Niels Van Velthoven

Yanhui Wang

Hannah Van Hees

Mickaël Henrion

Aram L Bugaev

Guillaume Gracy

Kassem Amro

Alexander V Soldatov

Johan G Alauzun

P Hubert Mutin

Dirk E De Vos

Heterogeneous Single-Site Catalysts for C-H Activation Reactions: Pd(II)-Loaded S,O-Functionalized Metal Oxide-Bisphosphonates

Keywords: single-site heterogeneous catalyst, oxidative Heck reaction, C-H activation, non-hydrolytic sol-gel, metal oxide-bisphosphonate

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INTRODUCTION

Over the past decades, homogeneous catalysis has been increasingly applied for the production of fine chemicals and especially active pharmaceutical ingredients (APIs), ensuring that reactions proceed with high efficiency, high yield, and minimal unwanted by-products. [START_REF] Torborg | Recent Applications of Palladium-Catalyzed Coupling Reactions in the Pharmaceutical, Agrochemical, and Fine Chemical Industries[END_REF][START_REF] Busacca | The Growing Impact of Catalysis in the Pharmaceutical Industry[END_REF] However, high costs and price volatility are generally associated with the transition metals that are commonly applied as homogeneous catalysts or catalyst precursors, and traces of noble metals in APIs are strictly regulated. [START_REF] Welch | Adsorbent Screening for Metal Impurity Removal in Pharmaceutical Process Research[END_REF][START_REF] Colacot | Nobel Prize in Chemistry: Palladium-Catalysed Cross-Coupling[END_REF] Hence, immobilization of the active complex by grafting the ligand on a solid support is an attractive strategy that generates a recyclable heterogeneous catalyst with isolated, well-defined active sites, while potentially preserving the structure-activity relationship of the homogeneous complex. [START_REF] De Vos | Ordered Mesoporous and Microporous Molecular Sieves Functionalized with Transition Metal Complexes as Catalysts for Selective Organic Transformations[END_REF][START_REF] Thomas | Single-Site Heterogeneous Catalysts[END_REF][START_REF] Copéret | Surface Organometallic and Coordination Chemistry toward Single-Site Heterogeneous Catalysts: Strategies, Methods, Structures, and Activities[END_REF][START_REF] Rogge | Metal-Organic and Covalent Organic Frameworks as Single-Site Catalysts[END_REF] As solid support, the use of inorganic materials offers various advantages over organic matrices, such as high chemical, thermal and mechanical stability, and also swelling can largely be avoided. [START_REF] De Vos | Ordered Mesoporous and Microporous Molecular Sieves Functionalized with Transition Metal Complexes as Catalysts for Selective Organic Transformations[END_REF] A powerful approach to convert such inorganic metal oxide supports into heterogeneous catalysts with well-defined active sites is by immobilizing molecular complexes on inorganic supports via ligands with silane, carboxylic acid, or phosphonic acid grafting groups. [START_REF] De Vos | Ordered Mesoporous and Microporous Molecular Sieves Functionalized with Transition Metal Complexes as Catalysts for Selective Organic Transformations[END_REF][START_REF] Copéret | Surface Organometallic and Coordination Chemistry toward Single-Site Heterogeneous Catalysts: Strategies, Methods, Structures, and Activities[END_REF][START_REF] Rogge | Metal-Organic and Covalent Organic Frameworks as Single-Site Catalysts[END_REF] Nevertheless, reaching similar activities with the immobilized complexes as with the analogous homogeneous catalysts can be particularly difficult. [START_REF] Cantillo | Immobilized Transition Metals as Catalysts for Cross-Couplings in Continuous Flow -A Critical Assessment of the Reaction Mechanism and Metal Leaching[END_REF][START_REF] Hübner | Why Does Industry Not Use Immobilized Transition Metal Complexes as Catalysts?[END_REF] Especially for new synthetic strategies that have only been explored fairly recently in homogeneous catalysis, the development of heterogeneous single-site catalysts remains challenging. [START_REF] Cui | Bridging Homogeneous and Heterogeneous Catalysis by Heterogeneous Single-Metal-Site Catalysts[END_REF] Metal-catalyzed coupling reactions via C-H activation have received increased research interest over the past decade as a novel synthesis methodology to couple organic fragments via a C-C bond without the need for prefunctionalization. [START_REF] Chen | Palladium(II)-Catalyzed C-H Activation/C-C Cross-Coupling Reactions: Versatility and Practicality[END_REF][START_REF] Bras | Intermolecular Dehydrogenative Heck Reactions[END_REF][START_REF] Zhou | Towards Ideal Synthesis: Alkenylation of Aryl C-H Bonds by a Fujiwara-Moritani Reaction[END_REF][START_REF] Yang | Oxidative C-H/C-H Coupling Reactions between Two (Hetero)Arenes[END_REF] Although many elegant synthesis procedures have already been proposed, they generally rely on substrates bearing directing groups that assist the challenging C-H activation step, which often limits the substrate scope. [START_REF] Lyons | Palladium-Catalyzed Ligand-Directed C-H Functionalization Reactions[END_REF][START_REF] Sambiagio | A Comprehensive Overview of Directing Groups Applied in Metal-Catalysed C-H Functionalisation Chemistry[END_REF] Alternatively, ancillary ligands can be employed to promote the metal-catalyzed cleavage of the C-H bond, even for substrates without directing group. [START_REF] Engle | Developing Ligands for Palladium(II)-Catalyzed C-H Functionalization: Intimate Dialogue between Ligand and Substrate[END_REF] In particular, ligands based on amino acids [START_REF] Engle | Ligand-Accelerated C-H Activation Reactions: Evidence for a Switch of Mechanism[END_REF][START_REF] Wang | Ligand-Enabled Reactivity and Selectivity in a Synthetically Versatile Aryl C-H Olefination[END_REF] , pyridines [START_REF] Zhang | Pd(II)-Catalyzed Olefination of Electron-Deficient Arenes Using 2,6-Dialkylpyridine Ligands[END_REF][START_REF] Kubota | Pyridine Ligands as Promoters in PdII/0-Catalyzed C-H Olefination Reactions[END_REF][START_REF] Chen | Dual Ligand-Enabled Nondirected C-H Olefination of Arenes[END_REF] , pyridones [START_REF] Wang | Ligand-Promoted Meta-C-H Arylation of Anilines, Phenols, and Heterocycles[END_REF][START_REF] Wang | Ligand-Accelerated Non-Directed C-H Functionalization of Arenes[END_REF] , phenanthrolines [START_REF] Ye | Ligand-Promoted C-3 Selective C-H Olefination of Pyridines with Pd Catalysts[END_REF][START_REF] Gao | Pd(II)-Catalyzed C3-Selective Arylation of Pyridine with (Hetero)Arenes[END_REF] , and 4,5-diazafluoren-9-one [START_REF] Campbell | Regiocontrolled Aerobic Oxidative Coupling of Indoles and Benzene Using Pd Catalysts with 4,5-Diazafluorene Ligands[END_REF][START_REF] Vasseur | Ligand-Promoted Reactivity of Alkenes in Dehydrogenative Heck Reactions of Furans and Thiophenes[END_REF] have been reported to increase the reaction rates of various metal-catalyzed undirected C-H activation reactions. Recently, the research teams of Fernández-Ibáñez and Carrow developed a new group of thioether-based ligands. [START_REF] Gorsline | Alkenylation of Heteroarenes: Mechanism, Rate, and Selectivity Changes Enabled by Thioether Ligands[END_REF][START_REF] Wang | Oligothiophene Synthesis by a General C-H Activation Mechanism: Electrophilic Concerted Metalation-Deprotonation (ECMD)[END_REF][START_REF] Naksomboon | O-Ligand-Promoted Palladium-Catalyzed C-H Functionalization Reactions of Nondirected Arenes[END_REF][START_REF] Naksomboon | O-Ligand-Promoted Palladium-Catalyzed C-H Olefination of Arenes with Allylic Substrates[END_REF][START_REF] Naksomboon | Para-Selective C-H Olefination of Aniline Derivatives via Pd/S,O-Ligand Catalysis[END_REF][START_REF] Álvarez-Casao | O-Ligand-Promoted Pd-Catalyzed C-H Olefination of Thiophenes[END_REF] These ligands contain a carboxyl group at the α-position of a thioether group and promote the Pd(II)-catalyzed oxidative C-H alkenylation [START_REF] Gorsline | Alkenylation of Heteroarenes: Mechanism, Rate, and Selectivity Changes Enabled by Thioether Ligands[END_REF][START_REF] Naksomboon | O-Ligand-Promoted Palladium-Catalyzed C-H Functionalization Reactions of Nondirected Arenes[END_REF][START_REF] Naksomboon | O-Ligand-Promoted Palladium-Catalyzed C-H Olefination of Arenes with Allylic Substrates[END_REF][START_REF] Naksomboon | Para-Selective C-H Olefination of Aniline Derivatives via Pd/S,O-Ligand Catalysis[END_REF][START_REF] Álvarez-Casao | O-Ligand-Promoted Pd-Catalyzed C-H Olefination of Thiophenes[END_REF] , arylation [START_REF] Wang | Oligothiophene Synthesis by a General C-H Activation Mechanism: Electrophilic Concerted Metalation-Deprotonation (ECMD)[END_REF] , and acetoxylation [START_REF] Naksomboon | O-Ligand-Promoted Palladium-Catalyzed C-H Functionalization Reactions of Nondirected Arenes[END_REF] of a wide selection of (hetero)arenes. However, even this new generation of Pd catalysts often needs high loadings (5-10 mol %) to reach excellent yields, which hinders their industrial breakthrough. [START_REF] Hayler | A Pharmaceutical Industry Perspective on Sustainable Metal Catalysis[END_REF] Nevertheless, few heterogeneous catalysts for oxidative C-H alkenylation and arylation reactions have been reported so far, [START_REF] Santoro | Heterogeneous Catalytic Approaches in C-H Activation Reactions[END_REF][START_REF] Dhakshinamoorthy | Formation of C-C and C-Heteroatom Bonds by C-H Activation by Metal Organic Frameworks as Catalysts or Supports[END_REF] of which only a handful are single-site catalysts with well-defined active sites. [START_REF] Duan | Single-Site Palladium(II) Catalyst for Oxidative Heck Reaction: Catalytic Performance and Kinetic Investigations[END_REF][START_REF] Zhang | Direct Aerobic Oxidative Homocoupling of Benzene to Biphenyl over Functional Porous Organic Polymer Supported Atomically Dispersed Palladium Catalyst[END_REF][START_REF] Van Velthoven | Single-Site Metal-Organic Framework Catalysts for the Oxidative Coupling of Arenes via C-H/C-H Activation[END_REF][START_REF] Otake | Enhanced Activity of Heterogeneous Pd(II) Catalysts on Acid-Functionalized Metal-Organic Frameworks[END_REF][START_REF] Van Velthoven | O-Functionalized Metal-Organic Frameworks as Heterogeneous Single-Site Catalysts for the Oxidative Alkenylation of Arenes via C-H Activation[END_REF] Only very recently, we reported the synthesis of a new family of hybrid organic-inorganic mesoporous materials with tunable porosity and functionality. [START_REF] Wang | Water-Stable, Nonsiliceous Hybrid Materials with Tunable Porosity and Functionality: Bridged Titania-Bisphosphonates[END_REF] These bridged metal oxidebisphosphonates could be promising catalytic supports given their high stability and outstanding textural properties. Herein, we report the first metal oxide-bisphosphonatebased solid single-site catalyst containing Pd(II)-thioether active sites that greatly accelerate C-H activation.

