Zúñiga César

Gabriel Loyola

Soledad Abarca

Carolina Ureta-Zañartu

Jose H Aliaga

Moulay Tahar Zagal

Frederic Sougrati

Walter Jaouen

Federico Orellana

Tasca

Loyola

Gabriel Abarca

Soledad Ureta-Zañartu

Carolina Aliaga

Jose H Zagal

Moulay Tahar Sougrati

Frédéric Jaouen

Walter Orellana

Federico Tasca
email: federico.tasca@usach.cl

Insights into the electronic structure of Fe penta-coordinated complexes. Spectroscopic examination and electrochemical analysis for the oxygen reduction and oxygen evolution reactions

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1.

Introduction.

With the ever-increasing demand for a less polluted atmosphere, the prices of platinum group metals (PGM) are rocketing due to the increasing demand for PGM-based catalysts [START_REF][END_REF]2 . Those catalysts are beneficial for the complete oxidation of hydrocarbons and the reduction of nitrogen oxides in the exhaust system of internal combustion engines, as well as in electrochemical devices where they facilitate key reactions for water electrolysers and fuel cells, namely the hydrogen and the oxygen evolution reactions (HER, OER) as well as the hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR). In the reduction of oxygen, the breaking of the O-O bond is a process that requires a high dissociation energy (118 kcal mol -1 ) 3 , while, in the OER the O-O bond formation is the rate-determining step 4 . The ORR can occur via indirect 2+2 or direct 4 electrons in both acidic and basic media, according to the following reactions:

In acidic media through 2+2 electrons the equation is:

+ 2 + 2 → E 0 = 0.682 V vs. SHE (1)
+ 2 + 2 → E 0 = 1.77 V vs. SHE (2)
In acidic media involving direct 4 electrons the equation is:

+ 4 + 4 → 2 E 0 = 1.23 vs. SHE (3)
In basic media, a similar process occurs to produce HO2 -through 2+2 electrons, and consecutively the reduction to it to OH -:

+ + 2 → + E 0 = -0.076 V vs. SHE (4)
+ + 2 → 3 E 0 = 0.88 V vs. SHE (5) For the direct 4 electrons mechanism the corresponding equation to produce OH -is:

+ 2 + 4
→ 4 E 0 = 0.401 vs. SHE (6) In the case of oxygen evolution reaction (OER) in alkaline media, the 4e -pathway proceeds according to the following equation:

4 → + 2 + 4 (7)
Also, OER involves multi-electron reactions where some elementary steps have an inverse correlation to the ORR, as in the following equation:

+ * → * + (8a) * + → * + 2 + (8b) * + → * + (8c) * + → + + ( 8d
)
Where * represent an active surface site.

For the ORR, the most active catalysts are based on Pt [5][6][7][8] , while for the OER the state of art catalysts are based on Ir, and/or Ru, depending on the pH, 9,10 where the most active catalyst is IrO2 11,12 . Pt was also initially studied for the OER by Rüetschi et al. in 1955 13 .

Because of the scarcity and the cost of PGMs, numerous researches have been done to replace them with more abundant metals, such as Fe or Mn, to develop non-precious metal catalysts (NPMC). In 1964, Jasinski proposed a non-precious metal macrocyclic complex as a catalyst for the ORR 14 . Since then, major improvements in the design of M-N-C catalysts with single-metal-atom sites have been achieved, leaning on pyrolytic or non-pyrolytic approaches. In particular, Fe-based catalysts have proved to be equal or even more active for the ORR than the industrial standard Pt-based catalyst in alkaline environment [15][16][17][18][19][20][21] . For the OER phosphines, and metal oxides of Co, Ni, Mn, and Fe have been proposed [22][23][24][25] . Among the best NPMC for the ORR in alkaline medium, single-atom catalysts (SAC) such as metal-phthalocyanines and metal-porphyrins (i.e. macrocycles containing metal and four pyrrolic nitrogen atoms (MN4)) belong to the most promising ones. To further increase the activity towards the ORR and the stability of well-defined MN4 sites (in particular in acid medium), two main strategies have been adopted: (i) the heat treatment of the catalyst [26][27][28][29][30][31] ; (ii) the synthesis of complexes with more positive redox potential by the addition of electron-withdrawing residues 32,33 . Although pyrolysis of MN4 leads to progress in catalytic activity towards ORR, [34][35][36][37][38] this approach has several drawbacks, such as reduced control of the electronic and structural environment of the metal center, formation of multitudinous metal centers during pyrolysis, and limited site density due to the increased propensity of forming metal particles during pyrolysis with increased metal content 39,40 .

In contrast, the exact coordination and ORR activity of MN4 macrocycles can be tuned by controlled synthetic approaches. For example, the introduction of electron-withdrawing groups to phthalocyanine ring has led to increased ORR activity 20,21,41 and clear electronic structure -activity correlations have been established for unpyrolyzed MN4 macrocycles, linking the formal redox potential of MN4 centers with the oxygen binding energy, and ORR activity, leading to well-known volcano plots 33,42,43 . Complexes with bio-inspired MN5 coordination from cytochrome c oxidase [44][45][46] have been developed to mimic the ORR active sites present in natural enzymes. The fifth nitrogen ligand, in axial position relative to the MN4 plane, can increase the redox potential and the stability of the metal center 18,[47][48][49][50][51][52][53] . Similar strategies have also been adopted to enhance the activity of the catalyst towards the OER [54][55][56] . While it has been clearly demonstrated that FeN5 coordination is superior to FeN4 coordination for the ORR in alkaline medium, it is yet unclear if the same trend is valid for the OER in the same environment.

Also, insights into the ORR and the OER at FeN5 active sites are still missing. For instance, while it is clear that during the reductive elimination bear by d 8 and OER (pH 13) and electrochemical impedance spectroscopy (EIS) have been carried out to compare our results with bifunctional catalysts reported in the literature 57 .

Finally, detailed spectroscopic studies of FePc-CNT (FeN4) and FePc-Py-CNT (FeN5) can be useful to better interpret the spectroscopic signatures of the Fe-based active sites in pyrolyzed catalysts 58,59 .

2. Experimental.

Materials and methods.

Fe(II) phthalocyanine was obtained from Porphy Chem (Dijon, France), 4-Amino pyridine, and NaNO2

were obtained from Sigma (St. Louis, USA). All aqueous solutions used Milli Q water (18.2 MΩ cm at 25ºC). Carbon nanotubes (CNT) were obtained from Cheaptubes inc (>90%) 50,60 . Dimethylformamide A modified edge-plane pyrolytic graphite disk, 5 mm of diameter (area: 0.194 cm 2 ), was used as working electrode, with the exception of the EIS experiments, where a glassy carbon (GC) disc of 0.077 cm 2 was used. In some experiments, indicated in the text, a film of commercial Pt-Vulcan 20 wt.% using 10 µL of a 1 mg mL -1 to obtain a catalyst loading of 0.05 mg cm -2 was deposited on the surface of the edge-plane pyrolytic graphite or GC disks, which were used as working electrode (electrodes were from Pine Instruments, Durham, NC, USA). In the experiments with rotating disc electrode a rotor system from PINE Instrument (Durham, NC, USA), was used.

Electrochemical processes were studied in 0.1 M KOH aqueous medium for alkaline (pH near 13) or 0.1 M H2SO4 (pH close to 1) for acidic medium. The electrochemical characterization and catalytic study were performed using an ultrapure N2 and O2 saturated atmosphere, at room temperature.

The determination of the number of electrons transferred during the ORR for each catalyst was done according to the Koutecky-Levich (K-L) equation [START_REF] Bard | Electrochemical Method. Fundamentals and Applications[END_REF] (Eq. 2), for which polarization curve at 5 mV s -1 were obtained at different rotation rates.

= + . / / / (Eq. 2)
Where jk: is the kinetic current density at a given potential (E); n: the number of transferred electrons; F: the Faraday constant (96.485 C mol -1 ); D: the O2 diffusion coefficient (1.9 × 10 -5 cm 2 s -1 ) [START_REF] Davis | [END_REF] ; ν: the kinematic viscosity of the solution (0.01 cm 2 s -1 ) 65 ; CO2: the O2 concentration (1.2 × 10 -6 mol cm -3 ) [START_REF] Davis | [END_REF] ; and ω: the electrode rotation rate (rad s -1 ).

In order to allow a better comparison among the electrodes, the rugosity factor FR, was determined based on the capacitive current density, according to the methodology reported in a previous work 66,67 = .

(Eq. 3)

Where jp, the peak current density is expressed in mA cm -2 , q1 correspond to the electrical charge density associated to the active specie process (C cm -2 ), R, F and T represent its usual meaning and v, the potential scan rate expressed in mV s -1 . 5.44 is a number that represent all the constant term considering the unities defined for Eq. 3. Thus, the parameter q1 can be determined from the slope in a j against v plot, with linear correlation passing by the origin. From the value is determined the  parameters 63 = (Eq. 4)

For Pt based electrodes the surface concentration of Pt H values were determined by extrapolation of the double-layer charging current and integrating the charge for hydrogen desorption using 0.210 mC cm -2 as the specific charge for a monolayer of adsorbed hydrogen on Pt [START_REF] Trasatti | [END_REF] Synthesis of pyridine functionalized CNT. Sodium nitrite (5 g) was dissolved into distilled water (10 mL) and cooled to 0°C in an ice bath. 7 g of 4-amino pyridine (Py) were carefully dissolved in 5 mL of 4M HCl solution and added carefully to the sodium nitrite solution, letting rest 30 min on ice. The carbon nanotubes (CNT) (0.1 g) were then added to the nitrite amino pyridine mixture and left at 0 °C for three hours, after which the solution was left stirring overnight at room temperature. The functionalized CNT (CNT-Py) are obtained by filtration and washed repeatedly with Millipore water, drying at 80 °C overnight.

Formation of FePc-Py-CNT. A sample of the functionalized CNT-Py (100 mg) were dispersed in 50 mL of DMF in a 200 mL 3-necked round-bottomed flask through 15 minutes sonication. To this, FePc (50 mg) was then added with stirring and nitrogen gas bubbling. The mixtures were then connected to a condenser apparatus and refluxed at approx. 120 °C for three hours with constant N2 flow. The resulting mixture was then filtered and repeatedly washed with Millipore water and dried overnight at 80 °C. After that, the ink preparation was carried out, dispersing 1 mg of the final mixture (FePc-Py-CNT) in 1 ml of 1:4 water-isopropanol (v/v ratio) to sonicate it for 30 minutes finally.

Formation of FePc-CNT. The FePc-CNT was obtained according to the previously reported protocol 21,49 . Briefly, CNT and FePc at 1:1 weight ratio was dispersed in 1:4 water-isopropanol (v/v ratio), then the dispersion was sonicated for 30 minutes. This ink was used to modify the electrode surface. Each ink (FePc-CNT and FePc-Py-CNT) was stored in a closed Eppendorf at room temperature, showing stability in this environmental condition after electrochemical studies.

