New Members of the Zig-Zag [MnIII 4] Clusters from Polydentate Schiff Base Ligands

Research Article 

New Members of the Zig-Zag [MnIII 4] Clusters from Polydentate Schiff Base Ligands

Corresponding author: Dr. Catherine P. Raptopoulou, Institute of Nanoscience and Nanotechnology, NCSR “Demokritos”, 15310 Aghia Paraskevi, Athens, Greece,Tel: +30 210 6503346; Fax: +30 210 6519430;E-mail: c.raptopoulou@inn.demokritos.gr

Abstract
New MnIII MnIII members of the family of zig-zag [MnIII 4] clusters are reported. Compounds [MnIII4(L1)2(HL1)2(H2O)2(MeOH)2](Br)2 (1) and [MnIII 4(L2)2(HL2)2(H2O)2(MeOH)2](Br)2 (2) were synthesized by the reaction of MnBr2∙4H2O with the Schiff base ligands OHC 6H4-CH=NC(R)(CH2OH)2 (R = CH3, H3L1; R = C2H5, H3L2) in MeOH. The molecular structures of 1-2 consist of centrosymmetric tetranuclear cations and bromide anions. The metal topology of the cations is described as three corner sharing [MnIII 2(μ-OR)2] units in trans orientation. The ligands chelate around the metal ions via the deprotonated phenolato oxygen, the imino nitrogen and one of the alkoxo oxygen atoms; the latter is bridging two adjacent MnIII ions. Two of the Schiff base ligands are doubly deprotonated and the other two are triply deprotonated and connect two and three metal ions, respectively.

Complexes 1 and 2 are isostructural to [MnIII4(L1)2(HL1)2(H2O)2(MeOH)2](ClO4)2(3) and [MnIII 4(HL2)(L2)22(H2O)2(MeOH)2] (ClO4)2 (4) reported previously by us. Magnetic studies of 1-3 revealed the presence of dominant antiferromagnetic interactions with exchange parameters comparable to those found for 4.

Keywords: Manganese (III) Clusters; Magnetic Studies; Schiff Base Ligands; Crystal Structures

Introduction

TThe intensive study of manganese complexes originates from its vital role in more than twenty enzymes and proteins [1].The biological role of manganese has been well documented and the use of Mnx clusters as models for the active sites ofcatalases and oxygen evolving complex (OEC) in Photosystem II (PSII) has been explored in great detail. Also the unique magnetic properties realized in Mnx clusters with x ranging from2 to 84[2], such as single-molecule and single-chain magnetic behavior, and the observation of new phenomena such as slowrelaxation, large hysteresis and quantum tunneling of the magnetization, has enlarged the interest for such clusters, whichhave been proposed for potential applications in information storage, spintronics, quantum computing and low temperaturemagnetic refrigeration [3].

The particular case of [Mn4Ox] clusters has been widely studied as models for the active site of OEC in PSII [4] and also in terms of both their physical and model-functioning properties. Forexample, a [MnIVMnIII3] cluster was reported as model for the active site of OEC, and later proposed as a qubit for quantum computing [5]. The complexes that contain the {Mn4Ox} core can be divided into six classes according to the atom connectivityof their core (Scheme 1). Clusters with the {Mn4O6} core adopt the adamantine [6], square [7] and linear (2,2,2) topologies[8,9], those with the {Mn4O5} core show the basket[10] and linear (2,1,2) topologies [11], those with the {Mn4O4} core preferthe cubane [12] and linear (1,2,1) topologies[13] and those with the {Mn4O2} core possess the common butterfly topology[14]. Recently, we have reported new complexes containing the {Mn4O6} core adopting the linear (2,2,2) topology[15] withtrans arrangement of the metal ions based on the Schiff base ligands, H3L1 and H3L2 (Scheme 2), which, at that time, represented the first examples of {Mn4O6} complexes of any topology containing exclusively MnIII ions. A congener [MnIII4] complex was later reported based on a similar Schiff base ligand [16]. Besides our previous work, H3L1 has been used in several 3d metal ions coordination chemistry [17, 18, 19, 20], whereas H3L2 was totally ignored. Recently, the family of clusters with the {Mn4O6} core was enriched with one [MnII 2MnIII 2] [21] and two [MnIII 4] [22] complexes which contain the planar diamond or butterfly-like core (Scheme 1). The latter [MnIII 4] complexes display a marked difference in the orientation of the Jahn-Teller axes of the respective MnIII ions, which has been shown to be crucial in determining the nature and magnitude of the magnetic exchange interaction. Similar magneto-structural correlations concerning the orientation of Jahn-Teller axes in MnIII dimers have been also established, both experimentally and theoretically [23].

We present herein, new members of the family of [MnIII 4O6] complexes adopting the linear (2,2,2) topology with trans arrangement of the metal ions or zig-zag, i.e. clusters [MnIII 4(L1)2(HL1)2(H2O)2(MeOH)2](Br)2(1) and [MnIII 4(L2)2(HL2)2(H2O)2(MeOH)2](Br)2 (2) (OH-C6H4-CH=NC(R)(CH2OH)2, R = CH3, H3L1; R = C2H5, H3L2) and compare their structural characteristics and magnetic behavior  with previously reported congener clusters.

Scheme 1. {Mn4Ox} core topologies.

Scheme 2. The ligands H3L1, H3L2.

