Mononuclear and One-Dimensional Cobalt(II) Complexes with the 3,6-Bis(picolylamino)-1,2,4,5-tetrazine Ligand

Abstract

We describe herein the synthesis and structural characterization of a tetrazine-based bis(picolylamine) ligand, namely, N3,N6-bis(pyridin-2-ylmethyl)-1,2,4,5-tetrazine-3,6-diamine (L, 1), which was prepared by the direct reaction between 2 equiv. of picolylamine and 3,6-dichloro-1,2,4,5-tetrazine. Its reaction with cobalt(II) hexafluoroacetylacetonate hydrate afforded the mononuclear complex [Co(hfac)2L]·CH3CN (2, Hhfac = hexafluoroacetylacetone) and the chain compound {[Co(hfac)2(µ-L)][Co(hfac)2(CH3OH)2]}n (3). The crystal structures of 1–3 were determined by X-ray structure analysis. The metal centre in 2 is trischelated by two hfac ligands and one picolylamine unit. The two crystallographically independent cobalt(II) ions in 3 (Co1 and Co2) are six-coordinate. Two bidentate hfac groups in equatorial positions and two trans-positioned pyridine nitrogen atoms build the coordination polyhedron around Co1 to form a neutral zigzag chain compound, which cocrystallizes with the mononuclear species trans-[Co2(hfac)2(CH3OH)2]. The magnetic properties of 2 and 3 were investigated in the temperature range 1.9–300 K. The decrease of the χMT values of 2 and 3 upon cooling is mainly due to the depopulation of the higher energy Kramers doublets of the magnetically isolated six-coordinate cobalt(II) ions. Alternating-current (ac) magnetic susceptibility measurements of 2 and 3 revealed the occurrence of frequency-dependent out-of-phase signals under applied direct-current (dc) fields at very low temperatures, and this result is indicative of single-ion magnet (SIM) behaviour.