The interplay of O–H⋯O hydrogen bonding in the generation of three new supramolecular complexes of CuII, NiII and CoIII: Syntheses, characterization and structural aspects

Inorganica Chimica Acta 362 (2009) 2828–2836

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The interplay of O–H  O hydrogen bonding in the generation of three new supramolecular complexes of CuII, NiII and CoIII: Syntheses, characterization and structural aspects Santarupa Thakurta a, Joy Chakraborty a, Georgina Rosair b, Ray J. Butcher c, Samiran Mitra a,* a

Department of Chemistry, Jadavpur University, Kolkata 700 032, West Bengal, India Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh, EH 14 4AS, UK c Department of Chemistry, Howard University, 2400 Sixth Street, NW Washington, DC 20059, USA b

a r t i c l e

i n f o

Article history: Received 3 June 2008 Received in revised form 19 November 2008 Accepted 3 January 2009 Available online 10 January 2009 Keywords: Schiff bases X-ray crystal structures O–H  O hydrogen bonding Ring motifs Supramolecular architectures

a b s t r a c t Three new coordination complexes, [Cu(L1)(H2O)] (1), [Ni(L2)2]CH3CN (2) and [Co(HL3)(L3)] (3) [where H2L1, N,N0 -bis(3-methoxysalicylidenimino)-1,3-diamino-propane; HL2, 2-((E)-(1,3-dihydroxy-2-methylpropan-2-ylimino)methyl)phenol; H2L3, 2-((E)-(2-hydroxyethylimino)methyl)-4-bromophenol] have been synthesized and systematically characterized by elemental analyses, FT-IR, electronic spectroscopy, cyclic voltammetry and thermogravimetric analyses. Single crystal X-ray diffraction studies confirm that the metal center in complex 1 has distorted square-pyramidal geometry while it is distorted octahedral in the other two complexes. In all the complexes O–H  O hydrogen bondings assemble the molecular units leading to ordered supramolecular architectures. While both complexes 1 and 2 form infinite one-dimensional arrays through the self organisation of hydrogen bonded ring motifs, complex 3 is a unique starshaped cyclic hexamer generated through intermolecular hydrogen bonding. Ó 2009 Elsevier B.V. All rights reserved.

1. Introduction Schiff base complexes of transition metals have acquired a special significance in the domain of metallo-organic and coordination chemistry because of their diverse range of applications, i.e. in organic synthesis, photochemistry, heterogeneous catalysis and as models of biological systems [1–5]. The emergence of supramolecular chemistry [6] of transition metal-coordination complexes has been recognized as one of the major developments in the field of molecule-based materials. Recent research on inorganic metal clusters, coordination complexes and organic molecules has revealed the fact that apart from the regular covalent bonds, weaker bonding forces such as hydrogen bonding, p–p interactions (faceto-face), C–H–p interactions (T-shaped geometry, edge-face or herringbone interactions), etc. [7–9] are quite efficient to produce polymeric assemblies. Among them, hydrogen bonding has inarguably become the masterkey interaction within the toolbox of supramolecular chemistry owing to its strength and directionality. It is able to control the way by which molecules self-recognize and aggregate in neutral molecular crystals, thereby offering new modes of chemical and biological recognition [10]. The multiple intertwined hydrogen bond patterns present in the solid state are

* Corresponding author. Tel.: +91 033 2414 6666x2505; fax: +91 033 2414 6414. E-mail address: [email protected] (S. Mitra). 0020-1693/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2009.01.002

usually well defined and often result in recognizable and recurring motifs in an aggregate. The use of graph-set notation as developed by Etter and co-workers [11] provides a new way to analyze and characterize different complicated patterns found in hydrogen bonded networks with the help of one or more of the four types of simplified graph sets (descriptions): chains, rings, finite complexes (dimers), and intramolecular (self) hydrogen bonds, assigned the pattern designators C, R, D, and S, respectively. Transition metal ions can be incorporated into H-bonding networks by employing bi-functional ligands that contain both efficient metal-coordination sites and peripheral hydrogen bonding functionalities. Hydrogen bonding interactions of such metal-coordination complexes are very interesting since they may yield a variety of penetrating or non-penetrating molecular frameworks of different dimensionality with specific architectural and functional behaviors [12]. Hydrogen bonding interactions play a crucial role in maintaining the integrity of biomolecular structure, information transfer, replication and catalysis in living organisms [10a]. The strength of hydrogen bonding arrays (typically 12– 120 kJ mol1), coupled with their high degree of directionality and selectivity, have been used to create efficient molecular hosts capable of selective binding to a wide variety of biological guests in aqueous and non-aqueous environments [13]. The current hotspot of research in contemporary supramolecular chemistry is to create hydrogen bonded host–guest systems with enormous potential applications [14].