EXPERIMENTAL SECTION

Synthesis of the Thioether Ligands S1 and S2.

The syntheses of thioether ligands S1 and S2 are described in detail in section 1. of the supporting information.

Synthesis of the Metal

Oxide-Bisphosphonate Supports.

TiOP.

The titanium oxide-bisphosphonate material was synthesized as described in the literature. [START_REF] Wang | Water-Stable, Nonsiliceous Hybrid Materials with Tunable Porosity and Functionality: Bridged Titania-Bisphosphonates[END_REF] Additional samples were prepared by changing the molar ratio of the linker (0.2 eq. of 4,4'-bis(diethylphosphonomethyl)biphenyl (L1) instead of 0.1 eq.). All other parameters were kept unchanged. 2.2.2. AlOP. The aluminum oxide oxide-bisphosphonate material was prepared similarly, using 0.63 g (3.08 mmol) aluminum (III) isopropoxide, 0.14 g (0.31 mmol, 0.1 eq.) L1, 10 mL toluene, and 0.47 g (0.46 mmol) acetic anhydride. 2.2.3. ZrOP. The zirconium oxide oxide-bisphosphonate material was prepared similarly, using 1.74 g (5.3 mmol) of purified Zr(O n Pr)4 (without 1-propanol), 0.24 g (0.53 mmol, 0.1 eq.) L1, and 10 mL 2-pentanone. The synthesis time in the autoclave at 200 °C was extended to 72 h.

Synthesis of the Metal Oxide Supports

2.3.1. TiO2. The TiO2 anatase support was prepared as described in the literature. [START_REF] Wang | Acetic Anhydride as an Oxygen Donor in the Non-Hydrolytic Sol-Gel Synthesis of Mesoporous TiO 2 with High Electrochemical Lithium Storage Performances[END_REF] 2.3.2. Al2O3. The amorphous Al2O3 support was prepared as described in the literature. [START_REF] Acosta | Preparation of Alumina Gels by a Non-Hydrolytic Sol-Gel Processing Method[END_REF] 2.3.3. ZrO2. The ZrO2 support was obtained via a non-hydrolytic sol-gel procedure. 10 mL 2-pentanone was added to 1.74 g (5.3 mmol) of purified Zr(O n Pr)4 (without 1-propanol). After stirring for 5 min, the resulting solution was transferred to a 23 mL stainless steel autoclave with a PTFE liner. The sealed autoclave was then heated at 240 °C for 72 h in an oven. After the reaction, the gel was thoroughly washed with ether (5 x 30 mL), EtOH (5 x 30 mL), and acetone (5 x 30 mL). Then, it was dried under vacuum (100 Pa) at 120 °C for 5 h. Finally, the material was ground into a fine white powder and was calcined in a muffle furnace at 500 °C (heating rate 10 °C/min) for 5 h in ambient air.

Grafting of the Thioether Ligand (S1) on the Supports.

2.4.1. TiOP-S1. 0.84 g (3 mmol) thioether ligand (S1) was added to 1 g TiOP in 30 mL deionized water. The resulting mixture was stirred at 30 °C for 48 h under an inert atmosphere. After the reaction, the precipitate was thoroughly washed with water (10 x 30 mL) and acetone (5 x 30 mL). Finally, it was dried overnight under vacuum (100 Pa) at room temperature and then ground into a fine powder. 2.4.2. AlOP-S1. S1 was grafted on the aluminum oxidebisphosphonate support similarly, using 1 g AlOP. 2.4.3. ZrOP-S1. S1 was grafted on the zirconium oxidebisphosphonate support similarly, using 1 g ZrOP. 2.4.4. TiO2-S1. S1 was grafted on the zirconium oxide support similarly, using 1 g TiO2. 2.4.5. Al2O3-S1. S1 was grafted on the aluminum oxide support similarly, using 1 g Al2O3. 2.4.6. ZrO2-S1. S1 was grafted on the aluminum oxide support similarly, using 1 g ZrO2.

Pd-loading of the functionalized supports

The functionalized supports were loaded with typically an equimolar amount of Pd(OAc)2 relative to S1. For instance, 14 mg (62.5 µmol) Pd(OAc)2 was added to 190 mg TiOP-S1 in 3.75 mL dichloromethane at room temperature. After stirring for 24 hours, the solution became transparent and a pale yellow powder was obtained, implying that the Pd(II) centers migrated to the functionalized titanium oxidebisphosphonate support. After removing the loading solvent under reduced pressure, the active Pd@TiOP-S1 catalyst was collected and stored under an inert atmosphere.

Characterization of the Heterogeneous Single-Site Catalysts

A Spectrum II Perkin-Elmer spectrometer was used to collect the Fourier-transform infrared (FTIR) spectra. The structure of the materials was analyzed via scanning electron microscopy (SEM) using a Hitachi S-4800 electron microscope equipped with an Oxford Instruments X-Max N SDD for energy dispersive X-ray spectroscopy (EDX). The high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) images were measured on a JEOL 2200FS-200 kV microscopy equipped with a JEOL CENTURIO detector for EDX. Nitrogen physisorption was conducted on a Micromeritics 3Flex surface analyzer at 77 K. The specific surface areas were calculated via the multipoint BET)method, the pore size distribution in the 1 to 5 nm range via the DFT method, and in the 2 to 100 nm range via the BJH method applied to the isotherm desorption branch. Solid-state [START_REF] Bras | Intermolecular Dehydrogenative Heck Reactions[END_REF] C CP-MAS NMR measurements were performed on a 300 MHz VARIAN VNMRS system using a 3.2 mm T3 probe with a sample spinning frequency of 12 kHz. Solid-state 31 P MAS NMR spectra were measured on 400 MHz Varian VNMRS spectrometer equipped with a 3.2 mm Varian T3 HXY probe. The spinning frequency was 20 kHz and a 90° excitation pulse with a duration of 3 μs was used in combination with 100 kHz spinal-64 1 H decoupling. Two hundred scans were recorded with a recycle delay of 30 s. Hydroxyapatite Ca10(PO4)6(OH)2 with a signal at 2.8 ppm (relative to 85 wt% H3PO4 in water) was used as an external reference to determine the 31 P chemical shift accurately. Powder X-ray diffraction (PXRD) data were recorded on a Malvern PANalytical Empyrean diffractometer equipped with at PIXcel 3D 1x1 detector. The patterns were recorded in transmission geometry within a 10° -70° 2Θrange with a step size of 0.013°. The theoretical patterns were simulated by Mercury 3.10 based on the corresponding CIF-files. XAS data were collected at the Structural Materials Science beamline of the Kurchatov Synchrotron Radiation Source (Moscow, Russia) in transmission mode for pelletized samples. The energy was selected by a Si(220) channel-cut monochromator. The spectra processing and analysis were conducted using Demeter software. Firstshell fitting was performed in R-space in the 1-2.6 Å region for k 2 -weighted data Fourier-transformed in the 2-8.2 Å -1 kregion. Theoretical phases and amplitudes were calculated by Feff 6 code.

Catalytic Reactions

Generally, 0.75 mL acetic acid (glacial, Fischer), 0.354 mL (2.50 mmol; 10 eq.) 2,6-dimethylanisole (98%, TCI), 36 µL (0.25 mmol; 1 eq.) n-butyl acrylate (99+%, Acros), 48 µL (0.25 mmol; 1 eq.) tert-butyl peroxybenzoate (TBPB) (98%, Alfa Aesar) and 50 µL hexadecane (internal standard) (>99%, J&K) were added to 40.8 mg (12.5 µmol; 5 mol %) Pd@TiOP-S1 in a 1.5 mL glass vial. The vials were closed and placed in a preheated 15-well aluminum heating block. The reactions were conducted at 100 °C for 2 hours while stirring. After the reaction, the vials were cooled on ice to quench the reaction and the catalyst was separated from the reaction medium via centrifugation. Product quantification was performed on a Shimadzu GC-2010 with a CP-Sil-5 CB column and a flame ionization detector. Product identification was carried out via GC-MS on an Agilent 6890 gas chromatograph with an HP-1 MS column and a 5973 MSD mass spectrometer.

