Assembling chiral salan-copper(II) complexes into a 2D-network with carboxylic acid functionalization

Article abstract of DOI:10.1016/j.inoche.2014.02.009

In this work, we aim to assemble a polymeric copper(II) network by ligand modifications with carboxylic acid functional groups. A chiral, reduced salen ligand, salan R,R-H₂1 with pendant ester functionalities has been synthesized. Its copper(II) complex, [Cu(R,R-1)], has been isolated, and its crystal structure determined by X-ray crystallography. Hydrolysing the ester groups of R,R-H₂1 to carboxylic acids yields R,R-H₄2 as the hydrochloride salt R,R-H₄2·2HCl. Treatment of R,R-H ₄2·2HCl with copper(II) acetate in the presence of triethylamine yields the complex [Cu(R,R-H₂2)]_n and X-ray structural analysis reveals that this consists of a 2D polymeric network. Each copper(II) center is bound within the near square-planar N₂O ₂ donor set of the Schiff base ligand [R,R-H₂2] ^{2 -} and pendant carboxylic acid groups of adjacent units coordinate through the CO₂H carbonyl units in the axial sites of the copper ion to complete an axially distorted octahedral coordination environment. The sheets are stacked via interdigitation of the cyclohexyl domains, giving an overall porous assembly containing carboxylic acid functionalities. This has proven to be an effective strategy for adding the dimensionality of metal-organic complexes by carboxylic acid functionalization of ligands.

Full text of DOI:10.1016/j.inoche.2014.02.009

Inorganic Chemistry Communications 43 (2014) 51–55
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Assembling chiral salan–copper(II) complexes into a 2D-network with
carboxylic acid functionalization
b
b
b
Guoqi Zhang a, , Edwin C. Constable , Catherine E. Housecroft , Jennifer A. Zampese
⁎
a
Department of Sciences, John Jay College, The City University of New York, New York, NY 10019, USA
Department of Chemistry, University of Basel, Spitalstrasse 51, CH 4055 Basel, Switzerland
b
a r t i c l e i n f o
a b s t r a c t
Article history:
In this work, we aim to assemble a polymeric copper(II) network by ligand modifications with carboxylic acid
functional groups. A chiral, reduced salen ligand, salan R,R-H21 with pendant ester functionalities has been syn-
thesized. Its copper(II) complex, [Cu(R,R-1)], has been isolated, and its crystal structure determined by X-ray
crystallography. Hydrolysing the ester groups of R,R-H21 to carboxylic acids yields R,R-H42 as the hydrochloride
salt R,R-H42·2HCl. Treatment of R,R-H42·2HCl with copper(II) acetate in the presence of triethylamine yields the
complex [Cu(R,R-H22)]n and X-ray structural analysis reveals that this consists of a 2D polymeric network. Each
copper(II) center is bound within the near square-planar N2O2 donor set of the Schiff base ligand [R,R-H22]2− and
pendant carboxylic acid groups of adjacent units coordinate through the CO2H carbonyl units in the axial sites of
the copper ion to complete an axially distorted octahedral coordination environment. The sheets are stacked via
interdigitation of the cyclohexyl domains, giving an overall porous assembly containing carboxylic acid function-
alities. This has proven to be an effective strategy for adding the dimensionality of metal–organic complexes by
carboxylic acid functionalization of ligands.
Received 12 December 2013
Accepted 10 February 2014
Available online 16 February 2014
Keywords:
Copper(II)
Salan ligand
Chirality
Self-assembly
Coordination network
© 2014 Elsevier B.V. All rights reserved.
Over the past decades, Schiff base type ligands have been a popular
choice for constructing versatile metal–organic compounds varying
from simple mononuclear complexes to complex supramolecular
frameworks through metal coordination, owing to their accessibility,
low-cost and highly efficient synthesis, facile derivatization, and ability
to bind a wide variety of metal ions [1–4]. Metal–Schiff base complexes
are attractive molecular architectures with promising applications in, for
example, catalysis [5], molecular magnetism [6], luminescent materials
[7] and molecular probes [8].
