Oussama Ouerghi

Mohammed H Geesi

Abdellah Kaiba

Hassane El

Abdul-Malek S Anouar

Philippe Al-Tamimi

Elmutasim O Guionneau

Rachid Ibnouf

Md Afroz Azzallou

Yassine Bakht

Riadi

El Hassane Anouar

Abdul-Malek S Al-Tamimi

Philippe Guionneau

Elmutasim O Ibnouf

Rachid Azzallou

Md Afroz Bakht

Yassine Riadi
email: y.riadi@psau.edu.sa

Synthesis

Keywords: antibacterial evaluation, crystal structure and molecular interactions analysis 6-Bromo-2-chloro-3-butylquinazolin-4(3H)-one, Anti-bacterial, Crystal structure, Hirshfeld surface analysis

come

Introduction

Quinazoline nuclei are diazotized heterocyclic compounds containing benzene and diazine cycles.

These compounds are of great importance in the pharmacological and biological fields. Indeed, the quinazoline scaffold is present in the structure of many synthetic and natural compounds. The quinazoline derivatives show an important role in the preparation of versatile compounds that are biologically and pharmacologically active, such as: insecticides, herbicides [1], antifungals, [START_REF] Cheeseman | The Chemisty of Heterocyclic Compounds[END_REF] antibacterials, antivirals. [START_REF] Porter | Comprehensive Heterocyclic Chemistry[END_REF] In fact, they are used as sedative agents [START_REF] Opitz | [END_REF], anthelmintics [5], potential radiosensitive agents [6], anti-tumor agents, [START_REF] Baguley | DNA intercalating antitumor agents[END_REF][START_REF] Brana | Benzimidazo[1,2-c]quinazolines: a new class of antitumor compounds[END_REF][START_REF] Werbet | Basically substituted ellipticine analogues as potential antitumor agents[END_REF] analgesic [START_REF] Alagarsamy | Synthesis and pharmacological evaluation of some 3-phenyl-2-substituted-3H -quinazolin-4-one asanalgesic, antiinflammatory agents[END_REF][START_REF] Jatav | Synthesis and antimicrobial activity of some new 3-[5-(4substituted)phenyl-1,3,4-oxadiazole-2yl]-2-styrylquinazoline-4(3H)-ones[END_REF], anti-virus [START_REF] Li | Synthesis and bioactivity of 4quinazoline oxime ethers[END_REF], anticytotoxin, anti-tuberculosis [START_REF] Chandrika | Synthesis leading to novel 2,4,6-trisubstitutedquinazoline derivatives, their antibacterial and cytotoxic activity againstTHP-1, HL-60 and A375 cell lines[END_REF][START_REF] Nandy | Synthesis and antitubercular activity of Mannich bases of 2-methyl-3H-quinazolin-4-ones[END_REF], antioxidant [START_REF] Saravanan | Synthesis and evaluation of antioxidant activities of novel quinazoline derivatives[END_REF], antihypertension, anti-diabetes [START_REF] Hess | Scriabine A: Antihypertensive 2-amino-4(3H)-quinazolinones[END_REF][START_REF] Malamas | Quinazolineacetic acids and related analogs as aldosereductase inhibitors[END_REF], etc.

Pharmaceutical chemists prepared versatile types of quinazoline molecules that have shown variety of biological activities as efficient anti-allergic and anti-inflammatory drugs [18]. In addition, many examples of drugs were found such as proquazone [19], DQO-501 [20], and fluproquazone [21].

Therefore, and in continuation of on our research on the development of new routes to synthesize novel heterocyclic compounds [START_REF] Riadi | Convenient synthesis of 2, 4-disubstituted pyrido [2, 3-d] pyrimidines via regioselective palladiumcatalyzed reactions[END_REF][START_REF] Geesi | Ultrasound-accelerated green synthesis of new quinolin-2-thione derivatives and antimicrobial evaluation against Escherichia coli and Staphylococcus aureus[END_REF][START_REF] Riadi | UV Light Mediated Palladium-Catalyzed Synthesis of 2-Substituedpyrido[2,3d]pyrimidines[END_REF][25][START_REF] Riadi | UV Light-mediated regioselective methylsulfanyl discrimination via Pdcatalyzed cross-coupling reactions of 2,4-dimethylsulfanylpyrido[2,3-d]pyrimidines[END_REF][START_REF] Riadi | Regioselective palladium-catalyzed Suzuki-Miyaura coupling reaction of 2,4,6-trihalogenopyrido[2,3-d]pyrimidines[END_REF][START_REF] Riadi | Novel animal-bone-meal-supported palladium as a green and efficient catalyst for Suzuki coupling reaction in water, under sunlight[END_REF][START_REF] Riadi | Photochemical route for the synthesis of novel 2monosubstituted pyrido [2, 3-d] pyrimidines by palladium-catalyzed cross-coupling reactions[END_REF][START_REF] Geesi | Synthesis, antibacterial evaluation, Crystal Structure and Hirshfeld surface analysis of a new 2-benzylsulfanyl-3-(4-fluoro-phenyl)-6-methyl-3H-quinazolin-4-one[END_REF]. Herein, we reported the preparation and the antibacterial activity assessement of novel quinazolinone molecule 6-bromo-2-chloro-3-butylquinazolin-4(3H)-one (6) with good yield (Scheme 1). Crystallographic investigations of the synthesized compound 6 were performed at room temperature, along with description of its molecular interactions, electrostatic surface potential (ESP) and fingerprint plots.

Synthesis

To prepare the targeted derivative, we used the procedure described in reference [START_REF] Chihiro | Palladium(0)-Catalyzed Carbon-Hydrogen Bond Functionalization for the Synthesis of Indoloquinazolinones[END_REF]. First, a mixture of urea (6 mmol) and 5-bromoanthranilic acid (1 mmol) was grinded in a mortar, then heated at 200 °C till total transfer of the mixture to liquid. This mixture was then cooled and NaOH solution (1N) was added, then the mixture was acidified using HCl solution (1N), until the total precipitation of intermediate 4. The mixture was filtered to obtain the compound 4. Then, compound 4 (17.6 mmol) was chlorinated in presence of phosphoryl chloride (176.4 mmol) and N,N-diisopropylethylamine (26.4 mmol) and the reaction was refluxed for 6 hours. Afterword, the reaction was cooled and the excess of phosphoryl chloride was removed in vacuo. Then, the resulting mixture was dissolved in dichloromethane DCM (25 mL) and washed using water and dried over magnesium sulfate. Finally, solvent removed by evaporation to obtain the 6-bromo-2chloro-4(1H)-quinazolinone 4. The intermediate 4 was reacted with an equimolar of 1butylbromide (1 mmol) in acetone (10 mL) and in presence of anhydrous potassium acetate (1.5 mmol) and under continuous stirring overnight. Reaction progression was controlled by thin layer 3 chromatography [TLC]). The resulting solid was extracted using dichoromethane (3 X 10 mL), then the solvent was removed under vacuo and the obtained solid was purified through chromatography column (100 % DCM) to achieve the title derivative 6 as a yellow crystal (yield 81%). Scheme 1. Strategy of the preparation of 6-bromo-2-chloro-3-Butylquinazolin-4(3H)-one (6)

Antibacterial activity

Synthesized molecule 6 was tested for the antibacterial activity against two gram-positive bacteria (Bacillus subtilis and methicillin resistant Staphylococcus aureus [MRSA]) and two gram-negative bacteria, (Escherichia coli and Pseudomonas aeruginosa) using a standard method documented in reference [START_REF] Simpson | Prevalence of penicillin-resistant Streptococcus pneumoniae-Connecticut[END_REF] and also described in detail in a previous article by our group [25].

Results of the antibacterial activity of the derivative 6 were cited in Table 1. This compound 6 showed good activity against two bacterial strains only.

Crystal structure

Crystal data and structure refinement details are summarized in Table2. The collection data was carried out on Bruker Apex II diffractometer using a Molybdenum anticathode. A structural hypothesis was provided by SIR97 program [START_REF] Altomare | SIR97: a new tool for crystal structure determination and refinement[END_REF]. The structure was solved by direct methods using SHELX97 program and all the atomic parameters are refined using a full-matrix least squares technique F 2 based on squared structure factor F 2 [START_REF] Sheldrick | A short history of SHELX[END_REF]. The non-hydrogen atoms were refined anisotropically but the hydrogen atoms were located theoretically. All the crystallography programs were used within the WINGX package [START_REF] Farrugia | WinGX suite for small-molecule single-crystal crystallography[END_REF]. The MERCURY program was used for Molecular graphics [START_REF] Macrae | Mercury CSD 2.0-new features for the visualization and investigation of crystal structures[END_REF].

CCDC 2012234 contains crystallographic data for the present work. These data can be obtained free of charge via the following link: www.ccdc.cam.ac.uk/data_request/cif.

The structural resolution reveals that our compound crystallises in an triclinic system with a P-1 space group and unit parameters cell: a

-2663( 5) 378( 5) 1252( 6)

-1519( 6) 3971( 5)

805(5)

-2881 [START_REF] Baguley | DNA intercalating antitumor agents[END_REF] -1758 [START_REF] Alagarsamy | Synthesis and pharmacological evaluation of some 3-phenyl-2-substituted-3H -quinazolin-4-one asanalgesic, antiinflammatory agents[END_REF] 9912( 1 The crystal structure is built via hydrogen bonds and π -π stacking interactions between neighboring molecules. These intermolecular interactions between molecules allows the crystal packing build up(Figure 4). Indeed, each molecule communicates with two other neighboring molecules through oxygen atom, butyl group and benzene via hydrogen bonds, π-stacking and trough π -π stacking interactions. The electrostatic surface potentials (ESP) of C12H12BrClN2O is shown in Figure 6. In ESP, the red color is attributed to the negative region, which corresponds to hydrogen bond acceptors. However, the blue color indicates the positive region, which corresponds to hydrogen bond donors. As can be seen from Figure 6, the lone pair of oxygen atom of carbonyl moiety of quinazoline is a hydrogen bond acceptor. This result is in accordance with the Hirshfeld surface analysis, which

show the formation of a strong intermolecular hydrogen bond between the one pair of oxygen atom of carbonyl moiety of quinazoline and the hydrogen atom of CH of the aromatic ring (Figure 5). The 2D fingerprint plots for the most intercontacts between the C12H12BrClN2O units are shown in Figure 7 and the ratio of their contributions are summarized in Table 4. The highest inter atomic contact contribution were found between hydrogen atoms H … H, about 39.6% (Figure 4), followed by Br…H/ H…Br, Cl…H/ H…Cl and O…H/ H…O about 13.5, 8.0 and 7.4%, respectively.

Conclusion

In summary, we have described the preparation of a new 6-bromo-2-chloro-3-buylquinazolin-4(3H)-one (6). Its anti -bacterial efficacy has been evaluated against different bacterial strains.

The single crystal XRD structural studies of the 6-bromo-2-chloro-3-butylquinazolin-4(3H)-one was performed. The compound crystallized in monoclinic crystal system with P-1 space group. It exhibited hydrogen bond interaction C=O…H, CH3CH2CH2CHH…C π-stacking interactions and slight π-π stacking interactions between rings of the molecule that assure the connection between molecules and stabilize the crystal. Moreover, the contribution of these interactions was also analysed by visualizing Hirshfeld surface. In addition, the electrostatic surface potential (ESP) was obtained using the DFT method.

