Metal-induced reductive ring opening of 1,2,4,5-tetrazines: three resulting coordination alternatives, including the new non-innocent 1,2-diiminohydrazido(2-) bridging ligand system

Abstract

Reaction of 3,6-diaryl-1,2,4,5-tetrazines (aryl = R = phenyl, 2-furyl or 2-thienyl) with 2 equiv of Ru(acac)₂(CH₃CN)₂ results in reductive tetrazine ring opening to yield diruthenium complexes [(acac)₂Ru^III(dih-R²-)Ru^III(acac)₂] bridged by the new 1,2-diiminohydrazido(2-) (dih-R^2- = HNC(R)NNC(R)NH^2-) ligands. rac/meso diastereoisomers could be detected and separated for the compounds with R = phenyl and 2-thienyl, all species are diamagnetic and were characterized by 1H NMR spectroscopy. Crystal structure determination of the meso isomers with R = phenyl and 2-thienyl confirmed the 1,2-diiminohydrazido formulation through long N-N (≈ 1.40 Å) and short C=N(H) bonds (≈ 1.31Å), implying two bridged ruthenium(III) centers at about 4.765 Å distance with strong antiferromagnetic coupling. The complexes undergo two reversible and well-separated one-electron reduction and oxidation processes, respectively. EPR Spectroscopy of the paramagnetic intermediates with comproportionation constants K_c > 10¹² and UV-vis-NIR spectroelectrochemistry were used to identify the accessible redox states as [(acac)₂Ru^II(dih-R²-)Ru^II(acac)₂]^2-, [(acac)₂Ru^II(dih-R^·-)Ru^II(acac)₂]-, [(acac)₂Ru^III(dih-R^2-)Ru^III(acac)₂], [(acac)₂Ru^III(dih-R·-)Ru^III(acac)₂]⁺, and [(acac)₂Ru^III(dih-R)Ru^III(acac)₂]²⁺. While the UV-vis-NIR spectroscopic response of [(acac)₂Ru(dih-R)Ru(acac)₂]^0/-/2- is very similar to that of [(bpy)2Ru(adc-R)Ru(bpy)₂]^4+/3+/2+, adc-R²- = 1,2-diacylhydrazido(2-), the EPR result indicating ligand-centered spin for [(acac)₂Ru^II(dih-R·-)Ru^II(acac)₂]- despite deceptive NIR absorptions around 1400 nm reveals distinct differences in the electronic structures.