Copper(II) Complexes with Bulky N-Substituted Diethanolamines: High-Field Electron Paramagnetic Resonance, Magnetic, and Catalytic Studies in Oxidative Cyclohexane Amidation

Article abstract of DOI:10.1021/acs.inorgchem.8b02145

The novel coordination compounds [Cu₂(H^tBuDea)₂(OAc)₂] (1) and [Cu₂(HⁿBuDea)₂Cl₂]·nH₂O (2) have been prepared through the reaction of the respective copper(II) salts with N-tert-butyldiethanolamine (H₂^tBuDea, for 1) or N-butyldiethanolamine (H₂ⁿBuDea, for 2) in methanol solution. Crystallographic analysis reveals that, in spite of the common binuclear {Cu₂(μ-O)₂} core, the supramolecular structures of the complexes are drastically different. In 1 binuclear molecules are linked together by H-bonds into 1D chains, while in 2 the neighboring pairs of binuclear molecules are H-bonded, forming tetranuclear aggregates. Variable-temperature (1.8-300 K) magnetic susceptibility measurements of 1 and 2 show a dominant antiferromagnetic behavior. Both complexes are also studied by HF-EPR spectroscopy. While the interaction between Cu(II) centers in 1 can be described by a single coupling constant J = 130.1(3) cm^-1 (using H = JS₁S₂), the crystallographically different {Cu₂(μ-O)₂} pairs in 2 are expected exchange from ferro- to antiferromagnetic behavior (with J ranging from -32 to 110 cm^-1, according to DFT calculations). Complexes 1 and 2 act as catalysts in the amidation of cyclohexane with benzamide, employing ^tBuOO^tBu as oxidant. The maximum achieved conversion of benzamide (20%, after 24 h reaction time) was observed in the 1/^tBuOO^tBu system. In the cases of ^tBuOO(O)CPh or ^tBuOOH oxidants, no significant amidation product was observed, while for ^tBuOO(O)CPh, the oxidative dehydrogenation of cyclohexane occurred, giving cyclohexene, to afford the allylic ester (cyclohex-2-en-1-yl benzoate) as the main reaction product.

Full text of DOI:10.1021/acs.inorgchem.8b02145

Article
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
Copper(II) Complexes with Bulky N‑Substituted Diethanolamines:
High-Field Electron Paramagnetic Resonance, Magnetic, and
Catalytic Studies in Oxidative Cyclohexane Amidation
Oksana V. Nesterova,† Dmytro S. Nesterov,* Julia Jezierska, Armando J. L. Pombeiro,*
,†
‡
,†
and Andrew Ozarowski§
†
Centro de Química Estrutural, Instituto Superior Tec
́
nico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal
‡
Faculty of Chemistry, University of Wroclaw, 14 Joliot-Curie Str., 50-383, Wroclaw, Poland
§
National High Magnetic Field Laboratory, Florida State University, 1800 East Paul Dirac Drive, Tallahassee, Florida 32310, United
States
*
S Supporting Information
ABSTRACT: The novel coordination compounds [Cu -
2
t
n
(
(
H BuDea) (OAc) ] (1) and [Cu (H BuDea) Cl ]·nH O
2) have been prepared through the reaction of the respective
copper(II) salts with N-tert-butyldiethanolamine (H BuDea,
for 1) or N-butyldiethanolamine (H BuDea, for 2) in
2 2 2 2 2 2
t
2
n
2
methanol solution. Crystallographic analysis reveals that, in
spite of the common binuclear {Cu (μ-O) } core, the
2
2
supramolecular structures of the complexes are drastically
different. In 1 binuclear molecules are linked together by H-
bonds into 1D chains, while in 2 the neighboring pairs of
binuclear molecules are H-bonded, forming tetranuclear
aggregates. Variable-temperature (1.8−300 K) magnetic
susceptibility measurements of 1 and 2 show a dominant
antiferromagnetic behavior. Both complexes are also studied by HF-EPR spectroscopy. While the interaction between Cu(II)
−
1
centers in 1 can be described by a single coupling constant J = 130.1(3) cm (using H = JS S ), the crystallographically
1
2
different {Cu (μ-O) } pairs in 2 are expected exchange from ferro- to antiferromagnetic behavior (with J ranging from −32 to
2
2
−
1
1
10 cm , according to DFT calculations). Complexes 1 and 2 act as catalysts in the amidation of cyclohexane with benzamide,
t
t
employing BuOO Bu as oxidant. The maximum achieved conversion of benzamide (20%, after 24 h reaction time) was
observed in the 1/ BuOO Bu system. In the cases of BuOO(O)CPh or BuOOH oxidants, no significant amidation product was
observed, while for BuOO(O)CPh, the oxidative dehydrogenation of cyclohexane occurred, giving cyclohexene, to afford the
t
t
t
t
t
allylic ester (cyclohex-2-en-1-yl benzoate) as the main reaction product.
INTRODUCTION
amine behaves as an efficient agent in stabilizing unique Cu8
■
7,8
paramagnetic clusters. It was proposed that the bulky tert-
butyl fragment promoted the formation and isolation of such a
polynuclear architecture. On the other hand, we anticipate that
these fragments may also facilitate the solubility of Cu(II)
coordination compounds in nonpolar solvents, like benzene,
toluene, or alkanes, which can be a crucial point in catalytic
studies.
Aliphatic aminoalcohols are organic compounds that are
broadly used for the construction of homo- and heterometallic
molecular complexes, which are of continuing interest in
1
modern chemistry. Such ligands show often a tripodal
coordination in a variety of metal complexes in which they
do not readily dissociate from the metal center. Moreover, in
the majority of cases, these ligands facilitate the preparation of
close-packed molecules and therefore are able to form
2
2
,3
Copper complexes are of special importance in a broad
1
,9−16
4
range of catalytic reactions,
including the radical
complexes with interesting magnetic properties, such a5s
3
functionalization of inert alkanes, where an sp C−H bond
undergoes hydrogen abstraction to produce a carbon-centered
alkyl radical. Alkyl radicals are reactive species, readily
single-molecule magnetism behavior, namely with copper.
Diethanolamines (H RDea) with an organic group (R) at the
2
17
nitrogen atom belong to a broadly explored family of aliphatic
18
19
20
1
,6
reacting with amides, amines, arylborates, carboxylic
aminoalcohol ligands. Although the coordination chemistry
of diethanolamines has a long history, the properties of such
compounds substituted with bulky neutral (aliphatic) frag-
ments are less studied, and almost unexplored for copper
complexes. Recently we have shown that N-tert-butyldiethanol-
21,22
23
acids,
or aliphatic groups, providing a powerful tool for
Received: July 29, 2018
©
XXXX American Chemical Society
A
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Article
Scheme 1. Schematic Representations of the Structures of Complexes 1 and 2, as Well as Their Syntheses
Figure 1. (Left) Crystal structure of 1, showing the atom numbering. H atoms are omitted for clarity. Color scheme: Cu, cyan; O, red; N, blue; C,
gray. (Right) Representation of the supramolecular chain in 1 with the enlarged fragment showing H-bonding interactions between bimetallic units.
The hydrogen atoms are omitted for clarity.
organic synthesis. However, the study of such catalytic
processes, being typically catalyzed by complexes of copper
or iron, is still rather limited and has been hampered by
solubility restrictions in suitable solvents. Considering the
recognized activity of aminoalcohol copper complexes in
radical oxidation of alkanes with peroxides in polar solvents
precipitation of the main product occurred in a few minutes
after the initiation of the reaction. After 40 min, the reaction
was stopped and the product was filtered off. Light-blue
microcrystals of 1 suitable for the X-ray crystallographic study
were formed in 2 days from the filtrate without any additional
procedures. For 2, precipitation was also observed immediately
after the start of the reaction, but during the reaction time (1
h) the formed powder was completely dissolved, and a clear
green-blue solution was obtained. This solution was filtered,
and green crystals of 2 suitable for X-ray crystallographic study
(see next section) were formed in 1 day. The general reactions
for the synthesis of 1 and 2 are expressed by eqs 1 and 2.
