Pierre Thuéry

Jack Harrowfield

Ni(2,2ʹ:6ʹ,2-terpyridine-4ʹ-carboxylate)2 Zwitterions and Carboxylate Polyanions in Mixed-Ligand Uranyl Ion Complexes with a Wide Range of Topologies

    Ni(2,2':6',2-Terpyridine-4'-carboxylate) 2 zwitterions and carboxylate polyanions in mixed-ligand uranyl ion complexes with a wide range of topologies Pierre Thuéry, Jack Harrowfield

INTRODUCTION

While mixed-ligand complexes of the uranyl ion UO2 2+ are abundant, in the majority of cases, as well exemplified with uranyl carboxylates, 14 they involve the association of a multidentate chelating or bridging ligand with rather simple anionic species such as oxide, hydroxide, fluoride, nitrate, formate or oxalate often arising inadvertently rather than as an intended component of the product. Simple neutral species such as H2O, N,N-dimethylformamide (DMF), dimethylsulfoxide (DMSO), N-methyl-2-pyrrolidone (NMP) and N,Ndimethylpropyleneurea (DMPU) are known co-ligands derived from the solvents used for synthesis and neutral chelates such as 1,10-phenanthroline, 2,2ʹ-bipyridine and 2,2ʹ;6ʹ,2terpyridine have, in a significant number of instances, 2,58 been used to block 2 or 3 equatorial coordination sites of the uranyl cation. None of these neutral species (4,4ʹ-bipyridine dioxide providing an exception [START_REF] Chorazy | Multi-Colour Uranyl Emission Efficiently Tuned by Hexacyanidometallates within Hybrid Coordination Frameworks[END_REF] ), however, is known to have a significant capacity to bridge uranyl centres and thus to facilitate polymerization in the same way as anionic ligands such as polycarboxylates. Mixed-ligand polycarboxylate complexes are an intriguing prospect for the development of new coordination polymers with uranyl ion but there are only rare instances where use of a mixture of dicarboxylate ligands does provide a coordination polymer containing both, with some examples involving stereoisomeric species. [START_REF] Thuéry | Uranyl-Organic Coordination Polymers with trans-1,2-, trans-1,4-and cis-1,4-Cyclohexanedicarboxylates: Effects of Bulky PPh4 + and PPh3Me + Counterions[END_REF][START_REF] Thuéry | Uranyl Ion-Containing Polymeric Assemblies with cis/trans Isomers of 1,2-, 1,3-, and 1,4-Cyclohexanedicarboxylates, Including a Helical Chain and a 6-Fold-Interpenetrated Framework[END_REF] Nevertheless, what is wellknown is that the neutral but zwitterionic ligand betaine (N,N,N-trimethylglycine) displays in the crystal structures of its uranyl ion complexes 1215 all the characteristics of an anionic carboxylate, including the ability to bridge uranyl centres in a  1 O 1 O' manner. A variety of doubly-and triply-zwitterionic organic carboxylate ligands has also been shown to provide coordination polymers or frameworks with various metal ions other than uranyl where the ligand mimics the behaviour of analogous di-or tricarboxylate anions. 1622 A particularly simple means of obtaining a dicarboxylato zwitterion is through the "expanded ligand" approach [START_REF] Constable | Expanded Ligands: bis(2,2ʹ:6ʹ,2-terpyridine carboxylic acid)ruthenium(II) Complexes as Metallosupramolecular Analogues of Dicarboxylic Acids[END_REF][START_REF] Constable | Expanded Ligands -An assembly Principle for Supramolecular Chemistry[END_REF] involving the formation of M II L2 complexes where M is a transition metal and L is a ditopic ligand such as 2,2ʹ;6ʹ,2-terpyridine-4ʹ-carboxylate (tpyc  ), which preferentially coordinates through the terpyridine unit. This is but one aspect of metalloligand chemistry, [START_REF] Garibay | The Use of Metalloligands in Metal-Organic Frameworks[END_REF][START_REF] Srivastava | Metalloligands to Material: Design Strategies and Network Topologies[END_REF] which has been exploited for a considerable variety of carboxylato-substituted species (e.g. cationic [START_REF] Donnelly | Chirality in Coordination Polymers: Homo-vs Hetero-chiral Strand Construction[END_REF] and anionic 2830 ) and which is recognized as a useful means of obtaining heterometallic coordination polymers and frameworks with varied functionality. [START_REF] Garibay | The Use of Metalloligands in Metal-Organic Frameworks[END_REF][START_REF] Srivastava | Metalloligands to Material: Design Strategies and Network Topologies[END_REF][START_REF] Costes | Metalloligands for Designing Single-Molecule and Single-Chain Magnets[END_REF] As a charge-neutral species, any zwitterionic ligand must be accompanied by anions in its crystalline complexes with metal cations, giving rise to the possibility that polymerization due to binding of a multiple carboxylato-zwitterion could be reinforced or usefully modified by binding of a polycarboxylate anion. As part of our continuing exploration of the utility of tpyc  in uranyl ion coordination chemistry, [START_REF] Thuéry | Uranyl Ion Complexes with 2,2ʹ:6ʹ,2-Terpyridine-4ʹcarboxylate. Interpenetration of Networks Involving "Expanded Ligands[END_REF][START_REF] Thuéry | Varying Structure-Directing Anions in Uranyl Ion Complexes with Ni(2,2ʹ:6ʹ,2-terpyridine-4ʹ-carboxylate)2[END_REF] this has been found in the monoperiodic mixed ligand complex of uranyl ion formed with Ni(tpyc)2 and trans-1,4-cyclohexane dicarboxylate (t-1,4chdc 2 ), [START_REF] Thuéry | Varying Structure-Directing Anions in Uranyl Ion Complexes with Ni(2,2ʹ:6ʹ,2-terpyridine-4ʹ-carboxylate)2[END_REF] a result complementing the preceding characterization of the uranyl ion complex of Ni(cptpy)2 (cptpy = 4ʹ-(4ʹ-carboxylatophenyl)-2,2ʹ;6ʹ,2-terpyridine) and 4,4ʹ-biphenyl dicarboxylate, [START_REF] Kong | Construction of Hybrid Bimetallic Uranyl Compounds Based on a Preassembled Terpyridine Metalloligand[END_REF] where the greater size of both the zwitterion and the dicarboxylate species leads to a diperiodic coordination polymer with hcb topology exhibiting 2D + 2D  3D polycatenation and remarkable catalytic properties. In the present work, we have focussed upon variation of the polycarboxylate accompanying the Ni(tpyc)2 zwitterion in their uranyl ion complexes, and we have isolated a series of 14 mixed-ligand complexes which have been crystallographically characterized. It appears that under the solvo-hydrothermal conditions of crystallization employed, di-and tri-carboxylate entities are the favoured companions of the zwitterion, leading to complexes covering all the periodicity range, from polynuclear, discrete species to a three-periodic framework, and displaying a wide array of topologies, with the "terpyridine embrace" (TE) [START_REF] Scudder | Crystal Supramolecular Motifs: Two-Dimensional Grids of terpy Embraces in [ML2] z Complexes (L = terpy or Aromatic N3-Tridentate Ligand)[END_REF][START_REF] Mcmurtrie | Engineering Grids of Metal Complexes: Development of the 2D M(terpy)2 Embrace Motif in Crystals[END_REF] involving both "face-to-face" and "edge-to-face" contacts of the aromatic units of the zwitterion units being a prominent feature of most of the structures.

EXPERIMENTAL SECTION

Syntheses. Caution! Uranium is a radioactive and chemically toxic element, and uranium-containing samples must be handled with suitable care and protection. Small quantities of reagents and solvents were employed to minimize any potential hazards arising both from the presence of uranium and the use of pressurized vessels for the syntheses.

[UO2(NO3)2(H2O)2]•4H2O (RP Normapur, 99%) and Ni(NO3)2•6H2O were purchased from Prolabo, and 2,2ʹ:6ʹ,2-terpyridine-4ʹ-carboxylic acid was from Alfa-Aesar. The carboxylic acids were from Aldrich, Fluka, or Alfa-Aesar, except for rac-trans-1,2cyclohexanedicarboxylic acid which was from Lancaster and for which the (1R,2R) enantiomer was isolated through crystallization with (R)-1-phenylethylamine as a resolving agent, as in the literature. [START_REF] Kimura | Crystal Structure of (1R,2R)-trans-1,2-Cyclohexanedicarboxylic Acid-(R)-1-Phenylethylamine Salt[END_REF] For all syntheses, a mixture of [UO2(NO3)2(H2O)2]•4H2O (25 mg, 0.05 mmol), Ni(NO3)2•6H2O (10 mg, 0.03 mmol), tpycH (14 mg, 0.05 mmol), and the additional carboxylic acid (0.05 mmol) were dissolved in a mixture of water (0.6 mL) and acetonitrile (0.2 mL). The solutions were placed in 10 mL tightly closed glass vessels (Pyrex ® culture tubes with SVL15 stoppers and teflon seals, provided by VWR) and heated at 140 °C in a sand bath. The crystals, of orange-yellow color, were grown in the hot, pressurized solutions (and not as a result of a final return to ambient conditions) and they were recovered after cooling down. A summary of the carboxylic acids used, represented in Scheme 1, and heating durations is given in Table 1.

Elemental analyses were performed by MEDAC Ltd., and the results are given in Table 2, together with the yields. Only a few crystals were obtained for compounds 3, 6 and 11, so that no elemental analysis could be performed. Several compounds contain solvent molecules which were not found in the crystal structures but whose presence is in agreement with the existence of voids in the structures (see below), while in the case of compounds 1 and 5, the acetonitrile molecules found from structure determination appear to have been lost during filtering and drying of the sample. In the case of compound 12, the number of extra water molecules may seem large, but it is in good agreement with the electron count for disordered solvent molecules determined from the crystal structure (316 electrons per formula unit). The solvent molecules indicated in Table 2 are in addition (or subtraction) to those indicated in the formulas. Scheme 1. The Polycarboxylic Acids Used as Coligands.

Table 1. Summary of the Synthesis Conditions

Compound

Additional carboxylic acid Duration Crystallography. The data were collected at 100(2) K on a Bruker D8 Quest diffractometer equipped with an Incoatec microfocus source (IS 3.0 Mo,  = 0.71073 Å) and a PHOTON III area detector, and operated through the APEX3 software. [START_REF]APEX3 Crystallography Software Suite, Ver[END_REF] The crystals were mounted on Mitegen micromounts with a protective coating of Paratone-N oil (Hampton Research). The data were processed with SAINT [START_REF] Saint | Ver. 8.40A; Bruker Nano[END_REF] and absorption corrections, either empirical (multi-scan) or based on crystal shape, were made with SADABS. [START_REF]SADABS, Bruker/Siemens Area Detector Absorption and Other Corrections, Ver. 2016/2[END_REF][START_REF] Krause | Comparison of Silver and Molybdenum Microfocus X-Ray Sources for Single-Crystal Structure Determination[END_REF] All structures were solved by intrinsic phasing with SHELXT, [START_REF] Sheldrick | SHELXT -Integrated Space-Group and Crystal-Structure Determination[END_REF] and refined by full-matrix least-squares on F 2 with SHELXL, [START_REF] Sheldrick | Crystal Structure Refinement with SHELXL[END_REF] using the ShelXle interface. [START_REF] Hübschle | ShelXle: a Qt Graphical User Interface for SHELXL[END_REF] All non-hydrogen atoms were refined with anisotropic displacement parameters. When possible, the hydrogen atoms bound to oxygen atoms were retrieved from residual electron density maps, and they were either refined with restraints or treated as riding atoms. The carbon-bound hydrogen atoms were introduced at calculated positions and were treated as riding atoms with an isotropic displacement parameter equal to 1.2 times that of the parent atom (1.5 for CH3, with optimized geometry). Some voids in the structures of compounds 1-3, 5, 6, and 8-14 contain disordered solvent molecules (water or acetonitrile, as indicated also by elemental analysis results) which could not be modelled satisfactorily, and the SQUEEZE software [START_REF] Spek | SQUEEZE: a Tool for the Calculation of the Disordered Solvent Contribution to the Calculated Structure Factors[END_REF] was used to subtract their contribution to the structure factors. Crystal data and structure refinement parameters are given in Table 3. The molecular plots were drawn with ORTEP-3, 46,[START_REF] Farrugia | WinGX and ORTEP for Windows: an Update[END_REF] and the polyhedral representations with VESTA. [START_REF] Momma | VESTA 3 for Three-Dimensional Visualization of Crystal, Volumetric and Morphology Data[END_REF] The topological analyses and nodal representations were made with ToposPro. [START_REF] Blatov | Applied Topological Analysis of Crystal Structures with the Program Package ToposPro[END_REF] Special details for some of the structures are as follows.

