Catena-[bis(o-aminobenzoato-κ³ N,O:O′)Mn(II)]

1 Introduction

In recent years research on coordination polymers has expanded rapidly because of their fascinating structural diversity and potential application as functional materials (Moulton and Zaworotko, 2001; Batten and Robson, 1998). Multifunctional carboxylate frameworks are of great interest due to their intriguing network topologies and their potential applications as microporous, magnetic, nonlinear optical and fluorescent materials (Zaworotko, 2000; Ward et al., 1997; Fujita et al., 1995; Armentano et al., 2001; Lin et al., 1998; Tong et al., 1999). The carboxylate group cannot only bridge two or more metal centers to produce a wide variety of complexes ranging from zero-dimensional discrete molecules to three dimensional architectures, but also can adopt various types of bridging conformation modes, such as the so-called syn-syn, syn-anti, and anti–anti modes (Scheme 1).

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Scheme 1 Schematic representations of the coordination modes for the carboxylate bridges.

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In terms of constructing the extended structural motifs, either di- and multi-carboxylato ligands, or multifunctional carboxylate-containing ligands incorporating other coordination groups containing N and O,O′ atoms, have been employed (Barthelet et al., 2002; Konar et al., 2002; Rueff et al., 2002). The N or O,O′-donor ligands adopt different coordination modes forming different kinds of building blocks, which are finally combined to generate different supramolecular architectures.

In this paper, we report the preparation and crystal structure of a new two dimensional coordination polymer formulated as [Mn(2-aba)₂]_n (1).

2 Experimental

A mixture of Mn(ClO₄)₂⋅4H₂O (150 mg, 0.46 mmol), H₂cp-Ala [2-(carboxyphenyl) imino-2-propanoic acid] (90 mg, 0.43 mmol), NaOH (47 mg, 1.18 mmol), ethanol (15 ml) and dichloromethane (15 ml) was refluxed for 30 min. The turbid solution was then filtered after being cooled to room temperature and slowly evaporated at ambient temperature to afford pale brown crystals of [Mn (2-aba)₂]_n after one month, Yield 23%.

3 Results and discussion

The title coordination polymer was prepared by reacting Mn(ClO₄)₂⋅4H₂O with Ala[2-(carboxyphenyl)-imino-2-propanoic acid] in the presence of base.

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The symmetric unit of (1) contains one manganese cation and four o-aminobenzoate ions. The Mn(II) atom is six-coordinate, forming a slightly distorted octahedral with MnN₂O₄ ∼ chromophore. In the coordination polyhedron, the equatorial plane is occupied by four O (O1, O1ⁱ, O2ⁱⁱ and O2ⁱⁱⁱ) atoms from four o-aminobenzoate anions and the axial sites situated two N (N1 and N1ⁱ) atoms from two o-aminobenzoate anions already associated with the equatorial plane (Fig. 1). The Mn(II) center is coordinated by two types of o-aminobenzoate anions, one behaves in a chelating mode [Mn—O1 = 2.1288(17) Å and Mn—N1 = 2.311(2) Å], the other one acts as a monodentate [Mn—O2 = 1.1801(19) Å] (Table 1) which is isostructural with Zn and Cu analogs (Haendler et al., 1983; Haendler and A lbanese, 1975) (Fig. 2).

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Figure 1 The ORTEP drawing of the title compound (1). Displacement ellipsoids are drawn at 50% probability level, symmetry codes: (i) −x + 1, −y, −z + 2, (ii) −x + 1, y − 0.5, −z + 5/2.

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Figure 2 View of the coordination environment around Mn(ii).

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The bond angles between the cis-donors around Mn(II) lie in the ranges 79.59(8)–90.87(8)°. The length of two manganese–carboxylate distances is none equivalent (Mn—O, 2.1288(17) and 2.1801(19) Å), while the manganese amino-bond (Mn—N, 2.311(2) Å) is slightly longer (Table 1). These bond angles and bond distances are slightly different from those observed in the [Mn(3-aba)2]_n complex (Ruihu et al., 2006). Hence the difference in Mn—O bond length in (1) may be attributed to the packing forces rather than differing in oxygen basicity. The 2-aba ligand acts as an exo-tridentate bridging mode through the amino nitrogen atom and μ₂,n²—carboxylate group (1). The double syn-anti mode μ_1,3-carboxylato–O,O bridges coming from two 2-aba ligands exist between neighboring Mn(II) ions forming an infinite Mn—O—C—O chain with the C—O—Mn bond angles being 128.09(17) and 132.461(17). The closest Mn⋯Mn distance within the chain is 5.4246(2) Å and the shortest inter-chain Mn⋯Mn distance is 5.2369(3) Å. The inter-chain distance is much smaller than the observed value of 8.129 Å between the Mn-atoms in the case of the related [Mn(3-aba)₂]_n complex, while the Mn⋯Mn distance within the chain of (1) is longer than the observed value of 4.607 Å in the same complex. This may be related to the mode of bonding of the carboxylate group syn–syn versus syn–anti. There exist hydrogen-bonding interactions in the two-dimensional network between the hydrogens of the amino group of one molecule and the oxygen of the carboxylate group of another molecule. The average D⋯A distances for N1—H2⋯O1 and N1—H1⋯O2 of (1) are within the range compared with 2.998 and 2.954 Å for 2-aba complexes of Co²⁺ and Ni²⁺ ions respectively (Wel You-Huan et al., 2006). The hydrogen bond geometry is listed in (Table 2). No hydrogen bonds are observed between 2D networks. The adjacent two-dimensional network is packed only by Van der Waals interactions (see Fig. 3).

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Figure 3 View of a pack diagram along the b axis showing the two-dimensional network.

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4 Conclusion

From all of these we conclude that by employing aromatic amino acids with different substituent positions (namely, ortho, metha or para) in addition to the type of metal cation one can easily vary the resultant supramolecular organization. By using 2-aminobenzoate as a ligand with different metal ions such as Mn²⁺ and Ca²⁺ or Sr²⁺ one can get different coordination modes especially with the amino group. In the case of Mn²⁺ ion (1) the amino group chelates the metal ion while in the case of Ca²⁺ and Sr²⁺ ions the amino group does not take part in the coordination in the former and while in the latter coordinates to the neighboring Sr²⁺ ion (Murugavel et al., 2000). By changing the position of the amino group in the aminobenzoate from 2 to 3 different supramolecular organizations will result, for example the 3-aminobenzoate ligand coordinates to the Mn²⁺ ion in different way compared to the title complex, but similar to Co²⁺ and Ni²⁺ complexes (Wel You-Huan et al., 2006).