An unprecedented CuII–Schiff base complex existing as two different trinuclear units with strong antiferromagnetic couplings

Polyhedron 28 (2009) 3542–3550

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An unprecedented CuII–Schiff base complex existing as two different trinuclear units with strong antiferromagnetic couplings Aurkie Ray a, Dipali Sadhukhan a, Georgina M. Rosair b, Carlos J. Gómez-García c, Samiran Mitra a,* a

Department of Chemistry, Jadavpur University, Raja S.C. Mullick Road, Kolkata 700 032, West Bengal, India School of Engineering and Physical Sciences, Heriot Watt University, Edinburgh, EH14 4AS, UK c Instituto de Ciencia Molecular ICMol, University of Valencia, Parque Científico, 46980 Paterna, Spain b

a r t i c l e

i n f o

Article history: Received 17 April 2009 Accepted 9 July 2009 Available online 14 July 2009 Keywords: CuII–Schiff base complex Linear Bent Trinuclear units Crystal structures Intra-trimer Inter-trimer Antiferromagnetic couplings

a b s t r a c t A new tetradentate N2O2 donor Schiff base ligand [OHC6H4CH@NCH2CH2CH(CH2CH3)N@CHC6H4OH = H2L] was obtained by 1:2 condensation of 1,3-diaminopentane with salicylaldehyde and has been used to synthesise an unusual copper(II) complex whose asymmetric unit presents two structurally different almost linear trinuclear units [Cu3(l-L)2(ClO4)2] [Cu3(l-L)2(H2O)(ClO4)2] (1). The ligand and the complex were characterised by elemental analysis, FT-IR, 1H NMR and UV–Vis spectroscopy in addition electrochemical and single crystal X-ray diffraction studies were performed for the complex. The magnetic properties of 1 reveal the presence of strong intra-trimer (J1 = 202(3) cm1 and J2 = 233(3) cm1) as well as very weak inter-trimer (zJ0 = 0.11(1) cm1) antiferromagnetic interactions. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction There has been considerable interest in synthesising trinuclear copper(II) complexes as such complexes can biomimic multicopper proteins and metalloenzymes such as the trinuclear active site in laccase [1]. Trinuclear copper(II) complexes act as model systems for understanding the type of magnetic interactions expected to operate through the orbitals of the bridging ligands connecting the metal centres. A search of the Cambridge Structural Database (November 2008) [2,3] revealed 46 copper(II) trinuclear complexes of which only a limited number of linear trinuclear compounds have been structurally and magnetically characterised [4–9]. Surprisingly of these 46 trinuclear copper(II) complexes only 12 are derived from Schiff bases. Our research group has mainly focussed on the Schiff base metal complexes which have been widely utilized in heterogeneous catalysis [10], molecular electronics, single molecule based magnetism and above all, photochemistry [11–15]. Tetradentate Schiff bases derived from 2 equiv. of salicylaldehyde and 1 equiv. of a variety of alkyl or aryl diamines have already been explored. Nathan and his group were successful in synthesising a series of copper(II) complexes derived from polymethylenediamines with a regular progression of chain lengths and showed that

* Corresponding author. Tel.: +91 033 2414 6666x2779; fax: +91 033 2414 6414. E-mail address: [email protected] (S. Mitra). 0277-5387/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2009.07.017

on increasing the chain length the symmetrical N2O2 donor ligand showed a conformational change (cis to trans) while coordinating the metal ion [16] but no trinuclear complexes were reported. Interestingly, till now there are only seven doubly phenoxo bridged trinuclear copper(II) Schiff base complexes known to us [17–22] with one of these reported by our group [17]. In these seven trinuclear complexes the phenoxo bridges of the Schiff base ligands are solely responsible for connecting the adjacent copper(II) centres and no other bridging ligand (acetate, nitrate) is present in the coordination environment of the copper(II) trimer. The Schiff bases responsible in generating such trinuclear species are mainly derived from 1,3-diamine or substituted 1,3-diamines with salicylaldehyde and earlier we reported similar trinuclear copper(II) Schiff base complex resulted from 1:2 condensation of 1,3-diamino propane with 2-hydroxy acetophenone [17]. As an extension of earlier work, in this paper we report an unusual CuII–Schiff base complex, 1 which exists as two independent almost linear trinuclear units. It is interesting to note that by introducing an ethyl substituent at the 3 position of the reported 1,3-diamine fragment of the Schiff base ligand (derived from 1,3diaminopropane and salicylaldehyde) [18] we obtained a new Schiff base ligand [H2L] (Scheme 1) which coordinated copper(II) ions, generating two structurally distinct and independent trinuclear crystallographic units. The coordination environment and geometry of the copper(II) centres in the two trinuclear units are significantly different from one another. The ligand and the

