Synthesis, crystal structure, and catalytic studies on dinuclear copper(II) mesocates

Article abstract of DOI:10.1016/j.ica.2011.04.033

Imine based bis-bidentate ligands H₂-m-xysal, (L ₁H₂); H₂-m-xysal-Cl, (L₂H ₂); H₂-m-xysal-Br, (L₃H₂); H ₂-m-xysal-OCH_3, (L₄

Full text of DOI:10.1016/j.ica.2011.04.033

Inorganica Chimica Acta 375 (2011) 106–113
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Inorganica Chimica Acta
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Synthesis, crystal structure, and catalytic studies on dinuclear copper(II) mesocates
P. Mosae Selvakumar a, Sandeep Nadella a, K. Jeya Prathap b, R.I. Kureshy b, E. Suresh a, P.S. Subramanian a,
⇑
a Analytical Science Division, Central Salt and Marine Chemicals Research Institute (CSIR), G.B. Marg, Bhavnagar 364 002, Gujarat, India
b Discipline of Inorganic Materials and Catalysis, Central Salt and Marine Chemicals Research Institute (CSIR), G.B. Marg, Bhavnagar 364 002, Gujarat, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Imine based bis-bidentate ligands H2-m-xysal, (L1H2); H2-m-xysal-Cl, (L2H2); H2-m-xysal-Br, (L3H2); H2-
m-xysal-OCH3, (L4H2); H2-m-xysal-(t-Bu)2, (L5H2) were synthesized and characterized. These substituted
1,3-bis(hydroxylbenzyl)-diaminoxylene dianion ligands upon treating with copper(II) acetate in 2:2
equivalent of L:M ratio, resulted in a series of binuclear [Cu2(m-xysal)2] neutral complexes 1–5. The crys-
Received 28 January 2011
Received in revised form 29 March 2011
Accepted 20 April 2011
Available online 28 April 2011
tal structures determined for the complexes 1 and 2 indicate a dinuclear association. The CHꢀ ꢀ ꢀ
p interac-
tion observed between the metal-chelate ring and the hydrogens associated with m-xylene spacer moiety
being first in this series of complexes, is demonstrated to stabilize the helical conformation through intra-
molecular self assembly process. The position of the resonance on the EPR spectra and the absence of
Keywords:
Copper(II)
Crystal structure
Catalysis
D
Ms = 1 feature for the complexes 2, 3, and 5 obtained for room temperature solid state samples
revealed that the metal centers though exist in the dinuclear unit, they are separated from each other
and possess a non-interacting monomer-type metal–metal association. The Cu(II) centers in all these
complexes possessing an intermediate geometry between tetrahedral and square planar, an appropriate
catalytic study converting 4-nitrobenzaldehye to corresponding nitroaldol was carried out using
complex 5.
Helicates
Henry reaction
Schiff base
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
though there are many metallohelicates, the preparation of
aa0-bis(salicylidinine)xylene as a mixture of meta and para isomer
Copper(II), an active site in several metalloproteins [1,2] and
metalloenzymes, is known to form complexes with variety of
molecular geometries [3] e.g. tetrahedral, square planar, square
pyramidal, and octahedral. Cu(II) in blue copper proteins possess-
ing an intermediate geometry between the tetrahedral and square
planar, several model systems including those with bidentate
Schiff base ligands had been reported for their structure–functional
activity [4–7]. Similarly, helicity [8] plays vital role on various bio-
reported by Yoshida et al. [15,16] and McNelis et al. [17] gained
significance. In continuation of that, Reedijk and coworkers [18]
have reported recently, the solution state studies along with its
crystal structures. Similarly, incorporating the isostructural spacer,
i.e., m-phenylenediamine(sal-PDAH2) and the m-xylenediamine
[H2-m-xysal], the respective bis(2-hydroxybenzoyl)diamino ben-
zene Schiff base ligands and dinuclear [Cu2(m-xysal)2] complexes
were already reported. In this regard the present study reports
the synthesis of a series of bis-bidentate Schiff-base ligands com-
posed with m-xylene spacer moiety and their respective binuclear
Cu(II) double helicates. While all these ligands and their Cu(II) heli-
cates are characterized using various spectroscopic techniques, the
crystal structures obtained for the complexes 1 and 2 are discussed
in detail. In this direction the reports by Potts [19–21], Constable
and coworkers [22–24] and Lehn [25,26] though demonstrated
some interesting Cu(I) and Cu(II) helicates, those studies with poly-
pyridyl ligands differ from the simple imine ligands used in the
present study. The coexistence of both the helical conformation
and the Cu(II) geometry with tetrahedrally distorted square planar
geometry preserves an added advantage in view of catalysis. In
addition, copper being catalytically important among the metals
used in the henry catalysts, it’s low cost, low toxicity and wide
utility in varieties of organic reactions, inspired us to examine their
components, such as enzymes, proteins, DNA, and a-amylase, etc.
Helicates thus gained paramount importance due to their unique
architectural aspect and hence the reports on varieties of helicates
[8–11] comprising single, double, triple, and multi-strands are rich
in the recent decade. Though the reports on helicates are rich,
researchers are yet to recognize some of their promising applica-
tions [12].
The helicates, thus possessing both helical conformation and
Cu(II) active centers, hold immense interest. Further, the helical
conformation influenced by various supramolecular interactions
and the consequent changes in the geometry are considered to
be important in view of their catalytic role [13,14]. In this direction
⇑
Corresponding author. Tel.: +91 278 2567760; fax: +91 278 2567562.
E-mail addresses: siva@csmcri.org, siva140@yahoo.co.in (P.S. Subramanian).
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.04.033

P. Mosae Selvakumar et al. / Inorganica Chimica Acta 375 (2011) 106–113
107
catalytic activity. Accordingly the catalytic investigation using the
complex 5 as representative catalyst was carried out following the
standard protocol used for nitroaldol catalysis (Henry reaction)
gentle heating for 3 h and then cooled to room temperature.