[(UO2)2(c-1,2-chdc)Ni(tpyc)2(NO3)2]24CH3CN (1) cis-1,2-cyclohexanedicarboxylic acid (c-1,2-chdcH2) w [(UO2)2(tdc)Ni(tpyc)2(NO3)2]2 (2) 2,5-thiophenedicarboxylic acid (tdcH2) d [(UO2)4(bdc)3Ni2(tpyc)4(NO3)2]2CH3CN2H2O (3) 1,4-benzenedicarboxylic acid (bdcH2) d [(UO2)2(tca)Ni(tpyc)2(NO3)]2CH3CN2H2O (4) tricarballylic acid (tcaH3) d [(UO2)2(1,2-pda)(1,2-pdaH)Ni(tpyc)2(NO3)]CH3CN (5) 1,2-phenylenediacetic acid (1,2-pdaH2) d [(UO2)2(ox)2Ni(tpyc)2] (6) oxalic acid (oxH2), generated in situ from malic acid d [Ni(tpycH)(H2O)3][UO2(1,2,3-btc)]22H2O (7) 1,2,3-benzenetricarboxylic acid (1,2,3-btcH3) w [UO2Ni2(tpyc)4][UO2(citH)]2 (8) citric acid (citH4) w [(UO2)2(btcH)Ni(tpyc)2(NO3)] (9) 1,2,3,4-butanetetracarboxylic acid (btcH4) d [(UO2)2(cbtcH)Ni(tpyc)2(NO3)] (10) cis,trans,cis-1,2,3,4-cyclobutanetetracarboxylic acid (cbtcH4) d [(UO2)2(ndc)Ni(tpyc)2(OH)(NO3)] (11) 2,6-naphthalenedicarboxylic acid (ndcH2) w [(UO2)4(t-R-1,2-chdc)4Ni2(tpyc)4] (12) trans-R,R-1,2-cyclohexanedicarboxylic acid (t-R-1,2-chdcH2) w [(UO2)4(1,3-pda)4Ni2(tpyc)4]CH3CN2H2O (13) 1,3-phenylenediacetic acid (1,3-pdaH2) w [(UO2)2(nta)2Ni2(tpyc)2] (14) nitrilotriacetic acid (ntaH3) w
Compound 8. The uranyl cation containing U2 is disordered over two very close positions related by an inversion centre. One complete cation has been refined with an occupancy parameter of 0.5. The associated disorder affecting the carboxylate groups bound to this cation has only been partially resolved; two positions have been found for O12, O13 and the attached atom C22, which were refined with occupancy parameters constrained to sum to unity, but notwithstanding the large and very anisotropic displacement parameters of O14, O15, and some of the carbon atoms of the aromatic ring bearing O12 and O13, the modelling of the disorder could not be extended further satisfactorily.

Compound 10. The uncoordinated carboxylic group is rotationally disordered over two positions which were refined with occupancy parameters constrained to sum to unity and restraints on displacement parameters and one bond length, and the associated hydrogen atoms were not found.

Compound 12. One cyclohexanedicarboxylate ligand is disordered over two positions with one common oxygen atom, which have been refined with occupancy parameters constrained to sum to unity. Other parts of the structure are seemingly affected by unresolved disorder, particularly those bound to this ligand. Restraints on some bond lengths and displacement parameters have been applied for the disordered atoms and other parts of the structure. The value of the refined Flack parameter is 0.006 (5).

Compound 13. Large residual electron density peaks are located close to atoms U2 and U4 (at less than 1 Å) in all the six experiments made with six different crystals of different sizes, and they remain whatever absorption correction method is applied. These peaks have been interpreted as due to minor components of disordered uranium atoms (U2A and U4A), the corresponding disorder on oxo atoms and ligands being unresolved but suggested by the rather large and anisotropic displacement parameters of many atoms. Restraints on several bond lengths and displacement parameters had to be applied. Checkcif suggests the possible space group Pca21, but, in addition to the large Rint value in the orthorhombic system, no satisfying refinement could be obtained in this space group, and the structure was refined as a 2component inversion twin in space group Pc, with a Flack parameter 0.422 (13).

RESULTS AND DISCUSSION

Prior to a description of each structure, it may be noted that, in all 14 compounds, the uranium coordination environment does not deviate significantly from its usual features. Uranium is either seven-or eight-coordinated, the coordination geometry being thus pentagonal-or hexagonal-bipyramidal in complexes 68, 14 and 15, 9, 12, 13, respectively, with two instances (10 and 11) are in the usual slightly distorted octahedral environment which will not be further described.

The complexes will be discussed in the order of increasing periodicity.

It is also notable that in no case is either of the hydrolysis products from the acetonitrile cosolvent (NH4 + and CH3CO2 -) found as part of the isolated crystals, quite unlike numerous systems where the use of DMF as the co-solvent results rather frequently in the incorporation of (CH3)2NH2 + and/or HCO2 -. Such hydrolytic "interference" in the syntheses of uranyl ion coordination polymers can be avoided by the use of NMP but an advantage in the use of solvents such as acetonitrile and DMF may well be that their hydrolysis results in buffering of the reaction medium at a higher pH than its initial value.

Polynuclear ring-shaped complexes. Two of the complexes in this series crystallize as discrete polynuclear complexes with analogous geometries,

[(UO2)2(c-1,2- chdc)Ni(tpyc)2(NO3)2]24CH3CN (1) and [(UO2)2(tdc)Ni(tpyc)2(NO3)2]2 (2)
, which include cis-1,2-cyclohexanedicarboxylate and 2,5-thiophenedicarboxylate, respectively. In the previously [START_REF] Thuéry | Varying Structure-Directing Anions in Uranyl Ion Complexes with Ni(2,2ʹ:6ʹ,2-terpyridine-4ʹ-carboxylate)2[END_REF] involving trans-1,4-cyclohexanedicarboxylate, the positional isomer of c-1,2-chdc 2-found in complex 1, the dicarboxylate and zwitterion units are present in perhaps the simplest form of coordination polymer that might be anticipated in that they act alternately as bridges between uranyl ions in a monoperiodic chain. Chelating nitrate ligands on each U VI centre appear to block dicarboxylate or zwitterion coordination which could produce extension into a second direction.

reported structure of [(UO2)2(t-1,4-chdc)Ni(tpyc)2(NO3)2](t-1,4-chdcH2)2CH3CN,
One or the other of these features is seen in most of the presently studied structures, though none shows the feature of the presence of the uncoordinated parent acid of the dicarboxylate acting as an hydrogen bonding bridge between polymer strands. Replacement of the divergent t-1,4-chdc 2-by the convergent and similarly bis- 2 O,Oʹ-chelated c-1,2-chdc 2-ligand has a conspicuous effect on the structure since, in complex

1, two curved (UO2)2(c-1,2-chdc) 2+
strands are connected by linear Ni(tpyc)2 units to form a centrosymmetric, hexanuclear metallacyclic assembly, with four chelating nitrate anions completing the coordination spheres of the four uranium atoms and preventing further polymerization (Figure 1). While c-1,2-chdc 2- can adopt a chiral conformation, the two units in each macrocycle are enantiomeric. The dinickelatetrauranacycle (denoted U4Ni2 ring in the following) is 42-membered and it has an elongated shape with the terpyridine (terpy) motifs protruding inside, so that the internal space is separated into two small apertures (Figure 1b). The rings stack into columns parallel to [100] (Figure 1c), the packing containing some free spaces (see Experimental Section With tdc 2-, a ligand of rather limited flexibility and a bent form with regard to its donor group orientations, complex 2 has a structure very similar to that of complex 1, apart from the absence of any well-ordered solvent. Again, the complex unit present, shown in Figure 2, is a [START_REF] Thuéry | Closed Uranyl-Dicarboxylate Oligomers: a Tetranuclear Metallatricycle with Uranyl Bridgeheads and 1,3-Adamantanediacetate Linkers[END_REF] or iminodiacetate, [START_REF] Unruh | Development of Metal-Organic Nanotubes Exhibiting Low-Temperature, Reversible Exchange of Confined "Ice Channels[END_REF] with further organization into nanotubules in the latter case. As shown in Figure 3, complex 2 appears as an expanded version of complex 1, the distances between the inequivalent U VI centres bridged by c-1,2chdc 2-or tdc since the cyclohexyl groups in 1 protrude on either side of the complex. The intracyclic spaces are slightly wider in 2 and, possibly as a consequence, the solvent molecules are heavily disordered in this case, while well-ordered acetonitrile molecules are located close to the cavities in 1. In both cases, the packing of the rings generates tube-like architectures, with however only two extremely narrow channels running inside. uranium atoms are tris- 2 O,Oʹ-chelated, U1 by one carboxylate group from tpyc -, one from tca 3-, and one nitrate anion, and U2 by one carboxylate from tpyc -and two from two tca 3- anions, U2 and tca 3-being thus three-coordinated (3-c) nodes. Hexanuclear rings are found here also, but they are 43-membered due to the different lengths of the carboxylate arms of the tca 3- ligand involved (Figure 5b). Each tricarboxylate pertains to two such rings, as seen in Figure 5e, and the concatenation of these rings gives rise to the formation of a nanotubular polymer phosphonates. [START_REF] Poojary | Structure Determination of a Complex Tubular Uranyl Phenylphosphonate, (UO2)3(HO3PC6H5)2(O3PC6H5)2H2O, from Conventional X-ray Powder Diffraction Data[END_REF][START_REF] Grohol | Alkali-Ion-Catalyzed Transformation of Two Linear Uranyl Phosphonates into a Tubular One[END_REF][START_REF] Adelani | Metal-Controlled Assembly of Uranyl Diphosphonates toward the Design of Functional Uranyl Nanotubules[END_REF][START_REF] Adelani | Incorporation of Cu 2+ Ions into Nanotubular Uranyl Diphosphonates[END_REF] Although nanotubular complexes involving only tricarballylate ligands are known, [START_REF] Thuéry | Variations on the Honeycomb Topology: from Triangular-and Square-Grooved Networks to Tubular Assemblies in Uranyl Tricarballylate Complexes[END_REF][START_REF] Thuéry | Uranyl Tricarballylate Triperiodic and Nanotubular Species. Counterion Control of Nanotube Diameter[END_REF] which show the potential of this three-pronged and curved ligand to build closed species, the shape of complex 4 is more reminiscent of the nanotubes obtained with 1,3-and 1,4-phenylenediacetates, [START_REF] Thuéry | Tubelike Uranyl-Phenylenediacetate Assemblies from Screening of Ligand Isomers and Structure-Directing Counterions[END_REF] in which the ligands adopt either a divergent, trans conformation and thus play a role analogous to Ni(tpyc)2 here, or a convergent cis conformation providing the required curvature, as tca 3-here (notwithstanding a different denticity). It is also notable that the topology of these two species is the same, both being 3-c, uninodal nets with the point symbol {4 2 .6}, but while 4 contains uranium and tca 3- Here also, some voids in the structure contain disordered solvent molecules (KPI, 0.68).