1
,9
(
acetonitrile/water), we propose N- and tert-butyldiethanol-
amines, with a relevant organic group, as suitable ligands for
radical alkane functionalization: while the bulky aliphatic
fragment provides solubility of the complex in a desired
nonpolar solvent, the diethanolamine moiety supports the
catalytically active copper species in solution. Herein we report
synthetic and structural features, magnetic, high-field electron
paramagnetic resonance (HF-EPR)/density functional theory
t
2
Cu(OAc) ·H O + 2H BuDea →
2
2
2
t
(
DFT) characterization, and catalytic properties in cyclo-
[
Cu (H BuDea) (OAc) ] + 2HOAc + 2H O
(1)
2
2
2
2
hexane amidation in a nonpolar solvent (benzene) of the novel
binuclear complexes [Cu (H BuDea) (OAc) ] (1) and [Cu -
t
2
2
2
2
n
2
CuCl ·2H O + 2H BuDea →
n
2
2
2
(
H BuDea) Cl ]·nH O (n = 0.037 or 0.5) (2) (Scheme 1).
2 2 2
n
[
Cu (H BuDea) Cl ] + 2HCl + 4H O
(2)
2
2
2
2
RESULTS AND DISCUSSION
■
Compound 2 typically crystallizes with one molecule of
water per four copper atoms, although we observed that in
some cases (structure 2c) the lattice may contain much smaller
amounts of water. Thus, the final composition of the complex
should be established by elemental or X-ray diffraction analysis
data.
Synthesis and IR Characterization. Compounds 1 and 2
are formed through the reaction of a copper(II) salt (acetate
for 1 and chloride for 2) with N-tert-butyldiethanolamine
t
n
(
H BuDea, for 1) or N-butyldiethanolamine (H BuDea, for
2
2
2
) in methanol solutions, using the molar ratio of CuX :L =
2
1
:2. Both reactions were initiated and brought to completion
The IR spectra of 1 and 2 show complex patterns in the
400−2000 cm region (Figure S1). Strong absorptions at
−
1
by heating and stirring in open air. In the case of 1,
B
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Article
564 and 1420 cm− in the spectrum of 1 can be associated
1
1
Table 1. Geometrical Parameters of H-Bonding in 1 and 2
with ν (COO) and ν (COO) vibrations of the coordinated
acetate group. The Δ value [ν (COO) − ν (COO)] of 144
cm is in accord with the bidentate function of the acetate
a
s
2
4
compd
H-bond
d(O···O), Å d(O−H), Å ∠(O−H···O), deg
a
s
a
−
1
1
O2H2···O4
2.563(2)
0.77(3)
175(4)
2
5
group. Considering these spectra as the characteristic ones,
we used them to monitor if the structures of 1 and 2 are
retained after recrystallization from nonpolar solvents. Benzene
was the solvent of primary interest (see below for the catalytic
properties). While complex 1 is sparingly soluble in benzene,
complex 2 dissolves, to some extent, even at room temper-
ature. The IR spectra of compounds obtained after
recrystallization reveal the identity of the compounds before
and after dissolution in hot benzene for both 1 and 2 (Figure
S2). Hence, we suppose that the structures of both complexes
are retained during the recrystallization from benzene. Slow
evaporation of a benzene solution of 2 afforded a crystalline
material which was subjected to single-crystal X-ray analysis.
The structure of the recrystallized species from benzene was
not solved due to poor crystal quality and probable twinning
2
2
2
a
b
c
O101H1···
2.679(6)
2.704(6)
2.673(6)
2.693(6)
0.84(2)
0.85(2)
0.85(2)
0.85(2)
172(7)
169(6)
160(6)
169(7)
b
O302
O301H3···
O102
O401H4···
O202
O201H2···
c
O402
O101H1···
2.683(4)
2.709(4)
2.678(4)
2.694(4)
0.72(4)
0.77(4)
0.82(4)
0.65(4)
172(5)
168(5)
170(4)
173(6)
b
O302
O301H3···
O102
O401H4···
O202
O201H2···
c
O402
problem (space group C was observed, instead of P2/c for the
2
un-recrystallized 2), but it was possible to see the dimeric
copper fragments with a Cu···Cu separation of 2.956 Å.
Crystal Structures. The single-crystal X-ray analysis
O101H1···
2.695(3)
2.729(3)
2.691(3)
2.707(3)
0.65(3)
0.72(3)
0.62(3)
0.73(3)
172(4)
176(3)
178(4)
171(3)
b
O302
O301H3···
t
O102
reveals that [Cu (H BuDea) (OAc) ] (1) is based on a
2
2
2
O401H4···
centrosymmetric binuclear {Cu (μ-O) } core (Figure 1, left).
2
2
O202
The complex molecules are linked together by strong
hydrogen-bonding, forming extended supramolecular chains
O201H2···
c
O402
(
Figure 1, right).
In the crystal structure of 1, the N-tert-butyldiethanol-
a
b
c
3
− x, 2 − y, −z. 2 − x, y, 1/2 − z. 1 − x, y, 1/2 − z.
aminate ligands are monodeprotonated and show tridentate
O,N,O) coordination, accounting for the formation of the
molecular-type structure and for partial compensation of the
(
binuclear molecules are H-bonded together, forming tetra-
nuclear supramolecular aggregates (Figure 2, right). Depend-
ing on the crystallization conditions, the complex 2 may
contain up to one water molecule per four copper dimers. The
crystal structure of 2 has been made in triplicate (see the
Experimental Section), showing n = 0.5 (for 2a and 2b) or
0.037 (for 2c). A similar motif with N-propyldiethanolamine
metal ion charge. The copper(II) ions in 1 have distorted
octahedral coordination environments, with an O N donor set
5
formed by oxygen and nitrogen atoms from tripodal N-tert-
butyldiethanolamine and bidentate-chelating acetate ligands.
The equatorial Cu−O distances lie in the range from
1
.9248(14) to 2.0313(16) Å, while apical Cu−O/N bond
(H PrpDea) was found earlier for the binuclear complex
2
26
lengths are 2.4960(17) and 2.7418(16) Å (Table S1). The cis
and trans O−Cu−O(N) bond angles vary from 53.03(5) to
[Cu (HPrpDea)Cl ].
2 2
Similar to the N-tert-butyldiethanolamines in 1, the N-
butyldiethanolamine ligands in 2 are monodeprotonated and
show the tripodal tridentate (O,N,O) coordination mode. The
overall crystal structure of the complex 2 consists of four
crystallographically independent molecules, where all copper-
1
22.14(5)° and from 149.22(5) to 174.14(6)°, respectively.
Such a deviation of the coordination geometry from a regular
octahedron is mainly caused by the asymmetric coordination
of the bidentate-chelating acetate ligands [Cu−O3
=
OAc
1
.9528(15) Å and Cu−O4
= 2.7418(16) Å], which are
(II) atoms are five-coordinated. The O NCl donor sets around
OAc
3
also involved in the formation of strong hydrogen bonds using
the weakly coordinated O4 atom. In the binuclear complex the
Cu···Cu distance is 2.9471(6) Å.