Electrode preparation. The electrode was prepared by drop coating in two steps. First, 5 L of the ink is drop cast on the electrode surface, and it was verified that it spread evenly before the operation is repeated once, resulting therefore in total of 10 L of dispersion drop cast on the GC electrode. Finally, the ink was dried by a flux of ultrapure N2.

Cyclic voltammetry (CV). Freshly prepared electrodes were characterized by CV in 0.1M KOH in order to determine its roughness factor and Fe(II)/Fe(III) surface concentration.

Electrochemical impedance spectroscopy (EIS). EIS measurements were carried out at constant potential in a one-compartment electrolytic cell with three-electrode configuration, in 0.1 M KOH electrolyte with a Luggin capillary for the reference electrode (inside a Luggin capillary closed with a Pt wire and filled with the supporting electrolyte) and a high area Pt helix or a carbon rode as counter electrode, using an Autolab PGstat 128N equipped with FRA module. A 5 mV sinusoidal modulation in the 1 kHz-10 mHz frequency range, with 10 frequencies per decade. The impedance spectra analysis was performed with the NOVA 2.1 software, including the Kramers-Kronig transformation (KKt) analyses. The experiments take about 17 minutes, the temperature was controlled at the beginning and the end of the measurement, observing an increase of 1°C. The measurements were made in triplicate.

Ex situ physicochemical characterization

Mössbauer Spectroscopy analysis. The 57 Fe Mossbauer spectra were obtained at room temperature (295 K) using 57 Co: Rh source embedded in a rhodium matrix ( 57 Co (Rh)). Triangular velocity waveform and a gas-filled proportional counter were used in order to detect the γ rays. Isomer shift was determined relative to α-Fe (20 μm, at 295 K). Samples were prepared by pressing ca. 100 mg of powder inside a pill shape (2 mm thick and 15 mm of diameter). The spectra were fitted with an appropriate combination of Lorentzian profiles representing quadrupole doublets and magnetic sextets by the least-squares methods using the program PC-Mos II [START_REF] Gro | PC-Mos II Version 1.0 Manual and Program Documentation[END_REF] .

X-Ray photoelectron spectroscopy (XPS). XPS measurements were carried out at the LNLS (Brazilian Synchrotron Light Laboratory) at the SXS beamline end-station. The spectra were collected using an InSb (111) double crystal monochromator at a fixed photon energy of 1840 eV. The O2 treatment at room temperature (RT) was accomplished in the preparation chamber using a pressure of 20 psi of 5% O2 + 95% He during 1 h. [START_REF] Pizarro | [END_REF] The sample was investigated using the survey scan, as well a with narrow scans in the Fe 2p, O 1s, and C 1s regions. The hemispherical electron analyser (PHOIBOS HSA500 150 R6) was used and set at a pass energy of 20 eV, and the energy step was 0.1 eV, with an acquisition time of 100 ms/point. The overall resolution was around 0.3 eV. The base pressure used inside the chamber was around 5.0x10 -9 mbar. The monochromator photon energy calibration was done at the Si K edge (1839 eV). The peaks were adjusted with an asymmetric Gaussian-Lorentzian sum function (30 % Lorentzian contribution). Also, all peaks were adjusted using a Shirley-type background. In the fitting procedure, the full width and half maximum (FWHM) value was kept constant over the adjustment procedure. The fitting at the Fe 2p electronic level was performed considering the well-known satellite peaks due to the shakeup effect associated with Fe. 72 Electron paramagnetic resonance spectroscopy analysis (EPR). EPR measurements were carried out for four different powder samples (a) FePc-Py-CNT, (b) FePc-CNT, (c) Py-CNT, and (d) CNT. The spectra were obtained with Bruker EMX-1572 spectrometer at 298K, working at 9.39 GHz (X-band).

Computational analysis.

Spin-polarized density functional theory (DFT) calculations were performed with the Vienna ab-initio simulation package (VASP) 73 with a similar set-up as reported by Cao et al 18 . The semi-local exchangecorrelation functional in the Perdew-Burke-Ernzerhof form was used 74 , including dispersion corrections for the van der Waals interaction, as proposed by Grimme (DFT-D2) 75 . A plane wave cut-off energy of 400 eV and the projector augmented-wave method for the core-valence interaction were employed 76 . A metallic single-walled carbon nanotube with (8,8) chiral indices, 11 nm in diameter and 288 total atoms, was simulated with a large supercell with periodic boundary conditions along the CNT axis. We consider a vacuum region of 20 Å to ensure negligible interaction between CNT lateral images. Due to the large size of the supercell, only the point for the Brillouin-zone sampling was considered.

3. Results and Discussion.

Physicochemical characterization.

EPR Analysis measurements. The EPR spectrum was determined for FePc-Py-CNT and FePc-CNT samples (Figure 1a). The g-factors determined from the spectra are giso: 2.032 and 2.072 for FePc-CNT and FePc-Py-CNT, respectively, where the EPR signal are related to the odd spin of Fe(III) centres in FePc exposed to O2, while Fe(II) centres would be EPR silent. The increase in g-factor for Fe(III) centres in FePc-Py-CNT vs. FePc-CNT could be associated with the possible electron withdrawing character of the Py axial ligand 18 . Similar increase in g-factor has been observed due to the presence of halogen groups in the periphery of the phthalocyanine ring 21 . A high spin delocalization in CNT could also be responsible for the high value of giso (i.e. giso: 2.072) 77 . The EPR spectra of Py-CNT and CNT were also recorded and are shown in the supporting information file (Figure S1) and the g-factors are giso: 2.083 and 2.194, respectively. For these samples, the EPR signal is specifically associated with surface defects 78,79 .

According to these EPR data, the presence of Py axial ligand in FePc-Py-CNT and its electron pulling character are corroborated 21,80 . 57 Fe Mössbauer Spectroscopy This technique can give information about the spin state and oxidation state of the Fe metal centre. In Fig. 1 (b) the 57 Fe Mössbauer spectrum is presented for FePc-Py-CNT catalyst. The spectrum shows three deconvoluted spectral components whose fitted isomeric shift (δiso), quadrupole splitting (∆EQ), and the full width at half maximum (FWHM) values are summarized in Table S1. First, it is clear that none of the three doublets can be assigned to the β-Fe(II)Pc, which has a distinct ∆EQ value of 2.6 mm s -1 81,82 , much higher than observed for any of the three fitted doublets with ∆EQ values ranging from 0.39 up to 1.04 mm s -1 (Table S1). Also, none of the three doublets can be assigned to Fe(II)Pc coordinated with two axial Py ligands, for which ∆EQ is also high, at 1.94 mm•s -1 . 81 In contrast, the doublet Db 82 has spectral parameters (δiso 0.22 mm s -1 , ∆EQ 1.04 mm s -1 ) that are similar to those reported for a FePc monolayer supported on Vulcan carbon black (δiso 0.37 mm s -1 , ∆EQ 0.91 mm s - 1 , reported in ref. 82,83 for the sample prepared via vacuum evaporation of FePc and deposition on carbon, without any annealing). The component Db might therefore be assigned to FePc molecules adsorbed on CNT, at locations where the surface was free of Py groups.

The lack of previous 57 Fe Mössbauer studies on well-defined FePc molecules axially grafted to carbon surfaces prevents a straightforward assignment of the other two spectral components, Da and Dc. Their lower δiso and ∆EQ values with respect to that for FePc is in general agreement with the trends reported for porphyrins with axial Py ligand relative to the same porphyrins without axial Py ligand. 84 For example, a decrease in ∆EQ from 1.95 to 0.67 mm/s was reported after addition of a Py axial ligand to Feoctaethylporphyrin. 84 A recent study from Cheng et. al. reported the axial ligation of FePc by N-groups from a N-doped carbon support. 85 The 57 Fe Mössbauer spectrum of that catalyst showed two doublets, the first one with (δiso, ∆EQ) of (0.45, 0.63 mm/s). That doublet has Mössbauer parameters similar to doublet Dc in the present work (δiso and ∆EQ of 0.39 and 0.54 mm/s, respectively, Table S1), and was assigned to an X-Fe(III)N4-Y octahedral coordination, with X, Y being axial ligands such as O or N. We therefore tentatively assign Dc in the present work to O-Fe(III)N4-N coordination, where N is from Py and O from adsorbed O2. The presence of adsorbed O2 on top of ORR-active Fe centres is expected when Mössbauer spectra are acquired in ambient air. This assignment is also supported by a study on oxygenated FePc crystals. Kuzmann et al. 86 showed that β-FePc crystals can adsorb O2 at near ambient temperature, and that this process modified the Mössbauer spectra with a strong decrease in the intensity of the doublet specific for β-Fe(II)Pc and the appearance of four new doublets, assigned to the adsorption of one or two O2 molecules per FeN4 centre, forming µ-peroxo or µ-oxo bridges between stacked FePc molecules. 86 Two new doublets had similar δiso of ca 0.4 mm/s at RT but different ∆EQ values (0.59 and 0.96 mm/s, for sample C in that work) while two other doublets had also similar δiso of ca 0.12 mm/s at RT but again different ∆EQ values (0.46 and 0.84 mm/s, for sample C in that work). These four doublet components were described as being consistent with an Fe(III) high-spin configuration, in this case an O-Fe(III)N4-O octahedral coordination. In particular, the Mössbauer parameters of species III in Ref. 86 S2. The differences in binding energies detected for Fe 2p3/2 peak is related to the alterations in the electronic structure, where the Fe centre can perform a rearrangement of the D4h local symmetry (Fe surrounded by four-coordinated nitrogen ligands) through a non-planar configuration supported by XPS Fe 2p core level signal 18 . Because of the presence of a site with five coordinated nitrogen, the Py can modulate the electronic environment of Fe 2+ /Fe 3 showing a shift towards high-binding energy values 50 .

The Fe 2+ /Fe 3+ ratio, for FePc-CNT and FePc-Py-CNT before exposure to O2, was determined by iron contribution summarized in Table S3, and S4. The contribution is related to the formal potential of the catalyst which is associated to the catalytic performance 21,87 . The XPS Fe 2p core level spectra show the peak position at 708.5 eV for the FePc-CNT composite (Figure 2 (a)). On the other hand, the Fe 2p3/2 core level spectra in FePc-Py-CNT is upshifted to 708.7 eV (Fig. 2 (b)). In this sense, the energy shift of 0.2 eV can be consider significant. Similar to what we reported before [START_REF] Pizarro | [END_REF] after O2 treatment, there is no significant energy shift for Fe 2p3/2 core level spectrum, indicating that no stable Fe-O2 adducts are formed in the gas phase. These results are expected since these molecules are ORR catalysts, not O2 carriers, and they should not bind O2 too strongly. In the mentioned case [START_REF] Pizarro | [END_REF] , we reported a Fe 2p3/2 shift after O2 exposure between 0.1 to 0.3 eV. Then, as Fe atom is coordinated with the Py group at the axial position in FePc-Py-CNT, the extra ligand leads to the change of electron density on Fe, indicating a higher oxidation state of the Fe than in the FePc-CNT. These results are in agreement with the values of the formal potentials observed (Table 1) 88 . Moreover, the presence of the pyridinium ligand has a strong impact on the core ionization energies of Fe (Fe 2p photoelectron peaks), associated with the ligand-to metal charge transfer process. The corresponding binding energies behaviour could be understood from back-bonding between Fe metal centre with strong sigma donation, from lonely nitrogen electron pairs of pyridine to half-filled dz 2 orbital, and consecutively metal-to-ligand charge transfer via weak  orbital to * unoccupied orbital in Py axial ligand, leading to a Fe 3+ character of the metallic centre [START_REF]IntechOpen[END_REF][START_REF] Lavallee | [END_REF][91] . In Figure 2 (c), and (d) are shown the XPS spectra for Fe 2p3/2 in FePc-CNT and FePc-Py-CNT after exposure to O2, where it is possible to observe an increasing Fe 2+ /Fe 3+ ratio for both samples (Table S2). As expected, a significant energy shift for the Fe 2p3/2 core level spectrum of FePc-Py-CNT was observed, suggesting that stable Fe-O2 adducts are formed during the O2 exposure. Indeed, after O2 exposure, an initial ratio of 0.33 was raised to 0.57 for FePc-Py-CNT (as for the Fe 2+ /Fe 3+ ratio). Therefore, an effective rearrangement of the dz 2 orbital, due to a change of local symmetry form of the five-coordination system to a hexacoordinate one, is observed for FePc-Py-CNT.

