Marta G Avello

Stéphane Golling

Lai Truong-Phuoc

Loic Vidal

Thierry Romero

Vasiliki Papaefthymiou

Nathalie Gruber

Michael Chetcuti

Frederic Leroux

Morgan Donnard

(NHC-olefin)-nickel(0) nanoparticles as catalysts for the (Z)-selective semi-hydrogenation of alkynes and ynamides

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

(NHC-olefin)-nickel(0) nanoparticles as catalysts for the (Z)-selective semi-hydrogenation of alkynes and ynamides

Metal nanoparticles (NPs) are particularly important in catalysis due to their large exposure surface area and because of the unique metal-metal interactions within their core. The major challenge in the application of NPs in catalysis is to design stable NPs under reaction conditions in order to avoid their agglomeration which ultimately results in the loss of their catalytic activity. To achieve that, different strategies have been explored, including the use of: surfactants and micellar catalysis, various supports (with different metal-support interactions) or organic ligands. 1 Whether the latter strategy was initially achieved through the use of ligands such as thiols, amines, cyanides, disulfides, thioethers or phosphines, 2 Nheterocyclic carbenes (NHC) ligands, which are now privileged ligands in organometallic chemistry and homogeneous catalysis, have been established as novel promising stabilisation agents for metal NPs. 3 Thus, several studies have demonstrated the versatility of N-heterocyclic carbene ligands in stabilising and structuring NPs of metals like Ru, 4 Rh, 5 or Pd 6 for significant applications in catalysis, such as the hydrogenation of alkenes and alkynes, the Buchwald-Hartwig amination or the oxidation of alcohols. However, NHC ligands have been much less applied to the stabilisation of NPs of Earth-abundant first-row transition metals, whose development is now a priority in catalysis for obvious economic, environmental and societal reasons. 7 To the best of our knowledge, there are only three reports on the use of NHC ligands to stabilise Ni NPs. 8,9 In the first two ones, such NHC-Ni NPs proved to be highly active catalysts for Kumada couplings 8a and the semi-hydrogenation of alkynes 8b and could be recycled a few times without significant loss of activity. Furthermore, in a recent report, we have shown that a half-sandwich Ni(II)-NHC-picolyl bromide complex effectively catalysed the hydrosilylation of a broad range of aldehydes without additive and of ketones in the presence of KOtBu at 100 °C. 9 Dynamic light scattering, scanning electron microscopy in transmission mode and X-ray photoelectron spectroscopy showed evidence for the involvement of NHC-picolyl Ni particles under the catalytic conditions used for the ketones' reductions.

Beside NHCs, alkenes have also been applied as ligands to low-valent metal species in the last decade and the resulting organometallic complexes and/or NPs displayed interesting catalytic properties. 10 Interestingly, the combination of NHCs with additional alkene ligands allowed the isolation of welldefined organometallics of Ni(0), 11 Pd(0) 12 and Pt(0) 13 which were active in various cross-coupling and reduction reactions.

Herein, we report that the reduction of the [NiCpBr(NHCcynamyl)] complex 1 with an excess of MeMgBr leads to coordinated Ni(0) nanoparticles (Figure 1). The combination of a strong -donor NHC ligand with -coordinating appended cinnamyl moiety likely prevents Ni(0) particles aggregation to larger inactive species and allows rapid exchanges with reaction substrates. The resulting NHC-olefin-Ni(0) particles effectively catalyse the (Z)-selective semi-hydrogenation of alkynes 8b,14 and ynamides, the latter being a rare example of direct hydrogenation of ynamides without the use of any salt or additive. 15 The catalytic semi-hydrogenation of internal or terminal alkynes is useful in numerous organic synthesis by providing alkene building blocks, both at laboratory and industrial scale. 16 In particular, such hydrogenation represents a critical step in polymerization reactions in order to eliminate alkynes and dienes from alkene raw materials.

Our study started with the synthesis of complex 1 from the corresponding imidazolium bromide and nickelocene in refluxing THF for 48 h (Scheme S1). Once isolated in 92% yield and fully characterised by NMR, HRMS, elemental analysis and single-crystal X-ray diffraction analysis (Figure S1, Table S1), complex 1 was readily reduced by reaction with 3 equiv. MeMgBr in toluene at 100 °C for 2 h (Scheme 1). The reduction was characterised by a striking colour change from pink to yellow then brown (Figure S2). The resulting species (3 mol%) was then applied to the (Z)selective semi-hydrogenation of various alkynes and ynamides under 10 bar H2, in toluene or ethanol and at temperatures ranging from 25 to 80 °C, depending on the substrate. After optimising all the reaction conditions (Tables S2,S3), all hydrogenations led to the corresponding (Z)-alkenes in good to excellent yields (Table 1). The temperature and the solvent were the key parameters to control the activity of these catalyses, ethanol leading to the highest activities due to a better solubility of the substrates and hydrogen. It is worth to note that the substitution of pre-catalyst 1 by nickelocene led to full hydrogenation of alkynes. Regarding terminal alkynes, phenylacetylene 2a, but-3-yn-1-ylbenzene 2b and 1phenylprop-2-yn-1-ol 2c were readily hydrogenated in 13 h at room temperature in toluene to afford the corresponding alkenes 3a-c with good selectivities (71 to 87%) and yields. By comparison, 1-ethynyl-3-nitrobenzene 2d required 62 h at 40°C to afford chemoselectively alkene 3d in high yield. Regarding internal alkynes, diphenylacetylene 2e1 required a 40 °C heat for 13 h to be fully consumed in toluene (Tables S2,S3), but afforded (Z)-stilbene 3e1 with a high selectivity of 91% and good isolated yield (78%). Noteworthy, the semi-hydrogenation of 2e1 proceeded also at the gram scale (5.6 mmol) in 80% yield within 30 h at 40°C in ethanol [3e1/4e1/5e1 88/2/10]. By comparison, chloro-or methoxy-substituted derivatives 2e2 and 2e3 needed 80°C for 13 or 48 h in toluene to lead selectively to the related (Z)-stilbenes 3e2 and 3e3 in good yields. In contrast, the hydrogenation of ethyl 3-phenylpropiolate 2g in ethanol was almost quantitative at room temperature for 13 h leading to (Z)-alkene 3g in a good selectivity (78%) with some alkane 5g (22%). Phenylpropyne 2f, 3-phenylprop-2-yn-1-ol 2h and 3-phenylpropiolonitrile 2i were also quantitatively hydrogenated in ethanol at 25 °C to afford the related (Z)alkenes 3f,h,i in good isolated yields (81-88 %) and selectivities (84-93 %) with only minor amounts of the (E)-isomers 4f and 4i (2 %) and alkanes 5f,h,i (5-14 %). On the whole, by comparison to "P-2 Ni" catalyst, our Ni catalyst was somehow less (Z)selective and universal but more chemoselective as aryl chloride moiety of 2e2 the nitro and nitrile functions of 2d and 2i were preserved. 16 In addition, it displayed a modular reactivity for semi-and full hydrogenations (see Table S3) and, to the best of our knowledge, allowed for the first time the effective and selective semi-hydrogenation of various ynamides without any additive. 15 Indeed, Ms, Ts, Boc and Moc protected derivatives 2j-m were hydrogenated to the desired (Z)-enamides in high selectivities (85-92 %) and good isolated yields (72-80 %) within 48 to 62 h in ethanol at 60 °C. Under similar conditions, the hydrogenation of the ynamide 2n bearing isoindolinone and phenylethyl moieties was completed in only 13 h with a good isolated yield (72 %) and a high selectivity in 3n (89 %) and only minor amounts of 4n and 5n.

