L Couëdel

A A Samarian

M Mikikian

L Boufendi

Dust density effect on complex plasma decay

Keywords: Complex plasma, dust, afterglow, decay PACS: 52.27.Lw

In this paper, the influence of dust particles on the plasma losses in a complex plasma afterglow is studied. It is shown that the dust particles can drastically shorten the plasma loss time by absorption-recombination onto their surfaces. The dust particle absorption frequency increases with the dust density but the dependence is not linear for high dust density. Finally, the possible use of dust absorption frequency measurements as a diagnostics for complex plasmas is mentioned and supported by comparison to existing experimental data.

Introduction

Complex plasmas (also called dusty plasmas) are partially ionised gases in which there are charged dust particles. They can be natural plasmas (such as near-Earth plasmas, combustion plasmas, comet tails, planetary rings, interstellar clouds, etc) or human-made plasmas (PECVD, etching, fusion reactor,etc). The dust particles are charged due to their interactions with the surrounding plasma electrons and ions [START_REF] Bouchoule | Dusty Plasmas: Physics, Chemistry and Technological impacts in Plasma Processing[END_REF][START_REF] Vladimirov | Physics and Applications of Complex Plasmas[END_REF]. The presence of this new charged species leads to interesting phenomena that distinguish complex plasma from classic dust-free plasma [START_REF] Barkan | Laboratory observation of the dust-acoustic wave mode[END_REF][START_REF] Cavarroc | Successive generations of dust in complex plasmas: A cyclic phenomenon in the void region[END_REF][START_REF] Mikikian | Self-excited void instability in dusty plasmas: plasma and dust cloud dynamics during the heartbeat instability[END_REF]. In laboratory discharges, the dust particles can be either grown directly in the plasma chamber (by sputtering [START_REF] Mikikian | Low frequency instabilities during dust particle growth in a radiofrequency plasma[END_REF][START_REF] Samsonov | Particle growth in a sputerring discharge[END_REF] or using reactive gases [START_REF] Cavarroc | Single-crystal silicon nanoparticles: An instability to check their synthesis[END_REF][START_REF] Hong | Growth precursors and dynamics of dust particle formation in the Ar/CH 4 and Ar/C 2 H 2 plasmas[END_REF]) or injected inside the plasma [START_REF] Arnas | High negative charge of a dust particle in a hot cathode discharge[END_REF][START_REF] Samarian | Sheath measurement in rfdischarge plasma with dust grains[END_REF]. Due to a higher mobility of the electrons the dust particles are negatively charged. Consequently, in running discharges, dust particles act as a sink for the electrons of the plasma [START_REF] Vladimirov | Physics and Applications of Complex Plasmas[END_REF]. The loss of charged species in a complex plasma afterglow is of major importance as it determines the charge carried by the dust particles during the decay process and is fundamental to explain the dust particle residual charges observed experimentally in the late afterglow [START_REF] Couëdel | Residual dust charges in discharge afterglow[END_REF]13]. Dust particle residual charge can be useful (for example, the cleaning of industrial or fusion reactors using electrostatic * lcouedel@physics.usyd.edu.au techniques) or harmful (for example, future single electron devices where residual charge on deposited nanocrystal would be the origin of malfunction). In a complex plasma afterglow, the dust particles will increase the plasma losses as ion and electron absorptionrecombination can occur at their surface. When the dust particle density is high the particle absorption time τ Q decreases. The plasma decay time τ L can thus be drastically shortened . This effect has been observed by Dimoff and Smy in 1970 [START_REF] Dimoff | Dust induced quenching of an afterglow plasma[END_REF] but their results were not properly explained, especially the effect of high dust density on the particle absorption frequency that was far from their theoretical prediction. In this paper, the influence of dust particle density on plasma losses due to recombination on to dust particle surfaces is discussed. It is shown that the dust particle absorption frequency does not grow linearly with the dust density. This effect is visible when the ratio of the total charge carried by the dust particles to the charge carried by the ions (Havnes parameter) is close to 1. Finally, by comparison of the model with existing experimental data, the possibility of using plasma decay time in a complex plasma as a diagnostic is discussed.

Theoretical approach

When a discharge is switched off, the ions and electrons of the plasma start to diffuse and the plasma density decreases as follows:

dn dt = - n τ L (1)
where n is the density of the plasma and τ L the plasma decay time. In a complex plasma, plasma losses are due to the diffusion of the ions and the electrons toward the wall of the reactor and by recombination onto the dust particle surfaces. Consequently, the plasma decay time τ L is:

τ -1 L = τ -1 D + τ -1 Q (2)
where τ D is the plasma diffusion time and τ Q the dust particle absorption time. As the dust particles are charged due to the fluxes of ions and electrons flowing to their surfaces, the dust particle absorption time will depend on the charge (and surface potential) of the dust particle. Using the orbital motion limited approach the charge on a dust particle is given by [START_REF] Vladimirov | Physics and Applications of Complex Plasmas[END_REF]:

dZ d dt = J e -J i = πr 2 d [n e v Te e -ϕ -n i v Ti (1 + T e T i ϕ)] (3)
where Z d is the dust particle charge number, J i(e) is the ion (electron) flux, r d is the dust particle radius, n i(e) is the ion (electron) density, v T i(e) is the ion (electron) thermal speed, T i(e) is the ion (electron) temperature and ϕ is the dimensionless dust surface potential. The latter can be linked do the dust charge number Z d by:

ϕ = Z d e 2 4πǫ 0 r d k B T e ( 4
)
where ǫ 0 is the vacuum dielectric permitivity and k B is the Boltzmann constant. It sould be noted that the influence of trapped ions on the dust particle charge is neglected. It has been shown that when T i << T e and (r d /λ D ) 2 << T i /T e where λ D is the Debye length, the trapped ions play a significant role in the charging process of dust particles [START_REF] Lampe | Trapped ion effect on shielding, current flow, and charging of a small object in a plasma[END_REF][START_REF] Zobnin | On the Charge of Dust Particles in a Low-Pressure Gas Discharge Plasma[END_REF] and Eq.3 has to be modified appropriately. However, for the conditions we are interested in, (r d /λ D ) 2 ∼ T i /T e . In a complex plasma afterglow, recombination on dust particle surfaces is directly linked to the flux of ion onto the dust particles as the dust particles are negatively charged in a laboratory plasma (Q d = -Z d e where Q d is the dust particle charge and e is the elementary charge). Consequently, the dust particle absorption time τ Q can be written as :

τ -1 Q = πr 2 d n d v T i (1 + T e T i ϕ) (5)
In a dust-free plasma (n e = n i ), the equilibrium surface potential of an isolated dust particle can be found by solving dZ d /dt = 0 and thus by solving:

T i T e m e m i 1/2 (1 + T e T i ϕ 0 ) exp(ϕ 0 ) = 1 (6)
In a complex plasma, however, the ion density n i and the electron density n e are different due to the charging of the dust particles. However, as the plasma is quasi-neutral, the electron density can be deduced using the neutrality equation:

n e = n i -Z d • n d (7)
The ratio of the electron density to the ion density can be thus written as:

n e n i = 1 - Z d n d n i = 1 -P H (8)
where P H = Z d n d /n i is the Havnes parameter. As it can be seen from Eq.7 and Eq.8, for very low dust density, the electron density is very close to the ion density (n e ≃ n i and P H ∼ 0). Consequently, the surface potential of the dust particle will be the same as for an isolated dust particle and the dust particle absorption time τ Q0 is inversely proportional to the dust density:

τ -1 Q0 = πr 2 d n d v T i (1 + T e T i ϕ 0 ) (9)
When the dust density increases, however, the electron and ion densities start to deviate from each other and the Havnes parameter increase. Consequently, solving dZ d /dt = 0, the dust surface potential follows:

T i T e m e m i 1/2 (1 + T e T i ϕ) exp(ϕ) = 1 -P H (10)
This equation has to be solved numerically and the value of the surface potential explicitly depends on the dust particle density. This will directly affect the dust particle absorption time. However, if the dust density is not too high, the deviation of the dimensionless dust surface potential from the isolated dust particle surface potential will be small so one can write ϕ = ϕ 0 + ∆ϕ. Using a first order approximation, Eq.10 can be rewritten as:

T i T e m e m i 1/2 1 + T e T i (ϕ 0 + ∆ϕ) exp(ϕ 0 )(1 + ∆ϕ) = 1 - Z d n d n i (11)
From Eq.6 and Eq.4, it can then be deduced that:

∆ϕ = -1 + T e T i m e m i 1/2 exp(ϕ 0 ) + 4πǫ 0 r d k B T e n d e 2 n i -1 × 4πǫ 0 r d k B T e e 2 ϕ 0 n d n i (12)
In typical laboratory plasma, (T e m e /T i m i ) 1/2 exp(ϕ 0 ) ∼ 10 -2 and (4πǫ 0 r d k B T e n d )/(e 2 n i ) ∼ 10 -1 , thus:

∆ϕ ≃ - 4πǫ 0 r d k B T e e 2 ϕ 0 n d n i ∼ -Z d0 n d n i ( 13
)
where Z d0 is the charge of an isloated dust particle. The dust particle absorption time can be rewritten as:

τ -1 Q ≃ πr 2 d n d v T i 1 + T e T i ϕ 0 -πr 2 d n 2 d v T i • T e T i 4πǫ 0 r d k B T e e 2 ϕ 0 n i (14)
The dust particle absorption time is no longer inversely proportional to the dust density. There is a term in n -2 d which tends to attenuate the effect of the dust particles on plasma losses. Indeed when the dust density is high the dust surface potential is lower as is the dust charge number Z d .