Cu1, Cu2, Cu3, and Cu4 atoms are formed by two oxygen
atoms and one nitrogen atom from aminoalcohol ligands and a
coordinated chlorine atom [Cu−O(N,Cl) = 1.941(4)−
2.2269(14) Å, Table S2] in the equatorial plane, and one
The strong H-bonds, OH···O, involving oxygen atoms
n
−
from asymmetric bidentate-chelating acetate and tridentate
oxygen atom from the H BuDea ligand [Cu−O
−
=
HBuDea
t
−
H BuDea ligands (Table 1), are responsible for the formation
of the overall 1D structure (Figure 1, right), where the eight-
membered {Cu O (OH) } supramolecular synthons (Figure 1,
2.277(3)−2.296(2) Å for O101; 2.288(3)−2.300(4) Å for
O201; 2.291(3)−2.304(4) Å for O301; 2.292(3)−2.320(2) Å
for O401] at the apical position. The cis and trans O−Cu−
O(N, Cl) bond angles vary from 79.02(16) to 103.65(7)° and
from 163.29(18) to 168.92(9)°, respectively (Table S3). In the
binuclear complexes, the Cu···Cu distances for 2a−2c are in
the ranges 2.9194(9)−2.9271(6) Å (for Cu1···Cu1),
2.9713(9)−2.9774(6) Å (for Cu2···Cu2), 2.9459(13)−
2.9498(5) Å (for Cu3···Cu3), and 2.9599(10)−2.9637(6) Å
(for Cu4···Cu4) (Table S2).
2
2
2
right, in the circle) play the role of chain-building fragments.
Further increase of the dimensionality of the H-bonded
polymer is not possible due to steric factors: the N-tert-butyl
groups of the aminoalcohol ligands prevent the formation of
hydrogen bonds between adjacent chains (Figure S3). The
shortest Cu···Cu separation within the supramolecular chain is
6
.10 Å.
The crystal structure of [Cu (H BuDea) Cl ]·nH O (2)
n
The binuclear molecules are joined together pairwise by a
set of strong O−H···O hydrogen bonds, involving all oxygen
atoms from both bimetallic parts (Table 1). Thereby, two
2
2
2
2
(Figure 2, left) is based, as for 1, on the same binuclear
{
Cu (μ-O) } core, but in the case of 2 the neighboring pairs of
2
2
C
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Figure 2. (Left) Crystal structure of the main structure of 2, showing the atom numbering. The uncoordinated water molecule and H atoms are
omitted for clarity. Color scheme: Cu, cyan; O, red; N, blue; C, gray. (Right) Representation of the supramolecular tetrametallic units based on
atoms Cu1/Cu3 and Cu2/Cu4, and detail showing H-bonding interactions with atom numbering.
Figure 3. Representation of the packing in 2 viewed down the b axis.
crystallographically independent tetranuclear supramolecular
aggregates are formed: one of them is based on the Cu1/Cu3
atom pair and the second one on Cu2/Cu4 (Figure 2, right).
In both cases the determinant supramolecular synthon that
accounts for such a type of structure formation is more
complicated than in the structure of 1 and consists of four
supramolecular eight-membered {Cu O (OH) } rings joined
groups of the aminoalcohol ligands, which sterically prevent H-
bonding between neighboring tetranuclear aggregates (Figure
S4). However, within the crystal structure packing, the
supramolecular tetranuclear aggregates based on Cu1/Cu3
atom pairs are H-bonded to the solvate H O molecules,
2
forming supramolecular polymeric chains (Figure 3, yellow
motifs) between which tetranuclear units based on Cu2/Cu4
pairs are located (Figure 3, blue motifs).
2
2
2
with two {Cu (μ-O) } parts, involving, in general, four
2
2 2
copper(II) and eight oxygen atoms (Figure 2, right). The
obtained tetranuclear aggregates reveal the “key-lock” basis of
the assembly, where the copper(II) atoms are located at a
vertexes of a distorted tetrahedron.
Formation of a complicated supramolecular structure with
participation of all tetranuclear units and solvate water
molecules is hampered by the presence of bulky N-butyl
Magnetic Properties of 1. The dependence of the
effective magnetic moment (μ ) on the temperature for the
eff
binuclear complex 1 (Figure 4) is indicative of antiferro-
magnetic exchange within the copper dimeric units. The
experimental data can be successfully fitted using the Bleaney−
Bowers equation, eq 3 (spin Hamiltonian H = JS S ), to get the
1
2
−
1
best fit values of g = 2.02(1), J = 130.1(3) cm , ρ = 0.0014(1),
D
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Figure 5. HF-EPR spectra of 1 recorded at 80 K with the microwave
frequencies as indicated. Blue: experimental; red: simulated using g =
x
−
1
−1
Figure 4. Dependences of the molar magnetic susceptibility (main
plot) and effective magnetic moment (inset) on the temperature for 1.
Open circles are experimental data related to one copper atom; solid
g = 2.063, g = 2.337, D = −1.327 cm , E = −0.065 cm .
y
z
Dxxdipole = −g μ /R
2
2
3
= −0.072 cm−1
−
1
B
Cu−Cu
lines are theoretical fits using g = 2.02(1), J = 130.1(3) cm , ρ =
x
−
6
3
−1
0
.0014(1), and TIP = 67(4) × 10 cm mol .
Dyydipole = g μ /2R
2
2
3
= 0.036 cm−1
= 0.046 cm−1
B
Cu−Cu
y
Dzzdipole = g μ /2R
2
2
3
Cu−Cu
(5)
z
B
and TIP = 67(4) × 10− cm3 mol , where ρ is the
paramagnetic impurity fraction and TIP is the temperature-
independent paramagnetism. The magnitude of TIP, obtained
from the fitting of experimental data, is close to that expected
6
−1
From the experimental spin Hamiltonian parameter set D =
1.327 cm , E = −0.065 cm , one can get the diagonal
elements of the zero-field-splitting tensor {D}:
−
1
−1
−
−
6
3
−1
II
(
60 × 10 cm mol ) for a Cu ion. The sign of exchange
D = −D/3 + E = 0.377 cm−
1
coupling constant J is in agreement with the relatively large
Cu−O−Cu angle of 99.3°. Antiferromagnetic interactions
are expected in systems like 1 when the CuOCu angle exceeds
xx
27
Dyy = −D/3 − E = 0.507 cm−1
Dzz = 2D/3 = −0.885 cm−1
9
7°.
2
2
−J/kT
2 2
Ng β
3
3e
Ng β 3
3kT 4
The experimental {D} tensor is the sum of the dipolar and
exchange-related contributions,
χ =
(1 − ρ) +
ρ + TIP
−
J/kT
kT 1 + 3e
(3)
dipolar
exchange
{D} = {D
} + {D
}
(6)
Solid-State EPR Spectra of 1. Relatively strong exchange
1
2+
interactions between two S = / Cu ions in 1 result in an S =
The diagonal elements of {Dexchange} can thus be calculated
2
0
ground state and a thermally accessible S = 1 excited state
ex
dipole
from Dxx = Dxx − Dxx
, etc.:
which is EPR active. A standard spin Hamiltonian for S = 1 was
therefore used in simulations of the powder spectra:
Dxxex = 0.449 cm−1, Dyyex = 0.471 cm−1,
Dzzex = −0.931 cm−1
2
z
1
3
2
x
2
̂
̂
̂
̂
̂
y
H = μ B{g}S + D S − S(S + 1) + E(S − S )
B
{
}
ex
From these diagonal elements, the scalar parameters D =
(
4)
−
1
ex
−1
−
1.39 cm and E = −0.011 cm are found from
D = (2D − D − D )/2, E = (D − D )/2
Not surprisingly, complex 1 exhibits EPR spectra similar to
those of other Cu dimers with alkoxo bridges of a similar
2
+
zz
xx
yy
xx
yy
(7)
structure. Simulation procedures led to the parameter set g =
x
−
1
−1
g = 2.063, g = 2.337, D = −1.327 cm , E = −0.065 cm . A
The superscript “ex” has been omitted because relations (7)
are valid for all tensors under discussion, that is, {D},
y
z
characteristic feature of such systems is that the g-matrix and
zero-field-splitting tensor must be assumed non-coaxial for a
successful simulation. In previous papers by some of us, a
tilting angle of 11° between the g and D directions was
exchange
dipolar
{D
}, and {D
}.
It is seen that the zero-field-splitting tensor is mainly
ex
exchange-related and is remarkably axial, meaning that D is
z
zz
xx
2
8,29
ex
found.
