Unusually short unsupported Au(III)⋯Au(III) aurophilic contacts in emissive lanthanide tetracyanoaurate(III) complexes

A series of [Au(CN)4]- salts with lanthanide 2,2'-bipyridine dioxide cations features Au(III) aurophilic interactions between [Au(CN)4]- groups, with Au⋯Au distances of 3.3603(4) Å and 3.4354(4) Å that are shorter than any previously reported. Computations predict the interactions to be weakly attractive; packing effects appear to also contribute to the close contacts. The materials are emissive: there is no Au(III)-based luminescence, but for Ln = Eu the PLQY of 29% is surprisingly high compared to related analogues.

Untangling ancillary ligand donation versus locus of oxidation effects on metal nitride reactivity

We detail the relative role of ancillary ligand electron-donating ability in comparison to the locus of oxidation (either metal or ligand) on the electrophilic reactivity of a series of oxidized Mn salen nitride complexes. The electron-donating ability of the ancillary salen ligand was tuned via the para-phenolate substituent (R = CF3, H, tBu, OiPr, NMe2, NEt2) in order to have minimal effect on the geometry at the metal center. Through a suite of experimental (electrochemistry, electron paramagnetic resonance spectroscopy, UV-vis-NIR spectroscopy) and theoretical (density functional theory) techniques, we have demonstrated that metal-based oxidation to [MnVI(SalR)N]+ occurs for R = CF3, H, tBu, OiPr, while ligand radical formation to [MnV(SalR)N]+˙ occurs with the more electron-donating substituents R = NMe2, NEt2. We next investigated the reactivity of the electrophilic nitride with triarylphosphines to form a MnIV phosphoraneiminato adduct and determined that the rate of reaction decreases as the electron-donating ability of the salen para-phenolate substituent is increased. Using a Hammett plot, we find a break in the Hammett relation between R = OiPr and R = NMe2, without a change in mechanism, consistent with the locus of oxidation exhibiting a dominant effect on nitride reactivity, and not the overall donating ability of the ancillary salen ligand. This work differentiates between the subtle and interconnected effects of ancillary ligand electron-donating ability, and locus of oxidation, on electrophilic nitride reactivity.

Targeting misfolding and aggregation of the amyloid-β peptide and mutant p53 protein using multifunctional molecules

Biomolecule misfolding and aggregation play a major role in human disease, spanning from neurodegeneration to cancer. Inhibition of these processes is of considerable interest, and due to the multifactorial nature of these diseases, the development of drugs that act on multiple pathways simultaneously is a promising approach. This Feature Article focuses on the development of multifunctional molecules designed to inhibit the misfolding and aggregation of the amyloid-β (Aβ) peptide in Alzheimer's disease (AD), and the mutant p53 protein in cancer. While for the former, the goal is to accelerate the removal of the Aβ peptide and associated aggregates, for the latter, the goal is reactivation via stabilization of the active folded form of mutant p53 protein and/or aggregation inhibition. Due to the similar aggregation pathway of the Aβ peptide and mutant p53 protein, a common therapeutic approach may be applicable.

Altering relative metal-binding affinities in multifunctional Metallochaperones for mutant p53 reactivation

The p53 protein plays a major role in cancer prevention, and over 50% of cancer diagnoses can be attributed to p53 malfunction. p53 incorporates a structural Zn site that is required for proper protein folding and function, and in many cases point mutations can result in loss of the Zn2+ ion, destabilization of the tertiary structure, and eventual amyloid aggregation. Herein, we report a series of compounds designed to act as small molecule stabilizers of mutant p53, and feature Zn-binding fragments to chaperone Zn2+ to the metal depleted site and restore wild-type (WT) function. Many Zn metallochaperones (ZMCs) have been shown to generate intracellular reactive oxygen species (ROS), likely by chelating redox-active metals such as Fe2+/3+ and Cu+/2+ and undergoing associated Fenton chemistry. High levels of ROS can result in off-target effects and general toxicity, and thus, careful tuning of ligand Zn2+ affinity, in comparison to the affinity for other endogenous metals, is important for selective mutant p53 targeting. In this work we show that by using carboxylate donors in place of pyridine we can change the relative Zn2+/Cu2+ binding ability in a series of ligands, and we investigate the impact of donor group changes on metallochaperone activity and overall cytotoxicity in two mutant p53 cancer cell lines (NUGC3 and SKGT2).