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CH3 +

1mmol H2N

NH2

Refluxed for 2 h

2mmol

CH3 N

OHC

N

OH OH

HO

H2 L

CH3 N

CH3

N

N

-2H

OH

HO

N

+

O

O

N2O2 donor set Scheme 1. Procedure of synthesis and the coordination mode of the N2O2 donor Schiff base ligand [H2L].

complex were spectroscopically characterised. A single crystal Xray diffraction study revealed the unusual structural aspects of 1. Unlike the previously reported antiferromagnetically coupled copper(II) trinuclear complexes, there is a more abrupt decrease in vmT product observed for 1 at very low temperatures and this is attributed to weak antiferromagnetic inter-trimer interactions. Thus the cryomagnetic investigation supported the presence of inter-trimer as well as intra-trimer antiferromagnetic interactions in 1. 2. Experimental 2.1. Materials All solvents were of reagent grade and used without further purification. 1,3-diaminopentane and salicylaldehyde were purchased from Aldrich Chemical Company and were used as received. Copper perchlorate hexahydrate was prepared by treatment of copper carbonate (E. Merck, India) with 60% perchloric acid (E. Merck, India) followed by the slow evaporation on the steam bath. It was then filtered through a fine glass-frit and preserved in a CaCl2 desiccator for further use. 2.2. Syntheses Caution! Perchlorate salts are potentially explosive and should be used in small quantity and with much care. 2.2.1. Synthesis of the Schiff base Ligand [H2L] The Schiff base ligand [H2L] was prepared by the reflux condensation of a methanolic solution of 1,3-diaminopentane (0.59 ml, 5 mmol) with 10 mmol salicylaldehyde (1.04 ml) shown in Scheme 1. The ligand was refluxed for 2 h when yellow coloured solution developed. The colour indicated the formation of the Schiff base. Slow evaporation of the resulting solution yielded shiny yellow crystalline Schiff base [H2L]. They were dried and stored in vacuo

over CaCl2 for subsequent use. Anal. Calc. for [C19H22N2O2]: C, 73.55; H, 7.09; N, 9.03. Found: C, 73.58; H, 7.11; N, 9.04%. 2.2.2. Synthesis of [Cu3(l-L)2(ClO4)2] [Cu3(l-L)2(H2O)(ClO4)2] (1) Cu(ClO4)26H2O (0.56 g, 1.5 mmol) was dissolved in 20 ml methanol and warmed. About 10 mL of yellow methanolic solution of the Schiff base [H2L] (0.354 g, 1 mmol), was added to it dropwise. The mixture was allowed to stir for 2 h at 50 °C. The dark green solution was filtered and maintained at 22 °C for crystallisation by slow evaporation. After one week, green block shaped crystals suitable for X-ray crystallography were obtained. Crystals were isolated by filtration and were air-dried. Anal. Calc. for [C37H40Cl2Cu3N4O14]: C, 43.26; H, 3.90; N, 19.10. Found: C, 43.30; H, 3.92; N, 19.13%. 2.3. Physical measurements The Fourier transform infrared spectra were recorded in the range 4000–400 cm1 on a Perkin–Elmer RX I FT-IR spectrophotometer with solid KBr disc. The electronic spectra were recorded at 300 K on a Perkin–Elmer Lambda 40 (UV–Vis) spectrometer using HPLC grade acetonitrile as solvent with a 1 cm quartz cuvette in the range 200–800 nm. 1H NMR spectra were recorded on a BRUKER 300 MHz FT-NMR spectrometer using trimethyl silane as an internal standard in CDCl3. C, H, N microanalysis was carried out with a Perkin–Elmer 2400 II elemental analyser. Electrochemical measurement was performed on a VersaStat-PotentioStat II cyclic voltammeter using HPLC grade acetonitrile as solvent where tetrabutylammonium perchlorate was used as supporting electrolyte at a scan rate 50 mV/s. Platinum and saturated calomel electrode (SCE) were the working and the reference electrodes in the process, respectively. The magnetic susceptibility measurements were carried out in the temperature range 2–300 K with an applied magnetic field of 0.1 T on a polycrystalline sample of 1 (mass = 31.71 mg) with a Quantum Design MPMS-XL-5 SQUID magnetometer. The isothermal magnetization was performed on

