Guillaume Roland

Alain Portavoce
email: alain.portavoce@im2np.fr

Maxime Bertoglio

Marion Descoins

Jacopo Remondina

Frédéric Lorut

Magali Putero
email: magali.putero@univ-amu.fr

Te and Ge solid-state reaction: comparison between the 2D and 3D growth of α-GeTe

come

Introduction

Phase Change Materials (PCM) are a class of materials, mostly composed of chalcogenides glasses, 1,2 which attract a lot of attention for their surprising phase change properties. Indeed, PCM can reversibly and quickly change between a high-resistivity amorphous (and in general low-reflectivity) state and a low-resistivity crystalline (and in general high-reflectivity) state through thermal annealing. [1][2][3] This property has been exploited for nonvolatile optical memories such as CDs, DVDs and Blu-ray discs, using the large reflectivity difference between the two states. [1][2][3][4] PCM's properties are also exploited for Phase Change Random Access Memories (PCRAM) applications. PCRAM are a new kind of non-volatile memories. In these memories, the bit of information is given by the state of the PCM: "0" if the PCM is in its highly resistive amorphous state, "1" if the PCM is in its low resistive crystalline state. The two most studied and prototypical materials for PCRAM applications are the compounds GeTe and Ge2Sb2Te5 (GST225). 1,2,4 However, the low crystallization temperature of GST225 ( 150-170°C 2 ) can be a limiting factor for high reliability applications, 5,6 such as automotive applications. On the contrary, GeTe has a higher crystallization temperature than GST225 ( 180-230 °C depending on oxidation 7 ) and exhibits ultra-fast switching. 8 GeTe is also studied for thermoelectric [9][10][11] and RF switches applications. [12][13][START_REF] Ghalem | IEEE MTT-S International Microwave Workshop Series on Advanced Materials and Processes for RF and THz Applications[END_REF] For all these applications, it is important to understand the GeTe growth mechanisms. Several studies have shown and studied the possibility to grow high quality epitaxial GeTe using either molecular beam epitaxy or pulsed laser deposition techniques, enabling to grow ultrathin (down to 1nm) epilayers for ferroelectricity applications [START_REF] Wang | [END_REF]16 or strain engineering 17 for example. However, most studies that seek to investigate GeTe focus on the crystallization of an amorphous homogeneous stoichiometric, or near-stoichiometric, thin film. These studies use a plethora of different techniques: Differential Scanning Calorimetry (DSC), 18,19 in situ resistivity measurements, [20][21][22] in situ reflectivity measurement, 19,23,24 Transmission Electron Microscopy (TEM) observations, 7,24-27 X-Ray Diffraction (XRD), 18,20,28,[START_REF] Mangelinck | IEEE International Interconnect Technology Conference and 2015 IEEE Materials for Advanced Metallization Conference[END_REF] X-ray Photoelectron Spectroscopy (XPS), 26,[START_REF] Yashina | [END_REF]31 etc. Even though these studies provide key insights into the crystallization mechanisms, they don't give information about selfdiffusion and interfacial reaction kinetics as reactive diffusion (RD) studies between two films in contact. RD is often used in the microelectronic industry in order to form silicide ohmic contacts on the source, gate and drain of Metal-Oxide-Semiconductor (MOS) transistors through the self-aligned-silicide (Salicide) process. [32][33][34] Since RD depends on self-diffusion and interfacial reaction, it is used to obtain information on the self-diffusion and interfacial reaction kinetics, which are of prime importance for process simulations and crystallization understanding. When a metal is deposited on a Ge or Si substrate, it is usual to observe a 3 -5 nm-thick, amorphous or crystalline, intermixing layer. [35][36][37][38] For germanides, the growth usually follows a parabolic law versus time. [39][40][41] This means that the growth is limited by atomic diffusion according to the linear-parabolic model. 42 In this work, GeTe growth kinetics is studied during the RD of a 90 nm-thick polycrystalline Te layer deposited by magnetron sputtering on a 60 nm-thick amorphous Ge layer (Te-on-Ge) using in situ XRD measurements, in situ TEM observations, as well as ex situ atom probe tomography (APT). We show that contrary to the reverse stack Ge-on-Te, 43 the growth of -GeTe during RD of the Te-on-Ge bilayer follows a 2D growth described by the linear-parabolic growth model, with higher crystallization temperature and nucleation

The sample was elaborated in a commercial magnetron sputtering system (base pressure of 10 -8 mbar). A 99.9999% pure Ar gas flow under a work pressure of 3.6 x 10 -3 mbar was used in order to sputter a 99.99% pure Te-target in RF mode and a 99.999% pure Ge-target in DC mode, using same sputtering powers and deposition time as in the previous study reported in ref. 43 . All the deposition have been made on the native oxide of Si(001) substrate at room temperature (RT) and both fluxes were calibrated separately by measuring the thickness of the sputtered films using X-ray reflectivity. The thin SiO2 layer (~ 2 nm) was kept in order to prevent diffusion of Si in the deposited layers. The sample was destined to the study of solid-state Te-on-Ge Reactive Diffusion (RD). Therefore, a 90 nm-thick Te layer was deposited on top of a 60 nm-thick Ge layer. After the entire bilayer has reacted, it is expected to have a GeTe layer with an extra 10 nmthick Te layer on the surface. This extra-layer was added in order to limit oxidation effects. After deposition (Fig. 1), the Ge layer is amorphous (a-Ge), whereas the Te layer is polycrystalline (poly-Te).

In situ X-ray Diffraction experiments

After deposition, the samples (6  6 cm 2 ) were cut into several pieces. 1.5  1.5 cm 2 specimens were used to perform in situ XRD measurements. All the XRD measurements were achieved in the Bragg-Brentano geometry (-2) in a Panalytical Empyrean diffractometer equipped with an PIXcel detector designed for high speed data collection, using a Cu K source ( = 0.154 nm). The specimens were annealed following a heating ramp (isochronal annealing) or at constant temperature (isothermal annealing) in situ in the XRD setup under a vacuum of ~ 10 -5 mbar. A temperature ramp identical to that of our previous work was used: 43 heating ramp was performed between 100 °C and 350 °C following a temperature ramp of 10 °C min -1 steps every 5 °C, separated by 4.5 minute-long XRD scans at constant temperature, thus corresponding to an average heating ramp of  0.9 °C min -1 . In addition to the ramp annealing, four isothermal annealing between 160 °C and 205 °C were performed on the sample.

In situ Transmission Electron Microscopy experiments

In situ TEM observations gave complementary information on the growth of the GeTe phase during Te-on-Ge RD. A TEM lamella was prepared using the in-situ lift out technique in a FIB-SEM system, Helios450 TM from ThermoFisher, and mounted onto a Molybdenum grid. Final thinning of the lamella was done using an 8 kV acceleration voltage, in order to limit possible Ga FIB induced amorphization. Once prepared, the grid was mounted onto a sample heating holder from Gatan, and then loaded into a Technai F20 TEM, from ThermoFisher, operating at a 200 kV accelerating voltage in TEM bright field mode. The in situ TEM annealing started at RT up to 350 °C following several isothermal steps. During each isothermal step, one image was taken every minute. The total annealing time was about 1 hour and 40 minutes.

Atom Probe Tomography experiments

APT sample preparation was achieved using a FEI Helios dualbeam focused ion beam (FIB) equipped with a micromanipulator and a gas injection system for Pt deposition. Prior to FIB preparation, the sample' surface was capped by a 100 nm-thick Ni protection layer deposited by sputtering. In the FIB set up, an additional 100 nm thick Pt protection layer was added on the region of interest, in order to prevent Ga contamination during the trench milling process. The region of interest was prepared in the cross-section geometry, retrieved using a micromanipulator, and placed on an APT support. The specimen was shaped as a sharp tip by focused Ga+ ion beam milling and the top of the specimen was cleaned by low energy (2 keV) ion beam milling, as a final step. The samples were analyzed in a LEAP 3000X HR instrument. The specimens' temperature was set to 50 K and the voltage pulse to 20% of the constant voltage. Fig. 2 presents the variation of the normalized integrated XRD intensities and the variation of the average GeTe grain size L along the normal of the sample surface versus temperature. The average grain size is deduced from the full width at half maximum (FWHM) of the α-GeTe (220) peak, using the Scherrer formula 44 and neglecting micro-strain contribution. The XRD integrated intensity is proportional to the corresponding diffracting volume. For the hex-Te phase, only the Te (110) is presented since it is the most intense peak correctly detected on Fig. 1. For the GeTe phase, only the most intense -GeTe (220) peak is shown, however, we have to note that the -GeTe (202) peak integrated intensity follows the same trend and will be shown for isothermal annealing. The α-GeTe phase appears at T ~ 200 °C (i.e. nucleation temperature) and grows until T ~ 220 °C. The nucleation temperature Tn ~ 200 °C is higher than that reported for the reverse bilayer Ge-on-Te with Tn = 175 °C. 43 Above 220 °C, the crystallized fraction of GeTe remains constant until the end of the temperature ramp. The appearance and the increase of the normalized integrated intensity of the α-GeTe (220) peak is concomitant with the decrease of the normalized integrated intensity of the hex-Te (110) peak. This is caused by the consumption of hex-Te phase by the α-GeTe growing phase, which is a typical behavior for a compound growth by RD. 45 The GeTe growth is not perfectly correlated with the decrease of Te in Fig. 2. Indeed, the hex-Te is consumed from 190 °C, but the α-GeTe phase is only detected from 200 °C. This could be caused by XRD detection limit, meaning that the diffracting volume of the α-GeTe phase is too small to be detected by XRD between 190 °C -200 °C. Another hypothesis is that the GeTe phase grows in an amorphous state between 190 °C and 200 °C, the nucleation of the crystalline α-GeTe starting at 200 °C. This amorphous growth as already been observed for several silicides. 37,38,46 Compared to the previous work on the reverse bilayer (Ge-on-Te, see 43 ), -GeTe growth rate seems to be different in these two samples, especially at the early growth stages, even if both should depend on atomic transport kinetics. 42,47 At the beginning of the growth, the average -GeTe (220) grain size is L ~ 10 nm. A first increase occurs rapidly up to L ~ 15 nm between 200 °C and 220 °C, then the grain size slowly increases further to reach L ~ 20 nm at the end of the annealing. The average α-GeTe grain size at the beginning of the growth is slightly smaller than that reported for the reverse stack (Ge-on-Te bilayer 43 ). Besides, at the end of annealing the grain size (~ 20 nm) is still smaller for the present sample Te-on-Ge compared to the Ge-on-Te bilayer (~ 35 nm, see 43 ). The texture is also different for both Te and GeTe phases. Indeed, in the present work (Te-on-Ge), the most intense peaks for the hex-Te phase correspond to the (100) and (110) Bragg reflections, and to the GeTe (220) peak for the α-GeTe phase. For the reverse stack (Ge-on-Te bilayer), almost the same peaks are observed, but the principal difference stands from their relative intensities: the most intense peaks are the Te (101) and the GeTe (202) Bragg reflections for respectively hex-Te and α-GeTe. The deposition power and time were kept the same for both samples, meaning that the only difference between the samples is the order of deposition. Indeed, the Te layer was deposited on the Si (001) substrate with a native oxide layer 43 for Ge-on-Te instead of on the a-Ge layer for Te-on-Ge. This is probably the reason of the hex-Te texture difference in the two samples. Usually, germanides and ARTICLE Please do not adjust margins Please do not adjust margins silicides RD studies are performed on the system metal deposited on semiconductor substrate. In this case, the substrate texture has been shown to have an impact on the nucleation temperature of the formed phase. 48 Therefore, it would not be surprising that the hex-Te texture impacts Tn and the texture of the phase α-GeTe.

