Indazole-Derived Mono-/Diruthenium and Heterotrinuclear Complexes: Switchable Binding Mode, Electronic Form, and Anion Sensing Events.

The article deals with the newer classes of mononuclear: [(acac) 2 Ru III (H-Iz)(Iz - )] 1 , [(acac) 2 Ru III (H-Iz) 2 ]ClO 4 [ 1 ]ClO 4 /[ 1' ]ClO 4 , and [(bpy) 2 Ru II (H-Iz)(Iz - )]ClO 4 [ 2 ]ClO 4 , mixed-valent unsymmetric dinuclear: [(acac) 2 Ru III (μ-Iz - ) 2 Ru II (bpy) 2 ]ClO 4 [ 3 ]ClO 4 , and heterotrinuclear: [(acac) 2 Ru III (μ-Iz - ) 2 M II (μ-Iz - ) 2 Ru III (acac) 2 ] (M = Co: 4a , Ni: 4b , Cu: 4c , and Zn: 4d ) complexes (H-Iz = indazole, Iz - = indazolate, acac = acetylacetonate, and bpy = 2,2'-bipyridine). Structural characterization of all the aforestated complexes established their molecular identities including varying binding modes (N a and N b donors and 1 H -indazole versus 2 H -indazole) of the heterocyclic H-Iz/Iz - in the complexes. Unlike [ 1' ]ClO 4 containing two NH protons at the backface of H-Iz units, the corresponding [ 1 ]ClO 4 was found to be unstable due to the deprotonation of its positively charged quaternary nitrogen center, and this resulted in the eventual formation of the parent complex 1 . A combination of experimental and density functional theory calculations indicated the redox noninnocent feature of Iz - in the complexes along the redox chain. The absence of intervalence charge transfer transition in the near-infrared region of the (Iz - ) 2 -bridged unsymmetric mixed-valent Ru III Ru II state in [ 3 ]ClO 4 suggested negligible intramolecular electronic coupling corresponding to a class I setup (Robin and Day classification). Heterotrinuclear complexes ( 4a-4d ) exhibited varying spin configurations due to spin-spin interactions between the terminal Ru(III) ions and the central M(II) ion. Though both [ 3 ]ClO 4 and 4a - 4d displayed ligand (Iz - /Iz • )-based oxidation, reductions were preferentially taken place at the bpy and metal (Ru III /Ru II ) centers, respectively. Unlike 1 or [ 2 ]ClO 4 containing one free NH proton at the backface of H-Iz, [ 1' ]ClO 4 with two H-Iz units could selectively and effectively recognize F - , OAc - , and CN - among the tested anions: F - , OAc - , CN - , Cl - , Br - , I - , SCN - , HSO 4 - , and Η 2 PΟ 4 - in CH 3 CN via intermolecular NH···anion hydrogen bonding interaction. The difference in the sensing feature between [ 1' ]ClO 4 and 1 /[ 2 ]ClO 4 could be rationalized by their p K a values of 8.4 and 11.3/10.8, respectively.