Clément Casalta

Samir Bouzbouz
email: samir.bouzbouz@univ-rouen.fr

Rhodium (III) Catalyzed Regioselective and Enantiospecific Allylic Arylation in Water by -Fluorine Elimination of the Allylic Fluoride: toward the Synthesis of Z-alkenyl-unsaturated amides

A direct coupling of arylboronic acids with allylic fluorides was carried out in water without additives using a rhodium (III) catalyst (Cp*RhCl2)2. The transformation proceeded with excellent γ-selectivity to afford major allyl-aryl coupling products (Z) -substituted -unsaturated amides. The reactions of -chiral allylic fluorides took place with excellent -to- chirality transfer to give allylated arenes with a stereogenic center at the benzylic and allylic position.

The development of original molecular architectures is still a major challenge for the organic chemist community. New criteria have been added to this challenge during the 21 st century, such as ecological friendly processes with atoms economy; it is obvious that catalytic processes are extremely demanded and developed to meet this challenge. However, despite the high ecological and economic demand, the major catalytic processes are still developed under drastic conditions (toxic organic solvents, additives oxidants, bases, acids ...). To overcome these drawbacks, the development of catalytic reactions in water 1 is one of the solutions to this problem, provided that the selected substrates and catalysts are compatible with water. Alkenes are an integral part of the most widely used organic compounds in the chemical industry and are present in a large number of bioactive molecules. Although the synthesis of E-alkenes is well established, the synthesis of Z-alkenes 2 is less depicted because of their low thermodynamic stability. Thus, the synthesis of Z-alkenes is a challenge and an important goal for synthetic chemists. To overcome the Wittig reaction, 3 which is the most general method of formation of Z-alkenes. Many different catalytic reactions have been developed, such as semi-hydrogenation of alkynes, 4 stereospecific cross-coupling reactions, 5 terminal-to-internal olefin isomerization 6 and Z-selective cross-metathesis. 7 Among the catalytic reactions, which make it possible to obtain a functionalized olefin, are the transition-metal catalyzed allylic substitutions with unstabilized carbon nucleophiles. Various approaches have been studied to allow allylic arylation using arylborane reagents with either prefunctionalized olefins 8-10 (bearing an appropriate leaving group) or nonfunctionalized olefins 11 (Scheme 1). In all cases, these methods described previously give E-aryls alkenes. Regarding the prefunctionalized olefins, the allylic arylations were carried out according to two wellestablished mechanisms, either via the (-allyl) metal intermediates [START_REF] Review: Pigge | Metal-Catalyzed Allylation of Organoboranes and Organoboronic Acids[END_REF] or a regioselective carbometallation of the allylic alkene followed by a -acetoxy elimination group rather than -hydride elimination. 13 Recently, we have described a Pt(0) catalyzed allylic substitution process of -fluoroenamides. The use of a soft nucleophile (amine) allowed us to generate an E-alkene via a (-allyl) metal intermediate [START_REF] Reddy | Efficient and direct synthesis of γ-amino-α,β-unsaturated amides by catalyzed allylic substitution of α-fluoroenamides: toward to synthesis of hybrid peptides and indolizidines[END_REF] no coordination was observed in this case. On the other hand, other metals such as rhodium can bond to the nitrogen atom of the amide to form amide-rhodium complexes. These complexes have been observed recently in allylation reactions via C-H activation. [START_REF] Mishra | Recent Advances in Catalytic C(sp 2 )-H Allylation C(sp3 )-H activation[END_REF] Knowing the potential for coordination that the amide function of our fluoroenamide substrates can have with the metallic atom of the catalyst, we thought that by using a boronic acid (hard nucleophile), we could easily and selectively obtain Z-arylated alkenes. Rhodium has been selected as transition metal because it is one of the metals that have been proven to coordinate with heteroatoms.

Scheme 1. Strategies to Accomplish of Allylic Arylations

States of art

This Work: Allylic Arylation from Allylic Fluoride via -F elimination

The challenge for us is to answer different questions: How could we orient the selectivity in favor of the thermodynamically less stable Z-isomer? How to avoid a second addition of aryl-boronics on the -unsaturated amide formed? How could we get total regioselectivity? Is there a transfer of chirality if we start from a chiral compound? Here we report an unprecedented catalytic process that allows generating both the stereocenter and the Z-olefin in one step via the creation of a Csp2-Csp3 bond (Scheme 1). Initially, -fluoroenamide 1a was examined as an allylating agent for phenylboronic acid in the presence of a variety of catalysts in water (Table 1). A control reaction conducted in the absence of catalyst with phenylboronic acid as a nucleophile, demonstrate that transformation is only induced by the presence of the catalyst (Table 1,entry 1). Although the reaction hardly proceeded with rhodium (I) catalysts (entries 2-5), the yield was greatly improved with a catalytic amount of rhodium (I) catalyst (entry 6). On the other hand, a significant quantity of the product resulting from the -H elimination was observed. The same result was observed using a rhodium (II) catalyst but with an increased ratio in favor of the Z-isomer (entry 7). Using rhodium trichloride no reaction was observed (entry 8). But, the use of the rhodium (III) (Cp*RhCl2)2 complex allowed us to obtain the product with excellent yield and a high Z/E ratio in favor of the Z isomer (entry 9). What must be noted is the absence of the product resulting from the -H elimination. By lowering the temperature, we observed a decrease of the ratio Z/E (entries 10-11). No significant improvement in the Z/E ratio was observed at 100 °C. Recently, the Sanford group [START_REF] Mantell | Improved Synthesis of [Cp R RhCl2]2 Complexes[END_REF] has described the preparation of (Cp*RhCl2)2 in situ in 2-propanol from RhCl3 and the pentamethylcyclopentadiene (Cp*H). We have tested these conditions in the presence of compound 1a and phenylboronic acid and no formation of desired product was observed. This lack of reactivity was confirmed using commercial (Cp*RhCl2)2. However, the association of RhCl3 and (Cp*H) in water allowed to obtain the arylated product. This result clearly shows that the catalyst (Cp*RhCl2)2 can be generated in situ in water and the use of isopropanol instead of water inhibits the reaction. Other organic solvents (THF, Dioxane, MeOH, and toluene) had the same inhibition effect . [START_REF]amide allylic fluoride (19) Review concerning C-F bond activation by fluorine activation[END_REF] The reactions afforded exclusively the -substitution products.

To generalize those conditions, a series of substituted N-benzylamide (1a, 1b, and 1c) were examined with Arylboronic acid, mono or di-substituted arylboronic acids bearing both electron-withdrawing and electron-donating groups (Cl, Br, F, CF3, COCH3, NHCOCH3, Me), in both the para, meta and ortho positions. The desired products 2ab to 2bb were obtained in good yield (75-95%) and a high Z/E selectivity (4.4/1 to 10.5/1). Using other boronic acids having substituents in the ortho position such as 2-bromo phenylboronic acid and 2,6 dimethyl phenyl boronic acid no reaction was observed using the substrate 1a presumably due to steric effects. On the other hand, the product yield decreased only for the product 2ca in 55% yield (R2 = CH2tBu), but with an excellent Z/E ratio 15:1 (Scheme 2). By contrast, the combination of (R2 = CH2tBu) with 3-bromophenylboronic acid gives only a ratio of 11.3/1 in favor of Z for the product 2cb in 63% yield. We showed that the reaction is sensitive to the electronic and steric effects of the aryl boronic acids and the substituent R2 in γ-position of 1, but also by the nature of the substituent R1 of the amide as shown in the results for a series N-isobutyl amide (1d-1f) and cyclohexyl amide 1g (Scheme 2). The desired products 2da-2gb were obtained in good yield (91-95%) and good ratio Z/E (4.5/1 to 14.9/1). On the other hand, the reactivity is better in the most sensitive case (R2 = CH2tBu) since the product 2fa (92%) is obtained after 15h of reaction while for the product 2ca the reaction is not complete after the same reaction time. It is important to note that for the amides de-rived from anilines, a decrease in reactivity was observed at long reaction times (14 h) and with a low Z/E ratio in favor of the Z isomer. Based on these results, we then investigated the case of primary amides to study the influence of a nonsubstituent amide on the reactivity and the stereoselectivity of the reaction. A series of primary -fluoroenamide substrates 1h-1k has been studied with a variety of phenylboronic acid, affording the corresponding products 2ha-2ka in moderate to good yield (up to 81%) and an excellent Z/E ratio (15.5/1 to 43/1) (Scheme 3). Finally, we investigated other arylboronic acids bearing either electron-withdrawing or electron-donating groups (3-Cl, 3-Br, 3-F, 3-Me, and 4-Br). The desired products 2hb-2kc were obtained in 49-88% yield and with an excellent Z/E ratio (15/1 to 43/1) (Scheme 3).

