Four New Trinuclear { Cu 3 ( μ 3OH ) ( oximate ) 3 } 2 + Clusters : Crystal Structure and Magnetic Behaviour . †

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b Departament de Cristal•lografia i Minerologia, Universitat de Barcelona, Martí i Franquès s/n, 08028 Barcelona, Spain.†  tert-butylphosphonic acid).In three of the new trinuclear compounds (1-3) there is a t BuPO 3 H - ligand axially coordinated to one of the copper atoms and one Cl -or Br -ligand bridging the other two copper atoms.In the fourth complex the hydrogenphosphonate is not present and the Cl - ligand doesn't bridge two copper atoms but instead there are two terminal Cl -ligands.
Compounds 1-3 show a known topology in copper oximato compounds: triangular systems with oximato bridging ligands containing a central µ 3 -hydroxo and two potentially chelating anions coordinated to the axial coordination sites of the copper atoms on opposite sides of the triangle faces, one of them bridging two copper atoms and the other as terminal.These anions are usually carboxylate ligands. 14,17,20,21In the case of compounds 1-3 the terminal ligand is a hydrogenphosphonate that is stabilized by hydrogen bond with the central µ 3 -hydroxo ligand and one Cl -or Br -ligand bridging the other two copper atoms on opposite sides.The other usual topology found in copper oximato chemistry are dinuclear compounds with very strong antiferromagnetic coupling (-J around 500 cm -1 ). 17,30om the magnetic point of view, in a copper(II) equilateral triangle (Scheme 1), taking into account the isotropic Hamiltonian ˆ, the derived equation for the magnetic susceptibility in function of the temperature is: where x = 3J/2kT.By using equation (1) to fit the experimental magnetic susceptibility values measured in triangular copper(II) complexes, it is usually found an obvious discrepancy mainly in the low-temperature magnetic data where the χ M T value is smaller than that for one unpaired electron.This discrepancy arises mainly from the non consideration in equation (1), derived from the isotropic Hamiltonian, of the intramolecular antisymmetric exchange. 22

Dalton Transactions Accepted Manuscript
Recently, F. Lloret and co-workers have published a new equation which includes the intramolecular antisymmetric exchange and the Zeeman interactions. 22This equation has been used to fit the experimental magnetic data of the new compounds 1-4.

Synthesis
Previous attempts to prepare copper(II)/tert-butylphosphonate/oximate compounds without halide salts were unsuccessful.The structural determination of 1 in a few crystals formed in an attempt without halide salts but due to the existence of a little quantity of chloride impurities in the starting copper methoxide salt decided us to attempt the synthesis by adding halide salts to the reaction mixture.The starting copper salt was copper(II) methoxide to avoid another anion to be present in the reaction and also to generate basic medium.The solvent was methanol.The stoichiometric equations for reactions 1-4 were: 1 Cu(II) methoxide + 1 oxime + 1 tertbutylphosphonic acid + 0.23 NaCl.Taking into account the formulae of compounds 1-3, the toichiometric coefficient of NaCl was minor to the necessary (0.33) to favour the tertbutylphosphonate as a ligand in the final compound.

Description of the structures of compounds 1 and 2.
The structures of compounds 1 and 2 are very similar and differ mainly in the bridging halide, chlorine in 1 and bromine in 2, and in the number of lattice water molecules.The structural parameters of complex 2 will be next to the corresponding ones of complex 1.
Insert Figures 1-4, Table 1 and 2 close to here Description of the structure of compound 3.
unit (Figure 5) and one and a half lattice methanol molecules.Selected bond distances and angles are listed in Table 3.The geometry around two of the copper(II) ions in the trimeric unit is best described as a distorted square pyramid (τ = 0.159 for Cu2 and 0.101 for Cu3).There is also a hydrogen bond involving the terminal methanol and the tert-butylphosphonate ligands [O8...O7 2.731(5) Å; O8-H8A...O7 174(7)º] (Figure 6).
Insert Figures 5 and 6, Table 3 close to here Description of the structure of compound 4.
The structure of 4 consists of a triangular [Cu 3 (µ 3 -OH)Cl 2 (PhPyCNO) 3 ] unit (Figure 7) and one half lattice water molecule.Selected bond distances and angles are listed in Table 4.The geometry around two of the copper(II) ions in the trimeric unit is best described as a distorted square pyramid (τ = 0.006 for Cu1, and 0.223 for Cu3) 31  unit, forming dimers of trimers not further connected (Figure 8).
Insert Figures 7 and 8, Table 4 close to here