Consequently the ion flux is reduced and the dust particle absorption time is increased compared to the value it would have if the dust particles can be considered as isolated. In this section, exact numerical solutions of Eq.5 and Eq.10 are presented and compared to the approximate solution of Eq.14. In Fig. 1, the absorption frequency τ -1 Q has been calculated for n i = 10 8 cm -3 , T e = 0.3 eV (close to experimental values of Ref. [START_REF] Dimoff | Dust induced quenching of an afterglow plasma[END_REF]) and a dust particle radius of r d = 10 µm. As it can be seen in Fig. 1(a), for low dust densities the dust absorption frequency τ -1 Q is increasing linearly as expected. On the contrary, for high dust density the absorption time τ -1

Numerical results

Q deviates from linearity. The exact solution and the first order approximation are in agreement even though some small discrepancies are observed at high dust density. In Fig. 1(b), the exact solution is presented for different ratio T e /T i . As can be seen, the absorption frequency τ -1 Q increases with increasing T e /T i . Indeed, when the electron temperature is higher than the ion temperature, ion and electron fluxes are greater at equilibrium (Eq.3) resulting in an increased dust particle absorption frequency. In Ref. [START_REF] Couëdel | Residual dust charges in discharge afterglow[END_REF], it is thus shown that the electron temperature relaxation in the plasma afterglow of a RF discharge results in a strong decrease of the dust particle charges. In Fig. 2, the ratio τ -1 Q /τ -1 Q0 as well as the Havnes parameter P H have been calculated for different dust densities and ion densities. As can be seen in Fig. 2(a), the deviation from τ -1

Q0 is marked at high dust density and low ion density. In Fig. 2(b) these conditions correspond to a Havnes parameter close to 1. For small Havnes parameters (i.e. small dust density and high ion density), it can be seen that τ -1 Q /τ -1 Q0 ∼ 1. This confirms that when dust particles can be considered as isolated inside the plasma the absorption frequency is directly proportional to the dust density while for high dust density the charge carried by a dust particle is reduced as the dust density increases and consequently the dust absorption frequency does not evolve linearly with the dust density. In a plasma afterglow, the plasma density decreases with time. Consequently the effect of the dust particles can become more and more important with time. In order to see this effect, the first moment of the decay of a complex plasma has been investigated using Eq.1, Eq.2, Eq.5 and Eq.8. The results are presented in Fig. 3. The plasma diffusion loss is taken into account by choosing a plasma diffusion frequency τ -1 D = 500 Hz (which is a typical value for a laboratory discharge with an argon pressure around 1 Torr and these plasma parameters). As can be seen in Fig. 3(a), the dust particles enhance plasma losses and the larger the dust density, the larger the effect on plasma losses. In Fig. 3(b), it can be seen that at the very beginning of the plasma afterglow, the absorption frequency is roughly proportional to the dust density. This is due to the high value of the plasma density at this time which allows a small Havnes parameter and thus dust particles can be considered as isolated. However, as the density decreases, the effect of the dust particles become more and more important and the particle absorption frequency decreases. Indeed the Havnes parameter increases with time as the plasma den-sity decreases. Consequently the dust particle charging is more and more affected by the presence of the other dust particles. The ion flux as well as the dust particle charge number decrease leading to a decrease of the dust absorption frequency. This effect is perfectly visible in Fig. 3(b) for high dust density.

Discussion and Conclusion

As we demonstrated in Sec.3, the dust density influences greatly the dust absorption-recombination process in an afterglow plasma. Thus, the dust absorption frequency does not increase linearly with the density, and for high density it is smaller than the absorption frequency that would occur if each dust particle could be considered as isolated in the plasma. Moreover, it was shown in Sec.2 and Sec.3 that if the dust density is not too high, a first order approximation of the absorption frequency matches very well with the exact solution. In Eq.14, there are only a few parameters to know (T e , T i , r d , ϕ 0 and n i ). Consequently, it is possible to use the measurement of the absorption frequency in the afterglow of a complex plasma in parallel with other diagnostics to deduce some important parameters of the plasma. For example, in Ref. [START_REF] Dimoff | Dust induced quenching of an afterglow plasma[END_REF], the dust particle absorption frequency has been measured in the afterglow of a linear pulsed discharge. The argon pressure was 1.9 Torr and the density of dust varied from 0 to 10 4 cm -3 . The dust particles had a mean radius r d ≃ 15µm. It is reported that the electron temperature was roughly equal to the ion temperature (T e ∼ T i ) [START_REF] Dimoff | Dust induced quenching of an afterglow plasma[END_REF]. Consequently, the influence of trapped ions on the dust particle charge can be neglected. In Fig. 4, experimental results from Ref. [START_REF] Dimoff | Dust induced quenching of an afterglow plasma[END_REF] are fitted using Eq.9 and Eq.14. The fit gives T i ≃ 0.042 eV , (T e /T i )ϕ 0 = 4.4 and thus assuming T i ≤ T e ≤ 5T i (depending when the measurement have been performed in the plasma afterglow), it gives 10 7 cm -3 ≤ n i ≤ 5 • 10 7 cm -3 . The range for the ion density and the ion temperature are much lower than those that were supposed in Ref. [START_REF] Dimoff | Dust induced quenching of an afterglow plasma[END_REF] but it could be due to the fact that their measurements were performed quite late in the plasma afterglow and temperature relaxation needed to be taken into account. To conclude, the influence of the dust density on plasma losses has been studied. It has been shown that the presence of dust particles can drastically shorten the plasma loss time and that the increase of the dust absorption frequency does not depend linearly on the dust particle density. Finally, by comparing our results with existing experimental data, the possible use of absorption frequency measurement as a diagnostic for complex plasma is mentioned. Comparison with existing experimental data [START_REF] Dimoff | Dust induced quenching of an afterglow plasma[END_REF] supports this idea.

Fig. 1 .

 1 Fig. 1. (Color Online) Particle absorption frequency as a function of the dust density. a) With or without taking into account the dust density effect. b) Numerical solution for different T e/T i

Fig. 2 . 1 Q

 21 Fig. 2. (Color Online) a) Ratio of the dust particle absorption frequency taking into account dust density effect τ -1 Q over the dust particle absorption frequency not taking into account dust density effect τ -1 Q 0 for different dust density and plasma density. b) Havnes parameter P H for different dust density and plasma density.

Fig. 3 . 1 Q

 31 Fig. 3. (Color Online) Top: evolution of ion density n i during the first microseconds of a plasma afterglow for different dust density. Bottom: Evolution of the dust particle absorption frequency τ -1 Q during the first microseconds of a plasma afterglow for different dust density. The plasma diffusion frequency is τ -1 D = 500 Hz and Te = T i = 0.3 eV

Fig. 4 .

 4 Fig. 4. (Color Online) Fit of experimental results of Ref.[14] using the first order approximation (Eq.14) the French-Australian integrated research program (FAST) under contract No.FR060169.

Acknowledgements

The authors would also like to thanks B. W. James for helpful discussions. This paper is partially supported by


Camille Bihanic

Sebastien Diliberto

Franck Pelissier

Eddy Petit

Clotilde Boulanger

Claude Grison

Eco-CaMnO x : A Greener Generation of Eco-catalysts for Eco-friendly Oxidation Processes

Keywords: catalyst, Ca-Mn cluster, sustainable oxidation, eco-catalysis 47

Owing to the alarming problems of depletion of mineral resources and ever-growing pollution by metallic elements, biosourced catalysts were prepared from Mn-accumulating plant species from a large-scale program of ecological rehabilitation of mining sites. This new generation of catalysts, "Eco-CaMnO x " catalysts, were obtained directly from a controlled thermal treatment of four species of Grevillea genus, a Mn-rich biomass, and without any further chemical treatment. Eco-CaMnO x catalysts were characterized by microwave plasma-atomic emission spectrometer, X-ray powder diffraction spectroscopy, high-resolution transmission electron microscopy, and scanning transmission electron microscopy-energy-dispersive X-ray spectroscopy. These analyses revealed a natural richness in two unusual and valuable mixed calciummanganese oxides: Ca 2 Mn 3 O 8 and CaMnO 3 . The high efficiency of Eco-CaMnO x catalysts was demonstrated via the epoxidation of various biosourced terpenes and lipids, using the mixture H 2 O 2 /NaHCO 3 as a green co-oxidant. This study demonstrated not only the high potential of Eco-CaMnO x catalysts as alternatives to oxidative reagents prohibited by REACH regulation but also the efficient synthesis of biosourced epoxides, which can be used as monomers for the preparation of biodegradable polymers. Finally, the original composition of Eco-CaMnO x catalysts led to distinctive catalytic behaviors compared to previously studied Eco-Mn and Pyr-Mn, allowing the preparation of sensitive epoxides such as linalool epoxide. KEYWORDS: rehabilitation of mining sites: phytoextraction, biosourced Mn

■ INTRODUCTION

Along with the increase of the world's population, which keeps demanding a higher standard of living, the claim for mineral resources is also rising. While several concerns have been raised over the elemental sustainability, the depletion of "trace metallic elements" (or "TEs") is intensifying. [START_REF] Hunt | Elemental Sustainability and the Importance of Scarce Element Recovery[END_REF] Growing exploitation of mineral resources is one of the main causes for the serious dissemination of TEs throughout the environment. TEs are among the most harmful pollutants. As they are not biodegradable, their wide dispersion into different ecosystems has raised concerns over their potential risks on human health and the environment. [START_REF]Ecology of Industrial Pollution[END_REF] New Caledonia provides a demonstrative case of this situation. The New Caledonian lagoon is the largest in the world, being classified as one of the UNESCO World Heritage Sites. New Caledonia is one of the highest biodiversity "hotspots" in the world. [START_REF] Myers | Threatened Biotas: "Hot Spots" in Tropical Forests[END_REF] This high biodiversity results from the unusual composition of its soil, which is rich in heavy metals, particularly in nickel. However, nickel in soils also implies intensive mining activities. 4 Nickel mines are opencast. Excavations to access deep nickel lodes require the removal of topsoil, upper soil horizons, and vegetation. The removal of the Eco-Mn revealed an original polymetallic composition 94 resulting in marked Brønsted and Lewis acidities and displayed 95 superior catalytic performances than conventional Mn 96 catalysts. For example, Eco-Mn 1 was successfully employed 97 in the aminoreduction of ketones 20 and constitutes the first 98 example of reduction promoted by a Mn-based catalyst. Eco-99 Mn 2 and Eco-MnO x led to a practical and elegant solution to the long-standing challenge of oxidative cleavage of 1,2-diols into aldehyde or ketones. [START_REF] Escande | a Biosourced Catalyst for Selective Aerobic Oxidative Cleavage of Activated 1,2-Diols[END_REF][START_REF] Escande | Switchable Alkene Epoxidation/Oxidative Cleavage with H 2 O 2 /NaHCO 3 : Efficient Heterogeneous Catalysis Derived from Biosourced Eco-Mn[END_REF] Supported Eco-Mn 1 provided also a huge potential as green catalysts for valuable epoxidation. [START_REF] Escande | Switchable Alkene Epoxidation/Oxidative Cleavage with H 2 O 2 /NaHCO 3 : Efficient Heterogeneous Catalysis Derived from Biosourced Eco-Mn[END_REF] Finally, Eco-Mn 1 acted as "Janus catalysts" by promoting tandem sequences of multicomponent type oxidations (synthesis of substituted pyridines) and cyclizations (synthesis of p-cymene). [START_REF] Escande | Acid Catalysis and Green Oxidations: Sequential Tandem Oxidation Processes Induced by Mn-Hyperaccumulating Plants[END_REF] Comparison of their properties to that of commercial salts highlighted the superior catalytic activities of Eco-Mn.

Recently we reported an extensive structural study of Eco-Mn 1 , characterized by inductive coupled plasma-mass spectroscopy, X-ray powder diffraction spectroscopy (XRPD), and X-ray absorption spectroscopy (XANES and EXAFS), completed with density functional theory calculations. 32 In a systematic analysis, we also analyzed the thermal residues, from New Caledonian Mn-accumulating plant species of Grevillea generated before the chemical activation, by XPRD (Figure 2). We detected a polymetallic oxide (a mixed manganesecalcium oxide, Ca 2 Mn 3 O 8 ) instead of polymetallic chlorides as for Eco-Mn 1 and Eco-Mn 2 .

