The structure of a coordination polymer constructed of manganese(II) biphenyl-4, 4′-dicarboxylate

1 Introduction

Metal-organic frameworks (MOFs) are one-, two-, or three dimensional micro-, meso-, or macro-porous coordination polymers which are constructed of metal cations as connectors and organic ligands as linker molecules [1–3]. They possess high surface areas (>3000 m² g^–1) and pores, the sizes of which cover the range from less than 20 Å to larger than 500 Å. Due to the broad applicability of metal-organic frameworks the interest in the development and design of new architectures is very high, and – as a consequence – numerous books and review articles have been published in the last decade. As examples, metal-organic frameworks are applied as hybrid materials for the purification of small gaseous molecules [4], as heterogeneous catalysts [5–7], in photonics [8], and for the design of new materials [9]. They have also been applied as separation membranes [10], for water adsorption [11], in device fabrications [12], in artificial photosynthesis and photocatalysis [13], and methane [14] as well as hydrogen storage [15–18].

Biphenyl-4,4′-dicarboxylate (bpdc) is a well suited organic linker of coordination polymers as well as metal-organic frameworks. Thus it has been applied very recently for the construction of a quaternary Zn₄O-carboxylate metal-organic framework for carbon dioxide and methane sorption [19], for the preparation of aluminum based coordination polymers possessing polysilane chains with hole-transporting abilities [20], and as building block of the robust Zr₆O₄(OH)₄(bpdc)₆ framework for the incorporation of a photosensitizer and proton reduction catalyst [21]. It was used to prepare the new flexible vanadium-containing metal-organic framework COMOC-2 for adsorption of light hydrocarbons [22] as well as new Ni, Co, and Cu-based coordination polymers for the adsorption of carbon dioxide, methanol, and water [23]. The Li salt of biphenyl-4,4′-dicarboxylate, Li₂(bpdc), was described as ultra-light metal-organic framework (ULMOF) which consists of alternating antifluorite type LiO layers connected by bpdc bridging units forming a 3D network [24]. The Zn and Cd coordination polymers {[Zn(bpdc)(H₂O)₂]· H₂O}_n and [Cd(bpdc)(H₂O)]_n have also been described [25].

In continuation of our interest in metal-organic frameworks [26, 27], adsorption of organic substrates on metal surfaces [28], catalysis [29], and heterocycle synthesis from carbenes and polycations [30–32] we report here on the structure of the Mn salt of biphenyl-4,4′-dicarboxylate.

2 Results and discussion

The manganese salt of biphenyl-4,4′-dicarboxylate 3 was obtained by slow crystallization of a mixture of biphenyl-4,4′-dicarboxylic acid 1, di(4-pyridyl)acetylene 2, and manganese(II) chloride in a mixture of dimethylformamide, ethanol and water in a closed vessel which was heated at 110 °C for three days (Scheme 1). After cooling to room temperature the product 3 was formed as complex {[Mn(bpdc)(H₂O)₂][Mn(bpdc)(H₂O)]} which crystallized as long yellow needles. Di(4-pyridyl)acetylene 2 played the role of a base under these conditions and remained in solution as salt 4. It has been found earlier that the coordination polymer consisting of cobalt(II) biphenyl-4,4′-dicarboxylate did neither react with pure water nor with pure pyridine, but it dissolved in pyridine-water mixtures and formed one-dimensional chains of ¹_∞[Co(bpdc)(py)₂(H₂O)₃] · 2 py or 2D networks of ²_∞[Co(bpdc)(py)₂] · H₂O [33]. In our case no crystallization or MOF formation under participation of the bipyridine present was observed. A thermogravimetric analysis of 3 under a nitrogen atmosphere showed the loss of three molecules of water (–8.66%) between 150 °C and 190 °C. At approximately 400 °C decomposition of the material begins.

Scheme 1:

Manganese salt formation from biphenyl-4,4′-dicarboxylate 1 and manganese(II) chloride in the presence of di(4-pyridyl)acetylene 2 as base.

The results of a single crystal X-ray analysis are given in Table 1 and are shown in Figs. 1, 2, and 3. Selected bond lengths and bond angles are presented in Table 2. The obtained species crystallized in the monoclinic space group C2/c with Z=4.