  = 4.91550(10) Å, b = 11.4764(2) Å, c = 12.1670(3) Å), volume (638.11(4) Å 3 ), α= 110.079(8)°, β= 93.8130(10)°, γ = 95.4580(10)° and Z = 2. The molecular structure of the title compound is illustrated in Figure 1.The Atomic positions (x, y, z) (x 10 4 ) and equivalent isotropic displacement parameters Ueq (Å 2 x10 3 ) are summarized in Table3

Table 2 :Figure 1 :

 21 Figure 1: View along b axis of asymmetric unit cell of the titled compound.

2 )

 2 are respectively 2.390 Å and 2.878 Å. The π-π stacking interactions between rings of molecules are presented in Figure. 3, the centroidcentroid distances between two neighboring molecules is about 4.915 Å. The distance between carbon atoms C-C is between 1.368(4) and 1.516(4) Å. The C-N distance is between 1.350(3) Å and 1.481(3) Å. The C-Br distance is about 1.892(2) Å. and Å Finally, C=O distance is approximately 1.210(3) Å. The torsion angle value C12-C11-C10-C9 and C11-C10-C9-N2 is about 177.23 (°) and 174.77 (°) respectively, this shows that the CH3-CH2-CH2-CH2-is slightly twisted (Figure1).

Figure 2 :Figure 3 :

 23 Figure 2: View showing the connection between the molecules by C=O…H hydrogen bonds and CH3CH2CH2CHH…C π-stacking interaction (dashed lines).

Figure 4 :4. Computational details 4 - 1 4 - 2

 44142 Figure 4: Packing of molecules in the crystal . a-Projection along a axis, b-Projection along b axis, c-Projection along c axis.

Figure 5 :

 5 Figure 5: dnorm mapped on the Hirshfeld surface for visualizing intermolecular interactions of the title compound.

Figure 6 :

 6 Figure 6: Electrostatic surface potential of C12H12BrClN2O obtained at the B3LYP/6-311+G(d,p) level of theory.

Figure 7 :

 7 Figure 7: The two-dimensional fingerprint plots for the title compound showing all (up), Br … H/H ... Br (middle) and O … H/H … H (bottom) closest intercontacts.

Table 1 .

 1 Activity of the derivative 6 against bacterial strains.

				Bacterial Strains
			Pa	Ec	MRSA	Bs
	Derivative 6	MIC (μg/ml) MBC (μg/ml)	-b -b	25 18.75	38.5 25	-b -b

a MIC: Minimum inhibitory concentration in μg/mL. MBC: Minimum bactericidal concentration b : No inhibition zone, no activity.

Table 4 :

 4 Summary of the major closest contacts and their percentage contributions to the Hirshfeld surface.

	Type of contact	Contribution (%)
	H … H	39.6
	Br … H/ H … Br	13.5
	Cl … H/ H … Cl	8.0

Acknowledgments

The authors would like to thank the Deanship of Scientific Research at Prince Sattam Bin Abdulaziz University for providing financial support for this project (grant no. 2019/03/11064).


Abdellah Kaiba
email: a.kaiba@psau.edu.sa

Mohammed H Geesi

Philippe Guionneau

Yassine Riadi

Talal A Aljohani

Ammar Elsanousi

Oussama Ouerghi

Synthesis, growth, and characterisation of a novel organic-inorganic perovskite-type hybrid system based on glycine

Keywords: Organic-inorganic hybrid, X-ray diffraction, Crystal structure, Micro-Raman spectroscopy, Vibrating sample magnetometer

1.

Introduction

Organic-inorganic layered perovskite compounds have been studied for over 80 years. These compounds are a sub-family of perovskites AMX3 structure, where A is an organic cation, M is a divalent ion which can be Pb 2 + , Cu 2 + , Sn 2+ , Co 2+ , Fe 2 + , Mn 2 + , Cr 2 + , Cd 2+ , or Yb 2+ and X is an oxygen atom in the case of oxides but it can be halogen or nitrogen in other cases; note that the electronegative atom is in its ionic form (halide) [START_REF] Mitzi | Organic-inorganic electronics[END_REF][START_REF] Allan | Atomistic lattice simulations of the ternary fluorides AMF3 (A= Li, Na, K, Rb, Cs; M= Mg[END_REF][START_REF] Papavassiliou | Three-and low-dimensional inorganic semiconductors[END_REF]. Many studies have been published with the formula [R-NH3]2MCl4. Remy et al synthesised a series of compounds containing copper chloride and short alkyl chains with amino groups [START_REF] Remy | Über Chlorokomplexsalze des zweiwertigen Kupfers[END_REF]. The organic-inorganic hybrids have received considerable interest in the recent years as they present unique thermal, magnetic, electronic, and optical properties offering opportunities for many technological applications. Most of the layered organic-inorganic hybrid materials have perovskite-type structures, where inorganic sheets consist of corner-sharing MX6 octahedral or tetrahedral as for Zinc and cobalt.

The ammonium function allows the organic molecule to be inserted between the octahedra, with ionic bonds between NH3 + and X -, which ensures the cohesion of the molecular crystal. There are two possible scenarios: one hydrogen atom can bond to the bridging halide and two hydrogen atoms to the terminal halides (terminal halogen configuration) or vice versa with two bridging and terminal hydrogen atoms (bridging halogen configuration). Various studies of organicinorganic hybrids based on ammonium chains with the general formula [(CnH2n+1NH3)2MX4] (n = 1-18; X = Cl, Br, I; M = Cd 2+ , Mn 2+ …) have been published [START_REF] Lemmerer | Synthesis, characterization and phase transitions of the inorganic-organic layered perovskite-type hybrids [(C n H 2n+ 1 NH 3) 2 PbI 4], n= 7, 8, 9 and[END_REF][START_REF] Abid | Phase transition in organic-inorganic perovskite (C9H19NH3) 2 PbI2Br2 of long-chain alkylammonium[END_REF][START_REF] Vaněk | Phase transitions in (CH3) 4NPbBr3[END_REF][START_REF] Mitzi | Structure and optical properties of several organic-inorganic hybrids containing corner-sharing chains of bismuth iodide octahedra[END_REF].

This family of compounds has a wide set of structures, properties, and applications; notably, some hybrids are used in solar energy storage field and photovoltaic cells. The first approved cell with an efficiency of 12% was manufactured; its performance increased rapidly and reached 20% in 2014 [START_REF] Zhou | Interface engineering of highly efficient perovskite solar cells[END_REF][START_REF] Green | The emergence of perovskite solar cells[END_REF][START_REF] Green | Photovoltaics: Perovskite cells charge forward[END_REF]. The challenge now is to develop photovoltaic cells using chemical methods that are inexpensive, reduce manufacturing time, and have a high conversion efficiency [START_REF] Zhang | Electrochemical design of nanostructured ZnO charge carrier layers for efficient solid-state perovskite-sensitized solar cells[END_REF].

Currently, the photovoltaic devices based on PbI3CH3NH3, which is a combination of lead halide and short chain (methylamine), present an unprecedentedly rapid rise in energy conversion efficiency. Theses composites with short organic chains provided the highest energy conversion efficiency [START_REF] Leguy | Reversible hydration of CH3NH3PbI3 in films, single crystals, and solar cells[END_REF][START_REF] Dang | Bulk crystal growth of hybrid perovskite material CH 3 NH 3 PbI 3[END_REF][START_REF] Heo | Efficient inorganic-organic hybrid heterojunction solar cells containing perovskite compound and polymeric hole conductors[END_REF][START_REF] Liu | Hybrid organic/inorganic nanocomposites for photovoltaic cells[END_REF][START_REF] Zhao | Efficient planar perovskite solar cells based on 1.8 eV band gap CH3NH3PbI2Br nanosheets via thermal decomposition[END_REF][START_REF] Peng | Hybrid organic-inorganic perovskites open a new era for low-cost, high efficiency solar cells[END_REF][START_REF] Prakash | Structural and optical studies of local disorder sensitivity in natural organic-inorganic self-assembled semiconductors[END_REF]. In this context, the layered organic inorganic hybrid like perovskite systems are an important family of functional materials. Among them those based on copper and cadmium halides have been widely studied due to their fascinating structures and versatile applications in solar energy, display devices, probes of biological systems. Particularly, cadmium halide based on organic inorganic hybrids have shown numerous attractive properties, such as photoluminescence, optical, electrical and catalytic properties. The combination of cadmium halide and organic cations have been previously highlighted [START_REF] Thorn | Crystal structure of [(C2H5) 2NH2] 2 [Cd3Br8 (CdBr2)]• H2O: An augmented ribbon structure[END_REF][START_REF] Kong | Unusual onedimensional branched-chain structures assembled by a novel imidazole-containing tripodal ligand with cadmium (II) salts and their fluorescent property[END_REF][START_REF] Gustafsson | Copper (I)-mediated quaternisation of 1, 4-diazabicyclo [2.2. 2octane (DABCO). Crystal structure of bis {(1-chloromethyl-4-aza-1-azoniabicyclo [2.2. 2octane)μ-chloro-chlorocopper (I)}[END_REF][START_REF] Kessentini | Synthesis, crystal structure, vibrational spectra, optical properties and theoretical investigation of bis (2-aminobenzimidazolium) tetraiodocadmate[END_REF][START_REF] Saber Lassoued | Structural, vibrational and optical properties of a new self-assembled organic-inorganic crystal (C4H7N2)[END_REF]. The first reported organicinorganic layered perovskite-type hybrids with carboxylic dimers were [(β-aluminium)2CuCl4] and [(β-aluminium)2CuBr4] [START_REF] Breneman | Diethylenetriammonium tetrachloromanganate (II) chloride[END_REF]. Other composites, [(HO2C(CH2)3NH3)2PbI4] and [(NH3-(CH2)3-CO2H)2CdCl4], were also studied [START_REF] Alshammari | Phase transitions, optical and electronic properties of the layered perovskite hybrid [NH3 (CH2) 2COOH] 2CdCl4 of Y-aminobutyric acid (GABA)[END_REF]. In this work, glycine (NH2-CH2-CO2H) considered as the organic moiety was combined to cadmium chloride considered as the inorganic moiety to prepare the [NH3-CH2-COOH]CdCl3.H2O composite. Glycine is a short chain amino acid ligand that substituted the commonly used monoamines such as methyl amines. The main purpose of this study is to investigate the structural, spectroscopic, magnetic and optical properties at room temperature of the synthesised composite.

Experimental Section

Single crystals of [NH3-CH2-COOH]CdCl3.H2O were obtained as follows: a solution of glycine (1g, 13.3 mmol) in distilled water was prepared and protonated by 1mL of hydrochloric acid (37%) added dropwise. The protonated solution was added dropwise to a solution of cadmium chloride (1.34g, 6.7 mmol) in distilled water. The transparent reaction mixture was stored at room temperature. The single crystals suitable for X-ray diffraction, were obtained after a for several days.

To evaluate the purity of the compound, X-ray photoelectron spectroscopy (XPS) analyses were performed using the K-Alpha XPS System (Thermo Fisher Scientific) equipped with a microfocused monochromatic Al Ka X-ray source (1486.6 eV). Raman spectroscopy investigations were performed using a SENTERRA II Compact Raman Microscope (Brucker) with a wavelength of 532 nm (green laser) and power of 0.25 mW in the wavelength range 50-4000 cm -1 . The optical absorbance was recorded on a JASCO V-570 instrument within the range of 200-800 nm.

