High Spin Entities

Abstract

The main aim of the work is the synthesis of new radicals and their organization into supramolecular architectures via H-bonding, Pi-stacking and metal coordination. Therefore within this thesis two new high spin ligands (1 and 2) carrying nitronylnitroxides, monoradicals 3-5, and a symmetrical triradical were synthesized and extensively characterized. The study of the absorption spectra of the radicals established that a pyridine component compared to benzene reduces the extinction coefficients in the visible region, and that the vibronic coupling is lost when the radicals are connected directly to the pyridine. From the ESR-spectra of the mono-, bi-, and triradicals in liquid solution a strong exchange coupling between the radical sites in the bi- and triradicals could be concluded (J >> An). Biradicals gave zerofield splitting value of D/hc ~ 0.24 ?10-4 cm-1, accounting for an averaged dipolar distance of r ~ 1.02 nm. This distance is much smaller than the one determined from X-ray structural analysis (r ~ 1.5 nm) demonstrating the spin delocalization into the conjugated systems. The helium temperature ESR measurements showed that the intensity of the Delta ms = 2 transition follow Curie?s law with positive exchange coupling value of about J ~ 10 ?15 K. Semi-empirical calculation supported the triplet ground state. In Chapter 4. are described the coordination chemistry of the radicals with metals. A centrosymmetric linear trinuclear copper complex 22 was crystallized with three Cu(hfac)2 and two radicals forming a S = 5/2 system. Linear coordination polymers were prepared by combining monoradical 4 with Cu(hfac)2 and Mn(hfac)2, for the alternative copper radical chain ferromagnetic coupling (J = 6 cm-1) was observed and the ferrimagnetic manganese-radical chain show antiferromagnetic coupling with interchain dipole exchange interaction (zJ? = -0.33 cm-1). Complexation of biradical 1 with Cu(hfac)2 gave a complex network structure upon crystallization, with seven copper centres for two biradical units. Magnetic susceptibility measurement show antiferromagnetic interaction from 300 K to 14 K and an increase of the susceptibility below 14 K, indicating a possible ferromagnetic interaction. The pure organic approaches towards supramoelcular organizations are described in Chapter 5. Here two H-bonding synthons based on radicals 26 and 27 were crystallized whose X-ray structure define H-bonding between the acetylene hydrogen and the radical oxygen. At temperatures below 30 K the interactions turn into antiferromagnetic behaviour. Finally also the stacking of the Pi- systems was studied. As first example was used monoradical 22 which led to a stacking distance of 3.7 ? with additional H-bonding of the water molecules. Although the angle between the molecules in the stack is 60 deg, which is good for ferromagnetic ordering, the magnetic measurement show strong decline of the susceptibility below 50 K and only an increase below 3 K. Finally an attempt was made to stack a Pi-structure using a symmetrical triradical on the basis of previous literature report of such moieties without radical sites. The overall yield, however, was too small for full elucidation of the solid state properties. Nevertheless, this triradical will definitely serve its purpose, if the reaction is well optimized for high yield.