Synthesis and Characterization of a Linear [Mn3(O2CMe)4(py)8]2+ Complex

Two new compounds that consist of the linear trinuclear manganese(II) cation [Mn3(O2CMe)4(py)8]2+ cocrystallizing with different counteranions (I3 −, [1]; ClO4 −, [2]) are reported. Complex 1 was prepared from the reaction of [Mn(O2CMe)2] · 4H2O with I2 in MeCO2H/py, whereas complex 2 was isolated from the reaction of [Mn3O(O2CMe)6(py)3] · py with [Mn(ClO4)2] · 6H2O in MeCN/py. The crystal structures of both compounds were determined by single crystal X-ray crystallography. Magnetic susceptibility studies that were performed in microcrystalline powder of 1 in the 2–300 K range revealed the presence of antiferromagnetic exchange interactions that resulted in an S = 5/2 ground spin state.

Herein, we report the synthesis, structural characterization, and magnetic properties of a new linear manganese(II) cation, [Mn 3 (O 2 CMe) 4 (py) 8 ] 2+ which cocrystallizes with two different counteranions (I 3 − , [1] and ClO 4 − , [2]). The cation of 1 and 2 represents the first linear trinuclear Mn II unit that contains only carboxylate and pyridine ligands and , and m and n are constants.
a rare example of a linear Mn II 3 cluster that is stabilized with carboxylate and terminal ligands without containing any polydentate chelates [30].

Experimental
2.1. Materials. All manipulations were performed under aerobic conditions using materials (reagent grade) and solvents as received; water was distilled in-house. [Mn 3 O (O 2 CMe) 6 (py) 3 ]·py was prepared as described elsewhere [27]. Warning: Although we encountered no problems, appropriate care should be taken in the use of the potentially explosives perchlorate anion.

[Mn
Method A. To a solution of [Mn 3 O(O 2 CMe) 6 (py) 3 ]·py (0.294 g, 0.345 mmol) in MeCN/py (10/2 mL) was added Mn(ClO 4 ) 2 ·6H 2 O (0.125 g, 0.345 mmol) and pdH 2 (0.10 mL, 0.105 g, 1.38 mmol) and the mixture was left under magnetic stirring for ∼30 minutes. The resulting dark redbrown slurry was filtered off and the dark red-brown filtrate was left undisturbed at room temperature. After few weeks yellow crystals appeared, suitable for X-ray structural determination. The crystals were isolated by filtration, washed with a copious amount of MeCN/py, and dried in vacuum; yield, ∼20% based on total ClO 4 − content. A sample for crystallography was maintained in contact with the mother liquor to prevent the loss of interstitial solvent. Anal. Calc  Mo-Kα radiation (λ = 0.71073Å). Suitable crystals were attached to glass fibers using paratone-N oil and transferred to a goniostat where they were cooled for data collection. Unit cell dimensions were determined and refined by using 12271 (3.07 ≤ θ ≤ 30.27 • ) and 5746 (3.06 ≤ θ ≤ 30.29 • ) reflections for 1 and 2, respectively. Empirical absorption corrections (multiscan based on symmetry-related measurements) were applied using CrysAlis RED software [31]. The structures were solved by direct methods using SIR92 [32], and refined on F 2 using full-matrix least squares with SHELXL97 [33]. Software packages used: CrysAlis CCD [31] for data collection, CrysAlis RED [31] for cell refinement and data reduction, WINGX for geometric calculations [34], and DIAMOND [35] and MERCURY [36] for molecular graphics. The non-H atoms were treated anisotropically, whereas the hydrogen atoms were placed in calculated, ideal positions and refined as riding on their respective carbon atoms. Unit cell data and structure refinement details are listed in Table 1.