While rare examples have described the use of mixed Mn-Ca oxide nanoparticles as oxidants, 33,34 no description was found for the use of Ca 2 Mn 3 O 8 in organic synthesis. However, mixed Mn-Ca oxides display biomimetic oxidative properties of the natural cofactor CaMn 4 O 5 of Photosystem II. Indeed, CaMn 4 O 5 performs the oxidation of water during the photosynthesis process, 35 and the complex Ca 2 Mn 3 O 8 presents similar oxidative activities in the electrochemical oxidation of water. [36][37][START_REF] Najafpour | Calcium Manganese-(IV) Oxides: Biomimetic and Efficient Catalysts for Water Oxidation[END_REF] The above facts prompted us to further structurally characterize these new potential oxidative eco-catalysts, which we named Eco-CaMnO x . We investigated the mineral composition of Eco-CaMnO x by microwave plasma-atomic emission spectrometer (MP-AES) and their crystalline composition by XRPD. The morphology of Eco-CaMnO x was analyzed by high-resolution transmission electron microscopy (HRTEM), and the structural distribution of In this study, two parameters were studied to investigate their influence on Eco-CaMnO x catalytic properties: the plant species, from which the catalysts were prepared, and the temperature of the thermal treatment. The dried leaves, from each of the four species of the Grevillea genus, were thermally treated in an oven, under air flow, at 300, 550, or 950 °C for 6 h, to destroy the organic matter. Indeed, these temperatures were chosen according to the literature, as different manganese oxides could be formed depending on the corresponding heating temperature: MnO 2 , Mn 2 O 3 , or Mn 3 O 4 . [START_REF] Dent Glasser | Oriented Transformations in the System MnO-O-H 2 O. Mineral[END_REF][START_REF] Buchel | Manganese Compounds and Manganese[END_REF] Eco-CaMnO x catalysts were then characterized by MP-AES, XRPD, HRTEM, and STEM-EDX mapping.

MP-AES Analysis of Eco-CaMnO x Catalysts. Metallic composition of intact Mn-accumulating plant leaves and of Eco-CaMnO x catalysts, resulting from different plant species and thermal treatments, was studied by MP-AES analyses.

Analyses were performed in triplicate for each sample to determine the standard deviation of the measurement (Table S1).

The results showed that Mn was the major transition metal in all eco-catalysts (1 to 16.1 wt %), except for Eco-CaMnO x f3 (G. exul ssp. exul 300 °C) (Figure 3). As Ca, Mg, Na, K, and Fe are essential mineral components of plants, they were found in Eco-CaMnO x catalysts, Ca being the major mineral element for every eco-catalyst. Interestingly, the composition of Eco-CaMnO x catalysts derived from different plants of the same species was quite similar, as shown by low RSD (Table S1).

This reproducibility of composition within species could be explained by the harvest timing. [START_REF] Losfeld | Leaf-Age and Soil-Plant Relationships: Key Factors for Reporting Trace-Elements Hyperaccumulation by Plants and Design Applications[END_REF] Indeed, leaves were harvested when they became a vegetal litter: they have the highest Mn concentration for each species. As a result of the accumulating specificity of each plant species, the chemical composition appeared different between Eco-CaMnO x catalysts.

Within a species the heating temperature at 550 and 950 °C did not have much influence on the chemical composition. But at 300 °C the elements were less concentrated. This might be C due to a partial degradation of the organic matter at 300 °C and a total degradation above 550 °C.

For further characterizations and experimentations, G. gillivrayi was chosen as the model species because the Eco-CaMnO x catalysts from G. gillivrayi present an average composition of minerals among the other species.

XRPD Analysis of Eco-CaMnO x Catalysts. XRPD was performed to deliver insights on the crystalline species present in Eco-CaMnO x catalysts and to compare with a reconstituted control of synthetic ashes. The synthetic ash control was obtained by mixing commercial metallic salts chosen so that the relative percentage of metallic elements in the starting mixture was exactly the same as in the leaves of G. exul ssp. rubiginosa, and was followed by a thermal treatment at 550 °C. The identified crystalline phases are summarized in Table 1. Mn was not present in a crystalline form in the eco-catalysts prepared at 300 °C.

Interestingly, for the other temperatures, Mn was present as mixed calcium-manganese oxides. Ca 2 Mn 3 O 8 was identified in all Eco-CaMnO x prepared at 550 °C, except for G. exul ssp. exul in which crystalline species of manganese was not observed. The absence of crystalline manganese species in G. exul ssp. exul could be linked to a low ratio of Mn/Ca (Table S1) preventing the crystallization process. CaMnO 3 was identified in all Eco-CaMnO x prepared at 950 °C. CaMnO 3 could result from the degradation of Ca 2 Mn 3 O 8 at high temperature, which was consistent with the calcination temperatures used by Han et al. to prepare these two complexes. [START_REF] Han | Porous Calcium-Manganese Oxide Microspheres for Electrocatalytic Oxygen Reduction with High Activity[END_REF] No crystalline mixed manganese-calcium oxide was observed in the synthetic ash control. It suggested that a crystallization mechanism is specific to plant-derived ecocatalysts and that it cannot be reproduced in a simple mixture of ashes. This difference in crystallization pathways illustrates the vegetal footprint of Eco-CaMnO x , which is in accordance with what we reported for Eco-Mn 1 . 32 The structure of Ca 2 Mn 3 O 8 has been reported as infinite sheets of Mn 3 O 8 4-held together by calcium ions. Mn ions are octahedrally coordinated by oxygen atoms and occupy threefourths of the octahedral layer sites. The other one-fourth is vacant. 37,[START_REF] Ansell | Structure of Calcium Manganese Oxide Ca2Mn3O8[END_REF] The negative charge is further compensated by the presence of Ca ions in trigonal prismatic sites located above and below layer vacancies. [START_REF] Najafpour | Calcium Manganese-(IV) Oxides: Biomimetic and Efficient Catalysts for Water Oxidation[END_REF] CaMnO 3 has a perovskite structure based on corner-shared MnO 6 -octahedra. Mn ions, present in the oxidation state IV, are located at the center of the octahedron, while each Ca ion occupies the interoctahedral sites. [START_REF] Najafpour | Calcium Manganese-(IV) Oxides: Biomimetic and Efficient Catalysts for Water Oxidation[END_REF][START_REF] Han | Porous Calcium-Manganese Oxide Microspheres for Electrocatalytic Oxygen Reduction with High Activity[END_REF] The layered structure of Ca 2 Mn 3 O 8 shares similarities with the cofactor CaMn 4 O 5 , the oxygen-evolving complex present in Photosystem II, which performs the photo-oxidation of water during photosynthesis. The incorporation of calcium into 281 manganese oxides greatly improves water oxidation capaci-282 ties. [START_REF] Brudvig | Water Oxidation Chemistry of Photosystem II[END_REF][START_REF] Najafpour | Nano-Sized Manganese-Calcium Cluster in Photosystem II[END_REF] Research on a series of manganese-calcium oxides

O 3 K 2 SO 4 K 3 Na(SO 4 ) 2 Eco-CaMnO x (G. gillivrayi 300 °C) a x Eco-CaMnO x (G. gillivrayi 550 °C) x x x x Eco-CaMnO x (G. gillivrayi 950 °C) x x x Eco-CaMnO x (G. exul ssp. exul 550 °C) a x x x Eco-CaMnO x (G. exul ssp. rubiginosa 550 °C) x x x x Eco-CaMnO x (G. meisneri 550 °C) x x x
x synthetic ash control (550 °C)

x x a Mn was not present in a crystalline form. Use of Eco-CaMnO x Catalysts in Synthesis. The 316 standard methods used in fine chemical industries can access 317 epoxidation of substitute alkenes by the use of stoichiometric 318 amounts of organic peroxides or percarboxylic acids, such as 319 meta-chloroperoxybenzoic acid. [START_REF] Sienel | Ullmann's Encyclopedia of Industrial Chemistry[END_REF] Other methods include the 320 use of dimethyldioxirane (also known as Murray's reagent), 321 formed in situ from the reaction between acetone and oxone, 46 322 or electrophilic halide source in the presence of a base. [START_REF] Bellucci | Facial Stereoselectivity of Two-Step Additions Initiated by Electrophilic Halogens to Methylenecyclohexanes. A Comparison with Epoxidation[END_REF] 323 Owing to safety concerns and/or the issue of large production 324 of wastes by these processes, there is a strong need to develop 325 new safer and eco-friendlier methods.