Scheme 3. Investigation of the primary amide substrates scope a,b,c

a Reaction conditions: 1a (1 equiv), ArB(OH)2 (2 equiv), (Cp*RhCl2)2 4-5 mol %, H2O (C = 0.3-0.8 M) see supporting information. b Isolated yield of Z-product. c ratio Z/E determined by 1H NMR analysis of the crude reaction.

The coordination of nitrogen rather than carbonyl has been demonstrated. No reaction occurs when the amide is replaced with an ester. On the other hand, with a tertiary amide (morpholine) the reaction occurs moderately in favor of the isomer E. [START_REF]amide allylic fluoride (19) Review concerning C-F bond activation by fluorine activation[END_REF] The origin of (Z)-stereoselectivity and regioselectivity can be explained by coordination of the rhodium catalyst with amide N-atom and the formation of the five-membered rhodacycle intermediate followed by -fluorine elimination (Scheme 6).

For these considerations, we chose to study the stereospecificity of the reaction with chiral allylic fluorides bearing a primary amide moiety. 18 The reaction of the (R)-(E)-1h (99%ee), which has a γ-CH2Pr substituent, with phenylboronic acid in the presence (Cp*RhCl2)2 gave (S)-(Z)-2ha (99%ee), indicating that the allyl-aryl coupling with an -chiral allylic fluorides took place with excellent -to-γ position chirality transfer. The reaction of (R)-(E)-1i and (R)-(E)-1j, which has respectively γ-CH2iPr and γ-CH2tBu substituents, with phenylboronic acid gave (S)-(Z)-2ia (97% ee) and (S)-(Z)-2ja (98% ee), suggesting that the efficiency of the chirality transfer is not significantly influenced by the steric demand of the γ-substituent (Scheme 4). Similarly, when using the 3-bromophenylboronic acid the chirality transfer in the arylation of the (R)-(E)-1h (99%ee), is not affected, the (S)-(Z)-2he is obtained with excellent ee 99%. This transfer of chirality was confirmed starting from chiral allylic fluorides of S-configuration (Scheme 4).

Scheme 4. Arylation of the Chiral allylic Fluorides

Subsequently, we conducted large-scale reactions of the Rh (III) catalyzed fluorinated alkene arylation method to demonstrate the practicality and operational simplicity of this methodology. This large-scale process was performed in racemic, and stereospecific versions giving high yield, good Z/E ratio and 99% ee (Scheme 5). The formation of the E-arylated product can be explained by the formation of the non-coordinated complex D, which then undergoes a regioselective insertion of the double bond C-C in the aryl-rhodium bond to form the complex F. The latter then undergoes a syn -fluorine elimination to form (E)-2 and regenerates A. At this stage, no precedent for -fluorine elimination from allyl fluoride in the presence of rhodium catalyst has been reported in the literature. 19 It is important to note that for our substrates, the -fluorine elimination process is very fast compared to -acetoxy elimination as shown by NMR spectra (see details for Supporting Information S24).

Similarly, the -fluorine elimination is not impacted by the presence of a second fluorine at the gem position, and the Z/E isomer ratio is even higher (see details for Supporting Information S66). This example is the first described from the gemdifluoro alkenes catalysis with Rh(III) (Scheme 7). The only examples in the literature dealing with the case of gemdifluoroalkenes have been studied in the presence of catalysts based on Co (III), Mn (I or 0) 20 and copper(I). 21

Scheme 7. Allylic Arylations of gem-difluorenamides

Finally, the functionalization of compound 2ac under the conditions of stille coupling at high temperature showed the high Z double bond stability, no isomerization was observed. 17

In summary, we have established a highly efficient, green and sustainable protocol for rhodium(III)-catalyzed -selective and stereospecific reaction between functionalized allylic fluoride and arylboronic acids, which gave -unsaturated amides with a high Z selectivity and a  stereogenic center via the creation of a Csp2-Csp3 bond.

Scheme 2 .

 2 Scheme 2. Investigation of the secondary amide substrates scope a,b,c

Scheme 5 .

 5 Scheme 5. Large-Scale allylic Arylations

Table 1 . Screening of Rhodium catalysts a

 1

	entry	catalyst	T °C (time)	yield %	Z/E ratio
	1	-	80 (16h)	N. R	-
	2	(S,S)-Et-DUPHOS-Rh	80 (1h)	<10 +SM	2.1:1
	3	Rh(COD)2BF4	80 (1h)	<10 +S.M	2.6:1
	4	RhCl(PPh3)3	80 (1h)	N.R.	-
	5	[RhCl(COD)]2	80 (1h)	<5 +S.M	1.5:1
	6	[Rh(OH)(COD)]2	80 (1h)	40 a	1.7:1
	7	Rh2(OOCCH3)4	80 (1h)	32 c	2.6:1
	8	RhCl3	80 (1h)	NR	-
	9	(Cp*RhCl2)2	80 (3h)	94 a	4.4:1
	10	(Cp*RhCl2)2	40 (24h)	80 c	1.6:1
	11	(Cp*RhCl2)2	25 (72h)	87 c	1:1
	a Reaction conditions: 1a (0.4 mmol), PhB(OH)2 (0.8 mmol), catalyst
	5 mol %, H2O (1 mL). b Isolated yield. c Yields determined by 1 H NMR
	analysis of the crude reaction mixture using CH2Br2 as internal standard.

ASSOCIATED CONTENT Supporting Information

The Supporting Information is available free of charge at https://pubs.acs.org. General information, experimental procedures, characterization data and NMR spectra (PDF).

AUTHOR INFORMATION Corresponding Author

*E-mail: samir.bouzbouz@univ-rouen.fr

ORCID

Samir Bouzbouz: 0000-0002-5544-7892

Notes

The authors declare no competing financial interest.


Lidia Martínez

Gonzalo Santoro

Pablo Merino

Mario Accolla

Koen Lauwaet

Jesús Sobrado

Hassan Sabbah

Ramón J Pelaez

Victor J Herrero

Isabel Tanarro

Marcelino Agúndez

Alberto Martín-Jimenez

Roberto Otero

Gary J Ellis

Christine Joblin
email: christine.joblin@irap.omp.eu

José Cernicharo
email: jose.cernicharo@csic.es

José A Martín-Gago

Non-aromatic carbonaceous species formed in the circumstellar innerlayers of evolved stars

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Stars, like our Sun, in their final stages of evolution reach the asymptotic giant branch (AGB) accompanied by a massive ejection of matter containing the vast chemical complexity that provides the building blocks of planets and the main ingredients necessary for the emergence of life [1][2][3] . The advent of powerful radio-telescopes like ALMA, with improved radial resolution and sensitivity, opens the possibility to outline the basic physical and chemical conditions in which stardust is formed in the circumstellar envelopes (CSE) [4][5][6] . This dust, which consists of silicates or carbonaceous material depending on the C/O elemental abundance ratio of the AGB star, will then be ejected into the interstellar medium (ISM) where, after processing, it becomes involved in the formation of new stars and planets. However, the chemical nature of the dust seeds and carbonaceous molecules produced by C-rich evolved stars, as well as the chemical routes leading to them, is still controversial. The dust seeds are first generated in the vicinity of the photosphere, at about 1-2 stellar radii 7 . In these regions, carbon nucleates into nanometersized grains in an equilibrium process with a low degree of ionization due to the absence of energetic radiation. The nucleation zone of an AGB star can be regarded as a homogeneous dense atomic cloud in the photosphere of the star which, contrary to the outer layers, is devoid of UV photons. Physical parameters in nucleation zones remain uncertain because they are difficult to derive from observations 4 . Moreover, there is a lack of experiments able to approach the relevant physical conditions, since most rely on energetic plasmas, combustion or decomposition of molecular precursors under conditions very unlike those in the dust formation regions of CSE 8 . Therefore, understanding the formation and nature of carbon stardust is complex and cannot be addressed solely by astronomical observations; dedicated laboratory simulations are required [9][10][11][12][13][14][15] .

A related issue is the formation of large carbonaceous molecules, polycyclic aromatic hydrocarbons (PAHs) and fullerenes. PAHs, identified by the emission of aromatic infrared bands (AIBs) 16 , are widespread in regions of massive star formation and in C-rich protoplanetary and planetary nebulae. Buckminsterfullerene C60 has been detected in some of these environments 6,17,18 and its cation is also present in the diffuse ISM 19 . A robust explanation of how these large molecules form is still lacking, especially in evolved stars where AIBs are only detected at late stages when the star evolves from red giant to white dwarf emitting ultraviolet (UV) radiation 20 .

