Synthesis and Density Functional Calculations of the New Molecule-Based Magnet Precursor [ Fe ( H 2 opba-i ) ( dmso ) 2 ] Cl

Um precursor inédito de sistemas magnéticos moleculares, [Fe(H 2 opba-i)(dmso) 2 ]Cl (1), onde opba = orto-fenilenobis(oxamato) na forma tautomérica iminoálcool, foi obtido como produto da reação entre H 4 opba e FeCl 3 . Os dados de análise elementar, espectroscopias de absorção no infravermelho e Mössbauer e medidas magnéticas indicam que este precursor é constituído por uma mistura de isômeros trans (83%) e cis (17%). O valor de χ M T a 298 K (2,1 emu K mol) corresponde ao Fe com estado de spin (S) entre 3/2 e 5/2. Cálculos teóricos (PBE/DZVP2) de transe cis-[Fe(H 2 opba-i)(dmso) 2 ] mostram que ambos os isômeros têm spin S = 1/2 no estado fundamental e S = 3/2 para o trans e S = 5/2 para o cis no primeiro estado excitado. A combinação destes resultados leva a valores de χ M T de 0,375 emu K mol e 2,3 emu K mol a baixas e altas temperaturas, respectivamente, os quais são concordantes com os dados experimentais para 1.


Introduction
The oxamato-based ligands have an important role in molecular magnetism.In the seventies, the proligand 1,3propylenebis(oxamato), pba, was synthesized, and some years later it was used in the preparation of molecule-based magnets. 1 In 1993, the new precursor [Bu 4 N] 2 [Cu(opba)], opba = ortho-phenylenebis(oxamato), was synthesized and largely used as a building block, due to its high solubility in common organic solvents. 2he majority of the work done with oxamato-based building blocks is with Cu II , most probably due to the stability of the compounds with this metal ion.Another reason for employing this metal ion resides on the ferrimagnetic strategy, which consists in the use of two metal ions with large difference in spin states and with antiferromagnetic coupling.3][4][5][6][7][8] Such systems are recognized by their potential applications in recording/ reading magnetic systems on the molecular scale.
The opba ligand has been used for stabilization of high oxidation states of transition metal ions such as Mn III and Fe III due to the high donor capacity of the deprotonated-amide group. 9Biomimetic systems such as the dimanganese(III) complex ][12] Unlike the usual trans geometry for the [M x (opba)] (x-4) complexes, in which two deprotonated-amide nitrogen and two carboxylate oxygen atoms of the ligand form a plane, in 1997 an iron(III)-carbonate monomeric complex with a cis-β geometry was described.This compound, [NMe 4 ] 3 [Fe(opba)(CO 3 )]•5H 2 O, has been shown to be a moderately efficient catalyst for the aerobic epoxidation of alkenes with co-oxidation of pivalaldehyde. 13here is no record in the literature on extended molecular systems or clusters built with precursors containing Fe III and oxamato ligands.Furthermore, few examples of Fe III compounds coordinated simultaneously by amide nitrogen atoms and carboxylate oxygen atoms of oxamato-based ligands have been reported. 11,13The present work reports the synthesis and characterization of a new Fe III complex, [Fe(H 2 opba-i)(dmso) 2 ]Cl (1), with opba = ortho-phenylenebis(oxamato) in an iminoalcohol tautomeric form.

Experimental
The proligand H 4 opba was prepared using an adaptation of a method described elsewhere. 2All reagents were used as purchased without further purification.Elemental analyses (C, H, N) were performed on a 2400 CHN-Perkin Elmer instrument.Iron content was determined on a Z-8200 Hitachi atomic absorption spectrophotometer.Infrared spectra (IR) were recorded on a Perkin-Elmer Spectrum GX FTIR spectrophotometer, using KBr discs.Mössbauer spectra were measured in the transmission geometry on a constant-acceleration conventional spectrometer by using a 57 Co/Rh source and a closed cycle He cryostat.Mössbauer isomer shifts are quoted relatively to the α-Fe at room temperature.The magnetic measurements were performed with a Quantum Design SQUID MPMS XL7 instrument.The diamagnetic correction of the sample was calculated through the constants of Pascal's constants. 14