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the same sample at 2 K with magnetic fields up to 5 T. The susceptibility data were corrected for the sample holders previously measured using the same conditions and for the diamagnetic contributions of the salt as deduced by using Pascal’s constant tables (vdia = 534  106 emu mol1). 2.4. X-ray crystallography The X-ray diffraction experiment was carried out at 100(2) K on a green block shaped single crystal (0.38  0.20  0.08 mm) of 1. The crystal was mounted on a Bruker X8 Apex 2 CCD area diffractometer equipped with Mo Ka radiation (k = 0.71073 Å). The lattice constants were refined by least-square refinement using 7542 reflections (2.35° < h < 21.31°), 9171 unique reflection (Rint = 0.0863). Structure solution and refinement based on 4644 observed reflections with I > 2r(I) and 579 parameters gave final R = 0.0716, wR = 0.1865 and S = 1.022. Data collection and data reduction were performed with the APEX 2 suite of programs [23]. All non hydrogen atoms were refined anisotropically by full-matrix least-squares based on F2. The H atoms were generated geometrically (apart from H atoms bound to O2W which were located in the difference map with O–H distances restrained to 0.90(2) Å and all were included in the refinement in the riding model approximation. The components of the disordered ethyl groups (C45 and C45A, C46 and C46A) had relative occupancies refined to 69%:31%, respectively. Restraints were applied to the displacement parameters. 50% Occupancy was assigned as an estimate for ethyl C atoms C22 and C23 because full occupancy resulted in unreasonable displacement ellipsoids. However location of the second disordered ethyl group fragment which would be expected to be bound to C12 was not discernable (highest residual peak 1.156 e Å3). C–C bond distances in the ethyl groups were restrained to 1.50(2) Å. In the disordered perchlorate ion four of the five O atoms were assigned occupancies of 75%. The fifth retained full occupancy. Selected crystallographic data, experimental conditions and some important features of the structural refinements of both the complexes are summarized in the Table 1.

Table 1 Crystal structure parameters of 1. Parameters

1

Empirical formula Formula weight Crystal System Space group a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z T (K) kMo Ka (Å) Dc [Mg m3] [l(mm1)] F(0 0 0) h Range for data collection (°) Total data Unique data Observed data [I > 2r(I)] R1[I > 2r(I)] wR2[I > 2r(I)] s Rint Dqmax (e Å3) Dqmin (e Å3)

C37H40Cl2Cu3N4O14 1026.25 monoclinic C2/c 32.7759(16) 8.9878(4) 28.3359(13) 90 90.532(2) 90 8346.9(7) 8 100(2) 0.71073 1.633 1.714 4184 2.35–21.31 58 860 9171 4644 0.0716 0.1865 1.022 0.0863 1.156 1.163

3. Results and discussion 3.1. Fourier transform IR spectroscopy The IR spectrum of 1 was analysed and compared with that of its free ligand H2L in the region 4000–400 cm1. A strong sharp absorption band around 1631 cm1 in the spectrum of the Schiff base ligand can be assigned to the C@N stretching. In the complex, this band is shifted to 1618 cm1 upon complexation with the metal, which can be attributed to the coordination of the imine nitrogen to the metal centre [24–26]. The ligand shows a well-defined band at 3537 cm1 due to O–H stretching which disappears in the complex, indicating the deprotonation of the Schiff base ligand upon complexation. The strong phenolic C–O absorption band at 1214 cm1 observed in the spectrum of H2L shifts to lower frequency at 1192 cm1, supporting the coordination of the deprotonated phenolic oxygen atoms to the metal centres in the complex [26]. A broad band at 3300–3450 cm1 indicates that water is present in 1. The characteristic bifurcated absorption band at 1098– 1121 cm1 in the spectrum of the complex indicates the presence of perchlorate anions. The ligand coordination to the metal centre is substantiated by a band appearing at 427 cm1 which is mainly attributed to mCu–N in the complex. 3.2. UV–Vis spectroscopy UV–Vis spectra of the free ligand and 1 were recorded at 300 K in HPLC grade acetonitrile solution. The UV–Vis spectrum of the Schiff base ligand [H2L] exhibits two charge transfer (CT) bands at 255 and 308 nm attributed to p ? p* and n ? p* transitions within the Schiff base ligand. In the spectrum of the complex the CT band at 255 nm remains intact, in agreement with the p ? p* transition of the Schiff base ligand. Another band at 340 nm is detected in the spectrum of 1. The band at 308 nm observed in the spectrum of the free ligand [H2L] is red shifted to 340 nm (in 1) in form of ligand to metal charge transfer (LMCT) transition. Unlike the spectrum of the free ligand, much weaker and less well-defined broad bands are found in the spectrum of the complex at 650 and 600 nm which are assigned to the d–d transitions. The two different d–d bands are in good agreement with the presence of two different coordination environments in 1. The band at 650 nm is associated with a copper(II) centre having square–pyramidal geometry and the d–d transition at 600 nm may be attributed to structurally well characterised square–planar copper(II) complexes [27]. 3.3. 1H NMR spectroscopy 1

