A new tetranuclear copper(II) complex with oximate bridges: Structure, magnetic properties and DFT study

Inorganica Chimica Acta 377 (2011) 99–104

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A new tetranuclear copper(II) complex with oximate bridges: Structure, magnetic properties and DFT study Sanjib Giri a, Debdulal Maity b, Jeffrey F. Godsell c, Saibal Roy c, Michael G.B. Drew d, Ashutosh Ghosh e, Gurucharan Mukhopadhyay b,⇑, Shyamal K. Saha a,⇑ a

Department of Materials Science, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India Department of Chemistry, Presidency College, Kolkata 700 073, India Microsystems Centre, Tyndall National Institute, University College Cork, Cork, Ireland d School of Chemistry, The University of Reading, P.O. Box 224, Whiteknights, Reading, RG6 6AD, UK e Department of Chemistry, University College of Science, University of Calcutta, 92, A.P.C. Road, Kolkata 700 009, India b c

a r t i c l e

i n f o

Article history: Received 26 April 2011 Received in revised form 11 July 2011 Accepted 27 July 2011 Available online 6 August 2011 Keywords: Tetranuclear copper complex Main group Oximato Magnetic property DFT calculations

a b s t r a c t A new tetranuclear complex, [Cu4L4](ClO4)42H2O (1), has been synthesized from the self-assembly of copper(II) perchlorate and the tridentate Schiff base ligand (2E,3E)-3-(2-aminopropylimino) butan-2-one oxime (HL). Single-crystal X-ray diffraction studies reveal that complex 1 consists of a Cu4(NO)4 core where the four copper(II) centers having square pyramidal environment are arranged in a distorted tetrahedral geometry. They are linked together by a rare bridging mode (l3-g1,g2,g1) of oximato ligands. Analysis of magnetic susceptibility data indicates moderate antiferromagnetic (J1 = 48 cm1, J2 = 40 cm1 and J3 = 52 cm1) exchange interaction through r-superexchange pathways (in-plane bridging) of the oxime group. Theoretical calculations based on DFT technique have been used to obtain the energy states of different spin configurations and estimate the coupling constants and to understand the exact magnetic exchange pathways. Ó 2011 Elsevier B.V. All rights reserved.

1. Introduction The chemistry of multinuclear metal complexes has been of increasing interest in recent years because of its relevance to the multimetal active sites of various metalloproteins [1]. The synthesis and characterization of polynuclear Cu(II) complexes have attracted tremendous attention over the past decades due to interest in investigating their structure and the role of copper centers in several catalytic systems and in understanding the magneto-structural correlations arising from the exchange coupling among Cu(II) centers with the aim of developing new molecular based magnets [2]. In this connection, our interest lies in the design, isolation, and characterization of polynuclear copper(II) complexes using Schiff base ligands. Owing to the flexibility of bridging ligands and the geometry around the copper centers those complexes can show ferromagnetic as well as antiferromagnetic exchange interactions depending on the bridging angles and bond distances. In this connection, density functional theory offers a promising tool which is applied to investigate magnetic exchange coupling by calculating the energy levels of different spin conformers [3]. Polydentate ligands containing oxime ⇑ Corresponding authors. E-mail addresses: [email protected] [email protected] (S.K. Saha).

(G.

Mukhopadhyay),

0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2011.07.048

groups are known to form a variety of hetero- and homo-polynuclear complexes, in which the oximate bridging group generally mediates very strong magnetic exchange interactions between metal ions [4]. The oximato group is very versatile and can adopt several types of bridging mode between metal ions as shown in Scheme 1 [5]. In contrast to tetranuclear copper(II) complexes, linear di- and trinuclear copper(II) complexes with oximate groups (C@N–O) have been widely studied during the last few years [6]. Among four bridging modes (Scheme 1) of the oxime group that have been identified in polynuclear compounds, the two-atom N/O bridging mode (Scheme 1a and b) is most commonly found, and it usually shows strong antiferromagnetic interactions through in-plane mode and weak antiferromagnetic interaction through out-of-plane bridging mode [7,8]. The three-atom O/N/O bridging mode (Scheme 1d) of the oxime group is rare especially in copper complexes. It appears from a search of the CCDC that this bridging mode has already been reported extensively in several manganese and nickel complexes [9], however it is still lacking in copper complex [7,10]. To the best of our knowledge, magnetic exchange interactions mediated through l3-g1,g2,g1 bridging of oximate in copper complex have never been explored. Therefore, in the present work, we report on the synthesis, spectroscopic characterization and structural aspects of a new tetranuclear with distorted tetrahedron-like copper complex derived from ONO donor Schiff base ligand HL.