Physical Measurements.
Elemental analyses were performed by the in-house facilities of the Chemistry Department, University of Cyprus. IR spectra were recorded on KBr pellets in the 4000-400 cm −1 range using a Shimadzu Prestige-21 spectrometer. Variable-temperature DC magnetic susceptibility data down to 1.80 K were collected on a Quantum Design MPMS-XL SQUID magnetometer equipped with a 70 kG (7 T) DC magnet. Diamagnetic corrections were applied to the observed paramagnetic susceptibilities using Pascal's constants. Samples were embedded in solid eicosane, unless otherwise stated, to prevent torquing.  [4,7,28]. Although the use of iodine as oxidant in Mn cluster chemistry has been reported in the past [4,7], the oxidation of Mn 2+ salts from iodine under various conditions is a rather unexplored synthetic method. Compound 1 was prepared during our investigations on reactions of Mn(O 2 CMe) 2 ·4H 2 O with iodine in MeCOOH/pyridine. A large amount of MeCOOH was used in order to avoid the formation of various Mn oxides/hydroxides that precipitate at basic conditions. Thus, the reaction of [Mn(O 2 CMe) 2 ]·4H 2 O with solid I 2 in a 1 : 1 ratio in MeCOOH/py (10/20 mL) resulted in the formation of dark brown crystals of 1 in ∼60% yield. The formation of 1 is summarized in (1)

Results and Discussions
Despite the presence of an oxidant (I 2 ) in the reaction mixture, the final product (compound 1) contains only Mn 2+ ions. We believe that species that contain Mn ions in higher oxidation states are also formed but are quite soluble and thus do not precipitate from the reaction solution.
Another synthetic method to new polynuclear Mn clusters employed recently by our group involves the use of aliphatic diols such as pdH 2 in Mn cluster chemistry. These studies have resulted in a number of new polynuclear clusters and coordination polymers with coordinated   3 ]·py as a starting material [37,38]. These studies, apart from compounds that contain coordinated pdH 2 ligands, have also resulted in complexes that do not include the diol in their asymmetric unit, with 2 being one of the members of this family. Thus, compound 2 was initially prepared from the reaction of [Mn 3 O(O 2 CMe) 6 (py) 3 ]·py with Mn(ClO 4 ) 2 ·6H 2 O in the presence of pdH 2 in a 1: 1: 4 ratio in MeCN/py (10/2 mL) in 20% yield. When the identity of 2 was established and known that it contained neither coordinated nor lattice pdH 2 /pd 2− ligands, the reaction resulted in the formation of 2 was repeated without including pdH 2 in the reaction mixture. This reaction gave a few crystals of 2. Various modifications were applied in this reaction in order to optimize its yield. Finally, the larger yield (achieved when no pdH 2 was included in the reaction mixture) was ∼9% and obtained when an extra amount of pyridine (4 more mL) was added to the reaction solution. The exact role of pdH 2 in the assembly of 2 and how its use results in larger reaction yield still remain unidentified.

Description of the Structures.
The molecular structure of complex 1 is presented in Figure 1 and selected interatomic distances and angles for 1 are listed in Table 2. Bond valence sum (BVS) calculations for the metal ions of 1 and 2 are given in Table 3. The crystal structures of 1 and 2 present a striking similarity with the main difference between them being their counter-ions and thus only that of 1 will be described here.
Compound 1 crystallizes in the monoclinic P2 1 /n space group and comprises the [Mn 3 (O 2 CMe) 4 (py) 8 ] 2+ cation and two I 3 − counteranions. The cation of 1 (Figure 1) consists of a linear array of three Mn II ions coordinated by four acetate groups and eight terminal pyridine molecules. The oxidation states of the Mn ions were determined by BVS calculations (Table 3), charge considerations, and inspection of metric parameters. The central metal ion of the trinuclear unit (Mn1), which is located on a crystallographic inversion center, is ligated by four oxygen atoms from four different acetate ligands and two molecules of pyridine adopting a distorted octahedral coordination geometry. All four acetate ligands bridge two Mn ions with two of them operating in the common syn-syn-η 1 : η 1 : μ 2 fashion, whereas the other two function in the less common monoatomically bridging η 2 : η 1 : μ 2 mode. The above mentioned carboxylate bridging modes have also been observed in several other linear trinuclear manganese (II) complexes [10][11][12][13][14][15][16][17][18][19][20][21][22][23]. However, in most linear Mn II 3 complexes each pair of Mn II ions is held together by at least three bridging ligands, whereas in 1 the neighboring Mn ions are connected through two bridging ligands only. One exception in this situation is the compound [Mn 3 (O 2 CMe) 6 (H 2 O)(phen) 2 ] where one pair of Mn ions is linked through two acetate ligands, whereas the second one is held together by three bridging MeCOO − ligands [16]. The consequence of the presence of less bridging ligands in 1 is the larger Mn· · · Mn separation (3.799 (2)Å) compared to the values observed in other linear trinuclear Mn II complexes which are within the range of 3.2-3.7Å [10][11][12][13][14][15][16][17][18][19][20][21][22][23]. The observed separation of 3.799Å is slightly smaller than that (3.868 (4)Å) between the Mn ions bridged by two acetate ligands in [Mn 3 (O 2 CMe) 6 (H 2 O)(phen) 2 ]. However, the Mn· · · Mn distance in the other pair of Mn ions of the latter is significantly shorter (3.489Å) and thus the average Mn· · · Mn separation falls within the range observed for the other linear trinuclear Mn II complexes.
The distorted octahedral coordination environment around each terminal metal ion (Mn2) is completed by three pyridine molecules. The Mn2N 3 O 3 octahedron is significantly distorted, with the main distortion arising from the acute O3-Mn2-O4 angle (58.24 (7) • ). The Mn1N 2 O 4 octahedron is almost perfect. All Mn-N and Mn-O bond lengths of the two crystallographically independent manganese ions are within the expected range for octahedral high-spin Mn II complexes. A close examination of the packing of 1 revealed that the trinuclear molecules are nearly perpendicular to each other ( Figure 2) and there are no significant hydrogen bonding interactions between neighboring units of 1.

