Transition metal oxo species are key intermediates for the activation of strong C‒H bonds. As such, there has been interest in understanding which structural or electronic parameters of metal oxo...
Biology employs exquisite control over proton, electron, H-atom, or H2 transfer. Similar control in synthetic systems has the potential to facilitate efficient and selective catalysis. Here we report a dihydrazonopyrrole...
The selective hydroxylation of aliphatic C–H bonds remains a challenging but broadly useful transformation. Nature has evolved systems that excel at this reaction, exemplified by cytochrome P450 enzymes, which use an iron-oxo intermediate to activate aliphatic C–H bonds with k 1 > 1400 s–1 at 4 °C. Many synthetic catalysts have been inspired by these enzymes and are similarly proposed to use transition metal-oxo intermediates. However, most examples of well-characterized transition metal-oxo species are not capable of reacting with strong, aliphatic C–H bonds, resulting in a lack of understanding of what factors facilitate this reactivity. Here, we report the isolation and characterization of a new terminal CoIII-oxo complex, PhB(AdIm)3CoIIIO. Upon oxidation, a transient CoIV-oxo intermediate is generated that is capable of hydroxylating aliphatic C–H bonds with an extrapolated k 1 for C–H activation >130 s–1 at 4 °C, comparable to values observed in cytochrome P450 enzymes. Experimental thermodynamic values and DFT analysis demonstrate that, although the initial C–H activation step in this reaction is endergonic, the overall reaction is driven by an extremely exergonic radical rebound step, similar to what has been proposed in cytochrome P450 enzymes. The rapid C–H hydroxylation reactivity displayed in this well-defined system provides insight into how hydroxylation is accomplished by biological systems and similarly potent synthetic oxidants.
There has been recent interest about how the rates of concerted proton electron transfer (CPET) are affected by the thermodynamic parameters of intermediates from stepwise PT or ET reactions. Semiclassical...
Transition metal−oxo complexes are key intermediates in a variety of oxidative transformations, notably C−H bond activation. The relative rate of C−H bond activation mediated by transition metal−oxo complexes is typically predicated on substrate bond dissociation free energy in cases with a concerted proton−electron transfer (CPET). However, recent work has demonstrated that alternative stepwise thermodynamic contributions such as acidity/basicity or redox potentials of the substrate/metal−oxo may dominate in some cases. In this context, we have found basicity-governed concerted activation of C−H bonds with the terminal Co III −oxo complex PhB( tBu Im) 3 Co III O. We have been interested in testing the limits of such basicity-dependent reactivity and have synthesized an analogous, more basic complex, PhB( Ad Im) 3 Co III O, and studied its reactivity with H-atom donors. This complex displays a higher degree of imbalanced CPET reactivity than PhB( tBu Im) 3 Co III O with C−H substrates, and O−H activation of phenol substrates displays mechanistic crossover to stepwise proton transfer−electron transfer (PTET) reactivity. Analysis of the thermodynamics of proton transfer (PT) and electron transfer (ET) reveals a distinct thermodynamic crossing point between concerted and stepwise reactivity. Furthermore, the relative rates of stepwise and concerted reactivity suggest that maximally imbalanced systems provide the fastest CPET rates up to the point of mechanistic crossover, which results in slower product formation.
Selective hydroxylation of aliphatic C–H bonds remains a challenging but broadly useful transformation. Nature has evolved systems that excel at this reaction, exemplified by cytochrome P450 enzymes which use an iron-oxo intermediate to activate aliphatic C–H bonds with k1 > 1400 s–1 at 4 °C. Many synthetic catalysts have been inspired by these enzymes and are similarly proposed to use transition metal-oxo intermediates. However, most examples of well-characterized transition metal-oxo species are not capable of reacting with strong, aliphatic C–H bonds, resulting in a lack of understanding of what factors facilitate this reactivity. Here, we report the isolation and characterization of a new terminal CoIII-oxo complex, PhB(AdIm)3CoIIIO. Upon oxidation a transient CoIV-oxo intermediate is generated that is capable of hydroxylating aliphatic C–H bonds with an extrapolated k1 for C–H activation >130 s–1 at 4 °C, comparable to values observed in cytochrome P450 enzymes. Experimental thermodynamic values and DFT analysis demonstrate that although the initial C–H activation step in this reaction is endergonic, the overall reaction is driven by an extremely exergonic radical rebound step, similar to what has been proposed in cytochrome P450 enzymes. The rapid C–H hydroxylation reactivity displayed in this well-defined system provides insight into how hydroxylation is accomplished by biological systems and similarly potent synthetic oxidants.
Recently there have been several experimental demonstrations of how concerted proton electron transfer (CPET) reaction rates are affected by off-diagonal energies, namely the stepwise thermodynamic parameters ΔG°PT and ΔG°ET. Semiclassical structure-activity relationships have been invoked to rationalize these linear free energy relationships despite the widely acknowledged importance of quantum effects such as nonadiabaticity and tunneling in CPET reactions. Here we report variable temperature kinetic isotope effect data for the asynchronous reactivity of a terminal Co-oxo complex with C–H bonds and find evidence of substantial quantum tunneling which is inconsistent with semiclassical models even when including tunneling corrections. This indicates substantial nonadiabatic tunneling in the CPET reactivity of this Co-oxo complex and further motivates the need for a quantum mechanical justification for the influence of ΔG°PT and ΔG°ET on reactivity. We include ΔG°PT and ΔG°ET in nonadiabatic models of CPET by modeling how they influence the anharmonicity and depth of proton potential energy surfaces, which we approximate with a four-state model. With this model we independently reproduce a dominant trend with ΔG°PT + ΔG°ET as well as a more subtle effect of ΔG°PT − ΔG°ET (equivalently η) in a nonadiabatic framework. The primary route through which these off-diagonal energies influence rates is through vibronic coupling. Our results reconcile predictions from semiclassical transition state theory with models that treat proton transfer quantum mechanically in CPET reactivity and suggest that similar treatments may be possible for other reactions with significant nuclear tunneling.
Late transition metal oxo and imide complexes play an important role in the catalytic functionalization and activation of small molecules. An emerging theme in this area over the past few decades has been the use of lower-coordination numbers, and pseudo-tetrahedral geometries in particular, to stabilize what would otherwise be highly reactive species. However, the bonding structure in d 6 oxo and imide complexes in this geometry is ambiguous. These species are typically depicted with a triple bond, however recent experimental evidence suggests significant empirical differences between these complexes and other triply bonded complexes with lower d-counts. Here we use a suite of computational orbital localization methods and electron density analyses to probe the bonding structure of isoelectronic d 6 Co(III) oxo and imide complexes. These analyses suggest that a triple bond description is inaccurate due to a dramatically weakened σ interaction.While the exact bond order in these cases is necessarily dependent on the model used, several metrics suggest that the strength of the metal-O/N bond is most similar to other formally doubly bonded complexes. File list (4) download file view on ChemRxiv Manuscript.pdf (2.81 MiB) download file view on ChemRxiv TOC.tif (858.29 KiB) download file view on ChemRxiv SI.pdf (1.36 MiB) download file view on ChemRxiv xyz Files.zip (24.38 KiB)
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