Results and discussion

GeTe growth during isochronal annealing

GeTe growth during isothermal annealing

Several in situ isothermal XRD annealing were performed between T = 160 °C and 205 °C. Fig. 3(a) presents the variation of the α-GeTe(202) XRD normalized integrated intensity versus time for four different temperatures. The signal is proportional to the crystallized volume (i.e; crystallized fraction). The GeTe growth seems to follow the 2D linear-parabolic law, 42 in contrast to the GeTe growth from the Ge-on-Te bilayer that surprisingly followed the JMAK model with a 3D growth. 43 For each isothermal temperature, the beginning of the growth could be fitted with a linear law versus time (shown in Fig. 3(b) for the annealing at the lowest temperature), indicating that the growth is limited by the reaction. This result is surprising for germanides, for which RD is usually governed by self-diffusion, exhibiting a parabolic growth law versus time. The slope of the linear fit allows determining the interface reaction rate 𝑲 𝒊 = 𝒅𝑳 𝒅𝒕 for each temperature, with L the thickness of the growing -GeTe layer, that is proportional to the normalized integrated intensity assuming a 2D linear growth. The results are summarized in Table 1. The interface reaction rate can be expressed as follows:

𝑲 𝒊 = 𝑲 𝟎 𝒆 - 𝑬 𝒊 𝑲 𝑩 𝑻
where kB is the Boltzmann constant, and T the temperature, while . The activation energy of interfacial reaction was found to be Ei = 1.3  0.1 eV. This energy is of the same order as the growth activation energy found for the reverse bilayer (1.2 eV for Geon-Te RD) 43 but higher than that of usual interface reaction of silicides or germanides (0.8 eV for Ni2Si, 49 0.9 eV for Ni5Ge3 45 ). The data presented in Fig. 3 can also be used to measure α-GeTe nucleation time t, which is defined as the time needed to detect for the first time the diffraction signal during isothermal annealing. Fig. 4(b) presents the variation of t versus 1/kBT using a logarithmic scale, and allowed a nucleation activation energy to be determined, using the following equation:

𝟏 ∆𝒕 = 𝛀 × 𝐞𝐱𝐩 (- 𝑬 𝒏 𝒌 𝑩 𝑻 )
The nucleation activation energy of the -GeTe phase was found to be En = 1.91  0.15 eV. This value is higher than that found for the reverse bilayer (En = 1.25 eV), 43 which reflects the difference ARTICLE Please do not adjust margins Please do not adjust margins between their nucleation temperatures. Besides, both the activation energies for nucleation and interfacial reaction are lower than the ones usually found for GeTe amorphous layer crystallization. [50][51][52] This is probably due to the fact that in this study, GeTe crystallization occurs through reactive diffusion at the Te-on-Ge interface, whereas it occurs homogeneously in the GeTe layer volume when GeTe crystallization is studied from an initially amorphous GeTe layer.

In situ TEM observations were performed in order to better understand -GeTe growth mechanisms. Four TEM images recorded during in situ observations are presented in Fig. 5. The annealing temperature goes from RT to 240 °C. After deposition (Fig. 5(a)), the sample is composed of a 91 nm-thick polycrystalline Te layer and a 49 nm-thick Ge amorphous layer. A 4 nm-thick amorphous intermixing layer can be observed at the Te/Ge interface. It is usual to observe a 3 to 5 nm-thick amorphous or crystalline intermixing layer between a metal and the Si or Ge substrate after sputtering. 35- 38 This layer acts usually as the initial stage of 2D growth of silicides or germanides. 47 During annealing (Fig. 5(b) to (d)), the intermixing layer grows as a continuous 2D layer, while voids form at the top of the Te layer. The void formation might be due to the Kirkendall effect, meaning that Te could be the main diffusive specie, or to Te desorption in the annealing conditions due to its high volatility under vacuum. One should note that even if the top Te layer is oxidized, it does not affect the GeTe growth that starts at the Te/Ge interface. In Fig. 5(b) recorded at 180 °C, the intermixing layer has grown compared to Fig. 5(a) (from ~ 4 nm to ~ 8 nm), but is still amorphous (Fig. 5(b)). From Fig. 5(c) (195 °C), the intermixing layer appears partially crystalline. These TEM observations show that the intermixing layer firstly grows amorphous (from ~ 4 to ~ 8 nm) and then nucleation takes place, in agreement with the XRD results obtained during isochronal annealing. The crystallized part of the intermixing layer is believed to be α-GeTe, as shown by XRD (Fig. 1). These observations are also in agreement with the in situ XRD isothermal annealing: -GeTe growth follows a 2D growth according to the linear-parabolic growth model, with a linear growth during the first stages, limited by reaction. The chemical composition of the amorphous intermixing layer was measured by APT after deposition. Fig. 6 shows a high resolution TEM image of the amorphous GeTe intermixing layer with its APT proximity histogram (proxigram). The proxigram shows the average variation in the concentration of Ge (red line) and Te (green line) atoms from the perpendicular direction at each point of the isosurface used to define the outline of the interface. The concentration profile shows the composition of the intermixing layer after deposition. The Ge (Te) concentration gradient varies linearly from 100 at% (0 at%) to 0 at% (100 at%) in a ~ 4.5 nm region corresponding to the intermixing layer. The measured intermixing layer's thickness is in good agreement with TEM observations (Fig. 5a).

6 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx

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Comparison between 2D and 3D -GeTe growth

Solid state reaction is significantly different in our two samples Ge-on-Te 43 and Te-on-Ge (this work) despite same sputtering conditions. Indeed, -GeTe formation in the Ge-on-Te sample was shown to be supported by both nucleation in Te grain boundaries (GBs) and at the a-Ge/poly-Te interface, and the growth was shown to be following the JMAK model due to a simultaneous 3D growth in Te GBs and in the a-Ge layer, supported by interfacial diffusion and bulk GB self-diffusion. In contrast, -GeTe formation in the Te-on-Ge sample is supported by nucleation in a concentration gradient 53 in a 2D amorphous layer located between a-Ge and poly-Te, and -GeTe increases linearly versus time as a 2D layer in agreement with linearparabolic models, meaning that the growth is limited by interfacial reaction instead of self-diffusion.

The difference of -GeTe nucleation mechanisms between the two systems Te-on-Ge and Ge-on-Te leads to a higher Tn = 200 °C (ramp annealing) and En = 1.91 eV (isothermal annealing) for Te-on-Ge reaction compared to Ge-on-Te reaction displaying Tn = 175 °C and En = 1.25 eV. 43 However, -GeTe activation energy of growth is found to be similar in the two samples, with Ei = 1.3 eV in the Te-on-Ge sample and Eg = 1.2 eV in the Ge-on-Te sample, despite a 2D growth in the Te-on-Ge sample and a 3D growth in the Ge-on-Te sample. This observation means that the reaction Ge + Te → -GeTe is almost independent of atomic diffusion mechanisms during 2D (GB self-diffusion) and 3D (interface and GB diffusion) growth, which agrees with the observation of a linear 2D growth in the Te-on-Ge sample. Indeed, -GeTe linear growth versus time suggests that Ge and Te self-diffusion in -GeTe GBs is unusually fast compared to reaction, contrasting with the growth of other germanides reported to date. [39][40][41] RD studies are mostly performed for microelectronic process improvement, and thus, more data are actually available about silicide RD than about germanide RD. Typically, Ei  1 eV (0.8 eV for Ni2Si 49 and 0.9 eV for Ni5Ge3, 45 for example) and the growth rate is limited by self-diffusion (parabolic growth) with an activation energy 1 < Eg < 2 eV (1.5 eV for Ni2Si, 49 for example) for silicide RD. One can note that indeed Ei in the case of Te-Ge RD is about 45% higher than expected, exhibiting a value close to that of silicide effective atomic self-diffusion, confirming the unusual specificity of the Ge-Te binary system characterized by a limiting slow interfacial reaction and fast self-diffusion.

