Preparation and X-ray Structures of the Dipyridyl-Terminated Macrobicyclic Iron, Cobalt, and Nickel(II) Tris-α-dioximates: Synthetic Strategies toward Rod-like 3d-Metal Complexes with Terminal N-Donor Groups Possessing an Optimal Ligand Aspect Ratio

Abstract

Dipyridyl-terminated cobalt and nickel(II) tris-nioximates were prepared by the template reaction of a given alicyclic α-dioxime with 4-pyridylboronic acid on the corresponding M²⁺ ion as a matrix. Attempts to prepare their tris-glyoximate analogues possessing an optimal ligand aspect ratio by the same synthetic approach were unsuccessful. Therefore, a rational multistage pathway toward them was elaborated. It is based on a postsynthetic functionalization of their reactive diiodoclathrochelate precursors and includes, in its first stage, palladium-catalyzed Sonogashira cross-coupling reactions with ethynyltrimethylsilane. Further cleavage of the trimethylsilyl groups in thus-obtained macrobicyclic products gave the bis(alkyne)-terminated clathrochelates, undergoing copper-catalyzed azide–alkyne cycloaddition reactions with the azidopyridine component to give the target clathrochelates. Thus, the prepared new diamagnetic iron(II) tris-glyoximate clathrochelates and paramagnetic cobalt- and nickel(II) tris-nioximates were characterized using elemental analysis, MALDI-TOF mass spectrometry, UV–vis, and ¹H NMR spectroscopies and by single-crystal XRD experiments for six of them. Nickel and cobalt(II) complexes have an almost trigonal-prismatic (TP) MN₆-coordination polyhedra, while the geometry of FeN₆-polyhedra is intermediate between a TP and a trigonal antiprism. Values of the ligand aspect ratio for them vary in a wide range from 1:1.3 up to approximately 1:4.