Hot filtration test

The reaction was performed under the standard reaction conditions. After 45 min, the reaction was stopped, quickly centrifuged, and the reaction solution was separated from the catalyst powder. The reaction solution was filtered and heated again to 100 °C for the remaining reaction time. Aliquots of 20 µL were taken during the reaction to assess the yield.

Pd leaching

The leached palladium content in the reaction solution was determined by ICP-OES. After reaction at the standard reaction conditions, the mixture was centrifugated and the reaction solution was separated from the catalyst powder. Next, the organics were evaporated and the remaining metal residue was redissolved in aqua regia. Finally, the aqua regia solution was diluted with a 5% HNO3 solution and the metal content was analyzed using a Varian 720-ES.

RESULTS AND DISCUSSION

Design and Synthesis of the Metal Oxide Bisphosphonate-based Single-Site Catalysts

Inspired by the bidentate S,O-ligands designed by Fernández-Ibáñez et al., [START_REF] Naksomboon | O-Ligand-Promoted Palladium-Catalyzed C-H Functionalization Reactions of Nondirected Arenes[END_REF] we synthesized a new thioether ligand with an additional phosphonic acid group (S1), which allows for grafting the ligand on a metal oxide support. As supports, bridged titanium oxide-bisphosphonate materials (TiOP) were synthesized by a one-step non-hydrolytic sol-gel route (Scheme 1a). [START_REF] Wang | Water-Stable, Nonsiliceous Hybrid Materials with Tunable Porosity and Functionality: Bridged Titania-Bisphosphonates[END_REF] These materials were developed only very recently by the group of Mutin and owe their high pore volumes, surface areas, and tunable mesoporosity to the combination of TiO2 nanodomains held together by rigid aromatic linkers with fully condensed bisphosphonate groups. [START_REF] Wang | Water-Stable, Nonsiliceous Hybrid Materials with Tunable Porosity and Functionality: Bridged Titania-Bisphosphonates[END_REF] In the present work, 4,4'bis(phosphonomethyl)biphenyl (L1) was used as a rigid bisphosphonate linker. As shown in Scheme 1b, the S1 ligands were grafted post-synthetically onto the resulting mesoporous titanium oxide-bisphosphonates and the structural formulas of the functionalized materials were determined from the Ti/P/S ratio measured by energy dispersive X-ray (EDX) spectroscopy and assuming complete condensation (Table S1). After grafting the S1 ligands, the BET surface area and pore volume decreased from 525 m 2 /g to 430 m 2 /g and 1.20 cm 3 /g to 0.80 cm 3 /g, respectively. Neverthe- less, the type IV isotherm of the functionalized material confirmed that the grafted support was still mesoporous (Figure 1a). The titanium oxide-bisphosphonate materials were also characterized by solid-state nuclear magnetic resonance (NMR) techniques to gain more information on the structure of the materials. The 13 C cross-polarization magic angle spinning (CP-MAS) NMR spectra of TiOP and TiOP-S1 confirm that the bisphosphonate linkers (L1) and thioether ligands (S1) remain intact after synthesis, while the lack of features around 60 ppm (O-CH2-CH3) and 75 ppm (O-CH-(CH3)2) implies that the P-OEt and Ti-O i Pr groups, respectively, are fully condensed (Figure 1b). In accordance with the literature, the broad signal centered at 21 ppm in the [START_REF] Wang | Oligothiophene Synthesis by a General C-H Activation Mechanism: Electrophilic Concerted Metalation-Deprotonation (ECMD)[END_REF] P MAS NMR spectra can be ascribed to the tridentate C-P-(OTi)3 sites of the bridging bisphosphonate linkers (Figure 1c). [START_REF] Wang | Water-Stable, Nonsiliceous Hybrid Materials with Tunable Porosity and Functionality: Bridged Titania-Bisphosphonates[END_REF] Furthermore, an additional signal at 12 ppm can be noted in the spectra of TiOP-S1 and TiO2-S1, which is ascribed to the tridentate C-P-(OTi)3 sites of the grafted S1 ligands (Figure S9). [START_REF] Brodard-Severac | High-Field 17O MAS NMR Investigation of Phosphonic Acid Monolayers on Titania[END_REF] More information on how the S1 ligands are grafted onto the titanium oxide-bisphosphonate materials was obtained via Fourier-transform infrared (FTIR) spectroscopy (Figure 1d). As for the pure S1 ligand, a clear carbonyl signal at 1700 cm -1 in the spectrum of TiOP-S1 can be noted, confirming that the S1 ligand is grafted via its phosphonate group rather than via its carboxylic acid group and that the active S,O-moiety (i.e., a carboxyl group at the α-position of a thioether group) thus remains preserved for Pd(II) anchoring. Finally, analysis of the X-ray diffraction pattern of the TiOP-S1 material revealed that it consists of anisotropic titanium dioxide anatase nanodomains, which are elongated along the c-axis given the more intense and narrower (004) reflection compared to the (200) reflection (Figure S1). The presence of such anatase nanorods was further evidenced via HR-TEM images, implying that the three-dimensional mesoporous structure of TiOP-S1 consists of anatase nanorods that are interconnected by the bisphosphonate linkers (Figure S3). The active Pd@TiOP-S1 catalyst was obtained by loading the functionalized titanium oxide-bisphosphonate support with Pd(OAc)2. The oxidation state of palladium in the preloaded Pd@TiOP-S1 catalyst is particularly important since only Pd(II) centers, rather than Pd(0), are catalytically active in oxidative C-H activation reactions. High angle annular dark field scanning transmission electron microscopy (HAADF-STEM) together with energy dispersive X-ray spectroscopy (EDX) indicated that the Pd(II) centers were uniformly distributed over the functionalized titanium oxidebisphosphonate support (Figure S15). Furthermore, Pd(0) nanoparticles could not be observed in the preloaded material, implying that the active sites of the catalyst are isolated Pd(II) sites. The single-site nature of the preloaded catalyst material was further evidenced by X-ray absorption spectroscopy (XAS). Analysis of the X-ray absorption near edge structure (XANES) revealed that the palladium sites in the preloaded catalyst are present as Pd(II) centers and not as Pd(0) species (Figure S17). In addition, no features resulting from Pd-Pd interactions could be observed in the Fourier transformed extended X-ray absorption fine structure (FT-EXAFS), excluding the presence of Pd(0) nanoparticles and confirming that the palladium centers are dispersed over the functionalized titanium oxide-bisphosphonate support as isolated Pd(II) active sites (Figure S18). Furthermore, the structure of the individual Pd(II) active sites could be determined via the FT-EXAFS data. The first shell fitting of the FT-EXAFS data reveals that the Pd(II) centers are surrounded by O-and S-neighbors (Figure 2a) and confirms the proposed structure of Pd@TiOP-S1 with Pd-O and Pd-S distances of 1.92 ± 0.02 Å and 2.27 ± 0.02 Å, respectively (Figure 2b).

Catalytic C-H activation reactions

The activity of the obtained Pd@TiOP-S1 single-site catalyst was tested in the oxidative C-H olefination of arenes, which has been extensively explored in recent years as a more environmentally benign alternative to the widely used traditional Heck cross-coupling reaction. [START_REF] Bras | Intermolecular Dehydrogenative Heck Reactions[END_REF][START_REF] Zhou | Towards Ideal Synthesis: Alkenylation of Aryl C-H Bonds by a Fujiwara-Moritani Reaction[END_REF] After optimizing the reaction conditions (see section 12. of the supporting information), a yield of 75% could be reached employing 5 mol % Pd@TiOP-S1 as catalyst in the oxidative C-H alkenylation of 2,6-dimethylanisole with nbutyl acrylate. This was significantly higher than when regular Pd(OAc)2 or Pd(II)-loaded unfunctionalized titanium oxide-bisphosphonate (Pd@TiOP) were used as catalyst (Table 1, entries 1-3), confirming that the tailormade S,O-binding sites of the immobilized S1 thioether ligands enhance the activity of the Pd(II) centers. When the catalyst loading was increased to 10 mol % or decreased to 2.5 mol %, the yield changed accordingly to 81% and 65%, respectively (Table 1, entries 4-5). Furthermore, when Pd(OAc)2 and TiOP-S1 were added separately, a comparable yield was achieved as with preloaded Pd@TiOP-S1 (74% vs. 75%, respectively) (Table 1, entries 3 and 6), indicating that the S,O-binding sites of the immobilized S1 thioether ligands bind the Pd(II) centers with significant affinity and readily combine to form the active Pd@TiOP-S1 single-site catalyst in situ. Finally, to compare the performance of our heterogeneous catalyst with the state-of-the-art homogeneous system, the original thioether ligand S2 without phosphonic acid group was added to 5 mol % Pd(OAc)2 under the same reaction conditions. A yield of 79% was obtained (Table 1, entry 7), which is only slightly higher than what is achieved using 5 mol % Pd@TiOP-S1 (75%). As a benchmark material for the titanium oxidebisphosphonate-based catalysts, a mesoporous anatase TiO2 support was synthesized and subsequently grafted with S1 ligands (Table S1). After loading TiO2-S1 with Pd(OAc)2 in a similar way as TiOP-S1, the catalytic performance of the resulting Pd@TiO2-S1 material was analyzed. Interestingly, the yield (43%) was considerably lower with 5 mol % Pd@TiO2-S1 than when 5 mol % Pd@TiOP-S1 was used (75%) and only slightly higher than with 5 mol % standard Pd(OAc)2 (Table 1, entry 8). Moreover, even when 5 mol % Pd(OAc)2 and TiO2-S1 were added separately (Table 1, entry 9), no significant increase in yield (41%) could be noted. Since the presence of S1 ligands on the TiO2-S1 material was confirmed by EDX, a Reaction conditions: 1a (2.5 mmol), 2a (0.25 mmol), Pd@TiOP-S1 (5 mol % Pd), TBPB (0.25 mmol), and AcOH (0.75 mL) at 100 °C for 2 h. b Reaction yield was measured via GC-FID. c 10 mol % Pd@TiOP-S1. d 2.5 mol % Pd@TiOP-S1.