In the present case, the spectra shown in Figure 5
close to Dyy (or |E | ≪ |D |).
ex
ex
were simulated with a tilting angle of 10°. The sign of D could
not be determined from the powder EPR spectra of the present
antiferromagnetic complex, but it was found to be negative in
both ferro- and antiferromagnetic dimers with a similar
The theory of the exchange-related zero-field splitting has
been formulated for the well-known dimeric “paddlewheel”
2
2
copper(II) carboxylates, in which Cu(II) has the d
state, like in our complex.
ground
x −y
30−33
28,29
CuOOCu bridge arrangement.
negative also in our case.
We will assume that D is
The exchange interaction anisotropy is caused by mixing of
the excited states of a system with its ground state via spin−
orbit coupling. In such excited dimer states, one of the copper
With the Cu···Cu distance of 2.947 Å, the contribution of
the magnetic dipolar interactions to the zero-field-splitting
tensor is as follows (x is the Cu−Cu direction, z is
perpendicular to CuOOCu plane):
2
2
ions remains in its ground state d , while another Cu is
x −y
promoted to one of its excited states. Additionally, a non-zero
matrix element of the L operator must exist between that
E
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excited state and the ground state. D and E can be expressed
as
that is, {Ddipolar} + {Dexchange}, and the g direction is observed.
ex
ex
3
0−33
Interestingly, this non-coaxiality is observed in all complexes
17,18
33
with the Cu(OR) Cu bridges,
including Cu(OH) Cu.
2
2
2
2
2
λ J
2
2
λ J
2
2
λ J
2
2
x −y ,xy
1
x −y ,xz
1
x −y ,yz
Neglecting this small but visible effect, we can summarize that
the dipolar contribution to the zero-field-splitting tensor is
axial about the Cu−Cu direction, while the contribution due to
the anisotropic exchange is axial about the direction
approximately perpendicular to the CuOOCu plane. Their
sum is therefore non-axial, resulting in the experimental E
Dex = 2 ΔE
−
−
2
4 ΔE2
4 ΔE2
2
2
2
2
2
2
x −y ,xy
x −y ,xz
x −y ,yz
2
2
λ J
ΔE
2
2
λ J
2
2
Eex = 14
x −y ,xz
1
4 ΔE2
x −y ,yz
−
2
2
2
2
2
x −y ,xz
x −y ,yz
(8)
−1
parameter of −0.065 cm . While E is much smaller than D, it
is much larger than that observed in the “paddlewheel” copper
carboxylates, in which both the dipolar and exchange
contributions are axial about the Cu−Cu direction.
2
2
Symbols J
represent the exchange integrals between the
x −y ,n
ground state of one copper ion and an excited state of another.
−
1
λ is the spin−orbit coupling constant, which equals −828 cm
2
+
for a free Cu ion but in a complex may be substantially
Frozen Solution EPR Spectra of 1. Since the catalysis
occurs in solution, a question arises whether the compound
retains its dimeric structure therein. The solubility of 1 in
benzene or toluene was insufficient for preparing good frozen
solution samples. We found that 1 dissolves sufficiently in
CHCl and in CH Cl , and frozen solution spectra were taken
reduced by the covalency effects. Figure 6 shows that the
3
2
2
in a 1:1 mixture of these solvents. The solvent mixture was
used to improve the glass quality, as pure solvents tend to
freeze into a crystalline “ice”, producing bad EPR spectra. At
8
0 K, a characteristic S = 1 spectrum was seen with the D and
E parameters different from those observed in the solid at 80 K
Figure 7).
(
orbital of a Cu2+ ion (left) versus
2
2
Figure 6. Arrangement of the d
x −y
the excited orbitals d , d , and d (right top, center, and bottom,
xz
yz
xy
respectively) in a CuOOCu bridge. The z-axis is perpendicular to
2
2
2
2
CuOOCu. The interactions d −dxz and d −dyz are expected to
x −y
x −y
be of similar magnitudes.
Figure 7. HF-EPR spectra recorded with 208 GHz for the solid 1 and
its frozen solution at the temperatures indicated. The solution
spectrum at 5 K was simulated with g = g = 2.055, g = 2.273, D =
−1.187 cm , E = −0.318 cm (red trace at the bottom). Opposite
to the spectrum of the solid, no tilting of the {D} tensor versus g was
required in simulations. Labels x, y, z indicate the molecular
orientations at which the observed resonances occur. Resonances
2
2
2
2
interactions d −d and d −d are expected to be of
x −y
xz
x −y
yz
ex
ex
similar magnitudes. The formulas for D and E involve the
same quantities λ/ΔE(d ,d ) which appear in the theory of
x
y
z
−
1
−1
2
2
x −y
n
2
+
g of Cu :
8
λ
gz = 2.0023 −
ΔE
2
2
labeled O are due to oxygen adsorbed on the sample, and a simulated
2
x −y ,xy
−
1
spectrum of O (S = 1, g = 2, D = 3.571 cm , E = 0) is shown as
2
xyz
2
λ
the upper red trace.
gx(y) = 2.0023 −
ΔE
2
2
x −y ,yz(xz)
(9)
Moreover, that spectrum persisted down to 5 K, while solid
samples exhibited at 5 K only signals due to monomeric
impurities plus a signal due to molecular oxygen adsorbed on
the sample. The S = 1 state spectrum of the copper dimer was
suppressed due to strong antiferromagnetic interactions in
solid 1. It is thus clear that the sign of J has changed, or at least
the antiferromagnetic interaction was very substantially
reduced. As far as the zero-field-splitting parameters are
Since g = gy in our complex, the expressions λ/
x
2
2
2
2
ΔE(d ,d ) and λ/ΔE(d ,d ) must also be of similar
x −y yz
x −y xz
exchange
magnitudes, and so must be the two {D
} tensor
ex
components, whose difference appears in eq 8 for E .
ex
Accordingly, E is expected to be small, as observed. Theory
predicts that the g-matrix in a centrosymmetric dimer should
exchange
30−32
be coaxial with the {D
} tensor.
The experimental
−
1
results presented here show that this is not exactly the case, as
the tilting angle of 10° between the total zero-field splitting,
concerned, there is 11% reduction of D from −1.327 cm in
−1
the solid to −1.187 cm in solution. The negative sign of D in
F
DOI: 10.1021/acs.inorgchem.8b02145
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Inorganic Chemistry
Article
solution has been experimentally determined (while it had to
be assumed in the solid state). Negative and positive D
produce very different intensity patterns in spectra simulated at
Table 2. Inter-copper Distances, Cu−O−Cu Angles, and
Calculated Exchange Coupling Constants for 1, 2a, and 2c
Cu···Cu
distance, Å
Cu−O−Cu
angle, °
J
,
calc
5
K, and only negative D is consistent with the 5 K spectrum in
−1
complex copper pair
cm
Figure 7. Much more pronounced is the change in E, from
1
Cu1···Cu1
2.947
99.3
230
−
1
−
0.065 to −0.318 cm . The molecular geometry of the dimer
must therefore have changed. The CuOCu angle in the solid 1
is 99.3° and squeezing it down to ∼97° should result in a
ferromagnetic interaction. The E parameter as large as −0.3
cm cannot be due to the dipolar interactions between the
two copper ions. The drastic increase in E must therefore be
due to the anisotropic exchange interactions. According to
formulas (8), splitting of the energies of the xz and yz orbitals
2
a
c
Cu1···Cu1
Cu3···Cu3
Cu4···Cu4
Cu2···Cu2
2.921
2.945
2.963
2.971
96.3
97.1
97.8
98.0
−32
58
90
27
−
1
74
2
Cu1···Cu1
Cu3···Cu3
Cu4···Cu4
Cu2···Cu2
2.927
2.950
2.964
2.977
96.6
97.6
98.1
98.3
6
86
108
110
ex
would produce non-zero E . However, this must be rejected,
as g remains equal to g (see formulas (9)). Presumably, a
x
y
lowering of the CuOOCu bridge symmetry causes differ-
2
2
2
2
−1
entiation of the d −d and d −d interactions depicted
86−110 cm , while one is very weakly antiferromagnetic,
x −y
xz
x −y
yz
ex
−1
−1
in Figure 6. This will result in non-zero E according to
formulas (8). It is worth emphasizing that 1 recrystallized from
a CH Cl +CHCl solution exhibits in solid state again spectra
with J just 6 cm , or even ferromagnetic, with J = −32 cm
(Table 2). Moreover, these four dimers in 2 are arranged into
two different tetramers (Figure 2).
