Bond elongation in the anion radical of coordinated tetrazine ligands: A crystallographic, spectroscopic and computational study of a reduced {Re(CO)3Cl} complex

The binuclear complex [(μ-Me2BPTZ)(Re(CO)3Cl)2] (1), where Me2BPTZ = 3,6-(5-methyl-pyridyl)-1,2,4,5-tetrazine, can be reduced by addition of bis(η5-pentamethylcyclopentadienyl) iron(II) (decamethylferrocene, Fc∗), to obtain a stable radical anion form 1−. A single-crystal X-ray diffraction study of the radical anion (1−)(Fc∗+) was conducted and compared with a computational model of the same compound in the neutral and reduced states. As such, this work presents the first structural analysis of a reduced diimine ligand that is coordinated to {Re(CO)3Cl} moieties. Bond-length changes within the tetrazine ring system were consistent with previously reported examples of tetrazine radicals and with calculated structures that show clear elongation of the azo-type NN bond. Consistently atomic charge calculations indicate that the extra electron in the radical anion resides largely at the tetrazine core. A negligible change in the Re–Cl bond length is observed and computed.