Self-assembled two-dimensional water-soluble dipicolinate Cu/Na coordination polymer: Structural features and catalytic activity for the mild peroxidative oxidation of cycloalkanes in acid-free medium

Article abstract of DOI:10.1002/ejic.200800355

The new water-soluble 2D Cu/Na coordination polymer [Cu(μ-dipic) ₂{Na₂(μ-H₂O)₄}] _n·2nH₂O (1) has been synthesized by self-assembly in aqueous medium from copper(II) nitrate, dipicolinic acid (H₂dipic) and sodium hydroxide in the presence of triethanolamine. It has been characterized by IR spectroscopy, FAB⁺-MS, elemental and single-crystal X-ray diffraction analyses, the latter featuring a layered 2D metal-organic structure that is extended to a 3D supramolecular assembly by extensive hydrogen bonding between adjacent layers and involving crystallization water molecules. Compound 1 has been shown to act as a catalyst precursor for the peroxidative oxidation of cyclohexane and cyclopentane to the corresponding cyclic ketones and alcohols by aqueous H₂O₂ in MeCN solution and in the absence of acid additives. Wiley-VCH Verlag GmbH & Co. KGaA, 2008.

Full text of DOI:10.1002/ejic.200800355

SHORT COMMUNICATION
DOI: 10.1002/ejic.200800355
Self-Assembled Two-Dimensional Water-Soluble Dipicolinate Cu/Na
Coordination Polymer: Structural Features and Catalytic Activity for the Mild
Peroxidative Oxidation of Cycloalkanes in Acid-Free Medium
[a]
[a]
[a,b]
Marina V. Kirillova, Alexander M. Kirillov,* M. Fátima C. Guedes da Silva,
Armando J. L. Pombeiro*[
and
a]
Keywords: Self-assembly / N,O ligands / Supramolecular chemistry / C–H activation / Homogeneous catalysis
The new water-soluble 2D Cu/Na coordination polymer
Cu(µ-dipic) {Na (µ-H O) }] ·2nH O (1) has been synthe-
sized by self-assembly in aqueous medium from copper(II)
nitrate, dipicolinic acid (H dipic) and sodium hydroxide in
cent layers and involving crystallization water molecules.
Compound 1 has been shown to act as a catalyst precursor
for the peroxidative oxidation of cyclohexane and cyclopen-
tane to the corresponding cyclic ketones and alcohols by
[
2
2
2
4
n
2
2
the presence of triethanolamine. It has been characterized by
IR spectroscopy, FAB -MS, elemental and single-crystal X-
aqueous H
additives.
2 2
O in MeCN solution and in the absence of acid
+
ray diffraction analyses, the latter featuring a layered 2D
metal–organic structure that is extended to a 3D supramolec- (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
ular assembly by extensive hydrogen bonding between adja- Germany, 2008)
ing[3b,4d,6a,b,d–f] and molecular magnetism.[6b,6e] Being also
Introduction
focused on the preparation of water-soluble com-
Pyridine-2,6-dicarboxylic acid (dipicolinic acid, H dipic)
[3b,6c,6g]
2
pounds,
we have synthesized a series of heteronuc-
is a well-known, versatile N,O-chelator in coordination
[4d]
lear aqua dipicolinate complexes of Co, Ni, Cu and Zn,
as well as a 2D Cu/Na coordination polymer constructed
[
1]
chemistry, which, apart from forming a rich family of
[6f]
[2]
mono- and polynuclear complexes, has also been a conve-
nient ligand for the design of various polymeric metal–or-
ganic frameworks and supramolecular assemblies.
particular, among an array of transition-metal compounds
derived from dipicolinic acid, copper dipicolinates represent
the major part, exhibiting interesting properties and ap-
plications in molecular magnetism, crystal engineering and
from copper triethanolaminate and aqua–sodium building
blocks and pyromellitate linkers. This compound appeared
to be the first highly water-soluble Cu coordination poly-
mer. As a continuation and potential merging of these stud-
ies, the current work aims at the synthesis of a related Cu/
Na polymeric compound derived from dipicolinic acid and
at probing its catalytic potential towards the peroxidative
oxidation of cycloalkanes, thus mimicking the multicopper
[
2,3]
In
[
2]
[4]
bioinorganic and aqueous chemistry. Nevertheless, al-
though dipicolinic acid features a recognized biological
[7]
particulate methane monooxygenase (pMMO).