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tated out immediately. It was filtered and washed with diethyl ether and then redissolved in CH3OH. Green parallelepiped-shaped crystals of complex 1 suitable for X-ray diffraction were obtained by slow-evaporation of the solvent. Yield: 82%. Anal. Calc. for C28H60Cl4Cu4N12O22: C, 25.62; N, 12.80; H, 4.61; Cu, 19.36. Found: C, 25.58; N, 12.74; H, 4.65; Cu, 19.22%. HRMS (ESI): Found m/z (M+H)+ = 912.12; Mcalcd = 912.19; IR: m(C@N), 1639 cm1, m(ClO4  ), 1108–1080 cm1. X-ray powder diffraction pattern of polycrystalline sample was measure to confirm the stability of complex.

2.2. Physical techniques

Scheme 1. Different bridging modes of oximate ligand.

Magnetic susceptibility analysis and DFT formalism reveals a suitable magneto-structure correlation and orbital interpretation of the complex. 2. Experimental

Elemental analyses (carbon, hydrogen and nitrogen) were performed using a Perkin-Elmer 240C elemental analyzer. IR spectra in KBr (4500–500 cm1) were recorded using a Perkin-Elmer RXI FT-IR spectrophotometer. Electronic spectra in acetonitrile (1200–350 nm) were recorded on a Hitachi U-3501 spectrophotometer. The magnetic susceptibility measurements were done with a MPMS XL5 SQUID magnetometer (QUANTUM DESIGN) and diamagnetic corrections were made using Pascal’s constants. Powder diffraction of bulk polycrystalline sample has been carried out at room temperature, using SEIFERT XRD 3000P. Caution! Metal perchlorates in the presence of organic ligands are potentially explosive. Only a small amount of the material should be prepared and it should then be handled with care.

2.1. Preparation of [Cu4L4](ClO4)4 A mixture of diacetylmonoxime (10 mmol, 1.011 g) and 1,2diaminopropane (5 mmol, 0.4 cm3) in methanol (25 cm3) was refluxed for about 3 h (Scheme 2). It was then cooled and a solution of Cu(ClO4)26H2O (5 mmol, 1.85 g) in methanol (20 cm3) was added with continuous stirring. A deep green compound precipi-

2.3. Crystallography Crystal data are given in Table 1. Intensity data were collected with Mo Ka radiation at 150 K using the Oxford Diffraction X-Calibur CCD System. The crystal was positioned at 50 mm from the CCD. 321 frames were measured with a counting time of 10 s. Data analysis was carried out with the CRYSALIS program [11]. The structure was solved using direct methods with the SHELXS97 program [12]. The non-hydrogen atoms were refined with anisotropic thermal parameters. The hydrogen atoms bonded to carbon were included in geometric positions and given thermal parameters equivalent to 1.2 times those of the atom to which they were attached. One of the perchlorates was disordered over two superimposed sites. An absorption correction was carried out using ABSPACK [13]. The hydrogen atoms on the water molecule were not located. The structure was refined on F2 using SHELXL97 to R1 0.0991, wR2 0.2511 for 5950 reflections with I > 2r(I).

Table 1 Crystal data and refinement details of complex 1.

Scheme 2. A schematic diagram for the synthetic procedure of complex 1.