The characteristic yellow precipitate obtained by Schiff base con-
densation reaction was filtered out. The yellow powder was dis-
solved in methanol:chloroform solvent mixture and allowed for
slow evaporation at room temperature and the resultant dark
yellow polycrystals were collected after 48 h.
[27]. Further the detailed study on the CHꢀ ꢀ ꢀ
p (metal-chelate ring)
interaction and its influence on the helical conformation involving
in tuning the Cu(II) geometry is also discussed in the present report.
(H2-m-xysal)
(L1H2).
2,20-(1,3-Phenylenebis
(methy-
lene))bis(azan-1-yl-1-ylidene)bis(methan-1-yl-1-ylidene)diphenol.
Yield (90%). Anal. Calc. for C22H20N2O2; C, 76.72; H, 5.85; N, 8.13.
Found: C, 76.61; H, 5.75; N, 86.18%. ESI-MS: Calc. for C22H20N2O2
(M+H)+ 345.16. Found: 345.56; 1H NMR (CD3CN, d, 200 MHz):
13.37 (Br s, –OH, 2H), 8.53 (s, CH@N, 2H), 7.36–7.23 (m, Ar phenyl,
8H), 6.92–6.83 (m, Ar, 4H), 4.75 (s, N–CH2, 4H). 13C NMR (50 MHz,
CD3CN): 62.5(CH2), 116.5(CH), 118.7(CH), 126.8(CH), 127.4(CH),
129.0(C), 131.8(CH), 132.3(CH), 139.2 (C), 160.9(C–O),
2. Experimental
2.1. Materials and general methods
All the chemicals were purchased from Aldrich & Co. Microanal-
ysis of the complexes was done using a Perkin-Elmer PE 2400 ser-
ies II CHNS/O elemental analyzer. IR spectra were recorded using
KBr pellets (1% w/w) on a Perkin-Elmer Spectrum GX FT-IR spec-
trophotometer. Electronic spectra were recorded on a Schimadzu
UV 3101PC spectrophotometer. Mass analyses were performed
using positive electron spray ionization (ESI+) technique on a
waters Q Tof-micro mass spectrometer for all these complexes
upon dissolving in 50:50 CH2Cl2:CH3OH solvent mixture. 1H NMR
spectra were recorded on a Bruker Avance II 200 FT-NMR spec-
trometer. Chemical shifts for proton resonances are reported in
ppm (d) relative to tetramethyl silane. Electron spin resonance
spectra were recorded using Bruker X-band electron paramagnetic
resonance spectrometer. The formation of nitroaldol was deter-
mined by HPLC (Shimadzu SCL-10AVP) using Chiracel columns
(AD, OD, OD-H).
166.4(CH@N). IR Spectra: (
1511, 1444, 1403, 1373, 1286, 1251, 1130, 754.
(H2-m-xysal-Cl) (L2H2).
2,20-(1,3-Phenylenebis
m
, cmꢁ1) 3405, 3062, 2647, 1724, 1610,
(methy-
lene))bis(azan-1-yl-1-ylidene)bis(methan-1-yl-1-ylidene)bis(4-
chlorophenol). Yield (96%). Anal. Calc. for C22H18Cl2N2O2; C, 63.93;
H, 4.39; N, 6.78. Found: C, 63.81; H, 4.38; N, 6.69%. ESI-MS: Calc. for
C
22H18Cl2N2O2 (M+H)+ 413.08. Found: 413.38; 1H NMR (CDCl3, d,
200 MHz): 13.31 (Br s, –OH, 2H), 8.35 (s, CH@N, 2H), 7.39–7.21
(m, Ar phenyl, 6H), 6.92–6.87 (m, Ar, 4H), 4.80 (s, N–CH2, 4H). 13C
NMR (50 MHz, CDCl3): 63.0(CH2), 118.6(CH), 119.5(C–Cl),
123.2(C), 127.0(CH), 129.2(CH), 130.5(CH), 132.2(CH), 138.2(C),
159.6(C–O), 164.5(CH@N). IR Spectra:
(m, ) 3433, 3020,
cmꢁ1
2904, 1613, 1515, 1450, 1414, 1370, 1311, 1240, 1165, 1130,
1034, 980, 871, 820.
2.2. X-ray crystallography
(H2-m-xysal-Br) (L3H2). 2,20-2,20(1,3-Phenylenebis (methy-
lene))bis(azan-1-yl-1-ylidene)bis(methan-1-yl-1-ylidene)bis(4-
bromophenol). Yield (95%). Anal. Calc. for C22H18Br2N2O2; C, 52.62;
H, 3.61; N, 5.58. Found: C, 52.64; H, 3.58; N, 5.51%. ESI-MS: Calc. for
In each case, a crystal of suitable size was selected from the
mother liquor and immersed in partone oil, then mounted on the
tip of a glass fiber, and cemented using epoxy resin. Intensity data
C
22H18Br2N2O2 (M+H)+ 500.98. Found: 500.88; 1H NMR (CDCl3, d,
for all three crystals were collected using Mo K
a (k = 0.71073 Å)
200 MHz): 13.35 (Br s, –OH, 2H), 8.35 (s, CH@N, 2H), 7.41–7.21
(m, Ar phenyl, 6H), 6.88–6.83 (m, Ar, 4H), 4.81 (s, N–CH2, 4H).
13C NMR (50 MHz, CDCl3): 63.0(CH2), 110.0(C–Br), 119.0(CH),
120.0(C), 127.0(CH), 129.2(CH), 133.5(CH), 135.0(CH), 138.2(C),
160.1(C–O), 164.4(CH@N). IR Spectra:
2640, 1718, 1617, 1515, 1450, 1400, 1365, 1270, 1230, 1118,
radiation on a Bruker SMART APEX diffractometer equipped with
CCD area detector. The data integration and reduction were pro-
cessed with SAINT [28] software. An empirical absorption correction
was applied to the collected reflections with SADABS [29] using XPREP
.