With the tosyl substituted ynamides 2o and 2p, the phenyl derivative 2o required more time to be reduced than the methyl derivative 2p and afforded the (Z)-enamide 3o with a higher selectivity of 94 % (vs. 74 % for 3p), isolated yields remaining good (84 % for 3o and 62 % for 3p). The ynamide 2q substituted by an indole fragment afforded almost exclusively 3q after 13 h reaction at 60 °C, with a fair isolated yield of 61 %. Finally, ynamide 2r led selectively to 3r', the ester moiety being changed from methyl to ethyl due to concomitant transesterification with the solvent.

To gain some insights into the nature of the probable Ni(0) active species, a sample solution was first analysed by DLS and particles with an average size of 110 nm ±21 were observed (Figure S3). Evaporation to dryness of a similar sample afforded a residue which was washed with dry pentane under argon and dried under vacuum. Noteworthy, the remaining solid, i.e.: our catalyst, proved still active although with a lower activity than an in situ-generated catalyst (Table S3). X-ray diffraction (XRD) in the reflexion mode was performed on the isolated catalyst placed on a glass support in open air but no diffraction patterns were observed, most likely due to the poor crystallinity of the sample (Figure S4). Therefore, no Ni(111) plan (i.e. Ni(0) species), nor NiO one could be observed. 17a As XRD is mainly a bulk technique, we then analysed our isolated catalyst by XPS in order to determine the chemical composition at the topmost surface (i.e.: 9-2 nm) of the particles. The survey scan and highresolution spectra of a freshly prepared sample revealed the core levels of C1s, Mg1s, O1s, N1s, Ni2p and Br3d elements (Figure S5). More specifically, the peaks values for Ni2p3/2 and 2p3/2 satellite of the Ni2p region were found at 854.8 and ca. 861.9 eV, respectively (Figure S6). These values are consistent with the presence of NiO, which likely results from the oxidation of the Ni(0) species during the transfer of the sample in air. 17 Furthermore, XPS provided the % surface ratio of all the elements and confirmed the presence of C, O, N, Si, Br and Ni elements in the catalyst sample (Table S6). The observed Ni/N ratio of ca. 1:3 suggested the presence of NHC-cinnamylcoordinated Ni particles. Noteworthy, Raman spectroscopy, carried out on a catalyst sample whose exposure to air was avoided, showed no evidence of Ni oxides (150-430 cm -1 ) and Ni hydroxides (3100-3650 cm -1 ) (Figure S7). The catalyst was then characterised by SEM, TEM and HR-TEM microscopies. At low magnification, piles with stacked layers of various sizes and thicknesses were observed (Figures 2a-d, S8 and S9). According to SEM-EDX chemical analysis, they could be attributed to the Ni-based material, as the observed piles were composed of C, N, O, Ni, Br and Mg elements, at least on their surface (Figure S8). At higher magnification, some domains appeared to be partially crystalline but no diffraction patterns were observed (Figures 3c,S9). The most visible NPs corresponded to a lattice spacing of 0.65 nm which is rather different from the uniform lattice spacing of 0.203 nm of the pure Ni(111) plane. 17a Other structures appeared to be finer but, due to the presence of organic matter, the contrast was too weak to allow other measurements. The dispersion of the elements was then studied by STEM EDS elemental mapping without quantification and sizing (Figure 2e-g, S10). A homogeneous dispersion of Ni, N, C and other observed elements was hereby confirmed. Finally, to get a reliable estimation of the number of NHCcinnamyl ligands that remain bound to the Ni, two samples were then analysed by ICP-AES and CHN elemental analyses. Mass percentages of 7.10±0.1 (Ni) and 3.51±0.2 (N) were measured, which corresponds to a Ni:N molar ratio of 1:2, thus to a Ni:NHC ratio of 1:1 as in the precursor complex 1. These results strongly suggest the coordination of NHC ligands and their appended -accepting cinnamyl moiety to the Ni(0) NPs. The reusability of the catalyst was next studied for the hydrogenation of alkynes 2b and 2f over 6 runs (Tables S4,S5). Though the terminal alkene 3b was recovered in good yields and selectivities until the 5 th run, a gradual deactivation of the catalyst over long reaction times resulted in a decrease of yields in internal alkene 3f while keeping a high selectivity. SEM and TEM analyses of the spent catalyst revealed features which were similar to the fresh catalyst but without crystalline domains and with lower Ni and N contents (Figures S11-12).