Complex 6, [(UO2)2(ox)2Ni(tpyc)2], was obtained from malic acid, oxalate (ox 2-) being generated in situ, a frequent occurrence when solvo-or hydrothermal synthetic procedures are used, the formation mechanism involved having been elucidated in some particular cases only. [START_REF] Andrews | In Situ Oxalate Formation During Hydrothermal Synthesis of Uranyl Hybrid Materials[END_REF][START_REF] Knope | Investigation of in Situ Oxalate Formation from 2,3-Pyrazinedicarboxylate under Hydrothermal Conditions Using Nuclear Magnetic Resonance Spectroscopy[END_REF] The absence of any nitrate and the presence of the particularly rigid and strongly coordinating oxalate ligand are notable features of the structure of complex 6. The unique uranyl cation is chelated by two oxalate ligands, one of them centrosymmetric and the other having twofold rotation symmetry, and it is also bound to one carboxylate oxygen atom from tpyc -(Figure 7). The Ni(tpyc)2 moiety has also twofold rotation symmetry. Bridging of uranyl centres through the formation of two 5-membered chelate rings, leaving one coordination site available for an additional donor, is usual with the oxalate ligand, [START_REF] Giesting | A Series of Sheet-Structured Alkali Metal Uranyl Oxalate Hydrates: Structures and IR Spectra[END_REF][START_REF] Thuéry | Uranyl-Copper(II) Heterometallic Oxalate Complexes: Coordination Polymers and Frameworks[END_REF][START_REF] Zou | The Influence of Piperazine Diamine Templates on the Synthesis, Structures and Properties of Uranyl Oxalate Complex[END_REF][START_REF] Meng | The Crystal Structure of Catena-poly[(1-(4-carboxybenzyl)pyridin-1-ium-4-carboxylato-κ 1 O)-(μ2-oxalatoκ 4 O:O′:O″:O‴)dioxidouranium(VI)], C16H11NO10[END_REF] and it leads here to the formation of a serpentine monoperiodic polymer running along [100] where the Ni(tpyc)2 units act as bridges between uranium centres separated by two others, thus filling the undulations of the polymer on both sides and defining U4Ni rings. The presence of the two oxalate chelate rings limits further coordination to that of a single donor from Ni(tpyc)2. This bis( 1 O) coordination mode of Ni(tpyc)2 has been frequently observed [START_REF] Thuéry | Uranyl Ion Complexes with 2,2ʹ:6ʹ,2-Terpyridine-4ʹcarboxylate. Interpenetration of Networks Involving "Expanded Ligands[END_REF][START_REF] Thuéry | Varying Structure-Directing Anions in Uranyl Ion Complexes with Ni(2,2ʹ:6ʹ,2-terpyridine-4ʹ-carboxylate)2[END_REF] (an analogous behaviour is known for the Ni(cptpy)2 species [START_REF] Kong | Construction of Hybrid Bimetallic Uranyl Compounds Based on a Preassembled Terpyridine Metalloligand[END_REF] ) but this is the first occurrence in the present series. The terpy moieties do not interact with one another within the polymer chain but the packing (KPI, Diperiodic coordination polymers. The presence of the citrate ligand Hcit 3-, in which the hydroxyl group has been deprotonated but one carboxylate group retains a proton, has consequences somewhat similar to that of 123btc 3-in that an anionic uranyl citrate polymer is associated with a cationic polymer involving Ni(tpyc)2 only as ligand, giving the complex [UO2Ni2(tpyc)4][UO2(citH)]2 (8) shown in Figure 9. Again, the monoperiodic polymer chains, directed along [100], are of a familiar form, closely related to that of those known in various other uranyl citrate complexes. [START_REF] Thuéry | Uranyl Ion Complexation by Citric and Citramalic Acids in the Presence of Diamines[END_REF][START_REF] Thuéry | Uranyl Ion Complexes with Malic, Citramalic, Citric and Tricarballylic Acids: Influence of Coligands and Additional Metal Cations[END_REF] The uranium atom bound to Ni(tpyc)2 is disordered and its environment somewhat uncertain, but it appears to be chelated by one carboxylate group of tpyc -and bound to three more carboxylate donors from three other ligands, and it is thus a 4-c node in the diperiodic cationic polymer formed, which is parallel to (100). This network has the point symbol {4 4 .6 2 } and the common sql topological type, as previously found in the complex [UO2Ni2(tpyc)4](I3)2. [START_REF] Thuéry | Varying Structure-Directing Anions in Uranyl Ion Complexes with Ni(2,2ʹ:6ʹ,2-terpyridine-4ʹ-carboxylate)2[END_REF] Due to the protruding terpyridine moieties, the size of the free space within the rings is however small, at 8  15 Å 2 , the elongated rings being arranged in herringbone fashion. The novelty of complex 8 is that it displays 1D + 2D semi-interpenetration (i.e. the two components can in principle be separated without the breakage of bonds) of the anionic polymers through the packed diperiodic networks (Figures 9b and9c). The uranyl citrate ribbons in adjacent rows along [010] are tilted with respect to one another so as to accommodate the elongated shape of the spaces in the herringbone arrangement. The two components are tethered to each other through hydrogen bonding of the carboxylic group of In passing to 1,2,3,4-butanetetracarboxylate (btc 4-), the complex formed, [(UO2)2(btcH)Ni(tpyc)2(NO3)] ( 9), contains this ligand in its monoprotonated form and shows a return to the involvement of nitrate coordination (Figure 10). One of the two independent uranium atoms is chelated by one nitrate anion and two carboxylate groups from the two different ligands, and the other is chelated by two groups from two btcH 3-ligands and one from tpyc -, a situation analogous to that found in complexes 3-5. Atom U2 and the btcH 3-ligand are thus 3-c nodes in the diperiodic network formed, which is parallel to (100) and has the {6 In the structure of the complex [(UO2)2(ndc)Ni(tpyc)2(OH)(NO3)] ( 11) obtained with the large, rigid aromatic 2,6-naphthalenedicarboxylate (ndc 2-), features seen in the previous complexes are augmented by the presence of bridging hydroxido ligands leading to the formation of dinuclear secondary building units (SBUs), which coexist with isolated uranyl moities. Both independent uranium atoms are chelated by one carboxylate group from one of the two centrosymmetric ndc 2-ligands, but U1 is further chelated by one carboxylate group from tpyc -and one nitrate anion, while U2 is bound to one monodentate carboxylate from tpyc - and two hydroxido anions, 7-and 8-coordination thus coexisting in the structure (Figure 12). If the dinuclear SBUs are considered as single, 4-c nodes, the diperiodic network formed, parallel to (10ī), has the sql topological type, with very elongated cells (50 Å between outermost SBUs). The cyclic units are 100-membered tetranickeladecauranacycles. Equally elongated decauranacycles (53 Å in length) have been found in a complex with sebacic acid, 7 but, to the best of our knowledge, cycles containing as many as 14 metal cations do not appear to have 12), has some points in common with complex 1 involving c-1,2-chdc 2-, but the stoichiometry is different and the absence of terminal nitrate anions leads to an increased periodicity. Complex 12 crystallizes in the Sohncke group C2221, and the four independent uranyl cations are all in similar environments, being  2 OOʹ-chelated by two carboxylate groups from t-R-1,2-chdc 2-ligands and one from tpyc -(Figure 13). Hexanuclear, 42-membered rings analogous to those in 1 are The complex [(UO2)4(1,3-pda)4Ni2(tpyc)4]CH3CN2H2O (13) involves 1,3phenylenediacetate (1,3-pda 2-), an isomer of 1,2-pda 2-giving the monoperiodic complex 5, of quite different composition; nitrate is no longer present here and the structure is considerably more complicated than that of 5. Complex 13 crystallizes in the Sohncke group Pc with four atoms are thus 3-c nodes while all the other components are simple links. The uninodal coordination polymer formed is diperiodic and parallel to (100), and it has the {8 2 .10} point symbol and the KIa topological type. The same topology has previously been found in other uranyl ion complexes with elongated ligands such as glutarate, [START_REF] Borkowski | Crystal Engineering with the Uranyl Cation II. Mixed Aliphatic Carboxylate/Aromatic Pyridyl Coordination Polymers: Synthesis, Crystal Structures, and Sensitized Luminescence[END_REF] 1,13-tridecanedioate, [START_REF] Harrowfield | Plumbing the Uncertainties of Solvothermal Synthesis Involving Uranyl Ion Carboxylate Complexes[END_REF] and 1,15-pentadecanedioate. [START_REF] Thuéry | The crystalline ,-Dicarboxylate Metal Complex with the Longest Aliphatic Chain to Date: Uranyl 1,15-Pentadecanedioate[END_REF] The layers are very thick (22 Å) and they display two sets of Triperiodic framework. While the oxophilicity of U VI results in its strong bonding to carboxylato donors, the azophilicity of Ni II may explain the particular nature of the complex isolated when nitrilotriacetate (nta 3-) was used as another functionalized tricarboxylate. Thus, the asymmetric unit in the complex [(UO2)2(nta)2Ni2(tpyc)2] ( 14) contains one uranyl cation,

cyclohexanedicarboxylate (t-R-1,2-chdc 2-), [(UO2)4(t-R-1,2-chdc)4Ni2(tpyc)4] (
The uranium atom is chelated by one carboxylate group from nta 3-and bound to three more oxygen donors from two tpyc -and one nta number of coordination sites, and involvement in weak interactions, the roles of the polycarboxylate ligands are more diverse. The dicarboxylates c-1,2-chdc 2-, tdc 2-, bdc 2-and t-R-1,2-chdc 2-lead to the formation of U4Ni2 rings which are either discrete in the first two cases, or part of mono-or diperiodic coordination polymers in the two last cases, respectively. In these complexes, the dicarboxylate ligand is always 2-c, and thus a simple link, while uranium is 2-c if chelated by nitrate, or a 3-c node if not, which results in periodicity increase. Only in some of these cases are intra-ring TE interactions present, which shows that these are not required for ring stabilization. Similar rings are found with tca 3-, but this tricarboxylate ligand is a 3-c node, and a ring-based nanotubular assembly is formed. A closely related semi-cyclic arrangement is found with 1,2-pda 2-, which is part of a meander-like chain. That closed species are obtained with convergent ligands such as c-1,2-chdc 2-, t-R-1,2-chdc 2-and tca 3-is not particularly surprising since these ligands have previously been shown to give uranyl ion complexes crystallizing as polynuclear cage-like or nanotubular species, [START_REF] Thuéry | Variations on the Honeycomb Topology: from Triangular-and Square-Grooved Networks to Tubular Assemblies in Uranyl Tricarballylate Complexes[END_REF][START_REF] Thuéry | Uranyl Tricarballylate Triperiodic and Nanotubular Species. Counterion Control of Nanotube Diameter[END_REF][START_REF] Thuéry | Coordination Polymers and Cage-Containing Frameworks in Uranyl Ion Complexes with rac-and (1R,2R)-trans-1,2-Cyclohexanedicarboxylates: Consequences of Chirality[END_REF][START_REF] Thuéry | Tetrahedral and Cuboidal Clusters in Complexes of Uranyl and Alkali or Alkaline-Earth Metal Ions with rac-and (1R,2R)-trans-1,2-Cyclohexanedicarboxylate[END_REF][START_REF] Thuéry | Counterion-Controlled Formation of an Octanuclear Uranyl Cage with cis-1,2-Cyclohexanedicarboxylate Ligands[END_REF][START_REF] Ou | Recent Advances in Structures and Applications of Coordination Polymers Based on Cyclohexanepolycarboxylate Ligands[END_REF] the Ni(tpyc)2 connector introducing a larger separation between metal atoms. The formation of rings in the case of the divergent bdc 2-is more unusual since this ligand is akin to t-1,4-chdc 2-, which gives a simple chain under similar conditions; 33 the presence of CH interactions in the case of bdc 2- possibly plays a role here. The rigid 1,2,3-btc 3-ligand yields a complex in which uranium and nickel are in separated, charged entities. Separation of charges is also found with citH 3-, but here semi-interpenetration of monoperiodic, anionic uranyl citrate chains into a diperiodic, cationic sql network incorporating uranyl and Ni(tpyc)2 is observed. The formation of rather similar uranyl-based monoperiodic polymers with 1,2,3-btc 3-and citH 3-is in keeping with the previously noted relatedness of these two ligands in their uranyl ion complexes. [START_REF] Thuéry | Uranyl Ion Coordination by Benzene-1,3,5-tricarboxylate: Building Chains and Networks from Binuclear Bricks[END_REF] More usual hcb diperiodic networks are found with btcH 3-and cbtcH 3- In all but two instances (complexes 13 and 14), a degree of association or interlocking of Ni(tpyc)2 units is apparent which resembles that described as the "terpyridine embrace" although here it does not extend beyond pairwise or extended linear arrays. In complex 13, stacking does occur but it associates tpyc -and 1,3-pda 2-units, whereas in complex 14 the uranyl oxo interaction with tpyc -blocks the approach of other tpyc -units. In 5, tpyc -units have a small degree of facial overlap accompanied by facial overlaps with both the 1,2-pda 2-and 1,2-pdaH - units in a structure which illustrates well how multiple weak interactions attain a unique balance in every crystal. The 4ʹ-carboxylate group of course renders tpyc -a different stacking entity to 2,2;6ʹ,2-terpyridine itself and in all but two of the present structures (that of 13, where tpyc - self-association is disrupted by that with 1,3-pda 2-