H NMR spectroscopy has been used to extract information regarding the coordination mode of the ligand H2L with the metal ion for 1. It is generally seen that 1H NMR spectra of the coordination complexes containing paramagnetic metal ions are broadened to some extent but for 1 fairly sharp and well-resolved spectrum was obtained which is probably due to sufficiently short electronic relaxation time [28–30]. Unlike the spectrum of H2L, the chemical shifts of the hyperfine-shifted signals of 1 appeared within a short range 0 to +15 ppm. For 1 the signal due to the protons closer to the respective metal centres experience stronger paramagnetic effects and consequently they are most downfield shifted. The NMR proton numbering scheme of H2L along with the 1H NMR spectra of the ligand and the complex are represented in Fig. 1a and b. The 1H NMR data are summarized in Table 2. In the NMR spectrum of the free ligand H2L no broad peak was observed due to free amine protons in the region 5.0–8.0 ppm indicating that, both –NH2 functions of the 1,3-diamine have undergone condensation with the

A. Ray et al. / Polyhedron 28 (2009) 3542–3550

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(a)

(b)

Fig. 1. (a) NMR proton numbering scheme of H2L. (b) 1H NMR spectra of H2L and 1.

Table 2 1 H NMR data for the H2L and 1. Schiff base/complex

d (in ppm)

H2L

0.89 (3H, t, H11); 2.24 (2H, m, H10); 2.5 (2H, m, H8); 3.42 (2H, t, H7); 3.50 (1H, m, H9); 3.8 (2H, s, H6); 5.26 (2H, s, H1); 6.35(2H, d, H5); 6.41 (2H, t, H3); 6.80 (2H, t, H4); 6.85 (2H, d, H2) 0.89 (3H, t, H11); 2.24 (2H, m, H10); 4.5 (2H, m, H8); 5.5 (2H, s, H2O); 8.52 (2H, t, H7); 8.8 (1H, m, H9); 10.55 (2H, d, H5); 11.91 (2H, t,H3); 12.90 (2H, t, H4); 13.75 (2H, d, H2); 13.9 (2H, s, H6)

1

carbonyl functional group of salicylaldehyde to result the Schiff base ligand H2L [31]. The 1H NMR spectra (Fig. 1) show that the signal due to the phenolic hydrogen (H1) of the free ligand H2L (5.26 ppm) is absent in the spectrum of 1. The imino proton (H6) is assumed to be significantly downfield shifted (10.1 ppm) due to the coordination of the Schiff base ligand in 1 through imine nitrogen atoms. The largest paramagnetic deshielding effect on

H6 indicates that it is in the closest vicinity to the metal centre. The phenyl hydrogens also show significant chemical shifts [4.2 ppm (H5), 5.5 ppm (H3), 6.1 ppm (H4), 6.90 ppm (H2)] indicating the participation of the deprotonated phenolato oxygen in the coordination with the metal centres [31]. The ortho (H2) and para (H4) hydrogens are significantly downfield shifted due to large spin delocalization effect through p-bonding of the aromatic

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ring. H2 being ortho to the coordinated phenolato oxygen, is closer to the metal centre and undergoes larger paramagnetic deshielding effect than H4. In 1 the methylene protons (H7) and methine proton (H9) both being very close to imine nitrogens show downfield shifts of 5.1 and 5.3 ppm, respectively from the free ligand values which are attributed to the coordination of the imine nitrogens to the metal centres. Unlike the spectrum of the free ligand the 1 H NMR spectrum of 1 contains a singlet at 5.5 ppm which can be assigned to the protons of the coordinated water molecules. The results obtained from the NMR spectra are in agreement with the structure obtained by the X-ray analysis. 3.4. Cyclic voltammetry The electrochemical behaviour of 1 was studied in HPLC grade acetonitrile medium with tetrabutylammonium perchlorate as