2
2
3
shown in Figure 5.
In a series of [Cu (RCOO) (H TEA) ] complexes, some
Although the interdimer interactions in these tetramers are
not expected to be strong, they are not vanishing, which may
be inferred from EPR; no characteristic copper dimer spectra
(like in 1) were observed (Figure 9). The sample also shows
28
2
2
2
2
species were relatively strongly ferromagnetic, while one was
relatively strongly antiferromagnetic. The zero-field-splitting
parameters D and E of the ferromagnetic and antiferro-
magnetic compounds were found to be not very different (and
similar to the parameters of 1). A similar effect may be
observed in the present workpresumably small structural
changes result in formation of ferromagnetic dimers in
solution, but the zero-field splitting remains similar to that
observed in the antiferromagnetic complex 1. It should be
emphasized that there is no proportionality between D and J,
as the magnitude of J is determined by the interactions
orbitals of two Cu2 ions, while
+
2
2
between the ground-state d
x −y
the zero-field splitting is associated with the exchange
interactions in excited states; see formulas (8) and Figure 6
above.
Magnetic Properties of 2. The magnetic properties of 2
Figures 8 and S5) appear to be non-interpretable. The system
Figure 9. Comparison of the EPR spectra of solid 1 and 2 recorded at
similar conditions.
(
contains four crystallographically different dimers (Figure 2),
and as the DFT calculations demonstrate (see below, Table 2),
three of them are expected to be relatively strongly
antiferromagnetic, with comparable coupling constants of
the presence of monomeric impurities, and there are too many
variables for a reliable fit. The higher temperature data (T > 20
K, Figure 8) indicate the presence of antiferromagnetic species
with J ≈ 100 cm , but the entire curve cannot be fitted either
by using the common model of a dimer plus monomeric
impurity or by assuming one of the four dimers to be very
weakly antiferromagnetic.
−
1
The DFT calculations were also performed for the four
different dimeric molecules present in complex 2. The Cu−O−
Cu angles in the dimers are close to the borderline (97−
3
4,35
9
8°)
separating ferromagnetic and antiferromagnetic
interactions in copper dinuclear systems. The calculations
predict strong antiferromagnetic coupling in 1 and in three
dimeric molecules present in 2, while weak antiferromagnetic
coupling is expected in the molecule with the smallest Cu−O−
Cu angle of 96.3−96.6° and the shortest Cu···Cu distance
(
Table 2). This, however, cannot be reconciled with the
magnetic susceptibility data.
Solid-State and Solution EPR Spectra of 2. Solid
samples exhibit, at temperatures ∼20 K and above, a
featureless broad resonance centered at the effective g value
of 2.11 (Figure 9). There is a shoulder on the low-field side of
Figure 8. Dependences of the molar magnetic susceptibility (main
plot) and effective magnetic moment (inset) on the temperature for
2c. The data are recalculated per one copper atom.
G
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Article
that broad band, which may be interpreted as a “parallel” S = 1
signal. The plot of the position of that shoulder versus the
species are no longer antiferromagnetic, similarly to the
observation in the frozen solution of 1. Interestingly, the
complicated spectrum observed in the frozen DMSO+py
solution of 2 indicates also the presence of species larger than
dimers.
microwave frequency allows the estimate of g = 2.25 and |D| =
z
−
1
0
.51 cm , but there is no better evidence of the S = 1 spectra.
Presumably, the weak interdimer and longer-range inter-
molecular exchange interactions blur the S = 1 spectra of
dimers but are not sufficient to produce S = 2 spectra of the
tetramers. The complex was dissolved in DMSO in a hope to
reduce the interdimer interactions, and frozen solution spectra
were recorded. The spectra indeed show at 5 K low-quality
Figure 11 shows an attempt to simulate that spectrum using
a dimer model (red trace). It is seen that the ΔM = 2 region
S
cannot be reproduced due to the presence of satellite signals.
Satellites are also present around a prominent transition at 5.6
T. Such signals may arise from weak interactions between two
dimers. Since we have no information on the structure of such
dimer pairs in solution, we will try to use the solid-state
structure. If we number the copper ions so that 1 and 2 are part
of one dimer and 3 and 4 belong to another one, the solid-state
structure implies that there will be two strong (and assumed
equal) interactions, 1−2 and 3−4. Interactions 1−3, 1−4, 2−3,
and 2−4 are expected to be weak and are assumed all equal. It
turns out that the magnitude of the latter ones determines the
positions of the satellite lines seen in the spectrum in Figure
resonances coming from the S = 1 state with g = 2.3, g
=
z
xy
−
1
2
.05, and D = −1.1 cm , thus similar to the parameters of 1
(
Figure 10). The ΔM = 2 resonances are clearly visible at ca.
S
11. The calculation details are given in the SI. Simulations
similar to that represented by the green lines in Figure 11 are
obtained over a broad range of the intradimer interactions 1−2
and 3−4, both ferromagnetic and weakly antiferromagnetic. J
1
2
−
1
=
J = −50 cm (ferromagnetic) was used in simulations. To
obtain correct positions of the satellites of the strong “parallel”
3
4
transition at 5.6 T, the isotropic interdimer interactions had to
−
1
be assumed equal to −0.3 cm . It appears thus possible that
pairs of dimers are present in solutions of 2, opposite to 1,
which does not exhibit intermolecular interactions in solution
or in the solid state.
Figure 10. Spectra of 2 in solid state and in frozen solutions. The
signal due to monomeric Cu species in the bottom spectrum is cut
2
+
off. The small feature at ∼7.4 T in the two upper spectra is due to
2+
traces of Mn .
Catalytic Properties. Complexes 1 and 2 were tested as
catalysts in the reaction of radical intermolecular amidation of
inert C−H bonds of alkanes. The reaction of amidation of
cyclohexane with benzamide, in benzene medium, was chosen
3
.2 T, but the entire spectrum cannot be simulated
satisfactorily. It seems that the interdimer interactions were
not eliminated in this solution. Finally, when a small amount of
pyridine was added to the DMSO solution of 2, complicated
spectra indicating a mixture of monomeric, dimeric, and
possibly tetrameric species were observed (Figure 10).
A solution of 2 in DMSO+py stored for a longer time
exhibits only monomeric species, with g = 2.06 and g = 2.27.
18
as a model (Scheme 2).
Scheme 2. Catalytic Amidation of Cyclohexane, Catalyzed
t
by 1 and 2 (R = Bu or H)
xy
z
Some features in the spectra of fresh solutions indicate the
presence of dimeric species (Figure 11). These spectra are
observed at the lowest temperaturesdown to 3 K; thus, the
Complexes 1 and 2 contain bulky tert-butyl (1) and normal
butyl (2) organic groups which facilitate their dissolution in
the catalytic medium to achieve homogeneous reaction
solutions. The reaction of 0.5 mmol of benzamide with 5
mmol of cyclohexane in the presence of 1 mmol of oxidant
t
t
(
BuOO Bu, di-tert-butylperoxide) and 12.5 μmol of catalyst (1
or 2, 2.5 mol% relative to benzamide) affords N-cyclohexyl-
benzamide (Scheme 2). The formation of this product was
t
t
observed only when BuOO Bu oxidant was used (Figure 12).
