A novel polymeric copper(II) compound containing peripheral nitro oxygen bridge and μ-OH core: An unprecedented tetradentate bonding mode of a potentially tridentate Schiff base

Article abstract of DOI:10.1016/j.inoche.2012.05.002

A copper(II) compound ([LCu(μ-OH)CuL(H₂O)]ClO ₄)_n (1) [where HL = 2-[(2-dimethylamino-ethylimino)- methyl]-4-nitro-phenol, is a well known Schiff base], has been synthesized and characterized by elemental analysis, IR and UV-Vis spectroscopy and single-crystal X-ray diffraction studies. The compound features a copper(II) polymeric coordination network composed of binuclear species [LCu(μ-OH)CuL(H₂O)]⁺ as building blocks. It crystallizes in monoclinic space group P2₁/c with cell dimensions a = 22.1453(9), b = 7.8314(3), c = 18.5626(8) and β=108.509(3). The potentially tridentate Schiff base shows a tetra-dentate bonding mode for the 1st time.

Full text of DOI:10.1016/j.inoche.2012.05.002

Inorganic Chemistry Communications 22 (2012) 14–17
Contents lists available at SciVerse ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
A novel polymeric copper(II) compound containing peripheral nitro oxygen bridge
and μ-OH core: An unprecedented tetradentate bonding mode of a potentially
tridentate Schiff base
⁎
Prasanta Kumar Bhaumik, Shouvik Chattopadhyay
Department of Chemistry, Inorganic Section, Jadavpur University, Kolkata – 700 032, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A copper(II) compound ([LCu(μ-OH)CuL(H2O)]ClO4)n (1) [where HL=2-[(2-dimethylamino-ethylimino)-
methyl]-4-nitro-phenol, is a well known Schiff base], has been synthesized and characterized by elemental
analysis, IR and UV–Vis spectroscopy and single-crystal X-ray diffraction studies. The compound features a
copper(II) polymeric coordination network composed of binuclear species [LCu(μ-OH)CuL(H2O)]+ as build-
ing blocks. It crystallizes in monoclinic space group P21/c with cell dimensions a=22.1453(9), b=7.8314(3),
c=18.5626(8) and β=108.509(3). The potentially tridentate Schiff base shows a tetra-dentate bonding
mode for the 1st time.
Received 13 March 2012
Accepted 1 May 2012
Available online 8 May 2012
Keywords:
Copper(II)
Polymer
Schiff base
μ-OH
© 2012 Elsevier B.V. All rights reserved.
Crystal structure
The study of metal coordination polymers with fascinating structur-
al frameworks has drawn considerable interest in the field of catalytic
biological activities, molecular-based magnets, enhanced electrical con-
ductivity etc. over the past decades [1–3]. The self-assembly of the li-
gands with versatile coordination modes assist the formation of the
coordination polymers [4]. Over the years, particular importance has
been placed on them because of their pivotal roles in biology [5]. More-
over, additional functional groups incorporated into the ligand moiety
have already been proved to be synthetically more useful as they may
interact with the guest molecules to form supra-molecular architecture
[6,7]. In the present work, we have utilized the nitro functionality of
5-nitrosalicylaldehyde to construct an extended network.
occupied by a bridging phenolato oxygen atom of another “Cu(Ligand)”
moiety, giving rise to dinuclear species [Cu2(Ligand)2]2+ [9,10]. Tri-
nuclear species [Cu3(Ligand)3(μ3-OH)]2+ may also be produced by
adding a base and thereby increasing the pH of the medium [11,12].
We have chosen the ligand HL {2-[(2-dimethylamino-ethylimino)-
methyl]-4-nitro-phenol} for preparing a dinclear species [LCu(μ-OH)
CuL(H2O)]+. This dinclear species underwent polymerization utilizing
the nitro functionality of the Schiff base. The tetradenticity of the poten-
tial tridentate ligand (HL) was not observed in a synthetic compound till
date. Herein, we report the synthesis and characterization of a unique
copper(II) polymer [LCu(μ-OH)CuL(H2O)]n(ClO4)n (1).