[
4d,5]
function,
to the best of our knowledge, the application
of its copper compounds as bioinspired catalysts have not
yet been reported.
We have recently developed various interfacing research
lines with relevance to self-assembly synthetic procedures in
Results and Discussion
The combination in aqueous solution, at ambient tem-
aqueous medium and synthesis of copper complexes, coor- perature and in air, of copper(II) nitrate, triethanolamine
dination polymers and hydrogen bonded supramolecular (H tea), sodium hydroxide and dipicolinic acid gives rise
3
[
3b,4d,6]
frameworks,
as well as their application in the mild to the formation by self-assembly of the new water soluble
[6a,d–h]
catalytic transformations of alkanes,
crystal engineer- polymeric product [Cu(µ-dipic) {Na (µ-H O) }] ·2nH O
2
2
2
4
n
2
(
1, Scheme 1). It has been isolated as a light blue crystalline
[
8]
+
solid and characterized by IR spectroscopy, FAB -MS,
elemental and single-crystal X-ray diffraction analyses.[
Compound 1 does not bear triethanolamine moieties, but
[
a] Centro de Química Estrutural, Complexo I, Instituto Superior
Técnico, TU Lisbon,
10]
Av. Rovisco Pais, 1049-001 Lisbon, Portugal
Fax: +351-21-846-4455
E-mail: pombeiro@ist.utl.pt
the use of H
3
tea is crucial for its synthesis, since, in the
kirillov@ist.utl.pt
absence of H tea, another product, [Cu(H O) Cu(dipic) ]·
3
2
5
2
[b] Universidade Lusófona de Humanidades e Tecnologias, ULHT
Lisbon,
[
4d,13]
2
H O,
is formed. Hence, triethanolamine appears to
2
Av. do Campo Grande, 376, 1749-024 Lisbon, Portugal
behave as a template towards the generation of 1.
Eur. J. Inorg. Chem. 2008, 3423–3427
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3423

A. M. Kirillov, A. J. L. Pombeiro et al.
SHORT COMMUNICATION
Scheme 1. Schematic representation of 1 (numbers correspond to
Figure 1. Structural fragment of 1 with partial atom labelling
scheme and intramolecular hydrogen bond (dashed line). Thermal
ellipsoids are drawn at the 30% probability level. Hydrogen atoms
and crystallization water molecules are omitted for clarity. Sym-
metry operators were used to generate equivalent atoms. Selected
bond lengths [Å] and angles [°]: Cu1–O11 2.1840(16), Cu1–O21
extensions of the polymeric network).
The IR spectrum of 1 exhibits a strong and broad band 2.2095(16), Cu1–N1 1.9278(17), Na1–O1 2.377(2), Na1–O3
–
1
–1
2
2
.5713(19), Na1–O12 2.3539(15), Na2–O1 2.441(2), Na2–O3
.393(2), Na2–O12 2.4726(15), O11–Cu1–O21 156.43(5), O11–
in the 3700–2800 cm region (max. at 3407 cm ) due to
the ν(H O) vibrations associated with both aqua–sodium
2
Cu1–N1 78.65(6), O21–Cu1–N1 77.85(6), Na1–O1–Na2 85.28(8),
Na1–O3–Na2 82.13(5), Na1–O12–Na2 85.07(5), O1–Na2–O12
units and crystallization water molecules. The broad char-
acter of this band results in the overlapping of weak ν(CH) 74.39(6), O3–Na2–O12 76.66(6), O1–Na2–O3 88.29(7). Hydrogen
vibrations and is indicative of an extensive hydrogen bond- bond D–H···A: for O1–H1A···O11i d(D···A)
Є(DHA) = 155(3)° [symmetry code: (i) –x, y, 1/2 – z].
= 2.898(2) Å;
ing. Other characteristic bands correspond to very strong
–
1
νas and ν vibrations (with maxima at 1621 and 1383 cm ,
s
[3b,4d]
respectively) of COO groups of dipic ligands.
