Mixed-Metal Coordination Polymers

The higher degree of heterogeneity of the target compounds opens yet another dimension in the already broad field of coordination chemistry. Quite obviously, the synthetic challenges mentioned in the Special Issue Information persist; commercially available ligands mostly lack the alternative donor sites beneficial for selective coordination. Our Special Issue has received five submissions which I will shortly summarize; my short overview is not intended to replace the original articles.

The contribution by Bhattacharyya et al. [1] deals with multi-component coordination compounds of Mn(II). The author team tackles the challenging task of understanding the interactions between the building blocks in these complex solids with DFT calculations, and they analyze the resulting electron density with Bader’s QTAIM [2]. One of the target compounds features complex cations and anions. According to the theoretical results, π stacking appears to be more relevant than hydrogen bonds in its crystal structure.

Sakhapov et al. [3] investigated bromo-substituted 4,4′-bipyridine, both in its uncoordinated form and as a ligand towards Co(II). Halogen bonds between nitrogen and bromine and inter-bromine contacts dominate the crystal structure of the uncoordinated compound. The bromo-substituted N donor ligand bridges octahedrally coordinated Co(II) ions to a chain polymer, also stabilized by halogen bonds.

Bao et al. [4] used the stepwise approach via a tripodal metalloligand to synthesize their mixed-metal phosphonates. Their systematic study comprised alkaline earth derivatives with Mg, Ca, Sr, and Ba dications, and the authors were able to link the crystal structures of their target compounds to proton conductivity in the solid state. For the heavier cations Sr²⁺ and Ba²⁺, the hydrogen bonds are localized and proton conductivity is low. The lighter congeners are isostructural, and the authors link the higher proton conductivity of the Mg²⁺ compound to its more pronounced Lewis acidity.

Rasmussen, Näther, and Bensch [5] discuss new derivatives of the rare [V₆As₈O₂₆]⁴⁻ cluster anion. They prepared and characterized the arsenato-polyoxovanadates of the divalent transition metal cations Co(II), Zn(II), and Cd(II). Although the three products are isotypic, the cluster anion in the latter compound exhibits a significantly smaller volume than in the derivatives of the 3d row. This explains the apparently counterintuitive smaller lattice parameters for the largest cation.

The contribution of van Terwingen et al. [6] features a pyrazole-substituted acetylacetone. This heteroditopic ligand ensures selective coordination based on different Pearson hardness of the donor atoms. The outcome of the reaction shows that even ligands designed and synthesized on purpose do not necessarily match all expectations: the mixed-metal product represents an oligomer rather than a polymer. Fortunately, the path towards higher nuclearity appears clear.

Despite the above-mentioned synthetic challenges, there is no reason to relent in our efforts: since our Special Issue has been completed, several contributions to internationally esteemed journals have highlighted the relevance of ordered mixed-metal materials in the field of luminescence and luminescence-based applications [7,8].

I personally see a second good reason to persevere in our attempts to design and characterize well-ordered mixed-metal species: catalysis often relies on a combination of different active centers, and I am optimistic to see more mixed-metal MOFs in the near future.