armando J. L. Pombeiro
*
The novel hexanuclear complexes [Cu4Fe2(OH)(Piv)4(tBuDea)4Cl]·0.5CH3CN (1) and [Cu4Mn2(OH)
(Piv)4(tBuDea)4Cl] (2) were prepared through one-pot self-assembly reactions of copper powder and iron(II)
or manganese(II) chloride with N-tert-butyldiethanolamine (H2tBuDea) and pivalic acid (HPiv) in acetonitrile.
Crystallographic studies revealed the uncommon molecular core type M6(µ-X)7(µ3-X)2 in 1 and 2, which can
be viewed as a combination of two trimetallic M3(µ-X)2(µ3-X) fragments joined by three bridging atoms. The
analysis and classification of the hexanuclear complexes having a M3(µ-X)2(µ3-X) moiety as a core forming
fragment using data from the Cambridge Structural Database (CSD) were performed. Variable-temperature
(1.8–300 K) magnetic susceptibility measurements of 1 showed a decrease of the effective magnetic
moment value at low temperature, indicative of antiferromagnetic coupling between the magnetic centres
(JFe–Cu/hc = −6.9 cm−1, JCu–Cu/hc = −4.1 cm−1, JFe–Fe/hc = −24.2 cm−1). Complex 1 acts as a catalyst in
the reaction of mild oxidation of cyclohexane with H2O2, showing the yields of products, cyclohexanol and
cyclohexanone, up to 17% using pyrazinecarboxylic acid as a promoter. In the oxidation of cis-1,2-dimethyl-
cyclohexane with m-chloroperoxybenzoic acid (m-CPBA), 70% of retention of stereoconfiguration was
observed for tertiary alcohols. Compound 1 also catalyses the amidation of cyclohexane with benzamide. In
all three catalytic reactions the by-products were investigated in detail and discussed.
Received 4th June 2018,
Accepted 10th July 2018
DOI: 10.1039/c8dt02290a
rsc.li/dalton
cule. Poly- and high-nuclear heterometallic coordination com-
pounds1a,2 represent a class of materials with recognized mag-
Introduction
The search towards novel molecule-based materials, able to netic3 and catalytic1a,4 effects, appearing from synergic inter-
combine useful physico-chemical properties, such as magnet- actions of a few different metals within one complex molecule.
ism and catalysis, has led to particular interest in polynuclear In pursuit of our research on heterometallic high-nuclear
coordination compounds.1 Apart from other possibilities, their coordination compounds, we applied the “direct synthesis”
multifunctionality can be realized by the involvement of two or approach,2 which is based on the spontaneous self-assembly
more dissimilar metal atoms into the final close-packed mole- and allows the use of metal powder(s) as the starting material
and simple flexible ligands, such as aliphatic aminoalco-
hols.1a,5 In this way, the formation of a complex high-nuclear
structure proceeds in a single-pot, easy and “straightforward”
method avoiding the separate steps of building block construc-
tion. Using this synthetic strategy a series of heterometallic
complexes of various nuclearities were obtained.1a,2 Many of
them show rather rare or even unique types of molecular struc-
ture cores and, moreover, can reveal a high catalytic activity in
the oxidation of alkanes with peroxides under mild condi-
tions1a,6 and an interesting magnetic behaviour.7
It has been shown that the simultaneous use of aminoalco-
hols and carboxylate ligands having bulky substituents (such
as tert-butyl groups) may lead to unexpected ligand coordi-
nation modes8 and polynuclear structures.9 In the present
study, we were interested to combine the “direct synthesis”
aCentro de Química Estrutural, Complexo I, Instituto Superior Técnico,
Universidade de Lisboa, Avenida Rovisco Pais, 1049-001 Lisboa, Portugal.
E-mail: pombeiro@tecnico.ulisboa.pt, dmytro.nesterov@tecnico.ulisboa.pt,
oksana.nesterova@tecnico.ulisboa.pt
bDepartment of Chemistry, FPV, University of SS Cyril and Methodius,
917 01 Trnava, Slovakia
†Electronic supplementary information (ESI) available: Selected geometrical
parameters (distances/Å and angles/°) for 1 and 2. The analysis of the {M6(µ-
X)7(µ3-X)2} Molecular Structure Type (MST). Figures (plots, chromatograms and
mass spectra). Details of 18O incorporation analysis; chromatograms of reaction
products for cyclohexane and cis-1,2-dimethylcyclohexane (cis-1,2-DMCH) oxi-
dation; accumulations of products for oxidation of cis-1,2-DMCH and [1]0 = 4.1 ×
10−4 M; EI mass spectra of normal and 18O-labeled products. CCDC 1447617
1816439. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c8dt02290a
This journal is © The Royal Society of Chemistry 2018
Dalton Trans.

View Article Online
Paper
Dalton Transactions
synthetic approach with the aminoalcohol/carboxylic acid were dissolved in CH3CN (30 mL) and magnetically stirred at
ligand system, expecting that both these powerful approaches 50–60 °C (5 h). The resulting dark-brown solution was filtered
would lead to novel polynuclear assemblies.
off and left untouched for 1 month for standing at r.t. After
A potential feature of the coordination compounds bearing this, iPrOH was added in small portions the next month,
bulky hydrophobic groups (such as tert-butyl ones) is their which afforded an oily substance with the inclusion of a small
solubility in low-polar solvents, such as benzene or aliphatic amount of dark-brown crystals suitable for X-ray crystallo-
hydrocarbons. This property is important for the studies of graphic study. The crystals were carefully removed from the oil,
homogeneous catalytic processes typically proceeding in such washed with methanol and dried. Anal. calc. for
solvents. As heterometallic complexes, particularly those invol- C52H105ClCu4Mn2N4O17 (M = 1457.88): C, 42.84; N, 3.84; H,
ving copper, manganese or iron, were shown as active catalysts 7.27%. Found: C, 43.0; N, 4.2; H, 7.4%.
in the reactions of alkane C–H functionalization,1a,4,10 hence
we aimed at the preparation of polynuclear heterometallic
Crystallography
compounds based on these metals with bulky N,O-donor
Crystal data for 1. C53H106.50ClCu4Fe2N4.50O17, M = 1480.23,
ligands and the investigation of their catalytic activity in valu- a = 16.296(3) Å, b = 23.480(5) Å, c = 36.268(8) Å, β = 99.460(8)°,
able C–H activation processes,11 such as hydroxylation1a,10a V = 13 688(5) Å3, T = 150(2) K, space group C2/c, Z = 8, MoKα,
and amidation12 of cycloalkanes and their derivatives in the 102 878 reflections measured, 14 044 independent reflections
presence of peroxide oxidants.
(Rint = 0.1117). The final R1 values were 0.0435 (I > 2σ(I)). The
In the present work, we have studied the interaction of final wR(F2) values were 0.0975 (all data). The goodness of fit
copper powder and iron(II) or manganese(II) chlorides with on F2 was 1.018.
N-tert-butyldiethanolamine (H2tBuDea) and pivalic acid (HPiv)
Crystal data for 2. C52H105ClCu4Mn2N4O17, M = 1457.88, a =
that afforded the novel heterometallic complexes [Cu4Fe2(OH) 22.8621(15) Å, b = 16.6480(15) Å, c = 19.7908(16) Å, β = 114.979
(Piv)4(tBuDea)4Cl]·0.5CH3CN (1) and [Cu4Mn2(OH)(Piv)4 (6)°, V = 6828.0(10) Å3, T = 150(2) K, space group Cc, Z = 4,
(tBuDea)4Cl] (2). The magnetic and alkane oxidation catalytic MoKα, 51 971 reflections measured, 14 023 independent reflec-
properties of 1 are discussed herein.
tions (Rint = 0.1286). The final R1 values were 0.0634 (I > 2σ(I)).
The final wR(F2) values were 0.1339 (all data). The goodness of
fit on F2 was 0.996.
The X-ray diffraction data for complexes 1 and 2 were col-
lected using a Bruker AXS KAPPA APEX II diffractometer with
graphite monochromated Mo-Kα radiation. Cell parameters
were retrieved using Bruker SMART and refined using Bruker
SAINT13 on all the observed reflections. Absorption corrections
were applied using SADABS.14 The structure was solved by
direct methods and refined against F2 using the program
SHELXL-2016/6.15 The structure of 2 was refined as an inver-
sion twin with 4.8% of the inverted component, leading to a
Flack parameter of 0.05(2).
The tert-Bu groups C44–C47 in both 1 and 2 were modelled
as being disordered over two positions, having A and B atom
labels, with populations 0.553(11) and 0.447(11) for 1, and
0.51(4) and 0.49(4) for 2, respectively. The atoms of the dis-
ordered tert-Bu groups were refined with isotropic displace-
ment parameters. The chlorine atom Cl1 in 2 was found to be
disordered over two positions, Cl1A and Cl1B, with occu-
pancies 0.324(9) and 0.676(9), respectively. The presence of a
difference peak of 0.96 e Å−3 in the Fourier map around the
ClB atom suggested the possibility of the presence of other
atoms, presumably water molecules, which coordinate with
Cu2. However, attempts to treat this peak as disordered water
were unsuccessful with unacceptable thermal parameters.