Theoretical calculations
All calculations were performed using the linear combination of Gaussian-type orbitals-Kohn-Sham density functional (LCGTO-KS-DFT) method implemented in the deMon program package. 15The exchange-correlation (XC) functional PBE was used to treat the systems; this XC functional uses the Perdew, Burke and Ernzerhof expression for exchange and correlation. 16We have used the DZVP basis set optimized explicitly for DFT by Godbout et al. 17 Automatically generated auxiliary basis sets (A2) have been used for fitting the charge density.An adaptive grid with a tolerance of 10 -6 for the numerical integration of the exchange-correlation and potential energy was used. 18,19ll structures have been fully optimized without symmetry constraint using the standard Broyden-Fletcher-Goldfarb-Shanno (BFGS) method.1][22][23][24][25][26] The solvation energies have been estimated using the Gaussian 98 program package. 27As described by Saracino et al. 28 and Barone et al., 29 we have used for all calculations the UAHF radii obtained by single point calculations at the HF/6-31G(d,p) level of theory using DFT optimized structures in gas phase.In the UAHF/PCM approach, the solute is placed in a polarizable cavity formed of spheres centered in the atomic groups.Inside the cavity, the dielectric constant is the same as in vacuum, and outside it takes the solvent value (ε = 46.7 for DMSO).