1D, 2D, and 3D frameworks. Such functional groups are typically
connected to the phenoxy unit of salen, and examples of this include
pendant hydroxy or alkoxy groups [16,23–26], pyridyl groups [27],
polyether macrocycles [28–31] and carboxylates [32–34]. Among
these, we are particularly interested in carboxylato functionalized
metal–salen complexes, because of the capability of aryl carboxylates
to function as versatile building blocks in metal–organic frameworks
(MOFs) [35]. Such compounds have potential for applications as porous
materials, heterogeneous catalysis and sensors [36–38].
Salen is an archetypal bis-Schiff base type ligand containing a N2O2
binding domain and it and its derivatives have become important
building blocks for the syntheses of a variety of metal complexes
[9–14]. Indeed, salen ligands form mononuclear M-N2O2 complexes
with a wide range of metal ions across the periodic table, and the geom-
etries encompass 4-, 5- and 6-coordination numbers [15]. Moreover, the
coordination versatility of the N2O2 binding sites in salen allows it to
form binuclear, trinuclear and polynuclear complexes [16]. For instance,
the O-atoms of the M-N2O2 motif exhibit the capability of binding another
metal center through μ2-O-bridging modes [17]. Moreover, the near
square planar M-N2O2 unit has a tendency to bind axial ligands, giving
the potential for extension of the complex assembly [18–22]. Additional
functional groups can also be introduced into the salen ligand, allowing
extension of the mononuclear complexes to multimetallic complexes or
An example of the carboxylato-functionalized salen ligand has been
reported in the literature to form a 3-D network while sequentially
reacting with copper(II) and zinc(II) ions [34]. However, the poor solu-
bility of the ligand itself has limited its application for the syntheses of a
broader range of crystalline materials, which is likely attributed to both
the rigid Schiff base backbone and the tethered carboxylic acid functional
groups that tend to form strong intermolecular hydrogen bonds. We as-
sumed that reducing the C_N bonds of a functionalized salen ligand
would improve the solubility of the ligand in common organic solvents,
facilitating the formation of new metallosupramolecular structures for
material application. In this paper, we report the synthesis of a carboxylic
acid functionalized reduced salen ligand, namely “salan” and its self-
assembly with copper(II) ions to generate a 2D network. Gao and
Zingaro [39] have recently reported the synthesis of the carboxylic
acid-functionalized ligand, H42, described in this work but, to the best
of our knowledge, its coordination chemistry has not previously been
described.
⁎
Corresponding author.
E-mail address: guzhang@jjay.cuny.edu (G. Zhang).
http://dx.doi.org/10.1016/j.inoche.2014.02.009
1387-7003/© 2014 Elsevier B.V. All rights reserved.

52
G. Zhang et al. / Inorganic Chemistry Communications 43 (2014) 51–55
To avoid handling what would be expected to be a poorly soluble
carboxylic acid tethered ligand, we first considered the analogous
ester. Schiff base condensation of methyl 3-formyl-4-hydroxybenzoate
[40] with (R,R)-1,2-diaminocyclohexane in methanol, followed by
reduction with sodium borohydride afforded the reduced Schiff base
R,R-H21 in good yield (Scheme 1) [41]. The base peak at m/z 443.4
from the electrospray ionization mass spectrum (ESI-MS) of R,R-
H21 is assigned to [M + H]+. R,R-H21 was used directly for the
next steps without further purification. Reaction of R,R-H21 with
Cu(OAc)2 in a CH2Cl2–MeOH solution led to the formation of green
crystals of [Cu(R,R-1)] in 49% isolated yield [42]. The copper(II) complex
was characterized by IR, ESI-MS and elemental analysis. The ESI mass
spectrum exhibited a base peak at m/z 526.2 and the highest peak at
m/z 1029.4, assigned to [Cu(1) + Na]+ and [(Cu(1))2 + Na]+, respec-
tively. The isotope patterns matched those simulated. Single-crystal
structural analysis unambiguously confirmed the formation of a
mononuclear complex [Cu(R,R-1)] (Fig. 1). [Cu(R,R-1)] crystallizes
in a chiral trigonal space group P32 [43]. The metal ion is 4-
coordinate within the salan N2O2 cavity in a slightly distorted
square-planar geometry. The bond lengths and angles are unexcep-
tional and very close to related complexes [13]. In each asymmetric
unit, one methanol molecule cocrystallized with the complex and is
hydrogen bonded to the phenoxy-O atom with an O\H…O distance
of 2.058 Å (O…O = 2.806(5) Å). Such hydrogen bonding interac-
tions between metal–salen (or salan) host and solvent molecules
(usually water or alcohols) have been extensively described in the liter-
ature, and multiple hydrogen bonds are often observed, in particular
when two pendant alkoxy functional groups were introduced to the
3-position of the phenoxy ring [13,16,26]. In the case of [Cu(R,R-1)],
only one hydrogen bond is formed, with the H…O separation being
shorter than those in analogs reported (O3…H100 = 2.058 Å vs
2.26–2.36 Å in analogous complexes) [26]. The Cu(R,R-1) molecules
are packed in the lattice so that ester carbonyl units of adjacent mol-
ecules interact weakly in the axial sites of each copper(II) center
(Cu1…O5i = 3.286(2) and Cu1…O2ii = 3.600(3) Å, symmetry codes
i = 2 − y, 2 + x − y, −1/3 + z, ii = −x + y, 1 − x, 1/3 + z,).