Experimental

General and spectroscopic measurements

All manipulations were performed under aerobic conditions using materials as received (Aldrich Co). All chemicals and solvents were of reagent grade. The Schiff base ligands H3L1 and H3L2 and complexes 3 and 4 were prepared as reported  previously [15]. Elemental analysis for carbon, hydrogen, and nitrogen was performed on a Perkin Elmer 2400/II automatic analyzer. Infrared spectra were recorded as KBr pellets in the range 4000 – 400 cm-1 on a Bruker Equinox 55/S FT-IR spectrophotometer. Variable-temperature magnetic susceptibility and field dependent magnetization measurements were carried out on polycrystalline sample 1 by using a SQUID magnetometer (Quantum Design, MPMS 5.5) and of 2 and 3 by using the extraction method of ACMS option of the Quantum Design PPMS 9T. Diamagnetic corrections were estimated from Pascal’s constants. The program PHI was use to fit the magnetic data [24].

Compound preparations

Synthesis of [Mn4(L1)2(HL1)2(H2O)2(MeOH)2](Br)2 (1). Solid H3L1 (1.00 mmol, 0.209 g) was added under stirring to
a colorless solution of MnBr2∙4H2O (1.00 mmol, 0.287 g) in MeOH (30 mL). The resulting solution was stirred for 2 hrs to obtain a brown solution. The final reaction solution was placed in a tube, layered with mixture of Et2O/n-hexane (20 mL, 1:1 v/v), sealed with a cork stopper and covered with parafilm to prevent evaporation. The two phases were mixed very slowly and the first crystals of 1 appeared in the interface between the two phases. After 2-3 days, the mixing of the two phases was complete and more crystals appeared in the solution. After almost 20 days the solution was almost colorless suggesting that the precipitation of 1 was complete. The crystals were filtered off and dried in air. (Yield: 0.18 g, ~55% based on the metal). C46H62Br2Mn4N4O16 requires: C, 42.29; H, 4.78; N, 4.29 %. Found: C, 42.16; H, 4.73; N, 4.25 %. FT-IR (KBr pellets, cm- 1): ν(OH), 3400(br, s); ν(C=N), 1610(vs); ν(C…O), 1540(s); ν(C…C), 1600-1400; δ(C-H), 760(s).

Synthesis of [Mn4(L2)2(HL2)2(MeOH)4](Br)2∙0.9H2(2∙0.9H2O). Solid H3L2 (1.00 mmol, 0.223 g) was added under stirring to a colorless solution of MnBr2∙4H2O (1.00 mmol, 0.287 g) in MeOH (30 mL). The resulting solution was stirred for 2 hrs to obtain a brown solution. The final reaction solution was placed in a tube, layered with mixture of Et2O/n-hexane (20 mL, 1:1 v/v), sealed with a cork stopper and covered with parafilm to prevent evaporation. The two phases were mixed very slowly and the first crystals of 2 appeared in the interface between the two phases. After 2-3 days, the mixing of the  two phases was complete and more crystals appeared in the solution. After almost 20 days the solution was almost colorless suggesting that the precipitation of 2 was complete. The crystals were filtered off and dried in air. (Yield: 0.19 g, ~56% based on the metal). C50H70Br2Mn4N4O16 requires: C, 44.07; H, 5.18; N, 4.11 %. Found: C, 43.89; H, 5.15; N, 4.08 %. FT-IR (KBr  pellets, cm-1): ν(OH), 3410(br, s); ν(C=N), 1609(vs); ν(C…O),1541(s); ν(C…C), 1600-1400; δ(C-H), 762(s).

Single crystal X-ray crystallography

A crystal of 1 (0.15 x 0.30 x 0.40 mm) and a crystal of 2∙0.9H2O (0.05 x 0.20 x 0.20 mm) were taken from the mother liquor and immediately cooled to -113 oC. Diffraction measurements were made on a Rigaku R-AXIS SPIDER Image Plate diffractometer using graphite monochromated Cu Kα radiation. Data collection (ω-scans) and processing (cell refinement, data reduction and empirical absorption correction) were performed using the CrystalClear program package[25]. The structures were solved by direct methods using SHELXS-97 and refined by full-matrix least-squares techniques on F2 with SHELXL-97[26]. Important crystallographic and refinement data are listed in Table 1. Further experimental crystallographic details for 1: 2θmax = 130°; reflections collected/unique/used, 17890/4455 [Rint = 0.0580]/4455; 339 parameters refined; (Δ/σ)max = 0.002; (Δρ)max/( Δρ)min = 0.723/-1.142 e/Å3; R1/wR2 (for all data), 0.0821/0.1581. Further experimental crystallographic details for 2∙0.9H2O: 2θmax = 130°; reflections collected/unique/used, 18176/4573 [Rint = 0.1514]/4573; 357 parameters refined; (Δ/σ)max = 0.003; (Δρ)max/( Δρ)min = 1.240/-0.713 e/Å3; R1/wR2 (for all data), 0.1438/0.2196. Hydrogen atoms were either located by difference maps and were refined isotropically or were introduced at calculated positions as riding on bonded atoms. All non-hydrogen atoms were refined anisotropically, except of the solvate molecules in 2∙0.9H2O which were refined isotropically with partial occupancy. Plots of the structures were drawn using the Diamond 3 program package[27].

