Ruthenium coordination preferences in imidazole‐containing systems revealed by electrospray ionization mass spectrometry and molecular modeling: possible cues for the surprising stability of the Ru (III)/tris (hydroxymethyl)‐aminomethane/imidazole complexes

Ruthenium is a platinoid that exhibits a range of unique chemical properties in solution, which are exploited in a variety of applications, including luminescent probes, anticancer therapies, and artificial photosynthesis. This paper focuses on a recently demonstrated ability of this metal in its +3 oxidation state to form highly stable complexes with tris (hydroxymethyl)aminomethane (H2 NC(CH2 OH)3 , Tris-base or T) and imidazole (Im) ligands, where a single Ru(III) cation is coordinated by two molecules of each T and Im. High-resolution electrospray ionization mass spectrometry (ESI MS) is used to characterize Ru(III) complexes formed by placing a Ru(II) complex [(NH3 )5 Ru(II) Cl]Cl in a Tris buffer under aerobic conditions. The most abundant ionic species in ESI MS represent mononuclear complexes containing an oxidized form of the metal, ie, [Xn Ru(III) T2 - 2H](+) , where X could be an additional T (n = 1) or NH3 (n = 0-2). Di- and tri-metal complexes also give rise to a series of abundant ions, with the highest mass ion representing a metal complex with an empirical formula Ru3 C24 O21 N6 H66 (interpreted as cyclo(T2 RuO)3 , a cyclic oxo-bridged structure, where the coordination sphere of each metal is completed by two T ligands). The empirical formulae of the binuclear species are consistent with the structures representing acyclic fragments of cyclo(T2 RuO)3 with addition of various combinations of ammonia and dioxygen as ligands. Addition of histidine in large molar excess to this solution results in complete disassembly of poly-nuclear complexes and gives rise to a variety of ionic species in the ESI mass spectrum with a general formula [Ru(III) Hisk Tm (NH3 )n - 2H](+) , where k = 0 to 2, m = 0 to 3, and n = 0 to 4. Ammonia adducts are present for all observed combinations of k and m, except k = m = 2, suggesting that [His2 Ru(III) T2 - 2H](+) represents a complex with a fully completed coordination sphere. The observed cornucopia of Ru(III) complexes formed in the presence of histidine is in stark contrast to the previously reported selective reactivity of imidazole, which interacts with the metal by preserving the RuT2 core and giving rise to a single abundant ruthenium complex (represented by [Im2 Ru(III) T2 - 2H](+) in ESI mass spectra). Surprisingly, the behavior of a hexa-histidine peptide (HHHHHH) is similar to that of a single imidazole, rather than a single histidine amino acid: The RuT2 core is preserved, with the following ionic species observed in ESI mass spectra: [HHHHHH.(Ru(III) T2 )m - (3m-1)H](+) (m = 1-3). The remarkable selectivity of the imidazole interaction with the Ru(III) T2 core is rationalized using energetic considerations at the quantum mechanical level of theory.