On the other hand, a Fe 0 contribution was included in Table S4 for the surface of the FePc-Py-CNT catalyst, only after the exposure to O2, because in the fitting procedure, the FWHM value was kept constant over the adjustment procedure. The fitting at the Fe 2p electronic level was performed considering the well-known satellite peaks due to the shakeup effect associated with Fe. Based on these considerations, we observed for FePc-Py-CNT after O2 exposure an increase of Fe 2+ /Fe 3+ showing a shift towards high-binding energy values. These results suggest that the Py can modulate the electronic environment of the Fe center, indicating a "metallic character" after O2 treatment for the Fe center.

Oxygen reduction reaction (ORR).

Cyclic voltammetry at 100 mV s -1 for FePc-CNT and FePc-Py-CNT modified electrodes in deaerated acidic and basic media are shown in Figure 3, black solid line. As can be observed, a well-defined faradic process appears for two reversible formal potentials Fe(II)/(I) and Fe(III)/(II) at more negative and positive values, respectively, according to literaturere 41,50,92 , where the focused process is the last one (see grey circles). The surface concentration () was obtained from cyclic voltammetry at different scan rates and considering the geometric electrode area (0.194 cm -2 ), according to Eq. ( 3) and ( 4). The  values are summarised in Table 1. The values range between 10 -9 to 10 -10 mol cm -2 , in agreement with the previous results reported for modified CNT 41,87,93 . The formal potential (E 0 ') of Fe(III/II) couple in FePc-CNT modified electrode are 0.657 and 0.801 V vs. RHE in acid and basic media, respectively, where the formal potential shifts to negative values in acidic media is associated with nitrogen protonation, similar to what happens with pyrolyzed catalysts 94 . Meanwhile for FePc-Py-CNT, the Fe(III/II) redox couple appears at 0.699 and 0.871 V vs. RHE in pH 1 and 13, respectively (Table 1). The formal potential Fe(III/II) redox couple is positively shifted by 42 mV in acid and 70 mV in basic electrolyte with respect to FePc-CNT, evidencing that the presence of pyridine in FePc-Py-CNT has an electron withdrawing effect as axial ligand, analogous to peripheral electron-withdrawing groups in phthalocyanines 33,87 . This is also supported with the changes observed in the Mossbauer and XPS spectra. This phenomenon, that has also been reported in previous works 18,50 , would act favouring a higher oxidation iron state due to inductive effect, which decreases the d electron density at the iron central metal 95 .

Linear scan voltammetry (LSV) was performed at 5 mV s -1 in a saturated O2 atmosphere (red curve in Fig. 3, using FePc-CNT and FePc-Py-CNT modified electrodes in alkaline and acidic medium). Also, a graphite electrode modified with a film of Pt/C 20 wt% catalyst has been tested under the same experimental conditions in both media for comparison (grey dashed line in Fig. 3). In alkaline media (Figure 3 (a) and (b)), the current density for the Pt/C 20% catalysts is lower than the theoretical value of -6.0 mAcm -2 . This is in accordance with values reported in the literature 96,97 , where the interference of an outer-sphere mechanism at pHs lower than 10 is considered as a consequence of the OH -ions adsorbed at the electrocatalytic surface 98,99 . In Figure 3 (b) we can observe that with FePc-Py-CNT at pH 13 the onset potential for the ORR is shifted to a more positive value than platinum based catalyst. In contrast, in Fig. positive values of the formal potential of Fe (III)/(II). This correlation has been proved also for some pyrolyzed NPMC catalysts. 34,100 The number of the electrons transferred has been estimated by the K-L equation ( 2) (Inset Fig. 3 (a-d)).

The results evidence that using FePc-CNT modified electrode, 2.6 electrons are transferred in acid media, indicating that O2 is reduced preferably by a 2 e -mechanism yielding mostly H2O2. The presence of the Py axial ligand improves the catalytic process, and O2 is almost completely reduced to H2O with a transfer of 3.6 e -. Both FePc-CNT and FePc-Py-CNT perform very well in basic media and 3.8 and 4.0 e - are transferred, respectively, reducing molecular oxygen to OH -, mainly. The Tafel slopes were determined according Butler-Volmer equation [START_REF] Bard | Electrochemical Method. Fundamentals and Applications[END_REF] from each polarization curves at 1600 rpm, with limiting current density correction [START_REF] Bard | Electrochemical Method. Fundamentals and Applications[END_REF]101 . Tafel slopes are summarized in Table 1. At pH 1, values of -0.086 and -0.097 V dec -1 were obtained for FePc-CNT and FePc-Py-CNT, respectively, suggesting that two parallel mechanisms are participating for ORR with different rate-determining step 33,50,98 .

In comparison, for basic media, FePc-CNT shows a Tafel slope of -0.044 V dec -1 , indicating that the ratedetermining step is the second-electron transfer which is preceded by a fast oxygen adsorption chemical step, and the fast first one-electron transfer step, 87,102,103 similar to Pt/C 20 wt% catalyst used to compare our measurement in both pH values. FePc-Py-CNT at pH 13 shows the lowest Tafel value, close to -0.022 V dec -1 , which is related to O2 -protonation as a rate-determining step influenced by Py axial coordination 18,104 . All the electrochemical parameters are summarized in Table 1. The turnover frequency values (TOF) (Table 1) were calculated at 0.8 V vs. RHE from the O2 reduction polarization curve and the respective electrode coverage (), (Eq. 4) 21 for the various electrodes.

The obtained TOF value for Pt/C 20% in alkaline media is lower than in acid media (0.85 and 3.85 respectively) because the surface concentration of the active sites (Γ, mol cm -2 ) in acid is lower than in basic media according to Table 1 and the TOF value is inversely proportional to the surface concentration of the active sites. In alkali, the TOF values range from 1.708 to 1.01, where the Py axial ligand in alkaline media allows for TOF numbers higher than the TOF value of Pt/C 20% (0.85 at 0.8 V vs. RHE).

Indeed, the low electron density on the Fe metal centre when in the presence of the axial coordination is strongly evidenced in acidic media, where the TOF value was one order magnitude higher for FePc-Py-CNT (0.02) concerning FePc-CNT (0.00).

Oxygen evolution reaction (OER).

The bifunctionality of redox catalysts for the ORR and the OER are very important because of the possibility to use those catalysts in metal-air batteries. The bifunctionality of FePc-Py-CNT has been tested in an N2 saturated atmosphere for the OER using 0.1 M KOH solutions and polarization curves at 5 mV s -1 . For those studies, a crucial parameter is the potential difference (ΔΕ) at which the ORR and the OER are performed, according to the following equation: 105

Δ = [ ] -[ / ] (Eq. 5)
Where Ej10 is the potential for the OER when the current achieves a value of 10 mA cm -2 and E1/2 is the half-wave potential for the ORR polarization curve.

The ΔΕ parameter allows us to compare the performance with several kinds of catalysts reported in the literature. Figure 4 (a) shows both polarization curves for ORR (blue line) and OER (red line) with ΔΕ value of 0.680 V for FePc-Py-CNT, which can be compared with the values reported in the literature for precious metals group (PMG) and summarized in Table S5. Furthermore, high bifunctional performance for FePc-Py-CNT has been observed even when it is compared to pyrolyzed catalysts, suggesting that pyridine axial coordination in FePc catalyst could be proposed as a convenient approach to develop bifunctional catalysts. The Tafel slope for OER on FePc-Py-CNT was 0.214 V dec -1 (Fig. 4 (b)), which well agrees with reported values for the OER [105][106][107][108] . According to this value, a four-electron pathway is probably involved during the OER to produce O2 in alkaline media, favouring the catalytic process according to the reaction (7) 55,[109][110][111] , where the active site is the Fe metal, with a predominant presence of Fe 3+ species, to perform the direct oxidation of adsorbed OH -species 112,113 . A range of Tafel values reported in the literature goes from 650 to 50 mV dec -1 105,114,115 suggesting different OER mechanisms 108 .

4 → + 2 + 4 (7)

Electrochemical impedance spectroscopy (EIS).

As pointed out in the Experimental Section the EIS data were obtained using a GC disc electrode because the experiments showed better reproducibility, probably due to a better current distribution. It is known that with too rough electrodes it is difficult to obtain good impedance measurements. Three working electrodes were used in order to separate the contributions of each component to the global process. This implies that the EIS data were obtained for (a) CNT/GC, (b) Py-CNT/GC, and (c) FePc-Py-CNT/GC electrodes, in 0.1 M KOH. The freshly prepared electrodes were stabilized by 10 CVs at 0.02 V s -1 in deaerated 0.1 M KOH. An example of the stabilized CV is plotted in Figure S2 for each kind of GC modified electrode, where is evidenced a large difference in the capacitance between the different electrodes (double layer region in the CV in Figure S2), fact that should be corroborated with the EIS measures. Roughness factor (FR) were obtained for all electrodes (see Tables S6 for the respective values). As was expected, independent of the support electrode, the CV with FePc-Py-CNT/GC electrode (Fig. S2, blue line) shows the same two couples of peak current associated to de Fe(II/I) and Fe(III/II) processes. In consequence, the coating by Fe-based species (Fe) were determined in the same way as described before, using the Eqs. ( 3) and ( 4). At higher applied potential than peak 2a, the OER is starting.