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As a part of the continuous exploration in this field, we have successfully obtained a series of well-ordered supramolecular architectures assembled through O–H  O hydrogen bonding interactions which enhance the stability of the complexes. We report herein the syntheses along with the spectroscopic, electrochemical, thermal and structural studies of three new Schiff base complexes [Cu(L1)(H2O)] (1), [Ni(L2)2]  CH3CN (2) and [Co(HL3)(L3)] (3). Investigation of their supramolecular structures reveals that both the copper and nickel complexes exhibit infinite one-dimensional arrays, generated by self repeating O–H  O hydrogen bonded ring motifs involving the synthon Rxx (12) [where x = 2 and 4 for complex 1 and 2, respectively]. In the supramolecular assembly of complex 3, hydrogen bonding interactions among the adjacent ligand functionalities of the neighboring molecules give rise to a 24-membered hexanuclear ring system of cobalt atoms. This unique star-shaped H-bonded macrocyclic ring, R66 (24) contains a vacant cavity within the aggregation.

2. Experimental 2.1. Materials Cu(NO3)2  3H2O, Ni(NO3)2  6H2O, Co(NO3)2  6H2O were purchased from Merck India Ltd and o-vanillin, salicylaldehyde, 5-bromosalicylaldehyde, 1,3-diaminopropane, ethanolamine and 2-amino-2-methyl-1,3-propanediol from Aldrich Co., USA. All the chemicals and solvents employed for the syntheses were of analytical grade and used as received without further purification. 2.2. Physical measurements The Fourier transform infrared spectra (4000–200 cm1) of the complexes were recorded on a Perkin Elmer Spectrum RX I FT-IR system with solid KBr disc. The electronic spectra were recorded on a Perkin Elmer Lambda 40 UV/Vis spectrometer using HPLC grade acetonitrile as solvent. C, H, N microanalyses were carried out with a Perkin Elmer 2400 II elemental analyser. Electrochemical studies were performed under Argon atmosphere on a Versastat-Potentiostat II electrochemical analyzer against a standard calomel electrode as reference and platinum as the working electrode. Room temperature magnetic susceptibilities were measured with a model 155 PAR vibrating sample magnetometer fitted with a Waker Scientific 175 FBAL magnet using Hg[Co(SCN)4] as the standard. The necessary diamagnetic corrections were done using Pascal’s table. The thermal analyses were done on a Mettler Toledo TGA/SDTA-851e thermal analyzer system in a dynamic atmosphere of dinitrogen (flow rate: 30 cm3 min1) in a platinum crucible at a heating rate of 10 °C min1. 2.3. Syntheses 2.3.1. Syntheses of the Schiff base ligands 2.3.1.1. Synthesis of H2L1 [N,N0 -bis(3-methoxysalicylidenimino)-1,3diaminopropane]. The ligand H2L1 was prepared by refluxing ovanillin (0.304 g, 2 mmol) and 1,3-diaminoprapane (0.08 ml, 1 mmol) in 20 ml of methanol at 65 °C for 45 min. Then a deep yellow solution was obtained which on slow evaporation yielded shinny yellow crystals of the ligand H2L1. They were dried and stored in vacuo for subsequent use. Yield: 0.308 g (90%). Anal. Calc. for C19H22N2O4: C, 66.65; H, 6.48; N, 8.18. Found: C, 66.49; H, 6.80; N, 8.08%. FT-IR (KBr, cm1): m(C@N) 1641. UV–Vis (k, nm): 260, 395. 2.3.1.2. Synthesis of HL2 [2-((E)-(1,3-dihydroxy-2-methylpropan-2ylimino) methyl)phenol]. Salicylaldehyde (0.20 ml, 2 mmol) was