Multifunctional metallochaperone modifications for targeting subsite cavities in mutant p53-Y220C

The p53 protein, known as the 'guardian of the genome', plays an important role in cancer prevention. Unfortunately, p53 mutations result in compromised activity with over 50% of cancers resulting from point mutations to p53. There is considerable interest in mutant p53 reactivation, with the development of small-molecule reactivators showing promise. We have focused our efforts on the common p53 mutation Y220C, which causes protein unfolding, aggregation, and can result in the loss of a structural Zn from the DNA-binding domain. In addition, the Y220C mutant creates a surface pocket that can be stabilized using small molecules. We previously reported the bifunctional ligand L5 as a Zn metallochaperone and reactivator of the p53-Y220C mutant. Herein we report two new ligands L5-P and L5-O that are designed to act as Zn metallochaperones and non-covalent binders in the Y220C mutant pocket. For L5-P the distance between the Zn-binding di-(2-picolyl)amine function and the pocket-binding diiodophenol was extended in comparison to L5, while for L5-O we extended the pocket-binding moiety via attachment of an alkyne function. While both new ligands displayed similar Zn-binding affinity to L5, neither acted as efficient Zn-metallochaperones. However, the new ligands exhibited significant cytotoxicity in the NCI-60 cell line screen as well as in the NUGC3 Y220C mutant cell line. We identified that the primary mode of cytotoxicity is likely reactive oxygen species (ROS) generation for L5-P and L5-O, in comparison to mutant p53 reactivation for L5, demonstrating that subtle changes to the ligand scaffold can change the toxicity pathway.

Chromium Nitride Umpolung Tuned by the Locus of Oxidation

Oxidation of a series of Cr^V nitride salen complexes (Cr^VNSal^R) with different para-phenolate substituents (R = CF₃, tBu, NMe₂) was investigated to determine how the locus of oxidation (either metal or ligand) dictates reactivity at the nitride. Para-phenolate substituents were chosen to provide maximum variation in the electron-donating ability of the tetradentate ligand at a site remote from the metal coordination sphere. We show that one-electron oxidation affords Cr^VI nitrides ([Cr^VINSal^R]⁺; R = CF₃, tBu) and a localized Cr^V nitride phenoxyl radical for the more electron-donating NMe₂ substituent ([Cr^VNSal^NMe2]^•+). The facile nitride homocoupling observed for the Mn^VI analogues was significantly attenuated for the Cr^VI complexes due to a smaller increase in nitride character in the M≡N π* orbitals for Cr relative to Mn. Upon oxidation, both the calculated nitride natural population analysis (NPA) charge and energy of molecular orbitals associated with the {Cr≡N} unit change to a lesser extent for the Cr^V ligand radical derivative ([Cr^VNSal^NMe2]^•+) in comparison to the Cr^VI derivatives ([Cr^VINSal^R]⁺; R = CF₃, tBu). As a result, [Cr^VNSal^NMe2]^•+ reacts with B(C₆F₅)₃, thus exhibiting similar nucleophilic reactivity to the neutral Cr^V nitride derivatives. In contrast, the Cr^VI derivatives ([Cr^VINSal^R]⁺; R = CF₃, tBu) act as electrophiles, displaying facile reactivity with PPh₃ and no reaction with B(C₆F₅)₃. Thus, while oxidation to the ligand radical does not change the reactivity profile, metal-based oxidation to Cr^VI results in umpolung, a switch from nucleophilic to electrophilic reactivity at the terminal nitride.

A role for bioinorganic chemistry in the reactivation of mutant p53 in cancer

Metal ion dysregulation has been implicated in a number of diseases from neurodegeneration to cancer. While defective metal ion transport mechanisms are known to cause specific diseases of genetic origin, the role of metal dysregulation in many diseases has yet to be elucidated due to the complicated function (both good and bad!) of metal ions in the body. A breakdown in metal ion speciation can manifest in several ways from increased reactive oxygen species (ROS) generation to an increase in protein misfolding and aggregation. In this review, we will discuss the role of Zn in the proper function of the p53 protein in cancer. The p53 protein plays a critical role in the prevention of genome mutations via initiation of apoptosis, DNA repair, cell cycle arrest, anti-angiogenesis, and senescence pathways to avoid propagation of damaged cells. p53 is the most frequently mutated protein in cancer and almost all cancers exhibit malfunction along the p53 pathway. Thus, there has been considerable effort dedicated to restoring normal p53 expression and activity to mutant p53. This includes understanding the relative populations of the Zn-bound and Zn-free p53 in wild-type and mutant forms, and the development of metallochaperones to re-populate the Zn binding site to restore mutant p53 activity. Parallels will be made to the development of multifunctional metal binding agents for modulating the aggregation of the amyloid-beta peptide in Alzheimer's Disease (AD).

Exciton Coupling in Redox-Active Salen based Self-Assembled Metallacycles

The incorporation of a redox-active nickel salen complex into supramolecular structures was explored via coordination-driven self-assembly with homobimetallic ruthenium complexes (bridged by oxalato or 5,8-dihydroxy-1,4-naphthoquinato ligands). The self-assembly resulted in the formation of a discrete rectangle using the oxalato complex and either a rectangle or a catenane employing the larger naphthoquinonato complex. The formation of the interlocked self-assembly was determined to be solvent and concentration dependent. The electronic structure and stability of the oxidized metallacycles was probed using electrochemical experiments, UV-Vis-NIR absorption, EPR spectroscopy and DFT calculations, confirming ligand radical formation. Exciton coupling of the intense near-infrared (NIR) ligand radical intervalence charge transfer (IVCT) bands provided further confirmation of the geometric and electronic structures in solution.

