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Polyhedron 24 (2005) 1922–1928

Effect of a pentadentate Schiff base on the helical supramolecularstructures of (l-alkoxo)(l-carboxylato)dicopper(II) complexes

Sunita Gupta, Arindam Mukherjee, Munirathinam Nethaji, Akhil R. Chakravarty *

Department of Inorganic and Physical Chemistry, Indian Institute of Science, Sir C.V. Raman Avenue, Bangalore 560012, India

Received 6 April 2005; accepted 2 May 2005Available online 19 August 2005

Abstract

Two new asymmetrically dibridged dicopper(II) complexes with a pentadentate Schiff base and p-hydroxycinnamate are preparedand structurally characterized by X-ray crystallography. The complexes have an asymmetrically dibridged (l-alkoxo)(l-carboxy-lato)dicopper(II) core with an alkoxo bridge from the Schiff base and the carboxylate showing a three-atom bridging mode. Vari-able-temperature magnetic studies show the complexes having an antiferromagnetically coupled spin system giving a singlet–tripletenergy separation of �160 and �132 cm�1 for 1 and 2, respectively. Complex 1, with a shorter Cu–OR–Cu angle, displays greaterantiferromagnetic spin coupling. Besides the Cu–OR–Cu angle, the role of the carboxylate ligand and the supramolecular structurein the spin coupling phenomena is observed. Complex 1 ÆMeOH shows the formation of intermolecular hydrogen bonds involvingthe axial methanol and the pentadentate Schiff base terminal oxygen atom. There is additional hydrogen bonding interactions involv-ing the p-hydroxy group of the carboxylate, the lattice methanol and the terminal oxygen atom. The crystal structure of complex2 ÆH2O displays the presence of a helical supramolecular structure due to hydrogen bonding interactions involving the pendantp-hydroxy group and the bridging oxygen atom of the carboxylate.� 2005 Elsevier Ltd. All rights reserved.

Keywords: Dicopper(II) complex; Schiff base; Crystal structure; Supramolecular helices; Magnetic properties; Hydrogen bonds

1. Introduction

Helical supramolecular architectures and polymericspecies formed by the process of molecular self-assem-bly are of interest for their novel structural featuresand for their ability to stabilize guest molecules of dif-ferent sizes and conformations [1–12]. We have re-cently shown that dicopper(II) complexes having apentadentate Schiff base N,N 0-1,3-diyl-bis(salicylaldi-mino)propan-2-ol (H3L

0) in its trianionic form andthe carboxylate ligands p-HOC6H4–X–CO2

�, whereX is a spacer like ACH@ CHA, –CH2– or –CH2–CH2–, form different types of supramolecular struc-tures [11,12]. Among the three complexes, the

0277-5387/$ - see front matter � 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.poly.2005.05.015

* Corresponding author. Tel.: +91 80 229 32533; fax: +91 80 23600683.

E-mail address: arc@ipc.iisc.erent.in (A.R. Chakravarty).

p-hydroxycinnamate complex [Cu2L0(O2CACH@CHA

C6H4-p-OH)] Æ2H2O forms a helical chain structuredue to hydrogen-bonding interactions involving thep-hydroxy group of the carboxylate and one phenoxooxygen atom of the Schiff base [11]. An importantstructural feature in this helical structure is the angleof �30� between the cinnamate ligand and the planeof the {Cu2L

0}+ unit. Two lattice water moleculesform an unprecedented helical one-dimensional chain,in which the alternate water molecules are anchored tothe supramolecular host structure. The present workstems from our interest to study the role of the Schiffbase in the formation of a helical host structure andthe ability of the supramolecular host to supportany guest solvent molecule(s). We have chosen apotentially pentadentate Schiff base N,N 0-1,3-diyl-bis(acetylacetoneimine)propan-2-ol (H3L) derived fromacetylacetone and 1,3-diaminopropan-2-ol for the

S. Gupta et al. / Polyhedron 24 (2005) 1922–1928 1923

synthesis of asymmetrically dibridged dicopper(II)complexes using p-hydroxycinnamic acid as the car-boxylate. The Schiff base H3L with two acetylacetonei-mine moieties is similar to H3L