Results and Discussion

Synthesis and spectroscopic characterization

The synthesis of 1 and 2 was achieved from the equimolar reaction of MnBr2∙4H2O with the respective Schiff base ligand, H3L1 (1) and H3L2 (2), in MeOH solution. Complexes 1 and 2 contain MnIII ions as a result of the aerial oxidation of the MnII ions in the precursor metal salt. Crystals of 1 and 2 suitable for X-ray crystal structure determination were isolated upon layering of the brown reaction solution with mixture of Et2O/nhexane. These studies revealed that 1 and 2 consist of [MnIII 4]2+ cations and Br- counterions. The presence of bromide anions in the precursor metal salt has not affected the identity of the [MnIII 4]2+ cations which are similar to those reported earlier by us from the use of perchlorate salts[15]. The IR spectra of both complexes showed ν(OH) vibrations at ~3400 cm-1 and ν(C=N) and ν(C…O) vibrations of the Schiff base ligands at ~1610 and ~1540 cm-1, respectively. The binding of the imino nitrogen was confirmed by the lowering of the frequency (~25 cm-1) [28].

Description of the structures

Complexes 1 and 2 crystallize in the monoclinic space group P21/n and can be considered isomorphous given the dissimilar unit cell parameters (Table 1). The molecular structures of 1 and 2 are shown in Figure 1; selected bond distances and angles are listed in Table 2. The molecular structures of 1 and 2 consist of tetranuclear MnIII dications and bromine anions; the structure of 2 contains two partially occupied H2O solvate molecules which will not be discussed further. The tetranuclear cations [MnIII 4] in 1 and 2 reside on inversion center lying in the middle of Mn(2)∙∙∙Mn(2’) distance. The core of [MnIII 4] cations consist of three corner sharing [MnIII 2(μ-OR)2] units in trans orientation as imposed by the symmetry of the metal core. The interatomic Mn∙∙∙Mn distances within each [MnIII 2(μ-OR)2] unit are Mn(1)∙∙∙Mn(2’) = 3.052 and Mn(2)∙∙∙Mn(2’) = 3.029 Å in 1, and Mn(1)∙∙∙Mn(2’) = 3.056 and Mn(2)∙∙∙Mn(2’) = 3.057 Å in 2. The [MnIII 2(μ-OR)2] units are planar. The central [MnIII 2(μ-OR)2] unit is almost perpendicular to both terminal [MnIII 2(μ-OR)2] units forming dihedral angles 86.0o (1) and 89.4o (2).

The four metal ions in the [MnIII 4] cations of 1 and 2 present distorted octahedral coordination geometry with Jahn-Teller distortion which is indicative of the presence of MnIII ions. Each of the terminal MnIII ions (Mn(1) and Mn(1’)) in 1 and 2 is coordinated to a doubly deprotonated Schiff base ligand through the Nimino and the deprotonated Ophenolato and one of the Oalkoxo atoms; the latter also acts as bridge to the neighboring central metal ion Mn(2’) or Mn(2). The six-coordination is completed by a bridging deprotonated Oalkoxo belonging to a triply deprotonated Schiff base ligand and two neutral solvate molecules, H2O and MeOH, which occupy the axial Jahn-Teller positions of the octahedron. Each of the central metal ions (Mn(2) and Mn(2’) in 1 and 2 is coordinated to a triply deprotonated Schiff base ligand through the Nimino and the deprotonated Ophenolato and one of the Oalkoxo atoms, whereas six-coordi nation is completed by three bridging Oalkoxo atoms, two from a neighboring triply deprotonated ligand and one from a doubly deprotonated ligand. The apical Jahn-Teller positions of Mn(2) and Mn(2’) are occupied by Oalkoxo atoms from the neighboring triply and doubly deprotonated ligands.

The doubly deprotonated Schiff base ligands chelate around the terminal MnIII ions and also bridge the terminal with the central metal ions adopting the coordination mode μ-ĸ2O:ĸO’:ĸN. The second pendant alkoxo group of the ligand remains protonated and is directed away of the coordination sphere of the manganese atoms. The triply deprotonated Schiff base ligands chelate around the central MnIIIions and also bridge the two central and one of the terminal metal ions adopting the coordination mode μ3-ĸ2O:ĸ2O’:ĸO’’:ĸN.

The six-membered ring Mn-O-C-C-C-N in the coordination sphere of the terminal metal ions is planar in 1 and 2, whereas the five-membered ring Mn-O-C-C-N presents the stable envelope conformation with the methylene carbon atom located 0.52 Å (1) and 0.57 Å (2) above the mean plane of the remaining four atoms. Similarly the six-membered ring Mn-O-C-C-C-N in the coordination sphere of the central metal ions is planar in 1 and 2, and the five-membered ring Mn-O-C-C-N adopts the envelope configuration with the methylene carbon at ~0.55 Å (1, 2) out of the best mean plane of the four atoms. The second six-membered ring Mn-O-C-C-C-O in the coordination sphere of the central MnIII ions adopts the half-chair conformation with one of the methylene carbon atoms located ~0.63 Å (1, 2) above.

The Mn-O/N bond distances in the equatorial plane of the distorted ochtahedron for all metal ions in 1 and 2 fall in the range 1.852(6)-1.995(5) Å, whereas those involving the apical Jahn-Teller positions (O(1w), O(1m) for Mn(1) and O(2’), O(12’) for Mn(2) and their symmetric in 1 and 2) are substantially longer; they fall in the range 2.153(6)-2.288(6) Å.

The tetranuclear cations in 1 and 2 are linked through hydrogen bonds O(1w)-H(1wB)∙∙∙O(3) (-1-x, -y, 2-z) and form 1D chains which extend parallel to the crystallographic a-axis (Figure S1, Table S1). The bromide counterions are also held via hydrogen bonds O(3)-H(3O)∙∙∙Br (x, y, z).