In summary, we report here the half-sandwich Ni(II)-NHC complex 1 bearing an appended cinnamyl moiety can readily be reduced by MeMgBr to lead to a first example of NHC-olefincoordinated Ni(0) nanoparticles. The resulting catalyst allowed the effective and (Z)-selective semi-hydrogenation of alkynes and, most impressively, of ynamides without any additive. Further developments on such NHC-olefin-Ni nanocatalysts are currently under study in our laboratories and will be reported in the future.

  Marta G. Avello, a Stéphane Golling, a Lai Truong-Phuoc, b Loïc Vidal, c Thierry Romero, b Vasiliki Papaefthimiou, b Nathalie Gruber, d Michael J. Chetcuti, a Frédéric R. Leroux, a Morgan Donnard, a Vincent Ritleng,* a Cuong Pham-Huu,* b and Christophe Michon* a a) Université de Strasbourg, Université de Haute-Alsace, Ecole européenne de Chimie, Polymères et Matériaux, CNRS, LIMA, UMR 7042, 25 rue Becquerel, 67087 Strasbourg, France. E-mail: cmichon@unistra.fr b) Université de Strasbourg, Ecole européenne de Chimie, Polymères et Matériaux, CNRS, ICPEES, UMR 7515, 25 rue Becquerel, 67087 Strasbourg, France. E-mail: cuong.pham-huu@unistra.fr c) Université de Haute-Alsace, CNRS, IS2M UMR UMR 7361, 15, rue Jean Starcky -BP 2488, 68057 Mulhouse, France d) Université de Strasbourg, Fédération de chimie Le Bel -FR2010, BP 296R8 1, rue Blaise Pascal, 67008 Strasbourg, France †This paper is dedicated to Dr Francine Agbossou-Niedercorn at the occasion of her retirement. ‡Electronic Supplementary Information (ESI) available: synthetic procedures, characterization data, CIF of 1, CCDC 2208403. See on the web. Nickel(0) nanoparticles coordinated to NHC ligands bearing a Ncoordinated cynnamyl moiety were readily prepared by reduction of a [NiCpBr(NHC-cynnamyl)] complex with methyl magnesium bromide. The combination of a strong -donor NHC ligand with a coordinating appended cinnamyl moiety likely prevents nickel(0) nanoparticles aggregation to larger inactive species, and allows the effective and (Z)-selective semi-hydrogenation of alkynes and ynamides.

Fig. 1 .

 1 Fig. 1. (Z)-selective semi-hydrogenation of alkyne derivatives catalysed by NHC-olefin-Ni(0) nanoparticles

Fig. 2

 2 Fig. 2 TEM analyses of reduced complex 1 used as catalyst. (a), (b), (c) and (d) TEM and HR-TEM images; (e), (f) and (g) elemental mapping for Ni, N and C by STEM-EDS

Table . 1

 . Catalysis scope -Yields and selectivities by GC and 1 H NMR for alkenes 3a-i and by 1 H NMR for enamides 3j-r. Isolated yields in parenthesis (average of at least 2 runs on a 0.5 mmol scale).

	Substrate 2	Solvent	T (°C)	t (h)	Yield (%)	Ratio 3/4/5
		toluene	25	13	100	71/0/29
		toluene	25	13	89	87/0/13
		toluene	25	13	100	75/0/25
		toluene	40	62	95	85(63)/0/15
		toluene	40	13	100	91/(78)/0/9
		toluene	80	13	100	90(75)/0/10
		toluene	80	48	87	100(69)/0/0
		ethanol	25	3	100	84/2/14
		ethanol	25	13	95	78(63)/0/22
		ethanol	25	4.5	100	88(81)/0/12
		ethanol	25	13	100	93(88)/2/5
		ethanol	60	48	100	88(74)/12/0
		ethanol	60	62	100	92(80)/6/2
		ethanol	60	62	100	92(72)/0/8
		ethanol	60	48	100	85(79)/15/0
		ethanol	60	13	100	89(72)/4/7
		ethanol	60	62	100	94(84)/6/0
		ethanol	60	36	100	74(62)/13/1 3
		ethanol	60	13	70	97(61)/0/3
						94(72)/6/0
		ethanol	70	13	81	(ethylester
						3r' and 4r')

Acknowledgements

This work has benefitted from the support provided by the University of Strasbourg Institute for Advanced Study (USIAS) for Fellowships, within the French national program "Investment for the future" (IdEx-Unistra) (C.M., C.P.-H. as well as M.G.A. for postdoc). CNRS is warmly acknowledged for his support. The authors thank Dr. E. Wasielewski and Mr. M. Chessé responsible for the research platforms and facilities (Strasbourg NMR plateform and Strasbourg chromatography facilities) of LIMA (UMR7042 CNRS-Unistra-UHA) who contributed, by their valuable technical and scientific support, to the achievement of this research project

Conflicts of interest

There are no conflicts to declare.


Preferential flow in soils : the Mobile-Immobile Model

 Orthic luvisol (FAO, 1998) Materials and Methods Under these assumptions:

 θC = θ m C m + θ im C im  ln(1-C/C 0 ) = -αt/θ im + ln(θ im /θ)

  the agricultural region of Beauce (France): contamination with several pesticides (C > 0.1 μg.L -1 the existence of preferential transport of the MIM type in a soil profile 2-Estimate the (hydraulic conductivity) parameters of the MIM model: the immobile water fraction θ im /θ and the mass exchange coefficient α of various materials from surface down to 1m-depth soil layers

  cm -3 ) , bulk density ρ b (g.cm -3 ) and the hydraulic conductivity, K(-0.1 kPa) (mm.h -1 ) -HPLC analysis -Determination of immobile water content θ im /θ, and the mass exchange coefficient α Assumptions of the model (Jaynes et al., 1995) -Initial tracers concentrations = 0 -C m =C 0 (no effect of the hydrodynamic dispersion in the mobile domain in the sampling volume )



  MIM type of preferential transport is a characteristic of the Beauce soilsHigh variability of θ im /θ the MIM type seems to be a characteristics of the Beauce soils in surface and subsurface Soil structure plays an important role on the hydraulic conductivity but also on the occurrence of MIM-type preferential flow Management of soil structure, through tillage practices, appears to be a possible strategy for acting on water and solutes transfer in soilConclusionThank you.