A scanning electron microscope (Quanta FEG 250) was used to determine the grain morphology of this compound. The magnetic measurements were recorded at room temperature using a vibrating sample magnetometer (EZ7; MicroSense) with a maximum applied field of 5 kOe. A regular single crystal was selected using an optical microscope. The Powder XRD Ultima IV Xray Diffractometer equipped with conventional X-ray source Cu Kα radiation (l: 1.54056 Å) was used to performed X-ray diffraction pattern with a 2θ range of 5-60°. A Nonius_Kappa_CCD diffractometer with a molybdenum anticathode was used for full data collection. The ϕ and ω scan modes were used, with a crystal-detector distance of 40 mm. The degree of mosaicity was (0.35°). DENZO-SMN program was used to reduce the data. The SIR 97 program allowed us to have a structural hypothesis [START_REF] Altomare | SIR97: a new tool for crystal structure determination and refinement[END_REF], and SHELX 97 allowed us to refine all the atomic parameters using the full-matrix least-squares technique on squared structure factor F 2 [START_REF] Gm | SHELX-97, release 97-2[END_REF]. Hydrogen atoms were located theoretically and refined isotropically. All these programs were used within the WINGX package [START_REF] Farrugia | WinGX suite for small-molecule single-crystal crystallography[END_REF]. The DIAMOND [START_REF] Brandenburg | DIAMOND 3.1 a[END_REF] and MERCURY [START_REF] Macrae | Mercury CSD 2.0-new features for the visualization and investigation of crystal structures[END_REF] programs were used for all the graphics. All the structural data in the Cambridge crystallographic data center Nos. CCDC 1985794 and can also be obtained freely at www.ccdc.cam.ac.uk/data_request/cif.

Results and Discussion

X-ray diffraction studies

The powder X-ray diffraction (PXRD) experiment was carried out to confirm the good crystallinity and homogeneity of the synthesized product [NH3-CH2-COOH]CdCl3.H2O. and Z = 8 parameters. The asymmetric unit contains one cation + NH3-(CH2)-COOH, one anion

[CdCl3] -, and one H2O molecule (Fig. 2a). In this structure, three general positions are occupied by cadmium and two equatorial chloride ions with a refined occupancy factor 0.5 (Site Occupation Factor SOF = 0.5). The site occupation factor (SOF) gives the probability to find an atom at a specific site. Its value ranges from nearly 0 (no atom) to a maximum of 1 (100%, site always occupied by this atom). The location that are not occupied are voids. In solids compounds, these can occur so that the charge on ions can be balanced [START_REF] Williams | Crystallography for chemists by Phillip E. Fanwick[END_REF].

This partial occupancy is a type of disorder encountered during the analysis of the crystal structure.

Moreover, the crystal structure is built by alternating between organic and inorganic parts stacked along the c-axis. The inorganic part consists of CdCl6 octahedrons linked with each other by chlorine atoms (Fig. 2b); the cohesion is ensured by hydrogen interactions. The hydrogen bond involves N-H…Cl connections established between the organic and inorganic layers (Fig. 2d) along with the first scenario (one hydrogen atom can bond to the bridging halide and two hydrogen atoms to the terminal halides (terminal halogen configuration), where the NH-Cl distances (Claxial, Claxial, and Clequatorial) 2.368, 2.467, and 2.507 Å, respectively. Moreover, the second type of hydrogen bond interactions ( + NH3(CH2)COOH…H2O) is formed between the acid function and three molecules of H2O with distances ranging between 1.800 Å and 3.030 Å (Figs. 2c,2d). There is another type of hydrogen bond interaction between the octahedron and H2O molecule (CdCl6….H2O), separated by about 2.616 Å (Fig. 2d). Indeed, H2O molecules play crucial role to ensure the cohesion of this compound. It is obvious that hydrogen bond interactions have an important effect on the stabilization of the packing in the crystal structure.

Despite complex hydrogen bond network, no twisted conformations of the organic chains are noted. The torsion angle NCCO is about 172 °.

The coordination geometry around Cd(II) is a distorted octahedron. Each Cd atom is coordinated by six chlorides, in which four chlorides occupy the basal plane while two chlorides occupy axial positions. The average Cd-Cl distances for the [CdCl6] octahedron for other Cd(II) salts, found in the Cambridge structural database (CSD), range between 2.45 Å and 2.47 Å [START_REF] Neve | Crystal architecture and mesophase structure of long-chain N-alkylpyridinium tetrachlorometallates[END_REF]. However, for this compound the Cd-Cl distances lie between 2.520 and 3.061 Å, the Cl-Cd-Cl angle values are between 82.99° and 100.28° and the bridging angles Cd-Cl-Cd are 147° and 164.01° (Table 2). The distance between neighbouring Cd…Cd in the inorganic sheet is 5.077 Å and 5.672 Å. In addition, the organic sheets display an alternation between long and short bonds Cd-Cl ( 2.666 and 3.061 Å) along with a non-linear bridging Cd-Cl-Cd angle (Fig. 2e). The distortion angle parameter ∑, referred to as the sum of all the deviations from 90° of the 12 cis Ф angles in the co-ordination sphere, is often used to estimate the coordination-sphere distortion

Table 2: Bond lengths [Å] and angles [°] for octahedral [CdCl6] of the organic-inorganic hybrid

C2H6Cd0.5Cl2NO3

X-ray photoelectron spectroscopy (XPS) analysis

The purity of the prepared compound was examined by using XPS. The XPS survey scan (Fig 3)

reveals the existence of characteristic peaks assigned to carbon, oxygen, cadmium, chloride, and nitrogen. The main peaks are O1s, Cd3d, N1s, C1s and Cl2p centred at 533eV, 406 eV, 400eV, 288 eV and 201 eV respectively. The atomic percentage is about 42% for carbon, 24.5% for oxygen, 11.5% for nitrogen, 17% for Chlorine and 5% for cadmium, % which emphasising the purity of our compound.

Bond lengths Cd-Cl [Å]

Angles VSM analysis was performed to evaluate the magnetic property of [NH3-CH2-COOH]CdCl3.H2O at room temperature. A ferromagnetic hysteresis loop can be clearly seen in Figure 5. Therefore, this compound exhibits a ferromagnetic behaviour with a remnant magnetisation Mr of approximately 2 x 10 -4 emu.g -1 and coercive field, referred to as as the reverse field needed to drive the magnetisation to zero after saturation, Hc=334 Oe. The magnetic saturation Ms is approximately 6.33 x 10 -4 emu.g -1 . Generally, in organic-inorganic hybrids, magnetism arises from the metal-ions transition in the inorganic moieties [START_REF] Estes | The magnetic properties of several quasi two-dimensional Heisenberg layer compounds: A new class of ferromagnetic insulators involving halocuprates[END_REF]. Even if, the copper is diamagnetic transition metal, it's become paramagnetic when present in compound. Cu(II) salts exhibit a strong Jahn-Teller distortion in [MX6] octahedra, with antiferrodistortive arrangement of the molecular axes in the plane of the perovskite sheet which leads to ferromagnetic interactions [START_REF] Halcrow | Jahn-Teller distortions in transition metal compounds, and their importance in functional molecular and inorganic materials[END_REF]. The Jahn-Teller distortion appearing in copper-based hybrids, where the alternation of long and short bonds due to Jahn-Teller effect leads to cooperative antiferro-distortive arrangements of the neighbouring octahedra, producing, hence, ferromagnetic interactions so that the spins become ferromagnetically ordered [START_REF] Sun | Reversible Thermochromism and Strong Ferromagnetism in Two-Dimensional Hybrid Perovskites[END_REF][START_REF] Nafday | 2D ferromagnetism in layered inorganic-organic hybrid perovskites[END_REF][START_REF] Park | Canted antiferromagnetism and spin reorientation transition in layered inorganic-organic perovskite (C 6 H 5 CH 2 CH 2 NH 3) 2 MnCl 4[END_REF]. The five d atomic orbitals are split into two degenerate sets when constructing a molecular orbital diagram. These are represented by the sets' symmetry labels: t2g(dxz, dyz, dxy) and eg(dz 2 and dx 2 -y 2 ). When a molecule possesses a degenerate electronic ground state, it will distort to remove the degeneracy and form a lower energy (and by consequence, lower symmetry) system. The octahedral complex will either elongate or compress the z ligand bonds. The cadmium Cd 2+ (d 10 ) belong to the case of transition metal ion in octahedral coordination the electronic configurations (d 3 , d 5 , d 8 , and d 10 ) do not exhibit Jahn-Teller distortions, even if the CdCl6 octahedron present significantly hard distortion (∑ =55.5°) as seen in structural description. As known cadmium transition metal is diamagnetic, but this 2D [NH3-CH2-COOH]CdCl3.H2O compound exhibit unusual ferromagnetic behaviour at room temperature (Figure 6). This let us think that there is presence of paramagnetic impurities.

This hypothesis is exclude because the XPS technique has shown that our compound is pure. The compound present a disorder in the level of the distorted CdCl6 octahedron caused by partial occupancy of atoms (Site Occupation Factor SOF = 0.5 for cadmium and equatorial chloride see structural part above). In this compound these can occur so that the charge on ions can be balanced.

The distortion and the partial occupancy let us think that Cd 2+ (d 10 ) (diamagnetic has been reduced to Cd + (d 9 ) which is paramagnetic, this allowed the compression of CdCl6 octahedron similar to Jahn -Teller distortion. So, the magnetism behaviour seems to originate from Cd + ions. In fact, it here is super-exchange interactions between neighbouring Cd + ions via no linear pathways Cd-Cl-Cd ( 147° and 167.01°). This interaction is mediated by the non-magnetic chlorine ion, with a key role in the magnetic properties. Actually, the compound showed significantly hard distortion in the level of the co-ordination spheres CdCl6 giving rise to changes in the Cd-Cl bond lengths and alternation of long and short bonds ( Cd-Cl). As known, the magnitude of the distortion of the octahedron has an effect on the intensity of the super-exchange interaction between ion, further leading to a weak or a strong ferromagnetism [START_REF] Sun | Reversible Thermochromism and Strong Ferromagnetism in Two-Dimensional Hybrid Perovskites[END_REF]. (2 x10 -4 emu.g -1 ), a coercive field approximately 334 Oe, and a saturation of approximately 6.33

x 10 -4 emu.g -1 . The energy gap value obtained from the optical data using Tauc plot method was 2.80 eV.

Figure 1 :

 1 Figure 1: experimental powder XRD patterns synthesized product [NH3-CH2-COOH]CdCl3.H2O

[ 34 ]

 34 . Herein, ∑ (55.5°) shows a significantly hard distortion of the co-ordination spheres of CdCl6 with respect to similar compound such as [NH3-(CH2)3-COOH]2CdCl4; (∑ =15.8°) [26]. This strong distortion of the octahedron CdCl6 is caused by the hydrogen bonds interactions of the ligand + NH3-CH2-COOH via + NH3 group and the H2O. Moreover, the interlayer spacing between two adjacent inorganic layers (dInt =12.672 Å) (Fig. 2b) was found to be less than the interlayer distance of [NH3-(CH2)3-COOH]2CdCl4 (dInt = 15.6 Å).

