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.

Enhancing Magnetic Communication between Metal Centres: The Role of s‐Tetrazine Based Radicals as Ligands

“…Although s ‐tetrazines have been known for over a century, their coordination chemistry and related coordination complexes were not well explored before 1985. In recent years, however, tz and its derivatives have become increasingly popular ligand frameworks, [43a–g, 48, 49, 52, 54] which may be attributed to their multiple metal binding sites (at least four nitrogen donor atoms within the conjugated ring system) and the availability of various redox states. Having said that, the use of the parent compound s ‐tetrazine in coordination chemistry has received relatively less attention compared to its 3,6‐disubstituted derivatives (e.g.…”

Mono- and Binuclear Copper(II) and Nickel(II) Complexes with the 3,6-Bis(picolylamino)-1,2,4,5-Tetrazine Ligand

“…Since L is a ditopic ligand containing two chelating units, its reaction with metal centers could, in principle, lead to discrete mono- and binuclear metal complexes or coordination polymers, depending on the number of available coordination sites on the metal center. As mentioned above, in the course of a previous study [ 31 ] we indeed obtained a mononuclear cobalt(II) complex [Co(hfac) 2 (L)], where L acts as a monotopic chelating ligand, and a one-dimensional coordination polymer [Co(hfac) 2 (µ-L)] n , in which L connects cobalt(II) centers through coordination of the pyridines only. We extend herein our study towards the paramagnetic centers copper(II) and nickel(II) by the use of the neutral [M(hfac) 2 ] precursors.…”

“…Copper(II) and nickel(II) hexafluoroacetylacetonate hydrates were purchased from Sigma Aldrich and dehydrated under high vacuum prior to their use. The L ligand was prepared as previously described [ 31 ].…”

“…Although the magnetic interaction between the metal centers was not mediated by the tetrazine ring, these results highlight the opportunities arising from its use as a platform for functional ligands, affording complexes provided with interesting magnetic properties. Besides functionalization with dipicolylamine, the reaction of 3,6-dichloro-TTZ with 2-picolylamine (2-PyCH 2 NH 2 ) in methyl- t -butylether provided either mono(picolylamine)-TTZ-Cl (pica-TTZ-Cl) [ 30 ] or bis(picolylamine)-TTZ (bis(pica)-TTZ L) [ 31 ] ( Scheme 1 ) depending on the reaction time and temperature.…”

“…Pica-TTZ-Cl proved its versatile coordination behavior as a neutral or monoanionic ligand in a series of copper(II) mononuclear and mixed-valence copper(II)/copper(I) binuclear complexes [ 30 ], while with ligand L we have previously reported the cobalt(II) complexes of the formula [Co(hfac) 2 (L)]·CH 3 CN and {[Co(hfac) 2 (µ-L)][Co(hfac) 2 (CH 3 OH) 2 ]} n (hfac = hexafluoroacetylacetonate), which have shown field-induced slow magnetic relaxation, indicative of the so-called single-ion magnet (SIM) behavior [ 31 ].…”