Chemical Physics Letters 649 (2016) 48–52 Contents lists available at ScienceDirect Chemical Physics Letters jou rn al h om epa ge: www.elsevier.com/locate/cplett Oxidative catalytic evolution of redox- and spin-states of a Fe-phthalocyanine studied by EPR Eleni Bletsa a , Maria Solakidou a , Maria Louloudi b , Yiannis Deligiannakis a,∗ a Laboratory of Physical Chemistry of Materials & Environment, Department of Physics, University of Ioannina, GR45110 Ioannina, Greece b Laboratory of Inorganic Chemistry, Department of Chemistry, University of Ioannina, GR45110 Ioannina, Greece a r t i c l e i n f o Article history: Received 28 December 2015 In final form 14 February 2016 Available online 17 February 2016 a b s t r a c t The catalytic-oxidative evolution of the redox/spin states of a Fe-phthalocyanine (Fe-Pc) catalyst was studied by electron paramagnetic resonance spectroscopy. Under oxidative catalytic conditions, Fe-Pc may evolve via multiple redox/spin conformations. Axial ligation of imidazole, O 2 or t-Bu-OOH as oxidant, results in a complex multipath redox/spin landscape that was determined in detail herein. The high-spin conformations of Fe-Pc/imidazole evolve more slowly than the low-spin conformations. Catalytically active vs. inactive conformations were distinguished. A unified physicochemical catalytic reaction mech- anism is discussed herein based on the distinct role of the various structural, spin and redox forms. © 2016 Published by Elsevier B.V. 1. Introduction Phthalocyanines (Pcs), formally known as tetrabenzo [5,10,15,20]-tetraazaporphyrins are conjugated 18p electron systems, with structure similar to porphyrins [1]. Phthalocyanines’ structural core consists of four iso-indole units, linked angularly by four meso-N bridges, leading to a macrocyclic aromatic molecule that can coordinate Fe in planar configuration [1]. Pcs possess remarkable thermal and chemical stability [2], while electronega- tive substituents on the Pc ring enhance chemical stability i.e. in comparison to porphyrins [3,4]. Due to their extended conjugation, metallo-phthalocyanines have attracted considerable interest by industry [1,4–6] e.g. as liquid crystals [7,8], electrocatalysts [9], gas and chemical sensors [10], electrochromic and electrolumi- nescent displays [11,12], non-linear optics [13], photovoltaics [14] and recently in photodynamic therapy [15]. The synthesis of Pc is cheaper than porphyrins, thus their large-scale availability together with their chemical and thermal stability make them viable candidates for industrial oxidation catalysis [5,16,17]. Previ- ously, Fe- or Mn-phthalocyanine catalysts were reported to be less active in oxidation reactions i.e. compared to the corresponding metalloporphyrins [17–19], within only some exceptions e.g. such as the water soluble iron tetra-sulfo-phthalocyanine, reported by Sorokin et al. [16,17] activated by t-Bu-OOH. Moreover, Fe-Pcs are well known for their high affinity to molecular O 2 , therefore their ∗ Corresponding author at: Department of Physics, University of Ioannina, Ioan- nina 45110, Greece. E-mail address: ideligia@cc.uoi.gr (Y. Deligiannakis). well-known O 2 reduction capacity [20–22]. Maroie et al. [22] have demonstrated that O 2 bonding on Fe-Pc depends on the nature of Fe species (monomers, dimers and polymers) [22], i.e. polymers have lower free energy of activation for [O 2 ] - formation [20]. However, so far the effect of O 2 on the redox/catalytic evolution of Fe-Pc has not been studied. As we show herein, O 2 may play a dual role i.e. as axial ligand and as oxidant, eventually competing with other oxidants that might be used in catalytic experiments. The redox properties of Fe-Pcs are more sensitive to the type and geometry of axial ligation than the Fe-porphyrins [23]. For exam- ple, imidazole coordination to the Fe-Pcs has trans effect to the fixed geometry of Fe-Pcs [24]. The oxidation state of the iron in Fe-Pcs can vary from Fe I to Fe IV depending on the number and nature of axial ligands and ring substituent [25]. Strong axial ligands display d 5- low-spin (S = 1/2) character i.e. as in the case of Fe-porphyrins [26], while five-coordinated Fe III -Pcs may have intermediate S = 3/2 character [25,27]. Under oxidative conditions, a Pc-Fe IV -oxo state may be formed, as shown by Sorokin et al. [28]. Thus, it is antic- ipated that under oxidative conditions the versatility of the spin and redox states of Fe-Pcs could result in multiple reaction paths. Indeed, Kobayashi et al. [29,30] were the first to comment that Fe- Pcs show a variety of high Fe III (S = 5/2) and low-spin Fe III (S = 1/2) conformations however their significance in catalytic-kinetics has not been studied so far. In this context, herein we present a detail study of the kinetics of the various redox/spin states of an Fe-Pc under catalytic oxida- tive conditions. A well-defined catalytic system [Fe-Pc/imidazole] was chosen as a model catalyst, for oxidation of pentachlorophe- nol, oxidised with t-Bu-OOH. The aims of the present work were [a] to identify the redox/spin conformations of the Fe-Pc/imidazole/ http://dx.doi.org/10.1016/j.cplett.2016.02.032 0009-2614/© 2016 Published by Elsevier B.V.