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Effect of a pentadentate Schiff base on the helical supramolecular structures of (μ-alkoxo)(μ-carboxylato)dicopper(II) complexes
Polyhedron 24 (2005) 1922–1928 www.elsevier.com/locate/poly
Effect of a pentadentate Schiff base on the helical supramolecular structures 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 2005 Available online 19 August 2005
Abstract Two new asymmetrically dibridged dicopper(II) complexes with a pentadentate Schiff base and p-hydroxycinnamate are prepared and structurally characterized by X-ray crystallography. The complexes have an asymmetrically dibridged (l-alkoxo)(l-carboxylato)dicopper(II) core with an alkoxo bridge from the Schiff base and the carboxylate showing a three-atom bridging mode. Variable-temperature magnetic studies show the complexes having an antiferromagnetically coupled spin system giving a singlet–triplet energy separation of 160 and 132 cm1 for 1 and 2, respectively. Complex 1, with a shorter Cu–OR–Cu angle, displays greater antiferromagnetic spin coupling. Besides the Cu–OR–Cu angle, the role of the carboxylate ligand and the supramolecular structure in the spin coupling phenomena is observed. Complex 1 Æ MeOH shows the formation of intermolecular hydrogen bonds involving the axial methanol and the pentadentate Schiff base terminal oxygen atom. There is additional hydrogen bonding interactions involving the p-hydroxy group of the carboxylate, the lattice methanol and the terminal oxygen atom. The crystal structure of complex 2 Æ H2O displays the presence of a helical supramolecular structure due to hydrogen bonding interactions involving the pendant p-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 polymeric species formed by the process of molecular self-assembly are of interest for their novel structural features and for their ability to stabilize guest molecules of different sizes and conformations [1–12]. We have recently shown that dicopper(II) complexes having a pentadentate Schiff base N,N 0 -1,3-diyl-bis(salicylaldimino)propan-2-ol (H3L 0 ) in its trianionic form and the carboxylate ligands p-HOC6 H4 –X–CO2 , where X is a spacer like ACH@ CHA, –CH2– or –CH2– CH2–, form different types of supramolecular structures [11,12]. Among the three complexes, the *
Corresponding author. Tel.: +91 80 229 32533; fax: +91 80 236 00683. E-mail address: [email protected] (A.R. Chakravarty). 0277-5387/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2005.05.015
p-hydroxycinnamate complex [Cu2L 0 (O2CACH@CHA C6H4-p-OH)] Æ 2H2O forms a helical chain structure due to hydrogen-bonding interactions involving the p-hydroxy group of the carboxylate and one phenoxo oxygen atom of the Schiff base [11]. An important structural feature in this helical structure is the angle of 30 between the cinnamate ligand and the plane of the {Cu2L 0 }+ unit. Two lattice water molecules form an unprecedented helical one-dimensional chain, in which the alternate water molecules are anchored to the supramolecular host structure. The present work stems from our interest to study the role of the Schiff base in the formation of a helical host structure and the ability of the supramolecular host to support any guest solvent molecule(s). We have chosen a potentially pentadentate Schiff base N,N 0 -1,3-diylbis(acetylacetoneimine)propan-2-ol (H3L) derived from acetylacetone and 1,3-diaminopropan-2-ol for the
S. Gupta et al. / Polyhedron 24 (2005) 1922–1928
synthesis of asymmetrically dibridged dicopper(II) complexes using p-hydroxycinnamic acid as the carboxylate. The Schiff base H3L with two acetylacetoneimine moieties is similar to H3L 0 that has two salicylaldimine groups. Herein, we report the synthesis, crystal structure and the magnetic properties of the complexes [Cu2L(O2CACH@CHAC6H4-p-OH)(MeOH)] Æ MeOH (1 Æ MeOH) and [Cu2L(O2CACH@ CHAC6H4-p-OH)] Æ H2O (2 Æ H2O). The significant observation of this study is the crystal structures of the complexes showing formation of different types of supramolecular structures. The complex 2 Æ H2O forms a helical structure through intermolecular hydrogen bonding interactions involving only the carboxylate ligands. The lattice water is non-covalently bound to the Schiff base of the dinuclear unit and does not take part in the supramolecular structure formation.
2. Experimental 2.1. Materials and measurements All reagents and chemicals were purchased from commercial sources and used as received. The solvents were purified following reported procedures [13]. The Schiff base N,N 0 -1,3-diyl-bis(acetylacetoneimine)propan-2-ol (H3L) and the dicopper(II) precursor complex, [Cu2L(O2CMe)] were prepared by literature methods [14]. The elemental analysis was done using a Thermo Finnigan FLASH EA 1112 CHN analyzer. The electronic and infrared spectral data were obtained from Lambda 55 and Perkin–Elmer Spectrum One FT-IR spectrometers, respectively. Variable temperature magnetic susceptibility data for the polycrystalline samples were obtained in the temperature range 18–300 K using a Model 300 Lewis-coil-force magnetometer of George Associates Inc. (Berkeley, USA) make equipped with a Cahn balance and a closed cycle cryostat (Air products). Hg[Co(NCS)4] was used as a standard. Experimental susceptibility data were corrected for diamagnetic contributions and temperature independent paramagnetism (Na = 60 · 106 cm3 M1 per copper). The molar magnetic susceptibilities were fitted to the modified Bleany–Bowers expression by means of a least-squares computer program [15,16]. The Hamiltonian and susceptibility equation used ^1 S ^ 2 Þ (S1 = S2 = 1/2 for a d9–d9 ^ ¼ 2J ðS are: H dicopper(II) core) and vCu ¼ ½Ng2 b2 =kðT hÞ½3 þ exp 1 ð2J =kðT hÞÞ ð1 qÞ þ ðNg21 b2 =4kT Þq þ N a , where q is the fraction of monomeric impurity and 2J is the singlet–triplet separation energy. The magnetic moments were calculated in lB unit. TGA measurements were made using a Mettler Toledo Star thermal analyzer.
