A novel type of heteroleptic Cu(I) complexes featuring nitrogen-rich tetrazine ligands: syntheses, crystal structures, spectral properties, cyclic voltammetry, and theoretical calculations.

Heteroleptic copper(I) complexes with the general formula [Cu(N^N)(P^P)]X constitute one of the most studied categories of 3d metal photosensitizers. Here, we examine using 1,2,4,5-tetrazine-based ligands to synthesize photoactive Cu(I) complexes. The newly prepared complexes were characterized by single-crystal X-ray analysis, which revealed the formation of dinuclear complexes [Cu2(μ-L1)(xantphos)2](ClO4)2 (1) and [Cu2(μ-L2)(xantphos)2](ClO4)2 (2), and mononuclear complexes [Cu(L3)(xantphos)]ClO4 (3) and [Cu(L4)(xantphos)]ClO4 (4), where L1 = 3,6-di(2'-pyridyl)-1,2,4,5-tetrazine (bptz), L2 = 3,6-bis-(3,5-dimethyl-pyrazol-1-yl)-1,2,4,5-tetrazine, L3 = 3-(2-pyridyl)-1,2,4,5-tetrazine, L4 = 3-(3,5-dimethyl-1H-pyrazol-1-yl)-1,2,4,5-tetrazine and xantphos = 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene. Solution stability assays were addressed by NMR spectroscopy showing that complexes are stable in dichloromethane over several days. The electronic excited states were investigated by UV-Vis and luminiscence spectroscopy and interpreted with the help of TD-DFT calculations. In the case of all the newly prepared complexes 1-4, the absorptions in the visible region were assigned to non-emissive MLCT transitions between the Cu(I) and the respective tetrazine ligand. Redox properties were probed by cyclic voltammetry and also supplemented by DFT calculations. Interestingly, tetrazine ligands L1-L4 show a shift of reduction potential to less negative values upon the formation of Cu(I) complexes 1-4. Moreover, the two complexes 3-4 represent the first reported case of mononuclear heteroleptic Cu(I)-tetrazine complexes.