CONCLUSION

We have reported the synthesis and crystal structure of 14 uranyl ion complexes involving the Ni(tpyc)2 zwitterionic assembler and a diverse set of polycarboxylate ligands intended to form neutral coordination polymers, an objective met in all but two cases in which separate charged species are obtained. The presence of the Ni(tpyc)2 unit in all but one of the present structures

shows that it can be used as a stable divergent dicarboxylate ligand, with a separation of 13 Å between the oxygen atoms of the two bonding sites, in varied coordination polymer systems and in particular can be employed to add variety to anionic polycarboxylate structures. One of its most notable properties is to organize most often into parallel arrangements with frequent occurrences of TE interactions. The use of di-, tri-and tetracarboxylate ligands displaying a large range of geometries has allowed the isolation of complexes with periodicities varying from zero to three and various geometrical or topological features. Under the particular solvohydrothermal conditions of synthesis employed, a preferred arrangement in this system appears to be in the form of U4Ni2 metallacycles, these being either discrete or part of mono-or diperiodic coordination polymers. Other metallacycles, U4Ni, U5Ni2 and U10Ni4, are also found

  ), as indicated by the Kitaigorodski packing index (KPI, calculated with PLATON 50 ) of 0.64. TE interactions are apparent both within the macrocycles [NiNi separation 8.6760(7) Å] and between those within the stacks down [100] [NiNi separation 8.9573(3) Å], with formation of two paralleldisplaced -stacking interactions [centroidcentroid distances, 3.6449(17) and 3.7821(16) Å; dihedral angles, 0.03(14) and 6.45(13)°] and three edge-to-face CH interactions [Hcentroid distances, 2.54-2.99 Å; C-Hcentroid angles, 134-170°]. The macrocycle stacks define channels that are occupied by the acetonitrile molecules, located close to the macrocycle cavity portal and involved in both CH(aromatic)N and CHO(carboxylate) hydrogen bonds, while adjacent stacks are closely packed as a result of both CH(aromatic)O(carboxylate) and CH(aromatic)O(nitrate) interactions as well as stacking of the nitrates with terpyridine rings.

Figure 1 .

 1 Figure 1. (a) View of compound 1. Displacement ellipsoids are drawn at the 50% probability level. Solvent molecules and hydrogen atoms are omitted. Symmetry code: i = 1 -x, 2 -y, 1 -z. (b, c) Two views of the packing with uranium coordination polyhedra yellow and those of nickel green.

Figure 2 .

 2 Figure 2. (a) View of compound 2. Displacement ellipsoids are drawn at the 50% probability level and hydrogen atoms are omitted. Symmetry code: i = 2 -x, 1 -y, 1 -z. (b, c) Two views of the packing with uranium coordination polyhedra yellow and those of nickel green.

Figure 3 .

 3 Figure 3. Space-filling representations of the rings in complexes 1 (a) and 2 (b) viewed down two orthogonal directions (uranium, orange; nickel, green; sulfur, yellow; oxygen, red; nitrogen, purple; carbon, blue; hydrogen atoms are omitted).

Figure 4 .Figure 5 .

 45 Figure 4. (a) View of compound 3. Displacement ellipsoids are drawn at the 50% probability level. The acetonitrile molecule and carbon-bound hydrogen atoms are omitted, and the hydrogen bond to a water molecule is shown as a dashed line. Symmetry codes: i = 2 -x, 1 -y, 1 -z; j = 1 -x, -y, -z. (b) View of the monoperiodic coordination polymer with uranium coordination polyhedra yellow and those of nickel green. (c) Packing with chains viewed sideways. (d) chain viewed down its axis. (e) Simplified representation showing the arrangement of chains (view down [101] with [010] horizontal; uranium, yellow; nickel, light blue; tpyc -ligands, dark blue; bdc 2-, red).

  directed along[010]. The projection of the tube along its axis looks rather similar to the ring in 1, although two uranyl cations are strongly tilted with respect to the ring plane. The relatively small size of the tca 3-bridge brings U1 and U2 of one ring within 8.5105(5) Å of one another, a significantly longer distance than the corresponding separation in complex 1, but the transannular UU distances of 18.8136(9) and 19.8134(9) Å are not far from those in the latter complex, the shape of the rings being somewhat distorted in 4. The intra-ring NiNi separation of 9.6747(17) Å does not allow for very significant intra-ring TE interactions, with however a possible CH contact [Hcentroid distance, 2.83 Å; C-Hcentroid angle, 120°], but one parallel-displaced -stacking interaction is found between adjoining rings with a NiNi separation of 8.6477(3) Å [centroidcentroid distance, 3.580(5) Å; dihedral angle, 5.0(4)°] as well as two CH interactions [Hcentroid distances, 2.83 and 2.93 Å; C-Hcentroid angles, 143 and 137°]. The tubes, having large apertures on their sides, pack in a slightly interdigitated fashion so as to give layers parallel to (100), as shown in Figure5d(KPI, 0.68), and the acetonitrile molecules are located within the tubes. Complex 4 belongs to the group of uranylbased nanotubular species, which includes several complexes with carboxylates55-62 and

  nodes, only uranium nodes are found in the phenylenediacetate complexes. Both species display an elongated, oblong section and wide side apertures, the [M(bipy)3] 2+ (M = Co, Ni, Zn, Ru; bipy = 2,2ʹ-bipyridine) counterions inserted within these spaces in the previous, anionic complexes being replaced by solvent molecules in the neutral complex 4. Another flexible aromatic dicarboxylic acid, 1,2-phenylenediacetate (1,2-pdaH2), gives the complex [(UO2)2(1,2-pda)(1,2-pdaH)Ni(tpyc)2(NO3)]CH3CN (5) shown in Figure 6. The

Figure 6 .

 6 Figure 6. (a) View of compound 5. Displacement ellipsoids are drawn at the 50% probability level. The solvent molecule and carbon-bound hydrogen atoms are omitted. Symmetry codes: i = 1 -x, y + 1/2, 1/2 -z; j = 1 -x, y -1/2, 1/2 -z. (b) Two views of the zigzag monoperiodic coordination polymer face-on or end-on, with uranium coordination polyhedra yellow and those of nickel green. (c) Packing with chains viewed end-on. (d) Simplified views showing the arrangement of chains, face-on or end-on (uranium, yellow; nickel, light blue; tpyc -ligands, dark blue; bridging 1,2-pda 2-ligands, red).

Figure 7 .

 7 Figure 7. (a) View of compound 6. Displacement ellipsoids are drawn at the 50% probability level and hydrogen atoms are omitted. Symmetry codes: i = 1 -x, 1 -y, -z; j = 3/2 -x, 1 -y, z; k = 1/2 -x, 1 -y, z. (b) View of the monoperiodic coordination polymer with uranium coordination polyhedra yellow and those of nickel green. (c) Packing with chains viewed end-on.

0. 62 )

 62 results in TE interactions between chains adjacent along [010] (NiNi distance, 8.3154(8) Å), with one parallel-displaced -stacking interaction [centroidcentroid distance, 3.611(3) Å; dihedral angle, 8.4(3)°] and one CH interaction [Hcentroid distance, 2.63 Å; C-Hcentroid angle, 149°].With the quite rigid 1,2,3-benzenetricarboxylate (1,2,3-btc 3-) ligand, a rather dramatic change in the nature of the product occurs, the complex [Ni(tpycH)(H2O)3][UO2(1,2,3btc)]22H2O (7) (Figure8) having an ionic composition. The two independent uranyl ions are in similar environments, each being chelated by two carboxylate groups from two 1,2,3-btc3- ligands forming two seven-membered rings and bound to a fifth carboxylate oxygen donor from a third ligand, thus giving a monoperiodic coordination polymer parallel to [10ī] of a form essentially identical to that known in association with various counterions or decorating cations.[START_REF] Thuéry | Uranyl Ion Coordination by Benzene-1,3,5-tricarboxylate: Building Chains and Networks from Binuclear Bricks[END_REF] This indication of a particular stability of the polymer may explain its resistance to incorporation of Ni(tpyc)2 and its crystallization in association with a cation of dipositive charge involving the undeprotonated form of tpycH. The polymeric anion chain has a step profile, such that the chains lie side by side in sheets parallel to (010) where aromatic ring stacking is involved to both sides. The cations form stacked pairs with significant overlap in projection of the terpy sections and reciprocal hydrogen bonding interactions of coordinated water with the tpycH carbonyl oxygen atom [OO, 2.711(4) Å; O-HO 152(5)°]. These pairs lie in sheets

Figure 8 .

 8 Figure 8. (a) View of compound 7. Displacement ellipsoids are drawn at the 50% probability level. Carbon-bound hydrogen atoms are omitted and hydrogen bonds are shown as dashed lines. Symmetry codes: i = 1 -x, -y, -z; j = -x, -y, 1 -z; k = x -1, y, z + 1; l = x + 1, y, z -1. (b) View of the monoperiodic coordination polymer. (c) Packing with uranium coordination polyhedra yellow and those of nickel green.

Hcit 3 -

 3 to the uncoordinated carboxylate oxygen atom of tpyc -[OO, 2.619(5) Å; O-HO 160(6)°]. The Ni(tpyc)2 units form undulating chains directed along [001] within the cationic sheets, in which TE interactions are found (NiNi distance, 8.7590(3) Å), as well as between units in different sheets (NiNi, 8.4733(2) Å) with in both cases one parallel-displaced stacking interaction [centroidcentroid distances, 3.906(3) and 3.495(2) Å; dihedral angles, 10.4(2) and 7.8(2)°] and one CH interaction [Hcentroid distances, 2.76 and 2.84 Å; C-Hcentroid angles, 147 and 145°]. Some voids are present in the packing (KPI, 0.61).

Figure 9 .

 9 Figure 9. (a) View of compound 8. Displacement ellipsoids are drawn at the 50% probability level. Carbon-bound hydrogen atoms are omitted and the hydrogen bond is shown as a dashed line. Only one position of the disordered atoms is shown. Symmetry codes: i = 1 -x, 1 -y, 1 -z; j = x + 1, y, z; k = -x, 1 -y, 1 -z; l = -x, y -1/2, 3/2 -z; m = -x, y + 1/2, 3/2 -z; n = -x, 1 -y, 2 -z; o = x, 1/2 -y, z + 1/2. (b) Diperiodic network viewed side-on and monoperiodic chains viewed end-on, with uranium coordination polyhedra yellow and those of nickel green (left), and slightly rotated simplified view (right; uranium, yellow; nickel, light blue; tpyc -ligands, dark blue; citH 3- ligands, red). (c) Packing with sheets viewed edge-on (left), and simplified view of the two assembled coordination polymers (right).

