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

Abstract

AbstractHerein, we report two newly synthesized salen‐type ligands, 2,3‐bis((3,5‐di‐tert‐butyl‐2‐hydroxybenzyl)thio)maleonitrile (H2L1) and 2,3‐bis((3,5‐di‐tert‐butyl‐2‐hydroxybenzyl)amino)malenonitrile (H4L2), bearing different coordination sites (sulfur vs amine) at maleonitrile tethered moiety to investigate metal mediated non‐innocence chemistry of these ligands. Upon metallation, ligand H2L1 did not yield simple metal‐ligand complex, rather ligand was split into two organic fragments, dithiolene moiety (mnt)2− and phenol moiety via C−S bond cleavage wherein (mnt)2− formed a stable metal complex [M(mnt)2]2−. The C−S bond cleavage was interpreted in terms of strong p(π) … d(π) interaction between metal and dithiolene moiety in H2L1 ligand that invoked the intramolecular rearrangement facilitating C−S bond cleavage. Interestingly, the phenol moiety further transformed to either unprecedented 2,4‐di‐tert‐butyl‐6‐methylenecyclohexa‐2,4‐dienone (i. e. spiro compound; 5) or 2,4‐di‐tert‐butyl‐6‐(hydroxymethyl)phenol (6) depending on the temperature of reaction and type of metal ion used which was further predicted using DFT calculation. On the other hand, combined experimental and DFT studies explained that upon metallation, ligand H4L2 yielded non‐cleavage [M(H2L2)] (3 for M=Cu(II) and 4 for M=Ni(II) ) complex, which slowly oxidized at −NH−CH2‐(amine) region to −N=CH− (imine) in H2L2 ligand under aerobic environment via C−H bond activation, yielding [CuII(L3)] (1) (or [NiII(L3)] (2) complex (H2L3=oxidatively dehydrogenated product of H4L2 ligand.