A Dual-Pronged Approach: A Ruthenium(III) Complex That Modulates Amyloid-ß Aggregation and Disrupts Its Formed Aggregates.

Alzheimer's disease (AD) is a devastating neurological disorder for which soluble oligomers of the peptide amyloid-ß (Aß) are now recognized as the neurotoxic species. Metal-based therapeutics are uniquely suited to target Aß, with ruthenium-based (Ru) complexes emerging as propitious candidates. Recently, azole-based Ru(III) complexes were observed to modulate the aggregation of Aß in solution, where the inclusion of a primary amine proximal to the ligand coordination site improved the activity of the complexes. To advance these structure-activity relationships, a series of oxazole-based Ru complexes were prepared and evaluated for their ability to modulate Aß aggregation. From these studies, a lead candidate, Oc, emerged that had superior activity relative to its azole predecessors in modulating the aggregation of soluble Aß and diminishing its cytotoxicity. Further evaluation of Oc demonstrated its ability to disrupt formed Aß aggregates, resulting in smaller amorphous species. Because altering both sides of the aggregation equilibrium for Aß has not been previously suggested for metal-based complexes for AD, this work represents an exciting new avenue for improved therapeutic success.

Finding the best location: Improving the anti-amyloid ability of ruthenium(III) complexes with pyridine ligands.

Alzheimer's disease (AD) is a devastating neurological disorder where one of the primary pathological hallmarks are aggregate deposits of the peptide amyloid-beta (Aß). Although the Food and Drug Administration (FDA) has recently approved therapeutics that specifically target Aß, resulting in the removal of these deposits, the associated costs of such treatments create a need for effective, yet cheaper, alternatives. Metal-based compounds are propitious therapeutic candidates as they exploit the metal-binding properties of Aß, forming stable interactions with the peptide, thereby limiting its aggregation and toxicity. Previously, ruthenium-based complexes have shown a strong ability to modulate the aggregation and cytotoxicity of Aß, where the incorporation of a primary amine on the coordinated heterocyclic ligand gave the greatest activity. To determine the importance of the location of the primary amine on the pyridine ligand, thereby establishing structure-activity relationships (SAR), four complexes (RuP1-4) were prepared and evaluated for their ability to coordinate and subsequently modulate the aggregation and cytotoxicity of Aß. Coordination to Aß was determined using three complementary spectroscopic methods: UV-Vis, 1H NMR, and circular dichroism (CD). Similarly, the impact of the complexes on Aß aggregation was evaluated using three sequential methods of turbidity, dynamic light scattering (DLS), and transmission electron microscopy (TEM). Overall, the location of the primary amine on the pyridine ligand did affect the resultant anti-Aß performance, with the 2-aminopyridine complex (RuP2) being the most active. This SAR will provide another guiding principle in the design of future metal-based anti-Aß complexes.

Ru(II)-arene azole complexes as anti-amyloid-ß agents.

With the recent clinical success of anti-amyloid-ß (Aß) monoclonal antibodies, there is a renewed interest in agents which target the Aß peptide of Alzheimer's disease (AD). Metal complexes are particularly well-suited for this development, given their structural versatility and ability to form stabile interactions with soluble Aß. In this report, a small series of ruthenium-arene complexes were evaluated for their respective ability to modulate both the aggregation and cytotoxicity of Aß. First, the stability of the complexes was evaluated in a variety of aqueous media where the complexes demonstrated exceptional stability. Next, the ability to coordinate and modulate the Aß peptide was evaluated using several spectroscopic methods, including thioflavin T (ThT) fluorescence, dynamic light scattering (DLS), and transmission electron microscopy (TEM). Overall, the complex RuBO consistently gave the greatest inhibitory action towards Aß aggregation, which correlated with its ability to coordinate to Aß in solution. Furthermore, RuBO also had the lowest affinity for serum albumin, which is a key consideration for a neurotherapeutic, as this protein does not cross the blood brain barrier. Lastly, RuBO also displayed promising neuroprotective properties, as it had the greatest inhibition of Aß-inducted cytotoxicity.

Increasing the bioavailability of Ru(III) anticancer complexes through hydrophobic albumin interactions.