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added to a solution of 2-amino-2-methyl-1,3-propanediol (0.210 g, 2 mmol) in methanol (20 ml) and the mixture was refluxed for 1 h at 65 °C. A yellow solid was formed which was filtered, washed with methanol and vacuum dried. Yield: 0.178 g (85%). Anal. Calc. for C11H15NO3: C, 63.14; H, 7.23; N, 6.69. Found: C, 63.08; H, 7.30; N, 6.80%. FT-IR (KBr, cm1): m(C@N) 1640. UV–Vis (k, nm): 263, 395. 2.3.1.3. Synthesis of H2L3 [2-((E)-(2-hydroxyethylimino)methyl)-4bromophenol]. The Schiff base H2L3 was prepared following the same procedure as adopted for the earlier except refluxing 5bromosalicylaldehyde (0.402 g, 2 mmol) with ethanolamine (0.12 ml, 2 mmol) in 20 ml methanol. Yield: 0.141 g (85%). Anal. Calc. for C9H10BrNO2: C, 44.29; H, 4.13; N, 5.74. Found: C, 44.32; H, 4.10; N, 5.81%. FT-IR (KBr, cm1): m(C@N) 1640. UV–Vis (k, nm): 261, 393. 2.3.2. Syntheses of the complexes 2.3.2.1. Synthesis of [Cu(L1)(H2O)] (1). 1 mmol (0.342 g) of the solid Schiff base ligand H2L1 was dissolved in 10 ml methanol. The solution was then added dropwise to a 10 ml gently warmed solution of Cu(NO3)2  3H2O (0.930 g, 1 mmol). The mixture was refluxed for 45 min at 65 °C. The resulting deep green solution was filtered and left undisturbed. After 5 days, greenish blue plate shaped X-ray diffraction quality single crystals were mechanically isolated from the bulk. Yield: 0.358 g (85%). Anal. Calc. for C19H22N2O5Cu: C, 54.09; H, 5.26; N, 6.64. Found: C, 54.18; H, 5.30; N, 6.91%. 2.3.2.2. Synthesis of [Ni(L2)2]  CH3CN (2). A solution of Ni(NO3)2  6H2O (0.291 g, 1 mmol) in methanol–acetonitrile solvent mixture (3:1 v/v, 15 ml) was added to a 20 ml methanolic solution of the ligand HL2 (0.418 g, 2 mmol) with stirring. The resulting green solution when kept at room temperature for two days yielded block shaped green single crystals suitable for X-ray analysis. Yield: 0.469 g (91%). Anal. Calc. for C24H31N3O6Ni: C, 55.84; H, 6.05; N, 8.14. Found: C, 56.02; H, 6.11; N, 8.35%. 2.3.2.3. Synthesis of [Co(HL3)(L3)] (3). 0.330 mg (2 mmol) of H2L3 was dissolved in 20 ml of methanol and 10 ml methanolic solution of Co(NO3)2  6H2O (0.291 g, 1 mmol) was added dropwise to this solution. After stirring for 1 h at 55 °C, the reaction mixture was left undisturbed. Blue block shaped single crystals appeared after two days. Yield: 0.424 g (78%). Anal. Calc. for C18H17Br2N2O4Co: C, 39.74; H, 3.15; N, 5.15. Found: C, 40.13; H, 3.19; N, 5.36%. 2.4. Crystallographic data collection and structure refinements A diffraction quality single crystal of 1 was mounted on an Oxford Diffraction Gemini-R diffractometer equipped with a graphite monochromator and Mo Ka radiation (k = 0.71073 Å). The crystallographic data collection was performed using multi scan technique at 173 K. Data collection and the unit cell refinement were performed using CrysAlisPRO software [15]. The structure of the complex was solved by direct method procedures with SHELXS [16a], and refined by full-matrix least squares based on F2 with SHELXL [16 b]. All calculations were carried out with the WinGX package program [17]. The non-hydrogen atoms were refined with anisotropic factors. All hydrogen atoms were positioned geometrically treated as riding on the bound atom and those of the water molecules were located in difference Fourier maps. Intensity data were collected on the single crystals of 2 and 3 at 100 K on a Bruker X8 Apex 2 CCD diffractometer equipped with a graphite monochromator and Mo Ka radiation (k = 0.71073 Å) using the Bruker Apex2 software [18]. Multiscan absorption correction was applied using SADABS [19a]. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were constrained

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Table 1 Crystallographic data and the refinement parameters for the complexes. Complex

1

2

3

Empirical formula Formula weight Crystal dimension (mm) Crystal system Space group a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z Temperature (K) kMo Ka (Å) Dc (g cm3) l (mm1) F(0 0 0) h (°) Total, unique data Observed data [I > 2r(I)] Rint Ra, Rwb Goodness-of-fit on F2 Dqmax, min (e Å3)

C19H22N2O5Cu 421.93 0.65  0.45  0.18 orthorhombic Pc21n (No. 33) 22.8743(6) 7.5649(2) 20.7140(6) 90 90 90 3584.39(17) 8 173(2) 0.71073 1.564 1.253 1752 4.85–30.77 18 311, 8674 5829 0.031 0.0360, 0.0862 0.993 0.811, 0.478

C24H31N3O6Ni 516.23 0.20  0.12  0.10 triclinic  (No. 2) P1 8.5850(15) 10.8029(19) 13.988(2) 77.287(9) 76.778(9) 75.296(9) 1203.3(4) 2 100(2) 0.71073 1.425 0.851 544 2.69–33.29 39 942, 9049 6723 0.049 0.0412, 0.0890 1.033 0.490, 0.462

C18H17Br2N2O4Co 544.09 0.20  0.20  0.20 hexagonal  (No. 146) R3 18.9716(14) 18.9716(14) 27.114(3) 90 90 120 8451.3(13) 18 100(2) 0.71073 1.924 5.196 4824 2.59–27.00 55 790, 4103 3348 0.060 0.0324, 0.0795 1.14 0.580, 0.840

a b

P P R = (|Fo  Fc|)/ |Fo|. P P Rw = { [w(|Fo  Fc|)2]/ [w|Fo|2]}½.