326

Hydrogen peroxide is considered as an ideal alternative 327 oxidant on both environmental and economic grounds. [START_REF] Noyori | Green Oxidation with Aqueous Hydrogen Peroxide[END_REF] Not 328 only is it relatively cheap, easily available, and stable at the 329 usual concentration (typically <60 wt %), but it is also a clean 330 oxidizing reagent, as it only forms water as a byproduct. 331 However, it requires an activation to catalyze the oxygen-332 transfer process. Many catalytic systems using H 2 O 2 as the 333 oxidant for epoxidation of alkenes were based on transition-334 metal catalysts. [START_REF] Joergensen | Transition-Metal-Catalyzed Epoxidations[END_REF][START_REF] Grigoropoulou | Recent Developments on the Epoxidation of Alkenes Using Hydrogen Peroxide as an Oxidant[END_REF][START_REF] Lane | Metal-Catalyzed Epoxidations of Alkenes with Hydrogen Peroxide[END_REF] However, the use of economically and 335 environmentally unsustainable metals, such as rhenium, 52-55 336 vanadium, [START_REF] Conte | Liquid Phase Oxidation Reactions by Peroxides in the Presence of Vanadium Complexes[END_REF]57 molybdenum, [START_REF] Dinoi | Olefin Epoxidation by H2O2/MeCN Catalysed by Cyclopentadienyloxidotungsten(VI) and Molybdenum(VI) Complexes: Experiments and Computations[END_REF][START_REF] Griffith | The Crystal Structures of[END_REF][START_REF] Gresley | Studies on Polyoxo and Polyperoxo-Metalates Part[END_REF] or tungsten, [START_REF] Venturello | Effective Catalytic System for Epoxidation of Olefins by Hydrogen Peroxide under Phase-Transfer Conditions[END_REF][START_REF] Sato | A Practical Method for Epoxidation of Terminal Olefins with 30% Hydrogen Peroxide under Halide-Free Conditions[END_REF][START_REF] Villa De | A Heterogeneous Tungsten Catalyst for Epoxidation of Terpenes and Tungsten-Catalyzed Synthesis of Acid-Sensitive Terpene Epoxides[END_REF][START_REF] Kamata | Highly Selective Epoxidation of Cycloaliphatic Alkenes with Aqueous Hydrogen Peroxide Catalyzed by [PO4WO(O2)24]3-/Imidazole[END_REF] prompted 337 the research toward inexpensive, abundant, and innocuous 338 transition metals. In particular, iron or manganese revealed to 339 be one of the more promising elements thanks to their 340 relatively low toxicity and their abundance in Earth's crust. 65,66 341 Porphyrin-Mn have been extensively investigated as a mimic 342 of cytochrome P450, [START_REF] Anelli | Synergistic Effect of Lipophilic Carboxylic Acids and Heterocyclic Axial Ligands in Alkene Epoxidation by Hydrogen Peroxide Catalysed by Manganese(III) Tetra-Aryl Porphyrins[END_REF][START_REF] Banfi | Biomimetic Models of Cytochrome P-450. A Doubly Tailed Manganese(III)-Tetraaryl Porphyrin; an Extremely Efficient Catalyst for Hydrocarbon Oxygenations Promoted by 30% H2O2[END_REF][START_REF] Maraval | Manganese(III) Porphyrin Catalysts for the Oxidation of Terpene Derivatives: A Comparative Study[END_REF][START_REF] Martins | Oxidation of Unsaturated Monoterpenes with Hydrogen Peroxide Catalysed by Manganese(III) Porphyrin Complexes[END_REF] but suffer from difficulties in 343 synthesis and purification. As they offer synthetical advantages 344 over porphyrin systems, considerable attention was given to 345 Salen-Mn, [START_REF] Berkessel | Pentacoordinated Manganese(III) Dihydrosalen Complexes as Biomimetic Oxidation Catalysts[END_REF][START_REF] Pietikaïnen | Convenient Asymmetric (Salen)Mn(III)-Catalyzed Epoxidation of Unfunctionalized Alkenes with Hydrogen Peroxide Using Carboxylate Salt Cocatalysts[END_REF] in particular to the Jacobsen asymmetric epoxidation reaction. [START_REF] Zhang | Enantioselective Epoxidation of Unfunctionalized Olefins Catalyzed by Salen Manganese Complexes[END_REF][START_REF] Katsuki | Mn-Salen Catalyst, Competitor of Enzymes, for Asymmetric Epoxidation[END_REF][START_REF] Ito | What Is the Origin of Highly Asymmetric Induction by a Chiral (Salen)Manganese(III) Complex? Design of a Conformationally Fixed Complex and Its Application to Asymmetric Epoxidation of 2, 2-Dimethylchromenes[END_REF] However, the homogeneous nature of such metalloporphyrin and Schiff base complexes leads not only to deactivation due to oxidative degradation of the ligand [START_REF] Serra | A View on the Mechanism of Metalloporphyrin Degradation in Hydrogen Peroxide Epoxidation Reactions[END_REF] but also to environmental costs concerning their nonrecyclability. To rectify both problems, heterogenization of these Mn-based catalysts was demonstrated, by grafting on a solid support or immobilization into porous solids, such as silica, polymers, nanoparticles, and zeolites. [77][78][79][80] For example, heterogeneous Ti silicates are nowadays used at an industrial scale with H 2 O 2 for conducting epoxidation reactions with very high catalytic and environmental performances. 81 In our previous works, we showed that montmorillonitesupported Eco-Mn 1 catalyzed the epoxidation of styrene. Eco-Mn 1 contained only 0.31 mol % of manganese, a much lower content of manganese than that reported in previously described Mn-derived heterogeneous catalysts. However, α-pinene oxide is a sensitive and reactive substrate, which rearranges easily in acid conditions. 87,88 The rearrangement of α-pinene oxide leads to the formation of a wide range of products, which depends largely on the acid-base properties of the catalytic system. Over 200 compounds were reported when the isomerization-reaction temperature was above 100 °C. 87 It is well described that Lewis acid sites favor the formation of campholenic aldehyde, pinocarveol, and isopinocamphone, whereas Brønsted acid sites result in the formation of trans-carveol, trans-sobrerol, and p-cymene. [89][90][91] For instance, one of the most desired products resulting from the α-pinene oxide is campholenic aldehyde because it is an intermediate for the manufacture of sandalwood fragrances.

The epoxidation of α-pinene into α-pinene oxide remains a challenge because of its isomerization into many different byproducts.

In a typical procedure, α-pinene, Eco-CaMnO x (G. gillivrayi 550 °C), and NaHCO 3 were stirred in a mixture of acetone:water (1:1) at room temperature for 10 min. Effect of H 2 O 2 Amount. The effect of the quantity of H 2 O 2 added into the reaction mixture was studied, as an excessive amount of H 2 O 2 could lead to a decrease in epoxide selectivity by overoxidation. [START_REF] Stekrova | Isomerization of α-Pinene Oxide Using Fe-Supported Catalysts: 1039 Selective Synthesis of Campholenic Aldehyde[END_REF] The conversion, yield, and selectivity t2 followed the same trend (Table 2). Increasing the quantity of H 2 O 2 increased the conversion, yield, and selectivity before reaching a threshold. Almost total conversion of α-pinene 1a E 409 and a yield of 64% in epoxide 2a were reached with 5 equiv of 410 H 2 O 2 (Table 2, entry 5). The addition time of H 2 O 2 in the 411 reaction mixture was also investigated. Although the selectivity 412 in epoxide remained similar by decreasing the addition time, 413 the conversion dropped from 98% in 1-2 h (Table 2, entries 6, 414 7) to 63% in 30 min (Table 2, entry 8). These results 415 supported the importance of a slow addition of H 2 O 2 in 416 oxidative reactions. Indeed, it was reported that a gradual 417 addition of H 2 O 2 decreased its self-decomposition. 93,94 418 The selectivity in α-pinene oxide gave some information 419 about Eco-CaMnO x . The oxygenated byproducts observed, 420 such as campholenic aldehyde, carveol, and iso-pinocamphone, 421 could be explained by the easy opening and rearrangement of 422 the epoxide by Lewis acid sites. This could reveal the presence 423 of many metallic species in our catalysts, like Fe, Ca, and Mg, 424 which could confer Lewis acid properties to Eco-CaMnO x 425 catalysts. As it was reported that α-pinene oxide could also 426 rearrange spontaneously in acid or neutral aqueous phase 427 media, 90 the epoxide was directly engaged in the reaction with 428 and without any catalyst, to test its stability. In the presence of 429 Eco-CaMnO x catalysts a conversion of 45% was observed, 430 whereas it was fully recovered without any catalyst. This result 431 indicated that the epoxide was stable in our conditions, even in 432 the presence of water, and that its rearrangement only occurred 433 in the presence of Eco-CaMnO x . This supports the presence of 434 Lewis acid sites in Eco-CaMnO x . Allylic oxidation is often a 435 competitive process of epoxidation in the oxidation of cyclic 436 olefins. [START_REF] Murphy | Allylic Oxofunctionalization of Cyclic Olefins with Homogeneous and Heterogeneous Catalysts[END_REF][START_REF] Menini | Cobalt-and Manganese-Substituted Ferrites as Efficient Single-Site Heterogeneous Catalysts for Aerobic Oxidation of Monoterpenic Alkenes Under Solvent-Free Conditions[END_REF] As both processes could occur simultaneously, it 437 could explain the formation of verbenone, identified among the 438 different byproducts.

439

For the following reactions, 5 equiv of H 2 O 2 was employed 440 as it represented the smallest amount affording total 441 conversion and high yield in epoxide. The addition time was 442 kept at 2 h.

443

Effect of NaHCO 3 Amount. The effect of the quantity of 444 NaHCO 3 added into the reaction mixture was investigated t3 445 (Table 3). No conversion of α-pinene was observed in the 446 absence of sodium hydrogen carbonate (Table 3, entry 1). 447 This result supported the essential presence of sodium 448 hydrogen carbonate allowing the epoxidation. It is assumed that NaHCO 3 and H 2 O 2 could produce in situ peroxymonocarbonate ions (HCO 4 -), [START_REF] Yao | Epoxidation of Alkenes with Bicarbonate-Activated Hydrogen Peroxide[END_REF][START_REF] Frank | Surprising Stereoselectivity in the Payne Epoxidation of Terpinen-4-Ol with Acetonitrile/Hydrogen Peroxide[END_REF] which would react with the Mneco-catalyst to form the active epoxidizing reagent. Indeed, a neutral or slightly basic pH (range from 7 to 9) is optimal for this bicarbonate-activated peroxide system, which corresponds to the pH values of the aqueous phase that ranged from 8.2 to 8.8 in our system.

An attempt to reduce NaHCO 3 quantity resulted in decreasing conversion, yield, and selectivity in epoxide.

Therefore, 5 equiv of NaHCO 3 seemed to be the lowest quantity giving high conversion and yield (Table 3, entry 5).

Effect of Catalyst Loading. The effect of the quantity of Eco-CaMnO x catalysts was then studied using α-pinene as the t4 substrate (Table 4). A low quantity of manganese gave high conversion rates. Almost total conversion and 64% yield were obtained with only 0.005 equiv in manganese (Table 4, entry 4). This result is remarkable compared to standard manganesebased catalysts, which usually require to be used in excess. [START_REF] Cahiez | Encyclopedia of Reagents for Organic Synthesis[END_REF] As a control, the reaction was carried out in the absence of Eco-CaMnO x catalysts. The reaction could not occur in the corresponding reaction time, in 2 h (Table 4, entry 1).

The oxidative capacity of Eco-CaMnO x was also compared to synthetic catalysts ( F Interestingly, Eco-CaMnO x catalysts gave a twice-higher 492 yield compared to synthetic Ca 2 Mn 3 O 8 (Table 4, entries 4 and 493 7). The enhanced reactivity of our catalyst might be due to a 494 different structural organization of manganese linked to its 495 vegetal footprint and/or due to the presence of traces of other 496 electroactive metal species.

497

Effect of the Plant Species and the Temperature of the 498 Thermal Treatment Used to Prepare Eco-CaMnO x . The 499 different Eco-CaMnO x catalysts prepared from the four species 500 of Grevillea were tested in the epoxidation of α-pinene by 501 using the optimized reaction conditions. The influence of the 502 heating temperature during the thermal treatment of biomass f6 503 was also investigated (Figure 6). For each plant species, each 504 corresponding Eco-CaMnO x catalyst showed a potent catalytic 505 activity. These results guarantee that every manganese-506 accumulating plant species used in rehabilitation could be 507 valorized afterward by this process. Within a species, the temperature of the thermal treatment did not seem to have much influence on the Eco-CaMnO x activities. Surprisingly, no loss of activity was observed for the eco-catalysts formed at 300 °C despite the residual organic matter. The Eco-CaMnO x catalysts of G. gillivrayi and G. meisneri prepared at 950 °C revealed similar activities as the corresponding Eco-CaMnO x catalysts prepared at lower temperatures (300 and 550 °C). The best catalyst in terms of activity and energy required for its preparation remained the one prepared at 550 °C.

The reaction conditions to perform the epoxidation of αpinene were optimized to reach a good compromise of the high efficiency of the reaction with sustainable and ecoresponsible use of chemical products and solvents. In these conditions, almost total conversion was accessed with 65% selectivity in pinene oxide. Longer reaction times were tested, but it only resulted in a decrease in selectivity, the conversion remaining total. This result was in accordance with the high reactivity of pinene oxide and thus its conversion into isomeric products. 100,101 Our system can be advantageously compared to other bicarbonate-activated peroxide systems based on Mn catalysts.

