Spectroscopic , Electrochemical , Magnetic and Structural Investigations of Dimanganese-( II / II ) and Mixed-Valence-( II / III )-Tetraiminodiphenolate Complexes

Neste trabalho descrevemos a preparação e a caracterização espectroscópica [ressonância paramagnética eletrônica (EPR), espectroscopia UV-Vis e espectroscopia de infravermelho com transformada de Fourier (FTIR)], magnética e espectroeletroquímica do complexo [Mn2(tidf)(OAc)(ClO4)(MeOH)] (tidf = ligante macrocíclico do tipo Robson, obtido pela condensação entre 2,6-diformil-4-metilfenol e 1,3-diaminopropano). O composto [Mn2(tidf) (OAc)(ClO4)(MeOH)] exibe antiferromagnetismo fraco e constante de acoplamento magnético J = −1.59(1) cm. A oxidação de [Mn2(tidf)(OAc)(ClO4)(MeOH)] monitorada por experimentos de espectroeletroquímica de UV-Vis e de EPR produziu espécies de valência mista Mn2(II/III) com perfil espectral semelhante ao do complexo [Mn Mn(tidf)Br3(H2O)2]. Tentativas de cristalização de [Mn2(tidf)(OAc)(ClO4)(MeOH)] produziram o complexo trinuclear de manganês [Mn3 (tidf)2(μ-OAc)2](ClO4)2. O composto possui duas unidades [Mn(tidf)] onde o íon metálico é pentacoordenado e conectadas a um íon central de Mn hexacoordenado através de pontes μ-fenolato e μ-acetato.


Synthesis
The following preparations were carried out under inert atmosphere using Schlenk techniques.

Physical measurements
UV-Vis spectra in the range 190-900 nm were obtained on a VARIAN Cary 100 spectrophotometer in acetonitrile.Infrared spectra were obtained with a FTS3500GX Bio-Rad Excalibur series spectrophotometer in the region 4000-400 cm −1 in KBr pellets.The electron paramagnetic resonance (EPR) spectra from powdered solid samples were recorded on a Bruker Elexsys E500 X-band spectrometer.The 77 K spectra were obtained employing a quartz finger Dewar.
Magnetic investigations were performed on powdered samples using a Quantum Design MPMS instrument equipped with a 5 T magnet.The temperature dependence of the magnetization (M) was followed from 1.8 to 300 K by applying a 10 kOe field (H) from 300 to 45 K and a 1 kOe field below 45 K to reduce magnetic saturation effects.Magnetic susceptibility per mole (χ M ) was then evaluated as χ M = M M /H.Magnetic data were corrected for the sample holder contribution and for the sample diamagnetism using Pascal's constants (c dia = 3.21 × 10 −4 emu mol −1 ). 25 Cyclic voltammetry was carried out with an IVIUM CompactStat potentiostat/galvanostat.A platinum disc electrode was employed for the measurements at I = 0.1 mol L −1 kept constant with tetrabutylammonium hexafluorophosphate (TBAPF 6 ).A Ag/AgNO 3 ([Ag + ] = 0.01 mol L −1 in a MeCN solution of TBAPF 6 0.1 mol L −1 ) along with a platinum wire were used as reference (0.503 V vs. SHE) and auxiliary electrodes, respectively.Typical experiments were conducted with a 3.0 × 10 −3 mol L −1 complex concentration in acetonitrile, DMF and methanolic solutions at ambient temperature.
UV-Vis spectroelectrochemistry measurements were performed with the IVIUM CompactStat potentiostat/ galvanostat attached to a Agilent 8453 diode-array spectrophotometer from ca. 1 mmol L −1 complex and 0.1 mol L −1 TBAPF 6 acetonitrile solutions.A three electrode system was used with a thin layer cell of 0.1 cm internal optical path length.A platinum minigrid was used as transparent working electrode, in the presence of a small Ag/AgNO 3 reference electrode and a platinum auxiliary electrode.
EPR spectroelectrochemistry was accomplished using the chronoamperometric mode in a two-electrode configuration glass cell containing platinum working and Ag/AgNO 3 reference electrodes.Approximately 1 mL of the complex solution was electrolyzed under stirring at 1.0 V for 30 min, after which no current was developed.After, 200 µL of the sample was transferred to a quartz tube and the EPR spectrum taken at 77 K.
The X-ray data were collected using a Bruker diffractometer, equipped with Cu Kα radiation (ImuS source).Cell refinement and data reduction were done using APEX2. 26Absorption corrections were applied using SADABS. 26Structure was solved by direct methods using SIR2004 27 and refined on F 2 by full-matrix least squares using SHELXL97. 28All non hydrogen atoms were refined anisotropically.The complex shows a disorder effect at the perchlorate anion and co-crystallizes with solvent molecules in the unit cell.The disordered anion molecule was refined using appropriate restraints.The structure refinement with the solvent molecules in the cell is not satisfactory.In consequence, the squeeze procedure was adopted. 29Drawings were made with the ORTEP-3 for Windows. 30More detailed information about the structure determinations is given in Table 1.Microanalyses were done in a CHN-2400 Perkin-Elmer analyzer.

