NOVEL 24-MEMBERED OCTANUCLEAR MANGANESE(III) METALLACROWN

A novel octanuclear manganese metalladiazamacrocycle (1) was synthesized employing a new pentadentate ligand N-methacryl-5-methylsalicylhydrazide (H3mamshz) by supramolecular self assembly. The backbone of this metal-organic assembly is a repeating unit of Mn–N–N–Mn linkage that extends to complete a 24-membered cyclic structure involving 8 manganese(III) centers. Successive manganese centers are in an octanuclear cyclic structure. The temperature-dependent magnetic properties show a typical weakly coupled antiferromagnetic behaviour.


INTRODUCTION
Metallamacrocycles have become important in recent years because of their interesting molecular architecture [1], multinuclear structures [2] and magnetic properties [3].They have also been used as building blocks for the construction of two-or three-dimensional network structures [4].A variety of metallamacrocycles and cages were found to form interesting hostguest systems with different metal ions of varying coordination and symmetry [5].Metal ions such as Ga, Co, Fe and Mn that can easily form stable octahedral coordination, are found to yield hexanuclear metallamacrocycles with trianionic pentadentate ligands [6].However manganese and iron have received special attention because of their ease of formation of metallamacrocycles and interesting magnetic properties [7].Controlling the size and nuclearity of metallamacrocycles and their properties has quite recently become of interest.Although there have been reports of metallamacrocycles and metallacrowns of high nuclearity containing iron and manganese, formation of stable cyclic high nuclearity structures has been an uphill task.

Materials and physical measurements
All chemicals used were of analytical grade and were used in this experiment without further purification.
Carbon, nitrogen and hydrogen analyses were determined using a Vario EL elemental analyzer.IR spectra were recorded on a Nicolet Avatar 360 FT-IR instrument using KBr discs in the 400-4000 cm -1 region. 1 H NMR spectra were measured on a FT-400A spectrometer in dmso-d 6 solution, with TMS as internal standard.The intensity data were collected at 296 K on a Bruker Smart APEX II diffractometer with graphite-monochromatized Mo Kα radiation (λ = 0.71073 Å).The structure was solved by direct methods and refined by full matrix least-squares techniques on F 2 with all non-hydrogen atoms treated anisotropically.All calculations were performed with the SHELXTL program package.Non-hydrogen atoms with geometrical rigidity were refined anisotropically, but non-hydrogen atoms with geometrical flexibility were refined isotropically; hydrogen atoms attached to the non-disordered part were assigned isotropic displacement coefficients U(H) = 1.2U(C) or 1.5U(C methyl ), and their coordinates were allowed to ride on their respective atoms.Temperature-dependent magnetic susceptibility measurements were carried out on powdered samples between 2 and 300 K using a Quantum Design MPMS-7XL SQUID magnetometer.Field-cooled magnetization data were collected at H = 1000 Oe.The diamagnetic correction for complex was calculated using Pascal's constants.

IR spectra
The IR spectra of H 3 mamshz and compound 1 are listed in Table 1.The data of compound 1 show the stretching vibration of the C=O band shifted to lower frequencies and the O-H (phenolic) shifted to higher frequencies compared with the ligand, suggesting the coordination of carbonyl oxygen atom and the O (phenolic) atom to Mn 3+ ion.The stretching vibration band (N-H) disappeared in the spectra of compound 1 indicating the deprotonation of the group and coordination of the hydrazide nitrogen atom to the Mn 3+ ion.The bands of C-N at 1286 and 1315 cm -1 are shifted to 1562 cm -1 due to the deprotonation of NH group.