Formula FW T (K) Crystal system Space group Crystal dimensions (mm) a (Å) b (Å) c (Å) b (°) V (Å3) Z dCalc (gm/cm3) l (mm1) No. of reflections measured Reflections with I > 2r(I) R1, wR2 (I > 2r(I))

C28H60Cl4Cu4N12O22 1312.84 150 monoclinic P21/n 0.19  0.17  0.03 12.1177(7) 20.6756(12) 21.9722(18) 99.818(6) 5424.3(6) 1 1.608 1.824 14 859 5950 0.0991, 0.2511

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3. Results and discussion 3.1. Spectral study FTIR spectrum for the present complex shows a band corresponding to azomethine (C@N) group at 1639 cm1. General lowering of this band substantiates the coordination of the CuII centre with the azomethine nitrogen. Usually the stretching vibration for water appears above 3400 cm1. Hence the appearance of a broad band in the range 3430 cm1 for this complex indicates the presence of water molecule. The nature and position of this band indicate that the H-atoms of water are involved in H-bonding. One peak at 3334 cm1 with a shoulder shows the overlapping of –NH2 and –OH peaks along with –NH2 bending at 1607 cm1. The pick splitting appeared in the range 1108–1080 cm1 indicates the presence of perchlorate anion somewhat distorted from the ideal tetrahedral geometry. This splitting also indicates that the perchlorate anion is involved in H-bonding. The electronic spectrum of the complex was recorded in CH3CN solution. It is consistent with square pyramidal geometry around the copper centre. Two d-d transitions appear between 550 and 650 nm with a shoulder around 526 nm. Other high energy bands are charge transfer in origin. 3.2. Crystal structure The structure determination by a single-crystal X-ray diffraction study reveals that the complex represents a tetranuclear species [Cu4L4]4+ as shown in Fig. 1a. Selected bond distances and bond angles are listed in Tables 2 and 3, respectively. In the cation, all four copper atoms have equivalent environments being five-coordinate with square pyramidal geometries. In the equatorial plane, each copper is bonded to three nitrogen atoms from one ligand. The fourth position in the coordination plane is occupied by the oxygen of a second ligand, resulting in bridging oxime groups. The axial fifth coordination position of the square pyramid is occupied by an oxygen atom of a third ligand. The bond lengths follow a similar pattern in the four coordination spheres. The bonds to N(n2), n = 1,4; of the N–O group are the longest Cu–N bonds falling in the range 1.970(6)– 0 2.012(6) Å A0 , next are the bonds to N(n8) in the range 1.985(8)– 0 2.025(7) Å A with Cu–N(n5) the shortest at 1.932(6)–1.976(7) Å A. The 0 axial bond lengths are much longer at 2.408(4)–2.492(5) Å A. The four donor atoms in the equatorial planes show r.m.s. deviations of 0.125, 0 0.114, 0.043, 0.113 Å A with the copper atom 0.178(1), 0.072(1),

Table 2 0 Selected bond lengths (Å A) for complex 1. Cu(2)–O(41) Cu(2)–N(25) Cu(2)–N(22) Cu(2)–O(31) Cu(4)–O(31) Cu(4)–N(45) Cu(4)–N(48) Cu(4)–N(42) Cu(4)–O(11)

1.946(4) 1.957(6) 2.012(6) 2.472(5) 1.898(5) 1.939(7) 1.985(8) 2.001(6) 2.475(6)

Cu(1)–O(21) Cu(1)–N(12) Cu(1)–N(15) Cu(1)–N(18) Cu(1)–O(41) Cu(3)–N(35) Cu(3)–N(32) Cu(3)–N(38) Cu(3)–O(21)

1.933(4) 1.970(6) 1.976(7) 1.990(7) 2.408(4) 1.932(6) 1.992(6) 2.025(7) 2.492(5)

Table 3 Selected and bond angles (o) for complex 1. O(21)–Cu(1)–O(41) O(41)–Cu(2)–O(31) N(12)–Cu(1)–O(41) N(15)–Cu(1)–O(41) N(18)–Cu(1)–O(41) O(11)–Cu(3)–N(35) O(11)–Cu(3)–N(32) O(11)–Cu(3)–N(38) N(42)–Cu(4)–O(11) N(48)–Cu(4)–O(11) O(11)–Cu(3)–O(21) N(28)–Cu(2)–O(31) N(22)–Cu(2)–O(31) O(31)–Cu(4)–O(11) N(45)–Cu(4)–O(11) N(25)–Cu(2)–O(31)

