Crystal Structure and Thermal Studies of Coordination Compounds

In recent decades, coordination compounds have been of great interest, thanks to their fascinating structures and functional properties. They can be used as catalytic, optical, magnetic, luminescence or electronic materials. The crystal structure and properties of these compounds are influenced by various factors, such as kind and coordination geometries of metal ions, nature of the ligands, kind of counter ions, ratio of using reagents, kind of solvent, etc. The X-ray analysis is often complemented by thermal and spectroscopic analyses. In the case of coordination compounds, the study of their thermal properties is very important because they provide information about physical (e.g., adsorption, chemisorption, crystallization, melting, crystal transition) and chemical (dehydration, desolvation, thermal decomposition, heterogeneous catalysis, etc.) processes occurring in a substance during its cooling or heating. These processes influence the properties and potential application of coordination compounds.

The papers presented in this Special Issue, “Crystal Structure and Thermal Studies of Coordination Compounds”, cover some aspects of coordination compounds from synthesis to characterisation, with an emphasis on descriptions of crystal structure as well as thermal properties.

Chen and co-workers reported the synthesis and properties of two zinc coordination polymers (CPSs) based on N-donor ligand (1,4-di(1H-imidazol-4-yl)benzene) and carboxylic acids (nitroterephthalic acid or 2,5-dibromoterephthalic acid). Single-crystal X-ray diffraction indicates that the nature of substituent groups of dicarboxylic acid plays a significant role in CPs formation. The complex {[Zn1(L)(NO₂pbda)]_n[Zn2(L)(NO₂pbda)]_n}, containing nitroterephthalic acid forms 2D+2D.

3D, inclined polycatenated framework, while the compound [Zn(L)(Brpbda)]_n shows a three-fold interpenetrating 4-connected dmp net. The difference in structures of synthesised compounds affects their thermal properties. The complex [Zn(L)(Brpbda)]_n has a higher thermal stability than {[Zn1(L)(NO₂pbda)]_n[Zn2(L)(NO₂pbda)]_n} [1]. In the next article, Chen et al. described two other zinc coordination polymers also based on the above-mentioned N-donor ligand (1,4-di(1H-imidazol-4-yl)benzene) and two carboxylic acid isomers (1,4-phenylenediacetic (H₂pphda) and 1,2-phenylenediacetic (H₂ophda) acids). In [Zn(L)(pphda)], the zinc ion lies on a glide. The coordination environment around the metal centre is described as a distorted tetrahedral. The N-donor ligands link Zn(II) ions to 1D zigzag chains which are further cross-linked by the pphda²⁻ anions. As a result of these interactions, a 3D four-fold interpenetrated diamond framework is created. The resulting complex is characterized by high thermal stability. Its decomposition process starts above 355 °C and is associated with a collapse of the 3D framework, as well as the combustion of the organic part of the complex. The replacement 1,4-phenylenediacetic acid by its isomer, i.e., 1,2-phenylenediacetic acid, results in the formation of [Zn(L)(ophda)]·H₂O. The central atom is coordinated via nitrogen atoms of two N-donor ligands and oxygen atoms of two ophda²⁻ anions, which leads to the formation of a 2D network. Layers are linked through intermolecular hydrogen bonds into a three-dimensional supramolecular network. The presence of water molecules in this complex lowers the decomposition temperature compared to [Zn(L)(pphda)]. The dehydration process starts at 95 °C, resulting in the formation a solvent-free compound which decomposes above 385 °C [2].

The coordination polymers were also characterised by Kirillov and co-workers. They described the results of a synthesis and characterization of two cadmium complexes prepared by the hydrothermal method from 2′,3′,4′,5′-diphenyl ether tetracarboxylic acid (H₄deta) and 2,2′-bipyridine (bpy). Using different amounts of bpy ligand resulted in the formation of two different CPs, i.e., two-dimensional (2D) [Cd₂(μ₆-deta)(bpy)(H₂O)]_n and three-dimensional (3D) [Cd₂(μ₅-deta)(bpy)₂(H₂O)]_n complexes. In the 2D coordination network, the asymmetric units contain two different cadmium(II) ions (Cd1 and Cd2). The Cd1 centre is seven coordinated with a CdO7 environment in a distorted pentagonal bipyramidal geometry, whereas the six-coordinated Cd2 atom has a distorted octahedral coordination geometry (CdN2O4). Both complexes show similar thermal stability, i.e., the first stage of thermal decomposition starts above 155 °C and is associated with the loss of water molecules. Solvent-free complexes decompose in different ways with the formation of CdO [3].