Elaboration on the Electronics of Salen Manganese Nitrides: Investigations into Alkoxy-Substituted Ligand Scaffolds

The ligand electronics of salen manganese nitride complexes directly influence the locus of oxidation and, thus, the reactivity of the resulting oxidized species. This work investigates the influence of tert-butoxy, isopropoxy, and methoxy substituents on the electronics of salen manganese nitride species and includes the first documentation of the para Hammett value for the tert-butoxy substituent (σ_para = -0.13 ± 0.03). Each alkoxy-substituted complex undergoes metal-based oxidation to form manganese(VI), and the kinetics of bimolecular homocoupling to form N₂ were assessed by cyclic voltammetry. Bis-oxidation of the manganese complexes was investigated at low temperature using cyclic voltammery and UV-vis-near-IR spectroscopy, and in combination with theoretical calculations, plausible electronic structures of the dications are provided.

Modification of amyloid-beta peptide aggregation via photoactivation of strained Ru(ii) polypyridyl complexes

Alzheimer's disease (AD) is a chronic neurodegenerative disorder characterized by progressive and irreversible damage to the brain. One of the hallmarks of the disease is the presence of both soluble and insoluble aggregates of the amyloid beta (Aβ) peptide in the brain, and these aggregates are considered central to disease progression. Thus, the development of small molecules capable of modulating Aβ peptide aggregation may provide critical insight into the pathophysiology of AD. In this work we investigate how photoactivation of three distorted Ru(ii) polypyridyl complexes (Ru1-3) alters the aggregation profile of the Aβ peptide. Photoactivation of Ru1-3 results in the loss of a 6,6'-dimethyl-2,2'-bipyridyl (6,6'-dmb) ligand, affording cis-exchangeable coordination sites for binding to the Aβ peptide. Both Ru1 and Ru2 contain an extended planar imidazo[4,5-f][1,10]phenanthroline ligand, as compared to a 2,2'-bipyridine ligand for Ru3, and we show that the presence of the phenanthroline ligand promotes covalent binding to Aβ peptide His residues, and in addition, leads to a pronounced effect on peptide aggregation immediately after photoactivation. Interestingly, all three complexes resulted in a similar aggregate size distribution at 24 h, forming insoluble amorphous aggregates as compared to significant fibril formation for peptide alone. Photoactivation of Ru1-3 in the presence of pre-formed Aβ1-42 fibrils results in a change to amorphous aggregate morphology, with Ru1 and Ru2 forming large amorphous aggregates immediately after activation. Our results show that photoactivation of Ru1-3 in the presence of either monomeric or fibrillar Aβ1-42 results in the formation of large amorphous aggregates as a common endpoint, with Ru complexes incorporating the extended phenanthroline ligand accelerating this process and thereby limiting the formation of oligomeric species in the initial stages of the aggregation process that are reported to show considerable toxicity.

Multifunctional compounds for the treatment of Alzheimer’s disease

Alzheimer’s disease (AD) is the most common form of dementia, and the prevalence of this currently untreatable disease is expected to rise in step with increased global life expectancy. AD is a multifaceted disorder commonly characterized by extracellular amyloid–beta (Aβ) aggregates, oxidative stress, metal ion dysregulation, and intracellular neurofibrillary tangles. This review will focus on medicinal inorganic chemistry strategies to target AD, with a focus on the Aβ peptide and its relation to metal ion dysregulation and oxidative stress. Multifunctional compounds designed to target multiple disease processes have emerged as promising therapeutic options, and recent reports detailing multifunctional metal-binding compounds, as well as discrete metal complexes, will be discussed.

Distorted copper(ii) radicals with sterically hindered salens: electronic structure and aerobic oxidation of alcohols

The sterically hindered salen ligands featuring biphenyl and tetramethyl putrescine linkers were synthesized and chelated to copper. The resulting complexes CuL^bp,tBu, CuL^bp,OMe, CuL^pu,tBu and CuL^pu,OMe were structurally characterized, showing a significanty tetrahedrally distorted metal center. The complexes show two reversible oxidation waves in the range 0.2 to 0.8 V vs. Fc⁺/Fc. A further reduction wave is detected in the range -1.4 to -1.7 V vs. Fc⁺/Fc. It is reversible for CuL^bp,tBu and CuL^bp,OMe and assigned to the Cu^II/Cu^I redox couple. One-electron oxidation of CuL^bp,OMe, CuL^pu,tBu and CuL^pu,OMe was performed chemically and electrochemically. It is accompanied by a quenching of the EPR resonances. Phenoxyl radical formation was established by X-Ray diffraction on the cations [CuL^bp,OMe]⁺ and [CuL^pu,OMe]⁺, whereby the coordination sphere is elongated upon oxidation with quinoidal distributions of bond distances. The cations exhibit a NIR band of moderate intensity in their optical spectrum, supporting their classification as class II mixed-valent radical species according to the Robin Day classification. The proposed electronic structures are supported by DFT calculations. The cations [CuL^bp,OMe]⁺, [CuL^pu,tBu]⁺ and [CuL^pu,OMe]⁺ were active towards aerobic oxidation of the unactivated alcohol 2-phenylethanol, with TON numbers up to 58 within 3 h.