For instance, Lane et al. obtained 89% yield but only in the presence of additives. 82 Qi et al. reached 40% selectivity using Mn 2+ -exchanged zeolites. [START_REF] Stekrova | Isomerization of α-Pinene Oxide Using Fe-Supported Catalysts: 1039 Selective Synthesis of Campholenic Aldehyde[END_REF]

Substrate Scope for the Epoxidation of Terpenes and

Lipids by Eco-CaMnO x Catalysts. The epoxidation of diverse terpenes and lipids was carried out to evaluate the scope of our t5 process (Table 5).

High conversions and yields were obtained with a large variety of terpenes, from cyclic to terminal substrates, which confirmed the efficiency of the method. In some cases, a longer reaction time was needed to reach higher conversions.

The chemoselectivity of Eco-CaMnO x catalysts was illustrated through the selective epoxidation of terpenes (Table 5, entries 2-8) bearing various functional groups, such as alcohol, ester, or enone, that remained unmodified.

The epoxidation of monoterpenes like terpineol, isopulegol, and citronellyl acetate (Table 4, entries 2, 3 and 5) gave total conversion and selectivity. The epoxidation of (R)-(+)-limonene, linalool, geranyl acetate, α-ionone, and β-ionone gave Table 5. Substrate Scope for the Epoxidation of Natural Substrates by Ecocatalysts (Eco-CaMnO x a , Eco-Mn 1 , [START_REF] Escande | Switchable Alkene Epoxidation/Oxidative Cleavage with H 2 O 2 /NaHCO 3 : Efficient Heterogeneous Catalysis Derived from Biosourced Eco-Mn[END_REF] and Pyr-Mn 120 ) H This indicated that our catalyst was not acidic, avoiding the intramolecular reaction. However, the rearrangement of the epoxide into the hydroxyfurane was easily observed when CDCl 3 was used for NMR assessments. As an example, only the hydroxyfurane was obtained with a 63% yield with the previous Eco-Mn, [START_REF] Escande | Switchable Alkene Epoxidation/Oxidative Cleavage with H 2 O 2 /NaHCO 3 : Efficient Heterogeneous Catalysis Derived from Biosourced Eco-Mn[END_REF] whereas Pyr-Mn yielded hydroxypyran. 108 Therefore, Eco-CaMnO x allowed the preparation of very sensitive epoxides. The higher chemoselectivity of Eco-CaMnO x is the direct consequence of its preparation by a sole thermal treatment. This result is of interest, as linalool epoxide is an important molecule in the communication of pollinating insects. 109 Thirty-four percent of diepoxides were also obtained. This proved the strong oxidizing properties of Eco-CaMnO x as it succeeded in epoxidizing a terminal double bond.

The monoepoxide was the only one identified with αand βionones. The only other byproduct formed during the reaction on both ionones was the product of allylic oxidation, with 12 and 30% of selectivity, respectively (Table 5, entries 7, 8). The other particularly noteworthy example was the epoxidation of oleic acid (Table 5, entry 9). Indeed, epoxides Table 5. continued a Reaction conditions: substrate (1.6 mmol, 1 equiv), catalyst (8 μmol, 0.005 equiv Mn), NaHCO 3 (8 mmol, 5 equiv), H 2 O 2 (30 wt %; 8 mmol, 5 equiv), and H 2 O (20 mL): acetone (20 mL), rt. b Conversion, yield, and selectivity were established by FID, using biphenyl as internal standard, and then confirmed by 1 H NMR from the isolated product. c Selectivity was calculated as the ratio of yield over conversion. d Catalyst: 0.5 equiv Mn. e Terpene (0.25 mmol, 1 equiv), supported catalyst (0.0031 equiv Mn), NaHCO 3 (0.05 mmol, 5 equiv), H 2 O 2 (30 wt %; 0.8 mmol, 3.2 equiv), and H 2 O/dimethylformamide (DMF) (2 mL), 0 °C, 4 h. f Terpene (0.18 mmol, 1 equiv), catalyst (0.00006 equiv Mn), NaHCO 3 (0.88 mmol, 5 equiv), H 2 O 2 (30 wt %; 1.75 mmol, 9.7 equiv), and H 2 O/acetone (2.6 mL), 30 °C, 2 h. of fatty acids from vegetal oils are very interesting as platform molecules for organic chemistry, but also as biodegradable lubricants, plasticizers, and stabilizers of polymers. [110][111][112] Sixty-six percent of conversion with a total selectivity in epoxide was reached in 4 h with Eco-CaMnO x (0.5 equiv Mn), while Eco-Mn was not able to catalyze this reaction. Pyr-Mn seemed more efficient than Eco-CaMnO x but required twice as much H 2 O 2 .

In addition to the higher reactivity of Eco-CaMnO x compared to Eco-Mn, the experimental conditions were also improved; epoxidations were conducted with acetone/H 2 O instead of DMF. In a context of growing environmental consciousness, it is motivating to be able to propose novel catalysts, which combine efficiency, use of renewable resources, and eco-friendly processes.

Recycling and Leaching Studies of Eco-CaMnO x Catalysts. Recycling of Eco-CaMnO x catalysts was investigated by t6 conducting the epoxidation of isopulegol (Table 6). Eco-CaMnO x catalysts were recycled by separation after centrifugation of the crude, washing, drying, and reuse for an identical experiment. Up to four successive reactions were conducted. Conversion and yield remained quantitative for the second run (Table 6, entries 1, 2), while a slight decrease was observed after the third run (Table 6, entries 3, 4), suggesting a small loss of activity of Eco-CaMnO x catalysts.

The small loss of activity was investigated by analyzing the composition and structure of Eco-CaMnO x catalysts after each run. No significant change of Mn and Ca contents was observed by MP-AES (Table S2), and the structure characterized by XRPD remained the same. Both results J 656 Mn, in short reaction times, at room temperature, using green 657 solvents and without any ligand or activation phase. Eco-658 CaMnO x catalysts afforded much higher conversions and 659 yields than conventional activated manganese dioxide or 660 synthetic Ca 2 Mn 3 O 8 . Not only did these results demonstrate 661 the vegetal footprint of Eco-CaMnO x catalysts but also they 662 proved that these catalysts could represent potential substitutes 663 to oxidative reagents inconsistent with the requirements of 664 REACH regulation. A wide range of mono-and diepoxides 665 were efficiently synthesized from terpenes, many of them 666 coming from abundant natural resources. Eco-CaMnO x opens 667 a new perspective for the valorization of Mn-rich biomass 668 through the preparation of biosourced monomers for new-669 biodegradable polymers. 105,106,114 This new generation of 670 ecocatalysts exhibits environmental and scientific benefits, f7 671 suggesting a promising life-cycle analysis (Figure 7). It can 672 drive a new approach of green chemistry with ecological, 673 economic, and social advantages.

Figure 1 .

 1 Figure 1. From the ecological restoration of New Caledonian mining sites to ecocatalysis.

Figure 2 .

 2 Figure 2. Steps for preparation of the previously studied Eco-Mn catalysts and of the new Eco-CaMnO x from Mn-accumulating plant species.

Figure 3 .

 3 Figure 3. Histogram representing the relative weight percentages of elements in Eco-CaMnO x catalysts, prepared from different plant species of Grevillea and thermal treatments, determined by MP-AES analyses.

  t1

283 showed that Ca 2

 2 Mn 3 O 8 and CaMnO 3 were efficient catalysts 284 for the oxygen evolution reaction and oxygen reduction 285 reaction. Indeed, manganese ions in Ca 2 Mn 3 O 8 and CaMnO 3 286 are in the oxidation state IV and they may present interesting 287 oxidative properties similar to MnO 2 . 288 HRTEM Analysis of Eco-CaMnO x Catalysts (G. gillivrayi, 289 550 °C). HRTEM images of Eco-CaMnO x (G. gillivrayi, 550 290 f4 °C) revealed different particles with various shapes (Figure 4). 291 Small round particles of about 10-50 nm in diameter, lamellar 292 particles of about 10 nm width over 100 nm length, as well as 293 thin layer-shape particles seemed blended together into a 294 matrix. 295 STEM-EDX Mapping of Eco-CaMnO x (G. gillivrayi, 550 °C). 296 The STEM-EDX analysis was used to establish the chemical 297 composition mapping of Eco-CaMnO x catalysts (G. gillivrayi, 298 f5 550 °C) (Figure 5). Manganese, calcium, magnesium, and 299 potassium were expectedly present, as shown by MP-AES 300 analyses. The polymetallic structure of Eco-CaMnO x catalysts 301 was supported by the aggregation of manganese, oxygen, and 302 all of the other elements in the round particles, the layer-shape 303 particles, and the matrix. However, it should be noticed that 304 the lamellar particles were exclusively composed of manganese, 305 oxygen, and calcium. The association of these only three 306 elements in a single well-defined shape of particles confirmed 307 the presence of the Ca 2 Mn 3 O 8 complex, and that this complex 308 is organized in lamellar sheets. 309 Extensive structural analyses of the different Eco-CaMnO x 310 catalysts showed the presence of two unusual and valuable 311 mixed calcium-manganese complexes: Ca 2 Mn 3 O 8 and 312 CaMnO 3 . The oxidative potential of the Eco-CaMnO x 313 catalysts was studied in the epoxidation of alkenes and 314 especially terpenes, as biosourced substrates.

Figure 4 .

 4 Figure 4. HRTEM image of Eco-CaMnO x catalysts (G. gillivrayi 550 °C).

  82,83 Comparison of Eco-Mn 1 properties to those of heterogeneous catalysts made by incorporation of commercial MnCl 2 •4H 2 O and FeCl 3 • 6H 2 O highlighted the superior catalytic activity of polymetallic species present in Eco-Mn 1 . These promising results prompted us to investigate the utilization of Eco-CaMnO x as an epoxidizing agent and to study the possible cooperative catalytic effect between Ca and Mn. Optimization of the Conditions for the Epoxidation of αpinene. The epoxidation of α-pinene was chosen as a probe reaction to optimize the reaction conditions. α-pinene is a monoterpene mostly found in pine trees, an abundant natural resource. The pinene oxide represents an inexpensive biosourced starting material for the synthesis of flavors, fragrances, and agrochemical and therapeutic substances. 84-86

H 2 O 2 (

 2 30% w/w in H 2 O) was added dropwise during 2 h in the mixture while stirring. After 2 h, the catalyst was filtered off and the reaction mixture was extracted with ethyl acetate. The organic layers were gathered and dried with Na 2 SO 4 . The solvent was then removed by evaporation and the reaction products analyzed by gas chromatography-mass spectrometry (GC-MS)/flame ionization detector (FID).

Figure 5 .

 5 Figure 5. STEM-EDX mapping of Eco-CaMnO x catalysts (G. gillivrayi, 550 °C).

Figure 6 .

 6 Figure 6. Effect of the plant species and of the temperature of the thermal treatment used to prepare Eco-CaMnO x on the epoxidation of α-pinene. Reaction conditions: α -pinene (1.6 mmol, 1 equiv), catalyst (8 mmol, 0.005 equiv Mn), NaHCO 3 (8 mmol, 5 equiv), H 2 O 2 (30 wt %; 8 mmol, 5 equiv), and H 2 O (20 mL): acetone (20 mL), rt, 2 h. Conversion, yield, and selectivity were established by FID using biphenyl as internal standard. Selectivity was calculated as the ratio of yield over conversion.