Figure 10 .

 10 Figure 10. (a) View of compound 9. Displacement ellipsoids are drawn at the 50% probability level and hydrogen atoms are omitted. Symmetry codes: i = x, 2 -y, z -1/2; j = x, y + 1, z; k = x, 2 -y, z + 1/2; l = x, y -1, z. (b) View of the diperiodic network with uranium coordination polyhedra yellow and those of nickel green. (c) Packing with layers viewed edge-on. (d) Simplified view of the network (uranium, yellow; nickel, light blue; tpyc -ligands, dark blue; btcH 3-ligands, red).

3 }Figure 11 .

 311 Figure 11. (a) View of compound 10. Displacement ellipsoids are drawn at the 50% probability level. Carbonbound hydrogen atoms are omitted and hydrogen bonds are shown as dashed lines. Only one position of the disordered atoms is shown. Symmetry codes: i = 1 -x, y + 1/2, 3/2 -z; j = x + 1, y, z + 1; k = x -1, y, z -1; l = 1 -x, y -1/2, 3/2 -z. (b) View of the diperiodic network with uranium coordination polyhedra yellow and those of nickel green. (c) Packing with layers viewed edge-on. (d) Simplified view of the network (uranium, yellow; nickel, light blue; tpyc -ligands, dark blue; cbtcH 3-ligands, red).

  been reported in uranyl-based complexes. When viewed down [010], the sheets are sawtoothshaped and terpy moieties pertaining to adjacent sheets are involved in a parallel-displaced stacking interaction [centroidcentroid distance, 3.632(6) Å; dihedral angle, 2.1(5)°], as well as a CH interaction [Hcentroid distance, 2.80 Å; C-Hcentroid angle, 146°], the distance between the nickel centres being 8.7884(6) Å. There are also intra-sheet double -stacking interactions between ndc 2-and two tpyc -moieties located on either side of it [centroidcentroid distances, 3.636(6) and 3.695(6) Å; dihedral angle, 11.0(5)° for both]. The KPI of 0.58 indicates the presence of large solvent-accessible voids.

Figure 12 .

 12 Figure 12. (a) View of compound 11. Displacement ellipsoids are drawn at the 50% probability level and carbonbound hydrogen atoms are omitted. Symmetry codes: i = 1 -x, 1 -y, -z; j = 2 -x, 3 -y, 1 -z; k = 1 -x, -y, -z. (b) View of the diperiodic network with uranium coordination polyhedra yellow and those of nickel green. (c) Packing with layers viewed edge-on. (d) View of one 14-nuclear ring. (e) Simplified view of the network (uranium, yellow; nickel, light blue; tpyc -ligands, dark blue; 2,6-ndc 2-ligands, red; hydroxide groups, orange).

Figure 13 .

 13 Figure 13. (a) View of compound 12. Displacement ellipsoids are drawn at the 30% probability level and hydrogen atoms are omitted. Only one position of the disordered atoms is shown. Symmetry codes: i = 1 -x, y, 3/2 -z; j = x -1/2, 1/2 -y, 1 -z; k = x + 1/2, 1/2 -y, 1 -z. (b) View of the diperiodic network with uranium coordination polyhedra yellow and those of nickel green (left), and slightly rotated simplified view (right; uranium, yellow; nickel, light blue; tpyc -ligands, dark blue; t-R-1,2-chdc 2-ligands, red). (c) Two layers viewed edge-on, with one intermediate layer omitted for clarity (left), and simplified view of four interlocked layers (right). (d) Another view of the packing with layers viewed edge-on.

UO2( 1 , 3 -

 13 pda) polymers on both surfaces, another set of such a polymer in the centre, and the bridging Ni(tpyc)2 units in between. The outermost UO2(1,3-pda) polymers, which include atoms U2, U4 and their symmetry equivalents, are chains with a square-wave profile running along [001], while the central polymers are zigzag or flattened helical chains directed along [010] and including U1 and U3. These two families of chains, with the lateral ones orthogonal to the central ones, are connected to one another by the Ni(tpyc)2 pillars, thus generating the diperiodic network. The KPI of 0.64 indicates that solvent molecules in excess of those found in the structure determination must be present (see Experimental Section), so that the role of hydrogen bonding interactions in the formation of the network is uncertain. There are no TE interactions here, the only parallel-displaced -stacking and CH interactions being between 1,3-pda 2-and tpyc -[centroidcentroid distances, 3.615(12)-3.855(14) Å; dihedral angles, 4-11°; Hcentroid distances, 2.90-2.97 Å; C-Hcentroid angles, 167-169°]. The -stacking interactions are simply pairwise for the central chains but they extend along [001] for the lateral chains, with the terpy aromatic rings involved inserted within the square wave part of the chains.

  , in which these ligands are 3-c nodes only due to the carboxylic group being uncoordinated, which prevents formation of a triperiodic framework. Other diperiodic networks, with the sql or KIa topologies, are formed with the dicarboxylates ndc 2-and 1,3-pda 2-acting as simple links. Due to the length of the Ni(tpyc)2 linkers, the rings formed in these species are generally very elongated, the maximum extent being found in the U10Ni5 rings in complex 11. The tricarboxylate nta 3-coordinates both uranium and nickel atoms, and is part of a Ni(nta)2 4-4-c node which, together with uranium and Ni(tpyc)2 4-c nodes, generates a triperiodic framework.

, and that of 7 ,

 7 where there are no Ni(tpyc)2 units) the O2CNiCO2 axes of the closest Ni(tpyc)2 pairs lie close to parallel. While details of the crystal formation mechanism are unknown, an [Ni(tpyc)2]2 pair with parallel axes places carboxylate group pairs with a separation ~7-8 Å, a distance compatible with bridging by various U-(bridging ligand)-U units, which might suggest the involvement of an embracing [Ni(tpyc)2]2 pair in the initial steps of formation of cyclic units, but not necessarily in their final form, as shown by the existence of such units devoid of intra-ring TE interactions.

Table 2 .

 2 Elemental Analysis Results and Yields

	Compound Solvent molecules	Yield (%)		Calculated (%)			Found (%)
				C	H	N	C	H	N
	1	4CH3CN, +10H2O 65	31.29	2.63	7.30	31.54	2.56	6.84
	2	+10H2O	57	29.73	2.10	7.30	29.76	2.04	6.95
	3		< 1
	4		56	33.53	2.34	8.38	32.50	2.13	7.79
	5	CH3CN, +10H2O	58	35.11	3.23	5.51	34.41	2.68	6.01
	6		< 1
	7		84	29.63	1.83	3.05	29.62	1.94	3.01
	8	+CH3CN, +5H2O	35	36.86	2.50	7.16	36.12	2.49	7.55
	9	+5H2O	80	31.31	2.43	6.39	31.74	2.36	6.91
	10		71	32.90	1.86	6.71	32.95	2.11	7.30
	11		< 1
	12	+30H2O	39	32.73	4.00	4.77	32.28	3.92	4.67
	13	+10H2O	24	38.25	3.00	5.47	38.52	2.83	6.04
	14	+10H2O	14	29.92	2.97	6.34	30.09	2.45	6.66

Table 3 .

 3 Crystal Data and Structure Refinement Details

		1	2	3	4	5	6	7
	chemical formula	C88H72N20Ni2O36U4 C76H44N16Ni2O36S2U4 C92H62N16Ni2O36U4 C42H35N9NiO19U2	C54H40N8NiO19U2	C36H20N6NiO16U2	C34H25N3NiO23U2
	M (g mol 1 )	3055.19	2890.93	3037.11	1504.56	1639.71	1327.35	1378.34
	cryst syst	triclinic	triclinic	triclinic	monoclinic	monoclinic	orthorhombic	triclinic
	space group	Pī	Pī	Pī	C2/c	P21/c	Pnna	Pī
	a (Å)	8.9573(3)	8.6617(3)	8.9799(2)	41.1941(15)	12.4860(6)	21.6394(5)	10.6595(6)
	b (Å)	17.0031(6)	15.9578(6)	15.2271(4)	8.6477(3)	15.7774(6)	14.7440(4)	12.4260(7)
	c (Å)	18.7001(7)	19.1458(7)	19.7173(5)	32.6050(11)	28.6893(11)	13.1652(3)	15.6728(8)
	 (deg)	76.7798(14)	104.8474(15)	72.1899(11)	90	90	90	78.465(3)
	 (deg)	79.6445(17)	90.2832(16)	84.8226(10)	123.2887(15)	101.992(2)	90	82.089(3)
	 (deg)	75.3637(14)	105.7054(15)	74.3191(11)	90	90	90	70.078(3)
	V (Å 3 )	2660.02(17)	2454.64(16)	2471.27(11)	9709.2(6)	5528.3(4)	4200.38(18)	1906.84(19)
	Z	1	1	1	8	4	4	2
	reflns collcd	214106	165265	155118	83642	108336	119365	151309
	indep reflns	13691	9329	12775	9218	10496	3990	9844
	obsd reflns [I > 2(I)] 12749	8822	11447	7580	8499	3375	9007
	Rint	0.050	0.045	0.057	0.068	0.090	0.088	0.057
	params refined	678	613	685	672	758	277	596
	R1	0.021	0.017	0.022	0.046	0.043	0.029	0.024
	wR2	0.054	0.041	0.046	0.110	0.084	0.055	0.056
	S	1.057	1.060	1.061	1.057	1.099	1.117	1.086
	min (e Å 3 )	0.88	0.85	1.68	2.09	1.72	1.22	1.43
	max (e Å 3 )	1.26	1.65	1.48	3.16	1.72	1.22	1.50
		8	9	10	11	12	13	14
	chemical formula	C76H50N12Ni2O28U3	C40H27N7NiO19U2	C40H27N7NiO20U2	C44H27N7NiO16U2	C96H80N12Ni2O32U4	C106H79N13Ni2O34U4 C44H32N8Ni2O20U2
	M (g mol 1 )	2410.79	1444.45	1460.45	1444.49	2983.26	3148.36	1586.25
	cryst syst	monoclinic	monoclinic	monoclinic	triclinic	orthorhombic	monoclinic	monoclinic
	space group	P21/c	C2/c	P21/c	Pī	C2221	Pc	C2/c
	a (Å)	8.4733(2)	23.3610(6)	24.0260(14)	8.7884(6)	20.8462(4)	24.3722(16)	9.9829(3)
	b (Å)	30.4417(6)	9.1845(2)	8.8432(5)	15.6675(11)	28.1089(5)	16.0289(9)	25.1302(8)
	c (Å)	16.7942(4)	46.5096(12)	25.4240(14)	19.8850(16)	45.8716(10)	14.2623(8)	21.9789(7)
	 (deg)	90	90	90	78.414(3)	90	90	90
	 (deg)	95.3601(12)	92.1370(13)	116.9437(16)	82.109(3)	90	90.626(2)	97.9405(18)
	 (deg)	90	90	90	80.438(2)	90	90	90
	V (Å 3 )	4312.98(17)	9972.1(4)	4815.4(5)	2629.6(3)	26879.1(9)	5571.4(6)	5461.0(3)
	Z	2	8	4	2	8	2	4
	reflns collcd	131445	96649	114065	22340	386454	133574	124005
	indep reflns	8202	9469	9132	9635	25507	21134	5192
	obsd reflns [I > 2(I)]	7319	8726	8114	5992	21708	18950	5020
	Rint	0.076	0.093	0.062	0.084	0.090	0.080	0.043
	params refined	590	622	658	631	1416	1442	345
	R1	0.030	0.049	0.029	0.055	0.034	0.069	0.022
	wR2	0.064	0.095	0.066	0.113	0.074	0.180	0.049
	S	1.071	1.268	1.085	0.965	1.058	1.058	1.159
	min (e Å 3 )	1.07	2.68	1.33	1.42	1.20	1.43	0.71
	max (e Å 3 )	2.63	1.59	1.46	1.61	1.47	3.79	1.92

  in which both geometries coexist. Uranium atoms are only bound to oxygen atoms from carboxylate or nitrate ligands, plus one instance of hydroxide anion in complex 11, and the bond lengths do not deviate in any remarkable way from their usual values