Complexes 1 and 2 belong to a small family of [Mn4] clusters containing the {Mn4O6} linear (2,2,2) or zig-zag metal  arrangement, which can take the cis or the trans conformation. Alternatively, the topology of the metal core can be described as three corner sharing [Mn2(μ-OR)2] units. Complexes 1 and 2 are isostructural to [Mn4(L1)2(HL1)2(H2O)2(MeOH)2](ClO4)2 (3) and [Mn4(L2)2(HL2)2(MeOH)4](ClO4)2 (4) previously reported by us[15].

Figure 1. Partially labeled plots of the cations of 1 (a) and 2 (b). Hydrogen atoms have been omitted for clarity. Primed atoms are generated by symmetry: (’) -x, -y, 2-z.

Magnetic measurements

Magnetic susceptibility measurements from powder samples of 1 and 2 were carried out in the 2-300 K temperature range in the presence of a magnetic field of 1 T (1) or 0.1 T (2). We have previously reported complexes [Mn4( L 1 ) 2( H L 1 ) 2(H2O ) 2(MeOH)2] ( C lO4 ) 2 ( 3 ) a n d [Mn4(L2)2(HL2)2(MeOH)4](ClO4)2 (4); however only the magnetic behavior of 4 was reported[15]. We present herein the magnetic study of 3 and compare our findings from the study of 1-4 with those of similar complexes from the literature. Magnetic susceptibility studies from powder sample of 3 were carried out in the 2.5-300 K temperature range under a magnetic field of 1 T. At room temperature, the product of the molar magnetic susceptibility, χM, with temperature, T, χMT, is 11.35 cm3Kmol-1 for (1), 11.19 cm3Kmol-1 for (2) and 11.35 cm3Kmol-1 for (3). These values are in good agreement with the theoretically calculated value of 12.00 cm3Kmol-1 for four non-interacting MnIII ions (S = 2). When the temperature is lowered,  he χMT product of 1 is decreasing smoothly down to ~80 K and then starts to decrease rapidly reaching the value of 2.91 cm3Kmol-1 at 3.4 K (Figure 2). For 2, the χMT product decreases gradually upon lowering the temperature reaching a value of

Table 2. Selected bond distances (Å) and angles (o) in 1 and 2. Symmetry operation: (‘) -x, -y, 2-z (1, 2).

3.76 cm3Kmol-1 at 2 K (Figure 3). The value of χMT product of 3 remains practically unchanged down to ~90 K and then decreases upon lowering the temperature, reaching the value of 1.42 cm3Kmol-1 at 2.5 K (Figure 4). The shape of the χMT vs T curves of 1-3 shown in Figures 2-4 are indicative of dominant antiferromagnetic interactions between the metal ions. The field dependence of the magnetization, M, at 2 K for 1, 2 and 3 is shown in the inset of Figures 2-4, respectively. In all cases, the magnetization increases without reaching saturation up to 8.5 T (1), 8 T (2) and 9 T (3) which indicates that the ground state is not well isolated from other spin states, also populated at low temperatures. This behavior is further corroborated by the large value of χMT product at low temperatures.

Considering the metal topology in 1-4, the magnetic exchange occurs between the MnIII ions of the successive [MnIII 2(μ-OR)2] units and given their centrosymmetric structure, the two terminal [MnIII 2(μ-OR)2] units are equivalent. Therefore we consider the exchange parameters J1 and J2 to describe the magnetic coupling between the metal ions within the terminal and central rhombic units, respectively (Scheme 3).

The experimental χMT vs T and M vs H data for 1-3 were fitted according to the spin Hamiltonian (1)

in which J1 and J2 are the isotropic exchange parameters defined above, S1 = S2 = S3 = S4 = 2 and g =2.0 for the MnIII ions, H is the applied magnetic field and β is the Bohr magneton.

The best fit gave J1 = +1.3(2) cm-1 and J2 = -7.0(2) cm-1 for 1 (solid line in Figure 2 and inset), J1 = +0.6(2) cm-1 and J2 = -7.6(2) cm-1 for 2 (solid line in Figure 3 and inset) and J1 = +2.0(2) cm-1 and J2 = -6.8(2) cm-1 for 3 (solid line in Figure 4 and inset); in all cases g was fixed to 2.0. These values are in good agreement with those found in 4, J1 = +1.7 cm-1 and J2 = -7.5 cm-1 (g = 2.0 fixed) [15].

In this simple approach we have neglected zero-field splitting effects from the MnIII (S = 2) ions, or possible intermolecular interactions; nevertheless the experimental χMT vs T and M vs H data were successfully reproduced.

Figure 2. Temperature dependence of χMT from a powder sample of 1 in the presence of a magnetic field of 1 T. Inset: Field dependence of magnetization at 2.0 K. Solid lines are theoretical curves obtained as described in the text.

Figure 3. Temperature dependence of χMT from a powder sample of 2 in the presence of a magnetic field of 0.1 T. Inset: Field dependence of magnetization at 2.0 K. Solid lines are theoretical curves obtained as described in the text.

Figure 4. Temperature dependence of χMT from a powder sample of 3 in the presence of a magnetic field of 1 T. Inset: Field dependence of magnetization at 2.5 K. Solid lines are theoretical curves obtained as described in the text.

Scheme 3. The {Mn4O6} linear (2,2,2) or zig-zag metal arrangementwith trans conformation in 1-2 showing the exchange coupling between the metal ions. Jahn-Teller axes of each MnIII ion are shown asorange bonds.