The growth mode difference between the two systems Te-on-Ge (2D) and Ge-on-Te (3D) is probably linked to the presence (case of Te-on-Ge) or not (case of Ge-on-Te) of the intermixing layer between a-Ge and poly-Te. Most silicide and germanide RD studies concern only the case of the metal deposited onto Si or Ge, and the presence after deposition of an intermixing layer between the metal and the semiconductor is generally reported. Therefore, our observations are difficult to compare to most of the bibliography. Nevertheless, Perrin-Toinin et al. 35 observed after sputtering the presence of an intermixing layer in both cases, Pd deposited on Ge and Ge deposited on Pd, and observed a 2D self-diffusion limited (parabolic) growth of Pd2Ge in the two cases. Using APT measurements, they showed that the thickness (~ 5 nm) and the composition (close to Pd2Ge stoichiometry) of the intermixing layer were about the same for the two systems Pd/Ge and Ge/Pd. These results suggest that the 3D regime observed for Ge-on-Te is due to the absence of intermixing layer in this case. In addition to a growth rate obeying the JMAK model, the signature of 3D growth in the Ge-on-Te sample is also observed on the average grain size. L is found to be about 75% larger in the Ge-on-Te sample (L ~ 35 nm) compared to in the Te-on-Ge sample (L ~ 20 nm) at the end of the growth, 3D growth promoting longer grains in the direction perpendicular to the sample surface.

Conclusion

Te-Ge reactive diffusion has been studied by in situ XRD, in situ TEM, and APT in a sample made of a poly-Te layer deposited on an a-Ge layer, and compared to the reverse case of an a-Ge layer deposited on a poly-Te layer in a same magnetron sputtering system in same conditions. -GeTe grows between poly-Te and a-Ge in the Te-on-Ge sample as a 2D layer with a linear growth rate versus time in agreement with the linear-parabolic model, contrasting with the 3D growth obeying the JMAK model previously reported in the Ge-on-Te sample. The growth mode difference appears to be related to the absence of an intermixing layer between a-Ge and poly-Te in the asdeposited Ge-on-Te sample. The nucleation temperature and the nucleation activation energy are found to be higher for the 2D growth in the Te-on-Ge sample. However, the growth activation energy during 2D linear growth in the Te-on-Ge sample and during Please do not adjust margins Please do not adjust margins 3D JMAK growth in the Ge-on-Te sample being similar, the overall observations coherently suggest that -GeTe growth is limited by unusually slow solid-state reaction combined with unusually fast GB (or interfacial) self-diffusion, compared to available literature data on silicides and germanides. This journal is © The Royal Society of Chemistry 20xx

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Fig. 1

 1 Fig. 1 is a series of XRD patterns recorded during the ramp annealing of the sample as a function of temperature. After deposition, 5 peaks are detected at 2θ = 23.04°, 27.497°, 38.26°, 40.369° and 43.485°. They respectively correspond to the (100), (101), (102), (110), and (111) Bragg reflections of the hexagonal Te phase, showing that the as-deposited Te layer is polycrystalline. No diffraction peak corresponding to the Ge phase is observed, meaning that the as-deposited Ge layer is amorphous. At T = 200 °C, five peaks are simultaneously detected. Those peaks are found at 2θ = 25.359°, 26.014°, 29.863°, 42.351° and 43.138° and they respectively correspond to the (003), (021), (202), (024), and (220) Bragg reflections of the rhombohedral α-GeTe phase. The hex-Te (100) and (110) Bragg reflections and the α-GeTe (220) peaks are the most intense peaks for the Te and GeTe phases.Fig.2presents the variation of the normalized integrated XRD intensities and the variation of the average GeTe grain size L along the normal of the sample surface versus temperature. The average grain size is deduced from the full width at half maximum (FWHM) of the α-GeTe (220) peak, using the Scherrer formula44 and neglecting micro-strain contribution. The XRD integrated intensity is proportional to the corresponding diffracting volume. For the hex-Te phase, only the Te (110) is presented since it is the most intense peak correctly detected on Fig.1. For the GeTe phase, only the most

Fig. 1

 1 Fig.1XRD patterns ( = 0.154 nm) as a function of temperature acquired during in situ isochronal annealing of ~ 0.9 K min -1 for Te-on-Ge bilayer. The color plot panel is in counts/s.

Fig. 2

 2 Fig. 2 Normalized integrated intensities of diffraction peaks and average grain size (L) during in situ XRD isochronal annealing of ~ 0.9 °C min -1 of Te-on-Ge bilayer: hex-Te (110) (red circles) and α-GeTe (220) (black squares).

Fig. 3

 3 Fig. 3 Normalized integrated intensity f(t) of the α-GeTe(202) diffraction peak versus time recorded during in situ XRD isothermal annealing of the Te-on-Ge bilayer at various temperatures: (a) versus time in log scale for temperatures between 160 °C and 205 °C; (b) versus time in linear scale at 160 °C, the solid line corresponds to a linear fit of the beginning of the growth.

K0

  and Ei are the pre-exponential factor and the activation energy of the interface reaction. Fig. 4(a) shows the variation of 𝑙𝑛(𝐾 𝑖 ) as a function of 1 𝐾 𝐵 𝑇

Fig. 4 (

 4 Fig. 4 (a) Variations of ln(Ki), with Ki the interface reaction rate, as a function of the inverse of temperature for -GeTe growth during the Te-on-Ge bilayer reactive diffusion; (b) Variations of the nucleation time ln(t) as a function of the inverse of temperature for GeTe nucleation during Te-on-Ge bilayer reactive diffusion.

Fig. 5

 5 Fig. 5 Cross-section TEM images acquired during in situ heating of a Te-on-Ge bilayer from RT to 240 °C: a) RT; b) 180 °C, the blue arrows indicate the voids formation; c) 195 °C; d) 240 °C. The added color highlights the new phase.

Fig. 6

 6 Fig. 6 High resolution TEM image of the GeTe amorphous intermixing layer, highlighted by the added color; the vertical white bar corresponds to 3 nm. The chemical composition of the intermixing layer is determined by a proximity histogram (proxigram) measured from a 85 at. % Te isoconcentration surface.

Table 1

 1 Interface reaction rate 𝐾 𝑖 deduced from the linear fit of the beginning of the growth for each isothermal annealing experiment.

	Temperature (°C)	Ki (a.u.)
	160	1.59 × 10 -5
	175	6.40 × 10 -5
	190	1.47 × 10 -4
	205	4.96 × 10 -4

Acknowledgements

This project was supported by the French National Association of Research and Technology (ANRT Project 2020/0907).

Author Contributions

Conflicts of interest

There are no conflicts to declare.


Maxime Michelas

Yasmine K Redjel

Jean-Claude Daran

Meriem Benslimane

Rinaldo Poli

Christophe Fliedel
email: christophe.fliedel@lcc-toulouse.fr

of tripodal tetradentate diamino-bis(phenolate) ligands: Synthesis, characterization, crystal structures and evaluation in radical polymerization processes

Keywords:

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Introduction

Cobalt is an essential physiological element (vitamin B12 and other enzymes) that is found only in low abundance in the Earth's crust and its complexes have found many applications in diverse fields of chemistry, such as biomedical areas (imaging, anticancer/antitumor therapy or as antiviral and antibacterial agents) [START_REF] Hernández-Romero | Firstrow transition metal compounds containing benzimidazole ligands: An overview of their anticancer and antitumor activity[END_REF][START_REF] Renfrew | Harnessing the properties of cobalt coordination complexes for biological application[END_REF][3], materials (e.g. single-molecule magnets [START_REF] Rechkemmer | A four-coordinate cobalt(II) single-ion magnet with coercivity and a very high energy barrier[END_REF][START_REF] Murrie | Cobalt(ii) single-molecule magnets[END_REF] and lithium-ion batteries [START_REF] Li | Cobalt in lithium-ion batteries[END_REF]), and catalysis (hydrogenation, cross-coupling, hydrofunctionalization, cycloaddition, water oxidation, etc.) [START_REF] Bera | Cobalt-N-Heterocyclic Carbene Complexes in Catalysis[END_REF][START_REF] Xiao | Bio-Inspired Molecular Catalysts for Water Oxidation[END_REF][START_REF] Matheu | The development of molecular water oxidation catalysts[END_REF][START_REF] Junge | Cobalt-Pincer Complexes in Catalysis[END_REF][START_REF] Liu | Cobalt Complexes as an Emerging Class of Catalysts for Homogeneous Hydrogenations[END_REF]. Polymerization is another important area in which cobalt complexes have proven to be very efficient. This includes diverse sub-areas such as coordination/insertion (living) ethylene polymerization [START_REF] Anderson-Wile | 3.23 -Living Transition Metal-Catalyzed Alkene Polymerization: Polyolefin Synthesis and New Polymer Architectures[END_REF], (stereoselective) ring-opening polymerization of epoxides or copolymerization of epoxides and carbon dioxide [START_REF] Fliedel | 9.14 -Metal Complexes as Catalysts/Moderators for Polymerization Reactions[END_REF][START_REF] Kozak | Copolymerization of carbon dioxide and epoxides by metal coordination complexes[END_REF][START_REF] Ajiro | 4.09 -Stereoselective Ring-Opening Polymerization of Epoxides[END_REF], and controlled radical polymerization by the organometallic approach (organometallic-mediated radical polymerization or OMRP) [START_REF] Fliedel | 9.14 -Metal Complexes as Catalysts/Moderators for Polymerization Reactions[END_REF][START_REF] Fliedel | Homolytically weak metal-carbon bonds make robust controlled radical polymerizations systems for "less-activated monomers[END_REF][START_REF] Debuigne | Organometallic-mediated radical polymerization of 'less activated monomers': Fundamentals, challenges and opportunities[END_REF][START_REF] Poli | Organometallic Mediated Radical Polymerization[END_REF][START_REF] Hurtgen | Insight into Organometallic-Mediated Radical Polymerization[END_REF][START_REF] Debuigne | Overview of cobalt-mediated radical polymerization: Roots, state of the art and future prospects[END_REF][START_REF] Poli | Relationship between one-electron transition-metal reactivity and radical polymerization processes[END_REF]. For the latter applications, the use of Schiff-bases as supporting ligands for the Co ion has long been predominant [START_REF] Kozak | Copolymerization of carbon dioxide and epoxides by metal coordination complexes[END_REF][START_REF] Peng | Reversible deactivation radical polymerization mediated by cobalt complexes: recent progress and perspectives[END_REF][START_REF] Kember | Catalysts for CO2/epoxide copolymerisation[END_REF][START_REF] Darensbourg | Making Plastics from Carbon Dioxide: Salen Metal Complexes as Catalysts for the Production of Polycarbonates from Epoxides and CO2[END_REF]. However, the use of other ligand scaffolds, particularly those incorporating aminophenolate moieties, led to efficient systems for the copolymerization of epoxides and CO2 [START_REF] Ambrose | Cobalt amino-bis(phenolate) complexes for coupling and copolymerization of epoxides with carbon dioxide[END_REF][START_REF] Reiter | Amine-bis(phenolato)cobalt(II) Catalysts for the Formation of Organic Carbonates from Carbon Dioxide and Epoxides[END_REF][START_REF] Kember | Di-cobalt(ii) catalysts for the copolymerisation of CO2 and cyclohexene oxide: support for a dinuclear mechanism?[END_REF].

