out-of-plane displacement (Ï) of the alkoxo group with respect to the molecular Cu2O2 plane. Large Ï values prevent an efficient overlap between the O 2p and ...
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Cite this: DOI: 10.1039/c3nj00000x
Received 00th XXXXX 2013, Accepted 00th XXXXX 2013 DOI: 10.1039/c3nj00000x www.rsc.org/njc
Magnetic behaviour vs. structural changes in a isomeric series of binuclear copper(II) complexes: an experimental and theoretical study Raj Pal Sharma*,a Anju Saini, a Paloth Venugopalan,a Valeria Ferretti, *b,c Federico Spizzo, d Celestino Angeli, b and Carmen J. Calzadoe In an endeavor to study how polydentate nitrogen donor ligand affects the magnetic properties of copper(II) methoxybenzoates, three novel complexes of copper(II) were investigated. [Cu 2(H2tea)2(o-methoxybenzoate) 2], [Cu 2(H2 tea)2(m-methoxybenzoate) 2].2H2O and [Cu 2(H2tea)2 (p-methoxybenzoate) 2].2H2 O, (where H2tea = mono-deprotonated triethanolamine) were synthesized by addition of triethanolamine (H 3tea) to the hydrated Cu(o,m-,p-methoxybenzoates) 2. The newly synthesized complexes have been characterized by elemental analyses, spectroscopic techniques (electronic and FT-IR), magnetic moment determination, molar conductance studies, TGA, and single crystal X-ray determination. The experimental characterization was integrated with the ab initio theoretical determination of the magnetic coupling constant value and with the analysis of the correlation between this value and the relevant geometrical parameters. Variable temperature solid state magnetization measurements and ab initio calculations indicate a remarkable ferromagnetic coupling of the unpaired electrons centered on the two Cu atoms for the m- and p-methoxybenzoate complexes (J=100.9 cm -1), while a non-negligible antiferromagnetic coupling is found for the third complex (J= -83.1 cm-1). This differential behaviour can be rationalized on the basis of the out-of-plane displacement () of the alkoxo group with respect to the molecular Cu2O2 plane. Large values prevent an efficient overlap between the O 2p and the magnetic Cu 3dx 2-y2, favouring a ferromagnetic coupling between the Cu sites.
Introduction Binuclear copper(II) complexes have attracted much attention during the past decades due to their diverse properties and reactivity. They can mimic the active sites of metallobiomolecules, display DNA binding and cleavage properties  or they can be used to investigate the mutual influence of two metal centers in terms of electronic, magnetic and redox properties. . Among the reported binuclear Cu(II) complexes, the dihydroxo- and dialkoxo bridged Cu(II) dinuclear compounds constitute a well-known family, structurally and magnetically characterized in detail from experimental  and theoretical  studies. The pioneering works of Hatfield and coworkers  on dihydroxo- and dialkoxo bridged dinuclear Cu(II) complexes established the correlation between the magnetic coupling constant J ˆ J Sˆ Sˆ , according to the Heisenberg-Dirac-Van Vleck (H 1 2 hamiltonian) and the structure, mainly as the Cu−O−Cu angle increases, the coupling J between Cu(II) ions switches from ferromagnetic (FM) with J > 0 to antiferromagnetic (AF) with J < 0, as shown in Figure 1.
Figure 1. J value (cm-1) vs bridging Cu-O-Cu angle (º) for a series of hydroxo- and alkoxo bridged Cu(II) dimers, Refs. [3, 4, 5,6]. The solid circles correspond to the calculated J values for systems 1-3.