Only traces of the target product were seen in the case of
t
BuOO(O)CPh (tert-butyl peroxybenzoate), while with
BuOOH no amidation product was detected. The maximum
t
achieved conversion of benzamide (in the case of the
t
t
1
/ BuOO Bu system) was 20% after 24 h reaction time.
Figure 11. Blue: Frozen solution EPR spectrum of 2 dissolved in
DMSO + pyridine. The signals attributed to a monomeric species
Fragments of chromatograms showing the characteristic peaks
of products are depicted in Figure 12.
with g = 2.26 and g = 2.05 are designated by “m”. A magnified part
z
xy
t
t
The 1/ BuOO Bu system was found to exhibit a higher
of the spectrum is also shown. The horizontal arrow indicates the
t
t
activity than 2/ BuOO Bu (Figure 12). The latter also revealed
chlorocyclohexane as a byproduct. A possible reason for
chlorocyclohexane formation could be the competition
ΔM = 2 region. Red: Simulation assuming a dimer with g = 2.04, g
S
xy
z
−
1
−1
=
2.265, D = −1.03 cm , E = −0.093 cm . Green: Simulation using
a model of two weakly interacting dimers (see text and SI).
H
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Article
Figure 12. Fragments of the chromatograms showing the reaction products of amidation of cyclohexane with benzamide in the presence of the
copper catalyst 1 or 2. The initial parts of the chromatograms containing large peaks of benzene, cyclohexane, and α,α,α-trifluorotoluene are
omitted for clarity.
t
t
Figure 13. Proposed reaction mechanism for the catalytic cyclohexane amidation system with BuOO Bu as oxidant. Copper complexes 1 and 2 are
schematically indicated as [Cu].
between benzamide and chloride, present in 2, in the reaction
with cyclohexyl radical Cy , which is expected to be one of the
These results suggested the use of alkyl radical traps, CCl Br
3
•
36,37
and 1,1-diphenylethylene,
to confirm the presence of the
main catalytic intermediates (see below). The presence of the
methylated byproduct, N-methylbenzamide, in the chromato-
grams (Figure 11) could be explained by the attack of
benzamide by methyl radical. The latter is expected to form in
cyclohexyl carbon-centered radical. In the competitive reaction
with equimolar amounts of benzamide and CCl Br (0.5 mmol
3
each), in the presence of the catalyst 2, bromocyclohexane was
observed as the main reaction product, while the formation of
all other derivatives of cyclohexane was totally suppressed
(Figures 13 and S7). When 1,1-diphenylethylene was applied
as a radical trap, no N-cyclohexyl benzamide was detected as
t
•
18
low quantities from the Bu-O radical. Also traces of
•
phenylcyclohexane, presumably formed upon attack of Cy to
benzene, were observed in all cases.
I
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Article
Figure 14. Comparison of the reaction products formed under copper-catalyzed (complex 1) and catalyst-free conditions.
t
t
Figure 15. Proposed scheme of allylic ester formation from cyclohexane for the cases of 1/ BuOO(O)CPh and 2/ BuOO(O)CPh systems.
well (Figures 13 and S8). Instead, the corresponding trapping
products were formed (Figure S8).
A blank test was performed to clarify if the copper catalyst is
2
0
t
t
required for splitting of the BuOO Bu oxidant and for the
formation of the amidation product. Toluene was used as a
solvent, for which the appearance of bibenzyl could serve as a
marker for a free-radical process. As described above, the
process catalyzed by 1 afforded N-cyclohexyl and N-benzyl-
benzamides as products. However, in the absence of the
catalyst, bibenzyl was formed as a single product (Figure 14),
while the N-benzylbenzamide was detected only at trace level
and N-cyclohexylbenzamide was not detected at all (Figure
S14). The appearance of bibenzyl (Figure 14) can be
understood as a result of the coupling of two benzyl radicals,
known to be relatively stable due to the resonance stabilization.
The use of benzene solvent was governed by its high
t
•
inertness toward attack by the Bu-O radical, which is a
proposed C−H attacking species (see below). We attempted
to replace benzene with other high-boiling solvents, such as
toluene, α,α,α-trifluorotoluene, and chlorobenzene. The
comparative reactions were done using equimolar amounts of
cyclohexane and solvent (10 mmol each) in order to evaluate
t
•
the reactivity of each solvent toward the Bu-O radical
relatively to cyclohexane. All the catalytic systems produced N-
cyclohexylbenzamide as the main reaction product, except with
toluene, for which equal amounts of N-cyclohexyl- and N-
benzyl-benzamides were formed (Figure S10). Bibenzyl
byproduct was also detected for the case of toluene. For
α,α,α-trifluorotoluene, no products attributable to its deriva-
tives were detected, while toluene and chlorobenzene resulted
in small amounts of the cross-coupling products of the solvent
and cyclohexane (Figure S10). The levels of N-methylbenz-
amide byproduct were much higher for all three solvents in
comparison with benzene (Figure S10). It is known that
t
•
Hence, assuming that benzyl radicals originate from the Bu-O
radical attack to toluene, one can conclude that, under the
t
t
conditions of the experiment, the BuOO Bu oxidant generates
t
•
Bu-O radicals in a noncatalytic pathway (Figure 13).
From these observations, as well as from previous reports,
18
one may assume a free radical reaction mechanism with O-
t
•
centered Bu-O radical as the main C−H attacking species.
The reaction starts with thermolysis of the oxidant to produce
•
t
•
t
•
benzene reacts with CH and Bu-O radicals in drastically
the Bu-O radical (Figure 12). This species is capable of
3
3
8,39
different ways.
It was proposed that, in the present case,
benzene is relatively unreactive toward the Bu-O radical but
acts as a scavenger of the CH3 radicals, in this manner
abstracting the hydrogen atom from a C−H bond of
t
•
40
•
cyclohexane to form the alkyl radical (Cy ), which reacts
with benzamide activated by coordination to copper (Figure
13). We tried to react 1 and 2 with benzamide to obtain
compounds that could be related to the catalytic intermediates,
but these attempts were unsuccessful (see the Supporting
Information).
•
reducing the yield of the N-methylbenzamide byproduct. The
presence of toluene, as a product of benzene methylation, was
proved by GC analysis (Figure S12). Interestingly, methyl-
cyclohexane was detected at only trace levels (Figure S12).
Further, only negligible amounts of methylated products were
found in the cases of the other solvents (Figure S13).
We attempted to use acetonitrile and cyclohexane (solvent-
free system) solvents but did detect the desired product (N-
J
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Inorganic Chemistry
Article
Table 3. Crystallographic Data for 1 and 2
compound
1
2a
2b
2c
C H Cl Cu N O
4.04
chemical formula
C H Cu N O
C H Cl Cu N O
C H Cl Cu N O
16 37 2 2 2 4.50
2
0
42
2
2
8
16 37
2
2
2
4.50
16 36.08
2
2
2
formula mass
565.63
527.45
527.45
519.19
crystal system
a/Å
b/Å
c/Å
monoclinic
8.2245(10)
8.5718(10)
17.369(2)
90
monoclinic
17.9978(19)
14.449(2)
19.249(2)
90
monoclinic
17.9899(6)
14.4314(4)
19.2451(7)
90
monoclinic
17.9777(8)
14.4447(8)
19.3836(10)
90
α/°
β/°
γ/°
93.384(6)
90
113.161(6)
90
113.2170(10)
90
112.739(2)
90
unit cell volume/Å3
temperature/K
space group
1222.3(2)
150(2)
4602.2(10)
150(2)
P2/c
4591.8(3)
150(2)
P2/c
4642.3(4)
296(2)
P2/c
P2 /n
1
no. of formula units per unit cell, Z
reflns measured
reflns independent
Rint
2
8
8
8
12 475
3734
24 165
7134
0.0860
0.0472
0.1074
1.018
33 597
9409
0.0759
0.0427
0.1009
1.059
69 088
9511
0.0643
0.0312
0.0784
1.046
0.0335
0.0363
0.0897
1.030
1487157
final R values (I > 2σ(I))
1
2
final wR(F ) values (all data)
GoF on F2
CCDC no.