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2.2. Preparation of [Cu2L(O2CACH@CHAC6H4-p-OH)(MeOH)] (1) Complex [Cu2L(O2CMe)] (0.15 g, 0.33 mmol) in MeOH (10 mL) was reacted with the sodium salt of p-hydroxycinnamic acid, prepared in situ by reacting the carboxylic acid (0.056 g, 0.33 mmol) with NaOH (0.013 g, 0.33 mmol) in methanol (5 mL), and the reaction mixture was refluxed for 30 min, followed by cooling to an ambient temperature. The solution was filtered and the filtrate on slow evaporation gave the green product in 70% yield. Single crystals of composition 1 Æ MeOH, suitable for X-ray diffraction, were obtained from 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, M1 cm1)]: 632 (350), 315 (126 000), 290 (109 000). FT-IR (KBr phase), cm1: 3400br, 2920w, 1612s, 1508s, 1400s, 1272s, 1169m, 1013m, 942m, 833m, 701m (br, broad; s, strong; m, medium; w, weak). Magnetic moment (leff) per copper in the dimeric unit: 1.63 lB at 300 K; 0.17 lB at 18 K; 2J = 160 cm1 from the theoretical fitting with g = 2.13, q = 0.004, g1 = 2.2, R = 8.5 · 103. 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 with the sodium salt of p-hydroxycinnamic acid, prepared in situ by reacting the carboxylic acid (0.082 g, 0.49 mmol) with NaOH (0.019 g, 0.47 mmol) in 1-propanol (10 mL) and the reaction mixture was refluxed for 30 min, followed by cooling to room temperature. The solution was filtered and the filtrate on slow evaporation 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 spectral data in DMSO [kmax, nm (e, M1 cm1)]: 630 (320), 315 (132 000), 285 (100 000). FT-IR (KBr phase, cm1): 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 cm1 from the theoretical fitting with g = 2.07, q = 0.005, g1 = 2.14, R = 2.8 · 102. Single crystals of 2 Æ H2O were obtained on slow evaporation of the mother liquor in the presence of a trace quantity of water. 2.4. X-ray crystallographic studies Single crystals of the complexes [Cu2L(O2CA CH@CHAC6H4-p-OH)(MeOH)] Æ MeOH (1 Æ MeOH) and [Cu2L(O2CACH@CHAC6H4-p-OH)] Æ H2O (2 Æ H2O) of the respective sizes 0.49 · 0.21 · 0.13 and
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S. Gupta et al. / Polyhedron 24 (2005) 1922–1928
0.26 · 0.18 · 0.11 mm3 were mounted on glass fibers with epoxy cement. All geometric and intensity data were collected using a Bruker SMART APEX CCD diffractometer having a fine focus 1.75 kW sealed tube Mo Ka X-ray source with increasing x (width of 0.3 per frame) at a scan speed of 5 s/frame for 1 Æ MeOH and 12 s/frame for 2 Æ H2O. A total of 20 555 reflections were collected for 1 in the range 1.47 6 h 6 27.18 of which 4356 reflections with I P 2r(I) were used for structure solution using 361 parameters. For 2 Æ H2O, a total of 16 283 reflections were collected in the range 2.0 6 h 6 24.71, of which 3457 reflections with I P 2r(I) were used for structure solution using 310 parameters. The hydrogen atoms attached to the carbon atoms, except two methanol molecules in 1 Æ MeOH, were fixed in their calculated positions and refined using a riding model for both the structures. The hydrogen atoms of the copper-bound and lattice methanol molecules were located in the difference Fourier maps and were refined isotropically. In both the structures, the hydrogen atom of the p-hydroxy group was located in the difference Fourier map and was refined isotropically. The hydrogen atoms of the lattice water in 2 Æ H2O were also located from the difference Fourier map and blocked after refining for the last few Table 1 Crystallographic data for (MeOH)] Æ MeOH (1 Æ MeOH) p-OH)] Æ H2O (2 Æ H2O) Empirical formula Crystal size (mm) Crystal morphology Crystal system Space group Formula weight Unit cell dimensions ˚) a (A ˚) b (A ˚) c (A a = c () b () ˚ 3) V (A Z dcalc. (g cm3) l(Mo) (mm1) F(0 0 0) ˚) k(Mo Ka) (A T (K) Reflections collected Independent reflections Observed reflections, [I > 2r(I)] Parameters Goodness of fit on F2 R (observed data) [R (all data)] Rw (observed data) [Rw(all data)] Largest difference peak ˚ 3) and hole (e A Weight factor: w ¼ 1=½r2 ðF 2o Þ þ ðAP Þ2 þ BP
[Cu2L(O2CACH@CHAC6H4-p-OH) and [Cu2L(O2CACH@CHAC6H41 Æ MeOH
2 Æ H2O
C24H34N2O8Cu2 0.25 · 0.19 · 0.09 cuboid monoclinic P21/n 605.61
C22H28N2O7Cu2 0.25 · 0.17 · 0.11 cuboid monoclinic P21/n 559.54
8.684(13) 11.316(17) 27.91(4) 90 95.98(3) 2728(7) 4 1.47 1.61 1256 0.71073 293(2) 20 555 5575 4356 361 1.020 0.050 [0.068] 0.1114 [0.1189]
10.472(4) 9.236(4) 25.156(9) 90 100.484(6) 2392.6(16) 4 1.55 1.82 1152 0.71073 293(2) 16 283 4074 3457 310 1.182 0.043 [0.052] 0.1011 [0.1051]
0.583 and 0.447 A = 0.0524, B = 1.5669
0.737 and 0.527 A = 0.0446, B = 2.1385
cycles to lower the shift/e.s.d. values. All the non-hydrogen atoms in the structures were refined anisotropically. The structure solution and refinement were made using the SHELX system of programs [17]. The perspective views of the molecules were obtained by ORTEP [18]. Selected crystallographic data are given in Table 1.
