Arylamino radical complexes of ruthenium and osmium: dual radical counter in a molecule

Abstract

Radical and non-radical ruthenium and osmium complexes of 1-amino-9,10-anthraquinone (AqNH₂), which is defined as a molecule of dual radical counter, are disclosed. 1-Amido-9,10-anthraquinone (AqNH⁻) complexes of the types trans-[Ru^II(AqNH⁻)(PPh₃)₂(CO)Cl] (1), trans-[Os^II(AqNH⁻)(PPh₃)₂(CO)Br] (2) and trans-[Ru^III(AqNH⁻)(PPh₃)₂Cl₂] (3) were isolated. AqNH⁻ of 1–3 is redox active and undergoes oxidation reversibly at +(0.05–0.35) V to the 1-amino-9,10-anthraquinone radical (AqNH˙) and reduction at -(0.86–1.60) V to the 1-amido-9,10-anthrasemiquinonate anion radical (Aq^NHSQ˙²⁻). The reaction of 2 with I₂ in CH₂Cl₂ afforded a crystalline AqNH˙ complex of the type trans-[Os^II(AqNH˙)(PPh₃)₂(CO)Br]⁺I₅⁻·½I₂ (2⁺I₅⁻·½I₂). AqNH˙ and Aq^NHSQ˙²⁻ complexes of the types trans-[Ru^II(AqNH˙)(PPh₃)₂(CO)Cl]⁺ (1⁺), trans-[Ru^III(AqNH˙)(PPh₃)₂Cl₂]⁺ (3⁺), trans-[Ru^II(Aq^NHSQ˙²⁻)(PPh₃)₂(CO)Cl]⁻ (1⁻) and trans-[Os^II(Aq^NHSQ˙²⁻)(PPh₃)₂(CO)Br]⁻ (2⁻) were generated chemically/electrochemically in solution. The electronic states of the complexes were authenticated by single crystal X-ray structure determinations of 1, 2·5/4 toluene, 3 and 2⁺I₅⁻·½I₂, EPR spectroscopy and density functional theory (DFT) calculations. AqNH˙ instigates a 2c–3e p_π–d_π interaction and the length in 2⁺I₅⁻·½I₂, 1.978(5) Å, is relatively shorter than the Os^II–NH_Aq⁻ length, 2.037(2) Å, while the Aq–NH˙ bond, 1.365(8) Å, is longer than the Aq–NH⁻ bond, 1.328(3) Å. DFT calculations predicted that the atomic spin is delocalized over the ligand backbone (1⁺, 56%) particularly in one of the p-orbitals of the nitrogen and the metal atoms of the 1⁺ and 2⁺ ions, while the spin is dominantly localized on the anthraquinone fragment of the 1⁻ and 2⁻ ions. TD DFT calculations were employed to elucidate the origins of the lower energy absorption bands of the neutral complexes. Hypsochromic shifts of the UV-vis-NIR absorption maximum during 1→1⁺, 2→2⁺ and 3→3⁺ conversions were recorded by spectroelectrochemical measurements.