supporting electrolyte at a scan rate of 50 mV/s. Complex 1 shows a reductive response at 0.608 V versus SCE, which is assigned to CuII ? CuI reduction and an oxidative response at 0.526 V versus SCE was assigned for CuI ? CuII oxidation (Fig. S1: provided as Supplementary data). This reduction and oxidation are quasi reversible, characterised by a peak to peak separation (DEp) of 82 mV which remain unchanged upon changing the scan rate. The anodic peak current (ipa) is almost equal to the cathodic peak current (ipc) as expected for a quasi reversible electron transfer process. The E1/2 value for 1 is 0.57 V. The E1/2 value and the quasi reversible nature of the cyclic voltammogram of 1 is in good agreement with that of the earlier reported doubly phenoxo bridged dicopper(II) complex [Cu2L1(ClO4)2]2CH3OH2H2O (0.52 V) [32]. 3.5. Description of the crystal structure of 1 The asymmetric unit of 1 comprises two structurally independent trinuclear units [Cu3(l-L)2(ClO4)2] (I) and [Cu3(l-L)2(H2O)(ClO4)2] (II) shown in Figs. 2 and 3, respectively. The main bond lengths and angles are summarised in Tables 3 and 4 and the Hbonding data are listed in Table 5. In the trinuclear units (I and II) there are two geometrically distinct copper(II) centres [Cu(1) and Cu(2) for I and Cu(3) and Cu(4) for II]. Cu(1) sits on a centre of inversion, while Cu(4) is positioned on a twofold axis. Thus I is constrained by crystallographic symmetry to be linear but II can be bent and is found in this case. The trinuclear unit I is very similar to the previously reported trinuclear copper(II) complex [Cu3(l-L0 )2(ClO4)2] (A) [L0 = OHC6H4CH@ NCH2–CH2–CH2N@CHC6H4OH] [17], where the three copper(II) centres are held together by two doubly phenoxo bridged oxygen atoms of the deprotonated tetradentate Schiff base ligands [L]2 and [L0 ]2, respectively (Fig. 2). In both trinuclear complexes (I and A) the respective Schiff bases behave as N2O2 donor ligands with two perchlorate ions coordinating with the two terminal copper atoms

Fig. 2. ORTEP diagram of I. The label suffix A denotes the symmetry operation x + 1/ 2,y + 3/2 used to generate this symmetry equivalent atom.

Table 4 Selected bond angles for 1 [°]. O(21)#1–Cu(1)–O(21) O(21)–Cu(1)–O(1) Cu(2)–Cu(1)–Cu(2)#1 N(9)–Cu(2)–N(13) N(9)–Cu(2)–O(21) N(13)–Cu(2)–O(21) O(1)–Cu(2)–O(6T) O(21)–Cu(2)–O(6T) O(44)–Cu(3)–N(36) O(44)–Cu(3)–N(32) N(36)–Cu(3)–N(32) O(44)–Cu(4)–O(1W) O(44)#2–Cu(4)–O(24) O(24)–Cu(4)–O(24)#2 Cu(2)–O(1)–Cu(1) Cu(3)–O(24)–Cu(4)

Fig. 3. ORTEP diagram of II. The label suffix A denotes the symmetry operation x + 1,y,z + 1/2 used to generate this symmetry equivalent atom.

180.0(3) 77.49(19) 180.0 96.7(3) 168.0(3) 91.5(3) 99.1(4) 91.5(3) 92.5(3) 166.6(3) 100.0(3) 89.83(15) 105.5(2) 110.6(3) 99.7(2) 98.1(2)

O(21)–Cu(1)–O(1)#1 O(1)#1–Cu(1)–O(1) N(9)–Cu(2)–O(1) O(1)–Cu(2)–N(13) O(1)–Cu(2)–O(21) N(9)–Cu(2)–O(6T) N(13)–Cu(2)–O(6T) O(44)–Cu(3)–O(24) O(24)–Cu(3)–N(36) O(24)–Cu(3)–N(32) O(44)–Cu(4)–O(44)#2 O(44)–Cu(4)–O(24) O(1W)–Cu(4)–O(24) Cu(3)–Cu(4)–Cu(3)#2 Cu(1)–O(21)–Cu(2) Cu(4)–O(44)–Cu(3)

102.51(19) 179.995(1) 93.3(3) 168.4(3) 77.72(19) 97.8(4) 85.3(5) 77.1(2) 167.3(2) 89.9(2) 179.7(3) 74.7(2) 124.68(14) 161.39(6) 99.6(2) 102.3(3)

Symmetry transformations used to generate equivalent atoms: #1 x + 1/2,y + 3/ 2,z #2 x + 1,y,z + 1/2.

Table 3 Selected bond lengths for 1 (Å). Cu(1)–O(21)#1 Cu(1)–Cu(2) Cu(2)–O(1) Cu(2)–O(6T) Cu(3)–O(24) Cu(3)–O(4S) Cu(4)–O(24)

1.952(5) 2.9849(9) 1.946(5) 2.395(9) 1.943(5) 2.529(6) 2.051(5)

Cu(1)–O(21) Cu(1)–O(5S) Cu(2)–N(13) O(1W)–Cu(4) Cu(3)–N(36) Cu(3)–Cu(4) Cu(4)–Cu(3)#2

1.952(5) 2.495(7) 1.949(8) 1.956(9) 1.954(7) 3.0179(9) 3.0180(9)

Symmetry transformations used to generate equivalent atoms: #1 x + 1/2,y + 3/2,z #2 x + 1,y,z + 1/2.