0 that has twosalicylaldimine groups. Herein, we report the synthe-sis, crystal structure and the magnetic properties ofthe complexes [Cu2L(O2CACH@CHAC6H4-p-OH)-(MeOH)] ÆMeOH (1 ÆMeOH) and [Cu2L(O2CACH@CHAC6H4-p-OH)] ÆH2O (2 ÆH2O). The significantobservation of this study is the crystal structures ofthe complexes showing formation of different typesof supramolecular structures. The complex 2 ÆH2Oforms a helical structure through intermolecularhydrogen bonding interactions involving only the car-boxylate ligands. The lattice water is non-covalentlybound to the Schiff base of the dinuclear unit anddoes not take part in the supramolecular structureformation.

2. Experimental

2.1. Materials and measurements

All reagents and chemicals were purchased fromcommercial sources and used as received. The solventswere purified following reported procedures [13]. TheSchiff base N,N 0-1,3-diyl-bis(acetylacetoneimine)pro-pan-2-ol (H3L) and the dicopper(II) precursor com-plex, [Cu2L(O2CMe)] were prepared by literaturemethods [14]. The elemental analysis was done usinga Thermo Finnigan FLASH EA 1112 CHN analyzer.The electronic and infrared spectral data were ob-tained from Lambda 55 and Perkin–Elmer SpectrumOne FT-IR spectrometers, respectively. Variable tem-perature magnetic susceptibility data for the polycrys-talline samples were obtained in the temperature range18–300 K using a Model 300 Lewis-coil-force magne-tometer of George Associates Inc. (Berkeley, USA)make equipped with a Cahn balance and a closed cy-cle cryostat (Air products). Hg[Co(NCS)4] was used asa standard. Experimental susceptibility data were cor-rected for diamagnetic contributions and temperatureindependent paramagnetism (Na = 60 · 10�6 cm3 M�1

per copper). The molar magnetic susceptibilities werefitted to the modified Bleany–Bowers expression bymeans of a least-squares computer program [15,16].The Hamiltonian and susceptibility equation usedare: H ¼ �2JðS1 � S2Þ (S1 = S2 = 1/2 for a d9–d9

dicopper(II) core) and vCu ¼ ½Ng2b2=kðT � hÞ�½3þ expð�2J=kðT � hÞÞ��1ð1� qÞ þ ðNg21b

2=4kT Þqþ N a, whereq is the fraction of monomeric impurity and 2J is thesinglet–triplet separation energy. The magnetic mo-ments were calculated in lB unit. TGA measurementswere made using a Mettler Toledo Star thermalanalyzer.

2.2. Preparation of [Cu2L(O2CACH@CHAC6H4-p-OH)-

(MeOH)] (1)

Complex [Cu2L(O2CMe)] (0.15 g, 0.33 mmol) inMeOH (10 mL) was reacted with the sodium salt ofp-hydroxycinnamic acid, prepared in situ by reactingthe carboxylic acid (0.056 g, 0.33 mmol) with NaOH(0.013 g, 0.33 mmol) in methanol (5 mL), and the reac-tion mixture was refluxed for 30 min, followed by cool-ing to an ambient temperature. The solution was filteredand the filtrate on slow evaporation gave the green prod-uct in �70% yield. Single crystals of composition1 ÆMeOH, suitable for X-ray diffraction, were obtainedfrom the mother liquor. Anal. Calc. for C23H30N2O7Cu2(1): C, 48.24; H, 5.10; N, 4.89. Found: C, 48.02; H, 5.24;N, 5.13%. Electronic spectral data in DMSO [kmax, nm(e, M�1 cm�1)]: 632 (350), 315 (126000), 290 (109000).FT-IR (KBr phase), cm�1: 3400br, 2920w, 1612s,1508s, 1400s, 1272s, 1169m, 1013m, 942m, 833m,701m (br, broad; s, strong; m, medium; w, weak). Mag-netic moment (leff) per copper in the dimeric unit:1.63 lB at 300 K; 0.17 lB at 18 K; 2J = �160 cm�1 fromthe theoretical fitting with g = 2.13, q = 0.004, g1 = 2.2,R = 8.5 · 10�3.