Spectroeletrochemistry
Figure 1a shows the UV-visible spectrum of complex 1 in acetonitrile.2][33] The small shoulder at 386 nm is due to either MLCT or a LMCT transition.The low intensity shoulder observed at 465 nm can be tentatively assigned to a spin-forbidden d-d band.
Cyclic voltammograms of 1 in acetonitrile exhibited a single pair of waves in the −2.0 to 2.0 V window with E pa = 0.14 V vs. Ag/Ag + and peak separation (∆E p ) of 112 mV at 100 mV s −1 (Figure 1b), assigned to the .1/2MeOHwith E 1/2 = 0.093 V vs. Ag/Ag + (L = is the dianion of the Schiff base condensation of 2 mol of 1,3-diaminopropane and 2 mol of 2,6-diformyl-4-tert-butylphenol). 34The magnesium complex [Mg 2 (tidf) 2 (CH 3 CN) 2 ] 2+ undergoes an irreversible reduction between −1.10 and −1.50 V vs. Ag/Ag + , which is related to the macrocyclic ligand tidf since the magnesium in not electroactive in that range of potential.Contrary to [Mg 2 (tidf) 2 (CH 3 CN) 2 ] 2+ , no intraligand redox processes were observed for [Mn 2 (tidf)(OAc)(H 2 O) 2 ] + in the same potential window, indicating a substantial electronic communication between the molecular orbitals of the manganese and the macrocycle ligand.This interaction is naturally reflected in the electronic spectra of the complex as we discuss below.
Spectroelectrochemical response of complex 1 between −0.40 and +1.50 V vs. Ag/Ag + , seen in Figures 1c and 1d, shows the decrease in the intensity of the band at 372 nm, which is an intraligand π→π* (diimine) transition.Since the ligand does not show any electrochemical activity in this range, we can conclude that the band at 372 also contains some contribution from MLCT transitions.The bands below 300 nm exhibited a slight decrease in intensity, also in accordance with an electronic metal-ligand delocalization.Hence, a change in the electron density on the manganese ion upon oxidation influences the UV-Vis spectrum.The oxidation Mn II → Mn III + e − was also followed by an increase of the absorbance at 425 nm (Figure 1c), most likely caused by a LMCT transition, pπ(tidf)→dπMn III , and at 570 nm (Figure 1d) assigned to a d-d transition.These features agree with the spectrum of the isolated mixedvalence complex [Mn II Mn III (tidf)Br 3 (H 2 O) 2 ] (2) as shown in Figure 2.
In order to validate the oxidation numbers of the manganese ions after oxidation of complex 1, we have carried out a spectroelectrochemical experiment based on EPR measurements in 0.1 mol L −1 TBAPF 6 methanolic solutions.In Figure 3a, the EPR spectra of complex 1 at 77 K showed six narrow and intense lines between 3000 and 3800 G with g = 2.0036 and isotropic hyperfine coupling constant A o = 95.4G typical of manganese(II) mononuclear complexes.Additionally, the spectrum of the dimeric manganese(II,II) species can be seen in the range 50-3000 G, with its clear isotropic hyperfine coupling constant signature of ca.45 G.This can be interpreted as the sum of allowed and forbidden transitions as a result of the zero field splitting for all possible states with total spin quantum number (S T ) different from zero.According to the Clebsh-Gordan equation, S T can assume values 5, 4, 3, 2, 1 and 0 for S 1 = S 2 = 5/2.Each S T component gives rise to an EPR absorption and each absorption has an intensity given by the Boltzmann thermal factors.
Similarly to the behavior in acetonitrile, complex 1 shows a single quasi-reversible electrochemical process in methanol with Ep a = 0.11 V vs. Ag/Ag + .It was then electrolyzed during 30 min at 1 V under argon atmosphere and its EPR spectrum recorded at 77 K as given in Figure 3b.Under these conditions and in agreement with the similar behavior previously reported, 34 it is expected the oxidation of a single manganese center to form the mixed-valence [Mn II Mn III (tidf)(OAc)(MeOH) 2 ] 2+ complex.Coordination of methanol can strongly reduce the magnetic exchange between the Mn II and Mn III and, consequently, the six-line pattern seen after the oxidation can be accounted to the uncoupled Mn II center of the mixed-valence compound.This is an interesting result as it relates to the magnetic behavior of the system.The EPR features also exclude the formation of Mn III -Mn IV , that would produce a spectrum with a spin ground S = ½ for a antiferromagnetic coupling and a very characteristic 16-line hyperfine pattern.
The other possibility is the presence of a small amount of a mononuclear Mn II -tidf complex as a contaminant.This would explain the typical six-line pattern of the EPR spectrum observed along with the broad features of the binuclear Mn(II)-Mn(II) species seen in Figure 3a.However, electrochemical oxidation at 1 V of the mononuclear Mn(II) complex would produce the mononuclear Mn III -complex, that would not show any EPR signals.Romain et al. 35 also reported a similar behavior after the electrochemical oxidation of [Mn II (L 2 )] 2+ (L = 6',2''-terpyridine).In that case, an one electron oxidation in CH 3 CN at 1.30 V vs. Ag/Ag + produced Mn III species, but the EPR spectrum at 100 K showed a 6-line signal, which was assigned to a small amount (estimated less than 5%) of the Mn II complex.However, this interpretation was discarded by the authors, since the 6-line feature could still be observed even after an exhaustive oxidation at 1.65 V.
Bearing in mind that the tidf 2− is a diphenolate ligand, another possible sequence to account for 6-line patter of the Mn II species in the EPR spectrum after oxidation,  would be the unimolecular decomposition of Mn III with the concomitant formation of a phenoxyl radical: [Mn III (tidf)] + → [Mn II (tidf − )] + .The phenoxyl radical was not detected as it possibly can react with H + ions from the solvent-methanol.Alternatively, the spectral pattern seen in the EPR after the electrochemical oxidation of the dimanganese(II)-complex (Figure 3b) could be due to the formation of the mixed valence trinuclear compound, [Mn III Mn II Mn III (tidf) 2 (µ-OAc) 2 ] + .This complex molecule was isolated and had its structure recognized by single-crystal X-ray diffraction as discussed below.
Although we cannot rule out this possibility, it seems less likely to occur because as we will present later, it has a very unusual structure and shows a high degree of tension of the coordinated macrocycle, a condition that might not exist in solution.

