Metal Ion (Ni^II vs Co^II)-Mediated Unusual Amine–Imine Interconversion in Conjugated Amine-ene-imine Ligand: Synthesis, Structure, and Characterization

Abstract

Two configurationally isomeric ligands, namely, 2-((E)benzylideneamino)-3-((pyridin-2-ylmethyl)amino)maleonitrile (HL 1 ) and 2-(benzylamino)-3-((E)-(pyridin-2-ylmethylene)amino)maleonitrile (HL 2 ), were synthesized and fully characterized, which are malenonitrile-tethered, N atom donors tridentate ligands. Structurally, they differ in the interchangeable position of amine and imine group only. Under the same reaction condition, Ni(II) invoked the transformation of (L 1 ) − to (L 2 ) − via simultaneous oxidation of amine to imine and the reduction of imine to amine. Two sequential 1,5-proton transfer pathways were anticipated for this type of unusual amine-imine oxidation/reduction process under acidic medium. In contrast, Co(II) was silent to such amine-imine interconversion reactions under both HL 1 and HL 2 ligand environment. The variation in electronic requirement might differentiate between both ligands such that HL 2 influenced the arial oxidation of Co(II) to Co(III) but HL 1 could not. The redox chemistry of Co(II)/Co(III) complexes with either HL 1 or HL 2 was explained using cyclic voltammetry and UV−vis spectroscopy.

Competitive electronic effect of ligand substitution over the role of metal ions (Ni and Co) on unusual amine–imine interconversion in conjugated amine–ene–imine ligands

“…The earlier reported 8 conversion of HL 1-H to HL 2-H simply meant the interconversion of the positions of amine and imine. Ligand HL 1-H was synthesized by condensation of picolinaldehyde and 2,3-diaminomaleonitrile followed by reduction of the imine group, and the final product was obtained from a condensation reaction with benzyaldehyde (Scheme 1).…”

“…We have chosen three different aryl groups which correspond to six ligand systems: (i) para -methoxyphenyl groups containing HL 1-OMe and HL 2-OMe (electron donating via the resonance effect) (ii) para -trifluorophenyl groups containing HL 1-CF3 and HL 2-CF3 (strong electron-withdrawing effect) and (iii) mesityl groups containing HL 1-Mes and HL 2-Mes (strong electron-donation effect). These new ligands were synthesized using similar methods 8 to those described for the parent ligands HL 1-H and HL 2-H (Scheme 2).…”

“…However, in an acidic environment, simultaneous oxidation of amine and reduction of imine is rare in the literature except for our recent discovery. 8 We recently reported an unusual phenomena where Ni( ii )-assisted simultaneous reduction of imine and oxidation of amine could occur within a conjugated malenonitrile-tethered amine–ene–imine ligand in an acidic medium (Scheme 1).…”

“…We also showed that Co( ii ) was unresponsive to such amine–imine interconversion reactions under the same reaction conditions. 8 It has also been proposed that two sequential 1,5-proton transfers in an acidic medium was the mechanism for this type of unusual amine–imine oxidation/reduction process; however a detailed analysis including the electronic effect on the periphery of the ligand and mechanistic aspects is essential. An understanding of the competitive role of the metal ion and the electronic effect upon substitution on the phenyl ring has yet to be explored.…”

M(II) (M=Cu, Ni) Assisted C−S Bond Cleavage and Oxidative Dehydrogenation of Amine on Non‐Innocent Salen Type Ligand Platforms by Varying Nitrogen vs. Sulfur Coordination Atoms

“…In this paper, we are presenting M(II) ions (Ni and Cu) assisted non‐innocence behavior (C−S bond cleavage and oxidative dehydrogenation of amine) of salen type ligands under the circumstances when diimine moiety of salen is changed into thioether or diamine moiety. We have recently reported [15] the unusual behavior of malenonitrile tethered tridented conjugated amine‐ene‐imine ligand where metal ion dependent amine‐imine inter‐conversion was observed. Thus, to understand the role of donor atoms towards the non‐innocence behavior of ligands, herein, we have purposely designed and reported the synthesis of two new tetradentate ligands which differ at malenonitrile tethered coordination sites; one is thio‐ether linked salen type ligand (2,3‐bis((3,5‐di‐tert‐butyl‐2‐hydroxybenzyl)thio)maleonitrile ( H 2 L 1 )) and another is NH linked reduced salen ligand (2,3‐bis((3,5‐di‐tert‐butyl‐2‐hydroxybenzyl)amino)malenonitrile (H 4 L 2 ) ).…”