to ideal geometry and treated as riding on the bound atom. All of the crystallographic computations were carried out using SHELXTL [19b], PLATON 99 [19c] and ORTEP [19d] programs. Details of the data collection parameters and crystallographic information for all the complexes are summarized in Table 1. 3. Results and discussion 3.1. Fourier transform infrared spectra The Fourier transform infrared spectra of the complexes 1–3 are consistent with the X-ray structural data. The strong absorption band occurring at 1603, 1593 and 1598 cm1 for 1, 2 and 3, respectively, can be assigned to the C@N stretching frequency of the coordinated ligand, whereas for the free ligand molecules the same band is observed at ca. 1640 cm1. The shift of this band towards lower frequency on complexation with the metal suggests coordination via imino nitrogen atom in all the complexes [20,21]. Ligand coordination to the metal center is substantiated by prominent bands appearing at 446–468 and 365–382 cm1 which can be attributed for m(M–N) and m(M–O), respectively, observed in all the complexes. The m(C–O) mode is present as a very strong band at about 1240 cm1. Several weak peaks observed for the complexes in the range 3180–2876 cm1 are likely to be due to the aromatic and aliphatic C–H stretches [22]. The IR spectrum of complex 1 confirms the presence of water molecule. The strong and broad band centering at 3325 and 1610 cm1 are attributable to mstr(O– H) and d(O–H) vibrations of the coordinated water molecule, respectively [22,23]. Generally, the O–H stretching vibration for water appears above 3400 cm1. The shift of this band towards lower wave number and its broadness indicate the presence of strong hydrogen bonding which is also confirmed from its crystal structure. 3.2. Electronic spectra The electronic spectral data for the complexes in HPLC grade acetonitrile solvent are in good agreement with their geometries.

Complex 1 shows a broad band centered at about 605 nm which results from a 2B1 ? 2E transition. This is a typical d–d band for copper(II) in the square-pyramidal environment [24]. Generally octahedral NiII complexes exhibit three distinct strong and broad absorption bands at 245, 380 and 645 nm due to the spin allowed transitions: 3A2g ? 3T2g, 3A2g ? 3T1g(F), 3A2g ? 3T1g(P), respectively. In the electronic spectrum of complex 2, a broad absorption band appears at kmax value of 640 nm which can be assigned to the spin allowed transition 3A2g ? 3T1g(P) of nickel(II) in an octahedral geometry [25]. The other two allowed transitions at the high energy region are probably obscured by the intense charge transfer transitions and therefore could not be observed prominently. Complex 3 exhibits a broad band at 550 nm (1A1g ? 1T1g) which is a characteristic d–d band of cobalt(III) in a distorted octahedral environment. A spin allowed transition nearly at 360 nm (1A1g ? 1T2g) was observed as a shoulder due to the overlapping charge transfer band in this region [26]. In the higher energy region of the spectra, each complex exhibits intense band at 350–385 nm region which can be attributed to the charge transfer transition from the coordinated unsaturated ligands to the respective metal ions (LMCT). For complexes 1, 2 and 3, the intense high energy bands at about 245 nm and 380 nm may be assigned to the intraligand p ? p* and n ? p* transitions, respectively. 3.3. Description of the crystal structures 3.3.1. [Cu(L1)(H2O)] (1) In the crystals of complex 1 two crystallographically independent complexes A and B, having Cu1 and Cu2 as central atoms, are present. Both complexes are five coordinated with very similar structural features. The perspective view of the complex A, together with the atom labeling scheme, is shown in Fig. 1. Selected bond lengths and angles for complexes A and B are given in Table 2a. The coordination number five is very common for CuII where it possesses either square-pyramidal (SP) or trigonal-bipyramidal (TBP) geometry. For a penta-coordinated metal center, the distortion of the metal center geometry from trigonal bipyramidal

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Fig. 1. ORTEP view of the monomeric unit of 1 (complex A) with atom labeling scheme. Thermal ellipsoids are drawn at the 40% probability level.