  627 indicated the high stability of Eco-CaMnO x catalysts, even in 628 the oxidizing conditions of the reaction. 629 Considering that the composition and structure of Eco-630 CaMnO x remained similar after four runs, leaching of Eco-631 CaMnO x was unlikely to occur but was still verified. Using 632 isopulegol as substrate, a specific experiment was conducted as 633 described by Sheldon et al. 113 After 30 min of reaction, Eco-634 CaMnO x was removed from the reaction mixture by filtration; 635 conversion of isopulegol was analyzed after this time at 35%. 636 The reaction mixture was stirred for another 4 h without Eco-637 CaMnO x ; conversion of isopulegol remained at 35%, 638 indicating that the reaction did not occur after removing 639 Eco-CaMnO x . As expected, the elemental composition of the 640 reaction mixture was analyzed by MP-AES and no trace of 641 metal was detected. These two results showed that the Eco-642 CaMnO x catalyst was a heterogeneous catalyst, in which 643 leaching of Mn can be excluded in the reported conditions.

  study, a new generation of ecocatalysts was easily 646 prepared from Mn-accumulating plant species, which are used 647 in ecological restoration. A controlled thermal treatment of 648 Mn-rich biomass led directly to green oxidative catalysts, Eco-649 CaMnO x , thus avoiding any further chemical activation. 650 Characterization by MP-AES, XRPD, HRTEM, and STEM-651 EDX revealed an original polymetallic system. Unusual mixed 652 manganese-calcium oxides, Ca 2 Mn 3 O 8 and CaMnO 3 , were 653 identified. All Eco-CaMnO x catalysts prepared from different 654 plant species demonstrated an excellent catalytic potential in 655 the epoxidation of terpenes and lipids, with only 0.005 equiv

Figure 7 .

 7 Figure 7. Qualitative environmental impact assessment of the catalyst preparation.

Table 1 .

 1 Crystalline Species Identified in Eco-CaMnO x Catalysts and in Synthetic Ash Control

	CaCO 3	CaSO 4	KCl	Ca 2 Mn 3 O 8	CaMnO 3	Mn 2

Table 4

 4

	, entries 7-9). The synthetic

experiments shows that calcium enhances the reactivity of manganese, suggesting a synergistic effect of mixed calciummanganese oxides toward epoxidation. These findings suggest that the addition of calcium does not change the oxidation potential of manganese, but might change the dispersion and/ or exposition of manganese, enhancing its reactivity as suggested by the works of Han et al. on mixed calciummanganese oxides.

[START_REF] Han | Porous Calcium-Manganese Oxide Microspheres for Electrocatalytic Oxygen Reduction with High Activity[END_REF]

Table 2 .

 2 Effect of the Amount of H 2 O 2 and of the Adding Time on the Epoxidation of α-pinene a Reaction conditions: α-pinene (1.6 mmol, 1 equiv), catalyst (8 μmol, 0.005 equiv Mn), NaHCO 3 (8 mmol, 5 equiv), H 2 O 2 (30 wt %), and H 2 O (20 mL): acetone (20 mL), rt, 2 h. b Conversion, yield, and selectivity were established by FID using biphenyl as internal standard. c Selectivity was calculated as the ratio of yield over conversion.

	entry	H 2 O 2 (equiv)	addition time (min)	conversion (%) b	yield (%) b	selectivity (%) b,c
	1	0	120	0	0	0
	2	1	120	62	21	34
	3	3	120	80	44	55
	4	4	120	77	47	61
	5	5	120	98	64	65
	6	10	120	99	62	63
	7	1 0	6 0	9 8	6 5	6 6
	8	1 0	3 0	6 3	4 1	6 5

a

Table 3 .

 3 Effect of the Amount of NaHCO 3 on the Epoxidation of α-pinene a

	entry NaHCO 3 (equiv) conversion (%) b yield (%) b selectivity (%) b,c
	1	0	0	0	0
	2	0.5	36	2	6
	3	1	47	7	15
	4	3	82	38	46
	5	5	98	64	65
	a Reaction conditions: α-pinene (1.6 mmol, 1 equiv), catalyst (8 μmol,
	0.005 equiv Mn), NaHCO 3 , H 2 O 2 (30 wt %; 8 mmol, 5 equiv), and H 2 O (20 mL): acetone (20 mL), rt, 2 h. b Conversion, yield, and
	selectivity were established by FID using biphenyl as internal standard. c Selectivity was calculated as the ratio of yield over
	conversion.

Table 4 .

 4 Effect of the Catalyst Loading on the Epoxidation of α-pinene a Conversion, yield, and selectivity were established by FID using biphenyl as the internal standard. c Selectivity was calculated as the ratio of yield over conversion. d 0.02 equiv Ca was used, corresponding to the quantity of Ca engaged by 0.005 equiv Mn with Eco-CaMnO x . e Traces of verbenol, carveol, and campholenic aldehyde were detected by GC-MS. f TON = number of mol of the product formed per mol of catalyst. g TOF = number of mol of the product formed per mol of catalyst and per unit of time.

	entry	catalyst	equiv Mn	conversion (%) b	yield (%) b	selectivity (%) b,c	TON f	TOF g (h -1 )
	1	Eco-CaMnO x	0	0	0	0	0	0
	2	Eco-CaMnO x	0.001	89	49	55	490	245
	3	Eco-CaMnO x	0.0025	97	48	49	192	96
	4	Eco-CaMnO x	0.005	97	62	64	124	62
	5	Eco-CaMnO x	0.01	94	55	59	55	28
	6	Eco-CaMnO x	0.1	91	61	67	6	3
	7	C a 2 Mn 3 O 8	0.005	51	32	63	64	32
	8	activated MnO 2	0.005	55	15	27	30	15
	9	CaO	d	7 e	0	0
	a Reaction conditions: α-pinene (1.6 mmol, 1 equiv), the catalyst, NaHCO 3 (8 mmol, 5 equiv), H 2 O 2 (30 wt %; 8 mmol, 5 equiv), and H 2 O (20
	mL): acetone (20 mL), rt, 2 h.

b

Table 6 .

 6 Recycling Studies of Eco-CaMnO x Catalysts

	entry	run	conversion (%)	yield (%)	selectivity (%)	post-run Mn content (wt % (±sd))	post-run Ca content (wt % (±sd))
	1	no 1	>99	>99	100	11.22 (0.40)	16.02 (0.61)
	2	no 2	>99	>99	100	11.82 (2.20)	16.78 (0.79)
	3	no 3	79	79	100	8.93 (1.80)	13.63 (0.76)
	4	no 4	73	73	100	10.80 (0.81)	14.78 (0.86)

  ASSOCIATED CONTENT 675 * sı Supporting Information 676 The Supporting Information is available free of charge at 677 https://pubs.acs.org/doi/10.1021/acssuschemeng.9b05444.

674 ■


Jesse L Peltier

Eder Tomás-Mendivil

Daniel R Tolentino

Max M Hansmann

Rodolphe Jazzar

Guy Ber- Trand

Realizing Metal-Free Carbene-Catalyzed Carbonylation Reactions with CO

Many organic and main group compounds, usually acids or bases, can accelerate chemical reactions when used in substoichiometric quantities, a process known as organocatalysis. In marked contrast, very few of these compounds are able to activate carbon monoxide and, until now, none of them catalyze its chemical transformation, a classical task for transition metals. Herein, we report that a stable singlet ambiphilic carbene activates CO and catalytically promotes the carbonylation of an ortho-quinone into a cyclic carbonate. These findings pave the way for the discovery of metal-free catalyzed carbonylation reactions.

At the end of the 19 th century, Sabatier discovered that thin slivers of reduced nickel could promote the addition of H2 to ethylene. Afterwards, it was widely accepted that a transition metal was required to activate small molecules in all chemical and biological systems [START_REF] Pilak | The Crystal Structure of the Apoenzyme of the Iron-Sulphur Cluster-Free Hydrogenase[END_REF] , and to promote their reaction with substrates. However, in the last decade, a handful of main group compounds have been shown to achieve the same task, 2 as exemplified by the hydrogenation of multiple bonds using a Frustrated Lewis Pair. 3 In 2007, we wrote that in possessing a lone pair of electrons and an accessible vacant orbital, singlet carbenes resemble transition metal centers, and thus could potentially mimic their chemical behavior. [START_REF] Frey | Facile splitting of hydrogen and ammonia by nucleophilic activation at a single carbon center[END_REF][START_REF] Martin | Stable singlet carbenes as mimics for transition metal centers[END_REF] Indeed, some stable electrophilic carbenes can split enthalpically strong bonds like BH, SiH, PH, NH, 6 and even CH [START_REF] Turner | Chemically non-innocent cyclic (alkyl)(amino)carbenes: Ligand rearrangement, C-H and C-F bond activation[END_REF] and CF bonds, [START_REF] Petrasiak | Activation of C-F, Si-F, and S-F Bonds by N-Heterocyclic Carbenes and Their Isoelectronic Analogues[END_REF] as well as challenging molecules like H2 and NH3. [START_REF] Frey | Facile splitting of hydrogen and ammonia by nucleophilic activation at a single carbon center[END_REF][START_REF] Moerdyk | Elucidation of carbene ambiphilicity leading to the discovery of reversible ammonia activation[END_REF] Additionally, it has recently been shown that a reductive elimination at carbon could also occur. 10 However, the third essential step of a classical transition metal catalytic cycle, namely the transformation of the reactants, is hampered by the lack of reactivity of CH and CF bonds. This analysis prompted us to turn our attention to catalytic carbonylation reactions with carbon monoxide, a task so far restricted to transition metals. 11 Although CO is one of the most inert molecules (bond-dissociation energy of 1072 vs 432 kJ/mol for H2), it is known to react with ambiphilic carbenes to give the corre-sponding ketenes. 12 The latter are very reactive, and thus the working hypothesis relied on the possibility that ketenes would react with a substrate, and that the resulting adduct would undergo a reductive elimination, releasing the product and regenerating the carbene. As a proof of principle, herein we report that selected singlet carbenes can promote the stoichiometric and even catalytic carbonylation of an ortho-quinone into a cyclic carbonate.

We started our investigation by performing stoichiometric reactions representing the different elementary steps of the putative catalytic cycle. First, the carbene must react with CO to form a ketene. From a computational study, Frenking et al. concluded that only carbenes featuring a sufficiently small singlet-triplet (S/T) gap could yield stable ketenes. [START_REF] Goedecke | When does carbonylation of carbenes yield ketenes? A theoretical study with implications for synthesis[END_REF] This result prompted us to consider the bicyclic (alkyl)(amino)carbene BiCAAC 14 and the cyclic (alkyl)(amino)carbene CAAC,[START_REF] Lavallo | Stable cyclic (alkyl)(amino)carbenes as rigid or flexible, bulky, electron-rich ligands for transition metal catalysts: A quaternary carbon makes the difference![END_REF][START_REF] Soleilhavoup | G. Cyclic (alkyl)(amino)carbenes (CAACs): Stable carbenes on the rise[END_REF] which feature a S/T gap of 45.7 and 49.2 kcal/mol, respectively. Indeed, these two carbenes instantaneously react with carbon monoxide in benzene or THF at room temperature to afford the corresponding ketenes Ket-1 and the previously reported [START_REF] Lavallo | CO fixation to stable acyclic and cyclic alkyl amino carbenes: Stable amino ketenes with a small HOMO-LUMO gap[END_REF] which were isolated in 85 and 90% yield, respectively (Scheme 1A).