In this work, using an unprecedented ultra-high vacuum (UHV) experimental setup 21 we investigate the basic carbonaceous chemistry taking place in the dust formation region of CSEs. Unlike previous experiments, our approach produces carbon dust seeds using exclusively gasphase C atoms and molecular hydrogen in a ratio close to that in the atmospheres of AGB stars without the requirement for introducing ad hoc C-bearing molecular precursors, such as acetylene or methane. Under these conditions, we show that carbon nanosized-particles, pure carbon clusters and carbon species of aliphatic nature are efficiently formed, whereas aromatics are only present at trace levels and no fullerenes are detected. Our results reproduce the relative abundances of simple molecules detected in CSEs, such as C2H2 and C2H4, which are directly generated by bottom-up chemistry and are simulated by a simple chemical kinetics model. Our data suggest that the pressures in circumstellar condensation environments are not sufficiently high for initiating cyclization mechanisms 22 and the formation of aromatic species must occur via other processes. We suggest that one pathway could be through thermal processing of aliphatic material on the surface of dust.

RESULTS

Ultra-high vacuum Gas Aggregation Sources for mimicking the Dust Formation Region of CSEs.

Our experimental set-up, called Stardust 21 , is an UHV machine specifically conceived to simulate in the laboratory the complex conditions of star-dust formation and processing in the environment of evolved stars. Stardust comprises six interconnected UHV modules, offering a high level of control over both the fabrication and processing of cosmic dust analogues. In addition, a collection of in situ characterization techniques are available. UHV technology is mandatory because of the high reactivity expected of the formed carbon nanostructures to atmospheric and residual gases 23 . A more detailed technical description is given in Supplementary Information 1.

Many different techniques have been proposed to form dust analogues, all with their own limitations and advantages. In Supplementary table 1 we present a detailed critical compilation of the most commonly used techniques. Among them, laser ablation is one of the best suited ones 10 which has been used, for instance, to simulate the reformation mechanisms of cosmic grains taking place at low temperature directly in the ISM 15 . In Stardust, we have chosen a sputter gas aggregation source (SGAS) because it is able to generate small clusters nano-sized particles by gas-phase aggregation of individual atoms in a weakly ionized environment 24 . Gas temperature during aggregation can be estimated to be lower than 1000 K 24 , in the range of the condensation temperatures of solids in CSEs (500-2000 K) 7,25 . Thus, the chemistry proceeds via atom aggregation under conditions in which most of the reactions that occur are neutralneutral, bimolecular and termolecular, closely resembling what happens in the dust formation region of CSEs.

The working principle of our experiment is the vaporization of C atoms from a graphite target, using Ar as the sputtering gas. Usually, standard growth methods that employ physical vapor deposition or magnetron sputtering produce nanoparticles or grains directly on the substrate as result of surface diffusion of the adsorbed atoms 26,27 . In the case of the SGAS, nanoparticles and clusters form from gas phase reactions as the sputtered material gets trapped in the aggregation zone of the machine.

In this work we unveil the formation of carbonaceous dust grains starting with a gas mixture containing the two most relevant species: carbon and molecular hydrogen. In carbon-rich CSEs oxygen is locked into CO and SiO and therefore is not available to form dust 28,29 . Fig. 1 shows a pictorial representation of a CSE where typical carbon and hydrogen densities are provided. Supplementary table 2 further compares the conditions achieved during the experiments in the gas aggregation zone with those in the dust formation zone of CSEs. A scheme of Stardust in the configuration used for these experiments is shown in Fig. 1. The experiments start with the vaporization of C atoms, followed by the injection of H2. The vaporized C atoms react with H2 and are subsequently dragged through a nozzle by differential pressure into the next chambers, where they are either analysed in gas phase or collected on inert surfaces for further analysis.

In our experiments, we have used different chemically-inert substrates to collect the carbonaceous stardust analogues for further studies, including Au, SiO2 or highly oriented pyrolytic graphite (HOPG). The results we show hereafter are unaffected by the chosen substrate. Once the formed species pass the nozzle of the first chamber, no further growth takes place in the expansion. The composition of the small gas-phase species is analysed in situ by mass spectrometry (MS) and Optical emission spectroscopy (OES). The material generated can be collected on a surface at position "SAMPLE-1" for ex situ studies, such as laser desorption ionization (LDI) mass spectroscopy or Atomic Force Microscopy (AFM). Finally, the deposit (at position Sample 2 in Fig. 1) is characterized in situ in a third chamber by Thermal Programmed Desorption (TPD). The initial gas-phase vaporized C atoms may interact with residual or injected molecular hydrogen at different densities, and there are no other species involved except Ar (the residual density before magnetron operation, excluding Ar and H2, is about 10 7 cm -3 ). The vaporized C atoms have a density of about 2.5x10 10 atoms•cm -3 as estimated from the sputtering energy (200 eV), the total drag current (200 mA) and the yield of emitted C atoms per impinging Ar + ion 30 . Under these conditions, sputtering of C atoms is preferential than C2 and C3 using Ar 31 . Although the estimated value is some orders of magnitude higher than that accepted for the dust nucleation zone of stars 28 , it provides an effective method to accelerate the chemistry, which takes years in a real AGB envelope. Setting higher C densities together with an Ar overpressure, which accelerates the number of three-body reactions, are the two strategies adopted to overcome the exposure time limitation. Regarding the hydrogen, two cases are considered: low and high densities. In the first case, H2 molecules and C atoms are in the same proportion, which can be achieved either using residual hydrogen or by introducing very low H2 densities that are estimated in the range of 1.5x10 10 mol•cm -3 , a value typically accepted for the dust nucleation zone at distances from the star of 2-3 R* 28 . In the second case, a high H2 density is achieved by introducing extra hydrogen at a concentration estimated at 1.5x10 12 mol•cm -3 , two orders of magnitude higher than that of vaporized C atoms. This situation provides a more appropriate concentration to simulate the chemistry in CSE in terms of H2/C abundance (≈1000) [START_REF] Cernicharo | The Polymerization of Acetylene, Hydrogen Cyanide, and Carbon Chains in the Neutral Layers of Carbon-rich Proto-planetary Nebulae[END_REF] .

Structure of the carbonaceous stardust analogues produced. Fig. 2a-b shows typical AFM images obtained on the C-dust collected on a SiOx surface at the position labelled "SAMPLE 1" in Fig. 1. Two different morphological features can be identified. Firstly, carbonaceous nanosized grains (encircled in white in Fig. 2a) are efficiently produced as the most abundant constituents 10,15 , with a surface density that increases with deposition time. These spherical grains exhibit an extremely narrow size distribution with an average size of 9 nm, and each is comprised of more than 10 5 C atoms (see Fig. 1a inset). Although larger grains (typically 100 nm) are thought to be present in space, in Stardust the nanoparticle size is conditioned by the geometry and experimental parameters and can be considered to represent the initial stages of carbonaceous dust growth. While the images of Fig. 2 were recorded for the case of low hydrogen concentration, similar structures can be found for the high H2 concentrations. Besides the carbonaceous nanograins, the other principal constituent of the C-dust analogues can be observed on the "flat" regions of Fig. 2b, where a thin mantle of material is observed, for instance in the area marked by a white square (see Supplementary Information 3). Structural resolution of these zones cannot be achieved using standard AFM, and more powerful in situ microscopies are required. Fig. 2c-d shows a scanning tunnelling microscope (STM) images obtained from the C analogues deposited on a Au surface. A STM-based detailed characterization of the carbon products obtained with the SGAS is complicated due to the large number of different species generated during the experiment and their sub-nanometric size. However, the capabilities of low temperature LT-STM allow us to resolve individual clusters in real-space with Å-scale resolution. At room temperature (300K) molecular clusters diffuse on the surface with high speed impeding their rigorous identification. At low temperatures (4K) the adsorbed products remain immobilized on the surface. Fig. 2c shows a wide plethora of individual nanometer-sized structures at the surface, with apparent heights ranging from 1 to 5 Å that correspond to small amorphous C-clusters, but are difficult to ascribe to a particular cluster type due to the varying shapes. Some of these small molecules can self-assemble on the surface forming 2D ordered islands (see Fig. 2d). This process is surface-mediated suggesting electrostatic interactions between the molecules upon adsorption on surfaces. The structures forming the ordered regions are no longer than 8 Å and with a maximum apparent height of 0.6 Å (see inset). The small size suggests that they correspond to individual molecules of only 3 to 5 C atoms. Unambiguous assignment to a precise chemical structure is difficult due to the fact that STM images show a topography that is convoluted with the electronic structure of the inspected element.

Aliphatic nature of the dust analogues.