Our group is especially interested in metal-mediated radical polymerizations such as atom transfer radical polymerization (ATRP) [START_REF] Matyjaszewski | Atom Transfer Radical Polymerization[END_REF][START_REF] Ribelli | Synthesis and Characterization of the Most Active Copper ATRP Catalyst Based on Tris[(4dimethylaminopyridyl)methyl[END_REF] and OMRP [START_REF] Poli | Relationship between one-electron transition-metal reactivity and radical polymerization processes[END_REF][START_REF] Poli | Radical Coordination Chemistry and Its Relevance to Metal-Mediated Radical Polymerization[END_REF] and in the study of interfering reactions and their mechanisms [START_REF] Thevenin | Impact of Catalyzed Radical Termination (CRT) and Reductive Radical Termination (RRT) in Metal-Mediated Radical Polymerization Processes[END_REF][START_REF] Fantin | Impact of Organometallic Intermediates on Copper-Catalyzed Atom Transfer Radical Polymerization[END_REF][START_REF] Thevenin | Reductive Termination of Cyanoisopropyl Radicals by Copper(I) Complexes and Proton Donors: Organometallic Intermediates or Coupled Proton-Electron Transfer?[END_REF][START_REF] Bellan | Catalyzed Chain Transfer in Vinyl Acetate Polymerization Mediated by 9-Oxyphenalenone Cobalt(II) Complexes[END_REF][START_REF] Ribelli | Effect of Ligand Structure on the CuII-R OMRP Dormant Species and Its Consequences for Catalytic Radical Termination in ATRP[END_REF][START_REF] Poli | New Phenomena in Organometallic-Mediated Radical Polymerization (OMRP) and Perspectives for Control of Less Active Monomers[END_REF]. Both ATRP and OMRP techniques belong to the family of reversibledeactivation radical polymerization (RDRP) techniques [START_REF] Corrigan | Reversible-deactivation radical polymerization (Controlled/living radical polymerization): From discovery to materials design and applications[END_REF][START_REF] Jenkins | Terminology for reversible-deactivation radical polymerization previously called "controlled" radical or "living" radical polymerization (IUPAC Recommendations[END_REF]. These techniques control radical polymerization processes by establishing a dynamic equilibrium between the active/propagating radical species (Pn/m • ) and a dormant species (Pn/m-Y), the latter resulting from the quenching of the radical by a moderator (Y), which favors the controlled polymeric chain growth versus bimolecular radical terminations (coupling and disproportionation). Scheme 1 (bottom) shows the dynamic equilibrium under reversible-termination (RT) and degenerative transfer (DT) modes for the specific case of OMRP, i.e. Y = L/Mt. Copper-mediated ATRP is now commonly applied in academic laboratories and in several industrial processes [START_REF] Destarac | Industrial development of reversible-deactivation radical polymerization: is the induction period over?[END_REF][START_REF] Destarac | Controlled Radical Polymerization: Industrial Stakes, Obstacles and Achievements[END_REF], allows the preparation of advanced materials [START_REF] Matyjaszewski | Advanced Materials by Atom Transfer Radical Polymerization[END_REF] and can be initiated in a variety of ways such as "normal", "reverse", "AGET" and "SARA" [START_REF] Ribelli | Atom Transfer Radical Polymerization: Billion Times More Active Catalysts and New Initiation Systems[END_REF], and run under various conditions (e.g. photo-or electronically induced, ppm levels of catalysts, aerobic conditions, etc.) [START_REF] Fliedel | 9.14 -Metal Complexes as Catalysts/Moderators for Polymerization Reactions[END_REF][START_REF] Ribelli | Atom Transfer Radical Polymerization: Billion Times More Active Catalysts and New Initiation Systems[END_REF][START_REF] Pan | Externally controlled atom transfer radical polymerization[END_REF]. In contrast, OMRP is much less investigated, but proved very efficient for the controlled (co)polymerization of so-called less-stabilized monomers (LAMs) such as vinyl acetate [START_REF] Debuigne | Highly efficient cobalt-mediated radical polymerization of vinyl acetate[END_REF], vinyl chloride [START_REF] Piette | Synthesis of poly(vinyl acetate)-b-poly(vinyl chloride) block copolymers by Cobalt-Mediated Radical Polymerization (CMRP)[END_REF] and vinylidene fluoride [START_REF] Fliedel | Homolytically weak metal-carbon bonds make robust controlled radical polymerizations systems for "less-activated monomers[END_REF][START_REF] Debuigne | Organometallic-mediated radical polymerization of 'less activated monomers': Fundamentals, challenges and opportunities[END_REF][START_REF] Banerjee | Organometallic-Mediated Radical Polymerization of Vinylidene Fluoride[END_REF], especially when using cobalt complexes as moderators [START_REF] Michelas | 1.03 -Reversible Homolysis of Metal-Carbon Bonds[END_REF]. In OMRP, "direct" and "reverse" initiation paths can be distinguished [START_REF] Fliedel | 9.14 -Metal Complexes as Catalysts/Moderators for Polymerization Reactions[END_REF]. A direct initiation occurs via homolysis of a [L/Mt x+1 -R0'] organometallic species, furnishing the primary radical • R0' and the [L/Mt x ] moderator. In contrast, a reverse initiation requires the use of a radical precursor, typically an azo-compound such as AIBN [2,2′-azobis(2-methylpropionitrile)], to generate the primary radical (R0) and the presence of the moderator (Scheme 1). While the reverse initiation necessitates the use of sensitive radical initiators and a suitable [L/Mt x ] complex, which is often subject to easy oxidation, the direct initiation appears more attractive. However, only very few unimolecular systems were reported; they are not easily accessible and are generally sensitive to heat or light [START_REF] Demarteau | Organocobalt Complexes as Sources of Carbon-Centered Radicals for Organic and Polymer Chemistries[END_REF][START_REF] Demarteau | Halomethyl-cobalt(bis-acetylacetonate) for the controlled synthesis of functional polymers[END_REF][START_REF] Zhao | A well-defined, versatile photoinitiator (salen)Co-CO2CH3 for visible light-initiated living/controlled radical polymerization[END_REF][START_REF] Debuigne | Mechanistic insights into the cobalt-mediated radical polymerization (CMRP) of vinyl acetate with cobalt(III) adducts as initiators[END_REF].

We are currently working on the development of air-stable and easy-to-handle unimolecular [L/Mt-R0] OMRP initiators to offer an alternative to the existing systems [START_REF] Demarteau | Organocobalt Complexes as Sources of Carbon-Centered Radicals for Organic and Polymer Chemistries[END_REF][START_REF] Morales-Cerrada | Fluoroalkyl Pentacarbonylmanganese(I) Complexes as Initiators for the Radical (co)Polymerization of Fluoromonomers[END_REF][START_REF] Morales-Cerrada | Fluoroalkyl Radical Generation by Homolytic Bond Dissociation in Pentacarbonylmanganese Derivatives[END_REF], with a special emphasis on cobalt-based systems due to their excellent performances in OMRP. In the present contribution, we report the synthesis of cobalt(III) carboxylate complexes of diamino bis-phenolate ligands, which were chosen on the basis of a recent work showing the ability of their iron(III) halide analogues to moderate the radical polymerization of styrene and methyl methacrylate under a cooperative ATRP/OMRP mechanism [START_REF] Allan | Controlled Radical Polymerization Mediated by Amine-Bis(phenolate) Iron(III) Complexes[END_REF]. Cobalt(II) counter parts were also synthesized and tested as potential moderators for processes with reverse initiation. Scheme 1. Direct and reverse initiation mechanisms (top) and control equilibria (bottom) of an organometallic-mediated radical polymerization (OMRP) process.

Experimental part 2.1. General considerations

The syntheses of the cobalt(II) complexes and the polymerization tests were conducted under an inert atmosphere (Ar), using standard Schlenk techniques or in an argon-filled glovebox, in flame-dried glassware and with dried and/or distilled monomers and solvents (see below). All other operations were carried out in air and using non-dried glassware and solvents.