Elemental analysis (carbon, hydrogen and nitrogen) was performed
on a Perkin–Elmer 240C elemental analyzer. Infrared spectrum in KBr
(4500–500 cm−1) was recorded using a Perkin–Elmer RXI FT-IR spectro-
photometer. Electronic spectrum in acetonitrile (900–200 nm) was
recorded in a Hitachi U-3501 spectrophotometer. Fluorescence spectrum
was obtained on HORIBA Jobin Yvon FluoroMax-P spectrophotometer at
room temperature. Cyclic voltammetry was performed on a CH instru-
ment (model CHI720D). The magnetic susceptibility measurement was
done with an EG & PAR vibrating sample magnetometer, model 155 at
room temperature and diamagnetic corrections were made using Pascal's
constants.
Copper(II), being a d9 system, likes better to undergo Jahn–Teller
distortion, leading to an elongated octahedral (4+2) coordination.
The strong-field ligands coordinate to the four equatorial sites, leav-
ing the two axial sites vacant. When one of the four coordination
sites of the square plane of the metal ion is occupied by a strongly co-
ordinating anion, the tridentate mono-condensed Schiff base is stabi-
lized through coordination to the three remaining sites, but with
weakly coordinating counter anions, all four equatorial sites of
copper(II) are to be coordinated preferably by strong-field ligands,
and this requirement may be fulfilled by the hydrolytic cleavage of
the tridentate Schiff base to yield two bidentate strong-field chelating
ligands which can occupy four equatorial sites [8]. In the absence of an
auxiliary ligand, the fourth coordination site of copper(II) may be
A clear yellow solution (25 ml) of the ligand, 2-[(2-dimethylamino-
ethylimino)-methyl]-4-nitro-phenol (HL), was synthesized by refluxing
a methanol solution of 5-nitrosalicylaldehyde (167 mg, 1 mmol) and N,
N-dimethyl-1,2-diaminoethane (0.1 ml, 1 mmol) in 1:1 ratio for 1 hr fol-
lowing the literature method [13]. It was not purified and was used di-
rectly for the preparation of the compound 1. The methanol solution of
HL was refluxed with copper(II) perchlorate under basic condition to
⁎
Corresponding author. Tel.: +91 9007777373.
E-mail address: shouvik.chem@gmail.com (S. Chattopadhyay).
1387-7003/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2012.05.002

P.K. Bhaumik, S. Chattopadhyay / Inorganic Chemistry Communications 22 (2012) 14–17
15
Scheme 1. Formation of the tridentate NNO Schiff base ligand, HL and the compound 1.
prepare compound 1 [14]. X-ray quality single crystals of 1 were obtained
from methanol solution by slow evaporation at room temperature [15].
The details of the synthesis are shown in Scheme 1.
of decomposition as reflected in the smooth curve, in the potential
range from −2 to 2 V and were uncorrected for junction contribution.
The value for the Fc–Fc+ couple under our conditions is 0.39 V.
Compound 1 crystallizes in a P21/c space group. The structure con-
sists of a one-dimensional infinite chain system with the binuclear re-
peating unit [LCu(μ-OH)CuL(H2O)]+ bridged by nitro groups. The
perspective view with the atom labelling scheme of the dimeric unit
[LCu(μ-OH)CuL(H2O)]+ is shown in Fig. 1. A view of the extended
chain is displayed in Fig. 2. Selected bond angles are summarized in
Table S1. The shortest Cu···Cu distance in the polymer is 3.137 Å.
Two molecules of the tridentate Schiff base HL, one coordinated
water molecule, one bridging hydroxide and two independent copper
atoms Cu(1) and Cu(2), having distorted octahedral and square-
pyramidal geometries respectively, are present in the binuclear unit.
