Coordination-Induced Spin-State Switching of an Aminyl-Radical-Bridged Nickel(II) Porphyrin Dimer between Doublet and Sextet States

A bis(Ni-II-porphyrinyl)aminyl radical with meso-C6F5 groups was prepared as a spin-delocalized stable aminyl radical with a doublet spin state. Upon addition of pyridine, both Ni-II centers became hexa-coordinated by accepting two axial pyridines, which triggered a spin-state change of the Ni-II centers from diamagnetic (S = 0) to paramagnetic (S = 1). The resulting high-spin Ni-II centers interact with the aminyl radical ferromagnetically to give rise to an overall sextet state (S = 5/2). Importantly, this coordination-induced spin-state switching can be conducted in a reversible manner, in that washing of the high-spin radical with aqueous hydrochloric acid regenerates the original doublet radical in good yield.