93.86(17) 89.23(18) 91.1(2) 103.9(2) 93.3(2) 172.0(3) 97.0(3) 99.2(3) 85.9(2) 95.8(2) 89.95(19) 92.1(2) 87.30(18) 91.39(19) 102.2(2) 101.5(2)

0

0.084(1), 0.118(1) Å A from the planes in the direction of the axial oxygen atom. Two of the copper atoms have further weak interactions to water molecules in the vacant axial position, namely Cu(2)–O(1) 0 0 2.63(1) Å A (x  1, y, z) and Cu(3)–O(3) 2.63(2) Å A. The extent of distortion of the coordination polyhedron from the square pyramid to the trigonal bipyramid has been calculated applying Addison’s parameter [14] (s) as an index of the degree of trigonality and s is defined as (b–a)/60 where b and a are the two trans-basal angles. For a perfectly square pyramid geometry s is equal to zero, while it becomes unity for perfectly trigonal-bipyramidal geometry. The Addison parameter values for Cu1(0.034), Cu2(0.113), Cu3(0.141) and Cu4(0.025) indicate that the penta co-ordinated geometry is square pyramidal with only a little distortion towards trigonal bipyramid. The amine nitrogen atoms N(18), N(28), N(38), N(48) all participate in hydrogen bonds (listed in Table 4) to water molecules or perchlo-

Fig. 1. (a) ORTEP-3 picture for the [Cu4L4]4+ cation in complex 1 with the atom numbering scheme. Thermal ellipsoids are shown at 25% probability. (b) Powder diffraction pattern of polycrystalline bulk sample.

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rate oxygen atoms. To analyze the purity of the polycrystalline sample we have measured the powder diffraction pattern (Fig. 1b) and found it to be consistent with the single crystal structure. 3.3. Magnetic property The magnetic behavior of the present complex is shown in Fig. 2a, where vMT is plotted against temperature. The vMT value at room temperature is 1.29 cm3 K mol1, which is low compared to that (1.5 cm3 K mol1) expected for four independent copper ions and decreases with decreasing temperature via a plateau (0.047 cm3 K mol1) between 20 and 5 K. The nature of the curve clearly indicates dominant antiferromagnetic interaction among the copper(II) ions through oximato bridges. The residual vMT value at 5 K is due to the presence of a monomeric impurity (S = 1/2) in the sample. To investigate the exchange interaction between the copper(II) atoms in the present complex, we have considered a quasi-tetrahedron arrangement of copper ions with oximato ligands in two different connecting modes (Scheme S1): (i) bridges through oxygen (l-1,1) (ii) bridges through –N–O–(l-1,2). Both bridging modes connect the pairs Cu1–Cu2, Cu2–Cu3, Cu3–Cu4, and Cu1–Cu4, with Cu–O–Cu angles in the range 102.16–104.76° (Table S2). A small change in Cu–O–Cu angle may cause large changes in exchange interactions. As the Cu–O–Cu angle of Cu1–Cu2 and Cu3–Cu4 pairs are very close, we have considered the same exchange interaction (J1) in spin Hamiltonian. Whereas Cu–O–Cu angle of Cu1–Cu3 and Cu2–Cu4 pairs differ widely, we have considered different exchange interaction J2 and J3, respectively in the Hamiltonian equation. On the other hand, there are two –N–O– bridging oxime group that link the pairs Cu1–Cu4 and Cu2–Cu3 with Cu  Cu distances 4.153 and 4.212 Å. These Cu  Cu distances are long enough so that it is not necessary to consider any magnetic exchange interactions through those copper pairs. Considering this scheme with exchange interactions as J1, J2 and J3, the magnetic data have been analyzed using isotropic spin Hamiltonian of four spins (S = 1/2) given by Eq. (1).