A balancing act: using small molecules for therapeutic intervention of the p53 pathway in cancer

Small molecules targeting various aspects of the p53 protein pathway have shown significant promise in the treatment of a number of cancer types.

Bifunctional ligand design for modulating mutant p53 aggregation in cancer

Protein misfolding and aggregation contributes to the development of a wide range of diseases. In cancer, over 50% of diagnoses are attributed to p53 malfunction due to missense mutations, many of which result in protein misfolding and accelerated aggregation. p53 mutations also frequently result in alteration or loss of zinc at the DNA-binding site, which increases aggregation via nucleation with zinc-bound p53. Herein, we designed two novel bifunctional ligands, LI and LH , to modulate mutant p53 aggregation and restore zinc binding using a metallochaperone approach. Interestingly, only the incorporation of iodine function in LI resulted in modulation of mutant p53 aggregation, both in recombinant and cellular environments. Native mass spectrometry shows a protein-ligand interaction for LI , as opposed to LH , which is hypothesized to lead to the distinct difference in the p53 aggregation profile for the two ligands. Incorporation of a di-2-picolylamine binding unit into the ligand design provided efficient intracellular zinc uptake, resulting in metallochaperone capability for both LI and LH . The ability of LI to reduce mutant p53 aggregation results in increased restoration of p53 transcriptional function and mediates both caspase-dependent and -independent cell death pathways. We further demonstrate that LI exhibits minimal toxicity in non-cancerous organoids, and that it is well tolerated in mice. These results demonstrate that iodination of our ligand framework restores p53 function by interacting with and inhibiting mutant p53 aggregation and highlights LI as a suitable candidate for comprehensive in vivo anticancer preclinical evaluations.

Modification of Aβ Peptide Aggregation via Covalent Binding of a Series of Ru(III) Complexes

Alzheimer's disease (AD) is the most common form of dementia, leading to loss of cognition, and eventually death. The disease is characterized by the formation of extracellular aggregates of the amyloid-beta (Aβ) peptide and neurofibrillary tangles of tau protein inside cells, and oxidative stress. In this study, we investigate a series of Ru(III) complexes (Ru-N) derived from NAMI-A in which the imidazole ligand has been substituted for pyridine derivatives, as potential therapeutics for AD. The ability of the Ru-N series to bind to Aβ was evaluated by NMR and ESI-MS, and their influence on the Aβ peptide aggregation process was investigated via electrophoresis gel/western blot, TEM, turbidity, and Bradford assays. The complexes were shown to bind covalently to the Aβ peptide, likely via a His residue. Upon binding, the complexes promote the formation of soluble high molecular weight aggregates, in comparison to peptide precipitation for peptide alone. In addition, TEM analysis supports both amorphous and fibrillar aggregate morphology for Ru-N treatments, while only large amorphous aggregates are observed for peptide alone. Overall, our results show that the Ru-N complexes modulate Aβ peptide aggregation, however, the change in the size of the pyridine ligand does not substantially alter the Aβ aggregation process.

Coordination-driven assembly of a supramolecular square and oxidation to a tetra-ligand radical species

The design and synthesis of a supramolecular square was achieved by coordination-driven assembly of redox-active nickel(ii) salen linkers and (ethylenediamine)palladium(ii) nodes. The tetrameric geometry of the supramolecular structure was confirmed via MS, NMR, and electrochemical experiments. While oxidation of the monomeric metalloligand Schiff-base affords a Ni(iii) species, oxidation of the coordination-driven assembly results in ligand radical formation.

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

The electronic structure of Co and V complexes of a tridentate benzoxazole-containing aminophenol ligand NNOH2 were characterized by both experimental and theoretical methods.

A catalytic antioxidant for limiting amyloid-beta peptide aggregation and reactive oxygen species generation

Alzheimer's disease (AD) is a multifaceted disease that is characterized by increased oxidative stress, metal-ion dysregulation, and the formation of intracellular neurofibrillary tangles and extracellular amyloid-β (Aβ) aggregates. In this work we report the large affinity binding of the iron(iii) 2,17-bis-sulfonato-5,10,15-tris(pentafluorophenyl)corrole complex FeL1 to the Aβ peptide (K _d ∼ 10^-7) and the ability of the bound FeL1 to act as a catalytic antioxidant in both the presence and absence of Cu(ii) ions. Specific findings are that: (a) an Aβ histidine residue binds axially to FeL1; (b) that the resulting adduct is an efficient catalase; (c) this interaction restricts the formation of high molecular weight peptide aggregates. UV-Vis and electron paramagnetic resonance (EPR) studies show that although the binding of FeL1 does not influence the Aβ-Cu(ii) interaction (K _d ∼ 10^-10), bound FeL1 still acts as an antioxidant thereby significantly limiting reactive oxygen species (ROS) generation from Aβ-Cu. Overall, FeL1 is shown to bind to the Aβ peptide, and modulate peptide aggregation. In addition, FeL1 forms a ternary species with Aβ-Cu(ii) and impedes ROS generation, thus showing the promise of discrete metal complexes to limit the toxicity pathways of the Aβ peptide.

