Synthesis, Characterization and Magnetic Studies of a Tetranuclear Manganese(II/IV) Compound Incorporating an Amino-Alcohol Derived Schiff Base

A new tetranuclear mixed-valence manganese(II/IV) compound [MnMn3(μ-Cl)3(μ3-O) (L)3] (1) (where H3L = (3E)-3-((Z)-4-hydroxy-4-phenylbut-3-en-2-ylideneamino)propane-1,2-diol) has been synthesized and characterized by different physicochemical methods. Single crystal X-ray diffraction analysis reveals that 1 is a tetrahedral cluster consisting of a Mn4Cl3O4 core in which the only Mn(II) ion is joined through three μ2-O bridges to an equilateral triangle of Mn(IV) ions, which are connected by a μ3-O and three μ2-Cl bridges. The redox behavior of 1 was studied by cyclic voltammetry. Variable temperature magnetic susceptibility measurements of 1 revealed predominant antiferromagnetic coupling inside the Mn4Cl3O4 cluster.


Introduction
Paramagnetic metal clusters are widely utilized in magnetic, electronic, optical, biological, and catalytic studies and applications [1,2].Though researchers are engaged in the development of several synthetic strategies to design such complexes with high nuclearity, their synthesis is still challenging [3][4][5].Polydentate ligands, functioning as chelators and bridges, are capable of coordinating with several metal centers and their pendant donating atoms can join other metal centers to construct various intricate structures [6,7].Schiff bases derived from amino alcohols have proven to possess the ability to form polynuclear complexes [1,2], which play a significant role as biomimetic models of metallobiomolecules [8,9].
Catalytic and magnetic behaviors of many mixed-valence manganese complexes have been investigated using different synthetic strategies [4,10].The most common oxidation state originates in these hetero-valence manganese complexes are Mn II , Mn III and Mn IV [11][12][13].Due to the presence of numerous possibilities in different magnetic coupling between pairs of ions in various oxidation states (II/II, II/III, II/IV, III/III, III/IV and IV/IV pairs), very high values of total molecular spin (S) [14][15][16][17] are an expected phenomenon in these complexes.The molecular magnetism of mixed-valence tetranuclear manganese clusters has been explored, and has been examined as biomimetic models of PS II [1,2,[14][15][16].
In this context, we have isolated a new mixed-valence manganese compound [Mn II Mn IV 3 (µ-Cl) 3 (µ 3 -O)(L) 3 ] (1), incorporating a new Schiff base ligand H 3 L (H 3 L = (3E)-3-((Z)-4-hydroxy-4-phenylbut-3-en-2-ylideneamino)propane-1,2-diol) (Scheme 1).The title compound has been characterized by micro-analytical analysis, FT-IR and UV/Visible spectroscopy, cyclic voltammetry and variable temperature magnetic measurements.The single crystal X-ray structure determination shows that 1 is a tetrahedral manganese cluster bearing a Mn 4 Cl 3 O 4 core.The bond valence sum analysis (BVS) reveals different valence states in the manganese centers, with one Mn(II) and three Mn(IV) ions.The electrochemical characterization of the complex reveals a two-step single electron redox process involving the Mn(IV) centers.Variable temperature magnetic susceptibility measurements show predominant antiferromagnetic intra-cluster coupling in compound 1.
In this context, we have isolated a new mixed-valence manganese compound [Mn II Mn IV 3(μ-Cl)3(μ3-O)(L)3] (1), incorporating a new Schiff base ligand H3L (H3L= (3E)-3-((Z)-4-hydroxy-4phenylbut-3-en-2-ylideneamino)propane-1,2-diol) (Scheme 1).The title compound has been characterized by micro-analytical analysis, FT-IR and UV/Visible spectroscopy, cyclic voltammetry and variable temperature magnetic measurements.The single crystal X-ray structure determination shows that 1 is a tetrahedral manganese cluster bearing a Mn4Cl3O4 core.The bond valence sum analysis (BVS) reveals different valence states in the manganese centers, with one Mn(II) and three Mn(IV) ions.The electrochemical characterization of the complex reveals a two-step single electron redox process involving the Mn(IV) centers.Variable temperature magnetic susceptibility measurements show predominant antiferromagnetic intra-cluster coupling in compound 1.Scheme 1.Schematic diagram for the synthesis of the Schiff base (H3L) and its deprotonated form (L 3− ).