One of the two copper atoms, Cu(2) is penta-coordinated, being
bonded to N(4), N(5) and O(4) of a tridentate Schiff base, another
oxygen O(100) of a hydroxide in the basal plane and one oxygen
atom, O(200), of a water molecule in the apical position. Cu(1), on
the other hand, is hexa-coordinated. The basal plane is constructed
by N(1), N(2) and O(1) of a tridentate Schiff base and another oxygen
O(100) of the hydroxide. One oxygen atom O(4) from the first Schiff
base unit is coordinated to the apical position. Another oxygen atom,
O(5)* from a symmetry-related (x,1/2-y,1/2+z) Schiff base unit
binds the sixth coordination site to form the polymer.
Fig. 1. A perspective view of the dimer of compound 1. Hydrogen atoms and perchlo-
rate anion are not shown for clarity. Selected bond distances (Å): Cu(1)–O(1)
1.944(5), Cu(1)–O(4) 2.432(5), Cu(1)–O(100) 1.908(5),Cu(1)–N(1) 2.058(6), Cu(1)–
N(2) 1.936(7), Cu(1)–O(5)* 2.794(5), Cu(2)–O(4) 1.939(5), Cu(2)–O(100) 1.892(5),
Cu(2)–O(200) 2.594(8), Cu(2)–N(4) 2.063(11) and Cu(2)–N(5) 1.928(8). Symmetry
elements *=x,1/2-y,1/2+z. Bond angles are given in supplementary Table S1.
The cyclic voltammogram was recorded in acetonitrile solution con-
taining TBAP as supporting electrolyte at a glassy carbon, Ag/Ag+ (non-
aquous) and a platinum wire as working, reference and auxiliary
electrodes respectively at ambient temperature (300 K) with no trace

16
P.K. Bhaumik, S. Chattopadhyay / Inorganic Chemistry Communications 22 (2012) 14–17
Fig. 2. 1-D polymeric structure of compound 1. Only the relevant atoms are shown.
The single-electron nature of the voltammogram has been confirmed
spectrum of compound 1, a broad band at 3430 cm−1 is observed and
may be assigned to OH stretching vibration (Figure S1). The electronic
spectrum of 1 in acetonitrile solution displays a single absorption band
at 617 nm (Fig. 4). Both the ligand (HL) and the compound (1) exhibit lu-
minescence in acetonitrile medium (Figure S2 and Fig. 5). The lumines-
cence data are listed in Table 2 (without solvent correction). These are
assigned to intra-ligand 1(π–π*) fluorescence [22]. Room temperature
magnetic susceptibility measurement showed that the compound has
χMT value 0.53 cm3 K mol−1 at 302 K, which is somewhat lower than
the value expected for similar systems [23]. The magnetic moment is
2.07 BM at that temperature.
by the comparison of current heights for compound 1 and that of a sim-
ple [Fe(bipy)3]2+ compound under identical conditions [18]. Cyclic
voltammetric study of the compound in acetonitrile solution under ni-
trogen atmosphere shows quasi-reversible electron transfer at 0.659 V
with ΔEP 273 mV (Fig. 3). The E1/2 and ΔEp values for the redox couple
are given in Table 1. The high E1/2 value for the latter case can be
The isolation and crystal structure of the compound unambiguously
show that the Schiff base, which is generally used as tridentate ligand,
can also act as tetradentate ligand under special circumstances. Such
bonding mode is unprecedented for these types of ligands. Thus the in-
troduction of free nitro functionality in the tridentate Schiff base is
proved to be an excellent strategy to construct an 1D polymer with
hydroxo-bridged dimeric copper(II) moiety, [LCu(μ-OH)CuL(H2O)]+ as
building units.
Acknowledgments
This work was supported by the University Grants Commission,
CAS-UGC, New Delhi. Crystallographic data were performed at the
DST-FIST, India-funded Single Crystal Diffractometer Facility at the
Department of Chemistry, Jadavpur University.