To further understand the nature of these carbon analogues we used gas-phase analysis techniques. The Swan C2 lines observed by OES are observed in Fig. 3a in the presence of low H2 density, but are not found in the case of higher H2 density. On the contrary, the Hα emission line is only observed at high H2 density. These observations suggest that excited H atoms are generated during the chemical evolution, involving C2 + H2 à CCH + H. The absence of the C2 lines at high H2 density indicates that most of the excited C2 formed is consumed in this reaction. Interestingly, C2 and H species are only detected in the regions close to the magnetron, which compares very well to observations from the photosphere of evolved stars where atomic hydrogen has been found and C2 bands are prominent [START_REF] Contreras | Echelle long-slit optical spectroscopy of evolved stars[END_REF] . See supplementary information 4 for complete OES spectra. Fig. 3b presents the evolution of the masses of some relevant species during the experiment recorded at position QMS of Fig. 1b. The ON region in the figure corresponds to the period where the magnetron is switched on, before and after this point we obtain the background values. Unfortunately, for these low masses a direct assignment with their parent molecule is not straightforward due to the electron ionization of the quadrupole that induces cracking patterns. Nevertheless, we discuss the most important contribution for every mass. The most striking change for the case of high H2 densities is the predominant formation of mass 26 amu, which mainly corresponds to C2H2 along with fragments of C2H4, C2H6 and larger aliphatic molecules. An increase of mass 16 amu is also observed, corresponding to methane. In the case of high H2 densities the formation of these species are more abundant and we also detect an increase in the main ionization fragments of ethane (C2H6), ethylene (C2H4), propane (C3H8) and larger alkanes (which contribute to most of the masses). Importantly, here these species were formed directly from C atoms and molecular hydrogen, without introducing them as precursors, and using ratios and aggregation processes similar to those reported to take place in the dust formation zone of CSEs 14 . Fig. 3b shows a decrease in the partial pressure of H2 when C evaporation is initiated, suggesting that H2 is partially consumed by chemical reactions that occur in the aggregation zone. It is important to remark that neither benzene (78 amu) nor toluene (91 amu) are detected, indicating that they are below the detection limit (3x10 3 mol cm - 3 ). Interestingly, masses 18 and 32, corresponding to water and oxygen respectively, are unchanged with respect to their background pressure, indicating that, although present as residual gases, they are not taking place in the reactions.

Fig. 3 | Spectroscopic characterization of the molecular species. a, Optical emission spectra recorded at position OES in Fig. 1b for low and high H2 densities. b, In situ gas-phase mass spectroscopy detection at position QMS of most important masses for low and high H2 densities. Vertical dashed lines indicate the time period where the magnetron was switched on and off. All masses have been offset for clarity.

We have performed ex situ laser desorption ionization mass-spectrometry (LDI-MS) in the socalled AROMA set-up [START_REF] Sabbah | Identification of PAH Isomeric Structure in Cosmic Dust Analogs: The AROMA Setup[END_REF] , a machine with high sensitivity for PAH detection. Fig. 4 provides the ion distribution obtained by desorption/ionization of representative samples grown with both H2 densities, showing that the molecular species generated have < 20 carbon atoms. C-clusters with a low degree of hydrogenation are observed for species containing 4 to 9 C atoms. Pure carbon clusters are observed from C8 + to C19 + , and above C9 + these are poorly hydrogenated, which can be understood by the formation of ring structures with lower reactivity [START_REF] Van Orden | Small Carbon Clusters: Spectroscopy, Structure, and Energetics[END_REF] . Above 300 amu the signal completely vanishes, indicating the absence of fullerenes, particularly C60 (see Supplementary Figure 4 for full mass range spectra).

The results from LDI-MS are summarized in the insets of Fig. 4 where the species generated are classified in families (see Supplementary Information 5 and methods). The analysis suggests a higher abundance of CnHm clusters for low H2 density, indicating the formation of more saturated aliphatic species at higher H2 density, which are not found by LDI-MS but evidenced by QMS (Fig. 3). Benzene or toluene, as key aromatic species, are marginally detected either in LDI-MS or in the QMS measurements. In LDI-MS, a few aromatic species are evidenced in very low abundance (total of 3% for both H2 densities), the largest one being C16H10 (Fig. 4). Therefore, it appears that the chemistry involved in the CSE does not favour the formation of PAHs, whose abundance can reach up to 18% of the total carbon species in the ISM [START_REF] Joblin | Contribution of polycyclic aromatic hydrocarbon molecules to the interstellar extinction curve[END_REF] . Fig. 4 shows that low masses, as C2H2, are more importantly detected for the case of low H2 density. However, this quantification shall not be considered as they correspond to volatile species that remain included in the clusters and their total abundance may change from one sample to another.

Fig. 4 | Ex situ laser desorption/ionization mass-spectrometry (LDI-MS) from samples grown with a low (upper panel) and high (lower panel) H2 densities. Data are normalized to the highest peak. Insets: Stacked bar graphs summarizing the family compositions by double bond equivalent (DBE) analysis on the full spectra (see Methods). The error bars correspond to the relative standard deviation obtained for the total ion signal over a mass range for each family of compounds.

DISCUSSION

Kinetic and energetic modelling. Considering the QMS and LDI-MS results we have constructed a time-dependent chemical kinetics model that computes the evolution of the chemical composition of a gas mixture. The model follows the formation of carbon-containing molecules from the precursor C + H2. That is, we aim to predict the main molecules that can be formed and to understand which are the underlying chemical reactions behind their formation.

We use a gas-phase chemical network consisting of about 100 neutral and ionic pure carbon clusters and hydrocarbons containing up to 10 carbon atoms. These species are linked by about 2000 chemical reactions, whose rate constants are extracted from the literature on combustion chemistry, atmospheric chemistry, astrochemistry, as well as from specific chemical kinetics studies [START_REF] Agúndez | The chemistry of disks around T Tauri and Herbig Ae/Be stars[END_REF] .

The initial conditions correspond to those at the gas aggregation zone in the high H2 dose experiment (see first section). The starting composition of the gas consists of a mixture of Ar/Ar + /H2/H/C with mole fractions 1.0/10 -4 /1.6x10 -6 /10 -7 /2.6x10 -8 . We consider that the gas is trapped in the aggregation zone at a constant pressure of 66 mbar (measured value) and a constant temperature of 500 K, which could be considered as an averaged representative value. In a second stage, the gas is neutralized and expands reaching a pressure of 0.1 mbar and a temperature of 300 K at a distance of 10 cm from the magnetron sources. At this stage, the sharp decrease in pressure makes the chemical reactions too slow in comparison with the dynamic time scale associated with the gas expansion, and the chemical composition remains quenched. Therefore, molecular synthesis takes place only during the first stage in the aggregation zone.

In Fig. 5 we show the results of the calculations for some relevant species. It is seen that after several tens of seconds, atomic carbon starts to be processed into different carbon-containing molecules. Pure carbon clusters such as C3, unsaturated hydrocarbons like C2H2 and C3H2, and saturated hydrocarbons such as CH4, C2H6, and C3H8 are abundantly formed. These results are qualitatively in agreement with the molecules detected in gas-phase (Fig. 3) in the Stardust machine, which indicates the disappearance of C2, together with the emergence of CH4 and C2H2. According to the model, the molecular synthesis takes place in various steps. The initial step, in which carbon is transformed from atoms to molecules, proceeds via threebody reactions with Ar as third body. In particular, C + C + Ar --> C2 + Ar C + H2 + Ar --> CH2 + Ar are key the reactions that constitute a bottleneck to the whole process of formation of Ccontaining molecules. For the pressure and temperature of the model, the time scale of these reactions is of the order of tens of seconds. The bimolecular reaction of atomic carbon with H2 to produce CH + H is endothermic by about 100 kJ/mol and thus is not efficient at a temperature of 500 K.

Once the above starting reactions have formed the first molecular species, the growth of molecules is driven by bimolecular reactions and three-body reactions involving neutral species. Routes involving ions are less efficient in our experimental conditions. Moreover, reactions of hydrocarbons with Ar + are fast and result in fragmentation [START_REF] Anicich | An index of the literature for bimolecular gas phase cation-molecule reaction kinetics[END_REF] , so that these reactions tend to produce a delay in the molecular synthesis.

According to the model, carbon growth mainly occurs by reactions of insertion of carbon atoms. For example, the reaction [START_REF] Clary | C + C2H2: A Key Reaction in Interstellar Chemistry[END_REF] and drives the transformation from species with two carbon atoms to molecules with three C atoms. Analogous reactions of polyacetylenes with atomic carbon allow the progressive processing of carbon into aliphatic hydrocarbons of increasing complexity.