2.2.FT-IRSpectroscopy
The solid state FT-IR spectrum of H3L and 1 is shown in the supplementary materials (Figure S1).These spectral studies reveal that the azomethine (ν-CH=N-) stretching frequency of the H3L (1602 cm −1 ) was shifted to a lower energy in the complex and appeared at 1591 cm −1 .The ν(Calk-O) stretching vibration of the H3L (1270cm −1 ) was also moved to lower energies in the complex and appeared at 1112 cm −1 .The stretching vibration corresponding to aliphatic-OH (ν-CH2-OH) of the H3L (centered a 3305 cm −1 ) was not observed in the spectrum of 1, confirming its deprotonation upon complexation.The binding of the ligand was further substantiated by the presence of a number of bands in the range 412-567 cm -1 , assignable to the Mn-O and Mn-N bonds [18].

FT-IR Spectroscopy
The solid state FT-IR spectrum of H 3 L and 1 is shown in the supplementary materials (Figure S1).These spectral studies reveal that the azomethine (ν -CH=N-) stretching frequency of the H 3 L (1602 cm −1 ) was shifted to a lower energy in the complex and appeared at 1591 cm −1 .The ν(C alk -O) stretching vibration of the H 3 L (1270 cm −1 ) was also moved to lower energies in the complex and appeared at 1112 cm −1 .The stretching vibration corresponding to aliphatic-OH (ν -CH2-OH ) of the H 3 L (centered a 3305 cm −1 ) was not observed in the spectrum of 1, confirming its deprotonation upon complexation.The binding of the ligand was further substantiated by the presence of a number of bands in the range 412-567 cm −1 , assignable to the Mn-O and Mn-N bonds [18].

Electronic Spectrum
Electronic absorption spectra of H 3 L (Figure S2) and 1 (Figure 1) were recorded in acetonitrile at 300K.The free ligand showed two peaks at 242 and 372 nm, assignable to π→π* and n→π* transitions, respectively, which on chelation underwent red shift and appeared at 248 and 374 nm in the spectrum of 1.An additional shoulder appeared at 473 nm in the spectrum of 1 that may be considered as a combination of ligand to metal charge transfer (LMCT) and ligand-field transitions involving the manganese(IV) ions.Weak d-d transition bands for Mn(IV) ions are found at 850 and 966 nm, corresponding to 2 T 1g → 4 A 2g and 2 E g → 4 A 2g transitions, respectively [19].

Electronic Spectrum
Electronic absorption spectra of H3L (Figure S2) and 1(Figure 1) were recorded in acetonitrile at 300K.The free ligand showed two peaks at 242 and 372 nm, assignable to π→π* and n→π* transitions, respectively, which on chelation underwent red shift and appeared at 248 and 374 nm in the spectrum of 1.An additional shoulder appeared at 473 nm in the spectrum of 1 that may be considered as a combination of ligand to metal charge transfer (LMCT) and ligand-field transitions involving the manganese(IV) ions.Weak d-d transition bands for Mn(IV) ions are found at 850 and 966 nm, corresponding to 2 T1g→ 4 A2g and 2 Eg→ 4 A2g transitions, respectively [19].

Crystal Structure Description
The single crystal X-ray diffraction study shows that compound 1 crystallizes in the trigonal space group R-3. Figure 2(left) shows a perspective view of 1 with atom labels.The compound possesses a tetra nuclear tetrahedral Mn4 structure (Figure 2(right)).Relevant bond distances and angles of 1 are listed in Table 1.

Crystal Structure Description
The single crystal X-ray diffraction study shows that compound 1 crystallizes in the trigonal space group R-3. Figure 2(left) shows a perspective view of 1 with atom labels.The compound possesses a tetra nuclear tetrahedral Mn 4 structure (Figure 2(right)).Relevant bond distances and angles of 1 are listed in Table 1.

Electronic Spectrum
Electronic absorption spectra of H3L (Figure S2) and 1(Figure 1) were recorded in acetonitrile at 300K.The free ligand showed two peaks at 242 and 372 nm, assignable to π→π* and n→π* transitions, respectively, which on chelation underwent red shift and appeared at 248 and 374 nm in the spectrum of 1.An additional shoulder appeared at 473 nm in the spectrum of 1 that may be considered as a combination of ligand to metal charge transfer (LMCT) and ligand-field transitions involving the manganese(IV) ions.Weak d-d transition bands for Mn(IV) ions are found at 850 and 966 nm, corresponding to 2 T1g→ 4 A2g and 2 Eg→ 4 A2g transitions, respectively [19].