Multifunctional Compounds for Activation of the p53-Y220C Mutant in Cancer

The p53 protein plays a major role in cancer prevention, and over 50 % of cancer diagnoses can be attributed to p53 malfunction. The common p53 mutation Y220C causes local protein unfolding, aggregation, and can result in a loss of Zn in the DNA-binding domain. Structural analysis has shown that this mutant creates a surface site that can be stabilized using small molecules, and herein a multifunctional approach to restore function to p53-Y220C is reported. A series of compounds has been designed that contain iodinated phenols aimed for interaction and stabilization of the p53-Y220C surface cavity, and Zn-binding fragments for metallochaperone activity. Their Zn-binding affinity was characterized using spectroscopic methods and demonstrate the ability of compounds L4 and L5 to increase intracellular levels of Zn²⁺ in a p53-Y220C-mutant cell line. The in vitro cytotoxicity of our compounds was initially screened by the National Cancer Institute (NCI-60), followed by testing in three stomach cancer cell lines with varying p53 status', including AGS (WTp53), MKN1 (V143A), and NUGC3 (Y220C). Our most promising ligand, L5, is nearly 3-fold more cytotoxic than cisplatin in a large number of cell lines. The impressive cytotoxicity of L5 is further maintained in a NUGC3 3D spheroid model. L5 also induces Y220C-specific apoptosis in a cleaved caspase-3 assay, reduces levels of unfolded mutant p53, and recovers p53 transcriptional function in the NUGC3 cell line. These results show that these multifunctional scaffolds have the potential to restore wild-type function in mutant p53-Y220C.

Electronic Structure and Reactivity Studies of a Nonsymmetric One-Electron Oxidized CuII Bis-phenoxide Complex

The tetradentate mixed imino/amino phenoxide ligand (N-(3,5-di-tert-butylsalicylidene)-N'-(2-hydroxyl-3,5-di-tert-butylbenzyl))-trans-1,2-cyclohexanediamine (salalen) was complexed with CuII, and the resulting Cu complex (2) was characterized by a number of experimental techniques and theoretical calculations. Two quasi-reversible redox processes for 2, as observed by cyclic voltammetry, demonstrated the potential stability of oxidized forms, and also the increased electron-donating ability of the salalen ligand in comparison to the salen analogue. The electronic structure of the one-electron oxidized [2]+ was then studied in detail, and Cu K-edge X-ray Absorption Spectroscopy (XAS) measurements confirmed a CuII-phenoxyl radical complex in solution. Subsequent resonance Raman (rR) and variable temperature 1H NMR studies, coupled with theoretical calculations, showed that [2• ]+ is a triplet (S = 1) CuII-phenoxyl radical species, with localization of the radical on the more electron-rich aminophenoxide. Attempted isolation of X-ray quality crystals of [2• ]+ afforded [2H]+, with a protonated phenol bonded to CuII, and an additional H-bonding interaction with the SbF6 - counterion. Stoichiometric reaction of dilute solutions of [2• ]+ with benzyl alcohol showed that the complex reacted in a similar manner as the oxidized CuII-salen analogue, and does not exhibit a substrate-binding pre-equilibrium as observed for the oxidized bisaminophenoxide CuII-salan derivative.

Evaluation of 99mTc-sulfonamide and sulfocoumarin derivatives for imaging carbonic anhydrase IX expression

With the aim to prepare hypoxia tumor imaging agents, technetium(I) and rhenium(I) tricarbonyl complexes with dipyridylamine (L1 = N-{[1-(2,2-dioxido-1,2-benzoxathiin-6-yl)-1H-1,2,3-triazol-4-yl]methyl}-N-(2-pyridinylmethyl)-2-pyridinemethanamine; L3 = N-{[1-[N-(4-aminosulfonylphenyl)]-1H-1,2,3-triazol-4-yl]methyl}-N-(2-pyridinyl-methyl)-2-pyridinemethanamine), and iminodiacetate (H₂L2 = N-{[1-(2,2-dioxido-1,2-benzoxathiin-6-yl)-1H-1,2,3-triazole-4-yl]methyl}-N-(carboxy-methyl)-glycine; H₂L4 = N-{[1-[N-(4-aminosulfonylphenyl)]-1H-1,2,3-triazole-4-yl]methyl}-N-(carboxymethyl)-glycine) ligands appended to sulfonamide or sulfocoumarin carbonic anhydrase inhibitors were synthesized. The Re(I) complexes were characterized using ¹H/¹³C NMR, MS, EA, and in one case the X-ray structure of [Et₃NH][Re(CO)₃(L2)] was obtained. As expected, the Re coordination geometry is distorted octahedral, with a tridentate iminodiacetate ligand in a fac arrangement dictated by the three strong-field CO ligands. Inhibition studies of human carbonic anhydrases (hCAs) showed that the Re sulfocoumarin derivatives were inactive against hCA-I, -II and -IV, but had moderate affinity for hCA-IX. The Re sulfonamides showed improved affinity against all tested hCAs, with [Re(CO)₃(L4)]^- being the most active and selective for the hCA-IX isoform. The corresponding ^99mTc complexes were synthesized from fac-[^99mTc(CO)₃(H₂O)₃]⁺, purified by HPLC, and obtained with average 41-76% decay-corrected radiochemical yields and with >99% radiochemical purity. Uptake in HT-29 tumors at 1 h post-injection was highest for [^99mTc(CO)₃(L4)]^- (0.14 ± 0.10%ID/g) in comparison to [^99mTc(CO)₃(L1)]⁺ (0.06 ± 0.01%ID/g), [^99mTc(CO)₃(L2)]^- (0.03 ± 0.00%ID/g), and [^99mTc(CO)₃(L3)]⁺ (0.07 ± 0.03%ID/g). The uptake in tumors was further reduced at 4 h post-injection. For potential imaging application with single photon emission computed tomography, further optimization is needed to improve the affinity to hCA-IX and uptake in hCA-IX expressing tumors.

