Nickel complexes have been widely employed as catalysts in C–C and C–heteroatom bond formation reactions. While Ni(0), Ni( i ), and Ni( ii ) intermediates are most relevant in these transformations, recently Ni( iii ) and Ni( iv ) species have also been proposed to play a role in catalysis. Reported herein is the synthesis, detailed characterization, and reactivity of a series of Ni( ii ) and Ni( iii ) metallacycle complexes stabilized by tetradentate pyridinophane ligands with various N-substituents. Interestingly, while the oxidation of the Ni( ii ) complexes with various other oxidants led to exclusive C–C bond formation in very good yields, the use of O 2 or H 2 O 2 as oxidants led to formation of appreciable amounts of C–O bond formation products, especially for the Ni( ii ) complex supported by an asymmetric pyridinophane ligand containing one tosyl N-substituent. Moreover, cryo-ESI-MS studies support the formation of several high-valent Ni species as key intermediates in this uncommon Ni-mediated oxygenase-type chemistry.
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Intermediates and mechanism in iron-catalyzed C–H methylation with trimethylaluminum
A mechanistic study is performed on the reaction method for iron-catalyzed C–H methylation with AlMe 3 reagent, previously proposed to involve cyclometalated iron( iii ) intermediates and an iron( iii )/( i ) reaction cycle. Detailed spectroscopic studies ( 57 Fe Mössbauer, EPR) during catalysis and in stoichiometric reactions identify iron( ii ) complexes, including cyclometalated iron( ii ) intermediates, as the major iron species formed in situ under catalytic reaction conditions. Reaction studies identify a cyclometalated iron( ii )-methyl species as the key intermediate leading to C–H methylated product upon reaction with oxidant, consistent with a previously proposed iron( ii )/iron( iii )/iron( i ) reaction manifold for C–H arylation.
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- Award ID(s):
- 1954480
- NSF-PAR ID:
- 10317963
- Date Published:
- Journal Name:
- Chemical Communications
- Volume:
- 57
- Issue:
- 95
- ISSN:
- 1359-7345
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract We investigate the CO oxidation into CO2catalyzed by a biomimetic Ni(II)‐iminothiolate complex in the presence of the sacrificial oxidizing agent methylviologen. We propose a catalytic mechanism supported by the density functional theory analysis of reaction intermediates that agrees with available experimental observations and kinetic data. We rule out a five‐coordinate Ni(II) species as well as a Ni(III) intermediate which was previously proposed and instead identify a key four‐coordinate Ni(II) carbonyl species. We find that the turnover‐limiting step is likely the formation of the Ni(II) carboxylic acid species, although if a worse oxidant were used or if the concentration was less, then the oxidation of the Ni(I) species would be the turnover‐limiting step. The reported findings should enable the design of better catalysts to favor one of the two competing pathways for CO to CO2conversion as catalyzed by this particular complex.
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Herein we report the direct observation of C–H bond activation at an isolated mononuclear Pd( iii ) center. The oxidation of the Pd( ii ) complex ( Me N4)Pd II (neophyl)Cl (neophyl = –CH 2 C(CH 3 ) 2 Ph; Me N4 = N , N ′-dimethyl-2,11-diaza[3.3](2,6)pyridinophane) using the mild oxidant ferrocenium hexafluorophosphate (FcPF 6 ) yields the stable Pd( iii ) complex [( Me N4)Pd III (neophyl)Cl]PF 6 . Upon the addition of an acetate source, [( Me N4)Pd III (neophyl)Cl]PF 6 undergoes Csp 2 –H bond activation to yield the cyclometalated product [( Me N4)Pd III (cycloneophyl)]PF 6 . This metalacycle can be independently prepared, allowing for a complete characterization of both the starting and final Pd( iii ) complexes. The C–H activation step can be monitored directly by EPR and UV-Vis spectroscopies, and kinetic isotope effect (KIE) studies suggest that either a pre-association step such as an agostic interaction may be rate limiting, or that the C–H activation is partially rate-limiting in conjunction with ligand rearrangement. Density functional theory calculations support that the reaction proceeds through a κ 3 ligand coordination and that the flexible ligand structure is important for this transformation. Overall, this study represents the first example of discrete C–H bond activation occurring at a Pd( iii ) center through a concerted metalation–deprotonation mechanism, akin to that observed for Pd( ii ) and Pd( iv ) centers.more » « less
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The density functional theory method is used to elucidate the elementary steps of Ni( ii )-catalyzed C(sp 2 )–H iodination with I 2 and substrates bearing N , N ′-bidentate directing centers, amide-oxazoline (AO) and 8-aminoquinoline (AQ). The relative stability of the lowest energy high- and low-spin electronic states of the catalyst and intermediates is found to be an important factor for all of the steps in the reaction. As a result, two-state reactivity for these systems is reported, where the reaction is initiated on the triplet surface and generates a high energy singlet nickelacycle. It is shown that the addition of Na 2 CO 3 base to the reaction mixture facilitates C–H activation. The presence of I 2 in the reaction provides the much needed driving force for the C–H activation and nickelacycle formation and ultimately reacts to form a new C–I bond through either a redox neutral electrophilic cleavage (EC) pathway or a one-electron reductive cleavage (REC) pathway. The previously proposed Ni( ii )/Ni( iv ) and homolytic cleavage pathways are found to be higher in energy. The nature of the substrate is found to have a large impact on the relative stability of the lowest electronic states and on the stability of the nickelacycle resulting from C–H activation.more » « less
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Abstract Half‐sandwich rhodium monohydrides are often proposed as intermediates in catalysis, but little is known regarding the redox‐induced reactivity accessible to these species. Herein, the bis(diphenylphosphino)ferrocene (dppf) ligand has been used to explore the reactivity that can be induced when a [Cp*Rh] monohydride undergoes remote (dppf‐centered) oxidation by 1e−. Chemical and electrochemical studies show that one‐electron redox chemistry is accessible to Cp*Rh(dppf), including a unique quasi‐reversible RhII/Iprocess at −0.96 V vs. ferrocenium/ferrocene (Fc+/0). This redox manifold was confirmed by isolation of an uncommon RhIIspecies, [Cp*Rh(dppf)]+, that was characterized by electron paramagnetic resonance (EPR) spectroscopy. Protonation of Cp*Rh(dppf) with anilinium triflate yielded an isolable and inert monohydride, [Cp*Rh(dppf)H]+, and this species was found to undergo a quasireversible electrochemical oxidation at +0.41 V vs. Fc+/0that corresponds to iron‐centered oxidation in the dppf backbone. Thermochemical analysis predicts that this dppf‐centered oxidation drives a dramatic increase in acidity of the Rh−H moiety by 23 p
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/D1CC05607G
