Copper(II) coordination polymers derived from triethanolamine and pyromellitic acid for bioinspired mild peroxidative oxidation of cyclohexane

Abstract

The new inorganic 1D coordination polymer [Cu2(H3tea)2(mu4-pma)]n has been prepared, via self-assembly in aqueous medium, from copper(II) nitrate, triethanolamine (H3tea), pyromellitic acid (H4pma) and lithium hydroxide, and characterized by IR spectroscopy, elemental and single-crystal X-ray diffraction analyses. This compound and the related 2D polymer [Cu2(mu-H2tea)(2){mu3-Na2(H2O)4}(mu6-pma)]n.10nH2O are shown to mimic the alkane partial oxidation activity of the multicopper particulate methane monooxygenase, acting as catalysts precursors for the peroxidative oxidation of cyclohexane into cyclohexanol and cyclohexanone, by hydrogen peroxide (as green oxidant) and at room temperature in acidic MeCN/H2O medium. An overall yield (based on cyclohexane) of 29% has been achieved.

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Copper(II) coordination polymers derived from triethanolamine
and pyromellitic acid for bioinspired mild peroxidative
oxidation of cyclohexane
Yauhen Y. Karabach
a
, Alexander M. Kirillov
a
, Matti Haukka
b
,
Maximilian N. Kopylovich
a
, Armando J.L. Pombeiro
a,*
a
Centro de Quı
´mica Estrutural, Complexo I, Instituto Superior Te
´cnico, TU Lisbon, Avenue Rovisco Pais, 1049–001 Lisbon, Portugal
b
University of Joensuu, Department of Chemistry, P.O. Box 111, FIN-80101 Joensuu, Finland
Received 19 September 2007; received in revised form 12 November 2007; accepted 19 November 2007
Available online 4 January 2008
Abstract
The new inorganic 1D coordination polymer [Cu
2
(H
3
tea)
2
(l
4
-pma)]
n
has been prepared, via self-assembly in aqueous medium, from
copper(II) nitrate, triethanolamine (H
3
tea), pyromellitic acid (H
4
pma) and lithium hydroxide, and characterized by IR spectroscopy, ele-
mental and single-crystal X-ray diffraction analyses. This compound and the related 2D polymer [Cu
2
(l-H
2
tea)
2
{l
3
-Na
2
(H
2
O)
4
}
(l
6
-pma)]
n
10nH
2
O are shown to mimic the alkane partial oxidation activity of the multicopper particulate methane monooxygenase,
acting as catalysts precursors for the peroxidative oxidation of cyclohexane into cyclohexanol and cyclohexanone, by hydrogen peroxide
(as green oxidant) and at room temperature in acidic MeCN/H
2
O medium. An overall yield (based on cyclohexane) of 29% has been
achieved.
Ó2007 Elsevier Inc. All rights reserved.
Keywords: Copper coordination polymers; Particulate methane monooxygenase; C–H activation; Hydrogen peroxide; Homogeneous catalysis
The search for new mild and efficient routes for
functionalization of alkanes to industrially valuable prod-
ucts constitutes a subject of high relevance in various areas
including environmental catalysis and green chemistry [1–
9]. A promising approach consists on the design and devel-
opment of new bioinspired catalytic materials [10–13].In
this respect, particulate methane monooxygenase (pMMO)
appears to be a unique copper enzyme that can catalyze the
hydroxylation of alkanes. It bears an active site composed
of a multinuclear Cu cluster possessing a NO-environment
[14–16]. However, although numerous examples of bioin-
spired copper compounds have been reported
[10–12,17,18], the synthetic and catalytic (with respect to
alkanes) studies on multinuclear copper complexes related
to pMMO remain rather scant [19–23].
Aiming at mimicking pMMO, we have recently reported
a series of multicopper compounds with NO-ligands and
applied them in the catalytic peroxidative oxidation of
alkanes [21–23]. In particular, the new aqua-soluble cop-
per–sodium 2D coordination polymer [Cu
2
(l-H
2
tea)
2
{l
3
-Na
2
(H
2
O)
4
}(l
6
-pma)]
n
10nH
2
O(1)(Scheme 1) has
been prepared [24] from triethanolamine (H
3
tea) and
pyromellitic acid (H
4
pma), and preliminary screened as a
potential bioinspired catalyst. In pursuit of that study,
the main objectives of the current work consist in (i) the
synthesis of another structurally distinct, although related,
copper coordination polymer, and (ii) a detailed evaluation
of the catalytic activity of both species towards the mild
peroxidative oxidation of cyclohexane to cyclohexanol
and cyclohexanone.
Hence, by following the previously reported [24] proce-
dure for 1, but adding lithium hydroxide instead of sodium
hydroxide as a pH regulator to an aqueous mixture
0162-0134/$ - see front matter Ó2007 Elsevier Inc. All rights reserved.
doi:10.1016/j.jinorgbio.2007.11.007
*
Corresponding author. Fax: +351 21 846 4455.
E-mail address: pombeiro@ist.utl.pt (A.J.L. Pombeiro).
www.elsevier.com/locate/jinorgbio
Available online at www.sciencedirect.com
Journal of Inorganic Biochemistry 102 (2008) 1190–1194
JOURNAL OF
Inorganic
Biochemistry

