TMPA‐based Cavitary Cobalt (II) Funnel Complexes

Abstract

The synthesis of a series of mononuclear, dicationic CoII funnel complexes is reported herein. Three ligands Calix‐TMPAX present a calix[6]arene cone closed at its small rim by a tris(2‐pyridylmethyl)amine (TMPA) unit and differ by the nature of three cavity walls, anisole, phenol or quinone. The X‐ray diffraction structure of [CoII(MeCN)Calix‐TMPAOMe](ClO4)2 displays a trigonal bipyramidal geometry, with Co bound to all 4 nitrogen atoms of the TMPA cap, and to one MeCN guest molecule buried inside the calixarene cavity. All complexes were fully characterized in solution as high spin 5‐coordinate species using various techniques, including 1H NMR spectroscopy. For comparison purpose, an analogous CoII complex based on the TMPACH2OH ligand, devoid of a calixarene core, was synthetized. Its X‐ray structure shows a dicationic 7‐coordinate cobalt(II) center in the N4O3 environment provided by the ligand, leaving no space for exogenous ligand binding. This contrasts with the 5‐coordinate complexes obtained with the calix‐ligands that allow guest‐ligand binding and exchange. A brief overview of the coordination properties of the calix‐complexes, compared to those obtained with TMPA ligands, devoid of a cavity, highlights major differences in terms of complexation kinetics, geometry, coordination to the labile site, anion affinity, nuclearity, and stability.