2.3. Preparation of [Cu2L(O2CACH@CHAC6H4-p-OH)]

(2)

The dimeric precursor [Cu2L(O2CMe)] (0.22 g,0.49 mmol) in 1-propanol (20 mL) was reacted withthe sodium salt of p-hydroxycinnamic acid, preparedin situ by reacting the carboxylic acid (0.082 g,0.49 mmol) with NaOH (0.019 g, 0.47 mmol) in 1-pro-panol (10 mL) and the reaction mixture was refluxedfor 30 min, followed by cooling to room temperature.The solution was filtered and the filtrate on slow evapo-ration gave the green product in �65% yield. Anal. Calc.for C22H28N2O7Cu2 (2): C, 47.22; H, 5.04; N, 5.00.Found: C, 47.29; H, 5.08; N, 5.12%. Electronic spectraldata in DMSO [kmax, nm (e, M�1 cm�1)]: 630 (320), 315(132000), 285 (100000). FT-IR (KBr phase, cm�1):3510br, 3001w, 1640s, 1606s, 1550s, 1514s, 1397s,1281s, 1250s, 1162s, 1015m, 986m, 831s, 761m, 712m.Magnetic moment (leff) per copper: 1.67 lB at 300 K;0.17 lB at 18 K; 2J = �132 cm�1 from the theoreticalfitting with g = 2.07, q = 0.005, g1 = 2.14,R = 2.8 · 10�2. Single crystals of 2 ÆH2O were obtainedon slow evaporation of the mother liquor in the presenceof a trace quantity of water.

2.4. X-ray crystallographic studies

Single crystals of the complexes [Cu2L(O2CACH@CHAC6H4-p-OH)(MeOH)] ÆMeOH (1 ÆMeOH)and [Cu2L(O2CACH@CHAC6H4-p-OH)] ÆH2O (2 ÆH2O)of the respective sizes 0.49 · 0.21 · 0.13 and

1924 S. Gupta et al. / Polyhedron 24 (2005) 1922–1928

0.26 · 0.18 · 0.11 mm3 were mounted on glass fibers withepoxy cement. All geometric and intensity data were col-lected using a Bruker SMART APEX CCD diffractome-ter having a fine focus 1.75 kW sealed tube Mo KaX-raysource with increasing x (width of 0.3� per frame) at ascan speed of 5 s/frame for 1 ÆMeOH and 12 s/framefor 2 ÆH2O. A total of 20555 reflections were collectedfor 1 in the range 1.47 6 h 6 27.18� of which 4356 reflec-tions with I P 2r(I) were used for structure solutionusing 361 parameters. For 2 ÆH2O, a total of 16283 reflec-tions were collected in the range 2.0 6 h 6 24.71�, ofwhich 3457 reflections with I P 2r(I) were used for struc-ture solution using 310 parameters. The hydrogen atomsattached to the carbon atoms, except two methanol mol-ecules in 1 ÆMeOH, were fixed in their calculated posi-tions and refined using a riding model for both thestructures. The hydrogen atoms of the copper-boundand lattice methanol molecules were located in the differ-ence Fourier maps and were refined isotropically. In boththe structures, the hydrogen atom of the p-hydroxygroup was located in the difference Fourier map andwas refined isotropically. The hydrogen atoms of the lat-tice water in 2 ÆH2O were also located from the differenceFourier map and blocked after refining for the last few

Table 1Crystallographic data for [Cu2L(O2CACH@CHAC6H4-p-OH)(MeOH)] ÆMeOH (1 ÆMeOH) and [Cu2L(O2CACH@CHAC6H4-p-OH)] ÆH2O (2 ÆH2O)

1 ÆMeOH 2 ÆH2O

Empirical formula C24H34N2O8Cu2 C22H28N2O7Cu2Crystal size (mm) 0.25 · 0.19 · 0.09 0.25 · 0.17 · 0.11Crystal morphology cuboid cuboidCrystal system monoclinic monoclinicSpace group P21/n P21/nFormula weight 605.61 559.54Unit cell dimensions

a (A) 8.684(13) 10.472(4)b (A) 11.316(17) 9.236(4)c (A) 27.91(4) 25.156(9)a = c (�) 90 90b (�) 95.98(3) 100.484(6)

