Abstract
Reaction of 3,6-diaryl-1,2,4,5-tetrazines (aryl = R = Ph, 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 sepd. for the compds. with R = Ph and 2-thienyl, all species are diamagnetic and were characterized by 1H NMR spectroscopy. Crystal structure detn. of the meso isomers with R = Ph and 2-thienyl confirmed the 1,2-diiminohydrazido formulation through long N-N ($\approx$1.40 \AA) and short C:N(H) bonds ($\approx$1.31 \AA), implying two bridged ruthenium(III) centers at $\sim$4.765 \AA distance with strong antiferromagnetic coupling. The complexes undergo two reversible and well-sepd. 1-electron redn. and oxidn. processes, resp. EPR Spectroscopy of the paramagnetic intermediates with comproportionation consts. Kc \textgreater 1012 and UV-visible-NIR spectroelectrochem. were used to identify the accessible redox states as (acac)2RuII(dih-R2-)RuII(acac)22-, (acac)2RuII(dih-R.bul.-)RuII(acac)2-, (acac)2RuIII(dih-R2-)RuIII(acac)2, (acac)2RuIII(dih-R.bul.-)RuIII(acac)2+, and (acac)2RuIII(dih-R)RuIII(acac)22+. While the UV-visible-NIR spectroscopic response of (acac)2Ru(dih-R)Ru(acac)20/-/2- is very similar to that of (bpy)2Ru(adc-R)Ru(bpy)24+/3+/2+, adc-R2- = 1,2-diacylhydrazido(2-), the EPR result indicating ligand-centered spin for (acac)2RuII(dih-R.bul.-)RuII(acac)2- despite deceptive NIR absorptions around 1400 nm reveals distinct differences in the electronic structures. on SciFinder(R)
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