Fig. 3. The cyclic voltammogram for compound 1 in acetonitrile medium.
explained on the basis of electron withdrawing power of the coordinated
nitro group which facilitates oxidation of copper(II). All the redox signals
remain virtually invariant under different scan rates (0.01–1.0 Vs−1) in
the temperature range 300–280 K.
The IR band, corresponding to the CN stretching vibration, appears at
1627 cm−1 [19]. The absorption band due to the perchlorate anion ap-
pears at around 1100 cm–1 [20]. An intense band appears at 1310 cm−1
due to the presence of the nitro group [21]. Interestingly, in the IR
Table 1
Electrochemical data for the copper(II) centre of the compound 1.
E
Pa(V)
EPc(V)
E1/2/V (ΔEP/mV)
CuII/I couple
0.796
0.523
0.659 (273)
E1/2 is calculated as the average of anodic (Epa) and cathodic (Epc) peak potentials;
ΔEp =(Epa −Epc).
Fig. 4. The UV–vis spectrum of compound 1 in acetonitrile medium.

P.K. Bhaumik, S. Chattopadhyay / Inorganic Chemistry Communications 22 (2012) 14–17
17
[8] B. Graham, M.T.W. Hearn, P.C. Junk, C.M. Kepert, F.E. Mabbs, B. Moubaraki, K.S.
Murray, L. Spiccia, Syntheses, crystal structures, magnetic properties, and EPR
spectra of tetranuclear copper(II) complexes featuring pairs of “roof-shaped”
Cu2X2 dimers with hydroxide, methoxide, and azide bridges, Inorg. Chem. 40
(2001) 1536–1543;
(b) J. Sletten, A. Soerensen, M. Julve, Y. Journaux, A tetranuclear hydroxo-
bridged copper(II) cluster of the cubane type. Preparation and structural
and magnetic characterization of tetrakis[(2,2′-bipyridyl)hydroxocopper(II)]
hexafluorophosphate, Inorg. Chem. 29 (1990) 5054–5058;
(c) K.S. Murray, The Magnetochemistry of homo and hetero-tetranuclear first-
row d-block complexes, Adv. Inorg. Chem. 43 (1995) 261–358;
(d) M. Melnik, Study of the relation between the structural data and magnetic
interaction in oxo-bridged bltnuclear copper(II) compounds, Coord. Chem.
Rev. 42 (1982) 259–293.
[9] W.H. Gu, X.-Y. Chen, L.-H. Yin, A. Yu, X.-Q. Fu, P. Cheng, A ferromagnetically
coupled tetranuclear pseudo-cubane copper(II) cluster: magnetic and ESR stud-
ies, Inorg. Chim. Acta 357 (2004) 4085–4090;
(b) Y. Song, C. Massera, O. Roubeau, P. Gamez, A.M.M. Lanfredi, J. Reedijk, An un-
usual open cubane structure in a μ1,1-azido- and alkoxo-bridged tetra-
nuclear copper(II) complex, [Cu4L2(μ1,1–N3)2]·5H2O (H3L
= N, N′-(2-
Hydroxylpropane-1,3-diyl)bis-salicylideneimine), Inorg. Chem. 43 (2004)
6842–6847.
[10] E.C. Alyea, A. Malek, Synthesis and characterization of some dibasic tridentate
schiff base ligands, Can. J. Chem. 53 (1975) 939–944;
Fig. 5. The fluorescent spectrum of compound 1 in acetonitrile medium.
(b) C.J. Carrano, C.M. Nunn, R. Quan, J.A. Boandies, V.L. Pecoraro, Monomeric and di-
meric vanadium(IV) and -(V) complexes of N-(hydroxyalkyl)salicylideneamines:
structures, magnetochemistry and reactivity, Inorg. Chem. 29 (1990) 944–951.
[11] A.W. Addison, T.N. Rao, J. Reedijk, J. van Rijn, G.C. Verchoor, Synthesis, struc-
ture, and spectroscopic properties of copper(II) compounds containing
nitrogen–sulphur donor ligands; the crystal and molecular structure of aqua
[1,7-bis(N-methylbenzimidazol-2′-yl)-2,6-dithiaheptane]copper(II) perchlorate, J.