3. Results and discussion 3.1. Synthesis and general aspects Dicopper(II) complexes 1 and 2 are prepared in good yield by substituting the acetate of the precursor complex [Cu2L(O2CMe)] having a pentadentate trianionic Schiff base N,N 0 -(2-hydroxypropane-1,3-diyl)bis(acetylacetoneimine) (H3L) by the carboxylate ligand, HO-p-C6 H4 ACH@CHACO2 in methanol or propanol solvent. We have prepared the complex from two different solvents with an objective to isolate supramolecular host structures stabilized by different lattice guest molecules (Scheme 1). The choice of propanol as a reaction medium is to isolate a complex with lattice water molecule(s) as propanol is less likely to support a supramolecular structure by hydrogen bonding interactions. The complexes are characterized from the analytical and spectral data. They are crystallized as [Cu2L(O2CACH@CHAC6H4-p-OH)(MeOH)] Æ MeOH (1 Æ MeOH) and [Cu2L(O2CACH@CHAC6H4p-OH)] Æ H2O (2 Æ H2O) and are structurally characterized by single crystal X-ray crystallography. The complexes are soluble in polar organic solvents and display a d–d band in the electronic spectra at 630 nm in DMSO. 3.2. Crystal structures Both the complexes crystallize in the centrosymmetric monoclinic space group P21/n with four dicopper units in the unit cell, having alternating chains of opposite helical chirality in the crystal. Selected bond distance and bond angle data are given in Table 2. The crystal structure of 1 Æ MeOH shows the presence of a dicopper(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.
S. Gupta et al. / Polyhedron 24 (2005) 1922–1928 Table 2 ˚ ) and angles () in 1 Æ MeOH and 2 Æ H2O Selected bond distances (A 1 Æ MeOH Cu(1) Cu(2) Cu(1)–O(1) Cu(1)–O(2) Cu(1)–O(5) Cu(1)–N(1) Cu(2)–O(2) Cu(2)–O(3) Cu(2)–O(4) Cu(2)–O(7) Cu(2)–N(2) Cu(1)–O(2)–Cu(2) O(1)–Cu(1)–O(2) O(1)–Cu(1)–O(5) O(1)–Cu(1)–N(1) O(2)–Cu(1)–O(5) O(2)–Cu(1)–N(1) O(5)–Cu(1)–N(1) O(2)–Cu(2)–O(3) O(2)–Cu(2)–O(4) O(2)–Cu(2)–N(2) O(2)–Cu(2)–O(7) O(3)–Cu(2)–O(4) O(3)–Cu(2)–O(7) O(3)–Cu(2)–N(2) O(4)–Cu(2)–O(7) O(4)–Cu(2)–N(2) O(7)–Cu(2)–N(2)
3.504(4) 1.918(3) 1.925(3) 1.946(3) 1.923(3) 1.931(3) 1.930(4) 1.973(3) 2.339(4) 1.961(3) 130.6(1) 177.4(1) 85.3(1) 95.0(2) 94.9(1) 84.8(2) 178.0(1) 169.1(1) 93.1(1) 85.2(1) 93.4(1) 87.1(1) 97.4(1) 92.4(1) 88.2(2) 168.8(1) 103.0(2)
2 Æ H2O 3.500(1) 1.900(3) 1.900(3) 1.922(3) 1.920(3) 1.922(3) 1.904(3) 1.957(3) 1.933(3) 132.8(1) 175.5(1) 84.9(1) 94.4(1) 95.3(1) 85.7(1) 175.1(1) 178.5(1) 93.9(1) 84.6(1) 87.2(1) 94.2(1) 178.3(1)
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planar giving an angle of 7.3(3) between the planes having the OOCACH@CHA group and the aromatic C6H4 ring. The cinnamate ligand forms an angle of 5.1(1) with the coordination plane of the {Cu2L}+ moiety. The complex self assembles into a supramolecular structure through hydrogen bonding interactions (Fig. 2). There are two types of intermolecular hydrogen bonding interactions observed in this crystal structure. The copper-bound methanol ligand is involved in the hydrogen bonding interactions with the terminal Schiff base 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 a supramolecular structure involving the lattice methanol molecule, the pendant hydroxy group of the carboxylate and the terminal oxygen atom of the Schiff base (Fig. 2(b)). The O(6) O(8) and O(8) O(3)#2 distances ˚ , respectively (#2: x + are 2.709(6) and 2.812(6) A 1/2 + 1, y + 1/2, z + 1/2 + 1). The hydrogen bonds are 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 Æ H2O has a (l-alkoxo) (l-carboxylate)dicopper(II) core formed by the pentadentate Schiff base (L) and the bridging carboxylate. The copper centers have a square planar CuNO3
planar CuNO3 coordination geometry and the Cu(2) atom has a square pyramidal (4 + 1) coordination ˚ geometry, giving a Cu Cu distance of 3.504(1) A (Fig. 1). The Schiff base ligand displays a pentadentate mode of coordination with the alkoxo oxygen atom bridging the copper centers. The Cu(2) atom is bonded to a methanol ligand at the elongated axial site. The basal planes at the metal centers are nearly coplanar, forming an angle of 10.1(1). The ACH@CHC6H4A moiety of the p-hydroxycinnamate in 1 Æ MeOH is nearly
Fig. 1. ORTEP view of the complex [Cu2L(O2CACH@CHAC6H4p-OH)(MeOH)] Æ MeOH (1 Æ MeOH).
Fig. 2. (a) The hydrogen bonding interactions involving the copperbound methanol ligand and the terminal oxygen atom of the Schiff base in 1 Æ MeOH. (b) The hydrogen bonding interactions involving the p-hydroxy group of the carboxylate, lattice methanol and the oxygen atom of the Schiff base in 1 Æ MeOH.
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Fig. 3. ORTEP view of the complex in [Cu2L(O2CACH@CHAC6H4p-OH)] Æ H2O (2 Æ H2O).