Furthermore, the presence of such water molecules would
result in a too short ClA–O(water) distance of less than 2 Å.
Hence this weak difference peak was ignored.
Experimental section
General
All chemicals were of reagent grade and used as received. All
experiments were carried out in air. Elemental analyses for C,
H and N were carried out by the Microanalytical Service of the
Instituto Superior Técnico. Infrared spectra (4000–400 cm−1
)
were recorded on a Vertex 70 (Bruker) instrument in KBr
pellets. Mass spectra were obtained using an LCQ Fleet mass
spectrometer with an ESI source (Thermo Scientific). Powder
X-ray data were collected at room temperature using a Bruker
D8 Advance diffractometer.
Synthesis of [Cu4Fe2(OH)(Piv)4(tBuDea)4Cl]·0.5CH3CN (1)
Copper powder (0.16 g, 2.5 mmol), FeCl2·4H2O (0.99 g,
5 mmol), pivalic acid (0.26 g, 2.5 mmol), N-tert-butyldiethano-
lamine (2.4 g, 15 mmol) and triethylamine (0.7 mL, 5 mmol)
were dissolved in CH3CN (30 mL) and magnetically stirred at
50–60 °C (2 h). The resulting dark-brown solution was filtered
off twice and dark-brown crystals suitable for X-ray crystallo-
graphic study were formed within four days. Yield: 0.58 g (63%
based on copper). Anal. calc. for C53H106.50ClCu4Fe2N4.50O17
(M = 1480.23): C, 43.00; N, 4.26; H, 7.25%. Found: C, 42.9; N,
4.5; H, 7.3%. The compound is sparingly soluble in DMSO,
DMF and insoluble in water.
Synthesis of [Cu4Mn2(OH)(Piv)4(tBuDea)4Cl] (2)
The H1 hydrogen atoms of the coordinated OH group in 1
Copper powder (0.16 g, 2.5 mmol), MnCl2·4H2O (0.99 g, and 2 were located in difference Fourier maps and refined
5 mmol), pivalic acid (0.26 g, 2.5 mmol), N-tert-butyldiethano- using a riding model using Uiso = 1.5Ueq (for 1) or refined
lamine (2.4 g, 15 mmol) and triethylamine (0.7 mL, 5 mmol) freely (for 2). All other hydrogen atoms were placed at calcu-
Dalton Trans.
This journal is © The Royal Society of Chemistry 2018

View Article Online
Dalton Transactions
Paper
lated positions and refined the same model with Uiso = nUeq ated temperatures may be explosive!) In all cases the final reac-
(n = 1.5 for H atoms of the methyl group and water H atoms, tion volume was 5 mL. Samples were quenched at room temp-
and n = 1.2 for other H atoms).
erature with an excess of solid PPh3 and directly analysed by
Crystallographic data for the reported structures can be GC and GC-MS techniques.
obtained by quoting the deposition numbers CCDC 1447617
Catalytic amidation of cyclohexane
(1) and 1816439 (2).†
The reactions were typically carried out under an N2 atmo-
sphere in a thermostated Schlenk tube under vigorous stirring.
Firstly the catalyst was introduced into the tube in the solid
form. Then benzene (1 mL) and cyclohexane (0.54 mL) were
added in this order. The oxidant (tBuOOtBu, 184 µL) was then
added at room temperature and the mixture was immediately
frozen with liquid nitrogen. The atmosphere was pumped and
filled with N2 a few times in order to remove air. The frozen
mixture was left to warm up under vacuum (to degasify) until
becoming liquid and the above procedure was repeated.
Finally the Schlenk tube was filled with N2 and the reaction
mixture was heated at 90 °C for 24 h. Then the reaction
mixture was cooled to room temperature and 10 mL of CH3CN
and 100 µL of α,α,α-trifluorotoluene (GC standard) were added.
The resulting mixture was directly analysed by GC/GC-MS
techniques.
Magnetic measurements
The magnetic data for 1 were obtained for a polycrystalline
sample with a SQUID magnetometer (MPMS-XL7, Quantum
Design) in the RSO mode of detection.
Gas chromatography and mass spectrometry
A PerkinElmer Clarus 500 gas chromatograph, equipped with
a polar capillary column (SGE BP-20; 30 m × 0.32 mm ×
25 μm) and a FID detector, was used for analyses of the reac-
tion products of cyclohexane oxidation. The following GC
conditions were used: 100 °C (1 min), 100–160 °C (10 degrees
per minute), 160 °C (1 min), 8 min total run time; 200 °C
injector temperature. A PerkinElmer Clarus 600 gas chro-
matograph, equipped with two non-polar capillary columns
(SGE BPX5; 30 m × 0.32 mm × 25 μm), one having an EI-MS
(electron impact) quadrupole detector and the other one with
a FID detector, was used for other analyses of the reaction
mixtures. The following GC conditions were used: 50 °C
(3 min), 50–120 °C (8 degrees per minute), 120–300 °C (35
degrees per minute), 300 °C (3.11 min), 20 min total run
Results and discussion
Synthesis and spectroscopic analysis
time; 200 °C injector temperature. For analysis of cyclohexane Compounds 1 and 2 were formed through the self-assembly
amidation products a different program was employed: 50 °C reactions of copper powder, iron(II) or manganese(II) chloride,
(3 min), 50–150 °C (30 degrees per minute), 150–300 °C (14 pivalic
acid
(HPiv)
and
N-tert-butyldiethanolamine
degrees per minute), 300 °C (2.95 min), 20 min total run (H2tBuDea) in acetonitrile solution in the presence of triethyl-
time; 200 °C injector temperature. Helium was used as the amine, using the molar synthetic ratio of Cu : FeCl2/
carrier gas (constant 14 psi pressure and constant 1 mL MnCl2 : HPiv : H2tBuDea = 1 : 2 : 1 : 6. Such a ratio was chosen
min−1 flow for Clarus 500 and Clarus 600 devices, respect- on the basis of our previous experience2 and also to prevent
ively). All EI mass spectra were recorded with 70 eV energy. the formation of very stable {Cu2(Piv)4} dimers. The reactions
The 16O/18O compositions of the oxygenated products were were brought to completion by heating and stirring until the
determined by the relative abundances of mass peaks at total dissolution of copper was observed (2 h for 1 and 5 h for
m/z = M+/(M+ + 2) molecular ion peaks. Estimation of low- 2). Dark-brown microcrystals of 1 were formed within four
level 18O incorporations into tertiary trans- and cis-alcohols days after standing of the resulting solution at r.t. In the case
required a special correction to exclude the systematic errors of 2, no product formation was observed after one month
of the mass spectrometer (see the ESI†).
standing of the resulting dark-brown solution at r.t. To
promote crystallization, iPrOH was added in small portions to
this solution the next month. A small amount of dark-brown
Catalytic oxidation of alkanes
The reactions were typically carried out in air in thermostated crystals, suitable for X-ray crystallographic study, was formed,
cylindrical vials with vigorous stirring. Firstly, CH3CN and but they were incorporated into the simultaneously formed
CH3NO2 (GC internal standard, final [CH3NO2] = 0.2 M) were dark-brown oil. The crystals were carefully removed from the
added to solid 1. Then, where applicable, the solution of the oil, washed with methanol and dried. A synthetic procedure
promoter was added under stirring, with the subsequent was repeated a few times aiming to improve the yield of the
addition of alkane. For oxidation of cyclohexane, a hydrogen product and trying to avoid the formation of the oily sub-
peroxide solution (50%, aqueous) was added dropwise within stance, but all of them were unsuccessful. We can assume
10 seconds to a hot (50 °C) solution of the other components. that in the case of 2, the heterometallic compound could not
For oxidation of cis-1,2-dimethylcyclohexane (cis-1,2-DMCH), a be isolated from the initial acetonitrile solution without the
i
solid oxidant, m-chloroperoxybenzoic acid (m-CPBA), was dis- addition of PrOH probably due to the high solubility of the
solved in CH3CN (typically 30 mg in 1 mL of CH3CN) and complex in this solvent. But, at the same time, such addition,
added dropwise in the same way as H2O2. (CAUTION: the com- apart from leading to the crystal formation, also led to the
bination of H2O2 or m-CPBA with organic compounds at elev- undesirable oil.
This journal is © The Royal Society of Chemistry 2018
Dalton Trans.

View Article Online
Paper
Dalton Transactions
The general reaction can be written as follows (M = Fe (1),
Mn (2); n = 0 (2); 0.5 (1)):
À
Á
4Cu0 þ 2 MIICl2 ꢀ 4H2O þ 4HPiv þ 4H2tBuDea
þ 3Et3N þ 2:5O2 þ nCH3CN !