Figure 2 .

 2 Figure 2. View showing the asymmetric unit (a). Crystal packing along the b-axis, clearly showing the alternation between the organic and inorganic layers along the c-axis (b). Connection between the organic chains and water by OH…H2O hydrogen bonds (dashed lines) (c). Cohesion between the organic and inorganic chains by N-H…Cl hydrogen bonds and organic chain and three H2O molecules (dashed lines) (d). View showing bridging angles Cd-Cl-Cd and alternation of long and short bonds Cd-Cl in inorganic sheet (e).

Figure 3 : 3 . 3

 333 Figure 3: The whole XPS spectrum of [NH3-CH2-COOH]CdCl3.H2O

Figure. 4 : 3 . 4 Figure 5 .

 4345 Figure.4: SEM micrograph of [NH3-CH2-COOH]CdCl3.H2O

Figure 6 : 3 . 6

 636 Figure 6: Vibrating sample magnetometer hysteresis loop measurement of [NH3-CH2-COOH]CdCl3.H2O at room temperature

Figure 7 :

 7 Figure 7: UV/vis diffuse absorbance plots of [NH3-CH2-COOH]CdCl3.H2O (a) and tauc plot from the UV/Vis analysis of [NH3-CH2-COOH]CdCl3.H2O (b)

Table 1 :

 1 X-ray diffraction and crystal data of the organic-inorganic hybrid [NH3-CH2-

	COOH]CdCl3.H2O
	Crystal data	C2H6Cd0.5Cl2NO3
	Empirical formula	C2 H8 Cd0.50 Cl2 N O3
	Formula weight	221.19
	Temperature	293(2) K
	Wavelength	0.71073 Å
	Crystal system	Orthorhombic
	Space group	Pc a m
	Unit cell dimensions	a = 7.43290(10) Å	α= 90°.
		b = 7.7433(2) Å	β= 90°.
		c = 25.3463(6) Å	γ = 90°.
	Volume Z Density (calculated) Absorption coefficient F(000) Crystal size Theta range for data collection	1458.81(5) Å -3 8 2.014 Mg/m 3 2.244 mm -1 872 0.17 x 0.14 x 0.12 mm
	Index ranges
	Reflections collected
	Independent reflections
	Completeness to theta = 25.242°
	Refinement method
	Data / restraints / parameters
	Goodness-of-fit on F 2
	Final R indices [I>2sigma(I)]
	R indices (all data)
	Largest diff. peak and hole

Acknowledgments

This project was supported by the Deanship of Scientific Research at Prince Sattam Bin Abdul Aziz University under the research project 2019/01/10463.


M N Kokorekin

D Mikhailov

V Gningue-Sall

Jouikov

Andrey S Mendkovich

Arona Ngom

Vladimir A Kokorekin

Mikhail N Mikhailov

Diariatou Gningue-Sall

Viatcheslav Jouikov

N D Zelinsky

Reversible dimerization of anion radicals of carbonyl compounds and the electrosynthesis of pinacols. The case of 9-fluorenone

Keywords: anion radical, dimerization, cyclic voltammetry, chronoamperometry, digital simulation

Reversible dimerization of anion radicals of carbonyl compounds and the electrosynthesis of pinacols. The case of 9-fluorenone

Introduction

Reductive dimerization of carbonyl compounds, first reported back in the mid-19 th century [1] (1859), is still of great actuality. One of the most interesting modern realizations of this method is electrochemical hydrodimerization of carbonyl compounds which attracts the interest as a "metal-free" process. Note that along with the processes of intermolecular electrodimerization, the examples of analogous reactions leading to vicinal cyclic diols were also reported [2,3].

Along with the great majority of works dealing with the mechanism of electrodimerization of carbonyl compounds in buffered solutions, there are also several reports on the study of this process in aprotic media and with the cations of supporting electrolyte that are not prone to the formation of close ion pairs with the anions (e.g. Alk 4 N + cations); this allows one to exclude the influence of these factors on electronic structure of the anion radicals and, in particular, on spin and charge distribution in these species. This permitted to reveal the relationship of the fundamental parameters of electronic structure, determined by means of quantum chemistry, with the reactivity of the anion radicals (in particular, with the rates of their dimerization [6, 10, 13 -15]), including the cases when different reaction centers are involved [11,13,[START_REF] Rusakov | Structure and Reactivity of Organic Radical Anions[END_REF]. Besides, general equations were derived relating rate constants of the formation of dimeric products in the reactions between two anion radicals, between an anion radical and a starting molecule, or between a dianion and a neutral molecule, etc [13 -15]. The issues of reversibility of dimerization of the anion radicals of carbonyl compounds were addressed less often, which is probably related to the fact that this process has irreversible character because of the formation of a C-C bond [2 -12]. At the same time, one can suppose that the anion radicals of carbonyl compounds showing no tendency to coupling can, in fact, be involved in a reversible dimerization whose the equilibrium is shifted towards the monomers. In order to test this hypothesis, 9-fluorenone (1) was taken as a model because it forms a remarkably persistent anion radical (1˙-) [START_REF] Empis | [END_REF]17]. The preference of 1 also relies on the fact that the electrochemical behavior of its reduction products, 9-fluorenol [18] and 9H,9'H-bifluorene-9,9'-diol (2) [19] (Scheme 1) has been studied previously in detail.

Scheme 1. Structures and designations of the compounds and intermediates studied

Experimental

General.

9H-Fluoren-9-one (1), phenol, (CH 3 ) 3 SiCl, (CH 3 ) 2 SiCl 2 , Bu 4 NClO 4 , Bu 4 NPF 6 , DMF and MeCN ("extra dry" grade) were supplied by Acros Organics. 1 H and 13 C NMR spectra of the products in CDCl 3 were recorded on a Bruker Avance 300 instrument (300.13 MHz for 1 H and 75.47 MHz for 13 C). The chemical shifts values (δ) are expressed relative to the chemical shifts of the solvent-d or TMS as an internal standard. FT-IR spectra were recorded using a Frontier Optica FTIR spectrophotometer (Perkin-Elmer). GC-MS was done at an HP-5973 MSD (EI mode, 70 kV, Agilent Technologies) station fitted with a 0.25mm×30m capillary column (OPTIMA-240, Mackerel-Nägel). The X-Ray structure was obtained using a D8 VENTURE Bruker AXS diffractometer equipped with a (CMOS) PHOTON 100 detector, Mo-Kα radiation (λ = 0.71073 Å, multilayer monochromator). The structure was solved by dual-space algorithm using the SHELXT program and then refined with full-matrix least-squares methods based on F 2 (SHELXL) [20]. All non-hydrogen atoms were refined with anisotropic atomic displacement . Under the conditions used, the potential of ferrocenium / ferrocene redox pair vs SCE was stable and equal to +0.468 ± 0.005 V. The solutions to be tested were thermostated at 25 ± 0.5 °С and deaerated by bubbling argon. To prevent any contact between the solution and the ambient air during the experiment, argon was continuously fed into the cell above the solution surface.

For CA and CV curves analyses, the current values in the presence of the substrate were corrected for the current of the supporting electrolyte at the given potential. The concentrations of fluorenone 1 (C 1 ) and phenol (C PhOH ) ranged from 5 to 30 mmol L -1 and the potential scan rates (v) varied from 0.016 to 5 V s -1 . The sampled current voltammograms [START_REF] Bard | Electrochemical Methods: Fundamentals and Applications[END_REF] (a subtype of steady-state voltammograms [START_REF] Mirkin | [END_REF]) were plotted using the values of the current from the CA curves at a transient time (t) of 2 s at the corresponding potentials. Cathodic (i 1c ) and anodic (i 1a ) peak currents and cathodic (i c ) and anodic (i a ) currents at t = 2 s and t = 4 s were used as the response functions in the case of CV and double step CA, respectively. The uncompensated resistance (R u = 400 ± 15 Ω) and active area of working electrode (0.067 cm 2 ) were determined by the procedure described earlier [23].

Electrolysis was carried out in a glass temperature-controlled (20 -25 °C) divided cell (V = 70 mL) equipped with trace-paper diaphragm and plane glassy carbon electrodes (S cathode = 8 cm 2 , In order to avoid the interference of ClO 4 ¯ with chlorosilanes during the electrolysis because of high oxophilicity of Si, the reduction of fluorenone 1 in the presence of (CH 3 ) 3 SiCl and (CH 3 ) 2 SiCl 2 was carried out using Bu 4 NPF 6 supporting electrolyte instead of perchlorate. Since chlorosilanes are extremely prone to hydrolysis by the residual water in the solution releasing protons that can alter the reduction mechanism, the target electrolysis was preceded by an in-situ electrolytic drying the catholyte [START_REF]Inorganic and Organometallic Polymers with Special Properties[END_REF]. For this, (CH 3 ) 3 SiCl (0.3 mmol, 32 mg) were added to the solution of fluorenone 1 (0.81 g, 4.5 mmol) in 0.2 M Bu 4 NPF 6 solution in CH 3 CN (V = 50 mL)

S anode = 4 cm 2
and the mixture was electrolyzed at a slightly negative cathodic potential (-0.3…-0.6 V) using an additional Pt electrode (S = 8 cm2 ) in order to reduce the protons formed from the hydrolysis.

The product of hydrolysis, hexamethyl disiloxane, is chemically and electrochemically inert and is being evaporated with the solvent during the workup. 1 After the current of the pre-electrolysis dropped to ca. 5 -7% of its initial value, the necessary amount of the corresponding freshly distilled (CH 3 ) 3 SiCl (1.22 g, 11.25 mmol) or (CH 3 ) 2 SiCl 2 (0.70 g, 5.4 mmol) was injected via a syringe and the potential was shifted to the -1.20…-1.40 V allowing to maintain current density of 7 mA cm -2 till the end of the electrolysis. The electrolysis was stopped after passing 1.2 F (522 C), acetonitrile was evaporated in vacuo and the resulting yellow solid has been extracted

with pentane (3 × 20 mL). The concentrating and flash chromatography (Silica gel, 60 -65 mesh, eluenthexane) gave colorless single crystals (suitable for X-ray diffractometry after slow evaporation of hexane) of 9,9'-bis[(trimethylsilyl)oxy]-9H,9'H-9,9'-bifluorene (0.95 g, 84%): m.p. 241 °C (240 -241 °C [START_REF] Luis | [END_REF]). FT-IR, cm -1 : 2950 v(C sp2 -H), 1519, 1464 v(C=C), 1277,

1251 (shoulder) δ(Si-CH 3 ), 1372, 1084, 888 v(Si-O), 842 v (C-Si), 762 v(Si-H), 738 δ(C sp2 -H).

Digital simulations

Digital simulations of CA and CV curves were carried out using DigiElch Professional, version 8F (Build 8.222), from ElchSoft. Computation of the model curves was performed taking into account the edge effect and solution uncompensated resistance. The rate constants were determined from CA and CV data using the procedure described previously [START_REF] Mirkin | [END_REF]18,26] that involved the variation of kinetic parameter λ [27,28], defined as λ = RTC PhOH C 1 /Fv and λ = C PhOH C 1 t for CV and CA, respectively.