In O2 saturated electrolyte, the CV shows the characteristic peak associated to the ORR (Figure S3), which occurs with lower overvoltage using FePc-Py-CNT/GC electrode and it is inhibited at Py-CNT/GC surface. In consequence, in principle impedance experiments for ORR were developed at two constant potentials, 0.89 and 0.79 V vs. RHE in deaerated and oxygen saturated solution. In Fig. S4 are shown as example the Nyquist plots obtained for FePc-Py-CNT/GC electrode. As can be seen, the curves are complex, and do not follow a unique equivalent circuit pattern during the process. No notable differences are observed in the Nyquist plots of the electrodes modified with FePc-Py-CNT/GC at these two potentials. In view, for the ORR (open symbols in Fig. S4) a typical Randles, Ru(RFCdl) circuit was used to determine the characteristic parameters, using the Fit and Simulation tool of Nova 2.2 software, considering valid the results with error lesser than 10%. Ru represent the non-compensated resistances, that at least include the solution resistance Rs and any other resistance to charge transport that may be generated by the film deposited on the electrode surface. RF correspond to the faradaic resistance associated to the electrochemical reaction and Cdl would be associated to the charge of the double layer.

In deaerated solution (closed symbol in Fig. S4), purely capacitive behaviour, a RuCdl equivalent circuit was used to fit. The results are given in Table S6. 3), whereas the electrode modified with Py-CNT/GC show the worst system for the electron transfer process since the charge transfer resistance is very high. See the values in Table S6. The Nyquist plot is more sensible a low frequencies, evidencing the possibility of a diffusion control using the FePc-Py-CNT/GC.

Respect to Table S6, there are an increase in the double layer capacitance at 0.88 V vs. RHE for ORR process using CNT/GC and FePc-Py-CNT/GC electrodes, which could be attributed to adsorption of reagents or intermediates of the reduction process favoured by Py, which blocks the reaction if the overpotential is increased by 0.1 V (E = 0.79 V vs. RHE). The system behaves as a capacitor at low frequencies. The Py-CNT/GC electrode shows a Phase angle that tends to -90°. Whereas for CNT/GC and FePc-Py-CNT/GC electrodes, the maximum value for phase angle is very similar, close to -68.8°, far of -45° expected for FePc-Py-CNT/GC electrode for a diffusion-controlled process, showing a complex electrochemical process.

EIS measurements were also carried out for the OER. In order to determine de potential region of interest, CVs increasing the positive limit were obtained at 20 mV s -1 in deaerated 0.1 M KOH using FePc-Py-CNT/GC electrode (Fig. S5). This Figure reflects for one side the stability of the electrode at higher applied potential and by the other, the effective increase in current density at not so high potential. The stability of these electrodes was determined evaluating the electrical charge Q, against potential, E, for repetitive potential cycles and after the EIS experiments. The plot shows a practically closed oval, evidencing the excellent reversibility of the process (not shown). In consequence, Nyquist plot at five potential values, between 0.78 and 1.43 V, were obtained in deaerated 0.1M KOH using FePc-Py-CNT/GC electrode (Fig. S6). As was expected the Faraday resistance decreases with the potential increases. The values are given in Table S7. Note that when the electrode surface is at 0.78 V vs. RHE Fe in FePc-Py-CNT is as Fe(II), and at 1.18 V vs. RHE and higher, Fe atoms are as Fe(III), and 1.38 V vs.

RHE potential is close to the Eonset,.

Call the attention that the Ru, the uncompensated resistance that is an intrinsic characteristic of the electrolyte and electrode material are near one tenth of those observed for ORR, which is probably a consequence of the reactive species being OH -, which must already be adsorbed at a much less positive potential. This agrees with the increase in double layer capacity near 10 times. However, the Ru values are higher at potentials where the OER reaction occurs, in agreement with an increase in the resistance to ionic transport by the film.

Returning to Fig. 5, in order to separate the different contributions to the impedance measurements for the OER, in Fig. 5 (b-1) are plotted the Nyquist plot for electrodes modified with CNT/GC, Py-CNT/GC, and FePc-Py-CNT/GC at 1.38 V. Again, Py-CNT/GC electrode appears highly resistive, whereas FePc-Py-CNT/GC electrode is somewhat less resistive than electrode CNT/GC (see the inset in Fig. 5b1). The impedance fitted parameters are tabulated in Table S8.

According to Table S8, variation in the values for the uncompensated resistance Ru with the components of the electrode agree with our previous supposition that this term does not only correspond to the resistance of the solution but also has a factor dependent on the ionic conductivity (mass transport) of the film deposited on the electrode surface. The Ru is more significant for Py-CNT/GC, despite it having the lowest roughness. The FePc-Py-CNT/GC shows the higher double layer capacity, evidencing a better disposition to adsorb the reaction components, similar to previously observed for ORR in Table S6.

Therefore, FePc-Py-CNT/GC exposes higher active site compare Py-CNT/GC and CNT/GC, respectively 116 , in total agreement with the explanations given in the first part of the experimental results of this work.

In conclusion, the impedance study, well agrees with a bifunctional behaviour in the FePc-Py-CNT/GC electrodes due to the fundamental role of the Fe(III) in the FePc-Py structure, the electrode activity increases by the increases in the OH -adsorption promoted by CNT.

3.5. Computational analysis.

DFT calculations were performed for FePc interacting with a single-walled CNT of 1.1 Å in diameter through physisorption (FePc-CNT), and chemisorption by an axially-coordinated pyridine (FePc-Py-CNT). Graphene was used to estimate the effect of the CNT curvature because experiments were performed with a larger CNT of 2 Å in diameter.

Figure 6 shows the FePc-CNT structure in the equilibrium geometry. We find that FePc is strongly attached on the CNT with binding energy of 1.692 eV, preserving its magnetic moment after adsorption (m=1.98 μB). The shortest distance between the CNT surface and the FePc molecule is found of 2.95 Å.

For the case of FePc on graphene (FePc-G), the FePc binding energy is even larger, -2.323 eV, and the distance between the graphene plane and FePc is found of 3.526 Å. Here, the FePc magnetic moment is also preserved (m=2.00 μB). Therefore, we can conclude that the FePc curvature, as induced by the interaction with the CNT, slightly alters its spin state. Table 2 shows results for the total energies of FePc physisorbed on the CNT and graphene for the allowed spin states, before and after the O2 adsorption. The four Fe-N bond distances are almost the same (1.93 Å), indicating the coordination Fe-N4 remained centrosymmetric. Once O2 is adsorbed on the Fe centre in the most stable end-on configuration, the magnetic moment changes to m=0 μB, suggesting an antiparallel coupling with the O2 molecule.

However, the m=2 μB spin state is very close in energy, about 0.03 eV higher, which indicates that both spin states are equally probable. After the O2 adsorption, the Fe atom moves out of the macrocycle plane about 0.2 Å, while two Fe-N bond distances increase, showing an asymmetric relaxation (1.93 and 1.95 Å, see Table S9). Also, the O-O bond distance increases by about 3.6% relative to the free O2 molecule.

The O2 binding energy is calculated to be of -0.809 eV. Figure 7 shows the equilibrium geometries of FePc-Py-CNT before and after the O2 adsorption. The FePc macrocycle also preserves its magnetic moment in the most stable state (m=2 μB). However, the spin states m=1 and 3 μB are close in energy, 0.04 and 0.03 eV higher, respectively. Therefore, three different spin configurations are equally probable. The four Fe-N bond distances are almost the same (1.94 Å),

showing a symmetric configuration. After the O2 on the Fe centre in the end-on configuration, the most stable spin state has a magnetic moment m=1 μB, follow closely by the spin states m=0 and 2 μB, with a difference in energy of 0.03 eV. Therefore, the O2-FePc-Py-CNT system (Fig. 7, and Table 3) might be found in different spin configurations (m=0, 1, 2 μB). Similarly, to the O2-FePc-CNT system, the Fe atom moves out of the macrocycle plane about 0.2 after the O2 adsorption, while two Fe-N bond distances increase, showing an asymmetric relaxation (1.94 and 1.96 Å, see Table S10). Also, the O-O bond distance increased about 4.1% with respect to the free O2 molecule. The O2 binding energy is calculated to be -0.758 eV. Comparing with O2-FePc-CNT, in the O2-FePc-Py-CNT system the Fe-O2 adduct is 0.051 eV less strong, while the O-O bond distance is slightly longer, suggesting better ORR catalytic activity for O2-FePc-Py-CNT.

Neya reported 117 that a more contracted coordination hole of Fe-N4 complexes generates an increase in energy splitting between dx 2y 2 and dz 2 where the last one orbital (dz 2 ) is stabilized, favouring the Fe-O2 binding strength. In this sense, we suggest that Py axial ligand in FePc-Py-CNT could slightly increase the dz 2 (due to more distance between N and Fe atoms, or a more expanded coordination hole of MN4) energy orbital in contrast to FePc-CNT, consecutively Fe(II)-O2 interaction is less intense, in agreement with theoretical studies.

Table 3. Total energy of the FePc-Py-CNT and FePc-Py-G with and without adsorbed O2 for the allowed spin states. E0 is the lowest energy state.

Finally, we obtained two main correlations (see graphical abstract) for FePc-CNT and FePc-Py-CNT when the catalytic activity where evaluated as log jk at the constant electrode potential value of 0.8 V vs.

the binding energy (M-O2) or vs. the formal potential E0'Fe(III)/(II), as proposed by several authors. 33,41,87,118 Both correlations agree with the previous reports 21,119,125 indicating that Py axial ligands directly affect the iron local electron density, changing the binding energy of the active site with O2 and modulating the catalytic activity for the ORR. In the ORR, the catalyst act with the O2 molecule with an effective rearrangement of the dz 2 orbital, due to a change of local symmetry form of the five-coordination system to a an hexacoordinate one, decreasing the energy associated with the ORR process. On the other hand, impedance measurements indicate that this catalyst has a lower resistance to the charge transfer process, which also favours the mass transfer through the film, necessary to maintain electroneutrality in it during the ORR process. and without the presence of molecular oxygen, and using blue, purple, red, and green colour for nitrogen, iron, oxygen, and carbon atoms respectively.

  and d 10 complexes, a transition occurs from octahedral coordination geometry (M(IV)M(II), M=Pd, Pt or M(III)M(I), M= Rh, Ir) or from a square planar geometry to a linear/bent, it is not clear what happens with FeN5 complexes. To obtain insights into the electronic structure of Fe centres in FeN5 sites, we coordinated Fe phthalocyanine (FePc)to pyridine-(Py) modified CNT to obtain FePc-Py-CNT, and we studied the formed complex with57 Fe Mössbauer spectroscopy, synchrotron X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR), and DFT calculations. Along with this, catalytic performance for ORR (pH 1 and 13)

