This work explores the mechanism whereby a cationic diimine Pd­(II) complex combines coordination insertion and radical polymerization to form polyolefin–polar block copolymers. The initial requirement involves the insertion of a single acrylate monomer into the Pd­(II)–polyolefin intermediates, which generate a stable polymeric chelate through a chain-walking mechanism. This thermodynamically stable chelate was also found to be photochemically inactive, and a unique mechanism was discovered which allows for radical polymerization. Rate-determining opening of the chelate by an ancillary ligand followed by additional chain walking allows the metal to migrate to the α-carbon of the acrylate moiety. Ultimately, the molecular parameters necessary for blue-light-triggered Pd–C bond homolysis from this α-carbon to form a carbon-centered macroradical species were established. This intermediate is understood to initiate free radical polymerization of acrylic monomers, thereby facilitating block copolymer synthesis from a single Pd­(II) complex. Key intermediates were isolated and comprehensively characterized through exhaustive analytical methods which detail the mechanism while confirming the structural integrity of the polyolefin–polar blocks. Chain walking combined with blue-light irradiation functions as the mechanistic switch from coordination insertion to radical polymerization. On the basis of these discoveries, robust di- and triblock copolymer syntheses have been demonstrated with olefins (ethylene and 1-hexene) which produce amorphous or crystalline blocks and acrylics (methyl acrylate, ethyl acrylate, n-butyl acrylate, and methyl methacrylate) in broad molecular weight ranges and compositions, yielding AB diblocks and BAB triblocks.

Reactive metal-ligand (M-L) multiply bonded complexes are ubiquitous intermediates in redox catalysis and have thus been long-standing targets of synthetic chemistry. The intrinsic reactivity of mid-to-late M-L multiply bonded complexes renders these structures challenging to isolate and structurally characterize. Although synthetic tuning of the ancillary ligand field can stabilize M-L multiply bonded complexes and result in isolable complexes, these efforts inevitably attenuate the reactivity of the M-L multiple bond. Here, we report the first direct characterization of a reactive Ru nitride intermediate by photocrystallography. Photogeneration of reactive M-L multiple bonds within crystalline matrices supports direct characterization of these critical intermediates without synthetic derivatization.

The fleeting lifetimes of reactive intermediates in C−H functionalization chemistry often prevent their direct characterization. For example, the critical nitrenoid intermediates that mediate Rh 2 -catalyzed C−H amination have eluded characterization for more than 40 years. In the absence of structural characterization of these species, methodological development is often computationally guided. Here we report the first X-ray crystal structure of a reactive Rh 2 nitrenoid, enabled by N 2 elimination from an organic azide ligand within a singlecrystal matrix. The resulting high-resolution structure displays metrical parameters consistent with a triplet nitrene complex of Rh 2 . The demonstration of facile access to reactive metal nitrenoids within a crystalline matrix provides a platform for structural characterization of the transient species at the heart of C−H functionalization.

Selective functionalization of aliphatic C–H bonds, ubiquitous in molecular structures, could allow ready access to diverse chemical products. While enzymatic oxygenation of C–H bonds is well established, the analogous enzymatic nitrogen functionalization is still unknown; nature is reliant on preoxidized compounds for nitrogen incorporation. Likewise, synthetic methods for selective nitrogen derivatization of unbiased C–H bonds remain elusive. In this work, new-to-nature heme-containing nitrene transferases were used as starting points for the directed evolution of enzymes to selectively aminate and amidate unactivated C­(sp3)–H sites. The desymmetrization of methyl- and ethylcyclohexane with divergent site selectivity is offered as demonstration. The evolved enzymes in these lineages are highly promiscuous and show activity toward a wide array of substrates, providing a foundation for further evolution of nitrene transferase function. Computational studies and kinetic isotope effects (KIEs) are consistent with a stepwise radical pathway involving an irreversible, enantiodetermining hydrogen atom transfer (HAT), followed by a lower-barrier diastereoselectivity-determining radical rebound step. In-enzyme molecular dynamics (MD) simulations reveal a predominantly hydrophobic pocket with favorable dispersion interactions with the substrate. By offering a direct path from saturated precursors, these enzymes present a new biochemical logic for accessing nitrogen-containing compounds.

