Coordination of weakly binding anions to [Ru2(μ-O2CCH3)4]+ in aqueous solution

• 5 [Ru2(O2CMe)4]+ complexes with various anions were characterized in aqueous media.
• 2 NO3--containing complexes show different binding modes in water despite low D.N.
• Comparisons are drawn to the limited literature examples in aqueous media.

Four new complexes involving the diruthenium(II,III) tetraacetate core, [Ru2(μ-O2CCH3)4]+, with three relatively weakly binding anions ClO4-, NO3- and CF3CO2- have been synthesized and structurally characterized in exclusively aqueous media. Despite their low, but still positive  , apparent donor numbers (D.N.) in water (according to previous literature by Linert et al.) both NO3- (D.N. = 0.21) and CF3CO2- (D.N. < 8.65) do coordinate axially to [Ru2(μ-O2CCH3)4]+ , with water having a D.N. = 19.5. NO3- forms both a polymer, [Ru2(μ-O2CCH3)4(NO3)]x, and a mixed axial adduct with water, [Ru2(μ-O2CCH3)4(H2O)(NO3)] depending on reaction stoichiometry. CF3CO2- forms a double salt of the form [Ru2(μ-O2CCH3)4(H2O)2] [Ru2(μ-O2CCH3)4(CF3COO)2]⋯2H2O. ClO4-, with a negative apparent D.N. in water of −12.4, only “binds” outer-sphere to form [Ru2(μ-O2CCH3)4(H2O)2](ClO4), at all reaction stoichiometries (as does the PF6- ion whose structure was also determined and found to be the same as a previous result). These results are compared to the very limited data of [Ru2(μ-O2CCH3)4]+ adduct salts in aqueous solution and it is concluded that despite the low donor numbers, as long as they are positive, the species formed in water can be every bit as varied and rich as those formed in other solvents.