Influence of the Disposition of the Anisotropy Axes into the Magnetic

Article pubs.acs.org/IC

Influence of the Disposition of the Anisotropy Axes into the Magnetic Properties of MnIII Dinuclear Compounds with Benzoato Derivative Bridges Beltzane Garcia-Cirera,† Silvia Gómez-Coca,*,†,‡,⊥ Mercè Font-Bardia,§ Eliseo Ruiz,†,‡ and Montserrat Corbella*,†,∥ †

Departament de Química Inorgànica i Orgànica, Universitat de Barcelona, Barcelona 08007, Spain Institut de Química Teórica i Computacional de la Universitat de Barcelona (IQTCUB), Barcelona 08007, Spain § Unitat de Difracció de Raigs X, Centres Científics i Tecnològics de la Universitat de Barcelona (CCiTUB) and Departament de Mineralogia, Petrologia i Geologia Aplicada, Facultat de Ciències de la Terra, Universitat de Barcelona, Barcelona 08007, Spain ∥ Institut de Nanociencia i Nanotecnologia de la Universitat de Barcelona (IN2UB), Barcelona 08007, Spain ‡

S Supporting Information *

ABSTRACT: The two new MnIII dinuclear compounds [{Mn(H2O)(phen)}2(μ-4-CH3C6H4COO)2(μ-O)](ClO4)2·3CH3CN· H2O (1·3CH3CN·H2O) and [{Mn(H2O)(phen)}(μ-O)(μ-2BrC6H4COO)2{Mn(NO3)(phen)}]NO3 (2) have been synthesized. Their structural data reveal significant differences in the shape of the coordination octahedron around the MnIII ions in both compounds. The different distortions from ideal geometry incite a very different magnetic behavior, affecting both the zero-field splitting parameters of the MnIII ions (DMn and EMn) and the magnetic interaction between them. Compound 1, with elongation in the monodentate ligand direction, shows antiferromagnetic coupling (ground state S = 0) and local DMn < 0, while compound 2, with compression in the oxo bridge direction, displays a ferromagnetic interaction (ground state S = 4) and local DMn > 0. Theoretical CASSCF and DFT calculations corroborate the different magnetic anisotropy and exchange coupling found in both compounds. Moreover, with the help of theoretical calculations, some interesting magneto-structural correlations have been found between the degree of distortion of the coordination octahedra and the magnetic coupling; it becomes more antiferromagnetic when the elongation parameter, Δ, in absolute value is increased.

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INTRODUCTION

compound reported in the literature with compressed octahedra around the MnIII ions, [{Mn(bpy)(N3)}2(μ-O)(μC6H5COO)2], which shows an important ferromagnetic coupling.10,11 Over the last years, we have focused our attention on the search of the structural parameters that control the magnetic behavior and, consequently, the ground state of the system (S = 0 or S = 4).4−7,12 Recently we reported the influence of the anisotropy of the MnIII ions and the relative orientation of the distortion axes on the magnetic behavior at low temperature.9 In addition, other authors have also analyzed the effect of the of the Jahn−Teller axis disposition on the magnetic properties of dinuclear Mn I I I complexes with [Mn 2 (OR) 2 ] 4+ , [Mn2(ONR)2]4+, and [Mn2(O)(ONR)]3+ cores.13−15 They classify them as a function of the relative orientation of the Jahn−Teller axes in each ion and their orientation with respect to the plane formed by the two bridging ligands. Between the

III

The magnetic properties of the Mn dinuclear compounds with the core [Mn2(μ-O)(μ-R′COO)2]2+ have been widely studied. The electronic configuration of the MnIII ions (d4) with an empty d orbital is responsible for the weak magnetic interaction between these ions, and the magnetic behavior can be either ferro- or antiferromagnetic.1 Several structural parameters influence the kind and strength of this interaction. One of the most important factors is the shape of the coordination octahedron, which can be elongated or compressed due to the Jahn−Teller effect. It was observed that compounds with tridentate blocking ligands show compressed octahedra around the MnIII ions, with the short distortion axis pointing to the oxo bridging ligand. When the capping ligand is bidentate, the sixth position is occupied by a monodentate ligand, and this fact provides a major flexibility to the coordination octahedra.2 Compounds with general formula [{Mn(L)(NN)}2(μ-O)(μ-n-RC6H4COO)2]n+ (NN = bidentate N-donor ligand) usually show elongated octahedra in the direction of the monodentate ligand (L).3−9 There is only one © XXXX American Chemical Society

Received: April 5, 2017

A

DOI: 10.1021/acs.inorgchem.7b00877 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Table 1. X-ray Crystallographic Data for Compounds 1 and 2 chem formula formula wt T/K λ(Mo Kα)/Å cryst syst space group cryst size/mm a/Å b/Å c/Å α/deg β/deg χ/deg V/Å3 Z ρcalcd/g cm−3 μ/mm−1 F(000) θ range/deg limiting indices no. of data/restraints/params goodness of fit on F2 final R indices (I > 2σ(I)) R indices (all data)

1·3CH3CN·H2O

2

C46H45Cl2Mn2N7O16 1132.67 100(2) 0.71073 P1̅ triclinic 0.1 × 0.09 × 0.08 11.734(5) 12.925(4) 18.325(6) 91.91(2) 108.60(2) 103.83(2) 2539.5(16) 2 1.481 0.678 1164 1.63−32.29 h = −14 to 16, k = −18 to 15, l = −26 to 26 12500/9/771 1.087 R1 = 0.0626, wR2 = 0.1729 R1 = 0.0786, wR2 = 0.1837

C38H26Br2Mn2N6O12 1028.35 104(2) 0.71073 Pna21 orthorhombic 0.74 × 0.17 × 0.126 18.0696(5) 20.3810(5) 10.5036(3) Å 90 90 90 3868.23(18) 4 1.766 2.792 2048 2.181−30.044 h = −25 to 25, k = −28 to 28, l = −14 to 12 10651/5/517 1.028 R1 = 0.0477, wR2 = 0.1136 R1 = 0.0613, wR2 = 0.1217

antiferromagnetic for a Mn−Obridge−Mn angle of 123.91° and an Mn−Obridge distance equal to 1.798 Å.18 Herein we report the synthesis, crystal structure, and magnetic study of two new MnIII dinuclear compounds, [{Mn(H2O)(phen)}2 (μ-4-CH 3C6H4 COO)2(μ-O)](ClO 4)2 · 3CH3CN·H2O (1·3CH3CN·H2O) and [{Mn(H2O)(phen)}(μO)(μ-2-BrC6H4COO)2{Mn(NO3)(phen)}]NO3 (2). The structural data reveal significant differences in the shape of the coordination octahedra of both compounds. Combined experimental and theoretical studies have been performed to analyze the magnetic properties of these compounds, the aim of this paper being to understand and rationalize the effect of the orientation of the Jahn−Teller axes on the magnetic behavior.

