Fine Tuning between Radical versus Nonradical States of Azoheteroarenes on Selective Osmium Platforms

The article highlights the cooperative impact of azoheteroarenes [abbt: 2,2′-azobis­(benzothiazole), L1–L3; bmpd: (E)-1,2-bis­(1-methyl-1H-pyrazole-3-yl) diazene, L4] and coligands [bpy: 2,2′-bipyridine; pap: 2-phenylazopyridine] in tuning radical (N–N^•–) versus nonradical (NN⁰) states of L on selective Os^II-platforms in structurally/spectroscopically characterized monomeric [1]­ClO₄–[6]­ClO₄ and [1]­(ClO₄)₂–[2]­(ClO₄)₂/[7]­(ClO₄)₂–[8]­(ClO₄)₂, respectively. The preferred syn-configuration of L in the complexes prevented obtaining ligand bridged dimeric species. It revealed that {Os­(bpy)₂} facilitated the stabilization of both nonradical ([1]­(ClO₄)₂–[2]­(ClO₄)₂) and radical ([1]­ClO₄–[2]­ClO₄) states of L1/L2, while it delivered exclusively the radical form for L3 in [3]­ClO₄. In contrast, {Os­(pap)₂} generated radical states of L1–L3 in [4]­ClO₄–[6]­ClO₄, respectively, without any alteration of the redox state of Os^II and azo (NN⁰) function of the pap coligand. The neutral state of L4 was, however, ascertained in [7]­(ClO₄)₂ or [8]­(ClO₄)₂ irrespective of the nature of the metal fragment {Os­(bpy)₂} or {Os­(pap)₂}, respectively. Switching between radical and nonradical forms of L in the complexes as a function L and coligand could be addressed based on their relative FMO (frontier molecular orbital) energies. Multiple close redox steps of the complexes extended a competitive electron transfer scenario between the redox active components including metal/L/bpy/pap, leading to delicate electronic forms in each case.