Bis(amido)ruthenium(IV) complexes with 2,3-diamino-2,3-dimethylbutane. Crystal structure and reversible Ru(IV) - Amide/Ru(III) - Amine and Ru(IV) - Amide/Ru(II)-amine redox couples in aqueous solution

Abstract

Two bis(amido)ruthenium(IV) complexes, [RuIv(bpy)(L-H)2]2+ and [RuIV(L)(L-H)2]2+ (bpy = 2,2′-bipyridine, L = 2,3-diamino-2,3-dimethylbutane, L-H = (H2NCMe2CMe2NH)-), were prepared by chemical oxidation of [RuII(bpy)(L)2]2+ and the reaction of [(n-Bu)4N][RuvINCl4] with L, respectively. The structures of [Ru(bpy)(L-H)2][ZnBr4]·CH3CN and [Ru(L)(L-H)2]Cl2·2H2O were determined by X-ray crystal analysis. [Ru(bpy)(L-H)2][ZnBr4]·CH3CN crystallizes in the monoclinic space group P21/n with a = 12.597(2) Å, b = 15.909(2) Å, c = 16.785(2) Å, β= 91.74(1)°, and Z = 4. [Ru(L)(L-H)2]Cl2·2H2O crystallizes in the tetragonal space group I41/a with a = 31.892(6) Å, c = 10.819(3) Å, and Z = 16. In both complexes, the two Ru-N(amide) bonds are cis to each other with bond distances ranging from 1.835(7) to 1.856(7) Å. The N(amide)-Ru-N(amide) angles are about 110°. The two Ru(IV) complexes are diamagnetic, and the chemical shifts of the amide protons occur at around 13 ppm. Both complexes display reversible metal-amide/metal-amine redox couples in aqueous solution with a pyrolytic graphite electrode. Depending on the pH of the media, reversible/ quasireversible le--2H+ Ru(IV)-amide/Ru(III)-amine and 2e--2H+ Ru(IV)-amide/Ru(II)-amine redox couples have been observed. At pH = 1.0, the E° is 0.46 V for [RuIv(bPy)(L-H)2]2+/[Ru III(bpy)(L)2]3+ and 0.29 V vs SCE for [RuIV(L)(L-H)2]2+/[RuIII(L) 3]3+. The difference in the E° values for the two Ru(IV)-amide complexes has been attributed to the fact that the chelating saturated diamine ligand is a better σ-donor than 2,2′-bipyridine.