Materials

Compounds cobalt(II) acetate (Sigma Aldrich, 99.99%), cobalt(II) acetylacetonate (Acros Organics, 99%), potassium hydroxide (Fisher Chemical, analytical reagent grade), triethylamine (Acros Organics, 99%), 2,2′-azobis(2-methylpropionitrile) (AIBN, Sigma Aldrich, 98%), 2,2'-Azobis(4-methoxy-2,4dimethylvaleronitrile) (V-70, Wako Chemicals), 1,3,5-trioxane (Sigma Aldrich, ≥99%) were used as received. Methyl methacrylate (MMA, Sigma Aldrich, 99%), methyl acrylate (MA, Acros Organics, 99%), styrene (St, Sigma Aldrich, ≥99%), vinyl acetate (VAc, Acros Organics, 99%) and tert-butyl acrylate (tBA, Sigma Aldrich, 98%) were passed through a column filled with neutral alumina, dried over calcium hydride (CaH2), distilled under reduced pressure, degassed by several freeze/pump/thaw cycles and stored in a freezer under argon. Anhydrous methanol and chloroform (≥99%, Sigma Aldrich), reagent grade pentane (≥99.5%, VWR Chemicals) and CDCl3 (99.8%D, Euriso-top) were used as received, without prior purification. Toluene was dried using an MBraun SPS system. Ligands H2L1 [START_REF] Shimazaki | A structural model for the galactose oxidase active site which shows counteranion-dependent phenoxyl radical formation by disproportionation[END_REF] and H2L2 [START_REF] Redjel | Acetylacetonato cobalt(III) and iron(III) complexes of picolylamine-and aminopropylamine-bis(phenolate) ligands: Synthesis, characterization and crystal structures[END_REF] were prepared according to literature methods.

Instrumentation

The NMR spectra were recorded on a Bruker Avance III 300 or 400 MHz spectrometer at ambient temperature. 1 H and 13 C chemical shifts (δ) are reported in ppm vs. SiMe4 and were determined using the residual 1 H and 13 C solvent peaks as internal standards. The coupling constants are reported in Hertz. The IR spectra were recorded in the 4000-100 cm -1 region on a PerkinElmer Frontier FT-IR spectrometer (ATR mode, diamond crystal). The electrospray ionization mass spectra (ESI-MS) were recorded on a Q-Tof Premier (Waters) instrument using nitrogen as drying agent and nebulizing gas and MeCN as solvent. The elemental analyses for all compounds were carried out by the analytical service of the LCC-Toulouse using a PerkinElmer 2400 CHNS/O Series II System (100V). The Mn and Mw/Mn of the polymers were determined by gel permeation chromatography (GPC) using a Shimadzu system equipped with a Shimadzu RID-20A refractive index detector and with two polymer standards service SDV analytical columns (1000 and 100 000 Å, 5 μm, 8 × 300 mm). Tetrahydrofuran (THF) was used as the eluent at a flow rate of 1 mL min -1 at 35 °C. Linear polystyrene standards were used for calibration.

X-ray structural analyses

A single crystal of each compound was mounted under inert perfluoropolyether at the tip of a glass fiber and cooled in the cryostream of either a Rigaku Oxford-Diffraction XCALIBUR Gemini EOS diffractometer for H2L1, a Nonius Bruker APEXII diffractometer for 4•CHCl3 and a Bruker APEXII for 5.

The structures were solved by using the integrated space-group and crystal structure determination SHELXT software [57] and refined by least-squares procedures on F 2 using SHELXL-2014 [58]. All H atoms attached to carbon or oxygen atoms were introduced in the calculations at idealised positions and treated according to the riding model. Compound 4•CHCl3 is a dinuclear complex arranged around an inversion centre, with half a molecule within the asymmetric unit. The two tert-butyl groups (C18 and C28) are statistically disordered. These disorders were treated using the tools (PART and SAME) available in SHELXL-2014. In compound 5, some residual electron density was difficult to model and therefore, the SQUEEZE function of PLATON [59] was used to eliminate the contribution of the electron density in the solvent region from the intensity data and the solvent-free model was employed from the final refinement. There are two cavities of 359 Å 3 per unit cell. PLATON estimated that each cavity contains 102 electrons which may correspond to a mixture of pentane and chloroform solvent molecules as suggested by chemical analyses. Crystal data and refinement parameters are shown in Table 1.

Synthesis of complex [Co II (L1)] (3)

In an argon-filled glovebox, solid [Co(acac)2] (944 mg, 3.7 mmol) was added in one portion to a solution of pro-ligand H2L1 (2 g, 3.7 mmol) and KOH (412 mg, 7.4 mmol) in a 1:1 mixture of dry MeOH/CHCl3 (50 mL) at room temperature. The resulting mixture was kept under inert atmosphere and refluxed with stirring for 18 h. The resulting purple suspension was filtered through a Celite pad and the volatiles were removed under reduced pressure. The crude product was washed with pentane (2 x 20 mL) and dried under vacuum, leading to complex 3 as a light purple powder (1.96 g, 88%). Anal. Calcd. for C36H50CoN2O2 (601.74): C, 71.86; H, 8.38; N, 4.66. Found: 71.33; H, 8.46; N, 4.60. FTIR: νmax(solid)/cm -1 : 2953br, 1579vs, 1516vs, 1460s, 1398vs, 1357s, 1317m, 1276w, 1256s, 1201w, 1169w, 1017m, 923m, 877w, 762s, 650m, 552s, 449s, 422m. MS (ESI): m/z 601.3 [M] + . UV-Vis: λmax(toluene)/nm: 506 (ε/dm 3 mol -1 cm -1 : 0.19); 529 (ε/dm 3 mol -1 cm -1 : 0.17).

The same reaction was carried out with [Co(OAc)2] (650 mg, 3.7 mmol) instead of [Co(acac)2], leading to complex 3 in 83% yield (1.85 g).

Synthesis of complex [Co II (L2)] (4)

The same procedure as for 3 was applied, starting from [Co(OAc)2] (830 mg, 4.7 mmol) or [Co(acac)2] (1.21 g, 4.7 mmol), pro-ligand H2L2 (2 g, 4.7 mmol) and KOH (526 mg, 9.4 mmol) in 50 mL of dry MeOH/CHCl3 (1:1). Complex 4 was isolated as a pink-purple powder (1.93 g, 85% with [Co(OAc)2]; 1.84 g, 81% with [Co(acac)2]). Suitable crystals for the X-ray diffraction analysis were grown by slow evaporation of a saturated CHCl3 solution of 4 at room temperature over two weeks. Anal. Calcd. for C27H40CoN2O2 (483.56): C, 67.06; H, 8.34; N, 5.79. Found: 66.76; H, 8.30; N, 5.86. FTIR: νmax(solid)/cm -1 : 2952br, 2867sh, 1645w, 1567s, 1557s, 1465vs, 1434s, 1412vs, 1361m, 1303m, 1235s, 1203w, 1165w, 1017w, 1004w, 931w, 875m, 863m, 806m, 761s, 738m, 646s, 624sh, 541m, 481m, 449m. MS (ESI): m/z 483.2 [M] + . UV-Vis: λmax(toluene)/nm: 512 (ε/dm 3 mol -1 cm -1 : 0.19); 532 (ε/dm 3 mol -1 cm -1 : 0.21).

Synthesis of complex [Co III (L1)(OAc)] (5)

Solid [Co(OAc)2] (32.5 mg, 0.18 mmol) was added in one portion to a solution of pro-ligand H2L1 (100 mg, 0.18 mmol) and excess of NEt3 (0.5 mL) in a 2:1 mixture of MeOH/CHCl3 (30 mL) at room temperature. The resulting mixture was stirred at room temperature in air during 24 h. The crude product obtained after removal of the volatiles was recrystallized from CHCl3, leading to complex 5 as a dark red powder (75.2 mg, 62%). Suitable crystals for the X-ray diffraction analysis were grown by slow diffusion of pentane into a saturated CHCl3 solution of 5 over a week at room temperature. Anal. Calcd. for C38H53CoN2O4 (660.79): C, 69.07; H, 8.08; N, 4.24. Found: 68.79; H, 8.30; N, 4.34. FTIR: νmax(solid)/cm -1 : 2950br, 1602w, 1515w, 1470vs, 1443s, 1411s, 1361m, 1277s, 1238m, 1203m, 1167m, 1130w, 1059w, 950w, 914w, 875w, 835m, 805w, 763s, 748m, 688vs, 625m, 554m, 496s, 438m. MS (ESI): m/z 601.3 [M -OAc] + . UV-Vis: λmax(toluene)/nm: 428 (ε/dm 3 mol -1 cm -1 : 2687). 1 H NMR (400 MHz, CDCl3): δ (ppm) 8.55 (d, 3 J = 5.7 Hz, 1H, Harom, Py), 7.39 (t, 3 J = 7.7 Hz, 1H, Harom, Py), 6.99 (t, 3 J = 6.7 Hz, 1H, Harom, Py), 6.94 (d, 4 J = 2.5 Hz, 2H, Harom, Phenol), 6.79 (d, 4 J = 2.5 Hz, 2H, Harom, Phenol), 6.63 (d, 3 J = 7.9 Hz, 1H, Harom, Py), 4.59 (d, 2 J = 13.3 Hz, 2H, CH2), 3.97 (s, 2H, CH2), 3.06 (d, 2 J = 13.2 Hz, 2H, CH2), 1.95 (s, 3H, OC(CH3)O), 1.31 (s, 18H, t Bu), 1.20 (s, 18H, t Bu). 13

General procedure for the polymerization tests

For the direct initiation. In the glove box, a tube was charged with complex 5 (5.7 mg, 8.6 µmol), trioxane (ca. 5 mg) as internal standard and MA (0.23 mL, 2.6 mmol, 300 eq), and then sealed with a septum. The resulting dark brown mixture was stirred at 60°C over a period of 24 h. The monomer conversion was determined by withdrawing aliquots of the reaction mixture for gravimetric and 1 H NMR analyses.

A similar procedure was used for the attempt to polymerize MMA, using the following quantities: complex 5 (11.4 mg, 17.2 µmol, 1 eq), MMA (0.54 mL, 5.1 mmol, 300 eq) and trioxane (ca. 5 mg).

