Novel non-symmetric POCOP-Ni(II) pincer complexes derived from 2,4-dihydroxybenzaldehyde

Abstract

Pincer ligands have long captivated chemists for their ability to merge rigidity, modularity, and fine-tuned electronic control into a single molecular scaffold. Within this class, POCOP-type ligands stand out for their capacity to stabilize reactive metal centers while enabling diverse catalytic and functional applications. Yet, the deliberate rupture of symmetry in these systems—particularly in non-symmetric POCOP frameworks—remains an underexplored route to unlocking new reactivity patterns. Herein, we report the synthesis of a family of non-symmetric Ni(II)–POCOP pincer complexes (2a-2c) and demonstrate that their formation is decisively governed by the presence or absence of base during the metalation step. Under basic conditions, selective formation of the aldehyde-type pincer complexes is achieved, whereas base-free conditions lead to acid-promoted ligand activation and the formation of imine-type addition complexes (3a-4c), which may partially hydrolyze during purification to yield aldehyde-type addition complexes (5a-5c). The structures and coordination modes of the isolated species were established by comprehensive spectroscopic methods and single-crystal X-ray diffraction. Subsequent optimization of the reaction conditions enabled the selective synthesis of the target non-symmetric Ni(II)–POCOP complexes in improved yield and purity, providing insight into the role of medium acidity in directing the stability and reactivity of phosphinite-based pincer systems.