The diffusion coefficient D of compound 1 in DMF was taken as 10 -5 cm 2 s -1 , based on the value found earlier for 1 [29]. The values of D for 2 and for hydrogen-bonded complex of 1, whose molecular volumes are approximately 2 and 1.5 times larger than that of 1, were estimated using the equation [30] as 7•10 -6 cm 2 s -1 and 8•10 -6 cm 2 s -1 , respectively. The diffusion coefficients in DMF-water mixtures were estimated using the values of the viscosities from [31]. The D of ions and the corresponding neutral species were taken equal [32].

Thus chosen values permitted to achieve a good agreement between the experimental and the theoretical peak currents in the entire range of concentrations and potential scan rates studied.

Electron transfer reaction (1) was considered to be fast and reversible. The values of the transfer coefficients (α) were taken as 0.5. Proton transfer reactions (4, 5) and ( 9) were considered to be fast and irreversible. Simulation of fast heterogeneous reactions was performed using the default values of the rate constants of electron transfer (k s = 10 4 cm s -1 ), as implemented in the software.

In the case of homogeneous processes, the rate constants of fast reactions were taken as an average value of diffusion-controlled rate constants in DMF, 4•10 9 M -1 s -1 [33]. Standard potentials (E 0 ) and homogeneous equilibrium and rates constants were determined using standard DigiElch Professional techniques optimizing the model curves to best fit with the experimental ones over the ranges of scan rates and concentrations indicated above with subsequent determination of the average values. The best agreement between the simulated and the experimental response functions was obtained for the following set of simulation parameters:

E 0 1 = -1.25 V, K 7 = 1.1 M -1 , k 7 ≥ 1•10 4 M -1 s -1
in the case of CV, and

K 3 = 5•10 -6 M -1 , k 3 ≥ 1•10 6 M -1 s -
1 in the case of CA. Variation of k 3 and k 7 above the indicated values does not affect the response function.

Quantum chemical calculations

The quantum chemical calculations were performed in the framework of density functional theory (DFT) with B3LYP exchange correlation functional [34 -36] using Gaussian 03 [START_REF] Gaussian | Gaussian[END_REF] and

Gaussian 09 [START_REF] Gaussian | [END_REF] program packages. As we have shown earlier [START_REF] Mikhailov | [END_REF], the calculated energies of the anion radicals of the compounds with π-bonds significantly depend on the inclusion of diffuse functions on hydrogen atoms in the basis set, whereas these effects are insignificant for the compounds without π-bonds. Therefore, all calculations in this work were performed using 6-311++G(d,p) basis set. Geometry optimization was done for all species considered. The nature of the stationary points (a minimum or a saddle point on the potential energy surface) was determined calculating the eigenvalues of the matrix of second derivatives of energy with respect to nuclear coordinates. In all optimizations, solvatation was taken into account within the reactive field continuum models: PCM [40 -42] and CSC-PCM [43] using the parameters for DMSO, which are similar to those of DMF used in the experiments.

Results and discussion

Cyclic voltammograms of 1 in DMF/0.1 M Bu 4 NClO 4 (Fig. 1) exhibit two cathodic peaks, first of which is related to the formation of an anion radical (1˙¯), while the second one corresponds to the formation of a dianion (1 2-) [START_REF] Empis | [END_REF]17]. From the ratio of first cathodic peak current (i 1c ) to the theoretical value of diffusion current of a one-electron process, the apparent number of electrons (n app ) was found to be 1.02 ± 0.02 over the whole interval of concentrations of 1 and the scan rates considered.

(

(3) As is seen from Fig. 2, the process occurring at the potentials of the first reduction peak is chemically reversible even at high values of с 1 and at slow scan rates v: the experimental parameter i 1a /i 1c corresponds to its theoretical value for a diffusion-controlled process noncomplicated by chemical reactions in the near-electrode layer. One-electron nature of the process and its chemical reversibility are also attested by chronoamperometry. Fig. 3 features a voltammogram plotted using the CA data. The limiting cathodic currents of these curves increases linearly with C 1 and corresponds to the observed electron stoichiometry (n app = 1.02 ± 0.03), while anodic-to-cathodic current ratio at t = 2 and 4 s (0.290 ± 0.005) is independent on C 1 and is close to the theoretical value for a diffusioncontrolled process (Fig. 4). These results might be considered as a formal indication that the anion radical 1˙-does not show any detectable reactivity under the conditions given. However, experimental data on the reactivity of anion radicals of aromatic carbonyl compounds under similar conditions [4 -12] and the corresponding theoretical considerations [6, 10, 13 -15] imply that the possibility of dimerization of 1˙-cannot be ruled out. In particular, the rate of dimerization of the anion radicals was shown to correlate with the spin density on the reaction center [13 -15]. Quantum chemical analysis of the spin density on carbon atom of the carbonyl group in 1˙-shows it to be 0.25. From the correlation of k 3 with the spin density [14,[START_REF] Rusakov | Structure and Reactivity of Organic Radical Anions[END_REF], the rate constant of dimerization of 1˙-was estimated as 10 4 M -1 s -1 . Therefore, we supposed reaction (3) to be reversible, with equilibrium shifted towards 1˙-. This hypothesis is in agreement with the results of quantum chemical calculations providing positive free enthalpy of reaction (3), ΔG = 62.3 kJ mol -1 .

1 + e 1 + e 1
The interactions with media components strongly affect kinetics and thermodynamics of the reactions of ion radicals for they modify the electronic structure of these species [11,[START_REF] Rusakov | Structure and Reactivity of Organic Radical Anions[END_REF]44,45].

When the anion radicals of carbonyl compounds are involved in ion pairing, spin density on the atoms of their carbonyl group increases [45,46], which in turn increases the rate of their radical dimerization [14,[START_REF] Rusakov | Structure and Reactivity of Organic Radical Anions[END_REF]. In addition, stabilization of dimer dianions by ion pairing is favorable for reducing the degree of reversibility of dimerization, as was previously observed for several acetyl derivatives [5] and activated olefins [47]. Since the influence of a cation increases when it decreases in size [44], the behavior of 1 has been scrutinized in lithium perchlorate solutions.

The addition of LiClO 4 to the solution of 1 does not affect electron stoichiometry of the process at the first peak and the number of electrons at this step remains unchanged, n app = 1.01 ± 0.02.

Instead, a new peak emerges at slightly more negative potentials, whose current grows as the concentration of Li + increases (Fig. 5). Concomitantly, the current of the reduction of 1˙-to 1 2- (i 2c ) decreases, though its chemical reversibility is maintained as is attested by the invariance of the i 2a /i 2c ratio. In DMF/0.1 M LiClO 4 solution, with LiClO 4 as the sole electrolyte, only the abovementioned new peak remains on the voltammograms (Fig. 5). The emergence of a new peak on the voltammograms of carbonyl compounds upon the addition of a lithium salt was observed previously and was rationalized as the formation of ion pairs between anion radicals and Li + [48]. However, unlike acetyl derivatives of arenes [5] for which chemical reversibility of the first reduction step decreases when increasing Li + concentration, the addition of LiClO 4 up to 4-fold excess does not alter the ratio of anodic to cathodic peaks of 1 (i 1a /i 1c = 0.63 ± 0.01) if the vertex potential does not exceed -1.70 V. Taking into account that in the case of 1 the addition of LiClO 4 practically does not affect the first peak, it should be assumed that the appearance of a new peak is caused by the formation of ion pairs Li + with 1 2-. The absence of any visible influence of Li + on the dimerization of 1˙-and on its equilibrium constant (K 3 ) can be rationalized that 1˙-and 2 2-are equally stabilized by the association with Li + cations.

(

) (5) 4
An alternative way for preventing dissociation of the dimer dianion might be protonating it by an appropriate proton donor. Expectedly, a pinacol can be obtained from the reduction of 1 in aprotic solvents in the presence of proton donors capable of protonating 2 2-and 2¯ (reactions 4 and 5) but not 1˙¯ (reaction 6). The results of quantum chemical calculations show water and phenol to meet this requirement (Table 1). Scan rate v = 0.1 V s -1 . The addition of up to 20% water to the solution does not lead to the appearance of any new peaks on the voltammograms of 1 and only causes the i 2a /i 2c ratio to decrease due to the protonation of dianion of 1 by water [17]. First reduction step keeps a one-electron character (n app = 1.06 ± 0.04), while the i 1a /i 1c ratio is still that typical of diffusion-controlled processes. Therefore, one might state that under these conditions the proton-donor ability of wateras expected from the quantum chemical calculations (Table 1)is insufficient for irreversible protonation of the dimer dianion. Furthermore, the rate of formation of the dimeric products does not increase as in case of activated olefins [28].

The irreversibility of dimerization can be achieved with phenol as a proton donor. In DMF, the pK a of phenol is 14 orders of magnitude lower than that of water [49] and unlike water, phenol greatly affects chemical reversibility of the first step of reduction of 1. Increasing the concentration of phenol causes the depletion of the anodic peak i 1a (Fig. 6) along with the appearance of the oxidation signal of phenolate anion at positive potentials; this anodic peak grows as the (C PhOH /C 1 ) ratio increases. Meanwhile, just as in the case with water, the oneelectron level of first reduction peak remains unchanged (n app = 1.01 ± 0.03) up to a 6-fold excess of phenol. This observation, as well as the data of Table 1, suggest that the acidity of phenol is insufficient for reaction (6) to run with a noticeable rate. Otherwise, the process would follow an ECE mechanism with the increased number of electrons involved, because quantum mechanical calculations have shown electron affinity of 3 • (3.39 eV) to be higher than that of 1 (3.19 eV). for mechanism (1, 7 -9). For simulation parameters see Section 2.3.

The ratio of anodic to cathodic currents in CA at the potential of limiting current of the first peak (at t = 2 and 4 s, respectively), as well as the ratio of peak currents in CV both decrease reciprocating the increase in C PhOH /C 1 (Fig. 7). Both methods thus attest that first reduction step of 1 in the presence of PhOH becomes irreversible leading to the formation of 2. Although the large scale electrolysis of 1 at the limiting current potential with the addition of phenol in fact gave pinacol 2 as a main product (75% yield, see Experimental), there is still an uncertainty regarding the exact pathway of this process.

Comparing the data of CA with the results of digital simulation (Fig. 7), one might conclude that the electroreduction of 1 under these conditions follows the mechanism including reactions (1, 3 -5). However, the dependencies i 1a /i 1c vs. C PhOH /C 1 (Fig. 8) from cyclic voltammetry differ from what was expected for this mechanism. Equilibrium constants K 3 obtained for the mechanism (1, 3 -5) are changing by almost two orders of magnitude depending on the value of C PhOH /C 1 .

Remembering of the proneness of PhOH to form hydrogen bonds [50], we considered an alternative mechanism (1, 7 -9), which was previously observed during electroreduction of activated olefins [28,51]. This mechanism is supported by the appearance of a new reduction peak at -1.55 V in the presence of phenol; this peak supposedly accounts for the reduction of a hydrogen-bonded complex of 1˙-with PhOH. Experimental (circles) and simulated for mechanism (1, 3 -5) (line). For simulation parameters, see Section 2.3.

One can see (Figs 6 and8) that in contrast to sequence (1, 3 -5), mechanism (1, 7 -9) adequately describes the electroreduction of 1 in phenol-containing solutions in the conditions of cyclic voltammetry. The obtained equilibrium constant of reaction (7) (K 7 ) has the same order of magnitude as those observed [50] for the complexes of PhOH with organic anions.

