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-R(2-))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 (approximately 1.40 A) and short C=N(H) bonds (approximately 1.31 A), implying two bridged ruthenium(III) centers at about 4.765 A 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)2Ru(II)(dih-R(2-))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-R(2-) = 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.

Unconventional Mixed‐Valent Complexes of Ruthenium and Osmium

“…Furthermore, owing to low-lying intraligand or charge-transfer transitions many radical complexes [82] will exhibit absorption features of variable intensity in the near-infrared (NIR) region, and such absorptions can easily be mistaken for intervalence charge transfer bands. [83,84] Frequently, the observation of an intermediate with a nearinfrared band has been taken as sufficient and convincing evidence for a mixed-valent situation although a highly noninnocent, for example, quinone-type bridging ligand was present.…”

“…The related new compounds from Scheme 4 (E = NH) are predominantly diruthenium(II)-radical species with correspondingly narrow EPR lines [84] although all the intermediates [(m-dih-R){Ru(acac) 2 …”

“…[91] Mixed O,N-donor bridging ligands are not uncommon both for mixed-valent and radical dinuclear complexes. [54,59,84,92] Of course, there are also established alloxygen coordinated diruthenium mixedvalent complexes with exclusively oxygendonor-containing bridges,…”

Structure, electrochemistry and spectroscopy of a new diacylhydrazido-bridged diruthenium complex with a strongly near-infrared absorbing RuIIIRuII intermediate

“…Azodicarboxylic esters and their anions and dianions as well as related molecular ions such as dih-R, diiminohydrazido(2−) [8], are easily available yet special ligands because they exhibit quinone type two-step redox behavior, a π conjugated bis-chelate function [7,9], a small 6 π center system of which 4 centers are coordinating, a radical intermediate stable against disproportionation [10], the resonance stabilization of the dianionic form, an "S frame" conformation which allows for a rather short M-M distances despite molecular bridging [9], a tuning potential through the substituents R at the noncoordinating carbon π centers [6], and the possibility to introduce additionally coordinating groups R, leading to bis-tridentate noninnocent ligands [11].…”

Valence‐State Distribution in the Ruthenium o‐Quinonoid Systems [Ru(trpy)(Cl)(L1)]+ and [Ru(trpy)(Cl)(L2)]+ (L1 = o‐Iminobenzoquinone, L2 = o‐Diiminobenzoquinone; trpy = 2,2′:6′,2″‐Terpyridine)

“…Sect.). [10] The synthesis of 2-ClO 4 by a different synthetic route and its electronic spectrum [in H 2 O; λ max (ε) = 506 nm (16000  -1 cm -1 ), 316 (22000), 280 (20000), 274 (21000), 232 (28000)] and metal oxidation potential (0.96 V vs. Ag/AgCl in CH 3 CN) have been reported earlier. [11] The structures of 1-ClO 4 and 2-ClO 4 were authenticated by their single-crystal X-ray structures (Figures 1 and 2, respectively, and Table 1).…”