A series of pyridine-based derivatives of the clinically successful Ru(III)-based complexes indazolium [trans-RuCl4(1H-indazole)2] (KP1019) and sodium [trans-RuCl4(1H-indazole)2] (KP1339) have been synthesized to probe the effect of hydrophobic interactions with human serum albumin (hsA) on anticancer activity. The solution behavior and protein interactions of the new compounds were characterized by using electron paramagnetic resonance (EPR) and UV/Vis spectroscopy. These studies have revealed that incorporation of hydrophobic substituents at the 4'-position of the axial pyridine ligand stabilizes non-coordinate interactions with hsA. As a consequence, direct coordination to the protein is inhibited, which is expected to increase the bioavailability of the complexes, thus potentially leading to improved anticancer activity. By using this approach, the lifetimes of hydrophobic protein interactions were extended from 2 h for the unsubstituted pyridine complex, to more than 24 h for several derivatives. Free complexes were tested for their anticancer activity against the SW480 human colon carcinoma cell line, exhibiting low cytotoxicity. Pre-treatment with hsA improved the solubility of every compound and led to some changes in activity. Particularly notable was the difference in activity between the methyl- and dibenzyl-functionalized complexes. The former shows reduced activity after incubation with hsA, indicating reduced bioavailability due to protein coordination. The latter exhibits little activity on its own but, following treatment with hsA, exhibited significant cytotoxicity, which is consistent with its ability to form non-coordinate interactions with the protein. Overall, our studies demonstrate that non-coordinate interactions with hsA are a viable target for enhancing the activity of Ru(III)-based complexes in vivo.

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.

Pyridine analogues of the antimetastatic Ru(III) complex NAMI-A targeting non-covalent interactions with albumin.

A series of pyridine-based derivatives of the antimetastatic Ru(III) complex imidazolium [trans-RuCl(4)(1H-imidazole)(DMSO-S)] (NAMI-A) have been synthesized along with their sodium-ion compensated analogues. These compounds have been characterized by X-ray crystallography, electron paramagnetic resonance (EPR), NMR, and electrochemistry, with the goal of probing their noncovalent interactions with human serum albumin (hsA). EPR studies show that the choice of imidazolium ligands and compensating ions does not strongly influence the rates of ligand exchange processes in aqueous buffer solutions. By contrast, the rate of formation and persistence of interactions of the complexes with hsA is found to be strongly dependent on the properties of the axial ligands. The stability of noncovalent binding is shown to correlate with the anticipated ability of the various pyridine ligands to interact with the hydrophobic binding domains of hsA. These interactions prevent the oligomerization of the complexes in solution and limit the rate of covalent binding to albumin amino acid side chains. Electrochemical studies demonstrate relatively high reduction potentials for these complexes, leading to the formation of Ru(II) species in aqueous solutions containing biological reducing agents, such as ascorbate. However, EPR measurements indicate that while noncovalent interactions with hsA do not prevent reduction, covalent binding produces persistent mononuclear Ru(III) species under these conditions.

A Ru(II)-arene-ferrocene complex with promising antibacterial activity.

The evolution of high virulence bacterial strains has necessitated the development of novel therapeutic agents to treat resistant infections. Metal-based therapeutics represent a promising avenue for advancement, given their structural variability and unique modes of action relative to classical organic molecules. One strategy that has seen marked success is the incorporation of ferrocene into the framework of established antibacterial agents, while ruthenium-based complexes have also shown promise as bioactive compounds. This work focused on the preparation of novel ruthenium(II)-arene complexes containing Schiff base ligands with an attached ferrocene, and evaluation of their antibacterial activity. Structure-activity relationships identified the importance of having a phenyl group between the Schiff base imine and the appended ferrocene. This complex, C2, showed prominent activity against several clinically relevant bacterial strains, including a minimum inhibitory concentration of 16 µg mL-1 for methicillin-resistant Staphylococcus aureus (MSRA). Overall, the results of this study represent a promising new lead for future development of novel antibacterial agents.

Importance of Hydrogen Bonding: Structure-Activity Relationships of Ruthenium(III) Complexes with Pyridine-Based Ligands for Alzheimer's Disease Therapy.

Alzheimer's disease (AD) is the most common form of dementia, where one of the pathological hallmarks of AD is extracellular protein deposits, the primary component of which is the peptide amyloid-ß (Aß). Recently, the soluble form of Aß has been recognized as the primary neurotoxic species, making it an important target for therapeutic development. Metal-based drugs are promising candidates to target Aß, as the interactions with the peptide can be tuned by ligand design. In the current study, 11 ruthenium complexes containing pyridine-based ligands were prepared, where the functional groups at the para position on the coordinated pyridine ligand were varied to determine structure-activity relationships. Overall, the complexes with terminal primary amines had the greatest impact on modulating the aggregation of Aß and diminishing its cytotoxicity. These results identify the importance of specific intermolecular interactions and are critical in the advancement of metal-based drugs for AD therapy.