The disagreement between the mechanisms of reduction of 1 revealed by CV and CA can be explained by different near-electrode concentrations of the intermediate products in these methods. In order to check over this hypothesis, we analyzed the concentration profiles of dimer products formed under the conditions of CV and CA via the mechanisms (1, 3 -5) and (1, 6 -9) (Fig. 9 and10). An inversed feature was indeed found for the formation of 2 in two methods. Another promising method for preventing the dissociation of the dimer dianion 2 2-might be involving it in the reactions of nucleophilic displacement with appropriate electrophilic substrates. Compared to C-electrophilic reagents most often used for alkoxide alkylation (AlkX, dimethylsulphate, dimethylcarbonate, methyl triflate , methoxymethyl ether etc [START_REF] Green | Protective Groups in Organic Synthesis[END_REF][START_REF] Selva | [END_REF]), halosilanes are generally more reactive due to the pronounced Lewis acidity of silicon [54] and its high affinity to oxygen [START_REF] Beserra | The Chemistry of Organic Silicon Compounds[END_REF]. Besides this, using haloalkanes for S N 2 alkylation of anion radicals raises the issue of single electron transfer interfering with the main process [START_REF] Saveant | Elements of Molecular and Biomolecular Electrochemistry[END_REF]; this factor is less important in the case of halosilanes given their more negative reduction potentials [START_REF] Soualmi | [END_REF]58]. Reducing ability of anion radical 1 - , formed at -1.40 V, is supposedly not strong enough to provide a sufficient driving force for the efficient ET to (CH 3 ) 3 SiCl or (CH 3 ) 2 SiCl 2

(with E red  -3.20…-2.80 V vs SCE [58 -60], ΔE (substrate-mediator) > -1.40 V). Therefore, trapping the dianion 2 2-was attempted using these two chlorosilanes.

Electroreduction of 1 in the presence of (CH 3 ) 3 SiCl was carried out with an excess of the chlorosilane (2.5 times) with both substances simultaneously loaded. Provided a large excess of (CH 3 ) 3 SiCl over the electrogenerated nucleophilic form of fluorenone (1 - or 2 2-) during the electrolysis, the silylation step can be considered as occurring at a maximal rate in a pseudo-firstorder regime.

Shortly after setting the working potential (E  |E red (1)|), the solution turned green from mixing colors of 1 (yellow) and 1 - (blue). If the hypothetical dimerization of similar to 3  silylated radicals had not consumed them all, they would have been immediately reduced at the applied potential. Indeed, assuming (as in the case of protonation of 1 - ) that the reduction potential of a silylated fluorene radical is less negative than E red (1) (set as the electrolysis potential), such radical species could not persist in the solution and be responsible for the observed blue coloration. This implies that 1 - remained in the solution even in the presence of an excess of (CH 3 ) 3 SiCl. Moreover, GC-MS monitoring of the electrolysis did not show the formation of 9trimethylsiloxo-9H-fluorene (silylated monomer of fluorene) even in a trace amount. This suggests as well ruling out direct silylation of the anion radical 1 - and ensuing reduction of the silylated radical to a silylated anion that would be protonated during the workup to give this product. Both arguments favor dianion 2 2-to be the active nucleophilic species of silylation.

The only O-silylated product, 9,9'-bis[(trimethylsilyl)oxy]-9H,9'H-9,9'-bifluorene (Fig. 11), was formed in a practically quantitative yield (96% by GC-MS, 84% isolated). The product has Zconfiguration when the two C-O bonds are eclipsed. This is in line with higher stability of an eclipsed than a staggered conformation in "clamped" polyarylethanes [61,62] that was observed, in particular, for 9,9'-bifluorenes [START_REF] Basindale | Reaction Mechanisms of Nucleophilic Attack at Silicon[END_REF]. Note that the formation of a -O-SiMe 2 -O-bridged bifluorene, when (CH 3 ) 2 SiCl 2 was used as an electrophile, also corroborates this feature. Addition of silicophilic co-reagents (HMPA, DMF or else of catalytic amounts of F - [START_REF] Basindale | Reaction Mechanisms of Nucleophilic Attack at Silicon[END_REF]), used to enhance the electrophilic reactivity of Si in chlorosilanes via the formation of a pentacoordinate transient [START_REF] Corriu | Hypervalent Species of Silicon: Structure and Reactivity[END_REF], had no visible effect neither on the yield nor on selectivity of silylation.

Interestingly, when using (CH 3 ) 2 SiCl 2 as an electrophile for trapping 2 2-, the formation of molecular silylated product was not so smooth: some polymer silylated products were formed along with fluorene, 9,9'-bifluorene and 9,9'-bifluorenylidene. This is probably caused by the fact that (CH 3 ) 2 SiCl 2 can react with two nucleophiles, which is possible either in a monomolecular (to give cyclic target product) or in an intermolecular (polymer products) manner. Therefore, avoiding an excess of the electrophile at any given moment of the electrolysis would supposedly favor intramolecular nucleophilic displacement in the chlorosilylated dimer anion. The origin of the non-silylated products also needs to be clarified. This work is under the progress and will be presented elsewhere.

Conclusion

In the case of the reversible nature of dimerization of the electrogenerated anion radicals of carbonyl compounds, high yields of pinacol-type products of electroreduction can be attained using proton donors and electrophiles whose acidity and electron affinity, respectively, are not sufficient for protonation of the anion radicals or for accepting an electron from them.

2 . 2 .

 22 atoms were finally included in their calculated positions and treated as riding on their parent atom with constrained thermal parameters. A final refinement on F 2 with 9939 unique intensities and 493 parameters converged at ωR F 2 = 0.1848 (R F = 0.0739) for 7396 observed reflections with I > 2σ(I). Electroanalytical and electrolysis instrumentation, experimental techniques Chronoamperometry (CA) and cyclic voltammetry (CV) were implemented using a computerassisted potentiostat P-30JM (Elins, Russia) (scan rate error 0.095 %, potential setting ±3000 mV). The experiments were performed in a glass electrochemical cell fitted with a water jacket for thermostating. Polarization curves were recorded in a three-electrode configuration. All electrodes were homemade. A glassy-carbon disc electrode (d = 1.7 mm) was used as the working electrode. The electrode surface was polished with chromium oxide paste (8 -17 μm) and the solution was agitated vigorously with argon before recording each CA and CV curve. A platinum plate (S = 3 cm 2 ) served as the auxiliary electrode. A saturated calomel electrode (SCE) was used as the reference electrode and was connected to the solution by a bridge with a porous glass frit diaphragm filled with the supporting electrolyte (0.1 M Bu 4 NClO 4 solution in N,Ndimethylformamide (DMF))

1 H

 1 NMR, δ: -0.29 (s, 18H, Me), 6.90 -7.05 and 7.19 -7.21 (m, 12H, Ph), 7.31 -7.33 (m, 4H, Ph); 13 C NMR δ: 1.2 (Si(CH 3 ) 3 ), 88.3 (C sp3 -O), 118.7, 126.3, 128.5, 140.7, 145.9. GC-MS, m/z: 506 (M + ), 491 (M + -Me), 433 (M + -TMS), 417 (M + -TMSO), 328 (M + -2TMSO), 253 (M + /2), 180 (M + /2 -TMS), 73 (TMS).

Figure 1 .

 1 Figure 1. Cyclic voltammograms of 1 (7 mmol L -1 ) in DMF/0.1 M Bu 4 NClO 4 at the potential scan rate of 0.1 V s -1 .

Figure 2 .

 2 Figure 2. Experimental (circles) and simulated for the diffusion-controlled process (line) dependences of i 1a /i 1c on C 1 and v -1 .

Figure 3 .

 3 Figure 3. Experimental CA currents (2 s after applying the potential pulse) of 1 (10 mmol L -1 ) in

Figure 4 .

 4 Figure 4. CA curves of 1 (20 mmol L -1 ) in DMF/0.1 M Bu 4 NClO 4 at the limiting current

Figure 5 .

 5 Figure 5. Cyclic voltammograms of 1 (7 mmol L -1 ) in DMF/0.1 M Bu 4 NClO 4 in the presence of 0 (1), 3.5 (2) and 10.0 (3) mmol L -1 LiClO 4 . Blue curve: 1 (7 mmol L -1 ) in DMF/0.1 M LiClO 4 .

Figure 6 .

 6 Figure 6. Cyclic voltammograms of 1 (10 mmol L -1 ) in DMF/0.1 M Bu 4 NClO 4 at v = 0.1 V s -1 in the presence of 0 (1), 5 (2) and 15 (3) mmol L -1 of phenol (lines), and simulated curves (circles)

Figure 7 .

 7 Figure 7. Dependence of i a /i c ratio on lg(λ) for CA of 1 (20 mmol L -1 ) in DMF/0.1 M Bu 4 NClO 4 .

Figures 9 and 10

 10 Figures 9 and 10 show that mechanism (1, 6 -9) dominates under conditions of CV when in case

Figure 8 .

 8 Figure 8. Equilibrium constants determined from CV data: K 3 for mechanism (1,3-5) (half-filled

Figure 9 .

 9 Figure 9. Evolution of surface concentration of 2 during the scan of potential in CV for two

Figure 10 .

 10 Figure 10. Evolution of surface concentration of 2, formed at the limiting current potential

Figure 11 .

 11 Figure 11. ORTEP drawing of 9,9'-bis[(trimethylsilyl)oxy]-9H,9'H-9,9'-bifluorene. Selected

Table 1 .

 1 Gibbs

	Reaction Proton donor HB
		H 2 O	PhOH
	4	-25.0	-101.8
	5	55.7	-21.1
	6	141.2	64.4

free energy (kJ mol -1 ) for protonation reactions (from DFT B3LYP/6-3-11++G(d,p) CSC-PCM).

When using (CH 3 )

SiCl for drying the solution of (CH

) 2 SiCl 2 , the remnants of non-hydrolyzed (CH 3 ) 2 SiCl account for the formation of some amount of di-trimethylsilylated product, attesting to higher efficiency of O-silylation with (CH 3 ) 2 SiCl compared to (CH 3 ) 2 SiCl 2 .

Author statement

Conflict of interest

The authors declare that there is no conflict of interest with any financial organization regarding the material discussed in the manuscript.


Ji Wang

Donglou Ren

Leilei Chen

Guian Man

Houyuan Zhang
email: zhgwang@impcas.ac.cn

Hongpeng Zhang

Laihui Luo

Weiping Li

Yanbo Pan

Peifeng Gao

Yabin Zhu
email: zhuyabin@impcas.ac.cn

Zhiguang Wang

Initial investigation of B 4 C-TiB 2 composites as neutron absorption material for nuclear reactors

Keywords: neutron absorber, B 4 C pellet, He irradiation, crack propagation, finite element

In this study, a specifically designed B 4 C-TiB 2 composite with the typical microstructural feature of a TiB 2 network (cages) that encapsulates a B 4 C matrix was fabricated by the molten-salt and spark plasma sintering (SPS) method. The finite-element (FE) calculation results show that the connected TiB 2 cages constitute a thermally conductive network, which effectively improves the overall thermal conductivity of the composite; these results agree well with the experimental results. Moreover, the Vickers indentation results reveal that the TiB 2 network (cages) can effectively impinge/block the propagation of cracks, which increases the composite toughness. The composite was subjected to helium (He) ion irradiation to simulate the situation in which the B 4 C-TiB 2 composites serve as neutron absorption material, and for which case a high quantity of He atoms is produced by the B 10 (n, ) Li 7 nuclear reaction. According to the transmission electron microscopy (TEM) results, the interfaces between TiB 2 and B 4 C act as effective sinks for He atoms, and are preferential nucleation sites for He bubbles. The theoretical and experimental results show that when the B 4 C-TiB 2 composites serve as neutron absorption pellets in nuclear reactors, they exhibit a better resistance to their disintegration than pure B 4 C pellets.

