Syntheses and electronic structures of one-electron-oxidized group 10 metal(II)-(disalicylidene)diamine complexes (metal = Ni, Pd, Pt)

Group 10 metal(II) complexes of H₂tbu-salen (H ₂tbu-salen = N,N′-bis(3′,5′-di-tert- butylsalicylidene)ethylenediamine) and H₂tbu-salcn (H ₂tbu-salcn = N,N′-bis(3′,5′-di-tert- butylsalicylidene)-1,2-cyclohexanediamine) containing two 2,4-di(tert-butyl) phenol moieties, [Ni(tbu-salen)] (1a), [Ni(tbu-salcn)] (1b), [Pd(tbusalen)] (2a), [Pd(tbu-salcn)] (2b), and [Pt(tbu-salen)] (3), were prepared and structurally characterized by X-ray diffraction, and the electronic structures of their one-electron-oxidized species were established by spectroscopic and electrochemical methods. All the complexes have a mononuclear structure with two phenolate oxygens coordinated in a very similar square-planar geometry. These complexes exhibited similar absorption spectra in CH₂Cl₂, indicating that they all have a similar structure in solution. Cyclic voltammograms of the complexes showed a quasi-reversible redox wave at E _1/2 = 0.82-1.05 V (vs Ag/AgCl), corresponding to formation of the relatively stable one-electron-oxidized species. The electrochemically oxidized or Ce(IV)-oxidized species of 1a, 2a, and 3 displayed a first-order decay with a half-life of 83, 20, and 148 min at -20 °C, respectively. Ni(II) complexes 1a and 1b were converted to the phenoxyl radicals upon one-electron oxidation in CH₂Cl₂ above -80 °C and to the Ni(III)-phenolate species below -120 °C. The temperature-dependent conversion was reversible with the Ni(III)-phenolate ground state and was found to be a valence tautomerism governed by the solvent. One-electron-oxidized 1 b was isolated as [Ni(tbu-salcn)]NO₃ (4) having the Ni(II)-phenoxyl radical ground state. One-electron-oxidized species of the Pd(II) complexes 2a and 2b were different from those of the Ni(II) complexes, the Pd(II)-phenoxyl radical species being the ground state in CH₂Cl₂ in the range 5-300 K. The one-electron-oxidized form of 2b, [Pd(tbu-salcn)]NO₃ (5), which was isolated as a dark green powder, was found to be a Pd(II)-phenoxyl radical complex. On the other hand, the ESR spectrum of the one-electron-oxidized species of Pt(II) complex 3 exhibited a temperature-independent large g anisotropy in CH₂Cl₂ below -80 °C, while its resonance Raman spectrum at -60 °C displayed ν_8a of the phenoxyl radical band at 1600 cm^-1. These results indicated that the ground state of the Pt(II)-phenoxyl radical species has a large distribution of the radical electron spin at the Pt center. One-electron oxidation of 3 gave [Pt(tbu-salen)]NO₃ (6) as a solid, where the oxidation state of the Pt center was determined to be ca, +2.5 from the XPS and XANES measurements.