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

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)2RuIII(dih-R2-)RuIII(acac)2] bridged by the new 1,2-diiminohydrazido(2−) (dih-R2- = HNC(R)NNC(R)NH2-) 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 (≈1.31 Å), 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)2RuII(dih-R2-)RuII(acac)2]2-, [(acac)2RuII(dih-R•-)RuII(acac)2]-, [(acac)2RuIII(dih-R2-)RuIII(acac)2], [(acac)2RuIII(dih-R•-)RuIII(acac)2]+, and [(acac)2RuIII(dih-R)RuIII(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)2RuII(dih-R•-)RuII(acac)2]- despite deceptive NIR absorptions around 1400 nm reveals distinct differences in the electronic structures.