N1
N2
O1
O6
Cu1
O3
O4
O2
O5
O100
Fig. 1. Molecular structure of [Cu(R,R-1)]·MeOH with ellipsoids plotted at the 40%
probability level, and atoms omitted. Selected bond parameters: Cu1\N1
2.0036(17), Cu1\N2 = 1.9974(18), Cu1\O3 = 1.8996(16), Cu1\O4 = 1.8964(17)
Å; O4\Cu1-O3 = 86.90(7), O4\Cu1\N2 = 93.36(7), O3\Cu1\N2 = 179.59(7),
O4\Cu1\N1 = 178.68(7), O3\Cu1\N1 = 93.98(7), N2\Cu1\N1 = 85.76(6)°.
H
=
two doublets (J = 12.9 Hz) at δ 4.46 and 4.32 ppm, reminiscent of
other chiral salan ligands we have previously reported [45], yet
these chemical shifts have moved dramatically to a lower field,
owing to the protonation of the adjacent amine groups.
We next examined the reaction of R,R-H42·2HCl with copper(II) ac-
etate. Treatment of R,R-H42·2HCl with Cu(OAc)2·2H2O in methanol in
the presence of two equivalents of triethylamine immediately gave a
blue suspension. A gray-blue precipitate was isolated from the reaction
mixture by filtration [46]. The product was poorly soluble in common
organic solvents, but soluble in DMF and DMSO. Blue blocks suitable
for single-crystal X-ray diffraction were grown by slow diffusion of
diethyl ether into a DMF solution of the complex. The bulk crystalline
material was also characterized by IR, ESI-MS and elemental analysis.
Only one peak (m/z 498.1) was observed in the ESI mass spectrum in a
dilute MeOH/DMF solution, and was assigned to [Cu(H22) + Na]+; the
isotope distribution matched that simulated. X-ray structural analysis re-
vealed the formation of a complex of stoichiometry [Cu(R,R-H22)], but
the quality of the structure prevented the solvent molecules from
being appropriately modeled, and so the data were treated using the
program SQUEEZE [47].
Hydrolysis of R,R-H21 in 6 M HCl under reflux affords R,R-H42
as a hydrochloride salt (R,R-H42·2HCl) in 66% yield [44]. The salt
R,R-H42·2HCl is soluble in polar protic solvents such as methanol,
ethanol and water, and was fully characterized by 1H and 13C
NMR, IR, ESI-MS and elemental analysis. The only peak in the ESI
mass spectrum appears at m/z 526.2, assigned to [H42 + H]+. 1H and
13C NMR spectra recorded in methanol-d4 were assigned by 2D
techniques and were in accordance with the structure shown in
Scheme 1. Of note is that the diastereotopic protons Ha appear as
[Cu(R,R-H22)]n crystallizes in the chiral tetragonal space group
P42212 [48]. In contrast to the discrete molecular structure of
[Cu(R,R-1)] (Fig. 1), [Cu(R,R-H22)]n forms a 2D network by virtue of
the fact that each peripheral carboxylic acid functionality of the
Scheme 1. Syntheses of R,R-H21, R,R-H42·2HCl and their copper(II) complexes, and labeling for NMR spectroscopic assignment of R,R-H42·2HCl.