coordination geometry, giving a Cu Cu distance of ˚ (Fig. 3). The basal planes at the copper(II) 3.501(1) A centers are nearly coplanar, forming an angle of 5.2(1). The ACH@CHC6H4A moiety of the p-hydroxycinnamate is nearly planar giving an angle of 4.2(3) between the ACH@CHA group and the aromatic C6H4 ring. The cinnamate ligand forms an angle of 5.8(1) with the coordination plane of the {Cu2L}+ moiety. This is in contrast to its analogous complex [Cu2L 0 (O2CACH@CHAC6H4-p-OH)] Æ 2H2O, having the Schiff base N,N 0 -(2-hydroxypropane-1,3-diyl)bis(salicylaldimine) (H3L 0 ), forming an angle of 30 between the plane of {Cu2L 0 } and the p-hydroxycinnamate ligand. The essentially planar structure in 2 Æ 2H2O could be due to the absence of any steric rigidity in comparison to H3L 0 that has two salicylaldimine moieties. Complex 2 Æ H2O self assembles in a unique way to form a helical supramolecular structure through bifurcated hydrogen bonding interactions involving the p-hydroxy group of the carboxylate and the terminal oxygen atom O(1) of the Schiff base and one oxygen atom of the bridging carboxylate O(5) [Fig. 4; O(6) O(1)#1, ˚ , O(6)–H(6) O(1)#1, 144.7; O(6) 2.899(4) A ˚ , O(6)–H(6) O(5)#1, 144.9; #1: O(5)#1, 2.975(4) A x + 1/2 + 1, +y + 1/2, z + 1/2 + 1]. This is in contrast to the helical structure of its H3L 0 analog that involves the p-hydroxy group of the carboxylate and the phenoxo atom of the Schiff base (L 0 ) in the hydrogen bonding interactions. The O(7) lattice water molecule in 2 Æ H2O is hydrogen bonded to the terminal oxygen atom O(3) of the Schiff base belonging to the binuclear ˚ ; O(7)–H(7B) O(3), complex [O(7) O(3), 2.902(6) A 178.9]. The lattice water in 2 Æ H2O does not take part in the helical structure formation but could be partially responsible for the overall planarity of the carboxylate and the {Cu2L}+ moieties.
Fig. 4. Perspective view of the self-assembled helical supramolecular structure of 2 Æ H2O. The bifurcated hydrogen bond is shown (- - -).
3.3. Magnetic properties The variable-temperature magnetic susceptibility data for the complexes in the temperature range 300– 18 K show an antiferromagnetic (AF) behavior of the complexes (Fig. 5). The magnetic moment values (per copper) of 1.7 lB at 300 K and 0.2 lB at 18 K indicate significant AF spin–spin coupling in the asymmetrically dibridged dicopper(II) core. The theoretical fittings of the magnetic susceptibility data, obtained by using the modified Bleaney–Bowers expression, gave the sin-
Fig. 5. vMT vs. T plots for the complexes 1 and 2. The solid lines show the theoretical fit to the experimental data (n, 1; s, 2).
S. Gupta et al. / Polyhedron 24 (2005) 1922–1928
glet–triplet energy separation (2J) value of 160 cm1 for 1 and 132 cm1 for 2, with the singlet as the ground state [16]. Complexes 1 and 2 belong to the class of asymmetrically dibridged dicopper(II) complexes in which the nature and magnitude of the intramolecular magnetic exchange interactions in the {Cu2(l-OR)(l-O2CR)} core primarily depend upon the Cu–O–Cu angle [14,19–28]. The lower magnitude of the 2J value, even with a large Cu–O–Cu angle of 130, is attributed to the counter-complimentary nature of overlap of the magnetic orbitals and is directly proportional to the energy separation of the symmetric and antisymmetric combinations of magnetic orbitals. It is interesting to note that complex 1 with a lower Cu–O–Cu angle of 130.6(1) has greater AF interaction than complex 2 that has a Cu–O–Cu angle of 132.7(1). The difference could be due to stronger Cu–O (carboxylate) bonds in 2 than in 1. Complexes 1 and 2 differ considerably in their supramolecular structures due to the involvement of the oxygen atoms in the non-covalent interactions. They also differ in the core structure where 1 has four and five coordinate copper centers, while the copper atoms in 2 have a four coordinate geometry. Again, the average Cu–O distance in the l-O2CR bridge of 1 Æ MeOH is ˚ and the same for 2 Æ H2O is 1.94 A ˚ . The coordi1.96 A nating planes having copper atoms Cu(1) and Cu(2) have a dihedral angle of 9.54(7) in 1 Æ MeOH and 5.14(6) for 2 Æ H2O. The observation of higher overall counter-complimentary effect in 2 in comparison to 1 could be due to the structural variations involving the carboxylate bridge and the supramolecular helices. 3.4. Thermogravimetric studies Complex 1 Æ MeOH shows a weight loss of 4.1% in the temperature range of 50–115 C, which is lower than the calculated percentage of 5.3% for the lattice methanol (Fig. 6). The discrepancy could be due to the labile nature of the lattice methanol at room temperature. Further, it is noticed that the copper bound methanol is not released in this temperature range, possibly due to the participation of the copper-bound methanol in the non-covalent ‘‘tetrameric’’ structure formation. We have observed a weight loss of 40% above 225 C. This could be due to the degradation of the complex above this temperature. Complex 2 Æ H2O shows the TGA graph with a weight loss of 3.0% in the temperature range of 110–140 C, which is in accordance of the theoretical value of 3.22% (Fig. 6). A further weight loss of 46% is observed above 240 C due to possible degradation of the complex. In summary, two new asymmetrically dibridged dicopper(II) complexes with a pentadentate Schiff base and p-hydroxycinnamate are prepared and structurally characterized by X-ray crystallography. The complexes form helical supramolecular structures involving hydro-
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Fig. 6. Thermogravimetric plots of (a) 1 Æ MeOH and (b) 2 Æ H2O showing weight loss of the sample with increasing temperature.
gen bonding interactions. The crystal structure of the complex with a lattice water molecule differs considerably from its analogous structure having the Schiff base N,N 0 -1,3-diyl-bis(salicylaldiminato)propan-2-ol (H3L 0 ). The angle between the plane of {Cu2L} with the carboxylate plane in 2 Æ H2O is 6, while the angle involving {Cu2L 0 } and the carboxylate in the reported structure is 30 [11]. The helix formation in 2 Æ H2O involves the bridging and pendant p-hydroxy group oxygen atoms of the carboxylate through hydrogen bonding interactions along with the terminal oxygen atom of the 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 the pentadentate Schiff base in the self-assembly process of supramolecular helix formation with different structural conformations.
Acknowledgements We thank the Department of Science and Technology, Government of India, for the financial support and for the CCD X-ray diffractometer facility. S.G. thanks CSIR for a fellowship.
Appendix A. Supplementary data Crystallographic data for the structural analysis of 1 Æ MeOH and 2 Æ H2O have been deposited with the Cambridge Crystallographic Data Center, CCDC Nos. 267777 and 267778. Copies of this information may be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (fax: +441223-366-033; e-mail: [email protected] or www: http://www.ccdc.cam.ac.uk). Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.poly.2005.05.015.