For the reverse initiation. In the glove box, a tube was charged with complex 3 (20 mg, 33.2 µmol, 1 eq), AIBN (3.3 mg, 20.0 µmol, 0.6 eq), the monomer (500 eq) and ca. 5 mg of trioxane as internal standard. The resulting mixture was stirred for 10 min at room temperature for homogenization and the tube was then immersed in a thermostatic oil bath at 65°C. For the polymerization of St and tBA, a hard block of polymer was formed in a few minutes. For VAc, aliquots were withdrawn with a degassed syringe at 0.6, 1, 1.5 and 2 h of reaction time and the monomer conversion was determined by 1 H NMR. The resulting residue was diluted with THF and the polymer was precipitated using an excess of pentane and dried under vacuum overnight at 80 °C before GPC characterization.

Results and discussion

Synthesis and characterization of the ligands and complexes

The H2L1 and H2L2 pro-ligands were synthesized according to previously published procedures by Yamauchi and coworkers [START_REF] Shimazaki | A structural model for the galactose oxidase active site which shows counteranion-dependent phenoxyl radical formation by disproportionation[END_REF] and our group [START_REF] Redjel | Acetylacetonato cobalt(III) and iron(III) complexes of picolylamine-and aminopropylamine-bis(phenolate) ligands: Synthesis, characterization and crystal structures[END_REF], respectively (Scheme 2). In our previous contribution, we reported that all attempts to crystalize H2L2 failed and we only succeeded in crystallizing its HCl adduct (H3L2 + Cl -) [START_REF] Redjel | Acetylacetonato cobalt(III) and iron(III) complexes of picolylamine-and aminopropylamine-bis(phenolate) ligands: Synthesis, characterization and crystal structures[END_REF]. To date, pro-ligand H2L1 was only structurally characterized in its [CuCl2] [START_REF] Shimazaki | A structural model for the galactose oxidase active site which shows counteranion-dependent phenoxyl radical formation by disproportionation[END_REF] and [Ni(OAc)2] [60] adducts. Along the course of the present work, we succeeded in growing single crystals of H2L1, suitable for X-ray diffraction studies, by slow diffusion of pentane into a saturated dichloromethane solution. Its molecular structure is depicted in Fig. 1. Pro-ligand H2L1 crystallized in the P21/c space group with four independent molecules in the unit cell. The "central" nitrogen atom (N1), which connects the three arms, has a nearly perfect tetrahedral geometry with the Cx-N1-Cy (x, y = 1, 2, 3) angles ranging between 109.51 [START_REF] Xiao | Bio-Inspired Molecular Catalysts for Water Oxidation[END_REF] and 111.46(8)°. Three intramolecular Hbonds are observed (Fig. 1), two of them involving N1 as a proton acceptor towards the O-H groups of both phenol rings (N1•••O2 2.7318(13) Å and N1•••O3 3.0768(12) Å). The third one connects the pyridine N16 atom as a proton acceptor and O3-H (2.8637(14) Å). Intramolecular H-bonds between the "central" nitrogen atom and the phenol moieties are regularly observed in the crystal structures of (di)amino bis-phenolate ligands [START_REF] Bowser | Diamine bis(phenolate) and pendant amine bis(phenolate) ligands: catalytic activity for the room temperature palladiumcatalyzed Suzuki-Miyauracoupling reaction[END_REF][START_REF] Taherimehr | New iron pyridylaminobis(phenolate) catalyst for converting CO2 into cyclic carbonates and cross-linked polycarbonates[END_REF][START_REF] Schroeder | A Synthetic and Multispectroscopic Speciation Analysis of Controlled Radical Polymerization Mediated by Amine-Bis(phenolate)iron Complexes[END_REF][START_REF] Chapurina | Scandium versus yttrium{amino-alkoxy-bis(phenolate)} complexes for the stereoselective ringopening polymerization of racemic lactide and beta-butyrolactone[END_REF][START_REF] Wichmann | Uranyl(VI) complexes with a diaminobisphenol from eugenol and N-(2-aminoethyl)morpholine: Syntheses, structures and extraction studies[END_REF]. Thus, the O3-H proton donor is involved in bifurcated hydrogen bonding with two proton acceptors. Scheme 2. Pro-ligands H2L1 and H2L2. In our previous contribution, we have shown that acetylacetonato cobalt(III) and iron(III) complexes of ligands L1 and L2, of general formula [M(acac)(L)] (M = Co or Fe; L = L1 or L2), do not behave as unimolecular initiators of a radical polymerization process [START_REF] Redjel | Acetylacetonato cobalt(III) and iron(III) complexes of picolylamine-and aminopropylamine-bis(phenolate) ligands: Synthesis, characterization and crystal structures[END_REF]. Therefore, in the present contribution, we aimed at assessing two points: 1) the behavior of the corresponding cobalt(II) complexes as moderators in OMRP, via reverse initiation (see Introduction) and 2) the ability of the corresponding acetato cobalt(III) complexes to initiate a radical polymerization. We focused on cobalt complexes and excluded from this study their iron analogues, because of their generally observed lower performances as moderators in radical polymerization [START_REF] Debuigne | Overview of cobalt-mediated radical polymerization: Roots, state of the art and future prospects[END_REF][START_REF] Poli | Iron-mediated reversible deactivation controlled radical polymerization[END_REF].