G. Zhang et al. / Inorganic Chemistry Communications 43 (2014) 51–55
53
a
N1
N1a
O2
O3
O2
O3a
Cu1
O1 O2b O1a
Cu1
O2b
b
H-bond
H-bond
Fig. 2. (a) The repeating structure unit of [Cu(R,R-H22)]n in the asymmetric unit with ellipsoids plotted at the 40% probability level, and (b) a highlighted view of the hydrogen bonds. Selected
bond parameters: Cu1\O1 = 1.9376(17), Cu1\N1 = 1.993(2), Cu1\O2 = 2.417(2) Å; O1\Cu1\O1a = 86.35(10), O1\Cu1\N1 = 92.82(8), O1a\Cu1\N1 = 177.20(12),
N1a\Cu1\N1 = 88.14(13), O1\Cu1\O2b = 98.25(9), O1a\Cu1\O2b = 87.14(9), N1\Cu1\O2b = 90.34(10), N1a\Cu1\O2b = 84.37(10)°. Symmetry codes: a = −x, −y,
z; b = x + 1/2, −y + 1/2, −z + 1/2.
coordinated ligand binds to the copper(II) center in an adjacent unit
(Fig. 2). Each CO2H coordinates through the carbonyl unit. An ORTEP-
type structure of the repeating unit in [Cu(R,R-H22)]n is shown in Fig. 2a,
and important bond parameters are given in the figure caption. Atom Cu1
resides on the special position 0, y, 1/2, indicating the Cu atom is on a 2-
fold axis. The asymmetric unit contains half of the coordination sphere
of the CuII ion, and the second half is generated by symmetry (with sym-
metry codes shown in the figure caption of Fig. 2). Similar to [Cu(R,R-1)],
the copper(II) ion in [Cu(R,R-H22)]n is bound in a near square planar en-
vironment by the N2O2 domain of the salan unit with Cu\O and Cu\N
bond lengths of 1.9376(17) and 1.993(2) Å, respectively. Compared to
[Cu(R,R-1)], the distinguishing feature of [Cu(R,R-H22)]n is that the
copper(II) center also binds axially to two carboxy oxygen atoms from ad-
jacent salan–CuII moieties. A relatively long Cu\Ocarbonyl distance of
2.417(2) Å is observed, and the carboxylic acid remains undeprotonated.
The O3\H3 unit of the free carboxylic acid forms a hydrogen bond to
the salan O1 atom with a very short O3\H3…O1 distance of 1.814 Å
(O1…O3 = 2.647(3) Å, angle O1…H3\O3 = 171°), indicating strong
hydrogen bonding interactions as shown in Fig. 2b. The coordination
and hydrogen bonding interactions between CuII centers and the
bis(carboxylic acid) units thus facilitate the formation of a 2D grid-like
network, with the size of each square grid being approximately
8.2 Å × 10.0 Å as shown in Fig. 3a. Furthermore, the 2D sheets are
interlocked by the interdigitation of cyclohexyl groups to form an
extended 3D porous framework with 1D channels parallel to the
crystallographic c-axis, that run through those large square grids
as seen in the 2D network. The channels are in reality occupied
with disordered solvent molecules (Fig. 3b and c).
In conclusion, we have synthesized two chiral salan-type ligands
functionalized by ester (R,R-H21) and carboxylic acid (R,R-H42) groups,
respectively. Upon reacting R,R-H21 with copper(II) acetate, the mono-
nuclear complex [Cu(R,R-1)] has been isolated. The crystal structure of
[Cu(R,R-1)] reveals that the copper(II) ion resides in a near square
planar environment in the salan N2O2 cavity. The copper(II) complex
of ligand R,R-H42 is the 2D coordination network [Cu(R,R-H22)]n.
Each copper(II) center is sited within the near square-planar N2O2
domain of the [R,R-H22]2− ligand and the pendant carboxylic acid
groups of adjacent units coordinate through the CO2H carbonyl units
in the axial sites to complete an axially distorted octahedral coordina-
tion environment. Stacking of sheets involves interdigitation of the
cyclohexyl domains giving a porous architecture featuring carboxylic
acid functionality. Although a few salen-based 3D networks are
known in the literature [33,34], the formation of the chiral 2D
[Cu(R,R-H22)]n structure is unusual. Further efforts to utilize this
copper(II) complex as a metalloligand to construct new metal–organic
materials and investigations of their applications in heterogeneous
catalysis are undergoing in our laboratory.