^ ¼ J ð^SCu1  ^SCu2 þ ^SCu3  ^SCu4 Þ  J ð^SCu1  ^SCu3 Þ  J ð^SCu2  ^SCu4 Þ H 1 2 3

ð1Þ

The experimental magnetic susceptibility data (shown in Fig. 2a) were fitted with the above spin Hamiltonian (Iso-subroutine of MAGPACK program [15]) and the best-fit parameters obtained with this model are g = 2.1, J1 = 52 cm1, J2 = 40 cm1, J3 = 48 cm1, q = 0.03 and R = 2.5  105 where q represents the fraction of paramagnetic impurity (S = 1/2) in the sample. In order to check whether the molar susceptibility data can be fitted to 1J model, we have fitted the magnetic data with J1 = J2 = J3 in the above spin Hamiltonian (Eq. (1)). The result shows a poor data fitting (R2 = 4  105) of molar susceptibility curve with J = 48 cm1, as shown in Fig. 2b. Based on the above structural and magnetic data, the following facts can be highlighted. 1. The magnetic interaction transmitted by in-plane bridging of oxime is much lower than that reported earlier [4]. Table 4 Hydrogen-bonding distances (Å) and angles (°) for the complex 1. N–H

H...O

N–H...O

N...O

Symmetry element

N(18)–H(18A) N(28)–H(28X) N(28)–H(28Y) N(38)–H(38A) N(38)–H(38B) N(48)–H(48A) N(48)–H(48B)

2.26 2.32 2.35 2.32 2.41 2.31 2.22

165 139 164 161 144 146 155

3.140(9) 3.052(19) 3.226(9) 3.190(13) 3.183(14) 3.101(18) 3.061(10)

O(1) (x  1, y, z) O(2) O(73) O(81) (1  x, 1  y, 1  z) O(81) O(3) O(71)

Fig. 2. Variation of vMT (cm3 mol1 K) as a function of temperature at 100 Oe field. Solid lines represent the theoretical curves corresponding to (a) 3  J model (b) 1  J model and (c) 4  J model. Here, black points are represents the experimental data.

2. The magnetic exchange coupling operated through out-of-plane bridging mode of oxime is negligible. 3. In the case of oxo-bridged or phenoxo-bridged copper complexes, magnetic interaction generally depends upon the bridging Cu–O–Cu angle and with an increase in angle, the interaction becomes more antiferromagnetic [16]. To understand these, we adopted DFT calculations using the experimental atomic coordinates. The DFT calculation leads to

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S. Giri et al. / Inorganica Chimica Acta 377 (2011) 99–104 Table 5 Geometrical parameters, calculated and experimental coupling constant corresponding to this complex. Cuij Cu12 Cu34 Cu13 Cu24 Cu23 Cu14

a(Cu–O– Cu) (°)

b(Cu–O–N– Cu) (°)

d2 (Cu. . .Cu) (Å)

JB3LYP/TZV (cm1)

J values extracted from Eq. (1) (cm1)

J values extracted from Eq. (2) (cm1)

J values extracted from Eq. (3) (cm1)

102.16 102.39 103.02 104.76 – –

36.09 35.08 47.49 36.79 80.79, 77.63 78.17, 69.15

3.400 3.442 3.481 3.478 4.212 4.153

60.3 (0.9) 52.1 (2.6) 62.9 (0.9) 62.9 (0.9) 0.45 (0.9) 0.45 (0.9)

52 52 40 48 – –

48 40 52 52 – –

44 40 53 53 2.5 2.5

the exchange interactions between different copper pairs are listed in Table 5. The experimentally obtained values of J1 = 52 cm1(Cu1–Cu2 and Cu3–Cu4), J2 = 40 cm1 (Cu1–Cu3) and J3 (Cu2–Cu4) = 48 cm1 do not agree with the theoretically calculated J values of Cu12 = 60.3 (0.9) cm1, Cu34 = 52.1 (2.6) cm1, Cu13 = 62.9 (0.9) cm1 and Cu24 = 62.9 (0.9) cm1, respectively. Here the numbers in parentheses indicate the maximum error values in computing the exchange energy. This indicates that the assumption to take the Cu–O–Cu bond angle as the most important parameter in the spin Hamiltonian is not correct and there must be some other structural parameters that control the exchange interaction. From the calculated J values using DFT, the spin Hamiltonian has been modified as