First, we explored the synthetic pathways to access the cobalt(II) complexes of the diamino bisphenolate ligands L1 and L2. Our initial attempts, which consisted in reacting the pro-ligands with a base (NEt3 or KO t Bu) followed by the addition of [CoCl2], or directly in the presence of [CoCl2], were unsuccessful (most of the metal precursor remained unreacted). Therefore, we turned our attention to the use of cobalt precursors incorporating an internal base, such as [Co(acac)2] and [Co(OAc)2] [START_REF] Thevenin | Cobalt complexes of an OSNSO-tetrapodal pentadentate ligand: Synthesis, structures and reactivity[END_REF]. The first tests, carried out under stoichiometric conditions (without additional base), with both [Co(acac)2] and [Co(OAc)2], led only to poor yields of the desired products [Co II (L1)] (3) and [Co II (L2)] (4). Further tests were performed with the addition of an external base (NEt3 or KOH). While the results with NEt3 were not satisfying, KOH improved the outcome of the reactions. Careful monitoring of the pro-ligand consumption by TLC highlighted that [Co(acac)2] is much more reactive. Optimized yields were obtained using H2L:Co(acac)2:KOH 1:1:2 (L = L1 or L2) in a mixture of toluene and methanol (1:1) at reflux for 18 h (Scheme 3). Whatever the nature of the pro-ligand (H2L1 or H2L2), the yields of the reactions were similar (80-90%, see Experimental part). Complexes 3 and 4 were characterized by elemental analysis (EA), infrared (FT-IR) and UV-Vis spectroscopic techniques and mass spectrometry (ESI-MS). For each complex, the most intense peak in the ESI mass spectrum corresponds to [M] + with the expected isotopic distribution (Figures S1 andS2, ESI). The UV-vis spectra of complexes 3 and 4 (Figure S8, ESI) exhibit two distinct absorption bands at similar wavelengths, i.e. 506 and 529 nm for 3 and 512 and 532 nm for 4, with ε vales in the same order of magnitude (0.17-0.21 dm 3 mol -1 cm -1 ). While all attempts to crystallize complex 3 failed, suitable single crystals of complex 4 for an X-ray diffraction study were grown by slow evaporation from a CHCl3 solution. As shown in Fig. 2, complex 4 crystalizes as a dimer with the two Co(L2) units connected via two µ-O bridges in a planar Co2O2 central core. The cobalt(II) center is five-coordinated with, as expected, ligand L2 acting as a tetradentate dianionic κ 4 -(O2,N2) ligand and the coordination sphere is completed by the coordination of a phenoxy group from the adjacent Co(L2) unit, forming the dimeric structure. In this type of five-coordinate metal complexes, the τ parameter allows a quantitative measure of the degree of distortion between the perfect trigonal bipyramid (TBP; τ = 1) and square pyramid (SP; τ = 0) extremes [START_REF] Addison | Synthesis, structure, and spectroscopic properties of copper(II) compounds containing nitrogen-sulphur donor ligands; the crystal and molecular structure of aqua[1,7-bis(N-methylbenzimidazol-2′-yl)-2,6dithiaheptane]copper(II) perchlorate[END_REF][START_REF]β -α)/60, where α and β are the angles that are opposite each other in the plane (e.g. N2O2 for Salen ligands), and by convention β is the most obtuse angle. The τ value ranges from 0 to 1; a value of 0 identifies a perfectly SP geometry and a value of 1 a perfectly TBP[END_REF]. The value of this parameter for 4 (τ = 0.17) indicates a slightly distorted square pyramidal geometry. The values of the Co-O-Co angles in 4 (101-103°) are quite close to those of the same Co2O2 motif observed in the crystal structures of several other cobalt(II) complexes of diamino-bis(phenolate) ligands [START_REF] Mukherjee | Diphenoxido-Bridged CoII and ZnII Complexes of Tripodal N2O2 Ligands: Stabilisation of MII-Coordinated Phenoxyl Radical Species[END_REF][START_REF] Labisbal | Electrochemical synthesis and structural characterization of Co(II), Ni(II) and Cu(II) complexes of N,N-bis(4,5-dimethyl-2-hydroxybenzyl)-N-(2-pyridylmethyl)amine[END_REF][START_REF] Mukherjee | Synthesis and Properties of Diphenoxo-Bridged CoII, NiII, CuII, and ZnII Complexes of a New Tripodal Ligand: Generation and Properties of MII-Coordinated Phenoxyl Radical Species[END_REF][START_REF] Schnieders | Zinc and Cobalt Complexes Derived from Tetradentate Tripodal Ligands with N2O2 Donorset as Model Compounds for Phosphatases[END_REF][START_REF] Rodríguez | Coordination Chemistry of Amine Bis(phenolate) Cobalt(II), Nickel(II), and Copper(II) Complexes[END_REF]. However, when sterically demanding groups are present in the ortho-positions of the phenolate moieties and/or on the pending N-based donor, preventing the formation of dimeric structures, or upon addition of Lewis bases or in coordinating solvents (L), the corresponding mononuclear [Co(O2,N2)(L)] adducts are obtained [START_REF] Das | Synthesis and structure of mono-, bi-and trimetallic amine-bis(phenolate) cobalt(II) complexes[END_REF][START_REF] Saunders | Structural variations in the coordination chemistry of amine-bis(phenolate) cobalt(II/III) complexes[END_REF]. Based on the dimeric structure reported for a cobalt(II) complex supported by a ligand analogous to L1 [START_REF] Das | Synthesis and structure of mono-, bi-and trimetallic amine-bis(phenolate) cobalt(II) complexes[END_REF], differing only by the presence of o-and p-tertpentyl in place of t Bu groups, we assume that complex 3 adopts a dimeric structure similar to 4. Scheme 3. Synthesis of complexes 3 and 4, and dimeric structure of 4 determined by XRD after crystallization from CHCl3. In contrast to the synthesis of the acetylacetonato cobalt(III) complexes with ligands L1 and L2 [START_REF] Redjel | Acetylacetonato cobalt(III) and iron(III) complexes of picolylamine-and aminopropylamine-bis(phenolate) ligands: Synthesis, characterization and crystal structures[END_REF], which used the commercially available [Co(acac)3] precursor, the same strategy is not conceivable for the synthesis of the corresponding acetate cobalt(III) complexes, due to non-availability of a [Co(OAc)3] precursor. Other common literature strategies consist of performing the reaction with the cobalt(II) precursor, i.e. [Co(OAc)2], under aerobic conditions, which induces the oxidation of the Co II ion to Co III , without formation of any oxo-(from O2) nor hydroxo-(from H2O in air) species [START_REF] Saunders | Structural variations in the coordination chemistry of amine-bis(phenolate) cobalt(II/III) complexes[END_REF], or of initially oxidizing the metal precursor using H2O2 followed by complexation [START_REF] Garai | Photorelease and Cellular Delivery of Mitocurcumin from Its Cytotoxic Cobalt(III) Complex in Visible Light[END_REF]. Given our objective to develop unimolecular initiators for radical polymerization processes, we focused our attention on the complex with the more sterically encumbering ligand, i.e. L1. Indeed, the [Co III (L1)(OAc)] complex [START_REF] Murrie | Cobalt(ii) single-molecule magnets[END_REF] was obtained in 62% yield after recrystallization, using the first strategy (Scheme 4). Complex 5 is diamagnetic, suggesting an octahedral environment for the Co III metal ion with a d 6 electronic configuration. Its 1 H and 13 C{ 1 H} NMR spectra (Figures S3-S6, ESI) exhibit all the signals expected for the diamino-bis(phenolate) ligand L1 [START_REF] Redjel | Acetylacetonato cobalt(III) and iron(III) complexes of picolylamine-and aminopropylamine-bis(phenolate) ligands: Synthesis, characterization and crystal structures[END_REF], and additional resonances for the OAc group, i.e. one 1 singlet at δ 1.95 ppm (CH3) [START_REF] Saunders | Structural variations in the coordination chemistry of amine-bis(phenolate) cobalt(II/III) complexes[END_REF] and two 13 C peaks at δ 24.0 (CH3) and 191.7 (OCO) ppm. The EA is also in agreement with the [Co(1)(OAc)] formula and the ESI-MS spectrum exhibited only one isotopic envelope (with the appropriate distribution) corresponding to the [M -OAc] + fragment, probably due to the fragility of the Co-OAc bond (Figures S7,ESI). The Co III complex 5 is intensely colored (dark brown), in contrast to its Co II analog (3, light purple), as shown by their respective UV-Vis spectra and ε values (see Experimental Part and Figure S8, ESI). Indeed, the UV-Vis spectrum of complex 5 exhibit a very strong absorption band at λmax = 428 nm (ε = 2 687 dm 3 mol -1 cm -1 ) blue shifted and much more intense compared to its Co II analog 3 (λmax = 506 and 529 nm (ε = 0.19 and 0.17 dm 3 mol -1 cm -1 , respectively, Figure S8, ESI). The molecular structure, obtained by an X-ray diffraction study, is depicted in Fig. 3. To the best to our knowledge, only one other compound of the type [Co III (O2,N2)(OAc)], with O2,N2 = diamino-bis(phenolate) ligand, was structurally characterized [START_REF] Saunders | Structural variations in the coordination chemistry of amine-bis(phenolate) cobalt(II/III) complexes[END_REF]. In complex 5, the Co III metal center is found in a slightly distorted octahedral environment, which is composed of one dianionic κ 4 -(O2,N2) diamino-bis(phenolate) ligand and one chelating acetate ligand. Unlike the O3-Co1-O4 angle of 67.41 [START_REF] Poli | Organometallic Mediated Radical Polymerization[END_REF]° imposed by the small OAc ligand bite, all other coordination angles around the Co III center are close to the ideal 90° (85.7(2)-95.26(18)°). Interestingly, the bond lengths involving the donor atoms of the supporting ligand L1 (O1, O2, N1, N2) and the Co III metal center are all slightly shorter than those observed in the analogous [Co III (L1)(acac)] complex [START_REF] Redjel | Acetylacetonato cobalt(III) and iron(III) complexes of picolylamine-and aminopropylamine-bis(phenolate) ligands: Synthesis, characterization and crystal structures[END_REF] and in an even more closely related [Co III (O2,N2)(OAc)] complex (O2,N2 = diamino-bis(phenolate) ligand) that differs only by the substitution of the -CH2Py arm by a -CH2CH2NMe2 arm [START_REF] Saunders | Structural variations in the coordination chemistry of amine-bis(phenolate) cobalt(II/III) complexes[END_REF]. However, the Co-O3 and Co-O4 bonds involving the OAc donor atoms are similar to these in the aforementioned related complex reported by Kozak and coll. [START_REF] Saunders | Structural variations in the coordination chemistry of amine-bis(phenolate) cobalt(II/III) complexes[END_REF], but significantly longer than those involving the acac group donor atoms of the [Co III (L1)(acac)] complex that we recently reported [START_REF] Redjel | Acetylacetonato cobalt(III) and iron(III) complexes of picolylamine-and aminopropylamine-bis(phenolate) ligands: Synthesis, characterization and crystal structures[END_REF].

Polymerization tests

As stated in the Introduction, our main motivation for carrying this work was to provide new air-, lightand temperature-stable, well-defined unimolecular initiators/moderators for controlled radical polymerization processes. We identified a contribution dating back to the 80's that mentioned the use of a [Co III (salen)(acac)] [Salen = N,N'-ethylenebis(salicylideneiminato)] complex, which was able to initiate the radical polymerization of methyl methacrylate [START_REF] Thiagarajan | Mixed ligand complexes in vinyl polymerization. I. [N,N′ethylenebis (salicylideneiminato)](acetylacetonato)cobalt(III) as an initiator[END_REF]. The polymerization initiation was proposed to occur via the thermal activation of the Co-acac bond (homolysis) and generation of an acac • radical. However, the propagation occurred as in a free-radical mechanism with no indication of a moderating effect by the metal complex, in contrast to a controlled radical polymerization (OMRP). This study inspired our recent work on the development of [Co III (L1/L2)(acac)] [N2,O2 = diaminobis(phenolate) ligand] for the same purpose. However, these complexes did not show any ability to initiate a radical polymerization [START_REF] Redjel | Acetylacetonato cobalt(III) and iron(III) complexes of picolylamine-and aminopropylamine-bis(phenolate) ligands: Synthesis, characterization and crystal structures[END_REF]. We attributed this lack of reactivity to the (too) strong chelation of the acac ligand, which prevented bond homolysis and departure as a free radical from the Co III center. For this reason, we moved to the synthesis of the analogous OAc complex 5, OAc supposedly forming a weaker chelate than acac.

Complex 5 was evaluated as initiator for the bulk polymerization of methyl methacrylate (MMA) and methyl acrylate (MA) at 60°C over a period of 24 h and aliquots were withdrawn regularly to monitor the conversion (see Experimental Part). Unfortunately, no polymer formation was observed under these conditions. While complex 5 was found ineffective as a unimolecular initiator, the potential of its Co II derivative, i.e. [Co II (L1)] (3), to behave as an OMRP moderator via reverse initiation remained possible. The bulk polymerization of styrene (St) and tert-butyl acrylate (tBA) in the presence of 3 or 4 at 65°C using AIBN as radical source (reaction conditions: 3 or 4/St or tBA/AIBN = 1/500/0.6) led to the rapid (few minutes) formation of a hard block of the corresponding polymers, indicating lack of control for these polymerizations. No better control was observed when performing the polymerizations at a lower temperature (30°C) using V-70 as radical initiator. In all cases, the cobalt complexes are fully soluble in the polymerization medium. This behavior is not surprising, since the active radicals of growing polystyrene and polyacrylate chains are known to form weak bonds with certain Co II complexes. Stronger bonds are expected for less-activated monomers. Thus, additional tests were carried out with vinyl acetate (VAc). Indeed, Peng and coll. very recently studied the performances of cobalt complexes of phenoxy-imine ligands towards the radical polymerization of VAc and MA [START_REF] Chen | Cobalt(II) phenoxy-imine complexes in radical polymerization of vinyl acetate: The interplay of catalytic chain transfer and controlled/living radical polymerization[END_REF]. For certain complexes, the VAc polymerization exhibited the features of controlled chain growth, i.e. OMRP, while for other complexes, extensive catalytic chain transfer appeared to take place. As in our case, the polymerization of MA, however, did not show any feature of a controlled process. By analyzing the polymerization results obtained with other four-coordinated cobalt-based systems, the authors concluded that the nature of the coordination geometry (square planar vs. tetrahedral) and the spinstate of the Co center were more decisive than the donor atom set (N4, N2,O2, O4) for the controlled polymerization of both LAMs and MAMs [START_REF] Chen | Cobalt(II) phenoxy-imine complexes in radical polymerization of vinyl acetate: The interplay of catalytic chain transfer and controlled/living radical polymerization[END_REF][START_REF] Benchaphanthawee | Organo-Cobalt Complexes in Reversible-Deactivation Radical Polymerization[END_REF]. The results of the bulk polymerization of vinyl acetate under the same conditions, i.e. 3/VAc/AIBN = 1/500/0.6 at 65°C, are presented in Table 2 and GPC traces are available in the ESI (Figure S9). Complex 3 is fully soluble in the polymerization medium under these conditions. Altogether, the fast polymerization rate, rapid formation of a polymeric material with much higher molecular weights (Mn(GPC)) than expected (Mn(theo)), which did not evolve much with the conversion, and dispersities between 1.36 and 1.86 imply a lack of control of the radical polymerization process. We observed a similar behavior during the radical polymerization of VAc in the presence of a bis(9-oxyphenalenone) cobalt(II) system, for which the occurrence of catalytic chain transfer (CCT) could be proven [START_REF] Bellan | Catalyzed Chain Transfer in Vinyl Acetate Polymerization Mediated by 9-Oxyphenalenone Cobalt(II) Complexes[END_REF]. The same polymerization carried out without cobalt complex led to the rapid formation of a highly viscous, nearly solid, mixture (Figure S10 in the ESI). The recorded data showed that the polymerization was much faster (35% conv in 30 min, vs. 28% in 60 min) and the MW of the PVAc formed much higher (300 000 vs 60 000 g/mol, Figure S10 in the ESI). The data are shown in the SI -Figure S10. As in the case of St and tBA, running the polymerization at 30°C, using V-70 as radical initiator, led to slower polymerization rate but did not positively affect the polymerization control. Under the optimized conditions, complex 4 did not perform better than 3.