FTIR, and 31 P NMR measurements (Table S1 and Figures S9, andS13), these catalytic results suggest that the grafted S1 ligands are not well accessible in TiO2-S1. Compared to the mesoporous TiOP-S1 material, the BET surface area of TiO2-S1 is 4 times lower, while the S1 ligand loading is only 1.7 times lower (Table 2, entries 1-2). As a result, the S1 surface density is more than two times higher on TiO2-S1 (1.04 S1/nm 2 ) compared to TiOP-S1 (0.46 S1/nm 2 ). Hence, compared to Pd@TiO2-S1, the active S,O-supported Pd(II) sites are more isolated and better accessible on Pd@TiOP-S1, resulting in a better single-site catalyst. Additionally, the presence of bisphosphonate linkers with a biphenyl backbone might also increase the affinity of the aromatic substrates towards the active sites on the surface of the supports. Besides Ti-based materials, functionalized Al and Zr oxidebisphosphonates and their respective metal oxides were synthesized similarly. EDX, FTIR, and solid-state NMR measurements confirmed that S1 was incorporated in all materials and that the active S,O-moieties were preserved (Table S1 and Figures S8, andS10-13). After loading with Pd(OAc)2, the obtained catalysts were tested in the oxidative C-H alkenylation reaction to explore the generality of this new catalyst system (Figure 3). While excellent yields could be achieved using Pd@ZrOP-S1 (73%), the activity of the Pd@AlOP-S1 catalyst (50%) was considerably lower and only slightly higher than when just Pd(OAc)2 was added (40%). Furthermore, for both metal oxide-based catalysts, Pd@Al2O3-S1 and Pd@ZrO2-S1, the yield was lower than 50%. Again, a clear inverse correlation between the activity and the S1 surface density of the single-site catalyst could be noted. While ZrOP-S1 displayed a low S1 surface density in line with the TiOP-S1 material, the ligand surface densities on AlOP-S1, Al2O3-S1, and ZrO2-S1 materials were all significantly higher (Table 2), confirming that a low S1 surface density ensures excellent accessibility to the isolated, S,O-supported Pd(II) active sites and ultimately high yields. The remarkably high Table 2. Comparison of the Structure Formula, BET Specific Surface Area (SBET), Total Pore Volume (Vp), Average Pore Diameter (Dp), S1 Loading, and S1 Surface Density of the Functionalized Metal Oxide and Metal Oxide-Bisphosphonate Supports a Structure formulas were determined based on the Ti/P/S EDX ratio assuming complete condensation. b BET specific surface area. c Total pore volume at P/P0 = 0.99. d BJH average pore diameter in the 2-80 nm range calculated from the desorption branch. e The S1 loadings of the supports were determined based on the structure formulas and molecular weights of the bisphosphonate linker L1 and thioether ligand S1. f The densities of the S1 ligands on the surface of the supports were calculated based on the S1 loadings and BET surface areas. g Twice the amount of L1 added during synthesis. 1). a The S1 surface densities were calculated based on the S1 loadings and BET surface areas (cfr. Table 2). b Twice the amount of L1 added during synthesis (cfr. Table 2). S1 surface density on AlOP-S1 is related to the low BET surface area of this support. While the BET surface area of the unfunctionalized AlOP material is high (550 m 2 /g), it decreases considerably after grafting with S1 (220 m 2 /g) (Table S1), indicating that the support is not stable under the acidic grafting conditions. Furthermore, varying the amount of bisphosphonate linker L1 in the TiOP-S1 support did not have a considerable influence on the activity of the corresponding catalyst (Table 2 entry 3 and Figure 3), as long as the S1 surface density remained low. Adding Pd(OAc)2 and the functionalized supports separately resulted again in similar yields as with preloading (Table S7), reaffirming that the differences in yield are not due to catalyst preparation but rather due to the intrinsic properties of the functionalized materials, such as the surface density of the S1 ligand. Finally, Pd(II)-loaded Ti-, Al-, and Zr-based oxide-bisphosphonates and metal oxides without S1 ligands were also tested as control experiments (Table S7). In all cases, only moderate yields were obtained, which further evidences that the presence of isolated, well-accessible S1 ligands is needed to obtain highly active single-site catalysts. Having established that the higher yield is due to the isolated, well-accessible S,O-supported Pd(II) centers, the yield was analyzed at several reaction times to determine if this is due to increased catalyst activity or rather to increased catalyst stability (Figure S19). The kinetic profiles clearly show that while all catalysts start to deactivate after 1 hour, already in the initial phase of the reaction, increased yields are achieved with Pd@TiOP-S1 compared to Pd@TiOP or standard Pd(OAc)2, confirming that the tailormade S,O-binding sites enhance the activity of Pd(II) for the rate-limiting C-H activation step. The slightly higher yield with Pd@TiOP compared to Pd(OAc)2, on the other hand, seems to originate from higher catalyst stability, which has been observed before for other porous supports. [START_REF] Van Velthoven | Single-Site Metal-Organic Framework Catalysts for the Oxidative Coupling of Arenes via C-H/C-H Activation[END_REF][START_REF] Anastasiou | C2-H Arylation of Indoles Catalyzed by Palladium-Containing Metal-Organic-Framework in γ-Valerolactone[END_REF] Finally, a TOF of 13 h -1 was reached with Pd@TiOP-S1 after 1 hour, which is among the highest values ever recorded for the non-directed oxidative C-H olefination of arenes. [START_REF] Van Velthoven | O-Functionalized Metal-Organic Frameworks as Heterogeneous Single-Site Catalysts for the Oxidative Alkenylation of Arenes via C-H Activation[END_REF]

Stability of the Catalyst

The stability of the titanium oxide-bisphosphonate support TiOP-S1 was assessed by comparing the X-ray diffraction patterns of the single-site catalysts before and after the reaction (Figure S1). As expected, no substantial change in the XRD pattern could be noticed, underlining the excellent structural stability of the mesoporous titanium oxidebisphosphonate supports. Furthermore, nitrogen physisorption measurements showed that the spent Pd@TiOP-S1 catalyst was still mesoporous. Despite the relatively strong affinity of the tailor-made S,O-binding sites for Pd centers, the presence of Pd(0) nanoparticles in the spent catalyst was evidenced by HAADF-STEM images (Figure S16). This is in line with the kinetic profiles, which show catalyst deactivation after 1 hour. Notably, no sign of aggregation of the sulfur-containing S1 thioether ligands could be observed, implying that S1 remained firmly attached to the surface of the titanium oxidebisphosphonate support during the reaction. Furthermore, inductively coupled plasma optical emission spectrometry (ICP-OES) measurements of the solution after reaction revealed that Pd leaching was minimized to 12 ppm, amounting to a mere 1% leaching. The heterogeneity of the single-site catalyst was also demonstrated by a hot filtration test (Figure S20), highlighting the feasibility of efficient recovery of the expensive Pd centers. Nevertheless, the activity of the single-site catalyst decreased considerably during consecutive runs (Figure S21). Given the minimal Pd leaching and the fact that using an excess of support compared to Pd does not improve the recyclability (Figure S22), this is likely due to the limited catalyst stability. The kinetic profile shows that the catalyst starts to deactivate after 1 hour, presumably due to the formation of Pd(0) aggregates that are apparent in the HAADF-STEM images of the catalyst after reaction and cannot efficiently be reoxidized by TBPB to the active Pd(II) centers. Moreover, no products could be observed in the control reaction with Pd(0) nanopowder (350 -800 nm nanoparticles) (Table S7), which supports our hypothesis that the poor recyclability is due to Pd(0) nanoparticle formation. Finally, it should be noted that several other Pd-based heterogeneous C-H activation catalyst systems encountered comparable recyclability problems. [START_REF] Otake | Enhanced Activity of Heterogeneous Pd(II) Catalysts on Acid-Functionalized Metal-Organic Frameworks[END_REF][START_REF] Van Velthoven | O-Functionalized Metal-Organic Frameworks as Heterogeneous Single-Site Catalysts for the Oxidative Alkenylation of Arenes via C-H Activation[END_REF]

Substrate Scope

The substrate scope was explored by testing a broad range of arenes and alkenes (Table 3). The oxidative alkenylation of functionalized arenes containing electron-donating groups furnished the products 3a -3d in excellent yields (81 -94%). Particularly, 2,6-dimethylphenol was converted in almost quantitative yields (94%), even when only a slight excess of 2,6-dimethylphenol was added, and a TON of 83 could be reached with 1 mol % Pd@TiOP-S1. In contrast, the alkenylation products of aromatics bearing alkyl groups (3e -3h) were obtained in only moderate yields (37 -54%), and even lower yields (6 -26%) were obtained using arenes without substituents or with electron-withdrawing substituents (3i and 3j). Since especially arenes with electron-donating groups are efficiently converted by the heterogeneous single-site catalyst, several electron-rich heteroarenes were also tested. While the alkenylation of 2methylfuran (3k) furnished the product in high yield (91%), lower yields were obtained using 2,5-dimethylfuran (3l), 2methylthiophene (3m), and 1-methylindole (3n) (64%, 40%, and 42%, respectively). Besides n-butyl acrylate, methyl acrylate and ethyl acrylate were also tested as olefins, furnishing the products 3o and 3p, respectively, in high yields (74 -77%). Finally, the applicability of our heterogeneous single-site catalyst for the pharmaceutical and specialty chemicals industry was demonstrated by converting industrially relevant reagents. Albeit in moderate regioselectivity for the desired para-isomers, excellent yields (81 -82%) were obtained for products 3q and 3r, which are extensively applied as sunscreen ingredients Amiloxate and Octinoxate, respectively.