Ruthenium(III) complexes with imidazole ligands that modulate the aggregation of the amyloid-ß peptide via hydrophobic interactions.

Alzheimer's disease (AD) is the most common form of dementia, characterized by extracellular protein deposits, comprised primarily of the peptide amyloid-beta (Aß), are a pathological indicator of the disease. Commonly known as Aß plaques, these deposits contain a relatively high concentration of metals, making metallotherapeutics uniquely suited to target soluble Aß, thereby limiting its aggregation and cytotoxicity. Ruthenium-based complexes are promising candidates for advancement, as the complex PMRU20 (2-aminothiazolium [trans-RuCl4(2-aminothiazole)2]) and several thiazole-based derivatives were found to prevent the aggregation of Aß, with hydrogen-bonding functional groups improving their performance. Further investigation into the impact of the heteroatom in the azole ring on the activity of Ru complexes was achieved through the synthesis and evaluation of a small set of imidazole-based compounds. The ability of the complexes to prevent the aggregation of Aß was determined where the same sample was subjected to analysis by three complementary methods: ThT fluorescence, dynamic light scattering (DLS), and transmission electron microscopy (TEM). It was found that hydrophobic interactions, along with hydrogen-bonding via the imidazole nitrogen heteroatom, promoted interactions with the Aß peptide, thereby limiting its aggregation. Furthermore, it was found that having rapid and sequential exchange proved detrimental as it resulted in a decreased association with Aß. These results highlight important considerations between a balance of intermolecular interactions and ligand exchange kinetics in the design of further therapeutic candidates.

Design and synthesis of vanadium hydrazide gels for Kubas-type hydrogen adsorption: a new class of hydrogen storage materials.

In this paper we demonstrate that the Kubas interaction, a nondissociative form of weak hydrogen chemisorption with binding enthalpies in the ideal 20-30 kJ/mol range for room-temperature hydrogen storage, can be exploited in the design of a new class of hydrogen storage materials which avoid the shortcomings of hydrides and physisorpion materials. This was accomplished through the synthesis of novel vanadium hydrazide gels that use low-coordinate V centers as the principal Kubas H(2) binding sites with only a negligible contribution from physisorption. Materials were synthesized at vanadium-to-hydrazine ratios of 4:3, 1:1, 1:1.5, and 1:2 and characterized by X-ray powder diffraction, X-ray photoelectron spectroscopy, nitrogen adsorption, elemental analysis, infrared spectroscopy, and electron paramagnetic resonance spectroscopy. The material with the highest capacity possesses an excess reversible storage of 4.04 wt % at 77 K and 85 bar, corresponding to a true volumetric adsorption of 80 kg H(2)/m(3) and an excess volumetric adsorption of 60.01 kg/m(3). These values are in the range of the ultimate U.S. Department of Energy goal for volumetric density (70 kg/m(3)) as well as the best physisorption material studied to date (49 kg H(2)/m(3) for MOF-177). This material also displays a surprisingly high volumetric density of 23.2 kg H(2)/m(3) at room temperature and 85 bar--roughly 3 times higher than that of compressed gas and approaching the DOE 2010 goal of 28 kg H(2)/m(3). These materials possess linear isotherms and enthalpies that rise on coverage and have little or no kinetic barrier to adsorption or desorption. In a practical system these materials would use pressure instead of temperature as a toggle and can thus be used in compressed gas tanks, currently employed in many hydrogen test vehicles, to dramatically increase the amount of hydrogen stored and therefore the range of any vehicle.

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

The compounds imidazolium [trans-[RuCl(4)(1H-imidazole)(2)] (KP418) and indazolium [trans-RuCl(4)(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.

Ruthenium(iii) complexes containing thiazole-based ligands that modulate amyloid-ß aggregation.

Alzheimer's Disease (AD) is a devastating neurodegenerative disorder where one of the commonly observed pathological hallmarks is extracellular deposits of the peptide amyloid-ß (Aß). These deposits contain a high concentration of metals and initially presented a promising target for therapy; however it has become increasingly evident that the soluble form of the peptide is neurotoxic, not the amyloidogenic species. Metal-based therapeutics are uniquely suited to target soluble Aß and have shown considerable promise to prevent the aggregation and induced cytotoxicity of the peptide in vitro. Herein, we have prepared a small series of derivatives of two promising Ru(iii) complexes NAMI-A (imidazolium [trans-RuCl4(1H-imidazole)(dimethyl sulfoxide-S)]) and PMRU20 (2-aminothiazolium [trans-RuCl4(2-aminothiazole)2]), to determine structure-activity relationships (SAR) for Ru(iii) therapeutics for AD. Using the three complementary methods of Thioflavin T fluorescence, dynamic light scattering (DLS), and transmission electron microscopy (TEM), it was determined that the symmetry around the metal center did not significantly impact the activity of the complexes, but rather the attached thiazole ligand(s) mitigated Aß aggregation. Across both families of Ru(iii) complexes the determined SAR for the functional groups on the thiazole ligands to modulate Aß aggregation were NH2 > CH3 > H. These results highlight the importance of secondary interactions between the metallotherapeutic and the Aß peptide where hydrogen-bonding has the greatest impact on modulating Aß aggregation.