The different nature of magnetic exchange, i.e. J1>0 for ferro- and J2<0 for antiferromagnetic coupling, can be explained based on magneto-structural correlations recently reported for bis-μ-alkoxo bridged MnIII dimers[23a] depending on the relative orientation of the Jahn-Teller axes within the dimer. Three different types were proposed concerning the relative orientation of the Jahn-Teller axes, i.e. type I exhibits co-linear orientation of the Jahn-Teller axes each of which are perpendicular to the bridging plane of the dimer, type II where the Jahn-Teller axes are parallel to each other as well as to the bridging plane, and type III where the Jahn-Teller axes are perpendicular to one another, one lies parallel to the bridging
plane and the second is perpendicular to it. According to experimental data and theoretical calculations it was suggested that type I complexes exhibit weak antiferromagnetic coupling with J values ranging from -8.2 cm-1 to -15.5 cm-1, type II complexes exhibit ferro- and/or antiferromagnetic coupling with J values in range -1.68 cm-1 to +6.3 cm-1, and type III complexes exhibit relatively strong ferromagnetic coupling due to the non-linearity of their respective elongated axes[23a]. As shown in Scheme 3, the terminal [MnIII 2(μ-OR)2] units are classified as type III, and the central [MnIII 2(μ-OR)2] units are classified as type II. Complexes 1-4 as well as the recently reported congener [MnIII 4(HL)2(H2L)2(MeOH)4](ClO4)2∙4MeOH (5) where H4L = 2-(((2-hydroxy-3-methoxyphenyl)methylene)amino)-2-(hydroxymethyl)- 1,3-propanediol[16], represent the only examples  of [Mn4] clusters with the {Mn4O6} linear (2,2,2) or zig-zag metal arrangement containing exclusively MnIII ions. In addition, 1-5 contain two terminal type III [MnIII 2 (μ-OR)2] units and one central type II [MnIII 2(μ-OR)2] unit. For 5, the magnetic studies revealed J1 = -1.46 cm-1 and J2 = -4.48 cm-1 with g = 2.11. Comparison of some geometrical characteristics of 1-4 and 5 with the J values are listed in Table 3, where Φ1 and Φ2 are the dihedral angles between the two Jahn-Teller axes in the terminal and central [MnIII 2(μ-OR)2] units (Scheme 4). Our findings for 1-4 concerning the sign and magnitude of J1 and J2 agree well with those observed experimentally and calculated theoretically for type II and type III bis-μ-alkoxo bridged [MnIII 2] dimers[23a]. For the latter, only two examples are reported so far, i.e. [Mn2(OMe)2(L)(MeOH)](ClO4) with Φ1 = 81.9 o, J(exp/calc) = +6.25/+9.45, and [Mn2(OMe)2(HL)4]  ith Φ1= 117.1 o, J(exp/calc) = +9.85/+9.8. In the case of 1-4, Φ1~90 o (88.9±0.7 o) and J1 = +1.4±0.6 cm-1. For type II structures with Φ1~0o, the competitive nature between the coexisting JF and JAF is determinant for the overall sign of the interaction.

Scheme 4. Definition of the dihedral angles Φ1 and Φ2 between the Jahn-Teller axes in the terminal and central [MnIII2(μ-OR)2] units, respectively. The Jahn-Teller axes of each MnIII ion is shown as thickblack lines.

Hence, examples with J2>0 or J2<0 are reported in the literature; in the case of 1-4, J2 = -7.2±0.4 cm-1.

Conclusions

The tetranuclear clusters [MnIII 4(L1)2(HL1)2(H2O)2(MeOH)2] (Br)2 (1) and [MnIII 4(L2)2(HL2)2(H2O)2(MeOH)2](Br)2 (2) which contain the Schiff base ligands OH-C6H4-CH=NC(R) (CH2OH)2 (R = CH3, H3L1; R = C2H5, H3L2) are reported.

Both complexes are isostructural to the previously reported perchlorate analogues 3-5, thus constitute a family of clusters suitable for comparative study of their magnetic behavior. All compounds consist of three corner sharing [MnIII 2(μ- OR)2] units in trans orientation. The relative orientation of the Jahn-Teller axes of the MnIII ions in the [MnIII 2(μ-OR)2] units with respect to the bridging plane is determinant for the ferro- or antiferromagnetic coupling between the metal ions. Weak  ferromagnetic exchange is observed between the metal ions in the two terminal type III [MnIII 2(μ-OR)2] units with J1 = 0.6(2)-2.0(2) cm-1, and antiferromagnetic coupling is observed between the metal ions in the central type II [MnIII 2(μ-OR)2] unit with J2 = -6.8(2)-7.6(2) cm-1 in 1-4. The relatively larger J2 values determine the overall antiferromagnetic behavior of 1-4. Theoretical calculations as well as further experimental studies are needed in order to establish magneto-structural correlations in this family of [MnIII 4] clusters and are under way in our lab.