1590488
1590489
1487156
cyclohexylbenzamide) even at a trace level, particularly because
these solvents do not allow the use of temperatures higher than
ation process, which thus can be classified as a cooperative
44
hydrogen atom transfer.
8
0 °C, which is not sufficient for thermal splitting of
t
t
BuOO Bu.
CONCLUSIONS
■
t
t
t
Replacement of BuOO Bu by BuOOH resulted in complete
suppression of the desired product formation for both 1 and 2
catalysts. Instead, elevated amounts of cyclohexanol and
cyclohexanone were seen (Figure 12), although N-methyl-
benzamide was still detectable.
In summary, we have successfully prepared novel coordination
t
compounds, [Cu (H BuDea) (OAc) ] (1) and [Cu -
2
2
2
2
n
(H BuDea) Cl ]·nH O (2). The structures of 1 and 2 reveal
2
2
2
different supramolecular organizations: in the complex 1 the
Cu cores form a H-bonded chain, while in the case of 2 the
Cu dimers are joined into supramolecular tetrametallic units
which either form H-bonded polymeric chains involving
solvent water molecules or remain isolated. The idea of
2
t
With BuOO(O)Ph as oxidant, the catalytic pathway was
2
t
t
considerably different from that for BuOO Bu. While only
traces of N-cyclohexylbenzamide were seen to arise after 24 h
reaction time with BuOO(O)Ph oxidant, the allylic ester
t
n
t
using the bulky aliphatic substituents ( Bu and Bu) in the
aminoalcohol ligands in the synthesis was to stimulate
formation of compounds with discrete polynuclear molecular
structures surrounded by bulky moieties, in contrast to the
formation of extended polymeric structures. Moreover, the
presence of these aliphatic substituents increases the solubility
in aprotic nonpolar solvents, which is of great importance for
catalytic applications. In the present case, the utilization of this
strategy afforded the isolation of 1 and 2, showing a good
solubility in benzene, and their successful application in the
above catalytic reaction.
(
cyclohex-2-en-1-yl benzoate) was found as the main reaction
product (Figure 12). The conversion of benzamide in this case
was lower than 5%, showing that the source of benzoyl group
in the ester comes from the oxidant, BuOO(O)Ph, but not
benzamide, which remains unreacted. From this fact, and also
from the observation of cyclohexene as a reaction byproduct
t
(
1
appears close to the cyclohexane peak, not shown in Figure
2), one may suppose that oxidative dehydrogenation of
cyclohexane occurs to produce cyclohexene, which then reacts
with BuOO(O)Ph to form the final allylic ester (Figure 15) in
a copper-catalyzed process, known as the Kharasch−Sosnovsky
reaction.
dehydrogenation of alkanes, such as organometallic dehydro-
genation, enzymatic desaturation, and cooperative hydrogen
atom transfer. The catalytic systems 1/ BuOO(O)Ph and
t
The magnetic investigations of 1 disclosed an antiferro-
magnetic coupling between copper centers, which can be fitted
with the Bleaney−Bowers model. HF-EPR spectroscopy
confirmed the presence of magnetically coupled spins / in
4
1−43
There are a few general mechanisms of mild
1
2
the solid state as well as in solution. In contrast to 1, the
magnetic properties of 2 could not be interpreted with this
model, showing a significant deviation at low temperatures.
Careful analysis of the crystal structure of 2 revealed the
presence of four geometrically different dicopper units, joined
into two different tetrametallic aggregates. This must result in a
superposition of magnetism due to four individual Cu−Cu
coupled pairs, or to two different tetranuclear units. Moreover,
the Cu−O−Cu angles in these pairs are close to the borderline
separating ferromagnetic and antiferromagnetic interactions in
copper dinuclear systems. DFT calculations of the magnetic
4
4
t
t
45
2
/ BuOO(O)Ph probably operate via the last route, where
t
•
the initial hydrogen abstraction occurs via attack of Bu-O
radical and then the cyclohexyl radical reacts with coordinated
benzoate (Figure 15) to form benzoic acid (observed by GC,
Figure 12) and cyclohexene. The latter is transformed to allylic
ester by means of the Kharasch−Sosnovsky reaction, catalyzed
by 1 or 2. Since no cyclohexene was detected with BuOO Bu
or BuOOH oxidants, one may conclude that the presence of
t
t
t
t
•
•
both Bu-O and PhC(O)O is essential for the dehydrogen-
K
DOI: 10.1021/acs.inorgchem.8b02145
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
couplings, performed for the structures 2a and 2c, predicted
data was sufficiently high for successful solution and refinement of the
structure (Table 3). The X-ray experiment was repeated resulting in
the crystal structure 2b, having full completeness. However, the
quality of the data 2b was not sufficient for full refinement of the H
atoms on the OH···O bonds. Thus, the data set 2c was collected
from the other sample of 2 and using greater redundancy (69 088
reflections against 33 597 for 2b, Table 3), finally allowing us to
obtain the most precise data for the H-bonding structure (Table 1).
The difference peaks in the Fourier maps that correspond to the
−
1
that JCuCu constants may vary from antiferro- (110 cm ) to
−
1
ferromagnetic (−32 cm ) couplings.
Furthermore, complexes 1 and 2 act as catalysts for oxidative
amidation of cyclohexane with benzamide, showing a moderate
t
t
activity in the case of the 1/ BuOO Bu system (20% of
benzamide conversion). From the experiments with radical
traps and various solvents, a free radical main reaction pathway
is proposed.
−
3
water molecules O1W have intensities of 5.92, 6.66, and 0.63 e Å
We expect that the results obtained within the present
research would improve the understanding of magnetic
interactions in spin-coupled systems as well as the exploration
of alkane functionalization catalytic processes. These results
facilitate further synthesis and investigation of polynuclear
coordination compounds with aliphatic-substituted amino-
alcohols as novel magnetic and catalytic materials.
for 2a, 2b, and 2c, respectively. Considering a very low intensity in the
case of 2c and also the elemental analysis data, the occupancy of the
water molecule in 2c was refined to be 0.073(7) (this number is per
four copper atoms).
Magnetic Measurements. Magnetic susceptibility data of
powdered samples were measured with a SQUID magnetometer
(
Quantum Design MPMSXL-5) over the temperature range 1.8−300
K at a magnetic induction of 0.5 T. Corrections for the sample holders
were applied. Diamagnetic corrections for the molecules were
determined from Pascal’s constants.
50
EXPERIMENTAL SECTION
All chemicals were of reagent grade and used as received. All
experiments were carried out in air. Elemental analyses for CHNS
were provided by the Microanalytical Service of the Instituto Superior
■
HF-EPR Spectroscopy. High-frequency EPR spectra were
recorded with a home-built spectrometer at the EMR facility of
NHMFL. The instrument is a transmission-type device in which
waves are propagated in cylindrical light-pipes. The microwaves were
generated by a phase-locked oscillator (Virginia Diodes) operating at
a frequency of 13 ± 1 GHz and generating its harmonics, of which the
−
1
Tecnico. Infrared spectra (4000−400 cm ) were recorded using a
́
PerkinElmer BX-FT IR instrument with KBr pellets.
t
Synthesis of [Cu (H BuDea) (OAc) ] (1). Cu(OAc) ·H O (0.5
2
2
2
2
2
4th, 8th, 12th, 16th, 24th, and 32nd were available. A superconducting
g, 2.5 mmol) and N-tert-butyldiethanolamine (0.81 g, 5 mmol) were
magnet (Oxford Instruments) capable of reaching a field of 17 T was
employed.
dissolved in CH OH (20 mL), forming a blue-green solution which
3
was magnetically stirred at 50−60 °C (40 min). Precipitation
occurred immediately after the start of the reaction. The resulting
mixture was filtered off, and the filtrate was kept at room temperature.