The pos-
+
itive FAB -MS spectrum of 1 allows the detection of
the following fragments: [Cu(dipic) + 3Na] (m/z = 462),
Cu(dipic) Na + H] (m/z = 440) and [Cu(dipic) + 3H]
+
2
+
+
[
2
2
2
(m/z 396), all in accord with the expected isotopic patterns.
The molecular structure of 1 (Figure 1, Figure 2) features
an infinite nonplanar 2D metal–organic network con-
2
–
structed from monomeric [Cu(dipic) ] units and poly-
2
2
+
meric {[Na (µ-H O) ] }
9
chains, alternating every
.7626(12) Å (the unit cell dimension a). The geometry
around the Cu1 atoms is significantly distorted octahedral
the main deviation involves the O11–Cu1–O21 angle of
56.43(5)°] and is formed by two nearly planar N,O,O-di-
2
2
4
n
[
1
picolinate ligands that adopt an arrangement in which they
are almost perpendicular to each other (Figure 1). The
2
–
connection of the [Cu(dipic)2] units with aqua–sodium
moieties is realized through the O12 carboxylate atoms,
simultaneously bridging the Na1 [2.3539(15) Å] and Na2
[
2.4726(15) Å] atoms and contributing to their distorted oc-
tahedral geometry. The octahedral geometry is completed
by two symmetry-equivalent O1 [avg. Na–O1 2.409(2) Å]
and O3 [avg. Na–O3 2.483(2) Å] bridging water molecules,
resulting in the generation of repeated up-down alternate
[
Na (µ-O
)(µ-H O) ] triangular bipyramid-like
2
carboxylate 2 2
[14]
cages of a rather unusual type. The Na1···Na2 separation
within those cages is 3.2640(5) Å, and the representative
Na–O–Na and O–Na–O angles are listed in Figure 1. In
general, most of the bonding parameters in 1 are compar-
able to those reported for other polymeric dipicolinate
Figure 2. Fragment of the crystal packing diagram of 1 along the
b axis (a) and the c axis (b) showing front (a) and side (b) views of
one 2D metal–organic layer. Hydrogen atoms and crystallization
2
– [4a,4e,4h]
compounds bearing either [Cu(dipic) ]
(
or [Na-
2
+
[4h,15]
H O) ]
moieties.
2
H O molecules are omitted for clarity.
2
n
3424
www.eurjic.org
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2008, 3423–3427

A 2D Water-Soluble Dipicolinate Cu/Na Coordination Polymer
i
Intramolecular O1–H1A···O11 [2.898(2) Å] and inter- solution at 50 °C to the corresponding cyclic alcohols and
ii
[16]
molecular O3–H3A···O22 [2.841(2) Å] hydrogen bonds ketones (Scheme 2).
Thus, 1 acts as a catalyst precursor
(the symmetry codes are those of Figure 1 and Figure 3) in these transformations, leading to the overall product
between the coordinated water molecules (O1, O3) and car- yields of 11.1 and 9.9% (molar yields based on cycloal-
boxylate groups (O11, O22) provide an additional linkage kanes) for cyclohexane and cyclopentane oxidation, respec-
2
–
2+
of [Cu(dipic)2] and {[Na (µ-H O) ] } building blocks, tively. Cyclic alcohols (cyclohexanol and cyclopentanol) are
2
2
4
n
thus contributing to the reinforcement of 2D metal–organic the major final products after the treatment of the final
layers and their direct interconnection, respectively. The reaction mixtures with triphenylphosphane, following a
[
17]
neighbouring layers alternate every 9.7626(12) Å (the unit method developed by Shul’pin.
The observed pro-
cell dimension b). In addition, the crystallization water nounced increase of the alcohol/ketone molar ratio upon
molecules O4 (two per formula unit) are located between this treatment with PPh indicates the formation of cyclo-
3
layers and participate in the formation of four intermo- alkylhydroperoxides (ROOH) as the main primary products
lecular hydrogen bonds (donating H atoms to carboxylate of the cycloalkane (RH) oxidation. The quantitative re-
O21 and O22 atoms, and accepting from O3 and O1 water duction of ROOH by PPh3 to the corresponding cyclic
ligands), thus giving rise to an extensive indirect linkage of alcohols (ROH) results in the elevated yields of the latter,
adjacent 2D layers and generation of a 3D supramolecular with concomitant reduction of the amount of ketone.[17]
assembly (Figure 3).