Author's personal copy
composed of copper(II) nitrate, triethanolamine and
pyromellitic acid, we have obtained, by self-assembly, the
polymeric compound [Cu
2
(H
3
tea)
2
(l
4
-pma)]
n
(2)(Scheme
1).
1
It has been isolated in ca. 50% yield as a blue air-stable
crystalline solid and characterized by IR spectroscopy,
elemental and single-crystal X-ray diffraction analyses.
2
A modification of the type of alkali metal hydroxide
(i.e. LiOH in 2vs. NaOH in 1) has resulted in a distinct
compound without incorporation of the alkali ions. Other
distinguishing features of 2consist in its insolubility in
common solvents, and in the presence of mononuclear
[Cu(H
3
tea)]
2+
subunits, in contrast to the good solubility
of 1in water and the dinuclear character of its [Cu
2
(l-
H
2
tea)
2
]
2+
core [24].
OH
Na
O
H2
H2
O
Na
OH2
OH2
O
Cu
N
HO
O
O
O
O
OO
OO
OH
O
Cu
NOH
1
1
2
23
3
4
4
(H2O)10
HO OH
Cu
N
HO
O
O
O
O
OO
OO
2
HO OH
Cu
N
HO
1
1
2
[Cu2(
µ
-H2tea)2{
µ
3-Na2(H2O)4
}(
µ
6-pma)]n·10nH2O (1) [Cu2(H3tea)2(
µ
4-pma)]n (2)
Scheme 1. Schematic representations (repeating units) of compounds 1and 2. Numbers indicate the corresponding extensions of polynuclear chains.
Fig. 1. Structural fragment of 2with the partial atom labelling scheme
and intramolecular hydrogen bonds (dashed lines). Various symmetry
transformations used to generate the equivalent non-hydrogen atoms.
Hydrogen atoms (apart from those involved in represented H-bonds) are
omitted for clarity. Displacement ellipsoids are drawn at the 30%
probability level. Cu green, N blue, O red, C grey. Selected bond lengths
(A
˚) and angles (°): Cu1–O1 1.945(3), Cu1–O3 2.365(3), Cu1–O4 1.992(3),
Cu1–O5 1.943(3), Cu1–O7 2.688(4), Cu1–N1 2.025(4), O1–Cu1–O5
89.20(13), O7–Cu1–O1 91.22(14), O4–Cu1–O5 91.78(13). Hydrogen bonds
DHA [d(DA) A
˚;\(DHA)°]: O4–H4O6 [2.587(4); 147.4], O7–
H7O2 [2.625(5); 156.2], O3–H3O7
i
[2.753(4); 171.8], symmetry code
(i): x+ 1/2, –y+ 1/2, z+ 1/2. (For interpretation of the references to color
in this figure legend, the reader is referred to the web version of this
article.)
1
Synthesis of [Cu
2
(H
3
tea)
2
(l
4
-pma)]
n
(2): To an aqueous solution
(10.0 mL) containing Cu(NO
3
)
2
2.5H
2
O (1.00 mmol) in HNO
3
(1.00 mmol) [the acid was added to avoid spontaneous hydrolysis of the
metal salt] were added dropwise triethanolamine (1.00 mmol, 130 lL), an
aqueous solution (3.0 mL) of LiOH (72 mg, 3.00 mmol) and pyromellitic
acid (0.50 mmol, 127 mg) in this order and with continuous stirring in air
at room temperature. The resulting reaction mixture was stirred overnight
and then filtered. The filtrate was left to evaporate in a beaker at ambient
(ca. 20–25 °C) temperature. Blue X-ray quality crystals in ca. 50% yield
(based on copper nitrate) were formed in one week, then collected and
dried in air. Anal. data for C
22
H
32
Cu
2
N
2
O
14
(675.6): Calc. C, 39.11; H,
4.77; N, 4.15%. Found: C, 38.74; H, 4.90; N 4.11%. IR (KBr pellet, band
type: vs. very strong; s, strong; m, medium; w, weak; br., broad; sh.,
shoulder): 3354 and 3264 s br. m(OH), 2992, 2899 and 2787 w m(CH), 1580
vs br. and 1494 m m
as
(COO), 1419 s (1385 sh.) and 1328 s (1286 sh.)
m
s
(COO), 1134 s, 1076 s, 1041 m, 1011 m, 900 s, 849 m, 802 m, 763 m, 680
w, 573 s, 541 m and 478 m (other bands) cm
1
. All chemicals were
obtained from commercial sources and used as received. All character-
ization procedures were performed on the instruments and according to
the techniques previously described [21,22]. Compound 1has been
obtained according to a published procedure [24].
2
X-ray crystal structure determination: An X-ray quality single crystal of
2was mounted in an inert oil within the cold dinitrogen stream of the
diffractometer. The X-ray diffraction data were collected with a Nonius
Kappa CCD diffractometer. The Denzo-Scalepack [32] program package
was used for cell refinements and data reduction. The structure was solved
by direct method using the SHELXS-97 program [33]. A multiscan
absorption correction based on equivalent reflections (XPREP in SHEL-
XTL) [34] was applied to all data. The structure was refined with
SHELXL-97 [35]. OH hydrogen atoms were located from the difference
Fourier map but not refined. Other H atoms were placed in idealized
positions and constrained to ride on their parent atoms. Crystal data for 2:
C
11
H
16
CuNO
7
,M= 337.79, monoclinic, space group P21/n,
a= 7.1237(4) A
˚,b= 14.5282(15) A
˚,c= 13.2247(14) A
˚,a=c= 90,
b= 104.139(6)°.V= 1327.2(2) A
˚
3
,Z=4, D
c
=1.690g/cm
3
,
l= 1.677 mm
1
,MoKaradiation, k= 0.71073 A
˚, 9915 reflections col-
lected, 2392 unique (R
int
= 0.0500), GOF = 1.077, R1 = 0.0453,
wR
2
= 0.1158, R1 = 0.0647 (all data), wR
2
= 0.1262 (all data).
Y.Y. Karabach et al. / Journal of Inorganic Biochemistry 102 (2008) 1190–1194 1191

