Isomeric Diruthenium Complexes of a Heterocyclic and Quinonoid Bridging Ligand: Valence and Spin Alternatives for the Metal/Ligand/Metal Arrangement

Abstract

5,7,12,14-Tetraazapentacene-6,13-quinone (L) reacts with 2 equiv of [Ru(acac)(CHCN)] to form two linkage isomeric bis(chelate) compounds, [{Ru(acac)}(μ-L)], blue 1, with 5,6;12,13 coordination and violet 2 with 5,6;13,14 coordination. The linkage isomers could be separated, structurally characterized in crystals as rac diastereomers (ΔΔ/ΛΛ), and studied by voltammetry (CV, DPV), EPR, and UV-vis-NIR spectroelectrochemistry (meso-1, rac-2). DFT and TD-DFT calculations support the structural and spectroscopic results and suggest a slight energy preference (ΔE = 263 cm) for the rac-isomer 1 as compared to 2. Starting from the Ru-(μ-L)-Ru configurations of 1 and 2 with low-lying metal-to-ligand charge transfer (MLCT) absorptions, the compounds undergo two reversible one-electron oxidation steps with open-shell intermediates 1 (K = 4 × 10) and 2 (K = 6 × 10). Both monocations display metal-centered spin according to EPR, but the DFT-calculated spin densities suggest a Ru(μ-L)Ru three-spin situation with opposite spin density at the bridging ligand for the meso form of 1, estimated to lie 1887 cm lower in energy than rac-1, which is calculated with a Class II mixed-valent situation Ru-(μ-L)-Ru. A three-spin arrangement Ru-(μ-L)-Ru with negative spin density at one metal site is suggested by DFT for rac-2 which is more stable by ΔE = 890 cm than rac-1. Reduction of 1 or 2 (K = 10-10) occurs mainly at the central bridging ligand with notable contributions (30%) from the metals in 1 and 2. The mixed-valent Ru(μ-L)Ru versus radical-bridged Ru(μ-L)Ru alternative is discussed comprehensively in comparison with related valence-ambiguous cases.

Diruthenium Complexes of p‐Benzoquinone–Imidazole Hybrid Ligands: Innocent or Noninnocent Behavior of the Quinone Moiety

“…The potential of the tricyclic fused benzoquinonodiimidazole heterocyclic system for variable metal binding and for noninnocent redox behavior may instigate the use of corresponding coordination compounds in receptor and sensor applications. The question also remains as to whether compounds of higher nuclearity can be obtained, as has been demonstrated for one other heteroquinonoid ligand recently . In summary, we have illustrated that the hitherto little‐used benzoquinonodiimidazolates are unusual bridging ligands with remarkably variable coordination and electron‐transfer properties.…”

“…The second reduction (R2) is separated from the first by 0.67 V, a typical value for quinone redox systems . Further reduction processes (R3, R4) involving multiple electron transfers are attributed to the coordinated bpy co‐ligands . In contrast to 1 2+ , the reduction of 2 2+ starts at a more negative potential, followed by several overlapping waves.…”

“…In the presence of a chelating support, ruthenium will also bind to quinone oxygen atoms, particularly when charge can be transferred to this acceptor moiety. Mixed N,O‐coordinated ruthenium complexes have been prepared as mono‐, di‐, tri‐, and tetranuclear species and have been studied with respect to their structural, electrochemical, and spectroscopic properties . Complexes with two (or more) metal atoms have attracted special interest because of their potential for mixed valency …”

“…In the presence of ac helating support, ruthenium will also bind to quinone oxygen atoms, [1] particularly when charge can be transferred to this acceptor moiety.M ixed N,O-coordinated ruthenium complexes have been prepared as mono-, di-, tri-, and tetranuclear species and have been studied with respect to their structural, electrochemical, and spectroscopicp roperties. [1][2][3] Complexes with two (or more) metal atoms have attracted special interest because of their potentialf or mixed valency. [2,3] Herein, we describe dinuclear bis(bipyridine)rutheniumc ompounds of the hybrid imidazolate/p-quinone/imidazolate heterocyclic ligandsL 1 2À and L2 2À ,o btainedf rom 2,6-bis(4-tolyl)-1,5-dihydro-1',4'-benzoquinono[2',3'-d:5',6'-d']diimidazole (I)a nd 2,6-bis(2-pyridyl)-1,5-dihydro-1',4'-benzoquinono[2',3'-d:5',6'd']diimidazole (II), which have aryl groups in the 2-and 6-positions.…”

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

“…The calculated lower energy of the broken symmetry singlet BS(1,1) S = 0a s compared to the triplet (S = 1) state for both 1 and 2 (Table S5 in the Supporting Information) also corroboratesa na ntiferromagnetically coupled situation. The structural data point to an only partially a-diimine-type chelation of Ru1 in 1 with ar elativelyl ong central CÀCb ond [11] while the trans isomer can be best described as containing two ketiminatoc helate functions [12] with partial doubleb ond character of the central CÀCb ond (Scheme 1). The bond lengthsC 1 ÀN1 and C1ÀC9 in the center do not show much variation (Table 2) because the twofold chelation restricts bond stretching movements.…”