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The Electron-Rich {Ru(acac)2} Directed Varying Configuration of the Deprotonated Indigo and Evidence for Its Bidirectional Noninnocence

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posted on 01.02.2016, 00:00 by Prasenjit Mondal, Ankita Das, Goutam Kumar Lahiri
This article highlights the hitherto unexplored varying binding modes of the deprotonated natural dye indigo (H2L) and its bidirectional noninnocent potential. The reaction of H2L with the selective metal precursor RuII(acac)2(CH3CN)2 (acac = acetylacetonate) leads to the simultaneous formation of paramagnetic RuIII(acac)2(HL) (1; blue solid) and diamagnetic RuII(acac)2(L) (2; red solid), which have been characterized by standard analytical, spectroscopic, and structural analysis. Crystal structures establish that the usual trans configurated and twisted monodeprotonated HL and unprecedented cis configurated nearly planar dehydroindigo (L) bind to the {Ru­(acac)2} metal fragment via the N,O and N,N donors, forming six- and five-membered chelates, respectively. It also reveals the existence of intramolecular N–H···O hydrogen-bonding interaction between the NH proton and CO group at the back face of the coordinated HL, in addition to an intermolecular N–H···O hydrogen bonding between the NH proton of HL of Molecule B and oxygen atom of the nearby acac of the second molecule (Molecule A) in the asymmetric unit of 1. The specific role of the electron-rich {Ru­(acac)2} metal fragment in stabilizing the cis-configuration of the electron-deficient L in 2 has been pointed out. Both 1 and 2 exhibit reversible one-electron oxidation and successive three reductions with varying Kc (comproportionation constant) values in the range of 1018–106. The potentials for the redox processes of 2 are positively shifted with respect to those of 1. The involvement of the metal or HL/L or mixed metal-HL/L-based orbitals in the accessible redox processes of 1n and 2n has been analyzed by spectroelectrochemistry, EPR at the paramagnetic states, and DFT calculated MO compositions/spin density distributions. The collective consideration of the experimental results and DFT/TD-DFT data has ascertained the participation of both the metal fragment {Ru­(acac)2} and the HL/L in the redox processes, which in effect result in mixed electronic structural forms of 1n and 2n (n = +1, 0, −1, −2, −3).