(

  DMF) and HCl were from Merck Inc. A typical three electrodes electrochemical cell was used to perform electrochemical measurements. Graphite bars were used as a counter electrode (supplied by Sigma-Aldrich) while an Ag/AgClsat was used as a reference electrode. The potentials reported in this work are given against the reversible hydrogen electrode (RHE), and the change of scale from Ag/AgCl was done based on the Nernst equation61,62 :

  match well those of Dc in the present work, while the low δiso and low ∆EQ values of species V match well those for Da.Based on these earlier works on FePc covalently attached to CNT or oxygenated FePc crystals, we tentatively assign both doublets Da and Dc to octahedral O-Fe(III)Pc-N coordination, with O from adsorbed O2, and the fifth N ligand coming from Py as a second axial ligand. However, one must keep in mind that the fitted δiso and ∆EQ values of doublet Da and/or Dc could also correspond to an iron centre in +II oxidation state and in low spin state. The existence of Fe(II)Pc state in ambient air with the present FePc-Py-CNT sample might be possible if some Fe centres are not accessible by air, e.g. via the stacking of two FePc molecules such as in β-FePc crystals, with one FePc being attached to CNT via a Py axial ligand (N-Fe(II)Pc-N coordination). Anyway, the presence of Fe(II) in the low spin state is not observed in XPS analysis (see below).X-Ray Photoelectron spectroscopy (XPS). High resolution XPS analysis for FePc-CNT and FePc-Py-CNT before exposure to O2 (Fig.2(a) and (b)), show a spin-orbit doublet of Fe 2p3/2 at 708.5 eV and 708.7 eV, respectively. The broad Fe 2p3/2 peak indicates the presence of Fe in different valence states according to the values summarized in Table

  3 (d), an increase in the overpotential for the ORR is observed at pH 1, if compared to the Pt catalyst, being this difference close to 110 mV. We must emphasize that the best catalytic performance was obtained in basic solution. In fact, FePc-Py-CNT at pH 13 (Figure3(b)) exhibits ∆Eonset of 38 mV and ∆E1/2 of 85 mV less negative than Pt/C 20 wt% commercial catalyst. This fact corroborates the notable influence of axial Py coordination, indicating a more efficient electronic configuration of the metal centre for the ORR. It is possible to associate the increment in the catalytic activity at pH 13 with a shift to more

Figure 5 (

 5 Figure 5 (a) shows in a comparative way the impedance results for the 3 types of electrodes studied in O2

  and FePc-Py-CNT catalysts were investigated for ORR in both acid and basic medium, whereas the OER was studied only in alkaline media. In both media, FePc-Py-CNT electrode performed as the best catalyst compared to FePc-CNT electrode for ORR, with a potential difference (∆E = Ej 10 OER -E1/2ORR) as bifunctional catalysts of ∆E =0.680 V, which well compares with pyrolyzed catalysts. The higher ORR and OER activity for FePc-Py-CNT vs FePc-CNT could be attributed to different factors which work synergically, as: (a) positive formal potential shift with respect to FePc-CNT due to the electron withdrawing effect of the Py axial ligand, (b) geometrical change in iron coordination hole size which modified dz 2 energy level, decreasing the strength of the Fe-O2 interaction for the FePc-Py-CNT catalyst, and (c) the 3dz 2 orbital electron availability. The DFT results also indicated that the Py axial ligand modified the Fe-N distance in the FeN4 plane. Consequently, an increase of 3dz 2 orbital energy level simultaneously decreases the strong Fe-O2 adduct interaction, improving the catalytic activity for ORR, in agreement with the ex-situ characterization of the catalyst by EPR, Mössbauer and XPS which indicate that there is a structural transformation without breaking the FePc molecule, where Py is in an axial location with respect to FePc, which generates an electron pulling character.

  78B. Corzilius, K. P. Dinse and K. Hata, Phys. Chem. Chem. Phys., 2007, 9, 6063-6072.

FiguresFigure 1 .Figure 2 .Figure 3 .Figure 5 .

 1235 Figures

Figure 6 .

 6 Figure 6. Image of calculated formation of FePc-CNT directly bound to a Carbon Nanotube, with blue,

Figure 7 :

 7 Figure 7: Computed structure of FePc bound to carbon nanotubes through amino pyridine linkages, with

Table 1 .

 1 ElecElectrochemical parameter for FePc-CNT, Fe-Py-CNT and Pt/C 20 wt% in acid and basic media.

			E 0'	Fe(III)/(II)					TOF@
					Eonset (*)
	Complex	Media	V vs RHE	V vs RE	Tafel	nº e -		(0.80 V vs. RHE) s -1
					RHE	(V dec -1 )	(K-L)	mol cm -2
	FePc-Py-CNT	0.1 M NaOH	0.871 ± 0.002	1.017 ± 0.001	-0.022 ± 0.001 4.0 ± 0.03 8.29x10 -10 ± 1x10 -11	1.78
	FePc-CNT	0.1 M NaOH	0.801 ± 0.003	0.916 ± 0.001	-0.044 ± 0.002 3.8 ± 0.10 7.94x10 -10 ± 1x10 -11	1.01
	FePc-Py-CNT	0.1 M H2SO4	0.699 ± 0.003	0.844 ± 0.002	-0.097 ± 0.003 3.6 ± 0.02	3.94x10 -9 ± 2x10 -10	0.02
	FePc-CNT	0.1 M H2SO4	0.657 ± 0.003	0.784 ± 0.001	-0.086 ± 0.001 2.6 ± 0.07	1.62x10 -8 ± 1x10 -10	-
	Pt 20%
	Vulcan	0.1 M NaOH		-	0.979 ± 0.001	-0.046 ± 0.001 4.0 ± 0.03	1.18x10 -9 ± 1x10 -10	0.85
	Pt 20%
	Vulcan	0.1 M H2SO4		-	0.954 ± 0.001	-0.047 ± 0.001 4.0 ± 0.03	2.8310 -10 ± 2x10 -11	3.82
	(*) The Onset potential (Eonset) was evaluated at 0.1mA cm -2

Table 2 .

 2 Total energy of the FePc-CNT and FePc-G with and without adsorbed O2 for the allowed spin states. E0 is the lowest energy state.

	m (μB)	FePc-CNT	FePc-G	O2-FePc-CNT	O2-FePc-G
		Etotal (eV)	Etotal (eV)	Etotal (eV)	Etotal (eV)
	0	E0 + 0.267	E0 + 0.256	E0	E0
	2	E0	E0	E0 + 0.032	E0 + 0.032
	4	E0 + 0.183	E0 + 0.231	E0 + 0.164	E0 + 0.162

Acknowledgements

The authors acknowledge the funding from Fondecyt Grants 1181840, 1181037, 11170879, Conicyt/Anid PIA Proyect ACT 192175, Proyecto POSTDOC_DICYT, 022142TG_POSTDOC, Vicerrectoría de Investigación, Desarrollo e Innovación. The authors are thankful to LNLS-CNPEM (Brazil) for the project 20170843 (SXS beamline).

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Author Information


Yann Spiesschaert

Jens Danneels

Niels Van Herck

Marc Guerre

Guillaume Acke

Johan Winne
email: johan.winne@ugent.be

Filip Du Prez
email: filip.duprez@ugent.be

Polyaddition synthesis using alkyne esters for the design of vinylogous urethane vitrimers

Keywords: Covalent Adaptable Networks, Vitrimers, Vinylogous Urethanes, Alkyne Esters, Water-free

Vitrimers are a subclass of covalent adaptable networks, which introduce reshapeability and recyclability in thermoset materials while maintaining a high degree of chemical resistance and dimensional stability. Vitrimer materials based on vinylogous urethane (VU) chemistry have drawn a lot of attention in this area. Classically, these are obtained by the polycondensation polymerization of acetoacetate and amine monomers. Unfortunately, this also releases water, often leading to porosity defects in the initially obtained non-reprocessed crosslinked materials. Here, we demonstrate that alkyne esters (AE) can be used as alternative building blocks for VU vitrimers by a polyaddition polymerization with amines, leading to water-free formulations and straightforward access to defect-free cured VU vitrimer materials.

 The vinylogous urethane (VU) chemistry was first introduced in vitrimer materials by our research group in 2015.32 In the latter studies, VU vitrimers were synthesised via the polycondensation reaction between acetoacetates (AA) and amines with the release of water (Scheme 1a). Scheme 1: Schematic overview of (a) the acetoacetate based VU platform, (b) the activated alkynone-based chemistry for CANs' preparation ( c) the herein described alkyne ester-based VU chemistry route.

 However, a principal technical bottleneck

Introduction

Polymer materials are generally divided into two classes depending on their thermal flow behaviour, i.e. either thermosets or thermoplastics. However, this distinction has been confounded by the introduction of covalent adaptable networks (CANs) [START_REF] Kloxin | Covalent adaptable networks: smart, reconfigurable and responsive network systems[END_REF][START_REF] Bowman | Covalent Adaptable Networks: Reversible Bond Structures Incorporated in Polymer Networks[END_REF][START_REF] Kloxin | Covalent Adaptable Networks (CANs): A Unique Paradigm in Cross-Linked Polymers[END_REF] and more in particular vitrimers, which defy classification into either class. [START_REF] Winne | Dynamic covalent chemistry in polymer networks: a mechanistic perspective[END_REF][START_REF] Denissen | Vitrimers: permanent organic networks with glass-like fluidity[END_REF][START_REF] Scheutz | Adaptable Crosslinks in Polymeric Materials: Resolving the Intersection of Thermoplastics and Thermosets[END_REF][START_REF] Van Zee | Permanently crosslinked polymers with dynamic network topology[END_REF][START_REF] Krishnakumar | Vitrimers: Associative dynamic covalent adaptive networks in thermoset polymers[END_REF][START_REF] Guerre | Vitrimers: directing chemical reactivity to control material properties[END_REF][START_REF] Montarnal | Silica-Like Malleable Materials from Permanent Organic Networks[END_REF] Indeed, vitrimers are permanent crosslinked materials that are able to change their network topology, thus introducing reshapeability and recyclability in thermosets, without losing network connectivity. This is made possible by the introduction of dynamic covalent bonds that follow a thermally responsive associative exchange mechanism (i.e. an addition/elimination pathway), meaning that first a new covalent in the application of this chemistry, is its polycondensation nature, often giving rise to defects in cured VU networks, due to the release of water and formation of bubbles in freshly cured samples, generating porosity throughout the material. This porosity can be avoided by prepolymerization approaches, [START_REF] Spiesschaert | Dynamic Curing Agents for Amine-Hardened Epoxy Vitrimers with Short (Re)processing Times[END_REF] or it can also be generally erased using an additional processing step, e.g. consisting of compression moulding. 32 Nonetheless, for certain applications, this additional processing step is not desired or even feasible (e.g., composite materials or 3D-printing) and thus, the VU chemistry in vitrimers can be troublesome for such specific applications. Prepolymerization provides an effective technical solution, as release of water and network formation can be separated, to obtain porosity-free networks by designing an oligomeric VU curing agent, as recently shown in our synthesis of VU-based epoxy vitrimers.33 However, the obtained oligomeric (VU prepolymer) epoxy curing agents display a high viscosity, again giving a technically troublesome epoxy curing process, as monomer mixing is difficult. For this and other reasons, we have long been interested in developing an alternative synthetic methodology to obtain water-free VU vitrimers via a more application-friendly polyaddition process for network design.