The second step involves the reaction between amino-ketenes and an ortho-quinone to give the corresponding lactones, akin to the reactivity of alkyl and aryl substituted ketenes (RR'C=C=O). [START_REF] Allen | Ketenes and other cumulenes as reactive intermediates[END_REF] Indeed, the reaction of Ket-1 and Ket-2 with 9,10phenanthrenequinone (Quin) at room temperature quickly and quantitatively led to the corresponding [4+2] cycloadducts Lact-1 and Lact-2 as single diastereomers (Scheme 1A), despite the introduction of a new chiral center (vide infra).

To check the feasibility of the reductive elimination, we first treated Lact-1 and Lact-2 with elemental sulfur, a classical trapping agent for singlet carbenes. 10a In the case of Lact-1, heating at 80 °C for 16 hours in THF was necessary to obtain Thio-1 along with the release of the cyclic carbonate Carbo-1 (Scheme 1B). With Lact-2 derived from CAAC, we were pleased to observe the clean formation of the thiolactam Thio-2 along with a quantitative amount of carbonate Carbo-1 after 16 hours at room temperature. Since Lact-1 and Lact-2 can be regarded as products resulting from the insertion of carbenes into the C-O single bond of the corresponding cyclic carbonates, we also investigated the reaction of BiCAAC and CAAC with Carbo-1 and observed the clean formation of the corresponding spirolactones Lact-1 and Lact-2, again as a single diastereomer (Scheme 1C). These results demonstrate the reversibility of the reductive elimination step. Scheme 1. Stoichiometric reactions mimicking the elementary steps of the postulated catalytic cycle.

To understand the diastereoselectivity of the process, we reacted BiCAAC with ethylene carbonate (Carbo-2) in C6D6 at room temperature. The 1 H NMR analysis showed the formation of a 74/26 mixture of Lact-3'/Lact-3 diastereomers (Scheme 1D). By heating the mixture at 80 °C the ratio was reversed towards the thermodynamic product (Lact-3). This mixture also reacted at 80 °C with elemental sulfur leading to Thio-1 and Carbo-2. Importantly, these observations strongly suggest that unlike in organometallic catalysis, [START_REF] Troisi | Synthesis of benzofused five-and six-membered heterocycles by palladium-catalyzed cyclocarbonylation[END_REF] the reductive elimination involves a polar stepwise process via opened (o-Bet) and closed betaines (c-Bet). [START_REF] Falivene | Guidelines to select the N-heterocyclic carbene for the organopolymerization of monomers with a polar group[END_REF] We believe that the diastereoselectivity observed with the orthoquinone is due to the better leaving group ability of the phenoxy anion, which facilitates the formation of the opened betaines, and thus hampered the observation of the kinetic diastereomers. To close the catalytic cycle, we reacted the spirolactones with carbon monoxide (Scheme 1E). With Lact-1, no reaction was observed at room temperature, even under 4 atm of carbon monoxide. However, heating a C6D6 solution of Lact-1 under 4 atm of CO at 80 °C for 16 hours led to Carbo-1 and a black residue from BiCAAC decomposition, which was not characterized. With Lact- 2, under 4 atm of CO, at room temperature, the solution turned slightly blue, indicating the formation of Ket-2, which was confirmed by the observation of the characteristic 13 C NMR CCO signal at 278.8 ppm. 12a Notably, 1 atm of CO was insufficient to observe any reaction, which demonstrates that this process is CO pressure-dependent. Scheme 2. Proposed mechanism and energy diagram. DFT calculations at B3LYP-D3/def2-TZVP level (Gibbs energies in kcal/mol). For clarity, ortho-benzoquinone is shown, although all calculations have been done for 9,10-phenanthrenequinone (Quin).

Encouraged by these results, we investigated the hypothetical catalytic process computationally (B3LYP-D3/def2-TZVP), as shown in Scheme 2, employing BiCAAC, sterically encumbered CAAC and small CAAC Me for comparison. The formation of ketenes is predicted to be exergonic in every case. Similarly, the reaction between the ketenes and 9,10-phenanthrenequinone (Quin) leading to spirolactones (Lact) are exergonic; the latter are the most stable intermediates, and thus can be regarded as the resting state of the catalytic cycle. Note that the relative energies between o-Bet, c-Bet and free carbonate plus CAAC are negligible and the overall transformation is exergonic. Importantly, calculations indicate that the reductive elimination step should be possible with all carbenes, although significantly easier with the bulky CAAC than with BiCAAC and CAAC Me . Together, the experimental and computational results discussed above suggested that a catalytic reaction should be achievable. We were delighted to find that indeed, with only 10 mol% CAAC in benzene after 40 hours at 60 °C under 4 atm of CO, the carbonylation of 9,10-phenanthrenequinone (Quin) was achieved and gave Carbo-1 in 80% isolated yield (Scheme 3). Scheme 3. Organocatalytic carbonylation of Quin-1.

On the other hand, all attempts to use BiCAAC and CAAC Me as catalysts failed, even upon using more forcing experimental conditions. We discovered that BiCAAC and the small CAAC Me readily react with Quin leading irreversibly to adducts I, 20 and II, 21 respectively, thus quenching the catalytic cycle (Scheme 4). As a control experiment, we confirmed that the bulky CAAC did not react with the o-quinone even at 80 o C and thereby leads to a viable catalyst.

Scheme 4. Deactivation of BiCAAC and CAAC Me

Until now, carbene organocatalysis was limited to weakly basic carbenes featuring large HOMO/LUMO gaps, allowing them to behave as good leaving groups (e.g., thiazol-2-ylidenes, [START_REF] Breslow | On the Mechanism of Thiamine Action. IV. Evidence from Studies on Model System[END_REF] 1,2,4triazol-5-ylidenes, 23 and 1,2,3-triazol-4-ylidenes 24 ). However, this work demonstrates that strongly ambiphilic carbenes such as CAACs are highly desirable in small molecule catalysis. Their electronic properties allow for mimicking all of the elementary steps involved in transition metal catalytic cycles, but their mechanism can be different, as shown by the reductive elimination process described herein, thus opening up new possibilities in catalytic transformations. TM-catalyzed carbonylation reactions are widely applied in industry in large scale employing CO gas as C1-source. 11 We are aware that the carbonylation of quinones is not of synthetic utility, but since not only carbenes, but also other low-valent maingroup compounds, such as silylenes, phosphinidenes and borylenes are able to bind CO, 25 these findings pave the way for the discovery of other metal-free catalyzed carbonylation reactions.

ASSOCIATED CONTENT Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/xxx. Metrical data for the solidstate structures of Lact-1, Lact-2, Carbo-1, I, and II are available free of charge from the Cambridge Crystallographic Data Centre under reference numbers CCDC-1999858, CCDC-1999855, CCDC-1999854, CCDC-1999857, and CCDC-1999856

ACKNOWLEDGMENTS

We are grateful to the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Catalysis Science Program, under Award # DE-SC0009376 for financial support of this work, the Tribal Membership Initiative fellowship administered by the UC San Diego Graduate Division for financial support of J.L.P. E.T.-M. is thankful to the Basque Government for a Doctoral Research Staff Improvement Program scholarship and to Dr. Zoraida Freixa for her support.

The authors declare no competing financial interests.


Glen P Junor

Jan Lorkowski

Dr Cory M Weinstein

Prof. Dr Rodolphe Jazzar
email: rjazzar@ucsd.edu

Cezary Pietraszuk

Guy Bertrand
email: gbertrand@ucsd.edu

The Influence of C(sp 3 )H-Selenium Interactions on the 77 Se NMR Quantification of the π-Accepting Properties of Carbenes

établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Since the isolation of a (phosphino)(silyl)carbene 1 and an Nheterocyclic carbene (NHC), 2 stable carbenes have become ubiquitous in chemical science. 3 Nowadays, a variety of stable carbenes, featuring very diverse electronic and steric properties, are known. [START_REF] Sau | For carbenes with different properties[END_REF] Accordingly, choosing the best carbene for a given application is not an easy task; 5 it requires a thorough understanding of the carbene stereoelectronic properties.