CONCLUSIONS

In this work, recently developed metal oxidebisphosphonate materials were grafted with thioether ligands and tested as supports to make Pd(II) single-site catalysts. Based on solid-state [START_REF] Bras | Intermolecular Dehydrogenative Heck Reactions[END_REF] C and 31 P NMR measurements and FTIR spectroscopy, it was established that the thioether ligands were successfully grafted on the metal oxide-bisphosphonate material, while preserving the active S,O-moiety of the ligands for Pd(II)-anchoring. Analysis of the resulting Pd@TiOP-S1 catalyst via HAADF-STEM, XANES, and FT-EXAFS indicated that the palladium centers were uniformly dispersed as isolated Pd(II) active sites and excluded the presence of Pd(0) nanoparticles in the catalyst before reaction. Moreover, the local environment of the S,O-supported Pd(II) single-sites could be determined via FT-EXAFS. The activity of the obtained Pd@TiOP-S1 single-site catalyst was tested for the oxidative C-H alkenylation of arenes. A TOF of 13 h -1 was achieved with Pd@TiOP-S1 after 1 h, which was approximately 2-times the TOF reached with standard Pd(OAc)2 and comparable to the activity of state-of-the-art homogeneous systems. Besides the presence of thioether ligands, also textural and surface properties of the support has a major influence on the activity. Significantly more active single-site catalysts were obtained using functionalized, mesoporous metal oxide-bisphosphonates with high specific surface areas and low S1 surface densities as catalyst support instead of the corresponding functionalized mesoporous metal oxides. This increase in activity seems to be associated with the highly porous nature of the metal-oxide bisphosphonate materials, which could facilitate the presence of isolated, well-accessible S,Osupported Pd(II) active sites. Finally, it was demonstrated that the optimal heterogeneous single-site catalyst furnishes the alkenylation products of several model substrates and industrially relevant arenes in good to excellent yields. However, further progress on the stability and recyclability is still needed to unlock the full potential of this new heterogeneous single-site catalyst for oxidative C-H activation reactions. Nevertheless, this study highlights that the use of such highly porous, organic-inorganic hybrid materials as supports can result in significantly more active single-site catalysts compared to conventional metal oxide supports, which might inspire future research in this rapidly growing domain.

ASSOCIATED CONTENT

Supporting Information. Additional characterization data, optimization of the reaction conditions, and product identification (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

Scheme 1 .

 1 Scheme 1. (a) Sol-gel synthesis of the TiOP support in non-aqueous media. (b) Graphical representation of the synthesis steps to transform the TiOP support into the active Pd@TiOP-S1 single-site catalyst.

Figure 1 .

 1 Figure 1. Analysis of the structural properties of TiOP (blue) and the functionalized TiOP-S1 material (orange): (a) N2 physisorption isotherms, (b) 13 C solid-state CP-MAS NMR spectra, (c) 31 P solid-state MAS NMR spectra, and (d) FTIR spectra with the reference spectrum of ligand S1 in green.

Figure 2 .

 2 Figure 2. (a) Experimental (solid black line) and best fit (dashed green line) data of the magnitude (top) and imaginary part (bottom) of the Fourier-transformed k 2 -weighted EXAFS spectrum of the fresh Pd@TiOP-S1 catalyst. The Pd-O and Pd-S contributions are shown by red and yellow dotted lines, respectively. (b) Structure model of the Pd(II) active site of Pd@TiOP-S1 that matches the experimental XAS data. The Ti, P, O, C, H, S, and Pd atoms are displayed as light gray, orange, red, dark gray, white, yellow, and dark blue spheres, respectively. The first and second shell around the Pd(II) center are shown as red and yellow dashed circles, respectively.

Figure 3 .

 3 Figure 3. Performance of the different Pd(II)-loaded metal oxide-bisphosphonate catalysts for the oxidative C-H alkenylation of 2,6dimethylanisole with n-butyl acrylate under the standard reaction conditions (cfr. Table1). a The S1 surface densities were calculated based on the S1 loadings and BET surface areas (cfr. Table2). b Twice the amount of L1 added during synthesis (cfr. Table2).

Table 1 . Comparison of the Activity of the different Cat- alysts for the Oxidative C-H Alkenylation of 2,6-Dime- thylanisole with n-Butyl Acrylate a

 1

ACKNOWLEDGMENTS

This research has received funding from the European Union's Horizon 2020 research and innovation program under grant agreement No. 720996. D. E. D. V. acknowledges the KU Leuven for support via a C3 project and a CASAS Metusalem project. N. V. V. WO for funding (1S32917N). A. L. B. acknowledges the President's Grant of Russian Federation for young scientists MK-2554.2019.2 for financial support of measurement and analysis of XAS data.

AUTHOR INFORMATION

Corresponding Authors * E-mail for D.E.D.V.: dirk.devos@kuleuven.be * E-mail for P.H.M.: hubert.mutin@univ-montp2.fr

Author Contributions

† These authors contributed equally. 3. Substrate Scope a a Standard reaction conditions: 1 (2.5 mmol), 2 (0.25 mmol), Pd@TiOP-S1 (5 mol %), TBPB (0.25 mmol), and AcOH (0.75 mL) at 100 °C for 4 h. Reaction yield was measured via GC-FID and the products were identified via GC-MS. b 2.5 mol % Pd@TiOP-S1. c 1 (0.5 mmol). d 1 (0.5 mmol), 1 mol % Pd@TiOP-S1. e Only one regioisomer was formed. f 1 (0.5 mmol), 70 °C. g 1 (0.5 mmol), 1 mol % Pd@TiOP-S1, 70 °C.

ORCID

Notes

The authors declare no competing financial interest.


Dimitra Koutroumpa
email: dimitra.koutroumpa@aerov.jussieu.fr

Rosine Lallement

Vasili Kharchenko

Alex Dalgarno

The Solar Wind Charge-eXchange contribution to the Local Soft X-ray Background Model to data comparison in the 0.1-1.0 keV band

Keywords: SWCX, Heliosphere, SXRB, ISM, Local Bubble D. Koutroumpa

The major sources of the Soft X-ray Background (SXRB), besides distinct structures as supernovae and superbubbles (e.g. Loop I), are: (i) an absorbed extragalactic emission following a power law, (ii) an absorbed thermal component (∼2×10 6 K) from the galactic disk and halo, (iii) an unabsorbed thermal component, supposedly at 10 6 K, attributed to the Local Bubble and (iv) the very recently identified unabsorbed Solar Wind Charge-eXchange (SWCX) emission from the heliosphere and the geocorona.

We study the SWCX heliospheric component and its contribution to observed data. In a first part, we apply a SWCX heliospheric simulation to model the oxygen lines (3/4 keV) local intensities during shadowing observations of the MBM 12 molecular cloud and a dense filament in the south galactic hemisphere with Chandra, XMM-Newton, and Suzaku telescopes. In a second part, we present a preliminary comparison of SWCX model results with ROSAT and Wisconsin surveys data in the 1/4 keV band.

We conclude that, in the 3/4 keV band, the total local intensity is entirely heliospheric, while in the 1/4 keV band, the heliospheric component seems to contribute significantly to the local SXRB intensity and has potentially a strong influence on the interpretation of the ROSAT and Wisconsin surveys data in terms of Local Bubble hot gas temperature.

Introduction

The surprising discovery of X-ray emission from comets (Lisse et al., 1996) led to establishing Charge-eXchange (CX) collisions between highly charged Solar Wind ions and solar system neutrals as a very efficient non-thermal mechanism of soft X-ray emission (Cravens, 1997). [START_REF] Cox | IAU Colloq. 166: The Local Bubble and Beyond[END_REF] suggested that X-ray emission induced in Solar Wind Charge-eXchange (SWCX) collisions with interstellar (IS) neutrals flowing across the Heliosphere would be an additional component of the diffuse soft X-ray background (SXRB).

Until that discovery, the SXRB was generally assumed to consist of an absorbed extragalactic emission following a power law, an absorbed thermal component (∼2×10 6 K) associated with the galactic disk and halo, and an unabsorbed thermal component, supposedly at 10 6 K, attributed to the Local Bubble (McCammon and Sanders, 1990;Kuntz and Snowden, 2000, and references within). The Local Bubble (or Local Cavity) is defined as the region within ∼100 pc of the solar system particularly devoid of dense (and cold) gas as suggested by absorption lines in nearby star spectra (Lallement et al., 2003).

The 1/4 keV background generally presents a negative correlation with the column density of Galactic neutral hydrogen (dominated by a Galactic plane to-pole variation) that strongly suggests an "absorption" model, in which the spatial structure of the SXRB is produced by absorption of a distant X-ray component (e.g., Bowyer et al., 1968;Marshall and Clark, 1984). However, the strong absorption predicted in such models was not confirmed by observation, neither was the large energy dependence of the absorption, so different solutions were needed.

