Mechanistic Insights into Amphoteric Reactivity of an Iron-Bispidine Complex

Metal Complexes of Bispidine Derivatives: Achievements and Prospects for the Future

Multidentate bispidine ligands, including tetra-, penta-, hexa-, hepta-, and octadentate variants, exhibit strong coordination tendencies due to their intrinsic rigidity, significant reorganization potential, and ability to efficiently encapsulate metal ions. These structural attributes profoundly influence the thermodynamic stability, metal ion selectivity, redox behavior, and spin-state configuration of the resulting complexes. Metal ions, in turn, serve as highly suitable candidates for coordination due to their remarkable kinetic inertness, rapid complex formation kinetics, and low redox potential. This review focuses on ligands incorporating the bispidine core (3,7-diazabicyclo[3.3.1]nonane) and provides an overview of advancements in the synthesis of metal complexes involving p-, d-, and f-block elements. Furthermore, the rationale behind the growing interest in bispidine-based complexes for applications in radiopharmaceuticals, medicinal chemistry, and organic synthesis is explored, particularly in the context of their potential for diagnostic and catalytic drug development.

Implications of non-native metal substitution in carbonic anhydrase - engineered enzymes and models

The enzyme carbonic anhydrase has been intensely studied over decades as a means to understand the role of zinc in hydrating CO2. The naturally occurring enzyme has also been immobilized on distinct heterogeneous platforms, which results in a different hybrid class of catalysts that are useful for the adsorption and hydration of CO2. However, the reusability and robustness of such natural and immobilized systems are substantially affected when tested under industrial conditions, such as high temperature and high flow rate. This led to the generation of model systems in the form of metal-coordination complexes, metal-organic frameworks, metallo-peptide self-assembled supramolecules and nanomaterials that mimic the primary, and, to some extent, secondary coordination sphere of the active site of the natural carbonic anhydrase enzymes. Furthermore, the effects of zinc-substitution by other relevant transition metals in both the naturally occurring enzymes and model systems has been reported. It has been observed that some other transition metal ions in the active site of carbonic anhydrase and its models can also accomplish similar activity, established by various reaction probes and ideas. Herein, we present a comprehensive highlight about substituting zinc in the active site of the modified enzymes and its biomimetic model systems with non-native metal ions and review how they affect the structural orientation and reactivity towards CO2 hydration. In addition, the utility of artificially engineered carbonic anhydrases towards a number of non-natural reactions is also discussed.

Unraveling Chlorite Oxidation Pathways in Equatorially Heteroatom-Substituted Nonheme Iron Complexes

The first-coordination sphere of catalysts is known to play a crucial role in reaction mechanisms, but details of how equatorial ligands influence the reactivity remain unknown. Heteroatom ligated to the equatorial position of iron centers in nonheme iron metalloenzymes modulates structure and reactivity. To investigate the impact of equatorial heteroatom substitution on chlorite oxidation, we synthesized and characterized three novel mononuclear nonheme iron(II) complexes with a pentadentate bispidine scaffold. These complexes feature systematic substitutions at the equatorial position in the bispidine ligand framework where the pyridine group is replaced with NMe2, SMe, and OMe groups. The three iron(II)-bispidine complexes were subjected to studies in chlorite oxidation reactions as a model pathway for oxygen atom transfer. Chlorine oxyanions, which have the halide in an oxidation state ranging from +1 to +7, have numerous applications but can contaminate water bodies, and this demands urgent environmental remediation. Chlorite, a common precursor to chlorine dioxide, is of particular interest due to the superior antimicrobial activity of chlorine dioxide. Moreover, its generation leads to fewer harmful byproducts in water treatment. Here, we demonstrate that these complexes can produce chlorine dioxide from chlorite in acetate buffer at room temperature and pH 5.0, oxidizing chlorite through the in situ formation of high-valent iron(IV)-oxo intermediates. This study establishes how subtle changes in the coordination sphere around iron can influence the reactivity.

Synthesis, Characterization, and Reactivity of Bispidine-Iron(IV)-Tosylimido Species

Reported are the synthesis and detailed studies of the iron(IV)-tosylimido complexes of two isomeric pentadentate bispidine ligands (bispidines are 3,7-diazabicyclo[3.3.1]nonane derivatives). This completes a series of five tosylimido complexes with comparable pentadentate amine/pyridine ligands, where the corresponding [(L)FeIV═O]2+ oxidants have been studied in detail. The characterization of the two new complexes in solution (UV-vis-NIR, Mössbauer, HR-ESI-MS) shows that these oxidants have an intermediate spin (S = 1) electronic ground state. The reactivities have been studied as oxidants in C-H activation at 1,3-cyclohexadiene and nitrogen atom transfer to thioanisole. For the latter substrate, the entire set of data for the five ligands and for both nitrogen and oxygen atom transfer is now available and the interesting observation is that oxygen atom transfer is, as expected, generally faster than nitrogen atom transfer, with the exception of the two ligands that have four and three pyridine groups oriented parallel to the Fe-O and Fe-N axes. A thorough DFT analysis indicates that this is due to steric effects in the case of the [(L)FeIV═O]2+ species, which are less important in the [(L)FeIV═NTs]2+ compounds due to partial electron transfer from the thioanisole substrate to the iron(IV)-tosylimido oxidant.

Amphoteric reactivity of a putative Cu(II)-mCPBA intermediate

In copper-based enzymes, Cu-hydroperoxo/alkylperoxo species are proposed as key intermediates for their biological activity. A vast amount of literature is available on the functional and structural mimics of enzymatic systems with heme and non-heme ligand frameworks to stabilize high valent metal intermediates, mostly at low temperatures. Herein, we report a reaction between [CuI(NCCH3)4]+ and meta-chloroperoxybenzoic acid (mCPBA) in CH3CN that produces a putative CuII(mCPBA) species (1). 1 was characterized by UV/Vis, resonance Raman, and EPR spectroscopies. 1 can catalyze both electrophilic and nucleophilic reactions, demonstrating its amphoteric behavior. Additionally, 1 can also conduct electron transfer reactions with a weak reducing agent such as diacetyl ferrocene, making it one of the reactive copper-based intermediates. One of the most important aspects of the current work is the easy synthesis of a CuII(mCPBA) adduct with no complicated ligands for stabilization. Over time, 1 decays to form a CuII paddle wheel complex (2) and is found to be unreactive towards substrate oxidation.