Light-blue crystals suitable for X-ray crystallographic study were
formed from the filtrate in 2 days. Yield: 0.6 g (85%). Anal. Calcd for
C H Cu N O (M = 565.63): C, 42.47; N, 4.95; H, 7.50. Found: C,
Thermogravimetric Measurements. A PerkinElmer STA-6000
model thermogravimetric analyzer was used for determination of the
thermal stability of complexes 1 and 2 (Figure S15). Samples
weighing ca. 10 mg were heated from 30 to 1000 °C at a heating rate
−
1
of 1−10 °C min under N atmosphere.
2
20
42
2
2
8
Catalytic Reactions. The reactions were typically carried out
4
2.6; N, 4.8; H, 7.8.
Synthesis of [Cu (H BuDea) Cl ]·nH O (2). CuCl ·2H O (0.43
n
under N atmosphere in a thermostated Schlenk tube under vigorous
2
2
2
2
2
2
2
stirring. First the catalyst (12.5 μmol, 7 and 6.5 mg for 1 and 2,
respectively) and benzamide (0.5 mmol, 60.6 mg) were introduced
into the tube in the solid form. Next benzene (1 mL) and cyclohexane
g, 2.5 mmol) and N-butyldiethanolamine (0.83 g, 5 mmol) were
dissolved in CH OH (20 mL), forming a blue-green solution which
3
was magnetically stirred at 50−60 °C (120 min). Precipitation
occurred immediately after the start of the reaction, but the
precipitate was completely dissolved after 1 h of reaction. The
resulting solution was filtered, and green crystals suitable for X-ray
crystallographic study were formed in 1 day. Yield: 0.25 g (39%).
(
5 mmol, 0.54 mL) were added in that order. The oxidant (1 mmol)
t
t
t
t
BuOO Bu, BuOOH (70% aq), or BuOO(O)CPh (184, 138, and
190 μL, respectively) was then added at room temperature, and the
mixture was immediately frozen with liquid nitrogen. The gas
Anal. Calcd for 2a and 2b (n = 0.5): C H Cl Cu N O (M =
atmosphere was pumped and filled with N a few times in order to
2
1
6
37
2
2
2
4.50
5
7
5
7
27.45): C, 36.43; N, 5.31; H, 7.09%. Found: C, 36.5; N, 5.2; H,
remove air. The frozen mixture was left to warm up under vacuum (to
degasify) until becoming a liquid, and the above procedure was
.2%. Anal. Calcd for 2c (n = 0.037): C H Cl Cu N O (M =
1
6
36.08
2
2
2
4.04
19.19): C, 37.01; N, 5.40; H, 7.02%. Found: C, 37.1; N, 5.4; H,
.0%.
Crystallography. The X-ray diffraction data for 1 and 2 were
repeated. Finally the Schlenk tube was filled with N , and the reaction
2
mixture was heated at 90 °C for 24 h with a possibility of gas escape
to compensate excessive pressure. The reaction mixtures were cooled
collected using a Bruker AXS KAPPA APEX II diffractometer with
graphite-monochromated Mo Kα radiation (Table 3). Data were
collected using omega scans of 0.5° per frame, and a full sphere of
data was obtained. Cell parameters were retrieved using Bruker
SMART software and refined using Bruker SAINT on all the observed
reflections. Absorption corrections were applied using SADABS.
The structures were solved by direct methods and refined against F
to room temperature, 10 mL of CH CN and 100 μL of α,α,α-
3
trifluorotoluene or 2,2,4-trimethylpentane (GC standard) were added,
and the resulting mixture was directly analyzed by GC/GC-MS
techniques.
Gas Chromatography. A PerkinElmer Clarus 600 gas chromato-
graph, equipped with two nonpolar capillary columns (SGE BPX5; 30
m × 0.22 mm × 25 μm), one having an EI-MS (electron impact)
detector and the other one with a FID detector, was used for analyses
of the reaction mixtures. The following GC conditions were applied:
50 °C (3 min), 50−150 °C (30° per minute), 150−300 °C (14° per
minute), 300 °C (4.95 min), 20 min total run time; 200 °C injector
temperature. Helium was used as the carrier gas (1 mL per minute
constant flow). All EI mass spectra were recorded with 70 eV energy.
The identification of peaks on the chromatograms was made on the
basis of comparison of respective EI mass spectra with those from the
NIST database v. 2.2 (PerkinElmer TurboMass v. 5.4.2.1617 software
was used).
46
2
47
using the programs SHELX-2014 (for 1) or SHELX-2016/6 (for 2).
The hydrogen atoms H2 (in 1) and H1−H4 (in 2) were located in
a difference Fourier map and refined with O−H distance restrained to
0
.85 Å (for 2a) or refined freely (in all other cases). The positions of
the H1W and H2W hydrogen atoms of the uncoordinated water
molecules in all structures of 2 were calculated on the basis of
geometry and force-field considerations using the CALC-OH
program within the WinGX package and not refined. All the
other hydrogen atoms in all the structures were placed at calculated
positions and refined using the model with Uiso = nUeq (n = 1.5 for H
atoms of methyl group and water H atoms, and n = 1.2 for methylenic
H atoms).
48
49
5
1−53
DFT Calculations. “Broken symmetry”
Density Functional
54,55
Theory calculations were performed by using the software ORCA
The first X-ray data set for compound 2 (2a) was occasionally
collected with low completeness of 0.778, although the quality of the
to get insight into the mechanism of the exchange interactions. In this
method, a self-consistent-field (SCF) calculation is first performed for
L
DOI: 10.1021/acs.inorgchem.8b02145
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
the S = 1 spin state of a dinuclear copper complex. Next, a “broken
symmetry” state is set up, with the unpaired electron being spin-up on
one copper ion and spin-down on the other, and another SCF
calculation is performed. The energies of the high-spin and broken
symmetry states are finally used to estimate the exchange integral
3967/2014 and UID/QUI/00100/2013; fellowships SFRH/
BPD/63710/2009 (O.V.N.) and SFRH/BPD/99533/2014
(
D.S.N.)). The high-field EPR spectra were recorded at the
NHMFL, which is funded by the NSF through the
Cooperative Agreement No. DMR-1157490 and the State of
Florida. J.J. is grateful to the Ministry of Science and Higher
Education of the Polish Republic for financial support in her
statutory activities of the Faculty of Chemistry of Wroclaw
University and for the purchase of the SQUID magnetometer.
̂
2
̂
̂
value, J (for Hamiltonian H = J S ·S ), based on the equation J =
1 2
2
2
(EHS − E )/(⟨S ⟩ − ⟨S ⟩ ), where E and EBS are the energies
of the high-spin (HS) and the broken-symmetry (BS) states and ⟨S⟩
BS
HS
BS
HS
2
are the expectation values of the spin-squared operator in the HS and
BS states. The equation is adapted to the +JS ·S convention used in
1
2
this paper. The original equation used in the ORCA program is J =
2
2
−
(EHS − E )/(⟨S ⟩ − ⟨S ⟩ ), since ORCA uses the Hamiltonian
BS
HS
BS
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−
2JS ·S . Ahlrichs-type basis set TZVPP for the metal and all
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1
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(
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″
4
(
51−53
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Supporting Information
The Supporting Information is available free of charge on the
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AUTHOR INFORMATION
Corresponding Authors
E-mail: dmytro.nesterov@tecnico.ulisboa.pt.
E-mail: pombeiro@tecnico.ulisboa.pt.
ORCID
Oksana V. Nesterova: 0000-0002-0114-6525
Dmytro S. Nesterov: 0000-0002-1095-6888
Andrew Ozarowski: 0000-0001-6225-9796
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Notes
The authors declare no competing financial interest.
1
ACKNOWLEDGMENTS
This work was supported by Fundaca
Tecnologia (FCT), Portugal (projects PTDC/QEQ-QIN/
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DOI: 10.1021/acs.inorgchem.8b02145
Inorg. Chem. XXXX, XXX, XXX−XXX