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Effect of a pentadentate Schiff base on the helical supramolecular structures 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 2005 Available online 19 August 2005
Abstract Two new asymmetrically dibridged dicopper(II) complexes with a pentadentate Schiff base and p-hydroxycinnamate are prepared and structurally characterized by X-ray crystallography. The complexes have an asymmetrically dibridged (l-alkoxo)(l-carboxylato)dicopper(II) core with an alkoxo bridge from the Schiff base and the carboxylate showing a three-atom bridging mode. Variable-temperature magnetic studies show the complexes having an antiferromagnetically coupled spin system giving a singlet–triplet energy separation of 160 and 132 cm1 for 1 and 2, respectively. Complex 1, with a shorter Cu–OR–Cu angle, displays greater antiferromagnetic spin coupling. Besides the Cu–OR–Cu angle, the role of the carboxylate ligand and the supramolecular structure in the spin coupling phenomena is observed. Complex 1 Æ MeOH shows the formation of intermolecular hydrogen bonds involving the axial methanol and the pentadentate Schiff base terminal oxygen atom. There is additional hydrogen bonding interactions involving the p-hydroxy group of the carboxylate, the lattice methanol and the terminal oxygen atom. The crystal structure of complex 2 Æ H2O displays the presence of a helical supramolecular structure due to hydrogen bonding interactions involving the pendant p-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 polymeric species formed by the process of molecular self-assembly are of interest for their novel structural features and for their ability to stabilize guest molecules of different sizes and conformations [1–12]. We have recently shown that dicopper(II) complexes having a pentadentate Schiff base N,N 0 -1,3-diyl-bis(salicylaldimino)propan-2-ol (H3L 0 ) in its trianionic form and the carboxylate ligands p-HOC6 H4 –X–CO2 , where X is a spacer like ACH@ CHA, –CH2– or –CH2– CH2–, form different types of supramolecular structures [11,12]. Among the three complexes, the *
Corresponding author. Tel.: +91 80 229 32533; fax: +91 80 236 00683. E-mail address: [email protected] (A.R. Chakravarty). 0277-5387/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2005.05.015
p-hydroxycinnamate complex [Cu2L 0 (O2CACH@CHA C6H4-p-OH)] Æ 2H2O forms a helical chain structure due to hydrogen-bonding interactions involving the p-hydroxy group of the carboxylate and one phenoxo oxygen atom of the Schiff base [11]. An important structural feature in this helical structure is the angle of 30 between the cinnamate ligand and the plane of the {Cu2L 0 }+ unit. Two lattice water molecules form an unprecedented helical one-dimensional chain, in which the alternate water molecules are anchored to the supramolecular host structure. The present work stems from our interest to study the role of the Schiff base in the formation of a helical host structure and the ability of the supramolecular host to support any guest solvent molecule(s). We have chosen a potentially pentadentate Schiff base N,N 0 -1,3-diylbis(acetylacetoneimine)propan-2-ol (H3L) derived from acetylacetone and 1,3-diaminopropan-2-ol for the
S. Gupta et al. / Polyhedron 24 (2005) 1922–1928
synthesis of asymmetrically dibridged dicopper(II) complexes using p-hydroxycinnamic acid as the carboxylate. The Schiff base H3L with two acetylacetoneimine moieties is similar to H3L 0 that has two salicylaldimine groups. Herein, we report the synthesis, crystal structure and the magnetic properties of the complexes [Cu2L(O2CACH@CHAC6H4-p-OH)(MeOH)] Æ MeOH (1 Æ MeOH) and [Cu2L(O2CACH@ CHAC6H4-p-OH)] Æ H2O (2 Æ H2O). The significant observation of this study is the crystal structures of the complexes showing formation of different types of supramolecular structures. The complex 2 Æ H2O forms a helical structure through intermolecular hydrogen bonding interactions involving only the carboxylate ligands. The lattice water is non-covalently bound to the Schiff base of the dinuclear unit and does not take part in the supramolecular structure formation.
2. Experimental 2.1. Materials and measurements All reagents and chemicals were purchased from commercial sources and used as received. The solvents were purified following reported procedures [13]. The Schiff base N,N 0 -1,3-diyl-bis(acetylacetoneimine)propan-2-ol (H3L) and the dicopper(II) precursor complex, [Cu2L(O2CMe)] were prepared by literature methods [14]. The elemental analysis was done using a Thermo Finnigan FLASH EA 1112 CHN analyzer. The electronic and infrared spectral data were obtained from Lambda 55 and Perkin–Elmer Spectrum One FT-IR spectrometers, respectively. Variable temperature magnetic susceptibility data for the polycrystalline samples were obtained in the temperature range 18–300 K using a Model 300 Lewis-coil-force magnetometer of George Associates Inc. (Berkeley, USA) make equipped with a Cahn balance and a closed cycle cryostat (Air products). Hg[Co(NCS)4] was used as a standard. Experimental susceptibility data were corrected for diamagnetic contributions and temperature independent paramagnetism (Na = 60 · 106 cm3 M1 per copper). The molar magnetic susceptibilities were fitted to the modified Bleany–Bowers expression by means of a least-squares computer program [15,16]. The Hamiltonian and susceptibility equation used ^1 S ^ 2 Þ (S1 = S2 = 1/2 for a d9–d9 ^ ¼ 2J ðS are: H dicopper(II) core) and vCu ¼ ½Ng2 b2 =kðT hÞ½3 þ exp 1 ð2J =kðT hÞÞ ð1 qÞ þ ðNg21 b2 =4kT Þq þ N a , where q is the fraction of monomeric impurity and 2J is the singlet–triplet separation energy. The magnetic moments were calculated in lB unit. TGA measurements were made using a Mettler Toledo Star thermal analyzer.
