H2BZmacropa-NCS: A Bifunctional Chelator for Actinium-225 Targeted Alpha Therapy.

Actinium-225 (225Ac) is one of the most promising radionuclides for targeted alpha therapy (TAT). With a half-life of 9.92 days and a decay chain that emits four high-energy α particles, 225Ac is well-suited for TAT when conjugated to macromolecular targeting vectors that exhibit extended in vivo circulation times. The implementation of 225Ac in these targeted constructs, however, requires a suitable chelator that can bind and retain this radionuclide in vivo. Previous work has demonstrated the suitability of a diaza-18-crown-6 macrocyclic chelator H2macropa for this application. Building upon these prior efforts, in this study, two rigid variants of H2macropa, which contain either one (H2BZmacropa) or two (H2BZ2macropa) benzene rings within the macrocyclic core, were synthesized and investigated for their potential use for 225Ac TAT. The coordination chemistry of these ligands with La3+, used as a nonradioactive model for Ac3+, was carried out. Both NMR spectroscopic and X-ray crystallographic studies of the La3+ complexes of these ligands revealed similar structural features to those found for the related complex of H2macropa. Thermodynamic stability constants of the La3+ complexes, however, were found to be 1 and 2 orders of magnitude lower than those of H2macropa for H2BZmacropa and H2BZ2macropa, respectively. The decrease in thermodynamic stability was rationalized via the use of density functional theory calculations. 225Ac radiolabeling and serum stability studies with H2BZmacropa showed that this chelator compares favorably with H2macropa. Based on these promising results, a bifunctional version of this chelator, H2BZmacropa-NCS, was synthesized and conjugated to the antibody codrituzumab (GC33), which targets the liver cancer biomarker glypican-3 (GPC3). The resulting GC33-BZmacropa conjugate and an analogous GC33-macropa conjugate were evaluated for their 225Ac radiolabeling efficiencies, antigen-binding affinities, and in vivo biodistribution in HepG2 liver cancer tumor-bearing mice. Although both conjugates were comparably effective in their radiolabeling efficiencies, [225Ac]Ac-GC33-BZmacropa showed slightly poorer serum stability and biodistribution than [225Ac]Ac-GC33-macropa. Together, these results establish H2BZmacropa-NCS as a new bifunctional chelator for the preparation of 225Ac radiopharmaceuticals.

Evaluation of the Effect of Macrocyclic Ring Size on [203Pb]Pb(II) Complex Stability in Pyridyl-Containing Chelators.

As an element-equivalent theranostic pair, lead-203 (203Pb, 100% EC, half-life = 51.92 h) and lead-212 (212Pb, 100% ß-, half-life = 10.64 h), through the emission of γ rays and an α particle in its decay chain, respectively, can aid in the development of personalized targeted radionuclide treatment for advanced and currently untreatable cancers. With these isotopes currently being used in clinical trials, an understanding of the relationship between the chelator structure, ability to incorporate the radiometal, and metal-complex stability is needed to help design appropriate chelators for clinical use. Herein, we report an investigation into the effect of ring size in macrocyclic chelators where pyridine, an intermediate Lewis base, acts as an electron donor toward lead. Crown-4Py (4,7,13,16-tetrakis(pyridin-2-ylmethyl)-1,10-dioxa-4,7,13,16-tetraazacyclooctadecane), cyclen-4Py (1,4,7,10-tetrakis(pyridin-2-ylmethyl)-1,4,7,10-tetraazacyclododecane), and NOON-2Py (7,16-bis(pyridin-2-ylmethyl)-1,4,10,13-tetraoxa-7,16-diazacyclooctadecane) were synthesized and analyzed for their ability to coordinate Pb2+. Metal complex stability was investigated via [203Pb]Pb2+ radiolabeling studies, 1H NMR spectroscopy, X-ray crystallography, and potentiometry. With the smallest macrocyclic backbone, cyclen-4Py had the highest radiochemical yield, while, in descending order, crown-4Py and NOON-2Py had the lowest. Thermodynamic stability constants (log KML) of 19.95(3), 13.29(5), and 11.67 for [Pb(Cyclen-4Py)]2+, [Pb(Crown-4Py)]2+, and [Pb(NOON-2Py)]2+, respectively, correlated with their radiochemical yields. The X-ray crystal structure of the least stable complexes [Pb(NOON-2Py)]2+ revealed a hemidirected Pb2+ center, as reflected by a void within the coordination sphere, and [Pb(Crown-4Py)]2+ showed an average Pb-N pyridine interatomic distance of >3 Å. By contrast, the crystal structure of [Pb(Cyclen-4Py)]2+ showed shorter Pb-N pyridine interactions, and in solution, only one highly symmetric isomer existed for this complex, whereas conformational flexibility was observed for both [Pb(Crown-4Py)]2+ and [Pb(NOON-2Py)]2+ at the NMR timescale. This study illustrates the importance of the macrocyclic backbone size when incorporating bulky electron-donor groups into the design of a macrocyclic chelator as it affects the accessibility of lead to the donor arms. Our results show that cyclen-4Py is a promising chelator for future studies with this theranostic pair.

