Redox‐Induced Spin‐State Switching and Mixed Valency in Quinonoid‐Bridged Dicobalt Complexes

Abstract

The complexes [{(tmpa)Co(II) }2 (μ-L(1) )(2-) ](2+) (1(2+) ) and [{(tmpa)Co(II) }2 (μ-L(2) )(2-) ](2+) (2(2+) ), with tmpa=tris(2-pyridylmethyl)amine, H2 L(1) =2,5-di-[2-(methoxy)-anilino]-1,4-benzoquinone, and H2 L(2) =2,5-di-[2-(trifluoromethyl)-anilino]-1,4-benzoquinone, were synthesized and characterized. Structural analysis of 2(2+) revealed a distorted octahedral coordination around the cobalt centers, and cobalt-ligand bond lengths that match with high-spin Co(II) centers. Superconducting quantum interference device (SQUID) magnetometric studies on 1(2+) and 2(2+) are consistent with the presence of two weakly exchange-coupled high-spin cobalt(II) ions, for which the nature of the coupling appears to depend on the substituents on the bridging ligand, being antiferromagnetic for 1(2+) and ferromagnetic for 2(2+) . Both complexes exhibit several one-electron redox steps, and these were investigated with cyclic voltammetry and UV/Vis/near-IR spectroelectrochemistry. For 1(2+) , it was possible to chemically isolate the pure forms of both the one-electron oxidized mixed-valent 1(3+) and the two-electron oxidized isovalent 1(4+) forms, and characterize them structurally as well as magnetically. This series thus provided an opportunity to investigate the effect of reversible electron transfers on the total spin-state of the molecule. In contrast to 2(2+) , for 1(4+) the metal-ligand distances and the distances within the quinonoid ligand point to the existence of two low-spin Co(III) centers, thus showing the innocence of the quintessential non-innocent ligands L. Magnetic data corroborate these observations by showing the decrease of the magnetic moment by roughly half (neglecting spin exchange effects) on oxidizing the molecules with one electron, and the disappearance of a paramagnetic response upon two-electron oxidation, which confirms the change in spin state associated with the electron-transfer steps.

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].…”

Direct measurement of dysprosium(III)˙˙˙dysprosium(III) interactions in a single-molecule magnet

“…Weak exchange can be better defined by EPR spectroscopy, even between orbitally degenerate ions 30,31 , but such studies on lanthanide pairs are rare outside minerals, due to fast spin-lattice relaxation. Compound 3, {Dy@Y 2 } and {Dy@Lu 2 } all give rich EPR spectra; the spectra of {Dy@Y 2 } and {Dy@Lu 2 } are very similar since in both cases we only see resonances for the Dy(III) ion in the NO 3 pocket.…”

Torque-Detected Electron Spin Resonance as a Tool to Investigate Magnetic Anisotropy in Molecular Nanomagnets

“…For a satisfactory The origin of magnetic anisotropy in Mn 12 Ac is the same as in Fe 4 , i.e., zero-field splitting of the ground spin state. To explore if other sources of anisotropy also suffice and allow for successful TDESR studies, we next focused on V 15 , which consists of fifteen S = ½ VO 2+ ions [30]. Magnetically, this compound has a layered structure consisting of two strongly antiferromagnetically coupled vanadyl hexagons ( Figure S2).…”

“…Importantly for the subject of this article, it allows experimental access to large energy splittings, such as those associated with magnetic anisotropy in MNMs [14]. It has also been used for the determination of anisotropic exchange interactions [15]. Finally, it can be operated in field and frequency domains [16].…”