Acknowledgments
We thank the University of Basel and the Novartis Foundation,
formerly Ciba-Geigy Jubilee Foundation for support. A Seed grant from

54
G. Zhang et al. / Inorganic Chemistry Communications 43 (2014) 51–55
the Office for the Advancement of Research at CUNY John Jay College to
G.Z. is also acknowledged for partial support to this work.
a
Appendix A. Supplementary materials
Crystallographic data for the structural analysis have been deposited
with the Cambridge Crystallographic Data Center as supplementary
publication nos. CCDC 973646 and 973647. Copies of the data can be
obtained free of charge via www.ccdc.ac.uk/conts/retrieving.html
(or from The Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK.
Fax: +44-1223-336-033. E-mail: deposit@ccdc.cam.ac.uk).
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[41] R,R-H21: A solution of (1R,2R)-1,2-diaminocyclohexane (0.571 g, 5.00 mmol)
Fig. 3. (a) The ball-stick representation of an extended 2D network, (b) a view of the
stacked 2D sheets along the crystallographic a axis, and (c) the space-filling representation
of the 3D stacking of sheets in [Cu(R,R-H22)]n viewed along the crystallographic c axis.
in MeOH (10 cm3) was added dropwise to
a stirred solution of methyl
3-formyl-4-hydroxybenzoate (1.80 g, 10.0 mmol) in MeOH (40 cm3). The resulting
mixture was stirred at room temperature for 4 h. Solid NaBH4 (1.90 g, 50.0 mmol)
was then added in small portions over a period of 1 h. The resulting colourless so-
lution was allowed to stir at room temperature for additional 3 h, after which, sat.

G. Zhang et al. / Inorganic Chemistry Communications 43 (2014) 51–55
55
NaHCO3 (20 cm3) was added, resulting in white precipitate, which was extracted
with CHCl3 (3 × 60 cm3). The combined organic solutions were washed with
water and brine, and dried over anhydrous Na2SO4. The solution was filtered and
concentrated under reduced pressure to give off-white solid (2.03 g, 92%). The crude
product was used directly for the next step. ESI-MS (MeOH) m/z 443.4 [M + H]+
(base peak, calc. 443.2), 465.3 [M + Na]+ (calc. 465.2), 907.5 [2M + Na]+ (the highest
peak, calc. 907.4).
HA4), 4.46 (d, J = 12.9 Hz, 2H, Ha), 4.32 (d, J = 12.9 Hz, 2H, Ha′), 3.68 (br, 2H, HB2),
2.43 (d, J = 13.6 Hz, 2H, HB3), 1.88 (br, 2H, HB4), 1.78 (br, 2H, HB3′), 1.51 (m, 2H,
HB4′). 13C NMR (126 MHz, CD3OD) δ 169.3 (CCOOH), 162.0 (CA2), 135.4 (CA6), 134.8
(CA3), 123.8 (CA1), 118.9 (CA5), 116.3 (CA4), 58.4 (CB2), 46.1 (Ca), 27.5 (CB3), 23.4
(CB4). FT-IR (solid, cm−1): 2948br, 1683s, 1652s, 1558s, 1374m, 1285m, 1176w,
1121w, 840m, 771m. ESI-MS (MeOH) m/z 415.2 [M + H]+ (base peak, calc. 415.2).
Found C 48.60, H 6.08, N 5.14; C22H28Cl2N2O6·3H2O requires C 48.81, H 6.33, N 5.17%.
[45] E.C. Constable, G. Zhang, C.E. Housecroft, M. Neuburger, S. Schaffner, W.-D. Woggon,
J.A. Zampese, New J. Chem. 33 (2009) 2166–2173.
