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-1 H -pyrazole-3-yl) diazene, L4] and coligands [bpy: 2,2'-bipyridine; pap: 2-phenylazopyridine] in tuning radical (N-N •- ) versus nonradical (N═N 0 ) states of L on selective Os II -platforms in structurally/spectroscopically characterized monomeric [ 1 ]ClO 4 -[ 6 ]ClO 4 and [ 1 ](ClO 4 ) 2 -[ 2 ](ClO 4 ) 2 /[ 7 ](ClO 4 ) 2 -[ 8 ](ClO 4 ) 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 ](ClO 4 ) 2 -[ 2 ](ClO 4 ) 2 ) and radical ([ 1 ]ClO 4 -[ 2 ]ClO 4 ) states of L1/L2, while it delivered exclusively the radical form for L3 in [ 3 ]ClO 4 . In contrast, {Os(pap) 2 } generated radical states of L1-L3 in [ 4 ]ClO 4 -[ 6 ]ClO 4 , respectively, without any alteration of the redox state of Os II and azo (N═N 0 ) function of the pap coligand. The neutral state of L4 was, however, ascertained in [ 7 ](ClO 4 ) 2 or [ 8 ](ClO 4 ) 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.