To evaluate the electronic properties of carbenes, several techniques have been developed, including calorimetric measurements of ruthenium complexes, 6 the 13 C NMR chemical shifts of palladium complexes, 7 the electrochemical E0 value for the Ru(III)/Ru(II) redox couples, 8 and the Tolman Electronic Parameter (TEP), which is the most popular. 9 Although all of these techniques provide a convenient way to evaluate the overall donor abilities of carbenes, they fail to deconvolute the sdonating and p-accepting properties. To determine their paccepting character, the 31 P{ 1 H} NMR 10 and 77 Se{ 1 H} NMR 11 chemical shifts of phenylphosphinidene-and selenium-carbeneadducts, respectively, have been exploited. The latter, developed by Ganter, relies on the ability of carbenes to engage in backbonding with the selenium lone pair (Figure 1). The more p-accepting carbenes favor resonance form I, which results in a downfield 77 Se{ 1 H} NMR signal, whereas the less π-accepting ones favor resonance form II resulting in an upfield shift. Compared with the phosphinidene scale (~400 ppm), 10 the selenium scale covers a wider spectral range (~1200 ppm) which should allow for a greater delineation of the π-accepting properties of closely matching carbenes. Moreover, it benefits from a simple experimental protocol i.e. addition of elemental selenium to an in-situ generated free carbene. Despite the popularity of this method, 12 careful examination of the literature reveals several inconsistencies in 77 Se{ 1 H} NMR data. For example, it appears that the chemical shift for the selenium adduct of NHCs bearing tertiary N-alkyl substituents such as IAd (197 ppm) and I t Bu (183 ppm) display unexpectedly downfield signals with respect to secondary alkyl analogues such as ICy (-22 ppm) and I i PrMe2 (-18 ppm) (Figure 2). As mentioned by Cavallo, Nolan et al, 12m such "difference amongst N,N-dialkylimidazol-2-ylidenes was very intriguing" since these data support, counterintuitively, a much stronger paccepting character for tertiary N-alkyl substituted NHCs. Similar observations were recently highlighted by Huynh across several other carbenes families, 13 stating caution should be taken when analyzing 77 Se{ 1 H} NMR data of carbene-selenium adducts. Rather short intramolecular Se-H distances, which are close to or within the sum of Van der Waals radii, were previously noted across several Se-NHC adducts by Cavallo and co-workers. 12m Herein, combining an experimental and theoretical approach, we demonstrate that non-classical hydrogen bonding (NCHB) 14 interactions resulting from negative hyperconjugation trigger a non-linear behavior of the 77 Se{ 1 H} NMR scale in carbeneselenium adducts. NCHB is well established with O and S, but is less common for selenium, which is less electronegative. 15 These interactions have been proposed with acidic protons in some oxazolylidene-selenium adducts 16 but are much more surprising with unactivated alkyl protons. To begin our study, we first considered the five-membered cyclic (alkyl)(amino) carbenes (CAAC-5). 17 We previously reported that the 77 Se{ 1 H} NMR signal of the selenium adduct of Et CAAC-5 1a is at 481 ppm. 12g As observed with the NHC series, we found that the 77 Se{ 1 H} NMR chemical shift of the selenium adducts of the more sterically hindered Menth CAAC-5 (635 ppm) 1b and Ad CAAC-5 (683 ppm) 1c (Figure 3) appeared considerably downfield. Going further, we synthesized the adducts of six-membered cyclic (alkyl)(amino)carbenes (CAAC-6), which provide more steric crowding than CAAC-5s as a result of a larger C-Ccarb-N angle. 18 Here also, we observed marked differences in the chemical shift of the small Et CAAC-6 (715 ppm) 2a and the large Ad CAAC-6 (863 ppm) 2b. Altogether, these results suggest, counterintuitively, that the bulkier CAACs 1b,c and 2b could be more p-accepting than their smaller variants 1a and 2a, respectively. Intrigued by these results, we verified by DFT that within the same family, these carbenes have comparable HOMO-LUMO gap and should therefore display similar electronic properties (Figure 4). [START_REF] Junor | All computational results are available for download free of charge from UCSD Library[END_REF] Furthermore, the X-ray crystallographic analysis showed short intramolecular Se … H distances for the adamantyl variants 1c and 2b (Figure 5), which are not present in 1a 12g and 2a. 18,[START_REF][END_REF] Thus, it appears, that same phenomenon is present in both CAACs and NHCs, although the substituent involved is attached to nitrogen for NHCs and on carbon for CAACs. To make a direct comparison, N-alkyl substituted CAACs were desirable, but until now they were hardly available using current synthetic methodologies. We found that they are readily accessible upon using a monomethylated instead of a dimethylated backbone (Scheme 1). Several CAAC-6 Me iminium salts 3a-e with various substituents on the nitrogen were prepared. 21 Contrary to the well-known dimethylated CAAC derivatives, the corresponding free CAAC-6 Me 4a-e are not stable at room temperature. However, using variable temperature 13 C{ 1 H} NMR, we confirmed that they can be generated at -80 o C and are persistent until -60 o C. Adducts 5a-e were prepared by deprotonation of 3a-e with KHMDS at -78 0 C in the presence of excess elemental selenium. The 77 Se{ 1 H} NMR chemical shifts for the room temperature stable 5a-e were explored to confirm the substitution pattern that leads to NCHB (Figure 6). Compared to the N-aryl substituted 5a (669 ppm), an upfield shift is observed for the N-alkyl substituted derivatives (5b: 521; 5c: 526 ppm). This is in good agreement with alkyl substituents increasing electron density on the nitrogen, thus raising the LUMO and reducing the carbene π-acidity. This should also be the case with N-t Bu 5d and N-Ad 5e but their signals (777 and 789 ppm, respectively), were downshifted by over 268 ppm compared 5b,c. As a reference, this range is larger than the difference between the DAC-Se adducts (846-856 ppm), 12l and Alder's acyclic diaminocarbene-Se (593 ppm), 22 which are two very distinct families of carbenes. Interestingly, DFT predicted that the p-accepting properties of Nalkyl carbenes 4b-e are within the same range, which conflicted with the 77 Se NMR data. Upon comparing the 1 H NMR of aldiminium 3b and carbeneselenium adduct 5b, we noticed a marked downfield shift of the exocyclic N-C-Ha hydrogen from 3.75 ppm to 5.85 ppm (Figure 7). This is an additional indication of the participation of C(sp 3 )-H bonds in non-classical C-H•••Se interactions. Note that recording the 1 H NMR of 5b up to 100 o C did not show a coalescence of the signal suggesting the presence of a rather strong Se-H interaction. The solid-state structures of compounds 5b-e also display short intramolecular Se-H distances, which is not the case for 5a (Figure 8). [START_REF][END_REF] Note, that these distances (2.46-2.66 Å) are well within the range of intramolecular hydrogen bonding in Silks' crystalline selenourea adduct [Se-H 2.51-2.60 Å], and in line with their proposed NCHB interactions. 16 Following geometry optimizations of 5c and 5d, based on the X-ray crystal structure data, 18 at the BP98/def2tzvpp level of theory, [START_REF]Solvent was modelled as a polarizable continuum of the experimentally used solvent (chloroform or acetone). NMR shielding calculations were done with the GIAO method. For NMR prediction[END_REF] and quantum theory of atoms in molecules (QTAIM) analysis we confirmed the existence of bond paths in both compounds (Figure 9). 24 Comparison of the electron density ρ(r) at the Se-H bond path critical points (BCP) showed that NHCB interactions are stronger when involving a b-hydrogen (5c) than a g-hydrogen to nitrogen (5d). 25 Thus, the strength of these interactions does not rationalize the observed upfield chemical shift of 5c compared to 5d. To investigate further, 77 Se NMR chemical shifts of 1c, 2b and 5b-e were calculated by DFT (Figure 10). The experimental value was well reproduced in the case of the conformationally rigid 1c and 2b. For the N-alkyl substituted 5b-e, which allow for a higher degree of rotation at nitrogen, calculations predicted upfield selenium NMR chemical shifts compared to those observed experimentally. We wondered, if in these cases, the accessibility of other conformational isomers, also stabilized through NCHB interactions, could explain the discrepancies. As an example, we considered 5c and its conformer 5c'. We confirmed that 5c' is energetically accessible (+2.3 kcal.mol -1 ) and more importantly predicts a significant downfield shift (+74 ppm) of the Se NMR signal. Consequently, we propose that the deshielding of the selenium atom in 5c' is favored by the formation of a quasi-cyclic 6-membered conformation stabilized by NCHB C(sp 3 )-H-Se interaction. In this case, the NCHB is better described as a negative hyperconjugative interaction between the lone pair of the Se atom (H-bond acceptor) and the σ * C-H orbital of the C(sp 3 )-H (H-bond donor); orbital overlap is maximized in a quasi 6-membered ring and facilitates electron transfer. 26,27 Note that the directionality of this type of interaction has already been shown to be a critical parameter for maximizing orbital overlap. 26,27 This effect is more pronounced in more constrained systems such as 5d-e, which have a higher probability of such 6-membered ring conformations than 5b,c. Moreover, it is particularly well exemplified in 1c and 2b in which the unusual distortion of the backbone, observed by X-ray, places the C-H bond in the right position with respect to the selenium atom (Figure 11). Experimental Predicted [c] 5b [b] 521 425 5c [a] 526 427 5d [a] 777 688 5e [a] 789 694 501 427 (CAAC)Se 5c 5c' 1c [a] 683 688

I II

Ad X CAAC-6 Et X 2.54 Å 2.64 Å 1c 2b 1a 2a d Se … H > 2.99 Å d Se … H > 2.82 Å N Me R 1 R 2 R 2 N Me R 1 R 2 R 2
2b [a] 863 883 In conclusion, we have provided experimental and computational evidence for the existence of non-classical hydrogen bonding interactions in CAAC-selenium adducts, causing major deviations from the expected trend in 77 Se NMR chemical shifts. Since the same reasoning can also be applied to I t Bu and IAd, it is likely that this phenomenon is also found in other carbene motifs not considered in the present study. These findings encourage caution when probing p-accepting properties within a carbene family, especially when bulky substituents are in proximity to the carbene center. However, the easy access to carbene-selenium adducts, the abundance of the 77 Se nucleus, and the wide range of chemical shifts makes the use of 77 Se NMR spectroscopy a very powerful method to compare different types of carbenes.

Entry for the Table of Contents

Non-classical hydrogen bonds (NCHB) in carbene-selenium adducts cause pronounced downfield shifts in 77 SeNMR spectra, perturbing the Se scale for probing p-accepting properties within a carbene family.

Figure 1 .

 1 Figure 1. Canonical Structures of Carbene-Se adducts and their relation to 77 Se{ 1 H} NMR Spectroscopy.

  12m

Figure 2 .

 2 Figure 2. Non-linear behavior of the 77 Se{ 1 H} NMR chemical shifts in NHC-Se adducts

Figure 3 .

 3 Figure 3. Non-linear behavior of the 77 Se{ 1 H} NMR chemical shifts in CAAC-Se adducts.

Figure 4 .

 4 Figure 4. Differences in 77 Se{ 1 H} NMR chemical shifts of 1a,c and 2a,b are not expected since the carbene HOMO-LUMO gaps are similar within each CAAC family.

Figure 5 .

 5 Figure 5. Short intramolecular Se-H distances are observed in 1c and 2b but not in 1a and 2a.

19

 19

Figure 6 .

 6 Figure 6. Non-linear behaviour of the 77 Se{ 1 H} NMR chemical shifts in CAAC-Se adducts. DAC-and Alder carbene-Se adducts included for comparison.

Figure 7 .

 7 Figure 7. Comparison of the 1 H NMR of 3b and 5b highlights NHCB Se-H interactions.

Figure 8 .

 8 Figure 8. Solid state structures of selenoamides 5a-e indicating NCHB Se-H interactions or absence thereof.

Figure 9 .

 9 Figure 9. QTAIM analysis highlighting the Se-H bond paths and the electron density ρ(r) at the bond path critical points (BCP) (values in 10 -3 a.u.)

Figure 10 .

 10 Figure 10. Experimental and predicted 77 Se{ 1 H} NMR chemical shift of 1c, 2b and 5b-e (left). Significant downfield-shift of 77 Se{ 1 H} NMR signal caused by rotation of substituent and formation of quasi 6-membered ring (right).

Figure 11 .

 11 Figure 11. Quasi-cyclic 6-membered conformation stabilized by NCHB C(sp 3 )-H-Se interaction in 1c and 2b maximizes orbital overlap with a significant backbone distortion.

Acknowledgements

Thanks are due to the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Catalysis Science Program, Award # DE-SC0009376 for financial support of this work. We are grateful to the Alfred P. Sloan Foundation's University Centre for Exemplary Mentoring (G.P.J.), the National Science Foundation Graduate Research Fellowship Program under Grant N o DGE-1650112 (G.P.J.). We also acknowledge the Kościuszko Foundation, Iwanowska program N o PPN/IWA/2018/1/00015 of Polish National Agency for Academic Exchange, the National Science Centre (Poland) project UMO-2015/19/N/ST5/00538 and grant N o POWR.03.02.00-00-I023/17 of the European union through the European Social Fund under the Operational Program Knowledge Education (J.L.), the Keck Foundation for computational resources and Dr. M. Gembicky for his crystallographic expertise. Keywords: Selenium scale • carbenes • NHC • CAAC • Hydrogen bonding