Geometric and Electronic Structures of Nickel(II) Complexes of Redox Noninnocent Tetradentate Phenylenediamine Ligands

• Published in: Inorganic chemistry 2016-01, Vol.55 (2), p.649-665
• Main Authors: Ciccione, Jérémie, Leconte, Nicolas, Luneau, Dominique, Philouze, Christian, Thomas, Fabrice
• Format: Article
• Language: English
• Subjects: Analytical chemistry; Chemical Sciences
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/acs.inorgchem.5b01947

Five tetradentate ligands based on the N,N′-bis­(2-amino-3,5-di-tert-butylphenyl)-o-phenylenediamine backbone were prepared, with different substituents at positions 4 and 5 (CH3 (3a), p-CH3O-C6H4 (3b), H (3c), Cl (3d), F (3e)). Their reaction with a nickel­(II) salt in air affords the neutral species 4­(a-e), which were isolated as single crystals. 4­(a-e) feature two antiferromagnetically exchange-coupled diiminosemiquinonate moieties, both located on peripheral rings, and a diamidobenzene bridging unit. Oxidation of 4­(a-e) with 1 equiv of AgSbF6 yields the cations 4­(a-e) + , which harbor a single diiminosemiquinonate radical. Significant structural differences were observed within the series. 4b + is mononuclear and contains a localized diiminosemiquinonate moiety. In contrast, 4c + is a dimer wherein the diiminosemiquinonate radical is rather delocalized over both peripheral rings. 4d + represents an intermediate case where the complex is mononuclear, but the radical is fully delocalized. Oxidation of 4­(a-e) with 2 equiv of AgSbF6 produces the corresponding mononuclear dications. X-ray diffraction data on 4­(b-d) 2+ reveals that the bridging ring retains its diamidobenzene character, whereas both peripheral rings have been oxidized into diiminobenzoquinone moieties. All the complexes were characterized by electrochemistry, EPR, and UV–vis–NIR spectroscopy. Remarkably, the electronic structures of the complexes differ from those reported by Wieghardt et al. for copper and zinc complexes of a related ligand involving a mixed N2O2 donor set (J. Am. Chem. Soc. 1999, 121, 9599). The easier oxidation of phenylenediamine moieties in comparison to aminophenols is proposed to account for the difference.