V (A3) 2728(7) 2392.6(16)Z 4 4dcalc. (g cm

�3) 1.47 1.55l(Mo) (mm�1) 1.61 1.82F(000) 1256 1152k(Mo Ka) (A) 0.71073 0.71073T (K) 293(2) 293(2)Reflections collected 20555 16283Independent reflections 5575 4074Observed reflections, [I > 2r(I)] 4356 3457Parameters 361 310Goodness of fit on F2 1.020 1.182R (observed data) [R (all data)] 0.050 [0.068] 0.043 [0.052]Rw (observed data)[Rw(all data)]

0.1114 [0.1189] 0.1011 [0.1051]

Largest difference peakand hole (e A�3)

0.583 and�0.447

0.737 and�0.527

Weight factor:w ¼ 1=½r2ðF 2

oÞ þ ðAPÞ2 þ BP �A = 0.0524,B = 1.5669

A = 0.0446,B = 2.1385

cycles to lower the shift/e.s.d. values. All the non-hydro-gen atoms in the structures were refined anisotropically.The structure solution and refinement were made usingthe SHELX system of programs [17]. The perspective viewsof the molecules were obtained by ORTEP [18]. Selectedcrystallographic data are given in Table 1.

3. Results and discussion

3.1. Synthesis and general aspects

Dicopper(II) complexes 1 and 2 are prepared ingood yield by substituting the acetate of the precursorcomplex [Cu2L(O2CMe)] having a pentadentate trian-ionic Schiff base N,N 0-(2-hydroxypropane-1,3-diyl)bis-(acetylacetoneimine) (H3L) by the carboxylate ligand,HO-p-C6H4ACH@CHACO2

� in methanol or propa-nol solvent. We have prepared the complex fromtwo different solvents with an objective to isolatesupramolecular host structures stabilized by differentlattice guest molecules (Scheme 1). The choice of pro-panol as a reaction medium is to isolate a complexwith lattice water molecule(s) as propanol is less likelyto support a supramolecular structure by hydrogenbonding interactions. The complexes are characterizedfrom the analytical and spectral data. They are crystal-lized as [Cu2L(O2CACH@CHAC6H4-p-OH)(MeOH)] ÆMeOH (1 ÆMeOH) and [Cu2L(O2CACH@CHAC6H4-p-OH)] ÆH2O (2 ÆH2O) and are structurally character-ized by single crystal X-ray crystallography. The com-plexes are soluble in polar organic solvents anddisplay a d–d band in the electronic spectra at�630 nm in DMSO.

3.2. Crystal structures

Both the complexes crystallize in the centrosymmet-ric monoclinic space group P21/n with four dicopperunits in the unit cell, having alternating chains of oppo-site helical chirality in the crystal. Selected bond dis-tance and bond angle data are given in Table 2. Thecrystal structure of 1 ÆMeOH shows the presence of adicopper(II) complex with a (l-alkoxo)(l-carboxylate)-dicopper(II) core in which the Cu(1) atom has a square

Scheme 1. Synthetic scheme for the complexes 1 and 2.