Table 2a Selected bond distances (Å) and angles (°) for 1. Bond lengths

(Å)

Bond angles

(°)

Cu1–O1A Cu1–O2A Cu1–N1A Cu1–N2A Cu1–O1W Cu2–O1B Cu2–O2B Cu2–N1B Cu2–N2B Cu2–O2W

1.9630(2) 1.9361(19) 1.9950(2) 1.9910(3) 2.3730(3) 1.933(2) 1.957(2) 1.974(3) 1.999(2) 2.363(3)

N2A–Cu1–N1A O1A–Cu1–O2A O2A–Cu1–N1A O1A–Cu1–N2A O1W–Cu1–O1A O1W–Cu1–N2A O2B–Cu2–N2B N1B–Cu2–N2B O2B–Cu2–N1B O1B–Cu2–N2B O2W–Cu2–O2B O2W–Cu2–N1B

92.44(10) 85.13(8) 174.93(8) 160.20(12) 103.14(9) 96.63(12) 89.96(9) 91.94(11) 160.22(13) 175.13(9) 105.39(9) 94.31(13)

(TBP) to square pyramidal (SP) can be evaluated by the Addison’s distortion index, s, defined as the ratio of the mean in-plane Cu– L bond distance to the out-of-plane Cu–L bond distance, s = [|h  U|/60], where h and U are the two largest coordination angles; s = 0 for perfect SP and 1 for ideal TBP [27]. Both Cu1 and Cu2 ions possess a distorted square-pyramidal geometry as reflected from their corresponding s values (s = 0.2455 and 0.2485 for Cu1 and Cu2, respectively). The equatorial plane is formed by the two imine nitrogen atoms (N1, N2) and two phenolic oxygen atoms (O1, O2) from the Schiff base ligand. The axial position is occupied by the oxygen atom from a coordinated water molecule. The central CuII ion is displaced from the mean equatorial plane by ca. 0.190 Å towards the axial oxygen. The equatorial bond lengths around the copper(II) centers [1.933(2)1.999(2) Å] are all shorter than the axial length [2.373(3) and 2.363(3) Å for A and B, respectively]. The six membered chelate ring CuC3N2 incorporating the trimethylene fragment from the starting diamine assumes a envelope conformation and both the CuOC3N chelate rings are almost planar. The two complexes A and B are mutually facing and connected through a pair of strong intermolecular hydrogen bonds to result in a dimeric aggregate as shown in Fig. 2. In this arrangement, two free methoxy oxygen atoms O3A and O4A of the Schiff base ligand chelated to the Cu1 center are engaged in the hydrogen bonding interactions with the hydrogen atoms H2W1 and H2W2 of the

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Fig. 2. Representation of the asymmetric unit of 1 containing the hydrogen bonded dimer of [Cu(L1)(H2O)]. Dashed lines indicate hydrogen bonds.

water molecule axially coordinated to the Cu2 center through O2W–H2W1  O3A and O2W–H2W2  O4A hydrogen bonds, respectively, thereby enclosing a twelve membered ring, assignable a graph-set notation of R22 (12). Again the hydrogen atoms of the water molecule coordinated to Cu1 act as hydrogen bond donors to the two methoxy oxygens of another adjacent [Cu(L1)(H2O)] moiety creating a similar ring motif. Repetition of this ring motif in either direction generates an extended hydrogen bonded array of complex 1 as depicted in Fig. 3. This infinite step-like polymeric network propagating along crystallographic c-axis is composed of two parallel layers interconnected by O–H  O bonding. Perpendicular distance between the two layers is found to be ca. 3.442 Å. The closest interlayer Cu  Cu separation is ca. 6.282 Å while the shortest intralayer separation is ca. 10.383 Å. The packing diagram of complex 1 is displayed in Fig. 4. All the H-bonding parameters are given in Table 3. 3.3.2. [Ni(L2)2]  CH3CN (2) Single crystal X-ray diffraction analysis reveals that complex 2 forms a one-dimensional polymeric chain due to the aggregation of the discrete monomeric entities through several classical intermolecular O–H  O hydrogen bonds. The asymmetric unit as illustrated in the ORTEP diagram (Fig. 5), consists of a monomeric Ni{C6H4(O)CH@N-C(CH3)(CH2OH)2}2 unit with a solvent acetonitrile molecule present in the lattice. In the complex the Schiff base ligand 2-((E)-(1,3-dihydroxy-2-methylpropan-2-ylimino)methyl)phenol behaves as a monoanionic, tridentate one with its ONO donor set which coordinates to the metal center through one imine nitrogen, one alkoxy oxygen atom and a deprotonated phenolic oxygen atom. The nickel(II) center adopts a distorted octahedral geometry being chelated by two units of such tridentate Schiff base ligands. The equatorial plane of the NiN2O4 chromophore consists of two imine nitrogen atoms (N9, N24), one phenolic oxygen atom O1 and one alkoxy oxygen O15, while the two axial sites are occupied by the phenolic and alkoxy oxygen atoms O16 and O30 respectively. The Ni–N(imine) distances are comparable to those observed for the similar kind of complexes present in the literature [28]. The three trans-angles at nickel(II) are very much close to 180°, varying from 169.96(4)° to 173.40(4)° while the cis-angles differ in the range 80.13(4)94.85(5)° being close to 90°. Selected bond lengths and angles for the complex 2 are given in Table 2b. A further insight to the structure of the complex 2 reveals the presence of several interesting inter- and intramolecular hydrogen

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Fig. 3. Diagram showing the propagation of the infinite array of complex 1 through self repeating ring motif. Dashed lines indicate hydrogen bonds.