At the time, the model most consistent with the data was the so-called displacement model [START_REF] Sanders | [END_REF]Snowden et al., 1990a). In this model, the hot gas is in the foreground (in the Local Hot Bubble) with respect to the absorbing cool-gas regions, and higher X-ray intensities are produced in directions where the cavity has a greater extent (towards the LB chimneys at high galactic latitudes) and thus, a greater emission measure of the plasma in it.

Since the discovery of SWCX emission, it became clear very quickly that it should contaminate at an unknown level all soft X-ray observations and it should depend strongly on SW flux and abundance variations as confirmed by the Long Term Enhancements detected during the ROSAT all-sky survey (Snowden et al., 1995) and recent X-ray observations (Snowden et al., 2004;Smith et al., 2005;Henley and Shelton, 2007a). The debate concerned the amount of contamination, and how the SWCX emission level would interfere with the Local Bubble thermal emission, since the two are most probably the major components of the unabsorbed fraction of the SXRB. Post-CX geocoronal emission can also contribute, but only during very intense SW enhancements and for short and easily identified intervals (Cravens et al., 2001).

The two components, although due to completely different mechanisms (collisional excitation for thermal plasma emission and electron capture for CX emission), produce spectra in the same energy range (E ≤ 1.5 keV) that are frequently confused. Cravens (2000) estimated that SWCX emission could be as much as the LB emission. Line ratios, though, are very sensitive to gas temperature (and abundance) for thermal emission. For example, a 10 6 keV plasma might be responsible for all IS diffuse emission at 0.25 keV, while at 2-4 × 10 6 K it might significantly contribute to the 3/4 keV diffuse emission as well [START_REF] Raymond | NATO ASIC Proc. 249: Hot Thin Plasmas in Astrophysics[END_REF]McCammon and Sanders, 1990, and references therein). CX line ratios mainly depend on heavy SW ion relative abundances and collision energy (Beiersdorfer et al., 2001;Kharchenko and Dalgarno, 2001;Wargelin et al., 2008). Detailed spectral comparison of SWCX and thermal emission needs a spectral resolution that should allow to resolve char-acteristic features as the O VII triplet at 0.57 keV and in general individual lines from the continuum. Such a comparison is beyond the scope of this paper.

In this paper we mainly focus on the intensity contribution of the SWCX emission to the observed (foreground) data in the 0.1 -1.0 keV range. In section 2 we briefly present the basic simulation model that calculates the SWCX emission.

In section 3 we analyse the simulation results for the 3/4 keV range and in particular the oxygen lines (O VII at 0.57 keV and O VIII at 0.65 keV). We present typical stationary maps of oxygen lines in section 3.1 and in section 3.2 we present the dynamic model we developed to account for short-scale temporal variations of SWCX emission , with an application on the MBM 12 Suzaku observation of February, 2006. Finaly, we conclude section 3 with a summary of shadowing observations on which we applied our SWCX dynamic model and discuss the results in the 3/4 keV range (sect. 3.3).

I section 4 we present a preliminary analysis of SWCX simulation results in the 1/4 keV range that we compare to observation data from the Wisconsin sounding rocket and ROSAT satellite surveys.

Finally, in section 5 we close our paper with a general discussion on our results.

SWCX model description

In Koutroumpa et al. (2006) we have presented the basic stationary model calculating the SWCX emission in the inner heliosphere. We calculate self-consistently the neutral H and He density distributions in the inner heliosphere (up to ∼100 AU), in response to solar gravity, radiation pressure and anisotropic ionization processes for the two neutral species. Ionization is mainly due to charge-exchange with SW protons for H atoms and to solar EUV photons for He atoms. We also consider the impact of CX on the solar wind ions distributions. This interaction is described in the following reaction:

X Q+ + [H, He] → X * (Q-1)+ + [H + , He + ] (1)
The collision rate per volume unit R X Q+ (cm -3 s -1 ) of X Q+ ions with the neutral heliospheric atoms is given by the equation:

R X Q+ (r) = N X Q+ (r) υ r (σ (H,X Q+ ) n H (r) + σ (He,X Q+ ) n He (r)) = R (X Q+ ,H) (r) + R (X Q+ ,He) (r) (2)
where σ (H,X Q+ ) and σ (He,X Q+ ) are the hydrogen and helium CX cross-sections, n H (r) and n He (r) are the hydrogen and helium density distributions respectively, ῡr = VS W -ῡn ≈ VS W the relative velocity between SW ions and IS neutrals in the inner heliosphere, and N X Q+ (r) is the self-consistent solution to the differential equation:

dN X Q+ dx = -N X Q+ (σ (H,X Q+ ) n H (x) + σ (He,X Q+ ) n He (x)) (3) + N X (Q+1)+ (σ (H,X (Q+1)+ ) n H (x) + σ (He,X (Q+1)+ ) n He (x))
expressing the evolution of the density distribution of ion X Q+ along SW streamlines due to production (from CX reactions of ion X (Q+1)+ ) and loss terms. Cross-section uncertainties are mainly due to instrumental systematic errors and most important to collision energy dependance of cross-sections. Detailed uncertainties for individual ions are not given in literature, but average uncertainties of ∼30% at most are reported (Wargelin et al., 2008). Fig. 1 SWCX model spectra in Line Units (photons cm -2 s -1 sr -1 ) for the 3/4 keV band. The emitting ion is noted above each line.

Then, we establish emissivity grids in units of (photons cm -3 s -1 ):

ε i (r) = R (X Q+ ,H) (r) Y (E i ,H) + R (X Q+ ,He) (r) Y (E i ,He) (4)
where Y (E i ,M) is the photon emission probability of spectral line E i following CX with the corresponding neutral species M (H or He individually). For any line of sight (LOS) and observation date, this spectral line is given by:

I E i (LU) = 1 4π ∼100AU 0 ε i (s) ds (5)
which defines the average level emission of the spectral line for the particular date and LOS, as well as the solar cycle phase (minimum or maximum) corresponding at this date.

SWCX in the 3/4 keV Band

An example of calculated SWCX emission spectra in the 3/4 keV (0.5-1.0 keV) range is presented in figure 1. The major emission lines contributing in this energy range are: (i) the He-like O VII multiplet, consisting of the following transitions:

-the 2 3 S 1 triplet at 560.9 eV, called forbidden (O6f) -the 2 3 P 1 triplet at 568.5 eV, called intercombination line (O6i) -the singlet state 2 1 P 1 at 574 eV, usually refered to as resonance line (O6r), and (ii) the H-like O VIII Lyman-α line at 653.1 eV.

In this list we can also add the He-like Ne IX multiplet (905.1 eV, 914.7 eV and 922.1 eV), but actual observing instruments do not always allow the detection of this line.

Stationary Model Maps

In figure 2 we present an example of calculated full-sky maps of heliospheric emission for line O VIII (0.65 keV) in ecliptic coordinates, on December, 5 (when the Earth is located downwind) of a typical solar maximum (upper panel) and solar minimum (lower panel) year. Colorscales represent O VIII line intensity given in units of 10 -9 erg cm -2 sr -1 s -1 , the red colour corresponding to minimum and the blue to maximum values. We have removed from every map a data portion of 20 • × 20 • around the solar disk where no instrument can observe. What we need to point out here are the striking differences between maximum and minimum solar conditions. At low latitudes, near the solar equator where slow SW dominates, X-ray emission is more intense for solar minimum than for solar maximum because the neutral atom content is higher during solar minimum. During solar minimum, photo-ionization is less efficient and H and He are less readily destroyed by photoionization. Since gravitational pressure exceeds radiation pressure, neutral H atoms have incoming trajectories that fill the ionization cavity.

On the other hand, at high ecliptic latitudes, LOS's are mostly affected by fast wind during solar minimum, and the X-ray emission is dominated by differences of ion relative abundances with respect to maximum cycle phase. Indeed, O 8+ is completely absent from the fast SW and the O VIII line is expected to be much fainter at solar minimum than solar maximum, as it is seen in the maps in fig. 2.

O 7+ has the same trend in relative abundance variations as O 8+ between solar minimum and solar maximum. Nevertheless, instead of being completely absent in the fast wind, it is only strongly depleted ([O 7+ / O] = 0.03) with respect to slow wind ([O 7+ / O] = 0.2). Therefore, we expect that O VII line (0.57 keV) variations from solar maximum to solar minimum will also have the same trend as O VIII shown in the maps.

Two excellent examples of such differences between solar minimum and maximum conditions for the same LOS, are the MBM 12 and South Galactic Filament (SGF) shadowing observations, analysed by Smith et al. (2005Smith et al. ( , 2007) ) and by Henley et al. (2007); Henley and Shelton (2007b,a) respectively. These fields were thoroughly analysed for their SWCX contamination in Koutroumpa et al. (2007) and the main outline and conclusions are presented in section 3.3.

The MBM 12 LOS points toward galactic coordinates (159.2 • , -34.47 • ), which translates to (47 • , 3 • ) in helioecliptic coordinates, very close to the ecliptic plane and inside the limits of the equatorial slow wind zone. Therefore, during solar minimum the MBM 12 exposures (Suzaku) will yield higher SWCX contamination than solar maximum exposures (Chandra) in the 3/4 keV range.

In the case of SGF, at very high southern ecliptic latitude (353 • , -73 • ) the effect is the opposite compared to MBM 12, because it is dominated by differences in the solar ions relative abundances. Therefore, the SWCX oxygen emission in the 3/4 keV range for the SGF field is expected to be much fainter at solar minimum (Suzaku observations) than solar maximum (XMM-Newton observations).