Â
Ã
CuI4IMI2IIðOHÞðPivÞ4ðtBuDeaÞ4Cl ꢀ nCH3CN
þ 3Et3N ꢀ HCl þ 12H2O
Fig. 2 The ball-and-stick representation of the hexanuclear {Cu4M2(µ-
O)7(µ3-O)2} (M = Fe, Mn) core showed on the example of 1. Colour
scheme: Fe, olive; Cy, cyan; O, red.
The IR spectra of complexes 1 and 2 are similar. The broad
band in the 3300–3500 cm−1 region is attributed to the ν(OH)
frequencies. In both cases, the very strong bands near
1550 cm−1 and 1420 cm−1 are assigned to the antisymmetric
and symmetric COO− stretching frequencies of pivalate,
respectively. The difference in wavenumbers between these
values, Δ = 130 cm−1, indicates the existence of bridging car-
boxylate ligands16 in the structures of the compounds, which
is in line with the X-ray structure determination.
mation and for the metal ion charge compensation. Both
crystal structures have the crystallographically independent
copper(II) metal atoms, Cu1, Cu3 and Cu4, which are five co-
ordinated and have NO4 donor sets. The Cu–O(N) bonds in 1
and 2 lie in the range from 1.877(2) to 2.438(10) Å (Table S1†),
while the O–Cu–O(N) bond angles vary from 77.9(3) to
174.1(3)°. In contrast, the Cu2 atom, in both 1 and 2 (for 2
with 68% because of the disorder of Cl1 atom), is six co-
ordinated and has a NO4Cl donor environment formed by the
nitrogen and oxygen atoms of the organic ligands [Cu2–O(N) =
1.935(8)–2.615(3) Å, Table S1†] and the chloride atom
[Cu2–Cl1 = 2.2548(11) and Cu2–Cl1B = 2.194(9) Å, for 1 and 2,
respectively]. Due to the disorder, in the case of 2, the five co-
ordinated Cu2 atom is also observed with 32% probability.
Each of the crystallographically independent Fe(III) atoms in 1,
Fe1 and Fe2, adopts a distorted octahedral geometry with the
Fe–O distances varying from 1.924(2) to 2.105(2) Å. The cis and
trans O–Fe–O bond angles range from 85.01(10) to 96.16(10)°
and from 171.48(10) to 178.73(10)°, respectively (Table S1†).
Similar to 1, both Mn1 and Mn2 in 2 are crystallographically
independent and show distorted octahedral environments
with the Mn–O bonds in the range of 1.854(7)–2.296(7) Å and
with cis and trans O–Mn–O angles in the ranges of 82.7(3)–97.6
(3)° and 171.4(3)–178.3(3)°, respectively (Table S1†). One of the
interesting features of both crystal structures is the hydroxido
bridge between the iron(III) or manganese(III) atoms with the
angle M1–O23–M2 equal to 140.15(12) and 140.5(5)°, for 1 and
2, respectively. Within the hexanuclear molecules of 1 and 2
the nearest (M–O–M) Cu⋯Cu distances vary from 2.94 to
3.22 Å, the nearest Cu⋯M (M = Fe, Mn) ones range from 2.99
to 3.46 Å and the Fe⋯Fe and Mn⋯Mn separations are 3.78
and 3.89 Å, for 1 and 2, respectively. For both structures, the
bulky N-tert-butyl groups in the organic ligands prevent the
formation of coordination or hydrogen bonds between adja-
cent hexanuclear molecules with a further increase of the
dimensionality of the final architecture (Fig. 3 and 4). In con-
trast to the crystal lattice of 2, in the structure of 1, acetonitrile
solvent molecules are located in the cavities between neigh-
bouring hexanuclear molecules (Fig. 3).
The phase purity of the bulk sample of 1 was confirmed by
powder X-ray analysis (Fig. S1†).
Crystal structures
The single crystal X-ray analysis reveals that both [Cu4Fe2(OH)
(Piv)4(tBuDea)4Cl]·0.5CH3CN (1) and [Cu4Mn2(OH)(Piv)4
(tBuDea)4Cl] (2) (Fig. 1 and S2†) are based on a hexanuclear
{Cu4M2(µ-O)7(µ3-O)2} (where M = Fe, Mn) core (Fig. 2), which
belongs to the general {M6(µ-X)7(µ3-X)2} (M = metal atom, X =
bridging atom) molecular structure type obtained by the exclu-
sion of all non-bridging non-metal atoms.
In the crystal structures all pivalate and N-tert-butyldietha-
nolamine ligands are completely deprotonated and show
bidentate (O,O) and tridentate (O,N,O) coordination, respect-
ively, thus accounting for the molecular structure type for-
The analysis of the Molecular Structure Type (MST)2,17
{M6(µ-X)7(µ3-X)2} of 1 and 2 (Fig. 2) reveals that it can be rep-
resented as a combination of two M3(µ-X)2(µ3-X) fragments
joined together by three bridging oxygen atoms (see the
detailed discussion and Tables S2 and S3 in the ESI†).
Fig. 1 Molecular structures of 1 (top) and 2 (bottom) in the ball-and-
stick representation with atom numbering. The hydrogen atoms are
omitted for clarity.
Dalton Trans.
This journal is © The Royal Society of Chemistry 2018

View Article Online
Dalton Transactions
Paper
Fig. 5 ESI-MS spectrum of an acetonitrile solution of 1, showing the
peak of its molecular ion (after the elimination of chloride ligand and
solvated acetonitrile).
Fig. 3 Polyhedral representation of the packing of hexanuclear mole-
cules in 1 viewed down the a axis. Colour scheme: Fe, olive; Cu, cyan; N,
blue; C, grey.
Magnetic properties
The magnetic functions (temperature dependence of the mag-
netic susceptibility corrected for the underlying diamagnetism
and transformed to the effective magnetic moment, and the
field dependence of the magnetization per formula unit) are
presented in Fig. 6. The effective magnetic moment at room
temperature adopts a value of µeff = 7.8µB that is much lower
than the spin-only value calculated as µeff(so)/µB = g{2·SFe(SFe
+
1) + 4·SCu(SCu + 1)}1/2 = 9.1. On cooling it gradually decreases
and at T = 2.0 K it is only µeff = 2.2µB.
This feature is typical for the exchange interaction of an
antiferromagnetic nature. The magnetization per formula unit
at T = 2.0 K and B = 7 T reads Mmol/NA = 1.9µB and it lies much
below the spin-only value of Mmol/NAµB = 2gFeSFe + 4gCuSCu
14 that again is a fingerprint of the sizable antiferromagnetic
coupling.
=
Fig. 4 Polyhedral representation of the packing of hexanuclear mole-
cules in 2 viewed down the b axis. Colour scheme: Mn, dark purple; Cu,
cyan; C, grey.
The coupling paths in complex 1 are rather complex and
described at least by the three different coupling constants
JFe–Cu, JCu–Cu and JFe–Fe between the adjacent centres connected
by one bridging oxygen atom. The angle Fe–O–Fe = 140.2°
induces a rather negative value of JFe–Fe. Two Cu–O–Cu angles
According to the Cambridge Structural Database (CSD,
v. 5.37),18 such a MST is rather rare. Although there are more
than 1700 structures of coordination compounds containing
the fragment M3(µ-X)2(µ3-X) in the CSD, only two structures
were found to fit exactly this MST (CSD refcodes TALPAX and
ZIHDID). Both these are polyoxometalate compounds of
tungsten.19
ESI-MS spectrometry
The electrospray mass spectra (ESI-MS) of acetonitrile solu-
tions of 1 were recorded to establish the structure of the com-
pound in solution. In the negative MS mode only a noisy, low-
intensity spectrum was seen. In contrast, the ESI-MS spectrum
in the positive mode revealed a strong peak at 1422.99 m/z,
which is close to the molecular weight of 1 (Fig. 5). The isoto-
pic distribution fits well with that expected for the cation
[1–Cl–CH3CN]+. Hence, the hexanuclear core of 1 retains its
integrity in solution.
Fig. 6 Magnetic functions for 1. Solid lines – calculated with the model
of isotropic exchange.
This journal is © The Royal Society of Chemistry 2018
Dalton Trans.

View Article Online
Paper
Dalton Transactions
are asymmetric (113.7° and 90.5° for one pair, and 103.6° and plexes into three groups based upon the “similarity distance”
93.9° for another one), which reveals that the ferromagnetic (Wards method, squared Euclidean norm) as shown in
portion is fully compensated by the antiferromagnetic one Fig. S3† – left. This assignment has been utilized in Fig. S3† –
yielding effectively JCu–Cu < 0. On the same basis also JFe–Cu < 0 right where the J vs. Fe–O–Fe angle correlation is displayed.
is expected.