Stabilization of different redox levels of a tridentate benzoxazole amidophenoxide ligand when bound to Co(iii) or V(v).

A tridentate benzoxazole-containing aminophenol ligand NNOH2 was coordinated to Co and V metal centers and the electronic structure of the resultant complexes characterized by both experimental and theoretical methods. The solid state structure of the Co complex exhibits a distorted octahedral geometry with two tridentate ligands bound in meridional fashion, and coordination-sphere bond lengths consistent with a Co(iii) oxidation state. EPR and magnetic data support a S = 1/2 ground state, and a formal electronic description of Co(iii)(NNOAP)(NNOISQ) where NNOAP corresponds to an amidophenoxide and NNOISQ to the iminosemiquinone redox level. However, the metrical parameters are similar for both ligands in the solid state, and DFT calculations support delocalization of the ligand radical over both ligands, affording an intermediate ligand redox level Co(iii)(NNO1.5-)(NNO1.5-). The vanadyl complex exhibits a distorted octahedral geometry in the solid state consistent with a V(v) metal center and amidophenoxide (NNOAP), acetylacetonate and oxo ligands. The ligand metrical parameters are consistent with significant amidophenoxide to V(v) π donation. Overall, our results highlight the roles of electron transfer, delocalization, and π bonding in the metal complexes under study, and thus the complexity in assignment of the electronic structure in these systems.

Albumin binding and ligand-exchange processes of the Ru(III) anticancer agent NAMI-A and its bis-DMSO analogue determined by ENDOR spectroscopy.

The ruthenium anticancer compound NAMI-A, imidazolium [trans-RuCl4(1H-imidazole)(DMSO-S)], is currently undergoing advanced clinical evaluation. As with other Ru(iii) chemotherapeutic candidates, interactions with human serum albumin (HSA) have been identified as a key component of the speciation of NAMI-A following intravenous administration. To characterize coordination to HSA, we have performed electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) spectroscopic analysis of deuterium-labelled isotopologues of both NAMI-A and its bis-DMSO analogue, [(DMSO)2H][trans-RuCl4(DMSO-S)2] (Ru-bis-DMSO). Samples were prepared using phosphate buffered saline, in the presence of HSA, and with the individual amino acids histidine, cysteine, and alanine. Analysis of (1)H ENDOR spectra shows characteristic hyperfine interactions from DMSO, water, and imidazole ligands. Furthermore, coordination of imidazole ligands was confirmed from diagnostic (14)N ENDOR signals. Combined with the EPR data from the complexes following incubation in the presence of histidine, the ENDOR data demonstrate that both complexes bind to HSA via histidine imidazoles. Furthermore, the protein-bound species are shown to have water ligands and, in the case of Ru-bis-DMSO, one species has a remaining coordinated DMSO.

Modulation of the Aß peptide aggregation pathway by KP1019 limits Aß-associated neurotoxicity.

Alzheimer's disease (AD) is a neurodegenerative disorder that is increasing worldwide due to increased life expectancy. AD is characterized by two pathological hallmarks in the brain: amyloid-ß (Aß) plaque deposits and neurofibrillary tangles. A focus of AD research has concentrated on either inhibiting Aß peptide aggregation that leads to plaque formation or breaking down pre-formed Aß peptide aggregates. An alternative approach is to modulate the Aß aggregation profile by facilitating the formation of Aß species that are off-pathway and non-toxic. Herein, we report the re-purposing of the widely studied Ru(iii) anti-cancer complex KP1019, towards regulating the aggregation profile of the Aß peptide. Using electron paramagnetic resonance (EPR) spectroscopy, we conclude that KP1019 binds to histidine residues, located at the N-terminus of the peptide, in a rapid and robust fashion. Native gels and transmission electron microscopy (TEM) analyses have provided insight into the species and structures that are generated by KP1019-Aß interactions. Finally, incubation in an in vitro human neuronal cell model has demonstrated that the formation of KP1019-Aß species rescues cell viability from Aß-associated neurotoxicity. Modulation of the Aß aggregation pathway via covalent interactions with small molecules is thus a promising AD therapeutic strategy.