Recently, our group reported the use of alkynones as monomers for novel thiol-ene based CAN materials. [START_REF] Van Herck | Covalent Adaptable Networks with Tunable Exchange Rates Based on Reversible Thiol-yne Cross-Linking[END_REF] In this work, thioacetals were obtained from the double thiol-Michael addition between activated alkynones and thiols (Scheme 1b). Such alkynone precursors allow the formation of a polymer network in a water-free manner as they react via an addition reaction instead of a condensation reaction. While alkyne esters (AE) are well-known precursors in organic chemistry for the preparation of VU compounds via an aza-Michael reactions, [START_REF] Hartman | Nucleophilic aromatic substitution by 3-amino-2-butenoates[END_REF][START_REF] Liu | Ag/CNT-catalyzed hydroamination of activated alkynes with aromatic amines[END_REF] this chemistry is also not readily applicable for polymer synthesis, as it is also known to undergo irreversible side reactions, such as swift amidation reactions, producing alkyne amides in high yields. [START_REF] Parkinson | Efficient NQO1 Substrates are Potent and Selective Anticancer Agents[END_REF][START_REF] Bair | Chemistry and Biology of Deoxynyboquinone, a Potent Inducer of Cancer Cell Death[END_REF] Alkyne esters are highly reactive monomers, and amines are highly reactive nucleophiles as well as bases, and their combination has so far proven troublesome in our ongoing studies in the VU area. From these initial studies, it became clear that a good reaction design and understanding of the reactivity parameters is necessary in order to supress side reactions and thus favour the VU-formation (Scheme 1c).

Here, we will report the first successful syntheses of VU vitrimers, using the attractive polyaddition methodology. First, small molecule reactivity studies have been performed to gain a better understanding of the possible reaction pathways between AE and amines and to evaluate the VU-precursor viability of AE. Furthermore, density functional theory (DFT) calculations were performed to further rationalise the experimental results obtained regarding VU formation in terms of steric and electronic influence and to investigate this influence on the transamination reaction. Following this, experimental transamination studies were also performed to corroborate the theoretical data. Based on our improved understanding of the underlying chemistry, we were able to tackle VU vitrimer synthesis via polyaddition polymerisations of AE monomer, including interesting VU-hardened epoxy vitrimers. The recycling potential and stress-relaxation behaviour of this new generation of vitrimers was also examined, and critically compared to that of their AA-based VU vitrimer counterparts.

Results and Discussion

Amine -Alkyne ester model study: determination of possible side reactions during VU formation

In the introduction, it was mentioned that alkyne esters can undergo an acyl substitution reaction leading to the formation of alkyne amides. Hergenrother and co-workers [START_REF] Parkinson | Efficient NQO1 Substrates are Potent and Selective Anticancer Agents[END_REF][START_REF] Bair | Chemistry and Biology of Deoxynyboquinone, a Potent Inducer of Cancer Cell Death[END_REF] demonstrated this by obtaining alkyne amides in high yield (up to 94 %) from reactions performed with a slight excess of amine compared to the alkyl ester (1.2:1 NH 2 :AE). Based on the reported reaction conditions (0°C; methanol as solvent), it was hypothesized that the acyl substitution might be favored in a protic solvent over the aza-Michael addition reaction. Since the herein envisioned VU synthesis procedures will be performed in bulk at elevated temperatures, this undesired substitution reaction was expected to exhibit a different reactivity profile. In order to investigate this, a model study was performed to determine the percentage of alkyne amide product formed on the one hand and whether the reaction efficiency was high enough to obtain VU materials on the other hand.

For this, 1-pentanol functionalized with an AE-function (1-Pentanol-AE) was reacted in bulk at 60 °C for 16h with three different butylamine/AE ratios (i.e. (1.1:1), (1:1) and (0.9:1)) (Figure 1a). Butylamine was chosen since it can be easily removed using rotary evaporation, thus allowing sample analysis by LCMS without the need for additional purification, which could remove possible side-products. In each case, the desired vinylogous urethane was found to be the major product, corroborating the stated hypothesis that a protic solvent assists the acyl substitution reaction. Nevertheless, besides trace amounts of the starting compound 1-Pentanol-AE and the expected VU (EVU), one side-product could also be detected in LCMS and NMR analysis (Figure 1b and Figure S.2), namely the alkyne amide (AmS). As expected, the reduction of the amount of butylamine resulted in a higher percentage of 1-pentanol-AE in the reaction mixture, up to a maximum of 18 % compared to EVU for the reaction using a 0.9:1 ratio (Figure 1b and Figure S.2). Interestingly, the ratio of the alkyne amide to the desired VU-compound remained constant throughout the different reactions, i.e. roughly 7 % (as determined by NMR).

The presence of such alkyne amide functions would most likely result in a fraction of network defects and thus a potentially higher soluble fraction of the resulting vitrimer materials.

Nevertheless, since no vinylogous urea signals were observed, the required excess of primary amines for transamination to the VU-functions is expected to be still present in the networks and vitrimer properties should thus be observed. Therefore, the AE moiety was still considered as a viable VU precursor for the synthesis of water-free VU vitrimers.

Computational study of the AE-based chemistry

In the above-described model study, it was observed that the reaction between amines and AEs was more exothermic than the reaction between amines and AAs. Due to the higher reactivity of the AE towards amines, previously discarded amines -such as aromatic amines -could now potentially be utilised to obtain VU vitrimers. To be able to test this hypothesis as efficiently as possible at the material level, we first computationally investigated the range of amines that can be used for the VU synthesis. All calculations were performed using the Gaussian 16 package 54 with the M06-2X/6-311+G(d,p) level of theory, as this functional performs well for thiol-Michael reactions, 55 which can be considered to be rather similar to the here investigated Aza-Michael reactions. The CPCM implicit solvent model [START_REF] Scalmani | Continuous surface charge polarizable continuum models of solvation. I. General formalism[END_REF] was utilised to take the effects of solvent -in this case chloroform -into account, as zwitterionic species in the gas phase are not possible without delocalisation. Gibbs free energy profiles for both the VU formation reaction (starting from AA and AE) and for the transamination reaction between an amine and a VU were obtained, directly leading to the relative Gibbs free energy (ΔΔG 0 ) and the relative Gibbs free energy of the transition state (ΔG 0, ‡ ) of the rate limiting step of the pathway. Scheme 2 represents a summary of the calculations that have been performed, including the different mechanistic pathways (Scheme 2 a-b) and the investigated amines (Scheme 2c). Scheme 2: General mechanisms of the vinylogous urethane formation starting from a) alkyne esters and b) acetoacetates in both the keto (1) and enol (2) tautomer and a primary amine. (c) Amines used during the computational study.

The broad range of different amines, ranging from methylamine to 4-nitroaniline, were chosen to investigate both steric and electronic influences, as to further quantify and rationalise their difference in reactivity. To reduce the complexity of the calculations, simplified versions of both the amines and the AA or AE, compared to their experimental used counterparts, such as methyl but-2-ynoate (Me-AE), methyl acetoacetate (Me-AA) and methylamine, were investigated. Furthermore, the reported results are related to the formation of VU in the cis-product, despite the fact that, theoretically and practically, the trans-product can also be formed. [START_REF] Haida | Acid-Mediated Autocatalysis in Vinylogous Urethane Vitrimers[END_REF] Nevertheless, the cis-product is always the major product, due to the extra stabilisation via an internal hydrogen bond, and was therefore chosen to represent the vinylogous urethane moiety in the performed calculations.

The calculation results concerning the VU formation based on Me-AE and Me-AA in its enol tautomer form, as well as those obtained for the transamination of ethyl VU with the amines (Scheme 2c), can be found in Table 1 and in the supporting information (section V.2). Based on the thermodynamic profile of each individual reaction pathway, one can already distinguish the following predictive trends for the VU formation for both Me-AE and Me-AA. First, thermodynamic driving force could be the sole cause of potential differences in reactivity as reactions between amines and Me-AE are all highly exergonic (ΔΔG 0 < -95 kJ/mol -1 ) (Table 1 and Figure 2). On the other hand, the reactions with Me-AA are mostly exergonic (-30 kJ/mol -1 < ΔΔG 0 < 15 kJ/mol -1 ) and thus have a much lower thermodynamic driving force (Table 1 andFigure S.34) and only minor differences in the ΔG 0, ‡ values between the two monomers were obtained. Second, increasing the steric hindrance of the primary amine (from methylamine to tert-butylamine) results in a higher kinetic barrier and reduced thermodynamic driving force for the VU formation. Third, tuning the nucleophilicity of the amine by using electron donating/withdrawing groups is expected to play an important role in the VU formation. Indeed, by introducing electron donating (eg methoxy group in p-anisidine) or withdrawing (eg nitro group in 4-nitroaniline) groups, the ΔG 0, ‡ value was lowered or increased respectively compared to the reaction with aniline. This can be explained by the fact that p-anisidine is more nucleophilic because of the delocalisation of an electron pair introduced by the methoxy group, whereas the nucleophilicity is lowered for 4-nitroaniline as the electrons are drawn towards the nitro group. Lastly, secondary amines display the lowest ΔG 0, ‡ , as expected from their higher nucleophilicity, but they also display the lowest thermodynamic driving force of all the investigated aliphatic amines, due to their inability to form internal hydrogen bonds for extra stabilisation. Therefore, they are expected to be under kinetic control. * Calculations did not converge or displayed wrong frequencies. Therefore, the Gibbs free energy could not be calculated.

VU Formation Transamination of Ethyl VU

Methyl but-2-ynoate Methyl acetoacetate (enol) Protic iminium Aprotic Aza-Michael

a Value of the first transition state, which is not necessarily the one of the rate determining step. and an iminium pathway (Figure 3 and Figure S.42), [START_REF] Denissen | Chemical control of the viscoelastic properties of vinylogous urethane vitrimers[END_REF] they were both investigated. The results of the calculations for the reaction between ethyl VU and the investigated amines via both pathways can be found in Table 1, Figure 3 andFigure S.39) The same calculations were also performed for the other VU, however, as similar trends were obtained, they will not be discussed in more detail and the results can be found in the supporting information (section V.2.2-V.2.4). In Table 1, one can see that the ΔG 0, ‡ values for Aza-Michael are much higher than the ones calculated for the iminium mechanism. These high values for the Aza-Michael pathway are in agreement with experimental results observed in previous studies when comparing the viscous flow activation energy. [START_REF] Spiesschaert | Influence of the polymer matrix on the viscoelastic behaviour of vitrimers[END_REF][START_REF] Denissen | Chemical control of the viscoelastic properties of vinylogous urethane vitrimers[END_REF][START_REF] Taplan | Fast processing of highly crosslinked, low-viscosity vitrimers[END_REF][START_REF] Guerre | Fluorinated vitrimer elastomers with a dual temperature response[END_REF] Although relating thermodynamic profiles of individual reaction pathways to experimental results is open to interpretation, the values calculated here provide a possible rationalization for the experimentally obtained results, which indicate that the Michaelpathway indeed has a high kinetic energy barrier.