After this innovative work, other magnetostructural correlations have been found for these systems, such as the out-of-plane shift of the C atom (H atom) of the bridging alkoxo(hydroxo) group described by Ruiz and coworkers [4e,6]. Although a large number of binuclear hydroxo- and alkoxo bridged Cu(II) complexes have been synthesized and characterized, only a few
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New J. Chem., 2013, 37, 1-3 | 1
of these complexes show ferromagnetic behavior, as clearly illustrated in Figure 1. The search for new systems with strong ferromagnetic coupling together with the versatility of alkoxo bridged Cu(II) dimers as building blocks in metallosupramolecular chemistry  explain the attention still paid to these systems. In this paper we report the synthesis and characterization of three alkoxo-bridged dinuclear copper(II) complexes, resulting from the reaction between triethanolamine (H3tea), methoxybenzoate and Cu(II) ions in basic solutions (Scheme 1). While [Cu2(H2tea)2(o-methoxybenzoate)2] (1) shows an antiferromagnetic behaviour, a remarkable ferromagnetic coupling is observed in the meta- and para- derivatives, the [Cu2(H2tea)2(m-methoxybenzoate)2].2H2O (2), and [Cu2(H2tea)2 (p-methoxybenzoate)2].2H2O (3), H2tea being the anion resulting from the loss of a proton in H 3tea. –O
OCH3 OCH3 o-methoxybenzoate
Scheme 1. Schematic representation of the ligands in systems 1, 2 and 3
The choice of these ligands was based on the fact that methoxybenzoates and its derivatives are of industrial and biological importance as they are used in chemical reactions (as intermediates) to obtain target materials such as dyes, pharmaceuticals, perfumes, photoinitiators and agrochemicals, whereas triethanolamine is a cheap, commercially available, environmentally tolerable polydentate ligand . Moreover, the triethanolamine copper(II) carboxylate complexes have been successfully utilized as selective and active catalyst for oxidation of alkanes . The addition of the ligand H 3tea ligand significantly changes the geometry of the carboxylate complexes, giving alkoxo-bridged copper(II) dinuclear units. In fact, aminoalcohols of lower denticity have been employed to spontaneously generate coordination polymers of high dimensionality, using the alkoxo-bridged Cu(II) dimers as building blocks [7,10]. The complexes 1-3 have been characterized by X-ray crystallography, thermogravimetric analysis, UV-vis and IR spectra, magnetization and variable temperature magnetic susceptibility measurements, while ab initio quantum chemistry calculations supply a detailed analysis of their electronic structures and independent evaluation of the magnetic coupling constants. A good correlation is found between the experimental and theoretical estimates of J. These value are in agreement with those observed for similar bis(hydroxo) and bis(alkoxo) dicopper(II) compounds and fit well on the magnetostructural J versus the bridging Cu-O-Cu angle curves (Figure 1)
Experimental Materials and Physical measurements
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Analytical grade reagents were used throughout this work without any further purification. Carbon, Hydrogen and Nitrogen were determined microanalytically by automatic Perkin Elmer 2400 CHN elemental analyzer and copper was determined by standard literature methods . FT-IR spectra were recorded as KBr pellets on PERKIN ELMER SPECTRUM RXFT-IR system. Electronic spectra were recorded in H2O using HITACHI 330 SPECTROPHOTOMETER. The thermogravimetric analysis (TGA) was conducted with a SDT Q600 instrument. The samples contained in alumina pan were heated from 25°C to 1000°C at a constant rate of 10°C min-1 under nitrogen environment with flux (flow rate) of 10mL/min. Synthesis of [Cu(H2tea)2(o-methoxybenzoate)2] (1): 1.64 g (6.6 mmol) of CuSO4 was dissolved in a minimum quantity of distilled water. Sodium salt of o-methoxybenzoate was prepared in situ by reacting 0.526 g (13.15 mmol) of NaOH with 2g (13.15 mmol) of o-methoxybenzoic acid in 20mL of water. Blue precipitated product was obtained with a good yield immediately on mixing the two solutions. The solution was filtered through a fine filter paper, washed with water and dried at room temperature. Triethanolamine was added dropwise at room temperature to the suspension of above blue colored product in water-methanol (4:1) mixture till a clear deep blue solution was obtained. The solution was allowed to stand at room temperature. On slow evaporation, green colored crystals appeared after second day. The newly synthesized complex is soluble in methanol but insoluble in chloroform and acetone. The complex salt decomposes at 166 οC. Yield 82%, Anal. Calcd for C28H24Cu2N2O12: C= 46.34, H= 3.31, N= 3.86, Cu=17.52%; Found: C= 46.17, H= 3.03, N= 3.39, Cu=17.18%. IR/cm-1 (KBr): 32330(m,br), 2966(w), 2936(w), 2882(w), 1607(s), 1572(s), 1488(m), 1462(m), 1387(s), 1296(m), 1244(ms), 1180(w), 1088(s), 1053(m), 1028(m), 911(m), 848(m), 753(s), 663(m), 612(m), 529(w), 469(m), 416(w). UV/Visible: λmax = 714 nm (εmax = 117.3 L mol-1 cm-1) and 280 nm (εmax = 9372.7 L mol-1 cm-1) Synthesis of [Cu(H2tea)2(m-methoxybenzoate)2].2H2O (2): The complex salt 2 was prepared in similar way as complex salt 1. When the solution was allowed to evaporate slowly at room temperature, green colored crystals appeared after five days. The newly synthesized complex is freely soluble methanol but insoluble in chloroform and acetone. The complex salt decomposes at 130 οC. Yield 82%, Anal. Calcd for C28H24Cu2N2O12. 2H2O; C = 44.15, H = 3.68, N = 3.68, Cu=16.69%; Found: C= 43.87, H= 3.34, N= 3.61, Cu= 16.18%. IR/cm-1 (KBr): 3410(m,br), 3187 (m), 1598(m), 1557(s), 1485(w), 1453(m), 1426(m), 1389(c), 1316(m), 1251(m), 1121(m), 1091(s), 1049(m), 1024(m), 1003(w), 952(w), 904(m), 773(s), 687(w), 666(m), 575(m), 499(w). UV/Vis: λmax = 714 nm (εmax = 100.4 L mol-1 cm-1) and 250 (εmax = 28479.5 L mol-1 cm-1). Synthesis of [Cu(H2tea)2(p-methoxybenzoate)2].2H2O (3): The complex salt 3 was prepared in similar way as complex salt 1. When the solution was allowed to evaporate slowly at room temperature, green colored crystals appeared after seven days. The newly synthesized complex is freely soluble methanol but insoluble in chloroform and acetone. The complex salt decomposes at 125 οC. Yield 82%, Anal. Calcd for C28H24Cu2N2O12. 2H2O C= 44.15, H= 3.68, N= 3.68, Cu=16.69 %; Found: C = 43.87, H = 3.34, N = 3.61, Cu=16.18%. IR/cm-1 (KBr): 3356(m,br), 3208(m), 2953(w), 386(w), 1604(s), 1542(s), 1456(m), 1390(s), 1253(s), 1171(m), 1091(s), 955(w), 906(m), 853(m), 829(m), 698(m), 621(m), 573(m), 498(w).