Supplementary material

Figure showing the supramolecular structures of 1 and 2 (Fig. S1) and Table of the hydrogen bonding interactions in 1 and 2 (Table S1). CCDC 1498033 and 1498034 contain the supplementary crystallographic data for 1 and 2∙0.9H2O. These data can be obtained free of charge via: http://www.ccdc.cam. ac.uk/conts/retrieving.html, or from the Cambridge  Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk. Supplementary data associated with this article can be found, in the online version, at doi:

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6. See for example:

(a) Dube C E, Wright D W, Pal S, Bonitatebus P J, Armstrong W H. Tetranuclear manganese-oxo aggregates relevant to the photosynthetic water oxidation center. Crystal structure, spectroscopic properties and reactivity of adamantane-shaped [Mn4O6(bpea)4]4+ and the reduced mixed-valence analog [Mn4O6(bpea)4]3+. J Am Chem Soc. 1998, 120(15): 3704-3716

(b) Wieghardt K, Bossek U, Nuber B, Weiss J, Bonvoisin J et al. Synthesis, crystal structures, reactivity, and magnetochemistry of a series of binuclear complexes of manganese(II),-(III), and -(IV) of biological relevance. The crystal structure of [L’MnIV(μ-O)3MnIVL’](PF6).2H2O containing an unprecedented short Mn…Mn distance of 2.296 Å. J Am Chem Soc. 1988, 110(22): 7398-7411

(c) Hagen K S, Westmoreland T D, Scott M J, Armstrong W H. Structural and electronic consequences of protonation in {Mn4O6}4+ cores: pH dependent properties of oxo-bridged manganese complexes, J. Am. Chem. Soc. 1989, 111(5): 1907-1709.

7. See for example: Chan M K, Armstrong W H. Support for a dimer of di-μ-oxo dimers model for the photosystem II manganese aggregate. Synthesis and properties of [(Mn2O2)2(tphpn) 2](ClO4)4. J Am.Chem Soc. 1991, 113(13): 5055-5057.

8. For cis arrangement examples see:

(a) Jeffrey J C, Thornton P, Ward MD. An unusual chainlike tetranuclear manganese(II) complex displaying ferromagnetic exchange. Inorg Chem. 1994, 33(16): 3612-3615

(b) Bardwell D A, Jeffrey J C, Ward MD. Coordination chemistry of mixed pyridine-phenol ligands: Polynuclear complexes of 6-(2-hydroxyphenyl)-2,2;-bipyridine with NiII, CdII, MnII and MnIIMnIII . J Chem Soc Dalton Trans. 1995, 18: 3071-3080

(c) Yoo J, Yamaguchi A, Nakano M, Krzystek J, Streib W E et al. Mixed-valence tetranuclear manganese single-molecule magnets. Inorg Chem. 2001, 40(18): 4604-4616 (d) Philouze C, Blondin G, Cirerd J J, Guilhem J, Pascard C et al. Aqueous chemistry of high-valent manganese. Structure, magnetic, and redox properties of a new type of Mn-oxo cluster, [MnIV 4O6(bpy)6]4+: Relevance to the oxygen evolving center in plants. J Am Chem Soc. 1994, 116(19): 8557-8565.

9. For trans arrangement examples see:

(a) Aromí G, Gamez P, Boldron C, Kooijman H, Spek A L et al. A zig-zag [MnII 4] cluster from a novel bis(μ-diketonate) ligand. Eur J Inorg Chem. 2006, 10(24): 1940-1944

(b) Aromí G, Gamez P, Krzustek J, Kooijman H, Spek A L et al. Novel linear transition metal clusters of a heptadentate bis-μ- diketone ligand. Inorg Chem. 2007, 46(7): 2519-2529

(c) Feltham H L C, Clérac R, Brooker S. A tetranuclear mixed-valence manganese complex of diimine ligand derived from 1,4-diformyl-2,3-dihydroxybenzene: Synthesis, structure, and magnetic properties. Aust J Chem. 2009, 62: 1119-1123

(d) Karotsis G, Jones L F, Papaefstathiou G S, Collins A, Parsons S et al. Rare tetranuclear mixed-valent [MnII 2MnIV 2] clusters as building blocks for extended networks. Dalton Trans. 2008: 4917-4925.

(e) Li D, Wang H, Wang S, Pan Y, Li C, et al. A linear tetranuclear single-molecule magnet of MnII 2MnIII 2 with the anion of 2-(hydroxymethyl) pyridine. Inorg Chem. 2010, 49(8): 3688-3690.

(f) Karlsson E A, Lee B L, Åkermark T, Johnston E V, Kärkäs MD et al. Photosensitized water oxidation by use of a bioinspired
manganese catalyst. Angew Chem Int Ed. 2011, 50(49): 11715- 11718.

(g) Bettle P J, Dawe L N, Anwar M U, Thompson L K. Dinuclear, tetranuclear and chain (MnII, CoII) complexes of multifunctional hydrazine ligands – Structural and magnetic studies. Eur J Inorg Chem point number 55. 2011: 5036-5042.

10. See for example: Mukhopadhyay S, Staples R J, Armstrong W H. Toward synthetic models for high oxidation state forms of the Photosystem II active site metal cluster: the first tetranuclear manganese cluster containing a [Mn4(μ-O)5]6+ core. J Chem Soc Chem Commun. 2002: 864-865.

11. See for example: Chen H Y, Faller J W, Crabtree R H, Brudvig G W. Dimer-of-dimers model for the oxygen-evolving complex of Photosystem II. Synthesis and properties of [MnIV 4O5(terpy-)4(H2O)2](ClO4)6. J Am Chem Soc. 2004, 126(23): 7345-7349.