^ ¼ J ð^SCu1  ^SCu3 þ ^SCu2  ^SCu4 Þ  J ð^SCu1  ^SCu2 Þ  J ð^SCu3  ^SCu4 Þ H 1 2 3

ð2Þ

As the exchange interactions are indistinguishable in spin Hamiltonian, the best fit of Eq. (2) also gives the same result of J values as that obtained earlier. The J values using the modified spin Hamiltonian are listed in Table 5. It is seen that these theoretical J values agree well with the experimental values, although the overestimation is still to be found in the DFT results. To check the magnetic exchange interaction for out-of-plane bridging of oximato ligand, we again simulate the magnetic data with 4  J model. This model includes an extra spin-spin interaction term (J4) in the modified spin Hamiltonian equation, given by Eq. (3). The result with the best fit (Fig. 2c) indicates a weak antiferromagnetic interaction transmitted by out of plane oximate bridging. The extracted fitting parameters are g = 2.1, q = 0.028, R = 3.5  105 and the J-values are listed in Table 5.

Fig. 3. Graphical representation of spin density at the low-spin state (-++-) for the present complex. Here, the hydrogen atoms are omitted for clarity. Positive and negative values are represented as blue and green surfaces, respectively. Isocontour value = 0.008. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this paper.)

^ ¼ J ð^SCu1  ^SCu3 þ ^SCu2  ^SCu4 Þ  J ð^SCu1  ^SCu2 Þ  J ð^SCu3  ^SCu4 Þ H 1 2 3  J 4 ð^SCu1  ^SCu4 þ ^SCu2  ^SCu3 Þ

ð3Þ

In our present complex, the magnetic exchange pathways are the resultant of two interactions – one, between the apical–equatorial (a–e) Cu centers bridging through oxygen of oximato ligands and the other between equatorial–equatorial (e–e) Cu centers bridging through –N–O– group (in-plane bridging of oxime). We also calculated the spin density distribution at low spin state to detect the exact magnetic exchange pathways shown in Fig. 3. As expected, the spin density distribution show the predominance of delocalization mechanism through a r-type exchange pathway involving dx2 y2 orbitals of CuII ions and the hybrid r-orbitals (sp2) of bridging oxygen and nitrogen atom. It is clear that the apical–equatorial (a–e) exchange pathways do not play any significant role to transmit magnetic interaction. In case of equatorial–equatorial (e–e) exchange pathways one can expect a strong antiferromagnetic to moderate antiferromagnetic interaction depending on the relative disposition of the planes that contain the magnetic orbitals [4,7,17]. So in this case, the dihedral angle (Cu–O–N–Cu) plays an important role in the val-

ues of J. Larger dihedral angles decreases the effective p-overlap between sp2 orbitals of nitrogen and oxygen of the oxime group. This non-co-planarity inhibits extension of delocalized spin as well as strong antiferromagnetism. Simply the perpendicular dx2 y2 orbitals lead to less overlap between magnetic centers and thus transmit relatively weaker interactions. But in our case, if we compare the J values from Table 5 with their dihedral angles then it is seen that there is no regular change of dihedral angle with exchange interaction. Apart from dihedral angle, there is another parameter viz. the Cu. . .Cu bond length that can affect to change the magnetic exchange coupling. In general, the magnetic exchange interaction for out-of-plane bridging of oximato ligand becomes small with positive or negative values and it is operated between dx2 y2 orbitals with apical and equatorial (a–e) position of copper ions through N–O of oximato ligands [8]. In our complex the magnetic orbitals of copper ions simultaneously participate in both in-plane (e–e) and out-of-plane (a–e) bridging but the out-of-plane bridged exchange interaction becomes negligible due to the superiority of the in-plane r-superexchange interaction.

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Table 6 parameters extracted from DFT calculation corresponding different spin conformations. No.