Magnetic properties
The magnetic properties of compounds 1-4 in the form of χ M T vs. T plot are shown in Figure 9.
ˆcan be used to describe the magnetic interactions through equation ( 1).An attempt to use this approach, however, failed to reproduce the low-temperature decrease in the χ M T vs. T plot for compounds 1-4.Different J and g values for the different magnetic centres do not improve the fit and lead to overparametrization.
The magnetic behaviour of the {Cu 3 (µ 3 -OH)} core has been extensively studied and S. Ferrer et al. have published a comprehensive review of the field. 22Taking into account this last paper, a new approach assuming the contribution of the antisymmetric exchange was considered.To fit the experimental data for compounds 1-4, we used the following Hamiltonian: where H iso is a Hamiltonian for isotropic exchange for an isosceles triangle with parameters J = J 12 = J 23 and j = J 13 ; H ASE is an axial Hamiltonian for the antisymmetric exchange with G Z parallel to the C 3 axis and G ⊥ = 0; H Zeem is an axial Hamiltonian for the Zeeman interaction with g ∥ = g 1z = g 2z = g 3z and g ⊥ = g 1x = g 2x = g 3x = g 1y = g 2y = g 3y .The exact analytical expression for the molar magnetic susceptibility in function of the temperature can be found in reference 22.
The best-fit parameters found in the fitting of the magnetic susceptibility experimental data for compounds 1-4 are listed in Table 5 and the theoretical curves calculated from these parameters are depicted as solid lines in Figure 9.
Insert Figure 9 and Table 5 close to here

Magnetostructural Correlations.
The more relevant structural parameters (bond lengths and angles) together with the exchange parameters for complexes 1-4 are listed in Table 6.These parameters are depicted in Scheme 2.  6, the J av , J, and j parameters depend on the α av , β, and γ angles, respectively: the larger the angle, the larger the magnetic coupling.So, given that β > γ, then |J| > |j|, except for compound 4, for which γ > β and |j| > |J|.It is worth noting that the Cu-O-Cu angle is directly related with the outof-plane shift of the hydroxo bridge from the plane defined by the three copper atoms: the larger the shift, the smaller the angles.In fact, for similar compounds, it has been suggested that the more flattened the Cu 3 O(H) bridge (i.e., Cu-O-Cu angles closer to 120°), the stronger the magnetic interaction. 18A plot of the Cu-O-Cu angle vs. the exchange coupling constant is shown in Figure 10.The best linear fit is expressed by equation ( 2), where J is given in cm −1 .J = -19.08θ+ 1656 (2) The equation ( 2) is valid for compounds with the {Cu 3 (µ 3 -OH)(oximate) 3 } 2+ fragment.Although the number of examples presented herein is hardly sufficient to establish a final accurate correlation, it may be concluded that the Cu-O-Cu bridgehead angle is one of the main factors governing the nature and magnitude of the magnetic coupling in the {Cu 3 (µ 3 -OH)(oximate) 3 } 2+ triangular tricopper(II) complexes.
Insert Figure 10, Table 6 and Scheme 2 close to here

Experimental Materials and physical measurements
All reagents, metal salt and ligands were used as obtained from Aldrich.Infrared spectra (4000-400 cm -1 ) were recorded from KBr pellets on a Perkin-Elmer 380-B spectrophotometer.Magnetic susceptibility measurements under magnetic fields of 0.3 T in the temperature range 2-300 K and magnetization measurements in the field range of 0-5 T were performed with a Quantum Design MPMS-XL SQUID magnetometer at the Magnetic Measurements Unit of the University of Barcelona.All measurements were performed on polycrystalline samples.Pascal's constants were used to estimate the diamagnetic corrections, which were subtracted from the experimental susceptibilities to give the corrected molar magnetic susceptibilities.

Crystallographic data collection and refinement.
The X-Ray single-crystal data of compound 1 was collected on a Bruker X8 Kappa APEX-II diffractometer with a graphite-monochromator utilizing Mo-Kα radiation (λ = 0.71073 Å), with ω and φ-scans at 100(1)K. 32Compound 2 was collected on a Bruker CCD SMART1000 diffractometer with a graphite-monochromator utilizing Mo-Kα radiation (λ = 0.71073 Å), with ω and φ-scans at 100(1)K 33 and compounds 3 and 4 were collected on a MAR345 diffractometer with an image plate detector and φ-scans at 110(2) K.The crystallographic data, conditions retained for the intensity data collection and some features of the structure refinements are listed in Table 7. Data processing, including Lorentz-polarization and absorption corrections were performed using the SADABS 34 computer programs.The structure was solved by direct methods and refined by full-matrix least-squares methods, using the SHELXTL program package. 35All non-hydrogen atoms were refined anisotropically.The H atoms attached to C and N atoms were added theoretically and treated as riding on the concerned parent atoms.H atoms attached to O atoms were located from difference Fourier maps and included in the final refinement cycles on fixed positions.
Insert Table 7 close to here

