Diruthenium and triruthenium compounds of the potential redox active non-chelated η 1 -N,η 1 -N-benzothiadiazole bridge.

In the present study, a series of non-chelated BTD (2,1,3-benzothiadiazole)-bridged diruthenium(II) ([{(CH 3 CN)(acac) 2 Ru II } 2 (μ-BTD)] 1, [{CH 3 CN(acac) 2 Ru II }(μ-BTD){Ru II (acac) 2 (η 1 -N-BTD)}] 2, [{(η 1 -N-BTD)(acac) 2 Ru II } 2 (μ-BTD)] 3), and triruthenium ([{(acac) 2 Ru II } 3 (μ-BTD) 2 (η 1 -N-BTD) 2 ] 4) complexes with varying ratios of η 1 -N and μ-bis-η 1 -N,η 1 -N modes of BTD were studied. Complexes 1-4 ( S = 0) were obtained via the one-pot reaction of electron-rich Ru(acac) 2 (CH 3 CN) 2 and electron-deficient BTD in refluxing acetone. The relatively low Ru(II)/Ru(III) potential of 1-4 (0.08-0.44 V versus SCE) further facilitated the isolation of the corresponding mixed valent Ru II Ru III ( S = 1/2) and Ru II Ru II Ru III ( S = 1/2)/Ru II Ru III Ru III ( S = 1) forms [1]ClO 4 -[3]ClO 4 and [4]ClO 4 /[4](ClO 4 ) 2 , respectively. The single-crystal X-ray structures of the representative mixed valent [1]ClO 4 and [3]ClO 4 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 4 as a function of its binding mode μ versus η 1 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 V ab of ≈2640-2890 cm -1 , as also corroborated by the K c values of 10 5 -10 8 , 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 1 and μ)-based orbitals in the reduction processes was evident by its free radical EPR feature.