Crystal Structure Description
The single crystal X-ray diffraction study shows that compound 1 crystallizes in the trigonal space group R-3.Table 1.Relevant bond distances (Å) and angles (°) for compound 1.
The tetranuclear mixed-valence manganese compound has a C 3 symmetry and possesses a Mn(II) center(Mn2) on the C 3 axis, three crystallographically equivalent Mn(IV) centers (Mn1, Mn1 1 and Mn1 2 ), three fully deprotonated crystallographically equivalent Schiff base ligands, three bridging Cl − ligands, and a central oxido anion connecting the three Mn(IV) centers located on the C 3 axis.The ligand L 3− exhibits a µ-bridging tetradentate coordination mode of the type O 3 N (Scheme 2) and is coordinated to both types of Mn ions.
The tetranuclear mixed-valence manganese compound has a C3 symmetry and possesses of a Mn(II) center(Mn2) on the C3 axis, three crystallographically equivalent Mn(IV) centers (Mn1, Mn1 1 and Mn1 2 ), three fully deprotonated crystallographically equivalent Schiff base ligands, three bridging Cl − ligands, and a central oxido anion connecting the three Mn(IV) centers located on the C3 axis.The ligand L 3− exhibits a μ-bridging tetradentate coordination mode of the type O3N (Scheme 2) and is coordinated to both types of Mn ions.Thus, each of the three crystallographically equivalent Schiff base anions coordinated to one of the three Mn(IV) metals in a tridentate O2N meridional mode through its enolato oxygen (O11), the imine nitrogen (N7), and the internal alkoxido oxygen atom (O2).The distorted octahedral geometry of the Mn(IV) ions is completed by two of the three equivalent μ-bridging chlorido ligands (Cl1 and Cl1 1 ; symmetry code: 1 = 1 − y, x−y, z) and the μ3-oxido ligand (O1).Mn2 is coordinated by the three Schiff base anions via their terminal (O3) and internal (O2) alkoxido oxygen atoms, giving rise to a slightly distorted trigonal prismatic geometry.The coordination geometry, analyzed by the program SHAPE [20,21], confirms that of Mn1 adopts a distorted octahedral geometry whereas Mn2 displays Thus, each of the three crystallographically equivalent Schiff base anions coordinated to one of the three Mn(IV) metals in a tridentate O 2 N meridional mode through its enolato oxygen (O11), the imine nitrogen (N7), and the internal alkoxido oxygen atom (O2).The distorted octahedral geometry of the Mn(IV) ions is completed by two of the three equivalent µ-bridging chlorido ligands (Cl1 and Cl1 1 ; symmetry code: 1 = 1 − y, x − y, z) and the µ 3 -oxido ligand (O1).Mn2 is coordinated by the three Schiff base anions via their terminal (O3) and internal (O2) alkoxido oxygen atoms, giving rise to a slightly distorted trigonal prismatic geometry.The coordination geometry, analyzed by the program SHAPE [20,21], confirms that of Mn1 adopts a distorted octahedral geometry whereas Mn2 displays a distorted trigonal prismatic geometry(Table 2).Each Mn(IV) is joined to the Mn(II) ion by the O2 atom with a Mn1-O2-Mn2 bond angle of 125.5(2) • .The three µ-bridging chlorido ligands connect two Mn(IV) atoms with a Mn1-Cl1-Mn1 2 (symmetry operation: 2 = 1 − x + y, 1 − x, z) bond angle of 74.20 (9) • and form an equilateral triangle as a result of the C 3 symmetry of the cluster.The oxido (O1) atom is located on the C 3 axis at the center of the triangle and connected with three Mn(IV) atoms with a Mn1 1 -O1-Mn1 2 bond angle of 119.00 (7) • .The central core can be described as a tetrahedral Mn 4 cluster formulated as Mn 4 Cl 3 O 4 , in which a Mn(II) ion is associated with threeµ 2 -O bridges to an equilateral triangle of Mn(IV) ions, which are connected by a µ 3 -O and three µ 2 -Cl bridges.The Mn(IV)-Mn(IV) and Mn(IV)-Mn(II) distances in the cluster are 3.2523(17) and 3.5801(17) Å, respectively.Crystal packing reveals that very weak non-classical intermolecular C-H•••Cl hydrogen bonding between aromatic hydrogen (H13) and chloride ligand (Cl1) with bond dimension C13-H13•••Cl1 = 3.742(13) Å leads to a 3D supramolecular structure in its solid state.Although there are hundreds of discrete multinuclear Mn cluster compounds with oxido and/or chlorido bridges, the Cambridge Crystallographic Data Center CCDC search showed that compound 1 is the first structurally characterized tetranuclear Mn cluster with an Mn IV 3 Mn II Cl 3 O 4 core involving Mn(II) and Mn(IV) ions.Although there is a similar tetranuclear cluster formulated as [Mn III  3 Mn II NaOCl 4 (HL 1 ) 3 ]•3MeCN (HL 1 = 2-((E)-(2,3-dihydroxypropylimino)methyl)phenol, CCDC code: LIQXOA), it contains one Mn(II) and three Mn(III) ions, along with a Na + cation connecting three Mn(III) atoms with a terminal Cl − ligand connected to the Na + ion [22].Additionally, there are two structures [Na 2 Mn 7 (µ 4 -O) 2 {µ 3 -C 9 H 7 NO 2 Ph(OMe)-κ 1 -N,κ 5  YEMBUQ) with a central cluster formed by two Mn 4 Cl 3 O 4 cores, again with one Mn(II) ion and three Mn(III) ions, which are connected by a triple µ-Cl bridge through the two equilateral triangular faces [24].Mn(IV)-Cl distances in the range 2.663(3)-2.719(3)Å observed in 1 are slightly higher in comparison to the Mn(III)-Cl distances found in the range 2.631(5)-2.712(3)Å for the reported mixed valence manganese complex (YEMBOK) [1] with a Mn II Mn III 3 (µ-Cl) 3 (µ 3 -O) triangular planar core.This can be attributed to the weak overlap between the Mn(IV) and Cl atom in the present compound, which probably arises due to the greater Mn(IV) . . .Mn(IV) separation (3.2523 (17) Å) within the Mn IV within the Mn III 3 core are 3.252(1) and 3.248(2) Å.However, no such structure was found in CCDC bearing a similar core involving Mn II and Mn IV atoms.
The oxidation state of the manganese ions has been determined by the bond valence sum (BVS) calculation based on the correlation between bond valence and bond distance.Here, valence between the two atoms i and j can be calculated using the empirical formula [25,26].
where bond valence between two atoms i and j is represented by the term S ij.The observed bond length between i and j is symbolized by the term R ij .R 0 and b are the tabulated bond valence parameters defined by Liu et al. [25].The algebraic sum of S ij values of all the bonds (n) around an atom i gives the oxidation number of the atom i and is symbolized by N ij , which is given by the equation With this equation, the values obtained for compound 1 are Mn1/Mn1 1 /Mn1 2 = 4.224 and Mn2 = 2.208, which clearly confirms the oxidation states of both types of manganese center.