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UV/Visible: λmax = 714 nm (ε max = 117 L mol-1 cm-1) and 250 nm (εmax = 28906.5 L mol-1 cm-1). Crystallography Single-crystal X-ray diffraction data for the three complexes were collected on a Nonius Kappa diffractometer equipped with a CCD detector with graphite-monochromatized MoKα radiation (λ = 0.71069 Å). Intensities were corrected for Lorentz, polarization and absorption effects . The structures were solved by direct methods with the SIR97 suite of programs  and refinement was performed on F2 by fullmatrix least-squares methods with all non-hydrogen atoms anisotropic. In 1, hydrogen atoms were included on calculated positions, riding on their carrier atoms, apart from those linked to O4 and O5 atoms of the tea ligand which were located in difference Fourier map. H4 was refined isotropically, while the coordinates of H5 were kept fixed. In 2 and in 3, all hydrogens were found in the difference Fourier map and refined isotropically. All calculations were performed using SHELXL97  implemented in the WINGX system of programs . Experimental details are given in Table 1. The ORTEPIII  diagrams of 1-3 complexes are shown in Figure 2.
Table 1. Crystal data and refinement parameters for complexes 1- 3. 1
Chemical formula Mr Crystal system, space group
C28H42Cu2N2O12. 2(H2O) 761.75 Triclinic, P¯1
C28H42Cu2N2O12. 2(H2O) 761.75 Triclinic, P¯1
a, b, c (Å)
10.9061(2), 8.5014(3), 16.9111(5) 90, 95.5220(12), 90 1560.67(8) 2 1.43 0.23 × 0.20 × 0.19 12691, 3751, 2598
7.5706(2), 8.3360(2), 15.0758(4) 96.122(1), 99.627(1), 114.943(1) 833.65(4) 1 1.34 0.35 × 0.14 × 0.11 13490, 4815, 4087
7.6094(1), 8.0937(2), 14.6853(4) 102.8580(8), 92.3380(8), 110.5400(13) 818.68(3) 1 1.37 0.30 × 0.26 × 0.20 13108, 4747, 4206
0.075 0.057, 0.166, 1.08 3751
0.039 0.036, 0.095, 0.95 4815
0.034 0.034, 0.091, 1.08 4747
α, β γ (°) V (Å3) Z μ (mm-1) Crystal size (mm) No. of measured, independent and observed [I > 2σ(I)] reflections Rint R[F2 > 2σ(F2)], wR(F2), S No. of reflections No. of parameters Δρmax, Δρmin (e Å-3)
725.72 Monoclinic, P21/c
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Figure 2. ORTEPIII view and atom numbering scheme for 1- 3. Thermal ellipsoids are drawn at the 50% probability level. Hydrogen bonds are drawn as dashed lines.