Author's personal copy
The crystal structure of 2(Fig. 1) is composed of
repeated symmetry equivalent [Cu(H
3
tea)]
2+
units inter-
connected by centrosymmetric l
4
-pyromellitate(4–) anions,
which act as spacers thus forming ladder-like 1D polymeric
chains (Fig. 2, see also Supplementary data). The fully pro-
tonated H
3
tea moiety acts as a tetradentate N,O,O,O-che-
lating ligand filling a distorted tetragonal-bipyramidal
geometry around the Cu1 atom that is completed by two
pyromellitate oxygen atoms (O1 and O5). The binding of
H
3
tea involves three nonequivalent Cu–O bonds with dis-
tances of 1.945(3) A
˚[Cu1–O1], 2.365(3) A
˚[Cu1–O3] and
2.688(4) A
˚[Cu1–O7], the latter being significantly elon-
gated relatively to the average bond of that type but com-
parable to related bond lengths in other Cu compounds
derived from triethanolamine [21,22,24]. The linkage of
[Cu(H
3
tea)]
2+
and pma moieties is further stabilized by
strong intramolecular O4–H4O6 [2.587(4) A
˚] and O7–
H7O2 [2.625(5) A
˚] H-bonds between two triethanola-
mine OH groups and carboxylate C@O oxygen atoms,
leading to the formation of six-membered nonplanar
Cu–O–HO–C–O rings with the O4–Cu1–O5 and O7–
Cu1–O1 bite angles of 91.78(13)°and 91.22(14)°, respec-
tively. The separations between collateral copper centres
(all lying in one plane) within the polymeric chain depicted
in Fig. 2 are 7.124(1) A
˚and 8.446(1) A
˚, the former being
the aunit-cell dimension. That 1D chain contains cavities
formed by repeated sixteen-membered Cu–pma–Cu–pma
rings (i.e. A, B, C in Fig. 2) trough connective O1–Cu1–
O5 [89.20(13)°] angles (Fig. 1). In the crystal cell of 2,
the neighbouring metal–organic chains are held together
via multiple intermolecular H-bonds [O3–H3O7
i
2.753(4) A
˚], thus resulting in the formation of a 3D hydro-
gen bonded supramolecular assembly (Supplementary
data).
Both 1and 2are catalysts precursors for the peroxida-
tive oxidation of cyclohexane, by aqueous H
2
O
2
, to cyclo-
hexanol and cyclohexanone (Scheme 2).
3
The reaction
proceeds in MeCN/H
2
O, at room temperature and atmo-
spheric pressure, and in the presence of an acid additive
(HNO
3
). Cyclohexane has been chosen as a recognized
substrate model [25], also in view of the significance of
the products [25,26]. The effects on the catalytic activity
of various factors (HNO
3
,H
2
O
2
, catalyst and MeCN
amounts) have been tested for 1(Fig. 3,Table 1) aiming
at the optimization of the conditions. The HNO
3
quantity
has a significant effect (Fig. 3a), and the acid-to-catalyst
precursor molar ratio of 10:1 is sufficient for the observed
high activity, although more acid leads to slightly higher
overall yields. The increase of oxidant amount also pro-
vides higher yields (Fig. 3b), growing from 1.7 to 29% on
changing the oxidant-to-catalyst molar ratio from 125 to
1000. A similar behaviour is observed for 2, although with
a slight yield drop for the high oxidant content conceivably
due to overoxidation (Table 1, entries 3,4). The optimal
amounts of 1(Fig. 3c) and acetonitrile (Fig. 3d) are 10.0
lmol and 3.0–3.5 mL, respectively.
