Copper(II) complexes bearing cyclobutanecarboxylate and pyridine ligands: a new series of dinuclear paddle-wheel complexes

Four members of a new series of paddle-wheel copper(II) complexes bearing cyclobutanecarboxylate as bridging ligand with pyridine derived ligands in axial positions are reported. They have been characterised by FTIR-ATR, UV–Vis spectroscopy, mass spectrometry, and single crystal X-ray diffraction. The synthesis is straight-forward by combining the carboxylic acid, copper(II) acetate, and a slight excess of a pyridine ligand. The molecular structures of three complexes reveal a coordination mode expected for such type of dinuclear copper(II) carboxylates.


Introduction
The combination of copper(II) salts with aromatic or alkyl carboxylic acids R-COOH often yield binculear paddlewheel complexes [1][2][3]. The vacant axial position can be occupied by various ligands L, whereas pyridines and its derivatives are most frequently used [4][5][6][7]. Due to the robustness and reliability of the preparation with numerous combinations of R and L (see Scheme 1) and due to advantageous coordination geometry in axial direction, such paddle-wheel structures have been used for the synthesis of MOFs, coordination polymers, and related materials [8][9][10].
Interestingly, cycloalkane carboxylates are extremely rarely used for binuclear copper complexes [11][12][13] and no crystal structures of such complexes have been determined so far. In this work we present paddle-wheel type copper(II) complexes bearing cyclobutanecarboxylate as bridging ligand, which is a carboxylic acid generally not frequently used in coordination chemistry. Pyridine derived ligands are used as axial ligands. The complexes were characterised by FTIR-ATR and UV-Vis spectroscopy as well as by single crystal X-ray diffraction.

Synthesis and characterisation
The complexes 1-4 were prepared by combining copper(II) acetate, an excess of the pyridine ligand, and cyclobutanecarboxylic acid in methanolic solutions at room temperature under aerobic conditions (Scheme 2). Upon stirring, the complexes precipitated out of solution as intense green powders. Crystals of all complexes could be obtained by re-crystallisation from methanol and are stable under ambient conditions. The compounds were characterised by FTIR-ATR, UV-Vis spectroscopy and mass spectrometry. The FTIR-ATR spectra are shown in the supplementary material (Fig.  S1). For the methyl-nicotinate ligands (3 and 4), a signal can be found at 1731 cm −1 , which is characteristic for the -COOMe group and thus is absent for 1 and 2. For the bridging carboxylate, characteristic symmetric and asymmetric vibrations, ν asym (COO -) and ν sym (COO -), can be found at ~ 1607 cm −1 and ~ 1421 cm −1 , respectively. The difference between these bands is 186 cm −1 which is indicative of a bridging binding mode of the carboxylate ligands [2,10,14].

Structural studies
All complexes were obtained as crystalline solids suitable for single crystal X-ray analysis by slow evaporation of their methanolic solutions. However, only the structures of 1, 3, and 4 could be solved. For 2, a unit cell of high symmetry (presumably tetragonal or orthorhombic) could be determined but no reliable structure solution could be found in numerous space groups maybe due to twinning of  the crystals. 1 The molecular structures of 1, 3, and 4 are depicted in Fig. 2 and selected bond lengths and bond angles are summarized in Table 2. Detailed crystallographic data are presented in Table 3. Complexes 1 and 3 were found to crystallise in the monoclinc space groups P2 1 /n and C2/c, respectively. Complex 4 crystallises in the triclinic space group P 1 . For 1 and 4 the asymmetric unit consist of one, for 3 it consists of a half formula unit. In all complexes, four carboxylates bridge two copper atoms forming a paddle-wheel like structure. In the axial positions the pyridine ligands are completing the coordination environment. Only 3 has a crystallographic imposed symmetry with an inversion centre bisecting the Cu-Cu bond. However, the other two complexes show a very similar geometry with just small deviation from inversion symmetry.

Conclusion
In conclusion, we have presented four members of a new series of paddle-wheel complexes bearing cyclobutanecarboxylate as bridging ligand. The synthesis is straight-forward with the combination of the carboxylic acid, copper(II) acetate, and a slight excess of the pyridine ligand. Although crystals could be obtained for all complexes, the crystal structure of only three complexes could be determined by singe-crystal X-ray diffraction. All attempts to solve the structure of the 4-ethylpyridine derivative failed so far. The other complexes reveal the coordination mode expected for such type of binuclaer copper(II) carboxylates.

Experimental
All solvents and reagents were commercially available and used as received. FTIR-ATR spectra were recorded on a Bruker Tensor 27 FT-IR with ATR unit. The spectra were recorded directly from dry powder of each complex.
HR-MS measurements were carried out on an Agilent 6520 QTOF mass spectrometer with an ESI source. For photophysical characterization, spectroscopic grade solvents were used throughout all measurements. Absorption spectra were recorded with a Cary 100 Bio UV-Vis spectrophotometer.
General procedure for the synthesis of complexes 1-4 The pyridine-type ligand ( The solution appears to be blue-green. Methanol is evaporated at room temperature until the complex precipitates. The complex is separated from the solvent by filtration over a Büchner-funnel and is washed with 10 cm 3 of cold MeOH. The compound is dried at room temperature under ambient condition. All compounds are green.  Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creat iveco mmons .org/licen ses/by/4.0/.