Octahedral Co(III) salen complexes: the role of peripheral ligand electronics on axial ligand release upon reduction

A series of octahedral CoIII salen complexes (where salen represents a N2O2 bis-Schiff-base bis-phenolate framework) were prepared with axial imidazole ligating groups. When using 1-methylimidazole (1-MeIm) axial ligands, the CoIII/CoII reduction potential could be altered by 220 mV via variation of the electron-donating ability of the para-ring substituents (R = H (1), OMe (2), tBu (3), Br (4), NO2 (5), and CF3 (6)). In addition, the irreversibility of the reduction process suggested substantial geometrical changes and axial ligand exchange upon reduction to the more labile CoII oxidation state. Installing an imidazole-coumarin conjugate as the axial ligands resulted in fluorescence quenching when bound to the CoIII centre (R = H (7), OMe (8), and CF3 (9)). The redox properties and fluorescence increase upon ligand release for 7–9 were studied under reducing conditions and in the presence of excess competing ligand (1-MeIm). It was determined that the Lewis acidity of the CoIII centre was the dominant factor in controlling axial ligand exchange for this series of complexes.

The structure of a one-electron oxidized Mn(iii)-bis(phenolate)dipyrrin radical complex and oxidation catalysis control via ligand-centered redox activity

The tetradentate ligand dppH3, which features a half-porphyrin and two electron-rich phenol moieties, was prepared and chelated to manganese. The mononuclear Mn(iii)-dipyrrophenolate complex 1 was structurally characterized. The metal ion lies in a square pyramidal environment, the apical position being occupied by a methanol molecule. Complex 1 displays two reversible oxidation waves at 0.00 V and 0.47 V vs. Fc⁺/Fc, which are assigned to ligand-centered processes. The one-electron oxidized species 1+ SbF6- was crystallized, showing an octahedral Mn(iii) center with two water molecules coordinated at both apical positions. The bond distance analysis and DFT calculations disclose that the radical is delocalized over the whole aromatic framework. Complex 1+ SbF6- exhibits an S_tot = 3/2 spin state due to the antiferromagnetic coupling between Mn(iii) and the ligand radical. The zero field splitting parameters are D = 1.6 cm^-1, E/D = 0.18(1), g_⊥ = 1.99 and g_∥ = 1.98. The dication 12+ is an integer spin system, which is assigned to a doubly oxidized ligand coordinated to a Mn(iii) metal center. Both 1 and 1+ SbF6- catalyze styrene oxidation in the presence of PhIO, but the nature of the main reaction product is different. Styrene oxide is the main reaction product when using 1, but phenylacetaldehyde is formed predominantly when using 1+ SbF6-. We examined the ability of complex 1+ SbF6- to catalyze the isomerization of styrene oxide and found that it is an efficient catalyst for the anti-Markovnikov opening of styrene oxide. The formation of phenylacetaldehyde from styrene therefore proceeds in a tandem E-I (epoxidation-isomerization) mechanism in the case of 1+ SbF6-. This is the first evidence of control of the reactivity for styrene oxidation by changing the oxidation state of a catalyst based on a redox-active ligand.

A small bifunctional chelator that modulates Aβ42 aggregation

Multifunctional compounds that can modulate amyloid-β (Aβ) aggregation and interact with metal ions hold considerable promise as therapeutic agents for Alzheimer’s disease (AD). Using the copper-catalyzed azide-alkyne cycloaddition reaction, a novel bifunctional chelator 2-(1-(4-(dimethylamino)benzyl)-1H-1,2,3-triazol-4-yl)phenol (L1) was synthesized. L1 contains a bidentate metal-binding unit and a pendant dimethylamino moiety. The product was characterized by 1H NMR, 13C NMR, and MS. The metal-binding properties of L1 were probed by UV–vis spectroscopy to determine Cu:L stoichiometry. L1 was determined to limit Aβ aggregation at 48 h via a ThT assay. In addition, L1 complies with Lipinski’s rules and calculated logBB values for potential drug likeness and BBB permeability. These results suggest that L1 is a suitable candidate for further study as a multifunctional compound to treat AD.