Cu(1)–O(1) Cu(2)–N(9) Cu(2)–O(21) Cu(3)–O(44) Cu(3)–N(32) Cu(4)–O(44)

1.960(4) 1.939(7) 1.956(5) 1.939(5) 1.956(6) 1.935(6)

A. Ray et al. / Polyhedron 28 (2009) 3542–3550 Table 5 Hydrogen bonds parameters of 1. D—HA

d(D–H) Å

d(HA) Å

d(DA) Å

\(DHA)°

O(1W)–H(1W)O(2W)#3 O(2W)–H(21W)O(1S)#4

0.84 0.901(10)

1.88 2.19(13)

2.676(8) 2.837(10)

157.6 128(9)

Symmetry transformations used to generate equivalent atoms: #3 x,y + 1,z #4 x,y + 1,z1/2.

[Cu(2), Cu(2A)] in a monodentate fashion in A, but in I the perchlorate ions appear bidentate, coordinating to the terminal Cu(2) and central Cu(1) centres. However, this perchlorate is disordered and it is one of the disordered atoms which bonds to Cu(2). The geometry of the terminal copper centres in both these complexes is best described as a distorted (4 + 1) (ONNO + O) square based pyramid with Addison parameters s = 0.057 for A and s = 0.007 for I (s = 0 corresponds to a perfect square pyramidal geometry and s = 1 to a perfect trigonal bipyramidal geometry). The four bonds constituting the basal plane of the external Cu(2) ions of I show very similar Cu–N and Cu–O bond distances [in the range 1.939(7)–1.956(5) Å]. The axial site is occupied by an oxygen atom of a coordinated perchlorate ion [Cu–O 2.395(9) Å]. The central copper atom in I is bound by four phenolato oxygens of the Schiff base ligands [Cu–O 1.952(5) and 1.960(4) Å] and can be described as having square planar coordination if the long Cu(1)–O(5s) [2.495(7) Å] contacts are not included. The asymmetric unit of 1 contains a second independent bent trinuclear unit [Cu3(l-L)2(H2O)(ClO4)2] (II) where the three copper centres are also linked by doubly bridging l-phenolato oxygen atoms of two units of the tetradentate N2O2 donor Schiff base ligands (Fig. 3). Unlike I, which is constrained to be linear by crystallographic symmetry, II is slightly distorted from linearity with a Cu(3)–Cu(4)–Cu(3) angle of 161.39(6)°. Additionally, the coordination environment of the two terminal Cu(3) centres and central Cu(4) centre in II are quite different to that in I. Although the terminal copper centres are very similar in I and II being square pyramidal and bound by the N2O2 donor atoms of the deprotonated Schiff bases and a terminally coordinating perchlorate anion, the central copper centres have different coordination environments.

3547

In II perchlorates do not bridge the terminal and central copper atoms, instead one water molecule coordinates to Cu(4) in addition to the four Schiff base oxygen atoms. The central Cu(4) atom has five coordinate trigonal bipyramidal geometry with an Addison parameter, s = 0.917. The equatorial sites of Cu(4) are occupied by two symmetry equivalent phenolato oxygen atoms [O(24)] and the coordinated water molecule [O(1W)]. The axial sites are occupied by the two remaining deprotonated phenolato oxygen atoms [O(44)] which also have the shortest Cu–O distances in that coordination sphere [Cu(4)–O(44), 1.935(6) Å; O(44)–Cu(4)– O(44)#2, 179.7(3)°, where #2 is the symmetry operation x + 1, y,z + 1/2]. A hydrogen bonded chain linking the trinuclear species II is formed between the bound water molecule and an uncoordinated solvent water molecule which in turn forms a OHO hydrogen bond to the perchlorate anion bound to Cu(3) resulting in a double stranded chain running parallel to the b axis. Alongside, the trinuclear unit I forms parallel stacks and the coordinated perchlorate forms C–HO hydrogen bonds with the Schiff base (Fig. 4). The benzene rings C(1)–C(6) overlap slightly with their symmetry equivalents in the neighbouring complex in the same stack but their centroids are too far apart for effective p–p-stacking. 3.6. Magnetic properties The thermal variation of the molar magnetic susceptibility per two copper(II) trimers times the temperature (vmT) for compound 1 shows a room temperature value of ca. 1.24 emu K mol1 (Fig. 5), much lower than the spin only expected one for six non interacting copper(II), S = 1/2 ions (2.25 emu K mol1). On lowering the temperature, the vmT product shows a fast decrease that softens at low temperatures to reach a value of ca. 0.80 emu K mol1 at ca. 50 K. At lower temperatures (below ca. 10 K) the vmT product shows a more pronounced decrease to reach a value of ca. 0.70 emu K mol1 at 2 K (inset in Fig. 5). This behaviour clearly indicates that compound 1 presents a strong antiferromagnetic coupling, as deduced form the decrease in the vmT product already observed at room temperatures. The softening observed at low

Fig. 4. Hydrogen bonded chains involving I alongside stacked molecules of II (hydrogen bonds drawn as dotted lines).