different orientations observed, the most common one has a parallel disposition of the Jahn−Teller axes, both parallel or perpendicular to the bridging plane. In contrast, compounds with a [Mn2(μ-O)(μ-R′COO)2]2+ core have three bridging ligands and usually show an orthogonal disposition of the elongation axes. With this topology, the parallel disposition of the Jahn−Teller axes perpendicular to the MnOMn plane is hindered. The magnetic anisotropy of the MnIII ions is quantified by the axial zero field splitting (ZFS) parameter, DMn, which depends on the spin−orbit coupling. This property has been widely studied over the last few years due to the presence of Mn ions in several polynuclear single-molecule magnets (SMMs). In fact, this ion was proposed as a source of magnetic anisotropy. To further understand its magnetic anisotropy, several theoretical studies have been devoted to mononuclear Mn compounds,16 including a systematic study of the magnetic anisotropy in mononuclear MnIII complexes.17 For these complexes the DMn values range between −5 and +4 cm−1, being negative for elongated systems and positive for compressed systems, showing that both usually employed approaches to calculate ZFS parameters (DFT and CASSCF) reproduce remarkably well the experimental values.16,17 For MnIII dinuclear compounds, it is worth noting the theoretical study of the compound [{Mn(L′)}2(μ-O)(μOAc)2](PF6) 2 (with a tridentate capping ligand L′ = trispyrrolidine-1,4,7-triazacyclononane). The MnIII ions are in a compressed octahedral environment, and the magnetic behavior is characterized by ferromagnetic exchange and positive DMn values.18 In addition, the authors performed simulations changing structural parameters. They showed that the coupling constant is highly sensitive to small structural variations, becoming antiferromagnetic for shorter Mn−Obridge distances and larger Mn−Obridge−Mn angles. It becomes

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EXPERIMENTAL SECTION

Synthesis. All manipulations were carried out under aerobic conditions. Reagents and solvents were obtained from commercial sources and used without further purification. NBu4MnO4 was prepared as described in the literature.19 Yields were calculated from stoichiometric reactions. Caution! Perchlorate salts of compounds containing organic ligands are potentially explosive. Only small quantities of these compounds should be prepared and handled behind suitable protective shields. [{Mn(H2O)(phen)}2(μ-4-CH3C6H4COO)2(μ-O)](ClO4)2·3CH3CN· H2O (1·3CH3CN·H2O). A mixture of 4-CH3C6H4COOH (0.22 g, 1.62 mmol) and Mn(ClO4)2·6H2O (0.45 g, 1.24 mmol) was dissolved in 15 mL of CH3CN. Then, 10 mL of an NBu4MnO4 (0.11 g, 0.32 mmol) solution and 5 mL of a 1,10-phenanthroline·H2O (phen) (0.29 g, 1.46 mmol) solution, both in CH3CN, were added at the same time to the aforementioned solution, and the mixture immediately turned dark brown. The mixture was stirred for 15 min, and no precipitate was formed. The solution was left undisturbed in the refrigerator. One month later dark brown crystals suitable for X-ray diffraction were obtained. The product was separated by filtration. Yield: 27%. Anal. Calcd for Mn2C40H34N4O15Cl2·0.5 H2O (mol wt 1000.51): C, 48.01; H, 3.52; N, 5.60. Found: C, 47.90; H, 3.56; N, 5.65. Selected IR data (KBr, main bands, cm−1): 3457 (m), 3065 (w), 1608 (m), 1629 (w), B


Article

Inorganic Chemistry

Figure 1. Crystal structures of the cationic complexes of compounds 1 (left) and 2 (right). Hydrogen atoms are omitted for clarity. (SADABS).22 All H atoms were computed and refined using a riding model with an isotropic temperature factor equal to 1.2 times the equivalent temperature factor of the atom that is linked to the H atom. Crystal data collection and refinement parameters are given in Table 1, and more details can be found in the Supporting Information. Computational Details. Due to the strong dependence of the magnetic coupling constant and ZFS parameters on small structural variations, all of the calculations were performed using the crystallographic geometries obtained by X-ray diffraction. The calculations of the ZFS parameters have been performed with the MOLCAS code (version 7.8)23 using the procedure described by Maurice et al.,24 which consists of performing a CASSCF calculation first, followed by the mixing of the energy of these CASSCF states within the SO-RASSI approach. After that, the axial (DMn) and rhombic (EMn) ZFS parameters together with the g and D tensors were extracted using the Single-Aniso software.25 The calculation of the magnetic anisotropy for each MnIII ion has been done using the geometry of the whole complex but replacing one of the MnIII ions for a GaIII ion. This procedure was done for both sites of the dinuclear entity. The electronic configuration of MnIII is d4; therefore, the selected active space CAS(4,5) contains four electrons in the five essentially atomic d orbitals. All of the quintet, triplet, and singlet states were included in the calculations. For the evaluation of the magnetic coupling constant of the dinuclear MnIII systems, the employed methodology has been previously described by Ruiz et al.26−28 The calculations were carried out using the density functional theory (DFT) with the Gaussian09 software (version g09d01),29 the hybrid B3LYP functional,30 and the triple-ζ basis set proposed by Schäfer et al.31 The initial wave function was generated previously with the Jaguar software (version 7.5),32 including the effects of the ligand field.33 Two spin distributions have been calculated for the MnIII dinuclear system, the high-spin distribution (ST = 4), in which the spin in both Mn ions are parallel, and the low-spin distribution (ST = 0), where the spin of Mn2 has been reversed. The exchange coupling constant has been obtained from the energy difference between both spin distributions (Els − Ehs = 10J). The influence of the structural parameters has been analyzed by the construction of three types of models based on the crystal structures of 1 and 2. (a) The angle formed by the Mn ions and the oxo bridge (Mn−Ob−Mn) has been modified in the 120−125° range; this variation provokes a slight change in the Mn−Ob distances (Tables S1 and S2 in the Supporting Information). (b) The Mn−L distance, L being the monodentate ligand, has been varied in the 2.1−2.3 Å range (Tables S3 and S4 in the Supporting Information), (c) The Mn−Nt distances, where Nt is the nitrogen atom trans to the oxo bridge, have

