Synthesis, 1H/13C NMR spectroscopic and structural studies of sterically induced internal dynamics in novel mono- and di-ruthenium(II) 2,2':6',2"-terpyridine complexes of 2,3-bis(2,2'- bipyridin-6-yl)pyrazine

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The mono- and di-metallic N6-ruthenium(II) complexes [Ru(terpy)L][PF6]2 1 and [{Ru(terpy)}2L][PF6]4 2, where L is 2,3-bis(2,2'-bipyridin-6-yl)pyrazine and terpy is 2,29:6',2''-terpyridine, have been prepared. Extensive and careful 1H and 13C NMR spectroscopic studies at ambient temperature showed that complexation induces steric strain in the scaffolding of L and thereby fluxional behaviour in both complexes. In 1, rotation of the free 2,29-bipyridyl (bpy) group of L relieves these interactions and results in two enantiomerically related conformations. In 2, a mutual twisting of both complexed bpy fragments results in helicene-like, P- and M-configured enantiomeric complexes. Dynamic low-temperature NMR studies of 1 and 2 in acetone-d6 solutions have provided activation energy data for both processes. The behaviour of 2 is paralleled by its crystal structure, which indicated a strong twisting of the bpy groups of L. Cyclic voltammetric and UV-spectroscopic studies conducted on both indicated a mild electrochemical and electronic coupling of the metal centres in 2.