Isovalent and Mixed-Valent Diruthenium Complexes [(acac)₂Ru^II(μ-bpytz)Ru^II(acac)₂] and [(acac)₂Ru^II(μ-bpytz)Ru^III(acac)₂](ClO₄) (acac = Acetylacetonate and bpytz = 3,6-Bis(3,5-dimethylpyrazolyl)-1,2,4,5-tetrazine):  Synthesis, Spectroelectrochemical, and EPR Investigation

Abstract

The title compounds involving the structurally characterized bridging ligand bpytz were characterized, showing very strong electrochemical stabilization of the mixed-valent RuIIRuIII state (Kc = 10(13.9)) but no detectable (epsilon < 20 M(-1) cm(-1)) intervalence charge-transfer band in the infrared region. In situ reduction of the neutral precursor produces a diruthenium(II) complex of the bpytz radical anion according to EPR spectroscopy, whereas oxidation of the mixed-valent form leads to a diruthenium(III) species.

2,5‐Dioxido‐1,4‐benzoquinonediimine (H2L2−), A Hydrogen‐Bonding Noninnocent Bridging Ligand Related to Aminated Topaquinone: Different Oxidation State Distributions in Complexes [{(bpy)2Ru}2(μ‐H2L)]n (n=0,+,2+,3+,4+) and [{(acac)2Ru}2(μ‐H2L)]m (m=2−,−,0,+,2+)

“…The species fails to show any distinct band in the near‐infrared region. It should be noted that very weak‐to‐undetectable IVCT bands have been reported previously for the Ru II Ru III mixed‐valent state for diruthenium– and triruthenium–acac systems1a,e, 22, 26 in spite of reasonably large K c values, and it was concluded that large K c values are independent of the IVCT band intensity 26b. However, 2 − displays a rhombic Ru III ‐type EPR spectrum (Figure 8b).…”

Mixed Donor–Acceptor‐Derived N,N′‐Diarylpyrazine‐2,5‐dicarboxamido‐Bridged Diruthenium Systems: Structures, Magnetic Properties, and Electronic Forms in Multiredox States

“…[3c] Thus, the change of the bridging ligand leads to a substantial low‐energy shift of the NIR band for the one‐electron‐reduced forms, as shown by a comparison of the data for 1 – / 2 – with that for 3 – . Such weak NIR absorptions in spite of the appreciably high K c values of 10 6 /10 7 for the mixed‐valent states in 1 – or 2 – are in line with similar observations for ligand‐bridged diruthenium(II)/(III) systems with very large to large K c values and negligible to weak IVCT transitions in the NIR region . The further reduction to the L 1/2 2– ‐bridged isovalent Ru II Ru II state in 1 2– / 2 2– resulted in two close redshifted bands at λ ≈ 700 nm with a twofold intensity enhancement in addition to relatively weak transitions at λ ≈ 500 nm.…”

New insights on surface‐enhanced Raman scattering based on controlled aggregation and spectroscopic studies, DFT calculations and symmetry analysis for 3,6‐bi‐2‐pyridyl‐1,2,4,5‐tetrazine adsorbed onto citrate‐stabilized gold nanoparticles

“…According to the theoretical calculations, this band corresponds to an n tz → π tz * transition involving the nonbonding orbitals localized at the N atoms of the central tetrazine ring (MO 41) to the anti‐bonding π* orbitals localized on same ring (MO 44). This band involves a n–π* transition, which is also observed in the tetrazine molecule 53, 54. In spite of being symmetry‐allowed in the C 2 V point group, this band is relatively weak since it involves orthogonal n(σ) and π orbitals.…”