(m, ) 3420, 3058,
cmꢁ1
The structures were solved by using SHELXTL [30] program and re-
fined on F2 by the full-matrix least-squares technique using SHELXL
[31]. Graphics were generated using ORTEP and packing and H-bond-
ing diagrams by PLATON [32] and MERCURY [33]. In all the three com-
plexes, non-hydrogen atoms were refined anisotropically till
convergence was reached and hydrogen atoms attached to all car-
bon atoms were geometrically fixed.
830.
(H2-m-xysal-OCH3) (L4H2). 2,20-6,60(1,3-Phenylenebis(methy-
lene))bis(azan-1-yl-1-ylidene)bis(methan-1-yl-1-ylidene)bis(2-
methoxy phenol). Yield (90%). Anal. Calc. for C26H28N2O4; C, 72.20;
H, 6.53; N, 6.48. Found: C, 72.11; H, 6.39; N, 6.32%. ESI-MS: Calc. for
C
24H24N2O4 (M+H)+ 433.21. Found: 433.35; 1H NMR (CDCl3, d,
200 MHz): 15.55 (Br s, –OH, 2H), 7.35–7.06 (m, Ar phenyl, 6H),
6.97–6.84 (m, Ar, 4H), 4.79 (s, N–CH2, 4H), 3.78 (s, O–CH3, 6H),
2.38 (s, C–CH3, 6H). 13C NMR (50 MHz, CDCl3): 15(CH3), 53.7
(O–CH3), 56.1(CH2), 112.8(CH), 118.8(CH), 119.1(CH), 126.3(CH),
129.1(C), 139.1(C), 150.9(C–OCH3), 157.3(C–O), 171.7(C@N). IR
2.3. General procedure for Henry reaction
Nitroaldol reactions were carried out in a magnetically stirred
screw cap vials. Copper (II) complex 5 was in situ generated by
mixing L5H2 (0.1 mmol) with copper acetate (0.1 mmol) in THF
(0.5 ml) at rt for 3 h and then appropriate aldehyde (1 mmol) and
nitromethane (1 mmol) were added to the resulting brownish
solution which was allowed to stir continuously for 35–48 h at rt
to 75 °C. The completion of the reaction was monitored by TLC.
The product was purified by column chromatography by using n-
hexane/EtOAc (90:10) ratio and confirmed by HPLC (Supplemen-
tary Fig. S4).
Spectra: (m
, cmꢁ1) 3430, 3018, 2901, 1610, 1517, 1458, 1423,
1384, 1316, 1243, 1209, 1174, 1131, 1037, 989, 875.
(H2-m-xysal-t-Bu) (L5H2). 2,20-6,60(1,3-Phenylenebis (methy-
lene))bis(azan-1-yl-1-ylidene)bis(methan-1-yl-1-ylidene)bis(2,4-
di-tert-butylphenol). Yield (92%). Anal. Calc. for C38H52N2O2; C,
80.24; H, 9.21; N, 4.92. Found: C, 80.11; H, 9.16; N, 4.87%. ESI-
MS: Calc. for C38H52N2O2 (M+H)+ 569.41; Found: 569.70; 1H NMR
(CDCl3, d, 200 MHz): 13.71 (Br s, –OH, 2H), 8.46 (s, CH@N, 2H),
7.38–7.11 (m, Ar phenyl, 8H), 4.78 (s, N–CH2, 4H), 1.43 (s, C–CH3,
18H)), 1.30 (s, C–CH3, 18H). 13C NMR (50 MHz, CDCl3): 30.3(CH3),
32.4(CH3), 35.0(C), 35.9(C), 64.1(CH2), 118.8(C), 126.9(CH),
127.8(CH), 127.9(CH), 128.3(CH), 129.9(CH), 137.6(C), 139.7(C),
2.4. Synthesis
2.4.1. Preparation of ligands
2.4.1.1. General procedure. Methanolic solution (75 ml) of alde-
141.0(C), 159.2(C–O), 167.7(CH@N). IR Spectra: (m
, cmꢁ1) 3400,
hyde
(0.002 mol)
and
1,3-bis-amino
methyl
benzene
3070, 2655, 1732, 1618, 1515, 1430, 1415, 1370, 1290, 1242,
(0.001 mol) were mixed and allowed for constant stirring with
1127, 842.

108
P. Mosae Selvakumar et al. / Inorganica Chimica Acta 375 (2011) 106–113
2.5. Preparation of complexes
2.5.1.5. [Cu2(m-xysal-t-Bu)2] (5). Yield (96%). Anal. Calc. for
76H100Cu2N4O4; C, 72.40; H, 7.99; N, 4.44. Found: C, 72.31; H,
7.78; N, 4.39%. ESI-MS: Calc. for C76H100Cu2N4O4 (M+H)+ 1259.64.
Found: 1260.71; UV–Vis [CH2Cl2, kmax nm,
Mꢁ1 cmꢁ1)]:
C
2.5.1. General procedure
To the H2-m-xysal ligand (0.001 mmol) dissolved in chloroform,
an ethanolic solution of [Cu(CH3COO)2ꢀH2O] (0.001 mmol) was
added drop by drop and allowed for constant stirring. During the
course of the reaction a color change from yellow to dark brownish
red indicate the formation of metal complex. In the case of com-
plexes 1 and 2, upon slow evaporation at room temperature, the
dark brown crystals were obtained in 48 h.
,
(e,
260(36 580), 318(19 800), 385(19 950), 660(392). IR Spectra: (m,
cmꢁ1) 3070, 2647, 1720, 1632, 1540, 1438, 1403, 1373, 1286,
1254, 1135, 754.