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2.2. Preparation of [Cu2L(O2CACH@CHAC6H4-p-OH)(MeOH)] (1) Complex [Cu2L(O2CMe)] (0.15 g, 0.33 mmol) in MeOH (10 mL) was reacted with the sodium salt of p-hydroxycinnamic acid, prepared in situ by reacting the carboxylic acid (0.056 g, 0.33 mmol) with NaOH (0.013 g, 0.33 mmol) in methanol (5 mL), and the reaction mixture was refluxed for 30 min, followed by cooling to an ambient temperature. The solution was filtered and the filtrate on slow evaporation gave the green product in 70% yield. Single crystals of composition 1 Æ MeOH, suitable for X-ray diffraction, were obtained from 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, M1 cm1)]: 632 (350), 315 (126 000), 290 (109 000). FT-IR (KBr phase), cm1: 3400br, 2920w, 1612s, 1508s, 1400s, 1272s, 1169m, 1013m, 942m, 833m, 701m (br, broad; s, strong; m, medium; w, weak). Magnetic moment (leff) per copper in the dimeric unit: 1.63 lB at 300 K; 0.17 lB at 18 K; 2J = 160 cm1 from the theoretical fitting with g = 2.13, q = 0.004, g1 = 2.2, R = 8.5 · 103. 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 with the sodium salt of p-hydroxycinnamic acid, prepared in situ by reacting the carboxylic acid (0.082 g, 0.49 mmol) with NaOH (0.019 g, 0.47 mmol) in 1-propanol (10 mL) and the reaction mixture was refluxed for 30 min, followed by cooling to room temperature. The solution was filtered and the filtrate on slow evaporation 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 spectral data in DMSO [kmax, nm (e, M1 cm1)]: 630 (320), 315 (132 000), 285 (100 000). FT-IR (KBr phase, cm1): 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 cm1 from the theoretical fitting with g = 2.07, q = 0.005, g1 = 2.14, R = 2.8 · 102. Single crystals of 2 Æ H2O were obtained on slow evaporation of the mother liquor in the presence of a trace quantity of water. 2.4. X-ray crystallographic studies Single crystals of the complexes [Cu2L(O2CA CH@CHAC6H4-p-OH)(MeOH)] Æ MeOH (1 Æ MeOH) and [Cu2L(O2CACH@CHAC6H4-p-OH)] Æ H2O (2 Æ H2O) of the respective sizes 0.49 · 0.21 · 0.13 and
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S. Gupta et al. / Polyhedron 24 (2005) 1922–1928
0.26 · 0.18 · 0.11 mm3 were mounted on glass fibers with epoxy cement. All geometric and intensity data were collected using a Bruker SMART APEX CCD diffractometer having a fine focus 1.75 kW sealed tube Mo Ka X-ray source with increasing x (width of 0.3 per frame) at a scan speed of 5 s/frame for 1 Æ MeOH and 12 s/frame for 2 Æ H2O. A total of 20 555 reflections were collected for 1 in the range 1.47 6 h 6 27.18 of which 4356 reflections with I P 2r(I) were used for structure solution using 361 parameters. For 2 Æ H2O, a total of 16 283 reflections were collected in the range 2.0 6 h 6 24.71, of which 3457 reflections with I P 2r(I) were used for structure solution using 310 parameters. The hydrogen atoms attached to the carbon atoms, except two methanol molecules in 1 Æ MeOH, were fixed in their calculated positions and refined using a riding model for both the structures. The hydrogen atoms of the copper-bound and lattice methanol molecules were located in the difference Fourier maps and were refined isotropically. In both the structures, the hydrogen atom of the p-hydroxy group was located in the difference Fourier map and was refined isotropically. The hydrogen atoms of the lattice water in 2 Æ H2O were also located from the difference Fourier map and blocked after refining for the last few Table 1 Crystallographic data for (MeOH)] Æ MeOH (1 Æ MeOH) p-OH)] Æ H2O (2 Æ H2O) Empirical formula Crystal size (mm) Crystal morphology Crystal system Space group Formula weight Unit cell dimensions ˚) a (A ˚) b (A ˚) c (A a = c () b () ˚ 3) V (A Z dcalc. (g cm3) l(Mo) (mm1) F(0 0 0) ˚) k(Mo Ka) (A T (K) Reflections collected Independent reflections Observed reflections, [I > 2r(I)] Parameters Goodness of fit on F2 R (observed data) [R (all data)] Rw (observed data) [Rw(all data)] Largest difference peak ˚ 3) and hole (e A Weight factor: w ¼ 1=½r2 ðF 2o Þ þ ðAP Þ2 þ BP
[Cu2L(O2CACH@CHAC6H4-p-OH) and [Cu2L(O2CACH@CHAC6H41 Æ MeOH
2 Æ H2O
C24H34N2O8Cu2 0.25 · 0.19 · 0.09 cuboid monoclinic P21/n 605.61
C22H28N2O7Cu2 0.25 · 0.17 · 0.11 cuboid monoclinic P21/n 559.54
8.684(13) 11.316(17) 27.91(4) 90 95.98(3) 2728(7) 4 1.47 1.61 1256 0.71073 293(2) 20 555 5575 4356 361 1.020 0.050 [0.068] 0.1114 [0.1189]
10.472(4) 9.236(4) 25.156(9) 90 100.484(6) 2392.6(16) 4 1.55 1.82 1152 0.71073 293(2) 16 283 4074 3457 310 1.182 0.043 [0.052] 0.1011 [0.1051]
0.583 and 0.447 A = 0.0524, B = 1.5669
0.737 and 0.527 A = 0.0446, B = 2.1385
cycles to lower the shift/e.s.d. values. All the non-hydrogen atoms in the structures were refined anisotropically. The structure solution and refinement were made using the SHELX system of programs [17]. The perspective views of the molecules were obtained by ORTEP [18]. Selected crystallographic data are given in Table 1.
3. Results and discussion 3.1. Synthesis and general aspects Dicopper(II) complexes 1 and 2 are prepared in good yield by substituting the acetate of the precursor complex [Cu2L(O2CMe)] having a pentadentate trianionic Schiff base N,N 0 -(2-hydroxypropane-1,3-diyl)bis(acetylacetoneimine) (H3L) by the carboxylate ligand, HO-p-C6 H4 ACH@CHACO2 in methanol or propanol solvent. We have prepared the complex from two different solvents with an objective to isolate supramolecular host structures stabilized by different lattice guest molecules (Scheme 1). The choice of propanol as a reaction medium is to isolate a complex with lattice water molecule(s) as propanol is less likely to support a supramolecular structure by hydrogen bonding interactions. The complexes are characterized from the analytical and spectral data. They are crystallized as [Cu2L(O2CACH@CHAC6H4-p-OH)(MeOH)] Æ MeOH (1 Æ MeOH) and [Cu2L(O2CACH@CHAC6H4p-OH)] Æ H2O (2 Æ H2O) and are structurally characterized by single crystal X-ray crystallography. The complexes are soluble in polar organic solvents and display a d–d band in the electronic spectra at 630 nm in DMSO. 3.2. Crystal structures Both the complexes crystallize in the centrosymmetric monoclinic space group P21/n with four dicopper units in the unit cell, having alternating chains of opposite helical chirality in the crystal. Selected bond distance and bond angle data are given in Table 2. The crystal structure of 1 Æ MeOH shows the presence of a dicopper(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.