An interesting feature of the active species generated
from 1consists in the almost full preservation of activity
even after three catalyst recycles,
3
i.e. 26% vs. 29% (original
yield) (Table 1, entries 5–7). The activity of 1and 2(Table
1) is only slightly lower than that (39% yield) reported for
the most active copper catalysts in the mild cyclohexane
oxidation [21,22], and is much higher than those exhibited
by other copper systems [22], including a related coordina-
tion polymer derived from H
3
tea and terephthalic acid
[21,22]. With respect to pMMO, the activities of the com-
Fig. 2. Partial representation of the ladder-like polymeric chain of 2
(projection down the caxis). Hydrogen atoms are omitted for clarity. Cu
green, N blue, O red, C grey. Letters A, B, C correspond to the equivalent
sixteen-membered Cu–pma–Cu–pma rings. (For interpretation of the
references to color in this figure legend, the reader is referred to the web
version of this article.)
Cu catalyst precursor
aq. H2O2
MeCN/H2O, r.t.
OH O
+
Scheme 2.
3
Catalytic activity studies: The reaction mixtures were prepared as
follows: to 1.25–30.0 lmol (typically 10.0 lmol) of catalyst precursor
(compound 1or 2) contained in the reaction flask were added 0.0–10.0 mL
(typically 3.0–3.5 mL) MeCN, 0.0–0.50 mmol (typically 0.1 mmol) HNO
3
,
0.625 mmol of C
6
H
12
and 1.25–10.0 mmol H
2
O
2
(30% in H
2
O), in this
order. The reaction mixture was stirred for 6 h at room temperature (ca.
25 °C) and air atmospheric pressure, then 90 lL of cycloheptanone (as
internal standard), typically 10.0 mL diethyl ether (to extract the substrate
and the products from the reaction mixture) and 0.5 g PPh
3
(to reduce the
cyclohexyl hydroperoxide, according to a method developed by Shul’pin
[28]) were added. The resulting mixture was stirred for 15 min and then a
sample taken from the organic phase was analyzed by GC using a
FISONS Instruments GC 8000 series gas chromatograph with a DB WAX
fused silica capillary column (P/N 123-7032) and the Jasco-Borwin v.1.50
software. The GC analyses of the aqueous phase showed the presence of
only traces (less than 0.05%) of oxidation products. Catalyst recycling
experiments were performed as follows. On completion of each batch, the
products were analyzed as usually (except that PPh
3
was added directly to
the GC sample and not to all the reaction mixture) and the catalytic
species was recovered by full evaporation of the reaction mixture under
vacuum. The subsequent batches were initiated upon addition of new
standard portions of all the other reagents and solvent [22]. In the
experiments with radical traps, the appropriate compounds e.g. 2,2,6,6-
tetramethylpiperidine-1-oxyl (TEMPO), CBrCl
3
or Ph
2
NH (0.625 mmol)
were also added to the reaction mixture. Blank experiments were
performed with different amounts of H
2
O
2
and other reagents, and
confirmed that no cyclohexane oxidation products (or only traces, below
0.3%) were obtained in the absence of the metal catalyst.
1192 Y.Y. Karabach et al. / Journal of Inorganic Biochemistry 102 (2008) 1190–1194