The title complex (No. 11 in Table 1) lies outside that corre-
The spin space consists of N = 62·24 = 576 functions which lation probably due to the fact that it consists of only a single
determine the dimension of the spin Hamiltonian matrices Fe–OH–Fe bridge.
subjected to manifold diagonalization. The situation is much
The principal component analysis (Fig. S4†) shows which
simplified when the spin symmetry is utilized which causes a variables correlate, anticorrelate or do not correlate: the J
factoring of the spin Hamiltonian matrices according to the values anticorrelate with the Fe–(OH)–Fe bond angle. Three
spin quantum number. Then the number of spin states with groups of objects are well separated. Surprisingly, the expected
Smin = 0 to Smax = 7 is 6, 14, 16, 16, 15, 11, 5 and 1. Thus NS = J (or –J) vs. P correlation (as outlined by Gorun and Lippard21)
16 is the maximum dimension of a block for S = 2 and S = does not exist in the given group of complexes as proven by the
3. However, the Zeeman term is diagonal only in the case of Pearson correlation analysis (Table 2).
uniform g-factors that might be a good approximation in the
The mathematical forms of the magnetostructural
present case where gCu = 2.2 and gFe = 2.0 are expected giving J-correlations in Fe(III) polynuclear complexes are still a devel-
rise to geff ∼ 2.1. The possible zero-field splitting is little oping task, from early simple21 to more sophisticated ones.22
effective in the present case.
Experimental data were fitted by exploiting an advanced
genetic algorithm with the error functional that combines rela-
tive errors of the susceptibility and magnetization, i.e. F =
Catalytic oxidation of cyclohexane with H2O2
The catalytic properties of complex 1 in the mild oxidation of
cyclohexane were investigated (Scheme 1). Cyclohexane was
w1·R(χ) + (1 − w1)·R(M). To this end: JFe–Cu/hc = −6.9, JCu–Cu/hc =
−4.1, JFe–Fe/hc = −24.2 (all in cm−1 conforming to the defi-
selected as a recognized model substrate for C–H activation
tests.1a Reaction of cyclohexane (0.2 M) with aq. H2O2 (1 M; 5
~
~
ˆ
nition HAB ¼ ꢁJðSA ꢀ SBÞ) and geff = 2.00. The minor corrections
are the temperature-independent magnetism χTIM = −0.5 ×
10−9 m3 mol−1 and the molecular-field correction (zj )/hc =
0.31 cm−1. The agreement factors are R(χ) = 0.034, R(M) = 0.049
and the calculated data are drawn as lines in Fig. 6. Some devi-
ations for the magnetization under a high field are attributed
to the effect of the zero-field splitting arising from the single-
ion anisotropy for Fe(III) centres.
equiv.) in the presence of complex 1 (1.4 × 10−4 M; 0.07 mol%)
in CH3CN, without any promoter, afforded cyclohexanol and
cyclohexanone at a very low yield of 0.6% (based on cyclo-
Table 2 Pair correlation coefficients for variables
Fe⋯Fe
Fe–O–Fe
Fe–O
P
J
The structural data for a number of hydroxido-bridged
Fe(III) complexes are compiled in Table 1 indicating that a
magnetostructural correlation J vs. Fe–OH–Fe angle might
exist. More detailed insight brought statistical multivariate
methods.20 The cluster analysis classifies the selected com-
Fe⋯Fe
0.9864
0.2124
0.0560
0.2339
0.1002
0.8454
−0.5663
−0.6105
0.1712
Fe–O–Fe
0.9864
0.2124
0.2339
Fe–O
P
J
0.0560
0.1002
−0.6105
0.8454
0.1712
0.1489
−0.5663
0.1489
Table 1 Structural data for hydroxido-bridged Fe(III) complexes
No Refcode
Fe⋯Fe/Å Fe–O–Fe/° Fe–O1/Å Fe–O2/Å Fe–Oav/Å P/Å
J/cm−1 Bridges
Complex
Ref.
1
2
3
4
5
6
7
8
9
AVOHID
AVOHAV
ZAQFAY
EQOFAS
XPYAFE
CAWNIX
CAHJEA
PICAFE
HXAPFE
3.066
3.128
3.085
3.072
3.078
3.162
3.155
3.089
3.118
102.36
100.4
100.7
100.0
103.2
107.0
102.8
103.6
105.3
106.3
140.2
123.0
2.004
2.085
2.002
2.008
1.989
1.98
2.055
1.993
1.986
1.980
2.029
1.960
1.930
1.985
2.005
2.002
1.938
1.95
1.980
1.937
1.937
1.941
1.989
1.952
1.967
2.035
2.004
2.005
1.964
1.965
2.017
1.965
1.962
1.985
2.009
1.977
1.930 −4.2 μ-(OH)2
1.985 −8.2 μ-(OH)2
2.002 −11.0 μ-(OH)2
2.002 −14.4 μ-(OH)2
1.938 −14.6 μ-(OH)2
[Fe2(OH)2(L1)2]
25
26
26
27
[Fe2(OH)2(L2)2]
[Fe2(OH)2(HL3)2](ClO4)2
[Fe2(OH)2(L4)2(H2O)4]·2H2O
[Fe2(OH)2(H2O)2(OH-L5)2]·4H2O 28
1.95 −14.8 μ-(OH) (OPh) [Fe2(OH)(L6)Cl2]·C4H8
29
30
28
31
1.980 −20.8 μ-(OH)2
1.937 −22.8 μ-(OH)2
1.937 −23.4 μ-(OH)2
[Fe2(OH)2(L7)2]·2H2O·2Py
[Fe2(OH)2(H2O)2(L8)2]
[Fe2(OH)2(H2O)2(L9)2]·2H2O
10 DEWPIE10 3.137
1.941 −24.0 μ-(OH) (OPh) [Fe2(OH)(H2O)2(L10)]·4H2O
32
11 1447617a
12 COCJIN
3.777
3.438
1.989 −24.2 μ-(OH)
1
This work
33
1.952 −34.0 μ-(OH) (OAc)2 [Fe2(OH)(OAc)2(L11)2]ClO4
a CCDC number, L1 = 2,2′-((methylimino)bis(methylene))bis(6-t-butyl-4-methylphenolato), L2 = 2,2′-((((pyridin-2-yl)methyl)imino)bis(methylene))
bis(4,6-dimethylphenolato), L3 = macrocyclic tetraaminodiphenol, H2L4 = 3,4-dihydroxycyclobutene-1,2-dione (squaric acid), L5 = 4-hydroxo-2,6-
pyridinedicarboxylato, L6 = trisalicylidenetriethylenetetramine, L7 = N,N′-ethylenebis(salicylamine), L8 = 2,6-pyridinedicarboxylato, L9 = 4-di-
methylamino-2,6-pyridinedicarboxylato, H5L10 = N,N′-(2-hydroxy-5-methyl-1,3-xylylene)bis[N-(carboxymethyl) glycine], L11 = hydrotris(1-pyrazolyl)
~
~
ˆ
borate. Convention: HAB ¼ ꢁJðSA ꢀ SBÞ. P is the shortest superexchange pathway defined as the shortest distance between the metal and the brid-
ging ligand(s).
Dalton Trans.
This journal is © The Royal Society of Chemistry 2018

View Article Online
Dalton Transactions
Paper
Scheme 1 Oxidation of cyclohexane with hydrogen peroxide catalysed
by 1.
hexane) after 2 h reaction time at 50 °C, with an A/K (cyclohex-
anol/cyclohexanone) mole ratio of 2.9 after reduction with
PPh3.23 No further yield increase with the time was observed.
The reaction rate was 1.9 × 10−7 M s−1
.
It is known that promoting agents (such as inorganic and
carboxylic acids) are able to enhance the catalytic activity of
coordination compounds in alkane oxidation.1a Considering
the reported data for the catalytic activities of copper and iron
complexes, we selected nitric, acetic, trifluoroacetic (TFA) and
pyrazinecarboxylic (PCA) acids to be tested as promoters for
the catalytic systems based on complex 1. The promoting
agents were used at a low concentration of 2.8 × 10−3 M. Acetic
and trifluoroacetic acids revealed the yields of the products
(1.1 and 0.2%, respectively, after 2 h reaction time, after
addition of PPh3) of the same level as that without using any
promoter. The reaction rates were determined as 3.2 × 10−7
and 6.6 × 10−8 M s−1 and the chromatograms recorded before
the addition of PPh3 allowed the detection24 of the peak of
cyclohexyl hydroperoxide (Fig. S6†) as the main reaction
product.