3. Results and discussion
The dianion ligands L1H2–L5H2 shown in Scheme 1 were pre-
pared following imine condensation between the substituted
salicylaldehydes and 1,3-bis-aminomethylbenzene. Keeping the
m-xylene spacer moiety same, the ligands L1H2, L2H2, and L3H2
were obtained with appropriate aldehydes such as 2-hydroxybenz-
aldehyde, 2-hydroxy-5-chloro-benzaldehyde, and 2-hydroxy-5-
bromo-benzaldehyde, respectively. Similarly, the ligands L4H2 and
L5H2 were obtained upon treating m-xylenediamine with 2-hydro-
xy-3-methoxy-acetophenone and 2-hydroxy-3,5-di-tert-butyl-
benzaldehyde, respectively. The Schiff base ligands L1H2–L5H2 are
yellow in color and are soluble in various organic solvents, such
as dichloromethane, chloroform, acetone, DMF, and DMSO. Upon
adapting this series of ligands with copper(II)acetate in 2:2 metal:-
ligand ratio, a series of binuclear Cu2 complexes [Cu2(L1)2]ꢀEtOH 1,
[Cu2(L2)2] 2, [Cu2(L3)2] (3), [Cu2(L4)2] 4, and [Cu2(L5)2] 5 were ob-
tained as represented in Scheme 1. All these Schiff base ligands
existing with two phenolate oxygen at their terminal phenyl rings
and two C@N nitrogens, they possess two bindentate binding do-
mains separated by m-xylene spacer moiety. The salicylaldehyde
head group in 1 was substituted appropriately by Cl, Br, OCH3,
and t-butyl in complex 2–5, respectively. Interestingly, though
the crystal structures obtained for complexes 1 and 2 supporting
the binuclear helical architecture, the UV–Vis and EPR spectral
studies carried out for these complexes explored the mononuclear
electronic features.
2.5.1.1. [Cu2(m-xysal)2]ꢀEtOH (1). Yield (87%). Anal. Calc. for
C46H42Cu2N4O5; C, 64.40; H, 4.93; N, 6.53. Found: C, 64.29; H,
4.87; N, 6.46%. ESI-MS: Calc. for C44H36Cu2N4O4 (M+H)+ 811.14.
Found: 811.47; (M+Na)+ 835.12 (835.47); UV–Vis [CH2Cl2, kmax
nm,
Mꢁ1 cmꢁ1)]: 257(36 726), 306(12 610), 385(13 045),
600(338). IR Spectra: (
, cmꢁ1): 3023, 2916, 1619, 1537, 1468,
,
(e,
m
1447, 1400, 1326, 1227, 1199, 1149, 10 951, 1037, 991, 905, 852.
2.5.1.2. [Cu2(m-xysal-Cl)2] (2). Yield (90%). Anal. Calc. for
C
44H32Cu2Cl4N4O4; C, 55.65; H, 3.40; N, 5.90. Found: C, 55.58; H,
3.31; N, 5.81%. ESI-MS: Calc. for
44H32Cu2Cl4N4O4 (M+H)+
950.66. Found: 951.05; UV–Vis [CH2Cl2, kmax, nm, (
, Mꢁ1 cmꢁ1)]:
C
e
255(36 480), 306(19 980), 385(20 092), 625(430). IR Spectra: (m,
cmꢁ1) 3025, 2908, 1622, 1524, 1458, 1423, 1384, 1316, 1243,
1209, 1174, 1131, 1037, 989, 875, 823.
2.5.1.3. [Cu2(m-xysal-Br)2] (3). Yield (89%). Anal. Calc. for
C
44H32Cu2Br4N4O4; C, 46.87; H, 2.86; N, 4.97. Found: C, 46.74; H,
2.81; N, 4.79%. ESI-MS: Calc. for C44H32Cu2Br4N4O4 (M+Na)+,
1150.76. Found: 1150.61; (M+H)+, 1126.78(1126.65). UV–Vis
[CH2Cl2, kmax
390(23 047), 630(410). IR Spectra: (
, nm, (e,
Mꢁ1 cmꢁ1)]: 260(36 550), 306(20 433),
m
, cmꢁ1) 3062, 2647, 1724,
1618, 1530, 1444, 1403, 1373, 1286, 1251, 1130, 750.
2.5.1.4. [Cu2(m-xysal-OCH3)2] (4). Yield (94%). Anal. Calc. for
C
48H44Cu2N4O8; C, 63.21; H, 5.30; N, 5.67. Found: C, 63.17; H,
5.28; N, 5.56%. ESI-MS: Calc. for C48H44Cu2N4O8 (M+H)+ 989.09.
Found: 989.30; UV–Vis [CH2Cl2, kmax nm,
Mꢁ1 cmꢁ1)]:
3.1. Spectral investigation
,
(e
,
The 1H NMR spectra recorded for all five ligands L1H2–L5H2 in
CDCl3 depicting the characteristic resonance at 8.35–8.46d, repre-
sent the formation of CH@N azomethine group except L4H2, which
did not have azomethine hydrogen. The characteristic IR bands for
270(36 600), 304(14 780), 367(18 090), 620(364). IR Spectra: (m,
cmꢁ1) 3058, 2645, 1729, 1628, 1538, 1440, 1400, 1371, 1284,
1250, 1127, 740.
Scheme 1. Synthesis of double stranded Cu2-helical complexes.

P. Mosae Selvakumar et al. / Inorganica Chimica Acta 375 (2011) 106–113
109
Fig. 1. Mass spectra for complexes 1–5 recorded in CH2Cl2:MeOH (50:50 v/v) solvent.
L1H2–L5H2 corresponded to (i) OH, (ii) C@N were elicited. The
broad signal centered at 3400 cmꢁ1 for all these free ligands, can
be ascribed to the OH functional group. The sharp and intense
peaks in the range of 1510–1517 and 1610–1618 cmꢁ1 can be
attributed to the mCH@N symmetric and anti-symmetric stretching
mode, respectively, in the case of free ligands. The disappearance
UV–Vis spectra arranged in the ascending order 1 (600 nm) < 4
(620 nm) < 2 (620 nm) < 3 (630 nm) < 5 (660 nm) indicate the
increasing distortion on the tetrahedrally distorted square planar
geometry of the complexes [38,39].
of the OH peak and the shift of 10–25 cmꢁ1 in the
mCH@N stretching
3.2. Crystal structure and molecular association
frequency in IR spectra of these complexes with respect to the cor-
responding free ligands indicates their complexation with Cu(II)
metal center.