Furthermore, similar to the VU formation results, increasing the steric hindrance of the amines increases the kinetic energy barrier (higher ΔG 0, ‡ values) and generally lowers the thermodynamic driving force (more positive values of ΔΔG 0 ) of both pathways. Especially the transamination with sterically hindered amines does not seem to be favoured, whereas nonsterically hindered amines (e.g. tert-butylamine, Table S.8 and S.16) could lead to an irreversible transamination reaction and therefore a non-dynamic system. Interestingly, despite the fact that the formation of aromatic VU is thermodynamically unfavoured, the transamination reaction via the protic iminium pathway seems to be highly preferred and this shows a very low kinetic energy barrier. The high exothermicity of the transamination reaction with aromatic amines (e.g.

aniline) can be explained by the fact that the ground states, as mentioned above, involve protonated amines. Since aniline and its derivatives are rather acidic, they prefer to donate a proton instead of accepting one. Therefore, it stands to reason that the low activation energy combined with the ease of proton transfer and aniline's aversion to be protonated could lead to potential exchange between aromatic amines and VU from aliphatic amines. This could be a possible way to introduce aromatic amines in a VU matrix, instead of directly synthesising aromatic VU from AA or AE precursors. Moreover, due to the high difference in ΔG 0, ‡ and ΔΔG 0 values, the exchange would indeed most likely follow an iminium pathway instead of an Aza-Michael mechanism, as there would be no thermodynamic driving force for the reaction to occur (ΔΔG 0 > 0 kJ.mol -1 ).

Additionally, the electronic influence also follows a similar trend as observed for the VU synthesis. Electron donating groups, which increase the amine's nucleophilicity, reduce the energy required to reach the transition state and lead to more favourable ΔΔG 0 values.

Alternatively, electron withdrawing groups raise both the ΔG 0, ‡ and ΔΔG 0 values, making it more difficult for transamination to occur. Lastly, despite the lower ΔG 0, ‡ value obtained for dimethylamine, no transamination will occur in a reaction with ethyl VU, as ΔΔG 0 value is not in favour of dimethyl VU formation, as could be expected due to the lack of internal hydrogen bond stabilisation effects. However, transamination reactions should be possible with aromatic VU following the Aza-Michael pathway, since this reaction is thermodynamically favoured (ΔΔG 0 = -3.3 kJ.mol -1 ) (Table S.6).

The obtained results are highly interesting as they suggest that mixing amines of different steric and electronic nature could be a way to tune the viscous flow behaviour of vinylogous urethane systems. Moreover, by smartly designing the system it might also be possible to deactivate the viscous flow after one processing step. This would allow the formation of a permanent network during usage, but having chemical recycling capability. In order to corroborate the obtained calculated results, some transamination and VU synthesis reactions were also performed, as described in the following section.

Steric and electronic influences on the transamination

As shown in the computational study, both the steric and electronic properties of the amines can have a large impact on their reactivity. Indeed, the reaction of aromatic amines, as well as amines on a tertiary carbon with AE and AA are clearly less thermodynamically favoured compared to aliphatic primary amines. Nevertheless, attempts were made to synthesise these VU's in bulk, preferably without the addition of a catalyst. The results showed no decent conversions when using AA. However, using AE functionalised 1-pentanol, it was possible to obtain a VU from tert-octylamine in high yield after a reaction carried out 24h at 100 °C and 72h at 120 °C under N 2 atmosphere (yield = 81% after purification). Following this, the preparation of an aromatic VU was attempted from the reaction between aniline and 1-pentanol-AE using similar conditions. The attempts were unsuccessful, even with harsher reaction conditions, as justified by the higher ΔG 0, ‡ . Therefore, the use of catalysts was investigated. The computational predictions are in agreement with these experimental results, providing further support for the plausibility of the performed calculations, and show that amines that were previously disregarded could finally be utilised for the formation of VU vitrimers with optimised reaction conditions (e.g. using solvent and appropriate catalyst). Following these model compound syntheses, transamination studies were performed (Figure 4a). First, four different VU were synthesised, each based on a different sterically hindered amine (Figure 4b). These were then mixed with a five-fold excess of a specified amine taken from the range shown in Figure 4c and heated to 120 °C in bulk (Figure 4a). In figures 4d-g and Table S.1, the different transamination reactions are represented as the remaining percentage of the starting VU as a function of time.

The theoretical calculations for the aliphatic primary amines (octylamine (A1), cyclohexylamine (A2) and tert-octylamine (A3)) were in good agreement with the experimental results, i.e. the least sterically hindered amine reacts the fastest and forms the most preferred VU, as clearly visible in Figure 4d-g. Indeed, both A1 and A2 readily perform transamination whereas no transamination product of A3 could be distinguished. A possible explanation for the lack of reaction could be linked to both kinetic (higher kinetic barrier to reach the transition state) and thermodynamic issues (unfavourable ΔΔG 0 values), related to the amine being greatly sterically hindered. Following this, the reaction between tert-octyl VU and A1 and A2 seems to be rather slow (Figure 4f), despite being clearly favoured towards the newly formed VU compounds according to the DFT calculations (Table S.8 and S.16). A possible explanation could be found in the steric hindrance of the tert-octyl group, which increases the kinetic energy barrier and could slow down possible attack of the amines and thus decreases the reaction rate, since equilibrium was not obtained after 4h. Furthermore, as predicted by the computational results, no transamination product of the reaction with dibutylamine (A4) could be observed as this would be a thermodynamically unfavoured reaction due to the lack of internal hydrogenbonding stabilisation (e.g. a ΔΔG 0 = 42.8 kJ.mol -1 for the transamination between ethyl VU and dimethylamine as shown in Table 1). However, it needs to be mentioned that these studies were followed via off-line NMR. This means that some exchange product could have been formed but, despite the high excess of amine, the backwards reaction into the more stable VU could have occurred before the NMR measurement was performed.

Furthermore, the aromatic amines (aniline (A5) and p-anisidine (A6)) did participate in exchange reactions, which is rather surprising as obtaining aromatic VU compounds was previously found to be quite difficult, [START_REF] Wright | Photoactivated Healable Vitrimeric Copolymers[END_REF] but was here anticipated based on the calculations.

Moreover, changing from octyl VU to cyclohexyl VU to tert-octyl VU (resulting in less thermodynamically stable VU) was also accompanied by increasing fractions of aromatic VU (Figure 4d-f and Table S.1), with the equilibrium fraction probably being even higher than 28 % (for A5) in the reaction with tert-octyl VU, as equilibrium was not yet reached after 4 hours.

Additionally, substituting A5 for its more nucleophilic counterpart of A6, also resulted in a larger equilibrium fraction of aromatic VU (Figure 4d), in line with the computational predictions as seen in Table 1 and SI section V.2.1.

In a next step, the activation energy of the transamination reaction between octyl VU and A2 (56.7 ± 4.6 kJ.mol -1 ) and vice-versa (80.3 ± 3.2 kJ.mol -1 ) was determined following the method described in the supporting information (Section III.15). As expected, a significant difference was obtained, which could potentially be explained by different stabilisation effects leading to the transition state, induced by the changes in the structure. Moreover, this difference further highlights the tune-ability of the transamination reaction by looking at the steric nature of the utilised amines. Following these model studies, the use of AE monomers to obtain VU vitrimers was investigated on material level.

Material properties

In a following stage, the AE chemistry was investigated at VU vitrimers level. Vitrimers based on either AE and AA should in principle be chemically identical and lead to similar material properties. However, due to the aforementioned side reaction observed for AE, it should be checked how this would impact the properties. Thus, a comparative study was made between materials prepared from a 1,6-hexanediol functionalised with either AE (Hex-AE) or AA moieties (Hex-AA). Moreover, in order to assess the possible formation of alkyne amide side product during the material preparation, Hex-AE was reacted with 2.5 eq. of butylamine (leading was confirmed via FT-IR measurements (Figure 5a). Indeed, the characteristic VU stretch (1640 cm -1 and 1580 cm -1 ) could be observed while the AE (2240 cm -1 and 1700 cm -1 ) or the AA (1720 cm -1 and 1700 cm -1 ) stretches were no longer visible. Following this, their thermal properties were investigated via DSC and TGA analysis and the results are shown in Table 2.

to
Both networks were found to possess very similar T g values (67 °C and 69 °C for VU-AE and VU-AA respectively). Moreover, they displayed good thermal stability with T d5% values above 280 °C (Figure 5b) and less than 1 % weight loss after 120 minutes at 160 °C (Figure 5d). These thermal stability values for VU-AE are promising, as it shows that the side-products formed during network formation do not greatly influence the service temperature. Then, the network integrity was investigated via swelling and solubility experiments in THF.

The swelling ratio of VU-AE was nearly 4 times larger with a value of 1010 % compared to 285 % for VU-AA (Table 2). This large difference could be related to several factors. It could be that a small percentage of side-product can have a drastic influence on the network connectivity or that the side-product is more prevalent on material level than the model studies suggest.

Another explanation could be that the VU formation reaction has a lower reaction efficiency when starting from AE monomers compared to starting from AA monomers. Moreover, the difference in network connectivity was also observed in the soluble fractions, as VU-AE lost over a quarter of its weight in THF, while VU-AA only lost 7 %. a Glass transition temperature (Tg) determined during second heating by differential scanning calorimetry (DSC) with a heating and cooling rate of 10 °C.min-1. b Temperature at which a mass loss of 5% is observed by thermogravimetric analysis (TGA). c Mass loss after an isothermal TGA measurement of 120 minutes at 160 °C. d Determined after 24h in THF via Equation S.2; e Determined via Equation S.1 after 24h in THF and dried for 24h in a vacuum oven at 40 °C. f Elongation at break (ε) and apparent Young's modulus (E') determined from tensile testing (0.05N preload and 10mm.min-1) with strain being measured as [actuator displacement]/[gauge length]. These values are relative and used for comparison purposes only. * Not performed.

In a next step, the tensile properties of both vitrimers were investigated. Unsurprisingly, VU-AE, with an elongation of 28.8 ± 20.5 %, turned out to be more flexible than VU-AA, which only displayed an elongation of 6.7 ± 2.0 % (Figure 5c and Table 2). This large difference comes from the fact that VU-AE exhibited a high amount of necking and drawing, most likely due to the plasticising effect of the side-product and the lower cross-link density. Furthermore, despite this lower cross-link density and longer elongation, the E' values for VU-AE and VU-AA were approximately the same (1.3 GPa).

Lastly, the viscoelastic properties of the materials were analysed via stress-relaxation. The stress-relaxation of VU-AE, as seen in Figure S.24a, occurred at a remarkably faster rate than the one of VU-AA (Figure S.24b). Indeed, relaxation times of 26.5 s and 120 s at 150 °C were obtained for VU-AE and VU-AA respectively. This faster relaxation rate is ascribed to a reduced cross-link density and the presence of low molecular weight compounds and thus a higher internal mobility of the chains for VU-AE. Additionally, hydroxyl groups have an accelerating effect on the stress-relaxation rate [START_REF] Spiesschaert | Dynamic Curing Agents for Amine-Hardened Epoxy Vitrimers with Short (Re)processing Times[END_REF] and, due to the transamidation side reaction, a small amount of these groups should be present in VU-AE. Furthermore, the Arrhenius plots (Figure 6a andb) display a highly linear behaviour and high activation energies were obtained (177 ± 6 and 140 ± 7 kJ.mol -1 for VU-AE and VU-AA respectively). These high activation energies could be the result of the used diamine. IPDA (Scheme 3) has two primary amines located on differently substituted carbons (primary vs secondary carbon), which were already found to behave differently in the VU synthesis and to have an influence on the transamination reaction.

