Valence‐State Distribution in the Ruthenium o‐Quinonoid Systems [Ru(trpy)(Cl)(L1)]+ and [Ru(trpy)(Cl)(L2)]+ (L1 = o‐Iminobenzoquinone, L2 = o‐Diiminobenzoquinone; trpy = 2,2′:6′,2″‐Terpyridine)

Abstract

AbstractValence‐state distributions in the ruthenium quinonoid (L) frameworks of [Ru(trpy)(Cl)(L1)]ClO4 (1‐ClO4) and [Ru(trpy)(Cl)(L2)]ClO4 (2‐ClO4) (L1 = o‐iminobenzoquinone, L2 = o‐diiminobenzoquinone, and trpy = 2,2′:6′,2″‐terpyridine) have been examined by structural, spectroelectrochemical, and density functional studies. The structural data, in corroboration with the DFT‐calculated bond lengths, suggest that the primary valence formulation of 1+ and 2+ is a spin‐coupled singlet configuration of [RuIII(trpy)(Cl)(LSq)]+ with a minority contribution from diamagnetic [RuII(trpy)(Cl)(LQ)]+. Consequently, the closely spaced successive two oxidation processes of 1+ and 2+ can be assigned to RuIII→RuIV and LSq→LQ, which involve the HOMO and HOMO–3 levels, respectively. The one‐electron‐oxidized species 12+ and 22+ display sharp EPR signals with g values of 2.011 and 2.014 at 77 K, respectively. The free radical EPR signal (g ≈ 2.0) of the one‐electron‐reduced species 1 or 2 signifies the preferential involvement of the ruthenium‐based orbitals in the first reduction process to yield [RuII(trpy)(Cl)(LSq)], although the LUMO is calculated to be a mixture of dπ(Ru) (≈24 %) and π*(L) (≈ 70 %). The subsequent second (1–/2–) and third (12–/22–) reduction steps in each case are associated simply with the terpyridyl‐based orbitals (≥90 %). The lowest energy charge‐transfer transitions of 1+ and 2+ at 556 and 509 nm are predicted to be HOMO → LUMO+1 and HOMO–1 → LUMO+1 transitions, respectively. In the successive oxidations 1+/2+ → 12+/22+ → 13+/23+ the lowest energy charge‐transfer transitions undergo a blue shift with a substantial reduction in intensity. The lowest energy charge‐transfer transitions, however, are red shifted with a reduction in intensity on going from (1+/2+) to 1/2. The origin of the transitions in the 12+/22+ and 1/2 systems is predicted by TDDFT analysis. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)