by copper(II) - NOPR

Indian Journal of Chemistry Vol. 50A, Sept-Oct 2011, pp. 1356-1362

Incorporation of lead(II) by copper(II)-salicylaldimine type ligand complexes: Cation–π interactions in controlling the distortion of coordination geometry around lead(II) Saptarshi Biswas & Ashutosh Ghosh* Department of Chemistry, University College of Science, University of Calcutta, 92 APC Road, Kolkata 700 009, India Email: [email protected] Received 17 May 2011; accepted 24 June 2011 Two tri-nuclear, hetero-metallic copper(II)-lead(II) complexes, [(CuL1)2Pb(NCS)2] (1) and [(CuL2)2Pb(NCS)2] (2) have been synthesized by reacting the “ligand-complex” [CuL1] and [CuL2] respectively with Pb(CH3COO)2·3H2O and ammonium thiocyanate (where H2L1 = N,N′-bis(salicylidene)-1,3-propanediamine and H2L2 = N,N′-bis(α-methylsalicylidene)-1,3-propanediamine are the tetradentate di-Schiff base ligands) in EtOH-water medium. Both complexes have been characterized by X-ray single crystal structural analyses. In both structures, the central Pb(II) is six-coordinate, being bonded to four oxygen atoms from two terminal square planar CuL units and a couple of thiocyanate nitrogen atoms. The geometry around Pb(II) in both complexes is far from ideal octahedron and the distortion is more severe in (2). The coordination environment around all Cu(II) centres are square planar with different degrees of tetrahedral distortions. There are significant intra- and intermolecular cation-π interactions between the phenyl rings of the Schiff base and Cu(II) that seem to control the distortion around Pb(II). Keywords: Coordination chemistry, Copper, Lead, Heterometallic complexes, Di-Schiff base ligands, Schiff bases, Crystal structure, Cation–π interactions

The design and synthesis of heterometallic coordination polymers have attracted much attention of the chemists for various reasons such as (i) the introduction of a second metal center may allow for the construction of new topologies1, (ii) the incorporation of unusual metal coordination environments may influence the physical properties of the materials, especially their catalytic, photoluminescent, and magnetic properties,2 (iii) they act as potential precursors for the formation of oxide materials allowing control over the stoichiometry of the individual metal species with relationship to one another in the final products3, etc. There are two main approaches towards the construction of heterometallic coordination polymers. The general one relies on the self-assembly of the different metal ions with organic linkers with different donor atoms and the different preference of the metal atoms towards different functional groups of a linker. The second strategy is to use preformed metal complexes as ligands that may coordinate to additional metal centers4. Recently, we have used neutral Cu(II)-chelate with salen [salen = N,N′-ethylenebis(salicylideneiminate)] type Schiff bases as a ‘‘ligand complex’’ to form heterometallic

tri- and tetranuclear complexes with Na+ and Cd2+ ions5. Lead is environmentally significant and its solid solutions with other oxides are of great importance in a number of ferroelectric and electronic devices6. The coordination sphere of lead is mainly dominated by oxygen7; sometimes it is bonded with N atoms8. A CSD search shows that some Cu-Pb heterometallic complexes with various types of ligands have been reported9. As a part of our ongoing studies on the development of the heterometallic coordination chemistry with the heavy main group elements we have used two copper-chelates, CuL1 and CuL2 (H2L1 =N,N′-bis(salicylidene)-1,3-propanediamine and H2L2 =N, N′-bis(α-methylsalicylidene)-1, 3-propanediamine) to incorporate Pb(II) ions with thiocyanate as anionic co-ligand. In this paper, we report the synthesis, spectral characterization and crystal structure of these two trinuclear complexes, [(CuL1)2Pb(NCS)2 (1) and (CuL2)2Pb(NCS)2] (2). Materials and Methods Salicylaldehyde, o-hydroxy acetophenone and 1,3-propanediamine were purchased from Lancaster

