Electronic, charge and magnetic interactions in three-centre systems

Abstract

Three-centre systems [QCuQ] n containing electron transfer-active and donor-substituted o-iminoquinone ligands were synthesised and studied as models for electronic, charge and magnetic coupling within materials involving coordinative bonding. The neutral precursors (n = 0) involve spin-bearing semiquinone radical ions, Q À , bridged by paramagnetic copper(II). Crystal structures and electron paramagnetic resonance (EPR) measurements reveal a delicate sensitivity of structures and of spin-spin coupling in these three-centre configurations, depending on weak secondary interactions involving substituents at the coordinatively ambivalent (hemilabile) ligands Q Â . Stepwise electron loss or uptake by [QCuQ] n is accompanied by considerable redox potential splitting and by spectroelectrochemically monitored change of UV-Vis-NIR absorption. Whereas the monoanions exhibit long-wavelength ligand-to-ligand intervalence charge transfer bands around 2000 nm for p(Q 2À )p*(Q À ) transitions, the monocations are formed in a reorganisation step, achieving coordinative saturation at the metal. Weak secondary interactions are thus shown to be sufficient to cause significant structural changes as well as qualitative differences in spin-spin interaction and in excited state structures.

Mononuclear and Dinuclear Ruthenium Complexes of cis- and trans-Thioindigo: Geometrical and Electronic Structure Analyses

“…Starting from the neutral mononuclear compound 1 with significant Ru III (L ·– ) character the one-electron oxidation to 1 + leads to a ruthenium­(III) complex of L. Two alternative formulations can be considered for the reduction to 1 – , because metal and ligand exhibit significant spin densities. On one side, in the dinuclear series, the oxidation of the native form 2 with Ru 2.5 (L ·– )­Ru 2.5 character can be envisaged to lead to a coupled three-spin situation of 2 + with one net spin remaining largely at the metal. On the other side, reduction to 2 – is interpreted as involving mostly ligand-centered spin with a possibly low-lying mixed-valent excited state.…”

Probing the Intramolecular Metal‐Selenoether Interaction in a Bis(iminosemiquinone)copper(II) Compound

“…In contrast to the coplanar systems (Q • – )Cu II (Q • – ), with dominant antiferromagnetic interaction between the two coplanar radical ligands and a resulting metal‐based spin the twisted complex 1b shows ligand‐centered spin, corresponding to a spin up‐down‐up situation of a three‐spin system . EPR investigations of the selenium analogue 1a at standard X‐band frequency (9.5 GHz) and at high frequency (350 GHz) (Figure ) reveal that the twisted 1a behaves similarly: The g components are marginally split and rather close to the free electron value of 2.0023, which is incompatible with copper‐based spin , , . Compound 1a is EPR silent at room temperature in fluid solution, probably due to increasing occupation of the S = 3/2 excited state of the three‐spin system.…”

“…The structure can thus be described as a highly distorted octahedron or as a copper(II) complex with 4+1+1 coordination . It should be noted that the methoxy analogue 1c of 1a and 1b displays no twisting (Scheme ) and no relevant copper–chalcogen interaction at d (Cu–O) > 3.4 Å , …”

“…the O 2 ‐caused spin crossover in heme proteins or the switching in molecular electronics . Introducing a small modification of the popular 4,6‐di‐ tert ‐butyl‐ N ‐aryl‐ o ‐iminosemiquinone ligand system, by making a thioether coordination available in ortho position,, we have previously discovered that the metal‐based spin (sequence: up‐up‐down ) of planar bis(iminosemiquinone)copper(II) complexes can be switched to ligand‐centered spin (sequence: up‐down‐up ), in association with structural changes such as NCuO/OCuN twisting and a very weak metal–chalcogen contact at about 3.2 Å, corresponding to the sum of the van der Waals radii according to Bondi at 1.43 (Cu) and 1.80 Å (S) ,…”

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

“…During those experimental and computational studies it turned out that new varieties of the one-electron-oxidized intermediates are conceivable, including classical mixed-valent systems Ru III (μ-L)­Ru II (Class II according to the Robin/Day classification) but also two kinds of three-spin situations, Ru III (μ-L • – )­Ru III , with spin sequences ↑↑↓ or ↑↓↑ . These charge and spin isomers are complementary to previously discussed pairs of alternatives Ru III (μ-L •– )­Ru II or Ru II (μ-L)­Ru II and Ru II (μ-L • – )­Ru II or Ru III (μ-L 2 – )­Ru II ; , related “resonance hybrids” have been discussed for Ru III (μ-Vd – )­Ru II or Ru II (μ-Vd • )­Ru II , where Vd • is the 1,5-diisopropyl-3-(4,6-dimethyl-2-pyrimidinyl)-6-oxoverdazyl radical …”