In contrast to acetic and trifluoroacetic acids, nitric and pyr-
azinecarboxylic ones led to a much higher activity with the
maximum achieved product yield of 11.9 and 16.7%, respect-
ively. The initial reaction rates W0 were estimated as 3.9 × 10−6
and 4.3 × 10−5 M s−1 for HNO3 and PCA promoters, respect-
ively. The accumulations of reaction products, measured after
the reduction of the samples with PPh3, revealed a pronounced
prevalence of cyclohexanol (Fig. 7) with cyclohexanol/cyclo-
hexanol (A/K) ratios up to 63 and 41, respectively. The
chromatograms recorded before the addition of PPh3 revealed
Fig. 8 Fragment of the chromatogram of the reaction (by-)products in
the oxidation of cyclohexane with H2O2 catalysed by complex 1 in the
presence of HNO3 in acetonitrile at room temperature and 24 h reaction
time.
the presence of cyclohexyl hydroperoxide as the main reaction
product (Fig. S6†), as expected for the oxidation of cyclohexane
with hydroxyl radicals as the main attacking species.1a,23
The assumption about a free radical pathway is in accord
with the overoxidation pattern (Fig. 8) recorded after 24 h (for
HNO3 promoter). The products of hydroxyl radical attack at
cyclohexanol, cyclohexanone and cyclohexyl hydroperoxide,
such as cyclohexanodiols, hydroxycyclohexanones and other
products, were observed (Fig. 8). This pattern is common34 for
catalytic systems oxidizing with hydroxyl radicals under con-
ditions close to those used in the present case.
Polynuclear complexes of iron and copper are known to be
efficient catalysts for the oxidation of alkanes with H2O2,
mostly showing a free radical reaction mechanism with the for-
mation of alkyl hydroperoxides as the main products.1a,35 In
such a case the maximum yield of primary products (alcohol,
ketone and hydroperoxide) is limited by overoxidation, being
at the ca. 50% level depending on the substrate.36 For the
cyclohexane substrate the best yields are shown by homo- and
heterometallic polynuclear complexes of iron and copper,
reaching 44%.1a,6a,b,37 In the present case the maximum yield
of 17% suggests that the catalytic system 1/PCA/H2O2 has a
moderate activity.
Catalytic oxidation of cis-1,2-dimethylcyclohexane with
m-CPBA
Oxidation of substituted cyclohexanes (Scheme 2) provides a
useful model in the tests for the regio- and/or stereoselectivity
properties of C–H activating catalytic systems.1a,38 Since the
Fig. 7 Accumulations of oxygenates with the time in cyclohexane
(0.2 M) oxidation with H2O2 (1 M, 50% aqueous), catalysed by complex 1
(1.4 × 10−4 M) in the presence of HNO3 (2.8 × 10−3 M) and PCA (2.8 ×
10−3 M), in acetonitrile at 50 °C.
Scheme 2 Oxidation of cis-1,2-dimethylcyclohexane (cis-1,2-DMCH;
X = –OH or vO) with m-CPBA catalysed by 1.
This journal is © The Royal Society of Chemistry 2018
Dalton Trans.

View Article Online
Paper
Dalton Transactions
hydrogen peroxide revealed a Fenton-like activity in the cyclo-
hexane oxidation catalysed by complex 1 (see above), no stereo-
selectivity in the oxidation of branched substrates is expected
in this case.38c Hence, we used a different oxidant, m-chloro-
peroxybenzoic acid (m-CPBA),39 as it may afford selective oxi-
dation of alkanes when cobalt,40 iron,40e,41 nickel42 or ruthe-
nium43 catalysts are used. In spite of the pronounced stereo-
selectivity and in some cases even showing asymmetric
hydroxylation,44 the typical yield of products shown by these
systems is below 15% based on the substrate.1a
Accumulations of the main reaction products, tertiary cis-
and trans-alcohols (here and further “cis” and “trans” denote
the stereoconfiguration of the methyl groups), in the course of
oxidation of cis-1,2-dimethylcyclohexane (cis-1,2-DMCH; 0.1 M)
with m-CPBA (0.027 M), catalysed by 1 (1.4 × 10−4 M) in the
absence of any promoter are shown in Fig. 9, top. The initial
reaction rates for cis- and trans-alcohols were determined to be
nearly equal with W0 = 7 × 10−7 M s−1. The tertiary cis-alcohol,
however, exhibits slightly higher final concentration, resulting
in a maximum cis/trans ratio of 1.3 (Fig. 9, top, inset). Such a
low cis/trans ratio corresponds to 14% of retention (see the
ESI† for details of the retention index estimate) of the stereo-
configuration of the cis-1,2-DMCH substrate. Also, rather low
3° : 2° normalized bond selectivities ranging from 4.6 : 1 to
6.7 : 1 are observed in the oxidation of cis-1,2-DMCH. The
overall yields of the products did not exceed 2% based on the
substrate and 7.4% based on the oxidant. Hence, complex 1,
in the absence of promoters, exhibits only a weak activity with
the m-CPBA oxidant.
Chlorobenzene is a common by-product in the reactions
where m-CPBA is used as an oxidant.45 The formation of chlor-
obenzene accounts for the presence of a m-chlorobenzoyl
radical, which undergoes decarboxylation.45 The following
hydrogen abstraction affords chlorobenzene. In the case of 1,
chlorobenzene was observed in quantities up to 4.6% of yield
based on m-CPBA (Fig. 9, middle).
The same test performed employing nitric acid as the pro-
moter with [HNO3]0 = 5.5 × 10−3 M (other conditions are as in
Fig. 9 caption) resulted in initial reaction rates W0 of 3.9 × 10−7
Fig. 9 Top and bottom: accumulations of the main products (tertiary
cis- and trans-alcohols) in the course of cis-1,2-DMCH oxidation (0.1 M)
and 1.6 × 10−7 M s−1 for accumulations of tertiary cis- and with m-CPBA (0.027 M) catalysed by complex 1 (1.4 × 10−4 M) in the
absence (top) or presence (bottom) of HNO3 (5.5 × 10−3 M) in aceto-
nitrile at 50 °C. The insets show the dependences of cis/trans ratios
(ratios of tertiary cis- and trans-alcohols) with the time. Middle: accumu-
lations of chlorobenzene in the absence or presence of the HNO3
trans-alcohols, respectively (Fig. 9, bottom). In spite of the
lower reaction rates compared to those for the tests without
the HNO3 promoter (Fig. 9, top), the higher maximum cis/trans
ratio of 5.3 was achieved (70% of retention of stereoconfigura-
tion, see the ESI†). The amount of chlorobenzene was lower
(Fig. 9, middle) in the presence of nitric acid (up to 1.9% of
yield based on m-CPBA, compared to 4.6% for the non-HNO3
promoter.
test). The 3° : 2° normalized selectivity after 5 h reaction time presence of high-valent metal-oxo species.46 It was found that
was 7.6 : 1, being higher than that for the test performed in the the incorporation of 18O into the tertiary cis-alcohol is at 2%
absence of nitric acid, 4.6 : 1. With three times higher concen- level (Fig. 10, inset). However, the intensity of the 130 m/z
tration of the catalyst, 4.1 × 10−4 M, and in the presence of signal (molecular ion of the 18O labeled species) in the mass
HNO3, the 3° : 2° normalized selectivity of 4.3 : 1 and the cis/ spectra of trans-alcohols was not sufficiently strong for a
trans ratio up to 2.0 were estimated (Fig. S7†).
reliable determination of the 18O incorporation level (Fig. S8†).
We performed the test with the HNO3 promoter and [1]0 = Although the incorporation of 18O into the alkane hydroxy-
1.4 × 10−4 M in the presence of 1 M of H218O, as is known that lation products (alcohols) has been undoubtedly detected, its
the incorporation of oxygen from water may account for the level (up to 2%) is too weak for a definite conclusion about the
Dalton Trans.
This journal is © The Royal Society of Chemistry 2018

View Article Online
Dalton Transactions
Paper
mate of the incorporation of 18O into the secondary alcohols
(X, XI and XIII, Fig. S10†) was not successful due to their
strong fragmentation upon electron impact and the absence of
molecular ion peaks in the respective mass spectra (although
the 43 → 45 m/z shift can be observed, Fig. S11†). 2,3- and 3,4-
dimethylcyclohexanones (IX and XII) were found to contain
more than half of 18O. This result is not surprising as ketones
can exchange oxygen with water via a non-catalytic pathway.34b,
c,49
The strong peak of the 2,7-octanedione (XV) by-product
(Fig. S10†), resulting from the C–C cleavage of the C6 ring of
the cis-1,2-DMCH substrate, was found to be labeled in the
4 : 34 : 62 ratio, corresponding to 16O16O : 16O18O : 18O18O com-
positions, respectively. The peak of 2-octanone (VI) contained
48% of 18O. The comparison of mass spectra made in the
absence and in the presence of H218O allowed the detection of
pronounced +2 m/z shifts in the peaks of the mass spectra of I,
III, VIII, XIV and XVI (Fig. S11†). The mass spectrum of the
peak at 6.99 min (IV) exhibits a 126 m/z signal, appearing as a
ketone derivative of cis-1,2-DMCH. However, the absence of +2
m/z shifts evidences against such an assignment. All the mass
spectra of the (by-)products I–XVI are depicted in Fig. S11.†
Fig. 10 Accumulations of the main products (tertiary cis- and trans-
alcohols) in the course of cis-1,2-DMCH oxidation with m-CPBA cata-
lysed by complex 1, in the presence of the HNO3 promoter and in the
presence (symbols and solid lines) or absence (dashed lines, for com-
parative purpose) of H218O (1 M). The inset shows the incorporation of
18O into the tertiary cis-alcohol.