Table 2Selected bond distances (A) and angles (�) in 1 ÆMeOH and 2 ÆH2O

1 ÆMeOH 2 ÆH2O

Cu(1)� � �Cu(2) 3.504(4) 3.500(1)Cu(1)–O(1) 1.918(3) 1.900(3)Cu(1)–O(2) 1.925(3) 1.900(3)Cu(1)–O(5) 1.946(3) 1.922(3)Cu(1)–N(1) 1.923(3) 1.920(3)Cu(2)–O(2) 1.931(3) 1.922(3)Cu(2)–O(3) 1.930(4) 1.904(3)Cu(2)–O(4) 1.973(3) 1.957(3)Cu(2)–O(7) 2.339(4)Cu(2)–N(2) 1.961(3) 1.933(3)Cu(1)–O(2)–Cu(2) 130.6(1) 132.8(1)O(1)–Cu(1)–O(2) 177.4(1) 175.5(1)O(1)–Cu(1)–O(5) 85.3(1) 84.9(1)O(1)–Cu(1)–N(1) 95.0(2) 94.4(1)O(2)–Cu(1)–O(5) 94.9(1) 95.3(1)O(2)–Cu(1)–N(1) 84.8(2) 85.7(1)O(5)–Cu(1)–N(1) 178.0(1) 175.1(1)O(2)–Cu(2)–O(3) 169.1(1) 178.5(1)O(2)–Cu(2)–O(4) 93.1(1) 93.9(1)O(2)–Cu(2)–N(2) 85.2(1) 84.6(1)O(2)–Cu(2)–O(7) 93.4(1)O(3)–Cu(2)–O(4) 87.1(1) 87.2(1)O(3)–Cu(2)–O(7) 97.4(1)O(3)–Cu(2)–N(2) 92.4(1) 94.2(1)O(4)–Cu(2)–O(7) 88.2(2)O(4)–Cu(2)–N(2) 168.8(1) 178.3(1)O(7)–Cu(2)–N(2) 103.0(2)

S. Gupta et al. / Polyhedron 24 (2005) 1922–1928 1925

planar CuNO3 coordination geometry and the Cu(2)atom has a square pyramidal (4 + 1) coordinationgeometry, giving a Cu� � �Cu distance of 3.504(1) A(Fig. 1). The Schiff base ligand displays a pentadentatemode of coordination with the alkoxo oxygen atombridging the copper centers. The Cu(2) atom is bondedto a methanol ligand at the elongated axial site. The ba-sal planes at the metal centers are nearly coplanar,forming an angle of 10.1(1)�. The ACH@CHC6H4Amoiety of the p-hydroxycinnamate in 1 ÆMeOH is nearly

Fig. 1. ORTEP view of the complex [Cu2L(O2CACH@CHAC6H4-p-OH)(MeOH)] ÆMeOH (1 ÆMeOH).

planar giving an angle of 7.3(3)� between the planeshaving the �OOCACH@CHA group and the aromaticC6H4 ring. The cinnamate ligand forms an angle of5.1(1)� with the coordination plane of the {Cu2L}

+ moi-ety. The complex self assembles into a supramolecularstructure through hydrogen bonding interactions (Fig.2). There are two types of intermolecular hydrogenbonding interactions observed in this crystal structure.The copper-bound methanol ligand is involved in thehydrogen bonding interactions with the terminal Schiffbase oxygen forming a non-covalently bound ‘‘tetra-meric’’ unit [Fig. 2(a); O(7)� � �O(1)#1, 2.969(5) A;O(7)–H(25)� � � O(1)#1, 162.0�; #1: �x + 1,�y + 1,�z + 2]. The other hydrogen-bonding network forms asupramolecular structure involving the lattice methanolmolecule, the pendant hydroxy group of the carboxylateand the terminal oxygen atom of the Schiff base (Fig.2(b)). The O(6)� � �O(8) and O(8)� � �O(3)#2 distancesare 2.709(6) and 2.812(6) A, respectively (#2: �x +1/2 + 1,�y + 1/2, �z + 1/2 + 1). The hydrogen bondsare nearly linear as evidenced from the O(6)–H(6)� � �O(8) and O(8)–H(26)� � �O(3)#2 angles of 167.5�and 163.8�, respectively.

The crystal structure of 2 ÆH2O has a (l-alkoxo)(l-carboxylate)dicopper(II) core formed by the pen-tadentate Schiff base (L) and the bridging carboxyl-ate. The copper centers have a square planar CuNO3

Fig. 2. (a) The hydrogen bonding interactions involving the copper-bound methanol ligand and the terminal oxygen atom of the Schiffbase in 1 ÆMeOH. (b) The hydrogen bonding interactions involving thep-hydroxy group of the carboxylate, lattice methanol and the oxygenatom of the Schiff base in 1 ÆMeOH.