BISWAS & GHOSH: INCORPORATION OF Pb(II) BY Cu(II)-SALICYLALDIMINE COMPLEXES

and were of reagent grade. They were used without further purification. Caution! Perchlorate salts of metal complexes with organic ligands are potentially explosive. Only a small amount of the material should be prepared and it should be handled with care. Elemental analyses (C, H and N) were performed using a Perkin-Elmer 2400 series II CHN analyzer. IR spectra in KBr pellets (4500-500 cm-1) were recorded using a Perkin-Elmer RXI FT-IR spectrophotometer. Electronic spectra in the solid state (800-250 nm) were recorded with a Hitachi U-3501 spectrophotometer. Synthesis of the Schiff base ligands, H2L1 and H2L2

The two di-Schiff-base ligands, H2L1 and H2L2, were prepared by standard methods. Briefly, 2 mmol of 1,3-propanediamine (0.16 mL) were mixed with 4 mmol of the required aldehyde (salicylaldehyde (0.41 mL) or 2-hydroxyacetophenone (0.48 mL) in ethanol (10 mL)]. The resulting solutions were refluxed for ca. 2 h, and allowed to cool. The yellow ethanolic solutions were used directly for complex formation. Synthesis of the complexes, [CuL1] and [CuL2]

To an ethanolic solution (20 mL) of Cu(ClO4)2·6H2O (1.852 g, 5 mmol), was added an ethanolic solution of H2L1 or H2L2 (5 mmol, 10 mL) to prepare the respective complexes CuL1 or CuL2 as reported earlier10,11. Synthesis of the complexes, [(CuL1)2Pb(NCS)2] (1) and [(CuL2)2Pb(NCS)2] (2)

Both complexes were synthesized by addition, with stirring, of an ethanolic solution (20 mL) of the ligand complex, [CuL1] (0.688 g, 2 mmol) for (1) and [CuL2] (0.784 g, 2 mmol) for (2), to 2 mL of an aqueous solution of Pb(CH3COO)2·3H2O (0.379 g, 1 mmol) followed by 2 mL of an aqueous solution of ammonium thiocyanate (0.159 g, 2 mmol). In both cases the microcrystalline green solid started to separate immediately. The stirring was continued for 30 min and the solid product was isolated by filtration, washed with ethanol and dried in vacuum desiccator containing anhydrous CaCl2. The filtrate was allowed to stand overnight to yield green (for 1) and brown (for 2) colored X-ray quality single crystals at the bottom of the vessel. Compound (1): Yield: 0.768 g. (76 %). Anal. (%): Calcd for C36H32Cu2PbN6O4S2 (1011.12): C, 42.72;

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H, 3.16; N, 8.31. Found: C, 42.68; H, 2.96; N, 8.42. λmax (solid, reflectance): 350, 595 nm. IR: υ(C=N), 1626 cm-1, υ(SCN), 2026 cm-1. Compound (2): Yield: 0.787 g. (74 %). Anal. (%): Calcd for C40H40PbCu2N6O4S2 (1067.22): C, 44.97; H, 3.75; N, 7.87. Found: C, 44.78; H, 3.86; N, 7.92. λmax (solid, reflectance): 358, 602 nm. IR: υ(C=N), 1626 cm-1, υ(SCN), 2009 cm-1. Crystallographic data collection and refinement

Suitable single crystals of both complexes were mounted on a Bruker-AXS SMART APEX II diffractometer equipped with a graphite monochromator and Mo-Kα (λ = 0.71073 Ǻ) radiation. The crystals were positioned at 60 mm from the CCD and 360 frames were measured with a counting time of 5 s. The structures were solved using Patterson method by using SHELXS 97. Subsequent difference Fourier synthesis and least-square refinement revealed the positions of the remaining non-hydrogen atoms. Non-hydrogen atoms were refined with independent anisotropic displacement parameters. Hydrogen atoms were placed in idealized positions and their displacement parameters were fixed to be 1.2 times larger than those of the attached non-hydrogen atom. Successful convergence was indicated by the maximum shift/error of 0.001 for the last cycle of the least squares refinement. In (1), one of the thiocyanate groups was disordered showing two positions of nitrogen atom (N6). The two sets of sites were refined with population parameters x and 1 - x, x refining to 0.55(1). Absorption corrections were carried out using the SADABS program12. All calculations were carried out using SHELXS 9713, SHELXL 9714, PLATON 9915, ORTEP-3216 and WinGX system ver. 1.6417. Data collection and structure refinement parameters and crystallographic data for the two complexes are given in Table 1. Results and Discussion Syntheses and spectral studies of the complexes

