Diruthenium and triruthenium compounds of the potential redox active non-chelated eta(1)-N,eta(1)-N-benzothiadiazole bridge

In the present study, a series of non-chelated BTD (2,1,3-benzothiadiazole)-bridged diruthenium(II) ([{(CH3CN)(acac)(2)Ru-II}(2)(mu-BTD)] 1, [{CH3CN(acac)(2)Ru-II}(mu-BTD){Ru-II(acac)(2)(eta(1)-N-BTD)}] 2, [{(eta(1)-N-BTD)(acac)(2)Ru-II}(2)(mu-BTD)] 3), and triruthenium ([{(acac)(2)Ru-II}(3)(mu-BTD)(2)(eta(1)-N-BTD)(2)] 4) complexes with varying ratios of eta(1)-N and mu-bis-eta(1)-N,eta(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)(CH3CN)(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 (RuRuIII)-Ru-II (S = 1/2) and (RuRuRuIII)-Ru-II-Ru-II (S = 1/2)/(RuRuRuIII)-Ru-II-Ru-III (S = 1) forms [1]ClO4 - [3]ClO4 and [4]ClO4/[4](ClO4)(2), respectively. The single-crystal X-ray structures of the representative mixed valent [1]ClO4 and [3]ClO4 established (i) Ru...Ru distances of 6.227 angstrom and 6.256 angstrom (molecule A)/6.184 angstrom (molecule B), respectively, (ii) a significant variation of the N-S distance of BTD in [3]ClO4 as a function of its binding mode mu versus eta(1) and (iii) similar Ru-N (mu-BTD) distances in each case corresponding to a valence delocalised situation. The mixed valent diruthenium (1(+)-3(+)) and triruthenium (4(+)/4(2+)) 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 approximate to 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 (eta(1) and mu)-based orbitals in the reduction processes was evident by its free radical EPR feature.