This work directly compares the spectroscopic and reactivity properties of an oxoiron(IV) and an oxoiron(V) complex that are supported by the same neutral tetradentate N-based PyNMe 3 ligand. A complete spectroscopic characterization of the oxoiron(IV) species (2) reveals that this compound exists as a mixture of two isomers. The reactivity of the thermodynamically more stable oxoiron(IV) isomer (2b) is directly compared to that exhibited by the previously reported 1e − -oxidized analogue [Fe V (O)(OAc)(PyNMe 3 )] 2+ (3). Our data indicates that 2b is 4 to 5 orders of magnitude slower than 3 in hydrogen atom transfer (HAT) from C−H bonds. The origin of this huge difference lies in the strength of the O−H bond formed after HAT by the oxoiron unit, the O−H bond derived from 3 being about 20 kcal•mol −1 stronger than that from 2b. The estimated bond strength of the Fe IV O−H bond of 100 kcal•mol −1 is very close to the reported values for highly active synthetic models of compound I of cytochrome P450. In addition, this comparative study provides direct experimental evidence that the lifetime of the carbon-centered radical that forms after the initial HAT by the high valent oxoiron complex depends on the oxidation state of the nascent Fe−OH complex. Complex 2b generates long-lived carbon-centered radicals that freely diffuse in solution, while 3 generates short-lived caged radicals that rapidly form product C−OH bonds, so only 3 engages in stereoretentive hydroxylation reactions. Thus, the oxidation state of the iron center modulates not only the rate of HAT but also the rate of ligand rebound.
C-F bonds are one of the most inert functionalities. Nevertheless, some [Cu2O2] 2+ species are able to defluorinatehydroxylate ortho-fluorophenolates in a chemoselective manner over other ortho-halophenolates. Albeit it is known that such reactivity is promoted by an electrophilic attack of a [Cu2O2] 2+ core over the arene ring, the crucial details of the mechanism that explain the chemo and regioselectivity of the reaction species remain unknown, and it has not being determined either if Cu II 2( 2 : 2 -O2) or Cu III 2(µ-O)2 species are responsible for the initial attack on the arene. Herein, we present a combined theoretical and experimental mechanistic study to unravel the origin of the chemo-selectivity of the ortho-defluorination-hydroxylation of 2-halophenolates by the [Cu2(O)2(DBED)2] 2+ complex. Our results show that the equilibria between (side-on)peroxo (P) and bis(μ-oxo) (O) isomers plays a key role in the mechanism, being the latter the reactive species. Furthermore, on the basis of quantum mechanical calculations, we have been able to rationalize the chemoselective preference of [Cu2(O)2(DBED)2] 2+ catalyst for the C-F activation over C-Cl and C-H activations.
The synthesis and spectroscopic and structural characterization of well-defined organometallic Ni(II) and Ni(III) complexes bearing the PyNMe3 ligand - a tetradentate N-based macrocyclic ligand which coordinates to the metal center...
Herein we explore the intrinsic organometallic reactivity of iron embedded in a tetradentate N 3 C macrocyclic ligand scaffold that allows the stabilization of aryl-Fe species, which are key intermediates in Fe-catalyzed cross-coupling and C−H functionalization processes. This study covers C−H activation reactions using Me L H and FeCl 2 , biaryl C−C coupling product formation through reaction with Grignard reagents, and crosscoupling reactions using Me L Br or H L Br in combination with Fe 0 (CO) 5 . Synthesis under light irradiation and moderate heating (50 °C) affords the aryl-Fe II complexes [Fe II (Br)( Me L)(CO)] (1 Me ) and [Fe II ( H L)(CO) 2 ]Br (1 H ). Exhaustive spectroscopic characterization of these rare low-spin diamagnetic species, including their crystal structures, allowed the investigation of their intrinsic reactivity.
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