Magnetic properties
The variable-temperature (2-300 K) magnetic measurements were collected for powdered samples of complex 1.The temperature dependence of molar magnetic susceptibility (χ M ) and of the χ M T product can be seen in Figure 4.The χ M T profile of complex 1 is typical of an antiferromagnetically coupled Mn II dimer.Its 300 K value (8.57emu K mol −1 ) is very close to the expected one for two uncoupled high-spin Mn II ions (8.75 emu K mol −1 ), lowering to 0.69 emu K mol −1 at 1.8 K, suggesting the presence of an unpaired remaining Mn II spin fraction.A quantitative estimation of the Mn II monomeric molar fraction, as well as of the magnetic exchange constant J can be obtained through a least squares fitting procedure of the χ(T) plot.The Heisenberg-Dirac-van Vleck Hamitonian used to fit the data has been , leading to the theoretical expression for χ Μ reported in the Supplementary Information. 36The exchange interaction constant J was found to be −1.59(1)cm −1 , in line with literature data about structurally similar Mn II dimmers. 37Small discrepancies with experimentally determined J values for the structurally related [Mn 2 (tidf)(OAc)](ClO 4 ) complex may indicate a different mode of coordination of acetate and perchlorate ligands.Unfortunately, without the single-crystal structure for both compounds, a definite conclusion could not be unequivocally established.
Structural description of the trimanganese mixed-valence complex [Mn 3 (tidf) 2 (µ-OAc) 2 ](ClO 4 ) 2 Figure 5 shows the representation of the molecular structure of the cation complex [Mn 3 (tidf) 2 (µ-OAc) 2 ] 2+ that crystallized in the presence of perchlorate anions in an overall composition C 52 H 58 Cl 2 Mn 3 N 8 O 16 and with molecular mass of 1287 g mol −1 .Many unsuccessful attempts were made to obtain single-crystals suitable for structure determination of complexes 1 and 2. As a result of the slow diffusion of toluene into a dichloromethane solution of 1, we were able to isolate a single batch of yellow crystals proper for diffraction.Unfortunately, the crystals were geminated and exhibited a high degree of disorder resulting in high values of R = 0.0977 and R w = 0.273.Despite our efforts, we were unable to reproduce the crystallization of similar batches of the compound.Considering the unusual mode of coordination of the macrocycle ligand and, most importantly, its elevated distortion round the metal ion, apparently this is not a stable molecule and we found no experimental evidence that the complex could maintain that structure in solution.

