Synthesis, Crystal Structures and Hirshfeld Surface Analysis of Coordination Compounds (3rd Edition)

1. Introduction

The third edition of the Special Issue of Crystals, entitled “Synthesis, Crystal Structures and Hirshfeld Surface Analysis of Coordination Compounds”, continues to serve as a high-profile forum for researchers to share their latest findings in the field of coordination chemistry. This edition brings together a collection of ten high-quality original research articles [1,2,3,4,5,6,7,8,9,10] that showcase the indispensable “triad” of modern structural chemistry: innovative chemical synthesis, precise X-ray structure determination, and insightful Hirshfeld surface analysis (HSA). The coordination compounds featured in this volume encompass a broad range of metal centers, including transition metals (Ni, Co, Cu, Mn, Zn, Cd, Ag, Fe) and lanthanides (Eu and Tb). In addition, the reported studies involve diverse ligand systems, including Schiff bases, carboxylates, N-donor ligands, and organophosphorus compounds. The articles collected in this Special Issue demonstrate that structural characterization is no longer limited to determining atom connectivity, but increasingly involves understanding the complex network of non-covalent interactions governing crystal packing. A recurring theme throughout this edition is the application of Hirshfeld surface analysis and two-dimensional fingerprint plots to quantify the contributions of various intermolecular contacts, including hydrogen bonding, π–π stacking, and van der Waals interactions. Hirshfeld surface analysis has become a cornerstone of this Special Issue series. Hirshfeld surface analysis has become one of the defining features of this Special Issue series. By visualizing the “molecular shape” within the crystal lattice, HSA enables researchers to move beyond traditional distance-based descriptions of intermolecular interactions. The fingerprint plots presented in these studies provide a valuable “barometer” for comparing structural analogs and understanding why certain compounds crystallize in specific motifs, thereby contributing to the rational design of crystalline materials.

