Noninnocence of the deprotonated 1,2-bis((1H-pyrrol-2-yl)methylene)hydrazine bridge in diruthenium frameworks – a function of co-ligands

Abstract

The article deals with the sensitive electronic forms in accessible redox states of structurally and spectroscopically authenticated deprotonated 1,2-bis((1H-pyrrol-2-yl)methylene)hydrazine (H₂L^R, R = H) or 1,2-bis((3,5-dimethyl-1H-pyrrol-2-yl)methylene)hydrazine (H₂L^R, R = Me), a BODIPY analogue bridged diruthenium complex as a function of varying ancillary ligands. It involved rac-(acac)₂Ru^III(μ-L^{R 2−})Ru^III(acac)₂1a, R = H; 1b, R = Me (S = 1, acac = acetylacetonate), rac-[(bpy)₂Ru^II(μ-L²⁻)Ru^II(bpy)₂](ClO₄)₂ [2](ClO₄)₂ (S = 0, bpy = 2,2′-bipyridine) and diastereomeric [(pap)₂Ru^II(μ-L²⁻)Ru^II(pap)₂](ClO₄)₂meso-[3a](ClO₄)₂/rac-[3b](ClO₄)₂ (S = 0, pap = phenylazopyridine). The crystal structure established the linkage of the conjugated –C5N2–N3C6– central unit with the two terminal deprotonated pyrrole units of coordinated L²⁻. The bridging L²⁻ in 1a, 1b, [2](ClO₄)₂, [3b](ClO₄)₂ and [3a](ClO₄)₂ was slightly twisted and planar with torsional angles of 41.54°, 42.91°, 37.38°, 35.33° and 0°, respectively, with regard to the central N2–N3 bond. The extent of twisting of the bridge followed an inverse relationship with the Ru⋯Ru separation: 4.935/4.934 Å 1a/1b < 5.141 Å [2](ClO₄)₂ < 5.201 Å [3b](ClO₄)₂ < 5.351 Å [3a](ClO₄)₂. This is also attributed to the intermolecular π⋯π/CH⋯π interactions between the nearby aromatic rings of L and bpy or pap in [2](ClO₄)₂ or [3](ClO₄)₂, respectively. The multiple redox steps of the complexes varied appreciably based on the σ-donating (acac) and π-acidic (bpy, pap) characteristics of the ancillary ligands. Experimental (structure, EPR) and theoretical (DFT) evaluation pertaining to the electronic forms of 1ⁿ, 2ⁿ and 3ⁿ demonstrated the preferential involvement of L based frontier orbitals in electron transfer processes even in combination with the redox facile ruthenium ion. This in turn highlighted its redox non-innocent feature as in the case of well-documented metal coordinated quinonoid, formazanate, diimine (bpy), azo (pap) and β-diketiminate functions.