1590 (s), 1550 (s), 1518 (vs), 1427 (s), 1385 (vs), 1374 (vs), 1341 (s), 1110 (vs), 849 (s), 762 (s), 721 (s), 624 (s). [{Mn(H2O)(phen)}(μ-O)(μ-2-BrC6H4COO)2{Mn(NO3)(phen)}]NO3 (2). A mixture of 2-BrC6H4COOH (0.13 g, 0.65 mmol) and Mn(NO3)2·xH2O (0.10 g, 0.56 mmol) were dissolved in 10 mL of CH3CN. Then, 5 mL of an NBu4MnO4 (0.03 g, 0.08 mmol) solution and 5 mL of a 1,10-phenanthroline·H2O (phen) (0.13 g, 0.66 mmol) solution, both in CH3CN, were added at the same time to the aforementioned solution and some precipitate was formed. The mixture was heated in a microwave device from 25 to 85 °C in 12 min. The final temperature was maintained for 5 min, and after that the mixture was slowly cooled to 25 °C. The obtained microcrystalline powder was removed by filtration, and the mother solution was left undisturbed at room temperature. After 1 month, dark brown crystals suitable for X-ray diffraction were obtained. Yield: 45%. Anal. Calcd for Mn2C38H26N6O12Br2 (1028.33): C, 44.38; H, 2.55; N, 8.17. Found: C, 44.21; H, 2.51; N, 8.01. Selected IR data (KBr, main bands, cm−1): 3433 (m), 1627 (w), 1594 (s), 1561 (w), 1518 (w), 1426 (s), 1384 (vs), 1104 (w), 1022 (s), 873 (w), 844 (m), 755 (m), 718 (s). Physical Measurements and Instruments. The synthesis of compound 2 was carried out in an Anton Paar Monowave 300 Synthesis Reactor. Chemical analyses of C, H, N were carried out by the “Servei de Microanàlisi” of the “Institut de Quimica Avançada de ́ Catalunya, (IQAC) Consell Superior d’Investigacions Cientifiques (CSIC)”. Infrared spectra were recorded on KBr pellets in the 4000− 400 cm−1 range with a Thermo Nicolet Avatar 330 FT-IR spectrometer. Magnetic susceptibility (χM) measurements between 2 and 300 K were carried out in a Quantum Design MPMS XL5 SQUID Magnetometer at the “Unitat de Mesures Magnètiques (Universitat de Barcelona)”, using a field of 200 G. Pascal’s constants were used to estimate the diamagnetic corrections for each compound. Magnetization measurements (M/Nμβ) were carried out in the range 1.8−6.8 K and at six different magnetic fields (0.5, 1.0, 2.0, 3.0, 4.0, and 5.0 T). The accuracy of the fit was calculated by the functions Rsus = ∑[(χMT)exp − (χMT)calc]2/∑[(χMT)exp]2 and Rmag = ∑[(M/Nμβ)exp − (M/Nμβ)calc]2/∑[(M/Nμβ)exp]2. Crystallographic Data Collection and Refinement. X-ray crystallographic data for both crystals (1 and 2) were collected at 100 K. Compound 1 was mounted on a MAR345 diffractometer with an image plate detector and compound 2 on a D8 Venture system equipped with a multilayer monochromator. For compounds 1 and 2 the structure was solved by direct methods, using the XT computer program,20 and refined by full-matrix least-squares methods with the SHELXL computer program.21 The final cell constants are based upon the refinement of the xyz centroids of reflections above 20 σ(I). Data were corrected for absorption effects using the Φ-scan method C


Article

Inorganic Chemistry Table 2. Selected Bond Lengths (Å) and Angles (deg) for Compounds 1·3CH3CN·H2O and 2 1·3CH3CN·H2O Mn(1)−O(1) Mn(1)−N(2) Mn(1)−O(5) Mn(1)−N(1) Mn(1)−O(3) Mn(1)−O(7) Mn(1)···Mn(2) Mn(2)−O(1) Mn(2)−N(4) Mn(2)−O(4) Mn(2)−N(3) Mn(2)−O(6) Mn(2)−O(8) Mn(1)−O(1)−Mn(2) O(1)−Mn(1)−N(2) O(5)−Mn(1)−N(1) O(3)−Mn(1)−O(7) O(1)−Mn(2)−N(4) O(4)−Mn(2)−N(3) O(6)−Mn(2)−O(8) O(7)−Mn(1)−Mn(2)−O(8) O(3)−C(26)−C(27)−C(28) O(6)−C(34)−C(35)-C(40)

2 1.803(2) 2.082(2) 1.967(2) 2.095(3) 2.1505(19) 2.233(2) 3.136(1) 1.779(2) 2.073(3) 1.952(2) 2.097(3) 2.157(2) 2.198(2) 122.16(12) 171.32(10) 168.70(9) 172.58(8) 170.10(10) 169.06(10) 173.16(8) 83.8(1) −0.3(4) −5.5(4)

Mn1−O1 Mn1−N1 Mn1−O2 Mn1−N2 Mn1−O5 Mn1−O9 Mn1···Mn2 Mn2−O1 Mn2−N4 Mn2−O4 Mn2−N3 Mn2−O3 Mn2−O6 Mn(1)−O(1)−Mn(2) O1−Mn1−N1 O2−Mn1−N2 O5−Mn1−O9 O1−Mn2−N4 O4−Mn2−N3 O3−Mn2−O6 O9−Mn1−Mn2−O6 O5−C32−C33−C38 O3−C25−C26-C27

1.778(4) 2.086(4) 2.028(4) 2.127(5) 2.105(5) 2.130(4) 3.1419(10) 1.777(4) 2.071(4) 1.999(4) 2.093(5) 2.154(4)0 2.164(4) 124.2(2) 167.74(18) 169.30(17) 170.91(17) 169.82(18) 168.03(17) 164.77(15) −96.3(2) −65.1(9) 35.8(8)

Description of Structures. The crystal structures of compounds 1·3CH 3 CN·H 2 O ([{Mn(H 2O)(phen)}2(μ-4CH 3 C 6 H 4 COO) 2 (μ-O)](ClO 4 ) 2 ·3CH 3 CN·H 2 O) and 2 ([{Mn(H2O)(phen)}(μ-2-BrC6H4COO)2(μ-O){Mn(NO3)(phen)}]NO3) are shown in Figure 1. In both compounds the two MnIII ions are bridged by one oxo and two μ1,3-nRC6H5COO− ligands (n-R = 4-Me for 1 and 2-Br for 2). Each manganese ion is chelated by a 1,10-phenanthroline (phen) ligand, and the hexacoordination is completed by a monodentate ligand. In compound 1 the monodentate ligands are water molecules, while in compound 2 one of the monodentate ligands is a nitrate anion and the other is a water molecule. The structural parameters of these compounds are in agreement with those reported for analogous compounds with the same [Mn2(μ-O)(μ-RC6H5COO)2]2+ core.2,5−7,9,11,12,35−37 Selected bond lengths and angles are given in Table 2. The Mn···Mn distance is ∼3.14 Å, and the Mn−O−Mn angle is ∼123°. The Mn−O bond lengths of the oxo bridge are ∼1.79 Å, and the Mn−N distances are ∼2.09 Å. The carboxylate bridging ligands are coordinated in a syn-syn conformation mode. One of the oxygen atoms is placed trans to the monodentate ligand, with an Mn−Ot distance of ∼2.15 Å for 1 and ∼2.13 Å for 2, whereas the other oxygen atom is placed in a cis position with a shorter Mn−Oc distance (∼1.96 Å for 1 and ∼2.01 Å for 2). A significant difference between these compounds can be found in the Mn−OL bond lengths: for compound 1, the Mn−Ow distances are 2.233 and 2.198 Å, whereas for compound 2 the Mn−Ow distance is 2.130 Å and the Mn−ONO3 distance is 2.164 Å. The carboxylate group and the aromatic ring of the benzoate derivative bridge are almost coplanar in compound 1; the twist angle ω(O−Ccarb−Car−Car) is close to 0°. In contrast, a significant twist of the aromatic ring is found in compound 2, probably due to the steric hindrance between phen ligands and

been modified between 2.025 and 2.1 Å; the shifts of the Nt atoms also shift the other nitrogen atom, Nc, of the ligand (Tables S5 and S6 in the Supporting Information).