C2H2 + C --> C3 + H2 --> C3H + H is fast
Apart from the increase of the number of carbon atoms, hydrogenation is also an important part of the chemical processing that takes place in the Stardust experiment. Hydrogenation essentially occurs via three-body reactions involving H atoms of the type CnHx + H + Ar --> CnHx+1 + Ar in which H atoms are successively added. This way, CH4 is formed from CH2, and C3H8 from less hydrogenated species like C3 and C3H. Under our experimental conditions, H atoms are abundantly formed through various reactions involving ions like Ar + + H2 --> ArH + + H There are some hydrogenation steps that mostly follow bimolecular reactions involving H2 rather than three-body reactions involving H atoms. This is the case of the conversion of C2 into C2H, which occurs by the reaction C2 + H2 --> C2H + H which could be rapid [START_REF] Pitts | Temperature dependence of the C2(X1Σg+) reaction with H2 and CH4 and C2(X1Σg+ and a 3Πu equilibrated states) with O2[END_REF] and more efficient than the three-body reaction, C2 + H + Ar.

The synthesis of hydrocarbons from atomic carbon is summarized in the scheme shown in Fig. 5. The scheme of carbon growth by C insertion and hydrogenation by three-body reactions with H can be extrapolated to larger species. However, the kinetics of reactions involving larger hydrocarbons is not so well constrained and it is likely that other routes, e.g., associations of hydrocarbon fragments of different size, will also be efficient. The calculations carried out at different temperatures indicate that for temperatures below 500 K (down to 300 K) the results are qualitatively similar to those obtained at 500 K, with small variation in the peak abundances. These slight deviations in abundance are caused by the dependence of the rate coefficients of the reactions involved in the molecular synthesis. At temperatures higher than 500 K the results gradually modify its behavior, with unsaturated hydrocarbons like polyacetylenic chains becoming more abundant at the expense of saturated hydrocarbons, like alkanes. At temperatures above 1000 K the model predicts that alkanes should have negligible abundances. This suggests that there could be a turning point for temperatures around 800-1000 K. Below these temperatures, hydrocarbon with high H/C ratios would form efficiently whilst at higher temperatures C-bearing molecules with low H/C ratios would dominate. This may have implications for the formation of carbonaceous material in Crich CSEs. At thermochemical equilibrium, graphite is predicted to condense at temperatures as high as 1500-2000 K [START_REF] Lodders | Condensation chemistry of carbon stars[END_REF] . However, the actual temperature at which carbonaceous dust forms in C-rich CSEs could be significantly lower because solid carbon may not be graphitic but rather in the form of less ordered materials like amorphous carbon [START_REF] Martin | Carbon grains in the envelope of IRC +10216[END_REF] and because the process of condensation is probably controlled by chemical kinetics rather than by thermochemical equilibrium.

Thermal induced formation of aromatics on surfaces. Fig. 6 shows a representative mass spectrum single scan at 430K from an in situ TPD experiment (see full TPD in Supplementary Figure 6). Although a precise assignment of masses is difficult to perform, families of CnHm hydrocarbons can be identified. The most abundant masses of 26 amu, 39 amu and 41 amu all belong to known fragments of larger aliphatic hydrocarbons 43 . Interestingly, in our experiment benzene and naphthalene were observed only after annealing the deposited carbon above 355K. These aromatic species are most likely formed after a catalytic recombination of small hydrocarbons on the gold surface, a process reported in surface chemistry [START_REF] Alyabyev | Gold as a catalyst. Part II. Alkynes in the reactions of carbon-carbon bond formation[END_REF][START_REF] Zaera | Surface chemistry of hydrocarbon fragments on transition metals: Towards understanding catalytic processes[END_REF] , since they were marginally (in the case of benzene) or not (in the case of naphthalene) observed in the LDI-MS analysis that is highly sensitive to aromatics. The fact that aromatics could be formed after annealing of carbonaceous aliphatics deposited on a C-rich surface needs to be further explored as a possible mechanism for the formation of these species in warm CSE environments. Indeed AIBs, the signature for PAHs, are not convincingly detected in AGBs but are observed at later stages when the star emits ultraviolet (UV) radiation that leads to photo-processing of the carbon-dust 20 .

Fig. 6 | Representative mass spectrum from a thermal desorption experiment on C-analogues formed using high H2 density on a Au surface. Temperature of 430 K.

Astrophysical implications. Carbon, nitrogen, oxygen (assumed to be in CO and SiOx in C-rich AGBs), and hydrogen are the main actors of the molecular universe, and the combination of these elements gives rise to a plethora of organic molecules. There are several models to account for the presence of aromatic species in the ejecta of evolved stars; one involves the polymerization of acetylene to benzene 5 and further growth by the hydrogen abstraction acetylene addition (HACA) mechanism 22,[START_REF] Zhao | Pyrene synthesis in circumstellar envelopes and its role in the formation of 2D nanostructures[END_REF] . Other scenarios involve the processing of SiC dust grains [START_REF] Merino | Graphene etching on SiC grains as a path to interstellar polycyclic aromatic hydrocarbons formation[END_REF] or the catalytic activity of C2H2 on silicate surfaces [START_REF] Tian | Catalytic conversion of acetylene to polycyclic aromatic hydrocarbons over particles of pyroxene and alumina[END_REF] . Although the HACA mechanism has been invoked to unravel the synthesis of PAHs in the outflows of carbon-rich AGB stars 22,[START_REF] Zhao | Pyrene synthesis in circumstellar envelopes and its role in the formation of 2D nanostructures[END_REF] , our results suggest an alternative scenario. We produce acetylene in a ratio H2/C2H2 of 50, compared to the 10 4 estimated for the inner layers of AGBs 4 . This is due to the fact that we have accelerated the chemistry by introducing higher C densities. Importantly, even in these favourable conditions we do not observe an efficient formation of PAHs. Earlier models of the formation of PAHs in CSE have shown that there is a narrow temperature window, between 900 and 1100K, in which PAHs could form 22,[START_REF] Cherchneff | Polycyclic aromatic hydrocarbon formation in carbon-rich stellar envelopes[END_REF] . However, the conversion of benzene to larger PAHs was found to be a bottleneck in the reaction scheme. The role of shocked regions close to the star photosphere in which temperatures higher than 1700 K could be reached have therefore been emphasized [START_REF] Cherchneff | The chemistry of carbon dust formation[END_REF] .

Our results also suggest the formation of a large amount of amorphous carbon nanosized grains that shall be considered the main component of the carbon stardust (see Fig. 3). Aliphatic species deposited on dust grains are susceptible to further react between themselves in a catalytically activated process leading to larger molecules or aromatic species upon thermal activation (see Fig. 6). Such a rise in temperature could be achieved in the highly UV-irradiated environments of protoplanetary nebulae where polymerization of gas species and dust chemistry could also lead to the formation of PAHs 5,[START_REF] Cernicharo | The Polymerization of Acetylene, Hydrogen Cyanide, and Carbon Chains in the Neutral Layers of Carbon-rich Proto-planetary Nebulae[END_REF] .

Finally, we are able to qualitatively reproduce the abundances of the C2H2 and C2H4 found in CSEs around AGB stars, indicating that SGAS, a technique not previously used in laboratory astrophysics, can be a very valuable tool to gain information on the chemistry operating in CSEs. When compared to other techniques SGASs can more closely resemble the delicate conditions of these cosmic environments (see Supplementary table 2). Most of the previously used techniques lead to the formation of aromatics, in addition to grains and clusters, either because of the use of much higher temperatures or because of the use of very high pressures of acetylene.

Conclusions

The combination of laboratory astrophysics, surface science and astronomical observations can unveil the chemical routes that operate in the inner layers of the envelope of evolved stars, providing new insights into the chemistry of carbonaceous stardust seed formation and fostering new observations. Our results suggest that PAHs might not be efficiently form during gas-phase growth in the CSE, which we show to generate aliphatic material and C-clusters together with carbonaceous nano-grains. However, the species that are deposited on dust grains are susceptible to further processing and dust can catalyse the formation of aromatic species upon thermal activation, which could take place as a result of the significant rise in dust temperature that occurs in highly UVirradiated environments.

Methods

Fabrication of the dust analogues. The carbon dust analogues were fabricated using a scaledup Multiple Ion Cluster Source (MICS) 21 , a type of sputtering gas aggregation source (SGAS), working under UHV conditions (base pressure 1x10 -9 mbar) from Oxford Applied Research Ltd.. The magnetrons were loaded with a graphite target of 99.95% purity. The Ar flux used for the experiments was 150 sccm. The typical power applied to the magnetron was 100 W and the working aggregation length (distance between the magnetron and the exit nozzle) was 374 mm. Extra-pure H2 (99.99% purity) was injected during fabrication through one of the lateral entrances of the aggregation zone of the MICS (see Supplementary Figure 1), using gas-dosing valves with flux-regulator to inject into the UHV system. The doses used for these experiments were: 0, 4x10 -4 and 1 sccm, with an Ar flux maintained at 150 sccm. The substrates employed for collecting the NPs are boron-doped Si(100) with its native oxide for AFM; polycrystalline Au for AROMA experiments; Au(111) and HOPG for TPD analysis.