Electrochemistry
Figure 3 represents the cyclic voltammograms of compound 1, measured within the potential range −2.0 to +2.0 V using n Bu 4 NClO 4 as the supporting electrolyte at the following scan rates: 50, 100, 150 and 200 mV s −1 .In the redox process, the tetranuclear compound 1 underwent two one-electron charge transfers, namely, (a) Mn II Mn IV ↔Mn II Mn III and (b) Mn II Mn III ↔Mn II Mn II .In the cathodic scan, the first peak, assigned to the Mn II Mn IV →Mn II Mn III reduction, appeared at 408, 386, 386 and 375 mV, vs. saturated calomel electrode(SCE), for the scan rates 50, 100, 150 and 200 mV s −1 , respectively.A second cathodic peak, attributed to the Mn II Mn III →Mn II Mn II reduction, was observed at −328, −369, −414 and −458 mV vs. SCE, for the scan rates 50, 100, 150 and 200 mV s −1 , respectively.The anodic scan showed one oxidation peak, attributed to the Mn II Mn II →Mn II Mn III oxidation, at −639, −561, −539 and −520 mV, vs. SCE, for the same respective scan rates [27,28].The reversibility of the redox processes is supported by the difference in the respective reduction to oxidation peaks which were found to be 311, 192, 125 and 69 mV when measured at scan rates: 50, 100, 150, 200 mVs −1 , respectively.It was also found that with the increase of scan rate, the corresponding cathodic peak shifted to more negative values while the anodic peak shifted to more positive values with an increase of peak currents for both scans and, as a result, the I pc /I pa being greater than 1, which also confirms the irreversible nature of the redox processes [28].