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RESULTS AND DISCUSSION Synthesis. These MnIII dinuclear compounds were obtained by a comproportionation reaction between MnX2 (X = ClO4 (1), NO3 (2)) and Bu4NMnO4, in the presence of the carboxylic acid (4-MeC6H4COOH for 1 and 2-BrC6H4COOH for 2) and 1,10-phenanthroline, leading to compounds with general formula [{Mn(L)(phen)}(μ-n-RC6H4COO)2(μ-O){Mn(L′)(phen)}]X2−m. L and L′ are monodentate ligands that can be H2O or X. For compound 1 L and L′ are H2O molecules, while for 2, one of the monodentate positions is occupied by the nitrate anion. Therefore, compound 1 has a divalent cation while for 2 the cationic complex is monovalent. The synthesis of compound 2 by the traditional method was unsuccessful due to the formation of subproducts and the low crystallinity of the desired compound. Therefore, the reaction was performed by heating the sample above the boiling point of the solvent, in a microwave device, on a closed vessel. The infrared spectra of these compounds show two characteristic bands of the carboxylate ligand at ∼1600 and ∼1380 cm−1, corresponding to the asymmetric and symmetric vibrations, respectively. The values of Δν = νa(COO) − νs(COO), being ∼200 cm−1, are indicative of carboxylate ligands coordinated in a bidentate bridging mode (μ1,3).34 The nitrate anions show a very strong band at ∼1384 cm−1, which appears overlapped with the carboxylate band. The perchlorate anions display broad bands at ∼1100 cm−1 and a band of moderate intensity at 623 cm−1. Bidentate phen ligands present characteristic bands in the 1600−1400 and 850−600 cm−1 regions. The Mn−Ob−Mn group usually displays a band at ∼730 cm−1 but is masked by 1,10-phenanthroline bands. D


Article

Inorganic Chemistry

Table 3. Length of x, y, and z Axes and Elongation (Δ) and Rhombicity (ρ) Parameters for Compounds 1 and 2 Considering the Axis Assignment of Figure 3

the bromo substituent in an ortho position. The relative disposition of the two coordination octahedra is almost perpendicular, with a torsion angle τ(L−Mn···Mn−L) smaller for compound 1 (83.8°) than for compound 2 (96.3°). Some hydrogen bonds connecting the monodentate ligand and the counteranion are present in the crystal structures of compounds 1 and 2. For 2, the net of hydrogen bonds generates a zigzag chain (Figures S1 and S2 and Table S7 in the Supporting Information). As was mentioned above, in these kinds of compounds, the MnIII ions show an elongated octahedron in the direction of the monodentate ligand; hence, the Jahn−Teller elongation axis should be approximately situated on the Ot−Mn−L direction (z axis). Moreover, there is some rhombic distortion, because the Mn−Ob distance is shorter than the Mn−Oc distance; arbitrarily the x axis could be considered in the Nt−Mn−Ob direction (Figure 2). Approximate values of the octahedron axis

Compound 1 Mn1 Mn2

z/ Å

y/Å

x/Å

Δ/%

ρ/%

4.384 4.355

4.062 4.049

3.885 3.852

10.32 10.24 10.28 (av)

4.56 5.11 4.84 (av)

Compounds 2 and Aa

a

x/Å

y/Å

z/Å

Mn1 Mn2

4.235 4.318

4.155 4.092

3.864 3.848

Mn

4.253

4.184

3.895

−7.89 −8.49 −8.19 (av) −7.68

−1.89 −5.23 −3.56 (av) −1.62

[{Mn(bpy)(N3)}2(μ-C6H5COO)2(μ-O)] (A).10,11

Figure 3. Schematic representation of the axes disposition for both compounds. The blue arrows indicate the Jahn−Teller axes. Figure 2. Schematic structure of the MnIII dinuclear complexes with the expected disposition of the axes of the coordination octahedron.

For compound 1, the Jahn−Teller axes are approximately orthogonal, but none of them are in the plane formed by the two MnIII ions and the oxo bridge, while in compound 2 both are in the plane and they share the end point. Although none of the studied compounds can be described using the classification reported by Brechin et al., it is evident that this difference in the orientation of the Jahn−Teller axes will determine their magnetic behavior, as they already observed before for other MnIII dinuclear complexes.13−15 The distortion parameters for both compounds are summarized in Table 3. The negative values of Δ and ρ for compound 2 agree with compressed octahedra in the z direction (Mn−Ob direction). For Mn2 it is clear that it exhibits an important rhombic distortion, but the magnitudes of the distortion parameters are similar for both assignations of the axes. Thus, the disposition of the Jahn−Teller axis from the structural parameters is ambiguous for Mn2. As indicated previously, to the best of our knowledge, there is only another compound reported in the literature that presents bidentate capping ligands and compressed octahedra. This is the neutral complex [{Mn(bpy)(N3)}2(μ-C6H5COO)2(μO)]10,11 (A), where both manganese ions are equivalent. This compound shows the same disposition of the Jahn−Teller axes as compound 2, and the elongation and rhombicity parameters are also negative (Table 3). These results confirm the validity of these parameters (Δ and ρ) for both kinds of distortions: elongation and compression of the octahedron. The axis length for the ideal octahedron can be calculated as ideal = (x + y + z)/3. For compound 1 the x and y axes are shorter than the ideal, while the z axis is longer, with an elongation of about 6.3% in this direction (Ot−Mn−L). For compound 2 the x and y axes are larger than the ideal, while z is shorter, with a compression of about 5.6% in the Nt−Mn−Ob direction.

lengths can be found by addition of Mn−ligand distances: x = d(Mn−Ob) + d(Mn−Nt), y = d(Mn−Oc) + d(Mn−Nc), and z = d(Mn−Ot) + d(Mn−OL). For both compounds we assume that the trend of the length axes is z > y > x, as was observed for analogous compounds. However, the analysis of the distortion of the coordination octahedra reveals significant differences between them. The kind and degree of distortion was evaluated by the elongation (Δ) and rhombicity (ρ) parameters, which are calculated using the formulas9 z − xy y−x Δ= and ρ = xy x where xy = (x + y)/2. Δ represents how different the Jahn− Teller axis is from the average length between x and y axes, and ρ represents the distortion within the xy plane. If the assignment of the axes is correct |Δ| should be greater than |ρ|.9 For compound 1, the parameters found follow the aforementioned trend (Table 3), and consequently coordination octahedra of the Mn ions are elongated in the monodentate ligand direction. However, for compound 2 the rhombicity parameter is greater than the elongation parameter (ρ > Δ) in Mn1 (Table S8 in the Supporting Information); this indicates that the axes are not correctly assigned. Moreover, Mn2 shows quite similar values of the elongation and rhombicity parameters, suggesting a great degree of rhombicity or a bad axis assignment. Therefore, for compound 2 the Jahn− Teller axes (z axis) should be in the Mn−Ob direction and arbitrarily we consider the x axis pointing to the monodentate ligand. Figure 3 shows comparatively the axes assignment for compounds 1 and 2. E