Fig. 4. Packing diagram showing the step-like polymeric network of complex 1 generated by hydrogen bonding interactions between the two layers.

bonds. In the monomeric asymmetric unit, the uncoordinated alkoxy OH groups (O13–H13, O28–H28) are found to be involved in the intramolecular hydrogen bondings with the neighbouring phenolic oxygen atoms through O13–H13  O16 and O28–H28  O1, respectively. They give rise to the formation of two isolated seven membered ring motifs which may be expressed through Etter’s graph-set notation as S(7), thereby providing stability to the system. It is interesting to mention here that the donor oxygen atoms (O13, O28) of these free alkoxyl groups involved in the intramolecular hydrogen bondings also behave as good acceptors forming two intermolecular hydrogen bonds (O15#–H15#  O13 and O30*– H30*  O28) [symmetry operations: # = x, y + 2, z + 1; * = x, y + 2, z] with coordinated alkoxy group of the adjacent monomeric units and vice-versa, thus affording an infinite one-dimensional hydrogen bonded polymeric chain propagating parallel to the crystallographic c-axis (Fig. 6). It makes the monomeric unit a good template which is an important factor in establishing an ordered supramolecular structure formed by self-assembly. As a consequence of the simultaneous interplay of such intra- and intermolecular O–H  O hydrogen bonds, a 12-membered ring motif of H-bonding can be envisaged involving the phenolic oxygens, free and coordinated alkoxy groups as well as any two adjacent metal centers. This ring motif can be expressed as R44 (12) by Etter’s graph-set notation. The translational repetition of this 12-membered ring motif mediated by the metal centers eventually helps to propagate the one-dimensional chain in either direction

Fig. 5. ORTEP view of 2 with atom labeling scheme. Thermal ellipsoids are drawn at the 40% probability level. Dashed lines indicate hydrogen bonds.

(Fig. 6). The most striking point is the dual nature of the free alkoxy oxygen atoms behaving as both donor and acceptor in the intra-

S. Thakurta et al. / Inorganica Chimica Acta 362 (2009) 2828–2836 Table 2b Selected bond distances (Å) and angles (°) for 2. Bond lengths

(Å)

Bond angles

(°)

Ni1–O1 Ni1–O15 Ni1–N9 Ni1–N24 Ni1–O16 Ni1–O30

2.0059(11) 2.1148(11) 2.0155(12) 2.0137(12) 2.0070(11) 2.1173(11)

O1– Ni1–N24 N24–Ni1–O15 O15–Ni1–N9 N9–Ni1–O1 O1–Ni1–O30 O15–Ni1–O16 N9–Ni1–O16 O1–Ni1–O15 N9–Ni1–N24 O16–Ni1–O30

94.85(5) 90.94(5) 80.13(4) 93.48(5) 89.75(4) 94.05(4) 91.99(4) 169.96(4) 170.34(5) 173.39(4)

and intermolecular hydrogen bonding, respectively. The Ni–O(phenolic) distances are considerably shorter than those of the Ni– O(alkoxy). It may be attributed to the fact that the phenolic oxygens act as intramolecular H-bond acceptors while the alkoxy oxygens are the donor atoms involved in the intermolecular hydrogen bondings. A few significant hydrogen bonding parameters are given in Table 3. 3.3.3. [Co(HL3)(L3)] (3) 2-((E)-(2-hydroxyethylimino)methyl)-4-bromophenol (H2L3) is a tridentate ligand with the phenolic-O, imine-N and alkoxy-O donor sites. The strong p acceptor ability of the Schiff base ligand favours oxidation of the cobalt(II) center in the presence of methanol, allowing the formation of [CoIII(HL3)(L3)] species. In the complex, the cobalt(III) center is coordinated to two tridentate Schiff base ligands of which one ligand molecule is in dianionic form (L3)2 after deprotonation of both the phenolic and alkoxy OH- groups while the other ligand is monoanionic (HL3) due to the deprotonation of phenolic OH only. X-ray crystal structure of 3 with atom numbering scheme is presented in Fig. 7 as an ORTEP plot. The geometry around the CoIII ion can be best described as a distorted octahedron with a CoN2O4 chromophore where the equatorial plane is constructed by two imine nitrogen atoms (N1, N2), phenolic oxygen O1 and alkoxy oxygen O2. In the equatorial plane