High metal substitution tolerance of anthrax lethal factor and characterization of its active copper-substituted analogue.

Anthrax lethal factor (LF) is a zinc-dependent metalloendopeptidase and a member of the gluzincin family. The current report demonstrates a high metal substitution tolerance of LF atypical of gluzincins and other zinc-dependent metalloproteases. Mn(2+), Co(2+), Ni(2+), Cu(2+) and Cd(2+) were found to reactivate the apoprotein of LF to a level either comparable to or significantly higher than that noted for the native zinc enzyme. The most active form of LF was obtained with Cu(2+), a surprising observation since most Cu(2+)-substituted zinc proteases display very low activity. Cu(2+)-substituted LF (CuLF), prepared by direct exchange and by apoprotein reconstitution methodologies, displayed a several-fold higher catalytic competence towards chromogenic and fluorogenic LF substrates than native LF. CuLF bound Cu(2+) tightly with a dissociation constant in the femtomolar range. The electron paramagnetic resonance spectrum of CuLF revealed the protein-bound metal ion to be coordinated to two nitrogen donor atoms, suggesting that Cu(2+) binds to both active site histidine residues. While ZnLF and CuLF (prepared by direct exchange) were capable of killing RAW 264.7 murine macrophage-like cells, apoLF and all metal-reconstituted apoprotein preparations failed to elicit a cytotoxic response. Competition experiments using apoLF/ZnLF mixtures demonstrated the propensity of apoLF to relieve ZnLF-induced cell death, suggesting that both protein forms can compete with each other for binding to protective antigen. The lack of cytotoxicity of apoLF and its metal-reconstituted variants likely originates from structural perturbations in these proteins which might prevent their translocation into the cytoplasm.

EPR as a probe of the intracellular speciation of ruthenium(III) anticancer compounds.

EPR (electron paramagnetic resonance) has been used to study interactions of the Ru(III) anticancer compounds imidazolium [trans-RuCl4(1H-imidazole)(DMSO-S)] (NAMI-A) and indazolium [trans-RuCl4(1H-indazole)2] (KP1019) with isolated subcellular components and whole cells of the yeast Saccharomyces cerevisiae. These studies are the first to probe the intracellular speciation of ruthenium using the EPR technique. Initially, NAMI-A and KP1019 were incubated at 30 °C, for time periods up to 24 hours with isolated cell wall, mitochondrial, cytoplasmic, and nuclear fractions of S. cerevisiae. EPR measurements demonstrate that NAMI-A initially forms non-coordinate interactions with each cell component. After longer incubation times these are replaced by coordinated species, particularly with cytoplasmic proteins. KP1019 shows a greater tendency to coordinate directly with cell components, demonstrating significant interactions with mitochondria and cytoplasmic proteins. Subsequently, each complex was incubated with whole cells of S. cerevisiae at 30 °C and whole-cell EPR measurements detected Ru(III) species in measurable concentrations even after 24 hours of incubation. Analysis of the resulting EPR spectra suggests NAMI-A interacts predominantly with cell walls, while KP1019 was found to be coordinating with both the mitochondrial and cytoplasmic protein fractions. Comparison of the signal intensity of these data with those from incubation with whole cells at 4 °C indicates different modes of transmembrane transport for each complex. These studies demonstrate that EPR can provide valuable insight into the oxidation state and speciation of ruthenium compounds in cellular environments.

Double oxidation localizes spin in a Ni bis-phenoxyl radical complex.

The electronic structure of a doubly oxidized Ni salen complex NiSal(tBu) (Sal(tBu) = N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexane-(1R,2R)-diamine) has been investigated by both experimental and theoretical methods. The doubly oxidized product was probed by resonance Raman spectroscopy, UV-vis-NIR, and EPR to determine the locus of oxidation as well as the spectroscopic signature of the complex. It was determined that double oxidation of NiSal(tBu) affords a bis-ligand radical species in solution via the presence of phenoxyl radical bands at ν(7a) (1504 cm(-1)) and ν(8a) (1579 cm(-1)) in the Raman spectrum, and the loss of the intense NIR transition reported for the mono-radical complex (Angew. Chem., Int. Ed., 2007, 46, 5198). Spectroscopic experiments, complemented by DFT calculations, show that the two radical spins are predominantly localized on the phenolate moieties, in opposition to the extensive delocalization over the ligand framework observed for the mono-radical analogue.

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.