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)(CH3-CN)(2) results in reductive tetrazine ring opening to yield diruthenium complexes [(acac)(2)Ru-III(dih-R2-)Ru-III(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 H-1 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 (approximate to 1.40 angstrom) and short C=N(H) bonds (approximate to 1.31 angstrom), implying two bridged ruthenium(III) centers at about 4.765 angstrom 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 > 10(12) and UV-vis-NIR spectroelectrochemistry were used to identify the accessible redox states as [(acac)(2)Ru-II(dih-R2-)Ru-II(acac)(2)](2-), [(acac)(2)Ru-II(dih-Rcenter dot-)Ru-II(acac)(2)](-), [(acac)(2)Ru-II(dih-R2-)Ru-III(acac)(2)], [(acac)(2)Ru-III(dih-Rcenter dot-)Ru-III(acac)(2)](+), and [(acac)(2)Ru-III(dih-R)Ru-III(acac)(2)](2+). While the UV-vis-NIR spectroscopic response of [(acac)(2)Ru(dih-R)Ru(acac)(2)](0/-/2-) is very similar to that of [(bpy)(2)Ru(adc-R)Ru(bpy)(2)](4+/3+/2+), adc-R-2- = 1,2-diacylhydrazido(2-), the EPR result indicating ligand-centered spin for [(acac)(2)Ru-II(dih-Rcenter dot-)Ru-II(acac)(2)](-) despite deceptive NIR absorptions around 1400 nm reveals distinct differences in the electronic structures.