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all the cis-angles fall within the range 85.75°95.03, while the two trans-angles are in the range 177.28–178.16° being very close to the ideal value of 180°. The axial sites are occupied by two deprotonated phenolic and alkoxy oxygen atoms O3 and O4, respectively with a trans- angle of 178.46(10)°. The average Co–N [1.892 Å] and Co–O [1.907 Å] bond lengths are in the same range as corresponding values in similar octahedral CoIII systems [29,30], and also significantly shorter than Co(II)–N and Co(II)–O bond lengths. In the reported complex, average N–Co–O bite angles for five and six membered chelate rings are found to be 86.11° and 94.98°, respectively. In general, bite angles of five membered and six membered chelate rings are 84–88° and 92–96°, respectively [31]. Therefore the values found are in nice agreement with the literature values. A detailed investigation of the crystal structure of complex 3 reveals that the donor alkoxy group O2–H2o of one moiety interacts with the acceptor alkoxy O4 atom of the adjacent moiety through O–H  O hydrogen bonds resulting in a 24-membered ring system which contains a hexanuclear core of six cobalt atoms. This Hbonded supramolecular organization may be designated by Etter’s graph-set notation R66 (24). The above assembly assumes a unique star-like shape and possesses a central round shaped cavity enclosed by the –CH2 groups from the ethanolamine fragment. A perspective view of the star-like assembly is given in Fig. 8, which also clearly demonstrates the finite cavity of the complex 3. The largest diameter at the center of the round shaped cavity is about 5.254 Å. In the macromolecular ring of complex 3, the neighboring Co  Co distance is ca. 5.425 Å, while the Co  Co distance across the cavity is ca. 10.207 Å. The average M  M  M interatomic angle is 105.64°, which is quite less than the interior angle in n-hexagon of 120°. A computation of the void volume of the network shows a value of 168.1 Å3 [19c]. The result suggests that the unfilled void has the potential to serve as host capable to trap suitable guest solvents. The cavities inside the star-shaped clusters are apparent from the crystal packing in space-filling model as shown in Fig. 9. This intriguing host framework may find diverse applications such as, for molecule recognition, solvent inclusion and solid-state catalysis. Selected bond lengths and bond angles are presented in Table 2c and the H-bond parameters are given in Table 3.

Fig. 6. Diagram showing the propagation of the polymeric chain of complex 2 through self repeating ring motif. Dashed lines indicate hydrogen bonds. Symmetry operation: # = x, y + 2, z + 1; * = x, y + 2, z.

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Fig. 7. ORTEP view of 3 with atom labeling scheme. Thermal ellipsoids are drawn at the 40% probability level.

Fig. 9. Packing diagram of complex 3 in space-filling model which shows the cavity.

Table 2c Selected bond distances (Å) and angles (°) for 3. Bond lengths

(Å)

Bond angles

(°)

Co1–O1 Co1–O2 Co1–N1 Co1–N2 Co1–O3 Co1–O4

1.879(2) 1.916(2) 1.893(2) 1.891(3) 1.879(2) 1.926(2)

O1–Co1–N2 N2–Co1–O2 O2–Co1–N1 N1–Co1–O1 O4–Co1–O1 O4–Co1–N2 O3–Co1–N2 O1–Co1–O2 N1–Co1–N2 O3–Co1–O4

95.03(10) 86.11(10) 93.05(10) 85.75(10) 88.46(9) 90.98(10) 87.66(10) 178.16(10) 177.28(11) 178.46(9)

Table 3 Hydrogen bonding parameters for 1, 2 and 3. D–H  A

Fig. 8. Perspective view of the star-shaped hydrogen bonded hexameric assembly of complex 3. Dotted lines indicate hydrogen bonds.

3.4. Cyclic voltammetry studies Electrochemical properties of complexes 1, 2 and 3 have been studied in HPLC grade acetonitrile medium with tetrabutylammonium perchlorate as supporting electrolyte at a scan rate 50 mV s1. The cyclic voltammogram of 1 shows one irreversible reductive response on the negative side of SCE at 0.59 V due to the CuII to CuI reduction couple. Several irreversible oxidation responses are also observed on the positive side of SCE at 0.71, 0.92, 1.14 V, that can be tentatively assigned to oxidation of the coordinated ligands for complex 1. CV scan of 2 shows one irreversible NiII–NiIII oxidation at E1/2 = 0.83 V versus SCE with DEp value of 200 mV, which suggests that the NiIII species is unstable and undergoes rapid decomposition. The electrochemical behavior of

1 O1W–H1W1  O3Ba O1W–H1W2  O4Ba O2W–H2W1  O3A O2W–H2W2  O4A 2 O13–H13  O16 O28-H28  O1 O15–H15  O13# O30–H30  O28* 3 O2–H2o  O4d

d(D–H) Å

d(H  A) Å

d(D  A) Å

\ðDHAÞ

0.832(16) 0.804(16) 0.870(16) 0.854(17)

2.037(19) 2.033(18) 1.993(16) 2.006(17)