A. Ray et al. / Polyhedron 28 (2009) 3542–3550

1.3

0.84 0.82

χmT (emu.K.mol-1)

1.2

0.80 0.78 0.76

1.1

0.74 0.72

1.0

0.70 0

20

40

60

80

100

0.9 0.8 0.7 0

50

100

150

200

250

300

T (K) Fig. 5. Thermal variation of the vmT product per two copper(II) trimers for 1. Inset shows the low temperature region. Solid and dashed lines represent the best fit to the S = 1/2 trimer model with and without inter-trimer interactions, respectively (see text).

temperatures suggests that this strong antiferromagnetic exchange does not lead to a diamagnetic spin ground state, in agreement with the expected behaviour for two independent antiferromagnetically coupled copper(II) trimers. Since each copper(II) trimer is expected to present an S = 1/2 spin ground state as a result of the intra-trimer antiferromagnetic coupling, the expected vmT value should be ca. 0.40 emu K mol1 per copper(II) trimer (i.e., 0.80 emu K mol1 for the two independent copper(II) trimers), in agreement with the experimental value. The more abrupt decrease observed at very low temperatures should therefore be attributed to weak antiferromagnetic inter-trimer interactions, as confirmed by the fitting results (see below). Since, as already mentioned, the structure of 1 shows the presence of two different copper(II) trimers, we have tried to fit the magnetic properties using a simple model containing the sum of two different S = 1/2 copper(II) linear trimers (the Hamiltonian is written as H = Jn[S1S2 + S2S3], where Jn is the intra-trimer exchange coupling constant of the trimer n, S2 is the spin of the central copper(II) ion and S1 and S3 correspond to the terminal ones): [33]

vtrim ¼

Ng 2 b2 1 þ e2x þ 10e3x 4kT 1 þ e2x þ 2e3x

with x ¼ J n =kT

ð1Þ

This model satisfactorily reproduces the magnetic data of 1 with g = 1.992(1), J1 = J2 = 327(2) K = 227(1) cm1 (dashed line in Fig. 5). Note that despite no restriction in the model used, the best values are obtained for J1 = J2. Although at high temperatures this simple model reproduces satisfactorily the magnetic properties of 1, the low temperature behaviour is not so well reproduced, especially the sharp decrease observed below ca. 10 K (inset in Fig. 5). In order to better reproduce the magnetic behaviour of 1 at low temperatures, we have included a weak inter-trimer exchange interaction by using the molecular field approximation [34].

v0trim ¼

vtrim 1  ð2zJ 0 =Ng 2 b2 Þvtrim

;

where vtrim is the susceptibility of the two independent S = 1/2 trimers (Eq. (1) with J1 and J2 for the two contributing trimers) and J0 is the inter-trimer exchange coupling of the z interacting trimers. This more complete model reproduces very satisfactorily the magnetic data of 1 over the whole temperature range with the following parameters: g = 2.0166(5), J1 = 291(4) K = 202(3) cm1, J2 = 336(5) K = 233(3) cm1 and zJ0 = 0.16(1) K = 0.11(1) cm1

(solid line in Fig. 5). This model reproduces very satisfactorily the decrease observed at very low temperatures and provides a more realistic g value than the isolated trimers model. However, it could be argued that this improvement may only be due to the increase in the number of adjustable parameters (from two/three to four), so we have run a fit using the second model with J1 and J2 fixed to the values obtained in the first one (J1 = J2 = 327 K). This more restricted model (with only two free parameters) also gives a very good agreement with the experimental data over the whole temperature range, including the decrease at low temperatures with g = 2.0082(8) and zJ0 = 0.13(1) K. In all cases the g value is lower than the values usually found for copper(II) complexes, suggesting the presence of a small diamagnetic impurity that reduces the magnetic signal of the sample. A confirmation of the S = 1/2 ground spin state arising from the antiferromagnetically coupled trimers in 1 is provided by the isothermal magnetization data at 2 K (Fig. 6) that shows a saturation value close to 2 lB, the expected value for two non interacting S = 1/2 ions and that can be very well reproduced with the sum of two identical Brillouin functions for S = 1/2 with a reduced g value of 1.967(1) as a consequence of the inter-trimer antiferromagnetic coupling (solid line in Fig. 6). The strong antiferromagnetic couplings observed in both copper(II) trimers in 1 can be easily rationalized from the similar structural features found in both trimers (Fig. 7). Thus, both trimers present a symmetrical distribution of the copper(II) ions with a perfect linear arrangement in trimer I (Fig. 7a) and an almost linear arrangement in trimer II (with a Cu3–Cu4–Cu3 angle of 161.4°, Fig. 7b). This arrangement implies that the two possible coupling constants inside each trimer are identical (given the symmetry of both trimers) and that the possible interaction between the terminal copper(II) atoms (Cu2 and Cu3) must be negligible, justifying the symmetrical linear trimer model used to fit the magnetic properties (see above). In both trimers the copper(II) ions are connected through double phenoxo bridges with very similar Cu–O bond distances and Cu–O–Cu bond angles (Fig. 7). Note that the possible perchlorate bridges imply a much longer Cu–O–Cl–O–Cu pathway with Cu–O bond distances ca. 0.5 Å longer than those of the bridging oxygen atoms and, therefore, they are negligible when compared with the much shorter double phenoxo bridges. From the several magneto-structural correlations established for double oxo bridged copper(II) complexes with the oxo bridges laying in the equatorial plane [35–38], the magnetic coupling in both trimers are expected to be strong and antiferromagnetic, in agreement with the observed behaviour. Thus, the correlation

