Valence‐State Analysis through Spectroelectrochemistry in a Series of Quinonoid‐Bridged Diruthenium Complexes [(acac)₂Ru(μ‐L)Ru(acac)₂]ⁿ (n=+2, +1, 0, −1, −2)

Abstract

The quinonoid ligand-bridged diruthenium compounds [(acac)(2)Ru(mu-L(2-))Ru(acac)(2)] (acac(-)=acetylacetonato=2,4-pentanedionato; L(2-)=2,5-dioxido-1,4-benzoquinone, 1; 3,6-dichloro-2,5-dioxido-1,4-benzoquinone, 2; 5,8-dioxido-1,4-naphthoquinone, 3; 2,3-dichloro-5,8-dioxido-1,4-naphthoquinone, 4; 1,5-dioxido-9,10-anthraquinone, 5; and 1,5-diimido-9,10-anthraquinone, 6) were prepared and characterized analytically. The crystal structure analysis of 5 in the rac configuration reveals two tris(2,4-pentanedionato)ruthenium moieties with an extended anthracenedione-derived bis(ketoenolate) pi-conjugated bridging ligand. The weakly antiferromagnetically coupled {Ru(III)(mu-L(2-))Ru(III)} configuration in 1-6 exhibits complicated overall magnetic and EPR responses. The simultaneous presence of highly redox-active quinonoid-bridging ligands and of two ruthenium centers capable of adopting the oxidation states +2, +3, and +4 creates a large variety of possible oxidation state combinations. Accordingly, the complexes 1-6 exhibit two reversible one-electron oxidation steps and at least two reversible reduction processes. Shifts to positive potentials were observed on introduction of Cl substituents (1-->2, 3-->4) or through replacement of NH by O (6-->5). The ligand-to-metal charge transfer (LMCT) absorptions in the visible region of the neutral molecules become more intense and shifted to lower energies on stepwise reduction with two electrons. On oxidation, the para-substituted systems 1-4 exhibit monocation intermediates with intervalence charge transfer (IVCT) transitions of Ru(III)Ru(IV) mixed-valent species. In contrast, the differently substituted systems 5 and 6 show no such near infrared (NIR) absorption. While the first reduction steps are thus assigned to largely ligand-centered processes, the oxidation appears to involve metal-ligand delocalized molecular orbitals with variable degrees of mixing.

Radical ligand-containing single-molecule magnets

“…Efforts to generate benzosemiquinonoid-containing transition metal complexes with a nuclearity of greater than one have resulted in numerous dinuclear compounds, including those containing metal centers bridged by derivatives of 2,5-dihydroxy-1,4-benzoquinonoids [172][173][174][175][176][177][178][179][180][181][182][183][184][185][186][187][188][189], bis(pyrazolyl)benzoquinonoids [190][191][192][193], bis(phosphine)benzoquinonoids [194,195], bis(amino) benzoquinonoids [196][197][198][199][200][201][202], and bis(amino)diiminobenzoquinonoids [86,[203][204][205][206]. Indeed, the radical form of these ligands has shown enhanced magnetic coupling compared to their non-radical redox isomers [174,180,182].…”

Recent Advances on Anilato-Based Molecular Materials with Magnetic and/or Conducting Properties

“…Since the first observation of a strong magnetic interaction between paramagnetic metal ions and the H 2 An x− (x = 3, 1) radical species, reported by Gatteschi et al [48] several types of metal complexes ranging from finite discrete homoleptic and heteroleptic mononuclear systems to extended homoleptic and heteroleptic polymeric systems showing a large variety of peculiar crystal structures and physical properties, have been obtained so far [7,[49][50][51][52][53][54][55][56][57][58][59][60]. Valence Tautomerism is an essential phenomenon in anilato-based systems [29,[61][62][63][64] and it has been observed for the first time by Sato et al [64] in the heteroleptic dinuclear complex, [(CoTPA) 2 (H 2 An)](PF 6 ) 3 , (TPA = tris(2-pyridylmethyl)amine) that exhibits a valence tautomeric transition with a distinct hysteresis effect (13 K) around room temperature and photoinduced valence tautomerism under low temperature.…”

Functional metal complexes based on bridging “imino”-quinonoid ligands

“…All three reduced forms have been described as three-spin systems suggesting a predominantly bridging ligand centered reduction. In such a situation, several predominantly Ru-centered spin as revealed by the large g-anisotropy of the signals [18,22].…”

“…Hence, the one-electron oxidized species has been formulated as a Ru III -Ru IV mixed-valent species. Whereas for [(acac) 2 Ru III (μ-dhbq)Ru IV (acac) 2 ] + the near-IR band appears at 1940 nm [22], for 8 + and 9 + that contain NR substituents on the bridging quinone ligand, the near-IR band appears at 1565 and 1515 nm respectively [13a,22], thus…”