Kataoka et al. reported the synthesis, crystal structure, thermal and spectroscopic properties of a novel heteroleptic paddlewheel-type coordination compound of rhodium [Rh₂(O₂CCH₃)₃(PABC)(DMF)]_n (where PABC = para-aminobenzenecarboxylate). The complex crystallises in the monoclinic system, space group P2₁/c. In the crystal structure, the Rh₂ core is coordinated by the bridging carboxylate groups of three CH₃COO⁻ ligands and one PABC ligand at the equatorial positions. The authors observed a reversible structural change (self-assembly and disassembly transformations) between the discrete species [Rh₂(O₂CCH₃)₃(PABC)(DMF)₂] (green solution) and the polymeric species (purple solid). The process was accompanied by a colour change, which easily occurred by the dissolution and evaporation procedures with DMF. The results of the thermal analysis gave the possibility to confirm the existence of DMF, CH₃COO⁻ and PABC ligands in the crystal structure of the compound. The complex decomposes in three steps. The first mass loss corresponds to the release of one DMF molecule. The mass loss recorded at higher temperature is connected with the decomposition of the CH₃COO⁻ and PABC ligands. In summary, they reported a new strategy for the construction of self-assembled CPs with paddlewheel-type Rh2-SBUs [4].

Powell et al. described the synthesis and structure of a large porous zeotype network. According to the authors, the slow in situ formation of the hexamethylenetetramine ligand (hmt) seems to be key in generating a µ₄-bridging mode of the hmt-node. The researchers synthesised four interesting structures of Cu(II) coordination polymers by carrying out the reaction in a different solvent system. The synthesis of a complex in a mixture of methanol and acetonitrile 1:1 (V:V) resulted in the formation of a 3D network of {Cu₂(piv)₄} paddlewheel species (piv = pivalate) linked by μ₄-hmt molecules showing the MTN zeolite topology (ZSM-39). The reaction conducted in the dichloromethane resulted in the formation of a compound consisting of distorted chair-shaped ten-membered rings which form a helical network. The presence of polyethyleneglycol leads to the formation of intertwined, DNA-like double-helix structures, while tetrahydrofuran favours the creation of a solvent-free complex with a 1D chain structure [5].

Osypiuk and co-workers reported the synthesis, crystal structure, thermal and spectral properties of new mononuclear [Cu(HL¹)]·H₂O, dinuclear [Cu₂(L¹)(OAc)(MeOH)]·2H₂O·MeOH and tetranuclear [Cu₄(L²)₂(OAc)₂]·4MeOH, [Cu₄(L²)₂(OAc)₂]·4H₂O·4MeOH complexes with hepta- and pentadentate Schiff base ligands. The mono- and dinuclear coordination compounds crystallise in the triclinic system, space group P-1. The tetranuclear complexes crystallise in the monoclinic system, space group P2₁/n and P2₁/c, respectively. In the crystal structure of the dinuclear compound, the metal(II) ions are linked by alkoxo- and carboxylato-bridges. The tetranuclear coordination compounda are formed from dinuclear species linkage through the phenoxo oxygen atoms of the fully deprotonated Schiff base ligand. The results of thermal analysis confirm that the complexes are thermally stable at room temperature. Their decomposition processes in air follow in a similar way. The first decomposition step concerns the release of solvent molecules (water and/or methanol). At a higher temperature, the organic part undergoes defragmentation and combustion. The main gaseous products resulting from the thermal degradation of complexes in N₂ are water, methanol, acetic acid, methane, carbon monoxide, carbon dioxide and ammonia. Copper oxide is the final decomposition solid product of the compounds in air [6].

This Special Issue, “Crystal Structure and Thermal Studies of Coordination Compounds” provides an overview of some recent research in this field. We hope that it will encourage readers to use both methods in their research.