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

Valence-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 [Ru^III(trpy)(Cl)(L_Sq)]⁺ with a minority contribution from diamagnetic [Ru^II(trpy)(Cl)(L_Q)]⁺. Consequently, the closely spaced successive two oxidation processes of 1⁺ and 2⁺ can be assigned to Ru^III→Ru^IV and L_Sq→L_Q, 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 [Ru^II(trpy)(Cl)(L_Sq)], although the LUMO is calculated to be a mixture of dπ(Ru) (≈24 %) and π^∗(L) (≈ 70 %). The subsequent second (1^-/2^-) and third (1^2-/2^2-) 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 ½. The origin of the transitions in the 1²⁺/2²⁺and ½ systems is predicted by TDDFT analysis.