Consequently, the performance of the control rods of nuclear reactors can be improved.

1. Introduction B 4 C is widely used as a neutron absorption material for control rods (CRs) in many nuclear reactor systems, in particular in fast reactor systems [START_REF] Donomae | Fast Spectrum Control Rod Materials[END_REF] (e.g., Phenix (France), BN (Russia), JOYO (Japan), ALMR, FFTF (United States)) owing to its high melting temperature, outstanding thermal stability, good mechanical properties, and the high neutron absorption cross section of 10 B; B 4 C neutron absorbers play an important role regarding the performance of CRs by, for example, adjusting the reactivity compensation during the normal operation cycles of nuclear reactors, initiating/terminating an operation cycle for refueling or reactor component maintenance, and rapidly shutting down the reactor if abnormal conditions occur. Therefore, B 4 C neutron absorbers are of primary importance for the safe and efficient operation of nuclear reactors.

In many CR designs (e.g., JOYO), B 4 C materials are usually presented in the form of pellets. However, during the operation lifetime of CRs, B 4 C pellets often fracture into pieces and even powders [START_REF] Maruyama | [END_REF]. These fractured B 4 C pellet pieces progressively fill the gaps between the pellets and cladding materials, which accelerates the absorber-cladding mechanical interaction [START_REF] Maruyama | [END_REF]. This leads to an incompatibility between the cladding materials and other components of reactors (e.g., induced absorber swelling cladding cracking (IASCC)) and shortens the CR lifetime. Moreover, the cracked materials are very sensitive to the subsequent radiation-assisted dissolution (i.e., radiolysis effects [3][4][START_REF] Gosset | Absorber materials for generation IV reactors[END_REF]).

Regarding water-cooled nuclear systems, when cooling water leaks into the pellet capsule owing to IASCC-induced cracking [START_REF]Control Assembly Materials for Water Reactors: Experience, Performance and Perspectives[END_REF], the radiolysis effect accelerates the dissolution of the absorber materials and promotes the disintegration of B 4 C pellets [START_REF] Gosset | Absorber materials for generation IV reactors[END_REF]. Both effects degrade the CRs and safe operation of nuclear reactors.

According to previous research studies, the disintegration of bulk B 4 C ceramic pellets is closely associated with the thermal-gradient-induced macrocracks and swelling-gradient-induced microcracks [START_REF] Hollenberg | [END_REF]. During the service lifetime of CRs, the (n, α) nuclear reaction releases an average energy of 2.78 MeV per event [START_REF] Donomae | Fast Spectrum Control Rod Materials[END_REF]; the most energy is deposited within the B 4 C matrix. For instance, at a CR burnup of 10 22 cap/cm 3 per year, a volume power of 1.5×10 2 W/cm 3 is deposited in the B 4 C pellets. Owing to the low thermal conductivity of B 4 C and the high heat-generation rate caused by the 10 B (n, α) 7 Li reaction, the temperature gradient of B 4 C pellets can reach up to 10 3 K/cm. Such a high temperature gradient causes a great thermal stress and results in thermal-stress-induced macrocracks. Moreover, a high quantity of He atoms produced by the transmutation reaction accumulate in the form of flat lenticular He bubbles [8,9]. These He bubbles cause significant volume swelling. When the neutron absorption pellets are placed in thermal neutron reactors, owing to the self-shielding effect [START_REF] Hollenberg | [END_REF][START_REF] Horn | Thermal Spectrum Control Rod Materials[END_REF], the burnup of the pellet periphery is usually stronger than that of the interior.

Because the He atom production is proportional to the burnup level, a gradient distribution of He bubbles leads to a steep swelling gradient of the B 4 C material and an abrupt local strain [START_REF] Hollenberg | [END_REF]: the consequence are swelling-gradient-induced microcracks. In general, the released heat and He atoms produced by the 10 B (n, α) 7 Li reaction are two factors that lead to the formation of cracks. Both factors promote the formation of macro-scale cracks and facilitate the crack propagation through pellets, which result in their disintegration [START_REF] Maruyama | [END_REF][START_REF] Gosset | [END_REF].

Therefore, it is of vital importance to improve the thermal conductivity and irradiation-induced swelling resistance of B 4 C absorber materials to guarantee the good long-term performance of CRs and a safe and efficient operation of nuclear reactors. B 4 C-diborides composites have been explored a long time ago to improve the performance of B 4 C in the irradiation field. For example, B 4 C-HfB 2 composite was designed to improve fracture toughness [START_REF] Decroix | Improvement of thermomechanical properties of ceramic materials for nuclear applications[END_REF][START_REF] Sairam | [END_REF] and the thermal-mechanical properties of the neutron absorber [START_REF] Decroix | Improvement of thermomechanical properties of ceramic materials for nuclear applications[END_REF]. In this study, a low economic cost and honeycomb-structured B 4 C-TiB 2 composite was elaborated to attain the aforementioned objective. The main feature of the composite is a highly thermally conductive cage-like TiB 2 network that encapsulates conventional B 4 C matrix grains. TiB 2 has a high melting point (3498 K), exceptional hardness (25-35 GPa), highly thermal conductivity, excellent wear resistance, a low thermal expansion coefficient (4.6 × 10 -6 K -1 , which is almost the same as that of B 4 C, 4.5 × 10 -6 K -1 ), and a high oxidation resistance [14][15][16][17][18]. Owning to the inimitable characteristics of

Results and discussions

Before the SEM analysis, the composites were mechanically polished with diamond papers (from 800 to 12500 grit) until mirror-like surfaces were obtained. interface, thereby proving that the isolated TiB 2 islands in the B 4 C matrix can redirect the crack propagation. A similar phenomenon was reported in [22]. In this study, the network (cages) topography in Fig. 8

(b) clearly

shows that the crack propagates through the TiB 2 walls (dashed arrows 2 and 3) and extends along the interface between the B 4 C and TiB 2 phases (dashed arrow 1), which facilitates the energy release of the crack tip. This is confirmed by the shorter crack length (12.2 ± 3.8 μm) of S3 than those of S1 (22.9 ± 2.8 μm) and S2 (16.1 ± 4.3 μm), as shown in Fig. 6.

It can be concluded that there are two underlying ways in which the planes by first-principle calculations. Moreover, they discovered that the activation energy of He ion diffusion is approximately 2 eV [28], which is consistent with the experimental results in [30]. The 2 eV migration energy means that the aggregation of He atoms in B 4 C materials is retarded and explains the observation of small-sized He bubbles in this study rather than large-sized He bubbles in metals under similar irradiation conditions [31].

The He bubbles that are distributed along the interfaces are bigger than the bubbles distributed in the B 4 C and TiB 2 grains (however, bubbles along the TiB 2 -TiB 2 interface are not investigated in this study), as shown in Fig. 10. This is due to the special structure of the interface. Misfits in the lattice structure of the interfaces contain plenty of open volume [32],

which provides adequate free space for the deposition of irradiation-induced point defects (e.g., vacancies, interstitials, transmuted elemental atoms) [33]. Owing to the high binding energy between vacancies and He atoms (~ 2 eV [27,28]), the open volume of interfaces can act as efficient trapping sites for He and preferential nucleation sites for He bubbles [8,9,34]. Therefore, once the He atoms are captured by the interfaces, they cannot easily be released within the temperature range investigated in this study. As shown in Fig. 10, no He bubbles depleted zone was detected, which is consistent with the results of Motte et al.'s work [28] that it was hard for He atoms to diffuse through or in the B 4 C grain boundaries below 1373 K. This might prove that long-range He diffusion is negligible at 1273 K. Moreover, TEM image in Fig. 10 shows that He bubbles deposition band in B 4 C is closer to the surface than that in TiB 2 , this phenomenon is also consistent with SRIM calculation as shown in Fig. 1 that the overlap of He distribution and damage profile is shallower in B 4 C than that in TiB 2 .

In a realistic environment of nuclear reactors, increasing the density of sinks for the precipitation of He atoms is an effective way to suppress the formation of large He bubbles [35], and reduce the He-induced property degradation (e.g. He embrittlement). Therefore, the network (cages) of the TiB 2 phases provides numerous sinks for He precipitation, which might inhibit the formation of large He bubbles, and mitigate the effects of He-accumulation-induced shear stress at interfaces, and the resulting microcracks, especially in the thermal neutron reactors.

However, other factors may affect performance of the B 4 C-TiB 2 composites, such as shear stress induced by He accumulation at the interfaces, different thermal dilation between B 4 C and TiB 2 . To have a better evaluation of the B 4 C-TiB 2 composites in a realistic environment, a detailed numerical evaluation will be carried out in the future study.

As an important component of the B 4 C-diborides composites, the behavior of diborides under irradiation environment is very crucial to the performance of composites. Cheminant et al. conducted neutron irradiation on HfB 2 , and found that porosity appeared at grain boundaries, and resulted in a decrease of the thermal diffusivity [36], which lead to the degradation of B 4 C-HfB 2 composites. Regarding the TiB 2 in this study, despite the inimitable merits of TiB 2 , however, its irradiation behavior was seldomly studied, which is very important for the application of TiB 2 as neutron absorbers. To the best of our knowledge, only two studies reported the microstructural evolution of TiB 2 under irradiation conditions [37,38]. Therefore, a systematic study on the irradiation tolerance of TiB 2 is needed, which will be part of our future work.

Conclusion

To improve the performance of show that the interfaces of B 4 C and TiB 2 are effective trapping sites for He atoms and nucleation sites for He bubbles. Note that in a fast neutron reactor, the ballistic damage induced by energetic neutrons scattering and the nuclear reaction products (e.g., He, Li) scattering are much higher than the sole He ions implantation, this leads to a higher vacancy/He ratio and the possible consequences on the defects dynamics [28].

Overall, this study provides a synthesis method for fabricating neutron absorption materials for the application in nuclear reactors. The special property-optimized B 4 C-TiB 2 composites with designed microstructure exhibit a better resistance to disintegration under irradiation and in the thermodynamic environment of nuclear reactors. In addition, they have a great potential for the application as neutron-absorbing pellets in nuclear reactors.

In the future study, other issues will be addressed to have a comprehensive evaluation of B 4 C-TiB 2 composites under irradiation conditions. For example, helium behavior in TiB The interconnected interfaces between TiB 2 network (cages) and B 4 C grains act as effective sinks for the aggregation of He atoms.

2. 2 .

 2 Fig.1; the Bragg peak with a He concentration of 8.92×10 21 cm -3 (7.02%) is located at a depth of ~ 1.38 μm in the B 4 C material, and 7.18×10 21 cm -3 (6.14%) at a depth of ~ 1.56 μm in TiB 2 material, respectively.2.3. Characterization

Fig. 2 Fig. 5 .