Our attempt to crystallize all these five complexes was partially
successful and we were fortunate to get crystals for complexes 1–
3. Though we obtained crystal for complex 3, our repeated at-
tempts failed to refine the structure and hence we have depicted
here only the respective crystallographic data.1 The complex 1
was crystallized in ethanol, while the complex 2 was obtained from
50:50 solvent mixture of dichloromethane and acetonitrile. Sum-
mary of the crystallographic data and selected bond distances, bond
angles for complexes 1 and 2 are depicted in Tables 1 and 2,
respectively.
Complexes 1 and 2 were crystallized in centrosymmetric space
group P21/c and Pbcn, respectively. An ORTEP view of the neutral
Cu(II) complexes 1 and 2 with atom numbering scheme is depicted
in Fig. 3. While the complex 1 crystallized with one ethanol mole-
cule, the complex 2 possess no solvent of crystallization. The Cu(II)
metal sitting on twofold axis, both the metal centers in the dinu-
clear unit possess a tetrahedrally distorted square planar geometry
coordinating with N2O2 donor atoms of deprotonated phenolate
oxygens and azomethine nitrogens from two different Schiff base
ligands in a symmetry related bidentate fashion. The Cu–N dis-
tances ranging 1.955–1.958 Å, 1.943–1.970 Å, and the Cu–O dis-
tances 1.896–1.916 Å, 1.881–1.891 Å, respectively, in complex 1
and 2, are well within the range reported for the related Schiff base
complexes [40,41]. The cis-angles subtended by the six-membered
chelate rings with Cu(II) in complexes 1 and 2 from each ligand
ranging 88.8–94.2°, 92.2–94.2°, respectively, are within the limit
of square planar value. However the trans-angles in the range of
152.5–164.3°, 150.7–151.1° are deviated from square planar and
The mass spectra obtained for complexes 1–5 are depicted in
Fig. 1. The mass spectral data showing characteristic positive
molecular ion peak, the mass values obtained are in accordance
with the formation of dinuclear Cu2 species. Thus an excellent
agreement between the experimental and calculated values con-
firmed the formation of the binuclear double stranded complexes.
All these complexes depicting a dominant peak with respect to
their monopositive ion (M+H)+, i.e., 811.47 for (1), 951.05, (2);
1126.65 (3); 989.30, (4); and 1260.71 for (5), the complexes 1
and 3 gave an additional peak correspond to their monopositive so-
dium ion (M+Na)+ adduct as illustrated in Fig. 1. All these mass
spectra thus supported the existence of two metal ions and two li-
gands without any ambiguity. The bunch of peaks appropriate for
the ESI species strongly supports, various isotopic as well as posi-
tive ion species, which clearly supports the formation of 2:2 metal
ligand dimeric associations. It may be noted that the mass values of
complexes with Na+ adduct [complex+Na+] is well-known phe-
nomenon in LC mass spectrometry [34] and hence it is not
uncommon.
The dinuclear metal complexes 1–5 are found to be dark brown
in color and are soluble in CH2Cl2 and CHCl3. The electronic spec-
trum for all these complexes behaving quite similar, they exhibit
two characteristic bands at 300–330 and 380–420 nm representing
various inter and intra ligand transitions as illustrated in Fig. 2. The
former band at 300–330 nm can be attributed to
p–
pꢂ and n–pꢂ
transitions. The band appeared at 380–420 nm can be assigned to
the ligand to metal charge transfer of non-bonding lone pair of
the phenolate oxygen to the d-orbitals of the Cu(II), i.e., LMCT
[35–37]. The broad spectral feature centered at 620–660 nm in
the visible region obtained for complexes 1–5 can be attributed
to the d–d transition. The red shift observed in the kmax of
1
Crystal data for complex 3: C44H32Br4Cu2N4O4, formula weight = 1127.45, Ortho-
rhombic, P212121; a = 12.7257(16), b = 23.687(3), c = 53.667(7) Å, U = 16177(3) Åꢁ3
,
T = 110(2) K, Z = 4, Dc = 1.852 mg mꢁ3
0.11 ꢃ 0.09 ꢃ 0.03 mm; 98 905 reflections measured of which 37 831 were unique
(Rint = 0.1455), 1049 parameters, wR2 = 0.1846, R1 = 0.1013 (with I P 2 (I)), S = 1.017.
, l(Mo Ka , F(0 0 0) = 8864,
) = 5.053 mmꢁ1
r

110
P. Mosae Selvakumar et al. / Inorganica Chimica Acta 375 (2011) 106–113
indicate the existence of a tetrahedrally distorted square planar
geometry. Each molecule existing in binuclear association, the
two symmetrically disposed ligand moieties are wrapped around
the M–M axis and generate a dinuclear double helical architecture.
The packing diagram with H-bonding interactions (Supplemen-
tary Figs. S1 and S2) indicated that the adjacent molecules, are
aligned in layers and are bridged via intermolecular C–Hꢀ ꢀ ꢀO
[O4ꢀ ꢀ ꢀH20–C20 = 3.394 Å] interaction in complex 1 and C–Hꢀ ꢀ ꢀCl
[Cl1ꢀ ꢀ ꢀH12–C12 = 3.628; \Cl1ꢀ ꢀ ꢀH12–C12 = 142.21°] contact in 2.
