| Abstract
| - The reaction of nickel(II) acetate and bulky N,N‘-disubstituted 1,2-phenylenediamine yields a tetrahedral, diamagnetic Ni(II) complex containing two π radicals. A broken symmetry (BS) DFT calculation supports the presence of two radicals strongly coupled in an antiferromagnetic fashion to two unpaired electrons of the metal center, as it is shown on the spin density plot (right).
- The reaction of 2 equiv of the bulky ligand N,N‘-bis(3,5-di-tert-butylphenyl)-1,2-phenylenediamine, H2[3LPDI], excesstriethylamine, and 1 equiv of M(CH3CO2)2·4H2O (M = Ni, Co) in the presence of air in CH3CN/CH2Cl2 solutionyields violet-black crystals of [NiII(3LISQ)2] CH3CN (1) or violet crystals of [Co(3L)2] (3). By using Pd(CH3CO2)2 asstarting material, green-blue crystals of [PdII(3LISQ)2]·CH3CN (2) were obtained. Single-crystal X-ray crystallographyrevealed that 1 and 3 contain (pseudo)tetrahedral neutral molecules [M(3L)2] (M = Ni, Co) whereas in 2 nearlysquare planar, neutral molecules [PdII(3LISQ)2] are present. Temperature-dependent susceptibility measurementsestablished that 1 and 2 are diamagnetic (S = 0) whereas 3 is paramagnetic with an S = 3/2 ground state. It isshown that 1 contains two π radical benzosemiquinonate(1−)-type monoanions, ((3LISQ)1-•, Srad = 1/2), and acentral Ni(II) ion (d8; S = 1) which are antiferromagnetically coupled yielding the observed St = 0 ground state.This result has been confirmed by broken symmetry DFT calculations of 1. In contrast, the St = 3/2 ground stateof 3 is more difficult to understand: the two resonance structures [CoIII(3LISQ)(3LPDI)] ↔ [CoII(3LPDI)(3LIBQ)] might beinvoked (for tetrahedral [CoII(3LISQ)2] containing an SCo = 3/2 with two antiferromagnetically coupled π-radical ligandsan St = 1/2 is anticipated). Complex 2 is diamagnetic (S = 0) containing a PdII ion (d8, SPd = 0 in an almostsquare planar ligand field) and two antiferromagnetically coupled ligand radicals (Srad = 1/2). The electrochemistryand spectroelectrochemistry of 1,2, and 3 have been studied, and electron-transfer series comprising the species[M(L)2]z (z = 2+, 1+, 0, 1−, 2−) have been established. All oxidations and reductions are ligand centered.
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