Article

Inorganic Chemistry Magnetic Properties. Magnetic susceptibility data were recorded for compounds 1 and 2 from room temperature to 2 K. For compound 1 the χMT product is ∼5.9 cm3 mol−1 K at room temperature, which is close to the expected value for two uncoupled MnIII ions (6.0 cm3 mol−1 K). The χMT values decrease slightly with the temperature until ∼50 K; below this temperature the χMT values decrease more abruptly on cooling, reaching a value of 2.4 cm3 mol−1 K at 2 K (Figure 4). This

Figure 5. M/NμB versus HT−1and χMT versus T (inset) plots for compound 2. The solid line is the best fit to the experimental data.

ferromagnetic interactions could contribute to the decreases of the χMT values at low temperature. Magnetization measurements were carried out at six different fields (0.5−5 T) in the 1.8−6.8 K range. M/Nμβ versus HT−1 plots are shown in Figure 5. The nonsuperposition of the lines indicates significant zero-field splitting within the ground state. The experimental data (M/NμB versus HT−1and χMT versus T) were fitted simultaneously with the PHI program.38 To achieve a good fit, it was necessary to include the term of intermolecular interactions, which was fixed to zJ′ = −0.015 cm−1. The best fit was obtained with positive DMn values. Note that the sign of the ZFS parameters is opposite to that found for compound 1 and those reported previously for analogous compounds.9 This fact agrees with the compressed distortion found in one of the manganese ions. Taking into consideration the disposition of the Jahn−Teller axes pointing to the Obridge, a rotation of the reference frame, of −60° for Mn1 and +60° for Mn2, through the Euler angle β was considered. The best fit leads to J = +11.8 cm−1, DMn = +3.2 cm−1, EMn = |0.7| cm−1, and g = 1.98, with Rsus = 4.0 × 10−4 and Rmag = 3.0 × 10−5 (values reported using the spin Hamiltonian H = −JS1·S2). In a previous work we reported some magnetostructural correlation for these kinds of dinuclear compounds (with general formula [{Mn(L)(NN)}2(μ-O)(μ-n-RC6H4COO)2]X2).9 As was indicated, all of those compounds show elongated octahedra with some rhombic distortion, except for compounds [{Mn(bpy)(N3)}2(μ-C6H5COO)2(μ-O)] (A)10,11 and 2, which present compressed octahedra. For the compounds with elongated/rhombic distortion, those with the most elongated octahedra display average parameter values of Δ = 15.6% and ρ = 3.5% (Δ/ρ = 4.5) and antiferromagnetic behavior.9 On the other extreme, the compound with a major degree of rhombicity presents Δ = 9.0% and ρ = 5.4% (Δ/ρ = 1.7) and ferromagnetic behavior.9,37 Compound 1, with Δ = 10.3% and ρ = 4.8% (Δ/ρ = 2.1), shows weak antiferromagnetic behavior, as expected for this structural distortion. Moreover, the correlation |EMn/DMn| versus ρ reported previously (|EMn/ DMn|= 0.06 + 0.03 ρ, where ρ is in percent)9 fits well for compound 1. On the basis of the ρ value calculated from the structural parameters, the expected |EMn/DMn|value should be 0.20, which is the same relationship found by the fit of the magnetic data. The three structural parameters that modulate the magnetic interaction are the elongation parameter (Δ), the twist of the

Figure 4. χMT versus T and M/NμB versus H (inset) plots for compound 1. The solid line is the best fit to the experimental data.

behavior is indicative of an antiferromagnetic coupling between the MnIII ions. However, the χMT value at low temperature and the shape of the M/Nμβ vs H plot at 2 K (Figure 4, inset) suggest the presence of more than one spin state populated at low temperature and a negative value of the MnIII ion axial zero-field splitting (ZFS). Recently there was a report of the importance of the disposition of the Jahn−Teller axes and the ZFS parameters of the MnIII ions (DMn and EMn) on the magnetic behavior of weak antiferromagnetic dinuclear systems.9 When the antiferromagnetic interaction is weak, the S = 1 state is populated, and if DMn < 0 MS = ±1 is at lower energy than for MS = 0; consequently, the χMT values at low temperatures do not fall to 0. The fit of the experimental data was performed with the PHI program38 assuming that the two MnIII ions have the same g, DMn, and EMn values. χMT and M/ Nμβ data were fitted simultaneously. Taking into consideration that the Jahn−Teller axes of the coordination octahedra are mostly orthogonal, a rotation of the reference frame, of −45° for Mn1 and +45° for Mn2, through the Euler angle β was considered. The best fit of χMT and M/Nμβ data was obtained with J = −1.8 cm−1, DMn = −4.1 cm−1, EMn = |0.8| cm−1 and g = 1.99, with Rsus = 5.3 × 10−5 and Rmag = 5.4 × 10−4 (values reported using the spin Hamiltonian H = −JS1·S2). For compound 2 the χMT value increases on cooling from ∼6.6 cm3 mol−1 K at room temperature to 9.25 cm3 mol−1 K at 17 K. Below this temperature χMT decreases, reaching a value of ∼7.5 cm3 mol−1 K at 2 K (Figure 5 inset). This behavior is characteristic of a ferromagnetic coupling. The χMT value expected for a system with a ground spin state S = 4 is 10 cm3 mol−1 K. The lower values found for the (χMT)max and the decrease in χMT below this temperature could be explained by the presence of zero-field splitting on the ground state and/or intermolecular antiferromagnetic interactions. As mentioned, this compound shows hydrogen bonds between dinuclear entities, through nitrate anions; thus, intermolecular antiF


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Table 4. Magnetic Coupling Constant (J) and ZFS Parameters for the MnIII Ions Obtained from the Fit of the Experimental Data, Calculated J Values Using DFT and the B3LYP Functional, and Calculated ZFS Parameters for Each Ion (Mn1 and Mn2) at the CASSCF+RASSI level (H = −JS1·S2) fit of the experimental data −1

−1

−1

CASSCF + RASSI −1

−1

J (cm )

DMn (cm )

|EMn| (cm )

|E/D|

B3LYP J (cm )

DMn1/DMn2 (cm )

|EMn1|/|EMn2| (cm−1)

(|EMn1/DMn1)/|(EMn2/DMn2|

1

−1.8

−4.1

0.8

0.20

−3.14

2

+11.8

+3.2

0.7

0.22

+12.89

−3.17 −3.12 +3.53 +3.35

0.89 0.94 0.66 0.83

0.28 0.30 0.19 0.25

Figure 6. Schematic orbital diagram for a d4 metal ion in octahedral geometry depending on the distortion: elongated versus compressed.