1/ 4 9 0-27 cm A p ploughed organo- mineral horizon 27-60 cm B t clay- enriched illuvial horizon 60-88 cm B heavy (red) clay horizon 88+ cm C weathered limestone

	Layer Depth	pH	Clay Silt Sand	OC CaCO 3
					g.kg -1
	A p	3	5.93	223	701	75	11.1	1.34
	B t	35	6.55	352	607	41	6.0	<1
	B	65	7.70	565	306	124	5.2	4.66
	C	92	8.44	339	57	16	3.5	564


Determine the IPU degradation properties of various materials sampled from surface (plough layer) and subsurface soil materials down to 1 m depth in a heterogeneous agricultural field of the Beauce region Evaluate the effects of water content and temperature on the kinetics of IPU degradation and on the proportions of the various molecular forms produced through IPU degradation 1 2 Context  The tertiary aquifer of the Beauce region (one of the largest groundwater resources in France) is polluted by several pesticides  The phenylurea herbicide isoproturon, 3-(4-isopropylphenyl)1-1-dimethylurea (IPU), used to control pre-and post-emergence of grass weeds and broad-leaved weeds in cereal crops is among the major pollutants.  The fate and behaviour of this herbicide is determined by the sorption and degradation capacities of the soil materials.

Materials and Methods

(g kg -1 ) (g kg -1 ) (g kg -1 ) (g g -1 ) (g kg -1 ) (g kg

Summary

The phenylurea herbicide isoproturon, 3-(4-isopropylphenyl)1-1-dimethylurea (IPU), is occasionally found in ground, drainage and surface waters. Degradation in soils is a key process for assessing the risk of water resources contamination by IPU. The degradation properties of various samples from surface and subsurface soil of a heterogeneous field in the Beauce agricultural region of France were studied using 14 C-IPU. Laboratory incubations were carried out at 22 and 10°C and at water contents representing 90 and 50% of the soil water holding capacity (WHC). Degradation was found to be more sensitive to water content variations than to temperature variations in the ranges that we used. For surface layers, at 10 and 22°C, the degradation half-life increased by a factor 10 and 15, respectively, when water content decreased from 90 to 50% WHC. Under optimal degradation conditions, monodemethyl-isoproturon was the main metabolite in surface samples. In subsurface samples, IPU half-lives were larger than 100 d, IPU was the main identified compound after 92 d of incubation and the main metabolite was an unidentified polar metabolite. These results suggest an alternative metabolic pathway probably involving hydroxylations in subsurface materials. Clay content seemed to play a major role in IPU degradation in both surface and subsurface horizons by decreasing IPU availability in soil solution and/or by limiting water availability for microorganisms.

Pesticide Behaviour in

N C H 3 H 3 C O N C H 3 H 3 C H N C H 3 H 3 C O N H C H 3 H N C H 3 H 3 C O N H 2 N H 2 H 3 C H 3 C 0.

-a

  Field site of 25 ha in the Beauce Region (France) -Main soil types: Orthic Luvisol, Eutric Cambisol and Calcic Cambisol -Soil samples from surface layers (LA1, LA2 and LA3) and subsurface layers (Water holding capacity estimated from Bruand et al. (2003) b Soil water matric potential, Ψ m , at eWHC, expressed in pF=log 10 |Ψ m |, where Ψ m is in hPa

  Characteristics of the surface (LA1, LA2, LA3) and subsurface soil depths (Bt1, B3, C1, C2, C3) of the three profiles (a, b and c) situated within the field site in the Beauce region References. Alletto, L., Coquet, Y., Benoit, P., Bergheaud, V., 2006. Effects of temperature and water content on degradation of isoproturon in three soil profiles. Chemosphere, in press. Bruand, A., Pérez Fernández, P., Duval, O., 2003. Use of class pedotransfer functions based on texture and bulk density of clods to generate water retention curves. Soil Use Manage. 19, 232-242. Sørensen, S., Bending, G., Jacobsen, G., Walker, A., Aamand, J., 2003. Microbial degradation of isoproturon and related phenylurea herbicides in and below agricultural fields. FEMS Microbiol. Ecol. 45, 1-11. Walker, A., Turner, I.J., Cullington, J.E., Welch, S.J., 1999. Aspects of the adsorption and degradation of isoproturon in a heavy clay soil. Soil Use Manage. 15, 9-13.Temperature and water content effects on the degradation of isoproturon in surface and subsurface soils Lionel Alletto a,b , Yves Coquet b , Pierre Benoit b , Valérie Bergheaud b a Purpan, Ecole Supérieure d'Agriculture, Agronomy Department -75, voie du TOEC -31 076 Toulouse cedex 3, France b UMR INRA/INAPG Environment and Arable Crops -BP 01 -78 850 Thiverval-Grignon, France

Fig 1 .

 1 Fig 1. Description of the three soil profiles situated within the agricultural field of the Beauce region. LA horizons correspond to ploughed organo-mineral layers, Bt horizons to clay-enriched illuvial horizon, B to a heavy red clay horizon and C to weathered limestone horizons

H

-

  In surface samples, T 1/2 ranged from 11 to 223 days depending on incubation conditions (Table 2) -In subsurface samples, T 1/2 were larger than 100 days (Walker et al., 1999) -Degradation was more sensitive to water content variations than to temperature variations Surface samples LA1: T 1/2 increased by a factor 15 at 22°C and 10 at 10°C -Clay content seemed to limit IPU availability and/or water availability to microorganisms Subsurface samples Bt1 and C1 (similar water potential, near pF 5.1, at 50% eWHC): T 1/2 increased with clay content  lower IPU availability due to sorption on clay particles -Mineralization decreased with depth and organic carbon content  Maximum of 16.5 % of applied 14 C after 92 d at 22 °C and 90 % eWHC in surface samples (Fig. 2a)  Minimum of 0.02 % of applied 14 C after 92 d at 10 °C and 50 % eWHC in subsurface samples -Mineralization was strongly affected by water availability in subsurface samples -Sensitivity of bound residues formation to soil water content -Correlations of non-extractable residues proportions with organic carbon content and mineralization Microbial activity is probably involved in the formation of nonextractable residues -Strong effect of the water content on the extractability of 14 C (Fig. 2a,b) -In surface samples, MDIPU was the main metabolite under optimal incubation conditions (i.e. 22 °C, 90 % eWHC) -In subsurface samples, IPU was the major molecular form after 92 d and the main metabolite was an unidentified polar metabolite probably a hydroxylated metabolite (Sørensen et al., 2003)  In subsurface layers, degradation of IPU was low and did not lead to the "usual" metabolites (MDIPU, DDIPU or 4IA)

Table 2 .

