Intramolecular Valence and Spin Interaction inmesoandracDiastereomers of ap-Quinonoid-Bridged Diruthenium Complex

Abstract

The complexes meso- and rac-[(acac)2Ru(mu-L)Ru(acac)2]n, 1 and 2, where L(2-) = 1,4-dioxido-2,3-bis(3,5-dimethylpyrazol-1'-yl)benzene and acac- = 2,4-pentanedionato, were characterized structurally, magnetically, electrochemically, and spectroscopically as well as spectroelectrochemically (UV-vis-NIR, EPR) in the accessible redox states (n = 0, +, -, 2-). Due to steric interference, the neutral compounds contain a severely twisted L(2-) bridging ligand with 43-48 degree dihedral angles between the planes of the hydroquinone dianion and those of the ortho positioned pyrazolyl substituents. The difference between meso and rac isomers is rather pronounced in terms of the redox potentials (easier oxidation and reduction of the rac form 2) and with respect to the absorption spectra of the oxidized states. Susceptibility and EPR measurements confirm the {Ru(III)(mu-L(2-))Ru(III)} configuration of the neutral species, showing J values of -37 and -21 cm(-1) for the spin-spin interaction between the ca. 7.75 A separated metal centers in 1 and 2, respectively. Two-step reduction involves the metals and produces Ru(III)Ru(II) mixed-valent monoanions with comproportionation constants of ca. 10(4), with Ru(III)-type EPR signals, and with broad intervalence charge transfer bands at about 1200-1500 nm absorption maximum, suggesting localized valence (class II). Oxidation produces intense near-infrared absorption at 892 (1+) or 1027 nm (2+) and narrow isotropic EPR spectra at g approximately 2.005, signifying unprecedented spin localization at the p-semiquinone bridge. These results are not compatible with an (L(2-))-bridged {Ru(IV)Ru(III)} situation nor with an {Ru(III)(mu-L(*-))Ru(III)} three-spin arrangement with up-down-up spin configuration in the ground state, which would result in metal-centered spin through antiferromagnetic coupling between the adjacent individual spins. Only the {Ru(III)(mu-L(*-))Ru(III)} situation, with up-up-down spin configuration, leads to ligand-centered resulting spin through the strong antiferromagnetic coupling between the remote metal spins, an unusual situation which is favored here because of weakened metal-radical coupling resulting from the pyrazolyl/p-semiquinone twist.

Straightforward Synthesis of Substituted p‐Quinones: Isolation of a Key Intermediate and Use as a Bridging Ligand in a Diruthenium Complex

“…This antiferromagnetic behavior is in agreement with previous reports [11,13] and is also visible from the nonsaturated value attained by the low-temperature M versus H curves. The obtained value is comparable to those previously reported for similar compounds and its slightly diminished magnitude can probably be attributed to the reduced twisting induced by the isopropyl substituents on the imino nitrogen atoms.…”

“…[4][5][6] The remarkable properties that such ligands impart to their metal complexes make such compounds useful in a variety of fields, such as homogenous catalysis, [7] supramolecular chemistry, [5,8] coordination polymers, [9,10] and as bridging ligands in combination with redox active metal centers such as ruthenium. [11][12][13][14] The last field has gained tremendous attention in recent years because of ambiguities arising in oxidation-state formulations also with "organometallic-type" non-innocent ligands. [15] In addition, the creation of quinone-based molecular magnets is a lively field of research that has produced many interesting results.…”

“…Redox processes involving multinuclear ruthenium complexes are known to display phenomena such as charge-transfer isomerism [27] and valence ambiguity when in combination with potentially non-innocent ligands. [11,13] Structural, electrochemical, magnetic, and UV/Vis-NIR and EPR spectroelectrochemical results are reported below.…”

“…[13,30] The magnetic susceptibility of compound 7 was measured with a SQUID setup (see Figure S3 in the Supporting Information). As in previously reported cases, [11,13,30] the spectrum was fitted by considering the system as composed of dimers with a small fraction, P, of defective paramagnetic sites [11,31] and using g, TIP, and the magnetic exchange, J, as free parameters; this yielded TIP = 0.01 AE 0.01 cm 3 mol…”

Isomeric Diruthenium Complexes Bridged by Deprotonated Indigo in cis and trans Configuration

“…The identityo ft he complexes corresponding to cis and trans forms of L 2À as well as the meso (DL)d iastereomeric configurations of the two tris chelated chiral metal units [9] in both 1 and 2 were established by singlec rystal X-ray structure determina-tion (see below). Although complex 2 (trans form) is found to be quite stable under ambient conditions, the complex 1 (cis form) undergoes slow degradation in solution.…”