A hemilabile manganese(I)-phenol complex and its coordination induced O-H bond weakening.

The known compound K[(PO)2Mn(CO)2] (PO = 2-((diphenylphosphino)methyl)-4,6-dimethylphenolate) (K[1]) was protonated to form the new Mn(i) complex (HPO)(PO)Mn(CO)2 (H1) and was determined to have a pKa approximately equal to tetramethylguanidine (TMG). The reduction potential of K[1] was determined to be -0.58 V vs. Fc/Fc+ in MeCN and allowed for an estimation of an experimental O-H bond dissociation free energy (BDFEO-H) of 73 kcal mol-1 according to the Bordwell equation. This value is in good agreement with a corrected DFT computed BDFEO-H of 68.0 kcal mol-1 (70.3 kcal mol-1 for intramolecular H-bonded isomer). The coordination of the protonated O-atom in the solid-state H1 was confirmed using FTIR spectroscopy and X-ray crystallography. The phenol moiety is hemilabile as evident from computation and experimental results. For instance, dissociation of the protonated O-atom in H1 is endergonic by only a few kcal mol-1 (DFT). Furthermore, [1]- and other Mn(i) compounds coordinated to PO and/or HPO do not react with MeCN, but H1 reacts with MeCN to form H1+MeCN. Experimental evidence for the solution-bound O-atoms of H1 was obtained from 1H NMR and UV-vis spectroscopy and by comparing the electronic spectra of bona fide 16-e- Mn(i) complexes such as [{PNP}Mn(CO)2] (PNP = -N{CH2CH2(PiPr2)}2) and [(Me3SiOP)(PO)Mn(CO)2] (Me3Si1). Compound H1 is only meta-stable (t1/2 0.5-1 day) and decomposes into products consistent with homolytic O-H bond cleavage. For instance, treatment of H1 with TEMPO resulted in formation of TEMPOH, free ligand, and [MnII{(PO)2Mn(CO)2}2]. Together with the experimental and calculated weakened BDFEO-H, these data provide strong evidence for the coordination and hemilability of the protonated O-atom in H1 and represents the first example of the phenolic Mn(i)-O linkage and a rare example of a "soft-homolysis" intermediate in the bond-weakening catalysis paradigm.

Resurgence of Organomanganese(I) Chemistry. Bidentate Manganese(I) Phosphine-Phenol(ate) Complexes.

As part of the United Nations 2019 celebration of the periodic table of elements, we are privileged to present our studies with the element manganese in this Forum Article series. Catalysis with organomanganese(I) complexes has recently emerged as an important area with the discovery that pincer manganese(I) complexes that can activate substrates through metal-ligand cooperative mechanisms are active (de)hydrogenation catalysts. However, this rapidly growing field faces several challenges, and we identify these in this Forum Article. Some of our efforts in addressing these challenges include using alternative precursors to Mn(CO)5Br to prepare manganese(I) dicarbonyl complexes, the latter of which is usually a component of active catalysts. Specifically, the synthesis of a new bidentate phosphine-phenol ligand along with its corresponding coordination chemistry of five new manganese(I) complexes is described. The complexes having two phenol-phenolate moieties interact with the secondary coordination sphere to enable facile loss of the bromido ligand and even one of the CO ligands to afford manganese(I) dicarbonyl centers.

Pentacarbonylmethylmanganese(i) as a synthon for Mn(i) pincer catalysts.

Mn(i) complexes that enable metal-ligand cooperative substrate activation catalyze a range of transformations. Use of MeMn(CO)5 as a synthon in place of typical Mn(CO)5Br was explored and found to be quite versatile, generating catalytically active species in situ by activation of O-H, N-H, and even C-H bonds.

CO-Photolysis-Induced H-Atom Transfer from MnIO-H Bonds.