[U-O(oxo), 1.713(8)-1.795(17) Å, U-O(carboxylato), 2.379(13)-2.57(3) Å for  2 O,Oʹchelating groups, 2.240(5)-2.430(4) Å for the other groups; U-O(nitrato), 2.483(2)-2.536(4) Å; U-O(hydroxido), 2.311(7) and 2.336(7) Å]. The nickel(II) cations in the Ni(tpyc)2 moieties

  centrosymmetric, uncharged U4Ni2 ring which is now 44-membered. The intra-annular NiNi separation in 2, 12.5892(7) Å, is too great to produce a significant TE.[START_REF] Scudder | Crystal Supramolecular Motifs: Two-Dimensional Grids of terpy Embraces in [ML2] z Complexes (L = terpy or Aromatic N3-Tridentate Ligand)[END_REF][START_REF] Mcmurtrie | Engineering Grids of Metal Complexes: Development of the 2D M(terpy)2 Embrace Motif in Crystals[END_REF] Instead, there is such a fourfold embrace [NiNi 8.6617(3) Å] between terpy moieties in separate rings which form

	stacked arrays running down [100], with two parallel-displaced -stacking interactions
	[centroidcentroid distances, 3.8112(17) and 3.6357(18) Å; dihedral angles, 0 and 5.07(14)°]
	and two CH interactions [Hcentroid distances, 2.77 and 2.83 Å; C-Hcentroid angles,
	140 and 142°]. These interactions result in the columns being held together by two rows of TE
	interactions along their length. There is also a partial stacking overlap of terpy and thiophene

rings pertaining to adjacent ring columns, [centroidcentroid distance, 3.9175(17) Å; dihedral angle, 18.95(

14

)°] and a clear CH(terpy)O(nitrate) interaction extending the association of the metallacycles. Here also, the KPI of 0.60 indicates the presence of voids occupied by disordered solvent molecules.

Complexes 1 and 2 belong to the rather restricted family of uranyl-based metallacycles, with known examples involving (2R,3R,4S,5S)-tetrahydrofurantetracarboxylate,

[START_REF] Thuéry | Uranyl-Based Metallamacrocycles: Tri-and Tetranuclear Complexes with (2R,3R,4S,5S)-Tetrahydrofurantetracarboxylic Acid[END_REF]

Kemp's tricarboxylate,

[START_REF] Thuéry | Increasing Complexity in the Uranyl Ion-Kemp's Triacid System: From Oneand Two-Dimensional Polymers to Uranyl-Copper(II) Dodeca-and Hexadecanuclear Species[END_REF][START_REF] Thuéry | Cavity Formation in Uranyl Ion Complexes with Kemp's Tricarboxylate: Grooved Diperiodic Nets and Polynuclear Cages[END_REF]

1,3-adamantanediacetate,

Table 4 .

 4 The carboxylate group of tpyc -is bridging two uranium atoms in the 2- 1 O: 1 O' mode, while one carboxylate group of nta 3-, directed away from the nickel atom, is  2 O,O'chelating uranium, another is bridging uranium and nickel centres in the anti/anti 2- 1 O: 1 O' mode, and the last is monodentate on the same nickel atom, each nta 3-being thus bound to three metal cations. If Ni(tpyc)2 and Ni(nta)2 4-are considered as 4-c nodes, together with uranium atoms which are also 4-c nodes, the framework is 3-nodal and it has the {4 2 .8 3 .10}{4 2 .8 4 }3 point symbol. There are no TE interactions here, all distances between aromatic ring centroids being larger than 4.5 Å and NiNi distances larger than 9.9 Å. An interaction between the uranyl oxo Principal Characteristics of the Complexes Reported

	3-anions. Ni2 is in a trans-N2O4 octahedral

is thus nearly ubiquitous in this family of complexes. Due to the variety of their geometry, a 4-c refers to dinuclear nodes. b Connectivity of the Ni(nta)2 4-node.

independent uranium atoms in the asymmetric unit, all of them  2 O,O'-chelated by two carboxylate groups from two 1,3-pda 2-ligands and one from tpyc -(Figure 14). The uranium one Ni(tpyc)2 moiety with twofold rotation symmetry, and one nta 3-anion, as expected, plus a second Ni II cation located on an inversion centre and bound to two nta 3-anions (Figure 15). within mono-or diperiodic polymers, the last one displaying a high degree of elongation. While large heterometallacycles have been found in the structures of a variety of other uranyl ion coordination polymers, [START_REF] Thuéry | Recent Advances in Structural Studies of Heterometallic Uranyl-Containing Coordination Polymers and Polynuclear Closed Species[END_REF] the adoption of a roughly rectangular form is unique to the present species and may be a consequence of the long, rod-like nature of the Ni(tpyc)2 links.

Although Ni(tpyc)2 has proven to be useful in defining the prospects in combining a neutral, zwitterionic dicarboxylate with polycarboxylate anions for the formation of uranyl ion coordination polymers, it has the disadvantage that the strong absorption of Ni(tpyc)2 at the wavelengths useful for uranyl ion excitation leads to the quenching of uranyl ion emission.

Replacement of Ni II by Zn II may solve this problem but the fact that only a single triperiodic structure with rather small internal cavities has been found so far in the Ni II systems indicates that either larger zwitterions or larger coligands must be found in order to generate solids with cavities present favouring potential applications as photo-oxidation catalysts.

ASSOCIATED CONTENT Accession Codes


Juliette Karpiel

Pierre Lonchambon

Frédéric Dappozze

Nathalie Herlin

Chantal Guillard

Graphene/TiO2 photocatalysts synthesis by laser pyrolysis for ethylene production

Keywords: Photocatalysis, TiO2, Graphene, Ethylene

Graphene/TiO2 Photocatalysts Synthesis By Laser Pyrolysis For Ethylene Production will be filled in by SPEA11

Juliette Karpiel 1,2 , Pierre Lonchambon 1 , Frédéric Dappozze 2 , Nathalie Herlin 1 , Chantal Guillard 2 (1) University of Paris-Saclay, CEA,CNRS, NIMBE, Gif-sur-Yvette Cedex, France. (2) University of Lyon 1, IRCELYON, CNRS, Villeurbanne, France. juliette.karpiel@cea.fr

In an effort to produce alkenes in a low-cost and less energyconsuming way, this study presents the possibility of a photocatalytic process permitting to obtain ethylene from propionic acid. To this end, TiO2 and graphene/TiO2 photocatalysts were successfully synthetized by an original method, the laser pyrolysis technique. Consisting on an interaction between a CO2 laser beam with liquid/gaseous precursors, this process provides great flexibility in terms of crystallinity, nanoparticles size etc. The obtained nanocomposites were mostly composed of anatase and graphene could be detected with RAMAN spectroscopy. TiO2 and graphene/TiO2 were compared for ethylene photoproduction from propionic acid, and ethylene yield did not seem to be significantly improved despite graphene addition.

Alkenes, and more particularly ethylene C2H4, are essential organic molecules for chemical and petrochemical industries and are needed in ton quantities [1]. New ways to produce ethylene without hydrocarbons are developed. They mainly focus on ethanol dehydration reaction. However, despite the use of heterogeneous acid catalysts, these processes require high temperatures reaching 200-550°C and consequent pressures, at least 0.3 MPa and consequently correspond to significant energy consumption [2][3].

In this context, we aim to synthesize environmental-friendly and low-cost photocatalysts, which could degrade organic compounds such as alcohols and acids into alkanes and alkenes at ambient temperature and atmospheric pressure. Particularly, the outcome sought here is to promote high selectivity and yield towards ethylene photoproduction using UVA as energy source.

To achieve this goal, graphene modified TiO2 nanostructures appear to be promising photocatalysts for their high stability, excellent electron mobility and large specific surface area [4][5]. Thus, TiO2-based photocatalysts were synthesized from TTIP (Titanium Tetra Isopropoxide) precursor by an original gas-phase method, the CO2 laser pyrolysis technique. This one-step method consists on an interaction between an infrared laser beam with gaseous and/or aerosolized liquid precursors. Such process, existing at industrial scale, offers a great flexibility in obtaining small and homogeneous nanoparticles (5-60 nm size ranging), with controlled crystallinity and carbon content. Addition of graphene (Graphene Nano Pellets -GNP, 120-150 m 2 /g) in TTIP allowed obtaining graphene-modified TiO2 composites (from 0.04 to 2.00 wt% regarding TTIP). Nanopowders were thus synthetized with a production rate of several grams per hour.

Obtained graphene/TiO2 nanocomposites were mostly formed of anatase (>95 wt%), indicating a low-temperature laser pyrolysis flame preserving graphene nanosheets from heat destruction. Moreover, GNP graphene could be detected by RAMAN spectroscopy. Graphene/TiO2 specific surface area tends to increase with graphene content. These photocatalysts were first used for photooxidation of formic acid solutions. At low graphene loadings, results shew slight improvements in formic acid degradation compared to bare TiO2. Besides, the composites were also tested for alkene photoproduction from 1 vol% of propionic acid photoreduction. Subsequent photo-Kolbe reaction permits producing CO2, ethane, ethylene, butane and H2. Graphene/TiO2 exhibit no significant ethylene production enhancement whatever graphene content. For both photocatalysts graphene-TiO2 and associated TiO2 reference, ethylene selectivities and yields remained low.

To conclude, graphene/TiO2 photocatalysts were successfully synthetized by laser pyrolysis. Despite slight improvements for formic acid degradation at low graphene loadings, these nanocomposites presented no significant differences in results compared to pristine TiO2 for ethylene photo-


Genevieve Gesan-Guiziou
email: genevieve.gesan-guiziou@inrae.fr

Baerbel Husing
email: baerbel.huesing@isi.fraunhofer.de

Karin Ostergren
email: karin.ostergren@ri.se

Andreas Papadakis
email: papadakis@synelixis.com

Theodore Zahariadis
email: zahariad@synelixis.com

Challenges and working practices for the application of Blockchain in Intermediate Dairy Value Chains

Working Challenges (1/2)

Challenge #1: Which data to register, so that they are meaningful?

• Select high-level attributes (taste, origin, integrity, milk quality) and associate with operational parameters (temperatures and pH, microbiological checks, milk mixing and percentages, maturation duration) Thank you for your attention

  Blockchain as Trustworthy info sharing bus (2/2)

	Outline Objectives	Plan
	1. Context and Rationale	1. Identify product traits appreciated by consumers
	2. Objectives and Methodology	(e.g. origin, taste, ..)
	3. Challenges 4. Discussion ongoing / future work 1. Support end-to-end transparency and traceability 2. Bring breeders and dairy in closer relationship with consumers 3. Enhance consumer trust 4. Amplify the competitive advantages of the stakeholders 5. Support the creation of new, differentiated dairy products	Proposed 2. Identify operational parameters (e.g., pH, temp) and rules 3. Retrieve, store and validate data per production in Blockchain 4. Include data on environmental aspects, animal welfare, support of local sociate
		5. Allow for verification using
		the Blockchain (app)
		6. Provide methodology and
		tools, for an open,
		extensible system

  Scenario verified as useful by dairy and breeders, to 'close the loop' in the dairy chain and enabler for increased responsibility and 'due credit' o Status: Implementation ongoing (stable application for the stakeholders, under design for consumers) o Further adapt with additional/modified info related a) to specific productions or b) sustainability aspects (environmental, animal welfare, support of local economy).