 25 Fig. 5. The heating rate in the B 4 C-TiB 2 composite with higher TiB 2

TiB 2

 2 phases affect the crack propagation in B 4 C-TiB 2 composites: first, when the volume fraction of the TiB 2 phase is low, TiB 2 distributes in the B 4 C matrix as isolated particles (shown in Fig. 8(a)), and when the propagation of a crack tip encounters a TiB 2 particle, the crack moves along the B 4 C/TiB 2 interface, prolongs the crack propagation length and dissipates more energy, finally results in an improvement of the fracture toughness of the B 4 C-TiB 2 composites. Similar dissipation mechanisms at the B 4 C/TiB 2 interfaces are also found in Refs.[23,24]. Second, when the volume fraction of the TiB 2 phase is high, the TiB 2 phase forms an enclosed cage-like structure that encapsulates the B 4 C grains (Fig.8(b)). Thus, the cracks have to penetrate the TiB 2 walls before extending along the B 4 C/TiB 2 interfaces or must propagate through the neighboring B 4 C grains. According to the model of Cook et al. on crack propagation in brittle systems [25], additional energy is required to deflect the crack TEM observation.The TEM images in Fig.9show the He bubble distribution and morphology in B 4 C phase of samples S1, S2, and S3. The images were recorded with the same underfocus of 2.8 μm. The He bubbles, which are small bright dots in the images, are located within a band area. The width of He bubbles deposition band is smaller than that of the implanted band as calculated by SRIM, similar phenomenon is observed by Motte et al.[8]. It is suggested that this is due to the different depths of the He ions induced damage profile and the He ions implanted profile. In the area of the damage profile, lot of vacancies were introduced by energetic ions collision, while in the area of He atoms implanted band, He atoms greatly outnumbered the vacancies. In the overlapped areas (the width of which was smaller than the implanted band as calculated by SRIM), abundant vacancies and He atoms promoted the formation of He-vacancy clusters and increased the density of He bubble germs. After annealing, He-vacancy clusters evolved into visible He bubbles under TEM observation and formed a bubbles band in the overlapped area. Moreover, Gillet et al. found that the dissociation energy of He-vacancy is ~ 2 eV, the high energy barrier inhibited the He diffusion by the formation of He-vacancy clusters[27], and this explained that no He bubbles outside the band area can be found (excepted the He bubbles in the grain boundaries).The He bubbles in the bubbles band are small and dense; such feature has been argued to arise partially from the diffusion behavior of He atoms in B 4 C. According to Refs. [27-30], He atoms must overcome a high energy barrier to migrate even through the most probable migration paths owing to the intrinsic chemical bond and lattice structure of B 4 C. Schneider et al. discovered that He atoms prefer to migrate between <111>

B 4 C

 4 neutron absorption material in nuclear reactors, a set of property-optimized B 4 C-TiB 2 composites were designed and fabricated through a combination of the molten-salt and SPS methods. The TiB 2 phase distributes along the B 4 C grain boundaries and forms a connected network (cages), which encapsulates the B 4 C matrix phase. The thermal conductivity and fracture toughness of the B 4 C-TiB 2 composites are greatly improved owing to the special cage-like structure. The FE analyses show that the connected TiB 2 network promotes the heat transfer and improves effectively the overall thermal conductivity of the B 4 C-TiB 2 composite. The improved thermal conductivity results in a lower thermal stress, which mitigates thermal-stress-induced crack propagation. Vickers indentation tests were performed on the composites to study the crack propagation behavior. The TiB 2 network (cages) can effectively impinge/block the crack propagation, thereby improving the composite fracture toughness. This is important for maintaining the integration of B 4 C-TiB 2 composites if the composite serves as neutron absorption material in nuclear reactors. Moreover, He ion irradiation was used to investigate the He behavior in the composites and to simulate the situation in which a high quantity of He is produced by the B 10 (n, ) Li 7 nuclear reaction. The TEM results

  2 and irradiation resistant property of TiB 2 , effects of the special cage-like structure and TiB 2 content on He diffusion and release during annealing, other synthesis routes to optimize the structure and properties of the B 4 C-TiB 2 composites, influence of He bubbles aggregation along the B 4 C-TiB 2 and TiB 2 -TiB 2 interface on the thermal conductivity of the composites and the crack propagation behavior, etc. 26-39.

Fig. 1 .

 1 Fig. 1. Displacement damage and He concentration as a function of

Fig. 2 .

 2 Fig. 2. SEM images of surface morphology of (a) 10.5 Vol.% TiB 2

Fig. 3 .Fig. 4 .

 34 Fig. 3. Geometric construction and boundary conditions of FE models: (a)

Fig. 5 .Fig. 6 .Fig. 7 .

 567 Fig. 5. Temperature variation at points P i (center of the right-end

Fig. 8 .

 8 Fig. 8. SEM images of Vickers indentation-induced crack propagation;

Fig. 9 .

 9 Fig. 9. TEM images of He bubble morphology in He deposition band in

Fig. 10 .

 10 Fig. 10. TEM image of He bubble morphology in He deposition band and

Fig. 1 .

 1 Fig. 1. Displacement damage and He concentration as a function of

Fig. 2 .

 2 Fig. 2. SEM images of surface morphology of (a) 10.5 Vol.% TiB 2

Fig. 3 .Fig. 4 .

 34 Fig. 3. Geometric construction and boundary conditions of FE models: (a)

Fig. 5 .

 5 Fig.5. Temperature variation at points P i (center of the right-end boundary

Fig. 6 .

 6 Fig. 6. Typical Vickers indentation impressions on (a) 10.5 Vol.% TiB 2

Fig. 7 .

 7 Fig. 7. Statistical results of Vickers indentation-induced crack length for

Fig. 8 .

 8 Fig. 8. SEM images of Vickers indentation-induced crack propagation;

Fig. 9 .

 9 Fig. 9. TEM images of He bubble morphology in He deposition band in

Fig. 10 .

 10 Fig. 10. TEM image of He bubble morphology in He deposition band and

  TiB 2 , it is therefore selected as the cage-like structure of the B 4 C-TiB 2 composites. The good features of conventional B 4 C material are maintained, and the thermal conductivity of the composite is significantly improved with respect to that of the pure B 4 C material. In addition, the composite exhibits many interfaces.

	uniform TiB 2 coating occurred on the B 4 C particles. The remnant NaCl, KCl, and other impurities were washed away with deionized water, and = = . (3)
	the samples were subsequently dried to obtain dry TiB 2 -coated B 4 C
	Where	and	are the volume of TiB 2 phases and the B 4 C-TiB 2
	powders. Note that the melt point of NaCl and KCl mixing is low (~ 933
	composites, respectively.	is the weight percent of TiB 2 measured
	K) and the liquid mass transfer mode between B 4 C and Ti in the molten
	by using XRD, the calculated weight percent of TiB 2 for S1, S2 and S3
	salts guarantees the uniform nucleation and growth of TiB 2 on the surface are 18 %, 26% and 44%, respectively. (4.5 g/cm 3 ) is the density of
	of B 4 C particles. Therefore, the NaCl and KCl were employed as the
	TiB 2 and	is the density of the B 4 C-TiB 2 composites (2.637, 2.796 and
	medium for reactions. 3.051 g/cm 3 for S1, S2 and S3, respectively). The density of the B 4 C-TiB 2
			3Ti + B 4 C TiC + 2TiB 2	(1)
	composites were measured by and Archimedes method. The chemical
			2TiC + B 4 C 2TiB 2 +3C	(2)
	composition and the calculated volume fraction of each sample are shown
	In the second step, the mixtures containing uniformly TiB 2 -coated B 4 C
	in Table 1.
	powders with different TiB 2 contents were sintered in a spark plasma
	sintering (SPS) system (SPS-20T-15, Chen Hua) with the same procedure
	to obtain three cylindrical bulk samples with diameters of 13 mm and
	heights of 2 mm (denoted as S1, S2, and S3). In the SPS procedure, the
	2. Experimental details heating rates are 50 K/min from RT to 1673 K, and 25 K/min from 1673
	2.1. Material synthesis K to 1973 K, respectively. The holding time and temperature are 10 min
	A two-step synthesis method was employed to synthesize the B 4 C-and 1973 K, respectively. The pressure during the holding stage is 45
	TiB 2 composite [19]. In the first step, Ti and B 4 C powders were mixed MPa. The cooling rate is 50 K/min from the holding temperature to RT.
	with NaCl and KCl in an Al 2 O 3 crucible. The mixtures with initial B 4 C/Ti Each bulk sample has a relative density of 100%. The volume fractions of
	molar ratios of 6:1, 4:1, and 2:1 were exposed to an Ar atmosphere at TiB 2 phases in B 4 C-TiB 2 composites are determined by the equation (3).
	1273 K. After the (1) and (2) in-situ reactions in the melted salt bath, a

The B 4 C-TiB 2 composites were subjected to He ion irradiation to simulate the situation in which the B 4 C-TiB 2 composite is exposed to a high neutron flux, and a high quantity of He atoms is produced by the (n, α) reaction. Moreover, a series of post-irradiation analyses and FE calculations were conducted to evaluate the behavior of the B 4 C-TiB 2 component in a nuclear reactor environment.

Table 1 .

 1 Molar ratio of raw B 4 C powder and Ti powder for molten-salt reaction, volume fraction of TiB 2 , weight percent of TiB 2 , B 4 C and C in synthesized bulks.

	Samples	Molar ratio (B 4 C:Ti)	Volume fraction of TiB 2	Weight percent of TiB 2	Weight percent of B 4 C	Weight percent of C
	S1	6:1	10.5%	18.0%	80.4%	1.6%
	S2	4:1	16.2%	26.0%	72.0%	2.0%
	S3	2:1	29.8%	44.0%	52.0%	4.0%

Table 2 .

 2 Material properties of B 4 C and TiB 2 used in heat transfer

			analysis [16, 19]
		Conductivity (W•m -1 •K -1 )	Specific Heat (J•kg -1 •K -1 )	Density (kg/m 3 )
	Temperature (K)	773	1273	773	1273	773	1273
	B 4 C	20.00	11.00	1680.80	2072.68	2550 2550
	TiB 2	81.00	78.10	1073.00	1186.00	4450 4390

Table 3 .

 3 Comparison of the results calculated from real composite structure models (CM), equivalent homogeneous models (HM) and the relative error (RE).

	Time		S1			S2			S3
	(+,)	CM(K) HM(K) RE(%) CM(K) HM(K) RE(%) CM(K) HM(K) RE(%)
	0.2	819	836	2.1	832	856	2.9	867	901	3.9

Acknowledgements

This research was supported by the National Natural Science Foundation of China (No. 11902129, 11805245), Strategic Priority Research Program of Chinese Academy of Sciences (No. XDA21010202), Natural Science Fund of Zhejiang Province (No. LQ20A050001), Natural Science Fund of Ningbo City (No. 2019A610183), and China

Data availability

The raw data required to reproduce these findings cannot be shared at this time as the data is part of an ongoing study. The processed data required to reproduce these findings cannot be shared at this time as the data is part of an ongoing study.

propagation and initiate secondary crack propagation across the interface; the primary crack requires much energy.

To quantitatively measure the fracture toughness of the B 4 C-TiB 2 composites, following equations are used [START_REF] Anstis | [END_REF]:

Declaration of interests

☑ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: