Ferro- or antiferromagnetism? Heisenberg chains in the crystal structures of verdazyl radicals

Metamagnetism and canted antiferromagnetic ordering in two monomeric CoII complexes with 1-(2-pyrimidyl)piperazine. Hirshfeld surface analysis and theoretical studies

Here we present the magnetic properties of two cobalt complexes formulated as: [Co(SCN)2(L)2] (1) and (H2L)2[Co(SCN)4]·H2O (2) (L = 1-(2-pyrimidyl)piperazine). The two compounds contain isolated tetrahedral CoII complexes with important intermolecular interactions that lead to the presence of a canted antiferromagnetic order below 11.5 and 10.0 K, with coercive fields at 2 K of 38 and 68 mT, respectively. Theoretical calculations have been used to explain this behaviour. Hirshfeld surface analysis shows the presence of strong intermolecular interactions in both compounds. The crystal geometries were used for geometry optimization using the DFT method. From the topological properties, electrostatic potential maps and molecular orbital analysis, information about the noncovalent interaction and chemical reactivity was obtained.

Synthesis and characterization of terthiophene bearing stable radicals

The synthesis and characterization of a new terthiophene bearing stable radical II is described. Through a cross coupling reaction between 2-tributylstannylthiophene and 6-(2,5-dibromo-thiophene-3-yl)pyridine-2-carboxaldehyde (2), 6-[2,2′:5′,2″]terthiophen-3′-ylpyridine-2-carboxaldehyde (3) was prepared. The condensation of 3 with 2, 4-diisopropylcarbonohydrazide bis-hydrochloride affords the heterocyclic tetrazane (4), which was oxidized with 1,4-benzoquinone to form the stable radical II. II characterized by IR, ESR, and cyclic voltammetry. Oxidative electropolymerization of II and its precursor 4 at oxidation peak potential of the terthiophene using cyclic voltammetric scans produces radical functionalized polyterthiophene on a platinum electrode (poly(II)-Pt).

Subsystem density-functional theory for interacting open-shell systems: spin densities and magnetic exchange couplings

We investigate the possibility of describing interacting open-shell systems in high-spin and broken-symmetry (BS) states with subsystem density-functional theory (sDFT). This subsystem method typically starts from the electronic-structure results obtained for individual systems, for which the spin states can be individually defined. Through the confining effect of the embedding potential and/or the use of monomer basis sets, these individual spin states can be preserved in sDFT calculations. This offers the possibility of easy convergence to broken-symmetry states with arbitrary local spin patterns. We show that the resulting spin densities are in very good agreement with successfully converged broken-symmetry Kohn-Sham density-functional theory (KS-DFT) calculations. Yet sDFT can even cure those BS cases where KS-DFT suffers from convergence problems or convergence to undesired spin states. In contrast to KS-DFT, the sDFT-results only show a mild exchange-correlation functional dependence. We also show that magnetic coupling constants from sDFT are not satisfactory with standard approximations for the non-additive kinetic energy. When this component is evaluated "exactly", i.e. based on potential reconstruction, however, the magnetic coupling constants derived from spin-state energy differences are greatly improved. Hence, the interacting radicals studied here represent cases where even (semi-)local approximations for the non-additive kinetic-energy potential work well, while the parent energy functionals do not yield satisfactory results for spin-state energy differences.