A variety of C–N bond-forming methods are enabled by the [2 + 2] cycloaddition reaction of a transition metal imide complex and an alkyne substrate to generate an azametallacyclobutene intermediate. This type of reactivity has been primarily limited to early transition metals like zirconium and titanium. Herein, we describe the preparation of an iron azametallacyclobutene complex by [2 + 2] cycloaddition of a β-diketiminate iron imide complex and an internal alkyne, 1-phenyl-1-propyne. The metallacycle reacts further upon exposure to a terminal alkyne, phenylacetylene, by a proposed protonation pathway that is distinct from the chemistry of its group 4 congeners and is in line with formation of an azametallacyclobutene intermediate in iron-catalyzed alkyne carboamination. The iron azametallacyclobutene complex also undergoes migratory insertion of aldehyde and nitrile substrates to the metal–nitrogen bond, in contrast to the exclusive metal–carbon insertion that has been observed for zirconium and titanium analogs.

Transition metal imide-mediated C–N bond formation is a powerful strategy for the introduction of nitrogen into organic compounds. We have discovered that the reaction of N-mesityl­(β-diketiminato)­iron imide complex tBuLFeNMes ( tBuL = 3,5-bis­(2,6-diisopropylphenylimino)-2,2,6,6-tetramethylheptyl and Mes = 2,4,6-trimethylphenyl) with a terminal alkyne substrate gives a β-alkynyl enamine product by a novel alkyne carboamination process. Stoichiometric experiments revealed a catalyst deactivation pathway involving generation of the acetylide complex, tBuLFeCCPh, and mesityl amine (MesNH2) from the acetylene complex, tBuLFe­(HCCPh), and mesityl azide (MesN3). This reactivity is suppressed in the presence of coordinating additive 4-tert-butylpyridine ( t BuPy), likely through formation of the four-coordinate complex tBuLFe­(HCCPh)­( t BuPy). These insights were instrumental in identifying reaction conditions that allow for turnover of the iron catalyst.

Herein we describe our investigation into the electronic structure of the first isolated monometallic iron azametallacyclobutene complex. Computational analysis through density functional theory calculations reveals electron delocalization throughout the four atoms of the ring system, in line with experimental observations and supporting the classification of this complex as a conjugated metallacycle. The results of this study also point to significant contribution from an imine-substituted iron carbene resonance structure to the overall bonding picture for the azametallacyclobutene. Accordingly, this complex participates in carbene-like reactivity in the presence of an isocyanide substrate to generate a ketenimine product. The related reaction with carbon monoxide leads to the isolation of a five-membered metallacycle that is analogous to the proposed intermediate in ketenimine formation, and confirms the α-carbon as the site of reactivity.

Herein we report a method for pyrrole synthesis via iron-catalyzed carboamination/copper-mediated cyclization that is completely regioselective for the formation of 1,2,4-trisubstituted products. This two-step, one-pot process offers significant improvements to previously reported conditions including the use of a readily available copper(I) source and a markedly less arduous experimental procedure. Exploration of the substrate scope reveals a variety of arylacetylenes undergo pyrrole formation to afford single isomer products. Isotopic labelling data points to a mechanism involving activation of the alkyne moiety by the copper(I) reagent during the cyclization step.

Compact visible wavelength achromats are essential for miniaturized and lightweight optics. However, fabrication of such achromats has proved to be exceptionally challenging. Here, using subsurface 3D printing inside mesoporous hosts we densely integrate aligned refractive and diffractive elements, forming thin high performance hybrid achromatic imaging micro-optics. Focusing efficiencies of 51–70% are achieved for 15μm thick, 90μm diameter, 0.3 numerical aperture microlenses. Chromatic focal length errors of less than 3% allow these microlenses to form high-quality images under broadband illumination (400–700 nm). Numerical apertures upwards of 0.47 are also achieved at the cost of some focusing efficiency, demonstrating the flexibility of this approach. Furthermore, larger area images are reconstructed from an array of hybrid achromatic microlenses, laying the groundwork for achromatic light-field imagers and displays. The presented approach precisely combines optical components within 3D space to achieve thin lens systems with high focusing efficiencies, high numerical apertures, and low chromatic focusing errors, providing a pathway towards achromatic micro-optical systems.