Thus the weak interaction facilitating the formation of a double
stranded helical chain, generates ‘‘P’’ and ‘‘M’’ sense of global helic-
ity. Interestingly, the close scrutiny of the crystal structure illus-
trated that the absolute configuration around the tetrahedrally
distorted Cu(II) cations possess opposite chirality, i.e., Delta (
and Lambda ( ). Further, the ligand wrapping around the ‘‘
and ‘‘ ’’ metal center, the global chirality can be assigned as ‘‘P’’
D)
K
D’’
K
(-positive) and ‘‘M’’ (-minus) based on the left and right handed
rotation. This unique arrangement can best be described as side-
by-side complex [42–45] according to Piguet et al. [9], while Albr-
echt [11,46] defines this as mesohelicates.
Fig. 2. UV Spectrum for 1–5 in CH2Cl2 (inset shows d–d band).
Table 1
3.3. C–Hꢀ ꢀ ꢀp (metal-chelate ring) influenced inter planar twist
Summary of the Crystallographic data for complexes 1 and 2.
The complexes 1 and 2 being helical, the existence of weak
Parameter
1
2
CHꢀ ꢀ ꢀ
p (metal-chelate ring) interaction [47,48] and its influence
on shaping the helical conformation is given special attention. Sur-
CCDC No.
Formula
MW
773239
773240
C44H32Cl4Cu2N4O4
949.62
C46H42Cu2N4O5
prisingly, the reports on these class of complexes, though suggest
857.92
Crystal system
Space group
a (Å)
b (Å)
c (Å)
monoclinic
P21/c
orthorhombic
Pbcn
12.891(12)
13.324(11)
23.68(2)
4067(6)
the presence of CHꢀ ꢀ ꢀ
p
and
p p stacking interactions, they mainly
ꢀ ꢀ ꢀ
focus on the interaction between the phenyl rings. Further, all such
16.4414(12)
9.8667(7)
24.3148(18)
3944.1(5)
4
100(2)
1.131
23 306
9210 [Rint = 0.0440]
contacts exceeding more than 4 Å (weak and long range interac-
tion), they ignored or unnoticed the existence of the CHꢀ ꢀ ꢀ
p (me-
V (Å3)
Z
tal-chelate ring) interaction [49–53]. Thus the present study
4
differing from the previous reports explores the existence of
T (K)
293(2)
1.358
9335
2577 [Rint = 0.1735]
CHꢀ ꢀ ꢀ
p interaction and its influence on helical conformation.
l(Mo K
a
) (mmꢁ1
)
Both these dinuclear complexes existing in C2 symmetry, they
possess four 6-membered metal-chelate rings around the Cu(II)
centers. An analysis regarding their contact with neighboring
hydrogens was carried out (Supplementary Fig. S3). In this regard
Reflections measured
Independent
reflections
Final R1, wR2 [I > 2
r
(I)] R1 = 0.0556,
R1 = 0.0798,
wR2 = 0.1223
wR2 = 0.1179
the above Scheme 2a and b illustrate the existence of CHꢀ ꢀ ꢀ
p con-
tact and interplanar twist angle() between the metal-chelate rings,
respectively. To simplify the understanding, the centroids of all
0
these four 6-membered chelate rings are assigned as Cga, Cga ,
0
Table 2
Cgb, Cgb as shown in Scheme 2a. The phenyl rings of the m-xylene
Selected bond distances and bond angles for complexes 1 and 2 with esd’s are in
parentheses.
spacer on the opposite strands are staggered with each other and
the respective mean planes measured in complex 1 and 2 are found
to be 34.5° and 32.0°. The hydrogens of m-xylene spacer are found
to be under the influence of the metal-chelate rings, as reflected
Complex 1
Bond lengths (Å)
Cu(2)–O(4)
Cu(2)–O(2)
Cu(2)–N(2)
Cu(2)–N(4)
Cu(2)–O(4)
1.896(2)
1.904(2)
1.955(2)
1.956(2)
1.896(2)
Cu(1)–O(3)
Cu(1)–O(1)
Cu(1)–N(3)
Cu(1)–N(1)
Cu(1)–O(3)
1.906(2)
1.916(2)
1.955(3)
1.958(3)
1.906(2)
from their respective C–Hꢀ ꢀ ꢀ
p distances (Table 3).
Based on the H-bonding distances, it is obvious that the hydro-
0
gens H12 [H12ꢀ ꢀ ꢀCgb = 3.00Å] and H14[Cgaꢀ ꢀ ꢀH14 = 3.42 Å] of m-
xylene in 1 are shorter than that of the respective hydrogens H32
Bond angles (°)
and H36 from the opposite strand. A similar approach in complex
O(4)–Cu(2)–O(2)
O(4)–Cu(2)–N(2)
O(2)–Cu(2)–N(2)
O(4)–Cu(2)–N(4)
O(2)–Cu(2)–N(4)
N(2)–Cu(2)–N(4)
152.55(10)
90.12(9)
94.28(10)
93.58(9)
91.06(10)
160.87(10)
O(3)–Cu(1)–O(1)
O(3)–Cu(1)–N(3)
O(1)–Cu(1)–N(3)
N(3)–Cu(1)–N(1)
O(1)–Cu(1)–N(1)
O(3)–Cu(1)–N(1)
155.11(9)
93.29(10)
92.60(9)
164.33(10)
91.97(10)
88.82(10)
2 produced only two set of identical C–Hꢀ ꢀ ꢀ
p contacts, due to its
crystallographic symmetry. The H10 of the m-xylene ring belong
0
to opposite strands, possess a stronger contact with Cgb and Cgb
b/b0
[H10ꢀ ꢀ ꢀCg
= 2.64 Å], while the H14 possess comparatively weak-
0
0
er contact with the Cga and Cga [H14ꢀ ꢀ ꢀCga/Cga = 3.38 Å].
