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Rational Design of Organic Magnets with Switchable Two-Way Couplers: Magnetic Modulation or Switching through Lactam–Lactim Tautomerization

Chen, Dongxiao, Deng, Tiange, Yang, Lu, Bu, Yuxiang
Journal of physical chemistry 2019 v.123 no.23 pp. 14152-14163
chemical bonding, ferrimagnetic materials, lactams, magnetism, moieties, solvents, tautomerization
Lactam–lactim tautomerism, which features a typical prototropic process, has been widely applied to various functional processes. Here, we use it to design the magnetism-tunable organic magnets. Taking experimentally reported phenazine-N,N′-dioxide (PDO) as template in which two nitroxide groups act as spin sources and two ortho-phendiyl groups act as two couplers, we further modify one coupler through replacing a −C═C– unit of an ortho-phendiyl by an amide (−CONH−) at three different positions, forming three switchable molecular magnets. Calculations reveal intriguing magnetic modulation or switching phenomena through tautomerization. That is, all lactim structures and PDO exhibit extremely strong antiferromagnetic spin couplings with magnetic coupling constants J ranging from −2485.7 to −2732.4 cm–¹, while the lactam structures are diamagnetic or moderately strong antiferromagnetic with a J of −1703.6 cm–¹, depending on different locations of the introduced amide/imide units. Strong spin couplings (including excessive couplings in diamagnetic cases) in all six molecules and PDO can be interpreted by proximity of two nitroxide groups, good planarity, and conjugation, while the reason for the differences in magnetic characteristics of three lactam forms is that the destruction of aromaticity induces two types of rearrangement of chemical bonds, which leads to opposite changes of the spin coupling between two radical units. Besides, the role of each coupler, ring aromaticity, Mulliken spin densities, and molecular orbitals are analyzed for the understanding of magnetic regulation. Double lactam–lactim tautomerization through modifying two couplers with amide/imide units, different orientations of hydroxyl H in lactim structures, and tautomerization energetics assisted by solvent molecules are also discussed. This work provides a promising strategy for rational design of organic molecular magnetic switches with two switchable spin communication channels.