	Discussion -Future Work
	Breeders (conditions, IoT, stable relationships, fair reimbursement)	Collection measurements (quantity, milk types, pH, temperature, physiological) microbiological,	Collection transport (mixing, quantities, types, cleaning -generation) residuals, LOT	Storage in dairy tanks (mixing, quantities, types, balance, dates of collections)
	Consumption (info validation)	Packaging (container generation, printing) types, final LOT	Production maturation measurements) duration, second maturation, phases (first and	Production initiation, (selection of milk types intermediate LOTs) centrifugation, pasteurization, /quantities, filtering,

o o Need to pilot (evaluate: process overhead, performance wrt Blockchain) o Work on business models (consumers appear willing to pay a premium price for trustworthy info)

FAIRCHAIN project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement n°101000723

Context and Rationale

Challenge #2, how to verify the cheese production procedure?

• Design a set of rules (e.g. value ranges for temperature and pH, milk mixing percentages, duration of maturation) based on the association of the operational values and the product traits • Implement verification in Blockchain (smart contracts) • Need to verify data at the edges (e.g. through patterns, anomaly detection) Challenge #3, how to rationalize Blockchain implications in terms of costs and performance?

• Heavy Blockchain data can affect performance and the transaction fees (depending also on the consensus algorithm) • Anchoring mechanism (from private to public Blockchains).

• Data granularity is an open question (trade-off needed).

Challenge #4, how to facilitate the involvement of stakeholders and build an application ecosystem?

• Facilitate usage on behalf of consumers (usage of QR)

• Offering of open APIs for new applications and scenarios e.g. public authorities for food certifications).


Houda Hafi
email: houda.hafi@univ-constantine2.dz

Wahabou Abdou
email: wahabou.abdou@u-bourgogne.fr

Salah Merniz
email: salah.merniz@univ-constantine2.dz

Impact of Concurrent Communications in Geographical Broadcasting Protocols for Vehicular Ad hoc Networks

Keywords: Vehicular ad hoc networks, IEEE 802.11p, Geographical broadcasting protocol, Segmentation, Waiting time adaptation

Bringing to the market intelligent vehicles is one of the current challenges faced by car manufacturers. These vehicles must be able to communicate in order to cooperate and be more effective. The issue of inter-vehicle communications is an active research topic. This paper proposes a reliable geographical broadcasting protocol which has a twofold goal: limiting the risk of interference and reducing the dissemination time. To achieve theses goals, two mechanisms are proposed. The first one divides the road (more precisely, each vehicle's coverage area) into several segments depending on the local density. Thereafter, the priority to relay a message is given to nodes that are in the farthest segment from the source node. The second mechanism allows to reduce the waiting time thanks to a periodic update process. This paper also analysis the performance of geographical broadcasting protocols in case of multiple simultaneous communications. The goal is to observe how these protocols behave when the radio channel becomes overloaded. The comparison study (in terms of packet loss and dissemination time) shows that the proposed protocol outperforms two other VANETs' broadcasting protocols.

I. INTRODUCTION

During the recent years, intelligent transport systems (ITS) have contributed to improve users' mobility. These systems are particularly interested in the safety and comfort of users. Some of the ITS specifically address issues related to intelligent vehicles: how to make them more autonomous and how to ensure the quality of communication between vehicles. This paper focuses on inter-vehicle communication issues.

Vehicles communicate using wireless technologies, including the IEEE 802.11p standard. The interconnection of vehicles forms a Vehicular ad hoc network (VANET). This network has three main objectives: improving road safety (eg. sending emergency messages), streamlining road traffic (preventing traffic jam) and providing more comfort (Internet access, online games, tourist attraction). These tasks are carried out using vehicle to road infrastructure communication (V2I), vehicle to vehicle communication (V2V), or vehicle to any other device communication (V2X). In V2V, each vehicle acts as a client and a relay (router) node. Indeed, to spread a message over a long distance, vehicles should collaborate and apply a relaying strategy between neighbors. The choice of the best relays in terms of number, quality, and reliability is not a trivial task. In high-density networks, too many relay nodes would increase the risk of interference, leading to the saturation of the bandwidth and a significant increase of the latency. Conversely, if there are not enough relay nodes in low-density networks the message may not be widely disseminated. In the literature, several works have proposed broadcast protocols to deal with this problem.

This paper introduces a novel geographic approach of broadcasting messages in VANETs. This method divides nodes' coverage area into segments and gives priority to farthest nodes. This novel segmentation method reacts to network density changes. Furthermore, it deals with the empty segment problem. The proposed approach also allows nodes to dynamically adjust their waiting time in order to shorten the dissemination time. This paper also aims to analyse the behaviour of geographic broadcasting protocols in VANETs when taking into account simultaneous communication (which may overload the radio channel).

The remaining of the paper is organized as follows. Section II presents the main families of broadcasting protocols. The proposed protocol and its main features are detailed in Section III . Sections IV and V give experimentation parameters and a comparative study of three broadcasting protocols. Section VI concludes this paper and points-out some future work.

II. RELATED WORK

Broadcasting consists in sending a message from one node to all other nodes within a network. In VANETs, wide dissemination of messages can only be ensured if some nodes relay the packets they receive. Moreover, the fact that nodes share the radio channel requires designing broadcasting strategies that minimize the risk of interference. This can be achieved by reducing the number of relays in high-density networks. This reduction should not lead to the interruption of the message propagation. Finding a good broadcasting strategy is complex in VANETs (in wireless ad hoc networks in general) because the decision to relay or not each message is taken in a decentralized way. This means none of the nodes have information on the overall network topology. Each decision is taken according to local information. Broadcasting protocols can be classified into deterministic and stochastic categories.

A. Deterministic methods

A broadcasting method is deterministic if its process is predictable. This category includes simple flooding and neighbor knowledge-based methods.

1) Simple flooding: it is the simplest broadcasting method. Every packet is relayed exactly once by each node. Any redundant copy of the packet received later is ignored. One drawback of this method is that it does not take into account the network density. In high-density networks, this method would generate many redundant copies of broadcasted packets, leading to the overuse of the radio resources.

2) Neighbor knowledge-based methods: they compare neighbor lists before relaying packets. Nodes exchange Hello packets in order to discover the local network topology and to build up their neighbor lists. The Flooding with Self Pruning [START_REF] Lim | Multicast tree construction and flooding in wireless ad hoc networks[END_REF] uses a 1-hop neighbor list. This list is inserted into the broadcast packets. This allows each receiver to compare its own list to the one included in this packet. If the lists are identical, the packet is dropped. Otherwise, the packet is relayed. Other methods such as Distributed Vehicular Broadcast (DV-CAST) [START_REF] Tonguz | Dv-cast: A distributed vehicular broadcast protocol for vehicular ad hoc networks[END_REF] and Least Common Neighbor (LCN) [START_REF] Yu | A selective flooding method for propagating emergency messages in vehicle safety communications[END_REF] also rely on 1-hop neighbor lists.

For static or low mobility networks, neighbor knowledgebased methods can achieve good performance. But in high mobility networks like VANETs, information about the neighbors become quickly inaccurate. Thus, this family of methods is hardly applicable for vehicular networks.

B. Stochastic methods

The stochastic methods statistically assess the gain that could be obtained if the packets are relayed by a given node. They include probabilistic scheme, counter-based and location-based methods.

1) Probabilistic methods: in these schemes, the receiver calculates a dissemination probability based on a defined parameter. For instance, in [START_REF] Wisitpongphan | Broadcast storm mitigation techniques in vehicular ad hoc networks[END_REF] the authors associate the forwarding probability of a node to its distance from the source so that the farthest node will have a high chance to rebroadcast the packet. In [START_REF] Wegener | Autocast: An adaptive data dissemination protocol for traffic information systems[END_REF], [START_REF] Mylonas | Speed adaptive probabilistic flooding for vehicular ad hoc networks[END_REF], [START_REF] Achour | Sead: A simple and efficient adaptive data dissemination protocol in vehicular ad-hoc networks[END_REF] the rebroadcasting probability depends on the node's local density, node's speed, and redundancy ratio respectively. In [START_REF] Lima | E-probt: a new approach to mitigate the broadcast storm problem in vanets[END_REF] the authors propose a protocol called E-ProbT that combines the number of common neighbors and the distance between the transmitter and the receiver to compute the probability of forwarding.

Smart-flooding [START_REF] Abdou | Designing smart adaptive flooding in manet using evolutionary algorithm[END_REF] aims to adapt the broadcasting probability to the local density. In addition to the probability, this protocol introduces three other parameters: the number of retransmissions for each packet, the delay between successive retransmissions and the TTL (time to live). To achieve good tuning of these parameters for various density levels, the authors have used a genetic algorithm.

2) Counter-based methods: they rely on a simple principle: the more a node receives copies of the same packet, the less likely it is useful to relay this packet. Upon reception of the first copy, the node initializes a counter C to 1 and sets a timeout RAD. During the waiting period, C is incremented upon reception of a new copy of the packet. When the RAD expires, C is compared to a threshold value C t . If C < C t , the packet is broadcasted. Otherwise, it is dropped. Like probabilistic methods, one challenge is to find an appropriate value of C t . Yassein et al. [START_REF] Bani Yassein | New counter based broadcast scheme using local neighborhood information in manets[END_REF] proposed the Smart Counter Based Broadcast Algorithm that adapts C t according to the network density. Thanks to Hello packets, the nodes build neighbor lists. The size of these lists allows to dynamically adjust C t . Karthikeyan et al. introduced in [START_REF] Karthikeyan | Optimum density based model for probabilistic flooding protocol in mobile ad hoc network[END_REF] a method named Density Based Flooding Algorithm. This method defines two categories of nodes according to their number of neighbors, with respect to a given threshold τ . Each node decides to relay each packet depending on its own category and the one of this packets last hop.

3) Geographical methods: they try to increase the additional coverage area that will result if the packet is forwarded. These methods do not consider whether nodes exist within that additional area or not. AckPBSM [START_REF] Ros | Reliable and efficient broadcasting in vehicular ad hoc networks[END_REF] and POCA [START_REF] Nakorn | Poca: Position-aware reliable broadcasting in vehicular ad-hoc networks[END_REF] use this approach and set lower RAD to nodes that are far from the source node. To evaluate the extra coverage area, the node can use the distance between itself and each node that has previously relayed the message (distance-based scheme) or the geographical coordinates (location-based scheme). In both distance-based and location-based schemes, a RAD timeout is set and the message is relayed if the additional coverage area is higher than a given threshold.

To resolve the broadcast storm problem [START_REF] Ni | The broadcast storm problem in a mobile ad hoc network[END_REF] and have a high reliability, authors in [START_REF] Korkmaz | Urban multihop broadcast protocol for inter-vehicle communication systems[END_REF], [START_REF] Fasolo | Smart broadcast algorithm for inter-vehicular communication[END_REF], [START_REF] Sahoo | Bpab: Binary partition assisted emergency broadcast protocol for vehicular ad hoc networks[END_REF] use the segment-based technique. They divide the road into multiple segments and vehicles within the farthest non-empty segment are in charge of relaying the packets. Its major disadvantage is the creation of empty segments since they are density-unaware methods.

III. A HYBRID SENDER AND RECEIVER ORIENTED BROADCASTING PROTOCOL

A broadcasting protocol is sender-oriented if the node that sends the packet chooses the relay nodes. For example, it can include the list of relay nodes in the packet header. In a receiver-oriented approach, each receiver node autonomously decides whether to relay the packet or not. It is worth noting that when packet propagation conditions are good, senderoriented protocols can give good results. However, if many packets are lost, it is possible that the relay nodes chosen by the source do not receive the packet. Such a situation could lead to the disruption of the broadcast. Therefore, when the risk of interference is high, the receiver-oriented approaches seem more interesting. This paper introduces a new novel broadcasting protocol that combines the sender-oriented and receiver-oriented methods. This protocol is named Segment-Delay Based Broadcasting Protocol (SDBP). SDBP is sender-oriented in the sense that the source helps the nodes with some information encapsulated in the packet header. On the other hand, it is a receiver-oriented protocol because each node acts autonomously after receiving the packet.

