A new metal–organic structure with alternating one-dimensional serpentine ribbon chains and two-dimensional layer networks of different topology

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Abstract

Alternating one-dimensional serpentine ribbon and two-dimensional layer networks have been found in [Cu(DPA)CO₃]·3H₂O. The serpentine ribbon network consists of hydrogen-bonded pentagons formed from well-ordered water molecules, whereas the two-dimensional layer displays bifunctional 2,2′-dipyridylamine-linked zigzag inorganic chains of copper(II) carbonate.

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Introduction

Crystal engineering of coordination polymers constructed via a net-based approach has been a successful synthetic route to new framework structures.¹ The deliberate selection of organic building blocks as well as metal centers, based on their geometric preference and different functionality, is the essential step of this strategy and has produced many fascinating metal–organic polymers.² The construction of extended metal–organic structures based on bifunctional organic ligands is very attractive since it utilizes both the covalently-bonded metal–ligand and hydrogen-bonded ligand–ligand properties, and the ligand functionality is retained in the network structures.³ The incorporation of hydrogen bonding into crystal structures may have interesting applications. For example, metal complexes with hydrogen bonding capabilities may be used to generate metal–organic nanofibers,^4a and also for use as chemotherapeutic reagents.^4b 2,2′-Dipyridylamine is a flexible ligand with bifunctionality, capable of forming both covalent- and hydrogen-bonds,⁵ and other interesting properties.⁶ While the bifunctionality of flexible 2,2′-dipyridylamine has been well utilized by mixing with rigid organic building blocks, such as oxalate, in order to construct two-dimensional frameworks,⁵ mixing of the flexible unit with carbonate–metal chain structures has been less well studied. Carbonate anions are simple inorganic units with tridentate capability and have been widely used as bridges or counter-anions in network complexes.⁷ Pure carbonate–metal extended inorganic chain structures may therefore be reasonably expected (see Scheme 1).⁷ The carbonates are good candidates for hydrogen bonding with bifunctional organic ligands in the network structures, and the inorganic chain in Scheme 1 can be expected to be linked by bifunctional 2,2′-dipyridylamine with both covalent and hydrogen bonds to form a two-dimensional metal–organic polymer. Here, we report a novel metal–organic polymeric structure with alternating one-dimensional serpentine ribbon chains and two-dimensional layer networks in [Cu(DPA)CO₃]·3H₂O 1 (DPA⊕=⊕2,2′-dipyridylamine).

Scheme 1 View of the carbonate–metal chain.

Results and discussion

Complex 1 was synthesized from a reaction of copper carbonate (basic) with 2,2′-dipyridylamine in a molar ratio of 1∶2 in a 23 ml acid digestion bomb under hydrothermal conditions. The obtained blue-plate crystals of 1 were suitable for single crystal X-ray diffraction analysis; details of the crystal structure solution and refinement are listed in Table 1.

Table 1 Crystal data and structure refinement for [Cu(DPA)CO₃]·3H₂O 1^a,b

Parameter
a Full-matrix, least squares refinement on F^2. b Click b106232h.txt for full crystallographic data (CCDC 157234).
Formula	[Cu(DPA)CO3]·3H2O
M	348.80
Crystal system	Monoclinic
Space group	P21/c
a/Å	11.209(1)
b/Å	7.107(1)
c/Å	17.339(1)
β/°	100.891(1)
V/Å^3	1356.37(10)
Z	4
T/K	223(2)
D c/g cm^−3	1.708
µ/mm^−1	1.643
Goodness-of-fit on F^2	1.076
Reflections collected	7092
Independent reflections	2605 [Rint⊕=⊕0.0227]
Final R indices [I⊕>⊕4σ(I)]	R 1⊕=⊕0.0212, wR2⊕=⊕0.0582
R indices (all data)	R 1⊕=⊕0.0246, wR2⊕=⊕0.0601

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The structure of 1 consists of square-pyramidal copper atoms coordinated by bidentate 2,2′-dipyridylamine and carbonato ligands at the equatorial positions, the apical position being occupied by another carbonato ligand in a monodentate fashion (Fig. 1).

Fig. 1 View of the coordination about Cu showing the atom numbering scheme. Thermal ellipsoids are 40% equiprobability envelopes, with hydrogen atoms as spheres of arbitrary diameter. A symmetry-related carbonate molecule has been added to complete the Cu coordination sphere.