nature of the C–H attacking species. Moreover, the presence of
1 M H2O caused kinetic changes in comparison with the
water-free test (Fig. 10): the maximum reached cis/trans value
is 2.0 (in contrast to 5.3 in the absence of water). The changes
are not likely to be from the 16O/18O kinetic isotope effect
(KIE) since only a weak incorporation of 18O is observed and
the presence of 18O causes the increase of the trans-product
amount (Fig. 10). Moreover, the 16O/18O KIE has low typical
values of 1.005–1.02,47 only reaching higher values (1.3) in
exceptional cases.48 Similar 18O incorporation levels were
observed for homo- and heterometallic complexes of cobalt
and cadmium with an aminoalcohol Schiff base ligand, where
the oxidation of cyclohexane with m-CPBA afforded up to 4%
of labeled alcohol.40a In this case the yield of products in the
oxidation of cis-1,2-DMCH was up to 14% based on the
substrate.
Catalytic amidation of cyclohexane with benzamide in the
presence of DTBP
Amide –NH–C(vO)– is a widespread fragment in natural and
synthetic compounds.50 Among the methods for the formation
of amides, those via amide functionalization of aliphatic
carbon to form R3C–NH–C(vO)– are still rare.12c,50a
Coordination compound 1 contains ligands having aliphatic
tert-butyl groups, thus making complex 1 soluble in non-polar
solvents, such as benzene. Taking this opportunity we tested
the catalytic activity of 1 in the reaction of radical amidation of
cyclohexane with benzamide (Scheme 3).
Oxidative amidation of cyclohexane (2.9 M) with benzamide
t
(0.3 M) in the presence of di-tert-butyl peroxide, BuOOtBu (0.6
M) and complex 1 (6 × 10−3 M) affords N-cyclohexyl benzamide
(peak at 12.95 min, Fig. S12†) as the main reaction product.
The conversion of benzamide was lower than 5% after 24 h, at
90 °C, disclosing a low catalytic activity of 1 under the con-
The chromatograms of the samples with H218O taken at 1
and 5 h reaction time were recorded twice, before and after the
addition of PPh3. While at 5 h time no significant differences
were observed (Fig. S9†), the chromatogram at 1 h time (before
the addition of PPh3) revealed lowered amounts of tertiary
trans-alcohol and secondary alcohols. The strong peak of chlor-
obenzene, observed before PPh3 addition at 1 h (Fig. S9†), may
account for the decomposition of non-reacted m-CPBA in the
hot (200 °C) GC injector. No peaks which could be assigned to
tertiary alkyl hydroperoxides have been detected, in contrast to
the observation of cyclohexyl hydroperoxide in the case of oxi-
dation of cyclohexane with the H2O2 oxidant (Fig. S6†).
However, the absence of such peaks cannot serve as sufficient
evidence for the absence of tertiary alkyl hydroperoxides. The
18O enrichment levels in the sample injected prior and after
the addition of PPh3, at 5 h time, were nearly equal.
ditions studied. Such
a conversion level indicates that
although complex 1 is well soluble in the reaction medium, it
acts as a catalyst to a limited extent. A possible explanation
could be the too high steric hindrance of the tBu groups of the
ligands. As far as we are aware, no polynuclear complexes have
been reported yet to catalyse such reactions, while from mono-
nuclear ones the most active are those of copper with substi-
The mass spectra of the by-products, secondary alcohols as
well as the respective ketones, were studied for the purpose of
Scheme 3 Oxidative amidation of cyclohexane with benzamide cata-
determination of their 16O/18O composition. The reliable esti- lysed by 1.
This journal is © The Royal Society of Chemistry 2018
Dalton Trans.

View Article Online
Paper
Dalton Transactions
tuted bipyridine ligands, showing almost quantitative benzamide in cyclohexane/benzene medium in the presence of
yields.12c,51
di-tert-butylperoxide. N-Cyclohexyl benzamide was formed as
The main reaction mechanism is assumed to proceed via the main reaction product, presumably via a radical attack of
the formation of free alkyl radicals, as suggested earlier for cyclohexane, as evidenced by the detection of chlorocyclo-
these types of reactions and copper catalysts.12c,52 This hexane, phenylcyclohexane and other by-products.
assumption is in accord with the by-products, typical for the
reactions of cyclohexyl radicals with chloro radicals24c and
with benzene radicals,53 according to the detection of chloro-
cyclohexane and phenylcyclohexane, respectively.
Conflicts of interest
The formation of toluene and N-methylbenzamide accounts There are no conflicts to declare.
for methyl radicals which are known to appear during the
t
thermal decomposition of BuOOtBu.12c A small peak of cyclo-
hex-2-en-1-yl benzoate (at 11.2 min, Fig. S12†) also suggests a
free-radical process, as such a product is known to appear
Acknowledgements
during the radical oxidation of cyclohexane with organic per- This work was supported by the Foundation for Science and
oxides, including tBuOOtBu.52 The weak peak of pivalic acid (at Technology (FCT), Portugal (projects PTDC/QEQ-QIN/3967/
4.37 min) should come from complex 1 where it serves as a 2014 and UID/QUI/00100/2013; fellowship SFRH/BPD/99533/
ligand.
2014 (D. S. N.)), and Slovak grant agencies (APVV 16-0039,
VEGA 1/0919/17 and VEGA 1/0013/18). The authors acknowl-
edge the IST Node of the Portuguese Network of Mass
Spectrometry for the ESI-MS measurements.
Conclusions
We have described the synthesis and crystal structures of the
new heterometallic compounds [Cu4Fe2(OH)(Piv)4(tBuDea)4Cl]·
0.5CH3CN (1) and [Cu4Mn2(OH)(Piv)4(tBuDea)4Cl] (2). These
complexes were prepared via a facile process, a one-pot open-
air reaction using zerovalent copper and iron or manganese
chloride as starting metal sources. Under the applied reaction
conditions, N-tert-butyldiethanolamine and pivalic acid,
having a bulky aliphatic substituent, support the formation of
complexes with a high nuclear discrete structures, preventing
the fusing into a coordination polymer. The obtained hexanuc-
lear compounds show an extremely rare type of molecular
core, {M6(µ-X)7(µ3-X)2}, where two M3(µ-X)2(µ3-X) fragments are
connected by three non-metal bridges, and, as far as we are
aware, 1 and 2 represent the first examples of heterometallic
complexes bearing it. The analysis via the Cambridge
Structural Database of hexanuclear structures with MSTs
based on the fragment M3(µ-X)2(µ3-X) allows one to classify
their main features.
For 1, the magnetic and catalytic studies were performed.
The magnetic investigation shows an antiferromagnetic coup-
ling between the paramagnetic centres. Catalytic investigations
disclosed that the activity of 1 in the oxidation of cyclohexane
with H2O2 can be efficiently promoted with PCA (pyrazinecar-
boxylic acid), while the other tested acid promoters were less
active or even inactive.
From the direct detection of the cyclohexyl hydroperoxide
in the oxidation of cyclohexane with H2O2 it may be assumed
that the hydroxyl radical attacks the C–H bond. In contrast, no
hydroperoxides were detected when m-CPBA (m-chloroperben-
zoic acid) was used as an oxidant. With the cis-1,2-dimethyl-
References
1 (a) D. S. Nesterov, O. V. Nesterova and A. J. L. Pombeiro,
Coord. Chem. Rev., 2018, 355, 199–222; (b) H. Li, Z. J. Yao,
D. Liu and G. X. Jin, Coord. Chem. Rev., 2015, 293, 139–157;
(c) D. Gatteschi, M. Fittipaldi, C. Sangregorio and L. Sorace,
Angew. Chem., Int. Ed., 2012, 51, 4792–4800;
(d) G. B. Shul’pin, Catalysts, 2016, 6, 50.
2 D. S. Nesterov, O. V. Nesterova, V. N. Kokozay and
A. J. L. Pombeiro, Eur. J. Inorg. Chem., 2014, 4496–4517.
3 (a) C. Papatriantafyllopoulou, E. E. Moushi, G. Christou
and A. J. Tasiopoulos, Chem. Soc. Rev., 2016, 45, 1597–1628;
(b) K. Liu, W. Shi and P. Cheng, Coord. Chem. Rev., 2015,
289, 74–122; (c) E. J. L. McInnes, G. A. Timco,
G. F. S. Whitehead and R. E. P. Winpenny, Angew. Chem.,
Int. Ed., 2015, 54, 14244–14269.
