Michael I. Webb

Serum-protein interactions with anticancer Ru(III) complexes KP1019 and KP418 characterized by EPR

The compounds imidazolium [trans-[RuCl4(1H-imidazole)2] (KP418) and indazolium [trans-RuCl4(1H-indazole)2] (KP1019) both show significant anticancer activity, with the latter recently having completed phase I clinical trials. An important component of this success has been associated with targeted delivery of the complexes to cancer cells by serum proteins. In this study, electron paramagnetic resonance (EPR) measurements, combined with incubation under physiological conditions, and separation of protein-bound fractions, have been used to characterize the interactions of these complexes with human serum albumin (hsA), human serum transferrin (hsTf) apoprotein, and whole human serum. The strong EPR signals observed in these experiments demonstrate that both complexes are primarily retained in the 3+ oxidation state in the presence of serum components. Rapid, noncovalent binding of KP1019 was observed in the presence of both hsA and serum, indicating that the predominant interactions occur within the hydrophobic binding sites of hsA. This sequestering process correlates with the low levels of side effects observed in clinical trials of the complex. At longer incubation times, the noncovalently bound complexes are converted slowly to a protein-coordinated form. Noncovalent interactions are not observed in the presence apo-hsTf, where only slow binding of KP1019 via ligand exchange with the protein occurs. By contrast, hydrophobic interactions of KP418 with hsA only occur with the aquated products of the complex, a process that also dominates in serum. In the presence of apo-hsTf, KP418 interacts directly with the protein through exchange of ligands, as observed with KP1019.

Control of ligand-exchange processes and the oxidation state of the antimetastatic Ru(III) complex NAMI-A by interactions with human serum albumin

The behaviour of the antimetastatic Ru(III) complex imidazolium [trans-RuCl₄(1H-imidazole)(DMSO-S)] (NAMI-A) under physiological conditions and its interactions with human serum albumin (hsA) have been studied using electron paramagnetic resonance spectroscopy (EPR). In physiological buffer at pH 7.4, these experiments demonstrate that the DMSO ligand is replaced rapidly by water, and spectra from the subsequent formation of five other Ru(III) complexes show further aquation processes. Although EPR spectra from mono-nuclear Ru(III) complexes are visible after 24 h in buffer, a significant decrease in the overall signal intensity following the first aquation step is consistent with the formation of oxo-bridged Ru(III) oligomers. Incubation with hsA reveals very rapid binding to the protein via hydrophobic interactions. This is followed by coordination through ligand exchange with protein side chains, likely with histidine imidazoles and at least one other specific site. Similar behaviour is observed when the complex is incubated in human serum, indicating that hsA binding dominates speciation in vivo. The addition of ascorbic acid to NAMI-A in buffer leads to quantitative reduction, producing EPR-silent Ru(II) complexes. However, this process is prevented when the complex binds coordinatively to hsA. Together, these results demonstrate the key role that hsA plays in defining the species found in vivo following intravenous treatment with NAMI-A, through prevention of oligomerization and maintenance of the oxidation state, to give protein-bound mono-nuclear Ru(III) species.

Class III Delocalization and Exciton Coupling in a Bimetallic Bis‐ligand Radical Complex

The geometric and electronic structure of an oxidized bimetallic Ni complex incorporating two redox-active Schiff-base ligands connected via a 1,2-phenylene linker has been investigated and compared to a monomeric analogue. Information from UV/Vis/NIR spectroscopy, electron paramagnetic resonance (EPR) spectroscopy, electrochemistry, and density functional theory (DFT) calculations provides important information on the locus of oxidation for the bimetallic complex. The neutral bimetallic complex is conformationally dynamic at room temperature, which complicates characterization of the oxidized forms. Comparison to an oxidized monomer analogue 1 provides critical insight into the electronic structure of the oxidized bimetallic complex 2. Oxidation of 1 provides [1˙](+), which is characterized as a fully delocalized ligand radical complex; the spectroscopic signature of this derivative includes an intense NIR band at 4500 cm(-1). Oxidation of 2 to the bis-oxidized form affords a bis-ligand radical species [2˙˙](2+). Variable temperature EPR spectroscopy of [2˙˙](2+) shows no evidence of coupling, and the triplet and broken symmetry solutions afforded by theoretical calculations are essentially isoenergetic. [2˙˙](2+) is thus best described as incorporating two non-interacting ligand radicals. Interestingly, the intense NIR intervalence charge transfer band observed for the delocalized ligand-radical [1˙](+) exhibits exciton splitting in [2˙˙](2+), due to coupling of the monomer transition dipoles in the enforced oblique dimer geometry. Evaluating the splitting of the intense intervalence charge transfer band can thus provide significant geometric and electronic information in less rigid bis-ligand radical systems. Addition of excess pyridine to [2˙˙](2+) results in a shift in the oxidation locus from a bis-ligand radical species to the Ni(III) /Ni(III) derivative [2(py)4](2+), demonstrating that the ligand system can incorporate significant bulk in the axial positions.