In order to ensure a wide dissemination of packets, efficient broadcasting protocols must take account of the network density. To this end, SDBP uses a segmentation method that relies on local density level and avoids creating empty segments. Furthermore, SDBP uses a dynamic adaptation of the waiting time for each node. SDBP is a geographical broadcasting protocol which consists of three main phases:

1) the neighbor discovery phase 2) the sending phase 3) the forwarding phase These key steps are illustrated on Figure 1. They are detailed in the Sections III-A to III-C. S e n d i n g p h a s e ( s o u r c e n o d e

A. Neighbor discovery phase

The neighbor discovery process is carried out by all nodes. This process is regularly repeated because the VANETs are highly mobile networks, resulting in frequent topology changes. SDBP uses periodic Hello packets to discover neighbors. These packets contain the following information: V ehicleID, P acketID, X-coordinate, Y -coordinate (see Figure 2). When a node receives a Hello packet, it adds the packet's source information into its neighborhood table. Each record in the table has an expiration time (to deal with inactive neighbors caused by a link failure, collision, etc.). This expiration time is three times the Hello packets frequency.

The neighborhood table will be used to estimate density and segment the sender's coverage area. The segmentation is performed before each sending or forwarding phase.

B. Sending phase

This phase corresponds to the initial sending of a packet. The source node efficiently chooses its neighbors that will relay the packet. Since SDBP seeks to avoid the drawbacks of sender-oriented protocols, it does not precisely identify the relays. Rather, it segments the network in such a way as to favor the farthest nodes. In literature, segmentation is generally done in a regular way. That means the source node's coverage area is divided into several segments of the same width. One of the disadvantages of this method is that it does not deal with the density (the number of nodes within the segments may vary). Moreover, if the nodes are not uniformly distributed, the farthest segments may be empty. This leads to the empty segment problem [START_REF] Wisitpongphan | Broadcast storm mitigation techniques in vehicular ad hoc networks[END_REF], [START_REF] Korkmaz | Urban multihop broadcast protocol for inter-vehicle communication systems[END_REF], [START_REF] Sahoo | Bpab: Binary partition assisted emergency broadcast protocol for vehicular ad hoc networks[END_REF]. SDBP introduces a novel approach that tackles this problem. In SDBP, the number of segment relies on the local density of each node (number of neighbors) and it does not merely depend on node's transmission range. In real world context, the transmission range is usually unknown since it is inconstant and varies swiftly depending on the signal propagation environment. SDBP divides the source node's coverage area into K segments, depending on the number of neighbors, say N (see Equation 1). N is estimated thanks to the neighborhood table (see Figure 1). All the segments contain the same number of nodes (M i ), except the farthest segment if N is not a multiple of K (see Equation 2).

K = ⌊ √ N ⌉ (1)
M i =          N K + (N mod K) if i=1 (the farthest segment) N K ∀i ∈ {2, 3, ..K} (2)
The boundaries of each segment are calculated using Equation 3. Figure 3 illustrates an example of the segmentation of the coverage area of vehicle S. Let us recall that the segmentation aims to reduce the interference and the dissemination time of packets. Therefore, priority of relaying messages should be given to nodes that are in the farthest segment. Consequently, each node must be aware of the segment it belongs to. For this purpose, the segments' boundaries are included in the packet header (see Figure 4).

               n i = (N + 1) - i j=1 M j Dmin i = n th i neighbor ′ s position Dmax i = F arthest neighbor ′ s position if i = 1 Dmin i-1 if i ∈ {2, 3, ..K} (3)

C. Forwarding phase

The forwarding phase starts when an intermediate node (say r) receives a packet. If it is the first copy of the packet, the node initialises a counter to compute the number of redundant copies. Then, it determines to which segment it belongs to and calculates its waiting time W T according to Equation 4.

W T r = i × SlotT ime × d(Source, F arthestN ode) d(r, Source) (4)
where i ∈ {1, 2, . . . , K} is the segment's number, d represents the distance between two nodes and SlotT ime is an approximate time for a packet to be thoroughly received by the other neighbors. In this work, we use the SlotT ime value proposed in [START_REF] Achour | Sead: A simple and efficient adaptive data dissemination protocol in vehicular ad-hoc networks[END_REF].

Afterwards, a timer is triggered which is the minimum between waiting time and SlotT ime. At the timer's expiration, four cases may be distinguished:

• If the node has received a redundant copy, the packet is dropped. • If no copy has been received by the node and its waiting time is expired (W T = 0), the node forwards the packet. • If the waiting time of the node has not expired yet and it is still greater than SlotT ime, the node updates its waiting time using the Equation 5. • Else, the minimum value between W T and SlotT ime is selected again and the process restarts. The waiting time adjustment helps hastening the dissemination process mostly in a network where there are many obstructions that prevent the good reception of packets. In actuality, when a node broadcasts a packet, it does not know which nodes will receive the packet and which ones will lose it. For instance, in a network where the message is lost by all nodes falling farther from the source, the nearest neighbors of the source must wait a lengthy time before broadcasting the message. This situation would result in an enormous increase of the end to end delay.

W T ′ r = W T r 2 (5)

IV. SIMULATION PARAMETERS AND PERFORMANCE

CRITERIA

A. Simulation parameters

This paper aims to study the performance of geographical broadcasting protocols, especially when there are several competing communications. This study is carried out using the Network Simulator 2 (NS2, version 2.35) [START_REF] Simulator | ns-2[END_REF]. We simulated a linear topology of 267 vehicles lined up over 8km. Vehicles' speeds vary from 40 km/h to 50 km/h (in the same direction). This scenario simulates mobility in an urban area.

Concurrent communications are simulated by increasing the number of packet sources. We start with 1% of the nodes that simultaneously send a packet. Then we gradually increase this value up to 25% simultaneous data traffic. The goal is to observe the behavior of the broadcasting protocols when the radio channel becomes overloaded. The simulation parameters are summarized in Table I.

To assess the performance of the protocols, we used the Nakagami radio propagation model. We tuned it to get a nondeterministic behavior in order to be as realistic as possible. The parameters of this propagation model are summarized in Table II.

B. Performance criteria

We compare two geographical broadcasting protocols: SDBP and FDP (Furthest Distance Protocol) [START_REF] Rehman | An adaptive relay nodes selection scheme for multi-hop broadcast in vanets[END_REF]. FDP is a sender-oriented distance-based dissemination technique where the twelve farthest nodes from the source are selected as forwarders. In addition to these two protocols, we present the results of the Simple Flooding Protocol (SFP), the well-known technique where nodes broadcast each received packet only once (the first time) without any waiting time. SFP is taken as a reference since it is the simplest broadcasting protocol.

In this study, we focus on the propagation of the message within a Zone Of Relevance (ZOR). Since in this paper we consider the dissemination of an emergency message in a VANET, the ZOR is defined as 2000m road section behind the source node. Indeed, the vehicles which present the greatest risk are those which are located in the vicinity of the vehicle that detects an unexpected road hazard.

The comparison study is carried out with respect to three performance metrics.

1) Unreachability ratio: the percentage of nodes in the ZOR that have not received the packet; 2) Number of dropped packets: number of packets discarded at the physical layer due to the following motives [START_REF] Hassan | IEEE 802.11 p performance evaluation in a city environment[END_REF]:

• Transmission Busy (TXB): this scenario occurs when a node in a transmission state receives another packet. Because a node can not send and receive at the same time, the packet which was supposed to be received will be dropped. The results presented in this section are the average values (of the total number of runs indicated in Table I) and the confidence interval (with a confidence level of 95%) for the performance metrics detailed in Section IV-B.

A. Packet loss

When an emergency message is broadcasted in a VANET, one of the first challenges is to insure that the packet will be received by all vehicles in the vicinity of the source node. A good reception in this area would limit the risk of accident or a multi-vehicle collision. Figure 5 shows the rate of vehicles that did not receive the message within the ZOR. When there are 1% source nodes, the three protocols fully cover the ZOR. Subsequently, the performance of SFP degrades very quickly. This behavior is due to the fact that all nodes relay the packets they receive. Therefore, in case of multiple competing data traffics, the risk of interference increases. Regarding the geographic protocols, we note that SDBP's results are not sensitive to the number of competing data flows. On the other hand, FDP's unreachability ratio increases with the number of sources. Indeed, since FDP uses an approach where the selection of relay nodes is done by the sources, this protocol is sometimes penalized when no selected node receives the message because of collision or mobility. It is worth noting that SDBP does not provide a list of potential relay nodes. Any neighbor could relay the packet, depending on the segment it belongs to. Packet losses within the ZOR can also be explained by their cause. Figure 6 shows that SDBP has a number of packet loss due to RXB slightly higher than FDP. This happens because SDBP reduces the waiting time in order to speed up the broadcast. The positive effects of this adaptation on the broadcasting time will be presented in Section V-B. The number of dropped packets due to RXB stabilizes. This shows that SDBP is able to adapt to the network load.

Usually, reducing the waiting time leads to overloading the network and to increasing the number of dropped packet. The most important thing to notice about the four histograms of Figure 6 is that the mechanism proposed by SDBP to update the wating time has a negligible effect on packet loss.

B. Dissemination time

Concerning the dissemination time (Figure 7), SFP's results are not relevant. Indeed, these low values are calculated on a small number of nodes, because of the low packet delivery ratio of this protocol. In general, the only nodes that receive the packet are 1 or 2 hops from the source. SDBP obtains a dissemination time which is generally not very sensitive to the number of source nodes because it uses a dynamic waiting time adjustment mechanism (which is not the case for FDP).

VI. CONCLUSION

This paper has introduced a new geographical broadcasting protocol for vehicular networks. This protocol, named SDBP, has two main mechanisms. The first mechanism segments the coverage area of each node depending on the local density level. It avoids the creation of empty segments. The second mechanism adjusts the waiting time in order to speed up the broadcasting process. The comparative study carried out shows that SDBP allows efficient packet delivery. It allows almost all vehicles within the zone of relevance to be reached in a very short time. The study also shows that method of reducing the waiting time introduced by SDBP has a negligible impact on the packet losses. As future work, we will investigate the benefits of the mechanisms proposed by SDBP on the number of relay nodes and packet redundancy. These two values will confirm the scalability of SDBP. It would also be interesting to adapt the waiting time with respect to the network density (not only to the segment the node belongs to).

) Phase 1 Fig. 1 .

 11 Fig. 1. Main steps of SDBP

Fig. 2 .

 2 Fig. 2. SDBP Hello Packet Header

Fig. 3 .

 3 Fig. 3. Example of the segmentation of a node's coverage area

Fig. 5 .

 5 Fig. 5. Unreachability ratio

Fig. 6 .Fig. 7 .

 67 Fig. 6. Number of dropped packets due to (a) TXB, (b) RXB, (c) SXB and (d) PXB in urban environment

•

  Reception Busy (RXB): in this case, a node receives a second packet while it is busy by receiving another packet. This situation will cause a collision.• Searching valid preamble (SXB): in this situation, the node which is in an Idle state drops the packet because it is searching for a valid preamble.

	• Receiving a frame preamble (PXB): as the previous
	case, the node is in an Idle state but it can not receive
	correctly the current packet because it is busy by
	receiving a valid preamble of another packet.
	3) Dissemination time: the required time in order that all
	nodes within the ZOR receive the packet;
	V. ANALYSIS OF THE IMPACT OF CONCURRENT
	COMMUNICATIONS

ACKNOWLEDGEMENT

This work was supported by the Algerian scholarship program PNE and the French Burgundy Regional Council.