Versatile coordination behaviour of the chloro-tetrazine-picolylamine ligand: mixed-valence binuclear Cu(i)/Cu(ii) complexes.

The 3-Cl-6-amino-(2'-picolyl)-1,2,4,5-tetrazine ligand HL has been synthesized and structurally characterized. Its versatile coordination behavior has been evidenced through reactions with Cu(Hfac) and Cu(triflate) precursors, which provided mixed-valence bimetallic CuCu complexes [Cu(μ-Cl)(L)] 2 and [Cu(μ-triflate)(L)] 5. Changing the Cu(ii) precursor and the solvent leads to the formation of mononuclear octahedral Cu(ii) complexes [CuCl(HL)]·2CHCN 3 and [Cu(Hfac)(HL)] 4, in which only the amino-pyridine unit is involved in the coordination of the metal center. In contrast, in complexes 2 and 5, the ligands are deprotonated and bridge the metal centers as pyridine-amido-tetrazine fragments, while a bridging chloride or triflate ligand completes the coordination sphere of the metal ions. The Cu-Cu distances of 2.4313(4) Å in 2 and 2.5198(10) Å in 5 lie among the shorter values within mixed-valence bimetallic Cu complexes. Mixed-valence character is strongly supported by DFT calculations, showing the equal repartition of the unpaired electron between the two metal centers.