Exploiting exciton coupling of ligand radical intervalence charge transfer transitions to tune NIR absorption

We detail the rational design of a series of bimetallic bis-ligand radical Ni salen complexes in which the relative orientation of the ligand radical chromophores provides a mechanism to tune the energy of intense intervalence charge transfer (IVCT) bands in the near infrared (NIR) region.

We detail the rational design of a series of bimetallic bis-ligand radical Ni salen complexes in which the relative orientation of the ligand radical chromophores provides a mechanism to tune the energy of intense intervalence charge transfer (IVCT) bands in the near infrared (NIR) region. Through a suite of experimental (electrochemistry, electron paramagnetic resonance spectroscopy, UV-vis-NIR spectroscopy) and theoretical (density functional theory) techniques, we demonstrate that bimetallic Ni salen complexes form bis-ligand radicals upon two-electron oxidation, whose NIR absorption energies depend on the geometry imposed in the bis-ligand radical complex. Relative to the oxidized monomer [1˙]⁺ (E = 4500 cm^–1, ε = 27 700 M^–1 cm^–1), oxidation of the cofacially constrained analogue 2 to [2˙˙]²⁺ results in a blue-shifted NIR band (E = 4830 cm^–1, ε = 42 900 M^–1 cm^–1), while oxidation of 5 to [5˙˙]²⁺, with parallel arrangement of chromophores, results in a red-shifted NIR band (E = 4150 cm^–1, ε = 46 600 M^–1 cm^–1); the NIR bands exhibit double the intensity in comparison to the monomer. Oxidation of the intermediate orientations results in band splitting for [3˙˙]²⁺ (E = 4890 and 4200 cm^–1; ε = 26 500 and 21 100 M^–1 cm^–1), and a red-shift for [4˙˙]²⁺ using ortho- and meta-phenylene linkers, respectively. This study demonstrates for the first time, the applicability of exciton coupling to ligand radical systems absorbing in the NIR region and shows that by simple geometry changes, it is possible to tune the energy of intense low energy absorption by nearly 400 nm.

Multi-target-directed phenol-triazole ligands as therapeutic agents for Alzheimer's disease

Alzheimer's disease (AD) is a multifactorial disease that is characterized by the formation of intracellular neurofibrillary tangles and extracellular amyloid-β (Aβ) plaque deposits. Increased oxidative stress, metal ion dysregulation, and the formation of toxic Aβ peptide oligomers are all considered to contribute to the etiology of AD. In this work we have developed a series of ligands that are multi-target-directed in order to address several disease properties. 2-(1-(3-Hydroxypropyl)-1H-1,2,3-triazol-4-yl)phenol (POH), 2-(1-(2-morpholinoethyl)-1H-1,2,3-triazol-4-yl)phenol (PMorph), and 2-(1-(2-thiomorpholinoethyl)-1H-1,2,3-triazol-4-yl)phenol (PTMorph) have been synthesized and screened for their antioxidant capacity, Cu-binding affinity, interaction with the Aβ peptide and modulation of Aβ peptide aggregation, and the ability to limit Aβ1-42-induced neurotoxicity in human neuronal culture. The synthetic protocol and structural variance incorporated via click chemistry, highlights the influence of R-group modification on ligand-Aβ interactions and neuroprotective effects. Overall, this study demonstrates that the phenol-triazole ligand scaffold can target multiple factors associated with AD, thus warranting further therapeutic development.

Mn(iv) and Mn(v)-radical species supported by the redox non-innocent bis(2-amino-3,5-di-tert-butylphenyl)amine pincer ligand

The electron-rich pincer ligand 1 has been synthesized and chelated to manganese. The octahedral Mn(iv) bis(diiminosemiquinonate) and Mn(v) (diiminobenzoquinone) (diiminosemiquinonate) radicals were structurally characterized.

Synthesis and evaluation of benzothiazole-triazole and benzothiadiazole-triazole scaffolds as potential molecular probes for amyloid-β aggregation

Fluorescent scaffolds that can be easily modifiedviaclick chemistry were investigated as probes for Aβ-plaque deposits in mouse tissue.

Synthesis and electronic structure determination of uranium(vi) ligand radical complexes

Pentagonal bipyramidal uranyl (UO₂²⁺) complexes of salen ligands were prepared and the electronic structure of the one-electron oxidized species[1a–c]+were investigated in solution.

