Doctor of Philosophy

Abstract

dissertationOrganic ligands have become an integral part of molecule-based magnetic systems. Incorporation of organic compounds into molecule-based magnets enables control of the physical properties and the blending of phenomena, e.g., photomagnetism. Understanding how to construct molecule-based magnetic systems with organic compounds should lead to new classes of readily functionalized materials. The magnetic behavior of the dimeric form of N,N'- ethylenebis(salicylideniminato)cobalt(II) (CoII(salen)) was reinvestigated as the dimer was seen to exhibit magnetic tristability associated with the dinuclear site. The singlet (S = 0) ground state arises from the antiferromagnetic coupled pair of the CoII ions, each being in the single-ion (S = 1/2) electron configuration. The triplet (S = 1) excited state arises from ferromagnetic coupling of the CoII S = 1/2 ions. As the temperature increases, the thermal energy populates both the singlet and triplet states. This leads to increases in the χT(T) approaching the Curie behavior for the two CoII S = 1/2 doublets at higher temperature. In contrast to salen-like compounds, spin-carrying radical organic ligands have been seen to enhance magnetic properties. Reactions of N,7,7-tricyanoquinonemethanimine (TCQMI), which forms a radical, with precedented cationic building-blocks of molecular magnets have shown mixed results. The expected structural motif of 1-D chains of alternating [TCQMI]•- and [FeCp*2]+ showed weak magnetic ferrimagnetic ordering at low temperatures with a Tc of 3.4. The reaction of TCQMI with MII(CO)x compounds leads to amorphous materials which showed either weak ferromagnetic coupling or paramagnetic behavior at low temperatures. These results indicate that TCQMI is incompatible with forming magnetic systems where direct bonding to metal centers is encountered. Further evidence of the unsuitability of TCQMI was encountered in the reaction with MnTPP(py) (TPP = Tetraphenylporphinato; py = pyridine). The resulting structure consisted of 1-D chains with MnNCNMn linkages of warped MnTPP and a σ-[TCQMI]22- dimer. The reaction of MnTPP(py) with TCQMI is in contrast with that of typical DCNQIs. The structure [MnIIITPP][Me2DCNQI] has been determined by Rietveld refinement of the X-ray powder diffraction method. The nonsolvated structure is composed of linear (1-D) chains of alternating [MnIIITPP]+ and μ-[Me2DCNQI]•-