Conclusion

We have presented the synthesis and characterization of cobalt(II) (3, 4) and acetato cobalt(III) (5) complexes of two tripodal tetradentate diamino-bis(phenolate) ligands (L1 and L2). The solid-state molecular structure of 4 has highlighted the formation of a dimeric structure containing a Co2O2 core, in which the cobalt(II) metal ions are five-coordinated in a slightly distorted square planar pyramidal geometry (τ = 0.17). In contrast, the crystal structure of complex 5 revealed, as expected, a monomeric complex in which the cobalt(III) center is six-coordinated, with ligand L1 acting as a tetradentate dianionic κ 4 -(O2,N2) chelating ligand and the chelating monoanionic OAc group completing the coordination sphere. The diamagnetic nature of 5 allowed its characterization by multinuclear NMR spectroscopic techniques. Complex 5 was evaluated as unimolecular initiator for the radical polymerization of MA nor MMA without success. The potential of its cobalt(II) analog (3) to moderate the radical polymerization of activated (St, tBA) or non-activated (VAc) monomers was also evaluated, and no features of a controlled polymerization process could be recorded. From this study and from our previous work on acetylacetonato cobalt(III) complexes of diamino-bis(phenolate) ligands [START_REF] Redjel | Acetylacetonato cobalt(III) and iron(III) complexes of picolylamine-and aminopropylamine-bis(phenolate) ligands: Synthesis, characterization and crystal structures[END_REF], it clearly appears that the combination of this ligand scaffold and cobalt is not best suited for the targeted application. We are therefore exploring other ligand architectures to develop original air-, light-and thermally stable unimolecular initiators of radical polymerization processes.

Fig. 1 .

 1 Fig. 1. View of the molecular structure of H2L1. Only the hydrogen atoms of the OH groups are represented for clarity. Ellipsoids are represented at the 50% probability level. Selected bond lengths (Å) and angles (°): N1-C1 1.4624(14), N1-C2 1.4777(13), N1-C3 1.4792(13), C22-O2 1.3730(13), C32-O3 1.3661(13), C1-N1-C2 111.46(8), C1-N1-C3 110.29(9), C2-N1-C3 109.51(8).

Fig. 2 .

 2 Fig. 2. View of the molecular structure of 4 in 4•CHCl3. The hydrogen atoms are omitted for clarity. Ellipsoids are represented at the 30% probability level. Selected bond lengths (Å) and angles (°): Co1-O1 1.9313(16), Co1-O2 1.9890(16), Co1-N1 2.2553(19), Co1-N2 2.127(2), Co1-O2' 2.1290(16), N1-Co1-N2 98.19(8), N1-Co1-O1 87.72(7), N1-Co1-O2 90.90(7), N1-Co1-O2' 162.43(7), O1-Co1-O2 152.51(8), Co1-O2-Co1' 101.50(7), C1-N1-C11 109.47(18), C1-N1-C21 110.28(18), C11-N1-C21 104.66(18), C3-N2-C4 110.2(2), C3-N2-C5 107.3(2), C4-N2-C5 108.0(2).

Scheme 4 .

 4 Scheme 4. Synthesis of complex 5.

Fig. 3 .

 3 Fig. 3. View of the molecular structure of 5. The hydrogen atoms are omitted for clarity. Ellipsoids are represented at the 50% probability level. Selected bond lengths (Å) and angles (°): Co1-O1 1.875(4), Co1-O2 1.901(4), Co1-N1 1.942(5), Co1-N2 1.892(5), Co1-O3 1.932(4), Co1-O4 1.970(4), O1-Co1-N1 90.72(18), O1-Co1-N2 85.89(19), O1-Co1-O3 91.14(17), O1-Co1-O4 88.36(17), O2-Co1-N1 95.26(18), O2-Co1-N2 93.96(18), O2-Co1-O3 88.10(17), O2-Co1-O4 85.85(16), N2-Co1-N1 85.7(2), O3-Co1-O4 67.41(18), O4-C41-O3 116.5(6).

Table 1 .

 1 Crystallographic data for compounds H2L1, 4•CHCl3 and 5.

	Compound	H2L1	4•CHCl3	5
	CCDC number	2208875	2208876	2208877
	Empirical formula	C36H52N2O2	C27H40CoN2O2, CHCl3	C38H53CoN2O4
	Formula weight	544.79	602.91	660.75
	Temperature, K	173(2)	173(2)	173(2)
	Wavelength, Å	1.54184	0.71073	0.71073
	Crystal system	Monoclinic	Triclinic	Monoclinic
	Space group	P 21/c	P -1	C 2/c
	a, Å	16.23240(10)	11.1896(4)	29.704(4)
	b, Å	8.764	11.8262(6)	17.917(3)
	c, Å	23.3993(2)	13.9587(5)	14.5301(19)
	, °	90.0	110.589(2)	90.0
	β, °	103.4480(10)	90.937(2)	94.423(4)
	, °	90.0	118.182(2)	90.0
	Volume, Å 3	3237.35(4)	1486.47(11)	7709.7(19)
	Z	4	2	8
	Density (calc), Mg m -3	1.118	1.347	1.139
	Abs. coefficient, mm -1	0.522	0.874	0.482
	F(000)	1192	634	2832
	Crystal size, mm 3	0.250 x 0.200 x 0.150 0.210 x 0.140 x 0.110	0.100 x 0.080 x 0.020
	Theta range, °	2.799 to 71.433	2.117 to 26.368	1.895 to 25,060°
	Reflections collected	61043	40512	69569
	Indpt reflections (Rint)	6285 (0.0379)	6074 (0.0371)	6830 (0.2454)
	Absorption correction	Multi-scan	Multi-scan	Multi-scan
	Max. / min. transmission	1.0 and 0.78295	0.7454 and 0.6903	0.7466 and 0.7156
	Refinement method	F 2	F 2	F 2
	Data /restraints/parameters 6285 / 0 / 374	6074 / 15 / 321	6830 / 0 / 419
	Goodness-of-fit on F 2	1.045	1.026	1.030
	R1, wR2 [I>2(I)]	0.0416, 0.1088	0.0425, 0.1022	0.0859, 0.1642
	R1, wR2 (all data)	0.0443, 0.1118	0.0524, 0.1095	0.1864, 0.2048
	Residual density, e Å -3	0.450 / -0.363	0.894 and -0.713	0.948 / -0.501

Table 2 .

 2 Bulk polymerization of VAc initiated by AIBN at 65 °C in presence complex 3. a Conditions: [3]/VAc/AIBN = 1/500/0.6. b Measured by 1 H NMR. c Calculated by conv x 500 x 86.09 (VAc). d Measured by GPC in THF (35°C) at a flow of 1 mL/min.

	t (h) Conv (%) b Mn(theo) c Mn(GPC) d Ð d
	0.6	19.7	8400	57 000 1.36
	1	28.0	12 040 60 000 1.45
	1.5	57.8	24 840 62 000 1.42
	2	75.0	32 270 65 000 1.86
	a

Acknowledgements

The authors thank the Agence Nationale de la Recherche, France (Grant ANR-19-CE07-0031-01, POLYSWITCH) for support of this work. Additional support by the Ministère de l'Enseignement Supérieur et de la Recherche Scientifique, France (PROFAS B+ fellowship to Y.K.R), the Centre National de la Recherche Scientifique (CNRS, France) and the Institut Universitaire de France (IUF, France) are also gratefully acknowledged.

Conflict of interest

The authors declare no conflict of interest.

Supplementary data

ESI-MS spectra of complexes 3-5; NMR spectra of complex 5; UV-Vis spectra of complexes 3-5; GPC traces of the bulk polymerization of VAc initiated by AIBN at 65 °C in presence or absence of complex 3 are available in the Electronic Supporting Information. CCDC 2208875 (H2L1), 2208876 (4•CHCl3) and 2208877 ( 5) contain the supplementary crystallographic data for this paper. These data are provided free of charge by the Cambridge Crystallographic Data Centre. Supplementary data to this article can be found online at https://doi.org/xxx.