Sensitivity of the Valence Structure in Diruthenium Complexes As a Function of Terminal and Bridging Ligands

The
compounds [(acac)2RuIII(μ-H2L2–)­RuIII(acac)2]
(rac, 1, and meso, 1′) and [(bpy)2RuII(μ-H2L•–)­RuII(bpy)2]­(ClO4)3 (meso, [2]­(ClO4)3) have been structurally, magnetically,
spectroelectrochemically, and computationally characterized (acac– = acetylacetonate, bpy = 2,2′-bipyridine, and
H4L = 1,4-diamino-9,10-anthraquinone). The N,O;N′,O′-coordinated
μ-H2Ln– forms
two β-ketiminato-type chelate rings, and 1 or 1′ are connected via NH···O hydrogen
bridges in the crystals. 1 exhibits a complex magnetic
behavior, while [2]­(ClO4)3 is a
radical species with mixed ligand/metal-based spin. The combination
of redox noninnocent bridge (H2L0 → →
→ →H2L4–) and {(acac)2RuII} → →{(acac)2RuIV} or {(bpy)2RuII} → {(bpy)2RuIII} in 1/1′
or 2 generates alternatives regarding the oxidation state
formulations for the accessible redox states (1n and 2n),
which have been assessed by UV–vis–NIR, EPR, and DFT/TD-DFT
calculations. The experimental and theoretical studies suggest variable
mixing of the frontier orbitals of the metals and the bridge, leading
to the following most appropriate oxidation state combinations: [(acac)2RuIII(μ-H2L•–)­RuIII(acac)2]+ (1+) → [(acac)2RuIII(μ-H2L2–)­RuIII(acac)2]
(1) → [(acac)2RuIII(μ-H2L•3–)­RuIII(acac)2]−/[(acac)2RuIII(μ-H2L2–)­RuII(acac)2]− (1–) → [(acac)2RuIII(μ-H2L4–)­RuIII(acac)2]2–/[(acac)2RuII(μ-H2L2–)­RuII(acac)2]2– (12–) and [(bpy)2RuIII(μ-H2L•–)­RuII(bpy)2]4+ (24+) → [(bpy)2RuII(μ-H2L•–)­RuII(bpy)2]3+/[(bpy)2RuII(μ-H2L2–)­RuIII(bpy)2]3+ (23+) → [(bpy)2RuII(μ-H2L2–)­RuII(bpy)2]2+ (22+). The favoring of RuIII by
σ-donating acac– and of RuII by
the π-accepting bpy coligands shifts the conceivable valence
alternatives accordingly. Similarly, the introduction of the NH donor
function in H2Ln as compared
to O causes a cathodic shift of redox potentials with corresponding
consequences for the valence structure.

本研究对化合物[(acac)₂RuIII(μ-H₂L²⁻)­RuIII(acac)₂]（外消旋体1与内消旋体1′）以及[(bpy)₂RuII(μ-H₂L•⁻)­RuII(bpy)₂]­(ClO₄)₃（内消旋体，[2]­(ClO₄)₃）开展了结构、磁学、光谱电化学与计算化学表征，其中acac⁻为乙酰丙酮根（acetylacetonate），bpy为2,2′-联吡啶（2,2′-bipyridine），H₄L为1,4-二氨基-9,10-蒽醌。N,O;N′,O′配位的μ-H₂Lⁿ⁻配体可形成两个β-酮亚胺型螯合环，晶体中1或1′通过NH···O氢键桥相互连接。1展现出复杂的磁学行为，而[2]­(ClO₄)₃属于配体与金属混合自旋的自由基物种。氧化还原非惰性配体桥（H₂L⁰ → → → →H₂L⁴⁻）与{(acac)₂RuII} → {(acac)₂RuIV}或{(bpy)₂RuII} → {(bpy)₂RuIII}在1/1′或2体系中的结合，为可及氧化态（1ⁿ与2ⁿ）的价态表述提供了多种可行方案，该问题已通过紫外-可见-近红外光谱（UV–vis–NIR）、电子顺磁共振谱（EPR）以及密度泛函理论（DFT）/含时密度泛函理论（TD-DFT）计算进行了系统评估。实验与理论研究表明，金属与桥配体的前线轨道存在可变混合模式，由此推导得到以下最合理的氧化态组合：[(acac)₂RuIII(μ-H₂L•⁻)­RuIII(acac)₂]⁺（1⁺）→ [(acac)₂RuIII(μ-H₂L²⁻)­RuIII(acac)₂]（1）→ [(acac)₂RuIII(μ-H₂L•³⁻)­RuIII(acac)₂]⁻/[(acac)₂RuIII(μ-H₂L²⁻)­RuII(acac)₂]⁻（1⁻）→ [(acac)₂RuIII(μ-H₂L⁴⁻)­RuIII(acac)₂]²⁻/[(acac)₂RuII(μ-H₂L²⁻)­RuII(acac)₂]²⁻（1²⁻），以及[(bpy)₂RuIII(μ-H₂L•⁻)­RuII(bpy)₂]⁴⁺（2⁴⁺）→ [(bpy)₂RuII(μ-H₂L•⁻)­RuII(bpy)₂]³⁺/[(bpy)₂RuII(μ-H₂L²⁻)­RuIII(bpy)₂]³⁺（2³⁺）→ [(bpy)₂RuII(μ-H₂L²⁻)­RuII(bpy)₂]²⁺（2²⁺）。σ给体配体acac⁻对RuIII的稳定作用，以及π受体配体bpy对RuII的稳定作用，会相应地调整可设想的价态变体。类似地，相较于氧供体位点，H₂Lⁿ中引入NH给体官能团会导致氧化还原电位发生阴极偏移，并对价态结构产生对应影响。