Radical architectures in two dimensions

Radical architectures in two dimensions is a thesis by Laura Voigt describing research conducted under the supervision of Assistant Professor Kasper Steen Pedersen submitted to the Department of Chemistry at the Technical University of Denmark. Synthesis of organic ligand-based coordination networks and metal-organic frameworks (MOFs) allows for the design of modular structures with components from the vast number of possible organic ligands and metal-based species.

The envisaged, and occasionally already realized, applications are of great variety, including gas sorption and storage, catalysis, electronics, magneto-electronics, photonics, and quantum technologies. A challenge in contemporary inorganic chemistry is to develop abilities to control the structural, electronic, magnetic, and chemical properties of MOFs and coordination networks.

To gain that proficiency, design and synthesis methodologies for the realization of coordination network and MOF architectures are to be developed. This work showcases some of such design principles displayed by a range of new coordination networks and MOFs with rich magnetic, electronic, structural, and chemical properties, using synthetic inorganic coordination chemistry and a range of physical characterization techniques.

As a joint theme the presented research revolves around two-dimensionality, i.e., strong bonds in two dimensions and weaker forces in the third dimension. The first approach pillars on ligand non-innocence to establish magnetic interactions and charge transport. Reducing metal ions can transfer electrons onto redox-active ligands leading to ligand radicals and non-innocence.

The spin density on the ligand mediates magnetic interactions between magnetic metal ion nodes and ligand mixed-valency may yield charge transport in form of electron hopping via the π-d conjugated orbitals. In the family of layered CrX2(pyz)2 (X = Cl, Br, I, NCS; pyz = pyrazine) the nature of the terminal halide or pseudo-halide ligand is decisive for the axial ligand field of Cr, and hence the redox properties of Cr and the activation of ligand non-innocence.

In that way, the ligand field can be used to regulate the electronic structure of the coordination network. Despite the structurally highly cognate structures, the properties of this family of coordination networks are exceedingly diverse depending on the placement of the ligand in the spectrochemical series.

Whereas CrI2(pyz)2 is an insulating antiferromagnet with an ordering temperature of 26 K, Cr(NCS)2(pyz)2 orders ferrimagnetically below 140 K and exhibits largely temperature-independent conductivity and a large hysteresis in the magnetoresistance. Single-crystal and powder X-ray diffractometry, X-ray absorption spectroscopy, magnetometry, resistivity measurements, and computational methods were used for the deconvolution of the properties of the compounds.

Additionally, the molecular compounds trans-Cr(acac)2L2 (with L = pyridine derivatives) were used to shed light on the ligand-field actuated non innocence in coordination compounds derive from Cr(II). The concept of ligand non-innocence through electron transfer from the metal during synthesis can be transferred to main group element chemistry, leading to a very rare example of a magnetic coordination network without any inner or outer transition metal ion in GaCl2(pyz)2.

To the contrary, it is shown that, with Ln(II) (Ln = lanthanide) ions, electron transfer is impeded, as neither Eu(II) nor the strongly reducing Yb(II) commence ligand reduction. The second approach makes use of the plasticity of Ln(II) ions arising from the deeply buried nature of the 4f orbitals and the large ionic radii to form pentagonal bipyramidal nodes rarely found in transition metal coordination chemistry.

An Yb(II) five-fold vertex node and 4,4¢-bipyridine linkers are used for the generation of a coordination network with layers exhibiting a rare semiregular Archimedean tessellation, which verges on quasicrystallinity. Archimedean tessellations are desirable motifs for materials science with promising applications within optoelectronics and magnetically frustrated systems.

The third approach employs the π-back-bonding nature of CO ligands for the stabilization of MOFs with zero-valent metal nodes. Using the common substitution reaction of homoleptic carbonyl compounds with neutral ligands, the reaction of hexacarbonyls of the group 6 metals with pyrazine led to the formation of MOFs with pore channels, in which the carbonyl groups line the pore walls of the framework.

Such MOFs with potentially novel chemical functionality grafted inside the pores constitute a future strategy for designing size-selective MOFs for heterogeneous catalysis.