Dynamic Simulations

In Koutroumpa et al. (2007) we presented a dynamic variant of the basic stationary SWCX model, used to simulate the heliospheric X-ray modulation due to SW proton flux and abundance variations. For each field simulated we use real-time flux and abundance measurements from in situ solar wind instruments (Wind, ACE-SWEPAM) and we model SW enhancements as simplified Corotating Interaction Regions (CIR) of a step function form. The total width of the CIR is defined such that the total duration of the step function is the same as the measured enhancement in SW instruments. The step function amplitude is defined so that its total area is equal to the integral of the measured flux during the same period of time.

At each instant we define the form of the CIR (as a Parker spiral) and its propagation in the interplanetary space, taking into account solid solar rotation (27-day period), the radial propagation speed, the "ignition" time on the solar disk towards each radial direction, and the total width of the CIR. Only in the cases of CMEs, we neglect the solar rotation, since radial propagation is dominating the CME structure. Depending on the CIR's width and propagation instant, we calculate the fragment of the LOS affected by the CIR and the local emissivity ε ′ i (r) modified by the SW proton flux and heavy ion abundance variations. We can then reproduce the temporal variation of the X-ray intensity levels during the periods of observation in simulated lightcurves for each of our targets. Abundance variations, can be correlated or anticorrelated with proton fluxes, so they can either emphasize or compensate for the influence of the SW proton flux enhancements.

The Case of the MBM 12 Suzaku Observation of February 2006

The observation of MBM 12 with Suzaku on February 3-8, 2006 was performed in two consecutive exposures, ON-CLOUD (3-6/02/2006, for a total of 231 ks) and immediately after that, OFF-CLOUD (6-8/02/2006, for a total of 168 ks) (Smith et al., 2007). A detailed description of the simulation we performed on this field is given in Koutroumpa et al. (2007).

As we already explained, the MBM 12 LOS is located at (47 • , 3 • ) of helioecliptic coordinates. In the February Suzaku observation this geometry is pointing directly toward the He cone, which is only ∼2 AU away from the observer, where the emissivity on the LOS is maximum. The observation geometry was presented in figure 1 of Koutroumpa et al. (2007). The cone, with a denser distribution of helium atoms, is acting as an amplifier to any SW flux variation.

The MBM 12 Suzaku observation was influenced by a short SW perturbation recorded at the end of the ON-CLOUD pointing. We model this enhancement as a step function in particle flux of 5.2 × 10 8 cm -2 s -1 propagating at a low speed (V S W = 350 km s -1 ) for 0.75

Summary of Shadowing Observations

In Table 1 we resume the MBM 12 and South Galactic Filament shadowing observations analysed in detail in Koutroumpa et al. (2007). All four observations were simulated with the SWCX model with SW conditions as close as possible to measurements in solar instruments, to account for the SWCX contamination of the oxygen line emission in each field.

Besides the basic differences due to solar maximum or solar minimum conditions during the observing period for each field, that we reported in section 3.1, three of the four observations (MBM 12 Chandra, Suzaku and SGF XMM observations respectively) were found to be highly contaminated by short-term SWCX emission variations. In particular, in our simulations we confirmed the contamination of the MBM 12 Chandra observation by a strong CME as suggested by Smith et al. (2005). We suggested that the SGF XMM observation was also highly contaminated, most probably by a CME, which was confirmed by Henley and Shelton (2007a) when they re-analysed and compared their Suzaku and XMM data.

The observation data represent unabsorbed (attributed to the LB) O VII and O VIII line intensities as derived from the authors (Smith et al., 2005(Smith et al., , 2007;;Henley et al., 2007;Henley and Shelton, 2007b,a). In particular, for the SGF field we give two sets of data: from the initial analysis published by Henley et al. (2007) and Henley and Shelton (2007b) (values we used in Koutroumpa et al., 2007) and the revised values published by Henley and Shelton (2007a).

The shadows block more or less efficiently the Galactic Halo oxygen emission, and in their analysis the authors derive the unabsorbed foreground emission of the oxygen lines, that we use to compare to the SWCX heliospheric emission. The only exception is in the Suzaku/MBM 12 observation (Smith et al., 2007) where the O VII and O VIII halo emission is added to the foreground emission for the OFF-CLOUD exposure. Contrary to the constant foreground assumption in the Smith et al. (2007) analysis, we demonstrated that there is a ∼30% and ∼55% increase in the OFF-CLOUD simulated SWCX O VII and O VII line intensities respectively, due to the brief SW enhancement at the end of the ON-CLOUD exposure (see §3.2.1). This increase was erroneously attributed to the Galactic Halo emission, which should be revised, and thus we include these values in the analysis as well. In figure 4 we linearly fit the data over the SWCX model results for the O VII (left panel) and O VIII (right panel) lines for the initial (plain lines) and revised (dashed lines) data sets. We did not use data errorbars as standard deviation to weight the fit, because not all data errorbars were communicated (Henley et al., 2007;Henley and Shelton, 2007b). The only exception is the O VII line fit, since Henley and Shelton (2007a) The first linear fits (in plain lines in fig. 4) we performed (Koutroumpa et al., 2007) had a slope of 1.19 (±0.14; 0.19 at 1σ for O VII and O VIII resp.) meaning that the SWCX model reproduced very well the local foreground emission measured in shadows. Moreover, the fit also gave an estimate of the residual foreground emission to be attributed to the LB. The foreground was found to be (-0.26 ± 0.43 at 1σ) LU for O VII and (-0.17 ± 0.24 at 1σ) LU for O VIII which means that with a high probability the LB O VII and O VIII emission is negligible compared to the heliospheric emission.

The new fits (in dashed lines, fig. 4), where we substituted the SGF observed foreground data with the revised values of Henley and Shelton (2007a) give slightly different results, although the correlation to the simulated values remains very satisfying (the correlation is even improved for the O VII line intensities) and the residual LB emission is still consistent to zero.

SWCX in the 1/keV Band

In this section we present a preliminary study of the SWCX background in the 1/4 keV (0.1-0.3 keV) band that we compare with the ROSAT and Wisconsin surveys data. In section 4.1 we describe briefly the SWCX simulation and in section 4.2 we compare the model results with the survey data sets.

Simulations

For the study on the 1/4 keV band, we updated our atomic database to include Fe, Si, S, Mg ions that emit intense lines in the 0.1-0.3 keV range. Individual radiative transition probabilities for these ions were calculated assuming that the ions are hydrogenic. Moreover, photon yields were calculated using a unique neutral species, which means that no distinction between H and He was made. The hydrogenic ion assumption and unique neutral species choice are only approximations, but the results give a good estimate of the order of magnitude of the X-ray intensity for this energy range. An example of calculated spectra in the 0.1-0.3 keV range is presented in figure 5, with the emitting ion identifying the most intense lines.

We have calculated spectra for two LOS, upwind (UW) and downwind (DW), for an observer assumed to be at 1 AU and in a crosswind position with respect to the LOS. This observing geometry was chosen to match the ROSAT satellite observing constraints. In the 1/4 keV (0.1-0.3 keV) range the model yields a total of 28.35 LU for the UW LOS and a total of 20.27 LU for the DW LOS. The intensity is probably underestimated because of the SW ion propagation in the heliosheath up to the heliopause, and in the heliotail up to ∼3 000 AU, where all ions are used up. The outer heliospheric region is neglected (for the moment) in our model, but preliminary estimates yield a maximum additional ∼20% contribution in the DW direction. In general, because of parallax effects as well, we can assume that within a 50% error bar, these values give an good estimate of average values of the heliospheric SWCX emission in the 1/4 keV range.

Model-Data Comparison

The sounding rocket survey of the University of Wisconsin and the ROSAT satellite survey have dominated the diffuse SXRB observations from 1972 to 1991. The Wisconsin survey allowed a full-sky mapping of the IS SXRB with a spatial resolution of ∼7 • (McCammon et al., 1983). The ROSAT survey improved the spatial resolution to ∼12' (Snowden et al., 1997). However, the instruments did not have any spectral resolution so they provided only total fluxes for each band.

In order to compare the modeled SWCX emission level to the measured SXRB intensities at 1/4 keV, we convolve the calculated spectra for the UW and DW LOS with each band response in this energy range of the instruments. The bands concerning our study are B (0.13 -0.188 keV), C (0.16 -0.284 keV) for the Wisconsin survey and R1 (0.08 -0.284 keV), R2 (0.14 -0.284 keV) for the ROSAT survey. The corresponding band responses are presented in figures: 1(a) of McCammon et al. (1983) for Wisconsin and 1 of Snowden et al. (2000) for ROSAT. In table 2 we summarize the total band fluxes derived from the SWCX spectra convolution with the band responses, and the measured data range for each band (evaluated from figures 6 and 7).

Wisconsin Sounding Rocket Survey

The SXRB survey of the University of Wisconsin was performed with a series of 10 sounding rocket flights between 1972 and 1979. The only spectral information in the 1/4 keV range is extracted by the boron (B) and carbon (C) filters incorporated into the detector windows that separate the two bands B and C presented in McCammon et al. (1983), figure 1(a).

Detailed maps derived from the complete survey were presented in McCammon et al. (1983). The maps present a clear negative correlation with the column density of galactic neutral hydrogen N HI . The X-ray intensity is lowest toward the galactic plane and highest at high galactic latitudes. Snowden et al. (1990b) investigated the band intensity ratio B/C (low to high energies) and found a variation between 0.25 to 0.46 in a dipole-like spatial correlation, aligned with a roughly galactic center-anticenter direction, that they attributed to an anisotropy of the Local Hot Bubble temperature ranging between 10 6.2 and 10 5.9 K respectively.