Computational Details The nature of both the ground state of the systems as well as the separation of the low-lying excited states were determined by means of Difference Dedicated Configuration Interaction (DDCI) calculations . In the three systems considered, each Cu atom bears one unpaired electron on a Cu 3dx 2-y2 orbital, which is placed on the xy plane containing both Cu and O bridging atoms. Depending on the nature of the interaction between these two unpaired electrons, the ground state can be a singlet or a triplet state. The difference energy between them is related to the magnetic coupling constant, J, ( Hˆ J Sˆ1Sˆ 2 ) through the expression:
J E S E T
in such a way that the ground state is the singlet state when the interaction between the Cu atoms is antiferromagnetic (J < 0) and the triplet state in the case of a ferromagnetic coupling (J > 0). This constant can be evaluated with accuracy from theoretical calculations. Among the different available approaches, the DDCI method can be considered as a reference method providing estimates of J in good agreement with experimental values for molecular and extended magnetic systems . In this approach, the energy of the singlet and
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ARTICLE triplet states are evaluated from configuration interaction calculations involving all the single and double excitations contributing to the singlet-triplet energy difference. The molecular orbitals employed in the DDCI approach are computed using the Complete Active Space Self-Consistent Field (CASSCF) method for the triplet state. The minimal active space was considered, CAS(2/2), where the two unpaired electrons occupy the two magnetic orbitals, i.e., the symmetric and antisymmetric combinations of the Cu 3dx 2-y2 orbitals. All CASSCF calculations were done with the MOLCAS suite of programs , while the CASDI suite of program  was employed for the DDCI calculations. The J value for complex 1 has been also computed in a MultiReference Perturbation Theory scheme, using the nelectron Valence State Perturbation Theory approach (NEVPT2)  on the CASCI(2/2) wavefunctions, on the basis of a set of state-average molecular orbitals, labeled as OMSA(7/10). These MOs are obtained from CASSCF(2/2) calculations on the magnetic singlet and the excited singlet of the same symmetry with weights 0.7 and 0.3, respectively. These MOs were shown to give reliable values of J from minimal active space multireference perturbative theory calculations for antiferromagnetic systems . In all calculations, the geometry from X-ray crystal structure for each system has been used. In order to make accessible the determination of the energy and wavefunction of the lowest electronic states with high accuracy, the external phenyl groups of the actual systems reported in Figure 2 have been replaced by hydrogen atoms (Figure 3).
Journal Name Atomic Natural Orbital (ANO) type basis functions  have been used for the other atoms, with contractions 4s3p1d for O and C atoms directly connected to metal atoms, and 3s2p for O, N and C atoms in the external shell, and 1s for H atoms. Magnetic susceptibility measurements Magnetic molar susceptibility measurements on 1 and 2 complexes were performed with a Quantum Design SQUID MPMS-XL magnetometer in the temperature range 2 – 300 K at an external magnetic field of 0.1 T. To reduce temperature thermalization effects, the data were collected in settling mode in the 2 – 35 K range, namely, fixing the temperature and allowing for thermal equilibrium, whilst in the 25 – 300 K range the data were collected in sweeping mode, i.e. changing the temperature with a rate of 2 K/min. In the range, where the temperatures overlap, the measured data overlap, as well. Magnetization loops on 1 and 2 complexes were collected at 2 K using a maximum applied field of 5 T. Background corrections for the sample holder and diamagnetic portions of the complex were applied.
Results and discussion To explore the chemistry of versatile methoxy substituted aryl carboxylate ligands in the presence of tripodal ligand triethanolamine, we have developed a new strategy to synthesize their copper (II) complexes. Moreover, a search in the Cambridge Crystallographic Database of Cu-complexes containing tea ligand (52 structures) revealed that in the vast majority of them the ligand bridges two metals to give dimers. This allowed us to plan the synthesis of the series of compounds presented here with the aim of studying how the use of different isomeric ligands can affect the structural and magnetic properties of the dimeric complexes. Synthesis and spectroscopic characterization Cu (II) methoxybenzoate complexes with triethanolamine were obtained in two steps: In the first step, bis(methoxybezoate) Cu(II) compounds were obtained by reacting CuSO 4.5H2O with aqueous solution of sodium salt of methoxybenzoates (o-,m-,p-) in 1:2 stoichiometric ratio resulting in immediate precipitation of the product (Eq.2) . Stirring was continued for an hour and the precipitated product was filtered, washed with hot water and air dried.
Figure 3. Model structures of the binuclear Cu(II) compounds (top) 1, 2/3 (bottom). Pink, red, blue, gray and white balls correspond, respectively, to Cu, O, N, C and H atoms.
Though this modification appears drastic, the experience gained from previous studies on similar systems  indicates that these external ligands do not play a role on the description of the magnetic interaction between Cu atoms, and only a quite small effect on the singlet-triplet energy difference is expected, if any. These external ligands, however, can have an important effect on the coordination geometry around the metal atom but these geometrical effects are explicitly taken into account in our calculations by incorporating geometrical parameters that were obtained from X-ray structural studies for the rest of system, in particular, for the closest neighbourhood to Cu(II) ions. In all the calculations, core electrons of Cu atoms have been represented by means of pseudopotentials , the valence ones by means of a basis with contraction [9s6p6d]/(3s3p4d).