S. Gupta et al. / Polyhedron 24 (2005) 1922–1928 Table 2 ˚ ) and angles () in 1 Æ MeOH and 2 Æ H2O Selected bond distances (A 1 Æ MeOH Cu(1) Cu(2) Cu(1)–O(1) Cu(1)–O(2) Cu(1)–O(5) Cu(1)–N(1) Cu(2)–O(2) Cu(2)–O(3) Cu(2)–O(4) Cu(2)–O(7) Cu(2)–N(2) Cu(1)–O(2)–Cu(2) O(1)–Cu(1)–O(2) O(1)–Cu(1)–O(5) O(1)–Cu(1)–N(1) O(2)–Cu(1)–O(5) O(2)–Cu(1)–N(1) O(5)–Cu(1)–N(1) O(2)–Cu(2)–O(3) O(2)–Cu(2)–O(4) O(2)–Cu(2)–N(2) O(2)–Cu(2)–O(7) O(3)–Cu(2)–O(4) O(3)–Cu(2)–O(7) O(3)–Cu(2)–N(2) O(4)–Cu(2)–O(7) O(4)–Cu(2)–N(2) O(7)–Cu(2)–N(2)
3.504(4) 1.918(3) 1.925(3) 1.946(3) 1.923(3) 1.931(3) 1.930(4) 1.973(3) 2.339(4) 1.961(3) 130.6(1) 177.4(1) 85.3(1) 95.0(2) 94.9(1) 84.8(2) 178.0(1) 169.1(1) 93.1(1) 85.2(1) 93.4(1) 87.1(1) 97.4(1) 92.4(1) 88.2(2) 168.8(1) 103.0(2)
2 Æ H2O 3.500(1) 1.900(3) 1.900(3) 1.922(3) 1.920(3) 1.922(3) 1.904(3) 1.957(3) 1.933(3) 132.8(1) 175.5(1) 84.9(1) 94.4(1) 95.3(1) 85.7(1) 175.1(1) 178.5(1) 93.9(1) 84.6(1) 87.2(1) 94.2(1) 178.3(1)
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planar giving an angle of 7.3(3) between the planes having the OOCACH@CHA group and the aromatic C6H4 ring. The cinnamate ligand forms an angle of 5.1(1) with the coordination plane of the {Cu2L}+ moiety. The complex self assembles into a supramolecular structure through hydrogen bonding interactions (Fig. 2). There are two types of intermolecular hydrogen bonding interactions observed in this crystal structure. The copper-bound methanol ligand is involved in the hydrogen bonding interactions with the terminal Schiff base 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 a supramolecular structure involving the lattice methanol molecule, the pendant hydroxy group of the carboxylate and the terminal oxygen atom of the Schiff base (Fig. 2(b)). The O(6) O(8) and O(8) O(3)#2 distances ˚ , respectively (#2: x + are 2.709(6) and 2.812(6) A 1/2 + 1, y + 1/2, z + 1/2 + 1). The hydrogen bonds are 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 Æ H2O has a (l-alkoxo) (l-carboxylate)dicopper(II) core formed by the pentadentate Schiff base (L) and the bridging carboxylate. The copper centers have a square planar CuNO3
planar CuNO3 coordination geometry and the Cu(2) atom has a square pyramidal (4 + 1) coordination ˚ geometry, giving a Cu Cu distance of 3.504(1) A (Fig. 1). The Schiff base ligand displays a pentadentate mode of coordination with the alkoxo oxygen atom bridging the copper centers. The Cu(2) atom is bonded to a methanol ligand at the elongated axial site. The basal planes at the metal centers are nearly coplanar, forming an angle of 10.1(1). The ACH@CHC6H4A moiety of the p-hydroxycinnamate in 1 Æ MeOH is nearly
Fig. 1. ORTEP view of the complex [Cu2L(O2CACH@CHAC6H4p-OH)(MeOH)] Æ MeOH (1 Æ MeOH).
Fig. 2. (a) The hydrogen bonding interactions involving the copperbound methanol ligand and the terminal oxygen atom of the Schiff base in 1 Æ MeOH. (b) The hydrogen bonding interactions involving the p-hydroxy group of the carboxylate, lattice methanol and the oxygen atom of the Schiff base in 1 Æ MeOH.
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S. Gupta et al. / Polyhedron 24 (2005) 1922–1928
Fig. 3. ORTEP view of the complex in [Cu2L(O2CACH@CHAC6H4p-OH)] Æ H2O (2 Æ H2O).