Atomic Force Microscopy (AFM) measurements were performed in the dynamic mode using a Cervantes AFM System equipped with the Dulcinea electronics from Nanotec Electronica S.L. All images were analyzed using WSxM software [START_REF] Horcas | WSXM: a software for scanning probe microscopy and a tool for nanotechnology[END_REF] . AFM images were recorded ex situ.

Scanning tunneling microscopy (STM).

Samples for scanning tunnelling microscopy (STM) were prepared in Stardust and transferred without exposing the samples to the atmosphere to the STM chamber, located in a different laboratory, using a UHV suitcase with an ion-getter pump (base pressure <1x10 -9 mbar) to avoid chemical reaction with environmental gases 23 . This procedure can be considered as in situ since the surface has never been at a pressure higher than 10 -9 mbar. Samples are deposited on an atomically flat single crystal Au(111) surface precleaned by repeated sputtering/annealing cycles, which consisted of sputtering for 10 min using Ar + ions at an energy of 1.25 kV and subsequent annealing at 800 K for 5 min. Clean Au(111) is later exposed to the carbon analogues for 5 minutes keeping the sample at room temperature, then immediately transferred to the UHV suitcase and to the STM chamber. The total time between sample growth and insertion in the STM cryostat was limited only by the pumping of the load lock volume. STM measurements were performed at 4K in a LT-STM (Scienta Omicron) with a base pressure <10 -11 mbar.

Optical emission spectroscopy (OES).

The light emitted by the plasma generated in front of the magnetron was collected through a fused silica window and an optical fibre of fused silica, and analysed by OES (see position in Fig. 1a) using a 193 mm focal length, motorized Czerny-Turner spectrograph (Andor, model Shamrock SR-193-i-A) equipped with a CCD camera (iDus DU420A-BVF). Two diffraction gratings with 1200 grooves/mm and 1800 grooves/mm, installed in a movable turret, provide spectral ranges of 300-1200 nm and 200-950 nm, respectively, and nominal spectral resolutions of 0.22 nm and 0.15 nm, respectively (for an input slit width of 20 µm). The relative spectral efficiencies of all the spectroscopic equipment was quantified for both diffraction gratings with a calibrated tungsten lamp.

In situ mass spectrometry (MS).

The gaseous species produced during the fabrication of the Cdust analogues were detected in situ with a quadrupole mass spectrometer (QMS) (0-100 amu) Prisma plus spectrometer from Pfeiffer. Lowest detectable partial pressure 10 -10 mbar. Includes Continuous Secondary Electron Multiplier.

Laser desorption/ionization (LDI) mass-spectrometry: the AROMA setup. AROMA (Aromatic Research of Organics with Molecular Analyzer) is dedicated to the analysis, with micron-scale resolution, of the carbonaceous molecular content of cosmic dust analogues. The experimental setup consists of a microprobe laser desorption ionization chamber and a segmented linear quadrupole ion trap connected to an orthogonal time of flight mass spectrometer (LQIT-oTOF). For each detected m/z peak with a signal-to-noise ratio (S/N) greater than 10, a chemical formula was assigned using the mMass software [START_REF] Strohalm | mMass 3: A Cross-Platform Software Environment for Precise Analysis of Mass Spectrometric Data[END_REF] , an open source mass spectrometry tool. The double bond equivalent (DBE) for each molecular formula was then calculated using the following equation for a given chemical formula of hydrocarbon species CcHh: DBE = c-(h/2)+1. This method allows sorting of the detected ions into families of compounds [START_REF] Marshall | Petroleomics: Chemistry of the underworld[END_REF] . This is done using empirical factors that delineate DBE boundaries in complex natural organic matter [START_REF] Koch | From mass to structure: An aromaticity index for highresolution mass data of natural organic matter[END_REF] . Hydrocarbons with 0.5 < DBE/C < 0.9 are considered to be aromatics. The line DBE/C ~1 corresponds to C clusters and the species falling in the zone 0.9≤DBE/C<1 are referred to as HC clusters, which includes CcH2 polyynes. Once all the detected species were sorted, the ion signal percentage was calculated for each family by dividing the sum of peak intensities detected in each family by the total ion intensity. The reported error bars in DBE analysis correspond to the relative standard deviation obtained for the total ion signal over a mass range of a family of compounds when the measurement was repeated several times (7 to 10) on natural samples. These error bars, of around 25%, are conservative since they have been derived from a complex matrix.

Thermal Programmed Desorption (TPD).

Experiments were performed in situ to determine the mass spectra of the carbon analogues deposited on low reactivity surfaces annealed at increasing temperatures. TPD was performed using a Pfeiffer HiQuad QMG 700 with QMA 400 mass spectrometer with a CP 400 ion counter preamplifier. The instrumentation allows measurement of masses between 0 and 512 amu with a detection limit of 10 -15 mbar. After carbon analogue deposition the samples were placed in front of the quadrupole at a short distance (>1 cm) ensuring that the majority of the collected gas species derive from the surface of the sample surface. Atomically flat Au(111) and HOPG substrates were used in the experiments, yielding comparable TPD spectra. Au(111) surfaces were cleaned in situ by the standard methodology of sputtering/annealing. HOPG was mechanically cleaved ex situ and consecutively introduced into the vacuum system, which ensures an atomically fresh graphite surface prior to deposition. The carbon analogues were deposited during 100 minutes, ensuring that there is sufficient carbon material to obtain well-defined signals in the TPD experiments. A temperature ramp of 12.5°K•min -1 was applied from room temperature to the maximum temperature examined (700K). 4367) through the Program of R&D activities between research groups in Technologies 2013, co-financed by European Structural Funds, is also recognised.

The stardust machine and the CSE: Technical details. The Stardust machine for mimicking the CircumStellar Envelope (CSE)

The Stardust machine is a novel experimental setup designed to simulate in the laboratory the complex conditions of cosmic dust formation and processing in the environment of evolved stars or supernova. It comprises six interconnected ultra-high-vacuum (UHV) modules, offering a high level of control over both the fabrication and processing of cosmic dust analogues. In addition, a collection of in situ characterization techniques is available. In Stardust the chemistry proceeds via atom aggregation under conditions in which most of the reactions that occur are neutralneutral, closely resembling what happens in the circumstellar envelopes (CSE) of Asymptotic Giant Branch (AGB) stars.

The Stardust machine was conceived as a multi-purpose experimental station. Thus, it has a versatile, modular design that allows easy reconfiguration of the machine based on the requirements of each specific experiment. Each module has been designed to simulate a given process in a given astrophysical environment. The first step corresponds to the generation of dust seeds, which can be produced at relatively low temperature in the first module, denominated Multiple Ion Cluster Source (MICS), where atoms are ejected from up to three targets. During their transit along this module atoms start to aggregate through collisions with other ejected atoms and the growth process occurs. After production, the seeds pass through a Diagnosis chamber, where it is possible to monitor, filter and perform experiments on these pristinely formed seeds. Subsequently, the seeds enter the Oven chamber, a passageway where the seeds can be heated with IR radiation during their transit. This can provide temperatures similar to those found in the dust formation zone of AGBs. Upon exiting the Oven chamber the grains can be accelerated in the Acceleration module, in order to simulate radiation pressure or the escape of the grains towards the interstellar medium (ISM), where the material will cool down. Furthermore, gas phase molecules (H2, CH4, C2H2…) can be introduced into any of the previously described chambers in desired proportions, in order to promote gas-grain interactions.

After the fabrication of the dust analogues, the next modules are dedicated to both analysing the properties of the generated pristine analogues, as well as tackling the question of dust evolution in various astrophysical environments ranging from later stages of evolved stars (protoplanetary and planetary nebulae), to the ISM and protoplanetary disks. The INFRA-ICE module is dedicated to performing IR spectroscopy of analogues formed at variable temperature down to 12 K, allowing on-substrate (by collecting the analogues) dust processing and molecular ice growth. The final module is the ANA (Analysis Module) that consists of a UHV chamber in which the dust analogues are collected on a substrate and further analysed by a collection of surface science techniques (XPS, Auger, UPS, LEIS…). Here the collected analogues can be heated to up to 1200 K combined with various dust processing techniques. All the processes occurring to the dust analogues during heating, irradiation and exposure can be monitored in situ by the available characterization techniques.

