Item

Abstract

The reaction of 2 equiv of the bulky ligand N,N‘-bis(3,5-di-tert-butylphenyl)-1,2-phenylenediamine, H₂[³L^PDI], excess triethylamine, and 1 equiv of M(CH₃CO₂)₂·4H₂O (M = Ni, Co) in the presence of air in CH₃CN/CH₂Cl₂ solution yields violet-black crystals of [Ni^II(³L^ISQ)₂] CH₃CN (1) or violet crystals of [Co(³L)₂] (3). By using Pd(CH₃CO₂)₂ as starting material, green-blue crystals of [Pd^II(³L^ISQ)₂]·CH₃CN (2) were obtained. Single-crystal X-ray crystallography revealed that 1 and 3 contain (pseudo)tetrahedral neutral molecules [M(³L)₂] (M = Ni, Co) whereas in 2 nearly square planar, neutral molecules [Pd^II(³L^ISQ)₂] are present. Temperature-dependent susceptibility measurements established that 1 and 2 are diamagnetic (S = 0) whereas 3 is paramagnetic with an S = 3/2 ground state. It is shown that 1 contains two π radical benzosemiquinonate(1−)-type monoanions, ((³L^ISQ)^1-•, S_rad = 1/2), and a central Ni(II) ion (d⁸; S = 1) which are antiferromagnetically coupled yielding the observed S_t = 0 ground state. This result has been confirmed by broken symmetry DFT calculations of 1. In contrast, the S_t = 3/2 ground state of 3 is more difficult to understand:  the two resonance structures [Co^III(³L^ISQ)(³L^PDI)] ↔ [Co^II(³L^PDI)(³L^IBQ)] might be invoked (for tetrahedral [Co^II(³L^ISQ)₂] containing an S_Co = 3/2 with two antiferromagnetically coupled π-radical ligands an S_t = 1/2 is anticipated). Complex 2 is diamagnetic (S = 0) containing a Pd^II ion (d⁸, S_Pd = 0 in an almost square planar ligand field) and two antiferromagnetically coupled ligand radicals (S_rad = 1/2). The electrochemistry and 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.