Diruthenium and triruthenium compounds of the potential redox active non-chelated η-N,η-N-benzothiadiazole bridge.

In the present study, a series of non-chelated BTD (2,1,3-benzothiadiazole)-bridged diruthenium(II) ([{(CHCN)(acac)Ru}(μ-BTD)] 1, [{CHCN(acac)Ru}(μ-BTD){Ru(acac)(η-N-BTD)}] 2, [{(η-N-BTD)(acac)Ru}(μ-BTD)] 3), and triruthenium ([{(acac)Ru}(μ-BTD)(η-N-BTD)] 4) complexes with varying ratios of η-N and μ-bis-η-N,η-N modes of BTD were studied. Complexes 1-4 ( = 0) were obtained the one-pot reaction of electron-rich Ru(acac)(CHCN) and electron-deficient BTD in refluxing acetone. The relatively low Ru(II)/Ru(III) potential of 1-4 (0.08-0.44 V SCE) further facilitated the isolation of the corresponding mixed valent RuRu ( = 1/2) and RuRuRu ( = 1/2)/RuRuRu ( = 1) forms [1]ClO-[3]ClO and [4]ClO/[4](ClO), respectively. The single-crystal X-ray structures of the representative mixed valent [1]ClO and [3]ClO established (i) Ru⋯Ru distances of 6.227 Å and 6.256 Å (molecule A)/6.184 Å (molecule B), respectively, (ii) a significant variation of the N-S distance of BTD in [3]ClO as a function of its binding mode μ η and (iii) similar Ru-N (μ-BTD) distances in each case corresponding to a valence delocalised situation. The mixed valent diruthenium (1+-3+) and triruthenium (4+/42+) complexes exhibited metal-based anisotropic electron paramagnetic resonance (EPR) and moderately intense low-energy intervalence charge-transfer (IVCT) transitions in the near-infrared region of 1730-1890 nm. Analysis of the IVCT band using the Hush treatment revealed a valence delocalised class III mixed valent state with the electronic coupling of ≈2640-2890 cm, as also corroborated by the values of 10-10, solvent independency of the IVCT band and uniform spin distribution between the metal ions in the singly occupied state(s). Furthermore, the involvement of the BTD (η and μ)-based orbitals in the reduction processes was evident by its free radical EPR feature.