2.758(3) 2.767(3) 2.800(3) 2.763(3)

144(3) 152(3) 154(3) 147(3)

0.788(19) 0.760(2) 0.728(19) 0.772(19)

1.85(2) 1.92(2) 1.94(2) 1.939(19)

2.6318(15) 2.6704(16) 2.6598(16) 2.6888(16)

171(2) 169(2) 170(2) 164(2)

0.903(18)

1.56(2)

2.4320(3)

162(4)

Symmetry transformations used to generate equivalent atoms. a x + ½, y, z + ½. # x, y + 2, z + 1. * x, y + 2, z. d x  y + 2/3, x + 1/3, z + 1/3.

complex 3 shows one irreversible wave at Epc = 0.54 to 0.69 V versus the SCE. This is attributed to the CoIII ? CoII reduction. No anodic peak could be observed even after applying a 200 mV s1 scan rate indicating the irreversible nature of the electrode process. An irreversible anodic peak at Epa = +0.29 to +0.46 V could be assigned to the oxidation of the ligand on the electrode surface.

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3.5. Magnetic study

Acknowledgements

The room temperature effective magnetic moment (leff) of complex 1 (1.85 B.M.) is typical for a CuII d9 system in a magnetically dilute environment. The magnetic moment for the polycrystalline sample of complex 2 is 2.92 B.M., which is compatible with a high-spin octahedral NiII complex. The observed magnetic moments are therefore in good agreement with the corresponding spin-only values indicating the presence of 1 and 2 unpaired electrons in complex 1 and 2, respectively [32,33]. The effective magnetic moment value 0.01 B.M. found for 3, if correlated with the electronic spectrum gives a clear indication of an octahedral CoIII complex in low spin ðt62g Þ configuration.

Santarupa Thakurta gratefully acknowledges the Council of Scientific and Industrial Research, New Delhi, India for the award of a Junior Research Fellowship (CSIR Sanction No.09/096(0519)/2007EMR-I). The work is also financially supported by the research grant from All India Council for Technical Education (AICTE), New Delhi, India.

3.6. Thermogravimetric analysis The thermal stabilities of the three complexes have been investigated using TGA over a temperature range of 25–800 °C in dynamic nitrogen atmosphere. The multiple steps in TGA curve of all the complexes show a gradual weight loss indicating fragmentwise decomposition with increasing temperature. The thermogram of complex 1 shows that it is stable up to 125 °C, after which it undergoes dehydration in the temperature range 125–150 °C with a total 1.83 % mass loss per formula unit. After completion of the dehydration process, the dirty-green coloured residue was investigated with FT-IR and no trace of stretching bands associated with the water molecules were observed in the spectrum. On further heating the deaquated species decomposes in the range 265–440 °C with 82.17% mass loss corresponding to the elimination of the Schiff base ligand. Complex 2 upon heating loses the lattice acetonitrile molecule in the first step (70–105 °C). The desolvated species is found to be stable up to 270 °C and thereafter it registers a mass loss of 87.56% in the range 270–415 °C, corresponding to the decomposition of two molecules of Schiff base ligand. Complex 3 is thermally stable up to 225 °C and then begins to decompose in two consecutive steps. The first step (225–300 °C) is probably due to a partial decomposition of the coordinated Schiff base ligand. The second thermal event occurring between 300 and 420 °C may be assigned to the removal of the rest of the ligand. The total mass loss up to 420 °C is found to be 88.63% which corroborates the loss of two molecules of Schiff base ligands. For all the complexes the thermal curves get stability after 600 °C after which no further mass loss was observed.

Appendix A. Supplementary material CCDC numbers 650260, 685501 and 685502 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.ica.2009.01.002. References [1] [2] [3] [4] [5]

[6] [7] [8] [9] [10]

[11]

[12] [13] [14] [15] [16]

4. Conclusion In the current work, we have successfully designed some interesting supramolecular architectures of coordination complexes by exploiting the O–H  O hydrogen bonding interactions involving the constituent materials. The strength, selectivity and directionality inherent in this type of hydrogen bonding processes have allowed molecular recognition leading to the formation of 1-D polymeric networks in complexes 1 and 2, as well as a cyclic aggregation containing a potential host framework in complex 3. Interestingly, in case of complexes 1 and 2, the self-repetition of the Hbonded ring motif Rxx (12) [where x = 2 and 4 for 1 and 2, respectively] has played a significant role in carrying out the propagation of the infinite arrays, whereas in complex 3 a H-bonded cyclic hexamer has been formed which can be designated by the synthon R66 (24). We present here the detailed structural descriptions of the three transition metal coordination complexes synthesized from different tri- or tetradentate Schiff base ligands along with the spectroscopic, electrochemical and thermal characterizations to compliment their crystal structures.

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