 2 Degradation half-lives (T 1/2 ) under the different incubation conditions tested

Mineralization Non-extractable residues Extractable fraction Degradation half-life

  Fig.2. Distribution kinetics of applied 14 C between bound residues, methanol extract, water extract and CO 2 in surface samples (LA1) and subsurface samples (Bt1, C1) and distribution kinetics of isoproturon and main metabolites in the pooled water and methanol extracts

	This

work was financially supported by the Ministry of Ecology and Sustainable Development of France


come

Variations of immobile water content and first-order mass exchange coefficient in three soil profiles

Yves Coquet, Lionel Alletto, Pierre Vachier


domain: shs shs

text:
Ayawo Roger Ekon

Mickael Petit
email: mickael.petit@lecnam.net

François Costa
email: francois.costa@ens-paris-saclay.fr

François Bouvet
email: francois.bouvet@synchrotron-soleil.fr

Eric Dupuy
email: eric.dupuy@synchrotron-soleil.fr

Impact of routing on the EMC behavior of a GaN HEMT-based full bridge DC-DC converter

Keywords: Power electronics, Routing, EMC, GaN components

In the perspective of upgrading the SOLEIL synchrotron accelerators, new electromagnet power supplies utilized for the control of the electron beam trajectory are being studied. For those implemented in the fastest beam orbit feedback loop, a new generation of low voltage, high frequency DC-DC converters based on Gallium Nitride (GaN) High Electron Mobility Transistors (HEMTs) is envisaged. One of the crucial points of their design concerns the routing of the switching cells, which is critical with respect to Electromagnetic Compatibility (EMC) and surge voltages, thus requiring a preliminary investigation. For this purpose, a routing model using the Ansys Q3D software is studied in this paper. The data from the Q3D simulation are compared to the experimental results for the prediction of the EMC performance of the DC-DC full-bridge converter. The EMC model resulting from this study allows analyzing the influence of the inter-cell parasitic inductances on the levels of the disturbances conducted in common and differential mode.

I. INTRODUCTION

The magnet power supplies foreseen for the new storage ring, currently under study, of the SOLEIL synchrotron light source require greater compactness, very high reliability, and minimization of the residual ripple of the output current. These increased constraints are contradictory [START_REF] Li | Comparison of four control methods to active power filters applied in accelerator power supplies[END_REF], especially for the power supplies involved in the fast orbit feedback system [START_REF] Hubert | Global orbit feedback systems down to dc using fast and slow correctors[END_REF], as they also request higher dynamics. Meeting these requirements means a technological and structural rethinking of the design of existing power supplies. In particular, an increase in switching frequency is necessary to higher the dynamic range and reduce the current ripple. The use of wide band gap components based on GaN allows meeting these constraints and enables enhancing the converter power density. Indeed, with increasingly steep switching edges [START_REF] Xiaoshan | Methodology for EMC Analysis in a GaN Based Power Module[END_REF], these components allow their use at high frequencies (0.1-1 MHz range), and thus an improvement in power density. Despite their switching and conduction performances, the EMC constraints of systems integrating these components are becoming extremely severe. Consequently, EMC must be taken into account during the design of the converter from the routing phase where certain rules must be respected without degrading the thermal performance [START_REF] Pace | Electrothermal Modeling of GaN Power Transistor for High Frequency Power Converter Design[END_REF]. A compromise between EMC and thermal management must be found. Achieving the EMC objectives essentially implies a reduction of the switching loops area. On the other hand, in order to better dissipate the losses, the distance between the active components is essential to limit the thermal coupling and spread the heat flows.

Achieving the first objective means mastering the various parasitic effects, and in particular, the parasitic inductances of the switching cells. In addition to this, the capacitive couplings between different variable potentials must also be controlled. A predictive study to minimize disturbances at the source, facilitating in a second time the sizing of the EMC filters, is thus essential from the design phase. The study of the impact of the switching cell area on the EMC performance of a Buck chopper has been proposed by [START_REF] Pascal | Study of the Impedance of the Bypassing Network of a Switching Cell -Influence of the Positioning of the Decoupling Capacitors[END_REF] [START_REF] Song | A Fast Calculation Model for Parasitic Inductance of SiC Power Devices[END_REF]. On the other hand, a study carried out by [START_REF] Liu | EMI investigation in a GaN HEMT power module[END_REF] shows the effect of an unbalanced capacitive coupling between the DC bus potentials and the ground on the EMC performance of a half bridge converter. However, these studies do not correspond to the topology chosen for the new magnet power supplies of the future SOLEIL fast orbit system, since it consists of a double switching cell. This makes the routing more complex than those presented in the literature do: the number of parasitic elements is clearly increased. Thus, this paper discusses the characterization of inductive and capacitive parasitic effects in a full bridge converter based on numerical simulation using Ansys Q3D software [8] [9] A comparison is made between the simulation data and measurements performed on a model. Intercellular parasitic inductances depending on the distance between the switching cells partially govern the EMC behavior of the converter, which is studied in the following.