Non-innocent ligand behaviour of a bimetallic Cu complex employing a bridging catecholate

The geometric and electronic structure of a bimetallic Cu Schiff-base complex and its one-electron oxidized form have been investigated. The two salen units in the neutral complex 1 are linked via a bridging catecholate function, and the coupling between the two Cu(II) d(9) centres was determined to be weakly antiferromagnetic on the basis of solid-state magnetic studies (J = -3 cm(-1)), and variable-temperature electron paramagnetic resonance (EPR) (J = -3 cm(-1)). Theoretical calculations (DFT) were in agreement with the experimental results (J = -7 cm(-1)), and provided insight into the coupling mechanism for the neutral system. One-electron oxidation provided [1](+) which was observed to have limited stability in solution. The oxidized complex was determined to be a ligand radical species in solution, with the electron hole potentially localized on the redox-active dioxolene, the phenolate ligands, or delocalized over the entire ligand system. Electrochemical experiments and UV-vis-NIR spectroscopy, in combination with density functional theory (DFT) calculations, provided insight into the locus of oxidation and the degree of delocalization in this system. The ligand radical for [1˙](+) was determined experimentally to be localized on the dioxolene bridge with a small amount of spin density on the outer phenolate moieties predicted by the calculations. This assignment was aided via comparison to data for the Ni analogue (Inorg. Chem., 2011, 50, 6746). The resonance Raman spectrum of [1˙](+) (λ(ex) = 413 nm) in CH(2)Cl(2) solution clearly exhibited a new band at 1308 cm(-1) in comparison to 1, supporting semiquinone formation. Variable-temperature EPR on the three-spin system [1˙](+) did not provide definitive information on the coupling interaction, possibly due to a very small difference in energy between the S = 3/2 and S = 1/2 states and/or a very small zero-field splitting, in combination with significant line-broadening. The data is consistent with a description of the overall electronic structure of [1˙](+) as a bimetallic Cu(II) complex with a bridge-localized semiquinone ligand radical species.

Merging the chemistry of electron-rich olefins with imidazolium ionic liquids: radicals and hydrogen-atom adducts

To probe the reactivity of ionic liquids relevant to their use in electrochemical applications, the ionic liquid 1-ethyl-3-methylimidazolium tetrachloroaluminate(III) was reacted with metallic lithium to produce a persistent radical, which can be considered a hydrogen-atom adduct of an electron-rich olefin (ERO). Reaction of tetrakis(dimethylamino)ethene, a bona fide ERO, with muonium, produces a structurally similar radical.

Direct Observation of Activated Hydrogen Binding to a Supported Organometallic Compound at Room Temperature

Current interest in the use of hydrogen as a transportation fuel has driven extensive research into novel gas storage materials. Although physisorption materials can possess technologically viable storage capacities, their isosteric heats are generally below 10 kJ mol , limiting these materials to cryogenic temperatures. Chemisorbers, such as metal hydrides or complex hydrides, can store large amounts of hydrogen but require elevated temperatures to release the gas; isosteric heats for hydride systems are typically larger than 40 kJ mol . The optimum conditions for viable room-temperature hydrogen storage require materials that possess isosteric heats of adsorption in between that of standard physisorbers and chemisorbers, typically in the 20– 30 kJ mol 1 regime. Theoretical work has shown that the incorporation of transition-metal atoms onto a porous support can provide such binding energies with multiple hydrogen molecules adsorbed. However, despite the very large number of theoretical papers, there is no direct experimental proof of these predictions yet. An early experimental example is the gas-phase hydrogen reaction with Ti–ethylene complexes, where the gravimetrically measured hydrogen uptake agrees well with theoretical predictions but details of structure, dynamics, and the local chemistry are absent. Herein, we present direct experimental evidence for dihydrogen–Ti binding on a silica-supported Ti organometallic complex (hereafter referred to as Ti-HMS) using detailed sorption and inelastic neutron scattering (INS) measurements. Our experimental findings are further supported by extensive first-principles DFT and reaction path calculations. We show that the Ti ion is essential for the formation of a dihydrogen complex, and its presence is confirmed by EPR spectroscopy (see Figure S1 in the Supporting Information). Surprisingly, we discover that the H2–Ti binding is a thermally activated process; exposing the supported organometallic to hydrogen below 150 K results in only physisorption, while near room temperature it forms H2–Ti moieties that are stable for extended periods of time. Such an activation barrier was missed in earlier DFT calculations, which predicted only the formation of dihydrogen complexes. Though this particular sample does not represent a viable storage material due to its modest uptake and nonoptimized support, it does offer a useful benchmark for understanding the underlying hydrogen coordination chemistries. The sorption performance of the activated Ti-HMS sample measured immediately prior to INS studies (Figure 1) shows an uptake of approximately 12 mg g 1 at 30 bar and 77 K, similar to the bare HMS. This is comparable but somewhat lower than that previously measured and indicates that some of the active titanium adsorption sites have been lost compared to the as-synthesized complex. This is not unexpected as Ti alkyls are highly reactive [a] Dr. J. M. Simmons, Dr. T. Yildirim NIST Center for Neutron Research National Institute of Standards and Technology Gaithersburg, MD 20899 (USA) Fax: (+1)301-921-9847 E-mail : jason.simmons@nist.gov taner@nist.gov [b] Dr. T. Yildirim Department of Materials Science and Engineering University of Pennsylvania Philadelphia, PA 19104 (USA) [c] Dr. A. Hamaed, Prof. D. M. Antonelli Department of Chemistry and Biochemistry University of Windsor, Windsor, ON, N9B 3P4 (Canada) [d] Prof. D. M. Antonelli Sustainable Environment Research Centre University of Glamorgan Pontypridd CF37 1DL (UK) [e] M. I. Webb, Prof. C. J. Walsby Department of Chemistry, Simon Fraser University Burnaby, BC, V5A 1S6 (Canada) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201102658. Figure 1. Excess hydrogen isotherms showing reversible physisorption for Tdose <150 K and irreversible adsorption at 300 K (inset).