S exp R R /b
(1 here bond valence between two atoms i and j is represented by the term Sij.The observed bon ngth between i and j is symbolized by the term Rij.R0 and b are the tabulated bond valen rameters defined by Liu et.Al. [25] The algebraic sum of Sij values of all the bonds (n) around a om i gives the oxidation number of the atom i and is symbolized by Nij, which is given by th uation With this equation, the values obtained for compound 1 are Mn1/Mn1 1 /Mn1 2 = 4.224 and Mn2 208, which clearly confirms the oxidation states of both types of manganese center.

Electrochemistry
Figure 3 represents the cyclic voltammograms of compound 1, measured within the potenti nge −2.0 to +2.0 V using n Bu4NClO4 as the supporting electrolyte at the following scan rates: 50, 10 0 and 200 mV s −1 .In the redox process, the tetranuclear compound 1 underwent two one-electro arge transfers, namely, (a) Mn II Mn IV ↔Mn II Mn III and (b) Mn II Mn III ↔Mn II Mn II .In the cathodic sca e first peak, assigned to the Mn II Mn IV →Mn II Mn III reduction, appeared at 408, 386, 386 and 375 m .saturated calomel electrode(SCE), for the scan rates 50, 100, 150 and 200 mV s −1 , respectively.cond cathodic peak, attributed to the Mn II Mn III →Mn II Mn II reduction, was observed at −328, −36 14 and −458 mV vs. SCE, for the scan rates 50, 100, 150 and 200 mV s −1 , respectively.The anod an showed one oxidation peak, attributed to the Mn II Mn II →Mn II Mn III oxidation, at −639, −561, −53 d −520 mV, vs. SCE, for the same respective scan rates.[27,28] Their reversibility of the redo ocesses is supported by the difference in the respective reduction to oxidation peaks which we und to be 311, 192, 125 and 69 mV when measured at scan rates: 50, 100, 150, 200 mVs −1 , respectivel was also found that with the increase of scan rate, the corresponding cathodic peak shifted to mo gative values while the anodic peak shifted to more positive values with an increase of pea rrents for both scans and, as a result, the Ipc/Ipa being greater than 1, which also confirms th eversible nature of the redox processes [28].

Magnetic Study
The thermal variation of the product of molar magnetic susceptibility per manganese tetram temperature and reaches a minimum value of ca.7.0 cm 3 K mol −1 at ca. 37 K (Figure 4).Below this temperature, χmT rises and reaches a maximum of ca.8.7 cm 3 K mol −1 at ca. 5 K. Below ca. 5 K, χmT sharply decreases and reaches a value of ca.8.1 cm 3 K mol −1 at 2 K.This behavior suggests that the predominant interaction inside the Mn4 cluster is antiferromagnetic, as shown by the continuous decrease of χmT, starting from room temperature to ca. 37 K. Since there are two different exchange coupling constants inside the Mn4 tetrahedron, if both are antiferromagnetic then they are expected to give rise to spin frustration, as not all pairwise interactions can present an antiferromagnetic interaction (see below).This spin frustration results in an intermediate spin ground state that gives rise to the observed increase of the magnetic moment below ca.37 K. Finally, the decrease observed at very low temperatures can be attributed to the presence of a zero field splitting of the spin ground state and/or to the presence of weak antiferromagnetic inter-cluster coupling.The structure of the Mn4Cl3O4 cluster in 1 shows that the manganese(II) ion is connected to another three manganese(IV) ions, and that there are two crystallographically independent manganese ions (three Mn1 atoms, corresponding to S = 3/2 manganese(IV) ions and one Mn2 atom, which is a S = 5/2 manganese(II) ion).A close look at the structure of the cluster also shows that there are two different exchange pathways (J1, connecting the three Mn1 ions through a μ3-oxido and a μ-chlorido bridge; and J2, connecting the Mn2 ion with the three Mn1 ions through a single μ-oxido bridge; see inset in Figure 4).Accordingly, we have used a model that takes into account the magnetic exchange scheme displayed in the inset in Figure 4 using the package MAGPACK [29,30]   The observed two weak antiferromagnetic exchange interactions inside the Mn4 tetramer leads to spin frustration.Thus, the stronger antiferromagnetic J2 coupling constant implies that the spins of the Mn2 ions must be antiparallel to the three Mn1 ions.Therefore, the spins of the three Mn1 ions should be parallel to each other, in conflict with the antiferromagnetic coupling observed for J1.
Finally, the weak couplings found for both exchange interactions are in agreement to those observed for other oxido-bridged Mn clusters and can be explained from previous magneto structural correlations that indicate that the coupling in this kind of Mn cluster with single μ-oxido bridges is weak and antiferromagnetic.[31][32][33] Unfortunately, the few related Mn4Cl3O clusters reported to date contain one Mn(II) and three Mn(III) ions and the magnetic couplings, although usually weak and antiferromagnetic, cannot be compared with those of 1.[22-24]The observed two weak antiferromagnetic exchange interactions inside the Mn 4 tetramer leads to spin frustration.Thus, the stronger antiferromagnetic J 2 coupling constant implies that the spins of the Mn2 ions must be antiparallel to the three Mn1 ions.Therefore, the spins of the three Mn1 ions should be parallel to each other, in conflict with the antiferromagnetic coupling observed for J 1 .