E/D values the sign of D loses its meaning. Although there are some differences between the experimental and calculated values, especially for compound 1, the same general trend is observed: |D| > 3 cm−1 and |E| < 1 cm−1 and considerable rhombicity. It was previously reported that the qualitative sign and magnitude of the D value of the single ion in mononuclear firstrow transition-metal compounds can be predicted on the basis of the coordination and electronic structure of the metal center.39 The DMn value is calculated from the elements of the diagonalized D tensor (D = Dzz − (Dxx + Dyy)/2). From that equation, it is possible to obtain the sign of the DMn value, as it will depend on the relative values of the Dzz and (Dxx + Dyy)/2 terms. If the former term is larger, the DMn value is negative, while in the opposite case (|(Dxx + Dyy)/2 | > |Dzz|) the DMn value is positive. The analysis of the components of the tensor Dii showed that the principal contributions to Dzz came from the excitation between a pair of orbitals with the same |ml| value. For Dxx and Dyy, the principal contributions came from the excitation betweena pair of orbitals with ml changes of ±1. In addition, the Dii value is inversely proportional to the energy difference between the orbitals involved in the excitation. The structural distortion due to the Jahn−Teller effect in the studied complexes will affect the splitting of the 3d orbitals (Figure 6) and consequently the DMn value. For an elongated octahedron, as in compound 1, the empty orbital is dx2−y2, while for compound 2 it should be dz2. For 1, the first excitation involves the dxy and dx2−y2 orbitals, with the same ml value, giving to a larger Dzz contribution and consequently a negative DMn value. However, for 2, the first excitation involves the dxz or dyz and dz2 orbitals, with a difference in ml value of ±1, giving to larger Dxx and Dyy contributions and consequently a positive DMn value. In both cases the first excitation is expected to involve large excitation energies from the nonbonding (dxy or dxz or dyz) to antibonding (dx2−y2 or dz2) orbitals, giving relatively small DMn values. However, it should be kept in mind that the relative energy of those orbitals can change depending

phenyl ring toward the carboxylate group (ω), and the relative disposition of the coordination octahedra (τ). Compound 1 shows a moderate Δ value, a nonrelevant ω angle, due to the substituent in a para position, and a small τ angle (83.8°). The relative disposition of the coordination octahedra (τ) for these kinds of compounds range from 73.3 to 120°.9,37 In general, compounds with small τ angle show antiferromagnetic behavior. Therefore, the weak magnetic behavior can be explained by the competition of both factors: the significant degree of rhombicity (Δ/ρ = 2.1) that promotes the ferromagnetic behavior and the small τ angle that works in the opposite sense. Theoretical Calculations. With the aim of finding a relationship between the structural parameters and the magnetic behavior of 1 and 2, two kinds of calculations were carried out. For the calculation of the ZFS parameter a multiconfigurational methodology was employed (CASSCF +RASSI), which has been used satisfactorily to obtain ZFS parameters in other MnIII complexes.16 Each MnIII was calculated independently, replacing the other MnIII ion with a GaIII ion and preserving the structural data of the whole complex. On the other hand, the magnetic coupling constants (J) were obtained using a procedure previously described in the literature (see Computational Details).26−28 The results of these calculations are collected in Table 4 together with the values obtained from the fit of the experimental data. The calculated ZFS parameters of each ion in these compounds corroborate the values obtained from the experimental fit, giving DMn < 0 for compound 1 and DMn > 0 for compound 2. It is worth noting that, for compound 2, the DMn values of both ions have a positive sign, indicative of a compressed octahedron. Moreover, the |E/D| ratio for Mn1 is 0.19 while that for Mn2 is 0.25, in agreement with the major rhombicity found for Mn2. For compound 1 the |E/D| ratios obtained from the calculated values are greater than the ratios obtained with the parameters obtained from the fit, in fact being close to the complete rhombicity (1/3). For these larger G


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Inorganic Chemistry

Figure 7. Principal axes of the Dii components of the D tensor for 1 (left) and 2 (right). Dxx, Dyy, and Dzz are represented with blue, green, and yellow arrows, respectively. Manganese, bromide, oxygen, nitrogen, and carbon atoms are represented in pink, yellow, red, blue, and gray, respectively.

on the coordinated ligands and the distortion from the octahedron. The calculated and experimental DMn values are in concordance with this qualitative model, with relatively small DMn values (around 3 cm−1) being negative for 1 and positive for 2. Figure 7 shows the orientation of the Dii components of the D tensor for 1 and 2. It can be observed that the anisotropy tensors are almost aligned along the metal−ligand bonds. For 1, the Dzz components are aligned with the Mn−monodentate ligand bonds, while the Dxx components are aligned with the Mn−oxo bonds. However, for 2 the Dzz components are aligned with the Mn−oxo bonds and the Dxx components with the Mn−monodentate ligand bonds. These results indicate that in compound 1 both MnIII ions have uniaxial anisotropy with the easy axis oriented almost in the Mn−monodentate ligand bonds. In the case of compound 2, both MnIII ions are in the easy plane with the easy plane perpendicular to the Mn−oxo bonds. Furthermore, these results corroborate that the geometry of the Mn2 in compound 2 can be better assigned as a compressed octahedron with major rhombicity, according to the second assignment performed in the description of structures (x in the monodentate ligand direction and z in the oxo bridge direction). Additionally, the angles between the Dzz components of the D tensors of Mn1 and Mn2 are 91° for compound 1 and 130° for 2, in agreement with the value of the Euler angle β considered in the fit of the experimental data, although the value is slightly larger for 2. The difference in the orientation of the axes between compounds 1 and 2 will affect the different coupling pathways through the bridging ligands. The magnitude and sign of the J values obtained from DFT calculations agree with those found from the fit of the experimental data, corroborating the obtained exchange coupling constants: antiferromagnetic for 1, with elongated geometry, and ferromagnetic for 2, with compressed geometry. The qualitative analysis of the exchange pathways shows similar π interactions through the μ-O bridge in both complexes (Figure 8a). Although there is a different disposition of the axes, the most important antiferromagnetic contribution is common in both complexes: for 1 it is the dxz(Mn1)/dxy(Mn2) interaction, while for 2 it is the dxz(Mn1)/dyz(Mn2) interaction. The other common π interaction, in the MnObMn plane

Figure 8. Schematic representation of the most relevant interaction pathways for compounds 1 and 2, taking in consideration the axes disposition shown in Figure 3.