[46] Cu(R,R-H22)n: To a stirred solution of R,R-H22·2HCl (54.0mg, 0.100 mmol) in MeOH
(5 cm3) was added triethylamine (20.2mg, 0.200 mmol), then a solution of
Cu(OAc)2·2H2O (21.7 mg, 0.10mmol) in MeOH (5 cm3) at room temperature, a
blue-green suspension formed upon stirring, which was continued for 30min,
then filtered by suction, washed with MeOH (5 cm3), and dried in vacuo. Yield:
40.0 mg (84.2%). Single-crystal suitable for X-ray analysis was grown by slow diffu-
sion of diethyl ether into a DMF solution of the complex over three days. ESI-MS
(MeOH) m/z 498.1 [M + Na+] (base peak, calc. 498.1), 973.3 [2M + Na+] (calc.
973.2), the peak envelopes matched those simulated. FT-IR (solid, cm−1): 3435br,
3203w, 2938w, 1652s, 1635s, 1601s, 1553w, 1490m, 1435w, 1410m, 1308s,
1300s, 1260s, 1244s, 1141m, 1127s, 1114m, 1055w, 934m, 911m, 836s, 782m,
650s. Found C 52.56, H 5.45, N 5.96; C22H24CuN2O6·1.5H2O requires C 52.53, H
5.41, N 5.57%.
[42] [Cu(R,R-1)]·MeOH: To a solution of H21 (44.2 mg, 0.10 mmol) in CH2Cl2 (10 cm3),
was added dropwise a solution of Cu(OAc)2 · 2H2O (21.7 mg, 0.10 mmol) in MeOH
(5.0 cm3) under stirring at room temperature, the resulting green solution was
stirred for 30 min, then filtered. The filtrate was allowed to slowly evaporate at
room temperature for 5 days, green blocks suitable for X-ray analysis were obtained,
the solvent was decanted and solid washed with MeOH, dried in the air. Yield:
26.2 mg (49%). FT-IR (solid, cm−1): 3216m, 1688s, 1596s, 1488s, 1429s,1320s,
1290s, 1259s, 1242s, 1190w, 1123s, 1043w, 978w, 913m, 842w, 771m, 651s.
ESI-MS (CH2Cl2/MeOH) m/z 526.2 [M + Na]+ (base peak, calc. 526.1), 1029.4
[2M + Na]+ (calc. 1029.2). Found C 54.17, H 5.83, N 5.30; C24H28CuN2O6·CH3-
OH·H2O requires C 54.19, H 6.18, N 5.06%.
[43] [Cu(R,R-1)]·MeOH: C25H32CuN2O7, M = 536.08, black block, trigonal, space
group P32, a = b = 14.267(3), c = 10.151(4) Å, U = 1789.4(8) Å3, Z = 3,
Dc = 1.492 Mg m−3, μ(Mo-K α) = 0.965 mm−1, T = 173 K. Total 37254 reflec-
tions, 5045 unique, Rint = 0.0504. Refinement of 5045 reflections (321 parameters)
with I N 2σ (I) converged at final R1 = 0.0222 (R1 all data = 0.0222), wR2 = 0.0603
(wR2 all data = 0.0603), gof = 1.061, Flack = 0.003 (7)
[47] A.L. Spek, Acta Crystallogr. Sect. D: Biol. Crystallogr. 65 (2009) 148–155.
[48] Cu(R,R-H22)n: C22H24CuN2O6, M = 475.98, blue block, tetragonal, space group P42212,
[44] R,R-H42·2HCl: H21 (0.550 g, 1.24 mmol) was dissolved in aqueous HCl (4 M, 20
cm3), the mixture was heated to reflux for 24 h, then cooled to room temperature.
White precipitate was collected by filtration, washed with acetone (5.0 cm3),
dried in vacuo over P2O5. Yield: 0.400 g (66%). 1H NMR (500 MHz, CD3OD) δ 8.15
(d, J = 2.1 Hz, 2H, HA6), 7.97 (dd, J = 2.2, 8.5 Hz, 2H, HA3), 6.99 (d, J = 8.6 Hz, 2H,
a = b = 14.052(2), c = 20.056(4) Å, U = 3960.3(12) Å3, Z = 4, Dc = 0.798 Mg m−3
,
μ(Mo-Kα) = 0.574 mm−1, T = 173(2) K. Total 3502 reflections, 2956 unique.
Refinement of 2956 reflections (142 parameters) with I N2σ (I) converged at
final R1 = 0.0431 (R1 all data = 0.0508), wR2 = 0.1072 (wR2 all data = 0.1104),
gof = 1.035, Flack = −0.06 (3).