Crystal structure, spectroscopy and ferromagnetostructural behavior of the complex [Cu II ( L )(Cl)( L ´)]·H 2 O ( L = 2-aminomethylbenzimidazole, L ´= L-isoleucinate)

[Cu II ( L )(Cl)( L ´)]·H 2 O 1 ( L = 2-aminomethylbenzimidazole, L ´ = L-isoleucinate) compound crystallizes in the orthorhombic space group P 2 1 2 1 2 1 . A short Cu—O...Cu


Introduction
2][3][4] These systems are found in biological molecules and carry out important functions as photosynthesis, 5 reversible dioxygen linking, 6 electron transfer, 7 peroxidation, 8 among others.On the other hand, the benzimidazole moiety and some of its derivatives present in molecules as vitamin B 12 5 and Cu II -benzimidazole complexes have been reported. 9,10The benzimidazole group coordinated to transition metal ions, allows planarity to the structures, and combined with other important aminoacid ligands, produce complexes of interest, which can be copper biosites models. 1,4In this work, we are reporting the synthesis, electronic studies and magnetostructural relations of a novel Cu II -five-coordinated complex.

Results and Discussion
Crystal structure Complex 1 crystallizes in an orthorhombic cell in the non-centrosymmetric space group P2 1 2 1 2 1 (Table 1), consistently with the chiral nature of the L-isoleucinate (L-Ile) ligand.The correctness of the absolute configuration for this ligand was confirmed by the refinement of a Flack parameter: x = -0.025(17) for 1310 Friedel pairs (64% of potential pairs have been measured).
The asymmetric unit in the crystal contains one monomeric molecular complex [Cu II (L)(L')Cl] and one water lattice molecule.Ligands L and L' are 2-aminomethylbenzimidazole and Lisoleucinate, respectively (Figure 1a).Chelating behavior for both ligands correspond to common coordination modes, which were previously observed in other transition metal complexes, including Cu II complexes.L-Ile coordinates through unprotonated amine function and one carboxylate O atom, as found for example in trans-bis(D,L-isoleucinato-N,O)copper(II). 11The conformation stabilized for L-Ile in 1 is slightly different from that observed in the free zwitterion, 12 allowing the amine group to coordinate to the metal centre: the torsion angle N12-C13-C14-O15 is 14.5(4)º in 1, while free ligand is twisted by -19.0(2) and -41.4(2)º (two conformations are stabilized in the solid state).Ligand L adopts an almost planar conformation, common for this ligand, and coordinates through two N atoms in a chelating mode.Such a coordination has been described in a Cu II complex including glycylglycinato as coligand, 13 and also in hexacoordinated Cu II complexes. 14he trigonality index about Cu site in 1 is τ = 0.27, close to the index expected for a square pyramidal environment, for which τ = 0 (τ = 1 for ideal trigonal bipyramidal geometry). 15,16The equatorial plane containing N12, O15, N3 and N11 atoms is distorted, the r.m.s.deviation from a least-squares plane containing these atoms being 0.142 Å. Significantly different bond lengths are found, in the range 1.948(2)-2.015(3)Å (Table 2).8][19][20] The angle between the normal to the basal plane N12/O15/N3/N11 and the line Cu-Cl is 3.7°.These features suggest that the covalency of the O and N atoms towards Cu II is very similar to each other (Table 2).

UV/Vis and NIR spectroscopy
The UV/Vis/NIR spectrum of 1 shows π-π* transitions at 270 nm (ε = 126 M -1 cm -1 ) and ligandmetal charge transfer transitions at 277 nm (ε = 129 M -1 cm -1 ).2][23] This band has been fitted to the combination of four Gaussian functions (Figure 2a).According with other experimental and theoretical results of Cu II -compounds reported, the Gaussian functions number expected for square pyramidal structure, would be three, corresponding to d x 2-y 2←d z 2, d x 2-y 2←d xy , d x 2-y 2←d xz , d yz , transitions. 24,25We attribute the presence of the four Gaussian functions to the low symmetry in the first coordination sphere around of Cu II , three nitrogen atoms, one chloro and one oxygen atoms.][26]  IR spectrum of 1 shows a band at 3496 cm -1 characteristic of complexes with water of crystallization.[30][31] These IR data are in full agreement with the functional groups present in the structure. 1H NMR spectroscopy 1 H NMR spectrum of 1 is typical of paramagnetic compounds showing a collection of lines broadening in a 35 to 8 ppm range and other collection of sharp lines in a 6.5 to 1 ppm range. 32[34] The assignment of the signals for specific protons is not possible and less yet its quantified.The 13 C NMR experiment does not show any useful signal, because of the large noise/signal ratio.

ESR spectroscopy
ESR X-band spectra of the polycrystalline powdered sample of 1 were recorded at 25 different temperatures between 300 K and 77 K, and at 77 K in methanol solution with concentration varying from 10 -2 M to 10 -3 M.These samples do not show any additional resonances at zero field or any other field at any gain.At 300/77 K complex 1 powder sample exhibits axial singlet spectra, 21,34,35 with the magnetic parameters obtained from the simulations of the experimental ESR spectra using ES-PRIT computational program of Jeol. 36The parameter values are g || = 2.150/2.150,g ⊥ = 2.040/2.0408][39][40][41] The linewidth parameter is of 76.30G/88.9G(Figure 3).The area under the resonance absorption, A, increases by a factor of 4.51, and the linewidth, Г, becomes 14% wider when the temperature is decreased from 300 K to 77 K spectrum.This ESR behavior of 1 is consistent with the presence of a weak ferromagnetic exchange interaction, in which the dipoledipole interaction is still dominant. 42,43 KAT USA, Inc. ESR spectra of 1 in solution at six different concentrations in the range ca 3 mM to 0.2 mM (Figure 4), show hyperfine interactions splitting, hfs, with A hfs value of 192 × 10 -4 cm -1 , 37 which is characteristic of normal tetragonal distorted Cu II -complexes. 38,44,45It is observed too a superhyperfine shfc structure with A shfc value of 9.24 × 10 -5 cm -1 .The zoom on Figure 4 shows the ESR spectrum at the lowest concentration with seven superhyperfine splitting, corresponding to three first neighbors at Cu II with I N = 1.Note that the Г values at all concentrations are identical, indicating that intermolecular dipolar interactions are not significant in this system, while intermolecular dipole-dipole interactions should be the main contribution to this width. 45

