Synthesis, Crystal Structures and Magnetic Properties of Mixed-Valent Tetranuclear Complexes with Y-Shaped MnII2MnIII2 Core

Tetranuclear MnII2MnIII2 complexes with 1,3-bis(5-bromo-3-metoxysalicylidenaminomethyl)-2propanol (H3bmsap) and 1,3-bis(5-chloro-3-methoxysalicylidenaminomethyl)-2-propanol (H3cmsap), [Mn4(bmsap)2(CH3CO2)3(CH3O)] (3) and [Mn4(cmsap)2(CH3CO2)3(CH3O)] (4), were synthesized and characterized by elemental analysis, infrared and diffused reflectance spectra and variable-temperature magnetic susceptibility measurements in the 2–300 K range. The crystal structures of 3 and 4 revealed a Y-shaped tetranuclear manganese cluster formed by the two Schiffbase ligands, three kinds of acetato ligands (bidentate, syn–anti-bridging, and syn–syn-bridging), and μ-methoxido ligand. The magnetic data showed the magnetic interactions among the four manganese atoms are antiferromagnetic as a whole within the tetranuclear cluster.


Electronic Spectra of Tetranuclear Manganese Complexes
The diffused reflectance spectra of the present complexes (Figure S1) may be characterized as broad spectral features with some peaks in the UV-vis-NIR region, as previously reported for 1 and 2 [28].The LMCT (Ligand-to-Metal Charge Transfer) bands may be responsible for the high-intensity absorptions in the UV region.In particular, the absorption band at the near-UV region (434sh nm for 3; 442sh for 4) can be assumed as a characteristic LMCT band of phenolato-oxygen to manganese(III) as found in some Schiff-base manganese(III) complexes [24,35].Furthermore, d-d bands due to sixor five-coordinated manganese(III) ions appeared and hid behind the strong near-UV bands in the visible and NIR regions until around 1000 nm [35].

Crystal Structures of Tetranuclear Manganese Complexes
Single crystals suitable for X-ray diffraction work were obtained for complexes 3 and 4, although the crystals of the latter complex were twinned.Crystal data and refinement parameters are given in Table 1.Selected bond lengths and angles are given in Tables 2 and 3.An ORTEP (Oak Ridge Thermal Ellipsoid Plot) drawing of the molecular structure of 3 is shown in Figure 3.One Schiff-base bmsap 3- ligand binds the Mn1, Mn2, and Mn3 atoms by the O1, N1, and O3 atoms, the O4, N2, and O3 atoms, and the O3 atom, respectively, with a folding structure of the Schiff-base ligand.On the other hand, the other bmsap 3− ligand binds the Mn1, Mn3, and Mn4 atoms by the O6, N3, and O8 atoms, the O8, N4, and O9 atoms, and the O9 and O10 atoms, respectively.The Mn1 atom is six-coordinated by the N2O4 donor set from the two Schiff-base ligands in an elongated octahedral geometry with the longer O1-Mn1-O3 axis.The Mn2 atom is five-coordinated by the NO4 donor set from one Schiff-base, one methoxido-oxygen, and one acetato-oxygen atom in a distorted square-pyramidal geometry, having the  value of 0.217 around the Mn2 atom [36].The Mn3 atom is six-coordinated by the NO5 donor set from two Schiff-base, one methoxido-oxygen atom, and one acetato-oxygen atom in a distorted octahedral geometry with relatively long bond distances.The Mn4 atom is also six-coordinated by the O6 donor set from one Schiff-base and four acetato-oxygen atoms in a distorted octahedron with longer Mn-O bonds.The charge balance and bond parameters around the manganese centers suggested the Mn1 and Mn2 atoms should be in a Mn(III) oxidation state, whereas the Mn 3 and Mn4 atoms should be in a Mn(II) oxidation state.The bond valence sum calculation supported this mixedvalent formula [37,38].The four manganese atoms are located at the corners of a Y-shaped core with distances between the four manganese atoms of 3.530(2) Å for Mn1•••Mn2, 3.237(1) Å for Mn2•••Mn3, 3.466(2) Å for Mn1•••Mn3, and 3.494(2) Å for Mn3•••Mn4, respectively, where the Mn1 and Mn2 atoms are bridged by the µ3-O3 alkoxido-oxygen atom of the Schiff-base ligand, the Mn1 and Mn3 atoms are bridged by the µ3-O3 alkoxido-oxygen atom and µ-O8 alkoxido-oxygen atom of the Schiff-base ligand, the Mn2 and Mn3 atoms are bridged by the µ3-O3 alkoxido-oxygen atom and methoxido-oxygen O17 atom, the Mn2 and Mn4 atoms are bridged by the acetato ligand, and the Mn3 and Mn4 atoms are bridged by the phenoxido-oxygen O9 atom and acetato ligand.It is to be noted that three kinds of coordination modes of acetato ligands, syn-syn-bridging, syn-anti-bridging, and bidentate, are found in the same molecule.A similar feature was observed in the hexanuclear copper(II) complex with 1,4,8,11-tetrakis(3methoxysalicylideneaminoethyl)-1,4,8,11-tetraazacycloteradecane (H4tmsaec), [Cu6(CH3CO2)8(tmsaec)], where monodentate, syn-syn-bridging, and bidentate modes were found for the acetato ligands [34].The molecular structure is similar to that of 1, but there is no coordinating water molecule nor monodentate acetato-ligand here.This structure is almost the same as that of 2. A packing diagram of 3 is shown in Figure 4.The tetranuclear molecules are separated each other in the crystal.