Materials and Methods
Finally, the weak couplings found for both exchange interactions are in agreement to those observed for other oxido-bridged Mn clusters and can be explained from previous magneto structural correlations that indicate that the coupling in this kind of Mn cluster with single µ-oxido bridges is weak and antiferromagnetic [31][32][33].Unfortunately, the few related Mn 4 Cl 3 O clusters reported to date contain one Mn(II) and three Mn(III) ions and the magnetic couplings, although usually weak and antiferromagnetic, cannot be compared with those of 1 [22][23][24].

Starting Materials
All solvents and chemicals used in the synthesis were of analytical grade.Benzoyl acetone and 3-aminopropane-1,2-diol were received from Aldrich Chemical Co. Inc. (Sigma-Aldrich, St. Louis, MO, USA).MnCl 2 •6H 2 O and LiOH were obtained from E. Merck, India.

Physical Measurements
Elemental analyses (carbon, hydrogen, and nitrogen) were performed in a Perkin Elmer 2400 II elemental analyzer.The FT-IR spectrum of H 3 L and 1 were recorded on a Perkin Elmer Spectrum RX I FT-IR system using a KBr pellet in the range 4000-400 cm −1 .The UV/Visible spectra of the H 3 L and compound 1 were recorded on a Perkin Elmer Lambda 40 UV/Vis spectrometer using HPLC grade acetonitrile in the range 200-1100 nm.Proton nuclear magnetic resonance spectrum of H 3 L was recorded in CDCl 3 on a 300 MHz Bruker Fourier transform NMR spectrometer using (CH 3 ) 4 Si(TMS) as internal standard.Electrochemical studies of compound 1 were carried out on a PAR VersaStat-potentiostat/Galvanostat II electrochemical analysis system using conventional three-electrode configurations in acetonitrile with n Bu 4 NClO 4 as the supporting electrolyte in dry argon atmosphere.Cyclic voltammetry was performed at scan rates of ν = 50, 100, 150 and 200 mV s −1 using platinized platinum millielectrode and saturated calomel electrode (SCE) as working and reference electrodes, respectively, along with a platinum counter electrode.Magnetic susceptibility measurements of polycrystalline sample of compound 1(with a mass of 44.76 mg) were performed in the temperature range 2-300 K with an applied magnetic field of 0.1 T on a Quantum Design

Scheme 1 .
Scheme 1. Schematic diagram for the synthesis of the Schiff base (H 3 L) and its deprotonated form (L 3− ).

Figure 1 .
Figure 1.UV/Visible spectrum of 1, recorded in acetonitrile at 300K.Inset: zoom of the high wavelength region showing the d-d band.

Figure 1 .
Figure 1.UV/Visible spectrum of 1, recorded in acetonitrile at 300K.Inset: zoom of the high wavelength region showing the d-d band.

Figure 1 .
Figure 1.UV/Visible spectrum of 1, recorded in acetonitrile at 300K.Inset: zoom of the high wavelength region showing the d-d band.

Figure 4 .
Figure 4. χmT (per Mn4 cluster) vs. T plot for compound 1.Solid line indicates the best fit to the tetramer model (see text).Inset shows exchange scheme of the Mn4 cluster.

Figure 4 .
Figure 4. χ m T (per Mn 4 cluster) vs. T plot for compound 1.Solid line indicates the best fit to the tetramer model (see text).Inset shows exchange scheme of the Mn 4 cluster.

Table 2 .
Shape values for the five possible coordination geometries found for coordination number six in Mn1 and Mn2 in compound 1.The minimum values are indicated in bold.