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2HL 2NaOH CuSO4.5H2O
L = o-/m-/p-methoxybenzoate
In the second step, air dried product of Cu (II) methoxybenzoate was suspended in MeOH/H 2O mixture and reacted with triethanolamine(dropwise) till a clear solution was obtained (Eq.3). CuL2.nH2O
H2tea = deprotonated triethanolamine, n = 0 for 1, n = 2 for 2 and 3
This solution was allowed to evaporate slowly at room temperature resulting in single crystals suitable for X-ray
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Journal Name structure determination after few days. They were filtered, washed with cold methanol-water mixture and dried in air. Vibrational spectra of newly synthesized complex salts have been recorded in the region 400-4000 cm-1. The band at 3410 cm-1 in 2 and at 3356 cm-1 in 3 were assigned to OH(H2O) stretching vibrations. The bands in the region 3209-3187 were ascribed to ν (O-H) of triethanolamine. The bands in the region 1607-1598 cm-1 were attributed to ν(C=C) of the benzoate moiety. The bands at 1572/1386 cm-1 in 1, 1557/1398 cm-1 in 2 and 1542/1390 cm-1 in 3 are attributed to νas(COO-) and νs(COO-) vibrations of the methoxybenzoate. The bands from 600-800 cm-1 are associated with deformation modes of benzoate groups. The medium to weak intensity bands at 873950 cm-1 in complex salts 1-3 may be attributed to C-C vibrations. The strong to medium bands in the region 1253-995 cm-1 may be attributed to the C-O stretching vibration of OCH3. The bands in the region 575-570 cm-1 and 499-469 cm-1 were ascribed to ν(Cu-N) and ν(Cu-O) vibrations, respectively. The peak assignments have been made in consultation with literature values . The electronic spectra of complex salts 1-3 were recorded in methanol. Electronic spectra of complex salts 1-3 showed absorption at around 714 nm (εmax/Lmol-1cm-1 = 117.3-104.4) due to d-d transition. The shoulder at wavelength 370 nm, typical for binuclear structure in which two copper atoms was bounded by three atomic centers [Cu-O-C-O-Cu] , was absent thus confirming the X-ray structure information. Strong absorption at 280 nm in 1 and at 250 nm in 2 and 3 was assigned to ligand-to-metal charge transfer (LMCT) transitions (from carboxylate group to Cu(II) atom) . The thermal stability of complexes 1-3 was examined by TGA, carried out under nitrogen atmosphere. In complex 1, only one step was clearly visible. TG curve of complex 1 continues to register weight loss from 150-475oC. The total weight loss (cal= 82.4%, expt =83.45%) corresponds to residual copper metal. In complex 2 the first step corresponds to the loss (cal= 4.73%, expt= 4.85%) of two water molecules of crystallization from temperature 60-135oC. The horizontal portion of the TG curve from 135-165oC corresponds to the formation of anhydrous [Cu(H2tea)2(m-methoxybenzoate)2]. Thereafter complex 2 continues to register weight loss till 396 oC. The total weight loss (calc = 83.3%, expt= 83.6%) corresponds to the copper metal. Similar to 2 in the TG curve of complex 3 the first step corresponds to the loss (cal= 4.73%, expt= 4.95%) of two water molecules of crystallization from temperature 68142oC. The horizontal portion of the TG curve from 142-155oC corresponds to the formation of anhydrous [Cu(H 2tea)2(pmethoxybenzoate)2]. Thereafter complex 3 continues to register weight loss till 396oC. The total weight loss (calc = 83.3%, expt= 84.9%) corresponds to the copper metal.
ARTICLE related Cu atoms with a Cu-O-Cu angle of 99.9(1)°; the Cu-Cu distance measures 2.9456(7) Å. The resulting coordination around each copper atom is square pyramidal, with the central Cu cation lying 0.03 Å above the mean-square plane formed by N1, O1, O3, O3’. The angle formed by this plane and the apical Cu-O4 line is 170.82(9)°. Conversely, in 2 and 3 H2tea ligand act as tetradentate ligand through the nitrogen atom and the three oxygen atoms, the non-protonated one forming a bridge between two centrosymmetrically related hexacoordinated Cu(II) atoms, giving origin to distorted octahedral coordination. The Cu-O-Cu angle and Cu-Cu distance are very similar in the two structures, being 97.51(6), 97.03(5)° and 2.9212(2), 2.9158(2) Å in 2 and 3, respectively. Selected bond distances and angles are given in Table 2. The packing architecture of all crystals is mainly controlled by the formation of O-H…O hydrogen bonds. In 1, the noncoordinated O5 atom acts both as H-bond donor (towards O2) and as an acceptor (from O4) with O…O distances of 2.668(5) and 2.695(4) Å , respectively, O2 and O4 belonging to the same complex. The overall packing can then be described as made of parallel layers of molecules running along the c direction (Figure 4). Table 2. Selected bond distances and angles (Å, º) Cu1 - O1 Cu1 - O3 Cu1 – O3’ Cu1 - O4 Cu1 - O5 Cu1 - N1 O1 - C7 O2 - C7 Cu1...Cu1’ Cu1-O3-Cu1’
1 1.923(3) 1.909(3) 1.940(3)i 2.365(4) 2.063(3) 1.272(5) 1.225(6) 2.9456(7)i 99.9(1)
2 1.952(1) 1.946(1) 1.940(1)i 2.498(2) 2.568(2) 2.049(2) 1.270(2) 1.244(3) 2.9212(2)i 97.51(6)
3 1.957(1) 1.947(1) 1.945(1)ii 2.558(2) 2.473(2) 2.058(1) 1.266(2) 1.250(3) 2.9158(2)ii 97.03(5)
Symmetry operations: (i) –x,1-y,-z; (ii) 1-x.1-y,1-z The packing of constituent unit in 2 and 3 is realized by the presence of co-crystallized water molecules, which bridge two adjacent complexes through the formation of a hydrogen bond with the central O3 atom at a O…O distance of 2.942(3) and 2.960(2) Å in 2 and 3, respectively.