Scheme 2.
For cyclohexane oxidation, the above-mentioned yield of
1
1.1% corresponds to a turnover number (TON, mol of
products/mol of 1) of 15, which can be increased up to 55
still with a reasonable overall product yield of 6.0%) upon
(
[
18]
modifying some reaction parameters. It should be noted
that the catalyst precursor 1 is active in the absence of any
acid additive, while the addition of only 5 equiv. of HNO3
relative to 1 leads to a full suppression of its catalytic ac-
[
16]
tivity.
Such behaviour contrasts with those of our pre-
II
[6a,6d–6h]
viously reported Cu systems,
for the peroxidative
oxidation of cycloalkanes, which exhibit high activity only
in the presence of an acid additive. Hence, the ability of 1
to catalyze cycloalkane oxidations in acid-free systems con-
stitutes an important feature, presumably related to the di-
II
picolinate environment around the Cu atom and to the
II
aqueous solubility. As observed for other Cu catalytic sys-
[
6a,6d–6h]
tems,
the introduction into the reaction mixture of
[
16]
a radical trap (CBrCl , BHT or Ph NH)
results in an
3
2
almost full suppression of the activity of 1. This behaviour,
along with the formation of cycloalkyl hydroperoxides (typ-
metal–organic layers shown by different shades of grey, which are ical intermediates in radical-type reactions, as discussed be-
Figure 3. Fragment of the crystal packing diagram of 1 (rotated
view along the c axis) representing the relative arrangement of three
extensively interlaced by intermolecular hydrogen bonds (dotted low), supports a free-radical mechanism for the cycloalkane
lines) between adjacent layers (direct linkage) and involve interca-
oxidation.
lated crystallization water molecules (represented as balls; indirect
Aiming to obtain further mechanistic information, we
linkage), resulting in the formation of a 3D supramolecular as-
[
19]
have performed EPR measurements, which reveal similar
sembly. Hydrogen atoms are omitted for clarity. Hydrogen bonds
ii
D–H···A {d(D···A) [Å]; Є(DHA) [°]}: O3–H3A···O22 {2.841(2); spectral features of 1 in both the solid state and in aqueous
iii
iv
1
{
67(4)}, O3–H3B···O4
{2.928(3); 164(3)}, O4–H4A···O21
solution, thus supporting the preservation of the [Cu-
dipic)2] cores upon dissolution of the compound. How-
ever, the intensity of the EPR signal dramatically decreases
(without a change in the g factor) upon treatment of an
v
2.946(3); 168(3)}, O4–H4B···O22 {2.798(3); 170(3)}, O1–
2–
(
H1B···O4 {3.095(3); 167(5)} [symmetry codes: (ii) –1 + x, 1 + y, z;
iii) x, 1 + y, z; (iv) 1 – x, y, 1/2 – z; (v) 1 – x, 1 – y, 1 – z]. For an
atom labelling scheme, see Figure 1.
(
aqueous solution of 1 with hydrogen peroxide, thus suggest-
I
In pursuit of our interest on the mild oxidation of al- ing the formation of a diamagnetic Cu species. The intro-
II
[6a,6d–6h]
kanes catalyzed by Cu centres,
we have prelimi- duction of a high excess of H O , however, does not lead
2 2
II
nary screened the catalytic potential of 1 for the peroxidat- to the complete disappearance of the Cu EPR signal, on
ive oxidation (i.e., by using a peroxide as an oxidant) of account of the reversibility of the Cu ↔ Cu transforma-
cyclohexane and cyclopentane by aqueous H O in MeCN tion as expected for a catalytic cycle that may include the
II
I
2
2
Eur. J. Inorg. Chem. 2008, 3423–3427
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
3425

A. M. Kirillov, A. J. L. Pombeiro et al.
SHORT COMMUNICATION
2
+
+
·
+
44, 911; e) S. K. Ghosh, P. K. Bharadwaj, Inorg. Chem. 2004,
43, 2293; f) S. K. Ghosh, P. K. Bharadwaj, Inorg. Chem. 2003,
following steps: (i) Cu + H O Ǟ Cu + HOO + H ; (ii)
2
+
2
2
+
·
+
+
Cu + HOO Ǟ Cu + H + O ; (iii) Cu + H O Ǟ
2
2
2
42, 8250.
