Item

Abstract

The reaction of three different 1-phenyl and 1,4-diphenyl substituted S-methylisothiosemicarbazides, H₂[L^1-6], with Ni(OAc)₂·4H₂O in ethanol in the presence of air yields six four-coordinate species [Ni(L^1-6•)₂] (1−6) where (L^1-6•)^1- represent the monoanionic π-radical forms. The crystal structures of the nickel complexes with 1-phenyl derivatives as in 1 reveal a square planar structure trans-[Ni(L^1-3•)₂], whereas the corresponding 1,4-diphenyl derivatives are distorted tetrahedral as is demonstrated by X-ray crystallography of [Ni(L^5•)₂] (5) and [Ni(L^6•)₂] (6). Both series of mononuclear complexes possess a diamagnetic ground state. The electronic structures of both series have been elucidated experimentally (electronic spectra magnetization data). The square planar complexes 1−3 consist of a diamagnetic central Ni(II) ion and two strongly antiferromagnetically coupled ligand π-radicals as has been deduced from correlated ab initio calculations; they are singlet diradicals. The tetrahedral complexes 4−6 consist of a paramagnetic high-spin Ni^II ion (S_Ni = 1), which is strongly antiferromagnetically coupled to two ligand π-radicals. This is clearly revealed by DFT and correlated ab initio calculations. Electrochemically, complexes 1−6 can be reduced to form stable, paramagnetic monoanions [1−6]^- (S = 1/2). The anions [1−3]^- are square planar Ni(II) (d,⁸ S_Ni = 0) species where the excess electron is delocalized over both ligands (class III, ligand mixed valency). In contrast, one-electron reduction of 4, 5, and 6 yields paramagnetic tetrahedral monoanions (S = 1/2). X-band EPR spectroscopy shows that there are two different isomers A and B of each monoanion present in solution. In these anions, the excess electron is localized on one ligand [Ni^II(L^4-6•)(L^4-6)]^- where (L^4-6)^2- is the closed shell dianion of the ligands H₂[L^4-6] as was deduced from their electronic spectra and broken symmetry DFT calculations. Oxidation of 1 and 5 with excess iodine yields octahedral complexes [Ni^II(L^1,ox)₂I₂] (7), [Ni^II(L^1,ox)₃](I₃)₂ (8), and trans-[Ni^II(L^5,ox)₂(I₃)₂] (9), which have been characterized by X-ray crystallography; (L^1-6,ox) represent the neutral, two-electron oxidized forms of the corresponding dianions (L^1-6)^2-. The room-temperature structures of complexes 1, 5, and 7 have been described previously in refs 1−5.