The Schiff-base ligands (H2L1 and H2L2) and their Cu(II) complexes, [CuL1 and CuL2] were synthesized using the reported procedure10,11. The Cu(II) complex on reaction with Pb(OAc)2·3H2O, in presence of ammonium thiocyanate in EtOH-H2O medium (5:1, v/v) resulted in the formation of two trinuclear complexes, [(CuL1)2Pb(NCS)2] (1) and [(CuL2)2Pb(NCS)2] (2) by self-assembly (Scheme 1).

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INDIAN J CHEM, SEC A, SEPT-OCT 2011

Table 1 – Crystal data of the complexes (1) and (2) (1) Formula Formula weight Space group Crystal system a (Å) b (Å) c (Å) β (deg.) V (Å3)

(2)

C36H32Cu2PbN6O4S2 C40H40Cu2PbN6O4S2 1011.12 1067.22 P21/c C2/c Monoclinic Monoclinic 14.155(5) 24.856(1) 11.451(5) 10.0153(4) 22.121(5) 16.315(1) 91.637(5) 109.084(1)

Z dcalcd (g cm-3) µ (mm-1) Rint No. of unique data Data with I > 2σ(I) R1 on I>2σ(I) wR2 on I>2σ(I) G-o-F value

3584(2)

3838.2(3)

4 1.874 6.027 0.0464 7609 5615 0.0409 0.0928 1.041

4 1.847 5.636 0.0394 5052 4215 0.0297 0.0655 1.040

Besides elemental analysis, both complexes were initially characterized by IR spectra. The appearance of the characteristic intense peaks for N-bonded thiocyanate at 2026 and 2009 cm-1 in IR spectra of complexes (1) and (2) respectively, clearly indicates the formation of the complexes. In both complexes, a strong and sharp band due to the azomethine υ(C=N) group of the Schiff base appears at 1613 and 1597 cm-1 for complexes (1) and (2) respectively. The solid-state reflectance spectra of both complexes show a broad single absorption bands near 595 and 602 nm for complexes (1) and (2) respectively. The positions of these bands are consistent with the observed square-based geometry around the copper centers. The intense broad band at about 350 and 358 nm for complexes (1) and (2) respectively is due to the ligand-to-metal charge transfer absorption bands.

Scheme 1

Structure of complex (1)

The structure of (1) consists of two CuL1 units, one lead(II) ion and two N-bonded thiocyanate ions with the formula [(CuL1)2Pb(NCS)2]. The ORTEP view is shown in Fig. 1 and the bond distances and angles are given in Table 2. The two terminal Cu(II) ions possess a tetra-coordinate, square-planar coordination sphere with a tetrahedral distortion being bonded to the four coordinating atoms from the deprotoned

Fig. 1 – ORTEP diagram of the complex (1). Ellipsoids are drawn at the 30 % probability. The minor fraction N6A (45 %) is omitted.


Table 2 – Bond distances and angles around the metal centers in complex (1) Bond distances (Å) Pb(1) - O(1) Pb(1) - O(2) Pb(1) - O(3) Pb(1) - O(4) Pb(1) - N(5) Pb(1) - N(6A) Pb(1) - N(6B) Cu(1) - O(3) Cu(1) - O(4) Cu(1) - N(1) Cu(1) - N(2) Cu(2) - O(1) Cu(2) - O(2) Cu(2) - N(3) Cu(2) - N(4)

2.536(4) 2.540(4) 2.452(4) 2.532(4) 2.794(8) 2.687(2) 2.656(2) 1.924(3) 1.927(4) 1.952(5) 1.966(4) 1.938(4) 1.913(4) 1.988(6) 1.965(5)