Treatment of (ArL)­CoBr (ArL = 5-mesityl-1,9-(2,4,6-Ph3C6H2)­dipyrrin) with a stoichiometric amount of 1-azido-4-(tert-butyl)­benzene N3(C6H4-p- t Bu) furnished the corresponding four-coordinate organoazide-bound complex (ArL)­CoBr­(N3(C6H4-p- t Bu)). Spectroscopic and structural characterization of the complex indicated redox innocent ligation of the organoazide. Slow expulsion of dinitrogen (N2) was observed at room temperature to afford a ligand functionalized product via a [3 + 2] annulation, which can be mediated by a high-valent nitrene intermediate such as a CoIII iminyl (ArL)­CoBr­(•N­(C6H4-p- t Bu)) or CoIV imido (ArL)­CoBr­(N­(C6H4-p- t Bu)) complex. The presence of the proposed intermediate and its viability as a nitrene group transfer reagent are supported by intermolecular C–H amination and aziridination reactivities. Unlike (ArL)­CoBr­(N3(C6H4-p- t Bu)), a series of alkyl azide-bound CoII analogues expel N2 only above 60 °C, affording paramagnetic intermediates that convert to the corresponding Co-imine complexes via α-H-atom abstraction. The corresponding N2-released structures were observed via single-crystal-to-crystal transformation, suggesting formation of a Co-nitrenoid intermediate in solid-state. Alternatively, the alkyl azide-bound congeners supported by a more sterically accessible dipyrrinato scaffold tBuL ( tBuL = 5-mesityl-(1,9-di-tert-butyl)­dipyrrin) facilitate intramolecular 1,3-dipolar cycloaddition as well as C–H amination to furnish 1,2,3-dihydrotriazole and substituted pyrrolidine products, respectively. For the C–H amination, we observe that the temperature required for azide activation varies depending on the presence of weak C–H bonds, suggesting that the alkyl azide adducts serve as viable species for C–H amination when the C–H bonds are (1) proximal to the azide moiety and (2) sufficiently weak to be activated.

Metal-organic frameworks (MOFs) have garnered substantial interest as platforms for site-isolated catalysis. Efficient diffusion of small-molecule substrates to interstitial lattice-confined catalyst sites is critical to leveraging unique opportunities of these materials as catalysts.Understanding the rates of substrate diffusion in MOFs is challenging,a nd few in situ chemical tools are available to evaluate substrate diffusion during interstitial MOF chemistry.H erein, we demonstrate nitrogen atom transfer (NAT)f rom al atticeconfined Ru 2 nitride to toluene to generate benzylamine.W e use the comparison of the intramolecular deuterium kinetic isotope effect (KIE), determined for amination of ap artially deuterated substrate,with the intermolecular KIE, determined by competitive amination of am ixture of perdeuterated and undeuterated substrates,t oe stablish the relative rates of substrate diffusion and interstitial chemistry.W ea nticipate that the developed KIE-based experiments will contribute to the development of porous materials for group-transfer catalysis.

Correlation of catalyst structure with activity is foundational to the rational design of transition metal catalysts. While X-ray crystallography routinely provides structural characterization of kinetically stable pre-catalysts and intermediates, experimental elucidation of the structures of reactive intermediates, which are the species intimately engaged in bond-breaking and-making in catalysis, is generally not possible due to the transient nature of these species. Here, we demonstrate in crystallo synthesis of Rh2 nitrenes that participate in catalytic C-H amination, and characterization of these transient intermediates as triplet adducts of Rh2. Further, we observe the impact of coordinating substrate, which is present in excess during catalysis, on the structure of transient Rh2 nitrenes involved in C-H amination. By providing structural characterization of authentic C-H functionalization intermediates, and not kinetically stabilized model complexes, these experiments provide the opportunity to define critical structure-activity relationships.

Rh-catalyzed C-H amination is a powerful method for nitrogenating organic molecules. While Rh nitrenoids are often invoked as reactive intermediates in these reactions, the exquisite reactivity and fleeting lifetime of these species has precluded their observation. Here, we report the photogeneration of a transient Rh nitrenoid that participates in C-H amination. The developed approach to Rh nitrenoids, based on photochemical cleavage of N-Cl bonds in N-chloroamido ligands, has enabled characterization of a reactive Rh nitrenoid by mass spectrometry and transient absorption spectroscopy. We anticipate that photogeneration of metal nitrenoids will contribute to the development of C-H amination catalysis by providing tools to directly study the structures of these critical intermediates.