Metal-Dependent Regiospecificity in the Exchange Coupling of the Magnetic Metal Ion with the Free Radical Substituent on Pyridine Base Ligands: Approaches to Photomagnetic Molecular Devices

Abstract

Whereas o- and p-quinodimethanes and their analogs, e.g., the Thiele hydrocarbon, have closed-shell electronic structures, m-quinodimethane and the Schlenk hydrocarbon have triplet ground states [1]. The same is true for the corresponding dicarbenes; whereas the “p-phenylenebis(phenylcarbene)” has a low-spin ground state [2a], the m-isomer is a high-spin quintet species in the ground state [2,3]. In the heteroatom-centered diradicals, p-benzo-quinonediimine N,N'-dioxide (p) [4a] and m-phenylenebis(N-tert-butylaminoxyl) (m) are obtained as stable solids with singlet and triplet ground states, respectively [4]. This regiospecificity in the exchange interaction between the two radical centers at the benzylic positions is derived from differences in the phase of the spin polarization of the π-electrons on the benzene ring and has served as an important guiding principle for designing super-high-spin organic molecules as prototypes for purely organic ferromagnets [4,5]. Connection of n carbene units through the m-phenylene, 1,3,5-benzenetriyl, and other ferromagnetic coupling units gives rise to various polycarbenes with S = n ground states [3] in which the highest record of the series reported so far is S = 9 [6]. Efforts to increase numbers of the aligned spins have been hampered by the development of antiferromagnetic intra- and/or interchain interactions between the carbene centers assembled in high local concentration; chemical bonds appear to be formed in the extreme case [7].