2. Overview and Analysis of the Contributions

The study by Roller et al. [1] reports a novel and efficient synthetic route toward neutral metal-functionalized heptaphospha-nortricyclane (P₇) cage compounds using a halodesilylation strategy. Starting from the heteroleptic precursor (hyp)₂(tms)P₇, the authors successfully prepared previously unknown coinage-metal derivatives of gold(I), silver(I), and copper(I), as well as a zinc(II) complex. A major finding of this work is that the resulting compounds exhibit distinct structural motifs depending on the metal center. The gold derivative forms a dinuclear complex, whereas the silver and copper analogs adopt tetrameric architectures with extended metal–phosphorus frameworks. All coinage-metal complexes display short metal–metal distances, indicating significant closed-shell metallophilic interactions, including aurophilic, argentophilic, and cuprophilic interactions. The study further demonstrates that the sterically demanding hypersilyl substituents effectively stabilize the P₇ cage and enable selective functionalization without degradation of the phosphorus framework, which had represented a significant challenge in earlier studies. Additionally, a neutral zinc complex was synthesized and fully characterized, confirming the broader applicability of this synthetic approach to other metal systems. In manuscript [2], the authors provide insights into the synthesis and comprehensive characterization of two novel copper(II) complexes containing pyridoxal-aminoguanidine (PLAG) ligands with different counterions (nitrate and sulfate). One of the key findings is that the nature of the counterion plays a decisive role in determining the coordination geometry. The nitrate-containing complex adopts a square-pyramidal structure, whereas the sulfate analog exhibits a square-planar geometry. Detailed crystallographic and Hirshfeld surface analyses revealed that intermolecular interactions and crystal packing are strongly influenced by the type of counterion, particularly through hydrogen bonding and electrostatic interactions. Complementary DFT and QTAIM calculations confirmed these observations and demonstrated that the Cu–N bonds possess partial covalent character, while the Cu–O interactions are generally weaker. Biological studies further demonstrated that both complexes interact effectively with human serum albumin (HSA) and DNA, with the nitrate-containing complex exhibiting significantly stronger binding affinity. Spectroscopic and thermodynamic analyses indicated that the binding process is spontaneous, entropy-driven, and governed primarily by hydrogen bonding and van der Waals interactions. The research team from the University of Münster [3] successfully addressed long-standing challenges in the coordination chemistry of copper(II) acetylacetonate ([Cu(acac)₂]). One of the principal achievements of this work was the structural elucidation of its pyridine adduct. Despite its apparent chemical simplicity, the adduct formed between [Cu(acac)₂] and pyridine had not previously been structurally characterized because of its high volatility and weak coordination ability. The authors successfully isolated this complex and performed single-crystal X-ray diffraction analysis. The study reports the synthesis and characterization of eight new compounds, including four pentacoordinate complexes, one unusual 3:2 oligomer, and two one-dimensional coordination polymers. These compounds were obtained using various pyridine derivatives, including 4-methylpyridine, 4-bromopyridine, and 4,4′-bipyridine. A significant outcome of the study is the established correlation between the donor strength of the pyridine ligands and the resulting coordination geometry. The authors demonstrated that stronger donor ligands lead to shorter Cu–N bonds, which in turn promote greater distortion from square-pyramidal toward trigonal-bipyramidal geometry. Using Hirshfeld surface analysis and fingerprint plots, the researchers quantified the supramolecular interactions stabilizing these volatile adducts in the solid state. They found that H···H contacts constitute the dominant contribution to crystal packing. Overall, the study provides a comprehensive overview of how substituents on the pyridine ring influence the stability and structural parameters of copper(II) complexes, offering valuable guidelines for the design of more sophisticated supramolecular architectures and metal–organic frameworks (MOFs). The research presented in paper [4] provides important insights into the coordination chemistry of redox-active ligands. The study reports the successful synthesis and characterization of twelve new coordination complexes derived from nickel(II), copper(II), and zinc(II) chloride salts with arylazoformamide (AAF) and arylazothioformamide (ATF) ligands. One of the most notable findings is that coordination of copper(II) chloride with ATF ligands induces a reduction process leading to the formation of copper(I) dimers, [ATF–CuCl]₂. In contrast, the nickel and zinc complexes, as well as all complexes containing AAF ligands, remained in the +2 oxidation state. Hirshfeld surface analysis was employed to quantify the intermolecular interactions present within the crystal structures. The results demonstrated that the crystal packing is dominated primarily by H···H contacts, followed by Cl···H/H···Cl interactions, whereas the metal centers contribute negligibly to intermolecular contacts. The findings enhance understanding of how these ligands may be utilized in oxidative metal dissolution and metal recovery processes, with potential applications in metal recycling and the separation of mixed-metal systems. The research team in study [5] successfully developed and characterized a series of novel two-dimensional coordination polymers. Specifically, the researchers synthesized three new porous metal–organic frameworks using a layer-diffusion technique involving tricyanomethanide (tcm) and 1,4-bis(imidazol-1-ylmethyl)benzene (bix) ligands. Single-crystal X-ray diffraction analysis revealed that all three compounds are isomorphous and crystallize in the triclinic crystal system. The metal centers exhibit octahedral coordination geometry, while the bix ligand adopts a trans configuration, leading to the formation of a two-dimensional porous layer structure. Thermogravimetric analysis (TGA) demonstrated that these frameworks exhibit remarkable thermal stability for 2D materials, making them promising candidates for applications in gas storage and catalysis. Hirshfeld surface analysis enabled the quantification of weak intermolecular interactions responsible for assembling