II. EMC MODEL AND ROUTING RULES

The studied converter is a four-quadrant DC-DC converter (H-bridge) based on 40 V / 29 A GaN components (ECP2055) Fig. 1-a. The capacitance of the input capacitor of the converter is Cxe = 47 µF. The DC bus voltage is Vdc = 24 V and the switching frequency is fsw = 400 kHz. The converter is designed for the power supply needs of the future SOLEIL fast orbit feedback system. In order to guarantee the symmetry of the converter, each switching loop has the same topology (length and loop area). For this reason, the capacitor Cxe has been distributed in two parts as shown in Fig. 1-b: -a part composed of two capacitors (Cb1 & Cb2) which play the role of decoupling capacitors close to the switching cells, -the other part is composed of the capacitor Cxe' realized by n capacitors of capacity (Cxe'/n) parallelized. The value of the equivalent parasitic inductance (ESL) is then minimized. In order to minimize the surface area and the total inductance of the switching loops, MLCC-SMD capacitors (Multilayer Ceramic Capacitor-Surface Mounted Device) are used: on the one hand, their equivalent series inductances (ESL) are lower and on the other hand, their reduced connections allow limiting the surface area of the switching loops. Scheme of a complete bridge chopper with its decoupling capacitor

A. Equivalent model

The equivalent EMC model of the converter is shown in Fig. 2. The internal parasitic inductances of the transistor packages, the different parasitic inductances on the control side as well as the different inductive couplings are not represented.

Concerning the power circuit, two switching loops can be identified:

-loop 1: composed of [Q1, Lb1+, Rs_Cb1, Cb1, Ls_Cb1, Lb1-, Q3 and Lp1]. -loop 2: composed of [Q2, Lb2+, Rs_Cb2, Cb2, Ls_Cb2, Lb2-, Q4 and Lp2].
This representation localizes the inductive effects that are actually distributed along the loop.

Based on model, the constraints that will apply to optimize the routing in order to limit overvoltages and minimize the propagation of parasitic currents at the source are explained as follows:

-the equivalent parasitic inductances in each switching loop are at the origin of overvoltages across the transistors when they are switched-off. Indeed, during the corresponding transient, the energy stored in these inductances is exchanged with the capacitance Coss of the transistor via a resonance mechanism. The equivalent parasitic impedance of each of these loops must be minimized. The values of ESL and ESR of the capacitors and transistors being fixed by their technology and size, the only elements we can minimize are those due to the routing (Lbi+, Lbi-and Lpi).

-the intercellular inductances (LM+ and LM-) do not directly contribute to the impedance of the switching loops. However, they do contribute to the degradation of EMC performance and to overvoltage at the transistor terminals. On the other hand, they must be minimized and balanced by routing and placement. Their effects are studied and further investigated in section IV. Diagram of the EMC equivalent model on the power side -the coupling capacitances between the DC bus potentials and ground (Cy+ and Cy-): they contribute to common mode filtering. They must be maximized and balanced also by the routing. This is also the case for Cx. Contributing to the differential mode filtering, the latter must be maximized. In [START_REF] Liu | EMI investigation in a GaN HEMT power module[END_REF] the effect of the imbalance between Cy+ and Cycapacitances in a Buck chopper has been studied.

-the coupling capacitances between the medium potentials of the switching cells and ground (Cp1 and Cp2): these floating potentials are the cause of common mode disturbances in the frequency range from 150 kHz to a few MHz [START_REF] Xiaoshan | Methodology for EMC Analysis in a GaN Based Power Module[END_REF]. These couplings must also be minimized by routing.

In general, inductive power/control power/power and control/control couplings are to be optimized by routing and placement.

B. Routing

Since there are many possible placements depending on the PCB technology, the one proposed here is suitable for a simple two-layer PCB. Thus, all components are surface mounted and placed on one side according to the schematic layout shown in (Fig. 1 b). Constrained by the small footprint of the active components, the thickness of copper that can be used is 35µm. In order to optimize the parasitic effects some routing rules have been respected:

-placing the DC bus potentials and the ground plane on adjacent layers to maximize the Cy-and Cy+ capacities.

-bringing the elements of a switching loop (switching cell and Cbi) as close as possible in order to minimize the equivalent impedance of the loop.

-reducing as much as possible the coupling surface between the ground plane and the planes representing the potentials at the midpoint of the cells.

-reserving an equivalent coupling surface between the midpoint potentials and the ground plane in order to guarantee the symmetry of the Cpi capacitances. Equation [START_REF] Li | Comparison of four control methods to active power filters applied in accelerator power supplies[END_REF] gives the coupling capacitance between two planes: [START_REF] Li | Comparison of four control methods to active power filters applied in accelerator power supplies[END_REF] where is the permittivity of vacuum ( 10) and is the relative permittivity of the dielectric (in this case FR4 ( 4)). is the distance between the two plates, in this case the thickness of the dielectric (i.e.1.5 mm on our PCB). S is the coupling area between the potentials.

Finally, the circuit was routed to ensure some symmetry between the two switching cells.

The top side of the routing is shown in Fig. 3 where Cxe' is reduced to a single capacitor. In order to improve the current density, the midpoint tracks (P1, P2) of the top layer are connected by vias to much larger planes on the bottom face. The capacitor Cxe' is placed at DX = 13 mm from Cb1. The distance contributing to determine the value of Lm separating the two arms is DM = 11 mm.

A. Instrumentation

The KEYSIGHT E4990A impedance analyzer is used. It is calibrated by Open/Short/Load standards and configured for a 'measurement time = 3', corresponding to the precision and speed of acquisition. The frequency sweep is performed between 100 kHz and 120 MHz. It is associated with KEYSIGHT 42941A probe. A SMA connector compensation kit is used for these measurements. Fig. 4 shows the test card. Photograph of the measuring bench B. Methodology for using Q3D extractor Q3D is a simulation tool produced by ANSYS allowing the extraction of parasitic resistances, inductances, capacitances, conductances (RLCG) of a routed circuit. It is equipped with a 3D quasi-static electromagnetic field solver based on the method of moments [START_REF]PCB design software Ansys Q3D Extractor[END_REF]. The elements extraction is done by a frequency sweep. Fig. 5 shows the variation of the inductance as a function of the frequency. The difference between the asymptotic AC and DC values is a consequence of the skin and proximity effect. It is the asymptotic value in the AC region that is taken into account for offsetting up the model since the frequency equivalent to the rise times (tr) of the transistors is high (freq~0.35/tr). This corresponds to about 70 MHz for a rise time of 5 ns for example.

In Q3D, the active and passive components are not simulated. Depending on the extraction, open circuits or short circuits represent them. The impedances to be extracted are on the other hand the partial track impedances between the pads represented under the software. The direction of current flow is necessary to determine which pad is source or sink. This method neglects any inductive coupling of the tracks with the passive components actually implemented on the PCB.