12. See for example:

(a) McKee V, Sheperd W B. X-ray structural analysis of a tetra manganese(II) complex of a new (4×4) Schiff-base macrocycle incorporating a cubane-like Mn4(alkoxy)4 core. J Chem Soc Chem Commun. 1985: 158-159

(b) Brooker S, McKee V, Sheperd W B, Parnell L K. Formation of a (4×4) Schiff-base macrocyclic ligand by a template rearrangement. Crystal and molecular structures of two tetranuclear manganese(II) complexes. J Chem Soc, Dalton Trans. 1987: 2555-2562

(c) Taft K L, Caneschi A, Delfs C D, Papeafthymiou G C, Lippard S J. Iron and manganese alkoxide cubes. J Am Chem Soc. 1993, 115(25): 11753-11766

(d) Aromí G, Bhaduri S, Artus P, Folting K, Christou G. Bridging nitrate groups in [Mn4O3(NO3)(O2CMe)3(R2dbm)3] (R = H, Et)
and [Mn4O2(NO3)(O2CEt)6(bpy)2](ClO4): Acidolysis routes to tetranuclear manganese carboxylate complexes. Inorg Chem. 2002, 41(4): 805-817

(e) Wu J Z, Sellitto E , Yap G P A, Shealts J, Dismukes J C. Trapping an elusive intermediate in manganese-oxo cubane chemistry. Inorg Chem. 2004, 43(19): 5795-5797

(f) Abrahams B F, Hudson T A, Robson R. In situ synthesis of trisubstituted methanol ligands and their potential as one-pot generators of cubane-like metal complexes. Chem.-Eur J. 2006, 12(27): 7095-7102.

13. See for example: Chen H, Collomb M N, Duboc C, Blondin G, Rivière E et al. New linear high-valent tetranuclear manganese- oxo cluster relevant to the Oxygen-Evolving complex of Photosystem II with oxo, hydroxo, and aqua coordinated to a single Mn(IV). Inorg Chem. 2005, 44(25): 9567-9573.

14. See for example:

(a) Vincent JB, Christmas C, Huffman JC, Christou G, Chang HR et al. Modelling the photosynthetic water oxidation centre: synthesis, structure, and magnetic properties of [Mn4O2(OAc)7(biphy) 2ClO4].3H2O (bipy=2,2’-bipyridine). J Chem Soc Chem Commun. 1987, (4): 236-238.

(b) Kalawiec RJ, Crabtree RH, Brudvig GW, Schulte GK. Modeling the oxygen-evolving center of photosystem II: synthesis and characterization of a tetranuclear manganese carboxylate complex. Inorg Chem. 1988, 27(8): 1309-1311.

(c) Vincent JB, Christmas C, Chang HR, Li Q, Boyd PDW et al. Modeling the photosynthetic water oxidation center. Preparation and properties of tetranuclear manganese complexes containing [Mn4O2]6+,7+,8+ cores, and the crystal structures of [Mn4O2(O2CMe)6(bipy)2] and [Mn4O2(O2CMe)7(bipy)2](ClO4). J Am Chem Soc. 1989, 111(6): 2086-2097.

(d) Wang S, Wemple MS, Yoo J, Folting K, Huffman JC et al. Tetranuclear manganese carboxylate complexes with a trigonal pyramidal metal topology via controlled potential electrolysis. Inorg Chem. 2000, 39(7): 1501-1513.

(e) Price DJ, Batten SR, Berry KJ, Moubaraki B, Murray KS. Structure and magnetism of trinuclear and tetranuclear mixed valent manganese clusters from dicyanonitrosomethanide derived ligands. Polyhedron. 2003, 22(1): 165-176.

(f) Tasiopoulos AJ, Wernsdorfer W, Abboud KA, Christou G. [Mn12O12(OMe)2(O2CPh)16(H2O)2]2- single-molecule magnets and other manganese compounds from a reductive aggregation procedure. Inorg Chem. 2005, 44(18): 6324-6338.

(g) Miyasaka H, Nakata K, Lecren L, Coulon C, Nakazawa Y et al. Two-dimensional networks based on Mn4 complex linked by
dicyanamide anion: From single-molecule magnet to classical magnet behavior. J Am Chem Soc. 2006, 128(11): 3770-3783.

(h) Wittick LM, Jones LF, Jensen P, Moubaraki B, Spiccia L et al. New mixed-valence MnII 2MnIII 2 clusters exhibiting an unprecedented MnII/III oxidation state distribution in their magnetically coupled cores. Dalton Trans. 2006, 22(12): 1534-1543.

15. Raptopoulou CP, Sanakis Y, Psycharis V, Pissas M. Zig-zag [MnIII 4] clusters from polydentate Schiff base ligands. Polyhedron. 2013, 64(12): 181-188.

16. Zhu W, Zhang S, Cui C, Bi F, Ke H et al. New dinuclear cobalt( II,III) and tetranuclear manganese(III) complexes assembled by a polydentate Schiff-base ligand: synthesis, structure and magnetic properties. Inorg Chem Commun. 2014, 46: 315- 319.

17. (a) Dey M, Rao CP, Saarenketo PK, Rissanen K. Mono-, di- and tri-nuclear Ni(II) complexes of N-, O-donor ligands : structural diversity and reactivity. Inorg Chem Commun. 2002, 5(11): 924-928;

(b) Raptopoulou CP, Papadopoulos AN, Malamatari DA, Ioannidis E, Moisidis G et al. Ni(II) and Cu(II) Schiff base complexes with an extended H-bond network. Inorg Chim Acta. 1998, 272(1-2): 283-290.