2
+
·
– [20]
Cu + HO + HO .
These steps are in accord with the
[
4] a) N.-H. Hu, Z.-G. Li, J.-W. Xu, H.-Q. Jia, J.-J. Niu, Cryst.
Growth Des. 2007, 7, 15; b) S. I. Kirin, K. Ohr, H. P. Yennawar,
C. M. Morgan, L. A. Levine, M. E. Williams, Inorg. Chem.
Commun. 2007, 10, 652; c) S. C. Manna, J. Ribas, E. Zang-
rando, N. R. Chaudhuri, Inorg. Chim. Acta 2007, 360, 2589;
d) M. V. Kirillova, M. F. C. G. da Silva, A. M. Kirillov, J. J. R.
Fraústo da Silva, A. J. L. Pombeiro, Inorg. Chim. Acta 2007,
proposed free-radical mechanism of cycloalkane oxidation,
which is further supported by the observed intense release
of dioxygen and colour change (from pale blue to greenish
yellow) on addition of hydrogen peroxide to an aqueous
solution of 1. Besides, the catalytic oxidation of cyclohex-
ane under dinitrogen (keeping the other reaction conditions
the same ) results in a drop in the total product yield from
1.1 to 5.1%. This fact confirms also the involvement of
atmospheric oxygen, by facilitating the generation of cyclo-
3
60, 506, and references cited therein; e) Y.-P. Cai, C.-Y. Su,
[
16]
G.-B. Li, Z.-W. Mao, C. Zhang, A.-W. Xu, B.-S. Kang, Inorg.
Chim. Acta 2005, 358, 1298; f) S. K. Ghosh, J. Ribas, P. K.
Bharadwaj, CrystEngComm 2004, 6, 250; g) B. Schmidt, J. Jir-
icek, A. Titz, G. F. Ye, K. Parang, Bioorg. Med. Chem. Lett.
2004, 14, 4203; h) L. C. Nathan, T. D. Mai, J. Chem. Crys-
tallogr. 2000, 30, 509.
1
·
·
·
alkylperoxyl (ROO ) radicals (R + O Ǟ ROO ), which lead
to the formation of cycloalkyl hydroperoxide primary
products (ROO + HOO Ǟ ROOH + O ; and ROO + RH [5] T. Douki, B. Setlow, P. Setlow, Photochem. Photobiol. Sci. 2005,
2
·
·
·
2
·
[17]
4, 591, and references cited therein.
Ǟ ROOH + R ). It should also be noted that the activity
of the heterometallic compound 1 is associated with copper
ions, while sodium cations have no direct involvement in
the observed catalysis.[
[
6] a) Y. Y. Karabach, A. M. Kirillov, M. Haukka, M. N. Kopylov-
ich, A. J. L. Pombeiro, J. Inorg. Biochem. 2008, 102, 1190; b)