Magnetic behavior
Molar magnetic susceptibility versus temperature, χ M -T, of powder samples of 1 were measured in the temperature range from 2 to 300 K. Figures a shows χ M -T plots fitted to eq (1) using a weak exchange interaction according to the Heisenberg model, with Hamiltonian, given by H ex = -2J S 1 .S 2 , for a pair of exchange-coupled S = 1/2 spins: [46][47][48] Here the magnetization, M, is: In Figure 5a the best fit is shown with solid line, where we used a exchange interaction about J = 0.103 cm -1 , g = 2.101, and R 2 = 0.99875.
The magnetic ordering could occur through the oxygen bridges linking the Cu II …Cu II ions, as can be observed in the crystalline structure of 1, shown in Scheme 1A, and it is through the coordinated O atom belonging to the carboxylate Cu-O…Cu.The distance O…Cu of 4.214(3) Ǻ is one of the shortest between neighboring molecules, and accordingly, to Scheme 1A the magnetic exchange communication is through an zig-zag infinite chain.However, we must remark that other possible magnetic pathway may be through of Cu-O-C=O…H-N-Cu, which is through five atoms and with a O…H distance of 2.643 Ǻ.This magnetic pathway is in a zig-zag infinite chain also (Scheme 1B).The results of the best fit, considering the Ising model, as shown (solid line) in Scheme 1 are: J = 0.99 cm -1 , g = 2.088, and R 2 = 0.99903 according to eq (2) (Figure 5b): 49 ( ) 1 2 The fitting data using the Myers and the Ising models, for our magnetic measurements are in complete correspondence to ESR results too.The weak interaction is expected in function of the ratio areas and the width of the ESR spectra at 77 and 300 K. On the other hand, the low concentrations of 1 in solution that show hfs and shfc interactions informs that the ferromagnetic exchange interaction Cu-Cu must be weak.Different concentrations of 1 in solution show that the width line remains unchanged, that: a) dipolar interaction is more important than the exchange interaction, and b) meaning that this last interaction is very weak and direct, without linking bridges.Compound 1 obtained in this work resulted with an asymmetry to accept a fifth ligand in the apical position, giving rise to a local structure around the Cu II ion which is typical of active sites of some Cu-proteins.

Experimental Section
General Procedures.Diffraction data for complex 1 were measured at room temperature on a Bruker P4 diffractometer (non-CCD detector) using standard procedures. 50Data were corrected for absorption effects on the basis of 10 ψ-scans and the structure solved and refined using SHELXTL-Plus software. 51A summary for data collection and refinement is given in Table 1, and details may be found in the archived CIF, along with complete geometric parameters. 52All C-bonded H atoms were placed in idealized positions and constrained to ride on their carrier atom.N-and O-bonded H atoms were found in a difference map and refined with free coordinates.The geometry of the lattice water molecule was however regularized through soft restraints on distances: O-H = 0.85(2) and H...H = 1.41(2)Å.The 1 H NMR spectra were recorded at 25 °C on a JEOL ECLIPSE 400 MHz spectrometer, using methanol-d solutions with ca 10 -2 M in the 100 to -100 ppm range.ESR measurements were carried out on polycrystalline samples at X-band frequency (9.4 Ghz) on a Jeol JES-RES3X spectrometer, at 300 and 77 K with 100 KHz modulation frequency.At 77 K in methanol solutions (ca 10 -2 M to 10 -3 M) were recorded the ESR spectra.The g values were accurately determined by DPPH pitch.Magnetic measurements were carried out with a Quantum Design Magnetometer MPMS-5S, at 2000 Gauss, as applied field in the temperature range from 2-300 K, using gelatin capsule as the container of the samples.The elemental analysis (C, H) was carried out at USAI UANM Laboratories.Infrared spectrum of CsI pellet of the complex was recorded on a Nicolet Magna IR Spectrometer 750, at rt in the range 4000-200 cm -1 .Electronic spectrum was recorded on a Shimadzu spectrophotometer UV-3100 on ca 0.1 × 10 -4 M, at rt, in methanolic solution in the range 190-1500 nm.Starting materials were purchased from Aldrich, Baker and Fermont laboratories.

Figure 1 .
Figure 1.(a) Molecular structure of 1 with displacement ellipsoids drawn at the 50% probability level for non-H atoms.The water molecule has been omitted for clarity.(b) Crystal structure of 1 viewed down [100].Dashed lines represent O...N contacts in hydrogen bonds involving the water lattice molecule.

Figure 2 .
Figure 2. (a) Electronic spectrum in visible region of 1. Experimental ( ___ ), adjusted by Gaussian curves and their sum (•••); (b) proposed d-d transitions in the low symmetry atomic d orbitals.

Figure 4 .
Figure 4. ESR spectra of 1 in solution at three different concentrations showing hfs and shfc interactions.Only three different concentrations are considered for clarity in the spectra change.

Figure 5 .
Figure 5. Temperature dependence of χ M vs T: (a) the experimental data open circles (ooo), solid line corresponds to the fitting using Myers expression, eq (1); (b) fit using Ising model.

Table 1 .
Crystallographic data for 1