4 P. Buchwalter, J. Rose and P. Braunstein, Chem. Rev., 2015,
115, 28–126.
5 J. W. Sharples and D. Collison, Coord. Chem. Rev., 2014,
260, 1–20.
6 (a) O. V. Nesterova, E. N. Chygorin, V. N. Kokozay,
V. V. Bon, I. V. Omelchenko, O. V. Shishkin, J. Titis,
R. Boca, A. J. L. Pombeiro and A. Ozarowski, Dalton Trans.,
2013, 42, 16909–16919; (b) D. S. Nesterov, E. N. Chygorin,
V. N. Kokozay, V. V. Bon, R. Boca, Y. N. Kozlov,
L. S. Shul’pina, J. Jezierska, A. Ozarowski, A. J. L. Pombeiro
and G. B. Shul’pin, Inorg. Chem., 2012, 51, 9110–9122;
(c) D. S. Nesterov, V. N. Kokozay, J. Jezierska, O. V. Pavlyuk,
R. Boca and A. J. L. Pombeiro, Inorg. Chem., 2011, 50, 4401–
4411; (d) D. S. Nesterov, V. N. Kokozay, V. V. Dyakonenko,
O. V. Shishkin, J. Jezierska, A. Ozarowski, A. M. Kirillov,
M. N. Kopylovich and A. J. L. Pombeiro, Chem. Commun.,
2006, 4605–4607.
cyclohexane substrate
a
moderate stereoselectivity was
observed with ca. 2% of incorporation of 18O from H218O. The
pronounced solubility of 1 in non-polar solvents allowed us to
perform the test for catalytic amidation of cyclohexane with
Dalton Trans.
This journal is © The Royal Society of Chemistry 2018

View Article Online
Dalton Transactions
Paper
7 (a) D. S. Nesterov, J. Jezierska, O. V. Nesterova,
A. J. L. Pombeiro and A. Ozarowski, Chem. Commun., 2014,
(c) K. J. Mitchell, K. A. Abboud and G. Christou, Inorg.
Chem., 2016, 55, 6597–6608.
50, 3431–3434; (b) D. S. Nesterov, E. C. B. A. Alegria and 23 G. B. Shul’pin, J. Mol. Catal. A: Chem., 2002, 189, 39–66.
J. Jezierska, Inorg. Chim. Acta, 2017, 460, 83–88; 24 (a) I. Gryca, K. Czerwinska, B. Machura, A. Chrobok,
(c) E. N. Chygorin, V. N. Kokozay, I. V. Omelchenko,
O. V. Shishkin, J. Titis, R. Boca and D. S. Nesterov, Dalton
Trans., 2015, 44, 10918–10922; (d) W. G. Wang, A. J. Zhou,
W. X. Zhang, M. L. Tong, X. M. Chen, M. Nakano,
C. C. Beedle and D. N. Hendrickson, J. Am. Chem. Soc.,
2007, 129, 1014–1015.
8 O. V. Nesterova, M. V. Kirillova, M. F. C. Guedes da Silva,
R. Boca and A. J. L. Pombeiro, CrystEngComm, 2014, 16,
775–783.
L. S. Shul’pina, M. L. Kuznetsov, D. S. Nesterov,
Y. N. Kozlov, A. J. L. Pombeiro, I. A. Varyan and
G. B. Shul’pin, Inorg. Chem., 2018, 57, 1824–1839;
(b) G. B. Shul’pin, D. S. Nesterov, L. S. Shul’pina and
A. J. L. Pombeiro, Inorg. Chim. Acta, 2017, 455, 666–676;
(c) O. V. Nesterova, D. S. Nesterov, A. Krogul-Sobczak,
M. F. C. Guedes da Silva and A. J. L. Pombeiro, J. Mol.
Catal. A: Chem., 2017, 426, 506–515.
25 T. Weyhermuller, R. Wagner and P. Chaudhuri,
Eur. J. Inorg. Chem., 2011, 2547–2557.
9 (a) A. M. Ako, V. Mereacre, R. Clerac, I. J. Hewitt, Y. H. Lan,
G. Buth, C. E. Anson and A. K. Powell, Inorg. Chem., 2009, 26 K. K. Nanda, S. K. Dutta, S. Baitalik, K. Venkatsubramanian
48, 6713–6723; (b) V. Mereacre, A. M. Ako, M. N. Akhtar, and K. Nag, J. Chem. Soc., Dalton Trans., 1995, 1239–1244.
A. Lindemann, C. E. Anson and A. K. Powell, Helv. Chim. 27 J. Carranza, J. Sletten, F. Lloret and M. Julve, Inorg. Chim.
Acta, 2009, 92, 2507–2524; (c) F. Klower, Y. H. Lan, Acta, 2011, 371, 13–19.
J. Nehrkorn, O. Waldmann, C. E. Anson and A. K. Powell, 28 J. A. Thich, C. C. Ou, D. Powers, B. Vasiliou,
Chem. – Eur. J., 2009, 15, 7413–7422.
10 (a) G. B. Shul’pin, Catalysts, 2016, 6, 50; (b) E. P. Talsi,
D. Mastropaolo, J. A. Potenza and H. J. Schugar, J. Am.
Chem. Soc., 1976, 98, 1425–1433.
R. V. Ottenbacher and K. P. Bryliakov, J. Organomet. Chem., 29 B. Chiari, O. Piovesana, T. Tarantelli and P. F. Zanazzi,
2015, 793, 102–107; (c) W. Liu and J. T. Groves, Acc. Chem. Inorg. Chem., 1983, 22, 2781–2784.
Res., 2015, 48, 1727–1735; (d) K. P. Bryliakov and E. P. Talsi, 30 L. Borer, L. Thalken, C. Ceccarelli, M. Glick, J. H. Zhang
Coord. Chem. Rev., 2014, 276, 73–96. and W. M. Reiff, Inorg. Chem., 1983, 22, 1719–1725.
11 (a) J. F. Hartwig and M. A. Larsen, ACS Cent. Sci., 2016, 2, 31 C. C. Ou, R. A. Lalancette, J. A. Potenza and H. J. Schugar,
281–292; (b) J. F. Hartwig, J. Am. Chem. Soc., 2016, 138, 2–
J. Am. Chem. Soc., 1978, 100, 2053–2057.
24.
32 B. P. Murch, F. C. Bradley, P. D. Boyle, V. Papaefthymiou
and L. Que, J. Am. Chem. Soc., 1987, 109, 7993–8003.
12 (a) J. P. Wan and Y. F. Jing, Beilstein J. Org. Chem., 2015, 11,
2209–2222; (b) H. T. Zeng and J. M. Huang, Org. Lett., 2015, 33 W. H. Armstrong and S. J. Lippard, J. Am. Chem. Soc., 1984,
17, 4276–4279; (c) B. L. Tran, B. J. Li, M. Driess and 106, 4632–4633.
J. F. Hartwig, J. Am. Chem. Soc., 2014, 136, 2555–2563; 34 (a) D. S. Nesterov, O. V. Nesterova, M. F. C. Guedes da Silva
(d) R. T. Gephart, D. L. Huang, M. J. B. Aguila, G. Schmidt,
A. Shahu and T. H. Warren, Angew. Chem., Int. Ed., 2012,
51, 6488–6492.
and A. J. L. Pombeiro, Catal. Sci. Technol., 2015, 5, 1801–
1812; (b) A. N. Bilyachenko, M. M. Levitsky, A. I. Yalymov,
A. A. Korlyukov, A. V. Vologzhanina, Y. N. Kozlov,
L. S. Shul’pina, D. S. Nesterov, A. J. L. Pombeiro, F. Lamaty,
X. Bantreil, A. Fetre, D. Y. Liu, J. Martinez, J. Long,
J. Larionova, Y. Guari, A. L. Trigub, Y. V. Zubavichus,
I. E. Golub, O. A. Filippov, E. S. Shubina and
G. B. Shul’pin, RSC Adv., 2016, 6, 48165–48180;
(c) M. M. Vinogradov, Y. N. Kozlov, A. N. Bilyachenko,
D. S. Nesterov, L. S. Shul’pina, Y. V. Zubavichus,
A. J. L. Pombeiro, M. M. Levitsky, A. I. Yalymov and
G. B. Shul’pin, New J. Chem., 2015, 39, 187–199;
(d) M. M. Vinogradov, Y. N. Kozlov, D. S. Nesterov,
L. S. Shul’pina, A. J. L. Pombeiro and G. B. Shul’pin, Catal.
Sci. Technol., 2014, 4, 3214–3226.
13 SAINT, Bruker AXS Inc., Madison, WI, 2004.
14 Bruker, APEX2 & SAINT, AXS Inc., Madison, WI, 2004.
15 G. M. Sheldrick, Acta Crystallogr., Sect. C: Struct. Chem.,
2015, 71, 3–8.