A. M. Kirillov, Y. Y. Karabach, M. Haukka, M. F. C. Gu-
edes da Silva, J. Sanchiz, M. N. Kopylovich, A. J. L. Pombeiro,
Inorg. Chem. 2008, 47, 162; c) A. M. Kirillov, P. Smole n´ ski,
M. F. C. Guedes da Silva, A. J. L. Pombeiro, Eur. J. Inorg.
Chem. 2007, 2686; d) C. Di Nicola, Y. Y. Karabach, A. M. Kir-
illov, M. Monari, L. Pandolfo, C. Pettinari, A. J. L. Pombeiro,
Inorg. Chem. 2007, 46, 221; e) D. S. Nesterov, V. N. Kokozay,
V. V. Dyakonenko, O. V. Shishkin, J. Jezierska, A. Ozarowski,
A. M. Kirillov, M. N. Kopylovich, A. J. L. Pombeiro, Chem.
Commun. 2006, 4605; f) Y. Y. Karabach, A. M. Kirillov,
M. F. C. G. da Silva, M. N. Kopylovich, A. J. L. Pombeiro,
Cryst. Growth Des. 2006, 6, 2200; g) A. M. Kirillov, M. N. Ko-
pylovich, M. V. Kirillova, E. Y. Karabach, M. Haukka,
M. F. C. G. da Silva, A. J. L. Pombeiro, Adv. Synth. Catal.
21]
Conclusions
We have prepared and fully characterized a novel 2D Cu/
Na coordination polymer [Cu(µ-dipic) {Na (µ-H O) }] ·
nH O, whose layered metal–organic network driven by
aqua–sodium chains bears a rare [M (µ-O
core and is further extended to a 3D supramolecular as-
sembly by means of multiple hydrogen bonds and guest
water molecules. This compound widens the still limited
of water-soluble copper(II) coordination poly-
mers. Being derived from the biologically relevant dipicolin-
2
2
2
4
n
2
2
)(µ-H O) ]
carboxylate 2 2
2
2
006, 348, 159; h) A. M. Kirillov, M. N. Kopylovich, M. V. Ki-
[
6f]
family
rillova, M. Haukka, M. F. C. G. da Silva, A. J. L. Pombeiro,
Angew. Chem. Int. Ed. 2005, 44, 4345.
[
4d,5]
ate ligand,
it acts as a promising bioinspired catalyst [7] a) R. L. Lieberman, A. C. Rosenzweig, Nature 2005, 434, 177;
precursor, related to the multicopper particulate methane
b) S. I. Chan, V. C.-C. Wang, J. C.-H. Lai, S. S.-F. Yu, P. P.-Y.
Chen, K. H.-C. Chen, C.-L. Chen, M. K. Chan, Angew. Chem.
Int. Ed. 2007, 46, 1992.
monooxygenase (pMMO),[7] for the mild peroxidative oxi-
dation of cycloalkanes to give the corresponding cyclic
alcohols and ketones, without requiring the presence of an
acid promoter. Further research towards the detailed explo-
ration of its catalytic potential and establishment of the
mechanism will be pursued.
[
8] Synthesis of [Cu(µ-dipic) {Na (µ-H O) }] ·2nH O (1): To an
2
2
2
4
n
2
aqueous solution (10.0 mL) containing Cu(NO
3
)
2
·2.5H
2
O
(
1.00 mmol, 233 mg) and 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 µL) and an
aqueous solution (4.0 mL) of NaOH (160 mg, 4.00 mmol), and
2
solid H dipic (84 mg, 0.50 mmol), in this order and with con-
tinuous stirring in air at ambient temperature. The resulting
clear blue solution was stirred overnight, then passed through
a filter, concentrated under reduced pressure to one half of its
volume, and kept in a beaker in air at ambient temperature.
Light blue X-ray quality crystals were formed in 2–3 weeks
from a deep blue oily solution, then collected, gently wiped
with a filter paper and dried in air to furnish compound 1 in
Acknowledgments
This work has been partially supported by the POCI 2010 (FEDER
funded) and “Science 2007” programs of the Foundation for Sci-
ence and Technology (FCT), Portugal. We gratefully thank Dr.
João Paulo Telo (IST) for his help with the EPR measurements.
ca. 70% yield based on H
2
dipic. Compound 1 is soluble in
O (S25 °C ≈ 15 mgmL ) and can be recrystallized from an
aqueous solution in the original form. C14 18CuN Na
–
1
H
2
[
[
[
1] For a review, see: L. C. Nathan, Trends Inorg. Chem. 1993, 3,
H
2
2 14
O
415.
(547.8): calcd. C 30.69, H 3.31, N 5.11; found C 30.28, H
+
2] See the Cambridge Structural Database (CSD, version 5.28,
Jan. 2008): F. H. Allen, Acta Crystallogr., Sect. B 2002, 58, 380.
