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Bimodal Reaction Sequences in Oxidation with Dioxygen in Photocatalysis
Published in Robert Bakhtchadjian, Bimodal Oxidation: Coupling of Heterogeneous and Homogeneous Reactions, 2019
Electron transfers between the catalyst and reactants occur by an outer-sphere electron transfer mechanism (for coordination-saturated complexes, the inner-sphere electron transfer cannot take place, because the formation of a new chemical bond through a ligand, bridging other (one more) metal center, is impossible). Note that the majority of the metalorganic complex photocatalysts in triplet state have a lifetime that is long enough for electron transfer rather than relaxing and returning to their ground state. In the case of homogeneous photocatalysis by complexes of different transition metals, the mechanism suggestions are reminiscent of the perceptions of redox catalysis. Thus, in homogeneous photocatalysis, the transition metal ion mediates the electron transfer from the oxidant to reductant compound.
Metalloprotein Electronics
Published in Sergey Edward Lyshevski, Nano and Molecular Electronics Handbook, 2018
Andrea Alessandrini, Paolo Facci
Generally speaking, coordination complexes that are relatively inert to ligand substitution employ outer-sphere electron transfer mechanisms, whereas those complexes in which coordinating ligands are more labile, are more prone to undergoing inner-sphere reactions. The requirement for formation of a ligand-bridged intermediate prior to electron transfer indicates that the energetic barrier to formation of the transition state is significantly greater in the inner-sphere electron transfer case. This difference in activation barriers between the two mechanisms reflects the Franck–Condon principle, which states that nuclear rearrangements are slower than electronic ones.
Controlled Radical Polymerization
Published in Samir H. Chikkali, Metal-Catalyzed Polymerization, 2017
Lifetime of propagating chains in FRP is short (~1 s) and at any time their concentration is at ppm level, and the concentration of dead chains is very high. However in RDRP, most of the chains are in dormant state that are reactivated for ~1 ms. The current thinking is focused on extending the life of propagating chains to more than 1 day, by inserting dormant periods of ~1 minute between the periods of activity.8 As mentioned earlier termination in RDRP is very low, however it is important to know how many dead chains exist. For this purpose, dead chain fraction (DCF) is defined as ratio of concentration of terminated chains to initial concentration of initiator. DCF depends on targeted degree of polymerization (DP = [M]0/[I]0), monomer conversion, propagation and termination rate constants, and reaction time. DCF values are low at slower rates of polymerization, lower monomer conversion, lower targeted DP, and higher initial monomer concentration, and for highly reactive monomers.8 Formation of radicals from dormant species in a Cu-catalyzed process, considering the mechanism, can take place by two pathways—the outer-sphere electron transfer (OSET) or the inner-sphere electron transfer (ISET). OSET can involve formation of radical anion intermediates in a stepwise or concerted fashion, whereas in ISET transfer of halogen atom from the halide to Cu(I) takes place via a Cu–X–C transition state, which is formally a single electron transfer process. The energy barrier for OSET as obtained by Marcus analysis is ~15 kcal/mol higher than the experimentally determined value, that is, OSET is ~1010 times slower than ISET.10 Therefore, it was concluded that Cu-catalyzed ATRP occurs via concerted homolytic dissociation of alkyl halide via ISET—an atom transfer process.
Electrochemical reduction of halogenated organic contaminants using carbon-based cathodes: A review
Published in Critical Reviews in Environmental Science and Technology, 2023
Jacob F. King, William A. Mitch
However, oxygenated functional groups are believed to catalyze reductive dehalogenation reactions in some cases when adsorption of contaminants results in formation of complexes with the oxygenated functional groups, enabling inner-sphere electron transfer to occur in addition to outer-sphere transfer. Chemisorption of TCE to C–O− groups on BDD electrodes was proposed to catalyze reductive dechlorination of TCE to form acetate and chloride (Mishra et al., 2008). The observed activation barrier was four-fold lower than that predicted by density functional theory (DFT) simulations for outer-sphere electron transfer but concurred with DFT simulations of an inner-sphere electron transfer via a complex between a deprotonated hydroxyl functional group on BDD and TCE. Catalysis can also be evidenced by a shift toward more positive potentials at which peak cathodic currents (Ep,c) are observed during cyclic voltammetry experiments, indicating a decline in the energy barrier for reduction (Durante et al., 2009; Isse, Berzi, et al., 2009; Isse, Mussini, et al., 2009; Isse, Sandona, et al., 2009). For example, catalysis was observed using Ag electrodes for reductive dehalogenation of halogenated alkanes, alkenes, benzyl chloride and brominated or iodinated aromatics, but not chlorinated aromatics (Durante et al., 2009; Y.-F. Huang et al., 2010; Isse et al., 2006; Isse, Berzi, et al., 2009; Isse, Mussini, et al., 2009; Isse, Sandona, et al., 2009; Klymenko et al., 2014; A. Wang et al., 2010), suggesting inner-sphere electron transfer after the formation of adducts between the halogenated organic and the electrode surface (Zhang et al., 2005).
Electrostatic bending and outer-sphere intervalence transfer in a flexible ligand-bridged ruthenium(III)-iron(II) complex
Published in Journal of Coordination Chemistry, 2018
Juan S. Aguirre-Araque, Reginaldo C. Rocha, Henrique E. Toma
The reported behavior provides a different view from the mixed valence interactions in bimetallic systems [35], combining outer-sphere excitation and electrostatic bending in a flexible ligand-bridged binuclear complex. It can also be employed for introducing outer-sphere/inner sphere electron transfer concepts in chemical education [36], or to explore possible applications in redox switching [37] or in molecular computing, encoding digital information by the charge configuration of the mixed-valence complex [38].