A detailed technical description of the overall machine is provided in reference 1 . The experiments presented in this article correspond to a short configuration of the machine, in which only some of the modules have been used. Given the aforementioned need for highly controllable and clean environments to undertake proper laboratory simulations, only ultra-high vacuum (UHV) compatible materials and seals were used in the construction of the machine.

Configuration of the Stardust set-up used in present experiments

Although the modular concept of the Stardust machine allows to be mounted in different configurations, for this work only three chambers were employed: MICS, Acceleration and ANA. The MICS module is a sputter gas aggregation source (SGAS) devoted to nanoparticle (NP) and cluster fabrication. It incorporates lateral entrances in the aggregation zone through which both Optical Emission Spectroscopy (OES) and gas (H2) injection is performed. The Acceleration module in this configuration is dedicated to monitor the NPs. In this work it was employed for sample collection for ex situ analysis and for Quadrupole Mass Spectrometry (QMS). Finally, the ANA module is devoted to in situ analysis of the NPs by electron spectroscopy techniques and thermal desorption spectroscopy. Further details of each module are given elsewhere 1 . Supplementary Figure 1 shows the configuration employed in this work. For the collection of the carbon star-dust analogues, two different fast entry points were used (in the Acceleration and ANA chambers). Unless otherwise specified, the samples were collected in the Acceleration chamber. The base pressure was 1x10 -9 in MICS, 5x10 -10 in Acceleration, and 1x10 -10 All the aforementioned techniques have been used to describe different conditions and regions of the cosmos. However, they present limitations as the experiments used to mimic astrophysical environments need to be scaled to laboratory standards, and on many occasions, it is very difficult to validate a technical set-up as representative of particular astronomical conditions. This is mostly true when trying to reproduce the chemistry of the inner layers of CSE of evolved stars. In most of these experiments the chemistry involves high-energetic or ionic content, out of equilibrium processes, or relies on molecular precursors that must be decomposed. These conditions are far removed from those of the inner layers of CSE, where clustering takes place mostly by neutral-neutral reactions in the absence of other sources of radiation. In this respect, the Stardust machine was specifically conceived for reproducing the inner layers of the CSE, and its differential advantage with respect to the rest of the dust analogue production techniques is that the basic constituents from which the material grows are individual thermalized atoms. This is critical in all the three steps of the dust growth process and enables us to obtain chemical synthesis routes from reactions in equilibrium, as observed in C-rich AGB stars and some protoplanetary nebulae.

nH2/nc2H2: Estimated: 10 4 Ref. 14 Detected: 10 7 Ref. 16 Acetylene is produced, not introduced : nH2/nC2H2: 50

We produce an excess of C2H2 with respect to the star, however, still low enough to initiate cyclization in these conditions Time in this area: 30 yr, to travel 10 15 cm at a expansion velocoty of 10 km/s. . Ref 11

Residence time in aggregation zones: seconds: Ref. 16 In Stardust the Ar pressure close to the magnetron enables 3-body reactions Sputter gas aggregation sources (SGAS) compared to Magnetron sputtering. Gas phase condensation techniques have been widely reported as good candidates for the formation of cosmic dust grain analogues 6 . Traditionally, in gas aggregation sources (GAS) atoms are evaporated to a region containing a cooling working gas. Upon reaching supersaturation and due to the collisions with the cooling gas, the atoms may nucleate to nanoparticles. These nanoparticles are afterwards dragged through a nozzle into another chamber with lower pressure, usually employed for deposition of the nanoparticle beam on a substrate 18 . Haberland and co-workers 19 combined this idea of the GAS with magnetron sputtering and developed the so-called SGAS. The SGAS have some characteristics that clearly differentiate the technique from sputtering techniques commonly used for the creation of dusty plasmas 20 . Biederman's group published a comparative study between both techniques 18 . One of the critical questions is the possibility of working at higher pressures of the working gas. Conventional sputtering processes usually operate in the 10 -2 -10 -3 mbar range, while the pressure in the aggregation zone of a SGAS is in the range of 0.1-1 mbar 21 . In these conditions, clustering is highly efficient as the sputtered vapour is rich in dimers (formed by condensation of atoms and not directedly sputtered out from the targe), which helps to overcome the energy barriers involved in the first stages of cluster nucleation, as this is considered to be a critical bottleneck in the clustering process 22 .

In SGAS, one chamber is used for sputtering and cluster growth and, subsequently, the nanoparticles are dragged by pressure difference into a secondary deposition chamber where the substrate is located. The SGAS process is not substrate dependent such that the nanoparticles, already formed in the first chamber, soft-land on the substrate instead of growing on the substrate as in conventional sputtering. Although the growth rates in SGAS are much slower than the sputtering processes (sputtering machines are designed to grow thick layers of a material on a substrate, whereas we focus on the first formed particles), the possibility to tune the size (typically 1-60 nm) 21 and the amount of nanoparticles generated with the variables of the process clearly represents and advantage in comparison to conventional sputtering, since lateral growth due to surface diffusion processes 23 is avoided.

Thus, in this article we propose the use of SGAS as a valuable and versatile technique for mimicking the conditions of CSE.

density and smaller features on the surface. c) Zoom of figure b). d) Zoom of figure c). e) Height Profiles obtained from the blue and green lines of c) and d), respectively, compared to the height profile in a SiOx substrate (dashed-black line). These images were recorded for a low hydrogen density.

4.-In situ optical emission spectroscopy

The emission spectra recorded during the production of carbon dust analogs in the aggregation zone of the stardust at different H2 densities show that near infrared Ar lines prevailed in all cases, as shown in Supplementary Figure 3 for the 690 -750 nm range. They correspond mainly to electronic transitions between the 4p-4s excited Ar levels, and extend up to 1000 nm. On the contrary, Ar emissions in the region 350-470 nm, corresponding to transitions 5p-4s that are usually very intense, have not been detected, due to the higher electron energy necessary to excite the upper level and to the fact that low Ar excitation temperatures (<1 eV) have been reported previously for this experimental system 1 . Furthermore, C2 and CH could be detected with low H2 densities (see insets in Supplementary Fig. 4 (a)), but these bands disappeared when a high H2 density was added. At the same time, Ar intensities decreased noticeably with increasing H2, while the (Ha, Hb, Hg) atomic lines of the Balmer series appeared.

Supplementary Figure 3. Optical emission spectrum of the plasma generated in the aggregation zone of Stardust for a low (a) and a high (b) H2 density. H2 flow rates of 4x10 -4 and 1 sccm were used, respectively. In order to obtain better insight into the molecular phase associated with the C-dust we have employed ex situ laser desorption ionization mass-spectrometry (LDI-MS) in the experimental set-up called AROMA 24 . The ion source offers the possibility of probing polycyclic aromatic hydrocarbons (PAHs), carbon clusters (C clusters), hydrogenated carbon clusters (HC clusters) and fullerenes in solid samples by performing by performing LDI in a single step 25 as well as using the laser desorption-laser ionization (L2MS) technique 24 , in which desorption and ionization are separated in time and space and each step is performed with a different laser. In the L2MS scheme, the approach used in these experiments, an infrared laser pulse desorbs the solid material into the gas phase, with minimal fragmentation, and a subsequent ultraviolet (UV) laser pulse selectively ionizes the gas phase aromatic organic molecules. The ions generated are stored and thermalized in the linear quadrupole ion trap (LQIT) and then monitored with Time Of Flight (TOF) spectrosmetry. This technique has many advantages including its sensitivity. However, one drawback is that it is biased towards species that efficiently respond to the excitation scheme, in particular regarding ionization. For example, with the lasers used in our experiments, it is not possible to detect alkanes. On the other hand, the scheme is very sensitive to the detection of PAHs and fullerenes 24 .

Supplementary Fig. 4 presents the full range mass spectra obtained with AROMA. CnH2 + species are observed together with CnH + species only for odd n values. The neutral precursors are expected to be CnH2. For even n, polyynes are the expected structure. For odd n, the situation is more complicated with a competition between cumulenic and polyacetylenic structures 26 , which most likely accounts for the fact that we observe both CnH + and CnH2 + species. Regarding smaller species, detection of C2H2 + and C3H3 + are notable. Both species are believed to be produced during the LDI analysis from the common C3H2 precursor. Indeed the latter species has a very high proton affinity 27 . In addition, a major dissociation channel of C3H2 + leads to C2H2 + + C 28 , which is a favourable production mechanism for C2H2 + in the AROMA conditions, whereas direct ionization of C2H2 is not a viable mechanism. Finally, evidence for oxidation was seen with a major peak corresponding to HC7O and a weak one that could be attributed to HC5O. It should be noted that these two stable species nor any other HCnO species have been detected in the ISM 29,30 . These were the only oxidized species that were detected and it is likely that they were formed by reaction of the deposited molecular precursors with environmental oxygen.

of each spectrum is marked on the left-hand side of the figure.. Spectral range from 20 to 150 a.m.u. The spectra are vertically shifted for clarity.