Bond angles (º) O(1) - Pb(1) - O(2) O(1) - Pb(1) - O(3) O(1) - Pb(1) - O(4) O(1) - Pb(1) - N(5) O(1) - Pb(1) - N(6B) O(1) - Pb(1) - N(6A) N(5) - Pb(1) - N(6A) O(2) - Pb(1) - O(3) O(2) - Pb(1) - O(4) O(2) - Pb(1) - N(5) O(2) - Pb(1) - N(6B) O(3) - Pb(1) - O(4) O(3) - Pb(1) - N(5) O(3) - Pb(1) - N(6B) O(4) - Pb(1) - N(5) O(4) - Pb(1) - N(6B) N(5) - Pb(1) - N(6B) O(2) - Pb(1) - N(6A) O(3) - Pb(1) - N(6A) O(4) - Pb(1) - N(6A) O(3) - Cu(1) - O(4) O(3) - Cu(1) - N(1) O(3) - Cu(1) - N(2) O(4) - Cu(1) - N(1) O(4) - Cu(1) - N(2) N(1) - Cu(1) - N(2) O(1) - Cu(2) - O(2) O(1) - Cu(2) - N(3) O(1) - Cu(2) - N(4) O(2) - Cu(2) - N(3) O(2) - Cu(2) - N(4) N(3) - Cu(2) - N(4)

57.9(1) 74.3(1) 104.1 (1) 142.3(2) 87.8(4) 104.7(5) 103.2(5) 100.7(1) 158.8(1) 94.5(2) 104.7(4) 61.1 (1) 88.1 (1) 134.3(4) 95.5(2) 84.2(4) 126.5(4) 95.9(5) 159.1(5) 99.9(5) 82.3 (2) 92.3(2) 163.5(2) 164.1(2) 91.8 (2) 97.3(2) 79.3 (2) 169.5(2) 92.0(2) 91.1(2) 170.9(2) 97.9(3)

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Schiff base. The rms deviations of the four coordinating atoms from the mean plane passing through them are 0.241 and 0.054 Å for Cu(1) and Cu(2) respectively. The Cu(1) and Cu(2) atoms are 0.004(1) and 0.014(1) Å respectively from this plane. The higher tetrahedral distortion in Cu(1) is also apparent from the dihedral angles (19.94° for Cu(1) and 4.60° for Cu(2) between the two planes, N(2)–Cu(1)–O(4), N(1)–Cu(1)–O(3) and N(3)–Cu(2)–O(2), N(4)–Cu(2)–O(1) for Cu(1) and Cu(2) respectively. This angle is 0º for a perfectly square-planar arrangement and 90° for a perfect tetrahedral arrangement. The avg. Cu-O bond distances around Cu(1) and Cu(2) are the same (1.925 Å), but are slightly greater than that in the mono-nuclear “ligand complex”10 (1.867 Å). The lengthening of the Cu–O bonds in the trinuclear complex seems to be due to the coordination of phenoxo group both to copper and to lead ions and is usual when such “ligand complex” coordinates to another metal ion5. The environment of lead atom is six-coordinate distorted octahedron, being bonded to four oxygen atoms from the two CuL1 moieties and two nitrogen atoms from two thiocyanate anions. One of the thiocyanate N (N6) is disordered over two positions, N6A and N6B. The Pb-O bond distances are in the range of 2.452–2.540 Å and the Pb-N distances are 2.794 and 2.671 Å (Table 2) for N5 and N6 (avg. of N6A and N6B) respectively. The distortion around Pb atom arises mostly due to the deviation of cisoid angles from 90° and transoid angle from 180°. The small bite angle of the “ligand complex” makes two of the cis angles at 57.9(1)° and 61.1(1)°, whereas the largest cis angle, N(5)-Pb(1)-N(6) = 114.8° is between the two coordinated thiocyanate group. The trans angles, which are in the range 142.3(2)° –159.1(5)° (Table 2), also indicate a significant distortion from ideal octahedral geometry. The conformation of the molecule and the distortion of coordination geometry around Pb(II) ion seem to be controlled by two intramolecular cation–π interacttions, which are associated through the centroid of the phenyl ring Cg···Cu(1) at a distance of 3.780 Å and another one involving Cg···Cu(2) at the distance of 3.621 Å (Fig. 2a). Cu(1), in which tetrahedral distortion is more prominent, shows weak interactions as expected. However, it is also involved in another weak intermolecular cation–π interaction with its neighbouring molecule with the centroid of the phenyl