Author's personal copy
pounds studied herein, based on mass and averaged over
the reaction time, i.e. 52 (for 1) and 72 (for 2) nmol of
products per min and mg of catalyst precursor (recalcu-
lated values from entries 1 and 2, Table 1), are higher than
that of pMMO although in different conditions, i.e. 17
nmol of EtOH per min and mg of protein, for the enzy-
matic hydroxylation of ethane, the most favourable sub-
strate for this enzyme [27].
Although the detailed mechanistic pathway is still to be
established, as for the other multinuclear copper catalysts
[21–23], it is believed to proceed through both C- and
O-centred radicals on account of the strong inhibition of
cyclohexane oxidation by the presence of traps for such
radicals (Supplementary data).
3
Cyclohexyl hydroperoxide
(CyOOH) is also detected (usually in ca. 3–5% yields, at the
end of the experiments)
3
as an intermediate [28]. The con-
version of CyOOH into the products can be metal-assisted
[22], conceivably involving the homolytic decomposition to
the alkyl peroxy CyOO

(upon O–H bond cleavage) and the
alkoxy CyO

(upon O–O bond rupture). Dismutation of
CyOO

would yield both cyclohexanol (CyOH) and cyclo-
hexanone with O
2
, while CyOH could also be derived upon
H-abstraction from cyclohexane (CyH) by CyO