Structures Description The crystal structures of 1-3 have been unambiguously established by single-crystal X-ray crystallography. Complex 1 consists of dimeric [Cu2(H2tea)2(omethoxybenzoate)2], while 2 and 3, the meta- and paramethoxybenzoate derivatives, are isomorphous isostructural and their asymmetric unit consists of one neutral [Cu2(H2tea)2(m/p-methoxybenzoate)2] complex and two cocrystallized water molecules. In all cases, the benzoate molecule is linked to Cu atom by only one oxygen. In 1, the deprotonated H3tea ligand i.e. H2tea acts as a tridentate ligand, one OH group being some 5 Å away from the metal, while the H2tea deprotonated oxygen bridges two centrosymmetrically
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Figure 4. Packing diagram of complex 1. Cyan, blue, red, grey and white balls correspond to Cu, N, O, C and H atoms, respectively.
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Journal Name (≈ 2/3) for temperatures lower than 30 K. At lower temperature, just a small decrease is found, suggesting that in this case the presence of intermolecular antiferromagnetic interactions may be considered negligible [4d]. Magnetization data collected at 2 K on compound 2 are presented in Figure 6b and, as expected, they are nicely described by a Brillouin function corresponding to a total angular momentum equal to 1 .
Figure 5. Packing diagram of complex 3. Hydrogen bonds are drawn as red lines. Same colour code than in Figure 3.
Table 3. Hydrogen bonding parameters; D= donor, A= acceptor (Å, °). D-H…A 1
O5-H5...O2i 0.87 2.668(5) 1.84 158 O4-H4...O5ii 0.85(3) 2.695(4) 1.89(4) 158(3) Symmetry codes: (i) -x,y-1/2,1/2-z; (ii) x,1/2-y,z-1/2
2 O5-H17...O2 0.81(3) 2.605(2) 1.81(3) O1W-H12...O5 0.76(4) 2.849(3) 2.11(4) O4-H4....O1Wi 0.70(3) 2.688(3) 1.99(3) O1W-H11...O3ii 0.76(4) 2.942(3) 2.22(4) Symmetry codes: (i) -x,1-y,-z; (ii) -x-1,1-y,-z 3
168(4) 164(3) 173(3) 157(4)
O1W0.67(3) 2.960(2) 2.35(4) 154(4) H1W...O3i ii O5-H21...O1W 0.69(3) 2.701(3) 2.01(3) 172(3) Symmetry codes: (i) -x,1-y,1-z; (ii) 1-x,1-y,1-z
This leads to the formation of ribbons of alternating water and complex molecules running along the a direction, as shown in Figure 5. As for the robustness of the structures, it is also noteworthy that the short intramolecular hydrogen bond between OH from H2tea and the non coordinated oxygen of the benzoate ligand, with a O…O distance of 2.605(2) and 2.596(2) Å in 2 and 3, respectively. Relevant hydrogen bonding parameters are listed in Table 3. Magnetic moments and susceptibility curves The data recorded for systems 1 and 2 are shown in Figure 6. Measurements were not carried out on 3 because it is structurally almost identical to complex 2, and similar behavior is expected as it is corroborated by the theoretical calculations reported in the next section. Plots of mT, expressed in reduced units, versus T show an antiferromagnetic behavior for system 1 (Figure 6a) and a ferromagnetic behavior for system 2 (Figure 6b). At high temperature, the mT values for both samples approach the 0.5 value, as expected for two uncoupled S=1/2 spins (with g = 2.00). For system 1, the mT value decreases as temperature is lowered, and eventually reaches a value close to zero at about 15 K. For systems 2, the mT continuously increases on cooling, and reaches a plateau value of about 0.66 6 | J. Name., 2012, 00, 1-3
Figure 6. Experimental mT vs. T data for compounds 1 (a) and 2 (b). The solid red line corresponds to the best fit. In the inset, the magnetic field dependence on magnetization of compound 2, normalized to the saturation value, is presented.