C. Experimental measurement vs. simulation

In a first step, we will compare the predictions from Q3D of a test card with measurements. For this purpose, the diagram in Fig. 6 representing the wiring of the converter is proposed. In this wiring, the transistors are disconnected and their location left in open circuit. The capacitor Cxe' is replaced by an SMA connector from which the measurement will be made. The Cbi capacitors are SMD capacitors 1.5 µF / 50V, X7R class insulation ref (CGA4J3X7R1H155K125AB). This capacitor has been previously characterized and integrated in the equivalent model.

The following expressions can be derived:

(2) (3)
The two resonance frequencies (fr1, fr2) and the antiresonance frequency (fa) resulting from the impedance curve are calculated as follows: The previous results allowed to plot the impedance curve of the diagram in Fig. 6 seen from the SMA connector and presented in Fig. 8. The curves of the model are not damped since the value of the resistances are not taken into account. Measured impedance (red), model (blue): variation of L1 to +/-20% at the turn of the asymptotic trend (black dotted line).

fr1 (2 * ( )) (4) fr2 (2 * ) (5) fa (2 * ( )/( * )) (5) # _%&' , # _%&) (6) # _%&' # _%&) (7)
The maximum relative error of the simulation compared to the measurement is less than 4%. This maximum relative error as well as the similarity of the measured and calculated impedance curves in Fig. 8 show that it is possible to have a satisfactory confidence in the results from the Q3D simulation of the parasitic components of a routing; the simulation accuracy is satisfactory. We will rely on these results from now on. These global measurements being validated, the different partial elements calculated are presented in TABLE II. Although DM < DX, LM+/-> LX+/-. This disparity is due to the fact that the two tracks Lx+/Lx-are much closer together than the tracks LM+/LM-. This closeness allows a reduction of the resulting inductances under the effect of the mutual. As discussed in Section 2, this analysis shows the importance of routing for noise optimization.

The equivalent inductance of each switching loop is about 2.2 nH. To this value, the parasitic packaging inductances of the transistors and capacitors will be added.

IV. EMC ANALYSIS

Upstream of the model presented in Fig. 2, a Line Impedance Stabilization Network (LISN) is added Fig. 9. In this case, it essentially allows fixing a propagation path to the conducted perturbations. It is simplified in order to measure directly the common mode (Icm) and differential mode (Idm) currents. In the following, Cxe' is a 47 µF / 50 V capacitor. The simulation includes numerical models of the transistors provided by the manufacturer (Ref EPC2055). The transistors are controlled on switching on/off by 2 Ω / 5 Ω gate resistors. The power circuit feeds an inductive load of 100 µH/ 1 Ω and is simulated under LTspcice. The average current in the load is 12 A.

A. Effect of LM+/-intercells inductances

LM+/-inductances are not part of the switching loop. In this section, their effect is studied. For this purpose, the other parasitic elements are kept constant according to the results in TABLE II. Only the value of the intercellular inductances LM+ and LM-is a parameter. This is a consequence of possible variations of the DM distance in Fig. 3. Fig. 10-a andb show the spectra of the common and differential mode currents as a function of LM values, for a variation of LM+/-from 0 nH to 10 nH (LM-= LM+) i.e. a variation of DM from 0 mm to about 43 mm.

An increase in the level of the disturbance spectra is recorded in common mode. This increase exists in the frequency band above MHz. For the chosen operating point, the FM resonance frequency remains constant (~130 MHz) as a function of LM+/-but its amplitude varies in the same direction as LM+/-(46dB to 62 dB). Indeed, this peak in the frequency spectrum corresponds to the resonance of the voltage across the transistors when they are switched-off. In differential mode, increasing LM+/-decreases the spectrum by a few MHz at the resonance frequency, which is similar to a contribution to differential mode filtering. This high amplitude, high frequency CM resonance could lead to severe radiated emissions that would require specific reduction methods.

For unbalanced LM+/-values, (LM+ ≠ LM-), Fig. 11 a andb allow us to observe the influence of this unbalance especially on the common mode. Thus, we can observe on this spectrum, an increase in the amplitude of harmonics in the frequency band between 10 and 90 MHz of the order of 5 dB for a deviation of 2 nH. However, in the differential mode, a slight difference between the spectra can be seen, even if it is not as obvious as in the common mode.

B. Discussion of the influence of LM+/-

The previous study is performed for a single operating point and for the specified load. The results presented above actually show the importance that must be given to the LM+/inductances. However, subject to experimental validation, all parasitic inductances inherent to the full bridge must be taken into account in the same way as the switching loop impedances during design.

Reducing these inductances means bringing the switching cells closer together by acting on the DM distance. Even if this work does not address the thermal aspect, a compromise must be found between EMC and thermal during the final routing. This will force to find an optimal value of DM in order to reach these two objectives.

V. CONCLUSION

We undertook this study in order to identify the main sources of disturbances and their effects from the design phase of the future power supplies of the SOLEIL Synchrotron and thus better understand their EMC constraints.

They are constituted by a full bridge DC-DC converter and based on the use of GaN components. The use of Q3D for the estimation of the parasitic by simulation has given results consistent with the experimental measurements. These measurements allowed building a partial EMC model of the power structure. Finally, a study on the effect of the intercellular parasitic inductances (specific to this structure because they are located between the switching cells) on the EMC behavior has been performed. This study shows that in order to improve the EMC behavior, the reduction of the inter-cell parasitic impedances is as important as the reduction of the switching loops areas. This reduction requiring the approach of the switching cells must not obviously degrade the thermal aspects. The developed model does not take into account the parasitic resistances of the routing; a better damping of the resonance peaks can be expected. The experimental validation of the effect of LM+/-and the damping of the impedance curves will be the subject of future work.

  Fig. 1.Scheme of a complete bridge chopper with its decoupling capacitor

Fig. 2 .

 2 Fig. 2. Diagram of the EMC equivalent model on the power side

Fig. 3 .

 3 Fig. 3. Full Bridge converter top side routingIII. SIMULATION AND EXPERIMENTAL VALIDATIONThis section describes the simulation tool used and the methodology of the experimental measurements. A comparison is made between the experimental results and those obtained by simulation.

