Versatility of Coordination Modes in Complexes of Monohydroxamic Acids with Half-Sandwich Type Ruthenium, Rhodium, Osmium and Iridium Cations

Complexation of the secondary N-methyl-acetohydroxamic acid (HMeaha) with [(eta(6)-p-cym)Ru(H2O)(3)](2+), [(eta(5)-Cp*)Rh(H2O)(3)](2+), [(eta(6)-p-cym)Os(H2O)(3)](2+) or [(eta(5)-Cp*)Ir(H2O)(3)](2+) cations and the primary acetohydroxamic acid (HAha) with the former two cations (first time in the literature) has been studied in aqueous solution using pH-potentiometry, NMR, ESI-MS and DFT calculations. Because the arene or arenyl ligands take three coordination sites of the metal ion, only three remain free for interaction with additional ligand(s). Meaha(-) forms five-membered hydroxamate type chelated [ML](+) and [M(OH)L], being the third coordination site occupied by a water molecule in the former complex and by a hydroxide ion in the latter one. While [ML](+) remains predominant even under basic conditions in the Ru- and Rh-containing systems, it is measurable only in slightly acidic region with the two 5d metal ions. Also the tendency for hydrolysis of the aqua ligand in [ML(H2O)](+) complexes follows the trend of the cations for hydrolysis, Os > Ir > Ru > Rh. The X-ray result did not show any role of the H substituent of Aha(-) in [{(eta(5)-Cp*)Rh}(2)((2)-Aha)(2)](2+), in solution, however, its significant role was supported especially with Ru. All the results support the formation of a stable trinuclear [M3H-2L3(H2O)](+) species in which doubly deprotonated (hydroxymate type) ligands bridge two metal centres by coordinating via the two oxygens to the one cation and via the N-donor to another one