Syntheses and Electronic Structures of One-Electron-Oxidized Group 10 Metal(II)−(Disalicylidene)diamine Complexes (Metal = Ni, Pd, Pt)

Abstract

Group 10 metal(II) complexes of H2tbu-salen (H2tbu-salen = N,N'-bis(3',5'-di-tert-butylsalicylidene)ethylenediamine) and H2tbu-salcn (H2tbu-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(tbu-salen)] (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 E1/2 = 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 degrees C, respectively. Ni(II) complexes 1a and 1b were converted to the phenoxyl radicals upon one-electron oxidation in CH2Cl2 above -80 degrees C and to the Ni(III)-phenolate species below -120 degrees 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 1b 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 degrees C, while its resonance Raman spectrum at -60 degrees C displayed nu8a 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.

Oxidation Chemistry of Metal(II) Salen-Type Complexes

“…These observations suggest that the complexes have the phenoxyl radical characteristics and that the radical electron is delocalized on the two phenolate moieties. Indeed, the XPS and K-edge XANES of an oxidized Ni complex showed the same binding energies and pre-edge peak of nickel ion as those of the complex before oxidation [40]. These results supported that the valence state of the nickel ion is +II.…”

“…Such a trend is also applied to the salen-type complexes, and most of the Cu(II) and group 10 metal(II) salen-type complexes exhibited two reversible redox waves in the range of 0 to 1.5 V vs NHE, due to having two phenolate moieties [29][30][31][39][40][41][42][43][44][45][46][47]. On the other hand, the metal centered one-and two-electron oxidized complexes may possibly be generated in the similar potential range.…”

“…The binding energies of the Pt ion in the oxidized complex were +0.2 eV higher, and LIII-edge XANES exhibited an increasing white line [59]. Such spectral features suggest that the oxidation state of the Pt ion in the oxidized complex is higher than +II but that the differences are rather small [40]. Therefore, [Pt(Salen)]SbF6 can be described mainly as the Pt(II)-phenoxyl radical species, but the radical electron is fully delocalized over the whole molecule including the central metal ion [40].…”

“…The present argument is focused on recent advances in the chemistry related with the synthesis, characterization, and reactivity of some of the one-electron oxidized metal(II)-salen type complexes, especially the complexes of group 10 metal(II) ions, Ni(II), Pd(II), and Pt(II) [39][40][41][42][43], and Cu(II) complexes [44][45][46][47] (Figure 3). Various useful and interesting salen-type complexes have been synthesized and characterized until now [48], and detailed electronic structural studies have recently been reported for a dinuclear chelating salen-type ligand having a catechol and a tetra(amino)tri(hydroxy)phenyl moiety [49,50].…”

“…The small potential differences are due to the substitution of the phenolate moiety and the ligand structure. However, there are some different electronic structures among the one-electron oxidized forms of the complexes in Table 1, i.e., a M III -phenolate ground state complex, a M II -phenoxyl radical where the radical electron is fully delocalized on two phenolate moieties, a relatively localized M II (phenolate)(phenoxyl) complex, and a M II (dinitrogen ligand radical)(phenolate)2 species [39][40][41][42][43][44][45][46][47]. The similarity of the potentials indicates that formation of all of the oxidized species is due to a simple electron transfer without significant structural changes.…”