X-ray diffraction data of compound 1
The molecular structure of the title azametallacrown is shown in Figure 1.Single-crystal X-ray analysis showed that compound 1 crystallizes in triclinic space group P-1.There are two octanuclear metallamacrocycle: [Mn 8 (mamshz) 8 (DMA) 8 ] (ring A) and £-1 [Mn 8 (mamshz) 8 (DMA) 6 (H 2 O) 2 ] (ring B), two DMA and four H 2 O molecules in an asymmetric unit (Table 1).Eight Mn(III) ions and eight deprotonated N-methacryl-5-methylsalicylhydrazidate (mamshz 3-) ligands assemble to form a planar 24-membered ring based on Mn-N-N-Mn linkage in the title azametallacrown.The ligand binds to the metal in a back-toback fashion, resulting in an octanuclear cyclic structure.The adoption of a sequence of chiralities by a certain kind of macrocycle is triggered by the tendency of the system to choose the lowest energy macrocycle (Figure 1, Table 2).The Mn…Mn…Mn interatomic angles in the 24-membered core rings are in the range 121.07(3)-134.38(2)° and 121.32(3)-130.27(4)° in the ring A and ring B, respectively.These values are quite close to the value of the interior angle in an n-octagon (135°).The bond distances for Mn-N (diazine), Mn-O (phenolate), and Mn-O (carbonyl) are in the ranges 1.92-2.36,1.85-1.89,and 1.94-1.99Å, respectively.The Mn(III)-O(DMA) bond distances are from 2.21 to 2.30 Å.The Mn(III)-O (H 2 O) bond distances are from 2.24 to 2.25 Å.The terminal groups of the tails of 4 ligands are directed to the top of the cyclic structure, while the rest are directed down the cycle.The disc like molecule offers a cavity with an entrance diameter ~ 3.6 Å and ~ 12.9 Å at the interior which may serve as potential molecular recognition site.There are no molecules in the 'host' cavity of the metallacrown.
The local environment of each metal centre of high-spin d 4 Mn(III) ion has the same Jahn-Teller elongated octahedral geometry as in other manganese metallamacrocycles [6].The Jahn-Teller distortion results in elongation of the axial manganese-oxygen/nitrogen bond lengths by ~ 0.3 Å relative to the other Mn-O/N bond lengths.The cyclic structure of the compound leaves a nano disc-shaped molecule (Table 3, Table 4).

Magnetic measurements
The temperature dependence of the magnetic susceptibility of compound 1 has been determined at a 1000 Oe field in 2-300 K temperature range.The diamagnetic correction was evaluated using Pascal's constants.As shown in Figure 2, the observed χ M T value for Compound 1 at 300 K is 58.67 cm 3 K mol -1 and upon cooling from 300 to 35 K, χ M T decreases slowly to approximately 30.24 cm 3 K mol -1 and then drops very rapidly to reach 2.41 cm 3 K mol -1 at 2 K. Figure 2 shows a typical weakly coupled antiferromagnetic behaviour.Thus, by fitting the magnetic susceptibility data at high temperatures, T > 35 K, to the Curie Weiss expression χ -1 (T) = C/(T+Θ), we obtained the Weiss constant, Θ = -32.61K for 1.The supplementary crystallographic data for (1) have been deposited under the number CCDC 796822 and can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Figure 1 .
Figure 1.Molecular structure of the ring A of the title compound 1 with Mn, N, O atoms labeling.H atoms and free solvent molecules are omitted for clarity.All manganese ions in 1 are in a distorted octahedral MnN 2 O 4 environment with a very distinct Jahn-Teller extension.The resulting octanuclear systems measure ~ 21.5 Å in diameter and ~ 10 Å in thickness.The neighboring Mn…Mn interatomic distances are 4.904(2)-4.983(2)Å and 4.871(2)-4.984(2)Å in ring A and ring B, respectively.The Mn…Mn…Mn interatomic angles in the 24-membered core rings are in the range 121.07(3)-134.38(2)° and 121.32(3)-130.27(4)° in the ring A and ring B, respectively.These values are quite close to the value of the interior angle in an n-octagon (135°).The bond distances for Mn-N (diazine), Mn-O (phenolate), and Mn-O (carbonyl) are in the ranges 1.92-

Figure 2 .
Figure 2. Plot of χ M T and χ -1 vs. T of polycrystalline sample of 1.

Table 2 .
Crystal data and structure refinement of compound 1.

Table 4 .
Selected bond angles of Compound 1.