Osmium(II) and Ruthenium(II) Arene Maltolato Complexes: Rapid Hydrolysis and Nucleobase Binding

Abstract

Density functional calculations show that aquation of [Os(eta6-arene)(XY)Cl]n+ complexes is more facile for complexes in which XY=an anionic O,O-chelated ligand compared to a neutral N,N-chelated ligand, and the mechanism more dissociative in character. The O,O-chelated XY=maltolato (mal) [M(eta6-p-cym)(mal)Cl] complexes, in which p-cym=p-cymene, M=OsII (1) and RuII (2), were synthesised and the X-ray crystal structures of 1 and 22 H2O determined. Their hydrolysis rates were rapid (too fast to follow by NMR spectroscopy). The aqua adduct of the OsII complex 1 was 1.6 pKa units more acidic than that of the RuII complex 2. Dynamic NMR studies suggested that O,O-chelate ring opening occurs on a millisecond timescale in coordinating proton-donor solvents, and loss of chelated mal in aqueous solution led to the formation of the hydroxo-bridged dimers [(eta6-p-cym)M(mu-OH)3M(eta6-p-cym)]+. The proportion of this dimer in solutions of the OsII complex 1 increased with dilution and it predominated at micromolar concentrations, even in the presence of 0.1 M NaCl (conditions close to those used for cytotoxicity testing). Although 9-ethylguanine (9-EtG) binds rapidly to Os(II) in 1 and more strongly (log K=4.4) than to RuII in 2 (log K=3.9), the OsII adduct [Os(eta6-p-cym)(mal)(9EtG)]+ was unstable with respect to formation of the hydroxo-bridged dimer at micromolar concentrations. Such insights into the aqueous solution chemistry of metal-arene complexes under biologically relevant conditions will aid the rational design of organometallic anticancer agents.

Crystal structure of bromido(η6-1-isopropyl-4-methylbenzene)(7-oxocyclohepta-1,3,5-trien-1-olato-κ2O,O′)osmium

“…The isobidentate nature of the OsO 2 C 2 moiety is evidence of delocalization of the C O bonds of the tropolone ligand upon coordination [C11-O1 = 1.303 (11), C17-O2 = 1.299 (11)Å ]. The aromatic ring of the p-cymene ligand appears almost planar, with the displacement of the arene ring centroid from the Os II center [1.676 Å ] being comparable with other similar complexes (Peacock et al, 2007;Kandioller et al 2013).…”

Crystal structure of dibromido μ-oxalato-κ2 O,O′:κ2 O′′,O′′′−η6-p-cymenediosmium(II), C22H28Br2O4Os2

“…the gure) is remarkably stable and can be utilized as a synthon for potential metallopharmaceuticals [6]. In addition to the potential of the complex as an anticancer agent, the non-innocent bridging oxalate ligand is of interest because of its potential for facilitating electronic interactions between redox centres due to its planarity and electron delocalization [7].…”

Crystal structure of (η 6-benzene)-(N-(2,6-dimethylphenyl)-1-(pyridin-2-yl)methanimine-κ2 N,N′)ruthenium(II) perchlorate monohydrate, C20H20Cl2N2O5Ru

“…There is an increasing interest in the chemistry of half-sandwich organo-ruthenium complexes due to their numerous applications in new catalytic systems [5]. In addition some ruthenium complexes with N,N′-bidentate ligands have also found application as anticancer agents [6]. The asymmetric unit contains one cationic ruthenium (II) complex, one perchlorate counter anion and one molecule of water.…”