The search for cobalt single-molecule magnets: A disk-like Co IIICo II6 cluster with a ligand derived from a novel transformation of 2-acetylpyridine

Abstract

The initial employment of 2-acetylpyridine, (py)(Me)CO, in Co cluster chemistry is reported, and the synthesis, crystal structure, and full magnetic study of [Co 7(OH) 6(L) 6](ClO 4) 3·1.6H 2O ( 1·1.6H 2O) are described; L − is the anion of 2-(pyridine-2-yl)pentane-2-ol-4-one, (py)(Me)C(CH 2COCH 3)(O) −, formed in situ through a crossed-aldol reaction in acetone under strongly basic conditions. The reaction of Co(ClO 4) 2·6H 2O, (py)(Me)CO and NBu n 4OMe (1:1:1.2) in acetone at room temperature under aerobic conditions affords 1·1.6H 2O in 35% yield. The mixed-valent cation possesses a wheel-shaped (or disk-like) structural motif comprising a central octahedral Co III atom linked to six peripheral distorted octahedral Co II atoms by six μ 3-ΟΗ − groups; the six Co II atoms on the rim are held together by six η 1:η 1:η 1:μ 2·L − ligands and the oxygen atoms of the hydroxo groups. Variable-temperature, solid-state dc and ac magnetic susceptibility studies were carried out on 1. The dc magnetic study of 1 shows a decline in the product χ Μ T with decreasing T. The observation of out-of phase ( χ″ Μ) ac susceptibility signals below ∼4 K suggests that the Co IIICo II 6 complex may be a single molecule magnet, and this was confirmed by single-crystal magnetization vs. dc field sweeps down to 0.04 K that exhibited hysteresis. The study reveals that each complex cation is a weak single-molecule magnet, but that there are also intermolecular interactions (also confirmed crystallographically) to create a 3D ordered lattice; this still gives some hysteresis at 5 K.