Tetrazine Chelate Ligands Bridging Two [Ru(acac) 2 ] Fragments: Mixed Valency and Radical Complex Formation

Using bis(3-methyl-2-pyridyl)-1,2,4,5-tetrazine 1 , 3-(2-pyrimidyl)- 6-methyl-1,2,4,5-tetrazine 2 and bis(2-pyrimidyl)-1,2,4,5-tetra-zine = bmtz as ligands, the complexes 3 = [Ru(acac) 2 ( 1 )], 4 = {[Ru(acac) 2 ] 2 ( 1 )], 5 = {[Ru(acac) 2 ] 2 (bmtz)], and 6 = {[Ru(acac) 2 ] 2 ( 2 )] were prepared and identified by structure analysis of crystal-lized material. The one-electron oxidized form 6 (PF 6 ) was also studied structurally, suggesting a Class II mixed-valent situation. The neutral dinuclear systems exhibit two reversible oxidation processes with comproportionation constants 10 9.2 < K c < 10 14.1 and one reduction which were analyzed UV/vis/NIR and EPR spectroscopically. Oxidation produces largely metal-based mixed-valent cations with very weak intervalence absorptions in the near IR whereas the electron uptake occurs at the tetrazine acceptor.

The complexes 3-6 were obtained from the ligands and the Ru II precursor [Ru(acac) 2 (MeCN) 2 ] as established for related examples. [13,19] Analytical and 1 H NMR data and the molecular structures of single crystals (Table S1) confirm the composition as detailed in the Introduction and below. In addition, the structural results provide information on isomerism, including rac/meso diastereomers (4-6), [17,18] configurational alternatives (5), and consequences of electron transfer (6, 6 + ). The reaction with 1 yields mononuclear 3 and the meso form of dinuclear 4. Whereas 5 was crystallized as the meso isomer from the rac/ meso mixture, the reaction of [Ru(acac) 2 (MeCN) 2 ] with 2 yielded rac and meso diastereomers of 6 in addition to some mononuclear material.
Compound 5. Several configurational isomers (1,2-, 1,3-or 1,4-positioning) can be conceived for a dinuclear complex of the potentially tetrakis-chelating bmtz. [7,8] Compounds of higher nuclearity were not detected in spite of the stoichiometric ratio of 1 : 4 for the reaction. Instead of the perhaps expected inversion-symmetric 1,3-configuration the alternative 1,4-arrangement with opposite Ru(acac) 2 groups was found (Tables 1, S10, S11; Figure 4), which exhibits a shorter metal-metal distance (5.424(2) Å) while maintaining small repulsion. The meso diasteromer has been established for the crystallized material although a further isomer was found by 1 H NMR spectroscopy. The free 2-pyrimidyl ring is twisted by 15.6(9)°in meso-5 relative to the plane containg the ruthenium centers. The NN and RuN bond parameters (Table 1) are reflecting stronger π back donation to the tetrazine ring. The  lengthening of the Ru-tetrazine bonds in dinuclear complexes may be due to the fact that the electron demand of the tetrazine ligand is satisfied by the presence of two donors instead of one. Compound 6. The situation of meso-5 is similarly observed for the meso-form of 6 without significant difference of the bond parameters (Tables 1, S12, S13; Figure 5). The RuN tz bonds are typically shorter at 1.95 Å than the RuN pym bonds of 2.04 Å (Table 1). Fortunately, it was also possible to crystallize the oneelectron oxidized form as the rac diastereomer 6(PF 6 ) from the reaction of rac/meso 6 with AgPF 6 (Tables 1, S14, S15; Figure 6). While the slight contraction of NN and RuN bonds (Table 1) on oxidation may be expected, the most significant result is the remarkable difference between the ruthenium-tetrazine bonds Ru2 N3 at 1.9370(17) Å and Ru1 N6 at 2.0448(17) Å (Table 1). This considerable disparity strongly suggests different oxidation states in the mixed-valent 6 + , corresponding to a Class II behavior [9,22] with the localized oxidation occurring on Ru1, in spite of the relatively short metal-metal distance of 5.431(3) Å.
A further notable result is the similarity of Ru2-N tz distances for 6 (1.941(7) Å) and 6 + (1.9370(17) Å).  6 . For the dinuclear compounds 4-6 reversible one-electron processes were observed for two oxidations and one reduction. The systems studied exhibit only small variation, caused by donor substituents as in 4 (shift to negative potentials) or acceptor substitution as in 5 (shift to positive potentials). Diastereoisomerism does not have a significant effect either ( Figure 7).

