Eric D. Bloch

The stabilization of low-coordinate and high-spin iron complexes plays a central role in enabling novel pathways for C–H functionalization and catalysis of organic reactions. Previous work from our group demonstrated that the bulky bis(carbene)borate ligand Ph2B(tBuIm)2– supports the three-coordinate iron(II) imido complex [Ph2B(tBuIm)2Fe=NDipp]−. In this study, we extend this platform by employing an even larger, more electron-withdrawing imido ligand to generate Ph2B(tBuIm)2Fe(N(terph)) (terph = 2,6-C6H5(C6H3)). This iron(III) imido complex was isolated and fully characterized, including by single-crystal X-ray diffraction. The complex reacts with 2-picoline to afford C–N bond functionalization at the benzylic position of the substrate. Mechanistic investigations by density functional theory calculations support a pathway in which intramolecular hydrogen atom transfer from the coordinated substrate to the imido ligand provides a carbon-centered radical. Subsequent intramolecular radical rebound forms a new C–N bond. Together, these two steps define a mechanistic sequence reminiscent of enzymatic C–H functionalization pathways at a well-defined, low-coordinate iron complex. In addition, DFT provides insight into the directing effect of the picoline ligand.

Zirconium-based coordination cages are versatile platforms for exploring host–guest chemistry and the design of advanced materials.

This study investigates the photochemical properties and stability of zirconium-based porous salts composed of coordination cages and redox-active polyoxometalates (POMs). While bulk porous salts demonstrated photoreduction confined to surface regions due to limited light penetration and scattering effects, thin films of cage-POM assemblies were fabricated to address these challenges. These optically transparent films allowed access to the catalyst core, enabling enhanced photochemical characterization. Comparative studies revealed that the thin films exhibited superior photochemical activity and maintained porosity under optimized conditions, whereas bulk powders suffered from surface-limited photoreduction and cage decomposition. This work highlights the utility of thin films as a platform for studying and advancing the photochemical applications of porous salts.

Exchanging triflate for alternative anions enhances fluorescence in porous biphenyl-based zirconium cages.

The iron(I) dinitrogen complex PhB(AdIm)₃FeN₂, which is supported by a very bulky 1-adamantyl-substituted tris(carbene)borate ligand, reacts with equimolar CO at low temperature to afford the high spin (S = 3/2) complex PhB(AdIm)₃Fe(CO). This monocarbonyl complex reacts with additional CO to afford the low spin (S = 1/2) dicarbonyl complex PhB(AdIm)₃Fe(CO)₂. By contrast, the high spin iron(I) tris(pyrazolyl)borate complex Tp^tBu,MeFe(CO) does not react with additional CO. Experimental and computational investigations reveal that coordination of CO to Tp^tBu,MeFe(CO) is spin blocked, whereas the stronger donor tris(carbene)borate ligand in PhB(AdIm)₃Fe(CO) sufficiently stabilizes the low spin state to enable a thermally accessible barrier for CO coordination. Kinetic measurements reveal that the rate of CO coordination reaction is many orders of magnitude slower than for a related cobalt complex, which is attributed in large part to the relative extent of geometric reorganization required to achieve the low spin state for iron(I) as opposed to cobalt(I). The barrier to the spin state change has allowed the mixed isotope complex PhB(AdIm)₃Fe(¹²CO)(¹³CO) to be prepared.

Post-synthetic metalation and metathesis chemistry are central to rational synthesis of metal–organic frameworks (MOFs) that are unavailable by direct self-assembly. The inherent microcrystallinity and heterogeneous nature of many MOFs renders characterization of the rate, extent, and distribution of post-synthetic modifications challenging. Here we describe the deposition of optically transparent, permanently porous thin films comprised of peripherally carboxylated free-base porphyrins and cationic porous molecular cages. The films are assembled via layer-by-layer growth controlled by coulombic charge pairing, which allows for systematic control over film thickness. The obtained thin films are optically transparent monoliths that retain the permanent porosity of the corresponding porous salts. Post-synthetic metalation of these films with Mn(HMDS)₂ affords the corresponding Mn(II) porphyrin-based materials (HMDS = hexamethyldisilazide). Access to thin films with systematically varied thickness (and thus optical density), combined with in situ spectroscopy, enables the kinetics and extent of metalation to be directly monitored. We demonstrate both structure- and thickness-dependence on metalation kinetics. These results provide a unique window into the molecular-scale mechanisms that underpin materials synthesis.

Porous zirconium-based coordination cages are promising materials for applications in gas adsorption, catalysis, and molecular separation due to their tunability and stability. However, their synthesis is often complicated by the formation of competing phases, including insoluble or poorly soluble byproducts that impact purity and composition. Moreover, product composition and solubility can vary widely due to factors such as humidity, seasonal fluctuations, and lab-to-lab variations, highlighting the inherent lack of robustness in these syntheses. In this work, we investigate how solvothermal synthesis conditions, particularly temperature and solvent decomposition, influence the formation and composition of these cages. We show that elevated temperatures accelerate solvent breakdown, leading to the incorporation of formate and acetate byproducts that alter the final cage structures and contribute to the formation of insoluble zirconium-based, amorphous solids. By systematically varying the reaction conditions, we optimized the composition of the isolated cage products, achieving improved phase purity. By optimizing synthetic parameters, we achieve control over cage formation and particle morphology while mitigating the effects of solvent decomposition. Our findings provide insights into the balance between ligand coordination and solvent effects, enabling the development of strategies to enhance the purity, porosity, and functionality of these molecular cages.