the 2D layers into a three-dimensional stacked structure. The authors identified a characteristic “saw-toothed” surface arrangement in which the tcm ligands penetrate the voids of neighboring layers. Because the overall 3D structure is maintained primarily through weak intermolecular interactions rather than strong covalent bonds, the materials appear to be promising candidates for exfoliation into ultrathin nanosheets analogous to graphene, potentially enabling future nanotechnology applications. Manuscript [6] provides a concise overview of major advances achieved over the past decade in medicinal inorganic chemistry, particularly focusing on osmium-based compounds as potential alternatives to traditional platinum-based anticancer drugs such as cisplatin. Researchers have successfully synthesized and characterized a broad range of osmium complexes in various oxidation states, including Os(II), Os(III), Os(IV), and Os(VI). In contrast to earlier arene-based systems, the compounds discussed in this review incorporate structurally diverse ligand systems, including polypyridyl, indazole, pyrazole, and nitrido ligands. One of the most significant achievements highlighted in this work is the development of osmium-based photosensitizers active in the near-infrared (NIR) region. Such compounds offer deeper tissue penetration and reduced systemic toxicity. Particularly noteworthy examples include heteronuclear iridium–osmium complexes and osmium polyazine compounds exhibiting high photocytotoxicity toward various cancer cell lines. The research conducted by Diksha et al. [7] investigates how the enantiopure or racemic nature of ligands, together with different counter-anions, influences the formation of silver(I) coordination compounds. The study demonstrates that ligand chirality exerts a stronger influence on structural speciation than the choice of counter-anion. In contrast, the anions mainly fine tune the supramolecular packing and hydrogen-bonding networks. This work contributes significantly to understanding the behavior of related chiral and achiral metallo-supramolecular systems and provides valuable insight into predicting their structural organization. The research team led by Christoph Janiak [8] successfully synthesized and characterized two new cobalt(II)-based metal–organic frameworks constructed from redox-active dihydrophenazine linkers. The study reports the successful solvothermal synthesis of two new coordination networks. A particularly important topological feature was identified in the tetracarboxylate framework, which exhibits a platinum sulfide (pts) topology with a rare inverse node arrangement. In this structure, the metal center functions as a tetrahedral node, whereas the linker acts as a square-planar node, representing the reverse of the node assignment typically observed in pts-type structures. The authors further demonstrated that framework formation strongly depends on both the metal ion and the counterion. Crystalline products were obtained exclusively from cobalt(II) nitrate, whereas other cobalt salts, such as chlorides and sulfates, failed to yield crystalline materials. By comparing these results with previously studied para-substituted isomers, the researchers showed that meta-positioning of the carboxylate groups enforces the angular geometry required for the formation of three-dimensional interpenetrated networks rather than lower-dimensional chains or layers. In manuscript [9], the authors successfully implemented a modular building-block strategy for the synthesis of phosphonated polyaromatic systems. This methodology represents an attractive alternative to conventional late-stage phosphonation procedures, which often require harsh reaction conditions and exhibit limited functional-group tolerance. The study reports the first synthesis of octaethyl(pyrene-tetrakis(biphenyl))tetrakis(phosphonate). The compound was comprehensively characterized using NMR spectroscopy and mass spectrometry. Photophysical investigations revealed promising properties for optoelectronic applications. In solution, the compound exhibits structured blue fluorescence, whereas in the solid state it displays aggregation-induced red-shifted turquoise luminescence with nanosecond lifetimes. Importantly, the study confirmed exclusively fluorescent behavior, with no evidence of phosphorescence even at low temperatures. In manuscript [10], the research team successfully synthesized and characterized a rare family of pyridine-coordinated iodobismuthate(III) salts. The study demonstrates that the architecture of bismuth-halide-pyridine anions can be precisely controlled through variation in reagent stoichiometry. Diffuse-reflectance measurements revealed narrow band gaps in the range of 1.9–2.2 eV, making these compounds promising lead-free alternatives to conventional iodoplumbates for next-generation photovoltaic applications. The authors employed single-crystal X-ray diffraction to determine twenty different crystal structures, including several solvates and polymorphs. Hirshfeld surface analysis was additionally used to quantify the dominant intermolecular interactions. Advanced computational methods, including KS-DFT, NLMO, and QTAIM calculations, were applied to elucidate the nature of chemical bonding within these systems. The results demonstrated that the bonding is predominantly ionic and that the light-absorption properties are governed by metal-centered charge transfer within the anionic units. Furthermore, the study revealed that the inner-sphere coordination environment remains remarkably robust regardless of the specific anion speciation, thereby providing a reliable structural platform for the future development of bismuth-based functional materials.

3. Conclusions

In summary, the ten publications included in this third edition clearly illustrate the vitality and continuing evolution of coordination chemistry. Together, they demonstrate that the synergy between innovative synthetic methodologies and advanced computational tools, such as Hirshfeld surface analysis, provides deeper insight into the solid-state behavior of metal–organic systems. As Guest Editor, I would like to express my sincere gratitude to all authors for their valuable contributions and to the reviewers for their careful evaluations, which ensured the high scientific quality of this Special Issue. I also thank the editorial staff of Crystals for their professional support throughout the editorial and publication process. I hope that this collection will inspire further research into the synthesis, structural characterization, and analysis of novel coordination compounds.