Cyclic voltammetry
As established by EPR (see below) the reduction occurs at the bridging ligands with unreduced tetrazine whereas the oxidation occurs stepwisely at the metals. The difference between the two oxidation potentials can be converted into an equilibrium constant  comproportionation, [9,10,18] which is one of the defining parameters for mixed-valent intermediates. The values obtained here (Table 2) of 10 9.2 -10 14.1 are typically [4,14,18] large; even the prototypical Creutz-Taube ion has only K c = 10 6 . [9]

EPR spectroscopy
In situ one-electron oxidation or reduction of the diamagnetic precursor compounds 3-6 yields EPR signals which can serve to identify the site of electron transfer. Table 3 and Figures 8, 9, S6, S7 illustrate a stark difference in EPR response: Electrochemical reduction produces single line signals near the free electron value of 2.0023 ( Figures S6, S7). This and the observability at room temperature suggest an organic radical species with very little contribution from the metals to the spin distribution. [23,24] A small g anisotropy can be noted in the frozen state but there is no hyperfine splitting visible as may be possible under favourable circumstances. [16] Oxidation produces EPR signals only at low temperatures in the frozen state with significant anisotropy of the g value. Axial 3 + and rhombic splittings (4 + -6 + , Figures 8, 9) have been observed, and the amount of g 1 -g 3 splitting in comparison [24] to other paramagnetic ruthenium complexes suggests major metal contributions to the spin distribution. The large spin orbit coupling [25] of the heavy element ruthenium is responsible for the g anisotropy and the rapid relaxation, allowing EPR signal observation only at low temperatures. [23,24] The different manifestations of g anisotropy in cations 3 + -6 + reflect the geometrical and electronic structures. Mononuclear 3 + exhibits an axial g splitting (Figure 8, left) as befits a single low-spin d 5 ion Ru 3 + . The corresponding dinuclear 4 + shows a slight rhombic behaviour (Figure 8, right), corresponding to the approximately centrosymmetric arrangement. The mixed valence in 4 + also results in rapid relaxation which leads to broad lines. Cations 5 + and 6 + are structurally similar and thus exhibit comparable EPR features ( Table 3). The considerable g anisotropy Δg from a rhombic splitting (Figure 9) reflects the low symmetry resulting from the strongly π accepting tetrazine and less accepting pyrimidine ring. The Class II behaviour of the mixed-valent cations produces close-lying frontier MOs as evident from large Δg values.
[b] g iso = isotropic g ¼ ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi p .
[e] Not determined. The dinuclear starting compounds 4-6 and the mononuclear complex 3 are distinguished by intense metal-to-ligand charge transfer (MLCT) absorptions in the visible (λ max = 500-700 nm, involving the low-lying π* orbital of tetrazine as target (LUMO)). The occurrence of more than one band reflects the presence of other π acceptor heterocycles (pyridine, pyrimidine) in addition to the stronger π accepting tetrazine. [17] Reduction to tetrazine radical anion complexes of ruthenium(II) produces rather small spectral changes. [23] The tetrazine anion radical components of the reduced ligands in the anionic complexes contribute to the rather limited shifts. The first one-electron oxidation to yield a mixed-valent (Ru III -Ru II ) intermediate should produce an inter-valence charge transfer (IVCT) absorption at long wavelengths, typically in the near infrared (NIR) region. [9,10] However, such transitions are known to be very weak for tetrazine-bridged systems [11,13c,14] which is also noted here. Figure 10 shows the emergent weak shoulder around 1500 nm for 4 + , and even less observable features (ɛ < 20 M À 1 cm À 1 ) were found for 5 + and 6 + in the same spectral region ( Table 4). The combination of very high K c values and very weak IVCT absorptions is apparently [13c] a hallmark of tetrazine bridged Ru III -Ru II mixed-valent intermediates, its origin will have to be further investigated through quantum chemical calculations. This observation relates to the concept that electron-poor ligands are poor conduits for hole transfer. [24b,c,28] The dications resulting from second oxidation (Figures 11, 13) display intense bands from ligand-to ligand and ligand-to-metal charge transfer around 700 nm between the visible and near infrared regions (Figures 11, 13). Compound 4 is distinguished by a weak infrared absorption at 1915 nm after the second oxidation ( Figure 11, Table 4). This transition is attributed to an LMCT (ligand-to-metal charge transfer) favoured by the presence of a donor substituted tetrazine bridge 1 and two oxidized metal centers. Redox systems 5 n + and 6 n + exhibit comparable spectra and spectral changes, as noted earlier for the very similar EPR response.

Conclusion
Three dinuclear ruthenium compounds have been synthesized and studied in 4 different oxidation states (À ,0, + ,2 +). The neutral precursors (which may be reduced to radical complexes) contain π accepting tetrazine bridges and electron rich ruthenium(II) metals, without major intramolecular electron transfer, as confirmed structurally and spectroscopically. Structural analysis of one of the mixed-valent intermediates (6 + ) suggests a Class II situation according to the Robin/Day classification scheme. [22] As in other tetrazine ligand bridged examples, [11,12,14,18] the mixed-valent species are thermodynamically stable over an unusually large potential range, as quantified by comproportionation constants 10 9.2 < K c < 10 14.1 , but exhibit only very weak IVCT absorptions in the near IR region.