Therefore, the higher E a could be the result of the difference in steric hindrance of the primary amines. Furthermore, VU-AE could also be recycled up to four times via compression moulding (150 °C for 35 minutes) after the material was broken into smaller pieces (Figure S.32). Tensile and thermal properties, as well as FT-IR were measured after each recycling step (Table 2 and Figure S.13, S.18 and S.29). The thermal properties and the chemical nature of the material remained practically unchanged. Furthermore, the apparent Young's modulus of VU-AE also stayed virtually the same with only a 50 MPa difference between the pristine and the four times recycled sample. However, the elongation was drastically reduced after the second recycling step, as the material seemed to have lost its ability for necking. Additionally, after four recycling steps, the swelling and soluble fraction were measured again, and lower swelling ratio (510 %) and soluble fraction (17 %) were observed. These latter changes indicate either the presence of some undesired side-reactions, changes in the matrix or the oxidation of pendent amines and seem to also result in further discolouration of the material (Figure S.32). Furthermore, the stress-relaxation rate also slowed down as a relaxation time of 78 s was obtained at 150 °C compared to 27 s for the unrecycled sample. Moreover, the calculated activation energy also increased to 218 ± 11 kJ.mol -1 and the linearity in the Arrhenius plot decreased (Figure S.25).

These results could be related to the oxidation of pendent amines and the potential presence of additional cross-linking reactions as this could affect the viscous flow behaviour of the material and the resulting viscous flow activation energy. Nevertheless, the VU-AE material is proven to be recyclable up to four times with only minor loss of material properties.

All these observations indicate that, despite the theoretical possibility of obtaining two chemically identical materials from AE and AA monomers, significant experimental differences are observed. However, this amide formation does not seem prohibitive for network synthesis, and may even be exploited to the benefit of the material design.

One-pot water-free VU epoxy vitrimers

Lastly, to showcase the possibilities of AE monomers to be used as additive in epoxy resins, some vinylogous urethane containing epoxy vitrimers were prepared. As previously mentioned, the advantage of AE type monomers is that they can be used as an additive in epoxy resins as a real drop-in technology to produce water-free VU containing epoxy vitrimers. Moreover, the material composition can be easily tuned with AE monomers regarding cross-link density and VU content, as it only needs to be mixed with the epoxy resin. First, an epoxy vitrimer (referred as OPN 42/5 -AE) was obtained by mixing AE functionalised ethylene glycol (EG-AE) together with EPIKOTE™ Resin MGS LR 135, TREN and 1,3-bis(aminomethyl)cyclohexane (BAC). This composition allowed the use of an epoxy vitrimer network (OPN 42/5 ) based on acetoacetylated ethylene glycol, previously reported by our group, as a reference material. The same synthetic procedure and ratios as the ones utilised for OPN 42/5 were also used for the synthesis of OPN 42/5 -AE. The latter was then fully characterised and compared to the reported OPN 42/5 (see Table 3). FT-IR spectra were also obtained and can be found in the SI (Figure S. 14-17). In line with the comparison between VU-AE and VU-AA, similar thermal properties (T g , T d5% and weight loss in isothermal TGA) were obtained for OPN 42/5 -AE and OPN 42/5 (Table 3, Figure S.19 and S.23). However, in contrast to the previous comparison, a lower soluble fraction for the AE network was obtained. This could be ascribed to multiple factors, such as a lower swelling due to slightly changed matrix or the occurrence of unexpected cross-linking reactions in combination with the epoxy matrix. Nevertheless, the lower soluble fraction is definitely a good sign for the potential of utilising AE monomers in combination with epoxy resins. Following this, similar trends in tensile properties are observed, in which the elongation decreases with recycling steps and the Young's modulus remained the same or increased slightly (Table 3 and These results clearly validate the use of AE monomers in order to obtain vinylogous urethane containing epoxy vitrimers in a water-free manner and further highlight the versatility of this novel functional handle for the formation of VU vitrimers. Additional materials were also synthesised to highlight the versatility of the AE approach and can be found in the supporting info (section IV).

Conclusion

This study demonstrated that alkyne esters can replace acetoacetate monomers for the preparation of vinylogous urethane vitrimers, leading to a more attractive polyaddition process of the dynamic networks, avoiding the release of water. Therefore, AE offers a solution for one of the main technical issues regarding the direct synthesis of VU vitrimer materials without waterinduced defects. Small molecule model studies, supported by DFT calculations, indicate that AE can indeed be used to synthesise VU, however, these also showed that an alkyne amide sideproduct is obtained next to the desired VU. Its formation can be reduced to below 5% in these model studies. Here, it was also noticed that AE have a much higher reactivity towards amines than acetoacetates as binding partners, and that VU bonds can also be obtained from less reactive amine monomers, not usable in classical VU polymerization, thus also increasing the scope of viable VU amine monomers. VU network synthesis also showed this side reaction, but nevertheless, besides a higher soluble fraction, the network obtained from an AE monomer displayed similar or better properties compared to a network obtained from an identical AA monomer. Moreover, VU-AE network displayed significantly faster stress-relaxation compared to VU-AA, possibly related to the hydrogen bond donating character of the amide side product as an embedded or internal catalyst. Additionally, polyaddition curing of epoxy vitrimers could be easily achieved with this novel approach, in a one-pot method, contacting amine, epoxy and AE monomers, giving defect-free epoxy networks. This also means that AE monomers can be seen as an easy drop-in method to obtain processable epoxy materials, as they can be easily added into epoxy resins as additives, with easy tuneability of the final VU content and cross-link density as a result.

Figure 1 :

 1 Figure 1: a) Model study reaction between 1-pentanol-AE and different ratios of butylamine

Table 1 :

 1 Calculated Gibbs free energies of the vinylogous urethane formation normalized to the respective starting compounds (neutral amine and neutral AA or AE) and of the transamination reaction, following the protic iminium pathway and aprotic Aza-Michael pathway of ethyl VU normalized to the respective starting compounds (for the iminium pathway: positively charged amine and neutral VU as shown in Figure 3; for the aprotic pathway: neutral amine and neutral VU as shown in Figure S.39).

Figure 2 :

 2 Figure 2: Calculated free energy diagram for the reaction between methyl but-2-ynoate and

Figure 3 :

 3 Figure 3: Calculated free energy profile for the transamination reaction of methyl-3-

  Using 1,5,7- triazabicyclo[4.4.0]dec-5-ene (TBD) as a catalyst at 100 °C for 24h led to the formation of the desired aromatic VU with a yield of 75% after purification. Other acid or basic catalysts (i.e. p-)) were also investigated without success.

Figure 4 :

 4 Figure 4: a) General reaction scheme of the investigated transamination model studies in d-g. b

Scheme 3 :

 3 Scheme 3:The used composition for both VU-AE and VU-AA.

Figure 5 :

 5 Figure 5: a) FT-IR spectrum of VU-AA and VU-AE. b) TGA of VU-AA and VU-AE with a

Figure 6 :

 6 Figure 6: a) and b) Arrhenius plot of the obtained relaxation times of VU-AE and VU-AA,

Figure S. 30

 30 Figure S.30). Lastly, the viscoelastic properties were compared (Figure S.26). As expected, the

Table 2 :

 2 Number of recycling steps, thermal properties, swelling ratio and soluble fractions, tensile properties, energy and characteristic relaxation times of the investigated vitrimers.

	Networks	Recycling steps	T g a (°C)	T d5% b (°C)	Iso TGA c (wt%)	Swelling Ratio d (%)	Soluble Fraction e (%)	ɛ f (%)	E' f (MPa)	E a (kJ.mol -1 )	τ * 150°C (s)
		Rx0	67	289	0.61	1010	26	28.8 ± 20.5 1300 ± 49	177 ± 6	27
		Rx1	65	287	0.76	*	*	15.2 ± 13.5 1280 ± 75	*	*
	VU-AE	Rx2	65	288	0.65	*	*	5.2 ± 0.9	1230 ± 40	*	*
		Rx3	66	288	0.58	*	*	2.7 ± 1.2	1140 ± 62	*	*
		Rx4	67	288	0.68	510	17	3.7 ± 2.0	1250 ± 37 218 ± 11	78
	VU-AA	Rx0	69	304	0.52	285	7	6.7 ± 2.0	1190 ± 87	140 ± 7	120

Table 3 :

 3 Ratio of bisamine:trisamine, number of recycling steps, thermal properties, swelling ratio and soluble fractions, tensile properties, activation energy and characteristic relaxation time of the investigated vitrimers. Glass transition temperature (T g ) determined during second heating by differential scanning calorimetry (DSC) with a heating and cooling rate of 10 °C.min-1. c Temperature at which a mass loss of 5% is observed by thermogravimetric analysis (TGA). d Mass loss after an isothermal TGA measurement of 120 minutes at 160 °C. e Determined by Soxhlet extraction in toluene at reflux temperature for 24 h via Equation V.3. f Elongation at break (ε) and apparent Young's modulus (E') determined from tensile testing (0.05N preload and 10mm.min-1) with strain being measured as [actuator displacement]/[gauge length]. These values are relative and used for comparison purposes only.

	Networks	Ratio amines a	Recycling steps	T g b (°C)	T d5% c (°C)	Iso TGA d (wt%)	Soluble Fraction e (%)	ɛ f (%)	E' f (MPa)	E a (kJ.mol -1 )	τ * 150°C (s)
				Rx0	69	249	0.70	2.4	10.7 ± 5.0	1520 ± 79	92 ± 1	4
	OPN 42/5 -AE	3.5:1	Rx1	72	242	0.89	*	5.3 ± 1.5	1710 ± 79	*	*
				Rx2	70	252	1.49	*	3.2 ± 0.3	1760 ± 43	*	*
				Rx0	74	250	0.9	5.0	10.6 ± 2.0	1680 ± 25	100 ± 1	7
	OPN 42/5	33	3.5:1	Rx1	74	*	*	*	3.9 ± 1.7	1710 ± 101	*	*
				Rx2	73	*	*	*	3.7 ± 0.8	1620 ± 36	*	*

a The ratio of the used bisamine:trisamine. b * Not performed.

Acknowledgements

Y.S (FWO.3F0.2016.0039.01) and N.V.H. (FWO.SPB.2016.0049.01) acknowledge the Research Foundation-Flanders (Fonds Wetenschappelijk Onderzoek-Vlaanderen) for Ph.D. and Postdoctoral fellowships. F.E.D.P and J.W. acknowledges UGent funding (BOF-GOA). B. De Meyer, J. Goeman, and T. Courtin are thanked for technical support. The computational resources (Stevin Supercomputer Infrastructure) and services used in this work were provided by the VSC (Flemish Supercomputer Center), funded by Ghent University, FWO and the Flemish Government-department EWI.

Associated content

Supporting Information. Supplemental figures, tables, schemes and pictures, additional material characterization and coordinates for the computational calculations.

Author information

Corresponding authors *E-mail: Filip.DuPrez@UGent.be and Johan.Winne@UGent.be

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes

The authors declare no competing financial interest.