Magnetic Data of Tetranuclear Manganese Complexes
The magnetic properties for the tetranuclear manganese complexes 3 and 4 are displayed in Figure 7 as the temperature variations of MT in the temperature range of 2-300 K.The effective magnetic moments of 3 and 4 are 9.77 µB and 10.17 µB, respectively, per Mn II 2Mn III 2 unit at 300 K, and a little lower than the theoretical value at room temperature.The calculated spin-only value is 10.86 µB for non-interacting two S = 5/2 spins and two S = 2 spins.When cooling, the magnetic moments of 3 and 4 steadily decrease from 300 K to around 100 K, and then abruptly diminish to a value of approximately 3.13 µB and 1.91 µB at 2 K, respectively.Overall, each tetranuclear manganese complex including 1 and 2 shows a similar pattern of magnetic moment decreasing with the lowering of temperature, suggesting that the magnetic coupling is antiferromagnetic as a whole.Therefore, the magnetic properties of the present complexes were described by the following model, taking account of the magnetic exchange interactions: J1 (Mn III -Mn III bridged by µ3-alkoxido-oxygen), J2 (Mn III -Mn II bridged by µ3-alkoxido-oxygen and µ-methoxido-oxygen), J3 (Mn III -Mn II bridged by µ3-alkoxidooxygen and µ-alkoxido-oxygen), J4 (Mn II -Mn II bridged by µ-phenoxido-oxygen and µ-acetato), J5 (Mn III -Mn II bridged by µ-acetato), and J6 (Mn III -Mn II without bridge) as shown in Figure 8.To determine the J1, J2, J3, J4, J5, and J6 values, the MT versus T data for 1, 2, 3, and 4, were fit to the theoretical expression based on the isotropic Heisenberg spin model given by Equation 1, using the program PHI [39].Mn III Mn III Mn II Mn II H = −2J1S(Mn III )•S(Mn III ) − 2J2S(Mn III )•S(Mn II ) − 2J3S(Mn III )•S(Mn II ) − 2J4S(Mn II )•S(Mn II ) − 2J5S(Mn III )•S(Mn II ) − 2J6S(Mn III )•S(Mn II ) ( In order to avoid overparameterization, the g values of the four manganese atoms were fixed to be 2.00 and J6 was set to be 0 cm -1 because of there being no intervening bridging group between these Mn atoms.Good fits were obtained for all four complexes, and the results are shown as solid lines in Figure 7.The fitting parameters are listed in Table 4.All of the J values except for J5 are negative, in accord with overall antiferromagnetic couplings in 1, 2, 3, and 4. Larger −J1 values are understandable, because antiferromagnetic coupling between Mn III and Mn III is usually stronger than that between Mn II and Mn III or Mn II [17,19].A ferromagnetic coupling was observed for J5.This may come from magnetic interaction via the syn-anti-bridging acetato ligand.-7.43 cm -1 -10.78 cm -1 -5.36 cm -1 J2 -1.77 cm -1 -2.56 cm -1 -7.01 cm -1 -2.30 cm -1 J3 -0.75 cm -1 -2.63 cm -1 -5.19 cm -1 -2.99 cm -1 J4 -2.20 cm -1 -2.07 cm -1 -2.14 cm -1 -3.29 cm -1 J5 7.84 cm -1 2.33 cm -1 0.61 cm -1 1.69 cm -1 J6 0 cm -1 0 cm -1 0 cm -1 0 cm −1

Materials and Methods
All the chemicals were commercial products and were used as supplied.The Schiff-base ligands H3bmsap and H3cmsp were prepared according to a method reported for H3msap in the literature [28].
Analyses of C, H, and N of the ligands and complexes were carried out with a Thermo-Finnigan FLASH EA1112 series CHNO-S analyzer (Thermo-Finnigan, USA).Infrared spectra were recorded on KBr pellets in the range 4000-600 cm -1 , with a JASCO MFT-2000 FT-IR Spectrophotometer (JASCO, Japan).Diffused reflectance spectrum was taken with a Shimadzu UV-vis-NIR Spectrophotometer Model UV-3100 (Shimadzu, Japan) in the range 200-1500 nm.Variable-temperature magnetic data (2-300 K) were obtained using a Quantum Design MPMS-XL7 SQUID magnetometer (Quantum Design, USA).
The crystal data of 3 and 4 were collected with a Bruker CCD diffractometer Smart APEX (Bruker, USA) fitted with Mo Kα radiation and a graphite monochromator.The structure of 3 was solved by an intrinsic phasing method, and refined by full-matrix least-squares methods.The structure of 4 was solved by an intrinsic method as a 2-component twin with only the nonoverlapping reflections of component 1.The structure was refined using the hklf 5 routine with all reflections of component 1 (including the overlapping ones).The hydrogen atoms were included in idealized positions based on a riding model.All calculations were performed using the SHELXT-2014/4 and SHELXTL-2014/7 [40,41].The CCDC numbers of 3 and 4 are 1887620 and 1887482, respectively.

Conclusions
In this study, two tetranuclear manganese complexes, [Mn4(bmsap)2(CH3CO2)3(CH3O)] (3) and [Mn4(cmsap)2(CH3CO2)3(CH3O)] (4), were prepared in a satisfactory yield as well as characterized.As we expected, the bromo-and chloro-substituent groups of the present ligands did not affect the tetranuclear features and the crystal structures of these complexes revealed that the present Schiffbase ligands also work with three kinds of acetato ligands (bidentate, syn-anti-bridging, and synsyn-bridging), and µ-methoxido ligand to construct a Y-shaped tetranuclear arrangement.The magnetic exchange interactions via the bridging ligands were found to be generally weak and mostly antiferromagnetic.

Figure 8 .
Figure 8. Magnetic coupling scheme in the present tetranuclear Mn II 2Mn III 2 complexes.

Table 1 .
Crystallographic data and refinement parameters of 3 and 4.