Fig. 3. ORTEP view of the complex in [Cu2L(O2CACH@CHAC6H4-p-OH)] ÆH2O (2 ÆH2O).

Fig. 4. Perspective view of the self-assembled helical supramolecularstructure of 2 ÆH2O. The bifurcated hydrogen bond is shown (- - -).

Fig. 5. vMT vs. T plots for the complexes 1 and 2. The solid lines showthe theoretical fit to the experimental data (n, 1; s, 2).

1926 S. Gupta et al. / Polyhedron 24 (2005) 1922–1928

coordination geometry, giving a Cu� � �Cu distance of3.501(1) A (Fig. 3). The basal planes at the copper(II)centers are nearly coplanar, forming an angle of5.2(1)�. The ACH@CHC6H4A moiety of the p-hydroxy-cinnamate is nearly planar giving an angle of 4.2(3)�between the ACH@CHA group and the aromaticC6H4 ring. The cinnamate ligand forms an angle of5.8(1)� with the coordination plane of the {Cu2L}

+

moiety. This is in contrast to its analogous complex[Cu2L

0(O2CACH@CHAC6H4-p-OH)] Æ2H2O, havingthe Schiff base N,N 0-(2-hydroxypropane-1,3-diyl)bis(sal-icylaldimine) (H3L

0), forming an angle of �30� betweenthe plane of {Cu2L

0} and the p-hydroxycinnamate li-gand. The essentially planar structure in 2 Æ2H2O couldbe due to the absence of any steric rigidity in compari-son to H3L

0 that has two salicylaldimine moieties. Com-plex 2 ÆH2O self assembles in a unique way to form ahelical supramolecular structure through bifurcatedhydrogen bonding interactions involving the p-hydroxygroup of the carboxylate and the terminal oxygen atomO(1) of the Schiff base and one oxygen atom of thebridging carboxylate O(5) [Fig. 4; O(6)� � �O(1)#1,2.899(4) A, O(6)–H(6)� � � O(1)#1, 144.7�; O(6)� � �O(5)#1, 2.975(4) A, O(6)–H(6)� � �O(5)#1, 144.9�; #1:�x + 1/2 + 1,+y + 1/2, �z + 1/2 + 1]. This is in con-trast to the helical structure of its H3L

0 analog that in-volves the p-hydroxy group of the carboxylate and thephenoxo atom of the Schiff base (L 0) in the hydrogenbonding interactions. The O(7) lattice water moleculein 2 ÆH2O is hydrogen bonded to the terminal oxygenatom O(3) of the Schiff base belonging to the binuclearcomplex [O(7)� � �O(3), 2.902(6) A; O(7)–H(7B)� � �O(3),178.9�]. The lattice water in 2 ÆH2O does not take partin the helical structure formation but could be partiallyresponsible for the overall planarity of the carboxylateand the {Cu2L}

+ moieties.

3.3. Magnetic properties

The variable-temperature magnetic susceptibilitydata for the complexes in the temperature range 300–18 K show an antiferromagnetic (AF) behavior of thecomplexes (Fig. 5). The magnetic moment values (percopper) of �1.7 lB at 300 K and �0.2 lB at 18 K indi-cate significant AF spin–spin coupling in the asymmetri-cally dibridged dicopper(II) core. The theoretical fittingsof the magnetic susceptibility data, obtained by usingthe modified Bleaney–Bowers expression, gave the sin-

Fig. 6. Thermogravimetric plots of (a) 1 ÆMeOH and (b) 2 ÆH2Oshowing weight loss of the sample with increasing temperature.