3] a) S. X. Cui, Y. L. Zhao, B. Li, J. P. Zhang, Q. Liu, Y. Zhang,
Polyhedron 2008, 27, 671; b) M. V. Kirillova, A. M. Kirillov,
M. F. C. Guedes da Silva, M. N. Kopylovich, J. J. R. Fraú-
sto da Silva, A. J. L. Pombeiro, Inorg. Chim. Acta 2008, 361,
3.04, N 5.09. FAB -MS (m-nitrobenzylalcohol): m/z = 462
+
+
[Cu(dipic)
2
+ 3Na] , 440 [Cu(dipic)
+ 3H] . IR (KBr, selected bands): ν˜ = 3407 (vs. br) and 3108
(w sh) ν(H O), 1685 (sh), 1621 (vs. br) and 1591 (sh) νas(COO)
+ δ(H O), 1421 (s) and 1383 (vs. br) ν (COO), 1279 (s), 1186
2 2 2
Na + H] , 396 [Cu(dipic)
+
2
2
s
(m), 1156 (w), 1081 (s), 1043 (w), 997 (w), 913 (s), 855 (w), 822
(m), 768 (s), 733 (s), 689 (m), 510 (m), 444 (w) and 420 (w)
1728, and references cited therein; c) X. Q. Zhao, B. Zhao, Y.
–
1
Ma, W. Shi, P. Cheng, Z. H. Jiang, D. Z. Liao, S. P. Yan, Inorg.
Chem. 2007, 46, 5832; d) B. Zhao, L. Yi, Y. Dai, X. Y. Chen,
P. Cheng, D. Z. Liao, S. P. Yan, Z. H. Jiang, Inorg. Chem. 2005,
(other bands) cm . All characterization procedures were per-
formed with the instruments and according to the techniques
previously described.[
9]
3426
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© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2008, 3423–3427

A 2D Water-Soluble Dipicolinate Cu/Na Coordination Polymer
[
[
9] A. M. Kirillov, M. Haukka, M. V. Kirillova, A. J. L. Pombeiro,
Adv. Synth. Catal. 2005, 347, 1435.
10] X-ray crystal structure determination: X-ray data were col-
lected with an Enraf–Nonius CAD4 diffractometer equipped
3
the reaction mixture, treated with an excess of solid PPh (for
prior reduction[ of cycloalkyl hydroperoxides that are the
main primary products) and then analyzed by GC by using a
Fisons Instruments GC 8000 series gas chromatograph with a
DB-624 (J&W) capillary column and the Jasco-Borwin v.1.50
software, revealing the formation of the following mixtures: cy-
clohexanol (9.3%) + cyclohexanone (1.8%) and cyclopentanol
(9.1%) + cyclopentanone (0.8%), for cyclohexane and cyclo-
pentane oxidations, respectively. Blank experiments were per-
formed for both cycloalkanes under similar reaction condi-
tions, which confirmed that no oxidation products were ob-
tained in the absence of precursor 1. Additional catalytic ex-
periments were performed in the presence of (i) either 5 or
17]
with graphite monochromator and by using Mo-K
λ = 0.71073 Å). The structure was solved by direct methods
α
radiation
(
[
11]
with the SHELXS-97 program. A semiempirical absorption
[
12]
correction was applied.
The structure was refined with the
SHELXL-97 program.[ All hydrogen atoms were located.
Crystal data for 1: C14 18CuN Na 14, M = 547.82, mono-
clinic, space group P2/c, a = 9.7626(12) Å, b = 7.9756(10) Å, c
11]
H
2
2
O
3
–3
=
F
12.9066(13) Å, V = 1004.9(2) Å , Z = 2, D
c
= 1.811 gcm ,
000 = 558, µ = 1.210 mm , T = 293 K, 2026 reflections col-
lected, 1624 reflections with IϾ2σ(I) (Rint = 0.0217), GoF =
.100, R = 0.0281, wR = 0.0736. CCDC-683529 contains the
–1
10 equiv. (relative to 1) of concentrated HNO
63% in H O), or (ii) radical traps CBrCl , 2,6-di-tert-butyl-4-
methylphenol (BHT) or Ph NH (2.50 mmol of each in a sepa-
3
(46–92 µmol,
1
1
2
2
3
supplementary crystallographic data for 1. These data can be
obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
11] G. M. Sheldrick, Acta Crystallogr., Sect. A 2008, 64, 112.
12] A. C. T. North, D. C. Phillips, F. S. Mathews, Acta Crystallogr.,
Sect. A 1968, 24, 351.
2
rate batch), displaying an almost complete suppression of cata-
lytic activity of 1 (only traces of products, below 0.3% were
observed).
G. B. Shul’pin, J. Mol. Catal. A 2002, 189, 39, and references
cited therein.