coordination geometry, giving a Cu Cu distance of ˚ (Fig. 3). The basal planes at the copper(II) 3.501(1) A centers are nearly coplanar, forming an angle of 5.2(1). The ACH@CHC6H4A moiety of the p-hydroxycinnamate is nearly planar giving an angle of 4.2(3) between the ACH@CHA group and the aromatic C6H4 ring. The cinnamate ligand forms an angle of 5.8(1) with the coordination plane of the {Cu2L}+ moiety. This is in contrast to its analogous complex [Cu2L 0 (O2CACH@CHAC6H4-p-OH)] Æ 2H2O, having the Schiff base N,N 0 -(2-hydroxypropane-1,3-diyl)bis(salicylaldimine) (H3L 0 ), forming an angle of 30 between the plane of {Cu2L 0 } and the p-hydroxycinnamate ligand. The essentially planar structure in 2 Æ 2H2O could be due to the absence of any steric rigidity in comparison to H3L 0 that has two salicylaldimine moieties. Complex 2 Æ H2O self assembles in a unique way to form a helical supramolecular structure through bifurcated hydrogen bonding interactions involving the p-hydroxy group of the carboxylate and the terminal oxygen atom O(1) of the Schiff base and one oxygen atom of the bridging carboxylate O(5) [Fig. 4; O(6) O(1)#1, ˚ , O(6)–H(6) O(1)#1, 144.7; O(6) 2.899(4) A ˚ , O(6)–H(6) O(5)#1, 144.9; #1: O(5)#1, 2.975(4) A x + 1/2 + 1, +y + 1/2, z + 1/2 + 1]. This is in contrast to the helical structure of its H3L 0 analog that involves the p-hydroxy group of the carboxylate and the phenoxo atom of the Schiff base (L 0 ) in the hydrogen bonding interactions. The O(7) lattice water molecule in 2 Æ H2O is hydrogen bonded to the terminal oxygen atom O(3) of the Schiff base belonging to the binuclear ˚ ; O(7)–H(7B) O(3), complex [O(7) O(3), 2.902(6) A 178.9]. The lattice water in 2 Æ H2O does not take part in the helical structure formation but could be partially responsible for the overall planarity of the carboxylate and the {Cu2L}+ moieties.
Fig. 4. Perspective view of the self-assembled helical supramolecular structure of 2 Æ H2O. The bifurcated hydrogen bond is shown (- - -).
3.3. Magnetic properties The variable-temperature magnetic susceptibility data for the complexes in the temperature range 300– 18 K show an antiferromagnetic (AF) behavior of the complexes (Fig. 5). The magnetic moment values (per copper) of 1.7 lB at 300 K and 0.2 lB at 18 K indicate significant AF spin–spin coupling in the asymmetrically dibridged dicopper(II) core. The theoretical fittings of the magnetic susceptibility data, obtained by using the modified Bleaney–Bowers expression, gave the sin-
Fig. 5. vMT vs. T plots for the complexes 1 and 2. The solid lines show the theoretical fit to the experimental data (n, 1; s, 2).
S. Gupta et al. / Polyhedron 24 (2005) 1922–1928
glet–triplet energy separation (2J) value of 160 cm1 for 1 and 132 cm1 for 2, with the singlet as the ground state [16]. Complexes 1 and 2 belong to the class of asymmetrically dibridged dicopper(II) complexes in which the nature and magnitude of the intramolecular magnetic exchange interactions in the {Cu2(l-OR)(l-O2CR)} core primarily depend upon the Cu–O–Cu angle [14,19–28]. The lower magnitude of the 2J value, even with a large Cu–O–Cu angle of 130, is attributed to the counter-complimentary nature of overlap of the magnetic orbitals and is directly proportional to the energy separation of the symmetric and antisymmetric combinations of magnetic orbitals. It is interesting to note that complex 1 with a lower Cu–O–Cu angle of 130.6(1) has greater AF interaction than complex 2 that has a Cu–O–Cu angle of 132.7(1). The difference could be due to stronger Cu–O (carboxylate) bonds in 2 than in 1. Complexes 1 and 2 differ considerably in their supramolecular structures due to the involvement of the oxygen atoms in the non-covalent interactions. They also differ in the core structure where 1 has four and five coordinate copper centers, while the copper atoms in 2 have a four coordinate geometry. Again, the average Cu–O distance in the l-O2CR bridge of 1 Æ MeOH is ˚ and the same for 2 Æ H2O is 1.94 A ˚ . The coordi1.96 A nating planes having copper atoms Cu(1) and Cu(2) have a dihedral angle of 9.54(7) in 1 Æ MeOH and 5.14(6) for 2 Æ H2O. The observation of higher overall counter-complimentary effect in 2 in comparison to 1 could be due to the structural variations involving the carboxylate bridge and the supramolecular helices. 3.4. Thermogravimetric studies Complex 1 Æ MeOH shows a weight loss of 4.1% in the temperature range of 50–115 C, which is lower than the calculated percentage of 5.3% for the lattice methanol (Fig. 6). The discrepancy could be due to the labile nature of the lattice methanol at room temperature. Further, it is noticed that the copper bound methanol is not released in this temperature range, possibly due to the participation of the copper-bound methanol in the non-covalent ‘‘tetrameric’’ structure formation. We have observed a weight loss of 40% above 225 C. This could be due to the degradation of the complex above this temperature. Complex 2 Æ H2O shows the TGA graph with a weight loss of 3.0% in the temperature range of 110–140 C, which is in accordance of the theoretical value of 3.22% (Fig. 6). A further weight loss of 46% is observed above 240 C due to possible degradation of the complex. In summary, two new asymmetrically dibridged dicopper(II) complexes with a pentadentate Schiff base and p-hydroxycinnamate are prepared and structurally characterized by X-ray crystallography. The complexes form helical supramolecular structures involving hydro-
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Fig. 6. Thermogravimetric plots of (a) 1 Æ MeOH and (b) 2 Æ H2O showing weight loss of the sample with increasing temperature.
gen bonding interactions. The crystal structure of the complex with a lattice water molecule differs considerably from its analogous structure having the Schiff base N,N 0 -1,3-diyl-bis(salicylaldiminato)propan-2-ol (H3L 0 ). The angle between the plane of {Cu2L} with the carboxylate plane in 2 Æ H2O is 6, while the angle involving {Cu2L 0 } and the carboxylate in the reported structure is 30 [11]. The helix formation in 2 Æ H2O involves the bridging and pendant p-hydroxy group oxygen atoms of the carboxylate through hydrogen bonding interactions along with the terminal oxygen atom of the 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 the pentadentate Schiff base in the self-assembly process of supramolecular helix formation with different structural conformations.
Acknowledgements We thank the Department of Science and Technology, Government of India, for the financial support and for the CCD X-ray diffractometer facility. S.G. thanks CSIR for a fellowship.
Appendix A. Supplementary data Crystallographic data for the structural analysis of 1 Æ MeOH and 2 Æ H2O have been deposited with the Cambridge Crystallographic Data Center, CCDC Nos. 267777 and 267778. Copies of this information may be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (fax: +441223-366-033; e-mail: [email protected] or www: http://www.ccdc.cam.ac.uk). Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.poly.2005.05.015.
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