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Fig. 2 – (a) Intra-molecular cation–π interaction in complex (1). (b) Inter-molcular cation–π interaction in complex (1). Table 3 – Bond distances and angles around the metal centers in complex (2)

Fig. 3 – ORTEP diagram of the complex (2). Ellipsoids are drawn at the 30 % probability.

ring Cg···Cu(1) distance of 3.792 Å that joins the two trinuclear units (Fig. 2b). Between the two N-bonded thiocyanate ions, one is almost linearly coordinated to Pb(II) with Pb(1)-N(6)-C(36) angle of 136.9°, while the other is significantly bent with Pb(1)-N(5)-C(35) angle of 103.8(7)°. Structure of complex (2)

The structure of (2), which contains a crystallographic two-fold axis, consists of two CuL units, one lead(II) ion and two N-bonded thiocyanate ions with the formula [(CuL2)2Pb(NCS)2]. The ORTEP view is shown in Fig. 3 and the bond distances and angles are given in Table 3. As in (1), the two terminal Cu(II) ions possess a tetracoordinate, square planar coordination sphere, being coordinated to the four donor atoms of the deprotonated Schiff base. The rms deviation (0.319 Å) of the four donor atoms from the mean plane passing through them as well as the dihedral angle 27.36° between the two planes, N(1)–Cu(1)–O(1) and N(2)–Cu(1)–O(2), indicate a

Bond distances(Å) Pb1 - O1 Pb1 - O2 Pb1 - N3 Cu1 - O1 Cu1 - O2 Cu1 - N1 Cu1 - N2

2.613(2) 2.518(2) 2.575(3) 1.886(2) 1.901(2) 1.940(2) 1.945(3)

Bond angles (º) O1 - Pb1 - O2 O1 - Pb1 - N3 O1 - Pb1 - O1a O1 - Pb1 - O2a O1 - Pb1 - N3a O2 - Pb1 - N3 O2 - Pb1 - O2a O2 - Pb1 - N3a N3 - Pb1 - N3a O1 - Cu1 - O2 O1 - Cu1 - N1 O1 - Cu1 - N2 O2 - Cu1 - N1 O2 - Cu1 - N2 N1 - Cu1 - N2

60.3(1) 94.5(1) 92.1(1) 83.0 (1) 159.0(1) 117.5(1) 127.2(1) 100.7(1) 86.4(1) 85.9(1) 94.0(1) 155.9(1) 164.5 (1) 91.3(1) 95.0 (1)

a

Symmetry operation = 2-x,y,3 / 2-z

higher tetrahedral distortion. The metal atom is 0.067(1) Å from this plane. The avg. Cu-O bond distance (1.894 Å) in (2) is shorter than that in any unit of compound (1) but is slightly greater that in the mono-nuclear complex11. The environment of the lead atom which sits on the two-fold axis is six-coordinate, being bonded to four


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(2) than in (1). Between the two N-bonded thiocyanate ions in (1), one is coordinated almost linearly but the other is considerably bent with an Pb(1)-N(5)-C(35) angle of 103.8(7)° whereas both the thiocyanate ions are equally bent in (2) as the molecule contains C2 symmetry element with an Pb(1)-N(3)-C(20) angle of 139.2(3)°. No obvious reason has been identified for this different degree of bending of thiocyanate ions in the compounds. Fig. 4 – Cation–π interactions in (2) to form a 1D polymeric chain.