Fig. 1:

Biphenyl-4,4′-dicarboxylate, manganese(II) and coordinated water molecules in the crystal structure of 3 (displacement parameters are drawn at the 50 % probability level, H atoms as spheres with arbitrary radii; crystallographic numbering; for symmetry operations used see footnote at Table 2).

Fig. 2:

Crystal structure of 3 as projected down the crystallographic b axis.

Fig. 3:

Crystal structure of 3 in projection down the crystallographic c axis.

The crystal structure of 3 is a 3D network containing Mn atoms in two different symmetry environments. The manganese atom Mn1 (occupation 0.5) is located in a 2 (C₂) symmetric environment with 5-fold coordination. Four bonds are formed to carboxylate groups (2×O1, 2×O2) and one bond is formed to a water molecule (O3). Mn1 and the oxygen atom of the water molecule (O3) reside on the crystallographic twofold axis. The Mn2 atom (occupation 0.5) is located in a crystallographic 1̅ (C_i) symmetric environment and is six-fold coordinated. Here, four carboxylate groups (2×O4, 2×O5) and two water molecules (2×O6) form an octahedron. Each Mn cation is bonded to the four biphenyl-4,4′-dicarboxylate anions via monodentate carboxylic oxygen and the aforementioned molecules of water. The C–O bond lengths of the carboxylate groups of bpdc are slightly different. Thus, the distance C1–O1 was determined to be 1.269(2) Å, whereas the bond length C1–O2 amounts to 1.251(3) Å. The corresponding bond lengths of O1 and O2 to manganese cations are 2.1558(16) Å and 2.0653(14) Å. The C–O bond lengths of the second carboxylate group were found to be 1.248(2) and 1.278(2) Å. Their bond lengths to manganese cations are 2.1795(14) and 2.1642(15) Å. The bond lengths of the manganese cations to oxygen atoms of the water molecules are 2.100(2) Å and 2.1761(14) Å. The two phenyl rings of the biphenyl-4,4′-dicarboxylate of the asymmetric unit are slightly tilted against each other as is exemplified by the dihedral (torsion) angle C4–C5–C8–C9=28.9(3)°. One of the carboxylate planes is virtually in plane with the phenyl ring to which it is attached as is evident from the dihedral angle C3–C2–C1–O1=0.3(3)°. The second carboxylate group is slightly more tilted with respect to the other phenyl ring (dihedral angle C10–C11–C14–O4=–13.8(3)°). For further dihedral angles see Table 2.

2.1 STM Results

To get real space images of the new compound, scanning tunneling microscopy (STM) measurements of thin films of 3 adsorbed on highly oriented pyrolytic graphite (HOPG) were performed. For thin film preparation highly diluted solutions (2×10^–9 mol L^–1, methanol) of manganese(II) chloride and the ligand were mixed and subsequently dropped onto the HOPG substrate which was used due to its inertness and its flat surface geometry. STM was carried out after the methanol was evaporated. Fig. 4 shows two STM images recorded at room temperature of 3 prepared on the HOPG surface. It becomes apparent that 3 forms molecular strands under these conditions. According to the measured line profile seen in Fig. 5, the averaged diameter of the strand amounts to 18±1 nm. The apparent height is about 0.5±0.02 nm. Considering the dimensions of the unit cell of 3 as measured by X-ray crystallography, the STM images suggest that the strands are built up by several agglomerated unit cells.

Fig. 4:

STM images of coordination polymer 3 prepared on HOPG sufaces. The images show a molecular strand adsorbed on the HOPG surface. (a) 217×217 nm², I_t=0.8 nA, U_t=0.5 V. (b) 87×87 nm², I_t=8 nA, U_t=0.5 V.

Fig. 5:

Averaged line profile of the molecular strand seen in Fig. 4 (see also inset). The averaged diameter amounts to 18±1 nm. The height of the strand is about 0.5±0.02 nm.

3 Experimental section

Chemicals used were obtained from commercial suppliers and used without further purification. Solvents were distilled before use.