The copper coordination sphere is distorted due to the geometrical limitations from the bidentate ligands. The angles around the copper atoms are N(1)–Cu–N(2), 93.57(7); N(1)–Cu–O(1), 98.76(6); N(2)–Cu–O(1), 161.99(6); N(1)–Cu–O(2), 162.13(6); N(2)–Cu–O(2), 99.43(6); O(1)–Cu–O(2), 65.96(6); N(1)–Cu–O(3)#1, 99.23(6); N(2)–Cu–O(3)#1, 92.37(6); O(1)–Cu–O(3)#1, 98.51(5) and O(2)–Cu–O(3)#1, 92.47(6)°. These distortions should be logically expected. The copper atoms are linked by carbonato ligands in both bidentate and monodentate fashions to form a zigzag, one-dimensional inorganic chain along the b axis (Fig. 2). The Cu–N bonds vary from 1.972(2) to 1.980(2) Å. The Cu–O distances range from 1.983(1) to 1.989(1) Å for the equatorial covalent bonds whereas the apical O–Cu distance is longer, 2.287(1) Å (Fig. 1), and these bonding properties from the square-pyramidal geometry are expected.

Fig. 2 View of the Cu–carbonato chain along the b axis with DPA omitted.

Similar bonding properties have been observed in copper square-pyramidal coordination structures of copper(II) oxidation states.⁸ The bifunctional 2,2′-dipyridylamine ligands covalently-bonded to copper atoms clearly favor the formation of hydrogen bonding with the open-NH groups (Fig. 3). The one-dimensional chains are then connected by mutual hydrogen-bonding linkages [N(3)–H(3N)⋯O(3), 2.785(2), 0.74 (N–H), 2.05 (H⋯O) Å, 168(3)° (N–H–O)] between the chains to result in a two-dimensional network lying parallel to the (1 0 0) plane (Fig. 4).

Fig. 3 View of the zigzag, one-dimensional chain with open-NH groups; only amine hydrogen atoms are shown.

Fig. 4 View down to the a axis of the two-dimensional network with mutual hydrogen bonds between the one-dimensional chain structures.

It is noteworthy that there are water molecules between the two-dimensional networks with well-ordered arrays of one-dimensional chains (Fig. 5).

Fig. 5 Space-filling view of the ordered water molecule array.

The arrays of water molecules linked by hydrogen bonds [O(4)–H(4A)⋯O(5)#1, 2.812(3), 0.95 (O–H), 1.87 (H⋯O) Å, 171.1° (O–H–O); O(5)–H(5A)⋯O(6), 2.747(3), 0.95 (O–H), 1.83 (H⋯O) Å, 160.9° (O–H–O); O(6)–H(6B)⋯O(4)#1, 2.730(2), 0.95 (O–H), 1.78 (H⋯O) Å, 172.6° (O–H–O); and O(6)–H(6A)⋯O(5)#3, 2.914(2), 0.95 (O–H), 2.01 (H⋯O) Å, 158.4° (O–H–O)] result in serpentine ribbon networks (Fig. 6).

Fig. 6 View of the serpentine ribbon network comprised of edge-sharing pentagons extended along the b axis; hydrogen atoms are omitted for clarity.

The serpentine ribbon network comprises edge-sharing pentagons extended along the b axis. The separation between two serpentine ribbon chains of adjacent layers is 11.209(1) Å along the a axis while the distance between two serpentine ribbon chains of the same layer is 8.669(1) Å. The one-dimensional serpentine ribbon chains and the two-dimensional layers are linked by hydrogen bonds [O(4)–H(4B)⋯O(2), 2.792(2), 0.95 (O–H), 1.87 (H⋯O) Å, 164.3° (O–H–O); and O(5)–H(5B)⋯O(1), 2.782(2), 0.95 (O–H), 1.84 (H⋯O) Å, 169.0° (O–H–O)] (Fig. 7). Many water molecule-inclusion coordination polymers have been synthesized where water molecules are often disordered in channeled structures of coordination networks.⁹ The non-ordered water molecules, in some structures, were used to fill in the voids in the interlayer region.¹⁰ The alternating one-dimensional serpentine ribbon chains and two-dimensional layer networks observed in 1 are unique and have never been reported.

Fig. 7 View perpendicular to the a–b plane showing the hydrogen bonding between adjacent stacks of serpentine ribbon chains and the layers; only water hydrogen atoms are shown. Click image or 7.htm to access a 3D representation.

Metal–organic network structures constructed from different chemical components and topologies have been a very attractive subject in the crystal engineering of coordination polymers. Given the fact that the parameters under hydrothermal conditions are complicated, unexpected structures may be obtained.⁸ The resulting new findings of the alternating one-dimensional serpentine ribbon and two-dimensional layer networks in 1 provide a new example of coordination polymers displaying different chemical and topological features, and we are currently investigating the conditions to utilize well-ordered water molecules in the crystal engineering of coordination polymers.

Acknowledgements

The authors thank the financial support from the Welch Foundation and assistance from Dr J. Korp with the crystallography. This work made use of MRSEC/TCSUH Shared Experimental Facilities supported by the National Science Foundation and the Texas Center for Superconductivity at the University of Houston.

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