Substituent Effects on Spin Density Distribution and Exchange Coupling in Semiquinone Complexes

Abstract

Design, synthesis, and characterization of novel high-spin species are critical in better understanding the field of molecular magnetism. It is also critical in advancing the field to a level of application, where new magnetic materials can be used in daily life. In this work, structure-property relationships of mono- and bis(semiquinone)s (SQs) were studied, substituent effects on exchange coupling and other physical properties were investigated, and a series of novel manganese(II), copper(II) and zinc complexes were synthesized and characterized.

Within an isostructural series of SQs and their Manganese(II) and Copper(II) complexes, substituents were found to modulate reduction potentials, UV-visible absorption maxima and exchange coupling. This work represents the first examination of whether an empirical relationship exists between these physical properties of these radical anionic systems and Hammett sigma parameters.

In a related investigation, SQ species with planar geometry and their Copper(II) complexes were prepared. Preliminary results indicate that substituent effects in these systems are nominal.

Finally, three meta-phenylene-type (MPL), ferromagnetically coupled bis(SQ)s (substituents = H, Ph, OCH₃) were prepared to continue an earlier project which determined that both donating (NMe₂) and withdrawing (NO₂) groups attenuate the singlet-triplet gap (&#916E_ST) in the ground-state triplet species. A relatively simple Hückel molecular orbital explanation described the effect. It is the first time that a singlet-triplet gap (Λ[subscript ST]) of a ground-state triplet biradical has been affected by substituents. The results of this research were found to be consistent with earlier work.