From Snowden et al. (1990b) we extracted a scatter plot (fig. 6) comparing the B and C band measurements in the Wisconsin survey. The data range is summarized in table 2 and gives ∼[20-100] cts s -1 for the B band and ∼[50-250] cts s -1 for the C band. We superposed, in this figure, the two points (wide circles) corresponding to our simulated spectra convolved with the B and C band responses (see table 2). Error bars are given only for information at

The ROSAT Survey

The ROSAT all-sky survey was performed in a scan mode with imaging telescopes in the soft X-ray band of 0.1 -2 keV, during the first year of the mission. The observing geometry constraints imposed to scan the sky in a big circle, during one orbit, perpendicular to the spin axis that coincided with the Sun-Earth direction. Therefore a complete scan of the sky was performed in a six-month period.

The ROSAT all-sky maps in the 1/4 keV range (Snowden et al., 1995(Snowden et al., , 1997) ) confirmed the Wisconsin results and the negative correlation with the H I column density. The improved spatial resolution of ROSAT, with respect to previous surveys, allowed a better use of shadows as tools to separate the foreground from distant components of the SXRB. An extensive catalog of shadows in the 1/4 keV band, with fitted foreground and distant emission values was presented in Snowden et al. (2000).

In figure 7 we present a scatter plot of foreground intensity I o measured in band R12, which is the sum of intensities in bands R1 and R2. Units are in 10 -6 cts s -1 arcmin -1 , better known as ROSAT Units (RU).The figure was extracted from Snowden et al. (2000) and the authors compare the data set analysed with two different methods. The exact comparison of the two methods is beyond the scope of this paper, since what we need is just the data intensity range (250-820 RU). In the figure y = x, thus the fitted slope is one. Simulated spectra convolved with R1 and R2 band responses and summed for the total result in R12 band yield 333.21 and 251.65 RU for the UW and DW direction respectively. These values, represented by the wide circles in fig. 7 with error bars at 50% (to give the SWCX flux range), are within the lower limits of observational data, that correspond to the low galactic latitude regions.

In the same paper (Snowden et al., 2000) confirm the dipole-like spatial correlation of the R2/R1 (high to low energies) flux ratio, although the variation is less pronounced than the one derived from the Wisconsin analysis, 1.25 to 1.04 from UW to DW. Our model results yield a constant R2/R1 ratio at an average value of 1.38.

Discussion

We have presented simulation results on the SWCX soft X-ray emission in the heliosphere in the 0.1-1.0 keV energy range, that we compared to data from past and present observing instruments. We separated our analysis into two energy ranges: the 3/4 keV, dominated by the oxygen (O VII and O VIII) lines and the 1/4 keV, where heavier ion lines as Mg, Si, S and Fe dominate the SWCX spectra. This analysis yields estimates of the SWCX heliospheric component within the diffuse Soft X-ray Background and confirms the large contamination of X-ray data by the heliospheric emission that needs to be taken into account in future determinations of the LB temperature and pressure.

The simulation results on shadowing clouds observed with Chandra, XMM-Newton and Suzaku suggest that the local 3/4 keV emission detected in front of shadows is entirely explained by the heliospheric SWCX emission (see table 1 and figure 4) and no emission from the LB is needed at these energies. Thus, combining our results in the 3/4 keV range and previous results published on the 3/4 and 1/4 keV ROSAT band ratios, the strict upper limit of the LB temperature is 10 6 K. Indeed, if the LB plasma is in collisional equilibrium at 10 6 K, then it mainly emits in the 1/4 keV range, and only very little emission is produced in the oxygen lines at 3/4 keV.

But, then, how does SWCX affect the data interpretation in the 1/4 keV range? We have compared SWCX simulation results with data from the ROSAT and Wisconsin surveys. Both sets of measured data were originally interpreted in terms of hot gas emission in the LB at a temperature of ∼ 10 6 K. Variations in the observed X-ray intensity were attributed to variations in the extent of the emission volume and therefore the emission measure of the plasma.

SWCX emission flux calculated for the R12 and C bands (which practically cover the same energy range) yielded average values that explain most of the observed emission in the galactic plane, previously attributed to the restricted regions of the LB. Indeed, according to Snowden et al. (1998), in the galactic center and anticenter directions (roughly UW and DW respectively), the LHB is found to have an extent of R LHB ∼ 50 pc (see upper left panel of figure 10 in Snowden et al., 1998), which corresponds to an unabsorbed 1/4 keV emission of I o = R LHB / 0.155 ≈ 322 RU measured in the R12 band, while our model predicts ∼[250-333] RU.

On the other hand, the SXRB intensity is typically brighter at higher Galactic latitudes, and SWCX cannot account for all the emission. This may be explained by the fact that at high latitudes the LOS's point through the chimneys allowing for non-absorbed halo emission to reach the observer. Moreover in the B band, which is the least overlapping in energy range with respect to the other 1/4 keV bands, the SWCX simulations predict about half of the observed lower limits. Besides, the SWCX model fails to reproduce the observed R2/R1 ratio by 10-33% and the B/C ratio by 40-67% .

The answer, then, to the local unabsorbed SXRB puzzle might lie in the proper mix of SWCX emission and warm (rather than hot) LB that will fill-in the gap in the B band and reproduce the observed band ratios (Koutroumpa et al., in preparation).

Fig. 2

 2 Fig. 2 Calculated full sky maps in ecliptic coordinates of the 0.65 keV O VIII line for solar maximum (upper panel) and solar minimum (lower panel) conditions. The observer is situated downwind (ecliptic longitude 75 • ). Colorscales are in units of 10 -9 erg cm -2 s -1 sr -1 .

Fig. 3

 3 Fig. 3 Top: O VII and O VIII line simulated lightcurves (in LU) for the MBM12 ON and OFF observations with Suzaku during the period 03-08/02/2006. Plain line stands for the O VII emission and dashed line stands for the O VIII emission. Dots represent the measured ON and OFF O VII line flux, and triangles represent the measured ON and OFF O VIII line flux(Smith et al., 2007). Bottom: Solar wind proton flux (dotted line) in units of 10 8 cm -2 s -1 for the same period. The step function simulating the SW enhancement is presented by the plain black line. In both panels, the vertical plain lines represent the start and end of the observation period, while the dashed vertical line is the separation between the ON and OFF exposures.

Fig. 4

 4 Fig. 4 Linear fits of oxygen line intensities. Foreground emission data from shadowing observations in litterature (MBM 12: Smith et al., 2005, 2007) and (SGF: Henley et al., 2007; Henley and Shelton, 2007b) versus SWCX model results for the O VII (left panel) and O VIII (right panel) line intensities. All linear fits are calculated with no error-bar weighting except for the O VII New SGF analysis fit (left panel, dashed line). See §3.3 for more details.

  provided errorbars for both XMM and Suzaku O VII detections. The old and new fit coefficients (y(LU) = (a + bx)(LU)) for the O VII and O VIII lines are also noted on the figure.

Fig. 5

 5 Fig. 5 Calculated SWCX spectra in Line Units for the 1/4 keV band. Emitting ions are marked above the most intense lines.

Fig. 6

 6 Fig. 6 Scatter plot of the total intensity calculated in the SWCX model (large circles) compared to total measured (crosses) values in B and C bands in the Wisconsin survey. The model reproduces the observed C band fluxes in the galactic plane, but not the B band fluxes. Figure extracted and adapted for our study from Snowden et al. (1990b).

Fig. 7

 7 Fig. 7 Same as fig. 6 but for the unabsorbed (I o ) emission of ROSAT shadowing observations in band R12 (R1 + R2). The model reproduces most of the R12 band flux in the galactic plane. Figure extracted and adapted for our study from Snowden et al. (2000).

Table 1

 1 Comparison of shadowing observation foreground data to SWCX model for O VII and O VIII line intensities. a Observation date and observing instrument: C for Chandra, X for XMM-Newton and S for Suzaku. b MBM 12 Suzaku-off exposure is included in the analysis, as foreground emission was not constant (see §3.2.1 for details). c Revised values for SGF foreground line intensities, fromHenley and Shelton (2007a).

			O VII (LU)		O VIII (LU)
	Target	Observation a	Data	Data c	SWCX	Data	Data c	SWCX
	MBM 12	Aug. '00, C	1.79 ± 0.55		1.49	2.34 ± 0.36		2.13
	MBM 12	Feb. '06, S-on	3.34 ± 0.26		3.56	0.24 ± 0.10		0.50
	MBM 12	Feb. '06, S-off b	5.68 ± 0.59		4.62	1.01 ± 0.26		0.77
	SGF	May, '02, X	3.40	6.2 +2.8 -2.9	3.16	1.00	...	1.02
	SGF	Mar. '06, S	0.13	1.1 +1.1 -1.4	0.34	N.C.	1.0±1.1	0.02

Table 2

 2 Summary of main band fluxes for the SWCX model in the upwind (UW) and downwind (DW) directions and measured data. For ROSAT data only the unabsorbed fraction I o is given. a RU = 10 -6 cts s -1 arcmin -1 .

Acknowledgements DK and RL aknowledge The Institute for Theoretical Atomic, Molecular and Optical Physics (ITAMP) for travel and living expenses support during a visit to ITAMP facilities. The authors would like to thank Dan McCammon for providing accurate values of the effective area functions of the Wisconsin and ROSAT bands. We are also thankful to Steve Snowden for useful (and lively) discussions. Finally, we are really greatful to our referee Jeffrey Linsky for the thorough report and constructive remarks, as well as for the organisation, along with the organising comittee of the ISSI Local Bubble Workshop, of this most interesting meeting.