oxygen atoms from the two CuL2 moieties and two nitrogen atoms from two thiocyanate anions. The bond distances around the Pb metal is given in Table 3. In this species, the “ligand complex” makes two of the cis angles at 60.3(1)° which is comparable to that of (1). However, unlike compound (1) the angle between the two thiocyanate groups is 86.4(1)°. On the other hand, angle O(2)-Pb(1)-N(3) = 117.5(1) is the largest among the cis angles in (2). The corresponding angles in (1) are in the range 91.0(4)°–101.0(4)°. The trans angle O(1)-Pb(1)-N(3)a = 159.0(1)° remains comparable but the other one O(2)-Pb(1)-O(2)a becomes unusually small at 127.2(1)° (Table 3). Unlike (1) there is no intramolecular cation–π interaction in this compound. However, the neighboring molecules are associated through rather weak cation–π interactions to forming a 1D polymeric chain with centroid of the phenyl ring Cg···Cu(1) distance of 3.896 Å (Fig. 4). The nonexistence of any intramolecular cation–π interaction seems to be due to the higher tetrahedral distortion in the copper atom. The Pb(1)-N(3)-C(20) angle 139.2(3)° of N-bonded thiocyanate ion shows a moderately bent coordination. Conclusions Both complexes (1) and (2) are 2:1 adducts of CuL and Pb(NCS)2, formed by the coordination of the phenoxo group of CuL units to the lead ion. The geometry of both complexes is six-coordinate but there is a significant difference in the orientations of the two CuL units around the Pb(II) ion. In complex (1), two CuL1 units slightly slide away from each other so as to be involved in intramolecular cation–π interaction (Fig. 2). The greater tetrahedral distortion of the Cu atoms in (2) seems unfavorable for such cation–π interactions. The geometry around the Pb ion is more severely distorted from ideal octahedron in

Supplementary Data CCDC 824246 and 824247 contain the supplementary crystallographic data for (1) and (2), respecttively. These data can be obtained free of charge via http://www.ccdc.cam. ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: (+44) 1223-336-033; Email: deposit @ccdc.cam.ac.uk. Acknowledgement One of the authors (SB) is thankful to Council of Scientific and Industrial Research, New Delhi, India, for award of Senior Research Fellowship (No.09/028 (0732)/2008-EMR-I). Crystallography was performed at the DST-FIST, India-funded Single Crystal Diffractometer Facility at the Department of Chemistry, University of Calcutta, Kolkata, India. References 1 (a) Vaz M G F, Pinheiro L M M, Stumpf H O, Alcântara A F C, Golhen S, Ouahab L, Cador O, Mathonière C & Kahn O, Chem Eur J, 5 (1999) 1486; (b) Gheorghe R, Cucos P, Andruh M, Costes J–P, Donnadieu B & Shova S, Chem–Eur J, 12 (2006) 187. 2 (a) Dincá M & Long J R, J Am Chem Soc, 127 (2005) 9376; (b) Berenguer J R, Lalinde E M & Moreno T, Coord Chem Rev, 254 (2010) 832; (c) Coronado E, Galán-Mascarós J R, Gómez-García C J & Laukhin V, Nature, 408 (2000) 447. 3 Kessler V G, Chem Commun, 11 (2003) 1213. 4 (a) O'Connor C J, Freyberg D P & Sinn E, Inorg Chem, 18 (1979) 1077; (b) Bencini A, Benelli C, Dei A & Gatteschi D, InorgChem, 24 (1985) 695; (c) Novitchi G, Shova S, Caneschi A, Costes J–P, Gdaniec M & Stanica N, Dalton Trans, (2004) 1194; (d) Chattopadhyay S, Bocelli G, Musatti A & Ghosh A, Inorg Chem Commun, 9 (2006) 1053; (e) Mukherjee P, Drew M G B, Tangoulis V, Estrader M, Diaz C & Ghosh A, Inorg Chem Commun, 12 (2009) 929. 5 (a) Biswas S, Naiya S, Drew M G B, Estarellas C, Frontera A & Ghosh A, Inorg Chim Acta, 366 (2011) 219; (b) Biswas S & Ghosh A, Polyhedron, 30 (2011) 676. 6 Lashgari K, Kritikos M, Lashgari K & Westin G, Acta Cryst, C54 (1998) 1794. 7 (a) Tahir M N,Ülkü D & Mövsümov E M, Acta Cryst, C52 (1996) 2436; (b) Schürman M & Huber F, Acta Cryst, C50 (1994) 1710.

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