The formation of TEMPOH from a mixture of [Mn(CO)3(µ3-OH)]4 (1) and (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) is shown to occur through a light-initiated CO photolysis from 1 (illumination at 300-375 nm). One hypothesis is that the loss of carbon monoxide (CO) causes significant O-H bond weakening to render proton-coupled electron transfer (PCET) to TEMPO favorable. For instance, the ground-state O-H bond dissociation free energy (BDFEO-H) of 1 (computed with density functional theory and estimated using effective BDFE reagents) is too high to transfer an H-atom to TEMPO. We also demonstrate that TEMPO and 1 interact in the dark through a hydrogen-bonded "precomplex" (1···TEMPO). We suggest that the PCET reaction that forms TEMPOH is the result of a H-atom-transfer reaction that occurs immediately after photolysis of a CO ligand(s).

Hole Extraction by Design in Photocatalytic Architectures Interfacing CdSe Quantum Dots with Topochemically Stabilized Tin Vanadium Oxide.

Tackling the complex challenge of harvesting solar energy to generate energy-dense fuels such as hydrogen requires the design of photocatalytic nanoarchitectures interfacing components that synergistically mediate a closely interlinked sequence of light-harvesting, charge separation, charge/mass transport, and catalytic processes. The design of such architectures requires careful consideration of both thermodynamic offsets and interfacial charge-transfer kinetics to ensure long-lived charge carriers that can be delivered at low overpotentials to the appropriate catalytic sites while mitigating parasitic reactions such as photocorrosion. Here we detail the theory-guided design and synthesis of nanowire/quantum dot heterostructures with interfacial electronic structure specifically tailored to promote light-induced charge separation and photocatalytic proton reduction. Topochemical synthesis yields a metastable ß-Sn0.23V2O5 compound exhibiting Sn 5s-derived midgap states ideally positioned to extract photogenerated holes from interfaced CdSe quantum dots. The existence of these midgap states near the upper edge of the valence band (VB) has been confirmed, and ß-Sn0.23V2O5/CdSe heterostructures have been shown to exhibit a 0 eV midgap state-VB offset, which underpins ultrafast subpicosecond hole transfer. The ß-Sn0.23V2O5/CdSe heterostructures are further shown to be viable photocatalytic architectures capable of efficacious hydrogen evolution. The results of this study underscore the criticality of precisely tailoring the electronic structure of semiconductor components to effect rapid charge separation necessary for photocatalysis.

Bisphosphine phenol and phenolate complexes of Mn(i): manganese(i) catalyzed Tishchenko reaction.

We synthesized new organomanganese complexes using the phenolic "pincer" type ligand H-POP. The coordination chemistry of H-POP with Mn(i) was explored, revealing a wide range of binding motifs. Finally, we found that complex 1 catalyzes the formation of benzyl benzoate from benzaldehyde in a Tishchenko reaction.

Photochemical Water-Splitting with Organomanganese Complexes.

Certain organometallic chromophores with water-derived ligands, such as the known [Mn(CO)3(µ3-OH)]4 (1) tetramer, drew our attention as possible platforms to study water-splitting reactions. Herein, we investigate the UV irradiation of various tricarbonyl organomanganese complexes, including 1, and demonstrate that dihydrogen, CO, and hydrogen peroxide form as products in a photochemical water-splitting decomposition reaction. The organic and manganese-containing side products are also characterized. Labeling studies with 18O-1 suggest that the source of oxygen atoms in H2O2 originates from free water that interacts with 1 after photochemical dissociation of CO (1-CO) constituting the oxidative half-reaction of water splitting mediated by 1. Hydrogen production from 1 is the result of several different processes, one of which involves the protons derived from the hydroxido ligands in 1 constituting the reductive half-reaction of water splitting mediated by 1. Other processes that generate H2 are also operative and are described. Collectively the results from the photochemical decomposition of 1 provide an opportunity to propose a mechanism, and it is discussed within the context of developing new strategies for water-splitting reactions with organomanganese complexes.

Exploring the Role of Carbonate in the Formation of an Organomanganese Tetramer.

The formation of metal-oxygen clusters is an important chemical transformation in biology and catalysis. For example, the biosynthesis of the oxygen-evolving complex in the enzyme photosystem II is a complicated stepwise process that assembles a catalytically active cluster. Herein we describe the role that carbonato ligands have in the formation of the known tetrameric complex [Mn(CO)3(µ3-OH)]4 (1). Complex 1 is synthesized in one step via the treatment of Mn2(CO)10 with excess Me3NO·2H2O. Alternatively, when anhydrous Me3NO is used, an OH-free synthetic intermediate (2) with carbonato ligands is produced. Complex 2 produces carbon dioxide, Me3NO·2H2O, and 1 when treated with water. Labeling studies reveal that the µ3-OH ligands in 1 are derived from the water and possibly the carbonato ligands in 2.