Conclusions
Four new trinuclear copper(II) complexes with the fragment {Cu 3 (µ 3 -OH)(oximate) 3 } 2+ (1-4) have been prepared from 2-pyridyl ketoxime derivatives and structurally characterized by X-ray crystallography.Their magnetic data have been analyzed by using an isotropic and antisymmetric exchange Hamiltonian.All these compounds show strong antiferromagnètic and antisymmetric exchange.The magnetostructural study presented here has shown a lineal correlation for complexes with the fragment {Cu 3 (µ 3 -OH)(oximate) 3 } 2+ between the Cu-O-Cu angle and the isotropic exchange parameters (J and j).Dalton Transactions
with a NNOOCl coordination environment, while the third copper ion (Cu2) has a NNOO coordination environment.The trimeric skeleton is created by the oximato nitrogen atoms of one PhPyCNO -ligand and the oxime oxygen atom of the adjacent PhPyCNO -ligand, whereas the O atom of the µ 3 -OH -ligand (O1) completes the square-planar bases of the three metal atoms, with Cu1-O1, Cu2-O1, and Cu3-O1 bond distances of 1.986(2), 1.944(2), and 1.989(3) Å, respectively.The apical position of Cu1 and Cu3 are occupied by two monodentate chloride ligands with Cu1-Cl1 and Cu3-Cl2 distances of 2.508(2) and 2.600(2) Å, respectively.The oximate bridges, Cu-O-N-Cu', deviate slightly from planarity, with torsion angles of 3.5(3)º (Cu1-O4-N6-Cu3), 32.7(3)º (Cu2-O2-N2-Cu1), and 17.7(3)º (Cu3-O3-N4-Cu2).The [Cu 3 ] unit can also be considered as an isosceles triangle with Cu1...Cu2, Cu1...Cu3, and Cu2...Cu3 distances of 3.207, 3.189, and 3.106 Å, respectively.The oxygen atom of the hydroxo ligand, which is trapped in the metallacrown ring, lies at 0.739 Å out of the plane defined by the copper atoms.There is an intermolecular H-bond.The capping µ 3 -OH hydrogen is engaged in a H-bond with a chloride ligand [O1•••Cl1 3.045(3) Å; O1-H1A•••Cl1 170(4)º; ': -x,y,1/2-z] of other trinuclear At room temperature, the χ M T values are in the range of 0.46-0.53cm3 •K•mol -1 per trinuclear unit.These values are appreciably lower than those expected for three noninteracting S = ½ ions (χ M T = 1.125 cm3 •K•mol -1 , g = 2.0), suggesting very strong antiferromagnetic coupling.When the samples are cooled, χ M T decreases continuously reaching values in the range of 0.26-0.34cm3 •K•mol -1 at 2 K.These χ M T vs. T curves clearly indicate strong intratrimer antiferromagnetic coupling.As first approximation, it is often assumed that the three metal ions are structurally equivalent and the isotropic spin Hamiltonian The Cu-N,O and Cu-OH bond lengths (d Cu-ox and d Cu-OH , respectively) are the mean values for each compound.The β angle is defined by the average of the two most similar Cu-O-Cu angles within the triangle, whereas the γ angle refers to the most different one and the α av angle is defined as (2β + γ)/3.Finally, the values of the exchange parameters are as follows: J = J 12 = J 13 , j = J 23 , and J av = (2J + j)/3.The magnetic interaction between two copper(II) ions within the triangle is mediated by both the diatomic N,O-(oxime) and the monatomic O-(hydroxo) bridges.The structural parameters associated with the oxime bridge are comparable in the four compounds.The hydroxo bridge also presents similar Cu-O distances (1.93−1.99Å) in 1-4 and no relation is appreciated between this small variation and the values of the exchange coupling parameters.The magnetostructural correlation involves mainly the Cu-O-Cu bridgehead angle.In this respect, as observed in Table

Figure 2 .
Figure 2. Dimer of trimers formed by H-bonds of compound 1.

Figure 4 .
Figure 4. Dimer of trimers formed by H-bonds of compound 2.

Figure 6 .
Figure 6.Dimer of trimers formed by H-bonds of compound 3.

Figure 8 .
Figure 8. Dimer of trimers formed by H-bonds of compound 4.

Figure 9 . 4 .
Figure 9. χ M T vs T plot in the 300-2 K range of temperatures for complexes 1-4.The solid lines are the best fit (see text).

Figure 10 . 1 .Scheme 2 .
Figure 10.Plot of the Cu-O-Cu angle vs. the exchange coupling constant.Scheme 1. Scheme 2. The angle β is defined by the average of the most similar Cu-OH-Cu angles within the triangle, whereas the angle γ refers to the most different one of them.The angle α is defined as the average of the three Cu-OH-Cu angles of the complex.

Table 1 .
Bond distances and angles of compound 1.

Table 2 .
Bond distances and angles of compound 2.

Table 3 .
Bond distances and angles of compound 3.

Table 4 .
Bond distances and angles of compound 4.

Table 7 .
Crystal data and structure refinement for complexes 1-4.

Table 5 Table 6 a
The parameters are defined in Scheme 2. The distances and bond angles are average values.ClCu 3 N 9 O 11 P C 74 H 84 Br 2 Cu 6 N 18 O 21 P 2 C 170 H 192 Br 4 Cu 12 N 24 O 38 P 4 C 144 H 116 Cl 8 Cu 12 N 24 O 18 J av θ δ