Chem. Soc. Dalton Trans. (1984) 1349–1356.
Table 2
Photophysical data for the Ligand (HL) and the compound 1.
Excitation (nm)
Emission (nm)
[12] S. Chattopadhyay, M.G.B. Drew, A. Ghosh, Methylene spacer-regulated structural
variation in cobalt(II/III) complexes with bridging acetate and salen- or salpn-
type schiff-base ligands, Eur. J. Inorg. Chem. (2008) 1693–1701.
[13] Z.L. You, H.L. Zhu, A novel thiocyanate-bridged dinuclear cadmium(II) complex:
di-μ-thiocyanato-bis((methanol){4-nitro-2-[2-(dimethylamino)
HL
352
375
400
250
357
390
470
470
471
421
432
442
1
ethyliminomethyl]phenolato} cadmium(II)), Acta Crystallogr. Sect. C: Cryst.
Struct. Commun. 61 (2005) m397–m399.
[14] A methanol solution (20 ml) of Cu(ClO4)2. 6H2O (370 mg, 1 mmol) was added to
a methanol solution (25 ml) of ligand HL (1 mmol). Then the mixture was
refluxed for 1 hr and cooled to room temperature. Single crystals, suitable for
X-ray diffraction, were grown on slow evaporation of the mother liquor in open
atmosphere. Yield: 320 mg (43.6 % ). Anal. Calcd. (%) for C22H31ClCu2N6O12: C,
36.00; H, 4.26; N, 11.45. Found: C, 36.3; H, 4.2; N, 11.2. λmax (nm) [εmax
(dm3mol–1 cm–1)] (acetonitrile) 617 (3.9×102), 360 (3.9×104) nm.
[15] Single crystals of the compound having suitable dimensions were used for data collec-
tion by means of a 'Bruker SMART APEX II' diffractometer equipped with graphite-
monochromated Mo-Kα radiation (λ = 0.71073 Å) at 293 K. The structure was
solved by direct methods and refined by full-matrix least squares on F2, using the
SHELX-97 package [16]. Non-hydrogen atoms were refined anisotropically. Hydrogen
atoms were placed in their geometrically idealized positions and constrained to ride
on the atoms to which they are attached. Empirical absorption corrections were car-
ried out with the ABSPACK program [17]. The structures were refined on F2 using
SHELXL-97 to R1 =0.0614 and wR2=0.1597 for 2170 reflections with I>2σ(I) re-
spectively. Crystal data for 1: Formula C22H31ClCu2N6O12 Formula Weight 734.08,
Crystal system Monoclinic, Space group P21/c, a(Å)=22.1453(9), b(Å)=7.8314(3),
c(Å)=18.5626(8), β(deg)=108.509(3), Z=4, dcalc(g cm-3)=1.597, F(000)=1504,
Total Reflections 15001, Unique Reflections 2663.
[16] G.M. Sheldrick, SHELXS-97 and SHELXL-97, University of Göttingen, Germany,
1997.
[17] ABSPACK, version 1, Oxford Diffraction, Abingdon, 2005.
[18] K. Mitra, S. Biswas, C.R. Lucas, B. Adhikary, Manganese(III) complexes of N2O2
donor 5-bromosalicylideneimine ligands: Combined effects of electron with-
drawing substituents and chelate ring size variations on electrochemical proper-
ties, Inorg. Chim. Acta 359 (2006) 1997–2003.
[19] V.P. Singh, P. Singh, A.K. Singh, Synthesis, structural and corrosion inhibition studies
on cobalt(II), nickel(II), copper(II) and zinc(II) complexes with 2-acetylthiophene
benzoylhydrazone, Inorg. Chim. Acta 379 (2011) 56–63.