To evaluate the magnetic coupling constant, the experimental susceptibility curve for both compounds has been fitted using the Bleaney-Bowers equation: m T
2N 2g 2 1 k BT 3 e J k BT
Best-fit parameters were obtained by minimization of the agreement factor R, 2 2 R T mT exp mT calc / T mT exp .
An additional temperature independent paramagnetic (TIP) contribution [4d] was at first included in the calculation. However, the corresponding results pointed out that its contribution is comparable or smaller than the instrumental resolution, so we considered it to be negligible and removed it from the calculation. Due to some uncertainty in the sample weight and in view of the well-known low accuracy involving the fitting of ferromagnetic systems with large molecular mass [4d,30], a fixed g=2.0 value is adopted in both fittings. The fitting procedure results in J = -83.1(3) cm-1 for system 1 with R= 4.16 .10-4, and remarkable ferromagnetic coupling constant J=100.9(7) cm-1 for system 2 with R= 1.2 .10 -5. These values are in the range of those reported for dialkoxo-bridged Cu(II) dinuclear with similar geometric structures [3-6]. Electronic structure and magnetic coupling System 1 contains two Cu(II) ions with a square-based pyramid coordination, while the Cu(II) ions in the two other systems are considered to have a pseudo-octahedral environment. Figure 3 represents the models employed in the theoretical calculations. They are essentially X-Ray structures where the phenyl groups have been replaced by H atoms with C-H distance of 1.07 Å (for details see ‘Computational details’ section). Figure 7 represents the magnetic active orbitals for system 3 (the magnetic orbitals of systems 1 and 2 present similar
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shapes) obtained from a CASSCF(2/2) calculation on the triplet state. They correspond to the symmetric and antisymmetric combinations of the Cu 3dx2-y2 orbitals with tails on the bridging O atoms and the external ligands.
Figure 7. Active magnetic orbitals in compound 3. Similar shapes are found for systems 1 and 2.
At this level of calculation, the three systems are ferromagnetic (Table 4), i.e., the triplet is the ground state. However, the dynamic electron correlation has different effect on these systems. Three different CI spaces can be considered: (i) CAS+S containing only single excitations on the top of the CAS, (ii) CAS+DDC2 including also double excitations involving two active orbitals, and (iii) CAS+DDCI space where also the double excitations involving only one active orbitals are included.
to the fitted ones. This behaviour is particularly unexpected for the AF complex 1, where DDCI evaluations are usually close to 80-90% of the experimental (fitted) J values. Besides the intrinsic limitations of the theoretical calculations, this discrepancy can also be related to the fact that a fixed g=2.00 value is assumed to fit the T curve, while g values as large as 2.1-2.2 can be found in literature for similar systems. It is possible to obtain fittings of similar quality with slightly larger g values (g=2.004) and smaller (in absolute value) J values (J=73 cm-1 for system 1, and J= 90 cm-1 for system 2), and then closer to the theoretical values. This puts in evidence that fittings are not univocal, an old problem in compounds with ferromagnetic ground states, but also for systems with an S=0 ground state and low symmetry [30,31]. An independent evaluation of g (when possible) should be necessary to help in assessing the error bar in J values, and the trustworthy agreement between ab initio and fitted J values. The nature of the interaction between Cu centers can be rationalized on the basis of two geometrical parameters: (i) the Cu-O-Cu angle and (ii) the out of plane displacement of the bridging alkoxo groups with respect to the Cu2O2 plane [3,4,5,6]. A small Cu-O-Cu angle prevents an efficient overlap with the 2p orbitals of the bridging O atoms, and then cancels the superexchange pathway (Scheme 2).
Table 4. Magnetic coupling constant J (cm-1) for binuclear Cu(II) complex 1, 2 and 3 1-3 at different levels of theory. Experimental values obtained from the best fit with g=2.0. CASCI CAS+S CAS+DDC2 CAS+DDCI (º) (º) Exp.