Partial conversion of thioamide into nitrile in a copper(II) complex of 2,6-diacetylpyridine bis(thiosemicarbazone), a drug prototype for Alzheimer's disease

This work reports the crystal structure of [(Z)-2-((E)-1-{6-[1-({[amino(sulfanidyl-κS)methylidene]amino}imino-κN)ethyl]pyridin-2-yl-κN}ethylidene)-1-cyanohydrazinido-κN(1)]copper(II), [Cu(C11H11N7S)], the first description of a copper(II) complex of 2,6-diacetylpyridine bis(thiosemicarbazone) showing partial conversion of a thioamide group to a nitrile group. The asymmetric ligand coordinates to the metal centre in an N,N',N'',S-tetradentate manner via the pyridine N atom, an imine N atom, the hydrazinide N atom and the sulfanidyl S atom, displaying a square-planar geometry. Ligand coordination results in two five-membered chelate rings and one six-membered chelate ring, and in crystal packing based on N-H···N hydrogen bonds of the cyanohydrazinide and hydrazinecarbothioamidate arms of the ligand. The correlation between the partial conversion upon metal complexation, H2S release and possible effects on the activity of bis(thiosemicarbazone)s as drug prototypes for Alzheimer's disease is also discussed.

Modulation of the Aβ peptide aggregation pathway by KP1019 limits Aβ-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-β (Aβ) plaque deposits and neurofibrillary tangles. A focus of AD research has concentrated on either inhibiting Aβ peptide aggregation that leads to plaque formation or breaking down pre-formed Aβ peptide aggregates. An alternative approach is to modulate the Aβ aggregation profile by facilitating the formation of Aβ 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 Aβ 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-Aβ interactions. Finally, incubation in an in vitro human neuronal cell model has demonstrated that the formation of KP1019-Aβ species rescues cell viability from Aβ-associated neurotoxicity. Modulation of the Aβ aggregation pathway via covalent interactions with small molecules is thus a promising AD therapeutic strategy.

Tuning ligand electronics and peripheral substitution on cobalt salen complexes: structure and polymerisation activity

A series of cobalt salen complexes, where salen represents an N2O2 bis-Schiff-base bis-phenolate framework, are prepared, characterised and investigated for reversible-termination organometallic mediated radical polymerisation (RT-OMRP). The salen ligands contain a cyclohexane diimine bridge and systematically altered para-substituted phenoxide moieties as a method to examine the electronic impact of the ligand on complex structure and reactivity. The complexes are characterised by single crystal X-ray diffraction, cyclic voltammetry, X-ray photoelectron spectroscopy, electron paramagnetic resonance spectroscopy and computational methods. Structural studies all support a tailorable metal centre reactivity altered by the electron-donating ability of the salen ligand. RT-OMRP of styrene, methyl methacrylate and vinyl acetate is reported and suggests that cobalt-carbon bond strength varies with the ligand substitution. Competing β-hydrogen abstraction affords long-chain olefin-terminated polymer chains and well controlled vinyl acetate polymerisations, contrasting with the lower temperature associative exchange mechanism of degenerative transfer OMRP.

Magnetostructural characterization of copper(II) hydroxide dimers and coordination polymers coordinated to apical isothiocyanate and cyanide-based counteranions

Five Cu(II) hydroxo-bridged complexes of the type [Cu2(μ-OH)2(L)2][A]x·yH2O (where L = 1,10-phenanthroline, A = NCS–, x = 2, y = 2 (1); L = 2,2′-bipyridine, A = NCS–, x = 2, y = 1 (2); L = 2,2′-bipyridine, A = [Au(CN)4]–, x = 2, y = 0 (3) and 2 (4); L = N,N,N′,N′-teteramethylethylenediamine, A = [Pt(SCN)4]2–, x = 1, y = 0 (5)) have been prepared and have been characterized by elemental analysis, IR and Raman spectroscopy, X-ray crystallography, and SQUID magnetometry. Complexes 1 and 2 consist of five-coordinate Cu(II) hydroxo-bridged dinuclear units bound to two NCS– anions in the apical positions, 3 and 4 are pseudo-polymorphs of a Cu(II) hydroxo-bridged dimer core with [Au(CN)4]– counteranions, and 5 is a coordination polymer of Cu(II) hydroxo-bridged dimers with trans-bridging [Pt(SCN)4]2– anions coordinated in the apical position of the five-coordinate Cu(II) centres. The magnetic susceptibility versus temperature for each system was measured, fit to obtain J-coupling values (which range from –0.37 to 17.8 cm−1), and qualitatively compared to known magnetostructural correlation parameters.

Influence of ligand flexibility on the electronic structure of oxidized Ni(III)-phenoxide complexes

One-electron-oxidized Ni(III)-phenoxide complexes with salen-type ligands, [Ni(salen)py2](2+) ([1(en)-py](2+)) and [Ni(1,2-salcn)py2](2+) ([1(cn)-py](2+)), with a five-membered chelate dinitrogen backbone and [Ni(salpn)py2](2+) ([2(pn)-py](2+)), with a six-membered chelate backbone, have been characterized with a combination of experimental and theoretical methods. The five-membered chelate complexes [1(en)-py](2+) and [1(cn)-py](2+) were assigned as Ni(III)-phenoxyl radical species, while the six-membered chelate complex [2(pn)-py](2+) was concluded to be a Ni(II)-bis(phenoxyl radical) species with metal-centered reduction in the course of the one-electron oxidation of the Ni(III)-phenoxide complex [2(pn)-py](+). Thus, the oxidation state of the one-electron-oxidized Ni(III) salen-type complexes depends on the chelate ring size of the dinitrogen backbone.