Fig. 1 |

 1 Fig. 1 | Production of stardust analogues. Upper part: pictorial representation of the CSE of a C rich AGB star, where typical carbon and hydrogen densities at given distances are provided following reference 28 . Lower part: Schematic of the Stardust machine configuration used in these experiments, with typical C and H2 densities considered. From left to right: there are three different vacuum environments: aggregation zone, where OES measurements are performed and H2 introduced; expansion area, where the quadrupole mass spectrometer (QMS) is placed and we collect samples for ex situ studies (sample 1 position), and analysis area (sample 2), where thermal programmed desorption (TPD) is performed.

Fig. 2 |

 2 Fig. 2 | Production and structure of the stardust analogues. a-b, AFM images recorded ex situ showing the presence of C-nanoparticles and small clusters forming an almost continuous layer. A single nanoparticle is encircled in fig. 2a. Inset of fig. 2a shows the size distribution in a bar-histogram performed over several images. The blue line represents the log-normal fitting of the size distribution c-d, in situ recorded STM images of a C-cluster film deposited on a Au surface. Scanned area: 25x25 nm 2 , recorded at

Fig. 5 |

 5 Fig. 5 |. Computed evolution of the molecular species formed Left: Evolution of the chemical composition computed with the chemical kinetics model as the gas flows in the aggregation zone, calculated using the values for high hydrogen densities (see text) and at T=500K. Right: Chemical scheme of synthesis of hydrocarbons from atomic carbon given by the chemical kinetics model. Calculations performed for the case of high hydrogen density.

5 .

 5 Laser Desorption/Ionization (LDI) mass-spectrometry: the AROMA set-up

Acknowledgements. We thank the European Research Council for funding support under Synergy Grant ERC-2013-SyG, G.A. 610256 (NANOCOSMOS). Also, partial support from the Spanish Research Agency (AEI) through grants MAT2017-85089-c2-1R, FIS2016-77578-R and FIS2016-77726-C3-1-P is acknowledged. Support from the FotoArt-CM Project (P2018/NMT-

Data availability. The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.

Author contributions. In-situ experiments in Stardust were performed by L.M, G.S, P.M, and M.Ac.; C.J. and H.S. performed LDI experiments. P.M.; K.L, R.O and A.M performed STM images. L.M. performed AFM images. RJ.P, V.J.H. and I.T. performed the OES experiments. M.Ag. made the kinetic calculations. G.J.E. and J.A.M-G wrote the first version of the manuscript. J.A.M-G supervised in-situ experiments, and C.J. and J.C the astrochemical interpretation. All authors discussed and contributed to the final version of the manuscript.

Competing interests. The authors declare no competing interests.

Additional information. Additional text, figures and references are provided.

Supplementary references, text and figures

Supplementary references

Supplementary Figure 1. Technical drawing (top) and photograph (bottom) of Stardust in the configuration employed for these experiments.

2.-Comparison between Stardust and other methods for fabricating carbon star-dust analogues.

Several methodologies for generating carbon star-dust analogues have been proposed in the literature resulting in a number of carbon products with different structure and hydrogen content. The main production techniques are summarized in Supplementary Table 1. This table collects the main physical parameters. Some of the parameters cannot be easily generalized as they are strongly dependent on the experimental conditions. This is the case of the total pressure, where a great deal of precision is needed to define this value (base pressure, pressure of the dragging or residual gas, partial pressure of the precursors, etc.). The list of references is representative and far from being exhaustive, as there are many of insightful experiments that have been performed in laboratory astrophysics. Stardust densities are higher to accelerate the chemistry. The high end of the H2/C ratio is close to that of the CSE.

Supplementary table 1: Comparison between typical techniques used to synthesize cosmic dust analogues.

Technique

Temperature range: 500-2000 K. Ref 11,12 Temperature range: 300-500K

The chemical kinetics model suggests that the effect of temperature is critical for the formation of alkanes, which are favored at low temperatures. Close to the magnetron surface, temperature can reach around 1000K, although at the aggregation area could be bellow 500K. 13 Absence of UV processing. Ref 14 Absence of UV photons.

Possible UV photons from the UV interstellar field in low mass loss rate AGB stars or in very clumpy envelopes Ionization degree: <10 -5. Ref 15 Ionization degree: 10 -5 . Ref. 13 The charge is found mainly on large particles after they are formed. Not playing important role in the chemistry of small molecules and clusters.

Atomic Force Microscope imaging (AFM).

AFM images were acquired ex situ, after fabrication of the samples using the fast entry point in the Acceleration chamber, marked as SAMPLE 1 position in Fig. 1 of the main text. Supplementary Figure 2a corresponds to a deposition carried out during 10 min under the conditions described in the main text, whereby a nanoparticle rate of 8 NPs/µm 2 •min can be derived. Most of the carbon is agglomerated in the form of nanometer size C-nanograins, as those that can be observed. Also, the substrate presents certain unresolved roughness comprised of smaller structures that may be related to the C/HC clusters detected with AROMA (white square in supplementary figure 2c). The features of the background and the C-nanograins can be easily differentiated from the line profiles. It is clear that on one hand there are Cnanograins of around 4 nm height (blue profile) and, even though they cannot be fully resolved using AFM, the structures depicted in the green profile present a height of around 1 nm. The former may be related to the structures presented in the STM analysis of Figure 2 in the main manuscript. The roughness of these small features is clearly larger than the roughness of the SiOx substrate (dashed-black line). The apparent trend of a larger abundance of C clusters relative to HC clusters with the H2 dose must be treated with caution, considering the error-bars. The reported error bars correspond to the relative standard deviation obtained for the total ion signal over a mass range of a family of compounds when the measurement was repeated several times (7 to 10) on natural samples. These error bars, of around 25%, are conservative since they have been derived from a complex matrix.

6.-Effect of the temperature in the kinetics models

Apart from the calculations presented in the main text carried out at 500 K, we have performed calculations at other temperatures to evaluate the sensitivity of the calculated abundances to the temperature. We have changed the pressure to use the same density of particles in all calculations and evaluate exclusively the influence of temperature. Supplementary Fig 6 presents an example. At lower temperatures (300 K) the results are qualitatively similar to those obtained at 500 K, with small variation in the peak abundances within the decade. Some masses are slightly higher whereas other decrease. These slight abundance deviations are caused by the dependence of the rate coefficients of the involved reactions in the molecular synthesis. At temperatures higher than 500 K the results gradually modify their trend, with unsaturated hydrocarbons like polyacetylenic chains becoming more abundant at the expense of saturated hydrocarbons, like alkanes. At temperatures above 1000 K the model predicts that the abundance of alkanes should be negligible.

Thermal Programmed Desorption

In Supplementary Fig. 6 we show a representative TPD experiment. The carbon analogues are deposited in ANA module on a Au(111) surface and annealed in situ at increasing temperatures. Mass spectrometry is recorded while increasing the temperature. As commented in the main text, consecutive families of small hydrocarbons with increasing carbon number of the family type CkHn, with 1<k<9 are identified at increasing temperatures (highlighted with blue background). The small hydrocarbons start to desorb from the surface at temperatures as low as 340 K. At higher temperatures, the desorption of small hydrocarbons has occurred and the intensity of the peaks of the associated compounds decreases compared with those obtained at lower temperatures. A good example can be seen in the family C3Hn which reaches its maximum intensity at a temperature of 433K. At higher temperatures benzene and naphthalene begin to be detected (highlighted with red background).

Importantly the intensity of the associated peak at 78 a.m.u increases with temperature. The increase is monotonous up to the maximum temperature examined of 700K. We also can distinguish mass 91, which can be attributed to toluene fragmentation. These results point towards the processing-induced formation of benzene after thermal processing of the analogues that is possibly mediated by the catalytic activity of the surface. However, we must point out that Au surfaces are notorious for their low-reactivity and low catalytic activity 31 . We have obtained similar results for TPD experiments on other inert substrates such as HOPG and SiOx. Therefore, aromatic formation from thermal processing of the analogues seems to be a robust process. In the real environment of AGB stars other processing mechanisms apart from temperature may be present, such as UV absorption or ionization radiation, which may further promote the conversion of non-aromatics into aromatic molecules and suggest an alternative route for the formation of PAHs. Supplementary Figure 6. Thermal programmed desorption of the carbon analogues formed with high hydrogen density deposited on Au(111) annealed at increasing temperatures The temperature at the start