Contents lists available at ScienceDirect Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett Research paper Ambient O 2 is a switch between [1-electron/1-radical] vs. [2–electron] oxidative catalytic path in Fe-Phthalocyanines Eleni Bletsa a , Maria Solakidou a,b , Maria Louloudi b , Yiannis Deligiannakis a, ⁎ a Laboratory of Physical Chemistry of Materials and Environment, Department of Physics, University of Ioannina, GR45110 Ioannina, Greece b Laboratory of Inorganic Chemistry, Department of Chemistry, University of Ioannina, GR45110 Ioannina, Greece HIGHLIGHTS • O 2 switches redox cycle of Fe-Phthalocyanine between [2e] or [1e+1 radical] path. • Distinct paths Fe IV ]O(S = 2) vs. Fe IV (S = 1)PC radical (S = 1/2) in absence/absence of O 2 . • Parallel & Perpendicular -mode EPR trace the two Fe-spin/redox paths. • The two paths result in distinct catalytic kinetic pentachlorophenol degradation. ARTICLE INFO Keywords: Fe-Phthalocyanine Oxygen Ferryl EPR Catalysis ABSTRACT The catalytic redox-intermediates of a Fe-Phthalocyanine (FePc) catalyst have been mapped using Dual-Mode EPR i.e. parallel-EPR to detect integer-spin and perpendicular-EPR to detect half-integer spin states. Molecular O 2 can switch the catalytic oxidative path of FePc catalyst between either a [two-electron, 2e] or an [1 electron +1 radical] path. In absence of O 2 the key reaction-intermediate is a Low-Spin [Fe IV (S = 1)-PC radical (S = 1/2)] state i.e. formed via a [2–electron path] vs. a High-Spin Fe IV ]O(S = 2) state, formed in the presence of O 2 via an [1-electron+1-radical] path, respectively resulting in distinct kinetic catalytic profiles at pentachlorophenol (PCP) degradation. 1. Introduction Fe-Phthalocyanines (FePcs), as industrial-dye products, can be considered as low-cost, readily available oxidation catalysts [1,2]. Strikingly, so far, in comparison to metalloporphyrins, all studied Fe- (or Mn)-phthalocyanine catalysts are reported to be less active than the corresponding porphyrins [3,4] i.e. when evaluated under comparable oxidative catalytic conditions. The reason for this inferiority of most Fe- Pc’s vs. homologous Fe-porphyrins, is not understood so far. Sorokin et al. [3] raised the possibility that O 2 might be implicated in the cat- alytic cycle of a Fe-phthalocyanine. This was postulated in an effort to explain a significant difference observed in catalytic conversion of tetrachlophenol (TCP) [3] in the presence of O 2 . However, so far, the molecular mechanism of the effect of O 2 on the redox/catalytic evolution of FePc remains unclear, although it was postulated to be related to different high-valent Fe-oxo or peroxo spe- cies generated by the primary oxidant i.e. KHSO 5 or H 2 O 2 [3]. In oxi- dations mediated by cytochrome P-450, the involvement of multiple reaction pathways, arguably can be explained by either the “two-oxi- dants” [5–7] or “two-spin-state” reactivity models [8]. In brief, the “two-oxidants” model considers that both [Fe]O] and [FeeOOH] (hydroperoxy-Fe) can be the two operating oxidants in the same system. This two-oxidants model is considered to correspond to the oxidative cycling of the archetypical P450 system, where multiple oxidation pathways are implicated [5–8]. As an alternative to the two-oxidants model, the multiple reaction pathways could be explained by only one oxidant [Fe]O], having two different spin states accessible i.e. there- fore the term “two-spin-states/one oxidant” [9,10]. In the two-spin- states model, the Fe-oxo unit is considered to have two closely-lying High-spin (HS) and Low-spin (LS) Fe-configurations. Due to the small energy difference between these HS & LS configurations, arguably, the Fe-oxo unit is able to switch between the, poorly bonding, High-spin state, and the strongly bonding, Low-spin state, thus allowing a low- energy catalytic path via a spin crossover along the reaction coordinate [9,10]. In the case of P450, the two-spin-states model has been dis- favored by theoretical studies [11,12]. https://doi.org/10.1016/j.cplett.2020.137180 Received 24 November 2019; Received in revised form 1 February 2020; Accepted 3 February 2020 ⁎ Corresponding author at: Department of Physics, University of Ioannina, Ioannina 45110, Greece. E-mail address: ideligia@uoi.gr (Y. Deligiannakis). Chemical Physics Letters 743 (2020) 137180 Available online 04 February 2020 0009-2614/ © 2020 Elsevier B.V. All rights reserved. T