Complex 2
In complex 1, one of the m-xylene spacer was dragged towards
Bond lengths (Å)
O(2)–Cu(1)#1
N(2)–Cu(1)#1
Cu(1)–O(1)
0
the metal-chelate ring Cga and Cgb through its hydrogens H14 and
1.891(7)
1.943(9)
1.881(7)
Cu(1)–O(2)#1
Cu(1)–N(2)#1
Cu(1)–N(1)
1.891(7)
1.943(9)
1.970(9)
H12, respectively. Similarly the other m-xylene spacer with H32
and H36 resides away, i.e., >3.7 Å. However a similar interaction
in complex 2 defined by H10 established a stronger influence with
Bond angles (°)
b0
the respective metal-chelate rings Cgb and Cg . Both these com-
O(1)–Cu(1)–O(2)#1
O(1)–Cu(1)–N(2)#1
O(2)#1–Cu(1)–N(2)#1
150.7(3)
94.2(4)
94.0(4)
O(2)#1–Cu(1)–N(1)
N(2)#1–Cu(1)–N(1)
92.2(4)
151.1(4)
plexes exploring the existence of C–Hꢀ ꢀ ꢀ
p (metal-chelate ring) con-
tacts [54–57] with opposite strands, this analysis further
#1
ꢁx + 1, y, ꢁz + 3/2.
strengthens their influence on shaping the helical conformation.

P. Mosae Selvakumar et al. / Inorganica Chimica Acta 375 (2011) 106–113
111
Fig. 3. ORTEP diagram depicting the complex (a) [Cu2(L1)2]ꢀEtOH (1), (b) [Cu2(L1)2] (2) with atom numbering scheme (40% probability factor for the thermal ellipsoid).
Scheme 2. (a) Centroid assignment for all chelate rings. (b) Interplanar twist angle.
Table 3
interplanar angle in complex 1 with 41.5° at Cu(1) and 44.8° at
CHꢀ ꢀ ꢀp contact measured in complexes 1 and 2.
Cu(2), and the Cu center in complex 2 exists with 44.0° undoubt-
edly support the existence of an intermediate geometry between
the square planar and tetrahedral thus leading to a tetrahedrally
distorted square planar geometry.
D–Hꢀ ꢀ ꢀA
Dist Hꢀ ꢀ ꢀ
(Å)
Dist. \D–Hꢀ ꢀ ꢀA
Angle
(°)
A
(Å)
Complex 1
C14–H14ꢀ ꢀ ꢀCga
3.78 H14ꢀ ꢀ ꢀCga 3.42 \C14H14ꢀ ꢀ ꢀCga
4.08 H32ꢀ ꢀ ꢀCgb 3.36 \C32H32ꢀ ꢀ ꢀCgb
105.7
135.9
106.0
139.0
C32–H32ꢀ ꢀ ꢀCgb
0
C36–H36ꢀ ꢀ ꢀCga
3.98 H36ꢀ ꢀ ꢀCga 3.62 \C36H36ꢀ ꢀ ꢀCga
0
0
3.4. EPR spectral investigation
0
0
0
C12–H12ꢀ ꢀ ꢀCgb
3.78 H12ꢀ ꢀ ꢀCgb 3.00 \C12-H12ꢀ ꢀ ꢀCgb
The complexes 1–5 were characteristically binuclear, which in-
spired us to investigate their M–M interaction. Accordingly the X-
band EPR spectra [62–66] of polycrystalline powder at room tem-
perature for complexes 2, 3, and 5 were recorded (Fig. 4). All these
spectra showing similar spectral pattern, their broad feature could
be attributed to the dipolar interaction. The position of the EPR sig-
Complex 2
C14–H14ꢀ ꢀ ꢀCga/Cg
3.80 H14ꢀ ꢀ ꢀCga 3.38 \C14-H14ꢀ ꢀ ꢀCga/ Cg
110.1
a
a
0
0
0
0
C10–H10ꢀ ꢀ ꢀCgb/Cgb 3.45 H10ꢀ ꢀ ꢀCgb 2.64 \C10-H10ꢀ ꢀ ꢀCgb/ Cgb 145.7
The six membered chelate rings existing with nitrogen and oxygen
atoms, which possess unpaired electrons may gain electron delo-
calization. The electron delocalization might have given the nucle-
ophilic nature and hence they tend to have contact with
electrophilic hydrogen of m-xylene spacer. The weak supramolec-
ular interaction between the m-xylene hydrogen and the metal-
chelate ring, thus, found varied depending upon their proximity.
nal and the absence of
DMs = 1 resonance strongly suggested, that
the molecules though assembled binuclear in nature, they still pos-
sess a non-interacting M–M association. The Cu–Cu distances mea-
sured as 6.377 and 6.591 Å, respectively, from the crystal
structures of 1 and 2, also supports the possibility of the existence
of non-interacting Cu(II) centers.
Depending upon the strength of the CHꢀ ꢀ ꢀ
p interaction, the metal
chelate rings may not be possible to retain their planarity. Accord-
ingly an interplanar twist measured between the metal-chelate
rings gives the possibility for the formation of square planar or tet-
rahedral geometry at Cu(II). Based on the classical method [58–61],
the interplanar twist angle (/) (Scheme 2b) 0, 180° defines square
planar geometry, while 90° defines the tetrahedral geometry. The
3.5. Catalytic studies
Copper(II) complexes with four-coordination can generally exist
in distorted square-planar or tetrahedral geometry. However on
comparison with square planar, the tetrahedral geometry was rel-
atively rare, while it was very common for Cu(I) complexes. The

112
P. Mosae Selvakumar et al. / Inorganica Chimica Acta 375 (2011) 106–113
Table 4
The catalytic activity of
methane.
5 in Henry reaction of 4-nitro benzaldehyde and nitro
Entry
Catalyst (mol%)
Temp. (°C)
Solvent
Time (h)
Yield (%)
1
2
3
4
5
6
7
8
5
70
70
70
70
50
RT
75
45
60
70
70
THF
THF
THF
THF
THF
THF
THF
DCM
CHCl3
THF
THF
35(48)
35(48)
35(48)
35(48)
48
48
35(48)
35
48
72
85
85
65
45
85
35
10
20
30
20
20
20
20
20
20
–
9
35
35(48)
48
40
75
Trace
10a
11
a
2-Flouro benzaldehyde as substrate.