S. Gupta et al. / Polyhedron 24 (2005) 1922–1928 1927

glet–triplet energy separation (2J) value of �160 cm�1

for 1 and �132 cm�1 for 2, with the singlet as theground state [16]. Complexes 1 and 2 belong to the classof asymmetrically dibridged dicopper(II) complexes inwhich the nature and magnitude of the intramolecularmagnetic exchange interactions in the {Cu2(l-OR)-(l-O2CR)} core primarily depend upon the Cu–O–Cuangle [14,19–28]. The lower magnitude of the 2J value,even with a large Cu–O–Cu angle of �130�, is attributedto the counter-complimentary nature of overlap of themagnetic orbitals and is directly proportional to the en-ergy separation of the symmetric and antisymmetriccombinations of magnetic orbitals. It is interesting tonote that complex 1 with a lower Cu–O–Cu angle of130.6(1)� has greater AF interaction than complex 2 thathas a Cu–O–Cu angle of 132.7(1)�. The difference couldbe due to stronger Cu–O (carboxylate) bonds in 2 thanin 1. Complexes 1 and 2 differ considerably in theirsupramolecular structures due to the involvement ofthe oxygen atoms in the non-covalent interactions. Theyalso differ in the core structure where 1 has four and fivecoordinate copper centers, while the copper atoms in 2have a four coordinate geometry. Again, the averageCu–O distance in the l-O2CR bridge of 1 ÆMeOH is1.96 A and the same for 2 ÆH2O is 1.94 A. The coordi-nating planes having copper atoms Cu(1) and Cu(2)have a dihedral angle of 9.54(7)� in 1 ÆMeOH and5.14(6)� for 2 ÆH2O. The observation of higher overallcounter-complimentary effect in 2 in comparison to 1

could be due to the structural variations involving thecarboxylate bridge and the supramolecular helices.

3.4. Thermogravimetric studies

Complex 1 ÆMeOH shows a weight loss of 4.1% in thetemperature range of 50–115 �C, which is lower than thecalculated percentage of 5.3% for the lattice methanol(Fig. 6). The discrepancy could be due to the labile nat-ure of the lattice methanol at room temperature. Fur-ther, it is noticed that the copper bound methanol isnot released in this temperature range, possibly due tothe participation of the copper-bound methanol in thenon-covalent ‘‘tetrameric’’ structure formation. Wehave observed a weight loss of �40% above 225 �C. Thiscould be due to the degradation of the complex abovethis temperature. Complex 2 ÆH2O shows the TGAgraph with a weight loss of 3.0% in the temperaturerange of 110–140 �C, which is in accordance of the the-oretical value of 3.22% (Fig. 6). A further weight loss of�46% is observed above 240 �C due to possible degrada-tion of the complex.

In summary, two new asymmetrically dibridgeddicopper(II) complexes with a pentadentate Schiff baseand p-hydroxycinnamate are prepared and structurallycharacterized by X-ray crystallography. The complexesform helical supramolecular structures involving hydro-

gen bonding interactions. The crystal structure of thecomplex with a lattice water molecule differs consider-ably from its analogous structure having the Schiff baseN,N 0-1,3-diyl-bis(salicylaldiminato)propan-2-ol (H3L

0).The angle between the plane of {Cu2L} with the carbox-ylate plane in 2 ÆH2O is �6�, while the angle involving{Cu2L

0} and the carboxylate in the reported structureis �30� [11]. The helix formation in 2 ÆH2O involvesthe bridging and pendant p-hydroxy group oxygenatoms of the carboxylate through hydrogen bondinginteractions along with the terminal oxygen atom ofthe Schiff base. Its analogous complex having the H3L

0

ligand forms a helix of a different structure [11,12].The present work shows the significant effect of thepentadentate Schiff base in the self-assembly process ofsupramolecular helix formation with different structuralconformations.

Acknowledgements

We thank the Department of Science and Technol-ogy, Government of India, for the financial supportand for the CCD X-ray diffractometer facility.S.G. thanks CSIR for a fellowship.

Appendix A. Supplementary data

Crystallographic data for the structural analysis of1 ÆMeOH and 2 ÆH2O have been deposited with theCambridge Crystallographic Data Center, CCDC Nos.267777 and 267778. Copies of this information may beobtained free of charge from The Director, CCDC, 12Union Road, Cambridge, CB2 1EZ, UK (fax: +44-1223-366-033; e-mail: deposit@ccdc.ac.uk or www:http://www.ccdc.cam.ac.uk). Supplementary data asso-ciated with this article can be found, in the online ver-sion, at doi:10.1016/j.poly.2005.05.015.

1928 S. Gupta et al. / Polyhedron 24 (2005) 1922–1928

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