Donor/acceptor neutral aggregation of a paddlewheel-type [Ru ₂^II,II] complex and TCNQ

The reaction of a paddlewheel-type diruthenium(II, II) complex, [Ru ₂(2,3,5,6-F₄-PhCO₂)₄(MeOH) ₂] (2,3,5,6-F₄-PhCO₂^- = 2,3,5,6-tetrafluorobenzoate), with acridine (acr) produces an acridine-capped discrete compound, [Ru₂(2,3,5,6-F₄-PhCO₂) ₄(acr)₂]·toluene (1). The Ru-Ru and Ru-N _ax (N_ax = axial N atom of acridine) bond lengths are 2.3087(5) Å and 2.450 Å (average), respectively, and the average Ru-O_eq length (O_eq = carboxylate oxygen atom) is 2.074 Å, indicating an oxidation state of [Ru₂^II,II]. The reaction of 1 with 7,7,8,8-tetracyano-p-quinodimethane (TCNQ) leads to a π-stacked aggregation with a formulation of [Ru₂(2,3,5,6-F ₄-PhCO₂)₄(acr)₂] ·TCNQ·2CH₂Cl₂ (1-TCNQ). Even in a donor/acceptor aggregated system of 1-TCNQ, the Ru-Ru and Ru-N_ax bond lengths are 2.3072(9) Å and 2.443(3) Å, respectively, similar to those of 1, and the average Ru-O_eq length is 2.072 Å, which replies an oxidation state of [Ru₂^II,II] as well as the case in 1. Indeed, the charge ρ of the TCNQ moiety in 1-TCNQ was evaluated to be almost zero (ρ = -0.063), and the infrared spectra of 1-TCNQ agree with the neutral state of compound. The DFT calculations for 1 have revealed that its HOMO (HOMO_[Ru2]) is associated with δ ^* orbital of Ru-Ru bond and lies at -5.088 eV, which is very close to the LUMO level for TCNQ (LUMO_TCNQ with -5.122 eV). The gap between the HOMO_[Ru2] and LUMO_TCNQ levels could be insufficient to induce CT in 1-TCNQ.