(Figure 8b), is the dxy(Mn1)/dxz(Mn2) interaction for 1 and dyz(Mn1)/dxz(Mn2) interaction for 2, which is sensitive to the Mn−Ob−Mn angle. However, there are three interactions that are only present in 1 due to the different arrangements of the magnetic axes: the dz2(Mn1)/dz2(Mn2) interaction (Figure 8c), which will depend on the angle, the dz2(Mn1)/dxz(Mn2) interaction (Figure 8d), and the dxy(Mn1)/dz2(Mn2) (Figure 8e) interaction. The three of them are expected to be antiferromagnetic. It has been previously observed that the exchange coupling constants are very sensitive to small changes in the geometry, and in this case, they are particularly sensitive to the angle between both Mn ions through the oxo bridging ligand due to the contribution of the μ-O bridge to the exchange pathways. To gain more insight into this, we have modified the Mn−Ob− H


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Inorganic Chemistry

S2 in the Supporting Information). Although 1 is antiferromagnetic, having a smaller angle, and 2 is ferromagnetic, having a larger angle, in both cases the same tendency is observed. When α is increased, the magnetic coupling becomes more antiferromagnetic (or less ferromagnetic). In fact, for the models derived from compound 1 when α is smaller than 121° a ferromagnetic coupling is obtained. A similar tendency was reported for dinuclear compounds with compressed octahedra and tridentate blocking ligands.18 However, although the variations in the Mn−Ob−Mn angle clearly affect the magnetic behavior and the observed tendency is the same for the models derived from both complexes (more antiferromagnetic when the angle increases), in the case of compound 2, and models derived from it, a larger ferromagnetic behavior in comparison to that in 1 is always observed. As described before in the qualitative analysis of the exchange pathways, this difference can be due to the presence of three extra antiferromagnetic interactions in 1 through the μ-O bridge. Nevertheless, it can also be related to the different geometry (elongation versus compression) and distortion from the ideal octahedron. To better understand how that will affect the exchange coupling, two different models have been created, changing the distance between the Mn ions and the monodentate ligand or the phen ligand (Figure 9). These two ligands have been chosen because they are not bridging ligands between both Mn ions (we are not altering the coupling pathway), and the modification of these Mn−ligand distances allows us to change the elongation/compression and rhombicity of the Mn ions. The Mn−monodentate ligand bond distance (dMn−L) in these types of compounds usually varies between 2.10 and 2.48 Å, being for the complexes studied here between 2.13 and 2.23 Å. We have chosen to vary this distance between 2.1 and 2.3 Å (see Tables S3 and S4 in the Supporting Information). An increase in dMn−L will enlarge the absolute value of the elongation parameter (Δ), while the rhombicity parameter (ρ) will be constant for 1 but not for 2 because of the different orientation of the axes. As was explained in the preceding section, the Δ parameter represents the elongation (Δ > 0) or compression (Δ < 0) of the octahedron and major |Δ| values mean that the coordination octahedron is further from the ideal geometry, independently of the orientation axes. As can be seen in Figure 11, in both cases when dMn−L increases (|Δ| increases), the coupling constant becomes more antiferromagnetic (less ferromagnetic). In the case of 1, for distances below

Mn angle (α) by moving the oxo bridge (Ob) atom along the axis formed by the O atom and the middle point between both Mn ions, the pseudo-C2 axis of the Mn2O core (dotted red line in Figure 9). In these kinds of dinuclear compounds, α usually

Figure 9. Schematic structure of the MnIII dinuclear complexes showing the geometric variations performed for the creation of the different models. The change in α has been done by moving Ob along the pseudo-C2 axis of the Mn2O core (red dotted line), the variation of dMn−L has been performed by moving the monodentate ligand along the Mn−L bond (blue dotted lines), and the change in dMn−Nt has been achieved by moving the phen ligand along the Mn−Nt bond (green dotted lines).

varies between 120 and 125°, this range being the one selected for the study. Moreover, the effect of the elongation or compression along the monodentate ligand direction and along the Obridge−Ntrans direction has been analyzed (Figure 9). Figure 10 shows the dependence of the calculated J value on the variation of the Mn−Ob−Mn angle (see also Tables S1 and

Figure 10. Dependence of the calculated J values for the different created models on the Mn−Ob−Mn angle. The square represents the calculated J value for the experimental geometry.

Figure 11. Dependence of the calculated J value on the average Δ parameter, in absolute value, for the different models created for compounds 1 (left) and 2 (right). Circles represent when the Mn−Ob−Mn angle (α) is changed, triangles represent the model where the Mn−monodentate ligand distance (dMn−L) is modified, and diamonds represent the model for which the Mn−Ntrans distance (dMn−Nt) is modified. I


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Inorganic Chemistry 2.13 Å (|Δ| ≤ 8%) the J values become positive (ferromagnetic coupling). For the model achieved moving the phen ligand, we have modified the distance between the Mn ions and the nitrogen atom trans to the oxo ligand (dMn−Nt; dotted green lines in Figure 9). We have kept the phen ligand unaltered, so when dMn−Nt changes it translates all of the phen ligand and consequently it modifies the distance between the Mn and the nitrogen atom cis to the oxo ligand (Mn−Nc). Typical Mn−N distances oscillate between 2.02 and 2.1 Å, an interval that we have chosen to vary dMn−Nt (see Tables S5 and S6 in the Supporting Information). An increase in this distance decreases the |Δ|and |ρ| parameters. As shown in Figure 11, the decrease in dMn‑Nt (|Δ| increase) makes the coupling more antiferromagnetic (less ferromagnetic). For all the created models the elongation and rhombocity parameters have been obtained using the same axis assignment as the described in the text for the original compound used to create the model. For compound 1 (z axis in the Mn−L direction) all of the created models are better described by these axes. However, for the models derived from compound 2, the axis assignment of 2 (z axis in the Mn−Ob direction) is correct for all of them, except for the model with the largest Mn−L distance (model 2-L7 in the Supporting Information), where the Δ/ρ ratio is larger for z in the Mn−L direction than in the Mn−Ob direction (Tables S9−S12 in the Supporting Information). By comparison of the obtained J values for the three models, it can be concluded that the structural parameter that has a stronger effect on the exchange coupling constant is α, the Mn−Ob−Mn angle. This was expected because the O atom is bridging both Mn atoms and we are directly altering the coupling pathway. In all the cases the increase in the |Δ| parameter makes the coupling more antiferromagnetic (less ferromagnetic). Therefore, in both cases the coupling is more antiferromagnetic when the Mn coordination environment is more distorted: more elongated for compound 1 and more compressed for 2. The modification of the Mn−ligand bond distances, dMn‑‑L or dMn−Nt, leads to a smaller variation in the magnetic coupling and very similar trends in all the cases, even when the variations in the rhombicity parameter (ρ) are different in each case. This indicates that the elongation/ compression is still affecting the magnetic coupling, even when the geometry of the ligands involved in the coupling pathway is unaltered, and the greater importance of the Δ parameter in the coupling constant in comparison with the ρ parameter.

example of a MnIII dinuclear compound with a capping bidentate ligand and compressed distortion inside a family of more than 25 compounds. The presence of a monodentate ligand in [{Mn(L)(NN)}2(μ-O)(μ-n-RC6H4COO)2]n+ compounds provides major flexibility to the coordination octahedra and, as experimentally observed, it usually favors the elongation distortion. However, 2 is a rare exception in this family. In fact, the synthesis and crystallization of compound 2 were unfavorable using traditional methods and they were only achieved using a synthetic strategy based on microwave radiation. Theoretical calculations corroborate the experimental findings. In the case of Mn2 in compound 2, it also shows larger |E/ D| ratio, which agrees with the major rhombicity found experimentally and helps in the correct assignment of the axes. The different exchange couplings observed for these compounds were also well reproduced. In addition, magnetostructural correlations show that the most important structural parameter studied is, as expected, the Mn−Ob−Mn angle. Moreover, they also show that, although the bridging ligands are unaltered, the increase in the |Δ| parameter (meaning a geometry further away from the ideal one) gives more antiferromagnetic character to the magnetic coupling and that the effect of the rhombicity parameter (ρ) is smaller in comparison. Although more examples and more studies of compressed systems are needed to further elucidate when this kind of geometry will be obtained and which geometric parameters will control their magnetic properties, the successful synthesis of compounds 1 and 2 provided us with the rare opportunity to study the magnetostructural correlations in these very similar, and at the same time very different, compounds. Despite the fact that they present distinct magnetic behavior, the magnetostructural correlations show that they follow the same trends.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00877. Details of the crystallographic data collection and refinement for compounds 1 and 2, details of the models used for the calculations, information about the hydrogen bonds present in the crystal structures of 1 and 2, distortion parameters for compound 2 with the axes of Figure 2, and distortion parameters for the differents models with both assignment axes (PDF)