Magnetic Properties.
Solid-state dc magnetic susceptibility studies were performed on a powdered crystalline sample of 1 in a 0.1 T field and in the 5.0-300 K temperature range. The obtained data are plotted as χ M T versus T in Figure 3.
The χ M T product at 300 K for 1 is 12.98 cm 3 mol −1 K, slightly smaller than the value expected for three Mn II (S = 5/2) noninteracting ions (13.125 cm 3 mol −1 K, g = 2) indicating the existence of antiferromagnetic exchange interactions. This is corroborated by the continuous decrease of χ M T upon cooling down to 10.63 cm 3 mol −1 K at ∼50 K. Below that temperature, the decrease is more abrupt, with χ M T reaching a value of 4.49 cm 3 mol −1 K at 5 K. The 5 K χ M T value is very close to the spin -only (g = 2) value of 4.375 cm 3 mol −1 K for a spin ground state S = 5/2. These results are indicative of antiferromagnetic exchange interactions between the Mn ions of 1 that lead to a spin ground state of S = 5/2.
The magnetic susceptibility was simulated taking into account only one isotropic intracluster magnetic interaction, J, between Mn1 and Mn2 centers since the exchange interaction between the terminal Mn ions of 1 and also of most of the known linear Mn II 3 complexes is negligible (J = 0) [10,11,15,16] because of the large Mn· · · Mn separation (for 1 Mn2· · · Mn2 = 7.598(1)Å). Application of the van Vleck equation [41] to the Kambe's vector coupling scheme [42] allows the determination of a theoretical χ M versus T expression for 1 from the following Hamiltonian: using the numbering scheme of Figure 1, where S 1 = S 2 = S 2 = 5/2. This expression was used to fit the experimental data giving J = −1.50 K and g = 2.00 (solid line, Figure 3).
A temperature-independent paramagnetism (TIP) term was held constant at 600 × 10 −6 cm 3 mol −1 K. The obtained J value is smaller than values reported in the literature for other linear Mn II 3 clusters with three bridging ligands per manganese pair which in most cases range from ∼−2.5 to ∼−7 K [18]. This behaviour could be rationalized on the basis of the existence of only two bridging ligands per manganese pair and larger Mn· · · Mn separations in 1 as was discussed in detail above (description of the structures). There are, however, examples of linear Mn II 3 clusters with J values comparable to that of 1, such as [Mn 3 (L 1 ) 2 (μ-O 2 CMe) 4 ]·2Et 2 O (HL 1 = (1-hydroxy-4-nitrobenzyl)((2-pyridyl)methyl)((1methylimidazol-2-yl)methyl)amine) (J = −1.7 K) [21]. clusters with only two bridging ligands linking each pair of Mn II ions. Variable temperature dc magnetic susceptibility studies revealed the existence of antiferomagnetic interactions between the Mn ions of 1 resulting in an S T = 5/2 spin ground state.