2.0

1.5

M (μB)

3548

1.0

0.5

0.0 0.0

0.5

1.0

1.5

2.0

2.5

-1

H/T (T.K ) Fig. 6. Isothermal magnetization at 2 K of 1. Solid line is the best fit to the sum of two identical S = 1/2 Brillouin functions (see text).

A. Ray et al. / Polyhedron 28 (2009) 3542–3550

3549

Fig. 7. Coordination environments of the two copper(II) trimers in 1 showing the bridging bond distances (Å) and angles (°) (colour code: Cu = light blue, N = dark blue, O = pink, Cl = green). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

established between the Cu–Cu distance (R) and the J parameter, predicts strong antiferromagnetic couplings of ca. 220 and 290 cm1 for trimers I and II (where the Cu–Cu distances are 2.985 and 3.018 Å, respectively). Furthermore, the Cu–O–Cu bond angle, which is the other structural parameter also used to estimate the J value, gives an estimation for the J values of ca. 160 cm1 for trimer I (where the Cu–O–Cu bond angles are 99.6 and 99.7°, Fig. 7 and Table 4) and ca. 210 cm1 for trimer II (where the Cu–O–Cu bond angles are ca. 98.1 and 102.3°, Fig. 7 and Table 4). Note that both magneto-structural correlations give J values in good agreement with the experimental ones even if these estimated values are approximate since these correlations do not include the torsion angle inside the Cu2O2 moieties and, even more important, in trimer II the central copper(II) ion presents a trigonal bipyramid geometry (s = 0.92). Nevertheless, these correlations allow us to suggest that in 1 the strongest antiferromagnetic coupling (J2 = 233(3) cm1) should be assigned to trimer II whereas the weakest coupling (J1 = 202(3) cm1) should be attributed to trimer I. The weak antiferromagnetic intermolecular coupling can be attributed to the presence of the H-bond network, as also observed in other similar copper(II) trimers [39]. 4. Conclusion In this paper, we have described the structural and magnetic properties of a novel and very original copper(II) complex presenting two structurally different copper(II) trinuclear units. It is really interesting to note that in 1 although the coordination environment of the central copper(II) centres in the two independent linear trinuclear units are quite different from one another, they exist in the same crystallographic asymmetric unit. From the structural point of view, the title compound represents, the first example of two chemically distinct, doubly phenoxo bridged copper(II) trinuclear units. The magnetic properties of the title compound clearly show that both the trinuclear units present similar strong antiferromagnetic coupling between the copper(II) ions through the doubly phenoxo bridges and is in good agreement with the expected value obtained from previous magneto-structural correlations for doubly phenoxo bridged copper(II) complexes. Acknowledgments A. Ray is thankful to the Council of Scientific and Industrial Research New Delhi, Govt. of India and Dipali Sadhukhan acknowledge University Grants Commission, New Delhi, Government of India for providing them the financial support to carry out the work. The authors also acknowledge the financial help obtained from All India Council for Technical Education, New Delhi, Government of India, the European Union (MAGMANet network of excellence) and the Spanish Ministerio de Educación y Ciencia (Projects

MAT2007-61584 and Consolider-Ingenio 2010 CSD 2007-00010 in Molecular Nanoscience). We also acknowledge use of the EPSRC Chemical Database Service.

Appendix A. Supplementary data CCDC 719841 contains the supplementary crystallographic data for 1. 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; or e-mail: [email protected]. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.poly.2009.07.017.

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