[2,28–31].
Despite of being distinct in structural and solubility
properties, 1and 2exhibit similar levels of activity (maxi-
mal overall yields of ca. 29%) what suggests a similarity
of the active catalytic species. Further studies towards wid-
ening the family of pMMO inspired multicopper complexes
and extending their catalytic application from the partial
oxidation of cyclohexane to other alkanes are in progress.
Fig. 3. Effects of acid-to-catalyst (a) and oxidant-to-catalyst (b) molar ratios, catalyst (c) and acetonitrile (d) amounts on the total yields of cyclohexanol
and cyclohexanone (% relatively to C
6
H
12
) in the peroxidative oxidation of cyclohexane with catalyst precursor 1. In (c) TONs (dashed line) correspond to
moles of both products per mol of precursor 1. Reaction conditions: 6 h reaction time, room temperature, C
6
H
12
= 0.625 mmol, compound 1= 10.0 (a, b,
d) or 0.0–30.0 lmol (c), n(HNO
3
)/n(catalyst) = 10:1 (b–d) or HNO
3
= 0.0–0.50 mmol (a), H
2
O
2
= 10.0 (a, c, d) or 1.25–10.0 mmol (b), MeCN = 3.0 (b, c),
5.0 (a) or 0.0–10.0 mL (d).
Table 1
Peroxidative oxidation of cyclohexane to cyclohexanol and cyclohexanone
in the presence of compounds 1and 2
a
Entry Catalyst
precursor
n(H
2
O
2
)/
n(catalyst
precursor)
Yield of products, %
b
Cyclo-
hexanol
Cyclo-
hexanone
Total
c,d
111000 12.9 16.1 29.0
22500 17.4 10.5 27.9
3
e
2500 16.7 9.6 26.3
4
e
21000 14.4 8.8 23.2
511000 13.4 15.7 29.1
61(after run 5)
f
1000 11.4 15.3 26.7
71(after run 6)
f
1000 11.2 15.0 26.2
a
Selected data; reaction conditions (unless stated otherwise): C
6
H
12
(0.625 mmol), catalyst precursor (10 lmol), aqueous 30% H
2
O
2
(10.0 mmol), MeCN (3.5 mL for entries 1–4, or 3.0 mL for entries 5–7),
HNO
3
(0.10 mmol), 6 h reaction time, room temperature.
b
Moles of product/100 moles of C
6
H
12
.
c
Cyclohexanol + cyclohexanone.
d
Overall TON values (moles of products/moles of catalyst) are equal to
the total % yields (entries 3, 4), or can be estimated as [total %
yield] 0.625 (entries 1, 2, 5–7).
e
C
6
H
12
(1.00 mmol).
f
The catalytic species used in the previous reaction batch (run) was
recovered,
3
and the subsequent batch was initiated upon addition of new
standard portions of all the other reagents and solvent.
Y.Y. Karabach et al. / Journal of Inorganic Biochemistry 102 (2008) 1190–1194 1193

Author's personal copy
Acknowledgments
This work has been partially supported by the Founda-
tion for Science and Technology (FCT), Portugal, and its
POCI 2010 programme (FEDER funded), and a HRTM
Marie Curie Research Training Network (AQUACHEM
Project, CMTN-CT-2003-503864).
Appendix A. Supplementary data
Crystallographic data for the structure reported in this
paper have been deposited with the Cambridge Crystallo-
graphic Data Centre as supplementary publication No.
CCDC-661341 (2). Copies of the data can be obtained free
of charge on application to the CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK (Fax: + 44 1223 336033; e-mail:
deposit@ccdc.cam.ac.uk or www: http://www.ccdc.cam.
ac.uk). Supplementary data associated with this article
can be found, in the online version, at doi:10.1016/
j.jinorgbio.2007.11.007.
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