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Accession Codes

CONCLUSIONS Two MnIII dinuclear compounds with 4-methylbenzoato (1) and 2-bromobenzoato (2) bridges and different counteranions (ClO4− for 1 and NO3− for 2) have been obtained in a crystalline form. Their structural data reveal significant differences in the shape of the coordination octahedron around the MnIII ions in both compounds and different dispositions of the Jahn−Teller axes. For 1 the elongation axis points to the mondentate ligand, while for 2 the coordination octahedron is compressed in the oxo-bridge direction. These factors affect the magnetic interaction and the zero-field splitting parameters of the MnIII ions (DMn and EMn). Compound 1 shows a ground state S = 0 and DMn < 0, while compound 2 shows a ground state S = 4 and DMn > 0. Remarkably, 2 is only the second

CCDC 1535702−1535703 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for S.G.-C: [email protected]. *E-mail for M.C.: [email protected]. ORCID

Silvia Gómez-Coca: 0000-0002-2299-4697 Eliseo Ruiz: 0000-0001-9097-8499 J


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(12) Corbella, M.; Fernandez, G.; Gonzalez, P.; Maestro, M.; FontBardia, M.; Stoeckli-Evans, H. Dinuclear MnIII Compounds [{Mn(bpy)(H2O)}2(μ-4-RC6H4COO)2(μ-O)](NO3)2 (R = Me, F, CF3, MeO, tBu): Effect of the R Group on the Magnetic Properties and the Catalase Activity. Eur. J. Inorg. Chem. 2012, 2012 (13), 2203−2212. (13) Berg, N.; Rajeshkumar, T.; Taylor, S. M.; Brechin, E. K.; Rajaraman, G.; Jones, L. F. What Controls the Magnetic Interaction in Bis-μ-Alkoxo MnIII Dimers? A Combined Experimental and Theoretical Exploration. Chem. - Eur. J. 2012, 18 (19), 5906−5918. (14) Barros, W. P.; Inglis, R.; Nichol, G. S.; Rajeshkumar, T.; Rajaraman, G.; Piligkos, S.; Stumpf, H. O.; Brechin, E. K. From Antiferromagnetic to Ferromagnetic Exchange in a Family of OximeBased MnIII Dimers: A Magneto-Structural Study. Dalt. Trans. 2013, 42 (47), 16510. (15) Comar, P.; Rajeshkumar, T.; Nichol, G. S.; Pitak, M. B.; Coles, S. J.; Rajaraman, G.; Brechin, E. K. Switching the Orientation of JahnTeller Axes in Oxime-Based MnIII Dimers and Its Effect upon Magnetic Exchange: A Combined Experimental and Theoretical Study. Dalt. Trans. 2015, 44 (46), 19805−19811. (16) Duboc, C. Determination and Prediction of the Magnetic Anisotropy of Mn Ions. Chem. Soc. Rev. 2016, 45, 5834−5847. (17) Duboc, C.; Ganyushin, D.; Sivalingam, K.; Collomb, M.-N.; Neese, F. Systematic Theoretical Study of the Zero-Field Splitting in Coordination Complexes of Mn(III). Density Functional Theory versus Multireference Wave Function Approaches. J. Phys. Chem. A 2010, 114 (39), 10750−10758. (18) Retegan, M.; Collomb, M.-N.; Neese, F.; Duboc, C. A Combined High-Field EPR and Quantum Chemical Study on a Weakly Ferromagnetically Coupled Dinuclear Mn(III) Complex. A Complete Analysis of the EPR Spectrum beyond the Strong Coupling Limit. Phys. Chem. Chem. Phys. 2013, 15 (1), 223−234. (19) Sala, T.; Sargent, M. V. Tetrabutylammonium Permanganate: An Efficient Oxidant for Organic Substrates. J. Chem. Soc., Chem. Commun. 1978, 253. (20) Sheldrick, G. XT, Program for the Solution of Crystal Structures; Bruker AXS Inc., Madison, WI. (21) Sheldrick, G. M. SHELXL Program for Small-Molecule Struture Refinement. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, 64, 112−122. (22) Sheldrick, G. M. SADABS Version 2008/1; Bruker AXS Inc., Madison, WI. (23) Aquilante, F.; De Vico, L.; Ferre, N.; Ghigo, G.; Malmqvist, P.A.; Neogrady, P.; Pedersen, T. B.; Pitonak, M.; Reiher, M.; Roos, B. O.; Serrano-Andres, L.; Urban, M.; Veryazov, V.; Lindh, R. MOLCAS 7: The next Generation. J. Comput. Chem. 2010, 31 (1), 224−247. (24) Maurice, R.; Guihéry, N.; Bastardis, R.; De Graaf, C. Rigorous Extraction of the Anisotropic Multispin Hamiltonian in Bimetallic Complexes from the Exact Electronic Hamiltonian. J. Chem. Theory Comput. 2010, 6 (1), 55−65. (25) Chibotaru, L. F.; Ungur, L. Ab initio calculation of anisotropic magnetic properties of complexes. I. Unique definition of pseudospin Hamiltonians and their derivation. J. Chem. Phys. 2012, 137, 064112. (26) Ruiz, E.; Alemany, P.; Alvarez, S.; Cano, J. Toward the Prediction of Magnetic Coupling in Molecular Systems: Hydroxo-and Alkoxo-Bridged Cu (Ii) Binuclear Complexes. J. Am. Chem. Soc. 1997, 119 (6), 1297−1303. (27) Ruiz, E.; Cano, J.; Alvarez, S. Density Functional Study of Exchange Coupling Constants in Single-Molecule Magnets: The Fe8 Complex. Chem. - Eur. J. 2005, 11 (16), 4767−4771. (28) Ruiz, E. Theoretical Study of the Exchange Coupling in Large Polynuclear Transition Metal Complexes Using DFT Methods. Struct. Bonding (Berlin, Ger.) 2004, 113, 71−102. (29) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin,

Montserrat Corbella: 0000-0002-7281-7576 Present Address ⊥

Department of Chemistry, King’s College London, London SE1 1DB, United Kingdom. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Ministerio de Economia y Competitividad of Spain (projects CTQ2012-30662, CTQ201563614-P, and CTQ2015-64579-C3-1-P). Allocation of computer facilities at CSUC is also acknowledged. E.R. thanks the Generalitat de Catalunya for an ICREA Academia grant.

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