Spin conformers (Cu1–Cu2– Cu3–Cu4)

Energy (Eh)

1 2 3 4 5 6 7 8

6.0122 2.9972 2.9967 2.9979 2.9974 1.9978 1.9971 1.9830

++++ +++ +++ +++ +++ ++ ++ ++

8703.415282 8703.415816 8703.415845 8703.415792 8703.415837 8703.415779 8703.415851 8703.416356

for awarding fellowship. S.K.S. acknowledges DST, New Delhi, for financial support. We thank the EPSRC (U.K.) and the University of Reading for funds for the diffractometer. Appendix A. Supplementary data

3.4. Computational methodology The BS-DFT is very useful in understanding the exchange pathways for polynuclear transition–metal complexes. Thus, in the present case, we have performed the broken symmetry DFT calculation to investigate the magnetic exchange pathways involved in the system. It has been reported in the literature that exchange coupling constants are often overestimated in DFT calculation [18]. In the present work, we used the spin flip approach for the calculation of broken symmetry states as implemented in the ORCA program package [19]. We have used the B3LYP [20] hybrid density functional and TZV basis set for all atoms [21] together with zeroth-order regular approximation (ZORA) to describe scalar relativistic effects. We have also taken advantage of the RI approximation with auxiliary TZV/J coulomb fitting basis sets to accelerate the calculations [22]. All of the energy calculations were performed including a tight SCF convergence criteria (Grid4). In the case of the present tetranuclear complex, we have calculated eight different spin conformations that correspond to three different spin configurations as shown in Table 6 and accordingly, the Ji values have been calculated using the method proposed by Ruiz and co-workers [23]. 4. Conclusions A tetranuclear CuII complex with oximate ligands has been synthesized and characterized by single crystal X-ray diffraction analysis. The 3D crystal structure shows that all copper ions occupy an equivalent environment with a square pyramidal geometry connected by rare l3-g1,g2,g1-oximato bridge. Interestingly, this l3g1,g2,g1-oximato bridge simultaneously adopts both in-plane and out-of-plane bridging mode of oxime group. Magnetic studies of this complex show moderate antiferromagnetic interactions (J1 = 48 cm1, J2 = 40 cm1 and J3 = 52 cm1) between the copper(II) centers mediated through in-plane bridging mode of oxime. We have performed DFT calculations to understand the magnetic exchange pathways and observed that, (i) the oxygen bridged exchange pathways (Cu–O–Cu) do not play any significant role in transmitting exchange interaction; instead, the in-plane bridged Cu–N–O–Cu dihedral angle and Cu. . .Cu distances are responsible for changing these moderate antiferromagnetic interactions. (ii) Magnetic exchange coupling operated through out-of-plane bridging mode of oxime is negligible. This tetranuclear copper complex shows a relatively weaker antiferromagnetic interaction through in-plane bridging of oximates as compared with the literature data. Acknowledgments S. Giri is grateful to the Council of Scientific and Industrial Research (Grant No. 09/080 (0639)/2009-EMR-I) New Delhi, India