1 18.0 20.1 7.3 -53.5 99.9 28 -83.1(3)
2 27.6 56.5 45.1 71.2 97.5 44 100.9(7)
3 28.1 59.2 48.2 83.3 97.0 44 ---
1’ 21.6 33.7 24.7 35.3 99.9 44
While the single excitations introduce a ferromagnetic contribution for the three systems (CAS+S entry in Table 4), a global antiferromagnetic effect is obtained at CAS+DDC2 level. At the best level of calculation (CAS+DDCI) system 1 becomes antiferromagnetic, while the electron correlation in systems 2 and 3 enhances the ferromagnetic coupling. That is, the extra excitations included in the CAS+DDCI space with respect to CAS+DDC2 one have a different effect on the magnetic constant, depending on the antiferromagnetic or ferromagnetic nature of the system. This behaviour has been previously noted in related binuclear Cu(II) systems . Our optimal CAS+DDCI J values are 71.2 and 83.3 cm-1, for complexes 2 and 3, respectively, while for complex 1, the coupling is antiferromagnetic, with a non negligible constant of -53.5 cm-1. The NEVPT2 estimate of J for complex 1 is -76.8 cm-1, which corroborates the AF nature of the interaction between the Cu centers. The main advantage of this NEVPT2 evaluation based on state-average MOs is the considerably reduced computational cost with respect to the DDCI one, in such a way that can be considered as a promising alternative to the always more expensive (both in time and resources) DDCI calculations on molecular magnetic systems. These results are in agreement with the fitted J values, although the theoretical values are slightly underestimated with respect
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The Cu-O-Cu angle for complexes 2 and 3 (=97.5º and 97.0º) falls in the region of ferromagnetic interactions for similar hydroxo- and alkoxo-bridged binuclear Cu(II) compounds while for complex 1 the angle =99.9º favours the superexchange through the oxygen ligands and then a larger contribution of the antiferromagnetic component of the coupling constant is expected [3-6]. This magnetostructural analysis is confirmed by the CAS+DDCI J values of 71.2 and 83.3 cm-1 for complexes 2 and 3, respectively (see Table 4). For complex 1, the coupling is antiferromagnetic, with a non negligible constant of -53.5 cm-1. These values are in agreement with those reported for similar -dihydroxo and -dialkoxo dicopper(II) compounds and fit well on the magnetostructural J versus the CuOCu angle curve (Figure 1). The same arguments can be used to explain the relationship between the magnetic coupling and value. This parameter is related to the relative orientation of the bridging O 2p orbitals with respect to the molecular plane. A large forces the 2p bridging orbitals to be tilted out of the plane of the magnetic Cu 3dx2-y2 orbitals, preventing an efficient overlap and then blocking the superexchange mechanism (Scheme 2). The larger
J. Name., 2012, 00, 1-3 | 7
ARTICLE the smaller the antiferromagnetic contribution to the coupling, and a dominant ferromagnetic coupling is expected [3-6]. The out of plane displacement of the alkoxo groups confirms this trend, since the displacement is larger for complexes 2 and 3 ( 44º), than for complex 1 ( 28º). Additional calculations for a model system, built from the geometrical structure of complex 1 with increasing values of prove the differential role of this geometrical parameter on the magnetic coupling of the three considered homologues. The J values obtained at CAS+DDCI level for angles of º (complex 1), 33º, 38º and 44º (model 1’ in Table 4) are -53.5, -15.5, 6.8 and 35.3 cm-1. These results indicate that so large a variation of the magnetic coupling constant in this series of parent dinuclear copper(II) complexes is in fact governed by the out-of-plane displacement of the alkoxo bridged groups.
Conclusions We have synthesized three new bis(alkoxo) dicopper(II) compounds resulting from the reaction of o-/m-/pmethoxybenzoate Cu(II) complexes with triethanolamine, H3tea. The crystal structures were solved and the magnetic properties studied by magnetic susceptibility measurements as a function of temperature. Also the UV/Vis and IR spectra have been recorded. The meta- (2) and para- (3) derivatives present similar features, resulting from very close geometrical structures. The deprotonated ligand H 2tea acts as a tetradentate ligand in 2 and 3, but as a tridentate ligand in the ortoderivative 1. This results in hexacoordinated Cu(II) atoms in 2 and 3, while they present a square-pyramidal coordination in system 1. The packing is driven by the formation of hydrogen bonds, particularly important for 2 and 3 due to the presence of crystallized water molecules. The magnetic studies yielded J values of -83.1 cm-1 and 100.9 cm-1 for 1 and 2, respectively, which can be correlated to the Cu-O-Cu angle and out-of-plane displacement of the alkoxo group. The magnetic coupling in these three compounds have been also examined by ab initio calculations, confirming the nature and amplitude of the interaction between the metallic centers.
Acknowledgements One of us (RPS) thanks the funding agency UGC, New Delhi (India) vide grant no. F. No. 40-60/2011 (SR) for financial support. Financial support has also been provided by the Spanish Ministry of Science and Innovation through Project CTQ200907767. CA has been financed by the Italian MIUR through its PRIN 2009 funds.
Notes and references a
Department of Chemistry, Panjab University, Chandigarh-160014, India. Dipartimento di Scienze Chimiche e Farmaceutiche and c Centro di Strutturistica Diffrattometrica, Università di Ferrara, Via Fossato di Mortara 17-27, I-44121, Ferrara, Italy. d Dipartimento di Fisica e Scienze della Terra, Università di Ferrara, Via Saragat 1, I-44122 Ferrara, Italy b
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Journal Name e
Departamento de Química Física, Universidad de Sevilla, c/Prof. García González s/n, E-41012 Sevilla, Spain † Electronic Supplementary Information (ESI) available: CCDC- 874506 (1) , 874507 (2) and 874508 (3), contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. See DOI: 10.1039/b000000x/
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