Results and Discussion
The elemental analysis data of the product from the reaction between H 4 opba and ferric chloride in KOH solution corresponded to the formulation [Fe(H 2 opba-i) (dmso) 2 ]Cl.This complex presents deprotonation of both O-carboxylic atoms as in the similar reaction of H 4 pba with KOH. 30 In the literature, there are several examples of deprotonation of both oxygen and nitrogen atoms of H 4 opba with copper(II), nickel(II) and manganese(II), resulting in the respective M x (opba) (x-4) compounds. 31In case of coordination with palladium(II), which affords the compound Na[Pd(Hpba)]•2H 2 O, the deprotonation of two O-carboxylic and only one nitrogen atom of H 4 pba has been observed. 30In fact, the present work is the first synthesis description using H 4 opba that has resulted in the formation of a complex of the type M x (H 2 opba) (x-2) .
The IR data of 1 show intense absorptions at 1693 and 1640 cm -1 , that can be attributed to the amide I band of secondary amides and C=O of carboxylate groups, respectively. 32,33The frequency shifts in relation to the proligand (registered in 1775 and 1672 cm -1 ) indicate the coordination of the two nitrogen and the two O-carboxylic atoms to the Fe III . 34The absorptions at 2913, 1013 and 1343 cm -1 (attributed to aliphatic C-H stretching, S=O stretching and methyl group vibrations, respectively) indicate the presence of DMSO molecules within the compound, as suggested by elemental analysis.
Figure 1 shows the 57 Fe Mössbauer spectrum of 1 at 70 K, least square fitted with two doublets (Lorentzian lines).One doublet (line 1, Figure 1) presents isomer shift δ 1 = 0.54(±0.05)mm s -1 and quadrupole splitting Δ 1 = 1.33(±0.04)mm s -1 ; the other doublet (line 2, Figure 1) presents isomer shift δ 2 = 0.52(±0.05)mm s -1 and quadrupole splitting Δ 2 = 0.66(±0.04)mm s -1 .The obtained Mössbauer parameters indicate the existence of two Fe III species within the product of the reaction, in similar chemical environments. 35,36The presence of iron oxide or hydroxide in the sample is ruled out.These parameters, in addition to the IR and elemental analysis results, suggest that Fe III is in an octahedral geometry coordinated to the nitrogen and oxygen atoms of the ligand H 2 opba-i.The other two coordination sites are filled with DMSO molecules.Indeed, the Mössbauer spectroscopy analysis indicates that the product of the reaction is composed of a mixture of two isomers.
According to the literature, Mössbauer spectra of octahedral complexes with trans isomers show quadrupole splittings twice as big as the ones given by complexes in the cis configuration. 37,38From these results, it can be proposed that the doublet 1 of Figure 1 (Δ 1 = 1.33 mm s -1 ) corresponds to the complex 1 in trans configuration, and the doublet 2 (Δ 2 = 0.66 mm s -1 ) corresponds to the same complex, but in cis configuration.Moreover, from the spectral areas, it can be proposed that the reaction product contains 83% of the trans and 17% of the cis isomers of 1 shown in Figure 2.
The magnetic properties of 1 have been investigated in the 2 -300 K temperature range using a polycrystalline powder.The temperature dependence of the dc magnetic susceptibility is shown in Figure 3 with a χ M T versus T plot, χ M being the molar magnetic susceptibility (diamagnetic correction from Pascal's constants: -80.0 10 -6 emu mol -1 ).At room temperature χ M T is equal to 2.1 emu K mol -1 .Using the spin only equation, which does not take into account either interactions among the spins or orbital contribution, the expected value of χ M T for a Fe III ion with a low spin state (S = 1/2), at room temperature, is equal to 0.375 emu K mol -1 . 14For the intermediate (S = 3/2) and high spin (S = 5/2) states, the values of χ M T are equal to 1.875 and 4.375 emu K mol -1 , respectively.Even though the isomeric shift values obtained by Mössbauer spectroscopy do not allow the determination of the spin multiplicity of 1, the experimental value of χ M T suggests that this compound presents an Fe III spin state intermediate between S = 3/2 and S = 5/2.χ M T decreases slowly as T is lowered until 15 K, and then it decreases more rapidly as T is further  lowered, reaching 0.34 emu K mol -1 at 2 K, the lowest available temperature.This value is approximately the one expected for a Fe III ion in a state of spin S = 1/2.The slight difference in the χ M T value and the slope change below 15 K can be explained by intermolecular antiferromagnetic interactions.
DFT calculations have been performed aiming to provide information about the electronic structure of the isomers of 1.0][41] Conformational analysis of the isolated H 4 opba proligand was performed considering the two tautomers H 4 opba (amide) and H 4 opba-i (iminoalcohol) shown in Figure 4.The H 4 opba tautomer is about 31.4 kcal mol -1 more stable than the H 4 opba-i tautomer according to the DFT results.The calculated C-2,C-1,N-1',C-2' and C-1,C-2,N-1",C-2" dihedral angles are 15.2 degrees in the H 4 opba tautomer.However, these dihedral angles decrease by about 6.6 degrees with the deprotonation of the carboxylic groups leading to H 2 opba 2-species.This indicates larger electronic π-delocalization in the oxamate groups, as expected.
Figure 5 shows the optimized geometry for the [Fe(H 2 opba-i)] + complex, which presents the iminoalcohol tautomer as a ligand.The [Fe(H 2 opba-i)] + complex has C 2v symmetry with Fe-O and Fe-N bond distances of 1.831 and 1.941 Å with a quadratic planar arrangement around the iron center.Calculations showed that the amide tautomer was not a minimum in the potential energy surface, leading to the intramolecular hydrogen transfer to the O-amide.The occurrence of this type of transfer was also verified for similar systems such as Na[Pd(Hpba)]•2H 2 O complex. 30ccording to DFT results, the ground state of [Fe(H 2 opba-i)] + is a quartet (S = 3/2) and the doublet (S = 1/2) and sextet (S = 5/ 2) lie 17.1 and 15.0 kcal mol -1 higher in energy, respectively.These results indicate that the Fe III ion coordinated to H 2 opbai in the absence of dmso presents intermediary crystal-field strength with a ground state of S = 3/2.
The effect of the solvent can lead to important changes in the geometry and electronic structure of metal complexes.The DMSO molecules can coordinate to the vacant sites of the iron center in the [Fe(H 2 opba-i)] + to form an octahedral complex.Figure 6 shows the optimized geometries of the cis-[Fe(H 2 opba-i)(dmso) 2 ] + and trans-[Fe(H 2 opba-i)(dmso) 2 ] + isomers.The optimized geometry of trans-[Fe(H 2 opba-i) (dmso) 2 ] + shows an octahedral surrounding containing oxamate groups in equatorial positions and the oxygen atoms of two dmso molecules in apical positions.In the case of cis-[Fe(H 2 opba-i)(dmso) 2 ] + , a distorted octahedral geometry with

Figure 2 .
Figure 2. The trans and cis isomers of 1.

Figure 3 .
Figure 3. Temperature dependence of the product χ M T versus temperature for 1.