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

We describe herein the synthesis and structural characterization of a tetrazine based bis(picolylamine) ligand (L, 1) that was prepared by direct reaction between two equivalents of picolylamine and 3,6-dichloro-1,2,4,5-tetrazine. Its reaction with cobalt(II) hexafluoroacetylacetonate afforded a mononuclear complex [Co(hfac)2L]·CH3CN (2) and a chain compound {[Co(hfac)2(-L)][Co(hfac)2(CH3OH)2]}n (3) (Hhfac = hexafluoroacetylacetone). The crystal structures of 1-3 were determined by X-ray structure analysis. The metal centre in 2 is tris-chelated by two hfac ligands and a picolylamine unit. The two crystallographically independent cobalt(II) ions that occur 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 leading to a neutral zig-zag chain which co-crystallizes with the mononuclear species trans-[Co2(hfac)2(CH3OH)2]. The magnetic properties of 2 and 3 have been investigated in the temperature range 1.9-300 K. The decrease of MT upon cooling which is observed for them is mainly due to the depopulation of the higher energy Kramers doublets of magnetically-isolated six-coordinate cobalt(II) ions. Alternating current (ac) magnetic susceptibility measurements of 2 and 3 show the occurrence of frequency-dependence of the out-of-phase signals under applied dc fields at very low temperatures which are indicative of Single-Ion Magnet (SIM) behaviour.

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