Conclusions
The binuclear complex [Mn II 2 (tidf)(OAc)(ClO 4 )(MeOH)] (1) was successfully prepared by template synthesis and its spectroscopic, electrochemical and magnetic properties investigated.From the magnetic point of view, 1 showed a weak antiferromagnetic behavior (J = −1.59(1)cm −1 ) and a remaining uncoupled mononuclear Mn II fraction of 3.5%.EPR signals from frozen methanolic solutions at 77 K are consistent with these findings, showing binuclear Mn II -Mn II to be the major compound, along with a small amount of a mononuclear Mn II species.UV-Vis and EPR spectroelectrochmical responses after oxidation of complex 1 at 1 V vs. Ag/Ag + reveal oxidation and stabilization of a probable valence-trapped mixed-valence complex Mn II -Mn III with close resemble of spectral features of complex 2. Crystallization of 1 produced a new and unexpected compound [Mn 3 (tidf) 2 (µ-OAc) 2 ](ClO 4 ) 2 (3) containing a trinuclear and complex Mn III Mn II Mn III structure.The complex can be seen as two pentacoordinated [Mn III (tidf)] + motifs connected to a central hexacoordinated Mn II ion through phenolate and acetate bridges.The severe distortion and uncommon mode of coordination of the macrocyclic ligand indicates an unstable compound, only detected in the solid state as a result of the self-assembly of its components.

Figure 4 .
Figure 4. Magnetic susceptibility data as a function of the temperature for complex 1.The red solid line is the best fit according to the theoretical expression discussed in the text, R 2 = 0.999.