Fig. 4. X-band spectra at 298 K of powder samples of complexes 2, 3, and 5.
substrate in presence of nitromethane at 70 °C using copper(II)tri-
flate and copper(I)iodide in place of copper(II)acetate. However,
the results are not encouraging in terms of yield of nitroalcohol
(yield <10%). As all these five helicates were possessing almost very
similar stereochemical structure, we have chosen the complex 5
for this catalytic study, since such sterically strained complexes
were demonstrated for better catalytic capability.
The metal-chelate ring existing with nucleophilic nature, it pos-
sesses strong contact with m-xylene ring hydrogen as demon-
strated in the crystal structure. A similar situation may arise,
when the substrate nitromethane approach the catalyst 5. It is ex-
pected that the hydrogen bonding contact with m-xylene hydrogen
will be weakened, when comparatively a stronger electrophile
comes into contact. Thus the nitromethane certainly being stron-
ger electrophile compared to the hydrogen of m-xylene ring, we
propose [68–72] that the removal of hydrogen from nitromethane
would be facilitated by the nucleophilic attack on aldehydic carbon
giving the nitroalcohol as an end product as shown in Scheme 3. In
general, researchers [73] use triethylamine to facilitate the depro-
tonation of nitromethane during the course of the catalytic reac-
tion. However, in the present case since the complex 5 itself
performing the deprotonation, this can be considered as an added
advantage of the catalyst.
present complexes with Cu(II) active sites existing in tetrahedrally
distorted square planar geometry, the helical architecture gains
added importance. Hence they were chosen for their catalytic
activity. In this study, we have carried out Henry reaction (Fig. 5)
using complex 5 as representative catalyst with 4-nitrobenzalde-
hyde as substrate in presence of nitromethane. In order to obtain
highest yield of the product nitroalcohol, an optimization of the
reaction conditions was carried out, varying the catalyst loading,
temperature etc through a series of experiments and the results
are presented in Table 4.
On increasing the catalyst loading from 5 to 30 mol% at 70 °C in
THF (Entry 1–4) we found that 20 mol% catalyst loading at 70 °C
(Entry 3) was optimum for achieving high yield (up to 85% from
48%) of corresponding nitroalcohol in 35 h. As temperature and
solvents played a crucial role for the catalytic reaction, the Henry
reaction was conducted at different temperatures, i.e., rt, 50, 70,
and 75 °C. Therefore on conducting the Henry reaction upon
decreasing the temperature from 70 °C to rt, the yield of nitroalco-
hol was adversely affected (Entries 5 and 6). However on increas-
ing the temperature from 70 to 75 °C (Entry 7) there was no
improvement in the product yield. Further attempts were made
to see the effect of solvents such as THF, CHCl3, CH2Cl2 (Entries 3,
8, and 9) on Henry reaction [67]. Among all the solvents used for
this study, THF was found to be the solvent of choice (Entry 3),
while the other two solvents were not found encouraging. Under
this optimized reaction conditions (Entry 3), the reaction was also
carried out with 2-flurobenzaldehyde as substrate, where 75%
yield (Entry 10) of corresponding nitroalcohol was obtained. A
blank reaction was conducted in the absence of catalyst 5 using
4-nitrobenzaldehyde as substrate in presence of nitromethane at
70 °C, where only trace quantity of nitroalcohol was obtained (En-
try 11). To confirm the role of the counter ions in the catalyst 5,
nitroaldol reaction was carried out with 4-nitrobenzaldehyde as
4. Conclusions
In summary, we have synthesized a series of dinuclear Cu2
‘‘metallohelicate’’ complexes, which retained the tetrahedrally dis-
torted square planar geometry. All these bis-bidentate Schiff base
ligands with m-xylene spacer possess two well separated binding
domains and formed stereochemically important Cu(II) binuclear
helicates is illustrated from the crystal structure. The crystal struc-
tures obtained for complexes 1 and 2 showing the helical architec-
ture, the influence of CHꢀ ꢀ ꢀ
p metal-chelate ring interaction and the
analysis on their consequent geometrical change demonstrate the
stabilization of tetrahedrally distorted square planar geometry at
Cu(II) metal center without any ambiguity. The tetrahedrally dis-
torted square planar Cu(II) in the binuclear association possess
both the
dimetallic association generates both P and M chirality which re-
sult the complexes as mesocates in every form. The CHꢀ ꢀ ꢀ contact
D and K chirality, and the ligand wrapped around the
p
in complex 1 being weak and long range, the respective interaction
in complex 2 was found strong. This supramolecular interaction
thus unequivocally established its influence in framing the helical
conformation and the geometrical rearrangement. The EPR spectra
obtained for complexes 2, 3, and 5 indicated that though the binu-
clear association existed in the helicate, both these metal ions were
isolated significantly from each other and possess a non-interact-
ing M–M association. The catalytic activity performed following
Fig. 5. Henry reaction followed for complex 5 as catalyst.

P. Mosae Selvakumar et al. / Inorganica Chimica Acta 375 (2011) 106–113
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Henry reaction by complex 5 catalyzing the 4-nitrobenzaldehyde
to the corresponding nitroalcohol gave 85% yield at 70 °C and sug-
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aldol reactions.
Acknowledgements
The authors P.S., P.M.S. and S.N. gratefully acknowledge the
financial support extended by DST (Project No. SR/S1/IC-19/2005)
and CSIR New Delhi (NWP 0010). Mr. Arun Kumar Das and Mr. Vi-
nod Agarwal are gratefully acknowledged for recording MS and IR
spectra, respectively.
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Appendix A. Supplementary material
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CCDC Nos. 773239 and 773240 for complexes 1 and 2 contains
the supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.ica.2011.04.033.
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