Rhodium assisted peri-C–H activation in benzothiazolyl-hydrazone derivatized pyrene

Abstract

Benzothiazolyl hydrazones incorporating polyaromatic pyrene moiety, 1 (H2LPyr), have been smartly employed as a directing group (DG) to bring about the rhodium assisted C–H bond activation at the peri position of pyrene. The formation of peri-metallated [RhIII(LPyr)(H)(PPh3)2] 3 is a logical consequence of its co-product, a dihalo complex [RhIII(HLPyr)Cl2(PPh3)2] 2, in due course of the reaction between the ligand and Wilkinson’s catalyst. The initial formation of the complex 2 in the initial stage of the reaction has been envisaged as the driving force for the generation of organometallic complex 3, where paucity of chloride ion triggers the tridentate coordination mode [LPyr]2−via in situ C–H activation. The underlying mechanism of formation of 3 has been observed to proceed via oxidative addition, involving a two electron transfer from the appropriate electron reservoir [RhI] to the ligand scaffold and this is accompanied by an intramolecular ligand to metal hydride transfer via a PCET pathway. Complexes 2 and 3 have been found to be redox active and are prone to oxidation at moderate potentials where the responses are analyzed to be exclusively ligand-centred in nature. Significantly, cyclometallated complex is more prone to oxidation relative to the non-activated compound, 2. The redox event has been meticulously scrutinized by DFT, revealing the destabilization of HOMO in 3 by ∼0.5 eV in comparison to 2. Both complexes provide rich optoelectronic features that have been analyzed to be predominantly 1ILCT in nature.