[
[
[17]
[
13] L. Wang, L. Duan, D. Xiao, E. Wang, C. Hu, J. Coord. Chem.
[
18] This activity has been achieved by using slightly different (rela-
2
004, 57, 1079.
14] It is interesting to note that according to a search in the CSD,
the encountered [M O) ] core, with bridg-
ing carboxylate oxygen and two µ-H O molecules completing
[16]
tive to those described ) reaction parameters, i.e., catalyst
[2]
[
precursor 1 (3.0 mg, 5.5 µmol), cyclohexane (10.0 mmol), ace-
2
(µ-Ocarboxylate)(µ-H
2
2
tonitrile (3.80 mL) and H
.68 mL).
2 2 2
O (10.0 mmol, 50% in H O,
2
0
the coordination sphere of two metal (M) atoms, appears to be
rarely observed [only 7 matches; CSD refcodes: ALOWUT
[
19] EPR spectra of 1 were recorded (following a reviewer’s sugges-
tion) on solid crystalline samples or frozen aqueous solutions
by using a Bruker ESP300E X-band spectrometer equipped
with an ER 4111 VT variable-temperature unit. Typical pat-
(
Ca), NOVJUD (Na), NOLGOK and RUHHUX (Ba),
KAMYON, VENDUN01 and YEHBUJ (K)], in contrast to
numerous structures (ca. 107 hits) bearing simpler [M (µ-
O)] fragments.
2
II
terns for Cu (I = 3/2) with g = 2.22 and A = 130 G (with
O
carboxylate)(µ-H
2
poorly resolved hyperfine coupling) were observed for both so-
lid (298 K) and solution (100 K) samples of 1. A broad signal
[
15] a) C. Ma, C. Chen, F. Chen, X. Zhang, H. Zhu, Q. Liu, D.
Liao, L. Li, Bull. Chem. Soc. Jpn. 2003, 76, 301; b) J. Limburg,
G. W. Brudvig, R. H. Crabtree, J. Am. Chem. Soc. 1997, 119,
(
g = 2.16) without hyperfine coupling was obtained for a solid
sample at 100 K.
2
761; c) Y. Dai, W. Shi, X.-J. Zhu, B. Zhao, P. Cheng, Inorg.
[
20] a) G. B. Shul’pin, J. Gradinaru, Y. N. Kozlov, Org. Biomol.
Chem. 2003, 1, 3611; b) G. B. Shul’pin, G. V. Nizova, Y. N.
Kozlov, L. G. Cuervo, G. Suss-Fink, Adv. Synth. Catal. 2004,
Chim. Acta 2006, 359, 3353; d) J. Chakraborty, H. Mayer-
Figge, W. S. Sheldrick, P. Banerjee, Polyhedron 2006, 25, 3138;
e) C.-Z. Xie, B. Yu, X.-Q. Wang, R.-J. Wang, G.-Q. Shen, D.-
Z. Shen, J. Coord. Chem. 2006, 59, 2005.
346, 317.
[
21] Additional blank experiments, carried out under similar reac-
[
16] Peroxidative oxidation of cyclohexane and cyclopentane: The
catalyst precursor 1 (5.0 mg, 9.2 µmol), used as a solid, was
tion conditions,[ but in the absence of compound 1 and in
16]
mixed with the cycloalkane (1.25 mmol), CH
standard for GC analysis, 0.50 mL of its 1.4  solution in
MeCN), acetonitrile (4.20 mL) and H (2.50 mmol, 50% in
O, 0.17 mL), in this order. The resulting reaction mixtures
3
NO
2
(internal
the presence of a catalytic amount (10 µmol) of Na
2 3
CO or
K
2
CO , reveal no activity in the oxidation of cyclohexane (i.e.
3
O
2
only traces of products, below 0.2% of the total yield, have
been detected). Moreover, the activity of 1 is not promoted to
any extent by the addition of potassium or sodium carbonate.
Received: April 7, 2008
2
H
2
were vigorously stirred in thermostatted (50 °C) Pyrex cylindri-
cal vessels for 5 h at atmospheric air pressure. The product
analysis was performed as follows: a sample was taken from
Published Online: July 7, 2008
Eur. J. Inorg. Chem. 2008, 3423–3427
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
3427