Appendix A. Supplementary data
Crystallographic data for the analysis have been deposited with
the Cambridge Crystallographic data Centre, CCDC No. 869694. Copies
of this information may be obtained free of charge from CCDC, 12
Union Road, Cambridge, CB21EZ, UK (fax: +44-1223-336033; e-
mail: deposit@ccdc.cam.ac.uk or www: http://www.ccdc.cam.ac.uk).
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.inoche.2012.05.002
References
[1] L. Toma, R. Lescouezec, J. Vaissermann, F.S. Delgado, C. Ruiz-Perez, R. Carrasco, J.
Cano, F. Lloret, M. Julve, Nuclearity controlled cyanide-bridged bimetallic CrIII
–
MnII compounds: synthesis, crystal structures, magnetic properties and theoreti-
cal calculations, Chem. Eur. J. 10 (2004) 6130–6145.
[2] M. Verdaguer, A. Bleuzen, V. Marvaud, J. Vaissermann, M. Seuleiman, C.
Desplanches, A. Scuiller, C. Train, R. Garde, G. Gelly, C. Lomenech, I. Rosenman,
P. Veillet, C. Cartier, F. Villain, Molecules to build solids: high TC molecule-based
magnets by design and recent revival of cyano complexes chemistry, Coord.
Chem. Rev. 190 (1999) 1023–1047.
[3] M. Ohba, H. Okawa, Synthesis and magnetism of multi-dimensional cyanide-
bridged bimetallic assemblies, Coord. Chem. Rev. 198 (2000) 313–328.
[4] Z.N. Chen, H.X. Zhang, K.B. Yu, K.C. Zheng, H. Cai, B.S. Kang, Extended network via hy-
drogen bond linkages of zig-zag coordination chains [{Cu2(trans-oxen)(μ-OH)(μ-H2-
O)}n]n+ or [Cu2(trans-oxen)(μ-OCN)2]n [H2oxen = N,N'-bis(2-aminoethyl)-oxamide],
J. Chem. Soc. Dalton Trans. (1998) 1133–1136.
[5] M. Fainerman-Melnikova, T. Szabo´-Pla´nka, A. Rockenbauer, R. Corrd, Coordina-
tion modes between copper(II) and N-Acetylneuraminic (Sialic) acid from a
2D-simulation analysis of EPR spectra. Implications for copper mediation of
sialoglycoconjugate chemistry relevant to human Biology, Inorg. Chem. 44
(2005) 2531–2543.
[20] J. Ma, X. Huang, R. Wei, L. Zhou, W. Liu, Hydrothermal synthesis and crystal struc-
tures of two lanthanide coordination polymers with novel tetradentate-
coordinated perchlorates, Inorg. Chim. Acta 362 (2009) 3440–3446.
[21] N.M. Karayannis, S.D. Sonsino, C.M. Mikulski, M.J. Strocko, L.L. Pytlewski, M.M.
Labes, Mono- and bi-dentate nitrato and cationic 4-ethoxypyridine N-oxide com-
plexes with metal nitrates, Inorg. Chim. Acta 4 (1970) 141–144.
[22] N.J. Turro, Energy transfer processes, Pure Appl. Chem. 49 (1977) 405–429.
[23] S. Mukhopadhyay, D. Mandal, P.B. Chatterjee, C. Desplanches, J.P. Sutter, R.J.
Butcher, M. Chaudhury, Bi- and trinuclear copper(II) complexes of a sterically
constrained phenol-based tetradentate ligand: syntheses, structures, and mag-
netic studies, Inorg. Chem. 43 (2004) 8501–8509.
[6] Z. Wang, S.M. Cohen, Postsynthetic modification of metal–organic frameworks,
Chem. Soc. Rev. 38 (2009) 1315–1329.
[7] H. Deng, C.J. Doonan, H. Furukawa, R.B. Ferreira, J. Towne, C.B. Knobler, B. Wang,
O.M. Yaghi, Multiple functional groups of varying ratios in metal–organic frame-
works, Science 327 (2010) 846–850.