18.(a) Kessissoglou DP, Butler WM, Pecoraro VL. Structural and spectroscopic characterization of the manganese(IV) Schiff base complex Mn(saladhp)2 (saladhp = 2-salicylideniminato- 1,3-didyfroxy-2-methylpropane). Chem Commun. 1986: 1253-1255;

(b) Li X, Kessissoglou DP, Kirk ML, Bender CJ, Pecoraro VL. Isolation of a mixed-valence trinuclear manganese complex potentially relevant to the photosynthetic oxygen-evolving com plex. Inorg Chem. 1988, 27(1): 1-3;

(c) Kessissoglou DP, Kirk ML, Lah MS, Li X, Raptopoulou C et al. Structural and magnetic characterization of trinuclear, mixed-valence manganese acetates. Inorg Chem. 1992, 31(26): 5424-5432;

(d) Kessissoglou DP, Kirk ML, Bender CA, Lah MS, Pecoraro VL. A bent mixed-valence manganese(III/II/III) complex: a new class of trinuclear, acetate bridged Schiff’s base compounds exhibiting a g=2 multiline e.s.r. signal. Chem Commun. 1989, (2): 84-86;

(e) Tangoulis V, Malamatari DA, Soulti K, Stergiou V, Raptopoulou CP et al. Manganese(II/II/II) and magnanese(III/II/III) trinuclear compounds. Structure and solid and solution behavior. Inorg Chem. 1996, 35(17): 4974-4983;

(f) Tangoulis V, Malamatari DA, Spyroulias GA, Raptopoulou CP, Terzis A et al. An EPR and 1H NMR active mixed-valence manganese(III/II/III) trinuclear compound. Inorg Chem. 2000, 39(12): 2621-2630;

(g) Malamatari DA, Hitou P, Hatzidimitriou AG, Inscore FE, Gourdon A et al. First example of a mixed valence MnIIIMnIIMnIII Schiff-base polymeric complex having a trimeric repeat unit. Crystal structure of [Mn3(Hsaladhp)2(acetato)2(5-Cl-salicylato)
2]n. Inorg Chem. 1995, 34 : 2493-2494.

19. Liimatainen J, Lehtonen A, Sillanpaa R. cis-dioxomolybdenum( IV) complexes with tridentate and tetradentate Schiff base ligands. Preparation, structures and inhibition of aerial oxidation of aldehydes. Polyhedron. 2000, 19(9): 1133-1138.

20. (a) Kessissoglou DP, Raptopoulou CP, Bakalbassis EG, Terzis A, Mrozinski J. Molubdenum(VI)-Copper(II) Schiff-base chain complexes having Cu2Mo2O4 cubane-like cores. Synthesis, crystal structure, and magnetic properties of [Cu2Mo2O4(saladhp) 2(MeO)2.2CH3CN]n [H3salahhp = 1,3-dihydroxy-2-methyl- 2-(salicylideneamino)propane]. Inorg Chem. 1992, 31: 4339-4345;

(b) Papadopoulos AN, Hatzidimitriou AG, Gourdon A, Kessissoglou DP. Synthesis and characterization of an oxomolybdate- copper(II) cluster containing coordinatively bound Schiff-base molecules. Inorg Chem. 1994, 33(10): 2073-2074.

21. (a) Karotsis G, Teat SJ, Wernsdorfer W, Piligkos S, Dalgarno SJ et al. Calix[4]arene-based single-molecule magnets. Angew Chem Int Ed. 2009, 48(44): 8285-8288;

(b) Taylor SM, Karotsis G, McIntosh RD, Kennedy S, Teat SJ et al. A family of calix[4]arene-supported [MnIII 2MnII 2] clusters. Chem Eur J. 2011, 17(27): 7521-7530.

22. (a) Habib F, Cook C, Korobkov I, Murugesu M. Novel in situ manganese-promoted double-aldol addition. Inorg Chim Acta.
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(b) McLellan R, Palacios MA, Brechin EK, Dalgarno SJ. Influencing the orientation of Jahn-Teller axes in butterfly-like MnIII 4
clusters. Chem Plus Chem. 2014, 79(5): 667-670.

23. (a) Berg N, Rajeshkumar T, Taylor SM, Brechin EK, Rajaraman G et al. 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;

(b) Barros WP, Inglis R, Nichol GS, Rajeshkumar T, Rajaraman G et al. From antiferromagnetic to ferromagnetic exchange in a family of oxime-based MnIII dimers: a magneto-structural study. Dalton Trans. 2013, 42(47): 16510-16517;

(c) Comar P, Rajeshkumar T, Nichol GS, Pitak MB, Coles SJ et al. Switching the orientation of Jahn-Teller axes in oxime-based MnIII dimers and its effect upon magnetic exchange: a combined experimental and theoretical study. Dalton Trans. 2015, 44(46): 19805-19811.

24. Chilton NF, Anderson RP, Turner LD, Soncini A, Murray KS. PHI: a powerfull new program for the analysis of anisotropic monomeric and exchange-coupled polynuclear d- and f-block complexes. J Comput Chem. 2013, 34(13): 1164-1175.

25. Rigaku/MSC. CrystalClear. Rigaku/MSC Inc., the Woodlands, Texas, USA. 2005.

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28. (a) Rao PV, Rao CP, Sreedhara A, Wegelius EK, Rissanen K et al. Synthesis, structure and reactivity of trans-UO2 2+ complexes of OH-containing ligands. Dalton Trans. 2000, (7): 1213-1218.

(b) Dey M, Rao CP, Saarenketo P, Rissanen K, Kolehmainen E. Four-, Five- and Six-coordinated ZnII complexes of OH-containing ligands: Syntheses, structure and reactivity. Eur J Inorg Chem. 2002, 2002(8): 2207-2215.

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