The description of the J values with structural units and a complete list of bond angles are available in the Supplementary information. CCDC 782627 contains the supplementary crystallographic data for complex 1. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via 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.2011.07.048. References [1] (a) K.J. Waldron, J.C. Rutherford, D. Ford, N.J. Robinson, Nature 460 (2009) 823; (b) K. Wieghardt, Angew. Chem. Int. Ed. Engl. 28 (1989) 1153. [2] (a) M. Mannini, F. Pineider, P. Sainctavit, C. Danieli, E. Otero, C. Sciancalepore, A.M. Talarico, M. Arrio, A. Cornia, D. Gatteschi, R. Sessoli, Nat. Mater. 8 (2009) 194; (b) S. Thakurta, P. Roy, R.J. Butcher, M.S. El Fallah, J. Tercero, E. Garribba, S. Mitra, Eur. J. Inorg. Chem. (2009) 4385. [3] (a) A. Ozarowski, I.B. Szyman´ska, T. Muziol, J. Jezierska, J. Am. Chem. Soc. 131 (2009) 10279; (b) C. Baffert, M. Orio, D.A. Pantazis, C. Duboc, A.G. Blackman, G. Blondin, F. Neese, A. Deronzier, M. Collomb, Inorg. Chem. 48 (2009) 10281; (c) S. Giri, S. Biswas, M.G.B. Drew, A. Ghosh, S.K. Saha, Inorg. Chim. Acta 368 (2011) 152. [4] E. Colacio, C. López-Magaña, V. McKee, A. Romerosa, J. Chem. Soc., Dalton Trans. (1999) 2923. [5] M. Chao, S. Kumaresan, Y. Wen, S. Lin, J.R. Hwu, K. Lu, Organometallics 19 (2000) 714. [6] (a) P. Chaudhuri, Coord. Chem. Rev. 243 (2003) 143; (b) R. Ruiz, F. Lloret, M. Julve, J. Faus, M.C. Munoz, X. Solans, Inorg. Chim. Acta 268 (1998) 263; (c) T. Lu, Y. Lin, H. Luh, F. Liao, C. Chung, Acta Crystallogr., Sect. C57 (2001) 1398. [7] J.A. Bertrand, J.H. Smith, D.G. VanDerveer, Inorg. Chem. 16 (1977) 1477. [8] B. Cervera, R. Ruiz, F. Lloret, M. Julve, J. Cano, J. Faus, C. Bois, J. Mrozinski, J. Chem. Soc., Dalton Trans. (1997) 395. [9] (a) L.F. Jones, M.E. Cochrane, B.D. Koivisto, D.A. Leigh, S.P. Perlepes, W. Wernsdorfer, E.K. Brechin, Inorg. Chim. Acta 361 (2008) 3420; (b) C.J. Milios, A. Vinslava, W. Wernsdorfer, S. Moggach, S. Parsons, S.P. Perlepes, G. Christou, E.K. Brechin, J. Am. Chem. Soc. 129 (2007) 2754; (c) Y. -B. Jiang, H. -Z. Kou, R. -J. Wang, A. -L. Cui, J. Ribas, Inorg. Chem. 44 (2005) 709. [10] I.O. Fritsky, H. Kozłowski, O.M. Kanderal, M. Haukka, J.S´. Kozłowska, E.G. Kontecka, F. Meyer, Chem. Commun. (2006) 4125. [11] CRYSALIS, Oxford Diffraction Ltd., Abingdon, 2006. [12] G.M. Sheldrick, Acta Crystallogr., Sect. A 64 (2008) 112. [13] ABSPACK, Oxford Diffraction Ltd., Oxford, 2005. [14] A.W. Addison, T.N. Rao, J. Reedjik, J. van Rijn, C.G. Verschoor, J. Chem. Soc., Dalton Trans. (1984) 1349. [15] (a) J.J. Borr´as-Almenar, J.M. Clemente-Juan, E. Coronado, B.S. Tsukerblat, Inorg. Chem. 38 (1999) 6081; (b) J.J. Borr´as-Almenar, J.M. Clemente-Juan, E. Coronado, B.S. Tsukerblat, J. Comput. Chem. 22 (2001) 985. [16] (a) E. Ruiz, P. Alemany, S. Alvarez, J. Cano, J. Am. Chem. Soc. 119 (1997) 1297; (b) E. Ruiz, P. Alemany, S. Alvarez, J. Cano, Inorg. Chem. 36 (1997) 3683. [17] P. Chaudhuri, M. Winter, U. Florke, H.-J. Haupt, Inorg. Chim. Acta 232 (1995) 125. [18] (a) C. Adamo, V. Barone, A. Bencini, F. Totti, I. Ciofini, Inorg. Chem. 38 (1999) 1996; (b) M.J. Prushan, D.M. Tomezsko, S. Lofland, M. Zeller, A.D. Hunter, Inorg. Chim. Acta 360 (2007) 2245. [19] F. Neese, ORCA, An ab initio, Density Functional and Semiempirical Program Package, Version 2.7, Universität Bonn: Bonn, Germany, 2009. [20] (a) A.D. Becke, J. Chem. Phys. 98 (1993) 5648; (b) C.T. Lee, W.T. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785; (c) A.D. Becke, Phys. Rev. A 38 (1988) 3098. [21] A. Schäfer, C. Huber, R. Ahlrichs, J. Chem. Phys. 100 (1994) 5829. [22] F. Weigend, Phys. Chem. Chem. Phys. 8 (2006) 1057. [23] E. Ruiz, J. Cano, S. Alvarez, P. Alemany, J. Comp. Chem. 20 (1999) 1391.