16 K. Nakamoto, Infrared and Raman Spectra of Inorganic and
Coordination Compounds, Part B, Applications in
Coordination, Organometallic and Bioinorganic Chemistry,
Wiley, Chichester, 5th edn, 1997.
17 V. G. Kessler, Chem. Commun., 2003, 1213–1222.
18 C. R. Groom and F. H. Allen, Angew. Chem., Int. Ed., 2014,
53, 662–671.
19 (a) W. P. Griffith, B. C. Parkin, A. J. P. White and
D. J. Williams, J. Chem. Soc., Chem. Commun., 1995, 2183– 35 G. B. Shul’pin, Dalton Trans., 2013, 42, 12794–12818.
2184; (b) M. Hashimoto, T. Iwamoto, H. Ichida and 36 G. B. Shul’pin, Y. N. Kozlov, L. S. Shul’pina, A. RKudinov
Y. Sasaki, Polyhedron, 1991, 10, 649–651.
and D. Mandelli, Inorg. Chem., 2009, 48, 10480–10482.
20 Statgraphics Centurion XV, ver. 15.1.02, StatPoint, Inc., 2006. 37 (a) A. N. Bilyachenko, M. M. Levitsky, A. I. Yalymov,
21 S. M. Gorun and S. J. Lippard, Inorg. Chem., 1991, 30,
1625–1630.
22 (a) H. Weihe and H. U. Gudel, J. Am. Chem. Soc., 1997, 119,
6539–6543; (b) R. Werner, S. Ostrovsky, K. Griesar and
W. Haase, Inorg. Chim. Acta, 2001, 326, 78–88;
A. A. Korlyukov, V. N. Khrustalev, A. V. Vologzhanina,
L. S. Shul’pina, N. S. Ikonnikov, A. E. Trigub,
P. V. Dorovatovskii, X. Bantreil, F. Lamaty, J. Long,
J. Larionova, I. E. Golub, E. S. Shubina and G. B. Shul’pin,
Angew. Chem., Int. Ed., 2016, 55, 15360–15363;
This journal is © The Royal Society of Chemistry 2018
Dalton Trans.

View Article Online
Paper
Dalton Transactions
(b) M. V. Kirillova, Y. N. Kozlov, L. S. Shul’pina,
Commun., 2006, 4016–4018; (d) T. Nagataki, K. Ishii,
Y. Tachi and S. Itoh, Dalton Trans., 2007, 1120–1128;
(e) T. Corona, F. F. Pfaff, F. Acuna-Pares, A. Draksharapu,
C. J. Whiteoak, V. Martin-Diaconescu, J. Lloret-Fillol,
W. R. Browne, K. Ray and A. Company, Chem. – Eur. J.,
2015, 21, 15029–15038.
O. Y. Lyakin, A. M. Kirillov, E. P. Talsi, A. J. L. Pombeiro
and G. B. Shul’pin, J. Catal., 2009, 268, 26–38;
(c) A. M. Kirillov, M. N. Kopylovich, M. V. Kirillova,
M. Haukka, M. F. C. Guedes da Silva and A. J. L. Pombeiro,
Angew. Chem., Int. Ed., 2005, 44, 4345–4349.
38 (a) J. Serrano-Plana, W. N. Oloo, L. Acosta-Rueda, 43 T. Kojima, K. I. Hayashi, S. Y. Iizuka, F. Tani, Y. Naruta,
K. K. Meier, B. Verdejo, E. Garcia-Espana, M. G. Basallote,
M. Kawano, Y. Ohashi, Y. Hirai, K. Ohkubold, Y. Matsuda
E. Munck, L. Que, A. Company and M. Costas, J. Am. Chem.
and S. Fukuzumi, Chem. – Eur. J., 2007, 13, 8212–8222.
Soc., 2015, 137, 15833–15842; (b) D. Font, M. Canta, 44 T. Nagataki, Y. Tachi and S. Itoh, J. Mol. Catal. A: Chem.,
M. Milan, O. Cusso, X. Ribas, R. J. M. Klein Gebbink and 2005, 225, 103–109.
M. Costas, Angew. Chem., Int. Ed., 2016, 55, 5776–5779; 45 A. Bravo, H. R. Bjorsvik, F. Fontana, F. Minisci and A. Serri,
(c) G. Olivo, O. Lanzalunga and S. Di Stefano, Adv. Synth.
Catal., 2016, 358, 843–863.
39 H. Hussain, A. Al-Harrasi, I. R. Green, I. Ahmed, G. Abbas
and N. U. Rehman, RSC Adv., 2014, 4, 12882–12917.
J. Org. Chem., 1996, 61, 9409–9416.
46 (a) K. A. Lee and W. Nam, J. Am. Chem. Soc., 1997, 119,
1916–1922; (b) W. W. Nam and J. S. Valentine, J. Am. Chem.
Soc., 1993, 115, 1772–1778.
40 (a) O. V. Nesterova, K. V. Kasyanova, V. G. Makhankova, 47 (a) S. Khan, K. R. Yang, M. Z. Ertem, V. S. Batista and
V. N. Kokozay, O. Y. Vassilyeva, B. W. Skelton,
D. S. Nesterov and A. J. L. Pombeiro, Appl. Catal., A, 2018,
560, 171–184; (b) O. V. Nesterova, M. N. Kopylovich and
D. S. Nesterov, RSC Adv., 2016, 6, 93756–93767;
(c) G. B. Shul’pin, D. A. Loginov, L. S. Shul’pina,
N. S. Ikonnikov, V. O. Idrisov, M. M. Vinogradov,
G. W. Brudvig, ACS Catal., 2015, 5, 7104–7113; (b) J. P. Roth
and C. J. Cramer, J. Am. Chem. Soc., 2008, 130, 7802–7803;
(c) L. M. Mirica and J. P. Klinman, Proc. Natl. Acad.
Sci. U. S. A., 2008, 105, 1814–1819; (d) L. M. Mirica,
K. P. McCusker, J. W. Munos, H. W. Liu and J. P. Klinman,
J. Am. Chem. Soc., 2008, 130, 8122–8123.
S. N. Osipov, Y. V. Nelyubina and P. M. Tyubaeva, 48 W. W. Y. Lam, W. L. Man, C. F. Leung, C. Y. Wong and
Molecules, 2016, 21, 1593; (d) J. Nakazawa, A. Yata, T. Hori, T. C. Lau, J. Am. Chem. Soc., 2007, 129, 13646–13652.
T. D. P. Stack, Y. Naruta and S. Hikichi, Chem. Lett., 2013, 49 R. V. Ottenbacher, D. G. Samsonenko, E. P. Talsi and
42, 1197–1199; (e) W. Nam, J. Y. Ryu, I. Kim and C. Kim, K. P. Bryliakov, ACS Catal., 2014, 4, 1599–1606.
Tetrahedron Lett., 2002, 43, 5487–5490; (f) W. Nam, I. Kim, 50 (a) C. Zhu, R. Wang and J. R. Falck, Chem. – Asian J., 2012,
Y. Kim and C. Kim, Chem. Commun., 2001, 1262–1263.
41 (a) W. Nam, M. H. Lim, S. K. Moon and C. Kim, J. Am.
7, 1502–1514; (b) J. Bariwal and E. Van der Eycken, Chem.
Soc. Rev., 2013, 42, 9283–9303.
Chem. Soc., 2000, 122, 10805–10809; (b) M. Kodera, 51 (a) Q. Michaudel, D. Thevenet and P. S. Baran, J. Am. Chem.
H. Shimakoshi and K. Kano, Chem. Commun., 1996, 1737–
1738; (c) R. Mayilmurugan, H. Stoeckli-Evans, E. Suresh
and M. Palaniandavar, Dalton Trans., 2009, 5101–5114.
Soc., 2012, 134, 2547–2550; (b) F. Teng, S. Sun, Y. Jiang,
J. T. Yu and J. Cheng, Chem. Commun., 2015, 51, 5902–
5905; (c) P. K. Chikkade, Y. Kuninobu and M. Kanai, Chem.
Sci., 2015, 6, 3195–3200.
42 (a) M. Balamurugan, R. Mayilmurugan, E. Suresh and
M. Palaniandavar, Dalton Trans., 2011, 40, 9413–9424; 52 B. L. Tran, M. Driess and J. F. Hartwig, J. Am. Chem. Soc.,
(b) M. Sankaralingam, M. Balamurugan, M. Palaniandavar, 2014, 136, 17292–17301.
P. Vadivelu and C. H. Suresh, Chem. – Eur. J., 2014, 20, 53 D. Liu, Y. X. Li, X. T. Qi, C. Liu, Y. Lan and A. W. Lei, Org.
11346–11361; (c) T. Nagataki, Y. Tachi and S. Itoh, Chem. Lett., 2015, 17, 998–1001.
Dalton Trans.
This journal is © The Royal Society of Chemistry 2018