Fig. 4 .

 4 Fig. 4. Photograph of the measuring bench

Fig. 5 .

 5 Fig. 5. Inductance of a conductor as a function of frequency [10]

Fig. 6 .

 6 Fig. 6. Equivalent diagram for the measurement of the parasitic inductances of the power tracks It is intuitively assumed that + and # _%&' # _%&) . The ESLs of the capacitors being known by their characterization, it is incumbent to measure L1 and L2. Fig. 7-a and b compare the L1 and (L1 + L2) inductances obtained by the experimental measurement and the Q3D simulation. To measure L1, Cb1 is shorted and Cb2 is disconnected from the schematic in Fig. 5. The reverse operation is applied to measure (L1+L2).

  Fig.7L1 inductance measurement (red) compared to the simulation (blue) and to the asymptotic value (black dotted line): L1 (a) and L1+L2 (b)

Fig. 8 .

 8 Fig.8.Measured impedance (red), model (blue): variation of L1 to +/-20% at the turn of the asymptotic trend (black dotted line).

Fig. 9 Complete

 9 Fig. 9 Complete power model with an upstream LISN (Ro= 27 Ω, R1 = 39 Ω, Co = 220 nF and Lo = 250µH)

Fig. 10 .

 10 Fig. 10. Common (a) and differential (b) mode current as a function of LM+/-

TABLE I

 I

	compares the asymptotic values in small signals
	of L1 and L1 + L2 obtained in simulation and experimentally.

TABLE I

 I

	: COMPARISON SIMULATION/MEASUREMENT OF ASYMPTOTIC
		VALUES IN SMALL SIGNALS
		L1(nH)	L1+L2(nH)
	Q3D	3.5	10.5
	Measure	3.6	10.9

TABLE II :

 II VALUES OF THE PARASITIC ELEMENTS OF THE EMC MODEL

				Partial inductances (nH)
	Lx+	Lx-	Lb1+	Lb1-	LM+	LM-	Lb2+	Lb2-	Lp1	Lbp2
	1.7	1.8	0.5	0.5	2.5	2.4	0.5	0.5	1.1	1.2
				Parasitic capacities (pF)
	Cy+		Cy-		Cp1		Cp2		Cx
	26		30		7.3		7.2		4.3


L Alletto

Y Coquet

C Labat

Introduction

Context of the study

The cropping systems Objectives of the study

Materials and methods

The experimental site: localisation and soil characteristics Isoxaflutole (IFT) properties Sampling procedure: soil Sampling procedure: water

Results and discussion

General data

Persistence of isoxaflutole

Water and herbicide leaching

Conclusion

National context: generalization of water resources pollution by pesticides… Economic context: Midi-Pyrénées region = 2 nd region for maize production in France Agronomic context: Typical maize production management in the region:

-80 % of the production is in continuous maize with more than 60 % irrigated.

-Tillage usually included a mouldboard ploughing ( 30

Main features of the Boulbènes soil types:

-  Degradation: formation of the diketonitrile (DKN) with a higher solubility (300 mg L -1 ), a lower retention (K OC : 92 L kg -1 ) and a higher persistence (DT 50 : 8-16 j)  Migration in soil was faster and more important under conventional technique (data not shown)

 Water drainage was two times higher under conventional technique  Herbicide leaching was between 2 and 7 times lower under conservation technique, with the lowest leaching under the cover-crop plot

Retention processes and/or degradation were modified by tillage and residues management ?



  -cm depth) at the end of the winter -Soils are unprotected during the inter-crop (from November to May) Environmental context:  In the region, this system of production has generated several environmental problems (nitrate, atrazine)  Now atrazine is forbidden: there is a need for development of new strategies to control weeds in continuous maize systems Organic carbon content  surface  pesticides sorption  (Locke et al., 1997) Desorption: very few studies… tend to increase under MT (Ding et al., 2002)  Degradation: highly contrasted results !  Runoff and erosion: MT are efficient to reduce erosion, but runoff depends on climatic conditions (Fawcett et al. 1994)  Leaching: contrasted results but for no-tillage systems leaching of pesticides increases (Watts & Hall, 1996) inside the maize residues mulch = maize residues in surface mixed with mineral compounds 1982; Roger-Estrade et al. ,2004)  Consequences on water dynamics and solutes transport Evaluate the effects of tillage practices (conventional vs. conservation tillage ; intercrop with or without cover-crop) on: 1-Isoxaflutole degradation and formation of diketonitrile 2-Leaching potential of isoxaflutole and its diketonitrile metabolite 9 Agricultural field of 15 ha  Continuous maize production  « Boulbènes » soils (Gleyic Luvisol)



  Annual grasses and broadleaves weeds, roots uptake  Pre-emergence of maize (75 g ha -1 )  Inhibitor of the biosynthesis of carotenoids  IFT: low solubility in water (6.2 mg L -1 ), rapidly degraded (DT 50 : 1.4-3 j), good retention on organic compounds (K OC : 122 L kg -1 )



  Ceramic cups: 4 by soil pit at 20 and 70 cm-depth  Fibreglass wick lysimeters: 2 by soil pit at 40 cm (25x25 cm). Fibreglass wick length: 70 cm  Sampling of soil water with ceramic cups and fibreglass wick lysimeters  Storage of frozen samples (-18°C) until analysis  Analysis by HPLC-MS/MS Sampling procedure: water General data  In soil samples: Limit of quantification (LOQ) ≈ 0.01 mg a.i. kg -1 soil  In water samples: LOQ depends on collected volumes (V)  If V < 50 ml  LOQ = 0.2 μg L -1  If V > 1000 ml  LOQ = 0.02 μg L -1 In 2005:  400 soil samples were analysed from treatment day to 28 DAT  73 water samples: (14 from the ceramic cups and 59 from the fibreglass wick lysimeters Rainfall data during the cropping season  Total precipitation: 599 mm  First rainfall: Tillage practices had no effect on the in-field degradation of isoxaflutole  Residues on soil surface seemed to slow down degradation rate of IFT Effect of interception and retention ?