Electron‐Rich Diruthenium Complexes with π‐Extended Alkenyl Ligands and Their F₄TCNQ Charge‐Transfer Salts**

Abstract

The synthesis of dinuclear ruthenium alkenyl complexes with {Ru(CO)(P i Pr 3 ) 2 (L)} entities (L = Cl À in complexes Ru 2 -3 and Ru 2 -7; L = acetylacetonate (acac À ) in complexes Ru 2 -4 and Ru 2 -8) and with π-conjugated 2,7-divinylphenanthrenediyl (Ru 2 -3, Ru 2 -4) or 5,8-divinylquinoxalinediyl (Ru 2 -7, Ru 2 -8) as bridging ligands are reported. The bridging ligands are laterally π-extended by anellating a pyrene (Ru 2 -7, Ru 2 -8) or a 6,7-benzoquinoxaline (Ru 2 -3, Ru 2 -4) π-perimeter. This was done with the hope that the open π-faces of the electron-rich complexes will foster association with planar electron acceptors via π-stacking. The dinuclear complexes were subjected to cyclic and square-wave voltammetry and were characterized in all accessible redox states by IR, UV/Vis/ NIR and, where applicable, by EPR spectroscopy. These studies signified the one-electron oxidized forms of divinylphenylene-bridged complexes Ru 2 -7, Ru 2 -8 as intrinsically delocalized mixed-valent species, and those of complexes Ru 2 -3 and Ru 2 -4 with the longer divinylphenanthrenediyl linker as partially localized on the IR, yet delocalized on the EPR timescale. The more electron-rich acac À congeners formed non-conductive 1 : 1 charge-transfer (CT) salts on treatment with the F 4 TCNQ electron acceptor. All spectroscopic techniques confirmed the presence of pairs of complex radical cations and F 4 TCNQ *À radical anions in these CT salts, but produced no firm evidence for the relevance of πstacking to their formation and properties.

Tetraruthenium Macrocycles with Laterally Extended Bis(alkenyl)quinoxaline Ligands and Their F4TCNQ•− Salts

“…A red shift of particularly the band at ca. 400 nm in the present macrocycles from its position of 351 nm in Ru 2 -3 is likely a consequence of the higher electron richness (hence, the higher energy of the relevant occupied donor levels) on coordinative saturation, and this was similarly observed for the corresponding acac − derivative of Ru 2 -3 [14].…”

“…A red shift of particularly the band at ca. 400 nm in the present macrocycles from its position of 351 nm in Ru2-3 is likely a consequence of the higher electron richness (hence, the higher energy of the relevant occupied donor levels) on coordinative saturation, and this was similarly observed for the corresponding acac − derivative of Ru2-3 [14]. Further oxidation to the associated tetracations leads to an additional blue shift of the Ru(CO) band to 1973 or 1952 cm −1 , respectively, as well as the bleach of the IVCT band.…”

“…Representative voltammograms in the THF/0.1 M NBu 4 + PF 6 − electrolyte are displayed in Figure 4. In this electrolyte, Ru 2 -3 displays two consecutive one-electron oxidations at E 1/2 = −68 and 77 mV, which are dominated by the electron-rich divinylphenylene diruthenium part of this molecule, as well as a chemically partially reversible reduction at E 1/2 = −2085 mV, which is biased toward the quinoxaline heterocycle [14]. Deviations of the closely spaced anodic waves from ideality seem to be due to some deposition of the oxidized forms of this complex onto the electrode surface of this electrolyte.…”

“…445 nm, and one close to 600 nm. All these bands have no equivalents in the simple divinylphenylene-bridged diruthenium complexes {(L)(P i Pr 3 ) 2 (CO)Ru} 2 (-CH=CH-C 6 H 4 -CH=CH) with L = Cl − , RCOO − , or acac − [14,44]; hence, they must originate from the lateral extension of the bridging arylene ligand. Quantum chemical calculations on a simplified PMe 3 model of complex Ru 2 -3 (further on denoted as Me Ru 2 -3) duly indicate that they are associated with intraligand charge transfer from either the electron-rich diruthenium divinylphenylene entities to the pyrazine and annulated pyrene rings, with varying contributions of these two units, or from pyrene to the quinoxaline heterocycle.…”

“…In previous work, we have demonstrated the ability of redox-active dinuclear ruthenium complexes with laterally π-extended bis(alkenyl)arylene ligands, such as they are present in compounds Ru 2 -3 and -4 of Figure 1, to form charge-transfer (CT) salts with the strong organic electron acceptor 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F 4 TCNQ, Figure 2). These salts showed enhanced stabilities, as compared to the ones prepared from complexes Ru 2 -1 and Ru 2 -2 with smaller arylene ligands [14]. We now wished to explore whether these dinuclear building blocks can be further elaborated into metallamacrocyclic structures.…”