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.